rfc7540 Varnish source references







Internet Engineering Task Force (IETF)                         M. Belshe
Request for Comments: 7540                                         BitGo
Category: Standards Track                                        R. Peon
ISSN: 2070-1721                                              Google, Inc
                                                         M. Thomson, Ed.
                                                                 Mozilla
                                                                May 2015


             Hypertext Transfer Protocol Version 2 (HTTP/2)

Abstract

   This specification describes an optimized expression of the semantics
   of the Hypertext Transfer Protocol (HTTP), referred to as HTTP
   version 2 (HTTP/2).  HTTP/2 enables a more efficient use of network
   resources and a reduced perception of latency by introducing header
   field compression and allowing multiple concurrent exchanges on the
   same connection.  It also introduces unsolicited push of
   representations from servers to clients.

   This specification is an alternative to, but does not obsolete, the
   HTTP/1.1 message syntax.  HTTP's existing semantics remain unchanged.

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7540.














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62
63	Copyright Notice
64
65	Copyright (c) 2015 IETF Trust and the persons identified as the
66	document authors. All rights reserved.
67
68	This document is subject to BCP 78 and the IETF Trust's Legal
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73	to this document. Code Components extracted from this document must
74	include Simplified BSD License text as described in Section 4.e of
75	the Trust Legal Provisions and are provided without warranty as
76	described in the Simplified BSD License.
77
78	Table of Contents
79
80	1. Introduction ....................................................4
81	2. HTTP/2 Protocol Overview ........................................5
82	2.1. Document Organization ......................................6
83	2.2. Conventions and Terminology ................................6
84	3. Starting HTTP/2 .................................................7
85	3.1. HTTP/2 Version Identification ..............................8
86	3.2. Starting HTTP/2 for "http" URIs ............................8
87	3.2.1. HTTP2-Settings Header Field .........................9
88	3.3. Starting HTTP/2 for "https" URIs ..........................10
89	3.4. Starting HTTP/2 with Prior Knowledge ......................10
90	3.5. HTTP/2 Connection Preface .................................11
91	4. HTTP Frames ....................................................12
92	4.1. Frame Format ..............................................12
93	4.2. Frame Size ................................................13
94	4.3. Header Compression and Decompression ......................14
95	5. Streams and Multiplexing .......................................15
96	5.1. Stream States .............................................16
97	5.1.1. Stream Identifiers .................................21
98	5.1.2. Stream Concurrency .................................22
99	5.2. Flow Control ..............................................22
100	5.2.1. Flow-Control Principles ............................23
101	5.2.2. Appropriate Use of Flow Control ....................24
102	5.3. Stream Priority ...........................................24
103	5.3.1. Stream Dependencies ................................25
104	5.3.2. Dependency Weighting ...............................26
105	5.3.3. Reprioritization ...................................26
106	5.3.4. Prioritization State Management ....................27
107	5.3.5. Default Priorities .................................28
108	5.4. Error Handling ............................................28
109	5.4.1. Connection Error Handling ..........................29
110	5.4.2. Stream Error Handling ..............................29
111
112
113
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118
119	5.4.3. Connection Termination .............................30
120	5.5. Extending HTTP/2 ..........................................30
121	6. Frame Definitions ..............................................31
122	6.1. DATA ......................................................31
123	6.2. HEADERS ...................................................32
124	6.3. PRIORITY ..................................................34
125	6.4. RST_STREAM ................................................36
126	6.5. SETTINGS ..................................................36
127	6.5.1. SETTINGS Format ....................................38
128	6.5.2. Defined SETTINGS Parameters ........................38
129	6.5.3. Settings Synchronization ...........................39
130	6.6. PUSH_PROMISE ..............................................40
131	6.7. PING ......................................................42
132	6.8. GOAWAY ....................................................43
133	6.9. WINDOW_UPDATE .............................................46
134	6.9.1. The Flow-Control Window ............................47
135	6.9.2. Initial Flow-Control Window Size ...................48
136	6.9.3. Reducing the Stream Window Size ....................49
137	6.10. CONTINUATION .............................................49
138	7. Error Codes ....................................................50
139	8. HTTP Message Exchanges .........................................51
140	8.1. HTTP Request/Response Exchange ............................52
141	8.1.1. Upgrading from HTTP/2 ..............................53
142	8.1.2. HTTP Header Fields .................................53
143	8.1.3. Examples ...........................................57
144	8.1.4. Request Reliability Mechanisms in HTTP/2 ...........60
145	8.2. Server Push ...............................................60
146	8.2.1. Push Requests ......................................61
147	8.2.2. Push Responses .....................................63
148	8.3. The CONNECT Method ........................................64
149	9. Additional HTTP Requirements/Considerations ....................65
150	9.1. Connection Management .....................................65
151	9.1.1. Connection Reuse ...................................66
152	9.1.2. The 421 (Misdirected Request) Status Code ..........66
153	9.2. Use of TLS Features .......................................67
154	9.2.1. TLS 1.2 Features ...................................67
155	9.2.2. TLS 1.2 Cipher Suites ..............................68
156	10. Security Considerations .......................................69
157	10.1. Server Authority .........................................69
158	10.2. Cross-Protocol Attacks ...................................69
159	10.3. Intermediary Encapsulation Attacks .......................70
160	10.4. Cacheability of Pushed Responses .........................70
161	10.5. Denial-of-Service Considerations .........................70
162	10.5.1. Limits on Header Block Size .......................71
163	10.5.2. CONNECT Issues ....................................72
164	10.6. Use of Compression .......................................72
165	10.7. Use of Padding ...........................................73
166	10.8. Privacy Considerations ...................................73
167
168
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   11. IANA Considerations ...........................................74
      11.1. Registration of HTTP/2 Identification Strings ............74
      11.2. Frame Type Registry ......................................75
      11.3. Settings Registry ........................................75
      11.4. Error Code Registry ......................................76
      11.5. HTTP2-Settings Header Field Registration .................77
      11.6. PRI Method Registration ..................................78
      11.7. The 421 (Misdirected Request) HTTP Status Code ...........78
      11.8. The h2c Upgrade Token ....................................78
   12. References ....................................................79
      12.1. Normative References .....................................79
      12.2. Informative References ...................................81
   Appendix A. TLS 1.2 Cipher Suite Black List .......................83
   Acknowledgements ..................................................95
   Authors' Addresses ................................................96

1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is a wildly successful
   protocol.  However, the way HTTP/1.1 uses the underlying transport
   ([RFC7230], Section 6) has several characteristics that have a
   negative overall effect on application performance today.

   In particular, HTTP/1.0 allowed only one request to be outstanding at
   a time on a given TCP connection.  HTTP/1.1 added request pipelining,
   but this only partially addressed request concurrency and still
   suffers from head-of-line blocking.  Therefore, HTTP/1.0 and HTTP/1.1
   clients that need to make many requests use multiple connections to a
   server in order to achieve concurrency and thereby reduce latency.

   Furthermore, HTTP header fields are often repetitive and verbose,
   causing unnecessary network traffic as well as causing the initial
   TCP [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 request and response messages on the same
   connection and uses an efficient coding for HTTP header fields.  It
   also allows prioritization of requests, letting more important
   requests complete more quickly, further improving performance.









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   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.

   Finally, HTTP/2 also enables more efficient processing of messages
   through use of binary message framing.

2.  HTTP/2 Protocol Overview

   HTTP/2 provides an optimized transport for HTTP semantics.  HTTP/2
   supports all of the core features of HTTP/1.1 but aims to be more
   efficient in several ways.

   The basic protocol unit in HTTP/2 is a frame (Section 4.1).  Each
   frame type serves a different purpose.  For example, HEADERS and DATA
   frames form the basis of HTTP requests and responses (Section 8.1);
   other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are
   used in support of other HTTP/2 features.

   Multiplexing of requests is achieved by having each HTTP request/
   response exchange associated with its own stream (Section 5).
   Streams are largely independent of each other, so a blocked or
   stalled request or response does not prevent progress on other
   streams.

   Flow control and prioritization ensure that it is possible to
   efficiently use multiplexed streams.  Flow control (Section 5.2)
   helps to ensure that only data that can be used by a receiver is
   transmitted.  Prioritization (Section 5.3) ensures that limited
   resources can be directed to the most important streams first.

   HTTP/2 adds a new interaction mode whereby a server can push
   responses to a client (Section 8.2).  Server push allows 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.

   Because HTTP header 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.






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2.1.  Document Organization

   The HTTP/2 specification is split into four parts:

   o  Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is
      initiated.

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

   o  Frame (Section 6) and error (Section 7) definitions include
      details of the frame and error types used in HTTP/2.

   o  HTTP mappings (Section 8) and additional requirements (Section 9)
      describe how HTTP semantics are expressed using frames and
      streams.

   While some of the frame and stream layer concepts are isolated from
   HTTP, this specification does not define a completely generic frame
   layer.  The frame and stream layers are tailored to the needs of the
   HTTP protocol and server push.

2.2.  Conventions and Terminology

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

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

   The following terms are used:

   client:  The endpoint that initiates an HTTP/2 connection.  Clients
      send HTTP requests and receive HTTP responses.

   connection:  A transport-layer connection between two endpoints.

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

   endpoint:  Either the client or server of the connection.






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   frame:  The smallest unit of communication within an HTTP/2
      connection, consisting of a header and a variable-length sequence
      of octets structured according to the frame type.

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

   receiver:  An endpoint that is receiving frames.

   sender:  An endpoint that is transmitting frames.

   server:  The endpoint that accepts an HTTP/2 connection.  Servers
      receive HTTP requests and send HTTP responses.

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

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

   Finally, the terms "gateway", "intermediary", "proxy", and "tunnel"
   are defined in Section 2.3 of [RFC7230].  Intermediaries act as both
   client and server at different times.

   The term "payload body" is defined in Section 3.3 of [RFC7230].

3.  Starting HTTP/2

   An HTTP/2 connection is an application-layer protocol running on top
   of a TCP connection ([TCP]).  The client is the TCP connection
   initiator.

   HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1.
   HTTP/2 shares the same default port numbers: 80 for "http" URIs and
   443 for "https" URIs.  As a result, implementations processing
   requests for target resource URIs like "http://example.org/foo" or
   "https://example.com/bar" are required to first discover whether the
   upstream server (the immediate peer to which the client wishes to
   establish a connection) supports HTTP/2.

   The means by which support for HTTP/2 is determined is different for
   "http" and "https" URIs.  Discovery for "http" URIs is described in
   Section 3.2.  Discovery for "https" URIs is described in Section 3.3.









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3.1.  HTTP/2 Version Identification

   The protocol defined in this document has two identifiers.

   o  The string "h2" identifies the protocol where HTTP/2 uses
      Transport Layer Security (TLS) [TLS12].  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.

   o  The string "h2c" identifies the protocol where HTTP/2 is run over
      cleartext TCP.  This identifier is used in the HTTP/1.1 Upgrade
      header field and in any place where HTTP/2 over TCP is identified.

      The "h2c" string is reserved from the ALPN identifier space but
      describes a protocol that does not use TLS.

   Negotiating "h2" or "h2c" implies the use of the transport, security,
   framing, and message semantics described in this document.

3.2.  Starting HTTP/2 for "http" URIs

   A client that makes a request for an "http" URI without prior
   knowledge about support for HTTP/2 on the next hop uses the HTTP
   Upgrade mechanism (Section 6.7 of [RFC7230]).  The client does so by
   making an HTTP/1.1 request that includes an Upgrade header field with
   the "h2c" token.  Such an HTTP/1.1 request MUST include exactly one
   HTTP2-Settings (Section 3.2.1) header field.

   For example:

     GET / HTTP/1.1
     Host: server.example.com
     Connection: Upgrade, HTTP2-Settings
     Upgrade: h2c
     HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>

   Requests that contain a payload body MUST be sent in their entirety
   before the client can send HTTP/2 frames.  This means that a large
   request can block the use of the connection until it is completely
   sent.

   If concurrency of an initial request with subsequent requests is
   important, an OPTIONS request can be used to perform the upgrade to
   HTTP/2, at the cost of an additional round trip.



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   A server that does not support HTTP/2 can respond to the request as
   though the Upgrade header field were absent:

     HTTP/1.1 200 OK
     Content-Length: 243
     Content-Type: text/html

     ...

   A server MUST ignore an "h2" token in an Upgrade header field.
   Presence of a token with "h2" implies HTTP/2 over TLS, which is
   instead negotiated as described in Section 3.3.

   A server that supports HTTP/2 accepts the upgrade with a 101
   (Switching Protocols) response.  After the empty line that terminates
   the 101 response, the server can begin sending HTTP/2 frames.  These
   frames MUST include a response to the request that initiated the
   upgrade.

   For example:

     HTTP/1.1 101 Switching Protocols
     Connection: Upgrade
     Upgrade: h2c

     [ HTTP/2 connection ...

   The first HTTP/2 frame sent by the server MUST be a server connection
   preface (Section 3.5) consisting of a SETTINGS frame (Section 6.5).
   Upon receiving the 101 response, the client MUST send a connection
   preface (Section 3.5), which includes a SETTINGS frame.

   The HTTP/1.1 request that is sent prior to upgrade is assigned a
   stream identifier of 1 (see Section 5.1.1) with default priority
   values (Section 5.3.5).  Stream 1 is implicitly "half-closed" from
   the client toward the server (see Section 5.1), since the request is
   completed as an HTTP/1.1 request.  After commencing the HTTP/2
   connection, stream 1 is used for the response.

3.2.1.  HTTP2-Settings Header Field

   A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly
   one "HTTP2-Settings" header field.  The HTTP2-Settings header field
   is a connection-specific header field that includes parameters that
   govern the HTTP/2 connection, provided in anticipation of the server
   accepting the request to upgrade.

     HTTP2-Settings    = token68



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   A server MUST NOT upgrade the connection to HTTP/2 if this header
   field is not present or if more than one is present.  A server MUST
   NOT send this header field.

   The content of the HTTP2-Settings header field is the payload of a
   SETTINGS frame (Section 6.5), encoded as a base64url string (that is,
   the URL- and filename-safe Base64 encoding described in Section 5 of
   [RFC4648], with any trailing '=' characters omitted).  The ABNF
   [RFC5234] production for "token68" is defined in Section 2.1 of
   [RFC7235].

   Since the upgrade is only intended to apply to the immediate
   connection, a client sending the HTTP2-Settings header field MUST
   also send "HTTP2-Settings" as a connection option in the Connection
   header field to prevent it from being forwarded (see Section 6.1 of
   [RFC7230]).

   A server decodes and interprets these values as it would any other
   SETTINGS frame.  Explicit acknowledgement of these settings
   (Section 6.5.3) is not necessary, since a 101 response serves as
   implicit acknowledgement.  Providing these values in the upgrade
   request gives a client an opportunity to provide parameters prior to
   receiving any frames from the server.

3.3.  Starting HTTP/2 for "https" URIs

   A client that makes a request to an "https" URI uses TLS [TLS12] with
   the application-layer protocol negotiation (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 3.5).

3.4.  Starting HTTP/2 with Prior Knowledge

   A client can learn that a particular server supports HTTP/2 by other
   means.  For example, [ALT-SVC] describes a mechanism for advertising
   this capability.

   A client MUST send the connection preface (Section 3.5) and then MAY
   immediately send HTTP/2 frames to such a server; servers can identify
   these connections by the presence of the connection preface.  This




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   only affects the establishment of HTTP/2 connections over cleartext
   TCP; implementations that support HTTP/2 over TLS MUST use protocol
   negotiation in TLS [TLS-ALPN].

   Likewise, the server MUST send a connection preface (Section 3.5).

   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.

3.5.  HTTP/2 Connection Preface

   In HTTP/2, each endpoint is required to send a connection preface as
   a final confirmation of the protocol in use and to establish the
   initial settings for the HTTP/2 connection.  The client and server
   each send a different connection preface.

   The client connection preface starts with a sequence of 24 octets,
   which in hex notation is:

     0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

   That is, the connection preface starts with the string "PRI *
   HTTP/2.0\r\n\r\nSM\r\n\r\n").  This sequence MUST be followed by a
   SETTINGS frame (Section 6.5), which MAY be empty.  The client sends
   the client connection preface immediately upon receipt of a 101
   (Switching Protocols) response (indicating a successful upgrade) or
   as the first application data octets of a TLS connection.  If
   starting an HTTP/2 connection with prior knowledge of server support
   for the protocol, the client connection preface is sent upon
   connection establishment.

      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.





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   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 parameters that
   necessarily alter how a client is expected to communicate with the
   server.  Upon receiving the SETTINGS frame, the client is expected to
   honor any parameters established.  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 payload.

    +-----------------------------------------------+
    |                 Length (24)                   |
    +---------------+---------------+---------------+
    |   Type (8)    |   Flags (8)   |
    +-+-------------+---------------+-------------------------------+
    |R|                 Stream Identifier (31)                      |
    +=+=============================================================+
    |                   Frame Payload (0...)                      ...
    +---------------------------------------------------------------+

                          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.  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.





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   Type:  The 8-bit type of the frame.  The frame type determines the
      format and semantics of the frame.  Implementations MUST ignore
      and discard any frame that has a type that is unknown.

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

      Flags are assigned semantics specific to the indicated frame type.
      Flags that have no defined semantics for a particular frame type
      MUST be ignored and MUST be left unset (0x0) when sending.

   R: A reserved 1-bit field.  The semantics of this bit are undefined,
      and the bit MUST remain unset (0x0) 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 0x0 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 is 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 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 header
   block (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and
   CONTINUATION), SETTINGS, and any frame with a stream identifier of 0.





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   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.  Header Compression and Decompression

   Just as in HTTP/1, a header field in HTTP/2 is a name with one or
   more associated values.  Header fields are used within HTTP request
   and response messages as well as in server push operations (see
   Section 8.2).

   Header lists are collections of zero or more header fields.  When
   transmitted over a connection, a header list is serialized into a
   header block using HTTP header compression [COMPRESSION].  The
   serialized header block is then divided into one or more octet
   sequences, called header block fragments, and transmitted within the
   payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6), or
   CONTINUATION (Section 6.10) frames.

   The Cookie header field [COOKIE] is treated specially by the HTTP
   mapping (see Section 8.1.2.5).

   A receiving endpoint reassembles the header block by concatenating
   its fragments and then decompresses the block to reconstruct the
   header list.

   A complete header block consists of either:

   o  a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag
      set, or

   o  a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared
      and one or more CONTINUATION frames, where the last CONTINUATION
      frame has the END_HEADERS flag set.

   Header compression is stateful.  One compression context and one
   decompression context are used for the entire connection.  A decoding
   error in a header block MUST be treated as a connection error
   (Section 5.4.1) of type COMPRESSION_ERROR.

   Each header block is processed as a discrete unit.  Header 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




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   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
   header block to be logically equivalent to a single frame.

   Header block fragments can only be sent as the 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 header 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 header block.

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:

   o  A single HTTP/2 connection can contain multiple concurrently open
      streams, with either endpoint interleaving frames from multiple
      streams.

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

   o  Streams can be closed by either endpoint.

   o  The order in which frames are sent on a stream is significant.
      Recipients process frames in the order they are received.  In
      particular, the order of HEADERS and DATA frames is semantically
      significant.

   o  Streams are identified by an integer.  Stream identifiers are
      assigned to streams by the endpoint initiating the stream.
















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

          send:   endpoint sends this frame
          recv:   endpoint receives this frame

          H:  HEADERS frame (with implied CONTINUATIONs)
          PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
          ES: END_STREAM flag
          R:  RST_STREAM frame

                          Figure 2: Stream States

   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.




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   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 or receiving a HEADERS frame 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)".

      *  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)".

      *  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.

      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.

   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 8.2).







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      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 MAY send a PRIORITY frame in this state to
      reprioritize the reserved stream.  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





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      END_STREAM flag causes the stream state to become "half-closed
      (local)"; an endpoint receiving an END_STREAM flag causes the
      stream state to become "half-closed (remote)".

      Either endpoint can send a RST_STREAM frame from this state,
      causing it to transition immediately to "closed".

   half-closed (local):
      A stream that is in the "half-closed (local)" state cannot be used
      for sending frames other than WINDOW_UPDATE, PRIORITY, and
      RST_STREAM.

      A stream transitions from this state to "closed" when a frame that
      contains an END_STREAM flag is received 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 bearing the END_STREAM
      flag is sent.

      PRIORITY frames received in this state are used to reprioritize
      streams that depend on the identified stream.

   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 that contains an END_STREAM flag or when either peer sends a
      RST_STREAM frame.







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

      An endpoint MUST NOT send frames other than PRIORITY on a closed
      stream.  An endpoint that receives any frame other than PRIORITY
      after receiving a RST_STREAM MUST treat that as a stream error
      (Section 5.4.2) of type STREAM_CLOSED.  Similarly, an endpoint
      that receives any frames after receiving a frame with the
      END_STREAM flag set MUST treat that as a connection error
      (Section 5.4.1) of type STREAM_CLOSED, unless the frame is
      permitted as described below.

      WINDOW_UPDATE or RST_STREAM frames can be received in this state
      for a short period after a DATA or HEADERS frame containing an
      END_STREAM flag is sent.  Until the remote peer receives and
      processes RST_STREAM or the frame bearing the END_STREAM flag, it
      might send frames of these types.  Endpoints MUST ignore
      WINDOW_UPDATE or RST_STREAM frames received in this state, though
      endpoints MAY choose to treat frames that arrive a significant
      time after sending END_STREAM as a connection error
      (Section 5.4.1) of type PROTOCOL_ERROR.

