draft-ietf-httpbis-http2-10.txt   draft-ietf-httpbis-http2-11.txt 
HTTPbis Working Group M. Belshe HTTPbis Working Group M. Belshe
Internet-Draft Twist Internet-Draft Twist
Intended status: Standards Track R. Peon Intended status: Standards Track R. Peon
Expires: August 17, 2014 Google, Inc Expires: October 5, 2014 Google, Inc
M. Thomson, Ed. M. Thomson, Ed.
Mozilla Mozilla
February 13, 2014 April 3, 2014
Hypertext Transfer Protocol version 2 Hypertext Transfer Protocol version 2
draft-ietf-httpbis-http2-10 draft-ietf-httpbis-http2-11
Abstract Abstract
This specification describes an optimized expression of the syntax of This specification describes an optimized expression of the syntax of
the Hypertext Transfer Protocol (HTTP). HTTP/2 enables a more the Hypertext Transfer Protocol (HTTP). HTTP/2 enables a more
efficient use of network resources and a reduced perception of efficient use of network resources and a reduced perception of
latency by introducing header field compression and allowing multiple latency by introducing header field compression and allowing multiple
concurrent messages on the same connection. It also introduces concurrent messages on the same connection. It also introduces
unsolicited push of representations from servers to clients. unsolicited push of representations from servers to clients.
This document is an alternative to, but does not obsolete, the This document is an alternative to, but does not obsolete, the
HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged. HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
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/>.
Working Group information and related documents can be found at Working Group information can be found at
<http://tools.ietf.org/wg/httpbis/> (Wiki) and <http://tools.ietf.org/wg/httpbis/>; that specific to HTTP/2 are at
<https://github.com/http2/http2-spec> (source code and issues <http://http2.github.io/>.
tracker).
The changes in this draft are summarized in Appendix A. The changes in this draft are summarized in Appendix A.
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
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 17, 2014. This Internet-Draft will expire on October 5, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Document Organization . . . . . . . . . . . . . . . . . . 5 2. HTTP/2 Protocol Overview . . . . . . . . . . . . . . . . . . . 5
1.2. Conventions and Terminology . . . . . . . . . . . . . . . 6 2.1. Document Organization . . . . . . . . . . . . . . . . . . 6
2. HTTP/2 Protocol Overview . . . . . . . . . . . . . . . . . . . 7 2.2. Conventions and Terminology . . . . . . . . . . . . . . . 7
2.1. HTTP Frames . . . . . . . . . . . . . . . . . . . . . . . 7 3. Starting HTTP/2 . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. HTTP Multiplexing . . . . . . . . . . . . . . . . . . . . 7
2.3. HTTP Semantics . . . . . . . . . . . . . . . . . . . . . . 7
3. Starting HTTP/2 . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. HTTP/2 Version Identification . . . . . . . . . . . . . . 8 3.1. HTTP/2 Version Identification . . . . . . . . . . . . . . 8
3.2. Starting HTTP/2 for "http" URIs . . . . . . . . . . . . . 9 3.2. Starting HTTP/2 for "http" URIs . . . . . . . . . . . . . 9
3.2.1. HTTP2-Settings Header Field . . . . . . . . . . . . . 10 3.2.1. HTTP2-Settings Header Field . . . . . . . . . . . . . 10
3.3. Starting HTTP/2 for "https" URIs . . . . . . . . . . . . . 11 3.3. Starting HTTP/2 for "https" URIs . . . . . . . . . . . . . 11
3.4. Starting HTTP/2 with Prior Knowledge . . . . . . . . . . . 11 3.4. Starting HTTP/2 with Prior Knowledge . . . . . . . . . . . 11
3.5. HTTP/2 Connection Header . . . . . . . . . . . . . . . . . 11 3.5. HTTP/2 Connection Preface . . . . . . . . . . . . . . . . 11
4. HTTP Frames . . . . . . . . . . . . . . . . . . . . . . . . . 12 4. HTTP Frames . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Frame Format . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Frame Format . . . . . . . . . . . . . . . . . . . . . . . 12
4.2. Frame Size . . . . . . . . . . . . . . . . . . . . . . . . 13 4.2. Frame Size . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Header Compression and Decompression . . . . . . . . . . . 14 4.3. Header Compression and Decompression . . . . . . . . . . . 14
5. Streams and Multiplexing . . . . . . . . . . . . . . . . . . . 15 5. Streams and Multiplexing . . . . . . . . . . . . . . . . . . . 15
5.1. Stream States . . . . . . . . . . . . . . . . . . . . . . 15 5.1. Stream States . . . . . . . . . . . . . . . . . . . . . . 16
5.1.1. Stream Identifiers . . . . . . . . . . . . . . . . . . 20 5.1.1. Stream Identifiers . . . . . . . . . . . . . . . . . . 20
5.1.2. Stream Concurrency . . . . . . . . . . . . . . . . . . 20 5.1.2. Stream Concurrency . . . . . . . . . . . . . . . . . . 20
5.2. Flow Control . . . . . . . . . . . . . . . . . . . . . . . 21 5.2. Flow Control . . . . . . . . . . . . . . . . . . . . . . . 21
5.2.1. Flow Control Principles . . . . . . . . . . . . . . . 21 5.2.1. Flow Control Principles . . . . . . . . . . . . . . . 21
5.2.2. Appropriate Use of Flow Control . . . . . . . . . . . 22 5.2.2. Appropriate Use of Flow Control . . . . . . . . . . . 22
5.3. Stream priority . . . . . . . . . . . . . . . . . . . . . 23
5.3.1. Priority Groups and Weighting . . . . . . . . . . . . 23
5.3.2. Stream Dependencies . . . . . . . . . . . . . . . . . 24
5.3.3. Reprioritization . . . . . . . . . . . . . . . . . . . 25
5.3.4. Prioritization State Management . . . . . . . . . . . 25
5.3.5. Default Priorities . . . . . . . . . . . . . . . . . . 26
5.4. Error Handling . . . . . . . . . . . . . . . . . . . . . . 26
5.4.1. Connection Error Handling . . . . . . . . . . . . . . 27
5.4.2. Stream Error Handling . . . . . . . . . . . . . . . . 27
5.4.3. Connection Termination . . . . . . . . . . . . . . . . 28
6. Frame Definitions . . . . . . . . . . . . . . . . . . . . . . 28
6.1. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
6.2. HEADERS . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.3. PRIORITY . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.4. RST_STREAM . . . . . . . . . . . . . . . . . . . . . . . . 34
6.5. SETTINGS . . . . . . . . . . . . . . . . . . . . . . . . . 35
6.5.1. SETTINGS Format . . . . . . . . . . . . . . . . . . . 36
6.5.2. Defined SETTINGS Parameters . . . . . . . . . . . . . 36
6.5.3. Settings Synchronization . . . . . . . . . . . . . . . 37
6.6. PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . . . 37
6.7. PING . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6.8. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.9. WINDOW_UPDATE . . . . . . . . . . . . . . . . . . . . . . 43
6.9.1. The Flow Control Window . . . . . . . . . . . . . . . 44
6.9.2. Initial Flow Control Window Size . . . . . . . . . . . 45
6.9.3. Reducing the Stream Window Size . . . . . . . . . . . 46
6.10. CONTINUATION . . . . . . . . . . . . . . . . . . . . . . . 46
6.11. ALTSVC . . . . . . . . . . . . . . . . . . . . . . . . . . 47
7. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . 49
8. HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 50
8.1. HTTP Request/Response Exchange . . . . . . . . . . . . . . 51
8.1.1. Informational Responses . . . . . . . . . . . . . . . 52
8.1.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 53
8.1.3. HTTP Header Fields . . . . . . . . . . . . . . . . . . 55
8.1.4. Request Reliability Mechanisms in HTTP/2 . . . . . . . 59
8.2. Server Push . . . . . . . . . . . . . . . . . . . . . . . 60
8.2.1. Push Requests . . . . . . . . . . . . . . . . . . . . 60
8.2.2. Push Responses . . . . . . . . . . . . . . . . . . . . 61
8.3. The CONNECT Method . . . . . . . . . . . . . . . . . . . . 62
9. Additional HTTP Requirements/Considerations . . . . . . . . . 63
9.1. Connection Management . . . . . . . . . . . . . . . . . . 63
9.2. Use of TLS Features . . . . . . . . . . . . . . . . . . . 64
9.3. GZip Content-Encoding . . . . . . . . . . . . . . . . . . 65
10. Security Considerations . . . . . . . . . . . . . . . . . . . 65
10.1. Server Authority . . . . . . . . . . . . . . . . . . . . . 65
10.2. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 66
10.3. Intermediary Encapsulation Attacks . . . . . . . . . . . . 66
10.4. Cacheability of Pushed Responses . . . . . . . . . . . . . 67
10.5. Denial of Service Considerations . . . . . . . . . . . . . 67
10.6. Use of Compression . . . . . . . . . . . . . . . . . . . . 68
10.7. Use of Padding . . . . . . . . . . . . . . . . . . . . . . 68
10.8. Privacy Considerations . . . . . . . . . . . . . . . . . . 69
5.3. Stream priority . . . . . . . . . . . . . . . . . . . . . 22 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 69
5.4. Error Handling . . . . . . . . . . . . . . . . . . . . . . 23 11.1. Registration of HTTP/2 Identification String . . . . . . . 70
5.4.1. Connection Error Handling . . . . . . . . . . . . . . 23 11.2. Error Code Registry . . . . . . . . . . . . . . . . . . . 70
5.4.2. Stream Error Handling . . . . . . . . . . . . . . . . 24 11.3. HTTP2-Settings Header Field Registration . . . . . . . . . 71
5.4.3. Connection Termination . . . . . . . . . . . . . . . . 24 11.4. PRI Method Registration . . . . . . . . . . . . . . . . . 71
6. Frame Definitions . . . . . . . . . . . . . . . . . . . . . . 24 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 71
6.1. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.2. HEADERS . . . . . . . . . . . . . . . . . . . . . . . . . 27 13.1. Normative References . . . . . . . . . . . . . . . . . . . 72
6.3. PRIORITY . . . . . . . . . . . . . . . . . . . . . . . . . 29 13.2. Informative References . . . . . . . . . . . . . . . . . . 74
6.4. RST_STREAM . . . . . . . . . . . . . . . . . . . . . . . . 29
6.5. SETTINGS . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.5.1. Setting Format . . . . . . . . . . . . . . . . . . . . 31
6.5.2. Defined Settings . . . . . . . . . . . . . . . . . . . 32
6.5.3. Settings Synchronization . . . . . . . . . . . . . . . 33
6.6. PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . . . 33
6.7. PING . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6.8. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6.9. WINDOW_UPDATE . . . . . . . . . . . . . . . . . . . . . . 38
6.9.1. The Flow Control Window . . . . . . . . . . . . . . . 39
6.9.2. Initial Flow Control Window Size . . . . . . . . . . . 40
6.9.3. Reducing the Stream Window Size . . . . . . . . . . . 40
6.10. CONTINUATION . . . . . . . . . . . . . . . . . . . . . . . 41
7. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . 42
8. HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 44
8.1. HTTP Request/Response Exchange . . . . . . . . . . . . . . 44
8.1.1. Informational Responses . . . . . . . . . . . . . . . 45
8.1.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 46
8.1.3. HTTP Header Fields . . . . . . . . . . . . . . . . . . 48
8.1.4. Request Reliability Mechanisms in HTTP/2 . . . . . . . 51
8.2. Server Push . . . . . . . . . . . . . . . . . . . . . . . 52
8.2.1. Push Requests . . . . . . . . . . . . . . . . . . . . 53
8.2.2. Push Responses . . . . . . . . . . . . . . . . . . . . 54
8.3. The CONNECT Method . . . . . . . . . . . . . . . . . . . . 54
9. Additional HTTP Requirements/Considerations . . . . . . . . . 56
9.1. Connection Management . . . . . . . . . . . . . . . . . . 56
9.2. Use of TLS Features . . . . . . . . . . . . . . . . . . . 56
9.3. GZip Content-Encoding . . . . . . . . . . . . . . . . . . 57
10. Security Considerations . . . . . . . . . . . . . . . . . . . 57
10.1. Server Authority and Same-Origin . . . . . . . . . . . . . 57
10.2. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 58
10.3. Intermediary Encapsulation Attacks . . . . . . . . . . . . 58
10.4. Cacheability of Pushed Resources . . . . . . . . . . . . . 58
10.5. Denial of Service Considerations . . . . . . . . . . . . . 59
10.6. Use of Padding . . . . . . . . . . . . . . . . . . . . . . 60
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 60
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 60
12.1. Registration of HTTP/2 Identification String . . . . . . . 61
12.2. Error Code Registry . . . . . . . . . . . . . . . . . . . 61
12.3. HTTP2-Settings Header Field Registration . . . . . . . . . 62
12.4. PRI Method Registration . . . . . . . . . . . . . . . . . 62
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 62
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 63
14.1. Normative References . . . . . . . . . . . . . . . . . . . 63
14.2. Informative References . . . . . . . . . . . . . . . . . . 65
Appendix A. Change Log (to be removed by RFC Editor before Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 65 publication) . . . . . . . . . . . . . . . . . . . . 74
A.1. Since draft-ietf-httpbis-http2-09 . . . . . . . . . . . . 65 A.1. Since draft-ietf-httpbis-http2-10 . . . . . . . . . . . . 74
A.2. Since draft-ietf-httpbis-http2-08 . . . . . . . . . . . . 66 A.2. Since draft-ietf-httpbis-http2-09 . . . . . . . . . . . . 75
A.3. Since draft-ietf-httpbis-http2-07 . . . . . . . . . . . . 66 A.3. Since draft-ietf-httpbis-http2-08 . . . . . . . . . . . . 75
A.4. Since draft-ietf-httpbis-http2-06 . . . . . . . . . . . . 66 A.4. Since draft-ietf-httpbis-http2-07 . . . . . . . . . . . . 75
A.5. Since draft-ietf-httpbis-http2-05 . . . . . . . . . . . . 66 A.5. Since draft-ietf-httpbis-http2-06 . . . . . . . . . . . . 75
A.6. Since draft-ietf-httpbis-http2-04 . . . . . . . . . . . . 67 A.6. Since draft-ietf-httpbis-http2-05 . . . . . . . . . . . . 76
A.7. Since draft-ietf-httpbis-http2-03 . . . . . . . . . . . . 67 A.7. Since draft-ietf-httpbis-http2-04 . . . . . . . . . . . . 76
A.8. Since draft-ietf-httpbis-http2-02 . . . . . . . . . . . . 67 A.8. Since draft-ietf-httpbis-http2-03 . . . . . . . . . . . . 76
A.9. Since draft-ietf-httpbis-http2-01 . . . . . . . . . . . . 68 A.9. Since draft-ietf-httpbis-http2-02 . . . . . . . . . . . . 77
A.10. Since draft-ietf-httpbis-http2-00 . . . . . . . . . . . . 68 A.10. Since draft-ietf-httpbis-http2-01 . . . . . . . . . . . . 77
A.11. Since draft-mbelshe-httpbis-spdy-00 . . . . . . . . . . . 69 A.11. Since draft-ietf-httpbis-http2-00 . . . . . . . . . . . . 78
A.12. Since draft-mbelshe-httpbis-spdy-00 . . . . . . . . . . . 78
1. Introduction 1. Introduction
The Hypertext Transfer Protocol (HTTP) is a wildly successful The Hypertext Transfer Protocol (HTTP) is a wildly successful
protocol. However, the HTTP/1.1 message format ([HTTP-p1], Section protocol. However, the HTTP/1.1 message format ([HTTP-p1], Section
3) is optimized for implementation simplicity and accessibility, not 3) was designed to be implemented with the tools at hand in the
application performance. As such it has several characteristics that 1990s, not modern Web application performance. As such it has
have a negative overall effect on application performance. several characteristics that have a negative overall effect on
application performance today.
In particular, HTTP/1.0 only allows one request to be outstanding at In particular, HTTP/1.0 only allows one request to be outstanding at
a time on a given connection. HTTP/1.1 pipelining only partially a time on a given connection. HTTP/1.1 pipelining only partially
addressed request concurrency and suffers from head-of-line blocking. addressed request concurrency and suffers from head-of-line blocking.
Therefore, clients that need to make many requests typically use Therefore, clients that need to make many requests typically use
multiple connections to a server in order to reduce latency. multiple connections to a server in order to reduce latency.
Furthermore, HTTP/1.1 header fields are often repetitive and verbose, Furthermore, HTTP/1.1 header fields are often repetitive and verbose,
which, in addition to generating more or larger network packets, can which, in addition to generating more or larger network packets, can
cause the small initial TCP congestion window to quickly fill. This cause the small initial TCP congestion window to quickly fill. This
skipping to change at page 5, line 33 skipping to change at page 5, line 34
single new TCP connection. single new TCP connection.
This document addresses these issues by defining an optimized mapping This document addresses these issues by defining an optimized mapping
of HTTP's semantics to an underlying connection. Specifically, it of HTTP's semantics to an underlying connection. Specifically, it
allows interleaving of request and response messages on the same allows interleaving of request and response messages on the same
connection and uses an efficient coding for HTTP header fields. It connection and uses an efficient coding for HTTP header fields. It
also allows prioritization of requests, letting more important also allows prioritization of requests, letting more important
requests complete more quickly, further improving performance. requests complete more quickly, further improving performance.
The resulting protocol is designed to be more friendly to the The resulting protocol is designed to be more friendly to the
network, because fewer TCP connections can be used, in comparison to network, because fewer TCP connections can be used in comparison to
HTTP/1.x. This means less competition with other flows, and longer- HTTP/1.x. This means less competition with other flows, and longer-
lived connections, which in turn leads to better utilization of lived connections, which in turn leads to better utilization of
available network capacity. available network capacity.
Finally, this encapsulation also enables more scalable processing of Finally, this encapsulation also enables more scalable processing of
messages through use of binary message framing. messages through use of binary message framing.
1.1. Document Organization 2. HTTP/2 Protocol Overview
HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2
supports all of the core features of HTTP/1.1, but aims to be more
efficient in several ways.
The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each
frame has a different type and purpose. For example, HEADERS and
DATA frames form the basis of HTTP requests and responses
(Section 8.1); other frame types like SETTINGS, WINDOW_UPDATE, and
PUSH_PROMISE are used in support of other HTTP/2 features.
Multiplexing of requests is achieved by having each HTTP request-
response exchanged assigned to a single stream (Section 5). Streams
are largely independent of each other, so a blocked or stalled
request does not prevent progress on other requests.
Flow control and prioritization ensure that it is possible to
properly use multiplexed streams. Flow control (Section 5.2) helps
to ensure that only data that can be used by a receiver is
transmitted. Prioritization (Section 5.3) ensures that limited
resources can be directed to the most important requests first.
HTTP/2 adds a new interaction mode, whereby a server can push
responses to a client (Section 8.2). Server push allows a server to
speculatively send a client data that the server anticipates the
client will need, trading off some network usage against a potential
latency gain. The server does this by synthesizing a request, which
it sends as a PUSH_PROMISE frame. The server is then able to send a
response to the synthetic request on an separate stream.
Frames that contain HTTP header fields are compressed (Section 4.3).
HTTP requests can be highly redundant, so compression can reduce the
size of requests and responses significantly.
HTTP/2 also supports HTTP Alternative Services (see [ALT-SVC]) using
the ALTSVC frame type (Section 6.11), to allow servers more control
over traffic to them.
2.1. Document Organization
The HTTP/2 specification is split into four parts: The HTTP/2 specification is split into four parts:
o Starting HTTP/2 (Section 3) covers how a HTTP/2 connection is o Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is
initiated. initiated.
o The framing (Section 4) and streams (Section 5) layers describe o The framing (Section 4) and streams (Section 5) layers describe
the way HTTP/2 frames are structured and formed into multiplexed the way HTTP/2 frames are structured and formed into multiplexed
streams. streams.
o Frame (Section 6) and error (Section 7) definitions include o Frame (Section 6) and error (Section 7) definitions include
details of the frame and error types used in HTTP/2. details of the frame and error types used in HTTP/2.
o HTTP mappings (Section 8) and additional requirements (Section 9) o HTTP mappings (Section 8) and additional requirements (Section 9)
describe how HTTP semantics are expressed using the mechanisms describe how HTTP semantics are expressed using frames and
defined. streams.
While some of the frame and stream layer concepts are isolated from While some of the frame and stream layer concepts are isolated from
HTTP, the intent is not to define a completely generic framing layer. HTTP, the intent is not to define a completely generic framing layer.
The framing and streams layers are tailored to the needs of the HTTP The framing and streams layers are tailored to the needs of the HTTP
protocol and server push. protocol and server push.
1.2. Conventions and Terminology 2.2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
All numeric values are in network byte order. Values are unsigned All numeric values are in network byte order. Values are unsigned
unless otherwise indicated. Literal values are provided in decimal unless otherwise indicated. Literal values are provided in decimal
or hexadecimal as appropriate. Hexadecimal literals are prefixed or hexadecimal as appropriate. Hexadecimal literals are prefixed
with "0x" to distinguish them from decimal literals. with "0x" to distinguish them from decimal literals.
skipping to change at page 6, line 43 skipping to change at page 7, line 32
connection error: An error that affects the entire HTTP/2 connection error: An error that affects the entire HTTP/2
connection. connection.
endpoint: Either the client or server of the connection. endpoint: Either the client or server of the connection.
frame: The smallest unit of communication within an HTTP/2 frame: The smallest unit of communication within an HTTP/2
connection, consisting of a header and a variable-length sequence connection, consisting of a header and a variable-length sequence
of bytes structured according to the frame type. of bytes structured according to the frame type.
intermediary: A "proxy", "gateway" or other intermediary as defined
in Section 2.3 of [HTTP-p1].
peer: An endpoint. When discussing a particular endpoint, "peer" peer: An endpoint. When discussing a particular endpoint, "peer"
refers to the endpoint that is remote to the primary subject of refers to the endpoint that is remote to the primary subject of
discussion. discussion.
receiver: An endpoint that is receiving frames. receiver: An endpoint that is receiving frames.
sender: An endpoint that is transmitting frames. sender: An endpoint that is transmitting frames.
server: The endpoint which did not initiate the HTTP/2 connection. server: The endpoint which did not initiate the HTTP/2 connection.
stream: A bi-directional flow of frames across a virtual channel stream: A bi-directional flow of frames across a virtual channel
within the HTTP/2 connection. within the HTTP/2 connection.
stream error: An error on the individual HTTP/2 stream. stream error: An error on the individual HTTP/2 stream.
2. HTTP/2 Protocol Overview 3. Starting HTTP/2
HTTP/2 provides an optimized transport for HTTP semantics.
An HTTP/2 connection is an application level protocol running on top An HTTP/2 connection is an application level protocol running on top
of a TCP connection ([TCP]). The client is the TCP connection of a TCP connection ([TCP]). The client is the TCP connection
initiator. initiator.
This document describes the HTTP/2 protocol using a logical structure
that is formed of three parts: framing, streams, and application
mapping. This structure is provided primarily as an aid to
specification, implementations are free to diverge from this
structure as necessary.
2.1. HTTP Frames
HTTP/2 provides an efficient serialization of HTTP semantics. HTTP
requests and responses are encoded into length-prefixed frames (see
Section 4.1).
HTTP header fields are compressed into a series of frames that
contain header block fragments (see Section 4.3).
2.2. HTTP Multiplexing
HTTP/2 provides the ability to multiplex HTTP requests and responses
over a single connection. Multiple requests or responses can be sent
concurrently on a connection using streams (Section 5). In order to
maintain independent streams, flow control and prioritization are
necessary.
