Subject: RE: [manet] hello protocols inappropriate for link sensing?

From: "John Mullen" <[email-masked]>
Date: Mon, 1 Mar 2004 09:37:42 -0700

Hi Dave,

I think your assumption is about right.  I just hedged my language
because so many people I've encountered don't appreciate the severity of
the problem or the necessity of solving it for MANET implementation.

:-)

We are about to do some field tests here, but from what I read in the
papers, it happens well over 50% of the time.  Because its solution
often involves timeouts, it leads to a very significant loss in
throughput.  I made the suggestion because we've noticed a number of
papers in which the false routes seem to be a difficulty, but the
practitioners attribute it to other sources and miss the essence of the
problem.  The severity of the problem would depend on such things as the
waveform in use, node density, data rates, and the fading factor.  For
example, if one selects a high fading factor to ensure high reliability
on usable links, one simultaneously increases the probability of RREPs
from nodes over unreliable links.  

According to our calculations, there are some things that can be done,
but I would say that at this point there is no silver bullet and the
ultimate solution will probably involve a coordinated effort on the part
of the three lowest OSI layers.

John Mullen

-----Original Message-----
From: David Young [mailto:[email-masked]] 
Sent: Friday, February 27, 2004 9:23 PM
To: John Mullen
Cc: 'Charles E. Perkins'; [email-masked]
Subject: Re: [manet] hello protocols inappropriate for link sensing?

On Thu, Feb 26, 2004 at 05:00:39PM -0700, John Mullen wrote:
> Hi David,
> 
> Your interpretation may be right, but I suggest you look at a log of 
> RREQ and RREP packets.  A great deal of capacity is lost due to 
> occasional successes over unreliable links, causing unreliable routes 
> to be chosen, which then fail.  That is, there may be another cause 
> for the symptoms you describe.

It is an OSPF network, but I know what you mean in the abstract about
RREQ/RREP succeeding over unreliable links.  The network in question has
no alternative routes, so I don't think that's the problem.

I am surprised if the RREQ/RREP packets' successes on marginal links are
only occasional.  I am interested to hear more about that. I have
pessimistically assumed that RREQ/RREP discovery frequently yields paths
that carry small RREQ/RREP packets fine, but which perform badly with
longish TCP segments.

Dave

-- 
David Young             OJC Technologies
[email-masked]      Urbana, IL * (217) 278-3933

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