HTTPbis Working Group                                          M. Belshe
Internet-Draft                                                     Twist
Intended status: Standards Track                                 R. Peon
Expires: October 5, November 30, 2013                                   Google, Inc
                                                         M. Thomson, Ed.
                                                               Microsoft
                                                        A. Melnikov, Ed.
                                                               Isode Ltd
                                                           April 3,
                                                            May 29, 2013

                Hypertext Transfer Protocol version 2.0
                      draft-ietf-httpbis-http2-02
                      draft-ietf-httpbis-http2-03

Abstract

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

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

Editorial Note (To be removed by RFC Editor)

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

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

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

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on October 5, November 30, 2013.

Copyright Notice

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

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Document Organization  . . . . . . . . . . . . . . . . . .  5
     1.2.  Conventions and Terminology  . . . . . . . . . . . . . . .  6
   2.  Starting HTTP/2.0  . . . . . . . . . . . . . . . . . . . . . .  7  6
     2.1.  HTTP/2.0 Version Identification  . . . . . . . . . . . . .  7
     2.2.  Starting HTTP/2.0 for "http:" URIs . . . . . . . . . . . .  8
     2.3.  Starting HTTP/2.0 for "https:" URIs  . . . . . . . . . . .  8
     2.4.  Starting HTTP/2.0 with Prior Knowledge . . . . . . . . . .  9
   3.  HTTP/2.0 Framing Layer . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Session  .  Connection . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Session  Connection Header  . . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.3.1.  Frame Header . . . . . . . . . . . . . . . . . . . . . 10
       3.3.2.  Frame Processing Size . . . . . . . . . . . . . . . . . . . 11 . . . 12
     3.4.  Streams  . . . . . . . . . . . . . . . . . . . . . . . . . 11 12
       3.4.1.  Stream Creation  . . . . . . . . . . . . . . . . . . . 12 13
       3.4.2.  Stream priority  . . . . . . . . . . . . . . . . . . . 12
       3.4.3.  Stream headers . . . . . . . . . . . . . . . . . . . . 13
       3.4.4.  Stream data exchange . . . . . . . . . . . . . . . . . 13
       3.4.5.
       3.4.3.  Stream half-close  . . . . . . . . . . . . . . . . . . 13
       3.4.6. 14
       3.4.4.  Stream close . . . . . . . . . . . . . . . . . . . . . 13 14
     3.5.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 14 15
       3.5.1.  Session  Connection Error Handling  . . . . . . . . . . . . . . . . 14 15
       3.5.2.  Stream Error Handling  . . . . . . . . . . . . . . . . 15 16
       3.5.3.  Error Codes  . . . . . . . . . . . . . . . . . . . . . 15 16
     3.6.  Stream Flow Control  . . . . . . . . . . . . . . . . . . . 16 17
       3.6.1.  Flow Control Principles  . . . . . . . . . . . . . . . 16 17
       3.6.2.  Appropriate Use of Flow Control  . . . . . . . . . . . 17 18
     3.7.  Frame Types  Header Blocks  . . . . . . . . . . . . . . . . . . . . . . 19
     3.8.  Frame Types  . . 18
       3.7.1.  DATA Frames . . . . . . . . . . . . . . . . . . . . . 18
       3.7.2.  HEADERS+PRIORITY 19
       3.8.1.  DATA Frames  . . . . . . . . . . . . . . . . . . . 18
       3.7.3.  RST_STREAM . . 20
       3.8.2.  HEADERS+PRIORITY . . . . . . . . . . . . . . . . . . . 20
       3.8.3.  RST_STREAM . 18
       3.7.4.  SETTINGS . . . . . . . . . . . . . . . . . . . . . 21
       3.8.4.  SETTINGS . . 19
       3.7.5.  PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . 22
       3.7.6.  PING 21
       3.8.5.  PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . 25
       3.8.6.  PING . . . . 23
       3.7.7.  GOAWAY . . . . . . . . . . . . . . . . . . . . . 26
       3.8.7.  GOAWAY . . . 23
       3.7.8.  HEADERS . . . . . . . . . . . . . . . . . . . . . 26
       3.8.8.  HEADERS  . . 24
       3.7.9.  WINDOW_UPDATE . . . . . . . . . . . . . . . . . . . . 25
       3.7.10. Header Block . 28
       3.8.9.  WINDOW_UPDATE  . . . . . . . . . . . . . . . . . . . . 28 29
   4.  HTTP Message Exchanges . . . . . . . . . . . . . . . . . . . . 28 32
     4.1.  Connection Management  . . . . . . . . . . . . . . . . . . 28
       4.1.1.  Use of GOAWAY  . . . . . . . . . . . . . . . . . . . . 29 32
     4.2.  HTTP Request/Response  . . . . . . . . . . . . . . . . . . 29 33
       4.2.1.  HTTP Header Fields and HTTP/2.0 Headers  . . . . . . . 29 33
       4.2.2.  Request  . . . . . . . . . . . . . . . . . . . . . . . 29 33
       4.2.3.  Response . . . . . . . . . . . . . . . . . . . . . . . 31 34
     4.3.  Server Push Transactions . . . . . . . . . . . . . . . . . 32 35
       4.3.1.  Server implementation  . . . . . . . . . . . . . . . . 33 36
       4.3.2.  Client implementation  . . . . . . . . . . . . . . . . 34 37
   5.  Design Rationale and Notes . . . . . . . . . . . . . . . . . . 35 38
     5.1.  Separation of Framing Layer and Application Layer  . . . . 35 38
     5.2.  Error handling - Framing Layer . . . . . . . . . . . . . . 35 39
     5.3.  One Connection Per per Domain  . . . . . . . . . . . . . . . . 36 39
     5.4.  Fixed vs Variable Length Fields  . . . . . . . . . . . . . 36 39
     5.5.  Server Push  . . . . . . . . . . . . . . . . . . . . . . . 36 40
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 37 40
     6.1.  Use of Same-origin constraints .  Server Authority and Same-Origin . . . . . . . . . . . . . 37 40
     6.2.  Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 37 40
     6.3.  Cacheability of Pushed Resources . . . . . . . . . . . . . 37 41
   7.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 37 41
     7.1.  Long Lived Connections . . . . . . . . . . . . . . . . . . 38 41
     7.2.  SETTINGS frame . . . . . . . . . . . . . . . . . . . . . . 38 41
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38 42
     8.1.  Frame Type Registry  . . . . . . . . . . . . . . . . . . . 38 42
     8.2.  Error Code Registry  . . . . . . . . . . . . . . . . . . . 39 43
     8.3.  Settings Registry  . . . . . . . . . . . . . . . . . . . . 39 43
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40 44
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41 44
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 41 44
     10.2. Informative References . . . . . . . . . . . . . . . . . . 42 45
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 42 46

