Subject: RE: [Ospf-manet] Re: evaluation plan for OSPF MANET

From: Emmanuel Baccelli <[email-masked]>
Date: Thu, 08 Dec 2005 01:17:13 +0100

Comments below.

Henderson, Thomas R wrote:

>> -----Original Message-----
>> From: Philippe Jacquet [mailto:[email-masked]] Sent: 
>> Tuesday, December 06, 2005 8:18 AM
>> To: Henderson, Thomas R
>> Cc: [email-masked]; Richard Ogier
>> Subject: Re: [Ospf-manet] Re: evaluation plan for OSPF MANET
>>
>> Dear Tom, dear Richard,
>>
>>
>> In both proposals the wireless flooding is based on a primary 
>> flooding mechanism and a backup mechanism to insure reliability. IMO 
>> the proposal whose primary mechanism is MPR should be considered as 
>> more mature since MPR has been deeply investigated. The primary 
>> flooding performance and robustness is certainly a key factor. Bad 
>> performance would affect delays and overhead because the backup 
>> activation has a certain cost.
>>   
>
>
> I cannot accept the contention that MPR-based approach to OSPF flooding
> is more robust or performant than the CDS-based approach without some
> justification, and the justification needs to consider how the whole
> proposed approach works together.  For example, I note that we observed
> that in Overlapping Relays simulations, there were a large number of
> non-active floods (three times the number of active floods-- see section
> 4.2.1 of our report). 
>  
>
I think Philippe was talking about the primary flooding performance, not 
primary+backup. Our study
in INRIA report 5609 shows that the primary performance of CDS-based 
approaches is more fragile than that
of MPR based approaches. We carried out this study because while a lot 
of study and experience had already
been gathered on MPR flooding over the years in MANET and elsewhere, it 
has not been the case for
CDS-based flooding. Now we have a few independent simulation+studies on 
CDS-based flooding, so we are starting to
get a better view on what prospects there are with such an approach. 
However, i am still puzzled why this
research debate has to happen here and delay things now, in a 
standardization process based on "rough
consensus and running code".

>> Regarding adjacencies, I mean that one should avoid advertizing 
>> adjacencies that are not synchronized, inside wired network portions, 
>> for example advertizing links such as those contained in "min-cost 
>> LSA", or those advertised with "U-bit". I am not making distinctions 
>> on specific proposals here. An ideal objective might be that a wired 
>> node could compute the shortest path to a mobile node based only on 
>> synchronized links. Of course this can be a debatable issue.
>>
>>   
>>
>
> I don't believe that my message was arguing for those points-- I was
> merely trying to reach agreement on the parameters of the debate, and to
> establish a process for it.
>
> Regards,
> Tom
>  
>
IMO this is rather important point. If only synchronized adjacencies are 
to be advertized and used for routing,
then we have to be very careful about route stretching if topology 
reduction is used based on a CDS backbone.
Basically, using only synchronized adjcencies for routing may invalidate 
CDS topology reduction approaches.
The only known topology reduction that can then be envisionned is to 
reduce topology to MPR selection adjacencies,
which may yield more adjacencies but will anyways keep optimal paths.

Emmanuel

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---

From: Richard Ogier <[email-masked]>
Date: Thu, 08 Dec 2005 23:14:15 -0800

Emmanuel,

Emmanuel Baccelli wrote:

>>
> IMO this is rather important point. If only synchronized adjacencies 
> are to be advertized and used for routing,
> Basically, using only synchronized adjcencies for routing may 
> invalidate CDS topology reduction approaches.

To see how this works, I would like to consider a simple example: A 
fully connected
(single-hop) MANET.  In such a network, there are no MPRs or MPR selectors.
The good news is that for this simple example your approach does NOT 
yield more
adjacencies than the CDS approach. The bad news is that your approach yields
ZERO adjacencies, so an OSPF extension based on your approach does not work.

For general topologies, your approach still does not work, since the 
resulting
adjacency graph is not guaranteed to be connected. I previously gave 
some examples
on the design team list.  I also described a variation of the approach 
that does
work, but my simulations showed that it resulted in many more 
adjacencies than
OSPF-MDR.  More importantly, it resulted in a much higher rate of new 
adjacencies
than OSPF-MDR.  This is important because database exchange is required 
for each
new adjacency.  Anyway, if you have a detailed spec for such a solution, 
then maybe
it can be implemented in GTNetS and evaluated.  Cisco is designing a 
solution based
on MPRs, but apparently they have decided not to take the approach you 
suggest
(maybe for the reasons I give above?).

Another relevant point is that OSPF already allows a non-adjacent 
neighbor to
be used as a next hop in a broadcast network (if the router and neighbor are
both fully adjacent with the DR). So if OSPF allows it, it is reasonable for
the OSPF extension to allow it.  We have made these points before on the
design team list.

Richard

---

From: Acee Lindem <[email-masked]>
Date: Mon, 12 Dec 2005 11:21:48 -0500

Richard, Emmanuel,

> ZERO adjacencies, so an OSPF extension based on your approach does not 

You'd definitely have to include the "disconnected" routers as well 
(i.e., routers with
common 2-hop neighbors).

> than OSPF-MDR.  This is important because database exchange is 
> required for each
> new adjacency.  Anyway, if you have a detailed spec for such a 
> solution, then maybe

We looked at it briefly (adjacencies among MPRs and routers that were 
otherwise
disconnected). However, in the amount of time that we spent on it we 
didn't get it
to perform better than smart peering. This isn't to say that it wouldn't 
yield better
results given sufficient resolve.