SPDY Protocol - Draft 3

SPDY Protocol - Draft 3
SPDY
SPDY Protocol
SPDY Protocol - Draft 3
1. Overview
One of the bottlenecks of HTTP implementations is that HTTP relies on multiple
connections for concurrency. This causes several problems, including additional
round trips for connection setup, slow-start delays, and connection rationing by
the client, where it tries to avoid opening too many connections to any single
server. HTTP pipelining helps some, but only achieves partial multiplexing. In
addition, pipelining has proven non-deployable in existing browsers due to
intermediary interference.
SPDY adds a framing layer for multiplexing multiple, concurrent streams across a
single TCP connection (or any reliable transport stream). The framing layer is
optimized for HTTP-like request-response streams, such that applications which
run over HTTP today can work over SPDY with little or no change on behalf of the
web application writer.
The SPDY session offers four improvements over HTTP:
Multiplexed requests: There is no limit to the number of requests
that can be issued concurrently over a single SPDY connection.
Prioritized requests: Clients can request certain resources to be
delivered first. This avoids the problem of congesting the network
channel with non-critical resources when a high-priority request is
pending.
Compressed headers: Clients today send a significant amount of
redundant data in the form of HTTP headers. Because a single web
page may require 50 or 100 subrequests, this data is significant.
Server pushed streams: Server Push enables content to be pushed from
servers to clients without a request.
SPDY attempts to preserve the existing semantics of HTTP. All features such as
cookies, ETags, Vary headers, Content-Encoding negotiations, etc work as they do
with HTTP; SPDY only replaces the way the data is written to the network.
1.1 Document Organization
The SPDY Specification is split into two parts: a framing layer (
Section
), which
multiplexes a TCP connection into independent, length-prefixed frames, and an
HTTP layer (
Section
),
which specifies the mechanism for overlaying HTTP request/response pairs on top
of the framing layer. While some of the framing layer concepts are isolated from
the HTTP layer, building a generic framing layer has not been a goal. The
framing layer is tailored to the needs of the HTTP protocol and server push.
1.2 Definitions
client: The endpoint initiating the SPDY session.
connection: A transport-level connection between two endpoints.
endpoint: Either the client or server of a connection.
frame: A header-prefixed sequence of bytes sent over a SPDY session.
server: The endpoint which did not initiate the SPDY session.
session: A synonym for a connection.
session error: An error on the SPDY session.
stream: A bi-directional flow of bytes across a virtual channel
within a SPDY session.
stream error: An error on an individual SPDY stream.
2. SPDY Framing Layer
2.1 Session (Connections)
The SPDY framing layer (or "session") runs atop a reliable transport layer such
as TCP. The client is the TCP connection initiator. SPDY connections are
persistent connections.
For best performance, it is expected that clients will not close open
connections until the user navigates away from all web pages referencing a
connection, or until the server closes the connection. Servers are encouraged to
leave connections open for as long as possible, but can terminate idle
connections if necessary. When either endpoint closes the transport-level
connection, it MUST first send a GOAWAY (
Section
2.6.6
) frame so that
the endpoints can reliably determine if requests finished before the close.
2.2 Framing
Once the connection is established, clients and servers exchange framed
messages. There are two types of frames: control frames (
Section
2.2.1
) and
data frames (
Section
2.2.2
). Frames
always have a common header which is 8 bytes in length.
The first bit is a control bit indicating whether a frame is a control frame or
data frame. Control frames carry a version number, a frame type, flags, and a
length. Data frames contain the stream ID, flags, and the length for the payload
carried after the common header. The simple header is designed to make reading
and writing of frames easy.
All integer values, including length, version, and type, are in network byte
order. SPDY does not enforce alignment of types in dynamically sized frames.
2.2.1 Control frames
+----------------------------------+ |C| Version(15bits) | Type(16bits) |
+----------------------------------+ | Flags (8) | Length (24 bits) |
+----------------------------------+ | Data |
+----------------------------------+
Control bit: The 'C' bit is a single bit indicating if this is a control
message. For control frames this value is always 1.
Version: The version number of the SPDY protocol. This document describes SPDY
version 3.
Type: The type of control frame. See Control Frames (
Section
2.6
) for
the complete list of control frames.
Flags: Flags related to this frame. Flags for control frames and data frames are
different.
Length: An unsigned 24-bit value representing the number of bytes after the
length field.
Data: data associated with this control frame. The format and length of this
data is controlled by the control frame type.
Control frame processing requirements:
Note that full length control frames (16MB) can be large for
implementations running on resource-limited hardware. In such cases,
implementations MAY limit the maximum length frame supported.
However, all implementations MUST be able to receive control frames
of at least 8192 octets in length.
2.2.2 Data frames
+----------------------------------+ |C| Stream-ID (31bits) |
+----------------------------------+ | Flags (8) | Length (24 bits) |
+----------------------------------+ | Data |
+----------------------------------+
Control bit: For data frames this value is always 0.
Stream-ID: A 31-bit value identifying the stream.
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - signifies that this frame represents the last
frame to be transmitted on this stream. See Stream Close (
Section
2.3.7
below.
Length: An unsigned 24-bit value representing the number of bytes after the
length field. The total size of a data frame is 8 bytes + length. It is valid to
have a zero-length data frame.
Data: The variable-length data payload; the length was defined in the length
field.
Data frame processing requirements:
If an endpoint receives a data frame for a stream-id which is not
open and the endpoint has not sent a GOAWAY (
Section
2.6.6
frame, it MUST issue a stream error (
Section
2.4.2
with the error code INVALID_STREAM for the stream-id.
If the endpoint which created the stream receives a data frame
before receiving a SYN_REPLY on that stream, it is a protocol error,
and the recipient MUST issue a stream error (
Section
2.4.2
with the status code PROTOCOL_ERROR for the stream-id.
Implementors note: If an endpoint receives multiple data frames for
invalid stream-ids, it MAY close the session.
2.3 Streams
Streams are independent sequences of bi-directional data divided into frames
with several properties:
Streams may be created by either the client or server.
Streams optionally carry a set of name/value header pairs.
Streams can concurrently send data interleaved with other streams.
Streams may be cancelled.
2.3.1 Stream frames
SPDY defines 3 control frames to manage the lifecycle of a stream:
SYN_STREAM - Open a new stream
SYN_REPLY - Remote acknowledgement of a new, open stream
RST_STREAM - Close a stream
2.3.2 Stream creation
A stream is created by sending a control frame with the type set to SYN_STREAM
Section
2.6.1
). If the
server is initiating the stream, the Stream-ID must be even. If the client is
initiating the stream, the Stream-ID must be odd. 0 is not a valid Stream-ID.
Stream-IDs from each side of the connection must increase monotonically as new
streams are created. E.g. Stream 2 may be created after stream 3, but stream 7
must not be created after stream 9. Stream IDs do not wrap: when a client or
server cannot create a new stream id without exceeding a 31 bit value, it MUST
NOT create a new stream.
The stream-id MUST increase with each new stream. If an endpoint receives a
SYN_STREAM with a stream id which is less than any previously received
SYN_STREAM, it MUST issue a session error (
Section
2.4.1
with the status PROTOCOL_ERROR.
It is a protocol error to send two SYN_STREAMs with the same stream-id. If a
recipient receives a second SYN_STREAM for the same stream, it MUST issue a
stream error (
Section
2.4.2
with the status code PROTOCOL_ERROR.
Upon receipt of a SYN_STREAM, the recipient can reject the stream by sending a
stream error (
Section
2.4.2
with the error code REFUSED_STREAM. Note, however, that the creating endpoint
may have already sent additional frames for that stream which cannot be
immediately stopped.
