a proposed modification to ARP


Dave_Katz@um.cc.umich.edu
Mon, 18 Jul 88 13:36:58 EDT


>Of course I could be a real killjoy and note that ARP won't be needed
>on ISO networks, routers will send you redirects instead.

Address resolution is normally done by the ES-IS protocol, ISO 9542.
Hosts (ESs) and routers (ISs) periodically multicast Hellos to one
another, providing the net-subnet mapping (NSAP-SNPA in the parlance).
Hosts can listen to the other hosts' ES hellos if desired, thus learning
the mapping directly; otherwise they can bounce the packets off of a
router and get a redirect.

The packet overhead of redirects vs. ARP is the same, and there are a couple
of advantages to the ISO approach. First, no broadcasts are necessary to
resolve the address (and hosts only have to listen to IS multicasts and
thus aren't bothered by other hosts' address resolution). Second, it is
unnecessary to hold on to the data packet while the address is being
resolved, which reduces implementation complexity and other fuzzy issues
(how long to hold onto the packet before giving up?).

For isolated nets with no routers, hosts multicast data packets with
unresolved net addresses to other hosts (essentially a broadcast since
there are only hosts on the wire), and the destination host sends an
ES hello directly to the sender for future reference.

Dave Katz
Merit/NSFnet



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