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
