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An Alt-Svc Parameter for QUIC Versions

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Martin Duke , Lucas Pardue
Last updated 2022-03-04
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HTTP                                                             M. Duke
Internet-Draft                                                    Google
Intended status: Standards Track                               L. Pardue
Expires: 5 September 2022                                     Cloudflare
                                                            4 March 2022

                 An Alt-Svc Parameter for QUIC Versions


   HTTP Alternative Services (Alt-Svc) describes how one origin's
   resource can be accessed via a different protocol/host/port
   combination.  Alternatives are advertised by servers using the Alt-
   Svc header field or the ALTSVC frame.  This includes a protocol name,
   which reuses Application Layer Protocol Negotiation (ALPN)
   codepoints.  The "h3" codepoint indicates the availability of HTTP/3.
   A client that uses such an alternative first makes a QUIC connection.
   However, without a priori knowledge of which QUIC version to use,
   clients might incur a round-trip latency penalty to complete QUIC
   version negotiation, or forfeit desirable properties of a QUIC
   version.  This document specifies a new Alt-Svc parameter that
   specifies alternative supported QUIC versions, which substantially
   reduces the chance of this penalty.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   duke-httpbis-quic-version-alt-svc.html.  Status information for this
   document may be found at

   Discussion of this document takes place on the HTTP Working Group
   mailing list (, which is archived at

   Source for this draft and an issue tracker can be found at

Status of This Memo

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

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   This Internet-Draft will expire on 5 September 2022.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  The quicv Parameter . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   HTTP Alternative Services (Alt-Svc) [ALTSVC] describes how one
   origin's resource can be accessed via a different protocol/host/port
   combination.  Alternatives are advertised by servers using the Alt-
   Svc header field or the ALTSVC frame.  This includes a protocol name,
   which reuses codepoints from the Application-Layer Protocol
   Negotiation (ALPN) TLS extension [RFC7301].  Servers can advertise
   multiple alternatives, in which case the order reflects the server's

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   preferences (the first value being the most preferred).

   Clients can ignore alternative services, or pick one at their
   discretion.  A client might use any details from the advertisement,
   in addition to out of band information, in determining if an
   alternative is suitable or preferred.

   While ALPN was originally intend to allow multiple applications to
   utilize TLS or DTLS on the same IP address and TCP or UDP port, ALPN
   can also usefully identify the transport in an Alt-Svc context.  The
   "h3" ALPN codepoint informs the client that it can use HTTP/3
   [I-D.ietf-quic-http] for access, which in turn requires the QUIC
   transport protocol [RFC8999].

   QUIC is versioned.  A client and server that both support a QUIC
   version can, through a negotiation process, generally agree on that
   version in no more than one round-trip.  However, to avoid that
   penalty clients might use the most commonly deployed QUIC version
   (e.g. version 1 [RFC9000] at the time of writing), rather than the
   version with the most desirable properties for the client's use case.

   To avoid the round-trip, one solution would be to register unique
   ALPN codepoints for each HTTP/3 and QUIC version combination.
   However, this might complicate deployment of new versions and
   deprecation of old ones: architecturally, an application should
   provide its ALPN to its QUIC implementation.  In this case, fully
   deploying a new version in that implementation would require updating
   all applications that use it.

   Instead, this document specifies an Alt-Svc parameter that lists the
   QUIC versions available to serve the resource.  Clients that do not
   understand this parameter will ignore it.  They might default to the
   most likely version, and/or incur a round-trip penalty in the event
   of a mismatch.  Clients that do process the parameter will connect
   successfully using the most desirable version with high probability.

2.  Conventions and Definitions

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

   This document uses the Augmented BNF defined in [RFC5234] and imports
   parameter from Section 3 of [ALTSVC].

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3.  The quicv Parameter

   This document specifies the "quicv" Alt-Svc parameter, which lists
   the QUIC versions supported by an endpoint, using the hexadecimal
   representation of the version field in a QUIC long header, as
   indicated in [RFC8999].  Senders MAY omit leading zeroes from version

   quicv         = version-list
   version-list  = DQUOTE version 1*( OWS, "," OWS version-number) DQUOTE
   version = 1*8 HEXDIG; hex-encoded QUIC version


   Alt-Svc: h3=":443"; quicv="1"
   Alt-Svc: h3=":443"; quicv="709a50c4,1"
   Alt-Svc: h3=":443"; quicv="709a50c4,1", h3=":1001"; quicv="709a50c4"

   The order of entries in version-list reflects the server's preference
   (with the first value being the most preferred alternative).

   Note that the quicv parameter applies to a single associated entry in
   the Alt-Svc list.  Servers MUST NOT provide a quicv parameter to an
   entry containing ALPN codepoint that does not potentially utilize

   If the Alt-Svc information resolves to a server pool that
   inconsistently supports different QUIC versions, the parameter SHOULD
   only advertise versions that are supported throughout the pool.

4.  Security Considerations

   This document inherits the security considerations of [ALTSVC],
   especially the implications of "Changing Protocols" in Section 9.3.
   There are few protocol properties guaranteed to hold across all QUIC
   versions, so endpoints should be aware what capabilities are
   intrinsic to the QUIC versions they are advertising.

   This parameter reveals capabilities of the described server, but this
   information is already available by inducing the server to generate a
   QUIC version negotiation packet.

5.  IANA Considerations

   Please add this entry ot the HTTP Alt-Svc Parameter Registry:

   Alt-Svc Parameter: quicv

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   Reference: This document

6.  References

6.1.  Normative References

   [ALTSVC]   Bishop, M. and M. Thomson, "HTTP Alternative Services",
              Work in Progress, Internet-Draft, draft-ietf-httpbis-
              rfc7838bis-00, 31 August 2021,

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

6.2.  Informative References

   [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, <>.

              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,

   [RFC8999]  Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,

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

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   TODO acknowledge.

Authors' Addresses

   Martin Duke

   Lucas Pardue

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