One document matched: draft-ietf-httpbis-http2-03.xml
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<rfc ipr="trust200902"
category="std"
docName="draft-ietf-httpbis-http2-03"
x:maturity-level="proposed"
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<front>
<title abbrev="HTTP/2.0">Hypertext Transfer Protocol version 2.0</title>
<author initials="M." surname="Belshe" fullname="Mike Belshe">
<organization>Twist</organization>
<address>
<email>mbelshe@chromium.org</email>
</address>
</author>
<author initials="R." surname="Peon" fullname="Roberto Peon">
<organization>Google, Inc</organization>
<address>
<email>fenix@google.com</email>
</address>
</author>
<author initials="M." surname="Thomson" fullname="Martin Thomson" role="editor">
<organization>Microsoft</organization>
<address>
<postal>
<street>3210 Porter Drive</street>
<city>Palo Alto</city>
<code>94304</code>
<country>US</country>
</postal>
<email>martin.thomson@skype.net</email>
</address>
</author>
<author initials="A." surname="Melnikov" fullname="Alexey Melnikov" role="editor">
<organization>Isode Ltd</organization>
<address>
<postal>
<street>5 Castle Business Village</street>
<street>36 Station Road</street>
<city>Hampton</city>
<region>Middlesex</region>
<code>TW12 2BX</code>
<country>UK</country>
</postal>
<email>Alexey.Melnikov@isode.com</email>
</address>
</author>
<date year="2013" />
<area>Applications</area>
<workgroup>HTTPbis Working Group</workgroup>
<keyword>HTTP</keyword>
<keyword>SPDY</keyword>
<keyword>Web</keyword>
<abstract>
<t>
This specification describes an optimized expression of the syntax of the Hypertext
Transfer Protocol (HTTP). The HTTP/2.0 encapsulation enables more efficient use of
network resources and reduced perception of latency by allowing header field
compression and multiple concurrent messages on the same connection. It also
introduces unsolicited push of representations from servers to clients.
</t>
<t>
This document is an alternative to, but does not obsolete the HTTP/1.1 message format
or protocol. HTTP's existing semantics remain unchanged.
</t>
</abstract>
<note title="Editorial Note (To be removed by RFC Editor)">
<t>
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
<eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
</t>
<t>
Working Group information and related documents can be found
at <eref target="http://tools.ietf.org/wg/httpbis/"/> (Wiki) and
<eref target="https://github.com/http2/http2-spec"/> (source code
and issues tracker).
</t>
<t>
The changes in this draft are summarized in <xref
target="changes.since.draft-ietf-httpbis-http2-02"/>.
</t>
</note>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>
The Hypertext Transfer Protocol (HTTP) is a wildly successful protocol. However, the HTTP/1.1 message
encapsulation (<xref target="HTTP-p1" x:fmt="," x:rel="#http.message"/>) 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.
</t>
<t>
In particular, HTTP/1.0 only allows one request to be delivered at a time on a given
connection. HTTP/1.1 pipelining only partially addressed request concurrency, and is not
widely deployed. Therefore, clients that need to make many requests (as is common on the
Web) typically use multiple connections to a server in order to reduce perceived latency.
</t>
<t>
Furthermore, HTTP/1.1 header fields are often repetitive and verbose, which, in
addition to generating more or larger network packets, can cause the small initial TCP
congestion window to quickly fill. This can result in excessive latency when multiple
requests are made on a single new TCP connection.
</t>
<t>
This document addresses these issues by defining an optimized mapping of HTTP's semantics to
an underlying connection. Specifically, it allows interleaving of request and response
messages on the same connection and uses an efficient coding for HTTP header fields. It
also allows prioritization of requests, letting more important requests complete more
quickly, further improving perceived performance.
</t>
<t>
The resulting protocol is designed to have be more friendly to the network, because fewer
TCP connections can be used, in comparison to HTTP/1.x. This means less competition with
other flows, and longer-lived connections, which in turn leads to better utilization of
available network capacity.
</t>
<t>
Finally, this encapsulation also enables more scalable processing of messages through use of
binary message framing.
</t>
<section title="Document Organization">
<t>
The HTTP/2.0 Specification is split into three parts: <xref target="starting">starting
HTTP/2.0</xref>, which covers how a HTTP/2.0 connection is initiated; <xref target="FramingLayer">a
framing layer</xref>, which multiplexes a single TCP connection into independent
frames of various types; and <xref target="HTTPLayer">an HTTP layer</xref>, which specifies the mechanism
for expressing HTTP interactions using the framing layer. While some of the
framing layer concepts are isolated from HTTP, 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.
</t>
</section>
<section title="Conventions and Terminology">
<t>
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 <xref target="RFC2119">RFC 2119</xref>.
</t>
<t>
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 <spanx style="verb">0x</spanx> to distinguish them
from decimal literals.
</t>
<t>
The following terms are used:
<list style="hanging">
<t hangText="client:">
The endpoint initiating the HTTP connection.
</t>
<t hangText="connection:">
A transport-level connection between two endpoints.
</t>
<t hangText="endpoint:">
Either the client or server of the connection.
</t>
<t hangText="frame:">
The smallest unit of communication within an HTTP/2.0 connection,
consisting of a header and a variable-length sequence of bytes
structured according to the frame type.
</t>
<t hangText="peer:">
An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint
that is remote to the primary subject of discussion.
</t>
<t hangText="receiver:">
An endpoint that is receiving frames.
</t>
<t hangText="sender:">
An endpoint that is transmitting frames.
</t>
<t hangText="server:">
The endpoint which did not initiate the HTTP connection.
</t>
<t hangText="connection error:">
An error on the HTTP/2.0 connection.
</t>
<t hangText="stream:">
A bi-directional flow of frames across a virtual channel within
the HTTP/2.0 connection.
</t>
<t hangText="stream error:">
An error on the individual HTTP/2.0 stream.
</t>
</list>
</t>
</section>
</section>
<section anchor="starting" title="Starting HTTP/2.0">
<t>HTTP/2.0 uses the same "http:" and "https:" URI schemes used by
HTTP/1.1. As a result, implementations processing requests for
target resource URIs like "http://example.org/foo" or
"https://example.com/bar" are required to first discover whether
the upstream server (the immediate peer to which the client wishes
to establish a connection) supports HTTP/2.0.</t>
<t>The means by which support for HTTP/2.0 is determined is different
for "http" and "https" URIs. Discovery for "https:" URIs is
described in <xref target="discover-https"/>. Discovery for
"http" URIs is described here.</t>
<section anchor="versioning" title="HTTP/2.0 Version Identification">
<t>
The protocol defined in this document is identified using the string "HTTP/2.0". This
identification is used in the HTTP/1.1 Upgrade header field, in the <xref
target="TLSALPN">TLS application layer protocol negotiation extension</xref> field and
other places where protocol identification is required.
</t>
<t>
Negotiating "HTTP/2.0" implies the use of the transport, security, framing and message
semantics described in this document.
</t>
<t>
<cref>Editor's Note: please remove the following text prior to the publication of a final
version of this document.</cref>
</t>
<t>
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".
</t>
<t>
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.
</t>
<t>
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".
</t>
<t>
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 to the "token" syntax defined in
<xref target="HTTP-p1" x:fmt="of" x:rel="#field.components"/>. Experimenters are
encouraged to coordinate their experiments on the ietf-http-wg@w3.org mailing list.
</t>
</section>
<section anchor="discover-http" title="Starting HTTP/2.0 for "http:" URIs">
<t>
A client that makes a request to an "http:" URI without prior knowledge about support for
HTTP/2.0 uses the HTTP Upgrade mechanism (<xref target="HTTP-p1" x:fmt="of"
x:rel="#header.upgrade"/>). The client makes an HTTP/1.1 request that includes an Upgrade
header field identifying HTTP/2.0.
</t>
<t>
For example:
</t>
<figure>
<artwork type="message/http; msgtype="request"" x:indent-with=" ">
GET /default.htm HTTP/1.1
Host: server.example.com
Connection: Upgrade
Upgrade: HTTP/2.0
</artwork>
</figure>
<t>
A server that does not support HTTP/2.0 can respond to the request as though the Upgrade
header field were absent:
</t>
<figure>
<artwork type="message/http; msgtype="responset"" x:indent-with=" ">
HTTP/1.1 200 OK
Content-length: 243
Content-type: text/html
...
</artwork>
</figure>
<t>
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.
</t>
<figure>
<artwork type="message/http; msgtype="response"" x:indent-with=" ">
HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: HTTP/2.0
[ HTTP/2.0 connection ...
</artwork>
</figure>
<t>
The first HTTP/2.0 frame sent by the server is a <xref target="SETTINGS">SETTINGS
frame</xref>. Upon receiving the 101 response, the client sends a <xref
target="ConnectionHeader">connection header</xref>, which includes a SETTINGS frame.
