Compatible Version Negotiation for QUIC
draft-ietf-quic-version-negotiation-03

QUIC Working Group                                           D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Informational                               E. Rescorla
Expires: 9 August 2021                                           Mozilla
                                                         5 February 2021

                Compatible Version Negotiation for QUIC
                 draft-ietf-quic-version-negotiation-03

Abstract

   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 a mutually supported version.  Optionally, if the original and
   negotiated version share a compatible first flight format, the
   negotiation can take place without incurring an extra round trip.

   Discussion of this work is encouraged to happen on the QUIC IETF
   mailing list quic@ietf.org (mailto:quic@ietf.org) or on the GitHub
   repository which contains the draft: https://github.com/quicwg/
   version-negotiation/ (https://github.com/quicwg/version-
   negotiation/).

Discussion Venues

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

   Source for this draft and an issue tracker can be found at
   https://github.com/quicwg/version-negotiation.

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

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   This Internet-Draft will expire on 9 August 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Compatible Versions . . . . . . . . . . . . . . . . . . . . .   3
   3.  Version Negotiation Mechanism . . . . . . . . . . . . . . . .   4
     3.1.  Connections and Version Negotiation . . . . . . . . . . .   5
     3.2.  Incompatible Version Negotiation  . . . . . . . . . . . .   5
     3.3.  Compatible Version Negotiation  . . . . . . . . . . . . .   5
   4.  Handshake Version Information . . . . . . . . . . . . . . . .   6
   5.  Version Downgrade Prevention  . . . . . . . . . . . . . . . .   8
   6.  Supported Versions  . . . . . . . . . . . . . . . . . . . . .   9
   7.  Client Choice of Original Version . . . . . . . . . . . . . .   9
   8.  Interaction with Retry  . . . . . . . . . . . . . . . . . . .   9
   9.  Interaction with 0-RTT  . . . . . . . . . . . . . . . . . . .  10
   10. Considerations for Future Versions  . . . . . . . . . . . . .  10
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  QUIC Transport Parameter . . . . . . . . . . . . . . . .  10
     12.2.  QUIC Transport Error Code  . . . . . . . . . . . . . . .  11
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The version-invariant properties of QUIC [INV] define a version
   negotiation (VN) packet but do not specify how an endpoint reacts
   when it receives one.  QUIC version 1 [QUIC] allows the server to use
   a VN packet to indicate that the version the client offered is
   unacceptable, but doesn't allow the client to safely make use of that
   information to create a new connection with a mutually supported

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   version.  With proper safety mechanisms in place, the VN packet can
   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, it is beneficial to avoid that cost
   whenever possible, especially given that most incremental versions
   are broadly similar to the the previous version.

   This specification describes a simple version negotiation mechanism
   which optionally leverages similarities between versions and can
   negotiate between the set of "compatible" versions in a single round
   trip.

   Discussion of this work is encouraged to happen on the QUIC IETF
   mailing list quic@ietf.org (mailto:quic@ietf.org) or on the GitHub
   repository which contains the draft: https://github.com/quicwg/
   version-negotiation/ (https://github.com/quicwg/version-
   negotiation/).

1.1.  Conventions and Definitions

   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.

2.  Compatible Versions

   If A and B are two distinct versions of QUIC, A is said to be
   "compatible" with B if it is possible to take a first flight of
   packets from version A and convert it into a first flight of packets
   from version B.  As an example, if versions A and B are absolutely
   equal in their wire image and behavior during the handshake but
   differ after the handshake, then A is compatible with B and B is
   compatible with A.

   Version compatibility is not bijective: it is possible for version A
   to be compatible with version B and for B not to be compatible with
   A.  This could happen for example if version B is a strict superset
   of version A.

   Note that version compatibility does not mean that every single
   possible instance of a first flight will succeed in conversion to the
   other version.  A first flight using version A is said to be
   "compatible" with version B if two conditions are met: first that
   version A is compatible with version B, and second that the
   conversion of this first flight to version B is well-defined.  For
   example, if version B is equal to A in all aspects except it

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   introduced a new frame in its first flight that version A cannot
   parse or even ignore, then B could still be compatible with A as
   conversions would succeed for connections where that frame is not
   used.  In this example, first flights using version B that carry this
   new frame would not be compatible with version A.

   When a new version of QUIC is defined, it is assumed to not be
   compatible with any other version unless otherwise specified.
   Similarly, no other version is compatible with the new version unless
   otherwise specified.  Implementations MUST NOT assume compatibility
   between versions unless explicitly specified.

