HTTP Semantics
Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for
distributed, collaborative, hypertext information systems. This document describes
the overall architecture of HTTP, establishes common terminology, and defines aspects
of the protocol that are shared by all versions. In this definition are core protocol
elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier
(URI) schemes.
This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538,
7615, 7694, and portions of 7230.
Editorial Note
This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org),
which is archived at
Working Group information can be found at
; source code and issues list for this draft can be found at
The changes in this draft are summarized in
Appendix C.1
Status of This Memo
This Internet-Draft is submitted in full conformance with the provisions of BCP 78
and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF).
Note that other groups may also distribute working documents as Internet-Drafts. The
list of current Internet-Drafts is at
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Internet-Drafts as reference material or to cite them other than as “work in progress”.
This Internet-Draft will expire on May 5, 2023.
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1.
Introduction
1.1.
Purpose
The Hypertext Transfer Protocol (HTTP) is a family of stateless, application-level,
request/response protocols that share a generic interface, extensible semantics, and
self-descriptive messages to enable flexible interaction with network-based hypertext
information systems.
HTTP hides the details of how a service is implemented by presenting a uniform interface
to clients that is independent of the types of resources provided. Likewise, servers
do not need to be aware of each client's purpose: a request can be considered in isolation
rather than being associated with a specific type of client or a predetermined sequence
of application steps. This allows general-purpose implementations to be used effectively
in many different contexts, reduces interaction complexity, and enables independent
evolution over time.
HTTP is also designed for use as an intermediation protocol, wherein proxies and gateways
can translate non-HTTP information systems into a more generic interface.
One consequence of this flexibility is that the protocol cannot be defined in terms
of what occurs behind the interface. Instead, we are limited to defining the syntax
of communication, the intent of received communication, and the expected behavior
of recipients. If the communication is considered in isolation, then successful actions
ought to be reflected in corresponding changes to the observable interface provided
by servers. However, since multiple clients might act in parallel and perhaps at cross-purposes,
we cannot require that such changes be observable beyond the scope of a single response.
1.2.
History and Evolution
HTTP has been the primary information transfer protocol for the World Wide Web since
its introduction in 1990. It began as a trivial mechanism for low-latency requests,
with a single method (GET) to request transfer of a presumed hypertext document identified
by a given pathname. As the Web grew, HTTP was extended to enclose requests and responses
within messages, transfer arbitrary data formats using MIME-like media types, and
route requests through intermediaries. These protocols were eventually defined as
HTTP/0.9 and HTTP/1.0 (see
[HTTP/1.0]
).
HTTP/1.1 was designed to refine the protocol's features while retaining compatibility
with the existing text-based messaging syntax, improving its interoperability, scalability,
and robustness across the Internet. This included length-based data delimiters for
both fixed and dynamic (chunked) content, a consistent framework for content negotiation,
opaque validators for conditional requests, cache controls for better cache consistency,
range requests for partial updates, and default persistent connections. HTTP/1.1 was
introduced in 1995 and published on the Standards Track in 1997
[RFC2068]
, revised in 1999
[RFC2616]
, and revised again in 2014 (
[RFC7230]
through
[RFC7235]
).
HTTP/2 (
[HTTP/2]
) introduced a multiplexed session layer on top of the existing TLS and TCP protocols
for exchanging concurrent HTTP messages with efficient field compression and server
push. HTTP/3 (
[HTTP/3]
) provides greater independence for concurrent messages by using QUIC as a secure
multiplexed transport over UDP instead of TCP.
All three major versions of HTTP rely on the semantics defined by this document. They
have not obsoleted each other because each one has specific benefits and limitations
depending on the context of use. Implementations are expected to choose the most appropriate
transport and messaging syntax for their particular context.
This revision of HTTP separates the definition of semantics (this document) and caching
[CACHING]
) from the current HTTP/1.1 messaging syntax (
[HTTP/1.1]
) to allow each major protocol version to progress independently while referring to
the same core semantics.
1.3.
Core Semantics
HTTP provides a uniform interface for interacting with a resource (
Section 3.1
) — regardless of its type, nature, or implementation — by sending messages that manipulate
or transfer representations (
Section 3.2
).
Each message is either a request or a response. A client constructs request messages
that communicate its intentions and routes those messages toward an identified origin
server. A server listens for requests, parses each message received, interprets the
message semantics in relation to the identified target resource, and responds to that
request with one or more response messages. The client examines received responses
to see if its intentions were carried out, determining what to do next based on the
status codes and content received.
HTTP semantics include the intentions defined by each request method (
Section 9
), extensions to those semantics that might be described in request header fields,
status codes that describe the response (
Section 15
), and other control data and resource metadata that might be given in response fields.
Semantics also include representation metadata that describe how content is intended
to be interpreted by a recipient, request header fields that might influence content
selection, and the various selection algorithms that are collectively referred to
as
content negotiation
Section 12
).
1.4.
Specifications Obsoleted by This Document
Table 1
Title
Reference
See
HTTP Over TLS
[RFC2818]
B.1
HTTP/1.1 Message Syntax and Routing [*]
[RFC7230]
B.2
HTTP/1.1 Semantics and Content
[RFC7231]
B.3
HTTP/1.1 Conditional Requests
[RFC7232]
B.4
HTTP/1.1 Range Requests
[RFC7233]
B.5
HTTP/1.1 Authentication
[RFC7235]
B.6
HTTP Status Code 308 (Permanent Redirect)
[RFC7538]
B.7
HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields
[RFC7615]
B.8
HTTP Client-Initiated Content-Encoding
[RFC7694]
B.9
[*] This document only obsoletes the portions of
RFC 7230
that are independent of the HTTP/1.1 messaging syntax and connection management; the
remaining bits of
RFC 7230
are obsoleted by "HTTP/1.1"
[HTTP/1.1]
2.
Conformance
2.1.
Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234]
, extended with the notation for case-sensitivity in strings defined in
[RFC7405]
It also uses a list extension, defined in
Section 5.6.1
, that allows for compact definition of comma-separated lists using a "#" operator
(similar to how the "*" operator indicates repetition).
Appendix A
shows the collected grammar with all list operators expanded to standard ABNF notation.
As a convention, ABNF rule names prefixed with "obs-" denote obsolete grammar rules
that appear for historical reasons.
The following core rules are included by reference, as defined in
Appendix B.1
of
[RFC5234]
: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal
0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab),
LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible
US-ASCII character).
Section 5.6
defines some generic syntactic components for field values.
This specification uses the terms "character", "character encoding scheme", "charset",
and "protocol element" as they are defined in
[RFC6365]
2.2.
Requirements Notation
The key words "
MUST
", "
MUST NOT
", "
REQUIRED
", "
SHALL
", "
SHALL NOT
", "
SHOULD
", "
SHOULD NOT
", "
RECOMMENDED
", "
NOT RECOMMENDED
", "
MAY
", and "
OPTIONAL
" in this document are to be interpreted as described in BCP 14
[RFC2119]
[RFC8174]
when, and only when, they appear in all capitals, as shown here.
This specification targets conformance criteria according to the role of a participant
in HTTP communication. Hence, requirements are placed on senders, recipients, clients,
servers, user agents, intermediaries, origin servers, proxies, gateways, or caches,
depending on what behavior is being constrained by the requirement. Additional requirements
are placed on implementations, resource owners, and protocol element registrations
when they apply beyond the scope of a single communication.
The verb "generate" is used instead of "send" where a requirement applies only to
implementations that create the protocol element, rather than an implementation that
forwards a received element downstream.
An implementation is considered conformant if it complies with all of the requirements
associated with the roles it partakes in HTTP.
A sender
MUST NOT
generate protocol elements that do not match the grammar defined by the corresponding
ABNF rules. Within a given message, a sender
MUST NOT
generate protocol elements or syntax alternatives that are only allowed to be generated
by participants in other roles (i.e., a role that the sender does not have for that
message).
Conformance to HTTP includes both conformance to the particular messaging syntax of
the protocol version in use and conformance to the semantics of protocol elements
sent. For example, a client that claims conformance to HTTP/1.1 but fails to recognize
the features required of HTTP/1.1 recipients will fail to interoperate with servers
that adjust their responses in accordance with those claims. Features that reflect
user choices, such as content negotiation and user-selected extensions, can impact
application behavior beyond the protocol stream; sending protocol elements that inaccurately
reflect a user's choices will confuse the user and inhibit choice.
When an implementation fails semantic conformance, recipients of that implementation's
messages will eventually develop workarounds to adjust their behavior accordingly.
A recipient
MAY
employ such workarounds while remaining conformant to this protocol if the workarounds
are limited to the implementations at fault. For example, servers often scan portions
of the User-Agent field value, and user agents often scan the Server field value,
to adjust their own behavior with respect to known bugs or poorly chosen defaults.
2.3.
Length Requirements
A recipient
SHOULD
parse a received protocol element defensively, with only marginal expectations that
the element will conform to its ABNF grammar and fit within a reasonable buffer size.
HTTP does not have specific length limitations for many of its protocol elements because
the lengths that might be appropriate will vary widely, depending on the deployment
context and purpose of the implementation. Hence, interoperability between senders
and recipients depends on shared expectations regarding what is a reasonable length
for each protocol element. Furthermore, what is commonly understood to be a reasonable
length for some protocol elements has changed over the course of the past three decades
of HTTP use and is expected to continue changing in the future.
At a minimum, a recipient
MUST
be able to parse and process protocol element lengths that are at least as long as
the values that it generates for those same protocol elements in other messages. For
example, an origin server that publishes very long URI references to its own resources
needs to be able to parse and process those same references when received as a target
URI.
Many received protocol elements are only parsed to the extent necessary to identify
and forward that element downstream. For example, an intermediary might parse a received
field into its field name and field value components, but then forward the field without
further parsing inside the field value.
2.4.
Error Handling
A recipient
MUST
interpret a received protocol element according to the semantics defined for it by
this specification, including extensions to this specification, unless the recipient
has determined (through experience or configuration) that the sender incorrectly implements
what is implied by those semantics. For example, an origin server might disregard
the contents of a received
Accept-Encoding
header field if inspection of the
User-Agent
header field indicates a specific implementation version that is known to fail on
receipt of certain content codings.
Unless noted otherwise, a recipient
MAY
attempt to recover a usable protocol element from an invalid construct. HTTP does
not define specific error handling mechanisms except when they have a direct impact
on security, since different applications of the protocol require different error
handling strategies. For example, a Web browser might wish to transparently recover
from a response where the
Location
header field doesn't parse according to the ABNF, whereas a systems control client
might consider any form of error recovery to be dangerous.
Some requests can be automatically retried by a client in the event of an underlying
connection failure, as described in
Section 9.2.2
2.5.
Protocol Version
HTTP's version number consists of two decimal digits separated by a "." (period or
decimal point). The first digit (major version) indicates the messaging syntax, whereas
the second digit (minor version) indicates the highest minor version within that major
version to which the sender is conformant (able to understand for future communication).
While HTTP's core semantics don't change between protocol versions, their expression
"on the wire" can change, and so the HTTP version number changes when incompatible
changes are made to the wire format. Additionally, HTTP allows incremental, backwards-compatible
changes to be made to the protocol without changing its version through the use of
defined extension points (
Section 16
).
The protocol version as a whole indicates the sender's conformance with the set of
requirements laid out in that version's corresponding specification(s). For example,
the version "HTTP/1.1" is defined by the combined specifications of this document,
"HTTP Caching"
[CACHING]
, and "HTTP/1.1"
[HTTP/1.1]
HTTP's major version number is incremented when an incompatible message syntax is
introduced. The minor number is incremented when changes made to the protocol have
the effect of adding to the message semantics or implying additional capabilities
of the sender.
The minor version advertises the sender's communication capabilities even when the
sender is only using a backwards-compatible subset of the protocol, thereby letting
the recipient know that more advanced features can be used in response (by servers)
or in future requests (by clients).
When a major version of HTTP does not define any minor versions, the minor version
"0" is implied. The "0" is used when referring to that protocol within elements that
require a minor version identifier.
3.
Terminology and Core Concepts
HTTP was created for the World Wide Web (WWW) architecture and has evolved over time
to support the scalability needs of a worldwide hypertext system. Much of that architecture
is reflected in the terminology used to define HTTP.
3.1.
Resources
The target of an HTTP request is called a
resource
. HTTP does not limit the nature of a resource; it merely defines an interface that
might be used to interact with resources. Most resources are identified by a Uniform
Resource Identifier (URI), as described in
Section 4
One design goal of HTTP is to separate resource identification from request semantics,
which is made possible by vesting the request semantics in the request method (
Section 9
) and a few request-modifying header fields. A resource cannot treat a request in
a manner inconsistent with the semantics of the method of the request. For example,
though the URI of a resource might imply semantics that are not safe, a client can
expect the resource to avoid actions that are unsafe when processing a request with
a safe method (see
Section 9.2.1
).
HTTP relies upon the Uniform Resource Identifier (URI) standard
[URI]
to indicate the target resource (
Section 7.1
) and relationships between resources.
3.2.
Representations
representation
is information that is intended to reflect a past, current, or desired state of a
given resource, in a format that can be readily communicated via the protocol. A representation
consists of a set of representation metadata and a potentially unbounded stream of
representation data (
Section 8
).
HTTP allows "information hiding" behind its uniform interface by defining communication
with respect to a transferable representation of the resource state, rather than transferring
the resource itself. This allows the resource identified by a URI to be anything,
including temporal functions like "the current weather in Laguna Beach", while potentially
providing information that represents that resource at the time a message is generated
[REST]
The uniform interface is similar to a window through which one can observe and act
upon a thing only through the communication of messages to an independent actor on
the other side. A shared abstraction is needed to represent ("take the place of")
the current or desired state of that thing in our communications. When a representation
is hypertext, it can provide both a representation of the resource state and processing
instructions that help guide the recipient's future interactions.
target resource
might be provided with, or be capable of generating, multiple representations that
are each intended to reflect the resource's current state. An algorithm, usually based
on
content negotiation
Section 12
), would be used to select one of those representations as being most applicable to
a given request. This
selected representation
provides the data and metadata for evaluating conditional requests (
Section 13
) and constructing the content for
200 (OK)
206 (Partial Content)
, and
304 (Not Modified)
responses to GET (
Section 9.3.1
).
3.3.
Connections, Clients, and Servers
HTTP is a client/server protocol that operates over a reliable transport- or session-layer
connection
An HTTP
client
is a program that establishes a connection to a server for the purpose of sending
one or more HTTP requests. An HTTP
server
is a program that accepts connections in order to service HTTP requests by sending
HTTP responses.
The terms client and server refer only to the roles that these programs perform for
a particular connection. The same program might act as a client on some connections
and a server on others.
HTTP is defined as a stateless protocol, meaning that each request message's semantics
can be understood in isolation, and that the relationship between connections and
messages on them has no impact on the interpretation of those messages. For example,
a CONNECT request (
Section 9.3.6
) or a request with the Upgrade header field (
Section 7.8
) can occur at any time, not just in the first message on a connection. Many implementations
depend on HTTP's stateless design in order to reuse proxied connections or dynamically
load balance requests across multiple servers.
As a result, a server
MUST NOT
assume that two requests on the same connection are from the same user agent unless
the connection is secured and specific to that agent. Some non-standard HTTP extensions
(e.g.,
[RFC4559]
) have been known to violate this requirement, resulting in security and interoperability
problems.
3.4.
Messages
HTTP is a stateless request/response protocol for exchanging
messages
across a
connection
. The terms
sender
and
recipient
refer to any implementation that sends or receives a given message, respectively.
A client sends requests to a server in the form of a
request
message with a method (
Section 9
) and request target (
Section 7.1
). The request might also contain header fields (
Section 6.3
) for request modifiers, client information, and representation metadata, content
Section 6.4
) intended for processing in accordance with the method, and trailer fields (
Section 6.5
) to communicate information collected while sending the content.
A server responds to a client's request by sending one or more
response
messages, each including a status code (
Section 15
). The response might also contain header fields for server information, resource
metadata, and representation metadata, content to be interpreted in accordance with
the status code, and trailer fields to communicate information collected while sending
the content.
3.5.
User Agents
The term
user agent
refers to any of the various client programs that initiate a request.
The most familiar form of user agent is the general-purpose Web browser, but that's
only a small percentage of implementations. Other common user agents include spiders
(web-traversing robots), command-line tools, billboard screens, household appliances,
scales, light bulbs, firmware update scripts, mobile apps, and communication devices
in a multitude of shapes and sizes.
Being a user agent does not imply that there is a human user directly interacting
with the software agent at the time of a request. In many cases, a user agent is installed
or configured to run in the background and save its results for later inspection (or
save only a subset of those results that might be interesting or erroneous). Spiders,
for example, are typically given a start URI and configured to follow certain behavior
while crawling the Web as a hypertext graph.
Many user agents cannot, or choose not to, make interactive suggestions to their user
or provide adequate warning for security or privacy concerns. In the few cases where
this specification requires reporting of errors to the user, it is acceptable for
such reporting to only be observable in an error console or log file. Likewise, requirements
that an automated action be confirmed by the user before proceeding might be met via
advance configuration choices, run-time options, or simple avoidance of the unsafe
action; confirmation does not imply any specific user interface or interruption of
normal processing if the user has already made that choice.
3.6.
Origin Server
The term
origin server
refers to a program that can originate authoritative responses for a given target
resource.
The most familiar form of origin server are large public websites. However, like user
agents being equated with browsers, it is easy to be misled into thinking that all
origin servers are alike. Common origin servers also include home automation units,
configurable networking components, office machines, autonomous robots, news feeds,
traffic cameras, real-time ad selectors, and video-on-demand platforms.
Most HTTP communication consists of a retrieval request (GET) for a representation
of some resource identified by a URI. In the simplest case, this might be accomplished
via a single bidirectional connection (===) between the user agent (UA) and the origin
server (O).
request >
UA
=======================================
< response
3.7.
Intermediaries
HTTP enables the use of intermediaries to satisfy requests through a chain of connections.
There are three common forms of HTTP
intermediary
: proxy, gateway, and tunnel. In some cases, a single intermediary might act as an
origin server, proxy, gateway, or tunnel, switching behavior based on the nature of
each request.
> > > >
UA
===========
===========
===========
===========
< < < <
The figure above shows three intermediaries (A, B, and C) between the user agent and
origin server. A request or response message that travels the whole chain will pass
through four separate connections. Some HTTP communication options might apply only
to the connection with the nearest, non-tunnel neighbor, only to the endpoints of
the chain, or to all connections along the chain. Although the diagram is linear,
each participant might be engaged in multiple, simultaneous communications. For example,
B might be receiving requests from many clients other than A, and/or forwarding requests
to servers other than C, at the same time that it is handling A's request. Likewise,
later requests might be sent through a different path of connections, often based
on dynamic configuration for load balancing.
The terms
upstream
and
downstream
are used to describe directional requirements in relation to the message flow: all
messages flow from upstream to downstream. The terms
inbound
and
outbound
are used to describe directional requirements in relation to the request route: inbound
means "toward the origin server", whereas outbound means "toward the user agent".
proxy
is a message-forwarding agent that is chosen by the client, usually via local configuration
rules, to receive requests for some type(s) of absolute URI and attempt to satisfy
those requests via translation through the HTTP interface. Some translations are minimal,
such as for proxy requests for "http" URIs, whereas other requests might require translation
to and from entirely different application-level protocols. Proxies are often used
to group an organization's HTTP requests through a common intermediary for the sake
of security services, annotation services, or shared caching. Some proxies are designed
to apply transformations to selected messages or content while they are being forwarded,
as described in
Section 7.7
gateway
(a.k.a.
reverse proxy
) is an intermediary that acts as an origin server for the outbound connection but
translates received requests and forwards them inbound to another server or servers.
Gateways are often used to encapsulate legacy or untrusted information services, to
improve server performance through
accelerator
caching, and to enable partitioning or load balancing of HTTP services across multiple
machines.
All HTTP requirements applicable to an origin server also apply to the outbound communication
of a gateway. A gateway communicates with inbound servers using any protocol that
it desires, including private extensions to HTTP that are outside the scope of this
specification. However, an HTTP-to-HTTP gateway that wishes to interoperate with third-party
HTTP servers needs to conform to user agent requirements on the gateway's inbound
connection.
tunnel
acts as a blind relay between two connections without changing the messages. Once
active, a tunnel is not considered a party to the HTTP communication, though the tunnel
might have been initiated by an HTTP request. A tunnel ceases to exist when both ends
of the relayed connection are closed. Tunnels are used to extend a virtual connection
through an intermediary, such as when Transport Layer Security (TLS,
[TLS13]
) is used to establish confidential communication through a shared firewall proxy.
The above categories for intermediary only consider those acting as participants in
the HTTP communication. There are also intermediaries that can act on lower layers
of the network protocol stack, filtering or redirecting HTTP traffic without the knowledge
or permission of message senders. Network intermediaries are indistinguishable (at
a protocol level) from an on-path attacker, often introducing security flaws or interoperability
problems due to mistakenly violating HTTP semantics.
For example, an
interception proxy
[RFC3040]
(also commonly known as a
transparent proxy
[RFC1919]
) differs from an HTTP proxy because it is not chosen by the client. Instead, an interception
proxy filters or redirects outgoing TCP port 80 packets (and occasionally other common
port traffic). Interception proxies are commonly found on public network access points,
as a means of enforcing account subscription prior to allowing use of non-local Internet
services, and within corporate firewalls to enforce network usage policies.
3.8.
Caches
cache
is a local store of previous response messages and the subsystem that controls its
message storage, retrieval, and deletion. A cache stores cacheable responses in order
to reduce the response time and network bandwidth consumption on future, equivalent
requests. Any client or server
MAY
employ a cache, though a cache cannot be used while acting as a tunnel.
The effect of a cache is that the request/response chain is shortened if one of the
participants along the chain has a cached response applicable to that request. The
following illustrates the resulting chain if B has a cached copy of an earlier response
from O (via C) for a request that has not been cached by UA or A.
> >
UA
===========
===========
- - - - - -
- - - - - -
< <
A response is
cacheable
if a cache is allowed to store a copy of the response message for use in answering
subsequent requests. Even when a response is cacheable, there might be additional
constraints placed by the client or by the origin server on when that cached response
can be used for a particular request. HTTP requirements for cache behavior and cacheable
responses are defined in
[CACHING]
There is a wide variety of architectures and configurations of caches deployed across
the World Wide Web and inside large organizations. These include national hierarchies
of proxy caches to save bandwidth and reduce latency, content delivery networks that
use gateway caching to optimize regional and global distribution of popular sites,
collaborative systems that broadcast or multicast cache entries, archives of pre-fetched
cache entries for use in off-line or high-latency environments, and so on.
3.9.
Example Message Exchange
The following example illustrates a typical HTTP/1.1 message exchange for a GET request
Section 9.3.1
) on the URI "http://www.example.com/hello.txt":
Client request:
GET /hello.txt HTTP/1.1
User-Agent: curl/7.64.1
Host: www.example.com
Accept-Language: en, mi
Server response:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes
Content-Length: 51
Vary: Accept-Encoding
Content-Type: text/plain
Hello World! My content includes a trailing CRLF.
4.
Identifiers in HTTP
Uniform Resource Identifiers (URIs)
[URI]
are used throughout HTTP as the means for identifying resources (
Section 3.1
).
4.1.
URI References
URI references are used to target requests, indicate redirects, and define relationships.
The definitions of "URI-reference", "absolute-URI", "relative-part", "authority",
"port", "host", "path-abempty", "segment", and "query" are adopted from the URI generic
syntax. An "absolute-path" rule is defined for protocol elements that can contain
a non-empty path component. (This rule differs slightly from the path-abempty rule
of RFC 3986, which allows for an empty path, and path-absolute rule, which does not
allow paths that begin with "//".) A "partial-URI" rule is defined for protocol elements
that can contain a relative URI but not a fragment component.
URI-reference
=
Section 4.1
absolute-URI
=
Section 4.3
relative-part
=
Section 4.2
authority
=
Section 3.2
uri-host
=
Section 3.2.2
port
=
Section 3.2.3
path-abempty
=
Section 3.3
segment
=
Section 3.3
query
=
Section 3.4
absolute-path
= 1*( "/" segment )
partial-URI
= relative-part [ "?" query ]
Each protocol element in HTTP that allows a URI reference will indicate in its ABNF
production whether the element allows any form of reference (URI-reference), only
a URI in absolute form (absolute-URI), only the path and optional query components
(partial-URI), or some combination of the above. Unless otherwise indicated, URI references
are parsed relative to the target URI (
Section 7.1
).
It is
RECOMMENDED
that all senders and recipients support, at a minimum, URIs with lengths of 8000 octets
in protocol elements. Note that this implies some structures and on-wire representations
(for example, the request line in HTTP/1.1) will necessarily be larger in some cases.
4.2.
HTTP-Related URI Schemes
IANA maintains the registry of URI Schemes
[BCP35]
at
. Although requests might target any URI scheme, the following schemes are inherent
to HTTP servers:
Table 2
URI Scheme
Description
Section
http
Hypertext Transfer Protocol
4.2.1
https
Hypertext Transfer Protocol Secure
4.2.2
Note that the presence of an "http" or "https" URI does not imply that there is always
an HTTP server at the identified origin listening for connections. Anyone can mint
a URI, whether or not a server exists and whether or not that server currently maps
that identifier to a resource. The delegated nature of registered names and IP addresses
creates a federated namespace whether or not an HTTP server is present.
4.2.1.
http URI Scheme
The "http" URI scheme is hereby defined for minting identifiers within the hierarchical
namespace governed by a potential HTTP origin server listening for TCP (
[TCP]
) connections on a given port.
http-URI
= "http" "://"
authority
path-abempty
[ "?"
query
The origin server for an "http" URI is identified by the
authority
component, which includes a host identifier (
[URI]
Section 3.2.2
) and optional port number (
[URI]
Section 3.2.3
). If the port subcomponent is empty or not given, TCP port 80 (the reserved port
for WWW services) is the default. The origin determines who has the right to respond
authoritatively to requests that target the identified resource, as defined in
Section 4.3.2
A sender
MUST NOT
generate an "http" URI with an empty host identifier. A recipient that processes such
a URI reference
MUST
reject it as invalid.
The hierarchical path component and optional query component identify the target resource
within that origin server's namespace.
4.2.2.
https URI Scheme
The "https" URI scheme is hereby defined for minting identifiers within the hierarchical
namespace governed by a potential origin server listening for TCP connections on a
given port and capable of establishing a TLS (
[TLS13]
) connection that has been secured for HTTP communication. In this context,
secured
specifically means that the server has been authenticated as acting on behalf of the
identified authority and all HTTP communication with that server has confidentiality
and integrity protection that is acceptable to both client and server.
https-URI
= "https" "://"
authority
path-abempty
[ "?"
query
The origin server for an "https" URI is identified by the
authority
component, which includes a host identifier (
[URI]
Section 3.2.2
) and optional port number (
[URI]
Section 3.2.3
). If the port subcomponent is empty or not given, TCP port 443 (the reserved port
for HTTP over TLS) is the default. The origin determines who has the right to respond
authoritatively to requests that target the identified resource, as defined in
Section 4.3.3
A sender
MUST NOT
generate an "https" URI with an empty host identifier. A recipient that processes
such a URI reference
MUST
reject it as invalid.
The hierarchical path component and optional query component identify the target resource
within that origin server's namespace.
A client
MUST
ensure that its HTTP requests for an "https" resource are secured, prior to being
communicated, and that it only accepts secured responses to those requests. Note that
the definition of what cryptographic mechanisms are acceptable to client and server
are usually negotiated and can change over time.
Resources made available via the "https" scheme have no shared identity with the "http"
scheme. They are distinct origins with separate namespaces. However, extensions to
HTTP that are defined as applying to all origins with the same host, such as the Cookie
protocol
[COOKIE]
, allow information set by one service to impact communication with other services
within a matching group of host domains. Such extensions ought to be designed with
great care to prevent information obtained from a secured connection being inadvertently
exchanged within an unsecured context.
4.2.3.
http(s) Normalization and Comparison
URIs with an "http" or "https" scheme are normalized and compared according to the
methods defined in
Section 6
of
[URI]
, using the defaults described above for each scheme.
HTTP does not require the use of a specific method for determining equivalence. For
example, a cache key might be compared as a simple string, after syntax-based normalization,
or after scheme-based normalization.
Scheme-based normalization (
Section 6.2.3
of
[URI]
) of "http" and "https" URIs involves the following additional rules:
If the port is equal to the default port for a scheme, the normal form is to omit
the port subcomponent.
When not being used as the target of an OPTIONS request, an empty path component is
equivalent to an absolute path of "/", so the normal form is to provide a path of
"/" instead.
The scheme and host are case-insensitive and normally provided in lowercase; all other
components are compared in a case-sensitive manner.
Characters other than those in the "reserved" set are equivalent to their percent-encoded
octets: the normal form is to not encode them (see Sections
2.1
and
2.2
of
[URI]
).
For example, the following three URIs are equivalent:
Two HTTP URIs that are equivalent after normalization (using any method) can be assumed
to identify the same resource, and any HTTP component
MAY
perform normalization. As a result, distinct resources
SHOULD NOT
be identified by HTTP URIs that are equivalent after normalization (using any method
defined in
Section 6.2
of
[URI]
).
4.2.4.
Deprecation of userinfo in http(s) URIs
The URI generic syntax for authority also includes a userinfo subcomponent (
[URI]
Section 3.2.1
) for including user authentication information in the URI. In that subcomponent,
the use of the format "user:password" is deprecated.
Some implementations make use of the userinfo component for internal configuration
of authentication information, such as within command invocation options, configuration
files, or bookmark lists, even though such usage might expose a user identifier or
password.
A sender
MUST NOT
generate the userinfo subcomponent (and its "@" delimiter) when an "http" or "https"
URI reference is generated within a message as a target URI or field value.
Before making use of an "http" or "https" URI reference received from an untrusted
source, a recipient
SHOULD
parse for userinfo and treat its presence as an error; it is likely being used to
obscure the authority for the sake of phishing attacks.
4.2.5.
http(s) References with Fragment Identifiers
Fragment identifiers allow for indirect identification of a secondary resource, independent
of the URI scheme, as defined in
Section 3.5
of
[URI]
. Some protocol elements that refer to a URI allow inclusion of a fragment, while
others do not. They are distinguished by use of the ABNF rule for elements where fragment
is allowed; otherwise, a specific rule that excludes fragments is used.
Note:
The fragment identifier component is not part of the scheme definition for a URI scheme
(see
Section 4.3
of
[URI]
), thus does not appear in the ABNF definitions for the "http" and "https" URI schemes
above.
4.3.
Authoritative Access
Authoritative access refers to dereferencing a given identifier, for the sake of access
to the identified resource, in a way that the client believes is authoritative (controlled
by the resource owner). The process for determining whether access is granted is defined
by the URI scheme and often uses data within the URI components, such as the authority
component when the generic syntax is used. However, authoritative access is not limited
to the identified mechanism.
Section 4.3.1
defines the concept of an origin as an aid to such uses, and the subsequent subsections
explain how to establish that a peer has the authority to represent an origin.
See
Section 17.1
for security considerations related to establishing authority.
4.3.1.
URI Origin
The
origin
for a given URI is the triple of scheme, host, and port after normalizing the scheme
and host to lowercase and normalizing the port to remove any leading zeros. If port
is elided from the URI, the default port for that scheme is used. For example, the
URI
would have the origin
{ "https", "example.com", "443" }
which can also be described as the normalized URI prefix with port always present:
Each origin defines its own namespace and controls how identifiers within that namespace
are mapped to resources. In turn, how the origin responds to valid requests, consistently
over time, determines the semantics that users will associate with a URI, and the
usefulness of those semantics is what ultimately transforms these mechanisms into
a resource for users to reference and access in the future.
Two origins are distinct if they differ in scheme, host, or port. Even when it can
be verified that the same entity controls two distinct origins, the two namespaces
under those origins are distinct unless explicitly aliased by a server authoritative
for that origin.
Origin is also used within HTML and related Web protocols, beyond the scope of this
document, as described in
[RFC6454]
4.3.2.
http Origins
Although HTTP is independent of the transport protocol, the "http" scheme (
Section 4.2.1
) is specific to associating authority with whomever controls the origin server listening
for TCP connections on the indicated port of whatever host is identified within the
authority component. This is a very weak sense of authority because it depends on
both client-specific name resolution mechanisms and communication that might not be
secured from an on-path attacker. Nevertheless, it is a sufficient minimum for binding
"http" identifiers to an origin server for consistent resolution within a trusted
environment.
If the host identifier is provided as an IP address, the origin server is the listener
(if any) on the indicated TCP port at that IP address. If host is a registered name,
the registered name is an indirect identifier for use with a name resolution service,
such as DNS, to find an address for an appropriate origin server.
When an "http" URI is used within a context that calls for access to the indicated
resource, a client
MAY
attempt access by resolving the host identifier to an IP address, establishing a TCP
connection to that address on the indicated port, and sending over that connection
an HTTP request message containing a request target that matches the client's target
URI (
Section 7.1
).
If the server responds to such a request with a non-interim HTTP response message,
as described in
Section 15
, then that response is considered an authoritative answer to the client's request.
Note, however, that the above is not the only means for obtaining an authoritative
response, nor does it imply that an authoritative response is always necessary (see
[CACHING]
). For example, the Alt-Svc header field
[ALTSVC]
allows an origin server to identify other services that are also authoritative for
that origin. Access to "http" identified resources might also be provided by protocols
outside the scope of this document.
4.3.3.
https Origins
The "https" scheme (
Section 4.2.2
) associates authority based on the ability of a server to use the private key corresponding
to a certificate that the client considers to be trustworthy for the identified origin
server. The client usually relies upon a chain of trust, conveyed from some prearranged
or configured trust anchor, to deem a certificate trustworthy (
Section 4.3.4
).
In HTTP/1.1 and earlier, a client will only attribute authority to a server when they
are communicating over a successfully established and secured connection specifically
to that URI origin's host. The connection establishment and certificate verification
are used as proof of authority.
In HTTP/2 and HTTP/3, a client will attribute authority to a server when they are
communicating over a successfully established and secured connection if the URI origin's
host matches any of the hosts present in the server's certificate and the client believes
that it could open a connection to that host for that URI. In practice, a client will
make a DNS query to check that the origin's host contains the same server IP address
as the established connection. This restriction can be removed by the origin server
sending an equivalent ORIGIN frame
[RFC8336]
The request target's host and port value are passed within each HTTP request, identifying
the origin and distinguishing it from other namespaces that might be controlled by
the same server (
Section 7.2
). It is the origin's responsibility to ensure that any services provided with control
over its certificate's private key are equally responsible for managing the corresponding
"https" namespaces or at least prepared to reject requests that appear to have been
misdirected (
Section 7.4
).
An origin server might be unwilling to process requests for certain target URIs even
when they have the authority to do so. For example, when a host operates distinct
services on different ports (e.g., 443 and 8000), checking the target URI at the origin
server is necessary (even after the connection has been secured) because a network
attacker might cause connections for one port to be received at some other port. Failing
to check the target URI might allow such an attacker to replace a response to one
target URI (e.g., "https://example.com/foo") with a seemingly authoritative response
from the other port (e.g., "https://example.com:8000/foo").
Note that the "https" scheme does not rely on TCP and the connected port number for
associating authority, since both are outside the secured communication and thus cannot
be trusted as definitive. Hence, the HTTP communication might take place over any
channel that has been secured, as defined in
Section 4.2.2
, including protocols that don't use TCP.
When an "https" URI is used within a context that calls for access to the indicated
resource, a client
MAY
attempt access by resolving the host identifier to an IP address, establishing a TCP
connection to that address on the indicated port, securing the connection end-to-end
by successfully initiating TLS over TCP with confidentiality and integrity protection,
and sending over that connection an HTTP request message containing a request target
that matches the client's target URI (
Section 7.1
).
If the server responds to such a request with a non-interim HTTP response message,
as described in
Section 15
, then that response is considered an authoritative answer to the client's request.
Note, however, that the above is not the only means for obtaining an authoritative
response, nor does it imply that an authoritative response is always necessary (see
[CACHING]
).
4.3.4.
https Certificate Verification
To establish a
secured
connection to dereference a URI, a client
MUST
verify that the service's identity is an acceptable match for the URI's origin server.
Certificate verification is used to prevent server impersonation by an on-path attacker
or by an attacker that controls name resolution. This process requires that a client
be configured with a set of trust anchors.
In general, a client
MUST
verify the service identity using the verification process defined in
Section 6
of
[RFC6125]
. The client
MUST
construct a reference identity from the service's host: if the host is a literal IP
address (
Section 4.3.5
), the reference identity is an IP-ID, otherwise the host is a name and the reference
identity is a DNS-ID.
A reference identity of type CN-ID
MUST NOT
be used by clients. As noted in
Section 6.2.1
of
[RFC6125]
, a reference identity of type CN-ID might be used by older clients.
A client might be specially configured to accept an alternative form of server identity
verification. For example, a client might be connecting to a server whose address
and hostname are dynamic, with an expectation that the service will present a specific
certificate (or a certificate matching some externally defined reference identity)
rather than one matching the target URI's origin.
In special cases, it might be appropriate for a client to simply ignore the server's
identity, but it must be understood that this leaves a connection open to active attack.
If the certificate is not valid for the target URI's origin, a user agent
MUST
either obtain confirmation from the user before proceeding (see
Section 3.5
) or terminate the connection with a bad certificate error. Automated clients
MUST
log the error to an appropriate audit log (if available) and
SHOULD
terminate the connection (with a bad certificate error). Automated clients
MAY
provide a configuration setting that disables this check, but
MUST
provide a setting which enables it.
4.3.5.
IP-ID Reference Identity
A server that is identified using an IP address literal in the "host" field of an
"https" URI has a reference identity of type IP-ID. An IP version 4 address uses the
"IPv4address" ABNF rule, and an IP version 6 address uses the "IP-literal" production
with the "IPv6address" option; see
Section 3.2.2
of
[URI]
. A reference identity of IP-ID contains the decoded bytes of the IP address.
An IP version 4 address is 4 octets, and an IP version 6 address is 16 octets. Use
of IP-ID is not defined for any other IP version. The iPAddress choice in the certificate
subjectAltName extension does not explicitly include the IP version and so relies
on the length of the address to distinguish versions; see
Section 4.2.1.6
of
[RFC5280]
A reference identity of type IP-ID matches if the address is identical to an iPAddress
value of the subjectAltName extension of the certificate.
5.
Fields
HTTP uses
fields
to provide data in the form of extensible name/value pairs with a registered key namespace.
Fields are sent and received within the header and trailer sections of messages (
Section 6
).
5.1.
Field Names
A field name labels the corresponding field value as having the semantics defined
by that name. For example, the
Date
header field is defined in
Section 6.6.1
as containing the origination timestamp for the message in which it appears.
field-name
token
Field names are case-insensitive and ought to be registered within the "Hypertext
Transfer Protocol (HTTP) Field Name Registry"; see
Section 16.3.1
The interpretation of a field does not change between minor versions of the same major
HTTP version, though the default behavior of a recipient in the absence of such a
field can change. Unless specified otherwise, fields are defined for all versions
of HTTP. In particular, the
Host
and
Connection
fields ought to be recognized by all HTTP implementations whether or not they advertise
conformance with HTTP/1.1.
New fields can be introduced without changing the protocol version if their defined
semantics allow them to be safely ignored by recipients that do not recognize them;
see
Section 16.3
A proxy
MUST
forward unrecognized header fields unless the field name is listed in the
Connection
header field (
Section 7.6.1
) or the proxy is specifically configured to block, or otherwise transform, such fields.
Other recipients
SHOULD
ignore unrecognized header and trailer fields. Adhering to these requirements allows
HTTP's functionality to be extended without updating or removing deployed intermediaries.
5.2.
Field Lines and Combined Field Value
Field sections are composed of any number of
field lines
, each with a
field name
(see
Section 5.1
) identifying the field, and a
field line value
that conveys data for that instance of the field.
When a field name is only present once in a section, the combined
field value
for that field consists of the corresponding field line value. When a field name is
repeated within a section, its combined field value consists of the list of corresponding
field line values within that section, concatenated in order, with each field line
value separated by a comma.
For example, this section:
Example-Field: Foo, Bar
Example-Field: Baz
contains two field lines, both with the field name "Example-Field". The first field
line has a field line value of "Foo, Bar", while the second field line value is "Baz".
The field value for "Example-Field" is the list "Foo, Bar, Baz".
5.3.
Field Order
A recipient
MAY
combine multiple field lines within a field section that have the same field name
into one field line, without changing the semantics of the message, by appending each
subsequent field line value to the initial field line value in order, separated by
a comma (",") and optional whitespace (
OWS
, defined in
Section 5.6.3
). For consistency, use comma SP.
The order in which field lines with the same name are received is therefore significant
to the interpretation of the field value; a proxy
MUST NOT
change the order of these field line values when forwarding a message.
This means that, aside from the well-known exception noted below, a sender
MUST NOT
generate multiple field lines with the same name in a message (whether in the headers
or trailers) or append a field line when a field line of the same name already exists
in the message, unless that field's definition allows multiple field line values to
be recombined as a comma-separated list (i.e., at least one alternative of the field's
definition allows a comma-separated list, such as an ABNF rule of #(values) defined
in
Section 5.6.1
).
Note:
In practice, the "Set-Cookie" header field (
[COOKIE]
) often appears in a response message across multiple field lines and does not use
the list syntax, violating the above requirements on multiple field lines with the
same field name. Since it cannot be combined into a single field value, recipients
ought to handle "Set-Cookie" as a special case while processing fields. (See Appendix
A.2.3 of
[Kri2001]
for details.)
The order in which field lines with differing field names are received in a section
is not significant. However, it is good practice to send header fields that contain
additional control data first, such as
Host
on requests and
Date
on responses, so that implementations can decide when not to handle a message as early
as possible.
A server
MUST NOT
apply a request to the target resource until it receives the entire request header
section, since later header field lines might include conditionals, authentication
credentials, or deliberately misleading duplicate header fields that could impact
request processing.
5.4.
Field Limits
HTTP does not place a predefined limit on the length of each field line, field value,
or on the length of a header or trailer section as a whole, as described in
Section 2
. Various ad hoc limitations on individual lengths are found in practice, often depending
on the specific field's semantics.
A server that receives a request header field line, field value, or set of fields
larger than it wishes to process
MUST
respond with an appropriate
4xx (Client Error)
status code. Ignoring such header fields would increase the server's vulnerability
to request smuggling attacks (
Section 11.2
of
[HTTP/1.1]
).
A client
MAY
discard or truncate received field lines that are larger than the client wishes to
process if the field semantics are such that the dropped value(s) can be safely ignored
without changing the message framing or response semantics.
5.5.
Field Values
HTTP field values consist of a sequence of characters in a format defined by the field's
grammar. Each field's grammar is usually defined using ABNF (
[RFC5234]
).
field-value
= *
field-content
field-content
field-vchar
[ 1*(
SP
HTAB
field-vchar
field-vchar
field-vchar
VCHAR
obs-text
obs-text
= %x80-FF
A field value does not include leading or trailing whitespace. When a specific version
of HTTP allows such whitespace to appear in a message, a field parsing implementation
MUST
exclude such whitespace prior to evaluating the field value.
Field values are usually constrained to the range of US-ASCII characters
[USASCII]
. Fields needing a greater range of characters can use an encoding, such as the one
defined in
[RFC8187]
. Historically, HTTP allowed field content with text in the ISO-8859-1 charset
[ISO-8859-1]
, supporting other charsets only through use of
[RFC2047]
encoding. Specifications for newly defined fields
SHOULD
limit their values to visible US-ASCII octets (VCHAR), SP, and HTAB. A recipient
SHOULD
treat other allowed octets in field content (i.e.,
obs-text
) as opaque data.
Field values containing CR, LF, or NUL characters are invalid and dangerous, due to
the varying ways that implementations might parse and interpret those characters;
a recipient of CR, LF, or NUL within a field value
MUST
either reject the message or replace each of those characters with SP before further
processing or forwarding of that message. Field values containing other CTL characters
are also invalid; however, recipients
MAY
retain such characters for the sake of robustness when they appear within a safe context
(e.g., an application-specific quoted string that will not be processed by any downstream
HTTP parser).
Fields that only anticipate a single member as the field value are referred to as
singleton fields
Fields that allow multiple members as the field value are referred to as
list-based fields
. The list operator extension of
Section 5.6.1
is used as a common notation for defining field values that can contain multiple members.
Because commas (",") are used as the delimiter between members, they need to be treated
with care if they are allowed as data within a member. This is true for both list-based
and singleton fields, since a singleton field might be erroneously sent with multiple
members and detecting such errors improves interoperability. Fields that expect to
contain a comma within a member, such as within an
HTTP-date
or
URI-reference
element, ought to be defined with delimiters around that element to distinguish any
comma within that data from potential list separators.
