RFC 9110 - HTTP Semantics
RFC 9110
HTTP Semantics
June 2022
Fielding, et al.
Standards Track
[Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
9110
STD:
97
Obsoletes:
2818
7230
7231
7232
7233
7235
7538
7615
7694
Updates:
3864
Category:
Standards Track
Published:
June 2022
ISSN:
2070-1721
Authors:
R. Fielding,
Ed.
Adobe
M. Nottingham,
Ed.
Fastly
J. Reschke,
Ed.
greenbytes
RFC 9110
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Revised BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Revised BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s)
controlling the copyright in such materials, this document may not
be modified outside the IETF Standards Process, and derivative
works of it may not be created outside the IETF Standards Process,
except to format it for publication as an RFC or to translate it
into languages other than English.
Table of Contents
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
A "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 ======================================= O
< response
Figure 1
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 =========== A =========== B =========== C =========== O
< < < <
Figure 2
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".
A "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
A "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.
A "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
A "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 =========== B - - - - - - C - - - - - - O
< <
Figure 3
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 =
absolute-URI =
relative-part =
authority =
uri-host =
port =
path-abempty =
segment =
query =
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
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 (
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 "
at least
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.
A "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 = 2DIGIT
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 = 4DIGIT
GMT = %s"GMT"
time-of-day = hour ":" minute ":" second
; 00:00:00 - 23:59:60 (leap second)
hour = 2DIGIT
minute = 2DIGIT
second = 2DIGIT
Obsolete formats:
obs-date = rfc850-date / asctime-date
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
date2 = day "-" month "-" 2DIGIT
; 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 ( 2DIGIT / ( SP 1DIGIT ))
; 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.
A "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
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
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 =
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.
A "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
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
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
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
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.
A "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 =
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 a
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
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
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.
A "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
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*3DIGIT ] )
/ ( "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 a
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 =
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 a
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
),
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 a
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
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
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"
STD 98
RFC 9111
DOI 10.17487/RFC9111
June 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
December 2005
Freed, N.
Klensin, J.
, and
T. Hansen
"Media Type Specifications and Registration Procedures"
BCP 13
RFC 6838
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
June 2012
[BCP35]
Thaler, D., Ed.
Hansen, T.
, and
T. Hardie
"Guidelines and Registration Procedures for URI Schemes"
BCP 35
RFC 7595
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"
In Proceedings of the IEEE 105(8)
DOI 10.1109/JPROC.2016.2637878
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"
In Proceedings of the 2012 ACM Conference on Computer and Communications Security (CCS '12), pp. 38-49
DOI 10.1145/2382196.2382204
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"
STD 99
RFC 9112
DOI 10.17487/RFC9112
June 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.T.
"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. C.
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 =
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 =
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 =
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 =
language-tag =
last-pos = 1*DIGIT
mailbox =
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 =
port =
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 =
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 =
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
second = 2DIGIT
segment =
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 =
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.
