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Versions: (draft-mbelshe-httpbis-spdy) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

HTTPbis Working Group                                          M. Belshe
Internet-Draft                                                     Twist
Intended status: Standards Track                                 R. Peon
Expires: November 30, 2013                                   Google, Inc
                                                         M. Thomson, Ed.
                                                               Microsoft
                                                        A. Melnikov, Ed.
                                                               Isode Ltd
                                                            May 29, 2013


                Hypertext Transfer Protocol version 2.0
                      draft-ietf-httpbis-http2-03

Abstract

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

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

Editorial Note (To be removed by RFC Editor)

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

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

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

Status of This Memo

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

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



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

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

   This Internet-Draft will expire on November 30, 2013.

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.  Starting HTTP/2.0  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  HTTP/2.0 Version Identification  . . . . . . . . . . . . .  7
     2.2.  Starting HTTP/2.0 for "http:" URIs . . . . . . . . . . . .  8
     2.3.  Starting HTTP/2.0 for "https:" URIs  . . . . . . . . . . .  8
     2.4.  Starting HTTP/2.0 with Prior Knowledge . . . . . . . . . .  9
   3.  HTTP/2.0 Framing Layer . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Connection . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Connection Header  . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.3.1.  Frame Header . . . . . . . . . . . . . . . . . . . . . 10
       3.3.2.  Frame Size . . . . . . . . . . . . . . . . . . . . . . 12
     3.4.  Streams  . . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.4.1.  Stream Creation  . . . . . . . . . . . . . . . . . . . 13
       3.4.2.  Stream priority  . . . . . . . . . . . . . . . . . . . 13
       3.4.3.  Stream half-close  . . . . . . . . . . . . . . . . . . 14
       3.4.4.  Stream close . . . . . . . . . . . . . . . . . . . . . 14
     3.5.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 15
       3.5.1.  Connection Error Handling  . . . . . . . . . . . . . . 15



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       3.5.2.  Stream Error Handling  . . . . . . . . . . . . . . . . 16
       3.5.3.  Error Codes  . . . . . . . . . . . . . . . . . . . . . 16
     3.6.  Stream Flow Control  . . . . . . . . . . . . . . . . . . . 17
       3.6.1.  Flow Control Principles  . . . . . . . . . . . . . . . 17
       3.6.2.  Appropriate Use of Flow Control  . . . . . . . . . . . 18
     3.7.  Header Blocks  . . . . . . . . . . . . . . . . . . . . . . 19
     3.8.  Frame Types  . . . . . . . . . . . . . . . . . . . . . . . 19
       3.8.1.  DATA Frames  . . . . . . . . . . . . . . . . . . . . . 20
       3.8.2.  HEADERS+PRIORITY . . . . . . . . . . . . . . . . . . . 20
       3.8.3.  RST_STREAM . . . . . . . . . . . . . . . . . . . . . . 21
       3.8.4.  SETTINGS . . . . . . . . . . . . . . . . . . . . . . . 21
       3.8.5.  PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . 25
       3.8.6.  PING . . . . . . . . . . . . . . . . . . . . . . . . . 26
       3.8.7.  GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . 26
       3.8.8.  HEADERS  . . . . . . . . . . . . . . . . . . . . . . . 28
       3.8.9.  WINDOW_UPDATE  . . . . . . . . . . . . . . . . . . . . 29
   4.  HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 32
     4.1.  Connection Management  . . . . . . . . . . . . . . . . . . 32
     4.2.  HTTP Request/Response  . . . . . . . . . . . . . . . . . . 33
       4.2.1.  HTTP Header Fields and HTTP/2.0 Headers  . . . . . . . 33
       4.2.2.  Request  . . . . . . . . . . . . . . . . . . . . . . . 33
       4.2.3.  Response . . . . . . . . . . . . . . . . . . . . . . . 34
     4.3.  Server Push Transactions . . . . . . . . . . . . . . . . . 35
       4.3.1.  Server implementation  . . . . . . . . . . . . . . . . 36
       4.3.2.  Client implementation  . . . . . . . . . . . . . . . . 37
   5.  Design Rationale and Notes . . . . . . . . . . . . . . . . . . 38
     5.1.  Separation of Framing Layer and Application Layer  . . . . 38
     5.2.  Error handling - Framing Layer . . . . . . . . . . . . . . 39
     5.3.  One Connection per Domain  . . . . . . . . . . . . . . . . 39
     5.4.  Fixed vs Variable Length Fields  . . . . . . . . . . . . . 39
     5.5.  Server Push  . . . . . . . . . . . . . . . . . . . . . . . 40
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 40
     6.1.  Server Authority and Same-Origin . . . . . . . . . . . . . 40
     6.2.  Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 40
     6.3.  Cacheability of Pushed Resources . . . . . . . . . . . . . 41
   7.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 41
     7.1.  Long Lived Connections . . . . . . . . . . . . . . . . . . 41
     7.2.  SETTINGS frame . . . . . . . . . . . . . . . . . . . . . . 41
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 42
     8.1.  Frame Type Registry  . . . . . . . . . . . . . . . . . . . 42
     8.2.  Error Code Registry  . . . . . . . . . . . . . . . . . . . 43
     8.3.  Settings Registry  . . . . . . . . . . . . . . . . . . . . 43
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 44
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 44
     10.2. Informative References . . . . . . . . . . . . . . . . . . 45
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 46