Once the stream is created, the creator may immediately send HEADERS or DATA
frames for that stream, without needing to wait for the recipient to
acknowledge.
2.3.2.1 Unidirectional streams
When an endpoint creates a stream with the FLAG_UNIDIRECTIONAL flag set, it
creates a unidirectional stream which the creating endpoint can use to send
frames, but the receiving endpoint cannot. The receiving endpoint is implicitly
already in the half-closed (
Section
2.3.6
state.
2.3.2.2 Bidirectional streams
SYN_STREAM frames which do not use the FLAG_UNIDIRECTIONAL flag are
bidirectional streams. Both endpoints can send data on a bi-directional stream.
2.3.3 Stream priority
The creator of a stream assigns a priority for that stream. Priority is
represented as an integer from 0 to 7. 0 represents the highest priority and 7
represents the lowest priority.
The sender and recipient SHOULD use best-effort to process streams in the order
of highest priority to lowest priority.
2.3.4 Stream headers
Streams carry optional sets of name/value pair headers which carry metadata
about the stream. After the stream has been created, and as long as the sender
is not closed (
Section
2.3.7
) or
half-closed (
Section
2.3.6
),
each side may send HEADERS frame(s) containing the header data. Header data can
be sent in multiple HEADERS frames, and HEADERS frames may be interleaved with
data frames.
2.3.5 Stream data exchange
Once a stream is created, it can be used to send arbitrary amounts of data.
Generally this means that a series of data frames will be sent on the stream
until a frame containing the FLAG_FIN flag is set. The FLAG_FIN can be set on a
SYN_STREAM (
Section
2.6.1
),
SYN_REPLY (
Section
2.6.2
), HEADERS
Section 2.6.7
) or
a DATA (
Section
2.2.2
) frame.
Once the FLAG_FIN has been sent, the stream is considered to be half-closed.
2.3.6 Stream half-close
When one side of the stream sends a frame with the FLAG_FIN flag set, the stream
is half-closed from that endpoint. The sender of the FLAG_FIN MUST NOT send
further frames on that stream. When both sides have half-closed, the stream is
closed.
If an endpoint receives a data frame after the stream is half-closed from the
sender (e.g. the endpoint has already received a prior frame for the stream with
the FIN flag set), it MUST send a RST_STREAM to the sender with the status
STREAM_ALREADY_CLOSED.
2.3.7 Stream close
There are 3 ways that streams can be terminated:
Normal termination: Normal stream termination occurs when both
sender and recipient have half-closed the stream by sending a
FLAG_FIN.
Abrupt termination: Either the client or server can send a
RST_STREAM control frame at any time. A RST_STREAM contains an error
code to indicate the reason for failure. When a RST_STREAM is sent
from the stream originator, it indicates a failure to complete the
stream and that no further data will be sent on the stream. When a
RST_STREAM is sent from the stream recipient, the sender, upon
receipt, should stop sending any data on the stream. The stream
recipient should be aware that there is a race between data already
in transit from the sender and the time the RST_STREAM is received.
See Stream Error Handling (
Section
2.4.2
TCP connection teardown: If the TCP connection is torn down while
un-closed streams exist, then the endpoint must assume that the
stream was abnormally interrupted and may be incomplete.
If an endpoint receives a data frame after the stream is closed, it must send a
RST_STREAM to the sender with the status PROTOCOL_ERROR.
2.4 Error Handling
The SPDY framing layer has only two types of errors, and they are always handled
consistently. Any reference in this specification to "issue a session error"
refers to
Section
2.4.1
Any reference to "issue a stream error" refers to
Section
2.4.2
2.4.1 Session Error Handling
A session error is any error which prevents further processing of the framing
layer or which corrupts the session compression state. When a session error
occurs, the endpoint encountering the error MUST first send a GOAWAY (
Section
2.6.6
frame with the stream id of most recently received stream from the remote
endpoint, and the error code for why the session is terminating. After sending
the GOAWAY frame, the endpoint MUST close the TCP connection.
Note that the session compression state is dependent upon both endpoints always
processing all compressed data. If an endpoint partially processes a frame
containing compressed data without updating compression state properly, future
control frames which use compression will be always be errored. Implementations
SHOULD always try to process compressed data so that errors which could be
handled as stream errors do not become session errors.
Note that because this GOAWAY is sent during a session error case, it is
possible that the GOAWAY will not be reliably received by the receiving
endpoint. It is a best-effort attempt to communicate with the remote about why
the session is going down.
2.4.2 Stream Error Handling
A stream error is an error related to a specific stream-id which does not affect
processing of other streams at the framing layer. Upon a stream error, the
endpoint MUST send a RST_STREAM (
Section
2.6.3
) frame
which contains the stream id of the stream where the error occurred and the
error status which caused the error. After sending the RST_STREAM, the stream is
closed to the sending endpoint. After sending the RST_STREAM, if the sender
receives any frames other than a RST_STREAM for that stream id, it will result
in sending additional RST_STREAM frames. An endpoint MUST NOT send a RST_STREAM
in response to an RST_STREAM, as doing so would lead to RST_STREAM loops.
Sending a RST_STREAM does not cause the SPDY session to be closed.
If an endpoint has multiple RST_STREAM frames to send in succession for the same
stream-id and the same error code, it MAY coalesce them into a single RST_STREAM
frame. (This can happen if a stream is closed, but the remote sends multiple
data frames. There is no reason to send a RST_STREAM for each frame in
succession).
2.5 Data flow
Because TCP provides a single stream of data on which SPDY multiplexes multiple
logical streams, clients and servers must intelligently interleave data messages
for concurrent sessions.
2.6 Control frame types
2.6.1 SYN_STREAM
The SYN_STREAM control frame allows the sender to asynchronously create a stream
between the endpoints. See Stream Creation (
Section
2.3.2
+------------------------------------+ |1| version | 1 |
+------------------------------------+ | Flags (8) | Length (24 bits) |
+------------------------------------+ |X| Stream-ID (31bits) |
+------------------------------------+ |X| Associated-To-Stream-ID (31bits) |
+------------------------------------+ | Pri|Unused | Slot | |
+-------------------+ | | Number of Name/Value pairs (int32) | <+
+------------------------------------+ | | Length of name (int32) | | This
section is the "Name/Value +------------------------------------+ | Header
Block", and is compressed. | Name (string) | |
+------------------------------------+ | | Length of value (int32) | |
+------------------------------------+ | | Value (string) | |
+------------------------------------+ | | (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
Section
2.3.6
state.
0x02 = FLAG_UNIDIRECTIONAL - a stream created with this flag puts
the recipient in the half-closed (
Section
2.3.6
state.
Length: The length is the number of bytes which follow the length field in the
frame. For SYN_STREAM frames, this is 10 bytes plus the length of the compressed
Name/Value block.
Stream-ID: The 31-bit identifier for this stream. This stream-id will be used in
frames which are part of this stream.
Associated-To-Stream-ID: The 31-bit identifier for a stream which this stream is
associated to. If this stream is independent of all other streams, it should be
0.
Priority: A 3-bit priority (
Section
2.3.3
field.
Unused: 5 bits of unused space, reserved for future use.
Slot: An 8 bit unsigned integer specifying the index in the server's CREDENTIAL
vector of the client certificate to be used for this request. see CREDENTIAL
frame (
Section
2.6.9
). The
value 0 means no client certificate should be associated with this stream.
Name/Value Header Block: A set of name/value pairs carried as part of the
SYN_STREAM. see Name/Value Header Block (
Section
2.6.10
).
