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

HTTPbis Working Group                                          M. Belshe
Internet-Draft                                                     Twist
Expires: July 26, 2013                                           R. Peon
                                                             Google, Inc
                                                         M. Thomson, Ed.
                                                               Microsoft
                                                        A. Melnikov, Ed.
                                                               Isode Ltd
                                                        January 22, 2013


                Hypertext Transfer Protocol version 2.0
                      draft-ietf-httpbis-http2-01

Abstract

   This document describes an optimised expression of the semantics of
   the HTTP protocol.  The HTTP/2.0 encapsulation enables more efficient
   transfer of resources over HTTP by providing compressed headers,
   simultaneous requests, and unsolicited push of resources from server
   to client.

   This document is an alternative to, but does not obsolete
   RFC{http-p1}.  The HTTP protocol semantics described in RFC{http-
   p2..p7} are unmodified.

Editorial Note (To be removed by RFC Editor)

   This draft is a work-in-progress, and does not yet reflect Working
   Group consensus.

   This draft contains features from the SPDY Protocol as a starting
   point, as per the Working Group's charter.  Future drafts will add,
   remove and change text, based upon the Working Group's decisions.

   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/>.

   The current issues list is at
   <http://tools.ietf.org/wg/httpbis/trac/report/21> and related
   documents (including fancy diffs) can be found at
   <http://tools.ietf.org/wg/httpbis/>.

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

Status of This Memo




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   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/.

   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 July 26, 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.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Starting HTTP/2.0  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  HTTP/2.0 Version Identification  . . . . . . . . . . . . .  6
     2.2.  Starting HTTP/2.0 for "http:" URIs . . . . . . . . . . . .  7
     2.3.  Starting HTTP/2.0 for "https:" URIs  . . . . . . . . . . .  8
   3.  HTTP/2.0 Framing Layer . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Session (Connections)  . . . . . . . . . . . . . . . . . .  8
     3.2.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Control frames . . . . . . . . . . . . . . . . . . . .  9
       3.2.2.  Data frames  . . . . . . . . . . . . . . . . . . . . . 10
     3.3.  Streams  . . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.3.1.  Stream frames  . . . . . . . . . . . . . . . . . . . . 11
       3.3.2.  Stream creation  . . . . . . . . . . . . . . . . . . . 11
       3.3.3.  Stream priority  . . . . . . . . . . . . . . . . . . . 12



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       3.3.4.  Stream headers . . . . . . . . . . . . . . . . . . . . 12
       3.3.5.  Stream data exchange . . . . . . . . . . . . . . . . . 13
       3.3.6.  Stream half-close  . . . . . . . . . . . . . . . . . . 13
       3.3.7.  Stream close . . . . . . . . . . . . . . . . . . . . . 13
     3.4.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 14
       3.4.1.  Session Error Handling . . . . . . . . . . . . . . . . 14
       3.4.2.  Stream Error Handling  . . . . . . . . . . . . . . . . 14
     3.5.  Stream Flow Control  . . . . . . . . . . . . . . . . . . . 15
       3.5.1.  Flow Control Principles  . . . . . . . . . . . . . . . 15
       3.5.2.  Basic Flow Control Algorithm . . . . . . . . . . . . . 16
     3.6.  Control frame types  . . . . . . . . . . . . . . . . . . . 16
       3.6.1.  SYN_STREAM . . . . . . . . . . . . . . . . . . . . . . 16
       3.6.2.  SYN_REPLY  . . . . . . . . . . . . . . . . . . . . . . 18
       3.6.3.  RST_STREAM . . . . . . . . . . . . . . . . . . . . . . 19
       3.6.4.  SETTINGS . . . . . . . . . . . . . . . . . . . . . . . 20
       3.6.5.  PING . . . . . . . . . . . . . . . . . . . . . . . . . 23
       3.6.6.  GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . 24
       3.6.7.  HEADERS  . . . . . . . . . . . . . . . . . . . . . . . 25
       3.6.8.  WINDOW_UPDATE  . . . . . . . . . . . . . . . . . . . . 26
       3.6.9.  CREDENTIAL . . . . . . . . . . . . . . . . . . . . . . 28
       3.6.10. Name/Value Header Block  . . . . . . . . . . . . . . . 30
   4.  HTTP Layering over HTTP/2.0  . . . . . . . . . . . . . . . . . 36
     4.1.  Connection Management  . . . . . . . . . . . . . . . . . . 36
       4.1.1.  Use of GOAWAY  . . . . . . . . . . . . . . . . . . . . 36
     4.2.  HTTP Request/Response  . . . . . . . . . . . . . . . . . . 37
       4.2.1.  Request  . . . . . . . . . . . . . . . . . . . . . . . 37
       4.2.2.  Response . . . . . . . . . . . . . . . . . . . . . . . 39
       4.2.3.  Authentication . . . . . . . . . . . . . . . . . . . . 39
     4.3.  Server Push Transactions . . . . . . . . . . . . . . . . . 40
       4.3.1.  Server implementation  . . . . . . . . . . . . . . . . 41
       4.3.2.  Client implementation  . . . . . . . . . . . . . . . . 42
   5.  Design Rationale and Notes . . . . . . . . . . . . . . . . . . 43
     5.1.  Separation of Framing Layer and Application Layer  . . . . 43
     5.2.  Error handling - Framing Layer . . . . . . . . . . . . . . 43
     5.3.  One Connection Per Domain  . . . . . . . . . . . . . . . . 44
     5.4.  Fixed vs Variable Length Fields  . . . . . . . . . . . . . 44
     5.5.  Compression Context(s) . . . . . . . . . . . . . . . . . . 45
     5.6.  Unidirectional streams . . . . . . . . . . . . . . . . . . 45
     5.7.  Data Compression . . . . . . . . . . . . . . . . . . . . . 45
     5.8.  Server Push  . . . . . . . . . . . . . . . . . . . . . . . 46
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 46
     6.1.  Use of Same-origin constraints . . . . . . . . . . . . . . 46
     6.2.  HTTP Headers and HTTP/2.0 Headers  . . . . . . . . . . . . 46
     6.3.  Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 46
     6.4.  Server Push Implicit Headers . . . . . . . . . . . . . . . 46
   7.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 47
     7.1.  Long Lived Connections . . . . . . . . . . . . . . . . . . 47
     7.2.  SETTINGS frame . . . . . . . . . . . . . . . . . . . . . . 47



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   8.  Requirements Notation  . . . . . . . . . . . . . . . . . . . . 47
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 48
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 49
     A.1.  Since draft-ietf-httpbis-http2-00  . . . . . . . . . . . . 49
     A.2.  Since draft-mbelshe-httpbis-spdy-00  . . . . . . . . . . . 49












































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1.  Introduction

   HTTP is a wildly successful protocol.  HTTP/1.1 message encapsulation
   [HTTP-p1] 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.

   Furthermore, HTTP/1.1 headers 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 headers.  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
   headers 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
   pairs on top of the framing layer.  While some of the framing layer



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   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.  Definitions

      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: A header-prefixed sequence of bytes sent over a HTTP/2.0
      session.

      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.

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 in using the string "HTTP/2.0".  This
   identification is used in the HTTP/1.1 Upgrade header, in the TLS-NPN
   [TLSNPN] [[TBD]] field and other places where protocol identification
   is required.

   [[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



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

   Non-compatible experiments that are based on these draft versions
   MUST include a further identifier.  For example, an experimental
   implementation of packet mood-based encoding based on
   draft-ietf-httpbis-http2-07 might identify itself as "HTTP-07-
   emo/2.0".  Note that any label MUST conform with the "token" syntax
   defined in Section 3.2.4 of [HTTP-p1].  Experimenters are encouraged
   to coordinate their experiments on the ietf-http-wg@w3.org mailing
   list.

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
   [HTTP-p2].  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.




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

      [ HTTP/2.0 frames ...

   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.  [[Open Issue: This is not
   definite.  We may yet choose to perform negotiation for every
   connection.  Reasons include intermediaries; phased upgrade of load-
   balanced server farms; etc...]]  [[Open Issue: We need to enumerate
   the ways that clients can learn of HTTP/2.0 support.]]

