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HTTPbis Working Group                                          M. Belshe
Internet-Draft                                                     Twist
Intended status: Standards Track                                 R. Peon
Expires: January 9, 2014                                     Google, Inc
                                                         M. Thomson, Ed.
                                                               Microsoft
                                                        A. Melnikov, Ed.
                                                               Isode Ltd
                                                            July 8, 2013


                Hypertext Transfer Protocol version 2.0
                      draft-ietf-httpbis-http2-04

Abstract

   This specification describes an optimized expression of the syntax of
   the Hypertext Transfer Protocol (HTTP).  The HTTP/2.0 encapsulation
   enables more efficient use of network resources and reduced
   perception of latency by allowing header field compression and
   multiple concurrent messages on the same connection.  It also
   introduces unsolicited push of representations from servers to
   clients.

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

   This version of the draft has been marked for implementation.
   Interoperability testing will occur in the HTTP/2.0 interim in
   Hamburg, DE, starting 2013-08-05.

Editorial Note (To be removed by RFC Editor)

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

   Working Group information and related documents can be found at
   <http://tools.ietf.org/wg/httpbis/> (Wiki) and
   <https://github.com/http2/http2-spec> (source code and issues
   tracker).

   The changes in this draft are summarized in Appendix A.1.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the



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   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
   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 January 9, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Document Organization  . . . . . . . . . . . . . . . . . .  5
     1.2.  Conventions and Terminology  . . . . . . . . . . . . . . .  6
   2.  HTTP/2.0 Protocol Overview . . . . . . . . . . . . . . . . . .  6
     2.1.  HTTP Frames  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  HTTP Multiplexing  . . . . . . . . . . . . . . . . . . . .  7
     2.3.  HTTP Semantics . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Starting HTTP/2.0  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  HTTP/2.0 Version Identification  . . . . . . . . . . . . .  8
     3.2.  Starting HTTP/2.0 for "http" URIs  . . . . . . . . . . . .  8
       3.2.1.  HTTP2-Settings Header Field  . . . . . . . . . . . . . 10
     3.3.  Starting HTTP/2.0 for "https" URIs . . . . . . . . . . . . 10
     3.4.  Starting HTTP/2.0 with Prior Knowledge . . . . . . . . . . 10
     3.5.  Connection Header  . . . . . . . . . . . . . . . . . . . . 11
   4.  HTTP Frames  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Frame Header . . . . . . . . . . . . . . . . . . . . . . . 12
     4.2.  Frame Size . . . . . . . . . . . . . . . . . . . . . . . . 13



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     4.3.  Header Compression and Decompression . . . . . . . . . . . 13
   5.  Streams and Multiplexing . . . . . . . . . . . . . . . . . . . 14
     5.1.  Stream States  . . . . . . . . . . . . . . . . . . . . . . 14
       5.1.1.  Stream Identifiers . . . . . . . . . . . . . . . . . . 18
       5.1.2.  Stream Concurrency . . . . . . . . . . . . . . . . . . 18
     5.2.  Flow Control . . . . . . . . . . . . . . . . . . . . . . . 18
       5.2.1.  Flow Control Principles  . . . . . . . . . . . . . . . 19
       5.2.2.  Appropriate Use of Flow Control  . . . . . . . . . . . 20
     5.3.  Stream priority  . . . . . . . . . . . . . . . . . . . . . 20
     5.4.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 21
       5.4.1.  Connection Error Handling  . . . . . . . . . . . . . . 21
       5.4.2.  Stream Error Handling  . . . . . . . . . . . . . . . . 22
       5.4.3.  Connection Termination . . . . . . . . . . . . . . . . 22
   6.  Frame Definitions  . . . . . . . . . . . . . . . . . . . . . . 22
     6.1.  DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     6.2.  HEADERS  . . . . . . . . . . . . . . . . . . . . . . . . . 23
     6.3.  PRIORITY . . . . . . . . . . . . . . . . . . . . . . . . . 24
     6.4.  RST_STREAM . . . . . . . . . . . . . . . . . . . . . . . . 25
     6.5.  SETTINGS . . . . . . . . . . . . . . . . . . . . . . . . . 26
       6.5.1.  Setting Format . . . . . . . . . . . . . . . . . . . . 26
       6.5.2.  Defined Settings . . . . . . . . . . . . . . . . . . . 27
     6.6.  PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . . . 27
     6.7.  PING . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.8.  GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.9.  WINDOW_UPDATE  . . . . . . . . . . . . . . . . . . . . . . 31
       6.9.1.  The Flow Control Window  . . . . . . . . . . . . . . . 32
       6.9.2.  Initial Flow Control Window Size . . . . . . . . . . . 33
       6.9.3.  Reducing the Stream Window Size  . . . . . . . . . . . 34
       6.9.4.  Ending Flow Control  . . . . . . . . . . . . . . . . . 34
   7.  Error Codes  . . . . . . . . . . . . . . . . . . . . . . . . . 35
   8.  HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 36
     8.1.  HTTP Request/Response Exchange . . . . . . . . . . . . . . 36
       8.1.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . 37
       8.1.2.  Request Header Fields  . . . . . . . . . . . . . . . . 38
       8.1.3.  Response Header Fields . . . . . . . . . . . . . . . . 39
       8.1.4.  GZip Content-Encoding  . . . . . . . . . . . . . . . . 40
       8.1.5.  Request Reliability Mechanisms in HTTP/2.0 . . . . . . 40
     8.2.  Server Push  . . . . . . . . . . . . . . . . . . . . . . . 41
   9.  Additional HTTP Requirements/Considerations  . . . . . . . . . 43
     9.1.  Frame Size Limits for HTTP . . . . . . . . . . . . . . . . 43
     9.2.  Connection Management  . . . . . . . . . . . . . . . . . . 43
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 43
     10.1. Server Authority and Same-Origin . . . . . . . . . . . . . 43
     10.2. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 44
     10.3. Cacheability of Pushed Resources . . . . . . . . . . . . . 44
   11. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 45
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45
     12.1. Frame Type Registry  . . . . . . . . . . . . . . . . . . . 45



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     12.2. Error Code Registry  . . . . . . . . . . . . . . . . . . . 46
     12.3. Settings Registry  . . . . . . . . . . . . . . . . . . . . 47
     12.4. HTTP2-Settings Header Field Registration . . . . . . . . . 47
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 48
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 48
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 48
     14.2. Informative References . . . . . . . . . . . . . . . . . . 50
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 50
     A.1.  Since draft-ietf-httpbis-http2-03  . . . . . . . . . . . . 50
     A.2.  Since draft-ietf-httpbis-http2-02  . . . . . . . . . . . . 50
     A.3.  Since draft-ietf-httpbis-http2-01  . . . . . . . . . . . . 50
     A.4.  Since draft-ietf-httpbis-http2-00  . . . . . . . . . . . . 51
     A.5.  Since draft-mbelshe-httpbis-spdy-00  . . . . . . . . . . . 51





































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1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is a wildly successful
   protocol.  However, the HTTP/1.1 message format ([HTTP-p1], Section
   3) is optimized for implementation simplicity and accessibility, not
   application performance.  As such it has several characteristics that
   have a negative overall effect on application performance.