If an endpoint receives a SYN_STREAM which is larger than the implementation
supports, it MAY send a RST_STREAM with error code FRAME_TOO_LARGE. All
implementations MUST support the minimum size limits defined in the Control
Frames section (
Section
2.2.1
).
2.6.2 SYN_REPLY
SYN_REPLY indicates the acceptance of a stream creation by the recipient of a
SYN_STREAM frame.
+------------------------------------+ |1| version | 2 |
+------------------------------------+ | Flags (8) | Length (24 bits) |
+------------------------------------+ |X| Stream-ID (31bits) |
+------------------------------------+ | Number of Name/Value pairs (int32) |
<+ +------------------------------------+ | | Length of name (int32) | | This
section is the "Name/Value +------------------------------------+ | Header
Block", and is compressed. | Name (string) | |
+------------------------------------+ | | Length of value (int32) | |
+------------------------------------+ | | Value (string) | |
+------------------------------------+ | | (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
Section
2.3.6
state.
Length: The length is the number of bytes which follow the length field in the
frame. For SYN_REPLY frames, this is 4 bytes plus the length of the compressed
Name/Value block.
Stream-ID: The 31-bit identifier for this stream.
If an endpoint receives multiple SYN_REPLY frames for the same active stream ID,
it MUST issue a stream error (
Section
2.4.2
with the error code STREAM_IN_USE.
Name/Value Header Block: A set of name/value pairs carried as part of the
SYN_STREAM. see Name/Value Header Block (
Section
2.6.10
).
If an endpoint receives a SYN_REPLY which is larger than the implementation
supports, it MAY send a RST_STREAM with error code FRAME_TOO_LARGE. All
implementations MUST support the minimum size limits defined in the Control
Frames section (
Section
2.2.1
).
2.6.3 RST_STREAM
The RST_STREAM frame allows for abnormal termination of a stream. When sent by
the creator of a stream, it indicates the creator wishes to cancel the stream.
When sent by the recipient of a stream, it indicates an error or that the
recipient did not want to accept the stream, so the stream should be closed.
+----------------------------------+ |1| version | 3 |
+----------------------------------+ | Flags (8) | 8 |
+----------------------------------+ |X| Stream-ID (31bits) |
+----------------------------------+ | Status code |
+----------------------------------+
Flags: Flags related to this frame. RST_STREAM does not define any flags. This
value must be 0.
Length: An unsigned 24-bit value representing the number of bytes after the
length field. For RST_STREAM control frames, this value is always 8.
Stream-ID: The 31-bit identifier for this stream.
Status code: (32 bits) An indicator for why the stream is being terminated.The
following status codes are defined:
1 - PROTOCOL_ERROR. This is a generic error, and should only be used
if a more specific error is not available.
2 - INVALID_STREAM. This is returned when a frame is received for a
stream which is not active.
3 - REFUSED_STREAM. Indicates that the stream was refused before any
processing has been done on the stream.
4 - UNSUPPORTED_VERSION. Indicates that the recipient of a stream
does not support the SPDY version requested.
5 - CANCEL. Used by the creator of a stream to indicate that the
stream is no longer needed.
6 - INTERNAL_ERROR. This is a generic error which can be used when
the implementation has internally failed, not due to anything in the
protocol.
7 - FLOW_CONTROL_ERROR. The endpoint detected that its peer violated
the flow control protocol.
8 - STREAM_IN_USE. The endpoint received a SYN_REPLY for a stream
already open.
9 - STREAM_ALREADY_CLOSED. The endpoint received a data or SYN_REPLY
frame for a stream which is half closed.
10 - INVALID_CREDENTIALS. The server received a request for a
resource whose origin does not have valid credentials in the client
certificate vector.
11 - FRAME_TOO_LARGE. The endpoint received a frame which this
implementation could not support. If FRAME_TOO_LARGE is sent for a
SYN_STREAM, HEADERS, or SYN_REPLY frame without fully processing the
compressed portion of those frames, then the compression state will
be out-of-sync with the other endpoint. In this case, senders of
FRAME_TOO_LARGE MUST close the session.
Note: 0 is not a valid status code for a RST_STREAM.
After receiving a RST_STREAM on a stream, the recipient must not send additional
frames for that stream, and the stream moves into the closed state.
2.6.4 SETTINGS
A SETTINGS frame contains a set of id/value pairs for communicating
configuration data about how the two endpoints may communicate. SETTINGS frames
can be sent at any time by either endpoint, are optionally sent, and are fully
asynchronous. When the server is the sender, the sender can request that
configuration data be persisted by the client across SPDY sessions and returned
to the server in future communications.
Persistence of SETTINGS ID/Value pairs is done on a per origin/IP pair (the
"origin" is the set of scheme, host, and port from the URI. See RFC6454). That
is, when a client connects to a server, and the server persists settings within
the client, the client SHOULD return the persisted settings on future
connections to the same origin AND IP address and TCP port. Clients MUST NOT
request servers to use the persistence features of the SETTINGS frames, and
servers MUST ignore persistence related flags sent by a client.
+----------------------------------+ |1| version | 4 |
+----------------------------------+ | Flags (8) | Length (24 bits) |
+----------------------------------+ | Number of entries |
+----------------------------------+ | ID/Value Pairs | | ... |
Control bit: The control bit is always 1 for this message.
Version: The SPDY version number.
Type: The message type for a SETTINGS message is 4.
Flags: FLAG_SETTINGS_CLEAR_SETTINGS (0x1): When set, the client should clear any
previously persisted SETTINGS ID/Value pairs. If this frame contains ID/Value
pairs with the FLAG_SETTINGS_PERSIST_VALUE set, then the client will first clear
its existing, persisted settings, and then persist the values with the flag set
which are contained within this frame. Because persistence is only implemented
on the client, this flag can only be used when the sender is the server.
Length: An unsigned 24-bit value representing the number of bytes after the
length field. The total size of a SETTINGS frame is 8 bytes + length.
Number of entries: A 32-bit value representing the number of ID/value pairs in
this message.
Each ID/value pair is as follows:
+----------------------------------+ | Flags(8) | ID (24 bits) |
+----------------------------------+ | Value (32 bits) |
+----------------------------------+
Flags: An 8 bit value. Defined Flags:
FLAG_SETTINGS_PERSIST_VALUE (0x1): When set, the sender of this
SETTINGS frame is requesting that the recipient persist the ID/Value
and return it in future SETTINGS frames sent from the sender to this
recipient. Because persistence is only implemented on the client,
this flag is only sent by the server.
FLAG_SETTINGS_PERSISTED (0x2): When set, the sender is notifying the
recipient that this ID/Value pair was previously sent to the sender
by the recipient with the FLAG_SETTINGS_PERSIST_VALUE, and the
sender is returning it. Because persistence is only implemented on
the client, this flag is only sent by the client.
ID: 24-bits in network byte order. Defined IDs:
1 - SETTINGS_UPLOAD_BANDWIDTH allows the sender to send its expected
upload bandwidth on this channel. This number is an estimate. The
value should be the integral number of kilobytes per second that the
sender predicts as an expected maximum upload channel capacity.
2 - SETTINGS_DOWNLOAD_BANDWIDTH allows the sender to send its
expected download bandwidth on this channel. This number is an
estimate. The value should be the integral number of kilobytes per
second that the sender predicts as an expected maximum download
channel capacity.
3 - SETTINGS_ROUND_TRIP_TIME allows the sender to send its expected
round-trip-time on this channel. The round trip time is defined as
the minimum amount of time to send a control frame from this client
to the remote and receive a response. The value is represented in
milliseconds.