2.3.  Starting HTTP/2.0 for "https:" URIs

   [[TBD, maybe NPN]]

3.  HTTP/2.0 Framing Layer

3.1.  Session (Connections)

   The HTTP/2.0 framing layer (or "session") runs atop a reliable
   transport layer such as 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 3.6.6) frame so that the endpoints can
   reliably determine if requests finished before the close.

3.2.  Framing

   Once the connection is established, clients and servers exchange
   framed messages.  There are two types of frames: control frames
   (Section 3.2.1) and data frames (Section 3.2.2).  Frames always have
   a common header which is 8 bytes in length.

   The first bit is a control bit indicating whether a frame is a
   control frame or data frame.  Control frames carry a version number,
   a frame type, flags, and a length.  Data frames contain the stream
   ID, flags, and the length for the payload carried after the common
   header.  The simple header is designed to make reading and writing of



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   frames easy.

   All integer values, including length, version, and type, are in
   network byte order.  HTTP/2.0 does not enforce alignment of types in
   dynamically sized frames.

3.2.1.  Control frames

   +----------------------------------+
   |C| Version(15bits) | Type(16bits) |
   +----------------------------------+
   | Flags (8)  |  Length (24 bits)   |
   +----------------------------------+
   |               Data               |
   +----------------------------------+

   Control bit: The 'C' bit is a single bit indicating if this is a
   control message.  For control frames this value is always 1.

   Version: The version number of the HTTP/2.0 protocol.  This document
   describes HTTP/2.0 version 3.

   Type: The type of control frame.  See Control Frames for the complete
   list of control frames.

   Flags: Flags related to this frame.  Flags for control frames and
   data frames are different.

   Length: An unsigned 24-bit value representing the number of bytes
   after the length field.

   Data: data associated with this control frame.  The format and length
   of this data is controlled by the control frame type.

   Control frame processing requirements:

      Note that full length control frames (16MB) can be large for
      implementations running on resource-limited hardware.  In such
      cases, implementations MAY limit the maximum length frame
      supported.  However, all implementations MUST be able to receive
      control frames of at least 8192 octets in length.










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3.2.2.  Data frames

   +----------------------------------+
   |C|       Stream-ID (31bits)       |
   +----------------------------------+
   | Flags (8)  |  Length (24 bits)   |
   +----------------------------------+
   |               Data               |
   +----------------------------------+

   Control bit: For data frames this value is always 0.

   Stream-ID: A 31-bit value identifying the stream.

   Flags: Flags related to this frame.  Valid flags are:

      0x01 = FLAG_FIN - signifies that this frame represents the last
      frame to be transmitted on this stream.  See Stream Close
      (Section 3.3.7) below.

      0x02 = FLAG_COMPRESS - indicates that the data in this frame has
      been compressed.

   Length: An unsigned 24-bit value representing the number of bytes
   after the length field.  The total size of a data frame is 8 bytes +
   length.  It is valid to have a zero-length data frame.

   Data: The variable-length data payload; the length was defined in the
   length field.

   Data frame processing requirements:

      If an endpoint receives a data frame for a stream-id which is not
      open and the endpoint has not sent a GOAWAY (Section 3.6.6) frame,
      it MUST send issue a stream error (Section 3.4.2) with the error
      code INVALID_STREAM for the stream-id.

      If the endpoint which created the stream receives a data frame
      before receiving a SYN_REPLY on that stream, it is a protocol
      error, and the recipient MUST issue a stream error (Section 3.4.2)
      with the status code PROTOCOL_ERROR for the stream-id.

      Implementors note: If an endpoint receives multiple data frames
      for invalid stream-ids, it MAY close the session.

      All HTTP/2.0 endpoints MUST accept compressed data frames.
      Compression of data frames is always done using zlib compression.
      Each stream initializes and uses its own compression context



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      dedicated to use within that stream.  Endpoints are encouraged to
      use application level compression rather than HTTP/2.0 stream
      level compression.

      Each HTTP/2.0 stream sending compressed frames creates its own
      zlib context for that stream, and these compression contexts MUST
      be distinct from the compression contexts used with SYN_STREAM/
      SYN_REPLY/HEADER compression.  (Thus, if both endpoints of a
      stream are compressing data on the stream, there will be two zlib
      contexts, one for sending and one for receiving).

3.3.  Streams

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

      Streams may be created by either the client or server.

      Streams optionally carry a set of name/value header pairs.

      Streams can concurrently send data interleaved with other streams.

      Streams may be cancelled.

3.3.1.  Stream frames

   HTTP/2.0 defines 3 control frames to manage the lifecycle of a
   stream:

      SYN_STREAM - Open a new stream

      SYN_REPLY - Remote acknowledgement of a new, open stream

      RST_STREAM - Close a stream

3.3.2.  Stream creation

   A stream is created by sending a control frame with the type set to
   SYN_STREAM (Section 3.6.1).  If the server is initiating the stream,
   the Stream-ID must be even.  If the client is initiating the stream,
   the Stream-ID must be odd. 0 is not a valid Stream-ID.  Stream-IDs
   from each side of the connection must increase monotonically as new
   streams are created.  E.g.  Stream 2 may be created after stream 3,
   but stream 7 must not be created after stream 9.  Stream IDs do not
   wrap: when a client or server cannot create a new stream id without
   exceeding a 31 bit value, it MUST NOT create a new stream.

   The stream-id MUST increase with each new stream.  If an endpoint



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   receives a SYN_STREAM with a stream id which is less than any
   previously received SYN_STREAM, it MUST issue a session error
   (Section 3.4.1) with the status PROTOCOL_ERROR.

   It is a protocol error to send two SYN_STREAMs with the same
   stream-id.  If a recipient receives a second SYN_STREAM for the same
   stream, it MUST issue a stream error (Section 3.4.2) with the status
   code PROTOCOL_ERROR.

   Upon receipt of a SYN_STREAM, the recipient can reject the stream by
   sending a stream error (Section 3.4.2) with the error code
   REFUSED_STREAM.  Note, however, that the creating endpoint may have
   already sent additional frames for that stream which cannot be
   immediately stopped.

   Once the stream is created, the creator may immediately send HEADERS
   or DATA frames for that stream, without needing to wait for the
   recipient to acknowledge.

3.3.2.1.  Unidirectional streams

   When an endpoint creates a stream with the FLAG_UNIDIRECTIONAL flag
   set, it creates a unidirectional stream which the creating endpoint
   can use to send frames, but the receiving endpoint cannot.  The
   receiving endpoint is implicitly already in the half-closed
   (Section 3.3.6) state.

3.3.2.2.  Bidirectional streams

   SYN_STREAM frames which do not use the FLAG_UNIDIRECTIONAL flag are
   bidirectional streams.  Both endpoints can send data on a bi-
   directional stream.

3.3.3.  Stream priority

   The creator of a stream assigns a priority for that stream.  Priority
   is represented as an integer from 0 to 7. 0 represents the highest
   priority and 7 represents the lowest priority.

   The sender and recipient SHOULD use best-effort to process streams in
   the order of highest priority to lowest priority.

3.3.4.  Stream headers

   Streams carry optional sets of name/value pair headers which carry
   metadata about the stream.  After the stream has been created, and as
   long as the sender is not closed (Section 3.3.7) or half-closed
   (Section 3.3.6), each side may send HEADERS frame(s) containing the



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   header data.  Header data can be sent in multiple HEADERS frames, and
   HEADERS frames may be interleaved with data frames.

3.3.5.  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 FLAG_FIN flag is set.  The
   FLAG_FIN can be set on a SYN_STREAM (Section 3.6.1), SYN_REPLY
   (Section 3.6.2), HEADERS (Section 3.6.7) or a DATA (Section 3.2.2)
   frame.  Once the FLAG_FIN has been sent, the stream is considered to
   be half-closed.

