path: root/doc/rfc/rfc9368.txt

Internet Engineering Task Force (IETF)                       D. Schinazi
Request for Comments: 9368                                    Google LLC
Updates: 8999                                                E. Rescorla
Category: Standards Track                                        Mozilla
ISSN: 2070-1721                                                 May 2023


                Compatible Version Negotiation for QUIC

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 chose is unacceptable.  This document describes a
   version negotiation mechanism that allows a client and server to
   select a mutually supported version.  Optionally, if the client's
   chosen version and the negotiated version share a compatible first
   flight format, the negotiation can take place without incurring an
   extra round trip.  This document updates RFC 8999.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9368.

Copyright Notice

   Copyright (c) 2023 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/license-info) 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Conventions
     1.2.  Definitions
   2.  Version Negotiation Mechanism
     2.1.  Incompatible Version Negotiation
     2.2.  Compatible Versions
     2.3.  Compatible Version Negotiation
     2.4.  Connections and Version Negotiation
     2.5.  Client Choice of Original Version
   3.  Version Information
   4.  Version Downgrade Prevention
   5.  Server Deployments of QUIC
   6.  Application-Layer Protocol Considerations
   7.  Considerations for Future Versions
     7.1.  Interaction with Retry
     7.2.  Interaction with TLS Resumption
     7.3.  Interaction with 0-RTT
   8.  Special Handling for QUIC Version 1
   9.  Security Considerations
   10. IANA Considerations
     10.1.  QUIC Transport Parameter
     10.2.  QUIC Transport Error Code
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   The version-invariant properties of QUIC [QUIC-INVARIANTS] define a
   Version Negotiation packet but do not specify how an endpoint reacts
   when it receives one.  QUIC version 1 [QUIC] allows the server to use
   a Version Negotiation packet to indicate that the version the client
   chose is unacceptable, but it doesn't allow the client to safely make
   use of that information to create a new connection with a mutually
   supported version.  This document updates [QUIC-INVARIANTS] by
   defining version negotiation mechanisms that leverage the Version
   Negotiation packet.

   With proper safety mechanisms in place, the Version Negotiation
   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 Version Negotiation
   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 previous version.  This specification also defines a simple
   version negotiation mechanism which leverages similarities between
   versions and can negotiate between "compatible" versions without
   additional round trips.

1.1.  Conventions

   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.

1.2.  Definitions

   The document uses the following terms:

   *  In the context of a given QUIC connection, the "first flight" of
      packets refers to the set of packets the client creates and sends
      to initiate the connection before it has heard back from the
      server.

   *  In the context of a given QUIC connection, the "client's Chosen
      Version" is the QUIC version of the connection's first flight.

   *  The "Original Version" is the QUIC version of the very first
      packet the client sends to the server.  If version negotiation
      spans multiple connections (see Section 2.4), the Original Version
      is equal to the client's Chosen Version of the first QUIC
      connection.

   *  The "Negotiated Version" is the QUIC version in use on the
      connection once the version negotiation process completes.

   *  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.  Note that
      the term "segment" here originated in Section 3.4.1 of [TCP].

2.  Version Negotiation Mechanism

   This document specifies two means of performing version negotiation:
   1) "incompatible", which requires a round trip and is applicable to
   all versions, and 2) "compatible", which allows saving the round trip
   but only applies when the versions are compatible (see Section 2.2).

   The client initiates a QUIC connection by choosing an Original
   Version and sending a first flight of QUIC packets with a long header
   to the server [QUIC-INVARIANTS].  The client's first flight includes
   Version Information (see Section 3), which will be used to optionally
   enable compatible version negotiation (see Section 2.3) and to
   prevent version downgrade attacks (see Section 4).

   Upon receiving this first flight, the server verifies whether it
   knows how to parse first flights from the Chosen Version (which is
   also the Original Version in this case).  If it does not, then it
   starts incompatible version negotiation (see Section 2.1), which
   causes the client to initiate a new connection with a different
   version.  For instance, if the client initiates a connection with
   version A that the server can't parse, the server starts incompatible
   version negotiation; then, when the client initiates a new connection
   with version B, we say that the first connection's client Chosen
   Version is A, the second connection's client Chosen Version is B, and
   the Original Version for the entire sequence is A.

