[ns] off_arp_

[Aman Singla <aman@atheros.com>]

Hi-

I notice that ns/tcl/lib/ns-mobilenode.tcl does:
ARPTable set off_arp_         0

and ns/tcl/lib/ns-cmutrace.tcl does:
CMUTrace/Drop set off_arp_ 0

This causes ARPTable::off_arp_ in the compiled
code to be set to 0 when the bind("off_arp_", &off_arp)
call is made (instead of 1040 - the value generated
by PacketHeaderManager in my config). This results in
the ARP module generating the arp header at offset
0 in the packet. The CMUTrace class does pick up the
right value of off_arp_ into its own protected variable,
looks for the ARP header at the right place (offset 1040)
and finds garbage, generating erroneous
values for ARP packets in the trace file.

I haven't confirmed if the CMUTrace/Drop class gets
the right offset for the arp packet or not..

I'm using a snapshot from the ns Aug 8 code.

My interpretation of the docs prompted me to comment
out the above two lines of code in the tcl files, which
resulted in the correct behavior (i.e. Trace file output
makes sense).

So - I'd assume this was a bug.. unless I'm missing
something (I'm new to ns) - in which case could someone
please explain the intent of this code and the behavior
I see in the generated trace.

thanks,

:a