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)
Section 15.2.1
100-continue (expect value)
Section 10.1.1
101 Switching Protocols (status code)
Section 15.2.2
1xx Informational (status code class)
Section 15.2
200 OK (status code)
Section 15.3.1
201 Created (status code)
Section 15.3.2
202 Accepted (status code)
Section 15.3.3
203 Non-Authoritative Information (status code)
Section 15.3.4
204 No Content (status code)
Section 15.3.5
205 Reset Content (status code)
Section 15.3.6
206 Partial Content (status code)
Section 15.3.7
2xx Successful (status code class)
Section 15.3
300 Multiple Choices (status code)
Section 15.4.1
301 Moved Permanently (status code)
Section 15.4.2
302 Found (status code)
Section 15.4.3
303 See Other (status code)
Section 15.4.4
304 Not Modified (status code)
Section 15.4.5
305 Use Proxy (status code)
Section 15.4.6
306 (Unused) (status code)
Section 15.4.7
307 Temporary Redirect (status code)
Section 15.4.8
308 Permanent Redirect (status code)
Section 15.4.9
3xx Redirection (status code class)
Section 15.4
400 Bad Request (status code)
Section 15.5.1
401 Unauthorized (status code)
Section 15.5.2
402 Payment Required (status code)
Section 15.5.3
403 Forbidden (status code)
Section 15.5.4
404 Not Found (status code)
Section 15.5.5
405 Method Not Allowed (status code)
Section 15.5.6
406 Not Acceptable (status code)
Section 15.5.7
407 Proxy Authentication Required (status code)
Section 15.5.8
408 Request Timeout (status code)
Section 15.5.9
409 Conflict (status code)
Section 15.5.10
410 Gone (status code)
Section 15.5.11
411 Length Required (status code)
Section 15.5.12
412 Precondition Failed (status code)
Section 15.5.13
413 Content Too Large (status code)
Section 15.5.14
414 URI Too Long (status code)
Section 15.5.15
415 Unsupported Media Type (status code)
Section 15.5.16
416 Range Not Satisfiable (status code)
Section 15.5.17
417 Expectation Failed (status code)
Section 15.5.18
418 (Unused) (status code)
Section 15.5.19
421 Misdirected Request (status code)
Section 15.5.20
422 Unprocessable Content (status code)
Section 15.5.21
426 Upgrade Required (status code)
Section 15.5.22
4xx Client Error (status code class)
Section 15.5
500 Internal Server Error (status code)
Section 15.6.1
501 Not Implemented (status code)
Section 15.6.2
502 Bad Gateway (status code)
Section 15.6.3
503 Service Unavailable (status code)
Section 15.6.4
504 Gateway Timeout (status code)
Section 15.6.5
505 HTTP Version Not Supported (status code)
Section 15.6.6
5xx Server Error (status code class)
Section 15.6
accelerator
Section 3.7, Paragraph 6
Accept header field
Section 12.5.1
Accept-Charset header field
Section 12.5.2
Accept-Encoding header field
Section 12.5.3
Accept-Language header field
Section 12.5.4
Accept-Ranges header field
Section 14.3
Allow header field
Section 10.2.1
Authentication-Info header field
Section 11.6.3
authoritative response
Section 17.1
Authorization header field
Section 11.6.2
browser
Section 3.5
cache
Section 3.8
cacheable
Section 3.8, Paragraph 4
client
Section 3.3
clock
Section 5.6.7
complete
Section 6.1
compress (Coding Format)
Section 8.4.1.1
compress (content coding)
Section 8.4.1
conditional request
Section 13
CONNECT method
Section 9.3.6
connection
Section 3.3
Connection header field
Section 7.6.1
content
Section 6.4
content coding
Section 8.4.1
content negotiation
Section 1.3, Paragraph 4
Content-Encoding header field
Section 8.4
Content-Language header field
Section 8.5
Content-Length header field
Section 8.6
