rfc9113 Varnish source references



Internet Engineering Task Force (IETF)                   M. Thomson, Ed.

Request for Comments: 9113                                       Mozilla

Obsoletes: 7540, 8740                                   C. Benfield, Ed.

Category: Standards Track                                     Apple Inc.

ISSN: 2070-1721                                                June 2022

                                 HTTP/2

Abstract

   This specification describes an optimized expression of the semantics

   of the Hypertext Transfer Protocol (HTTP), referred to as HTTP

   version 2 (HTTP/2).  HTTP/2 enables a more efficient use of network

   resources and a reduced latency by introducing field compression and

   allowing multiple concurrent exchanges on the same connection.

   This document obsoletes RFCs 7540 and 8740.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force

   (IETF).  It represents the consensus of the IETF community.  It has

   received public review and has been approved for publication by the

   Internet Engineering Steering Group (IESG).  Further information on

   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,

   and how to provide feedback on it may be obtained at

   https://www.rfc-editor.org/info/rfc9113.

Copyright Notice

   Copyright (c) 2022 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

   (https://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 Revised BSD License text as described in Section 4.e of the

   Trust Legal Provisions and are provided without warranty as described

   in the Revised BSD License.

Table of Contents

   1.  Introduction

   2.  HTTP/2 Protocol Overview

     2.1.  Document Organization

     2.2.  Conventions and Terminology

   3.  Starting HTTP/2

     3.1.  HTTP/2 Version Identification

     3.2.  Starting HTTP/2 for "https" URIs

     3.3.  Starting HTTP/2 with Prior Knowledge

     3.4.  HTTP/2 Connection Preface

   4.  HTTP Frames

     4.1.  Frame Format

     4.2.  Frame Size

     4.3.  Field Section Compression and Decompression

       4.3.1.  Compression State

   5.  Streams and Multiplexing

     5.1.  Stream States

       5.1.1.  Stream Identifiers

       5.1.2.  Stream Concurrency

     5.2.  Flow Control

       5.2.1.  Flow-Control Principles

       5.2.2.  Appropriate Use of Flow Control

       5.2.3.  Flow-Control Performance

     5.3.  Prioritization

       5.3.1.  Background on Priority in RFC 7540

       5.3.2.  Priority Signaling in This Document

     5.4.  Error Handling

       5.4.1.  Connection Error Handling

       5.4.2.  Stream Error Handling

       5.4.3.  Connection Termination

     5.5.  Extending HTTP/2

   6.  Frame Definitions

     6.1.  DATA

     6.2.  HEADERS

     6.3.  PRIORITY

     6.4.  RST_STREAM

     6.5.  SETTINGS

       6.5.1.  SETTINGS Format

       6.5.2.  Defined Settings

       6.5.3.  Settings Synchronization

     6.6.  PUSH_PROMISE

     6.7.  PING

     6.8.  GOAWAY

     6.9.  WINDOW_UPDATE

       6.9.1.  The Flow-Control Window

       6.9.2.  Initial Flow-Control Window Size

       6.9.3.  Reducing the Stream Window Size

     6.10. CONTINUATION

   7.  Error Codes

   8.  Expressing HTTP Semantics in HTTP/2

     8.1.  HTTP Message Framing

       8.1.1.  Malformed Messages

     8.2.  HTTP Fields

       8.2.1.  Field Validity

       8.2.2.  Connection-Specific Header Fields

       8.2.3.  Compressing the Cookie Header Field

     8.3.  HTTP Control Data

       8.3.1.  Request Pseudo-Header Fields

       8.3.2.  Response Pseudo-Header Fields

     8.4.  Server Push

       8.4.1.  Push Requests

       8.4.2.  Push Responses

     8.5.  The CONNECT Method

     8.6.  The Upgrade Header Field

     8.7.  Request Reliability

     8.8.  Examples

       8.8.1.  Simple Request

       8.8.2.  Simple Response

       8.8.3.  Complex Request

       8.8.4.  Response with Body

       8.8.5.  Informational Responses

   9.  HTTP/2 Connections

     9.1.  Connection Management

       9.1.1.  Connection Reuse

     9.2.  Use of TLS Features

       9.2.1.  TLS 1.2 Features

       9.2.2.  TLS 1.2 Cipher Suites

       9.2.3.  TLS 1.3 Features

   10. Security Considerations

     10.1.  Server Authority

     10.2.  Cross-Protocol Attacks

     10.3.  Intermediary Encapsulation Attacks

     10.4.  Cacheability of Pushed Responses

     10.5.  Denial-of-Service Considerations

       10.5.1.  Limits on Field Block Size

       10.5.2.  CONNECT Issues

     10.6.  Use of Compression

     10.7.  Use of Padding

     10.8.  Privacy Considerations

     10.9.  Remote Timing Attacks

   11. IANA Considerations

     11.1.  HTTP2-Settings Header Field Registration

     11.2.  The h2c Upgrade Token

   12. References

     12.1.  Normative References

     12.2.  Informative References

   Appendix A.  Prohibited TLS 1.2 Cipher Suites

   Appendix B.  Changes from RFC 7540

   Acknowledgments

   Contributors

   Authors' Addresses

1.  Introduction

   The performance of applications using the Hypertext Transfer Protocol

   (HTTP, [HTTP]) is linked to how each version of HTTP uses the

   underlying transport, and the conditions under which the transport

   operates.

   Making multiple concurrent requests can reduce latency and improve

   application performance.  HTTP/1.0 allowed only one request to be

   outstanding at a time on a given TCP [TCP] connection.  HTTP/1.1

   [HTTP/1.1] added request pipelining, but this only partially

   addressed request concurrency and still suffers from application-

   layer head-of-line blocking.  Therefore, HTTP/1.0 and HTTP/1.1

   clients use multiple connections to a server to make concurrent

   requests.

   Furthermore, HTTP fields are often repetitive and verbose, causing

   unnecessary network traffic as well as causing the initial TCP

   congestion window to quickly fill.  This can result in excessive

   latency when multiple requests are made on a new TCP connection.

   HTTP/2 addresses these issues by defining an optimized mapping of

   HTTP's semantics to an underlying connection.  Specifically, it

   allows interleaving of messages on the same connection and uses an

   efficient coding for HTTP fields.  It also allows prioritization of

   requests, letting more important requests complete more quickly,

   further improving performance.

   The resulting protocol is more friendly to the network because fewer

   TCP connections can be used in comparison to HTTP/1.x.  This means

   less competition with other flows and longer-lived connections, which

   in turn lead to better utilization of available network capacity.

   Note, however, that TCP head-of-line blocking is not addressed by

   this protocol.

   Finally, HTTP/2 also enables more efficient processing of messages

   through use of binary message framing.

   This document obsoletes RFCs 7540 and 8740.  Appendix B lists notable

   changes.

2.  HTTP/2 Protocol Overview

   HTTP/2 provides an optimized transport for HTTP semantics.  HTTP/2

   supports all of the core features of HTTP but aims to be more

   efficient than HTTP/1.1.

   HTTP/2 is a connection-oriented application-layer protocol that runs

   over a TCP connection ([TCP]).  The client is the TCP connection

   initiator.

   The basic protocol unit in HTTP/2 is a frame (Section 4.1).  Each

   frame type serves a different purpose.  For example, HEADERS and DATA

   frames form the basis of HTTP requests and responses (Section 8.1);

   other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are

   used in support of other HTTP/2 features.

   Multiplexing of requests is achieved by having each HTTP request/

   response exchange associated with its own stream (Section 5).

