Hypertext Transfer Protocol version 2.0Twistmbelshe@chromium.orgGoogle, Incfenix@google.comMicrosoft3210 Porter DrivePalo Alto94043USmartin.thomson@skype.netIsode Ltd5 Castle Business Village36 Station RoadHamptonMiddlesexTW12 2BXUKAlexey.Melnikov@isode.com
Applications
HTTPbis Working GroupHTTP
This specification describes an optimised expression of the syntax of the
Hypertext Transfer Protocol (HTTP). The HTTP/2.0 encapsulation enables more
efficient transfer of representations by providing compressed header fields,
simultaneous requests, and also introduces unsolicited push of representations
from server to client.
This document is an alternative to, but does not obsolete the HTTP message
format. HTTP semantics remain unchanged.
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
Working Group information and related documents can be found
at (Wiki) and
(source code
and issues tracker).
The changes in this draft are summarized in .
The Hypertext Transfer Protocol (HTTP) is a wildly successful protocol. The HTTP/1.1 message
encapsulation () is optimized for
implementation simplicity and accessibility, not application performance. As such it has
several characteristics that have a negative overall effect on application performance.
The HTTP/1.1 encapsulation ensures that only one request can be delivered at a time on a
given connection. HTTP/1.1 pipelining, which is not widely deployed, only partially
addresses these concerns. Clients that need to make multiple requests therefore use
commonly multiple connections to a server or servers in order to reduce the overall latency
of those requests. Need to tune the anti-pipelining comments here.
Furthermore, HTTP/1.1 header fields are represented in an inefficient fashion, which, in
addition to generating more or larger network packets, can cause the small initial TCP
window to fill more quickly than is ideal. This results in excessive latency where multiple
requests are made on a new TCP connection.
This document defines an optimized mapping of the HTTP semantics to a TCP connection. This
optimization reduces the latency costs of HTTP by allowing parallel requests on the same
connection and by using an efficient coding for HTTP header fields. Prioritization of
requests lets more important requests complete faster, further improving application
performance.
HTTP/2.0 applications have an improved impact on network congestion due to the use of fewer
TCP connections to achieve the same effect. Fewer TCP connections compete more fairly with
other flows. Long-lived connections are also more able to take better advantage of the
available network capacity, rather than operating in the slow start phase of TCP.
The HTTP/2.0 encapsulation also enables more efficient processing of messages by providing
efficient message framing. Processing of header fields in HTTP/2.0 messages is more
efficient (for entities that process many messages).
The HTTP/2.0 Specification is split into three parts: starting
HTTP/2.0, which covers how a HTTP/2.0 is started; a
framing layer, which multiplexes a TCP connection into independent, length-prefixed
frames; and an HTTP layer, 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.
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.
All numeric values are in network byte order. Values are unsigned unless otherwise
indicated. Literal values are provided in decimal or hexadecimal as appropriate.
Hexadecimal literals are prefixed with 0x to distinguish them
from decimal literals.
The following terms are used:
The endpoint initiating the HTTP/2.0 session.
A transport-level connection between two endpoints.
Either the client or server of a connection.
The smallest unit of communication, each containing a frame header.
A complete sequence of frames.
An endpoint that is receiving frames.
An endpoint that is transmitting frames.
The endpoint which did not initiate the HTTP/2.0 session.
A synonym for a connection.
An error on the HTTP/2.0 session.
A bi-directional flow of bytes across a virtual channel within a HTTP/2.0 session.
An error on an individual HTTP/2.0 stream.
Just as HTTP/1.1 does, HTTP/2.0 uses the "http:" and "https:" URI schemes. An
HTTP/2.0-capable client is therefore required to discover whether a server (or intermediary)
supports HTTP/2.0.
Different discovery mechanisms are defined for "http:" and "https:" URIs. Discovery for
"http:" URIs is described in ; discovery for "https:" URIs is
described in .
HTTP/2.0 is identified using the string "HTTP/2.0". This identification is used in the
HTTP/1.1 Upgrade header field, in the TLS-NPNTBD field and other places where protocol identification is required.
Negotiating "HTTP/2.0" implies the use of the transport, security, framing and message
semantics described in this document.
Editor's Note: please remove the following text prior to the publication of a final
version of this document.
Only implementations of the final, published RFC can identify themselves as "HTTP/2.0".
Until such an RFC exists, implementations MUST NOT identify themselves using "HTTP/2.0".
Examples and text throughout the rest of this document use "HTTP/2.0" as a matter of
editorial convenience only. Implementations of draft versions MUST NOT identify using
this string.
Implementations of draft versions of the protocol MUST add the string "-draft-" and the
corresponding draft number to the identifier before the separator ('/'). For example,
draft-ietf-httpbis-http2-03 is identified using the string "HTTP-draft-03/2.0".
Non-compatible experiments that are based on these draft versions MUST instead replace the
string "draft" with a different identifier. For example, an experimental implementation
of packet mood-based encoding based on draft-ietf-httpbis-http2-07 might identify itself
as "HTTP-emo-07/2.0". Note that any label MUST conform with the "token" syntax defined in
. Experimenters are
encouraged to coordinate their experiments on the ietf-http-wg@w3.org mailing list.
A client that makes a request to an "http:" URI without prior knowledge about support for
HTTP/2.0 uses the HTTP Upgrade mechanism (). The client makes an HTTP/1.1 request that includes an Upgrade
header field identifying HTTP/2.0.
For example:
GET /default.htm HTTP/1.1
Host: server.example.com
Connection: Upgrade
Upgrade: HTTP/2.0
A server that does not support HTTP/2.0 can respond to the request as though the Upgrade
header field were absent:
HTTP/1.1 200 OK
Content-length: 243
Content-type: text/html
...