   Note that both endpoints might disagree on whether two versions are
   compatible or not.  For example, two versions could have been defined
   concurrently and then specified as compatible in a third document
   much later - in that scenario one endpoint might be aware of the
   compatibility document while the other may not.

3.  Version Negotiation Mechanism

   This document specifies two means of performing version negotiation:
   one "incompatible" which requires a round trip and is applicable to
   all versions, and one "compatible" that allows saving the round trip
   but only applies when the versions are compatible.

   The client initiates a QUIC connection by sending a first flight of
   QUIC packets with a long header to the server [INV].  We'll refer to
   the version of those packets as the "original version".  The client's
   first flight includes handshake version information (see Section 4)
   which will be used to optionally enable compatible version negotation
   (see Section 3.3), and to prevent version downgrade attacks (see
   Section 5).

   Upon receiving this first flight, the server verifies whether it
   knows how to parse first flights from the original version.  If it
   does not, then it starts incompatible version negotiation, see
   Section 3.2.  If the server can parse the first flight, it can either
   establish the connection using the original version, or it MAY
   attempt compatible version negotiation, see Section 3.3.

   Note that it is possible for a server to have the ability to parse
   the first flight of a given version without fully supporting it, in
   the sense that it implements enough of the version's specification to
   parse first flight packets but not enough to fully establish a
   connection using that version.

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3.1.  Connections and Version Negotiation

   QUIC connections are shared state between a client and a server
   [INV].  The compatible version negotiation mechanism defined in this
   document (see Section 3.3) operates inside of a QUIC connection;
   i.e., the packets with the original version are part of the same
   connection as the packets with the negotiated version.  On the other
   hand, the incompatible version negotiation mechanism, which leverages
   QUIC Version Negotiation packets (see Section 3.2) conceptually
   operates across two QUIC connections: one before the Version
   Negotiation packet, and a distinct connection after.

3.2.  Incompatible Version Negotiation

   The server starts incompatible version negotiation by sending a VN
   packet, listing all the versions that it does support.

   Upon receiving the VN packet, the client will search for a version it
   supports in the list provided by the server.  If it doesn't find one,
   it aborts the connection attempt.  Otherwise, it selects a mutually
   supported version and sends a new first flight with that version - we
   refer to this version as the "negotiated version".

   The new first flight will allow the endpoints to establish a
   connection using the negotiated version.  The handshake of the
   negotiated version will exchange handshake version information (see
   Section 4) required to ensure that VN was genuine, i.e. that no
   attacker injected packets in order to influence the VN process, see
   Section 5.

3.3.  Compatible Version Negotiation

   When the server can parse the client's first flight using the
   original version, it can extract the client's handshake version
   information (see Section 4).  This contains the list of versions that
   the client thinks its first flight is compatible with.

   If the server supports one of the client's compatible versions, and
   the server also believes that the original version is compatible with
   this version, then the server converts the client's first flight to
   that version and replies to the client as if it had received the
   converted first flight.  The version used by the server in its reply
   is refered to as the "negotiated version".  The server MUST NOT reply
   with a version that is not present in the client's compatible
   versions, unless it is the original version.

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   If the server does not find a compatible version, it will use the
   original version if it supports it, and if it doesn't then the server
   will perform incompatible version negotiation instead, see
   Section 3.2.

   For the duration of the compatible version negotiation process,
   clients MUST use the same 5-tuple (source and destination IP
   addresses and UDP port numbers).  During that time, clients MUST also
   use the same Destination Connection ID, except if the server
   explicitly instructs the client to use a different Destination
   Connection ID (for example, a QUIC version 1 server can accomplish
   this by sending an INITIAL packet with a Source Connection ID that
   differed from the client's Destination Connection ID).  This allows
   load balancers to ensure that packets for a given connection are
   routed to the same server.

4.  Handshake Version Information

   During the handshake, endpoints will exchange handshake version
   information, which is a blob of data that is defined below.  In QUIC
   version 1, the handshake version information is transmitted using a
   new transport parameter, "version_negotiation".  The contents of
   handshake version information depend on whether the client or server
   is sending it, and are shown below (using the notation from the
   "Notational Conventions" section of [QUIC]):

   Client Handshake Version Information {
     Currently Attempted Version (32),
     Previously Attempted Version (32),
     Received Negotiation Version Count (i),
     Received Negotiation Version (32) ...,
     Compatible Version Count (i),
     Compatible Version (32) ...,
   }

               Figure 1: Client Handshake Version Information

   The content of each field is described below:

   Currently Attempted Version:  The version that the client is
      attempting to use.  This field MUST be equal to the value of the
      Version field in the long header that carries this data.