2.3. HTTP Semantics
HTTP/2 defines how HTTP requests and responses are mapped to streams
(see Section 8.1) and introduces a new interaction model, server push
(Section 8.2).
3. Starting HTTP/2
HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1. HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1.
HTTP/2 shares the same default port numbers: 80 for "http" URIs and HTTP/2 shares the same default port numbers: 80 for "http" URIs and
443 for "https" URIs. As a result, implementations processing 443 for "https" URIs. As a result, implementations processing
requests for target resource URIs like "http://example.org/foo" or requests for target resource URIs like "http://example.org/foo" or
"https://example.com/bar" are required to first discover whether the "https://example.com/bar" are required to first discover whether the
upstream server (the immediate peer to which the client wishes to upstream server (the immediate peer to which the client wishes to
establish a connection) supports HTTP/2. establish a connection) supports HTTP/2.
The means by which support for HTTP/2 is determined is different for The means by which support for HTTP/2 is determined is different for
"http" and "https" URIs. Discovery for "http" URIs is described in "http" and "https" URIs. Discovery for "http" URIs is described in
Section 3.2. Discovery for "https" URIs is described in Section 3.3. Section 3.2. Discovery for "https" URIs is described in Section 3.3.
3.1. HTTP/2 Version Identification 3.1. HTTP/2 Version Identification
The protocol defined in this document is identified using the string The protocol defined in this document has two identifiers.
"h2". This identification is used in the HTTP/1.1 Upgrade header
field, in the TLS application layer protocol negotiation extension
[TLSALPN] field, and other places where protocol identification is
required.
Negotiating "h2" implies the use of the transport, security, framing o The string "h2" identifies the protocol where HTTP/2 uses TLS
and message semantics described in this document. [TLS12]. This identifier is used in the TLS application layer
protocol negotiation extension [TLSALPN] field and any place that
HTTP/2 over TLS is identified.
[[anchor6: Editor's Note: please remove the remainder of this section When serialised into an ALPN protocol identifier (which is a
prior to the publication of a final version of this document.]] sequence of octets), the HTTP/2 protocol identifier string is
encoded using UTF-8 [UTF-8].
o The string "h2c" identifies the protocol where HTTP/2 is run over
cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade
header field and any place that HTTP/2 over TCP is identified.
Negotiating "h2" or "h2c" implies the use of the transport, security,
framing and message semantics described in this document.
[[anchor3: RFC Editor's Note: please remove the remainder of this
section prior to the publication of a final version of this
document.]]
Only implementations of the final, published RFC can identify Only implementations of the final, published RFC can identify
themselves as "h2". Until such an RFC exists, implementations MUST themselves as "h2" or "h2c". Until such an RFC exists,
NOT identify themselves using "h2". implementations MUST NOT identify themselves using these strings.
Examples and text throughout the rest of this document use "h2" as a Examples and text throughout the rest of this document use "h2" as a
matter of editorial convenience only. Implementations of draft 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 string Implementations of draft versions of the protocol MUST add the string
"-" and the corresponding draft number to the identifier. For "-" and the corresponding draft number to the identifier. For
example, draft-ietf-httpbis-http2-09 is identified using the string example, draft-ietf-httpbis-http2-11 over TLS is identified using the
"h2-09". string "h2-11".
Non-compatible experiments that are based on these draft versions Non-compatible experiments that are based on these draft versions
MUST append the string "-" and a experiment name to the identifier. MUST append the string "-" and an experiment name to the identifier.
For example, an experimental implementation of packet mood-based For example, an experimental implementation of packet mood-based
encoding based on draft-ietf-httpbis-http2-09 might identify itself encoding based on draft-ietf-httpbis-http2-09 might identify itself
as "h2-09-emo". Note that any label MUST conform to the "token" as "h2-09-emo". Note that any label MUST conform to the "token"
syntax defined in Section 3.2.6 of [HTTP-p1]. Experimenters are syntax defined in Section 3.2.6 of [HTTP-p1]. Experimenters are
encouraged to coordinate their experiments on the ietf-http-wg@w3.org encouraged to coordinate their experiments on the ietf-http-wg@w3.org
mailing list. mailing list.
3.2. Starting HTTP/2 for "http" URIs 3.2. Starting HTTP/2 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 uses the HTTP Upgrade mechanism knowledge about support for HTTP/2 uses the HTTP Upgrade mechanism
(Section 6.7 of [HTTP-p1]). The client makes an HTTP/1.1 request (Section 6.7 of [HTTP-p1]). The client makes an HTTP/1.1 request
that includes an Upgrade header field identifying HTTP/2 with the h2 that includes an Upgrade header field identifying HTTP/2 with the
token. The HTTP/1.1 request MUST include exactly one HTTP2-Settings "h2c" token. The HTTP/1.1 request MUST include exactly one HTTP2-
(Section 3.2.1) header field. Settings (Section 3.2.1) header field.
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, HTTP2-Settings Connection: Upgrade, HTTP2-Settings
Upgrade: h2 Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload> HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
Requests that contain an entity body MUST be sent in their entirety Requests that contain an entity body MUST be sent in their entirety
before the client can send HTTP/2 frames. This means that a large before the client can send HTTP/2 frames. This means that a large
request entity can block the use of the connection until it is request entity can block the use of the connection until it is
completely sent. completely sent.
If concurrency of an initial request with subsequent requests is If concurrency of an initial request with subsequent requests is
important, a small request can be used to perform the upgrade to important, a small request can be used to perform the upgrade to
HTTP/2, at the cost of an additional round-trip. HTTP/2, at the cost of an additional round-trip.
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Upgrade. Upgrade.
HTTP/1.1 101 Switching Protocols HTTP/1.1 101 Switching Protocols
Connection: Upgrade Connection: Upgrade
Upgrade: h2 Upgrade: h2
[ HTTP/2 connection ... [ HTTP/2 connection ...
The first HTTP/2 frame sent by the server is a SETTINGS frame The first HTTP/2 frame sent by the server is a SETTINGS frame
(Section 6.5). Upon receiving the 101 response, the client sends a (Section 6.5). Upon receiving the 101 response, the client sends a
connection header (Section 3.5), which includes a SETTINGS frame. connection preface (Section 3.5), which includes a SETTINGS frame.
The HTTP/1.1 request that is sent prior to upgrade is assigned stream The HTTP/1.1 request that is sent prior to upgrade is assigned stream
identifier 1 and is assigned the highest possible priority. Stream 1 identifier 1 and is assigned default priority values (Section 5.3.5).
is implicitly half closed from the client toward the server, since Stream 1 is implicitly half closed from the client toward the server,
the request is completed as an HTTP/1.1 request. After commencing since the request is completed as an HTTP/1.1 request. After
the HTTP/2 connection, stream 1 is used for the response. commencing the HTTP/2 connection, stream 1 is used for the response.
3.2.1. HTTP2-Settings Header Field 3.2.1. HTTP2-Settings Header Field
A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly
one "HTTP2-Settings" header field. The "HTTP2-Settings" header field one "HTTP2-Settings" header field. The "HTTP2-Settings" header field
is a hop-by-hop header field that includes settings that govern the is a hop-by-hop header field that includes parameters that govern the
HTTP/2 connection, provided in anticipation of the server accepting HTTP/2 connection, provided in anticipation of the server accepting
the request to upgrade. A server MUST reject an attempt to upgrade the request to upgrade. A server MUST reject an attempt to upgrade
if this header field is not present. if this header field is not present.
HTTP2-Settings = token68 HTTP2-Settings = token68
The content of the "HTTP2-Settings" header field is the payload of a The content of the "HTTP2-Settings" header field is the payload of a
SETTINGS frame (Section 6.5), encoded as a base64url string (that is, SETTINGS frame (Section 6.5), encoded as a base64url string (that is,
the URL- and filename-safe Base64 encoding described in Section 5 of the URL- and filename-safe Base64 encoding described in Section 5 of
[RFC4648], with any trailing '=' characters omitted). The ABNF [RFC4648], with any trailing '=' characters omitted). The ABNF
[RFC5234] production for "token68" is defined in Section 2.1 of [RFC5234] production for "token68" is defined in Section 2.1 of
[HTTP-p7]. [HTTP-p7].
The client MUST include values for the following settings
(Section 6.5.1):
o SETTINGS_MAX_CONCURRENT_STREAMS
o SETTINGS_INITIAL_WINDOW_SIZE
As a hop-by-hop header field, the "Connection" header field MUST As a hop-by-hop header field, the "Connection" header field MUST
include a value of "HTTP2-Settings" in addition to "Upgrade" when include a value of "HTTP2-Settings" in addition to "Upgrade" when
upgrading to HTTP/2. upgrading to HTTP/2.
A server decodes and interprets these values as it would any other A server decodes and interprets these values as it would any other
SETTINGS frame. Acknowledgement of the settings (Section 6.5.3) is SETTINGS frame. Acknowledgement of the SETTINGS parameters
not necessary, since a 101 response serves as implicit (Section 6.5.3) is not necessary, since a 101 response serves as
acknowledgment. Providing these values in the Upgrade request implicit acknowledgment. Providing these values in the Upgrade
ensures that the protocol does not require default values for the request ensures that the protocol does not require default values for
above settings, and gives a client an opportunity to provide other the above SETTINGS parameters, and gives a client an opportunity to
settings prior to receiving any frames from the server. provide other parameters prior to receiving any frames from the
server.
3.3. Starting HTTP/2 for "https" URIs 3.3. Starting HTTP/2 for "https" URIs
A client that makes a request to an "https" URI without prior A client that makes a request to an "https" URI without prior
knowledge about support for HTTP/2 uses TLS [TLS12] with the knowledge about support for HTTP/2 uses TLS [TLS12] with the
application layer protocol negotiation extension [TLSALPN]. application layer protocol negotiation extension [TLSALPN].
Once TLS negotiation is complete, both the client and the server send Once TLS negotiation is complete, both the client and the server send
a connection header (Section 3.5). a connection preface (Section 3.5).
3.4. Starting HTTP/2 with Prior Knowledge 3.4. Starting HTTP/2 with Prior Knowledge
A client can learn that a particular server supports HTTP/2 by other A client can learn that a particular server supports HTTP/2 by other
means. For example, [AltSvc] describes a mechanism for advertising means. For example, [ALT-SVC] describes a mechanism for advertising
this capability in an HTTP header field. A client MAY immediately this capability in an HTTP header field; the ALTSVC frame
send HTTP/2 frames to a server that is known to support HTTP/2, after (Section 6.11) describes a similar mechanism in HTTP/2.
the connection header (Section 3.5). A server can identify such a
connection by the use of the "PRI" method in the connection header. A client MAY immediately send HTTP/2 frames to a server that is known
This only affects the resolution of "http" URIs; servers supporting to support HTTP/2, after the connection preface (Section 3.5). A
HTTP/2 are required to support protocol negotiation in TLS [TLSALPN] server can identify such a connection by the use of the "PRI" method
for "https" URIs. in the connection preface. This only affects the resolution of
"http" URIs; servers supporting HTTP/2 are required to support
protocol negotiation in TLS [TLSALPN] for "https" URIs.
Prior support for HTTP/2 is not a strong signal that a given server Prior support for HTTP/2 is not a strong signal that a given server
will support HTTP/2 for future connections. It is possible for will support HTTP/2 for future connections. It is possible for
server configurations to change or for configurations to differ server configurations to change; for configurations to differ between
between instances in clustered server. Interception proxies (a.k.a. instances in clustered server; or network conditions to change.
"transparent" proxies) are another source of variability.
3.5. HTTP/2 Connection Header 3.5. HTTP/2 Connection Preface
Upon establishment of a TCP connection and determination that HTTP/2 Upon establishment of a TCP connection and determination that HTTP/2
will be used by both peers, each endpoint MUST send a connection will be used by both peers, each endpoint MUST send a connection
header as a final confirmation and to establish the initial settings preface as a final confirmation and to establish the initial SETTINGS
for the HTTP/2 connection. parameters for the HTTP/2 connection.
The client connection header starts with a sequence of 24 octets, The client connection preface starts with a sequence of 24 octets,
which in hex notation are: which in hex notation are:
505249202a20485454502f322e300d0a0d0a534d0d0a0d0a 0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
(the string "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"). This sequence is (the string "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"). This sequence is
followed by a SETTINGS frame (Section 6.5). The client sends the followed by a SETTINGS frame (Section 6.5). The SETTINGS frame MAY
client connection header immediately upon receipt of a 101 Switching be empty. The client sends the client connection preface immediately
Protocols response (indicating a successful upgrade), or as the first upon receipt of a 101 Switching Protocols response (indicating a
application data octets of a TLS connection. If starting an HTTP/2 successful upgrade), or as the first application data octets of a TLS
connection with prior knowledge of server support for the protocol, connection. If starting an HTTP/2 connection with prior knowledge of
the client connection header is sent upon connection establishment. server support for the protocol, the client connection preface is
sent upon connection establishment.
The client connection header is selected so that a large The client connection preface is selected so that a large
proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do
not attempt to process further frames. Note that this does not not attempt to process further frames. Note that this does not
address the concerns raised in [TALKING]. address the concerns raised in [TALKING].
The server connection header consists of just a SETTINGS frame The server connection preface consists of a potentially empty
(Section 6.5) that MUST be the first frame the server sends in the SETTINGS frame (Section 6.5) that MUST be the first frame the server
HTTP/2 connection. sends in the HTTP/2 connection.
To avoid unnecessary latency, clients are permitted to send To avoid unnecessary latency, clients are permitted to send
additional frames to the server immediately after sending the client additional frames to the server immediately after sending the client
connection header, without waiting to receive the server connection connection preface, without waiting to receive the server connection
header. It is important to note, however, that the server connection preface. It is important to note, however, that the server
header SETTINGS frame might include parameters that necessarily alter connection preface SETTINGS frame might include parameters that
how a client is expected to communicate with the server. Upon necessarily alter how a client is expected to communicate with the
receiving the SETTINGS frame, the client is expected to honor any server. Upon receiving the SETTINGS frame, the client is expected to
parameters established. honor any parameters established.
Clients and servers MUST terminate the TCP connection if either peer Clients and servers MUST terminate the TCP connection if either peer
does not begin with a valid connection header. A GOAWAY frame does not begin with a valid connection preface. A GOAWAY frame
(Section 6.8) MAY be omitted if it is clear that the peer is not (Section 6.8) MAY be omitted if it is clear that the peer is not
using HTTP/2. using HTTP/2.
4. HTTP Frames 4. HTTP Frames
Once the HTTP/2 connection is established, endpoints can begin Once the HTTP/2 connection is established, endpoints can begin
exchanging frames. exchanging frames.
4.1. Frame Format 4.1. Frame Format
skipping to change at page 13, line 16 skipping to change at page 13, line 30
and the bits MUST remain unset (0) when sending and MUST be and the bits MUST remain unset (0) when sending and MUST be
ignored when receiving. ignored when receiving.
Length: The length of the frame payload expressed as an unsigned 14- Length: The length of the frame payload expressed as an unsigned 14-
bit integer. The 8 octets of the frame header are not included in bit integer. The 8 octets of the frame header are not included in
this value. this value.
Type: The 8-bit type of the frame. The frame type determines how Type: The 8-bit type of the frame. The frame type determines how
the remainder of the frame header and payload are interpreted. the remainder of the frame header and payload are interpreted.
Implementations MUST treat the receipt of an unknown frame type Implementations MUST treat the receipt of an unknown frame type
(any frame type not defined in this document) as a connection (any frame types not defined in this document) as a connection
error (Section 5.4.1) of type PROTOCOL_ERROR. error (Section 5.4.1) of type PROTOCOL_ERROR.
Flags: An 8-bit field reserved for frame-type specific boolean Flags: An 8-bit field reserved for frame-type specific boolean
flags. flags.
Flags are assigned semantics specific to the indicated frame type. Flags are assigned semantics specific to the indicated frame type.
Flags that have no defined semantics for a particular frame type Flags that have no defined semantics for a particular frame type
MUST be ignored, and MUST be left unset (0) when sending. MUST be ignored, and MUST be left unset (0) when sending.
R: A reserved 1-bit field. The semantics of this bit are undefined R: A reserved 1-bit field. The semantics of this bit are undefined
skipping to change at page 13, line 40 skipping to change at page 14, line 8
Stream Identifier: A 31-bit stream identifier (see Section 5.1.1). Stream Identifier: A 31-bit stream identifier (see Section 5.1.1).
The value 0 is reserved for frames that are associated with the The value 0 is reserved for frames that are associated with the
connection as a whole as opposed to an individual stream. connection as a whole as opposed to an individual stream.
The structure and content of the frame payload is dependent entirely The structure and content of the frame payload is dependent entirely
on the frame type. on the frame type.
4.2. Frame Size 4.2. Frame Size
The maximum size of a frame payload varies by frame type. The The maximum size of a frame payload varies by frame type. The
absolute maximum size of a frame is 2^14-1 (16,383) octets. All absolute maximum size of a frame payload is 2^14-1 (16,383) octets,
meaning that the maximum frame size is 16,391 octets. All
implementations SHOULD be capable of receiving and minimally implementations SHOULD be capable of receiving and minimally
processing frames up to this maximum size. processing frames up to this maximum size.
Certain frame types, such as PING (see Section 6.7), impose Certain frame types, such as PING (see Section 6.7), impose
additional limits on the amount of payload data allowed. Likewise, additional limits on the amount of payload data allowed. Likewise,
additional size limits can be set by specific application uses (see additional size limits can be set by specific application uses (see
Section 9). Section 9).
If a frame size exceeds any defined limit, or is too small to contain If a frame size exceeds any defined limit, or is too small to contain
mandatory frame data, the endpoint MUST send a FRAME_SIZE_ERROR mandatory frame data, the endpoint MUST send a FRAME_SIZE_ERROR
error. A frame size error in a frame that affects connection-level error. A frame size error in a frame that could alter the state of
state MUST be treated as a connection error (Section 5.4.1). the entire connection MUST be treated as a connection error
(Section 5.4.1); this includes any frame carrying a header block
(Section 4.3) (that is, HEADERS, PUSH_PROMISE, and CONTINUATION),
SETTINGS, and any WINDOW_UPDATE frame with a stream identifier of 0.
4.3. Header Compression and Decompression 4.3. Header Compression and Decompression
A header field in HTTP/2 is a name-value pair with one or more A header field in HTTP/2 is a name-value pair with one or more
associated values. They are used within HTTP request and response associated values. They are used within HTTP request and response
messages as well as server push operations (see Section 8.2). messages as well as server push operations (see Section 8.2).
Header sets are collections of zero or more header fields. When Header sets are collections of zero or more header fields. When
transmitted over a connection, a header set is serialized into a transmitted over a connection, a header set is serialized into a
header block using HTTP Header Compression [COMPRESSION]. The header block using HTTP Header Compression [COMPRESSION]. The
serialized header block is then divided into one or more octet serialized header block is then divided into one or more octet
sequences, called header block fragments, and transmitted within the sequences, called header block fragments, and transmitted within the
payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6) or payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6) or
CONTINUATION (Section 6.10) frames. CONTINUATION (Section 6.10) frames.
HTTP Header Compression does not preserve the relative ordering of HTTP Header Compression does not preserve the relative ordering of
header fields. Header fields with multiple values are encoded into a header fields. Header fields with multiple values are encoded into a
single header field using a special delimiter, see Section 8.1.3.3. single header field using a special delimiter; see Section 8.1.3.3.
The Cookie header field [COOKIE] is treated specially by the HTTP The Cookie header field [COOKIE] is treated specially by the HTTP
mapping, see Section 8.1.3.4. mapping; see Section 8.1.3.4.
A receiving endpoint reassembles the header block by concatenating A receiving endpoint reassembles the header block by concatenating
the individual fragments, then decompresses the block to reconstruct its fragments, then decompresses the block to reconstruct the header
the header set. set.
A complete header block consists of either: A complete header block consists of either:
o a single HEADERS or PUSH_PROMISE frame each respectively with the o a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag
END_HEADERS or END_PUSH_PROMISE flag set, or set, or
o a HEADERS or PUSH_PROMISE frame with the END_HEADERS or o a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared
END_PUSH_PROMISE flag cleared and one or more CONTINUATION frames, and one or more CONTINUATION frames, where the last CONTINUATION
where the last CONTINUATION frame has the END_HEADERS flag set. frame has the END_HEADERS flag set.
Header blocks MUST be transmitted as a contiguous sequence of frames, Header compression is stateful, using a single compression context
with no interleaved frames of any other type, or from any other for the entire connection. Each header block is processed as a
stream. The last frame in a sequence of HEADERS or CONTINUATION discrete unit. Header blocks MUST be transmitted as a contiguous
frames MUST have the END_HEADERS flag set. The last frame in a sequence of frames, with no interleaved frames of any other type or
sequence of PUSH_PROMISE or CONTINUATION frames MUST have the from any other stream. The last frame in a sequence of HEADERS or
END_PUSH_PROMISE or END_HEADERS flag set (respectively). CONTINUATION frames MUST have the END_HEADERS flag set. The last
frame in a sequence of PUSH_PROMISE or CONTINUATION frames MUST have
the END_HEADERS flag set.
Header block fragments can only be sent as the payload of HEADERS, Header block fragments can only be sent as the payload of HEADERS,
PUSH_PROMISE or CONTINUATION frames. HEADERS, PUSH_PROMISE and PUSH_PROMISE or CONTINUATION frames, because these frames carry data
CONTINUATION frames carry data that can modify the compression that can modify the compression context maintained by a receiver. An
context maintained by a receiver. An endpoint receiving HEADERS, endpoint receiving HEADERS, PUSH_PROMISE or CONTINUATION frames MUST
PUSH_PROMISE or CONTINUATION frames MUST reassemble header blocks and reassemble header blocks and perform decompression even if the frames
perform decompression even if the frames are to be discarded. A are to be discarded. A receiver MUST terminate the connection with a
receiver MUST terminate the connection with a connection error connection error (Section 5.4.1) of type COMPRESSION_ERROR if it does
(Section 5.4.1) of type COMPRESSION_ERROR, if it does not decompress not decompress a header block.
a header block.
5. Streams and Multiplexing 5. Streams and Multiplexing
A "stream" is an independent, bi-directional sequence of HEADERS and A "stream" is an independent, bi-directional sequence of frames
DATA frames exchanged between the client and server within an HTTP/2 exchanged between the client and server within an HTTP/2 connection.
connection. Streams have several important characteristics: Streams have several important characteristics:
o A single HTTP/2 connection can contain multiple concurrently open o A single HTTP/2 connection can contain multiple concurrently open
streams, with either endpoint interleaving frames from multiple streams, with either endpoint interleaving frames from multiple
streams. streams.
o Streams can be established and used unilaterally or shared by o Streams can be established and used unilaterally or shared by
either the client or server. either the client or server.
o Streams can be closed by either endpoint. o Streams can be closed by either endpoint.
o The order in which frames are sent within a stream is significant. o The order in which frames are sent within a stream is significant.
Recipients process frames in the order they are received. Recipients process frames in the order they are received.
o Streams are identified by an integer. Stream identifiers are o Streams are identified by an integer. Stream identifiers are
assigned to streams by the initiating endpoint. assigned to streams by the endpoint initiating the stream.
5.1. Stream States 5.1. Stream States
The lifecycle of a stream is shown in Figure 1. The lifecycle of a stream is shown in Figure 1.
+--------+ +--------+
PP | | PP PP | | PP
,--------| idle |--------. ,--------| idle |--------.
