One document matched: draft-ietf-httpbis-http2-11.xml
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<rfc ipr="trust200902" category="std" docName="draft-ietf-httpbis-http2-11">
<front>
<title abbrev="HTTP/2">Hypertext Transfer Protocol version 2</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>Mozilla</organization>
<address>
<postal>
<street>Suite 300</street>
<street>650 Castro Street</street>
<city>Mountain View</city>
<region>CA</region>
<code>94041</code>
<country>US</country>
</postal>
<email>martin.thomson@gmail.com</email>
</address>
</author>
<date year="2014"/>
<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). HTTP/2 enables a more efficient use of network resources and a reduced
perception of latency by introducing header field compression and allowing 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 syntax.
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 can be found at <eref target="http://tools.ietf.org/wg/httpbis/"/>; that specific to HTTP/2 are at <eref target="http://http2.github.io/"/>.
</t>
<t>
The changes in this draft are summarized in <xref target="change.log"/>.
</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 format (<xref target="HTTP-p1"/>, Section 3) was
designed to be implemented with the tools at hand in the 1990s, not modern Web application
performance. As such it has several characteristics that have a negative overall effect on
application performance today.
</t>
<t>
In particular, HTTP/1.0 only allows one request to be outstanding at a time on a given
connection. HTTP/1.1 pipelining only partially addressed request concurrency and
suffers from head-of-line blocking. Therefore, clients that need to make many requests
typically use multiple connections to a server in order to reduce 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 performance.
</t>
<t>
The resulting protocol is designed to 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>
<section anchor="Overview" title="HTTP/2 Protocol Overview">
<t>
HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2 supports all of the core
features of HTTP/1.1, but aims to be more efficient in several ways.
</t>
<t>
The basic protocol unit in HTTP/2 is a <xref target="FrameHeader">frame</xref>. Each frame
has a different type and purpose. For example, <xref target="HEADERS" format="none">HEADERS</xref> and
<xref target="DATA" format="none">DATA</xref> frames form the basis of <xref target="HttpSequence">HTTP requests and
responses</xref>; other frame types like <xref target="SETTINGS" format="none">SETTINGS</xref>,
<xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref>, and <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> are used in support of other
HTTP/2 features.
</t>
<t>
Multiplexing of requests is achieved by having each HTTP request-response exchanged assigned
to a single <xref target="StreamsLayer">stream</xref>. Streams are largely independent of
each other, so a blocked or stalled request does not prevent progress on other requests.
</t>
<t>
Flow control and prioritization ensure that it is possible to properly use multiplexed
streams. <xref target="FlowControl">Flow control</xref> helps to ensure that only data that
can be used by a receiver is transmitted. <xref target="StreamPriority">Prioritization</xref> ensures that limited resources can be directed
to the most important requests first.
</t>
<t>
HTTP/2 adds a new interaction mode, whereby a server can <xref target="PushResources">push
responses to a client</xref>. Server push allows a server to speculatively send a client
data that the server anticipates the client will need, trading off some network usage
against a potential latency gain. The server does this by synthesizing a request, which it
sends as a <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame. The server is then able to send a response to
the synthetic request on an separate stream.
</t>
<t>
Frames that contain HTTP header fields are <xref target="HeaderBlock">compressed</xref>.
HTTP requests can be highly redundant, so compression can reduce the size of requests and
responses significantly.
</t>
<t>
HTTP/2 also supports HTTP Alternative Services (see <xref target="ALT-SVC"/>) using the <xref target="ALTSVC">ALTSVC frame type</xref>, to allow servers more control over traffic to
them.
</t>
<section title="Document Organization">
<t>
The HTTP/2 specification is split into four parts:
<list style="symbols">
<t>
<xref target="starting">Starting HTTP/2</xref> covers how an HTTP/2 connection is
initiated.
</t>
<t>
The <xref target="FramingLayer">framing</xref> and <xref target="StreamsLayer">streams</xref> layers describe the way HTTP/2 frames are
structured and formed into multiplexed streams.
</t>
<t>
<xref target="frame-types">Frame</xref> and <xref target="ErrorCodes">error</xref>
definitions include details of the frame and error types used in HTTP/2.
</t>
<t>
<xref target="HTTPLayer">HTTP mappings</xref> and <xref target="HttpExtra">additional
requirements</xref> describe how HTTP semantics are expressed using frames and
streams.
</t>
</list>
</t>
<t>
While some of the frame and stream layer concepts are isolated from HTTP, the intent is
not to define a completely generic framing layer. The framing and streams layers are
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/2 connection.
</t>
<t hangText="connection:">
A transport-level connection between two endpoints.
</t>
<t hangText="connection error:">
An error that affects the entire HTTP/2 connection.
</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 connection, consisting of a header
and a variable-length sequence of bytes structured according to the frame type.
</t>
<t hangText="intermediary:">
A "proxy", "gateway" or other intermediary as defined in Section 2.3 of <xref target="HTTP-p1"/>.
</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/2 connection.
</t>
<t hangText="stream:">
A bi-directional flow of frames across a virtual channel within the HTTP/2 connection.
</t>
<t hangText="stream error:">
An error on the individual HTTP/2 stream.
</t>
</list>
</t>
</section>
</section>
<section anchor="starting" title="Starting HTTP/2">
<t>
An HTTP/2 connection is an application level protocol running on top of a TCP connection
(<xref target="TCP"/>). The client is the TCP connection initiator.
</t>
<t>
HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1. HTTP/2 shares the same
default port numbers: 80 for "http" URIs and 443 for "https" URIs. As a result,
implementations processing requests for target resource URIs like <spanx style="verb">http://example.org/foo</spanx> or <spanx style="verb">https://example.com/bar</spanx> are required to first discover whether the
upstream server (the immediate peer to which the client wishes to establish a connection)
supports HTTP/2.
</t>
<t>
The means by which support for HTTP/2 is determined is different for "http" and "https"
URIs. Discovery for "http" URIs is described in <xref target="discover-http"/>. Discovery
for "https" URIs is described in <xref target="discover-https"/>.
</t>
<section anchor="versioning" title="HTTP/2 Version Identification">
<t>
The protocol defined in this document has two identifiers.
<list style="symbols">
<t>
The string "h2" identifies the protocol where HTTP/2 uses <xref target="TLS12">TLS</xref>. This identifier is used in the <xref target="TLSALPN">TLS application layer protocol negotiation extension</xref> field
and any place that HTTP/2 over TLS is identified.
<vspace blankLines="1"/>
When serialised into an ALPN protocol identifier (which is a sequence of octets), the
HTTP/2 protocol identifier string is encoded using <xref target="UTF-8">UTF-8</xref>.
</t>
<t>
The string "h2c" identifies the protocol where HTTP/2 is run over cleartext TCP. This
identifier is used in the HTTP/1.1 Upgrade header field and any place that HTTP/2 over
TCP is identified.
</t>
</list>
</t>
<t>
Negotiating "h2" or "h2c" implies the use of the transport, security, framing and message
semantics described in this document.
</t>
<t>
<cref>RFC Editor's Note: please remove the remainder of this section 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 "h2" or "h2c".
Until such an RFC exists, implementations MUST NOT identify themselves using these
strings.
</t>
<t>
Examples and text throughout the rest of this document use "h2" 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 "-" and the
corresponding draft number to the identifier. For example, draft-ietf-httpbis-http2-11
over TLS is identified using the string "h2-11".
</t>
<t>
Non-compatible experiments that are based on these draft versions MUST append the string
"-" and an experiment name to the identifier. For example, an experimental implementation
of packet mood-based encoding based on draft-ietf-httpbis-http2-09 might identify itself
as "h2-09-emo". Note that any label MUST conform to the "token" syntax defined in
Section 3.2.6 of <xref target="HTTP-p1"/>. 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 for "http" URIs">
<t>
A client that makes a request to an "http" URI without prior knowledge about support for
HTTP/2 uses the HTTP Upgrade mechanism (Section 6.7 of <xref target="HTTP-p1"/>). The client makes an HTTP/1.1 request that includes an Upgrade
header field identifying HTTP/2 with the "h2c" token. The HTTP/1.1 request MUST include
exactly one <xref target="Http2SettingsHeader">HTTP2-Settings</xref> header field.
</t>
<figure>
<preamble>For example:</preamble>
<artwork type="message/http; msgtype="request""><![CDATA[
GET /default.htm HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
]]></artwork>
</figure>
<t>
Requests that contain an entity body MUST be sent in their entirety before the client can
send HTTP/2 frames. This means that a large request entity can block the use of the
connection until it is completely sent.
</t>
<t>
If concurrency of an initial request with subsequent requests is important, a small
request can be used to perform the upgrade to HTTP/2, at the cost of an additional
round-trip.
</t>
<t>
A server that does not support HTTP/2 can respond to the request as though the Upgrade
header field were absent:
</t>
<figure>
<artwork type="message/http; msgtype="response""><![CDATA[
HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html
...
]]></artwork>
</figure>
<t>
A server that supports HTTP/2 can accept the upgrade with a 101 (Switching Protocols)
response. After the empty line that terminates the 101 response, the server can begin
sending HTTP/2 frames. These frames MUST include a response to the request that initiated
the Upgrade.
</t>
<figure>
<artwork type="message/http; msgtype="response""><![CDATA[
HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2
[ HTTP/2 connection ...
]]></artwork>
</figure>
<t>
The first HTTP/2 frame sent by the server is a <xref target="SETTINGS" format="none">SETTINGS</xref> frame (<xref target="SETTINGS"/>). Upon receiving the 101 response, the client sends a <xref target="ConnectionHeader">connection preface</xref>, which includes a
<xref target="SETTINGS" format="none">SETTINGS</xref> frame.
</t>
<t>
The HTTP/1.1 request that is sent prior to upgrade is assigned stream identifier 1 and is
assigned <xref target="pri-default">default priority values</xref>. Stream 1 is
implicitly half closed from the client toward the server, since the request is completed
as an HTTP/1.1 request. After commencing the HTTP/2 connection, stream 1 is used for the
response.
</t>
<section anchor="Http2SettingsHeader" title="HTTP2-Settings Header Field">
<t>
A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one <spanx style="verb">HTTP2-Settings</spanx> header field. The <spanx style="verb">HTTP2-Settings</spanx> header field is a hop-by-hop header field that
includes parameters that govern the HTTP/2 connection, provided in anticipation of the
server accepting the request to upgrade. A server MUST reject an attempt to upgrade if
this header field is not present.
</t>
<figure>
<artwork type="abnf"><![CDATA[
HTTP2-Settings = token68
]]></artwork>
</figure>
<t>
The content of the <spanx style="verb">HTTP2-Settings</spanx> header field is the
payload of a <xref target="SETTINGS" format="none">SETTINGS</xref> frame (<xref target="SETTINGS"/>), encoded as a
base64url string (that is, the URL- and filename-safe Base64 encoding described in Section 5 of <xref target="RFC4648"/>, with any trailing '=' characters omitted). The
<xref target="RFC5234">ABNF</xref> production for <spanx style="verb">token68</spanx> is
defined in Section 2.1 of <xref target="HTTP-p7"/>.
</t>
<t>
As a hop-by-hop header field, the <spanx style="verb">Connection</spanx> header field
MUST include a value of <spanx style="verb">HTTP2-Settings</spanx> in addition to <spanx style="verb">Upgrade</spanx> when upgrading to HTTP/2.
</t>
<t>
A server decodes and interprets these values as it would any other
<xref target="SETTINGS" format="none">SETTINGS</xref> frame. <xref target="SettingsSync">Acknowledgement of the
SETTINGS parameters</xref> is not necessary, since a 101 response serves as implicit
acknowledgment. Providing these values in the Upgrade request ensures that the protocol
does not require default values for the above SETTINGS parameters, and gives a client an
opportunity to provide other parameters prior to receiving any frames from the server.
</t>
</section>
</section>
<section anchor="discover-https" title="Starting HTTP/2 for "https" URIs">
<t>
A client that makes a request to an "https" URI without prior knowledge about support for
HTTP/2 uses <xref target="TLS12">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 preface</xref>.
</t>
</section>
<section anchor="known-http" title="Starting HTTP/2 with Prior Knowledge">
<t>
A client can learn that a particular server supports HTTP/2 by other means. For example,
<xref target="ALT-SVC"/> describes a mechanism for advertising this capability in an HTTP
header field; <xref target="ALTSVC">the ALTSVC frame</xref> describes a similar
mechanism in HTTP/2.
</t>
<t>
A client MAY immediately send HTTP/2 frames to a server that is known to support HTTP/2,
after the <xref target="ConnectionHeader">connection preface</xref>. A server can
identify such a connection by the use of the "PRI" method in the connection preface. This
only affects the resolution of "http" URIs; servers supporting HTTP/2 are required to
support <xref target="TLSALPN">protocol negotiation in TLS</xref> for "https" URIs.
</t>
<t>
Prior support for HTTP/2 is not a strong signal that a given server will support HTTP/2
for future connections. It is possible for server configurations to change; for
configurations to differ between instances in clustered server; or network conditions to
change.
</t>
</section>
<section anchor="ConnectionHeader" title="HTTP/2 Connection Preface">
<t>
Upon establishment of a TCP connection and determination that HTTP/2 will be used by both
peers, each endpoint MUST send a connection preface as a final confirmation and to
establish the initial SETTINGS parameters for the HTTP/2 connection.
</t>
<t>
The client connection preface starts with a sequence of 24 octets, which in hex notation
are:
</t>
<figure>
<artwork type="inline"><![CDATA[
0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
]]></artwork>
</figure>
<t>
(the string <spanx style="verb">PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n</spanx>). This sequence
is followed by a <xref target="SETTINGS" format="none">SETTINGS</xref> frame (<xref target="SETTINGS"/>). The
<xref target="SETTINGS" format="none">SETTINGS</xref> frame MAY be empty. The client sends the client connection
preface immediately upon receipt of a 101 Switching Protocols response (indicating a
successful upgrade), or as the first application data octets of a TLS connection. If
starting an HTTP/2 connection with prior knowledge of server support for the protocol, the
client connection preface is sent upon connection establishment.
</t>
<t>
<list>
<t>
The client connection preface 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 preface consists of a potentially empty <xref target="SETTINGS" format="none">SETTINGS</xref>
frame (<xref target="SETTINGS"/>) that MUST be the first frame the server sends in the
HTTP/2 connection.
</t>
<t>
To avoid unnecessary latency, clients are permitted to send additional frames to the
server immediately after sending the client connection preface, without waiting to receive
the server connection preface. It is important to note, however, that the server
connection preface <xref target="SETTINGS" format="none">SETTINGS</xref> frame might include parameters that necessarily
alter how a client is expected to communicate with the server. Upon receiving the
<xref target="SETTINGS" format="none">SETTINGS</xref> 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 preface. A <xref target="GOAWAY" format="none">GOAWAY</xref> frame (<xref target="GOAWAY"/>) MAY be
omitted if it is clear that the peer is not using HTTP/2.
</t>
</section>
</section>
<section anchor="FramingLayer" title="HTTP Frames">
<t>
Once the HTTP/2 connection is established, endpoints can begin exchanging frames.
</t>
<section anchor="FrameHeader" title="Frame Format">
<t>
All frames begin with an 8-octet header followed by a payload of between 0 and 16,383
octets.
</t>
<figure title="Frame Header">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R | Length (14) | Type (8) | Flags (8) |
+-+-+-----------+---------------+-------------------------------+
|R| Stream Identifier (31) |
+-+-------------------------------------------------------------+
| Frame Payload (0...) ...
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The fields of the frame header are defined as:
<list style="hanging">
<t hangText="R:">
A reserved 2-bit field. The semantics of these bits are undefined and the bits MUST
remain unset (0) when sending and MUST be ignored when receiving.
</t>
<t hangText="Length:">
The length of the frame payload expressed as an unsigned 14-bit integer. The 8 octets
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 payload are interpreted. Implementations MUST treat the receipt of
an unknown frame type (any frame types not defined in this document) as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t hangText="Flags:">
An 8-bit field reserved for frame-type specific boolean flags.
<vspace blankLines="1"/>
Flags are 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="StreamIdentifiers"/>). The 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 frame payload is dependent entirely on the frame type.
</t>
</section>
<section anchor="FrameSize" title="Frame Size">
<t>
The maximum size of a frame payload varies by frame type. The absolute maximum size of a
frame payload is 2^14-1 (16,383) octets, meaning that the maximum frame
size is 16,391 octets. All implementations SHOULD be capable of receiving and minimally
processing frames up to this maximum size.
</t>
<t>
Certain frame types, such as <xref target="PING" format="none">PING</xref> (see <xref target="PING"/>), impose
additional limits on the amount of payload data allowed. Likewise, additional size limits
can be set by specific application uses (see <xref target="HttpExtra"/>).
</t>
<t>
If a frame size exceeds any defined limit, or is too small to contain mandatory frame
data, the endpoint MUST send a <xref target="FRAME_SIZE_ERROR" format="none">FRAME_SIZE_ERROR</xref> error. A frame size error
in a frame that could alter the state of the entire connection MUST be treated as a <xref target="ConnectionErrorHandler">connection error</xref>; this includes any frame carrying
a <xref target="HeaderBlock">header block</xref> (that is, <xref target="HEADERS" format="none">HEADERS</xref>,
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref>, and <xref target="CONTINUATION" format="none">CONTINUATION</xref>), <xref target="SETTINGS" format="none">SETTINGS</xref>,
and any <xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref> frame with a stream identifier of 0.