   Previously Attempted Version:  If the client is sending this first

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      flight in response to a Version Negotiation packet, this field
      contains the version that the client used in the previous first
      flight that triggered the version negotiation packet.  If the
      client did not receive a Version Negotiation packet, this field
      SHALL be all-zeroes.

   Received Negotiation Version Count:  A variable-length integer
      specifying the number of Received Negotiation Version fields
      following it.  If the client is sending this first flight in
      response to a Version Negotiation packet, the subsequent versions
      SHALL include all the versions from that Version Negotiation
      packet in order, even if they are not supported by the client
      (even if the versions are reserved).  If the client has not
      received a Version Negotiation packet on this connection, this
      field SHALL be 0.

   Compatible Version Count:  A variable-length integer specifying the
      number of Compatible Version fields following it.  The client
      lists all versions that this first flight is compatible with in
      the subsequent Compatible Version fields, ordered by descending
      preference.  Note that the version in the Currently Attempted
      Version field MUST be included in the Compatible Version list to
      allow the client to communicate the currently attempted version's
      preference.  Note that this preference is only advisory, servers
      MAY choose to use their own preference instead.

   Server Handshake Version Information {
     Negotiated Version (32),
     Supported Version Count (i),
     Supported Version (32) ...,
   }

               Figure 2: Server Handshake Version Information

   The content of each field is described below:

   Negotiated Version:  The version that the server chose to use for the
      connection.  This field MUST be equal to the value of the Version
      field in the long header that carries this data.

   Supported Version Count:  A variable-length integer specifying the
      number of Supported Version fields following it.  The server
      encodes all versions it supports in the subsequent list, ordered
      by descending preference.  Note that the version in the Negotiated
      Version field MUST be included in the Supported Version list to
      allow the server to communicate the negotiated version's
      preference.  Note that this preference is only advisory, clients
      MAY choose to use their own preference instead.

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   Clients MAY include versions following the pattern "0x?a?a?a?a" in
   their "Compatible Version" list, and the server in their "Supported
   Version" list.  Those versions are reserved to exercise version
   negotiation (see the Versions section of [QUIC]), and MUST be ignored
   when parsing these fields.  On the other hand, the "Received
   Negotiation Version" list MUST be identical to the received Version
   Negotiation packet, so clients MUST NOT add or remove reserved
   version from that list.

5.  Version Downgrade Prevention

   Clients MUST ignore any received Version Negotiation packets that
   contain the version that they initially attempted.  Once a client has
   reacted to a Version Negotiation packet, it MUST drop all subsequent
   Version Negotiation packets on that connection.

   Servers MUST validate that the client's "Currently Attempted Version"
   matches the version in the long header that carried the handshake
   version information.  Similarly, clients MUST validate that the
   server's "Negotiated Version" matches the long header version.  If an
   endpoint's validation fails, it MUST close the connection.  If the
   connection was using QUIC version 1, it MUST be closed with a
   transport error of type "VERSION_NEGOTIATION_ERROR".

   When a server parses the client's handshake version information, if
   the "Received Negotiation Version Count" is not zero, the server MUST
   validate that it could have sent the Version Negotation packet
   described by the client in response to a first flight of version
   "Previously Attempted Version".  In particular, the server MUST
   ensure that there are no versions that it supports that are absent
   from the Received Negotiation Versions, and that the ordering matches
   the server's preference.  If this validation fails, the server MUST
   close the connection.  If the connection was using QUIC version 1, it
   MUST be closed with a transport error of type
   "VERSION_NEGOTIATION_ERROR".  This mitigates an attacker's ability to
   forge Version Negotiation packets to force a version downgrade.

   If a server operator is progressively deploying a new QUIC version
   throughout its fleet, it MAY perform a two-step process where it
   first progressively adds support for the new version, but without
   enforcing its presence in "Received Negotiation Versions".  Once all
   servers have been upgraded, the second step is to start enforcing
   that the new version is present in "Received Negotiation Versions".
   This opens connections to version downgrades during the upgrade
   window, since those could be due to clients communicating with both
   upgraded and non-upgraded servers.

