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Versions: (draft-mbelshe-httpbis-spdy) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 7540

HTTPbis Working Group                                          M. Belshe
Internet-Draft                                                     Twist
Intended status: Standards Track                                 R. Peon
Expires: October 5, 2013                                     Google, Inc
                                                         M. Thomson, Ed.
                                                               Microsoft
                                                        A. Melnikov, Ed.
                                                               Isode Ltd
                                                           April 3, 2013


                Hypertext Transfer Protocol version 2.0
                      draft-ietf-httpbis-http2-02

Abstract

   This specification describes an optimised expression of the syntax of
   the Hypertext Transfer Protocol (HTTP).  The HTTP/2.0 encapsulation
   enables more efficient transfer of representations by providing
   compressed header fields, simultaneous requests, and also introduces
   unsolicited push of representations from server to client.

   This document is an alternative to, but does not obsolete the HTTP
   message format.  HTTP 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.




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

   This Internet-Draft will expire on October 5, 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  . . . . . . . . . . . . . . . . . . . . . .  7
     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.  Session  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Session Header . . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.3.1.  Frame Header . . . . . . . . . . . . . . . . . . . . . 10
       3.3.2.  Frame Processing . . . . . . . . . . . . . . . . . . . 11
     3.4.  Streams  . . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.4.1.  Stream Creation  . . . . . . . . . . . . . . . . . . . 12
       3.4.2.  Stream priority  . . . . . . . . . . . . . . . . . . . 12
       3.4.3.  Stream headers . . . . . . . . . . . . . . . . . . . . 13
       3.4.4.  Stream data exchange . . . . . . . . . . . . . . . . . 13
       3.4.5.  Stream half-close  . . . . . . . . . . . . . . . . . . 13
       3.4.6.  Stream close . . . . . . . . . . . . . . . . . . . . . 13
     3.5.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 14
       3.5.1.  Session Error Handling . . . . . . . . . . . . . . . . 14
       3.5.2.  Stream Error Handling  . . . . . . . . . . . . . . . . 15



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       3.5.3.  Error Codes  . . . . . . . . . . . . . . . . . . . . . 15
     3.6.  Stream Flow Control  . . . . . . . . . . . . . . . . . . . 16
       3.6.1.  Flow Control Principles  . . . . . . . . . . . . . . . 16
       3.6.2.  Appropriate Use of Flow Control  . . . . . . . . . . . 17
     3.7.  Frame Types  . . . . . . . . . . . . . . . . . . . . . . . 18
       3.7.1.  DATA Frames  . . . . . . . . . . . . . . . . . . . . . 18
       3.7.2.  HEADERS+PRIORITY . . . . . . . . . . . . . . . . . . . 18
       3.7.3.  RST_STREAM . . . . . . . . . . . . . . . . . . . . . . 18
       3.7.4.  SETTINGS . . . . . . . . . . . . . . . . . . . . . . . 19
       3.7.5.  PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . 22
       3.7.6.  PING . . . . . . . . . . . . . . . . . . . . . . . . . 23
       3.7.7.  GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . 23
       3.7.8.  HEADERS  . . . . . . . . . . . . . . . . . . . . . . . 24
       3.7.9.  WINDOW_UPDATE  . . . . . . . . . . . . . . . . . . . . 25
       3.7.10. Header Block . . . . . . . . . . . . . . . . . . . . . 28
   4.  HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 28
     4.1.  Connection Management  . . . . . . . . . . . . . . . . . . 28
       4.1.1.  Use of GOAWAY  . . . . . . . . . . . . . . . . . . . . 29
     4.2.  HTTP Request/Response  . . . . . . . . . . . . . . . . . . 29
       4.2.1.  HTTP Header Fields and HTTP/2.0 Headers  . . . . . . . 29
       4.2.2.  Request  . . . . . . . . . . . . . . . . . . . . . . . 29
       4.2.3.  Response . . . . . . . . . . . . . . . . . . . . . . . 31
     4.3.  Server Push Transactions . . . . . . . . . . . . . . . . . 32
       4.3.1.  Server implementation  . . . . . . . . . . . . . . . . 33
       4.3.2.  Client implementation  . . . . . . . . . . . . . . . . 34
   5.  Design Rationale and Notes . . . . . . . . . . . . . . . . . . 35
     5.1.  Separation of Framing Layer and Application Layer  . . . . 35
     5.2.  Error handling - Framing Layer . . . . . . . . . . . . . . 35
     5.3.  One Connection Per Domain  . . . . . . . . . . . . . . . . 36
     5.4.  Fixed vs Variable Length Fields  . . . . . . . . . . . . . 36
     5.5.  Server Push  . . . . . . . . . . . . . . . . . . . . . . . 36
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 37
     6.1.  Use of Same-origin constraints . . . . . . . . . . . . . . 37
     6.2.  Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 37
     6.3.  Cacheability of Pushed Resources . . . . . . . . . . . . . 37
   7.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 37
     7.1.  Long Lived Connections . . . . . . . . . . . . . . . . . . 38
     7.2.  SETTINGS frame . . . . . . . . . . . . . . . . . . . . . . 38
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
     8.1.  Frame Type Registry  . . . . . . . . . . . . . . . . . . . 38
     8.2.  Error Code Registry  . . . . . . . . . . . . . . . . . . . 39
     8.3.  Settings Registry  . . . . . . . . . . . . . . . . . . . . 39
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     10.2. Informative References . . . . . . . . . . . . . . . . . . 42
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 42