</t>
</section>
<section anchor="HeaderBlock" title="Header Compression and Decompression">
<t>
A header field in HTTP/2 is a name-value pair with one or more associated values. They are
used within HTTP request and response messages as well as server push operations (see
<xref target="PushResources"/>).
</t>
<t>
Header sets are collections of zero or more header fields. When transmitted over a
connection, a header set is serialized into a header block using <xref target="COMPRESSION">HTTP Header Compression</xref>. The serialized header block is then
divided into one or more octet sequences, called header block fragments, and transmitted
within the payload of <xref target="HEADERS">HEADERS</xref>, <xref target="PUSH_PROMISE">PUSH_PROMISE</xref> or <xref target="CONTINUATION">CONTINUATION</xref> frames.
</t>
<t>
HTTP Header Compression does not preserve the relative ordering of header fields. Header
fields with multiple values are encoded into a single header field using a special
delimiter; see <xref target="HeaderOrdering"/>.
</t>
<t>
The <xref target="COOKIE">Cookie header field</xref> is treated specially by the HTTP
mapping; see <xref target="CompressCookie"/>.
</t>
<t>
A receiving endpoint reassembles the header block by concatenating its fragments, then
decompresses the block to reconstruct the header set.
</t>
<t>
A complete header block consists of either:
<list style="symbols">
<t>
a single <xref target="HEADERS" format="none">HEADERS</xref> or <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame,
with the END_HEADERS flag set, or
</t>
<t>
a <xref target="HEADERS" format="none">HEADERS</xref> or <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame with the END_HEADERS
flag cleared and one or more <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames,
where the last <xref target="CONTINUATION" format="none">CONTINUATION</xref> frame has the END_HEADERS flag set.
</t>
</list>
</t>
<t>
Header compression is stateful, using a single compression context for the entire
connection. Each header block is processed as a discrete unit. Header blocks MUST be
transmitted as a contiguous sequence of frames, with no interleaved frames of any other
type or from any other stream. The last frame in a sequence of <xref target="HEADERS" format="none">HEADERS</xref> or
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames MUST have the END_HEADERS flag set. The last frame in
a sequence of <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> or <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames MUST have
the END_HEADERS flag set.
</t>
<t>
Header block fragments can only be sent as the payload of <xref target="HEADERS" format="none">HEADERS</xref>,
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> or <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames, because these frames
carry data that can modify the compression context maintained by a receiver. An endpoint
receiving <xref target="HEADERS" format="none">HEADERS</xref>, <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> or
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames MUST reassemble header blocks and perform decompression
even if the frames are to be discarded. A receiver MUST terminate the connection with a
<xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="COMPRESSION_ERROR" format="none">COMPRESSION_ERROR</xref> if it does not decompress a header block.
</t>
</section>
</section>
<section anchor="StreamsLayer" title="Streams and Multiplexing">
<t>
A "stream" is an independent, bi-directional sequence of frames exchanged between the client
and server within an HTTP/2 connection. Streams have several important characteristics:
<list style="symbols">
<t>
A single HTTP/2 connection can contain multiple concurrently open streams, with either
endpoint interleaving frames from multiple streams.
</t>
<t>
Streams can be established and used unilaterally or shared by either the client or
server.
</t>
<t>
Streams can be closed by either endpoint.
</t>
<t>
The order in which frames are sent within a stream is significant. Recipients process
frames in the order they are received.
</t>
<t>
Streams are identified by an integer. Stream identifiers are assigned to streams by the
endpoint initiating the stream.
</t>
</list>
</t>
<section anchor="StreamStates" title="Stream States">
<t>
The lifecycle of a stream is shown in <xref target="StreamStatesFigure"/>.
</t>
<figure anchor="StreamStatesFigure" title="Stream States">
<artwork type="drawing"><![CDATA[
+--------+
PP | | PP
,--------| idle |--------.
/ | | \
v +--------+ v
+----------+ | +----------+
| | | H | |
,---| reserved | | | reserved |---.
| | (local) | v | (remote) | |
| +----------+ +--------+ +----------+ |
| | ES | | ES | |
| | H ,-------| open |-------. | H |
| | / | | \ | |
| v v +--------+ v v |
| +----------+ | +----------+ |
| | half | | | half | |
| | closed | | R | closed | |
| | (remote) | | | (local) | |
| +----------+ | +----------+ |
| | v | |
| | ES / R +--------+ ES / R | |
| `----------->| |<-----------' |
| R | closed | R |
`-------------------->| |<--------------------'
+--------+
H: HEADERS frame (with implied CONTINUATIONs)
PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
ES: END_STREAM flag
R: RST_STREAM frame
]]></artwork>
</figure>
<t>
Both endpoints have a subjective view of the state of a stream that could be different
when frames are in transit. Endpoints do not coordinate the creation of streams; they are
created unilaterally by either endpoint. The negative consequences of a mismatch in
states are limited to the "closed" state after sending <xref target="RST_STREAM" format="none">RST_STREAM</xref>, where
frames might be received for some time after closing.
</t>
<t>
Streams have the following states:
<list style="hanging">
<t hangText="idle:">
<vspace blankLines="0"/>
All streams start in the "idle" state. In this state, no frames have been
exchanged.
<vspace blankLines="1"/>
The following transitions are valid from this state:
<list style="symbols">
<t>
Sending or receiving a <xref target="HEADERS" format="none">HEADERS</xref> frame causes the stream to become
"open". The stream identifier is selected as described in <xref target="StreamIdentifiers"/>. The same <xref target="HEADERS" format="none">HEADERS</xref> frame can also
cause a stream to immediately become "half closed".
</t>
<t>
Sending a <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame marks the associated stream for
later use. The stream state for the reserved stream transitions to "reserved
(local)".
</t>
<t>
Receiving a <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame marks the associated stream as
reserved by the remote peer. The state of the stream becomes "reserved
(remote)".
</t>
</list>
</t>
<t hangText="reserved (local):">
<vspace blankLines="0"/>
A stream in the "reserved (local)" state is one that has been promised by sending a
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame. A <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame reserves an
idle stream by associating the stream with an open stream that was initiated by the
remote peer (see <xref target="PushResources"/>).
<vspace blankLines="1"/>
In this state, only the following transitions are possible:
<list style="symbols">
<t>
The endpoint can send a <xref target="HEADERS" format="none">HEADERS</xref> frame. This causes the stream to
open in a "half closed (remote)" state.
</t>
<t>
Either endpoint can send a <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame to cause the stream
to become "closed". This releases the stream reservation.
</t>
</list>
<vspace blankLines="1"/>
An endpoint MUST NOT send frames other than <xref target="HEADERS" format="none">HEADERS</xref> or
<xref target="RST_STREAM" format="none">RST_STREAM</xref> in this state.
<vspace blankLines="1"/>
A <xref target="PRIORITY" format="none">PRIORITY</xref> frame MAY be received in this state. Receiving any frames
other than <xref target="RST_STREAM" format="none">RST_STREAM</xref>, or <xref target="PRIORITY" format="none">PRIORITY</xref> MUST be treated as
a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t hangText="reserved (remote):">
<vspace blankLines="0"/>
A stream in the "reserved (remote)" state has been reserved by a remote peer.
<vspace blankLines="1"/>
In this state, only the following transitions are possible:
<list style="symbols">
<t>
Receiving a <xref target="HEADERS" format="none">HEADERS</xref> frame causes the stream to transition to
"half closed (local)".
</t>
<t>
Either endpoint can send a <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame to cause the stream
to become "closed". This releases the stream reservation.
</t>
</list>
<vspace blankLines="1"/>
An endpoint MAY send a <xref target="PRIORITY" format="none">PRIORITY</xref> frame in this state to reprioritize
the reserved stream. An endpoint MUST NOT send any other type of frame other than
<xref target="RST_STREAM" format="none">RST_STREAM</xref> or <xref target="PRIORITY" format="none">PRIORITY</xref>.
<vspace blankLines="1"/>
Receiving any other type of frame other than <xref target="HEADERS" format="none">HEADERS</xref> or
<xref target="RST_STREAM" format="none">RST_STREAM</xref> MUST be treated as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t hangText="open:">
<vspace blankLines="0"/>
A stream in the "open" state may be used by both peers to send frames of any type.
In this state, sending peers observe advertised <xref target="FlowControl">stream
level flow control limits</xref>.
<vspace blankLines="1"/>
From this state either endpoint can send a frame with an END_STREAM flag set, which
causes the stream to transition into one of the "half closed" states: an endpoint
sending an END_STREAM flag causes the stream state to become "half closed (local)"; an
endpoint receiving an END_STREAM flag causes the stream state to become "half closed
(remote)". A <xref target="HEADERS" format="none">HEADERS</xref> frame bearing an END_STREAM flag can be followed
by <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames.
<vspace blankLines="1"/>
Either endpoint can send a <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame from this state, causing it
to transition immediately to "closed".
</t>
<t hangText="half closed (local):">
<vspace blankLines="0"/>
A stream that is in the "half closed (local)" state cannot be used for sending frames.
<vspace blankLines="1"/>
A stream transitions from this state to "closed" when a frame that contains an
END_STREAM flag is received, or when either peer sends a <xref target="RST_STREAM" format="none">RST_STREAM</xref>
frame. A <xref target="HEADERS" format="none">HEADERS</xref> frame bearing an END_STREAM flag can be followed by
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames.
<vspace blankLines="1"/>
A receiver can ignore <xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref> or <xref target="PRIORITY" format="none">PRIORITY</xref> frames
in this state. These frame types might arrive for a short period after a frame
bearing the END_STREAM flag is sent.
</t>
<t hangText="half closed (remote):">
<vspace blankLines="0"/>
A stream that is "half closed (remote)" is no longer being used by the peer to send
frames. In this state, an endpoint is no longer obligated to maintain a receiver
flow control window if it performs flow control.
<vspace blankLines="1"/>
If an endpoint receives additional frames for a stream that is in this state, other
than <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames, it MUST respond with a <xref target="StreamErrorHandler">stream error</xref> of type
<xref target="STREAM_CLOSED" format="none">STREAM_CLOSED</xref>.
<vspace blankLines="1"/>
A stream can transition from this state to "closed" by sending a frame that contains
an END_STREAM flag, or when either peer sends a <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame.
</t>
<t hangText="closed:">
<vspace blankLines="0"/>
The "closed" state is the terminal state.
<vspace blankLines="1"/>
An endpoint MUST NOT send frames on a closed stream. An endpoint that receives any
frame after receiving a <xref target="RST_STREAM" format="none">RST_STREAM</xref> MUST treat that as a <xref target="StreamErrorHandler">stream error</xref> of type
<xref target="STREAM_CLOSED" format="none">STREAM_CLOSED</xref>. Similarly, an endpoint that receives any frames after
receiving a <xref target="DATA" format="none">DATA</xref> frame with the END_STREAM flag set, or any frames
except a <xref target="CONTINUATION" format="none">CONTINUATION</xref> frame after receiving a <xref target="HEADERS" format="none">HEADERS</xref>
frame with an END_STREAM flag set MUST treat that as a <xref target="StreamErrorHandler">stream error</xref> of type
<xref target="STREAM_CLOSED" format="none">STREAM_CLOSED</xref>.
<vspace blankLines="1"/>
<xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref>, <xref target="PRIORITY" format="none">PRIORITY</xref>, or <xref target="RST_STREAM" format="none">RST_STREAM</xref>
frames can be received in this state for a short period after a <xref target="DATA" format="none">DATA</xref>
or <xref target="HEADERS" format="none">HEADERS</xref> frame containing an END_STREAM flag is sent. Until the
remote peer receives and processes the frame bearing the END_STREAM flag, it might
send frame of any of these types. Endpoints MUST ignore
<xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref>, <xref target="PRIORITY" format="none">PRIORITY</xref>, or <xref target="RST_STREAM" format="none">RST_STREAM</xref>
frames received in this state, though endpoints MAY choose to treat frames that
arrive a significant time after sending END_STREAM as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
<vspace blankLines="1"/>
If this state is reached as a result of sending a <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame,
the peer that receives the <xref target="RST_STREAM" format="none">RST_STREAM</xref> might have already sent - or
enqueued for sending - frames on the stream that cannot be withdrawn. An endpoint
MUST ignore frames that it receives on closed streams after it has sent a
<xref target="RST_STREAM" format="none">RST_STREAM</xref> frame. An endpoint MAY choose to limit the period over
which it ignores frames and treat frames that arrive after this time as being in
error.
<vspace blankLines="1"/>
Flow controlled frames (i.e., <xref target="DATA" format="none">DATA</xref>) received after sending
<xref target="RST_STREAM" format="none">RST_STREAM</xref> are counted toward the connection flow control window.
Even though these frames might be ignored, because they are sent before the sender
receives the <xref target="RST_STREAM" format="none">RST_STREAM</xref>, the sender will consider the frames to count
against the flow control window.
<vspace blankLines="1"/>
An endpoint might receive a <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame after it sends
<xref target="RST_STREAM" format="none">RST_STREAM</xref>. <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> causes a stream to become
"reserved" even if the associated stream has been reset. Therefore, a
<xref target="RST_STREAM" format="none">RST_STREAM</xref> is needed to close an unwanted promised streams.
</t>
</list>
</t>
<t>
In the absence of more specific guidance elsewhere in this document, implementations
SHOULD treat the receipt of a message that is not expressly permitted in the description
of a state as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<section anchor="StreamIdentifiers" title="Stream Identifiers">
<t>
Streams are identified with an unsigned 31-bit integer. Streams initiated by a client
MUST use odd-numbered stream identifiers; those initiated by the server MUST use
even-numbered stream identifiers. A stream identifier of zero (0x0) is used for
connection control messages; the stream identifier zero MUST NOT be used to establish a
new stream.
</t>
<t>
HTTP/1.1 requests that are upgraded to HTTP/2 (see <xref target="discover-http"/>) are
responded to with a stream identifier of one (0x1). After the upgrade
completes, stream 0x1 is "half closed (local)" to the client. Therefore, stream 0x1
cannot be selected as a new stream identifier by a client that upgrades from HTTP/1.1.
</t>
<t>
The identifier of a newly established stream MUST be numerically greater than all
streams that the initiating endpoint has opened or reserved. This governs streams that
are opened using a <xref target="HEADERS" format="none">HEADERS</xref> frame and streams that are reserved using
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref>. An endpoint that receives an unexpected stream identifier
MUST respond with a <xref target="ConnectionErrorHandler">connection error</xref> of
type <xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
The first use of a new stream identifier implicitly closes all streams in the "idle"
state that might have been initiated by that peer with a lower-valued stream identifier.
For example, if a client sends a <xref target="HEADERS" format="none">HEADERS</xref> frame on stream 7 without ever
sending a frame on stream 5, then stream 5 transitions to the "closed" state when the
first frame for stream 7 is sent or received.
</t>
<t>
Stream identifiers cannot be reused. Long-lived connections can result in endpoint
exhausting 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>
</section>
<section title="Stream Concurrency">
<t>
A peer can limit the number of concurrently active streams using the
<xref target="SETTINGS_MAX_CONCURRENT_STREAMS" format="none">SETTINGS_MAX_CONCURRENT_STREAMS</xref> parameters within a
<xref target="SETTINGS" format="none">SETTINGS</xref> frame. The maximum concurrent streams setting is specific to
each endpoint and applies only to the peer that receives the setting. 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. Endpoints
MUST NOT exceed the limit set by their peer.
</t>
<t>
Streams that are in the "open" state, or either of the "half closed" states count toward
the maximum number of streams that an endpoint is permitted to open. Streams in any of
these three states count toward the limit advertised in the
<xref target="SETTINGS_MAX_CONCURRENT_STREAMS" format="none">SETTINGS_MAX_CONCURRENT_STREAMS</xref> setting (see <xref target="SettingValues"/>).
</t>
<t>
An endpoint that receives a <xref target="HEADERS" format="none">HEADERS</xref> frame that causes their advertised
concurrent stream limit to be exceeded MUST treat this as a <xref target="StreamErrorHandler">stream error</xref>.
</t>
<t>
Streams in either of the "reserved" states do not count as open.
</t>
</section>
</section>
<section anchor="FlowControl" title="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. Flow control is used for both
individual streams and for the connection as a whole.
</t>
<t>
HTTP/2 provides for flow control through use of the <xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref> frame
type.
</t>
<section anchor="fc-principles" title="Flow Control Principles">
<t>
HTTP/2 stream flow control aims to allow for future improvements to flow control
algorithms without requiring protocol changes. Flow control in HTTP/2 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 bytes
they are prepared to receive on a stream and for the entire connection. 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 window 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 65,535 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 <xref target="DATA" format="none">DATA</xref> 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 cannot be disabled.
