Subject: [Ospf-manet] Adjacency Reduction??

From: Russ White <[email-masked]>
Date: Tue, 31 Jan 2006 20:57:41 -0500

Y'all:

I've been talking to various folks interested in actually deploying OSPF 
MANET protocols, and I have to point to a logical assumption "we've" 
made without much considering whether it's a good assumption or not. 
Throughout this entire discussion on simulation scenarios, etc., it's 
always been assumed that adjacency reduction is a "good thing," because 
it reduces overhead.

The problem is, we're assuming that adjacency reduction is the _only_ 
way to reduce overhead to "acceptable levels," or to the "lowest 
levels," and--it seems--that adjacency reduction is the "right way" to 
reduce overhead.

I would like to challenge this assumption head on. First, I don't 
believe we should always look for the lowest possible overhead in some 
simulation. I have another email coming on simulations, so I'll hold 
that for a minute, and look at this from another angle.

Start with this question: Would anyone argue that OSPF, as it's 
currently designed, uses the lowest possible overhead mechanisms for any 
phase of its operation? If not, then what was the reason some lower 
overhead process wasn't used? I would argue lower overhead mechanisms 
were not used because they were not as "stable," or were not verifiably 
correct in operation. In other words, various mechanisms were analyzed 
for correct operation and flexibility, and the method with the best 
operational characteristics was chosen.

Now, as this applies to the MANET work specifically: A number of the 
people I've talked to who are considering deploying these solutions say 
that adjacency reduction is not an acceptable alternative, in their 
opinion, simply because when you reduce adjacencies, you increase 
convergence time, in all cases. There's no way around the fact that as 
you reduce adjacencies, you increase the time it takes for a new member 
of the network to begin routing packets, or a member of the network to 
recover from losing it's current adjacencies.

Second, I don't believe that adjacency reduction is the _only_ answer to 
reducing overhead. It's the simple/easy answer, perhaps, but there is a 
tradeoff that's not been considered.

I would suggest, in this line, two things:

- -- First, we should consider the pros and cons of adjacency reduction, 
from a protocol correctness perspective, rather than assuming that 
adjacency reduction is the "right way" to reduce overhead, or that the 
lowest possible overhead, at the cost of protocol correctness, is the 
"right solution."

- -- Second, if we _insist_ on building our world around simulations, we 
should include a scenarios where the number of "backups" is increased, 
to the point of building full adjacencies with every visible neighbor, 
to see how the proposed algorithms work in this situation. Regardless of 
how those in the WG feel about it, many deployments may be configured to 
have very high "backup counts," in the real world, because of real 
operational experence showing this is required in various situations. We 
should know how these proposed solutions react in these situations, and 
take it as a data point.

:-)

Russ

- -- 
[email-masked] CCIE <>< Grace Alone

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