4 - SETTINGS_MAX_CONCURRENT_STREAMS allows the sender to inform the
remote endpoint the maximum number of concurrent streams which it
will allow. By default there is no limit. For implementors it is
recommended that this value be no smaller than 100.
5 - SETTINGS_CURRENT_CWND allows the sender to inform the remote
endpoint of the current TCP CWND value.
6 - SETTINGS_DOWNLOAD_RETRANS_RATE allows the sender to inform the
remote endpoint the retransmission rate (bytes retransmitted / total
bytes transmitted).
7 - SETTINGS_INITIAL_WINDOW_SIZE allows the sender to inform the
remote endpoint the initial window size (in bytes) for new streams.
8 - SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE allows the server to
inform the client if the new size of the client certificate vector.
Value: A 32-bit value.
The message is intentionally extensible for future information which may improve
client-server communications. The sender does not need to send every type of
ID/value. It must only send those for which it has accurate values to convey.
When multiple ID/value pairs are sent, they should be sent in order of lowest id
to highest id. A single SETTINGS frame MUST not contain multiple values for the
same ID. If the recipient of a SETTINGS frame discovers multiple values for the
same ID, it MUST ignore all values except the first one.
A server may send multiple SETTINGS frames containing different ID/Value pairs.
When the same ID/Value is sent twice, the most recent value overrides any
previously sent values. If the server sends IDs 1, 2, and 3 with the
FLAG_SETTINGS_PERSIST_VALUE in a first SETTINGS frame, and then sends IDs 4 and
5 with the FLAG_SETTINGS_PERSIST_VALUE, when the client returns the persisted
state on its next SETTINGS frame, it SHOULD send all 5 settings (1, 2, 3, 4, and
5 in this example) to the server.
2.6.5 PING
The PING control frame is a mechanism for measuring a minimal round-trip time
from the sender. It can be sent from the client or the server. Recipients of a
PING frame should send an identical frame to the sender as soon as possible (if
there is other pending data waiting to be sent, PING should take highest
priority). Each ping sent by a sender should use a unique ID.
+----------------------------------+ |1| version | 6 |
+----------------------------------+ | 0 (flags) | 4 (length) |
+----------------------------------| | 32-bit ID |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The SPDY version number.
Type: The message type for a PING message is 6.
Length: This frame is always 4 bytes long.
ID: A unique ID for this ping, represented as an unsigned 32 bit value. When the
client initiates a ping, it must use an odd numbered ID. When the server
initiates a ping, it must use an even numbered ping. Use of odd/even IDs is
required in order to avoid accidental looping on PINGs (where each side
initiates an identical PING at the same time).
Note: If a sender uses all possible PING ids (e.g. has sent all 2^31 possible
IDs), it can wrap and start re-using IDs.
If a server receives an even numbered PING which it did not initiate, it must
ignore the PING. If a client receives an odd numbered PING which it did not
initiate, it must ignore the PING.
2.6.6 GOAWAY
The GOAWAY control frame is a mechanism to tell the remote side of the
connection to stop creating streams on this session. It can be sent from the
client or the server. Once sent, the sender will not respond to any new
SYN_STREAMs on this session. Recipients of a GOAWAY frame must not send
additional streams on this session, although a new session can be established
for new streams. The purpose of this message is to allow an endpoint to
gracefully stop accepting new streams (perhaps for a reboot or maintenance),
while still finishing processing of previously established streams.
There is an inherent race condition between an endpoint sending SYN_STREAMs and
the remote sending a GOAWAY message. To deal with this case, the GOAWAY contains
a last-stream-id indicating the stream-id of the last stream which was created
on the sending endpoint in this session. If the receiver of the GOAWAY sent new
SYN_STREAMs for sessions after this last-stream-id, they were not processed by
the server and the receiver may treat the stream as though it had never been
created at all (hence the receiver may want to re-create the stream later on a
new session).
Endpoints should always send a GOAWAY message before closing a connection so
that the remote can know whether a stream has been partially processed or not.
(For example, if an HTTP client sends a POST at the same time that a server
closes a connection, the client cannot know if the server started to process
that POST request if the server does not send a GOAWAY frame to indicate where
it stopped working).
After sending a GOAWAY message, the sender must ignore all SYN_STREAM frames for
new streams.
+----------------------------------+ |1| version | 7 |
+----------------------------------+ | 0 (flags) | 8 (length) |
+----------------------------------| |X| Last-good-stream-ID (31 bits) |
+----------------------------------+ | Status code |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The SPDY version number.
Type: The message type for a GOAWAY message is 7.
Length: This frame is always 8 bytes long.
Last-good-stream-Id: The last stream id which was accepted by the sender of the
GOAWAY message. If no streams were replied to, this value MUST be 0.
Status: The reason for closing the session.
0 - OK. This is a normal session teardown.
1 - PROTOCOL_ERROR. This is a generic error, and should only be used
if a more specific error is not available.
2 - INTERNAL_ERROR. This is a generic error which can be used when
the implementation has internally failed, not due to anything in the
protocol.
2.6.7 HEADERS
The HEADERS frame augments a stream with additional headers. It may be
optionally sent on an existing stream at any time. Specific application of the
headers in this frame is application-dependent. The name/value header block
within this frame is compressed.
+------------------------------------+ |1| version | 8 |
+------------------------------------+ | Flags (8) | Length (24 bits) |
+------------------------------------+ |X| Stream-ID (31bits) |
+------------------------------------+ | Number of Name/Value pairs (int32) |
<+ +------------------------------------+ | | Length of name (int32) | | This
section is the "Name/Value +------------------------------------+ | Header
Block", and is compressed. | Name (string) | |
+------------------------------------+ | | Length of value (int32) | |
+------------------------------------+ | | Value (string) | |
+------------------------------------+ | | (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
Section
2.3.6
state.
Length: An unsigned 24 bit value representing the number of bytes after the
length field. The minimum length of the length field is 4 (when the number of
name value pairs is 0).
Stream-ID: The stream this HEADERS block is associated with.
Name/Value Header Block: A set of name/value pairs carried as part of the
SYN_STREAM. see Name/Value Header Block (
Section
2.6.10
).
2.6.8 WINDOW_UPDATE
The WINDOW_UPDATE control frame is used to implement per stream flow control in
SPDY. Flow control in SPDY is per hop, that is, only between the two endpoints
of a SPDY connection. If there are one or more intermediaries between the client
and the origin server, flow control signals are not explicitly forwarded by the
intermediaries. (However, throttling of data transfer by any recipient may have
the effect of indirectly propagating flow control information upstream back to
the original sender.) Flow control only applies to the data portion of data
frames. Recipients must buffer all control frames. If a recipient fails to
buffer an entire control frame, it MUST issue a stream error (
Section
2.4.2
with the status code FLOW_CONTROL_ERROR for the stream.
Flow control in SPDY is implemented by a data transfer window kept by the sender
of each stream. The data transfer window is a simple uint32 that indicates how
many bytes of data the sender can transmit. After a stream is created, but
before any data frames have been transmitted, the sender begins with the initial
window size. This window size is a measure of the buffering capability of the
recipient. The sender must not send a data frame with data length greater than
the transfer window size. After sending each data frame, the sender decrements
its transfer window size by the amount of data transmitted. When the window size
becomes less than or equal to 0, the sender must pause transmitting data frames.
At the other end of the stream, the recipient sends a WINDOW_UPDATE control back
to notify the sender that it has consumed some data and freed up buffer space to
receive more data.
+----------------------------------+ |1| version | 9 |
+----------------------------------+ | 0 (flags) | 8 (length) |
+----------------------------------+ |X| Stream-ID (31-bits) |
+----------------------------------+ |X| Delta-Window-Size (31-bits) |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The SPDY version number.