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     A.1.  Since draft-ietf-httpbis-http2-01  . . . . . . . . . . . . 42
     A.2.  Since draft-ietf-httpbis-http2-00  . . . . . . . . . . . . 43
     A.3.  Since draft-mbelshe-httpbis-spdy-00  . . . . . . . . . . . 43
















































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1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is a wildly successful
   protocol.  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.

   The HTTP/1.1 encapsulation ensures that only one request can be
   delivered at a time on a given connection.  HTTP/1.1 pipelining,
   which is not widely deployed, only partially addresses these
   concerns.  Clients that need to make multiple requests therefore use
   commonly multiple connections to a server or servers in order to
   reduce the overall latency of those requests. [[anchor1: Need to tune
   the anti-pipelining comments here.]]

   Furthermore, HTTP/1.1 header fields are represented in an inefficient
   fashion, which, in addition to generating more or larger network
   packets, can cause the small initial TCP window to fill more quickly
   than is ideal.  This results in excessive latency where multiple
   requests are made on a new TCP connection.

   This document defines an optimized mapping of the HTTP semantics to a
   TCP connection.  This optimization reduces the latency costs of HTTP
   by allowing parallel requests on the same connection and by using an
   efficient coding for HTTP header fields.  Prioritization of requests
   lets more important requests complete faster, further improving
   application performance.

   HTTP/2.0 applications have an improved impact on network congestion
   due to the use of fewer TCP connections to achieve the same effect.
   Fewer TCP connections compete more fairly with other flows.  Long-
   lived connections are also more able to take better advantage of the
   available network capacity, rather than operating in the slow start
   phase of TCP.

   The HTTP/2.0 encapsulation also enables more efficient processing of
   messages by providing efficient message framing.  Processing of
   header fields in HTTP/2.0 messages is more efficient (for entities
   that process many messages).

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 is started; a
   framing layer (Section 3), which multiplexes a TCP connection into
   independent, length-prefixed frames; and an HTTP layer (Section 4),
   which specifies the mechanism for overlaying HTTP request/response



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   pairs on top of the framing layer.  While some of the framing layer
   concepts are isolated from the HTTP layer, building a generic framing
   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/2.0 session.

   connection:  A transport-level connection between two endpoints.

   endpoint:  Either the client or server of a connection.

   frame:  The smallest unit of communication, each containing a frame
      header.

   message:  A complete sequence of frames.

   receiver:  An endpoint that is receiving frames.

   sender:  An endpoint that is transmitting frames.

   server:  The endpoint which did not initiate the HTTP/2.0 session.

   session:  A synonym for a connection.

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

   stream:  A bi-directional flow of bytes across a virtual channel
      within a HTTP/2.0 session.

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








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2.  Starting HTTP/2.0

   Just as HTTP/1.1 does, HTTP/2.0 uses the "http:" and "https:" URI
   schemes.  An HTTP/2.0-capable client is therefore required to
   discover whether a server (or intermediary) supports HTTP/2.0.

   Different discovery mechanisms are defined for "http:" and "https:"
   URIs.  Discovery for "http:" URIs is described in Section 2.2;
   discovery for "https:" URIs is described in Section 2.3.

2.1.  HTTP/2.0 Version Identification

   HTTP/2.0 is identified using the string "HTTP/2.0".  This
   identification is used in the HTTP/1.1 Upgrade header field, in the
   TLS-NPN [TLSNPN] [[anchor4: TBD]] 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.

   [[anchor5: Editor's Note: please remove the following text prior to
   the publication of a final version of this document.]]

   Only implementations of the final, published RFC can identify
   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 with 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 session ...

   Once the server returns the 101 response, both the client and the
   server send a session header (Section 3.2).

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 TLS-NPN
   [TLSNPN] extension. [[anchor6: TBD, maybe ALPN]]

   Once TLS negotiation is complete, both the client and the server send
   a session header (Section 3.2).






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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 [TLSNPN].

   Prior support for HTTP/2.0 is not a strong signal that a given server
   will support HTTP/2.0 for future sessions.  It is possible for server
   configurations to change or for configurations to differ between
   instances in clustered server.  Different "transparent"
   intermediaries - intermediaries that are not explicitly selected by
   either client or server - are another source of variability.