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     A.1.  Since draft-ietf-httpbis-http2-02  . . . . . . . . . . . . 46
     A.2.  Since draft-ietf-httpbis-http2-01  . . . . . . . . . . . . 46
     A.3.  Since draft-ietf-httpbis-http2-00  . . . . . . . . . . . . 47
     A.4.  Since draft-mbelshe-httpbis-spdy-00  . . . . . . . . . . . 47















































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1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is a wildly successful
   protocol.  However, the HTTP/1.1 message encapsulation ([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 2), which covers how a HTTP/2.0 connection is
   initiated; a framing layer (Section 3), which multiplexes a single
   TCP connection into independent frames of various types; and an HTTP
   layer (Section 4), 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



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   layer has not been a goal.  The framing layer is tailored to the
   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.  Starting HTTP/2.0

   HTTP/2.0 uses the same "http:" and "https:" URI schemes used by
   HTTP/1.1.  As a result, implementations processing requests for
   target resource URIs like "http://example.org/foo" or



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   "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 "https:" URIs is
   described in Section 2.3.  Discovery for "http" URIs is described
   here.

2.1.  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.

   [[anchor3: 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.







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2.2.  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.

   For example:

     GET /default.htm HTTP/1.1
     Host: server.example.com
     Connection: Upgrade
     Upgrade: HTTP/2.0

   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 can accept 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 3.8.4).  Upon receiving the 101 response, the client sends a
   connection header (Section 3.2), which includes a SETTINGS frame.

2.3.  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 3.2).





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2.4.  Starting HTTP/2.0 with Prior Knowledge

   A client can learn that a particular server supports HTTP/2.0 by
   other means.  A client MAY immediately send HTTP/2.0 frames to a
   server that is known to support HTTP/2.0.  This only affects the
   resolution of "http:" URIs, servers supporting HTTP/2.0 are required
   to support protocol negotiation in TLS [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.  HTTP/2.0 Framing Layer

3.1.  Connection

   The HTTP/2.0 connection is an Application Level protocol running on
   top of a TCP connection ([RFC0793]).  The client is the TCP
   connection initiator.

   HTTP/2.0 connections are persistent.  That is, for best performance,
   it is expected a 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.

   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 3.8.7) frame so that both endpoints can reliably determine
   whether previously sent frames have been processed and gracefully
   complete or terminate any necessary remaining tasks.

3.2.  Connection Header

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

   The client connection header is a sequence of 24 octets (in hex
   notation)

   464f4f202a20485454502f322e300d0a0d0a42410d0a0d0a




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   (the string "FOO * HTTP/2.0\r\n\r\nBA\r\n\r\n") followed by a
   SETTINGS frame (Section 3.8.4).  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 3.8.4) 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.

   Clients and servers MUST terminate the TCP connection if either peer
   does not begin with a valid connection header.  A GOAWAY frame
   (Section 3.8.7) MAY be omitted if it is clear that the peer is not
   using HTTP/2.0.

3.3.  Framing

   Once the HTTP/2.0 connection is established, clients and servers can
   begin exchanging frames.

3.3.1.  Frame Header

   HTTP/2.0 frames share a common base format consisting of an 8-byte
   header followed by 0 to 65535 bytes of data.










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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Length (16)           |   Type (8)    |   Flags (8)   |
   +-+-------------+---------------+-------------------------------+
   |R|                 Stream Identifier (31)                      |
   +-+-------------------------------------------------------------+
   |                     Frame Data (0...)                       ...
   +---------------------------------------------------------------+

                               Frame Header

   The fields of the frame header are defined as:

   Length:  The length of the frame data expressed as an unsigned 16-bit
      integer.  The 8 bytes 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 data are interpreted.
      Implementations MUST ignore unsupported and unrecognized frame
      types.

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

      The least significant bit (0x1) - the FINAL bit - is defined for
      all frame types as an indication that this frame is the last the
      endpoint will send for the identified stream.  Setting this flag
      causes the stream to enter the half-closed state (Section 3.4.3).
      Implementations MUST process the FINAL bit for all frames whose
      stream identifier field is not 0x0.  The FINAL bit MUST NOT be set
      on frames that use a stream identifier of 0.

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

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

   Stream Identifier:  A 31-bit stream identifier (see Section 3.4.1).
      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 remaining frame data is dependent



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   entirely on the frame type.

3.3.2.  Frame Size

   Implementations with limited resources might not be capable of
   processing large frame sizes.  Such implementations MAY choose to
   place additional limits on the maximum frame size.  However, all
   implementations MUST be capable of receiving and processing frames
   containing at least 8192 octets of data. [[anchor6: Ed.  Question:
   Does this minimum include the 8-byte header or just the frame data?]]

   An implementation MUST terminate a stream immediately if it is unable
   to process a frame due it's size.  This is done by sending an
   RST_STREAM frame (Section 3.8.3) containing the FRAME_TOO_LARGE error
   code.

   [[anchor7: <https://github.com/http2/http2-spec/issues/28>: Need a
   way to signal the maximum frame size; no way to RST_STREAM on non-
   stream-related frames.]]

3.4.  Streams

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

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

   o  Streams can be rejected or cancelled by either endpoint.

   o  Multiple types of frames can be sent by either endpoint within a
      single stream.

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

   o  Streams optionally carry a set of name-value header pairs that are
      expressed within the headers block of HEADERS+PRIORITY, HEADERS,
      or PUSH_PROMISE frames.

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






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3.4.1.  Stream Creation

   There is no coordination or shared action between the client and
   server required to create a stream.  Rather, new streams are
   established by sending a frame whose stream identifier field
   references a previously unused stream identifier.