/ | | \ / | | \
v +--------+ v v +--------+ v
skipping to change at page 16, line 31 skipping to change at page 16, line 35
| | closed | | R | closed | | | | closed | | R | closed | |
| | (remote) | | | (local) | | | | (remote) | | | (local) | |
| +----------+ | +----------+ | | +----------+ | +----------+ |
| | v | | | | v | |
| | ES / R +--------+ ES / R | | | | ES / R +--------+ ES / R | |
| `----------->| |<-----------' | | `----------->| |<-----------' |
| R | closed | R | | R | closed | R |
`-------------------->| |<--------------------' `-------------------->| |<--------------------'
+--------+ +--------+
H: HEADERS frame (with implied CONTINUATIONs)
PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
ES: END_STREAM flag
R: RST_STREAM frame
Figure 1: Stream States Figure 1: Stream States
Both endpoints have a subjective view of the state of a stream that Both endpoints have a subjective view of the state of a stream that
could be different when frames are in transit. Endpoints do not could be different when frames are in transit. Endpoints do not
coordinate the creation of streams, they are created unilaterally by coordinate the creation of streams; they are created unilaterally by
either endpoint. The negative consequences of a mismatch in states either endpoint. The negative consequences of a mismatch in states
are limited to the "closed" state after sending RST_STREAM, where are limited to the "closed" state after sending RST_STREAM, where
frames might be received for some time after closing. frames might be received for some time after closing.
Streams have the following states: Streams have the following states:
idle: idle:
All streams start in the "idle" state. In this state, no frames All streams start in the "idle" state. In this state, no frames
have been exchanged. have been exchanged.
skipping to change at page 17, line 27 skipping to change at page 17, line 38
stream that was initiated by the remote peer (see Section 8.2). stream that was initiated by the remote peer (see Section 8.2).
In this state, only the following transitions are possible: In this state, only the following transitions are possible:
* The endpoint can send a HEADERS frame. This causes the stream * The endpoint can send a HEADERS frame. This causes the stream
to open in a "half closed (remote)" state. to open in a "half closed (remote)" state.
* Either endpoint can send a RST_STREAM frame to cause the stream * Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation. to become "closed". This releases the stream reservation.
An endpoint MUST NOT send frames other than than HEADERS or An endpoint MUST NOT send frames other than HEADERS or RST_STREAM
RST_STREAM in this state. in this state.
A PRIORITY frame MAY be received in this state. Receiving any A PRIORITY frame MAY be received in this state. Receiving any
frame other than RST_STREAM, or PRIORITY MUST be treated as a frames other than RST_STREAM, or PRIORITY MUST be treated as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR. connection error (Section 5.4.1) of type PROTOCOL_ERROR.
reserved (remote): reserved (remote):
A stream in the "reserved (remote)" state has been reserved by a A stream in the "reserved (remote)" state has been reserved by a
remote peer. remote peer.
In this state, only the following transitions are possible: In this state, only the following transitions are possible:
* Receiving a HEADERS frame causes the stream to transition to * Receiving a HEADERS frame causes the stream to transition to
"half closed (local)". "half closed (local)".
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A stream that is "half closed (remote)" is no longer being used by A stream that is "half closed (remote)" is no longer being used by
the peer to send frames. In this state, an endpoint is no longer the peer to send frames. In this state, an endpoint is no longer
obligated to maintain a receiver flow control window if it obligated to maintain a receiver flow control window if it
performs flow control. performs flow control.
If an endpoint receives additional frames for a stream that is in If an endpoint receives additional frames for a stream that is in
this state, other than CONTINUATION frames, it MUST respond with a this state, other than CONTINUATION frames, it MUST respond with a
stream error (Section 5.4.2) of type STREAM_CLOSED. stream error (Section 5.4.2) of type STREAM_CLOSED.
A stream can transition from this state to "closed" by sending a A stream can transition from this state to "closed" by sending a
frame that contains a END_STREAM flag, or when either peer sends a frame that contains an END_STREAM flag, or when either peer sends
RST_STREAM frame. a RST_STREAM frame.
closed: closed:
The "closed" state is the terminal state. The "closed" state is the terminal state.
An endpoint MUST NOT send frames on a closed stream. An endpoint An endpoint MUST NOT send frames on a closed stream. An endpoint
that receives any frame after receiving a RST_STREAM MUST treat that receives any frame after receiving a RST_STREAM MUST treat
that as a stream error (Section 5.4.2) of type STREAM_CLOSED. that as a stream error (Section 5.4.2) of type STREAM_CLOSED.
Similarly, an endpoint that receives any frame after receiving a Similarly, an endpoint that receives any frames after receiving a
DATA frame with the END_STREAM flag set, or any frame except a DATA frame with the END_STREAM flag set, or any frames except a
CONTINUATION frame after receiving a HEADERS frame with a CONTINUATION frame after receiving a HEADERS frame with an
END_STREAM flag set MUST treat that as a stream error END_STREAM flag set MUST treat that as a stream error
(Section 5.4.2) of type STREAM_CLOSED. (Section 5.4.2) of type STREAM_CLOSED.
WINDOW_UPDATE, PRIORITY, or RST_STREAM frames can be received in WINDOW_UPDATE, PRIORITY, or RST_STREAM frames can be received in
this state for a short period after a DATA or HEADERS frame this state for a short period after a DATA or HEADERS frame
containing an END_STREAM flag is sent. Until the remote peer containing an END_STREAM flag is sent. Until the remote peer
receives and processes the frame bearing the END_STREAM flag, it receives and processes the frame bearing the END_STREAM flag, it
might send frame of any of these types. Endpoints MUST ignore might send frame of any of these types. Endpoints MUST ignore
WINDOW_UPDATE, PRIORITY, or RST_STREAM frames received in this WINDOW_UPDATE, PRIORITY, or RST_STREAM frames received in this
state, though endpoints MAY choose to treat frames that arrive a state, though endpoints MAY choose to treat frames that arrive a
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5.1.1. Stream Identifiers 5.1.1. Stream Identifiers
Streams are identified with an unsigned 31-bit integer. Streams Streams are identified with an unsigned 31-bit integer. Streams
initiated by a client MUST use odd-numbered stream identifiers; those initiated by a client MUST use odd-numbered stream identifiers; those
initiated by the server MUST use even-numbered stream identifiers. A initiated by the server MUST use even-numbered stream identifiers. A
stream identifier of zero (0x0) is used for connection control stream identifier of zero (0x0) is used for connection control
messages; the stream identifier zero MUST NOT be used to establish a messages; the stream identifier zero MUST NOT be used to establish a
new stream. new stream.
A stream identifier of one (0x1) is used to respond to the HTTP/1.1 HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are
request which was specified during Upgrade (see Section 3.2). After responded to with a stream identifier of one (0x1). After the
the upgrade completes, stream 0x1 is "half closed (local)" to the upgrade completes, stream 0x1 is "half closed (local)" to the client.
client. Therefore, stream 0x1 cannot be selected as a new stream Therefore, stream 0x1 cannot be selected as a new stream identifier
identifier by a client that upgrades from HTTP/1.1. by a client that upgrades from HTTP/1.1.
The identifier of a newly established stream MUST be numerically The identifier of a newly established stream MUST be numerically
greater than all streams that the initiating endpoint has opened or greater than all streams that the initiating endpoint has opened or
reserved. This governs streams that are opened using a HEADERS frame reserved. This governs streams that are opened using a HEADERS frame
and streams that are reserved using PUSH_PROMISE. An endpoint that and streams that are reserved using PUSH_PROMISE. An endpoint that
receives an unexpected stream identifier MUST respond with a receives an unexpected stream identifier MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR. connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The first use of a new stream identifier implicitly closes all The first use of a new stream identifier implicitly closes all
streams in the "idle" state that might have been initiated by that streams in the "idle" state that might have been initiated by that
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can initiate, and servers specify the maximum number of concurrent can initiate, and servers specify the maximum number of concurrent
streams the client can initiate. Endpoints MUST NOT exceed the limit streams the client can initiate. Endpoints MUST NOT exceed the limit
set by their peer. set by their peer.
Streams that are in the "open" state, or either of the "half closed" Streams that are in the "open" state, or either of the "half closed"
states count toward the maximum number of streams that an endpoint is states count toward the maximum number of streams that an endpoint is
permitted to open. Streams in any of these three states count toward permitted to open. Streams in any of these three states count toward
the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting
(see Section 6.5.2). (see Section 6.5.2).
Streams in either of the "reserved" states do not count as open, even An endpoint that receives a HEADERS frame that causes their
if a small amount of application state is retained to ensure that the advertised concurrent stream limit to be exceeded MUST treat this as
promised stream can be successfully used. a stream error (Section 5.4.2).
Streams in either of the "reserved" states do not count as open.
5.2. Flow Control 5.2. Flow Control
Using streams for multiplexing introduces contention over use of the Using streams for multiplexing introduces contention over use of the
TCP connection, resulting in blocked streams. A flow control scheme TCP connection, resulting in blocked streams. A flow control scheme
ensures that streams on the same connection do not destructively ensures that streams on the same connection do not destructively
interfere with each other. Flow control is used for both individual interfere with each other. Flow control is used for both individual
streams and for the connection as a whole. streams and for the connection as a whole.
HTTP/2 provides for flow control through use of the WINDOW_UPDATE HTTP/2 provides for flow control through use of the WINDOW_UPDATE
skipping to change at page 22, line 29 skipping to change at page 22, line 39
5.2.2. Appropriate Use of Flow Control 5.2.2. Appropriate Use of Flow Control
Flow control is defined to protect endpoints that are operating under Flow control is defined to protect endpoints that are operating under
resource constraints. For example, a proxy needs to share memory resource constraints. For example, a proxy needs to share memory
between many connections, and also might have a slow upstream between many connections, and also might have a slow upstream
connection and a fast downstream one. Flow control addresses cases connection and a fast downstream one. Flow control addresses cases
where the receiver is unable process data on one stream, yet wants to where the receiver is unable process data on one stream, yet wants to
continue to process other streams in the same connection. continue to process other streams in the same connection.
Deployments that do not require this capability can advertise a flow Deployments that do not require this capability can advertise a flow
control of the maximum size, incrementing the available space when control window of the maximum size, incrementing the available space
new data is received. Sending data is always subject to the flow when new data is received. Sending data is always subject to the
control window advertised by the receiver. flow control window advertised by the receiver.
Deployments with constrained resources (for example, memory) MAY Deployments with constrained resources (for example, memory) MAY
employ flow control to limit the amount of memory a peer can consume. employ flow control to limit the amount of memory a peer can consume.
Note, however, that this can lead to suboptimal use of available Note, however, that this can lead to suboptimal use of available
network resources if flow control is enabled without knowledge of the network resources if flow control is enabled without knowledge of the
bandwidth-delay product (see [RFC1323]). bandwidth-delay product (see [RFC1323]).
Even with full awareness of the current bandwidth-delay product, Even with full awareness of the current bandwidth-delay product,
implementation of flow control can be difficult. When using flow implementation of flow control can be difficult. When using flow
control, the receiver MUST read from the TCP receive buffer in a control, the receiver MUST read from the TCP receive buffer in a
timely fashion. Failure to do so could lead to a deadlock when timely fashion. Failure to do so could lead to a deadlock when
critical frames, such as WINDOW_UPDATE, are not available to HTTP/2. critical frames, such as WINDOW_UPDATE, are not available to HTTP/2.
However, flow control can ensure that constrained resources are However, flow control can ensure that constrained resources are
protected without any reduction in connection utilization. protected without any reduction in connection utilization.
5.3. Stream priority 5.3. Stream priority
The endpoint establishing a new stream can assign a priority for the A client can assign a priority for a new stream by including
stream. Priority is represented as an unsigned 31-bit integer. 0 prioritization information in the HEADERS frame (Section 6.2) that
represents the highest priority and 2^31-1 represents the lowest opens the stream. For an existing stream, the PRIORITY frame
priority. (Section 6.3) can be used to change the priority.
The purpose of this value is to allow an endpoint to express the The purpose of prioritization is to allow an endpoint to express how
relative priority of a stream. An endpoint can use this information it would prefer its peer allocate resources when managing concurrent
to preferentially allocate resources to a stream. Within HTTP/2, streams. Most importantly, priority can be used to select streams
priority can be used to select streams for transmitting frames when for transmitting frames when there is limited capacity for sending.
there is limited capacity for sending. For instance, an endpoint
might enqueue frames for all concurrently active streams. As
transmission capacity becomes available, frames from higher priority
streams might be sent before lower priority streams.
Explicitly setting the priority for a stream does not guarantee any Each stream is prioritized into a group. Each group is identified
particular processing or transmission order for the stream relative using an identifier that is selected by the client. Each group is
to any other stream. Nor is there any mechanism provided by which assigned a relative weight, a number that is used to determine the
the initiator of a stream can force or require a receiving endpoint relative proportion of available resources that are assigned to that
to process concurrent streams in a particular order. group.
Unless explicitly specified in the HEADERS frame (Section 6.2) during Within a priority group, streams can also be marked as being
stream creation, the default stream priority is 2^30. dependent on the completion of other streams.
Pushed streams (Section 8.2) have a lower priority than their Explicitly setting the priority for a stream is input to a
associated stream. The promised stream inherits the priority value prioritization process. It does not guarantee any particular
of the associated stream plus one, up to a maximum of 2^31-1. processing or transmission order for the stream relative to any other
stream. An endpoint cannot force a peer to process concurrent
streams in a particular order using priority. Expressing priority is
therefore only ever a suggestion.
Prioritization information can be specified explicitly for streams as
they are created using the HEADERS frame, or changed using the
PRIORITY frame. Providing prioritization information is optional, so
default values are used if no explicit indicator is provided
(Section 5.3.5).
Explicit prioritization information can be provided for a stream to
either allocate the stream to a priority group (Section 5.3.1), or to
create a dependency on another stream (Section 5.3.2).
5.3.1. Priority Groups and Weighting
All streams are assigned a priority group. Each priority group is
allocated a 31-bit identifier and an integer weight between 1 to 256
(inclusive).
Specifying a priority group and weight for a stream causes the stream
to be assigned to the identified priority group and for the weight
for the group to be changed to the new value.
Resources are divided proportionally between priority groups based on
their weight. For example, a priority group with weight 4 ideally
receives one third of the resources allocated to a stream with weight
12.
5.3.2. Stream Dependencies
Each stream can be given an explicit dependency on another stream.
Including a dependency expresses a preference to allocate resources
to the identified stream rather than to the dependent stream.
A stream that is dependent on another stream becomes part of the
priority group of the stream it depends on. It belongs to the same
dependency tree as the stream it depends on.
A stream that is assigned directly to a priority group is not
dependent on any other stream. It is the root of a dependency tree
inside its priority group.
When assigning a dependency on another stream, by default, the stream
is added as a new dependency of the stream it depends on. For
example, if streams B and C are dependent on stream A, and if stream
D is created with a dependency on stream A, this results in a
dependency order of A followed by B, C, and D.
A A
/ \ ==> /|\
B C B D C
Example of Default Dependency Creation
An exclusive flag allows for the insertion of a new level of
dependencies. The exclusive flag causes the stream to become the
sole dependency of the stream it depends on, causing other
dependencies to become dependencies of the stream. In the previous
example, if stream D is created with an exclusive dependency on
stream A, this results in a dependency order of A followed by D
followed by B and C.
A
A |
/ \ ==> D
B C / \
B C
Example of Exclusive Dependency Creation
Streams are ordered into several dependency trees within their
priority group. Each dependency tree within a priority group SHOULD
be allocated the same amount of resources.
Inside a dependency tree, a dependent stream SHOULD only be allocated
resources if the streams that it depends on are either closed, or it
is not possible to make progress on them.
Streams with the same dependencies SHOULD be allocated the same
amount of resources. Thus, if streams B and C depend on stream A,
and if no progress can be made on A, streams B and C are given an
equal share of resources.
A stream MUST NOT depend on itself. An endpoint MAY either treat
this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR, or
assign default priority values (Section 5.3.5) to the stream.
5.3.3. Reprioritization
Stream priorities are changed using the PRIORITY frame. Setting a
priority group and weight causes a stream to become part of the
identified group, and not dependent on any other stream. Setting a
dependency causes a stream to become dependent on the identified
stream, which can cause the reprioritized stream to move to a new
priority group.
All streams that are dependent on a reprioritized stream move with
it. Setting a dependency with the exclusive flag for a reprioritized
stream moves all the dependencies of the stream it depends on to
become dependencies of the reprioritized stream.
5.3.4. Prioritization State Management
When a stream is closed, its dependencies can be moved to become
dependent on the stream the closed stream depends on, if any, or to
become new dependency tree roots otherwise.
It is possible for a stream to become closed while prioritization
information that creates a dependency on that stream is in transit.
If a stream identified in a dependency has been closed and any
associated priority information destroyed then the dependent stream
is instead assigned a default priority. This potentially creates
suboptimal prioritization, since the stream can be given an effective
priority that is higher than expressed by a peer.
To avoid this problem, endpoints SHOULD maintain prioritization state
for closed streams for a period after streams close. This could
create an large state burden for an endpoint, so this state MAY be
limited. The amount of additional state an endpoint maintains could
be dependent on load; under high load, prioritization state can be
discarded to limit resource commitments. In extreme cases, an
endpoint could even discard prioritization state for active or
reserved streams.
An endpoint SHOULD retain stream prioritization state for at least
one round trip, though maintaining state over longer periods reduces
the chance that default values have to be assigned to streams. An
endpoint MAY apply a fixed upper limit on the number of closed
streams for which prioritization state is tracked to limit state
exposure. If a fixed limit is applied, endpoints SHOULD maintain
state for at least as many streams as allowed by their setting for
SETTINGS_MAX_CONCURRENT_STREAMS.
An endpoint receiving a PRIORITY frame that changes the priority of a
closed stream SHOULD alter the weight of the priority group, or the
dependencies of the streams that depend on it, if it has retained
enough state to do so.
Priority group information is part of the priority state of a stream.
Priority groups that contain only closed streams can be assigned a
weight of zero.
The number of priority groups cannot exceed the number of non-closed
streams. This includes streams in the "reserved" state. Priority
state size for peer-initiated streams is limited by the value of
SETTINGS_MAX_CONCURRENT_STREAMS. Reserved streams do not count
toward the concurrent stream limit of either peer, but only the
endpoint that creates the reservation needs to maintain priority
information. Thus, the total amount of priority state for non-closed
streams can be limited by an endpoint.
5.3.5. Default Priorities
Providing priority information is optional. Streams are assigned to
a priority group with an identifier equal to the stream identifier
and a weight of 16.
Pushed streams (Section 8.2) initially depend on their associated
stream.
5.4. Error Handling 5.4. Error Handling
HTTP/2 framing permits two classes of error: HTTP/2 framing permits two classes of error:
o An error condition that renders the entire connection unusable is o An error condition that renders the entire connection unusable is
a connection error. a connection error.
o An error in an individual stream is a stream error. o An error in an individual stream is a stream error.
A list of error codes is included in Section 7. A list of error codes is included in Section 7.
5.4.1. Connection Error Handling 5.4.1. Connection Error Handling
A connection error is any error which prevents further processing of A connection error is any error which prevents further processing of
the framing layer or which corrupts any connection state. the framing layer, or which corrupts any connection state.
An endpoint that encounters a connection error SHOULD first send a An endpoint that encounters a connection error SHOULD first send a
GOAWAY frame (Section 6.8) with the stream identifier of the last GOAWAY frame (Section 6.8) with the stream identifier of the last
stream that it successfully received from its peer. The GOAWAY frame stream that it successfully received from its peer. The GOAWAY frame
includes an error code that indicates why the connection is includes an error code that indicates why the connection is
terminating. After sending the GOAWAY frame, the endpoint MUST close terminating. After sending the GOAWAY frame, the endpoint MUST close
the TCP connection. the TCP connection.
It is possible that the GOAWAY will not be reliably received by the It is possible that the GOAWAY will not be reliably received by the
receiving endpoint. In the event of a connection error, GOAWAY only receiving endpoint. In the event of a connection error, GOAWAY only
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frames if it receives frames on a closed stream after more than a frames if it receives frames on a closed stream after more than a
round-trip time. This behavior is permitted to deal with misbehaving round-trip time. This behavior is permitted to deal with misbehaving
implementations. implementations.
An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM
frame, to avoid looping. frame, to avoid looping.
5.4.3. Connection Termination 5.4.3. Connection Termination
If the TCP connection is torn down while streams remain in open or If the TCP connection is torn down while streams remain in open or
half closed states, then the endpoint MUST assume that the stream was half closed states, then the endpoint MUST assume that those streams
abnormally interrupted and could be incomplete. were abnormally interrupted and could be incomplete.
6. Frame Definitions 6. Frame Definitions
This specification defines a number of frame types, each identified This specification defines a number of frame types, each identified
by a unique 8-bit type code. Each frame type serves a distinct by a unique 8-bit type code. Each frame type serves a distinct
purpose either in the establishment and management of the connection purpose either in the establishment and management of the connection
as a whole, or of individual streams. as a whole, or of individual streams.
The transmission of specific frame types can alter the state of a The transmission of specific frame types can alter the state of a
connection. If endpoints fail to maintain a synchronized view of the connection. If endpoints fail to maintain a synchronized view of the
connection state, successful communication within the connection will connection state, successful communication within the connection will
no longer be possible. Therefore, it is important that endpoints no longer be possible. Therefore, it is important that endpoints
have a shared comprehension of how the state is affected by the use have a shared comprehension of how the state is affected by the use
any given frame. Accordingly, while it is expected that new frame any given frame.
types will be introduced by extensions to this protocol, only frames
defined by this document are permitted to alter the connection state.
6.1. DATA 6.1. DATA
DATA frames (type=0x0) convey arbitrary, variable-length sequences of DATA frames (type=0x0) convey arbitrary, variable-length sequences of
octets associated with a stream. One or more DATA frames are used, octets associated with a stream. One or more DATA frames are used,
for instance, to carry HTTP request or response payloads. for instance, to carry HTTP request or response payloads.
DATA frames MAY also contain arbitrary padding. Padding can be added DATA frames MAY also contain arbitrary padding. Padding can be added
to DATA frames to hide the size of messages. to DATA frames to hide the size of messages.
0 1 2 3 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Pad High(8)] | [Pad Low (8)] | Data (*) . | Pad High? (8) | Pad Low? (8) |
+---------------+---------------+-------------------------------+ +---------------+---------------+-------------------------------+
. Data (*) ... | Data (*) ...
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Padding (*) ... | Padding (*) ...
+---------------------------------------------------------------+ +---------------------------------------------------------------+
DATA Frame Payload DATA Frame Payload
The DATA frame contains the following fields: The DATA frame contains the following fields:
Pad High: An 8-bit field containing an amount of padding in units of Pad High: An 8-bit field containing an amount of padding in units of
256 octets. This field is optional and is only present if the 256 octets. This field is optional and is only present if the
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END_STREAM (0x1): Bit 1 being set indicates that this frame is the END_STREAM (0x1): Bit 1 being set indicates that this frame is the
last that the endpoint will send for the identified stream. last that the endpoint will send for the identified stream.
Setting this flag causes the stream to enter one of the "half Setting this flag causes the stream to enter one of the "half
closed" states or the "closed" state (Section 5.1). closed" states or the "closed" state (Section 5.1).
END_SEGMENT (0x2): Bit 2 being set indicates that this frame is the END_SEGMENT (0x2): Bit 2 being set indicates that this frame is the
last for the current segment. Intermediaries MUST NOT coalesce last for the current segment. Intermediaries MUST NOT coalesce
frames across a segment boundary and MUST preserve segment frames across a segment boundary and MUST preserve segment
boundaries when forwarding frames. boundaries when forwarding frames.