      PRIORITY frames can be sent on closed streams to prioritize
      streams that are dependent on the closed stream.  Endpoints SHOULD
      process PRIORITY frames, though they can be ignored if the stream
      has been removed from the dependency tree (see Section 5.3.4).

      If this state is reached as a result of sending a RST_STREAM
      frame, the peer that receives the RST_STREAM might have already
      sent -- or enqueued for sending -- frames on the stream that
      cannot be withdrawn.  An endpoint MUST ignore frames that it
      receives on closed streams after it has sent a RST_STREAM frame.
      An endpoint MAY choose to limit the period over which it ignores
      frames and treat frames that arrive after this time as being in
      error.

      Flow-controlled frames (i.e., DATA) received after sending
      RST_STREAM are counted toward the connection flow-control window.
      Even though these frames might be ignored, because they are sent
      before the sender receives the RST_STREAM, the sender will
      consider the frames to count against the flow-control window.

      An endpoint might receive a PUSH_PROMISE frame after it sends
      RST_STREAM.  PUSH_PROMISE causes a stream to become "reserved"
      even if the associated stream has been reset.  Therefore, a
      RST_STREAM is needed to close an unwanted promised stream.





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   In the absence of more specific guidance elsewhere in this document,
   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.  Frames of unknown types
   are ignored.

   An example of the state transitions for an HTTP request/response
   exchange can be found in Section 8.1.  An example of the state
   transitions for server push can be found in Sections 8.2.1 and 8.2.2.

5.1.1.  Stream Identifiers

   Streams are identified with 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 (0x0) is used for connection control
   messages; the stream identifier of zero cannot be used to establish a
   new stream.

   HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are
   responded to with a stream identifier of one (0x1).  After the
   upgrade completes, stream 0x1 is "half-closed (local)" to the client.
   Therefore, stream 0x1 cannot be selected as a new stream identifier
   by a client that upgrades from HTTP/1.1.

   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.

   The first use of a new stream identifier implicitly closes all
   streams in the "idle" state that might have been initiated by that
   peer with a lower-valued stream identifier.  For example, if a client
   sends a HEADERS frame on stream 7 without ever sending a frame on
   stream 5, then stream 5 transitions to the "closed" state when the
   first frame for stream 7 is sent or received.

   Stream identifiers cannot be reused.  Long-lived connections can
   result in an endpoint exhausting the available range of stream
   identifiers.  A client that is unable to establish a new stream
   identifier can establish a new connection for new streams.  A server
   that is unable to establish a new stream identifier can send a GOAWAY
   frame so that the client is forced to open a new connection for new
   streams.




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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 8.1.4) 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 for the connection as a whole.

   HTTP/2 provides for flow control through use of the WINDOW_UPDATE
   frame (Section 6.9).












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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.  Both types of flow
       control are between the endpoints of a single hop and not over
       the entire end-to-end path.

   2.  Flow control is based on WINDOW_UPDATE frames.  Receivers
       advertise how many octets they are prepared to receive on a
       stream and for the entire connection.  This is a credit-based
       scheme.

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

   4.  The initial value for the flow-control window is 65,535 octets
       for both new streams and the overall connection.

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

   6.  Flow control cannot be disabled.

   7.  HTTP/2 defines only the format and semantics of the WINDOW_UPDATE
       frame (Section 6.9).  This document does not stipulate how a
       receiver decides when to send this frame or the value that it
       sends, nor does it specify how a sender chooses to send packets.
       Implementations are able to select any algorithm that suits their
       needs.

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




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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.  When using flow
   control, the receiver MUST read from the TCP receive buffer in a
   timely fashion.  Failure to do so could lead to a deadlock when
   critical frames, such as WINDOW_UPDATE, are not read and acted upon.

5.3.  Stream Priority

   A client can assign a priority for a new stream by including
   prioritization information in the HEADERS frame (Section 6.2) that
   opens the stream.  At any other time, the PRIORITY frame
   (Section 6.3) can be used to change the priority of a stream.

   The purpose of prioritization is to allow an endpoint to express how
   it would prefer its peer to allocate resources when managing
   concurrent streams.  Most importantly, priority can be used to select
   streams for transmitting frames when there is limited capacity for
   sending.

   Streams can be prioritized by marking them as dependent on the
   completion of other streams (Section 5.3.1).  Each dependency is
   assigned a relative weight, a number that is used to determine the
   relative proportion of available resources that are assigned to
   streams dependent on the same stream.





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   Explicitly setting the priority for a stream is input to a
   prioritization process.  It does not guarantee any particular
   processing or transmission order for the stream relative to any other
   stream.  An endpoint cannot force a peer to process concurrent
   streams in a particular order using priority.  Expressing priority is
   therefore only a suggestion.

   Prioritization information can be omitted from messages.  Defaults
   are used prior to any explicit values being provided (Section 5.3.5).

5.3.1.  Stream Dependencies

   Each stream can be given an explicit dependency on another stream.
   Including a dependency expresses a preference to allocate resources
   to the identified stream rather than to the dependent stream.

   A stream that is not dependent on any other stream is given a stream
   dependency of 0x0.  In other words, the non-existent stream 0 forms
   the root of the tree.

   A stream that depends on another stream is a dependent stream.  The
   stream upon which a stream is dependent is a parent stream.  A
   dependency on a stream that is not currently in the tree -- such as a
   stream in the "idle" state -- results in that stream being given a
   default priority (Section 5.3.5).

   When assigning a dependency on another stream, the stream is added as
   a new dependency of the parent stream.  Dependent streams that share
   the same parent are not ordered with respect to each other.  For
   example, if streams B and C are dependent on stream A, and if stream
   D is created with a dependency on stream A, this results in a
   dependency order of A followed by B, C, and D in any order.

       A                 A
      / \      ==>      /|\
     B   C             B D C

             Figure 3: Example of Default Dependency Creation

   An exclusive flag allows for the insertion of a new level of
   dependencies.  The exclusive flag causes the stream to become the
   sole dependency of its parent stream, causing other dependencies to
   become dependent on the exclusive stream.  In the previous example,
   if stream D is created with an exclusive dependency on stream A, this
   results in D becoming the dependency parent of B and C.






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                         A
       A                 |
      / \      ==>       D
     B   C              / \
                       B   C

            Figure 4: Example of Exclusive Dependency Creation

   Inside the dependency tree, a dependent stream SHOULD only be
   allocated resources if either all of the streams that it depends on
   (the chain of parent streams up to 0x0) are closed or it is not
   possible to make progress on them.

   A stream cannot depend on itself.  An endpoint MUST treat this as a
   stream error (Section 5.4.2) of type PROTOCOL_ERROR.

5.3.2.  Dependency Weighting

   All dependent streams are allocated an integer weight between 1 and
   256 (inclusive).

   Streams with the same parent SHOULD be allocated resources
   proportionally based on their weight.  Thus, if stream B depends on
   stream A with weight 4, stream C depends on stream A with weight 12,
   and no progress can be made on stream A, stream B ideally receives
   one-third of the resources allocated to stream C.

5.3.3.  Reprioritization

   Stream priorities are changed using the PRIORITY frame.  Setting a
   dependency causes a stream to become dependent on the identified
   parent stream.

   Dependent streams move with their parent stream if the parent is
   reprioritized.  Setting a dependency with the exclusive flag for a
   reprioritized stream causes all the dependencies of the new parent
   stream to become dependent on the reprioritized stream.

   If a stream is made dependent on one of its own dependencies, the
   formerly dependent stream is first moved to be dependent on the
   reprioritized stream's previous parent.  The moved dependency retains
   its weight.

   For example, consider an original dependency tree where B and C
   depend on A, D and E depend on C, and F depends on D.  If A is made
   dependent on D, then D takes the place of A.  All other dependency
   relationships stay the same, except for F, which becomes dependent on
   A if the reprioritization is exclusive.



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       x                x                x                 x
       |               / \               |                 |
       A              D   A              D                 D
      / \            /   / \            / \                |
     B   C     ==>  F   B   C   ==>    F   A       OR      A
        / \                 |             / \             /|\
       D   E                E            B   C           B C F
       |                                     |             |
       F                                     E             E
                  (intermediate)   (non-exclusive)    (exclusive)

                Figure 5: Example of Dependency Reordering

5.3.4.  Prioritization State Management

   When a stream is removed from the dependency tree, its dependencies
   can be moved to become dependent on the parent of the closed stream.
   The weights of new dependencies are recalculated by distributing the
   weight of the dependency of the closed stream proportionally based on
   the weights of its dependencies.

   Streams that are removed from the dependency tree cause some
   prioritization information to be lost.  Resources are shared between
   streams with the same parent stream, which means that if a stream in
   that set closes or becomes blocked, any spare capacity allocated to a
   stream is distributed to the immediate neighbors of the stream.
   However, if the common dependency is removed from the tree, those
   streams share resources with streams at the next highest level.

   For example, assume streams A and B share a parent, and streams C and
   D both depend on stream A.  Prior to the removal of stream A, if
   streams A and D are unable to proceed, then stream C receives all the
   resources dedicated to stream A.  If stream A is removed from the
   tree, the weight of stream A is divided between streams C and D.  If
   stream D is still unable to proceed, this results in stream C
   receiving a reduced proportion of resources.  For equal starting
   weights, C receives one third, rather than one half, of available
   resources.

   It is possible for a stream to become closed while prioritization
   information that creates a dependency on that stream is in transit.
   If a stream identified in a dependency has no associated priority
   information, then the dependent stream is instead assigned a default
   priority (Section 5.3.5).  This potentially creates suboptimal
   prioritization, since the stream could be given a priority that is
   different from what is intended.





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   To avoid these problems, an endpoint SHOULD retain stream
   prioritization state for a period after streams become closed.  The
   longer state is retained, the lower the chance that streams are
   assigned incorrect or default priority values.

   Similarly, streams that are in the "idle" state can be assigned
   priority or become a parent of other streams.  This allows for the
   creation of a grouping node in the dependency tree, which enables
   more flexible expressions of priority.  Idle streams begin with a
   default priority (Section 5.3.5).

   The retention of priority information for streams that are not
   counted toward the limit set by SETTINGS_MAX_CONCURRENT_STREAMS could
   create a large state burden for an endpoint.  Therefore, the amount
   of prioritization state that is retained MAY be limited.

   The amount of additional state an endpoint maintains for
   prioritization could be dependent on load; under high load,
   prioritization state can be discarded to limit resource commitments.
   In extreme cases, an endpoint could even discard prioritization state
   for active or reserved streams.  If a limit is applied, endpoints
   SHOULD maintain state for at least as many streams as allowed by
   their setting for SETTINGS_MAX_CONCURRENT_STREAMS.  Implementations
   SHOULD also attempt to retain state for streams that are in active
   use in the priority tree.

   If it has retained enough state to do so, an endpoint receiving a
   PRIORITY frame that changes the priority of a closed stream SHOULD
   alter the dependencies of the streams that depend on it.

5.3.5.  Default Priorities

   All streams are initially assigned a non-exclusive dependency on
   stream 0x0.  Pushed streams (Section 8.2) initially depend on their
   associated stream.  In both cases, streams are assigned a default
   weight of 16.

5.4.  Error Handling

   HTTP/2 framing permits two classes of error:

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

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

   A list of error codes is included in Section 7.




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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 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 ([RFC7230], Section 6.6 describes how an immediate
   connection close can result in data loss).  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 header compression
   (Section 4.3) or flow control).

   Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame
   for any stream.  However, an endpoint MAY send additional RST_STREAM
   frames if it receives frames on a closed stream after more than a
   round-trip time.  This behavior is permitted to deal with misbehaving
   implementations.





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   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
   "open" or "half-closed" state, then the affected streams cannot be
   automatically retried (see Section 8.1.4 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 header fields.

   Extensions are permitted to use new frame types (Section 4.1), new
   settings (Section 6.5.2), or new error codes (Section 7).  Registries
   are established for managing these extension points: frame types
   (Section 11.2), settings (Section 11.3), and error codes
   (Section 11.4).

   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 header block (Section 4.3) are not permitted; these
   MUST be treated as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   Extensions that could change the semantics of existing protocol
   components 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.  Note that treating any frames
   other than DATA frames as flow controlled is such a change in
   semantics and 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



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   a setting is used for extension negotiation, the initial value MUST
   be defined in such a fashion that the extension is initially
   disabled.

6.  Frame Definitions

   This specification defines a number of frame types, each identified
   by a unique 8-bit type code.  Each frame type serves a distinct
   purpose in the establishment and management either of the connection
   as a whole or of 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
   any given frame.

6.1.  DATA

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

   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.

    +---------------+
    |Pad Length? (8)|
    +---------------+-----------------------------------------------+
    |                            Data (*)                         ...
    +---------------------------------------------------------------+
    |                           Padding (*)                       ...
    +---------------------------------------------------------------+

                       Figure 6: DATA Frame Payload

   The DATA frame contains the following fields:

   Pad Length:  An 8-bit field containing the length of the frame
      padding in units of octets.  This field is conditional (as
      signified by a "?" in the diagram) 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.



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   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:

   END_STREAM (0x1):  When set, bit 0 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).

   PADDED (0x8):  When set, bit 3 indicates that the Pad Length field
      and any padding that it describes are present.

   DATA frames MUST be associated with a stream.  If a DATA frame is
   received whose stream identifier field is 0x0, the recipient MUST
   respond with a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   DATA frames are subject to flow control and can only be sent when a
   stream is in the "open" or "half-closed (remote)" state.  The entire
   DATA frame payload is included in flow control, including the Pad
   Length and Padding fields if present.  If a DATA frame is received
   whose stream is not in "open" or "half-closed (local)" state, the
   recipient MUST respond with a stream error (Section 5.4.2) of type
   STREAM_CLOSED.

   The total number of padding octets is determined by the value of the
   Pad Length field.  If the length of the padding is the length of the
   frame payload or greater, the recipient MUST treat this as a
   connection error (Section 5.4.1) of type PROTOCOL_ERROR.

      Note: A frame can be increased in size by one octet by including a
      Pad Length field with a value of zero.

6.2.  HEADERS

   The HEADERS frame (type=0x1) is used to open a stream (Section 5.1),
   and additionally carries a header block fragment.  HEADERS frames can
   be sent on a stream in the "idle", "reserved (local)", "open", or
   "half-closed (remote)" state.









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    +---------------+
    |Pad Length? (8)|
    +-+-------------+-----------------------------------------------+
    |E|                 Stream Dependency? (31)                     |
    +-+-------------+-----------------------------------------------+
    |  Weight? (8)  |
    +-+-------------+-----------------------------------------------+
    |                   Header Block Fragment (*)                 ...
    +---------------------------------------------------------------+
    |                           Padding (*)                       ...
    +---------------------------------------------------------------+

                      Figure 7: HEADERS Frame Payload

   The HEADERS frame payload has the following 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.

   E: A single-bit flag indicating that the stream dependency is
      exclusive (see Section 5.3).  This field is only present if the
      PRIORITY flag is set.

   Stream Dependency:  A 31-bit stream identifier for the stream that
      this stream depends on (see Section 5.3).  This field is only
      present if the PRIORITY flag is set.

   Weight:  An unsigned 8-bit integer representing a priority weight for
      the stream (see Section 5.3).  Add one to the value to obtain a
      weight between 1 and 256.  This field is only present if the
      PRIORITY flag is set.

   Header Block Fragment:  A header block fragment (Section 4.3).

   Padding:  Padding octets.

   The HEADERS frame defines the following flags:

   END_STREAM (0x1):  When set, bit 0 indicates that the header block
      (Section 4.3) is the last that the endpoint will send for the
      identified stream.

      A HEADERS frame carries the END_STREAM flag that 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.




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   END_HEADERS (0x4):  When set, bit 2 indicates that this frame
      contains an entire header block (Section 4.3) and is not followed
      by any CONTINUATION frames.

      A HEADERS frame without the END_HEADERS flag set MUST be followed
      by a CONTINUATION frame for the same stream.  A receiver MUST
      treat the receipt of any other type of frame or a frame on a
      different stream as a connection error (Section 5.4.1) of type
      PROTOCOL_ERROR.

   PADDED (0x8):  When set, bit 3 indicates that the Pad Length field
      and any padding that it describes are present.

   PRIORITY (0x20):  When set, bit 5 indicates that the Exclusive Flag
      (E), Stream Dependency, and Weight fields are present; see
      Section 5.3.

   The payload of a HEADERS frame contains a header block fragment
   (Section 4.3).  A header 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 0x0, the recipient MUST
   respond with a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   The HEADERS frame changes the connection state as described in
   Section 4.3.

   The HEADERS frame can include padding.  Padding fields and flags are
   identical to those defined for DATA frames (Section 6.1).  Padding
   that exceeds the size remaining for the header block fragment MUST be
   treated as a PROTOCOL_ERROR.

   Prioritization information in a HEADERS frame is logically equivalent
   to a separate PRIORITY frame, but inclusion in HEADERS avoids the
   potential for churn in stream prioritization when new streams are
   created.  Prioritization fields in HEADERS frames subsequent to the
   first on a stream reprioritize the stream (Section 5.3.3).

6.3.  PRIORITY

   The PRIORITY frame (type=0x2) specifies the sender-advised priority
   of a stream (Section 5.3).  It can be sent in any stream state,
   including idle or closed streams.






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    +-+-------------------------------------------------------------+
    |E|                  Stream Dependency (31)                     |
    +-+-------------+-----------------------------------------------+
    |   Weight (8)  |
    +-+-------------+

                     Figure 8: PRIORITY Frame Payload

   The payload of a PRIORITY frame contains the following fields:

   E: A single-bit flag indicating that the stream dependency is
      exclusive (see Section 5.3).

   Stream Dependency:  A 31-bit stream identifier for the stream that
      this stream depends on (see Section 5.3).

   Weight:  An unsigned 8-bit integer representing a priority weight for
      the stream (see Section 5.3).  Add one to the value to obtain a
      weight between 1 and 256.

   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 0x0, the recipient MUST
   respond with a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   The PRIORITY frame can be sent on a stream in any state, though it
   cannot be sent between consecutive frames that comprise a single
   header block (Section 4.3).  Note that this frame could arrive after
   processing or frame sending has completed, which would cause it to
   have no effect on the identified stream.  For a stream that is in the
   "half-closed (remote)" or "closed" state, this frame can only affect
   processing of the identified stream and its dependent streams; it
   does not affect frame transmission on that stream.

   The PRIORITY frame can be sent for a stream in the "idle" or "closed"
   state.  This allows for the reprioritization of a group of dependent
   streams by altering the priority of an unused or closed parent
   stream.

   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.








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6.4.  RST_STREAM

   The RST_STREAM frame (type=0x3) 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.

    +---------------------------------------------------------------+
    |                        Error Code (32)                        |
    +---------------------------------------------------------------+

                    Figure 9: RST_STREAM Frame Payload

   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, with the exception of 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 0x0, the recipient MUST
   treat this as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   RST_STREAM frames MUST NOT be sent for a stream in the "idle" state.
   If a RST_STREAM frame identifying an idle stream is received, the
   recipient MUST treat this as a connection error (Section 5.4.1) of
   type PROTOCOL_ERROR.

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

6.5.  SETTINGS

   The SETTINGS frame (type=0x4) 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 parameters.  Individually, a SETTINGS parameter can
   also be referred to as a "setting".





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   SETTINGS parameters are not negotiated; they describe characteristics
   of the sending peer, which are used by the receiving peer.  Different
   values for the same parameter 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 parameters defined by this specification.

   Each parameter in a SETTINGS frame replaces any existing value for
   that parameter.  Parameters 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 its parameters.  Therefore,
   the value of a SETTINGS parameter is the last value that is seen by a
   receiver.

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

   ACK (0x1):  When set, bit 0 indicates that this frame acknowledges
      receipt and application of the peer's SETTINGS frame.  When this
      bit is set, the 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 (0x0).  If an
   endpoint receives a SETTINGS frame whose stream identifier field is
   anything other than 0x0, 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
   FRAME_SIZE_ERROR.









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6.5.1.  SETTINGS Format

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

    +-------------------------------+
    |       Identifier (16)         |
    +-------------------------------+-------------------------------+
    |                        Value (32)                             |
    +---------------------------------------------------------------+

                         Figure 10: Setting Format

6.5.2.  Defined SETTINGS Parameters

   The following parameters are defined:

   SETTINGS_HEADER_TABLE_SIZE (0x1):  Allows the sender to inform the
      remote endpoint of the maximum size of the header compression
      table used to decode header blocks, in octets.  The encoder can
      select any size equal to or less than this value by using
      signaling specific to the header compression format inside a
      header block (see [COMPRESSION]).  The initial value is 4,096
      octets.

   SETTINGS_ENABLE_PUSH (0x2):  This setting can be used to disable
      server push (Section 8.2).  An endpoint MUST NOT send a
      PUSH_PROMISE frame if it receives this parameter set to a value of
      0.  An endpoint 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 is 1, which indicates that server push is
      permitted.  Any value other than 0 or 1 MUST be treated as a
      connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   SETTINGS_MAX_CONCURRENT_STREAMS (0x3):  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.

      A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be
      treated as special by endpoints.  A zero value does prevent the
      creation of new streams; however, this can also happen for any




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      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 (0x4):  Indicates the sender's initial
      window size (in 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
      FLOW_CONTROL_ERROR.

