RFC 9113

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



   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

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
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   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
   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

   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

   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

   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

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

   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

   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

   *  The frame (Section 4) and stream (Section 5) layers describe the
      way HTTP/2 frames are structured and formed into multiplexed

   *  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

   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

2.2.  Conventions and Terminology

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

   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

   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:


   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

   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

   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

      |  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 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,

      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

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

   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

   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

   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

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

   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

   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

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

   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

   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

   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

   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

   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

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

   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

   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

   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.


   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

   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

   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.


   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

   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.


   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

   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

   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.


   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

   SETTINGS frames are acknowledged by the receiving peer.  To enable
   this, the SETTINGS frame defines the ACK flag:

   ACK (0x01):  When set, the ACK flag indicates that this frame
      acknowledges receipt and application of the peer's SETTINGS frame.
      When this bit is set, the frame payload of the SETTINGS frame MUST
      be empty.  Receipt of a SETTINGS frame with the ACK flag set and a
      length field value other than 0 MUST be treated as a connection
      error (Section 5.4.1) of type FRAME_SIZE_ERROR.  For more
      information, see Section 6.5.3 ("Settings Synchronization").

   SETTINGS frames always apply to a connection, never a single stream.
   The stream identifier for a SETTINGS frame MUST be zero (0x00).  If
   an endpoint receives a SETTINGS frame whose Stream Identifier field
   is anything other than 0x00, the endpoint MUST respond with a
   connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   The SETTINGS frame affects connection state.  A badly formed or
   incomplete SETTINGS frame MUST be treated as a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.

   A SETTINGS frame with a length other than a multiple of 6 octets MUST
   be treated as a connection error (Section 5.4.1) of type

6.5.1.  SETTINGS Format

   The frame payload of a SETTINGS frame consists of zero or more
   settings, each consisting of an unsigned 16-bit setting identifier
   and an unsigned 32-bit value.

   SETTINGS Frame {
     Length (24),
     Type (8) = 0x04,

     Unused Flags (7),
     ACK Flag (1),

     Reserved (1),
     Stream Identifier (31) = 0,

     Setting (48) ...,

   Setting {
     Identifier (16),
     Value (32),

                      Figure 7: SETTINGS Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
   fields are described in Section 4.  The frame payload of a SETTINGS
   frame contains any number of Setting fields, each of which consists

   Identifier:  A 16-bit setting identifier; see Section 6.5.2.

   Value:  A 32-bit value for the setting.

6.5.2.  Defined Settings

   The following settings are defined:

   SETTINGS_HEADER_TABLE_SIZE (0x01):  This setting allows the sender to
      inform the remote endpoint of the maximum size of the compression
      table used to decode field blocks, in units of octets.  The
      encoder can select any size equal to or less than this value by
      using signaling specific to the compression format inside a field
      block (see [COMPRESSION]).  The initial value is 4,096 octets.

   SETTINGS_ENABLE_PUSH (0x02):  This setting can be used to enable or
      disable server push.  A server MUST NOT send a PUSH_PROMISE frame
      if it receives this parameter set to a value of 0; see
      Section 8.4.  A client that has both set this parameter to 0 and
      had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame
      as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

      The initial value of SETTINGS_ENABLE_PUSH is 1.  For a client,
      this value indicates that it is willing to receive PUSH_PROMISE
      frames.  For a server, this initial value has no effect, and is
      equivalent to the value 0.  Any value other than 0 or 1 MUST be
      treated as a connection error (Section 5.4.1) of type

      A server MUST NOT explicitly set this value to 1.  A server MAY
      choose to omit this setting when it sends a SETTINGS frame, but if
      a server does include a value, it MUST be 0.  A client MUST treat
      receipt of a SETTINGS frame with SETTINGS_ENABLE_PUSH set to 1 as
      a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   SETTINGS_MAX_CONCURRENT_STREAMS (0x03):  This setting indicates the
      maximum number of concurrent streams that the sender will allow.
      This limit is directional: it applies to the number of streams
      that the sender permits the receiver to create.  Initially, there
      is no limit to this value.  It is recommended that this value be
      no smaller than 100, so as to not unnecessarily limit parallelism.

      treated as special by endpoints.  A zero value does prevent the
      creation of new streams; however, this can also happen for any
      limit that is exhausted with active streams.  Servers SHOULD only
      set a zero value for short durations; if a server does not wish to
      accept requests, closing the connection is more appropriate.

   SETTINGS_INITIAL_WINDOW_SIZE (0x04):  This setting indicates the
      sender's initial window size (in units of octets) for stream-level
      flow control.  The initial value is 2^16-1 (65,535) octets.

      This setting affects the window size of all streams (see
      Section 6.9.2).

      Values above the maximum flow-control window size of 2^31-1 MUST
      be treated as a connection error (Section 5.4.1) of type

   SETTINGS_MAX_FRAME_SIZE (0x05):  This setting indicates the size of
      the largest frame payload that the sender is willing to receive,
      in units of octets.

      The initial value is 2^14 (16,384) octets.  The value advertised
      by an endpoint MUST be between this initial value and the maximum
      allowed frame size (2^24-1 or 16,777,215 octets), inclusive.
      Values outside this range MUST be treated as a connection error
      (Section 5.4.1) of type PROTOCOL_ERROR.

   SETTINGS_MAX_HEADER_LIST_SIZE (0x06):  This advisory setting informs
      a peer of the maximum field section size that the sender is
      prepared to accept, in units of octets.  The value is based on the
      uncompressed size of field lines, including the length of the name
      and value in units of octets plus an overhead of 32 octets for
      each field line.

      For any given request, a lower limit than what is advertised MAY
      be enforced.  The initial value of this setting is unlimited.

   An endpoint that receives a SETTINGS frame with any unknown or
   unsupported identifier MUST ignore that setting.

6.5.3.  Settings Synchronization

   Most values in SETTINGS benefit from or require an understanding of
   when the peer has received and applied the changed parameter values.
   In order to provide such synchronization timepoints, the recipient of
   a SETTINGS frame in which the ACK flag is not set MUST apply the
   updated settings as soon as possible upon receipt.  SETTINGS frames
   are acknowledged in the order in which they are received.

   The values in the SETTINGS frame MUST be processed in the order they
   appear, with no other frame processing between values.  Unsupported
   settings MUST be ignored.  Once all values have been processed, the
   recipient MUST immediately emit a SETTINGS frame with the ACK flag
   set.  Upon receiving a SETTINGS frame with the ACK flag set, the
   sender of the altered settings can rely on the values from the oldest
   unacknowledged SETTINGS frame having been applied.

   If the sender of a SETTINGS frame does not receive an acknowledgment
   within a reasonable amount of time, it MAY issue a connection error
   (Section 5.4.1) of type SETTINGS_TIMEOUT.  In setting a timeout, some
   allowance needs to be made for processing delays at the peer; a
   timeout that is solely based on the round-trip time between endpoints
   might result in spurious errors.


   The PUSH_PROMISE frame (type=0x05) is used to notify the peer
   endpoint in advance of streams the sender intends to initiate.  The
   PUSH_PROMISE frame includes the unsigned 31-bit identifier of the
   stream the endpoint plans to create along with a field section that
   provides additional context for the stream.  Section 8.4 contains a
   thorough description of the use of PUSH_PROMISE frames.

     Length (24),
     Type (8) = 0x05,

     Unused Flags (4),
     PADDED Flag (1),
     END_HEADERS Flag (1),
     Unused Flags (2),

     Reserved (1),
     Stream Identifier (31),

     [Pad Length (8)],
     Reserved (1),
     Promised Stream ID (31),
     Field Block Fragment (..),
     Padding (..2040),

                    Figure 8: PUSH_PROMISE Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
   fields are described in Section 4.  The PUSH_PROMISE 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.

   Promised Stream ID:  An unsigned 31-bit integer that identifies the
      stream that is reserved by the PUSH_PROMISE.  The promised stream
      identifier MUST be a valid choice for the next stream sent by the
      sender (see "new stream identifier" in Section 5.1.1).

   Field Block Fragment:  A field block fragment (Section 4.3)
      containing the request control data and a header section.

   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 PUSH_PROMISE 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_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 PUSH_PROMISE 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

   PUSH_PROMISE frames MUST only be sent on a peer-initiated stream that
   is in either the "open" or "half-closed (remote)" state.  The stream
   identifier of a PUSH_PROMISE frame indicates the stream it is
   associated with.  If the Stream Identifier field specifies the value
   0x00, a recipient MUST respond with a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.

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

   the peer endpoint is set to 0.  An endpoint that has set this setting
   and has received acknowledgment MUST treat the receipt of a
   PUSH_PROMISE frame as a connection error (Section 5.4.1) of type

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

   A PUSH_PROMISE frame modifies the connection state in two ways.
   First, the inclusion of a field block (Section 4.3) potentially
   modifies the state maintained for field section compression.  Second,
   PUSH_PROMISE also reserves a stream for later use, causing the
   promised stream to enter the "reserved (local)" or "reserved
   (remote)" state.  A sender MUST NOT send a PUSH_PROMISE on a stream
   unless that stream is either "open" or "half-closed (remote)"; the
   sender MUST ensure that the promised stream is a valid choice for a
   new stream identifier (Section 5.1.1) (that is, the promised stream
   MUST be in the "idle" state).

   Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame
   causes the stream state to become indeterminate.  A receiver MUST
   treat the receipt of a PUSH_PROMISE on a stream that is neither
   "open" nor "half-closed (local)" as a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.  However, an endpoint that
   has sent RST_STREAM on the associated stream MUST handle PUSH_PROMISE
   frames that might have been created before the RST_STREAM frame is
   received and processed.

   A receiver MUST treat the receipt of a PUSH_PROMISE that promises an
   illegal stream identifier (Section 5.1.1) as a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.  Note that an illegal stream
   identifier is an identifier for a stream that is not currently in the
   "idle" state.

   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.

6.7.  PING

   The PING frame (type=0x06) is a mechanism for measuring a minimal
   round-trip time from the sender, as well as determining whether an
   idle connection is still functional.  PING frames can be sent from
   any endpoint.

   PING Frame {
     Length (24) = 0x08,
     Type (8) = 0x06,

     Unused Flags (7),
     ACK Flag (1),

     Reserved (1),
     Stream Identifier (31) = 0,

     Opaque Data (64),

                        Figure 9: PING Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
   fields are described in Section 4.

   In addition to the frame header, PING frames MUST contain 8 octets of
   opaque data in the frame payload.  A sender can include any value it
   chooses and use those octets in any fashion.

