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

From: "Henderson, Thomas R" <[email-masked]>
Date: Thu, 18 Dec 2003 15:57:31 -0800

> -----Original Message-----
> From: Richard Ogier [mailto:[email-masked]]
> Sent: Thursday, December 18, 2003 10:11 AM
> To: Yasuhiro Ohara
> Cc: [email-masked]; [email-masked]
> Subject: Re: [manet] Comment
> ondraft-spagnolo-manet-ospf-wireless-interface-00
> 
> 
> Yasuhiro Ohara wrote:
> 
> >
> >If you meant "How about to use normal (rfc2328's) stub area",
> >then I might just misunderstand what you said ...
> >
> >I wanted to say "defining new restriction for special case 
> (i.e. Manet)"
> >should be avoided if possible. We should utilize the existing scheme
> >as far as possible, shouldn't we ? (as it tends to lead some new
> >problems ...)
> >
> 
> I agree.  That is exactly the point I was making.   And this goes for 
> other aspects of OSPF, not just
> stub areas.  For example, it is not necessary to define a new message 
> type for flooding LSAs, as
> discussed in my draft 
>  
> http://www.ietf.org/internet-drafts/draft-ogier-manet-ospf-extension-00.txt
> 

I think we agree on the desirability of keeping OSPF as unchanged as 
possible.  That was why we made it a requirement in our project that all 
LSAs be unchanged-- we want backward compatibility with legacy OSPF routers.

But let's be clear about one thing.  There is a design space here for
optimization, ranging on one end from very minor changes to OSPF (e.g.
changing timer values) to very major (e.g., something that does not
look like OSPF at all, with, for example, route redistribution or LSA 
transcoding at the edges).  All things being equal, I think that everyone
will vote for "most like existing OSPF"-- but behavior of different
schemes is not guaranteed to be equal, of course.

I've said before that, in my opinion, defining a new message type for
flooding LSAs is not a big deal, if it accomplishes what is desired.  
The important issue in that particular case is getting the right 
flooding mechanism (MPR vs CDS, reliable vs unreliable).  If we take a
stance that we should not change message types unless absolutely 
necessary, then we are also disqualifying possibilities such as 
differential LSAs and modified LSRs (also suggested in the above draft).

Tom 

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---

From: Richard Ogier <[email-masked]>
Date: Fri, 19 Dec 2003 08:44:18 -0800

Tom,

 > schemes is not guaranteed to be equal, of course.

Agreed. A change to OSPF is justified if it results in a significant
improvement in performance (e.g., less overhead).

 > flooding LSAs is not a big deal, if it accomplishes what is desired.

That's true only if it results in a significant improvement that
cannot be achieved *without* defining a new message type for
flooding LSAs. In my opinion, a similar improvement *can* be
achieved without defining a new message type. This is something
that we can discuss and try to resolve.

 > differential LSAs and modified LSRs (also suggested in the above draft).

Of course (as I agreed above) we should *not* take the stance that
message types should be changed only when absolutely necessary.

But it is also important to distinguish between *required* features
(such as the new LSF message in your draft) and *optional* features
(such as the differential LSAs that I propose). Optional features
need not be implemented, in which case you have no change. But it is
nice to *have* the option, if it can be used in some situations to
significantly improve performance. (Sort of like icing on the cake.)

Richard

 > To

---

From: Richard Ogier <[email-masked]>
Date: Fri, 19 Dec 2003 10:28:21 -0800

Tom,

See my additional comments below.

 > that we can discuss and try to resolve.

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.  

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.

Richard

 > Of course (as I agreed above) we are *not* taking the stance that