   SETTINGS_MAX_FRAME_SIZE (0x5):  Indicates the size of the largest
      frame payload that the sender is willing to receive, in 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 (0x6):  This advisory setting informs a
      peer of the maximum size of header list that the sender is
      prepared to accept, in octets.  The value is based on the
      uncompressed size of header fields, including the length of the
      name and value in octets plus an overhead of 32 octets for each
      header field.

      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 parameters as soon as possible upon receipt.

   The values in the SETTINGS frame MUST be processed in the order they
   appear, with no other frame processing between values.  Unsupported
   parameters MUST be ignored.  Once all values have been processed, the



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   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 parameters can rely on the setting having been
   applied.

   If the sender of a SETTINGS frame does not receive an acknowledgement
   within a reasonable amount of time, it MAY issue a connection error
   (Section 5.4.1) of type SETTINGS_TIMEOUT.

6.6.  PUSH_PROMISE

   The PUSH_PROMISE frame (type=0x5) 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 set of headers that
   provide additional context for the stream.  Section 8.2 contains a
   thorough description of the use of PUSH_PROMISE frames.

    +---------------+
    |Pad Length? (8)|
    +-+-------------+-----------------------------------------------+
    |R|                  Promised Stream ID (31)                    |
    +-+-----------------------------+-------------------------------+
    |                   Header Block Fragment (*)                 ...
    +---------------------------------------------------------------+
    |                           Padding (*)                       ...
    +---------------------------------------------------------------+

                  Figure 11: PUSH_PROMISE Payload Format

   The PUSH_PROMISE frame payload has the following 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.

   R: A single reserved bit.

   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).

   Header Block Fragment:  A header block fragment (Section 4.3)
      containing request header fields.

   Padding:  Padding octets.




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   The PUSH_PROMISE frame defines the following flags:

   END_HEADERS (0x4):  When set, bit 2 indicates that this frame
      contains an entire header 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
      PROTOCOL_ERROR.

   PADDED (0x8):  When set, bit 3 indicates that the Pad Length field
      and any padding that it describes are present.

   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
   0x0, 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.

   PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of
   the peer endpoint is set to 0.  An endpoint that has set this setting
   and has received acknowledgement MUST treat the receipt of a
   PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   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 header block (Section 4.3) potentially
   modifies the state maintained for header compression.  Second,
   PUSH_PROMISE also reserves a stream for later use, causing the
   promised stream to enter the "reserved" 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).






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   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 PUSH_PROMISE frame can include padding.  Padding fields and flags
   are identical to those defined for DATA frames (Section 6.1).

6.7.  PING

   The PING frame (type=0x6) 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.

    +---------------------------------------------------------------+
    |                                                               |
    |                      Opaque Data (64)                         |
    |                                                               |
    +---------------------------------------------------------------+

                      Figure 12: PING Payload Format

   In addition to the frame header, PING frames MUST contain 8 octets of
   opaque data in the 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
   payload.  PING responses SHOULD be given higher priority than any
   other frame.

   The PING frame defines the following flags:

   ACK (0x1):  When set, bit 0 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.




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   PING frames are not associated with any individual stream.  If a PING
   frame is received with a stream identifier field value other than
   0x0, 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
   FRAME_SIZE_ERROR.

6.8.  GOAWAY

   The GOAWAY frame (type=0x7) 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 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 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
   connection.

   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.



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   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
   connection.

    +-+-------------------------------------------------------------+
    |R|                  Last-Stream-ID (31)                        |
    +-+-------------------------------------------------------------+
    |                      Error Code (32)                          |
    +---------------------------------------------------------------+
    |                  Additional Debug Data (*)                    |
    +---------------------------------------------------------------+

                     Figure 13: GOAWAY Payload Format

   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 0x0 as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.

   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,
   with the exception of idempotent actions like HTTP GET, PUT, or
   DELETE.  Any protocol activity that uses higher-numbered streams can
   be safely retried using a new connection.

   Activity on streams numbered lower or equal to the last stream
   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.



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   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 can 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 the state maintained for header 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 header 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 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.










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6.9.  WINDOW_UPDATE

   The WINDOW_UPDATE frame (type=0x8) 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.

    +-+-------------------------------------------------------------+
    |R|              Window Size Increment (31)                     |
    +-+-------------------------------------------------------------+

                  Figure 14: WINDOW_UPDATE Payload Format

   The 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 an
   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).





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   WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the
   END_STREAM flag.  This means that a receiver could receive a
   WINDOW_UPDATE frame on a "half-closed (remote)" or "closed" stream.
   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
   FRAME_SIZE_ERROR.

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
   counted.

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

   The receiver of a 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.

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





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   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 that forms part of the connection preface.  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 to 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





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   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.

6.10.  CONTINUATION

   The CONTINUATION frame (type=0x9) is used to continue a sequence of
   header 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.

    +---------------------------------------------------------------+
    |                   Header Block Fragment (*)                 ...
    +---------------------------------------------------------------+

                   Figure 15: CONTINUATION Frame Payload

   The CONTINUATION frame payload contains a header block fragment
   (Section 4.3).








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   The CONTINUATION frame defines the following flag:

   END_HEADERS (0x4):  When set, bit 2 indicates that this frame ends a
      header block (Section 4.3).

      If the END_HEADERS bit 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 whose stream identifier field is 0x0,
   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 (0x0):  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 (0x1):  The endpoint detected an unspecific protocol
      error.  This error is for use when a more specific error code is
      not available.

   INTERNAL_ERROR (0x2):  The endpoint encountered an unexpected
      internal error.

   FLOW_CONTROL_ERROR (0x3):  The endpoint detected that its peer
      violated the flow-control protocol.




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   SETTINGS_TIMEOUT (0x4):  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 (0x5):  The endpoint received a frame after a stream
      was half-closed.

   FRAME_SIZE_ERROR (0x6):  The endpoint received a frame with an
      invalid size.

   REFUSED_STREAM (0x7):  The endpoint refused the stream prior to
      performing any application processing (see Section 8.1.4 for
      details).

   CANCEL (0x8):  Used by the endpoint to indicate that the stream is no
      longer needed.

   COMPRESSION_ERROR (0x9):  The endpoint is unable to maintain the
      header compression context for the connection.

   CONNECT_ERROR (0xa):  The connection established in response to a
      CONNECT request (Section 8.3) was reset or abnormally closed.

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

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

   HTTP_1_1_REQUIRED (0xd):  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.  HTTP Message Exchanges

   HTTP/2 is intended to be as compatible as possible with current uses
   of HTTP.  This means that, from the application perspective, the
   features of the protocol are largely unchanged.  To achieve this, all
   request and response semantics are preserved, although the syntax of
   conveying those semantics has changed.

   Thus, the specification and requirements of HTTP/1.1 Semantics and
   Content [RFC7231], Conditional Requests [RFC7232], Range Requests
   [RFC7233], Caching [RFC7234], and Authentication [RFC7235] are
   applicable to HTTP/2.  Selected portions of HTTP/1.1 Message Syntax



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   and Routing [RFC7230], such as the HTTP and HTTPS URI schemes, are
   also applicable in HTTP/2, but the expression of those semantics for
   this protocol are defined in the sections below.

8.1.  HTTP Request/Response Exchange

   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.  for a response only, zero or more HEADERS frames (each followed
       by zero or more CONTINUATION frames) containing the message
       headers of informational (1xx) HTTP responses (see [RFC7230],
       Section 3.2 and [RFC7231], Section 6.2),

   2.  one HEADERS frame (followed by zero or more CONTINUATION frames)
       containing the message headers (see [RFC7230], Section 3.2),

   3.  zero or more DATA frames containing the payload body (see
       [RFC7230], Section 3.3), and

   4.  optionally, one HEADERS frame, followed by zero or more
       CONTINUATION frames containing the trailer-part, if present (see
       [RFC7230], Section 4.1.2).

   The last frame in the sequence bears an END_STREAM flag, noting that
   a HEADERS frame bearing the END_STREAM flag can be followed by
   CONTINUATION frames that carry any remaining portions of the header
   block.

   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 payloads.  The "chunked"
   transfer encoding defined in Section 4.1 of [RFC7230] MUST NOT be
   used in HTTP/2.

   Trailing header fields are carried in a header block that also
   terminates the stream.  Such a header block is a sequence starting
   with a HEADERS frame, followed by zero or more CONTINUATION frames,
   where the HEADERS frame bears an END_STREAM flag.  Header blocks
   after the first that do not terminate the stream are not part of an
   HTTP request or response.






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   A HEADERS frame (and associated CONTINUATION frames) can only appear
   at the start or end of a stream.  An endpoint that receives a HEADERS
   frame without the END_STREAM flag set after receiving a final (non-
   informational) status code MUST treat the corresponding request or
   response as malformed (Section 8.1.2.6).

   An HTTP request/response exchange fully consumes a single stream.  A
   request starts with the HEADERS frame that puts the stream into an
   "open" state.  The request ends with a frame bearing END_STREAM,
   which causes the stream to become "half-closed (local)" for the
   client and "half-closed (remote)" for the server.  A response starts
   with a HEADERS frame and ends with a frame bearing END_STREAM, which
   places the stream in the "closed" state.

   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 header 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).
   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.  Upgrading from HTTP/2

   HTTP/2 removes support for the 101 (Switching Protocols)
   informational status code ([RFC7231], Section 6.2.2).

   The semantics of 101 (Switching Protocols) aren't applicable to a
   multiplexed protocol.  Alternative protocols are able to use the same
   mechanisms that HTTP/2 uses to negotiate their use (see Section 3).

8.1.2.  HTTP Header Fields

   HTTP header fields carry information as a series of key-value pairs.
   For a listing of registered HTTP headers, see the "Message Header
   Field" registry maintained at <https://www.iana.org/assignments/
   message-headers>.

   Just as in HTTP/1.x, header field names are strings of ASCII
   characters that are compared in a case-insensitive fashion.  However,
   header field names MUST be converted to lowercase prior to their
   encoding in HTTP/2.  A request or response containing uppercase
   header field names MUST be treated as malformed (Section 8.1.2.6).



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8.1.2.1.  Pseudo-Header Fields

   While HTTP/1.x used the message start-line (see [RFC7230],
   Section 3.1) to convey the target URI, the method of the request, and
   the status code for the response, HTTP/2 uses special pseudo-header
   fields beginning with ':' character (ASCII 0x3a) for this purpose.

   Pseudo-header fields are not HTTP header fields.  Endpoints MUST NOT
   generate pseudo-header fields other than those defined in this
   document.

   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
   trailers.  Endpoints MUST treat a request or response that contains
   undefined or invalid pseudo-header fields as malformed
   (Section 8.1.2.6).

   All pseudo-header fields MUST appear in the header block before
   regular header fields.  Any request or response that contains a
   pseudo-header field that appears in a header block after a regular
   header field MUST be treated as malformed (Section 8.1.2.6).

8.1.2.2.  Connection-Specific Header Fields

   HTTP/2 does not use the Connection header field 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; any message containing connection-specific header fields MUST
   be treated as malformed (Section 8.1.2.6).

   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".

   This means that an intermediary transforming an HTTP/1.x message to
   HTTP/2 will need to remove any header fields nominated by the
   Connection header field, along with the Connection header field
   itself.  Such intermediaries SHOULD also remove other connection-
   specific header fields, such as Keep-Alive, Proxy-Connection,
   Transfer-Encoding, and Upgrade, even if they are not nominated by the
   Connection header field.

      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.



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8.1.2.3.  Request Pseudo-Header Fields

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

   o  The ":method" pseudo-header field includes the HTTP method
      ([RFC7231], Section 4).

   o  The ":scheme" pseudo-header field includes the scheme portion of
      the target URI ([RFC3986], Section 3.1).

      ":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.

   o  The ":authority" pseudo-header field includes the authority
      portion of the target URI ([RFC3986], Section 3.2).  The authority
      MUST NOT include the deprecated "userinfo" subcomponent for "http"
      or "https" schemed URIs.

      To ensure that the HTTP/1.1 request line can be reproduced
      accurately, this pseudo-header field MUST be omitted when
      translating from an HTTP/1.1 request that has a request target in
      origin or asterisk form (see [RFC7230], Section 5.3).  Clients
      that generate HTTP/2 requests directly SHOULD use the ":authority"
      pseudo-header field instead of the Host header field.  An
      intermediary that converts an HTTP/2 request to HTTP/1.1 MUST
      create a Host header field if one is not present in a request by
      copying the value of the ":authority" pseudo-header field.

   o  The ":path" pseudo-header field includes the path and query parts
      of the target URI (the "path-absolute" production and optionally a
      '?' character followed by the "query" production (see Sections 3.3
      and 3.4 of [RFC3986]).  A request in asterisk form 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 exception to this rule is 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 [RFC7230], Section 5.3.4).










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   All HTTP/2 requests MUST include exactly one valid value for the
   ":method", ":scheme", and ":path" pseudo-header fields, unless it is
   a CONNECT request (Section 8.3).  An HTTP request that omits
   mandatory pseudo-header fields is malformed (Section 8.1.2.6).

   HTTP/2 does not define a way to carry the version identifier that is
   included in the HTTP/1.1 request line.

8.1.2.4.  Response Pseudo-Header Fields

   For HTTP/2 responses, a single ":status" pseudo-header field is
   defined that carries the HTTP status code field (see [RFC7231],
   Section 6).  This pseudo-header field MUST be included in all
   responses; otherwise, the response is malformed (Section 8.1.2.6).

   HTTP/2 does not define a way to carry the version or reason phrase
   that is included in an HTTP/1.1 status line.

8.1.2.5.  Compressing the Cookie Header Field

   The Cookie header field [COOKIE] uses a semi-colon (";") to delimit
   cookie-pairs (or "crumbs").  This header field doesn't follow the
   list construction rules in HTTP (see [RFC7230], Section 3.2.2), which
   prevents cookie-pairs from being separated into different name-value
   pairs.  This can significantly reduce compression efficiency as
   individual cookie-pairs are updated.

   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








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8.1.2.6.  Malformed Requests and Responses

   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 header fields, the absence of mandatory
   header fields, or the inclusion of uppercase header field names.

   A request or response that includes a payload body 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 body.  A response
   that is defined to have no payload, as described in [RFC7230],
   Section 3.3.2, can 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
   PROTOCOL_ERROR.

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

8.1.3.  Examples

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

   An HTTP GET request includes request header fields and no payload
   body 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
                                          :path = /resource
                                          host = example.org
                                          accept = image/jpeg



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   Similarly, a response that includes only response header fields 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 ...

   An HTTP POST request that includes request header fields and payload
   data is transmitted as one HEADERS frame, followed by zero or more
   CONTINUATION frames containing the request header fields, 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
                                          :path = /resource
     {binary data}                        :scheme = https

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

                                      DATA
                                        + END_STREAM
                                      {binary data}

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

   A response that includes header fields and payload data 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 set:







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     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

                                      DATA
                                        + END_STREAM
                                      {binary data}

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

   Trailing header fields are sent as a header block after both the
   request or response header block and all the DATA frames have been
   sent.  The HEADERS frame starting the trailers header block 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 trailing header fields:

     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-length = 123
     123                                  content-type = image/jpeg
     {binary data}                        trailer = Foo
     0
     Foo: bar                         DATA
                                        - END_STREAM
                                      {binary data}

                                      HEADERS
                                        + END_STREAM
                                        + END_HEADERS
                                          foo = bar





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8.1.4.  Request Reliability Mechanisms in HTTP/2

   In HTTP/1.1, 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.  It is possible that some server processing
   occurred prior to the error, which could result in undesirable
   effects if the request were reattempted.

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

   o  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.

   o  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 as 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.

8.2.  Server Push

   HTTP/2 allows a server to pre-emptively send (or "push") responses
   (along with corresponding "promised" requests) to a client in
   association with a previous client-initiated request.  This can be
   useful when the server knows the client will need to have those
   responses available in order to fully process the response to the
   original request.






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   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 cacheable (see [RFC7231], Section 4.2.3),
   MUST be safe (see [RFC7231], Section 4.2.1), and MUST NOT include a
   request body.  Clients that receive a promised request that is not
   cacheable, that is not known to be safe, or that indicates the
   presence of a request body MUST reset the promised stream with a
   stream error (Section 5.4.2) of type PROTOCOL_ERROR.  Note 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 [RFC7234], Section 3) 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
   ([RFC7234], Section 5.2.2)) while the stream identified by the
   promised stream ID 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
   separately.

   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
   PROTOCOL_ERROR.

   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.  Clients MUST reject any attempt to change the
   SETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the
   message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

8.2.1.  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.




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   The PUSH_PROMISE frame includes a header block that contains 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 a request body.

   Pushed responses 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 8.1.2.3).  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 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 responses referenced by the header block (for instance, in
   Link header fields), sending a PUSH_PROMISE before sending the header
   block 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 in response to 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 header 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.




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8.2.2.  Push Responses

   After sending the PUSH_PROMISE frame, the server can begin delivering
   the pushed response as a response (Section 8.1.2.4) 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 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 disables
   server push by preventing the server from creating the necessary
   streams.  This does not prohibit a server from sending PUSH_PROMISE
   frames; clients need to reset any promised streams that are not
   wanted.

   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 or Common Name is not permitted to push a
   response for "https://www.example.org/doc".

   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 bearing END_STREAM, which places the stream in
   the "closed" state.

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








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8.3.  The CONNECT Method

   In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) 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 is used to establish a tunnel over a
   single HTTP/2 stream to a remote host for similar purposes.  The HTTP
   header field mapping works as defined in Section 8.1.2.3 ("Request
   Pseudo-Header Fields"), with a few differences.  Specifically:

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

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

   o  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 [RFC7230], Section 5.3)).

   A CONNECT request that does not conform to these restrictions is
   malformed (Section 8.1.2.6).

   A proxy that supports CONNECT establishes a TCP connection [TCP] to
   the server 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 [RFC7231], Section 4.3.6.

   After the initial HEADERS frame sent by each peer, all subsequent
   DATA frames correspond to data sent on the TCP connection.  The
   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 bearing the END_STREAM flag.  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.



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   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.

9.  Additional HTTP Requirements/Considerations

   This section outlines attributes of the HTTP protocol 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.








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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 8.3), 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.

   An origin server might offer a certificate with multiple
   "subjectAltName" attributes or names with wildcards, one of which is
   valid for the authority in the URI.  For example, a certificate with
   a "subjectAltName" of "*.example.com" might permit the use of the
   same connection for requests to URIs starting with
   "https://a.example.com/" and "https://b.example.com/".

   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 (SNI) [TLS-EXT] extension to select an
   origin server.  This means that it is possible for clients to send
   confidential information 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 9.1.2).

   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.

9.1.2.  The 421 (Misdirected Request) Status Code

   The 421 (Misdirected Request) status code indicates that the request
   was directed at a server that is not able to produce a response.
   This can be sent by a server that is not configured to produce
   responses for the combination of scheme and authority that are
   included in the request URI.




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   Clients receiving a 421 (Misdirected Request) response from a server
   MAY retry the request -- whether the request method is idempotent or
   not -- over a different connection.  This is possible if a connection
   is reused (Section 9.1.1) or if an alternative service is selected
   [ALT-SVC].

   This status code MUST NOT be generated by proxies.

   A 421 response is cacheable by default, i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [RFC7234]).

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.  HTTP/2 clients MUST indicate the target
   domain name when negotiating TLS.

   Deployments of HTTP/2 that negotiate TLS 1.3 or higher need only
   support and use the SNI extension; deployments of TLS 1.2 are subject
   to the requirements in the following sections.  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



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   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 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) [TLS12] and 224 bits for cipher suites that use
   ephemeral elliptic curve Diffie-Hellman (ECDHE) [RFC4492].  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 INADEQUATE_SECURITY.

9.2.2.  TLS 1.2 Cipher Suites

   A deployment of HTTP/2 over TLS 1.2 SHOULD NOT use any of the cipher
   suites that are listed in the cipher suite black list (Appendix A).

   Endpoints MAY choose to generate a connection error (Section 5.4.1)
   of type INADEQUATE_SECURITY if one of the cipher suites from the
   black list is negotiated.  A deployment that chooses to use a black-
   listed 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 on the black list.
   Consequently, when clients offer a cipher suite that is not on the
   black list, they have to be prepared to use that cipher suite with
   HTTP/2.

   The black list 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
   causing 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 [FIPS186].




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   Note that clients might advertise support of cipher suites that are
   on the black list 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 on the HTTP/2 black list.  However, this can
   result in HTTP/2 being negotiated with a black-listed cipher suite if
   the application protocol and cipher suite are independently selected.

10.  Security Considerations

10.1.  Server Authority

   HTTP/2 relies on the HTTP/1.1 definition of authority for determining
   whether a server is authoritative in providing a given response (see
   [RFC7230], Section 9.1).  This relies on local name resolution for
   the "http" URI scheme and the authenticated server identity for the
   "https" scheme (see [RFC2818], Section 3).

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
   protocols.

   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
   protocol.

   The cleartext version of HTTP/2 has minimal protection against cross-
   protocol attacks.  The connection preface (Section 3.5) contains a
   string that is designed to confuse HTTP/1.1 servers, but no special
   protection is offered for other protocols.  A server that is willing
   to ignore parts of an HTTP/1.1 request containing an Upgrade header
   field in addition to the client connection preface could be exposed
   to a cross-protocol attack.