   In particular, HTTP/1.0 only allows one request to be delivered at a
   time on a given connection.  HTTP/1.1 pipelining only partially
   addressed request concurrency, and is not widely deployed.
   Therefore, clients that need to make many requests (as is common on
   the Web) typically use multiple connections to a server in order to
   reduce perceived latency.

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

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

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

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

1.1.  Document Organization

   The HTTP/2.0 Specification is split into three parts: starting
   HTTP/2.0 (Section 3), which covers how a HTTP/2.0 connection is
   initiated; a framing layer (Section 4), which multiplexes a single
   TCP connection into independent frames of various types; and an HTTP
   layer (Section 8), which specifies the mechanism for expressing HTTP
   interactions using the framing layer.  While some of the framing
   layer concepts are isolated from HTTP, building a generic framing
   layer has not been a goal.  The framing layer is tailored to the



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   needs of the HTTP protocol and server push.

1.2.  Conventions and Terminology

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

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

   The following terms are used:

   client:  The endpoint initiating the HTTP connection.

   connection:  A transport-level connection between two endpoints.

   endpoint:  Either the client or server of the connection.

   frame:  The smallest unit of communication within an HTTP/2.0
      connection, consisting of a header and a variable-length sequence
      of bytes structured according to the frame type.

   peer:  An endpoint.  When discussing a particular endpoint, "peer"
      refers to the endpoint that is remote to the primary subject of
      discussion.

   receiver:  An endpoint that is receiving frames.

   sender:  An endpoint that is transmitting frames.

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

   connection error:  An error on the HTTP/2.0 connection.

   stream:  A bi-directional flow of frames across a virtual channel
      within the HTTP/2.0 connection.

   stream error:  An error on the individual HTTP/2.0 stream.

2.  HTTP/2.0 Protocol Overview

   HTTP/2.0 provides an optimized transport for HTTP semantics.

   An HTTP/2.0 connection is an application level protocol running on
   top of a TCP connection ([RFC0793]).  The client is the TCP



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   connection initiator.

   This document describes the HTTP/2.0 protocol using a logical
   structure that is formed of three parts: framing, streams, and
   application mapping.  This structure is provided primarily as an aid
   to specification, implementations are free to diverge from this
   structure as necessary.

2.1.  HTTP Frames

   HTTP/2.0 provides an efficient serialization of HTTP semantics.  HTTP
   requests and responses are encoded into length-prefixed frames (see
   Section 4.1).

   HTTP headers are compressed into a series of frames that contain
   header block fragments (see Section 4.3).

2.2.  HTTP Multiplexing

   HTTP/2.0 provides the ability to multiplex multiple HTTP requests and
   responses onto a single connection.  Multiple requests or responses
   can be sent concurrently on a connection using streams (Section 5).
   In order to maintain independent streams, flow control and
   prioritization are necessary.

2.3.  HTTP Semantics

   HTTP/2.0 defines how HTTP requests and responses are mapped to
   streams (see Section 8) and introduces a new interaction model,
   server push (Section 8.2).

3.  Starting HTTP/2.0

   HTTP/2.0 uses the same "http" and "https" URI schemes used by
   HTTP/1.1.  HTTP/2.0 shares the same default port numbers: 80 for
   "http" URIs and 443 for "https" URIs.  As a result, implementations
   processing requests for target resource URIs like
   "http://example.org/foo" or "https://example.com/bar" are required to
   first discover whether the upstream server (the immediate peer to
   which the client wishes to establish a connection) supports HTTP/2.0.

   The means by which support for HTTP/2.0 is determined is different
   for "http" and "https" URIs.  Discovery for "http" URIs is described
   in Section 3.2.  Discovery for "https" URIs is described in
   Section 3.3.






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3.1.  HTTP/2.0 Version Identification

   The protocol defined in this document is identified using the string
   "HTTP/2.0".  This identification is used in the HTTP/1.1 Upgrade
   header field, in the TLS application layer protocol negotiation
   extension [TLSALPN] field, and other places where protocol
   identification is required.

   Negotiating "HTTP/2.0" implies the use of the transport, security,
   framing and message semantics described in this document.

   [[anchor6: Editor's Note: please remove the following text prior to
   the publication of a final version of this document.]]

   Only implementations of the final, published RFC can identify
   themselves as "HTTP/2.0".  Until such an RFC exists, implementations
   MUST NOT identify themselves using "HTTP/2.0".

   Examples and text throughout the rest of this document use "HTTP/2.0"
   as a matter of editorial convenience only.  Implementations of draft
   versions MUST NOT identify using this string.

   Implementations of draft versions of the protocol MUST add the string
   "-draft-" and the corresponding draft number to the identifier before
   the separator ('/').  For example, draft-ietf-httpbis-http2-03 is
   identified using the string "HTTP-draft-03/2.0".

   Non-compatible experiments that are based on these draft versions
   MUST instead replace the string "draft" with a different identifier.
   For example, an experimental implementation of packet mood-based
   encoding based on draft-ietf-httpbis-http2-07 might identify itself
   as "HTTP-emo-07/2.0".  Note that any label MUST conform to the
   "token" syntax defined in Section 3.2.6 of [HTTP-p1].  Experimenters
   are encouraged to coordinate their experiments on the
   ietf-http-wg@w3.org mailing list.

3.2.  Starting HTTP/2.0 for "http" URIs

   A client that makes a request to an "http" URI without prior
   knowledge about support for HTTP/2.0 uses the HTTP Upgrade mechanism
   (Section 6.7 of [HTTP-p1]).  The client makes an HTTP/1.1 request
   that includes an Upgrade header field identifying HTTP/2.0.  The
   HTTP/1.1 request MUST include an HTTP2-Settings (Section 3.2.1)
   header field.







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   For example:

     GET /default.htm HTTP/1.1
     Host: server.example.com
     Connection: Upgrade, HTTP2-Settings
     Upgrade: HTTP/2.0
     HTTP2-Settings: <base64url encoding of HTTP/2.0 SETTINGS payload>

   Requests that contain a request entity body MUST be sent in their
   entirety before the client can send HTTP/2.0 frames.  This means that
   a large request entity can block the use of the connection until it
   is completely sent.

      If concurrency of an initial request with subsequent requests is
      important, a small request can be used to perform the upgrade to
      HTTP/2.0, at the cost of an additional round trip.

   A server that does not support HTTP/2.0 can respond to the request as
   though the Upgrade header field were absent:

     HTTP/1.1 200 OK
     Content-length: 243
     Content-type: text/html

     ...

   A server that supports HTTP/2.0 accepts the upgrade with a 101
   (Switching Protocols) status code.  After the empty line that
   terminates the 101 response, the server can begin sending HTTP/2.0
   frames.  These frames MUST include a response to the request that
   initiated the Upgrade.

     HTTP/1.1 101 Switching Protocols
     Connection: Upgrade
     Upgrade: HTTP/2.0

     [ HTTP/2.0 connection ...

   The first HTTP/2.0 frame sent by the server is a SETTINGS frame
   (Section 6.5).  Upon receiving the 101 response, the client sends a
   connection header (Section 3.5), which includes a SETTINGS frame.