3.3.6.  Stream half-close

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

   If an endpoint receives a data frame after the stream is half-closed
   from the sender (e.g. the endpoint has already received a prior frame
   for the stream with the FIN flag set), it MUST send a RST_STREAM to
   the sender with the status STREAM_ALREADY_CLOSED.

3.3.7.  Stream close

   There are 3 ways that streams can be terminated:

      Normal termination: Normal stream termination occurs when both
      sender and recipient have half-closed the stream by sending a
      FLAG_FIN.

      Abrupt termination: Either the client or server can send a
      RST_STREAM control frame at any time.  A RST_STREAM contains an
      error code to indicate the reason for failure.  When a RST_STREAM
      is sent from the stream originator, it indicates a failure to
      complete the stream and that no further data will be sent on the
      stream.  When a RST_STREAM is sent from the stream recipient, the
      sender, upon receipt, should stop sending any data on the stream.
      The stream recipient should be aware that there is a race between
      data already in transit from the sender and the time the
      RST_STREAM is received.  See Stream Error Handling (Section 3.4.2)

      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.




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

3.4.  Error Handling

   The HTTP/2.0 framing layer has only two types of errors, and they are
   always handled consistently.  Any reference in this specification to
   "issue a session error" refers to Section 3.4.1.  Any reference to
   "issue a stream error" refers to Section 3.4.2.

3.4.1.  Session Error Handling

   A session error is any error which prevents further processing of the
   framing layer or which corrupts the session compression state.  When
   a session error occurs, the endpoint encountering the error MUST
   first send a GOAWAY (Section 3.6.6) frame with the stream id of most
   recently received stream from the remote endpoint, and the error code
   for why the session is terminating.  After sending the GOAWAY frame,
   the endpoint MUST close the TCP connection.

   Note that the session compression state is dependent upon both
   endpoints always processing all compressed data.  If an endpoint
   partially processes a frame containing compressed data without
   updating compression state properly, future control frames which use
   compression will be always be errored.  Implementations SHOULD always
   try to process compressed data so that errors which could be handled
   as stream errors do not become session errors.

   Note that because this GOAWAY is sent during a session error case, it
   is possible that the GOAWAY will not be reliably received by the
   receiving endpoint.  It is a best-effort attempt to communicate with
   the remote about why the session is going down.

3.4.2.  Stream Error Handling

   A stream error is an error related to a specific stream-id which does
   not affect processing of other streams at the framing layer.  Upon a
   stream error, the endpoint MUST send a RST_STREAM (Section 3.6.3)
   frame which contains the stream id of the stream where the error
   occurred and the error status which caused the error.  After sending
   the RST_STREAM, the stream is closed to the sending endpoint.  After
   sending the RST_STREAM, if the sender receives any frames other than
   a RST_STREAM for that stream id, it will result in sending additional
   RST_STREAM frames.  An endpoint MUST NOT send a RST_STREAM in
   response to an RST_STREAM, as doing so would lead to RST_STREAM
   loops.  Sending a RST_STREAM does not cause the HTTP/2.0 session to
   be closed.




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   If an endpoint has multiple RST_STREAM frames to send in succession
   for the same stream-id and the same error code, it MAY coalesce them
   into a single RST_STREAM frame.  (This can happen if a stream is
   closed, but the remote sends multiple data frames.  There is no
   reason to send a RST_STREAM for each frame in succession).

3.5.  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.

3.5.1.  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.  The
   following principles guide the HTTP/2.0 design:

   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.

   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 [[TBD: ... and for the overall
       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.  Flow control can be disabled by a receiver.  A receiver can
       choose to either disable flow control, or to declare an infinite
       flow control limit.  [[TBD: determine whether just one mechanism
       is sufficient, and then which alternative]]





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   5.  HTTP/2.0 standardizes only the format of the window update
       message (Section 3.6.8).  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.  An example flow control algorithm is described in
       Section 3.5.2.

   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.

3.5.2.  Basic Flow Control Algorithm

   This section describes a basic flow control algorithm.  This
   algorithm is provided as an example, implementations can use any
   algorithm that complies with flow control requirements.

   [[Algorithm TBD]]

3.6.  Control frame types

3.6.1.  SYN_STREAM

   The SYN_STREAM control frame allows the sender to asynchronously
   create a stream between the endpoints.  See Stream Creation
   (Section 3.3.2)






















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   +------------------------------------+
   |1|    version    |         1        |
   +------------------------------------+
   |  Flags (8)  |  Length (24 bits)    |
   +------------------------------------+
   |X|           Stream-ID (31bits)     |
   +------------------------------------+
   |X| Associated-To-Stream-ID (31bits) |
   +------------------------------------+
   | Pri|Unused | Slot |                |
   +-------------------+                |
   | Number of Name/Value pairs (int32) |   <+
   +------------------------------------+    |
   |     Length of name (int32)         |    | This section is the
   +------------------------------------+    | "Name/Value Header
   |           Name (string)            |    | Block", and is
   +------------------------------------+    | compressed.
   |     Length of value  (int32)       |    |
   +------------------------------------+    |
   |          Value   (string)          |    |
   +------------------------------------+    |
   |           (repeats)                |   <+

   Flags: Flags related to this frame.  Valid flags are:

      0x01 = FLAG_FIN - marks this frame as the last frame to be
      transmitted on this stream and puts the sender in the half-closed
      (Section 3.3.6) state.

      0x02 = FLAG_UNIDIRECTIONAL - a stream created with this flag puts
      the recipient in the half-closed (Section 3.3.6) state.

   Length: The length is the number of bytes which follow the length
   field in the frame.  For SYN_STREAM frames, this is 10 bytes plus the
   length of the compressed Name/Value block.

   Stream-ID: The 31-bit identifier for this stream.  This stream-id
   will be used in frames which are part of this stream.

   Associated-To-Stream-ID: The 31-bit identifier for a stream which
   this stream is associated to.  If this stream is independent of all
   other streams, it should be 0.

   Priority: A 3-bit priority (Section 3.3.3) field.

   Unused: 5 bits of unused space, reserved for future use.

   Slot: An 8 bit unsigned integer specifying the index in the server's



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   CREDENTIAL vector of the client certificate to be used for this
   request. see CREDENTIAL frame (Section 3.6.9).  The value 0 means no
   client certificate should be associated with this stream.

   Name/Value Header Block: A set of name/value pairs carried as part of
   the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).

   If an endpoint receives a SYN_STREAM which is larger than the
   implementation supports, it MAY send a RST_STREAM with error code
   FRAME_TOO_LARGE.  All implementations MUST support the minimum size
   limits defined in the Control Frames section (Section 3.2.1).

3.6.2.  SYN_REPLY

   SYN_REPLY indicates the acceptance of a stream creation by the
   recipient of a SYN_STREAM frame.

   +------------------------------------+
   |1|    version    |         2        |
   +------------------------------------+
   |  Flags (8)  |  Length (24 bits)    |
   +------------------------------------+
   |X|           Stream-ID (31bits)     |
   +------------------------------------+
   | Number of Name/Value pairs (int32) |   <+
   +------------------------------------+    |
   |     Length of name (int32)         |    | This section is the
   +------------------------------------+    | "Name/Value Header
   |           Name (string)            |    | Block", and is
   +------------------------------------+    | compressed.
   |     Length of value  (int32)       |    |
   +------------------------------------+    |
   |          Value   (string)          |    |
   +------------------------------------+    |
   |           (repeats)                |   <+

   Flags: Flags related to this frame.  Valid flags are:

      0x01 = FLAG_FIN - marks this frame as the last frame to be
      transmitted on this stream and puts the sender in the half-closed
      (Section 3.3.6) state.

   Length: The length is the number of bytes which follow the length
   field in the frame.  For SYN_REPLY frames, this is 4 bytes plus the
   length of the compressed Name/Value block.

   Stream-ID: The 31-bit identifier for this stream.




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   If an endpoint receives multiple SYN_REPLY frames for the same active
   stream ID, it MUST issue a stream error (Section 3.4.2) with the
   error code STREAM_IN_USE.

   Name/Value Header Block: A set of name/value pairs carried as part of
   the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).