For example, a textual date and a URI (either of which might contain a comma) could
be safely carried in list-based field values like these:
Example-URIs: "http://example.com/a.html,foo",
"http://without-a-comma.example.com/"
Example-Dates: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
Note that double-quote delimiters are almost always used with the quoted-string production
Section 5.6.4
); using a different syntax inside double-quotes will likely cause unnecessary confusion.
Many fields (such as
Content-Type
, defined in
Section 8.3
) use a common syntax for parameters that allows both unquoted (token) and quoted
(quoted-string) syntax for a parameter value (
Section 5.6.6
). Use of common syntax allows recipients to reuse existing parser components. When
allowing both forms, the meaning of a parameter value ought to be the same whether
it was received as a token or a quoted string.
Note:
For defining field value syntax, this specification uses an ABNF rule named after
the field name to define the allowed grammar for that field's value (after said value
has been extracted from the underlying messaging syntax and multiple instances combined
into a list).
5.6.
Common Rules for Defining Field Values
5.6.1.
Lists (#rule ABNF Extension)
A #rule extension to the ABNF rules of
[RFC5234]
is used to improve readability in the definitions of some list-based field values.
A construct "#" is defined, similar to "*", for defining comma-delimited lists of
elements. The full form is "
elements, each separated by a single comma (",") and optional whitespace (
OWS
, defined in
Section 5.6.3
).
5.6.1.1.
Sender Requirements
In any production that uses the list construct, a sender
MUST NOT
generate empty list elements. In other words, a sender has to generate lists that
satisfy the following syntax:
1#element => element *( OWS "," OWS element )
and:
#element => [ 1#element ]
and for n >= 1 and m > 1:
Appendix A
shows the collected ABNF for senders after the list constructs have been expanded.
5.6.1.2.
Recipient Requirements
Empty elements do not contribute to the count of elements present. A recipient
MUST
parse and ignore a reasonable number of empty list elements: enough to handle common
mistakes by senders that merge values, but not so much that they could be used as
a denial-of-service mechanism. In other words, a recipient
MUST
accept lists that satisfy the following syntax:
#element => [ element ] *( OWS "," OWS [ element ] )
Note that because of the potential presence of empty list elements, the RFC 5234 ABNF
cannot enforce the cardinality of list elements, and consequently all cases are mapped
as if there was no cardinality specified.
For example, given these ABNF productions:
example-list = 1#example-list-elmt
example-list-elmt = token ; see
Section 5.6.2
Then the following are valid values for example-list (not including the double quotes,
which are present for delimitation only):
"foo,bar"
"foo ,bar,"
"foo , ,bar,charlie"
In contrast, the following values would be invalid, since at least one non-empty element
is required by the example-list production:
""
","
", ,"
5.6.2.
Tokens
Tokens are short textual identifiers that do not include whitespace or delimiters.
token
= 1*
tchar
tchar
= "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
DIGIT
ALPHA
; any
VCHAR
, except delimiters
Many HTTP field values are defined using common syntax components, separated by whitespace
or specific delimiting characters. Delimiters are chosen from the set of US-ASCII
visual characters not allowed in a
token
(DQUOTE and "(),/:;<=>?@[\]{}").
5.6.3.
Whitespace
This specification uses three rules to denote the use of linear whitespace: OWS (optional
whitespace), RWS (required whitespace), and BWS ("bad" whitespace).
The OWS rule is used where zero or more linear whitespace octets might appear. For
protocol elements where optional whitespace is preferred to improve readability, a
sender
SHOULD
generate the optional whitespace as a single SP; otherwise, a sender
SHOULD NOT
generate optional whitespace except as needed to overwrite invalid or unwanted protocol
elements during in-place message filtering.
The RWS rule is used when at least one linear whitespace octet is required to separate
field tokens. A sender
SHOULD
generate RWS as a single SP.
OWS and RWS have the same semantics as a single SP. Any content known to be defined
as OWS or RWS
MAY
be replaced with a single SP before interpreting it or forwarding the message downstream.
The BWS rule is used where the grammar allows optional whitespace only for historical
reasons. A sender
MUST NOT
generate BWS in messages. A recipient
MUST
parse for such bad whitespace and remove it before interpreting the protocol element.
BWS has no semantics. Any content known to be defined as BWS
MAY
be removed before interpreting it or forwarding the message downstream.
OWS
= *(
SP
HTAB
; optional whitespace
RWS
= 1*(
SP
HTAB
; required whitespace
BWS
OWS
; "bad" whitespace
5.6.4.
Quoted Strings
A string of text is parsed as a single value if it is quoted using double-quote marks.
quoted-string
DQUOTE
*(
qdtext
quoted-pair
DQUOTE
qdtext
HTAB
SP
/ %x21 / %x23-5B / %x5D-7E /
obs-text
The backslash octet ("\") can be used as a single-octet quoting mechanism within quoted-string
and comment constructs. Recipients that process the value of a quoted-string
MUST
handle a quoted-pair as if it were replaced by the octet following the backslash.
quoted-pair
= "\" (
HTAB
SP
VCHAR
obs-text
A sender
SHOULD NOT
generate a quoted-pair in a quoted-string except where necessary to quote DQUOTE and
backslash octets occurring within that string. A sender
SHOULD NOT
generate a quoted-pair in a comment except where necessary to quote parentheses ["("
and ")"] and backslash octets occurring within that comment.
5.6.5.
Comments
Comments can be included in some HTTP fields by surrounding the comment text with
parentheses. Comments are only allowed in fields containing "comment" as part of their
field value definition.
comment
= "(" *(
ctext
quoted-pair
comment
) ")"
ctext
HTAB
SP
/ %x21-27 / %x2A-5B / %x5D-7E /
obs-text
5.6.6.
Parameters
Parameters are instances of name/value pairs; they are often used in field values
as a common syntax for appending auxiliary information to an item. Each parameter
is usually delimited by an immediately preceding semicolon.
parameters
= *( OWS ";" OWS [
parameter
] )
parameter
parameter-name
"="
parameter-value
parameter-name
token
parameter-value
= (
token
quoted-string
Parameter names are case-insensitive. Parameter values might or might not be case-sensitive,
depending on the semantics of the parameter name. Examples of parameters and some
equivalent forms can be seen in media types (
Section 8.3.1
) and the Accept header field (
Section 12.5.1
).
A parameter value that matches the
token
production can be transmitted either as a token or within a quoted-string. The quoted
and unquoted values are equivalent.
Note:
Parameters do not allow whitespace (not even "bad" whitespace) around the "=" character.
5.6.7.
Date/Time Formats
Prior to 1995, there were three different formats commonly used by servers to communicate
timestamps. For compatibility with old implementations, all three are defined here.
The preferred format is a fixed-length and single-zone subset of the date and time
specification used by the Internet Message Format
[RFC5322]
HTTP-date
IMF-fixdate
obs-date
An example of the preferred format is
Sun, 06 Nov 1994 08:49:37 GMT ; IMF-fixdate
Examples of the two obsolete formats are
Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
A recipient that parses a timestamp value in an HTTP field
MUST
accept all three HTTP-date formats. When a sender generates a field that contains
one or more timestamps defined as HTTP-date, the sender
MUST
generate those timestamps in the IMF-fixdate format.
An HTTP-date value represents time as an instance of Coordinated Universal Time (UTC).
The first two formats indicate UTC by the three-letter abbreviation for Greenwich
Mean Time, "GMT", a predecessor of the UTC name; values in the asctime format are
assumed to be in UTC.
clock
is an implementation capable of providing a reasonable approximation of the current
instant in UTC. A clock implementation ought to use NTP (
[RFC5905]
), or some similar protocol, to synchronize with UTC.
Preferred format:
IMF-fixdate
day-name
","
SP
date1
SP
time-of-day
SP
GMT
; fixed length/zone/capitalization subset of the format
; see
Section 3.3
of
[RFC5322]
day-name
= %s"Mon" / %s"Tue" / %s"Wed"
/ %s"Thu" / %s"Fri" / %s"Sat" / %s"Sun"
date1
day
SP
month
SP
year
; e.g., 02 Jun 1982
day
= 2
DIGIT
month
= %s"Jan" / %s"Feb" / %s"Mar" / %s"Apr"
/ %s"May" / %s"Jun" / %s"Jul" / %s"Aug"
/ %s"Sep" / %s"Oct" / %s"Nov" / %s"Dec"
year
= 4
DIGIT
GMT
= %s"GMT"
time-of-day
hour
":"
minute
":"
second
; 00:00:00 - 23:59:60 (leap second)
hour
= 2
DIGIT
minute
= 2
DIGIT
second
= 2
DIGIT
Obsolete formats:
obs-date
rfc850-date
asctime-date
rfc850-date
day-name-l
","
SP
date2
SP
time-of-day
SP
GMT
date2
day
"-"
month
"-" 2
DIGIT
; e.g., 02-Jun-82
day-name-l
= %s"Monday" / %s"Tuesday" / %s"Wednesday"
/ %s"Thursday" / %s"Friday" / %s"Saturday"
/ %s"Sunday"
asctime-date
day-name
SP
date3
SP
time-of-day
SP
year
date3
month
SP
( 2
DIGIT
/ (
SP
DIGIT
))
; e.g., Jun 2
HTTP-date is case sensitive. Note that
Section 4.2
of
[CACHING]
relaxes this for cache recipients.
A sender
MUST NOT
generate additional whitespace in an HTTP-date beyond that specifically included as
SP in the grammar. The semantics of
day-name
day
month
year
, and
time-of-day
are the same as those defined for the Internet Message Format constructs with the
corresponding name (
[RFC5322]
Section 3.3
).
Recipients of a timestamp value in rfc850-date format, which uses a two-digit year,
MUST
interpret a timestamp that appears to be more than 50 years in the future as representing
the most recent year in the past that had the same last two digits.
Recipients of timestamp values are encouraged to be robust in parsing timestamps unless
otherwise restricted by the field definition. For example, messages are occasionally
forwarded over HTTP from a non-HTTP source that might generate any of the date and
time specifications defined by the Internet Message Format.
Note:
HTTP requirements for timestamp formats apply only to their usage within the protocol
stream. Implementations are not required to use these formats for user presentation,
request logging, etc.
6.
Message Abstraction
Each major version of HTTP defines its own syntax for communicating messages. This
section defines an abstract data type for HTTP messages based on a generalization
of those message characteristics, common structure, and capacity for conveying semantics.
This abstraction is used to define requirements on senders and recipients that are
independent of the HTTP version, such that a message in one version can be relayed
through other versions without changing its meaning.
message
consists of the following:
control data to describe and route the message,
a headers lookup table of name/value pairs for extending that control data and conveying
additional information about the sender, message, content, or context,
a potentially unbounded stream of content, and
a trailers lookup table of name/value pairs for communicating information obtained
while sending the content.
Framing and control data is sent first, followed by a header section containing fields
for the headers table. When a message includes content, the content is sent after
the header section, potentially followed by a trailer section that might contain fields
for the trailers table.
Messages are expected to be processed as a stream, wherein the purpose of that stream
and its continued processing is revealed while being read. Hence, control data describes
what the recipient needs to know immediately, header fields describe what needs to
be known before receiving content, the content (when present) presumably contains
what the recipient wants or needs to fulfill the message semantics, and trailer fields
provide optional metadata that was unknown prior to sending the content.
Messages are intended to be
self-descriptive
: everything a recipient needs to know about the message can be determined by looking
at the message itself, after decoding or reconstituting parts that have been compressed
or elided in transit, without requiring an understanding of the sender's current application
state (established via prior messages). However, a client
MUST
retain knowledge of the request when parsing, interpreting, or caching a corresponding
response. For example, responses to the
HEAD
method look just like the beginning of a response to
GET
but cannot be parsed in the same manner.
Note that this message abstraction is a generalization across many versions of HTTP,
including features that might not be found in some versions. For example, trailers
were introduced within the HTTP/1.1 chunked transfer coding as a trailer section after
the content. An equivalent feature is present in HTTP/2 and HTTP/3 within the header
block that terminates each stream.
6.1.
Framing and Completeness
Message framing indicates how each message begins and ends, such that each message
can be distinguished from other messages or noise on the same connection. Each major
version of HTTP defines its own framing mechanism.
HTTP/0.9 and early deployments of HTTP/1.0 used closure of the underlying connection
to end a response. For backwards compatibility, this implicit framing is also allowed
in HTTP/1.1. However, implicit framing can fail to distinguish an incomplete response
if the connection closes early. For that reason, almost all modern implementations
use explicit framing in the form of length-delimited sequences of message data.
A message is considered
complete
when all of the octets indicated by its framing are available. Note that, when no
explicit framing is used, a response message that is ended by the underlying connection's
close is considered complete even though it might be indistinguishable from an incomplete
response, unless a transport-level error indicates that it is not complete.
6.2.
Control Data
Messages start with control data that describe its primary purpose. Request message
control data includes a request method (
Section 9
), request target (
Section 7.1
), and protocol version (
Section 2.5
). Response message control data includes a status code (
Section 15
), optional reason phrase, and protocol version.
In HTTP/1.1 (
[HTTP/1.1]
) and earlier, control data is sent as the first line of a message. In HTTP/2 (
[HTTP/2]
) and HTTP/3 (
[HTTP/3]
), control data is sent as pseudo-header fields with a reserved name prefix (e.g.,
":authority").
Every HTTP message has a protocol version. Depending on the version in use, it might
be identified within the message explicitly or inferred by the connection over which
the message is received. Recipients use that version information to determine limitations
or potential for later communication with that sender.
When a message is forwarded by an intermediary, the protocol version is updated to
reflect the version used by that intermediary. The
Via
header field (
Section 7.6.3
) is used to communicate upstream protocol information within a forwarded message.
A client
SHOULD
send a request version equal to the highest version to which the client is conformant
and whose major version is no higher than the highest version supported by the server,
if this is known. A client
MUST NOT
send a version to which it is not conformant.
A client
MAY
send a lower request version if it is known that the server incorrectly implements
the HTTP specification, but only after the client has attempted at least one normal
request and determined from the response status code or header fields (e.g.,
Server
) that the server improperly handles higher request versions.
A server
SHOULD
send a response version equal to the highest version to which the server is conformant
that has a major version less than or equal to the one received in the request. A
server
MUST NOT
send a version to which it is not conformant. A server can send a
505 (HTTP Version Not Supported)
response if it wishes, for any reason, to refuse service of the client's major protocol
version.
A recipient that receives a message with a major version number that it implements
and a minor version number higher than what it implements
SHOULD
process the message as if it were in the highest minor version within that major version
to which the recipient is conformant. A recipient can assume that a message with a
higher minor version, when sent to a recipient that has not yet indicated support
for that higher version, is sufficiently backwards-compatible to be safely processed
by any implementation of the same major version.
6.3.
Header Fields
Fields (
Section 5
) that are sent or received before the content are referred to as "header fields"
(or just "headers", colloquially).
The
header section
of a message consists of a sequence of header field lines. Each header field might
modify or extend message semantics, describe the sender, define the content, or provide
additional context.
Note:
We refer to named fields specifically as a "header field" when they are only allowed
to be sent in the header section.
6.4.
Content
HTTP messages often transfer a complete or partial representation as the message
content
: a stream of octets sent after the header section, as delineated by the message framing.
This abstract definition of content reflects the data after it has been extracted
from the message framing. For example, an HTTP/1.1 message body (
Section 6
of
[HTTP/1.1]
) might consist of a stream of data encoded with the chunked transfer coding — a sequence
of data chunks, one zero-length chunk, and a trailer section — whereas the content
of that same message includes only the data stream after the transfer coding has been
decoded; it does not include the chunk lengths, chunked framing syntax, nor the trailer
fields (
Section 6.5
).
Note:
Some field names have a "Content-" prefix. This is an informal convention; while some
of these fields refer to the content of the message, as defined above, others are
scoped to the selected representation (
Section 3.2
). See the individual field's definition to disambiguate.
6.4.1.
Content Semantics
The purpose of content in a request is defined by the method semantics (
Section 9
).
For example, a representation in the content of a PUT request (
Section 9.3.4
) represents the desired state of the
target resource
after the request is successfully applied, whereas a representation in the content
of a POST request (
Section 9.3.3
) represents information to be processed by the target resource.
In a response, the content's purpose is defined by the request method, response status
code (
Section 15
), and response fields describing that content. For example, the content of a
200 (OK)
response to GET (
Section 9.3.1
) represents the current state of the
target resource
, as observed at the time of the message origination date (
Section 6.6.1
), whereas the content of the same status code in a response to POST might represent
either the processing result or the new state of the target resource after applying
the processing.
The content of a
206 (Partial Content)
response to GET contains either a single part of the selected representation or a
multipart message body containing multiple parts of that representation, as described
in
Section 15.3.7
Response messages with an error status code usually contain content that represents
the error condition, such that the content describes the error state and what steps
are suggested for resolving it.
Responses to the HEAD request method (
Section 9.3.2
) never include content; the associated response header fields indicate only what
their values would have been if the request method had been GET (
Section 9.3.1
).
2xx (Successful)
responses to a CONNECT request method (
Section 9.3.6
) switch the connection to tunnel mode instead of having content.
All
1xx (Informational)
204 (No Content)
, and
304 (Not Modified)
responses do not include content.
All other responses do include content, although that content might be of zero length.
6.4.2.
Identifying Content
When a complete or partial representation is transferred as message content, it is
often desirable for the sender to supply, or the recipient to determine, an identifier
for a resource corresponding to that specific representation. For example, a client
making a GET request on a resource for "the current weather report" might want an
identifier specific to the content returned (e.g., "weather report for Laguna Beach
at 20210720T1711"). This can be useful for sharing or bookmarking content from resources
that are expected to have changing representations over time.
For a request message:
If the request has a
Content-Location
header field, then the sender asserts that the content is a representation of the
resource identified by the Content-Location field value. However, such an assertion
cannot be trusted unless it can be verified by other means (not defined by this specification).
The information might still be useful for revision history links.
Otherwise, the content is unidentified by HTTP, but a more specific identifier might
be supplied within the content itself.
For a response message, the following rules are applied in order until a match is
found:
If the request method is HEAD or the response status code is
204 (No Content)
or
304 (Not Modified)
, there is no content in the response.
If the request method is GET and the response status code is
200 (OK)
, the content is a representation of the
target resource
Section 7.1
).
If the request method is GET and the response status code is
203 (Non-Authoritative Information)
, the content is a potentially modified or enhanced representation of the
target resource
as provided by an intermediary.
If the request method is GET and the response status code is
206 (Partial Content)
, the content is one or more parts of a representation of the target resource.
If the response has a
Content-Location
header field and its field value is a reference to the same URI as the target URI,
the content is a representation of the target resource.
If the response has a
Content-Location
header field and its field value is a reference to a URI different from the target
URI, then the sender asserts that the content is a representation of the resource
identified by the Content-Location field value. However, such an assertion cannot
be trusted unless it can be verified by other means (not defined by this specification).
Otherwise, the content is unidentified by HTTP, but a more specific identifier might
be supplied within the content itself.
6.5.
Trailer Fields
Fields (
Section 5
) that are located within a
trailer section
are referred to as "trailer fields" (or just "trailers", colloquially). Trailer fields
can be useful for supplying message integrity checks, digital signatures, delivery
metrics, or post-processing status information.
Trailer fields ought to be processed and stored separately from the fields in the
header section to avoid contradicting message semantics known at the time the header
section was complete. The presence or absence of certain header fields might impact
choices made for the routing or processing of the message as a whole before the trailers
are received; those choices cannot be unmade by the later discovery of trailer fields.
6.5.1.
Limitations on Use of Trailers
A trailer section is only possible when supported by the version of HTTP in use and
enabled by an explicit framing mechanism. For example, the chunked transfer coding
in HTTP/1.1 allows a trailer section to be sent after the content (
Section 7.1.2
of
[HTTP/1.1]
).
Many fields cannot be processed outside the header section because their evaluation
is necessary prior to receiving the content, such as those that describe message framing,
routing, authentication, request modifiers, response controls, or content format.
A sender
MUST NOT
generate a trailer field unless the sender knows the corresponding header field name's
definition permits the field to be sent in trailers.
Trailer fields can be difficult to process by intermediaries that forward messages
from one protocol version to another. If the entire message can be buffered in transit,
some intermediaries could merge trailer fields into the header section (as appropriate)
before it is forwarded. However, in most cases, the trailers are simply discarded.
A recipient
MUST NOT
merge a trailer field into a header section unless the recipient understands the corresponding
header field definition and that definition explicitly permits and defines how trailer
field values can be safely merged.
The presence of the keyword "trailers" in the TE header field (
Section 10.1.4
) of a request indicates that the client is willing to accept trailer fields, on behalf
of itself and any downstream clients. For requests from an intermediary, this implies
that all downstream clients are willing to accept trailer fields in the forwarded
response. Note that the presence of "trailers" does not mean that the client(s) will
process any particular trailer field in the response; only that the trailer section(s)
will not be dropped by any of the clients.
Because of the potential for trailer fields to be discarded in transit, a server
SHOULD NOT
generate trailer fields that it believes are necessary for the user agent to receive.
6.5.2.
Processing Trailer Fields
The "Trailer" header field (
Section 6.6.2
) can be sent to indicate fields likely to be sent in the trailer section, which allows
recipients to prepare for their receipt before processing the content. For example,
this could be useful if a field name indicates that a dynamic checksum should be calculated
as the content is received and then immediately checked upon receipt of the trailer
field value.
Like header fields, trailer fields with the same name are processed in the order received;
multiple trailer field lines with the same name have the equivalent semantics as appending
the multiple values as a list of members. Trailer fields that might be generated more
than once during a message
MUST
be defined as a list-based field even if each member value is only processed once
per field line received.
At the end of a message, a recipient
MAY
treat the set of received trailer fields as a data structure of name/value pairs,
similar to (but separate from) the header fields. Additional processing expectations,
if any, can be defined within the field specification for a field intended for use
in trailers.
6.6.
Message Metadata
Fields that describe the message itself, such as when and how the message has been
generated, can appear in both requests and responses.
6.6.1.
Date
The "Date" header field represents the date and time at which the message was originated,
having the same semantics as the Origination Date Field (orig-date) defined in
Section 3.6.1
of
[RFC5322]
. The field value is an HTTP-date, as defined in
Section 5.6.7
Date
HTTP-date
An example is
Date: Tue, 15 Nov 1994 08:12:31 GMT
A sender that generates a Date header field
SHOULD
generate its field value as the best available approximation of the date and time
of message generation. In theory, the date ought to represent the moment just before
generating the message content. In practice, a sender can generate the date value
at any time during message origination.
An origin server with a clock (as defined in
Section 5.6.7
MUST
generate a Date header field in all
2xx (Successful)
3xx (Redirection)
, and
4xx (Client Error)
responses, and
MAY
generate a Date header field in
1xx (Informational)
and
5xx (Server Error)
responses.
An origin server without a clock
MUST NOT
generate a Date header field.
A recipient with a clock that receives a response message without a Date header field
MUST
record the time it was received and append a corresponding Date header field to the
message's header section if it is cached or forwarded downstream.
A recipient with a clock that receives a response with an invalid Date header field
value
MAY
replace that value with the time that response was received.
A user agent
MAY
send a Date header field in a request, though generally will not do so unless it is
believed to convey useful information to the server. For example, custom applications
of HTTP might convey a Date if the server is expected to adjust its interpretation
of the user's request based on differences between the user agent and server clocks.
6.6.2.
Trailer
The "Trailer" header field provides a list of field names that the sender anticipates
sending as trailer fields within that message. This allows a recipient to prepare
for receipt of the indicated metadata before it starts processing the content.
Trailer
= #
field-name
For example, a sender might indicate that a signature will be computed as the content
is being streamed and provide the final signature as a trailer field. This allows
a recipient to perform the same check on the fly as it receives the content.
A sender that intends to generate one or more trailer fields in a message
SHOULD
generate a
Trailer
header field in the header section of that message to indicate which fields might
be present in the trailers.
If an intermediary discards the trailer section in transit, the
Trailer
field could provide a hint of what metadata was lost, though there is no guarantee
that a sender of Trailer will always follow through by sending the named fields.
7.
Routing HTTP Messages
HTTP request message routing is determined by each client based on the target resource,
the client's proxy configuration, and establishment or reuse of an inbound connection.
The corresponding response routing follows the same connection chain back to the client.
7.1.
Determining the Target Resource
Although HTTP is used in a wide variety of applications, most clients rely on the
same resource identification mechanism and configuration techniques as general-purpose
Web browsers. Even when communication options are hard-coded in a client's configuration,
we can think of their combined effect as a URI reference (
Section 4.1
).
A URI reference is resolved to its absolute form in order to obtain the
target URI
. The target URI excludes the reference's fragment component, if any, since fragment
identifiers are reserved for client-side processing (
[URI]
Section 3.5
).
To perform an action on a
target resource
, the client sends a request message containing enough components of its parsed target
URI to enable recipients to identify that same resource. For historical reasons, the
parsed target URI components, collectively referred to as the
request target
, are sent within the message control data and the
Host
header field (
Section 7.2
).
There are two unusual cases for which the request target components are in a method-specific
form:
For CONNECT (
Section 9.3.6
), the request target is the host name and port number of the tunnel destination,
separated by a colon.
For OPTIONS (
Section 9.3.7
), the request target can be a single asterisk ("*").
See the respective method definitions for details. These forms
MUST NOT
be used with other methods.
Upon receipt of a client's request, a server reconstructs the target URI from the
received components in accordance with their local configuration and incoming connection
context. This reconstruction is specific to each major protocol version. For example,
Section 3.3
of
[HTTP/1.1]
defines how a server determines the target URI of an HTTP/1.1 request.
Note:
Previous specifications defined the recomposed target URI as a distinct concept, the
effective request URI
7.2.
Host and :authority
The "Host" header field in a request provides the host and port information from the
target URI, enabling the origin server to distinguish among resources while servicing
requests for multiple host names.
In HTTP/2
[HTTP/2]
and HTTP/3
[HTTP/3]
, the Host header field is, in some cases, supplanted by the ":authority" pseudo-header
field of a request's control data.
Host
uri-host
[ ":"
port
] ;
Section 4
The target URI's authority information is critical for handling a request. A user
agent
MUST
generate a Host header field in a request unless it sends that information as an ":authority"
pseudo-header field. A user agent that sends Host
SHOULD
send it as the first field in the header section of a request.
For example, a GET request to the origin server for
would begin with:
GET /pub/WWW/ HTTP/1.1
Host: www.example.org
Since the host and port information acts as an application-level routing mechanism,
it is a frequent target for malware seeking to poison a shared cache or redirect a
request to an unintended server. An interception proxy is particularly vulnerable
if it relies on the host and port information for redirecting requests to internal
servers, or for use as a cache key in a shared cache, without first verifying that
the intercepted connection is targeting a valid IP address for that host.
7.3.
Routing Inbound Requests
Once the target URI and its origin are determined, a client decides whether a network
request is necessary to accomplish the desired semantics and, if so, where that request
is to be directed.
7.3.1.
To a Cache
If the client has a cache
[CACHING]
and the request can be satisfied by it, then the request is usually directed there
first.
7.3.2.
To a Proxy
If the request is not satisfied by a cache, then a typical client will check its configuration
to determine whether a proxy is to be used to satisfy the request. Proxy configuration
is implementation-dependent, but is often based on URI prefix matching, selective
authority matching, or both, and the proxy itself is usually identified by an "http"
or "https" URI.
If an "http" or "https" proxy is applicable, the client connects inbound by establishing
(or reusing) a connection to that proxy and then sending it an HTTP request message
containing a request target that matches the client's target URI.
7.3.3.
To the Origin
If no proxy is applicable, a typical client will invoke a handler routine (specific
to the target URI's scheme) to obtain access to the identified resource. How that
is accomplished is dependent on the target URI scheme and defined by its associated
specification.
Section 4.3.2
defines how to obtain access to an "http" resource by establishing (or reusing) an
inbound connection to the identified origin server and then sending it an HTTP request
message containing a request target that matches the client's target URI.
Section 4.3.3
defines how to obtain access to an "https" resource by establishing (or reusing) an
inbound secured connection to an origin server that is authoritative for the identified
origin and then sending it an HTTP request message containing a request target that
matches the client's target URI.
7.4.
Rejecting Misdirected Requests
Once a request is received by a server and parsed sufficiently to determine its target
URI, the server decides whether to process the request itself, forward the request
to another server, redirect the client to a different resource, respond with an error,
or drop the connection. This decision can be influenced by anything about the request
or connection context, but is specifically directed at whether the server has been
configured to process requests for that target URI and whether the connection context
is appropriate for that request.
For example, a request might have been misdirected, deliberately or accidentally,
such that the information within a received
Host
header field differs from the connection's host or port. If the connection is from
a trusted gateway, such inconsistency might be expected; otherwise, it might indicate
an attempt to bypass security filters, trick the server into delivering non-public
content, or poison a cache. See
Section 17
for security considerations regarding message routing.
Unless the connection is from a trusted gateway, an origin server
MUST
reject a request if any scheme-specific requirements for the target URI are not met.
In particular, a request for an "https" resource
MUST
be rejected unless it has been received over a connection that has been secured via
a certificate valid for that target URI's origin, as defined by
Section 4.2.2
The
421 (Misdirected Request)
status code in a response indicates that the origin server has rejected the request
because it appears to have been misdirected (
Section 15.5.20
).
7.5.
Response Correlation
A connection might be used for multiple request/response exchanges. The mechanism
used to correlate between request and response messages is version dependent; some
versions of HTTP use implicit ordering of messages, while others use an explicit identifier.
All responses, regardless of the status code (including
interim
responses) can be sent at any time after a request is received, even if the request
is not yet complete. A response can complete before its corresponding request is complete
Section 6.1
). Likewise, clients are not expected to wait any specific amount of time for a response.
Clients (including intermediaries) might abandon a request if the response is not
received within a reasonable period of time.
A client that receives a response while it is still sending the associated request
SHOULD
continue sending that request unless it receives an explicit indication to the contrary
(see, e.g.,
Section 9.5
of
[HTTP/1.1]
and
Section 6.4
of
[HTTP/2]
).
7.6.
Message Forwarding
As described in
Section 3.7
, intermediaries can serve a variety of roles in the processing of HTTP requests and
responses. Some intermediaries are used to improve performance or availability. Others
are used for access control or to filter content. Since an HTTP stream has characteristics
similar to a pipe-and-filter architecture, there are no inherent limits to the extent
an intermediary can enhance (or interfere) with either direction of the stream.
Intermediaries are expected to forward messages even when protocol elements are not
recognized (e.g., new methods, status codes, or field names) since that preserves
extensibility for downstream recipients.
An intermediary not acting as a tunnel
MUST
implement the
Connection
header field, as specified in
Section 7.6.1
, and exclude fields from being forwarded that are only intended for the incoming
connection.
An intermediary
MUST NOT
forward a message to itself unless it is protected from an infinite request loop.
In general, an intermediary ought to recognize its own server names, including any
aliases, local variations, or literal IP addresses, and respond to such requests directly.
An HTTP message can be parsed as a stream for incremental processing or forwarding
downstream. However, senders and recipients cannot rely on incremental delivery of
partial messages, since some implementations will buffer or delay message forwarding
for the sake of network efficiency, security checks, or content transformations.
7.6.1.
Connection
The "Connection" header field allows the sender to list desired control options for
the current connection.
Connection
= #
connection-option
connection-option
token
Connection options are case-insensitive.
When a field aside from Connection is used to supply control information for or about
the current connection, the sender
MUST
list the corresponding field name within the Connection header field. Note that some
versions of HTTP prohibit the use of fields for such information, and therefore do
not allow the Connection field.
Intermediaries
MUST
parse a received Connection header field before a message is forwarded and, for each
connection-option in this field, remove any header or trailer field(s) from the message
with the same name as the connection-option, and then remove the Connection header
field itself (or replace it with the intermediary's own control options for the forwarded
message).
Hence, the Connection header field provides a declarative way of distinguishing fields
that are only intended for the immediate recipient ("hop-by-hop") from those fields
that are intended for all recipients on the chain ("end-to-end"), enabling the message
to be self-descriptive and allowing future connection-specific extensions to be deployed
without fear that they will be blindly forwarded by older intermediaries.
Furthermore, intermediaries
SHOULD
remove or replace fields that are known to require removal before forwarding, whether
or not they appear as a connection-option, after applying those fields' semantics.
This includes but is not limited to:
Proxy-Connection (
Appendix C.2.2
of
[HTTP/1.1]
Keep-Alive (
Section 19.7.1
of
[RFC2068]
TE (
Section 10.1.4
Transfer-Encoding (
Section 6.1
of
[HTTP/1.1]
Upgrade (
Section 7.8
A sender
MUST NOT
send a connection option corresponding to a field that is intended for all recipients
of the content. For example,
Cache-Control
is never appropriate as a connection option (
Section 5.2
of
[CACHING]
).
Connection options do not always correspond to a field present in the message, since
a connection-specific field might not be needed if there are no parameters associated
with a connection option. In contrast, a connection-specific field received without
a corresponding connection option usually indicates that the field has been improperly
forwarded by an intermediary and ought to be ignored by the recipient.
When defining a new connection option that does not correspond to a field, specification
authors ought to reserve the corresponding field name anyway in order to avoid later
collisions. Such reserved field names are registered in the "Hypertext Transfer Protocol
(HTTP) Field Name Registry" (
Section 16.3.1
).
7.6.2.
Max-Forwards
The "Max-Forwards" header field provides a mechanism with the TRACE (
Section 9.3.8
) and OPTIONS (
Section 9.3.7
) request methods to limit the number of times that the request is forwarded by proxies.
This can be useful when the client is attempting to trace a request that appears to
be failing or looping mid-chain.
Max-Forwards
= 1*
DIGIT
The Max-Forwards value is a decimal integer indicating the remaining number of times
this request message can be forwarded.
Each intermediary that receives a TRACE or OPTIONS request containing a Max-Forwards
header field
MUST
check and update its value prior to forwarding the request. If the received value
is zero (0), the intermediary
MUST NOT
forward the request; instead, the intermediary
MUST
respond as the final recipient. If the received Max-Forwards value is greater than
zero, the intermediary
MUST
generate an updated Max-Forwards field in the forwarded message with a field value
that is the lesser of a) the received value decremented by one (1) or b) the recipient's
maximum supported value for Max-Forwards.
A recipient
MAY
ignore a Max-Forwards header field received with any other request methods.
7.6.3.
Via
The "Via" header field indicates the presence of intermediate protocols and recipients
between the user agent and the server (on requests) or between the origin server and
the client (on responses), similar to the "Received" header field in email (
Section 3.6.7
of
[RFC5322]
). Via can be used for tracking message forwards, avoiding request loops, and identifying
the protocol capabilities of senders along the request/response chain.
Via
= #(
received-protocol
RWS
received-by
RWS
comment
] )
received-protocol
= [
protocol-name
"/" ]
protocol-version
; see
Section 7.8
received-by
pseudonym
[ ":"
port
pseudonym
token
Each member of the Via field value represents a proxy or gateway that has forwarded
the message. Each intermediary appends its own information about how the message was
received, such that the end result is ordered according to the sequence of forwarding
recipients.
A proxy
MUST
send an appropriate Via header field, as described below, in each message that it
forwards. An HTTP-to-HTTP gateway
MUST
send an appropriate Via header field in each inbound request message and
MAY
send a Via header field in forwarded response messages.
For each intermediary, the received-protocol indicates the protocol and protocol version
used by the upstream sender of the message. Hence, the Via field value records the
advertised protocol capabilities of the request/response chain such that they remain
visible to downstream recipients; this can be useful for determining what backwards-incompatible
features might be safe to use in response, or within a later request, as described
in
Section 2.5
. For brevity, the protocol-name is omitted when the received protocol is HTTP.
The received-by portion is normally the host and optional port number of a recipient
server or client that subsequently forwarded the message. However, if the real host
is considered to be sensitive information, a sender
MAY
replace it with a pseudonym. If a port is not provided, a recipient
MAY
interpret that as meaning it was received on the default port, if any, for the received-protocol.
A sender
MAY
generate comments to identify the software of each recipient, analogous to the
User-Agent
and
Server
header fields. However, comments in Via are optional, and a recipient
MAY
remove them prior to forwarding the message.
For example, a request message could be sent from an HTTP/1.0 user agent to an internal
proxy code-named "fred", which uses HTTP/1.1 to forward the request to a public proxy
at p.example.net, which completes the request by forwarding it to the origin server
at www.example.com. The request received by www.example.com would then have the following
Via header field:
Via: 1.0 fred, 1.1 p.example.net
An intermediary used as a portal through a network firewall
SHOULD NOT
forward the names and ports of hosts within the firewall region unless it is explicitly
enabled to do so. If not enabled, such an intermediary
SHOULD
replace each received-by host of any host behind the firewall by an appropriate pseudonym
for that host.
An intermediary
MAY
combine an ordered subsequence of Via header field list members into a single member
if the entries have identical received-protocol values. For example,
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
A sender
SHOULD NOT
combine multiple list members unless they are all under the same organizational control
and the hosts have already been replaced by pseudonyms. A sender
MUST NOT
combine members that have different received-protocol values.
7.7.
Message Transformations
Some intermediaries include features for transforming messages and their content.
A proxy might, for example, convert between image formats in order to save cache space
or to reduce the amount of traffic on a slow link. However, operational problems might
occur when these transformations are applied to content intended for critical applications,
such as medical imaging or scientific data analysis, particularly when integrity checks
or digital signatures are used to ensure that the content received is identical to
the original.
An HTTP-to-HTTP proxy is called a
transforming proxy
if it is designed or configured to modify messages in a semantically meaningful way
(i.e., modifications, beyond those required by normal HTTP processing, that change
the message in a way that would be significant to the original sender or potentially
significant to downstream recipients). For example, a transforming proxy might be
acting as a shared annotation server (modifying responses to include references to
a local annotation database), a malware filter, a format transcoder, or a privacy
filter. Such transformations are presumed to be desired by whichever client (or client
organization) chose the proxy.
If a proxy receives a target URI with a host name that is not a fully qualified domain
name, it
MAY
add its own domain to the host name it received when forwarding the request. A proxy
MUST NOT
change the host name if the target URI contains a fully qualified domain name.
A proxy
MUST NOT
modify the "absolute-path" and "query" parts of the received target URI when forwarding
it to the next inbound server except as required by that forwarding protocol. For
example, a proxy forwarding a request to an origin server via HTTP/1.1 will replace
an empty path with "/" (
Section 3.2.1
of
[HTTP/1.1]
) or "*" (
Section 3.2.4
of
[HTTP/1.1]
), depending on the request method.
A proxy
MUST NOT
transform the content (
Section 6.4
) of a response message that contains a no-transform cache directive (
Section 5.2.2.6
of
[CACHING]
). Note that this does not apply to message transformations that do not change the
content, such as the addition or removal of transfer codings (
Section 7
of
[HTTP/1.1]
).
A proxy
MAY
transform the content of a message that does not contain a no-transform cache directive.
A proxy that transforms the content of a
200 (OK)
response can inform downstream recipients that a transformation has been applied by
changing the response status code to
203 (Non-Authoritative Information)
Section 15.3.4
).
A proxy
SHOULD NOT
modify header fields that provide information about the endpoints of the communication
chain, the resource state, or the
selected representation
(other than the content) unless the field's definition specifically allows such modification
or the modification is deemed necessary for privacy or security.
7.8.
Upgrade
The "Upgrade" header field is intended to provide a simple mechanism for transitioning
from HTTP/1.1 to some other protocol on the same connection.
A client
MAY
send a list of protocol names in the Upgrade header field of a request to invite the
server to switch to one or more of the named protocols, in order of descending preference,
before sending the final response. A server
MAY
ignore a received Upgrade header field if it wishes to continue using the current
protocol on that connection. Upgrade cannot be used to insist on a protocol change.
Upgrade
= #
protocol
protocol
protocol-name
["/"
protocol-version
protocol-name
token
protocol-version
token
Although protocol names are registered with a preferred case, recipients
SHOULD
use case-insensitive comparison when matching each protocol-name to supported protocols.
A server that sends a
101 (Switching Protocols)
response
MUST
send an Upgrade header field to indicate the new protocol(s) to which the connection
is being switched; if multiple protocol layers are being switched, the sender
MUST
list the protocols in layer-ascending order. A server
MUST NOT
switch to a protocol that was not indicated by the client in the corresponding request's
Upgrade header field. A server
MAY
choose to ignore the order of preference indicated by the client and select the new
protocol(s) based on other factors, such as the nature of the request or the current
load on the server.
A server that sends a
426 (Upgrade Required)
response
MUST
send an Upgrade header field to indicate the acceptable protocols, in order of descending
preference.
A server
MAY
send an Upgrade header field in any other response to advertise that it implements
support for upgrading to the listed protocols, in order of descending preference,
when appropriate for a future request.
The following is a hypothetical example sent by a client:
GET /hello HTTP/1.1
Host: www.example.com
Connection: upgrade
Upgrade: websocket, IRC/6.9, RTA/x11
The capabilities and nature of the application-level communication after the protocol
change is entirely dependent upon the new protocol(s) chosen. However, immediately
after sending the
101 (Switching Protocols)
response, the server is expected to continue responding to the original request as
if it had received its equivalent within the new protocol (i.e., the server still
has an outstanding request to satisfy after the protocol has been changed, and is
expected to do so without requiring the request to be repeated).
For example, if the Upgrade header field is received in a GET request and the server
decides to switch protocols, it first responds with a
101 (Switching Protocols)
message in HTTP/1.1 and then immediately follows that with the new protocol's equivalent
of a response to a GET on the target resource. This allows a connection to be upgraded
to protocols with the same semantics as HTTP without the latency cost of an additional
round trip. A server
MUST NOT
switch protocols unless the received message semantics can be honored by the new protocol;
an OPTIONS request can be honored by any protocol.
The following is an example response to the above hypothetical request:
HTTP/1.1 101 Switching Protocols
Connection: upgrade
Upgrade: websocket
[... data stream switches to websocket with an appropriate response
(as defined by new protocol) to the "GET /hello" request ...]
A sender of Upgrade
MUST
also send an "Upgrade" connection option in the
Connection
header field (
Section 7.6.1
) to inform intermediaries not to forward this field. A server that receives an Upgrade
header field in an HTTP/1.0 request
MUST
ignore that Upgrade field.
A client cannot begin using an upgraded protocol on the connection until it has completely
sent the request message (i.e., the client can't change the protocol it is sending
in the middle of a message). If a server receives both an Upgrade and an
Expect
header field with the "100-continue" expectation (
Section 10.1.1
), the server
MUST
send a
100 (Continue)
response before sending a
101 (Switching Protocols)
response.
The Upgrade header field only applies to switching protocols on top of the existing
connection; it cannot be used to switch the underlying connection (transport) protocol,
nor to switch the existing communication to a different connection. For those purposes,
it is more appropriate to use a
3xx (Redirection)
response (
Section 15.4
).
This specification only defines the protocol name "HTTP" for use by the family of
Hypertext Transfer Protocols, as defined by the HTTP version rules of
Section 2.5
and future updates to this specification. Additional protocol names ought to be registered
using the registration procedure defined in
Section 16.7
8.
Representation Data and Metadata
8.1.
Representation Data
The representation data associated with an HTTP message is either provided as the
content of the message or referred to by the message semantics and the target URI.
The representation data is in a format and encoding defined by the representation
metadata header fields.
The data type of the representation data is determined via the header fields
Content-Type
and
Content-Encoding
. These define a two-layer, ordered encoding model:
representation-data := Content-Encoding( Content-Type( data ) )
8.2.
Representation Metadata
Representation header fields provide metadata about the representation. When a message
includes content, the representation header fields describe how to interpret that
data. In a response to a HEAD request, the representation header fields describe the
representation data that would have been enclosed in the content if the same request
had been a GET.
8.3.
Content-Type
The "Content-Type" header field indicates the media type of the associated representation:
either the representation enclosed in the message content or the
selected representation
, as determined by the message semantics. The indicated media type defines both the
data format and how that data is intended to be processed by a recipient, within the
scope of the received message semantics, after any content codings indicated by
Content-Encoding
are decoded.
Content-Type
media-type
Media types are defined in
Section 8.3.1
. An example of the field is
Content-Type: text/html; charset=ISO-8859-4
A sender that generates a message containing content
SHOULD
generate a Content-Type header field in that message unless the intended media type
of the enclosed representation is unknown to the sender. If a Content-Type header
field is not present, the recipient
MAY
either assume a media type of "application/octet-stream" (
[RFC2046]
Section 4.5.1
) or examine the data to determine its type.