   The HTTP/1.1 request that is sent prior to upgrade is associated with
   stream 1 and is assigned the highest possible priority.  Stream 1 is
   implicitly half closed from the client toward the server, since the
   request is completed as an HTTP/1.1 request.  After commencing the
   HTTP/2.0 connection, stream 1 is used for the response.




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3.2.1.  HTTP2-Settings Header Field

   A client that upgrades from HTTP/1.1 to HTTP/2.0 MUST include an
   "HTTP2-Settings" header field.  The "HTTP2-Settings" header field is
   a hop-by-hop header field that includes settings that govern the
   HTTP/2.0 connection, provided in anticipation of the server accepting
   the request to upgrade.  A server MUST reject an attempt to upgrade
   if this header is not present.

     HTTP2-Settings    = token68

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

   The client MUST include values for the following settings
   (Section 6.5.1):

   o  SETTINGS_MAX_CONCURRENT_STREAMS

   o  SETTINGS_INITIAL_WINDOW_SIZE

   As a hop-by-hop header field, the "Connection" header field MUST
   include a value of "HTTP2-Settings" in addition to "Upgrade" when
   upgrading to HTTP/2.0.

   A server decodes and interprets these values as it would any other
   SETTINGS frame.  Providing these values in the Upgrade request
   ensures that the protocol does not require default values for the
   above settings, and gives a client an opportunity to provide other
   settings prior to receiving any frames from the server.

3.3.  Starting HTTP/2.0 for "https" URIs

   A client that makes a request to an "https" URI without prior
   knowledge about support for HTTP/2.0 uses TLS [RFC5246] with the
   application layer protocol negotiation extension [TLSALPN].

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

3.4.  Starting HTTP/2.0 with Prior Knowledge

   A client can learn that a particular server supports HTTP/2.0 by
   other means.  A client MAY immediately send HTTP/2.0 frames to a



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   server that is known to support HTTP/2.0, after the connection header
   (Section 3.5).  This only affects the resolution of "http" URIs;
   servers supporting HTTP/2.0 are required to support protocol
   negotiation in TLS [TLSALPN] for "https" URIs.

   Prior support for HTTP/2.0 is not a strong signal that a given server
   will support HTTP/2.0 for future connections.  It is possible for
   server configurations to change or for configurations to differ
   between instances in clustered server.  Interception proxies (a.k.a.
   "transparent" proxies) are another source of variability.

3.5.  Connection Header

   Upon establishment of a TCP connection and determination that
   HTTP/2.0 will be used by both peers, each endpoint MUST send a
   connection header as a final confirmation and to establish the
   initial settings for the HTTP/2.0 connection.

   The client connection header is a sequence of 24 octets, which in hex
   notation are:

     505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

   (the string "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n") followed by a
   SETTINGS frame (Section 6.5).  The client sends the client connection
   header immediately upon receipt of a 101 Switching Protocols response
   (indicating a successful upgrade), or after receiving a TLS Finished
   message from the server.  If starting an HTTP/2.0 connection with
   prior knowledge of server support for the protocol, the client
   connection header is sent upon connection establishment.

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

   The server connection header consists of just a SETTINGS frame
   (Section 6.5) that MUST be the first frame the server sends in the
   HTTP/2.0 connection.

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



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   Clients and servers MUST terminate the TCP connection if either peer
   does not begin with a valid connection header.  A GOAWAY frame
   (Section 6.8) MAY be omitted if it is clear that the peer is not
   using HTTP/2.0.

4.  HTTP Frames

   Once the HTTP/2.0 connection is established, endpoints can begin
   exchanging frames.

4.1.  Frame Header

   All frames begin with an 8-octet header followed by a payload of
   between 0 and 65,535 octets.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Length (16)           |   Type (8)    |   Flags (8)   |
    +-+-------------+---------------+-------------------------------+
    |R|                 Stream Identifier (31)                      |
    +-+-------------------------------------------------------------+
    |                   Frame Payload (0...)                      ...
    +---------------------------------------------------------------+

                               Frame Header

   The fields of the frame header are defined as:

   Length:  The length of the frame payload expressed as an unsigned 16-
      bit integer.  The 8 octets of the frame header are not included in
      this value.

   Type:  The 8-bit type of the frame.  The frame type determines how
      the remainder of the frame header and payload are interpreted.
      Implementations MUST ignore unsupported and unrecognized frame
      types.

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

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







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

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

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

4.2.  Frame Size

   The maximum size of a frame payload varies by frame type and use.
   The absolute maximum size is 65,535 octets.  All implementations
   SHOULD be capable of receiving and minimally processing frames up to
   this size.

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

   If a frame size exceeds any defined limit, or is too small to contain
   mandatory frame data, the endpoint MUST send a FRAME_TOO_LARGE error.
   Frame size errors in frames that affect connection-level state MUST
   be treated as a connection error (Section 5.4.1).

4.3.  Header Compression and Decompression

   A header in HTTP/2.0 is a name-value pair with one or more associated
   values.  They are used within HTTP request and response messages as
   well as server push operations (see Section 8.2).

   Header sets are logical collections of zero or more header fields
   arranged at the application layer.  When transmitted over a
   connection, the header set is serialized into a header block using
   HTTP Header Compression [COMPRESSION].  The serialized header block
   is then divided into one or more octet sequences, called header block
   fragments, and transmitted within the payload of HEADERS
   (Section 6.2) or PUSH_PROMISE (Section 6.6) frames.  The receiving
   endpoint reassembles the header block by concatenating the individual
   fragments, then decompresses the block to reconstruct the header set.

   Header block fragments can only be sent as the payload of HEADERS or
   PUSH_PROMISE frames.

   A compressed and encoded header block is transmitted in one or more



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   HEADERS or PUSH_PROMISE frames.  If the number of octets in the block
   is greater than the space remaining in the frame, the block is
   divided into multiple fragments, which are then transmitted in
   multiple frames.

   Header blocks MUST be transmitted as a contiguous sequence of frames,
   with no interleaved frames of any other type, or from any other
   stream.  The last frame in a sequence of HEADERS frames MUST have the
   END_HEADERS flag set.  The last frame in a sequence of PUSH_PROMISE
   frames MUST have the END_PUSH_PROMISE flag set.

   HEADERS and PUSH_PROMISE frames carry data that can modify the
   compression context maintained by a receiver.  An endpoint receiving
   HEADERS or PUSH_PROMISE frames MUST reassemble header blocks and
   perform decompression even if the frames are to be discarded, which
   is likely to occur after a stream is reset.  A receiver MUST
   terminate the connection with a connection error (Section 5.4.1) of
   type COMPRESSION_ERROR, if it does not decompress a header block.

5.  Streams and Multiplexing

   A "stream" is an independent, bi-directional sequence of HEADER and
   DATA frames exchanged between the client and server within an
   HTTP/2.0 connection.  Streams have several important characteristics:

   o  A single HTTP/2.0 connection can contain multiple concurrently
      active streams, with either endpoint interleaving frames from
      multiple streams.

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

   o  Streams can be closed by either endpoint.