</t>
</section>
<section anchor="discover-https" title="Starting HTTP/2.0 for "https:" URIs">
<t>
A client that makes a request to an "https:" URI without prior knowledge about support for
HTTP/2.0 uses <xref target="RFC5246">TLS</xref> with the <xref
target="TLSALPN">application layer protocol negotiation extension</xref>.
</t>
<t>
Once TLS negotiation is complete, both the client and the server send a <xref
target="ConnectionHeader">connection header</xref>.
</t>
</section>
<section anchor="known-http" title="Starting HTTP/2.0 with Prior Knowledge">
<t>
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 <xref target="TLSALPN">protocol negotiation in TLS</xref> for "https:" URIs.
</t>
<t>
Prior support for HTTP/2.0 is not a strong signal that a given server will support
HTTP/2.0 for future connections. It is possible for server configurations to change or for
configurations to differ between instances in clustered server. Interception proxies
(a.k.a. "transparent" proxies) are another source of variability.
</t>
</section>
</section>
<section anchor="FramingLayer" title="HTTP/2.0 Framing Layer">
<section title="Connection">
<t>
The HTTP/2.0 connection is an Application Level protocol running on top
of a TCP connection (<xref target="RFC0793"/>). The client is the TCP
connection initiator.
</t>
<t>HTTP/2.0 connections are persistent. That is, for best performance,
it is expected a clients will not close connections until it is
determined that no further communication with a server is necessary
(for example, when a user navigates away from a particular web page),
or until the server closes the connection.</t>
<t>Servers are encouraged to maintain open connections for as long as
possible, but are permitted to terminate idle connections if
necessary. When either endpoint chooses to close the transport-level
TCP connection, the terminating endpoint MUST first send a
<xref target="GOAWAY">GOAWAY</xref> frame so that both endpoints can
reliably determine whether previously sent frames have been processed
and gracefully complete or terminate any necessary remaining tasks.
</t>
</section>
<section anchor="ConnectionHeader" title="Connection Header">
<t>Upon establishment of a TCP connection and determination
that HTTP/2.0 will be used by both peers to communicate, each
endpoint MUST send a connection header as a final confirmation
and to establish the default parameters for the HTTP/2.0 connection.</t>
<t>
The client connection header is a sequence of 24 octets (in hex notation)
</t>
<figure><artwork type="inline">
464f4f202a20485454502f322e300d0a0d0a42410d0a0d0a</artwork></figure>
<t>
(the string <spanx style="verb">FOO * HTTP/2.0\r\n\r\nBA\r\n\r\n</spanx>) followed by a
<xref target="SETTINGS">SETTINGS frame</xref>. The client sends the client connection header
immediately upon receipt of a 101 Switching Protocols response (indicating a successful
upgrade), or after receiving a TLS Finished message from the server. If starting an
HTTP/2.0 connection with prior knowledge of server support for the protocol, the client
connection header is sent upon connection establishment.
</t>
<t>
<list>
<t>
The client connection 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. Note
that this does not address the concerns raised in <xref target="TALKING"/>.
</t>
</list>
</t>
<t>
The server connection header consists of just
a <xref target="SETTINGS">SETTINGS frame</xref> that MUST be the
first frame the server sends in the HTTP/2.0 connection.
</t>
<t>
To avoid unnecessary latency, clients are permitted to send
additional frames to the server immediately after sending the client
connection header, without waiting to receive the server connection header.
It is important to note, however, that the server connection header
SETTINGS frame might include parameters that necessarily alter
how a client is expected to communicate with the server. Upon
receiving the SETTINGS frame, the client is expected to honor any
parameters established.
</t>
<t>
Clients and servers MUST terminate the TCP connection if either
peer does not begin with a valid connection header. A
<xref target="GOAWAY">GOAWAY frame</xref> MAY be omitted if it is
clear that the peer is not using HTTP/2.0.
</t>
</section>
<section title="Framing">
<t>Once the HTTP/2.0 connection is established, clients and servers
can begin exchanging frames.</t>
<section anchor="FrameHeader" title="Frame Header">
<t>
HTTP/2.0 frames share a common base format consisting of
an 8-byte header followed by 0 to 65535 bytes of data.
</t>
<figure title="Frame Header">
<artwork type="inline">
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...) ...
+---------------------------------------------------------------+
</artwork>
</figure>
<t>
The fields of the frame header are defined as:
<list style="hanging">
<t hangText="Length:">
The length of the frame data expressed as an unsigned 16-bit
integer. The 8 bytes of the frame header are not included
in this value.
</t>
<t hangText="Type:">
The 8-bit type of the frame. The frame type determines how the remainder of the
frame header and data are interpreted. Implementations MUST ignore unsupported
and unrecognized frame types.
</t>
<t hangText="Flags:">
An 8-bit field reserved for frame-type specific boolean flags.
<vspace blankLines="1"/>
The least significant bit (0x1) - the FINAL bit - is defined for all frame types as
an indication that this frame is the last the endpoint will send for the identified
stream. Setting this flag causes the stream to enter the <xref
target="StreamHalfClose">half-closed state</xref>. Implementations MUST process the
FINAL bit for all frames whose stream identifier field is not 0x0. The FINAL bit
MUST NOT be set on frames that use a stream identifier of 0.
<vspace blankLines="1"/>
The remaining flags can be assigned semantics specific to the
indicated frame type. Flags that have no defined semantics for
a particular frame type MUST be ignored, and MUST be left
unset (0) when sending.
</t>
<t hangText="R:">
A reserved 1-bit field. The semantics of this bit are undefined
and the bit MUST remain unset (0) when sending and MUST be ignored
when receiving.
</t>
<t hangText="Stream Identifier:">
A 31-bit stream identifier (see <xref target="StreamCreation"/>). A value 0 is
reserved for frames that are associated with the connection as a
whole as opposed to an individual stream.
</t>
</list>
</t>
<t>The structure and content of the remaining frame data is
dependent entirely on the frame type.</t>
</section>
<section anchor="FrameSize" title="Frame Size">
<t>Implementations with limited resources might not be capable of
processing large frame sizes. Such implementations MAY choose
to place additional limits on the maximum frame size. However,
all implementations MUST be capable of receiving and processing
frames containing at least 8192 octets of data.
<cref>Ed. Question: Does this minimum include the 8-byte header
or just the frame data?</cref></t>
<t>An implementation MUST terminate a stream immediately if it is
unable to process a frame due it's size. This is done by sending
an <xref target="RST_STREAM">RST_STREAM frame</xref> containing
the FRAME_TOO_LARGE error code.
</t>
<t>
<cref><eref target="https://github.com/http2/http2-spec/issues/28">Issue 28</eref>: Need
a way to signal the maximum frame size; no way to RST_STREAM on non-stream-related
frames.</cref>
</t>
</section>
</section>
<section title="Streams">
<t>
A "stream" is an independent, bi-directional sequence of frames
exchanged between the client and server within an HTTP/2.0 connection.
Streams have several important characteristics:
<list style="symbols">
<t>
Streams can be established and used unilaterally or shared by
either the client or server.
</t>
<t>
Streams can be rejected or cancelled by either endpoint.
</t>
<t>
Multiple types of frames can be sent by either endpoint within
a single stream.
</t>
<t>
The order in which frames are sent within a stream is
significant. Recipients are required to process frames
in the order they are received.
</t>
<t>
Streams optionally carry a set of name-value header pairs
that are expressed within the headers block of HEADERS+PRIORITY,
HEADERS, or PUSH_PROMISE frames.
</t>
<t>
A single HTTP/2.0 connection can contain multiple concurrently
active streams, with either endpoint interleaving frames from
multiple streams.
</t>
</list>
</t>
<section anchor="StreamCreation" title="Stream Creation">
<t>
There is no coordination or shared action between the client and
server required to create a stream. Rather, new streams are
established by sending a frame whose stream identifier field
references a previously unused stream identifier.
</t>
<t>
All streams are identified by an unsigned 31-bit integer. Streams
initiated by a client use odd numbered stream identifiers; those
initiated by the server use even numbered stream identifiers.
A stream identifier of zero MUST NOT be used to establish a new
stream.
</t>
<t>
The identifier of a newly established stream MUST be numerically
greater than all previously established streams from that endpoint
within the HTTP/2.0 connection, unless the identifier has been reserved
using a <xref target="PUSH_PROMISE">PUSH_PROMISE</xref> frame.
An endpoint that receives an unexpected stream identifier MUST
respond with a <xref target="ConnectionErrorHandler">connection error</xref> of type
PROTOCOL_ERROR.
</t>
<t>
A peer can limit the total number of concurrently active streams
using the SETTINGS_MAX_CONCURRENT_STREAMS parameters within a
SETTINGS frame. The maximum concurrent streams setting is specific
to each endpoint and applies only to the peer. That is, clients
specify the maximum number of concurrent streams the server can
initiate, and servers specify the maximum number of concurrent
streams the client can initiate. Peer endpoints MUST NOT
exceed this limit. All concurrently active
streams initiated by an endpoint, including streams that are
<xref target="StreamHalfClose">half-open</xref> in any direction,
count toward that endpoint's limit.
