HTTPbis Working Group M. Belshe
Internet-Draft Twist
Intended status: Standards Track R. Peon
Expires: February 14, 2014 Google, Inc
M. Thomson, Ed.
Microsoft
A. Melnikov, Ed.
Isode Ltd
August 13, 2013
Hypertext Transfer Protocol version 2.0
draft-ietf-httpbis-http2-05
Abstract
This specification describes an optimized expression of the syntax of
the Hypertext Transfer Protocol (HTTP). The HTTP/2.0 encapsulation
enables more efficient use of network resources and reduced
perception of latency by allowing header field compression and
multiple concurrent messages on the same connection. It also
introduces unsolicited push of representations from servers to
clients.
This document is an alternative to, but does not obsolete the
HTTP/1.1 message format or protocol. HTTP's existing semantics
remain unchanged.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information and related documents can be found at
<http://tools.ietf.org/wg/httpbis/> (Wiki) and
<https://github.com/http2/http2-spec> (source code and issues
tracker).
The changes in this draft are summarized in Appendix A.1.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 14, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Document Organization . . . . . . . . . . . . . . . . . . 5
1.2. Conventions and Terminology . . . . . . . . . . . . . . . 6
2. HTTP/2.0 Protocol Overview . . . . . . . . . . . . . . . . . . 6
2.1. HTTP Frames . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. HTTP Multiplexing . . . . . . . . . . . . . . . . . . . . 7
2.3. HTTP Semantics . . . . . . . . . . . . . . . . . . . . . . 7
3. Starting HTTP/2.0 . . . . . . . . . . . . . . . . . . . . . . 7
3.1. HTTP/2.0 Version Identification . . . . . . . . . . . . . 8
3.2. Starting HTTP/2.0 for "http" URIs . . . . . . . . . . . . 8
3.2.1. HTTP2-Settings Header Field . . . . . . . . . . . . . 10
3.3. Starting HTTP/2.0 for "https" URIs . . . . . . . . . . . . 10
3.4. Starting HTTP/2.0 with Prior Knowledge . . . . . . . . . . 10
3.5. Connection Header . . . . . . . . . . . . . . . . . . . . 11
4. HTTP Frames . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Frame Format . . . . . . . . . . . . . . . . . . . . . . . 12
4.2. Frame Size . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3. Header Compression and Decompression . . . . . . . . . . . 13
5. Streams and Multiplexing . . . . . . . . . . . . . . . . . . . 14
5.1. Stream States . . . . . . . . . . . . . . . . . . . . . . 14
5.1.1. Stream Identifiers . . . . . . . . . . . . . . . . . . 18
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5.1.2. Stream Concurrency . . . . . . . . . . . . . . . . . . 19
5.2. Flow Control . . . . . . . . . . . . . . . . . . . . . . . 19
5.2.1. Flow Control Principles . . . . . . . . . . . . . . . 19
5.2.2. Appropriate Use of Flow Control . . . . . . . . . . . 21
5.3. Stream priority . . . . . . . . . . . . . . . . . . . . . 21
5.4. Error Handling . . . . . . . . . . . . . . . . . . . . . . 22
5.4.1. Connection Error Handling . . . . . . . . . . . . . . 22
5.4.2. Stream Error Handling . . . . . . . . . . . . . . . . 22
5.4.3. Connection Termination . . . . . . . . . . . . . . . . 23
6. Frame Definitions . . . . . . . . . . . . . . . . . . . . . . 23
6.1. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2. HEADERS . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.3. PRIORITY . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.4. RST_STREAM . . . . . . . . . . . . . . . . . . . . . . . . 26
6.5. SETTINGS . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.5.1. Setting Format . . . . . . . . . . . . . . . . . . . . 27
6.5.2. Defined Settings . . . . . . . . . . . . . . . . . . . 28
6.6. PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . . . 28
6.7. PING . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.8. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . . . 31
6.9. WINDOW_UPDATE . . . . . . . . . . . . . . . . . . . . . . 33
6.9.1. The Flow Control Window . . . . . . . . . . . . . . . 34
6.9.2. Initial Flow Control Window Size . . . . . . . . . . . 35
6.9.3. Reducing the Stream Window Size . . . . . . . . . . . 36
6.9.4. Ending Flow Control . . . . . . . . . . . . . . . . . 36
6.10. CONTINUATION . . . . . . . . . . . . . . . . . . . . . . . 36
7. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . 38
8. HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 38
8.1. HTTP Request/Response Exchange . . . . . . . . . . . . . . 39
8.1.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 39
8.1.2. HTTP Header Fields . . . . . . . . . . . . . . . . . . 41
8.1.3. Request Reliability Mechanisms in HTTP/2.0 . . . . . . 43
8.2. Server Push . . . . . . . . . . . . . . . . . . . . . . . 43
8.2.1. Push Requests . . . . . . . . . . . . . . . . . . . . 44
8.2.2. Push Responses . . . . . . . . . . . . . . . . . . . . 45
9. Additional HTTP Requirements/Considerations . . . . . . . . . 46
9.1. Connection Management . . . . . . . . . . . . . . . . . . 46
9.2. Use of TLS Features . . . . . . . . . . . . . . . . . . . 46
9.3. Frame Size Limits for HTTP . . . . . . . . . . . . . . . . 47
9.4. GZip Content-Encoding . . . . . . . . . . . . . . . . . . 47
10. Security Considerations . . . . . . . . . . . . . . . . . . . 47
10.1. Server Authority and Same-Origin . . . . . . . . . . . . . 47
10.2. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 47
10.3. Cacheability of Pushed Resources . . . . . . . . . . . . . 48
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 48
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
12.1. Frame Type Registry . . . . . . . . . . . . . . . . . . . 49
12.2. Error Code Registry . . . . . . . . . . . . . . . . . . . 49
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12.3. Settings Registry . . . . . . . . . . . . . . . . . . . . 50
12.4. HTTP2-Settings Header Field Registration . . . . . . . . . 51
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 51
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 51
14.1. Normative References . . . . . . . . . . . . . . . . . . . 51
14.2. Informative References . . . . . . . . . . . . . . . . . . 53
Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 53
A.1. Since draft-ietf-httpbis-http2-04 . . . . . . . . . . . . 53
A.2. Since draft-ietf-httpbis-http2-03 . . . . . . . . . . . . 54
A.3. Since draft-ietf-httpbis-http2-02 . . . . . . . . . . . . 54
A.4. Since draft-ietf-httpbis-http2-01 . . . . . . . . . . . . 54
A.5. Since draft-ietf-httpbis-http2-00 . . . . . . . . . . . . 55
A.6. Since draft-mbelshe-httpbis-spdy-00 . . . . . . . . . . . 55
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1. Introduction
The Hypertext Transfer Protocol (HTTP) is a wildly successful
protocol. However, the HTTP/1.1 message format ([HTTP-p1], Section
3) is optimized for implementation simplicity and accessibility, not
application performance. As such it has several characteristics that
have a negative overall effect on application performance.
In particular, HTTP/1.0 only allows one request to be delivered at a
time on a given connection. HTTP/1.1 pipelining only partially
addressed request concurrency, and is not widely deployed.
Therefore, clients that need to make many requests (as is common on
the Web) typically use multiple connections to a server in order to
reduce perceived latency.
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.
This document addresses these issues by defining an optimized mapping
of HTTP's semantics to an underlying connection. Specifically, it
allows interleaving of request and response messages on the same
connection and uses an efficient coding for HTTP header fields. It
also allows prioritization of requests, letting more important
requests complete more quickly, further improving perceived
performance.
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.
Finally, this encapsulation also enables more scalable processing of
messages through use of binary message framing.
1.1. Document Organization
The HTTP/2.0 Specification is split into three parts: starting
HTTP/2.0 (Section 3), which covers how a HTTP/2.0 connection is
initiated; a framing layer (Section 4), which multiplexes a single
TCP connection into independent frames of various types; and an HTTP
layer (Section 8), which specifies the mechanism for expressing HTTP
interactions using the framing layer. While some of the framing
layer concepts are isolated from HTTP, building a generic framing
layer has not been a goal. The framing layer is tailored to the
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needs of the HTTP protocol and server push.
1.2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
All numeric values are in network byte order. Values are unsigned
unless otherwise indicated. Literal values are provided in decimal
or hexadecimal as appropriate. Hexadecimal literals are prefixed
with "0x" to distinguish them from decimal literals.
The following terms are used:
client: The endpoint initiating the HTTP connection.
connection: A transport-level connection between two endpoints.
endpoint: Either the client or server of the connection.
frame: The smallest unit of communication within an HTTP/2.0
connection, consisting of a header and a variable-length sequence
of bytes structured according to the frame type.
peer: An endpoint. When discussing a particular endpoint, "peer"
refers to the endpoint that is remote to the primary subject of
discussion.
receiver: An endpoint that is receiving frames.
sender: An endpoint that is transmitting frames.
server: The endpoint which did not initiate the HTTP connection.
connection error: An error on the HTTP/2.0 connection.
stream: A bi-directional flow of frames across a virtual channel
within the HTTP/2.0 connection.
stream error: An error on the individual HTTP/2.0 stream.
2. HTTP/2.0 Protocol Overview
HTTP/2.0 provides an optimized transport for HTTP semantics.
An HTTP/2.0 connection is an application level protocol running on
top of a TCP connection ([RFC0793]). The client is the TCP
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connection initiator.
This document describes the HTTP/2.0 protocol using a logical
structure that is formed of three parts: framing, streams, and
application mapping. This structure is provided primarily as an aid
to specification, implementations are free to diverge from this
structure as necessary.
2.1. HTTP Frames
HTTP/2.0 provides an efficient serialization of HTTP semantics. HTTP
requests and responses are encoded into length-prefixed frames (see
Section 4.1).
HTTP headers are compressed into a series of frames that contain
header block fragments (see Section 4.3).
2.2. HTTP Multiplexing
HTTP/2.0 provides the ability to multiplex multiple HTTP requests and
responses onto a single connection. Multiple requests or responses
can be sent concurrently on a connection using streams (Section 5).
In order to maintain independent streams, flow control and
prioritization are necessary.
2.3. HTTP Semantics
HTTP/2.0 defines how HTTP requests and responses are mapped to
streams (see Section 8.1) and introduces a new interaction model,
server push (Section 8.2).
3. Starting HTTP/2.0
HTTP/2.0 uses the same "http" and "https" URI schemes used by
HTTP/1.1. HTTP/2.0 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
"http://example.org/foo" or "https://example.com/bar" are required to
first discover whether the upstream server (the immediate peer to
which the client wishes to establish a connection) supports HTTP/2.0.
The means by which support for HTTP/2.0 is determined is different
for "http" and "https" URIs. Discovery for "http" URIs is described
in Section 3.2. Discovery for "https" URIs is described in
Section 3.3.
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3.1. HTTP/2.0 Version Identification
The protocol defined in this document is identified using the string
"HTTP/2.0". This identification is used in the HTTP/1.1 Upgrade
header field, in the TLS application layer protocol negotiation
extension [TLSALPN] field, and other places where protocol
identification is required.
Negotiating "HTTP/2.0" implies the use of the transport, security,
framing and message semantics described in this document.
[[anchor6: Editor's Note: please remove the remainder of this section
prior to the publication of a final version of this document.]]
Only implementations of the final, published RFC can identify
themselves as "HTTP/2.0". Until such an RFC exists, implementations
MUST NOT identify themselves using "HTTP/2.0".
Examples and text throughout the rest of this document use "HTTP/2.0"
as a matter of editorial convenience only. Implementations of draft
versions MUST NOT identify using this string. The exception to this
rule is the string included in the connection header sent by clients
immediately after establishing an HTTP/2.0 connection (see
Section 3.5); this fixed length sequence of octets does not change.
Implementations of draft versions of the protocol MUST add the string
"-draft-" and the corresponding draft number to the identifier before
the separator ('/'). For example, draft-ietf-httpbis-http2-03 is
identified using the string "HTTP-draft-03/2.0".
Non-compatible experiments that are based on these draft versions
MUST instead replace the string "draft" with a different identifier.
For example, an experimental implementation of packet mood-based
encoding based on draft-ietf-httpbis-http2-07 might identify itself
as "HTTP-emo-07/2.0". Note that any label MUST conform to the
"token" syntax defined in Section 3.2.6 of [HTTP-p1]. Experimenters
are encouraged to coordinate their experiments on the
ietf-http-wg@w3.org mailing list.
3.2. Starting HTTP/2.0 for "http" URIs
A client that makes a request to an "http" URI without prior
knowledge about support for HTTP/2.0 uses the HTTP Upgrade mechanism
(Section 6.7 of [HTTP-p1]). The client makes an HTTP/1.1 request
that includes an Upgrade header field identifying HTTP/2.0. The
HTTP/1.1 request MUST include exactly one HTTP2-Settings
(Section 3.2.1) header field.
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For example:
GET /default.htm HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: HTTP/2.0
HTTP2-Settings: <base64url encoding of HTTP/2.0 SETTINGS payload>
Requests that contain a request entity body MUST be sent in their
entirety before the client can send HTTP/2.0 frames. This means that
a large request entity can block the use of the connection until it
is completely sent.
If concurrency of an initial request with subsequent requests is
important, a small request can be used to perform the upgrade to
HTTP/2.0, at the cost of an additional round trip.
A server that does not support HTTP/2.0 can respond to the request as
though the Upgrade header field were absent:
HTTP/1.1 200 OK
Content-length: 243
Content-type: text/html
...
A server that supports HTTP/2.0 accepts the upgrade with a 101
(Switching Protocols) status code. After the empty line that
terminates the 101 response, the server can begin sending HTTP/2.0
frames. These frames MUST include a response to the request that
initiated the Upgrade.
HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: HTTP/2.0
[ HTTP/2.0 connection ...
The first HTTP/2.0 frame sent by the server is a SETTINGS frame
(Section 6.5). Upon receiving the 101 response, the client sends a
connection header (Section 3.5), which includes a SETTINGS frame.
The HTTP/1.1 request that is sent prior to upgrade is associated with
stream 1 and is assigned the highest possible priority. 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/2.0 connection, stream 1 is used for the response.
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3.2.1. HTTP2-Settings Header Field
A client that upgrades from HTTP/1.1 to HTTP/2.0 MUST include exactly
one "HTTP2-Settings" header field. The "HTTP2-Settings" header field
is a hop-by-hop header field that includes settings that govern the
HTTP/2.0 connection, provided in anticipation of the server accepting
the request to upgrade. A server MUST reject an attempt to upgrade
if this header is not present.
HTTP2-Settings = token68
The content of the "HTTP2-Settings" header field is the payload of a
SETTINGS frame (Section 6.5), encoded as a base64url string (that is,
the URL- and filename-safe Base64 encoding described in Section 5 of
[RFC4648], with any trailing '=' characters omitted). The ABNF
[RFC5234] production for "token68" is defined in Section 2.1 of
[HTTP-p7].
The client MUST include values for the following settings
(Section 6.5.1):
o SETTINGS_MAX_CONCURRENT_STREAMS
o SETTINGS_INITIAL_WINDOW_SIZE
As a hop-by-hop header field, the "Connection" header field MUST
include a value of "HTTP2-Settings" in addition to "Upgrade" when
upgrading to HTTP/2.0.
A server decodes and interprets these values as it would any other
SETTINGS frame. Providing these values in the Upgrade request
ensures that the protocol does not require default values for the
above settings, and gives a client an opportunity to provide other
settings prior to receiving any frames from the server.
3.3. Starting HTTP/2.0 for "https" URIs
A client that makes a request to an "https" URI without prior
knowledge about support for HTTP/2.0 uses TLS [TLS12] with the
application layer protocol negotiation extension [TLSALPN].
Once TLS negotiation is complete, both the client and the server send
a connection header (Section 3.5).
3.4. Starting HTTP/2.0 with Prior Knowledge
A client can learn that a particular server supports HTTP/2.0 by
other means. A client MAY immediately send HTTP/2.0 frames to a
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server that is known to support HTTP/2.0, after the connection header
(Section 3.5). This only affects the resolution of "http" URIs;
servers supporting HTTP/2.0 are required to support protocol
negotiation in TLS [TLSALPN] for "https" URIs.
Prior support for HTTP/2.0 is not a strong signal that a given server
will support HTTP/2.0 for future connections. It is possible for
server configurations to change or for configurations to differ
between instances in clustered server. Interception proxies (a.k.a.
"transparent" proxies) are another source of variability.
3.5. Connection Header
Upon establishment of a TCP connection and determination that
HTTP/2.0 will be used by both peers, each endpoint MUST send a
connection header as a final confirmation and to establish the
initial settings for the HTTP/2.0 connection.
The client connection header is a sequence of 24 octets, which in hex
notation are:
505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
(the string "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n") followed by a
SETTINGS frame (Section 6.5). The client sends the client connection
header 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/2.0 connection with
prior knowledge of server support for the protocol, the client
connection header is sent upon connection establishment.
The client connection header is selected so that a large
proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do
not attempt to process further frames. Note that this does not
address the concerns raised in [TALKING].
The server connection header consists of just a SETTINGS frame
(Section 6.5) that MUST be the first frame the server sends in the
HTTP/2.0 connection.
To avoid unnecessary latency, clients are permitted to send
additional frames to the server immediately after sending the client
connection header, without waiting to receive the server connection
header. It is important to note, however, that the server connection
header SETTINGS frame might include parameters that necessarily alter
how a client is expected to communicate with the server. Upon
receiving the SETTINGS frame, the client is expected to honor any
parameters established.
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Clients and servers MUST terminate the TCP connection if either peer
does not begin with a valid connection header. A GOAWAY frame
(Section 6.8) MAY be omitted if it is clear that the peer is not
using HTTP/2.0.
4. HTTP Frames
Once the HTTP/2.0 connection is established, endpoints can begin
exchanging frames.
4.1. Frame Format
All frames begin with an 8-octet header followed by a payload of
between 0 and 65,535 octets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (16) | Type (8) | Flags (8) |
+-+-------------+---------------+-------------------------------+
|R| Stream Identifier (31) |
+-+-------------------------------------------------------------+
| Frame Payload (0...) ...
+---------------------------------------------------------------+
Frame Header
The fields of the frame header are defined as:
Length: The length of the frame payload expressed as an unsigned 16-
bit integer. The 8 octets of the frame header are not included in
this value.
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 ignore frames of unsupported or unrecognized
types.
Flags: An 8-bit field reserved for frame-type specific boolean
flags.
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.
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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.
Stream Identifier: A 31-bit stream identifier (see Section 5.1.1).
A value 0 is reserved for frames that are associated with the
connection as a whole as opposed to an individual stream.
The structure and content of the frame payload is dependent entirely
on the frame type.
4.2. Frame Size
The maximum size of a frame payload varies by frame type and use.
The absolute maximum size is 65,535 octets. All implementations
SHOULD be capable of receiving and minimally processing frames up to
this size.
Certain frame types, such as PING (see Section 6.7), impose
additional limits on the amount of payload data allowed. Likewise,
additional size limits can be set by specific application uses (see
Section 9).
If a frame size exceeds any defined limit, or is too small to contain
mandatory frame data, the endpoint MUST send a FRAME_TOO_LARGE error.
Frame size errors in frames that affect connection-level state MUST
be treated as a connection error (Section 5.4.1).
4.3. Header Compression and Decompression
A header in HTTP/2.0 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 Section 8.2).
Header sets are logical collections of zero or more header fields
arranged at the application layer. When transmitted over a
connection, the header set is serialized into a header block using
HTTP Header Compression [COMPRESSION]. The serialized header block
is then divided into one or more octet sequences, called header block
fragments, and transmitted within the payload of HEADERS
(Section 6.2), PUSH_PROMISE (Section 6.6) or CONTINUATION
(Section 6.10) frames. The receiving endpoint reassembles the header
block by concatenating the individual fragments, then decompresses
the block to reconstruct the header set.
Header block fragments can only be sent as the payload of HEADERS,
PUSH_PROMISE or CONTINUATION frames.
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A compressed and encoded header block is transmitted in a HEADERS or
PUSH_PROMISE frame, followed by zero or more CONTINUATION frames. If
the number of octets in the block is greater than the space remaining
in the frame, the block is divided into multiple fragments, which are
then transmitted in multiple frames.
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 HEADERS/CONTINUATION frames
MUST have the END_HEADERS flag set. The last frame in a sequence of
PUSH_PROMISE/CONTINUATION frames MUST have the END_PUSH_PROMISE/
END_HEADERS flag set (respectively).
HEADERS, PUSH_PROMISE and CONTINUATION frames carry data that can
modify the compression context maintained by a receiver. An endpoint
receiving HEADERS, PUSH_PROMISE or CONTINUATION frames MUST
reassemble header blocks and perform decompression even if the frames
are to be discarded, which is likely to occur after a stream is
reset. A receiver MUST terminate the connection with a connection
error (Section 5.4.1) of type COMPRESSION_ERROR, if it does not
decompress a header block.
5. Streams and Multiplexing
A "stream" is an independent, bi-directional sequence of HEADERS and
DATA frames exchanged between the client and server within an
HTTP/2.0 connection. Streams have several important characteristics:
o A single HTTP/2.0 connection can contain multiple concurrently
active streams, with either endpoint interleaving frames from
multiple streams.
o Streams can be established and used unilaterally or shared by
either the client or server.
o Streams can be closed by either endpoint.
o The order in which frames are sent within a stream is significant.
Recipients process frames in the order they are received.
o Streams are identified by an integer. Stream identifiers are
assigned to streams by the endpoint that initiates a stream.
5.1. Stream States
The lifecycle of a stream is shown in Figure 1.
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+--------+
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 |
`-------------------->| |<--------------------'
+--------+
Figure 1: Stream States
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 RST_STREAM, where
frames might be received for some time after closing.
Streams have the following states:
idle:
All streams start in the "idle" state. In this state, no frames
have been exchanged.
The following transitions are valid from this state:
* Sending or receiving a HEADERS frame causes the stream to
become "open". The stream identifier is selected as described
in Section 5.1.1.
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* Sending a PUSH_PROMISE frame marks the associated stream for
later use. The stream state for the reserved stream
transitions to "reserved (local)".
* Receiving a PUSH_PROMISE frame marks the associated stream as
reserved by the remote peer. The state of the stream becomes
"reserved (remote)".
reserved (local):
A stream in the "reserved (local)" state is one that has been
promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame
reserves an idle stream by associating the stream with an open
stream that was initiated by the remote peer (see Section 8.2).
In this state, only the following transitions are possible:
* The endpoint can send a HEADERS frame. This causes the stream
to open in a "half closed (remote)" state.
* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This also releases the stream reservation.
An endpoint MUST NOT send any other type of frame in this state.
Receiving any frame other than RST_STREAM or PRIORITY MUST be
treated as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
reserved (remote):
A stream in the "reserved (remote)" state has been reserved by a
remote peer.
In this state, only the following transitions are possible:
* Receiving a HEADERS frame causes the stream to transition to
"half closed (local)".
* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This also releases the stream reservation.
Receiving any other type of frame MUST be treated as a stream
error (Section 5.4.2) of type PROTOCOL_ERROR. An endpoint MAY
send RST_STREAM or PRIORITY frames in this state to cancel or
reprioritize the reserved stream.
open:
The "open" state is where both peers can send frames of any type.
In this state, sending peers observe advertised stream level flow
control limits (Section 5.2).
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From this state either endpoint can send a frame with a END_STREAM
flag set, which causes the stream to transition into one of the
"half closed" states: an endpoint sending a END_STREAM flag causes
the stream state to become "half closed (local)"; an endpoint
receiving a END_STREAM flag causes the stream state to become
"half closed (remote)".
Either endpoint can send a RST_STREAM frame from this state,
causing it to transition immediately to "closed".
half closed (local):
A stream that is "half closed (local)" cannot be used for sending
frames.
A stream transitions from this state to "closed" when a frame that
contains a END_STREAM flag is received, or when either peer sends
a RST_STREAM frame.
A receiver can ignore WINDOW_UPDATE or PRIORITY frames in this
state. These frame types might arrive for a short period after a
frame bearing the END_STREAM flag is sent.
half closed (remote):
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.
If an endpoint receives additional frames for a stream that is in
this state it MUST respond with a stream error (Section 5.4.2) of
type STREAM_CLOSED.
A stream can transition from this state to "closed" by sending a
frame that contains a END_STREAM flag, or when either peer sends a
RST_STREAM frame.
closed:
The "closed" state is the terminal state.
An endpoint MUST NOT send frames on a closed stream. An endpoint
that receives a frame after receiving a RST_STREAM or a frame
containing a END_STREAM flag on that stream MUST treat that as a
stream error (Section 5.4.2) of type STREAM_CLOSED.
WINDOW_UPDATE or PRIORITY frames can be received in this state for
a short period after a a 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 either frame type.
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Endpoints MUST ignore WINDOW_UPDATE frames received in this state,
though endpoints MAY choose to treat WINDOW_UPDATE frames that
arrive a significant time after sending END_STREAM as a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
If this state is reached as a result of sending a RST_STREAM
frame, the peer that receives the RST_STREAM 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 RST_STREAM 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.
Flow controlled frames (i.e., DATA) received after sending
RST_STREAM are counted toward the connection flow control window.
Even though these frames might be ignored, because they are sent
before the sender receives the RST_STREAM, the sender will
consider the frames to count against the flow control window.
An endpoint might receive a PUSH_PROMISE or a CONTINUATION frame
after it sends RST_STREAM. PUSH_PROMISE causes a stream to become
"reserved". If promised streams are not desired, a RST_STREAM can
be used to close any of those streams.
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 connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
5.1.1. Stream Identifiers
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
message; the stream identifier zero MUST NOT be used to establish a
new stream. A stream identifier of one (0x1) is used to respond to
the HTTP/1.1 request which was specified during Upgrade (see
Section 3.2); the stream identifier one MUST NOT be used to establish
a new stream.
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 HEADERS frame
and streams that are reserved using PUSH_PROMISE. An endpoint that
receives an unexpected stream identifier MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
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The first use of a new stream identifier implicitly closes all idle
streams that might have been initiated by that peer with a lower-
valued stream identifier.
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.
5.1.2. Stream Concurrency
A peer can limit the number of concurrently active streams using the
SETTINGS_MAX_CONCURRENT_STREAMS parameters within a SETTINGS frame.
The maximum concurrent streams setting is specific to each endpoint
and applies only to the peer that 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.
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 SETTINGS_MAX_CONCURRENT_STREAMS setting
(see Section 6.5.2).
Streams in either of the "reserved" states do not count as open, even
if a small amount of application state is retained to ensure that the
promised stream can be successfully used.
5.2. Flow Control
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.
HTTP/2.0 provides for flow control through use of the WINDOW_UPDATE
frame type.
5.2.1. Flow Control Principles
Experience with TCP congestion control has shown that algorithms can
evolve over time to become more sophisticated without requiring
protocol changes. TCP congestion control and its evolution is
clearly different from HTTP/2.0 flow control, though the evolution of
TCP congestion control algorithms shows that a similar approach could
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be feasible for HTTP/2.0 flow control.
HTTP/2.0 stream flow control aims to allow for future improvements to
flow control algorithms without requiring protocol changes. Flow
control in HTTP/2.0 has the following characteristics:
1. Flow control is hop-by-hop, not end-to-end.
2. 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.
3. Flow control is directional with overall control provided by the
receiver. A receiver MAY choose to set any window size that it
desires for each stream and for the entire connection. A sender
MUST respect flow control limits imposed by a receiver. Clients,
servers and intermediaries all independently advertise their flow
control preferences as a receiver and abide by the flow control
limits set by their peer when sending.
4. The initial value for the flow control window is 65535 bytes for
both new streams and the overall connection.
5. The frame type determines whether flow control applies to a
frame. Of the frames specified in this document, only DATA
frames are subject to flow control; all other frame types do not
consume space in the advertised flow control window. This
ensures that important control frames are not blocked by flow
control.
6. Flow control can be disabled by a receiver. A receiver can
choose to disable both forms of flow control by sending the
SETTINGS_FLOW_CONTROL_OPTIONS setting. See Ending Flow Control
(Section 6.9.4) for more details.
7. HTTP/2.0 standardizes only the format of the WINDOW_UPDATE frame
(Section 6.9). 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.
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.
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5.2.2. Appropriate Use of Flow Control
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.
Deployments that do not require this capability SHOULD disable flow
control for data that is being received. Note that flow control
cannot be disabled for sending. Sending data is always subject to
the flow control window advertised by the receiver.
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 [RFC1323]).
Even with full awareness of the current bandwidth-delay product,
implementation of flow control can be difficult. When using flow
control, the receive 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 WINDOW_UPDATE, are not available to
HTTP/2.0. However, flow control can ensure that constrained
resources are protected without any reduction in connection
utilization.
5.3. Stream priority
The endpoint establishing a new stream can assign a priority for the
stream. Priority is represented as an unsigned 31-bit integer. 0
represents the highest priority and 2^31-1 represents the lowest
priority.
The purpose of this value is to allow an endpoint to express the
relative priority of a stream. An endpoint can use this information
to preferentially allocate resources to a stream. Within HTTP/2.0,
priority can be used to select streams for transmitting frames when
there is limited capacity for sending. For instance, an endpoint
might enqueue frames for all concurrently active streams. As
transmission capacity becomes available, frames from higher priority
streams might be sent before lower priority streams.
Explicitly setting the priority for a stream does not guarantee any
particular processing or transmision order for the stream relative to
any other stream. Nor is there any mechanism provided by which the
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initiator of a stream can force or require a receiving endpoint to
process concurrent streams in a particular order.
Unless explicitly specified in the HEADERS frame (Section 6.2) during
stream creation, the default stream priority is 2^30.
Pushed streams (Section 8.2) have a lower priority than their
associated stream. The promised stream inherits the priority value
of the associated stream plus one, up to a maximum of 2^31-1.
5.4. Error Handling
HTTP/2.0 framing permits two classes of error:
o An error condition that renders the entire connection unusable is
a connection error.
o An error in an individual stream is a stream error.
A list of error codes is included in Section 7.
5.4.1. Connection Error Handling
A connection error is any error which prevents further processing of
the framing layer or which corrupts any connection state.
An endpoint that encounters a connection error SHOULD first send a
GOAWAY frame (Section 6.8) with the stream identifier of the last
stream that it successfully received from its peer. The GOAWAY frame
includes an error code that indicates why the connection is
terminating. After sending the GOAWAY frame, the endpoint MUST close
the TCP connection.
It is possible that the GOAWAY will not be reliably received by the
receiving endpoint. In the event of a connection error, GOAWAY only
provides a best-effort attempt to communicate with the peer about why
the connection is being terminated.
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 GOAWAY frame when ending a connection, as
long as circumstances permit it.
5.4.2. Stream Error Handling
A stream error is an error related to a specific stream identifier
that does not affect processing of other streams.
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An endpoint that detects a stream error sends a RST_STREAM frame
(Section 6.4) that contains the stream identifier of the stream where
the error occurred. The RST_STREAM frame includes an error code that
indicates the type of error.
A RST_STREAM is the last frame that an endpoint can send on a stream.
The peer that sends the RST_STREAM frame MUST be prepared to receive
any frames that were sent or enqueued for sending by the remote peer.
These frames can be ignored, except where they modify connection
state (such as the state maintained for header compression
(Section 4.3)).
Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame
for any stream. However, an endpoint MAY send additional RST_STREAM
frames if it receives frames on a closed stream after more than a
round trip time. This behavior is permitted to deal with misbehaving
implementations.
An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM
frame, to avoid looping.
5.4.3. Connection Termination
If the TCP connection is torn down while streams remain in open or
half closed states, then the endpoint MUST assume that the stream was
abnormally interrupted and could be incomplete.
6. Frame Definitions
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.
The transmission of specific frame types can alter the state of a
connection. If endpoints fail to maintain a synchronized view of the
connection state, successful communication within the connection will
no longer be possible. Therefore, it is important that endpoints
have a shared comprehension of how the state is affected by the use
any given frame. Accordingly, while it is expected that new frame
types will be introduced by extensions to this protocol, only frames
defined by this document are permitted to alter the connection state.
6.1. DATA
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.
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The DATA frame defines the following flags:
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 a "half closed" or
"closed" state (Section 5.1).
RESERVED (0x2): Bit 2 is reserved for future use.
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 connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
DATA frames are subject to flow control and can only be sent when a
stream is in the "open" or "half closed (remote)" states.
6.2. HEADERS
The HEADERS frame (type=0x1) carries name-value pairs. It is used to
open a stream (Section 5.1). HEADERS frames can be sent on a stream
in the "open" or "half closed (remote)" states.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Priority (31) |
+-+-------------------------------------------------------------+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
HEADERS Frame Payload
The HEADERS frame defines the following flags:
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 a "half closed" state
(Section 5.1).
RESERVED (0x2): Bit 2 is reserved for future use.
END_HEADERS (0x4): The END_HEADERS bit indicates that this frame
ends the sequence of header block fragments necessary to provide a
complete set of headers.
The payload for a complete header block is provided by a sequence
of that starts with a HEADERS frame, followed by zero or more
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CONTINUATION frames. The sequence is terminated by a frame with
the END_HEADERS flag set. Once the sequence terminates, the
payload of all HEADERS and CONTINUATION frames are concatenated
and interpreted as a single block.
A HEADERS 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 connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
PRIORITY (0x8): Bit 4 being set indicates that the first four octets
of this frame contain a single reserved bit and a 31-bit priority;
see Section 5.3. If this bit is not set, the four bytes do not
appear and the frame only contains a header block fragment.
The payload of a HEADERS frame contains a header block fragment
(Section 4.3). A header block that does not fit within a HEADERS
frame is continued in a CONTINUATION frame (Section 6.10).
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 connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
The HEADERS frame changes the connection state as described in
Section 4.3.
6.3. PRIORITY
The PRIORITY frame (type=0x2) specifies the sender-advised priority
of a stream. It can be sent at any time for an existing stream.
This enables reprioritisation of existing streams.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Priority (31) |
+-+-------------------------------------------------------------+
PRIORITY Frame Payload
The payload of a PRIORITY frame contains a single reserved bit and a
31-bit priority.
The PRIORITY frame does not define any flags.
The PRIORITY frame is associated with an existing stream. If a
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PRIORITY frame is received with a stream identifier of 0x0, the
recipient MUST respond with a connection error (Section 5.4.1) of
type PROTOCOL_ERROR.
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 header block (Section 4.3). 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.
6.4. RST_STREAM
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.
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) |
+---------------------------------------------------------------+
RST_STREAM Frame Payload
The RST_STREAM frame contains a single unsigned, 32-bit integer
identifying the error code (Section 7). The error code indicates why
the stream is being terminated.
The RST_STREAM frame does not define any flags.
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.
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 connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
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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 connection error (Section 5.4.1) of
type PROTOCOL_ERROR.
6.5. SETTINGS
The SETTINGS frame (type=0x4) conveys configuration parameters that
affect how endpoints communicate. The parameters are either
constraints on peer behavior or preferences.
SETTINGS frames MUST be sent at the start of a connection, and MAY be
sent at any other time by either endpoint over the lifetime of the
connection.
Implementations MUST support all of the settings defined by this
specification and MAY support additional settings defined by
extensions. Unsupported or unrecognized settings MUST be ignored.
New settings MUST NOT be defined or implemented in a way that
requires endpoints to understand them in order to communicate
successfully.
Each setting in a SETTINGS frame replaces the existing value for that
setting. Settings 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 settings. Therefore, the value
of a setting is the last value that is seen by a receiver. This
permits the inclusion of the same settings multiple times in the same
SETTINGS frame, though doing so does nothing other than waste
connection capacity.
The SETTINGS frame does not define any flags.
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 connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
The SETTINGS frame affects connection state. A badly formed or
incomplete SETTINGS frame MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
6.5.1. Setting Format
The payload of a SETTINGS frame consists of zero or more settings.
Each setting consists of an 8-bit reserved field, an unsigned 24-bit
setting identifier, and an unsigned 32-bit value.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (8) | Setting Identifier (24) |
+---------------+-----------------------------------------------+
| Value (32) |
+---------------------------------------------------------------+
Setting Format
6.5.2. Defined Settings
The following settings are defined:
SETTINGS_MAX_CONCURRENT_STREAMS (4): indicates the maximum number of
concurrent streams that the sender will allow. This limit is
directional: it applies to the number of streams that the sender
permits the receiver to create. By default there is no limit. It
is recommended that this value be no smaller than 100, so as to
not unnecessarily limit parallelism.
SETTINGS_INITIAL_WINDOW_SIZE (7): indicates the sender's initial
window size (in bytes) for stream level flow control.
This settings affects the window size of all streams, including
existing streams, see Section 6.9.2.
SETTINGS_FLOW_CONTROL_OPTIONS (10): indicates flow control options.
The least significant bit (0x1) of the value is set to indicate
that the sender has disabled all flow control. This bit cannot be
cleared once set, see Section 6.9.4.
All bits other than the least significant are reserved.
6.6. PUSH_PROMISE
The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint
in advance of streams the sender intends to initiate. The
PUSH_PROMISE frame includes the unsigned 31-bit identifier of the
stream the endpoint plans to create along with a minimal set of
headers that provide additional context for the stream. Section 8.2
contains a thorough description of the use of PUSH_PROMISE frames.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Promised-Stream-ID (31) |
+-+-------------------------------------------------------------+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
PUSH_PROMISE Payload Format
The payload of a PUSH_PROMISE includes a "Promised-Stream-ID". This
unsigned 31-bit integer identifies the stream the endpoint intends to
start sending frames for. The promised stream identifier MUST be a
valid choice for the next stream sent by the sender (see new stream
identifier (Section 5.1.1)).
Following the "Promised-Stream-ID" is a header block fragment
(Section 4.3).
PUSH_PROMISE frames MUST be associated with an existing, peer-
initiated stream. If the stream identifier field specifies the value
0x0, a recipient MUST respond with a connection error (Section 5.4.1)
of type PROTOCOL_ERROR.
The PUSH_PROMISE frame defines the following flags:
END_PUSH_PROMISE (0x1): The END_PUSH_PROMISE bit indicates that this
frame ends the sequence of header block fragments necessary to
provide a complete set of headers.
The payload for a complete header block is provided by a sequence
of PUSH_PROMISE frames, terminated by a PUSH_PROMISE frame with
the END_PUSH_PROMISE flag set. Once the sequence terminates, the
payload of all PUSH_PROMISE frames are concatenated and
interpreted as a single block.
A PUSH_PROMISE frame without the END_PUSH_PROMISE flag set MUST be
followed by a PUSH_PROMISE 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 connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
Promised streams are not required to be used in order promised. The
PUSH_PROMISE only reserves stream identifiers for later use.
Recipients of PUSH_PROMISE frames can choose to reject promised
streams by returning a RST_STREAM referencing the promised stream
identifier back to the sender of the PUSH_PROMISE.
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The PUSH_PROMISE frame modifies the connection state as defined in
Section 4.3.
A PUSH_PROMISE frame modifies the connection state in two ways. The
inclusion of a header block (Section 4.3) potentially modifies the
compression state. 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 new stream identifier
(Section 5.1.1) (that is, the promised stream MUST be in the "idle"
state).
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 connection error
(Section 5.4.1) of type PROTOCOL_ERROR. Similarly, a receiver MUST
treat the receipt of a PUSH_PROMISE that promises an illegal stream
identifier (Section 5.1.1) (that is, an identifier for a stream that
is not currently in the "idle" state) as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
6.7. PING
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.
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) |
| |
+---------------------------------------------------------------+
PING Payload Format
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.
Receivers of a PING frame that does not include a PONG flag MUST send
a PING frame with the PONG flag set in response, with an identical
payload. PING responses SHOULD given higher priority than any other
frame.
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The PING frame defines the following flags:
PONG (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.
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 connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
Receipt of a PING frame with a length field value other than 8 MUST
be treated as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
6.8. GOAWAY
The GOAWAY frame (type=0x7) informs the remote peer to stop creating
streams on this connection. It can be sent from the client or the
server. Once sent, the sender will ignore frames sent on new streams
for the remainder of the connection. Receivers of a GOAWAY frame
MUST NOT open additional streams on the connection, although a new
connection can be established for new streams. The purpose of this
frame is to allow an endpoint to gracefully stop accepting new
streams (perhaps for a reboot or maintenance), while still finishing
processing of previously established streams.
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).
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.
After sending a GOAWAY frame, the sender can discard frames for new
streams. However, any frames that alter connection state cannot be
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completely ignored. For instance, HEADERS, PUSH_PROMISE and
CONTINUATION frames MUST be minimally processed to ensure a
consistent compression state (see Section 4.3); similarly DATA frames
MUST be counted toward the connection flow control window.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Last-Stream-ID (31) |
+-+-------------------------------------------------------------+
| Error Code (32) |
+---------------------------------------------------------------+
| Additional Debug Data (*) |
+---------------------------------------------------------------+
GOAWAY Payload Format
The GOAWAY frame does not define any flags.
The GOAWAY frame applies to the connection, not a specific stream.
The stream identifier MUST be zero.
The last stream identifier in the GOAWAY frame contains the highest
numbered stream identifier for which the sender of the GOAWAY frame
has received frames on and might have taken some action on. All
streams up to and including the identified stream might have been
processed in some way. The last stream identifier is set to 0 if no
streams were processed.
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.
If a connection terminates without a GOAWAY frame, this value is
effectively the highest stream identifier.
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.
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.
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The last stream ID MUST be 0 if no streams were acted upon.
The GOAWAY frame also contains a 32-bit error code (Section 7) that
contains the reason for closing the connection.
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 data MUST NOT be persistently
stored, since it could contain sensitive information.
6.9. WINDOW_UPDATE
The WINDOW_UPDATE frame (type=0x9) is used to implement flow control.
Flow control operates at two levels: on each individual stream and on
the entire connection.
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.
Flow control only applies to frames that are identified as being
subject to flow control. Of the frame types defined in this
document, this includes only DATA frame. Frames that are exempt from
flow control MUST be accepted and processed, unless the receiver is
unable to assign resources to handling the frame. A receiver MAY
respond with a stream error (Section 5.4.2) or connection error
(Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable accept a
frame.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Window Size Increment (31) |
+-+-------------------------------------------------------------+
WINDOW_UPDATE Payload Format
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.
The WINDOW_UPDATE frame does not define any flags.
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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.
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 Section 5.1.
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 connection error
(Section 5.4.1). 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.
6.9.1. The Flow Control Window
Flow control in HTTP/2.0 is implemented using a window kept by each
sender on every stream. The flow control window is a simple integer
value that indicates how many bytes of data the sender is permitted
to transmit; as such, its size is a measure of the buffering
capability of the receiver.
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.
For flow control calculations, the 8 byte frame header is not
counted.
After sending a flow controlled frame, the sender reduces the space
available in both windows by the length of the transmitted frame.
The receiver of a 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.
A sender that receives a WINDOW_UPDATE frame updates the
corresponding window by the amount specified in the frame.
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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 RST_STREAM with the error code of FLOW_CONTROL_ERROR code;
for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code.
Flow controlled frames from the sender and WINDOW_UPDATE frames from
the receiver are completely asynchronous with respect to each other.
This property allows a receiver to aggressively update the window
size kept by the sender to prevent streams from stalling.
6.9.2. Initial Flow Control Window Size
When a HTTP/2.0 connection is first established, new streams are
created with an initial flow control window size of 65535 bytes. The
connection flow control window is 65535 bytes. Both endpoints can
adjust the initial window size for new streams by including a value
for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms
part of the connection header.
Prior to receiving a SETTINGS frame that sets a value for
SETTINGS_INITIAL_WINDOW_SIZE, 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.
A SETTINGS frame can alter the initial flow control window size for
all current streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE
changes, a receiver MUST adjust the size of all stream flow control
windows that it maintains by the difference between the new value and
the old value. A SETTINGS frame cannot alter the connection flow
control window.
A change to SETTINGS_INITIAL_WINDOW_SIZE could cause the available
space in a flow control window to become negative. A sender MUST
track the negative flow control window, and MUST NOT send new flow
controlled frames until it receives WINDOW_UPDATE frames that cause
the flow control window to become positive.
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 SETTINGS frame. The client retains a
negative flow control window until WINDOW_UPDATE frames restore the
window to being positive, after which the client can resume sending.
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6.9.3. Reducing the Stream Window Size
A receiver that wishes to use a smaller flow control window than the
current size can send a new SETTINGS frame. However, the receiver
MUST be prepared to receive data that exceeds this window size, since
the sender might send data that exceeds the lower limit prior to
processing the SETTINGS frame.
A receiver has two options for handling streams that exceed flow
control limits:
1. The receiver can immediately send RST_STREAM with
FLOW_CONTROL_ERROR error code for the affected streams.
2. The receiver can accept the streams and tolerate the resulting
head of line blocking, sending WINDOW_UPDATE frames as it
consumes data.
If a receiver decides to accept streams, both sides MUST recompute
the available flow control window based on the initial window size
sent in the SETTINGS.
6.9.4. Ending Flow Control
After a receiver reads in a frame that marks the end of a stream (for
example, a data stream with a END_STREAM flag set), it MUST cease
transmission of WINDOW_UPDATE frames for that stream. A sender is
not obligated to maintain the available flow control window for
streams that it is no longer sending on.
Flow control can be disabled the entire connection using the
SETTINGS_FLOW_CONTROL_OPTIONS setting. This setting ends all forms
of flow control. An implementation that does not wish to perform
flow control can use this in the initial SETTINGS exchange.
Flow control cannot be enabled again once disabled. Any attempt to
re-enable flow control - by sending a WINDOW_UPDATE or by clearing
the bits on the SETTINGS_FLOW_CONTROL_OPTIONS setting - MUST be
rejected with a FLOW_CONTROL_ERROR error code.
6.10. CONTINUATION
The CONTINUATION frame (type=0xA) is used to continue a sequence of
header block fragments (Section 4.3). Any number of CONTINUATION
frames can be sent on an existing stream, as long as the preceding
frame on the same stream is one of HEADERS, PUSH_PROMISE or
CONTINUATION.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
CONTINUATION Frame Payload
The CONTINUATION frame defines the following flags:
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 a "half closed" or
"closed" state (Section 5.1).
END_HEADERS (0x2): The END_HEADERS bit indicates that this frame
ends the sequence of header block fragments necessary to provide a
complete set of headers.
The payload for a complete header block is provided by a sequence
that starts with a HEADERS or PUSH_PROMISE frame and zero or more
CONTINUATION frames, terminated by a HEADERS, PUSH_PROMISE, or
CONTINUATION frame with the END_HEADERS flag set. Once the
sequence terminates, the payload of all frames in the sequence are
concatenated and interpreted as a single block.
A HEADERS, PUSH_PROMISE, or CONTINUATION 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 connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
The payload of a CONTINUATION frame contains a header block fragment
(Section 4.3).
The CONTINUATION frame changes the connection state as defined in
Section 4.3.
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 connection error (Section 5.4.1) of
type PROTOCOL_ERROR.
header block fragments (Section 4.3). A CONTINUATION frame MUST be
preceded by one of HEADERS, PUSH_PROMISE or CONTINUATION frame. A
recipient that observes violation of this rule MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
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7. Error Codes
Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY
frames to convey the reasons for the stream or connection error.
Error codes share a common code space. Some error codes only apply
to specific conditions and have no defined semantics in certain frame
types.
The following error codes are defined:
NO_ERROR (0): The associated condition is not as a result of an
error. For example, a GOAWAY might include this code to indicate
graceful shutdown of a connection.
PROTOCOL_ERROR (1): The endpoint detected an unspecific protocol
error. This error is for use when a more specific error code is
not available.
INTERNAL_ERROR (2): The endpoint encountered an unexpected internal
error.
FLOW_CONTROL_ERROR (3): The endpoint detected that its peer violated
the flow control protocol.
STREAM_CLOSED (5): The endpoint received a frame after a stream was
half closed.
FRAME_TOO_LARGE (6): The endpoint received a frame that was larger
than the maximum size that it supports.
REFUSED_STREAM (7): The endpoint refuses the stream prior to
performing any application processing, see Section 8.1.3 for
details.
CANCEL (8): Used by the endpoint to indicate that the stream is no
longer needed.
COMPRESSION_ERROR (9): The endpoint is unable to maintain the
compression context for the connection.
8. HTTP Message Exchanges
HTTP/2.0 is intended to be as compatible as possible with current
web-based applications. This means that, from the perspective of the
server business logic or application API, the features of HTTP are
unchanged. To achieve this, all of the application request and
response header semantics are preserved, although the syntax of
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conveying those semantics has changed. Thus, the rules from HTTP/1.1
([HTTP-p1], [HTTP-p2], [HTTP-p4], [HTTP-p5], [HTTP-p6], and
[HTTP-p7]) apply with the changes in the sections below.
8.1. HTTP Request/Response Exchange
A client sends an HTTP request on a new stream, using a previously
unused stream identifier (Section 5.1.1). A server sends an HTTP
response on the same stream as the request.
An HTTP request or response each consist of:
1. a HEADERS frame;
2. one contiguous sequence of zero or more CONTINUATION frames;
3. zero or more DATA frames; and
4. optionally, a contiguous sequence that starts with a HEADERS
frame, followed by zero or more CONTINUATION frames.
The last frame in the sequence bears an END_STREAM flag.
Other frames MAY be interspersed with these frames, but those frames
do not carry HTTP semantics. In particular, HEADERS frames (and any
CONTINUATION frames that follow) other than the first and optional
last frames in this sequence do not carry HTTP semantics.
Trailing header fields are carried in a header block that also
terminates the stream. That is, a sequence starting with a HEADERS
frame, followed by zero or more CONTINUATION frames, that carries an
END_STREAM flag on the last frame. Header blocks after the first
that do not terminate the stream are not part of an HTTP request or
response.
An HTTP request/response exchange fully consumes a single stream. A
request starts with the HEADERS 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 HEADERS frame and ends with a frame bearing END_STREAM, which
places the stream in the "closed" state.
8.1.1. Examples
For example, an HTTP GET request that includes request header fields
and no body, is transmitted as a single contiguous sequence of
HEADERS frames containing the serialized block of request header
fields. The last HEADERS frame in the sequence has both the
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END_HEADERS and END_STREAM flag set:
GET /resource HTTP/1.1 HEADERS
Host: example.org ==> + END_STREAM
Accept: image/jpeg + END_HEADERS
:method = GET
:scheme = https
:host = example.org
:path = /resource
accept = image/jpeg
Similarly, a response that includes only response header fields is
transmitted as a sequence of HEADERS frames containing the serialized
block of response header fields. The last HEADERS frame in the
sequence has both the END_HEADERS and END_STREAM flag set:
HTTP/1.1 204 No Content HEADERS
Content-Length: 0 ===> + END_STREAM
+ END_HEADERS
:status = 204
content-length: 0
An HTTP POST request that includes request header fields and payload
data is transmitted as one HEADERS frame, followed by zero or more
CONTINUATION frames, containing the request headers followed by one
or more DATA frames, with the last CONTINUATION (or HEADERS) frame
having the END_HEADERS flag set and the final DATA frame having the
END_STREAM flag set:
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} :host = example.org
:path = /resource
content-type = image/jpeg
content-length = 123
DATA
+ END_STREAM
{binary data}
A response that includes header fields and payload data is
transmitted as a HEADERS frame, followed by zero or more CONTINUATION
frames, followed by one or more DATA frames, with the last DATA frame
in the sequence having the END_STREAM flag set:
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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}
Trailing header fields are sent as a header block after both the
request or response header block and all the DATA frames have been
sent. The sequence of HEADERS/CONTINUATION frames that bears the
trailers includes a terminal frame that has both END_HEADERS and
END_STREAM flags set.
HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ===> - END_STREAM
Content-Length: 123 + END_HEADERS
TE: trailers :status = 200
123 content-type = image/jpeg
{binary data} content-length = 123
0
Foo: bar DATA
- END_STREAM
{binary data}
HEADERS
+ END_STREAM
+ END_HEADERS
foo: bar
8.1.2. HTTP Header Fields
HTTP/2.0 request and response header fields carry information as a
series of key-value pairs. This includes the target URI for the
request, the status code for the response, as well as HTTP header
fields.
HTTP header field names are strings of ASCII characters that are
compared in a case-insensitive fashion. Note that header compression
could cause case information to be lost.
The semantics of HTTP header fields are not altered by this
specification, though header fields relating to connection management
or request framing are no longer necessary. An HTTP/2.0 request MUST
NOT include any of the following header fields: Connection, Host,
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Keep-Alive, Proxy-Connection, TE, Transfer-Encoding, and Upgrade. A
server MUST treat the presence of any of these header fields as a
stream error (Section 5.4.2) of type PROTOCOL_ERROR.
8.1.2.1. Request Header Fields
HTTP/2.0 defines a number of headers starting with a ':' character
that carry information about the request target:
o The ":method" header field includes the HTTP method ([HTTP-p2],
Section 4).
o The ":scheme" header field includes the scheme portion of the
target URI ([RFC3986], Section 3.1).
o The ":host" header field includes the authority portion of the
target URI ([RFC3986], Section 3.2).
o The ":path" header field includes the path and query parts of the
target URI (the "path-absolute" production from [RFC3986] and
optionally a '?' character followed by the "query" production, see
[RFC3986], Section 3.3 and [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
for '*', in which case the ":path" header field MUST include '*'.
All HTTP/2.0 requests MUST include exactly one valid value for all of
these header fields. An intermediary MUST ensure that requests that
it forwards are correct. A server MUST treat the absence of any of
these header fields, presence of multiple values, or an invalid value
as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.
HTTP/2.0 does not define a way to carry the version identifier that
is included in the HTTP/1.1 request line.
All HTTP Requests that include a body can include a "content-length"
header field. If a server receives a request where the sum of the
DATA frame payload lengths does not equal the value of the
"content-length" header field, the server MUST return a 400 (Bad
Request) error.
8.1.2.2. Response Header Fields
A single ":status" header field is defined that carries the HTTP
status code field (see [HTTP-p2], Section 6). This header field MUST
be included in all responses. An intermediary MUST ensure that it
does not forward responses with absent or invalid values. A client
MUST treat the absence of the ":status"" header field, the presence
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of multiple values, or an invalid value as a stream error
(Section 5.4.2) of type PROTOCOL_ERROR.
HTTP/2.0 does not define a way to carry the version or reason phrase
that is included in an HTTP/1.1 status line.
8.1.3. Request Reliability Mechanisms in HTTP/2.0
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.
HTTP/2.0 provides two mechanisms for providing a guarantee to a
client that a request has not been processed:
o The GOAWAY 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.
o The REFUSED_STREAM error code can be included in a RST_STREAM
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.
In both cases, clients MAY automatically retry all requests,
including those with non-idempotent methods.
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
REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame
MUST include a stream identifier that is greater than or equal to the
given stream identifier.
In addition to these mechanisms, the PING 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 PING frame allows a client to safely test whether a connection is
still active without sending a request.
8.2. Server Push
HTTP/2.0 enables a server to pre-emptively send (or "push") multiple
associated resources to a client in response to a single request.
This feature becomes particularly helpful when the server knows the
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client will need to have those resources available in order to fully
process the originally requested resource.
Pushing additional resources is optional, and is negotiated only
between individual endpoints. For instance, an intermediary could
receive pushed resources from the server but is not required to
forward those on to the client. How to make use of the pushed
resources is up to that intermediary. Equally, the intermediary
might choose to push additional resources to the client, without any
action taken by the server.
8.2.1. Push Requests
Server push is semantically equivalent to a server responding to a
request. The PUSH_PROMISE frame, or frames, sent by the server
includes a header block that contains a complete set of request
headers that the server attributes to the request. It is not
possible to push a response to a request that includes a request
body.
Pushed resources are always associated with an explicit request from
a client. The PUSH_PROMISE frames sent by the server are sent on the
stream created for the original request. The PUSH_PROMISE frame
includes a promised stream identifier, chosen from the stream
identifiers available to the server (see Section 5.1.1).
The header fields in PUSH_PROMISE and any subsequent CONTINUATION
frames MUST be a valid and complete set of request headers
(Section 8.1.2.1). The server MUST include a method in the ":method"
header field that is safe (see [HTTP-p2], Section 4.2.1). If a
client receives a PUSH_PROMISE that does not include a complete and
valid set of header fields, or the ":method" header field identifies
a method that is not safe, it MUST respond with a stream error
(Section 5.4.2) of type PROTOCOL_ERROR.
The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to
sending any frames that reference the promised resources. This
avoids a race where clients issue requests for resources prior to
receiving any PUSH_PROMISE frames.
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 DATA frames that contain the image links ensure
that the client is able to see the promises before discovering the
resources. Similarly, if the server pushes resources referenced by
the header block (for instance, in Link header fields), sending the
push promises before sending the header block ensures that clients do
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not request those resources.
PUSH_PROMISE frames MUST NOT be sent by the client. PUSH_PROMISE
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)" to the server. PUSH_PROMISE frames are
interspersed with the frames that comprise a response, though they
cannot be interspersed with HEADERS and CONTINUATION frames that
comprise a single header block.
8.2.2. Push Responses
After sending the PUSH_PROMISE frame, the server can begin delivering
the pushed resource as a response (Section 8.1.2.2) 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 Section 8.1. This stream becomes
"half closed" to the client (Section 5.1) after the initial HEADERS
frame is sent.
Once a client receives a PUSH_PROMISE frame and chooses to accept the
pushed resource, the client SHOULD NOT issue any requests for the
promised resource until after the promised stream has closed.
If the client determines, for any reason, that it does not wish to
receive the pushed resource from the server, or if the server takes
too long to begin sending the promised resource, the client can send
an RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes,
and referencing the pushed stream's identifier.
A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit
the number of resources that can be concurrently pushed by a server.
Advertising a SETTINGS_MAX_CONCURRENT_STREAMS 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.
Clients receiving a pushed response MUST validate that the server is
authorized to push the resource using the same-origin policy
([RFC6454], Section 3). For example, a HTTP/2.0 connection to
"example.com" is generally [[anchor15: Ed: weaselly use of
"generally", needs better definition]] not permitted to push a
response for "www.example.org".
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9. Additional HTTP Requirements/Considerations
This section outlines attributes of the HTTP protocol that improve
interoperability, reduce exposure to known security vulnerabilities,
or reduce the potential for implementation variation.
9.1. Connection Management
HTTP/2.0 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.
Clients SHOULD NOT open more than one HTTP/2.0 connection to a given
origin ([RFC6454]) concurrently. A client can create additional
connections as replacements, either to replace connections that are
near to exhausting the available stream identifiers (Section 5.1.1),
or to replace connections that have encountered errors
(Section 5.4.1).
Servers are encouraged to maintain open connections for as long as
possible, but are permitted to terminate idle connections if
necessary. When either endpoint chooses to close the transport-level
TCP connection, the terminating endpoint MUST first send a GOAWAY
(Section 6.8) frame so that both endpoints can reliably determine
whether previously sent frames have been processed and gracefully
complete or terminate any necessary remaining tasks.
9.2. Use of TLS Features
Implementations of HTTP/2.0 MUST support TLS 1.1 [TLS11]. [[anchor18:
The working group intends to require at least the use of TLS 1.2
[TLS12] prior to publication of this document; negotiating TLS 1.1 is
permitted to enable the creation of interoperable implementations of
early drafts.]]
The TLS implementation MUST support the Server Name Indication (SNI)
[TLS-EXT] extension to TLS. HTTP/2.0 clients MUST indicate the
target domain name when negotiating TLS.
A server that receives a TLS handshake that does not include either
TLS 1.1 or SNI, MUST NOT negotiate HTTP/2.0. Removing HTTP/2.0
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
[TLSALPN].
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Implementations are encouraged not to negotiate TLS cipher suites
with known vulnerabilities, such as [RC4].
9.3. Frame Size Limits for HTTP
Frames used for HTTP messages MUST NOT exceed 2^14-1 (16383) octets
in length, not counting the 8 octet frame header. An endpoint MUST
treat the receipt of a larger frame as a FRAME_TOO_LARGE error (see
Section 4.2).
9.4. GZip Content-Encoding
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 or deflate encoding. A compressed
response MUST still bear an appropriate content-encoding header
field.
10. Security Considerations
10.1. Server Authority and Same-Origin
This specification uses the same-origin policy ([RFC6454], Section 3)
to determine whether an origin server is permitted to provide
content.
A server that is contacted using TLS is authenticated based on the
certificate that it offers in the TLS handshake (see [RFC2818],
Section 3). A server is considered authoritative for an "https"
resource if it has been successfully authenticated for the domain
part of the origin of the resource that it is providing.
A server is considered authoritative for an "http" resource if the
connection is established to a resolved IP address for the domain in
the origin of the resource.
A client MUST NOT use, in any way, resources provided by a server
that is not authoritative for those resources.
10.2. Cross-Protocol Attacks
When using TLS, we believe that HTTP/2.0 introduces no new cross-
protocol attacks. TLS encrypts the contents of all transmission
(except the handshake itself), making it difficult for attackers to
control the data which could be used in a cross-protocol attack.
[[anchor23: Issue: This is no longer true]]
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10.3. Cacheability of Pushed Resources
Pushed resources are responses without an explicit request; the
request for a pushed resource is synthesized from the request that
triggered the push, plus resource identification information provided
by the server. Request header fields are necessary for HTTP cache
control validations (such as the Vary header field) to work. For
this reason, caches MUST inherit request header fields from the
associated stream for the push. This includes the Cookie header
field.
Caching resources that are pushed is possible, based on the guidance
provided by the origin server in the Cache-Control header field.
However, this can cause issues if a single server hosts more than one
tenant. For example, a server might offer multiple users each a
small portion of its URI space.
Where multiple tenants share space on the same server, that server
MUST ensure that tenants are not able to push representations of
resources that they do not have authority over. Failure to enforce
this would allow a tenant to provide a representation that would be
served out of cache, overriding the actual representation that the
authoritative tenant provides.
Pushed resources for which an origin server is not authoritative are
never cached or used.
11. Privacy Considerations
HTTP/2.0 aims to keep connections open longer between clients and
servers in order to reduce the latency when a user makes a request.
The maintenance of these connections over time could be used to
expose private information. For example, a user using a browser
hours after the previous user stopped using that browser may be able
to learn about what the previous user was doing. This is a problem
with HTTP in its current form as well, however the short lived
connections make it less of a risk.
12. IANA Considerations
This document establishes registries for frame types, error codes and
settings. These new registries are entered in a new "Hypertext
Transfer Protocol (HTTP) 2.0 Parameters" section.
This document also registers the "HTTP2-Settings" header field for
use in HTTP.
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12.1. Frame Type Registry
This document establishes a registry for HTTP/2.0 frame types. The
"HTTP/2.0 Frame Type" registry operates under the "IETF Review"
policy [RFC5226].
Frame types are an 8-bit value. When reviewing new frame type
registrations, special attention is advised for any frame type-
specific flags that are defined. Frame flags can interact with
existing flags and could prevent the creation of globally applicable
flags.
Initial values for the "HTTP/2.0 Frame Type" registry are shown in
Table 1.
+--------+---------------+---------------------------+--------------+
| Frame | Name | Flags | Section |
| Type | | | |
+--------+---------------+---------------------------+--------------+
| 0 | DATA | END_STREAM(1) | Section 6.1 |
| 1 | HEADERS | END_STREAM(1), | Section 6.2 |
| | | END_HEADERS(4), | |
| | | PRIORITY(8) | |
| 2 | PRIORITY | - | Section 6.3 |
| 3 | RST_STREAM | - | Section 6.4 |
| 4 | SETTINGS | - | Section 6.5 |
| 5 | PUSH_PROMISE | END_PUSH_PROMISE(1) | Section 6.6 |
| 6 | PING | PONG(1) | Section 6.7 |
| 7 | GOAWAY | - | Section 6.8 |
| 9 | WINDOW_UPDATE | - | Section 6.9 |
| 10 | CONTINUATION | END_STREAM(1), | Section 6.10 |
| | | END_HEADERS(4) | |
+--------+---------------+---------------------------+--------------+
Table 1
12.2. Error Code Registry
This document establishes a registry for HTTP/2.0 error codes. The
"HTTP/2.0 Error Code" registry manages a 32-bit space. The "HTTP/2.0
Error Code" registry operates under the "Expert Review" policy
[RFC5226].
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.
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New registrations are advised to provide the following information:
Error Code: The 32-bit error code value.
Name: A name for the error code. Specifying an error code name is
optional.
Description: A description of the conditions where the error code is
applicable.
Specification: An optional reference for a specification that
defines the error code.
An initial set of error code registrations can be found in Section 7.
12.3. Settings Registry
This document establishes a registry for HTTP/2.0 settings. The
"HTTP/2.0 Settings" registry manages a 24-bit space. The "HTTP/2.0
Settings" registry operates under the "Expert Review" policy
[RFC5226].
Registrations for settings are required to include a description of
the setting. An expert reviewer is advised to examine new
registrations for possible duplication with existing settings. Use
of existing registrations is to be encouraged, but not mandated.
New registrations are advised to provide the following information:
Setting: The 24-bit setting value.
Name: A name for the setting. Specifying a name is optional.
Flags: Any setting-specific flags that apply, including their value
and semantics.
Description: A description of the setting. This might include the
range of values, any applicable units and how to act upon a value
when it is provided.
Specification: An optional reference for a specification that
defines the setting.
An initial set of settings registrations can be found in
Section 6.5.2.
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12.4. HTTP2-Settings Header Field Registration
This section registers the "HTTP2-Settings" header field in the
Permanent Message Header Field Registry [BCP90].
Header field name: HTTP2-Settings
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 3.2.1 of this document
Related information: This header field is only used by an HTTP/2.0
client for Upgrade-based negotiation.
13. Acknowledgements
This document includes substantial input from the following
individuals:
o 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).
o Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)
o William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro,
Jitu Padhye, Roberto Peon, Rob Trace (Flow control)
o Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner
(Substantial editorial contributions)
14. References
14.1. Normative References
[COMPRESSION] Ruellan, H. and R. Peon, "HTTP Header Compression",
draft-ietf-httpbis-header-compression-00 (work in
progress), June 2013.
[HTTP-p1] Fielding, R. and J. Reschke, "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-23 (work in progress),
July 2013.
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[HTTP-p2] Fielding, R. and J. Reschke, "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-23 (work in progress),
July 2013.
[HTTP-p4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Conditional Requests",
draft-ietf-httpbis-p4-conditional-23 (work in
progress), July 2013.
[HTTP-p5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range
Requests", draft-ietf-httpbis-p5-range-23 (work in
progress), July 2013.
[HTTP-p6] Fielding, R., Ed., Nottingham, M., Ed., and J.
Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1):
Caching", draft-ietf-httpbis-p6-cache-23 (work in
progress), July 2013.
[HTTP-p7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Authentication",
draft-ietf-httpbis-p7-auth-23 (work in progress),
July 2013.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
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December 2011.
[TLS-EXT] Eastlake, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
January 2011.
[TLS11] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.1", RFC 4346,
April 2006.
[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
[TLSALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application Layer
Protocol Negotiation Extension",
draft-ietf-tls-applayerprotoneg-01 (work in progress),
April 2013.
14.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90,
RFC 3864, September 2004.
[RC4] Rivest, R., "The RC4 encryption algorithm", RSA Data
Security, Inc. , March 1992.
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP
Extensions for High Performance", RFC 1323, May 1992.
[TALKING] Huang, L-S., Chen, E., Barth, A., Rescorla, E., and C.
Jackson, "Talking to Yourself for Fun and Profit",
2011, <http://w2spconf.com/2011/papers/websocket.pdf>.
Appendix A. Change Log (to be removed by RFC Editor before publication)
A.1. Since draft-ietf-httpbis-http2-04
Added CONTINUATION frame for HEADERS and PUSH_PROMISE.
PUSH_PROMISE is no longer implicitly prohibited if
SETTINGS_MAX_CONCURRENT_STREAMS is zero.
Push expanded to allow all safe methods without a request body.
Clarified the use of HTTP header fields in requests and responses.
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Prohibited HTTP/1.1 hop-by-hop header fields.
Requiring that intermediaries not forward requests with missing or
illegal routing :-headers.
Clarified requirements around handling different frames after stream
close, stream reset and GOAWAY.
Added more specific prohibitions for sending of different frame types
in various stream states.
Making the last received setting value the effective value.
Clarified requirements on TLS version, extension and ciphers.
A.2. Since draft-ietf-httpbis-http2-03
Committed major restructuring atrocities.
Added reference to first header compression draft.
Added more formal description of frame lifecycle.
Moved END_STREAM (renamed from FINAL) back to HEADERS/DATA.
Removed HEADERS+PRIORITY, added optional priority to HEADERS frame.
Added PRIORITY frame.
A.3. Since draft-ietf-httpbis-http2-02
Added continuations to frames carrying header blocks.
Replaced use of "session" with "connection" to avoid confusion with
other HTTP stateful concepts, like cookies.
Removed "message".
Switched to TLS ALPN from NPN.
Editorial changes.
A.4. Since draft-ietf-httpbis-http2-01
Added IANA considerations section for frame types, error codes and
settings.
Removed data frame compression.
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Added PUSH_PROMISE.
Added globally applicable flags to framing.
Removed zlib-based header compression mechanism.
Updated references.
Clarified stream identifier reuse.
Removed CREDENTIALS frame and associated mechanisms.
Added advice against naive implementation of flow control.
Added session header section.
Restructured frame header. Removed distinction between data and
control frames.
Altered flow control properties to include session-level limits.
Added note on cacheability of pushed resources and multiple tenant
servers.
Changed protocol label form based on discussions.
A.5. Since draft-ietf-httpbis-http2-00
Changed title throughout.
Removed section on Incompatibilities with SPDY draft#2.
Changed INTERNAL_ERROR on GOAWAY to have a value of 2 <https://
groups.google.com/forum/?fromgroups#!topic/spdy-dev/cfUef2gL3iU>.
Replaced abstract and introduction.
Added section on starting HTTP/2.0, including upgrade mechanism.
Removed unused references.
Added flow control principles (Section 5.2.1) based on <http://
tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>.
A.6. Since draft-mbelshe-httpbis-spdy-00
Adopted as base for draft-ietf-httpbis-http2.
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Updated authors/editors list.
Added status note.
Authors' Addresses
Mike Belshe
Twist
EMail: mbelshe@chromium.org
Roberto Peon
Google, Inc
EMail: fenix@google.com
Martin Thomson (editor)
Microsoft
3210 Porter Drive
Palo Alto 94304
US
EMail: martin.thomson@skype.net
Alexey Melnikov (editor)
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: Alexey.Melnikov@isode.com
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