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

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

5.1.  Stream States

   The lifecycle of a stream is shown in Figure 1.








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


                          Figure 1: Stream States

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

   Streams have the following states:

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

      The following transitions are valid from this state:

      *  Sending or receiving a HEADERS frame causes the stream to
         become "open".  The stream identifier is selected as described
         in Section 5.1.1.




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

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

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

      In this state, only the following transitions are possible:

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

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

      An endpoint MUST NOT send any other type of frame in this state.

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

      In this state, only the following transitions are possible:

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

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

      Receiving any other type of frame MUST be treated as a stream
      error (Section 5.4.2) of type PROTOCOL_ERROR.

   open:
      The "open" state is where both peers can send frames.  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 a END_STREAM
      flag set, which causes the stream to transition into one of the
      "half closed" states: an endpoint sending a END_STREAM flag causes
      the stream state to become "half closed (local)"; an endpoint



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      receiving a END_STREAM flag causes the stream state to become
      "half closed (remote)".

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

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

      A stream transitions from this state to "closed" when a frame that
      contains a END_STREAM flag is received, or when either peer sends
      a RST_STREAM frame.

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

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

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

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

      An endpoint MUST NOT send frames on a closed stream.  An endpoint
      that receives a frame after receiving a RST_STREAM or a frame
      containing a END_STREAM flag on that stream MUST treat that as a
      stream error (Section 5.4.2) of type STREAM_CLOSED.

      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 that sends a RST_STREAM frame 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.

      An endpoint might receive a PUSH_PROMISE frame after it sends
      RST_STREAM.  PUSH_PROMISE causes a stream to become "reserved".
      If promised streams are not desired, a RST_STREAM can be used to



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      close any of those streams.

5.1.1.  Stream Identifiers

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

   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.

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

5.1.2.  Stream Concurrency

   A peer can limit the number of concurrently active streams using the
   SETTINGS_MAX_CONCURRENT_STREAMS parameters within a SETTINGS frame.
   The maximum concurrent streams setting is specific to each endpoint
   and applies only to the peer that receives the setting.  That is,
   clients specify the maximum number of concurrent streams the server
   can initiate, and servers specify the maximum number of concurrent
   streams the client can initiate.  Endpoints MUST NOT exceed the limit
   set by their peer.

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

   Streams in either of the "reserved" states do not count as open, even
   if a small amount of application state is retained to ensure that the
   promised stream can be successfully used.

5.2.  Flow Control

   Using streams for multiplexing introduces contention over use of the
   TCP connection, resulting in blocked streams.  A flow control scheme



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   ensures that streams on the same connection do not destructively
   interfere with each other.  Flow control is used for both individual
   streams and for the connection as a whole.

   HTTP/2.0 provides for flow control through use of the WINDOW_UPDATE
   (Section 6.9) frame type.

5.2.1.  Flow Control Principles

   Experience with TCP congestion control has shown that algorithms can
   evolve over time to become more sophisticated without requiring
   protocol changes.  TCP congestion control and its evolution is
   clearly different from HTTP/2.0 flow control, though the evolution of
   TCP congestion control algorithms shows that a similar approach could
   be feasible for HTTP/2.0 flow control.

   HTTP/2.0 stream flow control aims to allow for future improvements to
   flow control algorithms without requiring protocol changes.  Flow
   control in HTTP/2.0 has the following characteristics:

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

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

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

   4.  The initial value for the flow control window is 65536 bytes for
       both new streams and the overall connection.

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

   6.  Flow control can be disabled by a receiver.  A receiver can
       choose to either disable flow control for a stream or connection
       by sending a window update frame with a specific flag.  See
       Ending Flow Control (Section 6.9.4) for more details.



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   7.  HTTP/2.0 standardizes only the format of the WINDOW_UPDATE frame
       (Section 6.9).  This does not stipulate how a receiver decides
       when to send this frame or the value that it sends.  Nor does it
       specify how a sender chooses to send packets.  Implementations
       are able to select any algorithm that suits their needs.

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

5.2.2.  Appropriate Use of Flow Control

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

   Deployments that do not require this capability SHOULD disable flow
   control for data that is being received.  Note that flow control
   cannot be disabled for sending.  Sending data is always subject to
   the flow control window advertised by the receiver.

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

   Even with full awareness of the current bandwidth-delay product,
   implementation of flow control is difficult.  However, it can ensure
   that constrained resources are protected without any reduction in
   connection utilization.

5.3.  Stream priority

   The endpoint establishing a new stream can assign a priority for the
   stream.  Priority is represented as an unsigned 31-bit integer. 0
   represents the highest priority and 2^31-1 represents the lowest
   priority.

   The purpose of this value is to allow the initiating endpoint to
   request that frames for the stream be processed with a specified
   priority relative to other concurrently active streams.  That is, if
   an endpoint receives interleaved frames for multiple streams, the



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   endpoint ought to make a best-effort attempt at processing frames for
   higher priority streams before processing those for lower priority
   streams.

   Explicitly setting the priority for a stream does not guarantee any
   particular processing order for the stream relative to any other
   stream.  Nor is there any mechanism provided by which the initiator
   of a stream can force or require a receiving endpoint to process
   frames from one stream before processing frames from another.

   Unless explicitly specified in the HEADERS frame (Section 6.2) during
   stream creation, the default stream priority is 2^30.  Pushed streams
   (Section 8.2) are assumed to inherit the priority of the associated
   stream plus one (or 2^31-1 if the the associated stream priority is
   2^31-1), i.e. they have priority one lower than the associated
   stream.

5.4.  Error Handling

   HTTP/2.0 framing permits two classes of error:

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

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

   A list of error codes is included in Section 7.

5.4.1.  Connection Error Handling

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

   An endpoint that encounters a connection error SHOULD first send a
   GOAWAY (Section 6.8) frame 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 if
   the error is recurrent.  Endpoints SHOULD send a GOAWAY frame when



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

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

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

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

5.4.3.  Connection Termination

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

6.  Frame Definitions

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

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



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   defined by this document are permitted to alter the connection state.

6.1.  DATA

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

   The DATA frame defines the following flags:

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

   RESERVED (0x2):  Bit 2 is reserved for future use.

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

6.2.  HEADERS

   The HEADERS frame (type=0x1) carries name-value pairs.  The HEADERS
   is used to open a stream (Section 5.1).  Any number of HEADERS frames
   can be sent on an existing stream at any time.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|                        Priority (31)                        |
    +-+-------------------------------------------------------------+
    |                   Header Block Fragment (*)                 ...
    +---------------------------------------------------------------+

                           HEADERS Frame Payload

   The HEADERS frame defines the following flags:

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







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   RESERVED (0x2):  Bit 2 is reserved for future use.

   END_HEADERS (0x4):  The END_HEADERS bit indicates that this frame
      ends the sequence of header block fragments necessary to provide a
      complete set of headers.

      The payload for a complete header block is provided by a sequence
      of HEADERS frames, terminated by a HEADERS frame with the
      END_HEADERS flag set.  Once the sequence terminates, the payload
      of all HEADERS frames are concatenated and interpreted as a single
      block.