3.  HTTP/2.0 Framing Layer

3.1.  Session

   The HTTP/2.0 session runs atop TCP ([RFC0793]).  The client is the
   TCP connection initiator.

   HTTP/2.0 connections are persistent connections.  For best
   performance, it is expected that clients will not close open
   connections until the user navigates away from all web pages
   referencing a connection, or until the server closes the connection.
   Servers are encouraged to leave connections open for as long as
   possible, but can terminate idle connections if necessary.  When
   either endpoint closes the transport-level connection, it MUST first
   send a GOAWAY (Section 3.7.7) frame so that the endpoints can
   reliably determine if requests finished before the close.

3.2.  Session Header

   After opening a TCP connection and performing either an HTTP/1.1
   Upgrade or TLS handshake, the client sends the client session header.
   The server replies with a server session header.

   The session header provides a final confirmation that both peers
   agree to use the HTTP/2.0 protocol.  The SETTINGS frame ensures that
   client or server configuration is known as quickly as possible.

   The client session header is the 25 byte sequence
   0x464f4f202a20485454502f322e300d0a0d0a4241520d0a0d0a (the string "FOO
   * HTTP/2.0\r\n\r\nBAR\r\n\r\n") followed by a SETTINGS frame
   (Section 3.7.4).  The client sends the client session header
   immediately after receiving an HTTP/1.1 Upgrade, or after receiving a
   TLS Finished message from the server.



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      The client session 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.  This doesn't address the concerns
      raised in [TALKING].

   The server session header is a SETTINGS frame (Section 3.7.4).  The
   server sends the server session header immediately after receiving
   and validating the client session header.

   The client sends requests immediately after sending the session
   header, without waiting to receive a server session header.  This
   ensures that confirming session headers does not add latency.

   Both client and server MUST close the connection if it does not begin
   with a valid session header.  A GOAWAY frame (Section 3.7.7) MAY be
   omitted if it is clear that the peer is not using HTTP/2.0.

3.3.  Framing

   Once the connection is established, clients and servers exchange
   HTTP/2.0 frames.  Frames are the basic unit of communication.

3.3.1.  Frame Header

   HTTP/2.0 frames share a common header format.  Frames have an 8 byte
   header with between 0 and 65535 bytes of data.

    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 16-bit length of the frame payload in bytes.  The length
      of the frame header is not included in this sum.

   Type:  The 8-bit type of the frame.  The frame type determines how
      the remainder of the frame header and payload are interpreted.
      Implementations MUST ignore frames that use types that they do not
      support.



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   Flags:  An 8-bit field reserved for flags.  Bits that have undefined
      semantics are reserved.  The following flags are defined for all
      frame types:

      FINAL (0x1):  Bit 1 (the least significant bit) indicates that
         this is the last frame in a stream.  This places the stream
         into a half-closed state (Section 3.4.5).  No further frames
         follow in the direction of the carrying frame.

      Frame types can define semantics for frame-specific flags.

   R: A reserved 1-bit field.  The semantics of this bit are not
      defined.

   Stream Identifier:  A 31-bit stream identifier (see Section 3.4.1).
      A value 0 is reserved for frames that are directed at the session
      as a whole instead of a single stream.

   Frame Data:  Frames contain between 0 and 65535 bytes of data.

   Reserved bits in the frame header MUST be set to zero when sending
   and MUST be ignored when receiving frames, unless the semantics of
   the bit are known.

3.3.2.  Frame Processing

   A frame of the maximum size might be too large for implementations
   with limited resources to process.  Implementations MAY choose to
   support frames smaller than the maximum possible size.  However,
   implementations MUST be able to receive frames containing at least
   8192 octets of payload.

   An implementation MUST immediately close a stream if it is unable to
   process a frame related to that stream due to it exceeding a size
   limit.  The implementation MUST send a RST_STREAM frame
   (Section 3.7.3) containing FRAME_TOO_LARGE error code if the frame
   size limit is exceeded.

   [[anchor9: <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

   Streams are independent sequences of bi-directional data divided into
   frames with several properties:





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   o  Streams can be created by either the client or server.

   o  Streams optionally carry a set of name-value header pairs.

   o  Streams can concurrently send data interleaved with other streams.

   o  Streams can be established and used unilaterally.

   o  Streams can be cancelled.

3.4.1.  Stream Creation

   Use of streams does not require negotiation.  A stream is not
   created, streams are used by sending a frame on the stream.

   Streams are identified by a 31-bit numeric identifier.  Streams
   initiated by a client use odd numbered stream identifiers.  Streams
   initiated by the server use odd numbered stream identifiers.  A
   stream identifier of zero MUST NOT be used to create a new stream.

