draft-ietf-quic-http.txt

QUIC                                                      M. Bishop, Ed.
Internet-Draft                                                    Akamai
Intended status: Standards Track                           June 07, 2019
Expires: December 9, 2019


             Hypertext Transfer Protocol Version 3 (HTTP/3)
                      draft-ietf-quic-http-latest

Abstract

   The QUIC transport protocol has several features that are desirable
   in a transport for HTTP, such as stream multiplexing, per-stream flow
   control, and low-latency connection establishment.  This document
   describes a mapping of HTTP semantics over QUIC.  This document also
   identifies HTTP/2 features that are subsumed by QUIC, and describes
   how HTTP/2 extensions can be ported to HTTP/3.

Note to Readers

   Discussion of this draft takes place on the QUIC working group
   mailing list (quic@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/search/?email_list=quic [1].

   Working Group information can be found at https://github.com/quicwg
   [2]; source code and issues list for this draft can be found at
   https://github.com/quicwg/base-drafts/labels/-http [3].

Status of This Memo

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

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

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

   This Internet-Draft will expire on December 9, 2019.







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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Prior versions of HTTP  . . . . . . . . . . . . . . . . .   4
     1.2.  Delegation to QUIC  . . . . . . . . . . . . . . . . . . .   4
   2.  HTTP/3 Protocol Overview  . . . . . . . . . . . . . . . . . .   5
     2.1.  Document Organization . . . . . . . . . . . . . . . . . .   6
     2.2.  Conventions and Terminology . . . . . . . . . . . . . . .   6
   3.  Connection Setup and Management . . . . . . . . . . . . . . .   8
     3.1.  Draft Version Identification  . . . . . . . . . . . . . .   8
     3.2.  Discovering an HTTP/3 Endpoint  . . . . . . . . . . . . .   8
       3.2.1.  QUIC Version Hints  . . . . . . . . . . . . . . . . .   9
     3.3.  Connection Establishment  . . . . . . . . . . . . . . . .   9
     3.4.  Connection Reuse  . . . . . . . . . . . . . . . . . . . .  10
   4.  HTTP Request Lifecycle  . . . . . . . . . . . . . . . . . . .  10
     4.1.  HTTP Message Exchanges  . . . . . . . . . . . . . . . . .  10
       4.1.1.  Header Formatting and Compression . . . . . . . . . .  12
       4.1.2.  Request Cancellation and Rejection  . . . . . . . . .  13
     4.2.  The CONNECT Method  . . . . . . . . . . . . . . . . . . .  13
     4.3.  Prioritization  . . . . . . . . . . . . . . . . . . . . .  15
       4.3.1.  Placeholders  . . . . . . . . . . . . . . . . . . . .  15
       4.3.2.  Priority Tree Maintenance . . . . . . . . . . . . . .  16
     4.4.  Server Push . . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Connection Closure  . . . . . . . . . . . . . . . . . . . . .  18
     5.1.  Idle Connections  . . . . . . . . . . . . . . . . . . . .  18
     5.2.  Connection Shutdown . . . . . . . . . . . . . . . . . . .  19
     5.3.  Immediate Application Closure . . . . . . . . . . . . . .  20
     5.4.  Transport Closure . . . . . . . . . . . . . . . . . . . .  21
   6.  Stream Mapping and Usage  . . . . . . . . . . . . . . . . . .  21
     6.1.  Bidirectional Streams . . . . . . . . . . . . . . . . . .  21
     6.2.  Unidirectional Streams  . . . . . . . . . . . . . . . . .  22
       6.2.1.  Control Streams . . . . . . . . . . . . . . . . . . .  23
       6.2.2.  Push Streams  . . . . . . . . . . . . . . . . . . . .  23



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       6.2.3.  Reserved Stream Types . . . . . . . . . . . . . . . .  24
   7.  HTTP Framing Layer  . . . . . . . . . . . . . . . . . . . . .  24
     7.1.  Frame Layout  . . . . . . . . . . . . . . . . . . . . . .  25
     7.2.  Frame Definitions . . . . . . . . . . . . . . . . . . . .  26
       7.2.1.  DATA  . . . . . . . . . . . . . . . . . . . . . . . .  26
       7.2.2.  HEADERS . . . . . . . . . . . . . . . . . . . . . . .  27
       7.2.3.  PRIORITY  . . . . . . . . . . . . . . . . . . . . . .  27
       7.2.4.  CANCEL_PUSH . . . . . . . . . . . . . . . . . . . . .  30
       7.2.5.  SETTINGS  . . . . . . . . . . . . . . . . . . . . . .  31
       7.2.6.  PUSH_PROMISE  . . . . . . . . . . . . . . . . . . . .  33
       7.2.7.  GOAWAY  . . . . . . . . . . . . . . . . . . . . . . .  34
       7.2.8.  MAX_PUSH_ID . . . . . . . . . . . . . . . . . . . . .  35
       7.2.9.  DUPLICATE_PUSH  . . . . . . . . . . . . . . . . . . .  36
       7.2.10. Reserved Frame Types  . . . . . . . . . . . . . . . .  37
   8.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  37
     8.1.  HTTP/3 Error Codes  . . . . . . . . . . . . . . . . . . .  37
   9.  Extensions to HTTP/3  . . . . . . . . . . . . . . . . . . . .  39
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  40
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  40
     11.1.  Registration of HTTP/3 Identification String . . . . . .  40
     11.2.  Registration of QUIC Version Hint Alt-Svc Parameter  . .  41
     11.3.  Frame Types  . . . . . . . . . . . . . . . . . . . . . .  41
     11.4.  Settings Parameters  . . . . . . . . . . . . . . . . . .  42
     11.5.  Error Codes  . . . . . . . . . . . . . . . . . . . . . .  43
     11.6.  Stream Types . . . . . . . . . . . . . . . . . . . . . .  46
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  47
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  47
     12.2.  Informative References . . . . . . . . . . . . . . . . .  48
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  48
   Appendix A.  Considerations for Transitioning from HTTP/2 . . . .  49
     A.1.  Streams . . . . . . . . . . . . . . . . . . . . . . . . .  49
     A.2.  HTTP Frame Types  . . . . . . . . . . . . . . . . . . . .  49
     A.3.  HTTP/2 SETTINGS Parameters  . . . . . . . . . . . . . . .  51
     A.4.  HTTP/2 Error Codes  . . . . . . . . . . . . . . . . . . .  52
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  53
     B.1.  Since draft-ietf-quic-http-19 . . . . . . . . . . . . . .  54
     B.2.  Since draft-ietf-quic-http-18 . . . . . . . . . . . . . .  54
     B.3.  Since draft-ietf-quic-http-17 . . . . . . . . . . . . . .  54
     B.4.  Since draft-ietf-quic-http-16 . . . . . . . . . . . . . .  54
     B.5.  Since draft-ietf-quic-http-15 . . . . . . . . . . . . . .  55
     B.6.  Since draft-ietf-quic-http-14 . . . . . . . . . . . . . .  55
     B.7.  Since draft-ietf-quic-http-13 . . . . . . . . . . . . . .  55
     B.8.  Since draft-ietf-quic-http-12 . . . . . . . . . . . . . .  56
     B.9.  Since draft-ietf-quic-http-11 . . . . . . . . . . . . . .  56
     B.10. Since draft-ietf-quic-http-10 . . . . . . . . . . . . . .  56
     B.11. Since draft-ietf-quic-http-09 . . . . . . . . . . . . . .  56
     B.12. Since draft-ietf-quic-http-08 . . . . . . . . . . . . . .  56
     B.13. Since draft-ietf-quic-http-07 . . . . . . . . . . . . . .  56



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     B.14. Since draft-ietf-quic-http-06 . . . . . . . . . . . . . .  57
     B.15. Since draft-ietf-quic-http-05 . . . . . . . . . . . . . .  57
     B.16. Since draft-ietf-quic-http-04 . . . . . . . . . . . . . .  57
     B.17. Since draft-ietf-quic-http-03 . . . . . . . . . . . . . .  57
     B.18. Since draft-ietf-quic-http-02 . . . . . . . . . . . . . .  57
     B.19. Since draft-ietf-quic-http-01 . . . . . . . . . . . . . .  57
     B.20. Since draft-ietf-quic-http-00 . . . . . . . . . . . . . .  58
     B.21. Since draft-shade-quic-http2-mapping-00 . . . . . . . . .  58
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  58
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  59

1.  Introduction

   HTTP semantics are used for a broad range of services on the
   Internet.  These semantics have commonly been used with two different
   TCP mappings, HTTP/1.1 and HTTP/2.  HTTP/3 supports the same
   semantics over a new transport protocol, QUIC.

1.1.  Prior versions of HTTP

   HTTP/1.1 is a TCP mapping which uses whitespace-delimited text fields
   to convey HTTP messages.  While these exchanges are human-readable,
   using whitespace for message formatting leads to parsing difficulties
   and workarounds to be tolerant of variant behavior.  Because each
   connection can transfer only a single HTTP request or response at a
   time in each direction, multiple parallel TCP connections are often
   used, reducing the ability of the congestion controller to accurately
   manage traffic between endpoints.

   HTTP/2 introduced a binary framing and multiplexing layer to improve
   latency without modifying the transport layer.  However, because the
   parallel nature of HTTP/2's multiplexing is not visible to TCP's loss
   recovery mechanisms, a lost or reordered packet causes all active
   transactions to experience a stall regardless of whether that
   transaction was impacted by the lost packet.

1.2.  Delegation to QUIC

   The QUIC transport protocol incorporates stream multiplexing and per-
   stream flow control, similar to that provided by the HTTP/2 framing
   layer.  By providing reliability at the stream level and congestion
   control across the entire connection, it has the capability to
   improve the performance of HTTP compared to a TCP mapping.  QUIC also
   incorporates TLS 1.3 at the transport layer, offering comparable
   security to running TLS over TCP, with the improved connection setup
   latency of TCP Fast Open [RFC7413]}.





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   This document defines a mapping of HTTP semantics over the QUIC
   transport protocol, drawing heavily on the design of HTTP/2.  While
   delegating stream lifetime and flow control issues to QUIC, a similar
   binary framing is used on each stream.  Some HTTP/2 features are
   subsumed by QUIC, while other features are implemented atop QUIC.

   QUIC is described in [QUIC-TRANSPORT].  For a full description of
   HTTP/2, see [HTTP2].

2.  HTTP/3 Protocol Overview

   HTTP/3 provides a transport for HTTP semantics using the QUIC
   transport protocol and an internal framing layer similar to HTTP/2.

   Once a client knows that an HTTP/3 server exists at a certain
   endpoint, it opens a QUIC connection.  QUIC provides protocol
   negotiation, stream-based multiplexing, and flow control.  An HTTP/3
   endpoint can be discovered using HTTP Alternative Services; this
   process is described in greater detail in Section 3.2.

   Within each stream, the basic unit of HTTP/3 communication is a frame
   (Section 7.2).  Each frame type serves a different purpose.  For
   example, HEADERS and DATA frames form the basis of HTTP requests and
   responses (Section 4.1).  Other frame types like SETTINGS, PRIORITY,
   and GOAWAY are used to manage the overall connection and
   relationships between streams.

   Multiplexing of requests is performed using the QUIC stream
   abstraction, described in Section 2 of [QUIC-TRANSPORT].  Each
   request and response consumes a single QUIC stream.  Streams are
   independent of each other, so one stream that is blocked or suffers
   packet loss does not prevent progress on other streams.

   Server push is an interaction mode introduced in HTTP/2 [HTTP2] which
   permits a server to push a request-response exchange to a client in
   anticipation of the client making the indicated request.  This trades
   off network usage against a potential latency gain.  Several HTTP/3
   frames are used to manage server push, such as PUSH_PROMISE,
   DUPLICATE_PUSH, MAX_PUSH_ID, and CANCEL_PUSH.

   As in HTTP/2, request and response headers are compressed for
   transmission.  Because HPACK [HPACK] relies on in-order transmission
   of compressed header blocks (a guarantee not provided by QUIC),
   HTTP/3 replaces HPACK with QPACK [QPACK].  QPACK uses separate
   unidirectional streams to modify and track header table state, while
   header blocks refer to the state of the table without modifying it.





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2.1.  Document Organization

   The HTTP/3 specification is split into seven parts.  The document
   begins with a detailed overview of the connection lifecycle and key
   concepts:

   o  Connection Setup and Management (Section 3) covers how an HTTP/3
      endpoint is discovered and a connection is established.

   o  HTTP Request Lifecycle (Section 4) describes how HTTP semantics
      are expressed using frames.

   o  Connection Closure (Section 5) describes how connections are
      terminated, either gracefully or abruptly.

   The details of the wire protocol and interactions with the transport
   are described in subsequent sections:

   o  Stream Mapping and Usage (Section 6) describes the way QUIC
      streams are used.

   o  HTTP Framing Layer (Section 7) describes the frames used on most
      streams.

   o  Error Handling (Section 8) describes how error conditions are
      handled and expressed, either on a particular stream or for the
      connection as a whole.