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   If an endpoint receives its peer's Handshake Version Information and
   fails to parse it (for example, if it is too short), then the
   endpoint MUST close the connection.  If the connection was using QUIC
   version 1, it MUST be closed with a transport error of type
   "TRANSPORT_PARAMETER_ERROR".

6.  Supported Versions

   The server's "Supported Version" list allows it to communicate the
   full list of versions it supports to the client.  In the case where
   clients initially attempt connections with the oldest version they
   support, this allows them to be notified of more recent versions the
   server supports.  If the client notices that the server supports a
   version that is more preferable that the one initially attempted by
   default, the client SHOULD cache that information and attempt the
   preferred version in subsequent connections.

7.  Client Choice of Original Version

   The client's first flight SHOULD be sent using the version that the
   server is most likely to support (in the absence of other
   information, this will often be the oldest version the client
   supports).

8.  Interaction with Retry

   QUIC version 1 features retry packets, which the server can send to
   validate the client's IP address before parsing the client's first
   flight.  This impacts compatible version negotiation because a server
   who wishes to send a retry packet before parsing the client's first
   flight won't have parsed the client's handshake version information
   yet.  If a future document wishes to define compatibility between two
   versions that support retry, that document MUST specify how version
   negotiation (both compatible and incompatible) interacts with retry
   during a handshake that requires both.  For example, that could be
   accomplished by having the server send a retry packet first and
   validating the client's IP address before starting version
   negotiation and deciding whether to use compatible version
   negotiation on that connection (in that scenario the retry packet
   would be sent using the original version).

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9.  Interaction with 0-RTT

   QUIC version 1 allows sending data from the client to the server
   during the handshake, by using 0-RTT packets.  If a future document
   wishes to define compatibility between two versions that support
   0-RTT, that document MUST address the scenario where there are 0-RTT
   packets in the client's first flight.  For example, this could be
   accomplished by defining which transformations are applied to 0-RTT
   packets.  Alternatively, that document could specify that compatible
   version negotiation causes 0-RTT data to be rejected by the server.

10.  Considerations for Future Versions

   In order to facilitate the deployment of future versions of QUIC,
   designers of future versions SHOULD attempt to design their new
   version such that commonly deployed versions are compatible with it.
   For example, a successor to QUIC version 1 may wish to design its
   transport parameters in a way that does not preclude compatibility.
   Additionally, frames in QUIC version 1 do not use a self-describing
   encoding, so unrecognized frame types cannot be parsed or ignored
   (see the Extension Frames section of [QUIC]); this means that new
   versions that wish to be very similar to QUIC version 1 and
   compatible with it should avoid introducing new frames in initial
   packets.

11.  Security Considerations

   The security of this version negotiation mechanism relies on the
   authenticity of the handshake version information exchanged during
   the handshake.  In QUIC version 1, transport parameters are
   authenticated ensuring the security of this mechanism.  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.

   The presence of the "Attempted Version" and "Negotiated Version"
   fields mitigates an attacker's ability to forge packets by altering
   the version.

12.  IANA Considerations

12.1.  QUIC Transport Parameter

   If this document is approved, IANA shall assign the following entry
   in the QUIC Transport Parameter Registry:

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     +--------+---------------------+---------------+
     | Value  |   Parameter Name    |   Reference   |
     +--------+---------------------+---------------+
     | 0x73DB | version_negotiation | This document |
     +--------+---------------------+---------------+

12.2.  QUIC Transport Error Code

   If this document is approved, IANA shall assign the following entry
   in the QUIC Transport Error Codes Registry:

     +--------+---------------------------+---------------+
     | Value  |      Parameter Name       |   Reference   |
     +--------+---------------------------+---------------+
     | 0x53F8 | VERSION_NEGOTIATION_ERROR | This document |
     +--------+---------------------------+---------------+

13.  Normative References

   [INV]      Thomson, M., "Version-Independent Properties of QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-
              invariants-13, 14 January 2021, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-invariants-13.txt>.

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-34, 14 January 2021,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-34.txt>.

   [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>.

   [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>.

Acknowledgments

   The authors would like to thank Martin Thomson, Mike Bishop, Nick
   Banks, Ryan Hamilton, and Roberto Peon for their input and
   contributions.

Authors' Addresses

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

   Email: dschinazi.ietf@gmail.com

   Eric Rescorla
   Mozilla

   Email: ekr@rtfm.com

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