   If an endpoint receives a SYN_REPLY which is larger than the
   implementation supports, it MAY send a RST_STREAM with error code
   FRAME_TOO_LARGE.  All implementations MUST support the minimum size
   limits defined in the Control Frames section (Section 3.2.1).

3.6.3.  RST_STREAM

   The RST_STREAM frame allows for abnormal termination of a stream.
   When sent by the creator of a stream, it indicates the 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.

   +----------------------------------+
   |1|   version    |         3       |
   +----------------------------------+
   | Flags (8)  |         8           |
   +----------------------------------+
   |X|          Stream-ID (31bits)    |
   +----------------------------------+
   |          Status code             |
   +----------------------------------+

   Flags: Flags related to this frame.  RST_STREAM does not define any
   flags.  This value must be 0.

   Length: An unsigned 24-bit value representing the number of bytes
   after the length field.  For RST_STREAM control frames, this value is
   always 8.

   Stream-ID: The 31-bit identifier for this stream.

   Status code: (32 bits) An indicator for why the stream is being
   terminated.The following status codes are defined:

      1 - PROTOCOL_ERROR.  This is a generic error, and should only be
      used if a more specific error is not available.

      2 - INVALID_STREAM.  This is returned when a frame is received for
      a stream which is not active.




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      3 - REFUSED_STREAM.  Indicates that the stream was refused before
      any processing has been done on the stream.

      4 - UNSUPPORTED_VERSION.  Indicates that the recipient of a stream
      does not support the HTTP/2.0 version requested.

      5 - CANCEL.  Used by the creator of a stream to indicate that the
      stream is no longer needed.

      6 - INTERNAL_ERROR.  This is a generic error which can be used
      when the implementation has internally failed, not due to anything
      in the protocol.

      7 - FLOW_CONTROL_ERROR.  The endpoint detected that its peer
      violated the flow control protocol.

      8 - STREAM_IN_USE.  The endpoint received a SYN_REPLY for a stream
      already open.

      9 - STREAM_ALREADY_CLOSED.  The endpoint received a data or
      SYN_REPLY frame for a stream which is half closed.

      10 - INVALID_CREDENTIALS.  The server received a request for a
      resource whose origin does not have valid credentials in the
      client certificate vector.

      11 - FRAME_TOO_LARGE.  The endpoint received a frame which this
      implementation could not support.  If FRAME_TOO_LARGE is sent for
      a SYN_STREAM, HEADERS, or SYN_REPLY frame without fully processing
      the compressed portion of those frames, then the compression state
      will be out-of-sync with the other endpoint.  In this case,
      senders of FRAME_TOO_LARGE MUST close the session.

      Note: 0 is not a valid status code for a RST_STREAM.

   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.

3.6.4.  SETTINGS

   A SETTINGS frame contains a set of id/value pairs for communicating
   configuration data about how the two endpoints may communicate.
   SETTINGS frames can be sent at any time by either endpoint, are
   optionally sent, and are fully asynchronous.  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.



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   Persistence of SETTINGS ID/Value pairs is done on a per origin/IP
   pair (the "origin" is the set of scheme, host, and port from the URI.
   See [RFC6454]).  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.

   +----------------------------------+
   |1|   version    |         4       |
   +----------------------------------+
   | Flags (8)  |  Length (24 bits)   |
   +----------------------------------+
   |         Number of entries        |
   +----------------------------------+
   |          ID/Value Pairs          |
   |             ...                  |

   Control bit: The control bit is always 1 for this message.

   Version: The HTTP/2.0 version number.

   Type: The message type for a SETTINGS message is 4.

   Flags: FLAG_SETTINGS_CLEAR_SETTINGS (0x1): When set, the client
   should clear any previously persisted SETTINGS ID/Value pairs.  If
   this frame contains ID/Value pairs with the
   FLAG_SETTINGS_PERSIST_VALUE set, then the client will first clear its
   existing, persisted settings, and then persist the values with the
   flag set which are contained within this frame.  Because persistence
   is only implemented on the client, this flag can only be used when
   the sender is the server.

   Length: An unsigned 24-bit value representing the number of bytes
   after the length field.  The total size of a SETTINGS frame is 8
   bytes + length.

   Number of entries: A 32-bit value representing the number of ID/value
   pairs in this message.

   ID: A 32-bit ID number, comprised of 8 bits of flags and 24 bits of
   unique ID.

      ID.flags:

         FLAG_SETTINGS_PERSIST_VALUE (0x1): When set, the sender of this
         SETTINGS frame is requesting that the recipient persist the ID/



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         Value and return it in future SETTINGS frames sent from the
         sender to this recipient.  Because persistence is only
         implemented on the client, this flag is only sent by the
         server.

         FLAG_SETTINGS_PERSISTED (0x2): When set, the sender is
         notifying the recipient that this ID/Value pair was previously
         sent to the sender by the recipient with the
         FLAG_SETTINGS_PERSIST_VALUE, and the sender is returning it.
         Because persistence is only implemented on the client, this
         flag is only sent by the client.

      Defined IDs:

         1 - SETTINGS_UPLOAD_BANDWIDTH 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.

         2 - SETTINGS_DOWNLOAD_BANDWIDTH 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.

         3 - SETTINGS_ROUND_TRIP_TIME 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.

         4 - SETTINGS_MAX_CONCURRENT_STREAMS allows the sender to inform
         the remote endpoint the maximum number of concurrent streams
         which it will allow.  By default there is no limit.  For
         implementors it is recommended that this value be no smaller
         than 100.

         5 - SETTINGS_CURRENT_CWND allows the sender to inform the
         remote endpoint of the current TCP CWND value.

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

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



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         8 - SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE allows the server
         to inform the client of the new size of the client certificate
         vector.

   Value: A 32-bit value.

   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
   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.

3.6.5.  PING

   The PING control frame is a mechanism for measuring a minimal round-
   trip time from the sender.  It can be sent from the client or the
   server.  Recipients of a PING frame should send an identical frame to
   the sender as soon as possible (if there is other pending data
   waiting to be sent, PING should take highest priority).  Each ping
   sent by a sender should use a unique ID.

   +----------------------------------+
   |1|   version    |         6       |
   +----------------------------------+
   | 0 (flags) |     4 (length)       |
   +----------------------------------|
   |            32-bit ID             |
   +----------------------------------+

   Control bit: The control bit is always 1 for this message.

   Version: The HTTP/2.0 version number.

   Type: The message type for a PING message is 6.

   Length: This frame is always 4 bytes long.



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   ID: A unique ID for this ping, represented as an unsigned 32 bit
   value.  When the client initiates a ping, it must use an odd numbered
   ID.  When the server initiates a ping, it must use an even numbered
   ping.  Use of odd/even IDs is required in order to avoid accidental
   looping on PINGs (where each side initiates an identical PING at the
   same time).

   Note: If a sender uses all possible PING ids (e.g. has sent all 2^31
   possible IDs), it can wrap and start re-using IDs.

   If a server receives an even numbered PING which it did not initiate,
   it must ignore the PING.  If a client receives an odd numbered PING
   which it did not initiate, it must ignore the PING.

3.6.6.  GOAWAY

   The GOAWAY control frame is a mechanism to tell 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 not
   respond to any new SYN_STREAMs on this session.  Recipients of a
   GOAWAY frame must not send additional streams on this 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 sending
   SYN_STREAMs and the remote sending a GOAWAY message.  To deal with
   this case, the GOAWAY contains a last-stream-id indicating the
   stream-id of the last stream which was created on the sending
   endpoint in this session.  If the receiver of the GOAWAY sent new
   SYN_STREAMs for sessions after this last-stream-id, they were not
   processed by the server and the receiver may treat the stream as
   though it had never been created at all (hence the receiver may want
   to re-create the stream 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
   server does not send a GOAWAY frame to indicate where it stopped
   working).

   After sending a GOAWAY message, the sender must ignore all SYN_STREAM
   frames for new streams.





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   +----------------------------------+
   |1|   version    |         7       |
   +----------------------------------+
   | 0 (flags) |     8 (length)       |
   +----------------------------------|
   |X|  Last-good-stream-ID (31 bits) |
   +----------------------------------+
   |          Status code             |
   +----------------------------------+

   Control bit: The control bit is always 1 for this message.

