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QUIC                                                      M. Bishop, Ed.
Internet-Draft                                                    Akamai
Intended status: Standards Track                             9 June 2020
Expires: 11 December 2020


             Hypertext Transfer Protocol Version 3 (HTTP/3)
                        draft-ietf-quic-http-29

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 (mailto:quic@ietf.org)), which is
   archived at https://mailarchive.ietf.org/arch/
   search/?email_list=quic.

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

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 11 December 2020.






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Copyright Notice

   Copyright (c) 2020 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  . . . . . . . . . . . . . . . . .   5
     1.2.  Delegation to QUIC  . . . . . . . . . . . . . . . . . . .   5
   2.  HTTP/3 Protocol Overview  . . . . . . . . . . . . . . . . . .   5
     2.1.  Document Organization . . . . . . . . . . . . . . . . . .   6
     2.2.  Conventions and Terminology . . . . . . . . . . . . . . .   7
   3.  Connection Setup and Management . . . . . . . . . . . . . . .   8
     3.1.  Draft Version Identification  . . . . . . . . . . . . . .   8
     3.2.  Discovering an HTTP/3 Endpoint  . . . . . . . . . . . . .   9
       3.2.1.  HTTP Alternative Services . . . . . . . . . . . . . .  10
       3.2.2.  Other Schemes . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Connection Establishment  . . . . . . . . . . . . . . . .  10
     3.4.  Connection Reuse  . . . . . . . . . . . . . . . . . . . .  11
   4.  HTTP Request Lifecycle  . . . . . . . . . . . . . . . . . . .  12
     4.1.  HTTP Message Exchanges  . . . . . . . . . . . . . . . . .  12
       4.1.1.  Field Formatting and Compression  . . . . . . . . . .  14
       4.1.2.  Request Cancellation and Rejection  . . . . . . . . .  17
       4.1.3.  Malformed Requests and Responses  . . . . . . . . . .  18
     4.2.  The CONNECT Method  . . . . . . . . . . . . . . . . . . .  19
     4.3.  HTTP Upgrade  . . . . . . . . . . . . . . . . . . . . . .  20
     4.4.  Server Push . . . . . . . . . . . . . . . . . . . . . . .  21
   5.  Connection Closure  . . . . . . . . . . . . . . . . . . . . .  23
     5.1.  Idle Connections  . . . . . . . . . . . . . . . . . . . .  23
     5.2.  Connection Shutdown . . . . . . . . . . . . . . . . . . .  23
     5.3.  Immediate Application Closure . . . . . . . . . . . . . .  25
     5.4.  Transport Closure . . . . . . . . . . . . . . . . . . . .  26
   6.  Stream Mapping and Usage  . . . . . . . . . . . . . . . . . .  26
     6.1.  Bidirectional Streams . . . . . . . . . . . . . . . . . .  26
     6.2.  Unidirectional Streams  . . . . . . . . . . . . . . . . .  27
       6.2.1.  Control Streams . . . . . . . . . . . . . . . . . . .  28
       6.2.2.  Push Streams  . . . . . . . . . . . . . . . . . . . .  29
       6.2.3.  Reserved Stream Types . . . . . . . . . . . . . . . .  29



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   7.  HTTP Framing Layer  . . . . . . . . . . . . . . . . . . . . .  30
     7.1.  Frame Layout  . . . . . . . . . . . . . . . . . . . . . .  31
     7.2.  Frame Definitions . . . . . . . . . . . . . . . . . . . .  31
       7.2.1.  DATA  . . . . . . . . . . . . . . . . . . . . . . . .  31
       7.2.2.  HEADERS . . . . . . . . . . . . . . . . . . . . . . .  32
       7.2.3.  CANCEL_PUSH . . . . . . . . . . . . . . . . . . . . .  32
       7.2.4.  SETTINGS  . . . . . . . . . . . . . . . . . . . . . .  33
       7.2.5.  PUSH_PROMISE  . . . . . . . . . . . . . . . . . . . .  36
       7.2.6.  GOAWAY  . . . . . . . . . . . . . . . . . . . . . . .  38
       7.2.7.  MAX_PUSH_ID . . . . . . . . . . . . . . . . . . . . .  38
       7.2.8.  Reserved Frame Types  . . . . . . . . . . . . . . . .  39
   8.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  39
     8.1.  HTTP/3 Error Codes  . . . . . . . . . . . . . . . . . . .  40
   9.  Extensions to HTTP/3  . . . . . . . . . . . . . . . . . . . .  41
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  42
     10.1.  Server Authority . . . . . . . . . . . . . . . . . . . .  42
     10.2.  Cross-Protocol Attacks . . . . . . . . . . . . . . . . .  42
     10.3.  Intermediary Encapsulation Attacks . . . . . . . . . . .  43
     10.4.  Cacheability of Pushed Responses . . . . . . . . . . . .  43
     10.5.  Denial-of-Service Considerations . . . . . . . . . . . .  43
       10.5.1.  Limits on Field Section Size . . . . . . . . . . . .  44
       10.5.2.  CONNECT Issues . . . . . . . . . . . . . . . . . . .  45
     10.6.  Use of Compression . . . . . . . . . . . . . . . . . . .  45
     10.7.  Padding and Traffic Analysis . . . . . . . . . . . . . .  46
     10.8.  Frame Parsing  . . . . . . . . . . . . . . . . . . . . .  46
     10.9.  Early Data . . . . . . . . . . . . . . . . . . . . . . .  46
     10.10. Migration  . . . . . . . . . . . . . . . . . . . . . . .  47
     10.11. Privacy Considerations . . . . . . . . . . . . . . . . .  47
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
     11.1.  Registration of HTTP/3 Identification String . . . . . .  47
     11.2.  New Registries . . . . . . . . . . . . . . . . . . . . .  48
       11.2.1.  Frame Types  . . . . . . . . . . . . . . . . . . . .  48
       11.2.2.  Settings Parameters  . . . . . . . . . . . . . . . .  49
       11.2.3.  Error Codes  . . . . . . . . . . . . . . . . . . . .  50
       11.2.4.  Stream Types . . . . . . . . . . . . . . . . . . . .  53
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  53
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  53
     12.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Appendix A.  Considerations for Transitioning from HTTP/2 . . . .  56
     A.1.  Streams . . . . . . . . . . . . . . . . . . . . . . . . .  57
     A.2.  HTTP Frame Types  . . . . . . . . . . . . . . . . . . . .  57
       A.2.1.  Prioritization Differences  . . . . . . . . . . . . .  58
       A.2.2.  Field Compression Differences . . . . . . . . . . . .  58
       A.2.3.  Guidance for New Frame Type Definitions . . . . . . .  58
       A.2.4.  Mapping Between HTTP/2 and HTTP/3 Frame Types . . . .  59
     A.3.  HTTP/2 SETTINGS Parameters  . . . . . . . . . . . . . . .  60
     A.4.  HTTP/2 Error Codes  . . . . . . . . . . . . . . . . . . .  61
       A.4.1.  Mapping Between HTTP/2 and HTTP/3 Errors  . . . . . .  62



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   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  62
     B.1.  Since draft-ietf-quic-http-28 . . . . . . . . . . . . . .  63
     B.2.  Since draft-ietf-quic-http-27 . . . . . . . . . . . . . .  63
     B.3.  Since draft-ietf-quic-http-26 . . . . . . . . . . . . . .  63
     B.4.  Since draft-ietf-quic-http-25 . . . . . . . . . . . . . .  63
     B.5.  Since draft-ietf-quic-http-24 . . . . . . . . . . . . . .  63
     B.6.  Since draft-ietf-quic-http-23 . . . . . . . . . . . . . .  63
     B.7.  Since draft-ietf-quic-http-22 . . . . . . . . . . . . . .  64
     B.8.  Since draft-ietf-quic-http-21 . . . . . . . . . . . . . .  64
     B.9.  Since draft-ietf-quic-http-20 . . . . . . . . . . . . . .  65
     B.10. Since draft-ietf-quic-http-19 . . . . . . . . . . . . . .  65
     B.11. Since draft-ietf-quic-http-18 . . . . . . . . . . . . . .  66
     B.12. Since draft-ietf-quic-http-17 . . . . . . . . . . . . . .  66
     B.13. Since draft-ietf-quic-http-16 . . . . . . . . . . . . . .  66
     B.14. Since draft-ietf-quic-http-15 . . . . . . . . . . . . . .  67
     B.15. Since draft-ietf-quic-http-14 . . . . . . . . . . . . . .  67
     B.16. Since draft-ietf-quic-http-13 . . . . . . . . . . . . . .  67
     B.17. Since draft-ietf-quic-http-12 . . . . . . . . . . . . . .  67
     B.18. Since draft-ietf-quic-http-11 . . . . . . . . . . . . . .  68
     B.19. Since draft-ietf-quic-http-10 . . . . . . . . . . . . . .  68
     B.20. Since draft-ietf-quic-http-09 . . . . . . . . . . . . . .  68
     B.21. Since draft-ietf-quic-http-08 . . . . . . . . . . . . . .  68
     B.22. Since draft-ietf-quic-http-07 . . . . . . . . . . . . . .  68
     B.23. Since draft-ietf-quic-http-06 . . . . . . . . . . . . . .  68
     B.24. Since draft-ietf-quic-http-05 . . . . . . . . . . . . . .  68
     B.25. Since draft-ietf-quic-http-04 . . . . . . . . . . . . . .  69
     B.26. Since draft-ietf-quic-http-03 . . . . . . . . . . . . . .  69
     B.27. Since draft-ietf-quic-http-02 . . . . . . . . . . . . . .  69
     B.28. Since draft-ietf-quic-http-01 . . . . . . . . . . . . . .  69
     B.29. Since draft-ietf-quic-http-00 . . . . . . . . . . . . . .  70
     B.30. Since draft-shade-quic-http2-mapping-00 . . . . . . . . .  70
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  70
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  72

1.  Introduction

   HTTP semantics [SEMANTICS] are used for a broad range of services on
   the Internet.  These semantics have most 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.











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1.1.  Prior versions of HTTP

   HTTP/1.1 [HTTP11] 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 complexity and motivates tolerance 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 effectively manage traffic between endpoints.