   Streams are largely independent of each other, so a blocked or

   stalled request or response does not prevent progress on other

   streams.

   Effective use of multiplexing depends on flow control and

   prioritization.  Flow control (Section 5.2) ensures that it is

   possible to efficiently use multiplexed streams by restricting data

   that is transmitted to what the receiver is able to handle.

   Prioritization (Section 5.3) ensures that limited resources are used

   most effectively.  This revision of HTTP/2 deprecates the priority

   signaling scheme from [RFC7540].

   Because HTTP fields used in a connection can contain large amounts of

   redundant data, frames that contain them are compressed

   (Section 4.3).  This has especially advantageous impact upon request

   sizes in the common case, allowing many requests to be compressed

   into one packet.

   Finally, HTTP/2 adds a new, optional interaction mode whereby a

   server can push responses to a client (Section 8.4).  This is

   intended to allow a server to speculatively send data to a client

   that the server anticipates the client will need, trading off some

   network usage against a potential latency gain.  The server does this

   by synthesizing a request, which it sends as a PUSH_PROMISE frame.

   The server is then able to send a response to the synthetic request

   on a separate stream.

2.1.  Document Organization

   The HTTP/2 specification is split into four parts:

   *  Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is

      initiated.

   *  The frame (Section 4) and stream (Section 5) layers describe the

      way HTTP/2 frames are structured and formed into multiplexed

      streams.

   *  Frame (Section 6) and error (Section 7) definitions include

      details of the frame and error types used in HTTP/2.

   *  HTTP mappings (Section 8) and additional requirements (Section 9)

      describe how HTTP semantics are expressed using frames and

      streams.

   While some of the frame- and stream-layer concepts are isolated from

   HTTP, this specification does not define a completely generic frame

   layer.  The frame and stream layers are tailored to the needs of

   HTTP.

2.2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and

   "OPTIONAL" in this document are to be interpreted as described in

   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all

   capitals, as shown here.

   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.

   This specification describes binary formats using the conventions

   described in Section 1.3 of RFC 9000 [QUIC].  Note that this format

   uses network byte order and that high-valued bits are listed before

   low-valued bits.

   The following terms are used:

   client:  The endpoint that initiates an HTTP/2 connection.  Clients

      send HTTP requests and receive HTTP responses.

   connection:  A transport-layer connection between two endpoints.

   connection error:  An error that affects the entire HTTP/2

      connection.

   endpoint:  Either the client or server of the connection.

   frame:  The smallest unit of communication within an HTTP/2

      connection, consisting of a header and a variable-length sequence

      of octets 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 that accepts an HTTP/2 connection.  Servers

      receive HTTP requests and send HTTP responses.

   stream:  A bidirectional flow of frames within the HTTP/2 connection.

   stream error:  An error on the individual HTTP/2 stream.

   Finally, the terms "gateway", "intermediary", "proxy", and "tunnel"

   are defined in Section 3.7 of [HTTP].  Intermediaries act as both

   client and server at different times.

   The term "content" as it applies to message bodies is defined in

   Section 6.4 of [HTTP].

3.  Starting HTTP/2

   Implementations that generate HTTP requests need to discover whether

   a server supports HTTP/2.

   HTTP/2 uses the "http" and "https" URI schemes defined in Section 4.2

   of [HTTP], with the same default port numbers as HTTP/1.1 [HTTP/1.1].

   These URIs do not include any indication about what HTTP versions an

   upstream server (the immediate peer to which the client wishes to

   establish a connection) supports.

   The means by which support for HTTP/2 is determined is different for

   "http" and "https" URIs.  Discovery for "https" URIs is described in

   Section 3.2.  HTTP/2 support for "http" URIs can only be discovered

   by out-of-band means and requires prior knowledge of the support as

   described in Section 3.3.

3.1.  HTTP/2 Version Identification

   The protocol defined in this document has two identifiers.  Creating

   a connection based on either implies the use of the transport,

   framing, and message semantics described in this document.

   *  The string "h2" identifies the protocol where HTTP/2 uses

      Transport Layer Security (TLS); see Section 9.2.  This identifier

      is used in the TLS Application-Layer Protocol Negotiation (ALPN)

      extension [TLS-ALPN] field and in any place where HTTP/2 over TLS

      is identified.

      The "h2" string is serialized into an ALPN protocol identifier as

      the two-octet sequence: 0x68, 0x32.

   *  The "h2c" string was previously used as a token for use in the

      HTTP Upgrade mechanism's Upgrade header field (Section 7.8 of

      [HTTP]).  This usage was never widely deployed and is deprecated

      by this document.  The same applies to the HTTP2-Settings header

      field, which was used with the upgrade to "h2c".

3.2.  Starting HTTP/2 for "https" URIs

   A client that makes a request to an "https" URI uses TLS [TLS13] with

   the ALPN extension [TLS-ALPN].

   HTTP/2 over TLS uses the "h2" protocol identifier.  The "h2c"

   protocol identifier MUST NOT be sent by a client or selected by a

   server; the "h2c" protocol identifier describes a protocol that does

   not use TLS.

   Once TLS negotiation is complete, both the client and the server MUST

   send a connection preface (Section 3.4).

3.3.  Starting HTTP/2 with Prior Knowledge

   A client can learn that a particular server supports HTTP/2 by other

   means.  For example, a client could be configured with knowledge that

   a server supports HTTP/2.

   A client that knows that a server supports HTTP/2 can establish a TCP

   connection and send the connection preface (Section 3.4) followed by

   HTTP/2 frames.  Servers can identify these connections by the

   presence of the connection preface.  This only affects the

   establishment of HTTP/2 connections over cleartext TCP; HTTP/2

   connections over TLS MUST use protocol negotiation in TLS [TLS-ALPN].

   Likewise, the server MUST send a connection preface (Section 3.4).

   Without additional information, prior support for HTTP/2 is not a

   strong signal that a given server will support HTTP/2 for future

   connections.  For example, it is possible for server configurations

   to change, for configurations to differ between instances in

   clustered servers, or for network conditions to change.

3.4.  HTTP/2 Connection Preface

   In HTTP/2, each endpoint is required to send a connection preface as

   a final confirmation of the protocol in use and to establish the

   initial settings for the HTTP/2 connection.  The client and server

   each send a different connection preface.

   The client connection preface starts with a sequence of 24 octets,

   which in hex notation is:

     0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

   That is, the connection preface starts with the string "PRI *

   HTTP/2.0\r\n\r\nSM\r\n\r\n".  This sequence MUST be followed by a

   SETTINGS frame (Section 6.5), which MAY be empty.  The client sends

   the client connection preface as the first application data octets of

   a connection.

      |  Note: The client connection preface is selected so that a large

      |  proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries

      |  do not attempt to process further frames.  Note that this does

      |  not address the concerns raised in [TALKING].

   The server connection preface consists of a potentially empty

   SETTINGS frame (Section 6.5) that MUST be the first frame the server

   sends in the HTTP/2 connection.

   The SETTINGS frames received from a peer as part of the connection

   preface MUST be acknowledged (see Section 6.5.3) after sending the

   connection preface.

   To avoid unnecessary latency, clients are permitted to send

   additional frames to the server immediately after sending the client

   connection preface, without waiting to receive the server connection

   preface.  It is important to note, however, that the server

   connection preface SETTINGS frame might include settings 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 settings established.  In some configurations, it is

   possible for the server to transmit SETTINGS before the client sends

   additional frames, providing an opportunity to avoid this issue.