   The stream identifier of a new stream MUST be greater than all other
   streams from that endpoint, unless the stream identifier was
   previously reserved (such as the promised stream identifier in a
   PUSH_PROMISE (Section 3.7.5) frame).  An endpoint that receives an
   unexpected stream identifier MUST treat this as a session error
   (Section 3.5.1) of type PROTOCOL_ERROR.

   A long-lived session can result in available stream identifiers being
   exhausted.  An endpoint that is unable to create a new stream
   identifier can establish a new session for any new streams.

   An endpoint cannot prevent the creation of a new stream, but it can
   request the early termination of an unwanted stream.  Upon receipt of
   a frame, the recipient can terminate the corresponding stream by
   sending a stream error (Section 3.5.2) of type REFUSED_STREAM.  This
   cannot prevent the initiating endpoint from sending frames for that
   stream prior to receiving this request.

3.4.2.  Stream priority

   The creator of a stream assigns a priority for that stream.  Priority
   is represented as a 31 bit integer. 0 represents the highest priority
   and 2^31-1 represents the lowest priority.

   The sender and recipient SHOULD use best-effort to process streams in
   the order of highest priority to lowest priority. [[anchor11: ED:
   toothless, useless "SHOULD": reword]]




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3.4.3.  Stream headers

   Streams carry optional sets of header fields which carry metadata
   about the stream.  After the stream has been created, and as long as
   the sender is not closed (Section 3.4.6) or half-closed
   (Section 3.4.5), each side may send HEADERS frame(s) containing the
   header data.  Header data can be sent in multiple HEADERS frames, and
   HEADERS frames may be interleaved with data frames.

3.4.4.  Stream data exchange

   Once a stream is created, it can be used to send arbitrary amounts of
   data.  Generally this means that a series of data frames will be sent
   on the stream until a frame containing the FINAL flag (Section 3.3.1)
   is set.  Once the FINAL flag has been set on any frame, the stream is
   considered to be half-closed.

3.4.5.  Stream half-close

   When one side of the stream sends a frame with the FINAL flag set,
   the stream is half-closed from that endpoint.  The sender of the
   FINAL flag MUST NOT send further frames on that stream.  When both
   sides have half-closed, the stream is closed.

   An endpoint MUST treat the receipt of a data frame on a half-closed
   stream as a stream error (Section 3.5.2) of type STREAM_CLOSED.

   Streams that have never received packets can be considered to be
   half-closed in the direction that is silent.  This allows either peer
   to create a unidirectional stream, which does not require an explicit
   close from the peer that does not transmit frames.

3.4.6.  Stream close

   Streams can be terminated in the following ways:

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

   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.






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

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

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

   If an endpoint receives a data frame after the stream is closed, it
   MAY send a RST_STREAM to the sender with the status PROTOCOL_ERROR.

3.5.  Error Handling

   HTTP/2.0 framing permits two classes of error:

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

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

3.5.1.  Session Error Handling

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

   An endpoint that encounters a session error MUST first send a GOAWAY
   (Section 3.7.7) 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 session 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 session error, GOAWAY only
   provides a best-effort attempt to communicate with the peer about why
   the session is going down.

   An endpoint can end a session at any time.  In particular, an
   endpoint MAY choose to treat a stream error as a session error if the



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   error is recurrent.  Endpoints SHOULD send a GOAWAY frame when ending
   a session, 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 3.7.3) frame that contains the stream identifier of the
   stream where the error occurred.  The RST_STREAM frame includes an
   error code that indicates the type of error.

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

   An endpoint SHOULD NOT send more than one RST_STREAM frame for any
   stream.  An endpoint MAY send additional RST_STREAM frames if it
   receives frames on a closed stream after more than a round trip time.
   This behaviour is permitted to deal with misbehaving implementations
   where treating this as a session error is inappropriate.

   An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM
   frame.  This could trigger infinite loops of RST_STREAM frames.

3.5.3.  Error Codes

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

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

   The following error codes are defined:

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

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





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

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

   INVALID_STREAM (4):  A frame was received for an inactive stream.

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

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

   REFUSED_STREAM (7):  Indicates that the stream was refused before any
      processing has been done on the stream.

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

3.6.  Stream Flow Control

   Multiplexing streams introduces contention for access to the shared
   TCP connection.  Stream contention can result in streams being
   blocked by other streams.  A flow control scheme ensures that streams
   do not destructively interfere with other streams on the same TCP
   connection.

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
   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 messages.  Receivers
       advertise how many octets they are prepared to receive on a
       stream.  This is a credit-based scheme.





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   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
       message (Section 3.7.9).  This does not stipulate how a receiver
       decides when to send this message 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 deployments (client, server or
   intermediary) that are operating under constraints.  For example, a
   proxy must share memory between many connections.  Flow control
   addresses cases where the receiver is unable process data on one
   stream, yet wants to be continue to process other streams.