   Version: The HTTP/2.0 version number.

   Type: The message type for a GOAWAY message is 7.

   Length: This frame is always 8 bytes long.

   Last-good-stream-Id: The last stream id which was replied to (with
   either a SYN_REPLY or RST_STREAM) by the sender of the GOAWAY
   message.  If no streams were replied to, this value MUST be 0.

   Status: The reason for closing the session.

      0 - OK.  This is a normal session teardown.

      1 - PROTOCOL_ERROR.  This is a generic error, and should only be
      used if a more specific error is not available.

      2 - INTERNAL_ERROR.  This is a generic error which can be used
      when the implementation has internally failed, not due to anything
      in the protocol.

3.6.7.  HEADERS

   The HEADERS frame augments a stream with additional headers.  It may
   be optionally sent on an existing stream at any time.  Specific
   application of the headers in this frame is application-dependent.
   The name/value header block within this frame is compressed.












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   +------------------------------------+
   |1|   version     |          8       |
   +------------------------------------+
   | Flags (8)  |   Length (24 bits)    |
   +------------------------------------+
   |X|          Stream-ID (31bits)      |
   +------------------------------------+
   | Number of Name/Value pairs (int32) |   <+
   +------------------------------------+    |
   |     Length of name (int32)         |    | This section is the
   +------------------------------------+    | "Name/Value Header
   |           Name (string)            |    | Block", and is
   +------------------------------------+    | compressed.
   |     Length of value  (int32)       |    |
   +------------------------------------+    |
   |          Value   (string)          |    |
   +------------------------------------+    |
   |           (repeats)                |   <+

   Flags: Flags related to this frame.  Valid flags are:

      0x01 = FLAG_FIN - marks this frame as the last frame to be
      transmitted on this stream and puts the sender in the half-closed
      (Section 3.3.6) state.

   Length: An unsigned 24 bit value representing the number of bytes
   after the length field.  The minimum length of the length field is 4
   (when the number of name value pairs is 0).

   Stream-ID: The stream this HEADERS block is associated with.

   Name/Value Header Block: A set of name/value pairs carried as part of
   the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).

3.6.8.  WINDOW_UPDATE

   The WINDOW_UPDATE control frame is used to implement per stream flow
   control in HTTP/2.0.  Flow control in HTTP/2.0 is per hop, that is,
   only between the two endpoints of a HTTP/2.0 connection.  If there
   are one or more intermediaries between the client and the origin
   server, flow control signals are not explicitly forwarded by the
   intermediaries.  (However, throttling of data transfer by any
   recipient may have the effect of indirectly propagating flow control
   information upstream back to the original sender.)  Flow control only
   applies to the data portion of data frames.  Recipients must buffer
   all control frames.  If a recipient fails to buffer an entire control
   frame, it MUST issue a stream error (Section 3.4.2) with the status
   code FLOW_CONTROL_ERROR for the stream.



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   Flow control in HTTP/2.0 is implemented by a data transfer window
   kept by the sender of each stream.  The data transfer window is a
   simple uint32 that indicates how many bytes of data the sender can
   transmit.  After a stream is created, but before any data frames have
   been transmitted, the sender begins with the initial window size.
   This window size is a measure of the buffering capability of the
   recipient.  The sender must not send a data frame with data length
   greater than the transfer window size.  After sending each data
   frame, the sender decrements its transfer window size by the amount
   of data transmitted.  When the window size becomes less than or equal
   to 0, the sender must pause transmitting data frames.  At the other
   end of the stream, the recipient sends a WINDOW_UPDATE control back
   to notify the sender that it has consumed some data and freed up
   buffer space to receive more data.

   +----------------------------------+
   |1|   version    |         9       |
   +----------------------------------+
   | 0 (flags) |     8 (length)       |
   +----------------------------------+
   |X|     Stream-ID (31-bits)        |
   +----------------------------------+
   |X|  Delta-Window-Size (31-bits)   |
   +----------------------------------+

   Control bit: The control bit is always 1 for this message.

   Version: The HTTP/2.0 version number.

   Type: The message type for a WINDOW_UPDATE message is 9.

   Length: The length field is always 8 for this frame (there are 8
   bytes after the length field).

   Stream-ID: The stream ID that this WINDOW_UPDATE control frame is
   for.

   Delta-Window-Size: The additional number of bytes that the sender can
   transmit in addition to existing remaining window size.  The legal
   range for this field is 1 to 2^31 - 1 (0x7fffffff) bytes.

   The window size as kept by the sender must never exceed 2^31
   (although it can become negative in one special case).  If a sender
   receives a WINDOW_UPDATE that causes the its window size to exceed
   this limit, it must send RST_STREAM with status code
   FLOW_CONTROL_ERROR to terminate the stream.

   When a HTTP/2.0 connection is first established, the default initial



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   window size for all streams is 64KB.  An endpoint can use the
   SETTINGS control frame to adjust the initial window size for the
   connection.  That is, its peer can start out using the 64KB default
   initial window size when sending data frames before receiving the
   SETTINGS.  Because SETTINGS is asynchronous, there may be a race
   condition if the recipient wants to decrease the initial window size,
   but its peer immediately sends 64KB on the creation of a new
   connection, before waiting for the SETTINGS to arrive.  This is one
   case where the window size kept by the sender will become negative.
   Once the sender detects this condition, it must stop sending data
   frames and wait for the recipient to catch up.  The recipient has two
   choices:

      immediately send RST_STREAM with FLOW_CONTROL_ERROR status code.

      allow the head of line blocking (as there is only one stream for
      the session and the amount of data in flight is bounded by the
      default initial window size), and send WINDOW_UPDATE as it
      consumes data.

   In the case of option 2, both sides must compute the window size
   based on the initial window size in the SETTINGS.  For example, if
   the recipient sets the initial window size to be 16KB, and the sender
   sends 64KB immediately on connection establishment, the sender will
   discover its window size is -48KB on receipt of the SETTINGS.  As the
   recipient consumes the first 16KB, it must send a WINDOW_UPDATE of
   16KB back to the sender.  This interaction continues until the
   sender's window size becomes positive again, and it can resume
   transmitting data frames.

   After the recipient reads in a data frame with FLAG_FIN that marks
   the end of the data stream, it should not send WINDOW_UPDATE frames
   as it consumes the last data frame.  A sender should ignore all the
   WINDOW_UPDATE frames associated with the stream after it send the
   last frame for the stream.

   The data frames from the sender and the WINDOW_UPDATE frames from the
   recipient are completely asynchronous with respect to each other.
   This property allows a recipient to aggressively update the window
   size kept by the sender to prevent the stream from stalling.

3.6.9.  CREDENTIAL

   The CREDENTIAL control frame is used by the client to send additional
   client certificates to the server.  A HTTP/2.0 client may decide to
   send requests for resources from different origins on the same
   HTTP/2.0 session if it decides that that server handles both origins.
   For example if the IP address associated with both hostnames matches



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   and the SSL server certificate presented in the initial handshake is
   valid for both hostnames.  However, because the SSL connection can
   contain at most one client certificate, the client needs a mechanism
   to send additional client certificates to the server.

   The server is required to maintain a vector of client certificates
   associated with a HTTP/2.0 session.  When the client needs to send a
   client certificate to the server, it will send a CREDENTIAL frame
   that specifies the index of the slot in which to store the
   certificate as well as proof that the client posesses the
   corresponding private key.  The initial size of this vector must be
   8.  If the client provides a client certificate during the first TLS
   handshake, the contents of this certificate must be copied into the
   first slot (index 1) in the CREDENTIAL vector, though it may be
   overwritten by subsequent CREDENTIAL frames.  The server must
   exclusively use the CREDENTIAL vector when evaluating the client
   certificates associated with an origin.  The server may change the
   size of this vector by sending a SETTINGS frame with the setting
   SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE value specified.  In the
   event that the new size is smaller than the current size, truncation
   occurs preserving lower-index slots as possible.

