Version negotiation: the bare minimum?

[Watson Ladd <[e-mail]>]

Dear WG,

I'd like to proffer the world's simplest version negotiation scheme,
based on comments heard during the meeting today from a number of
people.

The following weak assumptions are made: the client has a set of
versions. The server has a partial ordering on versions: this means
that versions are not necessarily preferred over each other (consider
experiments where we will do what the client offers first), but the
relation is transitive. Then the server selection is a function of the
client offered version and supported set.

The client transmits its supported versions and a proffered hello
version in the first packet. The server selects. If that selection is
incompatible they try again with the new selected version transmitted
in VN. If it is compatible, the server selects and proceeds.

The constraint on the handshake is that the supported versions and
offered version and server selection are incorporated on the handshake
in such a way that a mismatch triggers failure, and no two different
versions can derive the same keys. If we assume that e.g. SHA256 is
unbroken this is easy to get.

This only permits a downgrade to a version the server was willing to prefer.

Sincerely,
Watson Ladd