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




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

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

3.7.  Frame Types

3.7.1.  DATA Frames

   DATA frames (type=0) are used to convey HTTP message bodies.  The
   payload of a data frame contains either a request or response body.

   No frame-specific flags are defined for DATA frames.

3.7.2.  HEADERS+PRIORITY

   The HEADERS+PRIORITY frame (type=1) allows the sender to set header
   fields and stream priority at the same time.  This MUST be used for
   each stream that is created.

    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 3.7.8), with a 32-bit field containing priority included
   before the header block.

   The most significant bit of the priority is reserved.  The 31-bit
   priority indicates the priority for the stream, as assigned by the
   sender, see Section 3.4.2.

3.7.3.  RST_STREAM

   The RST_STREAM frame (type=3) allows for abnormal termination of a
   stream.  When sent by the creator of a stream, it indicates the



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   creator wishes to cancel the stream.  When sent by the recipient of a
   stream, it indicates an error or that the recipient did not want to
   accept the stream, so the stream should be closed.

    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 does not define any valid flags.

   The RST_STREAM frame contains a single 32-bit error code
   (Section 3.5.3).  The error code indicates why the stream is being
   terminated.

   After receiving a RST_STREAM on a stream, the recipient must not send
   additional frames for that stream, and the stream moves into the
   closed state.

3.7.4.  SETTINGS

   A SETTINGS frame (type=4) contains a set of id/value pairs for
   communicating configuration data about how the two endpoints may
   communicate.  SETTINGS frames MUST be sent at the start of a session,
   but they can be sent at any other time by either endpoint.  Settings
   are declarative, not negotiated, each peer indicates their own
   configuration.

   [[anchor17: Note that persistence of settings is under discussion in
   the WG and might be removed in a future version of this document.]]

   When the server is the sender, the sender can request that
   configuration data be persisted by the client across HTTP/2.0
   sessions and returned to the server in future communications.

   Clients persist settings on a per origin basis (see [RFC6454] for a
   definition of web origins).  That is, when a client connects to a
   server, and the server persists settings within the client, the
   client SHOULD return the persisted settings on future connections to
   the same origin AND IP address and TCP port.  Clients MUST NOT
   request servers to use the persistence features of the SETTINGS
   frames, and servers MUST ignore persistence related flags sent by a
   client.

   Valid frame-specific flags for the SETTINGS frame are:



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   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 session, never a single stream.
   The stream identifier for a settings frame MUST be zero.

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

                          SETTINGS ID/Value Pair

   The payload of a SETTINGS frame contains zero or more settings.  Each
   setting is comprised of the following

   Settings Flags:  An 8-bit flags field containing per-setting
      instructions.  The following flags are valid:

      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.

      All other settings flags are reserved.

   Setting Identifier:  A 24-bit field that identifies the setting.

   Value:  A 32-bit value for the setting.

   The following settings are defined:

   SETTINGS_UPLOAD_BANDWIDTH (1):  allows the sender to send its
      expected upload bandwidth on this channel.  This number is an
      estimate.  The value should be the integral number of kilobytes
      per second that the sender predicts as an expected maximum upload
      channel capacity.





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   SETTINGS_DOWNLOAD_BANDWIDTH (2):  allows the sender to send its
      expected download bandwidth on this channel.  This number is an
      estimate.  The value should be the integral number of kilobytes
      per second that the sender predicts as an expected maximum
      download channel capacity.

   SETTINGS_ROUND_TRIP_TIME (3):  allows the sender to send its expected
      round-trip-time on this channel.  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):  allows the sender to inform the
      remote endpoint the maximum number of concurrent streams which it
      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.  For implementers it is recommended
      that this value be no smaller than 100, so as to not unnecessarily
      limit parallelism.

   SETTINGS_CURRENT_CWND (5):  allows the sender to inform the remote
      endpoint of the current TCP CWND value.

   SETTINGS_DOWNLOAD_RETRANS_RATE (6):  allows the sender to inform the
      remote endpoint the retransmission rate (bytes retransmitted /
      total bytes transmitted).

   SETTINGS_INITIAL_WINDOW_SIZE (7):  allows the sender to inform the
      remote endpoint the initial window size (in bytes) for new
      streams.

   SETTINGS_FLOW_CONTROL_OPTIONS (10):  This setting allows an endpoint
      to indicate that streams directed to them will not be subject to
      flow control.  The least significant bit (0x1) is set to indicate
      that new streams are not flow controlled.  Bit 2 (0x2) is set to
      indicate that the session is not flow controlled.  All other bits
      are reserved.

      This setting applies to all streams, including existing streams.

      These bits cannot be cleared once set, see Section 3.7.9.4.