   Receivers of a PING frame that does not include an ACK flag MUST send
   a PING frame with the ACK flag set in response, with an identical
   frame payload.  PING responses SHOULD be given higher priority than
   any other frame.

   The PING frame defines the following flags:

   ACK (0x01):  When set, the ACK flag indicates that this PING frame is
      a PING response.  An endpoint MUST set this flag in PING
      responses.  An endpoint MUST NOT respond to PING frames containing
      this flag.

   PING frames are not associated with any individual stream.  If a PING
   frame is received with a Stream Identifier field value other than
   0x00, the recipient MUST respond with a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.

   Receipt of a PING frame with a length field value other than 8 MUST
   be treated as a connection error (Section 5.4.1) of type

6.8.  GOAWAY

   The GOAWAY frame (type=0x07) is used to initiate shutdown of a
   connection or to signal serious error conditions.  GOAWAY allows an
   endpoint to gracefully stop accepting new streams while still
   finishing processing of previously established streams.  This enables
   administrative actions, like server maintenance.

   There is an inherent race condition between an endpoint starting new
   streams and the remote peer sending a GOAWAY frame.  To deal with
   this case, the GOAWAY contains the stream identifier of the last
   peer-initiated stream that was or might be processed on the sending
   endpoint in this connection.  For instance, if the server sends a
   GOAWAY frame, the identified stream is the highest-numbered stream
   initiated by the client.

   Once the GOAWAY is sent, the sender will ignore frames sent on
   streams initiated by the receiver if the stream has an identifier
   higher than the included last stream identifier.  Receivers of a
   GOAWAY frame MUST NOT open additional streams on the connection,
   although a new connection can be established for new streams.

   If the receiver of the GOAWAY has sent data on streams with a higher
   stream identifier than what is indicated in the GOAWAY frame, those
   streams are not or will not be processed.  The receiver of the GOAWAY
   frame can treat the streams as though they had never been created at
   all, thereby allowing those streams to be retried later on a new

   Endpoints SHOULD always send a GOAWAY frame before closing a
   connection so that the remote peer 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 what streams it might
   have acted on.

   An endpoint might choose to close a connection without sending a
   GOAWAY for misbehaving peers.

   A GOAWAY frame might not immediately precede closing of the
   connection; a receiver of a GOAWAY that has no more use for the
   connection SHOULD still send a GOAWAY frame before terminating the

   GOAWAY Frame {
     Length (24),
     Type (8) = 0x07,

     Unused Flags (8),

     Reserved (1),
     Stream Identifier (31) = 0,

     Reserved (1),
     Last-Stream-ID (31),
     Error Code (32),
     Additional Debug Data (..),

                       Figure 10: GOAWAY Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
   fields are described in Section 4.

   The GOAWAY frame does not define any flags.

   The GOAWAY frame applies to the connection, not a specific stream.
   An endpoint MUST treat a GOAWAY frame with a stream identifier other
   than 0x00 as a connection error (Section 5.4.1) of type

   The last stream identifier in the GOAWAY frame contains the highest-
   numbered stream identifier for which the sender of the GOAWAY frame
   might have taken some action on or might yet take action on.  All
   streams up to and including the identified stream might have been
   processed in some way.  The last stream identifier can be set to 0 if
   no streams were processed.

      |  Note: In this context, "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.

   If a connection terminates without a GOAWAY frame, the last stream
   identifier is effectively the highest possible stream identifier.

   On streams with lower- or equal-numbered identifiers that were not
   closed completely prior to the connection being closed, reattempting
   requests, transactions, or any protocol activity is not possible,
   except for 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 than or equal to the last stream
   identifier might still complete successfully.  The sender of a GOAWAY
   frame might gracefully shut down a connection by sending a GOAWAY
   frame, maintaining the connection in an "open" state until all in-
   progress streams complete.

   An endpoint MAY send multiple GOAWAY frames if circumstances change.
   For instance, an endpoint that sends GOAWAY with NO_ERROR during
   graceful shutdown could subsequently encounter a condition that
   requires immediate termination of the connection.  The last stream
   identifier from the last GOAWAY frame received indicates which
   streams could have been acted upon.  Endpoints MUST NOT increase the
   value they send in the last stream identifier, since the peers might
   already have retried unprocessed requests on another connection.

   A client that is unable to retry requests loses all requests that are
   in flight when the server closes the connection.  This is especially
   true for intermediaries that might not be serving clients using
   HTTP/2.  A server that is attempting to gracefully shut down a
   connection SHOULD send an initial GOAWAY frame with the last stream
   identifier set to 2^31-1 and a NO_ERROR code.  This signals to the
   client that a shutdown is imminent and that initiating further
   requests is prohibited.  After allowing time for any in-flight stream
   creation (at least one round-trip time), the server MAY send another
   GOAWAY frame with an updated last stream identifier.  This ensures
   that a connection can be cleanly shut down without losing requests.

   After sending a GOAWAY frame, the sender can discard frames for
   streams initiated by the receiver with identifiers higher than the
   identified last stream.  However, any frames that alter connection
   state cannot be completely ignored.  For instance, HEADERS,
   PUSH_PROMISE, and CONTINUATION frames MUST be minimally processed to
   ensure that the state maintained for field section compression is
   consistent (see Section 4.3); similarly, DATA frames MUST be counted
   toward the connection flow-control window.  Failure to process these
   frames can cause flow control or field section compression state to
   become unsynchronized.

   The GOAWAY frame also contains a 32-bit error code (Section 7) that
   contains the reason for closing the connection.

   Endpoints MAY append opaque data to the frame payload of any GOAWAY
   frame.  Additional debug data is intended for diagnostic purposes
   only and carries no semantic value.  Debug information could contain
   security- or privacy-sensitive data.  Logged or otherwise
   persistently stored debug data MUST have adequate safeguards to
   prevent unauthorized access.


   The WINDOW_UPDATE frame (type=0x08) is used to implement flow
   control; see Section 5.2 for an overview.

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

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

   Flow control only applies to frames that are identified as being
   subject to flow control.  Of the frame types defined in this
   document, this includes only DATA frames.  Frames that are exempt
   from flow control MUST be accepted and processed, unless the receiver
   is unable to assign resources to handling the frame.  A receiver MAY
   respond with a stream error (Section 5.4.2) or connection error
   (Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept
   a frame.

     Length (24) = 0x04,
     Type (8) = 0x08,

     Unused Flags (8),

     Reserved (1),
     Stream Identifier (31),

     Reserved (1),
     Window Size Increment (31),

                   Figure 11: WINDOW_UPDATE Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
   fields are described in Section 4.  The frame payload of a
   WINDOW_UPDATE frame is one reserved bit plus an unsigned 31-bit
   integer indicating the number of octets that the sender can transmit
   in addition to the existing flow-control window.  The legal range for
   the increment to the flow-control window is 1 to 2^31-1
   (2,147,483,647) octets.

   The WINDOW_UPDATE frame does not define any flags.

   The WINDOW_UPDATE frame can be specific to a stream or to the entire
   connection.  In the former case, the frame's stream identifier
   indicates the affected stream; in the latter, the value "0" indicates
   that the entire connection is the subject of the frame.

   A receiver MUST treat the receipt of a WINDOW_UPDATE frame with a
   flow-control window increment of 0 as a stream error (Section 5.4.2)
   of type PROTOCOL_ERROR; errors on the connection flow-control window
   MUST be treated as a connection error (Section 5.4.1).

   WINDOW_UPDATE can be sent by a peer that has sent a frame with the
   END_STREAM flag set.  This means that a receiver could receive a
   WINDOW_UPDATE frame on a stream in a "half-closed (remote)" or
   "closed" state.  A receiver MUST NOT treat this as an error (see
   Section 5.1).

   A receiver that receives a flow-controlled frame MUST always account
   for its contribution against the connection flow-control window,
   unless the receiver treats this as a connection error
   (Section 5.4.1).  This is necessary even if the frame is in error.
   The sender counts the frame toward the flow-control window, but if
   the receiver does not, the flow-control window at the sender and
   receiver can become different.

   A WINDOW_UPDATE frame with a length other than 4 octets MUST be
   treated as a connection error (Section 5.4.1) of type

6.9.1.  The Flow-Control Window

   Flow control in HTTP/2 is implemented using a window kept by each
   sender on every stream.  The flow-control window is a simple integer
   value that indicates how many octets of data the sender is permitted
   to transmit; as such, its size is a measure of the buffering capacity
   of the receiver.

   Two flow-control windows are applicable: the stream flow-control
   window and the connection flow-control window.  The sender MUST NOT
   send a flow-controlled frame with a length that exceeds the space
   available in either of the flow-control windows advertised by the
   receiver.  Frames with zero length with the END_STREAM flag set (that
   is, an empty DATA frame) MAY be sent if there is no available space
   in either flow-control window.

   For flow-control calculations, the 9-octet frame header is not

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

   The receiver of a frame sends a WINDOW_UPDATE frame as it consumes
   data and frees up space in flow-control windows.  Separate
   WINDOW_UPDATE frames are sent for the stream- and connection-level
   flow-control windows.  Receivers are advised to have mechanisms in
   place to avoid sending WINDOW_UPDATE frames with very small
   increments; see Section of [RFC1122].

   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
   octets.  If a sender receives a WINDOW_UPDATE that causes a flow-
   control window to exceed this maximum, it MUST terminate either the
   stream or the connection, as appropriate.  For streams, the sender
   sends a RST_STREAM with an error code of FLOW_CONTROL_ERROR; for the
   connection, a GOAWAY frame with an error code of FLOW_CONTROL_ERROR
   is sent.

   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.

6.9.2.  Initial Flow-Control Window Size

   When an HTTP/2 connection is first established, new streams are
   created with an initial flow-control window size of 65,535 octets.
   The connection flow-control window is also 65,535 octets.  Both
   endpoints can adjust the initial window size for new streams by
   including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS
   frame.  The connection flow-control window can only be changed using
   WINDOW_UPDATE frames.

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

   In addition to changing the flow-control window for streams that are
   not yet active, a SETTINGS frame can alter the initial flow-control
   window size for streams with active flow-control windows (that is,
   streams in the "open" or "half-closed (remote)" state).  When the
   value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust
   the size of all stream flow-control windows that it maintains by the
   difference between the new value and the old value.

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

   For example, if the client sends 60 KB immediately on connection
   establishment and the server sets the initial window size to be 16
   KB, the client will recalculate the available flow-control window to
   be -44 KB 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.

   A SETTINGS frame cannot alter the connection flow-control window.

