Compatible Version Negotiation for QUIC
draft-ietf-quic-version-negotiation-05

QUIC                                                         D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                             E. Rescorla
Expires: 28 April 2022                                           Mozilla
                                                         25 October 2021

                Compatible Version Negotiation for QUIC
                 draft-ietf-quic-version-negotiation-05

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 Venues

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

   Discussion of this document takes place on the QUIC Working Group
   mailing list (quic@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/quic/.

   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
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   This Internet-Draft will expire on 28 April 2022.

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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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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 Simplified BSD License.

Table of Contents

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

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
   version.

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   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.  This document specifies
   version negotiation using VN packets, which adds an extra round trip
   to connection establishment if needed.

   It is beneficial to avoid additional round trips whenever possible,
   especially given that most incremental versions are broadly similar
   to the the previous version.  This specification also defines a
   simple version negotiation mechanism which leverages similarities
   between versions and can negotiate between the set of "compatible"
   versions without additional round trips.

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.

   In this document, the Maximum Segment Lifetime (MSL) represents the
   time a QUIC packet can exist in the network.  Implementations can
   make this configurable, and a RECOMMENDED value is one minute.

2.  Server Deployments of QUIC

   While this document mainly discusses a single QUIC server, it is
   common for deployments of QUIC servers to include a fleet of multiple
   server instances.  We therefore define the following terms:

   Acceptable Versions:  This is the set of versions supported by a
      given server instance.  More specifically, these are the versions
      that a given server instance will use if a client sends a first
      flight using them.

   Offered Versions:  This is the set of versions that a given server
      instance will send in a Version Negotiation packet if it receives
      a first flight from an unknown version.  This set will most often
      be equal to the Acceptaple Versions set, except during short
      transitions while versions are added or removed (see below).

   Fully-Deployed Versions:  This is the set of QUIC versions that is
      supported and negotiated by every single QUIC server instance in
      this deployment.  If a deployment only contains a single server
      instance, then this set is equal to the Offered Versions set,
      except during short transitions while versions are added or
      removed (see below).

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   If a deployment contains multiple server instances, software updates
   may not happen at exactly the same time on all server instances.
   Because of this, a client might receive a Version Negotiation packet
   from a server instance that has already been updated and the client's
   resulting connection attempt might reach a different server instance
   which hasn't been updated yet.

   However, even when there is only a single server instance, it is
   still possible to receive a stale Version Negotiation packet if the
   server performs its software update while the Version Negotiation
   packet is in flight.

   This could cause the version downgrade prevention mechanism described
   in Section 6 to falsely detect a downgrade attack.  To avoid that,
   server operators SHOULD perform a three-step process when they wish
   to add or remove support for a version:

   When adding support for a new version:

   *  The first step is to progressively add support for the new version
      to all server instances.  This step updates the Acceptable
      Versions but not the Offered Versions nor the Fully-Deployed
      Versions.  Once all server instances have been updated, operators
      wait for at least one MSL to allow any in-flight Version
      Negotiation packets to arrive.

   *  Then, the second step is to progressively add the new version to
      Offered Versions on all server instances.  Once complete,
      operators wait for at least another MSL.

   *  Finally, the third step is to progressively add the new version to
      Fully-Deployed Versions on all server instances.

   When removing support for a version:

   *  The first step is to progressively remove the version from Fully-
      Deployed Versions on all server instances.  Once it has been
      removed on all server instances, operators wait for at least one
      MSL to allow any in-flight Version Negotiation packets to arrive.

   *  Then, the second step is to progressively remove the version from
      Offered Versions on all server instances.  Once complete,
      operators wait for at least another MSL.

   *  Finally, the third step is to progressively remove support for the
      version from all server instances.  That step updates the
      Acceptable Versions.

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   Note that this opens connections to version downgrades (but only for
   partially-deployed versions) during the update window, since those
   could be due to clients communicating with both updated and non-
   updated server instances.

3.  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 symmetric: 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
   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.

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4.  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 Version Information (see Section 5) which will
   be used to optionally enable compatible version negotation (see
   Section 4.3), and to prevent version downgrade attacks (see
   Section 6).

   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 4.2.  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 4.3.

   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.

4.1.  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 4.3) is performed as part of a single QUIC
   connection; that is, the packets with the original version are part
   of the same connection as the packets with the negotiated version.

   In comparison, the incompatible version negotiation mechanism, which
   leverages QUIC Version Negotiation packets (see Section 4.2)
   conceptually operates across two QUIC connections: the connection
   attempt prior to receiving the Version Negotiation packet is distinct
   from the connection with the incompatible version that follows.

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4.2.  Incompatible Version Negotiation

   The server starts incompatible version negotiation by sending a
   Version Negotiation packet.  This packet SHALL include each entry
   from the server's set of Offered Versions (see Section 2) in a
   Supported Version field.  The server MAY add reserved versions (as
   defined in Section 6.3 of [QUIC]) in Supported Version fields.

   Clients will ignore a Version Negotiation packet if it contains the
   original version attempted by the client.  The client also ignores a
   Version Negotiation that contains incorrect connection ID fields; see
   Section 6 of [INV].

   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 version information (see Section 5)
   required to ensure that VN was genuine, i.e. that no attacker
   injected packets in order to influence the VN process, see Section 6.