</t>
<t>
HTTP/2 standardizes only the format of the <xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref> frame (<xref target="WINDOW_UPDATE"/>). 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 anchor="DisableFlowControl" title="Appropriate Use of Flow Control">
<t>
Flow control is defined to protect endpoints 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 can advertise a flow control window of
the maximum size, incrementing the available space when new data is received. 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 can be difficult. When using flow control, the receiver MUST read from the TCP
receive buffer in a timely fashion. Failure to do so could lead to a deadlock when
critical frames, such as <xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref>, are not available to HTTP/2.
However, flow control can ensure that constrained resources are protected without any
reduction in connection utilization.
</t>
</section>
</section>
<section anchor="StreamPriority" title="Stream priority">
<t>
A client can assign a priority for a new stream by including prioritization information in
the <xref target="HEADERS">HEADERS frame</xref> that opens the stream. For an existing
stream, the <xref target="PRIORITY">PRIORITY frame</xref> can be used to change the
priority.
</t>
<t>
The purpose of prioritization is to allow an endpoint to express how it would prefer its
peer allocate resources when managing concurrent streams. Most importantly, priority can
be used to select streams for transmitting frames when there is limited capacity for
sending.
</t>
<t>
Each stream is prioritized into a group. Each group is identified using an identifier
that is selected by the client. Each group is assigned a relative weight, a number that
is used to determine the relative proportion of available resources that are assigned to
that group.
</t>
<t>
Within a priority group, streams can also be marked as being dependent on the completion
of other streams.
</t>
<t>
Explicitly setting the priority for a stream is input to a prioritization process. It
does not guarantee any particular processing or transmission order for the stream relative
to any other stream. An endpoint cannot force a peer to process concurrent streams in a
particular order using priority. Expressing priority is therefore only ever a suggestion.
</t>
<t>
Prioritization information can be specified explicitly for streams as they are created
using the <xref target="HEADERS" format="none">HEADERS</xref> frame, or changed using the <xref target="PRIORITY" format="none">PRIORITY</xref>
frame. Providing prioritization information is optional, so default values are used if no
explicit indicator is provided (<xref target="pri-default"/>).
</t>
<t>
Explicit prioritization information can be provided for a stream to either allocate the
stream to a priority group (<xref target="pri-group"/>), or to create a dependency on
another stream (<xref target="pri-depend"/>).
</t>
<section title="Priority Groups and Weighting" anchor="pri-group">
<t>
All streams are assigned a priority group. Each priority group is allocated a 31-bit
identifier and an integer weight between 1 to 256 (inclusive).
</t>
<t>
Specifying a priority group and weight for a stream causes the stream to be assigned to
the identified priority group and for the weight for the group to be changed to the new
value.
</t>
<t>
Resources are divided proportionally between priority groups based on their weight. For
example, a priority group with weight 4 ideally receives one third of the resources
allocated to a stream with weight 12.
</t>
</section>
<section title="Stream Dependencies" anchor="pri-depend">
<t>
Each stream can be given an explicit dependency on another stream. Including a
dependency expresses a preference to allocate resources to the identified stream rather
than to the dependent stream.
</t>
<t>
A stream that is dependent on another stream becomes part of the priority group of the
stream it depends on. It belongs to the same dependency tree as the stream it depends
on.
</t>
<t>
A stream that is assigned directly to a priority group is not dependent on any other
stream. It is the root of a dependency tree inside its priority group.
</t>
<!-- <t>
Stream dependencies form a tree. Instead, streams that depend on another stream
also depend on all other streams at the same dependency level. For example, if both B
and C depend on A, then creating a dependency from D to B also implicitly creates a
dependency on C.
</t> -->
<t>
When assigning a dependency on another stream, by default, the stream is added as a new
dependency of the stream it depends on. For example, if streams B and C are dependent on
stream A, and if stream D is created with a dependency on stream A, this results in a
dependency order of A followed by B, C, and D.
</t>
<figure title="Example of Default Dependency Creation">
<artwork type="inline"><![CDATA[
A A
/ \ ==> /|\
B C B D C
]]></artwork>
</figure>
<t>
An exclusive flag allows for the insertion of a new level of dependencies. The
exclusive flag causes the stream to become the sole dependency of the stream it depends
on, causing other dependencies to become dependencies of the stream. In the previous
example, if stream D is created with an exclusive dependency on stream A, this results
in a dependency order of A followed by D followed by B and C.
</t>
<figure title="Example of Exclusive Dependency Creation">
<artwork type="inline"><![CDATA[
A
A |
/ \ ==> D
B C / \
B C
]]></artwork>
</figure>
<t>
Streams are ordered into several dependency trees within their priority group. Each
dependency tree within a priority group SHOULD be allocated the same amount of
resources.
</t>
<t>
Inside a dependency tree, a dependent stream SHOULD only be allocated resources if the
streams that it depends on are either closed, or it is not possible to make progress on
them.
</t>
<t>
Streams with the same dependencies SHOULD be allocated the same amount of resources.
Thus, if streams B and C depend on stream A, and if no progress can be made on A,
streams B and C are given an equal share of resources.
</t>
<t>
A stream MUST NOT depend on itself. An endpoint MAY either treat this as a <xref target="StreamErrorHandler">stream error</xref> of type <xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>,
or assign <xref target="pri-default">default priority values</xref> to the stream.
</t>
</section>
<section title="Reprioritization">
<t>
Stream priorities are changed using the <xref target="PRIORITY" format="none">PRIORITY</xref> frame. Setting a
priority group and weight causes a stream to become part of the identified group, and
not dependent on any other stream. Setting a dependency causes a stream to become
dependent on the identified stream, which can cause the reprioritized stream to move to
a new priority group.
</t>
<t>
All streams that are dependent on a reprioritized stream move with it. Setting a
dependency with the exclusive flag for a reprioritized stream moves all the dependencies
of the stream it depends on to become dependencies of the reprioritized stream.
</t>
</section>
<section title="Prioritization State Management">
<t>
When a stream is closed, its dependencies can be moved to become dependent on the stream
the closed stream depends on, if any, or to become new dependency tree roots otherwise.
</t>
<t>
It is possible for a stream to become closed while prioritization information that
creates a dependency on that stream is in transit. If a stream identified in a
dependency has been closed and any associated priority information destroyed then the
dependent stream is instead assigned a default priority. This potentially creates
suboptimal prioritization, since the stream can be given an effective priority that is
higher than expressed by a peer.
</t>
<t>
To avoid this problem, endpoints SHOULD maintain prioritization state for closed streams
for a period after streams close. This could create an large state burden for an
endpoint, so this state MAY be limited. The amount of additional state an endpoint
maintains could be dependent on load; under high load, prioritization state can be
discarded to limit resource commitments. In extreme cases, an endpoint could even
discard prioritization state for active or reserved streams.
</t>
<t>
An endpoint SHOULD retain stream prioritization state for at least one round trip,
though maintaining state over longer periods reduces the chance that default values have
to be assigned to streams. An endpoint MAY apply a fixed upper limit on the number of
closed streams for which prioritization state is tracked to limit state exposure. If a
fixed limit is applied, endpoints SHOULD maintain state for at least as many streams as
allowed by their setting for <xref target="SETTINGS_MAX_CONCURRENT_STREAMS" format="none">SETTINGS_MAX_CONCURRENT_STREAMS</xref>.
</t>
<t>
An endpoint receiving a <xref target="PRIORITY" format="none">PRIORITY</xref> frame that changes the priority of a
closed stream SHOULD alter the weight of the priority group, or the dependencies
of the streams that depend on it, if it has retained enough state to do so.
</t>
<t>
Priority group information is part of the priority state of a stream. Priority groups
that contain only closed streams can be assigned a weight of zero.
</t>
<t>
The number of priority groups cannot exceed the number of non-closed streams. This
includes streams in the "reserved" state. Priority state size for peer-initiated
streams is limited by the value of <xref target="SETTINGS_MAX_CONCURRENT_STREAMS" format="none">SETTINGS_MAX_CONCURRENT_STREAMS</xref>.
Reserved streams do not count toward the concurrent stream limit of either peer, but
only the endpoint that creates the reservation needs to maintain priority information.
Thus, the total amount of priority state for non-closed streams can be limited by an
endpoint.
</t>
</section>
<section title="Default Priorities" anchor="pri-default">
<t>
Providing priority information is optional. Streams are assigned to a priority group
with an identifier equal to the stream identifier and a weight of 16.
</t>
<t>
<xref target="PushResources">Pushed streams</xref> initially depend on their associated
stream.
</t>
</section>
</section>
<section title="Error Handling">
<t>
HTTP/2 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>
<t>
A list of error codes is included in <xref target="ErrorCodes"/>.
</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 SHOULD first send a <xref target="GOAWAY" format="none">GOAWAY</xref>
frame (<xref target="GOAWAY"/>) with the stream identifier of the last stream that it
successfully received from its peer. The <xref target="GOAWAY" format="none">GOAWAY</xref> frame includes an error
code that indicates why the connection is terminating. After sending the
<xref target="GOAWAY" format="none">GOAWAY</xref> frame, the endpoint MUST close the TCP connection.
</t>
<t>
It is possible that the <xref target="GOAWAY" format="none">GOAWAY</xref> will not be reliably received by the
receiving endpoint. In the event of a connection error, <xref target="GOAWAY" format="none">GOAWAY</xref> 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. Endpoints SHOULD send a
<xref target="GOAWAY" format="none">GOAWAY</xref> 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.
</t>
<t>
An endpoint that detects a stream error sends a <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame (<xref target="RST_STREAM"/>) that contains the stream identifier of the stream where the error
occurred. The <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame includes an error code that indicates the
type of error.
</t>
<t>
A <xref target="RST_STREAM" format="none">RST_STREAM</xref> is the last frame that an endpoint can send on a stream.
The peer that sends the <xref target="RST_STREAM" format="none">RST_STREAM</xref> 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 <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame for
any stream. However, an endpoint MAY send additional <xref target="RST_STREAM" format="none">RST_STREAM</xref> 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 <xref target="RST_STREAM" format="none">RST_STREAM</xref> in response to an
<xref target="RST_STREAM" format="none">RST_STREAM</xref> frame, to avoid looping.
</t>
</section>
<section title="Connection Termination">
<t>
If the TCP connection is torn down while streams remain in open or half closed states,
then the endpoint MUST assume that those streams were abnormally interrupted and could
be incomplete.
</t>
</section>
</section>
</section>
<section anchor="frame-types" title="Frame Definitions">
<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.
</t>
<section anchor="DATA" title="DATA">
<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>
DATA frames MAY also contain arbitrary padding. Padding can be added to DATA frames to
hide the size of messages.
</t>
<figure title="DATA Frame Payload">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pad High? (8) | Pad Low? (8) |
+---------------+---------------+-------------------------------+
| Data (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The DATA frame contains the following fields:
<list style="hanging">
<t hangText="Pad High:">
An 8-bit field containing an amount of padding in units of 256 octets. This field
is optional and is only present if the PAD_HIGH flag is set. This field, in
combination with Pad Low, determines how much padding there is on a frame.
</t>
<t hangText="Pad Low:">
An 8-bit field containing an amount of padding in units of single octets. This
field is optional and is only present if the PAD_LOW flag is set. This field, in
combination with Pad High, determines how much padding there is on a frame.
</t>
<t hangText="Data:">
Application data. The amount of data is the remainder of the frame payload after
subtracting the length of the other fields that are present.
</t>
<t hangText="Padding:">
Padding octets that contain no application semantic value. Padding octets MUST be
set to zero when sending and ignored when receiving.
</t>
</list>
</t>
<t>
The DATA frame defines the following flags:
<list style="hanging">
<t hangText="END_STREAM (0x1):">
Bit 1 being set indicates that this frame is the last that the endpoint will send
for the identified stream. Setting this flag causes the stream to enter one of
<xref target="StreamStates">the "half closed" states or the "closed" state</xref>.
</t>
<t hangText="END_SEGMENT (0x2):">
Bit 2 being set indicates that this frame is the last for the current segment.
Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve
segment boundaries when forwarding frames.
</t>
<t hangText="PAD_LOW (0x08):">
Bit 4 being set indicates that the Pad Low field is present.
</t>
<t hangText="PAD_HIGH (0x10):">
Bit 5 being set indicates that the Pad High field is present. This bit MUST NOT be
set unless the PAD_LOW flag is also set. Endpoints that receive a frame with
PAD_HIGH set and PAD_LOW cleared MUST treat this as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
</list>
</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
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
DATA frames are subject to flow control and can only be sent when a stream is in the
"open" or "half closed (remote)" states. Padding is included in flow control. If
a DATA frame is received whose stream is not in "open" or "half closed (local)" state,
the recipient MUST respond with a <xref target="StreamErrorHandler">stream error</xref>
of type <xref target="STREAM_CLOSED" format="none">STREAM_CLOSED</xref>.
</t>
<t>
The total number of padding octets is determined by multiplying the value of the Pad
High field by 256 and adding the value of the Pad Low field. Both Pad High and Pad Low
fields assume a value of zero if absent. If the length of the padding is greater than
the length of the remainder of the frame payload, the recipient MUST treat this as a
<xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
<list style="hanging">
<t hangText="Note:">
A frame can be increased in size by one octet by including a Pad Low field with a
value of zero.
</t>
</list>
</t>
<t>
Use of padding is a security feature; as such, its use demands some care, see <xref target="padding"/>.
</t>
</section>
<section anchor="HEADERS" title="HEADERS">
<t>
The HEADERS frame (type=0x1) carries name-value pairs. It is used to <xref target="StreamStates">open a stream</xref>. HEADERS frames can be sent on a stream in
the "open" or "half closed (remote)" states.
</t>
<figure title="HEADERS Frame Payload">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pad High? (8) | Pad Low? (8) |
+-+-------------+---------------+-------------------------------+
|R| Priority Group Identifier? (31) |
+-+-------------+-----------------------------------------------+
| Weight? (8) |
+-+-------------+-----------------------------------------------+
|E| Stream Dependency? (31) |
+-+-------------------------------------------------------------+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The HEADERS frame payload has the following fields:
<list style="hanging">
<t hangText="Pad High:">
Padding size high bits. This field is only present if the PAD_HIGH flag is set.
</t>
<t hangText="Pad Low:">
Padding size low bits. This field is only present if the PAD_LOW flag is set.
</t>
<t hangText="R:">
A single reserved bit. This field is optional and is only present if the
PRIORITY_GROUP flag is set.
</t>
<t hangText="Priority Group Identifier:">
A 31-bit identifier for a priority group, see <xref target="StreamPriority"/>. This
field is optional and is only present if the PRIORITY_GROUP flag is set.
</t>
<t hangText="Weight:">
An 8-bit weight for the identified priority group, see <xref target="StreamPriority"/>. This field is optional and is only present if the
PRIORITY_GROUP flag is set.
</t>
<t hangText="E:">
A single bit flag indicates that the stream dependency is exclusive, see <xref target="StreamPriority"/>. This field is optional and is only present if the
PRIORITY_DEPENDENCY flag is set.
</t>
<t hangText="Stream Dependency:">
A 31-bit stream identifier for the stream that this stream depends on, see <xref target="StreamPriority"/>. This field is optional and is only present if the
PRIORITY_DEPENDENCY flag is set.
</t>
<t hangText="Header Block Fragment:">
A <xref target="HeaderBlock">header block fragment</xref>.
</t>
<t hangText="Padding:">
Padding octets.
</t>
</list>
</t>
<t>
The HEADERS frame defines the following flags:
<list style="hanging">
<t hangText="END_STREAM (0x1):">
Bit 1 being set indicates that the <xref target="HeaderBlock">header block</xref>
is the last that the endpoint will send for the identified stream. Setting this
flag causes the stream to enter one of <xref target="StreamStates">"half closed"
states</xref>.
<vspace blankLines="1"/>
A HEADERS frame that is followed by <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames carries the
END_STREAM flag that signals the end of a stream. A <xref target="CONTINUATION" format="none">CONTINUATION</xref>
frame cannot be used to terminate a stream.
</t>
<t hangText="END_SEGMENT (0x2):">
Bit 2 being set indicates that this frame is the last for the current segment.
Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve
segment boundaries when forwarding frames.
</t>
<t hangText="END_HEADERS (0x4):">
Bit 3 being set indicates that this frame contains an entire <xref target="HeaderBlock">header block</xref> and is not followed by any
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames.
<vspace blankLines="1"/>
A HEADERS frame without the END_HEADERS flag set MUST be followed by a
<xref target="CONTINUATION" format="none">CONTINUATION</xref> 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
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t hangText="PAD_LOW (0x8):">
Bit 4 being set indicates that the Pad Low field is present.
</t>
<t hangText="PAD_HIGH (0x10):">
Bit 5 being set indicates that the Pad High field is present. This bit MUST NOT
be set unless the PAD_LOW flag is also set. Endpoints that receive a frame with
PAD_HIGH set and PAD_LOW cleared MUST treat this as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t hangText="PRIORITY_GROUP (0x20):">
Bit 6 being set indicates that the Priority Group Identifier and Weight fields are
present; see <xref target="StreamPriority"/>.