A server that supports HTTP/2.0 can accept the upgrade with a 101 (Switching Protocols)
status code. After the empty line that terminates the 101 response, the server can begin
sending HTTP/2.0 frames. These frames MUST include a response to the request that
initiated the Upgrade.
HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: HTTP/2.0
[ HTTP/2.0 session ...
Once the server returns the 101 response, both the client and the server send a session header.
A client that makes a request to an "https:" URI without prior knowledge about support for
HTTP/2.0 uses TLS with TLS-NPN
extension. TBD, maybe ALPN
Once TLS negotiation is complete, both the client and the server send a session header.
A client can learn that a particular server supports HTTP/2.0 by other means. A client
MAY immediately send HTTP/2.0 frames to a server that is known to support HTTP/2.0. This
only affects the resolution of "http:" URIs, servers supporting HTTP/2.0 are required to
support protocol negotiation in TLS.
Prior support for HTTP/2.0 is not a strong signal that a given server will support
HTTP/2.0 for future sessions. It is possible for server configurations to change or for
configurations to differ between instances in clustered server. Different "transparent"
intermediaries - intermediaries that are not explicitly selected by either client or
server - are another source of variability.
The HTTP/2.0 session runs atop TCP (). The client is the TCP
connection initiator.
HTTP/2.0 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 frame so that the endpoints can
reliably determine if requests finished before the close.
After opening a TCP connection and performing either an HTTP/1.1 Upgrade or TLS handshake,
the client sends the client session header. The server replies with a server session
header.
The session header provides a final confirmation that both peers agree to use the HTTP/2.0
protocol. The SETTINGS frame ensures that client or server configuration is known as
quickly as possible.
The client session header is the 25 byte sequence
0x464f4f202a20485454502f322e300d0a0d0a4241520d0a0d0a (the string FOO *
HTTP/2.0\r\n\r\nBAR\r\n\r\n) followed by a SETTINGS
frame. The client sends the client session header immediately after receiving an
HTTP/1.1 Upgrade, or after receiving a TLS Finished message from the server.
The client session header is selected so that a large proportion of HTTP/1.1 or
HTTP/1.0 servers and intermediaries do not attempt to process further frames. This
doesn't address the concerns raised in .
The server session header is a SETTINGS frame. The server
sends the server session header immediately after receiving and validating the client
session header.
The client sends requests immediately after sending the session header, without waiting to
receive a server session header. This ensures that confirming session headers does not
add latency.
Both client and server MUST close the connection if it does not begin with a valid session
header. A GOAWAY frame MAY be omitted if it is clear that
the peer is not using HTTP/2.0.
Once the connection is established, clients and servers exchange HTTP/2.0 frames. Frames
are the basic unit of communication.
HTTP/2.0 frames share a common header format. Frames have an 8 byte header with between
0 and 65535 bytes of data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (16) | Type (8) | Flags (8) |
+-+-------------+---------------+-------------------------------+
|R| Stream Identifier (31) |
+-+-------------------------------------------------------------+
| Frame Data (0...) ...
+---------------------------------------------------------------+
The fields of the frame header are defined as:
The 16-bit length of the frame payload in bytes. The length of the frame header is
not included in this sum.
The 8-bit type of the frame. The frame type determines how the remainder of the
frame header and payload are interpreted. Implementations MUST ignore frames that
use types that they do not support.
An 8-bit field reserved for flags. Bits that have undefined semantics are reserved.
The following flags are defined for all frame types:
Bit 1 (the least significant bit) indicates that this is the last frame in a
stream. This places the stream into a half-closed state. No further frames follow in
the direction of the carrying frame.
Frame types can define semantics for frame-specific flags.
A reserved 1-bit field. The semantics of this bit are not defined.
A 31-bit stream identifier (see ). A value 0 is
reserved for frames that are directed at the session as a whole instead of a single
stream.
Frames contain between 0 and 65535 bytes of data.
Reserved bits in the frame header MUST be set to zero when sending and MUST be ignored
when receiving frames, unless the semantics of the bit are known.
A frame of the maximum size might be too large for implementations with limited
resources to process. Implementations MAY choose to support frames smaller than the
maximum possible size. However, implementations MUST be able to receive frames
containing at least 8192 octets of payload.
An implementation MUST immediately close a stream if it is unable to process a frame
related to that stream due to it exceeding a size limit. The implementation MUST send a
RST_STREAM frame containing FRAME_TOO_LARGE error code
if the frame size limit is exceeded.
Issue 28: Need
a way to signal the maximum frame size; no way to RST_STREAM on non-stream-related
frames.
Streams are independent sequences of bi-directional data divided into frames with several
properties:
Streams can 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 can be established and used unilaterally.
Streams can be cancelled.
Use of streams does not require negotiation. A stream is not created, streams are used
by sending a frame on the stream.
Streams are identified by a 31-bit numeric identifier. Streams initiated by a client
use odd numbered stream identifiers. Streams initiated by the server use odd numbered
stream identifiers. A stream identifier of zero MUST NOT be used to create a new
stream.
The stream identifier of a new stream MUST be greater than all other streams from that
endpoint, unless the stream identifier was previously reserved (such as the promised
stream identifier in a PUSH_PROMISE frame). An
endpoint that receives an unexpected stream identifier MUST treat this as a session error of type PROTOCOL_ERROR.
A long-lived session can result in available stream identifiers being exhausted. An
endpoint that is unable to create a new stream identifier can establish a new session
for any new streams.
An endpoint cannot prevent the creation of a new stream, but it can request the early
termination of an unwanted stream. Upon receipt of a frame, the recipient can terminate
the corresponding stream by sending a stream
error of type REFUSED_STREAM. This cannot prevent the initiating endpoint
from sending frames for that stream prior to receiving this request.
