Path: utzoo!attcan!uunet!husc6!bbn!uwmcsd1!ig!agate!ucbvax!UM.CC.UMICH.EDU!Dave_Katz
From: Dave_Katz@UM.CC.UMICH.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  Dumb vs. smart host routing
Message-ID: <3046634@um.cc.umich.edu>
Date: 13 May 88 14:45:48 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 27

The OSI approach to this problem is to use the End System-Intermediate
System protocol (ISO 9542).  In this scheme the ES's (hosts) and IS's
(routers) periodically multicast "hello" packets to one another in order
to determine reachability.  The period used can ultimately be controlled
by the IS's.  This usually means that address mapping info is already
cached so the "hold on a minute while I find out where this packet
needs to go" approach that ARP uses is most often not needed.
 
Hosts will discover all routers on the subnet using this mechanism.
Redirects are used to teach the hosts which routers to use for
particular addresses.  The redirects can contain address masks defining
equivalence classes of destination addresses to redirect, as well as
possibly hinting that the host can algorithmically derive the MAC
layer address from the network layer address (OSI NSAP addresses are
big enough to embed MAC addresses in them if somebody wants to).
 
This protocol is purposely decoupled from IS-IS routing ("IGP") with
the philosophy that hosts should be kept insulated from the details
of what happens in the routers, and should be kept simple.  Thus the
only a priori information needed is the ES's own address.
 
This obviously doesn't help in the TCP/IP world, but it's worth
mentioning.
 
  --Dave Katz
    Merit Computer Network/NSFnet
    Dave_Katz@UM.CC.UMICH.EDU