Subject: RE: [manet] Comment on draft-spagnolo-manet-ospf-wireless-interface-00

From: "Henderson, Thomas R" <[email-masked]>
Date: Fri, 19 Dec 2003 11:20:07 -0800

> -----Original Message-----
> From: Richard Ogier [mailto:[email-masked]]
> Sent: Friday, December 19, 2003 10:28 AM
> To: Henderson, Thomas R
> Cc: [email-masked]
> Subject: Re: [manet] Comment on
> draft-spagnolo-manet-ospf-wireless-interface-00
> 
> 
> We can start this discussion now. If you want to achieve periodic
> unreliable flooding of LSAs, all you have to do is suppress LSA ACKs,
> as discussed in Section 5 of draft-ogier-manet-ospf-extension-00.txt.
> Moreover, a router can dynamically *decide* whether to suppress
> ACKs for a particular LSA, depending on how frequently it receives
> a new instance of the LSA (with a larger sequence number).
> (This approach can be used with a CDS or with MPRs.)
> I would be interested if you (or anyone) can give a good reason
> why this is not a good approach.  
> 

Richard,
I like this basic concept of yours because it offers the potential 
for hybrid operation without much complexity in the protocol
mechanism.   It also appears fairly robust to the case where 
routers are making inconsistent decisions on whether to suppress
ACKs-- retransmissions just keep occuring until the stingiest 
neighbor decides to ACK.  If we could allow nodes to suppress
ACKs for a short time, it would allow them to logically coalesce
ACKs of the same LSA received from many neighbors into a single 
transmission.   Further, it could perhaps be extended as Fred 
Baker has previously suggested that reflooding of an LSA can 
serve also as an implicit ACK to the upstream neighbor.  The 
main implementation cost seems to be to maintain and update 
the moving average for each LSA.

> Actually, I think the reliable flooding mechanism described in
> Section 6 of my draft is a better (more efficient) approach,
> but it requires more changes to OSPF, and therefore might not
> be as acceptable to the OSPF WG.
> 
Agreed.  It would be interesting to see how far one could get
by adopting the Section 5 approach by itself. 

Tom

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---

From: Richard Ogier <[email-masked]>
Date: Mon, 22 Dec 2003 22:02:10 -0800

Tom,

Please see my comments below.

>ACKs-- retransmissions just keep occuring until the stingiest

Good point. The only requirement for correctness is that a router
should not send an ACK for an LSA instance that it did not receive :-)

>ACKs of the same LSA received from many neighbors into a single

Right. OSPF allows LSA ACKs to be delayed, and the technique of
grouping several LSA ACKs into a single packet is mentioned in
Section 13.5 of RFC 2328, so this would not require an extension
to the standard, but perhaps a recommendation should be added
that this technique SHOULD be used on manet interfaces.

>serve also as an implicit ACK to the upstream neighbor.  The

RFC 2328 also allows implicit ACKs, so this also would not require an
extension. But there is still the question of whether implicit ACKs
should be used on manet interfaces. It probably wouldn't hurt,
but it wouldn't help much either, since only a small percentage of
nodes will forward LSAs (e.g., CDS nodes or MPRs).

>the moving average for each LSA.

I think this cost is negligible, since each LSA must be processed
anyway (and updating a moving average requires only a few
additions and multiplications).  It would be interesting to
determine the best values for the smoothing constant, hysteresis
thresholds (for deciding whether to suppress ACKs), and the ACK
delay when ACKs are not suppressed.

>by adopting the Section 5 approach by itself.

I agree.  But if reliable flooding is used, there still needs to
be a database exchange between adjacent neighbors.  As discussed
in my draft this exchange could occur between each CDS node and
each of its neighbors.  I suppose this could also be done between
each MPR and each of its neighbors, but this is not as
straightforward since each MPR is reponsible for relaying only
a subset of LSAs (which must somehow be defined).
So, to minimize the changes/additions to OSPF, the CDS approach
seems to be the best approach.

Richard