   HTTP/2 [HTTP2] 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 directly 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 [TLS13] at the transport layer, offering
   comparable security to running TLS over TCP, with the improved
   connection setup latency of TCP Fast Open [TFO].

   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.  Discovery
   of an HTTP/3 endpoint is described in Section 3.2.




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   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).

   Multiplexing of requests is performed using the QUIC stream
   abstraction, described in Section 2 of [QUIC-TRANSPORT].  Each
   request-response pair 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,
   MAX_PUSH_ID, and CANCEL_PUSH.

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

2.1.  Document Organization

   The following sections provide a detailed overview of the connection
   lifecycle and key concepts:

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

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

   *  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:

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

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



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   *  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:

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

   *  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].

   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,








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      consisting of a header and a variable-length sequence of bytes
      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 6.3.3 of [SEMANTICS].

   Finally, the terms "gateway", "intermediary", "proxy", and "tunnel"
   are defined in Section 2.2 of [SEMANTICS].  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".





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   Draft versions MUST use the corresponding draft transport version as
   their transport.  For example, the application protocol defined in
   draft-ietf-quic-http-25 uses the transport defined in draft-ietf-
   quic-transport-25.

   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 4.4.1.1 of [SEMANTICS].  Experimenters are encouraged to
   coordinate their experiments on the quic@ietf.org
   (mailto:quic@ietf.org) mailing list.

3.2.  Discovering an HTTP/3 Endpoint

   HTTP relies on the notion of an authoritative response: a response
   that has been determined to be the most appropriate response for that
   request given the state of the target resource at the time of
   response message origination by (or at the direction of) the origin
   server identified within the target URI.  Locating an authoritative
   server for an HTTP URL is discussed in Section 5.4 of [SEMANTICS].

   The "https" scheme associates authority with possession of a
   certificate that the client considers to be trustworthy for the host
   identified by the authority component of the URL.  If a server
   presents a certificate and proof that it controls the corresponding
   private key, then a client will accept a secured connection to that
   server as being authoritative for all origins with the "https" scheme
   and a host identified in the certificate.

   A client MAY attempt access to a resource with an "https" URI by
   resolving the host identifier to an IP address, establishing a QUIC
   connection to that address on the indicated port, and sending an
   HTTP/3 request message targeting the URI to the server over that
   secured connection.

   Connectivity problems (e.g., blocking UDP) can result in QUIC
   connection establishment failure; clients SHOULD attempt to use TCP-
   based versions of HTTP in this case.

   Servers MAY serve HTTP/3 on any UDP port; an alternative service
   advertisement always includes an explicit port, and URLs contain
   either an explicit port or a default port associated with the scheme.






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3.2.1.  HTTP Alternative Services

   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.

3.2.2.  Other Schemes

   Although HTTP is independent of the transport protocol, the "http"
   scheme associates authority with the ability to receive TCP
   connections on the indicated port of whatever host is identified
   within the authority component.  Because HTTP/3 does not use TCP,
   HTTP/3 cannot be used for direct access to the authoritative server
   for a resource identified by an "http" URI.  However, protocol
   extensions such as [ALTSVC] permit the authoritative server to
   identify other services which are also authoritative and which might
   be reachable over HTTP/3.

   Prior to making requests for an origin whose scheme is not "https",
   the client MUST ensure the server is willing to serve that scheme.
   If the client intends to make requests for an origin whose scheme is
   "http", this means that it MUST obtain a valid "http-opportunistic"
   response for the origin as described in [RFC8164] prior to making any
   such requests.  Other schemes might define other mechanisms.

3.3.  Connection Establishment

   HTTP/3 relies on QUIC version 1 as the underlying transport.  The use
   of other QUIC transport versions with HTTP/3 MAY be defined by future
   specifications.

   QUIC version 1 uses TLS version 1.3 or greater as its handshake
   protocol.  HTTP/3 clients MUST support a mechanism to indicate the
   target host to the server during the TLS handshake.  If the server is
   identified by a DNS name, clients MUST send the Server Name
   Indication (SNI) [RFC6066] TLS extension unless an alternative
   mechanism to indicate the target host is used.



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   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 7.2.4) 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

   HTTP/3 connections are persistent across multiple requests.  For best
   performance, it is expected that clients will not close connections
   until it is determined that no further communication with a server is
   necessary (for example, when a user navigates away from a particular
   web page) or until the server closes the connection.

   Once a connection exists to a server endpoint, this connection MAY be
   reused for requests with multiple different URI authority components.
   In general, a server is considered authoritative for all URIs with
   the "https" scheme for which the hostname in the URI is present in
   the authenticated certificate provided by the server, either as the
   CN field of the certificate subject or as a dNSName in the
   subjectAltName field of the certificate; see [RFC6125].  For a host
   that is an IP address, the client MUST verify that the address
   appears as an iPAddress in the subjectAltName field of the
   certificate.  If the hostname or address is not present in the
   certificate, the client MUST NOT consider the server authoritative
   for origins containing that hostname or address.  See Section 5.4 of
   [SEMANTICS] for more detail on authoritative access.

   Clients SHOULD NOT open more than one HTTP/3 connection to a given
   host and port pair, where the host is derived from a URI, a selected
   alternative service [ALTSVC], or a configured proxy.  A client MAY
   open multiple connections to the same IP address and UDP port using
   different transport or TLS configurations but SHOULD avoid creating
   multiple connections with the same configuration.










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   Servers are encouraged to maintain open connections for as long as
   possible but are permitted to terminate idle connections if
   necessary.  When either endpoint chooses to close the HTTP/3 session,
   the terminating endpoint SHOULD first send a GOAWAY frame
   (Section 5.2) so that both endpoints can reliably determine whether
   previously sent frames have been processed and gracefully complete or
   terminate any necessary remaining tasks.

   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].

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

   Pushed responses are sent on a server-initiated unidirectional QUIC
   stream; see Section 6.2.2.  A server sends zero or more interim HTTP
   responses, followed by a single final HTTP response, in the same
   manner as a standard response.  Push is described in more detail in
   Section 4.4.

   On a given stream, receipt of multiple requests or receipt of an
   additional HTTP response following a final HTTP response MUST be
   treated as malformed (Section 4.1.3).

   An HTTP message (request or response) consists of:

   1.  the header field section (see Section 4 of [SEMANTICS]), sent as
       a single HEADERS frame (see Section 7.2.2),

   2.  optionally, the payload body, if present (see Section 6.3.3 of
       [SEMANTICS]), sent as a series of DATA frames (see
       Section 7.2.1),

   3.  optionally, the trailer field section, if present (see
       Section 4.6 of [SEMANTICS]), sent as a single HEADERS frame.







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   Receipt of an invalid sequence of frames MUST be treated as a
   connection error of type H3_FRAME_UNEXPECTED (Section 8).  In
   particular, a DATA frame before any HEADERS frame, or a HEADERS or
   DATA frame after the trailing HEADERS frame is considered invalid.

   A server MAY send one or more PUSH_PROMISE frames (see Section 7.2.5)
   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.  These frames are not permitted in
   pushed responses; a pushed response which includes PUSH_PROMISE
   frames MUST be treated as a connection error of type
   H3_FRAME_UNEXPECTED.

   Frames of unknown types (Section 9), including reserved frames
   (Section 7.2.8) MAY be sent on a request or push stream before,
   after, or interleaved with other frames described in this section.

   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 7.1 of [HTTP11]
   MUST NOT be used.

   A response MAY consist of multiple messages when and only when one or
   more informational responses (1xx; see Section 9.2 of [SEMANTICS])
   precede a final response to the same request.  Interim responses do
   not contain a payload body or trailers.

   An HTTP request/response exchange fully consumes a client-initiated
   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
   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 H3_REQUEST_INCOMPLETE.

   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 the server does



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   not need to receive the remainder of the request, it MAY abort
   reading the request stream, send a complete response, and cleanly
   close the sending part of the stream.  The error code H3_NO_ERROR
   SHOULD be used when requesting that the client stop sending on the
   request 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.
   If the server sends a partial or complete response but does not abort
   reading, clients SHOULD continue sending the body of the request and
   close the stream normally.

4.1.1.  Field Formatting and Compression

   HTTP messages carry metadata as a series of key-value pairs, called
   HTTP fields.  For a listing of registered HTTP fields, see the
   "Hypertext Transfer Protocol (HTTP) Field Name Registry" maintained
   at https://www.iana.org/assignments/http-fields/.

   As in previous versions of HTTP, field names are strings containing a
   subset of ASCII characters that are compared in a case-insensitive
   fashion.  Properties of HTTP field names and values are discussed in
   more detail in Section 4.3 of [SEMANTICS].  As in HTTP/2, characters
   in field names MUST be converted to lowercase prior to their
   encoding.  A request or response containing uppercase characters in
   field names MUST be treated as malformed (Section 4.1.3).

   Like HTTP/2, HTTP/3 does not use the Connection header field to
   indicate connection-specific fields; in this protocol, connection-
   specific metadata is conveyed by other means.  An endpoint MUST NOT
   generate an HTTP/3 field section containing connection-specific
   fields; any message containing connection-specific fields MUST be
   treated as malformed (Section 4.1.3).

   The only exception to this is the TE header field, which MAY be
   present in an HTTP/3 request header; when it is, it MUST NOT contain
   any value other than "trailers".

   This means that an intermediary transforming an HTTP/1.x message to
   HTTP/3 will need to remove any fields nominated by the Connection
   field, along with the Connection field itself.  Such intermediaries
   SHOULD also remove other connection-specific fields, such as Keep-
   Alive, Proxy-Connection, Transfer-Encoding, and Upgrade, even if they
   are not nominated by the Connection field.








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4.1.1.1.  Pseudo-Header Fields

   Like HTTP/2, HTTP/3 employs a series of pseudo-header fields where
   the field name begins with the ':' character (ASCII 0x3a).  These
   pseudo-header fields convey the target URI, the method of the
   request, and the status code for the response.

   Pseudo-header fields are not HTTP fields.  Endpoints MUST NOT
   generate pseudo-header fields other than those defined in this
   document, except as negotiated via an extension; see Section 9.