In practice, resource owners do not always properly configure their origin server
to provide the correct Content-Type for a given representation. Some user agents examine
the content and, in certain cases, override the received type (for example, see
[Sniffing]
). This "MIME sniffing" risks drawing incorrect conclusions about the data, which
might expose the user to additional security risks (e.g., "privilege escalation").
Furthermore, distinct media types often share a common data format, differing only
in how the data is intended to be processed, which is impossible to distinguish by
inspecting the data alone. When sniffing is implemented, implementers are encouraged
to provide a means for the user to disable it.
Although Content-Type is defined as a singleton field, it is sometimes incorrectly
generated multiple times, resulting in a combined field value that appears to be a
list. Recipients often attempt to handle this error by using the last syntactically
valid member of the list, leading to potential interoperability and security issues
if different implementations have different error handling behaviors.
8.3.1.
Media Type
HTTP uses media types
[RFC2046]
in the
Content-Type
Section 8.3
) and
Accept
Section 12.5.1
) header fields in order to provide open and extensible data typing and type negotiation.
Media types define both a data format and various processing models: how to process
that data in accordance with the message context.
media-type
type
"/"
subtype
parameters
type
token
subtype
token
The type and subtype tokens are case-insensitive.
The type/subtype
MAY
be followed by semicolon-delimited parameters (
Section 5.6.6
) in the form of name/value pairs. The presence or absence of a parameter might be
significant to the processing of a media type, depending on its definition within
the media type registry. Parameter values might or might not be case-sensitive, depending
on the semantics of the parameter name.
For example, the following media types are equivalent in describing HTML text data
encoded in the UTF-8 character encoding scheme, but the first is preferred for consistency
(the "charset" parameter value is defined as being case-insensitive in
[RFC2046]
Section 4.1.2
):
text/html;charset=utf-8
Text/HTML;Charset="utf-8"
text/html; charset="utf-8"
text/html;charset=UTF-8
Media types ought to be registered with IANA according to the procedures defined in
[BCP13]
8.3.2.
Charset
HTTP uses
charset
names to indicate or negotiate the character encoding scheme (
[RFC6365]
Section 2
) of a textual representation. In the fields defined by this document, charset names
appear either in parameters (
Content-Type
), or, for
Accept-Encoding
, in the form of a plain
token
. In both cases, charset names are matched case-insensitively.
Charset names ought to be registered in the IANA "Character Sets" registry (
) according to the procedures defined in
Section 2
of
[RFC2978]
Note:
In theory, charset names are defined by the "mime-charset" ABNF rule defined in
Section 2.3
of
[RFC2978]
(as corrected in
[Err1912]
). That rule allows two characters that are not included in "token" ("{" and "}"),
but no charset name registered at the time of this writing includes braces (see
[Err5433]
).
8.3.3.
Multipart Types
MIME provides for a number of "multipart" types — encapsulations of one or more representations
within a single message body. All multipart types share a common syntax, as defined
in
Section 5.1.1
of
[RFC2046]
, and include a boundary parameter as part of the media type value. The message body
is itself a protocol element; a sender
MUST
generate only CRLF to represent line breaks between body parts.
HTTP message framing does not use the multipart boundary as an indicator of message
body length, though it might be used by implementations that generate or process the
content. For example, the "multipart/form-data" type is often used for carrying form
data in a request, as described in
[RFC7578]
, and the "multipart/byteranges" type is defined by this specification for use in
some
206 (Partial Content)
responses (see
Section 15.3.7
).
8.4.
Content-Encoding
The "Content-Encoding" header field indicates what content codings have been applied
to the representation, beyond those inherent in the media type, and thus what decoding
mechanisms have to be applied in order to obtain data in the media type referenced
by the
Content-Type
header field. Content-Encoding is primarily used to allow a representation's data
to be compressed without losing the identity of its underlying media type.
Content-Encoding
= #
content-coding
An example of its use is
Content-Encoding: gzip
If one or more encodings have been applied to a representation, the sender that applied
the encodings
MUST
generate a Content-Encoding header field that lists the content codings in the order
in which they were applied. Note that the coding named "identity" is reserved for
its special role in
Accept-Encoding
and thus
SHOULD NOT
be included.
Additional information about the encoding parameters can be provided by other header
fields not defined by this specification.
Unlike Transfer-Encoding (
Section 6.1
of
[HTTP/1.1]
), the codings listed in Content-Encoding are a characteristic of the representation;
the representation is defined in terms of the coded form, and all other metadata about
the representation is about the coded form unless otherwise noted in the metadata
definition. Typically, the representation is only decoded just prior to rendering
or analogous usage.
If the media type includes an inherent encoding, such as a data format that is always
compressed, then that encoding would not be restated in Content-Encoding even if it
happens to be the same algorithm as one of the content codings. Such a content coding
would only be listed if, for some bizarre reason, it is applied a second time to form
the representation. Likewise, an origin server might choose to publish the same data
as multiple representations that differ only in whether the coding is defined as part
of
Content-Type
or Content-Encoding, since some user agents will behave differently in their handling
of each response (e.g., open a "Save as ..." dialog instead of automatic decompression
and rendering of content).
An origin server
MAY
respond with a status code of
415 (Unsupported Media Type)
if a representation in the request message has a content coding that is not acceptable.
8.4.1.
Content Codings
Content coding values indicate an encoding transformation that has been or can be
applied to a representation. Content codings are primarily used to allow a representation
to be compressed or otherwise usefully transformed without losing the identity of
its underlying media type and without loss of information. Frequently, the representation
is stored in coded form, transmitted directly, and only decoded by the final recipient.
content-coding
token
All content codings are case-insensitive and ought to be registered within the "HTTP
Content Coding Registry", as described in
Section 16.6
Content-coding values are used in the
Accept-Encoding
Section 12.5.3
) and
Content-Encoding
Section 8.4
) header fields.
8.4.1.1.
Compress Coding
The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
[Welch]
that is commonly produced by the UNIX file compression program "compress". A recipient
SHOULD
consider "x-compress" to be equivalent to "compress".
8.4.1.2.
Deflate Coding
The "deflate" coding is a "zlib" data format
[RFC1950]
containing a "deflate" compressed data stream
[RFC1951]
that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and Huffman
coding.
Note:
Some non-conformant implementations send the "deflate" compressed data without the
zlib wrapper.
8.4.1.3.
Gzip Coding
The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy Check (CRC) that
is commonly produced by the gzip file compression program
[RFC1952]
. A recipient
SHOULD
consider "x-gzip" to be equivalent to "gzip".
8.5.
Content-Language
The "Content-Language" header field describes the natural language(s) of the intended
audience for the representation. Note that this might not be equivalent to all the
languages used within the representation.
Content-Language
= #
language-tag
Language tags are defined in
Section 8.5.1
. The primary purpose of Content-Language is to allow a user to identify and differentiate
representations according to the users' own preferred language. Thus, if the content
is intended only for a Danish-literate audience, the appropriate field is
Content-Language: da
If no Content-Language is specified, the default is that the content is intended for
all language audiences. This might mean that the sender does not consider it to be
specific to any natural language, or that the sender does not know for which language
it is intended.
Multiple languages
MAY
be listed for content that is intended for multiple audiences. For example, a rendition
of the "Treaty of Waitangi", presented simultaneously in the original Maori and English
versions, would call for
Content-Language: mi, en
However, just because multiple languages are present within a representation does
not mean that it is intended for multiple linguistic audiences. An example would be
a beginner's language primer, such as "A First Lesson in Latin", which is clearly
intended to be used by an English-literate audience. In this case, the Content-Language
would properly only include "en".
Content-Language
MAY
be applied to any media type — it is not limited to textual documents.
8.5.1.
Language Tags
A language tag, as defined in
[RFC5646]
, identifies a natural language spoken, written, or otherwise conveyed by human beings
for communication of information to other human beings. Computer languages are explicitly
excluded.
HTTP uses language tags within the
Accept-Language
and
Content-Language
header fields.
Accept-Language
uses the broader language-range production defined in
Section 12.5.4
, whereas
Content-Language
uses the language-tag production defined below.
language-tag
=
Section 2.1
A language tag is a sequence of one or more case-insensitive subtags, each separated
by a hyphen character ("-", %x2D). In most cases, a language tag consists of a primary
language subtag that identifies a broad family of related languages (e.g., "en" =
English), which is optionally followed by a series of subtags that refine or narrow
that language's range (e.g., "en-CA" = the variety of English as communicated in Canada).
Whitespace is not allowed within a language tag. Example tags include:
fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
See
[RFC5646]
for further information.
8.6.
Content-Length
The "Content-Length" header field indicates the associated representation's data length
as a decimal non-negative integer number of octets. When transferring a representation
as content, Content-Length refers specifically to the amount of data enclosed so that
it can be used to delimit framing (e.g.,
Section 6.2
of
[HTTP/1.1]
). In other cases, Content-Length indicates the selected representation's current
length, which can be used by recipients to estimate transfer time or to compare with
previously stored representations.
Content-Length
= 1*
DIGIT
An example is
Content-Length: 3495
A user agent
SHOULD
send Content-Length in a request when the method defines a meaning for enclosed content
and it is not sending
Transfer-Encoding
. For example, a user agent normally sends Content-Length in a POST request even when
the value is 0 (indicating empty content). A user agent
SHOULD NOT
send a Content-Length header field when the request message does not contain content
and the method semantics do not anticipate such data.
A server
MAY
send a Content-Length header field in a response to a HEAD request (
Section 9.3.2
); a server
MUST NOT
send Content-Length in such a response unless its field value equals the decimal number
of octets that would have been sent in the content of a response if the same request
had used the GET method.
A server
MAY
send a Content-Length header field in a
304 (Not Modified)
response to a conditional GET request (
Section 15.4.5
); a server
MUST NOT
send Content-Length in such a response unless its field value equals the decimal number
of octets that would have been sent in the content of a
200 (OK)
response to the same request.
A server
MUST NOT
send a Content-Length header field in any response with a status code of
1xx (Informational)
or
204 (No Content)
. A server
MUST NOT
send a Content-Length header field in any
2xx (Successful)
response to a CONNECT request (
Section 9.3.6
).
Aside from the cases defined above, in the absence of Transfer-Encoding, an origin
server
SHOULD
send a Content-Length header field when the content size is known prior to sending
the complete header section. This will allow downstream recipients to measure transfer
progress, know when a received message is complete, and potentially reuse the connection
for additional requests.
Any Content-Length field value greater than or equal to zero is valid. Since there
is no predefined limit to the length of content, a recipient
MUST
anticipate potentially large decimal numerals and prevent parsing errors due to integer
conversion overflows or precision loss due to integer conversion (
Section 17.5
).
Because Content-Length is used for message delimitation in HTTP/1.1, its field value
can impact how the message is parsed by downstream recipients even when the immediate
connection is not using HTTP/1.1. If the message is forwarded by a downstream intermediary,
a Content-Length field value that is inconsistent with the received message framing
might cause a security failure due to request smuggling or response splitting.
As a result, a sender
MUST NOT
forward a message with a Content-Length header field value that is known to be incorrect.
Likewise, a sender
MUST NOT
forward a message with a Content-Length header field value that does not match the
ABNF above, with one exception: a recipient of a Content-Length header field value
consisting of the same decimal value repeated as a comma-separated list (e.g, "Content-Length:
42, 42")
MAY
either reject the message as invalid or replace that invalid field value with a single
instance of the decimal value, since this likely indicates that a duplicate was generated
or combined by an upstream message processor.
8.7.
Content-Location
The "Content-Location" header field references a URI that can be used as an identifier
for a specific resource corresponding to the representation in this message's content.
In other words, if one were to perform a GET request on this URI at the time of this
message's generation, then a
200 (OK)
response would contain the same representation that is enclosed as content in this
message.
Content-Location
absolute-URI
partial-URI
The field value is either an
absolute-URI
or a
partial-URI
. In the latter case (
Section 4
), the referenced URI is relative to the target URI (
[URI]
Section 5
).
The Content-Location value is not a replacement for the target URI (
Section 7.1
). It is representation metadata. It has the same syntax and semantics as the header
field of the same name defined for MIME body parts in
Section 4
of
[RFC2557]
. However, its appearance in an HTTP message has some special implications for HTTP
recipients.
If Content-Location is included in a
2xx (Successful)
response message and its value refers (after conversion to absolute form) to a URI
that is the same as the target URI, then the recipient
MAY
consider the content to be a current representation of that resource at the time indicated
by the message origination date. For a GET (
Section 9.3.1
) or HEAD (
Section 9.3.2
) request, this is the same as the default semantics when no Content-Location is provided
by the server. For a state-changing request like PUT (
Section 9.3.4
) or POST (
Section 9.3.3
), it implies that the server's response contains the new representation of that resource,
thereby distinguishing it from representations that might only report about the action
(e.g., "It worked!"). This allows authoring applications to update their local copies
without the need for a subsequent GET request.
If Content-Location is included in a
2xx (Successful)
response message and its field value refers to a URI that differs from the target
URI, then the origin server claims that the URI is an identifier for a different resource
corresponding to the enclosed representation. Such a claim can only be trusted if
both identifiers share the same resource owner, which cannot be programmatically determined
via HTTP.
For a response to a GET or HEAD request, this is an indication that the target URI
refers to a resource that is subject to content negotiation and the Content-Location
field value is a more specific identifier for the
selected representation
For a
201 (Created)
response to a state-changing method, a Content-Location field value that is identical
to the
Location
field value indicates that this content is a current representation of the newly created
resource.
Otherwise, such a Content-Location indicates that this content is a representation
reporting on the requested action's status and that the same report is available (for
future access with GET) at the given URI. For example, a purchase transaction made
via a POST request might include a receipt document as the content of the
200 (OK)
response; the Content-Location field value provides an identifier for retrieving a
copy of that same receipt in the future.
A user agent that sends Content-Location in a request message is stating that its
value refers to where the user agent originally obtained the content of the enclosed
representation (prior to any modifications made by that user agent). In other words,
the user agent is providing a back link to the source of the original representation.
An origin server that receives a Content-Location field in a request message
MUST
treat the information as transitory request context rather than as metadata to be
saved verbatim as part of the representation. An origin server
MAY
use that context to guide in processing the request or to save it for other uses,
such as within source links or versioning metadata. However, an origin server
MUST NOT
use such context information to alter the request semantics.
For example, if a client makes a PUT request on a negotiated resource and the origin
server accepts that PUT (without redirection), then the new state of that resource
is expected to be consistent with the one representation supplied in that PUT; the
Content-Location cannot be used as a form of reverse content selection identifier
to update only one of the negotiated representations. If the user agent had wanted
the latter semantics, it would have applied the PUT directly to the Content-Location
URI.
8.8.
Validator Fields
Resource metadata is referred to as a
validator
if it can be used within a precondition (
Section 13.1
) to make a conditional request (
Section 13
). Validator fields convey a current validator for the
selected representation
Section 3.2
).
In responses to safe requests, validator fields describe the selected representation
chosen by the origin server while handling the response. Note that, depending on the
method and status code semantics, the selected representation for a given response
is not necessarily the same as the representation enclosed as response content.
In a successful response to a state-changing request, validator fields describe the
new representation that has replaced the prior
selected representation
as a result of processing the request.
For example, an ETag field in a
201 (Created)
response communicates the entity tag of the newly created resource's representation,
so that the entity tag can be used as a validator in later conditional requests to
prevent the "lost update" problem.
This specification defines two forms of metadata that are commonly used to observe
resource state and test for preconditions: modification dates (
Section 8.8.2
) and opaque entity tags (
Section 8.8.3
). Additional metadata that reflects resource state has been defined by various extensions
of HTTP, such as Web Distributed Authoring and Versioning
[WEBDAV]
, that are beyond the scope of this specification.
8.8.1.
Weak versus Strong
Validators come in two flavors: strong or weak. Weak validators are easy to generate
but are far less useful for comparisons. Strong validators are ideal for comparisons
but can be very difficult (and occasionally impossible) to generate efficiently. Rather
than impose that all forms of resource adhere to the same strength of validator, HTTP
exposes the type of validator in use and imposes restrictions on when weak validators
can be used as preconditions.
strong validator
is representation metadata that changes value whenever a change occurs to the representation
data that would be observable in the content of a
200 (OK)
response to GET.
A strong validator might change for reasons other than a change to the representation
data, such as when a semantically significant part of the representation metadata
is changed (e.g.,
Content-Type
), but it is in the best interests of the origin server to only change the value when
it is necessary to invalidate the stored responses held by remote caches and authoring
tools.
Cache entries might persist for arbitrarily long periods, regardless of expiration
times. Thus, a cache might attempt to validate an entry using a validator that it
obtained in the distant past. A strong validator is unique across all versions of
all representations associated with a particular resource over time. However, there
is no implication of uniqueness across representations of different resources (i.e.,
the same strong validator might be in use for representations of multiple resources
at the same time and does not imply that those representations are equivalent).
There are a variety of strong validators used in practice. The best are based on strict
revision control, wherein each change to a representation always results in a unique
node name and revision identifier being assigned before the representation is made
accessible to GET. A collision-resistant hash function applied to the representation
data is also sufficient if the data is available prior to the response header fields
being sent and the digest does not need to be recalculated every time a validation
request is received. However, if a resource has distinct representations that differ
only in their metadata, such as might occur with content negotiation over media types
that happen to share the same data format, then the origin server needs to incorporate
additional information in the validator to distinguish those representations.
In contrast, a
weak validator
is representation metadata that might not change for every change to the representation
data. This weakness might be due to limitations in how the value is calculated (e.g.,
clock resolution), an inability to ensure uniqueness for all possible representations
of the resource, or a desire of the resource owner to group representations by some
self-determined set of equivalency rather than unique sequences of data.
An origin server
SHOULD
change a weak entity tag whenever it considers prior representations to be unacceptable
as a substitute for the current representation. In other words, a weak entity tag
ought to change whenever the origin server wants caches to invalidate old responses.
For example, the representation of a weather report that changes in content every
second, based on dynamic measurements, might be grouped into sets of equivalent representations
(from the origin server's perspective) with the same weak validator in order to allow
cached representations to be valid for a reasonable period of time (perhaps adjusted
dynamically based on server load or weather quality). Likewise, a representation's
modification time, if defined with only one-second resolution, might be a weak validator
if it is possible for the representation to be modified twice during a single second
and retrieved between those modifications.
Likewise, a validator is weak if it is shared by two or more representations of a
given resource at the same time, unless those representations have identical representation
data. For example, if the origin server sends the same validator for a representation
with a gzip content coding applied as it does for a representation with no content
coding, then that validator is weak. However, two simultaneous representations might
share the same strong validator if they differ only in the representation metadata,
such as when two different media types are available for the same representation data.
Strong validators are usable for all conditional requests, including cache validation,
partial content ranges, and "lost update" avoidance. Weak validators are only usable
when the client does not require exact equality with previously obtained representation
data, such as when validating a cache entry or limiting a web traversal to recent
changes.
8.8.2.
Last-Modified
The "Last-Modified" header field in a response provides a timestamp indicating the
date and time at which the origin server believes the
selected representation
was last modified, as determined at the conclusion of handling the request.
Last-Modified
HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
8.8.2.1.
Generation
An origin server
SHOULD
send Last-Modified for any selected representation for which a last modification date
can be reasonably and consistently determined, since its use in conditional requests
and evaluating cache freshness (
[CACHING]
) can substantially reduce unnecessary transfers and significantly improve service
availability and scalability.
A representation is typically the sum of many parts behind the resource interface.
The last-modified time would usually be the most recent time that any of those parts
were changed. How that value is determined for any given resource is an implementation
detail beyond the scope of this specification.
An origin server
SHOULD
obtain the Last-Modified value of the representation as close as possible to the time
that it generates the
Date
field value for its response. This allows a recipient to make an accurate assessment
of the representation's modification time, especially if the representation changes
near the time that the response is generated.
An origin server with a clock (as defined in
Section 5.6.7
MUST NOT
generate a Last-Modified date that is later than the server's time of message origination
Date
Section 6.6.1
). If the last modification time is derived from implementation-specific metadata
that evaluates to some time in the future, according to the origin server's clock,
then the origin server
MUST
replace that value with the message origination date. This prevents a future modification
date from having an adverse impact on cache validation.
An origin server without a clock
MUST NOT
generate a Last-Modified date for a response unless that date value was assigned to
the resource by some other system (presumably one with a clock).
8.8.2.2.
Comparison
A Last-Modified time, when used as a validator in a request, is implicitly weak unless
it is possible to deduce that it is strong, using the following rules:
The validator is being compared by an origin server to the actual current validator
for the representation and,
That origin server reliably knows that the associated representation did not change
twice during the second covered by the presented validator;
or
The validator is about to be used by a client in an
If-Modified-Since
If-Unmodified-Since
, or
If-Range
header field, because the client has a cache entry for the associated representation,
and
That cache entry includes a
Date
value which is at least one second after the Last-Modified value and the client has
reason to believe that they were generated by the same clock or that there is enough
difference between the Last-Modified and Date values to make clock synchronization
issues unlikely;
or
The validator is being compared by an intermediate cache to the validator stored in
its cache entry for the representation, and
That cache entry includes a
Date
value which is at least one second after the Last-Modified value and the cache has
reason to believe that they were generated by the same clock or that there is enough
difference between the Last-Modified and Date values to make clock synchronization
issues unlikely.
This method relies on the fact that if two different responses were sent by the origin
server during the same second, but both had the same Last-Modified time, then at least
one of those responses would have a
Date
value equal to its Last-Modified time.
8.8.3.
ETag
The "ETag" field in a response provides the current entity tag for the
selected representation
, as determined at the conclusion of handling the request. An entity tag is an opaque
validator for differentiating between multiple representations of the same resource,
regardless of whether those multiple representations are due to resource state changes
over time, content negotiation resulting in multiple representations being valid at
the same time, or both. An entity tag consists of an opaque quoted string, possibly
prefixed by a weakness indicator.
ETag
entity-tag
entity-tag
= [
weak
opaque-tag
weak
= %s"W/"
opaque-tag
DQUOTE
etagc
DQUOTE
etagc
= %x21 / %x23-7E /
obs-text
VCHAR
except double quotes, plus obs-text
Note:
Previously, opaque-tag was defined to be a quoted-string (
[RFC2616]
Section 3.11
); thus, some recipients might perform backslash unescaping. Servers therefore ought
to avoid backslash characters in entity tags.
An entity tag can be more reliable for validation than a modification date in situations
where it is inconvenient to store modification dates, where the one-second resolution
of HTTP-date values is not sufficient, or where modification dates are not consistently
maintained.
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
An entity tag can be either a weak or strong validator, with strong being the default.
If an origin server provides an entity tag for a representation and the generation
of that entity tag does not satisfy all of the characteristics of a strong validator
Section 8.8.1
), then the origin server
MUST
mark the entity tag as weak by prefixing its opaque value with "W/" (case-sensitive).
A sender
MAY
send the ETag field in a trailer section (see
Section 6.5
). However, since trailers are often ignored, it is preferable to send ETag as a header
field unless the entity tag is generated while sending the content.
8.8.3.1.
Generation
The principle behind entity tags is that only the service author knows the implementation
of a resource well enough to select the most accurate and efficient validation mechanism
for that resource, and that any such mechanism can be mapped to a simple sequence
of octets for easy comparison. Since the value is opaque, there is no need for the
client to be aware of how each entity tag is constructed.
For example, a resource that has implementation-specific versioning applied to all
changes might use an internal revision number, perhaps combined with a variance identifier
for content negotiation, to accurately differentiate between representations. Other
implementations might use a collision-resistant hash of representation content, a
combination of various file attributes, or a modification timestamp that has sub-second
resolution.
An origin server
SHOULD
send an ETag for any selected representation for which detection of changes can be
reasonably and consistently determined, since the entity tag's use in conditional
requests and evaluating cache freshness (
[CACHING]
) can substantially reduce unnecessary transfers and significantly improve service
availability, scalability, and reliability.
8.8.3.2.
Comparison
There are two entity tag comparison functions, depending on whether or not the comparison
context allows the use of weak validators:
Strong comparison
two entity tags are equivalent if both are not weak and their opaque-tags match character-by-character.
Weak comparison
two entity tags are equivalent if their opaque-tags match character-by-character,
regardless of either or both being tagged as "weak".
The example below shows the results for a set of entity tag pairs and both the weak
and strong comparison function results:
Table 3
ETag 1
ETag 2
Strong Comparison
Weak Comparison
W/"1"
W/"1"
no match
match
W/"1"
W/"2"
no match
no match
W/"1"
"1"
no match
match
"1"
"1"
match
match
8.8.3.3.
Example: Entity Tags Varying on Content-Negotiated Resources
Consider a resource that is subject to content negotiation (
Section 12
), and where the representations sent in response to a GET request vary based on the
Accept-Encoding
request header field (
Section 12.5.3
):
>> Request:
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
In this case, the response might or might not use the gzip content coding. If it does
not, the response might look like:
>> Response:
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length: 70
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
An alternative representation that does use gzip content coding would be:
>> Response:
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-b"
Content-Length: 43
Vary: Accept-Encoding
Content-Type: text/plain
Content-Encoding: gzip
...binary data...
Note:
Content codings are a property of the representation data, so a strong entity tag
for a content-encoded representation has to be distinct from the entity tag of an
unencoded representation to prevent potential conflicts during cache updates and range
requests. In contrast, transfer codings (
Section 7
of
[HTTP/1.1]
) apply only during message transfer and do not result in distinct entity tags.
9.
Methods
9.1.
Overview
The request method token is the primary source of request semantics; it indicates
the purpose for which the client has made this request and what is expected by the
client as a successful result.
The request method's semantics might be further specialized by the semantics of some
header fields when present in a request if those additional semantics do not conflict
with the method. For example, a client can send conditional request header fields
Section 13.1
) to make the requested action conditional on the current state of the target resource.
HTTP is designed to be usable as an interface to distributed object systems. The request
method invokes an action to be applied to a
target resource
in much the same way that a remote method invocation can be sent to an identified
object.
method
token
The method token is case-sensitive because it might be used as a gateway to object-based
systems with case-sensitive method names. By convention, standardized methods are
defined in all-uppercase US-ASCII letters.
Unlike distributed objects, the standardized request methods in HTTP are not resource-specific,
since uniform interfaces provide for better visibility and reuse in network-based
systems
[REST]
. Once defined, a standardized method ought to have the same semantics when applied
to any resource, though each resource determines for itself whether those semantics
are implemented or allowed.
This specification defines a number of standardized methods that are commonly used
in HTTP, as outlined by the following table.
Table 4
Method Name
Description
Section
GET
Transfer a current representation of the target resource.
9.3.1
HEAD
Same as GET, but do not transfer the response content.
9.3.2
POST
Perform resource-specific processing on the request content.
9.3.3
PUT
Replace all current representations of the target resource with the request content.
9.3.4
DELETE
Remove all current representations of the target resource.
9.3.5
CONNECT
Establish a tunnel to the server identified by the target resource.
9.3.6
OPTIONS
Describe the communication options for the target resource.
9.3.7
TRACE
Perform a message loop-back test along the path to the target resource.
9.3.8
All general-purpose servers
MUST
support the methods GET and HEAD. All other methods are
OPTIONAL
The set of methods allowed by a target resource can be listed in an
Allow
header field (
Section 10.2.1
). However, the set of allowed methods can change dynamically. An origin server that
receives a request method that is unrecognized or not implemented
SHOULD
respond with the
501 (Not Implemented)
status code. An origin server that receives a request method that is recognized and
implemented, but not allowed for the target resource,
SHOULD
respond with the
405 (Method Not Allowed)
status code.
Additional methods, outside the scope of this specification, have been specified for
use in HTTP. All such methods ought to be registered within the "Hypertext Transfer
Protocol (HTTP) Method Registry", as described in
Section 16.1
9.2.
Common Method Properties
9.2.1.
Safe Methods
Request methods are considered
safe
if their defined semantics are essentially read-only; i.e., the client does not request,
and does not expect, any state change on the origin server as a result of applying
a safe method to a target resource. Likewise, reasonable use of a safe method is not
expected to cause any harm, loss of property, or unusual burden on the origin server.
This definition of safe methods does not prevent an implementation from including
behavior that is potentially harmful, that is not entirely read-only, or that causes
side effects while invoking a safe method. What is important, however, is that the
client did not request that additional behavior and cannot be held accountable for
it. For example, most servers append request information to access log files at the
completion of every response, regardless of the method, and that is considered safe
even though the log storage might become full and cause the server to fail. Likewise,
a safe request initiated by selecting an advertisement on the Web will often have
the side effect of charging an advertising account.
Of the request methods defined by this specification, the
GET
HEAD
OPTIONS
, and
TRACE
methods are defined to be safe.
The purpose of distinguishing between safe and unsafe methods is to allow automated
retrieval processes (spiders) and cache performance optimization (pre-fetching) to
work without fear of causing harm. In addition, it allows a user agent to apply appropriate
constraints on the automated use of unsafe methods when processing potentially untrusted
content.
A user agent
SHOULD
distinguish between safe and unsafe methods when presenting potential actions to a
user, such that the user can be made aware of an unsafe action before it is requested.
When a resource is constructed such that parameters within the target URI have the
effect of selecting an action, it is the resource owner's responsibility to ensure
that the action is consistent with the request method semantics. For example, it is
common for Web-based content editing software to use actions within query parameters,
such as "page?do=delete". If the purpose of such a resource is to perform an unsafe
action, then the resource owner
MUST
disable or disallow that action when it is accessed using a safe request method. Failure
to do so will result in unfortunate side effects when automated processes perform
a GET on every URI reference for the sake of link maintenance, pre-fetching, building
a search index, etc.
9.2.2.
Idempotent Methods
A request method is considered
idempotent
if the intended effect on the server of multiple identical requests with that method
is the same as the effect for a single such request. Of the request methods defined
by this specification,
PUT
DELETE
, and safe request methods are idempotent.
Like the definition of safe, the idempotent property only applies to what has been
requested by the user; a server is free to log each request separately, retain a revision
control history, or implement other non-idempotent side effects for each idempotent
request.
Idempotent methods are distinguished because the request can be repeated automatically
if a communication failure occurs before the client is able to read the server's response.
For example, if a client sends a PUT request and the underlying connection is closed
before any response is received, then the client can establish a new connection and
retry the idempotent request. It knows that repeating the request will have the same
intended effect, even if the original request succeeded, though the response might
differ.
A client
SHOULD NOT
automatically retry a request with a non-idempotent method unless it has some means
to know that the request semantics are actually idempotent, regardless of the method,
or some means to detect that the original request was never applied.
For example, a user agent can repeat a POST request automatically if it knows (through
design or configuration) that the request is safe for that resource. Likewise, a user
agent designed specifically to operate on a version control repository might be able
to recover from partial failure conditions by checking the target resource revision(s)
after a failed connection, reverting or fixing any changes that were partially applied,
and then automatically retrying the requests that failed.
Some clients take a riskier approach and attempt to guess when an automatic retry
is possible. For example, a client might automatically retry a POST request if the
underlying transport connection closed before any part of a response is received,
particularly if an idle persistent connection was used.
A proxy
MUST NOT
automatically retry non-idempotent requests. A client
SHOULD NOT
automatically retry a failed automatic retry.
9.2.3.
Methods and Caching
For a cache to store and use a response, the associated method needs to explicitly
allow caching and to detail under what conditions a response can be used to satisfy
subsequent requests; a method definition that does not do so cannot be cached. For
additional requirements see
[CACHING]
This specification defines caching semantics for GET, HEAD, and POST, although the
overwhelming majority of cache implementations only support GET and HEAD.
9.3.
Method Definitions
9.3.1.
GET
The GET method requests transfer of a current
selected representation
for the
target resource
. A successful response reflects the quality of "sameness" identified by the target
URI (
Section 1.2.2
of
[URI]
). Hence, retrieving identifiable information via HTTP is usually performed by making
a GET request on an identifier associated with the potential for providing that information
in a
200 (OK)
response.
GET is the primary mechanism of information retrieval and the focus of almost all
performance optimizations. Applications that produce a URI for each important resource
can benefit from those optimizations while enabling their reuse by other applications,
creating a network effect that promotes further expansion of the Web.
It is tempting to think of resource identifiers as remote file system pathnames and
of representations as being a copy of the contents of such files. In fact, that is
how many resources are implemented (see
Section 17.3
for related security considerations). However, there are no such limitations in practice.
The HTTP interface for a resource is just as likely to be implemented as a tree of
content objects, a programmatic view on various database records, or a gateway to
other information systems. Even when the URI mapping mechanism is tied to a file system,
an origin server might be configured to execute the files with the request as input
and send the output as the representation rather than transfer the files directly.
Regardless, only the origin server needs to know how each resource identifier corresponds
to an implementation and how that implementation manages to select and send a current
representation of the target resource.
A client can alter the semantics of GET to be a "range request", requesting transfer
of only some part(s) of the selected representation, by sending a
Range
header field in the request (
Section 14.2
).
Although request message framing is independent of the method used, content received
in a GET request has no generally defined semantics, cannot alter the meaning or target
of the request, and might lead some implementations to reject the request and close
the connection because of its potential as a request smuggling attack (
Section 11.2
of
[HTTP/1.1]
). A client
SHOULD NOT
generate content in a GET request unless it is made directly to an origin server that
has previously indicated, in or out of band, that such a request has a purpose and
will be adequately supported. An origin server
SHOULD NOT
rely on private agreements to receive content, since participants in HTTP communication
are often unaware of intermediaries along the request chain.
The response to a GET request is cacheable; a cache
MAY
use it to satisfy subsequent GET and HEAD requests unless otherwise indicated by the
Cache-Control header field (
Section 5.2
of
[CACHING]
).
When information retrieval is performed with a mechanism that constructs a target
URI from user-provided information, such as the query fields of a form using GET,
potentially sensitive data might be provided that would not be appropriate for disclosure
within a URI (see
Section 17.9
). In some cases, the data can be filtered or transformed such that it would not reveal
such information. In others, particularly when there is no benefit from caching a
response, using the POST method (
Section 9.3.3
) instead of GET can transmit such information in the request content rather than
within the target URI.
9.3.2.
HEAD
The HEAD method is identical to GET except that the server
MUST NOT
send content in the response. HEAD is used to obtain metadata about the
selected representation
without transferring its representation data, often for the sake of testing hypertext
links or finding recent modifications.
The server
SHOULD
send the same header fields in response to a HEAD request as it would have sent if
the request method had been GET. However, a server
MAY
omit header fields for which a value is determined only while generating the content.
For example, some servers buffer a dynamic response to GET until a minimum amount
of data is generated so that they can more efficiently delimit small responses or
make late decisions with regard to content selection. Such a response to GET might
contain
Content-Length
and
Vary
fields, for example, that are not generated within a HEAD response. These minor inconsistencies
are considered preferable to generating and discarding the content for a HEAD request,
since HEAD is usually requested for the sake of efficiency.
Although request message framing is independent of the method used, content received
in a HEAD request has no generally defined semantics, cannot alter the meaning or
target of the request, and might lead some implementations to reject the request and
close the connection because of its potential as a request smuggling attack (
Section 11.2
of
[HTTP/1.1]
). A client
SHOULD NOT
generate content in a HEAD request unless it is made directly to an origin server
that has previously indicated, in or out of band, that such a request has a purpose
and will be adequately supported. An origin server
SHOULD NOT
rely on private agreements to receive content, since participants in HTTP communication
are often unaware of intermediaries along the request chain.
The response to a HEAD request is cacheable; a cache
MAY
use it to satisfy subsequent HEAD requests unless otherwise indicated by the Cache-Control
header field (
Section 5.2
of
[CACHING]
). A HEAD response might also affect previously cached responses to GET; see
Section 4.3.5
of
[CACHING]
9.3.3.
POST
The POST method requests that the
target resource
process the representation enclosed in the request according to the resource's own
specific semantics. For example, POST is used for the following functions (among others):
Providing a block of data, such as the fields entered into an HTML form, to a data-handling
process;
Posting a message to a bulletin board, newsgroup, mailing list, blog, or similar group
of articles;
Creating a new resource that has yet to be identified by the origin server; and
Appending data to a resource's existing representation(s).
An origin server indicates response semantics by choosing an appropriate status code
depending on the result of processing the POST request; almost all of the status codes
defined by this specification could be received in a response to POST (the exceptions
being
206 (Partial Content)
304 (Not Modified)
, and
416 (Range Not Satisfiable)
).
If one or more resources has been created on the origin server as a result of successfully
processing a POST request, the origin server
SHOULD
send a
201 (Created)
response containing a
Location
header field that provides an identifier for the primary resource created (
Section 10.2.2
) and a representation that describes the status of the request while referring to
the new resource(s).
Responses to POST requests are only cacheable when they include explicit freshness
information (see
Section 4.2.1
of
[CACHING]
) and a
Content-Location
header field that has the same value as the POST's target URI (
Section 8.7
). A cached POST response can be reused to satisfy a later GET or HEAD request. In
contrast, a POST request cannot be satisfied by a cached POST response because POST
is potentially unsafe; see
Section 4
of
[CACHING]
If the result of processing a POST would be equivalent to a representation of an existing
resource, an origin server
MAY
redirect the user agent to that resource by sending a
303 (See Other)
response with the existing resource's identifier in the
Location
field. This has the benefits of providing the user agent a resource identifier and
transferring the representation via a method more amenable to shared caching, though
at the cost of an extra request if the user agent does not already have the representation
cached.
9.3.4.
PUT
The PUT method requests that the state of the
target resource
be created or replaced with the state defined by the representation enclosed in the
request message content. A successful PUT of a given representation would suggest
that a subsequent GET on that same target resource will result in an equivalent representation
being sent in a
200 (OK)
response. However, there is no guarantee that such a state change will be observable,
since the target resource might be acted upon by other user agents in parallel, or
might be subject to dynamic processing by the origin server, before any subsequent
GET is received. A successful response only implies that the user agent's intent was
achieved at the time of its processing by the origin server.
If the target resource does not have a current representation and the PUT successfully
creates one, then the origin server
MUST
inform the user agent by sending a
201 (Created)
response. If the target resource does have a current representation and that representation
is successfully modified in accordance with the state of the enclosed representation,
then the origin server
MUST
send either a
200 (OK)
or a
204 (No Content)
response to indicate successful completion of the request.
An origin server
SHOULD
verify that the PUT representation is consistent with its configured constraints for
the target resource. For example, if an origin server determines a resource's representation
metadata based on the URI, then the origin server needs to ensure that the content
received in a successful PUT request is consistent with that metadata. When a PUT
representation is inconsistent with the target resource, the origin server
SHOULD
either make them consistent, by transforming the representation or changing the resource
configuration, or respond with an appropriate error message containing sufficient
information to explain why the representation is unsuitable. The
409 (Conflict)
or
415 (Unsupported Media Type)
status codes are suggested, with the latter being specific to constraints on
Content-Type
values.
For example, if the target resource is configured to always have a
Content-Type
of "text/html" and the representation being PUT has a Content-Type of "image/jpeg",
the origin server ought to do one of:
reconfigure the target resource to reflect the new media type;
transform the PUT representation to a format consistent with that of the resource
before saving it as the new resource state; or,
reject the request with a
415 (Unsupported Media Type)
response indicating that the target resource is limited to "text/html", perhaps including
a link to a different resource that would be a suitable target for the new representation.
HTTP does not define exactly how a PUT method affects the state of an origin server
beyond what can be expressed by the intent of the user agent request and the semantics
of the origin server response. It does not define what a resource might be, in any
sense of that word, beyond the interface provided via HTTP. It does not define how
resource state is "stored", nor how such storage might change as a result of a change
in resource state, nor how the origin server translates resource state into representations.
Generally speaking, all implementation details behind the resource interface are intentionally
hidden by the server.
This extends to how header and trailer fields are stored; while common header fields
like
Content-Type
will typically be stored and returned upon subsequent GET requests, header and trailer
field handling is specific to the resource that received the request. As a result,
an origin server
SHOULD
ignore unrecognized header and trailer fields received in a PUT request (i.e., not
save them as part of the resource state).
An origin server
MUST NOT
send a validator field (
Section 8.8
), such as an
ETag
or
Last-Modified
field, in a successful response to PUT unless the request's representation data was
saved without any transformation applied to the content (i.e., the resource's new
representation data is identical to the content received in the PUT request) and the
validator field value reflects the new representation. This requirement allows a user
agent to know when the representation it sent (and retains in memory) is the result
of the PUT, and thus it doesn't need to be retrieved again from the origin server.
The new validator(s) received in the response can be used for future conditional requests
in order to prevent accidental overwrites (
Section 13.1
).
The fundamental difference between the POST and PUT methods is highlighted by the
different intent for the enclosed representation. The target resource in a POST request
is intended to handle the enclosed representation according to the resource's own
semantics, whereas the enclosed representation in a PUT request is defined as replacing
the state of the target resource. Hence, the intent of PUT is idempotent and visible
to intermediaries, even though the exact effect is only known by the origin server.
Proper interpretation of a PUT request presumes that the user agent knows which target
resource is desired. A service that selects a proper URI on behalf of the client,
after receiving a state-changing request,
SHOULD
be implemented using the POST method rather than PUT. If the origin server will not
make the requested PUT state change to the target resource and instead wishes to have
it applied to a different resource, such as when the resource has been moved to a
different URI, then the origin server
MUST
send an appropriate
3xx (Redirection)
response; the user agent
MAY
then make its own decision regarding whether or not to redirect the request.
A PUT request applied to the target resource can have side effects on other resources.
For example, an article might have a URI for identifying "the current version" (a
resource) that is separate from the URIs identifying each particular version (different
resources that at one point shared the same state as the current version resource).
A successful PUT request on "the current version" URI might therefore create a new
version resource in addition to changing the state of the target resource, and might
also cause links to be added between the related resources.
Some origin servers support use of the
Content-Range
header field (
Section 14.4
) as a request modifier to perform a partial PUT, as described in
Section 14.5
Responses to the PUT method are not cacheable. If a successful PUT request passes
through a cache that has one or more stored responses for the target URI, those stored
responses will be invalidated (see
Section 4.4
of
[CACHING]
).
9.3.5.
DELETE
The DELETE method requests that the origin server remove the association between the
target resource
and its current functionality. In effect, this method is similar to the "rm" command
in UNIX: it expresses a deletion operation on the URI mapping of the origin server
rather than an expectation that the previously associated information be deleted.
If the target resource has one or more current representations, they might or might
not be destroyed by the origin server, and the associated storage might or might not
be reclaimed, depending entirely on the nature of the resource and its implementation
by the origin server (which are beyond the scope of this specification). Likewise,
other implementation aspects of a resource might need to be deactivated or archived
as a result of a DELETE, such as database or gateway connections. In general, it is
assumed that the origin server will only allow DELETE on resources for which it has
a prescribed mechanism for accomplishing the deletion.
Relatively few resources allow the DELETE method — its primary use is for remote authoring
environments, where the user has some direction regarding its effect. For example,
a resource that was previously created using a PUT request, or identified via the
Location header field after a
201 (Created)
response to a POST request, might allow a corresponding DELETE request to undo those
actions. Similarly, custom user agent implementations that implement an authoring
function, such as revision control clients using HTTP for remote operations, might
use DELETE based on an assumption that the server's URI space has been crafted to
correspond to a version repository.
If a DELETE method is successfully applied, the origin server
SHOULD
send
202 (Accepted)
status code if the action will likely succeed but has not yet been enacted,
204 (No Content)
status code if the action has been enacted and no further information is to be supplied,
or
200 (OK)
status code if the action has been enacted and the response message includes a representation
describing the status.
Although request message framing is independent of the method used, content received
in a DELETE request has no generally defined semantics, cannot alter the meaning or
target of the request, and might lead some implementations to reject the request and
close the connection because of its potential as a request smuggling attack (
Section 11.2
of
[HTTP/1.1]
). A client
SHOULD NOT
generate content in a DELETE request unless it is made directly to an origin server
that has previously indicated, in or out of band, that such a request has a purpose
and will be adequately supported. An origin server
SHOULD NOT
rely on private agreements to receive content, since participants in HTTP communication
are often unaware of intermediaries along the request chain.
Responses to the DELETE method are not cacheable. If a successful DELETE request passes
through a cache that has one or more stored responses for the target URI, those stored
responses will be invalidated (see
Section 4.4
of
[CACHING]
).
9.3.6.
CONNECT
The CONNECT method requests that the recipient establish a tunnel to the destination
origin server identified by the request target and, if successful, thereafter restrict
its behavior to blind forwarding of data, in both directions, until the tunnel is
closed. Tunnels are commonly used to create an end-to-end virtual connection, through
one or more proxies, which can then be secured using TLS (Transport Layer Security,
[TLS13]
).