PAD_LOW (0x10): Bit 5 being set indicates that the Pad Low field is PAD_LOW (0x08): Bit 4 being set indicates that the Pad Low field is
present. present.
PAD_HIGH (0x20): Bit 6 being set indicates that the Pad High field PAD_HIGH (0x10): Bit 5 being set indicates that the Pad High field
is present. This bit MUST NOT be set unless the PAD_LOW flag is is present. This bit MUST NOT be set unless the PAD_LOW flag is
also set. Endpoints that receive a frame with PAD_HIGH set and also set. Endpoints that receive a frame with PAD_HIGH set and
PAD_LOW cleared MUST treat this as a connection error PAD_LOW cleared MUST treat this as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. (Section 5.4.1) of type PROTOCOL_ERROR.
DATA frames MUST be associated with a stream. If a DATA frame is DATA frames MUST be associated with a stream. If a DATA frame is
received whose stream identifier field is 0x0, the recipient MUST received whose stream identifier field is 0x0, the recipient MUST
respond with a connection error (Section 5.4.1) of type respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
DATA frames are subject to flow control and can only be sent when a DATA frames are subject to flow control and can only be sent when a
stream is in the "open" or "half closed (remote)" states. Padding is stream is in the "open" or "half closed (remote)" states. Padding is
not excluded from flow control. If a DATA frame is received whose included in flow control. If a DATA frame is received whose stream
stream is not in "open" or "half closed (local)" state, the recipient is not in "open" or "half closed (local)" state, the recipient MUST
MUST respond with a stream error (Section 5.4.2) of type respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.
STREAM_CLOSED.
The total number of padding octets is determined by multiplying the The total number of padding octets is determined by multiplying the
value of the Pad High field by 256 and adding the value of the Pad value of the Pad High field by 256 and adding the value of the Pad
Low field. Both Pad High and Pad Low fields assume a value of zero Low field. Both Pad High and Pad Low fields assume a value of zero
if absent. If the length of the padding is greater than the length if absent. If the length of the padding is greater than the length
of the remainder of the frame payload, the recipient MUST treat this of the remainder of the frame payload, the recipient MUST treat this
as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Note: A frame can be increased in size by one octet by including a Note: A frame can be increased in size by one octet by including a
Pad Low field with a value of zero. Pad Low field with a value of zero.
Use of padding is a security feature; as such, its use demands some Use of padding is a security feature; as such, its use demands some
care, see Section 10.6. care, see Section 10.7.
6.2. HEADERS 6.2. HEADERS
The HEADERS frame (type=0x1) carries name-value pairs. It is used to The HEADERS frame (type=0x1) carries name-value pairs. It is used to
open a stream (Section 5.1). HEADERS frames can be sent on a stream open a stream (Section 5.1). HEADERS frames can be sent on a stream
in the "open" or "half closed (remote)" states. in the "open" or "half closed (remote)" states.
0 1 2 3 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Pad High(8)] | [Pad Low (8)] |X| [Priority (31)] ... | Pad High? (8) | Pad Low? (8) |
+---------------+---------------+-+-----------------------------+ +-+-------------+---------------+-------------------------------+
...[Priority] | Header Block Fragment (*) ... |R| Priority Group Identifier? (31) |
+-------------------------------+-------------------------------+ +-+-------------+-----------------------------------------------+
| Weight? (8) |
+-+-------------+-----------------------------------------------+
|E| Stream Dependency? (31) |
+-+-------------------------------------------------------------+
| Header Block Fragment (*) ... | Header Block Fragment (*) ...
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Padding (*) ... | Padding (*) ...
+---------------------------------------------------------------+ +---------------------------------------------------------------+
HEADERS Frame Payload HEADERS Frame Payload
The HEADERS frame payload has the following fields: The HEADERS frame payload has the following fields:
Pad High: Padding size high bits. This field is only present if the Pad High: Padding size high bits. This field is only present if the
PAD_HIGH flag is set. PAD_HIGH flag is set.
Pad Low: Padding size low bits. This field is only present if the Pad Low: Padding size low bits. This field is only present if the
PAD_LOW flag is set. PAD_LOW flag is set.
X: A single reserved bit. This field is optional and is only present R: A single reserved bit. This field is optional and is only present
if the PRIORITY flag is set. if the PRIORITY_GROUP flag is set.
Priority: Prioritization information for the stream, see Priority Group Identifier: A 31-bit identifier for a priority group,
see Section 5.3. This field is optional and is only present if
the PRIORITY_GROUP flag is set.
Weight: An 8-bit weight for the identified priority group, see
Section 5.3. This field is optional and is only present if the Section 5.3. This field is optional and is only present if the
PRIORITY flag is set. PRIORITY_GROUP flag is set.
E: A single bit flag indicates that the stream dependency is
exclusive, see Section 5.3. This field is optional and is only
present if the PRIORITY_DEPENDENCY flag is set.
Stream Dependency: A 31-bit stream identifier for the stream that
this stream depends on, see Section 5.3. This field is optional
and is only present if the PRIORITY_DEPENDENCY flag is set.
Header Block Fragment: A header block fragment (Section 4.3). Header Block Fragment: A header block fragment (Section 4.3).
Padding: Padding octets. Padding: Padding octets.
The HEADERS frame defines the following flags: The HEADERS frame defines the following flags:
END_STREAM (0x1): Bit 1 being set indicates that the header block END_STREAM (0x1): Bit 1 being set indicates that the header block
(Section 4.3) is the last that the endpoint will send for the (Section 4.3) is the last that the endpoint will send for the
identified stream. Setting this flag causes the stream to enter identified stream. Setting this flag causes the stream to enter
skipping to change at page 28, line 21 skipping to change at page 32, line 8
END_HEADERS (0x4): Bit 3 being set indicates that this frame END_HEADERS (0x4): Bit 3 being set indicates that this frame
contains an entire header block (Section 4.3) and is not followed contains an entire header block (Section 4.3) and is not followed
by any CONTINUATION frames. by any CONTINUATION frames.
A HEADERS frame without the END_HEADERS flag set MUST be followed A HEADERS frame without the END_HEADERS flag set MUST be followed
by a CONTINUATION frame for the same stream. A receiver MUST by a CONTINUATION frame for the same stream. A receiver MUST
treat the receipt of any other type of frame or a frame on a treat the receipt of any other type of frame or a frame on a
different stream as a connection error (Section 5.4.1) of type different stream as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
PRIORITY (0x8): Bit 4 being set indicates that the first four octets PAD_LOW (0x8): Bit 4 being set indicates that the Pad Low field is
of this frame contain a single reserved bit and a 31-bit priority;
see Section 5.3. If this bit is not set, the four bytes do not
appear and the frame only contains a header block fragment.
PAD_LOW (0x10): Bit 5 being set indicates that the Pad Low field is
present. present.
PAD_HIGH (0x20): Bit 6 being set indicates that the Pad High field PAD_HIGH (0x10): Bit 5 being set indicates that the Pad High field
is present. This bit MUST NOT be set unless the PAD_LOW flag is is present. This bit MUST NOT be set unless the PAD_LOW flag is
also set. Endpoints that receive a frame with PAD_HIGH set and also set. Endpoints that receive a frame with PAD_HIGH set and
PAD_LOW cleared MUST treat this as a connection error PAD_LOW cleared MUST treat this as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. (Section 5.4.1) of type PROTOCOL_ERROR.
PRIORITY_GROUP (0x20): Bit 6 being set indicates that the Priority
Group Identifier and Weight fields are present; see Section 5.3.
PRIORITY_DEPENDENCY (0x40): Bit 7 being set indicates that the
Exclusive Flag (E) and Stream Dependency fields are present; see
Section 5.3.
The payload of a HEADERS frame contains a header block fragment The payload of a HEADERS frame contains a header block fragment
(Section 4.3). A header block that does not fit within a HEADERS (Section 4.3). A header block that does not fit within a HEADERS
frame is continued in a CONTINUATION frame (Section 6.10). frame is continued in a CONTINUATION frame (Section 6.10).
HEADERS frames MUST be associated with a stream. If a HEADERS frame HEADERS frames MUST be associated with a stream. If a HEADERS frame
is received whose stream identifier field is 0x0, the recipient MUST is received whose stream identifier field is 0x0, the recipient MUST
respond with a connection error (Section 5.4.1) of type respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
A HEADERS frame MUST NOT have both the PRIORITY_GROUP and
PRIORITY_DEPENDENCY flags set. Receipt of a HEADERS frame with both
these flags set MUST be treated as a stream error (Section 5.4.2) of
type PROTOCOL_ERROR.
The HEADERS frame changes the connection state as described in The HEADERS frame changes the connection state as described in
Section 4.3. Section 4.3.
The HEADERS frame includes optional padding. Padding fields and The HEADERS frame includes optional padding. Padding fields and
flags are identical to those defined for DATA frames (Section 6.1). flags are identical to those defined for DATA frames (Section 6.1).
6.3. PRIORITY 6.3. PRIORITY
The PRIORITY frame (type=0x2) specifies the sender-advised priority The PRIORITY frame (type=0x2) specifies the sender-advised priority
of a stream. It can be sent at any time for an existing stream. of a stream (Section 5.3). It can be sent at any time for an
This enables reprioritisation of existing streams. existing stream. This enables reprioritisation of existing streams.
0 1 2 3 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 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) | |R| Priority Group Identifier? (31) |
+-+-------------+-----------------------------------------------+
| Weight? (8) |
+-+-------------+-----------------------------------------------+
|E| Stream Dependency? (31) |
+-+-------------------------------------------------------------+ +-+-------------------------------------------------------------+
PRIORITY Frame Payload PRIORITY Frame Payload
The payload of a PRIORITY frame contains a single reserved bit and a The payload of a PRIORITY frame contains the following fields:
31-bit priority.
The PRIORITY frame does not define any flags. R: A single reserved bit. This field is optional and is only present
if the PRIORITY_GROUP flag is set.
Priority Group Identifier: A 31-bit identifier for a priority group,
see Section 5.3. This field is optional and is only present if
the PRIORITY_GROUP flag is set.
Weight: An 8-bit weight for the identified priority group, see
Section 5.3. This field is optional and is only present if the
PRIORITY_GROUP flag is set.
E: A single bit flag indicates that the stream dependency is
exclusive, see Section 5.3. This field is optional and is only
present if the PRIORITY_DEPENDENCY flag is set.
Stream Dependency: A 31-bit stream identifier for the stream that
this stream depends on, see Section 5.3. This field is optional
and is only present if the PRIORITY_DEPENDENCY flag is set.
The PRIORITY frame defines the following flags:
PRIORITY_GROUP (0x20): Bit 6 being set indicates that the Priority
Group Identifier and Weight fields are present; see Section 5.3.
PRIORITY_DEPENDENCY (0x40): Bit 7 being set indicates that the
Exclusive Flag (E) and Stream Dependency fields are present; see
Section 5.3.
A PRIORITY frame MUST have exactly one of the PRIORITY_GROUP and
PRIORITY_DEPENDENCY flags set. Receipt of a PRIORITY frame with
either none or both these flags set MUST be treated as a stream error
(Section 5.4.2) of type PROTOCOL_ERROR.
The PRIORITY frame is associated with an existing stream. If a The PRIORITY frame is associated with an existing stream. If a
PRIORITY frame is received with a stream identifier of 0x0, the PRIORITY frame is received with a stream identifier of 0x0, the
recipient MUST respond with a connection error (Section 5.4.1) of recipient MUST respond with a connection error (Section 5.4.1) of
type PROTOCOL_ERROR. type PROTOCOL_ERROR.
The PRIORITY frame can be sent on a stream in any of the "reserved The PRIORITY frame can be sent on a stream in any of the "reserved
(remote)", "open", "half-closed (local)", or "half closed (remote)" (remote)", "open", "half-closed (local)", or "half closed (remote)"
states, though it cannot be sent between consecutive frames that states, though it cannot be sent between consecutive frames that
comprise a single header block (Section 4.3). Note that this frame comprise a single header block (Section 4.3). Note that this frame
skipping to change at page 30, line 32 skipping to change at page 35, line 12
treat this as a connection error (Section 5.4.1) of type treat this as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
RST_STREAM frames MUST NOT be sent for a stream in the "idle" state. RST_STREAM frames MUST NOT be sent for a stream in the "idle" state.
If a RST_STREAM frame identifying an idle stream is received, the If a RST_STREAM frame identifying an idle stream is received, the
recipient MUST treat this as a connection error (Section 5.4.1) of recipient MUST treat this as a connection error (Section 5.4.1) of
type PROTOCOL_ERROR. type PROTOCOL_ERROR.
6.5. SETTINGS 6.5. SETTINGS
The SETTINGS frame (type=0x4) conveys configuration parameters that The SETTINGS frame (type=0x4) conveys configuration parameters (such
affect how endpoints communicate. The parameters are either as preferences and constraints on peer behavior) that affect how
constraints on peer behavior or preferences. endpoints communicate, and is also used to acknowledge the receipt of
those parameters. Individually, a SETTINGS parameter can also be
referred to as a "setting".
Settings are not negotiated. Settings describe characteristics of SETTINGS parameters are not negotiated; they describe characteristics
the sending peer, which are used by the receiving peer. Different of the sending peer, which are used by the receiving peer. Different
values for the same setting can be advertised by each peer. For values for the same parameter can be advertised by each peer. For
example, a client might set a high initial flow control window, example, a client might set a high initial flow control window,
whereas a server might set a lower value to conserve resources. whereas a server might set a lower value to conserve resources.
SETTINGS frames MUST be sent at the start of a connection, and MAY be A SETTINGS frame MUST be sent by both endpoints at the start of a
sent at any other time by either endpoint over the lifetime of the connection, and MAY be sent at any other time by either endpoint over
connection. the lifetime of the connection. Implementations MUST support all of
the parameters defined by this specification.
Implementations MUST support all of the settings defined by this
specification and MAY support additional settings defined by
extensions to this protocol. Unsupported or unrecognized settings
MUST be ignored. New settings MUST NOT be defined or implemented in
a way that requires endpoints to understand them in order to
communicate successfully.
Each setting in a SETTINGS frame replaces the existing value for that Each parameter in a SETTINGS frame replaces any existing value for
setting. Settings are processed in the order in which they appear, that parameter. Parameters are processed in the order in which they
and a receiver of a SETTINGS frame does not need to maintain any appear, and a receiver of a SETTINGS frame does not need to maintain
state other than the current value of settings. Therefore, the value any state other than the current value of its parameters. Therefore,
of a setting is the last value that is seen by a receiver. This the value of a SETTINGS parameter is the last value that is seen by a
permits the inclusion of the same settings multiple times in the same receiver.
SETTINGS frame, though doing so does nothing other than waste
connection capacity.
The SETTINGS frame defines the following flag: SETTINGS parameters are acknowledged by the receiving peer. To
enable this, the SETTINGS frame defines the following flag:
ACK (0x1): Bit 1 being set indicates that this frame acknowledges ACK (0x1): Bit 1 being set indicates that this frame acknowledges
receipt and application of the peer's SETTINGS frame. When this receipt and application of the peer's SETTINGS frame. When this
bit is set, the payload of the SETTINGS frame MUST be empty. bit is set, the payload of the SETTINGS frame MUST be empty.
Receipt of a SETTINGS frame with the ACK flag set and a length Receipt of a SETTINGS frame with the ACK flag set and a length
field value other than 0 MUST be treated as a connection error field value other than 0 MUST be treated as a connection error
(Section 5.4.1) of type FRAME_SIZE_ERROR. For more info, see (Section 5.4.1) of type FRAME_SIZE_ERROR. For more info, see
Settings Synchronization (Section 6.5.3). Settings Synchronization (Section 6.5.3).
SETTINGS frames always apply to a connection, never a single stream. SETTINGS frames always apply to a connection, never a single stream.
The stream identifier for a settings frame MUST be zero. If an The stream identifier for a SETTINGS frame MUST be zero. If an
endpoint receives a SETTINGS frame whose stream identifier field is endpoint receives a SETTINGS frame whose stream identifier field is
anything other than 0x0, the endpoint MUST respond with a connection anything other than 0x0, the endpoint MUST respond with a connection
error (Section 5.4.1) of type PROTOCOL_ERROR. error (Section 5.4.1) of type PROTOCOL_ERROR.
The SETTINGS frame affects connection state. A badly formed or The SETTINGS frame affects connection state. A badly formed or
incomplete SETTINGS frame MUST be treated as a connection error incomplete SETTINGS frame MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. (Section 5.4.1) of type PROTOCOL_ERROR.
6.5.1. Setting Format 6.5.1. SETTINGS Format
The payload of a SETTINGS frame consists of zero or more settings. The payload of a SETTINGS frame consists of zero or more parameters,
Each setting consists of an unsigned 8-bit setting identifier, and an each consisting of an unsigned 8-bit identifier and an unsigned 32-
unsigned 32-bit value. bit value.
0 1 2 3 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Identifier (8) | Value (32) ... | Identifier (8)|
+---------------+-----------------------------------------------+ +---------------+-----------------------------------------------+
...Value | | Value (32) |
+---------------+ +---------------------------------------------------------------+
Setting Format Setting Format
6.5.2. Defined Settings 6.5.2. Defined SETTINGS Parameters
The following settings are defined: The following parameters are defined:
SETTINGS_HEADER_TABLE_SIZE (1): Allows the sender to inform the SETTINGS_HEADER_TABLE_SIZE (1): Allows the sender to inform the
remote endpoint of the size of the header compression table used remote endpoint of the size of the header compression table used
to decode header blocks. The encoder can reduce this size by to decode header blocks. The encoder can reduce this size by
using signalling specific to the header compression format inside using signaling specific to the header compression format inside a
a header block. The initial value is 4,096 bytes. header block. The initial value is 4,096 bytes.
SETTINGS_ENABLE_PUSH (2): This setting can be use to disable server SETTINGS_ENABLE_PUSH (2): This setting can be use to disable server
push (Section 8.2). An endpoint MUST NOT send a PUSH_PROMISE push (Section 8.2). An endpoint MUST NOT send a PUSH_PROMISE
frame if it receives this setting set to a value of 0. An frame if it receives this parameter set to a value of 0. An
endpoint that has set this setting to 0 and had it acknowledged endpoint that has both set this parameter to 0 and had it
MUST treat the receipt of a PUSH_PROMISE frame as a connection acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a
error (Section 5.4.1) of type PROTOCOL_ERROR. connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The initial value is 1, which indicates that push is permitted. The initial value is 1, which indicates that push is permitted.
Any value other than 0 or 1 MUST be treated as a connection error Any value other than 0 or 1 MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. (Section 5.4.1) of type PROTOCOL_ERROR.
SETTINGS_MAX_CONCURRENT_STREAMS (3): Indicates the maximum number of SETTINGS_MAX_CONCURRENT_STREAMS (3): Indicates the maximum number of
concurrent streams that the sender will allow. This limit is concurrent streams that the sender will allow. This limit is
directional: it applies to the number of streams that the sender directional: it applies to the number of streams that the sender
permits the receiver to create. Initially there is no limit to permits the receiver to create. Initially there is no limit to
this value. It is recommended that this value be no smaller than this value. It is recommended that this value be no smaller than
100, so as to not unnecessarily limit parallelism. 100, so as to not unnecessarily limit parallelism.
A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be
treated as special by endpoints. A zero value does prevent the treated as special by endpoints. A zero value does prevent the
creation of new streams, however this can also happen for any creation of new streams, however this can also happen for any
limit that is exhausted with active streams. Servers SHOULD only limit that is exhausted with active streams. Servers SHOULD only
set a zero value for short durations; if a server does not wish to set a zero value for short durations; if a server does not wish to
accept requests, closing the connection could be preferable. accept requests, closing the connection could be preferable.
SETTINGS_INITIAL_WINDOW_SIZE (4): Indicates the sender's initial SETTINGS_INITIAL_WINDOW_SIZE (4): Indicates the sender's initial
window size (in bytes) for stream level flow control. window size (in bytes) for stream level flow control. The initial
value is 65,535.
This settings affects the window size of all streams, including This setting affects the window size of all streams, including
existing streams, see Section 6.9.2. existing streams, see Section 6.9.2.
Values above the maximum flow control window size of 2^31 - 1 MUST Values above the maximum flow control window size of 2^31 - 1 MUST
be treated as a connection error (Section 5.4.1) of type be treated as a connection error (Section 5.4.1) of type
FLOW_CONTROL_ERROR. FLOW_CONTROL_ERROR.
An endpoint that receives a SETTINGS frame with any other setting An endpoint that receives a SETTINGS frame with any other identifier
identifier MUST treat this as a connection error (Section 5.4.1) of MUST treat this as a connection error (Section 5.4.1) of type
type PROTOCOL_ERROR. PROTOCOL_ERROR.
6.5.3. Settings Synchronization 6.5.3. Settings Synchronization
Most values in SETTINGS benefit from or require an understanding of Most values in SETTINGS benefit from or require an understanding of
when the peer has received and applied the changed setting values. when the peer has received and applied the changed the communicated
In order to provide such synchronization timepoints, the recipient of parameter values. In order to provide such synchronization
a SETTINGS frame in which the ACK flag is not set MUST apply the timepoints, the recipient of a SETTINGS frame in which the ACK flag
updated settings as soon as possible upon receipt. is not set MUST apply the updated parameters as soon as possible upon
receipt.
The values in the SETTINGS frame MUST be applied in the order they The values in the SETTINGS frame MUST be applied in the order they
appear, with no other frame processing between values. Once all appear, with no other frame processing between values. Once all
values have been applied, the recipient MUST immediately emit a values have been applied, the recipient MUST immediately emit a
SETTINGS frame with the ACK flag set. The sender of altered settings SETTINGS frame with the ACK flag set. Upon receiving a SETTINGS
applies changes upon receiving a SETTINGS frame with the ACK flag frame with the ACK flag set, the sender of the altered parameters can
set. rely upon their application.
If the sender of a SETTINGS frame does not receive an acknowledgement If the sender of a SETTINGS frame does not receive an acknowledgement
within a reasonable amount of time, it MAY issue a connection error within a reasonable amount of time, it MAY issue a connection error
(Section 5.4.1) of type SETTINGS_TIMEOUT. (Section 5.4.1) of type SETTINGS_TIMEOUT.
6.6. PUSH_PROMISE 6.6. PUSH_PROMISE
The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint
in advance of streams the sender intends to initiate. The in advance of streams the sender intends to initiate. The
PUSH_PROMISE frame includes the unsigned 31-bit identifier of the PUSH_PROMISE frame includes the unsigned 31-bit identifier of the
stream the endpoint plans to create along with a set of headers that stream the endpoint plans to create along with a set of headers that
provide additional context for the stream. Section 8.2 contains a provide additional context for the stream. Section 8.2 contains a
thorough description of the use of PUSH_PROMISE frames. thorough description of the use of PUSH_PROMISE frames.
PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of
the peer endpoint is set to 0. the peer endpoint is set to 0.
0 1 2 3 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 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) | | Pad High? (8) | Pad Low? (8) |
+-+-------------------------------------------------------------+ +-+-------------+---------------+-------------------------------+
| Header Block Fragment (*) ... |R| Promised Stream ID (31) |
+-+-----------------------------+-------------------------------+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+ +---------------------------------------------------------------+
PUSH_PROMISE Payload Format PUSH_PROMISE Payload Format
The payload of a PUSH_PROMISE includes a "Promised-Stream-ID". This The PUSH_PROMISE frame payload has the following fields:
unsigned 31-bit integer identifies the stream the endpoint intends to
start sending frames for. The promised stream identifier MUST be a
valid choice for the next stream sent by the sender (see new stream
identifier (Section 5.1.1)).