The creator of a stream assigns a priority for that stream. Priority is represented as
a 31 bit integer. 0 represents the highest priority and 231-1 represents
the lowest priority.
The sender and recipient SHOULD use best-effort to process streams in the order of
highest priority to lowest priority. ED: toothless, useless "SHOULD":
reword
Streams carry optional sets of header fields which carry metadata about the stream.
After the stream has been created, and as long as the sender is not closed or half-closed,
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.
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 FINAL flag is set. Once the FINAL flag
has been set on any frame, the stream is considered to be half-closed.
When one side of the stream sends a frame with the FINAL flag set, the stream is
half-closed from that endpoint. The sender of the FINAL flag MUST NOT send further
frames on that stream. When both sides have half-closed, the stream is closed.
An endpoint MUST treat the receipt of a data frame on a half-closed stream as a stream error of type STREAM_CLOSED.
Streams that have never received packets can be considered to be half-closed in the
direction that is silent. This allows either peer to create a unidirectional stream,
which does not require an explicit close from the peer that does not transmit frames.
Streams can be terminated in the following ways:
Normal stream termination occurs when both sender and recipient have half-closed the
stream by sending a frame containing a FINAL flag.
A stream that only has frames sent in one direction can be tentatively considered to
be closed once a frame containing a FINAL flag is sent. The active sender on the
stream MUST be prepared to receive frames after closing the stream.
Either the peer can send a RST_STREAM control frame at any time to terminate an
active stream. RST_STREAM contains an error code to indicate the reason for
termination. A RST_STREAM indicates that the sender will transmit no further data
on the stream and that the receiver is requested to cease transmission.
The sender of a RST_STREAM frame MUST allow for frames that have already been sent
by the peer prior to the RST_STREAM being processed. If in-transit frames alter
session state, these frames cannot be safely discarded. See Stream Error Handling for more details.
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 MAY send a
RST_STREAM to the sender with the status PROTOCOL_ERROR.
HTTP/2.0 framing permits two classes of error:
An error condition that renders the entire session unusable is a session error.
An error in an individual stream is a stream error.
A session error is any error which prevents further processing of the framing layer or
which corrupts any session state.
An endpoint that encounters a session error MUST first send a GOAWAY frame with the stream identifier of the last stream that
it successfully received from its peer. The GOAWAY frame includes an error code that
indicates why the session is terminating. After sending the GOAWAY frame, the endpoint
MUST close the TCP connection.
It is possible that the GOAWAY will not be reliably received by the receiving endpoint.
In the event of a session error, GOAWAY only provides a best-effort attempt to
communicate with the peer about why the session is going down.
An endpoint can end a session at any time. In particular, an endpoint MAY choose to
treat a stream error as a session error if the error is recurrent. Endpoints SHOULD
send a GOAWAY frame when ending a session, as long as circumstances permit it.
A stream error is an error related to a specific stream identifier that does not affect
processing of other streams at the framing layer.
An endpoint that detects a stream error sends a RST_STREAM frame that contains the stream identifier of the
stream where the error occurred. The RST_STREAM frame includes an error code that
indicates the type of error.
A RST_STREAM is the last frame that an endpoint can send on a stream. The peer that
sends the RST_STREAM frame MUST be prepared to receive any frames that were sent or
enqueued for sending by the remote peer. These frames can be ignored, except where they
modify session state (such as the header compression state).
An endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. An endpoint
MAY send additional RST_STREAM frames if it receives frames on a closed stream after
more than a round trip time. This behaviour is permitted to deal with misbehaving
implementations where treating this as a session error is inappropriate.
An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM frame. This could
trigger infinite loops of RST_STREAM frames.
Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey
the reasons for the stream or session error.
Error codes share a common code space. Some error codes only apply to specific
conditions and have no defined semantics in certain frame types.
The following error codes are defined:
The associated condition is not as a result of an error. For example, a GOAWAY
might include this code to indicate graceful shutdown of a session.
An unspecific protocol error was detected. This error is for use when a more
specific error code is not available.
The implementation encountered an unexpected internal error.
The endpoint detected that its peer violated the flow control protocol.
A frame was received for an inactive stream.
The endpoint received a frame after a stream was half-closed.
The endpoint received a frame that was larger than the maximum size that it
supports.
Indicates that the stream was refused before any processing has been done on the
stream.
Used by the creator of a stream to indicate that the stream is no longer needed.
Multiplexing streams introduces contention for access to the shared TCP connection.
Stream contention can result in streams being blocked by other streams. A flow control
scheme ensures that streams do not destructively interfere with other streams on the same
TCP connection.
Experience with TCP congestion control has shown that algorithms can evolve over time to
become more sophisticated without requiring protocol changes. TCP congestion control
and its evolution is clearly different from HTTP/2.0 flow control, though the evolution
of TCP congestion control algorithms shows that a similar approach could be feasible for
HTTP/2.0 flow control.
HTTP/2.0 stream flow control aims to allow for future improvements to flow control
algorithms without requiring protocol changes. Flow control in HTTP/2.0 has the
following characteristics:
Flow control is hop-by-hop, not end-to-end.
Flow control is based on window update messages. Receivers advertise how many
octets they are prepared to receive on a stream. This is a credit-based scheme.
Flow control is directional with overall control provided by the receiver. A
receiver MAY choose to set any window size that it desires for each stream and for
the entire connection. A sender MUST respect flow control limits imposed by a
receiver. Clients, servers and intermediaries all independently advertise their
flow control preferences as a receiver and abide by the flow control limits set by
their peer when sending.
The initial value for the flow control window is 65536 bytes for both new streams
and the overall connection.
The frame type determines whether flow control applies to a frame. Of the frames
specified in this document, only data frames are subject to flow control; all other
frame types do not consume space in the advertised flow control window. This
ensures that important control frames are not blocked by flow control.
