Re: Dumb vs. smart host routing


Jeffrey I. Schiller (jis@BITSY.MIT.EDU)
Sat, 14 May 88 20:47:14 EDT


> ...When a host needs to send
>something off net and either A) does not have the IP Address of any
>router (if it had one address, it could send a packet to that router
>and get the redirect) or B) the host decides that the router it is
>using is dead (e.g. TCP has retransmitted too often) then the host
>broadcast a "Who can route to network X?" request and the routers on
>the local network would respond.

        Now be VERY careful with protocols that broadcast questions
that may get simultaneously answered by a large number of machines. In
a network with only one or two routers what you suggest would work
fine. However when you have about 20 routers on an Ethernet you are
likely to get a monster Ethernet collision when they all go to respond
to the broadcast. There have been several horror stories posted to
this mailing list of "monster collision" problems (I know, I was one
of the senders!). ARP works using broadcast questions, but in a
properly functioning network exactly one machine should respond, so no
collision.

        However you do bring up a problem with single threaded
machines. What if we modify your scheme so that all routers on a
network speak some (as yet undefined) protocol to elect one of them
the "advertised default router" and only this router would respond to
the "Who can route" message. If this router went down the other
routers would know and elect a new default router. [Note I have
simplified the question from "Who can route to network X?" to just a
simple request for a default router. That router can then send
redirects later when packets arrive.]

                        -Jeff



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