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10.3.  Intermediary Encapsulation Attacks

   The HTTP/2 header field encoding allows the expression of names that
   are not valid field names in the Internet Message Syntax used by
   HTTP/1.1.  Requests or responses containing invalid header field
   names MUST be treated as malformed (Section 8.1.2.6).  An
   intermediary therefore cannot translate an HTTP/2 request or response
   containing an invalid field name into an HTTP/1.1 message.

   Similarly, HTTP/2 allows header field values that are not valid.
   While most of the values that can be encoded will not alter header
   field parsing, carriage return (CR, ASCII 0xd), line feed (LF, ASCII
   0xa), and the zero character (NUL, ASCII 0x0) might be exploited by
   an attacker if they are translated verbatim.  Any request or response
   that contains a character not permitted in a header field value MUST
   be treated as malformed (Section 8.1.2.6).  Valid characters are
   defined by the "field-content" ABNF rule in Section 3.2 of [RFC7230].

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.  The use of header compression
   and flow control depend on a commitment of resources for storing a
   greater amount of state.  Settings for these features ensure that
   memory commitments for these features are strictly bounded.





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   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.

   Processing capacity cannot be guarded as effectively as state
   capacity.

   The SETTINGS frame can be abused to cause a peer to expend additional
   processing time.  This might be done by pointlessly changing SETTINGS
   parameters, setting multiple undefined parameters, or changing the
   same setting multiple times in the same frame.  WINDOW_UPDATE or
   PRIORITY frames can be abused to cause an unnecessary waste of
   resources.

   Large numbers of small or empty frames can be abused to cause a peer
   to expend time processing frame headers.  Note, however, that some
   uses are entirely legitimate, such as the sending of an empty DATA or
   CONTINUATION frame at the end of a stream.

   Header compression also offers some opportunities to waste processing
   resources; see Section 7 of [COMPRESSION] for more details on
   potential abuses.

   Limits in SETTINGS parameters 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.

   All these features -- i.e., SETTINGS changes, small frames, header
   compression -- have legitimate uses.  These features become a burden
   only when they are used unnecessarily or to excess.

   An endpoint that doesn't monitor this behavior exposes itself to a
   risk of denial-of-service attack.  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 type ENHANCE_YOUR_CALM.

10.5.1.  Limits on Header Block Size

   A large header block (Section 4.3) can cause an implementation to
   commit a large amount of state.  Header fields that are critical for
   routing can appear toward the end of a header block, which prevents
   streaming of header fields to their ultimate destination.  This
   ordering and other reasons, such as ensuring cache correctness, mean



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   that an endpoint might need to buffer the entire header block.  Since
   there is no hard limit to the size of a header block, some endpoints
   could be forced to commit a large amount of available memory for
   header fields.

   An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to advise peers
   of limits that might apply on the size of header blocks.  This
   setting is only advisory, so endpoints MAY choose to send header
   blocks that exceed this limit and risk having 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 obligated to do so.

   A server that receives a larger header 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 header 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 an
   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 header fields (Section 4.3); the
   following concerns also apply to the use of HTTP compressed content-
   codings ([RFC7231], Section 3.1.2.1).

   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.






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   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 might be provided by TLS [TLS12].  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 payloads to a fixed size exposes
   information as 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 value 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.



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   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 1.8 of [HTML5].

   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.

11.  IANA Considerations

   A string for identifying HTTP/2 is entered into the "Application-
   Layer Protocol Negotiation (ALPN) Protocol IDs" registry established
   in [TLS-ALPN].

   This document establishes a registry for frame types, settings, and
   error codes.  These new registries appear in the new "Hypertext
   Transfer Protocol version 2 (HTTP/2) Parameters" section.

   This document registers the HTTP2-Settings header field for use in
   HTTP; it also registers the 421 (Misdirected Request) status code.

   This document registers the "PRI" method for use in HTTP to avoid
   collisions with the connection preface (Section 3.5).

11.1.  Registration of HTTP/2 Identification Strings

   This document creates two registrations for the identification of
   HTTP/2 (see Section 3.3) in the "Application-Layer Protocol
   Negotiation (ALPN) Protocol IDs" registry established in [TLS-ALPN].

   The "h2" string identifies HTTP/2 when used over TLS:

   Protocol:  HTTP/2 over TLS

   Identification Sequence:  0x68 0x32 ("h2")

   Specification:  This document

   The "h2c" string identifies HTTP/2 when used over cleartext TCP:

   Protocol:  HTTP/2 over TCP






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   Identification Sequence:  0x68 0x32 0x63 ("h2c")

   Specification:  This document

11.2.  Frame Type Registry

   This document establishes a registry for HTTP/2 frame type codes.
   The "HTTP/2 Frame Type" registry manages an 8-bit space.  The "HTTP/2
   Frame Type" registry operates under either of the "IETF Review" or
   "IESG Approval" policies [RFC5226] for values between 0x00 and 0xef,
   with values between 0xf0 and 0xff being reserved for Experimental
   Use.

   New entries in this registry require the following information:

   Frame Type:  A name or label for the frame type.

   Code:  The 8-bit code assigned to the frame type.

   Specification:  A reference to a specification that includes a
      description of the frame layout, its semantics, and flags that the
      frame type uses, including any parts of the frame that are
      conditionally present based on the value of flags.

   The entries in the following table are registered by this document.

   +---------------+------+--------------+
   | Frame Type    | Code | Section      |
   +---------------+------+--------------+
   | DATA          | 0x0  | Section 6.1  |
   | HEADERS       | 0x1  | Section 6.2  |
   | PRIORITY      | 0x2  | Section 6.3  |
   | RST_STREAM    | 0x3  | Section 6.4  |
   | SETTINGS      | 0x4  | Section 6.5  |
   | PUSH_PROMISE  | 0x5  | Section 6.6  |
   | PING          | 0x6  | Section 6.7  |
   | GOAWAY        | 0x7  | Section 6.8  |
   | WINDOW_UPDATE | 0x8  | Section 6.9  |
   | CONTINUATION  | 0x9  | Section 6.10 |
   +---------------+------+--------------+

11.3.  Settings Registry

   This document establishes a registry for HTTP/2 settings.  The
   "HTTP/2 Settings" registry manages a 16-bit space.  The "HTTP/2
   Settings" registry operates under the "Expert Review" policy
   [RFC5226] for values in the range from 0x0000 to 0xefff, with values
   between and 0xf000 and 0xffff being reserved for Experimental Use.



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   New registrations are advised to provide the following information:

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

   Code:  The 16-bit code assigned to the setting.

   Initial Value:  An initial value for the setting.

   Specification:  An optional reference to a specification that
      describes the use of the setting.

   The entries in the following table are registered by this document.

   +------------------------+------+---------------+---------------+
   | Name                   | Code | Initial Value | Specification |
   +------------------------+------+---------------+---------------+
   | HEADER_TABLE_SIZE      | 0x1  | 4096          | Section 6.5.2 |
   | ENABLE_PUSH            | 0x2  | 1             | Section 6.5.2 |
   | MAX_CONCURRENT_STREAMS | 0x3  | (infinite)    | Section 6.5.2 |
   | INITIAL_WINDOW_SIZE    | 0x4  | 65535         | Section 6.5.2 |
   | MAX_FRAME_SIZE         | 0x5  | 16384         | Section 6.5.2 |
   | MAX_HEADER_LIST_SIZE   | 0x6  | (infinite)    | Section 6.5.2 |
   +------------------------+------+---------------+---------------+

11.4.  Error Code Registry

   This document establishes a registry for HTTP/2 error codes.  The
   "HTTP/2 Error Code" registry manages a 32-bit space.  The "HTTP/2
   Error Code" registry operates under the "Expert Review" policy
   [RFC5226].

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

   New registrations are advised to provide the following information:

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

   Code:  The 32-bit error code value.

   Description:  A brief description of the error code semantics, longer
      if no detailed specification is provided.





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

   The entries in the following table are registered by this document.

   +---------------------+------+----------------------+---------------+
   | Name                | Code | Description          | Specification |
   +---------------------+------+----------------------+---------------+
   | NO_ERROR            | 0x0  | Graceful shutdown    | Section 7     |
   | PROTOCOL_ERROR      | 0x1  | Protocol error       | Section 7     |
   |                     |      | detected             |               |
   | INTERNAL_ERROR      | 0x2  | Implementation fault | Section 7     |
   | FLOW_CONTROL_ERROR  | 0x3  | Flow-control limits  | Section 7     |
   |                     |      | exceeded             |               |
   | SETTINGS_TIMEOUT    | 0x4  | Settings not         | Section 7     |
   |                     |      | acknowledged         |               |
   | STREAM_CLOSED       | 0x5  | Frame received for   | Section 7     |
   |                     |      | closed stream        |               |
   | FRAME_SIZE_ERROR    | 0x6  | Frame size incorrect | Section 7     |
   | REFUSED_STREAM      | 0x7  | Stream not processed | Section 7     |
   | CANCEL              | 0x8  | Stream cancelled     | Section 7     |
   | COMPRESSION_ERROR   | 0x9  | Compression state    | Section 7     |
   |                     |      | not updated          |               |
   | CONNECT_ERROR       | 0xa  | TCP connection error | Section 7     |
   |                     |      | for CONNECT method   |               |
   | ENHANCE_YOUR_CALM   | 0xb  | Processing capacity  | Section 7     |
   |                     |      | exceeded             |               |
   | INADEQUATE_SECURITY | 0xc  | Negotiated TLS       | Section 7     |
   |                     |      | parameters not       |               |
   |                     |      | acceptable           |               |
   | HTTP_1_1_REQUIRED   | 0xd  | Use HTTP/1.1 for the | Section 7     |
   |                     |      | request              |               |
   +---------------------+------+----------------------+---------------+

11.5.  HTTP2-Settings Header Field Registration

   This section registers the HTTP2-Settings header field in the
   "Permanent Message Header Field Names" registry [BCP90].

   Header field name:  HTTP2-Settings

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF





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   Specification document(s):  Section 3.2.1 of this document

   Related information:  This header field is only used by an HTTP/2
      client for Upgrade-based negotiation.

11.6.  PRI Method Registration

   This section registers the "PRI" method in the "HTTP Method Registry"
   ([RFC7231], Section 8.1).

   Method Name:  PRI

   Safe:  Yes

   Idempotent:  Yes

   Specification document(s):  Section 3.5 of this document

   Related information:  This method is never used by an actual client.
      This method will appear to be used when an HTTP/1.1 server or
      intermediary attempts to parse an HTTP/2 connection preface.

11.7.  The 421 (Misdirected Request) HTTP Status Code

   This document registers the 421 (Misdirected Request) HTTP status
   code in the "HTTP Status Codes" registry ([RFC7231], Section 8.2).

   Status Code:  421

   Short Description:  Misdirected Request

   Specification:  Section 9.1.2 of this document

11.8.  The h2c Upgrade Token

   This document registers the "h2c" upgrade token in the "HTTP Upgrade
   Tokens" registry ([RFC7230], Section 8.6).

   Value:  h2c

   Description:  Hypertext Transfer Protocol version 2 (HTTP/2)

   Expected Version Tokens:  None

   Reference:  Section 3.2 of this document






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12.  References

12.1.  Normative References

   [COMPRESSION] Peon, R. and H. Ruellan, "HPACK: Header Compression for
                 HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
                 <http://www.rfc-editor.org/info/rfc7541>.

   [COOKIE]      Barth, A., "HTTP State Management Mechanism", RFC 6265,
                 DOI 10.17487/RFC6265, April 2011,
                 <http://www.rfc-editor.org/info/rfc6265>.

   [FIPS186]     NIST, "Digital Signature Standard (DSS)", FIPS PUB
                 186-4, July 2013,
                 <http://dx.doi.org/10.6028/NIST.FIPS.186-4>.

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
                 RFC2119, March 1997,
                 <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]     Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/
                 RFC2818, May 2000,
                 <http://www.rfc-editor.org/info/rfc2818>.

   [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,
                 <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]     Josefsson, S., "The Base16, Base32, and Base64 Data
                 Encodings", RFC 4648, DOI 10.17487/RFC4648, October
                 2006, <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5226]     Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section in RFCs", BCP 26,
                 RFC 5226, DOI 10.17487/RFC5226, May 2008,
                 <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5234]     Crocker, D., Ed. and P. Overell, "Augmented BNF for
                 Syntax Specifications: ABNF", STD 68, RFC 5234,
                 DOI 10.17487/ RFC5234, January 2008,
                 <http://www.rfc-editor.org/info/rfc5234>.

   [RFC7230]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Message Syntax and
                 Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014,
                 <http://www.rfc-editor.org/info/rfc7230>.



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   [RFC7231]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Semantics and Content",
                 RFC 7231, DOI 10.17487/RFC7231, June 2014,
                 <http://www.rfc-editor.org/info/rfc7231>.

   [RFC7232]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Conditional Requests",
                 RFC 7232, DOI 10.17487/RFC7232, June 2014,
                 <http://www.rfc-editor.org/info/rfc7232>.

   [RFC7233]     Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
                 "Hypertext Transfer Protocol (HTTP/1.1): Range
                 Requests", RFC 7233, DOI 10.17487/RFC7233, June 2014,
                 <http://www.rfc-editor.org/info/rfc7233>.

   [RFC7234]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
                 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                 RFC 7234, DOI 10.17487/RFC7234, June 2014,
                 <http://www.rfc-editor.org/info/rfc7234>.

   [RFC7235]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Authentication",
                 RFC 7235, DOI 10.17487/RFC7235, June 2014,
                 <http://www.rfc-editor.org/info/rfc7235>.

   [TCP]         Postel, J., "Transmission Control Protocol", STD 7, RFC
                 793, DOI 10.17487/RFC0793, September 1981,
                 <http://www.rfc-editor.org/info/rfc793>.

   [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,
                 <http://www.rfc-editor.org/info/rfc7301>.

   [TLS-ECDHE]   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,
                 <http://www.rfc-editor.org/info/rfc5289>.

   [TLS-EXT]     Eastlake 3rd, D., "Transport Layer Security (TLS)
                 Extensions: Extension Definitions", RFC 6066,
                 DOI 10.17487/RFC6066, January 2011,
                 <http://www.rfc-editor.org/info/rfc6066>.







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RFC 7540                         HTTP/2                         May 2015


   [TLS12]       Dierks, T. and E. Rescorla, "The Transport Layer
                 Security (TLS) Protocol Version 1.2", RFC 5246,
                 DOI 10.17487/ RFC5246, August 2008,
                 <http://www.rfc-editor.org/info/rfc5246>.

12.2.  Informative References

   [ALT-SVC]     Nottingham, M., McManus, P., and J. Reschke, "HTTP
                 Alternative Services", Work in Progress, draft-ietf-
                 httpbis-alt-svc-06, February 2015.

   [BCP90]       Klyne, G., Nottingham, M., and J. Mogul, "Registration
                 Procedures for Message Header Fields", BCP 90,
                 RFC 3864, September 2004,
                 <http://www.rfc-editor.org/info/bcp90>.

   [BREACH]      Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving
                 the CRIME Attack", July 2013,
                 <http://breachattack.com/resources/
                 BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.

   [HTML5]       Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
                 Doyle Navara, E., O'Connor, E., and S. Pfeiffer,
                 "HTML5", W3C Recommendation REC-html5-20141028, October
                 2014, <http://www.w3.org/TR/2014/REC-html5-20141028/>.

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

   [RFC4492]     Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and
                 B.  Moeller, "Elliptic Curve Cryptography (ECC) Cipher
                 Suites for Transport Layer Security (TLS)", RFC 4492,
                 DOI 10.17487/RFC4492, May 2006,
                 <http://www.rfc-editor.org/info/rfc4492>.

   [RFC6585]     Nottingham, M. and R. Fielding, "Additional HTTP Status
                 Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
                 <http://www.rfc-editor.org/info/rfc6585>.

   [RFC7323]     Borman, D., Braden, B., Jacobson, V., and R.
                 Scheffenegger, Ed., "TCP Extensions for High
                 Performance", RFC 7323, DOI 10.17487/RFC7323, September
                 2014, <http://www.rfc-editor.org/info/rfc7323>.

   [TALKING]     Huang, L., Chen, E., Barth, A., Rescorla, E., and C.
                 Jackson, "Talking to Yourself for Fun and Profit",
                 2011, <http://w2spconf.com/2011/papers/websocket.pdf>.



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   [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, <http://www.rfc-editor.org/info/rfc7525>.














































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Appendix A.  TLS 1.2 Cipher Suite Black List

   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:

   o  TLS_NULL_WITH_NULL_NULL

   o  TLS_RSA_WITH_NULL_MD5

   o  TLS_RSA_WITH_NULL_SHA

   o  TLS_RSA_EXPORT_WITH_RC4_40_MD5

   o  TLS_RSA_WITH_RC4_128_MD5

   o  TLS_RSA_WITH_RC4_128_SHA

   o  TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5

   o  TLS_RSA_WITH_IDEA_CBC_SHA

   o  TLS_RSA_EXPORT_WITH_DES40_CBC_SHA

   o  TLS_RSA_WITH_DES_CBC_SHA

   o  TLS_RSA_WITH_3DES_EDE_CBC_SHA

   o  TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA

   o  TLS_DH_DSS_WITH_DES_CBC_SHA

   o  TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA

   o  TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA

   o  TLS_DH_RSA_WITH_DES_CBC_SHA

   o  TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA

   o  TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA

   o  TLS_DHE_DSS_WITH_DES_CBC_SHA

   o  TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA

   o  TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA




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   o  TLS_DHE_RSA_WITH_DES_CBC_SHA

   o  TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA

   o  TLS_DH_anon_EXPORT_WITH_RC4_40_MD5

   o  TLS_DH_anon_WITH_RC4_128_MD5

   o  TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA

   o  TLS_DH_anon_WITH_DES_CBC_SHA

   o  TLS_DH_anon_WITH_3DES_EDE_CBC_SHA

   o  TLS_KRB5_WITH_DES_CBC_SHA

   o  TLS_KRB5_WITH_3DES_EDE_CBC_SHA

   o  TLS_KRB5_WITH_RC4_128_SHA

   o  TLS_KRB5_WITH_IDEA_CBC_SHA

   o  TLS_KRB5_WITH_DES_CBC_MD5

   o  TLS_KRB5_WITH_3DES_EDE_CBC_MD5

   o  TLS_KRB5_WITH_RC4_128_MD5

   o  TLS_KRB5_WITH_IDEA_CBC_MD5

   o  TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA

   o  TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA

   o  TLS_KRB5_EXPORT_WITH_RC4_40_SHA

   o  TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5

   o  TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5

   o  TLS_KRB5_EXPORT_WITH_RC4_40_MD5

   o  TLS_PSK_WITH_NULL_SHA

   o  TLS_DHE_PSK_WITH_NULL_SHA

   o  TLS_RSA_PSK_WITH_NULL_SHA




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   o  TLS_RSA_WITH_AES_128_CBC_SHA

   o  TLS_DH_DSS_WITH_AES_128_CBC_SHA

   o  TLS_DH_RSA_WITH_AES_128_CBC_SHA

   o  TLS_DHE_DSS_WITH_AES_128_CBC_SHA

   o  TLS_DHE_RSA_WITH_AES_128_CBC_SHA

   o  TLS_DH_anon_WITH_AES_128_CBC_SHA

   o  TLS_RSA_WITH_AES_256_CBC_SHA

   o  TLS_DH_DSS_WITH_AES_256_CBC_SHA

   o  TLS_DH_RSA_WITH_AES_256_CBC_SHA

   o  TLS_DHE_DSS_WITH_AES_256_CBC_SHA

   o  TLS_DHE_RSA_WITH_AES_256_CBC_SHA

   o  TLS_DH_anon_WITH_AES_256_CBC_SHA

   o  TLS_RSA_WITH_NULL_SHA256

   o  TLS_RSA_WITH_AES_128_CBC_SHA256

   o  TLS_RSA_WITH_AES_256_CBC_SHA256

   o  TLS_DH_DSS_WITH_AES_128_CBC_SHA256

   o  TLS_DH_RSA_WITH_AES_128_CBC_SHA256

   o  TLS_DHE_DSS_WITH_AES_128_CBC_SHA256

   o  TLS_RSA_WITH_CAMELLIA_128_CBC_SHA

   o  TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA

   o  TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA

   o  TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA

   o  TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA

   o  TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA




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   o  TLS_DHE_RSA_WITH_AES_128_CBC_SHA256

   o  TLS_DH_DSS_WITH_AES_256_CBC_SHA256

   o  TLS_DH_RSA_WITH_AES_256_CBC_SHA256

   o  TLS_DHE_DSS_WITH_AES_256_CBC_SHA256

   o  TLS_DHE_RSA_WITH_AES_256_CBC_SHA256

   o  TLS_DH_anon_WITH_AES_128_CBC_SHA256

   o  TLS_DH_anon_WITH_AES_256_CBC_SHA256

   o  TLS_RSA_WITH_CAMELLIA_256_CBC_SHA

   o  TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA

   o  TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA

   o  TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA

   o  TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA

   o  TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA

   o  TLS_PSK_WITH_RC4_128_SHA

   o  TLS_PSK_WITH_3DES_EDE_CBC_SHA

   o  TLS_PSK_WITH_AES_128_CBC_SHA

   o  TLS_PSK_WITH_AES_256_CBC_SHA

   o  TLS_DHE_PSK_WITH_RC4_128_SHA

   o  TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA

   o  TLS_DHE_PSK_WITH_AES_128_CBC_SHA

   o  TLS_DHE_PSK_WITH_AES_256_CBC_SHA

   o  TLS_RSA_PSK_WITH_RC4_128_SHA

   o  TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA

   o  TLS_RSA_PSK_WITH_AES_128_CBC_SHA




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   o  TLS_RSA_PSK_WITH_AES_256_CBC_SHA

