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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12

HTTPbis Working Group                                          M. Belshe
Internet-Draft                                                     Twist
Intended status: Standards Track                                 R. Peon
Expires: October 25, 2014                                    Google, Inc
                                                         M. Thomson, Ed.
                                                                 Mozilla
                                                          April 23, 2014


                 Hypertext Transfer Protocol version 2
                      draft-ietf-httpbis-http2-12

Abstract

   This specification describes an optimized expression of the syntax of
   the Hypertext Transfer Protocol (HTTP).  HTTP/2 enables a more
   efficient use of network resources and a reduced perception of
   latency by introducing header field compression and allowing multiple
   concurrent messages on the same connection.  It also introduces
   unsolicited push of representations from servers to clients.

   This document is an alternative to, but does not obsolete, the
   HTTP/1.1 message syntax.  HTTP's existing semantics remain unchanged.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft takes place on the HTTPBIS working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

   Working Group information can be found at
   <http://tools.ietf.org/wg/httpbis/>; that specific to HTTP/2 are at
   <http://http2.github.io/>.

   The changes in this draft are summarized in Appendix A.

   This version of HTTP/2, identified as "h2-12" or "h2c-12", is
   intended for implementation.  An interoperability event will be
   conducted 2014-06-05, see <https://github.com/http2/wg_materials/
   blob/master/interim-14-06/agenda.md>.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute



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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 25, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  HTTP/2 Protocol Overview . . . . . . . . . . . . . . . . . . .  5
     2.1.  Document Organization  . . . . . . . . . . . . . . . . . .  6
     2.2.  Conventions and Terminology  . . . . . . . . . . . . . . .  7
   3.  Starting HTTP/2  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  HTTP/2 Version Identification  . . . . . . . . . . . . . .  8
     3.2.  Starting HTTP/2 for "http" URIs  . . . . . . . . . . . . .  9
       3.2.1.  HTTP2-Settings Header Field  . . . . . . . . . . . . . 10
     3.3.  Starting HTTP/2 for "https" URIs . . . . . . . . . . . . . 11
     3.4.  Starting HTTP/2 with Prior Knowledge . . . . . . . . . . . 11
     3.5.  HTTP/2 Connection Preface  . . . . . . . . . . . . . . . . 11
   4.  HTTP Frames  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Frame Format . . . . . . . . . . . . . . . . . . . . . . . 12
     4.2.  Frame Size . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.3.  Header Compression and Decompression . . . . . . . . . . . 14
   5.  Streams and Multiplexing . . . . . . . . . . . . . . . . . . . 15
     5.1.  Stream States  . . . . . . . . . . . . . . . . . . . . . . 16
       5.1.1.  Stream Identifiers . . . . . . . . . . . . . . . . . . 20
       5.1.2.  Stream Concurrency . . . . . . . . . . . . . . . . . . 20
     5.2.  Flow Control . . . . . . . . . . . . . . . . . . . . . . . 21
       5.2.1.  Flow Control Principles  . . . . . . . . . . . . . . . 21



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       5.2.2.  Appropriate Use of Flow Control  . . . . . . . . . . . 22
     5.3.  Stream priority  . . . . . . . . . . . . . . . . . . . . . 23
       5.3.1.  Stream Dependencies  . . . . . . . . . . . . . . . . . 23
       5.3.2.  Dependency Weighting . . . . . . . . . . . . . . . . . 24
       5.3.3.  Reprioritization . . . . . . . . . . . . . . . . . . . 24
       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 . . . . . . . . . . . . . . . . . . . . . . . . 33
     6.5.  SETTINGS . . . . . . . . . . . . . . . . . . . . . . . . . 34
       6.5.1.  SETTINGS Format  . . . . . . . . . . . . . . . . . . . 35
       6.5.2.  Defined SETTINGS Parameters  . . . . . . . . . . . . . 35
       6.5.3.  Settings Synchronization . . . . . . . . . . . . . . . 37
     6.6.  PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . . . 37
     6.7.  PING . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     6.8.  GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     6.9.  WINDOW_UPDATE  . . . . . . . . . . . . . . . . . . . . . . 42
       6.9.1.  The Flow Control Window  . . . . . . . . . . . . . . . 43
       6.9.2.  Initial Flow Control Window Size . . . . . . . . . . . 44
       6.9.3.  Reducing the Stream Window Size  . . . . . . . . . . . 45
     6.10. CONTINUATION . . . . . . . . . . . . . . . . . . . . . . . 46
     6.11. ALTSVC . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     6.12. BLOCKED  . . . . . . . . . . . . . . . . . . . . . . . . . 49
   7.  Error Codes  . . . . . . . . . . . . . . . . . . . . . . . . . 49
   8.  HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 51
     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  . . . . . . . . . . . . . . . . . . . . 61
       8.2.2.  Push Responses . . . . . . . . . . . . . . . . . . . . 62
     8.3.  The CONNECT Method . . . . . . . . . . . . . . . . . . . . 63
   9.  Additional HTTP Requirements/Considerations  . . . . . . . . . 64
     9.1.  Connection Management  . . . . . . . . . . . . . . . . . . 64
     9.2.  Use of TLS Features  . . . . . . . . . . . . . . . . . . . 65
     9.3.  GZip Content-Encoding  . . . . . . . . . . . . . . . . . . 65
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 66
     10.1. Server Authority . . . . . . . . . . . . . . . . . . . . . 66
     10.2. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 66



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     10.3. Intermediary Encapsulation Attacks . . . . . . . . . . . . 67
     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 . . . . . . . . . . . . . . . . . . . . . . 69
     10.8. Privacy Considerations . . . . . . . . . . . . . . . . . . 70
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 70
     11.1. Registration of HTTP/2 Identification Strings  . . . . . . 70
     11.2. Error Code Registry  . . . . . . . . . . . . . . . . . . . 71
     11.3. HTTP2-Settings Header Field Registration . . . . . . . . . 71
     11.4. PRI Method Registration  . . . . . . . . . . . . . . . . . 72
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 72
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 73
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 73
     13.2. Informative References . . . . . . . . . . . . . . . . . . 74
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 75
     A.1.  Since draft-ietf-httpbis-http2-11  . . . . . . . . . . . . 75
     A.2.  Since draft-ietf-httpbis-http2-10  . . . . . . . . . . . . 75
     A.3.  Since draft-ietf-httpbis-http2-09  . . . . . . . . . . . . 76
     A.4.  Since draft-ietf-httpbis-http2-08  . . . . . . . . . . . . 76
     A.5.  Since draft-ietf-httpbis-http2-07  . . . . . . . . . . . . 76
     A.6.  Since draft-ietf-httpbis-http2-06  . . . . . . . . . . . . 76
     A.7.  Since draft-ietf-httpbis-http2-05  . . . . . . . . . . . . 77
     A.8.  Since draft-ietf-httpbis-http2-04  . . . . . . . . . . . . 77
     A.9.  Since draft-ietf-httpbis-http2-03  . . . . . . . . . . . . 77
     A.10. Since draft-ietf-httpbis-http2-02  . . . . . . . . . . . . 78
     A.11. Since draft-ietf-httpbis-http2-01  . . . . . . . . . . . . 78
     A.12. Since draft-ietf-httpbis-http2-00  . . . . . . . . . . . . 79
     A.13. Since draft-mbelshe-httpbis-spdy-00  . . . . . . . . . . . 79





















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1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is a wildly successful
   protocol.  However, the HTTP/1.1 message format ([HTTP-p1], Section
   3) was designed to be implemented with the tools at hand in the
   1990s, not modern Web application performance.  As such it has
   several characteristics that have a negative overall effect on
   application performance today.

   In particular, HTTP/1.0 only allows one request to be outstanding at
   a time on a given connection.  HTTP/1.1 pipelining only partially
   addressed request concurrency and suffers from head-of-line blocking.
   Therefore, clients that need to make many requests typically use
   multiple connections to a server in order to reduce latency.

   Furthermore, HTTP/1.1 header fields are often repetitive and verbose,
   which, in addition to generating more or larger network packets, can
   cause the small initial TCP congestion window to quickly fill.  This
   can result in excessive latency when multiple requests are made on a
   single new TCP connection.

   This document addresses these issues by defining an optimized mapping
   of HTTP's semantics to an underlying connection.  Specifically, it
   allows interleaving of request and response messages on the same
   connection and uses an efficient coding for HTTP header fields.  It
   also allows prioritization of requests, letting more important
   requests complete more quickly, further improving performance.

   The resulting protocol is designed to be more friendly to the
   network, because fewer TCP connections can be used in comparison to
   HTTP/1.x.  This means less competition with other flows, and longer-
   lived connections, which in turn leads to better utilization of
   available network capacity.

   Finally, this encapsulation also enables more scalable processing of
   messages through use of binary message framing.

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.



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   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 a 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:

   o  Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is
      initiated.

   o  The framing (Section 4) and streams (Section 5) layers describe
      the way HTTP/2 frames are structured and formed into multiplexed
      streams.

   o  Frame (Section 6) and error (Section 7) definitions include
      details of the frame and error types used in HTTP/2.

   o  HTTP mappings (Section 8) and additional requirements (Section 9)
      describe how HTTP semantics are expressed using frames and
      streams.

   While some of the frame and stream layer concepts are isolated from
   HTTP, the intent is not to define a completely generic framing layer.
   The framing and streams layers are tailored to the needs of the HTTP



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   protocol and server push.

2.2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   All numeric values are in network byte order.  Values are unsigned
   unless otherwise indicated.  Literal values are provided in decimal
   or hexadecimal as appropriate.  Hexadecimal literals are prefixed
   with "0x" to distinguish them from decimal literals.

   The following terms are used:

   client:  The endpoint initiating the HTTP/2 connection.

   connection:  A transport-level connection between two endpoints.

   connection error:  An error that affects the entire HTTP/2
      connection.

   endpoint:  Either the client or server of the connection.

   frame:  The smallest unit of communication within an HTTP/2
      connection, consisting of a header and a variable-length sequence
      of bytes structured according to the frame type.

   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"
      refers to the endpoint that is remote to the primary subject of
      discussion.

   receiver:  An endpoint that is receiving frames.

   sender:  An endpoint that is transmitting frames.

   server:  The endpoint which did not initiate the HTTP/2 connection.

   stream:  A bi-directional flow of frames across a virtual channel
      within the HTTP/2 connection.

   stream error:  An error on the individual HTTP/2 stream.






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3.  Starting HTTP/2

   An HTTP/2 connection is an application level protocol running on top
   of a TCP connection ([TCP]).  The client is the TCP connection
   initiator.

   HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1.
   HTTP/2 shares the same default port numbers: 80 for "http" URIs and
   443 for "https" URIs.  As a result, implementations processing
   requests for target resource URIs like "http://example.org/foo" or
   "https://example.com/bar" are required to first discover whether the
   upstream server (the immediate peer to which the client wishes to
   establish a connection) supports HTTP/2.

   The means by which support for HTTP/2 is determined is different for
   "http" and "https" URIs.  Discovery for "http" URIs is described in
   Section 3.2.  Discovery for "https" URIs is described in Section 3.3.

3.1.  HTTP/2 Version Identification

   The protocol defined in this document has two identifiers.

   o  The string "h2" identifies the protocol where HTTP/2 uses TLS
      [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.

      When serialised into an ALPN protocol identifier (which is a
      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
   themselves as "h2" or "h2c".  Until such an RFC exists,
   implementations MUST NOT identify themselves using these strings.

   Examples and text throughout the rest of this document use "h2" as a
   matter of editorial convenience only.  Implementations of draft



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   versions MUST NOT identify using this string.

   Implementations of draft versions of the protocol MUST add the string
   "-" and the corresponding draft number to the identifier.  For
   example, draft-ietf-httpbis-http2-11 over TLS is identified using the
   string "h2-11".

   Non-compatible experiments that are based on these draft versions
   MUST append the string "-" and an experiment name to the identifier.
   For example, an experimental implementation of packet mood-based
   encoding based on draft-ietf-httpbis-http2-09 might identify itself
   as "h2-09-emo".  Note that any label MUST conform to the "token"
   syntax defined in Section 3.2.6 of [HTTP-p1].  Experimenters are
   encouraged to coordinate their experiments on the ietf-http-wg@w3.org
   mailing list.

3.2.  Starting HTTP/2 for "http" URIs

   A client that makes a request to an "http" URI without prior
   knowledge about support for HTTP/2 uses the HTTP Upgrade mechanism
   (Section 6.7 of [HTTP-p1]).  The client makes an HTTP/1.1 request
   that includes an Upgrade header field identifying HTTP/2 with the
   "h2c" token.  The HTTP/1.1 request MUST include exactly one HTTP2-
   Settings (Section 3.2.1) header field.

   For example:

     GET /default.htm HTTP/1.1
     Host: server.example.com
     Connection: Upgrade, HTTP2-Settings
     Upgrade: h2c
     HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>


   Requests that contain an entity body MUST be sent in their entirety
   before the client can send HTTP/2 frames.  This means that a large
   request entity can block the use of the connection until it is
   completely sent.

   If concurrency of an initial request with subsequent requests is
   important, a small request can be used to perform the upgrade to
   HTTP/2, at the cost of an additional round-trip.

   A server that does not support HTTP/2 can respond to the request as
   though the Upgrade header field were absent:






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     HTTP/1.1 200 OK
     Content-Length: 243
     Content-Type: text/html

     ...

   A server that supports HTTP/2 can accept the upgrade with a 101
   (Switching Protocols) response.  After the empty line that terminates
   the 101 response, the server can begin sending HTTP/2 frames.  These
   frames MUST include a response to the request that initiated the
   Upgrade.

     HTTP/1.1 101 Switching Protocols
     Connection: Upgrade
     Upgrade: h2c

     [ HTTP/2 connection ...

   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
   connection preface (Section 3.5), which includes a SETTINGS frame.

   The HTTP/1.1 request that is sent prior to upgrade is assigned stream
   identifier 1 and is assigned default priority values (Section 5.3.5).
   Stream 1 is implicitly half closed from the client toward the server,
   since the request is completed as an HTTP/1.1 request.  After
   commencing the HTTP/2 connection, stream 1 is used for the response.

3.2.1.  HTTP2-Settings Header Field

   A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly
   one "HTTP2-Settings" header field.  The "HTTP2-Settings" header field
   is a hop-by-hop header field that includes parameters that govern the
   HTTP/2 connection, provided in anticipation of the server accepting
   the request to upgrade.  A server MUST reject an attempt to upgrade
   if this header field is not present.

     HTTP2-Settings    = token68

   The content of the "HTTP2-Settings" header field is the payload of a
   SETTINGS frame (Section 6.5), encoded as a base64url string (that is,
   the URL- and filename-safe Base64 encoding described in Section 5 of
   [RFC4648], with any trailing '=' characters omitted).  The ABNF
   [RFC5234] production for "token68" is defined in Section 2.1 of
   [HTTP-p7].

