Path: utzoo!attcan!uunet!mcvax!ukc!icdoc!qmc-cs!liam
From: liam@cs.qmc.ac.uk (William Roberts)
Newsgroups: comp.protocols.appletalk
Subject: Re: the comming appletalk crisis
Summary: how do you handle the logical net/physical net stuff
Message-ID: <895@sequent.cs.qmc.ac.uk>
Date: 20 Mar 89 19:59:35 GMT
References: <8902211606.AA11107@dsunx1.DSRD.ORNL.GOV> <28183@bu-cs.BU.EDU> <26176@apple.Apple.COM>
Reply-To: liam@cs.qmc.ac.uk (William Roberts)
Organization: Computer Science Dept, Queen Mary College, University of London, UK.
Lines: 37
Expires:
Sender:
Followup-To:
Distribution:
Keywords:

I agree that 10 packets per second is too many - the bridges
should not need to send that many RTMP packets; even one every
10 seconds is rather too frequent for my liking.

I'm mildly worried by the "logical net is same physical net"
suggestion, which sounds OK, but needs thinking through
carefully. The key question is

    How can you TELL that another node is on the same logical net?

The wrong answer is "use ARP"; this will generate a lot of
unnecessary ARP traffic, especially when talking to things like
LaserWriters which aren't on the Ethernet cable, but you'd use
ARP just in case they were.

A better method would be to extend the RTMP protocol to
indicate that two logical nets were actually the same - then
sophisticated nodes could work out the routing for themselves.
They'd probably have to anyway - it would be grotesque for a
bridge to broadcast routing information on every LOGICAL network.

There is also the question of how a node discovers its network
number - if this is done dynamically by broadcast on the local
physical network, how is a node to work out which LOGICAL net
it belongs to?

The basic idea is worth thinking about, but it will take a lot
more thinking... especially if you don't want to wreck the
otherwise fairly neat AppleTalk setup. I have always thought it
a brilliant insight that nodes and networks don't need numbers
unless there are others to be distinct from!
-- 

William Roberts         ARPA: liam@cs.qmc.ac.uk
Queen Mary College      UUCP: liam@qmc-cs.UUCP
190 Mile End Road       Tel:  01-975 5250
LONDON, E1 4NS, UK      Fax:  01-981 7517