legacy cookie field

[martinthomson]

I can't reconcile:

In this case, the cookie extension is omitted and the legacy_cookie field in the ClientHello message SHOULD be set to a zero length vector (i.e., a single zero byte length field) and MUST be ignored by a server negotiating DTLS 1.3.

... with:

If a DTLS 1.3 ClientHello is received with any other value in this field, the server MUST abort the handshake with an "illegal_parameter" alert.

The MUSTs in these appear to be pulling in opposite directions. I suspect that the latter is correct though. It is possible that a DTLS 1.3 client could get a legacy cookie from a DTLS 1.2 server, so that it is valid for the legacy cookie to appear when supported_versions includes DTLS 1.3. However, that represents an attack and a DTLS 1.3 server shouldn't continue to establish the connection. Maybe it isn't a bad attack because the client doesn't change its posture in ways that would materially affect the resulting session, but we have generally tried to avoid these.