   All streams are identified by an unsigned 31-bit integer.  Streams
   initiated by a client use odd numbered stream identifiers; those
   initiated by the server use even numbered stream identifiers.  A
   stream identifier of zero MUST NOT be used to establish a new stream.

   The identifier of a newly established stream MUST be numerically
   greater than all previously established streams from that endpoint
   within the HTTP/2.0 connection, unless the identifier has been
   reserved using a PUSH_PROMISE (Section 3.8.5) frame.  An endpoint
   that receives an unexpected stream identifier MUST respond with a
   connection error (Section 3.5.1) of type PROTOCOL_ERROR.

   A peer can limit the total 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 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.  Peer endpoints MUST NOT exceed this
   limit.  All concurrently active streams initiated by an endpoint,
   including streams that are half-open (Section 3.4.3) in any
   direction, count toward that endpoint's limit.

   Stream identifiers cannot be reused within a connection.  Long-lived
   connections can cause an endpoint to exhaust 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.

   Either endpoint can request the early termination of an unwanted
   stream by sending an RST_STREAM frame (Section 3.5.2) with an error
   code of either REFUSED_STREAM (if no frames have been processed) or
   CANCEL (if at least one frame has been processed).  Such termination
   might not take effect immediately as the peer might have sent
   additional frames on the stream prior to receiving the termination
   request.

3.4.2.  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



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   priority.

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

3.4.3.  Stream half-close

   When an endpoint sends a frame for a stream with the FINAL flag set,
   the stream is considered to be half-closed for that endpoint.
   Subsequent frames MUST NOT be sent by that endpoint for the half
   closed stream for the remaining duration of the HTTP/2.0 connection.
   When both endpoints have sent frames with the FINAL flag set, the
   stream is considered to be fully closed.

   If an endpoint receives additional frames for a stream that was
   previously half-closed by the sending peer, the recipient MUST
   respond with a stream error (Section 3.5.2) of type STREAM_CLOSED.

   An endpoint that has not yet half-closed a stream by sending the
   FINAL flag can continue sending frames on the stream.

   It is not necessary for an endpoint to half-close a stream for which
   it has not sent any frames.  This allows endpoints to use fully
   unidirectional streams that do not require explicit action or
   acknowledgement from the receiver.

3.4.4.  Stream close

   Streams can be terminated in the following ways:

   Normal termination:  Normal stream termination occurs when both
      client and server have half-closed the stream by sending a frame
      containing a FINAL flag (Section 3.3.1).







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   Half-close on unidirectional stream:  A stream that only has frames
      sent in one direction can be tentatively considered to be closed
      once a frame containing a FINAL flag is sent.  The active sender
      on the stream MUST be prepared to receive frames after closing the
      stream.

   Abrupt termination:  Either peer can send a RST_STREAM control frame
      at any time to terminate an active stream.  RST_STREAM contains an
      error code to indicate the reason for termination.  A RST_STREAM
      indicates that the sender will transmit no further data on the
      stream and that the receiver is advised to cease transmission on
      it.

      The sender of a RST_STREAM frame MUST allow for frames that have
      already been sent by the peer prior to the RST_STREAM being
      processed.  If in-transit frames alter connection state, these
      frames cannot be safely discarded.  See Stream Error Handling
      (Section 3.5.2) for more details.

   TCP connection teardown:  If the TCP connection is torn down while
      un-closed streams exist, then the endpoint MUST assume that the
      stream was abnormally interrupted and may be incomplete.

3.5.  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.

3.5.1.  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 MUST first send a
   GOAWAY (Section 3.8.7) 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.



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   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
   ending a connection, as long as circumstances permit it.

3.5.2.  Stream Error Handling

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

   An endpoint that detects a stream error sends a RST_STREAM
   (Section 3.8.3) frame that contains the stream identifier of the
   stream where the error occurred.  The RST_STREAM frame includes an
   error code that indicates the type of error.

   A RST_STREAM is the last frame that an endpoint can send on a stream.
   The peer that sends the RST_STREAM frame MUST be prepared to receive
   any frames that were sent or enqueued for sending by the remote peer.
   These frames can be ignored, except where they modify connection
   state (such as the state maintained for header compression
   (Section 3.7)).

   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.

3.5.3.  Error Codes

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





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   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.

   INVALID_STREAM (4):  The endpoint received a frame for an inactive
      stream.

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

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

   REFUSED_STREAM (7):  The endpoint is refusing the stream before
      processing its payload.

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

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

3.6.  Stream Flow Control

   Using streams for multiplexing introduces contention over use of the
   TCP connection, resulting in blocked streams.  A flow control scheme
   ensures that streams on the same connection do not destructively
   interfere with each other.

   HTTP/2.0 provides for flow control through use of the WINDOW_UPDATE
   (Section 3.8.9) frame type.

3.6.1.  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



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   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 octets they are prepared to receive on a
       stream.  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 declaring an infinite flow control limit.

   7.  HTTP/2.0 standardizes only the format of the window update frame
       (Section 3.8.9).  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.

3.6.2.  Appropriate Use of Flow Control

   Flow control is defined to protect endpoints (client, server or
   intermediary) that are operating under resource constraints.  For
   example, a proxy needs to share memory between many connections, and



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   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.

3.7.  Header Blocks

   The header block is found in the HEADERS, HEADERS+PRIORITY and
   PUSH_PROMISE frames.  The header block consists of a set of header
   fields, which are name-value pairs.  Headers are compressed using
   black magic.

   Compression of header fields is a work in progress, as is the format
   of this block.