</t>
<t>
Stream identifiers cannot be reused within a connection. Long-lived
connections can cause an endpoint to exhaust the available range of stream
identifiers. A client that is unable to establish a new stream identifier can
establish a new connection for new streams.
</t>
<t>
Either endpoint can request the early termination of an unwanted
stream by sending an <xref target="StreamErrorHandler">RST_STREAM
frame</xref> with an error code of either REFUSED_STREAM (if no
frames have been processed) or CANCEL (if at least one frame has
been processed). Such termination might not take effect immediately
as the peer might have sent additional frames on the stream prior
to receiving the termination request.
</t>
</section>
<section anchor="StreamPriority" title="Stream priority">
<t>
The endpoint establishing a new stream can assign a priority for
the stream. Priority is represented as an unsigned 31-bit integer.
0 represents the highest priority and 2<x:sup>31</x:sup>-1 represents
the lowest priority.
</t>
<t>
The purpose of this value is to allow the initiating
endpoint to request that frames for the stream be processed with
higher priority relative to any other concurrently active
streams. That is, if an endpoint receives interleaved frames
for multiple streams, the endpoint ought to make a best-effort
attempt at processing frames for higher priority streams before
processing those for lower priority streams.
</t>
<t>
Explicitly setting the priority for a stream does not
guarantee any particular processing order for the stream
relative to any other stream. Nor is there is any mechanism
provided by which the initiator of a stream can force or require
a receiving endpoint to process frames from one stream before
processing frames from another.
</t>
</section>
<section anchor="StreamHalfClose" title="Stream half-close">
<t>
When an endpoint sends a frame for a stream with the FINAL flag
set, the stream is considered to be half-closed for that endpoint.
Subsequent frames MUST NOT be sent by that endpoint for the half
closed stream for the remaining duration of the HTTP/2.0 connection.
When both endpoints have sent frames with the FINAL flag set, the
stream is considered to be fully closed.
</t>
<t>
If an endpoint receives additional frames for a stream that was
previously half-closed by the sending peer, the recipient MUST
respond with a <xref target="StreamErrorHandler">stream error</xref>
of type STREAM_CLOSED.
</t>
<t>
An endpoint that has not yet half-closed a stream by sending the
FINAL flag can continue sending frames on the stream.
</t>
<t>
It is not necessary for an endpoint to half-close a stream
for which it has not sent any frames. This allows endpoints to
use fully unidirectional streams that do not require explicit
action or acknowledgement from the receiver.
</t>
</section>
<section anchor="StreamClose" title="Stream close">
<t>
Streams can be terminated in the following ways:
<list style="hanging">
<t hangText="Normal termination:">
Normal stream termination occurs when both client and server have half-closed the
stream by sending a frame containing a <xref target="FrameHeader">FINAL flag</xref>.
</t>
<t hangText="Half-close on unidirectional stream:">
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.
</t>
<t hangText="Abrupt termination:">
Either 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 advised to cease transmission on it.
<vspace blankLines="1"/>
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
connection state, these frames cannot be safely discarded. See <xref
target="StreamErrorHandler">Stream Error Handling</xref> for more details.
</t>
<t hangText="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.
</t>
</list>
</t>
</section>
</section>
<section title="Error Handling">
<t>
HTTP/2.0 framing permits two classes of error:
<list style="symbols">
<t>
An error condition that renders the entire connection unusable is a connection error.
</t>
<t>
An error in an individual stream is a stream error.
</t>
</list>
</t>
<section anchor="ConnectionErrorHandler" title="Connection Error Handling">
<t>
A connection error is any error which prevents further processing of the framing layer or
which corrupts any connection state.
</t>
<t>
An endpoint that encounters a connection error MUST first send a <xref
target="GOAWAY">GOAWAY</xref> 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 connection is terminating. After sending the GOAWAY frame, the endpoint
MUST close the TCP connection.
</t>
<t>
It is possible that the GOAWAY will not be reliably received by the receiving endpoint.
In the event of a connection error, GOAWAY only provides a best-effort attempt to
communicate with the peer about why the connection is being terminated.
</t>
<t>
An endpoint can end a connection at any time. In particular, an endpoint MAY choose to
treat a stream error as a connection error if the error is recurrent. Endpoints SHOULD
send a GOAWAY frame when ending a connection, as long as circumstances permit it.
</t>
</section>
<section anchor="StreamErrorHandler" title="Stream Error Handling">
<t>
A stream error is an error related to a specific stream identifier that does not affect
processing of other streams at the framing layer.
</t>
<t>
An endpoint that detects a stream error sends a <xref
target="RST_STREAM">RST_STREAM</xref> 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.
</t>
<t>
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 connection state (such as the state maintained for <xref
target="HeaderBlock">header compression</xref>).
</t>
<t>
Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. However, an endpoint
MAY send additional RST_STREAM frames if it receives frames on a closed stream after
more than a round trip time. This behavior is permitted to deal with misbehaving
implementations.
</t>
<t>
An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM frame, to avoid looping.
</t>
</section>
<section anchor="ErrorCodes" title="Error Codes">
<t>
Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey
the reasons for the stream or connection error.
</t>
<t>
Error codes share a common code space. Some error codes only apply to specific
conditions and have no defined semantics in certain frame types.
</t>
<t>
The following error codes are defined:
<list style="hanging">
<t hangText="NO_ERROR (0):">
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 connection.
</t>
<t hangText="PROTOCOL_ERROR (1):">
The endpoint detected an unspecific protocol error. This error is for use when a more
specific error code is not available.
</t>
<t hangText="INTERNAL_ERROR (2):">
The endpoint encountered an unexpected internal error.
</t>
<t hangText="FLOW_CONTROL_ERROR (3):">
The endpoint detected that its peer violated the flow control protocol.
</t>
<t hangText="INVALID_STREAM (4):">
The endpoint received a frame for an inactive stream.
</t>
<t hangText="STREAM_CLOSED (5):">
The endpoint received a frame after a stream was half-closed.
</t>
<t hangText="FRAME_TOO_LARGE (6):">
The endpoint received a frame that was larger than the maximum size that it
supports.
</t>
<t hangText="REFUSED_STREAM (7):">
The endpoint is refusing the stream before processing its payload.
</t>
<t hangText="CANCEL (8):">
Used by the creator of a stream to indicate that the stream is no longer needed.
</t>
<t hangText="COMPRESSION_ERROR (9):">
The endpoint is unable to maintain the compression context for the connection.
</t>
</list>
</t>
</section>
</section>
<section anchor="flowcontrol" title="Stream Flow Control">
<t>
Using streams for multiplexing introduces contention over use of the TCP connection,
resulting in blocked streams. A flow control
scheme ensures that streams on the same connection do not destructively interfere with each other.
</t>
<t>
HTTP/2.0 provides for flow control through use of the
<xref target="WINDOW_UPDATE">WINDOW_UPDATE</xref> frame type.
</t>
<section anchor="fc-principles" title="Flow Control Principles">
<t>
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.
</t>
<t>
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:
<list style="numbers">
<t>
Flow control is hop-by-hop, not end-to-end.
</t>
<t>
Flow control is based on window update frames. Receivers advertise how many
octets they are prepared to receive on a stream. This is a credit-based scheme.
</t>
<t>
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.
</t>
<t>
The initial value for the flow control window is 65536 bytes for both new streams
and the overall connection.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
HTTP/2.0 standardizes only the format of the <xref target="WINDOW_UPDATE">window
update frame</xref>. This does not stipulate how a receiver decides when to send
this frame 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.
</t>
</list>
</t>
<t>
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.
</t>
</section>
<section title="Appropriate Use of Flow Control">
<t>
Flow control is defined to protect endpoints (client, server or intermediary) that are
operating under resource constraints. For example, a proxy needs to share memory between many
connections, and also might have a slow upstream connection and a fast downstream one.
Flow control addresses cases where the receiver is unable process data on
one stream, yet wants to continue to process other streams in the same connection.
</t>
<t>
Deployments that do not require 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.
</t>
<t>
Deployments with constrained resources (for example, memory) MAY employ flow control to
limit the amount of memory a peer can consume. Note, however, that this can lead to suboptimal use of
available network resources if flow control is enabled without knowledge of the
bandwidth-delay product (see <xref target="RFC1323"/>).
</t>
<t>
Even with full awareness of the current bandwidth-delay product, implementation of flow control
is difficult. However, it can ensure that constrained resources are protected without any
reduction in connection utilization.
</t>
</section>
</section>
<section anchor="HeaderBlock" title="Header Blocks">
<t>
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.
</t>
<t>
Compression of header fields is a work in progress, as is the format of this block.
</t>
<t>
The contents of header blocks MUST be processed by the compression context, even if stream
has been reset or the frame is discarded. If header blocks cannot be processed, the
receiver MUST treat the connection with a <xref target="ConnectionErrorHandler">connection
error</xref> of type COMPRESSION_ERROR.