   Additional resources are provided in the final sections:

   o  Extensions to HTTP/3 (Section 9) describes how new capabilities
      can be added in future documents.

   o  A more detailed comparison between HTTP/2 and HTTP/3 can be found
      in Appendix A.

2.2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Field definitions are given in Augmented Backus-Naur Form (ABNF), as
   defined in [RFC5234].





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   This document uses the variable-length integer encoding from
   [QUIC-TRANSPORT].

   The following terms are used:

   abort:  An abrupt termination of a connection or stream, possibly due
      to an error condition.

   client:  The endpoint that initiates an HTTP/3 connection.  Clients
      send HTTP requests and receive HTTP responses.

   connection:  A transport-layer connection between two endpoints,
      using QUIC as the transport protocol.

   connection error:  An error that affects the entire HTTP/3
      connection.

   endpoint:  Either the client or server of the connection.

   frame:  The smallest unit of communication on a stream in HTTP/3,
      consisting of a header and a variable-length sequence of octets
      structured according to the frame type.  Protocol elements called
      "frames" exist in both this document and [QUIC-TRANSPORT].  Where
      frames from [QUIC-TRANSPORT] are referenced, the frame name will
      be prefaced with "QUIC."  For example, "QUIC CONNECTION_CLOSE
      frames."  References without this preface refer to frames defined
      in Section 7.2.

   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/3 connection.  Servers
      receive HTTP requests and send HTTP responses.

   stream:  A bidirectional or unidirectional bytestream provided by the
      QUIC transport.

   stream error:  An error on the individual HTTP/3 stream.

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






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   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.

3.  Connection Setup and Management

3.1.  Draft Version Identification

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.

   HTTP/3 uses the token "h3" to identify itself in ALPN and Alt-Svc.
   Only implementations of the final, published RFC can identify
   themselves as "h3".  Until such an RFC exists, implementations MUST
   NOT identify themselves using this string.

   Implementations of draft versions of the protocol MUST add the string
   "-" and the corresponding draft number to the identifier.  For
   example, draft-ietf-quic-http-01 is identified using the string
   "h3-01".

   Non-compatible experiments that are based on these draft versions
   MUST append the string "-" and an experiment name to the identifier.
   For example, an experimental implementation based on draft-ietf-quic-
   http-09 which reserves an extra stream for unsolicited transmission
   of 1980s pop music might identify itself as "h3-09-rickroll".  Note
   that any label MUST conform to the "token" syntax defined in
   Section 3.2.6 of [RFC7230].  Experimenters are encouraged to
   coordinate their experiments on the quic@ietf.org mailing list.

3.2.  Discovering an HTTP/3 Endpoint

   An HTTP origin advertises the availability of an equivalent HTTP/3
   endpoint via the Alt-Svc HTTP response header field or the HTTP/2
   ALTSVC frame ([ALTSVC]), using the ALPN token defined in Section 3.3.

   For example, an origin could indicate in an HTTP response that HTTP/3
   was available on UDP port 50781 at the same hostname by including the
   following header field:

   Alt-Svc: h3=":50781"

   On receipt of an Alt-Svc record indicating HTTP/3 support, a client
   MAY attempt to establish a QUIC connection to the indicated host and
   port and, if successful, send HTTP requests using the mapping
   described in this document.





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   Connectivity problems (e.g. firewall blocking UDP) can result in QUIC
   connection establishment failure, in which case the client SHOULD
   continue using the existing connection or try another alternative
   endpoint offered by the origin.

   Servers MAY serve HTTP/3 on any UDP port, since an alternative always
   includes an explicit port.

3.2.1.  QUIC Version Hints

   This document defines the "quic" parameter for Alt-Svc, which MAY be
   used to provide version-negotiation hints to HTTP/3 clients.  QUIC
   versions are four-byte sequences with no additional constraints on
   format.  Leading zeros SHOULD be omitted for brevity.

   Syntax:

   quic = DQUOTE version-number [ "," version-number ] * DQUOTE
   version-number = 1*8HEXDIG; hex-encoded QUIC version

   Where multiple versions are listed, the order of the values reflects
   the server's preference (with the first value being the most
   preferred version).  Reserved versions MAY be listed, but unreserved
   versions which are not supported by the alternative SHOULD NOT be
   present in the list.  Origins MAY omit supported versions for any
   reason.

   Clients MUST ignore any included versions which they do not support.
   The "quic" parameter MUST NOT occur more than once; clients SHOULD
   process only the first occurrence.

   For example, suppose a server supported both version 0x00000001 and
   the version rendered in ASCII as "Q034".  If it also opted to include
   the reserved version (from Section 15 of [QUIC-TRANSPORT])
   0x1abadaba, it could specify the following header field:

   Alt-Svc: h3=":49288";quic="1,1abadaba,51303334"

   A client acting on this header field would drop the reserved version
   (not supported), then attempt to connect to the alternative using the
   first version in the list which it does support, if any.

3.3.  Connection Establishment

   HTTP/3 relies on QUIC as the underlying transport.  The QUIC version
   being used MUST use TLS version 1.3 or greater as its handshake
   protocol.  HTTP/3 clients MUST indicate the target domain name during




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   the TLS handshake.  This may be done using the Server Name Indication
   (SNI) [RFC6066] extension to TLS or using some other mechanism.

   QUIC connections are established as described in [QUIC-TRANSPORT].
   During connection establishment, HTTP/3 support is indicated by
   selecting the ALPN token "h3" in the TLS handshake.  Support for
   other application-layer protocols MAY be offered in the same
   handshake.

   While connection-level options pertaining to the core QUIC protocol
   are set in the initial crypto handshake, HTTP/3-specific settings are
   conveyed in the SETTINGS frame.  After the QUIC connection is
   established, a SETTINGS frame (Section 7.2.5) MUST be sent by each
   endpoint as the initial frame of their respective HTTP control stream
   (see Section 6.2.1).

3.4.  Connection Reuse

   Once a connection exists to a server endpoint, this connection MAY be
   reused for requests with multiple different URI authority components.
   The client MAY send any requests for which the client considers the
   server authoritative.

   An authoritative HTTP/3 endpoint is typically discovered because the
   client has received an Alt-Svc record from the request's origin which
   nominates the endpoint as a valid HTTP Alternative Service for that
   origin.  As required by [RFC7838], clients MUST check that the
   nominated server can present a valid certificate for the origin
   before considering it authoritative.  Clients MUST NOT assume that an
   HTTP/3 endpoint is authoritative for other origins without an
   explicit signal.

   A server that does not wish clients to reuse connections for a
   particular origin 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 of [HTTP2]).

   The considerations discussed in Section 9.1 of [HTTP2] also apply to
   the management of HTTP/3 connections.

4.  HTTP Request Lifecycle

4.1.  HTTP Message Exchanges

   A client sends an HTTP request on a client-initiated bidirectional
   QUIC stream.  A client MUST send only a single request on a given
   stream.  A server sends zero or more non-final HTTP responses on the




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   same stream as the request, followed by a single final HTTP response,
   as detailed below.

   An HTTP message (request or response) consists of:

   1.  the message header (see [RFC7230], Section 3.2), sent as a single
       HEADERS frame (see Section 7.2.2),

   2.  the payload body (see [RFC7230], Section 3.3), sent as a series
       of DATA frames (see Section 7.2.1),

   3.  optionally, one HEADERS frame containing the trailer-part, if
       present (see [RFC7230], Section 4.1.2).

   A server MAY send one or more PUSH_PROMISE frames (see Section 7.2.6)
   before, after, or interleaved with the frames of a response message.
   These PUSH_PROMISE frames are not part of the response; see
   Section 4.4 for more details.

   The HEADERS and PUSH_PROMISE frames might reference updates to the
   QPACK dynamic table.  While these updates are not directly part of
   the message exchange, they must be received and processed before the
   message can be consumed.  See Section 4.1.1 for more details.

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

   Trailing header fields are carried in an additional HEADERS frame
   following the body.  Senders MUST send only one HEADERS frame in the
   trailers section; receivers MUST discard any subsequent HEADERS
   frames.

   A response MAY consist of multiple messages when and only when one or
   more informational responses (1xx; see [RFC7231], Section 6.2)
   precede a final response to the same request.  Non-final responses do
   not contain a payload body or trailers.

   An HTTP request/response exchange fully consumes a bidirectional QUIC
   stream.  After sending a request, a client MUST close the stream for
   sending.  Unless using the CONNECT method (see Section 4.2), clients
   MUST NOT make stream closure dependent on receiving a response to
   their request.  After sending a final response, the server MUST close
   the stream for sending.  At this point, the QUIC stream is fully
   closed.

   When a stream is closed, this indicates the end of an HTTP message.
   Because some messages are large or unbounded, endpoints SHOULD begin
   processing partial HTTP messages once enough of the message has been



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   received to make progress.  If a client stream terminates without
   enough of the HTTP message to provide a complete response, the server
   SHOULD abort its response with the error code
   HTTP_INCOMPLETE_REQUEST.

   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 triggering a QUIC STOP_SENDING frame with error code
   HTTP_EARLY_RESPONSE, sending a complete response, and cleanly closing
   its stream.  Clients MUST NOT discard complete responses as a result
   of having their request terminated abruptly, though clients can
   always discard responses at their discretion for other reasons.

4.1.1.  Header Formatting and Compression

   HTTP message headers carry information as a series of key-value
   pairs, called header fields.  For a listing of registered HTTP header
   fields, see the "Message Header Field" registry maintained at
   https://www.iana.org/assignments/message-headers [4].

   Just as in previous versions of HTTP, header field names are strings
   of ASCII characters that are compared in a case-insensitive fashion.
   Properties of HTTP header field names and values are discussed in
   more detail in Section 3.2 of [RFC7230], though the wire rendering in
   HTTP/3 differs.  As in HTTP/2, header field names MUST be converted
   to lowercase prior to their encoding.  A request or response
   containing uppercase header field names MUST be treated as malformed.

   As in HTTP/2, HTTP/3 uses special pseudo-header fields beginning with
   the ':' character (ASCII 0x3a) to convey the target URI, the method
   of the request, and the status code for the response.  These pseudo-
   header fields are defined in Section 8.1.2.3 and 8.1.2.4 of [HTTP2].
   Pseudo-header fields are not HTTP header fields.  Endpoints MUST NOT
   generate pseudo-header fields other than those defined in [HTTP2].
   The restrictions on the use of pseudo-header fields in
   Section 8.1.2.1 of [HTTP2] also apply to HTTP/3.

   HTTP/3 uses QPACK header compression as described in [QPACK], a
   variation of HPACK which allows the flexibility to avoid header-
   compression-induced head-of-line blocking.  See that document for
   additional details.

   An HTTP/3 implementation MAY impose a limit on the maximum size of
   the header it will accept on an individual HTTP message; encountering
   a larger message header SHOULD be treated as a stream error of type
   "HTTP_EXCESSIVE_LOAD".  If an implementation wishes to advise its



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   peer of this limit, it can be conveyed as a number of bytes in the
   "SETTINGS_MAX_HEADER_LIST_SIZE" parameter.  The size of a header list
   is calculated based on the uncompressed size of header fields,
   including the length of the name and value in bytes plus an overhead
   of 32 bytes for each header field.

4.1.2.  Request Cancellation and Rejection

   Clients can cancel requests by aborting the stream (QUIC RESET_STREAM
   and/or STOP_SENDING frames, as appropriate) with an error code of
   HTTP_REQUEST_CANCELLED (Section 8.1).  When the client cancels a
   response, it indicates that this response is no longer of interest.
   Implementations SHOULD cancel requests by aborting both directions of
   a stream.

   When the server rejects a request without performing any application
   processing, it SHOULD abort its response stream with the error code
   HTTP_REQUEST_REJECTED.  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.  The client can treat
   requests rejected by the server as though they had never been sent at
   all, thereby allowing them to be retried later on a new connection.
   Servers MUST NOT use the HTTP_REQUEST_REJECTED error code for
   requests which were partially or fully processed.  When a server
   abandons a response after partial processing, it SHOULD abort its
   response stream with the error code HTTP_REQUEST_CANCELLED.

   When a client sends a STOP_SENDING with HTTP_REQUEST_CANCELLED, a
   server MAY send the error code HTTP_REQUEST_REJECTED in the
   corresponding RESET_STREAM if no processing was performed.  Clients
   MUST NOT reset streams with the HTTP_REQUEST_REJECTED error code
   except in response to a QUIC STOP_SENDING frame that contains the
   same code.

   If a stream is cancelled after receiving a complete response, the
   client MAY ignore the cancellation and use the response.  However, if
   a stream is cancelled after receiving a partial response, the
   response SHOULD NOT be used.  Automatically retrying such requests is
   not possible, unless this is otherwise permitted (e.g., idempotent
   actions like GET, PUT, or DELETE).