Following the "Promised-Stream-ID" is a header block fragment Pad High: Padding size high bits. This field is only present if the
(Section 4.3). PAD_HIGH flag is set.
PUSH_PROMISE frames MUST be associated with an existing, peer- Pad Low: Padding size low bits. This field is only present if the
initiated stream. If the stream identifier field specifies the value PAD_LOW flag is set.
0x0, a recipient MUST respond with a connection error (Section 5.4.1)
of type PROTOCOL_ERROR. R: A single reserved bit.
Promised Stream ID: This unsigned 31-bit integer identifies the
stream the endpoint intends to start sending frames for. The
promised stream identifier MUST be a valid choice for the next
stream sent by the sender (see new stream identifier
(Section 5.1.1)).
Header Block Fragment: A header block fragment (Section 4.3)
containing request header fields.
Padding: Padding octets.
The PUSH_PROMISE frame defines the following flags: The PUSH_PROMISE frame defines the following flags:
END_PUSH_PROMISE (0x4): Bit 3 being set indicates that this frame END_HEADERS (0x4): Bit 3 being set indicates that this frame
contains an entire header block (Section 4.3) and is not followed contains an entire header block (Section 4.3) and is not followed
by any CONTINUATION frames. by any CONTINUATION frames.
A PUSH_PROMISE frame without the END_PUSH_PROMISE flag set MUST be A PUSH_PROMISE frame without the END_HEADERS flag set MUST be
followed by a CONTINUATION frame for the same stream. A receiver followed by a CONTINUATION frame for the same stream. A receiver
MUST treat the receipt of any other type of frame or a frame on a MUST treat the receipt of any other type of frame or a frame on a
different stream as a connection error (Section 5.4.1) of type different stream as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
PAD_LOW (0x8): Bit 4 being set indicates that the Pad Low field is
present.
PAD_HIGH (0x10): Bit 5 being set indicates that the Pad High field
is present. This bit MUST NOT be set unless the PAD_LOW flag is
also set. Endpoints that receive a frame with PAD_HIGH set and
PAD_LOW cleared MUST treat this as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
PUSH_PROMISE frames MUST be associated with an existing, peer-
initiated stream. The stream identifier of a PUSH_PROMISE frame
indicates the stream it is associated with. If the stream identifier
field specifies the value 0x0, a recipient MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Promised streams are not required to be used in order promised. The Promised streams are not required to be used in order promised. The
PUSH_PROMISE only reserves stream identifiers for later use. PUSH_PROMISE only reserves stream identifiers for later use.
Recipients of PUSH_PROMISE frames can choose to reject promised Recipients of PUSH_PROMISE frames can choose to reject promised
streams by returning a RST_STREAM referencing the promised stream streams by returning a RST_STREAM referencing the promised stream
identifier back to the sender of the PUSH_PROMISE. identifier back to the sender of the PUSH_PROMISE.
The PUSH_PROMISE frame modifies the connection state as defined in The PUSH_PROMISE frame modifies the connection state as defined in
Section 4.3. Section 4.3.
A PUSH_PROMISE frame modifies the connection state in two ways. The A PUSH_PROMISE frame modifies the connection state in two ways. The
inclusion of a header block (Section 4.3) potentially modifies the inclusion of a header block (Section 4.3) potentially modifies the
compression state. PUSH_PROMISE also reserves a stream for later state maintained for header compression. PUSH_PROMISE also reserves
use, causing the promised stream to enter the "reserved" state. A a stream for later use, causing the promised stream to enter the
sender MUST NOT send a PUSH_PROMISE on a stream unless that stream is "reserved" state. A sender MUST NOT send a PUSH_PROMISE on a stream
either "open" or "half closed (remote)"; the sender MUST ensure that unless that stream is either "open" or "half closed (remote)"; the
the promised stream is a valid choice for a new stream identifier sender MUST ensure that the promised stream is a valid choice for a
(Section 5.1.1) (that is, the promised stream MUST be in the "idle" new stream identifier (Section 5.1.1) (that is, the promised stream
state). MUST be in the "idle" state).
Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame
causes the stream state to become indeterminate. A receiver MUST causes the stream state to become indeterminate. A receiver MUST
treat the receipt of a PUSH_PROMISE on a stream that is neither treat the receipt of a PUSH_PROMISE on a stream that is neither
"open" nor "half-closed (local)" as a connection error "open" nor "half-closed (local)" as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. Similarly, a receiver MUST (Section 5.4.1) of type PROTOCOL_ERROR. Similarly, a receiver MUST
treat the receipt of a PUSH_PROMISE that promises an illegal stream treat the receipt of a PUSH_PROMISE that promises an illegal stream
identifier (Section 5.1.1) (that is, an identifier for a stream that identifier (Section 5.1.1) (that is, an identifier for a stream that
is not currently in the "idle" state) as a connection error is not currently in the "idle" state) as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. (Section 5.4.1) of type PROTOCOL_ERROR.
The PUSH_PROMISE frame includes optional padding. Padding fields and
flags are identical to those defined for DATA frames (Section 6.1).
6.7. PING 6.7. PING
The PING frame (type=0x6) is a mechanism for measuring a minimal The PING frame (type=0x6) is a mechanism for measuring a minimal
round-trip time from the sender, as well as determining whether an round-trip time from the sender, as well as determining whether an
idle connection is still functional. PING frames can be sent from idle connection is still functional. PING frames can be sent from
any endpoint. any endpoint.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 36, line 31 skipping to change at page 41, line 39
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. An endpoint might choose to close a connection without working. An endpoint might choose to close a connection without
sending GOAWAY for misbehaving peers. sending GOAWAY for misbehaving peers.
After sending a GOAWAY frame, the sender can discard frames for new After sending a GOAWAY frame, the sender can discard frames for new
streams. However, any frames that alter connection state cannot be streams. However, any frames that alter connection state cannot be
completely ignored. For instance, HEADERS, PUSH_PROMISE and completely ignored. For instance, HEADERS, PUSH_PROMISE and
CONTINUATION frames MUST be minimally processed to ensure a CONTINUATION frames MUST be minimally processed to ensure the state
consistent compression state (see Section 4.3); similarly DATA frames maintained for header compression is consistent (see Section 4.3);
MUST be counted toward the connection flow control window. similarly DATA frames MUST be counted toward the connection flow
control window.
0 1 2 3 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 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) | |R| Last-Stream-ID (31) |
+-+-------------------------------------------------------------+ +-+-------------------------------------------------------------+
| Error Code (32) | | Error Code (32) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Additional Debug Data (*) | | Additional Debug Data (*) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
GOAWAY Payload Format GOAWAY Payload Format
The GOAWAY frame does not define any flags. The GOAWAY frame does not define any flags.
The GOAWAY frame applies to the connection, not a specific stream. The GOAWAY frame applies to the connection, not a specific stream.
An endpoint MUST treat a GOAWAY frame with a stream identifier other An endpoint MUST treat a GOAWAY frame with a stream identifier other
than 0x0 as a connection error (Section 5.4.1) of type than 0x0 as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
The last stream identifier in the GOAWAY frame contains the highest The last stream identifier in the GOAWAY frame contains the highest
numbered stream identifier for which the sender of the GOAWAY frame numbered stream identifier for which the sender of the GOAWAY frame
has received frames on and might have taken some action on. All has received frames and might have taken some action on. All streams
streams up to and including the identified stream might have been up to and including the identified stream might have been processed
processed in some way. The last stream identifier is set to 0 if no in some way. The last stream identifier is set to 0 if no streams
streams were processed. were processed.
Note: In this case, "processed" means that some data from the Note: In this case, "processed" means that some data from the
stream was passed to some higher layer of software that might have stream was passed to some higher layer of software that might have
taken some action as a result. taken some action as a result.
If a connection terminates without a GOAWAY frame, this value is If a connection terminates without a GOAWAY frame, this value is
effectively the highest stream identifier. effectively the highest stream identifier.
On streams with lower or equal numbered identifiers that were not On streams with lower or equal numbered identifiers that were not
closed completely prior to the connection being closed, re-attempting closed completely prior to the connection being closed, re-attempting
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If an endpoint maintains the connection and continues to exchange If an endpoint maintains the connection and continues to exchange
frames, ignored frames MUST be counted toward flow control limits frames, ignored frames MUST be counted toward flow control limits
(Section 5.2) or update header compression state (Section 4.3). (Section 5.2) or update header compression state (Section 4.3).
Otherwise, flow control or header compression state can become Otherwise, flow control or header compression state can become
unsynchronized. unsynchronized.
The GOAWAY frame also contains a 32-bit error code (Section 7) that The GOAWAY frame also contains a 32-bit error code (Section 7) that
contains the reason for closing the connection. contains the reason for closing the connection.
Endpoints MAY append opaque data to the payload of any GOAWAY frame. Endpoints MAY append opaque data to the payload of any GOAWAY frame.
Additional debug data is intended for diagnostic purposes only and Additional debug data is intended for diagnostic purposes only and
carries no semantic value. Debug information could contain security- carries no semantic value. Debug information could contain security-
or privacy-sensitive data. Logged or otherwise persistently stored or privacy-sensitive data. Logged or otherwise persistently stored
debug data MUST have adequate safeguards to prevent unauthorized debug data MUST have adequate safeguards to prevent unauthorized
access. access.
6.9. WINDOW_UPDATE 6.9. WINDOW_UPDATE
The WINDOW_UPDATE frame (type=0x8) is used to implement flow control. The WINDOW_UPDATE frame (type=0x8) is used to implement flow control;
see Section 5.2 for an overview.
Flow control operates at two levels: on each individual stream and on Flow control operates at two levels: on each individual stream and on
the entire connection. the entire connection.
Both types of flow control are hop by hop; that is, only between the Both types of flow control are hop-by-hop; that is, only between the
two endpoints. Intermediaries do not forward WINDOW_UPDATE frames two endpoints. Intermediaries do not forward WINDOW_UPDATE frames
between dependent connections. However, throttling of data transfer between dependent connections. However, throttling of data transfer
by any receiver can indirectly cause the propagation of flow control by any receiver can indirectly cause the propagation of flow control
information toward the original sender. information toward the original sender.
Flow control only applies to frames that are identified as being Flow control only applies to frames that are identified as being
subject to flow control. Of the frame types defined in this subject to flow control. Of the frame types defined in this
document, this includes only DATA frame. Frames that are exempt from document, this includes only DATA frame. Frames that are exempt from
flow control MUST be accepted and processed, unless the receiver is flow control MUST be accepted and processed, unless the receiver is
unable to assign resources to handling the frame. A receiver MAY unable to assign resources to handling the frame. A receiver MAY
respond with a stream error (Section 5.4.2) or connection error respond with a stream error (Section 5.4.2) or connection error
(Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable accept a (Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable accept a
frame. frame.
0 1 2 3 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| Window Size Increment (31) | |R| Window Size Increment (31) |
+-+-------------------------------------------------------------+ +-+-------------------------------------------------------------+
WINDOW_UPDATE Payload Format WINDOW_UPDATE Payload Format
The payload of a WINDOW_UPDATE frame is one reserved bit, plus an The payload of a WINDOW_UPDATE frame is one reserved bit, plus an
unsigned 31-bit integer indicating the number of bytes that the unsigned 31-bit integer indicating the number of bytes that the
sender can transmit in addition to the existing flow control window. sender can transmit in addition to the existing flow control window.
The legal range for the increment to the flow control window is 1 to The legal range for the increment to the flow control window is 1 to
2^31 - 1 (0x7fffffff) bytes. 2^31 - 1 (0x7fffffff) bytes.
skipping to change at page 40, line 8 skipping to change at page 45, line 15
sends a RST_STREAM with the error code of FLOW_CONTROL_ERROR code; sends a RST_STREAM with the error code of FLOW_CONTROL_ERROR code;
for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code. for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code.
Flow controlled frames from the sender and WINDOW_UPDATE frames from Flow controlled frames from the sender and WINDOW_UPDATE frames from
the receiver are completely asynchronous with respect to each other. the receiver are completely asynchronous with respect to each other.
This property allows a receiver to aggressively update the window This property allows a receiver to aggressively update the window
size kept by the sender to prevent streams from stalling. size kept by the sender to prevent streams from stalling.
6.9.2. Initial Flow Control Window Size 6.9.2. Initial Flow Control Window Size
When a HTTP/2 connection is first established, new streams are When an HTTP/2 connection is first established, new streams are
created with an initial flow control window size of 65,535 bytes. created with an initial flow control window size of 65,535 bytes.
The connection flow control window is 65,535 bytes. Both endpoints The connection flow control window is 65,535 bytes. Both endpoints
can adjust the initial window size for new streams by including a can adjust the initial window size for new streams by including a
value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that
forms part of the connection header. forms part of the connection preface. The connection flow control
window initial size cannot be changed.
Prior to receiving a SETTINGS frame that sets a value for Prior to receiving a SETTINGS frame that sets a value for
SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default
initial window size when sending flow controlled frames. Similarly, initial window size when sending flow controlled frames. Similarly,
the connection flow control window is set to the default initial the connection flow control window is set to the default initial
window size until a WINDOW_UPDATE frame is received. window size until a WINDOW_UPDATE frame is received.
A SETTINGS frame can alter the initial flow control window size for A SETTINGS frame can alter the initial flow control window size for
all current streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE all current streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE
changes, a receiver MUST adjust the size of all stream flow control changes, a receiver MUST adjust the size of all stream flow control
skipping to change at page 41, line 5 skipping to change at page 46, line 13
window to being positive, after which the client can resume sending. window to being positive, after which the client can resume sending.
6.9.3. Reducing the Stream Window Size 6.9.3. Reducing the Stream Window Size
A receiver that wishes to use a smaller flow control window than the A receiver that wishes to use a smaller flow control window than the
current size can send a new SETTINGS frame. However, the receiver current size can send a new SETTINGS frame. However, the receiver
MUST be prepared to receive data that exceeds this window size, since MUST be prepared to receive data that exceeds this window size, since
the sender might send data that exceeds the lower limit prior to the sender might send data that exceeds the lower limit prior to
processing the SETTINGS frame. processing the SETTINGS frame.
A receiver has two options for handling streams that exceed flow After sending a SETTINGS frame that reduces the initial flow control
control limits: window size, a receiver has two options for handling streams that
exceed flow control limits:
1. The receiver can immediately send RST_STREAM with 1. The receiver can immediately send RST_STREAM with
FLOW_CONTROL_ERROR error code for the affected streams. FLOW_CONTROL_ERROR error code for the affected streams.
2. The receiver can accept the streams and tolerate the resulting 2. The receiver can accept the streams and tolerate the resulting
head of line blocking, sending WINDOW_UPDATE frames as it head of line blocking, sending WINDOW_UPDATE frames as it
consumes data. consumes data.
If a receiver decides to accept streams, both sides MUST recompute
the available flow control window based on the initial window size
sent in the SETTINGS.
6.10. CONTINUATION 6.10. CONTINUATION
The CONTINUATION frame (type=0x9) is used to continue a sequence of The CONTINUATION frame (type=0x9) is used to continue a sequence of
header block fragments (Section 4.3). Any number of CONTINUATION header block fragments (Section 4.3). Any number of CONTINUATION
frames can be sent on an existing stream, as long as the preceding frames can be sent on an existing stream, as long as the preceding
frame on the same stream is one of HEADERS, PUSH_PROMISE or frame is on the same stream and is a HEADERS, PUSH_PROMISE or
CONTINUATION without the END_HEADERS or END_PUSH_PROMISE flag set. CONTINUATION frame without the END_HEADERS flag set.
0 1 2 3 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Pad High(8)] | [Pad Low (8)] | Header Block Fragment (*) . | Pad High? (8) | Pad Low? (8) |
+---------------+---------------+-------------------------------+ +---------------+---------------+-------------------------------+
| Header Block Fragment (*) ... | Header Block Fragment (*) ...
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Padding (*) ... | Padding (*) ...
+---------------------------------------------------------------+ +---------------------------------------------------------------+
CONTINUATION Frame Payload CONTINUATION Frame Payload
The CONTINUATION frame payload has the following fields: The CONTINUATION frame payload has the following fields:
skipping to change at page 42, line 13 skipping to change at page 47, line 19
The CONTINUATION frame defines the following flags: The CONTINUATION frame defines the following flags:
END_HEADERS (0x4): Bit 3 being set indicates that this frame ends a END_HEADERS (0x4): Bit 3 being set indicates that this frame ends a
header block (Section 4.3). header block (Section 4.3).
If the END_HEADERS bit is not set, this frame MUST be followed by If the END_HEADERS bit is not set, this frame MUST be followed by
another CONTINUATION frame. A receiver MUST treat the receipt of another CONTINUATION frame. A receiver MUST treat the receipt of
any other type of frame or a frame on a different stream as a any other type of frame or a frame on a different stream as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR. connection error (Section 5.4.1) of type PROTOCOL_ERROR.
PAD_LOW (0x10): Bit 5 being set indicates that the Pad Low field is PAD_LOW (0x8): Bit 4 being set indicates that the Pad Low field is
present. present.
PAD_HIGH (0x20): Bit 6 being set indicates that the Pad High field PAD_HIGH (0x10): Bit 5 being set indicates that the Pad High field
is present. This bit MUST NOT be set unless the PAD_LOW flag is is present. This bit MUST NOT be set unless the PAD_LOW flag is
also set. Endpoints that receive a frame with PAD_HIGH set and also set. Endpoints that receive a frame with PAD_HIGH set and
PAD_LOW cleared MUST treat this as a connection error PAD_LOW cleared MUST treat this as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. (Section 5.4.1) of type PROTOCOL_ERROR.
The payload of a CONTINUATION frame contains a header block fragment The payload of a CONTINUATION frame contains a header block fragment
(Section 4.3). (Section 4.3).
The CONTINUATION frame changes the connection state as defined in The CONTINUATION frame changes the connection state as defined in
Section 4.3. Section 4.3.
skipping to change at page 42, line 41 skipping to change at page 47, line 47
type PROTOCOL_ERROR. type PROTOCOL_ERROR.
A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or
CONTINUATION frame without the END_HEADERS flag set. A recipient CONTINUATION frame without the END_HEADERS flag set. A recipient
that observes violation of this rule MUST respond with a connection that observes violation of this rule MUST respond with a connection
error (Section 5.4.1) of type PROTOCOL_ERROR. error (Section 5.4.1) of type PROTOCOL_ERROR.
The CONTINUATION frame includes optional padding. Padding fields and The CONTINUATION frame includes optional padding. Padding fields and
flags are identical to those defined for DATA frames (Section 6.1). flags are identical to those defined for DATA frames (Section 6.1).
6.11. ALTSVC
The ALTSVC frame (type=0xA) advertises the availability of an
alternative service to the client. It can be sent at any time for an
existing client-initiated stream or stream 0, and is intended to
allow servers to load balance or otherwise segment traffic; see
[ALT-SVC] for details (in particular, Section 2.4, which outlines
client handling of alternative services).
An ALTSVC frame on a client-initiated stream indicates that the
conveyed alternative service is associated with the origin of that
stream.
An ALTSVC frame on stream 0 indicates that the conveyed alternative
service is associated with the origin contained in the Origin field
of the frame. An association with an origin that the client does not
consider authoritative for the current connection MUST be ignored.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max-Age (32) |
+-------------------------------+----------------+--------------+
| Port (16) | Reserved (8) | PID_LEN (8) |
+-------------------------------+----------------+--------------+
| Protocol-ID (*) |
+---------------+-----------------------------------------------+
| HOST_LEN (8) | Host (*) ...
+---------------+-----------------------------------------------+
| Origin? (*) ...
+---------------------------------------------------------------+
The ALTSVC frame contains the following fields:
Max-Age: An unsigned, 32-bit integer indicating the freshness
lifetime of the alternative service association, as per [ALT-SVC],
Section 2.2.
Port: An unsigned, 16-bit integer indicating the port that the
alternative service is available upon.
Reserved: For future use. Senders MUST set these bits to '0', and
recipients MUST ignore them.
PID_LEN: An unsigned, 8-bit integer indicating the length, in
octets, of the PROTOCOL-ID field.
Protocol-ID: A sequence of bytes (length determined by PID_LEN)
containing the ALPN protocol identifier of the alternative
service.
HOST_LEN: An unsigned, 8-bit integer indicating the length, in
octets, of the Host field.
Host: A sequence of characters (length determined by HOST_LEN)
containing an ASCII string indicating the host that the
alternative service is available upon. An internationalized
domain name [IDNA] MUST be expressed using A-labels.
Origin: An optional sequence of characters (length determined by
subtracting the length of all preceding fields from the frame
length) containing ASCII serialisation of an origin ([RFC6454],
Section 6.2) that the alternate service is applicable to.
The ALTSVC frame does not define any flags.
The ALTSVC frame is intended for receipt by clients; a server that
receives an ALTSVC frame MUST treat it as a connection error of type
PROTOCOL_ERROR.
The ALTSVC frame is processed hop-by-hop. An intermediary MUST NOT
forward ALTSVC frames, though it can use the information contained in
ALTSVC frames in forming new ALTSVC frames to send to its own
clients.
7. Error Codes 7. Error Codes
Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY
frames to convey the reasons for the stream or connection error. frames to convey the reasons for the stream or connection error.
Error codes share a common code space. Some error codes only apply Error codes share a common code space. Some error codes only apply
to specific conditions and have no defined semantics in certain frame to specific conditions and have no defined semantics in certain frame
types. types.
The following error codes are defined: The following error codes are defined:
skipping to change at page 44, line 7 skipping to change at page 50, line 41
ENHANCE_YOUR_CALM (11): The endpoint detected that its peer is ENHANCE_YOUR_CALM (11): The endpoint detected that its peer is
exhibiting a behavior over a given amount of time that has caused exhibiting a behavior over a given amount of time that has caused
it to refuse to process further frames. it to refuse to process further frames.
INADEQUATE_SECURITY (12): The underlying transport has properties INADEQUATE_SECURITY (12): The underlying transport has properties
that do not meet the minimum requirements imposed by this document that do not meet the minimum requirements imposed by this document
(see Section 9.2) or the endpoint. (see Section 9.2) or the endpoint.
8. HTTP Message Exchanges 8. HTTP Message Exchanges
HTTP/2 is intended to be as compatible as possible with current web- HTTP/2 is intended to be as compatible as possible with current uses
based applications. This means that, from the perspective of the of HTTP. This means that, from the perspective of the server and
server business logic or application API, the features of HTTP are client applications, the features of the protocol are unchanged. To
unchanged. To achieve this, all of the application request and achieve this, all request and response semantics are preserved,
response header semantics are preserved, although the syntax of although the syntax of conveying those semantics has changed.
conveying those semantics has changed. Thus, the rules from HTTP/1.1
([HTTP-p1], [HTTP-p2], [HTTP-p4], [HTTP-p5], [HTTP-p6], and Thus, the specification and requirements of HTTP/1.1 Semantics and
[HTTP-p7]) apply with the changes in the sections below. Content [HTTP-p2], Conditional Requests [HTTP-p4], Range Requests
[HTTP-p5], Caching [HTTP-p6] and Authentication [HTTP-p7] are
applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax
and Routing [HTTP-p1], such as the HTTP and HTTPS URI schemes, are
also applicable in HTTP/2, but the expression of those semantics for
this protocol are defined in the sections below.
8.1. HTTP Request/Response Exchange 8.1. HTTP Request/Response Exchange
A client sends an HTTP request on a new stream, using a previously A client sends an HTTP request on a new stream, using a previously
unused stream identifier (Section 5.1.1). A server sends an HTTP unused stream identifier (Section 5.1.1). A server sends an HTTP
response on the same stream as the request. response on the same stream as the request.