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

   PRIORITY (0x8):  Bit 4 being set indicates that the first four octets
      of this frame contain a single reserved bit and a 31-bit priority;
      see Section 5.3.  If this bit is not set, the four bytes do not
      appear and the frame only contains a header block fragment.

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

   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 defined in
   Section 4.3.

6.3.  PRIORITY

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|                        Priority (31)                        |
    +-+-------------------------------------------------------------+

                          PRIORITY Frame Payload




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   The payload of a PRIORITY frame contains a single reserved bit and a
   31-bit priority.

   The PRIORITY frame does not define any flags.

   The PRIORITY frame is associated with an existing stream.  If a
   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.

6.4.  RST_STREAM

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

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

                         RST_STREAM Frame Payload

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

   The RST_STREAM frame does not define any flags.

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

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






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6.5.  SETTINGS

   The SETTINGS frame (type=0x4) conveys configuration parameters that
   affect how endpoints communicate.  The parameters are either
   constraints on peer behavior or preferences.

   SETTINGS frames MUST be sent at the start of a connection, and MAY be
   sent at any other time by either endpoint over the lifetime of the
   connection.

   Implementations MUST support all of the settings defined by this
   specification and MAY support additional settings defined by
   extensions.  Unsupported or unrecognized settings MUST be ignored.
   New settings MUST NOT be defined or implemented in a way that
   requires endpoints to understand them in order to communicate
   successfully.

   A SETTINGS frame is not required to include every defined setting;
   senders can include only those parameters for which it has accurate
   values and a need to convey.  When multiple parameters are sent, they
   SHOULD be sent in order of numerically lowest ID to highest ID.  A
   single SETTINGS frame MUST NOT contain multiple values for the same
   ID.  If the receiver of a SETTINGS frame discovers multiple values
   for the same ID, it MUST ignore all values for that ID except the
   first one.

   Over the lifetime of a connection, an endpoint MAY send multiple
   SETTINGS frames containing previously unspecified parameters or new
   values for parameters whose values have already been established.
   Only the most recent provided setting value applies.

   The SETTINGS frame does not define any flags.

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

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

6.5.1.  Setting Format

   The payload of a SETTINGS frame consists of zero or more settings.
   Each setting consists of an 8-bit reserved field, an unsigned 24-bit
   setting identifier, and an unsigned 32-bit value.



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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Reserved (8) |            Setting Identifier (24)            |
    +---------------+-----------------------------------------------+
    |                        Value (32)                             |
    +---------------------------------------------------------------+

                              Setting Format

6.5.2.  Defined Settings

   The following settings are defined:

   SETTINGS_MAX_CONCURRENT_STREAMS (4):  indicates the maximum number of
      concurrent streams that the sender will allow.  This limit is
      directional: it applies to the number of streams that the sender
      permits the receiver to create.  By default there is no limit.  It
      is recommended that this value be no smaller than 100, so as to
      not unnecessarily limit parallelism.

   SETTINGS_INITIAL_WINDOW_SIZE (7):  indicates the sender's initial
      window size (in bytes) for stream level flow control.

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

   SETTINGS_FLOW_CONTROL_OPTIONS (10):  indicates that streams directed
      to the sender will not be subject to flow control.  The least
      significant bit (0x1) of the value is set to indicate that new
      streams are not flow controlled.  All other bits are reserved.

      This setting applies to all streams, including existing streams.

      These bits cannot be cleared once set, see Section 6.9.4.

6.6.  PUSH_PROMISE

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







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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|                Promised-Stream-ID (31)                      |
    +-+-------------------------------------------------------------+
    |                 Header Block Fragment (*)                   ...
    +---------------------------------------------------------------+

                        PUSH_PROMISE Payload Format

   The payload of a PUSH_PROMISE includes a "Promised-Stream-ID".  This
   unsigned 31-bit integer identifies the stream the endpoint intends to
   start sending frames for.  The promised stream identifier MUST be a
   valid choice for the next stream sent by the sender (see new stream
   identifier (Section 5.1.1)).

   Following the "Promised-Stream-ID" is a header block fragment
   (Section 4.3).

   PUSH_PROMISE frames MUST be associated with an existing, peer-
   initiated stream.  If the stream identifier field specifies the value
   0x0, a recipient MUST respond with a connection error (Section 5.4.1)
   of type PROTOCOL_ERROR.

   The PUSH_PROMISE frame defines the following flags:

   END_PUSH_PROMISE (0x1):  The END_PUSH_PROMISE bit indicates that this
      frame ends the sequence of header block fragments necessary to
      provide a complete set of headers.

      The payload for a complete header block is provided by a sequence
      of PUSH_PROMISE frames, terminated by a PUSH_PROMISE frame with
      the END_PUSH_PROMISE flag set.  Once the sequence terminates, the
      payload of all PUSH_PROMISE frames are concatenated and
      interpreted as a single block.

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

   Promised streams are not required to be used in order promised.  The
   PUSH_PROMISE only reserves stream identifiers for later use.

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



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

6.7.  PING

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

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

                            PING Payload Format

   In addition to the frame header, PING frames MUST contain 8 octets of
   data in the payload.  A sender can include any value it chooses and
   use those bytes in any fashion.

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

   The PING frame defines the following flags:

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

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

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

6.8.  GOAWAY

   The GOAWAY frame (type=0x7) informs the remote peer to stop creating
   streams on this connection.  It can be sent from the client or the



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

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

   Endpoints SHOULD always send a GOAWAY frame before closing a
   connection so that the remote can know whether a stream has been
   partially processed or not.  For example, if an HTTP client sends a
   POST at the same time that a server closes a connection, the client
   cannot know if the server started to process that POST request if the
   server does not send a GOAWAY frame to indicate where it stopped
   working.  An endpoint might choose to close a connection without
   sending GOAWAY for misbehaving peers.

   After sending a GOAWAY frame, the sender can discard frames for new
   streams.  However, any frames that alter connection state cannot be
   completely ignored.  For instance, HEADERS and PUSH_PROMISE frames
   MUST be minimally processed to ensure a consistent compression state
   (see Section 4.3).

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

                           GOAWAY Payload Format

   The GOAWAY frame does not define any flags.

   The GOAWAY frame applies to the connection, not a specific stream.



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   The stream identifier MUST be zero.

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

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

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

   Activity on streams numbered lower or equal to the last stream
   identifier might still complete successfully.  The sender of a GOAWAY
   frame might gracefully shut down a connection by sending a GOAWAY
   frame, maintaining the connection in an open state until all in-
   progress streams complete.

   The last stream ID MUST be 0 if no streams were acted upon.

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

   Endpoints MAY append opaque data to the payload of any GOAWAY frame.
   Additional debug data is intended for diagnostic purposes only and
   carries no semantic value.  Debug data MUST NOT be persistently
   stored, since it could contain sensitive information.

6.9.  WINDOW_UPDATE

   The WINDOW_UPDATE frame (type=0x9) is used to implement flow control.

   Flow control operates at two levels: on each individual stream and on
   the entire connection.