Flow control can be disabled by a receiver. A receiver can choose to either disable
flow control for a stream or connection by declaring an infinite flow control limit.
HTTP/2.0 standardizes only the format of the window
update message. This does not stipulate how a receiver decides when to send
this message or the value that it sends. Nor does it specify how a sender chooses
to send packets. Implementations are able to select any algorithm that suits their
needs.
Implementations are also responsible for managing how requests and responses are sent
based on priority; choosing how to avoid head of line blocking for requests; and
managing the creation of new streams. Algorithm choices for these could interact with
any flow control algorithm.
Flow control is defined to protect deployments (client, server or intermediary) that are
operating under constraints. For example, a proxy must share memory between many
connections. Flow control addresses cases where the receiver is unable process data on
one stream, yet wants to be continue to process other streams.
Deployments that do not rely on this capability SHOULD disable flow control for data
that is being received. Note that flow control cannot be disabled for sending.
Sending data is always subject to the flow control window advertised by the receiver.
Deployments with constrained resources (for example, memory), MAY employ flow control to
limit the amount of memory a peer can consume. This can lead to suboptimal use of
available network resources if flow control is enabled without knowledge of the
bandwidth-delay product (see ).
Implementation of flow control in full awareness of the current bandwidth-delay product
is difficult, but it can ensure that constrained resources are protected without any
reduction in connection utilization.
DATA frames (type=0) are used to convey HTTP message bodies. The payload of a data
frame contains either a request or response body.
No frame-specific flags are defined for DATA frames.
The HEADERS+PRIORITY frame (type=1) allows the sender to set header fields and stream
priority at the same time. This MUST be used for each stream that is created.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Priority (31) |
+-+-------------------------------------------------------------+
| Header Block (*) ...
+---------------------------------------------------------------+
The HEADERS+PRIORITY frame is identical to the HEADERS
frame, with a 32-bit field containing priority included before the header block.
The most significant bit of the priority is reserved. The 31-bit priority indicates the
priority for the stream, as assigned by the sender, see .
The RST_STREAM frame (type=3) 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code (32) |
+---------------------------------------------------------------+
The RST_STREAM frame does not define any valid flags.
The RST_STREAM frame contains a single 32-bit error
code. The error code indicates why the stream is being terminated.
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.
A SETTINGS frame (type=4) contains a set of id/value pairs for communicating
configuration data about how the two endpoints may communicate. SETTINGS frames MUST be
sent at the start of a session, but they can be sent at any other time by either
endpoint. Settings are declarative, not negotiated, each peer indicates their own
configuration.
Note that persistence of settings is under discussion in the WG and might be
removed in a future version of this document.
When the server is the sender, the sender can request that configuration data be
persisted by the client across HTTP/2.0 sessions and returned to the server in future
communications.
Clients persist settings on a per origin basis (see for a
definition of web origins). 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.
Valid frame-specific flags for the SETTINGS frame are:
Bit 2 being set indicates a request to clear any previously persisted settings
before processing the settings. Clients MUST NOT set this flag.
SETTINGS frames always apply to a session, never a single stream. The stream identifier
for a settings frame MUST be zero.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|SettingFlags(8)| Setting Identifier (24) |
+---------------+-----------------------------------------------+
| Value (32) |
+---------------------------------------------------------------+
The payload of a SETTINGS frame contains zero or more settings. Each setting is
comprised of the following
An 8-bit flags field containing per-setting instructions. The following flags are
valid:
Bit 1 (the least significant bit) being set indicates a request from the server
to the client to persist this setting. A client MUST NOT set this flag.
Bit 2 being set indicates that this setting is a persisted setting being
returned by the client to the server. This also indicates that this setting is
not a client setting, but a value previously set by the server. A server MUST
NOT set this flag.
All other settings flags are reserved.
A 24-bit field that identifies the setting.
A 32-bit value for the setting.
The following settings are defined:
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.
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.
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.
allows the sender to inform the remote endpoint the maximum number of concurrent
streams which it will allow. This limit is directional: it applies to the number
of streams that the sender permits the receiver to create. By default there is no
limit. For implementers it is recommended that this value be no smaller than 100,
so as to not unnecessarily limit parallelism.
allows the sender to inform the remote endpoint of the current TCP CWND value.
allows the sender to inform the remote endpoint the retransmission rate (bytes
retransmitted / total bytes transmitted).
allows the sender to inform the remote endpoint the initial window size (in bytes)
for new streams.
This setting allows an endpoint to indicate that streams directed to them will not
be subject to flow control. The least significant bit (0x1) is set to indicate
that new streams are not flow controlled. Bit 2 (0x2) is set to indicate that the
session is not flow controlled. All other bits are reserved.
This setting applies to all streams, including existing streams.
These bits cannot be cleared once set, see .
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.
The PUSH_PROMISE frame (type=5) allows the sender to signal a promise to create a stream
and serve the referenced resource. Minimal data allowing the receiver to understand
which resource(s) are to be pushed are to be included.
PUSH_PROMISE frames are sent on an existing stream. They declare the intent to use
another stream for the pushing of a resource. The PUSH_PROMISE allows the client an
opportunity to reject pushed resources.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Promised-Stream-ID (31) |
+-+-------------------------------------------------------------+
| Header Block (*) ...
+---------------------------------------------------------------+
There are no frame-specific flags for the PUSH_PROMISE frame.
The body of a PUSH_PROMISE includes a "Promised-Stream-ID". This 31-bit identifier
indicates the stream on which the resource will be pushed. The promised stream
identifier MUST be a valid choice for the next stream sent by the sender (see new stream identifier).
There is no requirement that the streams referred to by this frame are created in the
order referenced. The PUSH_PROMISE reserves stream identifiers for later use; these
reserved identifiers can be used as prioritization needs dictate.