   As a hop-by-hop header field, the "Connection" header field MUST
   include a value of "HTTP2-Settings" in addition to "Upgrade" when



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   upgrading to HTTP/2.

   A server decodes and interprets these values as it would any other
   SETTINGS frame.  Acknowledgement of the SETTINGS parameters
   (Section 6.5.3) is not necessary, since a 101 response serves as
   implicit acknowledgment.  Providing these values in the Upgrade
   request ensures that the protocol does not require default values for
   the above SETTINGS parameters, and gives a client an opportunity to
   provide other parameters prior to receiving any frames from the
   server.

3.3.  Starting HTTP/2 for "https" URIs

   A client that makes a request to an "https" URI without prior
   knowledge about support for HTTP/2 uses TLS [TLS12] with the
   application layer protocol negotiation extension [TLSALPN].

   Once TLS negotiation is complete, both the client and the server send
   a connection preface (Section 3.5).

3.4.  Starting HTTP/2 with Prior Knowledge

   A client can learn that a particular server supports HTTP/2 by other
   means.  For example, [ALT-SVC] describes a mechanism for advertising
   this capability in an HTTP header field; the ALTSVC frame
   (Section 6.11) describes a similar mechanism in HTTP/2.

   A client MAY immediately send HTTP/2 frames to a server that is known
   to support HTTP/2, after the connection preface (Section 3.5).  A
   server can identify such a connection by the use of the "PRI" method
   in the connection preface.  This only affects the resolution of
   "http" URIs; servers supporting HTTP/2 are required to support
   protocol negotiation in TLS [TLSALPN] for "https" URIs.

   Prior support for HTTP/2 is not a strong signal that a given server
   will support HTTP/2 for future connections.  It is possible for
   server configurations to change; for configurations to differ between
   instances in clustered server; or network conditions to change.

3.5.  HTTP/2 Connection Preface

   Upon establishment of a TCP connection and determination that HTTP/2
   will be used by both peers, each endpoint MUST send a connection
   preface as a final confirmation and to establish the initial SETTINGS
   parameters for the HTTP/2 connection.

   The client connection preface starts with a sequence of 24 octets,
   which in hex notation are:



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     0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

   (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 SETTINGS frame MAY
   be empty.  The client sends the client connection preface immediately
   upon receipt of a 101 Switching Protocols response (indicating a
   successful upgrade), or as the first application data octets of a TLS
   connection.  If starting an HTTP/2 connection with prior knowledge of
   server support for the protocol, the client connection preface is
   sent upon connection establishment.

      The client connection preface is selected so that a large
      proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do
      not attempt to process further frames.  Note that this does not
      address the concerns raised in [TALKING].

   The server connection preface consists of a potentially empty
   SETTINGS frame (Section 6.5) that MUST be the first frame the server
   sends in the HTTP/2 connection.

   To avoid unnecessary latency, clients are permitted to send
   additional frames to the server immediately after sending the client
   connection preface, without waiting to receive the server connection
   preface.  It is important to note, however, that the server
   connection preface SETTINGS frame might include parameters that
   necessarily alter how a client is expected to communicate with the
   server.  Upon receiving the SETTINGS frame, the client is expected to
   honor any parameters established.

   Clients and servers MUST terminate the TCP connection if either peer
   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
   using HTTP/2.

4.  HTTP Frames

   Once the HTTP/2 connection is established, endpoints can begin
   exchanging frames.

4.1.  Frame Format

   All frames begin with an 8-octet header followed by a payload of
   between 0 and 16,383 octets.








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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | R |     Length (14)           |   Type (8)    |   Flags (8)   |
    +-+-+-----------+---------------+-------------------------------+
    |R|                 Stream Identifier (31)                      |
    +-+-------------------------------------------------------------+
    |                   Frame Payload (0...)                      ...
    +---------------------------------------------------------------+

                               Frame Header

   The fields of the frame header are defined as:

   R: A reserved 2-bit field.  The semantics of these bits are undefined
      and the bits MUST remain unset (0) when sending and MUST be
      ignored when receiving.

   Length:  The length of the frame payload expressed as an unsigned 14-
      bit integer.  The 8 octets of the frame header are not included in
      this value.

   Type:  The 8-bit type of the frame.  The frame type determines how
      the remainder of the frame header and payload are interpreted.
      Implementations MUST treat the receipt of an unknown frame type
      (any frame types not defined in this document) as a connection
      error (Section 5.4.1) of type PROTOCOL_ERROR.

   Flags:  An 8-bit field reserved for frame-type specific boolean
      flags.

      Flags are assigned semantics specific to the indicated frame type.
      Flags that have no defined semantics for a particular frame type
      MUST be ignored, and MUST be left unset (0) when sending.

   R: A reserved 1-bit field.  The semantics of this bit are undefined
      and the bit MUST remain unset (0) when sending and MUST be ignored
      when receiving.

   Stream Identifier:  A 31-bit stream identifier (see Section 5.1.1).
      The value 0 is reserved for frames that are associated with the
      connection as a whole as opposed to an individual stream.

   The structure and content of the frame payload is dependent entirely
   on the frame type.






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4.2.  Frame Size

   The maximum size of a frame payload varies by frame type.  The
   absolute maximum size of a frame payload is 2^14-1 (16,383) octets,
   meaning that the maximum frame size is 16,391 octets.  All
   implementations SHOULD be capable of receiving and minimally
   processing frames up to this maximum size.

   Certain frame types, such as PING (see Section 6.7), impose
   additional limits on the amount of payload data allowed.  Likewise,
   additional size limits can be set by specific application uses (see
   Section 9).

   If a frame size exceeds any defined limit, or is too small to contain
   mandatory frame data, the endpoint MUST send a FRAME_SIZE_ERROR
   error.  A frame size error in a frame that could alter the state of
   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

   A header field in HTTP/2 is a name-value pair with one or more
   associated values.  They are used within HTTP request and response
   messages as well as server push operations (see Section 8.2).

   Header sets are collections of zero or more header fields.  When
   transmitted over a connection, a header set is serialized into a
   header block using HTTP Header Compression [COMPRESSION].  The
   serialized header block is then divided into one or more octet
   sequences, called header block fragments, and transmitted within the
   payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6) or
   CONTINUATION (Section 6.10) frames.

   HTTP Header Compression does not preserve the relative ordering of
   header fields.  Header fields with multiple values are encoded into a
   single header field using a special delimiter; see Section 8.1.3.3.

   The Cookie header field [COOKIE] is treated specially by the HTTP
   mapping; see Section 8.1.3.4.

   A receiving endpoint reassembles the header block by concatenating
   its fragments, then decompresses the block to reconstruct the header
   set.

   A complete header block consists of either:




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   o  a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag
      set, or

   o  a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared
      and one or more CONTINUATION frames, where the last CONTINUATION
      frame has the END_HEADERS flag set.

   Header compression is stateful, using a single compression context
   for the entire connection.  Each header block is processed as a
   discrete unit.  Header blocks MUST be transmitted as a contiguous
   sequence of frames, with no interleaved frames of any other type or
   from any other stream.  The last frame in a sequence of HEADERS or
   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,
   PUSH_PROMISE or CONTINUATION frames, because these frames carry data
   that can modify the compression context maintained by a receiver.  An
   endpoint receiving HEADERS, PUSH_PROMISE or CONTINUATION frames MUST
   reassemble header blocks and perform decompression even if the frames
   are to be discarded.  A receiver MUST terminate the connection with a
   connection error (Section 5.4.1) of type COMPRESSION_ERROR if it does
   not decompress a header block.

5.  Streams and Multiplexing

   A "stream" is an independent, bi-directional sequence of frames
   exchanged between the client and server within an HTTP/2 connection.
   Streams have several important characteristics:

   o  A single HTTP/2 connection can contain multiple concurrently open
      streams, with either endpoint interleaving frames from multiple
      streams.

   o  Streams can be established and used unilaterally or shared by
      either the client or server.

   o  Streams can be closed by either endpoint.

   o  The order in which frames are sent within a stream is significant.
      Recipients process frames in the order they are received.

   o  Streams are identified by an integer.  Stream identifiers are
      assigned to streams by the endpoint initiating the stream.






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5.1.  Stream States

   The lifecycle of a stream is shown in Figure 1.


                          +--------+
                    PP    |        |    PP
                 ,--------|  idle  |--------.
                /         |        |         \
               v          +--------+          v
        +----------+          |           +----------+
        |          |          | H         |          |
    ,---| reserved |          |           | reserved |---.
    |   | (local)  |          v           | (remote) |   |
    |   +----------+      +--------+      +----------+   |
    |      |          ES  |        |  ES          |      |
    |      | H    ,-------|  open  |-------.      | H    |
    |      |     /        |        |        \     |      |
    |      v    v         +--------+         v    v      |
    |   +----------+          |           +----------+   |
    |   |   half   |          |           |   half   |   |
    |   |  closed  |          | R         |  closed  |   |
    |   | (remote) |          |           | (local)  |   |
    |   +----------+          |           +----------+   |
    |        |                v                 |        |
    |        |  ES / R    +--------+  ES / R    |        |
    |        `----------->|        |<-----------'        |
    |  R                  | closed |                  R  |
    `-------------------->|        |<--------------------'
                          +--------+

      H:  HEADERS frame (with implied CONTINUATIONs)
      PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
      ES: END_STREAM flag
      R:  RST_STREAM frame


                          Figure 1: Stream States

   Both endpoints have a subjective view of the state of a stream that
   could be different when frames are in transit.  Endpoints do not
   coordinate the creation of streams; they are created unilaterally by
   either endpoint.  The negative consequences of a mismatch in states
   are limited to the "closed" state after sending RST_STREAM, where
   frames might be received for some time after closing.

   Streams have the following states:




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   idle:
      All streams start in the "idle" state.  In this state, no frames
      have been exchanged.

      The following transitions are valid from this state:

      *  Sending or receiving a HEADERS frame causes the stream to
         become "open".  The stream identifier is selected as described
         in Section 5.1.1.  The same HEADERS frame can also cause a
         stream to immediately become "half closed".

      *  Sending a PUSH_PROMISE frame marks the associated stream for
         later use.  The stream state for the reserved stream
         transitions to "reserved (local)".

      *  Receiving a PUSH_PROMISE frame marks the associated stream as
         reserved by the remote peer.  The state of the stream becomes
         "reserved (remote)".

   reserved (local):
      A stream in the "reserved (local)" state is one that has been
      promised by sending a PUSH_PROMISE frame.  A PUSH_PROMISE frame
      reserves an idle stream by associating the stream with an open
      stream that was initiated by the remote peer (see Section 8.2).

      In this state, only the following transitions are possible:

      *  The endpoint can send a HEADERS frame.  This causes the stream
         to open in a "half closed (remote)" state.

      *  Either endpoint can send a RST_STREAM frame to cause the stream
         to become "closed".  This releases the stream reservation.


      An endpoint MUST NOT send frames other than HEADERS or RST_STREAM
      in this state.

      A PRIORITY frame MAY be received in this state.  Receiving any
      frames other than RST_STREAM, or PRIORITY MUST be treated as a
      connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   reserved (remote):
      A stream in the "reserved (remote)" state has been reserved by a
      remote peer.

      In this state, only the following transitions are possible:





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      *  Receiving a HEADERS frame causes the stream to transition to
         "half closed (local)".

      *  Either endpoint can send a RST_STREAM frame to cause the stream
         to become "closed".  This releases the stream reservation.


      An endpoint MAY send a PRIORITY frame in this state to
      reprioritize the reserved stream.  An endpoint MUST NOT send any
      other type of frame other than RST_STREAM or PRIORITY.

      Receiving any other type of frame other than HEADERS or RST_STREAM
      MUST be treated as a connection error (Section 5.4.1) of type
      PROTOCOL_ERROR.

   open:
      A stream in the "open" state may be used by both peers to send
      frames of any type.  In this state, sending peers observe
      advertised stream level flow control limits (Section 5.2).

      From this state either endpoint can send a frame with an
      END_STREAM flag set, which causes the stream to transition into
      one of the "half closed" states: an endpoint sending an END_STREAM
      flag causes the stream state to become "half closed (local)"; an
      endpoint receiving an END_STREAM flag causes the stream state to
      become "half closed (remote)".  A HEADERS frame bearing an
      END_STREAM flag can be followed by CONTINUATION frames.

      Either endpoint can send a RST_STREAM frame from this state,
      causing it to transition immediately to "closed".

   half closed (local):
      A stream that is in the "half closed (local)" state cannot be used
      for sending frames.

      A stream transitions from this state to "closed" when a frame that
      contains an END_STREAM flag is received, or when either peer sends
      a RST_STREAM frame.  A HEADERS frame bearing an END_STREAM flag
      can be followed by CONTINUATION frames.

      A receiver can ignore WINDOW_UPDATE or PRIORITY frames in this
      state.  These frame types might arrive for a short period after a
      frame bearing the END_STREAM flag is sent.

   half closed (remote):
      A stream that is "half closed (remote)" is no longer being used by
      the peer to send frames.  In this state, an endpoint is no longer
      obligated to maintain a receiver flow control window if it



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      performs flow control.

      If an endpoint receives additional frames for a stream that is in
      this state, other than CONTINUATION frames, it MUST respond with a
      stream error (Section 5.4.2) of type STREAM_CLOSED.

      A stream can transition from this state to "closed" by sending a
      frame that contains an END_STREAM flag, or when either peer sends
      a RST_STREAM frame.

   closed:
      The "closed" state is the terminal state.

      An endpoint MUST NOT send frames on a closed stream.  An endpoint
      that receives any frame after receiving a RST_STREAM MUST treat
      that as a stream error (Section 5.4.2) of type STREAM_CLOSED.
      Similarly, an endpoint that receives any frames after receiving a
      DATA frame with the END_STREAM flag set, or any frames except a
      CONTINUATION frame after receiving a HEADERS frame with an
      END_STREAM flag set MUST treat that as a stream error
      (Section 5.4.2) of type STREAM_CLOSED.

      WINDOW_UPDATE, PRIORITY, or RST_STREAM frames can be received in
      this state for a short period after a DATA or HEADERS frame
      containing an END_STREAM flag is sent.  Until the remote peer
      receives and processes the frame bearing the END_STREAM flag, it
      might send frame of any of these types.  Endpoints MUST ignore
      WINDOW_UPDATE, PRIORITY, or RST_STREAM frames received in this
      state, though endpoints MAY choose to treat frames that arrive a
      significant time after sending END_STREAM as a connection error
      (Section 5.4.1) of type PROTOCOL_ERROR.

      If this state is reached as a result of sending a RST_STREAM
      frame, the peer that receives the RST_STREAM might have already
      sent - or enqueued for sending - frames on the stream that cannot
      be withdrawn.  An endpoint MUST ignore frames that it receives on
      closed streams after it has sent a RST_STREAM frame.  An endpoint
      MAY choose to limit the period over which it ignores frames and
      treat frames that arrive after this time as being in error.

