Re: Dumb vs. smart host routing


John Romkey (spdcc!kaos!romkey@husc6.harvard.edu)
13 May 88 06:22:54 GMT


In article <10285@ulysses.homer.nj.att.com> smb@ulysses.homer.nj.att.com (Steven Bellovin) writes:
>One problem I see is that ICMP Redirect is largely useless. It's only
>useful for the first gateway along the way to tell the originating host
>to use a different gateway; it can't be used to tell an intermediate
>gateway what the proper next hop is.

But that's not really a problem with ICMP redirects at all. Look at it
this way:
        ICMP redirects are the (okay, *a*) way routers communicate routing
        information to hosts

        XYZ (fill in your favorite routing protocol(s)) is the way
        routers communicate routing information to one another.

If the routers are doing their job properly then the routers on the
same subnet as the host will redirect it to the proper router via
ICMP; this router will then do XYZ things to figure out how to route
the packet from there. It's because of XYZ that the routers know when
to redirect in the first place.

>The conclusion of all this is that local gateways must be extremely
>smart. The current scheme, with EGP, works well enough in the current
>environment, where there's one central net (ARPANET+MILNET); it would
>fail miserably if there were a large number of interconnected backbone
>nets.
>
>I'm not certain what to do about the problem.

The example assumes that the routers are screwed up in the first
place. You don't necessarily have to have an incredible amount of
information in your routers that are used by your host - you just have
to have routing protocols that do the right thing (which may actually
require incredible amounts of information...oh well).

The thing to do about it is to refine the XYZ protocols (ancient GGP,
RIP, EGP) so that they work better for larger, more complicated
networks. Yes, the current system doesn't deal with complicated
networks very well. It doesn't handle redundant routes well. It
doesn't handle load-sharing very well. It doesn't route on Type of
Service very well (because not a lot of routers support it and
virtually no hosts set the TOS field in the IP header).

I believe there are people on an IETF task force or working group or
some such working on this problem.

--
			- 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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