OSPF-MDR Position draft-ogier-ospf-mdr-position-00.txt

1. IETF Meeting - OSPF WG OSPF-MDR Positiondraft-ogier-ospf-mdr-position-00.txt Richard Ogier Presented by Tom Henderson March 23, 2006

2. Basic Idea – Generalize Designated Router to MANET Designated Routers (MDRs) • In an OSPF broadcast network, a single DR is elected. • Each router becomes adjacent with the DR, forming a tree with n-1 edges. • The DR is the only interior node of the tree, and is a connected dominating set (CDS). • The node having the lexicographically largest value of (DR Level, RtrPri, RID) is elected as DR, where DR Level can be DR, BDR, or Other.

3. Basic Idea – Generalize Designated Router to MANET Designated Routers (MDRs) • In a multihop wireless network, the DR generalizes to multiple MDRs which form a CDS. • For fast convergence, the MDRs select themselves based on 2-hop neighbor information, still giving preference to nodes with largest (MDR Level, RtrPri, RID). • Adjacencies are formed between MDRs to form a connected backbone. • Each non-MDR becomes adjacent with an MDR neighbor called its parent.

4. Also Generalize Backup Designated Router for Biconnected Redundancy • In an OSPF broadcast network, a Backup DR is added for redundancy. • The adjacencies of the DR and Backup DR form a biconnected subgraph. • Each DR Other is adjacent with the DR and the Backup DR. • In a multihop wireless network, Backup MDRs are added so that each node is a neighbor of at least two (Backup) MDRs. • Additional adjacencies can then be added to form a biconnected backbone consisting of MDRs and Backup MDRs. • Each MDR Other selects two (Backup) MDR neighbors called parents, and forms adjacencies with its parents.

5. MDR Selection is Simple • Let Rmax be the neighbor with the lexicographically largest value of (MDR Level, RtrPri, RID). • A router selects itself as an MDR if it has a larger value of (MDR Level, RtrPri, RID) than all of its neighbors, or if there exists a neighbor that cannot be reached from Rmax in at most k hops via neighbors that have a larger value of (MDR Level, RtrPri, RID) than the router itself, where k = MDRConstraint. • A router selects itself as a Backup MDR if there exists a neighbor that cannot be reached from Rmax via two node-disjoint paths, using as intermediate hops only neighbor that have a larger value of (MDR Level, RtrPri, RID) than the router itself. • The MDR selection algorithm runs in O(n2) time, using breadth-first search. • Prioritizing routers according to (MDR Level, RtrPri, RID) • Allows neighbors to agree on which routers should become MDR. • Increases stability and lifetime of MDRs and adjacencies. • Achieves consistency with DR election algorithm of OSPF.

6. Similarities Between OSPF-MDR and OSPF • The MDR selection algorithm generalizes the DR election algorithm of OSPF, in that both select the same two routers as DR/MDR and Backup DR/MDR in a fully connected MANET. • Both OSPF and OSPF-MDR reduce the number of adjacencies by having each DR/MDR Other form adjacencies with two (Backup) DR/MDR neighbors. In both cases, these two neighbors are advertised in the DR and BDR fields of each Hello. • OSPF-MDR uses the same interface states as OSPF, with the "DR" and "Backup" states implying that the router is an MDR or Backup MDR. • In both OSPF and OSPF-MDR, the choice of DR/MDRs and adjacencies depends only on information obtained from Hellos, and the flooding mechanism is independent of the contents of LSA. • OSPF (in a broadcast network) and OSPF-MDR both allow a non-adjacent neighbor to be used as a next hop.

7. Advantages of OSPF-MDR over OR/SP (1 of 2) • OSPF-MDR reduces adjacencies in a manner similar to OSPF, whereas OR/SP reduces adjacencies in manner that is quite different from OSPF. • In OR/SP, selection of adjacencies and relays depends on router-LSAs, unlike OSPF and OSPF-MDR. • OR/SP requires the modification of router-LSAs, unlike OSPF-MDR. • In OSPF-MDR, the selection of adjacencies and relays depends only on 2-hop neighbor information from Hellos, implying: • Faster reponse to topology changes (Hellos are smaller than LSAs and can be sent more frequently especially if differential Hellos are used). • Better modularity. • Ability to build partial-topology LSAs based on 2-hop neighbor information. • Insulation from topology changes more than 2 hops away.

8. Advantages of OSPF-MDR over OR/SP (2 of 2) • Simulations show that OR/SP results in a larger number of adjacencies than OSPF-MDR (see simulation results). • Although OR/SP uses MPRs, they are restricted to the subgraph of synchronized adjacencies and therefore do not have the usual advantages of MPRs such as a stretch factor of 1. • The OR approach uses a large number of backup relays, which limits scalability. OSPF-MDR selects only enough Backup MDRs to provide biconnected coverage. • OSPF-MDR has not changed much since March 2005, whereas Smart Peering was introduced (but not correct) in July 2005, with major changes in November 2005 and February 2006 in an attempt to compete with OSPF-MDR.

9. Comparison to MPR Approaches • INRIA has proposed a solution in which each router becomes adjacent (synchronized) with each MPR and MPR selector. • INRIA’s proposal results in about 3 times as many adjacencies as uniconnected OSPF-MDR in a dense100-node network. • More importantly, it results in a much largerrate of new adjacencies, because MPRs must change frequently in order to maintain a stretch factor of 1. Simulations for a dense 100-node network show that INRIA’s approach results in about 13 times as many new adjacencies per second than uniconnected OSPF-MDR. (See draft for more details.) • OSPF-MDR can be modified to allow the option of selecting MDRs based on MPRs, forming an MPR-based CDS:draft-ogier-manet-ospf-mdr-ext-00.txt • However, using an MPR-based CDS is likely to result in more overhead due to a larger rate of new adjacencies (based on simulations). Such an approach also requires more complexity to select and advertise MPRs.

10. Simulation Comparison to OR/SP • OSPF-MDR with minimal LSAs (only Full neighbors) and min-cost LSAs was compared to OR/SP without unsynchronized adjacencies (LSAs include only Full neighbors), using GTNetS. • Scenario: 100 nodes, radio range 250 m, square grid with width 500 m, maximum node speed 10 m/s. • Conclusion: OR/SP creates almost 4 times as many adjacencies, resulting in much more overhead than OSPF-MDR.

11. Robustness of OSPF-MDR • OSPF-MDR was simulated with full-topology adjacencies, uniconnected adjacencies, and biconnected adjacencies, to determine how adjacency reduction affects delivery ratio. • Scenarios: 50 nodes, radio range 125 m and 250 m, square grid with width500 m, maximum node speed 10 m/s. • Conclusion: Using adjacency reduction does not impair delivery ratio.

12. Need for Adjacency Reduction • OSPF-MDR with partial-topology LSAs (only MDRs and BMDRs originated full LSAs) was simulated with uniconnected adjacencies and full-topology adjacencies, using GTNetS. • Scenarios: 100 nodes, radio range 125 m and 250 m, square grid with width500 m, maximum node speed 10 m/s. • The results show that using full-topology adjacencies prevents scalability to 100 nodes.

13. Conclusions • OSPF-MDR extends OSPF to MANETs in a natural way: • Generalizes notion of DR and Backup DR. • Reduces adjacencies based on MDRs, similar to OSPF. • Simulations show that OSPF-MDR achieves excellent performance and scalability to at least 100 nodes. • Improvements are possible, but the MDR approach appears to be the best choice for extending OSPF to support MANETs.