   The contents of header blocks MUST be processed by the compression
   context, even if stream has been reset or the frame is discarded.  If
   header blocks cannot be processed, the receiver MUST treat the
   connection with a connection error (Section 3.5.1) of type
   COMPRESSION_ERROR.

3.8.  Frame Types

   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



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   have a shared comprehension of how the state is affected by the use
   any given frame.  Accordingly, while it is expected that new frame
   types will be introduced by extensions to this protocol, only frames
   defined by this document are permitted to alter the connection state.

3.8.1.  DATA Frames

   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 does not define any type-specific flags.

   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 3.5.1) of type
   PROTOCOL_ERROR.

3.8.2.  HEADERS+PRIORITY

   The HEADERS+PRIORITY frame (type=0x1) allows the sender to set header
   fields and stream priority at the same 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 (*)                         ...
   +---------------------------------------------------------------+

                      HEADERS+PRIORITY Frame Payload

   The HEADERS+PRIORITY frame is identical to the HEADERS frame
   (Section 3.8.8), preceded by a single reserved bit and a 31-bit
   priority; see Section 3.4.2.

   HEADERS+PRIORITY uses the same flags as the HEADERS frame, except
   that a HEADERS+PRIORITY frame with a CONTINUES bit MUST be followed
   by another HEADERS+PRIORITY frame.  See HEADERS frame (Section 3.8.8)
   for any flags.

   HEADERS+PRIORITY frames MUST be associated with a stream.  If a
   HEADERS+PRIORITY frame is received whose stream identifier field is
   0x0, the recipient MUST respond with a connection error
   (Section 3.5.1) of type PROTOCOL_ERROR.

   The HEADERS+PRIORITY frame modifies the connection state as defined



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   in Section 3.7.

3.8.3.  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.  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 3.5.3).  The error code indicates
   why the stream is being terminated.

   No type-flags are defined.

   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 3.5.1) of type
   PROTOCOL_ERROR.

3.8.4.  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.




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   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 then 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 value provided setting value applies.

   The SETTINGS frame defines the following flag:

   CLEAR_PERSISTED (0x2):  Bit 2 being set indicates a request to clear
      any previously persisted settings before processing the settings.
      Clients MUST NOT set this flag.

   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 3.5.1) of type PROTOCOL_ERROR.

3.8.4.1.  Setting Format

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

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




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                              Setting Format

   Two flags are defined for the 8-bit flags field:

   PERSIST_VALUE (0x1):  Bit 1 (the least significant bit) being set
      indicates a request from the server to the client to persist this
      setting.  A client MUST NOT set this flag.

   PERSISTED (0x2):  Bit 2 being set indicates that this setting is a
      persisted setting being returned by the client to the server.
      This also indicates that this setting is not a client setting, but
      a value previously set by the server.  A server MUST NOT set this
      flag.

3.8.4.2.  Setting Persistence

   [[anchor12: Note that persistence of settings is under discussion in
   the WG and might be removed in a future version of this document.]]

   A server endpoint can request that configuration parameters sent to a
   client in a SETTINGS frame are to be persisted by the client across
   HTTP/2.0 connections and returned to the server in any new SETTINGS
   frame the client sends to the server in the current connection or any
   future connections.

   Persistence is requested on a per-setting basis by setting the
   PERSIST_VALUE flag (0x1).

   Client endpoints are not permitted to make such requests.  Servers
   MUST ignore any attempt by clients to request that a server persist
   configuration parameters.

   Persistence of configuration parameters is done on a per-origin basis
   (see [RFC6454]).  That is, when a client establishes a connection
   with a server, and the server requests that the client maintain
   persistent settings, the client SHOULD return the persisted settings
   on all future connections to the same origin, IP address and TCP
   port.

   Whenever the client sends a SETTINGS frame in the current connection,
   or establishes a new connection with the same origin, persisted
   configuration parameters are sent with the PERSISTED flag (0x2) set
   for each persisted parameter.

   Persisted settings accumulate until the server requests that all
   previously persisted settings are to be cleared by setting the
   CLEAR_PERSISTED (0x2) flag on the SETTINGS frame.




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   For example, if the server sends IDs 1, 2, and 3 with the
   FLAG_SETTINGS_PERSIST_VALUE in a first SETTINGS frame, and then sends
   IDs 4 and 5 with the FLAG_SETTINGS_PERSIST_VALUE in a subsequent
   SETTINGS frame, the client will return values for all 5 settings (1,
   2, 3, 4, and 5 in this example) to the server.

3.8.4.3.  Defined Settings

   The following settings are defined:

   SETTINGS_UPLOAD_BANDWIDTH (1):  indicates the sender's estimated
      upload bandwidth for this connection.  The value is an the
      integral number of kilobytes per second that the sender predicts
      as an expected maximum upload channel capacity.

   SETTINGS_DOWNLOAD_BANDWIDTH (2):  indicates the sender's estimated
      download bandwidth for this connection.  The value is an integral
      number of kilobytes per second that the sender predicts as an
      expected maximum download channel capacity.

   SETTINGS_ROUND_TRIP_TIME (3):  indicates the sender's estimated
      round-trip-time for this connection.  The round trip time is
      defined as the minimum amount of time to send a control frame from
      this client to the remote and receive a response.  The value is
      represented in milliseconds.

   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_CURRENT_CWND (5):  indicates the sender's current TCP CWND
      value.

   SETTINGS_DOWNLOAD_RETRANS_RATE (6):  indicates the sender's
      retransmission rate (bytes retransmitted / total bytes
      transmitted).

   SETTINGS_INITIAL_WINDOW_SIZE (7):  indicates the sender's initial
      stream window size (in bytes) for new streams.