Type: The message type for a WINDOW_UPDATE message is 9.
Length: The length field is always 8 for this frame (there are 8 bytes after the
length field).
Stream-ID: The stream ID that this WINDOW_UPDATE control frame is for.
Delta-Window-Size: The additional number of bytes that the sender can transmit
in addition to existing remaining window size. The legal range for this field is
1 to 2^31 - 1 (0x7fffffff) bytes.
The window size as kept by the sender must never exceed 2^31 (although it can
become negative in one special case). If a sender receives a WINDOW_UPDATE that
causes the its window size to exceed this limit, it must send RST_STREAM with
status code FLOW_CONTROL_ERROR to terminate the stream.
When a SPDY connection is first established, the default initial window size for
all streams is 64KB. An endpoint can use the SETTINGS control frame to adjust
the initial window size for the connection. That is, its peer can start out
using the 64KB default initial window size when sending data frames before
receiving the SETTINGS. Because SETTINGS is asynchronous, there may be a race
condition if the recipient wants to decrease the initial window size, but its
peer immediately sends 64KB on the creation of a new connection, before waiting
for the SETTINGS to arrive. This is one case where the window size kept by the
sender will become negative. Once the sender detects this condition, it must
stop sending data frames and wait for the recipient to catch up. The recipient
has two choices:
immediately send RST_STREAM with FLOW_CONTROL_ERROR status code.
allow the head of line blocking (as there is only one stream for the
session and the amount of data in flight is bounded by the default
initial window size), and send WINDOW_UPDATE as it consumes data.
In the case of option 2, both sides must compute the window size based on the
initial window size in the SETTINGS. For example, if the recipient sets the
initial window size to be 16KB, and the sender sends 64KB immediately on
connection establishment, the sender will discover its window size is -48KB on
receipt of the SETTINGS. As the recipient consumes the first 16KB, it must send
a WINDOW_UPDATE of 16KB back to the sender. This interaction continues until the
sender's window size becomes positive again, and it can resume transmitting data
frames.
After the recipient reads in a data frame with FLAG_FIN that marks the end of
the data stream, it should not send WINDOW_UPDATE frames as it consumes the last
data frame. A sender should ignore all the WINDOW_UPDATE frames associated with
the stream after it send the last frame for the stream.
The data frames from the sender and the WINDOW_UPDATE frames from the recipient
are completely asynchronous with respect to each other. This property allows a
recipient to aggressively update the window size kept by the sender to prevent
the stream from stalling.
2.6.9 CREDENTIAL
The CREDENTIAL control frame is used by the client to send additional client
certificates to the server. A SPDY client may decide to send requests for
resources from different origins on the same SPDY session if it decides that
that server handles both origins. For example if the IP address associated with
both hostnames matches and the SSL server certificate presented in the initial
handshake is valid for both hostnames. However, because the SSL connection can
contain at most one client certificate, the client needs a mechanism to send
additional client certificates to the server.
The server is required to maintain a vector of client certificates associated
with a SPDY session. When the client needs to send a client certificate to the
server, it will send a CREDENTIAL frame that specifies the index of the slot in
which to store the certificate as well as proof that the client posesses the
corresponding private key. The initial size of this vector must be 8. If the
client provides a client certificate during the first TLS handshake, the
contents of this certificate must be copied into the first slot (index 1) in the
CREDENTIAL vector, though it may be overwritten by subsequent CREDENTIAL frames.
The server must exclusively use the CREDENTIAL vector when evaluating the client
certificates associated with an origin. The server may change the size of this
vector by sending a SETTINGS frame with the setting
SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE value specified. In the event that the
new size is smaller than the current size, truncation occurs preserving
lower-index slots as possible.
TLS renegotiation with client authentication is incompatible with SPDY given the
multiplexed nature of SPDY. Specifically, imagine that the client has 2 requests
outstanding to the server for two different pages (in different tabs). When the
renegotiation + client certificate request comes in, the browser is unable to
determine which resource triggered the client certificate request, in order to
prompt the user accordingly.
+----------------------------------+ |1|000000000000011|0000000000001010|
+----------------------------------+ | flags (8) | Length (24 bits) |
+----------------------------------+ | Slot (16 bits) | | +-----------------+ |
| Proof Length (32 bits) | +----------------------------------+ | Proof |
+----------------------------------+ <+ | Certificate Length (32 bits) | |
+----------------------------------+ | Repeated until end of frame | Certificate
| | +----------------------------------+ <+
Slot: The index in the server's client certificate vector where this certificate
should be stored. If there is already a certificate stored at this index, it
will be overwritten. The index is one based, not zero based; zero is an invalid
slot index.
Proof: Cryptographic proof that the client has possession of the private key
associated with the certificate. The format is a TLS digitally-signed element
(http://tools.ietf.org/html/rfc5246#section-4.7). The signature algorithm must
be the same as that used in the CertificateVerify message. However, since the
MD5+SHA1 signature type used in TLS 1.0 connections can not be correctly encoded
in a digitally-signed element, SHA1 must be used when MD5+SHA1 was used in the
SSL connection. The signature is calculated over a 32 byte TLS extractor value
(http://tools.ietf.org/html/rfc5705) with a label of "EXPORTER SPDY certificate
proof" using the empty string as context. ForRSA certificates the signature
would be a PKCS#1 v1.5 signature. For ECDSA, it would be an ECDSA-Sig-Value
(http://tools.ietf.org/html/rfc5480#appendix-A). For a 1024-bit RSA key, the
CREDENTIAL message would be ~500 bytes.
Certificate: The certificate chain, starting with the leaf certificate. Each
certificate must be encoded as a 32 bit length, followed by a DER encoded
certificate. The certificate must be of the same type (RSA, ECDSA, etc) as the
client certificate associated with the SSL connection.
If the server receives a request for a resource with unacceptable credential
(either missing or invalid), it must reply with a RST_STREAM frame with the
status code INVALID_CREDENTIALS. Upon receipt of a RST_STREAM frame with
INVALID_CREDENTIALS, the client should initiate a new stream directly to the
requested origin and resend the request. Note, SPDY does not allow the server to
request different client authentication for different resources in the same
origin.
If the server receives an invalid CREDENTIAL frame, it MUST respond with a
GOAWAY frame and shutdown the session.
2.6.10 Name/Value Header Block
The Name/Value Header Block is found in the SYN_STREAM, SYN_REPLY and HEADERS
control frames, and shares a common format:
+------------------------------------+ | Number of Name/Value pairs (int32) |
+------------------------------------+ | Length of name (int32) |
+------------------------------------+ | Name (string) |
+------------------------------------+ | Length of value (int32) |
+------------------------------------+ | Value (string) |
+------------------------------------+ | (repeats) |
Number of Name/Value pairs: The number of repeating name/value pairs following
this field.
List of Name/Value pairs:
Length of Name: a 32-bit value containing the number of octets in
the name field. Note that in practice, this length must not exceed
2^24, as that is the maximum size of a SPDY frame.
Name: 0 or more octets, 8-bit sequences of data, excluding 0.
Length of Value: a 32-bit value containing the number of octets in
the value field. Note that in practice, this length must not exceed
2^24, as that is the maximum size of a SPDY frame.
Value: 0 or more octets, 8-bit sequences of data, excluding 0.
Each header name must have at least one value. Header names are encoded using
the
US-ASCII character
set
[ASCII] and must be all lower case. The length of each name must be greater
than zero. A recipient of a zero-length name MUST issue a stream error (
Section
2.4.2
with the status code PROTOCOL_ERROR for the stream-id.
