Subject: Proactive vs Reactive vs Hybrid

From: "A. Bruce McDonald" <[email-masked]>
Date: Fri, 7 Jul 2000 12:08:29 -0400 (EDT)

Hi,

I have been following the recent discussion and have a few observations.
It seems clear that the tradeoffs between proactive and reactive routing 
strategies are quite complex---which approach is best depends on many 
interacting factors. Perhaps more importantly, these factors themselves 
can be expected to be dynamic. Hence, to develop a technology that has 
any hope of long term commercial viability---or practical application,   
ad-hoc networks MUST be capable of adapting to work under a wide range of 
circumstances/scenarios. The must be work under a wide range of mobility 
conditions, they must be scalable and they must support many diverse 
applications. 

Hence, I strongly agree with Dr. Haas that the answer to the concerns 
lies in the study and development of hybrid routing strategies. The 
most viable solution to the problem is to build hybrid systems that can 
adapt in a way that dynamically manages the tradeoffs between proactive 
and reactive routing to meet 'local' conditions. That is, the 
appropriate routing strategy will depend on factors that are both 
temporally and spatially determined. These include the mobility patterns 
of the nodes, the number and distribution of nodes throughout a network, 
and the traffic patterns between the nodes. Other factors include the 
capabilities---processing, power etc.., of the nodes, and the 
environment---how does it affect the reliability of links. 

As I see it we can do one of two things: (1) Build 
ad-hoc systems that use a fixed approach and are only viable under a 
well-defined range of circumstances---very niche oriented. To do this we 
tune the networks to a particular scenario. Perhaps such systems can be 
tunable to work under different circumstances by changing system 
parameters; however, this destroys the spirit of ad-hoc networking, and 
is not likely to be a scalable alternative. (2) Build ad-hoc systems 
that dynamically adjust their routing strategy according to conditions 
that change in time and space.  I believe as a community we should 
concentrate on the later approach. 

A. Bruce McDonald

University of Pittsburgh and
Northeastern University

---

From: "Charles E. Perkins" <[email-masked]>
Date: Fri, 07 Jul 2000 14:33:38 -0700

Hello,

> interacting factors.

Moreover, the interactions between the factors are not well understood.

> lies in the study and development of hybrid routing strategies.

While I may not totally disagree with this statement, I fear that
it would lead to confusion in the short term.  One thing about
hybrid strategies is that they are almost guaranteed to be more
complicated than "purer" strategies.  Thus, gauging the effects
of parameter selection and effects from traffic patterns and mobility
rates and node populations and so on would, perhaps, become totally
impossible.  Already it is very, very difficult to get a clear
picture about how these interactions affect performance.

>                                                                   The
> temporally and spatially determined.

Those two statements are not the same, and I only agree with the
second one.  I think the most viable strategy is to come up with
some general principles for protocol design, and see if we can use
it to make performance predictions for particular choices of parameters,
environmental conditions, traffic patters from particular applications,
and so on.  If we cannot make any reliable predictions from the choices
we might have available, then we can't do any realistic protocol
design.  Thus, I am not at all sure that making things more complicated
by going into hybrid protocol designs right now is really viable.

Also note that a protocol can be made to be dynamic by measuring
external effects, and dynamically adjusting internal parameter
values.  This is not a hybrid approach, but it may be sufficient
to handle most time-varying scenarios.  Or maybe it will not be
sufficient.  Who knows?!

> concentrate on the later approach.

I would suggest instead that we follow another approach:
- Get a common simulation testbed
- Get some common basis for comparison between protocols
- Identify the viable protocol techniques that are likely to
  have wide applicability (e.g, local repair, expanding rings, ...)
- Create a repository of realistic traffic patterns, mobility
  traces, source code, and so on.
Of course, this isn't exactly the approach that we've been
taking, but on the other hand it is different than going into
hybrids as the next step.

Regards,
Charlie P.

---

From: Kenneth Brayer <[email-masked]>
Date: Mon, 10 Jul 2000 12:16:20 -0400

Mr. Perkins

"Charles E. Perkins" wrote:

I am the network engineer on the Air Force Global Grid project at The
MITRE Corporation at the Electronic Systems Center, Hanscom AFB, MA. The
approach that you have suggested above is exactly what I have been
proposing for use in picking a MANET protocol for the Global Grid. In
the end what we need is to evaluate protocols on a common basis against
parameters that are meaningful to our customers. Rapid delivery of
messages is what they generally want. We need standard traffic scenarios
and measures of performance. Twenty years ago when I published my Ad-Hoc
protocol, they didn't call it Ad-Hoc then, I used a testbed that I built
with Intel 8086 single board computers. That is all gone now but I still
have my software--real implementation on a bare machine in PL/M and ASM
86. I have noted that many researchers have implementations that run on
Linux. One approach that I am looking at is using real software in a
testbed. This would aviod the issue of demonstrating that the simulation
is correct and allow us to go up to reasonable numbers of nodes by, say,
using office computers over the weekend. The office computers also have
corporate network connections which could be used to collect statistics
of performance. P.S. I also know how to simulate errors on comm channels
which could be done after baseline performance was determined.

Kenneth Brayer

P.S.
References to my Ad-Hoc Protocol are enclosed in a word document for any
who are interested