   o  TLS_RSA_WITH_SEED_CBC_SHA

   o  TLS_DH_DSS_WITH_SEED_CBC_SHA

   o  TLS_DH_RSA_WITH_SEED_CBC_SHA

   o  TLS_DHE_DSS_WITH_SEED_CBC_SHA

   o  TLS_DHE_RSA_WITH_SEED_CBC_SHA

   o  TLS_DH_anon_WITH_SEED_CBC_SHA

   o  TLS_RSA_WITH_AES_128_GCM_SHA256

   o  TLS_RSA_WITH_AES_256_GCM_SHA384

   o  TLS_DH_RSA_WITH_AES_128_GCM_SHA256

   o  TLS_DH_RSA_WITH_AES_256_GCM_SHA384

   o  TLS_DH_DSS_WITH_AES_128_GCM_SHA256

   o  TLS_DH_DSS_WITH_AES_256_GCM_SHA384

   o  TLS_DH_anon_WITH_AES_128_GCM_SHA256

   o  TLS_DH_anon_WITH_AES_256_GCM_SHA384

   o  TLS_PSK_WITH_AES_128_GCM_SHA256

   o  TLS_PSK_WITH_AES_256_GCM_SHA384

   o  TLS_RSA_PSK_WITH_AES_128_GCM_SHA256

   o  TLS_RSA_PSK_WITH_AES_256_GCM_SHA384

   o  TLS_PSK_WITH_AES_128_CBC_SHA256

   o  TLS_PSK_WITH_AES_256_CBC_SHA384

   o  TLS_PSK_WITH_NULL_SHA256

   o  TLS_PSK_WITH_NULL_SHA384

   o  TLS_DHE_PSK_WITH_AES_128_CBC_SHA256




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RFC 7540                         HTTP/2                         May 2015


   o  TLS_DHE_PSK_WITH_AES_256_CBC_SHA384

   o  TLS_DHE_PSK_WITH_NULL_SHA256

   o  TLS_DHE_PSK_WITH_NULL_SHA384

   o  TLS_RSA_PSK_WITH_AES_128_CBC_SHA256

   o  TLS_RSA_PSK_WITH_AES_256_CBC_SHA384

   o  TLS_RSA_PSK_WITH_NULL_SHA256

   o  TLS_RSA_PSK_WITH_NULL_SHA384

   o  TLS_RSA_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_RSA_WITH_CAMELLIA_256_CBC_SHA256

   o  TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA256

   o  TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA256

   o  TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA256

   o  TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA256

   o  TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA256

   o  TLS_EMPTY_RENEGOTIATION_INFO_SCSV

   o  TLS_ECDH_ECDSA_WITH_NULL_SHA

   o  TLS_ECDH_ECDSA_WITH_RC4_128_SHA

   o  TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA

   o  TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA




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RFC 7540                         HTTP/2                         May 2015


   o  TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA

   o  TLS_ECDHE_ECDSA_WITH_NULL_SHA

   o  TLS_ECDHE_ECDSA_WITH_RC4_128_SHA

   o  TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA

   o  TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA

   o  TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA

   o  TLS_ECDH_RSA_WITH_NULL_SHA

   o  TLS_ECDH_RSA_WITH_RC4_128_SHA

   o  TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA

   o  TLS_ECDH_RSA_WITH_AES_128_CBC_SHA

   o  TLS_ECDH_RSA_WITH_AES_256_CBC_SHA

   o  TLS_ECDHE_RSA_WITH_NULL_SHA

   o  TLS_ECDHE_RSA_WITH_RC4_128_SHA

   o  TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA

   o  TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA

   o  TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA

   o  TLS_ECDH_anon_WITH_NULL_SHA

   o  TLS_ECDH_anon_WITH_RC4_128_SHA

   o  TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA

   o  TLS_ECDH_anon_WITH_AES_128_CBC_SHA

   o  TLS_ECDH_anon_WITH_AES_256_CBC_SHA

   o  TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA

   o  TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA

   o  TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA




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   o  TLS_SRP_SHA_WITH_AES_128_CBC_SHA

   o  TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA

   o  TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA

   o  TLS_SRP_SHA_WITH_AES_256_CBC_SHA

   o  TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA

   o  TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA

   o  TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256

   o  TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384

   o  TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256

   o  TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384

   o  TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256

   o  TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384

   o  TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256

   o  TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384

   o  TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256

   o  TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384

   o  TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256

   o  TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384

   o  TLS_ECDHE_PSK_WITH_RC4_128_SHA

   o  TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA

   o  TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA

   o  TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA

   o  TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256

   o  TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384




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   o  TLS_ECDHE_PSK_WITH_NULL_SHA

   o  TLS_ECDHE_PSK_WITH_NULL_SHA256

   o  TLS_ECDHE_PSK_WITH_NULL_SHA384

   o  TLS_RSA_WITH_ARIA_128_CBC_SHA256

   o  TLS_RSA_WITH_ARIA_256_CBC_SHA384

   o  TLS_DH_DSS_WITH_ARIA_128_CBC_SHA256

   o  TLS_DH_DSS_WITH_ARIA_256_CBC_SHA384

   o  TLS_DH_RSA_WITH_ARIA_128_CBC_SHA256

   o  TLS_DH_RSA_WITH_ARIA_256_CBC_SHA384

   o  TLS_DHE_DSS_WITH_ARIA_128_CBC_SHA256

   o  TLS_DHE_DSS_WITH_ARIA_256_CBC_SHA384

   o  TLS_DHE_RSA_WITH_ARIA_128_CBC_SHA256

   o  TLS_DHE_RSA_WITH_ARIA_256_CBC_SHA384

   o  TLS_DH_anon_WITH_ARIA_128_CBC_SHA256

   o  TLS_DH_anon_WITH_ARIA_256_CBC_SHA384

   o  TLS_ECDHE_ECDSA_WITH_ARIA_128_CBC_SHA256

   o  TLS_ECDHE_ECDSA_WITH_ARIA_256_CBC_SHA384

   o  TLS_ECDH_ECDSA_WITH_ARIA_128_CBC_SHA256

   o  TLS_ECDH_ECDSA_WITH_ARIA_256_CBC_SHA384

   o  TLS_ECDHE_RSA_WITH_ARIA_128_CBC_SHA256

   o  TLS_ECDHE_RSA_WITH_ARIA_256_CBC_SHA384

   o  TLS_ECDH_RSA_WITH_ARIA_128_CBC_SHA256

   o  TLS_ECDH_RSA_WITH_ARIA_256_CBC_SHA384

   o  TLS_RSA_WITH_ARIA_128_GCM_SHA256




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   o  TLS_RSA_WITH_ARIA_256_GCM_SHA384

   o  TLS_DH_RSA_WITH_ARIA_128_GCM_SHA256

   o  TLS_DH_RSA_WITH_ARIA_256_GCM_SHA384

   o  TLS_DH_DSS_WITH_ARIA_128_GCM_SHA256

   o  TLS_DH_DSS_WITH_ARIA_256_GCM_SHA384

   o  TLS_DH_anon_WITH_ARIA_128_GCM_SHA256

   o  TLS_DH_anon_WITH_ARIA_256_GCM_SHA384

   o  TLS_ECDH_ECDSA_WITH_ARIA_128_GCM_SHA256

   o  TLS_ECDH_ECDSA_WITH_ARIA_256_GCM_SHA384

   o  TLS_ECDH_RSA_WITH_ARIA_128_GCM_SHA256

   o  TLS_ECDH_RSA_WITH_ARIA_256_GCM_SHA384

   o  TLS_PSK_WITH_ARIA_128_CBC_SHA256

   o  TLS_PSK_WITH_ARIA_256_CBC_SHA384

   o  TLS_DHE_PSK_WITH_ARIA_128_CBC_SHA256

   o  TLS_DHE_PSK_WITH_ARIA_256_CBC_SHA384

   o  TLS_RSA_PSK_WITH_ARIA_128_CBC_SHA256

   o  TLS_RSA_PSK_WITH_ARIA_256_CBC_SHA384

   o  TLS_PSK_WITH_ARIA_128_GCM_SHA256

   o  TLS_PSK_WITH_ARIA_256_GCM_SHA384

   o  TLS_RSA_PSK_WITH_ARIA_128_GCM_SHA256

   o  TLS_RSA_PSK_WITH_ARIA_256_GCM_SHA384

   o  TLS_ECDHE_PSK_WITH_ARIA_128_CBC_SHA256

   o  TLS_ECDHE_PSK_WITH_ARIA_256_CBC_SHA384

   o  TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_CBC_SHA256




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   o  TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_CBC_SHA384

   o  TLS_ECDH_ECDSA_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_ECDH_ECDSA_WITH_CAMELLIA_256_CBC_SHA384

   o  TLS_ECDHE_RSA_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_ECDHE_RSA_WITH_CAMELLIA_256_CBC_SHA384

   o  TLS_ECDH_RSA_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_ECDH_RSA_WITH_CAMELLIA_256_CBC_SHA384

   o  TLS_RSA_WITH_CAMELLIA_128_GCM_SHA256

   o  TLS_RSA_WITH_CAMELLIA_256_GCM_SHA384

   o  TLS_DH_RSA_WITH_CAMELLIA_128_GCM_SHA256

   o  TLS_DH_RSA_WITH_CAMELLIA_256_GCM_SHA384

   o  TLS_DH_DSS_WITH_CAMELLIA_128_GCM_SHA256

   o  TLS_DH_DSS_WITH_CAMELLIA_256_GCM_SHA384

   o  TLS_DH_anon_WITH_CAMELLIA_128_GCM_SHA256

   o  TLS_DH_anon_WITH_CAMELLIA_256_GCM_SHA384

   o  TLS_ECDH_ECDSA_WITH_CAMELLIA_128_GCM_SHA256

   o  TLS_ECDH_ECDSA_WITH_CAMELLIA_256_GCM_SHA384

   o  TLS_ECDH_RSA_WITH_CAMELLIA_128_GCM_SHA256

   o  TLS_ECDH_RSA_WITH_CAMELLIA_256_GCM_SHA384

   o  TLS_PSK_WITH_CAMELLIA_128_GCM_SHA256

   o  TLS_PSK_WITH_CAMELLIA_256_GCM_SHA384

   o  TLS_RSA_PSK_WITH_CAMELLIA_128_GCM_SHA256

   o  TLS_RSA_PSK_WITH_CAMELLIA_256_GCM_SHA384

   o  TLS_PSK_WITH_CAMELLIA_128_CBC_SHA256




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   o  TLS_PSK_WITH_CAMELLIA_256_CBC_SHA384

   o  TLS_DHE_PSK_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_DHE_PSK_WITH_CAMELLIA_256_CBC_SHA384

   o  TLS_RSA_PSK_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_RSA_PSK_WITH_CAMELLIA_256_CBC_SHA384

   o  TLS_ECDHE_PSK_WITH_CAMELLIA_128_CBC_SHA256

   o  TLS_ECDHE_PSK_WITH_CAMELLIA_256_CBC_SHA384

   o  TLS_RSA_WITH_AES_128_CCM

   o  TLS_RSA_WITH_AES_256_CCM

   o  TLS_RSA_WITH_AES_128_CCM_8

   o  TLS_RSA_WITH_AES_256_CCM_8

   o  TLS_PSK_WITH_AES_128_CCM

   o  TLS_PSK_WITH_AES_256_CCM

   o  TLS_PSK_WITH_AES_128_CCM_8

   o  TLS_PSK_WITH_AES_256_CCM_8

      Note: This list was assembled from the set of registered TLS
      cipher suites at the time of writing.  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.














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RFC 7540                         HTTP/2                         May 2015


Acknowledgements

   This document includes substantial input from the following
   individuals:

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

   o  Gabriel Montenegro and Willy Tarreau (Upgrade mechanism).

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

   o  Mike Bishop (Extensibility).

   o  Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike
      Bishop, and Herve Ruellan (Substantial editorial contributions).

   o  Kari Hurtta, Tatsuhiro Tsujikawa, Greg Wilkins, Poul-Henning Kamp,
      and Jonathan Thackray.

   o  Alexey Melnikov, who was an editor of this document in 2013.

   A substantial proportion of Martin's contribution was supported by
   Microsoft during his employment there.

   The Japanese HTTP/2 community provided invaluable contributions,
   including a number of implementations as well as numerous technical
   and editorial contributions.




















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Authors' Addresses

   Mike Belshe
   BitGo

   EMail: mike@belshe.com


   Roberto Peon
   Google, Inc

   EMail: fenix@google.com


   Martin Thomson (editor)
   Mozilla
   331 E Evelyn Street
   Mountain View, CA  94041
   United States

   EMail: martin.thomson@gmail.com






























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285
286
287	2.1. Document Organization
288
289	The HTTP/2 specification is split into four parts:
290
291	o Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is
292	initiated.
293
294	o The frame (Section 4) and stream (Section 5) layers describe the
295	way HTTP/2 frames are structured and formed into multiplexed
296	streams.
297
298	o Frame (Section 6) and error (Section 7) definitions include
299	details of the frame and error types used in HTTP/2.
300
301	o HTTP mappings (Section 8) and additional requirements (Section 9)
302	describe how HTTP semantics are expressed using frames and
303	streams.
304
305	While some of the frame and stream layer concepts are isolated from
306	HTTP, this specification does not define a completely generic frame
307	layer. The frame and stream layers are tailored to the needs of the
308	HTTP protocol and server push.
309
310	2.2. Conventions and Terminology
311
312	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
313	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
314	document are to be interpreted as described in RFC 2119 [RFC2119].
315
316	All numeric values are in network byte order. Values are unsigned
317	unless otherwise indicated. Literal values are provided in decimal
318	or hexadecimal as appropriate. Hexadecimal literals are prefixed
319	with "0x" to distinguish them from decimal literals.
320
321	The following terms are used:
322
323	client: The endpoint that initiates an HTTP/2 connection. Clients
324	send HTTP requests and receive HTTP responses.
325
326	connection: A transport-layer connection between two endpoints.
327
328	connection error: An error that affects the entire HTTP/2
329	connection.
330
331	endpoint: Either the client or server of the connection.
332
333
334
335
336
337
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340	RFC 7540 HTTP/2 May 2015
341
342
343	frame: The smallest unit of communication within an HTTP/2
344	connection, consisting of a header and a variable-length sequence
345	of octets structured according to the frame type.
346
347	peer: An endpoint. When discussing a particular endpoint, "peer"
348	refers to the endpoint that is remote to the primary subject of
349	discussion.
350
351	receiver: An endpoint that is receiving frames.
352
353	sender: An endpoint that is transmitting frames.
354
355	server: The endpoint that accepts an HTTP/2 connection. Servers
356	receive HTTP requests and send HTTP responses.
357
358	stream: A bidirectional flow of frames within the HTTP/2 connection.
359
360	stream error: An error on the individual HTTP/2 stream.
361
362	Finally, the terms "gateway", "intermediary", "proxy", and "tunnel"
363	are defined in Section 2.3 of [RFC7230]. Intermediaries act as both
364	client and server at different times.
365
366	The term "payload body" is defined in Section 3.3 of [RFC7230].
367
368	3. Starting HTTP/2
369
370	An HTTP/2 connection is an application-layer protocol running on top
371	of a TCP connection ([TCP]). The client is the TCP connection
372	initiator.
373
374	HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1.
375	HTTP/2 shares the same default port numbers: 80 for "http" URIs and
376	443 for "https" URIs. As a result, implementations processing
377	requests for target resource URIs like "http://example.org/foo" or
378	"https://example.com/bar" are required to first discover whether the
379	upstream server (the immediate peer to which the client wishes to
380	establish a connection) supports HTTP/2.
381
382	The means by which support for HTTP/2 is determined is different for
383	"http" and "https" URIs. Discovery for "http" URIs is described in
384	Section 3.2. Discovery for "https" URIs is described in Section 3.3.
385
386
387
388
389
390
391
392
393
394	Belshe, et al. Standards Track [Page 7]

452	RFC 7540 HTTP/2 May 2015
453
454
455	A server that does not support HTTP/2 can respond to the request as
456	though the Upgrade header field were absent:
457
458	HTTP/1.1 200 OK
459	Content-Length: 243
460	Content-Type: text/html
461
462	...
463
464	A server MUST ignore an "h2" token in an Upgrade header field.
465	Presence of a token with "h2" implies HTTP/2 over TLS, which is
466	instead negotiated as described in Section 3.3.
467
468	A server that supports HTTP/2 accepts the upgrade with a 101
469	(Switching Protocols) response. After the empty line that terminates
470	the 101 response, the server can begin sending HTTP/2 frames. These
471	frames MUST include a response to the request that initiated the
472	upgrade.
473
474	For example:
475
476	HTTP/1.1 101 Switching Protocols
477	Connection: Upgrade
478	Upgrade: h2c
479
480	[ HTTP/2 connection ...
481
482	The first HTTP/2 frame sent by the server MUST be a server connection
483	preface (Section 3.5) consisting of a SETTINGS frame (Section 6.5).
484	Upon receiving the 101 response, the client MUST send a connection
485	preface (Section 3.5), which includes a SETTINGS frame.
486
487	The HTTP/1.1 request that is sent prior to upgrade is assigned a
488	stream identifier of 1 (see Section 5.1.1) with default priority
489	values (Section 5.3.5). Stream 1 is implicitly "half-closed" from	.../http2/cache_http2_session.c 297
490	the client toward the server (see Section 5.1), since the request is
491	completed as an HTTP/1.1 request. After commencing the HTTP/2
492	connection, stream 1 is used for the response.
493
494	3.2.1. HTTP2-Settings Header Field
495
496	A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly
497	one "HTTP2-Settings" header field. The HTTP2-Settings header field
498	is a connection-specific header field that includes parameters that
499	govern the HTTP/2 connection, provided in anticipation of the server
500	accepting the request to upgrade.
501
502	HTTP2-Settings = token68
503
504
505
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564	RFC 7540 HTTP/2 May 2015
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566
567	only affects the establishment of HTTP/2 connections over cleartext
568	TCP; implementations that support HTTP/2 over TLS MUST use protocol
569	negotiation in TLS [TLS-ALPN].
570
571	Likewise, the server MUST send a connection preface (Section 3.5).
572
573	Without additional information, prior support for HTTP/2 is not a
574	strong signal that a given server will support HTTP/2 for future
575	connections. For example, it is possible for server configurations
576	to change, for configurations to differ between instances in
577	clustered servers, or for network conditions to change.
578
579	3.5. HTTP/2 Connection Preface
580
581	In HTTP/2, each endpoint is required to send a connection preface as
582	a final confirmation of the protocol in use and to establish the
583	initial settings for the HTTP/2 connection. The client and server
584	each send a different connection preface.
585
586	The client connection preface starts with a sequence of 24 octets,
587	which in hex notation is:
588
589	0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
590
591	That is, the connection preface starts with the string "PRI *
592	HTTP/2.0\r\n\r\nSM\r\n\r\n"). This sequence MUST be followed by a
593	SETTINGS frame (Section 6.5), which MAY be empty. The client sends
594	the client connection preface immediately upon receipt of a 101
595	(Switching Protocols) response (indicating a successful upgrade) or
596	as the first application data octets of a TLS connection. If
597	starting an HTTP/2 connection with prior knowledge of server support
598	for the protocol, the client connection preface is sent upon
599	connection establishment.
600
601	Note: The client connection preface is selected so that a large
602	proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do
603	not attempt to process further frames. Note that this does not
604	address the concerns raised in [TALKING].
605
606	The server connection preface consists of a potentially empty
607	SETTINGS frame (Section 6.5) that MUST be the first frame the server
608	sends in the HTTP/2 connection.
609
610	The SETTINGS frames received from a peer as part of the connection
611	preface MUST be acknowledged (see Section 6.5.3) after sending the
612	connection preface.
613
614
615
616
617
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620	RFC 7540 HTTP/2 May 2015
621
622
623	To avoid unnecessary latency, clients are permitted to send
624	additional frames to the server immediately after sending the client
625	connection preface, without waiting to receive the server connection
626	preface. It is important to note, however, that the server
627	connection preface SETTINGS frame might include parameters that
628	necessarily alter how a client is expected to communicate with the
629	server. Upon receiving the SETTINGS frame, the client is expected to
630	honor any parameters established. In some configurations, it is
631	possible for the server to transmit SETTINGS before the client sends
632	additional frames, providing an opportunity to avoid this issue.
633
634	Clients and servers MUST treat an invalid connection preface as a
635	connection error (Section 5.4.1) of type PROTOCOL_ERROR. A GOAWAY
636	frame (Section 6.8) MAY be omitted in this case, since an invalid
637	preface indicates that the peer is not using HTTP/2.
638
639	4. HTTP Frames
640
641	Once the HTTP/2 connection is established, endpoints can begin
642	exchanging frames.
643
644	4.1. Frame Format
645
646	All frames begin with a fixed 9-octet header followed by a variable-
647	length payload.
648
649	+-----------------------------------------------+
650	\| Length (24) \|
651	+---------------+---------------+---------------+
652	\| Type (8) \| Flags (8) \|
653	+-+-------------+---------------+-------------------------------+
654	\|R\| Stream Identifier (31) \|
655	+=+=============================================================+
656	\| Frame Payload (0...) ...
657	+---------------------------------------------------------------+
658
659	Figure 1: Frame Layout
660
661	The fields of the frame header are defined as:
662
663	Length: The length of the frame payload expressed as an unsigned
664	24-bit integer. Values greater than 2^14 (16,384) MUST NOT be
665	sent unless the receiver has set a larger value for
666	SETTINGS_MAX_FRAME_SIZE.
667
668	The 9 octets of the frame header are not included in this value.
669
670
671
672
673
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1
2
3
4
5
6
7	Internet Engineering Task Force (IETF) M. Belshe
8	Request for Comments: 7540 BitGo
9	Category: Standards Track R. Peon
10	ISSN: 2070-1721 Google, Inc
11	M. Thomson, Ed.
12	Mozilla
13	May 2015
14
15
16	Hypertext Transfer Protocol Version 2 (HTTP/2)
17
18	Abstract
19
20	This specification describes an optimized expression of the semantics
21	of the Hypertext Transfer Protocol (HTTP), referred to as HTTP
22	version 2 (HTTP/2). HTTP/2 enables a more efficient use of network
23	resources and a reduced perception of latency by introducing header
24	field compression and allowing multiple concurrent exchanges on the
25	same connection. It also introduces unsolicited push of
26	representations from servers to clients.
27
28	This specification is an alternative to, but does not obsolete, the
29	HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged.
30
31	Status of This Memo
32
33	This is an Internet Standards Track document.
34
35	This document is a product of the Internet Engineering Task Force
36	(IETF). It represents the consensus of the IETF community. It has
37	received public review and has been approved for publication by the
38	Internet Engineering Steering Group (IESG). Further information on
39	Internet Standards is available in Section 2 of RFC 5741.
40
41	Information about the current status of this document, any errata,
42	and how to provide feedback on it may be obtained at
43	http://www.rfc-editor.org/info/rfc7540.
44
45
46
47
48
49
50
51
52
53
54
55
56
57
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172	RFC 7540 HTTP/2 May 2015
173
174
175	11. IANA Considerations ...........................................74
176	11.1. Registration of HTTP/2 Identification Strings ............74
177	11.2. Frame Type Registry ......................................75
178	11.3. Settings Registry ........................................75
179	11.4. Error Code Registry ......................................76
180	11.5. HTTP2-Settings Header Field Registration .................77
181	11.6. PRI Method Registration ..................................78
182	11.7. The 421 (Misdirected Request) HTTP Status Code ...........78
183	11.8. The h2c Upgrade Token ....................................78
184	12. References ....................................................79
185	12.1. Normative References .....................................79
186	12.2. Informative References ...................................81
187	Appendix A. TLS 1.2 Cipher Suite Black List .......................83
188	Acknowledgements ..................................................95
189	Authors' Addresses ................................................96
190
191	1. Introduction
192
193	The Hypertext Transfer Protocol (HTTP) is a wildly successful
194	protocol. However, the way HTTP/1.1 uses the underlying transport
195	([RFC7230], Section 6) has several characteristics that have a
196	negative overall effect on application performance today.
197
198	In particular, HTTP/1.0 allowed only one request to be outstanding at
199	a time on a given TCP connection. HTTP/1.1 added request pipelining,
200	but this only partially addressed request concurrency and still
201	suffers from head-of-line blocking. Therefore, HTTP/1.0 and HTTP/1.1
202	clients that need to make many requests use multiple connections to a
203	server in order to achieve concurrency and thereby reduce latency.
204
205	Furthermore, HTTP header fields are often repetitive and verbose,
206	causing unnecessary network traffic as well as causing the initial
207	TCP [TCP] congestion window to quickly fill. This can result in
208	excessive latency when multiple requests are made on a new TCP
209	connection.
210
211	HTTP/2 addresses these issues by defining an optimized mapping of
212	HTTP's semantics to an underlying connection. Specifically, it
213	allows interleaving of request and response messages on the same
214	connection and uses an efficient coding for HTTP header fields. It
215	also allows prioritization of requests, letting more important
216	requests complete more quickly, further improving performance.
217
218
219
220
221
222
223
224
225
226	Belshe, et al. Standards Track [Page 4]