4.3.  Compatible Version Negotiation

   When the server can parse the client's first flight using the
   original version, it can extract the client's Version Information
   structure (see Section 5).  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.

   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 4.2.

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   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.

5.  Version Information

   During the handshake, endpoints will exchange Version Information,
   which is a blob of data that is defined below.  In QUIC version 1,
   the Version Information is transmitted using a new transport
   parameter, version_information.  The contents of Version Information
   are shown below (using the notation from the "Notational Conventions"
   section of [QUIC]):

   Version Information {
     Chosen Version (32),
     Other Versions (32) ...,
   }

                    Figure 1: Version Information Format

   The content of each field is described below:

   Chosen Version:  The version that the sender has chosen to use for
      this connection.  In most cases, this field will be equal to the
      value of the Version field in the long header that carries this
      data.

   The contents of the Other Versions field depends on whether it is
   sent by the client or by the server.

   Client-Sent Other Versions:  When sent by a client, the Other
      Versions field lists all the versions that this first flight is
      compatible with, ordered by descending preference.  Note that the
      version in the Chosen Version field MUST be included in this list
      to allow the client to communicate the chosen version's
      preference.  Note that this preference is only advisory, servers
      MAY choose to use their own preference instead.

   Server-Sent Other Versions:  When sent by a server, the Other
      Versions field lists all the Fully-Deployed Versions of this
      server deployment, see Section 2.

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   Clients and servers MAY both include versions following the pattern
   0x?a?a?a?a in their Other Versions list.  Those versions are reserved
   to exercise version negotiation (see the Versions section of [QUIC]),
   and will never be selected when choosing a version to use.

6.  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.

   Both endpoints MUST parse their peer's Version Information during the
   handshake.  If parsing the Version Information failed (for example,
   if it is too short or if its length is not divisible by four), then
   the endpoint MUST close the connection; if the connection was using
   QUIC version 1, that connection closure MUST use a transport error of
   type TRANSPORT_PARAMETER_ERROR.

   If the Version Information was missing, the endpoints MAY complete
   the handshake if they have reason to believe the peer might not
   support this extension.  However, if a client has reacted to a
   Version Negotiation packet and the Version Information was missing,
   the client MUST close the connection; if the connection was using
   QUIC version 1, that connection closure MUST use a transport error of
   type VERSION_NEGOTIATION_ERROR.

   If the client received and acted on a Version Negotiation packet, the
   client MUST validate the server's Other Versions field.  The Other
   Versions field is validated by confirming that the client would have
   attempted the same version with knowledge of the versions the server
   supports.  That is, the client would have selected the same version
   if it received a Version Negotiation packet that listed the versions
   in the server's Other Versions field, plus the negotiated version.
   If the client would have selected a different version, the client
   MUST close the connection; if the connection was using QUIC version
   1, that connection closure MUST use a transport error of type
   VERSION_NEGOTIATION_ERROR.  This connection closure prevents an
   attacker from being able to use forged Version Negotiation packets to
   force a version downgrade.

   This validation of Other Versions is not sufficient to prevent
   downgrade.  Downgrade prevention also depends on the client ignoring
   Version Negotiation packets that contain the original version; see
   Section 4.2.

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   After the process of version negotiation in this document completes,
   the version in use for the connection is the version that the server
   sent in the Chosen Version field of its Version Information.  That
   remains true even if other versions were used in the Version field of
   long headers at any point in the lifetime of the connection;
   endpoints MUST NOT change the version that they consider to be in use
   based on the Version field of long headers as that field could be
   forged by attackers.

7.  Client Choice of Original Version

   The client's first connection attempt SHOULD be made using the
   version that the server is most likely to support.  The client
   selects the version it believes to be best supported from the
   versions that are compatible with the client's most preferred
   version.  Without additional information this could mean selecting
   the oldest version that 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.  A server that sends a Retry packet can do so before parsing
   the client's first flight.  A server that sends a Retry packet
   therefore might not have processed the client's Version Information
   before doing so.

   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 attempting compatible
   version negotiation.  In that scenario the Retry packet would be sent
   using the original version.

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.

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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 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.

12.  IANA Considerations

12.1.  QUIC Transport Parameter

   This document registers a new value in the QUIC Transport Parameter
   Registry maintained at https://www.iana.org/assignments/quic/
   quic.xhtml#quic-transport.

   Value:  0xFF73DB

   Parameter Name:  version_information

   Status:  provisional

   Specification:  This document

   When this document is approved, it will request permanent allocation
   of a codepoint in the 0-63 range to replace the provisional codepoint
   described above.

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12.2.  QUIC Transport Error Code

   This document registers a new value in the QUIC Transport Error Codes
   Registry maintained at https://www.iana.org/assignments/quic/
   quic.xhtml#quic-transport-error-codes.

   Value:  0x53F8

   Code:  VERSION_NEGOTIATION_ERROR

   Description:  Error negotiating version

   Status:  provisional

   Specification:  This document

   When this document is approved, it will request permanent allocation
   of a codepoint in the 0-63 range to replace the provisional codepoint
   described above.

13.  Normative References

   [INV]      Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,
              <https://www.rfc-editor.org/rfc/rfc8999>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [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/rfc/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/rfc/rfc8174>.

Acknowledgments

   The authors would like to thank Nick Banks, Mike Bishop, Ryan
   Hamilton, Roberto Peon, Anthony Rossi, and Martin Thomson 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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