CONNECT uses a special form of request target, unique to this method, consisting of
only the host and port number of the tunnel destination, separated by a colon. There
is no default port; a client
MUST
send the port number even if the CONNECT request is based on a URI reference that
contains an authority component with an elided port (
Section 4.1
). For example,
CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com
A server
MUST
reject a CONNECT request that targets an empty or invalid port number, typically by
responding with a 400 (Bad Request) status code.
Because CONNECT changes the request/response nature of an HTTP connection, specific
HTTP versions might have different ways of mapping its semantics into the protocol's
wire format.
CONNECT is intended for use in requests to a proxy. The recipient can establish a
tunnel either by directly connecting to the server identified by the request target
or, if configured to use another proxy, by forwarding the CONNECT request to the next
inbound proxy. An origin server
MAY
accept a CONNECT request, but most origin servers do not implement CONNECT.
Any
2xx (Successful)
response indicates that the sender (and all inbound proxies) will switch to tunnel
mode immediately after the response header section; data received after that header
section is from the server identified by the request target. Any response other than
a successful response indicates that the tunnel has not yet been formed.
A tunnel is closed when a tunnel intermediary detects that either side has closed
its connection: the intermediary
MUST
attempt to send any outstanding data that came from the closed side to the other side,
close both connections, and then discard any remaining data left undelivered.
Proxy authentication might be used to establish the authority to create a tunnel.
For example,
CONNECT server.example.com:443 HTTP/1.1
Host: server.example.com:443
Proxy-Authorization: basic aGVsbG86d29ybGQ=
There are significant risks in establishing a tunnel to arbitrary servers, particularly
when the destination is a well-known or reserved TCP port that is not intended for
Web traffic. For example, a CONNECT to "example.com:25" would suggest that the proxy
connect to the reserved port for SMTP traffic; if allowed, that could trick the proxy
into relaying spam email. Proxies that support CONNECT
SHOULD
restrict its use to a limited set of known ports or a configurable list of safe request
targets.
A server
MUST NOT
send any
Transfer-Encoding
or
Content-Length
header fields in a
2xx (Successful)
response to CONNECT. A client
MUST
ignore any Content-Length or Transfer-Encoding header fields received in a successful
response to CONNECT.
A CONNECT request message does not have content. The interpretation of data sent after
the header section of the CONNECT request message is specific to the version of HTTP
in use.
Responses to the CONNECT method are not cacheable.
9.3.7.
OPTIONS
The OPTIONS method requests information about the communication options available
for the target resource, at either the origin server or an intervening intermediary.
This method allows a client to determine the options and/or requirements associated
with a resource, or the capabilities of a server, without implying a resource action.
An OPTIONS request with an asterisk ("*") as the request target (
Section 7.1
) applies to the server in general rather than to a specific resource. Since a server's
communication options typically depend on the resource, the "*" request is only useful
as a "ping" or "no-op" type of method; it does nothing beyond allowing the client
to test the capabilities of the server. For example, this can be used to test a proxy
for HTTP/1.1 conformance (or lack thereof).
If the request target is not an asterisk, the OPTIONS request applies to the options
that are available when communicating with the target resource.
A server generating a successful response to OPTIONS
SHOULD
send any header that might indicate optional features implemented by the server and
applicable to the target resource (e.g.,
Allow
), including potential extensions not defined by this specification. The response
content, if any, might also describe the communication options in a machine or human-readable
representation. A standard format for such a representation is not defined by this
specification, but might be defined by future extensions to HTTP.
A client
MAY
send a
Max-Forwards
header field in an OPTIONS request to target a specific recipient in the request chain
(see
Section 7.6.2
). A proxy
MUST NOT
generate a Max-Forwards header field while forwarding a request unless that request
was received with a Max-Forwards field.
A client that generates an OPTIONS request containing content
MUST
send a valid
Content-Type
header field describing the representation media type. Note that this specification
does not define any use for such content.
Responses to the OPTIONS method are not cacheable.
9.3.8.
TRACE
The TRACE method requests a remote, application-level loop-back of the request message.
The final recipient of the request
SHOULD
reflect the message received, excluding some fields described below, back to the client
as the content of a
200 (OK)
response. The "message/http" format (
Section 10.1
of
[HTTP/1.1]
) is one way to do so. The final recipient is either the origin server or the first
server to receive a
Max-Forwards
value of zero (0) in the request (
Section 7.6.2
).
A client
MUST NOT
generate fields in a TRACE request containing sensitive data that might be disclosed
by the response. For example, it would be foolish for a user agent to send stored
user credentials (
Section 11
) or cookies
[COOKIE]
in a TRACE request. The final recipient of the request
SHOULD
exclude any request fields that are likely to contain sensitive data when that recipient
generates the response content.
TRACE allows the client to see what is being received at the other end of the request
chain and use that data for testing or diagnostic information. The value of the
Via
header field (
Section 7.6.3
) is of particular interest, since it acts as a trace of the request chain. Use of
the
Max-Forwards
header field allows the client to limit the length of the request chain, which is
useful for testing a chain of proxies forwarding messages in an infinite loop.
A client
MUST NOT
send content in a TRACE request.
Responses to the TRACE method are not cacheable.
10.
Message Context
10.1.
Request Context Fields
The request header fields below provide additional information about the request context,
including information about the user, user agent, and resource behind the request.
10.1.1.
Expect
The "Expect" header field in a request indicates a certain set of behaviors (expectations)
that need to be supported by the server in order to properly handle this request.
Expect
= #
expectation
expectation
token
[ "=" (
token
quoted-string
parameters
The Expect field value is case-insensitive.
The only expectation defined by this specification is "100-continue" (with no defined
parameters).
A server that receives an Expect field value containing a member other than
100-continue
MAY
respond with a
417 (Expectation Failed)
status code to indicate that the unexpected expectation cannot be met.
100-continue
expectation informs recipients that the client is about to send (presumably large)
content in this request and wishes to receive a
100 (Continue)
interim response if the method, target URI, and header fields are not sufficient to
cause an immediate success, redirect, or error response. This allows the client to
wait for an indication that it is worthwhile to send the content before actually doing
so, which can improve efficiency when the data is huge or when the client anticipates
that an error is likely (e.g., when sending a state-changing method, for the first
time, without previously verified authentication credentials).
For example, a request that begins with
PUT /somewhere/fun HTTP/1.1
Host: origin.example.com
Content-Type: video/h264
Content-Length: 1234567890987
Expect: 100-continue
allows the origin server to immediately respond with an error message, such as
401 (Unauthorized)
or
405 (Method Not Allowed)
, before the client starts filling the pipes with an unnecessary data transfer.
Requirements for clients:
A client
MUST NOT
generate a 100-continue expectation in a request that does not include content.
A client that will wait for a
100 (Continue)
response before sending the request content
MUST
send an
Expect
header field containing a 100-continue expectation.
A client that sends a 100-continue expectation is not required to wait for any specific
length of time; such a client
MAY
proceed to send the content even if it has not yet received a response. Furthermore,
since
100 (Continue)
responses cannot be sent through an HTTP/1.0 intermediary, such a client
SHOULD NOT
wait for an indefinite period before sending the content.
A client that receives a
417 (Expectation Failed)
status code in response to a request containing a 100-continue expectation
SHOULD
repeat that request without a 100-continue expectation, since the 417 response merely
indicates that the response chain does not support expectations (e.g., it passes through
an HTTP/1.0 server).
Requirements for servers:
A server that receives a 100-continue expectation in an HTTP/1.0 request
MUST
ignore that expectation.
A server
MAY
omit sending a
100 (Continue)
response if it has already received some or all of the content for the corresponding
request, or if the framing indicates that there is no content.
A server that sends a
100 (Continue)
response
MUST
ultimately send a final status code, once it receives and processes the request content,
unless the connection is closed prematurely.
A server that responds with a final status code before reading the entire request
content
SHOULD
indicate whether it intends to close the connection (e.g., see
Section 9.6
of
[HTTP/1.1]
) or continue reading the request content.
Upon receiving an HTTP/1.1 (or later) request that has a method, target URI, and complete
header section that contains a 100-continue expectation and an indication that request
content will follow, an origin server
MUST
send either:
an immediate response with a final status code, if that status can be determined by
examining just the method, target URI, and header fields, or
an immediate
100 (Continue)
response to encourage the client to send the request content.
The origin server
MUST NOT
wait for the content before sending the
100 (Continue)
response.
Upon receiving an HTTP/1.1 (or later) request that has a method, target URI, and complete
header section that contains a 100-continue expectation and indicates a request content
will follow, a proxy
MUST
either:
send an immediate response with a final status code, if that status can be determined
by examining just the method, target URI, and header fields, or
forward the request toward the origin server by sending a corresponding request-line
and header section to the next inbound server.
If the proxy believes (from configuration or past interaction) that the next inbound
server only supports HTTP/1.0, the proxy
MAY
generate an immediate
100 (Continue)
response to encourage the client to begin sending the content.
10.1.2.
From
The "From" header field contains an Internet email address for a human user who controls
the requesting user agent. The address ought to be machine-usable, as defined by "mailbox"
in
Section 3.4
of
[RFC5322]
From
mailbox
mailbox
=
Section 3.4
An example is:
From: spider-admin@example.org
The From header field is rarely sent by non-robotic user agents. A user agent
SHOULD NOT
send a From header field without explicit configuration by the user, since that might
conflict with the user's privacy interests or their site's security policy.
A robotic user agent
SHOULD
send a valid From header field so that the person responsible for running the robot
can be contacted if problems occur on servers, such as if the robot is sending excessive,
unwanted, or invalid requests.
A server
SHOULD NOT
use the From header field for access control or authentication, since its value is
expected to be visible to anyone receiving or observing the request and is often recorded
within logfiles and error reports without any expectation of privacy.
10.1.3.
Referer
The "Referer" [sic] header field allows the user agent to specify a URI reference
for the resource from which the
target URI
was obtained (i.e., the "referrer", though the field name is misspelled). A user agent
MUST NOT
include the fragment and userinfo components of the URI reference
[URI]
, if any, when generating the Referer field value.
Referer
absolute-URI
partial-URI
The field value is either an
absolute-URI
or a
partial-URI
. In the latter case (
Section 4
), the referenced URI is relative to the target URI (
[URI]
Section 5
).
The Referer header field allows servers to generate back-links to other resources
for simple analytics, logging, optimized caching, etc. It also allows obsolete or
mistyped links to be found for maintenance. Some servers use the Referer header field
as a means of denying links from other sites (so-called "deep linking") or restricting
cross-site request forgery (CSRF), but not all requests contain it.
Example:
Referer: http://www.example.org/hypertext/Overview.html
If the target URI was obtained from a source that does not have its own URI (e.g.,
input from the user keyboard, or an entry within the user's bookmarks/favorites),
the user agent
MUST
either exclude the Referer header field or send it with a value of "about:blank".
The Referer header field value need not convey the full URI of the referring resource;
a user agent
MAY
truncate parts other than the referring origin.
The Referer header field has the potential to reveal information about the request
context or browsing history of the user, which is a privacy concern if the referring
resource's identifier reveals personal information (such as an account name) or a
resource that is supposed to be confidential (such as behind a firewall or internal
to a secured service). Most general-purpose user agents do not send the Referer header
field when the referring resource is a local "file" or "data" URI. A user agent
SHOULD NOT
send a
Referer
header field if the referring resource was accessed with a secure protocol and the
request target has an origin differing from that of the referring resource, unless
the referring resource explicitly allows Referer to be sent. A user agent
MUST NOT
send a
Referer
header field in an unsecured HTTP request if the referring resource was accessed with
a secure protocol. See
Section 17.9
for additional security considerations.
Some intermediaries have been known to indiscriminately remove Referer header fields
from outgoing requests. This has the unfortunate side effect of interfering with protection
against CSRF attacks, which can be far more harmful to their users. Intermediaries
and user agent extensions that wish to limit information disclosure in Referer ought
to restrict their changes to specific edits, such as replacing internal domain names
with pseudonyms or truncating the query and/or path components. An intermediary
SHOULD NOT
modify or delete the Referer header field when the field value shares the same scheme
and host as the target URI.
10.1.4.
TE
The "TE" header field describes capabilities of the client with regard to transfer
codings and trailer sections.
As described in
Section 6.5
, a TE field with a "trailers" member sent in a request indicates that the client
will not discard trailer fields.
TE is also used within HTTP/1.1 to advise servers about which transfer codings the
client is able to accept in a response. As of publication, only HTTP/1.1 uses transfer
codings (see
Section 7
of
[HTTP/1.1]
).
The TE field value is a list of members, with each member (aside from "trailers")
consisting of a transfer coding name token with an optional weight indicating the
client's relative preference for that transfer coding (
Section 12.4.2
) and optional parameters for that transfer coding.
TE
= #
t-codings
t-codings
= "trailers" / (
transfer-coding
weight
] )
transfer-coding
token
*(
OWS
";"
OWS
transfer-parameter
transfer-parameter
token
BWS
"="
BWS
token
quoted-string
A sender of TE
MUST
also send a "TE" connection option within the
Connection
header field (
Section 7.6.1
) to inform intermediaries not to forward this field.
10.1.5.
User-Agent
The "User-Agent" header field contains information about the user agent originating
the request, which is often used by servers to help identify the scope of reported
interoperability problems, to work around or tailor responses to avoid particular
user agent limitations, and for analytics regarding browser or operating system use.
A user agent
SHOULD
send a User-Agent header field in each request unless specifically configured not
to do so.
User-Agent
product
*(
RWS
product
comment
) )
The User-Agent field value consists of one or more product identifiers, each followed
by zero or more comments (
Section 5.6.5
), which together identify the user agent software and its significant subproducts.
By convention, the product identifiers are listed in decreasing order of their significance
for identifying the user agent software. Each product identifier consists of a name
and optional version.
product
token
["/"
product-version
product-version
token
A sender
SHOULD
limit generated product identifiers to what is necessary to identify the product;
a sender
MUST NOT
generate advertising or other nonessential information within the product identifier.
A sender
SHOULD NOT
generate information in
product-version
that is not a version identifier (i.e., successive versions of the same product name
ought to differ only in the product-version portion of the product identifier).
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
A user agent
SHOULD NOT
generate a User-Agent header field containing needlessly fine-grained detail and
SHOULD
limit the addition of subproducts by third parties. Overly long and detailed User-Agent
field values increase request latency and the risk of a user being identified against
their wishes ("fingerprinting").
Likewise, implementations are encouraged not to use the product tokens of other implementations
in order to declare compatibility with them, as this circumvents the purpose of the
field. If a user agent masquerades as a different user agent, recipients can assume
that the user intentionally desires to see responses tailored for that identified
user agent, even if they might not work as well for the actual user agent being used.
10.2.
Response Context Fields
The response header fields below provide additional information about the response,
beyond what is implied by the status code, including information about the server,
about the
target resource
, or about related resources.
10.2.1.
Allow
The "Allow" header field lists the set of methods advertised as supported by the
target resource
. The purpose of this field is strictly to inform the recipient of valid request methods
associated with the resource.
Allow
= #
method
Example of use:
Allow: GET, HEAD, PUT
The actual set of allowed methods is defined by the origin server at the time of each
request. An origin server
MUST
generate an Allow header field in a
405 (Method Not Allowed)
response and
MAY
do so in any other response. An empty Allow field value indicates that the resource
allows no methods, which might occur in a 405 response if the resource has been temporarily
disabled by configuration.
A proxy
MUST NOT
modify the Allow header field — it does not need to understand all of the indicated
methods in order to handle them according to the generic message handling rules.
10.2.2.
Location
The "Location" header field is used in some responses to refer to a specific resource
in relation to the response. The type of relationship is defined by the combination
of request method and status code semantics.
Location
URI-reference
The field value consists of a single URI-reference. When it has the form of a relative
reference (
[URI]
Section 4.2
), the final value is computed by resolving it against the target URI (
[URI]
Section 5
).
For
201 (Created)
responses, the Location value refers to the primary resource created by the request.
For
3xx (Redirection)
responses, the Location value refers to the preferred target resource for automatically
redirecting the request.
If the Location value provided in a
3xx (Redirection)
response does not have a fragment component, a user agent
MUST
process the redirection as if the value inherits the fragment component of the URI
reference used to generate the target URI (i.e., the redirection inherits the original
reference's fragment, if any).
For example, a GET request generated for the URI reference "http://www.example.org/~tim"
might result in a
303 (See Other)
response containing the header field:
Location: /People.html#tim
which suggests that the user agent redirect to "http://www.example.org/People.html#tim"
Likewise, a GET request generated for the URI reference "http://www.example.org/index.html#larry"
might result in a
301 (Moved Permanently)
response containing the header field:
Location: http://www.example.net/index.html
which suggests that the user agent redirect to "http://www.example.net/index.html#larry",
preserving the original fragment identifier.
There are circumstances in which a fragment identifier in a Location value would not
be appropriate. For example, the Location header field in a
201 (Created)
response is supposed to provide a URI that is specific to the created resource.
Note:
Some recipients attempt to recover from Location header fields that are not valid
URI references. This specification does not mandate or define such processing, but
does allow it for the sake of robustness. A Location field value cannot allow a list
of members because the comma list separator is a valid data character within a URI-reference.
If an invalid message is sent with multiple Location field lines, a recipient along
the path might combine those field lines into one value. Recovery of a valid Location
field value from that situation is difficult and not interoperable across implementations.
Note:
The
Content-Location
header field (
Section 8.7
) differs from Location in that the Content-Location refers to the most specific resource
corresponding to the enclosed representation. It is therefore possible for a response
to contain both the Location and Content-Location header fields.
10.2.3.
Retry-After
Servers send the "Retry-After" header field to indicate how long the user agent ought
to wait before making a follow-up request. When sent with a
503 (Service Unavailable)
response, Retry-After indicates how long the service is expected to be unavailable
to the client. When sent with any
3xx (Redirection)
response, Retry-After indicates the minimum time that the user agent is asked to wait
before issuing the redirected request.
The Retry-After field value can be either an HTTP-date or a number of seconds to delay
after receiving the response.
Retry-After
HTTP-date
delay-seconds
A delay-seconds value is a non-negative decimal integer, representing time in seconds.
delay-seconds
= 1*
DIGIT
Two examples of its use are
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
10.2.4.
Server
The "Server" header field contains information about the software used by the origin
server to handle the request, which is often used by clients to help identify the
scope of reported interoperability problems, to work around or tailor requests to
avoid particular server limitations, and for analytics regarding server or operating
system use. An origin server
MAY
generate a Server header field in its responses.
Server
product
*(
RWS
product
comment
) )
The Server header field value consists of one or more product identifiers, each followed
by zero or more comments (
Section 5.6.5
), which together identify the origin server software and its significant subproducts.
By convention, the product identifiers are listed in decreasing order of their significance
for identifying the origin server software. Each product identifier consists of a
name and optional version, as defined in
Section 10.1.5
Example:
Server: CERN/3.0 libwww/2.17
An origin server
SHOULD NOT
generate a Server header field containing needlessly fine-grained detail and
SHOULD
limit the addition of subproducts by third parties. Overly long and detailed Server
field values increase response latency and potentially reveal internal implementation
details that might make it (slightly) easier for attackers to find and exploit known
security holes.
11.
HTTP Authentication
11.1.
Authentication Scheme
HTTP provides a general framework for access control and authentication, via an extensible
set of challenge-response authentication schemes, which can be used by a server to
challenge a client request and by a client to provide authentication information.
It uses a case-insensitive token to identify the authentication scheme:
auth-scheme =
token
Aside from the general framework, this document does not specify any authentication
schemes. New and existing authentication schemes are specified independently and ought
to be registered within the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry". For example, the "basic" and "digest" authentication schemes are defined
by
[RFC7617]
and
[RFC7616]
, respectively.
11.2.
Authentication Parameters
The authentication scheme is followed by additional information necessary for achieving
authentication via that scheme as either a comma-separated list of parameters or a
single sequence of characters capable of holding base64-encoded information.
token68 = 1*(
ALPHA
DIGIT
"-" / "." / "_" / "~" / "+" / "/" ) *"="
The token68 syntax allows the 66 unreserved URI characters (
[URI]
), plus a few others, so that it can hold a base64, base64url (URL and filename safe
alphabet), base32, or base16 (hex) encoding, with or without padding, but excluding
whitespace (
[RFC4648]
).
Authentication parameters are name/value pairs, where the name token is matched case-insensitively
and each parameter name
MUST
only occur once per challenge.
auth-param =
token
BWS
"="
BWS
token
quoted-string
Parameter values can be expressed either as "token" or as "quoted-string" (
Section 5.6
). Authentication scheme definitions need to accept both notations, both for senders
and recipients, to allow recipients to use generic parsing components regardless of
the authentication scheme.
For backwards compatibility, authentication scheme definitions can restrict the format
for senders to one of the two variants. This can be important when it is known that
deployed implementations will fail when encountering one of the two formats.
11.3.
Challenge and Response
401 (Unauthorized)
response message is used by an origin server to challenge the authorization of a user
agent, including a
WWW-Authenticate
header field containing at least one challenge applicable to the requested resource.
407 (Proxy Authentication Required)
response message is used by a proxy to challenge the authorization of a client, including
Proxy-Authenticate
header field containing at least one challenge applicable to the proxy for the requested
resource.
challenge
auth-scheme
[ 1*
SP
token68
/ #
auth-param
) ]
Note:
Many clients fail to parse a challenge that contains an unknown scheme. A workaround
for this problem is to list well-supported schemes (such as "basic") first.
A user agent that wishes to authenticate itself with an origin server — usually, but
not necessarily, after receiving a
401 (Unauthorized)
— can do so by including an
Authorization
header field with the request.
A client that wishes to authenticate itself with a proxy — usually, but not necessarily,
after receiving a
407 (Proxy Authentication Required)
— can do so by including a
Proxy-Authorization
header field with the request.
11.4.
Credentials
Both the
Authorization
field value and the
Proxy-Authorization
field value contain the client's credentials for the realm of the resource being requested,
based upon a challenge received in a response (possibly at some point in the past).
When creating their values, the user agent ought to do so by selecting the challenge
with what it considers to be the most secure auth-scheme that it understands, obtaining
credentials from the user as appropriate. Transmission of credentials within header
field values implies significant security considerations regarding the confidentiality
of the underlying connection, as described in
Section 17.16.1
credentials
auth-scheme
[ 1*
SP
token68
/ #
auth-param
) ]
Upon receipt of a request for a protected resource that omits credentials, contains
invalid credentials (e.g., a bad password) or partial credentials (e.g., when the
authentication scheme requires more than one round trip), an origin server
SHOULD
send a
401 (Unauthorized)
response that contains a
WWW-Authenticate
header field with at least one (possibly new) challenge applicable to the requested
resource.
Likewise, upon receipt of a request that omits proxy credentials or contains invalid
or partial proxy credentials, a proxy that requires authentication
SHOULD
generate a
407 (Proxy Authentication Required)
response that contains a
Proxy-Authenticate
header field with at least one (possibly new) challenge applicable to the proxy.
A server that receives valid credentials that are not adequate to gain access ought
to respond with the
403 (Forbidden)
status code (
Section 15.5.4
).
HTTP does not restrict applications to this simple challenge-response framework for
access authentication. Additional mechanisms can be used, such as authentication at
the transport level or via message encapsulation, and with additional header fields
specifying authentication information. However, such additional mechanisms are not
defined by this specification.
Note that various custom mechanisms for user authentication use the Set-Cookie and
Cookie header fields, defined in
[COOKIE]
, for passing tokens related to authentication.
11.5.
Establishing a Protection Space (Realm)
The
realm
authentication parameter is reserved for use by authentication schemes that wish to
indicate a scope of protection.
protection space
is defined by the origin (see
Section 4.3.1
) of the server being accessed, in combination with the realm value if present. These
realms allow the protected resources on a server to be partitioned into a set of protection
spaces, each with its own authentication scheme and/or authorization database. The
realm value is a string, generally assigned by the origin server, that can have additional
semantics specific to the authentication scheme. Note that a response can have multiple
challenges with the same auth-scheme but with different realms.
The protection space determines the domain over which credentials can be automatically
applied. If a prior request has been authorized, the user agent
MAY
reuse the same credentials for all other requests within that protection space for
a period of time determined by the authentication scheme, parameters, and/or user
preferences (such as a configurable inactivity timeout).
The extent of a protection space, and therefore the requests to which credentials
might be automatically applied, is not necessarily known to clients without additional
information. An authentication scheme might define parameters that describe the extent
of a protection space. Unless specifically allowed by the authentication scheme, a
single protection space cannot extend outside the scope of its server.
For historical reasons, a sender
MUST
only generate the quoted-string syntax. Recipients might have to support both token
and quoted-string syntax for maximum interoperability with existing clients that have
been accepting both notations for a long time.
11.6.
Authenticating Users to Origin Servers
11.6.1.
WWW-Authenticate
The "WWW-Authenticate" response header field indicates the authentication scheme(s)
and parameters applicable to the target resource.
WWW-Authenticate
= #
challenge
A server generating a
401 (Unauthorized)
response
MUST
send a WWW-Authenticate header field containing at least one challenge. A server
MAY
generate a WWW-Authenticate header field in other response messages to indicate that
supplying credentials (or different credentials) might affect the response.
A proxy forwarding a response
MUST NOT
modify any
WWW-Authenticate
header fields in that response.
User agents are advised to take special care in parsing the field value, as it might
contain more than one challenge, and each challenge can contain a comma-separated
list of authentication parameters. Furthermore, the header field itself can occur
multiple times.
For instance:
WWW-Authenticate: Basic realm="simple", Newauth realm="apps",
type=1, title="Login to \"apps\""
This header field contains two challenges, one for the "Basic" scheme with a realm
value of "simple" and another for the "Newauth" scheme with a realm value of "apps".
It also contains two additional parameters, "type" and "title".
Some user agents do not recognize this form, however. As a result, sending a WWW-Authenticate
field value with more than one member on the same field line might not be interoperable.
Note:
The challenge grammar production uses the list syntax as well. Therefore, a sequence
of comma, whitespace, and comma can be considered either as applying to the preceding
challenge, or to be an empty entry in the list of challenges. In practice, this ambiguity
does not affect the semantics of the header field value and thus is harmless.
11.6.2.
Authorization
The "Authorization" header field allows a user agent to authenticate itself with an
origin server — usually, but not necessarily, after receiving a
401 (Unauthorized)
response. Its value consists of credentials containing the authentication information
of the user agent for the realm of the resource being requested.
Authorization
credentials
If a request is authenticated and a realm specified, the same credentials are presumed
to be valid for all other requests within this realm (assuming that the authentication
scheme itself does not require otherwise, such as credentials that vary according
to a challenge value or using synchronized clocks).
A proxy forwarding a request
MUST NOT
modify any
Authorization
header fields in that request. See
Section 3.5
of
[CACHING]
for details of and requirements pertaining to handling of the Authorization header
field by HTTP caches.
11.6.3.
Authentication-Info
HTTP authentication schemes can use the "Authentication-Info" response field to communicate
information after the client's authentication credentials have been accepted. This
information can include a finalization message from the server (e.g., it can contain
the server authentication).
The field value is a list of parameters (name/value pairs), using the "auth-param"
syntax defined in
Section 11.3
. This specification only describes the generic format; authentication schemes using
Authentication-Info will define the individual parameters. The "Digest" Authentication
Scheme, for instance, defines multiple parameters in
Section 3.5
of
[RFC7616]
Authentication-Info
= #
auth-param
The Authentication-Info field can be used in any HTTP response, independently of request
method and status code. Its semantics are defined by the authentication scheme indicated
by the
Authorization
header field (
Section 11.6.2
) of the corresponding request.
A proxy forwarding a response is not allowed to modify the field value in any way.
Authentication-Info can be sent as a trailer field (
Section 6.5
) when the authentication scheme explicitly allows this.
11.7.
Authenticating Clients to Proxies
11.7.1.
Proxy-Authenticate
The "Proxy-Authenticate" header field consists of at least one challenge that indicates
the authentication scheme(s) and parameters applicable to the proxy for this request.
A proxy
MUST
send at least one Proxy-Authenticate header field in each
407 (Proxy Authentication Required)
response that it generates.
Proxy-Authenticate
= #
challenge
Unlike
WWW-Authenticate
, the Proxy-Authenticate header field applies only to the next outbound client on
the response chain. This is because only the client that chose a given proxy is likely
to have the credentials necessary for authentication. However, when multiple proxies
are used within the same administrative domain, such as office and regional caching
proxies within a large corporate network, it is common for credentials to be generated
by the user agent and passed through the hierarchy until consumed. Hence, in such
a configuration, it will appear as if Proxy-Authenticate is being forwarded because
each proxy will send the same challenge set.
Note that the parsing considerations for
WWW-Authenticate
apply to this header field as well; see
Section 11.6.1
for details.
11.7.2.
Proxy-Authorization
The "Proxy-Authorization" header field allows the client to identify itself (or its
user) to a proxy that requires authentication. Its value consists of credentials containing
the authentication information of the client for the proxy and/or realm of the resource
being requested.
Proxy-Authorization
credentials
Unlike
Authorization
, the Proxy-Authorization header field applies only to the next inbound proxy that
demanded authentication using the
Proxy-Authenticate
header field. When multiple proxies are used in a chain, the Proxy-Authorization header
field is consumed by the first inbound proxy that was expecting to receive credentials.
A proxy
MAY
relay the credentials from the client request to the next proxy if that is the mechanism
by which the proxies cooperatively authenticate a given request.
11.7.3.
Proxy-Authentication-Info
The "Proxy-Authentication-Info" response header field is equivalent to
Authentication-Info
, except that it applies to proxy authentication (
Section 11.3
) and its semantics are defined by the authentication scheme indicated by the Proxy-Authorization
header field (
Section 11.7.2
) of the corresponding request:
Proxy-Authentication-Info
= #
auth-param
However, unlike
Authentication-Info
, the Proxy-Authentication-Info header field applies only to the next outbound client
on the response chain. This is because only the client that chose a given proxy is
likely to have the credentials necessary for authentication. However, when multiple
proxies are used within the same administrative domain, such as office and regional
caching proxies within a large corporate network, it is common for credentials to
be generated by the user agent and passed through the hierarchy until consumed. Hence,
in such a configuration, it will appear as if Proxy-Authentication-Info is being forwarded
because each proxy will send the same field value.
Proxy-Authentication-Info can be sent as a trailer field (
Section 6.5
) when the authentication scheme explicitly allows this.
12.
Content Negotiation
When responses convey content, whether indicating a success or an error, the origin
server often has different ways of representing that information; for example, in
different formats, languages, or encodings. Likewise, different users or user agents
might have differing capabilities, characteristics, or preferences that could influence
which representation, among those available, would be best to deliver. For this reason,
HTTP provides mechanisms for
content negotiation
This specification defines three patterns of content negotiation that can be made
visible within the protocol: "proactive" negotiation, where the server selects the
representation based upon the user agent's stated preferences; "reactive" negotiation,
where the server provides a list of representations for the user agent to choose from;
and "request content" negotiation, where the user agent selects the representation
for a future request based upon the server's stated preferences in past responses.
Other patterns of content negotiation include "conditional content", where the representation
consists of multiple parts that are selectively rendered based on user agent parameters,
"active content", where the representation contains a script that makes additional
(more specific) requests based on the user agent characteristics, and "Transparent
Content Negotiation" (
[RFC2295]
), where content selection is performed by an intermediary. These patterns are not
mutually exclusive, and each has trade-offs in applicability and practicality.
Note that, in all cases, HTTP is not aware of the resource semantics. The consistency
with which an origin server responds to requests, over time and over the varying dimensions
of content negotiation, and thus the "sameness" of a resource's observed representations
over time, is determined entirely by whatever entity or algorithm selects or generates
those responses.
12.1.
Proactive Negotiation
When content negotiation preferences are sent by the user agent in a request to encourage
an algorithm located at the server to select the preferred representation, it is called
proactive negotiation
(a.k.a.,
server-driven negotiation
). Selection is based on the available representations for a response (the dimensions
over which it might vary, such as language, content coding, etc.) compared to various
information supplied in the request, including both the explicit negotiation header
fields below and implicit characteristics, such as the client's network address or
parts of the
User-Agent
field.
Proactive negotiation is advantageous when the algorithm for selecting from among
the available representations is difficult to describe to a user agent, or when the
server desires to send its "best guess" to the user agent along with the first response
(when that "best guess" is good enough for the user, this avoids the round-trip delay
of a subsequent request). In order to improve the server's guess, a user agent
MAY
send request header fields that describe its preferences.
Proactive negotiation has serious disadvantages:
It is impossible for the server to accurately determine what might be "best" for any
given user, since that would require complete knowledge of both the capabilities of
the user agent and the intended use for the response (e.g., does the user want to
view it on screen or print it on paper?);
Having the user agent describe its capabilities in every request can be both very
inefficient (given that only a small percentage of responses have multiple representations)
and a potential risk to the user's privacy;
It complicates the implementation of an origin server and the algorithms for generating
responses to a request; and,
It limits the reusability of responses for shared caching.
A user agent cannot rely on proactive negotiation preferences being consistently honored,
since the origin server might not implement proactive negotiation for the requested
resource or might decide that sending a response that doesn't conform to the user
agent's preferences is better than sending a
406 (Not Acceptable)
response.
Vary
header field (
Section 12.5.5
) is often sent in a response subject to proactive negotiation to indicate what parts
of the request information were used in the selection algorithm.
The request header fields
Accept
Accept-Charset
Accept-Encoding
, and
Accept-Language
are defined below for a user agent to engage in
proactive negotiation
of the response content. The preferences sent in these fields apply to any content
in the response, including representations of the target resource, representations
of error or processing status, and potentially even the miscellaneous text strings
that might appear within the protocol.
12.2.
Reactive Negotiation
With
reactive negotiation
(a.k.a.,
agent-driven negotiation
), selection of content (regardless of the status code) is performed by the user agent
after receiving an initial response. The mechanism for reactive negotiation might
be as simple as a list of references to alternative representations.
If the user agent is not satisfied by the initial response content, it can perform
a GET request on one or more of the alternative resources to obtain a different representation.
Selection of such alternatives might be performed automatically (by the user agent)
or manually (e.g., by the user selecting from a hypertext menu).
A server might choose not to send an initial representation, other than the list of
alternatives, and thereby indicate that reactive negotiation by the user agent is
preferred. For example, the alternatives listed in responses with the
300 (Multiple Choices)
and
406 (Not Acceptable)
status codes include information about available representations so that the user
or user agent can react by making a selection.
Reactive negotiation is advantageous when the response would vary over commonly used
dimensions (such as type, language, or encoding), when the origin server is unable
to determine a user agent's capabilities from examining the request, and generally
when public caches are used to distribute server load and reduce network usage.
Reactive negotiation suffers from the disadvantages of transmitting a list of alternatives
to the user agent, which degrades user-perceived latency if transmitted in the header
section, and needing a second request to obtain an alternate representation. Furthermore,
this specification does not define a mechanism for supporting automatic selection,
though it does not prevent such a mechanism from being developed.
12.3.
Request Content Negotiation
When content negotiation preferences are sent in a server's response, the listed preferences
are called
request content negotiation
because they intend to influence selection of an appropriate content for subsequent
requests to that resource. For example, the
Accept
Section 12.5.1
) and
Accept-Encoding
Section 12.5.3
) header fields can be sent in a response to indicate preferred media types and content
codings for subsequent requests to that resource.
Similarly,
Section 3.1
of
[RFC5789]
defines the "Accept-Patch" response header field, which allows discovery of which
content types are accepted in PATCH requests.
12.4.
Content Negotiation Field Features
12.4.1.
Absence
For each of the content negotiation fields, a request that does not contain the field
implies that the sender has no preference on that dimension of negotiation.
If a content negotiation header field is present in a request and none of the available
representations for the response can be considered acceptable according to it, the
origin server can either honor the header field by sending a
406 (Not Acceptable)
response or disregard the header field by treating the response as if it is not subject
to content negotiation for that request header field. This does not imply, however,
that the client will be able to use the representation.
Note:
A user agent sending these header fields makes it easier for a server to identify
an individual by virtue of the user agent's request characteristics (
Section 17.13
).
12.4.2.
Quality Values
The content negotiation fields defined by this specification use a common parameter,
named "q" (case-insensitive), to assign a relative "weight" to the preference for
that associated kind of content. This weight is referred to as a "quality value" (or
"qvalue") because the same parameter name is often used within server configurations
to assign a weight to the relative quality of the various representations that can
be selected for a resource.
The weight is normalized to a real number in the range 0 through 1, where 0.001 is
the least preferred and 1 is the most preferred; a value of 0 means "not acceptable".
If no "q" parameter is present, the default weight is 1.
weight
OWS
";"
OWS
"q="
qvalue
qvalue
= ( "0" [ "." 0*3
DIGIT
] )
/ ( "1" [ "." 0*3("0") ] )
A sender of qvalue
MUST NOT
generate more than three digits after the decimal point. User configuration of these
values ought to be limited in the same fashion.
12.4.3.
Wildcard Values
Most of these header fields, where indicated, define a wildcard value ("*") to select
unspecified values. If no wildcard is present, values that are not explicitly mentioned
in the field are considered unacceptable. Within
Vary
, the wildcard value means that the variance is unlimited.
Note:
In practice, using wildcards in content negotiation has limited practical value because
it is seldom useful to say, for example, "I prefer image/* more or less than (some
other specific value)". By sending Accept: */*;q=0, clients can explicitly request
406 (Not Acceptable)
response if a more preferred format is not available, but they still need to be able
to handle a different response since the server is allowed to ignore their preference.
12.5.
Content Negotiation Fields
12.5.1.
Accept
The "Accept" header field can be used by user agents to specify their preferences
regarding response media types. For example, Accept header fields can be used to indicate
that the request is specifically limited to a small set of desired types, as in the
case of a request for an in-line image.
When sent by a server in a response, Accept provides information about which content
types are preferred in the content of a subsequent request to the same resource.
Accept
= #(
media-range
weight
] )
media-range
= ( "*/*"
/ (
type
"/" "*" )
/ (
type
"/"
subtype
parameters
The asterisk "*" character is used to group media types into ranges, with "*/*" indicating
all media types and "type/*" indicating all subtypes of that type. The media-range
can include media type parameters that are applicable to that range.
Each media-range might be followed by optional applicable media type parameters (e.g.,
charset
), followed by an optional "q" parameter for indicating a relative weight (
Section 12.4.2
).
Previous specifications allowed additional extension parameters to appear after the
weight parameter. The accept extension grammar (accept-params, accept-ext) has been
removed because it had a complicated definition, was not being used in practice, and
is more easily deployed through new header fields. Senders using weights
SHOULD
send "q" last (after all media-range parameters). Recipients
SHOULD
process any parameter named "q" as weight, regardless of parameter ordering.
Note:
Use of the "q" parameter name to control content negotiation would interfere with
any media type parameter having the same name. Hence, the media type registry disallows
parameters named "q".
The example
Accept: audio/*; q=0.2, audio/basic
is interpreted as "I prefer audio/basic, but send me any audio type if it is the best
available after an 80% markdown in quality".
A more elaborate example is
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are the equally preferred
media types, but if they do not exist, then send the text/x-dvi representation, and
if that does not exist, send the text/plain representation".
Media ranges can be overridden by more specific media ranges or specific media types.
If more than one media range applies to a given type, the most specific reference
has precedence. For example,
Accept: text/*, text/plain, text/plain;format=flowed, */*
have the following precedence:
text/plain;format=flowed
text/plain
text/*
*/*
The media type quality factor associated with a given type is determined by finding
the media range with the highest precedence that matches the type. For example,
Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,
text/plain;format=fixed;q=0.4, */*;q=0.5
would cause the following values to be associated:
Table 5
Media Type
Quality Value
text/plain;format=flowed
text/plain
0.7
text/html
0.3
image/jpeg
0.5
text/plain;format=fixed
0.4
text/html;level=3
0.7
Note:
A user agent might be provided with a default set of quality values for certain media
ranges. However, unless the user agent is a closed system that cannot interact with
other rendering agents, this default set ought to be configurable by the user.
12.5.2.
Accept-Charset
The "Accept-Charset" header field can be sent by a user agent to indicate its preferences
for charsets in textual response content. For example, this field allows user agents
capable of understanding more comprehensive or special-purpose charsets to signal
that capability to an origin server that is capable of representing information in
those charsets.
Accept-Charset
= #( (
token
/ "*" ) [
weight
] )
Charset names are defined in
Section 8.3.2
. A user agent
MAY
associate a quality value with each charset to indicate the user's relative preference
for that charset, as defined in
Section 12.4.2
. An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
The special value "*", if present in the Accept-Charset header field, matches every
charset that is not mentioned elsewhere in the field.
Note:
Accept-Charset is deprecated because UTF-8 has become nearly ubiquitous and sending
a detailed list of user-preferred charsets wastes bandwidth, increases latency, and
makes passive fingerprinting far too easy (
Section 17.13
). Most general-purpose user agents do not send Accept-Charset unless specifically
configured to do so.
12.5.3.
Accept-Encoding
The "Accept-Encoding" header field can be used to indicate preferences regarding the
use of content codings (
Section 8.4.1
).
When sent by a user agent in a request, Accept-Encoding indicates the content codings
acceptable in a response.
When sent by a server in a response, Accept-Encoding provides information about which
content codings are preferred in the content of a subsequent request to the same resource.
An "identity" token is used as a synonym for "no encoding" in order to communicate
when no encoding is preferred.
Accept-Encoding
= #(
codings
weight
] )
codings
content-coding
/ "identity" / "*"
Each codings value
MAY
be given an associated quality value (weight) representing the preference for that
encoding, as defined in
Section 12.4.2
. The asterisk "*" symbol in an Accept-Encoding field matches any available content
coding not explicitly listed in the field.
Examples:
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
A server tests whether a content coding for a given representation is acceptable using
these rules:
If no Accept-Encoding header field is in the request, any content coding is considered
acceptable by the user agent.
If the representation has no content coding, then it is acceptable by default unless
specifically excluded by the Accept-Encoding header field stating either "identity;q=0"
or "*;q=0" without a more specific entry for "identity".
If the representation's content coding is one of the content codings listed in the
Accept-Encoding field value, then it is acceptable unless it is accompanied by a qvalue
of 0. (As defined in
Section 12.4.2
, a qvalue of 0 means "not acceptable".)
A representation could be encoded with multiple content codings. However, most content
codings are alternative ways to accomplish the same purpose (e.g., data compression).
When selecting between multiple content codings that have the same purpose, the acceptable
content coding with the highest non-zero qvalue is preferred.
An Accept-Encoding header field with a field value that is empty implies that the
user agent does not want any content coding in response. If a non-empty Accept-Encoding
header field is present in a request and none of the available representations for
the response have a content coding that is listed as acceptable, the origin server
SHOULD
send a response without any content coding unless the identity coding is indicated
as unacceptable.
When the Accept-Encoding header field is present in a response, it indicates what
content codings the resource was willing to accept in the associated request. The
field value is evaluated the same way as in a request.
Note that this information is specific to the associated request; the set of supported
encodings might be different for other resources on the same server and could change
over time or depend on other aspects of the request (such as the request method).
Servers that fail a request due to an unsupported content coding ought to respond
with a
415 (Unsupported Media Type)
status and include an Accept-Encoding header field in that response, allowing clients
to distinguish between issues related to content codings and media types. In order
to avoid confusion with issues related to media types, servers that fail a request
with a 415 status for reasons unrelated to content codings
MUST NOT
include the Accept-Encoding header field.
The most common use of Accept-Encoding is in responses with a
415 (Unsupported Media Type)
status code, in response to optimistic use of a content coding by clients. However,
the header field can also be used to indicate to clients that content codings are
supported in order to optimize future interactions. For example, a resource might
include it in a
2xx (Successful)
response when the request content was big enough to justify use of a compression coding
but the client failed do so.
12.5.4.
Accept-Language
The "Accept-Language" header field can be used by user agents to indicate the set
of natural languages that are preferred in the response. Language tags are defined
in
Section 8.5.1
Accept-Language
= #(
language-range
weight
] )
language-range
Section 2.1
Each language-range can be given an associated quality value representing an estimate
of the user's preference for the languages specified by that range, as defined in
Section 12.4.2
. For example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept British English and other types of English".
Note that some recipients treat the order in which language tags are listed as an
indication of descending priority, particularly for tags that are assigned equal quality
values (no value is the same as q=1). However, this behavior cannot be relied upon.
For consistency and to maximize interoperability, many user agents assign each language
tag a unique quality value while also listing them in order of decreasing quality.