228	RFC 7540 HTTP/2 May 2015
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230
231	The resulting protocol is more friendly to the network because fewer
232	TCP connections can be used in comparison to HTTP/1.x. This means
233	less competition with other flows and longer-lived connections, which
234	in turn lead to better utilization of available network capacity.
235
236	Finally, HTTP/2 also enables more efficient processing of messages
237	through use of binary message framing.
238
239	2. HTTP/2 Protocol Overview
240
241	HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2
242	supports all of the core features of HTTP/1.1 but aims to be more
243	efficient in several ways.
244
245	The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each
246	frame type serves a different purpose. For example, HEADERS and DATA
247	frames form the basis of HTTP requests and responses (Section 8.1);
248	other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are
249	used in support of other HTTP/2 features.
250
251	Multiplexing of requests is achieved by having each HTTP request/
252	response exchange associated with its own stream (Section 5).
253	Streams are largely independent of each other, so a blocked or
254	stalled request or response does not prevent progress on other
255	streams.
256
257	Flow control and prioritization ensure that it is possible to
258	efficiently use multiplexed streams. Flow control (Section 5.2)
259	helps to ensure that only data that can be used by a receiver is
260	transmitted. Prioritization (Section 5.3) ensures that limited
261	resources can be directed to the most important streams first.
262
263	HTTP/2 adds a new interaction mode whereby a server can push
264	responses to a client (Section 8.2). Server push allows a server to
265	speculatively send data to a client that the server anticipates the
266	client will need, trading off some network usage against a potential
267	latency gain. The server does this by synthesizing a request, which
268	it sends as a PUSH_PROMISE frame. The server is then able to send a
269	response to the synthetic request on a separate stream.
270
271	Because HTTP header fields used in a connection can contain large
272	amounts of redundant data, frames that contain them are compressed
273	(Section 4.3). This has especially advantageous impact upon request
274	sizes in the common case, allowing many requests to be compressed
275	into one packet.
276
277
278
279
280
281
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396	RFC 7540 HTTP/2 May 2015
397
398
399	3.1. HTTP/2 Version Identification
400
401	The protocol defined in this document has two identifiers.
402
403	o The string "h2" identifies the protocol where HTTP/2 uses
404	Transport Layer Security (TLS) [TLS12]. This identifier is used
405	in the TLS application-layer protocol negotiation (ALPN) extension
406	[TLS-ALPN] field and in any place where HTTP/2 over TLS is
407	identified.
408
409	The "h2" string is serialized into an ALPN protocol identifier as
410	the two-octet sequence: 0x68, 0x32.
411
412	o The string "h2c" identifies the protocol where HTTP/2 is run over
413	cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade
414	header field and in any place where HTTP/2 over TCP is identified.
415
416	The "h2c" string is reserved from the ALPN identifier space but
417	describes a protocol that does not use TLS.
418
419	Negotiating "h2" or "h2c" implies the use of the transport, security,
420	framing, and message semantics described in this document.
421
422	3.2. Starting HTTP/2 for "http" URIs
423
424	A client that makes a request for an "http" URI without prior
425	knowledge about support for HTTP/2 on the next hop uses the HTTP
426	Upgrade mechanism (Section 6.7 of [RFC7230]). The client does so by
427	making an HTTP/1.1 request that includes an Upgrade header field with
428	the "h2c" token. Such an HTTP/1.1 request MUST include exactly one
429	HTTP2-Settings (Section 3.2.1) header field.
430
431	For example:
432
433	GET / HTTP/1.1
434	Host: server.example.com
435	Connection: Upgrade, HTTP2-Settings
436	Upgrade: h2c
437	HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
438
439	Requests that contain a payload body MUST be sent in their entirety
440	before the client can send HTTP/2 frames. This means that a large
441	request can block the use of the connection until it is completely
442	sent.
443
444	If concurrency of an initial request with subsequent requests is
445	important, an OPTIONS request can be used to perform the upgrade to
446	HTTP/2, at the cost of an additional round trip.
447
448
449
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508	RFC 7540 HTTP/2 May 2015
509
510
511	A server MUST NOT upgrade the connection to HTTP/2 if this header
512	field is not present or if more than one is present. A server MUST
513	NOT send this header field.
514
515	The content of the HTTP2-Settings header field is the payload of a
516	SETTINGS frame (Section 6.5), encoded as a base64url string (that is,
517	the URL- and filename-safe Base64 encoding described in Section 5 of
518	[RFC4648], with any trailing '=' characters omitted). The ABNF
519	[RFC5234] production for "token68" is defined in Section 2.1 of
520	[RFC7235].
521
522	Since the upgrade is only intended to apply to the immediate
523	connection, a client sending the HTTP2-Settings header field MUST
524	also send "HTTP2-Settings" as a connection option in the Connection
525	header field to prevent it from being forwarded (see Section 6.1 of
526	[RFC7230]).
527
528	A server decodes and interprets these values as it would any other
529	SETTINGS frame. Explicit acknowledgement of these settings
530	(Section 6.5.3) is not necessary, since a 101 response serves as
531	implicit acknowledgement. Providing these values in the upgrade
532	request gives a client an opportunity to provide parameters prior to
533	receiving any frames from the server.
534
535	3.3. Starting HTTP/2 for "https" URIs
536
537	A client that makes a request to an "https" URI uses TLS [TLS12] with
538	the application-layer protocol negotiation (ALPN) extension
539	[TLS-ALPN].
540
541	HTTP/2 over TLS uses the "h2" protocol identifier. The "h2c"
542	protocol identifier MUST NOT be sent by a client or selected by a
543	server; the "h2c" protocol identifier describes a protocol that does
544	not use TLS.
545
546	Once TLS negotiation is complete, both the client and the server MUST
547	send a connection preface (Section 3.5).
548
549	3.4. Starting HTTP/2 with Prior Knowledge
550
551	A client can learn that a particular server supports HTTP/2 by other
552	means. For example, [ALT-SVC] describes a mechanism for advertising
553	this capability.
554
555	A client MUST send the connection preface (Section 3.5) and then MAY
556	immediately send HTTP/2 frames to such a server; servers can identify
557	these connections by the presence of the connection preface. This
558
559
560
561
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676	RFC 7540 HTTP/2 May 2015
677
678
679	Type: The 8-bit type of the frame. The frame type determines the	.../http2/cache_http2_proto.c 1491
680	format and semantics of the frame. Implementations MUST ignore
681	and discard any frame that has a type that is unknown.
682
683	Flags: An 8-bit field reserved for boolean flags specific to the
684	frame type.
685
686	Flags are assigned semantics specific to the indicated frame type.
687	Flags that have no defined semantics for a particular frame type	.../http2/cache_http2_proto.c 1510
688	MUST be ignored and MUST be left unset (0x0) when sending.
689
690	R: A reserved 1-bit field. The semantics of this bit are undefined,	.../http2/cache_http2_proto.c 1482
691	and the bit MUST remain unset (0x0) when sending and MUST be
692	ignored when receiving.
693
694	Stream Identifier: A stream identifier (see Section 5.1.1) expressed
695	as an unsigned 31-bit integer. The value 0x0 is reserved for
696	frames that are associated with the connection as a whole as
697	opposed to an individual stream.
698
699	The structure and content of the frame payload is dependent entirely
700	on the frame type.
701
702	4.2. Frame Size
703
704	The size of a frame payload is limited by the maximum size that a
705	receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This
706	setting can have any value between 2^14 (16,384) and 2^24-1
707	(16,777,215) octets, inclusive.
708
709	All implementations MUST be capable of receiving and minimally
710	processing frames up to 2^14 octets in length, plus the 9-octet frame
711	header (Section 4.1). The size of the frame header is not included
712	when describing frame sizes.
713
714	Note: Certain frame types, such as PING (Section 6.7), impose
715	additional limits on the amount of payload data allowed.
716
717	An endpoint MUST send an error code of FRAME_SIZE_ERROR if a frame
718	exceeds the size defined in SETTINGS_MAX_FRAME_SIZE, exceeds any
719	limit defined for the frame type, or is too small to contain
720	mandatory frame data. A frame size error in a frame that could alter
721	the state of the entire connection MUST be treated as a connection
722	error (Section 5.4.1); this includes any frame carrying a header
723	block (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and
724	CONTINUATION), SETTINGS, and any frame with a stream identifier of 0.
725
726
727
728
729
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732	RFC 7540 HTTP/2 May 2015
733
734
735	Endpoints are not obligated to use all available space in a frame.
736	Responsiveness can be improved by using frames that are smaller than
737	the permitted maximum size. Sending large frames can result in
738	delays in sending time-sensitive frames (such as RST_STREAM,
739	WINDOW_UPDATE, or PRIORITY), which, if blocked by the transmission of
740	a large frame, could affect performance.
741
742	4.3. Header Compression and Decompression
743
744	Just as in HTTP/1, a header field in HTTP/2 is a name with one or
745	more associated values. Header fields are used within HTTP request
746	and response messages as well as in server push operations (see
747	Section 8.2).
748
749	Header lists are collections of zero or more header fields. When
750	transmitted over a connection, a header list is serialized into a
751	header block using HTTP header compression [COMPRESSION]. The
752	serialized header block is then divided into one or more octet
753	sequences, called header block fragments, and transmitted within the
754	payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6), or
755	CONTINUATION (Section 6.10) frames.
756
757	The Cookie header field [COOKIE] is treated specially by the HTTP
758	mapping (see Section 8.1.2.5).
759
760	A receiving endpoint reassembles the header block by concatenating
761	its fragments and then decompresses the block to reconstruct the
762	header list.
763
764	A complete header block consists of either:
765
766	o a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag
767	set, or
768
769	o a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared
770	and one or more CONTINUATION frames, where the last CONTINUATION
771	frame has the END_HEADERS flag set.
772
773	Header compression is stateful. One compression context and one
774	decompression context are used for the entire connection. A decoding
775	error in a header block MUST be treated as a connection error
776	(Section 5.4.1) of type COMPRESSION_ERROR.
777
778	Each header block is processed as a discrete unit. Header blocks
779	MUST be transmitted as a contiguous sequence of frames, with no
780	interleaved frames of any other type or from any other stream. The
781	last frame in a sequence of HEADERS or CONTINUATION frames has the
782
783
784
785
786	Belshe, et al. Standards Track [Page 14]

788	RFC 7540 HTTP/2 May 2015
789
790
791	END_HEADERS flag set. The last frame in a sequence of PUSH_PROMISE
792	or CONTINUATION frames has the END_HEADERS flag set. This allows a
793	header block to be logically equivalent to a single frame.
794
795	Header block fragments can only be sent as the payload of HEADERS,
796	PUSH_PROMISE, or CONTINUATION frames because these frames carry data
797	that can modify the compression context maintained by a receiver. An
798	endpoint receiving HEADERS, PUSH_PROMISE, or CONTINUATION frames
799	needs to reassemble header blocks and perform decompression even if
800	the frames are to be discarded. A receiver MUST terminate the
801	connection with a connection error (Section 5.4.1) of type
802	COMPRESSION_ERROR if it does not decompress a header block.
803
804	5. Streams and Multiplexing
805
806	A "stream" is an independent, bidirectional sequence of frames
807	exchanged between the client and server within an HTTP/2 connection.
808	Streams have several important characteristics:
809
810	o A single HTTP/2 connection can contain multiple concurrently open
811	streams, with either endpoint interleaving frames from multiple
812	streams.
813
814	o Streams can be established and used unilaterally or shared by
815	either the client or server.
816
817	o Streams can be closed by either endpoint.
818
819	o The order in which frames are sent on a stream is significant.
820	Recipients process frames in the order they are received. In
821	particular, the order of HEADERS and DATA frames is semantically
822	significant.
823
824	o Streams are identified by an integer. Stream identifiers are
825	assigned to streams by the endpoint initiating the stream.
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842	Belshe, et al. Standards Track [Page 15]

844	RFC 7540 HTTP/2 May 2015
845
846
847	5.1. Stream States
848
849	The lifecycle of a stream is shown in Figure 2.
850
851	+--------+
852	send PP \| \| recv PP
853	,--------\| idle \|--------.
854	/ \| \| \
855	v +--------+ v
856	+----------+ \| +----------+
857	\| \| \| send H / \| \|
858	,------\| reserved \| \| recv H \| reserved \|------.
859	\| \| (local) \| \| \| (remote) \| \|
860	\| +----------+ v +----------+ \|
861	\| \| +--------+ \| \|
862	\| \| recv ES \| \| send ES \| \|
863	\| send H \| ,-------\| open \|-------. \| recv H \|
864	\| \| / \| \| \ \| \|
865	\| v v +--------+ v v \|
866	\| +----------+ \| +----------+ \|
867	\| \| half \| \| \| half \| \|
868	\| \| closed \| \| send R / \| closed \| \|
869	\| \| (remote) \| \| recv R \| (local) \| \|
870	\| +----------+ \| +----------+ \|
871	\| \| \| \| \|
872	\| \| send ES / \| recv ES / \| \|
873	\| \| send R / v send R / \| \|
874	\| \| recv R +--------+ recv R \| \|
875	\| send R / `----------->\| \|<-----------' send R / \|
876	\| recv R \| closed \| recv R \|
877	`----------------------->\| \|<----------------------'
878	+--------+
879
880	send: endpoint sends this frame
881	recv: endpoint receives this frame
882
883	H: HEADERS frame (with implied CONTINUATIONs)
884	PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
885	ES: END_STREAM flag
886	R: RST_STREAM frame
887
888	Figure 2: Stream States
889
890	Note that this diagram shows stream state transitions and the frames
891	and flags that affect those transitions only. In this regard,
892	CONTINUATION frames do not result in state transitions; they are
893	effectively part of the HEADERS or PUSH_PROMISE that they follow.
894
895
896
897
898	Belshe, et al. Standards Track [Page 16]

900	RFC 7540 HTTP/2 May 2015
901
902
903	For the purpose of state transitions, the END_STREAM flag is
904	processed as a separate event to the frame that bears it; a HEADERS
905	frame with the END_STREAM flag set can cause two state transitions.
906
907	Both endpoints have a subjective view of the state of a stream that
908	could be different when frames are in transit. Endpoints do not
909	coordinate the creation of streams; they are created unilaterally by
910	either endpoint. The negative consequences of a mismatch in states
911	are limited to the "closed" state after sending RST_STREAM, where
912	frames might be received for some time after closing.
913
914	Streams have the following states:
915
916	idle:
917	All streams start in the "idle" state.
918
919	The following transitions are valid from this state:
920
921	* Sending or receiving a HEADERS frame causes the stream to
922	become "open". The stream identifier is selected as described
923	in Section 5.1.1. The same HEADERS frame can also cause a
924	stream to immediately become "half-closed".
925
926	* Sending a PUSH_PROMISE frame on another stream reserves the
927	idle stream that is identified for later use. The stream state
928	for the reserved stream transitions to "reserved (local)".
929
930	* Receiving a PUSH_PROMISE frame on another stream reserves an
931	idle stream that is identified for later use. The stream state
932	for the reserved stream transitions to "reserved (remote)".
933
934	* Note that the PUSH_PROMISE frame is not sent on the idle stream
935	but references the newly reserved stream in the Promised Stream
936	ID field.
937
938	Receiving any frame other than HEADERS or PRIORITY on a stream in	include/tbl/h2_frames.h 57
939	this state MUST be treated as a connection error (Section 5.4.1)	include/tbl/h2_frames.h 66
940	of type PROTOCOL_ERROR.
941
942	reserved (local):
943	A stream in the "reserved (local)" state is one that has been
944	promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame
945	reserves an idle stream by associating the stream with an open
946	stream that was initiated by the remote peer (see Section 8.2).
947
948
949
950
951
952
953
954	Belshe, et al. Standards Track [Page 17]

956	RFC 7540 HTTP/2 May 2015
957
958
959	In this state, only the following transitions are possible:
960
961	* The endpoint can send a HEADERS frame. This causes the stream
962	to open in a "half-closed (remote)" state.
963
964	* Either endpoint can send a RST_STREAM frame to cause the stream
965	to become "closed". This releases the stream reservation.
966
967
968	An endpoint MUST NOT send any type of frame other than HEADERS,
969	RST_STREAM, or PRIORITY in this state.
970
971	A PRIORITY or WINDOW_UPDATE frame MAY be received in this state.
972	Receiving any type of frame other than RST_STREAM, PRIORITY, or
973	WINDOW_UPDATE on a stream in this state MUST be treated as a
974	connection error (Section 5.4.1) of type PROTOCOL_ERROR.
975
976	reserved (remote):
977	A stream in the "reserved (remote)" state has been reserved by a
978	remote peer.
979
980	In this state, only the following transitions are possible:
981
982	* Receiving a HEADERS frame causes the stream to transition to
983	"half-closed (local)".
984
985	* Either endpoint can send a RST_STREAM frame to cause the stream
986	to become "closed". This releases the stream reservation.
987
988	An endpoint MAY send a PRIORITY frame in this state to
989	reprioritize the reserved stream. An endpoint MUST NOT send any
990	type of frame other than RST_STREAM, WINDOW_UPDATE, or PRIORITY in
991	this state.
992
993	Receiving any type of frame other than HEADERS, RST_STREAM, or
994	PRIORITY on a stream in this state MUST be treated as a connection
995	error (Section 5.4.1) of type PROTOCOL_ERROR.
996
997	open:
998	A stream in the "open" state may be used by both peers to send
999	frames of any type. In this state, sending peers observe
1000	advertised stream-level flow-control limits (Section 5.2).
1001
1002	From this state, either endpoint can send a frame with an
1003	END_STREAM flag set, which causes the stream to transition into
1004	one of the "half-closed" states. An endpoint sending an
1005
1006
1007
1008
1009
1010	Belshe, et al. Standards Track [Page 18]