Content-Location header field
Section 8.7
Content-MD5 header field
Section 18.4, Paragraph 10
Content-Range header field
Section 14.4
Section 14.5
Content-Type header field
Section 8.3
control data
Section 6.2
Date header field
Section 6.6.1
deflate (Coding Format)
Section 8.4.1.2
deflate (content coding)
Section 8.4.1
DELETE method
Section 9.3.5
Delimiters
Section 5.6.2, Paragraph 3
downstream
Section 3.7, Paragraph 4
effective request URI
Section 7.1, Paragraph 8.1
ETag field
Section 8.8.3
Expect header field
Section 10.1.1
field
Section 5
Section 6.3
field line
Section 5.2, Paragraph 1
field line value
Section 5.2, Paragraph 1
field name
Section 5.2, Paragraph 1
field value
Section 5.2, Paragraph 2
Fields
Section 18.4, Paragraph 9
Accept
Section 12.5.1
Accept-Charset
Section 12.5.2
Accept-Encoding
Section 12.5.3
Accept-Language
Section 12.5.4
Accept-Ranges
Section 14.3
Allow
Section 10.2.1
Authentication-Info
Section 11.6.3
Authorization
Section 11.6.2
Connection
Section 7.6.1
Content-Encoding
Section 8.4
Content-Language
Section 8.5
Content-Length
Section 8.6
Content-Location
Section 8.7
Content-MD5
Section 18.4, Paragraph 10
Content-Range
Section 14.4
Section 14.5
Content-Type
Section 8.3
Date
Section 6.6.1
ETag
Section 8.8.3
Expect
Section 10.1.1
From
Section 10.1.2
Host
Section 7.2
If-Match
Section 13.1.1
If-Modified-Since
Section 13.1.3
If-None-Match
Section 13.1.2
If-Range
Section 13.1.5
If-Unmodified-Since
Section 13.1.4
Last-Modified
Section 8.8.2
Location
Section 10.2.2
Max-Forwards
Section 7.6.2
Proxy-Authenticate
Section 11.7.1
Proxy-Authentication-Info
Section 11.7.3
Proxy-Authorization
Section 11.7.2
Range
Section 14.2
Referer
Section 10.1.3
Retry-After
Section 10.2.3
Server
Section 10.2.4
TE
Section 10.1.4
Trailer
Section 6.6.2
Upgrade
Section 7.8
User-Agent
Section 10.1.5
Vary
Section 12.5.5
Via
Section 7.6.3
WWW-Authenticate
Section 11.6.1
Fragment Identifiers
Section 4.2.5
From header field
Section 10.1.2
gateway
Section 3.7, Paragraph 6
GET method
Section 9.3.1
Grammar
ALPHA
Section 2.1
Accept
Section 12.5.1
Accept-Charset
Section 12.5.2
Accept-Encoding
Section 12.5.3
Accept-Language
Section 12.5.4
Accept-Ranges
Section 14.3
Allow
Section 10.2.1
Authentication-Info
Section 11.6.3
Authorization
Section 11.6.2
BWS
Section 5.6.3
CR
Section 2.1
CRLF
Section 2.1
CTL
Section 2.1
Connection
Section 7.6.1
Content-Encoding
Section 8.4
Content-Language
Section 8.5
Content-Length
Section 8.6
Content-Location
Section 8.7
Content-Range
Section 14.4
Content-Type
Section 8.3
DIGIT
Section 2.1
DQUOTE
Section 2.1
Date
Section 6.6.1
ETag
Section 8.8.3
Expect
Section 10.1.1
From
Section 10.1.2
GMT
Section 5.6.7
HEXDIG
Section 2.1
HTAB
Section 2.1
HTTP-date
Section 5.6.7
Host
Section 7.2
IMF-fixdate
Section 5.6.7
If-Match
Section 13.1.1
If-Modified-Since
Section 13.1.3
If-None-Match
Section 13.1.2
If-Range
Section 13.1.5
If-Unmodified-Since
Section 13.1.4
LF
Section 2.1
Last-Modified
Section 8.8.2
Location
Section 10.2.2
Max-Forwards
Section 7.6.2
OCTET
Section 2.1
OWS
Section 5.6.3
Proxy-Authenticate
Section 11.7.1
Proxy-Authentication-Info
Section 11.7.3
Proxy-Authorization
Section 11.7.2
RWS
Section 5.6.3
Range
Section 14.2
Referer
Section 10.1.3
Retry-After
Section 10.2.3
SP
Section 2.1
Server
Section 10.2.4
TE
Section 10.1.4
Trailer
Section 6.6.2
URI-reference
Section 4.1
Upgrade
Section 7.8
User-Agent
Section 10.1.5
VCHAR
Section 2.1
Vary
Section 12.5.5
Via
Section 7.6.3
WWW-Authenticate
Section 11.6.1
absolute-URI
Section 4.1
absolute-path
Section 4.1
acceptable-ranges