Duplicate header names are not allowed. To send two identically named headers,
send a header with two values, where the values are separated by a single NUL
(0) byte. A header value can either be empty (e.g. the length is zero) or it can
contain multiple, NUL-separated values, each with length greater than zero. The
value never starts nor ends with a NUL character. Recipients of illegal value
fields MUST issue a stream error (
Section
2.4.2
with the status code PROTOCOL_ERROR for the stream-id.
2.6.10.1 Compression
The Name/Value Header Block is a section of the SYN_STREAM, SYN_REPLY, and
HEADERS frames used to carry header meta-data. This block is always compressed
using zlib compression. Within this specification, any reference to 'zlib' is
referring to the
ZLIB Compressed Data Format Specification Version 3.3 as part
of
RFC1950.
For each HEADERS compression instance, the initial state is initialized using
the following
dictionary
[UDELCOMPRESSION]:
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The entire contents of the name/value header block is compressed using zlib.
There is a single zlib stream for all name value pairs in one direction on a
connection. SPDY uses a SYNC_FLUSH between each compressed frame.
Implementation notes: the compression engine can be tuned to favor speed or
size. Optimizing for size increases memory use and CPU consumption. Because
header blocks are generally small, implementors may want to reduce the
window-size of the compression engine from the default 15bits (a 32KB window) to
more like 11bits (a 2KB window). The exact setting is chosen by the compressor,
the decompressor will work with any setting.
3. HTTP Layering over SPDY
SPDY is intended to be as compatible as possible with current web-based
applications. This means that, from the perspective of the server business logic
or application API, the features of HTTP are unchanged. To achieve this, all of
the application request and response header semantics are preserved, although
the syntax of conveying those semantics has changed. Thus, the rules from the
HTTP/1.1 specification in
RFC2616
apply with the changes in the sections below.
3.1 Connection Management
Clients SHOULD NOT open more than one SPDY session to a given
origin
concurrently.
Note that it is possible for one SPDY session to be finishing (e.g. a GOAWAY
message has been sent, but not all streams have finished), while another SPDY
session is starting.
3.1.1 Use of GOAWAY
SPDY provides a GOAWAY message which can be used when closing a connection from
either the client or server. Without a server GOAWAY message, HTTP has a race
condition where the client sends a request (a new SYN_STREAM) just as the server
is closing the connection, and the client cannot know if the server received the
stream or not. By using the last-stream-id in the GOAWAY, servers can indicate
to the client if a request was processed or not.
Note that some servers will choose to send the GOAWAY and immediately terminate
the connection without waiting for active streams to finish. The client will be
able to determine this because SPDY streams are determinstically closed. This
abrupt termination will force the client to heuristically decide whether to
retry the pending requests. Clients always need to be capable of dealing with
this case because they must deal with accidental connection termination cases,
which are the same as the server never having sent a GOAWAY.
More sophisticated servers will use GOAWAY to implement a graceful teardown.
They will send the GOAWAY and provide some time for the active streams to finish
before terminating the connection.
If a SPDY client closes the connection, it should also send a GOAWAY message.
This allows the server to know if any server-push streams were received by the
client.
If the endpoint closing the connection has not received any SYN_STREAMs from the
remote, the GOAWAY will contain a last-stream-id of 0.
3.2 HTTP Request/Response
3.2.1 Request
The client initiates a request by sending a SYN_STREAM frame. For requests which
do not contain a body, the SYN_STREAM frame MUST set the FLAG_FIN, indicating
that the client intends to send no further data on this stream. For requests
which do contain a body, the SYN_STREAM will not contain the FLAG_FIN, and the
body will follow the SYN_STREAM in a series of DATA frames. The last DATA frame
will set the FLAG_FIN to indicate the end of the body.
The SYN_STREAM Name/Value section will contain all of the HTTP headers which are
associated with an HTTP request. The header block in SPDY is mostly unchanged
from today's HTTP header block, with the following differences:
The first line of the request is unfolded into name/value pairs like
other HTTP headers and MUST be present:
":method" - the HTTP method for this request (e.g. "GET",
"POST", "HEAD", etc)
":path" - the url-path for this url with "/" prefixed. (See
RFC3986
).
For example, for "https://www.google.com/search?q=dogs" the path
would be "/search?q=dogs".
":version" - the HTTP version of this request (e.g. "HTTP/1.1")
In addition, the following two name/value pairs must also be present
in every request:
":host" - the hostport (See
RFC1738
portion of the URL for this request (e.g.
"www.google.com:1234"). This header is the same as the HTTP
'Host' header.
":scheme" - the scheme portion of the URL for this request (e.g.
"https"))
Header names are all lowercase.
The Connection, Host, Keep-Alive, Proxy-Connection, and
Transfer-Encoding headers are not valid and MUST not be sent.
User-agents MUST support gzip compression. Regardless of the
Accept-Encoding sent by the user-agent, the server may always send
content encoded with gzip or deflate encoding.
If a server receives a request where the sum of the data frame
payload lengths does not equal the size of the Content-Length
header, the server MUST return a 400 (Bad Request) error.
POST-specific changes:
Although POSTs are inherently chunked, POST requests SHOULD also
be accompanied by a Content-Length header. There are two reasons
for this: First, it assists with upload progress meters for an
improved user experience. But second, we know from early
versions of SPDY that failure to send a content length header is
incompatible with many existing HTTP server implementations.
Existing user-agents do not omit the Content-Length header, and
server implementations have come to depend upon this.
The user-agent is free to prioritize requests as it sees fit. If the user-agent
cannot make progress without receiving a resource, it should attempt to raise
the priority of that resource. Resources such as images, SHOULD generally use
the lowest priority.
If a client sends a SYN_STREAM without all of the method, host, path, scheme,
and version headers, the server MUST reply with a HTTP 400 Bad Request reply.
3.2.2 Response
The server responds to a client request with a SYN_REPLY frame. Symmetric to the
client's upload stream, server will send data after the SYN_REPLY frame via a
series of DATA frames, and the last data frame will contain the FLAG_FIN to
indicate successful end-of-stream. If a response (like a 202 or 204 response)
contains no body, the SYN_REPLY frame may contain the FLAG_FIN flag to indicate
no further data will be sent on the stream.
The response status line is unfolded into name/value pairs like
other HTTP headers and must be present:
":status" - The HTTP response status code (e.g. "200" or "200
OK")
":version" - The HTTP response version (e.g. "HTTP/1.1")
All header names must be lowercase.
The Connection, Keep-Alive, Proxy-Connection, and Transfer-Encoding
headers are not valid and MUST not be sent.
Responses MAY be accompanied by a Content-Length header for advisory
purposes. (e.g. for UI progress meters)
If a client receives a response where the sum of the data frame
payload lengths does not equal the size of the Content-Length
header, the client MUST ignore the content length header.
If a client receives a SYN_REPLY without a status or without a version header,
the client must reply with a RST_STREAM frame indicating a PROTOCOL ERROR.
3.2.3 Authentication
When a client sends a request to an origin server that requires authentication,
the server can reply with a "401 Unauthorized" response, and include a
WWW-Authenticate challenge header that defines the authentication scheme to be
used. The client then retries the request with an Authorization header
appropriate to the specified authentication scheme.
There are four options for proxy authentication, Basic, Digest, NTLM and
Negotiate (SPNEGO). The first two options were defined in
RFC2617
and are stateless. The second two options were developed by Microsoft and
specified in
RFC4559
and are stateful; otherwise known as multi-round authentication, or connection
authentication.
3.2.3.1 Stateless Authentication
Stateless Authentication over SPDY is identical to how it is performed over
HTTP. If multiple SPDY streams are concurrently sent to a single server, each
will authenticate independently, similar to how two HTTP connections would
independently authenticate to a proxy server.
