a proposed modification to ARP


Michael A. Patton (map@gaak.LCS.MIT.EDU)
Wed, 13 Jul 88 10:26:58 EDT


In Re:
> Date: Mon, 11 Jul 88 21:36:30 -0400 (EDT)
> From: Drew Daniel Perkins <ddp+@andrew.cmu.edu>
>
> I'm not sure if you are really suggesting using 2^24 multicast addresses or
> not. While quite precise, it would probably be be overkill.
I'm pretty sure that's what he had in mind, it's the obvious number
(see below). It may be overkill, but it doesn't hurt anything and
helps the "bother as few hosts as possible" philosophy of the original
message, in general only the intended host will actually have to
process each ARP "broadcast".
> ... Considering that many (most?) of
> the ethernet chips which support more than 1 multicast address
> actually support 64, a good compromise between only 1 address and
> 2^24 addresses might be to reserve 64.
But the point was that each host only needs to listen to ONE of these
addresses, the one it's IP address maps into, any ARPs sent to the
others would be ignored. With this algorithm you get the hardware to
do the ignoring, saving on CPU time and interrupt overhead, possibly
even on context switches.
> ... Some sort of fast hashing
> function could be used to map an IP address to one of 64 buckets.
> This probably rules out the hashing function (CRC's) used by many
> of the chips themselves. How hard is it to get lots of multicast
> addresses? Is it possible to buy 2^24 addresses?
It isn't possible to buy fewer than 2^24, that's the smallest package
available :-) When you buy Ethernet addresses, you buy a 24 bit
prefix, and you get to assign the other 24 bits as you see fit. This
seems like exactly the kind of scheme that fits this well.

> Drew

Mike



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