[xcp] Embedding XCP signalling in IP header

[Tim Shepard]

> - any "odd" IP thing, such as use of a per-hop next-header mechanism 
> borrowed from v6, will kick the packet out of the fast path.

A per-hop next header machanism borrowed from IPv6 and used over IPv4
would not, I expect, cause packets to be slow-pathed in non-XCP aware
routers.  (A different example: I would not expect SCTP/IPv4 packets
to be slow-pathed in non-SCTP-aware routers.   Likewise ESP/IPv4.)

If you make the router XCP-aware, so that it's looking for the XCP
protocol ID in the IPv4 header, then if it slow-paths that, that would
be a deliberate choice of the designer of that IPv4 router.  That
would be unfortunate.  I sure hope that doesn't happen.  If there was
reason to slow path in this case, then I would expect there would also
be a similar reason to slow-path the packets that carried the XCP
information in overloaded fragmentation fields.


			-Tim Shepard
			 shep at alum.mit.edu