     A.1.  Since draft-ietf-httpbis-http2-01 draft-ietf-httpbis-http2-02  . . . . . . . . . . . . 42 46
     A.2.  Since draft-ietf-httpbis-http2-00 draft-ietf-httpbis-http2-01  . . . . . . . . . . . . 43 46
     A.3.  Since draft-ietf-httpbis-http2-00  . . . . . . . . . . . . 47
     A.4.  Since draft-mbelshe-httpbis-spdy-00  . . . . . . . . . . . 43 47

1.  Introduction

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

   The HTTP/1.1 encapsulation ensures that

   In particular, HTTP/1.0 only allows one request can to be delivered at a
   time on a given connection.  HTTP/1.1 pipelining,
   which pipelining only partially
   addressed request concurrency, and is not widely deployed, only partially addresses these
   concerns.  Clients deployed.
   Therefore, clients that need to make multiple many requests therefore (as is common on
   the Web) typically use
   commonly multiple connections to a server or servers in order to
   reduce the overall latency of those requests. [[anchor1: Need to tune
   the anti-pipelining comments here.]] perceived latency.

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

   This document defines addresses these issues by defining an optimized mapping
   of the HTTP HTTP's semantics to a
   TCP an underlying connection.  This optimization reduces the latency costs  Specifically, it
   allows interleaving of HTTP
   by allowing parallel requests request and response messages on the same
   connection and by using uses an efficient coding for HTTP header fields.  Prioritization  It
   also allows prioritization of requests
   lets requests, letting more important
   requests complete faster, more quickly, further improving
   application perceived
   performance.

   HTTP/2.0 applications

   The resulting protocol is designed to have an improved impact on network congestion
   due be more friendly to the use of
   network, because fewer TCP connections can be used, in comparison to achieve the same effect.
   Fewer TCP connections compete more fairly
   HTTP/1.x.  This means less competition with other flows.  Long- flows, and longer-
   lived connections are also more able connections, which in turn leads to take better advantage utilization of the
   available network capacity, rather than operating in the slow start
   phase of TCP.

   The HTTP/2.0 capacity.

   Finally, this encapsulation also enables more efficient scalable processing of
   messages by providing efficient through use of binary message framing.  Processing of
   header fields in HTTP/2.0 messages is more efficient (for entities
   that process many messages).

1.1.  Document Organization

   The HTTP/2.0 Specification is split into three parts: starting
   HTTP/2.0 (Section 2), which covers how a HTTP/2.0 connection is started;
   initiated; a framing layer (Section 3), which multiplexes a single
   TCP connection into
   independent, length-prefixed frames; independent frames of various types; and an HTTP
   layer (Section 4), which specifies the mechanism for overlaying expressing HTTP request/response
   pairs on top of
   interactions using the framing layer.  While some of the framing
   layer concepts are isolated from the HTTP layer, HTTP, building a generic framing
   layer has not been a goal.  The framing layer is tailored to the
   needs of the HTTP protocol and server push.

1.2.  Conventions and Terminology

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

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

   The following terms are used:

   client:  The endpoint initiating the HTTP/2.0 session. HTTP connection.

   connection:  A transport-level connection between two endpoints.

   endpoint:  Either the client or server of a the connection.

   frame:  The smallest unit of communication, each containing communication within an HTTP/2.0
      connection, consisting of a frame
      header.

   message:  A complete header and a variable-length sequence
      of frames. bytes structured according to the frame type.

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

   receiver:  An endpoint that is receiving frames.

   sender:  An endpoint that is transmitting frames.

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

   session:  A synonym for a HTTP connection.

   session

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

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

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

2.  Starting HTTP/2.0

   Just as HTTP/1.1 does,

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

   Different discovery mechanisms are defined

   The means by which support for "http:" HTTP/2.0 is determined is different
   for "http" and "https:" "https" URIs.  Discovery for "http:" "https:" URIs is
   described in Section 2.2;
   discovery 2.3.  Discovery for "https:" "http" URIs is described in Section 2.3.
   here.

2.1.  HTTP/2.0 Version Identification

   HTTP/2.0

   The protocol defined in this document is identified using the string
   "HTTP/2.0".  This identification is used in the HTTP/1.1 Upgrade
   header field, in the
   TLS-NPN [TLSNPN] [[anchor4: TBD]] TLS application layer protocol negotiation
   extension [TLSALPN] field and other places where protocol
   identification is required.

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

   [[anchor5:

   [[anchor3: Editor's Note: please remove the following text prior to
   the publication of a final version of this document.]]

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

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

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

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

2.2.  Starting HTTP/2.0 for "http:" URIs

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

   For example:

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

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

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

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

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

     [ HTTP/2.0 session connection ...

   Once

   The first HTTP/2.0 frame sent by the server returns is a SETTINGS frame
   (Section 3.8.4).  Upon receiving the 101 response, both the client and the
   server send sends a session
   connection header (Section 3.2). 3.2), which includes a SETTINGS frame.

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

   A client that makes a request to an "https:" URI without prior
   knowledge about support for HTTP/2.0 uses TLS [RFC5246] with TLS-NPN
   [TLSNPN] extension. [[anchor6: TBD, maybe ALPN]] the
   application layer protocol negotiation extension [TLSALPN].

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

2.4.  Starting HTTP/2.0 with Prior Knowledge

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

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

3.  HTTP/2.0 Framing Layer

3.1.  Session  Connection

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

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

   Servers are encouraged to leave connections maintain open connections for as long as
   possible, but can are permitted to terminate idle connections if
   necessary.  When either endpoint closes chooses to close the transport-level
   TCP connection, it the terminating endpoint MUST first send a GOAWAY
   (Section 3.7.7) 3.8.7) frame so that the both endpoints can reliably determine if requests finished before the close.
   whether previously sent frames have been processed and gracefully
   complete or terminate any necessary remaining tasks.

3.2.  Session  Connection Header

   After opening

   Upon establishment of a TCP connection and performing either an HTTP/1.1
   Upgrade or TLS handshake, the client sends the client session header.
   The server replies with determination that
   HTTP/2.0 will be used by both peers to communicate, each endpoint
   MUST send a server session header.