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



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   Flow control only applies to frames that are identified as being
   subject to flow control.  Of the frame types defined in this
   document, this includes only DATA frame.  Frames that are exempt from
   flow control MUST be accepted and processed, unless the receiver is
   unable to assign resources to handling the frame.  A receiver MAY
   respond with a stream error (Section 5.4.2) or connection error
   (Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable accept a
   frame.

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

                       WINDOW_UPDATE Payload Format

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

   The WINDOW_UPDATE frame defines the following flags:

   END_FLOW_CONTROL (0x1):  Bit 1 being set indicates that flow control
      for the identified stream or connection has been ended; subsequent
      frames do not need to be flow controlled.

   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.

6.9.1.  The Flow Control Window

   Flow control in HTTP/2.0 is implemented using a window kept by each
   sender on every stream.  The flow control window is a simple integer
   value that indicates how many bytes of data the sender is permitted
   to transmit; as such, its size is a measure of the buffering
   capability of the receiver.

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



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   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.

6.9.2.  Initial Flow Control Window Size

   When a HTTP/2.0 connection is first established, new streams are
   created with an initial flow control window size of 65535 bytes.  The
   connection flow control window is 65535 bytes.  Both endpoints can
   adjust the initial window size for new streams by including a value
   for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms
   part of the connection header.

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

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



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   control window.

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

   For example, if the client sends 64KB 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 -48KB on receipt of the SETTINGS frame.  The client retains a
   negative flow control window until WINDOW_UPDATE frames restore the
   window to being positive, after which the client can resume sending.

6.9.3.  Reducing the Stream Window Size

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

   A receiver has two options for handling streams that exceed flow
   control limits:

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

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

   If a receiver decides to accept streams, both sides MUST recompute
   the available flow control window based on the initial window size
   sent in the SETTINGS.

6.9.4.  Ending Flow Control

   After a receiver reads in a frame that marks the end of a stream (for
   example, a data stream with a END_STREAM flag set), it MUST cease
   transmission of WINDOW_UPDATE frames for that stream.  A sender is
   not obligated to maintain the available flow control window for
   streams that it is no longer sending on.

   Flow control can be disabled for all streams on the connection using
   the SETTINGS_FLOW_CONTROL_OPTIONS setting.  An implementation that
   does not wish to perform stream flow control can use this in the



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   initial SETTINGS exchange.

   Flow control can be disabled for an individual stream or the overall
   connection by sending a WINDOW_UPDATE with the END_FLOW_CONTROL flag
   set.  The payload of a WINDOW_UPDATE frame that has the
   END_FLOW_CONTROL flag set is ignored.

   Flow control cannot be enabled again once disabled.  Any attempt to
   re-enable flow control - by sending a WINDOW_UPDATE or by clearing
   the bits on the SETTINGS_FLOW_CONTROL_OPTIONS setting - MUST be
   rejected with a FLOW_CONTROL_ERROR error code.

7.  Error Codes

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

   Error codes share a common code space.  Some error codes only apply
   to specific conditions and have no defined semantics in certain frame
   types.

   The following error codes are defined:

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

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

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

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

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

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

   REFUSED_STREAM (7):  The endpoint refuses the stream prior to
      performing any application processing, see Section 8.1.5 for
      details.





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

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

8.  HTTP Message Exchanges

   HTTP/2.0 is intended to be as compatible as possible with current
   web-based applications.  This means that, from the perspective of the
   server business logic or application API, the features of HTTP are
   unchanged.  To achieve this, all of the application request and
   response header semantics are preserved, although the syntax of
   conveying those semantics has changed.  Thus, the rules from HTTP/1.1
   ([HTTP-p1], [HTTP-p2], [HTTP-p4], [HTTP-p5], [HTTP-p6], and
   [HTTP-p7]) apply with the changes in the sections below.

8.1.  HTTP Request/Response Exchange

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

   An HTTP request or response each consist of:

   o  one contiguous sequence of HEADERS frames;

   o  zero or more DATA frames; and

   o  optionally, a contiguous sequence of HEADERS frames

   The last frame in the sequence bears an END_STREAM flag.

   Other frames, including HEADERS, MAY be interspersed with these
   frames, but those frames do not carry HTTP semantics.

   Trailing header fields are carried in a header block that also
   terminates the stream.  That is, a sequence of HEADERS frames that
   carries an END_STREAM flag on the last frame.  Header blocks after
   the first that do not terminate the stream are not part of an HTTP
   request or response.

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



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8.1.1.  Examples

   For example, an HTTP GET request that includes request header fields
   and no body, is transmitted as a single contiguous sequence of
   HEADERS frames containing the serialized block of request header
   fields.  The last HEADERS frame in the sequence has both the
   END_HEADERS and END_STREAM flag set:

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

   Similarly, a response that includes only response header fields is
   transmitted as a sequence of HEADERS frames containing the serialized
   block of response header fields.  The last HEADERS frame in the
   sequence has both the END_HEADERS and END_STREAM flag set:

     HTTP/1.1 204 No Content       HEADERS
     Content-Length: 0        ===>   + END_STREAM
                                     + END_HEADERS
                                       :status = 204
                                       content-length: 0

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

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

                                    DATA
                                      + END_STREAM
                                        {binary data}




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

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

                                    DATA
                                      + END_STREAM
                                        {binary data}

   Trailing header fields are sent as a header block after both the
   request or response header block and all the DATA frames have been
   sent.  The sequence of HEADERS frames that bears the trailers
   includes a terminal frame that has both END_HEADERS and END_STREAM
   flags set.

     HTTP/1.1 200 OK               HEADERS
     Content-Type: image/jpeg ===>   - END_STREAM
     Content-Length: 123             + END_HEADERS
     TE: trailers                      :status        = 200
     123                               content-type   = image/jpeg
     {binary data}                     content-length = 123
     0
     Foo: bar                      DATA
                                     - END_STREAM
                                       {binary data}

                                   HEADERS
                                     + END_STREAM
                                     + END_HEADERS
                                       foo: bar

8.1.2.  Request Header Fields

   The definitions of the request header fields are largely unchanged
   relative to HTTP/1.1, with a few notable exceptions:

   o  The HTTP/1.1 request-line has been split into two separate header
      fields named :method and :path, whose values specify the HTTP
      method for the request and the request-target, respectively.  The
      HTTP-version component of the request-line is removed entirely
      from the headers.



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   o  The host and optional port portions of the request URI (see
      [RFC3986], Section 3.2), are specified using the new :host header
      field. [[anchor13: Ed.  Note: it needs to be clarified whether or
      not this replaces the existing HTTP/1.1 Host header.]]

   o  A new :scheme header field has been added to specify the scheme
      portion of the request-target (e.g. "https")

   o  All header field names MUST be lowercased, and the definitions of
      all header field names defined by HTTP/1.1 are updated to be all
      lowercase.

   o  The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-
      Encoding header fields are no longer valid and MUST NOT be sent.
      [[anchor14: Ed.  Note: And "TE" I presume?]]

   All HTTP Requests MUST include the ":method", ":path", ":host", and
   ":scheme" header fields.

   Header fields whose names begin with ":" (whether defined in this
   document or future extensions to this document) MUST appear before
   any other header fields. [[anchor15: Ed.  Note: This requirement is
   currently pending review.  Consider it "on hold" for the moment.]]