</t>
</section>
<section title="Frame Types" anchor="frame-types">
<t>
This specification defines a number of frame types,
each identified by a unique 8-bit type code. Each frame
type serves a distinct purpose either in the establishment and
management of the connection as a whole, or of individual streams.
</t>
<t>
The transmission of specific frame types can alter the state of
a connection. If endpoints fail to maintain a synchronized view
of the connection state, successful communication within the connection
will no longer be possible. Therefore, it is important that endpoints
have a shared comprehension of how the state is affected by the use
any given frame. Accordingly, while it is expected that new frame
types will be introduced by extensions to this protocol, only frames
defined by this document are permitted to alter the connection state.
</t>
<section anchor="DataFrames" title="DATA Frames">
<t>
DATA frames (type=0x0) convey arbitrary, variable-length
sequences of octets associated with a stream. One or more
DATA frames are used, for instance, to carry HTTP request
or response payloads.
</t>
<t>
The DATA frame does not define any type-specific flags.
</t>
<t>
DATA frames MUST be associated with a stream. If a DATA frame is
received whose stream identifier field is 0x0, the recipient MUST
respond with a <xref target="ConnectionErrorHandler">connection error</xref>
of type PROTOCOL_ERROR.
</t>
</section>
<section anchor="HEADERS_PRIORITY" title="HEADERS+PRIORITY">
<t>
The HEADERS+PRIORITY frame (type=0x1) allows the sender to
set header fields and stream priority at the same time.
</t>
<figure title="HEADERS+PRIORITY Frame Payload">
<artwork type="inline">
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 (*) ...
+---------------------------------------------------------------+
</artwork>
</figure>
<t>
The HEADERS+PRIORITY frame is identical to the <xref target="HEADERS">HEADERS
frame</xref>, preceded by a single reserved bit and a 31-bit priority; see
<xref target="StreamPriority"/>.
</t>
<t>
HEADERS+PRIORITY uses the same flags as the HEADERS frame, except that a
HEADERS+PRIORITY frame with a CONTINUES bit MUST be followed by another HEADERS+PRIORITY
frame. See <xref target="HEADERS">HEADERS frame</xref> for any flags.
</t>
<t>
HEADERS+PRIORITY frames MUST be associated with a stream. If a
HEADERS+PRIORITY frame is received whose stream
identifier field is 0x0, the recipient MUST respond with a
<xref target="ConnectionErrorHandler">connection error</xref> of type
PROTOCOL_ERROR.
</t>
<t>
The HEADERS+PRIORITY frame modifies the connection state as
defined in <xref target="HeaderBlock" />.
</t>
</section>
<section anchor="RST_STREAM" title="RST_STREAM">
<t>
The RST_STREAM frame (type=0x3) allows for abnormal termination of a stream. When sent by
the initiator of a stream, it indicates that they wish to cancel the stream. When
sent by the receiver of a stream, it indicates that either the
receiver is rejecting the stream, requesting that the stream be
cancelled or that an error condition has occurred.
</t>
<figure title="RST_STREAM Frame Payload">
<artwork type="inline">
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) |
+---------------------------------------------------------------+
</artwork>
</figure>
<t>
The RST_STREAM frame contains a single unsigned,
32-bit integer identifying the <xref target="ErrorCodes">error code</xref>.
The error code indicates why the stream is being terminated.
</t>
<t>
No type-flags are defined.
</t>
<t>
The RST_STREAM frame fully terminates the referenced stream
and causes it to enter the closed state. After receiving a
RST_STREAM on a stream, the receiver MUST NOT send additional
frames for that stream. However, after sending the RST_STREAM,
the sending endpoint MUST be prepared to receive and process
additional frames sent on the stream that might have been sent
by the peer prior to the arrival of the RST_STREAM.
</t>
<t>
RST_STREAM frames MUST be associated with a stream. If a
RST_STREAM frame is received whose stream
identifier field is 0x0 the recipient MUST respond with a
<xref target="ConnectionErrorHandler">connection error</xref> of type
PROTOCOL_ERROR.
</t>
</section>
<section anchor="SETTINGS" title="SETTINGS">
<t>
The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints
communicate. The parameters are either constraints on peer behavior or preferences.
</t>
<t>
SETTINGS frames MUST be sent at the start of a connection, and MAY
be sent at any other time by either endpoint over the lifetime
of the connection.
</t>
<t>
Implementations MUST support all of the settings defined by this
specification and MAY support additional settings defined by
extensions. Unsupported or unrecognized settings MUST be ignored.
New settings MUST NOT be defined or implemented in a way that
requires endpoints to understand then in order to communicate
successfully.
</t>
<t>
A SETTINGS frame is not required to include every defined
setting; senders can include only those parameters for which it
has accurate values and a need to convey. When multiple parameters
are sent, they SHOULD be sent in order of numerically lowest ID to
highest ID. A single SETTINGS frame MUST NOT contain multiple values for the same ID. If the
receiver of a SETTINGS frame discovers multiple values for the same ID, it MUST ignore
all values for that ID except the first one.
</t>
<t>
Over the lifetime of a connection, an endpoint MAY send multiple SETTINGS frames
containing previously unspecified parameters or new values for parameters whose values
have already been established. Only the most recent value provided setting value
applies.
</t>
<t>
The SETTINGS frame defines the following flag:
<list style="hanging">
<t hangText="CLEAR_PERSISTED (0x2):">
Bit 2 being set indicates a request to clear any previously persisted settings
before processing the settings. Clients MUST NOT set this flag.
</t>
</list>
</t>
<t>
SETTINGS frames always apply to a connection, never a single stream.
The stream identifier for a settings frame MUST be zero. If an
endpoint receives a SETTINGS frame whose stream identifier field
is anything other than 0x0, the endpoint MUST respond with a
<xref target="ConnectionErrorHandler">connection error</xref> of type
PROTOCOL_ERROR.
</t>
<section title="Setting Format" anchor="SettingFormat">
<t>
The payload of a SETTINGS frame consists of zero or more settings. Each setting
consists of an 8-bit flags field specifying per-item instructions, an unsigned 24-bit
setting identifier, and an unsigned 32-bit value.
</t>
<figure title="Setting Format">
<artwork type="inline">
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) |
+---------------------------------------------------------------+
</artwork>
</figure>
<t>
Two flags are defined for the 8-bit flags field:
<list style="hanging">
<t hangText="PERSIST_VALUE (0x1):">
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.
</t>
<t hangText="PERSISTED (0x2):">
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.
</t>
</list>
</t>
</section>
<section title="Setting Persistence">
<t>
<cref>Note that persistence of settings is under discussion in the WG and might be
removed in a future version of this document.</cref>
</t>
<t>
A server endpoint can request that configuration parameters
sent to a client in a SETTINGS frame are to be persisted by
the client across HTTP/2.0 connections and returned to the server
in any new SETTINGS frame the client sends to the server
in the current connection or any future connections.
</t>
<t>
Persistence is requested on a per-setting basis by setting the
PERSIST_VALUE flag (0x1).
</t>
<t>
Client endpoints are not permitted to make such requests. Servers
MUST ignore any attempt by clients to request that a server
persist configuration parameters.
</t>
<t>
Persistence of configuration parameters is done on a per-origin
basis (see <xref target="RFC6454"/>). That is, when a client
establishes a connection with a server, and the server requests that
the client maintain persistent settings, the client SHOULD return
the persisted settings on all future connections to the same origin,
IP address and TCP port.
</t>
<t>
Whenever the client sends a SETTINGS frame in the current connection,
or establishes a new connection with the same origin, persisted
configuration parameters are sent with the PERSISTED
flag (0x2) set for each persisted parameter.
</t>
<t>
Persisted settings accumulate until the server requests that
all previously persisted settings are to be cleared by setting
the CLEAR_PERSISTED (0x2) flag on the SETTINGS frame.
</t>
<t>
For example, 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 in a
subsequent SETTINGS frame, the client will return values for
all 5 settings (1, 2, 3, 4, and 5 in this example) to the server.
</t>
</section>
<section anchor="SettingValues" title="Defined Settings">
<t>
The following settings are defined:
<list style="hanging">
<x:lt hangText="SETTINGS_UPLOAD_BANDWIDTH (1):">
<t>
indicates the sender's estimated upload bandwidth for this connection. The value
is an the integral number of kilobytes per
second that the sender predicts as an expected maximum upload channel capacity.
</t>
</x:lt>
<x:lt hangText="SETTINGS_DOWNLOAD_BANDWIDTH (2):">
<t>
indicates the sender's estimated download bandwidth for this connection. The value
is an integral number of kilobytes per
second that the sender predicts as an expected maximum download channel capacity.
</t>
</x:lt>
<x:lt hangText="SETTINGS_ROUND_TRIP_TIME (3):">
<t>
indicates the sender's estimated round-trip-time for this connection. 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.
</t>
</x:lt>
<x:lt hangText="SETTINGS_MAX_CONCURRENT_STREAMS (4):">
<t>
indicates the maximum number of concurrent streams that the sender 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. It is recommended that this value be no smaller than 100,
so as to not unnecessarily limit parallelism.