Section 14.3
asctime-date
Section 5.6.7
auth-param
Section 11.2
auth-scheme
Section 11.1
authority
Section 4.1
challenge
Section 11.3
codings
Section 12.5.3
comment
Section 5.6.5
complete-length
Section 14.4
connection-option
Section 7.6.1
content-coding
Section 8.4.1
credentials
Section 11.4
ctext
Section 5.6.5
date1
Section 5.6.7
day
Section 5.6.7
day-name
Section 5.6.7
day-name-l
Section 5.6.7
delay-seconds
Section 10.2.3
entity-tag
Section 8.8.3
etagc
Section 8.8.3
field-content
Section 5.5
field-name
Section 5.1
Section 6.6.2
field-value
Section 5.5
field-vchar
Section 5.5
first-pos
Section 14.1.1
Section 14.4
hour
Section 5.6.7
http-URI
Section 4.2.1
https-URI
Section 4.2.2
incl-range
Section 14.4
int-range
Section 14.1.1
language-range
Section 12.5.4
language-tag
Section 8.5.1
last-pos
Section 14.1.1
Section 14.4
media-range
Section 12.5.1
media-type
Section 8.3.1
method
Section 9.1
minute
Section 5.6.7
month
Section 5.6.7
obs-date
Section 5.6.7
obs-text
Section 5.5
opaque-tag
Section 8.8.3
other-range
Section 14.1.1
parameter
Section 5.6.6
parameter-name
Section 5.6.6
parameter-value
Section 5.6.6
parameters
Section 5.6.6
partial-URI
Section 4.1
port
Section 4.1
product
Section 10.1.5
product-version
Section 10.1.5
protocol-name
Section 7.6.3
protocol-version
Section 7.6.3
pseudonym
Section 7.6.3
qdtext
Section 5.6.4
query
Section 4.1
quoted-pair
Section 5.6.4
quoted-string
Section 5.6.4
qvalue
Section 12.4.2
range-resp
Section 14.4
range-set
Section 14.1.1
range-spec
Section 14.1.1
range-unit
Section 14.1
ranges-specifier
Section 14.1.1
received-by
Section 7.6.3
received-protocol
Section 7.6.3
rfc850-date
Section 5.6.7
second
Section 5.6.7
segment
Section 4.1
subtype
Section 8.3.1
suffix-length
Section 14.1.1
suffix-range
Section 14.1.1
t-codings
Section 10.1.4
tchar
Section 5.6.2
time-of-day
Section 5.6.7
token
Section 5.6.2
token68
Section 11.2
transfer-coding
Section 10.1.4
transfer-parameter
Section 10.1.4
type
Section 8.3.1
unsatisfied-range
Section 14.4
uri-host
Section 4.1
weak
Section 8.8.3
weight
Section 12.4.2
year
Section 5.6.7
gzip (Coding Format)
Section 8.4.1.3
gzip (content coding)
Section 8.4.1
HEAD method
Section 9.3.2
Header Fields
Accept
Section 12.5.1
Accept-Charset
Section 12.5.2
Accept-Encoding
Section 12.5.3
Accept-Language
Section 12.5.4
Accept-Ranges
Section 14.3
Allow
Section 10.2.1
Authentication-Info
Section 11.6.3
Authorization
Section 11.6.2
Connection
Section 7.6.1
Content-Encoding
Section 8.4
Content-Language
Section 8.5
Content-Length
Section 8.6
Content-Location
Section 8.7
Content-MD5
Section 18.4, Paragraph 10
Content-Range
Section 14.4
Section 14.5
Content-Type
Section 8.3
Date
Section 6.6.1
ETag
Section 8.8.3
Expect
Section 10.1.1
From
Section 10.1.2
Host
Section 7.2
If-Match
Section 13.1.1
If-Modified-Since
Section 13.1.3
If-None-Match
Section 13.1.2
If-Range
Section 13.1.5
If-Unmodified-Since
Section 13.1.4
Last-Modified
Section 8.8.2
Location
Section 10.2.2
Max-Forwards
Section 7.6.2
Proxy-Authenticate
Section 11.7.1
Proxy-Authentication-Info
Section 11.7.3
Proxy-Authorization
Section 11.7.2
Range
Section 14.2
Referer
Section 10.1.3
Retry-After
Section 10.2.3
Server
Section 10.2.4
TE
Section 10.1.4
Trailer
Section 6.6.2
Upgrade
Section 7.8
User-Agent
Section 10.1.5
Vary
Section 12.5.5
Via
Section 7.6.3
WWW-Authenticate
Section 11.6.1
header section
Section 6.3
Host header field
Section 7.2
http URI scheme
Section 4.2.1
https URI scheme
Section 4.2.2
idempotent
Section 9.2.2
If-Match header field
Section 13.1.1
