Subject: RE: overhead tradeoff between on-demand and generic link state ro uting protocols

From: "Josephson, William" <[email-masked]>
Date: Wed, 29 Mar 2000 11:21:04 -0500

It's really not fair to describe any one approach to be optimal based solely
on the amount of overhead it generates.  In some situations, one desires
more to minimize delay to setup the topology and effect information transfer
more than minimizing overhead.  Usually
both count.  As a simple example, an on-demand protocol in which its nodes
caches its routes for a very long periods of node idleness will perform
quicker when all the nodes need to wake up
and react (to the event not the link change). This is a situation of a
fairly immobile but ad
hoc network like bluetooth attempts to solve.

-----Original Message-----
From: Zygmunt Haas [mailto:[email-masked]]
Sent: Wednesday, March 29, 2000 6:57 AM
To: John Zavgren
Cc: M. Scott Corson; [email-masked]; Zygmunt Haas
Subject: Re: overhead tradeoff between on-demand and generic link state
routing protocols

Hi John,

Actually, such an analysis should take into the account not only the
traffic distribution, but the mobility of the nodes as well. Let me
explain further.

Another way of looking at the issue that you have raised is to see whether
some mix of proactive vs. reactive (on-demand) behavior of the routing 
protocol would be more efficient for a specific network operational
condition. 
In other words, if the network load is heavy and uniform, proactive
protocols 
should do better, while when the load is low and highly localized, reactive
protocols should be preferred. In the middle, there is a broad range of
traffic mixes that would justify some degree of proactivity/reactivity.

You see where I am getting at ... the ZRP is exactly such a protocol. It
allows to adjust the degree of reactivity/proactivity in the protocol
behavior based on a single parameter - the Zone Radius (ZR). It has been
shown that for a highly dynamic network (i.e., high mobility nodes) and
for low traffic load, small ZR leads to optimal performance - of course
small ZR corresponds to a purely reactive protocol. On the other hand, for
a stationary, highly loaded network, large ZR is much better, corresponding 
to purely proactive behavior.

The ZR is like a "knob" (Joe's analogy (:-)) that allows to adjust the
behavior of the protocol to the current network conditions (mobility and
traffic load). More information on the behavior of this "knob" could be
found in:
M.R. Pearlman and Z.J. Haas, "Determining the Optimal Configuration of for
the Zone Routing Protocol," IEEE JSAC, special issue on Ad-Hoc Networks,
vol. 17, no.8, August 1999

I believe that if there is to be a single (or a small set) of MANET
routing protocols, some mechanism to adapt to the current network
conditions (again, traffic load and nodal mobility) is essential, since
the ad hoc networking environment is so braodly defined.

Cheers,

Zygmunt.
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Wireless Networks Laboratory		                  
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> On Tue, 28 Mar 2000, John Zavgren wrote:
>
> > Greetings:
> > 
> >  I have been reading about on-demand routing protocols that are being
proposed by the manet group.  Is there any analysis that compares this class
of routing protocol with the class of protocol that maintains routes "all
the time", i.e., generic link state, or distance vector? 
> > 
> > Of course, this comparison would need to be made for carefully chosen
end-to-end traffic profiles. If there is no traffic being offered to the
network, then the on-demand protocol will generate no routing update
traffic. And I suspect that if a uniform, heavy end-to-end traffic load is
presented to an on-demand routing protocol, the protocol will look pretty
bad in comparison.
> > Thanks
> > John Zavgren
> > [email-masked]
> > 617-873-2280

---

From: Zygmunt Haas <[email-masked]>
Date: Wed, 29 Mar 2000 13:14:10 -0500 (EST)

Hi Bill,

Yes, I agree that other considerations, besides the volume of control
traffic, need to be taken while considering the proactive vs. reactive 
protocol behavior. As you have rightfully stated, delay is one such a
consideration. (I do not, however, agree that on-demand protocols
(even with extensively long caching) would perform better than proactive
protocols, as in the proactive protocols routes are always available
immediately when they are needed.
 
To add my "3 cents" again - a hybrid (proactive/reactive) protocol could
be useful in "optimizing" the response time. Maybe, a correct approach
would be to consider a "cost" function that contains both communication
overhead and delay, weighted by some coefficient of importance to a
specific application.

Zygmunt.
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