   TLS renegotiation with client authentication is incompatible with
   HTTP/2.0 given the multiplexed nature of HTTP/2.0.  Specifically,
   imagine that the client has 2 requests outstanding to the server for
   two different pages (in different tabs).  When the renegotiation +
   client certificate request comes in, the browser is unable to
   determine which resource triggered the client certificate request, in
   order to prompt the user accordingly.

   +----------------------------------+
   |1|000000000000001|0000000000001011|
   +----------------------------------+
   | flags (8)  |  Length (24 bits)   |
   +----------------------------------+
   |  Slot (16 bits) |                |
   +-----------------+                |
   |      Proof Length (32 bits)      |
   +----------------------------------+
   |               Proof              |
   +----------------------------------+ <+
   |   Certificate Length (32 bits)   |  |
   +----------------------------------+  | Repeated until end of frame
   |            Certificate           |  |
   +----------------------------------+ <+

   Slot: The index in the server's client certificate vector where this
   certificate should be stored.  If there is already a certificate



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   stored at this index, it will be overwritten.  The index is one
   based, not zero based; zero is an invalid slot index.

   Proof: Cryptographic proof that the client has possession of the
   private key associated with the certificate.  The format is a TLS
   digitally-signed element ([RFC5246], Section 4.7).  The signature
   algorithm must be the same as that used in the CertificateVerify
   message.  However, since the MD5+SHA1 signature type used in TLS 1.0
   connections can not be correctly encoded in a digitally-signed
   element, SHA1 must be used when MD5+SHA1 was used in the SSL
   connection.  The signature is calculated over a 32 byte TLS extractor
   value (http://tools.ietf.org/html/rfc5705) with a label of "EXPORTER
   HTTP/2.0 certificate proof" using the empty string as context.
   ForRSA certificates the signature would be a PKCS#1 v1.5 signature.
   For ECDSA, it would be an ECDSA-Sig-Value
   (http://tools.ietf.org/html/rfc5480#appendix-A).  For a 1024-bit RSA
   key, the CREDENTIAL message would be ~500 bytes.

   Certificate: The certificate chain, starting with the leaf
   certificate.  Each certificate must be encoded as a 32 bit length,
   followed by a DER encoded certificate.  The certificate must be of
   the same type (RSA, ECDSA, etc) as the client certificate associated
   with the SSL connection.

   If the server receives a request for a resource with unacceptable
   credential (either missing or invalid), it must reply with a
   RST_STREAM frame with the status code INVALID_CREDENTIALS.  Upon
   receipt of a RST_STREAM frame with INVALID_CREDENTIALS, the client
   should initiate a new stream directly to the requested origin and
   resend the request.  Note, HTTP/2.0 does not allow the server to
   request different client authentication for different resources in
   the same origin.

   If the server receives an invalid CREDENTIAL frame, it MUST respond
   with a GOAWAY frame and shutdown the session.

3.6.10.  Name/Value Header Block

   The Name/Value Header Block is found in the SYN_STREAM, SYN_REPLY and
   HEADERS control frames, and shares a common format:











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   +------------------------------------+
   | Number of Name/Value pairs (int32) |
   +------------------------------------+
   |     Length of name (int32)         |
   +------------------------------------+
   |           Name (string)            |
   +------------------------------------+
   |     Length of value  (int32)       |
   +------------------------------------+
   |          Value   (string)          |
   +------------------------------------+
   |           (repeats)                |

   Number of Name/Value pairs: The number of repeating name/value pairs
   following this field.

   List of Name/Value pairs:

      Length of Name: a 32-bit value containing the number of octets in
      the name field.  Note that in practice, this length must not
      exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.

      Name: 0 or more octets, 8-bit sequences of data, excluding 0.

      Length of Value: a 32-bit value containing the number of octets in
      the value field.  Note that in practice, this length must not
      exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.

      Value: 0 or more octets, 8-bit sequences of data, excluding 0.

   Each header name must have at least one value.  Header names are
   encoded using the US-ASCII character set [ASCII] and must be all
   lower case.  The length of each name must be greater than zero.  A
   recipient of a zero-length name MUST issue a stream error
   (Section 3.4.2) with the status code PROTOCOL_ERROR for the
   stream-id.

   Duplicate header names are not allowed.  To send two identically
   named headers, send a header with two values, where the values are
   separated by a single NUL (0) byte.  A header value can either be
   empty (e.g. the length is zero) or it can contain multiple, NUL-
   separated values, each with length greater than zero.  The value
   never starts nor ends with a NUL character.  Recipients of illegal
   value fields MUST issue a stream error (Section 3.4.2) with the
   status code PROTOCOL_ERROR for the stream-id.






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3.6.10.1.  Compression

   The Name/Value Header Block is a section of the SYN_STREAM,
   SYN_REPLY, and HEADERS frames used to carry header meta-data.  This
   block is always compressed using zlib compression.  Within this
   specification, any reference to 'zlib' is referring to the ZLIB
   Compressed Data Format Specification Version 3.3 as part of RFC1950.
   [RFC1950]

   For each HEADERS compression instance, the initial state is
   initialized using the following dictionary [UDELCOMPRESSION]:

   <CODE BEGINS>

   const unsigned char http2_dictionary_txt[] = {
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     0x00, 0x00, 0x00, 0x0e, 0x63, 0x6f, 0x6e, 0x74,  \\ - - - - c o n t



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     0x65, 0x6e, 0x74, 0x2d, 0x6c, 0x65, 0x6e, 0x67,  \\ e n t - l e n g
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     0x61, 0x6e, 0x73, 0x66, 0x65, 0x72, 0x2d, 0x65,  \\ a n s f e r - e
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     0x31, 0x20, 0x55, 0x6e, 0x61, 0x75, 0x74, 0x68,  \\ 1 - U n a u t h



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     0x6f, 0x72, 0x69, 0x7a, 0x65, 0x64, 0x34, 0x30,  \\ o r i z e d 4 0
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     0x69, 0x66, 0x2c, 0x61, 0x70, 0x70, 0x6c, 0x69,  \\ i f - a p p l i
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     0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x78,  \\ c a t i o n - x
     0x68, 0x74, 0x6d, 0x6c, 0x2b, 0x78, 0x6d, 0x6c,  \\ h t m l - x m l
     0x2c, 0x74, 0x65, 0x78, 0x74, 0x2f, 0x70, 0x6c,  \\ - t e x t - p l
     0x61, 0x69, 0x6e, 0x2c, 0x74, 0x65, 0x78, 0x74,  \\ a i n - t e x t
     0x2f, 0x6a, 0x61, 0x76, 0x61, 0x73, 0x63, 0x72,  \\ - j a v a s c r
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     0x69, 0x63, 0x70, 0x72, 0x69, 0x76, 0x61, 0x74,  \\ i c p r i v a t
     0x65, 0x6d, 0x61, 0x78, 0x2d, 0x61, 0x67, 0x65,  \\ e m a x - a g e
     0x3d, 0x67, 0x7a, 0x69, 0x70, 0x2c, 0x64, 0x65,  \\ - g z i p - d e
     0x66, 0x6c, 0x61, 0x74, 0x65, 0x2c, 0x73, 0x64,  \\ f l a t e - s d
     0x63, 0x68, 0x63, 0x68, 0x61, 0x72, 0x73, 0x65,  \\ c h c h a r s e
     0x74, 0x3d, 0x75, 0x74, 0x66, 0x2d, 0x38, 0x63,  \\ t - u t f - 8 c
     0x68, 0x61, 0x72, 0x73, 0x65, 0x74, 0x3d, 0x69,  \\ h a r s e t - i
     0x73, 0x6f, 0x2d, 0x38, 0x38, 0x35, 0x39, 0x2d,  \\ s o - 8 8 5 9 -
     0x31, 0x2c, 0x75, 0x74, 0x66, 0x2d, 0x2c, 0x2a,  \\ 1 - u t f - - -



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     0x2c, 0x65, 0x6e, 0x71, 0x3d, 0x30, 0x2e         \\ - e n q - 0 -
   };

   <CODE ENDS>

   The entire contents of the name/value header block is compressed
   using zlib.  There is a single zlib stream for all name value pairs
   in one direction on a connection.  HTTP/2.0 uses a SYNC_FLUSH between
   each compressed frame.