   Clients and servers MUST treat an invalid connection preface as a

   connection error (Section 5.4.1) of type PROTOCOL_ERROR.  A GOAWAY

   frame (Section 6.8) MAY be omitted in this case, since an invalid

   preface indicates that the peer is not using HTTP/2.

4.  HTTP Frames

   Once the HTTP/2 connection is established, endpoints can begin

   exchanging frames.

4.1.  Frame Format

   All frames begin with a fixed 9-octet header followed by a variable-

   length frame payload.

   HTTP Frame {

     Length (24),

     Type (8),

     Flags (8),

     Reserved (1),

     Stream Identifier (31),

     Frame Payload (..),

   }

                           Figure 1: Frame Layout

   The fields of the frame header are defined as:

   Length:  The length of the frame payload expressed as an unsigned

      24-bit integer in units of octets.  Values greater than 2^14

      (16,384) MUST NOT be sent unless the receiver has set a larger

      value for SETTINGS_MAX_FRAME_SIZE.

      The 9 octets of the frame header are not included in this value.

   Type:  The 8-bit type of the frame.  The frame type determines the

      format and semantics of the frame.  Frames defined in this

      document are listed in Section 6.  Implementations MUST ignore and

      discard frames of unknown types.

   Flags:  An 8-bit field reserved for boolean flags specific to the

      frame type.

      Flags are assigned semantics specific to the indicated frame type.

      Unused flags are those that have no defined semantics for a

      particular frame type.  Unused flags MUST be ignored on receipt

      and MUST be left unset (0x00) when sending.

   Reserved:  A reserved 1-bit field.  The semantics of this bit are

      undefined, and the bit MUST remain unset (0x00) when sending and

      MUST be ignored when receiving.

   Stream Identifier:  A stream identifier (see Section 5.1.1) expressed

      as an unsigned 31-bit integer.  The value 0x00 is reserved for

      frames that are associated with the connection as a whole as

      opposed to an individual stream.

   The structure and content of the frame payload are dependent entirely

   on the frame type.

4.2.  Frame Size

   The size of a frame payload is limited by the maximum size that a

   receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting.  This

   setting can have any value between 2^14 (16,384) and 2^24-1

   (16,777,215) octets, inclusive.

   All implementations MUST be capable of receiving and minimally

   processing frames up to 2^14 octets in length, plus the 9-octet frame

   header (Section 4.1).  The size of the frame header is not included

   when describing frame sizes.

      |  Note: Certain frame types, such as PING (Section 6.7), impose

      |  additional limits on the amount of frame payload data allowed.

   An endpoint MUST send an error code of FRAME_SIZE_ERROR if a frame

   exceeds the size defined in SETTINGS_MAX_FRAME_SIZE, exceeds any

   limit defined for the frame type, or is too small to contain

   mandatory frame data.  A frame size error in a frame that could alter

   the state of the entire connection MUST be treated as a connection

   error (Section 5.4.1); this includes any frame carrying a field block

   (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and CONTINUATION), a

   SETTINGS frame, and any frame with a stream identifier of 0.

   Endpoints are not obligated to use all available space in a frame.

   Responsiveness can be improved by using frames that are smaller than

   the permitted maximum size.  Sending large frames can result in

   delays in sending time-sensitive frames (such as RST_STREAM,

   WINDOW_UPDATE, or PRIORITY), which, if blocked by the transmission of

   a large frame, could affect performance.

4.3.  Field Section Compression and Decompression

   Field section compression is the process of compressing a set of

   field lines (Section 5.2 of [HTTP]) to form a field block.  Field

   section decompression is the process of decoding a field block into a

   set of field lines.  Details of HTTP/2 field section compression and

   decompression are defined in [COMPRESSION], which, for historical

   reasons, refers to these processes as header compression and

   decompression.

   Each field block carries all of the compressed field lines of a

   single field section.  Header sections also include control data

   associated with the message in the form of pseudo-header fields

   (Section 8.3) that use the same format as a field line.

      |  Note: RFC 7540 [RFC7540] used the term "header block" in place

      |  of the more generic "field block".

   Field blocks carry control data and header sections for requests,

   responses, promised requests, and pushed responses (see Section 8.4).

   All these messages, except for interim responses and requests

   contained in PUSH_PROMISE (Section 6.6) frames, can optionally

   include a field block that carries a trailer section.

   A field section is a collection of field lines.  Each of the field

   lines in a field block carries a single value.  The serialized field

   block is then divided into one or more octet sequences, called field

   block fragments.  The first field block fragment is transmitted

   within the frame payload of HEADERS (Section 6.2) or PUSH_PROMISE

   (Section 6.6), each of which could be followed by CONTINUATION

   (Section 6.10) frames to carry subsequent field block fragments.

   The Cookie header field [COOKIE] is treated specially by the HTTP

   mapping (see Section 8.2.3).

   A receiving endpoint reassembles the field block by concatenating its

   fragments and then decompresses the block to reconstruct the field

   section.

   A complete field section consists of either:

   *  a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag

      set, or

   *  a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag unset

      and one or more CONTINUATION frames, where the last CONTINUATION

      frame has the END_HEADERS flag set.

   Each field block is processed as a discrete unit.  Field blocks MUST

   be transmitted as a contiguous sequence of frames, with no

   interleaved frames of any other type or from any other stream.  The

   last frame in a sequence of HEADERS or CONTINUATION frames has the

   END_HEADERS flag set.  The last frame in a sequence of PUSH_PROMISE

   or CONTINUATION frames has the END_HEADERS flag set.  This allows a

   field block to be logically equivalent to a single frame.

   Field block fragments can only be sent as the frame payload of

   HEADERS, PUSH_PROMISE, or CONTINUATION frames because these frames

   carry data that can modify the compression context maintained by a

   receiver.  An endpoint receiving HEADERS, PUSH_PROMISE, or

   CONTINUATION frames needs to reassemble field blocks and perform

   decompression even if the frames are to be discarded.  A receiver

   MUST terminate the connection with a connection error (Section 5.4.1)

   of type COMPRESSION_ERROR if it does not decompress a field block.

   A decoding error in a field block MUST be treated as a connection

   error (Section 5.4.1) of type COMPRESSION_ERROR.

4.3.1.  Compression State

   Field compression is stateful.  Each endpoint has an HPACK encoder

   context and an HPACK decoder context that are used for encoding and

   decoding all field blocks on a connection.  Section 4 of

   [COMPRESSION] defines the dynamic table, which is the primary state

   for each context.

   The dynamic table has a maximum size that is set by an HPACK decoder.

   An endpoint communicates the size chosen by its HPACK decoder context

   using the SETTINGS_HEADER_TABLE_SIZE setting; see Section 6.5.2.

   When a connection is established, the dynamic table size for the

   HPACK decoder and encoder at both endpoints starts at 4,096 bytes,

   the initial value of the SETTINGS_HEADER_TABLE_SIZE setting.

   Any change to the maximum value set using SETTINGS_HEADER_TABLE_SIZE

   takes effect when the endpoint acknowledges settings (Section 6.5.3).

   The HPACK encoder at that endpoint can set the dynamic table to any

   size up to the maximum value set by the decoder.  An HPACK encoder

   declares the size of the dynamic table with a Dynamic Table Size

   Update instruction (Section 6.3 of [COMPRESSION]).