   An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that
   causes any flow-control window to exceed the maximum size as a
   connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.

6.9.3.  Reducing the Stream Window Size

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

   After sending a SETTINGS frame that reduces the initial flow-control
   window size, a receiver MAY continue to process streams that exceed
   flow-control limits.  Allowing streams to continue does not allow the
   receiver to immediately reduce the space it reserves for flow-control
   windows.  Progress on these streams can also stall, since
   WINDOW_UPDATE frames are needed to allow the sender to resume
   sending.  The receiver MAY instead send a RST_STREAM with an error
   code of FLOW_CONTROL_ERROR for the affected streams.


   The CONTINUATION frame (type=0x09) is used to continue a sequence of
   field block fragments (Section 4.3).  Any number of CONTINUATION
   frames can be sent, as long as the preceding frame is on the same
   stream and is a HEADERS, PUSH_PROMISE, or CONTINUATION frame without
   the END_HEADERS flag set.

     Length (24),
     Type (8) = 0x09,

     Unused Flags (5),
     END_HEADERS Flag (1),
     Unused Flags (2),

     Reserved (1),
     Stream Identifier (31),

     Field Block Fragment (..),

                    Figure 12: CONTINUATION Frame Format

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
   fields are described in Section 4.  The CONTINUATION frame payload
   contains a field block fragment (Section 4.3).

   The CONTINUATION frame defines the following flag:

   END_HEADERS (0x04):  When set, the END_HEADERS flag indicates that
      this frame ends a field block (Section 4.3).

      If the END_HEADERS flag is not set, this frame MUST be followed by
      another CONTINUATION frame.  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.

   The CONTINUATION frame changes the connection state as defined in
   Section 4.3.

   CONTINUATION frames MUST be associated with a stream.  If a
   CONTINUATION frame is received with a Stream Identifier field of
   0x00, the recipient MUST respond with a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.

   A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or
   CONTINUATION frame without the END_HEADERS flag set.  A recipient
   that observes violation of this rule MUST respond with a connection
   error (Section 5.4.1) of type PROTOCOL_ERROR.

7.  Error Codes

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

   Error codes share a common code space.  Some error codes apply only
   to either streams or the entire connection and have no defined
   semantics in the other context.

   The following error codes are defined:

   NO_ERROR (0x00):  The associated condition is not a result of an
      error.  For example, a GOAWAY might include this code to indicate
      graceful shutdown of a connection.

   PROTOCOL_ERROR (0x01):  The endpoint detected an unspecific protocol
      error.  This error is for use when a more specific error code is
      not available.

   INTERNAL_ERROR (0x02):  The endpoint encountered an unexpected
      internal error.

   FLOW_CONTROL_ERROR (0x03):  The endpoint detected that its peer
      violated the flow-control protocol.

   SETTINGS_TIMEOUT (0x04):  The endpoint sent a SETTINGS frame but did
      not receive a response in a timely manner.  See Section 6.5.3
      ("Settings Synchronization").

   STREAM_CLOSED (0x05):  The endpoint received a frame after a stream
      was half-closed.

   FRAME_SIZE_ERROR (0x06):  The endpoint received a frame with an
      invalid size.

   REFUSED_STREAM (0x07):  The endpoint refused the stream prior to
      performing any application processing (see Section 8.7 for

   CANCEL (0x08):  The endpoint uses this error code to indicate that
      the stream is no longer needed.

   COMPRESSION_ERROR (0x09):  The endpoint is unable to maintain the
      field section compression context for the connection.

   CONNECT_ERROR (0x0a):  The connection established in response to a
      CONNECT request (Section 8.5) was reset or abnormally closed.

   ENHANCE_YOUR_CALM (0x0b):  The endpoint detected that its peer is
      exhibiting a behavior that might be generating excessive load.

   INADEQUATE_SECURITY (0x0c):  The underlying transport has properties
      that do not meet minimum security requirements (see Section 9.2).

   HTTP_1_1_REQUIRED (0x0d):  The endpoint requires that HTTP/1.1 be
      used instead of HTTP/2.

   Unknown or unsupported error codes MUST NOT trigger any special
   behavior.  These MAY be treated by an implementation as being
   equivalent to INTERNAL_ERROR.

8.  Expressing HTTP Semantics in HTTP/2

   HTTP/2 is an instantiation of the HTTP message abstraction (Section 6
   of [HTTP]).

8.1.  HTTP Message Framing

   A client sends an HTTP request on a new stream, using a previously
   unused stream identifier (Section 5.1.1).  A server sends an HTTP
   response on the same stream as the request.

   An HTTP message (request or response) consists of:

   1.  one HEADERS frame (followed by zero or more CONTINUATION frames)
       containing the header section (see Section 6.3 of [HTTP]),

   2.  zero or more DATA frames containing the message content (see
       Section 6.4 of [HTTP]), and

   3.  optionally, one HEADERS frame (followed by zero or more
       CONTINUATION frames) containing the trailer section, if present
       (see Section 6.5 of [HTTP]).

   For a response only, a server MAY send any number of interim
   responses before the HEADERS frame containing a final response.  An
   interim response consists of a HEADERS frame (which might be followed
   by zero or more CONTINUATION frames) containing the control data and
   header section of an interim (1xx) HTTP response (see Section 15 of
   [HTTP]).  A HEADERS frame with the END_STREAM flag set that carries
   an informational status code is malformed (Section 8.1.1).

   The last frame in the sequence bears an END_STREAM flag, noting that
   a HEADERS frame with the END_STREAM flag set can be followed by
   CONTINUATION frames that carry any remaining fragments of the field

   Other frames (from any stream) MUST NOT occur between the HEADERS
   frame and any CONTINUATION frames that might follow.

   HTTP/2 uses DATA frames to carry message content.  The chunked
   transfer encoding defined in Section 7.1 of [HTTP/1.1] cannot be used
   in HTTP/2; see Section 8.2.2.

   Trailer fields are carried in a field block that also terminates the
   stream.  That is, trailer fields comprise a sequence starting with a
   HEADERS frame, followed by zero or more CONTINUATION frames, where
   the HEADERS frame bears an END_STREAM flag.  Trailers MUST NOT
   include pseudo-header fields (Section 8.3).  An endpoint that
   receives pseudo-header fields in trailers MUST treat the request or
   response as malformed (Section 8.1.1).

   An endpoint that receives a HEADERS frame without the END_STREAM flag
   set after receiving the HEADERS frame that opens a request or after
   receiving a final (non-informational) status code MUST treat the
   corresponding request or response as malformed (Section 8.1.1).

   An HTTP request/response exchange fully consumes a single stream.  A
   request starts with the HEADERS frame that puts the stream into the
   "open" state.  The request ends with a frame with the END_STREAM flag
   set, which causes the stream to become "half-closed (local)" for the
   client and "half-closed (remote)" for the server.  A response stream
   starts with zero or more interim responses in HEADERS frames,
   followed by a HEADERS frame containing a final status code.

   An HTTP response is complete after the server sends -- or the client
   receives -- a frame with the END_STREAM flag set (including any
   CONTINUATION frames needed to complete a field block).  A server can
   send a complete response prior to the client sending an entire
   request if the response does not depend on any portion of the request
   that has not been sent and received.  When this is true, a server MAY
   request that the client abort transmission of a request without error
   by sending a RST_STREAM with an error code of NO_ERROR after sending
   a complete response (i.e., a frame with the END_STREAM flag set).
   Clients MUST NOT discard responses as a result of receiving such a
   RST_STREAM, though clients can always discard responses at their
   discretion for other reasons.

8.1.1.  Malformed Messages

   A malformed request or response is one that is an otherwise valid
   sequence of HTTP/2 frames but is invalid due to the presence of
   extraneous frames, prohibited fields or pseudo-header fields, the
   absence of mandatory pseudo-header fields, the inclusion of uppercase
   field names, or invalid field names and/or values (in certain
   circumstances; see Section 8.2).

   A request or response that includes message content can include a
   content-length header field.  A request or response is also malformed
   if the value of a content-length header field does not equal the sum
   of the DATA frame payload lengths that form the content, unless the
   message is defined as having no content.  For example, 204 or 304
   responses contain no content, as does the response to a HEAD request.
   A response that is defined to have no content, as described in
   Section 6.4.1 of [HTTP], MAY have a non-zero content-length header
   field, even though no content is included in DATA frames.

   Intermediaries that process HTTP requests or responses (i.e., any
   intermediary not acting as a tunnel) MUST NOT forward a malformed
   request or response.  Malformed requests or responses that are
   detected MUST be treated as a stream error (Section 5.4.2) of type

   For malformed requests, a server MAY send an HTTP response prior to
   closing or resetting the stream.  Clients MUST NOT accept a malformed

   Endpoints that progressively process messages might have performed
   some processing before identifying a request or response as
   malformed.  For instance, it might be possible to generate an
   informational or 404 status code without having received a complete
   request.  Similarly, intermediaries might forward incomplete messages
   before detecting errors.  A server MAY generate a final response
   before receiving an entire request when the response does not depend
   on the remainder of the request being correct.

   These requirements are intended to protect against several types of
   common attacks against HTTP; they are deliberately strict because
   being permissive can expose implementations to these vulnerabilities.

8.2.  HTTP Fields

   HTTP fields (Section 5 of [HTTP]) are conveyed by HTTP/2 in the
   HEADERS, CONTINUATION, and PUSH_PROMISE frames, compressed with HPACK

   Field names MUST be converted to lowercase when constructing an
   HTTP/2 message.

8.2.1.  Field Validity

   The definitions of field names and values in HTTP prohibit some
   characters that HPACK might be able to convey.  HTTP/2
   implementations SHOULD validate field names and values according to
   their definitions in Sections 5.1 and 5.5 of [HTTP], respectively,
   and treat messages that contain prohibited characters as malformed
   (Section 8.1.1).

   Failure to validate fields can be exploited for request smuggling
   attacks.  In particular, unvalidated fields might enable attacks when
   messages are forwarded using HTTP/1.1 [HTTP/1.1], where characters
   such as carriage return (CR), line feed (LF), and COLON are used as
   delimiters.  Implementations MUST perform the following minimal
   validation of field names and values:

   *  A field name MUST NOT contain characters in the ranges 0x00-0x20,
      0x41-0x5a, or 0x7f-0xff (all ranges inclusive).  This specifically
      excludes all non-visible ASCII characters, ASCII SP (0x20), and
      uppercase characters ('A' to 'Z', ASCII 0x41 to 0x5a).