</t>
</x:lt>
<x:lt hangText="SETTINGS_CURRENT_CWND (5):">
<t>
indicates the sender's current TCP CWND value.
</t>
</x:lt>
<x:lt hangText="SETTINGS_DOWNLOAD_RETRANS_RATE (6):">
<t>
indicates the sender's retransmission rate (bytes retransmitted / total bytes transmitted).
</t>
</x:lt>
<x:lt hangText="SETTINGS_INITIAL_WINDOW_SIZE (7):">
<t>
indicates the sender's initial stream window size (in bytes) for new streams.
</t>
</x:lt>
<x:lt hangText="SETTINGS_FLOW_CONTROL_OPTIONS (10):">
<t>
indicates that streams directed to the sender will not
be subject to flow control. The least significant bit (0x1) is set to indicate
that new streams are not flow controlled. All other bits are reserved.
</t>
<t>
This setting applies to all streams, including existing streams.
</t>
<t>
These bits cannot be cleared once set, see <xref target="EndFlowControl"/>.
</t>
</x:lt>
</list>
</t>
</section>
</section>
<section anchor="PUSH_PROMISE" title="PUSH_PROMISE">
<t>
The PUSH_PROMISE frame (type=0x5) is used to notify the peer
endpoint in advance of streams the sender intends to initiate.
The PUSH_PROMISE frame includes the unsigned 31-bit identifier
of the stream the endpoint plans to create along with a minimal
set of headers that provide additional context for the stream.
<xref target="PushResources"/> contains a thorough description
of the use of PUSH_PROMISE frames.
</t>
<figure title="PUSH_PROMISE Payload Format">
<artwork type="inline">
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 (*) ...
+---------------------------------------------------------------+
</artwork>
</figure>
<t>
The payload of a PUSH_PROMISE includes a "Promised-Stream-ID". This
unsigned 31-bit integer identifies the stream the endpoint
intends to start sending frames for. The promised stream
identifier MUST be a valid choice for the next stream sent by the sender (see <xref
target="StreamCreation">new stream identifier</xref>).
</t>
<t>
PUSH_PROMISE frames MUST be associated with an existing stream. If
the stream identifier field specifies the value 0x0, a recipient
MUST respond with a <xref target="ConnectionErrorHandler">connection
error</xref> of type PROTOCOL_ERROR.
</t>
<t>
The state of promised streams is bound to the state of the
original associated stream on which the PUSH_PROMISE frame
were sent. If the originating stream state changes to
fully closed, all associated promised streams fully close as well.
<cref>Ed. Note: We need clarification on this point. How synchronized
are the lifecycles of streams and associated promised streams?</cref>
</t>
<t>
PUSH_PROMISE uses the same flags as the HEADERS frame, except that a PUSH_PROMISE frame
with a CONTINUES bit MUST be followed by another PUSH_PROMISE frame. See <xref
target="HEADERS">HEADERS frame</xref> for any flags.
</t>
<t>
Promised streams are not required to be used in order promised.
The PUSH_PROMISE only reserves stream identifiers for later use.
</t>
<t>
Recipients of PUSH_PROMISE frames can choose to reject promised
streams by returning a RST_STREAM referencing the promised stream
identifier back to the sender of the PUSH_PROMISE.
</t>
<t>
The PUSH_PROMISE frame modifies the connection state as
defined in <xref target="HeaderBlock" />.
</t>
</section>
<section anchor="PING" title="PING">
<t>
The PING frame (type=0x6) is a mechanism for measuring a minimal round-trip time from the
sender, as well as determining whether an idle connection is still functional.
PING frames can be sent from any endpoint.
</t>
<t>
PING frames consist of an arbitrary, variable-length sequence of
octets. Receivers of a PING send a response PING frame with the PONG
flag set and precisely the same sequence of octets back to the
sender as soon as possible.
</t>
<t>
Processing of PING frames SHOULD be performed with the highest
priority if there are additional frames waiting to be processed.
</t>
<t>
The PING frame defines one type-specific flag:
<list style="hanging">
<t hangText="PONG (0x2):">
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.
</t>
</list>
</t>
<t>
PING frames are not associated with any individual stream. If a
PING frame is received with a stream identifier field value other
than 0x0, the recipient MUST respond with a
<xref target="ConnectionErrorHandler">connection error</xref> of type
PROTOCOL_ERROR.
</t>
</section>
<section anchor="GOAWAY" title="GOAWAY">
<t>
The GOAWAY frame (type=0x7) informs the remote peer to stop creating
streams on this connection. 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
connection. Receivers of a GOAWAY frame MUST NOT open additional streams on the connection,
although a new connection can be established for new streams. The purpose of this frame
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.
</t>
<t>
There is an inherent race condition between an endpoint starting new streams and the
remote sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream
identifier of the last stream which was processed on the sending endpoint in this
connection. 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 connection).
</t>
<t>
Endpoints should always send a GOAWAY frame 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).
</t>
<t>
After sending a GOAWAY frame, the sender can ignore frames for new streams.
</t>
<t>
<cref>Issue: connection state that is established by those "ignored" frames cannot be
ignored without the state in the two peers becoming unsynchronized.</cref>
</t>
<figure title="GOAWAY Payload Format">
<artwork type="inline">
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) |
+---------------------------------------------------------------+
</artwork>
</figure>
<t>
The GOAWAY frame does not define any type-specific flags.
</t>
<t>
The GOAWAY frame applies to the connection, not a specific stream. The stream identifier
MUST be zero.
</t>
<t>
The last stream identifier in the GOAWAY frame contains the highest numbered stream
identifier for which the sender of the GOAWAY frame has received frames on and might
have taken some action on. All streams up to and including the identified stream might
have been processed in some way. The last stream identifier is set to 0 if no streams
were processed.
<list>
<t>
Note: In this case, "processed" means that some data from the stream was passed to
some higher layer of software that might have taken some action as a result.
</t>
</list>
</t>
<t>
On streams with lower or equal numbered identifiers that do not close completely prior
to the connection being closed, re-attempting requests, transactions, or any protocol
activity is not possible (with the exception of idempotent actions like HTTP GET, PUT,
or DELETE). Any protocol activity that uses higher numbered streams can be safely
retried using a new connection.
</t>
<t>
Activity on streams numbered lower or equal to the last stream identifier might still complete
successfully. The sender of a GOAWAY frame gracefully shut down a connection by sending
a GOAWAY frame, maintaining the connection in an open state until all in-progress
streams complete.
</t>
<t>
The last stream ID MUST be 0 if no streams were acted upon.
</t>
<t>
The GOAWAY frame also contains a 32-bit <xref target="ErrorCodes">error code</xref> that
contains the reason for closing the connection.
</t>
</section>
<section anchor="HEADERS" title="HEADERS">
<t>
The HEADERS frame (type=0x8) provides header fields for a stream.
Any number of HEADERS frames can may be sent on an existing stream
at any time.
</t>
<t>
Additional type-specific flags for the HEADERS frame are:
<list style="hanging">
<t hangText="CONTINUES (0x2):">
The CONTINUES bit indicates that this frame does not contain the entire payload
necessary to provide a complete set of headers.
<vspace blankLines="1"/>
The payload for a complete set of headers is provided by a sequence of HEADERS
frames, terminated by a HEADERS frame without the CONTINUES bit. Once the sequence
terminates, the payload of all HEADERS frames are concatenated and interpreted as a
single block.
<vspace blankLines="1"/>
A HEADERS frame that includes a CONTINUES bit MUST be followed by a HEADERS frame
for the same stream. A receiver MUST treat the receipt of any other type of frame
or a frame on a different stream as a <xref
target="ConnectionErrorHandler">connection error</xref> of type PROTOCOL_ERROR.
</t>
</list>
</t>
<t>
The payload of a HEADERS frame contains a <xref target="HeaderBlock">Headers Block</xref>.
</t>
<t>
The HEADERS frame is associated with an existing stream. If
a HEADERS frame is received with a stream identifier of 0x0,
the recipient MUST respond with a
<xref target="StreamErrorHandler">stream error</xref> of type
PROTOCOL_ERROR.
</t>
<t>
The HEADERS frame changes the connection state as defined in
<xref target="HeaderBlock" />.
</t>
</section>
<section anchor="WINDOW_UPDATE" title="WINDOW_UPDATE">
<t>
The WINDOW_UPDATE frame (type=0x9) is used to implement flow control.
</t>
<t>
Flow control operates at two levels: on each individual stream and on the
entire connection.
</t>
<t>
Both types of flow control are hop by hop; that is, only between the two endpoints.
Intermediaries do not forward WINDOW_UPDATE frames between
dependent connections. However, throttling of data transfer by any receiver can
indirectly cause the propagation of flow control information toward the original
sender.
</t>
<t>
Flow control only applies to frames that are identified as being subject to flow
control. Of the frame types defined in this document, this includes only DATA frame.