      Flow controlled frames (i.e., DATA) received after sending
      RST_STREAM are counted toward the connection flow control window.
      Even though these frames might be ignored, because they are sent
      before the sender receives the RST_STREAM, the sender will
      consider the frames to count against the flow control window.

      An endpoint might receive a PUSH_PROMISE frame after it sends
      RST_STREAM.  PUSH_PROMISE causes a stream to become "reserved"



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      even if the associated stream has been reset.  Therefore, a
      RST_STREAM is needed to close an unwanted promised streams.

   In the absence of more specific guidance elsewhere in this document,
   implementations SHOULD treat the receipt of a message that is not
   expressly permitted in the description of a state as a connection
   error (Section 5.4.1) of type PROTOCOL_ERROR.

5.1.1.  Stream Identifiers

   Streams are identified with an unsigned 31-bit integer.  Streams
   initiated by a client MUST use odd-numbered stream identifiers; those
   initiated by the server MUST use even-numbered stream identifiers.  A
   stream identifier of zero (0x0) is used for connection control
   messages; the stream identifier zero MUST NOT be used to establish a
   new stream.

   HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are
   responded to with a stream identifier of one (0x1).  After the
   upgrade completes, stream 0x1 is "half closed (local)" to the client.
   Therefore, stream 0x1 cannot be selected as a new stream identifier
   by a client that upgrades from HTTP/1.1.

   The identifier of a newly established stream MUST be numerically
   greater than all streams that the initiating endpoint has opened or
   reserved.  This governs streams that are opened using a HEADERS frame
   and streams that are reserved using PUSH_PROMISE.  An endpoint that
   receives an unexpected stream identifier MUST respond with a
   connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   The first use of a new stream identifier implicitly closes all
   streams in the "idle" state that might have been initiated by that
   peer with a lower-valued stream identifier.  For example, if a client
   sends a HEADERS frame on stream 7 without ever sending a frame on
   stream 5, then stream 5 transitions to the "closed" state when the
   first frame for stream 7 is sent or received.

   Stream identifiers cannot be reused.  Long-lived connections can
   result in endpoint exhausting the available range of stream
   identifiers.  A client that is unable to establish a new stream
   identifier can establish a new connection for new streams.

5.1.2.  Stream Concurrency

   A peer can limit the number of concurrently active streams using the
   SETTINGS_MAX_CONCURRENT_STREAMS parameters within a SETTINGS frame.
   The maximum concurrent streams setting is specific to each endpoint
   and applies only to the peer that receives the setting.  That is,



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   clients specify the maximum number of concurrent streams the server
   can initiate, and servers specify the maximum number of concurrent
   streams the client can initiate.  Endpoints MUST NOT exceed the limit
   set by their peer.

   Streams that are in the "open" state, or either of the "half closed"
   states count toward the maximum number of streams that an endpoint is
   permitted to open.  Streams in any of these three states count toward
   the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting
   (see Section 6.5.2).

   An endpoint that receives a HEADERS frame that causes their
   advertised concurrent stream limit to be exceeded MUST treat this as
   a stream error (Section 5.4.2).

   Streams in either of the "reserved" states do not count as open.

5.2.  Flow Control

   Using streams for multiplexing introduces contention over use of the
   TCP connection, resulting in blocked streams.  A flow control scheme
   ensures that streams on the same connection do not destructively
   interfere with each other.  Flow control is used for both individual
   streams and for the connection as a whole.

   HTTP/2 provides for flow control through use of the WINDOW_UPDATE
   frame type.

5.2.1.  Flow Control Principles

   HTTP/2 stream flow control aims to allow for future improvements to
   flow control algorithms without requiring protocol changes.  Flow
   control in HTTP/2 has the following characteristics:

   1.  Flow control is hop-by-hop, not end-to-end.

   2.  Flow control is based on window update frames.  Receivers
       advertise how many bytes they are prepared to receive on a stream
       and for the entire connection.  This is a credit-based scheme.

   3.  Flow control is directional with overall control provided by the
       receiver.  A receiver MAY choose to set any window size that it
       desires for each stream and for the entire connection.  A sender
       MUST respect flow control limits imposed by a receiver.  Clients,
       servers and intermediaries all independently advertise their flow
       control window as a receiver and abide by the flow control limits
       set by their peer when sending.




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   4.  The initial value for the flow control window is 65,535 bytes for
       both new streams and the overall connection.

   5.  The frame type determines whether flow control applies to a
       frame.  Of the frames specified in this document, only DATA
       frames are subject to flow control; all other frame types do not
       consume space in the advertised flow control window.  This
       ensures that important control frames are not blocked by flow
       control.

   6.  Flow control cannot be disabled.

   7.  HTTP/2 standardizes only the format of the WINDOW_UPDATE frame
       (Section 6.9).  This does not stipulate how a receiver decides
       when to send this frame or the value that it sends.  Nor does it
       specify how a sender chooses to send packets.  Implementations
       are able to select any algorithm that suits their needs.

   Implementations are also responsible for managing how requests and
   responses are sent based on priority; choosing how to avoid head of
   line blocking for requests; and managing the creation of new streams.
   Algorithm choices for these could interact with any flow control
   algorithm.

5.2.2.  Appropriate Use of Flow Control

   Flow control is defined to protect endpoints that are operating under
   resource constraints.  For example, a proxy needs to share memory
   between many connections, and also might have a slow upstream
   connection and a fast downstream one.  Flow control addresses cases
   where the receiver is unable process data on one stream, yet wants to
   continue to process other streams in the same connection.

   Deployments that do not require this capability can advertise a flow
   control window of the maximum size, incrementing the available space
   when new data is received.  Sending data is always subject to the
   flow control window advertised by the receiver.

   Deployments with constrained resources (for example, memory) MAY
   employ flow control to limit the amount of memory a peer can consume.
   Note, however, that this can lead to suboptimal use of available
   network resources if flow control is enabled without knowledge of the
   bandwidth-delay product (see [RFC1323]).

   Even with full awareness of the current bandwidth-delay product,
   implementation of flow control can be difficult.  When using flow
   control, the receiver MUST read from the TCP receive buffer in a
   timely fashion.  Failure to do so could lead to a deadlock when



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   critical frames, such as WINDOW_UPDATE, are not available to HTTP/2.
   However, flow control can ensure that constrained resources are
   protected without any reduction in connection utilization.

5.3.  Stream priority

   A client can assign a priority for a new stream by including
   prioritization information in the HEADERS frame (Section 6.2) that
   opens the stream.  For an existing stream, the PRIORITY frame
   (Section 6.3) can be used to change the priority.

   The purpose of prioritization is to allow an endpoint to express how
   it would prefer its peer allocate resources when managing concurrent
   streams.  Most importantly, priority can be used to select streams
   for transmitting frames when there is limited capacity for sending.

   Streams can be prioritized by marking them as dependent on the
   completion of other streams (Section 5.3.1).  Each dependency is
   assigned a relative weight, a number that is used to determine the
   relative proportion of available resources that are assigned to
   streams dependent on the same stream.

   Explicitly setting the priority for a stream is input to a
   prioritization process.  It does not guarantee any particular
   processing or transmission order for the stream relative to any other
   stream.  An endpoint cannot force a peer to process concurrent
   streams in a particular order using priority.  Expressing priority is
   therefore only ever a suggestion.

   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).

5.3.1.  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 not dependent on any other stream can given a stream
   dependency of 0x0.

   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



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   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

   Inside the 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.

5.3.2.  Dependency Weighting

   Each dependency is allocated an integer weight between 1 to 256
   (inclusive).

   Streams with the same dependencies SHOULD be allocated resources
   proportionally based on their weight.  Thus, if stream B depends on
   stream A with weight 4, and C depends on stream A with weight 12, and
   if no progress can be made on A, stream B ideally receives one third
   of the resources allocated to stream C.

   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
   dependency causes a stream to become dependent on the identified
   stream.



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   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.

   If a stream is made dependent on one of its own dependencies, the
   formerly dependent stream is first moved to be depedent on the
   reprioritized streams previous dependency, retaining its weight.

   For example, for an original dependency tree where B and C depend on
   A, D and E depend on C, and F depends on D; if A is made dependent on
   D, then D takes the place of A with A dependent on D and all other
   dependency relationships staying the same.

                        0
       A               / \               D                 D
      / \             D   A             / \        OR      |
     B   C     ==>   /   / \    ==>    F   A       ==>     A
        / \         F   B   C             / \             /|\
       D   E                |            B   C           B C F
       |                    E                |             |
       F                                     E             F
                  (intermediate)   (non-exclusive)    (exclusive)

                     Example of Dependency Reordering

5.3.4.  Prioritization State Management

   When a stream is removed from the dependency tree, its dependencies
   can be moved to become dependent on the stream the closed stream
   depends on.  The weights of new dependencies SHOULD be assigned by
   distributing the weight of the dependency of the closed stream
   proportionally based on the weights of its dependencies.

   Streams that are removed from the dependency tree cause some
   prioritization information to be lost.  Resources are shared between
   streams that depend on the same stream, which means that if a stream
   in that set closes or becomes blocked, any spare capacity allocated
   to a stream is distributed to the immediate neighbors of the stream.
   However, if the common dependency is removed from the tree, those
   streams share resources with streams at the next highest level.  For
   example, assume streams A and B share a dependency, and C and D both
   depend on A. Prior to the removal of A, if stream A and D are unable
   to proceed, then C receives all the resources dedicated to A. If A is
   removed from the tree, the weight of A is divided equally between D
   and E, which results in stream C receiving a reduced proportion of
   resources (one third, rather than one half).




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   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 these problems, endpoints SHOULD maintain prioritization
   state for closed streams for a period after streams close.

   An endpoint SHOULD retain stream prioritization state for a period
   after streams become closed.  The longer state is retained, the lower
   the chance that streams are assigned incorrect or default priority
   values.

   This could create a large state burden for an endpoint, so this state
   MAY be limited.  An endpoint MAY apply a fixed upper limit on the
   number of closed streams for which prioritization state is tracked to
   limit state exposure.  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.  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 dependencies of the streams that
   depend on it, if it has retained enough state to do so.

5.3.5.  Default Priorities

   Providing priority information is optional.  Streams are assigned a
   dependency on stream 0x0.  Pushed streams (Section 8.2) initially
   depend on their associated stream.  In both cases, streams are
   assigned a default weight of 16.

5.4.  Error Handling

   HTTP/2 framing permits two classes of error:

   o  An error condition that renders the entire connection unusable is
      a connection error.

   o  An error in an individual stream is a stream error.

   A list of error codes is included in Section 7.



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5.4.1.  Connection Error Handling

   A connection error is any error which prevents further processing of
   the framing layer, or which corrupts any connection state.

   An endpoint that encounters a connection error SHOULD first send a
   GOAWAY frame (Section 6.8) with the stream identifier of the last
   stream that it successfully received from its peer.  The GOAWAY frame
   includes an error code that indicates why the connection is
   terminating.  After sending the GOAWAY frame, the endpoint MUST close
   the TCP connection.

   It is possible that the GOAWAY will not be reliably received by the
   receiving endpoint.  In the event of a connection error, GOAWAY only
   provides a best-effort attempt to communicate with the peer about why
   the connection is being terminated.

   An endpoint can end a connection at any time.  In particular, an
   endpoint MAY choose to treat a stream error as a connection error.
   Endpoints SHOULD send a GOAWAY frame when ending a connection, as
   long as circumstances permit it.

5.4.2.  Stream Error Handling

   A stream error is an error related to a specific stream identifier
   that does not affect processing of other streams.

   An endpoint that detects a stream error sends a RST_STREAM frame
   (Section 6.4) that contains the stream identifier of the stream where
   the error occurred.  The RST_STREAM frame includes an error code that
   indicates the type of error.

   A RST_STREAM is the last frame that an endpoint can send on a stream.
   The peer that sends the RST_STREAM frame MUST be prepared to receive
   any frames that were sent or enqueued for sending by the remote peer.
   These frames can be ignored, except where they modify connection
   state (such as the state maintained for header compression
   (Section 4.3)).

   Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame
   for any stream.  However, an endpoint MAY send additional RST_STREAM
   frames if it receives frames on a closed stream after more than a
   round-trip time.  This behavior is permitted to deal with misbehaving
   implementations.

   An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM
   frame, to avoid looping.




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5.4.3.  Connection Termination

   If the TCP connection is torn down while streams remain in open or
   half closed states, then the endpoint MUST assume that those streams
   were abnormally interrupted and could be incomplete.

6.  Frame Definitions

   This specification defines a number of frame types, each identified
   by a unique 8-bit type code.  Each frame type serves a distinct
   purpose either in the establishment and management of the connection
   as a whole, or of individual streams.

   The transmission of specific frame types can alter the state of a
   connection.  If endpoints fail to maintain a synchronized view of the
   connection state, successful communication within the connection will
   no longer be possible.  Therefore, it is important that endpoints
   have a shared comprehension of how the state is affected by the use
   any given frame.

6.1.  DATA

   DATA frames (type=0x0) convey arbitrary, variable-length sequences of
   octets associated with a stream.  One or more DATA frames are used,
   for instance, to carry HTTP request or response payloads.

   DATA frames MAY also contain arbitrary padding.  Padding can be added
   to DATA frames to hide the size of messages.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Pad High? (8) |  Pad Low? (8) |
    +---------------+---------------+-------------------------------+
    |                            Data (*)                         ...
    +---------------------------------------------------------------+
    |                           Padding (*)                       ...
    +---------------------------------------------------------------+

                            DATA Frame Payload

   The DATA frame contains the following fields:

   Pad High:  An 8-bit field containing an amount of padding in units of
      256 octets.  This field is optional and is only present if the
      PAD_HIGH flag is set.  This field, in combination with Pad Low,
      determines how much padding there is on a frame.




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   Pad Low:  An 8-bit field containing an amount of padding in units of
      single octets.  This field is optional and is only present if the
      PAD_LOW flag is set.  This field, in combination with Pad High,
      determines how much padding there is on a frame.

   Data:  Application data.  The amount of data is the remainder of the
      frame payload after subtracting the length of the other fields
      that are present.

   Padding:  Padding octets that contain no application semantic value.
      Padding octets MUST be set to zero when sending and ignored when
      receiving.

   The DATA frame defines the following flags:

   END_STREAM (0x1):  Bit 1 being set indicates that this frame is the
      last that the endpoint will send for the identified stream.
      Setting this flag causes the stream to enter one of the "half
      closed" states or the "closed" state (Section 5.1).

   END_SEGMENT (0x2):  Bit 2 being set indicates that this frame is the
      last for the current segment.  Intermediaries MUST NOT coalesce
      frames across a segment boundary and MUST preserve segment
      boundaries when forwarding frames.

   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.

   COMPRESSED (0x20):  Bit 6 being set indicates that the data in the
      frame has been compressed with GZIP compression ([GZIP]).

   DATA frames MUST be associated with a stream.  If a DATA frame is
   received whose stream identifier field is 0x0, the recipient MUST
   respond with a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   Data frames are optionally compressed using GZip compression [GZIP].
   Each frame is individually compressed; the state of the compressor is
   reset for each frame.