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




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      This setting applies to all streams, including existing streams.

      These bits cannot be cleared once set, see Section 3.8.9.4.

3.8.5.  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 4.3
   contains a thorough description of the use of PUSH_PROMISE frames.

    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 (*)                         ...
   +---------------------------------------------------------------+

                        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 3.4.1)).

   PUSH_PROMISE frames MUST be associated with an existing stream.  If
   the stream identifier field specifies the value 0x0, a recipient MUST
   respond with a connection error (Section 3.5.1) of type
   PROTOCOL_ERROR.

   The state of promised streams is bound to the state of the original
   associated stream on which the PUSH_PROMISE frame were sent.  If the
   originating stream state changes to fully closed, all associated
   promised streams fully close as well. [[anchor13: Ed.  Note: We need
   clarification on this point.  How synchronized are the lifecycles of
   streams and associated promised streams?]]

   PUSH_PROMISE uses the same flags as the HEADERS frame, except that a
   PUSH_PROMISE frame with a CONTINUES bit MUST be followed by another
   PUSH_PROMISE frame.  See HEADERS frame (Section 3.8.8) for any flags.

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




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

   The PUSH_PROMISE frame modifies the connection state as defined in
   Section 3.7.

3.8.6.  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.

   PING frames consist of an arbitrary, variable-length sequence of
   octets.  Receivers of a PING send a response PING frame with the PONG
   flag set and precisely the same sequence of octets back to the sender
   as soon as possible.

   Processing of PING frames SHOULD be performed with the highest
   priority if there are additional frames waiting to be processed.

   The PING frame defines one type-specific flag:

   PONG (0x2):  Bit 2 being set indicates that this PING frame is a PING
      response.  An endpoint MUST set this flag in PING responses.  An
      endpoint MUST NOT respond to PING frames containing this flag.

   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 3.5.1) of type PROTOCOL_ERROR.

3.8.7.  GOAWAY

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

   There is an inherent race condition between an endpoint starting new
   streams and the remote sending a GOAWAY frame.  To deal with this
   case, the GOAWAY contains the stream identifier of the last stream



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   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).

   After sending a GOAWAY frame, the sender can ignore frames for new
   streams.

   [[anchor14: Issue: connection state that is established by those
   "ignored" frames cannot be ignored without the state in the two peers
   becoming unsynchronized.]]

    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)                          |
   +---------------------------------------------------------------+

                           GOAWAY Payload Format

   The GOAWAY frame does not define any type-specific flags.

   The GOAWAY frame applies to the connection, not a specific stream.
   The stream identifier MUST be zero.

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

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




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   On streams with lower or equal numbered identifiers that do not close
   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 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 3.5.3)
   that contains the reason for closing the connection.

3.8.8.  HEADERS

   The HEADERS frame (type=0x8) provides header fields for a stream.
   Any number of HEADERS frames can may be sent on an existing stream at
   any time.

   Additional type-specific flags for the HEADERS frame are:

   CONTINUES (0x2):  The CONTINUES bit indicates that this frame does
      not contain the entire payload necessary to provide a complete set
      of headers.

      The payload for a complete set of headers is provided by a
      sequence of HEADERS frames, terminated by a HEADERS frame without
      the CONTINUES bit.  Once the sequence terminates, the payload of
      all HEADERS frames are concatenated and interpreted as a single
      block.

      A HEADERS frame that includes a CONTINUES bit 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 3.5.1) of type
      PROTOCOL_ERROR.

   The payload of a HEADERS frame contains a Headers Block
   (Section 3.7).

   The HEADERS frame is associated with an existing stream.  If a
   HEADERS frame is received with a stream identifier of 0x0, the
   recipient MUST respond with a stream error (Section 3.5.2) of type



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   PROTOCOL_ERROR.

   The HEADERS frame changes the connection state as defined in
   Section 3.7.

3.8.9.  WINDOW_UPDATE

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

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

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

   Flow control only applies to frames that are identified as being
   subject to flow control.  Of the frame types defined in this
   document, this includes only DATA frame.  Frames that are exempt from
   flow control MUST be accepted and processed, unless the receiver is
   unable to assign resources to handling the frame.  A receiver MAY
   respond with a stream error (Section 3.5.2) or connection error
   (Section 3.5.1) of type FLOW_CONTROL_ERROR if it is unable accept a
   frame.

   The following additional flags are defined for the WINDOW_UPDATE
   frame:

   END_FLOW_CONTROL (0x2):  Bit 2 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.

   The payload of a WINDOW_UPDATE frame is a 32-bit value indicating the
   additional number of bytes that the sender can transmit in addition
   to the existing flow control window.  The legal range for this field
   is 1 to 2^31 - 1 (0x7fffffff) bytes; the most significant bit of this
   value is reserved.







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3.8.9.1.  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 FINAL flag set (for
   example, an empty data frame) MAY be sent if there is no available
   space in either flow control window.

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

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

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

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

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

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

3.8.9.2.  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 65536 bytes.  Both endpoints can
   adjust the initial window size for new streams by including a value



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   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, a client 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 flow control windows
   that it maintains by the difference between the new value and the old
   value.

   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 server sets the initial window size to be 16KB,
   and the client sends 64KB immediately on connection establishment,
   the client will recalculate the available flow control window to be
   -48KB on receipt of the SETTINGS frame.  The client retains a
   negative flow control window until WINDOW_UPDATE frames restore the
   window to being positive, after which the client can resume sending.

3.8.9.3.  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



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   sent in the SETTINGS.