   The message is intentionally extensible for future information which
   may improve client-server communications.  The sender does not need
   to send every type of ID/value.  It must only send those for which it
   has accurate values to convey.  When multiple ID/value pairs are
   sent, they should be sent in order of lowest id to highest id.  A
   single SETTINGS frame MUST not contain multiple values for the same



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   ID.  If the recipient of a SETTINGS frame discovers multiple values
   for the same ID, it MUST ignore all values except the first one.

   A server may send multiple SETTINGS frames containing different ID/
   Value pairs.  When the same ID/Value is sent twice, the most recent
   value overrides any previously sent values.  If the server sends IDs
   1, 2, and 3 with the FLAG_SETTINGS_PERSIST_VALUE in a first SETTINGS
   frame, and then sends IDs 4 and 5 with the
   FLAG_SETTINGS_PERSIST_VALUE, when the client returns the persisted
   state on its next SETTINGS frame, it SHOULD send all 5 settings (1,
   2, 3, 4, and 5 in this example) to the server.

3.7.5.  PUSH_PROMISE

   The PUSH_PROMISE frame (type=5) allows the sender to signal a promise
   to create a stream and serve the referenced resource.  Minimal data
   allowing the receiver to understand which resource(s) are to be
   pushed are to be included.

   PUSH_PROMISE frames are sent on an existing stream.  They declare the
   intent to use another stream for the pushing of a resource.  The
   PUSH_PROMISE allows the client an opportunity to reject pushed
   resources.

    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

   There are no frame-specific flags for the PUSH_PROMISE frame.

   The body of a PUSH_PROMISE includes a "Promised-Stream-ID".  This 31-
   bit identifier indicates the stream on which the resource will be
   pushed.  The promised stream identifier MUST be a valid choice for
   the next stream sent by the sender (see new stream identifier
   (Section 3.4.1)).

   There is no requirement that the streams referred to by this frame
   are created in the order referenced.  The PUSH_PROMISE reserves
   stream identifiers for later use; these reserved identifiers can be
   used as prioritization needs dictate.

   The PUSH_PROMISE also includes a header block (Section 3.7.10), which



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   describes the resource that will be pushed.

3.7.6.  PING

   The PING frame (type=6) is a mechanism for measuring a minimal round-
   trip time from the sender.  PING frames can be sent from the client
   or the server.

   Recipients of a PING frame send an identical frame to the sender as
   soon as possible.  PING should take highest priority if there is
   other data waiting to be sent.

   The PING frame defines a frame-specific flag:

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

   The payload of a PING frame contains any value.  A PING response MUST
   contain the contents of the PING request.

3.7.7.  GOAWAY

   The GOAWAY frame (type=7) informs the remote side of the connection
   to stop creating streams on this session.  It can be sent from the
   client or the server.  Once sent, the sender will ignore frames sent
   on new streams for the remainder of the session.  Recipients of a
   GOAWAY frame MUST NOT open additional streams on the session,
   although a new session can be established for new streams.  The
   purpose of this message 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 message.  To deal with this
   case, the GOAWAY contains the stream identifier of the last stream
   which was processed on the sending endpoint in this session.  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 session).

   Endpoints should always send a GOAWAY message 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



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   server does not send a GOAWAY frame to indicate where it stopped
   working).

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

   [[anchor18: Issue: session state that is established by those
   "ignored" messages 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 valid flags.

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

   The GOAWAY frame contains an identifier of the last stream that the
   sender of the GOAWAY is prepared to act upon, which can include
   processing and replies.  This allows an endpoint to discover what
   streams might have had some effect or what might be safe to
   automatically retry.  If no streams were acted upon, the last stream
   ID MUST be 0.

   The GOAWAY frame contains a 32-bit error code (Section 3.5.3) that
   contains the reason for closing the session.

3.7.8.  HEADERS

   The HEADERS frame (type=8) provides header fields for a stream.  It
   may be optionally sent on an existing stream at any time.  Specific
   application of the headers in this frame is application-dependent.

   No frame-specific flags are defined for the HEADERS frame.

   The body of a HEADERS frame contains a Headers Block
   (Section 3.7.10).






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3.7.9.  WINDOW_UPDATE

   The WINDOW_UPDATE frame (type=9) is used to implement flow control in
   HTTP/2.0.

   Flow control in HTTP/2.0 operates at two levels: on each individual
   stream and on the entire session.

   Flow control in HTTP/2.0 is hop by hop, that is, only between the two
   endpoints of a HTTP/2.0 connection.  Intermediaries do not forward
   WINDOW_UPDATE messages between dependent sessions.  However,
   throttling of data transfer by any recipient 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 frames defined in this document,
   only data frames are subject to flow control.  Receivers MUST either
   buffer or process all other frames, terminate the corresponding
   stream, or terminate the session.  The stream or session is
   terminated with a FLOW_CONTROL_ERROR code.