   Once an endpoint acknowledges a change to SETTINGS_HEADER_TABLE_SIZE

   that reduces the maximum below the current size of the dynamic table,

   its HPACK encoder MUST start the next field block with a Dynamic

   Table Size Update instruction that sets the dynamic table to a size

   that is less than or equal to the reduced maximum; see Section 4.2 of

   [COMPRESSION].  An endpoint MUST treat a field block that follows an

   acknowledgment of the reduction to the maximum dynamic table size as

   a connection error (Section 5.4.1) of type COMPRESSION_ERROR if it

   does not start with a conformant Dynamic Table Size Update

   instruction.

      |  Implementers are advised that reducing the value of

      |  SETTINGS_HEADER_TABLE_SIZE is not widely interoperable.  Use of

      |  the connection preface to reduce the value below the initial

      |  value of 4,096 is somewhat better supported, but this might

      |  fail with some implementations.

5.  Streams and Multiplexing

   A "stream" is an independent, bidirectional sequence of frames

   exchanged between the client and server within an HTTP/2 connection.

   Streams have several important characteristics:

   *  A single HTTP/2 connection can contain multiple concurrently open

      streams, with either endpoint interleaving frames from multiple

      streams.

   *  Streams can be established and used unilaterally or shared by

      either endpoint.

   *  Streams can be closed by either endpoint.

   *  The order in which frames are sent is significant.  Recipients

      process frames in the order they are received.  In particular, the

      order of HEADERS and DATA frames is semantically significant.

   *  Streams are identified by an integer.  Stream identifiers are

      assigned to streams by the endpoint initiating the stream.

5.1.  Stream States

   The lifecycle of a stream is shown in Figure 2.

                                +--------+

                        send PP |        | recv PP

                       ,--------+  idle  +--------.

                      /         |        |         \

                     v          +--------+          v

              +----------+          |           +----------+

              |          |          | send H /  |          |

       ,------+ reserved |          | recv H    | reserved +------.

       |      | (local)  |          |           | (remote) |      |

       |      +---+------+          v           +------+---+      |

       |          |             +--------+             |          |

       |          |     recv ES |        | send ES     |          |

       |   send H |     ,-------+  open  +-------.     | recv H   |

       |          |    /        |        |        \    |          |

       |          v   v         +---+----+         v   v          |

       |      +----------+          |           +----------+      |

       |      |   half-  |          |           |   half-  |      |

       |      |  closed  |          | send R /  |  closed  |      |

       |      | (remote) |          | recv R    | (local)  |      |

       |      +----+-----+          |           +-----+----+      |

       |           |                |                 |           |

       |           | send ES /      |       recv ES / |           |

       |           |  send R /      v        send R / |           |

       |           |  recv R    +--------+   recv R   |           |

       | send R /  `----------->|        |<-----------'  send R / |

       | recv R                 | closed |               recv R   |

       `----------------------->|        |<-----------------------'

                                +--------+

                          Figure 2: Stream States

   send:  endpoint sends this frame

   recv:  endpoint receives this frame

   H:  HEADERS frame (with implied CONTINUATION frames)

   ES:  END_STREAM flag

   R:  RST_STREAM frame

   PP:  PUSH_PROMISE frame (with implied CONTINUATION frames); state

      transitions are for the promised stream

   Note that this diagram shows stream state transitions and the frames

   and flags that affect those transitions only.  In this regard,

   CONTINUATION frames do not result in state transitions; they are

   effectively part of the HEADERS or PUSH_PROMISE that they follow.

   For the purpose of state transitions, the END_STREAM flag is

   processed as a separate event to the frame that bears it; a HEADERS

   frame with the END_STREAM flag set can cause two state transitions.

   Both endpoints have a subjective view of the state of a stream that

   could be different when frames are in transit.  Endpoints do not

   coordinate the creation of streams; they are created unilaterally by

   either endpoint.  The negative consequences of a mismatch in states

   are limited to the "closed" state after sending RST_STREAM, where

   frames might be received for some time after closing.

   Streams have the following states:

   idle:  All streams start in the "idle" state.

      The following transitions are valid from this state:

      *  Sending a HEADERS frame as a client, or receiving a HEADERS

         frame as a server, causes the stream to become "open".  The

         stream identifier is selected as described in Section 5.1.1.

         The same HEADERS frame can also cause a stream to immediately

         become "half-closed".

      *  Sending a PUSH_PROMISE frame on another stream reserves the

         idle stream that is identified for later use.  The stream state

         for the reserved stream transitions to "reserved (local)".

         Only a server may send PUSH_PROMISE frames.

      *  Receiving a PUSH_PROMISE frame on another stream reserves an

         idle stream that is identified for later use.  The stream state

         for the reserved stream transitions to "reserved (remote)".

         Only a client may receive PUSH_PROMISE frames.

      *  Note that the PUSH_PROMISE frame is not sent on the idle stream

         but references the newly reserved stream in the Promised Stream

         ID field.

      *  Opening a stream with a higher-valued stream identifier causes

         the stream to transition immediately to a "closed" state; note

         that this transition is not shown in the diagram.

      Receiving any frame other than HEADERS or PRIORITY on a stream in

      this state MUST be treated as a connection error (Section 5.4.1)

      of type PROTOCOL_ERROR.  If this stream is initiated by the

      server, as described in Section 5.1.1, then receiving a HEADERS

      frame MUST also be treated as a connection error (Section 5.4.1)

      of type PROTOCOL_ERROR.

   reserved (local):  A stream in the "reserved (local)" state is one

      that has been promised by sending a PUSH_PROMISE frame.  A

      PUSH_PROMISE frame reserves an idle stream by associating the

      stream with an open stream that was initiated by the remote peer

      (see Section 8.4).

      In this state, only the following transitions are possible:

      *  The endpoint can send a HEADERS frame.  This causes the stream

         to open in a "half-closed (remote)" state.

      *  Either endpoint can send a RST_STREAM frame to cause the stream

         to become "closed".  This releases the stream reservation.

      An endpoint MUST NOT send any type of frame other than HEADERS,

      RST_STREAM, or PRIORITY in this state.

      A PRIORITY or WINDOW_UPDATE frame MAY be received in this state.

      Receiving any type of frame other than RST_STREAM, PRIORITY, or

      WINDOW_UPDATE on a stream in this state MUST be treated as a

      connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   reserved (remote):  A stream in the "reserved (remote)" state has

      been reserved by a remote peer.

      In this state, only the following transitions are possible:

      *  Receiving a HEADERS frame causes the stream to transition to

         "half-closed (local)".

      *  Either endpoint can send a RST_STREAM frame to cause the stream

         to become "closed".  This releases the stream reservation.

      An endpoint MUST NOT send any type of frame other than RST_STREAM,

      WINDOW_UPDATE, or PRIORITY in this state.

      Receiving any type of frame other than HEADERS, RST_STREAM, or

      PRIORITY on a stream in this state MUST be treated as a connection

      error (Section 5.4.1) of type PROTOCOL_ERROR.

   open:  A stream in the "open" state may be used by both peers to send

      frames of any type.  In this state, sending peers observe

      advertised stream-level flow-control limits (Section 5.2).

      From this state, either endpoint can send a frame with an

      END_STREAM flag set, which causes the stream to transition into

      one of the "half-closed" states.  An endpoint sending an

      END_STREAM flag causes the stream state to become "half-closed

      (local)"; an endpoint receiving an END_STREAM flag causes the

      stream state to become "half-closed (remote)".