   If the server can parse the first flight, it can establish the
   connection using the client's Chosen Version, or it MAY select any
   other compatible version, as described in Section 2.3.

   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.

2.1.  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 5) 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, as required by Section 4.
   The client also ignores a Version Negotiation packet that contains
   incorrect connection ID fields, as required by Section 6 of
   [QUIC-INVARIANTS].

   Upon receiving the Version Negotiation packet, the client SHALL
   search for a version it supports in the list provided by the server.
   If it doesn't find one, it SHALL abort the connection attempt.
   Otherwise, it SHALL select a mutually supported version and send a
   new first flight with that version -- this version is now 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 3)
   that is required to ensure that version negotiation was genuine,
   i.e., that no attacker injected packets in order to influence the
   version negotiation process (see Section 4).

   Only servers can start incompatible version negotiation.  Clients
   MUST NOT send Version Negotiation packets and servers MUST ignore all
   received Version Negotiation packets.

2.2.  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.  Note that the conversion of the first flight can
   be lossy; some data, such as QUIC version 1 0-RTT packets, could be
   ignored during conversion and retransmitted later.

   Version compatibility is not symmetric.  It is possible for version A
   to be compatible with version B and for version B not to be
   compatible with version A.  This could happen, for example, if
   version B is a strict superset of version A, i.e., if version A
   includes the concept of streams and STREAM frames and version B
   includes the concept of streams and the hypothetical concept of tubes
   along with STREAM and TUBE frames, then A would be compatible with B,
   but B would not be compatible with 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: (1) version A
   is compatible with version B and (2) the conversion of this first
   flight to version B is well defined.  For example, if version B is
   equal to version A in all aspects except it introduced a new frame in
   its first flight that version A cannot parse or even ignore, then
   version B could still be compatible with version 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.

2.3.  Compatible Version Negotiation

   When the server can parse the client's first flight using the
   client's Chosen Version, it can extract the client's Version
   Information structure (see Section 3).  This contains the list of
   versions that the client knows its first flight is compatible with.

   In order to perform compatible version negotiation, the server MUST
   select one of these versions that it (1) supports and (2) knows the
   client's Chosen Version is compatible with.  This selected version is
   now the Negotiated Version.  After selecting it, the server attempts
   to convert the client's first flight into that version and replies to
   the client as if it had received the converted first flight.

   If those formats are identical, as in cases where the Negotiated
   Version is the same as the client's Chosen Version, then this will be
   the identity transformation.  If the first flight is correctly
   formatted, then this conversion process cannot fail by definition of
   the first flight being compatible; if the server is unable to convert
   the first flight, it MUST abort the handshake.

   If a document specifies that a QUIC version is compatible with
   another, that document MUST specify the mechanism by which clients
   are made aware of the Negotiated Version.  An example of such a
   mechanism is to have the client determine the server's Negotiated
   Version by examining the QUIC long header Version field.  Note that,
   in this example mechanism, it is possible for the server to initially
   send packets with the client's Chosen Version before switching to the
   Negotiated Version (this can happen when the client's Version
   Information structure spans multiple packets; in that case, the
   server might acknowledge the first packet in the client's Chosen
   Version and later switch to a different Negotiated Version).
   Mutually compatible versions SHOULD use the same mechanism.

   Note that, after the first flight is converted to the Negotiated
   Version, the handshake completes in the Negotiated Version.  If the
   Negotiated Version has requirements that apply during the handshake,
   those requirements apply to the entire handshake, including the
   converted first flight.  In particular, if the Negotiated Version
   mandates that endpoints perform validations on Handshake packets,
   endpoints MUST also perform such validations on the converted first
   flight.  For instance, if the Negotiated Version requires that the
   5-tuple remain stable for the entire handshake (as QUIC version 1
   does), then both endpoints need to validate the 5-tuple of all
   packets received during the handshake, including the converted first
   flight.

   Note also that the client can disable compatible version negotiation
   by only including the Chosen Version in the Available Versions field
   of the Version Information (see Section 3).