An HTTP request or response each consist of: An HTTP message (request or response) consists of:
1. a HEADERS frame;
2. one contiguous sequence of zero or more CONTINUATION frames; 1. one HEADERS frame, followed by zero or more CONTINUATION frames
(containing the message headers; see [HTTP-p1], Section 3.2), and
3. zero or more DATA frames; and 2. zero or more DATA frames (containing the message payload; see
[HTTP-p1], Section 3.3), and
4. optionally, a contiguous sequence that starts with a HEADERS 3. optionally, one HEADERS frame, followed by zero or more
frame, followed by zero or more CONTINUATION frames. CONTINUATION frames (containing the trailer-part, if present; see
[HTTP-p1], Section 4.1.2).
The last frame in the sequence bears an END_STREAM flag, though a The last frame in the sequence bears an END_STREAM flag, though a
HEADERS frame bearing the END_STREAM flag can be followed by HEADERS frame bearing the END_STREAM flag can be followed by
CONTINUATION frames that carry any remaining portions of the header CONTINUATION frames that carry any remaining portions of the header
block. block.
Other frames MAY be interspersed with these frames, but those frames Other frames (from any stream) MUST NOT occur between either HEADERS
do not carry HTTP semantics. In particular, HEADERS frames (and any frame and the following CONTINUATION frames (if present), nor between
CONTINUATION frames that follow) other than the first and optional CONTINUATION frames.
last frames in this sequence do not carry HTTP semantics.
Otherwise, frames MAY be interspersed on the stream between these
frames, but those frames do not carry HTTP semantics. In particular,
HEADERS frames (and any CONTINUATION frames that follow) other than
the first and optional last frames in this sequence do not carry HTTP
semantics.
Trailing header fields are carried in a header block that also Trailing header fields are carried in a header block that also
terminates the stream. That is, a sequence starting with a HEADERS terminates the stream. That is, a sequence starting with a HEADERS
frame, followed by zero or more CONTINUATION frames, where the frame, followed by zero or more CONTINUATION frames, where the
HEADERS frame bears an END_STREAM flag. Header blocks after the HEADERS frame bears an END_STREAM flag. Header blocks after the
first that do not terminate the stream are not part of an HTTP first that do not terminate the stream are not part of an HTTP
request or response. request or response.
An HTTP request/response exchange fully consumes a single stream. A An HTTP request/response exchange fully consumes a single stream. A
request starts with the HEADERS frame that puts the stream into an request starts with the HEADERS frame that puts the stream into an
skipping to change at page 45, line 14 skipping to change at page 52, line 11
the stream to become "half closed" for the client. A response starts the stream to become "half closed" for the client. A response starts
with a HEADERS frame and ends with a frame bearing END_STREAM, with a HEADERS frame and ends with a frame bearing END_STREAM,
optionally followed by CONTINUATION frames, which places the stream optionally followed by CONTINUATION frames, which places the stream
in the "closed" state. in the "closed" state.
8.1.1. Informational Responses 8.1.1. Informational Responses
The 1xx series of HTTP response status codes ([HTTP-p2], Section 6.2) The 1xx series of HTTP response status codes ([HTTP-p2], Section 6.2)
are not supported in HTTP/2. are not supported in HTTP/2.
The most common use case for 1xx is using a Expect header field with The most common use case for 1xx is using an Expect header field with
a "100-continue" token (colloquially, "Expect/continue") to indicate a "100-continue" token (colloquially, "Expect/continue") to indicate
that the client expects a 100 (Continue) non-final response status that the client expects a 100 (Continue) non-final response status
code, receipt of which indicates that the client should continue code, receipt of which indicates that the client should continue
sending the request body if it has not already done so. sending the request body if it has not already done so.
Typically, Expect/continue is used by clients wishing to avoid Typically, Expect/continue is used by clients wishing to avoid
sending a large amount of data in a request body, only to have the sending a large amount of data in a request body, only to have the
request rejected by the origin server. request rejected by the origin server (thus leaving the connection
potentially unusable).
HTTP/2 does not enable the Expect/continue mechanism; if the server HTTP/2 does not enable the Expect/continue mechanism; if the server
sends a final status code to reject the request, it can do so without sends a final status code to reject the request, it can do so without
making the underlying connection unusable. making the underlying connection unusable.
Note that this means HTTP/2 clients sending requests with bodies may Note that this means HTTP/2 clients sending requests with bodies may
waste at least one round trip of sent data when the request is waste at least one round trip of sent data when the request is
rejected. This can be mitigated by restricting the amount of data rejected. This can be mitigated by restricting the amount of data
sent for the first round trip by bandwidth-constrained clients, in sent for the first round trip by bandwidth-constrained clients, in
anticipation of a final status code. anticipation of a final status code.
Other defined 1xx status codes are not applicable to HTTP/2; the Other defined 1xx status codes are not applicable to HTTP/2. For
semantics of 101 (Switching Protocols) is better expressed using a example, the semantics of 101 (Switching Protocols) aren't suitable
distinct frame type, since they apply to the entire connection, not to a multiplexed protocol. Likewise, 102 (Processing) is no longer
just one stream. Likewise, 102 (Processing) is no longer necessary, necessary, because HTTP/2 has a separate means of keeping the
because HTTP/2 has a separate means of keeping the connection alive. connection alive.
This difference between protocol versions necessitates special This difference between protocol versions necessitates special
handling by intermediaries that translate between them: handling by intermediaries that translate between them:
o An intermediary that gateways HTTP/1.1 to HTTP/2 MUST generate a o An intermediary that gateways HTTP/1.1 to HTTP/2 MUST generate a
100 (Continue) response if a received request includes and Expect 100 (Continue) response if a received request includes and Expect
header field with a "100-continue" token ([HTTP-p2], Section header field with a "100-continue" token ([HTTP-p2], Section
5.1.1), unless it can immediately generate a final status code. 5.1.1), unless it can immediately generate a final status code.
It MUST NOT forward the "100-continue" expectation in the request It MUST NOT forward the "100-continue" expectation in the request
header fields. header fields.
skipping to change at page 46, line 16 skipping to change at page 53, line 14
o An intermediary that gateways HTTP/2 to HTTP/1.1 MUST discard all o An intermediary that gateways HTTP/2 to HTTP/1.1 MUST discard all
other 1xx informational responses. other 1xx informational responses.
8.1.2. Examples 8.1.2. Examples
This section shows HTTP/1.1 requests and responses, with This section shows HTTP/1.1 requests and responses, with
illustrations of equivalent HTTP/2 requests and responses. illustrations of equivalent HTTP/2 requests and responses.
An HTTP GET request includes request header fields and no body and is An HTTP GET request includes request header fields and no body and is
therefore transmitted as a single contiguous sequence of HEADERS and therefore transmitted as a single HEADERS frame, followed by zero or
CONTINUATION frames containing the serialized block of request header more CONTINUATION frames containing the serialized block of request
fields. The last HEADERS frame in the sequence has both the header fields. The last HEADERS frame in the sequence has both the
END_HEADERS and END_STREAM flags set: END_HEADERS and END_STREAM flags set:
GET /resource HTTP/1.1 HEADERS GET /resource HTTP/1.1 HEADERS
Host: example.org ==> + END_STREAM Host: example.org ==> + END_STREAM
Accept: image/jpeg + END_HEADERS Accept: image/jpeg + END_HEADERS
:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.org :path = /resource
:path = /resource host = example.org
accept = image/jpeg accept = image/jpeg
Similarly, a response that includes only response header fields is Similarly, a response that includes only response header fields is
transmitted as a sequence of HEADERS frames containing the serialized transmitted as a HEADERS frame (again, followed by zero or more
block of response header fields. The last HEADERS frame in the CONTINUATION frames) containing the serialized block of response
sequence has both the END_HEADERS and END_STREAM flag set: header fields. The last HEADERS frame in the sequence has both the
END_HEADERS and END_STREAM flag set:
HTTP/1.1 304 Not Modified HEADERS HTTP/1.1 304 Not Modified HEADERS
ETag: "xyzzy" ===> + END_STREAM ETag: "xyzzy" ==> + END_STREAM
Expires: Thu, 23 Jan ... + END_HEADERS Expires: Thu, 23 Jan ... + END_HEADERS
:status = 304 :status = 304
etag: "xyzzy" etag: "xyzzy"
expires: Thu, 23 Jan ... expires: Thu, 23 Jan ...
An HTTP POST request that includes request header fields and payload An HTTP POST request that includes request header fields and payload
data is transmitted as one HEADERS frame, followed by zero or more data is transmitted as one HEADERS frame, followed by zero or more
CONTINUATION frames containing the request header fields, followed by CONTINUATION frames containing the request header fields, followed by
one or more DATA frames, with the last CONTINUATION (or HEADERS) one or more DATA frames, with the last CONTINUATION (or HEADERS)
frame having the END_HEADERS flag set and the final DATA frame having frame having the END_HEADERS flag set and the final DATA frame having
the END_STREAM flag set: the END_STREAM flag set:
POST /resource HTTP/1.1 HEADERS POST /resource HTTP/1.1 HEADERS
Host: example.org ==> - END_STREAM Host: example.org ==> - END_STREAM
Content-Type: image/jpeg + END_HEADERS Content-Type: image/jpeg + END_HEADERS
Content-Length: 123 :method = POST Content-Length: 123 :method = POST
:scheme = https :scheme = https
{binary data} :authority = example.org {binary data} :path = /resource
:path = /resource :authority = example.org
content-type = image/jpeg content-type = image/jpeg
content-length = 123 content-length = 123
DATA DATA
+ END_STREAM + END_STREAM
{binary data} {binary data}
A response that includes header fields and payload data is A response that includes header fields and payload data is
transmitted as a HEADERS frame, followed by zero or more CONTINUATION transmitted as a HEADERS frame, followed by zero or more CONTINUATION
frames, followed by one or more DATA frames, with the last DATA frame frames, followed by one or more DATA frames, with the last DATA frame
in the sequence having the END_STREAM flag set: in the sequence having the END_STREAM flag set:
HTTP/1.1 200 OK HEADERS HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> - END_STREAM Content-Type: image/jpeg ==> - END_STREAM
Content-Length: 123 + END_HEADERS Content-Length: 123 + END_HEADERS
:status = 200 :status = 200
{binary data} content-type = image/jpeg {binary data} content-type = image/jpeg
content-length = 123 content-length = 123
DATA DATA
+ END_STREAM + END_STREAM
{binary data} {binary data}
Trailing header fields are sent as a header block after both the Trailing header fields are sent as a header block after both the
request or response header block and all the DATA frames have been request or response header block and all the DATA frames have been
sent. The sequence of HEADERS/CONTINUATION frames that bears the sent. The sequence of HEADERS/CONTINUATION frames that bears the
trailers includes a terminal frame that has both END_HEADERS and trailers includes a terminal frame that has both END_HEADERS and
END_STREAM flags set. END_STREAM flags set.
HTTP/1.1 200 OK HEADERS HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ===> - END_STREAM Content-Type: image/jpeg ==> - END_STREAM
Transfer-Encoding: chunked + END_HEADERS Transfer-Encoding: chunked + END_HEADERS
TE: trailers :status = 200 Trailer: Foo :status = 200
content-length = 123 content-length = 123
123 content-type = image/jpeg 123 content-type = image/jpeg
{binary data} {binary data} trailer = Foo
0 DATA 0
Foo: bar - END_STREAM Foo: bar DATA
{binary data} - END_STREAM
{binary data}
HEADERS HEADERS
+ END_STREAM + END_STREAM
+ END_HEADERS + END_HEADERS
foo: bar foo: bar
8.1.3. HTTP Header Fields 8.1.3. HTTP Header Fields
HTTP/2 request and response header fields carry information as a HTTP header fields carry information as a series of key-value pairs.
series of key-value pairs. This includes the target URI for the For a listing of registered HTTP headers, see the Message Header
request, the status code for the response, as well as HTTP header Field Registry maintained at
fields. <http://www.iana.org/assignments/message-headers>.
HTTP header field names are strings of ASCII characters that are While HTTP/1.x used the message start-line (see [HTTP-p1], Section
compared in a case-insensitive fashion. Header field names MUST be 3.1) to convey the target URI and method of the request, and the
converted to lowercase prior to their encoding in HTTP/2. A request status code for the response, HTTP/2 uses special pseudo-headers
or response containing uppercase header field names MUST be treated beginning with ":" for these tasks.
as malformed (Section 8.1.3.5).
Just as in HTTP/1.x, header field names are strings of ASCII
characters that are compared in a case-insensitive fashion. However,
header field names MUST be converted to lowercase prior to their
encoding in HTTP/2. A request or response containing uppercase
header field names MUST be treated as malformed (Section 8.1.3.5).
HTTP/2 does not use the Connection header field to indicate "hop-by- HTTP/2 does not use the Connection header field to indicate "hop-by-
hop" header fields; in this protocol, connection-specific metadata is hop" header fields; in this protocol, connection-specific metadata is
conveyed by other means. As such, a HTTP/2 message containing conveyed by other means. As such, a HTTP/2 message containing
Connection MUST be treated as malformed (Section 8.1.3.5). Connection MUST be treated as malformed (Section 8.1.3.5).
This means that an intermediary transforming a HTTP/1.x message to This means that an intermediary transforming an HTTP/1.x message to
HTTP/2 will need to remove any header fields nominated by the HTTP/2 will need to remove any header fields nominated by the
Connection header field, along with the Connection header field Connection header field, along with the Connection header field
itself. Such intermediaries SHOULD also remove other connection- itself. Such intermediaries SHOULD also remove other connection-
specific header fields, such as Keep-Alive, Proxy-Connection, specific header fields, such as Keep-Alive, Proxy-Connection,
Transfer-Encoding and Upgrade, even if they are not nominated by Transfer-Encoding and Upgrade, even if they are not nominated by
Connection. Connection.
One exception to this is the TE header field, which MAY be present in One exception to this is the TE header field, which MAY be present in
a HTTP/2 request, but when it is MUST NOT contain any value other an HTTP/2 request, but when it is MUST NOT contain any value other
than "trailers". than "trailers".
Note: HTTP/2 purposefully does not support upgrade to another Note: HTTP/2 purposefully does not support upgrade to another
protocol. The handshake methods described in Section 3 are protocol. The handshake methods described in Section 3 are
believed sufficient to negotiate the use of alternative protocols. believed sufficient to negotiate the use of alternative protocols.
8.1.3.1. Request Header Fields 8.1.3.1. Request Header Fields
HTTP/2 defines a number of header fields starting with a colon ':' HTTP/2 defines a number of header fields starting with a colon ':'
character that carry information about the request target: character that carry information about the request target:
o The ":method" header field includes the HTTP method ([HTTP-p2], o The ":method" header field includes the HTTP method ([HTTP-p2],
Section 4). Section 4).
o The ":scheme" header field includes the scheme portion of the o The ":scheme" header field includes the scheme portion of the
target URI ([RFC3986], Section 3.1). target URI ([RFC3986], Section 3.1).
":scheme" is not restricted to "http" and "https" schemed URIs. A
proxy or gateway can translate requests for non-HTTP schemes,
enabling the use of HTTP to interact with non-HTTP services.
o The ":authority" header field includes the authority portion of o The ":authority" header field includes the authority portion of
the target URI ([RFC3986], Section 3.2). The authority MUST NOT the target URI ([RFC3986], Section 3.2). The authority MUST NOT
include the deprecated "userinfo" subcomponent for "http:" or include the deprecated "userinfo" subcomponent for "http" or
"https:" URIs. "https" schemed URIs.
To ensure that the HTTP/1.1 request line can be reproduced To ensure that the HTTP/1.1 request line can be reproduced
accurately, this header field MUST be omitted when translating accurately, this header field MUST be omitted when translating
from an HTTP/1.1 request that has a request target in origin or from an HTTP/1.1 request that has a request target in origin or
asterisk form (see [HTTP-p1], Section 5.3). Clients that generate asterisk form (see [HTTP-p1], Section 5.3). Clients that generate
HTTP/2 requests directly SHOULD instead omit the "Host" header HTTP/2 requests directly SHOULD instead omit the "Host" header
field. An intermediary that converts a request to HTTP/1.1 MUST field. An intermediary that converts an HTTP/2 request to
create a "Host" header field if one is not present in a request by HTTP/1.1 MUST create a "Host" header field if one is not present
copying the value of the ":authority" header field. in a request by copying the value of the ":authority" header
field.
o The ":path" header field includes the path and query parts of the o The ":path" header field includes the path and query parts of the
target URI (the "path-absolute" production from [RFC3986] and target URI (the "path-absolute" production from [RFC3986] and
optionally a '?' character followed by the "query" production, see optionally a '?' character followed by the "query" production, see
[RFC3986], Section 3.3 and [RFC3986], Section 3.4). This field [RFC3986], Section 3.3 and [RFC3986], Section 3.4). This field
MUST NOT be empty; URIs that do not contain a path component MUST MUST NOT be empty; URIs that do not contain a path component MUST
include a value of '/', unless the request is an OPTIONS request include a value of '/', unless the request is an OPTIONS request
in asterisk form, in which case the ":path" header field MUST in asterisk form, in which case the ":path" header field MUST
include '*'. include '*'.
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status code field (see [HTTP-p2], Section 6). This header field MUST status code field (see [HTTP-p2], Section 6). This header field MUST
be included in all responses, otherwise the response is malformed be included in all responses, otherwise the response is malformed
(Section 8.1.3.5). (Section 8.1.3.5).
HTTP/2 does not define a way to carry the version or reason phrase HTTP/2 does not define a way to carry the version or reason phrase
that is included in an HTTP/1.1 status line. that is included in an HTTP/1.1 status line.
8.1.3.3. Header Field Ordering 8.1.3.3. Header Field Ordering
HTTP Header Compression [COMPRESSION] does not preserve the order of HTTP Header Compression [COMPRESSION] does not preserve the order of
header fields. The relative order of header fields with different header fields, because the relative order of header fields with
names is not important. However, the same header field can be different names is not important. However, the same header field can
repeated to form a comma-separated list (see [HTTP-p1], Section be repeated to form a list (see [HTTP-p1], Section 3.2.2), where the
3.2.2), where the relative order of header field values is relative order of header field values is significant. This
significant. This repetition can occur either as a single header repetition can occur either as a single header field with a comma-
field with a comma-separated list of values, or as several header separated list of values, or as several header fields with a single
fields with a single value, or any combination thereof. value, or any combination thereof. Therefore, in the latter case,
ordering needs to be preserved before compression takes place.
To preserve the order of a comma-separated list, the ordered values To preserve the order of multiple occurrences of a header field with
for a single header field name appearing in different header fields the same name, its ordered values are concatenated into a single
are concatenated into a single value. A zero-valued octet (0x0) is value using a zero-valued octet (0x0) to delimit them.
used to delimit multiple values.
After decompression, header fields that have values containing zero After decompression, header fields that have values containing zero
octets (0x0) MUST be split into multiple header fields before being octets (0x0) MUST be split into multiple header fields before being
processed. processed.
For example, the following HTTP/1.x header block:
Content-Type: text/html
Cache-Control: max-age=60, private
Cache-Control: must-revalidate
contains three Cache-Control directives; two in the first Cache-
Control header field, and the last one in the second Cache-Control
field. Before compression, they would need to be converted to a form
similar to this (with 0x0 represented as "\0"):
cache-control: max-age=60, private\0must-revalidate
content-type: text/html
Note here that the ordering between Content-Type and Cache-Control is
not preserved, but the relative ordering of the Cache-Control
directives -- as well as the fact that the first two were comma-
separated, while the last was on a different line -- is.
Header fields containing multiple values MUST be concatenated into a Header fields containing multiple values MUST be concatenated into a
single value unless the ordering of that header field is known to be single value unless the ordering of that header field is known to be
not significant. insignificant.
The special case of "set-cookie" - which does not form a comma- The special case of "set-cookie" - which does not form a comma-
separated list, but can have multiple values - does not depend on separated list, but can have multiple values - does not depend on
ordering. The "set-cookie" header field MAY be encoded as multiple ordering. The "set-cookie" header field MAY be encoded as multiple
header field values, or as a single concatenated value. header field values, or as a single concatenated value.
8.1.3.4. Compressing the Cookie Header Field 8.1.3.4. Compressing the Cookie Header Field
The Cookie header field [COOKIE] can carry a significant amount of The Cookie header field [COOKIE] can carry a significant amount of
redundant data. redundant data.
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To allow for better compression efficiency, the Cookie header field To allow for better compression efficiency, the Cookie header field
MAY be split into separate header fields, each with one or more MAY be split into separate header fields, each with one or more
cookie-pairs. If there are multiple Cookie header fields after cookie-pairs. If there are multiple Cookie header fields after
decompression, these MUST be concatenated into a single octet string decompression, these MUST be concatenated into a single octet string
using the two octet delimiter of 0x3B, 0x20 (the ASCII string "; "). using the two octet delimiter of 0x3B, 0x20 (the ASCII string "; ").
The Cookie header field MAY be split using a zero octet (0x0), as The Cookie header field MAY be split using a zero octet (0x0), as
defined in Section 8.1.3.3. When decoding, zero octets MUST be defined in Section 8.1.3.3. When decoding, zero octets MUST be
replaced with the cookie delimiter ("; "). replaced with the cookie delimiter ("; ").
8.1.3.5. Malformed Requests and Responses 8.1.3.5. Malformed Messages
A malformed request or response is one that uses a valid sequence of A malformed request or response is one that uses a valid sequence of
HTTP/2 frames, but is otherwise invalid due to the presence of HTTP/2 frames, but is otherwise invalid due to the presence of
prohibited header fields, the absence of mandatory header fields, or prohibited header fields, the absence of mandatory header fields, or
the inclusion of uppercase header field names. the inclusion of uppercase header field names.
A request or response that includes an entity body can include a A request or response that includes an entity body can include a
"content-length" header field. A request or response is also "content-length" header field. A request or response is also
malformed if the value of a "content-length" header field does not malformed if the value of a "content-length" header field does not
equal the sum of the DATA frame payload lengths that form the body. equal the sum of the DATA frame payload lengths that form the body.
Intermediaries that process HTTP requests or responses (i.e., all Intermediaries that process HTTP requests or responses (i.e., all
intermediaries other than those acting as tunnels) MUST NOT forward a intermediaries other than those acting as tunnels) MUST NOT forward a
malformed request or response. malformed request or response.
Implementations that detect malformed requests or responses need to Implementations that detect malformed requests or responses need to
ensure that the stream ends. For malformed requests, a server MAY ensure that the stream ends. For malformed requests, a server MAY
send an HTTP response prior to closing or resetting the stream. send an HTTP response prior to closing or resetting the stream.
Clients MUST NOT accept a malformed response. Clients MUST NOT accept a malformed response. Note that these
requirements are intended to protect against several types of common
attacks against HTTP; they are deliberately strict, because being
permissive can expose implementations to these vulnerabilites.
8.1.4. Request Reliability Mechanisms in HTTP/2 8.1.4. Request Reliability Mechanisms in HTTP/2
In HTTP/1.1, an HTTP client is unable to retry a non-idempotent In HTTP/1.1, an HTTP client is unable to retry a non-idempotent
request when an error occurs, because there is no means to determine request when an error occurs, because there is no means to determine
the nature of the error. It is possible that some server processing the nature of the error. It is possible that some server processing
occurred prior to the error, which could result in undesirable occurred prior to the error, which could result in undesirable
effects if the request were reattempted. effects if the request were reattempted.