   Implementation notes: the compression engine can be tuned to favor
   speed or size.  Optimizing for size increases memory use and CPU
   consumption.  Because header blocks are generally small, implementors
   may want to reduce the window-size of the compression engine from the
   default 15bits (a 32KB window) to more like 11bits (a 2KB window).
   The exact setting is chosen by the compressor, the decompressor will
   work with any setting.

4.  HTTP Layering over HTTP/2.0

   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 the
   HTTP/1.1 specification in RFC2616 [RFC2616] 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.

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
   (a new SYN_STREAM) 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.




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   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 determinstically 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 any
   SYN_STREAMs from the remote, the GOAWAY will contain a last-stream-id
   of 0.

4.2.  HTTP Request/Response

4.2.1.  Request

   The client initiates a request by sending a SYN_STREAM frame.  For
   requests which do not contain a body, the SYN_STREAM frame MUST set
   the FLAG_FIN, indicating that the client intends to send no further
   data on this stream.  For requests which do contain a body, the
   SYN_STREAM will not contain the FLAG_FIN, and the body will follow
   the SYN_STREAM in a series of DATA frames.  The last DATA frame will
   set the FLAG_FIN to indicate the end of the body.

   The SYN_STREAM Name/Value section will contain all of the HTTP
   headers which 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 first line of the request is unfolded into name/value pairs
      like other HTTP headers and MUST be present:

         ":method" - the HTTP method for this request (e.g.  "GET",
         "POST", "HEAD", etc)

         ":path" - the url-path for this url with "/" prefixed.  (See
         RFC1738 [RFC1738]).  For example, for
         "http://www.google.com/search?q=dogs" the path would be
         "/search?q=dogs".



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         ":version" - the HTTP version of this request (e.g.
         "HTTP/1.1")

      In addition, the following two name/value pairs must also be
      present in every request:

         ":host" - the hostport (See RFC1738 [RFC1738]) portion of the
         URL for this request (e.g. "www.google.com:1234").  This header
         is the same as the HTTP 'Host' header.

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

      Header names are all lowercase.

      The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-
      Encoding headers 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.

      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, the server MUST return a 400 (Bad Request) error.

      POST-specific changes:

         Although POSTs are inherently chunked, POST requests SHOULD
         also be accompanied by a Content-Length header.  There are two
         reasons for this: First, it assists with upload progress meters
         for an improved user experience.  But second, we know from
         early versions of HTTP/2.0 that failure to send a content
         length header is incompatible with many existing HTTP server
         implementations.  Existing user-agents do not omit the Content-
         Length header, and server implementations have come to depend
         upon this.

   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 SYN_STREAM without all of the method, host, path,
   scheme, and version headers, the server MUST reply with a HTTP 400
   Bad Request reply.





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4.2.2.  Response

   The server responds to a client request with a SYN_REPLY frame.
   Symmetric to the client's upload stream, server will send data after
   the SYN_REPLY frame via a series of DATA frames, and the last data
   frame will contain the FLAG_FIN to indicate successful end-of-stream.
   If a response (like a 202 or 204 response) contains no body, the
   SYN_REPLY frame may contain the FLAG_FIN 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 headers and must be present:

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

         ":version" - The HTTP response version (e.g.  "HTTP/1.1")

      All header names must be lowercase.

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

      Responses MAY be accompanied by a Content-Length header for
      advisory purposes. (e.g. for UI progress meters)

      If a client receives a response where the sum of the data frame
      payload lengths does not equal the size of the Content-Length
      header, the client MUST ignore the content length header.

   If a client receives a SYN_REPLY without a status or without a
   version header, the client must reply with a RST_STREAM frame
   indicating a PROTOCOL ERROR.

4.2.3.  Authentication

   When a client sends a request to an origin server that requires
   authentication, the server can reply with a "401 Unauthorized"
   response, and include a WWW-Authenticate challenge header that
   defines the authentication scheme to be used.  The client then
   retries the request with an Authorization header appropriate to the
   specified authentication scheme.

   There are four options for proxy authentication, Basic, Digest, NTLM
   and Negotiate (SPNEGO).  The first two options were defined in
   RFC2617 [RFC2617], and are stateless.  The second two options were
   developed by Microsoft and specified in RFC4559 [RFC4559], and are
   stateful; otherwise known as multi-round authentication, or



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   connection authentication.

4.2.3.1.  Stateless Authentication

   Stateless Authentication over HTTP/2.0 is identical to how it is
   performed over HTTP.  If multiple HTTP/2.0 streams are concurrently
   sent to a single server, each will authenticate independently,
   similar to how two HTTP connections would independently authenticate
   to a proxy server.

4.2.3.2.  Stateful Authentication

   Unfortunately, the stateful authentication mechanisms were
   implemented and defined in a such a way that directly violates
   RFC2617 - they do not include a "realm" as part of the request.  This
   is problematic in HTTP/2.0 because it makes it impossible for a
   client to disambiguate two concurrent server authentication
   challenges.

   To deal with this case, HTTP/2.0 servers using Stateful
   Authentication MUST implement one of two changes:

      Servers can add a "realm=<desired realm>" header so that the two
      authentication requests can be disambiguated and run concurrently.
      Unfortunately, given how these mechanisms work, this is probably
      not practical.

      Upon sending the first stateful challenge response, the server
      MUST buffer and defer all further frames which are not part of
      completing the challenge until the challenge has completed.
      Completing the authentication challenge may take multiple round
      trips.  Once the client receives a "401 Authenticate" response for
      a stateful authentication type, it MUST stop sending new requests
      to the server until the authentication has completed by receiving
      a non-401 response on at least one stream.

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.



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   Browsers receiving a pushed response MUST validate that the server is
   authorized to push the URL using the browser same-origin [RFC6454]
   policy.  For example, a HTTP/2.0 connection to www.foo.com is
   generally not permitted to push a response for www.evil.com.

   If the browser accepts a pushed response (e.g. it does not send a
   RST_STREAM), the browser MUST attempt to cache the pushed response in
   same way that it would cache any other response.  This means
   validating the response headers and inserting into the disk cache.

   Because pushed responses have no request, they have no request
   headers associated with them.  At the framing layer, HTTP/2.0 pushed
   streams contain an "associated-stream-id" which indicates the
   requested stream for which the pushed stream is related.  The pushed
   stream inherits all of the headers from the associated-stream-id with
   the exception of ":host", ":scheme", and ":path", which are provided
   as part of the pushed response stream headers.  The browser MUST
   store these inherited and implied request headers 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.

4.3.1.  Server implementation

   When the server intends to push a resource to the user-agent, it
   opens a new stream by sending a unidirectional SYN_STREAM.  The
   SYN_STREAM MUST include an Associated-To-Stream-ID, and MUST set the
   FLAG_UNIDIRECTIONAL flag.  The SYN_STREAM MUST include headers for
   ":scheme", ":host", ":path", which represent the URL for the resource
   being pushed.  Subsequent headers may follow in HEADERS frames.  The
   purpose of the association is so that the user-agent can
   differentiate which request induced the pushed stream; without it, if
   the user-agent had two tabs open to the same page, each pushing
   unique content under a fixed URL, the user-agent would not be able to
   differentiate the requests.

   The Associated-To-Stream-ID must be the ID of an existing, open
   stream.  The reason for this restriction is to have a clear endpoint
   for pushed content.  If the user-agent requested a resource on stream
   11, the server replies on stream 11.  It can push any number of
   additional streams to the client before sending a FLAG_FIN on stream
   11.  However, once the originating stream is closed no further push
   streams may be associated with it.  The pushed streams do not need to
   be closed (FIN set) before the originating stream is closed, they
   only need to be created before the originating stream closes.



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   It is illegal for a server to push a resource with the Associated-To-
   Stream-ID of 0.

