Subject: Re: [Ospf-wireless-design] Another example for self-selected MPRs

From: Emmanuel Baccelli <[email-masked]>
Date: Wed, 13 Apr 2005 06:30:45 +0200

See comments bellow...

Richard Ogier wrote:

> I want to mention another design goal of of the MDR/BMDR flooding
> procedure.  I wanted it to be useful for other MANET applications
> that do not require reliable flooding and therefore do not use ACKs.
> Thus, I designed the flooding procedure so that it provides redundancy
> (via the BMDRs) to improve the delivery ratio even if ACKs are not used.
> Thus, a BMDR forwards an LSA not based on ACKs, but based on
> whether the neighboring relays that transmitted the LSA cover
> all neighbors.  Thus, the flooding procedure is separate from
> the reliability procedure (ACKs and retransmissions).
>
> Non-active overlapping relays (NAORs) require that ACKs be
> used, since all NAORs will eventually forward each LSA if it
> is never ACKed.  (If we disable the NAORs in applications that
> do not use ACKs, so that only active MPRs forward LSAs, then
> reliability will be poor, especially if it can take 6 seconds
> to select a new MPR following a link failure.)
>

The way i see it is that Overlapping Relays are largely based on 
MPRs. The difference between them is enssentially that Overlapping Relays 
also make use of OSPF Acks as an additionnal signalling mechanism to 
trigger some re-transmissions if needed. That is, if LSAs are not generated
frequently enough to provide reliability through periodicity.

This said, i think it remains to be seen wether MPRs are more, or less 
reliable than a source independent approach in case of dense networks or 
networks with a "sufficient" link failure rates.

Actually, having MPRs should be an advantage in such cases: if a node X 
fails to re-transmit a broadcast, some neighbor node Y should receive it 
from an mpr selector (instead of from X) and will thus retransmit, 
naturally covering up X's failure. Whereas with a source independent
approach, if node X fails to emit, then basically nobody will.

Having some backup (basically doubling up the CDS) helps a little to go in the 
direction of the MPR approach, but the generalization of the backing-up in a
decentralized environment like an ad-hoc network may indeed be much better off
with MPRs.

I think some real experience needs to be gathered with source-independent
approaches, in order to compare it fairly with the real experience that 
has been gathered over the years and on the field with ad-hoc networking with
MPRs. 

Until then, we can also compare simulations. I am curious to see some results 
comparing, say, the delivery rate in case of mobility (i.e. "sufficient"
link failure rates) for an MPR approach and a source independent approach?

Emmanuel

----------------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.

_______________________________________________
Ospf-wireless-design mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/ospf-wireless-design

---

From: "Henderson, Thomas R" <[email-masked]>
Date: Wed, 13 Apr 2005 10:25:49 -0700

> -----Original Message-----
> From: Emmanuel Baccelli [mailto:[email-masked]] 
> Sent: Tuesday, April 12, 2005 9:31 PM
> To: [email-masked]
> Subject: Re: [Ospf-wireless-design] Another example for 
> self-selected MPRs
> 
> 
> This said, i think it remains to be seen wether MPRs are 
> more, or less reliable than a source independent approach in 
> case of dense networks or networks with a "sufficient" link 
> failure rates.
> 
> Actually, having MPRs should be an advantage in such cases: 
> if a node X fails to re-transmit a broadcast, some neighbor 
> node Y should receive it from an mpr selector (instead of 
> from X) and will thus retransmit, naturally covering up X's 
> failure. Whereas with a source independent approach, if node 
> X fails to emit, then basically nobody will.
> 

I don't think the last statement accurately depicts the situation with
the approach proposed by Richard.   There are two related mechanisms at
work:  flooding, and retransmissions along adjacencies.  MDRs and MBDRs
are used during the initial flood (with MBDRs waiting for a short
backoff interval to flood, while suppressing if the MDRs are sensed to
have done their job).  These LSAs are also put on the retransmission
list and retransmitted if needed.  The adjacencies in Richard's proposal
also line up with the MDRs and MBDRs for convenience, although fuller
topologies are possible.  So an LSA is flooded by DRs, the flood is
backed up by BDR, and retransmissions along adjacencies back up those
floods.  If for some reason the floods fail and the adjacencies fail
simultaneously, the receiver will form new adjacencies and recover the
LSA via synchronization.

Tom

---

From: Emmanuel Baccelli <[email-masked]>
Date: Thu, 14 Apr 2005 07:54:52 +0200

Comments in below

>simultaneously, the receiver will form new adjacencies and recover the

Yes. I was talking about the base algorithms (CDS, MPR) behind the specific
mechanisms proposed by Richard and by Cisco for the purpose of OSPF on
wireless. 

Emmanuel