   Pseudo-header fields are only valid in the context in which they are
   defined.  Pseudo-header fields defined for requests MUST NOT appear
   in responses; pseudo-header fields defined for responses MUST NOT
   appear in requests.  Pseudo-header fields MUST NOT appear in
   trailers.  Endpoints MUST treat a request or response that contains
   undefined or invalid pseudo-header fields as malformed
   (Section 4.1.3).

   All pseudo-header fields MUST appear in the header field section
   before regular header fields.  Any request or response that contains
   a pseudo-header field that appears in a header field section after a
   regular header field MUST be treated as malformed (Section 4.1.3).

   The following pseudo-header fields are defined for requests:

   ":method":  Contains the HTTP method (Section 7 of [SEMANTICS])

   ":scheme":  Contains the scheme portion of the target URI
      (Section 3.1 of [RFC3986])

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

   ":authority":  Contains the authority portion of the target URI
      (Section 3.2 of [RFC3986]).  The authority MUST NOT include the
      deprecated "userinfo" subcomponent for "http" or "https" schemed
      URIs.

      To ensure that the HTTP/1.1 request line can be reproduced










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      accurately, this pseudo-header field MUST be omitted when
      translating from an HTTP/1.1 request that has a request target in
      origin or asterisk form; see Section 3.2 of [HTTP11].  Clients
      that generate HTTP/3 requests directly SHOULD use the ":authority"
      pseudo-header field instead of the Host field.  An intermediary
      that converts an HTTP/3 request to HTTP/1.1 MUST create a Host
      field if one is not present in a request by copying the value of
      the ":authority" pseudo-header field.

   ":path":  Contains the path and query parts of the target URI (the
      "path-absolute" production and optionally a '?' character followed
      by the "query" production; see Sections 3.3 and 3.4 of [URI].  A
      request in asterisk form includes the value '*' for the ":path"
      pseudo-header field.

      This pseudo-header field MUST NOT be empty for "http" or "https"
      URIs; "http" or "https" URIs that do not contain a path component
      MUST include a value of '/'.  The exception to this rule is an
      OPTIONS request for an "http" or "https" URI that does not include
      a path component; these MUST include a ":path" pseudo-header field
      with a value of '*'; see Section 3.2.4 of [HTTP11].

   All HTTP/3 requests MUST include exactly one value for the ":method",
   ":scheme", and ":path" pseudo-header fields, unless it is a CONNECT
   request; see Section 4.2.

   If the ":scheme" pseudo-header field identifies a scheme which has a
   mandatory authority component (including "http" and "https"), the
   request MUST contain either an ":authority" pseudo-header field or a
   "Host" header field.  If these fields are present, they MUST NOT be
   empty.  If both fields are present, they MUST contain the same value.
   If the scheme does not have a mandatory authority component and none
   is provided in the request target, the request MUST NOT contain the
   ":authority" pseudo-header and "Host" header fields.

   An HTTP request that omits mandatory pseudo-header fields or contains
   invalid values for those pseudo-header fields is malformed
   (Section 4.1.3).

   HTTP/3 does not define a way to carry the version identifier that is
   included in the HTTP/1.1 request line.

   For responses, a single ":status" pseudo-header field is defined that
   carries the HTTP status code; see Section 9 of [SEMANTICS].  This
   pseudo-header field MUST be included in all responses; otherwise, the
   response is malformed (Section 4.1.3).





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   HTTP/3 does not define a way to carry the version or reason phrase
   that is included in an HTTP/1.1 status line.

4.1.1.2.  Field Compression

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

   To allow for better compression efficiency, the "Cookie" field
   [RFC6265] MAY be split into separate field lines, each with one or
   more cookie-pairs, before compression.  If a decompressed field
   section contains multiple cookie field lines, these MUST be
   concatenated into a single octet string using the two-octet delimiter
   of 0x3B, 0x20 (the ASCII string "; ") before being passed into a
   context other than HTTP/2 or HTTP/3, such as an HTTP/1.1 connection,
   or a generic HTTP server application.

4.1.1.3.  Header Size Constraints

   An HTTP/3 implementation MAY impose a limit on the maximum size of
   the message header it will accept on an individual HTTP message.  A
   server that receives a larger header section than it is willing to
   handle can send an HTTP 431 (Request Header Fields Too Large) status
   code ([RFC6585]).  A client can discard responses that it cannot
   process.  The size of a field list is calculated based on the
   uncompressed size of fields, including the length of the name and
   value in bytes plus an overhead of 32 bytes for each field.

   If an implementation wishes to advise its peer of this limit, it can
   be conveyed as a number of bytes in the
   SETTINGS_MAX_FIELD_SECTION_SIZE parameter.  An implementation which
   has received this parameter SHOULD NOT send an HTTP message header
   which exceeds the indicated size, as the peer will likely refuse to
   process it.  However, because this limit is applied at each hop,
   messages below this limit are not guaranteed to be accepted.

4.1.2.  Request Cancellation and Rejection

   Clients can cancel requests by resetting and aborting the request
   stream with an error code of H3_REQUEST_CANCELLED (Section 8.1).
   When the client aborts reading a response, it indicates that this
   response is no longer of interest.  Implementations SHOULD cancel
   requests by abruptly terminating any directions of a stream that are
   still open.





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   When the server rejects a request without performing any application
   processing, it SHOULD abort its response stream with the error code
   H3_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 H3_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 H3_REQUEST_CANCELLED.

   When a client resets a request with the error code
   H3_REQUEST_CANCELLED, a server MAY abruptly terminate the response
   using the error code H3_REQUEST_REJECTED if no processing was
   performed.  Clients MUST NOT use the H3_REQUEST_REJECTED error code,
   except when a server has requested closure of the request stream with
   this error 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).

4.1.3.  Malformed Requests and Responses

   A malformed request or response is one that is an otherwise valid
   sequence of frames but is invalid due to:

   *  the presence of prohibited fields or pseudo-header fields,

   *  the absence of mandatory pseudo-header fields,

   *  invalid values for pseudo-header fields,

   *  pseudo-header fields after fields,

   *  an invalid sequence of HTTP messages,

   *  the inclusion of uppercase field names, or

   *  the inclusion of invalid characters in field names or values

   A request or response that includes a payload body can include a
   Content-Length header field.  A request or response is also malformed
   if the value of a content-length header field does not equal the sum



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   of the DATA frame payload lengths that form the body.  A response
   that is defined to have no payload, as described in Section 6.3.3 of
   [SEMANTICS] can have a non-zero content-length field, even though no
   content is included in DATA frames.

   Intermediaries that process HTTP requests or responses (i.e., any
   intermediary not acting as a tunnel) MUST NOT forward a malformed
   request or response.  Malformed requests or responses that are
   detected MUST be treated as a stream error (Section 8) of type
   H3_GENERAL_PROTOCOL_ERROR.

   For malformed requests, a server MAY send an HTTP response prior to
   closing or resetting the stream.  Clients MUST NOT accept a malformed
   response.  Note that these requirements are intended to protect
   against several types of common attacks against HTTP; they are
   deliberately strict because being permissive can expose
   implementations to these vulnerabilities.

4.2.  The CONNECT Method

   The CONNECT method requests that the recipient establish a tunnel to
   the destination origin server identified by the request-target
   (Section 3.2 of [HTTP11]).  It 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 and HTTP/3, the CONNECT
   method is used to establish a tunnel over a single stream.

   A CONNECT request MUST be constructed as follows:

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

   *  The ":scheme" and ":path" pseudo-header fields are omitted

   *  The ":authority" pseudo-header field contains the host and port to
      connect to (equivalent to the authority-form of the request-target
      of CONNECT requests; see Section 3.2.3 of [HTTP11])

   The request stream remains open at the end of the request to carry
   the data to be transferred.  A CONNECT request that does not conform
   to these restrictions is malformed; see Section 4.1.3.








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   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 Section 9.3 of [SEMANTICS].

   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
   H3_FRAME_UNEXPECTED.

   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 by abruptly terminating the
   stream.  A proxy treats any error in the TCP connection, which
   includes receiving a TCP segment with the RST bit set, as a stream
   error of type H3_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.

4.3.  HTTP Upgrade

   HTTP/3 does not support the HTTP Upgrade mechanism (Section 9.9 of
   [HTTP11]) or 101 (Switching Protocols) informational status code
   (Section 9.2.2 of [SEMANTICS]).









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4.4.  Server Push

   Server push is an interaction mode 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 one or more PUSH_PROMISE frames (see Section 7.2.5) that
   carry the request fields, then included with the push stream which
   ultimately fulfills those promises.  When the same Push ID is
   promised on multiple request streams, the decompressed request field
   sections MUST contain the same fields in the same order, and both the
   name and the value in each field MUST be exact matches.

   Server push is only enabled on a connection when a client sends a
   MAX_PUSH_ID frame; see Section 7.2.7.  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 H3_ID_ERROR.

   The header section of the request message is carried by a
   PUSH_PROMISE frame (see Section 7.2.5) on the request stream which
   generated the push.  This allows the server push to be associated
   with a client request.

   Not all requests can be pushed.  A server MAY push requests which
   have the following properties:

   *  cacheable; see Section 7.2.3 of [SEMANTICS]

   *  safe; see Section 7.2.1 of [SEMANTICS]

   *  does not include a request body or trailer section

   The server MUST include a value in the ":authority" pseudo-header
   field for which the server is authoritative; see Section 3.4.

   Clients SHOULD send a CANCEL_PUSH frame upon receipt of a
   PUSH_PROMISE frame carrying a request which is not cacheable, is not
   known to be safe, that indicates the presence of a request body, or
   for which it does not consider the server authoritative.





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   Each pushed response is associated with one or more client requests.
   The push is associated with the request stream on which the
   PUSH_PROMISE frame was received.  The same server push can be
   associated with additional client requests using a PUSH_PROMISE frame
   with the same Push ID on multiple request streams.  These
   associations do not affect the operation of the protocol, but MAY be
   considered by user agents when deciding how to use pushed resources.