   Valid flags for the WINDOW_UPDATE frame are:

   END_FLOW_CONTROL (0x2):  Bit 2 being set indicates that flow control
      for the identified stream or session is ended and subsequent
      frames do not need to be flow controlled.

   The WINDOW_UPDATE frame can be stream related or session related.
   The stream identifier in the WINDOW_UPDATE frame header identifies
   the affected stream, or includes a value of 0 to indicate that the
   session flow control window is updated.

   The payload of a WINDOW_UPDATE frame contains a 32-bit value.  This
   value is 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.

3.7.9.1.  The Flow Control Window

   Flow control in HTTP/2.0 is implemented by a flow control window kept
   by the sender of each stream.  The flow control window is a simple
   integer value that indicates how many bytes of data the sender is
   permitted to transmit.  The flow control window size is a measure of
   the buffering capability of the recipient.

   Two flow control windows apply to the sending of every message: the
   stream flow control window and the session flow control window.  The



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   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 message sends a WINDOW_UPDATE frame as it consumes
   data and frees up space in flow control windows.  Separate
   WINDOW_UPDATE messages are sent for the stream and session 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 session, as appropriate.  For streams, the sender sends
   a RST_STREAM with the error code of FLOW_CONTROL_ERROR code; for the
   session, a GOAWAY message 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.

3.7.9.2.  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
   session flow control window is 65536 bytes.  Both endpoints can
   adjust the initial window size for new streams by including a value
   for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms
   part of the session header.

   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 session flow control window is set to the default initial window
   size until a WINDOW_UPDATE message is received.

   A SETTINGS frame can alter the initial flow control window size for



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   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 not send new flow
   controlled frames until it receives WINDOW_UPDATE messages 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.

3.7.9.3.  Reducing the Stream Window Size

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

   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 messages as it
       consumes data.

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

3.7.9.4.  Ending Flow Control

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




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   Flow control can be disabled for all streams or the session 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
   session by sending a WINDOW_UPDATE with the END_FLOW_CONTROL flag
   set.  The payload of a WINDOW_UPDATE frame that has the
   END_FLOW_CONTROL flag set is ignored.

   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.

3.7.10.  Header Block

   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.

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 session to a given
   origin ([RFC6454]) concurrently.

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







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4.1.1.  Use of GOAWAY

   HTTP/2.0 provides a GOAWAY message which can be used when closing a
   connection from either the client or server.  Without a server GOAWAY
   message, HTTP has a race condition where the client sends a request
   just as the server is closing the connection, and the client cannot
   know if the server received the stream or not.  By using the last-
   stream-id in the GOAWAY, servers can indicate to the client if a
   request was processed or not.

   Note that some servers will choose to send the GOAWAY and immediately
   terminate the connection without waiting for active streams to
   finish.  The client will be able to determine this because HTTP/2.0
   streams are deterministically closed.  This abrupt termination will
   force the client to heuristically decide whether to retry the pending
   requests.  Clients always need to be capable of dealing with this
   case because they must deal with accidental connection termination
   cases, which are the same as the server never having sent a GOAWAY.

   More sophisticated servers will use GOAWAY to implement a graceful
   teardown.  They will send the GOAWAY and provide some time for the
   active streams to finish before terminating the connection.

   If a HTTP/2.0 client closes the connection, it should also send a
   GOAWAY message.  This allows the server to know if any server-push
   streams were received by the client.

   If the endpoint closing the connection has not received frames on any
   stream, the GOAWAY will contain a last-stream-id of 0.

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



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   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 that are associated with an HTTP request.
   The header block in HTTP/2.0 is mostly unchanged from today's HTTP
   header block, with the following differences:

      The following fields that are carried in the request line in
      HTTP/1.1 ([HTTP-p1], Section 3.1.1) are defined as special-valued
      name-value pairs:

      ":method":  the HTTP method for this request (e.g.  "GET", "POST",
         "HEAD", etc) ([HTTP-p2], Section 4)

      ":path":  ":path" - the request-target for this URI with "/"
         prefixed (see [HTTP-p1], Section 3.1.1).  For example, for
         "http://www.google.com/search?q=dogs" the path would be
         "/search?q=dogs". [[anchor26: what forms of the HTTPbis
         request-target are allowed here?]]

      These header fields MUST be present in HTTP requests.

      In addition, the following two name-value pairs MUST be present in
      every request:

      ":host":  the host and optional port portions (see [RFC3986],
         Section 3.2) of the URI for this request (e.g. "www.google.com:
         1234").  This header field is the same as the HTTP 'Host'
         header field ([HTTP-p1], Section 5.4).

      ":scheme":  the scheme portion of the URI for this request (e.g.
         "https")

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

      Header field names MUST be all lowercase.

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

      User-agents MUST support gzip compression.  Regardless of the
      Accept-Encoding sent by the user-agent, the server may always send
      content encoded with gzip or deflate encoding. [[anchor27: Still



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      valid?]]

