Re: Dumb vs. smart host routing


William Westfield (BILLW@MATHOM.CISCO.COM)
Fri 13 May 88 13:16:09-PDT


Well, I might as well throw some more ideas into the fire. At cisco
(where we make routers), we also believe that hosts should know nothing
about routing protocols. This becomes much more obvious when your router
is running 4 different routing protocols simultaneously, and translating
metrics from one to another is both difficult and dangerous.

Of course, it ought to be possible to use any of the gateways you've
learned about via redirects as new default gateways, but this is probably
not easy to add to existing code. However, John Romkey said:

    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.

We believe that such a protocol already exists which provides this
function, and is implemented by nearly all tcp/ip vendors. It is called
ARP. A router can respond to ARP requests that it recognizes as being for
hosts not connected to the local cable if it knows a route to that network.
This technique is being called "Proxy ARP", and is frequently used to make
subnetting transparent to hosts. It works equally well to make any network
topology transparent to hosts. Redundancy is handled automatically if you
convince TCP and other high level protocols to flush ARP entries when they
are about to time out.

Of course, this only works with hosts on networks where ARP is used for
address resolution (broadcasts are possible). But if a host has only a
point-to-point link to the network, it might as well use whatever gateway
is at the other end of that link as its default gateway. This leaves only
networks like X.25 and the ARPANet (non-broadcast, but not really point-to-
point either), and it might not be such a bad idea to have those hosts know
about routing, at least to the extent of having multiple default gateways.

Bill Westfield
cisco Systems.
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