Two frames

[martinthomson]

It might be easier to present this as follows:

OBSERVED_ADDRESS_V4 Frame {
    Type (i) = 0x9f81a6,
    Sequence Number (i),
    IPv4 (32),
    Port (16),
}
OBSERVED_ADDRESS_V6 Frame {
    Type (i) = 0x9f81a7,
    Sequence Number (i),
    IPv6 (128),
    Port (16),
}

Yes, this raises the question about whether the sequence numbers are taken from the same space or not (can you send a v4 frame with the same sequence number as a v6 frame), but I think that's already an open question. I just find this less ambiguous, because you don't have to think about whether you might have none (or both) of the IP addresses.

[huitema]

An endpoint using multiple paths could well use a different source IP address for each path. The observed address can be associated with a path, see the discussion in 5, address discovery, "endpoints SHOULD bundle the OBSERVED_ADDRESS frame with probing packets". This get a bit more complex with NAT, because we can have several successive mappings for the same path. The sequence number allows endpoint distinguish the latest reflexive address for a given path. With potential 6to4 NAT, it might be better to keep a single sequence for both V4 and V6.

[martinthomson]

That's what I figured as well. The issue exists so that you can write that into the spec :)