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

   The client session connection header is the 25 byte a sequence
   0x464f4f202a20485454502f322e300d0a0d0a4241520d0a0d0a of 24 octets (in hex
   notation)

   464f4f202a20485454502f322e300d0a0d0a42410d0a0d0a
   (the string "FOO * HTTP/2.0\r\n\r\nBAR\r\n\r\n") HTTP/2.0\r\n\r\nBA\r\n\r\n") followed by a
   SETTINGS frame (Section 3.7.4). 3.8.4).  The client sends the client session
   connection header immediately after receiving an HTTP/1.1 Upgrade, upon receipt of a 101 Switching
   Protocols response (indicating a successful upgrade), or after
   receiving a TLS Finished message from the server.  If starting an
   HTTP/2.0 connection with prior knowledge of server support for the
   protocol, the client connection header is sent upon connection
   establishment.

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

   The server session connection header is consists of just a SETTINGS frame
   (Section 3.7.4).  The 3.8.4) that MUST be the first frame the server sends in the server session header immediately after receiving
   and validating
   HTTP/2.0 connection.

   To avoid unnecessary latency, clients are permitted to send
   additional frames to the client session header.

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

   Both the server connection
   header SETTINGS frame might include parameters that necessarily alter
   how a client is expected to communicate with the server.  Upon
   receiving the SETTINGS frame, the client is expected to honor any
   parameters established.

   Clients and server servers MUST close terminate the TCP connection if it either peer
   does not begin with a valid session connection header.  A GOAWAY frame
   (Section 3.7.7) 3.8.7) MAY be omitted if it is clear that the peer is not
   using HTTP/2.0.

3.3.  Framing

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

3.3.1.  Frame Header

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

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

                               Frame Header

   The fields of the frame header are defined as:

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

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

   Flags:  An 8-bit field reserved for frame-type specific boolean
      flags.  Bits that have undefined
      semantics are reserved.

      The following flags are least significant bit (0x1) - the FINAL bit - is defined for
      all frame types:

      FINAL (0x1):  Bit 1 (the least significant bit) indicates types as an indication that this frame is the last frame in a the
      endpoint will send for the identified stream.  This places  Setting this flag
      causes the stream
         into a to enter the half-closed state (Section 3.4.5).  No further frames
         follow in 3.4.3).
      Implementations MUST process the direction FINAL bit for all frames whose
      stream identifier field is not 0x0.  The FINAL bit MUST NOT be set
      on frames that use a stream identifier of the carrying frame.

      Frame types 0.

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

   R: A reserved 1-bit field.  The semantics of this bit are not
      defined. undefined
      and the bit MUST remain unset (0) when sending and MUST be ignored
      when receiving.

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

   Frame Data:  Frames contain between 0

   The structure and 65535 bytes content of data.

   Reserved bits in the remaining frame header MUST be set to zero when sending
   and MUST be ignored when receiving frames, unless the semantics of data is dependent
   entirely on the bit are known. frame type.

3.3.2.  Frame Processing

   A frame of the maximum size Size

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

   An implementation MUST immediately close terminate a stream immediately if it is unable
   to process a frame related to that stream due to it exceeding a size
   limit.  The implementation MUST send a it's size.  This is done by sending an
   RST_STREAM frame (Section 3.7.3) 3.8.3) containing the FRAME_TOO_LARGE error code if the frame
   size limit is exceeded.

   [[anchor9:
   code.

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

3.4.  Streams

   Streams are independent sequences of

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

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

   o  Streams can be rejected or cancelled by either endpoint.

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

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

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

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

   o  Streams can be established and used unilaterally.

   o  Streams can be cancelled.

3.4.1.  Stream Creation

   Use of streams does not require negotiation.  A stream

   There is not
   created, no coordination or shared action between the client and
   server required to create a stream.  Rather, new streams are used
   established by sending a frame on the stream.

   Streams whose stream identifier field
   references a previously unused stream identifier.

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

   The stream identifier of a new newly established stream MUST be numerically
   greater than all other previously established streams from that endpoint, endpoint
   within the HTTP/2.0 connection, unless the stream identifier was
   previously has been
   reserved (such as the promised stream identifier in using a PUSH_PROMISE (Section 3.7.5) frame). 3.8.5) frame.  An endpoint
   that receives an unexpected stream identifier MUST treat this as respond with a session
   connection error (Section 3.5.1) of type PROTOCOL_ERROR.

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

   An endpoint cannot prevent limit the creation total number of a new stream, but it can
   request concurrently active streams
   using the early termination of an unwanted stream.  Upon receipt of SETTINGS_MAX_CONCURRENT_STREAMS parameters within a frame,
   SETTINGS frame.  The maximum concurrent streams setting is specific
   to each endpoint and applies only to the recipient peer.  That is, clients
   specify the maximum number of concurrent streams the server can terminate
   initiate, and servers specify the corresponding stream maximum number of concurrent
   streams the client can initiate.  Peer endpoints MUST NOT exceed this
   limit.  All concurrently active streams initiated by
   sending an endpoint,
   including streams that are half-open (Section 3.4.3) in any
   direction, count toward that endpoint's limit.

   Stream identifiers cannot be reused within a connection.  Long-lived
   connections can cause an endpoint to exhaust the available range of
   stream identifiers.  A client that is unable to establish a new
   stream error identifier can establish a new connection for new streams.

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

3.4.2.  Stream priority

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

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

3.4.3.  Stream headers

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

3.4.4.  Stream data exchange

   Once a stream is created, it can be used higher priority
   relative to send arbitrary amounts of
   data.  Generally this means that any other concurrently active streams.  That is, if an
   endpoint receives interleaved frames for multiple streams, the
   endpoint ought to make a series of data best-effort attempt at processing frames will be sent
   on for
   higher priority streams before processing those for lower priority
   streams.

   Explicitly setting the stream until priority for a frame containing stream does not guarantee any
   particular processing order for the FINAL flag (Section 3.3.1) stream relative to any other
   stream.  Nor is there is set.  Once the FINAL flag has been set on any frame, mechanism provided by which the
   initiator of a stream is
   considered can force or require a receiving endpoint to be half-closed.

3.4.5.
   process frames from one stream before processing frames from another.

3.4.3.  Stream half-close

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

   An

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

   Streams

   An endpoint that have never received packets can be considered to be has not yet half-closed in a stream by sending the direction that
   FINAL flag can continue sending frames on the stream.

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

3.4.6. receiver.

3.4.4.  Stream close

   Streams can be terminated in the following ways:

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

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

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

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

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

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

3.5.  Error Handling

   HTTP/2.0 framing permits two classes of error:

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

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

3.5.1.  Session  Connection Error Handling

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

   An endpoint that encounters a session connection error MUST first send a
   GOAWAY (Section 3.7.7) 3.8.7) frame with the stream identifier of the last
   stream that it successfully received from its peer.  The GOAWAY frame
   includes an error code that indicates why the session connection is
   terminating.  After sending the GOAWAY frame, the endpoint MUST close
   the TCP connection.