Frames that are exempt from flow control MUST be accepted and processed, unless the
receiver is unable to assign resources to handling the frame. A receiver MAY respond
with a <xref target="StreamErrorHandler">stream error</xref> or <xref
target="ConnectionErrorHandler">connection error</xref> of type FLOW_CONTROL_ERROR if it
is unable accept a frame.
</t>
<t>
The following additional flags are defined for the WINDOW_UPDATE frame:
<list style="hanging">
<t hangText="END_FLOW_CONTROL (0x2):">
Bit 2 being set indicates that flow control for the identified stream or connection has been
ended; subsequent frames do not need to be flow controlled.
</t>
</list>
</t>
<t>
The WINDOW_UPDATE frame can be specific to a stream or to the entire connection.
In the former case, the frame's stream identifier indicates the affected stream;
in the latter, the value "0" indicates that the entire connection is the subject of
the frame.
</t>
<t>
The payload of a WINDOW_UPDATE frame is a 32-bit value indicating 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 2<x:sup>31</x:sup> - 1
(0x7fffffff) bytes; the most significant bit of this value is reserved.
</t>
<section title="The Flow Control Window">
<t>
Flow control in HTTP/2.0 is implemented using a window kept by each sender on every
stream. The flow control window is a simple integer value that indicates how many
bytes of data the sender is permitted to transmit; as such, its size is a
measure of the buffering capability of the receiver.
</t>
<t>
Two flow control windows are applicable; the stream flow
control window and the connection 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.
</t>
<t>
For flow control calculations, the 8 byte frame header is not counted.
</t>
<t>
After sending a flow controlled frame, the sender reduces the space available in both
windows by the length of the transmitted frame.
</t>
<t>
The receiver of a frame sends a WINDOW_UPDATE frame as it consumes data and frees up
space in flow control windows. Separate WINDOW_UPDATE frames are sent for the
stream and connection level flow control windows.
</t>
<t>
A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the
amount specified in the frame.
</t>
<t>
A sender MUST NOT allow a flow control window to exceed 2<x:sup>31</x:sup> - 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 connection, as appropriate. For
streams, the sender sends a RST_STREAM with the error code of FLOW_CONTROL_ERROR code;
for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code.
</t>
<t>
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.
</t>
</section>
<section title="Initial Flow Control Window Size">
<t>
When a HTTP/2.0 connection is first established, new streams are created with an
initial flow control window size of 65535 bytes. The connection 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 connection header.
</t>
<t>
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 connection flow control window is
set to the default initial window size until a WINDOW_UPDATE frame is received.
</t>
<t>
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.
</t>
<t>
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 MUST NOT send new flow controlled frames until it receives WINDOW_UPDATE
frames that cause the flow control window to become positive.
</t>
<t>
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.
</t>
</section>
<section title="Reducing the Stream Window Size">
<t>
A receiver that wishes to use a smaller flow control window than the current size
can send 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.
</t>
<t>
A receiver has two options for handling streams that exceed flow control limits:
<list style="numbers">
<t>
The receiver can immediately send RST_STREAM with FLOW_CONTROL_ERROR error code
for the affected streams.
</t>
<t>
The receiver can accept the streams and tolerate the resulting head of line
blocking, sending WINDOW_UPDATE frames as it consumes data.
</t>
</list>
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.
</t>
</section>
<section anchor="EndFlowControl" title="Ending Flow Control">
<t>
After a receiver reads in a frame that marks the end of a stream (for example, a data
stream with a FINAL flag set), it MUST cease transmission of WINDOW_UPDATE frames for
that stream. A sender is not obligated to maintain the available flow control window
for streams that it is no longer sending on.
</t>
<t>
Flow control can be disabled for all streams or the connection 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.
</t>
<t>
Flow control can be disabled for an individual stream or the overall connection 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.
</t>
<t>
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.
</t>
</section>
</section>
</section>
</section>
<section anchor="HTTPLayer" title="HTTP Message Exchanges">
<t>
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 (<xref target="HTTP-p1"/>, <xref
target="HTTP-p2"/>, <xref target="HTTP-p4"/>, <xref target="HTTP-p5"/>, <xref
target="HTTP-p6"/>, and <xref target="HTTP-p7"/>) apply with the changes in the sections
below.
</t>
<section title="Connection Management">
<t>
Clients SHOULD NOT open more than one HTTP/2.0 connection to a given origin
(<xref target="RFC6454"/>) concurrently.
</t>
<t>
Note that it is possible for one HTTP/2.0 connection to be finishing (e.g. a GOAWAY frame
has been sent, but not all streams have finished), while another HTTP/2.0 connection is
starting.
</t>
</section>
<section title="HTTP Request/Response">
<section title="HTTP Header Fields and HTTP/2.0 Headers">
<t>
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.
</t>
</section>
<section anchor="HttpRequest" title="Request">
<t>
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 <xref
target="PushResources"/>). 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.
</t>
<t>
The header fields included in the HEADERS+PRIORITY frame contain
all of the HTTP header fields associated with an HTTP request.
The definitions of these headers are largely unchanged relative
to HTTP/1.1, with a few notable exceptions:
<list style="symbols">
<t>The HTTP/1.1 request-line has been split into two separate
header fields named :method and :path, whose values
specify the HTTP method for the request and the request-target,
respectively. The HTTP-version component of the request-line
is removed entirely from the headers.</t>
<t>The host and optional port portions of the request URI
(see <xref target="RFC3986"/>, Section 3.2), is specified
using the new :host header field. <cref>Ed. Note: it
needs to be clarified whether or not this replaces the
existing HTTP/1.1 Host header.</cref></t>
<t>A new :scheme header field has been added to specify the
scheme portion of the request-target (e.g. "https")</t>
<t>All header field names MUST be lowercased, and the
definitions of all header field names defined by
HTTP/1.1 are updated to be all lowercase.</t>
<t>The Connection, Host, Keep-Alive, Proxy-Connection, and
Transfer-Encoding header fields are no longer valid and
MUST not be sent.</t>
</list>
</t>
<t>All HTTP Requests MUST include the ":method", ":path", ":host",
and ":scheme" header fields.</t>
<t>Header fields whose names begin with ":" (whether defined in this
document or future extensions to this document) MUST appear before
any other header fields.</t>
<t>
If a client sends a HEADERS+PRIORITY frame that omits a mandatory
header, the server MUST reply with a HTTP 400 Bad Request reply.
<cref>Ed: why PROTOCOL_ERROR on missing ":status" in the response,
but HTTP 400 here?</cref>
</t>
<t>
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.
</t>
<t>
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 use 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, and some server implementations have come to
depend upon its presence.
</t>
<t>
A client provides priority in requests as a hint to the server. A server SHOULD attempt
to provide responses to higher priority requests before lower priority requests. A
server could send lower priority responses during periods that higher priority responses
are unavailable to ensure better utilization of a connection.
</t>
<t>
If the server receives a data frame prior to a HEADERS+PRIORITY frame the server MUST
treat this as a <xref target="StreamErrorHandler">stream error</xref> of type
PROTOCOL_ERROR.
</t>
</section>
<section anchor="HttpResponse" title="Response">
<t>
The server responds to a client request using the same stream identifier that was used
by the request. An HTTP response begins with a HEADERS frame. An HTTP response body
consists of a series of DATA frames. The last data frame contains a FINAL flag to
indicate 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.
</t>
<t>
<list>
<t>
The response status line is unfolded into name-value pairs like other HTTP header
fields and must be present:
<list style="hanging">
<t hangText="":status":">
The HTTP response status code (e.g. "200" or "200 OK")
</t>
</list>
</t>
<t>
All header field names starting with ":" (whether defined in this document or future
extensions to this document) MUST appear before any other header fields.
</t>
<t>
All header field names MUST be all lowercase.
</t>
<t>
The Connection, Keep-Alive, Proxy-Connection, and Transfer-Encoding header fields
are not valid and MUST not be sent.
</t>
<t>
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.
</t>
<t>
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. <cref>Ed: See <eref
target="https://github.com/http2/http2-spec/issues/46">issue 46</eref>.</cref>
</t>
</list>
</t>
<t>
If a client receives a response with an absent or duplicated status header, the client
MUST treat this as a <xref target="StreamErrorHandler">stream error</xref> of type
PROTOCOL_ERROR.
</t>
<t>
If the client receives a data frame prior to a HEADERS frame the client MUST treat this
as a <xref target="StreamErrorHandler">stream error</xref> of type PROTOCOL_ERROR.
</t>
<t>
Clients MUST support gzip compression. Regardless of the value of the Accept-Encoding
header field, a server MAY send responses with gzip or deflate encoding. A compressed
response MUST still bear an appropriate Content-Encoding header field.
</t>
</section>
</section>
<section anchor="PushResources" title="Server Push Transactions">
<t>
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.