   If the server does not find a compatible version (including the
   client's Chosen Version), it will perform incompatible version
   negotiation instead (see Section 2.1).

   Note that it is possible to have incompatible version negotiation
   followed by compatible version negotiation.  For instance, if version
   A is compatible with version B and version C is compatible with
   version D, the following scenario could occur:

   Client                                          Server

   Chosen = A, Available Versions = (A, B) ------------->
   <------------------------ Version Negotiation = (D, C)

   Chosen = C, Available Versions = (C, D) ------------->
   <------------- Chosen = D, Available Versions = (D, C)

                   Figure 1: Combined Negotiation Example

   In this example, the client selected C from the server's Version
   Negotiation packet, but the server preferred D and then selected it
   from the client's offer.

2.4.  Connections and Version Negotiation

   QUIC connections are shared state between a client and a server
   [QUIC-INVARIANTS].  The compatible version negotiation mechanism
   defined in this document (see Section 2.3) is performed as part of a
   single QUIC connection; that is, the packets with the client's Chosen
   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 2.1),
   conceptually operates across two QUIC connections, i.e., the
   connection attempt prior to receiving the Version Negotiation packet
   is distinct from the connection with the incompatible version that
   follows.

   Note that this separation across two connections is conceptual, i.e.,
   it applies to normative requirements on QUIC connections, but it does
   not require implementations to internally use two distinct connection
   objects.

2.5.  Client Choice of Original Version

   When the client picks its Original Version, it SHOULD try to avoid
   incompatible version negotiation to save a round trip.  Therefore,
   the client SHOULD pick an Original Version to maximize the combined
   probability that both:

   *  the server knows how to parse first flights from the Original
      Version and

   *  the Original Version is compatible with the client's preferred
      version.

   Without additional information, this could mean selecting the oldest
   version that the client supports while advertising newer compatible
   versions in the client's first flight.

3.  Version Information

   During the handshake, endpoints will exchange Version Information,
   which consists of a Chosen Version and a list of Available Versions.
   Any version of QUIC that supports this mechanism MUST provide a
   mechanism to exchange Version Information in both directions during
   the handshake, such that this data is authenticated.

   In QUIC version 1, the Version Information is transmitted using a new
   version_information transport parameter (see Section 7.4 of [QUIC]).
   The contents of Version Information are shown below (using the
   notation from Section 1.3 of [QUIC]):

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

                    Figure 2: 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; however, future versions or extensions can choose to set
      different values in the long header Version field.

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

   Client-Sent Available Versions:  When sent by a client, the Available
      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 Available Versions:  When sent by a server, the Available
      Versions field lists all the Fully Deployed Versions of this
      server deployment (see Section 5).  The ordering of the versions
      in this field does not carry any semantics.  Note that the version
      in the Chosen Version field is not necessarily included in this
      list, because the server operator could be in the process of
      removing support for this version.  For the same reason, the
      Available Versions field MAY be empty.

   Clients and servers MAY both include versions following the pattern
   0x?a?a?a?a in their Available Versions list.  Those versions are
   reserved to exercise version negotiation (see Section 15 of [QUIC])
   and will never be selected when choosing a version to use.

4.  Version Downgrade Prevention

   A version downgrade is an attack where a malicious entity manages to
   make the QUIC endpoints negotiate a QUIC version different from the
   one they would have negotiated in the absence of the attack.  The
   mechanism described in this document is designed to prevent downgrade
   attacks.

   Clients MUST ignore any received Version Negotiation packets that
   contain the Original Version.  A client that makes a connection
   attempt based on information received from a Version Negotiation
   packet MUST ignore any Version Negotiation packets it receives in
   response to that connection attempt.

   Both endpoints MUST parse their peer's Version Information during the
   handshake.  If that leads to a parsing failure (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 an endpoint receives a Chosen Version
   equal to zero, or any Available Version equal to zero, it MUST treat
   it as a parsing failure.  If a server receives Version Information
   where the Chosen Version is not included in Available Versions, it
   MUST treat it as a parsing failure.