4.2.  The CONNECT Method

   The pseudo-method CONNECT ([RFC7231], Section 4.3.6) 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/1.x, CONNECT is used to convert an entire HTTP connection into a
   tunnel to a remote host.  In HTTP/2, the CONNECT method is used to



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   establish a tunnel over a single HTTP/2 stream to a remote host for
   similar purposes.

   A CONNECT request in HTTP/3 functions in the same manner as in
   HTTP/2.  The request MUST be formatted as described in [HTTP2],
   Section 8.3.  A CONNECT request that does not conform to these
   restrictions is malformed.  The request stream MUST NOT be closed at
   the end of the request.

   A proxy that supports CONNECT establishes a TCP connection
   ([RFC0793]) 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.

   All DATA frames on the stream correspond to data sent or received on
   the TCP connection.  Any DATA frame sent by the client is transmitted
   by the proxy to the TCP server; data received from the TCP server is
   packaged into DATA frames by the proxy.  Note that the size and
   number of TCP segments is not guaranteed to map predictably to the
   size and number of HTTP DATA or QUIC STREAM frames.

   Once the CONNECT method has completed, only DATA frames are permitted
   to be sent on the stream.  Extension frames MAY be used if
   specifically permitted by the definition of the extension.  Receipt
   of any other frame type MUST be treated as a connection error of type
   HTTP_UNEXPECTED_FRAME.

   The TCP connection can be closed by either peer.  When the client
   ends the request stream (that is, the receive stream at the proxy
   enters the "Data Recvd" state), the proxy will set the FIN bit on its
   connection to the TCP server.  When the proxy receives a packet with
   the FIN bit set, it will terminate the send stream that it sends to
   the client.  TCP connections which remain half-closed in a single
   direction are not invalid, but are often handled poorly by servers,
   so clients SHOULD NOT close a stream for sending while they still
   expect to receive data from the target of the CONNECT.

   A TCP connection error is signaled with QUIC RESET_STREAM frame.  A
   proxy treats any error in the TCP connection, which includes
   receiving a TCP segment with the RST bit set, as a stream error of
   type HTTP_CONNECT_ERROR (Section 8.1).  Correspondingly, if a proxy
   detects an error with the stream or the QUIC connection, it MUST
   close the TCP connection.  If the underlying TCP implementation
   permits it, the proxy SHOULD send a TCP segment with the RST bit set.






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4.3.  Prioritization

   HTTP/3 uses a priority scheme similar to that described in [HTTP2],
   Section 5.3.  In this priority scheme, a given element can be
   designated as dependent upon another element.  This information is
   expressed in the PRIORITY frame Section 7.2.3 which identifies the
   element and the dependency.  The elements that can be prioritized
   are:

   o  Requests, identified by the ID of the request stream

   o  Pushes, identified by the Push ID of the promised resource
      (Section 7.2.6)

   o  Placeholders, identified by a Placeholder ID

   Taken together, the dependencies across all prioritized elements in a
   connection form a dependency tree.  An element can depend on another
   element or on the root of the tree.  A reference to an element which
   is no longer in the tree is treated as a reference to the root of the
   tree.  The structure of the dependency tree changes as PRIORITY
   frames modify the dependency links between prioritized elements.

   Due to reordering between streams, an element can also be prioritized
   which is not yet in the tree.  Such elements are added to the tree
   with the requested priority.

   When a prioritized element is first created, it has a default initial
   weight of 16 and a default dependency.  Requests and placeholders are
   dependent on the root of the priority tree; pushes are dependent on
   the client request on which the PUSH_PROMISE frame was sent.

   Requests may override the default initial values by including a
   PRIORITY frame (see Section 7.2.3) at the beginning of the stream.
   These priorities can be updated by sending a PRIORITY frame on the
   control stream.

4.3.1.  Placeholders

   In HTTP/2, certain implementations used closed or unused streams as
   placeholders in describing the relative priority of requests.  This
   created confusion as servers could not reliably identify which
   elements of the priority tree could be discarded safely.  Clients
   could potentially reference closed streams long after the server had
   discarded state, leading to disparate views of the prioritization the
   client had attempted to express.





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   In HTTP/3, a number of placeholders are explicitly permitted by the
   server using the "SETTINGS_NUM_PLACEHOLDERS" setting.  Because the
   server commits to maintaining these placeholders in the
   prioritization tree, clients can use them with confidence that the
   server will not have discarded the state.  Clients MUST NOT send the
   "SETTINGS_NUM_PLACEHOLDERS" setting; receipt of this setting by a
   server MUST be treated as a connection error of type
   "HTTP_WRONG_SETTING_DIRECTION".

   Placeholders are identified by an ID between zero and one less than
   the number of placeholders the server has permitted.

   Like streams, placeholders have priority information associated with
   them.

4.3.2.  Priority Tree Maintenance

   Because placeholders will be used to "root" any persistent structure
   of the tree which the client cares about retaining, servers can
   aggressively prune inactive regions from the priority tree.  For
   prioritization purposes, a node in the tree is considered "inactive"
   when the corresponding stream has been closed for at least two round-
   trip times (using any reasonable estimate available on the server).
   This delay helps mitigate race conditions where the server has pruned
   a node the client believed was still active and used as a Stream
   Dependency.

   Specifically, the server MAY at any time:

   o  Identify and discard branches of the tree containing only inactive
      nodes (i.e. a node with only other inactive nodes as descendants,
      along with those descendants)

   o  Identify and condense interior regions of the tree containing only
      inactive nodes, allocating weight appropriately

       x                x                 x
       |                |                 |
       P                P                 P
      / \               |                 |
     I   I     ==>      I      ==>        A
        / \             |                 |
       A   I            A                 A
       |                |
       A                A

                Figure 1: Example of Priority Tree Pruning




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   In the example in Figure 1, "P" represents a Placeholder, "A"
   represents an active node, and "I" represents an inactive node.  In
   the first step, the server discards two inactive branches (each a
   single node).  In the second step, the server condenses an interior
   inactive node.  Note that these transformations will result in no
   change in the resources allocated to a particular active stream.

   Clients SHOULD assume the server is actively performing such pruning
   and SHOULD NOT declare a dependency on a stream it knows to have been
   closed.

4.4.  Server Push

   Server push is an interaction mode introduced in HTTP/2 [HTTP2] which
   permits a server to push a request-response exchange to a client in
   anticipation of the client making the indicated request.  This trades
   off network usage against a potential latency gain.  HTTP/3 server
   push is similar to what is described in HTTP/2 [HTTP2], but uses
   different mechanisms.

   Each server push is identified by a unique Push ID.  This Push ID is
   used in a single PUSH_PROMISE frame (see Section 7.2.6) which carries
   the request headers, possibly included in one or more DUPLICATE_PUSH
   frames (see Section 7.2.9), then included with the push stream which
   ultimately fulfills those promises.

   Server push is only enabled on a connection when a client sends a
   MAX_PUSH_ID frame (see Section 7.2.8).  A server cannot use server
   push until it receives a MAX_PUSH_ID frame.  A client sends
   additional MAX_PUSH_ID frames to control the number of pushes that a
   server can promise.  A server SHOULD use Push IDs sequentially,
   starting at 0.  A client MUST treat receipt of a push stream with a
   Push ID that is greater than the maximum Push ID as a connection
   error of type HTTP_LIMIT_EXCEEDED.

   The header of the request message is carried by a PUSH_PROMISE frame
   (see Section 7.2.6) on the request stream which generated the push.
   This allows the server push to be associated with a client request.
   Promised requests MUST conform to the requirements in Section 8.2 of
   [HTTP2].

   The same server push can be associated with additional client
   requests using a DUPLICATE_PUSH frame (see Section 7.2.9).

   Ordering of a PUSH_PROMISE or DUPLICATE_PUSH in relation to certain
   parts of the response is important.  The server SHOULD send
   PUSH_PROMISE or DUPLICATE_PUSH frames prior to sending HEADERS or
   DATA frames that reference the promised responses.  This reduces the



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   chance that a client requests a resource that will be pushed by the
   server.

   When a server later fulfills a promise, the server push response is
   conveyed on a push stream (see Section 6.2.2).  The push stream
   identifies the Push ID of the promise that it fulfills, then contains
   a response to the promised request using the same format described
   for responses in Section 4.1.

   Due to reordering, DUPLICATE_PUSH frames or push stream data can
   arrive before the corresponding PUSH_PROMISE frame.  When a client
   receives a DUPLICATE_PUSH frame for an as-yet-unknown Push ID, the
   request headers of the push are not immediately available.  The
   client can either delay generating new requests for content
   referenced following the DUPLICATE_PUSH frame until the request
   headers become available, or can initiate requests for discovered
   resources and cancel the requests if the requested resource is
   already being pushed.  When a client receives a new push stream with
   an as-yet-unknown Push ID, both the associated client request and the
   pushed request headers are unknown.  The client can buffer the stream
   data in expectation of the matching PUSH_PROMISE.  The client can use
   stream flow control (see section 4.1 of [QUIC-TRANSPORT]) to limit
   the amount of data a server may commit to the pushed stream.

   If a promised server push is not needed by the client, the client
   SHOULD send a CANCEL_PUSH frame.  If the push stream is already open
   or opens after sending the CANCEL_PUSH frame, a QUIC STOP_SENDING
   frame with an appropriate error code can also be used (e.g.,
   HTTP_PUSH_REFUSED, HTTP_PUSH_ALREADY_IN_CACHE; see Section 8).  This
   asks the server not to transfer additional data and indicates that it
   will be discarded upon receipt.

5.  Connection Closure

   Once established, an HTTP/3 connection can be used for many requests
   and responses over time until the connection is closed.  Connection
   closure can happen in any of several different ways.

5.1.  Idle Connections

   Each QUIC endpoint declares an idle timeout during the handshake.  If
   the connection remains idle (no packets received) for longer than
   this duration, the peer will assume that the connection has been
   closed.  HTTP/3 implementations will need to open a new connection
   for new requests if the existing connection has been idle for longer
   than the server's advertised idle timeout, and SHOULD do so if
   approaching the idle timeout.




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   HTTP clients are expected to request that the transport keep
   connections open while there are responses outstanding for requests
   or server pushes, as described in Section 19.2 of [QUIC-TRANSPORT].
   If the client is not expecting a response from the server, allowing
   an idle connection to time out is preferred over expending effort
   maintaining a connection that might not be needed.  A gateway MAY
   maintain connections in anticipation of need rather than incur the
   latency cost of connection establishment to servers.  Servers SHOULD
   NOT actively keep connections open.

5.2.  Connection Shutdown

   Even when a connection is not idle, either endpoint can decide to
   stop using the connection and let the connection close gracefully.
   Since clients drive request generation, clients perform a connection
   shutdown by not sending additional requests on the connection;
   responses and pushed responses associated to previous requests will
   continue to completion.  Servers perform the same function by
   communicating with clients.

   Servers initiate the shutdown of a connection by sending a GOAWAY
   frame (Section 7.2.7).  The GOAWAY frame indicates that client-
   initiated requests on lower stream IDs were or might be processed in
   this connection, while requests on the indicated stream ID and
   greater were rejected.  This enables client and server to agree on
   which requests were accepted prior to the connection shutdown.  This
   identifier MAY be zero if no requests were processed.  Servers SHOULD
   NOT increase the QUIC MAX_STREAMS limit after sending a GOAWAY frame.

   Clients MUST NOT send new requests on the connection after receiving
   GOAWAY; a new connection MAY be established to send additional
   requests.

   Some requests might already be in transit.  If the client has already
   sent requests on streams with a Stream ID greater than or equal to
   that indicated in the GOAWAY frame, those requests will not be
   processed and MAY be retried by the client on a different connection.
   The client MAY cancel these requests.  It is RECOMMENDED that the
   server explicitly reject such requests (see Section 4.1.2) in order
   to clean up transport state for the affected streams.

   Requests on Stream IDs less than the Stream ID in the GOAWAY frame
   might have been processed; their status cannot be known until a
   response is received, the stream is reset individually, or the
   connection terminates.  Servers MAY reject individual requests on
   streams below the indicated ID if these requests were not processed.





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   Servers SHOULD send a GOAWAY frame when the closing of a connection
   is known in advance, even if the advance notice is small, so that the
   remote peer can know whether a request has been partially processed
   or not.  For example, if an HTTP client sends a POST at the same time
   that a server closes a QUIC 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.

   A client that is unable to retry requests loses all requests that are
   in flight when the server closes the connection.  A server MAY send
   multiple GOAWAY frames indicating different stream IDs, but MUST NOT
   increase the value they send in the last Stream ID, since clients
   might already have retried unprocessed requests on another
   connection.  A server that is attempting to gracefully shut down a
   connection SHOULD send an initial GOAWAY frame with the last Stream
   ID set to the maximum value allowed by QUIC's MAX_STREAMS and SHOULD
   NOT increase the MAX_STREAMS limit thereafter.  This signals to the
   client that a shutdown is imminent and that initiating further
   requests is prohibited.  After allowing time for any in-flight
   requests (at least one round-trip time), the server MAY send another
   GOAWAY frame with an updated last Stream ID.  This ensures that a
   connection can be cleanly shut down without losing requests.

