Subject: Proposal for OSPF MANET Extensions Design Points

From: Acee Lindem <[email-masked]>
Date: Fri, 16 Jan 2004 09:49:24 -0500

I believe that for the foreseeable future the vast majority of
networks will be fixed (although they may support wireless hosts).
Based on this belief and the fact that what is being proposed for
MANET is very different from base OSPFv3 I'd like to propose the
following design points for the work:

    1. A fixed network OSPFv3 implementation shouldn't require
       changes to fully interoperate with an OSPFv3 implementation
       supporting the MANET extensions.
    2. The MANET extensions should be limited to wireless interfaces (or
       whatever level of segregation we end up with).
    3. As much as possible, an OSPFv3 implementation with the MANET
       extensions should be a proper subset of an OSPFv3 implementation
       without the extensions. In other words, we should be extremely
       suspect of changes to basic OSPFv3 mechanisms.

Thanks,
--
Acee

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From: Richard Ogier <[email-masked]>
Date: Fri, 16 Jan 2004 11:07:57 -0800

>       suspect of changes to basic OSPFv3 mechanisms.

I agree with Acee and Thomas regarding points 1 and 2,
and I agree with Thomas regarding point 3.
The extension should include all mechanisms of OSPFv3,
but some of the mechanisms must be modified to
support multi-hop (ad hoc) wireless interfaces.
I agree that OSPFv3 mechanisms should be changed only
when necessary, or if the change results in a substantial
improvement in performance.

The main need is to support the multi-hop wireless interface type,
whether or not the routers are mobile.
Since OSPF already supports topology changes, this will also
support mobile routers, although some additional changes may
be needed to do this more efficiently.
I think it is good to support mobile routers, since these can exist
in vehicle networks, robot networks, military networks, etc.

I think the extensions that I proposed
  draft-ogier-manet-ospf-extension-00.txt
satisfy points 1 and 2, and also minimize the changes to OSPF.
This may also be true for the other proposal
  draft-spagnolo-manet-ospf-wireless-interface-00
but as I discussed previously, the latter proposal has more
changes than are necessary.

In particular, my proposal allows an OSPF area to include both
non-MANET routers running unmodified OSPFv3, and MANET routers
running the extended OSPF. A router with no MANET interface
runs unmodified OSPFv3, but can have a neighbor that runs
the extended OSPF.

The main extension that is needed to support MANET interfaces
is to generalize the concept of a Designated Router to
support multi-hop wireless networks.  As I have discussed,
a simple and natural extension is to use a connected
dominating set (CDS).  Each node of the CDS is a generalized DR
(called "overlapping relay" in Fred Baker's presentation).
An adjacency would then be formed only between CDS nodes and
their neighbors.

In addition, as I have stated (see my message of January 6), it is
possible to suppress ACKs for LSAs that are updated frequently,
to reduce overhead when topology changes are frequent.
This is a very simple modification to OSPFv3, and in fact, since
ACKs can be delayed anyway, it might not require any change
to the spec.

I also proposed a more efficient mechanism for reliable flooding,
based on periodic Database Description packets, but whether such
a change is justified depends on the network parameters (mobility,
bandwidth, etc) that we want to support.

Richard