   To minimize race conditions with the client, the SYN_STREAM for the
   pushed resources MUST be sent prior to sending any content which
   could allow the client to discover the pushed resource and request
   it.

   The server MUST only push resources which would have been returned
   from a GET request.

   Note: If the server does not have all of the Name/Value Response
   headers available at the time it issues the HEADERS frame for the
   pushed resource, it may later use an additional HEADERS frame to
   augment the name/value pairs to be associated with the pushed stream.
   The subsequent HEADERS frame(s) must not contain a header for
   ':host', ':scheme', or ':path' (e.g. the server can't change the
   identity of the resource to be pushed).  The HEADERS frame must not
   contain duplicate headers with a previously sent HEADERS frame.  The
   server must send a HEADERS frame including the scheme/host/port
   headers before sending any data frames on the stream.

4.3.2.  Client implementation

   When fetching a resource the client has 3 possibilities:

      the resource is not being pushed

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

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

   When a SYN_STREAM and HEADERS frame which contains an Associated-To-
   Stream-ID is received, the client must not issue GET requests for the
   resource in the pushed stream, and instead wait for the pushed stream
   to arrive.

   If a client receives a server push stream with stream-id 0, it MUST
   issue a session error (Section 3.4.1) with the status code
   PROTOCOL_ERROR.

   When a client receives a SYN_STREAM from the server without a the
   ':host', ':scheme', and ':path' headers in the Name/Value section, it
   MUST reply with a RST_STREAM with error code HTTP_PROTOCOL_ERROR.

   To cancel individual server push streams, the client can issue a
   stream error (Section 3.4.2) with error code CANCEL.  Upon receipt,
   the server MUST stop sending on this stream immediately (this is an



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   Abrupt termination).

   To cancel all server push streams related to a request, the client
   may issue a stream error (Section 3.4.2) with error code 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.

   If the server sends a HEADER frame containing duplicate headers with
   a previous HEADERS frame for the same stream, the client must issue a
   stream error (Section 3.4.2) with error code 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 headers
   (http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.40).

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, the definition of the HEADERS and compression contexts 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



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   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 occuring 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.

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 choses 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



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   interactions, we believe this is completely mitigated.

5.5.  Compression Context(s)

   When isolating the compression contexts used for communicating with
   multiple origins, we had a few choices to make.  We could have
   maintained a map (or list) of compression contexts usable for each
   origin.  The basic case is easy - each HEADERS frame would need to
   identify the context to use for that frame.  However, compression
   contexts are not cheap, so the lifecycle of each context would need
   to be bounded.  For proxy servers, where we could churn through many
   contexts, this would be a concern.  We considered using a static set
   of contexts, say 16 of them, which would bound the memory use.  We
   also considered dynamic contexts, which could be created on the fly,
   and would need to be subsequently destroyed.  All of these are
   complicated, and ultimately we decided that such a mechanism creates
   too many problems to solve.

   Alternatively, we've chosen the simple approach, which is to simply
   provide a flag for resetting the compression context.  For the common
   case (no proxy), this fine because most requests are to the same
   origin and we never need to reset the context.  For cases where we
   are using two different origins over a single HTTP/2.0 session, we
   simply reset the compression state between each transition.

5.6.  Unidirectional streams

   Many readers notice that unidirectional streams are both a bit
   confusing in concept and also somewhat redundant.  If the recipient
   of a stream doesn't wish to send data on a stream, it could simply
   send a SYN_REPLY with the FLAG_FIN bit set.  The FLAG_UNIDIRECTIONAL
   is, therefore, not necessary.

   It is true that we don't need the UNIDIRECTIONAL markings.  It is
   added because it avoids the recipient of pushed streams from needing
   to send a set of empty frames (e.g. the SYN_STREAM w/ FLAG_FIN) which
   otherwise serve no purpose.

5.7.  Data Compression

   Generic compression of data portion of the streams (as opposed to
   compression of the headers) without knowing the content of the stream
   is redundant.  There is no value in compressing a stream which is
   already compressed.  Because of this, HTTP/2.0 does allow data
   compression to be optional.  We included it because study of existing
   websites shows that many sites are not using compression as they
   should, and users suffer because of it.  We wanted a mechanism where,
   at the HTTP/2.0 layer, site administrators could simply force



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   compression - it is better to compress twice than to not compress.

   Overall, however, with this feature being optional and sometimes
   redundant, it is unclear if it is useful at all.  We will likely
   remove it from the specification.

5.8.  Server Push

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

6.  Security Considerations

6.1.  Use of Same-origin constraints

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

6.2.  HTTP Headers and HTTP/2.0 Headers

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

6.3.  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.

6.4.  Server Push Implicit Headers

   Pushed resources do not have an associated request.  In order for
   existing HTTP cache control validations (such as the Vary header) to
   work, however, all cached resources must have a set of request
   headers.  For this reason, browsers MUST be careful to inherit
   request headers from the associated stream for the push.  This
   includes the 'Cookie' header.



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7.  Privacy Considerations

7.1.  Long Lived Connections

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

7.2.  SETTINGS frame

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

   Clients implementing privacy modes, such as Google Chrome's
   "incognito mode", may wish to disable client-persisted SETTINGS
   storage.

   Clients MUST clear persisted SETTINGS information when clearing the
   cookies.

   TODO: Put range maximums on each type of setting to limit
   inappropriate uses.

8.  Requirements Notation

   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].

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).





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   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 principles)

   o  Mark Nottingham and Julian Reschke

10.  Normative References

   [ASCII]            "US-ASCII. Coded Character Set - 7-Bit American
                      Standard Code for Information Interchange.
                      Standard ANSI X3.4-1986, ANSI, 1986.".

   [HTTP-p1]          Fielding, R. and J. Reschke, "Hypertext Transfer
                      Protocol (HTTP/1.1): Message Syntax and Routing",
                      draft-ietf-httpbis-p1-messaging-21 (work in
                      progress), October 2012.

   [HTTP-p2]          Fielding, R. and J. Reschke, "Hypertext Transfer
                      Protocol (HTTP/1.1): Semantics and Content",
                      draft-ietf-httpbis-p2-semantics-21 (work in
                      progress), October 2012.

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

   [RFC1738]          Berners-Lee, T., Masinter, L., and M. McCahill,
                      "Uniform Resource Locators (URL)", RFC 1738,
                      December 1994.

   [RFC1950]          Deutsch, L. and J. Gailly, "ZLIB Compressed Data
                      Format Specification version 3.3", RFC 1950,
                      May 1996.

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

   [RFC2616]          Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                      Masinter, L., Leach, P., and T. Berners-Lee,
                      "Hypertext Transfer Protocol -- HTTP/1.1",
                      RFC 2616, June 1999.

   [RFC2617]          Franks, J., Hallam-Baker, P., Hostetler, J.,
                      Lawrence, S., Leach, P., Luotonen, A., and L.
                      Stewart, "HTTP Authentication: Basic and Digest
                      Access Authentication", RFC 2617, June 1999.




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   [RFC4559]          Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-
                      based Kerberos and NTLM HTTP Authentication in
                      Microsoft Windows", RFC 4559, June 2006.

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

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

   [TLSNPN]           Langley, A., "TLS Next Protocol Negotiation",
                      draft-agl-tls-nextprotoneg-01 (work in progress),
                      August 2010.

   [UDELCOMPRESSION]  Yang, F., Amer, P., and J. Leighton, "A
                      Methodology to Derive SPDY's Initial Dictionary
                      for Zlib Compression", <http://www.eecis.udel.edu/
                      ~amer/PEL/poc/pdf/SPDY-Fan.pdf>.

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

A.1.  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 3.5.1) based on <http://
   tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>.

A.2.  Since draft-mbelshe-httpbis-spdy-00

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

   Updated authors/editors list.

   Added status note.




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Authors' Addresses

   Mike Belshe
   Twist

   EMail: mbelshe@chromium.org


   Roberto Peon
   Google, Inc

   EMail: fenix@google.com


   Martin Thomson (editor)
   Microsoft
   3210 Porter Drive
   Palo Alto  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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