Subject: [Ospf-manet] out-of-track routes

From: Emmanuel Baccelli <[email-masked]>
Date: Fri, 23 Jun 2006 11:44:14 +0200

Hi all,

Allowing "out-of-track" routing that does not change the predicted 
end-to-end cost is of course compatible with RFC 2328. But out-of-track 
routing that changes the predicted end-to-end cost is in contradiction 
with RFC 2328, and with other related documents that built on the fact 
that actual route costs can be calculated. So concerning the design of a 
solution for OSPF operation on MANETs, I think we should evaluate this 
issue further.

RFC 2328 allows implicitely to route over non-adjacencies on broadcast 
network because a broadcast network implies de facto mesh connectivity 
between routers on that network, and thus a router can predict the cost 
of the route that will actually be taken by packets, even if 
non-adjacencies are used on broadcast networks.

However, mesh connectivity cannot be assumed on MANETs and therefore 
allowing to route over non-adjacencies in this environment makes it 
impossible for a router to predict the cost of the route that will 
actually be taken by packets through a MANET. There may be an exception 
with the first hop of a route originated inside a MANET, which, even if 
it is not an adjacency, does not prevent the router to predict the cost 
of route taken further on. However, approaches going further than this 
exception, that allow unpredictable routing over non-advertized links, 
should be carefully examined in that respect.

Emmanuel

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From: Acee Lindem <[email-masked]>
Date: Fri, 23 Jun 2006 09:29:15 -0400

Hi Emmanuel,

Emmanuel Baccelli wrote:

> end-to-end cost is of course compatible with RFC 2328. But 
> out-of-track routing that changes the predicted end-to-end cost is in 
> contradiction with RFC 2328, and with other related documents that 
> built on the fact that actual route costs can be calculated. So 
> concerning the design of a solution for OSPF operation on MANETs, I 
> think we should evaluate this issue further.
>
> between routers on that network, and thus a router can predict the 
> cost of the route that will actually be taken by packets, even if 
> actually be taken by packets through a MANET. There may be an 
> exception with the first hop of a route originated inside a MANET, 
> which, even if it is not an adjacency, does not prevent the router to 
> predict the cost of route taken further on.

I don't see the problem here. You don't need an adjacency to know the 
cost of
a p2mp link (whether it be static or derived dynamically based on the radio
characteristics).

> However, approaches going further than this exception, that allow 
> unpredictable routing over non-advertized links, should be carefully 
> examined in that respect.

Are you referring to LSA pruning techniques? I believe someone (may have
been me ;^) did bring up the fact that you may not take the optimal path 
when
a nodes has multiple interfaces (radio or other). Richard had some ideas on
how to solve the problem but I think we wanted to solve the 
flooding/adjacency
reduction problem without the additional optimization.

Thanks,
Acee

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From: Emmanuel Baccelli <[email-masked]>
Date: Thu, 29 Jun 2006 17:38:34 +0200

Acee Lindem wrote:

>> RFC 2328 allows implicitely to route over non-adjacencies on 
>> broadcast network because a broadcast network implies de facto mesh 
>> connectivity between routers on that network, and thus a router can 
>> predict the cost of the route that will actually be taken by packets, 
>> even if non-adjacencies are used on broadcast networks.
>>
What I am talking about is the fact that in RFC 2328, even though a 
route may use some links that are not an adjacencies, we have the 
assurance that the cost of the route will anyways be the one calculated 
from the start. I think this is an important fact. If we choose an 
approach that changes this fact, we should be aware of the consequences, 
and there is a risk that MDR changes this fact.

> been me ;^) did bring up the fact that you may not take the optimal 
> path when

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From: "Henderson, Thomas R" <[email-masked]>
Date: Thu, 29 Jun 2006 10:40:09 -0700

Emmanuel, some comments inline below:

> -----Original Message-----
> From: Emmanuel Baccelli [mailto:[email-masked]] 
> Sent: Thursday, June 29, 2006 8:39 AM
> To: [email-masked]
> Subject: Re: [Ospf-manet] out-of-track routes
> 
> Acee Lindem wrote:
> 
> >>
> >>
> > a p2mp link (whether it be static or derived dynamically 
> based on the 
> > radio
> > characteristics).
> >

I feel that two issues are being confounded here.  There are two
mechanisms that we are considering for OSPF MANET.  The first allows a
reduced set of adjacencies, which can be thought of as control-plane
topology control.  The second allows a reduced set of advertised
neighbors in router-LSAs, which can be thought of as data-plane topology
control.  

The cost of a path results from the links advertised in the LSAs and
used by routers to build their forwarding tables, without regard for
whether such advertised links are directly adjacent.  This is the point
that Acee made in his reply.  Philippe previously raised on this list
some concern about the overall quality (not cost) of paths that include
non-adjacent links, under the assumption that a link that is in Full is
more likely to be of higher quality than one that can only reach 2-Way.
But I think a good implementation can make the effort to ensure that
only good quality neighbors are advertised and used.  In any case, both
the MPR-based and MDR-based drafts assume that some links used for
forwarding packets are non-adjacent, and this concept is used in OSPF
broadcast interface, as has been pointed out by Richard many times.

Now, considering costs of paths, we should be discussing the partial
topology LSAs and leave adjacencies out of the discussion.  The concern
you seem to be raising is that a downstream node could choose to use a
link that was not advertised in its router-LSA, thereby changing the
path and cost that was computed upstream.  In your MPR-based draft, if
link costs are uniform, this will not happen by nature of the MPR
selection property and the use of hybrid nodes (which synchronize with
every neighbor) at the MANET boundary.  This was previously discussed on
the list.  Now, there are two points to consider: i) in what cases, can
MPR- or MDR-based approaches change the path (cost) as predicted by an
upstream node, and ii) whether this makes any difference.  

For the first point, a changing path cost may occur for MPR-based
approaches if link costs are not uniform, because in that case, you lose
the property that MPR relationships are aligned with shortest paths
(unless you change how MPRs are selected).  The same thing can also
occur in MDR when link costs are non-uniform, but if the neighbors could
advertise their costs also in Hellos (extending the existing LLS TLV in
Hellos), then Richard's proposed min-cost LSAs could guarantee min-cost
paths even in the case of non-uniform link costs.

For the second point, whether this makes any difference, I see above
that you say this is an important fact, but I am skeptical; can you
provide an example where a problem might arise from downstream nodes
improving upon the path computed upstream?   Moreover, it seems to me
that both drafts perform similarly in this regard; that there is no
greater "risk" with MDR.

Tom

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From: Richard Ogier <[email-masked]>
Date: Thu, 29 Jun 2006 12:52:15 -0700

I was about to reply to Emmanuel, but I see that Tom already said
everything I would have said.  In particular, I agree that downstream
nodes would only improve upon the path computed upstream, which
should not cause a problem.  (Also, this can be avoided by
using a version of min-cost LSAs that provides min-cost
paths, or an MPR selection algorithm that provides min-cost
paths.)  Let's not forget that even MPR-based routing can
use intermediate hops that are not advertised in LSAs.

Richard