   Every QUIC version that supports version negotiation MUST define a
   method for closing the connection with a version negotiation error.
   For QUIC version 1, version negotiation errors are signaled using a
   transport error of type VERSION_NEGOTIATION_ERROR (see Section 10.2).

   When a server receives a client's first flight, the server will first
   establish which QUIC version is in use for this connection in order
   to properly parse the first flight.  This may involve examining data
   that is not part of the handshake transcript, such as parts of the
   packet header.  When the server then processes the client's Version
   Information, the server MUST validate that the client's Chosen
   Version matches the version in use for the connection.  If the two
   differ, the server MUST close the connection with a version
   negotiation error.

   In the specific case of QUIC version 1, the server determines that
   version 1 is in use by observing that the Version field of the first
   Long Header packet it receives is set to 0x00000001.  Subsequently,
   if the server receives the client's Version Information over QUIC
   version 1 (as indicated by the Version field of the Long Header
   packets that carried the transport parameters) and the client's
   Chosen Version is not set to 0x00000001, the server MUST close the
   connection with a version negotiation error.

   Servers MAY complete the handshake even if the Version Information is
   missing.  Clients MUST NOT complete the handshake if they are
   reacting to a Version Negotiation packet and the Version Information
   is missing, but MAY do so otherwise.

   If a client receives Version Information where the server's Chosen
   Version was not sent by the client as part of its Available Versions,
   the client MUST close the connection with a version negotiation
   error.  If a client has reacted to a Version Negotiation packet and
   the server's Version Information was missing, the client MUST close
   the connection with a version negotiation error.

   If the client received and acted on a Version Negotiation packet, the
   client MUST validate the server's Available Versions field.  The
   Available 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 Available Versions field, plus the
   Negotiated Version.  If the client would have selected a different
   version, the client MUST close the connection with a version
   negotiation error.  In particular, if the client reacted to a Version
   Negotiation packet and the server's Available Versions field is
   empty, the client MUST close the connection with a version
   negotiation error.  These connection closures prevent an attacker
   from being able to use forged Version Negotiation packets to force a
   version downgrade.

   As an example, let's assume a client supports hypothetical QUIC
   versions 10, 12, and 14 with a preference for higher versions.  The
   client initiates a connection attempt with version 12.  Let's explore
   two independent example scenarios:

   *  In the first scenario, the server supports versions 10, 13, and
      14, but only 13 and 14 are Fully Deployed (see Section 5).  The
      server sends a Version Negotiation packet with versions 10, 13,
      and 14.  This triggers an incompatible version negotiation, and
      the client initiates a new connection with version 14.  Then, the
      server's Available Versions field contains 13 and 14.  In that
      scenario, the client would have also picked 14 if it had received
      a Version Negotiation packet with versions 13 and 14; therefore,
      the handshake succeeds using Negotiated Version 14.

   *  In the second scenario, the server supports versions 10, 13, and
      14, and they are all Fully Deployed.  However, the attacker forges
      a Version Negotiation packet with versions 10 and 13.  This
      triggers an incompatible version negotiation, and the client
      initiates a new connection with version 10.  Then, the server's
      Available Versions field contains 10, 13, and 14.  In that
      scenario, the client would have picked 14 instead of 10 if it had
      received a Version Negotiation packet with versions 10, 13, and
      14; therefore, the client aborts the handshake with a version
      negotiation error.

   This validation of Available 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 2.1).

   After the process of version negotiation described 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.  In particular, since the client can be made aware of the
   Negotiated Version by the QUIC long header version during compatible
   version negotiation (see Section 2.3), clients MUST validate that the
   server's Chosen Version is equal to the Negotiated Version; if they
   do not match, the client MUST close the connection with a version
   negotiation error.  This prevents an attacker's ability to influence
   version negotiation by forging the long header Version field.

5.  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.  Therefore, we 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 Acceptable 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).

   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 4 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, as described below.

   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.

   Note that, during the update window, connections are vulnerable to
   downgrade attacks for Acceptable Versions that are not Fully
   Deployed.  This is because a client cannot distinguish such a
   downgrade attack from legitimate exchanges with both updated and non-
   updated server instances.