   Once all accepted requests have been processed, the server can permit
   the connection to become idle, or MAY initiate an immediate closure
   of the connection.  An endpoint that completes a graceful shutdown
   SHOULD use the HTTP_NO_ERROR code when closing the connection.

   If a client has consumed all available bidirectional stream IDs with
   requests, the server need not send a GOAWAY frame, since the client
   is unable to make further requests.

5.3.  Immediate Application Closure

   An HTTP/3 implementation can immediately close the QUIC connection at
   any time.  This results in sending a QUIC CONNECTION_CLOSE frame to
   the peer; the error code in this frame indicates to the peer why the
   connection is being closed.  See Section 8 for error codes which can
   be used when closing a connection.

   Before closing the connection, a GOAWAY MAY be sent to allow the
   client to retry some requests.  Including the GOAWAY frame in the
   same packet as the QUIC CONNECTION_CLOSE frame improves the chances
   of the frame being received by clients.







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5.4.  Transport Closure

   For various reasons, the QUIC transport could indicate to the
   application layer that the connection has terminated.  This might be
   due to an explicit closure by the peer, a transport-level error, or a
   change in network topology which interrupts connectivity.

   If a connection terminates without a GOAWAY frame, clients MUST
   assume that any request which was sent, whether in whole or in part,
   might have been processed.

6.  Stream Mapping and Usage

   A QUIC stream provides reliable in-order delivery of bytes, but makes
   no guarantees about order of delivery with regard to bytes on other
   streams.  On the wire, data is framed into QUIC STREAM frames, but
   this framing is invisible to the HTTP framing layer.  The transport
   layer buffers and orders received QUIC STREAM frames, exposing the
   data contained within as a reliable byte stream to the application.
   Although QUIC permits out-of-order delivery within a stream HTTP/3
   does not make use of this feature.

   QUIC streams can be either unidirectional, carrying data only from
   initiator to receiver, or bidirectional.  Streams can be initiated by
   either the client or the server.  For more detail on QUIC streams,
   see Section 2 of [QUIC-TRANSPORT].

   When HTTP headers and data are sent over QUIC, the QUIC layer handles
   most of the stream management.  HTTP does not need to do any separate
   multiplexing when using QUIC - data sent over a QUIC stream always
   maps to a particular HTTP transaction or connection context.

6.1.  Bidirectional Streams

   All client-initiated bidirectional streams are used for HTTP requests
   and responses.  A bidirectional stream ensures that the response can
   be readily correlated with the request.  This means that the client's
   first request occurs on QUIC stream 0, with subsequent requests on
   stream 4, 8, and so on.  In order to permit these streams to open, an
   HTTP/3 client SHOULD send non-zero values for the QUIC transport
   parameters "initial_max_stream_data_bidi_local".  An HTTP/3 server
   SHOULD send non-zero values for the QUIC transport parameters
   "initial_max_stream_data_bidi_remote" and "initial_max_bidi_streams".
   It is recommended that "initial_max_bidi_streams" be no smaller than
   100, so as to not unnecessarily limit parallelism.

   These streams carry frames related to the request/response (see
   Section 4.1).  When a stream terminates cleanly, if the last frame on



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   the stream was truncated, this MUST be treated as a connection error
   (see HTTP_MALFORMED_FRAME in Section 8.1).  Streams which terminate
   abruptly may be reset at any point in the frame.

   HTTP/3 does not use server-initiated bidirectional streams; clients
   MUST omit or specify a value of zero for the QUIC transport parameter
   "initial_max_bidi_streams".

6.2.  Unidirectional Streams

   Unidirectional streams, in either direction, are used for a range of
   purposes.  The purpose is indicated by a stream type, which is sent
   as a variable-length integer at the start of the stream.  The format
   and structure of data that follows this integer is determined by the
   stream type.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream Type (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: Unidirectional Stream Header

   Some stream types are reserved (Section 6.2.3).  Two stream types are
   defined in this document: control streams (Section 6.2.1) and push
   streams (Section 6.2.2).  Other stream types can be defined by
   extensions to HTTP/3; see Section 9 for more details.

   The performance of HTTP/3 connections in the early phase of their
   lifetime is sensitive to the creation and exchange of data on
   unidirectional streams.  Endpoints that set low values for the QUIC
   transport parameters "initial_max_uni_streams" and
   "initial_max_stream_data_uni" will increase the chance that the
   remote peer reaches the limit early and becomes blocked.  In
   particular, the value chosen for "initial_max_uni_streams" should
   consider that remote peers may wish to exercise reserved stream
   behavior (Section 6.2.3).  To reduce the likelihood of blocking, both
   clients and servers SHOULD send a value of three or greater for the
   QUIC transport parameter "initial_max_uni_streams", and a value of
   1,024 or greater for the QUIC transport parameter
   "initial_max_stream_data_uni".

   If the stream header indicates a stream type which is not supported
   by the recipient, the remainder of the stream cannot be consumed as
   the semantics are unknown.  Recipients of unknown stream types MAY
   trigger a QUIC STOP_SENDING frame with an error code of




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   HTTP_UNKNOWN_STREAM_TYPE, but MUST NOT consider such streams to be a
   connection error of any kind.

   Implementations MAY send stream types before knowing whether the peer
   supports them.  However, stream types which could modify the state or
   semantics of existing protocol components, including QPACK or other
   extensions, MUST NOT be sent until the peer is known to support them.

   A sender can close or reset a unidirectional stream unless otherwise
   specified.  A receiver MUST tolerate unidirectional streams being
   closed or reset prior to the reception of the unidirectional stream
   header.

6.2.1.  Control Streams

   A control stream is indicated by a stream type of "0x00".  Data on
   this stream consists of HTTP/3 frames, as defined in Section 7.2.

   Each side MUST initiate a single control stream at the beginning of
   the connection and send its SETTINGS frame as the first frame on this
   stream.  If the first frame of the control stream is any other frame
   type, this MUST be treated as a connection error of type
   HTTP_MISSING_SETTINGS.  Only one control stream per peer is
   permitted; receipt of a second stream which claims to be a control
   stream MUST be treated as a connection error of type
   HTTP_WRONG_STREAM_COUNT.  The sender MUST NOT close the control
   stream, and the receiver MUST NOT request that the sender close the
   control stream.  If either control stream is closed at any point,
   this MUST be treated as a connection error of type
   HTTP_CLOSED_CRITICAL_STREAM.

   A pair of unidirectional streams is used rather than a single
   bidirectional stream.  This allows either peer to send data as soon
   they are able.  Depending on whether 0-RTT is enabled on the
   connection, either client or server might be able to send stream data
   first after the cryptographic handshake completes.

6.2.2.  Push Streams

   Server push is an optional feature introduced in HTTP/2 that allows a
   server to initiate a response before a request has been made.  See
   Section 4.4 for more details.

   A push stream is indicated by a stream type of "0x01", followed by
   the Push ID of the promise that it fulfills, encoded as a variable-
   length integer.  The remaining data on this stream consists of HTTP/3
   frames, as defined in Section 7.2, and fulfills a promised server
   push.  Server push and Push IDs are described in Section 4.4.



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   Only servers can push; if a server receives a client-initiated push
   stream, this MUST be treated as a connection error of type
   HTTP_WRONG_STREAM_DIRECTION.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           0x01 (i)                          ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Push ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: Push Stream Header

   Each Push ID MUST only be used once in a push stream header.  If a
   push stream header includes a Push ID that was used in another push
   stream header, the client MUST treat this as a connection error of
   type HTTP_DUPLICATE_PUSH.

6.2.3.  Reserved Stream Types

   Stream types of the format "0x1f * N + 0x21" for integer values of N
   are reserved to exercise the requirement that unknown types be
   ignored.  These streams have no semantics, and can be sent when
   application-layer padding is desired.  They MAY also be sent on
   connections where no data is currently being transferred.  Endpoints
   MUST NOT consider these streams to have any meaning upon receipt.

   The payload and length of the stream are selected in any manner the
   implementation chooses.

7.  HTTP Framing Layer

   HTTP frames are carried on QUIC streams, as described in Section 6.
   HTTP/3 defines three stream types: control stream, request stream,
   and push stream.  This section describes HTTP/3 frame formats and the
   streams types on which they are permitted; see Table 1 for an
   overview.  A comparison between HTTP/2 and HTTP/3 frames is provided
   in Appendix A.2.












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   +----------------+------------+------------+-----------+------------+
   | Frame          | Control    | Request    | Push      | Section    |
   |                | Stream     | Stream     | Stream    |            |
   +----------------+------------+------------+-----------+------------+
   | DATA           | No         | Yes        | Yes       | Section 7. |
   |                |            |            |           | 2.1        |
   |                |            |            |           |            |
   | HEADERS        | No         | Yes        | Yes       | Section 7. |
   |                |            |            |           | 2.2        |
   |                |            |            |           |            |
   | PRIORITY       | Yes        | Yes (1)    | No        | Section 7. |
   |                |            |            |           | 2.3        |
   |                |            |            |           |            |
   | CANCEL_PUSH    | Yes        | No         | No        | Section 7. |
   |                |            |            |           | 2.4        |
   |                |            |            |           |            |
   | SETTINGS       | Yes (1)    | No         | No        | Section 7. |
   |                |            |            |           | 2.5        |
   |                |            |            |           |            |
   | PUSH_PROMISE   | No         | Yes        | No        | Section 7. |
   |                |            |            |           | 2.6        |
   |                |            |            |           |            |
   | GOAWAY         | Yes        | No         | No        | Section 7. |
   |                |            |            |           | 2.7        |
   |                |            |            |           |            |
   | MAX_PUSH_ID    | Yes        | No         | No        | Section 7. |
   |                |            |            |           | 2.8        |
   |                |            |            |           |            |
   | DUPLICATE_PUSH | No         | Yes        | No        | Section 7. |
   |                |            |            |           | 2.9        |
   +----------------+------------+------------+-----------+------------+

              Table 1: HTTP/3 frames and stream type overview

   Certain frames can only occur as the first frame of a particular
   stream type; these are indicated in Table 1 with a (1).  Specific
   guidance is provided in the relevant section.

   Note that, unlike QUIC frames, HTTP/3 frames can span multiple
   packets.

7.1.  Frame Layout

   All frames have the following format:







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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Type (i)                          ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Length (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Frame Payload (*)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 4: HTTP/3 frame format

   A frame includes the following fields:

   Type:  A variable-length integer that identifies the frame type.

   Length:  A variable-length integer that describes the length of the
      Frame Payload.

   Frame Payload:  A payload, the semantics of which are determined by
      the Type field.

   Each frame's payload MUST contain exactly the fields identified in
   its description.  A frame payload that contains additional bytes
   after the identified fields or a frame payload that terminates before
   the end of the identified fields MUST be treated as a connection
   error of type HTTP_MALFORMED_FRAME.

7.2.  Frame Definitions

7.2.1.  DATA

   DATA frames (type=0x0) convey arbitrary, variable-length sequences of
   bytes associated with an HTTP request or response payload.

   DATA frames MUST be associated with an HTTP request or response.  If
   a DATA frame is received on either control stream, the recipient MUST
   respond with a connection error (Section 8) of type
   HTTP_WRONG_STREAM.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Payload (*)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 5: DATA frame payload




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7.2.2.  HEADERS

   The HEADERS frame (type=0x1) is used to carry a header block,
   compressed using QPACK.  See [QPACK] for more details.

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

                      Figure 6: HEADERS frame payload

   HEADERS frames can only be sent on request / push streams.

7.2.3.  PRIORITY

   The PRIORITY (type=0x2) frame specifies the client-advised priority
   of a request, server push or placeholder.

   A PRIORITY frame identifies an element to prioritize, and an element
   upon which it depends.  A Prioritized ID or Dependency ID identifies
   a client-initiated request using the corresponding stream ID, a
   server push using a Push ID (see Section 7.2.6), or a placeholder
   using a Placeholder ID (see Section 4.3.1).

   When a client initiates a request, a PRIORITY frame MAY be sent as
   the first frame of the stream, creating a dependency on an existing
   element.  In order to ensure that prioritization is processed in a
   consistent order, any subsequent PRIORITY frames for that request
   MUST be sent on the control stream.  A PRIORITY frame received after
   other frames on a request stream MUST be treated as a connection
   error of type HTTP_UNEXPECTED_FRAME.

   If, by the time a new request stream is opened, its priority
   information has already been received via the control stream, the
   PRIORITY frame sent on the request stream MUST be ignored.














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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PT |DT | Empty |         [Prioritized Element ID (i)]        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                [Element Dependency ID (i)]                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Weight (8)  |
   +-+-+-+-+-+-+-+-+

                     Figure 7: PRIORITY frame payload

   The PRIORITY frame payload has the following fields:

   PT (Prioritized Element Type):  A two-bit field indicating the type
      of element being prioritized (see Table 2).  When sent on a
      request stream, this MUST be set to "11".  When sent on the
      control stream, this MUST NOT be set to "11".