      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 used 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,
      which caused server implementations have come to depend upon its
      presence.

   The user-agent is free to prioritize requests as it sees fit.  If the
   user-agent cannot make progress without receiving a resource, it
   should attempt to raise the priority of that resource.  Resources
   such as images, SHOULD generally use the lowest priority.

   If a client sends a HEADERS+PRIORITY frame that omits a mandatory
   header, the server MUST reply with a HTTP 400 Bad Request reply.
   [[anchor28: Ed: why PROTOCOL_ERROR on missing ":status" in the
   response, but HTTP 400 here?]]

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

4.2.3.  Response

   The server responds to a client request with a HEADERS frame.
   Symmetric to the client's upload stream, server will send any
   response body in a series of DATA frames.  The last data frame will
   contain the FINAL flag to indicate the end of the stream and the end
   of the response.  A response that contains no body (such as a 204 or
   304 response) consists only of a HEADERS frame that contains the
   FINAL flag to indicate no further data will be sent on the stream.

      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.



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      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. [[anchor29: 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 or HEADERS+
   PRIORITY frame the client MUST treat this as a stream error
   (Section 3.5.2) of type PROTOCOL_ERROR.

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.  Pushing of resources avoids the
   round-trip delay, but also creates a potential race where a server
   can be pushing content which a user-agent is in the process of
   requesting.  The following mechanics attempt to prevent the race
   condition while enabling the performance benefit.

   Server push is an optional feature.  Server push can be disabled by
   clients that do not wish to receive pushed resources by advertising a
   SETTINGS_MAX_CONCURRENT_STREAMS SETTING (Section 3.7.4) of zero.
   This prevents servers from creating the streams necessary to push
   resources.

   Browsers receiving a pushed response MUST validate that the server is
   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 [[anchor30: Ed: weaselly use of
   "generally", needs better definition]] not permitted to push a



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   response for "www.example.org".

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

   Pushed responses are associated with a request at the HTTP/2.0
   framing layer.  The PUSH_PROMISE includes a stream identifier for an
   associated request/response exchange that supplies request header
   fields.  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.

   Implementation note: With server push, it is theoretically possible
   for servers to push unreasonable amounts of content or resources to
   the user-agent.  Browsers MUST implement throttles to protect against
   unreasonable push attacks. [[anchor31: Ed: insufficiently specified
   to implement; would like to remove]]

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 ":port").

   A server can push multiple resources in response to a request, but
   these can only be sent while the response stream remains open.  A
   server MUST NOT send a PUSH_PROMISE on a half-closed stream.

   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



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   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

   When a HEADERS+PRIORITY frame that contains an
   Associated-To-Stream-ID is received, the client MUST NOT[[anchor34:
   SHOULD NOT?]] issue GET requests for the resource in the pushed
   stream, and instead wait for the pushed stream to arrive.

   A server MUST NOT push a resource with an Associated-To-Stream-ID of
   0.  Clients MUST treat this as a session error (Section 3.5.1) of
   type PROTOCOL_ERROR.

   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.  Upon receipt, the
   server ceases transmission of the pushed data.

   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. [[anchor35: 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.]]



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   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
   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
   in tact, 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 session is hosed.  In this case, the endpoint
   detecting the error should initiate a connection close.




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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
   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



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   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.  Use of Same-origin constraints

   This specification uses the same-origin policy ([RFC6454], Section 3)
   in all cases where verification of content is required.

6.2.  Cross-Protocol Attacks

   By utilizing 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.
   [[anchor45: Issue: This is no longer true]]

6.3.  Cacheability of Pushed Resources

   Pushed resources do not have an associated request.  In order for
   existing HTTP cache control validations (such as the Vary header
   field) to work, all cached resources must have a set of request
   header fields.  For this reason, caches MUST be careful to 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






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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
   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.







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

                                  Table 1

8.2.  Error Code Registry

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

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

   New registrations are advised to provide the following information:

   Error Code:  The 32-bit error code value.

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

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

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

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

8.3.  Settings Registry

   This document establishes a registry for HTTP/2.0 settings.  The
   "HTTP/2.0 Settings" registry manages a 24-bit space.  The "HTTP/2.0



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   Settings" registry operates under the "Expert Review" policy
   [RFC5226].

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

   New registrations are advised to provide the following information:

   Setting:  The 24-bit setting value.

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

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

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

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

   An initial set of settings registrations can be found in
   Section 3.7.4.

9.  Acknowledgements

   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 and Julian Reschke

10.  References






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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.

   [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.

   [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.



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

   [TLSNPN]   Langley, A., "Transport Layer Security (TLS) Next Protocol
              Negotiation Extension", draft-agl-tls-nextprotoneg-04
              (work in progress), May 2012.

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.
              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-http2-01

   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.2.  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.3.  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  94043
   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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