Subject: [OSPF] Two new drafts comparing the MANET extensions

From: Richard Ogier <[email-masked]>
Date: Mon, 22 Sep 2008 09:59:15 -0800

Hello,

I have posted two drafts that compare the three proposed MANET
extensions of OSPF, including simulation comparisons.  This is
independent of the Last Call just announced.  (I was planning to
announce this today without knowing when the Last Call for the
OSPF MANET drafts would finally begin.)
However, the timing might be beneficial, since the comparison drafts
might provide a better understanding of the three proposals and
how they differ, at a time when there is some focus on the drafts.

Comparison of OSPF-MDR and OSPF-OR
www.ietf.org/internet-drafts/draft-ogier-ospf-manet-mdr-or-comparison-00.txt

Comparison of OSPF-MDR and OSPF-MPR
www.ietf.org/internet-drafts/draft-ogier-ospf-manet-mdr-mpr-comparison-00.txt

A slide presentation that compares all three proposals can be found at
www.manet-routing.org

The simulation results were obtained using the most recent available
GTNetS code for each protocol.  Let me know if more recent code
is available.  The conclusions of the two comparison drafts are
summarized below.  Comments are welcome.

Richard

The conclusions of draft-ogier-ospf-manet-mdr-or-comparison-00
can be summarized as follows:

   o  OSPF-MDR provides an option for partial-topology LSAs that
      provides routing along shortest (minimum-cost) paths, and is
      scalable to at least 160 nodes.  OSPF-OR does not provide such an
      option, and therefore does not provide a scalable solution for
      shortest-path routing.

   o  OSPF-OR requires a modification to the router-LSA packet format
      when adjacency reduction is used, whereas OSPF-MDR does not.

   o  Simulations show that OSPF-OR forms a much larger number of new
      adjacencies per second than OSPF-MDR in large mobile networks
      when both protocols use adjacency reduction.  In a scenario with
      120 nodes, OSPF-OR forms 5.6 times as many adjacencies as
      OSPF-MDR, resulting in 5.8 times as much overhead from Database
      Description (DD) packets.  Overhead from DD packets can be very
      substantial if there is a large number of inter-area-prefix-LSAs
      or AS-external-LSAs.

   o  OSPF-OR has a much larger number of backup relays (called non-
      active ORs) than OSPF-MDR, which results in greater overhead
      in large, dense networks due to redundant backup flooding.

   o  When both protocols use adjacency reduction and minimal LSAs
      (which provide suboptimal routing), simulations show that OSPF-MDR
      generates much less overhead and is much more scalable than
      OSPF-OR, achieving good performance in mobile networks with up to
      200 nodes.  In dense, mobile networks, OSPF-MDR generates less
      overhead with 200 nodes than OSPF-OR generates with 100 nodes.

   o  When both protocols use full-topology LSAs with adjacency
      reduction, OSPF-MDR still generates significantly less overhead
      than OSPF-OR with 40 or more nodes, even if OSPF-OR is modified
      to omit backup flooding.

   o  When both protocols use full-topology LSAs without adjacency
      reduction, OSPF-MDR performs about the same as OSPF-OR when
      OSPF-OR is modified to omit backup flooding.

The conclusions of draft-ogier-ospf-manet-mdr-mpr-comparison-00
can be summarized as follows:

   o  Simulations show that OSPF-MPR forms a much larger number of new
      adjacencies per second than OSPF-MDR in large mobile networks
      when both protocols use adjacency reduction.  In a scenario with
      100 nodes, OSPF-MPR formed about 12 times as many adjacencies as
      OSPF-MDR, resulting in about 11 times as much overhead from
      Database Description (DD) packets.  Overhead from DD packets can
      be very substantial if there is a large number of
      inter-area-prefix-LSAs or AS-external-LSAs.

   o  OSPF-MDR with adjacency reduction and min-cost LSAs generates
      much less overhead and is much more scalable than OSPF-MPR
      (see simulation results).  For example, OSPF-MDR with min-cost
      LSAs generates less overhead with 160 nodes than OSPF-MPR
      generates with 100 nodes.  OSPF-MDR with minimal LSAs (which
      OSPF-MPR does not provide) generates less overhead with 200
      nodes than OSPF-MPR generates with 80 nodes.

   o  OSPF-MDR is more flexible than OSPF-MPR.  For example, OSPF-MDR
      provides two LSA options that OSPF-MPR does not provide:

      (1) Minimal LSAs, which allow scalability to 200 nodes while
          providing routing along nearly shortest paths.
      (2) Full-topology LSAs with adjacency reduction, which allows
          full-topology LSAs with lower overhead.  OSPF-MPR does not
          provide this option since it only advertises adjacent
          neighbors in LSAs.

   o  OSPF-MPR does not provide any backup flooding; thus flooding of
      LSAs can be delayed while MPRs are being updated.

   o  OSPF-MPR uses MPR flooding while OSPF-MDR uses MDR/CDS flooding.
      MPRs are source-dependent and neighbor-selected, while MDRs
      are source-independent and self-selected.  Each method has
      advantages, so it is important to compare the whole protocols
      rather than compare MPR versus MDR flooding in isolation.

   o  In OSPF-MPR, the router with the largest Router ID is given an
      unfair burden, since as a "synch router" it must form an adjacency
      with each of its neighbors.  This is true regardless of its router
      priority.  (OSPF-MDR uses router priority when selecting MDRs.)

   o  In OSPF-MPR, the next hop of a route need not be synchronized or
      adjacent, and therefore can be far from synchronized.  OSPF-MDR
      requires that the next hop be Full or routable, which ensures some
      degree of synchronization.

   o  OSPF-MPR does not provide differential Hellos.

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