Subject: Problems Simulating LMR

From: Rae Harbird <[email-masked]>
Date: Thu, 28 Aug 1997 17:02:41 +0100

Dear Scott or anyone else in the MANET group,

I am simulating LMR (as described in 'A distributed routing algorithm
for mobile wireless networks', Corson and Ephremides 1994) and I would
very much appreciate it if you could help with a problem I have
regarding the algorithm.

I am simulating LMR but have not yet implemented a channel access
scheme. As far as I can tell this may cause the following situation to
arise:

	A node will never store multiple routes because as a node
	retransmits the first RPY it receives, it will change all its
	UN assigned links as UP links. (This is to ensure loop
	freedom.)

	If only one route is ever stored, the FQ mechanism will be
	activated more frequently.

	In a situation where two neighbouring nodes i and j
	independently receive a route for k, both may broadcast the route
	received to each other and mark the link as UP. If i's link to k
	subsequently fails, i will mark link i,j as UN_W and broadcast a
	FQ.
	
	Node j will not participate in the FQ process since it
	correctly, previously ignored the route from i. From node i's
	point of view, i will never receive an RPY down the UN_W link
	i,j. Node i must wait until the FQ timeout has expired and then
	delete the UN_W link with j for DEST k.

	Node j will never reply to a subsequent QRY for k from i, because 
	it still has the link marked as UP and QRYs are not processed over
	UP links.

Is there a solution to this? What is the problem with j processing a new
QRY over it's UP link with i for DEST as long as i has waited for a FQ
timeout before transmitting a QRY?

Any enlightenment you can provide would be very much appreciated!

Rae Harbird

---

From: "M. Scott Corson" <[email-masked]>
Date: Fri, 29 Aug 1997 10:51:46 -0400 (EDT)

> 	UN assigned links as UP links. (This is to ensure loop

Not true.  A node may receive multiple RPYs before it has retransmitted
a RPY (this RPY may be in its transmission queue).  
Each reception causes creation of a DN, so a node may have multiple DNs.

> 	received to each other and mark the link as UP.

Not true.  If you look closely at the paper, you'll notice that it
specifies half-duplex link operation.  In other words, a node can either
transmit or receive--it cannot do both simultaneously.  So, the 
inconsistency to which you refer above--where each node marks the link
as UP from its perspective--cannot arise in that fashion.

> Any enlightenment you can provide would be very much appreciated!

The only enlightenment I can give is that while LMR was fun to create,
the result was a purely academic, and never intended for use in a real
system.  There's a subtle instability in the protocol--which the paper 
mentions--which occurs too frequently for my liking, and hurts its
performance.

TORA was originally developed to remove the instability problem of LMR.

-Scott

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M. Scott Corson                                 Institute for Systems Research
[email-masked]                              A.V. Williams Bldg. (115)
Phone: 301-405-6630 (Rm. 2205 AVW)              University of Maryland     
Fax:   301-314-8586                             College Park, MD  20742

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