1012	RFC 7540 HTTP/2 May 2015
1013
1014
1015	END_STREAM flag causes the stream state to become "half-closed
1016	(local)"; an endpoint receiving an END_STREAM flag causes the
1017	stream state to become "half-closed (remote)".
1018
1019	Either endpoint can send a RST_STREAM frame from this state,
1020	causing it to transition immediately to "closed".
1021
1022	half-closed (local):
1023	A stream that is in the "half-closed (local)" state cannot be used
1024	for sending frames other than WINDOW_UPDATE, PRIORITY, and
1025	RST_STREAM.
1026
1027	A stream transitions from this state to "closed" when a frame that
1028	contains an END_STREAM flag is received or when either peer sends
1029	a RST_STREAM frame.
1030
1031	An endpoint can receive any type of frame in this state.
1032	Providing flow-control credit using WINDOW_UPDATE frames is
1033	necessary to continue receiving flow-controlled frames. In this
1034	state, a receiver can ignore WINDOW_UPDATE frames, which might
1035	arrive for a short period after a frame bearing the END_STREAM
1036	flag is sent.
1037
1038	PRIORITY frames received in this state are used to reprioritize
1039	streams that depend on the identified stream.
1040
1041	half-closed (remote):
1042	A stream that is "half-closed (remote)" is no longer being used by
1043	the peer to send frames. In this state, an endpoint is no longer
1044	obligated to maintain a receiver flow-control window.
1045
1046	If an endpoint receives additional frames, other than
1047	WINDOW_UPDATE, PRIORITY, or RST_STREAM, for a stream that is in
1048	this state, it MUST respond with a stream error (Section 5.4.2) of
1049	type STREAM_CLOSED.
1050
1051	A stream that is "half-closed (remote)" can be used by the
1052	endpoint to send frames of any type. In this state, the endpoint
1053	continues to observe advertised stream-level flow-control limits
1054	(Section 5.2).
1055
1056	A stream can transition from this state to "closed" by sending a
1057	frame that contains an END_STREAM flag or when either peer sends a
1058	RST_STREAM frame.
1059
1060
1061
1062
1063
1064
1065
1066	Belshe, et al. Standards Track [Page 19]

1068	RFC 7540 HTTP/2 May 2015
1069
1070
1071	closed:
1072	The "closed" state is the terminal state.
1073
1074	An endpoint MUST NOT send frames other than PRIORITY on a closed
1075	stream. An endpoint that receives any frame other than PRIORITY
1076	after receiving a RST_STREAM MUST treat that as a stream error
1077	(Section 5.4.2) of type STREAM_CLOSED. Similarly, an endpoint
1078	that receives any frames after receiving a frame with the
1079	END_STREAM flag set MUST treat that as a connection error
1080	(Section 5.4.1) of type STREAM_CLOSED, unless the frame is
1081	permitted as described below.
1082
1083	WINDOW_UPDATE or RST_STREAM frames can be received in this state
1084	for a short period after a DATA or HEADERS frame containing an
1085	END_STREAM flag is sent. Until the remote peer receives and
1086	processes RST_STREAM or the frame bearing the END_STREAM flag, it
1087	might send frames of these types. Endpoints MUST ignore
1088	WINDOW_UPDATE or RST_STREAM frames received in this state, though
1089	endpoints MAY choose to treat frames that arrive a significant
1090	time after sending END_STREAM as a connection error
1091	(Section 5.4.1) of type PROTOCOL_ERROR.
1092
1093	PRIORITY frames can be sent on closed streams to prioritize
1094	streams that are dependent on the closed stream. Endpoints SHOULD
1095	process PRIORITY frames, though they can be ignored if the stream
1096	has been removed from the dependency tree (see Section 5.3.4).
1097
1098	If this state is reached as a result of sending a RST_STREAM
1099	frame, the peer that receives the RST_STREAM might have already
1100	sent -- or enqueued for sending -- frames on the stream that
1101	cannot be withdrawn. An endpoint MUST ignore frames that it
1102	receives on closed streams after it has sent a RST_STREAM frame.
1103	An endpoint MAY choose to limit the period over which it ignores
1104	frames and treat frames that arrive after this time as being in
1105	error.
1106
1107	Flow-controlled frames (i.e., DATA) received after sending
1108	RST_STREAM are counted toward the connection flow-control window.
1109	Even though these frames might be ignored, because they are sent
1110	before the sender receives the RST_STREAM, the sender will
1111	consider the frames to count against the flow-control window.
1112
1113	An endpoint might receive a PUSH_PROMISE frame after it sends
1114	RST_STREAM. PUSH_PROMISE causes a stream to become "reserved"
1115	even if the associated stream has been reset. Therefore, a
1116	RST_STREAM is needed to close an unwanted promised stream.
1117
1118
1119
1120
1121
1122	Belshe, et al. Standards Track [Page 20]

1124	RFC 7540 HTTP/2 May 2015
1125
1126
1127	In the absence of more specific guidance elsewhere in this document,
1128	implementations SHOULD treat the receipt of a frame that is not
1129	expressly permitted in the description of a state as a connection
1130	error (Section 5.4.1) of type PROTOCOL_ERROR. Note that PRIORITY can
1131	be sent and received in any stream state. Frames of unknown types
1132	are ignored.
1133
1134	An example of the state transitions for an HTTP request/response
1135	exchange can be found in Section 8.1. An example of the state
1136	transitions for server push can be found in Sections 8.2.1 and 8.2.2.
1137
1138	5.1.1. Stream Identifiers
1139
1140	Streams are identified with an unsigned 31-bit integer. Streams	.../http2/cache_http2_proto.c 1284
1141	initiated by a client MUST use odd-numbered stream identifiers; those
1142	initiated by the server MUST use even-numbered stream identifiers. A
1143	stream identifier of zero (0x0) is used for connection control
1144	messages; the stream identifier of zero cannot be used to establish a
1145	new stream.
1146
1147	HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are
1148	responded to with a stream identifier of one (0x1). After the
1149	upgrade completes, stream 0x1 is "half-closed (local)" to the client.
1150	Therefore, stream 0x1 cannot be selected as a new stream identifier
1151	by a client that upgrades from HTTP/1.1.
1152
1153	The identifier of a newly established stream MUST be numerically	.../http2/cache_http2_proto.c 729
1154	greater than all streams that the initiating endpoint has opened or	.../http2/cache_http2_proto.c 1285
1155	reserved. This governs streams that are opened using a HEADERS frame
1156	and streams that are reserved using PUSH_PROMISE. An endpoint that
1157	receives an unexpected stream identifier MUST respond with a
1158	connection error (Section 5.4.1) of type PROTOCOL_ERROR.
1159
1160	The first use of a new stream identifier implicitly closes all
1161	streams in the "idle" state that might have been initiated by that
1162	peer with a lower-valued stream identifier. For example, if a client
1163	sends a HEADERS frame on stream 7 without ever sending a frame on
1164	stream 5, then stream 5 transitions to the "closed" state when the
1165	first frame for stream 7 is sent or received.
1166
1167	Stream identifiers cannot be reused. Long-lived connections can
1168	result in an endpoint exhausting the available range of stream
1169	identifiers. A client that is unable to establish a new stream
1170	identifier can establish a new connection for new streams. A server
1171	that is unable to establish a new stream identifier can send a GOAWAY
1172	frame so that the client is forced to open a new connection for new
1173	streams.
1174
1175
1176
1177
1178	Belshe, et al. Standards Track [Page 21]

1180	RFC 7540 HTTP/2 May 2015
1181
1182
1183	5.1.2. Stream Concurrency
1184
1185	A peer can limit the number of concurrently active streams using the
1186	SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within
1187	a SETTINGS frame. The maximum concurrent streams setting is specific
1188	to each endpoint and applies only to the peer that receives the
1189	setting. That is, clients specify the maximum number of concurrent
1190	streams the server can initiate, and servers specify the maximum
1191	number of concurrent streams the client can initiate.
1192
1193	Streams that are in the "open" state or in either of the "half-
1194	closed" states count toward the maximum number of streams that an
1195	endpoint is permitted to open. Streams in any of these three states
1196	count toward the limit advertised in the
1197	SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the
1198	"reserved" states do not count toward the stream limit.
1199
1200	Endpoints MUST NOT exceed the limit set by their peer. An endpoint	.../http2/cache_http2_proto.c 739
1201	that receives a HEADERS frame that causes its advertised concurrent
1202	stream limit to be exceeded MUST treat this as a stream error
1203	(Section 5.4.2) of type PROTOCOL_ERROR or REFUSED_STREAM. The choice
1204	of error code determines whether the endpoint wishes to enable
1205	automatic retry (see Section 8.1.4) for details).
1206
1207	An endpoint that wishes to reduce the value of
1208	SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current
1209	number of open streams can either close streams that exceed the new
1210	value or allow streams to complete.
1211
1212	5.2. Flow Control
1213
1214	Using streams for multiplexing introduces contention over use of the
1215	TCP connection, resulting in blocked streams. A flow-control scheme
1216	ensures that streams on the same connection do not destructively
1217	interfere with each other. Flow control is used for both individual
1218	streams and for the connection as a whole.
1219
1220	HTTP/2 provides for flow control through use of the WINDOW_UPDATE
1221	frame (Section 6.9).
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234	Belshe, et al. Standards Track [Page 22]

1236	RFC 7540 HTTP/2 May 2015
1237
1238
1239	5.2.1. Flow-Control Principles
1240
1241	HTTP/2 stream flow control aims to allow a variety of flow-control
1242	algorithms to be used without requiring protocol changes. Flow
1243	control in HTTP/2 has the following characteristics:
1244
1245	1. Flow control is specific to a connection. Both types of flow
1246	control are between the endpoints of a single hop and not over
1247	the entire end-to-end path.
1248
1249	2. Flow control is based on WINDOW_UPDATE frames. Receivers
1250	advertise how many octets they are prepared to receive on a
1251	stream and for the entire connection. This is a credit-based
1252	scheme.
1253
1254	3. Flow control is directional with overall control provided by the
1255	receiver. A receiver MAY choose to set any window size that it
1256	desires for each stream and for the entire connection. A sender
1257	MUST respect flow-control limits imposed by a receiver. Clients,
1258	servers, and intermediaries all independently advertise their
1259	flow-control window as a receiver and abide by the flow-control
1260	limits set by their peer when sending.
1261
1262	4. The initial value for the flow-control window is 65,535 octets
1263	for both new streams and the overall connection.
1264
1265	5. The frame type determines whether flow control applies to a	include/tbl/h2_frames.h 40
1266	frame. Of the frames specified in this document, only DATA
1267	frames are subject to flow control; all other frame types do not
1268	consume space in the advertised flow-control window. This
1269	ensures that important control frames are not blocked by flow
1270	control.
1271
1272	6. Flow control cannot be disabled.
1273
1274	7. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE
1275	frame (Section 6.9). This document does not stipulate how a
1276	receiver decides when to send this frame or the value that it
1277	sends, nor does it specify how a sender chooses to send packets.
1278	Implementations are able to select any algorithm that suits their
1279	needs.
1280
1281	Implementations are also responsible for managing how requests and
1282	responses are sent based on priority, choosing how to avoid head-of-
1283	line blocking for requests, and managing the creation of new streams.
1284	Algorithm choices for these could interact with any flow-control
1285	algorithm.
1286
1287
1288
1289
1290	Belshe, et al. Standards Track [Page 23]

1292	RFC 7540 HTTP/2 May 2015
1293
1294
1295	5.2.2. Appropriate Use of Flow Control
1296
1297	Flow control is defined to protect endpoints that are operating under
1298	resource constraints. For example, a proxy needs to share memory
1299	between many connections and also might have a slow upstream
1300	connection and a fast downstream one. Flow-control addresses cases
1301	where the receiver is unable to process data on one stream yet wants
1302	to continue to process other streams in the same connection.
1303
1304	Deployments that do not require this capability can advertise a flow-
1305	control window of the maximum size (2^31-1) and can maintain this
1306	window by sending a WINDOW_UPDATE frame when any data is received.
1307	This effectively disables flow control for that receiver.
1308	Conversely, a sender is always subject to the flow-control window
1309	advertised by the receiver.
1310
1311	Deployments with constrained resources (for example, memory) can
1312	employ flow control to limit the amount of memory a peer can consume.
1313	Note, however, that this can lead to suboptimal use of available
1314	network resources if flow control is enabled without knowledge of the
1315	bandwidth-delay product (see [RFC7323]).
1316
1317	Even with full awareness of the current bandwidth-delay product,
1318	implementation of flow control can be difficult. When using flow
1319	control, the receiver MUST read from the TCP receive buffer in a
1320	timely fashion. Failure to do so could lead to a deadlock when
1321	critical frames, such as WINDOW_UPDATE, are not read and acted upon.
1322
1323	5.3. Stream Priority
1324
1325	A client can assign a priority for a new stream by including
1326	prioritization information in the HEADERS frame (Section 6.2) that
1327	opens the stream. At any other time, the PRIORITY frame
1328	(Section 6.3) can be used to change the priority of a stream.
1329
1330	The purpose of prioritization is to allow an endpoint to express how
1331	it would prefer its peer to allocate resources when managing
1332	concurrent streams. Most importantly, priority can be used to select
1333	streams for transmitting frames when there is limited capacity for
1334	sending.
1335
1336	Streams can be prioritized by marking them as dependent on the
1337	completion of other streams (Section 5.3.1). Each dependency is
1338	assigned a relative weight, a number that is used to determine the
1339	relative proportion of available resources that are assigned to
1340	streams dependent on the same stream.
1341
1342
1343
1344
1345
1346	Belshe, et al. Standards Track [Page 24]

1348	RFC 7540 HTTP/2 May 2015
1349
1350
1351	Explicitly setting the priority for a stream is input to a
1352	prioritization process. It does not guarantee any particular
1353	processing or transmission order for the stream relative to any other
1354	stream. An endpoint cannot force a peer to process concurrent
1355	streams in a particular order using priority. Expressing priority is
1356	therefore only a suggestion.
1357
1358	Prioritization information can be omitted from messages. Defaults
1359	are used prior to any explicit values being provided (Section 5.3.5).
1360
1361	5.3.1. Stream Dependencies
1362
1363	Each stream can be given an explicit dependency on another stream.
1364	Including a dependency expresses a preference to allocate resources
1365	to the identified stream rather than to the dependent stream.
1366
1367	A stream that is not dependent on any other stream is given a stream
1368	dependency of 0x0. In other words, the non-existent stream 0 forms
1369	the root of the tree.
1370
1371	A stream that depends on another stream is a dependent stream. The
1372	stream upon which a stream is dependent is a parent stream. A
1373	dependency on a stream that is not currently in the tree -- such as a
1374	stream in the "idle" state -- results in that stream being given a
1375	default priority (Section 5.3.5).
1376
1377	When assigning a dependency on another stream, the stream is added as
1378	a new dependency of the parent stream. Dependent streams that share
1379	the same parent are not ordered with respect to each other. For
1380	example, if streams B and C are dependent on stream A, and if stream
1381	D is created with a dependency on stream A, this results in a
1382	dependency order of A followed by B, C, and D in any order.
1383
1384	A A
1385	/ \ ==> /\|\
1386	B C B D C
1387
1388	Figure 3: Example of Default Dependency Creation
1389
1390	An exclusive flag allows for the insertion of a new level of
1391	dependencies. The exclusive flag causes the stream to become the
1392	sole dependency of its parent stream, causing other dependencies to
1393	become dependent on the exclusive stream. In the previous example,
1394	if stream D is created with an exclusive dependency on stream A, this
1395	results in D becoming the dependency parent of B and C.
1396
1397
1398
1399
1400
1401
1402	Belshe, et al. Standards Track [Page 25]

1404	RFC 7540 HTTP/2 May 2015
1405
1406
1407	A
1408	A \|
1409	/ \ ==> D
1410	B C / \
1411	B C
1412
1413	Figure 4: Example of Exclusive Dependency Creation
1414
1415	Inside the dependency tree, a dependent stream SHOULD only be
1416	allocated resources if either all of the streams that it depends on
1417	(the chain of parent streams up to 0x0) are closed or it is not
1418	possible to make progress on them.
1419
1420	A stream cannot depend on itself. An endpoint MUST treat this as a
1421	stream error (Section 5.4.2) of type PROTOCOL_ERROR.
1422
1423	5.3.2. Dependency Weighting
1424
1425	All dependent streams are allocated an integer weight between 1 and
1426	256 (inclusive).
1427
1428	Streams with the same parent SHOULD be allocated resources
1429	proportionally based on their weight. Thus, if stream B depends on
1430	stream A with weight 4, stream C depends on stream A with weight 12,
1431	and no progress can be made on stream A, stream B ideally receives
1432	one-third of the resources allocated to stream C.
1433
1434	5.3.3. Reprioritization
1435
1436	Stream priorities are changed using the PRIORITY frame. Setting a
1437	dependency causes a stream to become dependent on the identified
1438	parent stream.
1439
1440	Dependent streams move with their parent stream if the parent is
1441	reprioritized. Setting a dependency with the exclusive flag for a
1442	reprioritized stream causes all the dependencies of the new parent
1443	stream to become dependent on the reprioritized stream.
1444
1445	If a stream is made dependent on one of its own dependencies, the
1446	formerly dependent stream is first moved to be dependent on the
1447	reprioritized stream's previous parent. The moved dependency retains
1448	its weight.
1449
1450	For example, consider an original dependency tree where B and C
1451	depend on A, D and E depend on C, and F depends on D. If A is made
1452	dependent on D, then D takes the place of A. All other dependency
1453	relationships stay the same, except for F, which becomes dependent on
1454	A if the reprioritization is exclusive.
1455
1456
1457
1458	Belshe, et al. Standards Track [Page 26]

1460	RFC 7540 HTTP/2 May 2015
1461
1462
1463	x x x x
1464	\| / \ \| \|
1465	A D A D D
1466	/ \ / / \ / \ \|
1467	B C ==> F B C ==> F A OR A
1468	/ \ \| / \ /\|\
1469	D E E B C B C F
1470	\| \| \|
1471	F E E
1472	(intermediate) (non-exclusive) (exclusive)
1473
1474	Figure 5: Example of Dependency Reordering
1475
1476	5.3.4. Prioritization State Management
1477
1478	When a stream is removed from the dependency tree, its dependencies
1479	can be moved to become dependent on the parent of the closed stream.
1480	The weights of new dependencies are recalculated by distributing the
1481	weight of the dependency of the closed stream proportionally based on
1482	the weights of its dependencies.
1483
1484	Streams that are removed from the dependency tree cause some
1485	prioritization information to be lost. Resources are shared between
1486	streams with the same parent stream, which means that if a stream in
1487	that set closes or becomes blocked, any spare capacity allocated to a
1488	stream is distributed to the immediate neighbors of the stream.
1489	However, if the common dependency is removed from the tree, those
1490	streams share resources with streams at the next highest level.
1491
1492	For example, assume streams A and B share a parent, and streams C and
1493	D both depend on stream A. Prior to the removal of stream A, if
1494	streams A and D are unable to proceed, then stream C receives all the
1495	resources dedicated to stream A. If stream A is removed from the
1496	tree, the weight of stream A is divided between streams C and D. If
1497	stream D is still unable to proceed, this results in stream C
1498	receiving a reduced proportion of resources. For equal starting
1499	weights, C receives one third, rather than one half, of available
1500	resources.
1501
1502	It is possible for a stream to become closed while prioritization
1503	information that creates a dependency on that stream is in transit.
1504	If a stream identified in a dependency has no associated priority
1505	information, then the dependent stream is instead assigned a default
1506	priority (Section 5.3.5). This potentially creates suboptimal
1507	prioritization, since the stream could be given a priority that is
1508	different from what is intended.
1509
1510
1511
1512
1513
1514	Belshe, et al. Standards Track [Page 27]