</t>
<t hangText="PRIORITY_DEPENDENCY (0x40):">
Bit 7 being set indicates that the Exclusive Flag (E) and Stream
Dependency fields are present; see <xref target="StreamPriority"/>.
</t>
</list>
</t>
<t>
The payload of a HEADERS frame contains a <xref target="HeaderBlock">header block
fragment</xref>. A header block that does not fit within a HEADERS frame is continued
in a <xref target="CONTINUATION">CONTINUATION frame</xref>.
</t>
<t>
HEADERS frames MUST be associated with a stream. If a HEADERS frame is received whose
stream identifier field is 0x0, the recipient MUST respond with a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
A HEADERS frame MUST NOT have both the PRIORITY_GROUP and PRIORITY_DEPENDENCY flags set.
Receipt of a HEADERS frame with both these flags set MUST be treated as a <xref target="StreamErrorHandler">stream error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
The HEADERS frame changes the connection state as described in <xref target="HeaderBlock"/>.
</t>
<t>
The HEADERS frame includes optional padding. Padding fields and flags are identical to
those defined for <xref target="DATA">DATA frames</xref>.
</t>
</section>
<section anchor="PRIORITY" title="PRIORITY">
<t>
The PRIORITY frame (type=0x2) specifies the <xref target="StreamPriority">sender-advised
priority of a stream</xref>. It can be sent at any time for an existing stream. This
enables reprioritisation of existing streams.
</t>
<figure title="PRIORITY Frame Payload">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Priority Group Identifier? (31) |
+-+-------------+-----------------------------------------------+
| Weight? (8) |
+-+-------------+-----------------------------------------------+
|E| Stream Dependency? (31) |
+-+-------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The payload of a PRIORITY frame contains the following fields:
<list style="hanging">
<t hangText="R:">
A single reserved bit. This field is optional and is only present if the
PRIORITY_GROUP flag is set.
</t>
<t hangText="Priority Group Identifier:">
A 31-bit identifier for a priority group, see <xref target="StreamPriority"/>. This
field is optional and is only present if the PRIORITY_GROUP flag is set.
</t>
<t hangText="Weight:">
An 8-bit weight for the identified priority group, see <xref target="StreamPriority"/>. This field is optional and is only present if the
PRIORITY_GROUP flag is set.
</t>
<t hangText="E:">
A single bit flag indicates that the stream dependency is exclusive, see <xref target="StreamPriority"/>. This field is optional and is only present if the
PRIORITY_DEPENDENCY flag is set.
</t>
<t hangText="Stream Dependency:">
A 31-bit stream identifier for the stream that this stream depends on, see <xref target="StreamPriority"/>. This field is optional and is only present if
the PRIORITY_DEPENDENCY flag is set.
</t>
</list>
</t>
<t>
The PRIORITY frame defines the following flags:
<list style="hanging">
<t hangText="PRIORITY_GROUP (0x20):">
Bit 6 being set indicates that the Priority Group Identifier and Weight fields are
present; see <xref target="StreamPriority"/>.
</t>
<t hangText="PRIORITY_DEPENDENCY (0x40):">
Bit 7 being set indicates that the Exclusive Flag (E) and Stream
Dependency fields are present; see <xref target="StreamPriority"/>.
</t>
</list>
</t>
<t>
A PRIORITY frame MUST have exactly one of the PRIORITY_GROUP and PRIORITY_DEPENDENCY
flags set. Receipt of a PRIORITY frame with either none or both these flags set MUST be
treated as a <xref target="StreamErrorHandler">stream error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
The PRIORITY frame is associated with an existing stream. If a PRIORITY frame is
received with a stream identifier of 0x0, the recipient MUST respond with a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
The PRIORITY frame can be sent on a stream in any of the "reserved (remote)", "open",
"half-closed (local)", or "half closed (remote)" states, though it cannot be sent
between consecutive frames that comprise a single <xref target="HeaderBlock">header
block</xref>. Note that this frame could arrive after processing or frame sending has
completed, which would cause it to have no effect. For a stream that is in the "half
closed (remote)" state, this frame can only affect processing of the stream and not
frame transmission.
</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 or that
an error condition has occurred. 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"><![CDATA[
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>
The RST_STREAM frame does not define any flags.
</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
with a stream identifier of 0x0, the recipient MUST treat this as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
RST_STREAM frames MUST NOT be sent for a stream in the "idle" state. If a RST_STREAM
frame identifying an idle stream is received, the recipient MUST treat this as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
</section>
<section anchor="SETTINGS" title="SETTINGS">
<t>
The SETTINGS frame (type=0x4) conveys configuration parameters (such as preferences and
constraints on peer behavior) that affect how endpoints communicate, and is also used
to acknowledge the receipt of those parameters. Individually, a SETTINGS parameter can
also be referred to as a "setting".
</t>
<t>
SETTINGS parameters are not negotiated; they describe characteristics of the sending
peer, which are used by the receiving peer. Different values for the same parameter can
be advertised by each peer. For example, a client might set a high initial flow control
window, whereas a server might set a lower value to conserve resources.
</t>
<t>
A SETTINGS frame MUST be sent by both endpoints at the start of a connection, and MAY
be sent at any other time by either endpoint over the lifetime of the connection.
Implementations MUST support all of the parameters defined by this specification.
</t>
<t>
Each parameter in a SETTINGS frame replaces any existing value for that parameter.
Parameters are processed in the order in which they appear, and a receiver of a
SETTINGS frame does not need to maintain any state other than the current value of its
parameters. Therefore, the value of a SETTINGS parameter is the last value that is seen
by a receiver.
</t>
<t>
SETTINGS parameters are acknowledged by the receiving peer. To enable this, the SETTINGS
frame defines the following flag:
<list style="hanging">
<t hangText="ACK (0x1):">
Bit 1 being set indicates that this frame acknowledges receipt and application of
the peer's SETTINGS frame. When this bit is set, the payload of the SETTINGS frame
MUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a length field
value other than 0 MUST be treated as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="FRAME_SIZE_ERROR" format="none">FRAME_SIZE_ERROR</xref>. For more info, see <xref target="SettingsSync">Settings Synchronization</xref>.
</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
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
The SETTINGS frame affects connection state. A badly formed or incomplete SETTINGS
frame MUST be treated as a <xref target="ConnectionErrorHandler">connection error</xref>
of type <xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<section title="SETTINGS Format" anchor="SettingFormat">
<t>
The payload of a SETTINGS frame consists of zero or more parameters, each consisting
of an unsigned 8-bit identifier and an unsigned 32-bit value.
</t>
<figure title="Setting Format">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier (8)|
+---------------+-----------------------------------------------+
| Value (32) |
+---------------------------------------------------------------+
]]></artwork>
</figure>
</section>
<section anchor="SettingValues" title="Defined SETTINGS Parameters">
<t>
The following parameters are defined:
<list style="hanging">
<t hangText="SETTINGS_HEADER_TABLE_SIZE (1):" anchor="SETTINGS_HEADER_TABLE_SIZE">
Allows the sender to inform the remote endpoint of the size of the
header compression table used to decode header blocks. The encoder can reduce
this size by using signaling specific to the header compression format inside a
header block. The initial value is 4,096 bytes.
</t>
<t hangText="SETTINGS_ENABLE_PUSH (2):" anchor="SETTINGS_ENABLE_PUSH">
This setting can be use to disable <xref target="PushResources">server
push</xref>. An endpoint MUST NOT send a <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame if
it receives this parameter set to a value of 0. An endpoint that has both set
this parameter to 0 and had it acknowledged MUST treat the receipt of a
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
<vspace blankLines="1"/>
The initial value is 1, which indicates that push is permitted. Any value other
than 0 or 1 MUST be treated as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t hangText="SETTINGS_MAX_CONCURRENT_STREAMS (3):" anchor="SETTINGS_MAX_CONCURRENT_STREAMS">
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. Initially there is no limit to this value. It
is recommended that this value be no smaller than 100, so as to not
unnecessarily limit parallelism.
<vspace blankLines="1"/>
A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be treated as
special by endpoints. A zero value does prevent the creation of new streams,
however this can also happen for any limit that is exhausted with active
streams. Servers SHOULD only set a zero value for short durations; if a server
does not wish to accept requests, closing the connection could be preferable.
</t>
<t hangText="SETTINGS_INITIAL_WINDOW_SIZE (4):" anchor="SETTINGS_INITIAL_WINDOW_SIZE">
Indicates the sender's initial window size (in bytes) for stream level flow
control. The initial value is 65,535.
<vspace blankLines="1"/>
This setting affects the window size of all streams, including existing
streams, see <xref target="InitialWindowSize"/>.
<vspace blankLines="1"/>
Values above the maximum flow control window size of 2^31 - 1 MUST
be treated as a <xref target="ConnectionErrorHandler">connection error</xref> of
type <xref target="FLOW_CONTROL_ERROR" format="none">FLOW_CONTROL_ERROR</xref>.
</t>
</list>
</t>
<t>
An endpoint that receives a SETTINGS frame with any other identifier MUST
treat this as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
</section>
<section anchor="SettingsSync" title="Settings Synchronization">
<t>
Most values in SETTINGS benefit from or require an understanding of when the peer has
received and applied the changed the communicated parameter values. In order to
provide such synchronization timepoints, the recipient of a SETTINGS frame in which
the ACK flag is not set MUST apply the updated parameters as soon as possible upon
receipt.
</t>
<t>
The values in the SETTINGS frame MUST be applied in the order they appear, with no
other frame processing between values. Once all values have been applied, the
recipient MUST immediately emit a SETTINGS frame with the ACK flag set. Upon
receiving a SETTINGS frame with the ACK flag set, the sender of the altered parameters
can rely upon their application.
</t>
<t>
If the sender of a SETTINGS frame does not receive an acknowledgement within a
reasonable amount of time, it MAY issue a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="SETTINGS_TIMEOUT" format="none">SETTINGS_TIMEOUT</xref>.
</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 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>
<t>
PUSH_PROMISE MUST NOT be sent if the <xref target="SETTINGS_ENABLE_PUSH" format="none">SETTINGS_ENABLE_PUSH</xref> setting of the
peer endpoint is set to 0.
</t>
<figure title="PUSH_PROMISE Payload Format">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pad High? (8) | Pad Low? (8) |
+-+-------------+---------------+-------------------------------+
|R| Promised Stream ID (31) |
+-+-----------------------------+-------------------------------+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The PUSH_PROMISE frame payload has the following fields:
<list style="hanging">
<t hangText="Pad High:">
Padding size high bits. This field is only present if the PAD_HIGH flag is set.
</t>
<t hangText="Pad Low:">
Padding size low bits. This field is only present if the PAD_LOW flag is set.
</t>
<t hangText="R:">
A single reserved bit.
</t>
<t hangText="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="StreamIdentifiers">new stream
identifier</xref>).
</t>
<t hangText="Header Block Fragment:">
A <xref target="HeaderBlock">header block fragment</xref> containing request header
fields.
</t>
<t hangText="Padding:">
Padding octets.
</t>
</list>
</t>
<t>
The PUSH_PROMISE frame defines the following flags:
<list style="hanging">
<t hangText="END_HEADERS (0x4):">
Bit 3 being set indicates that this frame contains an entire <xref target="HeaderBlock">header block</xref> and is not followed by any
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames.
<vspace blankLines="1"/>
A PUSH_PROMISE frame without the END_HEADERS flag set MUST be followed by a
CONTINUATION 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
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t hangText="PAD_LOW (0x8):">
Bit 4 being set indicates that the Pad Low field is present.
</t>
<t hangText="PAD_HIGH (0x10):">
Bit 5 being set indicates that the Pad High field is present. This bit MUST NOT
be set unless the PAD_LOW flag is also set. Endpoints that receive a frame with
PAD_HIGH set and PAD_LOW cleared MUST treat this as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
</list>
</t>
<t>
PUSH_PROMISE frames MUST be associated with an existing, peer-initiated stream. The
stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with.
If the stream identifier field specifies the value 0x0, a recipient MUST respond with a
<xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</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
<xref target="RST_STREAM" format="none">RST_STREAM</xref> 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>
<t>
A PUSH_PROMISE frame modifies the connection state in two ways. The inclusion of a
<xref target="HeaderBlock">header block</xref> potentially modifies the state maintained
for header compression. PUSH_PROMISE also reserves a stream for later use, causing the
promised stream to enter the "reserved" state. A sender MUST NOT send a PUSH_PROMISE on
a stream unless that stream is either "open" or "half closed (remote)"; the sender MUST
ensure that the promised stream is a valid choice for a <xref target="StreamIdentifiers">new stream identifier</xref> (that is, the promised stream
MUST be in the "idle" state).
</t>
<t>
Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame causes the stream
state to become indeterminate. A receiver MUST treat the receipt of a PUSH_PROMISE on a
stream that is neither "open" nor "half-closed (local)" as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>. Similarly, a receiver MUST treat the receipt of a
PUSH_PROMISE that promises an <xref target="StreamIdentifiers">illegal stream
identifier</xref> (that is, an identifier for a stream that is not currently in the
"idle" state) as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
The PUSH_PROMISE frame includes optional padding. Padding fields and flags are
identical to those defined for <xref target="DATA">DATA frames</xref>.
</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>
<figure title="PING Payload Format">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Opaque Data (64) |
| |
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
In addition to the frame header, PING frames MUST contain 8 octets of data in the
payload. A sender can include any value it chooses and use those bytes in any fashion.
</t>
<t>
Receivers of a PING frame that does not include a ACK flag MUST send a PING frame with
the ACK flag set in response, with an identical payload. PING responses SHOULD be given
higher priority than any other frame.
</t>
<t>
The PING frame defines the following flags:
<list style="hanging">
<t hangText="ACK (0x1):">
Bit 1 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
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
Receipt of a PING frame with a length field value other than 8 MUST be treated as a
<xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="FRAME_SIZE_ERROR" format="none">FRAME_SIZE_ERROR</xref>.
</t>
</section>
<section anchor="GOAWAY" title="GOAWAY">
<t>
The GOAWAY frame (type=0x7) informs the remote peer to stop creating streams on this
connection. GOAWAY can be sent by either 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. An endpoint might choose
to close a connection without sending GOAWAY for misbehaving peers.
</t>
<t>
After sending a GOAWAY frame, the sender can discard frames for new streams. However,
any frames that alter connection state cannot be completely ignored. For instance,
<xref target="HEADERS" format="none">HEADERS</xref>, <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> and <xref target="CONTINUATION" format="none">CONTINUATION</xref>
frames MUST be minimally processed to ensure the state maintained for header compression
is consistent (see <xref target="HeaderBlock"/>); similarly DATA frames MUST be counted
toward the connection flow control window.
</t>
<figure title="GOAWAY Payload Format">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Last-Stream-ID (31) |
+-+-------------------------------------------------------------+
| Error Code (32) |
+---------------------------------------------------------------+
| Additional Debug Data (*) |
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The GOAWAY frame does not define any flags.
</t>
<t>
The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST
treat a <xref target="GOAWAY" format="none">GOAWAY</xref> frame with a stream identifier other than 0x0 as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</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 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>
If a connection terminates without a GOAWAY frame, this value is effectively the highest
stream identifier.
</t>
<t>
On streams with lower or equal numbered identifiers that were not closed 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 might 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>
If an endpoint maintains the connection and continues to exchange frames, ignored frames
MUST be counted toward <xref target="FlowControl">flow control limits</xref> or update
<xref target="HeaderBlock">header compression state</xref>. Otherwise, flow control or
header compression state can become unsynchronized.
</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>
<t>
Endpoints MAY append opaque data to the payload of any GOAWAY frame. Additional debug
data is intended for diagnostic purposes only and carries no semantic value. Debug
information could contain security- or privacy-sensitive data. Logged or otherwise
persistently stored debug data MUST have adequate safeguards to prevent unauthorized
access.
</t>
</section>
<section anchor="WINDOW_UPDATE" title="WINDOW_UPDATE">
<t>
The WINDOW_UPDATE frame (type=0x8) is used to implement flow control; see <xref target="FlowControl"/> for an overview.
</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
<xref target="DATA" format="none">DATA</xref> 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
<xref target="FLOW_CONTROL_ERROR" format="none">FLOW_CONTROL_ERROR</xref> if it is unable accept a frame.
</t>
<figure title="WINDOW_UPDATE Payload Format">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Window Size Increment (31) |
+-+-------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The payload of a WINDOW_UPDATE frame is one reserved bit, plus an unsigned 31-bit
integer indicating the number of bytes that the sender can transmit in addition to the
existing flow control window. The legal range for the increment to the flow control
window is 1 to 2^31 - 1 (0x7fffffff) bytes.
</t>
<t>
The WINDOW_UPDATE frame does not define any flags.
</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>
WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the END_STREAM flag.
This means that a receiver could receive a WINDOW_UPDATE frame on a "half closed
(remote)" or "closed" stream. A receiver MUST NOT treat this as an error, see <xref target="StreamStates"/>.
</t>
<t>
A receiver that receives a flow controlled frame MUST always account for its
contribution against the connection flow control window, unless the receiver treats this
as a <xref target="ConnectionErrorHandler">connection error</xref>. This is necessary
even if the frame is in error. Since the sender counts the frame toward the flow
control window, if the receiver does not, the flow control window at sender and receiver
can become different.
</t>
<section title="The Flow Control Window">
<t>
Flow control in HTTP/2 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 END_STREAM 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 2^31 - 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 <xref target="RST_STREAM" format="none">RST_STREAM</xref> with the error code of
<xref target="FLOW_CONTROL_ERROR" format="none">FLOW_CONTROL_ERROR</xref> code; for the connection, a <xref target="GOAWAY" format="none">GOAWAY</xref>
frame with a <xref target="FLOW_CONTROL_ERROR" format="none">FLOW_CONTROL_ERROR</xref> 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 anchor="InitialWindowSize" title="Initial Flow Control Window Size">
<t>
When an HTTP/2 connection is first established, new streams are created with an
initial flow control window size of 65,535 bytes. The connection flow control window
is 65,535 bytes. Both endpoints can adjust the initial window size for new streams by
including a value for <xref target="SETTINGS_INITIAL_WINDOW_SIZE" format="none">SETTINGS_INITIAL_WINDOW_SIZE</xref> in the
<xref target="SETTINGS" format="none">SETTINGS</xref> frame that forms part of the connection preface. The
connection flow control window initial size cannot be changed.
</t>
<t>
Prior to receiving a <xref target="SETTINGS" format="none">SETTINGS</xref> frame that sets a value for
<xref target="SETTINGS_INITIAL_WINDOW_SIZE" format="none">SETTINGS_INITIAL_WINDOW_SIZE</xref>, an endpoint 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 <xref target="SETTINGS" format="none">SETTINGS</xref> frame can alter the initial flow control window size for all
current streams. When the value of <xref target="SETTINGS_INITIAL_WINDOW_SIZE" format="none">SETTINGS_INITIAL_WINDOW_SIZE</xref>
changes, a receiver MUST adjust the size of all stream flow control windows that it
maintains by the difference between the new value and the old value. A
<xref target="SETTINGS" format="none">SETTINGS</xref> frame cannot alter the connection flow control window.
</t>
<t>
An endpoint MUST treat a change to <xref target="SETTINGS_INITIAL_WINDOW_SIZE" format="none">SETTINGS_INITIAL_WINDOW_SIZE</xref> that
causes any flow control window to exceed the maximum size as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="FLOW_CONTROL_ERROR" format="none">FLOW_CONTROL_ERROR</xref>.
</t>
<t>
A change to <xref target="SETTINGS_INITIAL_WINDOW_SIZE" format="none">SETTINGS_INITIAL_WINDOW_SIZE</xref> can 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 client sends 60KB immediately on connection establishment, and the
server sets the initial window size to be 16KB, the client will recalculate the
available flow control window to be -44KB on receipt of the <xref target="SETTINGS" format="none">SETTINGS</xref>
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 <xref target="SETTINGS" format="none">SETTINGS</xref> 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 <xref target="SETTINGS" format="none">SETTINGS</xref> frame.
</t>
<t>
After sending a SETTINGS frame that reduces the initial flow control window size, a
receiver has two options for handling streams that exceed flow control limits:
<list style="numbers">
<t>
The receiver can immediately send <xref target="RST_STREAM" format="none">RST_STREAM</xref> with
<xref target="FLOW_CONTROL_ERROR" format="none">FLOW_CONTROL_ERROR</xref> 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>
</t>
</section>
</section>
<section anchor="CONTINUATION" title="CONTINUATION">
<t>
The CONTINUATION frame (type=0x9) is used to continue a sequence of <xref target="HeaderBlock">header block fragments</xref>. Any number of CONTINUATION frames
can be sent on an existing stream, as long as the preceding frame is on the same stream
and is a <xref target="HEADERS" format="none">HEADERS</xref>, <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> or CONTINUATION frame
without the END_HEADERS flag set.
</t>
<figure title="CONTINUATION Frame Payload">
<artwork type="inline"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pad High? (8) | Pad Low? (8) |
+---------------+---------------+-------------------------------+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The CONTINUATION frame payload has the following fields:
<list style="hanging">
<t hangText="Pad High:">
Padding size high bits. This field is only present if the PAD_HIGH flag is set.
</t>
<t hangText="Pad Low:">
Padding size low bits. This field is only present if the PAD_LOW flag is set.
</t>
<t hangText="Header Block Fragment:">
A <xref target="HeaderBlock">header block fragment</xref>.
</t>
<t hangText="Padding:">
Padding octets.
</t>
</list>
</t>
<t>
The CONTINUATION frame defines the following flags:
<list style="hanging">
<t hangText="END_HEADERS (0x4):">
Bit 3 being set indicates that this frame ends a <xref target="HeaderBlock">header block</xref>.
<vspace blankLines="1"/>
If the END_HEADERS bit is not set, this frame MUST be followed by another
CONTINUATION frame. 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
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t hangText="PAD_LOW (0x8):">
Bit 4 being set indicates that the Pad Low field is present.
</t>
<t hangText="PAD_HIGH (0x10):">
Bit 5 being set indicates that the Pad High field is present. This bit MUST NOT
be set unless the PAD_LOW flag is also set. Endpoints that receive a frame with
PAD_HIGH set and PAD_LOW cleared MUST treat this as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
</list>
</t>
<t>
The payload of a CONTINUATION frame contains a <xref target="HeaderBlock">header block
fragment</xref>.
</t>
<t>
The CONTINUATION frame changes the connection state as defined in <xref target="HeaderBlock"/>.
</t>
<t>
CONTINUATION frames MUST be associated with a stream. If a CONTINUATION 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>
A CONTINUATION frame MUST be preceded by a <xref target="HEADERS" format="none">HEADERS</xref>,
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> or CONTINUATION frame without the END_HEADERS flag set.
A recipient that observes violation of this rule MUST respond with a
<xref target="ConnectionErrorHandler"> connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
The CONTINUATION frame includes optional padding. Padding fields and flags are
identical to those defined for <xref target="DATA">DATA frames</xref>.
</t>
</section>
<section anchor="ALTSVC" title="ALTSVC">
<t>
The ALTSVC frame (type=0xA) advertises the availability of an alternative service to the
client. It can be sent at any time for an existing client-initiated stream or stream 0,
and is intended to allow servers to load balance or otherwise segment traffic; see <xref target="ALT-SVC"/> for details (in particular, Section 2.4, which outlines client
handling of alternative services).
</t>
<t>
An ALTSVC frame on a client-initiated stream indicates that the conveyed alternative
service is associated with the origin of that stream.
</t>
<t>
An ALTSVC frame on stream 0 indicates that the conveyed alternative service is
associated with the origin contained in the Origin field of the frame. An association
with an origin that the client does not consider authoritative for the current
connection MUST be ignored.
</t>
<figure>
<artwork><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max-Age (32) |
+-------------------------------+----------------+--------------+
| Port (16) | Reserved (8) | PID_LEN (8) |
+-------------------------------+----------------+--------------+
| Protocol-ID (*) |
+---------------+-----------------------------------------------+
| HOST_LEN (8) | Host (*) ...
+---------------+-----------------------------------------------+
| Origin? (*) ...
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The ALTSVC frame contains the following fields:
<list style="hanging">
<t hangText="Max-Age:">
An unsigned, 32-bit integer indicating the freshness lifetime of the alternative
service association, as per <xref target="ALT-SVC"/>, Section 2.2.
</t>
<t hangText="Port:">
An unsigned, 16-bit integer indicating the port that the alternative service is
available upon.
</t>
<t hangText="Reserved:">
For future use. Senders MUST set these bits to '0', and recipients MUST ignore them.
</t>
<t hangText="PID_LEN:">
An unsigned, 8-bit integer indicating the length, in octets, of the PROTOCOL-ID
field.
</t>
<t hangText="Protocol-ID:">
A sequence of bytes (length determined by PID_LEN) containing the ALPN protocol
identifier of the alternative service.
</t>
<t hangText="HOST_LEN:">
An unsigned, 8-bit integer indicating the length, in octets, of the Host field.
</t>
<t hangText="Host:">
A sequence of characters (length determined by HOST_LEN) containing an ASCII string
indicating the host that the alternative service is available upon. An <xref target="IDNA">internationalized domain name</xref> MUST be expressed using A-labels.
</t>
<t hangText="Origin:">
An optional sequence of characters (length determined by subtracting the length of
all preceding fields from the frame length) containing ASCII serialisation of an
origin (<xref target="RFC6454"/>, Section 6.2) that the alternate service is
applicable to.
</t>
</list>
</t>
<t>The ALTSVC frame does not define any flags.</t>
<t>
The ALTSVC frame is intended for receipt by clients; a server that receives an ALTSVC
frame MUST treat it as a connection error of type PROTOCOL_ERROR.
</t>
<t>
The ALTSVC frame is processed hop-by-hop. An intermediary MUST NOT forward ALTSVC
frames, though it can use the information contained in ALTSVC frames in forming new
ALTSVC frames to send to its own clients.
</t>
</section>
</section>
<section anchor="ErrorCodes" title="Error Codes">
<t>
Error codes are 32-bit fields that are used in <xref target="RST_STREAM" format="none">RST_STREAM</xref> and
<xref target="GOAWAY" format="none">GOAWAY</xref> 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):" anchor="NO_ERROR">
The associated condition is not as a result of an error. For example, a
<xref target="GOAWAY" format="none">GOAWAY</xref> might include this code to indicate graceful shutdown of a
connection.
</t>
<t hangText="PROTOCOL_ERROR (1):" anchor="PROTOCOL_ERROR">
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):" anchor="INTERNAL_ERROR">
The endpoint encountered an unexpected internal error.
</t>
<t hangText="FLOW_CONTROL_ERROR (3):" anchor="FLOW_CONTROL_ERROR">
The endpoint detected that its peer violated the flow control protocol.
</t>
<t hangText="SETTINGS_TIMEOUT (4):" anchor="SETTINGS_TIMEOUT">
The endpoint sent a SETTINGS frame, but did not receive a response in a
timely manner. See <xref target="SettingsSync">Settings Synchronization</xref>.
</t>
<t hangText="STREAM_CLOSED (5):" anchor="STREAM_CLOSED">
The endpoint received a frame after a stream was half closed.
</t>
<t hangText="FRAME_SIZE_ERROR (6):" anchor="FRAME_SIZE_ERROR">
The endpoint received a frame that was larger than the maximum size that it
supports.
</t>
<t hangText="REFUSED_STREAM (7):" anchor="REFUSED_STREAM">
The endpoint refuses the stream prior to performing any application processing, see
<xref target="Reliability"/> for details.
</t>
<t hangText="CANCEL (8):" anchor="CANCEL">
Used by the endpoint to indicate that the stream is no longer needed.
</t>
<t hangText="COMPRESSION_ERROR (9):" anchor="COMPRESSION_ERROR">
The endpoint is unable to maintain the compression context for the connection.
</t>
<t hangText="CONNECT_ERROR (10):" anchor="CONNECT_ERROR">
The connection established in response to a <xref target="CONNECT">CONNECT
request</xref> was reset or abnormally closed.
</t>
<t hangText="ENHANCE_YOUR_CALM (11):" anchor="ENHANCE_YOUR_CALM">
The endpoint detected that its peer is exhibiting a behavior over a given amount of time
that has caused it to refuse to process further frames.
</t>
<t hangText="INADEQUATE_SECURITY (12):" anchor="INADEQUATE_SECURITY">
The underlying transport has properties that do not meet the minimum requirements
imposed by this document (see <xref target="TLSUsage"/>) or the endpoint.
</t>
</list>
</t>
</section>
<section anchor="HTTPLayer" title="HTTP Message Exchanges">
<t>
HTTP/2 is intended to be as compatible as possible with current uses of HTTP. This means
that, from the perspective of the server and client applications, the features of the
protocol are unchanged. To achieve this, all request and response semantics are preserved,
although the syntax of conveying those semantics has changed.
</t>
<t>
Thus, the specification and requirements of HTTP/1.1 Semantics and Content <xref target="HTTP-p2"/>, Conditional Requests <xref target="HTTP-p4"/>, Range Requests <xref target="HTTP-p5"/>, Caching <xref target="HTTP-p6"/> and Authentication <xref target="HTTP-p7"/> are applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax
and Routing <xref target="HTTP-p1"/>, such as the HTTP and HTTPS URI schemes, are also
applicable in HTTP/2, but the expression of those semantics for this protocol are defined
in the sections below.
</t>
<section anchor="HttpSequence" title="HTTP Request/Response Exchange">
<t>
A client sends an HTTP request on a new stream, using a previously unused <xref target="StreamIdentifiers">stream identifier</xref>. A server sends an HTTP response on
the same stream as the request.
</t>
<t>
An HTTP message (request or response) consists of:
<list style="numbers">
<t>
one <xref target="HEADERS" format="none">HEADERS</xref> frame, followed by zero or more
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames (containing the message headers; see <xref target="HTTP-p1"/>, Section 3.2), and
</t>
<t>
zero or more <xref target="DATA" format="none">DATA</xref> frames (containing the message payload; see <xref target="HTTP-p1"/>, Section 3.3), and
</t>
<t>
optionally, one <xref target="HEADERS" format="none">HEADERS</xref> frame, followed by zero or more
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames (containing the trailer-part, if present; see
<xref target="HTTP-p1"/>, Section 4.1.2).
</t>
</list>
The last frame in the sequence bears an END_STREAM flag, though a <xref target="HEADERS" format="none">HEADERS</xref>
frame bearing the END_STREAM flag can be followed by <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames
that carry any remaining portions of the header block.
</t>
<t>
Other frames (from any stream) MUST NOT occur between either <xref target="HEADERS" format="none">HEADERS</xref> frame
and the following <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames (if present), nor between
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames.
</t>
<t>
Otherwise, frames MAY be interspersed on the stream between these frames, but those
frames do not carry HTTP semantics. In particular, <xref target="HEADERS" format="none">HEADERS</xref> frames (and any
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames that follow) other than the first and optional last
frames in this sequence do not carry HTTP semantics.
</t>
<t>
Trailing header fields are carried in a header block that also terminates the stream.
That is, a sequence starting with a <xref target="HEADERS" format="none">HEADERS</xref> frame, followed by zero or more
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames, where the <xref target="HEADERS" format="none">HEADERS</xref> frame bears an
END_STREAM flag. Header blocks after the first that do not terminate the stream are not
part of an HTTP request or response.
</t>
<t>
An HTTP request/response exchange fully consumes a single stream. A request starts with
the <xref target="HEADERS" format="none">HEADERS</xref> frame that puts the stream into an "open" state and ends with a
frame bearing END_STREAM, which causes the stream to become "half closed" for the client.
A response starts with a <xref target="HEADERS" format="none">HEADERS</xref> frame and ends with a frame bearing
END_STREAM, optionally followed by <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames, which places the
stream in the "closed" state.
</t>
<section anchor="informational-responses" title="Informational Responses">
<t>
The 1xx series of HTTP response status codes (<xref target="HTTP-p2"/>, Section 6.2) are not supported in HTTP/2.
</t>
<t>
The most common use case for 1xx is using an Expect header field with a <spanx style="verb">100-continue</spanx> token (colloquially, "Expect/continue") to indicate
that the client expects a 100 (Continue) non-final response status code, receipt of
which indicates that the client should continue sending the request body if it has not
already done so.
</t>
<t>
Typically, Expect/continue is used by clients wishing to avoid sending a large amount
of data in a request body, only to have the request rejected by the origin server (thus
leaving the connection potentially unusable).
</t>
<t>
HTTP/2 does not enable the Expect/continue mechanism; if the server sends a final status
code to reject the request, it can do so without making the underlying connection
unusable.
</t>
<t>
Note that this means HTTP/2 clients sending requests with bodies may waste at least one
round trip of sent data when the request is rejected. This can be mitigated by
restricting the amount of data sent for the first round trip by bandwidth-constrained
clients, in anticipation of a final status code.
</t>
<t>
Other defined 1xx status codes are not applicable to HTTP/2. For example, the semantics
of 101 (Switching Protocols) aren't suitable to a multiplexed protocol. Likewise, 102
(Processing) is no longer necessary, because HTTP/2 has a separate means of keeping the
connection alive.
</t>
<t>
This difference between protocol versions necessitates special handling by
intermediaries that translate between them:
<list style="symbols">
<t>
An intermediary that gateways HTTP/1.1 to HTTP/2 MUST generate a 100 (Continue)
response if a received request includes and Expect header field with a <spanx style="verb">100-continue</spanx> token (<xref target="HTTP-p2"/>, Section 5.1.1), unless it can immediately generate a final status code.
It MUST NOT forward the <spanx style="verb">100-continue</spanx> expectation in the
request header fields.
</t>
<t>
An intermediary that gateways HTTP/2 to HTTP/1.1 MAY add an Expect header field with
a <spanx style="verb">100-continue</spanx> expectation when forwarding a request
that has a body; see <xref target="HTTP-p2"/>, Section 5.1.1 for
specific requirements.
</t>
<t>
An intermediary that gateways HTTP/2 to HTTP/1.1 MUST discard all other 1xx
informational responses.
</t>
</list>
</t>
</section>
<section title="Examples">
<t>
This section shows HTTP/1.1 requests and responses, with illustrations of equivalent
HTTP/2 requests and responses.
</t>
<t>
An HTTP GET request includes request header fields and no body and is therefore
transmitted as a single <xref target="HEADERS" format="none">HEADERS</xref> frame, followed by zero or more
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames containing the serialized block of request header
fields. The last <xref target="HEADERS" format="none">HEADERS</xref> frame in the sequence has both the END_HEADERS
and END_STREAM flags set:
</t>
<figure>
<artwork type="inline"><![CDATA[
GET /resource HTTP/1.1 HEADERS
Host: example.org ==> + END_STREAM
Accept: image/jpeg + END_HEADERS
:method = GET
:scheme = https
:path = /resource
host = example.org
accept = image/jpeg
]]></artwork>
</figure>
<t>
Similarly, a response that includes only response header fields is transmitted as a
<xref target="HEADERS" format="none">HEADERS</xref> frame (again, followed by zero or more
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames) containing the serialized block of response header
fields. The last <xref target="HEADERS" format="none">HEADERS</xref> frame in the sequence has both the END_HEADERS
and END_STREAM flag set:
</t>
<figure>
<artwork type="inline"><![CDATA[
HTTP/1.1 304 Not Modified HEADERS
ETag: "xyzzy" ==> + END_STREAM
Expires: Thu, 23 Jan ... + END_HEADERS
:status = 304
etag: "xyzzy"
expires: Thu, 23 Jan ...
]]></artwork>
</figure>
<t>
An HTTP POST request that includes request header fields and payload data is
transmitted as one <xref target="HEADERS" format="none">HEADERS</xref> frame, followed by zero or more
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames containing the request header fields, followed by
one or more <xref target="DATA" format="none">DATA</xref> frames, with the last <xref target="CONTINUATION" format="none">CONTINUATION</xref> (or
<xref target="HEADERS" format="none">HEADERS</xref>) frame having the END_HEADERS flag set and the final
<xref target="DATA" format="none">DATA</xref> frame having the END_STREAM flag set:
</t>
<figure>
<artwork type="inline"><![CDATA[
POST /resource HTTP/1.1 HEADERS
Host: example.org ==> - END_STREAM
Content-Type: image/jpeg + END_HEADERS
Content-Length: 123 :method = POST
:scheme = https
{binary data} :path = /resource
:authority = example.org
content-type = image/jpeg
content-length = 123
DATA
+ END_STREAM
{binary data}
]]></artwork>
</figure>
<t>
A response that includes header fields and payload data is transmitted as a
<xref target="HEADERS" format="none">HEADERS</xref> frame, followed by zero or more <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames,
followed by one or more <xref target="DATA" format="none">DATA</xref> frames, with
the last <xref target="DATA" format="none">DATA</xref> frame in the sequence having the END_STREAM flag set:
</t>
<figure>
<artwork type="inline"><![CDATA[
HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> - END_STREAM
Content-Length: 123 + END_HEADERS
:status = 200
{binary data} content-type = image/jpeg
content-length = 123
DATA
+ END_STREAM
{binary data}
]]></artwork>
</figure>
<t>
Trailing header fields are sent as a header block after both the request or response
header block and all the <xref target="DATA" format="none">DATA</xref> frames have been sent. The sequence of
<xref target="HEADERS" format="none">HEADERS</xref>/<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames that bears the trailers
includes a terminal frame that has both END_HEADERS and END_STREAM flags set.
</t>
<figure>
<artwork type="inline"><![CDATA[
HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> - END_STREAM
Transfer-Encoding: chunked + END_HEADERS
Trailer: Foo :status = 200
content-length = 123
123 content-type = image/jpeg
{binary data} trailer = Foo
0
Foo: bar DATA
- END_STREAM
{binary data}
HEADERS
+ END_STREAM
+ END_HEADERS
foo: bar
]]></artwork>
</figure>
</section>
<section anchor="HttpHeaders" title="HTTP Header Fields">
<t>
HTTP header fields carry information as a series of key-value pairs. For a listing of
registered HTTP headers, see the Message Header Field Registry maintained at
<eref target="http://www.iana.org/assignments/message-headers"/>.
</t>
<t>
While HTTP/1.x used the message start-line (see <xref target="HTTP-p1"/>, Section 3.1) to convey the target URI and method of
the request, and the status code for the response, HTTP/2 uses special pseudo-headers
beginning with ":" for these tasks.
</t>
<t>
Just as in HTTP/1.x, header field names are strings of ASCII characters that are compared
in a case-insensitive fashion. However, header field names MUST be converted to lowercase
prior to their encoding in HTTP/2. A request or response containing uppercase header
field names MUST be treated as <xref target="malformed">malformed</xref>.
</t>
<t>
HTTP/2 does not use the Connection header field to indicate "hop-by-hop" header fields;
in this protocol, connection-specific metadata is conveyed by other means. As such, a
HTTP/2 message containing Connection MUST be treated as <xref target="malformed">malformed</xref>.
</t>
<t>
This means that an intermediary transforming an HTTP/1.x message to
HTTP/2 will need to remove any header fields nominated by the Connection header field,
along with the Connection header field itself. Such intermediaries SHOULD also remove
other connection-specific header fields, such as Keep-Alive, Proxy-Connection,
Transfer-Encoding and Upgrade, even if they are not nominated by Connection.
</t>
<t>
One exception to this is the TE header field, which MAY be present in an HTTP/2 request,
but when it is MUST NOT contain any value other than "trailers".
</t>
<t>
<list style="hanging">
<t hangText="Note:">
HTTP/2 purposefully does not support upgrade to another protocol. The
handshake methods described in <xref target="starting"/> are believed sufficient to
negotiate the use of alternative protocols.
</t>
</list>
</t>
<section anchor="HttpRequest" title="Request Header Fields">
<t>
HTTP/2 defines a number of header fields starting with a colon ':' character that carry
information about the request target:
<list style="symbols">
<t>
The <spanx style="verb">:method</spanx> header field includes the HTTP method
(<xref target="HTTP-p2"/>, Section 4).
</t>
<t>
The <spanx style="verb">:scheme</spanx> header field includes the scheme portion
of the target URI (<xref target="RFC3986"/>, Section 3.1).
<vspace blankLines="1"/>
<spanx style="verb">:scheme</spanx> is not restricted to <spanx style="verb">http</spanx> and <spanx style="verb">https</spanx> schemed URIs. A
proxy or gateway can translate requests for non-HTTP schemes, enabling the use of
HTTP to interact with non-HTTP services.
</t>
<t>
The <spanx style="verb">:authority</spanx> header field includes the authority
portion of the target URI (<xref target="RFC3986"/>, Section 3.2). The
authority MUST NOT include the deprecated <spanx style="verb">userinfo</spanx>
subcomponent for <spanx style="verb">http</spanx> or <spanx style="verb">https</spanx> schemed URIs.
<vspace blankLines="1"/>
To ensure that the HTTP/1.1 request line can be reproduced accurately, this
header field MUST be omitted when translating from an HTTP/1.1 request that has a
request target in origin or asterisk form (see <xref target="HTTP-p1"/>, Section 5.3). Clients that generate HTTP/2 requests directly SHOULD
instead omit the <spanx style="verb">Host</spanx> header field. An intermediary
that converts an HTTP/2 request to HTTP/1.1 MUST create a <spanx style="verb">Host</spanx> header field if one is not present in a request by
copying the value of the <spanx style="verb">:authority</spanx> header field.
</t>
<t>
The <spanx style="verb">:path</spanx> header field includes the path and query
parts of the target URI (the <spanx style="verb">path-absolute</spanx> production
from <xref target="RFC3986"/> and optionally a '?' character followed by the
<spanx style="verb">query</spanx> production, see <xref target="RFC3986"/>, Section 3.3 and <xref target="RFC3986"/>, Section 3.4). This field
MUST NOT be empty; URIs that do not contain a path component MUST include a value
of '/', unless the request is an OPTIONS request in asterisk form, in which case
the <spanx style="verb">:path</spanx> header field MUST include '*'.
</t>
</list>
All HTTP/2 requests MUST include exactly one valid value for the <spanx style="verb">:method</spanx>, <spanx style="verb">:scheme</spanx>, and <spanx style="verb">:path</spanx> header fields, unless this is a <xref target="CONNECT">CONNECT request</xref>. An HTTP request that omits mandatory header
fields is <xref target="malformed">malformed</xref>.
</t>
<t>
Header field names that start with a colon are only valid in the HTTP/2 context. These
are not HTTP header fields. Implementations MUST NOT generate header fields that start
with a colon, but they MUST ignore any header field that starts with a colon. In
particular, header fields with names starting with a colon MUST NOT be exposed as HTTP
header fields.
</t>
<t>
HTTP/2 does not define a way to carry the version identifier that is included in the
HTTP/1.1 request line.
</t>
</section>
<section anchor="HttpResponse" title="Response Header Fields">
<t>
A single <spanx style="verb">:status</spanx> header field is defined that carries the
HTTP status code field (see <xref target="HTTP-p2"/>, Section 6).
This header field MUST be included in all responses, otherwise the response is <xref target="malformed">malformed</xref>.
</t>
<t>
HTTP/2 does not define a way to carry the version or reason phrase that is included in
an HTTP/1.1 status line.
</t>
</section>
<section anchor="HeaderOrdering" title="Header Field Ordering">
<t>
<xref target="COMPRESSION">HTTP Header Compression</xref> does not preserve the order of
header fields, because the relative order of header fields with different names is not
important. However, the same header field can be repeated to form a
list (see <xref target="HTTP-p1"/>, Section 3.2.2), where the relative
order of header field values is significant. This repetition can occur either as a
single header field with a comma-separated list of values, or as several header fields
with a single value, or any combination thereof. Therefore, in the latter case, ordering
needs to be preserved before compression takes place.
</t>
<t>
To preserve the order of multiple occurrences of a header field with the same name, its
ordered values are concatenated into a single value using a zero-valued octet (0x0) to
delimit them.
</t>
<t>
After decompression, header fields that have values containing zero octets (0x0) MUST be
split into multiple header fields before being processed.
</t>
<t>For example, the following HTTP/1.x header block:</t>
<figure>
<artwork type="inline"><![CDATA[
Content-Type: text/html
Cache-Control: max-age=60, private
Cache-Control: must-revalidate
]]></artwork></figure>
<t>
contains three Cache-Control directives; two in the first Cache-Control header field,
and the last one in the second Cache-Control field. Before compression, they would need
to be converted to a form similar to this (with 0x0 represented as "\0"):
</t>
<figure>
<artwork type="inline"><![CDATA[
cache-control: max-age=60, private\0must-revalidate
content-type: text/html
]]></artwork></figure>
<t>
Note here that the ordering between Content-Type and Cache-Control is not preserved, but
the relative ordering of the Cache-Control directives -- as well as the fact that the
first two were comma-separated, while the last was on a different line -- is.
</t>
<t>
Header fields containing multiple values MUST be concatenated into a single value unless
the ordering of that header field is known to be insignificant.
</t>
<t>
The special case of <spanx style="verb">set-cookie</spanx> - which does not form a
comma-separated list, but can have multiple values - does not depend on ordering. The
<spanx style="verb">set-cookie</spanx> header field MAY be encoded as multiple header
field values, or as a single concatenated value.
</t>
</section>
<section anchor="CompressCookie" title="Compressing the Cookie Header Field">
<t>
The <xref target="COOKIE">Cookie header field</xref> can carry a significant amount of
redundant data.
</t>
<t>
The Cookie header field uses a semi-colon (";") to delimit cookie-pairs (or "crumbs").
This header field doesn't follow the list construction rules in HTTP (see <xref target="HTTP-p1"/>, Section 3.2.2), which prevents cookie-pairs from
being separated into different name-value pairs. This can significantly reduce
compression efficiency as individual cookie-pairs are updated.
</t>
<t>
To allow for better compression efficiency, the Cookie header field MAY be split into
separate header fields, each with one or more cookie-pairs. If there are multiple
Cookie header fields after decompression, these MUST be concatenated into a single octet
string using the two octet delimiter of 0x3B, 0x20 (the ASCII string "; ").
</t>
<t>
The Cookie header field MAY be split using a zero octet (0x0), as defined in <xref target="HeaderOrdering"/>. When decoding, zero octets MUST be replaced with the cookie
delimiter ("; ").
</t>
</section>
<section anchor="malformed" title="Malformed Messages">
<t>
A malformed request or response is one that uses a valid sequence of HTTP/2 frames, but
is otherwise invalid due to the presence of prohibited header fields, the absence of
mandatory header fields, or the inclusion of uppercase header field names.
</t>
<t>
A request or response that includes an entity body can include a <spanx style="verb">content-length</spanx> header field. A request or response is also
malformed if the value of a <spanx style="verb">content-length</spanx> header field does
not equal the sum of the <xref target="DATA" format="none">DATA</xref> frame payload lengths that form the body.
</t>
<t>
Intermediaries that process HTTP requests or responses (i.e., all intermediaries other
than those acting as tunnels) MUST NOT forward a malformed request or response.
</t>
<t>
Implementations that detect malformed requests or responses need to ensure that the
stream ends. For malformed requests, a server MAY send an HTTP response prior to
closing or resetting the stream. Clients MUST NOT accept a malformed response. Note
that these requirements are intended to protect against several types of common attacks
against HTTP; they are deliberately strict, because being permissive can expose
implementations to these vulnerabilites.
</t>
</section>
</section>
<section anchor="Reliability" title="Request Reliability Mechanisms in HTTP/2">
<t>
In HTTP/1.1, an HTTP client is unable to retry a non-idempotent request when an error
occurs, because there is no means to determine the nature of the error. It is possible
that some server processing occurred prior to the error, which could result in
undesirable effects if the request were reattempted.
</t>
<t>
HTTP/2 provides two mechanisms for providing a guarantee to a client that a request has
not been processed:
<list style="symbols">
<t>
The <xref target="GOAWAY" format="none">GOAWAY</xref> frame indicates the highest stream number that might have
been processed. Requests on streams with higher numbers are therefore guaranteed to
be safe to retry.
</t>
<t>
The <xref target="REFUSED_STREAM" format="none">REFUSED_STREAM</xref> error code can be included in a
<xref target="RST_STREAM" format="none">RST_STREAM</xref> frame to indicate that the stream is being closed prior to
any processing having occurred. Any request that was sent on the reset stream can
be safely retried.
</t>
</list>
</t>
<t>
Requests that have not been processed have not failed; clients MAY automatically retry
them, even those with non-idempotent methods.
</t>
<t>
A server MUST NOT indicate that a stream has not been processed unless it can guarantee
that fact. If frames that are on a stream are passed to the application layer for any
stream, then <xref target="REFUSED_STREAM" format="none">REFUSED_STREAM</xref> MUST NOT be used for that stream, and a
<xref target="GOAWAY" format="none">GOAWAY</xref> frame MUST include a stream identifier that is greater than or
equal to the given stream identifier.
</t>
<t>
In addition to these mechanisms, the <xref target="PING" format="none">PING</xref> frame provides a way for a
client to easily test a connection. Connections that remain idle can become broken as
some middleboxes (for instance, network address translators, or load balancers) silently
discard connection bindings. The <xref target="PING" format="none">PING</xref> frame allows a client to safely
test whether a connection is still active without sending a request.
</t>
</section>
</section>
<section anchor="PushResources" title="Server Push">
<t>
HTTP/2 enables a server to pre-emptively send (or "push") one or more associated
responses to a client in response to a single request. This feature becomes particularly
helpful when the server knows the client will need to have those responses available in
order to fully process the response to the original request.
</t>
<t>
Pushing additional responses is optional, and is negotiated between individual
endpoints. The <xref target="SETTINGS_ENABLE_PUSH" format="none">SETTINGS_ENABLE_PUSH</xref> setting can be set to 0 to indicate
that server push is disabled.
</t>
<t>
Because pushing responses is effectively hop-by-hop, an intermediary could receive pushed
responses from the server and choose not to forward those on to the client. In other
words, how to make use of the pushed responses is up to that intermediary. Equally, the
intermediary might choose to push additional responses to the client, without any action
taken by the server.
</t>
<t>
A client cannot push. Thus, servers MUST treat the receipt of a
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame as a <xref target="ConnectionErrorHandler">connection
error</xref>. Clients MUST reject any attempt to change the
<xref target="SETTINGS_ENABLE_PUSH" format="none">SETTINGS_ENABLE_PUSH</xref> setting to a value other than "0" by treating the
message as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
A server can only push responses that are cacheable (see <xref target="HTTP-p6"/>, Section 3); promised requests MUST be safe (see <xref target="HTTP-p2"/>, Section 4.2.1) and MUST NOT include a request body.
</t>
<section title="Push Requests">
<t>
Server push is semantically equivalent to a server responding to a request; however, in
this case that request is also sent by the server, as a <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref>
frame.
</t>
<t>
The <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame includes a header block that contains a complete
set of request header fields that the server attributes to the request. It is not
possible to push a response to a request that includes a request body.