3.2.3.2 Stateful Authentication
Unfortunately, the stateful authentication mechanisms were implemented and
defined in a such a way that directly violates RFC2617 - they do not include a
"realm" as part of the request. This is problematic in SPDY because it makes it
impossible for a client to disambiguate two concurrent server authentication
challenges.
To deal with this case, SPDY servers using Stateful Authentication MUST
implement one of two changes:
Servers can add a "realm=" header so that the
two authentication requests can be disambiguated and run
concurrently. Unfortunately, given how these mechanisms work, this
is probably not practical.
Upon sending the first stateful challenge response, the server MUST
buffer and defer all further frames which are not part of completing
the challenge until the challenge has completed. Completing the
authentication challenge may take multiple round trips. Once the
client receives a "401 Authenticate" response for a stateful
authentication type, it MUST stop sending new requests to the server
until the authentication has completed by receiving a non-401
response on at least one stream.
3.3 Server Push Transactions
SPDY enables a server to send multiple replies to a client for a single request.
The rationale for this feature is that sometimes a server knows that it will
need to send multiple resources in response to a single request. Without server
push features, the client must first download the primary resource, then
discover the secondary resource(s), and request them. Pushing of resources
avoids the round-trip delay, but also creates a potential race where a server
can be pushing content which a user-agent is in the process of requesting. The
following mechanics attempt to prevent the race condition while enabling the
performance benefit.
Browsers receiving a pushed response MUST validate that the server is authorized
to push the URL using the
browser
same-origin
policy. For example, a SPDY connection to www.foo.com is generally not permitted
to push a response for www.evil.com.
If the browser accepts a pushed response (e.g. it does not send a RST_STREAM),
the browser MUST attempt to cache the pushed response in same way that it would
cache any other response. This means validating the response headers and
inserting into the cache.
Because pushed responses have no request, they have no request headers
associated with them. At the framing layer, SPDY pushed streams contain an
"associated-stream-id" which indicates the requested stream for which the pushed
stream is related. The pushed stream inherits all of the headers from the
associated-stream-id with the exception of ":host", ":scheme", and ":path",
which are provided as part of the pushed response stream headers. The browser
MUST store these inherited and implied request headers with the cached resource.
Implementation note: With server push, it is theoretically possible for servers
to push unreasonable amounts of content or resources to the user-agent. Browsers
MUST implement throttles to protect against unreasonable push attacks.
3.3.1 Server implementation
When the server intends to push a resource to the user-agent, it opens a new
stream by sending a unidirectional SYN_STREAM. The SYN_STREAM MUST include an
Associated-To-Stream-ID, and MUST set the FLAG_UNIDIRECTIONAL flag. The
SYN_STREAM MUST include headers for ":scheme", ":host", ":path", which represent
the URL for the resource being pushed. Subsequent headers may follow in HEADERS
frames. The purpose of the association is so that the user-agent can
differentiate which request induced the pushed stream; without it, if the
user-agent had two tabs open to the same page, each pushing unique content under
a fixed URL, the user-agent would not be able to differentiate the requests.
The Associated-To-Stream-ID must be the ID of an existing, open stream. The
reason for this restriction is to have a clear endpoint for pushed content. If
the user-agent requested a resource on stream 11, the server replies on stream
11. It can push any number of additional streams to the client before sending a
FLAG_FIN on stream 11. However, once the originating stream is closed no further
push streams may be associated with it. The pushed streams do not need to be
closed (FIN set) before the originating stream is closed, they only need to be
created before the originating stream closes.
It is illegal for a server to push a resource with the Associated-To-Stream-ID
of 0.
To minimize race conditions with the client, the SYN_STREAM for the pushed
resources MUST be sent prior to sending any content which could allow the client
to discover the pushed resource and request it.
The server MUST only push resources which would have been returned from a GET
request.
Note: If the server does not have all of the Name/Value Response headers
available at the time it issues the HEADERS frame for the pushed resource, it
may later use an additional HEADERS frame to augment the name/value pairs to be
associated with the pushed stream. The subsequent HEADERS frame(s) must not
contain a header for ':host', ':scheme', or ':path' (e.g. the server can't
change the identity of the resource to be pushed). The HEADERS frame must not
contain duplicate headers with a previously sent HEADERS frame. The server must
send a HEADERS frame including the scheme/host/port headers before sending any
data frames on the stream.
3.3.2 Client implementation
When fetching a resource the client has 3 possibilities:
the resource is not being pushed
the resource is being pushed, but the data has not yet arrived
the resource is being pushed, and the data has started to arrive
When a SYN_STREAM and HEADERS frame which contains an Associated-To-Stream-ID is
received, the client must not issue GET requests for the resource in the pushed
stream, and instead wait for the pushed stream to arrive.
If a client receives a server push stream with stream-id 0, it MUST issue a
session error (
Section
2.4.1
with the status code PROTOCOL_ERROR.
When a client receives a SYN_STREAM from the server without a the ':host',
':scheme', and ':path' headers in the Name/Value section, it MUST reply with a
RST_STREAM with error code HTTP_PROTOCOL_ERROR.
To cancel individual server push streams, the client can issue a stream error
Section
2.4.2
with error code CANCEL. Upon receipt, the server MUST stop sending on this
stream immediately (this is an Abrupt termination).
To cancel all server push streams related to a request, the client may issue a
stream error (
Section
2.4.2
with error code CANCEL on the associated-stream-id. By cancelling that stream,
the server MUST immediately stop sending frames for any streams with
in-association-to for the original stream.
If the server sends a HEADER frame containing duplicate headers with a previous
HEADERS frame for the same stream, the client must issue a stream error
Section
2.4.2
with error code PROTOCOL ERROR.
If the server sends a HEADERS frame after sending a data frame for the same
stream, the client MAY ignore the HEADERS frame. Ignoring the HEADERS frame
after a data frame prevents handling of HTTP's trailing headers
(https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.40).
4. Design Rationale and Notes
Authors' notes: The notes in this section have no bearing on the SPDY protocol
as specified within this document, and none of these notes should be considered
authoritative about how the protocol works. However, these notes may prove
useful in future debates about how to resolve protocol ambiguities or how to
evolve the protocol going forward. They may be removed before the final draft.
4.1 Separation of Framing Layer and Application Layer
Readers may note that this specification sometimes blends the framing layer
Section
with requirements of a specific application - HTTP (
Section
).
This is reflected in the request/response nature of the streams, the definition
of the HEADERS and compression contexts which are very similar to HTTP, and
other areas as well.
This blending is intentional - the primary goal of this protocol is to create a
low-latency protocol for use with HTTP. Isolating the two layers is convenient
for description of the protocol and how it relates to existing HTTP
implementations. However, the ability to reuse the SPDY framing layer is a non
goal.
4.2 Error handling - Framing Layer
Error handling at the SPDY layer splits errors into two groups: Those that
affect an individual SPDY stream, and those that do not.
When an error is confined to a single stream, but general framing is in tact,
SPDY attempts to use the RST_STREAM as a mechanism to invalidate the stream but
move forward without aborting the connection altogether.
For errors occuring outside of a single stream context, SPDY assumes the entire
session is hosed. In this case, the endpoint detecting the error should initiate
a connection close.
4.3 One Connection Per Domain
SPDY attempts to use fewer connections than other protocols have traditionally
used. The rationale for this behavior is because it is very difficult to provide
a consistent level of service (e.g. TCP slow-start), prioritization, or optimal
compression when the client is connecting to the server through multiple
channels.
