[xcp] Embedding XCP signalling in IP header

[Tim Shepard]

OK, why not use some mechanism (IPv4 option or TCP option, it doesn't
seem to matter) to query the other end's ability to use XCP and then
when you get an affirmative response, switching to an XCP header that
sits between IPv4 and IPv6?  That seems much simpler to me.

I've also heard that some VPN tunnels ignore the DF bit and fragment
anyways.  (I believe this was mentioned in the pmtud WG at IETF60.)

			-Tim Shepard
			 shep at alum.mit.edu