      Either endpoint can send a RST_STREAM frame from this state,

      causing it to transition immediately to "closed".

   half-closed (local):  A stream that is in the "half-closed (local)"

      state cannot be used for sending frames other than WINDOW_UPDATE,

      PRIORITY, and RST_STREAM.

      A stream transitions from this state to "closed" when a frame is

      received with the END_STREAM flag set or when either peer sends a

      RST_STREAM frame.

      An endpoint can receive any type of frame in this state.

      Providing flow-control credit using WINDOW_UPDATE frames is

      necessary to continue receiving flow-controlled frames.  In this

      state, a receiver can ignore WINDOW_UPDATE frames, which might

      arrive for a short period after a frame with the END_STREAM flag

      set is sent.

      PRIORITY frames can be received in this state.

   half-closed (remote):  A stream that is "half-closed (remote)" is no

      longer being used by the peer to send frames.  In this state, an

      endpoint is no longer obligated to maintain a receiver flow-

      control window.

      If an endpoint receives additional frames, other than

      WINDOW_UPDATE, PRIORITY, or RST_STREAM, for a stream that is in

      this state, it MUST respond with a stream error (Section 5.4.2) of

      type STREAM_CLOSED.

      A stream that is "half-closed (remote)" can be used by the

      endpoint to send frames of any type.  In this state, the endpoint

      continues to observe advertised stream-level flow-control limits

      (Section 5.2).

      A stream can transition from this state to "closed" by sending a

      frame with the END_STREAM flag set or when either peer sends a

      RST_STREAM frame.

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

      A stream enters the "closed" state after an endpoint both sends

      and receives a frame with an END_STREAM flag set.  A stream also

      enters the "closed" state after an endpoint either sends or

      receives a RST_STREAM frame.

      An endpoint MUST NOT send frames other than PRIORITY on a closed

      stream.  An endpoint MAY treat receipt of any other type of frame

      on a closed stream as a connection error (Section 5.4.1) of type

      STREAM_CLOSED, except as noted below.

      An endpoint that sends a frame with the END_STREAM flag set or a

      RST_STREAM frame might receive a WINDOW_UPDATE or RST_STREAM frame

      from its peer in the time before the peer receives and processes

      the frame that closes the stream.

      An endpoint that sends a RST_STREAM frame on a stream that is in

      the "open" or "half-closed (local)" state could receive any type

      of frame.  The peer might have sent or enqueued for sending these

      frames before processing the RST_STREAM frame.  An endpoint MUST

      minimally process and then discard any frames it receives in this

      state.  This means updating header compression state for HEADERS

      and PUSH_PROMISE frames.  Receiving a PUSH_PROMISE frame also

      causes the promised stream to become "reserved (remote)", even

      when the PUSH_PROMISE frame is received on a closed stream.

      Additionally, the content of DATA frames counts toward the

      connection flow-control window.

      An endpoint can perform this minimal processing for all streams

      that are in the "closed" state.  Endpoints MAY use other signals

      to detect that a peer has received the frames that caused the

      stream to enter the "closed" state and treat receipt of any frame

      other than PRIORITY as a connection error (Section 5.4.1) of type

      PROTOCOL_ERROR.  Endpoints can use frames that indicate that the

      peer has received the closing signal to drive this.  Endpoints

      SHOULD NOT use timers for this purpose.  For example, an endpoint

      that sends a SETTINGS frame after closing a stream can safely

      treat receipt of a DATA frame on that stream as an error after

      receiving an acknowledgment of the settings.  Other things that

      might be used are PING frames, receiving data on streams that were

      created after closing the stream, or responses to requests created

      after closing the stream.

   In the absence of more specific rules, implementations SHOULD treat

   the receipt of a frame that is not expressly permitted in the

   description of a state as a connection error (Section 5.4.1) of type

   PROTOCOL_ERROR.  Note that PRIORITY can be sent and received in any

   stream state.

   The rules in this section only apply to frames defined in this

   document.  Receipt of frames for which the semantics are unknown

   cannot be treated as an error, as the conditions for sending and

   receiving those frames are also unknown; see Section 5.5.

   An example of the state transitions for an HTTP request/response

   exchange can be found in Section 8.8.  An example of the state

   transitions for server push can be found in Sections 8.4.1 and 8.4.2.

5.1.1.  Stream Identifiers

   Streams are identified by an unsigned 31-bit integer.  Streams

   initiated by a client MUST use odd-numbered stream identifiers; those

   initiated by the server MUST use even-numbered stream identifiers.  A

   stream identifier of zero (0x00) is used for connection control

   messages; the stream identifier of zero cannot be used to establish a

   new stream.

   The identifier of a newly established stream MUST be numerically

   greater than all streams that the initiating endpoint has opened or

   reserved.  This governs streams that are opened using a HEADERS frame

   and streams that are reserved using PUSH_PROMISE.  An endpoint that

   receives an unexpected stream identifier MUST respond with a

   connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   A HEADERS frame will transition the client-initiated stream

   identified by the stream identifier in the frame header from "idle"

   to "open".  A PUSH_PROMISE frame will transition the server-initiated

   stream identified by the Promised Stream ID field in the frame

   payload from "idle" to "reserved (local)" or "reserved (remote)".

   When a stream transitions out of the "idle" state, all streams in the

   "idle" state that might have been opened by the peer with a lower-

   valued stream identifier immediately transition to "closed".  That

   is, an endpoint may skip a stream identifier, with the effect being

   that the skipped stream is immediately closed.

   Stream identifiers cannot be reused.  Long-lived connections can

   result in an endpoint exhausting the available range of stream

   identifiers.  A client that is unable to establish a new stream

   identifier can establish a new connection for new streams.  A server

   that is unable to establish a new stream identifier can send a GOAWAY

   frame so that the client is forced to open a new connection for new

   streams.

5.1.2.  Stream Concurrency

   A peer can limit the number of concurrently active streams using the

   SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within

   a SETTINGS frame.  The maximum concurrent streams setting is specific

   to each endpoint and applies only to the peer that receives the

   setting.  That is, clients specify the maximum number of concurrent

   streams the server can initiate, and servers specify the maximum

   number of concurrent streams the client can initiate.

   Streams that are in the "open" state or in either of the "half-

   closed" states count toward the maximum number of streams that an

   endpoint is permitted to open.  Streams in any of these three states

   count toward the limit advertised in the

   SETTINGS_MAX_CONCURRENT_STREAMS setting.  Streams in either of the

   "reserved" states do not count toward the stream limit.

   Endpoints MUST NOT exceed the limit set by their peer.  An endpoint

   that receives a HEADERS frame that causes its advertised concurrent

   stream limit to be exceeded MUST treat this as a stream error

   (Section 5.4.2) of type PROTOCOL_ERROR or REFUSED_STREAM.  The choice

   of error code determines whether the endpoint wishes to enable

   automatic retry (see Section 8.7 for details).

   An endpoint that wishes to reduce the value of

   SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current

   number of open streams can either close streams that exceed the new

   value or allow streams to complete.

5.2.  Flow Control

   Using streams for multiplexing introduces contention over use of the

   TCP connection, resulting in blocked streams.  A flow-control scheme

   ensures that streams on the same connection do not destructively

   interfere with each other.  Flow control is used for both individual

   streams and the connection as a whole.

   HTTP/2 provides for flow control through use of the WINDOW_UPDATE

   frame (Section 6.9).

5.2.1.  Flow-Control Principles

   HTTP/2 stream flow control aims to allow a variety of flow-control

   algorithms to be used without requiring protocol changes.  Flow

   control in HTTP/2 has the following characteristics:

   1.  Flow control is specific to a connection.  HTTP/2 flow control

       operates between the endpoints of a single hop and not over the

       entire end-to-end path.

