Dumb vs. smart host routing


John Romkey (ima!spdcc!kaos!romkey@decvax.dec.com)
12 May 88 01:01:00 GMT


In article <8805102321.AA26819@hogg.cc.uoregon.edu> jqj@HOGG.CC.UOREGON.EDU writes:
>>You REALLY DON'T want hosts to know about routing.
>At issue here is a critical point: how smart is it desirable for hosts
>to be? Braden argues that they should be very dumb. I would argue
>that they can be dumb if they don't really need connectivity off their
>network, but should be a little bit smarter if possible.

As the network grows the part which will give out is the routing
substrate. The Internet has already had this happen a few times. We
went from GGP to EGP, then EGP broke and it had to support multiple
packet updates, and now we're in the process of scrapping EGP for the
next step. Each of these steps was good enough for networks of a
certain size and broke as the network grew.

If we can keep the hosts as stupid as possible with regard to the
routing algorithms and bottle the complexity up in the routers then
we're in a better position to deal with the next time the Internet
hits a level where the existing routing protocols break. Whatever the
next solution is, it may not hold for 10,000 networks. Or 100,000
networks. With one or two hundred hosts per network.

[I've been working with TCP/IP long enough that when I started, we
called routers 'gateways', so know that I'm using them as synonyms in
this message.]

I'd rather treat the network as a black box from the host's point of
view. For a host with a single network interface, I think the best
way for it to route is to have a list of default gateways which it
cycles through, caching a route with a 'connection'. If TCP notices
problems with lots of retransmissions, it could call IP and request
that it reroute the connection (IP flushes the route and uses a
different default gateway). Since the gateways know the TRUTH they can
send ICMP redirects when they believe there is a better route.

If some new protocols were cooked up to allow hosts to query routers
for some useful information WITHOUT the hosts understanding how the
routers worked, that would be okay as far as I'm concerned.

Multihomed hosts are a more substantial problem, and right now they
probably do have to at least listen to routing protocols in order to
figure out the best routes to use.

This way, the next time the routing substrate breaks because the
Internetwork has grown too much, only the routing substrate needs to
be changed. Just fixing this will be a bad enough problem without
having to change all the host software in the world. We'll have a lot
more flexibility to change the routing substrate then.

--
			- john romkey
UUCP: UUCP: romkey@kaos.uucp			ARPA: romkey@xx.lcs.mit.edu
 ...harvard!spdcc!kaos!romkey		Telephone: (617) 776-3121



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