Through lab measurements, we have seen consistent latency benefits by using
fewer connections from the client. The overall number of packets sent by SPDY
can be as much as 40% less than HTTP. Handling large numbers of concurrent
connections on the server also does become a scalability problem, and SPDY
reduces this load.
The use of multiple connections is not without benefit, however. Because SPDY
multiplexes multiple, independent streams onto a single stream, it creates a
potential for head-of-line blocking problems at the transport level. In tests so
far, the negative effects of head-of-line blocking (especially in the presence
of packet loss) is outweighed by the benefits of compression and prioritization.
4.4 Fixed vs Variable Length Fields
SPDY favors use of fixed length 32bit fields in cases where smaller, variable
length encodings could have been used. To some, this seems like a tragic waste
of bandwidth. SPDY choses the simple encoding for speed and simplicity.
The goal of SPDY is to reduce latency on the network. The overhead of SPDY
frames is generally quite low. Each data frame is only an 8 byte overhead for a
1452 byte payload (~0.6%). At the time of this writing, bandwidth is already
plentiful, and there is a strong trend indicating that bandwidth will continue
to increase. With an average worldwide bandwidth of 1Mbps, and assuming that a
variable length encoding could reduce the overhead by 50%, the latency saved by
using a variable length encoding would be less than 100 nanoseconds. More
interesting are the effects when the larger encodings force a packet boundary,
in which case a round-trip could be induced. However, by addressing other
aspects of SPDY and TCP interactions, we believe this is completely mitigated.
4.5 Compression Context(s)
When isolating the compression contexts used for communicating with multiple
origins, we had a few choices to make. We could have maintained a map (or list)
of compression contexts usable for each origin. The basic case is easy - each
HEADERS frame would need to identify the context to use for that frame. However,
compression contexts are not cheap, so the lifecycle of each context would need
to be bounded. For proxy servers, where we could churn through many contexts,
this would be a concern. We considered using a static set of contexts, say 16 of
them, which would bound the memory use. We also considered dynamic contexts,
which could be created on the fly, and would need to be subsequently destroyed.
All of these are complicated, and ultimately we decided that such a mechanism
creates too many problems to solve.
Alternatively, we've chosen the simple approach, which is to simply provide a
flag for resetting the compression context. For the common case (no proxy), this
fine because most requests are to the same origin and we never need to reset the
context. For cases where we are using two different origins over a single SPDY
session, we simply reset the compression state between each transition.
4.6 Unidirectional streams
Many readers notice that unidirectional streams are both a bit confusing in
concept and also somewhat redundant. If the recipient of a stream doesn't wish
to send data on a stream, it could simply send a SYN_REPLY with the FLAG_FIN bit
set. The FLAG_UNIDIRECTIONAL is, therefore, not necessary.
It is true that we don't need the UNIDIRECTIONAL markings. It is added because
it avoids the recipient of pushed streams from needing to send a set of empty
frames (e.g. the SYN_STREAM w/ FLAG_FIN) which otherwise serve no purpose.
4.7 Data Compression
Generic compression of data portion of the streams (as opposed to compression of
the headers) without knowing the content of the stream is redundant. There is no
value in compressing a stream which is already compressed. Because of this, SPDY
initially allowed data compression to be optional. We included it because study
of existing websites shows that many sites are not using compression as they
should, and users suffer because of it. We wanted a mechanism where, at the SPDY
layer, site administrators could simply force compression - it is better to
compress twice than to not compress.
Overall, however, with this feature being optional and sometimes redundant, it
was unclear if it was useful at all. We removed it from the specification.
4.8 Server Push
A subtle but important point is that server push streams must be declared before
the associated stream is closed. The reason for this is so that proxies have a
lifetime for which they can discard information about previous streams. If a
pushed stream could associate itself with an already-closed stream, then
endpoints would not have a specific lifecycle for when they could disavow
knowledge of the streams which went before.
5. Security Considerations
5.1 Use of Same-origin constraints
This specification uses the
same-origin
policy
in all cases where verification of content is required.
5.2 HTTP Headers and SPDY Headers
At the application level, HTTP uses name/value pairs in its headers. Because
SPDY merges the existing HTTP headers with SPDY headers, there is a possibility
that some HTTP applications already use a particular header name. To avoid any
conflicts, all headers introduced for layering HTTP over SPDY are prefixed with
":". ":" is not a valid sequence in HTTP header naming, preventing any possible
conflict.
5.3 Cross-Protocol Attacks
By utilizing TLS, we believe that SPDY introduces no new cross-protocol attacks.
TLS encrypts the contents of all transmission (except the handshake itself),
making it difficult for attackers to control the data which could be used in a
cross-protocol attack.
5.4 Server Push Implicit Headers
Pushed resources do not have an associated request. In order for existing HTTP
cache control validations (such as the Vary header) to work, however, all cached
resources must have a set of request headers. For this reason, browsers MUST be
careful to inherit request headers from the associated stream for the push. This
includes the 'Cookie' header.
6. Privacy Considerations
6.1 Long Lived Connections
SPDY aims to keep connections open longer between clients and servers in order
to reduce the latency when a user makes a request. The maintenance of these
connections over time could be used to expose private information. For example,
a user using a browser hours after the previous user stopped using that browser
may be able to learn about what the previous user was doing. This is a problem
with HTTP in its current form as well, however the short lived connections make
it less of a risk.
6.2 SETTINGS frame
The SPDY SETTINGS frame allows servers to store out-of-band transmitted
information about the communication between client and server on the client.
Although this is intended only to be used to reduce latency, renegade servers
could use it as a mechanism to store identifying information about the client in
future requests.
Clients implementing privacy modes, such as Google Chrome's "incognito mode",
may wish to disable client-persisted SETTINGS storage.
Clients MUST clear persisted SETTINGS information when clearing the cookies.
TODO: Put range maximums on each type of setting to limit inappropriate uses.
7. Incompatibilities with SPDY draft #2
Here is a list of the major changes between this draft and draft #2.
Addition of flow control
Increased 16 bit length fields in SYN_STREAM and SYN_REPLY to 32
bits.
Changed definition of compression for DATA frames
Updated compression dictionary
Fixed off-by-one on the compression dictionary for headers
Increased priority field from 2bits to 3bits.
Removed NOOP frame
Split the request "url" into "scheme", "host", and "path"
Added the requirement that POSTs contain content-length.
Removed wasted 16bits of unused space from the end of the SYN_REPLY
and HEADERS frames.
Fixed bug: Priorities were described backward (0 was lowest instead
of highest).
Fixed bug: Name/Value header counts were duplicated in both the Name
Value header block and also the containing frame.
Fixed endianness issues with SETTINGS ids (which were little
endian). Now everything is big endian.
Header values can now be empty.
8. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in
RFC
2119
9. Acknowledgements
Many individuals have contributed to the design and evolution of SPDY: Adam
Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa Wilk, Costin
Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam Barth, Ryan Hamilton,
Gavin Peters, Kent Alstad, Kevin Lindsay, Paul Amer, Fan Yang, Jonathan Leighton
10. Normative References
[TLSNPN]
Langley, A., “
TLS Next Protocol Negotiation
”, <
>.
[ASCII]
“US-ASCII. Coded Character Set - 7-Bit American Standard Code for Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.”.
[UDELCOMPRESSION]
Yang, F., Amer, P., and J. Leighton, “
A Methodology to Derive SPDY’s Initial Dictionary for Zlib Compression
”, <
>.
11. Errata
Section 2.6.6 originally listed INTERNAL_ERROR as status code 11.
Authors' Addresses
Mike Belshe Twist EMail:
mbelshe@chromium.org
Roberto Peon Google, Inc EMail:
fenix@google.com