   2.  Flow control is based on WINDOW_UPDATE frames.  Receivers

       advertise how many octets they are prepared to receive on a

       stream and for the entire connection.  This is a credit-based

       scheme.

   3.  Flow control is directional with overall control provided by the

       receiver.  A receiver MAY choose to set any window size that it

       desires for each stream and for the entire connection.  A sender

       MUST respect flow-control limits imposed by a receiver.  Clients,

       servers, and intermediaries all independently advertise their

       flow-control window 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 65,535 octets

       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.  An endpoint can choose to disable its own flow control, but an

       endpoint cannot ignore flow-control signals from its peer.

   7.  HTTP/2 defines only the format and semantics of the WINDOW_UPDATE

       frame (Section 6.9).  This document does not stipulate how a

       receiver decides when to send this frame or the value that it

       sends, nor does it specify how a sender chooses to send packets.

       Implementations are able to select any algorithm that suits their

       needs.

   Implementations are also responsible for prioritizing the sending of

   requests and responses, choosing how to avoid head-of-line blocking

   for requests, and managing the creation of new streams.  Algorithm

   choices for these could interact with any flow-control algorithm.

5.2.2.  Appropriate Use of Flow Control

   Flow control is defined to protect endpoints that are operating under

   resource constraints.  For example, a proxy needs to share memory

   between many connections and also might have a slow upstream

   connection and a fast downstream one.  Flow control addresses cases

   where the receiver is unable to process data on one stream yet wants

   to continue to process other streams in the same connection.

   Deployments that do not require this capability can advertise a flow-

   control window of the maximum size (2^31-1) and can maintain this

   window by sending a WINDOW_UPDATE frame when any data is received.

   This effectively disables flow control for that receiver.

   Conversely, a sender is always subject to the flow-control window

   advertised by the receiver.

   Deployments with constrained resources (for example, memory) can

   employ flow control to limit the amount of memory a peer can consume.

   Note, however, that this can lead to suboptimal use of available

   network resources if flow control is enabled without knowledge of the

   bandwidth * delay product (see [RFC7323]).

   Even with full awareness of the current bandwidth * delay product,

   implementation of flow control can be difficult.  Endpoints MUST read

   and process HTTP/2 frames from the TCP receive buffer as soon as data

   is available.  Failure to read promptly could lead to a deadlock when

   critical frames, such as WINDOW_UPDATE, are not read and acted upon.

   Reading frames promptly does not expose endpoints to resource

   exhaustion attacks, as HTTP/2 flow control limits resource

   commitments.

5.2.3.  Flow-Control Performance

   If an endpoint cannot ensure that its peer always has available flow-

   control window space that is greater than the peer's bandwidth *

   delay product on this connection, its receive throughput will be

   limited by HTTP/2 flow control.  This will result in degraded

   performance.

   Sending timely WINDOW_UPDATE frames can improve performance.

   Endpoints will want to balance the need to improve receive throughput

   with the need to manage resource exhaustion risks and should take

   careful note of Section 10.5 in defining their strategy to manage

   window sizes.

5.3.  Prioritization

   In a multiplexed protocol like HTTP/2, prioritizing allocation of

   bandwidth and computation resources to streams can be critical to

   attaining good performance.  A poor prioritization scheme can result

   in HTTP/2 providing poor performance.  With no parallelism at the TCP

   layer, performance could be significantly worse than HTTP/1.1.

   A good prioritization scheme benefits from the application of

   contextual knowledge such as the content of resources, how resources

   are interrelated, and how those resources will be used by a peer.  In

   particular, clients can possess knowledge about the priority of

   requests that is relevant to server prioritization.  In those cases,

   having clients provide priority information can improve performance.

5.3.1.  Background on Priority in RFC 7540

   RFC 7540 defined a rich system for signaling priority of requests.

   However, this system proved to be complex, and it was not uniformly

   implemented.

   The flexible scheme meant that it was possible for clients to express

   priorities in very different ways, with little consistency in the

   approaches that were adopted.  For servers, implementing generic

   support for the scheme was complex.  Implementation of priorities was

   uneven in both clients and servers.  Many server deployments ignored

   client signals when prioritizing their handling of requests.

   In short, the prioritization signaling in RFC 7540 [RFC7540] was not

   successful.

5.3.2.  Priority Signaling in This Document

   This update to HTTP/2 deprecates the priority signaling defined in

   RFC 7540 [RFC7540].  The bulk of the text related to priority signals

   is not included in this document.  The description of frame fields

   and some of the mandatory handling is retained to ensure that

   implementations of this document remain interoperable with

   implementations that use the priority signaling described in RFC

   7540.

   A thorough description of the RFC 7540 priority scheme remains in

   Section 5.3 of [RFC7540].

   Signaling priority information is necessary to attain good

   performance in many cases.  Where signaling priority information is

   important, endpoints are encouraged to use an alternative scheme,

   such as the scheme described in [HTTP-PRIORITY].

   Though the priority signaling from RFC 7540 was not widely adopted,

   the information it provides can still be useful in the absence of

   better information.  Endpoints that receive priority signals in

   HEADERS or PRIORITY frames can benefit from applying that

   information.  In particular, implementations that consume these

   signals would not benefit from discarding these priority signals in

   the absence of alternatives.

   Servers SHOULD use other contextual information in determining

   priority of requests in the absence of any priority signals.  Servers

   MAY interpret the complete absence of signals as an indication that

   the client has not implemented the feature.  The defaults described

   in Section 5.3.5 of [RFC7540] are known to have poor performance

   under most conditions, and their use is unlikely to be deliberate.

5.4.  Error Handling

   HTTP/2 framing permits two classes of errors:

   *  An error condition that renders the entire connection unusable is

      a connection error.

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

   A list of error codes is included in Section 7.

   It is possible that an endpoint will encounter frames that would

   cause multiple errors.  Implementations MAY discover multiple errors

   during processing, but they SHOULD report at most one stream and one

   connection error as a result.

   The first stream error reported for a given stream prevents any other

   errors on that stream from being reported.  In comparison, the

   protocol permits multiple GOAWAY frames, though an endpoint SHOULD

   report just one type of connection error unless an error is

   encountered during graceful shutdown.  If this occurs, an endpoint

   MAY send an additional GOAWAY frame with the new error code, in

   addition to any prior GOAWAY that contained NO_ERROR.

   If an endpoint detects multiple different errors, it MAY choose to

   report any one of those errors.  If a frame causes a connection

   error, that error MUST be reported.  Additionally, an endpoint MAY

   use any applicable error code when it detects an error condition; a

   generic error code (such as PROTOCOL_ERROR or INTERNAL_ERROR) can

   always be used in place of more specific error codes.

5.4.1.  Connection Error Handling

   A connection error is any error that prevents further processing of

   the frame layer or corrupts any connection state.

   An endpoint that encounters a connection error SHOULD first send a

   GOAWAY frame (Section 6.8) with the stream identifier of the last

   stream that it successfully received from its peer.  The GOAWAY frame

   includes an error code (Section 7) that indicates why the connection

   is terminating.  After sending the GOAWAY frame for an error

   condition, the endpoint MUST close the TCP connection.

   It is possible that the GOAWAY will not be reliably received by the

   receiving endpoint.  In the event of a connection error, GOAWAY only

   provides a best-effort attempt to communicate with the peer about why

   the connection is being terminated.