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

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

3.5.2.  Stream Error Handling

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

   An endpoint that detects a stream error sends a RST_STREAM
   (Section 3.7.3) 3.8.3) frame that contains the stream identifier of the
   stream where the error occurred.  The RST_STREAM frame includes an
   error code that indicates the type of error.

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

   An
   (Section 3.7)).

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

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

3.5.3.  Error Codes

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

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

   The following error codes are defined:

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

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

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

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

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

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

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

   REFUSED_STREAM (7):  Indicates that  The endpoint is refusing the stream was refused before any
      processing has been done on the stream. its payload.

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

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

3.6.  Stream Flow Control

   Multiplexing

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

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

3.6.1.  Flow Control Principles

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

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

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

   2.  Flow control is based on window update messages. frames.  Receivers
       advertise how many octets they are prepared to receive on a
       stream.  This is a credit-based scheme.

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

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

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

   6.  Flow control can be disabled by a receiver.  A receiver can
       choose to either disable flow control for a stream or connection
       by declaring an infinite flow control limit.

   7.  HTTP/2.0 standardizes only the format of the window update
       message frame
       (Section 3.7.9). 3.8.9).  This does not stipulate how a receiver decides
       when to send this message frame or the value that it sends.  Nor does it
       specify how a sender chooses to send packets.  Implementations
       are able to select any algorithm that suits their needs.

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

3.6.2.  Appropriate Use of Flow Control

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

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

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

   Implementation of flow control in

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

3.7.  Frame Types

3.7.1.  DATA Frames

   DATA frames (type=0) are used to convey HTTP message bodies.  Header Blocks

   The
   payload of a data frame contains either a request or response body.

   No frame-specific flags are defined for DATA frames.

3.7.2. header block is found in the HEADERS, HEADERS+PRIORITY and
   PUSH_PROMISE frames.  The HEADERS+PRIORITY frame (type=1) allows the sender to header block consists of a set of header
   fields, which are name-value pairs.  Headers are compressed using
   black magic.

   Compression of header fields and stream priority at the same time.  This MUST be used for
   each stream that is created.

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

                      HEADERS+PRIORITY Frame Payload

   The HEADERS+PRIORITY frame is identical to the HEADERS frame
   (Section 3.7.8), with a 32-bit field containing priority included
   before work in progress, as is the header format
   of this block.

   The most significant bit contents of header blocks MUST be processed by the priority is reserved.  The 31-bit
   priority indicates compression
   context, even if stream has been reset or the priority for frame is discarded.  If
   header blocks cannot be processed, the stream, as assigned by receiver MUST treat the
   sender, see Section 3.4.2.

3.7.3.  RST_STREAM

   The RST_STREAM frame (type=3) allows for abnormal termination
   connection with a connection error (Section 3.5.1) of type
   COMPRESSION_ERROR.

3.8.  Frame Types

   This specification defines a
   stream.  When sent number of frame types, each identified
   by a unique 8-bit type code.  Each frame type serves a distinct
   purpose either in the creator establishment and management of a stream, it indicates the
   creator wishes to cancel the stream.  When sent by connection
   as a whole, or of individual streams.

   The transmission of specific frame types can alter the recipient state of a
   stream,
   connection.  If endpoints fail to maintain a synchronized view of the
   connection state, successful communication within the connection will
   no longer be possible.  Therefore, it indicates an error or is important that endpoints
   have a shared comprehension of how the recipient did not want state is affected by the use
   any given frame.  Accordingly, while it is expected that new frame
   types will be introduced by extensions to
   accept this protocol, only frames
   defined by this document are permitted to alter the stream, so connection state.

3.8.1.  DATA Frames

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

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

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

3.8.2.  HEADERS+PRIORITY

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |X|                   Priority (31)                             |                         Error Code (32)
   +-+-------------------------------------------------------------+
   |                    Header Block (*)                         ...
   +---------------------------------------------------------------+

                         RST_STREAM

                      HEADERS+PRIORITY Frame Payload

   The RST_STREAM HEADERS+PRIORITY frame does not define any valid flags.

   The RST_STREAM is identical to the HEADERS frame contains
   (Section 3.8.8), preceded by a single 32-bit error code
   (Section 3.5.3).  The error code indicates why reserved bit and a 31-bit
   priority; see Section 3.4.2.

   HEADERS+PRIORITY uses the stream is being
   terminated.

   After receiving same flags as the HEADERS frame, except
   that a RST_STREAM on HEADERS+PRIORITY frame with a stream, the recipient must not send
   additional frames CONTINUES bit MUST be followed
   by another HEADERS+PRIORITY frame.  See HEADERS frame (Section 3.8.8)
   for that stream, and the any flags.

   HEADERS+PRIORITY frames MUST be associated with a stream.  If a
   HEADERS+PRIORITY frame is received whose stream moves into identifier field is
   0x0, the
   closed state.

3.7.4.  SETTINGS

   A SETTINGS frame (type=4) contains recipient MUST respond with a set connection error
   (Section 3.5.1) of id/value pairs for
   communicating configuration data about how type PROTOCOL_ERROR.

   The HEADERS+PRIORITY frame modifies the two endpoints may
   communicate.  SETTINGS frames MUST be connection state as defined
   in Section 3.7.

3.8.3.  RST_STREAM

   The RST_STREAM frame (type=0x3) allows for abnormal termination of a
   stream.  When sent at by the start initiator of a session,
   but stream, it indicates that
   they can be wish to cancel the stream.  When sent at any other time by either endpoint.  Settings
   are declarative, not negotiated, each peer the receiver of a
   stream, it indicates their own
   configuration.

   [[anchor17: Note that persistence of settings either the receiver is under discussion in rejecting the WG and might
   stream, requesting that the stream be removed in cancelled or that an error
   condition has occurred.

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

                         RST_STREAM Frame Payload

   The RST_STREAM frame contains a future version of this document.]]

   When the server is single unsigned, 32-bit integer
   identifying the sender, error code (Section 3.5.3).  The error code indicates
   why the sender can request that
   configuration data be persisted by stream is being terminated.

   No type-flags are defined.

   The RST_STREAM frame fully terminates the client across HTTP/2.0
   sessions referenced stream and returned
   causes it to enter the server in future communications.