   *  With the exception of pseudo-header fields (Section 8.3), which
      have a name that starts with a single colon, field names MUST NOT
      include a colon (ASCII COLON, 0x3a).

   *  A field value MUST NOT contain the zero value (ASCII NUL, 0x00),
      line feed (ASCII LF, 0x0a), or carriage return (ASCII CR, 0x0d) at
      any position.

   *  A field value MUST NOT start or end with an ASCII whitespace
      character (ASCII SP or HTAB, 0x20 or 0x09).

      |  Note: An implementation that validates fields according to the
      |  definitions in Sections 5.1 and 5.5 of [HTTP] only needs an
      |  additional check that field names do not include uppercase
      |  characters.

   A request or response that contains a field that violates any of
   these conditions MUST be treated as malformed (Section 8.1.1).  In
   particular, an intermediary that does not process fields when
   forwarding messages MUST NOT forward fields that contain any of the
   values that are listed as prohibited above.

   When a request message violates one of these requirements, an
   implementation SHOULD generate a 400 (Bad Request) status code (see
   Section 15.5.1 of [HTTP]), unless a more suitable status code is
   defined or the status code cannot be sent (e.g., because the error
   occurs in a trailer field).

      |  Note: Field values that are not valid according to the
      |  definition of the corresponding field do not cause a request to
      |  be malformed; the requirements above only apply to the generic
      |  syntax for fields as defined in Section 5 of [HTTP].

8.2.2.  Connection-Specific Header Fields

   HTTP/2 does not use the Connection header field (Section 7.6.1 of
   [HTTP]) to indicate connection-specific header fields; in this
   protocol, connection-specific metadata is conveyed by other means.
   An endpoint MUST NOT generate an HTTP/2 message containing
   connection-specific header fields.  This includes the Connection
   header field and those listed as having connection-specific semantics
   in Section 7.6.1 of [HTTP] (that is, Proxy-Connection, Keep-Alive,
   Transfer-Encoding, and Upgrade).  Any message containing connection-
   specific header fields MUST be treated as malformed (Section 8.1.1).

   The only exception to this is the TE header field, which MAY be
   present in an HTTP/2 request; when it is, it MUST NOT contain any
   value other than "trailers".

   An intermediary transforming an HTTP/1.x message to HTTP/2 MUST
   remove connection-specific header fields as discussed in
   Section 7.6.1 of [HTTP], or their messages will be treated by other
   HTTP/2 endpoints as malformed (Section 8.1.1).

      |  Note: HTTP/2 purposefully does not support upgrade to another
      |  protocol.  The handshake methods described in Section 3 are
      |  believed sufficient to negotiate the use of alternative
      |  protocols.

8.2.3.  Compressing the Cookie Header Field

   The Cookie header field [COOKIE] uses a semicolon (";") to delimit
   cookie-pairs (or "crumbs").  This header field contains multiple
   values, but does not use a COMMA (",") as a separator, thereby
   preventing cookie-pairs from being sent on multiple field lines (see
   Section 5.2 of [HTTP]).  This can significantly reduce compression
   efficiency, as updates to individual cookie-pairs would invalidate
   any field lines that are stored in the HPACK table.

   To allow for better compression efficiency, the Cookie header field
   MAY be split into separate header fields, each with one or more
   cookie-pairs.  If there are multiple Cookie header fields after
   decompression, these MUST be concatenated into a single octet string
   using the two-octet delimiter of 0x3b, 0x20 (the ASCII string "; ")
   before being passed into a non-HTTP/2 context, such as an HTTP/1.1
   connection, or a generic HTTP server application.

   Therefore, the following two lists of Cookie header fields are
   semantically equivalent.

   cookie: a=b; c=d; e=f

   cookie: a=b
   cookie: c=d
   cookie: e=f

8.3.  HTTP Control Data

   HTTP/2 uses special pseudo-header fields beginning with a ':'
   character (ASCII 0x3a) to convey message control data (see
   Section 6.2 of [HTTP]).

   Pseudo-header fields are not HTTP header fields.  Endpoints MUST NOT
   generate pseudo-header fields other than those defined in this
   document.  Note that an extension could negotiate the use of
   additional pseudo-header fields; see Section 5.5.

   Pseudo-header fields are only valid in the context in which they are
   defined.  Pseudo-header fields defined for requests MUST NOT appear
   in responses; pseudo-header fields defined for responses MUST NOT
   appear in requests.  Pseudo-header fields MUST NOT appear in a
   trailer section.  Endpoints MUST treat a request or response that
   contains undefined or invalid pseudo-header fields as malformed
   (Section 8.1.1).

   All pseudo-header fields MUST appear in a field block before all
   regular field lines.  Any request or response that contains a pseudo-
   header field that appears in a field block after a regular field line
   MUST be treated as malformed (Section 8.1.1).

   The same pseudo-header field name MUST NOT appear more than once in a
   field block.  A field block for an HTTP request or response that
   contains a repeated pseudo-header field name MUST be treated as
   malformed (Section 8.1.1).

8.3.1.  Request Pseudo-Header Fields

   The following pseudo-header fields are defined for HTTP/2 requests:

   *  The ":method" pseudo-header field includes the HTTP method
      (Section 9 of [HTTP]).

   *  The ":scheme" pseudo-header field includes the scheme portion of
      the request target.  The scheme is taken from the target URI
      (Section 3.1 of [RFC3986]) when generating a request directly, or
      from the scheme of a translated request (for example, see
      Section 3.3 of [HTTP/1.1]).  Scheme is omitted for CONNECT
      requests (Section 8.5).

      ":scheme" is not restricted to "http" and "https" schemed URIs.  A
      proxy or gateway can translate requests for non-HTTP schemes,
      enabling the use of HTTP to interact with non-HTTP services.

   *  The ":authority" pseudo-header field conveys the authority portion
      (Section 3.2 of [RFC3986]) of the target URI (Section 7.1 of
      [HTTP]).  The recipient of an HTTP/2 request MUST NOT use the Host
      header field to determine the target URI if ":authority" is

      Clients that generate HTTP/2 requests directly MUST use the
      ":authority" pseudo-header field to convey authority information,
      unless there is no authority information to convey (in which case
      it MUST NOT generate ":authority").

      Clients MUST NOT generate a request with a Host header field that
      differs from the ":authority" pseudo-header field.  A server
      SHOULD treat a request as malformed if it contains a Host header
      field that identifies an entity that differs from the entity in
      the ":authority" pseudo-header field.  The values of fields need
      to be normalized to compare them (see Section 6.2 of [RFC3986]).
      An origin server can apply any normalization method, whereas other
      servers MUST perform scheme-based normalization (see Section 6.2.3
      of [RFC3986]) of the two fields.

      An intermediary that forwards a request over HTTP/2 MUST construct
      an ":authority" pseudo-header field using the authority
      information from the control data of the original request, unless
      the original request's target URI does not contain authority
      information (in which case it MUST NOT generate ":authority").
      Note that the Host header field is not the sole source of this
      information; see Section 7.2 of [HTTP].

      An intermediary that needs to generate a Host header field (which
      might be necessary to construct an HTTP/1.1 request) MUST use the
      value from the ":authority" pseudo-header field as the value of
      the Host field, unless the intermediary also changes the request
      target.  This replaces any existing Host field to avoid potential
      vulnerabilities in HTTP routing.

      An intermediary that forwards a request over HTTP/2 MAY retain any
      Host header field.

      Note that request targets for CONNECT or asterisk-form OPTIONS
      requests never include authority information; see Sections 7.1 and
      7.2 of [HTTP].

      ":authority" MUST NOT include the deprecated userinfo subcomponent
      for "http" or "https" schemed URIs.

   *  The ":path" pseudo-header field includes the path and query parts
      of the target URI (the absolute-path production and, optionally, a
      '?' character followed by the query production; see Section 4.1 of
      [HTTP]).  A request in asterisk form (for OPTIONS) includes the
      value '*' for the ":path" pseudo-header field.

      This pseudo-header field MUST NOT be empty for "http" or "https"
      URIs; "http" or "https" URIs that do not contain a path component
      MUST include a value of '/'.  The exceptions to this rule are:

      -  an OPTIONS request for an "http" or "https" URI that does not
         include a path component; these MUST include a ":path" pseudo-
         header field with a value of '*' (see Section 7.1 of [HTTP]).

      -  CONNECT requests (Section 8.5), where the ":path" pseudo-header
         field is omitted.

   All HTTP/2 requests MUST include exactly one valid value for the
   ":method", ":scheme", and ":path" pseudo-header fields, unless they
   are CONNECT requests (Section 8.5).  An HTTP request that omits
   mandatory pseudo-header fields is malformed (Section 8.1.1).

   Individual HTTP/2 requests do not carry an explicit indicator of
   protocol version.  All HTTP/2 requests implicitly have a protocol
   version of "2.0" (see Section 6.2 of [HTTP]).

8.3.2.  Response Pseudo-Header Fields

   For HTTP/2 responses, a single ":status" pseudo-header field is
   defined that carries the HTTP status code field (see Section 15 of
   [HTTP]).  This pseudo-header field MUST be included in all responses,
   including interim responses; otherwise, the response is malformed
   (Section 8.1.1).

   HTTP/2 responses implicitly have a protocol version of "2.0".

8.4.  Server Push

   HTTP/2 allows a server to preemptively send (or "push") responses
   (along with corresponding "promised" requests) to a client in
   association with a previous client-initiated request.

   Server push was designed to allow a server to improve client-
   perceived performance by predicting what requests will follow those
   that it receives, thereby removing a round trip for them.  For
   example, a request for HTML is often followed by requests for
   stylesheets and scripts referenced by that page.  When these requests
   are pushed, the client does not need to wait to receive the
   references to them in the HTML and issue separate requests.

   In practice, server push is difficult to use effectively, because it
   requires the server to correctly anticipate the additional requests
   the client will make, taking into account factors such as caching,
   content negotiation, and user behavior.  Errors in prediction can
   lead to performance degradation, due to the opportunity cost that the
   additional data on the wire represents.  In particular, pushing any
   significant amount of data can cause contention issues with responses
   that are more important.

   A client can request that server push be disabled, though this is
   negotiated for each hop independently.  The SETTINGS_ENABLE_PUSH
   setting can be set to 0 to indicate that server push is disabled.

   Promised requests MUST be safe (see Section 9.2.1 of [HTTP]) and
   cacheable (see Section 9.2.3 of [HTTP]).  Promised requests cannot
   include any content or a trailer section.  Clients that receive a
   promised request that is not cacheable, that is not known to be safe,
   or that indicates the presence of request content MUST reset the
   promised stream with a stream error (Section 5.4.2) of type
   PROTOCOL_ERROR.  Note that this could result in the promised stream
   being reset if the client does not recognize a newly defined method
   as being safe.