   DT (Element Dependency Type):  A two-bit field indicating the type of
      element being depended on (see Table 3).

   Empty:  A four-bit field which MUST be zero when sent and has no
      semantic value on receipt.

   Prioritized Element ID:  A variable-length integer that identifies
      the element being prioritized.  Depending on the value of
      Prioritized Type, this contains the Stream ID of a request stream,
      the Push ID of a promised resource, a Placeholder ID of a
      placeholder, or is absent.

   Element Dependency ID:  A variable-length integer that identifies the
      element on which a dependency is being expressed.  Depending on
      the value of Dependency Type, this contains the Stream ID of a
      request stream, the Push ID of a promised resource, the
      Placeholder ID of a placeholder, or is absent.  For details of
      dependencies, see Section 4.3 and [HTTP2], Section 5.3.

   Weight:  An unsigned 8-bit integer representing a priority weight for
      the prioritized element (see [HTTP2], Section 5.3).  Add one to
      the value to obtain a weight between 1 and 256.

   The values for the Prioritized Element Type (Table 2) and Element
   Dependency Type (Table 3) imply the interpretation of the associated
   Element ID fields.






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     +---------+------------------+---------------------------------+
     | PT Bits | Type Description | Prioritized Element ID Contents |
     +---------+------------------+---------------------------------+
     | 00      | Request stream   | Stream ID                       |
     |         |                  |                                 |
     | 01      | Push stream      | Push ID                         |
     |         |                  |                                 |
     | 10      | Placeholder      | Placeholder ID                  |
     |         |                  |                                 |
     | 11      | Current stream   | Absent                          |
     +---------+------------------+---------------------------------+

                    Table 2: Prioritized Element Types

      +---------+------------------+--------------------------------+
      | DT Bits | Type Description | Element Dependency ID Contents |
      +---------+------------------+--------------------------------+
      | 00      | Request stream   | Stream ID                      |
      |         |                  |                                |
      | 01      | Push stream      | Push ID                        |
      |         |                  |                                |
      | 10      | Placeholder      | Placeholder ID                 |
      |         |                  |                                |
      | 11      | Root of the tree | Absent                         |
      +---------+------------------+--------------------------------+

                     Table 3: Element Dependency Types

   Note that unlike in [HTTP2], the root of the tree cannot be
   referenced using a Stream ID of 0, as in QUIC stream 0 carries a
   valid HTTP request.  The root of the tree cannot be reprioritized.

   The PRIORITY frame can express relationships which might not be
   permitted based on the stream on which it is sent or its position in
   the stream.  These situations MUST be treated as a connection error
   of type HTTP_MALFORMED_FRAME.  The following situations are examples
   of invalid PRIORITY frames:

   o  A PRIORITY frame sent on a request stream with the Prioritized
      Element Type set to any value other than "11"

   o  A PRIORITY frame sent on a request stream which expresses a
      dependency on a request with a greater Stream ID than the current
      stream

   o  A PRIORITY frame sent on a control stream with the Prioritized
      Element Type set to "11"




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   o  A PRIORITY frame which claims to reference a request, but the
      associated ID does not identify a client-initiated bidirectional
      stream

   A PRIORITY frame with Empty bits not set to zero MAY be treated as a
   connection error of type HTTP_MALFORMED_FRAME.

   A PRIORITY frame that references a non-existent Push ID, a
   Placeholder ID greater than the server's limit, or a Stream ID the
   client is not yet permitted to open MUST be treated as a connection
   error of type HTTP_LIMIT_EXCEEDED.

   A PRIORITY frame received on any stream other than a request or
   control stream MUST be treated as a connection error of type
   HTTP_WRONG_STREAM.

   PRIORITY frames received by a client MUST be treated as a connection
   error of type HTTP_UNEXPECTED_FRAME.

7.2.4.  CANCEL_PUSH

   The CANCEL_PUSH frame (type=0x3) is used to request cancellation of a
   server push prior to the push stream being received.  The CANCEL_PUSH
   frame identifies a server push by Push ID (see Section 7.2.6),
   encoded as a variable-length integer.

   When a server receives this frame, it aborts sending the response for
   the identified server push.  If the server has not yet started to
   send the server push, it can use the receipt of a CANCEL_PUSH frame
   to avoid opening a push stream.  If the push stream has been opened
   by the server, the server SHOULD send a QUIC RESET_STREAM frame on
   that stream and cease transmission of the response.

   A server can send the CANCEL_PUSH frame to indicate that it will not
   be fulfilling a promise prior to creation of a push stream.  Once the
   push stream has been created, sending CANCEL_PUSH has no effect on
   the state of the push stream.  A QUIC RESET_STREAM frame SHOULD be
   used instead to abort transmission of the server push response.

   A CANCEL_PUSH frame is sent on the control stream.  Receiving a
   CANCEL_PUSH frame on a stream other than the control stream MUST be
   treated as a connection error of type HTTP_WRONG_STREAM.









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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Push ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 8: CANCEL_PUSH frame payload

   The CANCEL_PUSH frame carries a Push ID encoded as a variable-length
   integer.  The Push ID identifies the server push that is being
   cancelled (see Section 7.2.6).

   If the client receives a CANCEL_PUSH frame, that frame might identify
   a Push ID that has not yet been mentioned by a PUSH_PROMISE frame.

7.2.5.  SETTINGS

   The SETTINGS frame (type=0x4) conveys configuration parameters that
   affect how endpoints communicate, such as preferences and constraints
   on peer behavior.  Individually, a SETTINGS parameter can also be
   referred to as a "setting"; the identifier and value of each setting
   parameter can be referred to as a "setting identifier" and a "setting
   value".

   SETTINGS frames always apply to a connection, never a single stream.
   A SETTINGS frame MUST be sent as the first frame of each control
   stream (see Section 6.2.1) by each peer, and MUST NOT be sent
   subsequently.  If an endpoint receives a second SETTINGS frame on the
   control stream, the endpoint MUST respond with a connection error of
   type HTTP_UNEXPECTED_FRAME.

   SETTINGS frames MUST NOT be sent on any stream other than the control
   stream.  If an endpoint receives a SETTINGS frame on a different
   stream, the endpoint MUST respond with a connection error of type
   HTTP_WRONG_STREAM.

   SETTINGS parameters are not negotiated; they describe characteristics
   of the sending peer, which can be used by the receiving peer.
   However, a negotiation can be implied by the use of SETTINGS - each
   peer uses SETTINGS to advertise a set of supported values.  The
   definition of the setting would describe how each peer combines the
   two sets to conclude which choice will be used.  SETTINGS does not
   provide a mechanism to identify when the choice takes effect.

   Different values for the same parameter can be advertised by each
   peer.  For example, a client might be willing to consume a very large
   response header, while servers are more cautious about request size.




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   Parameters MUST NOT occur more than once in the SETTINGS frame.  A
   receiver MAY treat the presence of the same parameter more than once
   as a connection error of type HTTP_MALFORMED_FRAME.

   The payload of a SETTINGS frame consists of zero or more parameters.
   Each parameter consists of a setting identifier and a value, both
   encoded as QUIC variable-length integers.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Identifier (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Value (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 9: SETTINGS parameter format

   An implementation MUST ignore the contents for any SETTINGS
   identifier it does not understand.

7.2.5.1.  Defined SETTINGS Parameters

   The following settings are defined in HTTP/3:

   SETTINGS_MAX_HEADER_LIST_SIZE (0x6):  The default value is unlimited.
      See Section 4.1.1 for usage.

   SETTINGS_NUM_PLACEHOLDERS (0x9):  The default value is 0.  However,
      this value SHOULD be set to a non-zero value by servers.  See
      Section 4.3.1 for usage.

   Setting identifiers of the format "0x1f * N + 0x21" for integer
   values of N are reserved to exercise the requirement that unknown
   identifiers be ignored.  Such settings have no defined meaning.
   Endpoints SHOULD include at least one such setting in their SETTINGS
   frame.  Endpoints MUST NOT consider such settings to have any meaning
   upon receipt.

   Because the setting has no defined meaning, the value of the setting
   can be any value the implementation selects.

   Additional settings can be defined by extensions to HTTP/3; see
   Section 9 for more details.







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7.2.5.2.  Initialization

   An HTTP implementation MUST NOT send frames or requests which would
   be invalid based on its current understanding of the peer's settings.
   All settings begin at an initial value, and are updated upon receipt
   of a SETTINGS frame.  For servers, the initial value of each client
   setting is the default value.

   For clients using a 1-RTT QUIC connection, the initial value of each
   server setting is the default value.  When a 0-RTT QUIC connection is
   being used, the initial value of each server setting is the value
   used in the previous session.  Clients MUST store the settings the
   server provided in the session being resumed and MUST comply with
   stored settings until the current server settings are received.  A
   client uses these initial values in a situation where it wants to
   send requests before the server's SETTINGS frame has arrived.  This
   removes the need for a client to wait for the SETTINGS frame before
   sending requests.

   A server can remember the settings that it advertised, or store an
   integrity-protected copy of the values in the ticket and recover the
   information when accepting 0-RTT data.  A server uses the HTTP/3
   settings values in determining whether to accept 0-RTT data.

   A server MAY accept 0-RTT and subsequently provide different settings
   in its SETTINGS frame.  If 0-RTT data is accepted by the server, its
   SETTINGS frame MUST NOT reduce any limits or alter any values that
   might be violated by the client with its 0-RTT data.  The server MAY
   omit settings from its SETTINGS frame which are unchanged from the
   initial value.

7.2.6.  PUSH_PROMISE

   The PUSH_PROMISE frame (type=0x5) is used to carry a promised request
   header set from server to client on a request stream, as in HTTP/2.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Push ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Header Block (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 10: PUSH_PROMISE frame payload

   The payload consists of:




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   Push ID:  A variable-length integer that identifies the server push
      operation.  A Push ID is used in push stream headers
      (Section 4.4), CANCEL_PUSH frames (Section 7.2.4), DUPLICATE_PUSH
      frames (Section 7.2.9), and PRIORITY frames (Section 7.2.3).

   Header Block:  QPACK-compressed request header fields for the
      promised response.  See [QPACK] for more details.

   A server MUST NOT use a Push ID that is larger than the client has
   provided in a MAX_PUSH_ID frame (Section 7.2.8) and MUST NOT use the
   same Push ID in multiple PUSH_PROMISE frames.  A client MUST treat
   receipt of a PUSH_PROMISE that contains a larger Push ID than the
   client has advertised or a Push ID which has already been promised as
   a connection error of type HTTP_MALFORMED_FRAME.

   If a PUSH_PROMISE frame is received on the control stream, the client
   MUST respond with a connection error (Section 8) of type
   HTTP_WRONG_STREAM.

   A client MUST NOT send a PUSH_PROMISE frame.  A server MUST treat the
   receipt of a PUSH_PROMISE frame as a connection error of type
   HTTP_UNEXPECTED_FRAME.

   See Section 4.4 for a description of the overall server push
   mechanism.

7.2.7.  GOAWAY

   The GOAWAY frame (type=0x7) is used to initiate graceful shutdown of
   a connection by a server.  GOAWAY allows a server to stop accepting
   new requests while still finishing processing of previously received
   requests.  This enables administrative actions, like server
   maintenance.  GOAWAY by itself does not close a connection.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Stream ID (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 11: GOAWAY frame payload

   The GOAWAY frame is always sent on the control stream.  It carries a
   QUIC Stream ID for a client-initiated bidirectional stream encoded as
   a variable-length integer.  A client MUST treat receipt of a GOAWAY
   frame containing a Stream ID of any other type as a connection error
   of type HTTP_MALFORMED_FRAME.




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   Clients do not need to send GOAWAY to initiate a graceful shutdown;
   they simply stop making new requests.  A server MUST treat receipt of
   a GOAWAY frame on any stream as a connection error (Section 8) of
   type HTTP_UNEXPECTED_FRAME.

   The GOAWAY frame applies to the connection, not a specific stream.  A
   client MUST treat a GOAWAY frame on a stream other than the control
   stream as a connection error (Section 8) of type HTTP_WRONG_STREAM.

   See Section 5.2 for more information on the use of the GOAWAY frame.

7.2.8.  MAX_PUSH_ID

   The MAX_PUSH_ID frame (type=0xD) is used by clients to control the
   number of server pushes that the server can initiate.  This sets the
   maximum value for a Push ID that the server can use in a PUSH_PROMISE
   frame.  Consequently, this also limits the number of push streams
   that the server can initiate in addition to the limit set by the QUIC
   MAX_STREAMS frame.

   The MAX_PUSH_ID frame is always sent on the control stream.  Receipt
   of a MAX_PUSH_ID frame on any other stream MUST be treated as a
   connection error of type HTTP_WRONG_STREAM.

   A server MUST NOT send a MAX_PUSH_ID frame.  A client MUST treat the
   receipt of a MAX_PUSH_ID frame as a connection error of type
   HTTP_UNEXPECTED_FRAME.