HTTP/2 provides two mechanisms for providing a guarantee to a client HTTP/2 provides two mechanisms for providing a guarantee to a client
skipping to change at page 52, line 14 skipping to change at page 59, line 49
o The GOAWAY frame indicates the highest stream number that might o The GOAWAY frame indicates the highest stream number that might
have been processed. Requests on streams with higher numbers are have been processed. Requests on streams with higher numbers are
therefore guaranteed to be safe to retry. therefore guaranteed to be safe to retry.
o The REFUSED_STREAM error code can be included in a RST_STREAM o The REFUSED_STREAM error code can be included in a RST_STREAM
frame to indicate that the stream is being closed prior to any frame to indicate that the stream is being closed prior to any
processing having occurred. Any request that was sent on the processing having occurred. Any request that was sent on the
reset stream can be safely retried. reset stream can be safely retried.
Clients MUST NOT treat requests that have not been processed as Requests that have not been processed have not failed; clients MAY
having failed. Clients MAY automatically retry these requests, automatically retry them, even those with non-idempotent methods.
including those with non-idempotent methods.
A server MUST NOT indicate that a stream has not been processed A server MUST NOT indicate that a stream has not been processed
unless it can guarantee that fact. If frames that are on a stream unless it can guarantee that fact. If frames that are on a stream
are passed to the application layer for any stream, then are passed to the application layer for any stream, then
REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame
MUST include a stream identifier that is greater than or equal to the MUST include a stream identifier that is greater than or equal to the
given stream identifier. given stream identifier.
In addition to these mechanisms, the PING frame provides a way for a In addition to these mechanisms, the PING frame provides a way for a
client to easily test a connection. Connections that remain idle can client to easily test a connection. Connections that remain idle can
become broken as some middleboxes (for instance, network address become broken as some middleboxes (for instance, network address
translators, or load balancers) silently discard connection bindings. translators, or load balancers) silently discard connection bindings.
The PING frame allows a client to safely test whether a connection is The PING frame allows a client to safely test whether a connection is
still active without sending a request. still active without sending a request.
8.2. Server Push 8.2. Server Push
HTTP/2 enables a server to pre-emptively send (or "push") multiple HTTP/2 enables a server to pre-emptively send (or "push") one or more
associated resources to a client in response to a single request. associated responses to a client in response to a single request.
This feature becomes particularly helpful when the server knows the This feature becomes particularly helpful when the server knows the
client will need to have those resources available in order to fully client will need to have those responses available in order to fully
process the originally requested resource. process the response to the original request.
Pushing additional resources is optional, and is negotiated only Pushing additional responses is optional, and is negotiated between
between individual endpoints. The SETTINGS_ENABLE_PUSH setting can individual endpoints. The SETTINGS_ENABLE_PUSH setting can be set to
be set to 0 to indicate that server push is disabled. Even if 0 to indicate that server push is disabled.
enabled, an intermediary could receive pushed resources from the
server but could choose not to forward those on to the client. How
to make use of the pushed resources is up to that intermediary.
Equally, the intermediary might choose to push additional resources
to the client, without any action taken by the server.
A client cannot push resources. Clients and servers MUST operate as Because pushing responses is effectively hop-by-hop, an intermediary
though the server has disabled PUSH_PROMISE by setting the could receive pushed responses from the server and choose not to
SETTINGS_ENABLE_PUSH to 0. As a consequence, servers MUST treat the forward those on to the client. In other words, how to make use of
receipt of a PUSH_PROMISE frame as a connection error the pushed responses is up to that intermediary. Equally, the
(Section 5.4.1). Clients MUST reject any attempt to change this intermediary might choose to push additional responses to the client,
setting by treating the message as a connection error (Section 5.4.1) without any action taken by the server.
of type PROTOCOL_ERROR.
A server can only push requests that are safe (see [HTTP-p2], Section A client cannot push. Thus, servers MUST treat the receipt of a
4.2.1), cacheable (see [HTTP-p6], Section 3) and do not include a PUSH_PROMISE frame as a connection error (Section 5.4.1). Clients
request body. MUST reject any attempt to change the SETTINGS_ENABLE_PUSH setting to
a value other than "0" by treating the message as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
A server can only push responses that are cacheable (see [HTTP-p6],
Section 3); promised requests MUST be safe (see [HTTP-p2], Section
4.2.1) and MUST NOT include a request body.
8.2.1. Push Requests 8.2.1. Push Requests
Server push is semantically equivalent to a server responding to a Server push is semantically equivalent to a server responding to a
request. The PUSH_PROMISE frame, or frames, sent by the server request; however, in this case that request is also sent by the
includes a header block that contains a complete set of request server, as a PUSH_PROMISE frame.
header fields that the server attributes to the request. It is not
possible to push a response to a request that includes a request
body.
Pushed resources are always associated with an explicit request from The PUSH_PROMISE frame includes a header block that contains a
a client. The PUSH_PROMISE frames sent by the server are sent on the complete set of request header fields that the server attributes to
stream created for the original request. The PUSH_PROMISE frame the request. It is not possible to push a response to a request that
includes a promised stream identifier, chosen from the stream includes a request body.
identifiers available to the server (see Section 5.1.1).
Pushed responses are always associated with an explicit request from
the client. The PUSH_PROMISE frames sent by the server are sent on
that explicit request's stream. The PUSH_PROMISE frame also includes
a promised stream identifier, chosen from the stream identifiers
available to the server (see Section 5.1.1).
The header fields in PUSH_PROMISE and any subsequent CONTINUATION The header fields in PUSH_PROMISE and any subsequent CONTINUATION
frames MUST be a valid and complete set of request header fields frames MUST be a valid and complete set of request header fields
(Section 8.1.3.1). The server MUST include a method in the ":method" (Section 8.1.3.1). The server MUST include a method in the ":method"
header field that is safe and cacheable. If a client receives a header field that is safe and cacheable. If a client receives a
PUSH_PROMISE that does not include a complete and valid set of header PUSH_PROMISE that does not include a complete and valid set of header
fields, or the ":method" header field identifies a method that is not fields, or the ":method" header field identifies a method that is not
safe, it MUST respond with a stream error (Section 5.4.2) of type safe, it MUST respond with a stream error (Section 5.4.2) of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to
sending any frames that reference the promised resources. This sending any frames that reference the promised responses. This
avoids a race where clients issue requests for resources prior to avoids a race where clients issue requests prior to receiving any
receiving any PUSH_PROMISE frames. PUSH_PROMISE frames.
For example, if the server receives a request for a document For example, if the server receives a request for a document
containing embedded links to multiple image files, and the server containing embedded links to multiple image files, and the server
chooses to push those additional images to the client, sending push chooses to push those additional images to the client, sending push
promises before the DATA frames that contain the image links ensures promises before the DATA frames that contain the image links ensures
that the client is able to see the promises before discovering the that the client is able to see the promises before discovering
resources. Similarly, if the server pushes resources referenced by embedded links. Similarly, if the server pushes responses referenced
the header block (for instance, in Link header fields), sending the by the header block (for instance, in Link header fields), sending
push promises before sending the header block ensures that clients do the push promises before sending the header block ensures that
not request those resources. clients do not request them.
PUSH_PROMISE frames MUST NOT be sent by the client. PUSH_PROMISE PUSH_PROMISE frames MUST NOT be sent by the client. PUSH_PROMISE
frames can be sent by the server on any stream that was opened by the frames can be sent by the server on any stream that was opened by the
client. They MUST be sent on a stream that is in either the "open" client. They MUST be sent on a stream that is in either the "open"
or "half closed (remote)" state to the server. PUSH_PROMISE frames or "half closed (remote)" state to the server. PUSH_PROMISE frames
are interspersed with the frames that comprise a response, though are interspersed with the frames that comprise a response, though
they cannot be interspersed with HEADERS and CONTINUATION frames that they cannot be interspersed with HEADERS and CONTINUATION frames that
comprise a single header block. comprise a single header block.
8.2.2. Push Responses 8.2.2. Push Responses
After sending the PUSH_PROMISE frame, the server can begin delivering After sending the PUSH_PROMISE frame, the server can begin delivering
the pushed resource as a response (Section 8.1.3.2) on a server- the pushed response as a response (Section 8.1.3.2) on a server-
initiated stream that uses the promised stream identifier. The initiated stream that uses the promised stream identifier. The
server uses this stream to transmit an HTTP response, using the same server uses this stream to transmit an HTTP response, using the same
sequence of frames as defined in Section 8.1. This stream becomes sequence of frames as defined in Section 8.1. This stream becomes
"half closed" to the client (Section 5.1) after the initial HEADERS "half closed" to the client (Section 5.1) after the initial HEADERS
frame is sent. frame is sent.
Once a client receives a PUSH_PROMISE frame and chooses to accept the Once a client receives a PUSH_PROMISE frame and chooses to accept the
pushed resource, the client SHOULD NOT issue any requests for the pushed response, the client SHOULD NOT issue any requests for the
promised resource until after the promised stream has closed. promised response until after the promised stream has closed.
If the client determines, for any reason, that it does not wish to If the client determines, for any reason, that it does not wish to
receive the pushed resource from the server, or if the server takes receive the pushed response from the server, or if the server takes
too long to begin sending the promised resource, the client can send too long to begin sending the promised response, the client can send
an RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes, an RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes,
and referencing the pushed stream's identifier. and referencing the pushed stream's identifier.
A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit
the number of resources that can be concurrently pushed by a server. the number of responses that can be concurrently pushed by a server.
Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables
server push by preventing the server from creating the necessary server push by preventing the server from creating the necessary
streams. This does not prohibit a server from sending PUSH_PROMISE streams. This does not prohibit a server from sending PUSH_PROMISE
frames; clients need to reset any promised streams that are not frames; clients need to reset any promised streams that are not
wanted. wanted.
Clients receiving a pushed response MUST validate that the server is Clients receiving a pushed response MUST validate that the server is
authorized to push the resource using the same-origin policy authorized to provide the response, see Section 10.1. For example,
([RFC6454], Section 3). For example, a HTTP/2 connection to an server that offers a certificate for only the "example.com" DNS-ID
"example.com" is generally [[anchor15: Ed: weaselly use of or Common Name is not permitted to push a response for
"generally", needs better definition]] not permitted to push a "https://www.example.org/doc".
response for "www.example.org".
8.3. The CONNECT Method 8.3. The CONNECT Method
The HTTP pseudo-method CONNECT ([HTTP-p2], Section 4.3.6) is used to In HTTP/1.x, the pseudo-method CONNECT ([HTTP-p2], Section 4.3.6) is
convert an HTTP/1.1 connection into a tunnel to a remote host. used to convert an HTTP connection into a tunnel to a remote host.
CONNECT is primarily used with HTTP proxies to establish a TLS CONNECT is primarily used with HTTP proxies to establish a TLS
session with a server for the purposes of interacting with "https" session with an origin server for the purposes of interacting with
resources. "https" resources.
In HTTP/2, the CONNECT method is used to establish a tunnel over a In HTTP/2, the CONNECT method is used to establish a tunnel over a
single HTTP/2 stream to a remote host. The HTTP header field mapping single HTTP/2 stream to a remote host, for similar purposes. The
works as mostly as defined in Request Header Fields HTTP header field mapping works as mostly as defined in Request
(Section 8.1.3.1), with a few differences. Specifically: Header Fields (Section 8.1.3.1), with a few differences.
Specifically:
o The ":method" header field is set to "CONNECT". o The ":method" header field is set to "CONNECT".
o The ":scheme" and ":path" header fields MUST be omitted. o The ":scheme" and ":path" header fields MUST be omitted.
o The ":authority" header field contains the host and port to o The ":authority" header field contains the host and port to
connect to (equivalent to the authority-form of the request-target connect to (equivalent to the authority-form of the request-target
of CONNECT requests, see [HTTP-p1], Section 5.3). of CONNECT requests, see [HTTP-p1], Section 5.3).
A proxy that supports CONNECT, establishes a TCP connection [TCP] to A proxy that supports CONNECT establishes a TCP connection [TCP] to
the server identified in the ":authority" header field. Once this the server identified in the ":authority" header field. Once this
connection is successfully established, the proxy sends a HEADERS connection is successfully established, the proxy sends a HEADERS
frame containing a 2xx series status code, as defined in [HTTP-p2], frame containing a 2xx series status code to the client, as defined
Section 4.3.6. in [HTTP-p2], Section 4.3.6.
After the initial HEADERS frame sent by each peer, all subsequent After the initial HEADERS frame sent by each peer, all subsequent
DATA frames correspond to data sent on the TCP connection. The DATA frames correspond to data sent on the TCP connection. The
payload of any DATA frames sent by the client are transmitted by the payload of any DATA frames sent by the client are transmitted by the
proxy to the TCP server; data received from the TCP server is proxy to the TCP server; data received from the TCP server is
assembled into DATA frames by the proxy. Frame types other than DATA assembled into DATA frames by the proxy. Frame types other than DATA
or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY) or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY)
MUST NOT be sent on a connected stream, and MUST be treated as a MUST NOT be sent on a connected stream, and MUST be treated as a
stream error (Section 5.4.2) if received. stream error (Section 5.4.2) if received.
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9.1. Connection Management 9.1. Connection Management
HTTP/2 connections are persistent. For best performance, it is HTTP/2 connections are persistent. For best performance, it is
expected clients will not close connections until it is determined expected clients will not close connections until it is determined
that no further communication with a server is necessary (for that no further communication with a server is necessary (for
example, when a user navigates away from a particular web page), or example, when a user navigates away from a particular web page), or
until the server closes the connection. until the server closes the connection.
Clients SHOULD NOT open more than one HTTP/2 connection to a given Clients SHOULD NOT open more than one HTTP/2 connection to a given
origin ([RFC6454]) concurrently. A client can create additional destination, where a destination is the IP address and port that is
connections as replacements, either to replace connections that are derived from a URI, a selected alternative service [ALT-SVC], or a
near to exhausting the available stream identifiers (Section 5.1.1), configured proxy. A client can create additional connections as
or to replace connections that have encountered errors replacements, either to replace connections that are near to
(Section 5.4.1). exhausting the available stream identifier space (Section 5.1.1), or
to replace connections that have encountered errors (Section 5.4.1).
Clients MAY use a single connection for more than one origin when A client MAY open multiple connections to the same IP address and TCP
each origin's hostname resolves to the same IP address, and they port using different Server Name Indication [TLS-EXT] values or to
share the same port. For "https" scheme origins, the server's provide different TLS client certificates, but SHOULD avoid creating
certificate MUST be valid for each origin's hostname. The multiple connections with the same configuration. [[anchor15: Need
considerations in RFC 6125 [RFC6125] for verification of identity more text on how client certificates relate here, see issue #363.]]
apply.
Clients MAY use a single server connection to send requests for URIs
with multiple different authority components as long as the server is
authoritative (Section 10.1).
Servers are encouraged to maintain open connections for as long as Servers are encouraged to maintain open connections for as long as
possible, but are permitted to terminate idle connections if possible, but are permitted to terminate idle connections if
necessary. When either endpoint chooses to close the transport-level necessary. When either endpoint chooses to close the transport-level
TCP connection, the terminating endpoint SHOULD first send a GOAWAY TCP connection, the terminating endpoint SHOULD first send a GOAWAY
(Section 6.8) frame so that both endpoints can reliably determine (Section 6.8) frame so that both endpoints can reliably determine
whether previously sent frames have been processed and gracefully whether previously sent frames have been processed and gracefully
complete or terminate any necessary remaining tasks. complete or terminate any necessary remaining tasks.
9.2. Use of TLS Features 9.2. Use of TLS Features
skipping to change at page 57, line 9 skipping to change at page 64, line 49
[TLS-EXT] extension to TLS. HTTP/2 clients MUST indicate the target [TLS-EXT] extension to TLS. HTTP/2 clients MUST indicate the target
domain name when negotiating TLS. domain name when negotiating TLS.
The TLS implementation MUST disable compression. TLS compression can The TLS implementation MUST disable compression. TLS compression can
lead to the exposure of information that would not otherwise be lead to the exposure of information that would not otherwise be
revealed [RFC3749]. Generic compression is unnecessary since HTTP/2 revealed [RFC3749]. Generic compression is unnecessary since HTTP/2
provides compression features that are more aware of context and provides compression features that are more aware of context and
therefore likely to be more appropriate for use for performance, therefore likely to be more appropriate for use for performance,
security or other reasons. security or other reasons.
Implementations MUST negotiate ephemeral cipher suites (DHE or ECDHE) Implementations MUST negotiate - and therefore use - ephemeral cipher
with a minimum size of 2048 bits (DHE) or security level of 128 bits suites, such as ephemeral Diffie-Hellman (DHE) or the elliptic curve
(ECDHE). Clients MUST accept DHE sizes of up to 4096 bits. variant (ECDHE) with a minimum size of 2048 bits (DHE) or security
level of 128 bits (ECDHE). Clients MUST accept DHE sizes of up to
4096 bits.
Implementations are encouraged not to negotiate TLS cipher suites
with known vulnerabilities, such as [RC4].
An implementation that negotiates a TLS connection that does not meet An implementation that negotiates a TLS connection that does not meet
the requirements in this section, or any policy-based constraints, the requirements in this section, or any policy-based constraints,
SHOULD NOT negotiate HTTP/2. Removing HTTP/2 protocols from SHOULD NOT negotiate HTTP/2. Removing HTTP/2 protocols from
consideration could result in the removal of all protocols from the consideration could result in the removal of all protocols from the
set of protocols offered by the client. This causes protocol set of protocols offered by the client. This causes protocol
negotiation failure, as described in Section 3.2 of [TLSALPN]. negotiation failure, as described in Section 3.2 of [TLSALPN].
Due to implementation limitations, it might not be possible to fail Due to implementation limitations, it might not be possible to fail
TLS negotiation based on all of these requirements. An endpoint MUST TLS negotiation based on all of these requirements. An endpoint MUST
terminate a HTTP/2 connection that is opened on a TLS session that terminate an HTTP/2 connection that is opened on a TLS session that
does not meet these minimum requirements with a connection error does not meet these minimum requirements with a connection error
(Section 5.4.1) of type INADEQUATE_SECURITY. (Section 5.4.1) of type INADEQUATE_SECURITY.
Implementations are encouraged not to negotiate TLS cipher suites
with known vulnerabilities, such as [RC4].
9.3. GZip Content-Encoding 9.3. GZip Content-Encoding
Clients MUST support gzip compression for HTTP response bodies. Clients MUST support gzip compression for HTTP response bodies.
Regardless of the value of the accept-encoding header field, a server Regardless of the value of the accept-encoding header field, a server
MAY send responses with gzip or deflate encoding. A compressed MAY send responses with gzip encoding. A compressed response MUST
response MUST still bear an appropriate content-encoding header still bear an appropriate content-encoding header field.
field.
10. Security Considerations This effectively changes the implicit value of the Accept-Encoding
header field ([HTTP-p2], Section 5.3.4) from "identity" to "identity,
gzip", however gzip encoding cannot be suppressed by including
";q=0". Intermediaries that perform translation from HTTP/2 to
HTTP/1.1 MUST decompress payloads unless the request includes an
Accept-Encoding value that includes "gzip".
10.1. Server Authority and Same-Origin 10. Security Considerations
This specification uses the same-origin policy ([RFC6454], Section 3) 10.1. Server Authority
to determine whether an origin server is permitted to provide
content.
A server that is contacted using TLS is authenticated based on the A client is only able to accept HTTP/2 responses from servers that
certificate that it offers in the TLS handshake (see [RFC2818], are authoritative for those resources. This is particularly
Section 3). A server is considered authoritative for an "https" important for server push (Section 8.2), where the client validates
resource if it has been successfully authenticated for the domain the PUSH_PROMISE before accepting the response.
part of the origin of the resource that it is providing.
A server is considered authoritative for an "http" resource if the HTTP/2 relies on the HTTP/1.1 definition of authority for determining
connection is established to a resolved IP address for the domain in whether a server is authoritative in providing a given response, see
the origin of the resource. [HTTP-p1], Section 9.1). This relies on local name resolution for
the "http" URI scheme, and the offered server identity for the
"https" scheme (see [RFC2818], Section 3).
A client MUST NOT use, in any way, resources provided by a server A client MUST NOT use, in any way, resources provided by a server
that is not authoritative for those resources. that is not authoritative for those resources.
10.2. Cross-Protocol Attacks 10.2. Cross-Protocol Attacks
When using TLS, we believe that HTTP/2 introduces no new cross- In a cross-protocol attack, an attacker causes a client to initiate a
protocol attacks. TLS encrypts the contents of all transmission transaction in one protocol toward a server that understands a
(except the handshake itself), making it difficult for attackers to different protocol. An attacker might be able to cause the
control the data which could be used in a cross-protocol attack. transaction to appear as valid transaction in the second protocol.
[[anchor19: Issue: This is no longer true]] In combination with the capabilities of the web context, this can be
used to interact with poorly protected servers in private networks.
Completing a TLS handshake with an ALPN identifier for HTTP/2 can be
considered sufficient. ALPN provides a positive indication that a
server is willing to proceed with HTTP/2, which prevents attacks on
other TLS-based protocols.
The encryption in TLS makes it difficult for attackers to control the
data which could be used in a cross-protocol attack on a cleartext
protocol.
The cleartext version of HTTP/2 has minimal protection against cross-
protocol attacks. The connection preface (Section 3.5) contains a
string that is designed to confuse HTTP/1.1 servers, but no special
protection is offered for other protocols. A server that is willing
to ignore parts of an HTTP/1.1 request containing an Upgrade header
field could be exposed to a cross-protocol attack.
10.3. Intermediary Encapsulation Attacks 10.3. Intermediary Encapsulation Attacks
HTTP/2 header field names and values are encoded as sequences of HTTP/2 header field names and values are encoded as sequences of
octets with a length prefix. This enables HTTP/2 to carry any string octets with a length prefix. This enables HTTP/2 to carry any string
of octets as the name or value of a header field. An intermediary of octets as the name or value of a header field. An intermediary
that translates HTTP/2 requests or responses into HTTP/1.1 directly that translates HTTP/2 requests or responses into HTTP/1.1 directly
could permit the creation of corrupted HTTP/1.1 messages. An could permit the creation of corrupted HTTP/1.1 messages. An
attacker might exploit this behavior to cause the intermediary to attacker might exploit this behavior to cause the intermediary to
create HTTP/1.1 messages with illegal header fields, extra header create HTTP/1.1 messages with illegal header fields, extra header
fields, or even new messages that are entirely falsified. fields, or even new messages that are entirely falsified.
An intermediary that performs translation into HTTP/1.1 cannot alter Header field names or values that contain characters not permitted by
the semantics of requests or responses. In particular, header field HTTP/1.1, including carriage return (U+000D) or line feed (U+000A)
names or values that contain characters not permitted by HTTP/1.1, MUST NOT be translated verbatim by an intermediary, as stipulated in
including carriage return (U+000D) or line feed (U+000A) MUST NOT be [HTTP-p1], Section 3.2.4.
translated verbatim, as stipulated in [HTTP-p1], Section 3.2.4.
Translation from HTTP/1.x to HTTP/2 does not produce the same Translation from HTTP/1.x to HTTP/2 does not produce the same
opportunity to an attacker. Intermediaries that perform translation opportunity to an attacker. Intermediaries that perform translation
to HTTP/2 MUST remove any instances of the "obs-fold" production from to HTTP/2 MUST remove any instances of the "obs-fold" production from
header field values. header field values.
10.4. Cacheability of Pushed Resources 10.4. Cacheability of Pushed Responses
Pushed resources are responses without an explicit request from the Pushed responses do not have an explicit request from the client; the
client. Request header fields are provided by the server in the request is provided by the server in the PUSH_PROMISE frame.