   Pushed responses that are cacheable (see Section 3 of [CACHING]) can
   be stored by the client, if it implements an HTTP cache.  Pushed
   responses are considered successfully validated on the origin server
   (e.g., if the "no-cache" cache response directive is present; see
   Section of [CACHING]) while the stream identified by the
   promised stream identifier is still open.

   Pushed responses that are not cacheable MUST NOT be stored by any
   HTTP cache.  They MAY be made available to the application

   The server MUST include a value in the ":authority" pseudo-header
   field for which the server is authoritative (see Section 10.1).  A
   client MUST treat a PUSH_PROMISE for which the server is not
   authoritative as a stream error (Section 5.4.2) of type

   An intermediary can receive pushes from the server and choose not to
   forward them on to the client.  In other words, how to make use of
   the pushed information is up to that intermediary.  Equally, the
   intermediary might choose to make additional pushes to the client,
   without any action taken by the server.

   A client cannot push.  Thus, servers MUST treat the receipt of a
   PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.  A server cannot set the SETTINGS_ENABLE_PUSH setting
   to a value other than 0 (see Section 6.5.2).

8.4.1.  Push Requests

   Server push is semantically equivalent to a server responding to a
   request; however, in this case, that request is also sent by the
   server, as a PUSH_PROMISE frame.

   The PUSH_PROMISE frame includes a field block that contains control
   data and a complete set of request header fields that the server
   attributes to the request.  It is not possible to push a response to
   a request that includes message content.

   Promised requests are always associated with an explicit request from
   the client.  The PUSH_PROMISE frames sent by the server are sent on
   that explicit request's stream.  The PUSH_PROMISE frame also includes
   a promised stream identifier, chosen from the stream identifiers
   available to the server (see Section 5.1.1).

   The header fields in PUSH_PROMISE and any subsequent CONTINUATION
   frames MUST be a valid and complete set of request header fields
   (Section 8.3.1).  The server MUST include a method in the ":method"
   pseudo-header field that is safe and cacheable.  If a client receives
   a PUSH_PROMISE that does not include a complete and valid set of
   header fields or the ":method" pseudo-header field identifies a
   method that is not safe, it MUST respond on the promised stream with
   a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

   The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to
   sending any frames that reference the promised responses.  This
   avoids a race where clients issue requests prior to receiving any
   PUSH_PROMISE frames.

   For example, if the server receives a request for a document
   containing embedded links to multiple image files and the server
   chooses to push those additional images to the client, sending
   PUSH_PROMISE frames before the DATA frames that contain the image
   links ensures that the client is able to see that a resource will be
   pushed before discovering embedded links.  Similarly, if the server
   pushes resources referenced by the field block (for instance, in Link
   header fields), sending a PUSH_PROMISE before sending the header
   ensures that clients do not request those resources.

   PUSH_PROMISE frames MUST NOT be sent by the client.

   PUSH_PROMISE frames can be sent by the server on any client-initiated
   stream, but the stream MUST be in either the "open" or "half-closed
   (remote)" state with respect to the server.  PUSH_PROMISE frames are
   interspersed with the frames that comprise a response, though they
   cannot be interspersed with HEADERS and CONTINUATION frames that
   comprise a single field block.

   Sending a PUSH_PROMISE frame creates a new stream and puts the stream
   into the "reserved (local)" state for the server and the "reserved
   (remote)" state for the client.

8.4.2.  Push Responses

   After sending the PUSH_PROMISE frame, the server can begin delivering
   the pushed response as a response (Section 8.3.2) on a server-
   initiated stream that uses the promised stream identifier.  The
   server uses this stream to transmit an HTTP response, using the same
   sequence of frames as that defined in Section 8.1.  This stream
   becomes "half-closed" to the client (Section 5.1) after the initial
   HEADERS frame is sent.

   Once a client receives a PUSH_PROMISE frame and chooses to accept the
   pushed response, the client SHOULD NOT issue any requests for the
   promised response until after the promised stream has closed.

   If the client determines, for any reason, that it does not wish to
   receive the pushed response from the server or if the server takes
   too long to begin sending the promised response, the client can send
   a RST_STREAM frame, using either the CANCEL or REFUSED_STREAM code
   and referencing the pushed stream's identifier.

   A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit
   the number of responses that can be concurrently pushed by a server.
   Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero prevents
   the server from opening the streams necessary to push responses.
   However, this does not prevent the server from reserving streams
   using PUSH_PROMISE frames, because reserved streams do not count
   toward the concurrent stream limit.  Clients that do not wish to
   receive pushed resources need to reset any unwanted reserved streams
   or set SETTINGS_ENABLE_PUSH to 0.

   Clients receiving a pushed response MUST validate that either the
   server is authoritative (see Section 10.1) or the proxy that provided
   the pushed response is configured for the corresponding request.  For
   example, a server that offers a certificate for only the example.com
   DNS-ID (see [RFC6125]) is not permitted to push a response for

   The response for a PUSH_PROMISE stream begins with a HEADERS frame,
   which immediately puts the stream into the "half-closed (remote)"
   state for the server and "half-closed (local)" state for the client,
   and ends with a frame with the END_STREAM flag set, which places the
   stream in the "closed" state.

      |  Note: The client never sends a frame with the END_STREAM flag
      |  set for a server push.

8.5.  The CONNECT Method

   The CONNECT method (Section 9.3.6 of [HTTP]) is used to convert an
   HTTP connection into a tunnel to a remote host.  CONNECT is primarily
   used with HTTP proxies to establish a TLS session with an origin
   server for the purposes of interacting with "https" resources.

   In HTTP/2, the CONNECT method establishes a tunnel over a single
   HTTP/2 stream to a remote host, rather than converting the entire
   connection to a tunnel.  A CONNECT header section is constructed as
   defined in Section 8.3.1 ("Request Pseudo-Header Fields"), with a few
   differences.  Specifically:

   *  The ":method" pseudo-header field is set to CONNECT.

   *  The ":scheme" and ":path" pseudo-header fields MUST be omitted.

   *  The ":authority" pseudo-header field contains the host and port to
      connect to (equivalent to the authority-form of the request-target
      of CONNECT requests; see Section 3.2.3 of [HTTP/1.1]).

   A CONNECT request that does not conform to these restrictions is
   malformed (Section 8.1.1).

   A proxy that supports CONNECT establishes a TCP connection [TCP] to
   the host and port identified in the ":authority" pseudo-header field.
   Once this connection is successfully established, the proxy sends a
   HEADERS frame containing a 2xx-series status code to the client, as
   defined in Section 9.3.6 of [HTTP].

   After the initial HEADERS frame sent by each peer, all subsequent
   DATA frames correspond to data sent on the TCP connection.  The frame
   payload of any DATA frames sent by the client is transmitted by the
   proxy to the TCP server; data received from the TCP server is
   assembled into DATA frames by the proxy.  Frame types other than DATA
   or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY)
   MUST NOT be sent on a connected stream and MUST be treated as a
   stream error (Section 5.4.2) if received.

   The TCP connection can be closed by either peer.  The END_STREAM flag
   on a DATA frame is treated as being equivalent to the TCP FIN bit.  A
   client is expected to send a DATA frame with the END_STREAM flag set
   after receiving a frame with the END_STREAM flag set.  A proxy that
   receives a DATA frame with the END_STREAM flag set sends the attached
   data with the FIN bit set on the last TCP segment.  A proxy that
   receives a TCP segment with the FIN bit set sends a DATA frame with
   the END_STREAM flag set.  Note that the final TCP segment or DATA
   frame could be empty.

   A TCP connection error is signaled with RST_STREAM.  A proxy treats
   any error in the TCP connection, which includes receiving a TCP
   segment with the RST bit set, as a stream error (Section 5.4.2) of
   type CONNECT_ERROR.  Correspondingly, a proxy MUST send a TCP segment
   with the RST bit set if it detects an error with the stream or the
   HTTP/2 connection.

8.6.  The Upgrade Header Field

   HTTP/2 does not support the 101 (Switching Protocols) informational
   status code (Section 15.2.2 of [HTTP]).

   The semantics of 101 (Switching Protocols) aren't applicable to a
   multiplexed protocol.  Similar functionality might be enabled through
   the use of extended CONNECT [RFC8441], and other protocols are able
   to use the same mechanisms that HTTP/2 uses to negotiate their use
   (see Section 3).

8.7.  Request Reliability

   In general, an HTTP client is unable to retry a non-idempotent
   request when an error occurs because there is no means to determine
   the nature of the error (see Section 9.2.2 of [HTTP]).  It is
   possible that some server processing occurred prior to the error,
   which could result in undesirable effects if the request were

   HTTP/2 provides two mechanisms for providing a guarantee to a client
   that a request has not been processed:

   *  The GOAWAY frame indicates the highest stream number that might
      have been processed.  Requests on streams with higher numbers are
      therefore guaranteed to be safe to retry.

   *  The REFUSED_STREAM error code can be included in a RST_STREAM
      frame to indicate that the stream is being closed prior to any
      processing having occurred.  Any request that was sent on the
      reset stream can be safely retried.

   Requests that have not been processed have not failed; clients MAY
   automatically retry them, even those with non-idempotent methods.

   A server MUST NOT indicate that a stream has not been processed
   unless it can guarantee that fact.  If frames that are on a stream
   are passed to the application layer for any stream, then
   REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame
   MUST include a stream identifier that is greater than or equal to the
   given stream identifier.

   In addition to these mechanisms, the PING frame provides a way for a
   client to easily test a connection.  Connections that remain idle can
   become broken, because some middleboxes (for instance, network
   address translators or load balancers) silently discard connection
   bindings.  The PING frame allows a client to safely test whether a
   connection is still active without sending a request.

8.8.  Examples

   This section shows HTTP/1.1 requests and responses, with
   illustrations of equivalent HTTP/2 requests and responses.

8.8.1.  Simple Request

   An HTTP GET request includes control data and a request header with
   no message content and is therefore transmitted as a single HEADERS
   frame, followed by zero or more CONTINUATION frames containing the
   serialized block of request header fields.  The HEADERS frame in the
   following has both the END_HEADERS and END_STREAM flags set; no
   CONTINUATION frames are sent.