   An endpoint MUST NOT send a DATA frame with the COMPRESSED flag set
   unless the SETTINGS_COMPRESS_DATA setting is enabled, that is, set to



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   1.  An endpoint that has not enabled DATA frame compression MUST
   treat the receipt of a DATA frame with the COMPRESSED flag set as a
   connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   DATA frames are subject to flow control and can only be sent when a
   stream is in the "open" or "half closed (remote)" states.  Padding is
   included in flow control.  If a DATA frame is received whose stream
   is not in "open" or "half closed (local)" state, the recipient MUST
   respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.

   The total number of padding octets is determined by multiplying the
   value of the Pad High field by 256 and adding the value of the Pad
   Low field.  Both Pad High and Pad Low fields assume a value of zero
   if absent.  If the length of the padding is greater than the length
   of the remainder of the frame payload, the recipient MUST treat this
   as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   Note:  A frame can be increased in size by one octet by including a
      Pad Low field with a value of zero.

   Use of padding is a security feature; as such, its use demands some
   care, see Section 10.7.

6.2.  HEADERS

   The HEADERS frame (type=0x1) carries name-value pairs.  It is used to
   open a stream (Section 5.1).  HEADERS frames can be sent on a stream
   in the "open" or "half closed (remote)" states.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Pad High? (8) |  Pad Low? (8) |
    +-+-------------+---------------+-------------------------------+
    |E|                 Stream Dependency? (31)                     |
    +-+-------------+-----------------------------------------------+
    |  Weight? (8)  |
    +-+-------------+-----------------------------------------------+
    |                   Header Block Fragment (*)                 ...
    +---------------------------------------------------------------+
    |                           Padding (*)                       ...
    +---------------------------------------------------------------+

                           HEADERS Frame Payload

   The HEADERS frame payload has the following fields:





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   Pad High:  Padding size high bits.  This field is only present if the
      PAD_HIGH flag is set.

   Pad Low:  Padding size low bits.  This field is only present if the
      PAD_LOW 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 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 flag is set.

   Weight:  An 8-bit weight for the stream, see Section 5.3.  Add one to
      the value to obtain a weight between 1 and 256.  This field is
      optional and is only present if the PRIORITY flag is set.

   Header Block Fragment:  A header block fragment (Section 4.3).

   Padding:  Padding octets.

   The HEADERS frame defines the following flags:

   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
      identified stream.  Setting this flag causes the stream to enter
      one of "half closed" states (Section 5.1).

      A HEADERS frame that is followed by CONTINUATION frames carries
      the END_STREAM flag that signals the end of a stream.  A
      CONTINUATION frame cannot be used to terminate a stream.

   END_SEGMENT (0x2):  Bit 2 being set indicates that this frame is the
      last for the current segment.  Intermediaries MUST NOT coalesce
      frames across a segment boundary and MUST preserve segment
      boundaries when forwarding frames.

   END_HEADERS (0x4):  Bit 3 being set indicates that this frame
      contains an entire header block (Section 4.3) and is not followed
      by any CONTINUATION frames.

      A HEADERS frame without the END_HEADERS flag set MUST be followed
      by a CONTINUATION frame for the same stream.  A receiver MUST
      treat the receipt of any other type of frame or a frame on a
      different stream as a connection error (Section 5.4.1) of type
      PROTOCOL_ERROR.




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   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.

   PRIORITY (0x20):  Bit 6 being set indicates that the Exclusive Flag
      (E), Stream Dependency, and Weight fields are present; see
      Section 5.3.

   The payload of a HEADERS frame contains a header block fragment
   (Section 4.3).  A header block that does not fit within a HEADERS
   frame is continued in a CONTINUATION frame (Section 6.10).

   HEADERS frames MUST be associated with a stream.  If a HEADERS frame
   is received whose stream identifier field is 0x0, the recipient MUST
   respond with a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   The HEADERS frame changes the connection state as described in
   Section 4.3.

   The HEADERS frame includes optional padding.  Padding fields and
   flags are identical to those defined for DATA frames (Section 6.1).

6.3.  PRIORITY

   The PRIORITY frame (type=0x2) specifies the sender-advised priority
   of a stream (Section 5.3).  It can be sent at any time for an
   existing stream.  This enables reprioritization of existing streams.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |E|                  Stream Dependency (31)                     |
    +-+-------------+-----------------------------------------------+
    |   Weight (8)  |
    +-+-------------+

                          PRIORITY Frame Payload

   The payload of a PRIORITY frame contains the following fields:






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   E: A single bit flag indicates that the stream dependency is
      exclusive, see Section 5.3.

   Stream Dependency:  A 31-bit stream identifier for the stream that
      this stream depends on, see Section 5.3.

   Weight:  An 8-bit weight for the identified stream dependency, see
      Section 5.3.  Add one to the value to obtain a weight between 1
      and 256.

   The PRIORITY frame does not define any flags.

   The PRIORITY frame is associated with an existing stream.  If a
   PRIORITY frame is received with a stream identifier of 0x0, the
   recipient MUST respond with a connection error (Section 5.4.1) of
   type PROTOCOL_ERROR.

   The PRIORITY frame can be sent on a stream in any of the "reserved
   (remote)", "open", "half-closed (local)", or "half closed (remote)"
   states, though it cannot be sent between consecutive frames that
   comprise a single header block (Section 4.3).  Note that this frame
   could arrive after processing or frame sending has completed, which
   would cause it to have no effect.  For a stream that is in the "half
   closed (remote)" state, this frame can only affect processing of the
   stream and not frame transmission.

6.4.  RST_STREAM

   The RST_STREAM frame (type=0x3) allows for abnormal termination of a
   stream.  When sent by the initiator of a stream, it indicates that
   they wish to cancel the stream or that an error condition has
   occurred.  When sent by the receiver of a stream, it indicates that
   either the receiver is rejecting the stream, requesting that the
   stream be cancelled or that an error condition has occurred.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Error Code (32)                        |
    +---------------------------------------------------------------+

                         RST_STREAM Frame Payload

   The RST_STREAM frame contains a single unsigned, 32-bit integer
   identifying the error code (Section 7).  The error code indicates why
   the stream is being terminated.

   The RST_STREAM frame does not define any flags.



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   The RST_STREAM frame fully terminates the referenced stream and
   causes it to enter the closed state.  After receiving a RST_STREAM on
   a stream, the receiver MUST NOT send additional frames for that
   stream.  However, after sending the RST_STREAM, the sending endpoint
   MUST be prepared to receive and process additional frames sent on the
   stream that might have been sent by the peer prior to the arrival of
   the RST_STREAM.

   RST_STREAM frames MUST be associated with a stream.  If a RST_STREAM
   frame is received with a stream identifier of 0x0, the recipient MUST
   treat this as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   RST_STREAM frames MUST NOT be sent for a stream in the "idle" state.
   If a RST_STREAM frame identifying an idle stream is received, the
   recipient MUST treat this as a connection error (Section 5.4.1) of
   type PROTOCOL_ERROR.

6.5.  SETTINGS

   The SETTINGS frame (type=0x4) conveys configuration parameters (such
   as preferences and constraints on peer behavior) that affect how
   endpoints communicate, and is also used to acknowledge the receipt of
   those parameters.  Individually, a SETTINGS parameter can also be
   referred to as a "setting".

   SETTINGS parameters are not negotiated; they describe characteristics
   of the sending peer, which are used by the receiving peer.  Different
   values for the same parameter can be advertised by each peer.  For
   example, a client might set a high initial flow control window,
   whereas a server might set a lower value to conserve resources.

   A SETTINGS frame MUST be sent by both endpoints at the start of a
   connection, and MAY be sent at any other time by either endpoint over
   the lifetime of the connection.  Implementations MUST support all of
   the parameters defined by this specification.

   Each parameter in a SETTINGS frame replaces any existing value for
   that parameter.  Parameters are processed in the order in which they
   appear, and a receiver of a SETTINGS frame does not need to maintain
   any state other than the current value of its parameters.  Therefore,
   the value of a SETTINGS parameter is the last value that is seen by a
   receiver.

   SETTINGS parameters are acknowledged by the receiving peer.  To
   enable this, the SETTINGS frame defines the following flag:





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   ACK (0x1):  Bit 1 being set indicates that this frame acknowledges
      receipt and application of the peer's SETTINGS frame.  When this
      bit is set, the payload of the SETTINGS frame MUST be empty.
      Receipt of a SETTINGS frame with the ACK flag set and a length
      field value other than 0 MUST be treated as a connection error
      (Section 5.4.1) of type FRAME_SIZE_ERROR.  For more info, see
      Settings Synchronization (Section 6.5.3).

   SETTINGS frames always apply to a connection, never a single stream.
   The stream identifier for a SETTINGS frame MUST be zero.  If an
   endpoint receives a SETTINGS frame whose stream identifier field is
   anything other than 0x0, the endpoint MUST respond with a connection
   error (Section 5.4.1) of type PROTOCOL_ERROR.

   The SETTINGS frame affects connection state.  A badly formed or
   incomplete SETTINGS frame MUST be treated as a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.

6.5.1.  SETTINGS Format

   The payload of a SETTINGS frame consists of zero or more parameters,
   each consisting of an unsigned 8-bit identifier and an unsigned 32-
   bit value.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Identifier (8)|
    +---------------+-----------------------------------------------+
    |                        Value (32)                             |
    +---------------------------------------------------------------+

                              Setting Format

6.5.2.  Defined SETTINGS Parameters

   The following parameters are defined:

   SETTINGS_HEADER_TABLE_SIZE (1):  Allows the sender to inform the
      remote endpoint of the size of the header compression table used
      to decode header blocks.  The encoder can reduce this size by
      using signaling specific to the header compression format inside a
      header block.  The initial value is 4,096 bytes.

   SETTINGS_ENABLE_PUSH (2):  This setting can be use to disable server
      push (Section 8.2).  An endpoint MUST NOT send a PUSH_PROMISE
      frame if it receives this parameter set to a value of 0.  An
      endpoint that has both set this parameter to 0 and had it



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      acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a
      connection error (Section 5.4.1) of type PROTOCOL_ERROR.

      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
      (Section 5.4.1) of type PROTOCOL_ERROR.

   SETTINGS_MAX_CONCURRENT_STREAMS (3):  Indicates the maximum number of
      concurrent streams that the sender will allow.  This limit is
      directional: it applies to the number of streams that the sender
      permits the receiver to create.  Initially there is no limit to
      this value.  It is recommended that this value be no smaller than
      100, so as to not unnecessarily limit parallelism.

      A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be
      treated as special by endpoints.  A zero value does prevent the
      creation of new streams, however this can also happen for any
      limit that is exhausted with active streams.  Servers SHOULD only
      set a zero value for short durations; if a server does not wish to
      accept requests, closing the connection could be preferable.

   SETTINGS_INITIAL_WINDOW_SIZE (4):  Indicates the sender's initial
      window size (in bytes) for stream level flow control.  The initial
      value is 65,535.

      This setting affects the window size of all streams, including
      existing streams, see Section 6.9.2.

      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
      FLOW_CONTROL_ERROR.

   SETTINGS_COMPRESS_DATA (5):  This setting is used to enable GZip
      compression of DATA frames.

      A value of 1 indicates that DATA frames MAY be compressed.  A
      value of 0 indicates that compression is not permitted.  The
      initial value is 0.

      Values other than 0 or 1 are invalid.  An endpoint MUST treat the
      receipt of any other value as a connection error (Section 5.4.1)
      of type PROTOCOL_ERROR.

   An endpoint that receives a SETTINGS frame with any other identifier
   MUST treat this as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.





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6.5.3.  Settings Synchronization

   Most values in SETTINGS benefit from or require an understanding of
   when the peer has received and applied the changed the communicated
   parameter values.  In order to provide such synchronization
   timepoints, the recipient of a SETTINGS frame in which the ACK flag
   is not set MUST apply the updated parameters as soon as possible upon
   receipt.

   The values in the SETTINGS frame MUST be applied in the order they
   appear, with no other frame processing between values.  Once all
   values have been applied, the recipient MUST immediately emit a
   SETTINGS frame with the ACK flag set.  Upon receiving a SETTINGS
   frame with the ACK flag set, the sender of the altered parameters can
   rely upon their application.

   If the sender of a SETTINGS frame does not receive an acknowledgement
   within a reasonable amount of time, it MAY issue a connection error
   (Section 5.4.1) of type SETTINGS_TIMEOUT.

6.6.  PUSH_PROMISE

   The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint
   in advance of streams the sender intends to initiate.  The
   PUSH_PROMISE frame includes the unsigned 31-bit identifier of the
   stream the endpoint plans to create along with a set of headers that
   provide additional context for the stream.  Section 8.2 contains a
   thorough description of the use of PUSH_PROMISE frames.

   PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of
   the peer endpoint is set to 0.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Pad High? (8) | Pad Low? (8)  |
    +-+-------------+---------------+-------------------------------+
    |R|                  Promised Stream ID (31)                    |
    +-+-----------------------------+-------------------------------+
    |                   Header Block Fragment (*)                 ...
    +---------------------------------------------------------------+
    |                           Padding (*)                       ...
    +---------------------------------------------------------------+

                        PUSH_PROMISE Payload Format

   The PUSH_PROMISE frame payload has the following fields:




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   Pad High:  Padding size high bits.  This field is only present if the
      PAD_HIGH flag is set.

   Pad Low:  Padding size low bits.  This field is only present if the
      PAD_LOW flag is set.

   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:

   END_HEADERS (0x4):  Bit 3 being set indicates that this frame
      contains an entire header block (Section 4.3) and is not followed
      by any CONTINUATION frames.

      A PUSH_PROMISE frame without the END_HEADERS flag set MUST be
      followed by a CONTINUATION frame for the same stream.  A receiver
      MUST treat the receipt of any other type of frame or a frame on a
      different stream as a connection error (Section 5.4.1) of type
      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
   PUSH_PROMISE only reserves stream identifiers for later use.



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   Recipients of PUSH_PROMISE frames can choose to reject promised
   streams by returning a RST_STREAM referencing the promised stream
   identifier back to the sender of the PUSH_PROMISE.

   The PUSH_PROMISE frame modifies the connection state as defined in
   Section 4.3.

   A PUSH_PROMISE frame modifies the connection state in two ways.  The
   inclusion of a header block (Section 4.3) potentially modifies the
   state maintained for header compression.  PUSH_PROMISE also reserves
   a stream for later use, causing the promised stream to enter the
   "reserved" state.  A sender MUST NOT send a PUSH_PROMISE on a stream
   unless that stream is either "open" or "half closed (remote)"; the
   sender MUST ensure that the promised stream is a valid choice for a
   new stream identifier (Section 5.1.1) (that is, the promised stream
   MUST be in the "idle" state).

   Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame
   causes the stream state to become indeterminate.  A receiver MUST
   treat the receipt of a PUSH_PROMISE on a stream that is neither
   "open" nor "half-closed (local)" as a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.  Similarly, a receiver MUST
   treat the receipt of a PUSH_PROMISE that promises an illegal stream
   identifier (Section 5.1.1) (that is, an identifier for a stream that
   is not currently in the "idle" state) as a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.

   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

   The PING frame (type=0x6) is a mechanism for measuring a minimal
   round-trip time from the sender, as well as determining whether an
   idle connection is still functional.  PING frames can be sent from
   any endpoint.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                      Opaque Data (64)                         |
    |                                                               |
    +---------------------------------------------------------------+

                            PING Payload Format

   In addition to the frame header, PING frames MUST contain 8 octets of



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   data in the payload.  A sender can include any value it chooses and
   use those bytes in any fashion.

   Receivers of a PING frame that does not include an ACK flag MUST send
   a PING frame with the ACK flag set in response, with an identical
   payload.  PING responses SHOULD be given higher priority than any
   other frame.

   The PING frame defines the following flags:

   ACK (0x1):  Bit 1 being set indicates that this PING frame is a PING
      response.  An endpoint MUST set this flag in PING responses.  An
      endpoint MUST NOT respond to PING frames containing this flag.

   PING frames are not associated with any individual stream.  If a PING
   frame is received with a stream identifier field value other than
   0x0, the recipient MUST respond with a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.

   Receipt of a PING frame with a length field value other than 8 MUST
   be treated as a connection error (Section 5.4.1) of type
   FRAME_SIZE_ERROR.

6.8.  GOAWAY

   The GOAWAY frame (type=0x7) informs the remote peer to stop creating
   streams on this connection.  GOAWAY can be sent by either the client
   or the server.  Once sent, the sender will ignore frames sent on new
   streams for the remainder of the connection.  Receivers of a GOAWAY
   frame MUST NOT open additional streams on the connection, although a
   new connection can be established for new streams.  The purpose of
   this frame is to allow an endpoint to gracefully stop accepting new
   streams (perhaps for a reboot or maintenance), while still finishing
   processing of previously established streams.

   There is an inherent race condition between an endpoint starting new
   streams and the remote sending a GOAWAY frame.  To deal with this
   case, the GOAWAY contains the stream identifier of the last stream
   which was processed on the sending endpoint in this connection.  If
   the receiver of the GOAWAY used streams that are newer than the
   indicated stream identifier, they were not processed by the sender
   and the receiver may treat the streams as though they had never been
   created at all (hence the receiver may want to re-create the streams
   later on a new connection).

   Endpoints SHOULD always send a GOAWAY frame before closing a
   connection so that the remote can know whether a stream has been
   partially processed or not.  For example, if an HTTP client sends a



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   POST at the same time that a server closes a connection, the client
   cannot know if the server started to process that POST request if the
   server does not send a GOAWAY frame to indicate where it stopped
   working.  An endpoint might choose to close a connection without
   sending GOAWAY for misbehaving peers.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |R|                  Last-Stream-ID (31)                        |
    +-+-------------------------------------------------------------+
    |                      Error Code (32)                          |
    +---------------------------------------------------------------+
    |                  Additional Debug Data (*)                    |
    +---------------------------------------------------------------+

                           GOAWAY Payload Format

   The GOAWAY frame does not define any flags.

   The GOAWAY frame applies to the connection, not a specific stream.
   An endpoint MUST treat a GOAWAY frame with a stream identifier other
   than 0x0 as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   The last stream identifier in the GOAWAY frame contains the highest
   numbered stream identifier for which the sender of the GOAWAY frame
   has received frames and might have taken some action on.  All streams
   up to and including the identified stream might have been processed
   in some way.  The last stream identifier can be set to 0 if no
   streams were processed.

      Note: In this context, "processed" means that some data from the
      stream was passed to some higher layer of software that might have
      taken some action as a result.

   If a connection terminates without a GOAWAY frame, this value is
   effectively the highest possible stream identifier.

   On streams with lower or equal numbered identifiers that were not
   closed completely prior to the connection being closed, re-attempting
   requests, transactions, or any protocol activity is not possible
   (with the exception of idempotent actions like HTTP GET, PUT, or
   DELETE).  Any protocol activity that uses higher numbered streams can
   be safely retried using a new connection.

   Activity on streams numbered lower or equal to the last stream
   identifier might still complete successfully.  The sender of a GOAWAY



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   frame might gracefully shut down a connection by sending a GOAWAY
   frame, maintaining the connection in an open state until all in-
   progress streams complete.

   An endpoint MAY send multiple GOAWAY frames if circumstances change.
   For instance, an endpoint that sends GOAWAY with NO_ERROR during
   graceful shutdown could subsequently encounter an condition that
   requires immediate termination of the connection.  The last stream
   identifier from the last GOAWAY frame received applies.

   After sending a GOAWAY frame, the sender can discard frames for
   streams with identifiers higher than the identified last stream.
   However, any frames that alter connection state cannot be completely
   ignored.  For instance, HEADERS, PUSH_PROMISE and CONTINUATION frames
   MUST be minimally processed to ensure the state maintained for header
   compression is consistent (see Section 4.3); similarly DATA frames
   MUST be counted toward the connection flow control window.  Failure
   to process these frames can cause flow control or header compression
   state to become unsynchronized.

   The GOAWAY frame also contains a 32-bit error code (Section 7) that
   contains the reason for closing the connection.

   Endpoints MAY append opaque data to the payload of any GOAWAY frame.
   Additional debug data is intended for diagnostic purposes only and
   carries no semantic value.  Debug information could contain security-
   or privacy-sensitive data.  Logged or otherwise persistently stored
   debug data MUST have adequate safeguards to prevent unauthorized
   access.

6.9.  WINDOW_UPDATE

   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
   the entire connection.

   Both types of flow control are hop-by-hop; that is, only between the
   two endpoints.  Intermediaries do not forward WINDOW_UPDATE frames
   between dependent connections.  However, throttling of data transfer
   by any receiver can indirectly cause the propagation of flow control
   information toward the original sender.

   Flow control only applies to frames that are identified as being
   subject to flow control.  Of the frame types defined in this
   document, this includes only DATA frame.  Frames that are exempt from
   flow control MUST be accepted and processed, unless the receiver is



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   unable to assign resources to handling the frame.  A receiver MAY
   respond with a stream error (Section 5.4.2) or connection error
   (Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept
   a frame.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |R|              Window Size Increment (31)                     |
    +-+-------------------------------------------------------------+

                       WINDOW_UPDATE Payload Format

   The payload of a WINDOW_UPDATE frame is one reserved bit, plus an
   unsigned 31-bit integer indicating the number of bytes that the
   sender can transmit in addition to the existing flow control window.
   The legal range for the increment to the flow control window is 1 to
   2^31 - 1 (0x7fffffff) bytes.

   The WINDOW_UPDATE frame does not define any flags.

   The WINDOW_UPDATE frame can be specific to a stream or to the entire
   connection.  In the former case, the frame's stream identifier
   indicates the affected stream; in the latter, the value "0" indicates
   that the entire connection is the subject of the frame.

   WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the
   END_STREAM flag.  This means that a receiver could receive a
   WINDOW_UPDATE frame on a "half closed (remote)" or "closed" stream.
   A receiver MUST NOT treat this as an error, see Section 5.1.

   A receiver that receives a flow controlled frame MUST always account
   for its contribution against the connection flow control window,
   unless the receiver treats this as a connection error
   (Section 5.4.1).  This is necessary even if the frame is in error.
   Since the sender counts the frame toward the flow control window, if
   the receiver does not, the flow control window at sender and receiver
   can become different.

6.9.1.  The Flow Control Window

   Flow control in HTTP/2 is implemented using a window kept by each
   sender on every stream.  The flow control window is a simple integer
   value that indicates how many bytes of data the sender is permitted
   to transmit; as such, its size is a measure of the buffering
   capability of the receiver.

   Two flow control windows are applicable: the stream flow control



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   window and the connection flow control window.  The sender MUST NOT
   send a flow controlled frame with a length that exceeds the space
   available in either of the flow control windows advertised by the
   receiver.  Frames with zero length with the END_STREAM flag set (for
   example, an empty data frame) MAY be sent if there is no available
   space in either flow control window.

   For flow control calculations, the 8 byte frame header is not
   counted.

   After sending a flow controlled frame, the sender reduces the space
   available in both windows by the length of the transmitted frame.

   The receiver of a frame sends a WINDOW_UPDATE frame as it consumes
   data and frees up space in flow control windows.  Separate
   WINDOW_UPDATE frames are sent for the stream and connection level
   flow control windows.

   A sender that receives a WINDOW_UPDATE frame updates the
   corresponding window by the amount specified in the frame.

   A sender MUST NOT allow a flow control window to exceed 2^31 - 1
   bytes.  If a sender receives a WINDOW_UPDATE that causes a flow
   control window to exceed this maximum it MUST terminate either the
   stream or the connection, as appropriate.  For streams, the sender
   sends a RST_STREAM with the error code of FLOW_CONTROL_ERROR code;
   for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code.

   Flow controlled frames from the sender and WINDOW_UPDATE frames from
   the receiver are completely asynchronous with respect to each other.
   This property allows a receiver to aggressively update the window
   size kept by the sender to prevent streams from stalling.

   A sender that is unable to send data on a stream due to either flow
   control window being zero or lower MAY send a BLOCKED frame
   (Section 6.12) in order to inform the receiver of a potential flow
   control problem.

6.9.2.  Initial Flow Control Window Size

   When an HTTP/2 connection is first established, new streams are
   created with an initial flow control window size of 65,535 bytes.
   The connection flow control window is 65,535 bytes.  Both endpoints
   can adjust the initial window size for new streams by including a
   value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that
   forms part of the connection preface.  The connection flow control
   window initial size cannot be changed.




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   Prior to receiving a SETTINGS frame that sets a value for
   SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default
   initial window size when sending flow controlled frames.  Similarly,
   the connection flow control window is set to the default initial
   window size until a WINDOW_UPDATE frame is received.

   A SETTINGS frame can alter the initial flow control window size for
   all current streams.  When the value of SETTINGS_INITIAL_WINDOW_SIZE
   changes, a receiver MUST adjust the size of all stream flow control
   windows that it maintains by the difference between the new value and
   the old value.  A SETTINGS frame cannot alter the connection flow
   control window.

   An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that
   causes any flow control window to exceed the maximum size as a
   connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.

   A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available
   space in a flow control window to become negative.  A sender MUST
   track the negative flow control window, and MUST NOT send new flow
   controlled frames until it receives WINDOW_UPDATE frames that cause
   the flow control window to become positive.

   For example, if the client sends 60KB immediately on connection
   establishment, and the server sets the initial window size to be
   16KB, the client will recalculate the available flow control window
   to be -44KB on receipt of the SETTINGS frame.  The client retains a
   negative flow control window until WINDOW_UPDATE frames restore the
   window to being positive, after which the client can resume sending.

6.9.3.  Reducing the Stream Window Size

   A receiver that wishes to use a smaller flow control window than the
   current size can send a new SETTINGS frame.  However, the receiver
   MUST be prepared to receive data that exceeds this window size, since
   the sender might send data that exceeds the lower limit prior to
   processing the SETTINGS frame.

   After sending a SETTINGS frame that reduces the initial flow control
   window size, a receiver has two options for handling streams that
   exceed flow control limits:

   1.  The receiver can immediately send RST_STREAM with
       FLOW_CONTROL_ERROR error code for the affected streams.

   2.  The receiver can accept the streams and tolerate the resulting
       head of line blocking, sending WINDOW_UPDATE frames as it
       consumes data.



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6.10.  CONTINUATION

   The CONTINUATION frame (type=0x9) is used to continue a sequence of
   header block fragments (Section 4.3).  Any number of CONTINUATION
   frames can be sent on an existing stream, as long as the preceding
   frame is on the same stream and is a HEADERS, PUSH_PROMISE or
   CONTINUATION frame without the END_HEADERS flag set.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Pad High? (8) | Pad Low? (8)  |
    +---------------+---------------+-------------------------------+
    |                   Header Block Fragment (*)                 ...
    +---------------------------------------------------------------+
    |                           Padding (*)                       ...
    +---------------------------------------------------------------+

                        CONTINUATION Frame Payload

   The CONTINUATION frame payload has the following fields:

   Pad High:  Padding size high bits.  This field is only present if the
      PAD_HIGH flag is set.

   Pad Low:  Padding size low bits.  This field is only present if the
      PAD_LOW flag is set.

   Header Block Fragment:  A header block fragment (Section 4.3).

   Padding:  Padding octets.

   The CONTINUATION frame defines the following flags:

   END_HEADERS (0x4):  Bit 3 being set indicates that this frame ends a
      header block (Section 4.3).

      If the END_HEADERS bit is not set, this frame MUST be followed by
      another CONTINUATION frame.  A receiver MUST treat the receipt of
      any other type of frame or a frame on a different stream as a
      connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   PAD_LOW (0x8):  Bit 4 being set indicates that the Pad Low field is
      present.







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   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.

   The payload of a CONTINUATION frame contains a header block fragment
   (Section 4.3).

   The CONTINUATION frame changes the connection state as defined in
   Section 4.3.

   CONTINUATION frames MUST be associated with a stream.  If a
   CONTINUATION frame is received whose stream identifier field is 0x0,
   the recipient MUST respond with a connection error (Section 5.4.1) of
   type PROTOCOL_ERROR.

   A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or
   CONTINUATION frame without the END_HEADERS flag set.  A recipient
   that observes violation of this rule MUST respond with a connection
   error (Section 5.4.1) of type PROTOCOL_ERROR.

   The CONTINUATION frame includes optional padding.  Padding fields and
   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.









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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Max-Age (32)                         |
    +-------------------------------+---------------+---------------+
    |            Port (16)          | Reserved (8)  | Proto-Len (8) |
    +-------------------------------+---------------+---------------+
    |                        Protocol-ID (*)                        |
    +---------------+-----------------------------------------------+
    | Host-Len (8)  |                   Host (*)                  ...
    +---------------+-----------------------------------------------+
    |                          Origin? (*)                        ...
    +---------------------------------------------------------------+

                           ALTSVC Frame Payload

   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.

   Proto-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 "Proto-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 the ASCII serialisation of an origin
      ([RFC6454], Section 6.2) that the alternate service is applicable
      to.



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   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
   (Section 5.4.1) 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.

6.12.  BLOCKED

   The BLOCKED frame (type=0xB) indicates that the sender is unable to
   send data due to a closed flow control window.

   [[anchor12: The BLOCKED frame is included in this draft version to
   facilitate experimentation.  If the results of the experiment do not
   provide positive feedback, it could be removed.]]

   The BLOCKED frame is used to provide feedback about the performance
   of flow control for the purposes of performance tuning and debugging.
   The BLOCKED frame can be sent by a peer when flow controlled data
   cannot be sent due to the connection- or stream-level flow control.
   This frame MUST NOT be sent if there are other reasons preventing
   data from being sent, either a lack of available data, or the
   underlying transport being blocked.