   Ordering of a PUSH_PROMISE in relation to certain parts of the
   response is important.  The server SHOULD send PUSH_PROMISE frames
   prior to sending HEADERS or DATA frames that reference the promised
   responses.  This reduces the 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, push stream data can arrive before the
   corresponding PUSH_PROMISE frame.  When a client receives a new push
   stream with an as-yet-unknown Push ID, both the associated client
   request and the pushed request header fields 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, the client can abort
   reading the stream with an error code of H3_REQUEST_CANCELLED.  This
   asks the server not to transfer additional data and indicates that it
   will be discarded upon receipt.

   Pushed responses that are cacheable (see Section 3 of [CACHING]) can
   be stored by the client, if it implements an HTTP cache.  Pushed
   responses are considered successfully validated on the origin server
   (e.g., if the "no-cache" cache response directive is present
   (Section 5.2.2.3 of [CACHING])) at the time the pushed response is
   received.

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





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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.

   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 10.2.2 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 initiate a graceful connection close.
   Endpoints initiate the graceful shutdown of a connection by sending a
   GOAWAY frame (Section 7.2.6).  The GOAWAY frame contains an
   identifier that indicates to the receiver the range of requests or
   pushes that were or might be processed in this connection.  The
   server sends a client-initiated bidirectional Stream ID; the client
   sends a Push ID.  Requests or pushes with the indicated identifier or
   greater are rejected by the sender of the GOAWAY.  This identifier
   MAY be zero if no requests or pushes were processed.

   The information in the GOAWAY frame enables a client and server to
   agree on which requests or pushes were accepted prior to the
   connection shutdown.  Upon sending a GOAWAY frame, the endpoint
   SHOULD explicitly cancel (see Section 4.1.2 and Section 7.2.3) any
   requests or pushes that have identifiers greater than or equal to
   that indicated, in order to clean up transport state for the affected
   streams.  The endpoint SHOULD continue to do so as more requests or
   pushes arrive.




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   Endpoints MUST NOT initiate new requests or promise new pushes on the
   connection after receipt of a GOAWAY frame from the peer.  Clients
   MAY establish a new connection to send additional requests.

   Some requests or pushes might already be in transit:

   *  Upon receipt of a GOAWAY frame, if the client has already sent
      requests with a Stream ID greater than or equal to the identifier
      received in a GOAWAY frame, those requests will not be processed.
      Clients can safely retry unprocessed requests on a different
      connection.

      Requests on Stream IDs less than the Stream ID in a GOAWAY frame
      from the server might have been processed; their status cannot be
      known until a response is received, the stream is reset
      individually, another GOAWAY is received, or the connection
      terminates.

      Servers MAY reject individual requests on streams below the
      indicated ID if these requests were not processed.

   *  If a server receives a GOAWAY frame after having promised pushes
      with a Push ID greater than or equal to the identifier received in
      a GOAWAY frame, those pushes will not be accepted.

   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.  An endpoint MAY
   send multiple GOAWAY frames indicating different identifiers, but the
   identifier in each frame MUST NOT be greater than the identifier in
   any previous frame, since clients might already have retried
   unprocessed requests on another connection.  Receiving a GOAWAY
   containing a larger identifier than previously received MUST be
   treated as a connection error of type H3_ID_ERROR.










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   An endpoint that is attempting to gracefully shut down a connection
   can send a GOAWAY frame with a value set to the maximum possible
   value (2^62-4 for servers, 2^62-1 for clients).  This ensures that
   the peer stops creating new requests or pushes.  After allowing time
   for any in-flight requests or pushes to arrive, the endpoint can send
   another GOAWAY frame indicating which requests or pushes it might
   accept before the end of the connection.  This ensures that a
   connection can be cleanly shut down without losing requests.

   A client has more flexibility in the value it chooses for the Push ID
   in a GOAWAY that it sends.  A value of 2^62 - 1 indicates that the
   server can continue fulfilling pushes which have already been
   promised, and the client can continue granting push credit as needed;
   see Section 7.2.7.  A smaller value indicates the client will reject
   pushes with Push IDs greater than or equal to this value.  Like the
   server, the client MAY send subsequent GOAWAY frames so long as the
   specified Push ID is strictly smaller than all previously sent
   values.

   Even when a GOAWAY indicates that a given request or push will not be
   processed or accepted upon receipt, the underlying transport
   resources still exist.  The endpoint that initiated these requests
   can cancel them to clean up transport state.

   Once all accepted requests and pushes have been processed, the
   endpoint 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 H3_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 indicating that the application layer has terminated the
   connection.  The application 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 in HTTP/3.

   Before closing the connection, a GOAWAY frame 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 fields 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 server SHOULD configure non-zero minimum values for the number
   of permitted streams and the initial stream flow control window.  So
   as to not unnecessarily limit parallelism, at least 100 requests
   SHOULD be permitted at a time.








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   HTTP/3 does not use server-initiated bidirectional streams, though an
   extension could define a use for these streams.  Clients MUST treat
   receipt of a server-initiated bidirectional stream as a connection
   error of type H3_STREAM_CREATION_ERROR unless such an extension has
   been negotiated.

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.

   Unidirectional Stream Header {
     Stream Type (i),
   }

                   Figure 1: 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).  [QPACK] defines two additional stream
   types.  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 excessively restrict the
   number of streams or the flow control window of these streams will
   increase the chance that the remote peer reaches the limit early and
   becomes blocked.  In particular, implementations should consider that
   remote peers may wish to exercise reserved stream behavior
   (Section 6.2.3) with some of the unidirectional streams they are
   permitted to use.  To avoid blocking, the transport parameters sent
   by both clients and servers MUST allow the peer to create at least
   one unidirectional stream for the HTTP control stream plus the number
   of unidirectional streams required by mandatory extensions (three
   being the minimum number required for the base HTTP/3 protocol and
   QPACK), and SHOULD provide at least 1,024 bytes of flow control
   credit to each stream.










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   Note that an endpoint is not required to grant additional credits to
   create more unidirectional streams if its peer consumes all the
   initial credits before creating the critical unidirectional streams.
   Endpoints SHOULD create the HTTP control stream as well as the
   unidirectional streams required by mandatory extensions (such as the
   QPACK encoder and decoder streams) first, and then create additional
   streams as allowed by their peer.

   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
   abort reading of the stream with an error code of
   H3_STREAM_CREATION_ERROR, 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
   H3_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 H3_STREAM_CREATION_ERROR.
   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 H3_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
   as it is 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.




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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 by zero or more interim HTTP responses followed by a single
   final HTTP response, as defined in Section 4.1.  Server push and Push
   IDs are described in Section 4.4.

   Only servers can push; if a server receives a client-initiated push
   stream, this MUST be treated as a connection error of type
   H3_STREAM_CREATION_ERROR.

   Push Stream Header {
     Stream Type (i) = 0x01,
     Push ID (i),
   }

                        Figure 2: 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 H3_ID_ERROR.

6.2.3.  Reserved Stream Types

   Stream types of the format "0x1f * N + 0x21" for non-negative 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.  Implementations MAY terminate these streams
   cleanly, or MAY abruptly terminate them.  When terminating abruptly,
   the error code H3_NO_ERROR or a reserved error code (Section 8.1)
   SHOULD be used.







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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.

   +--------------+----------------+----------------+--------+---------+
   | Frame        | Control Stream | Request        | Push   | Section |
   |              |                | Stream         | Stream |         |
   +==============+================+================+========+=========+
   | DATA         | No             | Yes            | Yes    | Section |
   |              |                |                |        | 7.2.1   |
   +--------------+----------------+----------------+--------+---------+
   | HEADERS      | No             | Yes            | Yes    | Section |
   |              |                |                |        | 7.2.2   |
   +--------------+----------------+----------------+--------+---------+
   | CANCEL_PUSH  | Yes            | No             | No     | Section |
   |              |                |                |        | 7.2.3   |
   +--------------+----------------+----------------+--------+---------+
   | SETTINGS     | Yes (1)        | No             | No     | Section |
   |              |                |                |        | 7.2.4   |
   +--------------+----------------+----------------+--------+---------+
   | PUSH_PROMISE | No             | Yes            | No     | Section |
   |              |                |                |        | 7.2.5   |
   +--------------+----------------+----------------+--------+---------+
   | GOAWAY       | Yes            | No             | No     | Section |
   |              |                |                |        | 7.2.6   |
   +--------------+----------------+----------------+--------+---------+
   | MAX_PUSH_ID  | Yes            | No             | No     | Section |
   |              |                |                |        | 7.2.7   |
   +--------------+----------------+----------------+--------+---------+
   | Reserved     | Yes            | Yes            | Yes    | Section |
   |              |                |                |        | 7.2.8   |
   +--------------+----------------+----------------+--------+---------+

              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.





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7.1.  Frame Layout

   All frames have the following format:

   HTTP/3 Frame Format {
     Type (i),
     Length (i),
     Frame Payload (..),
   }

                       Figure 3: 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 in bytes
      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 H3_FRAME_ERROR.

   When a stream terminates cleanly, if the last frame on the stream was
   truncated, this MUST be treated as a connection error (Section 8) of
   type H3_FRAME_ERROR.  Streams which terminate abruptly may be reset
   at any point in a 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 a control stream, the recipient MUST
   respond with a connection error (Section 8) of type
   H3_FRAME_UNEXPECTED.







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   DATA Frame {
     Type (i) = 0x0,
     Length (i),
     Data (..),
   }

                            Figure 4: DATA Frame

7.2.2.  HEADERS

   The HEADERS frame (type=0x1) is used to carry an HTTP field section,
   encoded using QPACK.  See [QPACK] for more details.

   HEADERS Frame {
     Type (i) = 0x1,
     Length (i),
     Encoded Field Section (..),
   }

                          Figure 5: HEADERS Frame

   HEADERS frames can only be sent on request / push streams.  If a
   HEADERS frame is received on a control stream, the recipient MUST
   respond with a connection error (Section 8) of type
   H3_FRAME_UNEXPECTED.

7.2.3.  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 7.2.5),
   encoded as a variable-length integer.

   When a client sends CANCEL_PUSH, it is indicating that it does not
   wish to receive the promised resource.  The server SHOULD abort
   sending the resource, but the mechanism to do so depends on the state
   of the corresponding push stream.  If the server has not yet created
   a push stream, it does not create one.  If the push stream is open,
   the server SHOULD abruptly terminate that stream.  If the push stream
   has already ended, the server MAY still abruptly terminate the stream
   or MAY take no action.

   When a server sends CANCEL_PUSH, it is indicating that it will not be
   fulfilling a promise.  The client cannot expect the corresponding
   promise to be fulfilled, unless it has already received and processed
   the promised response.  A server SHOULD send a CANCEL_PUSH even if it
   has opened the corresponding stream.




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   Sending CANCEL_PUSH has no direct effect on the state of existing
   push streams.  A client SHOULD NOT send a CANCEL_PUSH when it has
   already received a corresponding push stream.  A push stream could
   arrive after a client has sent CANCEL_PUSH, because a server might
   not have processed the CANCEL_PUSH.  The client SHOULD abort reading
   the stream with an error code of H3_REQUEST_CANCELLED.

   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 H3_FRAME_UNEXPECTED.

   CANCEL_PUSH Frame {
     Type (i) = 0x3,
     Length (i),
     Push ID (..),
   }

                        Figure 6: CANCEL_PUSH Frame

   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 7.2.5.  If a CANCEL_PUSH frame is received
   which references a Push ID greater than currently allowed on the
   connection, this MUST be treated as a connection error of type
   H3_ID_ERROR.

   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 due
   to reordering.  If a server receives a CANCEL_PUSH frame for a Push
   ID that has not yet been mentioned by a PUSH_PROMISE frame, this MUST
   be treated as a connection error of type H3_ID_ERROR.

7.2.4.  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 H3_FRAME_UNEXPECTED.




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   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
   H3_FRAME_UNEXPECTED.

   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 field section, while servers are more cautious about request
   size.

   The same setting identifier MUST NOT occur more than once in the
   SETTINGS frame.  A receiver MAY treat the presence of duplicate
   setting identifiers as a connection error of type H3_SETTINGS_ERROR.

   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.

   Setting {
     Identifier (i),
     Value (i),
   }

   SETTINGS Frame {
     Type (i) = 0x4,
     Length (i),
     Setting (..) ...,
   }

                          Figure 7: SETTINGS Frame

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

7.2.4.1.  Defined SETTINGS Parameters

   The following settings are defined in HTTP/3:

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



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   Setting identifiers of the format "0x1f * N + 0x21" for non-negative
   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.

7.2.4.2.  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.  Each endpoint SHOULD use
   these initial values to send messages before the peer's SETTINGS
   frame has arrived, as packets carrying the settings can be lost or
   delayed.  When the SETTINGS frame arrives, any settings are changed
   to their new values.

   This removes the need to wait for the SETTINGS frame before sending
   messages.  Endpoints MUST NOT require any data to be received from
   the peer prior to sending the SETTINGS frame; settings MUST be sent
   as soon as the transport is ready to send data.

   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.  1-RTT keys will always become
   available prior to SETTINGS arriving, even if the server sends
   SETTINGS immediately.  Clients SHOULD NOT wait indefinitely for
   SETTINGS to arrive before sending requests, but SHOULD process
   received datagrams in order to increase the likelihood of processing
   SETTINGS before sending the first request.












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   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
   SHOULD store the settings the server provided in the connection where
   resumption information was provided, but MAY opt not to store
   settings in certain cases (e.g., if the session ticket is received
   before the SETTINGS frame).  A client MUST comply with stored
   settings - or default values, if no values are stored - when
   attempting 0-RTT.  Once a server has provided new settings, clients
   MUST comply with those values.

   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.  If the
   server cannot determine that the settings remembered by a client are
   compatible with its current settings, it MUST NOT accept 0-RTT data.
   Remembered settings are compatible if a client complying with those
   settings would not violate the server's current settings.

   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 MUST
   include all settings which differ from their default values.  If a
   server accepts 0-RTT but then sends settings that are not compatible
   with the previously specified settings, this MUST be treated as a
   connection error of type H3_SETTINGS_ERROR.  If a server accepts
   0-RTT but then sends a SETTINGS frame that omits a setting value that
   the client understands (apart from reserved setting identifiers) that
   was previously specified to have a non-default value, this MUST be
   treated as a connection error of type H3_SETTINGS_ERROR.

7.2.5.  PUSH_PROMISE

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

   PUSH_PROMISE Frame {
     Type (i) = 0x5,
     Length (i),
     Push ID (i),
     Encoded Field Section (..),
   }

                        Figure 8: PUSH_PROMISE Frame

   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 7.2.3).

   Encoded Field Section:  QPACK-encoded 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 7.2.7).  A client MUST treat
   receipt of a PUSH_PROMISE frame that contains a larger Push ID than
   the client has advertised as a connection error of H3_ID_ERROR.

   A server MAY use the same Push ID in multiple PUSH_PROMISE frames.
   If so, the decompressed request header sets MUST contain the same
   fields in the same order, and both the name and the value in each
   field MUST be exact matches.  Clients SHOULD compare the request
   header sections for resources promised multiple times.  If a client
   receives a Push ID that has already been promised and detects a
   mismatch, it MUST respond with a connection error of type
   H3_GENERAL_PROTOCOL_ERROR.  If the decompressed field sections match
   exactly, the client SHOULD associate the pushed content with each
   stream on which a PUSH_PROMISE was received.

   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 PUSH_PROMISE that uses a Push ID that they
   have already consumed and discarded are forced to ignore the
   PUSH_PROMISE.

   If a PUSH_PROMISE frame is received on the control stream, the client
   MUST respond with a connection error (Section 8) of type
   H3_FRAME_UNEXPECTED.

   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
   H3_FRAME_UNEXPECTED.

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










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7.2.6.  GOAWAY

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

   GOAWAY Frame {
     Type (i) = 0x7,
     Length (i),
     Stream ID/Push ID (..),
   }

                           Figure 9: GOAWAY Frame

   The GOAWAY frame is always sent on the control stream.  In the server
   to client direction, 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 H3_ID_ERROR.

   In the client to server direction, the GOAWAY frame carries a Push ID
   encoded as a variable-length integer.

   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 H3_FRAME_UNEXPECTED.

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

7.2.7.  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 PUSH_PROMISE
   and CANCEL_PUSH frames.  Consequently, this also limits the number of
   push streams that the server can initiate in addition to the limit
   maintained by the QUIC transport.

   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 H3_FRAME_UNEXPECTED.

   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
   H3_FRAME_UNEXPECTED.



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   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.

   MAX_PUSH_ID Frame {
     Type (i) = 0x1,
     Length (i),
     Push ID (..),
   }

                    Figure 10: 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 7.2.5.  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
   H3_ID_ERROR.

7.2.8.  Reserved Frame Types

   Frame types of the format "0x1f * N + 0x21" for non-negative 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 on any open stream 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.

   Frame types which were used in HTTP/2 where there is no corresponding
   HTTP/3 frame have also been reserved (Section 11.2.1).  These frame
   types MUST NOT be sent, and receipt MAY be treated as an error of
   type H3_FRAME_UNEXPECTED.

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].






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   An endpoint MAY choose to treat a stream error as a connection error
   under certain circumstances.  Implementations need to consider the
   impact on outstanding requests before making this choice.

   Because new error codes can be defined without negotiation (see
   Section 9), use of an error code in an unexpected context or receipt
   of an unknown error code MUST be treated as equivalent to
   H3_NO_ERROR.  However, closing a stream can have other effects
   regardless of the error code; see Section 4.1.

   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 when abruptly
   terminating streams, aborting reading of streams, or immediately
   closing connections.

   H3_NO_ERROR (0x100):  No error.  This is used when the connection or
      stream needs to be closed, but there is no error to signal.

   H3_GENERAL_PROTOCOL_ERROR (0x101):  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.

   H3_INTERNAL_ERROR (0x102):  An internal error has occurred in the
      HTTP stack.

   H3_STREAM_CREATION_ERROR (0x103):  The endpoint detected that its
      peer created a stream that it will not accept.

   H3_CLOSED_CRITICAL_STREAM (0x104):  A stream required by the
      connection was closed or reset.

   H3_FRAME_UNEXPECTED (0x105):  A frame was received which was not
      permitted in the current state or on the current stream.

   H3_FRAME_ERROR (0x106):  A frame that fails to satisfy layout
      requirements or with an invalid size was received.

   H3_EXCESSIVE_LOAD (0x107):  The endpoint detected that its peer is
      exhibiting a behavior that might be generating excessive load.

   H3_ID_ERROR (0x108):  A Stream ID or Push ID was used incorrectly,
      such as exceeding a limit, reducing a limit, or being reused.

   H3_SETTINGS_ERROR (0x109):  An endpoint detected an error in the



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      payload of a SETTINGS frame.

   H3_MISSING_SETTINGS (0x10A):  No SETTINGS frame was received at the
      beginning of the control stream.

   H3_REQUEST_REJECTED (0x10B):  A server rejected a request without
      performing any application processing.

   H3_REQUEST_CANCELLED (0x10C):  The request or its response (including
      pushed response) is cancelled.

   H3_REQUEST_INCOMPLETE (0x10D):  The client's stream terminated
      without containing a fully-formed request.

   H3_CONNECT_ERROR (0x10F):  The connection established in response to
      a CONNECT request was reset or abnormally closed.

   H3_VERSION_FALLBACK (0x110):  The requested operation cannot be
      served over HTTP/3.  The peer should retry over HTTP/1.1.

   Error codes of the format "0x1f * N + 0x21" for non-negative integer
   values of N are reserved to exercise the requirement that unknown
   error codes be treated as equivalent to H3_NO_ERROR (Section 9).
   Implementations SHOULD select an error code from this space with some
   probability when they would have sent H3_NO_ERROR.

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 fields.

   Extensions are permitted to use new frame types (Section 7.2), new
   settings (Section 7.2.4.1), new error codes (Section 8), or new
   unidirectional stream types (Section 6.2).  Registries are
   established for managing these extension points: frame types
   (Section 11.2.1), settings (Section 11.2.2), error codes
   (Section 11.2.3), and stream types (Section 11.2.4).

   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



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   extensions without prior arrangement or negotiation.  However, where
   a known frame type is required to be in a specific location, such as
   the SETTINGS frame as the first frame of the control stream (see
   Section 6.2.1), an unknown frame type does not satisfy that
   requirement and SHOULD be treated as an error.

   Extensions that could change the semantics of existing protocol
   components MUST be negotiated before being used.  For example, an
   extension that changes the layout of the HEADERS frame cannot be used
   until the peer has given a positive signal that this is acceptable.
   Coordinating when such a revised layout comes into effect could prove
   complex.  As such, allocating new identifiers for new definitions of
   existing protocol elements is likely to be more effective.

   This document doesn't mandate a specific method for negotiating the
   use of an extension but notes that a setting (Section 7.2.4.1) 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
   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.  However, many of the considerations from
   Section 10 of [HTTP2] apply to [QUIC-TRANSPORT] and are discussed in
   that document.

10.1.  Server Authority

   HTTP/3 relies on the HTTP definition of authority.  The security
   considerations of establishing authority are discussed in
   Section 11.1 of [SEMANTICS].

10.2.  Cross-Protocol Attacks

   The use of ALPN in the TLS and QUIC handshakes establishes the target
   application protocol before application-layer bytes are processed.
   Because all QUIC packets are encrypted, it is difficult for an
   attacker to control the plaintext bytes of an HTTP/3 connection which
   could be used in a cross-protocol attack on a plaintext protocol.









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10.3.  Intermediary Encapsulation Attacks

   The HTTP/3 field encoding allows the expression of names that are not
   valid field names in the syntax used by HTTP (Section 4.3 of
   [SEMANTICS]).  Requests or responses containing invalid field names
   MUST be treated as malformed (Section 4.1.3).  An intermediary
   therefore cannot translate an HTTP/3 request or response containing
   an invalid field name into an HTTP/1.1 message.

   Similarly, HTTP/3 allows field values that are not valid.  While most
   of the values that can be encoded will not alter field parsing,
   carriage return (CR, ASCII 0xd), line feed (LF, ASCII 0xa), and the
   zero character (NUL, ASCII 0x0) might be exploited by an attacker if
   they are translated verbatim.  Any request or response that contains
   a character not permitted in a field value MUST be treated as
   malformed (Section 4.1.3).  Valid characters are defined by the
   "field-content" ABNF rule in Section 4.4 of [SEMANTICS].

10.4.  Cacheability of Pushed Responses

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

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

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

   Pushed responses for which an origin server is not authoritative (see
   Section 3.4) MUST NOT be used or cached.

10.5.  Denial-of-Service Considerations

   An HTTP/3 connection can demand a greater commitment of resources to
   operate than an HTTP/1.1 or HTTP/2 connection.  The use of field
   compression and flow control depend on a commitment of resources for
   storing a greater amount of state.  Settings for these features
   ensure that memory commitments for these features are strictly
   bounded.




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   The number of PUSH_PROMISE frames is constrained in a similar
   fashion.  A client that accepts server push SHOULD limit the number
   of Push IDs it issues at a time.

   Processing capacity cannot be guarded as effectively as state
   capacity.

   The ability to send undefined protocol elements which the peer is
   required to ignore can be abused to cause a peer to expend additional
   processing time.  This might be done by setting multiple undefined
   SETTINGS parameters, unknown frame types, or unknown stream types.
   Note, however, that some uses are entirely legitimate, such as
   optional-to-understand extensions and padding to increase resistance
   to traffic analysis.

   Compression of field sections also offers some opportunities to waste
   processing resources; see Section 7 of [QPACK] for more details on
   potential abuses.

   All these features - i.e., server push, unknown protocol elements,
   field compression - have legitimate uses.  These features become a
   burden only when they are used unnecessarily or to excess.

   An endpoint that doesn't monitor this behavior exposes itself to a
   risk of denial-of-service attack.  Implementations SHOULD track the
   use of these features and set limits on their use.  An endpoint MAY
   treat activity that is suspicious as a connection error (Section 8)
   of type H3_EXCESSIVE_LOAD, but false positives will result in
   disrupting valid connections and requests.

10.5.1.  Limits on Field Section Size

   A large field section (Section 4.1) can cause an implementation to
   commit a large amount of state.  Header fields that are critical for
   routing can appear toward the end of a header field section, which
   prevents streaming of the header field section to its ultimate
   destination.  This ordering and other reasons, such as ensuring cache
   correctness, mean that an endpoint likely needs to buffer the entire
   header field section.  Since there is no hard limit to the size of a
   field section, some endpoints could be forced to commit a large
   amount of available memory for header fields.

   An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE
   (Section 7.2.4.1) setting to advise peers of limits that might apply
   on the size of field sections.  This setting is only advisory, so
   endpoints MAY choose to send field sections that exceed this limit
   and risk having the request or response being treated as malformed.
   This setting is specific to a connection, so any request or response



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   could encounter a hop with a lower, unknown limit.  An intermediary
   can attempt to avoid this problem by passing on values presented by
   different peers, but they are not obligated to do so.

   A server that receives a larger field section than it is willing to
   handle can send an HTTP 431 (Request Header Fields Too Large) status
   code [RFC6585].  A client can discard responses that it cannot
   process.

10.5.2.  CONNECT Issues

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

10.6.  Use of Compression

   Compression can allow an attacker to recover secret data when it is
   compressed in the same context as data under attacker control.
   HTTP/3 enables compression of fields (Section 4.1.1); the following
   concerns also apply to the use of HTTP compressed content-codings;
   see Section 6.1.2 of [SEMANTICS].

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

   Implementations communicating on a secure channel MUST NOT compress
   content that includes both confidential and attacker-controlled data
   unless separate compression dictionaries are used for each source of
   data.  Compression MUST NOT be used if the source of data cannot be
   reliably determined.

   Further considerations regarding the compression of fields sections
   are described in [QPACK].









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10.7.  Padding and Traffic Analysis

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

   Where HTTP/2 employs PADDING frames and Padding fields in other
   frames to make a connection more resistant to traffic analysis,
   HTTP/3 can either rely on transport-layer padding or employ the
   reserved frame and stream types discussed in Section 7.2.8 and
   Section 6.2.3.  These methods of padding produce different results in
   terms of the granularity of padding, how padding is arranged in
   relation to the information that is being protected, whether padding
   is applied in the case of packet loss, and how an implementation
   might control padding.  Redundant padding could even be
   counterproductive.

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

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

10.8.  Frame Parsing

   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.

10.9.  Early Data

   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.







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10.10.  Migration

   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.

10.11.  Privacy Considerations

   Several characteristics of HTTP/3 provide an observer an opportunity
   to correlate actions of a single client or server over time.  These
   include the value of settings, the timing of reactions to stimulus,
   and the handling of any features that are controlled by settings.

   As far as these create observable differences in behavior, they could
   be used as a basis for fingerprinting a specific client.

   HTTP/3's preference for using a single QUIC connection allows
   correlation of a user's activity on a site.  Reusing connections for
   different origins allows for correlation of activity across those
   origins.

   Several features of QUIC solicit immediate responses and can be used
   by an endpoint to measure latency to their peer; this might have
   privacy implications in certain scenarios.

11.  IANA Considerations

   This document registers a new ALPN protocol ID (Section 11.1) and
   creates new registries that manage the assignment of codepoints in
   HTTP/3.

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.  New Registries

   New registries created in this document operate under the QUIC
   registration policy documented in Section 22.1 of [QUIC-TRANSPORT].
   These registries all include the common set of fields listed in
   Section 22.1.1 of [QUIC-TRANSPORT].

   The initial allocations in these registries created in this document
   are all assigned permanent status and list as contact both the IESG
   (iesg@ietf.org) and the HTTP working group (ietf-http-wg@w3.org).

11.2.1.  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
   registry follows the QUIC registry policy; see Section 11.2.
   Permanent registrations in this registry are assigned using the
   Specification Required policy [RFC8126], except for values between
   0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using
   Standards Action or IESG Approval as defined in Section 4.9 and 4.10
   of [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.

   In addition to common fields as described in Section 11.2, permanent
   registrations in this registry MUST include the following field:

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

   Specifications of frame types MUST include a description of the frame
   layout and its semantics, including any parts of the frame that are
   conditionally present.

   The entries in Table 2 are registered by this document.













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

                   Table 2: Initial HTTP/3 Frame Types

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

11.2.2.  Settings Parameters

   This document establishes a registry for HTTP/3 settings.  The
   "HTTP/3 Settings" registry governs a 62-bit space.  This registry
   follows the QUIC registry policy; see Section 11.2.  Permanent
   registrations in this registry are assigned using the Specification
   Required policy [RFC8126], except for values between 0x00 and 0x3f
   (in hexadecimal; inclusive), which are assigned using Standards
   Action or IESG Approval as defined in Section 4.9 and 4.10 of
   [RFC8126].

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



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   In addition to common fields as described in Section 11.2, permanent
   registrations in this registry MUST include the following fields:

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

   Default:  The value of the setting unless otherwise indicated.  A
      default SHOULD be the most restrictive possible value.

   The entries in Table 3 are registered by this document.

     +------------------------+-------+-----------------+-----------+
     | Setting Name           | Value | Specification   | Default   |
     +========================+=======+=================+===========+
     | Reserved               |  0x2  | N/A             | N/A       |
     +------------------------+-------+-----------------+-----------+
     | Reserved               |  0x3  | N/A             | N/A       |
     +------------------------+-------+-----------------+-----------+
     | Reserved               |  0x4  | N/A             | N/A       |
     +------------------------+-------+-----------------+-----------+
     | Reserved               |  0x5  | N/A             | N/A       |
     +------------------------+-------+-----------------+-----------+
     | MAX_FIELD_SECTION_SIZE |  0x6  | Section 7.2.4.1 | Unlimited |
     +------------------------+-------+-----------------+-----------+

                     Table 3: Initial HTTP/3 Settings

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

11.2.3.  Error Codes

   This document establishes a registry for HTTP/3 error codes.  The
   "HTTP/3 Error Code" registry manages a 62-bit space.  This registry
   follows the QUIC registry policy; see Section 11.2.  Permanent
   registrations in this registry are assigned using the Specification
   Required policy [RFC8126], except for values between 0x00 and 0x3f
   (in hexadecimal; inclusive), which are assigned using Standards
   Action or IESG Approval as defined in Section 4.9 and 4.10 of
   [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.





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   In addition to common fields as described in Section 11.2, permanent
   registrations in this registry MUST include the following fields:

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

   Description:  A brief description of the error code semantics.

   The entries in the Table 4 are registered by this document.

   +---------------------------+--------+--------------+---------------+
   | Name                      | Value  | Description  | Specification |
   +===========================+========+==============+===============+
   | H3_NO_ERROR               | 0x0100 | No error     | Section 8.1   |
   +---------------------------+--------+--------------+---------------+
   | H3_GENERAL_PROTOCOL_ERROR | 0x0101 | General      | Section 8.1   |
   |                           |        | protocol     |               |
   |                           |        | error        |               |
   +---------------------------+--------+--------------+---------------+
   | H3_INTERNAL_ERROR         | 0x0102 | Internal     | Section 8.1   |
   |                           |        | error        |               |
   +---------------------------+--------+--------------+---------------+
   | H3_STREAM_CREATION_ERROR  | 0x0103 | Stream       | Section 8.1   |
   |                           |        | creation     |               |
   |                           |        | error        |               |
   +---------------------------+--------+--------------+---------------+
   | H3_CLOSED_CRITICAL_STREAM | 0x0104 | Critical     | Section 8.1   |
   |                           |        | stream was   |               |
   |                           |        | closed       |               |
   +---------------------------+--------+--------------+---------------+
   | H3_FRAME_UNEXPECTED       | 0x0105 | Frame not    | Section 8.1   |
   |                           |        | permitted    |               |
   |                           |        | in the       |               |
   |                           |        | current      |               |
   |                           |        | state        |               |
   +---------------------------+--------+--------------+---------------+
   | H3_FRAME_ERROR            | 0x0106 | Frame        | Section 8.1   |
   |                           |        | violated     |               |
   |                           |        | layout or    |               |
   |                           |        | size rules   |               |
   +---------------------------+--------+--------------+---------------+
   | H3_EXCESSIVE_LOAD         | 0x0107 | Peer         | Section 8.1   |
   |                           |        | generating   |               |
   |                           |        | excessive    |               |
   |                           |        | load         |               |
   +---------------------------+--------+--------------+---------------+
   | H3_ID_ERROR               | 0x0108 | An           | Section 8.1   |
   |                           |        | identifier   |               |



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   |                           |        | was used     |               |
   |                           |        | incorrectly  |               |
   +---------------------------+--------+--------------+---------------+
   | H3_SETTINGS_ERROR         | 0x0109 | SETTINGS     | Section 8.1   |
   |                           |        | frame        |               |
   |                           |        | contained    |               |
   |                           |        | invalid      |               |
   |                           |        | values       |               |
   +---------------------------+--------+--------------+---------------+
   | H3_MISSING_SETTINGS       | 0x010A | No SETTINGS  | Section 8.1   |
   |                           |        | frame        |               |
   |                           |        | received     |               |
   +---------------------------+--------+--------------+---------------+
   | H3_REQUEST_REJECTED       | 0x010B | Request not  | Section 8.1   |
   |                           |        | processed    |               |
   +---------------------------+--------+--------------+---------------+
   | H3_REQUEST_CANCELLED      | 0x010C | Data no      | Section 8.1   |
   |                           |        | longer       |               |
   |                           |        | needed       |               |
   +---------------------------+--------+--------------+---------------+
   | H3_REQUEST_INCOMPLETE     | 0x010D | Stream       | Section 8.1   |
   |                           |        | terminated   |               |
   |                           |        | early        |               |
   +---------------------------+--------+--------------+---------------+
   | H3_CONNECT_ERROR          | 0x010F | TCP reset    | Section 8.1   |
   |                           |        | or error on  |               |
   |                           |        | CONNECT      |               |
   |                           |        | request      |               |
   +---------------------------+--------+--------------+---------------+
   | H3_VERSION_FALLBACK       | 0x0110 | Retry over   | Section 8.1   |
   |                           |        | HTTP/1.1     |               |
   +---------------------------+--------+--------------+---------------+

                    Table 4: Initial HTTP/3 Error Codes

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













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11.2.4.  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 registry follows the QUIC registry policy; see Section 11.2.
   Permanent registrations in this registry are assigned using the
   Specification Required policy [RFC8126], except for values between
   0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using
   Standards Action or IESG Approval as defined in Section 4.9 and 4.10
   of [RFC8126].

   In addition to common fields as described in Section 11.2, permanent
   registrations in this registry MUST include the following fields:

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

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

   Specifications for permanent registrations MUST include a description
   of the stream type, including the layout semantics of the stream
   contents.

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

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

                                  Table 5

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

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>.





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   [CACHING]  Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-08, 26 May 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-httpbis-cache-08.txt>.

   [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>.

   [QPACK]    Krasic, C., Bishop, M., and A. Frindell, Ed., "QPACK:
              Header Compression for HTTP over QUIC", Work in Progress,
              Internet-Draft, draft-ietf-quic-qpack-16, 9 June 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-qpack-16>.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", Work in Progress,
              Internet-Draft, draft-ietf-quic-transport-28, 9 June 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-transport-
              28>.

   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

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

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

   [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>.

   [RFC8164]  Nottingham, M. and M. Thomson, "Opportunistic Security for
              HTTP/2", RFC 8164, DOI 10.17487/RFC8164, May 2017,
              <https://www.rfc-editor.org/info/rfc8164>.

   [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>.

   [SEMANTICS]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-08, 26 May 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-httpbis-semantics-08.txt>.

12.2.  Informative References

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

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

   [HTTP11]   Fielding, R., Nottingham, M., and J. Reschke, "HTTP/1.1
              Messaging", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-08, 26 May 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-httpbis-messaging-08.txt>.






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   [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>.

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

   [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>.

   [TFO]      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>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

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.







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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.

   In contrast to HTTP/2, stream concurrency in HTTP/3 is managed by
   QUIC.  QUIC considers a stream closed when all data has been received
   and sent data has been acknowledged by the peer.  HTTP/2 considers a
   stream closed when the frame containing the END_STREAM bit has been
   committed to the transport.  As a result, the stream for an
   equivalent exchange could remain "active" for a longer period of
   time.  HTTP/3 servers might choose to permit a larger number of
   concurrent client-initiated bidirectional streams to achieve
   equivalent concurrency to HTTP/2, depending on the expected usage
   patterns.

   Due to the presence of other unidirectional stream types, HTTP/3 does
   not rely exclusively on the number of concurrent unidirectional
   streams to control the number of concurrent in-flight pushes.
   Instead, HTTP/3 clients use the MAX_PUSH_ID frame to control the
   number of pushes received from an HTTP/3 server.

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.







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

   Some examples of feature adaptations are described below, as well as
   general guidance to extension frame implementors converting an HTTP/2
   extension to HTTP/3.

A.2.1.  Prioritization Differences

   HTTP/2 specifies priority assignments in PRIORITY frames and
   (optionally) in HEADERS frames.  HTTP/3 does not provide a means of
   signaling priority.

   Note that while there is no explicit signaling for priority, this
   does not mean that prioritization is not important for achieving good
   performance.

A.2.2.  Field Compression Differences

   HPACK was designed with the assumption of in-order delivery.  A
   sequence of encoded field sections 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.

   Because this total ordering is not provided by QUIC, HTTP/3 uses a
   modified version of HPACK, called QPACK.  QPACK uses a single
   unidirectional stream to make all modifications to the dynamic table,
   ensuring a total order of updates.  All frames which contain encoded
   fields merely reference the table state at a given time without
   modifying it.

   [QPACK] provides additional details.

A.2.3.  Guidance for New Frame Type Definitions

   Frame type definitions in HTTP/3 often use the QUIC variable-length
   integer encoding.  In particular, Stream IDs use this encoding, which
   allows 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).  Redefinition of the encoding of
   extension frame types might be necessary if the encoding includes a
   Stream ID.






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   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.

A.2.4.  Mapping Between HTTP/2 and HTTP/3 Frame Types

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

   HEADERS (0x1):  The PRIORITY region of HEADERS is not defined in
      HTTP/3 frames.  Padding is not defined in HTTP/3 frames.  See
      Section 7.2.2.

   PRIORITY (0x2):  As described in Appendix A.2.1, HTTP/3 does not
      provide a means of signaling priority.

   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 7.2.3).

   SETTINGS (0x4):  SETTINGS frames are sent only at the beginning of
      the connection.  See Section 7.2.4 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 7.2.5.

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

   GOAWAY (0x7):  GOAWAY does not contain an error code.  In the client
      to server direction, it carries a Push ID instead of a server
      initiated stream ID.  See Section 7.2.6.

   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.






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   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 11.2.1.

A.3.  HTTP/2 SETTINGS Parameters

   An important difference from HTTP/2 is that settings are sent once,
   as the first frame of the control stream, 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.

   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_FIELD_SECTION_SIZE:  See Section 7.2.4.1.

   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 their value to limit it
   to 30 bits for more efficient encoding, or to make use of the 62-bit
   space if more than 30 bits are required.




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   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 11.2.2.

   As QUIC streams might arrive out-of-order, endpoints are advised to
   not wait for the peers' settings to arrive before responding to other
   streams.  See Section 7.2.4.2.

A.4.  HTTP/2 Error Codes

   QUIC has the same concepts of "stream" and "connection" errors that
   HTTP/2 provides.  However, the differences between HTTP/2 and HTTP/3
   mean that error codes are not directly portable between versions.

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

   NO_ERROR (0x0):  H3_NO_ERROR in Section 8.1.

   PROTOCOL_ERROR (0x1):  This is mapped to H3_GENERAL_PROTOCOL_ERROR
      except in cases where more specific error codes have been defined.
      This includes H3_FRAME_UNEXPECTED and H3_CLOSED_CRITICAL_STREAM
      defined in Section 8.1.

   INTERNAL_ERROR (0x2):  H3_INTERNAL_ERROR in Section 8.1.

   FLOW_CONTROL_ERROR (0x3):  Not applicable, since QUIC handles flow
      control.

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

   STREAM_CLOSED (0x5):  Not applicable, since QUIC handles stream
      management.

   FRAME_SIZE_ERROR (0x6):  H3_FRAME_ERROR error code defined in
      Section 8.1.

   REFUSED_STREAM (0x7):  H3_REQUEST_REJECTED (in Section 8.1) is used
      to indicate that a request was not processed.  Otherwise, not
      applicable because QUIC handles stream management.

   CANCEL (0x8):  H3_REQUEST_CANCELLED in Section 8.1.

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



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   CONNECT_ERROR (0xa):  H3_CONNECT_ERROR in Section 8.1.

   ENHANCE_YOUR_CALM (0xb):  H3_EXCESSIVE_LOAD in Section 8.1.

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

   H3_1_1_REQUIRED (0xd):  H3_VERSION_FALLBACK in Section 8.1.

   Error codes need to be defined for HTTP/2 and HTTP/3 separately.  See
   Section 11.2.3.

A.4.1.  Mapping Between HTTP/2 and HTTP/3 Errors

   An intermediary that converts between HTTP/2 and HTTP/3 may encounter
   error conditions from either upstream.  It is useful to communicate
   the occurrence of error to the downstream but error codes largely
   reflect connection-local problems that generally do not make sense to
   propagate.

   An intermediary that encounters an error from an upstream origin can
   indicate this by sending an HTTP status code such as 502, which is
   suitable for a broad class of errors.

   There are some rare cases where it is beneficial to propagate the
   error by mapping it to the closest matching error type to the
   receiver.  For example, an intermediary that receives an HTTP/2
   stream error of type REFUSED_STREAM from the origin has a clear
   signal that the request was not processed and that the request is
   safe to retry.  Propagating this error condition to the client as an
   HTTP/3 stream error of type H3_REQUEST_REJECTED allows the client to
   take the action it deems most appropriate.  In the reverse direction
   the intermediary might deem it beneficial to pass on client request
   cancellations that are indicated by terminating a stream with
   H3_REQUEST_CANCELLED.

   Conversion between errors is described in the logical mapping.  The
   error codes are defined in non-overlapping spaces in order to protect
   against accidental conversion that could result in the use of
   inappropriate or unknown error codes for the target version.  An
   intermediary is permitted to promote stream errors to connection
   errors but they should be aware of the cost to the connection for
   what might be a temporary or intermittent error.

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-28

   *  CANCEL_PUSH is recommended even when the stream is reset (#3698,
      #3700)

   *  Use H3_ID_ERROR when GOAWAY contains a larger identifier (#3631,
      #3634)

B.2.  Since draft-ietf-quic-http-27

   *  Updated text to refer to latest HTTP revisions

   *  Use the HTTP definition of authority for establishing and
      coalescing connections (#253, #2223, #3558)

   *  Define use of GOAWAY from both endpoints (#2632, #3129)

   *  Require either :authority or Host if the URI scheme has a
      mandatory authority component (#3408, #3475)

B.3.  Since draft-ietf-quic-http-26

   *  No changes

B.4.  Since draft-ietf-quic-http-25

   *  Require QUICv1 for HTTP/3 (#3117, #3323)

   *  Remove DUPLICATE_PUSH and allow duplicate PUSH_PROMISE (#3275,
      #3309)

   *  Clarify the definition of "malformed" (#3352, #3345)

B.5.  Since draft-ietf-quic-http-24

   *  Removed H3_EARLY_RESPONSE error code; H3_NO_ERROR is recommended
      instead (#3130,#3208)

   *  Unknown error codes are equivalent to H3_NO_ERROR (#3276,#3331)

   *  Some error codes are reserved for greasing (#3325,#3360)

B.6.  Since draft-ietf-quic-http-23

   *  Removed "quic" Alt-Svc parameter (#3061,#3118)

   *  Clients need not persist unknown settings for use in 0-RTT
      (#3110,#3113)



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   *  Clarify error cases around CANCEL_PUSH (#2819,#3083)

B.7.  Since draft-ietf-quic-http-22

   *  Removed priority signaling (#2922,#2924)

   *  Further changes to error codes (#2662,#2551):

      -  Error codes renumbered

      -  HTTP_MALFORMED_FRAME replaced by HTTP_FRAME_ERROR,
         HTTP_ID_ERROR, and others

   *  Clarify how unknown frame types interact with required frame
      sequence (#2867,#2858)

   *  Describe interactions with the transport in terms of defined
      interface terms (#2857,#2805)

   *  Require the use of the "http-opportunistic" resource (RFC 8164)
      when scheme is "http" (#2439,#2973)

   *  Settings identifiers cannot be duplicated (#2979)

   *  Changes to SETTINGS frames in 0-RTT (#2972,#2790,#2945):

      -  Servers must send all settings with non-default values in their
         SETTINGS frame, even when resuming

      -  If a client doesn't have settings associated with a 0-RTT
         ticket, it uses the defaults

      -  Servers can't accept early data if they cannot recover the
         settings the client will have remembered

   *  Clarify that Upgrade and the 101 status code are prohibited
      (#2898,#2889)

   *  Clarify that frame types reserved for greasing can occur on any
      stream, but frame types reserved due to HTTP/2 correspondence are
      prohibited (#2997,#2692,#2693)

   *  Unknown error codes cannot be treated as errors (#2998,#2816)

B.8.  Since draft-ietf-quic-http-21

   No changes




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B.9.  Since draft-ietf-quic-http-20

   *  Prohibit closing the control stream (#2509, #2666)

   *  Change default priority to use an orphan node (#2502, #2690)

   *  Exclusive priorities are restored (#2754, #2781)

   *  Restrict use of frames when using CONNECT (#2229, #2702)

   *  Close and maybe reset streams if a connection error occurs for
      CONNECT (#2228, #2703)

   *  Encourage provision of sufficient unidirectional streams for QPACK
      (#2100, #2529, #2762)

   *  Allow extensions to use server-initiated bidirectional streams
      (#2711, #2773)

   *  Clarify use of maximum header list size setting (#2516, #2774)

   *  Extensive changes to error codes and conditions of their sending

      -  Require connection errors for more error conditions (#2511,
         #2510)

      -  Updated the error codes for illegal GOAWAY frames (#2714,
         #2707)

      -  Specified error code for HEADERS on control stream (#2708)

      -  Specified error code for servers receiving PUSH_PROMISE (#2709)

      -  Specified error code for receiving DATA before HEADERS (#2715)

      -  Describe malformed messages and their handling (#2410, #2764)

      -  Remove HTTP_PUSH_ALREADY_IN_CACHE error (#2812, #2813)

      -  Refactor Push ID related errors (#2818, #2820)

      -  Rationalize HTTP/3 stream creation errors (#2821, #2822)

B.10.  Since draft-ietf-quic-http-19

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





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

B.11.  Since draft-ietf-quic-http-18

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

   *  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

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

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

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

B.12.  Since draft-ietf-quic-http-17

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

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

B.13.  Since draft-ietf-quic-http-16

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

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

      -  Permitted as first frame of request streams

      -  Remove exclusive reprioritization

      -  Changes to Prioritized Element Type bits

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

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



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

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

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

B.14.  Since draft-ietf-quic-http-15

   Substantial editorial reorganization; no technical changes.

B.15.  Since draft-ietf-quic-http-14

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

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

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

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

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

B.16.  Since draft-ietf-quic-http-13

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

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

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

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

B.17.  Since draft-ietf-quic-http-12

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

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

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




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

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

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

B.18.  Since draft-ietf-quic-http-11

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

B.19.  Since draft-ietf-quic-http-10

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

B.20.  Since draft-ietf-quic-http-09

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

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

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

B.21.  Since draft-ietf-quic-http-08

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

B.22.  Since draft-ietf-quic-http-07

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

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

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

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

B.23.  Since draft-ietf-quic-http-06

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

B.24.  Since draft-ietf-quic-http-05





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

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

   *  Expanded text on GOAWAY and cancellation (#757)

B.25.  Since draft-ietf-quic-http-04

   *  Cite RFC 5234 (#404)

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

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

   *  SETTINGS_ENABLE_PUSH instead of SETTINGS_DISABLE_PUSH (#477)

   *  Restored GOAWAY (#696)

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

   *  DATA frames cannot be empty (#700)

B.26.  Since draft-ietf-quic-http-03

   None.

B.27.  Since draft-ietf-quic-http-02

   *  Track changes in transport draft

B.28.  Since draft-ietf-quic-http-01

   *  SETTINGS changes (#181):

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

      -  SETTINGS_ACK removed

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

      -  Boolean format updated

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




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

   *  HPACK Sequence counter can wrap (#173)

   *  0-RTT guidance added

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

B.29.  Since draft-ietf-quic-http-00

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

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

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

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

   *  Described CONNECT pseudo-method (#95)

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

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

B.30.  Since draft-shade-quic-http2-mapping-00

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

   *  Updated authors/editors list

Acknowledgements

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

   The IETF QUIC Working Group received an enormous amount of support
   from many people.  Among others, the following people provided
   substantial contributions to this document:

   *  Bence Beky

   *  Daan De Meyer




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   *  Martin Duke

   *  Roy Fielding

   *  Alan Frindell

   *  Alessandro Ghedini

   *  Nick Harper

   *  Ryan Hamilton

   *  Christian Huitema

   *  Subodh Iyengar

   *  Robin Marx

   *  Patrick McManus

   *  Luca Nicco

   *  奥 一穂 (Kazuho Oku)

   *  Lucas Pardue

   *  Roberto Peon

   *  Julian Reschke

   *  Eric Rescorla

   *  Martin Seemann

   *  Ben Schwartz

   *  Ian Swett

   *  Willy Taureau

   *  Martin Thomson

   *  Dmitri Tikhonov

   *  Tatsuhiro Tsujikawa

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



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Author's Address

   Mike Bishop (editor)
   Akamai

   Email: mbishop@evequefou.be













































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