   The maximum Push ID is unset when a connection is created, meaning
   that a server cannot push until it receives a MAX_PUSH_ID frame.  A
   client that wishes to manage the number of promised server pushes can
   increase the maximum Push ID by sending MAX_PUSH_ID frames as the
   server fulfills or cancels server pushes.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Push ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 12: MAX_PUSH_ID frame payload

   The MAX_PUSH_ID frame carries a single variable-length integer that
   identifies the maximum value for a Push ID that the server can use
   (see Section 7.2.6).  A MAX_PUSH_ID frame cannot reduce the maximum
   Push ID; receipt of a MAX_PUSH_ID that contains a smaller value than
   previously received MUST be treated as a connection error of type
   HTTP_MALFORMED_FRAME.



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7.2.9.  DUPLICATE_PUSH

   The DUPLICATE_PUSH frame (type=0xE) is used by servers to indicate
   that an existing pushed resource is related to multiple client
   requests.

   The DUPLICATE_PUSH frame is always sent on a request stream.  Receipt
   of a DUPLICATE_PUSH frame on any other stream MUST be treated as a
   connection error of type HTTP_WRONG_STREAM.

   A client MUST NOT send a DUPLICATE_PUSH frame.  A server MUST treat
   the receipt of a DUPLICATE_PUSH frame as a connection error of type
   HTTP_UNEXPECTED_FRAME.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Push ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 13: DUPLICATE_PUSH frame payload

   The DUPLICATE_PUSH frame carries a single variable-length integer
   that identifies the Push ID of a resource that the server has
   previously promised (see Section 7.2.6), though that promise might
   not be received before this frame.  A server MUST NOT use a Push ID
   that is larger than the client has provided in a MAX_PUSH_ID frame
   (Section 7.2.8).  A client MUST treat receipt of a DUPLICATE_PUSH
   that contains a larger Push ID than the client has advertised as a
   connection error of type HTTP_MALFORMED_FRAME.

   This frame allows the server to use the same server push in response
   to multiple concurrent requests.  Referencing the same server push
   ensures that a promise can be made in relation to every response in
   which server push might be needed without duplicating request headers
   or pushed responses.

   Allowing duplicate references to the same Push ID is primarily to
   reduce duplication caused by concurrent requests.  A server SHOULD
   avoid reusing a Push ID over a long period.  Clients are likely to
   consume server push responses and not retain them for reuse over
   time.  Clients that see a DUPLICATE_PUSH that uses a Push ID that
   they have since consumed and discarded are forced to ignore the
   DUPLICATE_PUSH.







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7.2.10.  Reserved Frame Types

   Frame types of the format "0x1f * N + 0x21" for integer values of N
   are reserved to exercise the requirement that unknown types be
   ignored (Section 9).  These frames have no semantics, and can be sent
   when application-layer padding is desired.  They MAY also be sent on
   connections where no data is currently being transferred.  Endpoints
   MUST NOT consider these frames to have any meaning upon receipt.

   The payload and length of the frames are selected in any manner the
   implementation chooses.

8.  Error Handling

   QUIC allows the application to abruptly terminate (reset) individual
   streams or the entire connection when an error is encountered.  These
   are referred to as "stream errors" or "connection errors" and are
   described in more detail in [QUIC-TRANSPORT].  An endpoint MAY choose
   to treat a stream error as a connection error.

   This section describes HTTP/3-specific error codes which can be used
   to express the cause of a connection or stream error.

8.1.  HTTP/3 Error Codes

   The following error codes are defined for use in QUIC RESET_STREAM
   frames, STOP_SENDING frames, and CONNECTION_CLOSE frames when using
   HTTP/3.

   HTTP_NO_ERROR (0x00):  No error.  This is used when the connection or
      stream needs to be closed, but there is no error to signal.

   HTTP_WRONG_SETTING_DIRECTION (0x01):  A client-only setting was sent
      by a server, or a server-only setting by a client.

   HTTP_PUSH_REFUSED (0x02):  The server has attempted to push content
      which the client will not accept on this connection.

   HTTP_INTERNAL_ERROR (0x03):  An internal error has occurred in the
      HTTP stack.

   HTTP_PUSH_ALREADY_IN_CACHE (0x04):  The server has attempted to push
      content which the client has cached.

   HTTP_REQUEST_CANCELLED (0x05):  The request or its response is
      cancelled.





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   HTTP_INCOMPLETE_REQUEST (0x06):  The client's stream terminated
      without containing a fully-formed request.

   HTTP_CONNECT_ERROR (0x07):  The connection established in response to
      a CONNECT request was reset or abnormally closed.

   HTTP_EXCESSIVE_LOAD (0x08):  The endpoint detected that its peer is
      exhibiting a behavior that might be generating excessive load.

   HTTP_VERSION_FALLBACK (0x09):  The requested operation cannot be
      served over HTTP/3.  The peer should retry over HTTP/1.1.

   HTTP_WRONG_STREAM (0x0A):  A frame was received on a stream where it
      is not permitted.

   HTTP_LIMIT_EXCEEDED (0x0B):  A Stream ID, Push ID, or Placeholder ID
      greater than the current maximum for that identifier was
      referenced.

   HTTP_DUPLICATE_PUSH (0x0C):  A Push ID was referenced in two
      different stream headers.

   HTTP_UNKNOWN_STREAM_TYPE (0x0D):  A unidirectional stream header
      contained an unknown stream type.

   HTTP_WRONG_STREAM_COUNT (0x0E):  A unidirectional stream type was
      used more times than is permitted by that type.

   HTTP_CLOSED_CRITICAL_STREAM (0x0F):  A stream required by the
      connection was closed or reset.

   HTTP_WRONG_STREAM_DIRECTION (0x0010):  A unidirectional stream type
      was used by a peer which is not permitted to do so.

   HTTP_EARLY_RESPONSE (0x0011):  The remainder of the client's request
      is not needed to produce a response.  For use in STOP_SENDING
      only.

   HTTP_MISSING_SETTINGS (0x0012):  No SETTINGS frame was received at
      the beginning of the control stream.

   HTTP_UNEXPECTED_FRAME (0x0013):  A frame was received which was not
      permitted in the current state.

   HTTP_REQUEST_REJECTED (0x0014):  A server rejected a request without
      performing any application processing.





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   HTTP_GENERAL_PROTOCOL_ERROR (0x00FF):  Peer violated protocol
      requirements in a way which doesn't match a more specific error
      code, or endpoint declines to use the more specific error code.

   HTTP_MALFORMED_FRAME (0x01XX):  An error in a specific frame type.
      If the frame type is "0xfe" or less, the type is included as the
      last byte of the error code.  For example, an error in a
      MAX_PUSH_ID frame would be indicated with the code (0x10D).  The
      last byte "0xff" is used to indicate any frame type greater than
      "0xfe".

9.  Extensions to HTTP/3

   HTTP/3 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/3 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 7.2), new
   settings (Section 7.2.5.1), new error codes (Section 8), or new
   unidirectional stream types (Section 6.2).  Registries are
   established for managing these extension points: frame types
   (Section 11.3), settings (Section 11.4), error codes (Section 11.5),
   and stream types (Section 11.6).

   Implementations MUST ignore unknown or unsupported values in all
   extensible protocol elements.  Implementations MUST discard frames
   and unidirectional streams 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.

   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.

   This document doesn't mandate a specific method for negotiating the
   use of an extension but notes that a setting (Section 7.2.5.1) could
   be used for that purpose.  If both peers set a value that indicates
   willingness to use the extension, then the extension can be used.  If
   a setting is used for extension negotiation, the default value MUST




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   be defined in such a fashion that the extension is disabled if the
   setting is omitted.

10.  Security Considerations

   The security considerations of HTTP/3 should be comparable to those
   of HTTP/2 with TLS.  Note that where HTTP/2 employs PADDING frames
   and Padding fields in other frames to make a connection more
   resistant to traffic analysis, HTTP/3 can rely on QUIC PADDING frames
   or employ the reserved frame and stream types discussed in
   Section 7.2.10 and Section 6.2.3.

   When HTTP Alternative Services is used for discovery for HTTP/3
   endpoints, the security considerations of [ALTSVC] also apply.

   Several protocol elements contain nested length elements, typically
   in the form of frames with an explicit length containing variable-
   length integers.  This could pose a security risk to an incautious
   implementer.  An implementation MUST ensure that the length of a
   frame exactly matches the length of the fields it contains.

   The use of 0-RTT with HTTP/3 creates an exposure to replay attack.
   The anti-replay mitigations in [HTTP-REPLAY] MUST be applied when
   using HTTP/3 with 0-RTT.

   Certain HTTP implementations use the client address for logging or
   access-control purposes.  Since a QUIC client's address might change
   during a connection (and future versions might support simultaneous
   use of multiple addresses), such implementations will need to either
   actively retrieve the client's current address or addresses when they
   are relevant or explicitly accept that the original address might
   change.

11.  IANA Considerations

11.1.  Registration of HTTP/3 Identification String

   This document creates a new registration for the identification of
   HTTP/3 in the "Application Layer Protocol Negotiation (ALPN) Protocol
   IDs" registry established in [RFC7301].

   The "h3" string identifies HTTP/3:

   Protocol:  HTTP/3

   Identification Sequence:  0x68 0x33 ("h3")

   Specification:  This document



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11.2.  Registration of QUIC Version Hint Alt-Svc Parameter

   This document creates a new registration for version-negotiation
   hints in the "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter"
   registry established in [RFC7838].

   Parameter:  "quic"

   Specification:  This document, Section 3.2.1

11.3.  Frame Types

   This document establishes a registry for HTTP/3 frame type codes.
   The "HTTP/3 Frame Type" registry governs a 62-bit space.  This space
   is split into three spaces that are governed by different policies.
   Values between "0x00" and "0x3f" (in hexadecimal) are assigned via
   the Standards Action or IESG Review policies [RFC8126].  Values from
   "0x40" to "0x3fff" operate on the Specification Required policy
   [RFC8126].  All other values are assigned to Private Use [RFC8126].

   While this registry is separate from the "HTTP/2 Frame Type" registry
   defined in [HTTP2], it is preferable that the assignments parallel
   each other where the code spaces overlap.  If an entry is present in
   only one registry, every effort SHOULD be made to avoid assigning the
   corresponding value to an unrelated operation.

   New entries in this registry require the following information:

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

   Code:  The 62-bit code assigned to the frame type.

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

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














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                 +----------------+------+---------------+
                 | Frame Type     | Code | Specification |
                 +----------------+------+---------------+
                 | DATA           | 0x0  | Section 7.2.1 |
                 |                |      |               |
                 | HEADERS        | 0x1  | Section 7.2.2 |
                 |                |      |               |
                 | PRIORITY       | 0x2  | Section 7.2.3 |
                 |                |      |               |
                 | CANCEL_PUSH    | 0x3  | Section 7.2.4 |
                 |                |      |               |
                 | SETTINGS       | 0x4  | Section 7.2.5 |
                 |                |      |               |
                 | PUSH_PROMISE   | 0x5  | Section 7.2.6 |
                 |                |      |               |
                 | Reserved       | 0x6  | N/A           |
                 |                |      |               |
                 | GOAWAY         | 0x7  | Section 7.2.7 |
                 |                |      |               |
                 | Reserved       | 0x8  | N/A           |
                 |                |      |               |
                 | Reserved       | 0x9  | N/A           |
                 |                |      |               |
                 | MAX_PUSH_ID    | 0xD  | Section 7.2.8 |
                 |                |      |               |
                 | DUPLICATE_PUSH | 0xE  | Section 7.2.9 |
                 +----------------+------+---------------+

   Additionally, each code of the format "0x1f * N + 0x21" for integer
   values of N (that is, "0x21", "0x40", ..., through
   "0x3FFFFFFFFFFFFFFE") MUST NOT be assigned by IANA.

11.4.  Settings Parameters

   This document establishes a registry for HTTP/3 settings.  The
   "HTTP/3 Settings" registry governs a 62-bit space.  This space is
   split into three spaces that are governed by different policies.
   Values between "0x00" and "0x3f" (in hexadecimal) are assigned via
   the Standards Action or IESG Review policies [RFC8126].  Values from
   "0x40" to "0x3fff" operate on the Specification Required policy
   [RFC8126].  All other values are assigned to Private Use [RFC8126].
   The designated experts are the same as those for the "HTTP/2
   Settings" registry defined in [HTTP2].

   While this registry is separate from the "HTTP/2 Settings" registry
   defined in [HTTP2], it is preferable that the assignments parallel
   each other.  If an entry is present in only one registry, every




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   effort SHOULD be made to avoid assigning the corresponding value to
   an unrelated operation.

   New registrations are advised to provide the following information:

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

   Code:  The 62-bit code assigned to 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.

