An Alt-Svc Parameter for QUIC Versions

Introduction HTTP Alternative Services (Alt-Svc) describes how one origin's resource can be accessed via a different protocol/host/port combination. Alternatives are advertised by servers using the Alt-Svc header field or the ALTSVC frame. This includes a protocol name, which reuses codepoints from the Application-Layer Protocol Negotiation (ALPN) TLS extension . Servers can advertise multiple alternatives, in which case the order reflects the server's preferences (the first value being the most preferred). Clients can ignore alternative services, or pick one at their discretion. A client might use any details from the advertisement, in addition to out of band information, in determining if an alternative is suitable or preferred. While ALPN was originally intend to allow multiple applications to utilize TLS or DTLS on the same IP address and TCP or UDP port, ALPN can also usefully identify the transport in an Alt-Svc context. The "h3" ALPN codepoint informs the client that it can use HTTP/3 for access, which in turn requires the QUIC transport protocol . QUIC is versioned. A client and server that both support a QUIC version can, through a negotiation process, generally agree on that version in no more than one round-trip. However, to avoid that penalty clients might use the most commonly deployed QUIC version (e.g. version 1 at the time of writing), rather than the version with the most desirable properties for the client's use case. To avoid the round-trip, one solution would be to register unique ALPN codepoints for each HTTP/3 and QUIC version combination. However, this might complicate deployment of new versions and deprecation of old ones: architecturally, an application should provide its ALPN to its QUIC implementation. In this case, fully deploying a new version in that implementation would require updating all applications that use it. Instead, this document specifies an Alt-Svc parameter that lists the QUIC versions available to serve the resource. Clients that do not understand this parameter will ignore it. They might default to the most likely version, and/or incur a round-trip penalty in the event of a mismatch. Clients that do process the parameter will connect successfully using the most desirable version with high probability.

The quicv Parameter This document specifies the "quicv" Alt-Svc parameter, which lists the QUIC versions supported by an endpoint, using the hexadecimal representation of the version field in a QUIC long header, as indicated in . Senders MAY omit leading zeroes from version numbers. Examples: The order of entries in version-list reflects the server's preference (with the first value being the most preferred alternative). Note that the quicv parameter applies to a single associated entry in the Alt-Svc list. Servers MUST NOT provide a quicv parameter to an entry containing ALPN codepoint that does not potentially utilize QUIC. If the Alt-Svc information resolves to a server pool that inconsistently supports different QUIC versions, the parameter SHOULD only advertise versions that are supported throughout the pool.

Security Considerations This document inherits the security considerations of , especially the implications of "Changing Protocols" in Section 9.3. There are few protocol properties guaranteed to hold across all QUIC versions, so endpoints should be aware what capabilities are intrinsic to the QUIC versions they are advertising. This parameter reveals capabilities of the described server, but this information is already available by inducing the server to generate a QUIC version negotiation packet.