   Clients persist settings on a per origin basis (see [RFC6454] for a
   definition of web origins).  That is, when a client connects to closed state.  After receiving a
   server, and the server persists settings within the client, the
   client SHOULD return the persisted settings RST_STREAM on future connections to
   a stream, the same origin AND IP address and TCP port.  Clients receiver MUST NOT
   request servers to use send additional frames for that
   stream.  However, after sending the persistence features of RST_STREAM, the SETTINGS
   frames, and servers sending endpoint
   MUST ignore persistence related flags be prepared to receive and process additional frames sent on the
   stream that might have been sent by a
   client.

   Valid frame-specific flags for the SETTINGS frame are:

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

   SETTINGS arrival of
   the RST_STREAM.

   RST_STREAM frames always apply to a session, never MUST be associated with a single stream.
   The stream identifier for  If a settings RST_STREAM
   frame is received whose stream identifier field is 0x0 the recipient
   MUST be zero.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |SettingFlags(8)|             Setting Identifier (24)           |
   +---------------+-----------------------------------------------+
   |                        Value (32)                             |
   +---------------------------------------------------------------+ respond with a connection error (Section 3.5.1) of type
   PROTOCOL_ERROR.

3.8.4.  SETTINGS ID/Value Pair

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

   Settings Flags:  An 8-bit flags field containing per-setting
      instructions. (type=0x4) conveys configuration parameters that
   affect how endpoints communicate.  The following flags parameters are valid:

      PERSIST_VALUE (0x1):  Bit 1 (the least significant bit) being set
         indicates either
   constraints on peer behavior or preferences.

   SETTINGS frames MUST be sent at the start of a request from connection, and MAY be
   sent at any other time by either endpoint over the server to lifetime of the client to persist
   connection.

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

   A client SETTINGS frame is not required to include every defined setting;
   senders can include only those parameters for which it has accurate
   values and a need to convey.  When multiple parameters are sent, they
   SHOULD be sent in order of numerically lowest ID to highest ID.  A
   single SETTINGS frame MUST NOT set this flag.

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

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

   The SETTINGS frame defines the following flag:

   CLEAR_PERSISTED (0x2):  Bit 2 being set indicates that this setting is a
         persisted setting being returned by the client request to the server.
         This also indicates that this setting is not a client setting,
         but a value clear
      any previously set by persisted settings before processing the server.  A server settings.
      Clients MUST NOT set this flag.

      All other settings flags are reserved.

   Setting Identifier:  A 24-bit

   SETTINGS frames always apply to a connection, never a single stream.
   The stream identifier for a settings frame MUST be zero.  If an
   endpoint receives a SETTINGS frame whose stream identifier field that identifies is
   anything other than 0x0, the endpoint MUST respond with a connection
   error (Section 3.5.1) of type PROTOCOL_ERROR.

3.8.4.1.  Setting Format

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

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

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

   PERSIST_VALUE (0x1):  Bit 1 (the least significant bit) being set
      indicates a request from the server to the client to persist this
      setting.

   Value:  A 32-bit client MUST NOT set this flag.

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

3.8.4.2.  Setting Persistence

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

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

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

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

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

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

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

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

3.8.4.3.  Defined Settings

   The following settings are defined:

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

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

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

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

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

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

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

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

      This setting applies to all streams, including existing streams.

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

   The message is intentionally extensible for future information which
   may improve client-server communications. 3.8.9.4.

3.8.5.  PUSH_PROMISE

   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 PUSH_PROMISE 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 (type=0x5) 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 used to notify the persisted
   state on its next SETTINGS frame, it SHOULD send all 5 settings (1,
   2, 3, 4, and 5 peer endpoint
   in this example) to advance of streams the server.

3.7.5.  PUSH_PROMISE sender intends to initiate.  The
   PUSH_PROMISE frame (type=5) allows includes the sender to signal a promise unsigned 31-bit identifier of the
   stream the endpoint plans to create along with a stream and serve the referenced resource.  Minimal data
   allowing minimal set of
   headers that provide additional context for the receiver to understand which resource(s) are to be
   pushed are to be included.

   PUSH_PROMISE frames are sent on an existing stream.  They declare  Section 4.3
   contains a thorough description of the
   intent to use another stream for the pushing of a resource.  The PUSH_PROMISE allows the client an opportunity to reject pushed
   resources. frames.

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

                        PUSH_PROMISE Payload Format

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

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

   There is no requirement that

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

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

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

   Promised streams are created not required to be used in the order referenced. promised.  The
   PUSH_PROMISE only reserves stream identifiers for later use; these reserved identifiers use.

   Recipients of PUSH_PROMISE frames can be
   used as prioritization needs dictate. choose to reject promised
   streams by returning a RST_STREAM referencing the promised stream
   identifier back to the sender of the PUSH_PROMISE.

   The PUSH_PROMISE also includes a header block (Section 3.7.10), which
   describes frame modifies the resource that will be pushed.

3.7.6. connection state as defined in
   Section 3.7.

3.8.6.  PING

   The PING frame (type=6) (type=0x6) is a mechanism for measuring a minimal round-
   trip
   round-trip time from the sender. sender, as well as determining whether an
   idle connection is still functional.  PING frames can be sent from the client
   or the server.

   Recipients
   any endpoint.

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

   Processing of PING should take frames SHOULD be performed with the highest
   priority if there is
   other data are additional frames waiting to be sent. processed.

   The PING frame defines a frame-specific one type-specific flag:

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

   The payload of a

   PING frame contains frames are not associated with any value.  A individual stream.  If a PING response MUST
   contain
   frame is received with a stream identifier field value other than
   0x0, the contents recipient MUST respond with a connection error
   (Section 3.5.1) of the PING request.

3.7.7. type PROTOCOL_ERROR.

3.8.7.  GOAWAY

   The GOAWAY frame (type=7) (type=0x7) informs the remote side of the connection peer to stop creating
   streams on this session. connection.  It can be sent from the client or the
   server.  Once sent, the sender will ignore frames sent on new streams
   for the remainder of the session.  Recipients connection.  Receivers of a GOAWAY frame
   MUST NOT open additional streams on the session, connection, although a new session
   connection can be established for new streams.  The purpose of this message
   frame is to allow an endpoint to gracefully stop accepting new
   streams (perhaps for a reboot or maintenance), while still finishing
   processing of previously established streams.

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

   Endpoints should always send a GOAWAY message frame before closing a
   connection so that the remote can know whether a stream has been
   partially processed or not.  (For example, if an HTTP client sends a
   POST at the same time that a server closes a connection, the client
   cannot know if the server started to process that POST request if the
   server does not send a GOAWAY frame to indicate where it stopped
   working).

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

   [[anchor18:

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

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

                           GOAWAY Payload Format

   The GOAWAY frame does not define any valid flags.