             +----------------------+------+-----------------+
             | Setting Name         | Code | Specification   |
             +----------------------+------+-----------------+
             | Reserved             | 0x2  | N/A             |
             |                      |      |                 |
             | Reserved             | 0x3  | N/A             |
             |                      |      |                 |
             | Reserved             | 0x4  | N/A             |
             |                      |      |                 |
             | Reserved             | 0x5  | N/A             |
             |                      |      |                 |
             | MAX_HEADER_LIST_SIZE | 0x6  | Section 7.2.5.1 |
             |                      |      |                 |
             | NUM_PLACEHOLDERS     | 0x9  | Section 7.2.5.1 |
             +----------------------+------+-----------------+

   Additionally, each code of the format "0x1f * N + 0x21" for integer
   values of N (that is, "0x21", "0x40", ..., through
   "0x3FFFFFFFFFFFFFFE") MUST NOT be assigned by IANA.

11.5.  Error Codes

   This document establishes a registry for HTTP/3 error codes.  The
   "HTTP/3 Error Code" registry manages a 62-bit space.  The "HTTP/3
   Error Code" registry operates under the "Expert Review" policy
   [RFC8126].

   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:



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

   Code:  The 62-bit error code value.

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

   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   | Specificatio |
   |                           |        |               | n            |
   +---------------------------+--------+---------------+--------------+
   | HTTP_NO_ERROR             | 0x0000 | No error      | Section 8.1  |
   |                           |        |               |              |
   | HTTP_WRONG_SETTING_DIRECT | 0x0001 | Setting sent  | Section 8.1  |
   | ION                       |        | in wrong      |              |
   |                           |        | direction     |              |
   |                           |        |               |              |
   | HTTP_PUSH_REFUSED         | 0x0002 | Client        | Section 8.1  |
   |                           |        | refused       |              |
   |                           |        | pushed        |              |
   |                           |        | content       |              |
   |                           |        |               |              |
   | HTTP_INTERNAL_ERROR       | 0x0003 | Internal      | Section 8.1  |
   |                           |        | error         |              |
   |                           |        |               |              |
   | HTTP_PUSH_ALREADY_IN_CACH | 0x0004 | Pushed        | Section 8.1  |
   | E                         |        | content       |              |
   |                           |        | already       |              |
   |                           |        | cached        |              |
   |                           |        |               |              |
   | HTTP_REQUEST_CANCELLED    | 0x0005 | Data no       | Section 8.1  |
   |                           |        | longer needed |              |
   |                           |        |               |              |
   | HTTP_INCOMPLETE_REQUEST   | 0x0006 | Stream        | Section 8.1  |
   |                           |        | terminated    |              |
   |                           |        | early         |              |
   |                           |        |               |              |
   | HTTP_CONNECT_ERROR        | 0x0007 | TCP reset or  | Section 8.1  |
   |                           |        | error on      |              |
   |                           |        | CONNECT       |              |
   |                           |        | request       |              |
   |                           |        |               |              |



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   | HTTP_EXCESSIVE_LOAD       | 0x0008 | Peer          | Section 8.1  |
   |                           |        | generating    |              |
   |                           |        | excessive     |              |
   |                           |        | load          |              |
   |                           |        |               |              |
   | HTTP_VERSION_FALLBACK     | 0x0009 | Retry over    | Section 8.1  |
   |                           |        | HTTP/1.1      |              |
   |                           |        |               |              |
   | HTTP_WRONG_STREAM         | 0x000A | A frame was   | Section 8.1  |
   |                           |        | sent on the   |              |
   |                           |        | wrong stream  |              |
   |                           |        |               |              |
   | HTTP_LIMIT_EXCEEDED       | 0x000B | An identifier | Section 8.1  |
   |                           |        | limit was     |              |
   |                           |        | exceeded      |              |
   |                           |        |               |              |
   | HTTP_DUPLICATE_PUSH       | 0x000C | Push ID was   | Section 8.1  |
   |                           |        | fulfilled     |              |
   |                           |        | multiple      |              |
   |                           |        | times         |              |
   |                           |        |               |              |
   | HTTP_UNKNOWN_STREAM_TYPE  | 0x000D | Unknown unidi | Section 8.1  |
   |                           |        | rectional     |              |
   |                           |        | stream type   |              |
   |                           |        |               |              |
   | HTTP_WRONG_STREAM_COUNT   | 0x000E | Too many unid | Section 8.1  |
   |                           |        | irectional    |              |
   |                           |        | streams       |              |
   |                           |        |               |              |
   | HTTP_CLOSED_CRITICAL_STRE | 0x000F | Critical      | Section 8.1  |
   | AM                        |        | stream was    |              |
   |                           |        | closed        |              |
   |                           |        |               |              |
   | HTTP_WRONG_STREAM_DIRECTI | 0x0010 | Unidirectiona | Section 8.1  |
   | ON                        |        | l stream in   |              |
   |                           |        | wrong         |              |
   |                           |        | direction     |              |
   |                           |        |               |              |
   | HTTP_EARLY_RESPONSE       | 0x0011 | Remainder of  | Section 8.1  |
   |                           |        | request not   |              |
   |                           |        | needed        |              |
   |                           |        |               |              |
   | HTTP_MISSING_SETTINGS     | 0x0012 | No SETTINGS   | Section 8.1  |
   |                           |        | frame         |              |
   |                           |        | received      |              |
   |                           |        |               |              |
   | HTTP_UNEXPECTED_FRAME     | 0x0013 | Frame not     | Section 8.1  |
   |                           |        | permitted in  |              |



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   |                           |        | the current   |              |
   |                           |        | state         |              |
   |                           |        |               |              |
   | HTTP_REQUEST_REJECTED     | 0x0014 | Request not   | Section 8.1  |
   |                           |        | processed     |              |
   |                           |        |               |              |
   | HTTP_MALFORMED_FRAME      | 0x01XX | Error in      | Section 8.1  |
   |                           |        | frame         |              |
   |                           |        | formatting    |              |
   +---------------------------+--------+---------------+--------------+

11.6.  Stream Types

   This document establishes a registry for HTTP/3 unidirectional stream
   types.  The "HTTP/3 Stream Type" registry governs a 62-bit space.
   This space is split into three spaces that are governed by different
   policies.  Values between "0x00" and 0x3f (in hexadecimal) are
   assigned via the Standards Action or IESG Review policies [RFC8126].
   Values from "0x40" to "0x3fff" operate on the Specification Required
   policy [RFC8126].  All other values are assigned to Private Use
   [RFC8126].

   New entries in this registry require the following information:

   Stream Type:  A name or label for the stream type.

   Code:  The 62-bit code assigned to the stream type.

   Specification:  A reference to a specification that includes a
      description of the stream type, including the layout semantics of
      its payload.

   Sender:  Which endpoint on a connection may initiate a stream of this
      type.  Values are "Client", "Server", or "Both".

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

            +----------------+------+---------------+--------+
            | Stream Type    | Code | Specification | Sender |
            +----------------+------+---------------+--------+
            | Control Stream | 0x00 | Section 6.2.1 | Both   |
            |                |      |               |        |
            | Push Stream    | 0x01 | Section 4.4   | Server |
            +----------------+------+---------------+--------+

   Additionally, each code of the format "0x1f * N + 0x21" for integer
   values of N (that is, "0x21", "0x40", ..., through
   "0x3FFFFFFFFFFFFFFE") MUST NOT be assigned by IANA.



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12.  References

12.1.  Normative References

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

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

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [QPACK]    Krasic, C., Bishop, M., and A. Frindell, Ed., "QPACK:
              Header Compression for HTTP over QUIC", draft-ietf-quic-
              qpack (work in progress), June 2019.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport (work in progress), June 2019.

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

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

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

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






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   [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,
              <https://www.rfc-editor.org/info/rfc7230>.

   [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,
              <https://www.rfc-editor.org/info/rfc7231>.

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

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

12.2.  Informative References

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

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

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

12.3.  URIs

   [1] https://mailarchive.ietf.org/arch/search/?email_list=quic

   [2] https://github.com/quicwg

   [3] https://github.com/quicwg/base-drafts/labels/-http

   [4] https://www.iana.org/assignments/message-headers



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Appendix A.  Considerations for Transitioning from HTTP/2

   HTTP/3 is strongly informed by HTTP/2, and bears many similarities.
   This section describes the approach taken to design HTTP/3, points
   out important differences from HTTP/2, and describes how to map
   HTTP/2 extensions into HTTP/3.

   HTTP/3 begins from the premise that similarity to HTTP/2 is
   preferable, but not a hard requirement.  HTTP/3 departs from HTTP/2
   where QUIC differs from TCP, either to take advantage of QUIC
   features (like streams) or to accommodate important shortcomings
   (such as a lack of total ordering).  These differences make HTTP/3
   similar to HTTP/2 in key aspects, such as the relationship of
   requests and responses to streams.  However, the details of the
   HTTP/3 design are substantially different than HTTP/2.

   These departures are noted in this section.

A.1.  Streams

   HTTP/3 permits use of a larger number of streams (2^62-1) than
   HTTP/2.  The considerations about exhaustion of stream identifier
   space apply, though the space is significantly larger such that it is
   likely that other limits in QUIC are reached first, such as the limit
   on the connection flow control window.

A.2.  HTTP Frame Types

   Many framing concepts from HTTP/2 can be elided on QUIC, because the
   transport deals with them.  Because frames are already on a stream,
   they can omit the stream number.  Because frames do not block
   multiplexing (QUIC's multiplexing occurs below this layer), the
   support for variable-maximum-length packets can be removed.  Because
   stream termination is handled by QUIC, an END_STREAM flag is not
   required.  This permits the removal of the Flags field from the
   generic frame layout.

   Frame payloads are largely drawn from [HTTP2].  However, QUIC
   includes many features (e.g., flow control) which are also present in
   HTTP/2.  In these cases, the HTTP mapping does not re-implement them.
   As a result, several HTTP/2 frame types are not required in HTTP/3.
   Where an HTTP/2-defined frame is no longer used, the frame ID has
   been reserved in order to maximize portability between HTTP/2 and
   HTTP/3 implementations.  However, even equivalent frames between the
   two mappings are not identical.

   Many of the differences arise from the fact that HTTP/2 provides an
   absolute ordering between frames across all streams, while QUIC



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   provides this guarantee on each stream only.  As a result, if a frame
   type makes assumptions that frames from different streams will still
   be received in the order sent, HTTP/3 will break them.

   For example, implicit in the HTTP/2 prioritization scheme is the
   notion of in-order delivery of priority changes (i.e., dependency
   tree mutations): since operations on the dependency tree such as
   reparenting a subtree are not commutative, both sender and receiver
   must apply them in the same order to ensure that both sides have a
   consistent view of the stream dependency tree.  HTTP/2 specifies
   priority assignments in PRIORITY frames and (optionally) in HEADERS
   frames.  To achieve in-order delivery of priority changes in HTTP/3,
   PRIORITY frames are sent as the first frame on a request stream or on
   the control stream and exclusive prioritization has been removed.
   HTTP/3 permits the prioritization of requests, pushes and
   placeholders that each exist in separate identifier spaces.  The
   HTTP/3 PRIORITY frame replaces the stream dependency field with
   fields that can identify the element of interest and its dependency.

   Likewise, HPACK was designed with the assumption of in-order
   delivery.  A sequence of encoded header blocks must arrive (and be
   decoded) at an endpoint in the same order in which they were encoded.
   This ensures that the dynamic state at the two endpoints remains in
   sync.  As a result, HTTP/3 uses a modified version of HPACK,
   described in [QPACK].

   Frame type definitions in HTTP/3 often use the QUIC variable-length
   integer encoding.  In particular, Stream IDs use this encoding, which
   allow for a larger range of possible values than the encoding used in
   HTTP/2.  Some frames in HTTP/3 use an identifier rather than a Stream
   ID (e.g.  Push IDs in PRIORITY frames).  Redefinition of the encoding
   of extension frame types might be necessary if the encoding includes
   a Stream ID.

   Because the Flags field is not present in generic HTTP/3 frames,
   those frames which depend on the presence of flags need to allocate
   space for flags as part of their frame payload.

   Other than this issue, frame type HTTP/2 extensions are typically
   portable to QUIC simply by replacing Stream 0 in HTTP/2 with a
   control stream in HTTP/3.  HTTP/3 extensions will not assume
   ordering, but would not be harmed by ordering, and would be portable
   to HTTP/2 in the same manner.

   Below is a listing of how each HTTP/2 frame type is mapped:

   DATA (0x0):  Padding is not defined in HTTP/3 frames.  See
      Section 7.2.1.



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   HEADERS (0x1):  The PRIORITY region of HEADERS is not defined in
      HTTP/3 frames.  A separate PRIORITY frame is used in all cases.
      Padding is not defined in HTTP/3 frames.  See Section 7.2.2.

   PRIORITY (0x2):  As described above, the PRIORITY frame references a
      variety of identifiers.  It is sent as the first frame on a
      request streams or on the control stream.  See Section 7.2.3.

   RST_STREAM (0x3):  RST_STREAM frames do not exist, since QUIC
      provides stream lifecycle management.  The same code point is used
      for the CANCEL_PUSH frame (Section 7.2.4).