</t>
<t>
Server push is an optional feature. The SETTINGS_MAX_CONCURRENT_STREAMS setting from the
client limits the number of resources that can be concurrently pushed by a server. Server
push can be disabled by clients that do not wish to receive pushed resources by
advertising a SETTINGS_MAX_CONCURRENT_STREAMS <xref target="SETTINGS">SETTING</xref> of
zero. This prevents servers from creating the streams necessary to push resources.
</t>
<t>
Clients receiving a pushed response MUST validate that the server is authorized to push
the resource using the same-origin policy (<xref target="RFC6454" x:fmt="," x:sec="3"/>).
For example, a HTTP/2.0 connection to <spanx style="verb">example.com</spanx> is generally
<cref>Ed: weaselly use of "generally", needs better definition</cref> not permitted to
push a response for <spanx style="verb">www.example.org</spanx>.
</t>
<t>
A client that accepts pushed resources caches those resources as though they were
responses to GET requests.
</t>
<t>
Pushing of resources avoids the round-trip delay, but also creates a potential race where
a server can be pushing content which a client is in the process of requesting. The
PUSH_PROMISE frame reduces the chances of this condition occurring, while retaining the
performance benefit.
</t>
<t>
Pushed responses are associated with a request at the HTTP/2.0 framing layer. The
PUSH_PROMISE is sent on the stream for the associated request, which allows a receiver to
correlate the pushed resource with a request. The pushed stream inherits all of the
request header fields from the associated stream with the exception of resource
identification header fields (<spanx style="verb">:host</spanx>, <spanx
style="verb">:scheme</spanx>, and <spanx style="verb">:path</spanx>), which are provided
as part of the PUSH_PROMISE frame.
</t>
<t>
Pushed resources always have an associated <spanx style="verb">:method</spanx> of <spanx
style="verb">GET</spanx>. A cache MUST store these inherited and implied request header
fields with the cached resource.
</t>
<section title="Server implementation">
<t>
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 (<spanx style="verb">:scheme</spanx>, <spanx
style="verb">:host</spanx>, and <spanx style="verb">:path</spanx>).
</t>
<t>
A server can push multiple resources in response to a request, but all pushed resources
MUST be promised on the response stream for the associated request. A server cannot
send a PUSH_PROMISE on a new stream or a half-closed stream.
</t>
<t>
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 <xref target="HTTP-p6" x:fmt=","
x:sec="4"/>).
</t>
<t>
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 <xref target="HttpResponse">HTTP response</xref>: a HEADERS
frame followed by DATA frames.
</t>
<t>
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.
</t>
<t>
The server MUST only push resources that could have been returned from a GET request.
</t>
<t>
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.
</t>
</section>
<section title="Client implementation">
<t>
When fetching a resource the client has 3 possibilities:
<list style="numbers">
<t>
the resource is not being pushed
</t>
<t>
the resource is being pushed, but the data has not yet arrived
</t>
<t>
the resource is being pushed, and the data has started to arrive
</t>
</list>
</t>
<t>
A client SHOULD NOT issue GET requests for a resource that has been promised. A client
is instead advised to wait for the pushed resource to arrive.
</t>
<t>
When a client receives a PUSH_PROMISE frame from the server without a the <spanx
style="verb">:host</spanx>, <spanx style="verb">:scheme</spanx>, and <spanx
style="verb">:path</spanx> header fields, it MUST treat this as a <xref
target="StreamErrorHandler">stream error</xref> of type PROTOCOL_ERROR.
</t>
<t>
To cancel individual server push streams, the client can issue a <xref
target="StreamErrorHandler">stream error</xref> of type CANCEL. After receiving a
PUSH_PROMISE frame, the client is able to cancel the pushed resource before receiving
any frames on the promised stream. The server ceases transmission of the pushed
resource; if the server has not commenced transmission, it does not start.
</t>
<t>
To cancel all server push streams related to a request, the client may issue a <xref
target="StreamErrorHandler">stream error</xref> 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.
<cref>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.</cref>
</t>
<t>
A client can choose to time out pushed streams if the server does not provide the
resource in a timely fashion. A <xref target="StreamErrorHandler">stream error</xref>
of type CANCEL can be used to stop a timed out push.
</t>
<t>
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 <xref target="StreamErrorHandler">stream error</xref> of type PROTOCOL_ERROR.
</t>
<t>
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 (<xref target="HTTP-p1" x:fmt="of"
x:rel="#header.trailer"/>).
</t>
</section>
</section>
</section>
<section title="Design Rationale and Notes">
<t>
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.
</t>
<section title="Separation of Framing Layer and Application Layer">
<t>
Readers may note that this specification sometimes blends the <xref
target="FramingLayer">framing layer</xref> with requirements of a specific application -
<xref target="HTTPLayer">HTTP</xref>. 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.
</t>
<t>
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.
</t>
</section>
<section title="Error handling - Framing Layer">
<t>
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.
</t>
<t>
When an error is confined to a single stream, but general framing is intact, HTTP/2.0
attempts to use the RST_STREAM as a mechanism to invalidate the stream but move forward
without aborting the connection altogether.
</t>
<t>
For errors occurring outside of a single stream context, HTTP/2.0 assumes the entire
connection is hosed. In this case, the endpoint detecting the error should initiate a
connection close.
</t>
</section>
<section title="One Connection per Domain">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Fixed vs Variable Length Fields">
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Server Push">
<t>
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.
</t>
</section>
</section>
<section title="Security Considerations">
<section title="Server Authority and Same-Origin">
<t>
This specification uses the same-origin policy (<xref target="RFC6454" x:fmt=","
x:sec="3"/>) to determine whether an origin server is permitted to provide content.
</t>
<t>
A server that is contacted using TLS is authenticated based on the certificate that it
offers in the TLS handshake (see <xref target="RFC2818" x:fmt="," x:sec="3"/>). A server
is considered authoritative for an "https:" resource if it has been successfully
authenticated for the domain part of the origin of the resource that it is providing.
</t>
<t>
A server is considered authoritative for an "http:" resource if the connection is
established to a resolved IP address for the domain in the origin of the resource.
</t>
<t>
A client MUST NOT use, in any way, resources provided by a server that is not
authoritative for those resources.
</t>
</section>
<section title="Cross-Protocol Attacks">
<t>
When using 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. <cref>Issue: This is no longer true</cref>
</t>
</section>
<section title="Cacheability of Pushed Resources">
<t>
Pushed resources are synthesized responses without an explicit request; the request for a
pushed resource is synthesized from the request that triggered the push, plus resource
identification information provided by the server. Request header fields are necessary
for HTTP cache control validations (such as the Vary header field) to work. For this
reason, caches MUST inherit request header fields from the associated stream for the push.
This includes the Cookie header field.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
Pushed resources for which an origin server is not authoritative are never cached or used.
</t>
</section>
</section>
<section title="Privacy Considerations">
<section title="Long Lived Connections">
<t>
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.
</t>
</section>
<section title="SETTINGS frame">
<t>
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.
</t>
<t>
Clients implementing privacy modes can disable client-persisted SETTINGS storage.
</t>
<t>
Clients MUST clear persisted SETTINGS information when clearing the cookies.
</t>
</section>
</section>
<section title="IANA Considerations">
<t>
This document establishes registries for frame types, error codes and settings.
</t>
<section title="Frame Type Registry">
<t>
This document establishes a registry for HTTP/2.0 frame types. The "HTTP/2.0 Frame Type"
registry operates under the <xref target="RFC5226">"IETF Review" policy</xref>.
</t>
<t>
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.
</t>
<t>
Initial values for the "HTTP/2.0 Frame Type" registry are shown in <xref
target="IanaInitialFrameType"/>.
</t>
<texttable anchor="IanaInitialFrameType">
<ttcol>Frame Type</ttcol><ttcol>Name</ttcol><ttcol>Flags</ttcol>
<c>0</c><c>DATA</c><c>-</c>
<c>1</c><c>HEADERS+PRIORITY</c><c>-</c>
<c>3</c><c>RST_STREAM</c><c>-</c>
<c>4</c><c>SETTINGS</c><c>CLEAR_PERSISTED(2)</c>
<c>5</c><c>PUSH_PROMISE</c><c>-</c>
<c>6</c><c>PING</c><c>PONG(2)</c>
<c>7</c><c>GOAWAY</c><c>-</c>
<c>8</c><c>HEADERS</c><c>-</c>
<c>9</c><c>WINDOW_UPDATE</c><c>END_FLOW_CONTROL(2)</c>
</texttable>
</section>
<section title="Error Code Registry">
<t>
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
<xref target="RFC5226">"Expert Review" policy</xref>.
</t>
<t>
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.
</t>
<t>
New registrations are advised to provide the following information:
<list style="hanging">
<t hangText="Error Code:">
The 32-bit error code value.
</t>
<t hangText="Name:">
A name for the error code. Specifying an error code name is optional.
</t>
<t hangText="Description:">
A description of the conditions where the error code is applicable.
</t>
<t hangText="Specification:">
An optional reference for a specification that defines the error code.
