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Compatible Version Negotiation for QUIC

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Document Type This is an older version of an Internet-Draft whose latest revision is Replaced
Authors David Schinazi , Eric Rescorla
Last updated 2019-09-10 (Latest revision 2019-03-11)
Replaced by draft-ietf-quic-version-negotiation
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QUIC Working Group                                           D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Informational                               E. Rescorla
Expires: March 13, 2020                                          Mozilla
                                                      September 10, 2019

                Compatible Version Negotiation for QUIC


   QUIC does not provide a complete version negotiation mechanism but
   instead only provides a way for the server to indicate that the
   version the client offered is unacceptable.  This document describes
   a version negotiation mechanism that allows a client and server to
   select from a set of QUIC versions which share a compatible Initial
   format without incurring an extra round trip.

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
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   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 March 13, 2020.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   3.  Version Negotiation Mechanism . . . . . . . . . . . . . . . .   3
   4.  Compatible Versions Transport Parameter . . . . . . . . . . .   3
   5.  Compatible Versions . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   QUIC [I-D.ietf-quic-transport] does not provide a complete version
   negotiation (VN) mechanism; the VN packet only allows the server to
   indicate that the version the client offered is unacceptable, but
   doesn't allow the client to safely make use of that information.  In
   principle the VN packet could be part of a mechanism to allow two
   QUIC implementations to negotiate between two totally disjoint
   versions of QUIC (at the cost of an extra round trip).  However,
   experience with negotiation of previous IETF protocols indicates that
   this is probably not the most common scenario:

   1.  Implementations do not generally want to incur an extra round
       trip to negotiate versions.

   2.  Most incremental versions are broadly similar to the the previous
       version, and so the version negotiation mechanism can be built on
       the assumption that the version advertisement and selection is
       common to the versions to be negotiated.

   This specification describes a simple version negotiation mechanism
   which exploits property (2) and can negotiate between the set of
   "compatible" versions in a single round trip.  Negotiation between
   totally disjoint versions - if it ever proves to be necessary - is
   left as a topic for future work.

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.

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3.  Version Negotiation Mechanism

   The mechanism defined in this document is straightforward: the client
   maintains a list of QUIC versions it supports, ordered by preference.
   Its Initial packet is sent using the version that the server is most
   likely to support (in practice, this will generally be the oldest
   version the client supports); that Initial packet then lists all of
   the other compatible versions (Section 5) that the client supports in
   the supported_compatible_versions field of its transport parameters
   (Section 4).  The server then selects its preferred version and
   responds with that version in all of its future packets (except for
   Retry, as below).  It also inserts the selected version in the
   negotiated_compatible_version field of its transport parameters.

   The server MUST NOT select a version not offered by the client.  The
   client MUST validate that the version in the server's packets is one
   of the versions that it offered and that it matches the value in the
   server's transport parameters.

   If the server sends a Retry, it MUST use the same version that the
   client provided in its Initial.  Version negotiation takes place
   after the retry cycle is over.

   In order for negotiation to complete successfully, the client's
   Initial packet (and initial CRYPTO frames) MUST be interpretable by
   the server.  This implies that servers must retain the ability to
   process the Initial packet from older versions as long as they are
   reasonably popular.  This is not generally an issue in practice as
   long as the the overall structure of the protocol remains similar.

   If the server receives an Initial packet with a version it does not
   understand this will cause a connection failure and the server SHOULD
   send a Version Negotiation packet as defined in

4.  Compatible Versions Transport Parameter

   This document adds a new transport parameter, CompatibleVersions:

   struct {
         select (Handshake.msg_type) {
            case client_hello:
               QuicVersion supported_compatible_versions<4..2^8-4>;

            case encrypted_extensions:
               QuicVersion negotiated_compatible_version;
   } CompatibleVersions;

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   The client's "supported_compatible_versions" parameter lists the
   versions it supports in decreasing order of preference.  The server's
   "negotiated_compatible_version" parameter lists the version it has
   selected.  If the client does not send this transport parameter, the
   server MUST assume that the client only supports the version it used
   for the Initial packet and MUST NOT send its own parameter.

   Clients MAY include versions following the pattern 0x?a?a?a?a in
   their supported_compatible_versions.  Those versions are reserved to
   exercise version negotiation (see the Versions section of
   [I-D.ietf-quic-transport]), and MUST be ignored by the server when
   parsing supported_compatible_versions.

5.  Compatible Versions

   Two versions of QUIC A and B are "compatible" if a version A Initial
   can be used to negotiate version B and vice versa.  The most common
   scenario is a sequence of versions 1, 2, 3, etc. in which all the
   Initial packets have the same basic structure but might include
   specific extensions (especially inside the crypto handshake) that are
   only meaningful in some subset of versions and are ignored in others.
   Note that it is not possible to add new frame types in Initial
   packets because QUIC frames do not use a self-describing encoding, so
   unrecognized frame types cannot be parsed or ignored (see the
   Extension Frames section of [I-D.ietf-quic-transport]).

   When a new version of QUIC is defined, it is assumed to not be
   compatible with any other version unless otherwise specified.
   Implementations MUST NOT assume compatibility between version unless
   explicitly specified.

6.  Security Considerations

   The crypto handshake is already required to guarantee agreement on
   the supported parameters, so negotiation between compatible versions
   will have the security of the weakest common version.

   The requirement that versions not be assumed compatible mitigates the
   possibility of cross-protocol attacks, but more analysis is still
   needed here.

7.  IANA Considerations

   If this document is approved, IANA shall assign the identifier TBD
   for the "compatible_versions" transport parameter.

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8.  Normative References

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-22 (work
              in progress), July 2019.

   [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, <>.

Authors' Addresses

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043
   United States of America


   Eric Rescorla


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