3.8.9.4.  Ending Flow Control

   After a receiver reads in a frame that marks the end of a stream (for
   example, a data stream with a FINAL 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 or the connection using
   the SETTINGS_FLOW_CONTROL_OPTIONS setting.  An implementation that
   does not wish to perform flow control can use this in the 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.

4.  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.

4.1.  Connection Management

   Clients SHOULD NOT open more than one HTTP/2.0 connection to a given
   origin ([RFC6454]) concurrently.

   Note that it is possible for one HTTP/2.0 connection to be finishing
   (e.g. a GOAWAY frame has been sent, but not all streams have
   finished), while another HTTP/2.0 connection is starting.







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4.2.  HTTP Request/Response

4.2.1.  HTTP Header Fields and HTTP/2.0 Headers

   At the application level, HTTP uses name-value pairs in its header
   fields.  Because HTTP/2.0 merges the existing HTTP header fields with
   HTTP/2.0 headers, there is a possibility that some HTTP applications
   already use a particular header field name.  To avoid any conflicts,
   all header fields introduced for layering HTTP over HTTP/2.0 are
   prefixed with ":". ":" is not a valid sequence in HTTP/1.* header
   field naming, preventing any possible conflict.

4.2.2.  Request

   The client initiates a request by sending a HEADERS+PRIORITY frame.
   Requests that do not contain a body MUST set the FINAL flag,
   indicating that the client intends to send no further data on this
   stream, unless the server intends to push resources (see
   Section 4.3).  HEADERS+PRIORITY frame does not contain the FINAL flag
   for requests that contain a body.  The body of a request follows as a
   series of DATA frames.  The last DATA frame sets the FINAL flag to
   indicate the end of the body.

   The header fields included in the HEADERS+PRIORITY frame contain all
   of the HTTP header fields associated with an HTTP request.  The
   definitions of these headers 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.

   o  The host and optional port portions of the request URI (see
      [RFC3986], Section 3.2), is specified using the new :host header
      field. [[anchor21: 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.



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   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.

   If a client sends a HEADERS+PRIORITY frame that omits a mandatory
   header, the server MUST reply with a HTTP 400 Bad Request reply.
   [[anchor22: Ed: why PROTOCOL_ERROR on missing ":status" in the
   response, but HTTP 400 here?]]

   If a server receives a request where the sum of the data frame
   payload lengths does not equal the size of the Content-Length header
   field, the server MUST return a 400 (Bad Request) error.

   Although POSTs are inherently chunked, POST requests SHOULD also be
   accompanied by a Content-Length header field.  First, it informs the
   server of how much data to expect, which the server can use to track
   overall progress and provide appropriate user feedback.  More
   importantly, some HTTP server implementations fail to correctly
   process requests that omit the Content-Length header field.  Many
   existing clients send a Content-Length header field, and some server
   implementations have come to depend upon its presence.

   A client provides priority in requests as a hint to the server.  A
   server SHOULD attempt to provide responses to higher priority
   requests before lower priority requests.  A server could send lower
   priority responses during periods that higher priority responses are
   unavailable to ensure better utilization of a connection.

   If the server receives a data frame prior to a HEADERS+PRIORITY frame
   the server MUST treat this as a stream error (Section 3.5.2) of type
   PROTOCOL_ERROR.

4.2.3.  Response

   The server responds to a client request using the same stream
   identifier that was used by the request.  An HTTP response begins
   with a HEADERS frame.  An HTTP response body consists of a series of
   DATA frames.  The last data frame contains a FINAL flag to indicate
   the end of the response.  A response that contains no body (such as a
   204 or 304 response) consists only of a HEADERS frame that contains
   the FINAL flag to indicate no further data will be sent on the
   stream.






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      The response status line is unfolded into name-value pairs like
      other HTTP header fields and must be present:

      ":status":  The HTTP response status code (e.g. "200" or "200 OK")

      All header field names starting with ":" (whether defined in this
      document or future extensions to this document) MUST appear before
      any other header fields.

      All header field names MUST be all lowercase.

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

      Responses MAY be accompanied by a Content-Length header field for
      advisory purposes.  This allows clients to learn the full size of
      an entity prior to receiving all the data frames.  This can help
      in, for example, reporting progress.

      If a client receives a response where the sum of the data frame
      payload length does not equal the size of the Content-Length
      header field, the client MUST ignore the content length header
      field. [[anchor23: Ed: See
      <https://github.com/http2/http2-spec/issues/46>.]]

   If a client receives a response with an absent or duplicated status
   header, the client MUST treat this as a stream error (Section 3.5.2)
   of type PROTOCOL_ERROR.

   If the client receives a data frame prior to a HEADERS frame the
   client MUST treat this as a stream error (Section 3.5.2) of type
   PROTOCOL_ERROR.

   Clients MUST support gzip compression.  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.

4.3.  Server Push Transactions

   HTTP/2.0 enables a server to send multiple replies to a client for a
   single request.  The rationale for this feature is that sometimes a
   server knows that it will need to send multiple resources in response
   to a single request.  Without server push features, the client must
   first download the primary resource, then discover the secondary
   resource(s), and request them.

   Server push is an optional feature.  The



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   SETTINGS_MAX_CONCURRENT_STREAMS setting from the client limits the
   number of resources that can be concurrently pushed by a server.
   Server push can be disabled by clients that do not wish to receive
   pushed resources by advertising a SETTINGS_MAX_CONCURRENT_STREAMS
   SETTING (Section 3.8.4) of zero.  This prevents servers from creating
   the streams necessary to push resources.