Additional discussion of language priority lists can be found in
Section 2.3
of
[RFC4647]
For matching,
Section 3
of
[RFC4647]
defines several matching schemes. Implementations can offer the most appropriate matching
scheme for their requirements. The "Basic Filtering" scheme (
[RFC4647]
Section 3.3.1
) is identical to the matching scheme that was previously defined for HTTP in
Section 14.4
of
[RFC2616]
It might be contrary to the privacy expectations of the user to send an Accept-Language
header field with the complete linguistic preferences of the user in every request
Section 17.13
).
Since intelligibility is highly dependent on the individual user, user agents need
to allow user control over the linguistic preference (either through configuration
of the user agent itself or by defaulting to a user controllable system setting).
A user agent that does not provide such control to the user
MUST NOT
send an Accept-Language header field.
Note:
User agents ought to provide guidance to users when setting a preference, since users
are rarely familiar with the details of language matching as described above. For
example, users might assume that on selecting "en-gb", they will be served any kind
of English document if British English is not available. A user agent might suggest,
in such a case, to add "en" to the list for better matching behavior.
12.5.5.
Vary
The "Vary" header field in a response describes what parts of a request message, aside
from the method and target URI, might have influenced the origin server's process
for selecting the content of this response.
Vary
= #( "*" /
field-name
A Vary field value is either the wildcard member "*" or a list of request field names,
known as the selecting header fields, that might have had a role in selecting the
representation for this response. Potential selecting header fields are not limited
to fields defined by this specification.
A list containing the member "*" signals that other aspects of the request might have
played a role in selecting the response representation, possibly including aspects
outside the message syntax (e.g., the client's network address). A recipient will
not be able to determine whether this response is appropriate for a later request
without forwarding the request to the origin server. A proxy
MUST NOT
generate "*" in a Vary field value.
For example, a response that contains
Vary: accept-encoding, accept-language
indicates that the origin server might have used the request's
Accept-Encoding
and
Accept-Language
header fields (or lack thereof) as determining factors while choosing the content
for this response.
A Vary field containing a list of field names has two purposes:
To inform cache recipients that they
MUST NOT
use this response to satisfy a later request unless the later request has the same
values for the listed header fields as the original request (
Section 4.1
of
[CACHING]
) or reuse of the response has been validated by the origin server. In other words,
Vary expands the cache key required to match a new request to the stored cache entry.
To inform user agent recipients that this response was subject to content negotiation
Section 12
) and a different representation might be sent in a subsequent request if other values
are provided in the listed header fields (
proactive negotiation
).
An origin server
SHOULD
generate a Vary header field on a cacheable response when it wishes that response
to be selectively reused for subsequent requests. Generally, that is the case when
the response content has been tailored to better fit the preferences expressed by
those selecting header fields, such as when an origin server has selected the response's
language based on the request's
Accept-Language
header field.
Vary might be elided when an origin server considers variance in content selection
to be less significant than Vary's performance impact on caching, particularly when
reuse is already limited by cache response directives (
Section 5.2
of
[CACHING]
).
There is no need to send the Authorization field name in Vary because reuse of that
response for a different user is prohibited by the field definition (
Section 11.6.2
). Likewise, if the response content has been selected or influenced by network region,
but the origin server wants the cached response to be reused even if recipients move
from one region to another, then there is no need for the origin server to indicate
such variance in Vary.
13.
Conditional Requests
A conditional request is an HTTP request with one or more request header fields that
indicate a precondition to be tested before applying the request method to the target
resource.
Section 13.2
defines when to evaluate preconditions and their order of precedence when more than
one precondition is present.
Conditional GET requests are the most efficient mechanism for HTTP cache updates
[CACHING]
. Conditionals can also be applied to state-changing methods, such as PUT and DELETE,
to prevent the "lost update" problem: one client accidentally overwriting the work
of another client that has been acting in parallel.
13.1.
Preconditions
Preconditions are usually defined with respect to a state of the target resource as
a whole (its current value set) or the state as observed in a previously obtained
representation (one value in that set). If a resource has multiple current representations,
each with its own observable state, a precondition will assume that the mapping of
each request to a
selected representation
Section 3.2
) is consistent over time. Regardless, if the mapping is inconsistent or the server
is unable to select an appropriate representation, then no harm will result when the
precondition evaluates to false.
Each precondition defined below consists of a comparison between a set of validators
obtained from prior representations of the target resource to the current state of
validators for the selected representation (
Section 8.8
). Hence, these preconditions evaluate whether the state of the target resource has
changed since a given state known by the client. The effect of such an evaluation
depends on the method semantics and choice of conditional, as defined in
Section 13.2
Other preconditions, defined by other specifications as extension fields, might place
conditions on all recipients, on the state of the target resource in general, or on
a group of resources. For instance, the "If" header field in WebDAV can make a request
conditional on various aspects of multiple resources, such as locks, if the recipient
understands and implements that field (
[WEBDAV]
Section 10.4
).
Extensibility of preconditions is only possible when the precondition can be safely
ignored if unknown (like
If-Modified-Since
), when deployment can be assumed for a given use case, or when implementation is
signaled by some other property of the target resource. This encourages a focus on
mutually agreed deployment of common standards.
13.1.1.
If-Match
The "If-Match" header field makes the request method conditional on the recipient
origin server either having at least one current representation of the target resource,
when the field value is "*", or having a current representation of the target resource
that has an entity tag matching a member of the list of entity tags provided in the
field value.
An origin server
MUST
use the strong comparison function when comparing entity tags for If-Match (
Section 8.8.3.2
), since the client intends this precondition to prevent the method from being applied
if there have been any changes to the representation data.
If-Match
= "*" / #
entity-tag
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
If-Match is most often used with state-changing methods (e.g., POST, PUT, DELETE)
to prevent accidental overwrites when multiple user agents might be acting in parallel
on the same resource (i.e., to prevent the "lost update" problem). In general, it
can be used with any method that involves the selection or modification of a representation
to abort the request if the
selected representation
's current entity tag is not a member within the If-Match field value.
When an origin server receives a request that selects a representation and that request
includes an If-Match header field, the origin server
MUST
evaluate the If-Match condition per
Section 13.2
prior to performing the method.
To evaluate a received If-Match header field:
If the field value is "*", the condition is true if the origin server has a current
representation for the target resource.
If the field value is a list of entity tags, the condition is true if any of the listed
tags match the entity tag of the selected representation.
Otherwise, the condition is false.
An origin server that evaluates an If-Match condition
MUST NOT
perform the requested method if the condition evaluates to false. Instead, the origin
server
MAY
indicate that the conditional request failed by responding with a
412 (Precondition Failed)
status code. Alternatively, if the request is a state-changing operation that appears
to have already been applied to the selected representation, the origin server
MAY
respond with a
2xx (Successful)
status code (i.e., the change requested by the user agent has already succeeded, but
the user agent might not be aware of it, perhaps because the prior response was lost
or an equivalent change was made by some other user agent).
Allowing an origin server to send a success response when a change request appears
to have already been applied is more efficient for many authoring use cases, but comes
with some risk if multiple user agents are making change requests that are very similar
but not cooperative. For example, multiple user agents writing to a common resource
as a semaphore (e.g., a nonatomic increment) are likely to collide and potentially
lose important state transitions. For those kinds of resources, an origin server is
better off being stringent in sending 412 for every failed precondition on an unsafe
method. In other cases, excluding the ETag field from a success response might encourage
the user agent to perform a GET as its next request to eliminate confusion about the
resource's current state.
A client
MAY
send an If-Match header field in a
GET
request to indicate that it would prefer a
412 (Precondition Failed)
response if the selected representation does not match. However, this is only useful
in range requests (
Section 14
) for completing a previously received partial representation when there is no desire
for a new representation.
If-Range
Section 13.1.5
) is better suited for range requests when the client prefers to receive a new representation.
A cache or intermediary
MAY
ignore If-Match because its interoperability features are only necessary for an origin
server.
Note that an If-Match header field with a list value containing "*" and other values
(including other instances of "*") is syntactically invalid (therefore not allowed
to be generated) and furthermore is unlikely to be interoperable.
13.1.2.
If-None-Match
The "If-None-Match" header field makes the request method conditional on a recipient
cache or origin server either not having any current representation of the target
resource, when the field value is "*", or having a
selected representation
with an entity tag that does not match any of those listed in the field value.
A recipient
MUST
use the weak comparison function when comparing entity tags for If-None-Match (
Section 8.8.3.2
), since weak entity tags can be used for cache validation even if there have been
changes to the representation data.
If-None-Match
= "*" / #
entity-tag
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
If-None-Match is primarily used in conditional GET requests to enable efficient updates
of cached information with a minimum amount of transaction overhead. When a client
desires to update one or more stored responses that have entity tags, the client
SHOULD
generate an If-None-Match header field containing a list of those entity tags when
making a GET request; this allows recipient servers to send a
304 (Not Modified)
response to indicate when one of those stored responses matches the selected representation.
If-None-Match can also be used with a value of "*" to prevent an unsafe request method
(e.g., PUT) from inadvertently modifying an existing representation of the target
resource when the client believes that the resource does not have a current representation
Section 9.2.1
). This is a variation on the "lost update" problem that might arise if more than
one client attempts to create an initial representation for the target resource.
When an origin server receives a request that selects a representation and that request
includes an If-None-Match header field, the origin server
MUST
evaluate the If-None-Match condition per
Section 13.2
prior to performing the method.
To evaluate a received If-None-Match header field:
If the field value is "*", the condition is false if the origin server has a current
representation for the target resource.
If the field value is a list of entity tags, the condition is false if one of the
listed tags matches the entity tag of the selected representation.
Otherwise, the condition is true.
An origin server that evaluates an If-None-Match condition
MUST NOT
perform the requested method if the condition evaluates to false; instead, the origin
server
MUST
respond with either a) the
304 (Not Modified)
status code if the request method is GET or HEAD or b) the
412 (Precondition Failed)
status code for all other request methods.
Requirements on cache handling of a received If-None-Match header field are defined
in
Section 4.3.2
of
[CACHING]
Note that an If-None-Match header field with a list value containing "*" and other
values (including other instances of "*") is syntactically invalid (therefore not
allowed to be generated) and furthermore is unlikely to be interoperable.
13.1.3.
If-Modified-Since
The "If-Modified-Since" header field makes a GET or HEAD request method conditional
on the
selected representation
's modification date being more recent than the date provided in the field value.
Transfer of the selected representation's data is avoided if that data has not changed.
If-Modified-Since
HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient
MUST
ignore If-Modified-Since if the request contains an
If-None-Match
header field; the condition in
If-None-Match
is considered to be a more accurate replacement for the condition in If-Modified-Since,
and the two are only combined for the sake of interoperating with older intermediaries
that might not implement
If-None-Match
A recipient
MUST
ignore the If-Modified-Since header field if the received field value is not a valid
HTTP-date, the field value has more than one member, or if the request method is neither
GET nor HEAD.
A recipient
MUST
ignore the If-Modified-Since header field if the resource does not have a modification
date available.
A recipient
MUST
interpret an If-Modified-Since field value's timestamp in terms of the origin server's
clock.
If-Modified-Since is typically used for two distinct purposes: 1) to allow efficient
updates of a cached representation that does not have an entity tag and 2) to limit
the scope of a web traversal to resources that have recently changed.
When used for cache updates, a cache will typically use the value of the cached message's
Last-Modified
header field to generate the field value of If-Modified-Since. This behavior is most
interoperable for cases where clocks are poorly synchronized or when the server has
chosen to only honor exact timestamp matches (due to a problem with Last-Modified
dates that appear to go "back in time" when the origin server's clock is corrected
or a representation is restored from an archived backup). However, caches occasionally
generate the field value based on other data, such as the
Date
header field of the cached message or the clock time at which the message was received,
particularly when the cached message does not contain a
Last-Modified
header field.
When used for limiting the scope of retrieval to a recent time window, a user agent
will generate an If-Modified-Since field value based on either its own clock or a
Date
header field received from the server in a prior response. Origin servers that choose
an exact timestamp match based on the selected representation's
Last-Modified
header field will not be able to help the user agent limit its data transfers to only
those changed during the specified window.
When an origin server receives a request that selects a representation and that request
includes an If-Modified-Since header field without an
If-None-Match
header field, the origin server
SHOULD
evaluate the If-Modified-Since condition per
Section 13.2
prior to performing the method.
To evaluate a received If-Modified-Since header field:
If the selected representation's last modification date is earlier or equal to the
date provided in the field value, the condition is false.
Otherwise, the condition is true.
An origin server that evaluates an If-Modified-Since condition
SHOULD NOT
perform the requested method if the condition evaluates to false; instead, the origin
server
SHOULD
generate a
304 (Not Modified)
response, including only those metadata that are useful for identifying or updating
a previously cached response.
Requirements on cache handling of a received If-Modified-Since header field are defined
in
Section 4.3.2
of
[CACHING]
13.1.4.
If-Unmodified-Since
The "If-Unmodified-Since" header field makes the request method conditional on the
selected representation
's last modification date being earlier than or equal to the date provided in the
field value. This field accomplishes the same purpose as
If-Match
for cases where the user agent does not have an entity tag for the representation.
If-Unmodified-Since
HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient
MUST
ignore If-Unmodified-Since if the request contains an
If-Match
header field; the condition in
If-Match
is considered to be a more accurate replacement for the condition in If-Unmodified-Since,
and the two are only combined for the sake of interoperating with older intermediaries
that might not implement
If-Match
A recipient
MUST
ignore the If-Unmodified-Since header field if the received field value is not a valid
HTTP-date (including when the field value appears to be a list of dates).
A recipient
MUST
ignore the If-Unmodified-Since header field if the resource does not have a modification
date available.
A recipient
MUST
interpret an If-Unmodified-Since field value's timestamp in terms of the origin server's
clock.
If-Unmodified-Since is most often used with state-changing methods (e.g., POST, PUT,
DELETE) to prevent accidental overwrites when multiple user agents might be acting
in parallel on a resource that does not supply entity tags with its representations
(i.e., to prevent the "lost update" problem). In general, it can be used with any
method that involves the selection or modification of a representation to abort the
request if the
selected representation
's last modification date has changed since the date provided in the If-Unmodified-Since
field value.
When an origin server receives a request that selects a representation and that request
includes an If-Unmodified-Since header field without an
If-Match
header field, the origin server
MUST
evaluate the If-Unmodified-Since condition per
Section 13.2
prior to performing the method.
To evaluate a received If-Unmodified-Since header field:
If the selected representation's last modification date is earlier than or equal to
the date provided in the field value, the condition is true.
Otherwise, the condition is false.
An origin server that evaluates an If-Unmodified-Since condition
MUST NOT
perform the requested method if the condition evaluates to false. Instead, the origin
server
MAY
indicate that the conditional request failed by responding with a
412 (Precondition Failed)
status code. Alternatively, if the request is a state-changing operation that appears
to have already been applied to the selected representation, the origin server
MAY
respond with a
2xx (Successful)
status code (i.e., the change requested by the user agent has already succeeded, but
the user agent might not be aware of it, perhaps because the prior response was lost
or an equivalent change was made by some other user agent).
Allowing an origin server to send a success response when a change request appears
to have already been applied is more efficient for many authoring use cases, but comes
with some risk if multiple user agents are making change requests that are very similar
but not cooperative. In those cases, an origin server is better off being stringent
in sending 412 for every failed precondition on an unsafe method.
A client
MAY
send an If-Unmodified-Since header field in a
GET
request to indicate that it would prefer a
412 (Precondition Failed)
response if the selected representation has been modified. However, this is only useful
in range requests (
Section 14
) for completing a previously received partial representation when there is no desire
for a new representation.
If-Range
Section 13.1.5
) is better suited for range requests when the client prefers to receive a new representation.
A cache or intermediary
MAY
ignore If-Unmodified-Since because its interoperability features are only necessary
for an origin server.
13.1.5.
If-Range
The "If-Range" header field provides a special conditional request mechanism that
is similar to the
If-Match
and
If-Unmodified-Since
header fields but that instructs the recipient to ignore the
Range
header field if the validator doesn't match, resulting in transfer of the new
selected representation
instead of a
412 (Precondition Failed)
response.
If a client has a partial copy of a representation and wishes to have an up-to-date
copy of the entire representation, it could use the
Range
header field with a conditional GET (using either or both of
If-Unmodified-Since
and
If-Match
.) However, if the precondition fails because the representation has been modified,
the client would then have to make a second request to obtain the entire current representation.
The "If-Range" header field allows a client to "short-circuit" the second request.
Informally, its meaning is as follows: if the representation is unchanged, send me
the part(s) that I am requesting in Range; otherwise, send me the entire representation.
If-Range
entity-tag
HTTP-date
A valid
entity-tag
can be distinguished from a valid
HTTP-date
by examining the first three characters for a DQUOTE.
A client
MUST NOT
generate an If-Range header field in a request that does not contain a
Range
header field. A server
MUST
ignore an If-Range header field received in a request that does not contain a
Range
header field. An origin server
MUST
ignore an If-Range header field received in a request for a target resource that does
not support Range requests.
A client
MUST NOT
generate an If-Range header field containing an entity tag that is marked as weak.
A client
MUST NOT
generate an If-Range header field containing an
HTTP-date
unless the client has no entity tag for the corresponding representation and the date
is a strong validator in the sense defined by
Section 8.8.2.2
A server that receives an If-Range header field on a Range request
MUST
evaluate the condition per
Section 13.2
prior to performing the method.
To evaluate a received If-Range header field containing an
HTTP-date
If the
HTTP-date
validator provided is not a strong validator in the sense defined by
Section 8.8.2.2
, the condition is false.
If the
HTTP-date
validator provided exactly matches the
Last-Modified
field value for the selected representation, the condition is true.
Otherwise, the condition is false.
To evaluate a received If-Range header field containing an
entity-tag
If the
entity-tag
validator provided exactly matches the
ETag
field value for the selected representation using the strong comparison function (
Section 8.8.3.2
), the condition is true.
Otherwise, the condition is false.
A recipient of an If-Range header field
MUST
ignore the
Range
header field if the If-Range condition evaluates to false. Otherwise, the recipient
SHOULD
process the
Range
header field as requested.
Note that the If-Range comparison is by exact match, including when the validator
is an
HTTP-date
, and so it differs from the "earlier than or equal to" comparison used when evaluating
an
If-Unmodified-Since
conditional.
13.2.
Evaluation of Preconditions
13.2.1.
When to Evaluate
Except when excluded below, a recipient cache or origin server
MUST
evaluate received request preconditions after it has successfully performed its normal
request checks and just before it would process the request content (if any) or perform
the action associated with the request method. A server
MUST
ignore all received preconditions if its response to the same request without those
conditions, prior to processing the request content, would have been a status code
other than a
2xx (Successful)
or
412 (Precondition Failed)
. In other words, redirects and failures that can be detected before significant processing
occurs take precedence over the evaluation of preconditions.
A server that is not the origin server for the target resource and cannot act as a
cache for requests on the target resource
MUST NOT
evaluate the conditional request header fields defined by this specification, and
it
MUST
forward them if the request is forwarded, since the generating client intends that
they be evaluated by a server that can provide a current representation. Likewise,
a server
MUST
ignore the conditional request header fields defined by this specification when received
with a request method that does not involve the selection or modification of a
selected representation
, such as CONNECT, OPTIONS, or TRACE.
Note that protocol extensions can modify the conditions under which preconditions
are evaluated or the consequences of their evaluation. For example, the immutable
cache directive (defined by
[RFC8246]
) instructs caches to forgo forwarding conditional requests when they hold a fresh
response.
Although conditional request header fields are defined as being usable with the HEAD
method (to keep HEAD's semantics consistent with those of GET), there is no point
in sending a conditional HEAD because a successful response is around the same size
as a
304 (Not Modified)
response and more useful than a
412 (Precondition Failed)
response.
13.2.2.
Precedence of Preconditions
When more than one conditional request header field is present in a request, the order
in which the fields are evaluated becomes important. In practice, the fields defined
in this document are consistently implemented in a single, logical order, since "lost
update" preconditions have more strict requirements than cache validation, a validated
cache is more efficient than a partial response, and entity tags are presumed to be
more accurate than date validators.
A recipient cache or origin server
MUST
evaluate the request preconditions defined by this specification in the following
order:
When recipient is the origin server and
If-Match
is present, evaluate the
If-Match
precondition:
if true, continue to step
if false, respond
412 (Precondition Failed)
unless it can be determined that the state-changing request has already succeeded
(see
Section 13.1.1
When recipient is the origin server,
If-Match
is not present, and
If-Unmodified-Since
is present, evaluate the
If-Unmodified-Since
precondition:
if true, continue to step
if false, respond
412 (Precondition Failed)
unless it can be determined that the state-changing request has already succeeded
(see
Section 13.1.4
When
If-None-Match
is present, evaluate the
If-None-Match
precondition:
if true, continue to step
if false for GET/HEAD, respond
304 (Not Modified)
if false for other methods, respond
412 (Precondition Failed)
When the method is GET or HEAD,
If-None-Match
is not present, and
If-Modified-Since
is present, evaluate the
If-Modified-Since
precondition:
if true, continue to step
if false, respond
304 (Not Modified)
When the method is GET and both
Range
and
If-Range
are present, evaluate the
If-Range
precondition:
if true and the
Range
is applicable to the
selected representation
, respond
206 (Partial Content)
otherwise, ignore the
Range
header field and respond
200 (OK)
Otherwise,
perform the requested method and respond according to its success or failure.
Any extension to HTTP that defines additional conditional request header fields ought
to define the order for evaluating such fields in relation to those defined in this
document and other conditionals that might be found in practice.
14.
Range Requests
Clients often encounter interrupted data transfers as a result of canceled requests
or dropped connections. When a client has stored a partial representation, it is desirable
to request the remainder of that representation in a subsequent request rather than
transfer the entire representation. Likewise, devices with limited local storage might
benefit from being able to request only a subset of a larger representation, such
as a single page of a very large document, or the dimensions of an embedded image.
Range requests are an
OPTIONAL
feature of HTTP, designed so that recipients not implementing this feature (or not
supporting it for the target resource) can respond as if it is a normal GET request
without impacting interoperability. Partial responses are indicated by a distinct
status code to not be mistaken for full responses by caches that might not implement
the feature.
14.1.
Range Units
Representation data can be partitioned into subranges when there are addressable structural
units inherent to that data's content coding or media type. For example, octet (a.k.a.
byte) boundaries are a structural unit common to all representation data, allowing
partitions of the data to be identified as a range of bytes at some offset from the
start or end of that data.
This general notion of a
range unit
is used in the
Accept-Ranges
Section 14.3
) response header field to advertise support for range requests, the
Range
Section 14.2
) request header field to delineate the parts of a representation that are requested,
and the
Content-Range
Section 14.4
) header field to describe which part of a representation is being transferred.
range-unit
token
All range unit names are case-insensitive and ought to be registered within the "HTTP
Range Unit Registry", as defined in
Section 16.5.1
Range units are intended to be extensible, as described in
Section 16.5
14.1.1.
Range Specifiers
Ranges are expressed in terms of a range unit paired with a set of range specifiers.
The range unit name determines what kinds of range-spec are applicable to its own
specifiers. Hence, the following grammar is generic: each range unit is expected to
specify requirements on when
int-range
suffix-range
, and
other-range
are allowed.
A range request can specify a single range or a set of ranges within a single representation.
ranges-specifier
range-unit
"="
range-set
range-set
= 1#
range-spec
range-spec
int-range
suffix-range
other-range
An
int-range
is a range expressed as two non-negative integers or as one non-negative integer through
to the end of the representation data. The range unit specifies what the integers
mean (e.g., they might indicate unit offsets from the beginning, inclusive numbered
parts, etc.).
int-range
first-pos
"-" [
last-pos
first-pos
= 1*
DIGIT
last-pos
= 1*
DIGIT
An
int-range
is invalid if the
last-pos
value is present and less than the
first-pos
suffix-range
is a range expressed as a suffix of the representation data with the provided non-negative
integer maximum length (in range units). In other words, the last N units of the representation
data.
suffix-range
= "-"
suffix-length
suffix-length
= 1*
DIGIT
To provide for extensibility, the
other-range
rule is a mostly unconstrained grammar that allows application-specific or future
range units to define additional range specifiers.
other-range
= 1*( %x21-2B / %x2D-7E )
; 1*(VCHAR excluding comma)
ranges-specifier
is invalid if it contains any
range-spec
that is invalid or undefined for the indicated
range-unit
A valid
ranges-specifier
is
satisfiable
if it contains at least one
range-spec
that is satisfiable, as defined by the indicated
range-unit
. Otherwise, the
ranges-specifier
is
unsatisfiable
14.1.2.
Byte Ranges
The "bytes" range unit is used to express subranges of a representation data's octet
sequence. Each byte range is expressed as an integer range at some offset, relative
to either the beginning (
int-range
) or end (
suffix-range
) of the representation data. Byte ranges do not use the
other-range
specifier.
The
first-pos
value in a bytes
int-range
gives the offset of the first byte in a range. The
last-pos
value gives the offset of the last byte in the range; that is, the byte positions
specified are inclusive. Byte offsets start at zero.
If the representation data has a content coding applied, each byte range is calculated
with respect to the encoded sequence of bytes, not the sequence of underlying bytes
that would be obtained after decoding.
Examples of bytes range specifiers:
The first 500 bytes (byte offsets 0-499, inclusive):
bytes=0-499
The second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-999
A client can limit the number of bytes requested without knowing the size of the
selected representation
. If the
last-pos
value is absent, or if the value is greater than or equal to the current length of
the representation data, the byte range is interpreted as the remainder of the representation
(i.e., the server replaces the value of
last-pos
with a value that is one less than the current length of the selected representation).
A client can refer to the last N bytes (N > 0) of the selected representation using
suffix-range
. If the selected representation is shorter than the specified
suffix-length
, the entire representation is used.
Additional examples, assuming a representation of length 10000:
The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500
Or:
bytes=9500-
The first and last bytes only (bytes 0 and 9999):
bytes=0-0,-1
The first, middle, and last 1000 bytes:
bytes= 0-999, 4500-5499, -1000
Other valid (but not canonical) specifications of the second 500 bytes (byte offsets
500-999, inclusive):
bytes=500-600,601-999
bytes=500-700,601-999
For a
GET
request, a valid bytes
range-spec
is
satisfiable
if it is either:
an
int-range
with a
first-pos
that is less than the current length of the selected representation or
suffix-range
with a non-zero
suffix-length
When a selected representation has zero length, the only
satisfiable
form of
range-spec
in a
GET
request is a
suffix-range
with a non-zero
suffix-length
In the byte-range syntax,
first-pos
last-pos
, and
suffix-length
are expressed as decimal number of octets. Since there is no predefined limit to the
length of content, recipients
MUST
anticipate potentially large decimal numerals and prevent parsing errors due to integer
conversion overflows.
14.2.
Range
The "Range" header field on a GET request modifies the method semantics to request
transfer of only one or more subranges of the selected representation data (
Section 8.1
), rather than the entire
selected representation
Range
ranges-specifier
A server
MAY
ignore the Range header field. However, origin servers and intermediate caches ought
to support byte ranges when possible, since they support efficient recovery from partially
failed transfers and partial retrieval of large representations.
A server
MUST
ignore a Range header field received with a request method that is unrecognized or
for which range handling is not defined. For this specification,
GET
is the only method for which range handling is defined.
An origin server
MUST
ignore a Range header field that contains a range unit it does not understand. A proxy
MAY
discard a Range header field that contains a range unit it does not understand.
A server that supports range requests
MAY
ignore or reject a
Range
header field that contains an invalid
ranges-specifier
Section 14.1.1
), a
ranges-specifier
with more than two overlapping ranges, or a set of many small ranges that are not
listed in ascending order, since these are indications of either a broken client or
a deliberate denial-of-service attack (
Section 17.15
). A client
SHOULD NOT
request multiple ranges that are inherently less efficient to process and transfer
than a single range that encompasses the same data.
A server that supports range requests
MAY
ignore a
Range
header field when the selected representation has no content (i.e., the selected representation's
data is of zero length).
A client that is requesting multiple ranges
SHOULD
list those ranges in ascending order (the order in which they would typically be received
in a complete representation) unless there is a specific need to request a later part
earlier. For example, a user agent processing a large representation with an internal
catalog of parts might need to request later parts first, particularly if the representation
consists of pages stored in reverse order and the user agent wishes to transfer one
page at a time.
The Range header field is evaluated after evaluating the precondition header fields
defined in
Section 13.1
, and only if the result in absence of the Range header field would be a
200 (OK)
response. In other words, Range is ignored when a conditional GET would result in
304 (Not Modified)
response.
The If-Range header field (
Section 13.1.5
) can be used as a precondition to applying the Range header field.
If all of the preconditions are true, the server supports the Range header field for
the target resource, the received Range field-value contains a valid
ranges-specifier
with a
range-unit
supported for that target resource, and that
ranges-specifier
is
satisfiable
with respect to the selected representation, the server
SHOULD
send a
206 (Partial Content)
response with content containing one or more partial representations that correspond
to the satisfiable
range-spec
(s) requested.
The above does not imply that a server will send all requested ranges. In some cases,
it may only be possible (or efficient) to send a portion of the requested ranges first,
while expecting the client to re-request the remaining portions later if they are
still desired (see
Section 15.3.7
).
If all of the preconditions are true, the server supports the Range header field for
the target resource, the received Range field-value contains a valid
ranges-specifier
, and either the
range-unit
is not supported for that target resource or the
ranges-specifier
is unsatisfiable with respect to the selected representation, the server
SHOULD
send a
416 (Range Not Satisfiable)
response.
14.3.
Accept-Ranges
The "Accept-Ranges" field in a response indicates whether an upstream server supports
range requests for the target resource.
Accept-Ranges
acceptable-ranges
acceptable-ranges
= 1#
range-unit
For example, a server that supports byte-range requests (
Section 14.1.2
) can send the field
Accept-Ranges: bytes
to indicate that it supports byte range requests for that target resource, thereby
encouraging its use by the client for future partial requests on the same request
path. Range units are defined in
Section 14.1
A client
MAY
generate range requests regardless of having received an Accept-Ranges field. The
information only provides advice for the sake of improving performance and reducing
unnecessary network transfers.
Conversely, a client
MUST NOT
assume that receiving an Accept-Ranges field means that future range requests will
return partial responses. The content might change, the server might only support
range requests at certain times or under certain conditions, or a different intermediary
might process the next request.
A server that does not support any kind of range request for the target resource
MAY
send
Accept-Ranges: none
to advise the client not to attempt a range request on the same request path. The
range unit "none" is reserved for this purpose.
The Accept-Ranges field
MAY
be sent in a trailer section, but is preferred to be sent as a header field because
the information is particularly useful for restarting large information transfers
that have failed in mid-content (before the trailer section is received).
14.4.
Content-Range
The "Content-Range" header field is sent in a single part
206 (Partial Content)
response to indicate the partial range of the
selected representation
enclosed as the message content, sent in each part of a multipart 206 response to
indicate the range enclosed within each body part (
Section 14.6
), and sent in
416 (Range Not Satisfiable)
responses to provide information about the selected representation.
Content-Range
range-unit
SP
range-resp
unsatisfied-range
range-resp
incl-range
"/" (
complete-length
/ "*" )
incl-range
first-pos
"-"
last-pos
unsatisfied-range
= "*/"
complete-length
complete-length
= 1*
DIGIT
If a
206 (Partial Content)
response contains a
Content-Range
header field with a
range unit
Section 14.1
) that the recipient does not understand, the recipient
MUST NOT
attempt to recombine it with a stored representation. A proxy that receives such a
message
SHOULD
forward it downstream.
Content-Range might also be sent as a request modifier to request a partial PUT, as
described in
Section 14.5
, based on private agreements between client and origin server. A server
MUST
ignore a Content-Range header field received in a request with a method for which
Content-Range support is not defined.
For byte ranges, a sender
SHOULD
indicate the complete length of the representation from which the range has been extracted,
unless the complete length is unknown or difficult to determine. An asterisk character
("*") in place of the complete-length indicates that the representation length was
unknown when the header field was generated.
The following example illustrates when the complete length of the selected representation
is known by the sender to be 1234 bytes:
Content-Range: bytes 42-1233/1234
and this second example illustrates when the complete length is unknown:
Content-Range: bytes 42-1233/*
A Content-Range field value is invalid if it contains a
range-resp
that has a
last-pos
value less than its
first-pos
value, or a
complete-length
value less than or equal to its
last-pos
value. The recipient of an invalid
Content-Range
MUST NOT
attempt to recombine the received content with a stored representation.
A server generating a
416 (Range Not Satisfiable)
response to a byte-range request
SHOULD
send a Content-Range header field with an
unsatisfied-range
value, as in the following example:
Content-Range: bytes */1234
The complete-length in a 416 response indicates the current length of the selected
representation.
The Content-Range header field has no meaning for status codes that do not explicitly
describe its semantic. For this specification, only the
206 (Partial Content)
and
416 (Range Not Satisfiable)
status codes describe a meaning for Content-Range.
The following are examples of Content-Range values in which the selected representation
contains a total of 1234 bytes:
The first 500 bytes:
Content-Range: bytes 0-499/1234
The second 500 bytes:
Content-Range: bytes 500-999/1234
All except for the first 500 bytes:
Content-Range: bytes 500-1233/1234
The last 500 bytes:
Content-Range: bytes 734-1233/1234
14.5.
Partial PUT
Some origin servers support
PUT
of a partial representation when the user agent sends a
Content-Range
header field (
Section 14.4
) in the request, though such support is inconsistent and depends on private agreements
with user agents. In general, it requests that the state of the
target resource
be partly replaced with the enclosed content at an offset and length indicated by
the Content-Range value, where the offset is relative to the current selected representation.
An origin server
SHOULD
respond with a
400 (Bad Request)
status code if it receives
Content-Range
on a PUT for a target resource that does not support partial PUT requests.
Partial PUT is not backwards compatible with the original definition of PUT. It may
result in the content being written as a complete replacement for the current representation.
Partial resource updates are also possible by targeting a separately identified resource
with state that overlaps or extends a portion of the larger resource, or by using
a different method that has been specifically defined for partial updates (for example,
the PATCH method defined in
[RFC5789]
).
14.6.
Media Type multipart/byteranges
When a
206 (Partial Content)
response message includes the content of multiple ranges, they are transmitted as
body parts in a multipart message body (
[RFC2046]
Section 5.1
) with the media type of "multipart/byteranges".
The "multipart/byteranges" media type includes one or more body parts, each with its
own
Content-Type
and
Content-Range
fields. The required boundary parameter specifies the boundary string used to separate
each body part.
Implementation Notes:
Additional CRLFs might precede the first boundary string in the body.
Although
[RFC2046]
permits the boundary string to be quoted, some existing implementations handle a quoted
boundary string incorrectly.
A number of clients and servers were coded to an early draft of the byteranges specification
that used a media type of "multipart/x-byteranges",
which is almost (but not quite) compatible with this type.
Despite the name, the "multipart/byteranges" media type is not limited to byte ranges.
The following example uses an "exampleunit" range unit:
HTTP/1.1 206 Partial Content
Date: Tue, 14 Nov 1995 06:25:24 GMT
Last-Modified: Tue, 14 July 04:58:08 GMT
Content-Length: 2331785
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-Type: video/example
Content-Range: exampleunit 1.2-4.3/25
...the first range...
--THIS_STRING_SEPARATES
Content-Type: video/example
Content-Range: exampleunit 11.2-14.3/25
...the second range
--THIS_STRING_SEPARATES--
The following information serves as the registration form for the "multipart/byteranges"
media type.
Type name:
multipart
Subtype name:
byteranges
Required parameters:
boundary
Optional parameters:
N/A
Encoding considerations:
only "7bit", "8bit", or "binary" are permitted
Security considerations:
see
Section 17
Interoperability considerations:
N/A
Published specification:
RFC 9110 (see
Section 14.6
Applications that use this media type:
HTTP components supporting multiple ranges in a single request
Fragment identifier considerations:
N/A
Additional information:
Deprecated alias names for this type:
N/A
Magic number(s):
N/A
File extension(s):
N/A
Macintosh file type code(s):
N/A
Person and email address to contact for further information:
See Authors' Addresses section.
Intended usage:
COMMON
Restrictions on usage:
N/A
Author:
See Authors' Addresses section.
Change controller:
IESG
15.
Status Codes
The status code of a response is a three-digit integer code that describes the result
of the request and the semantics of the response, including whether the request was
successful and what content is enclosed (if any). All valid status codes are within
the range of 100 to 599, inclusive.
The first digit of the status code defines the class of response. The last two digits
do not have any categorization role. There are five values for the first digit:
1xx (Informational)
: The request was received, continuing process
2xx (Successful)
: The request was successfully received, understood, and accepted
3xx (Redirection)
: Further action needs to be taken in order to complete the request
4xx (Client Error)
: The request contains bad syntax or cannot be fulfilled
5xx (Server Error)
: The server failed to fulfill an apparently valid request
HTTP status codes are extensible. A client is not required to understand the meaning
of all registered status codes, though such understanding is obviously desirable.
However, a client
MUST
understand the class of any status code, as indicated by the first digit, and treat
an unrecognized status code as being equivalent to the x00 status code of that class.
For example, if a client receives an unrecognized status code of 471, it can see from
the first digit that there was something wrong with its request and treat the response
as if it had received a
400 (Bad Request)
status code. The response message will usually contain a representation that explains
the status.
Values outside the range 100..599 are invalid. Implementations often use three-digit
integer values outside of that range (i.e., 600..999) for internal communication of
non-HTTP status (e.g., library errors). A client that receives a response with an
invalid status code
SHOULD
process the response as if it had a
5xx (Server Error)
status code.
A single request can have multiple associated responses: zero or more
interim
(non-final) responses with status codes in the "informational" (
1xx
) range, followed by exactly one
final
response with a status code in one of the other ranges.
15.1.
Overview of Status Codes
The status codes listed below are defined in this specification. The reason phrases
listed here are only recommendations — they can be replaced by local equivalents or
left out altogether without affecting the protocol.
Responses with status codes that are defined as heuristically cacheable (e.g., 200,
203, 204, 206, 300, 301, 308, 404, 405, 410, 414, and 501 in this specification) can
be reused by a cache with heuristic expiration unless otherwise indicated by the method
definition or explicit cache controls
[CACHING]
; all other status codes are not heuristically cacheable.
Additional status codes, outside the scope of this specification, have been specified
for use in HTTP. All such status codes ought to be registered within the "Hypertext
Transfer Protocol (HTTP) Status Code Registry", as described in
Section 16.2
15.2.
Informational 1xx
The 1xx (Informational) class of status code indicates an interim response for communicating
connection status or request progress prior to completing the requested action and
sending a final response. Since HTTP/1.0 did not define any 1xx status codes, a server
MUST NOT
send a 1xx response to an HTTP/1.0 client.
A 1xx response is terminated by the end of the header section; it cannot contain content
or trailers.
A client
MUST
be able to parse one or more 1xx responses received prior to a final response, even
if the client does not expect one. A user agent
MAY
ignore unexpected 1xx responses.
A proxy
MUST
forward 1xx responses unless the proxy itself requested the generation of the 1xx
response. For example, if a proxy adds an "Expect: 100-continue" header field when
it forwards a request, then it need not forward the corresponding
100 (Continue)
response(s).
15.2.1.
100 Continue
The 100 (Continue) status code indicates that the initial part of a request has been
received and has not yet been rejected by the server. The server intends to send a
final response after the request has been fully received and acted upon.
When the request contains an
Expect
header field that includes a
100-continue
expectation, the 100 response indicates that the server wishes to receive the request
content, as described in
Section 10.1.1
. The client ought to continue sending the request and discard the 100 response.
If the request did not contain an
Expect
header field containing the
100-continue
expectation, the client can simply discard this interim response.
15.2.2.
101 Switching Protocols
The 101 (Switching Protocols) status code indicates that the server understands and
is willing to comply with the client's request, via the
Upgrade
header field (
Section 7.8
), for a change in the application protocol being used on this connection. The server
MUST
generate an Upgrade header field in the response that indicates which protocol(s)
will be in effect after this response.
It is assumed that the server will only agree to switch protocols when it is advantageous
to do so. For example, switching to a newer version of HTTP might be advantageous
over older versions, and switching to a real-time, synchronous protocol might be advantageous
when delivering resources that use such features.
15.3.
Successful 2xx
The 2xx (Successful) class of status code indicates that the client's request was
successfully received, understood, and accepted.
15.3.1.
200 OK
The 200 (OK) status code indicates that the request has succeeded. The content sent
in a 200 response depends on the request method. For the methods defined by this specification,
the intended meaning of the content can be summarized as:
Table 6
Request Method
Response content is a representation of:
GET
the
target resource
HEAD
the
target resource
, like GET, but without transferring the representation data
POST
the status of, or results obtained from, the action
PUT, DELETE
the status of the action
OPTIONS
communication options for the target resource
TRACE
the request message as received by the server returning the trace
Aside from responses to CONNECT, a 200 response is expected to contain message content
unless the message framing explicitly indicates that the content has zero length.
If some aspect of the request indicates a preference for no content upon success,
the origin server ought to send a
204 (No Content)
response instead. For CONNECT, there is no content because the successful result is
a tunnel, which begins immediately after the 200 response header section.
A 200 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
In 200 responses to GET or HEAD, an origin server
SHOULD
send any available validator fields (
Section 8.8
) for the
selected representation
, with both a strong entity tag and a
Last-Modified
date being preferred.
In 200 responses to state-changing methods, any validator fields (
Section 8.8
) sent in the response convey the current validators for the new representation formed
as a result of successfully applying the request semantics. Note that the PUT method
Section 9.3.4
) has additional requirements that might preclude sending such validators.
15.3.2.
201 Created
The 201 (Created) status code indicates that the request has been fulfilled and has
resulted in one or more new resources being created. The primary resource created
by the request is identified by either a
Location
header field in the response or, if no
Location
header field is received, by the target URI.
The 201 response content typically describes and links to the resource(s) created.
Any validator fields (
Section 8.8
) sent in the response convey the current validators for a new representation created
by the request. Note that the PUT method (
Section 9.3.4
) has additional requirements that might preclude sending such validators.
15.3.3.
202 Accepted
The 202 (Accepted) status code indicates that the request has been accepted for processing,
but the processing has not been completed. The request might or might not eventually
be acted upon, as it might be disallowed when processing actually takes place. There
is no facility in HTTP for re-sending a status code from an asynchronous operation.
The 202 response is intentionally noncommittal. Its purpose is to allow a server to
accept a request for some other process (perhaps a batch-oriented process that is
only run once per day) without requiring that the user agent's connection to the server
persist until the process is completed. The representation sent with this response
ought to describe the request's current status and point to (or embed) a status monitor
that can provide the user with an estimate of when the request will be fulfilled.
15.3.4.
203 Non-Authoritative Information
The 203 (Non-Authoritative Information) status code indicates that the request was
successful but the enclosed content has been modified from that of the origin server's
200 (OK)
response by a transforming proxy (
Section 7.7
). This status code allows the proxy to notify recipients when a transformation has
been applied, since that knowledge might impact later decisions regarding the content.
For example, future cache validation requests for the content might only be applicable
along the same request path (through the same proxies).
A 203 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
15.3.5.
204 No Content
The 204 (No Content) status code indicates that the server has successfully fulfilled
the request and that there is no additional content to send in the response content.
Metadata in the response header fields refer to the
target resource
and its
selected representation
after the requested action was applied.
For example, if a 204 status code is received in response to a PUT request and the
response contains an
ETag
field, then the PUT was successful and the ETag field value contains the entity tag
for the new representation of that target resource.
The 204 response allows a server to indicate that the action has been successfully
applied to the target resource, while implying that the user agent does not need to
traverse away from its current "document view" (if any). The server assumes that the
user agent will provide some indication of the success to its user, in accord with
its own interface, and apply any new or updated metadata in the response to its active
representation.
For example, a 204 status code is commonly used with document editing interfaces corresponding
to a "save" action, such that the document being saved remains available to the user
for editing. It is also frequently used with interfaces that expect automated data
transfers to be prevalent, such as within distributed version control systems.
A 204 response is terminated by the end of the header section; it cannot contain content
or trailers.
A 204 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
15.3.6.
205 Reset Content
The 205 (Reset Content) status code indicates that the server has fulfilled the request
and desires that the user agent reset the "document view", which caused the request
to be sent, to its original state as received from the origin server.
This response is intended to support a common data entry use case where the user receives
content that supports data entry (a form, notepad, canvas, etc.), enters or manipulates
data in that space, causes the entered data to be submitted in a request, and then
the data entry mechanism is reset for the next entry so that the user can easily initiate
another input action.
Since the 205 status code implies that no additional content will be provided, a server
MUST NOT
generate content in a 205 response.
15.3.7.