   An endpoint can end a connection at any time.  In particular, an

   endpoint MAY choose to treat a stream error as a connection error.

   Endpoints SHOULD send a GOAWAY frame when ending a connection,

   providing that circumstances permit it.

5.4.2.  Stream Error Handling

   A stream error is an error related to a specific stream that does not

   affect processing of other streams.

   An endpoint that detects a stream error sends a RST_STREAM frame

   (Section 6.4) that contains the stream identifier of the stream where

   the error occurred.  The RST_STREAM frame includes an error code that

   indicates the type of error.

   A RST_STREAM is the last frame that an endpoint can send on a stream.

   The peer that sends the RST_STREAM frame MUST be prepared to receive

   any frames that were sent or enqueued for sending by the remote peer.

   These frames can be ignored, except where they modify connection

   state (such as the state maintained for field section compression

   (Section 4.3) or flow control).

   Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame

   for any stream.  However, an endpoint MAY send additional RST_STREAM

   frames if it receives frames on a closed stream after more than a

   round-trip time.  This behavior is permitted to deal with misbehaving

   implementations.

   To avoid looping, an endpoint MUST NOT send a RST_STREAM in response

   to a RST_STREAM frame.

5.4.3.  Connection Termination

   If the TCP connection is closed or reset while streams remain in the

   "open" or "half-closed" states, then the affected streams cannot be

   automatically retried (see Section 8.7 for details).

5.5.  Extending HTTP/2

   HTTP/2 permits extension of the protocol.  Within the limitations

   described in this section, protocol extensions can be used to provide

   additional services or alter any aspect of the protocol.  Extensions

   are effective only within the scope of a single HTTP/2 connection.

   This applies to the protocol elements defined in this document.  This

   does not affect the existing options for extending HTTP, such as

   defining new methods, status codes, or fields (see Section 16 of

   [HTTP]).

   Extensions are permitted to use new frame types (Section 4.1), new

   settings (Section 6.5), or new error codes (Section 7).  Registries

   for managing these extension points are defined in Section 11 of

   [RFC7540].

   Implementations MUST ignore unknown or unsupported values in all

   extensible protocol elements.  Implementations MUST discard frames

   that have unknown or unsupported types.  This means that any of these

   extension points can be safely used by extensions without prior

   arrangement or negotiation.  However, extension frames that appear in

   the middle of a field block (Section 4.3) are not permitted; these

   MUST be treated as a connection error (Section 5.4.1) of type

   PROTOCOL_ERROR.

   Extensions SHOULD avoid changing protocol elements defined in this

   document or elements for which no extension mechanism is defined.

   This includes changes to the layout of frames, additions or changes

   to the way that frames are composed into HTTP messages (Section 8.1),

   the definition of pseudo-header fields, or changes to any protocol

   element that a compliant endpoint might treat as a connection error

   (Section 5.4.1).

   An extension that changes existing protocol elements or state MUST be

   negotiated before being used.  For example, an extension that changes

   the layout of the HEADERS frame cannot be used until the peer has

   given a positive signal that this is acceptable.  In this case, it

   could also be necessary to coordinate when the revised layout comes

   into effect.  For example, treating frames other than DATA frames as

   flow controlled requires a change in semantics that both endpoints

   need to understand, so this can only be done through negotiation.

   This document doesn't mandate a specific method for negotiating the

   use of an extension but notes that a setting (Section 6.5.2) could be

   used for that purpose.  If both peers set a value that indicates

   willingness to use the extension, then the extension can be used.  If

   a setting is used for extension negotiation, the initial value MUST

   be defined in such a fashion that the extension is initially

   disabled.

6.  Frame Definitions

   This specification defines a number of frame types, each identified

   by a unique 8-bit type code.  Each frame type serves a distinct

   purpose in the establishment and management of either the connection

   as a whole or individual streams.

   The transmission of specific frame types can alter the state of a

   connection.  If endpoints fail to maintain a synchronized view of the

   connection state, successful communication within the connection will

   no longer be possible.  Therefore, it is important that endpoints

   have a shared comprehension of how the state is affected by the use

   of any given frame.

6.1.  DATA

   DATA frames (type=0x00) 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 message

   contents.

   DATA frames MAY also contain padding.  Padding can be added to DATA

   frames to obscure the size of messages.  Padding is a security

   feature; see Section 10.7.

   DATA Frame {

     Length (24),

     Type (8) = 0x00,

     Unused Flags (4),

     PADDED Flag (1),

     Unused Flags (2),

     END_STREAM Flag (1),

     Reserved (1),

     Stream Identifier (31),

     [Pad Length (8)],

     Data (..),

     Padding (..2040),

   }

                        Figure 3: DATA Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier

   fields are described in Section 4.  The DATA frame contains the

   following additional fields:

   Pad Length:  An 8-bit field containing the length of the frame

      padding in units of octets.  This field is conditional and is only

      present if the PADDED flag is set.

   Data:  Application data.  The amount of data is the remainder of the

      frame payload after subtracting the length of the other fields

      that are present.

   Padding:  Padding octets that contain no application semantic value.

      Padding octets MUST be set to zero when sending.  A receiver is

      not obligated to verify padding but MAY treat non-zero padding as

      a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   The DATA frame defines the following flags:

   PADDED (0x08):  When set, the PADDED flag indicates that the Pad

      Length field and any padding that it describes are present.

   END_STREAM (0x01):  When set, the END_STREAM flag indicates that this

      frame is the last that the endpoint will send for the identified

      stream.  Setting this flag causes the stream to enter one of the

      "half-closed" states or the "closed" state (Section 5.1).

      |  Note: An endpoint that learns of stream closure after sending

      |  all data can close a stream by sending a STREAM frame with a

      |  zero-length Data field and the END_STREAM flag set.  This is

      |  only possible if the endpoint does not send trailers, as the

      |  END_STREAM flag appears on a HEADERS frame in that case; see

      |  Section 8.1.

   DATA frames MUST be associated with a stream.  If a DATA frame is

   received whose Stream Identifier field is 0x00, the recipient MUST

   respond with a connection error (Section 5.4.1) of type

   PROTOCOL_ERROR.

   DATA frames are subject to flow control and can only be sent when a

   stream is in the "open" or "half-closed (remote)" state.  The entire

   DATA frame payload is included in flow control, including the Pad

   Length and Padding fields if present.  If a DATA frame is received

   whose stream is not in the "open" or "half-closed (local)" state, the

   recipient MUST respond with a stream error (Section 5.4.2) of type

   STREAM_CLOSED.

   The total number of padding octets is determined by the value of the

   Pad Length field.  If the length of the padding is the length of the

   frame payload or greater, the recipient MUST treat this as a

   connection error (Section 5.4.1) of type PROTOCOL_ERROR.

      |  Note: A frame can be increased in size by one octet by

      |  including a Pad Length field with a value of zero.

6.2.  HEADERS

   The HEADERS frame (type=0x01) is used to open a stream (Section 5.1),

   and additionally carries a field block fragment.  Despite the name, a

   HEADERS frame can carry a header section or a trailer section.

   HEADERS frames can be sent on a stream in the "idle", "reserved

   (local)", "open", or "half-closed (remote)" state.

