Subject: Re: TORA and multiple paths

From: "Vincent D. Park" <[email-masked]>
Date: Tue, 7 Oct 1997 11:12:00 -0400

>Hi Vincent,
>
>I was recently reading your paper in the proceedings of INFOCOM'97
>outlining TORA and was wondering if you could clarify the following
>point for me.
>
>Your paper mentions that TORA is capable of determining multiple paths to
>a given destination. My question is this, does TORA distribute the load
>over the multiple paths and if so how does it distribute the traffic ?
>
>My apologoies if this is a straight forward question, but I can't find any
>information about this in the paper.
>
>Kind Regards,
>
>Kieran Power.
>
>
>************************************************************************
>Kieran Power.                        Room : 63 - G21
>Department of Digital Systems,       Tel : +61 3 9905-3234
>Monash University, Clayton Campus,   Fax : +61 3 9905-3574
>Wellington Road, Clayton 3168,       E-mail :[email-masked]
>AUSTRALIA.    			     WWW : www.dgs.monash.edu.au/~kpower
>************************************************************************

Sorry, I was a little slow responding to your earlier message.

TORA often provides multiple paths to a destination, in that nodes often
have multiple links marked as "downstream" (all of which lead to the
destination). Thus, from a global (external) perspective there may be
multiple paths leading from a given source to a given destination. However,
from an internal perspective each node has only "one-hop" knowledge.
Therefore, a node does not know the full path to the destination, it only
knows that it can forward any messages for a given destination over any of
the links marked as downstream.

Given the above, I think your question pertains to how the "next-hop"
forwarding decision is made by a node. This (in some sense) is orthogonal
to the TORA protocol (i.e. the forwarding decision can be made any number
of ways without changing TORA). Since the INFOCOM '97 paper, we have
simulated the protocol and completed a performance comparison. More
information and a paper on this topic can be found on our web site
(http://tonnant.itd.nrl.navy.mil/tora/tora.html).

For the performance comparison we tried two different methods for making
the forwarding decision. In the first method, the next-hop was selected
randomly (from the set of downstream links) for each individual message
packet. This was a simple attempt to distribute the traffic load. However,
since in the simulations all nodes were generating message traffic (at
equivalent rates) for all other nodes, I suspect the traffic was already
well distributed (from a global perspective). This method would have made
more sense given a different traffic pattern scenario. For example, a
relatively small number of source/destination pairs (i.e. compared to the
number of nodes in the network) generating relatively heavy traffic
streams. In the second method, all message packets were forwarded to the
neighboring node with the "lowest" height. For the scenarios and traffic
patterns we tested, this method actually performed better. It would be
interesting to test a scenario where distributing the load would seem
advantageous.

Hope this was helpful,

-Vince

-----------------------------------------------------------------------------
Vincent D. Park
Information Technology Division
Naval Research Laboratory
4555 Overlook Ave.
Washington, DC 20375-5000
------------------------------------------------------------------------------