</t>
<t>
Pushed responses are always associated with an explicit request from the client. The
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frames sent by the server are sent on that explicit
request's stream. The <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame also includes a promised stream
identifier, chosen from the stream identifiers available to the server (see <xref target="StreamIdentifiers"/>).
</t>
<t>
The header fields in <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> and any subsequent
<xref target="CONTINUATION" format="none">CONTINUATION</xref> frames MUST be a valid and complete set of <xref target="HttpRequest">request header fields</xref>. The server MUST include a
method in the <spanx style="verb">:method</spanx> header field that is safe and
cacheable. If a client receives a <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> that does not include a
complete and valid set of header fields, or the <spanx style="verb">:method</spanx>
header field identifies a method that is not safe, it MUST respond with a <xref target="StreamErrorHandler">stream error</xref> of type
<xref target="PROTOCOL_ERROR" format="none">PROTOCOL_ERROR</xref>.
</t>
<t>
The server SHOULD send <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> (<xref target="PUSH_PROMISE"/>)
frames prior to sending any frames that reference the promised responses. This avoids a
race where clients issue requests prior to receiving any <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref>
frames.
</t>
<t>
For example, if the server receives a request for a document containing embedded links
to multiple image files, and the server chooses to push those additional images to the
client, sending push promises before the <xref target="DATA" format="none">DATA</xref> frames that contain the
image links ensures that the client is able to see the promises before discovering
embedded links. Similarly, if the server pushes responses referenced by the header
block (for instance, in Link header fields), sending the push promises before sending
the header block ensures that clients do not request them.
</t>
<t>
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frames MUST NOT be sent by the client.
<xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frames can be sent by the server on any stream that was
opened by the client. They MUST be sent on a stream that is in either the "open" or
"half closed (remote)" state to the server. <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frames are
interspersed with the frames that comprise a response, though they cannot be
interspersed with <xref target="HEADERS" format="none">HEADERS</xref> and <xref target="CONTINUATION" format="none">CONTINUATION</xref> frames that
comprise a single header block.
</t>
</section>
<section title="Push Responses">
<t>
After sending the <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame, the server can begin delivering the
pushed response as a <xref target="HttpResponse">response</xref> on a server-initiated
stream that uses the promised stream identifier. The server uses this stream to transmit
an HTTP response, using the same sequence of frames as defined in <xref target="HttpSequence"/>. This stream becomes <xref target="StreamStates">"half closed" to
the client</xref> after the initial <xref target="HEADERS" format="none">HEADERS</xref> frame is sent.
</t>
<t>
Once a client receives a <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame and chooses to accept the
pushed response, the client SHOULD NOT issue any requests for the promised response
until after the promised stream has closed.
</t>
<t>
If the client determines, for any reason, that it does not wish to receive the pushed
response from the server, or if the server takes too long to begin sending the promised
response, the client can send an <xref target="RST_STREAM" format="none">RST_STREAM</xref> frame, using either the
<xref target="CANCEL" format="none">CANCEL</xref> or <xref target="REFUSED_STREAM" format="none">REFUSED_STREAM</xref> codes, and referencing the pushed
stream's identifier.
</t>
<t>
A client can use the <xref target="SETTINGS_MAX_CONCURRENT_STREAMS" format="none">SETTINGS_MAX_CONCURRENT_STREAMS</xref> setting to limit the
number of responses that can be concurrently pushed by a server. Advertising a
<xref target="SETTINGS_MAX_CONCURRENT_STREAMS" format="none">SETTINGS_MAX_CONCURRENT_STREAMS</xref> value of zero disables server push by
preventing the server from creating the necessary streams. This does not prohibit a
server from sending PUSH_PROMISE frames; clients need to reset any promised streams that
are not wanted.
</t>
<t>
Clients receiving a pushed response MUST validate that the server is authorized to
provide the response, see <xref target="authority"/>. For example, an server that
offers a certificate for only the <spanx style="verb">example.com</spanx> DNS-ID or
Common Name is not permitted to push a response for <spanx style="verb">https://www.example.org/doc</spanx>.
</t>
</section>
</section>
<section anchor="CONNECT" title="The CONNECT Method">
<t>
In HTTP/1.x, the pseudo-method CONNECT (<xref target="HTTP-p2"/>, Section 4.3.6) is used to convert an HTTP connection into a tunnel to a remote host.
CONNECT is primarily used with HTTP proxies to establish a TLS session with an origin
server for the purposes of interacting with <spanx style="verb">https</spanx> resources.
</t>
<t>
In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream
to a remote host, for similar purposes. The HTTP header field mapping works as mostly as
defined in <xref target="HttpRequest">Request Header Fields</xref>, with a few
differences. Specifically:
<list style="symbols">
<t>
The <spanx style="verb">:method</spanx> header field is set to <spanx style="verb">CONNECT</spanx>.
</t>
<t>
The <spanx style="verb">:scheme</spanx> and <spanx style="verb">:path</spanx> header
fields MUST be omitted.
</t>
<t>
The <spanx style="verb">:authority</spanx> header field contains the host and port
to connect to (equivalent to the authority-form of the request-target of CONNECT
requests, see <xref target="HTTP-p1"/>, Section 5.3).
</t>
</list>
</t>
<t>
A proxy that supports CONNECT establishes a <xref target="TCP">TCP connection</xref> to
the server identified in the <spanx style="verb">:authority</spanx> header field. Once
this connection is successfully established, the proxy sends a <xref target="HEADERS" format="none">HEADERS</xref>
frame containing a 2xx series status code to the client, as defined in <xref target="HTTP-p2"/>, Section 4.3.6.
</t>
<t>
After the initial <xref target="HEADERS" format="none">HEADERS</xref> frame sent by each peer, all subsequent
<xref target="DATA" format="none">DATA</xref> frames correspond to data sent on the TCP connection. The payload of
any <xref target="DATA" format="none">DATA</xref> frames sent by the client are transmitted by the proxy to the TCP
server; data received from the TCP server is assembled into <xref target="DATA" format="none">DATA</xref> frames by
the proxy. Frame types other than <xref target="DATA" format="none">DATA</xref> or stream management frames
(<xref target="RST_STREAM" format="none">RST_STREAM</xref>, <xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref>, and <xref target="PRIORITY" format="none">PRIORITY</xref>)
MUST NOT be sent on a connected stream, and MUST be treated as a <xref target="StreamErrorHandler">stream error</xref> if received.
</t>
<t>
The TCP connection can be closed by either peer. The END_STREAM flag on a
<xref target="DATA" format="none">DATA</xref> frame is treated as being equivalent to the TCP FIN bit. A client is
expected to send a <xref target="DATA" format="none">DATA</xref> frame with the END_STREAM flag set after receiving
a frame bearing the END_STREAM flag. A proxy that receives a <xref target="DATA" format="none">DATA</xref> frame
with the END_STREAM flag set sends the attached data with the FIN bit set on the last TCP
segment. A proxy that receives a TCP segment with the FIN bit set sends a
<xref target="DATA" format="none">DATA</xref> frame with the END_STREAM flag set. Note that the final TCP segment
or <xref target="DATA" format="none">DATA</xref> frame could be empty.
</t>
<t>
A TCP connection error is signaled with <xref target="RST_STREAM" format="none">RST_STREAM</xref>. A proxy treats any
error in the TCP connection, which includes receiving a TCP segment with the RST bit set,
as a <xref target="StreamErrorHandler">stream error</xref> of type
<xref target="CONNECT_ERROR" format="none">CONNECT_ERROR</xref>. Correspondingly, a proxy MUST send a TCP segment with the
RST bit set if it detects an error with the stream or the HTTP/2 connection.
</t>
</section>
</section>
<section anchor="HttpExtra" title="Additional HTTP Requirements/Considerations">
<t>
This section outlines attributes of the HTTP protocol that improve interoperability, reduce
exposure to known security vulnerabilities, or reduce the potential for implementation
variation.
</t>
<section title="Connection Management">
<t>
HTTP/2 connections are persistent. For best performance, it is expected 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>
Clients SHOULD NOT open more than one HTTP/2 connection to a given destination, where a
destination is the IP address and port that is derived from a URI, a selected <xref target="ALT-SVC">alternative service</xref>, or a configured proxy. A client can create
additional connections as replacements, either to replace connections that are near to
exhausting the available <xref target="StreamIdentifiers">stream identifier space</xref>,
or to replace connections that have encountered <xref target="ConnectionErrorHandler">errors</xref>.
</t>
<t>
A client MAY open multiple connections to the same IP address and TCP port using different
<xref target="TLS-EXT">Server Name Indication</xref> values or to provide different TLS
client certificates, but SHOULD avoid creating multiple connections with the same
configuration. <cref>Need more text on how client certificates relate here, see issue
#363.</cref>
</t>
<t>
Clients MAY use a single server connection to send requests for URIs with multiple
different authority components as long as the server is <xref target="authority">authoritative</xref>.
</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 SHOULD first send a
<xref target="GOAWAY" format="none">GOAWAY</xref> (<xref target="GOAWAY"/>) 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 title="Use of TLS Features" anchor="TLSUsage">
<t>
Implementations of HTTP/2 MUST support <xref target="TLS12">TLS 1.2</xref>. The general
TLS usage guidance in <xref target="TLSBCP"/> SHOULD be followed, with some additional
restrictions that are specific to HTTP/2.
</t>
<t>
The TLS implementation MUST support the <xref target="TLS-EXT">Server Name Indication
(SNI)</xref> extension to TLS. HTTP/2 clients MUST indicate the target domain name when
negotiating TLS.
</t>
<t>
The TLS implementation MUST disable compression. TLS compression can lead to the exposure
of information that would not otherwise be revealed <xref target="RFC3749"/>. Generic
compression is unnecessary since HTTP/2 provides compression features that are more aware
of context and therefore likely to be more appropriate for use for performance, security
or other reasons.
</t>
<t>
Implementations MUST negotiate - and therefore use - ephemeral cipher suites, such as
ephemeral Diffie-Hellman (DHE) or the elliptic curve variant (ECDHE) with a minimum size
of 2048 bits (DHE) or security level of 128 bits (ECDHE). Clients MUST accept DHE sizes of
up to 4096 bits.
</t>
<t>
Implementations are encouraged not to negotiate TLS cipher suites with known
vulnerabilities, such as <xref target="RC4"/>.
</t>
<t>
An implementation that negotiates a TLS connection that does not meet the requirements in
this section, or any policy-based constraints, SHOULD NOT negotiate HTTP/2. Removing
HTTP/2 protocols from consideration could result in the removal of all protocols from the
set of protocols offered by the client. This causes protocol negotiation failure, as
described in Section 3.2 of <xref target="TLSALPN"/>.
</t>
<t>
Due to implementation limitations, it might not be possible to fail TLS negotiation based
on all of these requirements. An endpoint MUST terminate an HTTP/2 connection that is
opened on a TLS session that does not meet these minimum requirements with a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="INADEQUATE_SECURITY" format="none">INADEQUATE_SECURITY</xref>.
</t>
</section>
<section anchor="Compression" title="GZip Content-Encoding">
<t>
Clients MUST support gzip compression for HTTP response bodies. Regardless of the value
of the accept-encoding header field, a server MAY send responses with gzip encoding. A
compressed response MUST still bear an appropriate content-encoding header field.
</t>
<t>
This effectively changes the implicit value of the Accept-Encoding header field (<xref target="HTTP-p2"/>, Section 5.3.4) from "identity" to
"identity, gzip", however gzip encoding cannot be suppressed by including ";q=0".
Intermediaries that perform translation from HTTP/2 to HTTP/1.1 MUST decompress payloads
unless the request includes an Accept-Encoding value that includes "gzip".
</t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<section title="Server Authority" anchor="authority">
<t>
A client is only able to accept HTTP/2 responses from servers that are authoritative for
those resources. This is particularly important for <xref target="PushResources">server
push</xref>, where the client validates the <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> before accepting
the response.
</t>
<t>
HTTP/2 relies on the HTTP/1.1 definition of authority for determining whether a server is
authoritative in providing a given response, see <xref target="HTTP-p1"/>, Section 9.1). This relies on local name resolution for the "http"
URI scheme, and the offered server identity for the "https" scheme (see <xref target="RFC2818"/>, Section 3).
</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>
In a cross-protocol attack, an attacker causes a client to initiate a transaction in one
protocol toward a server that understands a different protocol. An attacker might be able
to cause the transaction to appear as valid transaction in the second protocol. In
combination with the capabilities of the web context, this can be used to interact with
poorly protected servers in private networks.
</t>
<t>
Completing a TLS handshake with an ALPN identifier for HTTP/2 can be considered
sufficient. ALPN provides a positive indication that a server is willing to proceed with
HTTP/2, which prevents attacks on other TLS-based protocols.
</t>
<t>
The encryption in TLS makes it difficult for attackers to control the data which could be
used in a cross-protocol attack on a cleartext protocol.
</t>
<t>
The cleartext version of HTTP/2 has minimal protection against cross-protocol attacks.
The <xref target="ConnectionHeader">connection preface</xref> contains a string that is
designed to confuse HTTP/1.1 servers, but no special protection is offered for other
protocols. A server that is willing to ignore parts of an HTTP/1.1 request containing an
Upgrade header field could be exposed to a cross-protocol attack.
</t>
</section>
<section title="Intermediary Encapsulation Attacks">
<t>
HTTP/2 header field names and values are encoded as sequences of octets with a length
prefix. This enables HTTP/2 to carry any string of octets as the name or value of a
header field. An intermediary that translates HTTP/2 requests or responses into HTTP/1.1
directly could permit the creation of corrupted HTTP/1.1 messages. An attacker might
exploit this behavior to cause the intermediary to create HTTP/1.1 messages with illegal
header fields, extra header fields, or even new messages that are entirely falsified.
</t>
<t>
Header field names or values that contain characters not permitted by HTTP/1.1, including
carriage return (U+000D) or line feed (U+000A) MUST NOT be translated verbatim by an
intermediary, as stipulated in <xref target="HTTP-p1"/>, Section 3.2.4.
</t>
<t>
Translation from HTTP/1.x to HTTP/2 does not produce the same opportunity to an attacker.
Intermediaries that perform translation to HTTP/2 MUST remove any instances of the <spanx style="verb">obs-fold</spanx> production from header field values.
</t>
</section>
<section title="Cacheability of Pushed Responses">
<t>
Pushed responses do not have an explicit request from the client; the request
is provided by the server in the <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref> frame.
</t>
<t>
Caching responses 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 responses for which an origin server is not authoritative (see
<xref target="authority"/>) are never cached or used.
</t>
</section>
<section title="Denial of Service Considerations">
<t>
An HTTP/2 connection can demand a greater commitment of resources to operate than a
HTTP/1.1 connection. The use of header compression and flow control depend on a
commitment of resources for storing a greater amount of state. Settings for these
features ensure that memory commitments for these features are strictly bounded.
Processing capacity cannot be guarded in the same fashion.
</t>
<t>
The <xref target="SETTINGS" format="none">SETTINGS</xref> frame can be abused to cause a peer to expend additional
processing time. This might be done by pointlessly changing SETTINGS parameters, setting
multiple undefined parameters, or changing the same setting multiple times in the same
frame. <xref target="WINDOW_UPDATE" format="none">WINDOW_UPDATE</xref> or <xref target="PRIORITY" format="none">PRIORITY</xref> frames can be abused to
cause an unnecessary waste of resources. A server might erroneously issue
<xref target="ALTSVC" format="none">ALTSVC</xref> frames for origins on which it cannot be authoritative to generate
excess work for clients.
</t>
<t>
Large numbers of small or empty frames can be abused to cause a peer to expend time
processing frame headers. Note however that some uses are entirely legitimate, such as
the sending of an empty <xref target="DATA" format="none">DATA</xref> frame to end a stream.
</t>
<t>
Header compression also offers some opportunities to waste processing resources; see <xref target="COMPRESSION"/> for more details on potential abuses.
</t>
<t>
Limits in <xref target="SETTINGS" format="none">SETTINGS</xref> parameters cannot be reduced instantaneously, which
leaves an endpoint exposed to behavior from a peer that could exceed the new limits. In
particular, immediately after establishing a connection, limits set by a server are not
known to clients and could be exceeded without being an obvious protocol violation.
</t>
<t>
All these features - i.e., <xref target="SETTINGS" format="none">SETTINGS</xref> changes, small frames, header
compression - have legitimate uses. These features become a burden only when they are
used unnecessarily or to excess.
</t>
<t>
An endpoint that doesn't monitor this behavior exposes itself to a risk of denial of
service attack. Implementations SHOULD track the use of these features and set limits on
their use. An endpoint MAY treat activity that is suspicious as a <xref target="ConnectionErrorHandler">connection error</xref> of type
<xref target="ENHANCE_YOUR_CALM" format="none">ENHANCE_YOUR_CALM</xref>.
</t>
</section>
<section title="Use of Compression">
<t>
HTTP/2 enables greater use of compression for both header fields (<xref target="HeaderBlock"/>) and response bodies (<xref target="Compression"/>). Compression
can allow an attacker to recover secret data when it is compressed in the same context
as data under attacker control.