   The BLOCKED frame is sent on the stream that is blocked, that is, the
   stream with a non-positive number of bytes available in the flow
   control window.  A BLOCKED frame can be sent on stream 0x0 to
   indicate that connection-level flow control is blocked.

   An endpoint MUST NOT send any subsequent BLOCKED frames until the
   affected flow control window becomes positive.  This means that
   WINDOW_UPDATE frames are received or SETTINGS_INITIAL_WINDOW_SIZE is
   increased before more BLOCKED frames can be sent.

   The BLOCKED frame defines no flags and contains no payload.  A
   receiver MUST treat the receipt of a BLOCKED frame with a payload as
   a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.

7.  Error Codes

   Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY
   frames to convey the reasons for the stream or connection error.

   Error codes share a common code space.  Some error codes only apply



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   to specific conditions and have no defined semantics in certain frame
   types.

   The following error codes are defined:

   NO_ERROR (0):  The associated condition is not as a result of an
      error.  For example, a GOAWAY might include this code to indicate
      graceful shutdown of a connection.

   PROTOCOL_ERROR (1):  The endpoint detected an unspecific protocol
      error.  This error is for use when a more specific error code is
      not available.

   INTERNAL_ERROR (2):  The endpoint encountered an unexpected internal
      error.

   FLOW_CONTROL_ERROR (3):  The endpoint detected that its peer violated
      the flow control protocol.

   SETTINGS_TIMEOUT (4):  The endpoint sent a SETTINGS frame, but did
      not receive a response in a timely manner.  See Settings
      Synchronization (Section 6.5.3).

   STREAM_CLOSED (5):  The endpoint received a frame after a stream was
      half closed.

   FRAME_SIZE_ERROR (6):  The endpoint received a frame that was larger
      than the maximum size that it supports.

   REFUSED_STREAM (7):  The endpoint refuses the stream prior to
      performing any application processing, see Section 8.1.4 for
      details.

   CANCEL (8):  Used by the endpoint to indicate that the stream is no
      longer needed.

   COMPRESSION_ERROR (9):  The endpoint is unable to maintain the
      compression context for the connection.

   CONNECT_ERROR (10):  The connection established in response to a
      CONNECT request (Section 8.3) was reset or abnormally closed.

   ENHANCE_YOUR_CALM (11):  The endpoint detected that its peer is
      exhibiting a behavior over a given amount of time that has caused
      it to refuse to process further frames.






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   INADEQUATE_SECURITY (12):  The underlying transport has properties
      that do not meet the minimum requirements imposed by this document
      (see Section 9.2) or the endpoint.

8.  HTTP Message Exchanges

   HTTP/2 is intended to be as compatible as possible with current uses
   of HTTP.  This means that, from the perspective of the server and
   client applications, the features of the protocol are unchanged.  To
   achieve this, all request and response semantics are preserved,
   although the syntax of conveying those semantics has changed.

   Thus, the specification and requirements of HTTP/1.1 Semantics and
   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

   A client sends an HTTP request on a new stream, using a previously
   unused stream identifier (Section 5.1.1).  A server sends an HTTP
   response on the same stream as the request.

   An HTTP message (request or response) consists of:

   1.  one HEADERS frame, followed by zero or more CONTINUATION frames
       (containing the message headers; see [HTTP-p1], Section 3.2), and

   2.  zero or more DATA frames (containing the message payload; see
       [HTTP-p1], Section 3.3), and

   3.  optionally, one HEADERS frame, followed by zero or more
       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
   HEADERS frame bearing the END_STREAM flag can be followed by
   CONTINUATION frames that carry any remaining portions of the header
   block.

   Other frames (from any stream) MUST NOT occur between either HEADERS
   frame and the following CONTINUATION frames (if present), nor between
   CONTINUATION frames.

   Otherwise, frames MAY be interspersed on the stream between these



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   frames, but those frames do not carry HTTP semantics.  In particular,
   HEADERS frames (and any CONTINUATION frames that follow) other than
   the first and optional last frames in this sequence do not carry HTTP
   semantics.

   Trailing header fields are carried in a header block that also
   terminates the stream.  That is, a sequence starting with a HEADERS
   frame, followed by zero or more CONTINUATION frames, where 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
   request or response.

   An HTTP request/response exchange fully consumes a single stream.  A
   request starts with the HEADERS frame that puts the stream into an
   "open" state and ends with a frame bearing END_STREAM, which causes
   the stream to become "half closed" for the client.  A response starts
   with a HEADERS frame and ends with a frame bearing END_STREAM,
   optionally followed by CONTINUATION frames, which places the stream
   in the "closed" state.

8.1.1.  Informational Responses

   The 1xx series of HTTP response status codes ([HTTP-p2], Section 6.2)
   are not supported in HTTP/2.

   The most common use case for 1xx is using an Expect header field with
   a "100-continue" token (colloquially, "Expect/continue") to indicate
   that the client expects a 100 (Continue) non-final response status
   code, receipt of which indicates that the client should continue
   sending the request body if it has not already done so.

   Typically, Expect/continue is used by clients wishing to avoid
   sending a large amount of data in a request body, only to have the
   request rejected by the origin server (thus leaving the connection
   potentially unusable).

   HTTP/2 does not enable the Expect/continue mechanism; if the server
   sends a final status code to reject the request, it can do so without
   making the underlying connection unusable.

   Note that this means HTTP/2 clients sending requests with bodies may
   waste at least one round trip of sent data when the request is
   rejected.  This can be mitigated by restricting the amount of data
   sent for the first round trip by bandwidth-constrained clients, in
   anticipation of a final status code.

   Other defined 1xx status codes are not applicable to HTTP/2.  For
   example, the semantics of 101 (Switching Protocols) aren't suitable



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   to a multiplexed protocol.  Likewise, 102 (Processing) is no longer
   necessary, because HTTP/2 has a separate means of keeping the
   connection alive.

   This difference between protocol versions necessitates special
   handling by intermediaries that translate between them:

   o  An intermediary that gateways HTTP/1.1 to HTTP/2 MUST generate a
      100 (Continue) response if a received request includes and Expect
      header field with a "100-continue" token ([HTTP-p2], Section
      5.1.1), unless it can immediately generate a final status code.
      It MUST NOT forward the "100-continue" expectation in the request
      header fields.

   o  An intermediary that gateways HTTP/2 to HTTP/1.1 MAY add an Expect
      header field with a "100-continue" expectation when forwarding a
      request that has a body; see [HTTP-p2], Section 5.1.1 for specific
      requirements.

   o  An intermediary that gateways HTTP/2 to HTTP/1.1 MUST discard all
      other 1xx informational responses.

8.1.2.  Examples

   This section shows HTTP/1.1 requests and responses, with
   illustrations of equivalent HTTP/2 requests and responses.

   An HTTP GET request includes request header fields and no body and is
   therefore transmitted as a single HEADERS frame, followed by zero or
   more CONTINUATION frames containing the serialized block of request
   header fields.  The HEADERS frame in the following has both the
   END_HEADERS and END_STREAM flags set; no CONTINUATION frames are
   sent:

     GET /resource HTTP/1.1           HEADERS
     Host: example.org          ==>     + END_STREAM
     Accept: image/jpeg                 + END_HEADERS
                                          :method = GET
                                          :scheme = https
                                          :path = /resource
                                          host = example.org
                                          accept = image/jpeg

   Similarly, a response that includes only response header fields is
   transmitted as a HEADERS frame (again, followed by zero or more
   CONTINUATION frames) containing the serialized block of response
   header fields.




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     HTTP/1.1 304 Not Modified        HEADERS
     ETag: "xyzzy"              ==>     + END_STREAM
     Expires: Thu, 23 Jan ...           + END_HEADERS
                                          :status = 304
                                          etag: "xyzzy"
                                          expires: Thu, 23 Jan ...

   An HTTP POST request that includes request header fields and payload
   data is transmitted as one HEADERS frame, followed by zero or more
   CONTINUATION frames containing the request header fields, followed by
   one or more DATA frames, with the last CONTINUATION (or HEADERS)
   frame having the END_HEADERS flag set and the final DATA frame having
   the END_STREAM flag set:

     POST /resource HTTP/1.1          HEADERS
     Host: example.org          ==>     - END_STREAM
     Content-Type: image/jpeg           - END_HEADERS
     Content-Length: 123                  :method = POST
                                          :path = /resource
     {binary data}                        content-type = image/jpeg

                                      CONTINUATION
                                        + END_HEADERS
                                          :authority = example.org
                                          :scheme = https
                                          content-length = 123

                                      DATA
                                        + END_STREAM
                                      {binary data}

   Note that data contributing to any given header field could be spread
   between header block fragments.  The allocation of header fields to
   frames in this example is illustrative only.

   A response that includes header fields and payload data is
   transmitted as a HEADERS frame, followed by zero or more CONTINUATION
   frames, followed by one or more DATA frames, with the last DATA frame
   in the sequence having the END_STREAM flag set:












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     HTTP/1.1 200 OK                  HEADERS
     Content-Type: image/jpeg   ==>     - END_STREAM
     Content-Length: 123                + END_HEADERS
                                          :status = 200
     {binary data}                        content-type = image/jpeg
                                          content-length = 123

                                      DATA
                                        + END_STREAM
                                      {binary data}

   Trailing header fields are sent as a header block after both the
   request or response header block and all the DATA frames have been
   sent.  The HEADERS frame starting the trailers header block has the
   END_STREAM flag set.

     HTTP/1.1 200 OK                  HEADERS
     Content-Type: image/jpeg   ==>     - END_STREAM
     Transfer-Encoding: chunked         + END_HEADERS
     Trailer: Foo                         :status        = 200
                                          content-length = 123
     123                                  content-type   = image/jpeg
     {binary data}                        trailer        = Foo
     0
     Foo: bar                         DATA
                                        - END_STREAM
                                      {binary data}

                                      HEADERS
                                        + END_STREAM
                                        + END_HEADERS
                                          foo: bar

8.1.3.  HTTP Header Fields

   HTTP header fields carry information as a series of key-value pairs.
   For a listing of registered HTTP headers, see the Message Header
   Field Registry maintained at
   <http://www.iana.org/assignments/message-headers>.

   While HTTP/1.x used the message start-line (see [HTTP-p1], Section
   3.1) to convey the target URI and method of the request, and the
   status code for the response, HTTP/2 uses special pseudo-headers
   beginning with ":" for these tasks.

   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



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   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-
   hop" header fields; in this protocol, connection-specific metadata is
   conveyed by other means.  As such, a HTTP/2 message containing
   Connection MUST be treated as malformed (Section 8.1.3.5).

   This means that an intermediary transforming an HTTP/1.x message to
   HTTP/2 will need to remove any header fields nominated by the
   Connection header field, along with the Connection header field
   itself.  Such intermediaries SHOULD also remove other connection-
   specific header fields, such as Keep-Alive, Proxy-Connection,
   Transfer-Encoding and Upgrade, even if they are not nominated by
   Connection.

   One exception to this is the TE header field, which MAY be present in
   an HTTP/2 request, but when it is MUST NOT contain any value other
   than "trailers".

   Note:  HTTP/2 purposefully does not support upgrade to another
      protocol.  The handshake methods described in Section 3 are
      believed sufficient to negotiate the use of alternative protocols.

8.1.3.1.  Request Header Fields

   HTTP/2 defines a number of header fields starting with a colon ':'
   character that carry information about the request target:

   o  The ":method" header field includes the HTTP method ([HTTP-p2],
      Section 4).

   o  The ":scheme" header field includes the scheme portion of the
      target URI ([RFC3986], Section 3.1).

      ":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
      the target URI ([RFC3986], Section 3.2).  The authority MUST NOT
      include the deprecated "userinfo" subcomponent for "http" or
      "https" schemed URIs.

      To ensure that the HTTP/1.1 request line can be reproduced
      accurately, this header field MUST be omitted when translating
      from an HTTP/1.1 request that has a request target in origin or
      asterisk form (see [HTTP-p1], Section 5.3).  Clients that generate



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      HTTP/2 requests directly SHOULD instead omit the "Host" header
      field.  An intermediary that converts an HTTP/2 request to
      HTTP/1.1 MUST create a "Host" header field if one is not present
      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
      target URI (the "path-absolute" production from [RFC3986] and
      optionally a '?' character followed by the "query" production, see
      [RFC3986], Section 3.3 and [RFC3986], Section 3.4).  This field
      MUST NOT be empty; URIs that do not contain a path component MUST
      include a value of '/', unless the request is an OPTIONS request
      in asterisk form, in which case the ":path" header field MUST
      include '*'.

   All HTTP/2 requests MUST include exactly one valid value for the
   ":method", ":scheme", and ":path" header fields, unless this is a
   CONNECT request (Section 8.3).  An HTTP request that omits mandatory
   header fields is malformed (Section 8.1.3.5).

   Header field names that start with a colon are only valid in the
   HTTP/2 context.  These are not HTTP header fields.  Implementations
   MUST NOT generate header fields that start with a colon, but they
   MUST ignore any header field that starts with a colon.  In
   particular, header fields with names starting with a colon MUST NOT
   be exposed as HTTP header fields.

   HTTP/2 does not define a way to carry the version identifier that is
   included in the HTTP/1.1 request line.

8.1.3.2.  Response Header Fields

   A single ":status" header field is defined that carries the HTTP
   status code field (see [HTTP-p2], Section 6).  This header field MUST
   be included in all responses, otherwise the response is malformed
   (Section 8.1.3.5).

   HTTP/2 does not define a way to carry the version or reason phrase
   that is included in an HTTP/1.1 status line.

8.1.3.3.  Header Field Ordering

   HTTP Header Compression [COMPRESSION] does not preserve the order of
   header fields, because the relative order of header fields with
   different names is not important.  However, the same header field can
   be repeated to form a list (see [HTTP-p1], Section 3.2.2), where the
   relative order of header field values is significant.  This
   repetition can occur either as a single header field with a comma-



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   separated list of values, or as several header fields with a single
   value, or any combination thereof.  Therefore, in the latter case,
   ordering needs to be preserved before compression takes place.

   To preserve the order of multiple occurrences of a header field with
   the same name, its ordered values are concatenated into a single
   value using a zero-valued octet (0x0) to delimit them.

   After decompression, header fields that have values containing zero
   octets (0x0) MUST be split into multiple header fields before being
   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
   single value unless the ordering of that header field is known to be
   insignificant.

   The special case of "set-cookie" - which does not form a comma-
   separated list, but can have multiple values - does not depend on
   ordering.  The "set-cookie" header field MAY be encoded as multiple
   header field values, or as a single concatenated value.

8.1.3.4.  Compressing the Cookie Header Field

   The Cookie header field [COOKIE] can carry a significant amount of
   redundant data.

   The Cookie header field uses a semi-colon (";") to delimit cookie-
   pairs (or "crumbs").  This header field doesn't follow the list
   construction rules in HTTP (see [HTTP-p1], Section 3.2.2), which



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   prevents cookie-pairs from being separated into different name-value
   pairs.  This can significantly reduce compression efficiency as
   individual cookie-pairs are updated.