The PUSH_PROMISE also includes a header block, which
describes the resource that will be pushed.
The PING frame (type=6) is a mechanism for measuring a minimal round-trip time from the
sender. PING frames can be sent from the client or the server.
Recipients of a PING frame send an identical frame to the sender as soon as possible.
PING should take highest priority if there is other data waiting to be sent.
The PING frame defines a frame-specific flag:
Bit 2 being set indicates that this ping frame is a ping response. An endpoint MUST
set this flag in ping responses. An endpoint MUST NOT respond to ping frames
containing this flag.
The payload of a PING frame contains any value. A PING response MUST contain the
contents of the PING request.
The GOAWAY frame (type=7) informs 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 ignore frames sent on new streams for the remainder of the
session. Recipients of a GOAWAY frame MUST NOT open additional streams on the 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 starting new streams and the
remote sending a GOAWAY message. To deal with this case, the GOAWAY contains the stream
identifier of the last stream which was processed on the sending endpoint in this
session. If the receiver of the GOAWAY used streams that are newer than the indicated
stream identifier, they were not processed by the sender and the receiver may treat the
streams as though they had never been created at all (hence the receiver may want to
re-create the streams 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 can ignore frames for new streams.
Issue: session state that is established by those "ignored" messages cannot be
ignored without the state in the two peers becoming unsynchronized.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Last-Stream-ID (31) |
+-+-------------------------------------------------------------+
| Error Code (32) |
+---------------------------------------------------------------+
The GOAWAY frame does not define any valid flags.
The GOAWAY frame applies to the session, not a specific stream. The stream identifier
MUST be zero.
The GOAWAY frame contains an identifier of the last stream that the sender of the GOAWAY
is prepared to act upon, which can include processing and replies. This allows an
endpoint to discover what streams might have had some effect or what might be safe to
automatically retry. If no streams were acted upon, the last stream ID MUST be 0.
The GOAWAY frame contains a 32-bit error code that
contains the reason for closing the session.
The HEADERS frame (type=8) provides header fields for a stream. It may be optionally
sent on an existing stream at any time. Specific application of the headers in this
frame is application-dependent.
No frame-specific flags are defined for the HEADERS frame.
The body of a HEADERS frame contains a Headers Block.
The WINDOW_UPDATE frame (type=9) is used to implement flow control in HTTP/2.0.
Flow control in HTTP/2.0 operates at two levels: on each individual stream and on the
entire session.
Flow control in HTTP/2.0 is hop by hop, that is, only between the two endpoints of a
HTTP/2.0 connection. Intermediaries do not forward WINDOW_UPDATE messages between
dependent sessions. However, throttling of data transfer by any recipient can
indirectly cause the propagation of flow control information toward the original
sender.
Flow control only applies to frames that are identified as being subject to flow
control. Of the frames defined in this document, only data frames are subject to flow
control. Receivers MUST either buffer or process all other frames, terminate the
corresponding stream, or terminate the session. The stream or session is terminated
with a FLOW_CONTROL_ERROR code.
Valid flags for the WINDOW_UPDATE frame are:
Bit 2 being set indicates that flow control for the identified stream or session is
ended and subsequent frames do not need to be flow controlled.
The WINDOW_UPDATE frame can be stream related or session related. The stream
identifier in the WINDOW_UPDATE frame header identifies the affected stream, or
includes a value of 0 to indicate that the session flow control window is updated.
The payload of a WINDOW_UPDATE frame contains a 32-bit value. This value is the
additional number of bytes that the sender can transmit in addition to the existing flow
control window. The legal range for this field is 1 to 231 - 1
(0x7fffffff) bytes; the most significant bit of this value is reserved.
Flow control in HTTP/2.0 is implemented by a flow control window kept by the sender of
each stream. The flow control window is a simple integer value that indicates how many
bytes of data the sender is permitted to transmit. The flow control window size is a
measure of the buffering capability of the recipient.
Two flow control windows apply to the sending of every message: the stream flow
control window and the session flow control window. The sender MUST NOT send a flow
controlled frame with a length that exceeds the space available in either of the flow
control windows advertised by the receiver. Frames with zero length with the FINAL
flag set (for example, an empty data frame) MAY be sent if there is no available space
in either flow control window.
For flow control calculations, the 8 byte frame header is not counted.
After sending a flow controlled frame, the sender reduces the space available in both
windows by the length of the transmitted frame.
The receiver of a message sends a WINDOW_UPDATE frame as it consumes data and frees up
space in flow control windows. Separate WINDOW_UPDATE messages are sent for the
stream and session level flow control windows.
A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the
amount specified in the frame.
A sender MUST NOT allow a flow control window to exceed 231 - 1 bytes.
If a sender receives a WINDOW_UPDATE that causes a flow control window to exceed this
maximum it MUST terminate either the stream or the session, as appropriate. For
streams, the sender sends a RST_STREAM with the error code of FLOW_CONTROL_ERROR code;
for the session, a GOAWAY message with a FLOW_CONTROL_ERROR code.
Flow controlled frames from the sender and WINDOW_UPDATE frames from the receiver are
completely asynchronous with respect to each other. This property allows a receiver to
aggressively update the window size kept by the sender to prevent streams from
stalling.
When a HTTP/2.0 connection is first established, new streams are created with an
initial flow control window size of 65535 bytes. The session flow control window is
65536 bytes. Both endpoints can adjust the initial window size for new streams by
including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms
part of the session header.
Prior to receiving a SETTINGS frame that sets a value for
SETTINGS_INITIAL_WINDOW_SIZE, a client can only use the default initial window size
when sending flow controlled frames. Similarly, the session flow control window is
set to the default initial window size until a WINDOW_UPDATE message is received.
