Subject: RE: [Ospf-manet] Update to OSPF-MDR draft

From: "Henderson, Thomas R" <[email-masked]>
Date: Mon, 23 Jan 2006 08:30:44 -0800

> -----Original Message-----
> From: Philippe Jacquet [mailto:[email-masked]] 
> Sent: Monday, January 23, 2006 5:36 AM
> To: Richard Ogier
> Cc: [email-masked]
> Subject: Re: [Ospf-manet] Update to OSPF-MDR draft
> 
> Dear Richard,
> 
> I am sure that there are very good reasons why OSPF needs 
> that advertized links be synchronized. If you don't do so 
> then this will be a strong exception to the fact that wired 
> OSPF should not be affected by the wireless extension, since 
> wired nodes would receive non-synchronized link advertizements.

Philippe, perhaps you can clarify, but I am reading your response to
possibly indicate that you do not believe that OSPF should advertise any
Type 1 links in router-LSAs unless they are FULL adjacencies.  Or
perhaps you are agreeing with Richard and Abhay Roy that advertised
links need to be either directly or indirectly (through more than one
hop) synchronized.

I also do not know what you mean by "should not be affected"-- partial
topology LSAs do not affect backward compatibility.

> 
> I agree that synchronizing links may create overhead when the 
> network is changing fast. It means that wireless OSPF 
> networks will not be allowed to change as fast as MANETs may 
> do, but I understood that this restriction was already in the 
> spec of wireless OSPF (you already acknowledged that fast 
> speed was out of spec). By the way, we have shown that MPR 
> links will generate a number of synchronizations that is 
> similar to your bi-connected CDS, and in theory not more than 
> two or three times a minimal tree CDS (but the later has a 
> tremendously larger stretch). 

Can you point me to a reference where you have shown this?  Is this in
the context of OSPF synchronization or some other synchronization?

> 
> If synchronization is the main problem gainst fast moving 
> extensions, then the solution will be less in a further 
> reduction of the number of synchronized links (we are close 
> to minimal), than in a reducion of the overhead of each 
> synchronization process, where there is plenty room for 
> optimization. 

Our data has consistently shown that synchronization is not the main
problem for fast moving OSPF MANETs-- it is the sheer number of flooded
LSA updates.  For instance, see the results in Figure 7 of the below
draft:
http://hipserver.mct.phantomworks.org/ietf/drafts/draft-spagnolo-manet-o
spf-design-00.txt

We are not "close to minimal" in the number of synchronized links unless
we use topology reduction strategies for dense MANETs.

Regards,
Tom

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From: Philippe Jacquet <[email-masked]>
Date: Thu, 26 Jan 2006 11:22:00 +0100

>topology LSAs do not affect backward compatibility.

Tom, undirect synchronization and direct synchronization are not 
equivalent, see my last post to Richard. Advertizing unsynchronized 
links is making ospf going backward in quality and it is not clear that 
it may not affect  the stability of the wired part based on classic ospf.

>the context of OSPF synchronization or some other synchronization?
The number of MPR links is in average three times the number of minimal 
tree adjacencies (n-1) but with an optimal stretch factor of 1. In the 
following I have simulated an uni-connected MDR the following way: I 
have developped a minimal spanning tree on the MDR cds, and then added 
the non cds node as leaves. The stretch factors are the following.

MDR tree (uni-connected) stretch factor

20:       1.730830417

30:       1.950088600

40:       2.135779520

50:      2.009307827

This mean that view from an external node the route length are twice 
their optimal length and the overhead will be 100%.

>http://hipserver.mct.phantomworks.org/ietf/drafts/draft-spagnolo-manet-o

You simulated a wireless ospf alone like a manet not connected to a 
classic (and potentially large) wired ospf network in background . As I 
said in a previous post, the discussion has too much focalised on an 
isolated wireless ospf which is nothing more than a manet. You did not 
include in your simulation the actual interaction with the wired ospf 
which is the most important.

In passing flooding on MPR links LSAs containing only MPR links seems to 
have very good performance, better than those obtained with MDR links 
(bi-connected) even in average condition scenarios.

>we use topology reduction strategies for dense MANETs.
Best regards,
Philippe