Subject: RE: [manet] Dumber Question?

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Wed, 15 May 2002 14:42:58 -0600

Hi Richard,

I'm not so sure this is a clear advantage, at least not in highly dynamic
MANETs.

If link status changes rapidly, say every minute or so, a proactive protocol
would have to have nodes advertise about twice a minute in order to have
routing tables that are reasonably accurate.  Because of the possibility of
data collisions, this will tie up the MANET, at least locally, for a sizable
chunk of the available time, leading to significantly less bandwidth
available for the transmission of data.  

Also, if the MANET is sizable, not only will there be more bandwidth lost to
advertisements, but the propagation the advertisements to all nodes in the
net will take a considerable amount of additional time.  This may be long
enough that at no time do all nodes have an accurate picture of the network.

Another complication arises in the case of asymmetric links.  In this case,
the fact that Node A can hear B does not mean Node B can hear Node A.  So,
if Node A receives an advertisement from Node B, it is not assurance that
Node B is reachable directly from Node A.  Node A must respond to Node B's
advertisement, in which case both A and B will know they can reach each
other.  This means we need about twice as many packets to establish the
proactive routing.

Of course, both problems become worse rapidly with the size of the net, so
one might be able to have a proactive protocol locally (involving just a few
nodes), but a reactive protocol globally.

John Mullen
-----Original Message-----
From: [email-masked] [mailto:[email-masked]]
Sent: Wednesday, May 15, 2002 12:16 PM
To: [email-masked]
Cc: [email-masked]
Subject: Re: [manet] Dumber Question? 

> <snip> 

Yes, proactive protocols have a lot of advantages over on-demand
protocols that have not been discussed much.
For example, if one node wants to send a message to another
node that is currently unreachable, an on-demand protocol will
try to discover a route a few times and then give up (and then perhaps 
try periodically).  If proactive routing is used, the sending node 
will be informed as soon as the unreachable node becomes reachable.

Another advantage of proactive protocols is that more efficient flooding 
is possible at no extra cost, using the topology information provided 
by the proactive protocol (e.g., parents in TBRPF or MPRs in OLSR.)

Of course there are other advantages (e.g., proactive protocols are 
more efficient when the number of sources is large, proactive protocols 
can ensure that an alternate route is always maintained, etc.).

Richard

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From: "Tao Lin" <[email-masked]>
Date: Wed, 15 May 2002 17:44:44 -0400

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.

----- Original Message -----
From: "Mullen, John P. (Contractor)"
<[email-masked]>
To: <[email-masked]>
Sent: Wednesday, May 15, 2002 4:42 PM
Subject: RE: [manet] Dumber Question?

---

From: "Tao Lin" <[email-masked]>
Date: Wed, 15 May 2002 17:44:44 -0400