Dumb vs. smart host routing


mar@ATHENA.MIT.EDU
Fri, 13 May 88 13:27:33 EDT


You are right in that ICMP redirects are only good in indicating the
first hop from a host. But that does not make them largely useless,
since this is their intended use, and is a valuble part of not
requiring hosts to understand complicated routing. This way a host
only needs to know about one local gateway *which is currently
functioning* and redirects will do the rest.

Gateways, on the other hand, must understand routing within their own
autonomous system, and to other systems. It is expected that gateways
will make use of other protocols for this, not try to use ICMP for
something it was not designed for.

As in the discussion about how hosts find out about the different
first-hop gateways available: the hosts shouldn't listen to RIP, a
gateway routing protocol, just as gateways don't for the most part
receive ICMP redirects. We will get nothing but trouble if we try to
use the same protocols between gateways and for hosts to communicate
with gateways. A gateway-to-gateway protocol is necessarily going to
be much more complicated than what a host needs, and *will* evolve
over time as the internet grows. It should be possible to specify a
gateway-to-host protocol in such a way that it is simple and not
likely to change so that it will get implemented by all vendors for
all operating systems.
                                        -Mark Rosenstein



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