> i was thinking of the link layer address.  there's currently no way to
> get that from the network layer in the kernel on a per-packet basis,
> unlike, say, the receiving ip address.

What makes you think the link layer address is unique?
It only needs to be unique on a (bridged) LAN segment.
Indeed some of the sBus ethernet cards sold by sun do not have a MAC
address PROM.  The solaris driver (and I assume the netbsd one) uses
the system mac address for these add-on cards.

    David