206 Partial Content
The 206 (Partial Content) status code indicates that the server is successfully fulfilling
a range request for the target resource by transferring one or more parts of the
selected representation
A server that supports range requests (
Section 14
) will usually attempt to satisfy all of the requested ranges, since sending less
data will likely result in another client request for the remainder. However, a server
might want to send only a subset of the data requested for reasons of its own, such
as temporary unavailability, cache efficiency, load balancing, etc. Since a 206 response
is self-descriptive, the client can still understand a response that only partially
satisfies its range request.
A client
MUST
inspect a 206 response's
Content-Type
and
Content-Range
field(s) to determine what parts are enclosed and whether additional requests are
needed.
A server that generates a 206 response
MUST
generate the following header fields, in addition to those required in the subsections
below, if the field would have been sent in a
200 (OK)
response to the same request:
Date
Cache-Control
ETag
Expires
Content-Location
, and
Vary
Content-Length
header field present in a 206 response indicates the number of octets in the content
of this message, which is usually not the complete length of the selected representation.
Each
Content-Range
header field includes information about the selected representation's complete length.
A sender that generates a 206 response to a request with an
If-Range
header field
SHOULD NOT
generate other representation header fields beyond those required because the client
already has a prior response containing those header fields. Otherwise, a sender
MUST
generate all of the representation header fields that would have been sent in a
200 (OK)
response to the same request.
A 206 response is heuristically cacheable; i.e., unless otherwise indicated by explicit
cache controls (see
Section 4.2.2
of
[CACHING]
).
15.3.7.1.
Single Part
If a single part is being transferred, the server generating the 206 response
MUST
generate a
Content-Range
header field, describing what range of the selected representation is enclosed, and
a content consisting of the range. For example:
HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Range: bytes 21010-47021/47022
Content-Length: 26012
Content-Type: image/gif
... 26012 bytes of partial image data ...
15.3.7.2.
Multiple Parts
If multiple parts are being transferred, the server generating the 206 response
MUST
generate "multipart/byteranges" content, as defined in
Section 14.6
, and a
Content-Type
header field containing the "multipart/byteranges" media type and its required boundary
parameter. To avoid confusion with single-part responses, a server
MUST NOT
generate a
Content-Range
header field in the HTTP header section of a multiple part response (this field will
be sent in each part instead).
Within the header area of each body part in the multipart content, the server
MUST
generate a
Content-Range
header field corresponding to the range being enclosed in that body part. If the selected
representation would have had a
Content-Type
header field in a
200 (OK)
response, the server
SHOULD
generate that same
Content-Type
header field in the header area of each body part. For example:
HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Length: 1741
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-Type: application/pdf
Content-Range: bytes 500-999/8000
...the first range...
--THIS_STRING_SEPARATES
Content-Type: application/pdf
Content-Range: bytes 7000-7999/8000
...the second range
--THIS_STRING_SEPARATES--
When multiple ranges are requested, a server
MAY
coalesce any of the ranges that overlap, or that are separated by a gap that is smaller
than the overhead of sending multiple parts, regardless of the order in which the
corresponding range-spec appeared in the received
Range
header field. Since the typical overhead between each part of a "multipart/byteranges"
is around 80 bytes, depending on the selected representation's media type and the
chosen boundary parameter length, it can be less efficient to transfer many small
disjoint parts than it is to transfer the entire selected representation.
A server
MUST NOT
generate a multipart response to a request for a single range, since a client that
does not request multiple parts might not support multipart responses. However, a
server
MAY
generate a "multipart/byteranges" response with only a single body part if multiple
ranges were requested and only one range was found to be satisfiable or only one range
remained after coalescing. A client that cannot process a "multipart/byteranges" response
MUST NOT
generate a request that asks for multiple ranges.
A server that generates a multipart response
SHOULD
send the parts in the same order that the corresponding range-spec appeared in the
received
Range
header field, excluding those ranges that were deemed unsatisfiable or that were coalesced
into other ranges. A client that receives a multipart response
MUST
inspect the
Content-Range
header field present in each body part in order to determine which range is contained
in that body part; a client cannot rely on receiving the same ranges that it requested,
nor the same order that it requested.
15.3.7.3.
Combining Parts
A response might transfer only a subrange of a representation if the connection closed
prematurely or if the request used one or more Range specifications. After several
such transfers, a client might have received several ranges of the same representation.
These ranges can only be safely combined if they all have in common the same strong
validator (
Section 8.8.1
).
A client that has received multiple partial responses to GET requests on a target
resource
MAY
combine those responses into a larger continuous range if they share the same strong
validator.
If the most recent response is an incomplete
200 (OK)
response, then the header fields of that response are used for any combined response
and replace those of the matching stored responses.
If the most recent response is a
206 (Partial Content)
response and at least one of the matching stored responses is a
200 (OK)
, then the combined response header fields consist of the most recent 200 response's
header fields. If all of the matching stored responses are 206 responses, then the
stored response with the most recent header fields is used as the source of header
fields for the combined response, except that the client
MUST
use other header fields provided in the new response, aside from
Content-Range
, to replace all instances of the corresponding header fields in the stored response.
The combined response content consists of the union of partial content ranges within
the new response and all of the matching stored responses. If the union consists of
the entire range of the representation, then the client
MUST
process the combined response as if it were a complete
200 (OK)
response, including a
Content-Length
header field that reflects the complete length. Otherwise, the client
MUST
process the set of continuous ranges as one of the following: an incomplete
200 (OK)
response if the combined response is a prefix of the representation, a single
206 (Partial Content)
response containing "multipart/byteranges" content, or multiple
206 (Partial Content)
responses, each with one continuous range that is indicated by a
Content-Range
header field.
15.4.
Redirection 3xx
The 3xx (Redirection) class of status code indicates that further action needs to
be taken by the user agent in order to fulfill the request. There are several types
of redirects:
Redirects that indicate this resource might be available at a different URI, as provided
by the
Location
header field, as in the status codes
301 (Moved Permanently)
302 (Found)
307 (Temporary Redirect)
, and
308 (Permanent Redirect)
Redirection that offers a choice among matching resources capable of representing
this resource, as in the
300 (Multiple Choices)
status code.
Redirection to a different resource, identified by the
Location
header field, that can represent an indirect response to the request, as in the
303 (See Other)
status code.
Redirection to a previously stored result, as in the
304 (Not Modified)
status code.
Note:
In HTTP/1.0, the status codes
301 (Moved Permanently)
and
302 (Found)
were originally defined as method-preserving (
[HTTP/1.0]
Section 9.3
) to match their implementation at CERN;
303 (See Other)
was defined for a redirection that changed its method to GET. However, early user
agents split on whether to redirect POST requests as POST (according to then-current
specification) or as GET (the safer alternative when redirected to a different site).
Prevailing practice eventually converged on changing the method to GET.
307 (Temporary Redirect)
and
308 (Permanent Redirect)
[RFC7538]
were later added to unambiguously indicate method-preserving redirects, and status
codes
301
and
302
have been adjusted to allow a POST request to be redirected as GET.
If a
Location
header field (
Section 10.2.2
) is provided, the user agent
MAY
automatically redirect its request to the URI referenced by the Location field value,
even if the specific status code is not understood. Automatic redirection needs to
be done with care for methods not known to be
safe
, as defined in
Section 9.2.1
, since the user might not wish to redirect an unsafe request.
When automatically following a redirected request, the user agent
SHOULD
resend the original request message with the following modifications:
Replace the target URI with the URI referenced by the redirection response's
Location
header field value after resolving it relative to the original request's target URI.
Remove header fields that were automatically generated by the implementation, replacing
them with updated values as appropriate to the new request. This includes:
Connection-specific header fields (see
Section 7.6.1
),
Header fields specific to the client's proxy configuration, including (but not limited
to)
Proxy-Authorization
Origin-specific header fields (if any), including (but not limited to)
Host
Validating header fields that were added by the implementation's cache (e.g.,
If-None-Match
If-Modified-Since
), and
Resource-specific header fields, including (but not limited to)
Referer
, Origin,
Authorization
, and Cookie.
Consider removing header fields that were not automatically generated by the implementation
(i.e., those present in the request because they were added by the calling context)
where there are security implications; this includes but is not limited to
Authorization
and Cookie.
Change the request method according to the redirecting status code's semantics, if
applicable.
If the request method has been changed to GET or HEAD, remove content-specific header
fields, including (but not limited to)
Content-Encoding
Content-Language
Content-Location
Content-Type
Content-Length
, Digest,
Last-Modified
A client
SHOULD
detect and intervene in cyclical redirections (i.e., "infinite" redirection loops).
Note:
An earlier version of this specification recommended a maximum of five redirections
[RFC2068]
Section 10.3
). Content developers need to be aware that some clients might implement such a fixed
limitation.
15.4.1.
300 Multiple Choices
The 300 (Multiple Choices) status code indicates that the
target resource
has more than one representation, each with its own more specific identifier, and
information about the alternatives is being provided so that the user (or user agent)
can select a preferred representation by redirecting its request to one or more of
those identifiers. In other words, the server desires that the user agent engage in
reactive negotiation to select the most appropriate representation(s) for its needs
Section 12
).
If the server has a preferred choice, the server
SHOULD
generate a
Location
header field containing a preferred choice's URI reference. The user agent
MAY
use the Location field value for automatic redirection.
For request methods other than HEAD, the server
SHOULD
generate content in the 300 response containing a list of representation metadata
and URI reference(s) from which the user or user agent can choose the one most preferred.
The user agent
MAY
make a selection from that list automatically if it understands the provided media
type. A specific format for automatic selection is not defined by this specification
because HTTP tries to remain orthogonal to the definition of its content. In practice,
the representation is provided in some easily parsed format believed to be acceptable
to the user agent, as determined by shared design or content negotiation, or in some
commonly accepted hypertext format.
A 300 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
Note:
The original proposal for the 300 status code defined the URI header field as providing
a list of alternative representations, such that it would be usable for 200, 300,
and 406 responses and be transferred in responses to the HEAD method. However, lack
of deployment and disagreement over syntax led to both URI and Alternates (a subsequent
proposal) being dropped from this specification. It is possible to communicate the
list as a Link header field value
[RFC8288]
whose members have a relationship of "alternate", though deployment is a chicken-and-egg
problem.
15.4.2.
301 Moved Permanently
The 301 (Moved Permanently) status code indicates that the
target resource
has been assigned a new permanent URI and any future references to this resource ought
to use one of the enclosed URIs. The server is suggesting that a user agent with link-editing
capability can permanently replace references to the target URI with one of the new
references sent by the server. However, this suggestion is usually ignored unless
the user agent is actively editing references (e.g., engaged in authoring content),
the connection is secured, and the origin server is a trusted authority for the content
being edited.
The server
SHOULD
generate a
Location
header field in the response containing a preferred URI reference for the new permanent
URI. The user agent
MAY
use the Location field value for automatic redirection. The server's response content
usually contains a short hypertext note with a hyperlink to the new URI(s).
Note:
For historical reasons, a user agent
MAY
change the request method from POST to GET for the subsequent request. If this behavior
is undesired, the
308 (Permanent Redirect)
status code can be used instead.
A 301 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
15.4.3.
302 Found
The 302 (Found) status code indicates that the target resource resides temporarily
under a different URI. Since the redirection might be altered on occasion, the client
ought to continue to use the target URI for future requests.
The server
SHOULD
generate a
Location
header field in the response containing a URI reference for the different URI. The
user agent
MAY
use the Location field value for automatic redirection. The server's response content
usually contains a short hypertext note with a hyperlink to the different URI(s).
Note:
For historical reasons, a user agent
MAY
change the request method from POST to GET for the subsequent request. If this behavior
is undesired, the
307 (Temporary Redirect)
status code can be used instead.
15.4.4.
303 See Other
The 303 (See Other) status code indicates that the server is redirecting the user
agent to a different resource, as indicated by a URI in the
Location
header field, which is intended to provide an indirect response to the original request.
A user agent can perform a retrieval request targeting that URI (a GET or HEAD request
if using HTTP), which might also be redirected, and present the eventual result as
an answer to the original request. Note that the new URI in the Location header field
is not considered equivalent to the target URI.
This status code is applicable to any HTTP method. It is primarily used to allow the
output of a POST action to redirect the user agent to a different resource, since
doing so provides the information corresponding to the POST response as a resource
that can be separately identified, bookmarked, and cached.
A 303 response to a GET request indicates that the origin server does not have a representation
of the
target resource
that can be transferred by the server over HTTP. However, the
Location
field value refers to a resource that is descriptive of the target resource, such
that making a retrieval request on that other resource might result in a representation
that is useful to recipients without implying that it represents the original target
resource. Note that answers to the questions of what can be represented, what representations
are adequate, and what might be a useful description are outside the scope of HTTP.
Except for responses to a HEAD request, the representation of a 303 response ought
to contain a short hypertext note with a hyperlink to the same URI reference provided
in the
Location
header field.
15.4.5.
304 Not Modified
The 304 (Not Modified) status code indicates that a conditional GET or HEAD request
has been received and would have resulted in a
200 (OK)
response if it were not for the fact that the condition evaluated to false. In other
words, there is no need for the server to transfer a representation of the target
resource because the request indicates that the client, which made the request conditional,
already has a valid representation; the server is therefore redirecting the client
to make use of that stored representation as if it were the content of a
200 (OK)
response.
The server generating a 304 response
MUST
generate any of the following header fields that would have been sent in a
200 (OK)
response to the same request:
Content-Location
Date
ETag
, and
Vary
Cache-Control
and
Expires
(see
[CACHING]
Since the goal of a 304 response is to minimize information transfer when the recipient
already has one or more cached representations, a sender
SHOULD NOT
generate representation metadata other than the above listed fields unless said metadata
exists for the purpose of guiding cache updates (e.g.,
Last-Modified
might be useful if the response does not have an
ETag
field).
Requirements on a cache that receives a 304 response are defined in
Section 4.3.4
of
[CACHING]
. If the conditional request originated with an outbound client, such as a user agent
with its own cache sending a conditional GET to a shared proxy, then the proxy
SHOULD
forward the 304 response to that client.
A 304 response is terminated by the end of the header section; it cannot contain content
or trailers.
15.4.6.
305 Use Proxy
The 305 (Use Proxy) status code was defined in a previous version of this specification
and is now deprecated (
Appendix B
of
[RFC7231]
).
15.4.7.
306 (Unused)
The 306 status code was defined in a previous version of this specification, is no
longer used, and the code is reserved.
15.4.8.
307 Temporary Redirect
The 307 (Temporary Redirect) status code indicates that the
target resource
resides temporarily under a different URI and the user agent
MUST NOT
change the request method if it performs an automatic redirection to that URI. Since
the redirection can change over time, the client ought to continue using the original
target URI for future requests.
The server
SHOULD
generate a
Location
header field in the response containing a URI reference for the different URI. The
user agent
MAY
use the Location field value for automatic redirection. The server's response content
usually contains a short hypertext note with a hyperlink to the different URI(s).
15.4.9.
308 Permanent Redirect
The 308 (Permanent Redirect) status code indicates that the
target resource
has been assigned a new permanent URI and any future references to this resource ought
to use one of the enclosed URIs. The server is suggesting that a user agent with link-editing
capability can permanently replace references to the target URI with one of the new
references sent by the server. However, this suggestion is usually ignored unless
the user agent is actively editing references (e.g., engaged in authoring content),
the connection is secured, and the origin server is a trusted authority for the content
being edited.
The server
SHOULD
generate a
Location
header field in the response containing a preferred URI reference for the new permanent
URI. The user agent
MAY
use the Location field value for automatic redirection. The server's response content
usually contains a short hypertext note with a hyperlink to the new URI(s).
A 308 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
Note:
This status code is much younger (June 2014) than its sibling codes and thus might
not be recognized everywhere. See
Section 4
of
[RFC7538]
for deployment considerations.
15.5.
Client Error 4xx
The 4xx (Client Error) class of status code indicates that the client seems to have
erred. Except when responding to a HEAD request, the server
SHOULD
send a representation containing an explanation of the error situation, and whether
it is a temporary or permanent condition. These status codes are applicable to any
request method. User agents
SHOULD
display any included representation to the user.
15.5.1.
400 Bad Request
The 400 (Bad Request) status code indicates that the server cannot or will not process
the request due to something that is perceived to be a client error (e.g., malformed
request syntax, invalid request message framing, or deceptive request routing).
15.5.2.
401 Unauthorized
The 401 (Unauthorized) status code indicates that the request has not been applied
because it lacks valid authentication credentials for the target resource. The server
generating a 401 response
MUST
send a
WWW-Authenticate
header field (
Section 11.6.1
) containing at least one challenge applicable to the target resource.
If the request included authentication credentials, then the 401 response indicates
that authorization has been refused for those credentials. The user agent
MAY
repeat the request with a new or replaced
Authorization
header field (
Section 11.6.2
). If the 401 response contains the same challenge as the prior response, and the
user agent has already attempted authentication at least once, then the user agent
SHOULD
present the enclosed representation to the user, since it usually contains relevant
diagnostic information.
15.5.3.
402 Payment Required
The 402 (Payment Required) status code is reserved for future use.
15.5.4.
403 Forbidden
The 403 (Forbidden) status code indicates that the server understood the request but
refuses to fulfill it. A server that wishes to make public why the request has been
forbidden can describe that reason in the response content (if any).
If authentication credentials were provided in the request, the server considers them
insufficient to grant access. The client
SHOULD NOT
automatically repeat the request with the same credentials. The client
MAY
repeat the request with new or different credentials. However, a request might be
forbidden for reasons unrelated to the credentials.
An origin server that wishes to "hide" the current existence of a forbidden
target resource
MAY
instead respond with a status code of
404 (Not Found)
15.5.5.
404 Not Found
The 404 (Not Found) status code indicates that the origin server did not find a current
representation for the
target resource
or is not willing to disclose that one exists. A 404 status code does not indicate
whether this lack of representation is temporary or permanent; the
410 (Gone)
status code is preferred over 404 if the origin server knows, presumably through some
configurable means, that the condition is likely to be permanent.
A 404 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
15.5.6.
405 Method Not Allowed
The 405 (Method Not Allowed) status code indicates that the method received in the
request-line is known by the origin server but not supported by the
target resource
. The origin server
MUST
generate an
Allow
header field in a 405 response containing a list of the target resource's currently
supported methods.
A 405 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
15.5.7.
406 Not Acceptable
The 406 (Not Acceptable) status code indicates that the
target resource
does not have a current representation that would be acceptable to the user agent,
according to the
proactive negotiation
header fields received in the request (
Section 12.1
), and the server is unwilling to supply a default representation.
The server
SHOULD
generate content containing a list of available representation characteristics and
corresponding resource identifiers from which the user or user agent can choose the
one most appropriate. A user agent
MAY
automatically select the most appropriate choice from that list. However, this specification
does not define any standard for such automatic selection, as described in
Section 15.4.1
15.5.8.
407 Proxy Authentication Required
The 407 (Proxy Authentication Required) status code is similar to
401 (Unauthorized)
, but it indicates that the client needs to authenticate itself in order to use a
proxy for this request. The proxy
MUST
send a
Proxy-Authenticate
header field (
Section 11.7.1
) containing a challenge applicable to that proxy for the request. The client
MAY
repeat the request with a new or replaced
Proxy-Authorization
header field (
Section 11.7.2
).
15.5.9.
408 Request Timeout
The 408 (Request Timeout) status code indicates that the server did not receive a
complete request message within the time that it was prepared to wait.
If the client has an outstanding request in transit, it
MAY
repeat that request. If the current connection is not usable (e.g., as it would be
in HTTP/1.1 because request delimitation is lost), a new connection will be used.
15.5.10.
409 Conflict
The 409 (Conflict) status code indicates that the request could not be completed due
to a conflict with the current state of the target resource. This code is used in
situations where the user might be able to resolve the conflict and resubmit the request.
The server
SHOULD
generate content that includes enough information for a user to recognize the source
of the conflict.
Conflicts are most likely to occur in response to a PUT request. For example, if versioning
were being used and the representation being PUT included changes to a resource that
conflict with those made by an earlier (third-party) request, the origin server might
use a 409 response to indicate that it can't complete the request. In this case, the
response representation would likely contain information useful for merging the differences
based on the revision history.
15.5.11.
410 Gone
The 410 (Gone) status code indicates that access to the
target resource
is no longer available at the origin server and that this condition is likely to be
permanent. If the origin server does not know, or has no facility to determine, whether
or not the condition is permanent, the status code
404 (Not Found)
ought to be used instead.
The 410 response is primarily intended to assist the task of web maintenance by notifying
the recipient that the resource is intentionally unavailable and that the server owners
desire that remote links to that resource be removed. Such an event is common for
limited-time, promotional services and for resources belonging to individuals no longer
associated with the origin server's site. It is not necessary to mark all permanently
unavailable resources as "gone" or to keep the mark for any length of time — that
is left to the discretion of the server owner.
A 410 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
15.5.12.
411 Length Required
The 411 (Length Required) status code indicates that the server refuses to accept
the request without a defined
Content-Length
Section 8.6
). The client
MAY
repeat the request if it adds a valid Content-Length header field containing the length
of the request content.
15.5.13.
412 Precondition Failed
The 412 (Precondition Failed) status code indicates that one or more conditions given
in the request header fields evaluated to false when tested on the server (
Section 13
). This response status code allows the client to place preconditions on the current
resource state (its current representations and metadata) and, thus, prevent the request
method from being applied if the target resource is in an unexpected state.
15.5.14.
413 Content Too Large
The 413 (Content Too Large) status code indicates that the server is refusing to process
a request because the request content is larger than the server is willing or able
to process. The server
MAY
terminate the request, if the protocol version in use allows it; otherwise, the server
MAY
close the connection.
If the condition is temporary, the server
SHOULD
generate a
Retry-After
header field to indicate that it is temporary and after what time the client
MAY
try again.
15.5.15.
414 URI Too Long
The 414 (URI Too Long) status code indicates that the server is refusing to service
the request because the target URI is longer than the server is willing to interpret.
This rare condition is only likely to occur when a client has improperly converted
a POST request to a GET request with long query information, when the client has descended
into an infinite loop of redirection (e.g., a redirected URI prefix that points to
a suffix of itself) or when the server is under attack by a client attempting to exploit
potential security holes.
A 414 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
15.5.16.
415 Unsupported Media Type
The 415 (Unsupported Media Type) status code indicates that the origin server is refusing
to service the request because the content is in a format not supported by this method
on the
target resource
The format problem might be due to the request's indicated
Content-Type
or
Content-Encoding
, or as a result of inspecting the data directly.
If the problem was caused by an unsupported content coding, the
Accept-Encoding
response header field (
Section 12.5.3
) ought to be used to indicate which (if any) content codings would have been accepted
in the request.
On the other hand, if the cause was an unsupported media type, the
Accept
response header field (
Section 12.5.1
) can be used to indicate which media types would have been accepted in the request.
15.5.17.
416 Range Not Satisfiable
The 416 (Range Not Satisfiable) status code indicates that the set of ranges in the
request's
Range
header field (
Section 14.2
) has been rejected either because none of the requested ranges are satisfiable or
because the client has requested an excessive number of small or overlapping ranges
(a potential denial of service attack).
Each range unit defines what is required for its own range sets to be satisfiable.
For example,
Section 14.1.2
defines what makes a bytes range set satisfiable.
A server that generates a 416 response to a byte-range request
SHOULD
generate a
Content-Range
header field specifying the current length of the selected representation (
Section 14.4
).
For example:
HTTP/1.1 416 Range Not Satisfiable
Date: Fri, 20 Jan 2012 15:41:54 GMT
Content-Range: bytes */47022
Note:
Because servers are free to ignore
Range
, many implementations will respond with the entire selected representation in a
200 (OK)
response. That is partly because most clients are prepared to receive a
200 (OK)
to complete the task (albeit less efficiently) and partly because clients might not
stop making an invalid range request until they have received a complete representation.
Thus, clients cannot depend on receiving a
416 (Range Not Satisfiable)
response even when it is most appropriate.
15.5.18.
417 Expectation Failed
The 417 (Expectation Failed) status code indicates that the expectation given in the
request's
Expect
header field (
Section 10.1.1
) could not be met by at least one of the inbound servers.
15.5.19.
418 (Unused)
[RFC2324]
was an April 1 RFC that lampooned the various ways HTTP was abused; one such abuse
was the definition of an application-specific 418 status code, which has been deployed
as a joke often enough for the code to be unusable for any future use.
Therefore, the 418 status code is reserved in the IANA HTTP Status Code Registry.
This indicates that the status code cannot be assigned to other applications currently.
If future circumstances require its use (e.g., exhaustion of 4NN status codes), it
can be re-assigned to another use.
15.5.20.
421 Misdirected Request
The 421 (Misdirected Request) status code indicates that the request was directed
at a server that is unable or unwilling to produce an authoritative response for the
target URI. An origin server (or gateway acting on behalf of the origin server) sends
421 to reject a target URI that does not match an
origin
for which the server has been configured (
Section 4.3.1
) or does not match the connection context over which the request was received (
Section 7.4
).
A client that receives a 421 (Misdirected Request) response
MAY
retry the request, whether or not the request method is idempotent, over a different
connection, such as a fresh connection specific to the target resource's origin, or
via an alternative service
[ALTSVC]
A proxy
MUST NOT
generate a 421 response.
15.5.21.
422 Unprocessable Content
The 422 (Unprocessable Content) status code indicates that the server understands
the content type of the request content (hence a
415 (Unsupported Media Type)
status code is inappropriate), and the syntax of the request content is correct, but
it was unable to process the contained instructions. For example, this status code
can be sent if an XML request content contains well-formed (i.e., syntactically correct),
but semantically erroneous XML instructions.
15.5.22.
426 Upgrade Required
The 426 (Upgrade Required) status code indicates that the server refuses to perform
the request using the current protocol but might be willing to do so after the client
upgrades to a different protocol. The server
MUST
send an
Upgrade
header field in a 426 response to indicate the required protocol(s) (
Section 7.8
).
Example:
HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length: 53
Content-Type: text/plain
This service requires use of the HTTP/3.0 protocol.
15.6.
Server Error 5xx
The 5xx (Server Error) class of status code indicates that the server is aware that
it has erred or is incapable of performing the requested method. Except when responding
to a HEAD request, the server
SHOULD
send a representation containing an explanation of the error situation, and whether
it is a temporary or permanent condition. A user agent
SHOULD
display any included representation to the user. These status codes are applicable
to any request method.
15.6.1.
500 Internal Server Error
The 500 (Internal Server Error) status code indicates that the server encountered
an unexpected condition that prevented it from fulfilling the request.
15.6.2.
501 Not Implemented
The 501 (Not Implemented) status code indicates that the server does not support the
functionality required to fulfill the request. This is the appropriate response when
the server does not recognize the request method and is not capable of supporting
it for any resource.
A 501 response is heuristically cacheable; i.e., unless otherwise indicated by the
method definition or explicit cache controls (see
Section 4.2.2
of
[CACHING]
).
15.6.3.
502 Bad Gateway
The 502 (Bad Gateway) status code indicates that the server, while acting as a gateway
or proxy, received an invalid response from an inbound server it accessed while attempting
to fulfill the request.
15.6.4.
503 Service Unavailable
The 503 (Service Unavailable) status code indicates that the server is currently unable
to handle the request due to a temporary overload or scheduled maintenance, which
will likely be alleviated after some delay. The server
MAY
send a
Retry-After
header field (
Section 10.2.3
) to suggest an appropriate amount of time for the client to wait before retrying
the request.
Note:
The existence of the 503 status code does not imply that a server has to use it when
becoming overloaded. Some servers might simply refuse the connection.
15.6.5.
504 Gateway Timeout
The 504 (Gateway Timeout) status code indicates that the server, while acting as a
gateway or proxy, did not receive a timely response from an upstream server it needed
to access in order to complete the request.
15.6.6.
505 HTTP Version Not Supported
The 505 (HTTP Version Not Supported) status code indicates that the server does not
support, or refuses to support, the major version of HTTP that was used in the request
message. The server is indicating that it is unable or unwilling to complete the request
using the same major version as the client, as described in
Section 2.5
, other than with this error message. The server
SHOULD
generate a representation for the 505 response that describes why that version is
not supported and what other protocols are supported by that server.
16.
Extending HTTP
HTTP defines a number of generic extension points that can be used to introduce capabilities
to the protocol without introducing a new version, including methods, status codes,
field names, and further extensibility points within defined fields, such as authentication
schemes and cache directives (see Cache-Control extensions in
Section 5.2.3
of
[CACHING]
). Because the semantics of HTTP are not versioned, these extension points are persistent;
the version of the protocol in use does not affect their semantics.
Version-independent extensions are discouraged from depending on or interacting with
the specific version of the protocol in use. When this is unavoidable, careful consideration
needs to be given to how the extension can interoperate across versions.
Additionally, specific versions of HTTP might have their own extensibility points,
such as transfer codings in HTTP/1.1 (
Section 6.1
of
[HTTP/1.1]
) and HTTP/2 SETTINGS or frame types (
[HTTP/2]
). These extension points are specific to the version of the protocol they occur within.
Version-specific extensions cannot override or modify the semantics of a version-independent
mechanism or extension point (like a method or header field) without explicitly being
allowed by that protocol element. For example, the CONNECT method (
Section 9.3.6
) allows this.
These guidelines assure that the protocol operates correctly and predictably, even
when parts of the path implement different versions of HTTP.
16.1.
Method Extensibility
16.1.1.
Method Registry
The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained by IANA at
, registers
method
names.
HTTP method registrations
MUST
include the following fields:
Method Name (see
Section 9
Safe ("yes" or "no", see
Section 9.2.1
Idempotent ("yes" or "no", see
Section 9.2.2
Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC8126]
Section 4.8
).
16.1.2.
Considerations for New Methods
Standardized methods are generic; that is, they are potentially applicable to any
resource, not just one particular media type, kind of resource, or application. As
such, it is preferred that new methods be registered in a document that isn't specific
to a single application or data format, since orthogonal technologies deserve orthogonal
specification.
Since message parsing (
Section 6
) needs to be independent of method semantics (aside from responses to HEAD), definitions
of new methods cannot change the parsing algorithm or prohibit the presence of content
on either the request or the response message. Definitions of new methods can specify
that only a zero-length content is allowed by requiring a Content-Length header field
with a value of "0".
Likewise, new methods cannot use the special host:port and asterisk forms of request
target that are allowed for
CONNECT
and
OPTIONS
, respectively (
Section 7.1
). A full URI in absolute form is needed for the target URI, which means either the
request target needs to be sent in absolute form or the target URI will be reconstructed
from the request context in the same way it is for other methods.
A new method definition needs to indicate whether it is safe (
Section 9.2.1
), idempotent (
Section 9.2.2
), cacheable (
Section 9.2.3
), what semantics are to be associated with the request content (if any), and what
refinements the method makes to header field or status code semantics. If the new
method is cacheable, its definition ought to describe how, and under what conditions,
a cache can store a response and use it to satisfy a subsequent request. The new method
ought to describe whether it can be made conditional (
Section 13.1
) and, if so, how a server responds when the condition is false. Likewise, if the
new method might have some use for partial response semantics (
Section 14.2
), it ought to document this, too.
Note:
Avoid defining a method name that starts with "M-", since that prefix might be misinterpreted
as having the semantics assigned to it by
[RFC2774]
16.2.
Status Code Extensibility
16.2.1.
Status Code Registry
The "Hypertext Transfer Protocol (HTTP) Status Code Registry", maintained by IANA
at
, registers
status code
numbers.
A registration
MUST
include the following fields:
Status Code (3 digits)
Short Description
Pointer to specification text
Values to be added to the HTTP status code namespace require IETF Review (see
[RFC8126]
Section 4.8
).
16.2.2.
Considerations for New Status Codes
When it is necessary to express semantics for a response that are not defined by current
status codes, a new status code can be registered. Status codes are generic; they
are potentially applicable to any resource, not just one particular media type, kind
of resource, or application of HTTP. As such, it is preferred that new status codes
be registered in a document that isn't specific to a single application.
New status codes are required to fall under one of the categories defined in
Section 15
. To allow existing parsers to process the response message, new status codes cannot
disallow content, although they can mandate a zero-length content.
Proposals for new status codes that are not yet widely deployed ought to avoid allocating
a specific number for the code until there is clear consensus that it will be registered;
instead, early drafts can use a notation such as "4NN", or "3N0" .. "3N9", to indicate
the class of the proposed status code(s) without consuming a number prematurely.
The definition of a new status code ought to explain the request conditions that would
cause a response containing that status code (e.g., combinations of request header
fields and/or method(s)) along with any dependencies on response header fields (e.g.,
what fields are required, what fields can modify the semantics, and what field semantics
are further refined when used with the new status code).
By default, a status code applies only to the request corresponding to the response
it occurs within. If a status code applies to a larger scope of applicability — for
example, all requests to the resource in question or all requests to a server — this
must be explicitly specified. When doing so, it should be noted that not all clients
can be expected to consistently apply a larger scope because they might not understand
the new status code.
The definition of a new final status code ought to specify whether or not it is heuristically
cacheable. Note that any response with a final status code can be cached if the response
has explicit freshness information. A status code defined as heuristically cacheable
is allowed to be cached without explicit freshness information. Likewise, the definition
of a status code can place constraints upon cache behavior if the must-understand
cache directive is used. See
[CACHING]
for more information.
Finally, the definition of a new status code ought to indicate whether the content
has any implied association with an identified resource (
Section 6.4.2
).
16.3.
Field Extensibility
HTTP's most widely used extensibility point is the definition of new header and trailer
fields.
New fields can be defined such that, when they are understood by a recipient, they
override or enhance the interpretation of previously defined fields, define preconditions
on request evaluation, or refine the meaning of responses.
However, defining a field doesn't guarantee its deployment or recognition by recipients.
Most fields are designed with the expectation that a recipient can safely ignore (but
forward downstream) any field not recognized. In other cases, the sender's ability
to understand a given field might be indicated by its prior communication, perhaps
in the protocol version or fields that it sent in prior messages, or its use of a
specific media type. Likewise, direct inspection of support might be possible through
an OPTIONS request or by interacting with a defined well-known URI
[RFC8615]
if such inspection is defined along with the field being introduced.
16.3.1.
Field Name Registry
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines the namespace
for HTTP field names.
Any party can request registration of an HTTP field. See
Section 16.3.2
for considerations to take into account when creating a new HTTP field.
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is located at
. Registration requests can be made by following the instructions located there or
by sending an email to the "ietf-http-wg@w3.org" mailing list.
Field names are registered on the advice of a designated expert (appointed by the
IESG or their delegate). Fields with the status 'permanent' are Specification Required
[RFC8126]
Section 4.6
).
Registration requests consist of the following information:
Field name:
The requested field name. It
MUST
conform to the field-name syntax defined in
Section 5.1
, and it
SHOULD
be restricted to just letters, digits, and hyphen ('-') characters, with the first
character being a letter.
Status:
"permanent", "provisional", "deprecated", or "obsoleted".
Specification document(s):
Reference to the document that specifies the field, preferably including a URI that
can be used to retrieve a copy of the document. Optional but encouraged for provisional
registrations. An indication of the relevant section(s) can also be included, but
is not required.
And optionally:
Comments:
Additional information, such as about reserved entries.
The expert(s) can define additional fields to be collected in the registry, in consultation
with the community.
Standards-defined names have a status of "permanent". Other names can also be registered
as permanent if the expert(s) finds that they are in use, in consultation with the
community. Other names should be registered as "provisional".
Provisional entries can be removed by the expert(s) if — in consultation with the
community — the expert(s) find that they are not in use. The expert(s) can change
a provisional entry's status to permanent at any time.
Note that names can be registered by third parties (including the expert(s)) if the
expert(s) determines that an unregistered name is widely deployed and not likely to
be registered in a timely manner otherwise.
16.3.2.
Considerations for New Fields
HTTP header and trailer fields are a widely used extension point for the protocol.
While they can be used in an ad hoc fashion, fields that are intended for wider use
need to be carefully documented to ensure interoperability.
In particular, authors of specifications defining new fields are advised to consider
and, where appropriate, document the following aspects:
Under what conditions the field can be used; e.g., only in responses or requests,
in all messages, only on responses to a particular request method, etc.
Whether the field semantics are further refined by their context, such as their use
with certain request methods or status codes.
The scope of applicability for the information conveyed. By default, fields apply
only to the message they are associated with, but some response fields are designed
to apply to all representations of a resource, the resource itself, or an even broader
scope. Specifications that expand the scope of a response field will need to carefully
consider issues such as content negotiation, the time period of applicability, and
(in some cases) multi-tenant server deployments.
Under what conditions intermediaries are allowed to insert, delete, or modify the
field's value.
If the field is allowable in trailers; by default, it will not be (see
Section 6.5.1
).
Whether it is appropriate or even required to list the field name in the
Connection
header field (i.e., if the field is to be hop-by-hop; see
Section 7.6.1
).
Whether the field introduces any additional security considerations, such as disclosure
of privacy-related data.
Request header fields have additional considerations that need to be documented if
the default behavior is not appropriate:
If it is appropriate to list the field name in a
Vary
response header field (e.g., when the request header field is used by an origin server's
content selection algorithm; see
Section 12.5.5
).
If the field is intended to be stored when received in a PUT request (see
Section 9.3.4
).
If the field ought to be removed when automatically redirecting a request due to security
concerns (see
Section 15.4
).
16.3.2.1.
Considerations for New Field Names
Authors of specifications defining new fields are advised to choose a short but descriptive
field name. Short names avoid needless data transmission; descriptive names avoid
confusion and "squatting" on names that might have broader uses.
To that end, limited-use fields (such as a header confined to a single application
or use case) are encouraged to use a name that includes that use (or an abbreviation)
as a prefix; for example, if the Foo Application needs a Description field, it might
use "Foo-Desc"; "Description" is too generic, and "Foo-Description" is needlessly
long.
While the field-name syntax is defined to allow any token character, in practice some
implementations place limits on the characters they accept in field-names. To be interoperable,
new field names
SHOULD
constrain themselves to alphanumeric characters, "-", and ".", and
SHOULD
begin with a letter. For example, the underscore ("_") character can be problematic
when passed through non-HTTP gateway interfaces (see
Section 17.10
).
Field names ought not be prefixed with "X-"; see
[BCP178]
for further information.
Other prefixes are sometimes used in HTTP field names; for example, "Accept-" is used
in many content negotiation headers, and "Content-" is used as explained in
Section 6.4
. These prefixes are only an aid to recognizing the purpose of a field and do not
trigger automatic processing.
16.3.2.2.
Considerations for New Field Values
A major task in the definition of a new HTTP field is the specification of the field
value syntax: what senders should generate, and how recipients should infer semantics
from what is received.
Authors are encouraged (but not required) to use either the ABNF rules in this specification
or those in
[RFC8941]
to define the syntax of new field values.
Authors are advised to carefully consider how the combination of multiple field lines
will impact them (see
Section 5.3
). Because senders might erroneously send multiple values, and both intermediaries
and HTTP libraries can perform combination automatically, this applies to all field
values — even when only a single value is anticipated.
Therefore, authors are advised to delimit or encode values that contain commas (e.g.,
with the
quoted-string
rule of
Section 5.6.4
, the String data type of
[RFC8941]
, or a field-specific encoding). This ensures that commas within field data are not
confused with the commas that delimit a list value.
For example, the
Content-Type
field value only allows commas inside quoted strings, which can be reliably parsed
even when multiple values are present. The
Location
field value provides a counter-example that should not be emulated: because URIs can
include commas, it is not possible to reliably distinguish between a single value
that includes a comma from two values.
Authors of fields with a singleton value (see
Section 5.5
) are additionally advised to document how to treat messages where the multiple members
are present (a sensible default would be to ignore the field, but this might not always
be the right choice).
16.4.
Authentication Scheme Extensibility
16.4.1.
Authentication Scheme Registry
The "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" defines the
namespace for the authentication schemes in challenges and credentials. It is maintained
at
Registrations
MUST
include the following fields:
Authentication Scheme Name
Pointer to specification text
Notes (optional)
Values to be added to this namespace require IETF Review (see
[RFC8126]
Section 4.8
).
16.4.2.
Considerations for New Authentication Schemes
There are certain aspects of the HTTP Authentication framework that put constraints
on how new authentication schemes can work:
HTTP authentication is presumed to be stateless: all of the information necessary
to authenticate a request
MUST
be provided in the request, rather than be dependent on the server remembering prior
requests. Authentication based on, or bound to, the underlying connection is outside
the scope of this specification and inherently flawed unless steps are taken to ensure
that the connection cannot be used by any party other than the authenticated user
(see
Section 3.3
).
The authentication parameter "realm" is reserved for defining protection spaces as
described in
Section 11.5
. New schemes
MUST NOT
use it in a way incompatible with that definition.
The "token68" notation was introduced for compatibility with existing authentication
schemes and can only be used once per challenge or credential. Thus, new schemes ought
to use the auth-param syntax instead, because otherwise future extensions will be
impossible.
The parsing of challenges and credentials is defined by this specification and cannot
be modified by new authentication schemes. When the auth-param syntax is used, all
parameters ought to support both token and quoted-string syntax, and syntactical constraints
ought to be defined on the field value after parsing (i.e., quoted-string processing).
This is necessary so that recipients can use a generic parser that applies to all
authentication schemes.
Note:
The fact that the value syntax for the "realm" parameter is restricted to quoted-string
was a bad design choice not to be repeated for new parameters.
Definitions of new schemes ought to define the treatment of unknown extension parameters.
In general, a "must-ignore" rule is preferable to a "must-understand" rule, because
otherwise it will be hard to introduce new parameters in the presence of legacy recipients.
Furthermore, it's good to describe the policy for defining new parameters (such as
"update the specification" or "use this registry").
Authentication schemes need to document whether they are usable in origin-server authentication
(i.e., using
WWW-Authenticate
), and/or proxy authentication (i.e., using
Proxy-Authenticate
).
The credentials carried in an
Authorization
header field are specific to the user agent and, therefore, have the same effect on
HTTP caches as the "private" cache response directive (
Section 5.2.2.7
of
[CACHING]
), within the scope of the request in which they appear.
Therefore, new authentication schemes that choose not to carry credentials in the
Authorization
header field (e.g., using a newly defined header field) will need to explicitly disallow
caching, by mandating the use of cache response directives (e.g., "private").
Schemes using
Authentication-Info
Proxy-Authentication-Info
, or any other authentication related response header field need to consider and document
the related security considerations (see
Section 17.16.4
).
16.5.
Range Unit Extensibility
16.5.1.
Range Unit Registry
The "HTTP Range Unit Registry" defines the namespace for the range unit names and
refers to their corresponding specifications. It is maintained at
Registration of an HTTP Range Unit
MUST
include the following fields:
Name
Description
Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC8126]
Section 4.8
).
16.5.2.
Considerations for New Range Units
Other range units, such as format-specific boundaries like pages, sections, records,
rows, or time, are potentially usable in HTTP for application-specific purposes, but
are not commonly used in practice. Implementors of alternative range units ought to
consider how they would work with content codings and general-purpose intermediaries.
16.6.
Content Coding Extensibility
16.6.1.
Content Coding Registry
The "HTTP Content Coding Registry", maintained by IANA at
, registers
content-coding
names.
Content coding registrations
MUST
include the following fields:
Name
Description
Pointer to specification text
Names of content codings
MUST NOT
overlap with names of transfer codings (per the "HTTP Transfer Coding Registry" located
at
) unless the encoding transformation is identical (as is the case for the compression
codings defined in
Section 8.4.1
).
Values to be added to this namespace require IETF Review (see
Section 4.8
of
[RFC8126]
) and
MUST
conform to the purpose of content coding defined in
Section 8.4.1
16.6.2.
Considerations for New Content Codings
New content codings ought to be self-descriptive whenever possible, with optional
parameters discoverable within the coding format itself, rather than rely on external
metadata that might be lost during transit.
16.7.
Upgrade Token Registry
The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry" defines the namespace
for protocol-name tokens used to identify protocols in the
Upgrade
header field. The registry is maintained at
Each registered protocol name is associated with contact information and an optional
set of specifications that details how the connection will be processed after it has
been upgraded.
Registrations happen on a "First Come First Served" basis (see
Section 4.4
of
[RFC8126]
) and are subject to the following rules:
A protocol-name token, once registered, stays registered forever.
A protocol-name token is case-insensitive and registered with the preferred case to
be generated by senders.
The registration
MUST
name a responsible party for the registration.
The registration
MUST
name a point of contact.
The registration
MAY
name a set of specifications associated with that token. Such specifications need
not be publicly available.
The registration
SHOULD
name a set of expected "protocol-version" tokens associated with that token at the
time of registration.
The responsible party
MAY
change the registration at any time. The IANA will keep a record of all such changes,
and make them available upon request.