   To allow for better compression efficiency, the Cookie header field
   MAY be split into separate header fields, each with one or more
   cookie-pairs.  If there are multiple Cookie header fields after
   decompression, these MUST be concatenated into a single octet string
   using the two octet delimiter of 0x3B, 0x20 (the ASCII string "; ").

   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
   replaced with the cookie delimiter ("; ").

8.1.3.5.  Malformed Messages

   A malformed request or response is one that uses a valid sequence of
   HTTP/2 frames, but is otherwise invalid due to the presence of
   prohibited header fields, the absence of mandatory header fields, or
   the inclusion of uppercase header field names.

   A request or response that includes an entity body can include a
   "content-length" header field.  A request or response is also
   malformed if the value of a "content-length" header field does not
   equal the sum of the uncompressed DATA frame payload lengths that
   form the body.

   Note:  The "Content-Length" header field is set to the length of an
      entity body.  Compression of DATA frames is a function of HTTP/2
      that does not alter entities.

   Intermediaries that process HTTP requests or responses (i.e., all
   intermediaries other than those acting as tunnels) MUST NOT forward a
   malformed request or response.

   Implementations that detect malformed requests or responses need to
   ensure that the stream ends.  For malformed requests, a server MAY
   send an HTTP response prior to closing or resetting the stream.
   Clients MUST NOT accept a malformed response.  Note that these
   requirements are intended to protect against several types of common
   attacks against HTTP; they are deliberately strict, because being
   permissive can expose implementations to these vulnerabilites.

8.1.4.  Request Reliability Mechanisms in HTTP/2

   In HTTP/1.1, an HTTP client is unable to retry a non-idempotent
   request when an error occurs, because there is no means to determine
   the nature of the error.  It is possible that some server processing



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   occurred prior to the error, which could result in undesirable
   effects if the request were reattempted.

   HTTP/2 provides two mechanisms for providing a guarantee to a client
   that a request has not been processed:

   o  The GOAWAY frame indicates the highest stream number that might
      have been processed.  Requests on streams with higher numbers are
      therefore guaranteed to be safe to retry.

   o  The REFUSED_STREAM error code can be included in a RST_STREAM
      frame to indicate that the stream is being closed prior to any
      processing having occurred.  Any request that was sent on the
      reset stream can be safely retried.

   Requests that have not been processed have not failed; clients MAY
   automatically retry them, even those with non-idempotent methods.

   A server MUST NOT indicate that a stream has not been processed
   unless it can guarantee that fact.  If frames that are on a stream
   are passed to the application layer for any stream, then
   REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame
   MUST include a stream identifier that is greater than or equal to the
   given stream identifier.

   In addition to these mechanisms, the PING frame provides a way for a
   client to easily test a connection.  Connections that remain idle can
   become broken as some middleboxes (for instance, network address
   translators, or load balancers) silently discard connection bindings.
   The PING frame allows a client to safely test whether a connection is
   still active without sending a request.

8.2.  Server Push

   HTTP/2 enables a server to pre-emptively send (or "push") one or more
   associated responses to a client in response to a single request.
   This feature becomes particularly helpful when the server knows the
   client will need to have those responses available in order to fully
   process the response to the original request.

   Pushing additional responses is optional, and is negotiated between
   individual endpoints.  The SETTINGS_ENABLE_PUSH setting can be set to
   0 to indicate that server push is disabled.

   Because pushing responses is effectively hop-by-hop, an intermediary
   could receive pushed responses from the server and choose not to
   forward those on to the client.  In other words, how to make use of
   the pushed responses is up to that intermediary.  Equally, the



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   intermediary might choose to push additional responses to the client,
   without any action taken by the server.

   A client cannot push.  Thus, servers MUST treat the receipt of a
   PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.  Clients 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

   Server push is semantically equivalent to a server responding to a
   request; however, in this case that request is also sent by the
   server, as a PUSH_PROMISE frame.

   The PUSH_PROMISE frame includes a header block that contains a
   complete set of request header fields that the server attributes to
   the request.  It is not possible to push a response to a request that
   includes a request body.

   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
   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"
   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
   fields, or the ":method" header field identifies a method that is not
   safe, it MUST respond with a stream error (Section 5.4.2) of type
   PROTOCOL_ERROR.

   The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to
   sending any frames that reference the promised responses.  This
   avoids a race where clients issue requests prior to receiving any
   PUSH_PROMISE frames.

   For example, if the server receives a request for a document
   containing embedded links to multiple image files, and the server
   chooses to push those additional images to the client, sending push



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   promises before the DATA frames that contain the image links ensures
   that the client is able to see the promises before discovering
   embedded links.  Similarly, if the server pushes responses referenced
   by the header block (for instance, in Link header fields), sending
   the push promises before sending the header block ensures that
   clients do not request them.

   PUSH_PROMISE frames MUST NOT be sent by the client.  PUSH_PROMISE
   frames can be sent by the server on any stream that was opened by the
   client.  They MUST be sent on a stream that is in either the "open"
   or "half closed (remote)" state to the server.  PUSH_PROMISE frames
   are interspersed with the frames that comprise a response, though
   they cannot be interspersed with HEADERS and CONTINUATION frames that
   comprise a single header block.

8.2.2.  Push Responses

   After sending the PUSH_PROMISE frame, the server can begin delivering
   the pushed response as a response (Section 8.1.3.2) on a server-
   initiated stream that uses the promised stream identifier.  The
   server uses this stream to transmit an HTTP response, using the same
   sequence of frames as defined in Section 8.1.  This stream becomes
   "half closed" to the client (Section 5.1) after the initial HEADERS
   frame is sent.

   Once a client receives a PUSH_PROMISE frame and chooses to accept the
   pushed response, the client SHOULD NOT issue any requests for the
   promised response until after the promised stream has closed.

   If the client determines, for any reason, that it does not wish to
   receive the pushed response from the server, or if the server takes
   too long to begin sending the promised response, the client can send
   an RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes,
   and referencing the pushed stream's identifier.

   A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit
   the number of responses that can be concurrently pushed by a server.
   Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables
   server push by preventing the server from creating the necessary
   streams.  This does not prohibit a server from sending PUSH_PROMISE
   frames; clients need to reset any promised streams that are not
   wanted.

   Clients receiving a pushed response MUST validate that the server is
   authorized to provide the response, see Section 10.1.  For example, a
   server that offers a certificate for only the "example.com" DNS-ID or
   Common Name is not permitted to push a response for
   "https://www.example.org/doc".



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8.3.  The CONNECT Method

   In HTTP/1.x, the pseudo-method CONNECT ([HTTP-p2], Section 4.3.6) is
   used to convert an HTTP connection into a tunnel to a remote host.
   CONNECT is primarily used with HTTP proxies to establish a TLS
   session with an origin server for the purposes of interacting with
   "https" resources.

   In HTTP/2, the CONNECT method is used to establish a tunnel over a
   single HTTP/2 stream to a remote host, for similar purposes.  The
   HTTP header field mapping works as mostly as defined in Request
   Header Fields (Section 8.1.3.1), with a few differences.
   Specifically:

   o  The ":method" header field is set to "CONNECT".

   o  The ":scheme" and ":path" header fields MUST be omitted.

   o  The ":authority" header field contains the host and port to
      connect to (equivalent to the authority-form of the request-target
      of CONNECT requests, see [HTTP-p1], Section 5.3).

   A proxy that supports CONNECT establishes a TCP connection [TCP] to
   the server identified in the ":authority" header field.  Once this
   connection is successfully established, the proxy sends a HEADERS
   frame containing a 2xx series status code to the client, as defined
   in [HTTP-p2], Section 4.3.6.

   After the initial HEADERS frame sent by each peer, all subsequent
   DATA frames correspond to data sent on the TCP connection.  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
   assembled into DATA frames by the proxy.  Frame types other than DATA
   or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY)
   MUST NOT be sent on a connected stream, and MUST be treated as a
   stream error (Section 5.4.2) if received.

   The TCP connection can be closed by either peer.  The END_STREAM flag
   on a DATA frame is treated as being equivalent to the TCP FIN bit.  A
   client is expected to send a DATA frame with the END_STREAM flag set
   after receiving a frame bearing the END_STREAM flag.  A proxy that
   receives a DATA frame with the END_STREAM flag set sends the attached
   data with the FIN bit set on the last TCP segment.  A proxy that
   receives a TCP segment with the FIN bit set sends a DATA frame with
   the END_STREAM flag set.  Note that the final TCP segment or DATA
   frame could be empty.

   A TCP connection error is signaled with RST_STREAM.  A proxy treats



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   any error in the TCP connection, which includes receiving a TCP
   segment with the RST bit set, as a stream error (Section 5.4.2) of
   type CONNECT_ERROR.  Correspondingly, a proxy MUST send a TCP segment
   with the RST bit set if it detects an error with the stream or the
   HTTP/2 connection.

9.  Additional HTTP Requirements/Considerations

   This section outlines attributes of the HTTP protocol that improve
   interoperability, reduce exposure to known security vulnerabilities,
   or reduce the potential for implementation variation.

9.1.  Connection Management

   HTTP/2 connections are persistent.  For best performance, it is
   expected clients will not close connections until it is determined
   that no further communication with a server is necessary (for
   example, when a user navigates away from a particular web page), or
   until the server closes the connection.

   Clients SHOULD NOT open more than one HTTP/2 connection to a given
   destination, where a destination is the IP address and port that is
   derived from a URI, a selected alternative service [ALT-SVC], or a
   configured proxy.  A client can create additional connections as
   replacements, either to replace connections that are near to
   exhausting the available stream identifier space (Section 5.1.1), or
   to replace connections that have encountered errors (Section 5.4.1).

   A client MAY open multiple connections to the same IP address and TCP
   port using different Server Name Indication [TLS-EXT] values or to
   provide different TLS client certificates, but SHOULD avoid creating
   multiple connections with the same configuration. [[anchor17: Need
   more text on how client certificates relate here, see issue #363.]]

   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
   possible, but are permitted to terminate idle connections if
   necessary.  When either endpoint chooses to close the transport-level
   TCP connection, the terminating endpoint SHOULD first send a GOAWAY
   (Section 6.8) frame so that both endpoints can reliably determine
   whether previously sent frames have been processed and gracefully
   complete or terminate any necessary remaining tasks.






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9.2.  Use of TLS Features

   Implementations of HTTP/2 MUST support TLS 1.2 [TLS12].  The general
   TLS usage guidance in [TLSBCP] SHOULD be followed, with some
   additional restrictions that are specific to HTTP/2.

   The TLS implementation MUST support the Server Name Indication (SNI)
   [TLS-EXT] extension to TLS.  HTTP/2 clients MUST indicate the target
   domain name when negotiating TLS.

   The TLS implementation MUST disable compression.  TLS compression can
   lead to the exposure of information that would not otherwise be
   revealed [RFC3749].  Generic compression is unnecessary since HTTP/2
   provides compression features that are more aware of context and
   therefore likely to be more appropriate for use for performance,
   security or other reasons.

   Implementations MUST negotiate - and therefore use - ephemeral cipher
   suites, such as ephemeral Diffie-Hellman (DHE) or the elliptic curve
   variant (ECDHE) with a minimum size of 2048 bits (DHE) or security
   level of 128 bits (ECDHE).  Clients MUST accept DHE sizes of up to
   4096 bits.

   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
   the requirements in this section, or any policy-based constraints,
   SHOULD NOT negotiate HTTP/2.  Removing HTTP/2 protocols from
   consideration could result in the removal of all protocols from the
   set of protocols offered by the client.  This causes protocol
   negotiation failure, as described in Section 3.2 of [TLSALPN].

   Due to implementation limitations, it might not be possible to fail
   TLS negotiation based on all of these requirements.  An endpoint MUST
   terminate an HTTP/2 connection that is opened on a TLS session that
   does not meet these minimum requirements with a connection error
   (Section 5.4.1) of type INADEQUATE_SECURITY.

9.3.  GZip Content-Encoding

   Clients MUST support gzip compression for HTTP response bodies.
   Regardless of the value of the accept-encoding header field, a server
   MAY send responses with gzip encoding.  A compressed response MUST
   still bear an appropriate content-encoding header field.

   This effectively changes the implicit value of the Accept-Encoding
   header field ([HTTP-p2], Section 5.3.4) from "identity" to "identity,



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   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.  Security Considerations

10.1.  Server Authority

   A client is only able to accept HTTP/2 responses from servers that
   are authoritative for those resources.  This is particularly
   important for server push (Section 8.2), where the client validates
   the PUSH_PROMISE before accepting the response.

   HTTP/2 relies on the HTTP/1.1 definition of authority for determining
   whether a server is authoritative in providing a given response, see
   [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
   that is not authoritative for those resources.

10.2.  Cross-Protocol Attacks

   In a cross-protocol attack, an attacker causes a client to initiate a
   transaction in one protocol toward a server that understands a
   different protocol.  An attacker might be able to cause the
   transaction to appear as valid transaction in the second protocol.
   In combination with the capabilities of the web context, this can be
   used to interact with poorly protected servers in private networks.

   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.




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10.3.  Intermediary Encapsulation Attacks

   HTTP/2 header field names and values are encoded as sequences of
   octets with a length prefix.  This enables HTTP/2 to carry any string
   of octets as the name or value of a header field.  An intermediary
   that translates HTTP/2 requests or responses into HTTP/1.1 directly
   could permit the creation of corrupted HTTP/1.1 messages.  An
   attacker might exploit this behavior to cause the intermediary to
   create HTTP/1.1 messages with illegal header fields, extra header
   fields, or even new messages that are entirely falsified.

   Header field names or values that contain characters not permitted by
   HTTP/1.1, including carriage return (U+000D) or line feed (U+000A)
   MUST NOT be translated verbatim by an intermediary, as stipulated in
   [HTTP-p1], Section 3.2.4.

   Translation from HTTP/1.x to HTTP/2 does not produce the same
   opportunity to an attacker.  Intermediaries that perform translation
   to HTTP/2 MUST remove any instances of the "obs-fold" production from
   header field values.

10.4.  Cacheability of Pushed Responses

   Pushed responses do not have an explicit request from the client; the
   request is provided by the server in the PUSH_PROMISE frame.

   Caching responses that are pushed is possible based on the guidance
   provided by the origin server in the Cache-Control header field.
   However, this can cause issues if a single server hosts more than one
   tenant.  For example, a server might offer multiple users each a
   small portion of its URI space.

   Where multiple tenants share space on the same server, that server
   MUST ensure that tenants are not able to push representations of
   resources that they do not have authority over.  Failure to enforce
   this would allow a tenant to provide a representation that would be
   served out of cache, overriding the actual representation that the
   authoritative tenant provides.

   Pushed responses for which an origin server is not authoritative (see
   Section 10.1) are never cached or used.

