Subject: [Ospf-wireless-design] Algorithms and optimizations

From: Padma Pillay-Esnault <[email-masked]>
Date: Tue, 05 Apr 2005 16:16:10 -0700

Looking at the multiple threads lately, it seems that the discussions 
are drifting to evaluating the optimizations
of one proposal as an argument versus another. We think that we should 
bear in mind that it is the base algorithm that is of prime importance.

While some optimizations are required for scalability, they can be 
applied to both base algorithms and hence they should have a limited 
weight on evaluation of a proposal.

Another essential foundation is OSPFv3, changes that impact the basic 
protocol have to be evaluated.
In the real world, existing stacks will need to be changed to 
incorporate wireless interface. How much
of these changes can it really absorb and still function correctly? The 
extent of change has to be evaluated carefully so as not to jeopardise 
stability and  uncover  corner cases that have a nagging habit of popping
up pretty late in the game. We think that changes should be limited to 
what is really essential for manet to
perform.

Most of the optimizations are applicable to both base algorithms and 
including them only adds more
parameters that blur what should really be evaluated. Some optimizations 
can be palliatives and hence
they cannot be used to validate the base algorithms.

IMHO, evaluation should be done thru a number of clearly defined 
criteria. A first attempt
could look like the following
- Comparison of base algorithm without any optimization.
- Evaluation of the changes required to the OSPFv3 protocol , bearing in 
mind extensive changes
usually spell instability and newly discovered corner cases.
- Leverage of existing proven mechanisms of the OSPFv3 protocol.
- Clearly define in which circumstances the optimizations that really 
make sense. For instance, limiting adjacencies is an interesting concept 
only if there is a high number of nodes potentially going out of range. 
However, it adds unnecessary complexity if the nodes have a wide range 
and less likely to drop out. In other words, do these scenarios really 
matter or are they unlikely in the particular nework we are addressing.  
Also, optimizations should retain their optional character.
- Last but not the least, identify the sector most likely to deployment 
of this technology and tackle their specific
problems and perks first.

Padma

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From: Richard Ogier <[email-masked]>
Date: Wed, 06 Apr 2005 20:49:34 -0700

Padma,

A given technique (or optimization) may be more beneficial to one
approach than to another approach. For example, if you want to compare
using a jet versus using a propeller on aircraft, it would be foolish
to insist that both aircraft use the same body. A body that works well
with a propeller might break apart if a jet were used, and the design
team might come to the false conclusion that propellers are superior
to jets.

Regarding OSPF extensions, both approaches are actually optimizations
of OSPF, so when you say to compare the approaches without any
optimizations, it might not be easy to define what that means.

The technique of reducing the set of adjacencies is an example of a
technique that *might* work better in the MDR (Manet Designated Router)
approach than the MPR approach.  In fact, reducing the number of
adjacencies is one of the main design goals of the MPR approach.
So, if we are forced to compare the two approaches assuming that all
neighbors are adjacent (which could result in more than 2000
adjacencies in a dense 100-node network), the overhead due to forming
adjacencies might dominate the total overhead, and one might conclude
that both methods perform about the same. On the other hand, if we
allow both approaches to employ whatever technique it can to reduce
the number of adjacencies, the two approaches might perform quite
differently.

> make sense. For instance, limiting adjacencies is an interesting 
> concept only if there is a high number of nodes potentially going out 
> of range. However, it adds unnecessary complexity if the nodes have a 
> wide range and less likely to drop out. In other words, do these 
> scenarios really matter or are they unlikely in the particular nework 
> we are addressing.  Also, optimizations should retain their optional 
> character.

Limiting adjacencies is interesting even if all nodes are always in
range of each other. In fact, that is the reason why adjacencies are
limited in an OSPF broadcast network. Instead of n(n-1) adjacencies,
there are only 2n - 3 adjacencies, since each router is adjacent only
to the DR and BDR. The main idea of the MDR approach is to generalize
this technique to multi-hop networks.

However, I agree that (in most cases) optimizations should be optional.

Richard

Padma Pillay-Esnault wrote:

> of these changes can it really absorb and still function correctly? 
> The extent of change has to be evaluated carefully so as not to 
> jeopardise stability and  uncover  corner cases that have a nagging 
> habit of popping
> parameters that blur what should really be evaluated. Some 
> optimizations can be palliatives and hence
> - Evaluation of the changes required to the OSPFv3 protocol , bearing 
> in mind extensive changes
> make sense. For instance, limiting adjacencies is an interesting 
> concept only if there is a high number of nodes potentially going out 
> of range. However, it adds unnecessary complexity if the nodes have a 
> wide range and less likely to drop out. In other words, do these 
> scenarios really matter or are they unlikely in the particular nework 
> we are addressing.  Also, optimizations should retain their optional 
> character.
> - Last but not the least, identify the sector most likely to 
> deployment of this technology and tackle their specific