   All HTTP Requests that include a body SHOULD include the "content-
   length" header field.  If a server receives a request where the sum
   of the DATA frame payload lengths does not equal the value of the
   "content-length" header field, the server MUST return a 400 (Bad
   Request) error.

   If a client omits a mandatory header field from the request, the
   server MUST reply with a HTTP 400 Bad Request reply.

8.1.3.  Response Header Fields

   The definitions of the response header fields are largely unchanged
   relative to HTTP/1.1, with a few notable exceptions:

   o  The response status line has been reduced to a single ":status"
      header field whose value specifies only the numeric response
      status code.  The status text component of the HTTP/1.1 response
      has been dropped entirely.

   o  The response MUST contain exactly one :status header field with
      exactly one response status value.  If the client receives an HTTP
      response that does not include the :status field, or provides
      multiple response status code values, it MUST respond with a
      stream error (Section 5.4.2) of type PROTOCOL_ERROR.



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   o  All header field names MUST be lowercased, and the definitions of
      all header field names defined by HTTP/1.1 are updated to be all
      lowercase.

   o  The Connection, Keep-Alive, Proxy-Connection, and Transfer-
      Encoding header fields are not valid and MUST NOT be sent.

   Header fields whose names begin with ":" (whether defined in this
   document or future extensions to this document) MUST appear before
   any other header fields. [[anchor16: Ed.  Note: This requirement is
   currently pending review.  Consider it "on hold" for the moment.]]

8.1.4.  GZip Content-Encoding

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

8.1.5.  Request Reliability Mechanisms in HTTP/2.0

   In HTTP/1.1, an HTTP client is unable to retry a non-idempotent
   request when an error occurs, because there is no means to determine
   the nature of the error.  It is possible that some server processing
   occurred prior to the error, which could result in undesirable
   effects if the request were reattempted.

   HTTP/2.0 provides two mechanisms for providing a guarantee to a
   client that a request has not been processed:

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

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

   In both cases, clients MAY automatically retry all requests,
   including those with non-idempotent methods.

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



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   given stream identifier.

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

8.2.  Server Push

   HTTP/2.0 enables a server to pre-emptively send (or "push") multiple
   associated resources to a client in response to a single request.
   This feature becomes particularly helpful when the server knows the
   client will need to have those resources available in order to fully
   process the originally requested resource.

   Pushing additional resources is optional, and is negotiated only
   between individual endpoints.  For instance, an intermediary could
   receive pushed resources from the server but is not required to
   forward those on to the client.  How to make use of the pushed
   resources is up to that intermediary.  Equally, the intermediary
   might choose to push additional resources to the client, without any
   action taken by the server.

   Server push is semantically equivalent to a server responding to a
   GET request for that resource.  The PUSH_PROMISE frame, or frames,
   sent by the server includes a header block that contains the request
   headers that the server has assumed.

   Pushed resources are always associated with an explicit request from
   a client.  The PUSH_PROMISE frames sent by the server are sent on the
   stream created for the original request.  The PUSH_PROMSE frame
   includes a promised stream identifier, chosen from the stream
   identifiers available to the server (see Section 5.1.1).  Any header
   fields that are not specified in the PUSH_PROMISE frames sent by the
   server are inherited from the original request sent by the client.

   The header fields in PUSH_PROMISE MUST include the ":scheme", ":host"
   and ":path" header fields that identify the resource that is being
   pushed.  A PUSH_PROMISE always implies an HTTP method of GET.  If a
   client receives a PUSH_PROMISE that does not include these header
   fields, or a value for the ":method" header field, it MUST respond
   with a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

   After sending the PUSH_PROMISE frame, the server can begin delivering
   the pushed resource on a new, server-initiated stream that uses the
   promised stream identifier.  This stream is already implicitly "half



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   closed" to the client (Section 5.1).  The server uses this stream to
   transmit an HTTP response, using the same sequence of frames as
   defined in Section 8.1.

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

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

   For example, if the server receives a request for a document
   containing embedded links to multiple image files, and the server
   chooses to push those additional images to the client, sending push
   promises before the DATA frames that contain the image links ensure
   that the client is able to see the promises before discovering the
   resources.  Likewise, if the server pushes resources referenced by
   the header block (for instance, in Link header fields), sending the
   push promises before sending the header block ensures that clients do
   not request those resources.

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

   A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit
   the number of resources that can be concurrently pushed by a server.
   Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables
   server push by preventing the server from creating the necessary
   streams.

   The request header fields provided in the PUSH_PROMISE frame SHOULD
   include enough information for a client to determine whether a cached
   representation of the resource is already available.  If the client
   determines, for any reason, that it does not wish to receive the
   pushed resource from the server, or if the server takes too long to
   begin sending the promised resource, the client can send an
   RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes,
   and referencing the pushed stream's identifier.

   Clients receiving a pushed response MUST validate that the server is



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   authorized to push the resource using the same-origin policy
   ([RFC6454], Section 3).  For example, a HTTP/2.0 connection to
   "example.com" is generally [[anchor17: Ed: weaselly use of
   "generally", needs better definition]] not permitted to push a
   response for "www.example.org".

9.  Additional HTTP Requirements/Considerations

   TODO: SNI, gzip and deflate Content-Encoding, etc..

9.1.  Frame Size Limits for HTTP

   Frames used for HTTP messages MUST NOT exceed 2^14-1 (16383) octets
   in length, not counting the 8 octet frame header.  An endpoint MUST
   treat the receipt of a larger frame as a FRAME_TOO_LARGE error (see
   Section 4.2).

9.2.  Connection Management

   HTTP/2.0 connections are persistent.  For best performance, it is
   expected clients will not close connections until it is determined
   that no further communication with a server is necessary (for
   example, when a user navigates away from a particular web page), or
   until the server closes the connection.

   Clients SHOULD NOT open more than one HTTP/2.0 connection to a given
   origin ([RFC6454]) concurrently.  A client can create additional
   connections as replacements, either to replace connections that are
   near to exhausting the available stream identifiers (Section 5.1.1),
   or to replace connections that have encountered errors
   (Section 5.4.1).

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

10.  Security Considerations

10.1.  Server Authority and Same-Origin

   This specification uses the same-origin policy ([RFC6454], Section 3)
   to determine whether an origin server is permitted to provide
   content.




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   A server that is contacted using TLS is authenticated based on the
   certificate that it offers in the TLS handshake (see [RFC2818],
   Section 3).  A server is considered authoritative for an "https"
   resource if it has been successfully authenticated for the domain
   part of the origin of the resource that it is providing.

   A server is considered authoritative for an "http" resource if the
   connection is established to a resolved IP address for the domain in
   the origin of the resource.

   A client MUST NOT use, in any way, resources provided by a server
   that is not authoritative for those resources.

