Subject: RE: [Ospf-wireless-design] Design team outbrief (draft slides) posted

From: "Henderson, Thomas R" <[email-masked]>
Date: Sun, 6 Nov 2005 10:54:50 -0800

> This bullet on page 17 of your slides bothers me:
> 
> > Emmanuel Baccelli stated that MPR flooding offers better properties 
> > than CDS flooding, with at least as good topology reduction 
> > capabilities, and better routing stretch performance
> 
> 
> Emmanuel may have "stated" such things, but Phil and I 
> clearly pointed out that some of his statements were 
> incorrect and based on a lack of understanding of the MDR 
> approach. Therefore, one may say that your bullet is the 
> truth, but not the whole truth.  You could add something like:
> "Phil and Richard pointed out that Emmanuel's statements were 
> not justified and Emmanuel did not dispute this."  OK, so 
> this may be biased in the other direction, but I am making a point.
> 

I am trying to represent the range of opinions expressed without passing
too much judgement on whether claims have been substantiated.

> The following bullet is from your last slide:
> 
> > To move forward on one approach, first need to make a decision to 
> > pursue either source-independent or source-dependent flooding
> 
> 
> With recent improvements to the Smart Peering approach, and 
> with improvements also possible to the MDR approach, it looks 
> like we need to continue to compare the two approaches (and 
> improvements of them) for a few more months.
> (I will discuss possible improvements to MDR in another 
> email.) I am not opposed to "moving forward" with *two* 
> approaches, at least for a few more months, with the 
> understanding that a decision must be made at some point. 
> This would mean that the WG would provide 
> comments/suggestions for *both* approaches, and help to 
> decide which approach to use.
> As you mentioned, the accepted approach can borrow techniques 
> from the other approach if appropriate.
> 

This is to be discussed at the WG meeting-- whether to continue forward
with two approaches, and if the goal remains to have one agreed upon
approach, what is the process and timetable by which one is selected.

Since you'll not be there, I'll take your post above as a preference to
continue to study both approaches for another meeting cycle.  

Thanks for the feedback,
Tom

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From: Richard Ogier <[email-masked]>
Date: Sun, 06 Nov 2005 12:03:15 -0800

>too much judgement on whether claims have been substantiated.

Still, I think it would be misleading not to at least point out the fact
that the min-cost LSAs of OSPF-MDR also avoid route stretch, just as 
MPRs do.
This is not passing judgement, but is a simple, clear fact that Phil has
expressed, and which Emmanuel was not aware of.  Or you can say it is
another relevant claim made by Phil and me (without passing judgement).

I have some additional comments on Cisco's simulation results,
and some comments on consensus (below).

In the simulations for Smart Peering, what neighbors were included in
the router-LSAs?  If only adjacent neighbors were included,
then that could explain the lower overhead obtained for Smart Peering,
and would also result in highly suboptimal paths
(much longer than shortest paths).  That is why simulation
results are not very meaningful unless other measures such
as average path length and delivery ratio are also presented.
(The average path length can be obtained from the numbers of UDP
packets sent/forwarded/received, but I did not see those numbers.)

Also, since Cisco is running SPF twice, once for real adjacencies
and once for all acceptable links, I would like to know how
a router knows which non-local links are real adjacencies.
Does the LSA somehow indicate this?  Is a different Link State ID
used for real adjacenies versus non-adjacencies?

Also, I want to point out that other methods for selecting
MDRs are being considered (including MPR pruning as has been
discussed) which might result in fewer MDRs and adjacencies,
especially in sparse networks.

For example, as I suggested to Tom and Phil a few months ago,
the idea of Smart Peering can be used to reduce the number of
adjacencies in OSPF-MDR.  In this modification, another TLV
is defined for Hellos which indicates which neighbors are adjacent,
to provide 2-hop adjacency information.  Adjacencies are still formed
only between (Backup) MDRs and their neighbors, but a new adjacency is
suppressed if the 2-hop adjacency information indicates that an
adjacency path of at most 3 hops exists to the neighbor under
consideration.  I have not yet incorporated this enhancement, because
I was not sure it would be worth the added complexity, and didn't
know that Cisco was working on an enhancement to Smart Peering.
Note, however, that the enhancement to Smart Peering also adds
complexity, since it requires running SPF on two different
topology graphs.

