Subject: Ns and IP_BROADCAST

From: Sanket Nesargi <[email-masked]>
Date: Fri, 13 Apr 2001 15:34:00 -0500 (CDT)

Hi,
 	We had been working on some issues on MANET configuration and ran
into questions regarding broadcast and multicast (a lot of which have
been answered in the discussion on MANET and IP Layering).
	In the current implementation of the routing protocols in ns, any
packet with the destination address IP_BROADCAST (255.255.255.255) and a
port number other than the router port (255) is not received at even the
one hop neighbors of the originator (we were using ns2.1b6a for our
simulations). Depending upon the routing protocol used, the packet either
gets dropped or crashes the software. We had to make modifications to
ensure reception at atleast the one hop neighbors. I guess the actual
semantics of to which all nodes this packet gets delivered to would
depend upon the consensus reached on how such destination addresses need
to be handled.
	Also, when ARP requests are broadcast, since the IP header is
absent, and the broadcast in question is a layer 2 broadcast, it should be
responded to only by the one hop neighbors (since, in any case, host
routes need to be maintained for all nodes inside the MANET). If the link
between the  ARP source and target is unidirectional (A -> B), the ARP
response (the target L2 address of which is A's L2 address), would not be
delivered to A. Hence, ARP would fail in any case (unless something
like L2TP based protocols are used to overcome this). This leads us to
believe that additional effort would be required to achieve synergy
between the routing protocols and ARP cache in the presence of
unidirectional links.

	Thanks,
	Sanket

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	Standing on a hill in my mountain of dreams,
	telling myself its not as hard as it seems
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Sanket S. Nesargi
Distributed Systems Lab
University of Texas at Dallas,		http://www.utdallas.edu/~sanket
Richardson TX 75082			<[email-masked]>
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