   HEADERS Frame {

     Length (24),

     Type (8) = 0x01,

     Unused Flags (2),

     PRIORITY Flag (1),

     Unused Flag (1),

     PADDED Flag (1),

     END_HEADERS Flag (1),

     Unused Flag (1),

     END_STREAM Flag (1),

     Reserved (1),

     Stream Identifier (31),

     [Pad Length (8)],

     [Exclusive (1)],

     [Stream Dependency (31)],

     [Weight (8)],

     Field Block Fragment (..),

     Padding (..2040),

   }

                       Figure 4: HEADERS Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier

   fields are described in Section 4.  The HEADERS frame payload has the

   following additional fields:

   Pad Length:  An 8-bit field containing the length of the frame

      padding in units of octets.  This field is only present if the

      PADDED flag is set.

   Exclusive:  A single-bit flag.  This field is only present if the

      PRIORITY flag is set.  Priority signals in HEADERS frames are

      deprecated; see Section 5.3.2.

   Stream Dependency:  A 31-bit stream identifier.  This field is only

      present if the PRIORITY flag is set.

   Weight:  An unsigned 8-bit integer.  This field is only present if

      the PRIORITY flag is set.

   Field Block Fragment:  A field block fragment (Section 4.3).

   Padding:  Padding octets that contain no application semantic value.

      Padding octets MUST be set to zero when sending.  A receiver is

      not obligated to verify padding but MAY treat non-zero padding as

      a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   The HEADERS frame defines the following flags:

   PRIORITY (0x20):  When set, the PRIORITY flag indicates that the

      Exclusive, Stream Dependency, and Weight fields are present.

   PADDED (0x08):  When set, the PADDED flag indicates that the Pad

      Length field and any padding that it describes are present.

   END_HEADERS (0x04):  When set, the END_HEADERS flag indicates that

      this frame contains an entire field block (Section 4.3) and is not

      followed by any CONTINUATION frames.

      A HEADERS frame without the END_HEADERS flag set MUST be followed

      by a CONTINUATION frame for the same stream.  A receiver MUST

      treat the receipt of any other type of frame or a frame on a

      different stream as a connection error (Section 5.4.1) of type

      PROTOCOL_ERROR.

   END_STREAM (0x01):  When set, the END_STREAM flag indicates that the

      field block (Section 4.3) is the last that the endpoint will send

      for the identified stream.

      A HEADERS frame with the END_STREAM flag set signals the end of a

      stream.  However, a HEADERS frame with the END_STREAM flag set can

      be followed by CONTINUATION frames on the same stream.  Logically,

      the CONTINUATION frames are part of the HEADERS frame.

   The frame payload of a HEADERS frame contains a field block fragment

   (Section 4.3).  A field block that does not fit within a HEADERS

   frame is continued in a CONTINUATION frame (Section 6.10).

   HEADERS frames MUST be associated with a stream.  If a HEADERS frame

   is received whose Stream Identifier field is 0x00, the recipient MUST

   respond with a connection error (Section 5.4.1) of type

   PROTOCOL_ERROR.

   The HEADERS frame changes the connection state as described in

   Section 4.3.

   The total number of padding octets is determined by the value of the

   Pad Length field.  If the length of the padding is the length of the

   frame payload or greater, the recipient MUST treat this as a

   connection error (Section 5.4.1) of type PROTOCOL_ERROR.

      |  Note: A frame can be increased in size by one octet by

      |  including a Pad Length field with a value of zero.

6.3.  PRIORITY

   The PRIORITY frame (type=0x02) is deprecated; see Section 5.3.2.  A

   PRIORITY frame can be sent in any stream state, including idle or

   closed streams.

   PRIORITY Frame {

     Length (24) = 0x05,

     Type (8) = 0x02,

     Unused Flags (8),

     Reserved (1),

     Stream Identifier (31),

     Exclusive (1),

     Stream Dependency (31),

     Weight (8),

   }

                      Figure 5: PRIORITY Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier

   fields are described in Section 4.  The frame payload of a PRIORITY

   frame contains the following additional fields:

   Exclusive:  A single-bit flag.

   Stream Dependency:  A 31-bit stream identifier.

   Weight:  An unsigned 8-bit integer.

   The PRIORITY frame does not define any flags.

   The PRIORITY frame always identifies a stream.  If a PRIORITY frame

   is received with a stream identifier of 0x00, the recipient MUST

   respond with a connection error (Section 5.4.1) of type

   PROTOCOL_ERROR.

   Sending or receiving a PRIORITY frame does not affect the state of

   any stream (Section 5.1).  The PRIORITY frame can be sent on a stream

   in any state, including "idle" or "closed".  A PRIORITY frame cannot

   be sent between consecutive frames that comprise a single field block

   (Section 4.3).

   A PRIORITY frame with a length other than 5 octets MUST be treated as

   a stream error (Section 5.4.2) of type FRAME_SIZE_ERROR.

6.4.  RST_STREAM

   The RST_STREAM frame (type=0x03) allows for immediate termination of

   a stream.  RST_STREAM is sent to request cancellation of a stream or

   to indicate that an error condition has occurred.

   RST_STREAM Frame {

     Length (24) = 0x04,

     Type (8) = 0x03,

     Unused Flags (8),

     Reserved (1),

     Stream Identifier (31),

     Error Code (32),

   }

                     Figure 6: RST_STREAM Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier

   fields are described in Section 4.  Additionally, the RST_STREAM

   frame contains a single unsigned, 32-bit integer identifying the

   error code (Section 7).  The error code indicates why the stream is

   being terminated.

   The RST_STREAM frame does not define any flags.

   The RST_STREAM frame fully terminates the referenced stream and

   causes it to enter the "closed" state.  After receiving a RST_STREAM

   on a stream, the receiver MUST NOT send additional frames for that

   stream, except for PRIORITY.  However, after sending the RST_STREAM,

   the sending endpoint MUST be prepared to receive and process

   additional frames sent on the stream that might have been sent by the

   peer prior to the arrival of the RST_STREAM.

   RST_STREAM frames MUST be associated with a stream.  If a RST_STREAM

   frame is received with a stream identifier of 0x00, the recipient

   MUST treat this as a connection error (Section 5.4.1) of type

   PROTOCOL_ERROR.

   RST_STREAM frames MUST NOT be sent for a stream in the "idle" state.

   If a RST_STREAM frame identifying an idle stream is received, the

   recipient MUST treat this as a connection error (Section 5.4.1) of

   type PROTOCOL_ERROR.

   A RST_STREAM frame with a length other than 4 octets MUST be treated

   as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.

6.5.  SETTINGS

   The SETTINGS frame (type=0x04) conveys configuration parameters that

   affect how endpoints communicate, such as preferences and constraints

   on peer behavior.  The SETTINGS frame is also used to acknowledge the

   receipt of those settings.  Individually, a configuration parameter

   from a SETTINGS frame is referred to as a "setting".

   Settings are not negotiated; they describe characteristics of the

   sending peer, which are used by the receiving peer.  Different values

   for the same setting can be advertised by each peer.  For example, a

   client might set a high initial flow-control window, whereas a server

   might set a lower value to conserve resources.

   A SETTINGS frame MUST be sent by both endpoints at the start of a

   connection and MAY be sent at any other time by either endpoint over

   the lifetime of the connection.  Implementations MUST support all of

   the settings defined by this specification.

   Each parameter in a SETTINGS frame replaces any existing value for

   that parameter.  Settings are processed in the order in which they

   appear, and a receiver of a SETTINGS frame does not need to maintain

   any state other than the current value of each setting.  Therefore,

   the value of a SETTINGS parameter is the last value that is seen by a

   receiver.

   SETTINGS frames are acknowledged by the receiving peer.  To enable

   this, the SETTINGS frame defines the ACK flag: