draft-ietf-quic-http.txt

QUIC                                                      M. Bishop, Ed.
Internet-Draft                                                    Akamai
Intended status: Standards Track                         27 January 2021
Expires: 31 July 2021


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

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.

DO NOT DEPLOY THIS VERSION OF HTTP

   DO NOT DEPLOY THIS VERSION OF HTTP/3 UNTIL IT IS IN AN RFC.  This
   version is still a work in progress.  For trial deployments, please
   use earlier versions.

Note to Readers

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

   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 31 July 2021.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction
     1.1.  Prior versions of HTTP
     1.2.  Delegation to QUIC
   2.  HTTP/3 Protocol Overview
     2.1.  Document Organization
     2.2.  Conventions and Terminology
   3.  Connection Setup and Management
     3.1.  Discovering an HTTP/3 Endpoint
       3.1.1.  HTTP Alternative Services
       3.1.2.  Other Schemes
     3.2.  Connection Establishment
     3.3.  Connection Reuse
   4.  HTTP Request Lifecycle
     4.1.  HTTP Message Exchanges
       4.1.1.  Field Formatting and Compression
       4.1.2.  Request Cancellation and Rejection
       4.1.3.  Malformed Requests and Responses
     4.2.  The CONNECT Method
     4.3.  HTTP Upgrade
     4.4.  Server Push
   5.  Connection Closure
     5.1.  Idle Connections
     5.2.  Connection Shutdown
     5.3.  Immediate Application Closure
     5.4.  Transport Closure
   6.  Stream Mapping and Usage
     6.1.  Bidirectional Streams
     6.2.  Unidirectional Streams
       6.2.1.  Control Streams
       6.2.2.  Push Streams
       6.2.3.  Reserved Stream Types
   7.  HTTP Framing Layer
     7.1.  Frame Layout
     7.2.  Frame Definitions
       7.2.1.  DATA
       7.2.2.  HEADERS
       7.2.3.  CANCEL_PUSH
       7.2.4.  SETTINGS
       7.2.5.  PUSH_PROMISE
       7.2.6.  GOAWAY
       7.2.7.  MAX_PUSH_ID
       7.2.8.  Reserved Frame Types
   8.  Error Handling
     8.1.  HTTP/3 Error Codes
   9.  Extensions to HTTP/3
   10. Security Considerations
     10.1.  Server Authority
     10.2.  Cross-Protocol Attacks
     10.3.  Intermediary Encapsulation Attacks
     10.4.  Cacheability of Pushed Responses
     10.5.  Denial-of-Service Considerations
       10.5.1.  Limits on Field Section Size
       10.5.2.  CONNECT Issues
     10.6.  Use of Compression
     10.7.  Padding and Traffic Analysis
     10.8.  Frame Parsing
     10.9.  Early Data
     10.10. Migration
     10.11. Privacy Considerations
   11. IANA Considerations
     11.1.  Registration of HTTP/3 Identification String
     11.2.  New Registries
       11.2.1.  Frame Types
       11.2.2.  Settings Parameters
       11.2.3.  Error Codes
       11.2.4.  Stream Types
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Appendix A.  Considerations for Transitioning from HTTP/2
     A.1.  Streams
     A.2.  HTTP Frame Types
       A.2.1.  Prioritization Differences
       A.2.2.  Field Compression Differences
       A.2.3.  Flow Control Differences
       A.2.4.  Guidance for New Frame Type Definitions
       A.2.5.  Comparison Between HTTP/2 and HTTP/3 Frame Types
     A.3.  HTTP/2 SETTINGS Parameters
     A.4.  HTTP/2 Error Codes
       A.4.1.  Mapping Between HTTP/2 and HTTP/3 Errors
   Appendix B.  Change Log
     B.1.  Since draft-ietf-quic-http-32
     B.2.  Since draft-ietf-quic-http-31
     B.3.  Since draft-ietf-quic-http-30
     B.4.  Since draft-ietf-quic-http-29
     B.5.  Since draft-ietf-quic-http-28
     B.6.  Since draft-ietf-quic-http-27
     B.7.  Since draft-ietf-quic-http-26
     B.8.  Since draft-ietf-quic-http-25
     B.9.  Since draft-ietf-quic-http-24
     B.10. Since draft-ietf-quic-http-23
     B.11. Since draft-ietf-quic-http-22
     B.12. Since draft-ietf-quic-http-21
     B.13. Since draft-ietf-quic-http-20
     B.14. Since draft-ietf-quic-http-19
     B.15. Since draft-ietf-quic-http-18
     B.16. Since draft-ietf-quic-http-17
     B.17. Since draft-ietf-quic-http-16
     B.18. Since draft-ietf-quic-http-15
     B.19. Since draft-ietf-quic-http-14
     B.20. Since draft-ietf-quic-http-13
     B.21. Since draft-ietf-quic-http-12
     B.22. Since draft-ietf-quic-http-11
     B.23. Since draft-ietf-quic-http-10
     B.24. Since draft-ietf-quic-http-09
     B.25. Since draft-ietf-quic-http-08
     B.26. Since draft-ietf-quic-http-07
     B.27. Since draft-ietf-quic-http-06
     B.28. Since draft-ietf-quic-http-05
     B.29. Since draft-ietf-quic-http-04
     B.30. Since draft-ietf-quic-http-03
     B.31. Since draft-ietf-quic-http-02
     B.32. Since draft-ietf-quic-http-01
     B.33. Since draft-ietf-quic-http-00
     B.34. Since draft-shade-quic-http2-mapping-00
   Acknowledgments
   Author's Address

1.  Introduction

   HTTP semantics ([SEMANTICS]) are used for a broad range of services
   on the Internet.  These semantics have most commonly been used with
   HTTP/1.1 and HTTP/2.  HTTP/1.1 has been used over a variety of
   transport and session layers, while HTTP/2 has been used primarily
   with TLS over TCP.  HTTP/3 supports the same semantics over a new
   transport protocol, QUIC.

1.1.  Prior versions of HTTP

   HTTP/1.1 ([HTTP11]) 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
   excessive tolerance of variant behavior.

   Because HTTP/1.1 does not include a multiplexing layer, multiple TCP
   connections are often used to service requests in parallel.  However,
   that has a negative impact on congestion control and network
   efficiency, since TCP does not share congestion control across
   multiple connections.

   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, QUIC 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 confidentiality and integrity to running TLS over TCP,
   with the improved connection setup latency of TCP Fast Open ([TFO]).

   This document defines HTTP/3, a mapping of HTTP semantics over the
   QUIC transport protocol, drawing heavily on the design of HTTP/2.
   HTTP/3 relies on QUIC to provide confidentiality and integrity
   protection of data; peer authentication; and reliable, in-order, per-
   stream delivery.  While delegating stream lifetime and flow control
   issues to QUIC, a binary framing similar to the HTTP/2 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 3.1.

   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).  Frames that apply to the entire connection
   are conveyed on a dedicated control stream.

   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])
   that 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 lifecycle
   of an HTTP/3 connection:

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

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

   *  Connection Closure (Section 5) describes how HTTP/3 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.

   *  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.

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

   The following terms are used:

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

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

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

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

   endpoint:  Either the client or server of the connection.

   frame:  The smallest unit of communication on a stream in HTTP/3,
      consisting of a header and a variable-length sequence of 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.

   HTTP/3 connection:  A QUIC connection where the negotiated
      application protocol is HTTP/3.

   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.  All streams within an HTTP/3 connection can be
      considered "HTTP/3 streams," but multiple stream types are defined
      within HTTP/3.

   stream error:  An application-level error on the individual stream.

   The term "content" is defined in Section 6.4 of [SEMANTICS].

   Finally, the terms "resource", "message", "user agent", "origin
   server", "gateway", "intermediary", "proxy", and "tunnel" are defined
   in Section 3 of [SEMANTICS].

   Packet diagrams in this document use the format defined in
   Section 1.3 of [QUIC-TRANSPORT] to illustrate the order and size of
   fields.

3.  Connection Setup and Management

3.1.  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 URI is discussed in Section 4.3 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 URI.  Upon receiving a
   server certificate in the TLS handshake, the client MUST verify that
   the certificate is an acceptable match for the URI's origin server
   using the process described in Section 4.3.4 of [SEMANTICS].  If the
   certificate cannot be verified with respect to the URI's origin
   server, the client MUST NOT consider the server authoritative for
   that origin.

   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 (including
   validation of the server certificate as described above), and sending
   an HTTP/3 request message targeting the URI to the server over that
   secured connection.  Unless some other mechanism is used to select
   HTTP/3, the token "h3" is used in the Application Layer Protocol
   Negotiation (ALPN; see [RFC7301]) extension during the TLS handshake.

   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 URIs contain
   either an explicit port or a default port associated with the scheme.

3.1.1.  HTTP Alternative Services

   An HTTP origin can advertise 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 "h3" ALPN token.

   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; if this connection is successful, the client can send HTTP
   requests using the mapping described in this document.

3.1.2.  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 that are also authoritative and that 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.
   For origins whose scheme is "http", an experimental method to
   accomplish this is described in [RFC8164].  Other mechanisms might be
   defined for various schemes in the future.

3.2.  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 domain name ([DNS-TERMS]), clients MUST send the
   Server Name Indication (SNI; [RFC6066]) TLS extension unless an
   alternative mechanism to indicate the target host is used.

   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.

3.3.  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.
   To use an existing connection for a new origin, clients MUST validate
   the certificate presented by the server for the new origin server
   using the process described in Section 4.3.4 of [SEMANTICS].  This
   implies that clients will need to retain the server certificate and
   any additional information needed to verify that certificate; clients
   which do not do so will be unable to reuse the connection for
   additional origins.

   If the certificate is not acceptable with regard to the new origin
   for any reason, the connection MUST NOT be reused and a new
   connection SHOULD be established for the new origin.  If the reason
   the certificate cannot be verified might apply to other origins
   already associated with the connection, the client SHOULD re-validate
   the server certificate for those origins.  For instance, if
   validation of a certificate fails because the certificate has expired
   or been revoked, this might be used to invalidate all other origins
   for which that certificate was used to establish authority.

   Clients SHOULD NOT open more than one HTTP/3 connection to a given IP
   address and UDP port, where the IP address and port might be derived
   from a URI, a selected alternative service ([ALTSVC]), a configured
   proxy, or name resolution of any of these.  A client MAY open
   multiple HTTP/3 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.

   Servers are encouraged to maintain open HTTP/3 connections for as
   long as possible but are permitted to terminate idle connections if
   necessary.  When either endpoint chooses to close the HTTP/3
   connection, 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 HTTP/3 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 7.4 of [SEMANTICS].

4.  HTTP Request Lifecycle

4.1.  HTTP Message Exchanges

   A client sends an HTTP request on a request stream, which is a
   client-initiated bidirectional QUIC stream; see Section 6.1.  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.
   See Section 15 of [SEMANTICS] for a description of interim and final
   HTTP responses.

   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 section, sent as a single HEADERS frame (see
       Section 7.2.2),

   2.  optionally, the content, if present, sent as a series of DATA
       frames (see Section 7.2.1), and

   3.  optionally, the trailer section, if present, sent as a single
       HEADERS frame.

   Header and trailer sections are described in Sections 6.3 and 6.5 of
   [SEMANTICS]; the content is described in Section 6.4 of [SEMANTICS].

   Receipt of an invalid sequence of frames MUST be treated as a
   connection error of type H3_FRAME_UNEXPECTED; see 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.
   Other frame types, especially unknown frame types, might be permitted
   subject to their own rules; see Section 9.

   A server MAY send one or more PUSH_PROMISE frames (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.  PUSH_PROMISE frames are not permitted
   on push streams; a pushed response that includes PUSH_PROMISE frames
   MUST be treated as a connection error of type H3_FRAME_UNEXPECTED;
   see Section 8.

   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.

   Transfer codings (see Section 6.1 of [HTTP11]) are not defined for
   HTTP/3; the Transfer-Encoding header field MUST NOT be used.

   A response MAY consist of multiple messages when and only when one or
   more interim responses (1xx; see Section 15.2 of [SEMANTICS]) precede
   a final response to the same request.  Interim responses do not
   contain content or trailer sections.

   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 the final 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-initiated
   stream terminates without enough of the HTTP message to provide a
   complete response, the server SHOULD abort its response stream with
   the error code H3_REQUEST_INCOMPLETE; see Section 8.

   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
   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 the request, 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; see Sections 6.3 and 6.5 of [SEMANTICS].  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/.

      *Note:* This registry will not exist until [SEMANTICS] is
      approved. *RFC Editor*, please remove this note prior to
      publication.

   Field names are strings containing a subset of ASCII characters.
   Properties of HTTP field names and values are discussed in more
   detail in Section 5.1 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".

   An intermediary transforming an HTTP/1.x message to HTTP/3 MUST
   remove connection-specific header fields as discussed in
   Section 7.6.1 of [SEMANTICS], or their messages will be treated by
   other HTTP/3 endpoints as malformed (Section 4.1.3).

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; however, an extension could negotiate a modification of
   this restriction; 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 trailer
   sections.  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 section before
   regular header fields.  Any request or response that contains a
   pseudo-header field that appears in a header 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 9 of [SEMANTICS])

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

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

      See Section 3.1.2 for guidance on using a scheme other than
      "https".

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

      To ensure that the HTTP/1.1 request line can be reproduced
      accurately, this pseudo-header field MUST be omitted when
      translating from an HTTP/1.1 request that has a request target in
      origin or asterisk form; see Section 7.1 of [SEMANTICS].  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 7.1 of [SEMANTICS].

   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 that 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 or "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 15 of [SEMANTICS].  This
   pseudo-header field MUST be included in all responses; otherwise, the
   response is malformed (Section 4.1.3).

   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

   [QPACK] describes a variation of HPACK that gives an encoder some
   control over how much head-of-line blocking can be caused by
   compression.  This allows an encoder to balance compression
   efficiency with latency.  HTTP/3 uses QPACK to compress header and
   trailer sections, including the pseudo-header fields present in the
   header section.

   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 byte string using the two-byte 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 that
   has received this parameter SHOULD NOT send an HTTP message header
   that exceeds the indicated size, as the peer will likely refuse to
   process it.  However, an HTTP message can traverse one or more
   intermediaries before reaching the origin server; see Section 3.7 of
   [SEMANTICS].  Because this limit is applied separately by each
   implementation which processes the message, messages below this limit
   are not guaranteed to be accepted.

4.1.2.  Request Cancellation and Rejection

   Once a request stream has been opened, the request MAY be cancelled
   by either endpoint.  Clients cancel requests if the response is no
   longer of interest; servers cancel requests if they are unable to or
   choose not to respond.  When possible, it is RECOMMENDED that servers
   send an HTTP response with an appropriate status code rather than
   canceling a request it has already begun processing.

   Implementations SHOULD cancel requests by abruptly terminating any
   directions of a stream that are still open.  This means resetting the
   sending parts of streams and aborting reading on receiving parts of
   streams; see Section 2.4 of [QUIC-TRANSPORT].

   When the server cancels a request without performing any application
   processing, the request is considered "rejected."  The server 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.

   Servers MUST NOT use the H3_REQUEST_REJECTED error code for requests
   that 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.

   Client SHOULD use the error code H3_REQUEST_CANCELLED to cancel
   requests.  Upon receipt of this error code, 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 canceled 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.  Only idempotent actions such as GET,
   PUT, or DELETE can be safely retried; a client SHOULD NOT
   automatically retry a request with a non-idempotent method unless it
   has some means to know that the request semantics are idempotent
   independent of the method or some means to detect that the original
   request was never applied.  See Section 9.2.2 of [SEMANTICS] for more
   details.

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 is defined as having content when it
   contains a Content-Length header field (Section 6.4.1 of
   [SEMANTICS]), is malformed if the value of a Content-Length header
   field does not equal the sum of the DATA frame lengths received.  A
   response that is defined as never having content, even when a
   Content-Length is present, 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_MESSAGE_ERROR.

   For malformed requests, a server MAY send an HTTP response indicating
   the error 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; see
   Section 9.3.6 of [SEMANTICS].  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 7.1 of [SEMANTICS])

   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.

   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 15.3 of [SEMANTICS].

   All DATA frames on the stream correspond to data sent or received on
   the TCP connection.  The payload of 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 known frame type MUST be treated as a connection error
   of type H3_FRAME_UNEXPECTED; see Section 8.

   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 close the send stream that it sends to the
   client.  TCP connections that 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; see Section 8.  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.

   Since CONNECT creates a tunnel to an arbitrary server, proxies that
   support CONNECT SHOULD restrict its use to a set of known ports or a
   list of safe request targets; see Section 9.3.6 of [SEMANTICS] for
   more detail.

4.3.  HTTP Upgrade

   HTTP/3 does not support the HTTP Upgrade mechanism (Section 7.8 of
   [SEMANTICS]) or 101 (Switching Protocols) informational status code
   (Section 15.2.2 of [SEMANTICS]).

4.4.  Server Push

   Server push is an interaction mode that 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 Section 8.2 of [HTTP2], but uses different mechanisms.

   Each server push is assigned a unique Push ID by the server.  The
   Push ID is used to refer to the push in various contexts throughout
   the lifetime of the HTTP/3 connection.

   The Push ID space begins at zero, and ends at a maximum value set by
   the MAX_PUSH_ID frame; see Section 7.2.7.  In particular, a server is
   not able to push until after the client sends a MAX_PUSH_ID frame.  A
   client sends MAX_PUSH_ID frames to control the number of pushes that
   a server can promise.  A server SHOULD use Push IDs sequentially,
   beginning from zero.  A client MUST treat receipt of a push stream as
   a connection error of type H3_ID_ERROR (Section 8) when no
   MAX_PUSH_ID frame has been sent or when the stream references a Push
   ID that is greater than the maximum Push ID.

   The Push ID is used in one or more PUSH_PROMISE frames
   (Section 7.2.5) that carry the header section of the request message.
   These frames are sent on the request stream that generated the push.
   This allows the server push to be associated with a client request.
   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
   identical.

   The Push ID is then included with the push stream that ultimately
   fulfills those promises; 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 as described in Section 4.1.

   Finally, the Push ID can be used in CANCEL_PUSH frames; see
   Section 7.2.3.  Clients use this frame to indicate they do not wish
   to receive a promised resource.  Servers use this frame to indicate
   they will not be fulfilling a previous promise.

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

   *  cacheable; see Section 9.2.3 of [SEMANTICS]

   *  safe; see Section 9.2.1 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.  If the client has not
   yet validated the connection for the origin indicated by the pushed
   request, it MUST perform the same verification process it would do
   before sending a request for that origin on the connection; see
   Section 3.3.  If this verification fails, the client MUST NOT
   consider the server authoritative for that origin.

   Clients SHOULD send a CANCEL_PUSH frame upon receipt of a
   PUSH_PROMISE frame carrying a request that 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.  Any
   corresponding responses MUST NOT be used or cached.

   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 frame 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.

   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.

   Push stream data can also arrive after a client has canceled a push.
   In this case, 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; see
   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.

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 QUIC 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 HTTP/3
   connection for new requests if the existing connection has been idle
   for longer than the idle timeout negotiated during the QUIC
   handshake, and SHOULD do so if approaching the idle timeout; see
   Section 10.1 of [QUIC-TRANSPORT].

   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 10.1.2 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 an HTTP/3 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 (Section 4.4).  Requests or pushes with the indicated
   identifier or greater are rejected (Section 4.1.2) 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 shutdown
   of the HTTP/3 connection.  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.

   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
      contained in the GOAWAY frame, those requests will not be
      processed.  Clients can safely retry unprocessed requests on a
      different HTTP connection.  A client that is unable to retry
      requests loses all requests that are in flight when the server
      closes the 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 with a lower Stream ID
      than that of the request in question, 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 contained
      in the 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.

   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 HTTP connection.
   Receiving a GOAWAY containing a larger identifier than previously
   received MUST be treated as a connection error of type H3_ID_ERROR;
   see Section 8.

   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 that have already been
   promised.  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 no greater than any previously sent value.

   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 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 that 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.

   If there are open streams that have not been explicitly closed, they
   are implicitly closed when the connection is closed; see Section 10.2
   of [QUIC-TRANSPORT].

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 that interrupts connectivity.

   If a connection terminates without a GOAWAY frame, clients MUST
   assume that any request that 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, the stream data containing HTTP frames is
   carried by 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 to the entire HTTP/3
   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.  These streams are referred
   to as request streams.

   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 request streams SHOULD be permitted at a
   time.

   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 (Section 8) 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

   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.  Some stream
   types are reserved (Section 6.2.3).

   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.

   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 that 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
   or a reserved error code (Section 8.1), 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 that 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 claiming 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.  Connection errors are
   described in Section 8.

   Because the contents of the control stream are used to manage the
   behavior of other streams, endpoints SHOULD provide enough flow
   control credit to keep the peer's control stream from becoming
   blocked.

   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 available on the QUIC
   connection, either client or server might be able to send stream data
   first.

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; see Section 8.

   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; see Section 8.

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
   sending implementation chooses.  When sending a reserved stream type,
   the implementation MAY either terminate the stream cleanly or reset
   it.  When resetting the stream, either the H3_NO_ERROR error code or
   a reserved error code (Section 8.1) SHOULD be used.

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
   their permitted stream types; 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

   The SETTINGS frame can only occur as the first frame of a Control
   stream; this is indicated in Table 1 with a (1).  Specific guidance
   is provided in the relevant section.

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

7.1.  Frame Layout

   All frames have the following format:

   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; see Section 8.  In particular,
   redundant length encodings MUST be verified to be self-consistent;
   see Section 10.8.

   When a stream terminates cleanly, if the last frame on the stream was
   truncated, this MUST be treated as a connection error of type
   H3_FRAME_ERROR; see Section 8.  Streams that 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 HTTP request or response content.

   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 of type H3_FRAME_UNEXPECTED; see
   Section 8.

   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 or 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 4.4), 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.

   A server sends CANCEL_PUSH to indicate that it will not be fulfilling
   a promise which was previously sent.  The client cannot expect the
   corresponding promise to be fulfilled, unless it has already received
   and processed the promised response.  Regardless of whether a push
   stream has been opened, a server SHOULD send a CANCEL_PUSH frame when
   it determines that promise will not be fulfilled.  If a stream has
   already been opened, the server can abort sending on the stream with
   an error code of H3_REQUEST_CANCELLED.

   Sending a CANCEL_PUSH frame has no direct effect on the state of
   existing push streams.  A client SHOULD NOT send a CANCEL_PUSH frame
   when it has already received a corresponding push stream.  A push
   stream could arrive after a client has sent a CANCEL_PUSH frame,
   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 (i),
   }

                        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 4.4.  If a CANCEL_PUSH frame is received that
   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 an entire HTTP/3 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.

   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 that 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 any parameter with an 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 4.1.1.3 for usage.

   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.

   Setting identifiers which were defined in [HTTP2] where there is no
   corresponding HTTP/3 setting have also been reserved
   (Section 11.2.2).  These reserved settings MUST NOT be sent, and
   their receipt MUST be treated as a connection error of type
   H3_SETTINGS_ERROR.

   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 that 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 the packet containing SETTINGS being processed by
   QUIC, 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.

   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 HTTP/3
   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 that 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 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:

   Push ID:  A variable-length integer that identifies the server push
      operation.  A Push ID is used in push stream headers (Section 4.4)
      and 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 frame 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 frame that uses a Push ID that
   they have already consumed and discarded are forced to ignore the
   promise.

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

   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 8.

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

7.2.6.  GOAWAY

   The GOAWAY frame (type=0x7) is used to initiate graceful shutdown of
   an HTTP/3 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 entire connection, not a specific
   stream.  A client MUST treat a GOAWAY frame on a stream other than
   the control stream as a connection error of type H3_FRAME_UNEXPECTED;
   see Section 8.

   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.

   The maximum Push ID is unset when an HTTP/3 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) = 0xd,
     Length (i),
     Push ID (i),
   }

                        Figure 10: MAX_PUSH_ID Frame

   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 4.4.  A MAX_PUSH_ID frame cannot reduce the maximum Push
   ID; receipt of a MAX_PUSH_ID frame 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
   MAY be sent on any stream where frames are allowed to be sent.  This
   enables their use for application-layer padding.  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 that 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 their receipt MUST be treated as a
   connection error of type H3_FRAME_UNEXPECTED.

8.  Error Handling

   When a stream cannot be completed successfully, QUIC allows the
   application to abruptly terminate (reset) that stream and communicate
   a reason; see Section 2.4 of [QUIC-TRANSPORT].  This is referred to
   as a "stream error."  An HTTP/3 implementation can decide to close a
   QUIC stream and communicate the type of error.  Wire encodings of
   error codes are defined in Section 8.1.  Stream errors are distinct
   from HTTP status codes which indicate error conditions.  Stream
   errors indicate that the sender did not transfer or consume the full
   request or response, while HTTP status codes indicate the result of a
   request that was successfully received.

   If an entire connection needs to be terminated, QUIC similarly
   provides mechanisms to communicate a reason; see Section 5.3 of
   [QUIC-TRANSPORT].  This is referred to as a "connection error."
   Similar to stream errors, an HTTP/3 implementation can terminate a
   QUIC connection and communicate the reason using an error code from
   Section 8.1.

   Although the reasons for closing streams and connections are called
   "errors," these actions do not necessarily indicate a problem with
   the connection or either implementation.  For example, a stream can
   be reset if the requested resource is no longer needed.

   An endpoint MAY choose to treat a stream error as a connection error
   under certain circumstances, closing the entire connection in
   response to a condition on a single stream.  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; for example, see Section 4.1.

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 HTTP/3 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 that does not 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 HTTP/3
      connection was closed or reset.

   H3_FRAME_UNEXPECTED (0x105):  A frame was received that 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
      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_MESSAGE_ERROR (0x10e):  An HTTP message was malformed and cannot
      be processed.

   H3_CONNECT_ERROR (0x10f):  The TCP 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 abort reading on unidirectional streams that have unknown or
   unsupported types.  This means that any of these extension points can
   be safely used by extensions without prior arrangement or
   negotiation.  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 does not 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 17.1 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.
   This ensures that endpoints have strong assurances that peers are
   using the same protocol.

   This does not guarantee protection from all cross-protocol attacks.
   Section 21.5 of [QUIC-TRANSPORT] describes some ways in which the
   plaintext of QUIC packets can be used to perform request forgery
   against endpoints that don't use authenticated transports.

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 5.1 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 can transport field values that are not valid.
   While most 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 5.5 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.

   Clients are required to reject pushed responses for which an origin
   server is not authoritative; see Section 4.4.

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.

   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 that 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 does not monitor such 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 of type
   H3_EXCESSIVE_LOAD (Section 8), 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 section, which prevents
   streaming of the header 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 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_FIELD_SECTION_SIZE
   (Section 4.1.1.3) 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 an HTTP/3 connection, so any request or
   response could encounter a hop with a lower, unknown limit.  An
   intermediary can attempt to avoid this problem by passing on values
   presented by different peers, but they are not obligated to do so.

   A server that receives a larger 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 a
   proxy, since stream creation is relatively inexpensive when compared
   to the creation and maintenance of a TCP connection.  Therefore, a
   proxy that supports CONNECT might be more conservative in the number
   of simultaneous requests it accepts.

   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.  To
   account for this, a proxy might delay increasing the QUIC stream
   limits for some time after a TCP connection terminates.

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 8.4.1 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 contexts 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 field sections
   are described in [QPACK].

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.

   Reserved stream types can be used to give the appearance of sending
   traffic even when the connection is idle.  Because HTTP traffic often
   occurs in bursts, apparent traffic can be used to obscure the timing
   or duration of such bursts, even to the point of appearing to send a
   constant stream of data.  However, as such traffic is still flow
   controlled by the receiver, a failure to promptly drain such streams
   and provide additional flow control credit can limit the sender's
   ability to send real traffic.

   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.  Redundant padding could even be
   counterproductive.  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.  When applying [HTTP-REPLAY] to HTTP/3,
   references to the TLS layer refer to the handshake performed within
   QUIC, while all references to application data refer to the contents
   of streams.

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

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].  These registries [SHALL be/are]
   collected under a "Hypertext Transfer Protocol version 3 (HTTP/3)
   Parameters" heading.

   The initial allocations in these registries created in this document
   are all assigned permanent status and list a change controller of the
   IETF and a contact of 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.  Expert reviewers MAY
   reject unrelated registrations which would conflict with the same
   value in the corresponding registry.

   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.

                 +==============+=======+===============+
                 | 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

   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 and MUST NOT appear in the listing of
   assigned values.

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.  Expert reviewers MAY reject unrelated
   registrations which would conflict with the same value in the
   corresponding registry.

   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               |  0x0  | N/A             | N/A       |
     +------------------------+-------+-----------------+-----------+
     | 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

   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 and MUST NOT appear in the listing of
   assigned values.

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.
   Use of values that are registered in the "HTTP/2 Error Code" registry
   is discouraged, and expert reviewers MAY reject such registrations.

   In addition to common fields as described in Section 11.2, this
   registry includes two additional fields.  Permanent registrations in
   this registry MUST include the following field:

   Name:  A name for the error code.

   Description:  A brief description of the error code semantics.

   The entries in Table 4 are registered by this document.  These error
   codes were selected from the range that operates on a Specification
   Required policy to avoid collisions with HTTP/2 error codes.

   +===========================+=======+==============+===============+
   | Name                      | Value | Description  | Specification |
   +===========================+=======+==============+===============+
   | H3_NO_ERROR               | 0x100 | No error     | Section 8.1   |
   +---------------------------+-------+--------------+---------------+
   | H3_GENERAL_PROTOCOL_ERROR | 0x101 | General      | Section 8.1   |
   |                           |       | protocol     |               |
   |                           |       | error        |               |
   +---------------------------+-------+--------------+---------------+
   | H3_INTERNAL_ERROR         | 0x102 | Internal     | Section 8.1   |
   |                           |       | error        |               |
   +---------------------------+-------+--------------+---------------+
   | H3_STREAM_CREATION_ERROR  | 0x103 | Stream       | Section 8.1   |
   |                           |       | creation     |               |
   |                           |       | error        |               |
   +---------------------------+-------+--------------+---------------+
   | H3_CLOSED_CRITICAL_STREAM | 0x104 | Critical     | Section 8.1   |
   |                           |       | stream was   |               |
   |                           |       | closed       |               |
   +---------------------------+-------+--------------+---------------+
   | H3_FRAME_UNEXPECTED       | 0x105 | Frame not    | Section 8.1   |
   |                           |       | permitted in |               |
   |                           |       | the current  |               |
   |                           |       | state        |               |
   +---------------------------+-------+--------------+---------------+
   | H3_FRAME_ERROR            | 0x106 | Frame        | Section 8.1   |
   |                           |       | violated     |               |
   |                           |       | layout or    |               |
   |                           |       | size rules   |               |
   +---------------------------+-------+--------------+---------------+
   | H3_EXCESSIVE_LOAD         | 0x107 | Peer         | Section 8.1   |
   |                           |       | generating   |               |
   |                           |       | excessive    |               |
   |                           |       | load         |               |
   +---------------------------+-------+--------------+---------------+
   | H3_ID_ERROR               | 0x108 | An           | Section 8.1   |
   |                           |       | identifier   |               |
   |                           |       | was used     |               |
   |                           |       | incorrectly  |               |
   +---------------------------+-------+--------------+---------------+
   | H3_SETTINGS_ERROR         | 0x109 | SETTINGS     | Section 8.1   |
   |                           |       | frame        |               |
   |                           |       | contained    |               |
   |                           |       | invalid      |               |
   |                           |       | values       |               |
   +---------------------------+-------+--------------+---------------+
   | H3_MISSING_SETTINGS       | 0x10a | No SETTINGS  | Section 8.1   |
   |                           |       | frame        |               |
   |                           |       | received     |               |
   +---------------------------+-------+--------------+---------------+
   | H3_REQUEST_REJECTED       | 0x10b | Request not  | Section 8.1   |
   |                           |       | processed    |               |
   +---------------------------+-------+--------------+---------------+
   | H3_REQUEST_CANCELLED      | 0x10c | Data no      | Section 8.1   |
   |                           |       | longer       |               |
   |                           |       | needed       |               |
   +---------------------------+-------+--------------+---------------+
   | H3_REQUEST_INCOMPLETE     | 0x10d | Stream       | Section 8.1   |
   |                           |       | terminated   |               |
   |                           |       | early        |               |
   +---------------------------+-------+--------------+---------------+
   | H3_MESSAGE_ERROR          | 0x10e | Malformed    | Section 8.1   |
   |                           |       | message      |               |
   +---------------------------+-------+--------------+---------------+
   | H3_CONNECT_ERROR          | 0x10f | TCP reset or | Section 8.1   |
   |                           |       | error on     |               |
   |                           |       | CONNECT      |               |
   |                           |       | request      |               |
   +---------------------------+-------+--------------+---------------+
   | H3_VERSION_FALLBACK       | 0x110 | Retry over   | Section 8.1   |
   |                           |       | HTTP/1.1     |               |
   +---------------------------+-------+--------------+---------------+

                   Table 4: Initial HTTP/3 Error Codes

   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 and MUST NOT appear in the listing of
   assigned values.

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 an HTTP/3 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 and 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

   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 and MUST NOT appear in the listing of
   assigned values.

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>.

   [CACHING]  Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-14, 12 January 2021, <http://www.ietf.org/
              internet-drafts/draft-ietf-httpbis-cache-14.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, 27 January 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-qpack>.

   [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, 27 January
              2021,
              <https://tools.ietf.org/html/draft-ietf-quic-transport>.

   [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>.

   [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>.

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

   [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>.

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

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

   [SEMANTICS]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-14, 12 January 2021,
              <http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
              semantics-14.txt>.

   [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>.

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>.

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

   [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",
              Work in Progress, Internet-Draft, draft-ietf-httpbis-
              messaging-14, 12 January 2021, <http://www.ietf.org/
              internet-drafts/draft-ietf-httpbis-messaging-14.txt>.

   [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>.

   [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>.

   [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>.

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 from HTTP/2.

   Some important departures are noted in this section.

A.1.  Streams

   HTTP/3 permits use of a larger number of streams (2^62-1) than
   HTTP/2.  The same 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.

   In HTTP/2, only request and response bodies (the frame payload of
   DATA frames) are subject to flow control.  All HTTP/3 frames are sent
   on QUIC streams, so all frames on all streams are flow-controlled in
   HTTP/3.

   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) that 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 frame types that appear in
   both mappings do not have identical semantics.

   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 that contain encoded
   fields merely reference the table state at a given time without
   modifying it.

   [QPACK] provides additional details.

A.2.3.  Flow Control Differences

   HTTP/2 specifies a stream flow control mechanism.  Although all
   HTTP/2 frames are delivered on streams, only the DATA frame payload
   is subject to flow control.  QUIC provides flow control for stream
   data and all HTTP/3 frame types defined in this document are sent on
   streams.  Therefore, all frame headers and payload are subject to
   flow control.

A.2.4.  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 other 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.

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

   Other than these issues, 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 are expected to be
   portable to HTTP/2.

A.2.5.  Comparison 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 in HTTP/3, 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 frame 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 in HTTP/3, as 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 in HTTP/3,
      since QUIC provides flow control.

   CONTINUATION (0x9):  CONTINUATION frames do not exist in HTTP/3;
      instead, larger HEADERS/PUSH_PROMISE frames than HTTP/2 are
      permitted.

   Frame types defined by extensions to HTTP/2 need to be separately
   registered for HTTP/3 if still applicable.  The IDs of frames defined
   in [HTTP2] have been reserved for simplicity.  Note that the frame
   type space in HTTP/3 is substantially larger (62 bits versus 8 bits),
   so many HTTP/3 frame types have no equivalent HTTP/2 code points.
   See Section 11.2.1.

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 setting that is retained in HTTP/3 has the same value as
   in HTTP/2.  The superseded settings are reserved, and their receipt
   is an error.  See Section 7.2.4.1 for discussion of both the retained
   and reserved values.

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

   SETTINGS_HEADER_TABLE_SIZE (0x1):  See [QPACK].

   SETTINGS_ENABLE_PUSH (0x2):  This is removed in favor of the
      MAX_PUSH_ID frame, which provides a more granular control over
      server push.  Specifying a setting with the identifier 0x2
      (corresponding to the SETTINGS_ENABLE_PUSH parameter) in the
      HTTP/3 SETTINGS frame is an error.

   SETTINGS_MAX_CONCURRENT_STREAMS (0x3):  QUIC controls the largest
      open Stream ID as part of its flow control logic.  Specifying a
      setting with the identifier 0x3 (corresponding to the
      SETTINGS_MAX_CONCURRENT_STREAMS parameter) in the HTTP/3 SETTINGS
      frame is an error.

   SETTINGS_INITIAL_WINDOW_SIZE (0x4):  QUIC requires both stream and
      connection flow control window sizes to be specified in the
      initial transport handshake.  Specifying a setting with the
      identifier 0x4 (corresponding to the SETTINGS_INITIAL_WINDOW_SIZE
      parameter) in the HTTP/3 SETTINGS frame is an error.

   SETTINGS_MAX_FRAME_SIZE (0x5):  This setting has no equivalent in
      HTTP/3.  Specifying a setting with the identifier 0x5
      (corresponding to the SETTINGS_MAX_FRAME_SIZE parameter) in the
      HTTP/3 SETTINGS frame is an error.

   SETTINGS_MAX_HEADER_LIST_SIZE (0x6):  This setting identifier has
      been renamed SETTINGS_MAX_FIELD_SECTION_SIZE.

   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 that 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.

   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 not
   to 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.
      Such cases include H3_FRAME_UNEXPECTED, H3_MESSAGE_ERROR, 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 acknowledgment 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].

   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.

   HTTP_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; see Section 4.1.2.

   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 HTTP/3 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.

B.1.  Since draft-ietf-quic-http-32

   *  Removed draft version guidance; added final version string

   *  Added H3_MESSAGE_ERROR for malformed messages

B.2.  Since draft-ietf-quic-http-31

   Editorial changes only.

B.3.  Since draft-ietf-quic-http-30

   Editorial changes only.

B.4.  Since draft-ietf-quic-http-29

   *  Require a connection error if a reserved frame type that
      corresponds to a frame in HTTP/2 is received (#3991, #3993)

   *  Require a connection error if a reserved setting that corresponds
      to a setting in HTTP/2 is received (#3954, #3955)

B.5.  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.6.  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.7.  Since draft-ietf-quic-http-26

   *  No changes

B.8.  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.9.  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.10.  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)

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

B.11.  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.12.  Since draft-ietf-quic-http-21

   No changes

B.13.  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.14.  Since draft-ietf-quic-http-19

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

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

B.15.  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.16.  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.17.  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)

   *  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.18.  Since draft-ietf-quic-http-15

   Substantial editorial reorganization; no technical changes.

B.19.  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.20.  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.21.  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)

   *  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.22.  Since draft-ietf-quic-http-11

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

B.23.  Since draft-ietf-quic-http-10

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

B.24.  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.25.  Since draft-ietf-quic-http-08

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

B.26.  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.27.  Since draft-ietf-quic-http-06

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

B.28.  Since draft-ietf-quic-http-05

   *  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.29.  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.30.  Since draft-ietf-quic-http-03

   None.

B.31.  Since draft-ietf-quic-http-02

   *  Track changes in transport draft

B.32.  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)

   *  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.33.  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.34.  Since draft-shade-quic-http2-mapping-00

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

   *  Updated authors/editors list

Acknowledgments

   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

   *  Martin Duke

   *  Roy Fielding

   *  Alan Frindell

   *  Alessandro Ghedini

   *  Nick Harper

   *  Ryan Hamilton

   *  Christian Huitema

   *  Subodh Iyengar

   *  Robin Marx

   *  Patrick McManus

   *  Luca Niccolini

   *  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.

Author's Address

   Mike Bishop (editor)
   Akamai

   Email: mbishop@evequefou.be