   The GOAWAY frame applies define any type-specific flags.

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

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

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

   On streams with lower or equal numbered identifiers that do not close
   completely prior to the session, connection being closed, re-attempting
   requests, transactions, or any protocol activity is not a specific stream.  The
   stream identifier MUST be zero.

   The GOAWAY frame contains an identifier possible
   (with the exception of idempotent actions like HTTP GET, PUT, or
   DELETE).  Any protocol activity that uses higher numbered streams can
   be safely retried using a new connection.

   Activity on streams numbered lower or equal to the last stream that the
   identifier might still complete successfully.  The sender of the a GOAWAY is prepared to act upon, which can include
   processing and replies.  This allows
   frame gracefully shut down a connection by sending a GOAWAY frame,
   maintaining the connection in an endpoint to discover what
   streams might have had some effect or what might be safe to
   automatically retry.  If no open state until all in-progress
   streams were acted upon, the complete.

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

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

3.7.8. connection.

3.8.8.  HEADERS

   The HEADERS frame (type=8) (type=0x8) provides header fields for a stream.  It
   Any number of HEADERS frames can may be optionally sent on an existing stream at
   any time.  Specific
   application

   Additional type-specific flags for the HEADERS frame are:

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

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

      A HEADERS frame that includes a CONTINUES bit MUST be followed by
      a HEADERS frame for the same stream.  A receiver MUST treat the
      receipt of any other type of frame or a frame on a different
      stream as a connection error (Section 3.5.1) of the headers in this frame is application-dependent.

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

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

3.7.9. 3.7).

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

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

3.8.9.  WINDOW_UPDATE

   The WINDOW_UPDATE frame (type=9) (type=0x9) is used to implement flow control in
   HTTP/2.0. control.

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

   Flow connection.

   Both types of flow control in HTTP/2.0 is are hop by hop, hop; that is, only between the
   two
   endpoints of a HTTP/2.0 connection. endpoints.  Intermediaries do not forward WINDOW_UPDATE messages frames
   between dependent sessions. connections.  However, throttling of data transfer
   by any recipient receiver can indirectly cause the propagation of flow control
   information toward the original sender.

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

   Valid
   frame.

   The following additional flags are defined for the WINDOW_UPDATE frame are:
   frame:

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

   The WINDOW_UPDATE frame can be specific to a stream related or session related.
   The to the entire
   connection.  In the former case, the frame's stream identifier
   indicates the affected stream; in the WINDOW_UPDATE frame header identifies latter, the affected stream, or includes a value of 0 to indicate "0" indicates
   that the
   session flow control window entire connection is updated. the subject of the frame.

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

3.7.9.1.

3.8.9.1.  The Flow Control Window

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

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

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

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

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

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

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

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

3.7.9.2.

3.8.9.2.  Initial Flow Control Window Size

   When a HTTP/2.0 connection is first established, new streams are
   created with an initial flow control window size of 65535 bytes.  The
   session
   connection flow control window is 65536 bytes.  Both endpoints can
   adjust the initial window size for new streams by including a value
   for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms
   part of the session connection header.

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

   A SETTINGS frame can alter the initial flow control window size for
   all current streams.  When the value of SETTINGS_INITIAL_WINDOW_SIZE
   changes, a receiver MUST adjust the size of all flow control windows
   that it maintains by the difference between the new value and the old
   value.

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

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

3.7.9.3.  Reducing the Stream Window Size

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

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

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

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

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

3.7.9.4.  Ending Flow Control

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

   Flow control can be disabled for all streams or client sends 64KB immediately on connection establishment,
   the session using client will recalculate the
   SETTINGS_FLOW_CONTROL_OPTIONS setting.  An implementation that does
   not wish to perform available flow control can use this in the initial SETTINGS
   exchange.

   Flow control can window to be disabled for an individual stream or the overall
   session by sending a WINDOW_UPDATE with
   -48KB on receipt of the END_FLOW_CONTROL flag
   set. SETTINGS frame.  The payload of client retains a
   negative flow control window until WINDOW_UPDATE frame that has frames restore the
   END_FLOW_CONTROL flag set is ignored.

   Flow control cannot be enabled again once disabled.  Any attempt
   window to
   re-enable being positive, after which the client can resume sending.

3.8.9.3.  Reducing the Stream Window Size

   A receiver that wishes to use a smaller flow control - by sending a WINDOW_UPDATE or by clearing window than the bits on
   current size can send a new SETTINGS frame.  However, the SETTINGS_FLOW_CONTROL_OPTIONS setting - receiver
   MUST be
   rejected with a FLOW_CONTROL_ERROR error code.

3.7.10.  Header Block

   The header block is found in prepared to receive data that exceeds this window size, since
   the HEADERS, HEADERS+PRIORITY and
   PUSH_PROMISE frames.  The header block consists of a set of header
   fields, which are name-value pairs.  Headers are compressed using
   black magic.

   Compression of header fields is a work in progress, as is sender might send data that exceeds the format
   of this block.

4.  HTTP Message Exchanges

   HTTP/2.0 is intended lower limit prior to be as compatible as possible with current
   web-based applications.  This means that, from the perspective of
   processing the
   server business logic or application API, SETTINGS frame.

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

   1.  The receiver can immediately send RST_STREAM with
       FLOW_CONTROL_ERROR error code for the features of HTTP are
   unchanged.  To achieve this, all of affected streams.

   2.  The receiver can accept the application request streams and
   response header semantics are preserved, although tolerate the syntax resulting
       head of
   conveying those semantics has changed.  Thus, line blocking, sending WINDOW_UPDATE frames as it
       consumes data.

   If a receiver decides to accept streams, both sides MUST recompute
   the rules from HTTP/1.1
   ([HTTP-p1], [HTTP-p2], [HTTP-p4], [HTTP-p5], [HTTP-p6], and
   [HTTP-p7]) apply with available flow control window based on the changes initial window size
   sent in the sections below.

4.1.  Connection Management

   Clients SHOULD NOT open more than one HTTP/2.0 session to SETTINGS.

3.8.9.4.  Ending Flow Control

   After a given
   origin ([RFC6454]) concurrently.

   Note receiver reads in a frame that marks the end of a stream (for
   example, a data stream with a FINAL flag set), it is possible MUST cease
   transmission of WINDOW_UPDATE frames for one HTTP/2.0 session to be finishing
   (e.g. a GOAWAY message has been sent, but that stream.  A sender is
   not all obligated to maintain the available flow control window for
   streams have
   finished), while another HTTP/2.0 session that it is starting.

4.1.1.  Use of GOAWAY

   HTTP/2.0 provides a GOAWAY message which no longer sending on.

