Original text:
14 RFC2119 RFC8174 when, and only when, they appear in all capitals, as shown here.  2.  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 much later - in that scenario one endpoint might be aware of the compatibility document while the other may not. <del> 3. </del>  This document specifies two means of performing version negotiation: one "incompatible" which requires a round trip and is applicable to 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 <del> first flight includes handshake version information (see hs-vers- info) which will be used to optionally enable compatible version negotation (see compat-vn), and to prevent version downgrade attacks (see downgrade). </del>  Upon receiving this first flight, the server verifies whether it knows how to parse first flights from the original version.  If it parse first flight packets but not enough to fully establish a connection using that version. <del> 3.1. </del>  QUIC connections are shared state between a client and a server INV. The compatible version negotiation mechanism defined in this document across two QUIC connections: one before the Version Negotiation packet, and a distinct connection after. <del> 3.2. </del>  <del> The server starts incompatible version negotiation by sending a VN packet, listing all the versions that it does support. </del>  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, The new first flight will allow the endpoints to establish a connection using the negotiated version.  The handshake of the <del> negotiated version will exchange handshake version information (see hs-vers-info) required to ensure that VN was genuine, i.e. that no attacker injected packets in order to influence the VN process, see downgrade. </del>  <del> 3.3. </del>  When the server can parse the client's first flight using the <del> original version, it can extract the client's handshake version information (see hs-vers-info).  This contains the list of versions that the client thinks its first flight is compatible with. </del>  If the server supports one of the client's compatible versions, and the server also believes that the original version is compatible with load balancers to ensure that packets for a given connection are routed to the same server. <del> 4. </del>  <del> 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): </del>  The content of each field is described below: <del> 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. If the client is sending this first 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. 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. 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. </del>  <del> The content of each field is described below: </del>  <del> 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. 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 </del>  preference instead. <del> 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 will 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. </del>  <del> 5. </del>  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. <del> 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. 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. 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. </del>  7. 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 <del> 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 </del>  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 11. The security of this version negotiation mechanism relies on the <del> 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. </del>  The requirement that versions not be assumed compatible mitigates the possibility of cross-protocol attacks, but more analysis is still needed here. <del> The presence of the "Attempted Version" and "Negotiated Version" fields mitigates an attacker's ability to forge packets by altering the version. </del> 12. 12.1.

Reviewer comments:
This PR simplifies the document's design. The new simplified design was by NAME . It removes the indication of server versions that everyone seemed to think was useless, but apart from that has very similar properties to the previous design. One important difference is that it is now the client's responsibility to detect VN-packet-based downgrade attacks instead of the server's. .
In , NAME points out that the backticks don't render nicely in the TXT version, so let's remove them
In , NAME mentioned that having the client communicate preference ordering between compatible versions isn't absolutely necessary. This issue tracks whether we want to keep this ordering or not.
During the IETF QUIC WG meeting there was (weak-ish) consensus that this useful to keep.
The strongest argument against this was that it doesn't give the server a full ordering, because it doesn't tell the server where the offered version falls in the preference list. There's a possible answer to that, which is that the client should only offer compatible versions which are more preferred than the currently offered version. However, we should explicitly state that the server makes the actual decision and MAY factor client preferences into that. I suspect most won't.
NAME that's incorrect. The offered version is present in the list.
Ah, thanks for correcting me. Then we're spending bytes to repeat the version number, which we don't expect servers will typically use. Not the end of the world either way.
Consensus established. Closing this issue with no action.
Almost there, but I think we need clarity on the clients verification-and-drop requirements.

Revised text:
14 RFC2119 RFC8174 when, and only when, they appear in all capitals, as shown here. <ins> 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. </ins> 2. <ins> 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: 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. 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). 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 downgrade 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. 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. </ins> 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 much later - in that scenario one endpoint might be aware of the compatibility document while the other may not.  <ins> 4. </ins> This document specifies two means of performing version negotiation: one "incompatible" which requires a round trip and is applicable to 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  <ins> first flight includes Version Information (see vers-info) which will be used to optionally enable compatible version negotation (see compat-vn), and to prevent version downgrade attacks (see downgrade). </ins> Upon receiving this first flight, the server verifies whether it knows how to parse first flights from the original version.  If it parse first flight packets but not enough to fully establish a connection using that version.  <ins> 4.1. </ins> QUIC connections are shared state between a client and a server INV. The compatible version negotiation mechanism defined in this document across two QUIC connections: one before the Version Negotiation packet, and a distinct connection after.  <ins> 4.2. </ins>  <ins> 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 server-fleet) in a Supported Version field.  The server MAY add reserved versions (as defined in the Versions section of QUIC) in Supported Version fields. </ins> 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, The new first flight will allow the endpoints to establish a connection using the negotiated version.  The handshake of the  <ins> negotiated version will exchange version information (see vers-info) required to ensure that VN was genuine, i.e. that no attacker injected packets in order to influence the VN process, see downgrade. </ins>  <ins> 4.3. </ins> When the server can parse the client's first flight using the  <ins> original version, it can extract the client's Version Information structure (see vers-info).  This contains the list of versions that the client thinks its first flight is compatible with. </ins> If the server supports one of the client's compatible versions, and the server also believes that the original version is compatible with load balancers to ensure that packets for a given connection are routed to the same server.  <ins> 5. </ins>  <ins> 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): </ins> The content of each field is described below:  <ins> 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. </ins>  <ins> The contents of the "Other Versions" field depends on whether it is sent by the client or by the server. </ins>  <ins> 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 </ins> preference instead.  <ins> When sent by a server, the "Other Versions" field lists all the Fully-Deployed Versions of this server deployment, see server- fleet. </ins>  <ins> 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. </ins> 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.  <ins> Both endpoints MUST parse their peer's Version Information during the handshake.  If the Version Information was missing or if parsing it 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 a client has reacted to a Version Negotiation packet, it MUST validate that the server's "Other Versions" field does not contain the client's original version, and that the client would have selected the same negotiated version if it had received a Version Negotiation packet whose Supported Versions field had the same contents as the server's "Other Versions" field.  If any of these checks fail, 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. 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. </ins> 7. 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  <ins> flight won't have parsed the client's Version Information yet.  If a future document wishes to define compatibility between two versions that support retry, that document MUST specify how version </ins> 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 11. The security of this version negotiation mechanism relies on the  <ins> 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. </ins> The requirement that versions not be assumed compatible mitigates the possibility of cross-protocol attacks, but more analysis is still needed here.  12. 12.1.