</t>
</list>
</t>
<t>
An initial set of error code registrations can be found in <xref target="ErrorCodes"/>.
</t>
</section>
<section title="Settings Registry">
<t>
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
<xref target="RFC5226">"Expert Review" policy</xref>.
</t>
<t>
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.
</t>
<t>
New registrations are advised to provide the following information:
<list style="hanging">
<t hangText="Setting:">
The 24-bit setting value.
</t>
<t hangText="Name:">
A name for the setting. Specifying a name is optional.
</t>
<t hangText="Flags:">
Any setting-specific flags that apply, including their value and semantics.
</t>
<t hangText="Description:">
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.
</t>
<t hangText="Specification:">
An optional reference for a specification that defines the setting.
</t>
</list>
</t>
<t>
An initial set of settings registrations can be found in <xref target="SettingValues"/>.
</t>
</section>
</section>
<section title="Acknowledgements">
<t>
This document includes substantial input from the following individuals:
<list style="symbols">
<t>
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).
</t>
<t>
Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)
</t>
<t>
William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto
Peon, Rob Trace (Flow control)
</t>
<t>
Mark Nottingham, Julian Reschke, James Snell (Editorial)
</t>
</list>
</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="RFC0793">
<front>
<title abbrev='Transmission Control Protocol'>
Transmission Control Protocol
</title>
<author initials='J.' surname='Postel' fullname='Jon Postel'>
<organization>University of Southern California (USC)/Information Sciences
Institute</organization>
</author>
<date year='1981' month='September' />
</front>
<seriesInfo name='STD' value='7' />
<seriesInfo name='RFC' value='793' />
</reference>
<reference anchor="RFC2119">
<front>
<title>
Key words for use in RFCs to Indicate Requirement Levels
</title>
<author initials="S." surname="Bradner" fullname="Scott Bradner">
<organization>Harvard University</organization>
<address><email>sob@harvard.edu</email></address>
</author>
<date month="March" year="1997"/>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="2119"/>
</reference>
<reference anchor="RFC2818">
<front>
<title>
HTTP Over TLS
</title>
<author initials="E." surname="Rescorla" fullname="Eric Rescorla">
<organization/>
</author>
<date month="May" year="2000"/>
</front>
<seriesInfo name="RFC" value="2818"/>
</reference>
<reference anchor="RFC3986">
<front>
<title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic
Syntax</title>
<author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'></author>
<author initials='R.' surname='Fielding' fullname='Roy T. Fielding'></author>
<author initials='L.' surname='Masinter' fullname='Larry Masinter'></author>
<date year='2005' month='January' />
</front>
<seriesInfo name='STD' value='66' />
<seriesInfo name='RFC' value='3986' />
</reference>
<reference anchor="RFC5226">
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials="T." surname="Narten" fullname="T. Narten">
<organization /></author>
<author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
<organization /></author>
<date year="2008" month="May" />
</front>
<seriesInfo name="BCP" value="26" />
<seriesInfo name="RFC" value="5226" />
</reference>
<reference anchor="RFC5246">
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
<author initials="T." surname="Dierks" fullname="Tim Dierks"/>
<author initials="E." surname="Rescorla" fullname="Eric Rescorla"/>
<date year="2008" month="August" />
</front>
<seriesInfo name="RFC" value="5246" />
</reference>
<reference anchor="RFC6454">
<front>
<title>The Web Origin Concept</title>
<author initials='A.' surname='Barth' fullname='A. Barth'/>
<date year='2011' month='December' />
</front>
<seriesInfo name='RFC' value='6454' />
</reference>
<reference anchor="TLSALPN">
<front>
<title>Transport Layer Security (TLS) Application Layer Protocol Negotiation Extension</title>
<author initials="S.F." surname="Friedl" fullname="Stephan Friedl"></author>
<author initials="A." surname="Popov" fullname="Andrei Popov"></author>
<author initials="A." surname="Langley" fullname="Adam Langley"></author>
<author initials="E." surname="Stephan" fullname="Emile Stephan"></author>
<date month="April" year="2013" />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-tls-applayerprotoneg-01" />
</reference>
<reference anchor='HTTP-p1'>
<front>
<title>
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
<author initials='R.' surname='Fielding' fullname='Roy Fielding'></author>
<author initials='J.' surname='Reschke' fullname='Julian Reschke'></author>
<date month='February' year='2013' />
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-httpbis-p1-messaging-22' />
<x:source href="refs/draft-ietf-httpbis-p1-messaging-22.xml" basename="https://svn.tools.ietf.org/svn/wg/httpbis/draft-ietf-httpbis/22/p1-messaging"/>
</reference>
<reference anchor='HTTP-p2'>
<front>
<title>
Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
<author initials='R.' surname='Fielding' fullname='Roy Fielding'></author>
<author initials='J.' surname='Reschke' fullname='Julian Reschke'></author>
<date month='February' year='2013' />
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-httpbis-p2-semantics-22' />
<x:source href="refs/draft-ietf-httpbis-p2-semantics-22.xml" basename="https://svn.tools.ietf.org/svn/wg/httpbis/draft-ietf-httpbis/22/p2-semantics"/>
</reference>
<reference anchor="HTTP-p4">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month='February' year='2013' />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-22" />
</reference>
<reference anchor="HTTP-p5">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
<organization abbrev="W3C">World Wide Web Consortium</organization>
<address><email>ylafon@w3.org</email></address>
</author>
<author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month='February' year='2013' />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-22"/>
</reference>
<reference anchor="HTTP-p6">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
<organization>Akamai</organization>
<address><email>mnot@mnot.net</email></address>
</author>
<author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month='February' year='2013' />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-22"/>
</reference>
<reference anchor="HTTP-p7">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month='February' year='2013' />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-22"/>
</reference>
</references>
<references title="Informative References">
<reference anchor="RFC1323">
<front>
<title>
TCP Extensions for High Performance
</title>
<author initials='V.' surname='Jacobson' fullname='Van Jacobson'></author>
<author initials='B.' surname='Braden' fullname='Bob Braden'></author>
<author initials='D.' surname='Borman' fullname='Dave Borman'></author>
<date year='1992' month='May' />
</front>
<seriesInfo name='RFC' value='1323' />
</reference>
<reference anchor="TALKING" target="http://w2spconf.com/2011/papers/websocket.pdf">
<front>
<title>
Talking to Yourself for Fun and Profit
</title>
<author initials="L-S." surname="Huang"/>
<author initials="E." surname="Chen"/>
<author initials="A." surname="Barth"/>
<author initials="E." surname="Rescorla"/>
<author initials="C." surname="Jackson"/>
<date year="2011" />
</front>
</reference>
</references>
<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
<section title="Since draft-ietf-httpbis-http2-02" anchor="changes.since.draft-ietf-httpbis-http2-02">
<t>
Added continuations to frames carrying header blocks.
</t>
<t>
Replaced use of "session" with "connection" to avoid confusion with other HTTP stateful
concepts, like cookies.
</t>
<t>
Removed "message".
</t>
<t>
Switched to TLS ALPN from NPN.
</t>
<t>
Editorial changes.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-01" anchor="changes.since.draft-ietf-httpbis-http2-01">
<t>
Added IANA considerations section for frame types, error codes and settings.
</t>
<t>
Removed data frame compression.
</t>
<t>
Added PUSH_PROMISE.
</t>
<t>
Added globally applicable flags to framing.
</t>
<t>
Removed zlib-based header compression mechanism.
</t>
<t>
Updated references.
</t>
<t>
Clarified stream identifier reuse.
</t>
<t>
Removed CREDENTIALS frame and associated mechanisms.
</t>
<t>
Added advice against naive implementation of flow control.
</t>
<t>
Added session header section.
</t>
<t>
Restructured frame header. Removed distinction between data and control frames.
</t>
<t>
Altered flow control properties to include session-level limits.
</t>
<t>
Added note on cacheability of pushed resources and multiple tenant servers.
</t>
<t>
Changed protocol label form based on discussions.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-00" anchor="changes.since.draft-ietf-httpbis-http2-00">
<t>
Changed title throughout.
</t>
<t>
Removed section on Incompatibilities with SPDY draft#2.
</t>
<t>
Changed INTERNAL_ERROR on GOAWAY to have a value of 2 <eref
target="https://groups.google.com/forum/?fromgroups#!topic/spdy-dev/cfUef2gL3iU"/>.
</t>
<t>
Replaced abstract and introduction.
</t>
<t>
Added section on starting HTTP/2.0, including upgrade mechanism.
</t>
<t>
Removed unused references.
</t>
<t>
Added <xref target="fc-principles">flow control principles</xref> based on <eref
target="http://tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01"/>.
</t>
</section>
<section title="Since draft-mbelshe-httpbis-spdy-00" anchor="changes.since.draft-mbelshe-httpbis-spdy-00">
<t>
Adopted as base for draft-ietf-httpbis-http2.
</t>
<t>
Updated authors/editors list.
</t>
<t>
Added status note.
</t>
</section>
</section>
</back>
</rfc>
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