routing question

[root@network.ucsd.edu (Rooty-Toot-Toot)]

Path: ucsd.edu!brian
From: brian@ucsd.edu (Brian Kantor)
Newsgroups: mail.tcp-group
Subject: Re: routing question
Date: 18 Jan 1992 14:39:25 GMT
Organization: The Avant-Garde of the Now, Ltd.
Lines: 57
Message-ID: <kngf0tINN844@network.ucsd.edu>
References: <29778a0b.skyld@skyld.UUCP>
NNTP-Posting-Host: ucsd.edu

"Jeff Angus" <jangus@skyld.sr.com> writes:
>there a provision in NOS that allows it to know where to try and send some-
>thing or does this all have to be done by hand everytime for everyone?

There are several ways to solve this problem.

        1. subnet by frequency (ecch)
        2. fixed routes for each station (impractical in a large network)
        3. OSPF/RSPF
        4. modified ARP (not yet implemented)
        5. RIP (hacked)

In order to accomodate the most people with the least hassle, at least
now, I've chosen to use RIP.  Here's how:

First step is an administrative one: the local packet Gestapo has to
designate some channel as the default route.  Typically this will be
easy to divine: it's the channel that most of the tcp/ip'ers hang out
on.  Usually it's also the slowest channel.

Each station runs a rip receiver ("start rip") to allow rip responses to
build temporary routing entries.

Each station which wishes to communicate on other than the default
channel sends an unsolicited RIP response to the other stations its
trying to talk to, naming itself as a route to itself.  This can be
directed or broadcast.

To broadcast RIP responses, you have to wedge an entry for the broadcast
address into your arp table - "arp add 44.0.0.0 ax25 qst-0" will do.
Then start rip - "rip add 44.0.0.0 60 2", for example.

Note that rip is broadcasting once a minute.  That's because the
expiration time of the rip entries in the routing table is currently 4
minutes - reasonable for an Ethernet, where RIP is primarily used.
That's a bit impatient for radio.  However, to change it requires
recompiling NOS, and should include the agreement of all concerned.
[Personally, I think the RIP-route table TTL should be derived from
a per-interface parameter.]  This once a minute broadcast by each
station in the area is one of the reasons why you don't want to do
this on your slow channels, which is why you don't want to designate
a slow channel as non-default.  At 9600 bps, it's tolerable, given the
low number of stations on in most areas.  At 56kb, it's nothing.

Note also that the "rip add" command given above has a flags of '2'.
This applies to NOS1229 and later only, and causes the RIP packet
to contain a route to itself.  The last few versions of NOS have not
allowed you to enter a route to yourself in the routing table, so
RIP couldn't advertise that.  Versions earlier than that DID allow
you to enter such a reflexive route, which will also work, but has
the potential for causing routing loops.

Finally, I don't know of any reasonable way to do this if your IP paths
are running over some kludge lower-level transport network.  I guess
you'll have to use fixed routes.  Consider it additional incentive to do
it right.
        - Brian