Subject: [Ospf-wireless-design] draft-ogier-manet-ospf-extension-04.txt

From: Richard Ogier <[email-masked]>
Date: Mon, 18 Jul 2005 09:30:38 -0700

All,

An update to the OSPF-MDR draft has been submitted and can be
found here until it is available at the IETF site:
http://home.earthlink.net/~ogier/draft-ogier-manet-ospf-extension-04.txt

The main changes are as follows:

   o  The draft has been rewritten to specify complete details.
   o  Packet formats are now specified.
   o  The term MANET Designated Router (MDR) is now used instead of
      Designated Router (DR) for MANET interfaces.
   o  Only a single parametrized MDR selection algorithm is now
      specified (previously called the MPN CDS algorithm), which
      includes the Essential CDS algorithm as a special case.  This
      algorithm runs in O(d^2) time, where d is the number of neighbors.
   o  The optional ANP CDS algorithm has been omitted from the draft.
   o  A procedure for selecting the MDR Parent and Backup MDR Parent has
      been added as Phase 4 of the MDR selection algorithm.
   o  The term "synchronized neighbor" has been changed to "routable
      neighbor", to reflect that such a neighbor is not perfectly
      synchronized, but is sufficiently synchronized to be advertised in
      router-LSAs and used as a next hop.
   o  A new option for partial-topology LSAs, called min-cost LSAs, has
      been added, which provides minimum cost routes under certain
      assumptions.

One thing I hope will be clear when you all read this draft, is that
the (Backup) MDR is a natural generalization of the (Backup) DR of OSPF.
The (Backup) MDR Parent selected by a router replaces the (Backup) DR
selected by a router, and both have very similar meanings:

(1) If a router is an MDR, then its MDR Parent will be the router itself.
(2) If a router is a Backup MDR, then its Backup MDR Parent will be the
    router itself.
(3) If a router is an MDR Other, then it forms adjacencies with its
    MDR Parent and Backup MDR Parent.

Regarding the following report recently posted by Emmanuel:
http://www.inria.fr/rrrt/rr-5619.html

The approach summarized in Section 3 (Approach without Route Stretch)
provides min-hop paths by advertising only links between MPRs and
their selectors.  This does not provide min-cost (shortest) paths
unless all link costs are equal.  The min-cost LSAs described
in the updated draft for OSPF-MDR also provide min-hop paths
if all link costs are equal, but also provide min-cost paths
if the metric that a router advertises to all neighbors is equal
to the cost for the interface (which for example can be smaller
if the router has larger bandwidth).  A similar extension is
also possible with MPRs, but you would have to select MPRs
based on interface cost, which might result in more MPRs and
less stable adjacencies if the cost changes.  This is one
reason to decouple the choice of LSAs from the flooding
mechanism, and the min-cost LSAs achieve this.
In fact they achieve this without requiring additional information
(e.g., selected MPRs) to be reported to neighbors, which can
also incur additional delay.  I want to avoid adding additional
mechanisms that add complexity without much benefit.

Also, if an adjacency is formed between each MPR and its selectors,
which has been discussed, this would result in a large number of
adjacencies.  In addition, such a set of adjacencies is not
guaranteed to form a connected subgraph, as also been discussed.

Regarding the following report:
http://www.inria.fr/rrrt/rr-5609.html

First of all, the model used seems to be abstract and may not
reflect a realistic model such as used in the GTNetS simulation.
Any proposed solutions should be implemented in GTNetS and compared
with the existing solutions.

Also, regarding your results for delayed 2-hop neighbor information,
I want to point out that the differential Hellos in the
OSPF-MDR solution ensure that 2-hop neighbor information is
learned very quickly, typically within 2 seconds.  So any
results that assume a longer delay in obtaining 2-hop neighbor
information don't have much practical significance if such
a delay is easily avoided.

Your model, being abstract, also does not seem to consider the
possibility that it will sometimes take longer for 2-hop
neighbor information to reach an MPR selector, than to reach
a self-selected CDS node, as in the example I posted here
on 3/29/2005.  In summary, we could spend a lot of time making
theoretical arguments and getting nowhere (as we did previously
in another WG).  Fortunately, we now have a way to compare
different approaches on a fair basis, i.e., using GTNetS.

Richard

_______________________________________________
Ospf-wireless-design mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/ospf-wireless-design