Regarding consensus, it appears to me that we have a deadlock,
since each team has their own solution that they want to win.
One way to break the deadlock is to trust the opinion of the
person that we chose to lead the team because of his fairness
and neutrality.
Or we can continue the race for a few more months (perhaps
with a deadline) and hope it will become more clear which
approach is preferable.  Performance is an important criteria,
but other qualities are also important, such as minimizing
the changes to OSPF and maintaining the same philisophy as OSPF.

Richard

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From: Richard Ogier <[email-masked]>
Date: Sun, 06 Nov 2005 18:26:10 -0800

>continue to study both approaches for another meeting cycle.

Actually, my preference is to focus on the MDR approach while
evaluating possible enhancements to the MDR approach using MPRs
(e.g., selecting MDRs by pruning MPRs and defining LSAs based
on MPRs), a variation of Smart Peering to reduce the number of
adjacencies (although I doubt that the improvement in efficiency
will be worth the additional complexity), and other enhancements
to the MDR approach.

I believe that ORs with Smart Peering can result in a performance
similar to MDRs (after both are implemented in the best possible
way), but in this case the approach that results in the most
natural extension to OSPF (minimizing changes and keeping the
same philosophy) should be preferred.  After all, we are extending
OSPF, not developing a completely new protocol.

Also, the Smart Peering approach was introduced late in the game
and I would not want its consideration to cause even more delays
in achieving our ultimate goal.

My second choice would be to continue to study both approaches,
but preferably in a manner that gets us closer to an acceptable
design.  (For example, I don't think it would be good to have
another surprise just before the next meeting.)

Richard

---

From: Richard Ogier <[email-masked]>
Date: Mon, 07 Nov 2005 08:38:02 -0800

> capabilities, and better routing stretch performance

After further consideration, I partly agree with the above statement
depending on how it is interpreted.
The part about "better route stretch performance" is true if we are
talking only about *flooding* and not *routing* (since routing can
occur along non-adjacencies).  Min-hop routing can be achieved either
with MPRs or with the min-cost LSAs described in the MDR draft.

The part about "good topology reduction capabilities" depends on how
adjacencies are defined. (By topology reduction I am talking about
adjacencies, keeping in mind that LSAs can also include non-adjacent
neighbors.) In our simulations, Phil and I showed that this
is certainly not true if each node forms an adjacency with each of
its MPRs and MPR selectors, and Emmanuel has agreed with this:

> On the other hand using
> MPR/MPRselector adjacencies does yield more adjacencies,
> but no route stretching whatsoever.

If adjacencies are defined in another way, then the method needs
to be specified. One possible way (which I plan to investigate by
modifying OSPF-MDR) is to select the CDS/MDRs by pruning MPRs
and then defining adjacencies as in the MDR draft, but Emmanuel
did not specify such a method.

Richard

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From: Emmanuel Baccelli <[email-masked]>
Date: Mon, 07 Nov 2005 18:45:07 +0100

I think there is something rather confusing in the latest discussions: 
the use of the term adjacency, especially when compared to its 
traditional meaning in OSPF. For example, some simulation results tend 
to show that the MDR approach features a reduced number of adjacencies 
without featuring any route stretching -- if "min-hop" LSAs a used.  
However, this supposes that routing is not only done over adjacencies 
(indeed, if routing was only done over adjacencies, the MDR approach 
leads to more than 50% route stretching in most cases). This is thus 
very confusing, since in traditional OSPF routing is supposed to happen 
only over adjacencies. So there is here at least a vocabulary issue, 
that needs to be clarified in my opinion.

Emmanuel