The IESG
MAY
reassign responsibility for a protocol token. This will normally only be used in the
case when a responsible party cannot be contacted.
17.
Security Considerations
This section is meant to inform developers, information providers, and users of known
security concerns relevant to HTTP semantics and its use for transferring information
over the Internet. Considerations related to caching are discussed in
Section 7
of
[CACHING]
, and considerations related to HTTP/1.1 message syntax and parsing are discussed
in
Section 11
of
[HTTP/1.1]
The list of considerations below is not exhaustive. Most security concerns related
to HTTP semantics are about securing server-side applications (code behind the HTTP
interface), securing user agent processing of content received via HTTP, or secure
use of the Internet in general, rather than security of the protocol. The security
considerations for URIs, which are fundamental to HTTP operation, are discussed in
Section 7
of
[URI]
. Various organizations maintain topical information and links to current research
on Web application security (e.g.,
[OWASP]
).
17.1.
Establishing Authority
HTTP relies on the notion of an
authoritative response
: a response that has been determined by (or at the direction of) the origin server
identified within the target URI to be the most appropriate response for that request
given the state of the target resource at the time of response message origination.
When a registered name is used in the authority component, the "http" URI scheme (
Section 4.2.1
) relies on the user's local name resolution service to determine where it can find
authoritative responses. This means that any attack on a user's network host table,
cached names, or name resolution libraries becomes an avenue for attack on establishing
authority for "http" URIs. Likewise, the user's choice of server for Domain Name Service
(DNS), and the hierarchy of servers from which it obtains resolution results, could
impact the authenticity of address mappings; DNS Security Extensions (DNSSEC,
[RFC4033]
) are one way to improve authenticity, as are the various mechanisms for making DNS
requests over more secure transfer protocols.
Furthermore, after an IP address is obtained, establishing authority for an "http"
URI is vulnerable to attacks on Internet Protocol routing.
The "https" scheme (
Section 4.2.2
) is intended to prevent (or at least reveal) many of these potential attacks on establishing
authority, provided that the negotiated connection is secured and the client properly
verifies that the communicating server's identity matches the target URI's authority
component (
Section 4.3.4
). Correctly implementing such verification can be difficult (see
[Georgiev]
).
Authority for a given origin server can be delegated through protocol extensions;
for example,
[ALTSVC]
. Likewise, the set of servers for which a connection is considered authoritative
can be changed with a protocol extension like
[RFC8336]
Providing a response from a non-authoritative source, such as a shared proxy cache,
is often useful to improve performance and availability, but only to the extent that
the source can be trusted or the distrusted response can be safely used.
Unfortunately, communicating authority to users can be difficult. For example,
phishing
is an attack on the user's perception of authority, where that perception can be misled
by presenting similar branding in hypertext, possibly aided by userinfo obfuscating
the authority component (see
Section 4.2.1
). User agents can reduce the impact of phishing attacks by enabling users to easily
inspect a target URI prior to making an action, by prominently distinguishing (or
rejecting) userinfo when present, and by not sending stored credentials and cookies
when the referring document is from an unknown or untrusted source.
17.2.
Risks of Intermediaries
HTTP intermediaries are inherently situated for on-path attacks. Compromise of the
systems on which the intermediaries run can result in serious security and privacy
problems. Intermediaries might have access to security-related information, personal
information about individual users and organizations, and proprietary information
belonging to users and content providers. A compromised intermediary, or an intermediary
implemented or configured without regard to security and privacy considerations, might
be used in the commission of a wide range of potential attacks.
Intermediaries that contain a shared cache are especially vulnerable to cache poisoning
attacks, as described in
Section 7
of
[CACHING]
Implementers need to consider the privacy and security implications of their design
and coding decisions, and of the configuration options they provide to operators (especially
the default configuration).
Intermediaries are no more trustworthy than the people and policies under which they
operate; HTTP cannot solve this problem.
17.3.
Attacks Based on File and Path Names
Origin servers frequently make use of their local file system to manage the mapping
from target URI to resource representations. Most file systems are not designed to
protect against malicious file or path names. Therefore, an origin server needs to
avoid accessing names that have a special significance to the system when mapping
the target resource to files, folders, or directories.
For example, UNIX, Microsoft Windows, and other operating systems use ".." as a path
component to indicate a directory level above the current one, and they use specially
named paths or file names to send data to system devices. Similar naming conventions
might exist within other types of storage systems. Likewise, local storage systems
have an annoying tendency to prefer user-friendliness over security when handling
invalid or unexpected characters, recomposition of decomposed characters, and case-normalization
of case-insensitive names.
Attacks based on such special names tend to focus on either denial-of-service (e.g.,
telling the server to read from a COM port) or disclosure of configuration and source
files that are not meant to be served.
17.4.
Attacks Based on Command, Code, or Query Injection
Origin servers often use parameters within the URI as a means of identifying system
services, selecting database entries, or choosing a data source. However, data received
in a request cannot be trusted. An attacker could construct any of the request data
elements (method, target URI, header fields, or content) to contain data that might
be misinterpreted as a command, code, or query when passed through a command invocation,
language interpreter, or database interface.
For example, SQL injection is a common attack wherein additional query language is
inserted within some part of the target URI or header fields (e.g.,
Host
Referer
, etc.). If the received data is used directly within a SELECT statement, the query
language might be interpreted as a database command instead of a simple string value.
This type of implementation vulnerability is extremely common, in spite of being easy
to prevent.
In general, resource implementations ought to avoid use of request data in contexts
that are processed or interpreted as instructions. Parameters ought to be compared
to fixed strings and acted upon as a result of that comparison, rather than passed
through an interface that is not prepared for untrusted data. Received data that isn't
based on fixed parameters ought to be carefully filtered or encoded to avoid being
misinterpreted.
Similar considerations apply to request data when it is stored and later processed,
such as within log files, monitoring tools, or when included within a data format
that allows embedded scripts.
17.5.
Attacks via Protocol Element Length
Because HTTP uses mostly textual, character-delimited fields, parsers are often vulnerable
to attacks based on sending very long (or very slow) streams of data, particularly
where an implementation is expecting a protocol element with no predefined length
Section 2.3
).
To promote interoperability, specific recommendations are made for minimum size limits
on fields (
Section 5.4
). These are minimum recommendations, chosen to be supportable even by implementations
with limited resources; it is expected that most implementations will choose substantially
higher limits.
A server can reject a message that has a target URI that is too long (
Section 15.5.15
) or request content that is too large (
Section 15.5.14
). Additional status codes related to capacity limits have been defined by extensions
to HTTP
[RFC6585]
Recipients ought to carefully limit the extent to which they process other protocol
elements, including (but not limited to) request methods, response status phrases,
field names, numeric values, and chunk lengths. Failure to limit such processing can
result in arbitrary code execution due to buffer or arithmetic overflows, and increased
vulnerability to denial-of-service attacks.
17.6.
Attacks Using Shared-Dictionary Compression
Some attacks on encrypted protocols use the differences in size created by dynamic
compression to reveal confidential information; for example,
[BREACH]
. These attacks rely on creating a redundancy between attacker-controlled content
and the confidential information, such that a dynamic compression algorithm using
the same dictionary for both content will compress more efficiently when the attacker-controlled
content matches parts of the confidential content.
HTTP messages can be compressed in a number of ways, including using TLS compression,
content codings, transfer codings, and other extension or version-specific mechanisms.
The most effective mitigation for this risk is to disable compression on sensitive
data, or to strictly separate sensitive data from attacker-controlled data so that
they cannot share the same compression dictionary. With careful design, a compression
scheme can be designed in a way that is not considered exploitable in limited use
cases, such as HPACK (
[HPACK]
).
17.7.
Disclosure of Personal Information
Clients are often privy to large amounts of personal information, including both information
provided by the user to interact with resources (e.g., the user's name, location,
mail address, passwords, encryption keys, etc.) and information about the user's browsing
activity over time (e.g., history, bookmarks, etc.). Implementations need to prevent
unintentional disclosure of personal information.
17.8.
Privacy of Server Log Information
A server is in the position to save personal data about a user's requests over time,
which might identify their reading patterns or subjects of interest. In particular,
log information gathered at an intermediary often contains a history of user agent
interaction, across a multitude of sites, that can be traced to individual users.
HTTP log information is confidential in nature; its handling is often constrained
by laws and regulations. Log information needs to be securely stored and appropriate
guidelines followed for its analysis. Anonymization of personal information within
individual entries helps, but it is generally not sufficient to prevent real log traces
from being re-identified based on correlation with other access characteristics. As
such, access traces that are keyed to a specific client are unsafe to publish even
if the key is pseudonymous.
To minimize the risk of theft or accidental publication, log information ought to
be purged of personally identifiable information, including user identifiers, IP addresses,
and user-provided query parameters, as soon as that information is no longer necessary
to support operational needs for security, auditing, or fraud control.
17.9.
Disclosure of Sensitive Information in URIs
URIs are intended to be shared, not secured, even when they identify secure resources.
URIs are often shown on displays, added to templates when a page is printed, and stored
in a variety of unprotected bookmark lists. Many servers, proxies, and user agents
log or display the target URI in places where it might be visible to third parties.
It is therefore unwise to include information within a URI that is sensitive, personally
identifiable, or a risk to disclose.
When an application uses client-side mechanisms to construct a target URI out of user-provided
information, such as the query fields of a form using GET, potentially sensitive data
might be provided that would not be appropriate for disclosure within a URI. POST
is often preferred in such cases because it usually doesn't construct a URI; instead,
POST of a form transmits the potentially sensitive data in the request content. However,
this hinders caching and uses an unsafe method for what would otherwise be a safe
request. Alternative workarounds include transforming the user-provided data prior
to constructing the URI or filtering the data to only include common values that are
not sensitive. Likewise, redirecting the result of a query to a different (server-generated)
URI can remove potentially sensitive data from later links and provide a cacheable
response for later reuse.
Since the
Referer
header field tells a target site about the context that resulted in a request, it
has the potential to reveal information about the user's immediate browsing history
and any personal information that might be found in the referring resource's URI.
Limitations on the Referer header field are described in
Section 10.1.3
to address some of its security considerations.
17.10.
Application Handling of Field Names
Servers often use non-HTTP gateway interfaces and frameworks to process a received
request and produce content for the response. For historical reasons, such interfaces
often pass received field names as external variable names, using a name mapping suitable
for environment variables.
For example, the Common Gateway Interface (CGI) mapping of protocol-specific meta-variables,
defined by
Section 4.1.18
of
[RFC3875]
, is applied to received header fields that do not correspond to one of CGI's standard
variables; the mapping consists of prepending "HTTP_" to each name and changing all
instances of hyphen ("-") to underscore ("_"). This same mapping has been inherited
by many other application frameworks in order to simplify moving applications from
one platform to the next.
In CGI, a received
Content-Length
field would be passed as the meta-variable "CONTENT_LENGTH" with a string value matching
the received field's value. In contrast, a received "Content_Length" header field
would be passed as the protocol-specific meta-variable "HTTP_CONTENT_LENGTH", which
might lead to some confusion if an application mistakenly reads the protocol-specific
meta-variable instead of the default one. (This historical practice is why
Section 16.3.2.1
discourages the creation of new field names that contain an underscore.)
Unfortunately, mapping field names to different interface names can lead to security
vulnerabilities if the mapping is incomplete or ambiguous. For example, if an attacker
were to send a field named "Transfer_Encoding", a naive interface might map that to
the same variable name as the "Transfer-Encoding" field, resulting in a potential
request smuggling vulnerability (
Section 11.2
of
[HTTP/1.1]
).
To mitigate the associated risks, implementations that perform such mappings are advised
to make the mapping unambiguous and complete for the full range of potential octets
received as a name (including those that are discouraged or forbidden by the HTTP
grammar). For example, a field with an unusual name character might result in the
request being blocked, the specific field being removed, or the name being passed
with a different prefix to distinguish it from other fields.
17.11.
Disclosure of Fragment after Redirects
Although fragment identifiers used within URI references are not sent in requests,
implementers ought to be aware that they will be visible to the user agent and any
extensions or scripts running as a result of the response. In particular, when a redirect
occurs and the original request's fragment identifier is inherited by the new reference
in
Location
Section 10.2.2
), this might have the effect of disclosing one site's fragment to another site. If
the first site uses personal information in fragments, it ought to ensure that redirects
to other sites include a (possibly empty) fragment component in order to block that
inheritance.
17.12.
Disclosure of Product Information
The
User-Agent
Section 10.1.5
),
Via
Section 7.6.3
), and
Server
Section 10.2.4
) header fields often reveal information about the respective sender's software systems.
In theory, this can make it easier for an attacker to exploit known security holes;
in practice, attackers tend to try all potential holes regardless of the apparent
software versions being used.
Proxies that serve as a portal through a network firewall ought to take special precautions
regarding the transfer of header information that might identify hosts behind the
firewall. The
Via
header field allows intermediaries to replace sensitive machine names with pseudonyms.
17.13.
Browser Fingerprinting
Browser fingerprinting is a set of techniques for identifying a specific user agent
over time through its unique set of characteristics. These characteristics might include
information related to how it uses the underlying transport protocol, feature capabilities,
and scripting environment, though of particular interest here is the set of unique
characteristics that might be communicated via HTTP. Fingerprinting is considered
a privacy concern because it enables tracking of a user agent's behavior over time
[Bujlow]
) without the corresponding controls that the user might have over other forms of
data collection (e.g., cookies). Many general-purpose user agents (i.e., Web browsers)
have taken steps to reduce their fingerprints.
There are a number of request header fields that might reveal information to servers
that is sufficiently unique to enable fingerprinting. The
From
header field is the most obvious, though it is expected that From will only be sent
when self-identification is desired by the user. Likewise, Cookie header fields are
deliberately designed to enable re-identification, so fingerprinting concerns only
apply to situations where cookies are disabled or restricted by the user agent's configuration.
The
User-Agent
header field might contain enough information to uniquely identify a specific device,
usually when combined with other characteristics, particularly if the user agent sends
excessive details about the user's system or extensions. However, the source of unique
information that is least expected by users is
proactive negotiation
Section 12.1
), including the
Accept
Accept-Charset
Accept-Encoding
, and
Accept-Language
header fields.
In addition to the fingerprinting concern, detailed use of the
Accept-Language
header field can reveal information the user might consider to be of a private nature.
For example, understanding a given language set might be strongly correlated to membership
in a particular ethnic group. An approach that limits such loss of privacy would be
for a user agent to omit the sending of Accept-Language except for sites that have
been explicitly permitted, perhaps via interaction after detecting a
Vary
header field that indicates language negotiation might be useful.
In environments where proxies are used to enhance privacy, user agents ought to be
conservative in sending proactive negotiation header fields. General-purpose user
agents that provide a high degree of header field configurability ought to inform
users about the loss of privacy that might result if too much detail is provided.
As an extreme privacy measure, proxies could filter the proactive negotiation header
fields in relayed requests.
17.14.
Validator Retention
The validators defined by this specification are not intended to ensure the validity
of a representation, guard against malicious changes, or detect on-path attacks. At
best, they enable more efficient cache updates and optimistic concurrent writes when
all participants are behaving nicely. At worst, the conditions will fail and the client
will receive a response that is no more harmful than an HTTP exchange without conditional
requests.
An entity tag can be abused in ways that create privacy risks. For example, a site
might deliberately construct a semantically invalid entity tag that is unique to the
user or user agent, send it in a cacheable response with a long freshness time, and
then read that entity tag in later conditional requests as a means of re-identifying
that user or user agent. Such an identifying tag would become a persistent identifier
for as long as the user agent retained the original cache entry. User agents that
cache representations ought to ensure that the cache is cleared or replaced whenever
the user performs privacy-maintaining actions, such as clearing stored cookies or
changing to a private browsing mode.
17.15.
Denial-of-Service Attacks Using Range
Unconstrained multiple range requests are susceptible to denial-of-service attacks
because the effort required to request many overlapping ranges of the same data is
tiny compared to the time, memory, and bandwidth consumed by attempting to serve the
requested data in many parts. Servers ought to ignore, coalesce, or reject egregious
range requests, such as requests for more than two overlapping ranges or for many
small ranges in a single set, particularly when the ranges are requested out of order
for no apparent reason. Multipart range requests are not designed to support random
access.
17.16.
Authentication Considerations
Everything about the topic of HTTP authentication is a security consideration, so
the list of considerations below is not exhaustive. Furthermore, it is limited to
security considerations regarding the authentication framework, in general, rather
than discussing all of the potential considerations for specific authentication schemes
(which ought to be documented in the specifications that define those schemes). Various
organizations maintain topical information and links to current research on Web application
security (e.g.,
[OWASP]
), including common pitfalls for implementing and using the authentication schemes
found in practice.
17.16.1.
Confidentiality of Credentials
The HTTP authentication framework does not define a single mechanism for maintaining
the confidentiality of credentials; instead, each authentication scheme defines how
the credentials are encoded prior to transmission. While this provides flexibility
for the development of future authentication schemes, it is inadequate for the protection
of existing schemes that provide no confidentiality on their own, or that do not sufficiently
protect against replay attacks. Furthermore, if the server expects credentials that
are specific to each individual user, the exchange of those credentials will have
the effect of identifying that user even if the content within credentials remains
confidential.
HTTP depends on the security properties of the underlying transport- or session-level
connection to provide confidential transmission of fields. Services that depend on
individual user authentication require a
secured
connection prior to exchanging credentials (
Section 4.2.2
).
17.16.2.
Credentials and Idle Clients
Existing HTTP clients and user agents typically retain authentication information
indefinitely. HTTP does not provide a mechanism for the origin server to direct clients
to discard these cached credentials, since the protocol has no awareness of how credentials
are obtained or managed by the user agent. The mechanisms for expiring or revoking
credentials can be specified as part of an authentication scheme definition.
Circumstances under which credential caching can interfere with the application's
security model include but are not limited to:
Clients that have been idle for an extended period, following which the server might
wish to cause the client to re-prompt the user for credentials.
Applications that include a session termination indication (such as a "logout" or
"commit" button on a page) after which the server side of the application "knows"
that there is no further reason for the client to retain the credentials.
User agents that cache credentials are encouraged to provide a readily accessible
mechanism for discarding cached credentials under user control.
17.16.3.
Protection Spaces
Authentication schemes that solely rely on the "realm" mechanism for establishing
a protection space will expose credentials to all resources on an origin server. Clients
that have successfully made authenticated requests with a resource can use the same
authentication credentials for other resources on the same origin server. This makes
it possible for a different resource to harvest authentication credentials for other
resources.
This is of particular concern when an origin server hosts resources for multiple parties
under the same origin (
Section 11.5
). Possible mitigation strategies include restricting direct access to authentication
credentials (i.e., not making the content of the
Authorization
request header field available), and separating protection spaces by using a different
host name (or port number) for each party.
17.16.4.
Additional Response Fields
Adding information to responses that are sent over an unencrypted channel can affect
security and privacy. The presence of the
Authentication-Info
and
Proxy-Authentication-Info
header fields alone indicates that HTTP authentication is in use. Additional information
could be exposed by the contents of the authentication-scheme specific parameters;
this will have to be considered in the definitions of these schemes.
18.
IANA Considerations
The change controller for the following registrations is: "IETF (iesg@ietf.org) -
Internet Engineering Task Force".
18.1.
URI Scheme Registration
IANA has updated the "Uniform Resource Identifier (URI) Schemes" registry
[BCP35]
at
with the permanent schemes listed in
Table 2
in
Section 4.2
18.2.
Method Registration
IANA has updated the "Hypertext Transfer Protocol (HTTP) Method Registry" at
with the registration procedure of
Section 16.1.1
and the method names summarized in the following table.
Table 7
Method
Safe
Idempotent
Section
CONNECT
no
no
9.3.6
DELETE
no
yes
9.3.5
GET
yes
yes
9.3.1
HEAD
yes
yes
9.3.2
OPTIONS
yes
yes
9.3.7
POST
no
no
9.3.3
PUT
no
yes
9.3.4
TRACE
yes
yes
9.3.8
no
no
18.2
The method name "*" is reserved because using "*" as a method name would conflict
with its usage as a wildcard in some fields (e.g., "Access-Control-Request-Method").
18.3.
Status Code Registration
IANA has updated the "Hypertext Transfer Protocol (HTTP) Status Code Registry" at
with the registration procedure of
Section 16.2.1
and the status code values summarized in the following table.
Table 8
Value
Description
Section
100
Continue
15.2.1
101
Switching Protocols
15.2.2
200
OK
15.3.1
201
Created
15.3.2
202
Accepted
15.3.3
203
Non-Authoritative Information
15.3.4
204
No Content
15.3.5
205
Reset Content
15.3.6
206
Partial Content
15.3.7
300
Multiple Choices
15.4.1
301
Moved Permanently
15.4.2
302
Found
15.4.3
303
See Other
15.4.4
304
Not Modified
15.4.5
305
Use Proxy
15.4.6
306
(Unused)
15.4.7
307
Temporary Redirect
15.4.8
308
Permanent Redirect
15.4.9
400
Bad Request
15.5.1
401
Unauthorized
15.5.2
402
Payment Required
15.5.3
403
Forbidden
15.5.4
404
Not Found
15.5.5
405
Method Not Allowed
15.5.6
406
Not Acceptable
15.5.7
407
Proxy Authentication Required
15.5.8
408
Request Timeout
15.5.9
409
Conflict
15.5.10
410
Gone
15.5.11
411
Length Required
15.5.12
412
Precondition Failed
15.5.13
413
Content Too Large
15.5.14
414
URI Too Long
15.5.15
415
Unsupported Media Type
15.5.16
416
Range Not Satisfiable
15.5.17
417
Expectation Failed
15.5.18
418
(Unused)
15.5.19
421
Misdirected Request
15.5.20
422
Unprocessable Content
15.5.21
426
Upgrade Required
15.5.22
500
Internal Server Error
15.6.1
501
Not Implemented
15.6.2
502
Bad Gateway
15.6.3
503
Service Unavailable
15.6.4
504
Gateway Timeout
15.6.5
505
HTTP Version Not Supported
15.6.6
18.4.
Field Name Registration
This specification updates the HTTP-related aspects of the existing registration procedures
for message header fields defined in
[RFC3864]
. It replaces the old procedures as they relate to HTTP by defining a new registration
procedure and moving HTTP field definitions into a separate registry.
IANA has created a new registry titled "Hypertext Transfer Protocol (HTTP) Field Name
Registry" as outlined in
Section 16.3.1
IANA has moved all entries in the "Permanent Message Header Field Names" and "Provisional
Message Header Field Names" registries (see
) with the protocol 'http' to this registry and has applied the following changes:
The 'Applicable Protocol' field has been omitted.
Entries that had a status of 'standard', 'experimental', 'reserved', or 'informational'
have been made to have a status of 'permanent'.
Provisional entries without a status have been made to have a status of 'provisional'.
Permanent entries without a status (after confirmation that the registration document
did not define one) have been made to have a status of 'provisional'. The expert(s)
can choose to update the entries' status if there is evidence that another is more
appropriate.
IANA has annotated the "Permanent Message Header Field Names" and "Provisional Message
Header Field Names" registries with the following note to indicate that HTTP field
name registrations have moved:
Note
HTTP field name registrations have been moved to [
] per [RFC9110].
IANA has updated the "Hypertext Transfer Protocol (HTTP) Field Name Registry" with
the field names listed in the following table.
Table 9
Field Name
Status
Section
Comments
Accept
permanent
12.5.1
Accept-Charset
deprecated
12.5.2
Accept-Encoding
permanent
12.5.3
Accept-Language
permanent
12.5.4
Accept-Ranges
permanent
14.3
Allow
permanent
10.2.1
Authentication-Info
permanent
11.6.3
Authorization
permanent
11.6.2
Connection
permanent
7.6.1
Content-Encoding
permanent
8.4
Content-Language
permanent
8.5
Content-Length
permanent
8.6
Content-Location
permanent
8.7
Content-Range
permanent
14.4
Content-Type
permanent
8.3
Date
permanent
6.6.1
ETag
permanent
8.8.3
Expect
permanent
10.1.1
From
permanent
10.1.2
Host
permanent
7.2
If-Match
permanent
13.1.1
If-Modified-Since
permanent
13.1.3
If-None-Match
permanent
13.1.2
If-Range
permanent
13.1.5
If-Unmodified-Since
permanent
13.1.4
Last-Modified
permanent
8.8.2
Location
permanent
10.2.2
Max-Forwards
permanent
7.6.2
Proxy-Authenticate
permanent
11.7.1
Proxy-Authentication-Info
permanent
11.7.3
Proxy-Authorization
permanent
11.7.2
Range
permanent
14.2
Referer
permanent
10.1.3
Retry-After
permanent
10.2.3
Server
permanent
10.2.4
TE
permanent
10.1.4
Trailer
permanent
6.6.2
Upgrade
permanent
7.8
User-Agent
permanent
10.1.5
Vary
permanent
12.5.5
Via
permanent
7.6.3
WWW-Authenticate
permanent
11.6.1
permanent
12.5.5
(reserved)
The field name "*" is reserved because using that name as an HTTP header field might
conflict with its special semantics in the
Vary
header field (
Section 12.5.5
).
IANA has updated the "Content-MD5" entry in the new registry to have a status of 'obsoleted'
with references to
Section 14.15
of
[RFC2616]
(for the definition of the header field) and
Appendix B
of
[RFC7231]
(which removed the field definition from the updated specification).
18.5.
Authentication Scheme Registration
IANA has updated the "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry"
at
with the registration procedure of
Section 16.4.1
. No authentication schemes are defined in this document.
18.6.
Content Coding Registration
IANA has updated the "HTTP Content Coding Registry" at
with the registration procedure of
Section 16.6.1
and the content coding names summarized in the table below.
Table 10
Name
Description
Section
compress
UNIX "compress" data format
[Welch]
8.4.1.1
deflate
"deflate" compressed data (
[RFC1951]
) inside the "zlib" data format (
[RFC1950]
8.4.1.2
gzip
GZIP file format
[RFC1952]
8.4.1.3
identity
Reserved
12.5.3
x-compress
Deprecated (alias for compress)
8.4.1.1
x-gzip
Deprecated (alias for gzip)
8.4.1.3
18.7.
Range Unit Registration
IANA has updated the "HTTP Range Unit Registry" at
with the registration procedure of
Section 16.5.1
and the range unit names summarized in the table below.
Table 11
Range Unit Name
Description
Section
bytes
a range of octets
14.1.2
none
reserved as keyword to indicate range requests are not supported
14.3
18.8.
Media Type Registration
IANA has updated the "Media Types" registry at
with the registration information in
Section 14.6
for the media type "multipart/byteranges".
IANA has updated the registry note about "q" parameters with a link to
Section 12.5.1
of this document.
18.9.
Port Registration
IANA has updated the "Service Name and Transport Protocol Port Number Registry" at
for the services on ports 80 and 443 that use UDP or TCP to:
use this document as "Reference", and
when currently unspecified, set "Assignee" to "IESG" and "Contact" to "IETF_Chair".
18.10.
Upgrade Token Registration
IANA has updated the "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry" at
with the registration procedure described in
Section 16.7
and the upgrade token names summarized in the following table.
Table 12
Name
Description
Expected Version Tokens
Section
HTTP
Hypertext Transfer Protocol
any DIGIT.DIGIT (e.g., "2.0")
2.5
19.
References
19.1.
Normative References
[CACHING]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “
HTTP Caching
”, November 2022.
[RFC1950]
Deutsch, P. and J-L. Gailly, “
ZLIB Compressed Data Format Specification version 3.3
”, RFC 1950,
DOI 10.17487/RFC1950
, May 1996, <
>.
[RFC1951]
Deutsch, P., “
DEFLATE Compressed Data Format Specification version 1.3
”, RFC 1951,
DOI 10.17487/RFC1951
, May 1996, <
>.
[RFC1952]
Deutsch, P., “
GZIP file format specification version 4.3
”, RFC 1952,
DOI 10.17487/RFC1952
, May 1996, <
>.
[RFC2046]
Freed, N. and N. Borenstein, “
Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types
”, RFC 2046,
DOI 10.17487/RFC2046
, November 1996, <
>.
[RFC2119]
Bradner, S., “
Key words for use in RFCs to Indicate Requirement Levels
”,
BCP 14
, RFC 2119,
DOI 10.17487/RFC2119
, March 1997, <
>.
[RFC4647]
Phillips, A., Ed. and M. Davis, Ed., “
Matching of Language Tags
”,
BCP 47
, RFC 4647,
DOI 10.17487/RFC4647
, September 2006, <
>.
[RFC4648]
Josefsson, S., “
The Base16, Base32, and Base64 Data Encodings
”, RFC 4648,
DOI 10.17487/RFC4648
, October 2006, <
>.
[RFC5234]
Crocker, D., Ed. and P. Overell, “
Augmented BNF for Syntax Specifications: ABNF
”,
STD 68
, RFC 5234,
DOI 10.17487/RFC5234
, January 2008, <
>.
[RFC5280]
Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “
Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List
(CRL) Profile
”, RFC 5280,
DOI 10.17487/RFC5280
, May 2008, <
>.
[RFC5322]
Resnick, P., Ed., “
Internet Message Format
”, RFC 5322,
DOI 10.17487/RFC5322
, October 2008, <
>.
[RFC5646]
Phillips, A., Ed. and M. Davis, Ed., “
Tags for Identifying Languages
”,
BCP 47
, RFC 5646,
DOI 10.17487/RFC5646
, September 2009, <
>.
[RFC6125]
Saint-Andre, P. and J. Hodges, “
Representation and Verification of Domain-Based Application Service Identity within
Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context
of Transport Layer Security (TLS)
”, RFC 6125,
DOI 10.17487/RFC6125
, March 2011, <
>.
[RFC6365]
Hoffman, P. and J. Klensin, “
Terminology Used in Internationalization in the IETF
”,
BCP 166
, RFC 6365,
DOI 10.17487/RFC6365
, September 2011, <
>.
[RFC7405]
Kyzivat, P., “
Case-Sensitive String Support in ABNF
”, RFC 7405,
DOI 10.17487/RFC7405
, December 2014, <
>.
[RFC8174]
Leiba, B., “
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
”,
BCP 14
, RFC 8174,
DOI 10.17487/RFC8174
, May 2017, <
>.
[TCP]
Postel, J., “
Transmission Control Protocol
”,
STD 7
, RFC 793,
DOI 10.17487/RFC0793
, September 1981, <
>.
[TLS13]
Rescorla, E., “
The Transport Layer Security (TLS) Protocol Version 1.3
”, RFC 8446,
DOI 10.17487/RFC8446
, August 2018, <
>.
[URI]
Berners-Lee, T., Fielding, R., and L. Masinter, “
Uniform Resource Identifier (URI): Generic Syntax
”,
STD 66
, RFC 3986,
DOI 10.17487/RFC3986
, January 2005, <
>.
[USASCII]
American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI X3.4, 1986.
[Welch]
Welch, T., “
A Technique for High-Performance Data Compression
”, IEEE Computer 17(6),
DOI 10.1109/MC.1984.1659158
, June 1984, <
>.
19.2.
Informative References
[ALTSVC]
Nottingham, M., McManus, P., and J. Reschke, “
HTTP Alternative Services
”, RFC 7838,
DOI 10.17487/RFC7838
, April 2016, <
>.
[BCP13]
Freed, N. and J. Klensin, “
Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures
”,
BCP 13
, RFC 4289,
DOI 10.17487/RFC4289
, December 2005.
Freed, N., Klensin, J., and T. Hansen, “
Media Type Specifications and Registration Procedures
”,
BCP 13
, RFC 6838,
DOI 10.17487/RFC6838
, January 2013.
[BCP178]
Saint-Andre, P., Crocker, D., and M. Nottingham, “
Deprecating the "X-" Prefix and Similar Constructs in Application Protocols
”,
BCP 178
, RFC 6648,
DOI 10.17487/RFC6648
, June 2012.
[BCP35]
Thaler, D., Ed., Hansen, T., and T. Hardie, “
Guidelines and Registration Procedures for URI Schemes
”,
BCP 35
, RFC 7595,
DOI 10.17487/RFC7595
, June 2015.
[BREACH]
Gluck, Y., Harris, N., and A. Prado, “
BREACH: Reviving the CRIME Attack
”, July 2013, <
>.
[Bujlow]
Bujlow, T., Carela-Español, V., Solé-Pareta, J., and P. Barlet-Ros, “A Survey on Web Tracking: Mechanisms, Implications, and Defenses”,
DOI 10.1109/JPROC.2016.2637878
, In Proceedings of the IEEE 105(8), August 2017.
[COOKIE]
Barth, A., “
HTTP State Management Mechanism
”, RFC 6265,
DOI 10.17487/RFC6265
, April 2011, <
>.
[Err1912]
RFC Errata,
Erratum ID 1912
, RFC 2978, <
>.
[Err5433]
RFC Errata,
Erratum ID 5433
, RFC 2978, <
>.
[Georgiev]
Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh, D., and V. Shmatikov, “The Most Dangerous Code in the World: Validating SSL Certificates in Non-Browser Software”,
DOI 10.1145/2382196.2382204
, In Proceedings of the 2012 ACM Conference on Computer and Communications Security
(CCS '12), pp. 38-49, October 2012.
[HPACK]
Peon, R. and H. Ruellan, “
HPACK: Header Compression for HTTP/2
”, RFC 7541,
DOI 10.17487/RFC7541
, May 2015, <
>.
[HTTP/1.0]
Berners-Lee, T., Fielding, R., and H. Frystyk, “
Hypertext Transfer Protocol -- HTTP/1.0
”, RFC 1945,
DOI 10.17487/RFC1945
, May 1996, <
>.
[HTTP/1.1]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “
HTTP/1.1
”, November 2022.
[HTTP/2]
Thomson, M., Ed. and C. Benfield, Ed., “
HTTP/2
”, RFC 9113,
DOI 10.17487/RFC9113
, June 2022, <
>.
[HTTP/3]
Bishop, M., Ed., “
HTTP/3
”, RFC 9114,
DOI 10.17487/RFC9114
, June 2022, <
>.
[ISO-8859-1]
International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1:
Latin alphabet No. 1”, ISO/IEC 8859-1:1998, 1998.
[Kri2001]
Kristol, D., “
HTTP Cookies: Standards, Privacy, and Politics
”, ACM Transactions on Internet Technology 1(2), November 2001, <
>.
[OWASP]
The Open Web Application Security Project
, <
>.
[REST]
Fielding, R., “
Architectural Styles and the Design of Network-based Software Architectures
”, Doctoral Dissertation, University of California, Irvine, September 2000, <
>.
[RFC1919]
Chatel, M., “
Classical versus Transparent IP Proxies
”, RFC 1919,
DOI 10.17487/RFC1919
, March 1996, <
>.
[RFC2047]
Moore, K., “
MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions
for Non-ASCII Text
”, RFC 2047,
DOI 10.17487/RFC2047
, November 1996, <
>.
[RFC2068]
Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T. Berners-Lee, “
Hypertext Transfer Protocol -- HTTP/1.1
”, RFC 2068,
DOI 10.17487/RFC2068
, January 1997, <
>.
[RFC2145]
Mogul, J., Fielding, R., Gettys, J., and H. Frystyk, “
Use and Interpretation of HTTP Version Numbers
”, RFC 2145,
DOI 10.17487/RFC2145
, May 1997, <
>.
[RFC2295]
Holtman, K. and A. Mutz, “
Transparent Content Negotiation in HTTP
”, RFC 2295,
DOI 10.17487/RFC2295
, March 1998, <
>.
[RFC2324]
Masinter, L., “
Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)
”, RFC 2324,
DOI 10.17487/RFC2324
, 1 April 1998, <
>.
[RFC2557]
Palme, J., Hopmann, A., and N. Shelness, “
MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)
”, RFC 2557,
DOI 10.17487/RFC2557
, March 1999, <
>.
[RFC2616]
Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “
Hypertext Transfer Protocol -- HTTP/1.1
”, RFC 2616,
DOI 10.17487/RFC2616
, June 1999, <
>.
[RFC2617]
Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “
HTTP Authentication: Basic and Digest Access Authentication
”, RFC 2617,
DOI 10.17487/RFC2617
, June 1999, <
>.
[RFC2774]
Nielsen, H., Leach, P., and S. Lawrence, “
An HTTP Extension Framework
”, RFC 2774,
DOI 10.17487/RFC2774
, February 2000, <
>.
[RFC2818]
Rescorla, E., “
HTTP Over TLS
”, RFC 2818,
DOI 10.17487/RFC2818
, May 2000, <
>.
[RFC2978]
Freed, N. and J. Postel, “
IANA Charset Registration Procedures
”,
BCP 19
, RFC 2978,
DOI 10.17487/RFC2978
, October 2000, <
>.
[RFC3040]
Cooper, I., Melve, I., and G. Tomlinson, “
Internet Web Replication and Caching Taxonomy
”, RFC 3040,
DOI 10.17487/RFC3040
, January 2001, <
>.
[RFC3864]
Klyne, G., Nottingham, M., and J. Mogul, “
Registration Procedures for Message Header Fields
”,
BCP 90
, RFC 3864,
DOI 10.17487/RFC3864
, September 2004, <
>.
[RFC3875]
Robinson, D. and K. Coar, “
The Common Gateway Interface (CGI) Version 1.1
”, RFC 3875,
DOI 10.17487/RFC3875
, October 2004, <
>.
[RFC4033]
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “
DNS Security Introduction and Requirements
”, RFC 4033,
DOI 10.17487/RFC4033
, March 2005, <
>.
[RFC4559]
Jaganathan, K., Zhu, L., and J. Brezak, “
SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows
”, RFC 4559,
DOI 10.17487/RFC4559
, June 2006, <
>.
[RFC5789]
Dusseault, L. and J. Snell, “
PATCH Method for HTTP
”, RFC 5789,
DOI 10.17487/RFC5789
, March 2010, <
>.
[RFC5905]
Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, “
Network Time Protocol Version 4: Protocol and Algorithms Specification
”, RFC 5905,
DOI 10.17487/RFC5905
, June 2010, <
>.
[RFC6454]
Barth, A., “
The Web Origin Concept
”, RFC 6454,
DOI 10.17487/RFC6454
, December 2011, <
>.
[RFC6585]
Nottingham, M. and R. Fielding, “
Additional HTTP Status Codes
”, RFC 6585,
DOI 10.17487/RFC6585
, April 2012, <
>.
[RFC7230]
Fielding, R., Ed. and J. Reschke, Ed., “
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
”, RFC 7230,
DOI 10.17487/RFC7230
, June 2014, <
>.
[RFC7231]
Fielding, R., Ed. and J. Reschke, Ed., “
Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
”, RFC 7231,
DOI 10.17487/RFC7231
, June 2014, <
>.
[RFC7232]
Fielding, R., Ed. and J. Reschke, Ed., “
Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
”, RFC 7232,
DOI 10.17487/RFC7232
, June 2014, <
>.
[RFC7233]
Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “
Hypertext Transfer Protocol (HTTP/1.1): Range Requests
”, RFC 7233,
DOI 10.17487/RFC7233
, June 2014, <
>.
[RFC7234]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “
Hypertext Transfer Protocol (HTTP/1.1): Caching
”, RFC 7234,
DOI 10.17487/RFC7234
, June 2014, <
>.
[RFC7235]
Fielding, R., Ed. and J. Reschke, Ed., “
Hypertext Transfer Protocol (HTTP/1.1): Authentication
”, RFC 7235,
DOI 10.17487/RFC7235
, June 2014, <
>.
[RFC7538]
Reschke, J., “
The Hypertext Transfer Protocol Status Code 308 (Permanent Redirect)
”, RFC 7538,
DOI 10.17487/RFC7538
, April 2015, <
>.
[RFC7540]
Belshe, M., Peon, R., and M. Thomson, Ed., “
Hypertext Transfer Protocol Version 2 (HTTP/2)
”, RFC 7540,
DOI 10.17487/RFC7540
, May 2015, <
>.
[RFC7578]
Masinter, L., “
Returning Values from Forms: multipart/form-data
”, RFC 7578,
DOI 10.17487/RFC7578
, July 2015, <
>.
[RFC7615]
Reschke, J., “
HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields
”, RFC 7615,
DOI 10.17487/RFC7615
, September 2015, <
>.
[RFC7616]
Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, “
HTTP Digest Access Authentication
”, RFC 7616,
DOI 10.17487/RFC7616
, September 2015, <
>.
[RFC7617]
Reschke, J., “
The 'Basic' HTTP Authentication Scheme
”, RFC 7617,
DOI 10.17487/RFC7617
, September 2015, <
>.
[RFC7694]
Reschke, J., “
Hypertext Transfer Protocol (HTTP) Client-Initiated Content-Encoding
”, RFC 7694,
DOI 10.17487/RFC7694
, November 2015, <
>.
[RFC8126]
Cotton, M., Leiba, B., and T. Narten, “
Guidelines for Writing an IANA Considerations Section in RFCs
”,
BCP 26
, RFC 8126,
DOI 10.17487/RFC8126
, June 2017, <
>.
[RFC8187]
Reschke, J., “
Indicating Character Encoding and Language for HTTP Header Field Parameters
”, RFC 8187,
DOI 10.17487/RFC8187
, September 2017, <
>.
[RFC8246]
McManus, P., “
HTTP Immutable Responses
”, RFC 8246,
DOI 10.17487/RFC8246
, September 2017, <
>.
[RFC8288]
Nottingham, M., “
Web Linking
”, RFC 8288,
DOI 10.17487/RFC8288
, October 2017, <
>.
[RFC8336]
Nottingham, M. and E. Nygren, “
The ORIGIN HTTP/2 Frame
”, RFC 8336,
DOI 10.17487/RFC8336
, March 2018, <
>.
[RFC8615]
Nottingham, M., “
Well-Known Uniform Resource Identifiers (URIs)
”, RFC 8615,
DOI 10.17487/RFC8615
, May 2019, <
>.
[RFC8941]
Nottingham, M. and P-H. Kamp, “
Structured Field Values for HTTP
”, RFC 8941,
DOI 10.17487/RFC8941
, February 2021, <
>.
[Sniffing]
WHATWG, “
MIME Sniffing
”, <
>.
[WEBDAV]
Dusseault, L., Ed., “
HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)
”, RFC 4918,
DOI 10.17487/RFC4918
, June 2007, <
>.
Appendix A.
