Re: MIT pcip and 3com board


Thomas Narten (narten@purdue.edu)
Thu, 02 Oct 86 10:35:54 EST


This is probably not the problem, but....

We recently discovered that the implementation of ARP under pcip is not
quite the same as that on other machines, such as 4.2/4.3 Unix. The ARP
we were running under 4.2 on our VAXes came from Sun; we were not
running the vanilla 4.2, so I can't say what 4.2 ARP does.

When pcip recieves an ARP packet, the first thing it does is check the
target protocol address against its own. If they do not match, the
packet is tossed without further processing. 4.3 Unix on the other
hand, checks first to see if the sender protocol address in the reply
packet matches an entry in its ARP cache. If it does, it updates the
entry.

In our case, we had a machine that sent ARP replies, using the same
addresses in the sender and target fields. On Unix machines, this
worked. Unix would add an entry (marked incomplete) for the Internet
address it wanted, send out an ARP request, and the machine would
answer. Even though the ARP reply that was received did not have the
requesting machines address in the target fields, the entry was
correctly installed.

The pcip machine in the other hand would toss the reply, thinking it
was not the reply is was looking for.

Thomas
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