   SETTINGS (0x4):  SETTINGS frames are sent only at the beginning of
      the connection.  See Section 7.2.5 and Appendix A.3.

   PUSH_PROMISE (0x5):  The PUSH_PROMISE does not reference a stream;
      instead the push stream references the PUSH_PROMISE frame using a
      Push ID.  See Section 7.2.6.

   PING (0x6):  PING frames do not exist, since QUIC provides equivalent
      functionality.

   GOAWAY (0x7):  GOAWAY is sent only from server to client and does not
      contain an error code.  See Section 7.2.7.

   WINDOW_UPDATE (0x8):  WINDOW_UPDATE frames do not exist, since QUIC
      provides flow control.

   CONTINUATION (0x9):  CONTINUATION frames do not exist; instead,
      larger HEADERS/PUSH_PROMISE frames than HTTP/2 are permitted.

   Frame types defined by extensions to HTTP/2 need to be separately
   registered for HTTP/3 if still applicable.  The IDs of frames defined
   in [HTTP2] have been reserved for simplicity.  Note that the frame
   type space in HTTP/3 is substantially larger (62 bits versus 8 bits),
   so many HTTP/3 frame types have no equivalent HTTP/2 code points.
   See Section 11.3.

A.3.  HTTP/2 SETTINGS Parameters

   An important difference from HTTP/2 is that settings are sent once,
   at the beginning of the connection, and thereafter cannot change.
   This eliminates many corner cases around synchronization of changes.

   Some transport-level options that HTTP/2 specifies via the SETTINGS
   frame are superseded by QUIC transport parameters in HTTP/3.  The
   HTTP-level options that are retained in HTTP/3 have the same value as
   in HTTP/2.



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   Below is a listing of how each HTTP/2 SETTINGS parameter is mapped:

   SETTINGS_HEADER_TABLE_SIZE:  See [QPACK].

   SETTINGS_ENABLE_PUSH:  This is removed in favor of the MAX_PUSH_ID
      which provides a more granular control over server push.

   SETTINGS_MAX_CONCURRENT_STREAMS:  QUIC controls the largest open
      Stream ID as part of its flow control logic.  Specifying
      SETTINGS_MAX_CONCURRENT_STREAMS in the SETTINGS frame is an error.

   SETTINGS_INITIAL_WINDOW_SIZE:  QUIC requires both stream and
      connection flow control window sizes to be specified in the
      initial transport handshake.  Specifying
      SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame is an error.

   SETTINGS_MAX_FRAME_SIZE:  This setting has no equivalent in HTTP/3.
      Specifying it in the SETTINGS frame is an error.

   SETTINGS_MAX_HEADER_LIST_SIZE:  See Section 7.2.5.1.

   In HTTP/3, setting values are variable-length integers (6, 14, 30, or
   62 bits long) rather than fixed-length 32-bit fields as in HTTP/2.
   This will often produce a shorter encoding, but can produce a longer
   encoding for settings which use the full 32-bit space.  Settings
   ported from HTTP/2 might choose to redefine the format of their
   settings to avoid using the 62-bit encoding.

   Settings need to be defined separately for HTTP/2 and HTTP/3.  The
   IDs of settings defined in [HTTP2] have been reserved for simplicity.
   Note that the settings identifier space in HTTP/3 is substantially
   larger (62 bits versus 16 bits), so many HTTP/3 settings have no
   equivalent HTTP/2 code point.  See Section 11.4.

A.4.  HTTP/2 Error Codes

   QUIC has the same concepts of "stream" and "connection" errors that
   HTTP/2 provides.  However, there is no direct portability of HTTP/2
   error codes.

   The HTTP/2 error codes defined in Section 7 of [HTTP2] map to the
   HTTP/3 error codes as follows:

   NO_ERROR (0x0):  HTTP_NO_ERROR in Section 8.1.

   PROTOCOL_ERROR (0x1):  No single mapping.  See new
      HTTP_MALFORMED_FRAME error codes defined in Section 8.1.




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   INTERNAL_ERROR (0x2):  HTTP_INTERNAL_ERROR in Section 8.1.

   FLOW_CONTROL_ERROR (0x3):  Not applicable, since QUIC handles flow
      control.  Would provoke a QUIC_FLOW_CONTROL_RECEIVED_TOO_MUCH_DATA
      from the QUIC layer.

   SETTINGS_TIMEOUT (0x4):  Not applicable, since no acknowledgement of
      SETTINGS is defined.

   STREAM_CLOSED (0x5):  Not applicable, since QUIC handles stream
      management.  Would provoke a QUIC_STREAM_DATA_AFTER_TERMINATION
      from the QUIC layer.

   FRAME_SIZE_ERROR (0x6):  HTTP_MALFORMED_FRAME error codes defined in
      Section 8.1.

   REFUSED_STREAM (0x7):  HTTP_REQUEST_REJECTED (in Section 8.1) is used
      to indicate that a request was not processed.  Otherwise, not
      applicable because QUIC handles stream management.  A
      STREAM_ID_ERROR at the QUIC layer is used for streams that are
      improperly opened.

   CANCEL (0x8):  HTTP_REQUEST_CANCELLED in Section 8.1.

   COMPRESSION_ERROR (0x9):  Multiple error codes are defined in
      [QPACK].

   CONNECT_ERROR (0xa):  HTTP_CONNECT_ERROR in Section 8.1.

   ENHANCE_YOUR_CALM (0xb):  HTTP_EXCESSIVE_LOAD in Section 8.1.

   INADEQUATE_SECURITY (0xc):  Not applicable, since QUIC is assumed to
      provide sufficient security on all connections.

   HTTP_1_1_REQUIRED (0xd):  HTTP_VERSION_FALLBACK in Section 8.1.

   Error codes need to be defined for HTTP/2 and HTTP/3 separately.  See
   Section 11.5.

Appendix B.  Change Log

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.








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B.1.  Since draft-ietf-quic-http-19

   o  SETTINGS_NUM_PLACEHOLDERS is 0x9 (#2443,#2530)

   o  Non-zero bits in the Empty field of the PRIORITY frame MAY be
      treated as an error (#2501)

B.2.  Since draft-ietf-quic-http-18

   o  Resetting streams following a GOAWAY is recommended, but not
      required (#2256,#2457)

   o  Use variable-length integers throughout (#2437,#2233,#2253,#2275)

      *  Variable-length frame types, stream types, and settings
         identifiers

      *  Renumbered stream type assignments

      *  Modified associated reserved values

   o  Frame layout switched from Length-Type-Value to Type-Length-Value
      (#2395,#2235)

   o  Specified error code for servers receiving DUPLICATE_PUSH (#2497)

   o  Use connection error for invalid PRIORITY (#2507, #2508)

B.3.  Since draft-ietf-quic-http-17

   o  HTTP_REQUEST_REJECTED is used to indicate a request can be retried
      (#2106, #2325)

   o  Changed error code for GOAWAY on the wrong stream (#2231, #2343)

B.4.  Since draft-ietf-quic-http-16

   o  Rename "HTTP/QUIC" to "HTTP/3" (#1973)

   o  Changes to PRIORITY frame (#1865, #2075)

      *  Permitted as first frame of request streams

      *  Remove exclusive reprioritization

      *  Changes to Prioritized Element Type bits





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   o  Define DUPLICATE_PUSH frame to refer to another PUSH_PROMISE
      (#2072)

   o  Set defaults for settings, allow request before receiving SETTINGS
      (#1809, #1846, #2038)

   o  Clarify message processing rules for streams that aren't closed
      (#1972, #2003)

   o  Removed reservation of error code 0 and moved HTTP_NO_ERROR to
      this value (#1922)

   o  Removed prohibition of zero-length DATA frames (#2098)

B.5.  Since draft-ietf-quic-http-15

   Substantial editorial reorganization; no technical changes.

B.6.  Since draft-ietf-quic-http-14

   o  Recommend sensible values for QUIC transport parameters
      (#1720,#1806)

   o  Define error for missing SETTINGS frame (#1697,#1808)

   o  Setting values are variable-length integers (#1556,#1807) and do
      not have separate maximum values (#1820)

   o  Expanded discussion of connection closure (#1599,#1717,#1712)

   o  HTTP_VERSION_FALLBACK falls back to HTTP/1.1 (#1677,#1685)

B.7.  Since draft-ietf-quic-http-13

   o  Reserved some frame types for grease (#1333, #1446)

   o  Unknown unidirectional stream types are tolerated, not errors;
      some reserved for grease (#1490, #1525)

   o  Require settings to be remembered for 0-RTT, prohibit reductions
      (#1541, #1641)

   o  Specify behavior for truncated requests (#1596, #1643)








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B.8.  Since draft-ietf-quic-http-12

   o  TLS SNI extension isn't mandatory if an alternative method is used
      (#1459, #1462, #1466)

   o  Removed flags from HTTP/3 frames (#1388, #1398)

   o  Reserved frame types and settings for use in preserving
      extensibility (#1333, #1446)

   o  Added general error code (#1391, #1397)

   o  Unidirectional streams carry a type byte and are extensible
      (#910,#1359)

   o  Priority mechanism now uses explicit placeholders to enable
      persistent structure in the tree (#441,#1421,#1422)

B.9.  Since draft-ietf-quic-http-11

   o  Moved QPACK table updates and acknowledgments to dedicated streams
      (#1121, #1122, #1238)

B.10.  Since draft-ietf-quic-http-10

   o  Settings need to be remembered when attempting and accepting 0-RTT
      (#1157, #1207)

B.11.  Since draft-ietf-quic-http-09

   o  Selected QCRAM for header compression (#228, #1117)

   o  The server_name TLS extension is now mandatory (#296, #495)

   o  Specified handling of unsupported versions in Alt-Svc (#1093,
      #1097)

B.12.  Since draft-ietf-quic-http-08

   o  Clarified connection coalescing rules (#940, #1024)

B.13.  Since draft-ietf-quic-http-07

   o  Changes for integer encodings in QUIC (#595,#905)

   o  Use unidirectional streams as appropriate (#515, #240, #281, #886)

   o  Improvement to the description of GOAWAY (#604, #898)



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   o  Improve description of server push usage (#947, #950, #957)

B.14.  Since draft-ietf-quic-http-06

   o  Track changes in QUIC error code usage (#485)

B.15.  Since draft-ietf-quic-http-05

   o  Made push ID sequential, add MAX_PUSH_ID, remove
      SETTINGS_ENABLE_PUSH (#709)

   o  Guidance about keep-alive and QUIC PINGs (#729)

   o  Expanded text on GOAWAY and cancellation (#757)

B.16.  Since draft-ietf-quic-http-04

   o  Cite RFC 5234 (#404)

   o  Return to a single stream per request (#245,#557)

   o  Use separate frame type and settings registries from HTTP/2 (#81)

   o  SETTINGS_ENABLE_PUSH instead of SETTINGS_DISABLE_PUSH (#477)

   o  Restored GOAWAY (#696)

   o  Identify server push using Push ID rather than a stream ID
      (#702,#281)

   o  DATA frames cannot be empty (#700)

B.17.  Since draft-ietf-quic-http-03

   None.

B.18.  Since draft-ietf-quic-http-02

   o  Track changes in transport draft

B.19.  Since draft-ietf-quic-http-01

   o  SETTINGS changes (#181):

      *  SETTINGS can be sent only once at the start of a connection; no
         changes thereafter

      *  SETTINGS_ACK removed



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      *  Settings can only occur in the SETTINGS frame a single time

      *  Boolean format updated

   o  Alt-Svc parameter changed from "v" to "quic"; format updated
      (#229)

   o  Closing the connection control stream or any message control
      stream is a fatal error (#176)

   o  HPACK Sequence counter can wrap (#173)

   o  0-RTT guidance added

   o  Guide to differences from HTTP/2 and porting HTTP/2 extensions
      added (#127,#242)

B.20.  Since draft-ietf-quic-http-00

   o  Changed "HTTP/2-over-QUIC" to "HTTP/QUIC" throughout (#11,#29)

   o  Changed from using HTTP/2 framing within Stream 3 to new framing
      format and two-stream-per-request model (#71,#72,#73)

   o  Adopted SETTINGS format from draft-bishop-httpbis-extended-
      settings-01

   o  Reworked SETTINGS_ACK to account for indeterminate inter-stream
      order (#75)

   o  Described CONNECT pseudo-method (#95)

   o  Updated ALPN token and Alt-Svc guidance (#13,#87)

   o  Application-layer-defined error codes (#19,#74)

B.21.  Since draft-shade-quic-http2-mapping-00

   o  Adopted as base for draft-ietf-quic-http

   o  Updated authors/editors list

Acknowledgements

   The original authors of this specification were Robbie Shade and Mike
   Warres.





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   A substantial portion of Mike's contribution was supported by
   Microsoft during his employment there.

Author's Address

   Mike Bishop (editor)
   Akamai

   Email: mbishop@evequefou.be










































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