Subject: RE: [Ospf-wireless-design] Re: Plan for deciding between the twoapproaches

From: "Henderson, Thomas R" <[email-masked]>
Date: Mon, 28 Mar 2005 08:52:35 -0800

> 
> 1. Selecting neighbors as MPRs allows better efficiency by avoiding
>    redundant MPRs. However, as the following example shows,
>    this possible advantage is completely erased by the use of
>    (self-selected) non-active overlapping relays.
> 
>           1
>         /   \
>        /     \
>       2       3
>        \     /
>         \   /
>           4
> 
> 
>   Node 1 will select only node 3 as an MPR.
>   However, node 2 will select itself as a non-active overlapping relay
>   and will therefore forward each LSA flooded by node 1,
>   since it cannot hear that node 3 is also forwarding each LSA.
> 

My reading of section 3.4.8 of the draft suggests that node 2 will not
necessarily forward each LSA, because node 4 will either send an
explicit or implicit ACK to node 3, which should also be received by
node 2.  The third-from-last paragraph in this section says "Note that a
node can determine whether its further flooding a LSA will only result
in a redundant transmission by already having heard link state
acknowledgments (ACKs) or floods for the LSA from all of its peers."

If 2 could hear 3, then not that much would change-- 2 would reset its
PushBackInterval timer upon 3's transmission, but even if it does not
(i.e., can't hear 3), the PushBackInterval should be set to large enough
value such that there would be opportunity for 2 to hear 4's
ACK/reflood.

Tom

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From: "Madhavi W. Chandra" <[email-masked]>
Date: Mon, 28 Mar 2005 12:28:53 -0500

> value such that there would be opportunity for 2 to hear 4's

This is absolutely correct.  Node 2 would delay it's transmission
initially (Pushback timer), and then suppress altogther upon hearing
4's ACK.  Node 2 would not further flood once it deems its transmission
as redundant.

The behavior of non-overlapping relays is important to understand 
properly in order to see the benefit of the overall scheme, and also
to see that the changes to OSPF flooding are at a minimum.

-Madhavi

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From: Richard Ogier <[email-masked]>
Date: Mon, 28 Mar 2005 10:56:27 -0800

>value such that there would be opportunity for 2 to hear 4's
My mistake. I didn't realize that PushBackInterval was larger than
AckInterval. I do this differently in my draft.
But there is a tradeoff (between efficiency and robustness):
If nodes 2 and 3 are neighbors, and node 3 does not flood the LSA,
then it would be nice for node 2 to flood the LSA soon,
rather than waiting for a time larger than AckInterval.
That is what a BDR does in my draft.

Richard