   Flow control can be used when closing a
   connection from either the client disabled for all streams or server.  Without a server GOAWAY
   message, HTTP has a race condition where the client sends a request
   just as the server is closing the connection, and the client cannot
   know if connection using
   the server received SETTINGS_FLOW_CONTROL_OPTIONS setting.  An implementation that
   does not wish to perform flow control can use this in the initial
   SETTINGS exchange.

   Flow control can be disabled for an individual stream or not.  By using the last-
   stream-id in the GOAWAY, servers can indicate to overall
   connection by sending a WINDOW_UPDATE with the client if END_FLOW_CONTROL flag
   set.  The payload of a
   request was processed or not.

   Note WINDOW_UPDATE frame that some servers will choose to send the GOAWAY and immediately
   terminate has the connection without waiting for active streams to
   finish.  The client will
   END_FLOW_CONTROL flag set is ignored.

   Flow control cannot be able enabled again once disabled.  Any attempt to determine this because HTTP/2.0
   streams are deterministically closed.  This abrupt termination will
   force
   re-enable flow control - by sending a WINDOW_UPDATE or by clearing
   the client to heuristically decide whether to retry bits on the pending
   requests.  Clients always need to SETTINGS_FLOW_CONTROL_OPTIONS setting - MUST be capable of dealing with this
   case because they must deal
   rejected with accidental connection termination
   cases, which are the same as the server never having sent a GOAWAY.

   More sophisticated servers will use GOAWAY FLOW_CONTROL_ERROR error code.

4.  HTTP Message Exchanges

   HTTP/2.0 is intended to implement a graceful
   teardown.  They will send be as compatible as possible with current
   web-based applications.  This means that, from the GOAWAY and provide some time for perspective of the
   active streams to finish before terminating
   server business logic or application API, the connection.

   If a HTTP/2.0 client closes features of HTTP are
   unchanged.  To achieve this, all of the connection, it should also send a
   GOAWAY message.  This allows application request and
   response header semantics are preserved, although the server to know if any server-push
   streams were received by syntax of
   conveying those semantics has changed.  Thus, the client.

   If rules from HTTP/1.1
   ([HTTP-p1], [HTTP-p2], [HTTP-p4], [HTTP-p5], [HTTP-p6], and
   [HTTP-p7]) apply with the endpoint closing changes in the sections below.

4.1.  Connection Management

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

   Note that it is possible for one HTTP/2.0 connection to be finishing
   (e.g. a GOAWAY frame has been sent, but not received frames on any
   stream, the GOAWAY will contain a last-stream-id of 0. all streams have
   finished), while another HTTP/2.0 connection is starting.

4.2.  HTTP Request/Response

4.2.1.  HTTP Header Fields and HTTP/2.0 Headers

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

4.2.2.  Request

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

   The header fields included in the HEADERS+PRIORITY frame contain all
   of the HTTP header fields that are associated with an HTTP request.  The header block in HTTP/2.0 is mostly
   definitions of these headers are largely unchanged from today's HTTP
   header block, relative to
   HTTP/1.1, with the following differences: a few notable exceptions:

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

      ":method": request-line has been split into two separate header
      fields named :method and :path, whose values specify the HTTP
      method for this request (e.g.  "GET", "POST",
         "HEAD", etc) ([HTTP-p2], Section 4)

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

      These header fields MUST be present in HTTP requests.

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

      ":host": request-line is removed entirely
      from the headers.

   o  The host and optional port portions (see [RFC3986],
         Section 3.2) of the URI for this request (e.g. "www.google.com:
         1234").  This header field URI (see
      [RFC3986], Section 3.2), is specified using the same as new :host header
      field. [[anchor21: Ed.  Note: it needs to be clarified whether or
      not this replaces the HTTP 'Host' existing HTTP/1.1 Host header.]]

   o  A new :scheme header field ([HTTP-p1], Section 5.4).

      ":scheme": has been added to specify the scheme
      portion of the URI for this request request-target (e.g. "https")

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

   o  The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-
      Encoding header fields are no longer valid and MUST not be sent.

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

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

      Header field names MUST be all lowercase.

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

      User-agents MUST support gzip compression.  Regardless of the
      Accept-Encoding sent by the user-agent,

   If a client sends a HEADERS+PRIORITY frame that omits a mandatory
   header, the server may always send
      content encoded MUST reply with gzip or deflate encoding. [[anchor27: Still
      valid?]] a HTTP 400 Bad Request reply.
   [[anchor22: Ed: why PROTOCOL_ERROR on missing ":status" in the
   response, but HTTP 400 here?]]

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

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

   The user-agent is free to prioritize

   A client provides priority in requests as it sees fit.  If the
   user-agent cannot make progress without receiving a resource, it
   should attempt hint to raise the priority of that resource.  Resources
   such as images, server.  A
   server SHOULD generally use the lowest priority.

   If a client sends a HEADERS+PRIORITY frame that omits a mandatory
   header, the attempt to provide responses to higher priority
   requests before lower priority requests.  A server MUST reply with could send lower
   priority responses during periods that higher priority responses are
   unavailable to ensure better utilization of a HTTP 400 Bad Request reply.
   [[anchor28: Ed: why PROTOCOL_ERROR on missing ":status" in the
   response, but HTTP 400 here?]] connection.

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

4.2.3.  Response

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

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

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

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

      All header field names MUST be all lowercase.

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

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

      If a client receives a response where the sum of the data frame
      payload length does not equal the size of the Content-Length
      header field, the client MUST ignore the content length header
      field. [[anchor29: [[anchor23: Ed: See
      <https://github.com/http2/http2-spec/issues/46>.]]

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

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

   Clients MUST support gzip compression.  Regardless of the value of
   the Accept-Encoding header field, a server MAY send responses with
   gzip or deflate encoding.  A compressed response MUST still bear an
   appropriate Content-Encoding header field.

4.3.  Server Push Transactions

   HTTP/2.0 enables a server to send multiple replies to a client for a
   single request.  The rationale for this feature is that sometimes a
   server knows that it will need to send multiple resources in response
   to a single request.  Without server push features, the client must
   first download the primary resource, then discover the secondary
   resource(s), and request them.  Pushing of resources avoids the
   round-trip delay, but also creates a potential race where a server
   can be pushing content which a user-agent is in the process of
   requesting.  The following mechanics attempt to prevent the race
   condition while enabling the performance benefit.

