Re: workstation != gateway


J. Noel Chiappa (JNC@XX.LCS.MIT.EDU)
Tue 25 Feb 86 01:00:07-EST


        Mike:

        I never intended to saddle Berkeley with the blame for EGP! I
will also say in Berkeley's defense, that I'm not sure that *anyone*
recognized that IGP's would have to cart around info as to the source
of routes until long after Routed was done. Still, there's *no*
upwardly compatible way to extend it that we could find. (Defining a
whole new protocol with a different version doesn't count.)

        One issue that people over look with all these different
broadcast addresses is that they can be fairly expensive to check for
in gateways. I recognize all the same formats as you do, and in our
(additedly hyper-optimized) gateway it's *expensive*! I have figured
out some optimization recently, but it's still going to cost a fair
chunk. Right now, routing is about 5-10% of fowarding costs. Was it
wise to slow down basic packet forwarding, by e.g. 5%, to make broadcast
(used on %.01 of packets) work?
        Unless you want to assume that broadcast packets are flagged
by a hardware boardcast address (not a good assumption for a variety
of reasons) checking for broadcast will be somewhat expensive.
Depending on how multi-cast was added, it might also wind up as
somewhat expensive. (BTW, I wish people wwould start using FFFFFFFF
for 'local wire broadcast' like the spec recommends; it is so much
easier to deal with, and you don't have to think about what parts to
mask off and on.)

        I can't believe that BBN gateways discard input packets sent
to the broadcast address. (My gateways discards packets sent to an
address that happens to be the local broadcast address, unless
specifically told to broadcast the packet, but don't do anything like
that to an incoming packet.) What on earth do you gain? Any packet that
algorithm would discard could have been sent directly to the gateway
in any case, and it prevents some really useful functions.

        Noel
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