Subject: RE: [manet] Dumber Question?

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Thu, 16 May 2002 08:23:40 -0600

Hi Mike,

By "packet," I meant any packet, data or control.

All I know about OLSR is what I've read at: 

http://menetou.inria.fr/olsr/draft-ietf-manet-olsr-04.txt

>From that, I gather that the primary advantage of OLSR is that it reduces
the number of nodes retransmitting packets to a subset, called Multipoint
Relays.  In a sense, the protocol defines a subset of nodes to be routers
according to the conditions in effect.  This is, of course, most useful when
the net is both large and dense.  At times, this is true of the nets I work
with, so this could be a useful strategy.

The problem is that since the links are not symmetric, it is often true that
Node A receiving a HELLO from Node B does not mean Node B can receive a
packet from Node A.  If the HELLO message does not contain A's address, Node
A knows that it can receive packets from B, but it is probable that it
cannot send packets directly to B.  It can send B's address in its own
HELLO, but, as I read the protocol, unless B is a two-hop neighbor, Node B
will never know that Node A can receive its packets.

Of course, this would also be a problem for a reactive protocol.  The point
is that one may more HELLO packets per unit time to maintain the routing
tables in the presence of asymmetric links than with symmetric ones.  If the
bandwidth is limited, this can lead to very high overhead.

In general, the nets I work with are uneven in density.  For example, if a
company is attempting to cross a minefield using a cleared path, its units
will be near each other and the net will be locally dense.  However, if the
same company is involved in scouting, the net is liable to be sparse.  The
OLSR might be advantageous in the first case, but in the second, the added
overhead, together with the reduced link capacity (capacity is a decreasing
function of the distance between nodes), might make it a liability.  An
adaptive protocol could switch to OLSR when and where the net is dense and
switch to a different protocol when and where it is not.

John Mullen

-----Original Message-----
From: Mike Christman [mailto:[email-masked]]
Sent: Wednesday, May 15, 2002 5:47 PM
To: Mullen, John P. (Contractor)
Cc: Tao Lin; [email-masked]
Subject: RE: [manet] Dumber Question?

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! :)

 On Wed, 15 May 2002, Mullen, John P. (Contractor) wrote:

> 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.
> 
> _______________________________________________
> manet mailing list
> [email-masked]
> https://www1.ietf.org/mailman/listinfo/manet
> 

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From: "Amir Qayyum <Amir Qayyum" <[email-masked]>
Date: Fri, 17 May 2002 10:11:41 +0500

Hi John,

 > http://menetou.inria.fr/olsr/draft-ietf-manet-olsr-04.txt

Version 6 of the draft is now available on IETF web site and also at
http://hipercom.inria.fr/olsr/draft-ietf-manet-olsr-06.txt. (However,
what I write below is there in OLSR since its very first draft).

 > ...

In OLSR, a node includes all of its one-hop neighbor nodes along with
their link status (symmetric or asymmetric) in its HELLO messages. It
helps to determine the bi-directional links between one-hop neighbor
nodes. Therefore in OLSR, a node A does know after receiving a HELLO
message from node B whether node B can also hear it or not.

Moreover, the HELLO messages provide information about two-hop
neighborhood which helps to compute MPRs "locally" at each node. A
node selects its MPRs among the neighbor nodes with bi-directional
connectivity only.

Amir

----

Amir Qayyum
Avaz Networks Inc.

---

From: "Amir Qayyum <Amir Qayyum" <[email-masked]>
Date: Fri, 17 May 2002 10:11:41 +0500

Hi John,

Amir

----

Amir Qayyum
Avaz Networks Inc.