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OSPF Database ExchangeSummary List Optimization

draft-ietf-ospf-dbex-opt-00.txt

Richard Ogier

Presented by Acee Lindem

March 19, 2007

IETF 68 - OSPF WG

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Database Exchange Optimization

• Basic idea: Router does not need to list an LSA in a Database Description (DD) packet if the neighbor already has the same instance or a newer instance of the LSA.

– Reason: The neighbor will not request the LSA from the router.

• Mechanism: When the router accepts a received DD packet as the next in sequence from a neighbor in state Exchange, the following additional step is performed for each LSA listed in the packet:

– If the Database summary list for the neighbor contains an instance of the LSA that is the same or less recent than the one listed in the packet, the LSA is removed from the Database summary list.

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Benefits of Optimization

• Fully backward compatible with OSPFv2 and OSPFv3.

• Does not affect synchronization, since the LSAs omitted from DD packets are unnecessary.

• Reduces the overhead of DD packets by about 50% in large networks.

– Reason: It reduces the number of LSA headers exchanged by about 50% when the two routers are already nearly synchronized (see example).

• Especially useful in MANETs, in which an adjacency is often formed between two routers that are already nearly synchronized.

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Changes since IETF 66

• Draft clarifies that the additional step must be performed for every LSA listed in the received DD packet (to fully update the summary list) before the next DD packet is sent in response.

• For faster lookup of LSAs in the summary list, the draft suggests that LSAs be listed in lexicographically increasing order of(LS type, Link State ID, Advertising Router) for OSPFv2 and(LS type, Advertising Router, Link State ID) for OSPFv3.