> 	I totally agree that it is cleaner to add field to struct mbuf,
> 	however, I really hesitate to add member into mbuf, as in many
> 	case the usable data size in mbuf matters.  on architectures
> 	with sizeof(void *) == 4, MHLEN will be 100 and this captures
> 	short packets, which are commonly used in tcp ack and others.
> 	We may want to increase MSIZE into 256 (even on non-64bit arch) if
> 	we need to add more fields.

If size is a concern, how about using a union and a flag bit?
(if the flag is clear, use the rcvif arm of the union; if set, use the
sndso arm of the union).

One other note.. it occurs to me that a complete policy engine may
also want to know the receiving interface for forwarded packets, so
keeping the old value around might make sense.

					- Bill