If-Modified-Since header field
Section 13.1.3
If-None-Match header field
Section 13.1.2
If-Range header field
Section 13.1.5
If-Unmodified-Since header field
Section 13.1.4
inbound
Section 3.7, Paragraph 4
incomplete
Section 6.1
interception proxy
Section 3.7, Paragraph 10
intermediary
Section 3.7
Last-Modified header field
Section 8.8.2
list-based field
Section 5.5, Paragraph 7
Location header field
Section 10.2.2
Max-Forwards header field
Section 7.6.2
Media Type
multipart/byteranges
Section 14.6
multipart/x-byteranges
Section 14.6, Paragraph 4, Item 3
message
Section 3.4
Section 6
message abstraction
Section 6
messages
Section 3.4
metadata
Section 8.8
Method
Section 18.2, Paragraph 3
CONNECT
Section 9.3.6
DELETE
Section 9.3.5
GET
Section 9.3.1
HEAD
Section 9.3.2
OPTIONS
Section 9.3.7
POST
Section 9.3.3
PUT
Section 9.3.4
TRACE
Section 9.3.8
multipart/byteranges Media Type
Section 14.6
multipart/x-byteranges Media Type
Section 14.6, Paragraph 4, Item 3
non-transforming proxy
Section 7.7
OPTIONS method
Section 9.3.7
origin
Section 4.3.1
Section 11.5
origin server
Section 3.6
outbound
Section 3.7, Paragraph 4
phishing
Section 17.1
POST method
Section 9.3.3
Protection Space
Section 11.5
proxy
Section 3.7, Paragraph 5
Proxy-Authenticate header field
Section 11.7.1
Proxy-Authentication-Info header field
Section 11.7.3
Proxy-Authorization header field
Section 11.7.2
PUT method
Section 9.3.4
Range header field
Section 14.2
Realm
Section 11.5
recipient
Section 3.4
Referer header field
Section 10.1.3
representation
Section 3.2
request
Section 3.4
request target
Section 7.1
resource
Section 3.1
Section 4
response
Section 3.4
Retry-After header field
Section 10.2.3
reverse proxy
Section 3.7, Paragraph 6
safe
Section 9.2.1
satisfiable range
Section 14.1.1
secured
Section 4.2.2
selected representation
Section 3.2, Paragraph 4
Section 8.8
Section 13.1
self-descriptive
Section 6
sender
Section 3.4
server
Section 3.3
Server header field
Section 10.2.4
singleton field
Section 5.5, Paragraph 6
spider
Section 3.5
Status Code
Section 15
Status Codes
Final
Section 15, Paragraph 7
Informational
Section 15, Paragraph 7
Interim
Section 15, Paragraph 7
Status Codes Classes
1xx Informational
Section 15.2
2xx Successful
Section 15.3
3xx Redirection
Section 15.4
4xx Client Error
Section 15.5
5xx Server Error
Section 15.6
target resource
Section 7.1
target URI
Section 7.1
TE header field
Section 10.1.4
TRACE method
Section 9.3.8
Trailer Fields
Section 6.5
ETag
Section 8.8.3
Trailer header field
Section 6.6.2
trailer section
Section 6.5
trailers
Section 6.5
transforming proxy
Section 7.7
transparent proxy
Section 3.7, Paragraph 10
tunnel
Section 3.7, Paragraph 8
unsatisfiable range
Section 14.1.1
Upgrade header field
Section 7.8
upstream
Section 3.7, Paragraph 4
URI
Section 4
origin
Section 4.3.1
URI reference
Section 4.1
URI scheme
http
Section 4.2.1
https
Section 4.2.2
user agent
Section 3.5
User-Agent header field
Section 10.1.5
validator
Section 8.8
strong
Section 8.8.1
weak
Section 8.8.1
Vary header field
Section 12.5.5
Via header field
Section 7.6.3
WWW-Authenticate header field
Section 11.6.1
x-compress (content coding)
Section 8.4.1
x-gzip (content coding)
Section 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:
Datatracker
RFC 9110
RFC
- Internet Standard
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RFC
- Internet Standard
June 2022
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Mark Nottingham
Julian Reschke
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