1516	RFC 7540 HTTP/2 May 2015
1517
1518
1519	To avoid these problems, an endpoint SHOULD retain stream
1520	prioritization state for a period after streams become closed. The
1521	longer state is retained, the lower the chance that streams are
1522	assigned incorrect or default priority values.
1523
1524	Similarly, streams that are in the "idle" state can be assigned
1525	priority or become a parent of other streams. This allows for the
1526	creation of a grouping node in the dependency tree, which enables
1527	more flexible expressions of priority. Idle streams begin with a
1528	default priority (Section 5.3.5).
1529
1530	The retention of priority information for streams that are not
1531	counted toward the limit set by SETTINGS_MAX_CONCURRENT_STREAMS could
1532	create a large state burden for an endpoint. Therefore, the amount
1533	of prioritization state that is retained MAY be limited.
1534
1535	The amount of additional state an endpoint maintains for
1536	prioritization could be dependent on load; under high load,
1537	prioritization state can be discarded to limit resource commitments.
1538	In extreme cases, an endpoint could even discard prioritization state
1539	for active or reserved streams. If a limit is applied, endpoints
1540	SHOULD maintain state for at least as many streams as allowed by
1541	their setting for SETTINGS_MAX_CONCURRENT_STREAMS. Implementations
1542	SHOULD also attempt to retain state for streams that are in active
1543	use in the priority tree.
1544
1545	If it has retained enough state to do so, an endpoint receiving a
1546	PRIORITY frame that changes the priority of a closed stream SHOULD
1547	alter the dependencies of the streams that depend on it.
1548
1549	5.3.5. Default Priorities
1550
1551	All streams are initially assigned a non-exclusive dependency on
1552	stream 0x0. Pushed streams (Section 8.2) initially depend on their
1553	associated stream. In both cases, streams are assigned a default
1554	weight of 16.
1555
1556	5.4. Error Handling
1557
1558	HTTP/2 framing permits two classes of error:
1559
1560	o An error condition that renders the entire connection unusable is
1561	a connection error.
1562
1563	o An error in an individual stream is a stream error.
1564
1565	A list of error codes is included in Section 7.
1566
1567
1568
1569
1570	Belshe, et al. Standards Track [Page 28]

1572	RFC 7540 HTTP/2 May 2015
1573
1574
1575	5.4.1. Connection Error Handling
1576
1577	A connection error is any error that prevents further processing of
1578	the frame layer or corrupts any connection state.
1579
1580	An endpoint that encounters a connection error SHOULD first send a
1581	GOAWAY frame (Section 6.8) with the stream identifier of the last
1582	stream that it successfully received from its peer. The GOAWAY frame
1583	includes an error code that indicates why the connection is
1584	terminating. After sending the GOAWAY frame for an error condition,
1585	the endpoint MUST close the TCP connection.
1586
1587	It is possible that the GOAWAY will not be reliably received by the
1588	receiving endpoint ([RFC7230], Section 6.6 describes how an immediate
1589	connection close can result in data loss). In the event of a
1590	connection error, GOAWAY only provides a best-effort attempt to
1591	communicate with the peer about why the connection is being
1592	terminated.
1593
1594	An endpoint can end a connection at any time. In particular, an
1595	endpoint MAY choose to treat a stream error as a connection error.
1596	Endpoints SHOULD send a GOAWAY frame when ending a connection,
1597	providing that circumstances permit it.
1598
1599	5.4.2. Stream Error Handling
1600
1601	A stream error is an error related to a specific stream that does not
1602	affect processing of other streams.
1603
1604	An endpoint that detects a stream error sends a RST_STREAM frame
1605	(Section 6.4) that contains the stream identifier of the stream where
1606	the error occurred. The RST_STREAM frame includes an error code that
1607	indicates the type of error.
1608
1609	A RST_STREAM is the last frame that an endpoint can send on a stream.
1610	The peer that sends the RST_STREAM frame MUST be prepared to receive
1611	any frames that were sent or enqueued for sending by the remote peer.
1612	These frames can be ignored, except where they modify connection
1613	state (such as the state maintained for header compression
1614	(Section 4.3) or flow control).
1615
1616	Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame
1617	for any stream. However, an endpoint MAY send additional RST_STREAM
1618	frames if it receives frames on a closed stream after more than a
1619	round-trip time. This behavior is permitted to deal with misbehaving
1620	implementations.
1621
1622
1623
1624
1625
1626	Belshe, et al. Standards Track [Page 29]

1628	RFC 7540 HTTP/2 May 2015
1629
1630
1631	To avoid looping, an endpoint MUST NOT send a RST_STREAM in response
1632	to a RST_STREAM frame.
1633
1634	5.4.3. Connection Termination
1635
1636	If the TCP connection is closed or reset while streams remain in
1637	"open" or "half-closed" state, then the affected streams cannot be
1638	automatically retried (see Section 8.1.4 for details).
1639
1640	5.5. Extending HTTP/2
1641
1642	HTTP/2 permits extension of the protocol. Within the limitations
1643	described in this section, protocol extensions can be used to provide
1644	additional services or alter any aspect of the protocol. Extensions
1645	are effective only within the scope of a single HTTP/2 connection.
1646
1647	This applies to the protocol elements defined in this document. This
1648	does not affect the existing options for extending HTTP, such as
1649	defining new methods, status codes, or header fields.
1650
1651	Extensions are permitted to use new frame types (Section 4.1), new
1652	settings (Section 6.5.2), or new error codes (Section 7). Registries
1653	are established for managing these extension points: frame types
1654	(Section 11.2), settings (Section 11.3), and error codes
1655	(Section 11.4).
1656
1657	Implementations MUST ignore unknown or unsupported values in all
1658	extensible protocol elements. Implementations MUST discard frames
1659	that have unknown or unsupported types. This means that any of these
1660	extension points can be safely used by extensions without prior
1661	arrangement or negotiation. However, extension frames that appear in
1662	the middle of a header block (Section 4.3) are not permitted; these
1663	MUST be treated as a connection error (Section 5.4.1) of type
1664	PROTOCOL_ERROR.
1665
1666	Extensions that could change the semantics of existing protocol
1667	components MUST be negotiated before being used. For example, an
1668	extension that changes the layout of the HEADERS frame cannot be used
1669	until the peer has given a positive signal that this is acceptable.
1670	In this case, it could also be necessary to coordinate when the
1671	revised layout comes into effect. Note that treating any frames
1672	other than DATA frames as flow controlled is such a change in
1673	semantics and can only be done through negotiation.
1674
1675	This document doesn't mandate a specific method for negotiating the
1676	use of an extension but notes that a setting (Section 6.5.2) could be
1677	used for that purpose. If both peers set a value that indicates
1678	willingness to use the extension, then the extension can be used. If
1679
1680
1681
1682	Belshe, et al. Standards Track [Page 30]

1684	RFC 7540 HTTP/2 May 2015
1685
1686
1687	a setting is used for extension negotiation, the initial value MUST
1688	be defined in such a fashion that the extension is initially
1689	disabled.
1690
1691	6. Frame Definitions
1692
1693	This specification defines a number of frame types, each identified
1694	by a unique 8-bit type code. Each frame type serves a distinct
1695	purpose in the establishment and management either of the connection
1696	as a whole or of individual streams.
1697
1698	The transmission of specific frame types can alter the state of a
1699	connection. If endpoints fail to maintain a synchronized view of the
1700	connection state, successful communication within the connection will
1701	no longer be possible. Therefore, it is important that endpoints
1702	have a shared comprehension of how the state is affected by the use
1703	any given frame.
1704
1705	6.1. DATA
1706
1707	DATA frames (type=0x0) convey arbitrary, variable-length sequences of
1708	octets associated with a stream. One or more DATA frames are used,
1709	for instance, to carry HTTP request or response payloads.
1710
1711	DATA frames MAY also contain padding. Padding can be added to DATA
1712	frames to obscure the size of messages. Padding is a security
1713	feature; see Section 10.7.
1714
1715	+---------------+
1716	\|Pad Length? (8)\|
1717	+---------------+-----------------------------------------------+
1718	\| Data (*) ...
1719	+---------------------------------------------------------------+
1720	\| Padding (*) ...
1721	+---------------------------------------------------------------+
1722
1723	Figure 6: DATA Frame Payload
1724
1725	The DATA frame contains the following fields:
1726
1727	Pad Length: An 8-bit field containing the length of the frame
1728	padding in units of octets. This field is conditional (as
1729	signified by a "?" in the diagram) and is only present if the
1730	PADDED flag is set.
1731
1732	Data: Application data. The amount of data is the remainder of the
1733	frame payload after subtracting the length of the other fields
1734	that are present.
1735
1736
1737
1738	Belshe, et al. Standards Track [Page 31]

1740	RFC 7540 HTTP/2 May 2015
1741
1742
1743	Padding: Padding octets that contain no application semantic value.
1744	Padding octets MUST be set to zero when sending. A receiver is
1745	not obligated to verify padding but MAY treat non-zero padding as
1746	a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
1747
1748	The DATA frame defines the following flags:
1749
1750	END_STREAM (0x1): When set, bit 0 indicates that this frame is the	include/tbl/h2_frames.h 51
1751	last that the endpoint will send for the identified stream.
1752	Setting this flag causes the stream to enter one of the "half-
1753	closed" states or the "closed" state (Section 5.1).
1754
1755	PADDED (0x8): When set, bit 3 indicates that the Pad Length field
1756	and any padding that it describes are present.
1757
1758	DATA frames MUST be associated with a stream. If a DATA frame is	include/tbl/h2_frames.h 46
1759	received whose stream identifier field is 0x0, the recipient MUST
1760	respond with a connection error (Section 5.4.1) of type
1761	PROTOCOL_ERROR.
1762
1763	DATA frames are subject to flow control and can only be sent when a
1764	stream is in the "open" or "half-closed (remote)" state. The entire
1765	DATA frame payload is included in flow control, including the Pad
1766	Length and Padding fields if present. If a DATA frame is received	.../http2/cache_http2_proto.c 860
1767	whose stream is not in "open" or "half-closed (local)" state, the
1768	recipient MUST respond with a stream error (Section 5.4.2) of type
1769	STREAM_CLOSED.
1770
1771	The total number of padding octets is determined by the value of the
1772	Pad Length field. If the length of the padding is the length of the
1773	frame payload or greater, the recipient MUST treat this as a
1774	connection error (Section 5.4.1) of type PROTOCOL_ERROR.
1775
1776	Note: A frame can be increased in size by one octet by including a
1777	Pad Length field with a value of zero.
1778
1779	6.2. HEADERS
1780
1781	The HEADERS frame (type=0x1) is used to open a stream (Section 5.1),
1782	and additionally carries a header block fragment. HEADERS frames can
1783	be sent on a stream in the "idle", "reserved (local)", "open", or
1784	"half-closed (remote)" state.
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794	Belshe, et al. Standards Track [Page 32]

1796	RFC 7540 HTTP/2 May 2015
1797
1798
1799	+---------------+
1800	\|Pad Length? (8)\|
1801	+-+-------------+-----------------------------------------------+
1802	\|E\| Stream Dependency? (31) \|
1803	+-+-------------+-----------------------------------------------+
1804	\| Weight? (8) \|
1805	+-+-------------+-----------------------------------------------+
1806	\| Header Block Fragment (*) ...
1807	+---------------------------------------------------------------+
1808	\| Padding (*) ...
1809	+---------------------------------------------------------------+
1810
1811	Figure 7: HEADERS Frame Payload
1812
1813	The HEADERS frame payload has the following fields:
1814
1815	Pad Length: An 8-bit field containing the length of the frame
1816	padding in units of octets. This field is only present if the
1817	PADDED flag is set.
1818
1819	E: A single-bit flag indicating that the stream dependency is
1820	exclusive (see Section 5.3). This field is only present if the
1821	PRIORITY flag is set.
1822
1823	Stream Dependency: A 31-bit stream identifier for the stream that
1824	this stream depends on (see Section 5.3). This field is only
1825	present if the PRIORITY flag is set.
1826
1827	Weight: An unsigned 8-bit integer representing a priority weight for
1828	the stream (see Section 5.3). Add one to the value to obtain a
1829	weight between 1 and 256. This field is only present if the
1830	PRIORITY flag is set.
1831
1832	Header Block Fragment: A header block fragment (Section 4.3).
1833
1834	Padding: Padding octets.
1835
1836	The HEADERS frame defines the following flags:
1837
1838	END_STREAM (0x1): When set, bit 0 indicates that the header block
1839	(Section 4.3) is the last that the endpoint will send for the
1840	identified stream.
1841
1842	A HEADERS frame carries the END_STREAM flag that signals the end
1843	of a stream. However, a HEADERS frame with the END_STREAM flag
1844	set can be followed by CONTINUATION frames on the same stream.
1845	Logically, the CONTINUATION frames are part of the HEADERS frame.
1846
1847
1848
1849
1850	Belshe, et al. Standards Track [Page 33]

1852	RFC 7540 HTTP/2 May 2015
1853
1854
1855	END_HEADERS (0x4): When set, bit 2 indicates that this frame	include/tbl/h2_frames.h 60
1856	contains an entire header block (Section 4.3) and is not followed
1857	by any CONTINUATION frames.
1858
1859	A HEADERS frame without the END_HEADERS flag set MUST be followed	.../http2/cache_http2_proto.c 1299
1860	by a CONTINUATION frame for the same stream. A receiver MUST
1861	treat the receipt of any other type of frame or a frame on a
1862	different stream as a connection error (Section 5.4.1) of type
1863	PROTOCOL_ERROR.
1864
1865	PADDED (0x8): When set, bit 3 indicates that the Pad Length field
1866	and any padding that it describes are present.
1867
1868	PRIORITY (0x20): When set, bit 5 indicates that the Exclusive Flag
1869	(E), Stream Dependency, and Weight fields are present; see
1870	Section 5.3.
1871
1872	The payload of a HEADERS frame contains a header block fragment
1873	(Section 4.3). A header block that does not fit within a HEADERS
1874	frame is continued in a CONTINUATION frame (Section 6.10).
1875
1876	HEADERS frames MUST be associated with a stream. If a HEADERS frame	include/tbl/h2_frames.h 55
1877	is received whose stream identifier field is 0x0, the recipient MUST
1878	respond with a connection error (Section 5.4.1) of type
1879	PROTOCOL_ERROR.
1880
1881	The HEADERS frame changes the connection state as described in
1882	Section 4.3.
1883
1884	The HEADERS frame can include padding. Padding fields and flags are	.../http2/cache_http2_proto.c 776
1885	identical to those defined for DATA frames (Section 6.1). Padding
1886	that exceeds the size remaining for the header block fragment MUST be
1887	treated as a PROTOCOL_ERROR.
1888
1889	Prioritization information in a HEADERS frame is logically equivalent
1890	to a separate PRIORITY frame, but inclusion in HEADERS avoids the
1891	potential for churn in stream prioritization when new streams are
1892	created. Prioritization fields in HEADERS frames subsequent to the
1893	first on a stream reprioritize the stream (Section 5.3.3).
1894
1895	6.3. PRIORITY
1896
1897	The PRIORITY frame (type=0x2) specifies the sender-advised priority
1898	of a stream (Section 5.3). It can be sent in any stream state,
1899	including idle or closed streams.
1900
1901
1902
1903
1904
1905
1906	Belshe, et al. Standards Track [Page 34]

1908	RFC 7540 HTTP/2 May 2015
1909
1910
1911	+-+-------------------------------------------------------------+
1912	\|E\| Stream Dependency (31) \|
1913	+-+-------------+-----------------------------------------------+
1914	\| Weight (8) \|
1915	+-+-------------+
1916
1917	Figure 8: PRIORITY Frame Payload
1918
1919	The payload of a PRIORITY frame contains the following fields:
1920
1921	E: A single-bit flag indicating that the stream dependency is
1922	exclusive (see Section 5.3).
1923
1924	Stream Dependency: A 31-bit stream identifier for the stream that
1925	this stream depends on (see Section 5.3).
1926
1927	Weight: An unsigned 8-bit integer representing a priority weight for
1928	the stream (see Section 5.3). Add one to the value to obtain a
1929	weight between 1 and 256.
1930
1931	The PRIORITY frame does not define any flags.
1932
1933	The PRIORITY frame always identifies a stream. If a PRIORITY frame	include/tbl/h2_frames.h 64
1934	is received with a stream identifier of 0x0, the recipient MUST
1935	respond with a connection error (Section 5.4.1) of type
1936	PROTOCOL_ERROR.
1937
1938	The PRIORITY frame can be sent on a stream in any state, though it
1939	cannot be sent between consecutive frames that comprise a single
1940	header block (Section 4.3). Note that this frame could arrive after
1941	processing or frame sending has completed, which would cause it to
1942	have no effect on the identified stream. For a stream that is in the
1943	"half-closed (remote)" or "closed" state, this frame can only affect
1944	processing of the identified stream and its dependent streams; it
1945	does not affect frame transmission on that stream.
1946
1947	The PRIORITY frame can be sent for a stream in the "idle" or "closed"
1948	state. This allows for the reprioritization of a group of dependent
1949	streams by altering the priority of an unused or closed parent
1950	stream.
1951
1952	A PRIORITY frame with a length other than 5 octets MUST be treated as
1953	a stream error (Section 5.4.2) of type FRAME_SIZE_ERROR.
1954
1955
1956
1957
1958
1959
1960
1961
1962	Belshe, et al. Standards Track [Page 35]

1964	RFC 7540 HTTP/2 May 2015
1965
1966
1967	6.4. RST_STREAM
1968
1969	The RST_STREAM frame (type=0x3) allows for immediate termination of a
1970	stream. RST_STREAM is sent to request cancellation of a stream or to
1971	indicate that an error condition has occurred.
1972
1973	+---------------------------------------------------------------+
1974	\| Error Code (32) \|
1975	+---------------------------------------------------------------+
1976
1977	Figure 9: RST_STREAM Frame Payload
1978
1979	The RST_STREAM frame contains a single unsigned, 32-bit integer
1980	identifying the error code (Section 7). The error code indicates why
1981	the stream is being terminated.
1982
1983	The RST_STREAM frame does not define any flags.
1984
1985	The RST_STREAM frame fully terminates the referenced stream and
1986	causes it to enter the "closed" state. After receiving a RST_STREAM
1987	on a stream, the receiver MUST NOT send additional frames for that
1988	stream, with the exception of PRIORITY. However, after sending the
1989	RST_STREAM, the sending endpoint MUST be prepared to receive and
1990	process additional frames sent on the stream that might have been
1991	sent by the peer prior to the arrival of the RST_STREAM.
1992
1993	RST_STREAM frames MUST be associated with a stream. If a RST_STREAM	include/tbl/h2_frames.h 73
1994	frame is received with a stream identifier of 0x0, the recipient MUST
1995	treat this as a connection error (Section 5.4.1) of type
1996	PROTOCOL_ERROR.
1997
1998	RST_STREAM frames MUST NOT be sent for a stream in the "idle" state.
1999	If a RST_STREAM frame identifying an idle stream is received, the
2000	recipient MUST treat this as a connection error (Section 5.4.1) of
2001	type PROTOCOL_ERROR.
2002
2003	A RST_STREAM frame with a length other than 4 octets MUST be treated	.../http2/cache_http2_proto.c 377
2004	as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.
2005
2006	6.5. SETTINGS
2007
2008	The SETTINGS frame (type=0x4) conveys configuration parameters that
2009	affect how endpoints communicate, such as preferences and constraints
2010	on peer behavior. The SETTINGS frame is also used to acknowledge the
2011	receipt of those parameters. Individually, a SETTINGS parameter can
2012	also be referred to as a "setting".
2013
2014
2015
2016
2017
2018	Belshe, et al. Standards Track [Page 36]

2020	RFC 7540 HTTP/2 May 2015
2021
2022
2023	SETTINGS parameters are not negotiated; they describe characteristics
2024	of the sending peer, which are used by the receiving peer. Different
2025	values for the same parameter can be advertised by each peer. For
2026	example, a client might set a high initial flow-control window,
2027	whereas a server might set a lower value to conserve resources.
2028
2029	A SETTINGS frame MUST be sent by both endpoints at the start of a
2030	connection and MAY be sent at any other time by either endpoint over
2031	the lifetime of the connection. Implementations MUST support all of
2032	the parameters defined by this specification.
2033
2034	Each parameter in a SETTINGS frame replaces any existing value for
2035	that parameter. Parameters are processed in the order in which they
2036	appear, and a receiver of a SETTINGS frame does not need to maintain
2037	any state other than the current value of its parameters. Therefore,
2038	the value of a SETTINGS parameter is the last value that is seen by a
2039	receiver.
2040
2041	SETTINGS parameters are acknowledged by the receiving peer. To
2042	enable this, the SETTINGS frame defines the following flag:
2043
2044	ACK (0x1): When set, bit 0 indicates that this frame acknowledges
2045	receipt and application of the peer's SETTINGS frame. When this
2046	bit is set, the payload of the SETTINGS frame MUST be empty.
2047	Receipt of a SETTINGS frame with the ACK flag set and a length	.../http2/cache_http2_proto.c 577
2048	field value other than 0 MUST be treated as a connection error
2049	(Section 5.4.1) of type FRAME_SIZE_ERROR. For more information,
2050	see Section 6.5.3 ("Settings Synchronization").
2051
2052	SETTINGS frames always apply to a connection, never a single stream.	include/tbl/h2_frames.h 83
2053	The stream identifier for a SETTINGS frame MUST be zero (0x0). If an
2054	endpoint receives a SETTINGS frame whose stream identifier field is
2055	anything other than 0x0, the endpoint MUST respond with a connection
2056	error (Section 5.4.1) of type PROTOCOL_ERROR.
2057
2058	The SETTINGS frame affects connection state. A badly formed or
2059	incomplete SETTINGS frame MUST be treated as a connection error
2060	(Section 5.4.1) of type PROTOCOL_ERROR.
2061
2062	A SETTINGS frame with a length other than a multiple of 6 octets MUST	.../http2/cache_http2_proto.c 581
2063	be treated as a connection error (Section 5.4.1) of type
2064	FRAME_SIZE_ERROR.
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074	Belshe, et al. Standards Track [Page 37]