A SETTINGS frame can alter the initial flow control window size for all current
streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST
adjust the size of all flow control windows that it maintains by the difference
between the new value and the old value.
A change to SETTINGS_INITIAL_WINDOW_SIZE could cause the available space in a flow
control window to become negative. A sender MUST track the negative flow control
window and not send new flow controlled frames until it receives WINDOW_UPDATE
messages that cause the flow control window to become positive.
For example, if the server sets the initial window size to be 16KB, and the client
sends 64KB immediately on connection establishment, the client will recalculate the
available flow control window to be -48KB on receipt of the SETTINGS frame. The
client retains a negative flow control window until WINDOW_UPDATE frames restore the
window to being positive, after which the client can resume sending.
A receiver that wishes to use a smaller flow control window than the current size
sends a new SETTINGS frame. However, the receiver MUST be prepared to receive data
that exceeds this window size, since the sender might send data that exceeds the lower
limit prior to processing the SETTINGS frame.
A receiver has two options for handling streams that exceed flow control limits:
The receiver can immediately send RST_STREAM with FLOW_CONTROL_ERROR error code
for the affected streams.
The receiver can accept the streams and tolerate the resulting head of line
blocking, sending WINDOW_UPDATE messages as it consumes data.
If a receiver decides to accept streams, both sides must recompute the available flow
control window based on the initial window size sent in the SETTINGS.
After a recipient reads in a frame that marks the end of a stream (for example, a data
stream with a FINAL flag set), it ceases transmission of WINDOW_UPDATE frames. A
sender is not required to maintain the available flow control window for streams that
it is no longer sending on.
Flow control can be disabled for all streams or the session using the
SETTINGS_FLOW_CONTROL_OPTIONS setting. An implementation that does not wish to
perform flow control can use this in the initial SETTINGS exchange.
Flow control can be disabled for an individual stream or the overall session by
sending a WINDOW_UPDATE with the END_FLOW_CONTROL flag set. The payload of a
WINDOW_UPDATE frame that has the END_FLOW_CONTROL flag set is ignored.
Flow control cannot be enabled again once disabled. Any attempt to re-enable flow
control - by sending a WINDOW_UPDATE or by clearing the bits on the
SETTINGS_FLOW_CONTROL_OPTIONS setting - MUST be rejected with a FLOW_CONTROL_ERROR
error code.
The header block is found in the HEADERS, HEADERS+PRIORITY and PUSH_PROMISE frames. The
header block consists of a set of header fields, which are name-value pairs. Headers
are compressed using black magic.
Compression of header fields is a work in progress, as is the format of this block.
HTTP/2.0 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 HTTP/1.1 (, , , , , and ) apply with the changes in the sections
below.
Clients SHOULD NOT open more than one HTTP/2.0 session to a given origin
() concurrently.
Note that it is possible for one HTTP/2.0 session to be finishing (e.g. a GOAWAY message
has been sent, but not all streams have finished), while another HTTP/2.0 session is
starting.
HTTP/2.0 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 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 HTTP/2.0 streams are deterministically 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 HTTP/2.0 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 frames on any stream, the GOAWAY
will contain a last-stream-id of 0.
At the application level, HTTP uses name-value pairs in its header fields. Because
HTTP/2.0 merges the existing HTTP header fields with HTTP/2.0 headers, there is a
possibility that some HTTP applications already use a particular header field name. To
avoid any conflicts, all header fields introduced for layering HTTP over HTTP/2.0 are
prefixed with ":". ":" is not a valid sequence in HTTP/1.* header field naming,
preventing any possible conflict.
The client initiates a request by sending a HEADERS+PRIORITY frame. Requests that do
not contain a body MUST set the FINAL flag, indicating that the client intends to send
no further data on this stream, unless the server intends to push resources (see ). HEADERS+PRIORITY frame does not contain the FINAL flag for
requests that contain a body. The body of a request follows as a series of DATA
frames. The last DATA frame sets the FINAL flag to indicate the end of the body.
The header fields included in the HEADERS+PRIORITY frame contain all of the HTTP header
fields that are associated with an HTTP request. The header block in HTTP/2.0 is mostly
unchanged from today's HTTP header block, with the following differences:
The following fields that are carried in the request line in HTTP/1.1 () are defined as special-valued
name-value pairs:
the HTTP method for this request (e.g. "GET", "POST", "HEAD", etc)
()
":path" - the request-target for this URI with "/" prefixed (see ). For example, for
"http://www.google.com/search?q=dogs" the path would be
"/search?q=dogs". what forms of the HTTPbis request-target are allowed
here?
These header fields MUST be present in HTTP requests.
In addition, the following two name-value pairs MUST be present in every request:
the host and optional port portions (see ) of the URI for this request (e.g. "www.google.com:1234"). This
header field is the same as the HTTP 'Host' header field ().
the scheme portion of the URI for this request (e.g. "https")
All header field names starting with ":" (whether defined in this document or future
extensions to this document) MUST appear before any other header fields.
Header field names MUST be all lowercase.
The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-Encoding header
fields 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. Still valid?
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 field, the server MUST return a 400
(Bad Request) error.
Although POSTs are inherently chunked, POST requests SHOULD also be accompanied by a
Content-Length header field. First, it informs the server of how much data to
expect, which the server can used to track overall progress and provide appropriate
user feedback. More importantly, some HTTP server implementations fail to correctly
process requests that omit the Content-Length header field. Many existing clients
send a Content-Length header field, which caused server implementations have come to
depend upon its presence.
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 HEADERS+PRIORITY frame that omits a mandatory header, the server
MUST reply with a HTTP 400 Bad Request reply. Ed: why PROTOCOL_ERROR on missing
":status" in the response, but HTTP 400 here?
If the server receives a data frame prior to a HEADERS or HEADERS+PRIORITY frame the
server MUST treat this as a stream error of
type PROTOCOL_ERROR.
The server responds to a client request with a HEADERS frame. Symmetric to the client's
upload stream, server will send any response body in a series of DATA frames. The last
data frame will contain the FINAL flag to indicate the end of the stream and the end of
the response. A response that contains no body (such as a 204 or 304 response) consists
only of a HEADERS frame that contains the FINAL 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 header
fields and must be present:
The HTTP response status code (e.g. "200" or "200 OK")
All header field names starting with ":" (whether defined in this document or future
extensions to this document) MUST appear before any other header fields.
All header field names MUST be all lowercase.
The Connection, Keep-Alive, Proxy-Connection, and Transfer-Encoding header fields
are not valid and MUST not be sent.
Responses MAY be accompanied by a Content-Length header field for advisory purposes.
This allows clients to learn the full size of an entity prior to receiving all the
data frames. This can help in, for example, reporting progress.
If a client receives a response where the sum of the data frame payload length does
not equal the size of the Content-Length header field, the client MUST ignore the
content length header field. Ed: See issue 46.
If a client receives a response with an absent or duplicated status header, the client
MUST treat this as a stream error of type
PROTOCOL_ERROR.
If the client receives a data frame prior to a HEADERS or HEADERS+PRIORITY frame the
client MUST treat this as a stream error of
type PROTOCOL_ERROR.
HTTP/2.0 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.
Server push is an optional feature. Server push can be disabled by clients that do not
wish to receive pushed resources by advertising a SETTINGS_MAX_CONCURRENT_STREAMS SETTING of zero. This prevents servers from creating the streams
necessary to push resources.
Browsers receiving a pushed response MUST validate that the server is authorized to push
the resource using the same-origin policy ().
For example, a HTTP/2.0 connection to example.com is generally
Ed: weaselly use of "generally", needs better definition not permitted to
push a response for www.example.org.
A client that accepts pushed resources caches those resources as though they were
responses to GET requests.
Pushed responses are associated with a request at the HTTP/2.0 framing layer. The
PUSH_PROMISE includes a stream identifier for an associated request/response exchange
that supplies request header fields. The pushed stream inherits all of the request
header fields from the associated stream with the exception of resource identification
header fields (:host, :scheme,
and :path), which are provided as part of the PUSH_PROMISE
frame. Pushed resources always have an associated :method of
GET. A cache MUST store these inherited and implied request
header fields 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. Ed: insufficiently
specified to implement; would like to remove
A server pushes resources in association with a request from the client. Prior to
closing the response stream, the server sends a PUSH_PROMISE for each resource that it
intends to push. The PUSH_PROMISE includes header fields that allow the client to
identify the resource (:scheme, :host, and :port).
A server can push multiple resources in response to a request, but these can only be
sent while the response stream remains open. A server MUST NOT send a PUSH_PROMISE on a
half-closed stream.
The server SHOULD include any header fields in a PUSH_PROMISE that would allow a cache
to determine if the resource is already cached (see ).
After sending a PUSH_PROMISE, the server commences transmission of a pushed resource. A
pushed resource uses a server-initiated stream. The server sends frames on this stream
in the same order as an HTTP response: a HEADERS
frame followed by DATA frames.
Many uses of server push are to send content that a client is likely to discover a need
for based on the content of a response representation. To minimize the chances that a
client will make a request for resources that are being pushed - causing duplicate
copies of a resource to be sent by the server - a PUSH_PROMISE frame SHOULD be sent
prior to any content in the response representation that might allow a client to
discover the pushed resource and request it.
The server MUST only push resources that could have been returned from a GET request.
Note: A server does not need to have all response header fields available at the time it
issues a PUSH_PROMISE frame. All remaining header fields are included in the HEADERS
frame. The HEADERS frame MUST NOT duplicate header fields from the PUSH_PROMISE frames.
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 HEADERS+PRIORITY frame that contains an Associated-To-Stream-ID is received, the
client MUST NOTSHOULD NOT? issue GET requests for the resource in the
pushed stream, and instead wait for the pushed stream to arrive.
A server MUST NOT push a resource with an Associated-To-Stream-ID of 0. Clients MUST
treat this as a session error of type
PROTOCOL_ERROR.
When a client receives a PUSH_PROMISE frame from the server without a the :host, :scheme, and :path header fields, it MUST treat this as a stream error of type PROTOCOL_ERROR.
To cancel individual server push streams, the client can issue a stream error of type CANCEL. Upon receipt, the
server ceases transmission of the pushed data.
To cancel all server push streams related to a request, the client may issue a stream error of type 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.
Ed: Triggering side-effects on stream reset is going to be problematic for the
framing layer. Purely from a design perspective, it's a layering violation. More
practically speaking, the base request stream might already be removed. Special
handling logic would be required.
If the server sends a HEADERS frame containing header fields that duplicate values on a
previous HEADERS or PUSH_PROMISE frames on the same stream, the client MUST treat this
as a stream error of type 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 header fields ().
Authors' notes: The notes in this section have no bearing on the HTTP/2.0 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.
Readers may note that this specification sometimes blends the framing layer with requirements of a specific application -
HTTP. This is reflected in the request/response nature of
the streams and the definition of the HEADERS 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 HTTP/2.0 framing layer is a non goal.
Error handling at the HTTP/2.0 layer splits errors into two groups: Those that affect an
individual HTTP/2.0 stream, and those that do not.
When an error is confined to a single stream, but general framing is in tact, HTTP/2.0
attempts to use the RST_STREAM as a mechanism to invalidate the stream but move forward
without aborting the connection altogether.
For errors occurring outside of a single stream context, HTTP/2.0 assumes the entire
session is hosed. In this case, the endpoint detecting the error should initiate a
connection close.
HTTP/2.0 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 HTTP/2.0 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 HTTP/2.0 reduces this load.
The use of multiple connections is not without benefit, however. Because HTTP/2.0
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.
HTTP/2.0 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.
HTTP/2.0 chooses the simple encoding for speed and simplicity.
The goal of HTTP/2.0 is to reduce latency on the network. The overhead of HTTP/2.0 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 HTTP/2.0 and TCP interactions, we believe this is completely mitigated.
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.
This specification uses the same-origin policy () in all cases where verification of content is required.
By utilizing TLS, we believe that HTTP/2.0 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. Issue: This is no longer true
Pushed resources do not have an associated request. In order for existing HTTP cache
control validations (such as the Vary header field) to work, all cached resources must
have a set of request header fields. For this reason, caches MUST be careful to inherit
request header fields from the associated stream for the push. This includes the Cookie
header field.
Caching resources that are pushed is possible, based on the guidance provided by the
origin server in the Cache-Control header field. However, this can cause issues if a
single server hosts more than one tenant. For example, a server might offer multiple
users each a small portion of its URI space.
Where multiple tenants share space on the same server, that server MUST ensure that
tenants are not able to push representations of resources that they do not have authority
over. Failure to enforce this would allow a tenant to provide a representation that would
be served out of cache, overriding the actual representation that the authoritative tenant
provides.
Pushed resources for which an origin server is not authoritative are never cached or used.
HTTP/2.0 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.
The HTTP/2.0 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 can disable client-persisted SETTINGS storage.
Clients MUST clear persisted SETTINGS information when clearing the cookies.
This document establishes registries for frame types, error codes and settings.
This document establishes a registry for HTTP/2.0 frame types. The "HTTP/2.0 Frame Type"
registry operates under the "IETF Review" policy.
Frame types are an 8-bit value. When reviewing new frame type registrations, special
attention is advised for any frame type-specific flags that are defined. Frame flags can
interact with existing flags and could prevent the creation of globally applicable flags.
Initial values for the "HTTP/2.0 Frame Type" registry are shown in .
Frame TypeNameFlags0DATA-1HEADERS+PRIORITY-3RST_STREAM-4SETTINGSCLEAR_PERSISTED(2)5PUSH_PROMISE-6PINGPONG(2)7GOAWAY-8HEADERS-9WINDOW_UPDATEEND_FLOW_CONTROL(2)
This document establishes a registry for HTTP/2.0 error codes. The "HTTP/2.0 Error Code"
registry manages a 32-bit space. The "HTTP/2.0 Error Code" registry operates under the
"Expert Review" policy.
Registrations for error codes are required to include a description of the error code. An
expert reviewer is advised to examine new registrations for possible duplication with
existing error codes. Use of existing registrations is to be encouraged, but not
mandated.
New registrations are advised to provide the following information:
The 32-bit error code value.
A name for the error code. Specifying an error code name is optional.
A description of the conditions where the error code is applicable.
An optional reference for a specification that defines the error code.
An initial set of error code registrations can be found in .
This document establishes a registry for HTTP/2.0 settings. The "HTTP/2.0 Settings"
registry manages a 24-bit space. The "HTTP/2.0 Settings" registry operates under the
"Expert Review" policy.
Registrations for settings are required to include a description of the setting. An
expert reviewer is advised to examine new registrations for possible duplication with
existing settings. Use of existing registrations is to be encouraged, but not mandated.
New registrations are advised to provide the following information:
The 24-bit setting value.
A name for the setting. Specifying a name is optional.
Any setting-specific flags that apply, including their value and semantics.
A description of the setting. This might include the range of values, any applicable
units and how to act upon a value when it is provided.
An optional reference for a specification that defines the setting.
An initial set of settings registrations can be found in .
This document includes substantial input from the following individuals:
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 (SPDY
contributors).
Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)
William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto
Peon, Rob Trace (Flow control)
Mark Nottingham and Julian Reschke
Transmission Control Protocol
University of Southern California (USC)/Information Sciences
Institute
Key words for use in RFCs to Indicate Requirement Levels
Harvard Universitysob@harvard.eduUniform Resource Identifier (URI): Generic
SyntaxGuidelines for Writing an IANA Considerations Section in RFCsThe Transport Layer Security (TLS) Protocol Version 1.2The Web Origin ConceptTransport Layer Security (TLS) Next Protocol Negotiation Extension
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
Hypertext Transfer Protocol (HTTP/1.1): Semantics and ContentHypertext Transfer Protocol (HTTP/1.1): Conditional RequestsAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Range RequestsAdobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): CachingAdobe Systems Incorporatedfielding@gbiv.comAkamaimnot@mnot.netgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): AuthenticationAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.de
TCP Extensions for High Performance
Talking to Yourself for Fun and Profit
Added IANA considerations section for frame types, error codes and settings.
Removed data frame compression.
Added PUSH_PROMISE.
Added globally applicable flags to framing.
Removed zlib-based header compression mechanism.
Updated references.
Clarified stream identifier reuse.
Removed CREDENTIALS frame and associated mechanisms.
Added advice against naive implementation of flow control.
Added session header section.
Restructured frame header. Removed distinction between data and control frames.
Altered flow control properties to include session-level limits.
Added note on cacheability of pushed resources and multiple tenant servers.
Changed protocol label form based on discussions.
Changed title throughout.
Removed section on Incompatibilities with SPDY draft#2.
Changed INTERNAL_ERROR on GOAWAY to have a value of 2 .
Replaced abstract and introduction.
Added section on starting HTTP/2.0, including upgrade mechanism.
Removed unused references.
Added flow control principles based on .
Adopted as base for draft-ietf-httpbis-http2.
Updated authors/editors list.
Added status note.