Collected ABNF
In the collected ABNF below, list rules are expanded per
Section 5.6.1
Accept
= [ ( media-range [ weight ] ) *( OWS "," OWS ( media-range [
weight ] ) ) ]
Accept-Charset
= [ ( ( token / "*" ) [ weight ] ) *( OWS "," OWS ( (
token / "*" ) [ weight ] ) ) ]
Accept-Encoding
= [ ( codings [ weight ] ) *( OWS "," OWS ( codings [
weight ] ) ) ]
Accept-Language
= [ ( language-range [ weight ] ) *( OWS "," OWS (
language-range [ weight ] ) ) ]
Accept-Ranges
= acceptable-ranges
Allow
= [ method *( OWS "," OWS method ) ]
Authentication-Info
= [ auth-param *( OWS "," OWS auth-param ) ]
Authorization
= credentials
BWS
= OWS
Connection
= [ connection-option *( OWS "," OWS connection-option )
Content-Encoding
= [ content-coding *( OWS "," OWS content-coding )
Content-Language
= [ language-tag *( OWS "," OWS language-tag ) ]
Content-Length
= 1*DIGIT
Content-Location
= absolute-URI / partial-URI
Content-Range
= range-unit SP ( range-resp / unsatisfied-range )
Content-Type
= media-type
Date
= HTTP-date
ETag
= entity-tag
Expect
= [ expectation *( OWS "," OWS expectation ) ]
From
= mailbox
GMT
= %x47.4D.54 ; GMT
HTTP-date
= IMF-fixdate / obs-date
Host
= uri-host [ ":" port ]
IMF-fixdate
= day-name "," SP date1 SP time-of-day SP GMT
If-Match
= "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]
If-Modified-Since
= HTTP-date
If-None-Match
= "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]
If-Range
= entity-tag / HTTP-date
If-Unmodified-Since
= HTTP-date
Last-Modified
= HTTP-date
Location
= URI-reference
Max-Forwards
= 1*DIGIT
OWS
= *( SP / HTAB )
Proxy-Authenticate
= [ challenge *( OWS "," OWS challenge ) ]
Proxy-Authentication-Info
= [ auth-param *( OWS "," OWS auth-param )
Proxy-Authorization
= credentials
RWS
= 1*( SP / HTAB )
Range
= ranges-specifier
Referer
= absolute-URI / partial-URI
Retry-After
= HTTP-date / delay-seconds
Server
= product *( RWS ( product / comment ) )
TE
= [ t-codings *( OWS "," OWS t-codings ) ]
Trailer
= [ field-name *( OWS "," OWS field-name ) ]
URI-reference
=
Section 4.1
Upgrade
= [ protocol *( OWS "," OWS protocol ) ]
User-Agent
= product *( RWS ( product / comment ) )
Vary
= [ ( "*" / field-name ) *( OWS "," OWS ( "*" / field-name ) )
Via
= [ ( received-protocol RWS received-by [ RWS comment ] ) *( OWS
"," OWS ( received-protocol RWS received-by [ RWS comment ] ) ) ]
WWW-Authenticate
= [ challenge *( OWS "," OWS challenge ) ]
absolute-URI
=
Section 4.3
absolute-path
= 1*( "/" segment )
acceptable-ranges
= range-unit *( OWS "," OWS range-unit )
asctime-date
= day-name SP date3 SP time-of-day SP year
auth-param
= token BWS "=" BWS ( token / quoted-string )
auth-scheme
= token
authority
=
Section 3.2
challenge
= auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","
OWS auth-param ) ] ) ]
codings
= content-coding / "identity" / "*"
comment
= "(" *( ctext / quoted-pair / comment ) ")"
complete-length
= 1*DIGIT
connection-option
= token
content-coding
= token
credentials
= auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","
OWS auth-param ) ] ) ]
ctext
= HTAB / SP / %x21-27 ; '!'-'''
/ %x2A-5B ; '*'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
date1
= day SP month SP year
date2
= day "-" month "-" 2DIGIT
date3
= month SP ( 2DIGIT / ( SP DIGIT ) )
day
= 2DIGIT
day-name
= %x4D.6F.6E ; Mon
/ %x54.75.65 ; Tue
/ %x57.65.64 ; Wed
/ %x54.68.75 ; Thu
/ %x46.72.69 ; Fri
/ %x53.61.74 ; Sat
/ %x53.75.6E ; Sun
day-name-l
= %x4D.6F.6E.64.61.79 ; Monday
/ %x54.75.65.73.64.61.79 ; Tuesday
/ %x57.65.64.6E.65.73.64.61.79 ; Wednesday
/ %x54.68.75.72.73.64.61.79 ; Thursday
/ %x46.72.69.64.61.79 ; Friday
/ %x53.61.74.75.72.64.61.79 ; Saturday
/ %x53.75.6E.64.61.79 ; Sunday
delay-seconds
= 1*DIGIT
entity-tag
= [ weak ] opaque-tag
etagc
= "!" / %x23-7E ; '#'-'~'
/ obs-text
expectation
= token [ "=" ( token / quoted-string ) parameters ]
field-content
= field-vchar [ 1*( SP / HTAB / field-vchar )
field-vchar ]
field-name
= token
field-value
= *field-content
field-vchar
= VCHAR / obs-text
first-pos
= 1*DIGIT
hour
= 2DIGIT
http-URI
= "http://" authority path-abempty [ "?" query ]
https-URI
= "https://" authority path-abempty [ "?" query ]
incl-range
= first-pos "-" last-pos
int-range
= first-pos "-" [ last-pos ]
language-range
=
Section 2.1
language-tag
=
Section 2.1
last-pos
= 1*DIGIT
mailbox
=
Section 3.4
media-range
= ( "*/*" / ( type "/*" ) / ( type "/" subtype ) )
parameters
media-type
= type "/" subtype parameters
method
= token
minute
= 2DIGIT
month
= %x4A.61.6E ; Jan
/ %x46.65.62 ; Feb
/ %x4D.61.72 ; Mar
/ %x41.70.72 ; Apr
/ %x4D.61.79 ; May
/ %x4A.75.6E ; Jun
/ %x4A.75.6C ; Jul
/ %x41.75.67 ; Aug
/ %x53.65.70 ; Sep
/ %x4F.63.74 ; Oct
/ %x4E.6F.76 ; Nov
/ %x44.65.63 ; Dec
obs-date
= rfc850-date / asctime-date
obs-text
= %x80-FF
opaque-tag
= DQUOTE *etagc DQUOTE
other-range
= 1*( %x21-2B ; '!'-'+'
/ %x2D-7E ; '-'-'~'
parameter
= parameter-name "=" parameter-value
parameter-name
= token
parameter-value
= ( token / quoted-string )
parameters
= *( OWS ";" OWS [ parameter ] )
partial-URI
= relative-part [ "?" query ]
path-abempty
=
Section 3.3
port
=
Section 3.2.3
product
= token [ "/" product-version ]
product-version
= token
protocol
= protocol-name [ "/" protocol-version ]
protocol-name
= token
protocol-version
= token
pseudonym
= token
qdtext
= HTAB / SP / "!" / %x23-5B ; '#'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
query
=
Section 3.4
quoted-pair
= "\" ( HTAB / SP / VCHAR / obs-text )
quoted-string
= DQUOTE *( qdtext / quoted-pair ) DQUOTE
qvalue
= ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
range-resp
= incl-range "/" ( complete-length / "*" )
range-set
= range-spec *( OWS "," OWS range-spec )
range-spec
= int-range / suffix-range / other-range
range-unit
= token
ranges-specifier
= range-unit "=" range-set
received-by
= pseudonym [ ":" port ]
received-protocol
= [ protocol-name "/" ] protocol-version
relative-part
=
Section 4.2
rfc850-date
= day-name-l "," SP date2 SP time-of-day SP GMT
second
= 2DIGIT
segment
=
Section 3.3
subtype
= token
suffix-length
= 1*DIGIT
suffix-range
= "-" suffix-length
t-codings
= "trailers" / ( transfer-coding [ weight ] )
tchar
= "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
"^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
time-of-day
= hour ":" minute ":" second
token
= 1*tchar
token68
= 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" )
*"="
transfer-coding
= token *( OWS ";" OWS transfer-parameter )
transfer-parameter
= token BWS "=" BWS ( token / quoted-string )
type
= token
unsatisfied-range
= "*/" complete-length
uri-host
=
Section 3.2.2
weak
= %x57.2F ; W/
weight
= OWS ";" OWS "q=" qvalue
year
= 4DIGIT
Appendix B.
Changes from Previous RFCs
B.1.
Changes from RFC 2818
None.
B.2.
Changes from RFC 7230
The sections introducing HTTP's design goals, history, architecture, conformance criteria,
protocol versioning, URIs, message routing, and header fields have been moved here.
The requirement on semantic conformance has been replaced with permission to ignore
or work around implementation-specific failures. (
Section 2.2
The description of an origin and authoritative access to origin servers has been extended
for both "http" and "https" URIs to account for alternative services and secured connections
that are not necessarily based on TCP. (Sections
4.2.1
4.2.2
4.3.1
, and
7.3.3
Explicit requirements have been added to check the target URI scheme's semantics and
reject requests that don't meet any associated requirements. (
Section 7.4
Parameters in media type, media range, and expectation can be empty via one or more
trailing semicolons. (
Section 5.6.6
"Field value" now refers to the value after multiple field lines are combined with
commas — by far the most common use. To refer to a single header line's value, use
"field line value". (
Section 6.3
Trailer field semantics now transcend the specifics of chunked transfer coding. The
use of trailer fields has been further limited to allow generation as a trailer field
only when the sender knows the field defines that usage and to allow merging into
the header section only if the recipient knows the corresponding field definition
permits and defines how to merge. In all other cases, implementations are encouraged
either to store the trailer fields separately or to discard them instead of merging.
Section 6.5.1
The priority of the absolute form of the request URI over the Host header field by
origin servers has been made explicit to align with proxy handling. (
Section 7.2
The grammar definition for the Via field's "received-by" was expanded in RFC 7230
due to changes in the URI grammar for host
[URI]
that are not desirable for Via. For simplicity, we have removed uri-host from the
received-by production because it can be encompassed by the existing grammar for pseudonym.
In particular, this change removed comma from the allowed set of characters for a
host name in received-by. (
Section 7.6.3
B.3.
Changes from RFC 7231
Minimum URI lengths to be supported by implementations are now recommended. (
Section 4.1
The following have been clarified: CR and NUL in field values are to be rejected or
mapped to SP, and leading and trailing whitespace needs to be stripped from field
values before they are consumed. (
Section 5.5
Parameters in media type, media range, and expectation can be empty via one or more
trailing semicolons. (
Section 5.6.6
An abstract data type for HTTP messages has been introduced to define the components
of a message and their semantics as an abstraction across multiple HTTP versions,
rather than in terms of the specific syntax form of HTTP/1.1 in
[HTTP/1.1]
, and reflect the contents after the message is parsed. This makes it easier to distinguish
between requirements on the content (what is conveyed) versus requirements on the
messaging syntax (how it is conveyed) and avoids baking limitations of early protocol
versions into the future of HTTP. (
Section 6
The terms "payload" and "payload body" have been replaced with "content", to better
align with its usage elsewhere (e.g., in field names) and to avoid confusion with
frame payloads in HTTP/2 and HTTP/3. (
Section 6.4
The term "effective request URI" has been replaced with "target URI". (
Section 7.1
Restrictions on client retries have been loosened to reflect implementation behavior.
Section 9.2.2
The fact that request bodies on GET, HEAD, and DELETE are not interoperable has been
clarified. (Sections
9.3.1
9.3.2
, and
9.3.5
The use of the Content-Range header field (
Section 14.4
) as a request modifier on PUT is allowed. (
Section 9.3.4
A superfluous requirement about setting
Content-Length
has been removed from the description of the OPTIONS method. (
Section 9.3.7
The normative requirement to use the "message/http" media type in TRACE responses
has been removed. (
Section 9.3.8
List-based grammar for
Expect
has been restored for compatibility with RFC 2616. (
Section 10.1.1
Accept
and
Accept-Encoding
are allowed in response messages; the latter was introduced by
[RFC7694]
. (
Section 12.3
"Accept Parameters" (accept-params and accept-ext ABNF production) have been removed
from the definition of the Accept field. (
Section 12.5.1
The Accept-Charset field is now deprecated. (
Section 12.5.2
The semantics of "*" in the
Vary
header field when other values are present was clarified. (
Section 12.5.5
Range units are compared in a case-insensitive fashion. (
Section 14.1
The use of the Accept-Ranges field is not restricted to origin servers. (
Section 14.3
The process of creating a redirected request has been clarified. (
Section 15.4
Status code 308 (previously defined in
[RFC7538]
) has been added so that it's defined closer to status codes 301, 302, and 307. (
Section 15.4.9
Status code 421 (previously defined in
Section 9.1.2
of
[RFC7540]
) has been added because of its general applicability. 421 is no longer defined as
heuristically cacheable since the response is specific to the connection (not the
target resource). (
Section 15.5.20
Status code 422 (previously defined in
Section 11.2
of
[WEBDAV]
) has been added because of its general applicability. (
Section 15.5.21
B.4.
Changes from RFC 7232
Previous revisions of HTTP imposed an arbitrary 60-second limit on the determination
of whether Last-Modified was a strong validator to guard against the possibility that
the Date and Last-Modified values are generated from different clocks or at somewhat
different times during the preparation of the response. This specification has relaxed
that to allow reasonable discretion. (
Section 8.8.2.2
An edge-case requirement on If-Match and If-Unmodified-Since has been removed that
required a validator not to be sent in a 2xx response if validation fails because
the change request has already been applied. (Sections
13.1.1
and
13.1.4
The fact that If-Unmodified-Since does not apply to a resource without a concept of
modification time has been clarified. (
Section 13.1.4
Preconditions can now be evaluated before the request content is processed rather
than waiting until the response would otherwise be successful. (
Section 13.2
B.5.
Changes from RFC 7233
Refactored the range-unit and ranges-specifier grammars to simplify and reduce artificial
distinctions between bytes and other (extension) range units, removing the overlapping
grammar of other-range-unit by defining range units generically as a token and placing
extensions within the scope of a range-spec (other-range). This disambiguates the
role of list syntax (commas) in all range sets, including extension range units, for
indicating a range-set of more than one range. Moving the extension grammar into range
specifiers also allows protocol specific to byte ranges to be specified separately.
It is now possible to define Range handling on extension methods. (
Section 14.2
Described use of the
Content-Range
header field (
Section 14.4
) as a request modifier to perform a partial PUT. (
Section 14.5
B.6.
Changes from RFC 7235
None.
B.7.
Changes from RFC 7538
None.
B.8.
Changes from RFC 7615
None.
B.9.
Changes from RFC 7694
This specification includes the extension defined in
[RFC7694]
but leaves out examples and deployment considerations.
Appendix C.
Change Log
This section is to be removed before publishing as an RFC.
See
for changes up to version 19 of this document.
C.1.
Since draft-ietf-httpbis-semantics-19
This (unpublished) draft contains changes that were made after draft 19 was approved
by the IESG. Most changes are editorial only. Furthermore:
In
Section 16.3.1
, add states 'obsoleted' and 'deprecated'; in
Section 18.4
, change status 'standard' to 'permanent' (
In
Section 12.5.3
, slightly relax requirements for handling Accept-Encoding field values (
In
Section 18.4
, update IANA instructions based on received feedback (
In
Section 14.1
, clarified handling of invalid or unsatisfiable range requests (
Cite revised HTTP/2 spec where applicable (
Acknowledgements
Aside from the current editors, the following individuals deserve special recognition
for their contributions to early aspects of HTTP and its core specifications: Marc Andreessen, Tim Berners-Lee, Robert Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jim Gettys, Jean-François Groff, Phillip M. Hallam-Baker, Koen Holtman, Jeffery L. Hostetler, Shel Kaphan, Dave Kristol, Yves Lafon, Scott D. Lawrence, Paul J. Leach, Håkon W. Lie, Ari Luotonen, Larry Masinter, Rob McCool, Jeffrey C. Mogul, Lou Montulli, David Morris, Henrik Frystyk Nielsen, Dave Raggett, Eric Rescorla, Tony Sanders, Lawrence C. Stewart, Marc VanHeyningen, and Steve Zilles.
This document builds on the many contributions that went into past specifications
of HTTP, including
[HTTP/1.0]
[RFC2068]
[RFC2145]
[RFC2616]
[RFC2617]
[RFC2818]
[RFC7230]
[RFC7231]
[RFC7232]
[RFC7233]
[RFC7234]
, and
[RFC7235]
. The acknowledgements within those documents still apply.
Since 2014, the following contributors have helped improve this specification by reporting
bugs, asking smart questions, drafting or reviewing text, and evaluating issues:
Alan Egerton, Alex Rousskov, Amichai Rothman, Amos Jeffries, Anders Kaseorg, Andreas Gebhardt, Anne van Kesteren, Armin Abfalterer, Aron Duby, Asanka Herath, Asbjørn Ulsberg, Asta Olofsson, Attila Gulyas, Austin Wright, Barry Pollard, Ben Burkert, Benjamin Kaduk, Björn Höhrmann, Brad Fitzpatrick, Chris Pacejo, Colin Bendell, Cory Benfield, Cory Nelson, Daisuke Miyakawa, Dale Worley, Daniel Stenberg, Danil Suits, David Benjamin, David Matson, David Schinazi, Дилян Палаузов (Dilyan Palauzov), Eric Anderson, Eric Rescorla, Éric Vyncke, Erik Kline, Erwin Pe, Etan Kissling, Evert Pot, Evgeny Vrublevsky, Florian Best, Francesca Palombini, Igor Lubashev, James Callahan, James Peach, Jeffrey Yasskin, Kalin Gyokov, Kannan Goundan, 奥 一穂 (Kazuho Oku), Ken Murchison, Krzysztof Maczyński, Lars Eggert, Lucas Pardue, Martin Duke, Martin Dürst, Martin Thomson, Martynas Jusevičius, Matt Menke, Matthias Pigulla, Mattias Grenfeldt, Michael Osipov, Mike Bishop, Mike Pennisi, Mike Taylor, Mike West, Mohit Sethi, Murray Kucherawy, Nathaniel J. Smith, Nicholas Hurley, Nikita Prokhorov, Patrick McManus, Piotr Sikora, Poul-Henning Kamp, Rick van Rein, Robert Wilton, Roberto Polli, Roman Danyliw, Samuel Williams, Semyon Kholodnov, Simon Pieters, Simon Schüppel, Stefan Eissing, Taylor Hunt, Todd Greer, Tommy Pauly, Vasiliy Faronov, Vladimir Lashchev, Wenbo Zhu, William A. Rowe Jr., Willy Tarreau, Xingwei Liu, Yishuai Li, and Zaheduzzaman Sarker.
Index
100 Continue (status code)
15.2.1
18.3
100-continue (expect value)
10.1.1
101 Switching Protocols (status code)
15.2.2
18.3
1xx Informational (status code class)
15.2
200 OK (status code)
15.3.1
18.3
201 Created (status code)
15.3.2
18.3
202 Accepted (status code)
15.3.3
18.3
203 Non-Authoritative Information (status code)
7.7
15.3.4
18.3
204 No Content (status code)
15.3.5
18.3
205 Reset Content (status code)
15.3.6
18.3
206 Partial Content (status code)
6.4.1
8.3.3
14.2
15.3.7
18.3
2xx Successful (status code class)
15.3
300 Multiple Choices (status code)
15.4.1
15.5.7
18.3
301 Moved Permanently (status code)
15.4.2
18.3
302 Found (status code)
15.4.3
18.3
303 See Other (status code)
15.4.4
18.3
304 Not Modified (status code)
8.6
15.4.5
18.3
305 Use Proxy (status code)
15.4.6
18.3
306 (Unused) (status code)
15.4.7
18.3
307 Temporary Redirect (status code)
15.4.8
18.3
308 Permanent Redirect (status code)
15.4.9
18.3
B.3
3xx Redirection (status code class)
7.8
15.4
16.3.2
B.3
400 Bad Request (status code)
15.5.1
18.3
401 Unauthorized (status code)
15.5.2
18.3
402 Payment Required (status code)
15.5.3
18.3
403 Forbidden (status code)
11.4
15.5.4
18.3
404 Not Found (status code)
15.5.5
18.3
405 Method Not Allowed (status code)
15.5.6
18.3
406 Not Acceptable (status code)
15.5.7
18.3
407 Proxy Authentication Required (status code)
15.5.8
18.3
408 Request Timeout (status code)
15.5.9
18.3
409 Conflict (status code)
15.5.10
18.3
410 Gone (status code)
15.5.11
18.3
411 Length Required (status code)
15.5.12
18.3
412 Precondition Failed (status code)
15.5.13
18.3
413 Content Too Large (status code)
15.5.14
17.5
18.3
414 URI Too Long (status code)
15.5.15
17.5
18.3
415 Unsupported Media Type (status code)
15.5.16
18.3
416 Range Not Satisfiable (status code)
15.5.17
18.3
417 Expectation Failed (status code)
15.5.18
18.3
418 (Unused) (status code)
15.5.19
18.3
421 Misdirected Request (status code)
7.4
15.5.20
18.3
B.3
422 Unprocessable Content (status code)
15.5.21
18.3
B.3
426 Upgrade Required (status code)
15.5.22
18.3
4xx Client Error (status code class)
15.5
500 Internal Server Error (status code)
15.6.1
18.3
501 Not Implemented (status code)
15.6.2
18.3
502 Bad Gateway (status code)
15.6.3
18.3
503 Service Unavailable (status code)
15.6.4
18.3
504 Gateway Timeout (status code)
15.6.5
18.3
505 HTTP Version Not Supported (status code)
15.6.6
18.3
5xx Server Error (status code class)
15.6
accelerator
3.7
Accept header field
5.6.6
8.3.1
12.3
12.5.1
15.5.16
18.4
18.8
B.3
Accept-Charset header field
12.5.2
18.4
B.3
Accept-Encoding header field
8.4.1
8.8.3.3
12.3
12.5.3
15.5.16
18.4
18.6
C.1
Accept-Language header field
8.5.1
12.5.4
18.4
Accept-Ranges header field
14.1
14.3
18.4
18.7
B.3
Allow header field
9.1
10.2.1
18.4
ALTSVC
4.3.2
15.5.20
17.1
19.2
Authentication-Info header field
11.6.3
18.4
authoritative response
17.1
Authorization header field
11.6.2
11.6.3
12.5.5
15.5.2
18.4
BREACH
17.6
19.2
browser
3.5
Bujlow
17.13
19.2
cache
3.8
cacheable
3.8
CACHING
1.2
2.5
3.8
4.3.2
4.3.3
5.6.7
7.3.1
7.6.1
7.7
8.8.2.1
8.8.3.1
9.2.3
9.3.1
9.3.2
9.3.2
9.3.3
9.3.3
9.3.4
9.3.5
11.6.2
12.5.5
12.5.5
13
13.1.2
13.1.3
15.1
15.3.1
15.3.4
15.3.5
15.3.7
15.4.1
15.4.2
15.4.5
15.4.5
15.4.9
15.5.5
15.5.6
15.5.11
15.5.15
15.6.2
16
16.2.2
16.4.2
17
17.2
19.1
Section 3.5
11.6.2
Section 4
9.3.3
Section 4.1
12.5.5
Section 4.2
5.6.7
Section 4.2.1
9.3.3
Section 4.2.2
15.3.1
15.3.4
15.3.5
15.3.7
15.4.1
15.4.2
15.4.9
15.5.5
15.5.6
15.5.11
15.5.15
15.6.2
Section 4.3.2
13.1.2
13.1.3
Section 4.3.4
15.4.5
Section 4.3.5
9.3.2
Section 4.4
9.3.4
9.3.5
Section 5.2
7.6.1
9.3.1
9.3.2
12.5.5
Section 5.2.2.6
7.7
Section 5.2.2.7
16.4.2
Section 5.2.3
16
Section 7
17
17.2
client
3.3
clock
5.6.7
complete
6.1
compress (Coding Format)
8.4.1.1
compress (content coding)
8.4.1
conditional request
13
CONNECT method
3.3
6.4.1
7.1
8.6
9.1
9.3.6
16
16.1.2
18.2
connection
3.3
Connection header field
5.1
7.6
7.6.1
7.8
10.1.4
15.4
16.3.2
18.4
content
6.4
content coding
8.4.1
content negotiation
1.3
Content-Encoding header field
8.4
8.4.1
18.4
Content-Language header field
8.5
18.4
Content-Length header field
8.6
15.5.12
18.4
Content-Location header field
8.7
9.3.3
10.2.2
18.4
Content-MD5 header field
18.4
Content-Range header field
9.3.4
14.1
14.4
14.5
14.5
15.5.17
18.4
B.3
B.5
Content-Type header field
5.5
8.3
8.3.1
18.4
control data
6.2
4.2.2
5.3
9.3.8
11.4
19.2
Date header field
5.1
6.4.1
6.6.1
8.8.2.1
18.4
deflate (Coding Format)
8.4.1.2
deflate (content coding)
8.4.1
DELETE method
9.1
9.2.2
9.3.5
18.2
B.3
Delimiters
5.6.2
downstream
3.7
effective request URI
7.1
Err1912
8.3.2
19.2
Err5433
8.3.2
19.2
ETag field
8.8
8.8.3
18.4
Expect header field
7.8
10.1.1
15.2.1
15.5.18
18.4
B.3
field
6.3
field line
5.2
field line value
5.2
field name
5.2
field value
5.2
Fields
18.4
Accept
5.6.6
8.3.1
12.3
12.5.1
15.5.16
18.4
18.8
B.3
Accept-Charset
12.5.2
18.4
B.3
Accept-Encoding
8.4.1
8.8.3.3
12.3
12.5.3
15.5.16
18.4
18.6
C.1
Accept-Language
8.5.1
12.5.4
18.4
Accept-Ranges
14.1
14.3
18.4
18.7
B.3
Allow
9.1
10.2.1
18.4
Authentication-Info
11.6.3
18.4
Authorization
11.6.2
11.6.3
12.5.5
15.5.2
18.4
Connection
5.1
7.6
7.6.1
7.8
10.1.4
15.4
16.3.2
18.4
Content-Encoding
8.4
8.4.1
18.4
Content-Language
8.5
18.4
Content-Length
8.6
15.5.12
18.4
Content-Location
8.7
9.3.3
10.2.2
18.4
Content-MD5
18.4
Content-Range
9.3.4
14.1
14.4
14.5
14.5
15.5.17
18.4
B.3
B.5
Content-Type
5.5
8.3
8.3.1
18.4
Date
5.1
6.4.1
6.6.1
8.8.2.1
18.4
ETag
8.8
8.8.3
18.4
Expect
7.8
10.1.1
15.2.1
15.5.18
18.4
B.3
From
10.1.2
18.4
Host
4.3.3
7.1
7.2
18.4
B.2
If-Match
13.1.1
13.2.2
18.4
B.4
If-Modified-Since
13.1.3
18.4
If-None-Match
13.1.2
18.4
If-Range
13.1.1
13.1.4
13.1.5
14.2
18.4
If-Unmodified-Since
13.1.4
13.2.2
18.4
B.4
B.4
Last-Modified
8.8
8.8.2
18.4
Location
9.3.3
10.2.2
15.4
17.11
18.4
Max-Forwards
7.6.2
9.3.7
9.3.8
18.4
Proxy-Authenticate
11.7.1
15.5.8
18.4
Proxy-Authentication-Info
11.7.3
18.4
Proxy-Authorization
11.7.2
11.7.3
15.5.8
18.4
Range
9.3.1
14.1
14.2
15.5.17
16.1.2
18.4
B.5
Referer
10.1.3
17.9
18.4
Retry-After
10.2.3
15.6.4
18.4
Server
10.2.4
17.12
18.4
TE
6.5.1
7.6.1
10.1.4
18.4
Trailer
6.5.2
6.6.2
18.4
Upgrade
3.3
7.6.1
7.6.3
7.8
15.2.2
15.5.22
18.4
User-Agent
10.1.5
10.2.4
17.12
18.4
Vary
12.1
12.5.5
16.3.2
18.4
18.4
18.4
B.3
Via
6.2
7.6.3
9.3.8
17.12
18.4
B.2
WWW-Authenticate
11.6.1
11.7.1
15.5.2
18.4
Fragment Identifiers
4.2.5
From header field
10.1.2
18.4
gateway
3.7
Georgiev
17.1
19.2
GET method
3.2
3.9
6.4.1
6.4.1
8.7
9.1
9.2.1
9.3.1
13.1.1
13.1.4
14.1.2
14.1.2
14.2
18.2
B.3
Grammar
absolute-path
4.1
absolute-URI
4.1
Accept
12.5.1
Accept-Charset
12.5.2
Accept-Encoding
12.5.3
Accept-Language
12.5.4
Accept-Ranges
14.3
acceptable-ranges
14.3
Allow
10.2.1
ALPHA
2.1
asctime-date
5.6.7
auth-param
11.2
auth-scheme
11.1
Authentication-Info
11.6.3
authority
4.1
Authorization
11.6.2
BWS
5.6.3
challenge
11.3
codings
12.5.3
comment
5.6.5
complete-length
14.4
Connection
7.6.1
connection-option
7.6.1
content-coding
8.4.1
Content-Encoding
8.4
Content-Language
8.5
Content-Length
8.6
Content-Location
8.7
Content-Range
14.4
Content-Type
8.3
CR
2.1
credentials
11.4
CRLF
2.1
ctext
5.6.5
CTL
2.1
Date
6.6.1
date1
5.6.7
day
5.6.7
day-name
5.6.7
day-name-l
5.6.7
delay-seconds
10.2.3
DIGIT
2.1
DQUOTE
2.1
entity-tag
8.8.3
ETag
8.8.3
etagc
8.8.3
Expect
10.1.1
field-content
5.5
field-name
5.1
6.6.2
field-value
5.5
field-vchar
5.5
first-pos
14.1.1
14.4
From
10.1.2
GMT
5.6.7
HEXDIG
2.1
Host
7.2
hour
5.6.7
HTAB
2.1
HTTP-date
5.6.7
http-URI
4.2.1
https-URI
4.2.2
If-Match
13.1.1
If-Modified-Since
13.1.3
If-None-Match
13.1.2
If-Range
13.1.5
If-Unmodified-Since
13.1.4
IMF-fixdate
5.6.7
incl-range
14.4
int-range
14.1.1
language-range
12.5.4
language-tag
8.5.1
Last-Modified
8.8.2
last-pos
14.1.1
14.4
LF
2.1
Location
10.2.2
Max-Forwards
7.6.2
media-range
12.5.1
media-type
8.3.1
method
9.1
minute
5.6.7
month
5.6.7
obs-date
5.6.7
obs-text
5.5
OCTET
2.1
opaque-tag
8.8.3
other-range
14.1.1
OWS
5.6.3
parameter
5.6.6
parameter-name
5.6.6
parameter-value
5.6.6
parameters
5.6.6
partial-URI
4.1
port
4.1
product
10.1.5
product-version
10.1.5
protocol-name
7.6.3
protocol-version
7.6.3
Proxy-Authenticate
11.7.1
Proxy-Authentication-Info
11.7.3
Proxy-Authorization
11.7.2
pseudonym
7.6.3
qdtext
5.6.4
query
4.1
quoted-pair
5.6.4
quoted-string
5.6.4
qvalue
12.4.2
Range
14.2
range-resp
14.4
range-set
14.1.1
range-spec
14.1.1
range-unit
14.1
ranges-specifier
14.1.1
received-by
7.6.3
received-protocol
7.6.3
Referer
10.1.3
Retry-After
10.2.3
rfc850-date
5.6.7
RWS
5.6.3
second
5.6.7
segment
4.1
Server
10.2.4
SP
2.1
subtype
8.3.1
suffix-length
14.1.1
suffix-range
14.1.1
t-codings
10.1.4
tchar
5.6.2
TE
10.1.4
time-of-day
5.6.7
token
5.6.2
token68
11.2
Trailer
6.6.2
transfer-coding
10.1.4
transfer-parameter
10.1.4
type
8.3.1
unsatisfied-range
14.4
Upgrade
7.8
uri-host
4.1
URI-reference
4.1
User-Agent
10.1.5
Vary
12.5.5
VCHAR
2.1
Via
7.6.3
weak
8.8.3
weight
12.4.2
WWW-Authenticate
11.6.1
year
5.6.7
gzip (Coding Format)
8.4.1.3
gzip (content coding)
8.4.1
HEAD method
6.4.1
8.6
8.7
9.1
9.2.1
9.3.2
18.2
B.3
Header Fields
Accept
5.6.6
8.3.1
12.3
12.5.1
15.5.16
18.4
18.8
B.3
Accept-Charset
12.5.2
18.4
B.3
Accept-Encoding
8.4.1
8.8.3.3
12.3
12.5.3
15.5.16
18.4
18.6
C.1
Accept-Language
8.5.1
12.5.4
18.4
Accept-Ranges
14.1
14.3
18.4
18.7
B.3
Allow
9.1
10.2.1
18.4
Authentication-Info
11.6.3
18.4
Authorization
11.6.2
11.6.3
12.5.5
15.5.2
18.4
Connection
5.1
7.6
7.6.1
7.8
10.1.4
15.4
16.3.2
18.4
Content-Encoding
8.4
8.4.1
18.4
Content-Language
8.5
18.4
Content-Length
8.6
15.5.12
18.4
Content-Location
8.7
9.3.3
10.2.2
18.4
Content-MD5
18.4
Content-Range
9.3.4
14.1
14.4
14.5
14.5
15.5.17
18.4
B.3
B.5
Content-Type
5.5
8.3
8.3.1
18.4
Date
5.1
6.4.1
6.6.1
8.8.2.1
18.4
ETag
8.8
8.8.3
18.4
Expect
7.8
10.1.1
15.2.1
15.5.18
18.4
B.3
From
10.1.2
18.4
Host
4.3.3
7.1
7.2
18.4
B.2
If-Match
13.1.1
13.2.2
18.4
B.4
If-Modified-Since
13.1.3
18.4
If-None-Match
13.1.2
18.4
If-Range
13.1.1
13.1.4
13.1.5
14.2
18.4
If-Unmodified-Since
13.1.4
13.2.2
18.4
B.4
B.4
Last-Modified
8.8
8.8.2
18.4
Location
9.3.3
10.2.2
15.4
17.11
18.4
Max-Forwards
7.6.2
9.3.7
9.3.8
18.4
Proxy-Authenticate
11.7.1
15.5.8
18.4
Proxy-Authentication-Info
11.7.3
18.4
Proxy-Authorization
11.7.2
11.7.3
15.5.8
18.4
Range
9.3.1
14.1
14.2
15.5.17
16.1.2
18.4
B.5
Referer
10.1.3
17.9
18.4
Retry-After
10.2.3
15.6.4
18.4
Server
10.2.4
17.12
18.4
TE
6.5.1
7.6.1
10.1.4
18.4
Trailer
6.5.2
6.6.2
18.4
Upgrade
3.3
7.6.1
7.6.3
7.8
15.2.2
15.5.22
18.4
User-Agent
10.1.5
10.2.4
17.12
18.4
Vary
12.1
12.5.5
16.3.2
18.4
18.4
18.4
B.3
Via
6.2
7.6.3
9.3.8
17.12
18.4
B.2
WWW-Authenticate
11.6.1
11.7.1
15.5.2
18.4
header section
6.3
Host header field
4.3.3
7.1
7.2
18.4
B.2
HPACK
17.6
19.2
http URI scheme
4.2.1
HTTP/1.0
1.2
15.4
19.2
"Acknowledgements"
Section 9.3
15.4
HTTP/1.1
1.2
1.4
2.5
5.4
6.2
6.4
6.5.1
7.1
7.5
7.6.1
7.6.1
7.7
7.7
7.7
8.4
8.6
8.8.3.3
9.3.1
9.3.2
9.3.5
9.3.8
10.1.1
10.1.4
16
17
17.10
19.2
B.3
Section 3.2.1
7.7
Section 3.2.4
7.7
Section 6
6.4
Section 6.1
7.6.1
8.4
16
Section 6.2
8.6
Section 7
7.7
8.8.3.3
10.1.4
Section 7.1.2
6.5.1
Section 9.5
7.5
Section 9.6
10.1.1
Section 10.1
9.3.8
Section 11
17
Section 11.2
5.4
9.3.1
9.3.2
9.3.5
17.10
Appendix C.2.2
7.6.1
HTTP/2
1.2
6.2
7.2
7.5
16
19.2
HTTP/3
1.2
6.2
7.2
19.2
https URI scheme
4.2.2
idempotent
9.2.2
If-Match header field
13.1.1
13.2.2
18.4
B.4
If-Modified-Since header field
13.1.3
18.4
If-None-Match header field
13.1.2
18.4
If-Range header field
13.1.1
13.1.4
13.1.5
14.2
18.4
If-Unmodified-Since header field
13.1.4
13.2.2
18.4
B.4
B.4
inbound
3.7
incomplete
6.1
interception proxy
3.7
intermediary
3.7
ISO-8859-1
5.5
19.2
Kri2001
5.3
19.2
Last-Modified header field
8.8
8.8.2
18.4
list-based field
5.5
Location header field
9.3.3
10.2.2
15.4
17.11
18.4
Max-Forwards header field
7.6.2
9.3.7
9.3.8
18.4
Media Type
multipart/byteranges
14.6
multipart/x-byteranges
14.6
message
3.4
message abstraction
messages
3.4
metadata
8.8
Method
18.2
CONNECT
3.3
6.4.1
7.1
8.6
9.1
9.3.6
16
16.1.2
18.2
DELETE
9.1
9.2.2
9.3.5
18.2
B.3
GET
3.2
3.9
6.4.1
6.4.1
8.7
9.1
9.2.1
9.3.1
13.1.1
13.1.4
14.1.2
14.1.2
14.2
18.2
B.3
HEAD
6.4.1
8.6
8.7
9.1
9.2.1
9.3.2
18.2
B.3
OPTIONS
7.1
7.6.2
9.1
9.2.1
9.3.7
16.1.2
18.2
B.3
POST
6.4.1
8.7
9.1
9.3.1
9.3.3
18.2
PUT
6.4.1
8.7
9.1
9.2.2
9.3.4
14.5
15.3.1
15.3.2
16.3.2
18.2
B.3
TRACE
7.6.2
9.1
9.2.1
9.3.8
18.2
B.3
multipart/byteranges Media Type
14.6
multipart/x-byteranges Media Type
14.6
non-transforming proxy
7.7
OPTIONS method
7.1
7.6.2
9.1
9.2.1
9.3.7
16.1.2
18.2
B.3
origin
4.3.1
11.5
origin server
3.6
outbound
3.7
OWASP
17
17.16
19.2
phishing
17.1
POST method
6.4.1
8.7
9.1
9.3.1
9.3.3
18.2
Protection Space
11.5
proxy
3.7
Proxy-Authenticate header field
11.7.1
15.5.8
18.4
Proxy-Authentication-Info header field
11.7.3
18.4
Proxy-Authorization header field
11.7.2
11.7.3
15.5.8
18.4
PUT method
6.4.1
8.7
9.1
9.2.2
9.3.4
14.5
15.3.1
15.3.2
16.3.2
18.2
B.3
Range header field
9.3.1
14.1
14.2
15.5.17
16.1.2
18.4
B.5
Realm
11.5
recipient
3.4
Referer header field
10.1.3
17.9
18.4
representation
3.2
request
3.4
request target
7.1
resource
3.1
response
3.4
REST
3.2
9.1
19.2
Retry-After header field
10.2.3
15.6.4
18.4
reverse proxy
3.7
RFC1919
3.7
19.2
RFC1950
8.4.1.2
18.6
19.1
RFC1951
8.4.1.2
18.6
19.1
RFC1952
8.4.1.3
18.6
19.1
RFC2046
8.3
8.3.1
8.3.1
8.3.3
14.6
14.6
19.1
Section 4.1.2
8.3.1
Section 4.5.1
8.3
Section 5.1
14.6
Section 5.1.1
8.3.3
RFC2047
5.5
19.2
RFC2068
1.2
7.6.1
15.4
19.2
"Acknowledgements"
Section 10.3
15.4
Section 19.7.1
7.6.1
RFC2119
2.2
19.1
RFC2145
19.2
"Acknowledgements"
RFC2295
12
19.2
RFC2324
15.5.19
19.2
RFC2557
8.7
19.2
Section 4
8.7
RFC2616
1.2
8.8.3
12.5.4
18.4
19.2
"Acknowledgements"
Section 3.11
8.8.3
Section 14.4
12.5.4
Section 14.15
18.4
RFC2617
19.2
"Acknowledgements"
RFC2774
16.1.2
19.2
RFC2818
1.4
19.2
"Acknowledgements"
RFC2978
8.3.2
8.3.2
19.2
Section 2
8.3.2
Section 2.3
8.3.2
RFC3040
3.7
19.2
RFC3864
18.4
19.2
RFC3875
17.10
19.2
Section 4.1.18
17.10
RFC4033
17.1
19.2
RFC4289
RFC4559
3.3
19.2
RFC4647
12.5.4
12.5.4
12.5.4
12.5.4
19.1
Section 2.1
12.5.4
Section 2.3
12.5.4
Section 3
12.5.4
Section 3.3.1
12.5.4
RFC4648
11.2
19.1
RFC5234
2.1
2.1
5.5
5.6.1
19.1
Appendix B.1
2.1
RFC5280
4.3.5
19.1
Section 4.2.1.6
4.3.5
RFC5322
5.6.7
5.6.7
5.6.7
6.6.1
7.6.3
10.1.2
10.1.2
19.1
Section 3.3
5.6.7
5.6.7
Section 3.4
10.1.2
10.1.2
Section 3.6.1
6.6.1
Section 3.6.7
7.6.3
RFC5646
8.5.1
8.5.1
8.5.1
19.1
Section 2.1
8.5.1
RFC5789
12.3
14.5
19.2
Section 3.1
12.3
RFC5905
5.6.7
19.2
RFC6125
4.3.4
4.3.4
19.1
Section 6
4.3.4
Section 6.2.1
4.3.4
RFC6365
2.1
8.3.2
19.1
RFC6454
4.3.1
19.2
RFC6585
17.5
19.2
RFC6648
RFC6838
RFC7230
1.2
1.4
1.4
1.4
19.2
"Acknowledgements"
RFC7231
1.4
15.4.6
18.4
19.2
"Acknowledgements"
Appendix B
15.4.6
18.4
RFC7232
1.4
19.2
"Acknowledgements"
RFC7233
1.4
19.2
"Acknowledgements"
RFC7234
19.2
"Acknowledgements"
RFC7235
1.2
1.4
19.2
"Acknowledgements"
RFC7405
2.1
19.1
RFC7538
1.4
15.4
15.4.9
19.2
B.3
Section 4
15.4.9
RFC7540
19.2
B.3
RFC7578
8.3.3
19.2
RFC7595
RFC7615
1.4
19.2
RFC7616
11.1
11.6.3
19.2
Section 3.5
11.6.3
RFC7617
11.1
19.2
RFC7694
1.4
19.2
B.3
B.9
RFC8126
16.1.1
16.2.1
16.3.1
16.4.1
16.5.1
16.6.1
16.7
19.2
Section 4.4
16.7
Section 4.6
16.3.1
Section 4.8
16.1.1
16.2.1
16.4.1
16.5.1
16.6.1
RFC8174
2.2
19.1
RFC8187
5.5
19.2
RFC8246
13.2.1
19.2
RFC8288
15.4.1
19.2
RFC8336
4.3.3
17.1
19.2
RFC8615
16.3
19.2
RFC8941
16.3.2.2
16.3.2.2
19.2
safe
9.2.1
satisfiable range
14.1.1
secured
4.2.2
selected representation
3.2
8.8
13.1
self-descriptive
sender
3.4
server
3.3
Server header field
10.2.4
17.12
18.4
singleton field
5.5
Sniffing
8.3
19.2
spider
3.5
Status Code
15
Status Codes
Final
15
Informational
15
Interim
15
Status Codes Classes
1xx Informational
15.2
2xx Successful
15.3
3xx Redirection
7.8
15.4
16.3.2
B.3
4xx Client Error
15.5
5xx Server Error
15.6
target resource
7.1
target URI
7.1
TCP
4.2.1
19.1
TE header field
6.5.1
7.6.1
10.1.4
18.4
TLS13
3.7
4.2.2
9.3.6
19.1
TRACE method
7.6.2
9.1
9.2.1
9.3.8
18.2
B.3
Trailer Fields
6.5
ETag
8.8
8.8.3
18.4
Trailer header field
6.5.2
6.6.2
18.4
trailer section
6.5
trailers
6.5
transforming proxy
7.7
transparent proxy
3.7
tunnel
3.7
unsatisfiable range
14.1.1
Upgrade header field
3.3
7.6.1
7.6.3
7.8
15.2.2
15.5.22
18.4
upstream
3.7
URI
origin
4.3.1
URI
3.1
4.1
4.1
4.1
4.1
4.1
4.1
4.1
4.1
4.1
4.2.1
4.2.1
4.2.2
4.2.2
4.2.3
4.2.3
4.2.3
4.2.3
4.2.3
4.2.3
4.2.4
4.2.5
4.2.5
4.3.5
7.1
8.7
9.3.1
10.1.3
10.1.3
10.2.2
10.2.2
11.2
17
19.1
B.2
Section 2.1
4.2.3
Section 2.2
4.2.3
Section 3.2
4.1
Section 3.2.1
4.2.4
Section 3.2.2
4.1
4.2.1
4.2.2
4.3.5
Section 3.2.3
4.1
4.2.1
4.2.2
Section 3.3
4.1
4.1
Section 3.4
4.1
Section 3.5
4.2.5
7.1
Section 4.1
4.1
Section 4.2
4.1
10.2.2
Section 4.3
4.1
4.2.5
Section 5
8.7
10.1.3
10.2.2
Section 6
4.2.3
Section 6.2
4.2.3
Section 6.2.3
4.2.3
Section 7
17
URI reference
4.1
URI scheme
http
4.2.1
https
4.2.2
USASCII
5.5
19.1
user agent
3.5
User-Agent header field
10.1.5
10.2.4
17.12
18.4
validator
8.8
strong
8.8.1
weak
8.8.1
Vary header field
12.1
12.5.5
16.3.2
18.4
18.4
18.4
B.3
Via header field
6.2
7.6.3
9.3.8
17.12
18.4
B.2
WEBDAV
8.8
13.1
19.2
B.3
Section 10.4
13.1
Section 11.2
B.3
Welch
8.4.1.1
18.6
19.1
WWW-Authenticate header field
11.6.1
11.7.1
15.5.2
18.4
x-compress (content coding)
8.4.1
x-gzip (content coding)
8.4.1
Authors' Addresses
Roy T. Fielding
(editor)
Adobe
345 Park Ave
San Jose, CA 95110
United States of America
Email: fielding@gbiv.com
URI:
Mark Nottingham
(editor)
Fastly
Prahran
Australia
Email: mnot@mnot.net
URI:
Julian Reschke
(editor)
greenbytes GmbH
Hafenweg 16
48155 Münster
Germany
Email: julian.reschke@greenbytes.de
URI: