draft-ietf-httpbis-http2-01.txt   draft-ietf-httpbis-http2-02.txt 
HTTPbis Working Group M. Belshe HTTPbis Working Group M. Belshe
Internet-Draft Twist Internet-Draft Twist
Expires: July 26, 2013 R. Peon Intended status: Standards Track R. Peon
Google, Inc Expires: October 5, 2013 Google, Inc
M. Thomson, Ed. M. Thomson, Ed.
Microsoft Microsoft
A. Melnikov, Ed. A. Melnikov, Ed.
Isode Ltd Isode Ltd
January 22, 2013 April 3, 2013
Hypertext Transfer Protocol version 2.0 Hypertext Transfer Protocol version 2.0
draft-ietf-httpbis-http2-01 draft-ietf-httpbis-http2-02
Abstract Abstract
This document describes an optimised expression of the semantics of This specification describes an optimised expression of the syntax of
the HTTP protocol. The HTTP/2.0 encapsulation enables more efficient the Hypertext Transfer Protocol (HTTP). The HTTP/2.0 encapsulation
transfer of resources over HTTP by providing compressed headers, enables more efficient transfer of representations by providing
simultaneous requests, and unsolicited push of resources from server compressed header fields, simultaneous requests, and also introduces
to client. unsolicited push of representations from server to client.
This document is an alternative to, but does not obsolete This document is an alternative to, but does not obsolete the HTTP
RFC{http-p1}. The HTTP protocol semantics described in RFC{http- message format. HTTP semantics remain unchanged.
p2..p7} are unmodified.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
This draft is a work-in-progress, and does not yet reflect Working
Group consensus.
This draft contains features from the SPDY Protocol as a starting
point, as per the Working Group's charter. Future drafts will add,
remove and change text, based upon the Working Group's decisions.
Discussion of this draft takes place on the HTTPBIS working group Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>. <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at Working Group information and related documents can be found at
<http://tools.ietf.org/wg/httpbis/trac/report/21> and related <http://tools.ietf.org/wg/httpbis/> (Wiki) and
documents (including fancy diffs) can be found at <https://github.com/http2/http2-spec> (source code and issues
<http://tools.ietf.org/wg/httpbis/>. tracker).
The changes in this draft are summarized in Appendix A.1. The changes in this draft are summarized in Appendix A.1.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on July 26, 2013. This Internet-Draft will expire on October 5, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Document Organization . . . . . . . . . . . . . . . . . . 5 1.1. Document Organization . . . . . . . . . . . . . . . . . . 5
1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Conventions and Terminology . . . . . . . . . . . . . . . 6
2. Starting HTTP/2.0 . . . . . . . . . . . . . . . . . . . . . . 6 2. Starting HTTP/2.0 . . . . . . . . . . . . . . . . . . . . . . 7
2.1. HTTP/2.0 Version Identification . . . . . . . . . . . . . 6 2.1. HTTP/2.0 Version Identification . . . . . . . . . . . . . 7
2.2. Starting HTTP/2.0 for "http:" URIs . . . . . . . . . . . . 7 2.2. Starting HTTP/2.0 for "http:" URIs . . . . . . . . . . . . 8
2.3. Starting HTTP/2.0 for "https:" URIs . . . . . . . . . . . 8 2.3. Starting HTTP/2.0 for "https:" URIs . . . . . . . . . . . 8
3. HTTP/2.0 Framing Layer . . . . . . . . . . . . . . . . . . . . 8 2.4. Starting HTTP/2.0 with Prior Knowledge . . . . . . . . . . 9
3.1. Session (Connections) . . . . . . . . . . . . . . . . . . 8 3. HTTP/2.0 Framing Layer . . . . . . . . . . . . . . . . . . . . 9
3.2. Framing . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Session . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Control frames . . . . . . . . . . . . . . . . . . . . 9 3.2. Session Header . . . . . . . . . . . . . . . . . . . . . . 9
3.2.2. Data frames . . . . . . . . . . . . . . . . . . . . . 10 3.3. Framing . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Streams . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.3.1. Frame Header . . . . . . . . . . . . . . . . . . . . . 10
3.3.1. Stream frames . . . . . . . . . . . . . . . . . . . . 11 3.3.2. Frame Processing . . . . . . . . . . . . . . . . . . . 11
3.3.2. Stream creation . . . . . . . . . . . . . . . . . . . 11 3.4. Streams . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3.3. Stream priority . . . . . . . . . . . . . . . . . . . 12 3.4.1. Stream Creation . . . . . . . . . . . . . . . . . . . 12
3.3.4. Stream headers . . . . . . . . . . . . . . . . . . . . 12 3.4.2. Stream priority . . . . . . . . . . . . . . . . . . . 12
3.3.5. Stream data exchange . . . . . . . . . . . . . . . . . 13 3.4.3. Stream headers . . . . . . . . . . . . . . . . . . . . 13
3.3.6. Stream half-close . . . . . . . . . . . . . . . . . . 13 3.4.4. Stream data exchange . . . . . . . . . . . . . . . . . 13
3.3.7. Stream close . . . . . . . . . . . . . . . . . . . . . 13 3.4.5. Stream half-close . . . . . . . . . . . . . . . . . . 13
3.4. Error Handling . . . . . . . . . . . . . . . . . . . . . . 14 3.4.6. Stream close . . . . . . . . . . . . . . . . . . . . . 13
3.4.1. Session Error Handling . . . . . . . . . . . . . . . . 14 3.5. Error Handling . . . . . . . . . . . . . . . . . . . . . . 14
3.4.2. Stream Error Handling . . . . . . . . . . . . . . . . 14 3.5.1. Session Error Handling . . . . . . . . . . . . . . . . 14
3.5. Stream Flow Control . . . . . . . . . . . . . . . . . . . 15 3.5.2. Stream Error Handling . . . . . . . . . . . . . . . . 15
3.5.1. Flow Control Principles . . . . . . . . . . . . . . . 15 3.5.3. Error Codes . . . . . . . . . . . . . . . . . . . . . 15
3.5.2. Basic Flow Control Algorithm . . . . . . . . . . . . . 16 3.6. Stream Flow Control . . . . . . . . . . . . . . . . . . . 16
3.6. Control frame types . . . . . . . . . . . . . . . . . . . 16 3.6.1. Flow Control Principles . . . . . . . . . . . . . . . 16
3.6.1. SYN_STREAM . . . . . . . . . . . . . . . . . . . . . . 16 3.6.2. Appropriate Use of Flow Control . . . . . . . . . . . 17
3.6.2. SYN_REPLY . . . . . . . . . . . . . . . . . . . . . . 18 3.7. Frame Types . . . . . . . . . . . . . . . . . . . . . . . 18
3.6.3. RST_STREAM . . . . . . . . . . . . . . . . . . . . . . 19 3.7.1. DATA Frames . . . . . . . . . . . . . . . . . . . . . 18
3.6.4. SETTINGS . . . . . . . . . . . . . . . . . . . . . . . 20 3.7.2. HEADERS+PRIORITY . . . . . . . . . . . . . . . . . . . 18
3.6.5. PING . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.7.3. RST_STREAM . . . . . . . . . . . . . . . . . . . . . . 18
3.6.6. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . 24 3.7.4. SETTINGS . . . . . . . . . . . . . . . . . . . . . . . 19
3.6.7. HEADERS . . . . . . . . . . . . . . . . . . . . . . . 25 3.7.5. PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . 22
3.6.8. WINDOW_UPDATE . . . . . . . . . . . . . . . . . . . . 26 3.7.6. PING . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.6.9. CREDENTIAL . . . . . . . . . . . . . . . . . . . . . . 28 3.7.7. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . 23
3.6.10. Name/Value Header Block . . . . . . . . . . . . . . . 30 3.7.8. HEADERS . . . . . . . . . . . . . . . . . . . . . . . 24
4. HTTP Layering over HTTP/2.0 . . . . . . . . . . . . . . . . . 36 3.7.9. WINDOW_UPDATE . . . . . . . . . . . . . . . . . . . . 25
4.1. Connection Management . . . . . . . . . . . . . . . . . . 36 3.7.10. Header Block . . . . . . . . . . . . . . . . . . . . . 28
4.1.1. Use of GOAWAY . . . . . . . . . . . . . . . . . . . . 36 4. HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 28
4.2. HTTP Request/Response . . . . . . . . . . . . . . . . . . 37 4.1. Connection Management . . . . . . . . . . . . . . . . . . 28
4.2.1. Request . . . . . . . . . . . . . . . . . . . . . . . 37 4.1.1. Use of GOAWAY . . . . . . . . . . . . . . . . . . . . 29
4.2.2. Response . . . . . . . . . . . . . . . . . . . . . . . 39 4.2. HTTP Request/Response . . . . . . . . . . . . . . . . . . 29
4.2.3. Authentication . . . . . . . . . . . . . . . . . . . . 39 4.2.1. HTTP Header Fields and HTTP/2.0 Headers . . . . . . . 29
4.3. Server Push Transactions . . . . . . . . . . . . . . . . . 40 4.2.2. Request . . . . . . . . . . . . . . . . . . . . . . . 29
4.3.1. Server implementation . . . . . . . . . . . . . . . . 41 4.2.3. Response . . . . . . . . . . . . . . . . . . . . . . . 31
4.3.2. Client implementation . . . . . . . . . . . . . . . . 42 4.3. Server Push Transactions . . . . . . . . . . . . . . . . . 32
5. Design Rationale and Notes . . . . . . . . . . . . . . . . . . 43 4.3.1. Server implementation . . . . . . . . . . . . . . . . 33
5.1. Separation of Framing Layer and Application Layer . . . . 43 4.3.2. Client implementation . . . . . . . . . . . . . . . . 34
5.2. Error handling - Framing Layer . . . . . . . . . . . . . . 43 5. Design Rationale and Notes . . . . . . . . . . . . . . . . . . 35
5.3. One Connection Per Domain . . . . . . . . . . . . . . . . 44 5.1. Separation of Framing Layer and Application Layer . . . . 35
5.4. Fixed vs Variable Length Fields . . . . . . . . . . . . . 44 5.2. Error handling - Framing Layer . . . . . . . . . . . . . . 35
5.5. Compression Context(s) . . . . . . . . . . . . . . . . . . 45 5.3. One Connection Per Domain . . . . . . . . . . . . . . . . 36
5.6. Unidirectional streams . . . . . . . . . . . . . . . . . . 45 5.4. Fixed vs Variable Length Fields . . . . . . . . . . . . . 36
5.7. Data Compression . . . . . . . . . . . . . . . . . . . . . 45 5.5. Server Push . . . . . . . . . . . . . . . . . . . . . . . 36
5.8. Server Push . . . . . . . . . . . . . . . . . . . . . . . 46 6. Security Considerations . . . . . . . . . . . . . . . . . . . 37
6. Security Considerations . . . . . . . . . . . . . . . . . . . 46 6.1. Use of Same-origin constraints . . . . . . . . . . . . . . 37
6.1. Use of Same-origin constraints . . . . . . . . . . . . . . 46 6.2. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 37
6.2. HTTP Headers and HTTP/2.0 Headers . . . . . . . . . . . . 46 6.3. Cacheability of Pushed Resources . . . . . . . . . . . . . 37
6.3. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 46 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 37
6.4. Server Push Implicit Headers . . . . . . . . . . . . . . . 46 7.1. Long Lived Connections . . . . . . . . . . . . . . . . . . 38
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 47 7.2. SETTINGS frame . . . . . . . . . . . . . . . . . . . . . . 38
7.1. Long Lived Connections . . . . . . . . . . . . . . . . . . 47 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
7.2. SETTINGS frame . . . . . . . . . . . . . . . . . . . . . . 47 8.1. Frame Type Registry . . . . . . . . . . . . . . . . . . . 38
8.2. Error Code Registry . . . . . . . . . . . . . . . . . . . 39
8. Requirements Notation . . . . . . . . . . . . . . . . . . . . 47 8.3. Settings Registry . . . . . . . . . . . . . . . . . . . . 39
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
10. Normative References . . . . . . . . . . . . . . . . . . . . . 48 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
10.1. Normative References . . . . . . . . . . . . . . . . . . . 41
10.2. Informative References . . . . . . . . . . . . . . . . . . 42
Appendix A. Change Log (to be removed by RFC Editor before Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 49 publication) . . . . . . . . . . . . . . . . . . . . 42
A.1. Since draft-ietf-httpbis-http2-00 . . . . . . . . . . . . 49
A.2. Since draft-mbelshe-httpbis-spdy-00 . . . . . . . . . . . 49 A.1. Since draft-ietf-httpbis-http2-01 . . . . . . . . . . . . 42
A.2. Since draft-ietf-httpbis-http2-00 . . . . . . . . . . . . 43
A.3. Since draft-mbelshe-httpbis-spdy-00 . . . . . . . . . . . 43
1. Introduction 1. Introduction
HTTP is a wildly successful protocol. HTTP/1.1 message encapsulation The Hypertext Transfer Protocol (HTTP) is a wildly successful
[HTTP-p1] is optimized for implementation simplicity and protocol. The HTTP/1.1 message encapsulation ([HTTP-p1], Section 3)
accessibility, not application performance. As such it has several is optimized for implementation simplicity and accessibility, not
characteristics that have a negative overall effect on application application performance. As such it has several characteristics that
performance. have a negative overall effect on application performance.
The HTTP/1.1 encapsulation ensures that only one request can be The HTTP/1.1 encapsulation ensures that only one request can be
delivered at a time on a given connection. HTTP/1.1 pipelining, delivered at a time on a given connection. HTTP/1.1 pipelining,
which is not widely deployed, only partially addresses these which is not widely deployed, only partially addresses these
concerns. Clients that need to make multiple requests therefore use concerns. Clients that need to make multiple requests therefore use
commonly multiple connections to a server or servers in order to commonly multiple connections to a server or servers in order to
reduce the overall latency of those requests. reduce the overall latency of those requests. [[anchor1: Need to tune
the anti-pipelining comments here.]]
Furthermore, HTTP/1.1 headers are represented in an inefficient Furthermore, HTTP/1.1 header fields are represented in an inefficient
fashion, which, in addition to generating more or larger network fashion, which, in addition to generating more or larger network
packets, can cause the small initial TCP window to fill more quickly packets, can cause the small initial TCP window to fill more quickly
than is ideal. This results in excessive latency where multiple than is ideal. This results in excessive latency where multiple
requests are made on a new TCP connection. requests are made on a new TCP connection.
This document defines an optimized mapping of the HTTP semantics to a This document defines an optimized mapping of the HTTP semantics to a
TCP connection. This optimization reduces the latency costs of HTTP TCP connection. This optimization reduces the latency costs of HTTP
by allowing parallel requests on the same connection and by using an by allowing parallel requests on the same connection and by using an
efficient coding for HTTP headers. Prioritization of requests lets efficient coding for HTTP header fields. Prioritization of requests
more important requests complete faster, further improving lets more important requests complete faster, further improving
application performance. application performance.
HTTP/2.0 applications have an improved impact on network congestion HTTP/2.0 applications have an improved impact on network congestion
due to the use of fewer TCP connections to achieve the same effect. due to the use of fewer TCP connections to achieve the same effect.
Fewer TCP connections compete more fairly with other flows. Long- Fewer TCP connections compete more fairly with other flows. Long-
lived connections are also more able to take better advantage of the lived connections are also more able to take better advantage of the
available network capacity, rather than operating in the slow start available network capacity, rather than operating in the slow start
phase of TCP. phase of TCP.
The HTTP/2.0 encapsulation also enables more efficient processing of The HTTP/2.0 encapsulation also enables more efficient processing of
messages by providing efficient message framing. Processing of messages by providing efficient message framing. Processing of
headers in HTTP/2.0 messages is more efficient (for entities that header fields in HTTP/2.0 messages is more efficient (for entities
process many messages). that process many messages).
1.1. Document Organization 1.1. Document Organization
The HTTP/2.0 Specification is split into three parts: starting The HTTP/2.0 Specification is split into three parts: starting
HTTP/2.0 (Section 2), which covers how a HTTP/2.0 is started; a HTTP/2.0 (Section 2), which covers how a HTTP/2.0 is started; a
framing layer (Section 3), which multiplexes a TCP connection into framing layer (Section 3), which multiplexes a TCP connection into
independent, length-prefixed frames; and an HTTP layer (Section 4), independent, length-prefixed frames; and an HTTP layer (Section 4),
which specifies the mechanism for overlaying HTTP request/response which specifies the mechanism for overlaying HTTP request/response
pairs on top of the framing layer. While some of the framing layer pairs on top of the framing layer. While some of the framing layer
concepts are isolated from the HTTP layer, building a generic framing concepts are isolated from the HTTP layer, building a generic framing
layer has not been a goal. The framing layer is tailored to the layer has not been a goal. The framing layer is tailored to the
needs of the HTTP protocol and server push. needs of the HTTP protocol and server push.
1.2. Definitions 1.2. Conventions and Terminology
client: The endpoint initiating the HTTP/2.0 session. 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].
connection: A transport-level connection between two endpoints. 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.
endpoint: Either the client or server of a connection. The following terms are used:
frame: A header-prefixed sequence of bytes sent over a HTTP/2.0 client: The endpoint initiating the HTTP/2.0 session.
session.
server: The endpoint which did not initiate the HTTP/2.0 session. connection: A transport-level connection between two endpoints.
session: A synonym for a connection. endpoint: Either the client or server of a connection.
session error: An error on the HTTP/2.0 session. frame: The smallest unit of communication, each containing a frame
header.
stream: A bi-directional flow of bytes across a virtual channel message: A complete sequence of frames.
receiver: An endpoint that is receiving frames.
sender: An endpoint that is transmitting frames.
server: The endpoint which did not initiate the HTTP/2.0 session.
session: A synonym for a connection.
session error: An error on the HTTP/2.0 session.
stream: A bi-directional flow of bytes across a virtual channel
within a HTTP/2.0 session. within a HTTP/2.0 session.
stream error: An error on an individual HTTP/2.0 stream. stream error: An error on an individual HTTP/2.0 stream.
2. Starting HTTP/2.0 2. Starting HTTP/2.0
Just as HTTP/1.1 does, HTTP/2.0 uses the "http:" and "https:" URI Just as HTTP/1.1 does, HTTP/2.0 uses the "http:" and "https:" URI
schemes. An HTTP/2.0-capable client is therefore required to schemes. An HTTP/2.0-capable client is therefore required to
discover whether a server (or intermediary) supports HTTP/2.0. discover whether a server (or intermediary) supports HTTP/2.0.
Different discovery mechanisms are defined for "http:" and "https:" Different discovery mechanisms are defined for "http:" and "https:"
URIs. Discovery for "http:" URIs is described in Section 2.2; URIs. Discovery for "http:" URIs is described in Section 2.2;
discovery for "https:" URIs is described in Section 2.3. discovery for "https:" URIs is described in Section 2.3.
2.1. HTTP/2.0 Version Identification 2.1. HTTP/2.0 Version Identification
HTTP/2.0 is identified in using the string "HTTP/2.0". This HTTP/2.0 is identified using the string "HTTP/2.0". This
identification is used in the HTTP/1.1 Upgrade header, in the TLS-NPN identification is used in the HTTP/1.1 Upgrade header field, in the
[TLSNPN] [[TBD]] field and other places where protocol identification TLS-NPN [TLSNPN] [[anchor4: TBD]] field and other places where
is required. protocol identification is required.
[[Editor's Note: please remove the following text prior to the Negotiating "HTTP/2.0" implies the use of the transport, security,
publication of a final version of this document.]] framing and message semantics described in this document.
[[anchor5: Editor's Note: please remove the following text prior to
the publication of a final version of this document.]]
Only implementations of the final, published RFC can identify Only implementations of the final, published RFC can identify
themselves as "HTTP/2.0". Until such an RFC exists, implementations themselves as "HTTP/2.0". Until such an RFC exists, implementations
MUST NOT identify themselves using "HTTP/2.0". MUST NOT identify themselves using "HTTP/2.0".
Examples and text throughout the rest of this document use "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 as a matter of editorial convenience only. Implementations of draft
versions MUST NOT identify using this string. versions MUST NOT identify using this string.
Implementations of draft versions of the protocol MUST add the Implementations of draft versions of the protocol MUST add the string
corresponding draft number to the identifier before the separator "-draft-" and the corresponding draft number to the identifier before
('/'). For example, draft-ietf-httpbis-http2-03 is identified using the separator ('/'). For example, draft-ietf-httpbis-http2-03 is
the string "HTTP-03/2.0". identified using the string "HTTP-draft-03/2.0".
Non-compatible experiments that are based on these draft versions Non-compatible experiments that are based on these draft versions
MUST include a further identifier. For example, an experimental MUST instead replace the string "draft" with a different identifier.
implementation of packet mood-based encoding based on For example, an experimental implementation of packet mood-based
draft-ietf-httpbis-http2-07 might identify itself as "HTTP-07- encoding based on draft-ietf-httpbis-http2-07 might identify itself
emo/2.0". Note that any label MUST conform with the "token" syntax as "HTTP-emo-07/2.0". Note that any label MUST conform with the
defined in Section 3.2.4 of [HTTP-p1]. Experimenters are encouraged "token" syntax defined in Section 3.2.6 of [HTTP-p1]. Experimenters
to coordinate their experiments on the ietf-http-wg@w3.org mailing are encouraged to coordinate their experiments on the
list. ietf-http-wg@w3.org mailing list.
2.2. Starting HTTP/2.0 for "http:" URIs 2.2. Starting HTTP/2.0 for "http:" URIs
A client that makes a request to an "http:" URI without prior A client that makes a request to an "http:" URI without prior
knowledge about support for HTTP/2.0 uses the HTTP Upgrade mechanism knowledge about support for HTTP/2.0 uses the HTTP Upgrade mechanism
[HTTP-p2]. The client makes an HTTP/1.1 request that includes an (Section 6.7 of [HTTP-p1]). The client makes an HTTP/1.1 request
Upgrade header field identifying HTTP/2.0. that includes an Upgrade header field identifying HTTP/2.0.
For example: For example:
GET /default.htm HTTP/1.1 GET /default.htm HTTP/1.1
Host: server.example.com Host: server.example.com
Connection: Upgrade Connection: Upgrade
Upgrade: HTTP/2.0 Upgrade: HTTP/2.0
A server that does not support HTTP/2.0 can respond to the request as A server that does not support HTTP/2.0 can respond to the request as
though the Upgrade header field were absent: though the Upgrade header field were absent:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-length: 243 Content-length: 243
Content-type: text/html Content-type: text/html
... ...
A server that supports HTTP/2.0 can accept the upgrade with a 101 A server that supports HTTP/2.0 can accept the upgrade with a 101
(Switching Protocols) status code. After the empty line that (Switching Protocols) status code. After the empty line that
terminates the 101 response, the server can begin sending HTTP/2.0 terminates the 101 response, the server can begin sending HTTP/2.0
frames. These frames MUST include a response to the request that frames. These frames MUST include a response to the request that
initiated the Upgrade. initiated the Upgrade.
HTTP/1.1 101 Switching Protocols HTTP/1.1 101 Switching Protocols
Connection: Upgrade Connection: Upgrade
Upgrade: HTTP/2.0 Upgrade: HTTP/2.0
[ HTTP/2.0 frames ... [ HTTP/2.0 session ...
A client can learn that a particular server supports HTTP/2.0 by Once the server returns the 101 response, both the client and the
other means. A client MAY immediately send HTTP/2.0 frames to a server send a session header (Section 3.2).
server that is known to support HTTP/2.0. [[Open Issue: This is not
definite. We may yet choose to perform negotiation for every
connection. Reasons include intermediaries; phased upgrade of load-
balanced server farms; etc...]] [[Open Issue: We need to enumerate
the ways that clients can learn of HTTP/2.0 support.]]
2.3. Starting HTTP/2.0 for "https:" URIs 2.3. Starting HTTP/2.0 for "https:" URIs
[[TBD, maybe NPN]] A client that makes a request to an "https:" URI without prior
knowledge about support for HTTP/2.0 uses TLS [RFC5246] with TLS-NPN
[TLSNPN] extension. [[anchor6: TBD, maybe ALPN]]
Once TLS negotiation is complete, both the client and the server send
a session header (Section 3.2).
2.4. 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
server that is known to support HTTP/2.0. This only affects the
resolution of "http:" URIs, servers supporting HTTP/2.0 are required
to support protocol negotiation in TLS [TLSNPN].
Prior support for HTTP/2.0 is not a strong signal that a given server
will support HTTP/2.0 for future sessions. It is possible for server
configurations to change or for configurations to differ between
instances in clustered server. Different "transparent"
intermediaries - intermediaries that are not explicitly selected by
either client or server - are another source of variability.
3. HTTP/2.0 Framing Layer 3. HTTP/2.0 Framing Layer
3.1. Session (Connections) 3.1. Session
The HTTP/2.0 framing layer (or "session") runs atop a reliable The HTTP/2.0 session runs atop TCP ([RFC0793]). The client is the
transport layer such as TCP [RFC0793]. The client is the TCP TCP connection initiator.
connection initiator. HTTP/2.0 connections are persistent
connections.
For best performance, it is expected that clients will not close open HTTP/2.0 connections are persistent connections. For best
performance, it is expected that clients will not close open
connections until the user navigates away from all web pages connections until the user navigates away from all web pages
referencing a connection, or until the server closes the connection. referencing a connection, or until the server closes the connection.
Servers are encouraged to leave connections open for as long as Servers are encouraged to leave connections open for as long as
possible, but can terminate idle connections if necessary. When possible, but can terminate idle connections if necessary. When
either endpoint closes the transport-level connection, it MUST first either endpoint closes the transport-level connection, it MUST first
send a GOAWAY (Section 3.6.6) frame so that the endpoints can send a GOAWAY (Section 3.7.7) frame so that the endpoints can
reliably determine if requests finished before the close. reliably determine if requests finished before the close.
3.2. Framing 3.2. Session Header
Once the connection is established, clients and servers exchange
framed messages. There are two types of frames: control frames
(Section 3.2.1) and data frames (Section 3.2.2). Frames always have
a common header which is 8 bytes in length.
The first bit is a control bit indicating whether a frame is a After opening a TCP connection and performing either an HTTP/1.1
control frame or data frame. Control frames carry a version number, Upgrade or TLS handshake, the client sends the client session header.
a frame type, flags, and a length. Data frames contain the stream The server replies with a server session header.
ID, flags, and the length for the payload carried after the common
header. The simple header is designed to make reading and writing of
frames easy.
All integer values, including length, version, and type, are in The session header provides a final confirmation that both peers
network byte order. HTTP/2.0 does not enforce alignment of types in agree to use the HTTP/2.0 protocol. The SETTINGS frame ensures that
dynamically sized frames. client or server configuration is known as quickly as possible.
3.2.1. Control frames The client session header is the 25 byte sequence
0x464f4f202a20485454502f322e300d0a0d0a4241520d0a0d0a (the string "FOO
* HTTP/2.0\r\n\r\nBAR\r\n\r\n") followed by a SETTINGS frame
(Section 3.7.4). The client sends the client session header
immediately after receiving an HTTP/1.1 Upgrade, or after receiving a
TLS Finished message from the server.
+----------------------------------+ The client session header is selected so that a large proportion
|C| Version(15bits) | Type(16bits) | of HTTP/1.1 or HTTP/1.0 servers and intermediaries do not attempt
+----------------------------------+ to process further frames. This doesn't address the concerns
| Flags (8) | Length (24 bits) | raised in [TALKING].
+----------------------------------+
| Data |
+----------------------------------+
Control bit: The 'C' bit is a single bit indicating if this is a The server session header is a SETTINGS frame (Section 3.7.4). The
control message. For control frames this value is always 1. server sends the server session header immediately after receiving
and validating the client session header.
Version: The version number of the HTTP/2.0 protocol. This document The client sends requests immediately after sending the session
describes HTTP/2.0 version 3. header, without waiting to receive a server session header. This
ensures that confirming session headers does not add latency.
Type: The type of control frame. See Control Frames for the complete Both client and server MUST close the connection if it does not begin
list of control frames. with a valid session header. A GOAWAY frame (Section 3.7.7) MAY be
omitted if it is clear that the peer is not using HTTP/2.0.
Flags: Flags related to this frame. Flags for control frames and 3.3. Framing
data frames are different.
Length: An unsigned 24-bit value representing the number of bytes Once the connection is established, clients and servers exchange
after the length field. HTTP/2.0 frames. Frames are the basic unit of communication.
Data: data associated with this control frame. The format and length 3.3.1. Frame Header
of this data is controlled by the control frame type.
Control frame processing requirements: HTTP/2.0 frames share a common header format. Frames have an 8 byte
header with between 0 and 65535 bytes of data.
Note that full length control frames (16MB) can be large for 0 1 2 3
implementations running on resource-limited hardware. In such 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
cases, implementations MAY limit the maximum length frame +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
supported. However, all implementations MUST be able to receive | Length (16) | Type (8) | Flags (8) |
control frames of at least 8192 octets in length. +-+-------------+---------------+-------------------------------+
|R| Stream Identifier (31) |
+-+-------------------------------------------------------------+
| Frame Data (0...) ...
+---------------------------------------------------------------+
3.2.2. Data frames Frame Header
+----------------------------------+ The fields of the frame header are defined as:
|C| Stream-ID (31bits) |
+----------------------------------+
| Flags (8) | Length (24 bits) |
+----------------------------------+
| Data |
+----------------------------------+
Control bit: For data frames this value is always 0. Length: The 16-bit length of the frame payload in bytes. The length
of the frame header is not included in this sum.
Stream-ID: A 31-bit value identifying the stream. 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 that use types that they do not
support.
Flags: Flags related to this frame. Valid flags are: Flags: An 8-bit field reserved for flags. Bits that have undefined
semantics are reserved. The following flags are defined for all
frame types:
0x01 = FLAG_FIN - signifies that this frame represents the last FINAL (0x1): Bit 1 (the least significant bit) indicates that
frame to be transmitted on this stream. See Stream Close this is the last frame in a stream. This places the stream
(Section 3.3.7) below. into a half-closed state (Section 3.4.5). No further frames
follow in the direction of the carrying frame.
0x02 = FLAG_COMPRESS - indicates that the data in this frame has Frame types can define semantics for frame-specific flags.
been compressed.
Length: An unsigned 24-bit value representing the number of bytes R: A reserved 1-bit field. The semantics of this bit are not
after the length field. The total size of a data frame is 8 bytes + defined.
length. It is valid to have a zero-length data frame.
Data: The variable-length data payload; the length was defined in the Stream Identifier: A 31-bit stream identifier (see Section 3.4.1).
length field. A value 0 is reserved for frames that are directed at the session
as a whole instead of a single stream.
Data frame processing requirements: Frame Data: Frames contain between 0 and 65535 bytes of data.
If an endpoint receives a data frame for a stream-id which is not Reserved bits in the frame header MUST be set to zero when sending
open and the endpoint has not sent a GOAWAY (Section 3.6.6) frame, and MUST be ignored when receiving frames, unless the semantics of
it MUST send issue a stream error (Section 3.4.2) with the error the bit are known.
code INVALID_STREAM for the stream-id.
If the endpoint which created the stream receives a data frame 3.3.2. Frame Processing
before receiving a SYN_REPLY on that stream, it is a protocol
error, and the recipient MUST issue a stream error (Section 3.4.2)
with the status code PROTOCOL_ERROR for the stream-id.
Implementors note: If an endpoint receives multiple data frames A frame of the maximum size might be too large for implementations
for invalid stream-ids, it MAY close the session. with limited resources to process. Implementations MAY choose to
support frames smaller than the maximum possible size. However,
implementations MUST be able to receive frames containing at least
8192 octets of payload.
All HTTP/2.0 endpoints MUST accept compressed data frames. An implementation MUST immediately close a stream if it is unable to
Compression of data frames is always done using zlib compression. process a frame related to that stream due to it exceeding a size
Each stream initializes and uses its own compression context limit. The implementation MUST send a RST_STREAM frame
dedicated to use within that stream. Endpoints are encouraged to (Section 3.7.3) containing FRAME_TOO_LARGE error code if the frame
use application level compression rather than HTTP/2.0 stream size limit is exceeded.
level compression.
Each HTTP/2.0 stream sending compressed frames creates its own [[anchor9: <https://github.com/http2/http2-spec/issues/28>: Need a
zlib context for that stream, and these compression contexts MUST way to signal the maximum frame size; no way to RST_STREAM on non-
be distinct from the compression contexts used with SYN_STREAM/ stream-related frames.]]
SYN_REPLY/HEADER compression. (Thus, if both endpoints of a
stream are compressing data on the stream, there will be two zlib
contexts, one for sending and one for receiving).
3.3. Streams 3.4. Streams
Streams are independent sequences of bi-directional data divided into Streams are independent sequences of bi-directional data divided into
frames with several properties: frames with several properties:
Streams may be created by either the client or server. o Streams can be created by either the client or server.
Streams optionally carry a set of name/value header pairs.
Streams can concurrently send data interleaved with other streams.
Streams may be cancelled.
3.3.1. Stream frames
HTTP/2.0 defines 3 control frames to manage the lifecycle of a
stream:
SYN_STREAM - Open a new stream
SYN_REPLY - Remote acknowledgement of a new, open stream
RST_STREAM - Close a stream
3.3.2. Stream creation o Streams optionally carry a set of name-value header pairs.
A stream is created by sending a control frame with the type set to o Streams can concurrently send data interleaved with other streams.
SYN_STREAM (Section 3.6.1). If the server is initiating the stream,
the Stream-ID must be even. If the client is initiating the stream,
the Stream-ID must be odd. 0 is not a valid Stream-ID. Stream-IDs
from each side of the connection must increase monotonically as new
streams are created. E.g. Stream 2 may be created after stream 3,
but stream 7 must not be created after stream 9. Stream IDs do not
wrap: when a client or server cannot create a new stream id without
exceeding a 31 bit value, it MUST NOT create a new stream.
The stream-id MUST increase with each new stream. If an endpoint o Streams can be established and used unilaterally.
receives a SYN_STREAM with a stream id which is less than any
previously received SYN_STREAM, it MUST issue a session error
(Section 3.4.1) with the status PROTOCOL_ERROR.
It is a protocol error to send two SYN_STREAMs with the same o Streams can be cancelled.
stream-id. If a recipient receives a second SYN_STREAM for the same
stream, it MUST issue a stream error (Section 3.4.2) with the status
code PROTOCOL_ERROR.
Upon receipt of a SYN_STREAM, the recipient can reject the stream by 3.4.1. Stream Creation
sending a stream error (Section 3.4.2) with the error code
REFUSED_STREAM. Note, however, that the creating endpoint may have
already sent additional frames for that stream which cannot be
immediately stopped.
Once the stream is created, the creator may immediately send HEADERS Use of streams does not require negotiation. A stream is not
or DATA frames for that stream, without needing to wait for the created, streams are used by sending a frame on the stream.
recipient to acknowledge.
3.3.2.1. Unidirectional streams Streams are identified by a 31-bit numeric identifier. Streams
initiated by a client use odd numbered stream identifiers. Streams
initiated by the server use odd numbered stream identifiers. A
stream identifier of zero MUST NOT be used to create a new stream.
When an endpoint creates a stream with the FLAG_UNIDIRECTIONAL flag The stream identifier of a new stream MUST be greater than all other
set, it creates a unidirectional stream which the creating endpoint streams from that endpoint, unless the stream identifier was
can use to send frames, but the receiving endpoint cannot. The previously reserved (such as the promised stream identifier in a
receiving endpoint is implicitly already in the half-closed PUSH_PROMISE (Section 3.7.5) frame). An endpoint that receives an
(Section 3.3.6) state. unexpected stream identifier MUST treat this as a session error
(Section 3.5.1) of type PROTOCOL_ERROR.
3.3.2.2. Bidirectional streams A long-lived session can result in available stream identifiers being
exhausted. An endpoint that is unable to create a new stream
identifier can establish a new session for any new streams.
SYN_STREAM frames which do not use the FLAG_UNIDIRECTIONAL flag are An endpoint cannot prevent the creation of a new stream, but it can
bidirectional streams. Both endpoints can send data on a bi- request the early termination of an unwanted stream. Upon receipt of
directional stream. a frame, the recipient can terminate the corresponding stream by
sending a stream error (Section 3.5.2) of type REFUSED_STREAM. This
cannot prevent the initiating endpoint from sending frames for that
stream prior to receiving this request.
3.3.3. Stream priority 3.4.2. Stream priority
The creator of a stream assigns a priority for that stream. Priority The creator of a stream assigns a priority for that stream. Priority
is represented as an integer from 0 to 7. 0 represents the highest is represented as a 31 bit integer. 0 represents the highest priority
priority and 7 represents the lowest priority. and 2^31-1 represents the lowest priority.
The sender and recipient SHOULD use best-effort to process streams in The sender and recipient SHOULD use best-effort to process streams in
the order of highest priority to lowest priority. the order of highest priority to lowest priority. [[anchor11: ED:
toothless, useless "SHOULD": reword]]
3.3.4. Stream headers 3.4.3. Stream headers
Streams carry optional sets of name/value pair headers which carry Streams carry optional sets of header fields which carry metadata
metadata about the stream. After the stream has been created, and as about the stream. After the stream has been created, and as long as
long as the sender is not closed (Section 3.3.7) or half-closed the sender is not closed (Section 3.4.6) or half-closed
(Section 3.3.6), each side may send HEADERS frame(s) containing the (Section 3.4.5), each side may send HEADERS frame(s) containing the
header data. Header data can be sent in multiple HEADERS frames, and header data. Header data can be sent in multiple HEADERS frames, and
HEADERS frames may be interleaved with data frames. HEADERS frames may be interleaved with data frames.
3.3.5. Stream data exchange 3.4.4. Stream data exchange
Once a stream is created, it can be used to send arbitrary amounts of Once a stream is created, it can be used to send arbitrary amounts of
data. Generally this means that a series of data frames will be sent data. Generally this means that a series of data frames will be sent
on the stream until a frame containing the FLAG_FIN flag is set. The on the stream until a frame containing the FINAL flag (Section 3.3.1)
FLAG_FIN can be set on a SYN_STREAM (Section 3.6.1), SYN_REPLY is set. Once the FINAL flag has been set on any frame, the stream is
(Section 3.6.2), HEADERS (Section 3.6.7) or a DATA (Section 3.2.2) considered to be half-closed.
frame. Once the FLAG_FIN has been sent, the stream is considered to
be half-closed.
3.3.6. Stream half-close 3.4.5. Stream half-close
When one side of the stream sends a frame with the FLAG_FIN flag set, When one side of the stream sends a frame with the FINAL flag set,
the stream is half-closed from that endpoint. The sender of the the stream is half-closed from that endpoint. The sender of the
FLAG_FIN MUST NOT send further frames on that stream. When both FINAL flag MUST NOT send further frames on that stream. When both
sides have half-closed, the stream is closed. sides have half-closed, the stream is closed.
If an endpoint receives a data frame after the stream is half-closed An endpoint MUST treat the receipt of a data frame on a half-closed
from the sender (e.g. the endpoint has already received a prior frame stream as a stream error (Section 3.5.2) of type STREAM_CLOSED.
for the stream with the FIN flag set), it MUST send a RST_STREAM to
the sender with the status STREAM_ALREADY_CLOSED.
3.3.7. Stream close Streams that have never received packets can be considered to be
half-closed in the direction that is silent. This allows either peer
to create a unidirectional stream, which does not require an explicit
close from the peer that does not transmit frames.
There are 3 ways that streams can be terminated: 3.4.6. Stream close
Normal termination: Normal stream termination occurs when both Streams can be terminated in the following ways:
Normal termination: Normal stream termination occurs when both
sender and recipient have half-closed the stream by sending a sender and recipient have half-closed the stream by sending a
FLAG_FIN. frame containing a FINAL flag (Section 3.3.1).
Abrupt termination: Either the client or server can send a Half-close on unidirectional stream: A stream that only has frames
RST_STREAM control frame at any time. A RST_STREAM contains an sent in one direction can be tentatively considered to be closed
error code to indicate the reason for failure. When a RST_STREAM once a frame containing a FINAL flag is sent. The active sender
is sent from the stream originator, it indicates a failure to on the stream MUST be prepared to receive frames after closing the
complete the stream and that no further data will be sent on the stream.
stream. When a RST_STREAM is sent from the stream recipient, the
sender, upon receipt, should stop sending any data on the stream.
The stream recipient should be aware that there is a race between
data already in transit from the sender and the time the
RST_STREAM is received. See Stream Error Handling (Section 3.4.2)
TCP connection teardown: If the TCP connection is torn down while Abrupt termination: Either the peer can send a RST_STREAM control
frame at any time to terminate an active stream. RST_STREAM
contains an error code to indicate the reason for termination. A
RST_STREAM indicates that the sender will transmit no further data
on the stream and that the receiver is requested to cease
transmission.
The sender of a RST_STREAM frame MUST allow for frames that have
already been sent by the peer prior to the RST_STREAM being
processed. If in-transit frames alter session state, these frames
cannot be safely discarded. See Stream Error Handling
(Section 3.5.2) for more details.
TCP connection teardown: If the TCP connection is torn down while
un-closed streams exist, then the endpoint must assume that the un-closed streams exist, then the endpoint must assume that the
stream was abnormally interrupted and may be incomplete. stream was abnormally interrupted and may be incomplete.
If an endpoint receives a data frame after the stream is closed, it If an endpoint receives a data frame after the stream is closed, it
must send a RST_STREAM to the sender with the status PROTOCOL_ERROR. MAY send a RST_STREAM to the sender with the status PROTOCOL_ERROR.
3.4. Error Handling 3.5. Error Handling
The HTTP/2.0 framing layer has only two types of errors, and they are HTTP/2.0 framing permits two classes of error:
always handled consistently. Any reference in this specification to
"issue a session error" refers to Section 3.4.1. Any reference to
"issue a stream error" refers to Section 3.4.2.
3.4.1. Session Error Handling o An error condition that renders the entire session unusable is a
session error.
o An error in an individual stream is a stream error.
3.5.1. Session Error Handling
A session error is any error which prevents further processing of the A session error is any error which prevents further processing of the
framing layer or which corrupts the session compression state. When framing layer or which corrupts any session state.
a session error occurs, the endpoint encountering the error MUST
first send a GOAWAY (Section 3.6.6) frame with the stream id of most
recently received stream from the remote endpoint, and the error code
for why the session is terminating. After sending the GOAWAY frame,
the endpoint MUST close the TCP connection.
Note that the session compression state is dependent upon both An endpoint that encounters a session error MUST first send a GOAWAY
endpoints always processing all compressed data. If an endpoint (Section 3.7.7) frame with the stream identifier of the last stream
partially processes a frame containing compressed data without that it successfully received from its peer. The GOAWAY frame
updating compression state properly, future control frames which use includes an error code that indicates why the session is terminating.
compression will be always be errored. Implementations SHOULD always After sending the GOAWAY frame, the endpoint MUST close the TCP
try to process compressed data so that errors which could be handled connection.
as stream errors do not become session errors.
Note that because this GOAWAY is sent during a session error case, it It is possible that the GOAWAY will not be reliably received by the
is possible that the GOAWAY will not be reliably received by the receiving endpoint. In the event of a session error, GOAWAY only
receiving endpoint. It is a best-effort attempt to communicate with provides a best-effort attempt to communicate with the peer about why
the remote about why the session is going down. the session is going down.
3.4.2. Stream Error Handling An endpoint can end a session at any time. In particular, an
endpoint MAY choose to treat a stream error as a session error if the
error is recurrent. Endpoints SHOULD send a GOAWAY frame when ending
a session, as long as circumstances permit it.
A stream error is an error related to a specific stream-id which does 3.5.2. Stream Error Handling
not affect processing of other streams at the framing layer. Upon a
stream error, the endpoint MUST send a RST_STREAM (Section 3.6.3)
frame which contains the stream id of the stream where the error
occurred and the error status which caused the error. After sending
the RST_STREAM, the stream is closed to the sending endpoint. After
sending the RST_STREAM, if the sender receives any frames other than
a RST_STREAM for that stream id, it will result in sending additional
RST_STREAM frames. An endpoint MUST NOT send a RST_STREAM in
response to an RST_STREAM, as doing so would lead to RST_STREAM
loops. Sending a RST_STREAM does not cause the HTTP/2.0 session to
be closed.
If an endpoint has multiple RST_STREAM frames to send in succession A stream error is an error related to a specific stream identifier
for the same stream-id and the same error code, it MAY coalesce them that does not affect processing of other streams at the framing
into a single RST_STREAM frame. (This can happen if a stream is layer.
closed, but the remote sends multiple data frames. There is no
reason to send a RST_STREAM for each frame in succession).
3.5. Stream Flow Control An endpoint that detects a stream error sends a RST_STREAM
(Section 3.7.3) frame that contains the stream identifier of the
stream where the error occurred. The RST_STREAM frame includes an
error code that indicates the type of error.
A RST_STREAM is the last frame that an endpoint can send on a stream.
The peer that sends the RST_STREAM frame MUST be prepared to receive
any frames that were sent or enqueued for sending by the remote peer.
These frames can be ignored, except where they modify session state
(such as the header compression state).
An endpoint SHOULD NOT send more than one RST_STREAM frame for any
stream. An endpoint MAY send additional RST_STREAM frames if it
receives frames on a closed stream after more than a round trip time.
This behaviour is permitted to deal with misbehaving implementations
where treating this as a session error is inappropriate.
An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM
frame. This could trigger infinite loops of RST_STREAM frames.
3.5.3. 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 session error.
Error codes share a common code space. Some error codes only apply
to specific conditions and have no defined semantics in certain frame
types.
The following error codes are defined:
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 session.
PROTOCOL_ERROR (1): An unspecific protocol error was detected. This
error is for use when a more specific error code is not available.
INTERNAL_ERROR (2): The implementation encountered an unexpected
internal error.
FLOW_CONTROL_ERROR (3): The endpoint detected that its peer violated
the flow control protocol.
INVALID_STREAM (4): A frame was received for an inactive stream.
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): Indicates that the stream was refused before any
processing has been done on the stream.
CANCEL (8): Used by the creator of a stream to indicate that the
stream is no longer needed.
3.6. Stream Flow Control
Multiplexing streams introduces contention for access to the shared Multiplexing streams introduces contention for access to the shared
TCP connection. Stream contention can result in streams being TCP connection. Stream contention can result in streams being
blocked by other streams. A flow control scheme ensures that streams blocked by other streams. A flow control scheme ensures that streams
do not destructively interfere with other streams on the same TCP do not destructively interfere with other streams on the same TCP
connection. connection.
3.5.1. Flow Control Principles 3.6.1. Flow Control Principles
Experience with TCP congestion control has shown that algorithms can Experience with TCP congestion control has shown that algorithms can
evolve over time to become more sophisticated without requiring evolve over time to become more sophisticated without requiring
protocol changes. TCP congestion control and its evolution is protocol changes. TCP congestion control and its evolution is
clearly different from HTTP/2.0 flow control, though the evolution of clearly different from HTTP/2.0 flow control, though the evolution of
TCP congestion control algorithms shows that a similar approach could TCP congestion control algorithms shows that a similar approach could
be feasible for HTTP/2.0 flow control. be feasible for HTTP/2.0 flow control.
HTTP/2.0 stream flow control aims to allow for future improvements to HTTP/2.0 stream flow control aims to allow for future improvements to
flow control algorithms without requiring protocol changes. The flow control algorithms without requiring protocol changes. Flow
following principles guide the HTTP/2.0 design: control in HTTP/2.0 has the following characteristics:
1. Flow control is hop-by-hop, not end-to-end. 1. Flow control is hop-by-hop, not end-to-end.
2. Flow control is based on window update messages. Receivers 2. Flow control is based on window update messages. Receivers
advertise how many octets they are prepared to receive on a advertise how many octets they are prepared to receive on a
stream. This is a credit-based scheme. stream. This is a credit-based scheme.
3. Flow control is directional with overall control provided by the 3. Flow control is directional with overall control provided by the
receiver. A receiver MAY choose to set any window size that it receiver. A receiver MAY choose to set any window size that it
desires for each stream [[TBD: ... and for the overall desires for each stream and for the entire connection. A sender
connection]]. A sender MUST respect flow control limits imposed MUST respect flow control limits imposed by a receiver. Clients,
by a receiver. Clients, servers and intermediaries all servers and intermediaries all independently advertise their flow
independently advertise their flow control preferences as a control preferences as a receiver and abide by the flow control
receiver and abide by the flow control limits set by their peer limits set by their peer when sending.
when sending.
4. Flow control can be disabled by a receiver. A receiver can 4. The initial value for the flow control window is 65536 bytes for
choose to either disable flow control, or to declare an infinite both new streams and the overall connection.
flow control limit. [[TBD: determine whether just one mechanism
is sufficient, and then which alternative]]
5. HTTP/2.0 standardizes only the format of the window update 5. The frame type determines whether flow control applies to a
message (Section 3.6.8). This does not stipulate how a receiver 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 either disable flow control for a stream or connection
by declaring an infinite flow control limit.
7. HTTP/2.0 standardizes only the format of the window update
message (Section 3.7.9). This does not stipulate how a receiver
decides when to send this message or the value that it sends. decides when to send this message or the value that it sends.
Nor does it specify how a sender chooses to send packets. Nor does it specify how a sender chooses to send packets.
Implementations are able to select any algorithm that suits their Implementations are able to select any algorithm that suits their
needs. An example flow control algorithm is described in needs.
Section 3.5.2.
Implementations are also responsible for managing how requests and Implementations are also responsible for managing how requests and
responses are sent based on priority; choosing how to avoid head of responses are sent based on priority; choosing how to avoid head of
line blocking for requests; and managing the creation of new streams. line blocking for requests; and managing the creation of new streams.
Algorithm choices for these could interact with any flow control Algorithm choices for these could interact with any flow control
algorithm. algorithm.
3.5.2. Basic Flow Control Algorithm 3.6.2. Appropriate Use of Flow Control
This section describes a basic flow control algorithm. This
algorithm is provided as an example, implementations can use any
algorithm that complies with flow control requirements.
[[Algorithm TBD]]
3.6. Control frame types
3.6.1. SYN_STREAM
The SYN_STREAM control frame allows the sender to asynchronously
create a stream between the endpoints. See Stream Creation
(Section 3.3.2)
+------------------------------------+
|1| version | 1 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
|X| Associated-To-Stream-ID (31bits) |
+------------------------------------+
| Pri|Unused | Slot | |
+-------------------+ |
| Number of Name/Value pairs (int32) | <+
+------------------------------------+ |
| Length of name (int32) | | This section is the
+------------------------------------+ | "Name/Value Header
| Name (string) | | Block", and is
+------------------------------------+ | compressed.
| Length of value (int32) | |
+------------------------------------+ |
| Value (string) | |
+------------------------------------+ |
| (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
(Section 3.3.6) state.
0x02 = FLAG_UNIDIRECTIONAL - a stream created with this flag puts
the recipient in the half-closed (Section 3.3.6) state.
Length: The length is the number of bytes which follow the length
field in the frame. For SYN_STREAM frames, this is 10 bytes plus the
length of the compressed Name/Value block.
Stream-ID: The 31-bit identifier for this stream. This stream-id
will be used in frames which are part of this stream.
Associated-To-Stream-ID: The 31-bit identifier for a stream which
this stream is associated to. If this stream is independent of all
other streams, it should be 0.
Priority: A 3-bit priority (Section 3.3.3) field.
Unused: 5 bits of unused space, reserved for future use.
Slot: An 8 bit unsigned integer specifying the index in the server's
CREDENTIAL vector of the client certificate to be used for this
request. see CREDENTIAL frame (Section 3.6.9). The value 0 means no
client certificate should be associated with this stream.
Name/Value Header Block: A set of name/value pairs carried as part of
the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).
If an endpoint receives a SYN_STREAM which is larger than the
implementation supports, it MAY send a RST_STREAM with error code
FRAME_TOO_LARGE. All implementations MUST support the minimum size
limits defined in the Control Frames section (Section 3.2.1).
3.6.2. SYN_REPLY
SYN_REPLY indicates the acceptance of a stream creation by the
recipient of a SYN_STREAM frame.
+------------------------------------+
|1| version | 2 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
| Number of Name/Value pairs (int32) | <+
+------------------------------------+ |
| Length of name (int32) | | This section is the
+------------------------------------+ | "Name/Value Header
| Name (string) | | Block", and is
+------------------------------------+ | compressed.
| Length of value (int32) | |
+------------------------------------+ |
| Value (string) | |
+------------------------------------+ |
| (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
(Section 3.3.6) state.
Length: The length is the number of bytes which follow the length Flow control is defined to protect deployments (client, server or
field in the frame. For SYN_REPLY frames, this is 4 bytes plus the intermediary) that are operating under constraints. For example, a
length of the compressed Name/Value block. proxy must share memory between many connections. Flow control
addresses cases where the receiver is unable process data on one
stream, yet wants to be continue to process other streams.
Stream-ID: The 31-bit identifier for this stream. Deployments that do not rely on this capability SHOULD disable flow
control for data that is being received. Note that flow control
cannot be disabled for sending. Sending data is always subject to
the flow control window advertised by the receiver.
If an endpoint receives multiple SYN_REPLY frames for the same active Deployments with constrained resources (for example, memory), MAY
stream ID, it MUST issue a stream error (Section 3.4.2) with the employ flow control to limit the amount of memory a peer can consume.
error code STREAM_IN_USE. This can lead to suboptimal use of available network resources if
flow control is enabled without knowledge of the bandwidth-delay
product (see [RFC1323]).
Name/Value Header Block: A set of name/value pairs carried as part of Implementation of flow control in full awareness of the current
the SYN_STREAM. see Name/Value Header Block (Section 3.6.10). bandwidth-delay product is difficult, but it can ensure that
constrained resources are protected without any reduction in
connection utilization.
If an endpoint receives a SYN_REPLY which is larger than the 3.7. Frame Types
implementation supports, it MAY send a RST_STREAM with error code
FRAME_TOO_LARGE. All implementations MUST support the minimum size
limits defined in the Control Frames section (Section 3.2.1).
3.6.3. RST_STREAM 3.7.1. DATA Frames
The RST_STREAM frame allows for abnormal termination of a stream. DATA frames (type=0) are used to convey HTTP message bodies. The
When sent by the creator of a stream, it indicates the creator wishes payload of a data frame contains either a request or response body.
to cancel the stream. When sent by the recipient of a stream, it
indicates an error or that the recipient did not want to accept the
stream, so the stream should be closed.
+----------------------------------+ No frame-specific flags are defined for DATA frames.
|1| version | 3 |
+----------------------------------+
| Flags (8) | 8 |
+----------------------------------+
|X| Stream-ID (31bits) |
+----------------------------------+
| Status code |
+----------------------------------+
Flags: Flags related to this frame. RST_STREAM does not define any 3.7.2. HEADERS+PRIORITY
flags. This value must be 0.
Length: An unsigned 24-bit value representing the number of bytes The HEADERS+PRIORITY frame (type=1) allows the sender to set header
after the length field. For RST_STREAM control frames, this value is fields and stream priority at the same time. This MUST be used for
always 8. each stream that is created.
Stream-ID: The 31-bit identifier for this stream. 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 (*) ...
+---------------------------------------------------------------+
Status code: (32 bits) An indicator for why the stream is being HEADERS+PRIORITY Frame Payload
terminated.The following status codes are defined:
1 - PROTOCOL_ERROR. This is a generic error, and should only be The HEADERS+PRIORITY frame is identical to the HEADERS frame
used if a more specific error is not available. (Section 3.7.8), with a 32-bit field containing priority included
before the header block.
2 - INVALID_STREAM. This is returned when a frame is received for The most significant bit of the priority is reserved. The 31-bit
a stream which is not active. priority indicates the priority for the stream, as assigned by the
sender, see Section 3.4.2.
3 - REFUSED_STREAM. Indicates that the stream was refused before 3.7.3. RST_STREAM
any processing has been done on the stream.
4 - UNSUPPORTED_VERSION. Indicates that the recipient of a stream The RST_STREAM frame (type=3) allows for abnormal termination of a
does not support the HTTP/2.0 version requested. stream. When sent by the creator of a stream, it indicates the
creator wishes to cancel the stream. When sent by the recipient of a
stream, it indicates an error or that the recipient did not want to
accept the stream, so the stream should be closed.
5 - CANCEL. Used by the creator of a stream to indicate that the 0 1 2 3
stream is no longer needed. 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) |
+---------------------------------------------------------------+
6 - INTERNAL_ERROR. This is a generic error which can be used RST_STREAM Frame Payload
when the implementation has internally failed, not due to anything
in the protocol.
7 - FLOW_CONTROL_ERROR. The endpoint detected that its peer The RST_STREAM frame does not define any valid flags.
violated the flow control protocol.
8 - STREAM_IN_USE. The endpoint received a SYN_REPLY for a stream The RST_STREAM frame contains a single 32-bit error code
already open. (Section 3.5.3). The error code indicates why the stream is being
terminated.
9 - STREAM_ALREADY_CLOSED. The endpoint received a data or After receiving a RST_STREAM on a stream, the recipient must not send
SYN_REPLY frame for a stream which is half closed. additional frames for that stream, and the stream moves into the
closed state.
10 - INVALID_CREDENTIALS. The server received a request for a 3.7.4. SETTINGS
resource whose origin does not have valid credentials in the
client certificate vector.
11 - FRAME_TOO_LARGE. The endpoint received a frame which this A SETTINGS frame (type=4) contains a set of id/value pairs for
implementation could not support. If FRAME_TOO_LARGE is sent for communicating configuration data about how the two endpoints may
a SYN_STREAM, HEADERS, or SYN_REPLY frame without fully processing communicate. SETTINGS frames MUST be sent at the start of a session,
the compressed portion of those frames, then the compression state but they can be sent at any other time by either endpoint. Settings
will be out-of-sync with the other endpoint. In this case, are declarative, not negotiated, each peer indicates their own
senders of FRAME_TOO_LARGE MUST close the session. configuration.
Note: 0 is not a valid status code for a RST_STREAM. [[anchor17: Note that persistence of settings is under discussion in
the WG and might be removed in a future version of this document.]]
After receiving a RST_STREAM on a stream, the recipient must not send When the server is the sender, the sender can request that
additional frames for that stream, and the stream moves into the configuration data be persisted by the client across HTTP/2.0
closed state. sessions and returned to the server in future communications.
3.6.4. SETTINGS Clients persist settings on a per origin basis (see [RFC6454] for a
definition of web origins). That is, when a client connects to a
server, and the server persists settings within the client, the
client SHOULD return the persisted settings on future connections to
the same origin AND IP address and TCP port. Clients MUST NOT
request servers to use the persistence features of the SETTINGS
frames, and servers MUST ignore persistence related flags sent by a
client.
A SETTINGS frame contains a set of id/value pairs for communicating Valid frame-specific flags for the SETTINGS frame are:
configuration data about how the two endpoints may communicate.
SETTINGS frames can be sent at any time by either endpoint, are
optionally sent, and are fully asynchronous. When the server is the
sender, the sender can request that configuration data be persisted
by the client across HTTP/2.0 sessions and returned to the server in
future communications.
Persistence of SETTINGS ID/Value pairs is done on a per origin/IP CLEAR_PERSISTED (0x2): Bit 2 being set indicates a request to clear
pair (the "origin" is the set of scheme, host, and port from the URI. any previously persisted settings before processing the settings.
See [RFC6454]). That is, when a client connects to a server, and the Clients MUST NOT set this flag.
server persists settings within the client, the client SHOULD return
the persisted settings on future connections to the same origin AND
IP address and TCP port. Clients MUST NOT request servers to use the
persistence features of the SETTINGS frames, and servers MUST ignore
persistence related flags sent by a client.
+----------------------------------+ SETTINGS frames always apply to a session, never a single stream.
|1| version | 4 | The stream identifier for a settings frame MUST be zero.
+----------------------------------+
| Flags (8) | Length (24 bits) |
+----------------------------------+
| Number of entries |
+----------------------------------+
| ID/Value Pairs |
| ... |
Control bit: The control bit is always 1 for this message. 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|SettingFlags(8)| Setting Identifier (24) |
+---------------+-----------------------------------------------+
| Value (32) |
+---------------------------------------------------------------+
Version: The HTTP/2.0 version number. SETTINGS ID/Value Pair
Type: The message type for a SETTINGS message is 4. The payload of a SETTINGS frame contains zero or more settings. Each
setting is comprised of the following
Flags: FLAG_SETTINGS_CLEAR_SETTINGS (0x1): When set, the client Settings Flags: An 8-bit flags field containing per-setting
should clear any previously persisted SETTINGS ID/Value pairs. If instructions. The following flags are valid:
this frame contains ID/Value pairs with the
FLAG_SETTINGS_PERSIST_VALUE set, then the client will first clear its
existing, persisted settings, and then persist the values with the
flag set which are contained within this frame. Because persistence
is only implemented on the client, this flag can only be used when
the sender is the server.
Length: An unsigned 24-bit value representing the number of bytes PERSIST_VALUE (0x1): Bit 1 (the least significant bit) being set
after the length field. The total size of a SETTINGS frame is 8 indicates a request from the server to the client to persist
bytes + length. this setting. A client MUST NOT set this flag.
Number of entries: A 32-bit value representing the number of ID/value PERSISTED (0x2): Bit 2 being set indicates that this setting is a
pairs in this message. persisted setting being returned by the client to the server.
This also indicates that this setting is not a client setting,
but a value previously set by the server. A server MUST NOT
set this flag.
ID: A 32-bit ID number, comprised of 8 bits of flags and 24 bits of All other settings flags are reserved.
unique ID.
ID.flags: Setting Identifier: A 24-bit field that identifies the setting.
FLAG_SETTINGS_PERSIST_VALUE (0x1): When set, the sender of this Value: A 32-bit value for the setting.
SETTINGS frame is requesting that the recipient persist the ID/
Value and return it in future SETTINGS frames sent from the
sender to this recipient. Because persistence is only
implemented on the client, this flag is only sent by the
server.
FLAG_SETTINGS_PERSISTED (0x2): When set, the sender is The following settings are defined:
notifying the recipient that this ID/Value pair was previously
sent to the sender by the recipient with the
FLAG_SETTINGS_PERSIST_VALUE, and the sender is returning it.
Because persistence is only implemented on the client, this
flag is only sent by the client.
Defined IDs: SETTINGS_UPLOAD_BANDWIDTH (1): allows the sender to send its
expected upload bandwidth on this channel. This number is an
estimate. The value should be the integral number of kilobytes
per second that the sender predicts as an expected maximum upload
channel capacity.
1 - SETTINGS_UPLOAD_BANDWIDTH allows the sender to send its SETTINGS_DOWNLOAD_BANDWIDTH (2): allows the sender to send its
expected upload bandwidth on this channel. This number is an expected download bandwidth on this channel. This number is an
estimate. The value should be the integral number of kilobytes estimate. The value should be the integral number of kilobytes
per second that the sender predicts as an expected maximum per second that the sender predicts as an expected maximum
upload channel capacity. download channel capacity.
2 - SETTINGS_DOWNLOAD_BANDWIDTH allows the sender to send its SETTINGS_ROUND_TRIP_TIME (3): allows the sender to send its expected
expected download bandwidth on this channel. This number is an round-trip-time on this channel. The round trip time is defined
estimate. The value should be the integral number of kilobytes as the minimum amount of time to send a control frame from this
per second that the sender predicts as an expected maximum client to the remote and receive a response. The value is
download channel capacity. represented in milliseconds.
3 - SETTINGS_ROUND_TRIP_TIME allows the sender to send its SETTINGS_MAX_CONCURRENT_STREAMS (4): allows the sender to inform the
expected round-trip-time on this channel. The round trip time remote endpoint the maximum number of concurrent streams which it
is defined as the minimum amount of time to send a control will allow. This limit is directional: it applies to the number
frame from this client to the remote and receive a response. of streams that the sender permits the receiver to create. By
The value is represented in milliseconds. default there is no limit. For implementers it is recommended
that this value be no smaller than 100, so as to not unnecessarily
limit parallelism.
4 - SETTINGS_MAX_CONCURRENT_STREAMS allows the sender to inform SETTINGS_CURRENT_CWND (5): allows the sender to inform the remote
the remote endpoint the maximum number of concurrent streams endpoint of the current TCP CWND value.
which it will allow. By default there is no limit. For
implementors it is recommended that this value be no smaller
than 100.
5 - SETTINGS_CURRENT_CWND allows the sender to inform the SETTINGS_DOWNLOAD_RETRANS_RATE (6): allows the sender to inform the
remote endpoint of the current TCP CWND value. remote endpoint the retransmission rate (bytes retransmitted /
total bytes transmitted).
6 - SETTINGS_DOWNLOAD_RETRANS_RATE allows the sender to inform SETTINGS_INITIAL_WINDOW_SIZE (7): allows the sender to inform the
the remote endpoint the retransmission rate (bytes remote endpoint the initial window size (in bytes) for new
retransmitted / total bytes transmitted). streams.
7 - SETTINGS_INITIAL_WINDOW_SIZE allows the sender to inform SETTINGS_FLOW_CONTROL_OPTIONS (10): This setting allows an endpoint
the remote endpoint the initial window size (in bytes) for new to indicate that streams directed to them will not be subject to
streams. flow control. The least significant bit (0x1) is set to indicate
that new streams are not flow controlled. Bit 2 (0x2) is set to
indicate that the session is not flow controlled. All other bits
are reserved.
8 - SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE allows the server This setting applies to all streams, including existing streams.
to inform the client of the new size of the client certificate
vector.
Value: A 32-bit value. These bits cannot be cleared once set, see Section 3.7.9.4.
The message is intentionally extensible for future information which The message is intentionally extensible for future information which
may improve client-server communications. The sender does not need may improve client-server communications. The sender does not need
to send every type of ID/value. It must only send those for which it to send every type of ID/value. It must only send those for which it
has accurate values to convey. When multiple ID/value pairs are has accurate values to convey. When multiple ID/value pairs are
sent, they should be sent in order of lowest id to highest id. A sent, they should be sent in order of lowest id to highest id. A
single SETTINGS frame MUST not contain multiple values for the same single SETTINGS frame MUST not contain multiple values for the same
ID. If the recipient of a SETTINGS frame discovers multiple values ID. If the recipient of a SETTINGS frame discovers multiple values
for the same ID, it MUST ignore all values except the first one. for the same ID, it MUST ignore all values except the first one.
A server may send multiple SETTINGS frames containing different ID/ A server may send multiple SETTINGS frames containing different ID/
Value pairs. When the same ID/Value is sent twice, the most recent Value pairs. When the same ID/Value is sent twice, the most recent
value overrides any previously sent values. If the server sends IDs value overrides any previously sent values. If the server sends IDs
1, 2, and 3 with the FLAG_SETTINGS_PERSIST_VALUE in a first SETTINGS 1, 2, and 3 with the FLAG_SETTINGS_PERSIST_VALUE in a first SETTINGS
frame, and then sends IDs 4 and 5 with the frame, and then sends IDs 4 and 5 with the
FLAG_SETTINGS_PERSIST_VALUE, when the client returns the persisted FLAG_SETTINGS_PERSIST_VALUE, when the client returns the persisted
state on its next SETTINGS frame, it SHOULD send all 5 settings (1, state on its next SETTINGS frame, it SHOULD send all 5 settings (1,
2, 3, 4, and 5 in this example) to the server. 2, 3, 4, and 5 in this example) to the server.
3.6.5. PING 3.7.5. PUSH_PROMISE
The PING control frame is a mechanism for measuring a minimal round- The PUSH_PROMISE frame (type=5) allows the sender to signal a promise
trip time from the sender. It can be sent from the client or the to create a stream and serve the referenced resource. Minimal data
server. Recipients of a PING frame should send an identical frame to allowing the receiver to understand which resource(s) are to be
the sender as soon as possible (if there is other pending data pushed are to be included.
waiting to be sent, PING should take highest priority). Each ping
sent by a sender should use a unique ID.
+----------------------------------+ PUSH_PROMISE frames are sent on an existing stream. They declare the
|1| version | 6 | intent to use another stream for the pushing of a resource. The
+----------------------------------+ PUSH_PROMISE allows the client an opportunity to reject pushed
| 0 (flags) | 4 (length) | resources.
+----------------------------------|
| 32-bit ID |
+----------------------------------+
Control bit: The control bit is always 1 for this message. 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 (*) ...
+---------------------------------------------------------------+
Version: The HTTP/2.0 version number. PUSH_PROMISE Payload Format
Type: The message type for a PING message is 6. There are no frame-specific flags for the PUSH_PROMISE frame.
Length: This frame is always 4 bytes long. The body of a PUSH_PROMISE includes a "Promised-Stream-ID". This 31-
bit identifier indicates the stream on which the resource will be
pushed. The promised stream identifier MUST be a valid choice for
the next stream sent by the sender (see new stream identifier
(Section 3.4.1)).
ID: A unique ID for this ping, represented as an unsigned 32 bit There is no requirement that the streams referred to by this frame
value. When the client initiates a ping, it must use an odd numbered are created in the order referenced. The PUSH_PROMISE reserves
ID. When the server initiates a ping, it must use an even numbered stream identifiers for later use; these reserved identifiers can be
ping. Use of odd/even IDs is required in order to avoid accidental used as prioritization needs dictate.
looping on PINGs (where each side initiates an identical PING at the
same time).
Note: If a sender uses all possible PING ids (e.g. has sent all 2^31 The PUSH_PROMISE also includes a header block (Section 3.7.10), which
possible IDs), it can wrap and start re-using IDs. describes the resource that will be pushed.
If a server receives an even numbered PING which it did not initiate, 3.7.6. PING
it must ignore the PING. If a client receives an odd numbered PING
which it did not initiate, it must ignore the PING.
3.6.6. GOAWAY The PING frame (type=6) is a mechanism for measuring a minimal round-
trip time from the sender. PING frames can be sent from the client
or the server.
The GOAWAY control frame is a mechanism to tell the remote side of Recipients of a PING frame send an identical frame to the sender as
the connection to stop creating streams on this session. It can be soon as possible. PING should take highest priority if there is
sent from the client or the server. Once sent, the sender will not other data waiting to be sent.
respond to any new SYN_STREAMs on this session. Recipients of a
GOAWAY frame must not send additional streams on this session, The PING frame defines a frame-specific flag:
PONG (0x2): Bit 2 being set indicates that this ping frame is a ping
response. An endpoint MUST set this flag in ping responses. An
endpoint MUST NOT respond to ping frames containing this flag.
The payload of a PING frame contains any value. A PING response MUST
contain the contents of the PING request.
3.7.7. GOAWAY
The GOAWAY frame (type=7) informs the remote side of the connection
to stop creating streams on this session. It can be sent from the
client or the server. Once sent, the sender will ignore frames sent
on new streams for the remainder of the session. Recipients of a
GOAWAY frame MUST NOT open additional streams on the session,
although a new session can be established for new streams. The although a new session can be established for new streams. The
purpose of this message is to allow an endpoint to gracefully stop purpose of this message is to allow an endpoint to gracefully stop
accepting new streams (perhaps for a reboot or maintenance), while accepting new streams (perhaps for a reboot or maintenance), while
still finishing processing of previously established streams. still finishing processing of previously established streams.
There is an inherent race condition between an endpoint sending There is an inherent race condition between an endpoint starting new
SYN_STREAMs and the remote sending a GOAWAY message. To deal with streams and the remote sending a GOAWAY message. To deal with this
this case, the GOAWAY contains a last-stream-id indicating the case, the GOAWAY contains the stream identifier of the last stream
stream-id of the last stream which was created on the sending which was processed on the sending endpoint in this session. If the
endpoint in this session. If the receiver of the GOAWAY sent new receiver of the GOAWAY used streams that are newer than the indicated
SYN_STREAMs for sessions after this last-stream-id, they were not stream identifier, they were not processed by the sender and the
processed by the server and the receiver may treat the stream as receiver may treat the streams as though they had never been created
though it had never been created at all (hence the receiver may want at all (hence the receiver may want to re-create the streams later on
to re-create the stream later on a new session). a new session).
Endpoints should always send a GOAWAY message before closing a Endpoints should always send a GOAWAY message before closing a
connection so that the remote can know whether a stream has been connection so that the remote can know whether a stream has been
partially processed or not. (For example, if an HTTP client sends a 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 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 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 server does not send a GOAWAY frame to indicate where it stopped
working). working).
After sending a GOAWAY message, the sender must ignore all SYN_STREAM After sending a GOAWAY message, the sender can ignore frames for new
frames for new streams. streams.
+----------------------------------+
|1| version | 7 |
+----------------------------------+
| 0 (flags) | 8 (length) |
+----------------------------------|
|X| Last-good-stream-ID (31 bits) |
+----------------------------------+
| Status code |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The HTTP/2.0 version number.
Type: The message type for a GOAWAY message is 7. [[anchor18: Issue: session state that is established by those
"ignored" messages cannot be ignored without the state in the two
peers becoming unsynchronized.]]
Length: This frame is always 8 bytes long. 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) |
+---------------------------------------------------------------+
Last-good-stream-Id: The last stream id which was replied to (with GOAWAY Payload Format
either a SYN_REPLY or RST_STREAM) by the sender of the GOAWAY
message. If no streams were replied to, this value MUST be 0.
Status: The reason for closing the session. The GOAWAY frame does not define any valid flags.
0 - OK. This is a normal session teardown. The GOAWAY frame applies to the session, not a specific stream. The
stream identifier MUST be zero.
1 - PROTOCOL_ERROR. This is a generic error, and should only be The GOAWAY frame contains an identifier of the last stream that the
used if a more specific error is not available. sender of the GOAWAY is prepared to act upon, which can include
processing and replies. This allows an endpoint to discover what
streams might have had some effect or what might be safe to
automatically retry. If no streams were acted upon, the last stream
ID MUST be 0.
2 - INTERNAL_ERROR. This is a generic error which can be used The GOAWAY frame contains a 32-bit error code (Section 3.5.3) that
when the implementation has internally failed, not due to anything contains the reason for closing the session.
in the protocol.
3.6.7. HEADERS 3.7.8. HEADERS
The HEADERS frame augments a stream with additional headers. It may The HEADERS frame (type=8) provides header fields for a stream. It
be optionally sent on an existing stream at any time. Specific may be optionally sent on an existing stream at any time. Specific
application of the headers in this frame is application-dependent. application of the headers in this frame is application-dependent.
The name/value header block within this frame is compressed.
+------------------------------------+ No frame-specific flags are defined for the HEADERS frame.
|1| version | 8 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
| Number of Name/Value pairs (int32) | <+
+------------------------------------+ |
| Length of name (int32) | | This section is the
+------------------------------------+ | "Name/Value Header
| Name (string) | | Block", and is
+------------------------------------+ | compressed.
| Length of value (int32) | |
+------------------------------------+ |
| Value (string) | |
+------------------------------------+ |
| (repeats) | <+
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - marks this frame as the last frame to be
transmitted on this stream and puts the sender in the half-closed
(Section 3.3.6) state.
Length: An unsigned 24 bit value representing the number of bytes
after the length field. The minimum length of the length field is 4
(when the number of name value pairs is 0).
Stream-ID: The stream this HEADERS block is associated with.
Name/Value Header Block: A set of name/value pairs carried as part of
the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).
3.6.8. WINDOW_UPDATE
The WINDOW_UPDATE control frame is used to implement per stream flow
control in HTTP/2.0. Flow control in HTTP/2.0 is per hop, that is,
only between the two endpoints of a HTTP/2.0 connection. If there
are one or more intermediaries between the client and the origin
server, flow control signals are not explicitly forwarded by the
intermediaries. (However, throttling of data transfer by any
recipient may have the effect of indirectly propagating flow control
information upstream back to the original sender.) Flow control only
applies to the data portion of data frames. Recipients must buffer
all control frames. If a recipient fails to buffer an entire control
frame, it MUST issue a stream error (Section 3.4.2) with the status
code FLOW_CONTROL_ERROR for the stream.
Flow control in HTTP/2.0 is implemented by a data transfer window
kept by the sender of each stream. The data transfer window is a
simple uint32 that indicates how many bytes of data the sender can
transmit. After a stream is created, but before any data frames have
been transmitted, the sender begins with the initial window size.
This window size is a measure of the buffering capability of the
recipient. The sender must not send a data frame with data length
greater than the transfer window size. After sending each data
frame, the sender decrements its transfer window size by the amount
of data transmitted. When the window size becomes less than or equal
to 0, the sender must pause transmitting data frames. At the other
end of the stream, the recipient sends a WINDOW_UPDATE control back
to notify the sender that it has consumed some data and freed up
buffer space to receive more data.
+----------------------------------+
|1| version | 9 |
+----------------------------------+
| 0 (flags) | 8 (length) |
+----------------------------------+
|X| Stream-ID (31-bits) |
+----------------------------------+
|X| Delta-Window-Size (31-bits) |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The HTTP/2.0 version number.
Type: The message type for a WINDOW_UPDATE message is 9.
Length: The length field is always 8 for this frame (there are 8 The body of a HEADERS frame contains a Headers Block
bytes after the length field). (Section 3.7.10).
Stream-ID: The stream ID that this WINDOW_UPDATE control frame is 3.7.9. WINDOW_UPDATE
for.
Delta-Window-Size: The additional number of bytes that the sender can The WINDOW_UPDATE frame (type=9) is used to implement flow control in
transmit in addition to existing remaining window size. The legal HTTP/2.0.
range for this field is 1 to 2^31 - 1 (0x7fffffff) bytes.
The window size as kept by the sender must never exceed 2^31 Flow control in HTTP/2.0 operates at two levels: on each individual
(although it can become negative in one special case). If a sender stream and on the entire session.
receives a WINDOW_UPDATE that causes the its window size to exceed
this limit, it must send RST_STREAM with status code
FLOW_CONTROL_ERROR to terminate the stream.
When a HTTP/2.0 connection is first established, the default initial Flow control in HTTP/2.0 is hop by hop, that is, only between the two
window size for all streams is 64KB. An endpoint can use the endpoints of a HTTP/2.0 connection. Intermediaries do not forward
SETTINGS control frame to adjust the initial window size for the WINDOW_UPDATE messages between dependent sessions. However,
connection. That is, its peer can start out using the 64KB default throttling of data transfer by any recipient can indirectly cause the
initial window size when sending data frames before receiving the propagation of flow control information toward the original sender.
SETTINGS. Because SETTINGS is asynchronous, there may be a race
condition if the recipient wants to decrease the initial window size,
but its peer immediately sends 64KB on the creation of a new
connection, before waiting for the SETTINGS to arrive. This is one
case where the window size kept by the sender will become negative.
Once the sender detects this condition, it must stop sending data
frames and wait for the recipient to catch up. The recipient has two
choices:
immediately send RST_STREAM with FLOW_CONTROL_ERROR status code. Flow control only applies to frames that are identified as being
subject to flow control. Of the frames defined in this document,
only data frames are subject to flow control. Receivers MUST either
buffer or process all other frames, terminate the corresponding
stream, or terminate the session. The stream or session is
terminated with a FLOW_CONTROL_ERROR code.
allow the head of line blocking (as there is only one stream for Valid flags for the WINDOW_UPDATE frame are:
the session and the amount of data in flight is bounded by the
default initial window size), and send WINDOW_UPDATE as it
consumes data.
In the case of option 2, both sides must compute the window size END_FLOW_CONTROL (0x2): Bit 2 being set indicates that flow control
based on the initial window size in the SETTINGS. For example, if for the identified stream or session is ended and subsequent
the recipient sets the initial window size to be 16KB, and the sender frames do not need to be flow controlled.
sends 64KB immediately on connection establishment, the sender will
discover its window size is -48KB on receipt of the SETTINGS. As the
recipient consumes the first 16KB, it must send a WINDOW_UPDATE of
16KB back to the sender. This interaction continues until the
sender's window size becomes positive again, and it can resume
transmitting data frames.
After the recipient reads in a data frame with FLAG_FIN that marks The WINDOW_UPDATE frame can be stream related or session related.
the end of the data stream, it should not send WINDOW_UPDATE frames The stream identifier in the WINDOW_UPDATE frame header identifies
as it consumes the last data frame. A sender should ignore all the the affected stream, or includes a value of 0 to indicate that the
WINDOW_UPDATE frames associated with the stream after it send the session flow control window is updated.
last frame for the stream.
The data frames from the sender and the WINDOW_UPDATE frames from the The payload of a WINDOW_UPDATE frame contains a 32-bit value. This
recipient are completely asynchronous with respect to each other. value is the additional number of bytes that the sender can transmit
This property allows a recipient to aggressively update the window in addition to the existing flow control window. The legal range for
size kept by the sender to prevent the stream from stalling. this field is 1 to 2^31 - 1 (0x7fffffff) bytes; the most significant
bit of this value is reserved.
3.6.9. CREDENTIAL 3.7.9.1. The Flow Control Window
The CREDENTIAL control frame is used by the client to send additional Flow control in HTTP/2.0 is implemented by a flow control window kept
client certificates to the server. A HTTP/2.0 client may decide to by the sender of each stream. The flow control window is a simple
send requests for resources from different origins on the same integer value that indicates how many bytes of data the sender is
HTTP/2.0 session if it decides that that server handles both origins. permitted to transmit. The flow control window size is a measure of
For example if the IP address associated with both hostnames matches the buffering capability of the recipient.
and the SSL server certificate presented in the initial handshake is
valid for both hostnames. However, because the SSL connection can
contain at most one client certificate, the client needs a mechanism
to send additional client certificates to the server.
The server is required to maintain a vector of client certificates Two flow control windows apply to the sending of every message: the
associated with a HTTP/2.0 session. When the client needs to send a stream flow control window and the session flow control window. The
client certificate to the server, it will send a CREDENTIAL frame sender MUST NOT send a flow controlled frame with a length that
that specifies the index of the slot in which to store the exceeds the space available in either of the flow control windows
certificate as well as proof that the client posesses the advertised by the receiver. Frames with zero length with the FINAL
corresponding private key. The initial size of this vector must be flag set (for example, an empty data frame) MAY be sent if there is
8. If the client provides a client certificate during the first TLS no available space in either flow control window.
handshake, the contents of this certificate must be copied into the
first slot (index 1) in the CREDENTIAL vector, though it may be
overwritten by subsequent CREDENTIAL frames. The server must
exclusively use the CREDENTIAL vector when evaluating the client
certificates associated with an origin. The server may change the
size of this vector by sending a SETTINGS frame with the setting
SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE value specified. In the
event that the new size is smaller than the current size, truncation
occurs preserving lower-index slots as possible.
TLS renegotiation with client authentication is incompatible with For flow control calculations, the 8 byte frame header is not
HTTP/2.0 given the multiplexed nature of HTTP/2.0. Specifically, counted.
imagine that the client has 2 requests outstanding to the server for
two different pages (in different tabs). When the renegotiation +
client certificate request comes in, the browser is unable to
determine which resource triggered the client certificate request, in
order to prompt the user accordingly.
+----------------------------------+ After sending a flow controlled frame, the sender reduces the space
|1|000000000000001|0000000000001011| available in both windows by the length of the transmitted frame.
+----------------------------------+
| flags (8) | Length (24 bits) |
+----------------------------------+
| Slot (16 bits) | |
+-----------------+ |
| Proof Length (32 bits) |
+----------------------------------+
| Proof |
+----------------------------------+ <+
| Certificate Length (32 bits) | |
+----------------------------------+ | Repeated until end of frame
| Certificate | |
+----------------------------------+ <+
Slot: The index in the server's client certificate vector where this The receiver of a message sends a WINDOW_UPDATE frame as it consumes
certificate should be stored. If there is already a certificate data and frees up space in flow control windows. Separate
stored at this index, it will be overwritten. The index is one WINDOW_UPDATE messages are sent for the stream and session level flow
based, not zero based; zero is an invalid slot index. control windows.
Proof: Cryptographic proof that the client has possession of the A sender that receives a WINDOW_UPDATE frame updates the
private key associated with the certificate. The format is a TLS corresponding window by the amount specified in the frame.
digitally-signed element ([RFC5246], Section 4.7). The signature
algorithm must be the same as that used in the CertificateVerify
message. However, since the MD5+SHA1 signature type used in TLS 1.0
connections can not be correctly encoded in a digitally-signed
element, SHA1 must be used when MD5+SHA1 was used in the SSL
connection. The signature is calculated over a 32 byte TLS extractor
value (http://tools.ietf.org/html/rfc5705) with a label of "EXPORTER
HTTP/2.0 certificate proof" using the empty string as context.
ForRSA certificates the signature would be a PKCS#1 v1.5 signature.
For ECDSA, it would be an ECDSA-Sig-Value
(http://tools.ietf.org/html/rfc5480#appendix-A). For a 1024-bit RSA
key, the CREDENTIAL message would be ~500 bytes.
Certificate: The certificate chain, starting with the leaf A sender MUST NOT allow a flow control window to exceed 2^31 - 1
certificate. Each certificate must be encoded as a 32 bit length, bytes. If a sender receives a WINDOW_UPDATE that causes a flow
followed by a DER encoded certificate. The certificate must be of control window to exceed this maximum it MUST terminate either the
the same type (RSA, ECDSA, etc) as the client certificate associated stream or the session, as appropriate. For streams, the sender sends
with the SSL connection. a RST_STREAM with the error code of FLOW_CONTROL_ERROR code; for the
session, a GOAWAY message with a FLOW_CONTROL_ERROR code.
If the server receives a request for a resource with unacceptable Flow controlled frames from the sender and WINDOW_UPDATE frames from
credential (either missing or invalid), it must reply with a the receiver are completely asynchronous with respect to each other.
RST_STREAM frame with the status code INVALID_CREDENTIALS. Upon This property allows a receiver to aggressively update the window
receipt of a RST_STREAM frame with INVALID_CREDENTIALS, the client size kept by the sender to prevent streams from stalling.
should initiate a new stream directly to the requested origin and
resend the request. Note, HTTP/2.0 does not allow the server to
request different client authentication for different resources in
the same origin.
If the server receives an invalid CREDENTIAL frame, it MUST respond 3.7.9.2. Initial Flow Control Window Size
with a GOAWAY frame and shutdown the session.
3.6.10. Name/Value Header Block When a HTTP/2.0 connection is first established, new streams are
created with an initial flow control window size of 65535 bytes. The
session flow control window is 65536 bytes. Both endpoints can
adjust the initial window size for new streams by including a value
for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms
part of the session header.
The Name/Value Header Block is found in the SYN_STREAM, SYN_REPLY and Prior to receiving a SETTINGS frame that sets a value for
HEADERS control frames, and shares a common format: SETTINGS_INITIAL_WINDOW_SIZE, a client can only use the default
initial window size when sending flow controlled frames. Similarly,
the session flow control window is set to the default initial window
size until a WINDOW_UPDATE message is received.
+------------------------------------+ A SETTINGS frame can alter the initial flow control window size for
| Number of Name/Value pairs (int32) | all current streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE
+------------------------------------+ changes, a receiver MUST adjust the size of all flow control windows
| Length of name (int32) | that it maintains by the difference between the new value and the old
+------------------------------------+ value.
| Name (string) |
+------------------------------------+
| Length of value (int32) |
+------------------------------------+
| Value (string) |
+------------------------------------+
| (repeats) |
Number of Name/Value pairs: The number of repeating name/value pairs A change to SETTINGS_INITIAL_WINDOW_SIZE could cause the available
following this field. space in a flow control window to become negative. A sender MUST
track the negative flow control window and not send new flow
controlled frames until it receives WINDOW_UPDATE messages that cause
the flow control window to become positive.
List of Name/Value pairs: For example, if the server sets the initial window size to be 16KB,
and the client sends 64KB immediately on connection establishment,
the client will recalculate the available flow control window to be
-48KB on receipt of the SETTINGS frame. The client retains a
negative flow control window until WINDOW_UPDATE frames restore the
window to being positive, after which the client can resume sending.
Length of Name: a 32-bit value containing the number of octets in 3.7.9.3. Reducing the Stream Window Size
the name field. Note that in practice, this length must not
exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.
Name: 0 or more octets, 8-bit sequences of data, excluding 0. A receiver that wishes to use a smaller flow control window than the
current size sends a new SETTINGS frame. However, the receiver MUST
be prepared to receive data that exceeds this window size, since the
sender might send data that exceeds the lower limit prior to
processing the SETTINGS frame.
Length of Value: a 32-bit value containing the number of octets in A receiver has two options for handling streams that exceed flow
the value field. Note that in practice, this length must not control limits:
exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.
Value: 0 or more octets, 8-bit sequences of data, excluding 0. 1. The receiver can immediately send RST_STREAM with
FLOW_CONTROL_ERROR error code for the affected streams.
Each header name must have at least one value. Header names are 2. The receiver can accept the streams and tolerate the resulting
encoded using the US-ASCII character set [ASCII] and must be all head of line blocking, sending WINDOW_UPDATE messages as it
lower case. The length of each name must be greater than zero. A consumes data.
recipient of a zero-length name MUST issue a stream error
(Section 3.4.2) with the status code PROTOCOL_ERROR for the
stream-id.
Duplicate header names are not allowed. To send two identically If a receiver decides to accept streams, both sides must recompute
named headers, send a header with two values, where the values are the available flow control window based on the initial window size
separated by a single NUL (0) byte. A header value can either be sent in the SETTINGS.
empty (e.g. the length is zero) or it can contain multiple, NUL-
separated values, each with length greater than zero. The value
never starts nor ends with a NUL character. Recipients of illegal
value fields MUST issue a stream error (Section 3.4.2) with the
status code PROTOCOL_ERROR for the stream-id.
3.6.10.1. Compression 3.7.9.4. Ending Flow Control
The Name/Value Header Block is a section of the SYN_STREAM, After a recipient reads in a frame that marks the end of a stream
SYN_REPLY, and HEADERS frames used to carry header meta-data. This (for example, a data stream with a FINAL flag set), it ceases
block is always compressed using zlib compression. Within this transmission of WINDOW_UPDATE frames. A sender is not required to
specification, any reference to 'zlib' is referring to the ZLIB maintain the available flow control window for streams that it is no
Compressed Data Format Specification Version 3.3 as part of RFC1950. longer sending on.
[RFC1950]
For each HEADERS compression instance, the initial state is Flow control can be disabled for all streams or the session using the
initialized using the following dictionary [UDELCOMPRESSION]: SETTINGS_FLOW_CONTROL_OPTIONS setting. An implementation that does
not wish to perform flow control can use this in the initial SETTINGS
exchange.
<CODE BEGINS> Flow control can be disabled for an individual stream or the overall
session by sending a WINDOW_UPDATE with the END_FLOW_CONTROL flag
set. The payload of a WINDOW_UPDATE frame that has the
END_FLOW_CONTROL flag set is ignored.
const unsigned char http2_dictionary_txt[] = { Flow control cannot be enabled again once disabled. Any attempt to
0x00, 0x00, 0x00, 0x07, 0x6f, 0x70, 0x74, 0x69, \\ - - - - o p t i re-enable flow control - by sending a WINDOW_UPDATE or by clearing
0x6f, 0x6e, 0x73, 0x00, 0x00, 0x00, 0x04, 0x68, \\ o n s - - - - h the bits on the SETTINGS_FLOW_CONTROL_OPTIONS setting - MUST be
0x65, 0x61, 0x64, 0x00, 0x00, 0x00, 0x04, 0x70, \\ e a d - - - - p rejected with a FLOW_CONTROL_ERROR error code.
0x6f, 0x73, 0x74, 0x00, 0x00, 0x00, 0x03, 0x70, \\ o s t - - - - p
0x75, 0x74, 0x00, 0x00, 0x00, 0x06, 0x64, 0x65, \\ u t - - - - d e
0x6c, 0x65, 0x74, 0x65, 0x00, 0x00, 0x00, 0x05, \\ l e t e - - - -
0x74, 0x72, 0x61, 0x63, 0x65, 0x00, 0x00, 0x00, \\ t r a c e - - -
0x06, 0x61, 0x63, 0x63, 0x65, 0x70, 0x74, 0x00, \\ - a c c e p t -
0x00, 0x00, 0x0e, 0x61, 0x63, 0x63, 0x65, 0x70, \\ - - - a c c e p
0x74, 0x2d, 0x63, 0x68, 0x61, 0x72, 0x73, 0x65, \\ t - c h a r s e
0x74, 0x00, 0x00, 0x00, 0x0f, 0x61, 0x63, 0x63, \\ t - - - - a c c
0x65, 0x70, 0x74, 0x2d, 0x65, 0x6e, 0x63, 0x6f, \\ e p t - e n c o
0x64, 0x69, 0x6e, 0x67, 0x00, 0x00, 0x00, 0x0f, \\ d i n g - - - -
0x61, 0x63, 0x63, 0x65, 0x70, 0x74, 0x2d, 0x6c, \\ a c c e p t - l
0x61, 0x6e, 0x67, 0x75, 0x61, 0x67, 0x65, 0x00, \\ a n g u a g e -
0x00, 0x00, 0x0d, 0x61, 0x63, 0x63, 0x65, 0x70, \\ - - - a c c e p
0x74, 0x2d, 0x72, 0x61, 0x6e, 0x67, 0x65, 0x73, \\ t - r a n g e s
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0x00, 0x00, 0x00, 0x0d, 0x61, 0x75, 0x74, 0x68, \\ - - - - a u t h
0x6f, 0x72, 0x69, 0x7a, 0x61, 0x74, 0x69, 0x6f, \\ o r i z a t i o
0x6e, 0x00, 0x00, 0x00, 0x0d, 0x63, 0x61, 0x63, \\ n - - - - c a c
0x68, 0x65, 0x2d, 0x63, 0x6f, 0x6e, 0x74, 0x72, \\ h e - c o n t r
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0x65, 0x6e, 0x74, 0x2d, 0x65, 0x6e, 0x63, 0x6f, \\ e n t - e n c o
0x64, 0x69, 0x6e, 0x67, 0x00, 0x00, 0x00, 0x10, \\ d i n g - - - -
0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, 0x2d, \\ c o n t e n t -
0x6c, 0x61, 0x6e, 0x67, 0x75, 0x61, 0x67, 0x65, \\ l a n g u a g e
0x00, 0x00, 0x00, 0x0e, 0x63, 0x6f, 0x6e, 0x74, \\ - - - - c o n t
0x65, 0x6e, 0x74, 0x2d, 0x6c, 0x65, 0x6e, 0x67, \\ e n t - l e n g
0x74, 0x68, 0x00, 0x00, 0x00, 0x10, 0x63, 0x6f, \\ t h - - - - c o
0x6e, 0x74, 0x65, 0x6e, 0x74, 0x2d, 0x6c, 0x6f, \\ n t e n t - l o
0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x00, 0x00, \\ c a t i o n - -
0x00, 0x0b, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, \\ - - c o n t e n
0x74, 0x2d, 0x6d, 0x64, 0x35, 0x00, 0x00, 0x00, \\ t - m d 5 - - -
0x0d, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, \\ - c o n t e n t
0x2d, 0x72, 0x61, 0x6e, 0x67, 0x65, 0x00, 0x00, \\ - r a n g e - -
0x00, 0x0c, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, \\ - - c o n t e n
0x74, 0x2d, 0x74, 0x79, 0x70, 0x65, 0x00, 0x00, \\ t - t y p e - -
0x00, 0x04, 0x64, 0x61, 0x74, 0x65, 0x00, 0x00, \\ - - d a t e - -
0x00, 0x04, 0x65, 0x74, 0x61, 0x67, 0x00, 0x00, \\ - - e t a g - -
0x00, 0x06, 0x65, 0x78, 0x70, 0x65, 0x63, 0x74, \\ - - e x p e c t
0x00, 0x00, 0x00, 0x07, 0x65, 0x78, 0x70, 0x69, \\ - - - - e x p i
0x72, 0x65, 0x73, 0x00, 0x00, 0x00, 0x04, 0x66, \\ r e s - - - - f
0x72, 0x6f, 0x6d, 0x00, 0x00, 0x00, 0x04, 0x68, \\ r o m - - - - h
0x6f, 0x73, 0x74, 0x00, 0x00, 0x00, 0x08, 0x69, \\ o s t - - - - i
0x66, 0x2d, 0x6d, 0x61, 0x74, 0x63, 0x68, 0x00, \\ f - m a t c h -
0x00, 0x00, 0x11, 0x69, 0x66, 0x2d, 0x6d, 0x6f, \\ - - - i f - m o
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};
<CODE ENDS> 3.7.10. Header Block
The entire contents of the name/value header block is compressed The header block is found in the HEADERS, HEADERS+PRIORITY and
using zlib. There is a single zlib stream for all name value pairs PUSH_PROMISE frames. The header block consists of a set of header
in one direction on a connection. HTTP/2.0 uses a SYNC_FLUSH between fields, which are name-value pairs. Headers are compressed using
each compressed frame. black magic.
Implementation notes: the compression engine can be tuned to favor Compression of header fields is a work in progress, as is the format
speed or size. Optimizing for size increases memory use and CPU of this block.
consumption. Because header blocks are generally small, implementors
may want to reduce the window-size of the compression engine from the
default 15bits (a 32KB window) to more like 11bits (a 2KB window).
The exact setting is chosen by the compressor, the decompressor will
work with any setting.
4. HTTP Layering over HTTP/2.0 4. HTTP Message Exchanges
HTTP/2.0 is intended to be as compatible as possible with current HTTP/2.0 is intended to be as compatible as possible with current
web-based applications. This means that, from the perspective of the web-based applications. This means that, from the perspective of the
server business logic or application API, the features of HTTP are server business logic or application API, the features of HTTP are
unchanged. To achieve this, all of the application request and unchanged. To achieve this, all of the application request and
response header semantics are preserved, although the syntax of response header semantics are preserved, although the syntax of
conveying those semantics has changed. Thus, the rules from the conveying those semantics has changed. Thus, the rules from HTTP/1.1
HTTP/1.1 specification in RFC2616 [RFC2616] apply with the changes in ([HTTP-p1], [HTTP-p2], [HTTP-p4], [HTTP-p5], [HTTP-p6], and
the sections below. [HTTP-p7]) apply with the changes in the sections below.
4.1. Connection Management 4.1. Connection Management
Clients SHOULD NOT open more than one HTTP/2.0 session to a given Clients SHOULD NOT open more than one HTTP/2.0 session to a given
origin [RFC6454] concurrently. origin ([RFC6454]) concurrently.
Note that it is possible for one HTTP/2.0 session to be finishing Note that it is possible for one HTTP/2.0 session to be finishing
(e.g. a GOAWAY message has been sent, but not all streams have (e.g. a GOAWAY message has been sent, but not all streams have
finished), while another HTTP/2.0 session is starting. finished), while another HTTP/2.0 session is starting.
4.1.1. Use of GOAWAY 4.1.1. Use of GOAWAY
HTTP/2.0 provides a GOAWAY message which can be used when closing a HTTP/2.0 provides a GOAWAY message which can be used when closing a
connection from either the client or server. Without a server GOAWAY connection from either the client or server. Without a server GOAWAY
message, HTTP has a race condition where the client sends a request message, HTTP has a race condition where the client sends a request
(a new SYN_STREAM) just as the server is closing the connection, and just as the server is closing the connection, and the client cannot
the client cannot know if the server received the stream or not. By know if the server received the stream or not. By using the last-
using the last-stream-id in the GOAWAY, servers can indicate to the stream-id in the GOAWAY, servers can indicate to the client if a
client if a request was processed or not. request was processed or not.
Note that some servers will choose to send the GOAWAY and immediately Note that some servers will choose to send the GOAWAY and immediately
terminate the connection without waiting for active streams to terminate the connection without waiting for active streams to
finish. The client will be able to determine this because HTTP/2.0 finish. The client will be able to determine this because HTTP/2.0
streams are determinstically closed. This abrupt termination will streams are deterministically closed. This abrupt termination will
force the client to heuristically decide whether to retry the pending force the client to heuristically decide whether to retry the pending
requests. Clients always need to be capable of dealing with this requests. Clients always need to be capable of dealing with this
case because they must deal with accidental connection termination case because they must deal with accidental connection termination
cases, which are the same as the server never having sent a GOAWAY. cases, which are the same as the server never having sent a GOAWAY.
More sophisticated servers will use GOAWAY to implement a graceful More sophisticated servers will use GOAWAY to implement a graceful
teardown. They will send the GOAWAY and provide some time for the teardown. They will send the GOAWAY and provide some time for the
active streams to finish before terminating the connection. active streams to finish before terminating the connection.
If a HTTP/2.0 client closes the connection, it should also send a If a HTTP/2.0 client closes the connection, it should also send a
GOAWAY message. This allows the server to know if any server-push GOAWAY message. This allows the server to know if any server-push
streams were received by the client. streams were received by the client.
If the endpoint closing the connection has not received any If the endpoint closing the connection has not received frames on any
SYN_STREAMs from the remote, the GOAWAY will contain a last-stream-id stream, the GOAWAY will contain a last-stream-id of 0.
of 0.
4.2. HTTP Request/Response 4.2. HTTP Request/Response
4.2.1. Request 4.2.1. HTTP Header Fields and HTTP/2.0 Headers
The client initiates a request by sending a SYN_STREAM frame. For At the application level, HTTP uses name-value pairs in its header
requests which do not contain a body, the SYN_STREAM frame MUST set fields. Because HTTP/2.0 merges the existing HTTP header fields with
the FLAG_FIN, indicating that the client intends to send no further HTTP/2.0 headers, there is a possibility that some HTTP applications
data on this stream. For requests which do contain a body, the already use a particular header field name. To avoid any conflicts,
SYN_STREAM will not contain the FLAG_FIN, and the body will follow all header fields introduced for layering HTTP over HTTP/2.0 are
the SYN_STREAM in a series of DATA frames. The last DATA frame will prefixed with ":". ":" is not a valid sequence in HTTP/1.* header
set the FLAG_FIN to indicate the end of the body. field naming, preventing any possible conflict.
The SYN_STREAM Name/Value section will contain all of the HTTP 4.2.2. Request
headers which are associated with an HTTP request. The header block
in HTTP/2.0 is mostly unchanged from today's HTTP header block, with
the following differences:
The first line of the request is unfolded into name/value pairs The client initiates a request by sending a HEADERS+PRIORITY frame.
like other HTTP headers and MUST be present: Requests that do not contain a body MUST set the FINAL flag,
indicating that the client intends to send no further data on this
stream, unless the server intends to push resources (see
Section 4.3). HEADERS+PRIORITY frame does not contain the FINAL flag
for requests that contain a body. The body of a request follows as a
series of DATA frames. The last DATA frame sets the FINAL flag to
indicate the end of the body.
":method" - the HTTP method for this request (e.g. "GET", The header fields included in the HEADERS+PRIORITY frame contain all
"POST", "HEAD", etc) of the HTTP header fields that are associated with an HTTP request.
The header block in HTTP/2.0 is mostly unchanged from today's HTTP
header block, with the following differences:
":path" - the url-path for this url with "/" prefixed. (See The following fields that are carried in the request line in
RFC1738 [RFC1738]). For example, for HTTP/1.1 ([HTTP-p1], Section 3.1.1) are defined as special-valued
name-value pairs:
":method": the HTTP method for this request (e.g. "GET", "POST",
"HEAD", etc) ([HTTP-p2], Section 4)
":path": ":path" - the request-target for this URI with "/"
prefixed (see [HTTP-p1], Section 3.1.1). For example, for
"http://www.google.com/search?q=dogs" the path would be "http://www.google.com/search?q=dogs" the path would be
"/search?q=dogs". "/search?q=dogs". [[anchor26: what forms of the HTTPbis
request-target are allowed here?]]
":version" - the HTTP version of this request (e.g. These header fields MUST be present in HTTP requests.
"HTTP/1.1")
In addition, the following two name/value pairs must also be In addition, the following two name-value pairs MUST be present in
present in every request: every request:
":host" - the hostport (See RFC1738 [RFC1738]) portion of the ":host": the host and optional port portions (see [RFC3986],
URL for this request (e.g. "www.google.com:1234"). This header Section 3.2) of the URI for this request (e.g. "www.google.com:
is the same as the HTTP 'Host' header. 1234"). This header field is the same as the HTTP 'Host'
header field ([HTTP-p1], Section 5.4).
":scheme" - the scheme portion of the URL for this request ":scheme": the scheme portion of the URI for this request (e.g.