   Clients receiving a pushed response MUST validate that the server is
   authorized to push the resource using the same-origin policy
   ([RFC6454], Section 3).  For example, a HTTP/2.0 connection to
   "example.com" is generally [[anchor24: Ed: weaselly use of
   "generally", needs better definition]] not permitted to push a
   response for "www.example.org".

   A client that accepts pushed resources caches those resources as
   though they were responses to GET requests.

   Pushing of resources avoids the round-trip delay, but also creates a
   potential race where a server can be pushing content which a client
   is in the process of requesting.  The PUSH_PROMISE frame reduces the
   chances of this condition occurring, while retaining the performance
   benefit.

   Pushed responses are associated with a request at the HTTP/2.0
   framing layer.  The PUSH_PROMISE is sent on the stream for the
   associated request, which allows a receiver to correlate the pushed
   resource with a request.  The pushed stream inherits all of the
   request header fields from the associated stream with the exception
   of resource identification header fields (":host", ":scheme", and
   ":path"), which are provided as part of the PUSH_PROMISE frame.

   Pushed resources always have an associated ":method" of "GET".  A
   cache MUST store these inherited and implied request header fields
   with the cached resource.

4.3.1.  Server implementation

   A server pushes resources in association with a request from the
   client.  Prior to closing the response stream, the server sends a
   PUSH_PROMISE for each resource that it intends to push.  The
   PUSH_PROMISE includes header fields that allow the client to identify
   the resource (":scheme", ":host", and ":path").

   A server can push multiple resources in response to a request, but
   all pushed resources MUST be promised on the response stream for the
   associated request.  A server cannot send a PUSH_PROMISE on a new
   stream or a half-closed stream.




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   The server SHOULD include any header fields in a PUSH_PROMISE that
   would allow a cache to determine if the resource is already cached
   (see [HTTP-p6], Section 4).

   After sending a PUSH_PROMISE, the server commences transmission of a
   pushed resource.  A pushed resource uses a server-initiated stream.
   The server sends frames on this stream in the same order as an HTTP
   response (Section 4.2.3): a HEADERS frame followed by DATA frames.

   Many uses of server push are to send content that a client is likely
   to discover a need for based on the content of a response
   representation.  To minimize the chances that a client will make a
   request for resources that are being pushed - causing duplicate
   copies of a resource to be sent by the server - a PUSH_PROMISE frame
   SHOULD be sent prior to any content in the response representation
   that might allow a client to discover the pushed resource and request
   it.

   The server MUST only push resources that could have been returned
   from a GET request.

   Note: A server does not need to have all response header fields
   available at the time it issues a PUSH_PROMISE frame.  All remaining
   header fields are included in the HEADERS frame.  The HEADERS frame
   MUST NOT duplicate header fields from the PUSH_PROMISE frames.

4.3.2.  Client implementation

   When fetching a resource the client has 3 possibilities:

   1.  the resource is not being pushed

   2.  the resource is being pushed, but the data has not yet arrived

   3.  the resource is being pushed, and the data has started to arrive

   A client SHOULD NOT issue GET requests for a resource that has been
   promised.  A client is instead advised to wait for the pushed
   resource to arrive.

   When a client receives a PUSH_PROMISE frame from the server without a
   the ":host", ":scheme", and ":path" header fields, it MUST treat this
   as a stream error (Section 3.5.2) of type PROTOCOL_ERROR.

   To cancel individual server push streams, the client can issue a
   stream error (Section 3.5.2) of type CANCEL.  After receiving a
   PUSH_PROMISE frame, the client is able to cancel the pushed resource
   before receiving any frames on the promised stream.  The server



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   ceases transmission of the pushed resource; if the server has not
   commenced transmission, it does not start.

   To cancel all server push streams related to a request, the client
   may issue a stream error (Section 3.5.2) of type CANCEL on the
   associated-stream-id.  By cancelling that stream, the server MUST
   immediately stop sending frames for any streams with
   in-association-to for the original stream. [[anchor27: Ed: Triggering
   side-effects on stream reset is going to be problematic for the
   framing layer.  Purely from a design perspective, it's a layering
   violation.  More practically speaking, the base request stream might
   already be removed.  Special handling logic would be required.]]

   A client can choose to time out pushed streams if the server does not
   provide the resource in a timely fashion.  A stream error
   (Section 3.5.2) of type CANCEL can be used to stop a timed out push.

   If the server sends a HEADERS frame containing header fields that
   duplicate values on a previous HEADERS or PUSH_PROMISE frames on the
   same stream, the client MUST treat this as a stream error
   (Section 3.5.2) of type PROTOCOL_ERROR.

   If the server sends a HEADERS frame after sending a data frame for
   the same stream, the client MAY ignore the HEADERS frame.  Ignoring
   the HEADERS frame after a data frame prevents handling of HTTP's
   trailing header fields (Section 4.1.1 of [HTTP-p1]).

5.  Design Rationale and Notes

   Authors' notes: The notes in this section have no bearing on the
   HTTP/2.0 protocol as specified within this document, and none of
   these notes should be considered authoritative about how the protocol
   works.  However, these notes may prove useful in future debates about
   how to resolve protocol ambiguities or how to evolve the protocol
   going forward.  They may be removed before the final draft.

5.1.  Separation of Framing Layer and Application Layer

   Readers may note that this specification sometimes blends the framing
   layer (Section 3) with requirements of a specific application - HTTP
   (Section 4).  This is reflected in the request/response nature of the
   streams and the definition of the HEADERS which are very similar to
   HTTP, and other areas as well.

