3600's & Internet|0.0.0.0

[Rich Cohen <HI.COHEN@mcc.com>]

This is for general information, should anyone else stumble across
this themselves.

Tidbit:

	Symbolics 3600's with IO Rev 2 (i.e., non-PE Video, no audio
	capability) running IP-TCP sometimes broadcast seemingly pointless
	ethernet ARP (Address Resolution Protocol) packets asking about
	internet address 0.0.0.0.

There.  We told you.  Now you know.

For those who aren't into network protocols...

ARP is a protocol that allows a host to determine the appropriate
hardware address (e.g., physical ethernet address) to use to
communicate with another machine of at a known higher level protocol
address (e.g., an internet address).  Since internet address 0.0.0.0
is restricted, and cannot be assigned to any host, ARPing for this
address is bogus.

Ok.  So, what's going on, you might ask?  Well, if you have Symbolics
sources, and you look in the machine dependent 3600 ethernet interface
code (SYS: IP-TCP;IP.LISP), you will see a defun-in-flavor named
IP-UNWEDGE-3600-ETHERNET-INTERFACE.  This function is called on 3600's
with IO Rev 2 boards when the length of an incoming IP packet is odd
and greater than 46.  IP-UNWEDGE-3600-ETHERNET-INTERFACE just sends
out an ARP request for internet address 0.0.0.0, possibly unwedging
the 3600 ethernet interface along the way.  Presumably there is
some magic going on here.

Anyway, if you ever look at the traffic on your ethernet, and notice
some random 3600 ARPing for 0.0.0.0, don't worry.  It's a feature.

-- Rich Cohen & Clive Dawson
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