     GET /resource HTTP/1.1           HEADERS
     Host: example.org          ==>     + END_STREAM
     Accept: image/jpeg                 + END_HEADERS
                                          :method = GET
                                          :scheme = https
                                          :authority = example.org
                                          :path = /resource
                                          host = example.org
                                          accept = image/jpeg

8.8.2.  Simple Response

   Similarly, a response that includes only control data and a response
   header is transmitted as a HEADERS frame (again, followed by zero or
   more CONTINUATION frames) containing the serialized block of response
   header fields.

     HTTP/1.1 304 Not Modified        HEADERS
     ETag: "xyzzy"              ==>     + END_STREAM
     Expires: Thu, 23 Jan ...           + END_HEADERS
                                          :status = 304
                                          etag = "xyzzy"
                                          expires = Thu, 23 Jan ...

8.8.3.  Complex Request

   An HTTP POST request that includes control data and a request header
   with message content is transmitted as one HEADERS frame, followed by
   zero or more CONTINUATION frames containing the request header,
   followed by one or more DATA frames, with the last CONTINUATION (or
   HEADERS) frame having the END_HEADERS flag set and the final DATA
   frame having the END_STREAM flag set:

     POST /resource HTTP/1.1          HEADERS
     Host: example.org          ==>     - END_STREAM
     Content-Type: image/jpeg           - END_HEADERS
     Content-Length: 123                  :method = POST
                                          :authority = example.org
                                          :path = /resource
     {binary data}                        :scheme = https

                                        + END_HEADERS
                                          content-type = image/jpeg
                                          host = example.org
                                          content-length = 123

                                        + END_STREAM
                                      {binary data}

   Note that data contributing to any given field line could be spread
   between field block fragments.  The allocation of field lines to
   frames in this example is illustrative only.

8.8.4.  Response with Body

   A response that includes control data and a response header with
   message content is transmitted as a HEADERS frame, followed by zero
   or more CONTINUATION frames, followed by one or more DATA frames,
   with the last DATA frame in the sequence having the END_STREAM flag

     HTTP/1.1 200 OK                  HEADERS
     Content-Type: image/jpeg   ==>     - END_STREAM
     Content-Length: 123                + END_HEADERS
                                          :status = 200
     {binary data}                        content-type = image/jpeg
                                          content-length = 123

                                        + END_STREAM
                                      {binary data}

8.8.5.  Informational Responses

   An informational response using a 1xx status code other than 101 is
   transmitted as a HEADERS frame, followed by zero or more CONTINUATION

   A trailer section is sent as a field block after both the request or
   response field block and all the DATA frames have been sent.  The
   HEADERS frame starting the field block that comprises the trailer
   section has the END_STREAM flag set.

   The following example includes both a 100 (Continue) status code,
   which is sent in response to a request containing a "100-continue"
   token in the Expect header field, and a trailer section:

     HTTP/1.1 100 Continue            HEADERS
     Extension-Field: bar       ==>     - END_STREAM
                                        + END_HEADERS
                                          :status = 100
                                          extension-field = bar

     HTTP/1.1 200 OK                  HEADERS
     Content-Type: image/jpeg   ==>     - END_STREAM
     Transfer-Encoding: chunked         + END_HEADERS
     Trailer: Foo                         :status = 200
                                          content-type = image/jpeg
     123                                  trailer = Foo
     {binary data}
     0                                DATA
     Foo: bar                           - END_STREAM
                                      {binary data}

                                        + END_STREAM
                                        + END_HEADERS
                                          foo = bar

9.  HTTP/2 Connections

   This section outlines attributes of HTTP that improve
   interoperability, reduce exposure to known security vulnerabilities,
   or reduce the potential for implementation variation.

9.1.  Connection Management

   HTTP/2 connections are persistent.  For best performance, it is
   expected that clients will not close connections until it is
   determined that no further communication with a server is necessary
   (for example, when a user navigates away from a particular web page)
   or until the server closes the connection.

   Clients SHOULD NOT open more than one HTTP/2 connection to a given
   host and port pair, where the host is derived from a URI, a selected
   alternative service [ALT-SVC], or a configured proxy.

   A client can create additional connections as replacements, either to
   replace connections that are near to exhausting the available stream
   identifier space (Section 5.1.1), to refresh the keying material for
   a TLS connection, or to replace connections that have encountered
   errors (Section 5.4.1).

   A client MAY open multiple connections to the same IP address and TCP
   port using different Server Name Indication [TLS-EXT] values or to
   provide different TLS client certificates but SHOULD avoid creating
   multiple connections with the same configuration.

   Servers are encouraged to maintain open connections for as long as
   possible but are permitted to terminate idle connections if
   necessary.  When either endpoint chooses to close the transport-layer
   TCP connection, the terminating endpoint SHOULD first send a GOAWAY
   (Section 6.8) frame so that both endpoints can reliably determine
   whether previously sent frames have been processed and gracefully
   complete or terminate any necessary remaining tasks.

9.1.1.  Connection Reuse

   Connections that are made to an origin server, either directly or
   through a tunnel created using the CONNECT method (Section 8.5), MAY
   be reused for requests with multiple different URI authority
   components.  A connection can be reused as long as the origin server
   is authoritative (Section 10.1).  For TCP connections without TLS,
   this depends on the host having resolved to the same IP address.

   For "https" resources, connection reuse additionally depends on
   having a certificate that is valid for the host in the URI.  The
   certificate presented by the server MUST satisfy any checks that the
   client would perform when forming a new TLS connection for the host
   in the URI.  A single certificate can be used to establish authority
   for multiple origins.  Section 4.3 of [HTTP] describes how a client
   determines whether a server is authoritative for a URI.

   In some deployments, reusing a connection for multiple origins can
   result in requests being directed to the wrong origin server.  For
   example, TLS termination might be performed by a middlebox that uses
   the TLS Server Name Indication [TLS-EXT] extension to select an
   origin server.  This means that it is possible for clients to send
   requests to servers that might not be the intended target for the
   request, even though the server is otherwise authoritative.

   A server that does not wish clients to reuse connections can indicate
   that it is not authoritative for a request by sending a 421
   (Misdirected Request) status code in response to the request (see
   Section 15.5.20 of [HTTP]).

   A client that is configured to use a proxy over HTTP/2 directs
   requests to that proxy through a single connection.  That is, all
   requests sent via a proxy reuse the connection to the proxy.

9.2.  Use of TLS Features

   Implementations of HTTP/2 MUST use TLS version 1.2 [TLS12] or higher
   for HTTP/2 over TLS.  The general TLS usage guidance in [TLSBCP]
   SHOULD be followed, with some additional restrictions that are
   specific to HTTP/2.

   The TLS implementation MUST support the Server Name Indication (SNI)
   [TLS-EXT] extension to TLS.  If the server is identified by a domain
   name [DNS-TERMS], clients MUST send the server_name TLS extension
   unless an alternative mechanism to indicate the target host is used.

   Requirements for deployments of HTTP/2 that negotiate TLS 1.3 [TLS13]
   are included in Section 9.2.3.  Deployments of TLS 1.2 are subject to
   the requirements in Sections 9.2.1 and 9.2.2.  Implementations are
   encouraged to provide defaults that comply, but it is recognized that
   deployments are ultimately responsible for compliance.

9.2.1.  TLS 1.2 Features

   This section describes restrictions on the TLS 1.2 feature set that
   can be used with HTTP/2.  Due to deployment limitations, it might not
   be possible to fail TLS negotiation when these restrictions are not
   met.  An endpoint MAY immediately terminate an HTTP/2 connection that
   does not meet these TLS requirements with a connection error
   (Section 5.4.1) of type INADEQUATE_SECURITY.

   A deployment of HTTP/2 over TLS 1.2 MUST disable compression.  TLS
   compression can lead to the exposure of information that would not
   otherwise be revealed [RFC3749].  Generic compression is unnecessary,
   since HTTP/2 provides compression features that are more aware of
   context and therefore likely to be more appropriate for use for
   performance, security, or other reasons.

   A deployment of HTTP/2 over TLS 1.2 MUST disable renegotiation.  An
   endpoint MUST treat a TLS renegotiation as a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.  Note that disabling
   renegotiation can result in long-lived connections becoming unusable
   due to limits on the number of messages the underlying cipher suite
   can encipher.

   An endpoint MAY use renegotiation to provide confidentiality
   protection for client credentials offered in the handshake, but any
   renegotiation MUST occur prior to sending the connection preface.  A
   server SHOULD request a client certificate if it sees a renegotiation
   request immediately after establishing a connection.

   This effectively prevents the use of renegotiation in response to a
   request for a specific protected resource.  A future specification
   might provide a way to support this use case.  Alternatively, a
   server might use an error (Section 5.4) of type HTTP_1_1_REQUIRED to
   request that the client use a protocol that supports renegotiation.

   Implementations MUST support ephemeral key exchange sizes of at least
   2048 bits for cipher suites that use ephemeral finite field Diffie-
   Hellman (DHE) (Section 8.1.2 of [TLS12]) and 224 bits for cipher
   suites that use ephemeral elliptic curve Diffie-Hellman (ECDHE)
   [RFC8422].  Clients MUST accept DHE sizes of up to 4096 bits.
   Endpoints MAY treat negotiation of key sizes smaller than the lower
   limits as a connection error (Section 5.4.1) of type

9.2.2.  TLS 1.2 Cipher Suites

   A deployment of HTTP/2 over TLS 1.2 SHOULD NOT use any of the
   prohibited cipher suites listed in Appendix A.

   Endpoints MAY choose to generate a connection error (Section 5.4.1)
   of type INADEQUATE_SECURITY if one of the prohibited cipher suites is
   negotiated.  A deployment that chooses to use a prohibited cipher
   suite risks triggering a connection error unless the set of potential
   peers is known to accept that cipher suite.

   Implementations MUST NOT generate this error in reaction to the
   negotiation of a cipher suite that is not prohibited.  Consequently,
   when clients offer a cipher suite that is not prohibited, they have
   to be prepared to use that cipher suite with HTTP/2.

   The list of prohibited cipher suites includes the cipher suite that
   TLS 1.2 makes mandatory, which means that TLS 1.2 deployments could
   have non-intersecting sets of permitted cipher suites.  To avoid this
   problem, which causes TLS handshake failures, deployments of HTTP/2
   that use TLS 1.2 MUST support TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
   [TLS-ECDHE] with the P-256 elliptic curve [RFC8422].

   Note that clients might advertise support of cipher suites that are
   prohibited in order to allow for connection to servers that do not
   support HTTP/2.  This allows servers to select HTTP/1.1 with a cipher
   suite that is prohibited in HTTP/2.  However, this can result in
   HTTP/2 being negotiated with a prohibited cipher suite if the
   application protocol and cipher suite are independently selected.

