Subject: RE: [manet] Dumber Question?

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Wed, 15 May 2002 16:08:19 -0600

Hello Lin,

Perhaps, we are not discussing the same sorts of MANETs.

In general, Node A does not know it is reachable from Node B until it
receives a packet from Node B.  Node B does not know that it can reach Node
A until it receives an ACK from Node A, indicating that it received the
packet from Node B.  Whether the protocol is proactive or not, data
collisions, etc. can cause the impression that a node is not reachable.  

If Node A's status changes from reachable to unreachable from Node B, it
will not know, unless it is expecting a packet from Node B, that such a
thing has happened.  Node B, however, will know, since its packet was not
ACKed, that either Node A did not get the packet or Node B could not hear
Node A's reply. 

If Node A later becomes reachable, it will still not know until Node B sends
it a packet.  These control packets are as likely to suffer a data collision
or be damaged by temporary interference as those of a reactive protocol, so
I see no inherent advantage.

The only difference I see in the protocols is their impact on overhead.
This would include the re-transmission of packets directed onto obsolete
paths, those lost in data collisions, and those lost in buffer overflows, as
well as the packets needed to inform each node about the others.  Since
these would depend on the traffic load, network density, propagation
conditions, and operational factors, it would seem that an adaptive hybrid
protocol would be best.

However, I would have to say that giving the protocol more degrees of
freedom also introduces the problem of how to control those settings and
admits the possibility of instability.

John Mullen

-----Original Message-----
From: Tao Lin [mailto:[email-masked]]
Sent: Wednesday, May 15, 2002 3:45 PM
To: Mullen, John P. (Contractor)
Cc: [email-masked]
Subject: Re: [manet] Dumber Question?

I agree that proactive protocols may have higher overhead if a topology
change does not affect a in-use route. We have to admit that this is a
disadvantage for proactive protocols in MANET. However, proactive protocols
have many advantages, e.g. they can respond to route changes faster, they
provide immediate notification of reachable or unreachable destinations
(reactive protocols can not decide whether the destination is unreachable or
not because there could be packet collisions when broadcasting route
requests or destination unreachable.), in general they build shorter paths,
and an overhead with proactive protocols is independent of traffic profiles.
If we use a mixed protocol, we may suffer both disadvanatges. So my point is
that proactive protocols are good approaches for MANET routing, especially
with a low mobility MANET. When the network mobility increases, (AND with
certain traffic profiles), we should introduce some reactive features. We
are looking into the performance (in term of overhead) of MANET routing
protocols given several network and traffic profiles. Hopefully these
results will be available soon.

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From: Mike Christman <[email-masked]>
Date: Wed, 15 May 2002 19:47:23 -0400 (EDT)

Mr. Mullen,

Perhaps I am not understanding your point of view here or I have not read 
the earlier threads closely enough.  It would seem like a proactive algorithm, 
like OLSR, would not have the problem that you described.  When you 
say that "Node A does not know it is reachable from Node B until it
 receives a packet from Node B." I assume that you mean a data packet.  
However, it would seem as if using periodic "hello" messages, as OLSR 
does, would avoid this problem.  If a collisions can prevent "hello" 
messages from being received, but I believe that OLSR allows for 3 missed 
messages before timing out.  In this case Nodes A and B are ALWAYS 
expecting a packet from other nodes.  But, as we all know by know, this 
consumes more bandwidth.  I hope that this comment is relevant! :)

---

From: Mike Christman <[email-masked]>
Date: Wed, 15 May 2002 19:47:23 -0400 (EDT)

Mr. Mullen,