(e.g. "https")) "https")
Header names are all lowercase. All header field names starting with ":" (whether defined in this
document or future extensions to this document) MUST appear before
any other header fields.
Header field names MUST be all lowercase.
The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer- The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-
Encoding headers are not valid and MUST not be sent. Encoding header fields are not valid and MUST not be sent.
User-agents MUST support gzip compression. Regardless of the User-agents MUST support gzip compression. Regardless of the
Accept-Encoding sent by the user-agent, the server may always send Accept-Encoding sent by the user-agent, the server may always send
content encoded with gzip or deflate encoding. content encoded with gzip or deflate encoding. [[anchor27: Still
valid?]]
If a server receives a request where the sum of the data frame If a server receives a request where the sum of the data frame
payload lengths does not equal the size of the Content-Length payload lengths does not equal the size of the Content-Length
header, the server MUST return a 400 (Bad Request) error. header field, the server MUST return a 400 (Bad Request) error.
POST-specific changes:
Although POSTs are inherently chunked, POST requests SHOULD Although POSTs are inherently chunked, POST requests SHOULD also
also be accompanied by a Content-Length header. There are two be accompanied by a Content-Length header field. First, it
reasons for this: First, it assists with upload progress meters informs the server of how much data to expect, which the server
for an improved user experience. But second, we know from can used to track overall progress and provide appropriate user
early versions of HTTP/2.0 that failure to send a content feedback. More importantly, some HTTP server implementations fail
length header is incompatible with many existing HTTP server to correctly process requests that omit the Content-Length header
implementations. Existing user-agents do not omit the Content- field. Many existing clients send a Content-Length header field,
Length header, and server implementations have come to depend which caused server implementations have come to depend upon its
upon this. presence.
The user-agent is free to prioritize requests as it sees fit. If the The user-agent is free to prioritize requests as it sees fit. If the
user-agent cannot make progress without receiving a resource, it user-agent cannot make progress without receiving a resource, it
should attempt to raise the priority of that resource. Resources should attempt to raise the priority of that resource. Resources
such as images, SHOULD generally use the lowest priority. such as images, SHOULD generally use the lowest priority.
If a client sends a SYN_STREAM without all of the method, host, path, If a client sends a HEADERS+PRIORITY frame that omits a mandatory
scheme, and version headers, the server MUST reply with a HTTP 400 header, the server MUST reply with a HTTP 400 Bad Request reply.
Bad Request reply. [[anchor28: Ed: why PROTOCOL_ERROR on missing ":status" in the
response, but HTTP 400 here?]]
4.2.2. Response If the server receives a data frame prior to a HEADERS or HEADERS+
PRIORITY frame the server MUST treat this as a stream error
(Section 3.5.2) of type PROTOCOL_ERROR.
The server responds to a client request with a SYN_REPLY frame. 4.2.3. Response
Symmetric to the client's upload stream, server will send data after
the SYN_REPLY frame via a series of DATA frames, and the last data
frame will contain the FLAG_FIN to indicate successful end-of-stream.
If a response (like a 202 or 204 response) contains no body, the
SYN_REPLY frame may contain the FLAG_FIN flag to indicate no further
data will be sent on the stream.
The response status line is unfolded into name/value pairs like The server responds to a client request with a HEADERS frame.
other HTTP headers and must be present: Symmetric to the client's upload stream, server will send any
response body in a series of DATA frames. The last data frame will
contain the FINAL flag to indicate the end of the stream and the end
of the response. A response that contains no body (such as a 204 or
304 response) consists only of a HEADERS frame that contains the
FINAL flag to indicate no further data will be sent on the stream.
":status" - The HTTP response status code (e.g. "200" or "200 The response status line is unfolded into name-value pairs like
OK") other HTTP header fields and must be present:
":version" - The HTTP response version (e.g. "HTTP/1.1") ":status": The HTTP response status code (e.g. "200" or "200 OK")
All header names must be lowercase. All header field names starting with ":" (whether defined in this
document or future extensions to this document) MUST appear before
any other header fields.
All header field names MUST be all lowercase.
The Connection, Keep-Alive, Proxy-Connection, and Transfer- The Connection, Keep-Alive, Proxy-Connection, and Transfer-
Encoding headers are not valid and MUST not be sent. Encoding header fields are not valid and MUST not be sent.
Responses MAY be accompanied by a Content-Length header for Responses MAY be accompanied by a Content-Length header field for
advisory purposes. (e.g. for UI progress meters) advisory purposes. This allows clients to learn the full size of
an entity prior to receiving all the data frames. This can help
in, for example, reporting progress.
If a client receives a response where the sum of the data frame If a client receives a response where the sum of the data frame
payload lengths does not equal the size of the Content-Length payload length does not equal the size of the Content-Length
header, the client MUST ignore the content length header. header field, the client MUST ignore the content length header
field. [[anchor29: Ed: See
If a client receives a SYN_REPLY without a status or without a <https://github.com/http2/http2-spec/issues/46>.]]
version header, the client must reply with a RST_STREAM frame
indicating a PROTOCOL ERROR.
4.2.3. Authentication
When a client sends a request to an origin server that requires
authentication, the server can reply with a "401 Unauthorized"
response, and include a WWW-Authenticate challenge header that
defines the authentication scheme to be used. The client then
retries the request with an Authorization header appropriate to the
specified authentication scheme.
There are four options for proxy authentication, Basic, Digest, NTLM
and Negotiate (SPNEGO). The first two options were defined in
RFC2617 [RFC2617], and are stateless. The second two options were
developed by Microsoft and specified in RFC4559 [RFC4559], and are
stateful; otherwise known as multi-round authentication, or
connection authentication.
4.2.3.1. Stateless Authentication
Stateless Authentication over HTTP/2.0 is identical to how it is
performed over HTTP. If multiple HTTP/2.0 streams are concurrently
sent to a single server, each will authenticate independently,
similar to how two HTTP connections would independently authenticate
to a proxy server.
4.2.3.2. Stateful Authentication
Unfortunately, the stateful authentication mechanisms were
implemented and defined in a such a way that directly violates
RFC2617 - they do not include a "realm" as part of the request. This
is problematic in HTTP/2.0 because it makes it impossible for a
client to disambiguate two concurrent server authentication
challenges.
To deal with this case, HTTP/2.0 servers using Stateful
Authentication MUST implement one of two changes:
Servers can add a "realm=<desired realm>" header so that the two If a client receives a response with an absent or duplicated status
authentication requests can be disambiguated and run concurrently. header, the client MUST treat this as a stream error (Section 3.5.2)
Unfortunately, given how these mechanisms work, this is probably of type PROTOCOL_ERROR.
not practical.
Upon sending the first stateful challenge response, the server If the client receives a data frame prior to a HEADERS or HEADERS+
MUST buffer and defer all further frames which are not part of PRIORITY frame the client MUST treat this as a stream error
completing the challenge until the challenge has completed. (Section 3.5.2) of type PROTOCOL_ERROR.
Completing the authentication challenge may take multiple round
trips. Once the client receives a "401 Authenticate" response for
a stateful authentication type, it MUST stop sending new requests
to the server until the authentication has completed by receiving
a non-401 response on at least one stream.
4.3. Server Push Transactions 4.3. Server Push Transactions
HTTP/2.0 enables a server to send multiple replies to a client for a HTTP/2.0 enables a server to send multiple replies to a client for a
single request. The rationale for this feature is that sometimes a single request. The rationale for this feature is that sometimes a
server knows that it will need to send multiple resources in response server knows that it will need to send multiple resources in response
to a single request. Without server push features, the client must to a single request. Without server push features, the client must
first download the primary resource, then discover the secondary first download the primary resource, then discover the secondary
resource(s), and request them. Pushing of resources avoids the resource(s), and request them. Pushing of resources avoids the
round-trip delay, but also creates a potential race where a server round-trip delay, but also creates a potential race where a server
can be pushing content which a user-agent is in the process of can be pushing content which a user-agent is in the process of
requesting. The following mechanics attempt to prevent the race requesting. The following mechanics attempt to prevent the race
condition while enabling the performance benefit. condition while enabling the performance benefit.
Server push is an optional feature. Server push can be disabled by
clients that do not wish to receive pushed resources by advertising a
SETTINGS_MAX_CONCURRENT_STREAMS SETTING (Section 3.7.4) of zero.
This prevents servers from creating the streams necessary to push
resources.
Browsers receiving a pushed response MUST validate that the server is Browsers receiving a pushed response MUST validate that the server is
authorized to push the URL using the browser same-origin [RFC6454] authorized to push the resource using the same-origin policy
policy. For example, a HTTP/2.0 connection to www.foo.com is ([RFC6454], Section 3). For example, a HTTP/2.0 connection to
generally not permitted to push a response for www.evil.com. "example.com" is generally [[anchor30: Ed: weaselly use of
"generally", needs better definition]] not permitted to push a
response for "www.example.org".
If the browser accepts a pushed response (e.g. it does not send a A client that accepts pushed resources caches those resources as
RST_STREAM), the browser MUST attempt to cache the pushed response in though they were responses to GET requests.
same way that it would cache any other response. This means
validating the response headers and inserting into the disk cache.
Because pushed responses have no request, they have no request Pushed responses are associated with a request at the HTTP/2.0
headers associated with them. At the framing layer, HTTP/2.0 pushed framing layer. The PUSH_PROMISE includes a stream identifier for an
streams contain an "associated-stream-id" which indicates the associated request/response exchange that supplies request header
requested stream for which the pushed stream is related. The pushed fields. The pushed stream inherits all of the request header fields
stream inherits all of the headers from the associated-stream-id with from the associated stream with the exception of resource
the exception of ":host", ":scheme", and ":path", which are provided identification header fields (":host", ":scheme", and ":path"), which
as part of the pushed response stream headers. The browser MUST are provided as part of the PUSH_PROMISE frame. Pushed resources
store these inherited and implied request headers with the cached always have an associated ":method" of "GET". A cache MUST store
these inherited and implied request header fields with the cached
resource. resource.
Implementation note: With server push, it is theoretically possible Implementation note: With server push, it is theoretically possible
for servers to push unreasonable amounts of content or resources to for servers to push unreasonable amounts of content or resources to
the user-agent. Browsers MUST implement throttles to protect against the user-agent. Browsers MUST implement throttles to protect against
unreasonable push attacks. unreasonable push attacks. [[anchor31: Ed: insufficiently specified
to implement; would like to remove]]
4.3.1. Server implementation 4.3.1. Server implementation
When the server intends to push a resource to the user-agent, it A server pushes resources in association with a request from the
opens a new stream by sending a unidirectional SYN_STREAM. The client. Prior to closing the response stream, the server sends a
SYN_STREAM MUST include an Associated-To-Stream-ID, and MUST set the PUSH_PROMISE for each resource that it intends to push. The
FLAG_UNIDIRECTIONAL flag. The SYN_STREAM MUST include headers for PUSH_PROMISE includes header fields that allow the client to identify
":scheme", ":host", ":path", which represent the URL for the resource the resource (":scheme", ":host", and ":port").
being pushed. Subsequent headers may follow in HEADERS frames. The
purpose of the association is so that the user-agent can
differentiate which request induced the pushed stream; without it, if
the user-agent had two tabs open to the same page, each pushing
unique content under a fixed URL, the user-agent would not be able to
differentiate the requests.
The Associated-To-Stream-ID must be the ID of an existing, open A server can push multiple resources in response to a request, but
stream. The reason for this restriction is to have a clear endpoint these can only be sent while the response stream remains open. A
for pushed content. If the user-agent requested a resource on stream server MUST NOT send a PUSH_PROMISE on a half-closed stream.
11, the server replies on stream 11. It can push any number of
additional streams to the client before sending a FLAG_FIN on stream
11. However, once the originating stream is closed no further push
streams may be associated with it. The pushed streams do not need to
be closed (FIN set) before the originating stream is closed, they
only need to be created before the originating stream closes.
It is illegal for a server to push a resource with the Associated-To- The server SHOULD include any header fields in a PUSH_PROMISE that
Stream-ID of 0. would allow a cache to determine if the resource is already cached
(see [HTTP-p6], Section 4).
To minimize race conditions with the client, the SYN_STREAM for the After sending a PUSH_PROMISE, the server commences transmission of a
pushed resources MUST be sent prior to sending any content which pushed resource. A pushed resource uses a server-initiated stream.
could allow the client to discover the pushed resource and request The server sends frames on this stream in the same order as an HTTP
response (Section 4.2.3): a HEADERS frame followed by DATA frames.
Many uses of server push are to send content that a client is likely
to discover a need for based on the content of a response
representation. To minimize the chances that a client will make a
request for resources that are being pushed - causing duplicate
copies of a resource to be sent by the server - a PUSH_PROMISE frame
SHOULD be sent prior to any content in the response representation
that might allow a client to discover the pushed resource and request
it. it.
The server MUST only push resources which would have been returned The server MUST only push resources that could have been returned
from a GET request. from a GET request.
Note: If the server does not have all of the Name/Value Response Note: A server does not need to have all response header fields
headers available at the time it issues the HEADERS frame for the available at the time it issues a PUSH_PROMISE frame. All remaining
pushed resource, it may later use an additional HEADERS frame to header fields are included in the HEADERS frame. The HEADERS frame
augment the name/value pairs to be associated with the pushed stream. MUST NOT duplicate header fields from the PUSH_PROMISE frames.
The subsequent HEADERS frame(s) must not contain a header for
':host', ':scheme', or ':path' (e.g. the server can't change the
identity of the resource to be pushed). The HEADERS frame must not
contain duplicate headers with a previously sent HEADERS frame. The
server must send a HEADERS frame including the scheme/host/port
headers before sending any data frames on the stream.
4.3.2. Client implementation 4.3.2. Client implementation
When fetching a resource the client has 3 possibilities: When fetching a resource the client has 3 possibilities:
the resource is not being pushed 1. the resource is not being pushed
the resource is being pushed, but the data has not yet arrived 2. the resource is being pushed, but the data has not yet arrived
the resource is being pushed, and the data has started to arrive 3. the resource is being pushed, and the data has started to arrive
When a SYN_STREAM and HEADERS frame which contains an Associated-To- When a HEADERS+PRIORITY frame that contains an
Stream-ID is received, the client must not issue GET requests for the Associated-To-Stream-ID is received, the client MUST NOT[[anchor34:
resource in the pushed stream, and instead wait for the pushed stream SHOULD NOT?]] issue GET requests for the resource in the pushed
to arrive. stream, and instead wait for the pushed stream to arrive.
If a client receives a server push stream with stream-id 0, it MUST A server MUST NOT push a resource with an Associated-To-Stream-ID of
issue a session error (Section 3.4.1) with the status code 0. Clients MUST treat this as a session error (Section 3.5.1) of
PROTOCOL_ERROR. type PROTOCOL_ERROR.
When a client receives a SYN_STREAM from the server without a the When a client receives a PUSH_PROMISE frame from the server without a
':host', ':scheme', and ':path' headers in the Name/Value section, it the ":host", ":scheme", and ":path" header fields, it MUST treat this
MUST reply with a RST_STREAM with error code HTTP_PROTOCOL_ERROR. as a stream error (Section 3.5.2) of type PROTOCOL_ERROR.
To cancel individual server push streams, the client can issue a To cancel individual server push streams, the client can issue a
stream error (Section 3.4.2) with error code CANCEL. Upon receipt, stream error (Section 3.5.2) of type CANCEL. Upon receipt, the
the server MUST stop sending on this stream immediately (this is an server ceases transmission of the pushed data.
Abrupt termination).
To cancel all server push streams related to a request, the client To cancel all server push streams related to a request, the client
may issue a stream error (Section 3.4.2) with error code CANCEL on may issue a stream error (Section 3.5.2) of type CANCEL on the
the associated-stream-id. By cancelling that stream, the server MUST associated-stream-id. By cancelling that stream, the server MUST
immediately stop sending frames for any streams with immediately stop sending frames for any streams with
in-association-to for the original stream. in-association-to for the original stream. [[anchor35: Ed: Triggering
side-effects on stream reset is going to be problematic for the
If the server sends a HEADER frame containing duplicate headers with framing layer. Purely from a design perspective, it's a layering
a previous HEADERS frame for the same stream, the client must issue a violation. More practically speaking, the base request stream might
stream error (Section 3.4.2) with error code PROTOCOL ERROR. already be removed. Special handling logic would be required.]]
If the server sends a HEADERS frame containing header fields that
duplicate values on a previous HEADERS or PUSH_PROMISE frames on the
same stream, the client MUST treat this as a stream error
(Section 3.5.2) of type PROTOCOL_ERROR.
If the server sends a HEADERS frame after sending a data frame for If the server sends a HEADERS frame after sending a data frame for
the same stream, the client MAY ignore the HEADERS frame. Ignoring the same stream, the client MAY ignore the HEADERS frame. Ignoring
the HEADERS frame after a data frame prevents handling of HTTP's the HEADERS frame after a data frame prevents handling of HTTP's
trailing headers trailing header fields (Section 4.1.1 of [HTTP-p1]).
(http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.40).
5. Design Rationale and Notes 5. Design Rationale and Notes
Authors' notes: The notes in this section have no bearing on the Authors' notes: The notes in this section have no bearing on the
HTTP/2.0 protocol as specified within this document, and none of HTTP/2.0 protocol as specified within this document, and none of
these notes should be considered authoritative about how the protocol these notes should be considered authoritative about how the protocol
works. However, these notes may prove useful in future debates about works. However, these notes may prove useful in future debates about
how to resolve protocol ambiguities or how to evolve the protocol how to resolve protocol ambiguities or how to evolve the protocol
going forward. They may be removed before the final draft. going forward. They may be removed before the final draft.