   Server push is an optional feature.  The
   SETTINGS_MAX_CONCURRENT_STREAMS setting from the client limits the
   number of resources that can be concurrently pushed by a server.
   Server push can be disabled by clients that do not wish to receive
   pushed resources by advertising a SETTINGS_MAX_CONCURRENT_STREAMS
   SETTING (Section 3.7.4) 3.8.4) of zero.  This prevents servers from creating
   the streams necessary to push resources.

   Browsers

   Clients receiving a pushed response MUST validate that the server is
   authorized to push the resource using the same-origin policy
   ([RFC6454], Section 3).  For example, a HTTP/2.0 connection to
   "example.com" is generally [[anchor30: [[anchor24: Ed: weaselly use of
   "generally", needs better definition]] not permitted to push a
   response for "www.example.org".

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

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

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

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

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

4.3.1.  Server implementation

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

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

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

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

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

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

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

4.3.2.  Client implementation

   When fetching a resource the client has 3 possibilities:

   1.  the resource is not being pushed

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

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

   When a HEADERS+PRIORITY frame that contains an
   Associated-To-Stream-ID is received, the

   A client MUST NOT[[anchor34: SHOULD NOT?]] NOT issue GET requests for the a resource in the pushed
   stream, and that has been
   promised.  A client is instead advised to wait for the pushed stream
   resource to arrive.

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

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

   To cancel individual server push streams, the client can issue a
   stream error (Section 3.5.2) of type CANCEL.  Upon receipt,  After receiving a
   PUSH_PROMISE frame, the client is able to cancel the pushed resource
   before receiving any frames on the promised stream.  The server
   ceases transmission of the pushed data. resource; if the server has not
   commenced transmission, it does not start.

   To cancel all server push streams related to a request, the client
   may issue a stream error (Section 3.5.2) of type CANCEL on the
   associated-stream-id.  By cancelling that stream, the server MUST
   immediately stop sending frames for any streams with
   in-association-to for the original stream. [[anchor35: [[anchor27: Ed: Triggering
   side-effects on stream reset is going to be problematic for the
   framing layer.  Purely from a design perspective, it's a layering
   violation.  More practically speaking, the base request stream might
   already be removed.  Special handling logic would be required.]]

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

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

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

5.  Design Rationale and Notes

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

5.1.  Separation of Framing Layer and Application Layer

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

   This blending is intentional - the primary goal of this protocol is
   to create a low-latency protocol for use with HTTP.  Isolating the
   two layers is convenient for description of the protocol and how it
   relates to existing HTTP implementations.  However, the ability to
   reuse the HTTP/2.0 framing layer is a non goal.

5.2.  Error handling - Framing Layer

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

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

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

5.3.  One Connection Per per Domain

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

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

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

5.4.  Fixed vs Variable Length Fields

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

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

5.5.  Server Push

   A subtle but important point is that server push streams must be
   declared before the associated stream is closed.  The reason for this
   is so that proxies have a lifetime for which they can discard
   information about previous streams.  If a pushed stream could
   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  Server Authority and Same-Origin

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

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

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

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

6.2.  Cross-Protocol Attacks

   By utilizing

   When using TLS, we believe that HTTP/2.0 introduces no new cross-
   protocol attacks.  TLS encrypts the contents of all transmission
   (except the handshake itself), making it difficult for attackers to
   control the data which could be used in a cross-protocol attack.
   [[anchor45:

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

6.3.  Cacheability of Pushed Resources

   Pushed resources do not have are synthesized responses without an associated request.  In order explicit
   request; the request for a pushed resource is synthesized from the
   request that triggered the push, plus resource identification
   information provided by the server.  Request header fields are
   necessary for
   existing HTTP cache control validations (such as the Vary header
   field) to work, all cached resources must have a set of request
   header fields. work.  For this reason, caches MUST be careful to inherit request header
   fields from the associated stream for the push.  This includes the
   Cookie header field.

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

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

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

7.  Privacy Considerations

7.1.  Long Lived Connections

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

7.2.  SETTINGS frame

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

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

   Clients MUST clear persisted SETTINGS information when clearing the
   cookies.

8.  IANA Considerations

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

8.1.  Frame Type Registry

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

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

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

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

                                  Table 1

8.2.  Error Code Registry

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

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

   New registrations are advised to provide the following information:

   Error Code:  The 32-bit error code value.

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

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

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

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

8.3.  Settings Registry

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

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

   New registrations are advised to provide the following information:

   Setting:  The 24-bit setting value.

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

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

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

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

   An initial set of settings registrations can be found in
   Section 3.7.4. 3.8.4.3.

9.  Acknowledgements

   This document includes substantial input from the following
   individuals:

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

   o  Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)

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

   o  Mark Nottingham and Nottingham, Julian Reschke Reschke, James Snell (Editorial)

10.  References

10.1.  Normative References

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

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

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

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

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

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

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

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

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

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

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

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

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

   [TLSNPN]

   [TLSALPN]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Next Application Layer Protocol
              Negotiation Extension", draft-agl-tls-nextprotoneg-04 draft-ietf-tls-applayerprotoneg-01
              (work in progress), May 2012. April 2013.

10.2.  Informative References

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

   [TALKING]  Huang, L-S., Chen, E., Barth, A., Rescorla, E., and C.

              Jackson, "Talking to Yourself for Fun and Profit", 2011,
              <http://w2spconf.com/2011/papers/websocket.pdf>.

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

A.1.  Since draft-ietf-httpbis-http2-02

   Added continuations to frames carrying header blocks.

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

   Removed "message".

   Switched to TLS ALPN from NPN.

   Editorial changes.

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

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

   Removed data frame compression.

   Added PUSH_PROMISE.

   Added globally applicable flags to framing.

   Removed zlib-based header compression mechanism.

   Updated references.

   Clarified stream identifier reuse.

   Removed CREDENTIALS frame and associated mechanisms.

   Added advice against naive implementation of flow control.

   Added session header section.

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

   Altered flow control properties to include session-level limits.

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

   Changed protocol label form based on discussions.

A.2.

A.3.  Since draft-ietf-httpbis-http2-00

   Changed title throughout.

   Removed section on Incompatibilities with SPDY draft#2.

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

   Replaced abstract and introduction.

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

   Removed unused references.

   Added flow control principles (Section 3.6.1) based on <http://
   tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>.

A.3.

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

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

   Updated authors/editors list.

   Added status note.

Authors' Addresses

   Mike Belshe
   Twist

   EMail: mbelshe@chromium.org

   Roberto Peon
   Google, Inc

   EMail: fenix@google.com
   Martin Thomson (editor)
   Microsoft
   3210 Porter Drive
   Palo Alto  94043  94304
   US

   EMail: martin.thomson@skype.net

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

   EMail: Alexey.Melnikov@isode.com