10.2.  Cross-Protocol Attacks

   When using TLS, we believe that HTTP/2.0 introduces no new cross-
   protocol attacks.  TLS encrypts the contents of all transmission
   (except the handshake itself), making it difficult for attackers to
   control the data which could be used in a cross-protocol attack.
   [[anchor23: Issue: This is no longer true]]

10.3.  Cacheability of Pushed Resources

   Pushed resources are responses without an explicit request; the
   request for a pushed resource is synthesized from the request that
   triggered the push, plus resource identification information provided
   by the server.  Request header fields are necessary for HTTP cache
   control validations (such as the Vary header field) to work.  For
   this reason, caches MUST inherit request header fields from the
   associated stream for the push.  This includes the Cookie header
   field.

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

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

   Pushed resources for which an origin server is not authoritative are
   never cached or used.




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11.  Privacy Considerations

   HTTP/2.0 aims to keep connections open longer between clients and
   servers in order to reduce the latency when a user makes a request.
   The maintenance of these connections over time could be used to
   expose private information.  For example, a user using a browser
   hours after the previous user stopped using that browser may be able
   to learn about what the previous user was doing.  This is a problem
   with HTTP in its current form as well, however the short lived
   connections make it less of a risk.

12.  IANA Considerations

   This document establishes registries for frame types, error codes and
   settings.  These new registries are entered in a new "Hypertext
   Transfer Protocol (HTTP) 2.0 Parameters" section.

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

12.1.  Frame Type Registry

   This document establishes a registry for HTTP/2.0 frame types.  The
   "HTTP/2.0 Frame Type" registry operates under the "IETF Review"
   policy [RFC5226].

   Frame types are an 8-bit value.  When reviewing new frame type
   registrations, special attention is advised for any frame type-
   specific flags that are defined.  Frame flags can interact with
   existing flags and could prevent the creation of globally applicable
   flags.

   Initial values for the "HTTP/2.0 Frame Type" registry are shown in
   Table 1.

















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   +-----------+---------------+---------------------------------------+
   | Frame     | Name          | Flags                                 |
   | Type      |               |                                       |
   +-----------+---------------+---------------------------------------+
   | 0         | DATA          | END_STREAM(1)                         |
   | 1         | HEADERS       | END_STREAM(1), END_HEADERS(4),        |
   |           |               | PRIORITY(8)                           |
   | 2         | PRIORITY      | -                                     |
   | 3         | RST_STREAM    | -                                     |
   | 4         | SETTINGS      | -                                     |
   | 5         | PUSH_PROMISE  | END_PUSH_PROMISE(1)                   |
   | 6         | PING          | PONG(1)                               |
   | 7         | GOAWAY        | -                                     |
   | 9         | WINDOW_UPDATE | END_FLOW_CONTROL(1)                   |
   +-----------+---------------+---------------------------------------+

                                  Table 1

12.2.  Error Code Registry

   This document establishes a registry for HTTP/2.0 error codes.  The
   "HTTP/2.0 Error Code" registry manages a 32-bit space.  The "HTTP/2.0
   Error Code" registry operates under the "Expert Review" policy
   [RFC5226].

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

   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.







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12.3.  Settings Registry

   This document establishes a registry for HTTP/2.0 settings.  The
   "HTTP/2.0 Settings" registry manages a 24-bit space.  The "HTTP/2.0
   Settings" registry operates under the "Expert Review" policy
   [RFC5226].

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

   New registrations are advised to provide the following information:

   Setting:  The 24-bit setting value.

   Name:  A name for the setting.  Specifying a name is optional.

   Flags:  Any setting-specific flags that apply, including their value
      and semantics.

   Description:  A description of the setting.  This might include the
      range of values, any applicable units and how to act upon a value
      when it is provided.

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

   An initial set of settings registrations can be found in
   Section 6.5.2.

12.4.  HTTP2-Settings Header Field Registration

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

   Header field name:  HTTP2-Settings

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF

   Specification document(s):  RFC XXXX (this document)






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   Related information:  This header field is only used by an HTTP/2.0
      client for Upgrade-based negotiation.

13.  Acknowledgements

   This document includes substantial input from the following
   individuals:

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

   o  Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)

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

   o  Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner
      (Substantial editorial contributions)

14.  References

14.1.  Normative References

   [COMPRESSION]  Ruellan, H. and R. Peon, "HTTP Header Compression",
                  draft-ietf-httpbis-header-compression-00 (work in
                  progress), June 2013.

   [HTTP-p1]      Fielding, R. and J. Reschke, "Hypertext Transfer
                  Protocol (HTTP/1.1): Message Syntax and Routing",
                  draft-ietf-httpbis-p1-messaging-22 (work in progress),
                  February 2013.

   [HTTP-p2]      Fielding, R. and J. Reschke, "Hypertext Transfer
                  Protocol (HTTP/1.1): Semantics and Content",
                  draft-ietf-httpbis-p2-semantics-22 (work in progress),
                  February 2013.

   [HTTP-p4]      Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Conditional Requests",
                  draft-ietf-httpbis-p4-conditional-22 (work in
                  progress), February 2013.

   [HTTP-p5]      Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke,
                  Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range
                  Requests", draft-ietf-httpbis-p5-range-22 (work in
                  progress), February 2013.



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   [HTTP-p6]      Fielding, R., Ed., Nottingham, M., Ed., and J.
                  Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1):
                  Caching", draft-ietf-httpbis-p6-cache-22 (work in
                  progress), February 2013.

   [HTTP-p7]      Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Authentication",
                  draft-ietf-httpbis-p7-auth-22 (work in progress),
                  February 2013.

   [RFC0793]      Postel, J., "Transmission Control Protocol", STD 7,
                  RFC 793, September 1981.

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

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

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

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

   [RFC5246]      Dierks, T. and E. Rescorla, "The Transport Layer
                  Security (TLS) Protocol Version 1.2", RFC 5246,
                  August 2008.

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

   [TLSALPN]      Friedl, S., Popov, A., Langley, A., and E. Stephan,
                  "Transport Layer Security (TLS) Application Layer
                  Protocol Negotiation Extension",
                  draft-ietf-tls-applayerprotoneg-01 (work in progress),
                  April 2013.







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14.2.  Informative References

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

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

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

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

A.1.  Since draft-ietf-httpbis-http2-03

   Committed major restructuring atrocities.

   Added reference to first header compression draft.

   Added more formal description of frame lifecycle.

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

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

   Added PRIORITY frame.

A.2.  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.3.  Since draft-ietf-httpbis-http2-01

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

   Removed data frame compression.



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   Added PUSH_PROMISE.

   Added globally applicable flags to framing.

   Removed zlib-based header compression mechanism.

   Updated references.

   Clarified stream identifier reuse.

   Removed CREDENTIALS frame and associated mechanisms.

   Added advice against naive implementation of flow control.

   Added session header section.

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

   Altered flow control properties to include session-level limits.

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

   Changed protocol label form based on discussions.

A.4.  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.5.  Since draft-mbelshe-httpbis-spdy-00

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




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   Updated authors/editors list.

   Added status note.

Authors' Addresses

   Mike Belshe
   Twist

   EMail: mbelshe@chromium.org


   Roberto Peon
   Google, Inc

   EMail: fenix@google.com


   Martin Thomson (editor)
   Microsoft
   3210 Porter Drive
   Palo Alto  94304
   US

   EMail: martin.thomson@skype.net


   Alexey Melnikov (editor)
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   EMail: Alexey.Melnikov@isode.com
















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