9.2.3.  TLS 1.3 Features

   TLS 1.3 includes a number of features not available in earlier
   versions.  This section discusses the use of these features.

   HTTP/2 servers MUST NOT send post-handshake TLS 1.3
   CertificateRequest messages.  HTTP/2 clients MUST treat a TLS post-
   handshake CertificateRequest message as a connection error
   (Section 5.4.1) of type PROTOCOL_ERROR.

   The prohibition on post-handshake authentication applies even if the
   client offered the "post_handshake_auth" TLS extension.  Post-
   handshake authentication support might be advertised independently of
   ALPN [TLS-ALPN].  Clients might offer the capability for use in other
   protocols, but inclusion of the extension cannot imply support within

   [TLS13] defines other post-handshake messages, NewSessionTicket and
   KeyUpdate, which can be used as they have no direct interaction with
   HTTP/2.  Unless the use of a new type of TLS message depends on an
   interaction with the application-layer protocol, that TLS message can
   be sent after the handshake completes.

   TLS early data MAY be used to send requests, provided that the
   guidance in [RFC8470] is observed.  Clients send requests in early
   data assuming initial values for all server settings.

10.  Security Considerations

   The use of TLS is necessary to provide many of the security
   properties of this protocol.  Many of the claims in this section do
   not hold unless TLS is used as described in Section 9.2.

10.1.  Server Authority

   HTTP/2 relies on the HTTP definition of authority for determining
   whether a server is authoritative in providing a given response (see
   Section 4.3 of [HTTP]).  This relies on local name resolution for the
   "http" URI scheme and the authenticated server identity for the
   "https" scheme.

10.2.  Cross-Protocol Attacks

   In a cross-protocol attack, an attacker causes a client to initiate a
   transaction in one protocol toward a server that understands a
   different protocol.  An attacker might be able to cause the
   transaction to appear as a valid transaction in the second protocol.
   In combination with the capabilities of the web context, this can be
   used to interact with poorly protected servers in private networks.

   Completing a TLS handshake with an ALPN identifier for HTTP/2 can be
   considered sufficient protection against cross-protocol attacks.
   ALPN provides a positive indication that a server is willing to
   proceed with HTTP/2, which prevents attacks on other TLS-based

   The encryption in TLS makes it difficult for attackers to control the
   data that could be used in a cross-protocol attack on a cleartext

   The cleartext version of HTTP/2 has minimal protection against cross-
   protocol attacks.  The connection preface (Section 3.4) contains a
   string that is designed to confuse HTTP/1.1 servers, but no special
   protection is offered for other protocols.

10.3.  Intermediary Encapsulation Attacks

   HPACK permits encoding of field names and values that might be
   treated as delimiters in other HTTP versions.  An intermediary that
   translates an HTTP/2 request or response MUST validate fields
   according to the rules in Section 8.2 before translating a message to
   another HTTP version.  Translating a field that includes invalid
   delimiters could be used to cause recipients to incorrectly interpret
   a message, which could be exploited by an attacker.

   Section 8.2 does not include specific rules for validation of pseudo-
   header fields.  If the values of these fields are used, additional
   validation is necessary.  This is particularly important where
   ":scheme", ":authority", and ":path" are combined to form a single
   URI string [RFC3986].  Similar problems might occur when that URI or
   just ":path" is combined with ":method" to construct a request line
   (as in Section 3 of [HTTP/1.1]).  Simple concatenation is not secure
   unless the input values are fully validated.

   An intermediary can reject fields that contain invalid field names or
   values for other reasons -- in particular, those fields that do not
   conform to the HTTP ABNF grammar from Section 5 of [HTTP].
   Intermediaries that do not perform any validation of fields other
   than the minimum required by Section 8.2 could forward messages that
   contain invalid field names or values.

   An intermediary that receives any fields that require removal before
   forwarding (see Section 7.6.1 of [HTTP]) MUST remove or replace those
   header fields when forwarding messages.  Additionally, intermediaries
   should take care when forwarding messages containing Content-Length
   fields to ensure that the message is well-formed (Section 8.1.1).
   This ensures that if the message is translated into HTTP/1.1 at any
   point, the framing will be correct.

10.4.  Cacheability of Pushed Responses

   Pushed responses do not have an explicit request from the client; the
   request is provided by the server in the PUSH_PROMISE frame.

   Caching responses 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 responses for which an origin server is not authoritative (see
   Section 10.1) MUST NOT be used or cached.

10.5.  Denial-of-Service Considerations

   An HTTP/2 connection can demand a greater commitment of resources to
   operate than an HTTP/1.1 connection.  Both field section compression
   and flow control depend on a commitment of a greater amount of state.
   Settings for these features ensure that memory commitments for these
   features are strictly bounded.

   The number of PUSH_PROMISE frames is not constrained in the same
   fashion.  A client that accepts server push SHOULD limit the number
   of streams it allows to be in the "reserved (remote)" state.  An
   excessive number of server push streams can be treated as a stream
   error (Section 5.4.2) of type ENHANCE_YOUR_CALM.

   A number of HTTP/2 implementations were found to be vulnerable to
   denial of service [NFLX-2019-002].  Below is a list of known ways
   that implementations might be subject to denial-of-service attacks:

   *  Inefficient tracking of outstanding outbound frames can lead to
      overload if an adversary can cause large numbers of frames to be
      enqueued for sending.  A peer could use one of several techniques
      to cause large numbers of frames to be generated:

      -  Providing tiny increments to flow control in WINDOW_UPDATE
         frames can cause a sender to generate a large number of DATA

      -  An endpoint is required to respond to a PING frame.

      -  Each SETTINGS frame requires acknowledgment.

      -  An invalid request (or server push) can cause a peer to send
         RST_STREAM frames in response.

   *  An attacker can provide large amounts of flow-control credit at
      the HTTP/2 layer but withhold credit at the TCP layer, preventing
      frames from being sent.  An endpoint that constructs and remembers
      frames for sending without considering TCP limits might be subject
      to resource exhaustion.

   *  Large numbers of small or empty frames can be abused to cause a
      peer to expend time processing frame headers.  Caution is required
      here as some uses of small frames are entirely legitimate, such as
      the sending of an empty DATA or CONTINUATION frame at the end of a

   *  The SETTINGS frame might also be abused to cause a peer to expend
      additional processing time.  This might be done by pointlessly
      changing settings, sending multiple undefined settings, or
      changing the same setting multiple times in the same frame.

   *  Handling reprioritization with PRIORITY frames can require
      significant processing time and can lead to overload if many
      PRIORITY frames are sent.

   *  Field section compression also provides opportunities for an
      attacker to waste processing resources; see Section 7 of
      [COMPRESSION] for more details on potential abuses.

   *  Limits in SETTINGS cannot be reduced instantaneously, which leaves
      an endpoint exposed to behavior from a peer that could exceed the
      new limits.  In particular, immediately after establishing a
      connection, limits set by a server are not known to clients and
      could be exceeded without being an obvious protocol violation.

   Most of the features that might be exploited for denial of service --
   such as SETTINGS changes, small frames, field section compression --
   have legitimate uses.  These features become a burden only when they
   are used unnecessarily or to excess.

   An endpoint that doesn't monitor use of these features exposes itself
   to a risk of denial of service.  Implementations SHOULD track the use
   of these features and set limits on their use.  An endpoint MAY treat
   activity that is suspicious as a connection error (Section 5.4.1) of

10.5.1.  Limits on Field Block Size

   A large field block (Section 4.3) can cause an implementation to
   commit a large amount of state.  Field lines that are critical for
   routing can appear toward the end of a field block, which prevents
   streaming of fields to their ultimate destination.  This ordering and
   other reasons, such as ensuring cache correctness, mean that an
   endpoint might need to buffer the entire field block.  Since there is
   no hard limit to the size of a field block, some endpoints could be
   forced to commit a large amount of available memory for field blocks.

   An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to advise peers
   of limits that might apply on the size of uncompressed field blocks.
   This setting is only advisory, so endpoints MAY choose to send field
   blocks that exceed this limit and risk the request or response being
   treated as malformed.  This setting is specific to a connection, so
   any request or response could encounter a hop with a lower, unknown
   limit.  An intermediary can attempt to avoid this problem by passing
   on values presented by different peers, but they are not obliged to
   do so.

   A server that receives a larger field block than it is willing to
   handle can send an HTTP 431 (Request Header Fields Too Large) status
   code [RFC6585].  A client can discard responses that it cannot
   process.  The field block MUST be processed to ensure a consistent
   connection state, unless the connection is closed.

10.5.2.  CONNECT Issues

   The CONNECT method can be used to create disproportionate load on a
   proxy, since stream creation is relatively inexpensive when compared
   to the creation and maintenance of a TCP connection.  A proxy might
   also maintain some resources for a TCP connection beyond the closing
   of the stream that carries the CONNECT request, since the outgoing
   TCP connection remains in the TIME_WAIT state.  Therefore, a proxy
   cannot rely on SETTINGS_MAX_CONCURRENT_STREAMS alone to limit the
   resources consumed by CONNECT requests.

10.6.  Use of Compression

   Compression can allow an attacker to recover secret data when it is
   compressed in the same context as data under attacker control.
   HTTP/2 enables compression of field lines (Section 4.3); the
   following concerns also apply to the use of HTTP compressed content-
   codings (Section 8.4.1 of [HTTP]).

   There are demonstrable attacks on compression that exploit the
   characteristics of the Web (e.g., [BREACH]).  The attacker induces
   multiple requests containing varying plaintext, observing the length
   of the resulting ciphertext in each, which reveals a shorter length
   when a guess about the secret is correct.

   Implementations communicating on a secure channel MUST NOT compress
   content that includes both confidential and attacker-controlled data
   unless separate compression dictionaries are used for each source of
   data.  Compression MUST NOT be used if the source of data cannot be
   reliably determined.  Generic stream compression, such as that
   provided by TLS, MUST NOT be used with HTTP/2 (see Section 9.2).

   Further considerations regarding the compression of header fields are
   described in [COMPRESSION].

10.7.  Use of Padding

   Padding within HTTP/2 is not intended as a replacement for general
   purpose padding, such as that provided by TLS [TLS13].  Redundant
   padding could even be counterproductive.  Correct application can
   depend on having specific knowledge of the data that is being padded.

   To mitigate attacks that rely on compression, disabling or limiting
   compression might be preferable to padding as a countermeasure.

