Re: weird arps


Bill Northlich (zehntel!kinetics!billn@ucbvax.Berkeley.EDU)
20 Oct 88 16:18:07 GMT


The 806 arps with no bits in the protocol address except the rightmost 8 bits
(0.0.0.x) are what we call "old-style Appletalk Arps (AARP)".

With "Ethertalk" Apple specifies a version of arping to use for Appletalk.
Prior to Ethertalk, we (I personnaly must take the blame) threw together
some mods to standard if_ether.c to allow our in-kernel Appletalk code to talk
to others of it's ilk. Hence the rule "if the left 3 bytes of the 0800
(IP) protocol address are zeros, then it's an Appletalk ARP".

Our plan was to "do it right" in a future release, and send an 806 packet
with an 809b (Appletalk) protocol type. Unfortunately we knew of systems which
did not check the protocol type in their ARP modules, and reasoned that it would
be less damaging to the world for some machines to wind up with wierd IP
addresses in their caches, than to risk crashing machines with broken ARP
modules just because we wanted to do Appletalk on ethernet.

Now Apple has come out with AARP on a completely different ethernet
type (80f3). They used a new type primairily as another sop to reality, ie,
on a given machine, if some other bunch of software has reserved type 806, it
may be hard to "get into the pants" of the existing 806 code to add Appletalk
stuff. This is especially true in VMS.

Currently the only system we know of which uses these old AARP's is the Sequent
running Appletalk. They refuse to upgrade. In existing networks, there may
be some Pyramids and/or some Vaxes which have not been upgraded. Also, our
Appletalk router boxes can be configured to do old-style AARP, but they don't
as a normal thing. In general, there is no need to have old AARP unless you
have a Sequent. We have a Sequent. Hardware-wise, the most reliable UNIX
machine I've ever used.
/bill



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