PUSH_PROMISE frame. These header fields are provided so that
existing HTTP semantics can be applied.
Caching resources that are pushed is possible based on the guidance Caching responses that are pushed is possible based on the guidance
provided by the origin server in the Cache-Control header field. provided by the origin server in the Cache-Control header field.
However, this can cause issues if a single server hosts more than one However, this can cause issues if a single server hosts more than one
tenant. For example, a server might offer multiple users each a tenant. For example, a server might offer multiple users each a
small portion of its URI space. small portion of its URI space.
Where multiple tenants share space on the same server, that server Where multiple tenants share space on the same server, that server
MUST ensure that tenants are not able to push representations of MUST ensure that tenants are not able to push representations of
resources that they do not have authority over. Failure to enforce resources that they do not have authority over. Failure to enforce
this would allow a tenant to provide a representation that would be this would allow a tenant to provide a representation that would be
served out of cache, overriding the actual representation that the served out of cache, overriding the actual representation that the
authoritative tenant provides. authoritative tenant provides.
Pushed resources for which an origin server is not authoritative are Pushed responses for which an origin server is not authoritative (see
never cached or used. Section 10.1) are never cached or used.
10.5. Denial of Service Considerations 10.5. Denial of Service Considerations
An HTTP/2 connection can demand a greater commitment of resources to An HTTP/2 connection can demand a greater commitment of resources to
operate than a HTTP/1.1 connection. The use of header compression operate than a HTTP/1.1 connection. The use of header compression
and flow control depend on a commitment of resources for storing a and flow control depend on a commitment of resources for storing a
greater amount of state. Settings for these features ensure that greater amount of state. Settings for these features ensure that
memory commitments for these features are strictly bounded. memory commitments for these features are strictly bounded.
Processing capacity cannot be guarded in the same fashion. Processing capacity cannot be guarded in the same fashion.
The SETTINGS frame can be abused to cause a peer to expend additional The SETTINGS frame can be abused to cause a peer to expend additional
processing time. This might be done by pointlessly changing processing time. This might be done by pointlessly changing SETTINGS
settings, setting multiple undefined settings, or changing the same parameters, setting multiple undefined parameters, or changing the
setting multiple times in the same frame. Similarly, WINDOW_UPDATE same setting multiple times in the same frame. WINDOW_UPDATE or
or PRIORITY frames can be abused to cause an unnecessary waste of PRIORITY frames can be abused to cause an unnecessary waste of
resources. resources. A server might erroneously issue ALTSVC frames for
origins on which it cannot be authoritative to generate excess work
for clients.
Large numbers of small or empty frames can be abused to cause a peer Large numbers of small or empty frames can be abused to cause a peer
to expend time processing frame headers. Note however that some uses to expend time processing frame headers. Note however that some uses
are entirely legitimate, such as the sending of an empty DATA frame are entirely legitimate, such as the sending of an empty DATA frame
to end a stream. to end a stream.
Header compression also offers some opportunities to waste processing Header compression also offers some opportunities to waste processing
resources, see [COMPRESSION] for more details on potential abuses. resources; see [COMPRESSION] for more details on potential abuses.
Limits in settings cannot be reduced instantaneously, which leaves an Limits in SETTINGS parameters cannot be reduced instantaneously,
endpoint exposed to behavior from a peer that could exceed the new which leaves an endpoint exposed to behavior from a peer that could
limits. In particular, immediately after establishing a connection, exceed the new limits. In particular, immediately after establishing
limits set by a server are not known to clients and could be exceeded a connection, limits set by a server are not known to clients and
without being an obvious protocol violation. could be exceeded without being an obvious protocol violation.
In all these cases, there are legitimate reasons to use these All these features - i.e., SETTINGS changes, small frames, header
protocol mechanisms. These features become a burden only when they compression - have legitimate uses. These features become a burden
are used unnecessarily or to excess. only when they are used unnecessarily or to excess.
An endpoint that doesn't monitor this behavior exposes itself to a An endpoint that doesn't monitor this behavior exposes itself to a
risk of denial of service attack. Implementations SHOULD track the risk of denial of service attack. Implementations SHOULD track the
use of these features and set limits on their use. An endpoint MAY use of these features and set limits on their use. An endpoint MAY
treat activity that is suspicious as a connection error treat activity that is suspicious as a connection error
(Section 5.4.1) of type ENHANCE_YOUR_CALM. (Section 5.4.1) of type ENHANCE_YOUR_CALM.
10.6. Use of Padding 10.6. Use of Compression
HTTP/2 enables greater use of compression for both header fields
(Section 4.3) and response bodies (Section 9.3). Compression can
allow an attacker to recover secret data when it is compressed in the
same context as data under attacker control.
There are demonstrable attacks on compression that exploit the
characteristics of the web (e.g., [BREACH]). The attacker induces
multiple requests containing varying plaintext, observing the length
of the resulting ciphertext in each, which reveals a shorter length
when a guess about the secret is correct.
Implementations communicating on a secure channel MUST NOT compress
content that includes both confidential and attacker-controlled data
unless separate compression dictionaries are used for each source of
data. Compression MUST NOT be used if the source of data cannot be
reliably determined.
Further considerations regarding the compression of header fields are
described in [COMPRESSION].
10.7. Use of Padding
Padding within HTTP/2 is not intended as a replacement for general Padding within HTTP/2 is not intended as a replacement for general
purpose padding, such as might be provided by TLS [TLS12]. Redundant purpose padding, such as might be provided by TLS [TLS12]. Redundant
padding could even be counterproductive. Correct application can padding could even be counterproductive. Correct application can
depend on having specific knowledge of the data that is being padded. depend on having specific knowledge of the data that is being padded.
To mitigate attacks that rely on compression, disabling compression To mitigate attacks that rely on compression, disabling compression
might be preferable to padding as a countermeasure. might be preferable to padding as a countermeasure.
Padding can be used to obscure the exact size of frame content. Padding can be used to obscure the exact size of frame content, and
Padding is provided to mitigate specific attacks within HTTP. For is provided to mitigate specific attacks within HTTP. For example,
example, attacks where compressed content includes both attacker- attacks where compressed content includes both attacker-controlled
controlled plaintext and secret data (see for example, [BREACH]). plaintext and secret data (see for example, [BREACH]).
Use of padding can result in less protection than might seem Use of padding can result in less protection than might seem
immediately obvious. At best, padding only makes it more difficult immediately obvious. At best, padding only makes it more difficult
for an attacker to infer length information by increasing the number for an attacker to infer length information by increasing the number
of frames an attacker has to observe. Incorrectly implemented of frames an attacker has to observe. Incorrectly implemented
padding schemes can be easily defeated. In particular, randomized padding schemes can be easily defeated. In particular, randomized
padding with a predictable distribution provides very little padding with a predictable distribution provides very little
protection; or padding payloads to a fixed size exposes information protection; or padding payloads to a fixed size exposes information
as payload sizes cross the fixed size boundary, which could be as payload sizes cross the fixed size boundary, which could be
possible if an attacker can control plaintext. possible if an attacker can control plaintext.
Intermediaries SHOULD NOT remove padding; though an intermediary Intermediaries SHOULD NOT remove padding, though an intermediary MAY
could remove padding and add differing amounts if the intent is to remove padding and add differing amounts if the intent is to improve
improve the protections padding affords. the protections padding affords.
11. Privacy Considerations 10.8. Privacy Considerations
HTTP/2 aims to keep connections open longer between clients and Several characteristics of HTTP/2 provide an observer an opportunity
servers in order to reduce the latency when a user makes a request. to correlate actions of a single client or server over time. This
The maintenance of these connections over time could be used to includes the value of settings, the manner in which flow control
expose private information. For example, a user using a browser windows are managed, the way priorities are allocated to streams,
hours after the previous user stopped using that browser may be able timing of reactions to stimulus, and handling of any optional
to learn about what the previous user was doing. This is a problem features.
with HTTP in its current form as well, however the short lived
connections make it less of a risk.
12. IANA Considerations As far as this creates observable differences in behavior, they could
be used as a basis for fingerprinting a specific client, as defined
in <http://www.w3.org/TR/html5/introduction.html#fingerprint>.
11. IANA Considerations
A string for identifying HTTP/2 is entered into the "Application A string for identifying HTTP/2 is entered into the "Application
Layer Protocol Negotiation (ALPN) Protocol IDs" registry established Layer Protocol Negotiation (ALPN) Protocol IDs" registry established
in [TLSALPN]. in [TLSALPN].
This document establishes registries for error codes. This new This document establishes a registry for error codes. This new
registry is entered into a new "Hypertext Transfer Protocol (HTTP) 2 registry is entered into a new "Hypertext Transfer Protocol (HTTP) 2
Parameters" section. Parameters" section.
This document registers the "HTTP2-Settings" header field for use in This document registers the "HTTP2-Settings" header field for use in
HTTP. HTTP.
This document registers the "PRI" method for use in HTTP, to avoid This document registers the "PRI" method for use in HTTP, to avoid
collisions with the connection header (Section 3.5). collisions with the connection preface (Section 3.5).
12.1. Registration of HTTP/2 Identification String 11.1. Registration of HTTP/2 Identification String
This document creates a registration for the identification of HTTP/2 This document creates two registrations for the identification of
in the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" HTTP/2 in the "Application Layer Protocol Negotiation (ALPN) Protocol
registry established in [TLSALPN]. IDs" registry established in [TLSALPN].
Protocol: HTTP/2 The "h2" string identifies HTTP/2 when used over TLS:
Protocol: HTTP/2 over TLS
Identification Sequence: 0x68 0x32 ("h2") Identification Sequence: 0x68 0x32 ("h2")
Specification: This document (RFCXXXX) Specification: This document (RFCXXXX)
12.2. Error Code Registry The "h2c" string identifies HTTP/2 when used over cleartext TCP:
Protocol: HTTP/2 over TCP
Identification Sequence: 0x68 0x32 0x63 ("h2c")
Specification: This document (RFCXXXX)
11.2. Error Code Registry
This document establishes a registry for HTTP/2 error codes. The This document establishes a registry for HTTP/2 error codes. The
"HTTP/2 Error Code" registry manages a 32-bit space. The "HTTP/2 "HTTP/2 Error Code" registry manages a 32-bit space. The "HTTP/2
Error Code" registry operates under the "Expert Review" policy Error Code" registry operates under the "Expert Review" policy
[RFC5226]. [RFC5226].
Registrations for error codes are required to include a description Registrations for error codes are required to include a description
of the error code. An expert reviewer is advised to examine new of the error code. An expert reviewer is advised to examine new
registrations for possible duplication with existing error codes. registrations for possible duplication with existing error codes.
Use of existing registrations is to be encouraged, but not mandated. Use of existing registrations is to be encouraged, but not mandated.
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optional. optional.
Description: A description of the conditions where the error code is Description: A description of the conditions where the error code is
applicable. applicable.
Specification: An optional reference for a specification that Specification: An optional reference for a specification that
defines the error code. defines the error code.
An initial set of error code registrations can be found in Section 7. An initial set of error code registrations can be found in Section 7.
12.3. HTTP2-Settings Header Field Registration 11.3. HTTP2-Settings Header Field Registration
This section registers the "HTTP2-Settings" header field in the This section registers the "HTTP2-Settings" header field in the
Permanent Message Header Field Registry [BCP90]. Permanent Message Header Field Registry [BCP90].
Header field name: HTTP2-Settings Header field name: HTTP2-Settings
Applicable protocol: http Applicable protocol: http
Status: standard Status: standard
Author/Change controller: IETF Author/Change controller: IETF
Specification document(s): Section 3.2.1 of this document Specification document(s): Section 3.2.1 of this document
Related information: This header field is only used by an HTTP/2 Related information: This header field is only used by an HTTP/2
client for Upgrade-based negotiation. client for Upgrade-based negotiation.
12.4. PRI Method Registration 11.4. PRI Method Registration
This section registers the "PRI" method in the HTTP Method Registry This section registers the "PRI" method in the HTTP Method Registry
[HTTP-p2]. [HTTP-p2].
Method Name: PRI Method Name: PRI
Safe No Safe No
Idempotent No Idempotent No
Specification document(s) Section 3.5 of this document Specification document(s) Section 3.5 of this document
Related information: This method is never used by an actual client. Related information: This method is never used by an actual client.
This method will appear to be used when an HTTP/1.1 server or This method will appear to be used when an HTTP/1.1 server or
intermediary attempts to parse an HTTP/2 connection header. intermediary attempts to parse an HTTP/2 connection preface.
13. Acknowledgements 12. 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).
skipping to change at page 63, line 16 skipping to change at page 72, line 16
Jitu Padhye, Roberto Peon, Rob Trace (Flow control). Jitu Padhye, Roberto Peon, Rob Trace (Flow control).
o Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike o Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike
Bishop, Herve Ruellan (Substantial editorial contributions). Bishop, Herve Ruellan (Substantial editorial contributions).
o Alexey Melnikov was an editor of this document during 2013. o Alexey Melnikov was an editor of this document during 2013.
o A substantial proportion of Martin's contribution was supported by o A substantial proportion of Martin's contribution was supported by
Microsoft during his employment there. Microsoft during his employment there.
14. References 13. References
14.1. Normative References 13.1. Normative References
[ALT-SVC] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", draft-ietf-httpbis-alt-svc-01
(work in progress), April 2014.
[COMPRESSION] Ruellan, H. and R. Peon, "HPACK - Header Compression [COMPRESSION] Ruellan, H. and R. Peon, "HPACK - Header Compression
for HTTP/2", draft-ietf-httpbis-header-compression-06 for HTTP/2", draft-ietf-httpbis-header-compression-07
(work in progress), February 2014. (work in progress), April 2014.
[COOKIE] Barth, A., "HTTP State Management Mechanism", [COOKIE] Barth, A., "HTTP State Management Mechanism",
RFC 6265, April 2011. RFC 6265, April 2011.
[HTTP-p1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext [HTTP-p1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Message Syntax and Transfer Protocol (HTTP/1.1): Message Syntax and
Routing", draft-ietf-httpbis-p1-messaging-26 (work in Routing", draft-ietf-httpbis-p1-messaging-26 (work in
progress), February 2014. progress), February 2014.
[HTTP-p2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext [HTTP-p2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
skipping to change at page 64, line 27 skipping to change at page 73, line 32
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006. Encodings", RFC 4648, October 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008. RFC 5226, May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service
Identity within Internet Public Key Infrastructure
Using X.509 (PKIX) Certificates in the Context of
Transport Layer Security (TLS)", RFC 6125, March 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011. December 2011.
[TCP] Postel, J., "Transmission Control Protocol", STD 7, [TCP] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981. RFC 793, September 1981.
[TLS-EXT] Eastlake, D., "Transport Layer Security (TLS) [TLS-EXT] Eastlake, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, Extensions: Extension Definitions", RFC 6066,
January 2011. January 2011.
[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer [TLS12] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246, Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008. August 2008.
[TLSALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan, [TLSALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application Layer "Transport Layer Security (TLS) Application Layer
Protocol Negotiation Extension", Protocol Negotiation Extension",
draft-ietf-tls-applayerprotoneg-04 (work in progress), draft-ietf-tls-applayerprotoneg-05 (work in progress),
January 2014. March 2014.
14.2. Informative References [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[AltSvc] Nottingham, M., "HTTP Alternate Services", 13.2. Informative References
draft-nottingham-httpbis-alt-svc-01 (work in
progress), December 2013.
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, Procedures for Message Header Fields", BCP 90,
RFC 3864, September 2004. RFC 3864, September 2004.
[BREACH] Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving [BREACH] Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving
the CRIME Attack", July 2013, <http:// the CRIME Attack", July 2013, <http://
breachattack.com/resources/ breachattack.com/resources/
BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>. BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.
[IDNA] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[RC4] Rivest, R., "The RC4 encryption algorithm", RSA Data [RC4] Rivest, R., "The RC4 encryption algorithm", RSA Data
Security, Inc. , March 1992. Security, Inc. , March 1992.
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP [RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP
Extensions for High Performance", RFC 1323, May 1992. Extensions for High Performance", RFC 1323, May 1992.
[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol [RFC3749] Hollenbeck, S., "Transport Layer Security Protocol
Compression Methods", RFC 3749, May 2004. Compression Methods", RFC 3749, May 2004.
[TALKING] Huang, L-S., Chen, E., Barth, A., Rescorla, E., and C. [TALKING] Huang, L-S., Chen, E., Barth, A., Rescorla, E., and C.
Jackson, "Talking to Yourself for Fun and Profit", Jackson, "Talking to Yourself for Fun and Profit",
2011, <http://w2spconf.com/2011/papers/websocket.pdf>. 2011, <http://w2spconf.com/2011/papers/websocket.pdf>.
[TLSBCP] Sheffer, Y. and R. Holz, "Recommendations for Secure [TLSBCP] Sheffer, Y., Holz, R., and P. Saint-Andre,
Use of TLS and DTLS", draft-sheffer-tls-bcp-01 (work "Recommendations for Secure Use of TLS and DTLS",
in progress), September 2013. draft-sheffer-tls-bcp-02 (work in progress),
February 2014.
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-http2-09 A.1. Since draft-ietf-httpbis-http2-10
Changed "connection header" to "connection preface" to avoid
confusion.
Added dependency-based stream prioritization.
Added "h2c" identifier to distinguish between cleartext and secured
HTTP/2.
Adding missing padding to PUSH_PROMISE.
Integrate ALTSVC frame and supporting text.
Dropping requirement on "deflate" Content-Encoding.
Improving security considerations around use of compression.
A.2. Since draft-ietf-httpbis-http2-09
Adding padding for data frames. Adding padding for data frames.
Renumbering frame types, error codes, and settings. Renumbering frame types, error codes, and settings.
Adding INADEQUATE_SECURITY error code. Adding INADEQUATE_SECURITY error code.
Updating TLS usage requirements to 1.2; forbidding TLS compression. Updating TLS usage requirements to 1.2; forbidding TLS compression.
Removing extensibility for frames and settings. Removing extensibility for frames and settings.
skipping to change at page 66, line 16 skipping to change at page 75, line 36
Changing the protocol identification token to "h2". Changing the protocol identification token to "h2".
Changing the use of :authority to make it optional and to allow Changing the use of :authority to make it optional and to allow
userinfo in non-HTTP cases. userinfo in non-HTTP cases.
Allowing split on 0x0 for Cookie. Allowing split on 0x0 for Cookie.
Reserved PRI method in HTTP/1.1 to avoid possible future collisions. Reserved PRI method in HTTP/1.1 to avoid possible future collisions.
A.2. Since draft-ietf-httpbis-http2-08 A.3. Since draft-ietf-httpbis-http2-08
Added cookie crumbling for more efficient header compression. Added cookie crumbling for more efficient header compression.
Added header field ordering with the value-concatenation mechanism. Added header field ordering with the value-concatenation mechanism.
A.3. Since draft-ietf-httpbis-http2-07 A.4. Since draft-ietf-httpbis-http2-07
Marked draft for implementation. Marked draft for implementation.
A.4. Since draft-ietf-httpbis-http2-06 A.5. Since draft-ietf-httpbis-http2-06
Adding definition for CONNECT method. Adding definition for CONNECT method.
Constraining the use of push to safe, cacheable methods with no Constraining the use of push to safe, cacheable methods with no
request body. request body.
Changing from :host to :authority to remove any potential confusion. Changing from :host to :authority to remove any potential confusion.
Adding setting for header compression table size. Adding setting for header compression table size.
Adding settings acknowledgement. Adding settings acknowledgement.
Removing unnecessary and potentially problematic flags from Removing unnecessary and potentially problematic flags from
CONTINUATION. CONTINUATION.
Added denial of service considerations. Added denial of service considerations.
A.5. Since draft-ietf-httpbis-http2-05 A.6. Since draft-ietf-httpbis-http2-05
Marking the draft ready for implementation. Marking the draft ready for implementation.
Renumbering END_PUSH_PROMISE flag. Renumbering END_PUSH_PROMISE flag.
Editorial clarifications and changes. Editorial clarifications and changes.
A.6. Since draft-ietf-httpbis-http2-04 A.7. Since draft-ietf-httpbis-http2-04
Added CONTINUATION frame for HEADERS and PUSH_PROMISE. Added CONTINUATION frame for HEADERS and PUSH_PROMISE.
PUSH_PROMISE is no longer implicitly prohibited if PUSH_PROMISE is no longer implicitly prohibited if
SETTINGS_MAX_CONCURRENT_STREAMS is zero. SETTINGS_MAX_CONCURRENT_STREAMS is zero.
Push expanded to allow all safe methods without a request body. Push expanded to allow all safe methods without a request body.
Clarified the use of HTTP header fields in requests and responses. Clarified the use of HTTP header fields in requests and responses.
Prohibited HTTP/1.1 hop-by-hop header fields. Prohibited HTTP/1.1 hop-by-hop header fields.
skipping to change at page 67, line 30 skipping to change at page 76, line 49
Clarified requirements around handling different frames after stream Clarified requirements around handling different frames after stream
close, stream reset and GOAWAY. close, stream reset and GOAWAY.
Added more specific prohibitions for sending of different frame types Added more specific prohibitions for sending of different frame types
in various stream states. in various stream states.
Making the last received setting value the effective value. Making the last received setting value the effective value.
Clarified requirements on TLS version, extension and ciphers. Clarified requirements on TLS version, extension and ciphers.
A.7. Since draft-ietf-httpbis-http2-03 A.8. Since draft-ietf-httpbis-http2-03
Committed major restructuring atrocities. Committed major restructuring atrocities.
Added reference to first header compression draft. Added reference to first header compression draft.
Added more formal description of frame lifecycle. Added more formal description of frame lifecycle.
Moved END_STREAM (renamed from FINAL) back to HEADERS/DATA. Moved END_STREAM (renamed from FINAL) back to HEADERS/DATA.
Removed HEADERS+PRIORITY, added optional priority to HEADERS frame. Removed HEADERS+PRIORITY, added optional priority to HEADERS frame.
Added PRIORITY frame. Added PRIORITY frame.
A.8. Since draft-ietf-httpbis-http2-02 A.9. Since draft-ietf-httpbis-http2-02
Added continuations to frames carrying header blocks. Added continuations to frames carrying header blocks.
Replaced use of "session" with "connection" to avoid confusion with Replaced use of "session" with "connection" to avoid confusion with
other HTTP stateful concepts, like cookies. other HTTP stateful concepts, like cookies.
Removed "message". Removed "message".
Switched to TLS ALPN from NPN. Switched to TLS ALPN from NPN.
Editorial changes. Editorial changes.
A.9. Since draft-ietf-httpbis-http2-01 A.10. Since draft-ietf-httpbis-http2-01
Added IANA considerations section for frame types, error codes and Added IANA considerations section for frame types, error codes and
settings. settings.
Removed data frame compression. Removed data frame compression.
Added PUSH_PROMISE. Added PUSH_PROMISE.
Added globally applicable flags to framing. Added globally applicable flags to framing.
skipping to change at page 68, line 42 skipping to change at page 78, line 12
Restructured frame header. Removed distinction between data and Restructured frame header. Removed distinction between data and
control frames. control frames.
Altered flow control properties to include session-level limits. Altered flow control properties to include session-level limits.
Added note on cacheability of pushed resources and multiple tenant Added note on cacheability of pushed resources and multiple tenant
servers. servers.
Changed protocol label form based on discussions. Changed protocol label form based on discussions.
A.10. Since draft-ietf-httpbis-http2-00 A.11. 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 5.2.1) based on <http:// Added flow control principles (Section 5.2.1) 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.11. Since draft-mbelshe-httpbis-spdy-00 A.12. 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. 247 change blocks. 
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