   Padding can be used to obscure the exact size of frame content and is
   provided to mitigate specific attacks within HTTP -- for example,
   attacks where compressed content includes both attacker-controlled
   plaintext and secret data (e.g., [BREACH]).

   Use of padding can result in less protection than might seem
   immediately obvious.  At best, padding only makes it more difficult
   for an attacker to infer length information by increasing the number
   of frames an attacker has to observe.  Incorrectly implemented
   padding schemes can be easily defeated.  In particular, randomized
   padding with a predictable distribution provides very little
   protection; similarly, padding frame payloads to a fixed size exposes
   information as frame payload sizes cross the fixed-sized boundary,
   which could be possible if an attacker can control plaintext.

   Intermediaries SHOULD retain padding for DATA frames but MAY drop
   padding for HEADERS and PUSH_PROMISE frames.  A valid reason for an
   intermediary to change the amount of padding of frames is to improve
   the protections that padding provides.

10.8.  Privacy Considerations

   Several characteristics of HTTP/2 provide an observer an opportunity
   to correlate actions of a single client or server over time.  These
   include the values of settings, the manner in which flow-control
   windows are managed, the way priorities are allocated to streams, the
   timing of reactions to stimulus, and the handling of any features
   that are controlled by settings.

   As far as these create observable differences in behavior, they could
   be used as a basis for fingerprinting a specific client, as defined
   in Section 3.2 of [PRIVACY].

   HTTP/2's preference for using a single TCP connection allows
   correlation of a user's activity on a site.  Reusing connections for
   different origins allows tracking across those origins.

   Because the PING and SETTINGS frames solicit immediate responses,
   they can be used by an endpoint to measure latency to their peer.
   This might have privacy implications in certain scenarios.

10.9.  Remote Timing Attacks

   Remote timing attacks extract secrets from servers by observing
   variations in the time that servers take when processing requests
   that use secrets.  HTTP/2 enables concurrent request creation and
   processing, which can give attackers better control over when request
   processing commences.  Multiple HTTP/2 requests can be included in
   the same IP packet or TLS record.  HTTP/2 can therefore make remote
   timing attacks more efficient by eliminating variability in request
   delivery, leaving only request order and the delivery of responses as
   sources of timing variability.

   Ensuring that processing time is not dependent on the value of a
   secret is the best defense against any form of timing attack.

11.  IANA Considerations

   This revision of HTTP/2 marks the HTTP2-Settings header field and the
   h2c upgrade token, both defined in [RFC7540], as obsolete.

   Section 11 of [RFC7540] registered the h2 and h2c ALPN identifiers
   along with the PRI HTTP method.  RFC 7540 also established a registry
   for frame types, settings, and error codes.  These registrations and
   registries apply to HTTP/2, but are not redefined in this document.

   IANA has updated references to RFC 7540 in the following registries
   to refer to this document: "TLS Application-Layer Protocol
   Negotiation (ALPN) Protocol IDs", "HTTP/2 Frame Type", "HTTP/2
   Settings", "HTTP/2 Error Code", and "HTTP Method Registry".  The
   registration of the PRI method has been updated to refer to
   Section 3.4; all other section numbers have not changed.

   IANA has changed the policy on those portions of the "HTTP/2 Frame
   Type" and "HTTP/2 Settings" registries that were reserved for
   Experimental Use in RFC 7540.  These portions of the registries shall
   operate on the same policy as the remainder of each registry.

11.1.  HTTP2-Settings Header Field Registration

   This section marks the HTTP2-Settings header field registered by
   Section 11.5 of [RFC7540] in the "Hypertext Transfer Protocol (HTTP)
   Field Name Registry" as obsolete.  This capability has been removed:
   see Section 3.1.  The registration is updated to include the details
   as required by Section 18.4 of [HTTP]:

   Field Name:  HTTP2-Settings

   Status:  obsoleted

   Reference:  Section 3.2.1 of [RFC7540]

   Comments:  Obsolete; see Section 11.1 of this document.

11.2.  The h2c Upgrade Token

   This section records the h2c upgrade token registered by Section 11.8
   of [RFC7540] in the "Hypertext Transfer Protocol (HTTP) Upgrade Token
   Registry" as obsolete.  This capability has been removed: see
   Section 3.1.  The registration is updated as follows:

   Value:  h2c

   Description:  (OBSOLETE) Hypertext Transfer Protocol version 2

   Expected Version Tokens:  None

   Reference:  Section 3.1 of this document

12.  References

12.1.  Normative References

   [CACHING]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,

              Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,

   [COOKIE]   Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8422]  Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
              Curve Cryptography (ECC) Cipher Suites for Transport Layer
              Security (TLS) Versions 1.2 and Earlier", RFC 8422,
              DOI 10.17487/RFC8422, August 2018,

   [RFC8470]  Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <https://www.rfc-editor.org/info/rfc8470>.

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [TLS-ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

              Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
              256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              DOI 10.17487/RFC5289, August 2008,

   [TLS-EXT]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [TLS12]    Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [TLSBCP]   Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

12.2.  Informative References

   [ALT-SVC]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/info/rfc7838>.

   [BREACH]   Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the
              CRIME Attack", 12 July 2013,

              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

              Oku, K. and L. Pardue, "Extensible Prioritization Scheme
              for HTTP", RFC 9218, DOI 10.17487/RFC9218, June 2022,

   [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <https://www.rfc-editor.org/info/rfc9112>.

              Netflix, "HTTP/2 Denial of Service Advisory", 13 August
              2019, <https://github.com/Netflix/security-

   [PRIVACY]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC3749]  Hollenbeck, S., "Transport Layer Security Protocol
              Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May
              2004, <https://www.rfc-editor.org/info/rfc3749>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC8441]  McManus, P., "Bootstrapping WebSockets with HTTP/2",
              RFC 8441, DOI 10.17487/RFC8441, September 2018,

   [RFC8740]  Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740,
              DOI 10.17487/RFC8740, February 2020,

   [TALKING]  Huang, L., Chen, E., Barth, A., Rescorla, E., and C.
              Jackson, "Talking to Yourself for Fun and Profit", 2011,

Appendix A.  Prohibited TLS 1.2 Cipher Suites

   An HTTP/2 implementation MAY treat the negotiation of any of the
   following cipher suites with TLS 1.2 as a connection error
   (Section 5.4.1) of type INADEQUATE_SECURITY:

   *  TLS_RSA_WITH_RC4_128_MD5
   *  TLS_RSA_WITH_RC4_128_SHA
   *  TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
   *  TLS_DH_anon_WITH_RC4_128_MD5
   *  TLS_KRB5_WITH_RC4_128_SHA
   *  TLS_KRB5_WITH_RC4_128_MD5
   *  TLS_DH_anon_WITH_AES_128_CBC_SHA
   *  TLS_DH_anon_WITH_AES_256_CBC_SHA
   *  TLS_DH_anon_WITH_AES_128_CBC_SHA256
   *  TLS_DH_anon_WITH_AES_256_CBC_SHA256
   *  TLS_PSK_WITH_RC4_128_SHA
   *  TLS_DH_anon_WITH_AES_128_GCM_SHA256
   *  TLS_DH_anon_WITH_AES_256_GCM_SHA384
   *  TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA256
   *  TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA256
   *  TLS_ECDH_anon_WITH_RC4_128_SHA
   *  TLS_ECDH_anon_WITH_AES_128_CBC_SHA
   *  TLS_ECDH_anon_WITH_AES_256_CBC_SHA
   *  TLS_DH_anon_WITH_ARIA_128_CBC_SHA256
   *  TLS_DH_anon_WITH_ARIA_256_CBC_SHA384
   *  TLS_DH_anon_WITH_ARIA_128_GCM_SHA256
   *  TLS_DH_anon_WITH_ARIA_256_GCM_SHA384
   *  TLS_DH_anon_WITH_CAMELLIA_128_GCM_SHA256
   *  TLS_DH_anon_WITH_CAMELLIA_256_GCM_SHA384
   *  TLS_RSA_WITH_AES_128_CCM_8
   *  TLS_RSA_WITH_AES_256_CCM_8
   *  TLS_PSK_WITH_AES_128_CCM_8
   *  TLS_PSK_WITH_AES_256_CCM_8

      |  Note: This list was assembled from the set of registered TLS
      |  cipher suites when [RFC7540] was developed.  This list includes
      |  those cipher suites that do not offer an ephemeral key exchange
      |  and those that are based on the TLS null, stream, or block
      |  cipher type (as defined in Section 6.2.3 of [TLS12]).
      |  Additional cipher suites with these properties could be
      |  defined; these would not be explicitly prohibited.

   For more details, see Section 9.2.2.

Appendix B.  Changes from RFC 7540

   This revision includes the following substantive changes:

   *  Use of TLS 1.3 was defined based on [RFC8740], which this document

   *  The priority scheme defined in RFC 7540 is deprecated.
      Definitions for the format of the PRIORITY frame and the priority
      fields in the HEADERS frame have been retained, plus the rules
      governing when PRIORITY frames can be sent and received, but the
      semantics of these fields are only described in RFC 7540.  The
      priority signaling scheme from RFC 7540 was not successful.  Using
      the simpler signaling in [HTTP-PRIORITY] is recommended.

   *  The HTTP/1.1 Upgrade mechanism is deprecated and no longer
      specified in this document.  It was never widely deployed, with
      plaintext HTTP/2 users choosing to use the prior-knowledge
      implementation instead.

   *  Validation for field names and values has been narrowed.  The
      validation that is mandatory for intermediaries is precisely
      defined, and error reporting for requests has been amended to
      encourage sending 400-series status codes.

   *  The ranges of codepoints for settings and frame types that were
      reserved for Experimental Use are now available for general use.

   *  Connection-specific header fields -- which are prohibited -- are
      more precisely and comprehensively identified.

   *  Host and ":authority" are no longer permitted to disagree.

   *  Rules for sending Dynamic Table Size Update instructions after
      changes in settings have been clarified in Section 4.3.1.

   Editorial changes are also included.  In particular, changes to
   terminology and document structure are in response to updates to core
   HTTP semantics [HTTP].  Those documents now include some concepts
   that were first defined in RFC 7540, such as the 421 status code or
   connection coalescing.


   Credit for non-trivial input to this document is owed to a large
   number of people who have contributed to the HTTP Working Group over
   the years.  [RFC7540] contains a more extensive list of people that
   deserve acknowledgment for their contributions.


   Mike Belshe and Roberto Peon authored the text that this document is
   based on.

Authors' Addresses

   Martin Thomson (editor)
   Email: mt@lowentropy.net

   Cory Benfield (editor)
   Apple Inc.
   Email: cbenfield@apple.com