6.  Application-Layer Protocol Considerations

   When a client creates a QUIC connection, its goal is to use an
   application-layer protocol.  Therefore, when considering which
   versions are compatible, clients will only consider versions that
   support one of the intended application-layer protocols.  If the
   client's first flight advertises multiple Application-Layer Protocol
   Negotiation (ALPN) [ALPN] tokens and multiple compatible versions, it
   is possible for some application-layer protocols to not be able to
   run over some of the offered compatible versions.  It is the server's
   responsibility to only select an ALPN token that can run over the
   compatible QUIC version that it selects.

   A given ALPN token MUST NOT be used with a new QUIC version that is
   different from the version for which the ALPN token was originally
   defined, unless all the following requirements are met:

   *  The new QUIC version supports the transport features required by
      the application protocol.

   *  The new QUIC version supports ALPN.

   *  The version of QUIC for which the ALPN token was originally
      defined is compatible with the new QUIC version.

   When incompatible version negotiation is in use, the second
   connection that is created in response to the received Version
   Negotiation packet MUST restart its application-layer protocol
   negotiation process without taking into account the Original Version.

7.  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.

   QUIC version 1 defines multiple features which are not documented in
   the QUIC invariants.  Since, at the time of writing, QUIC version 1
   is widely deployed, this section discusses considerations for future
   versions to help with compatibility with QUIC version 1.

7.1.  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.  Therefore, a server that sends a Retry
   packet 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 first send a Retry packet in the
   Original Version, thereby validating the client's IP address before
   attempting compatible version negotiation.  If both versions support
   authenticating Retry packets, the compatibility definition needs to
   define how to authenticate the Retry in the Negotiated Version
   handshake even though the Retry itself was sent using the client's
   Chosen Version.

7.2.  Interaction with TLS Resumption

   QUIC version 1 uses TLS 1.3, which supports session resumption by
   sending session tickets in one connection that can be used in a later
   connection (see Section 2.2 of [TLS]).  New versions that also use
   TLS 1.3 SHOULD mandate that their session tickets are tightly scoped
   to one version of QUIC, i.e., require that clients not use them
   across multiple version and that servers validate this client
   requirement.  This helps mitigate cross-protocol attacks.

7.3.  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.  That document could specify that compatible version
   negotiation causes 0-RTT data to be rejected by the server.

8.  Special Handling for QUIC Version 1

   Because QUIC version 1 was the only QUIC version that was published
   on the IETF Standards Track before this document, it is handled
   specially as follows: if a client is starting a QUIC version 1
   connection in response to a received Version Negotiation packet and
   the version_information transport parameter is missing from the
   server's transport parameters, then the client SHALL proceed as if
   the server's transport parameters contained a version_information
   transport parameter with a Chosen Version set to 0x00000001 and an
   Available Version list containing exactly one version set to
   0x00000001.  This allows version negotiation to work with servers
   that only support QUIC version 1.  Note that implementations that
   wish to use version negotiation to negotiate versions other than QUIC
   version 1 MUST implement the version negotiation mechanism defined in
   this document.

9.  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.  That analysis is out of scope for this document.

10.  IANA Considerations

10.1.  QUIC Transport Parameter

   IANA has registered the following value in the "QUIC Transport
   Parameters" registry maintained at <https://www.iana.org/assignments/
   quic>.

   Value:  0x11
   Parameter Name:  version_information
   Status:  permanent
   Specification:  RFC 9368

10.2.  QUIC Transport Error Code

   IANA has registered the following value in the "QUIC Transport Error
   Codes" registry maintained at <https://www.iana.org/assignments/
   quic>.

   Value:  0x11
   Code:  VERSION_NEGOTIATION_ERROR
   Description:  Error negotiating version
   Status:  permanent
   Specification:  RFC 9368

11.  References

11.1.  Normative References

   [ALPN]     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>.

   [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/info/rfc9000>.

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

   [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>.

   [TLS]      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>.

11.2.  Informative References

   [TCP]      Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/info/rfc9293>.

Acknowledgments

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

Authors' Addresses

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America
   Email: dschinazi.ietf@gmail.com


   Eric Rescorla
   Mozilla
   Email: ekr@rtfm.com