10.5.  Denial of Service Considerations

   An HTTP/2 connection can demand a greater commitment of resources to
   operate than a HTTP/1.1 connection.  The use of header compression
   and flow control depend on a commitment of resources for storing a
   greater amount of state.  Settings for these features ensure that



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   memory commitments for these features are strictly bounded.
   Processing capacity cannot be guarded in the same fashion.

   The SETTINGS frame can be abused to cause a peer to expend additional
   processing time.  This might be done by pointlessly changing SETTINGS
   parameters, setting multiple undefined parameters, or changing the
   same setting multiple times in the same frame.  WINDOW_UPDATE,
   PRIORITY, or BLOCKED frames can be abused to cause an unnecessary
   waste of 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
   to expend time processing frame headers.  Note however that some uses
   are entirely legitimate, such as the sending of an empty DATA frame
   to end a stream.

   Header compression also offers some opportunities to waste processing
   resources; see [COMPRESSION] for more details on potential abuses.

   Limits in SETTINGS parameters cannot be reduced instantaneously,
   which leaves an endpoint exposed to behavior from a peer that could
   exceed the new limits.  In particular, immediately after establishing
   a connection, limits set by a server are not known to clients and
   could be exceeded without being an obvious protocol violation.

   All these features - i.e., SETTINGS changes, small frames, header
   compression - have legitimate uses.  These features become a burden
   only when they are used unnecessarily or to excess.

   An endpoint that doesn't monitor this behavior exposes itself to a
   risk of denial of service attack.  Implementations SHOULD track the
   use of these features and set limits on their use.  An endpoint MAY
   treat activity that is suspicious as a connection error
   (Section 5.4.1) of type ENHANCE_YOUR_CALM.

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.



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   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.

   Intermediaries MUST NOT alter the compression of DATA frames unless
   additional information is available that allows the intermediary to
   identify the source of data.  In particular, frames that are not
   compressed cannot be compressed, and frames that are separately
   compressed cannot be merged into a single frame.  Compressed frames
   MAY be decompressed or split into multiple frames.

   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
   purpose padding, such as might be provided by TLS [TLS12].  Redundant
   padding could even be counterproductive.  Correct application can
   depend on having specific knowledge of the data that is being padded.

   To mitigate attacks that rely on compression, disabling compression
   might be preferable to padding as a countermeasure.

   Padding can be used to obscure the exact size of frame content, and
   is provided to mitigate specific attacks within HTTP.  For example,
   attacks where compressed content includes both attacker-controlled
   plaintext and secret data (see for example, [BREACH]).

   Use of padding can result in less protection than might seem
   immediately obvious.  At best, padding only makes it more difficult
   for an attacker to infer length information by increasing the number
   of frames an attacker has to observe.  Incorrectly implemented
   padding schemes can be easily defeated.  In particular, randomized
   padding with a predictable distribution provides very little
   protection; or padding payloads to a fixed size exposes information
   as payload sizes cross the fixed size boundary, which could be
   possible if an attacker can control plaintext.

   Intermediaries SHOULD NOT remove padding, though an intermediary MAY
   remove padding and add differing amounts if the intent is to improve
   the protections padding affords.







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10.8.  Privacy Considerations

   Several characteristics of HTTP/2 provide an observer an opportunity
   to correlate actions of a single client or server over time.  This
   includes the value of settings, the manner in which flow control
   windows are managed, the way priorities are allocated to streams,
   timing of reactions to stimulus, and handling of any optional
   features.

   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
   Layer Protocol Negotiation (ALPN) Protocol IDs" registry established
   in [TLSALPN].

   This document establishes a registry for error codes.  This new
   registry is entered into a new "Hypertext Transfer Protocol (HTTP) 2
   Parameters" section.

   This document registers the "HTTP2-Settings" header field for use in
   HTTP.

   This document registers the "PRI" method for use in HTTP, to avoid
   collisions with the connection preface (Section 3.5).

11.1.  Registration of HTTP/2 Identification Strings

   This document creates two registrations for the identification of
   HTTP/2 in the "Application Layer Protocol Negotiation (ALPN) Protocol
   IDs" registry established in [TLSALPN].

   The "h2" string identifies HTTP/2 when used over TLS:

   Protocol:  HTTP/2 over TLS

   Identification Sequence:  0x68 0x32 ("h2")

   Specification:  This document (RFCXXXX)

   The "h2c" string identifies HTTP/2 when used over cleartext TCP:







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   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
   "HTTP/2 Error Code" registry manages a 32-bit space.  The "HTTP/2
   Error Code" registry operates under the "Expert Review" policy
   [RFC5226].

   Registrations for error codes are required to include a description
   of the error code.  An expert reviewer is advised to examine new
   registrations for possible duplication with existing error codes.
   Use of existing registrations is to be encouraged, but not mandated.

   New registrations are advised to provide the following information:

   Error Code:  The 32-bit error code value.

   Name:  A name for the error code.  Specifying an error code name is
      optional.

   Description:  A description of the conditions where the error code is
      applicable.

   Specification:  An optional reference for a specification that
      defines the error code.

   An initial set of error code registrations can be found in Section 7.

11.3.  HTTP2-Settings Header Field Registration

   This section registers the "HTTP2-Settings" header field in the
   Permanent Message Header Field Registry [BCP90].

   Header field name:  HTTP2-Settings

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF






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   Specification document(s):  Section 3.2.1 of this document

   Related information:  This header field is only used by an HTTP/2
      client for Upgrade-based negotiation.

11.4.  PRI Method Registration

   This section registers the "PRI" method in the HTTP Method Registry
   [HTTP-p2].

   Method Name:  PRI

   Safe  No

   Idempotent  No

   Specification document(s)  Section 3.5 of this document

   Related information:  This method is never used by an actual client.
      This method will appear to be used when an HTTP/1.1 server or
      intermediary attempts to parse an HTTP/2 connection preface.

12.  Acknowledgements

   This document includes substantial input from the following
   individuals:

   o  Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa
      Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam
      Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay,
      Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors).

   o  Gabriel Montenegro and Willy Tarreau (Upgrade mechanism).

   o  William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro,
      Jitu Padhye, Roberto Peon, Rob Trace (Flow control).

   o  Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike
      Bishop, Herve Ruellan (Substantial editorial contributions).

   o  Alexey Melnikov was an editor of this document during 2013.

   o  A substantial proportion of Martin's contribution was supported by
      Microsoft during his employment there.

13.  References





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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
                  for HTTP/2", draft-ietf-httpbis-header-compression-07
                  (work in progress), April 2014.

   [COOKIE]       Barth, A., "HTTP State Management Mechanism",
                  RFC 6265, April 2011.

   [GZIP]         Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and
                  G. Randers-Pehrson, "GZIP file format specification
                  version 4.3", RFC 1952, May 1996.

   [HTTP-p1]      Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Message Syntax and
                  Routing", draft-ietf-httpbis-p1-messaging-26 (work in
                  progress), February 2014.

   [HTTP-p2]      Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Semantics and Content",
                  draft-ietf-httpbis-p2-semantics-26 (work in progress),
                  February 2014.

   [HTTP-p4]      Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Conditional Requests",
                  draft-ietf-httpbis-p4-conditional-26 (work in
                  progress), February 2014.

   [HTTP-p5]      Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke,
                  Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range
                  Requests", draft-ietf-httpbis-p5-range-26 (work in
                  progress), February 2014.

   [HTTP-p6]      Fielding, R., Ed., Nottingham, M., Ed., and J.
                  Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1):
                  Caching", draft-ietf-httpbis-p6-cache-26 (work in
                  progress), February 2014.

   [HTTP-p7]      Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Authentication",
                  draft-ietf-httpbis-p7-auth-26 (work in progress),
                  February 2014.

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate



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                  Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2818]      Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3986]      Berners-Lee, T., Fielding, R., and L. Masinter,
                  "Uniform Resource Identifier (URI): Generic Syntax",
                  STD 66, RFC 3986, January 2005.

   [RFC4648]      Josefsson, S., "The Base16, Base32, and Base64 Data
                  Encodings", RFC 4648, October 2006.

   [RFC5226]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                  an IANA Considerations Section in RFCs", BCP 26,
                  RFC 5226, May 2008.

   [RFC5234]      Crocker, D. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC6454]      Barth, A., "The Web Origin Concept", RFC 6454,
                  December 2011.

   [TCP]          Postel, J., "Transmission Control Protocol", STD 7,
                  RFC 793, September 1981.

   [TLS-EXT]      Eastlake, D., "Transport Layer Security (TLS)
                  Extensions: Extension Definitions", RFC 6066,
                  January 2011.

   [TLS12]        Dierks, T. and E. Rescorla, "The Transport Layer
                  Security (TLS) Protocol Version 1.2", RFC 5246,
                  August 2008.

   [TLSALPN]      Friedl, S., Popov, A., Langley, A., and E. Stephan,
                  "Transport Layer Security (TLS) Application Layer
                  Protocol Negotiation Extension",
                  draft-ietf-tls-applayerprotoneg-05 (work in progress),
                  March 2014.

   [UTF-8]        Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, November 2003.

13.2.  Informative References

   [BCP90]        Klyne, G., Nottingham, M., and J. Mogul, "Registration
                  Procedures for Message Header Fields", BCP 90,
                  RFC 3864, September 2004.

   [BREACH]       Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving



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                  the CRIME Attack", July 2013, <http://
                  breachattack.com/resources/
                  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
                  Security, Inc. , March 1992.

   [RFC1323]      Jacobson, V., Braden, B., and D. Borman, "TCP
                  Extensions for High Performance", RFC 1323, May 1992.

   [RFC3749]      Hollenbeck, S., "Transport Layer Security Protocol
                  Compression Methods", RFC 3749, May 2004.

   [TALKING]      Huang, L-S., Chen, E., Barth, A., Rescorla, E., and C.
                  Jackson, "Talking to Yourself for Fun and Profit",
                  2011, <http://w2spconf.com/2011/papers/websocket.pdf>.

   [TLSBCP]       Sheffer, Y., Holz, R., and P. Saint-Andre,
                  "Recommendations for Secure Use of TLS and DTLS",
                  draft-sheffer-tls-bcp-02 (work in progress),
                  February 2014.

Appendix A.  Change Log (to be removed by RFC Editor before publication)

A.1.  Since draft-ietf-httpbis-http2-11

   Added BLOCKED frame (at risk).

   Simplified priority scheme.

   Added DATA per-frame GZip compression.

A.2.  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.




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   Integrate ALTSVC frame and supporting text.

   Dropping requirement on "deflate" Content-Encoding.

   Improving security considerations around use of compression.

A.3.  Since draft-ietf-httpbis-http2-09

   Adding padding for data frames.

   Renumbering frame types, error codes, and settings.

   Adding INADEQUATE_SECURITY error code.

   Updating TLS usage requirements to 1.2; forbidding TLS compression.

   Removing extensibility for frames and settings.

   Changing setting identifier size.

   Removing the ability to disable flow control.

   Changing the protocol identification token to "h2".

   Changing the use of :authority to make it optional and to allow
   userinfo in non-HTTP cases.

   Allowing split on 0x0 for Cookie.

   Reserved PRI method in HTTP/1.1 to avoid possible future collisions.

A.4.  Since draft-ietf-httpbis-http2-08

   Added cookie crumbling for more efficient header compression.

   Added header field ordering with the value-concatenation mechanism.

A.5.  Since draft-ietf-httpbis-http2-07

   Marked draft for implementation.

A.6.  Since draft-ietf-httpbis-http2-06

   Adding definition for CONNECT method.

   Constraining the use of push to safe, cacheable methods with no
   request body.




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   Changing from :host to :authority to remove any potential confusion.

   Adding setting for header compression table size.

   Adding settings acknowledgement.

   Removing unnecessary and potentially problematic flags from
   CONTINUATION.

   Added denial of service considerations.

A.7.  Since draft-ietf-httpbis-http2-05

   Marking the draft ready for implementation.

   Renumbering END_PUSH_PROMISE flag.

   Editorial clarifications and changes.

A.8.  Since draft-ietf-httpbis-http2-04

   Added CONTINUATION frame for HEADERS and PUSH_PROMISE.

   PUSH_PROMISE is no longer implicitly prohibited if
   SETTINGS_MAX_CONCURRENT_STREAMS is zero.

   Push expanded to allow all safe methods without a request body.

   Clarified the use of HTTP header fields in requests and responses.
   Prohibited HTTP/1.1 hop-by-hop header fields.

   Requiring that intermediaries not forward requests with missing or
   illegal routing :-headers.

   Clarified requirements around handling different frames after stream
   close, stream reset and GOAWAY.

   Added more specific prohibitions for sending of different frame types
   in various stream states.

   Making the last received setting value the effective value.

   Clarified requirements on TLS version, extension and ciphers.

A.9.  Since draft-ietf-httpbis-http2-03

   Committed major restructuring atrocities.




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   Added reference to first header compression draft.

   Added more formal description of frame lifecycle.

   Moved END_STREAM (renamed from FINAL) back to HEADERS/DATA.

   Removed HEADERS+PRIORITY, added optional priority to HEADERS frame.

   Added PRIORITY frame.

A.10.  Since draft-ietf-httpbis-http2-02

   Added continuations to frames carrying header blocks.

   Replaced use of "session" with "connection" to avoid confusion with
   other HTTP stateful concepts, like cookies.

   Removed "message".

   Switched to TLS ALPN from NPN.

   Editorial changes.

A.11.  Since draft-ietf-httpbis-http2-01

   Added IANA considerations section for frame types, error codes and
   settings.

   Removed data frame compression.

   Added PUSH_PROMISE.

   Added globally applicable flags to framing.

   Removed zlib-based header compression mechanism.

   Updated references.

   Clarified stream identifier reuse.

   Removed CREDENTIALS frame and associated mechanisms.

   Added advice against naive implementation of flow control.

   Added session header section.

   Restructured frame header.  Removed distinction between data and
   control frames.



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   Altered flow control properties to include session-level limits.

   Added note on cacheability of pushed resources and multiple tenant
   servers.

   Changed protocol label form based on discussions.

A.12.  Since draft-ietf-httpbis-http2-00

   Changed title throughout.

   Removed section on Incompatibilities with SPDY draft#2.

   Changed INTERNAL_ERROR on GOAWAY to have a value of 2 <https://
   groups.google.com/forum/?fromgroups#!topic/spdy-dev/cfUef2gL3iU>.

   Replaced abstract and introduction.

   Added section on starting HTTP/2.0, including upgrade mechanism.

   Removed unused references.

   Added flow control principles (Section 5.2.1) based on <http://
   tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>.

A.13.  Since draft-mbelshe-httpbis-spdy-00

   Adopted as base for draft-ietf-httpbis-http2.

   Updated authors/editors list.

   Added status note.

Authors' Addresses

   Mike Belshe
   Twist

   EMail: mbelshe@chromium.org


   Roberto Peon
   Google, Inc

   EMail: fenix@google.com






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   Martin Thomson (editor)
   Mozilla
   Suite 300
   650 Castro Street
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   US

   EMail: martin.thomson@gmail.com











































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