5.1. Separation of Framing Layer and Application Layer 5.1. Separation of Framing Layer and Application Layer
Readers may note that this specification sometimes blends the framing Readers may note that this specification sometimes blends the framing
layer (Section 3) with requirements of a specific application - HTTP layer (Section 3) with requirements of a specific application - HTTP
(Section 4). This is reflected in the request/response nature of the (Section 4). This is reflected in the request/response nature of the
streams, the definition of the HEADERS and compression contexts which streams and the definition of the HEADERS which are very similar to
are very similar to HTTP, and other areas as well. HTTP, and other areas as well.
This blending is intentional - the primary goal of this protocol is This blending is intentional - the primary goal of this protocol is
to create a low-latency protocol for use with HTTP. Isolating the to create a low-latency protocol for use with HTTP. Isolating the
two layers is convenient for description of the protocol and how it two layers is convenient for description of the protocol and how it
relates to existing HTTP implementations. However, the ability to relates to existing HTTP implementations. However, the ability to
reuse the HTTP/2.0 framing layer is a non goal. reuse the HTTP/2.0 framing layer is a non goal.
5.2. Error handling - Framing Layer 5.2. Error handling - Framing Layer
Error handling at the HTTP/2.0 layer splits errors into two groups: Error handling at the HTTP/2.0 layer splits errors into two groups:
Those that affect an individual HTTP/2.0 stream, and those that do Those that affect an individual HTTP/2.0 stream, and those that do
not. not.
When an error is confined to a single stream, but general framing is When an error is confined to a single stream, but general framing is
in tact, HTTP/2.0 attempts to use the RST_STREAM as a mechanism to in tact, HTTP/2.0 attempts to use the RST_STREAM as a mechanism to
invalidate the stream but move forward without aborting the invalidate the stream but move forward without aborting the
connection altogether. connection altogether.
For errors occuring outside of a single stream context, HTTP/2.0 For errors occurring outside of a single stream context, HTTP/2.0
assumes the entire session is hosed. In this case, the endpoint assumes the entire session is hosed. In this case, the endpoint
detecting the error should initiate a connection close. detecting the error should initiate a connection close.
5.3. One Connection Per Domain 5.3. One Connection Per Domain
HTTP/2.0 attempts to use fewer connections than other protocols have HTTP/2.0 attempts to use fewer connections than other protocols have
traditionally used. The rationale for this behavior is because it is traditionally used. The rationale for this behavior is because it is
very difficult to provide a consistent level of service (e.g. TCP very difficult to provide a consistent level of service (e.g. TCP
slow-start), prioritization, or optimal compression when the client slow-start), prioritization, or optimal compression when the client
is connecting to the server through multiple channels. is connecting to the server through multiple channels.
skipping to change at page 44, line 38 skipping to change at page 36, line 31
single stream, it creates a potential for head-of-line blocking single stream, it creates a potential for head-of-line blocking
problems at the transport level. In tests so far, the negative problems at the transport level. In tests so far, the negative
effects of head-of-line blocking (especially in the presence of effects of head-of-line blocking (especially in the presence of
packet loss) is outweighed by the benefits of compression and packet loss) is outweighed by the benefits of compression and
prioritization. prioritization.
5.4. Fixed vs Variable Length Fields 5.4. Fixed vs Variable Length Fields
HTTP/2.0 favors use of fixed length 32bit fields in cases where HTTP/2.0 favors use of fixed length 32bit fields in cases where
smaller, variable length encodings could have been used. To some, smaller, variable length encodings could have been used. To some,
this seems like a tragic waste of bandwidth. HTTP/2.0 choses the this seems like a tragic waste of bandwidth. HTTP/2.0 chooses the
simple encoding for speed and simplicity. simple encoding for speed and simplicity.
The goal of HTTP/2.0 is to reduce latency on the network. The The goal of HTTP/2.0 is to reduce latency on the network. The
overhead of HTTP/2.0 frames is generally quite low. Each data frame overhead of HTTP/2.0 frames is generally quite low. Each data frame
is only an 8 byte overhead for a 1452 byte payload (~0.6%). At the is only an 8 byte overhead for a 1452 byte payload (~0.6%). At the
time of this writing, bandwidth is already plentiful, and there is a time of this writing, bandwidth is already plentiful, and there is a
strong trend indicating that bandwidth will continue to increase. strong trend indicating that bandwidth will continue to increase.
With an average worldwide bandwidth of 1Mbps, and assuming that a With an average worldwide bandwidth of 1Mbps, and assuming that a
variable length encoding could reduce the overhead by 50%, the variable length encoding could reduce the overhead by 50%, the
latency saved by using a variable length encoding would be less than latency saved by using a variable length encoding would be less than
100 nanoseconds. More interesting are the effects when the larger 100 nanoseconds. More interesting are the effects when the larger
encodings force a packet boundary, in which case a round-trip could encodings force a packet boundary, in which case a round-trip could
be induced. However, by addressing other aspects of HTTP/2.0 and TCP be induced. However, by addressing other aspects of HTTP/2.0 and TCP
interactions, we believe this is completely mitigated. interactions, we believe this is completely mitigated.
5.5. Compression Context(s) 5.5. Server Push
When isolating the compression contexts used for communicating with
multiple origins, we had a few choices to make. We could have
maintained a map (or list) of compression contexts usable for each
origin. The basic case is easy - each HEADERS frame would need to
identify the context to use for that frame. However, compression
contexts are not cheap, so the lifecycle of each context would need
to be bounded. For proxy servers, where we could churn through many
contexts, this would be a concern. We considered using a static set
of contexts, say 16 of them, which would bound the memory use. We
also considered dynamic contexts, which could be created on the fly,
and would need to be subsequently destroyed. All of these are
complicated, and ultimately we decided that such a mechanism creates
too many problems to solve.
Alternatively, we've chosen the simple approach, which is to simply
provide a flag for resetting the compression context. For the common
case (no proxy), this fine because most requests are to the same
origin and we never need to reset the context. For cases where we
are using two different origins over a single HTTP/2.0 session, we
simply reset the compression state between each transition.
5.6. Unidirectional streams
Many readers notice that unidirectional streams are both a bit
confusing in concept and also somewhat redundant. If the recipient
of a stream doesn't wish to send data on a stream, it could simply
send a SYN_REPLY with the FLAG_FIN bit set. The FLAG_UNIDIRECTIONAL
is, therefore, not necessary.
It is true that we don't need the UNIDIRECTIONAL markings. It is
added because it avoids the recipient of pushed streams from needing
to send a set of empty frames (e.g. the SYN_STREAM w/ FLAG_FIN) which
otherwise serve no purpose.
5.7. Data Compression
Generic compression of data portion of the streams (as opposed to
compression of the headers) without knowing the content of the stream
is redundant. There is no value in compressing a stream which is
already compressed. Because of this, HTTP/2.0 does allow data
compression to be optional. We included it because study of existing
websites shows that many sites are not using compression as they
should, and users suffer because of it. We wanted a mechanism where,
at the HTTP/2.0 layer, site administrators could simply force
compression - it is better to compress twice than to not compress.
Overall, however, with this feature being optional and sometimes
redundant, it is unclear if it is useful at all. We will likely
remove it from the specification.
5.8. Server Push
A subtle but important point is that server push streams must be A subtle but important point is that server push streams must be
declared before the associated stream is closed. The reason for this declared before the associated stream is closed. The reason for this
is so that proxies have a lifetime for which they can discard is so that proxies have a lifetime for which they can discard
information about previous streams. If a pushed stream could information about previous streams. If a pushed stream could
associate itself with an already-closed stream, then endpoints would associate itself with an already-closed stream, then endpoints would
not have a specific lifecycle for when they could disavow knowledge not have a specific lifecycle for when they could disavow knowledge
of the streams which went before. of the streams which went before.
6. Security Considerations 6. Security Considerations
6.1. Use of Same-origin constraints 6.1. Use of Same-origin constraints
This specification uses the same-origin policy [RFC6454] in all cases This specification uses the same-origin policy ([RFC6454], Section 3)
where verification of content is required. in all cases where verification of content is required.
6.2. HTTP Headers and HTTP/2.0 Headers
At the application level, HTTP uses name/value pairs in its headers.
Because HTTP/2.0 merges the existing HTTP headers with HTTP/2.0
headers, there is a possibility that some HTTP applications already
use a particular header name. To avoid any conflicts, all headers
introduced for layering HTTP over HTTP/2.0 are prefixed with ":". ":"
is not a valid sequence in HTTP header naming, preventing any
possible conflict.
6.3. Cross-Protocol Attacks 6.2. Cross-Protocol Attacks
By utilizing TLS, we believe that HTTP/2.0 introduces no new cross- By utilizing TLS, we believe that HTTP/2.0 introduces no new cross-
protocol attacks. TLS encrypts the contents of all transmission protocol attacks. TLS encrypts the contents of all transmission
(except the handshake itself), making it difficult for attackers to (except the handshake itself), making it difficult for attackers to
control the data which could be used in a cross-protocol attack. control the data which could be used in a cross-protocol attack.
[[anchor45: Issue: This is no longer true]]
6.4. Server Push Implicit Headers 6.3. Cacheability of Pushed Resources
Pushed resources do not have an associated request. In order for Pushed resources do not have an associated request. In order for
existing HTTP cache control validations (such as the Vary header) to existing HTTP cache control validations (such as the Vary header
work, however, all cached resources must have a set of request field) to work, all cached resources must have a set of request
headers. For this reason, browsers MUST be careful to inherit header fields. For this reason, caches MUST be careful to inherit
request headers from the associated stream for the push. This request header fields from the associated stream for the push. This
includes the 'Cookie' header. includes the Cookie header field.
7. Privacy Considerations 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.
7. Privacy Considerations
7.1. Long Lived Connections 7.1. Long Lived Connections
HTTP/2.0 aims to keep connections open longer between clients and 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. servers in order to reduce the latency when a user makes a request.
The maintenance of these connections over time could be used to The maintenance of these connections over time could be used to
expose private information. For example, a user using a browser expose private information. For example, a user using a browser
hours after the previous user stopped using that browser may be able 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 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 with HTTP in its current form as well, however the short lived
connections make it less of a risk. connections make it less of a risk.
7.2. SETTINGS frame 7.2. SETTINGS frame
The HTTP/2.0 SETTINGS frame allows servers to store out-of-band The HTTP/2.0 SETTINGS frame allows servers to store out-of-band
transmitted information about the communication between client and transmitted information about the communication between client and
server on the client. Although this is intended only to be used to server on the client. Although this is intended only to be used to
reduce latency, renegade servers could use it as a mechanism to store reduce latency, renegade servers could use it as a mechanism to store
identifying information about the client in future requests. identifying information about the client in future requests.
Clients implementing privacy modes, such as Google Chrome's Clients implementing privacy modes can disable client-persisted
"incognito mode", may wish to disable client-persisted SETTINGS SETTINGS storage.
storage.
Clients MUST clear persisted SETTINGS information when clearing the Clients MUST clear persisted SETTINGS information when clearing the
cookies. cookies.
TODO: Put range maximums on each type of setting to limit 8. IANA Considerations
inappropriate uses.
8. Requirements Notation This document establishes registries for frame types, error codes and
settings.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 8.1. Frame Type Registry
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. 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 Type | Name | Flags |
+------------+------------------+---------------------+
| 0 | DATA | - |
| 1 | HEADERS+PRIORITY | - |
| 3 | RST_STREAM | - |
| 4 | SETTINGS | CLEAR_PERSISTED(2) |
| 5 | PUSH_PROMISE | - |
| 6 | PING | PONG(2) |
| 7 | GOAWAY | - |
| 8 | HEADERS | - |
| 9 | WINDOW_UPDATE | END_FLOW_CONTROL(2) |
+------------+------------------+---------------------+
Table 1
8.2. 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.
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 3.5.3.
8.3. 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 3.7.4.
9. Acknowledgements 9. Acknowledgements
This document includes substantial input from the following This document includes substantial input from the following
individuals: individuals:
o Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa o Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa
Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam
Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay, Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay,
Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors). Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors).
o Gabriel Montenegro and Willy Tarreau (Upgrade mechanism) o Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)
o William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, o William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro,
Jitu Padhye, Roberto Peon, Rob Trace (Flow control principles) Jitu Padhye, Roberto Peon, Rob Trace (Flow control)
o Mark Nottingham and Julian Reschke o Mark Nottingham and Julian Reschke
10. Normative References 10. References
10.1. Normative References
[ASCII] "US-ASCII. Coded Character Set - 7-Bit American [HTTP-p1] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
Standard Code for Information Interchange. (HTTP/1.1): Message Syntax and Routing",
Standard ANSI X3.4-1986, ANSI, 1986.". draft-ietf-httpbis-p1-messaging-22 (work in progress),
February 2013.
[HTTP-p1] Fielding, R. and J. Reschke, "Hypertext Transfer [HTTP-p2] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
Protocol (HTTP/1.1): Message Syntax and Routing", (HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p1-messaging-21 (work in draft-ietf-httpbis-p2-semantics-22 (work in progress),
progress), October 2012. February 2013.
[HTTP-p2] Fielding, R. and J. Reschke, "Hypertext Transfer [HTTP-p4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", Protocol (HTTP/1.1): Conditional Requests",
draft-ietf-httpbis-p2-semantics-21 (work in draft-ietf-httpbis-p4-conditional-22 (work in progress),
progress), October 2012. February 2013.
[RFC0793] Postel, J., "Transmission Control Protocol", [HTTP-p5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
STD 7, RFC 793, September 1981. "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
draft-ietf-httpbis-p5-range-22 (work in progress),
February 2013.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, [HTTP-p6] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
"Uniform Resource Locators (URL)", RFC 1738, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
December 1994. draft-ietf-httpbis-p6-cache-22 (work in progress),
February 2013.
[RFC1950] Deutsch, L. and J. Gailly, "ZLIB Compressed Data [HTTP-p7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Format Specification version 3.3", RFC 1950, Protocol (HTTP/1.1): Authentication",
May 1996. draft-ietf-httpbis-p7-auth-22 (work in progress),
February 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
Indicate Requirement Levels", BCP 14, RFC 2119, RFC 793, September 1981.
March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Masinter, L., Leach, P., and T. Berners-Lee, Requirement Levels", BCP 14, RFC 2119, March 1997.
"Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Lawrence, S., Leach, P., Luotonen, A., and L. Resource Identifier (URI): Generic Syntax", STD 66,
Stewart, "HTTP Authentication: Basic and Digest RFC 3986, January 2005.
Access Authentication", RFC 2617, June 1999.
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO- [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
based Kerberos and NTLM HTTP Authentication in IANA Considerations Section in RFCs", BCP 26, RFC 5226,
Microsoft Windows", RFC 4559, June 2006. May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
Security (TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246, August 2008.
August 2008.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011. December 2011.
[TLSNPN] Langley, A., "TLS Next Protocol Negotiation", [TLSNPN] Langley, A., "Transport Layer Security (TLS) Next Protocol
draft-agl-tls-nextprotoneg-01 (work in progress), Negotiation Extension", draft-agl-tls-nextprotoneg-04
August 2010. (work in progress), May 2012.
[UDELCOMPRESSION] Yang, F., Amer, P., and J. Leighton, "A 10.2. Informative References
Methodology to Derive SPDY's Initial Dictionary
for Zlib Compression", <http://www.eecis.udel.edu/ [RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
~amer/PEL/poc/pdf/SPDY-Fan.pdf>. 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) Appendix A. Change Log (to be removed by RFC Editor before publication)
A.1. Since draft-ietf-httpbis-http2-00 A.1. Since draft-ietf-httpbis-http2-01
Added IANA considerations section for frame types, error codes and
settings.
Removed data frame compression.
Added PUSH_PROMISE.
Added globally applicable flags to framing.
Removed zlib-based header compression mechanism.
Updated references.
Clarified stream identifier reuse.
Removed CREDENTIALS frame and associated mechanisms.
Added advice against naive implementation of flow control.
Added session header section.
Restructured frame header. Removed distinction between data and
control frames.
Altered flow control properties to include session-level limits.
Added note on cacheability of pushed resources and multiple tenant
servers.
Changed protocol label form based on discussions.
A.2. Since draft-ietf-httpbis-http2-00
Changed title throughout. Changed title throughout.
Removed section on Incompatibilities with SPDY draft#2. Removed section on Incompatibilities with SPDY draft#2.
Changed INTERNAL_ERROR on GOAWAY to have a value of 2 <https:// Changed INTERNAL_ERROR on GOAWAY to have a value of 2 <https://
groups.google.com/forum/?fromgroups#!topic/spdy-dev/cfUef2gL3iU>. groups.google.com/forum/?fromgroups#!topic/spdy-dev/cfUef2gL3iU>.
Replaced abstract and introduction. Replaced abstract and introduction.
Added section on starting HTTP/2.0, including upgrade mechanism. Added section on starting HTTP/2.0, including upgrade mechanism.
Removed unused references. Removed unused references.
Added flow control principles (Section 3.5.1) based on <http:// Added flow control principles (Section 3.6.1) based on <http://
tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>. tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>.
A.2. Since draft-mbelshe-httpbis-spdy-00 A.3. Since draft-mbelshe-httpbis-spdy-00
Adopted as base for draft-ietf-httpbis-http2. Adopted as base for draft-ietf-httpbis-http2.
Updated authors/editors list. Updated authors/editors list.
Added status note. Added status note.
Authors' Addresses Authors' Addresses
Mike Belshe Mike Belshe
 End of changes. 319 change blocks. 
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