   This blending is intentional - the primary goal of this protocol is
   to create a low-latency protocol for use with HTTP.  Isolating the
   two layers is convenient for description of the protocol and how it
   relates to existing HTTP implementations.  However, the ability to



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   reuse the HTTP/2.0 framing layer is a non goal.

5.2.  Error handling - Framing Layer

   Error handling at the HTTP/2.0 layer splits errors into two groups:
   Those that affect an individual HTTP/2.0 stream, and those that do
   not.

   When an error is confined to a single stream, but general framing is
   intact, HTTP/2.0 attempts to use the RST_STREAM as a mechanism to
   invalidate the stream but move forward without aborting the
   connection altogether.

   For errors occurring outside of a single stream context, HTTP/2.0
   assumes the entire connection is hosed.  In this case, the endpoint
   detecting the error should initiate a connection close.

5.3.  One Connection per Domain

   HTTP/2.0 attempts to use fewer connections than other protocols have
   traditionally used.  The rationale for this behavior is because it is
   very difficult to provide a consistent level of service (e.g.  TCP
   slow-start), prioritization, or optimal compression when the client
   is connecting to the server through multiple channels.

   Through lab measurements, we have seen consistent latency benefits by
   using fewer connections from the client.  The overall number of
   packets sent by HTTP/2.0 can be as much as 40% less than HTTP.
   Handling large numbers of concurrent connections on the server also
   does become a scalability problem, and HTTP/2.0 reduces this load.

   The use of multiple connections is not without benefit, however.
   Because HTTP/2.0 multiplexes multiple, independent streams onto a
   single stream, it creates a potential for head-of-line blocking
   problems at the transport level.  In tests so far, the negative
   effects of head-of-line blocking (especially in the presence of
   packet loss) is outweighed by the benefits of compression and
   prioritization.

5.4.  Fixed vs Variable Length Fields

   HTTP/2.0 favors use of fixed length 32bit fields in cases where
   smaller, variable length encodings could have been used.  To some,
   this seems like a tragic waste of bandwidth.  HTTP/2.0 chooses the
   simple encoding for speed and simplicity.

   The goal of HTTP/2.0 is to reduce latency on the network.  The
   overhead of HTTP/2.0 frames is generally quite low.  Each data frame



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   is only an 8 byte overhead for a 1452 byte payload (~0.6%).  At the
   time of this writing, bandwidth is already plentiful, and there is a
   strong trend indicating that bandwidth will continue to increase.
   With an average worldwide bandwidth of 1Mbps, and assuming that a
   variable length encoding could reduce the overhead by 50%, the
   latency saved by using a variable length encoding would be less than
   100 nanoseconds.  More interesting are the effects when the larger
   encodings force a packet boundary, in which case a round-trip could
   be induced.  However, by addressing other aspects of HTTP/2.0 and TCP
   interactions, we believe this is completely mitigated.

5.5.  Server Push

   A subtle but important point is that server push streams must be
   declared before the associated stream is closed.  The reason for this
   is so that proxies have a lifetime for which they can discard
   information about previous streams.  If a pushed stream could
   associate itself with an already-closed stream, then endpoints would
   not have a specific lifecycle for when they could disavow knowledge
   of the streams which went before.

6.  Security Considerations

6.1.  Server Authority and Same-Origin

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

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

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

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

6.2.  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.



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   [[anchor37: Issue: This is no longer true]]

6.3.  Cacheability of Pushed Resources

   Pushed resources are synthesized 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.

7.  Privacy Considerations

7.1.  Long Lived Connections

   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.

7.2.  SETTINGS frame

   The HTTP/2.0 SETTINGS frame allows servers to store out-of-band
   transmitted information about the communication between client and
   server on the client.  Although this is intended only to be used to
   reduce latency, renegade servers could use it as a mechanism to store



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   identifying information about the client in future requests.

   Clients implementing privacy modes can disable client-persisted
   SETTINGS storage.

   Clients MUST clear persisted SETTINGS information when clearing the
   cookies.

8.  IANA Considerations

   This document establishes registries for frame types, error codes and
   settings.

8.1.  Frame Type Registry

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

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

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

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

                                  Table 1








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8.2.  Error Code Registry

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

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

   New registrations are advised to provide the following information:

   Error Code:  The 32-bit error code value.

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

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

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

   An initial set of error code registrations can be found in
   Section 3.5.3.

8.3.  Settings Registry

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

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

   New registrations are advised to provide the following information:

   Setting:  The 24-bit setting value.

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






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

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

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

   An initial set of settings registrations can be found in
   Section 3.8.4.3.

9.  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 (Editorial)

10.  References

10.1.  Normative References

   [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.



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   [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.

   [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.

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

10.2.  Informative References

   [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.



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              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-http2-02

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

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

   Removed data frame compression.

   Added PUSH_PROMISE.

   Added globally applicable flags to framing.

   Removed zlib-based header compression mechanism.

   Updated references.

   Clarified stream identifier reuse.

   Removed CREDENTIALS frame and associated mechanisms.

   Added advice against naive implementation of flow control.

   Added session header section.

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

   Altered flow control properties to include session-level limits.

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



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   Changed protocol label form based on discussions.

A.3.  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 3.6.1) based on <http://
   tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>.

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

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

   Updated authors/editors list.

   Added status note.

Authors' Addresses

   Mike Belshe
   Twist

   EMail: mbelshe@chromium.org


   Roberto Peon
   Google, Inc

   EMail: fenix@google.com











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   Martin Thomson (editor)
   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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