Compatible Version Negotiation for QUIC

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Table of Contents

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 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.¶

2. 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 (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 original version. 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 and the server starts incompatible version negotiation and the client then 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 either 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.¶

Client                                          Server

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

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

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 the "Notational Conventions" section of [QUIC]):¶

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

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 depends 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 the Versions section 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 a 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. For example, the server determines that QUIC version 1 is in use by observing that the Version field of the first Long Header packet it receives is set to 0x00000001. 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. For example, if a 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 will close the connection with a version negotiation error.¶

If a client receives a 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 the Version Information was missing, the endpoints MAY complete the handshake. However, if a client has reacted to a Version Negotiation packet and the 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 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 during compatible version negotiation the client is made aware of the negotiated version by the QUIC long header version (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 Version long header 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. 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 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:¶

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 partially-deployed versions. 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 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 which 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.¶

8. Special Handling for QUIC Version 1

Because QUIC version 1 was the only IETF Standards Track version of QUIC published 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 which wish to use version negotiation to negotiate versions other than QUIC version 1 will need to 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

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