RE: [Ospf-wireless-design] Design Team slides for IETF 63

> -----Original Message-----
> From: Richard Ogier [mailto:rich.ogier@earthlink.net] 
> Sent: Thursday, August 04, 2005 6:40 PM
> To: Henderson, Thomas R
> Cc: ospf-wireless-design@ietf.org
> Subject: Re: [Ospf-wireless-design] Design Team slides for IETF 63
> 
> 
> I would like to comment on the smart peering approach, 
> studied in Section 4.5 of the report.
> I agree that "smart peering" has a circular dependency 
> problem if non-adjacent neighbors are included in LSAs.
> (A simple 4-node example can be constructed to show this.)
> 
> To avoid this problem, I considered a variation in which each 
> router lists its adjacent neighbors in a new Hello TLV.
> Then the router can avoid forming an adjacency with a 
> neighbor if there already exists a path of adjacencies from 
> the router to that neighbor based on the adjacency 
> information obtained from Hellos.  

I think that this may improve the performance, but one challenge that
remains is that it is not clear whether the loss of an adjacency
somewhere causes nodes to have to reconsider their previous decisions to
defer forming adjacencies with their neighbors.  If the degree of the
node is not high, however, it probably will not be burdensome to revisit
these decisions when this Hello TLV changes.  But since the adjacency
decisions are just being made on this localized information, it will
probably drive up the number of adjacencies.

> This can be used in 
> conjunction with the MDR extension, so that a router selects 
> adjacent neighbors as before except that it avoids forming an 
> adjacency with a neighbor if there already exists a path of 
> adjacencies to that neighbor.
> An adjacent neighbor still remains adjacent only as long as 
> either the router or the neighbor is a (Backup) MDR, so that 
> adjacencies are still aligned with (Backup) MDRs.
> This will reduce the number of adjacencies (at least a 
> little), but I have not run simulations to determine how this 
> affects performance.

This explanation wasn't completely clear to me.  Are you suggesting that
the DR-other nodes would then tend to form only one adjacency to the
backbone in this case?

If one suppresses adjacencies between DR-other neighbors, then one might
expect that these synchronized DR-other neighbors are often two hops
away from one another (adjacent to the same (Backup) MDR).  But it is
not guaranteed; in a dense network, they may have become adjacent to
(Backup) MDRs that are not topologically close.  In this case, your
suggestion to include adjacency info in the Hellos might help avoid
"synchronized" neighbors with long adjacency paths between them, but I
am not sure that it is going to be worth it-- I am not sure that such
scenarios are common (plus, again, it might cause more adjacencies). 

Tom

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