Subject: [Ospf-manet] Re: Ospf-manet Digest, Vol 6, Issue 5

From: [email-masked]
Date: Sun, 23 Apr 2006 20:14:49 -0400 (EDT)

Hello everyone,

This is the first time I am sending an email to this mailing list. So
greetings:) There is much I have learned simply by following the
discussion on this list.

I would first like to point out another argument to favor reduced set of
adjacencies: retransmissions. If we allowed the adjacency graph to be 
denser, redundant retransmissions would have an adverse effect. Also, we
should consider that battery life is an important parameter in MANET
routers, and relieving nodes to maintain expensive state information would
prove beneficial. Thus it seems to me that "minimal set of adjacencies" is
perhaps more essential than has been discussed. Also as Richard Ogier
correctly pointed out, "rate of change of adjacencies" (which would
determine database description traffic) is very important. Finally, nodes
having too many adjacencies would result in nodes generating too many LSA
(at the behest of every change in adjacency), which would have a further
effect in nodes having to share too many LSA headers in database
synchronization. The thing about MDR solution is that MDR selection is
persistent and as long as nodes are within reach of each other, MDR-other
nodes will not change their (backup) parent frequently, and will not
generate unnecessary LSA. Since in denser networks MDR-other nodes
constiture almost 80% of nodes, this feature is of great importance. It
offers both things: reduced set of adjacencies (helps retransmissions),
and reduced rate of LSA generation (which helps everything - LSU traffic,
database description traffic and LSU acknowledgement traffic).

If I understand correctly, the argument against the strategy of "minimal
adjacency graph and maximal routing graph" is that of routing loops. But
consider the fact that according to Ogier's published results (which I
have verified), the rate of change of adjacencies over the whole network
is not more than a few adjacencies (I remember figures such as 4-5
adjacencies for a 50 node biconnected network, which averages to about 10
seconds per node per adjacency change). Thus even if routing loops
existed, they could be temporary. But this should ofcourse be evaluated
more closely by introducing some advanced statistics.

However, if the network is too dynamic, obviously the strategies that
would boggle it down with protocol overhead are going to perform subpar.
In that case, we should evaluate whether delivery ratio would be more
adversely affected by a) possible routing loops, or b) more protocol
overhead. If the network is not too dynamic, then most of the 2-way
neighbors will be indirectly synchronized almost all the time, and minimal
adjacency graph solution would be superior without question. Thus an
evaluation in the spectrum towards more dynamic networks would be
extremely helpful.

Of course if we have on the order of 50 MBPS datarate (wimax), then a
protocol overhead of 100 KBPS versus 1 MBPS will not matter that much.
However a) OSPF should not make any assumption about layer 2 and should
only perform to the best of its ability to guarantee best service at
minimal overhead, and b) Battery life will still be a factor in high
datarate environments and nodes having to maintain more adjacencies and
perform more operations will drain their batteries that much faster.

Best regards,

Aniket Desai

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> Today's Topics:
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>    1. advertising non-adjacent links (was RE: [Ospf-manet] How can
>       we get	GTNet software?) (Henderson, Thomas R)
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> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 21 Apr 2006 15:40:23 -0700
> From: "Henderson, Thomas R" <[email-masked]>
> Subject: advertising non-adjacent links (was RE: [Ospf-manet] How can
> 	we get	GTNet software?)
> To: "Philippe Jacquet" <[email-masked]>
> Cc: [email-masked]
> Message-ID:
> 	<[email-masked]>
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>
>> -----Original Message-----
>> From: Philippe Jacquet [mailto:[email-masked]]
>> Sent: Friday, April 21, 2006 8:15 AM
>> To: Richard Ogier
>> Cc: [email-masked]
>> Subject: Re: [Ospf-manet] How can we get GTNet software?
>>
>>
>> I recognize that the concept of MDR has interesting feature
>> in providing
>> a reduced synchronizing backbone, but one must investigate
>> the effect of
>> advertizing non synchronized links in the core OSPF. Maybe we
>> could give
>> less synchronization priority degree to other synchronized
>> links. Maybe
>> some interesting combinations are possible.
>
> This comment causes me to ask (perhaps more broadly of the OSPF MANET
> list than just of Philippe) how one should evaluate the effect of
> advertising these non-adjacent neighbors?  I have heard several people
> declare general discomfort with this strategy without being able to
> precisely describe why there is a problem.
>
> Of course, in an unconstrained situation, we can form adjacencies with
> everyone and advertise full topology and run sub-second Hellos, but this
> is OSPF MANET and we have to trade off some of these items to achieve
> better scalability.
>
> I think that two separate issues are being conflated in this discussion.
> Adjacencies are used to preserve database synchronization in an OSPF
> network.  Type 1 links are advertised in LSAs to provide a graph upon
> which to build paths.  I fail to see why it should be the case that all
> advertised links should be adjacent links.  If anything, it should be
> the link or neighbor quality that governs whether the link is included
> in the data plane, not whether it exists in the OSPF control plan.  What
> is the problem here?
>
>>
>> My first rule of the thumb evaluation let me think that
>> advertizing full
>> LSA's everytime there is a change may lead to a tremendous overhead,
>> comparable to the of cost of synchronizing all links.
>
> Exactly.
>
>> Or we should
>> accept that wireless OSPF would work on slow moving or quasi static
>> wireless networks, and not on all kinds of MANETs.
>>
>
> I don't think we have to accept that.  However, I do think that if we
> declare that OSPF MANET will only work on quasi-static or sparse MANETs,
> and we do not allow for strategies to intelligently prune dense
> topologies when they arise, OSPF MANET will have been a wasted effort.
>
> Tom
>
>
>
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> End of Ospf-manet Digest, Vol 6, Issue 5
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