</t>
<t>
There are demonstrable attacks on compression that exploit the characteristics of the web
(e.g., <xref target="BREACH"/>). The attacker induces multiple requests containing
varying plaintext, observing the length of the resulting ciphertext in each, which
reveals a shorter length when a guess about the secret is correct.
</t>
<t>
Implementations communicating on a secure channel MUST NOT compress content that includes
both confidential and attacker-controlled data unless separate compression dictionaries
are used for each source of data. Compression MUST NOT be used if the source of data
cannot be reliably determined.
</t>
<t>
Further considerations regarding the compression of header fields are described in <xref target="COMPRESSION"/>.
</t>
</section>
<section title="Use of Padding" anchor="padding">
<t>
Padding within HTTP/2 is not intended as a replacement for general purpose padding, such
as might be provided by <xref target="TLS12">TLS</xref>. Redundant padding could even be
counterproductive. Correct application can depend on having specific knowledge of the
data that is being padded.
</t>
<t>
To mitigate attacks that rely on compression, disabling compression might be preferable to
padding as a countermeasure.
</t>
<t>
Padding can be used to obscure the exact size of frame content, and is provided to
mitigate specific attacks within HTTP. For example, attacks where compressed content
includes both attacker-controlled plaintext and secret data (see for example, <xref target="BREACH"/>).
</t>
<t>
Use of padding can result in less protection than might seem immediately obvious. At
best, padding only makes it more difficult for an attacker to infer length information by
increasing the number of frames an attacker has to observe. Incorrectly implemented
padding schemes can be easily defeated. In particular, randomized padding with a
predictable distribution provides very little protection; or padding payloads to a fixed
size exposes information as payload sizes cross the fixed size boundary, which could be
possible if an attacker can control plaintext.
</t>
<t>
Intermediaries SHOULD NOT remove padding, though an intermediary MAY remove padding and
add differing amounts if the intent is to improve the protections padding affords.
</t>
</section>
<section title="Privacy Considerations">
<t>
Several characteristics of HTTP/2 provide an observer an opportunity to correlate actions
of a single client or server over time. This includes the value of settings, the manner
in which flow control windows are managed, the way priorities are allocated to streams,
timing of reactions to stimulus, and handling of any optional features.
</t>
<t>
As far as this creates observable differences in behavior, they could be used as a basis
for fingerprinting a specific client, as defined in <eref target="http://www.w3.org/TR/html5/introduction.html#fingerprint"/>.
</t>
</section>
</section>
<section anchor="iana" title="IANA Considerations">
<t>
A string for identifying HTTP/2 is entered into the "Application Layer Protocol Negotiation
(ALPN) Protocol IDs" registry established in <xref target="TLSALPN"/>.
</t>
<t>
This document establishes a registry for error codes. This new registry is entered into a
new "Hypertext Transfer Protocol (HTTP) 2 Parameters" section.
</t>
<t>
This document registers the <spanx style="verb">HTTP2-Settings</spanx> header field for
use in HTTP.
</t>
<t>
This document registers the <spanx style="verb">PRI</spanx> method for use in HTTP, to avoid
collisions with the <xref target="ConnectionHeader">connection preface</xref>.
</t>
<section anchor="iana-alpn" title="Registration of HTTP/2 Identification String">
<t>
This document creates two registrations for the identification of HTTP/2 in the "Application
Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in <xref target="TLSALPN"/>.
</t>
<t>
The "h2" string identifies HTTP/2 when used over TLS:
<list style="hanging">
<t hangText="Protocol:">HTTP/2 over TLS</t>
<t hangText="Identification Sequence:">0x68 0x32 ("h2")</t>
<t hangText="Specification:">This document (RFCXXXX)</t>
</list>
</t>
<t>
The "h2c" string identifies HTTP/2 when used over cleartext TCP:
<list style="hanging">
<t hangText="Protocol:">HTTP/2 over TCP</t>
<t hangText="Identification Sequence:">0x68 0x32 0x63 ("h2c")</t>
<t hangText="Specification:">This document (RFCXXXX)</t>
</list>
</t>
</section>
<section title="Error Code Registry">
<t>
This document establishes a registry for HTTP/2 error codes. The "HTTP/2 Error Code"
registry manages a 32-bit space. The "HTTP/2 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="HTTP2-Settings Header Field Registration">
<t>
This section registers the <spanx style="verb">HTTP2-Settings</spanx> header field in the
<xref target="BCP90">Permanent Message Header Field Registry</xref>.
<list style="hanging">
<t hangText="Header field name:">
HTTP2-Settings
</t>
<t hangText="Applicable protocol:">
http
</t>
<t hangText="Status:">
standard
</t>
<t hangText="Author/Change controller:">
IETF
</t>
<t hangText="Specification document(s):">
<xref target="Http2SettingsHeader"/> of this document
</t>
<t hangText="Related information:">
This header field is only used by an HTTP/2 client for Upgrade-based negotiation.
</t>
</list>
</t>
</section>
<section title="PRI Method Registration">
<t>
This section registers the <spanx style="verb">PRI</spanx> method in the
<xref target="HTTP-p2">HTTP Method Registry</xref>.
<list style="hanging">
<t hangText="Method Name:">
PRI
</t>
<t hangText="Safe">
No
</t>
<t hangText="Idempotent">
No
</t>
<t hangText="Specification document(s)">
<xref target="ConnectionHeader"/> of this document
</t>
<t hangText="Related information:">
This method is never used by an actual client. This method will appear to be used
when an HTTP/1.1 server or intermediary attempts to parse an HTTP/2 connection
preface.
</t>
</list>
</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, Jeff Pinner, Mike Bishop, Herve Ruellan
(Substantial editorial contributions).
</t>
<t>
Alexey Melnikov was an editor of this document during 2013.
</t>
<t>
A substantial proportion of Martin's contribution was supported by Microsoft during his
employment there.
</t>
</list>
</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="COMPRESSION">
<front>
<title>HPACK - Header Compression for HTTP/2</title>
<author initials="H." surname="Ruellan" fullname="Herve Ruellan"/>
<author initials="R." surname="Peon" fullname="Roberto Peon"/>
<date month="April" year="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-header-compression-07"/>
</reference>
<reference anchor="TCP">
<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"/>
<date month="May" year="2000"/>
</front>
<seriesInfo name="RFC" value="2818"/>
</reference>
<reference anchor="UTF-8">
<front>
<title abbrev="UTF-8">UTF-8, a transformation format of ISO 10646</title>
<author initials="F." surname="Yergeau" fullname="Francois Yergeau"/>
<date year="2003" month="November"/>
</front>
<seriesInfo name="STD" value="63"/>
<seriesInfo name="RFC" value="3629"/>
</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 initials="R." surname="Fielding" fullname="Roy T. Fielding"/>
<author initials="L." surname="Masinter" fullname="Larry Masinter"/>
<date year="2005" month="January"/>
</front>
<seriesInfo name="STD" value="66"/>
<seriesInfo name="RFC" value="3986"/>
</reference>
<reference anchor="RFC4648">
<front>
<title>The Base16, Base32, and Base64 Data Encodings</title>
<author fullname="S. Josefsson" initials="S." surname="Josefsson"/>
<date year="2006" month="October"/>
</front>
<seriesInfo value="4648" name="RFC"/>
</reference>
<reference anchor="RFC5226">
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials="T." surname="Narten" fullname="T. Narten"/>
<author initials="H." surname="Alvestrand" fullname="H. Alvestrand"/>
<date year="2008" month="May"/>
</front>
<seriesInfo name="BCP" value="26"/>
<seriesInfo name="RFC" value="5226"/>
</reference>
<reference anchor="RFC5234">
<front>
<title>Augmented BNF for Syntax Specifications: ABNF</title>
<author initials="D." surname="Crocker" fullname="D. Crocker"/>
<author initials="P." surname="Overell" fullname="P. Overell"/>
<date year="2008" month="January"/>
</front>
<seriesInfo name="STD" value="68"/>
<seriesInfo name="RFC" value="5234"/>
</reference>
<reference anchor="RFC6454">
<front>
<title>The Web Origin Concept</title>
<author initials="A." surname="Barth" fullname="Adam Barth"/>
<date year="2011" month="December"/>
</front>
<seriesInfo name="RFC" value="6454"/>
</reference>
<reference anchor="TLS12">
<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="TLS-EXT">
<front>
<title>
Transport Layer Security (TLS) Extensions: Extension Definitions
</title>
<author initials="D." surname="Eastlake" fullname="D. Eastlake"/>
<date year="2011" month="January"/>
</front>
<seriesInfo name="RFC" value="6066"/>
</reference>
<reference anchor="TLSALPN">
<front>
<title>Transport Layer Security (TLS) Application Layer Protocol Negotiation Extension</title>
<author initials="S." surname="Friedl" fullname="Stephan Friedl"/>
<author initials="A." surname="Popov" fullname="Andrei Popov"/>
<author initials="A." surname="Langley" fullname="Adam Langley"/>
<author initials="E." surname="Stephan" fullname="Emile Stephan"/>
<date month="March" year="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-tls-applayerprotoneg-05"/>
</reference>
<reference anchor="HTTP-p1">
<front>
<title>
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</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="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-26"/>
</reference>
<reference anchor="HTTP-p2">
<front>
<title>
Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</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="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-26"/>
</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="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-26"/>
</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="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-26"/>
</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 fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
<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="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-26"/>
</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="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-26"/>
</reference>
<reference anchor="COOKIE">
<front>
<title>HTTP State Management Mechanism</title>
<author initials="A." surname="Barth" fullname="A. Barth"/>
<date year="2011" month="April"/>
</front>
<seriesInfo name="RFC" value="6265"/>
</reference>
<reference anchor="ALT-SVC">
<front>
<title>
HTTP Alternative Services
</title>
<author initials="M." surname="Nottingham" fullname="Mark Nottingham">
<organization>Akamai</organization>
</author>
<author initials="P." surname="McManus" fullname="Patrick McManus">
<organization>Mozilla</organization>
</author>
<author initials="J." surname="Reschke" fullname="Julian Reschke">
<organization>greenbytes</organization>
</author>
<date year="2014" month="April"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-alt-svc-01"/>
</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 initials="B." surname="Braden" fullname="Bob Braden"/>
<author initials="D." surname="Borman" fullname="Dave Borman"/>
<date year="1992" month="May"/>
</front>
<seriesInfo name="RFC" value="1323"/>
</reference>
<reference anchor="RFC3749">
<front>
<title>Transport Layer Security Protocol Compression Methods</title>
<author initials="S." surname="Hollenbeck" fullname="S. Hollenbeck"/>
<date year="2004" month="May"/>
</front>
<seriesInfo name="RFC" value="3749"/>
</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>
<reference anchor="BREACH" target="http://breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf">
<front>
<title>
BREACH: Reviving the CRIME Attack
</title>
<author initials="Y." surname="Gluck"/>
<author initials="N." surname="Harris"/>
<author initials="A." surname="Prado"/>
<date year="2013" month="July" day="12"/>
</front>
</reference>
<reference anchor="RC4">
<front>
<title>
The RC4 encryption algorithm
</title>
<author initials="R." surname="Rivest" fullname="Ron Rivest"/>
<date year="1992" month="March"/>
</front>
<seriesInfo name="RSA Data Security, Inc." value=""/>
</reference>
<reference anchor="BCP90">
<front>
<title>Registration Procedures for Message Header Fields</title>
<author initials="G." surname="Klyne" fullname="G. Klyne">
<organization>Nine by Nine</organization>
<address><email>GK-IETF@ninebynine.org</email></address>
</author>
<author initials="M." surname="Nottingham" fullname="M. Nottingham">
<organization>BEA Systems</organization>
<address><email>mnot@pobox.com</email></address>
</author>
<author initials="J." surname="Mogul" fullname="J. Mogul">
<organization>HP Labs</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<date year="2004" month="September"/>
</front>
<seriesInfo name="BCP" value="90"/>
<seriesInfo name="RFC" value="3864"/>
</reference>
<reference anchor="TLSBCP">
<front>
<title>
Recommendations for Secure Use of TLS and DTLS
</title>
<author initials="Y." surname="Sheffer" fullname="Yaron Sheffer">
<organization>Porticor</organization>
</author>
<author initials="R." surname="Holz" fullname="Ralph Holz">
<organization>Technische Universitaet Muenchen</organization>
</author>
<author initials="P." surname="Saint-Andre" fullname="Peter Saint-Andre">
<organization>&yet</organization>
</author>
<date year="2014" month="February"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-sheffer-tls-bcp-02"/>
</reference>
<reference anchor="IDNA">
<front>
<title>
Internationalized Domain Names for Applications (IDNA): Definitions and Document
Framework
</title>
<author initials="J." surname="Klensin" fullname="John Klensin"/>
<date year="2010" month="August"/>
</front>
<seriesInfo name="RFC" value="5890"/>
</reference>
</references>
<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
<section title="Since draft-ietf-httpbis-http2-10" anchor="changes.since.draft-ietf-httpbis-http2-10">
<t>
Changed "connection header" to "connection preface" to avoid confusion.
</t>
<t>
Added dependency-based stream prioritization.
</t>
<t>
Added "h2c" identifier to distinguish between cleartext and secured HTTP/2.
</t>
<t>
Adding missing padding to <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref>.
</t>
<t>
Integrate ALTSVC frame and supporting text.
</t>
<t>
Dropping requirement on "deflate" Content-Encoding.
</t>
<t>
Improving security considerations around use of compression.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-09" anchor="changes.since.draft-ietf-httpbis-http2-09">
<t>
Adding padding for data frames.
</t>
<t>
Renumbering frame types, error codes, and settings.
</t>
<t>
Adding INADEQUATE_SECURITY error code.
</t>
<t>
Updating TLS usage requirements to 1.2; forbidding TLS compression.
</t>
<t>
Removing extensibility for frames and settings.
</t>
<t>
Changing setting identifier size.
</t>
<t>
Removing the ability to disable flow control.
</t>
<t>
Changing the protocol identification token to "h2".
</t>
<t>
Changing the use of :authority to make it optional and to allow userinfo in non-HTTP
cases.
</t>
<t>
Allowing split on 0x0 for Cookie.
</t>
<t>
Reserved PRI method in HTTP/1.1 to avoid possible future collisions.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-08" anchor="changes.since.draft-ietf-httpbis-http2-08">
<t>
Added cookie crumbling for more efficient header compression.
</t>
<t>
Added header field ordering with the value-concatenation mechanism.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-07" anchor="changes.since.draft-ietf-httpbis-http2-07">
<t>
Marked draft for implementation.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-06" anchor="changes.since.draft-ietf-httpbis-http2-06">
<t>
Adding definition for CONNECT method.
</t>
<t>
Constraining the use of push to safe, cacheable methods with no request body.
</t>
<t>
Changing from :host to :authority to remove any potential confusion.
</t>
<t>
Adding setting for header compression table size.
</t>
<t>
Adding settings acknowledgement.
</t>
<t>
Removing unnecessary and potentially problematic flags from CONTINUATION.
</t>
<t>
Added denial of service considerations.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-05" anchor="changes.since.draft-ietf-httpbis-http2-05">
<t>
Marking the draft ready for implementation.
</t>
<t>
Renumbering END_PUSH_PROMISE flag.
</t>
<t>
Editorial clarifications and changes.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-04" anchor="changes.since.draft-ietf-httpbis-http2-04">
<t>
Added CONTINUATION frame for HEADERS and PUSH_PROMISE.
</t>
<t>
PUSH_PROMISE is no longer implicitly prohibited if SETTINGS_MAX_CONCURRENT_STREAMS is
zero.
</t>
<t>
Push expanded to allow all safe methods without a request body.
</t>
<t>
Clarified the use of HTTP header fields in requests and responses. Prohibited HTTP/1.1
hop-by-hop header fields.
</t>
<t>
Requiring that intermediaries not forward requests with missing or illegal routing
:-headers.
</t>
<t>
Clarified requirements around handling different frames after stream close, stream reset
and <xref target="GOAWAY" format="none">GOAWAY</xref>.
</t>
<t>
Added more specific prohibitions for sending of different frame types in various stream
states.
</t>
<t>
Making the last received setting value the effective value.
</t>
<t>
Clarified requirements on TLS version, extension and ciphers.
</t>
</section>
<section title="Since draft-ietf-httpbis-http2-03" anchor="changes.since.draft-ietf-httpbis-http2-03">
<t>
Committed major restructuring atrocities.
</t>
<t>
Added reference to first header compression draft.
</t>
<t>
Added more formal description of frame lifecycle.
</t>
<t>
Moved END_STREAM (renamed from FINAL) back to <xref target="HEADERS" format="none">HEADERS</xref>/<xref target="DATA" format="none">DATA</xref>.
</t>
<t>
Removed HEADERS+PRIORITY, added optional priority to <xref target="HEADERS" format="none">HEADERS</xref> frame.
</t>
<t>
Added <xref target="PRIORITY" format="none">PRIORITY</xref> frame.
</t>
</section>
<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 <xref target="PUSH_PROMISE" format="none">PUSH_PROMISE</xref>.
</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 <xref target="INTERNAL_ERROR" format="none">INTERNAL_ERROR</xref> on <xref target="GOAWAY" format="none">GOAWAY</xref> 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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