OSPF Database Exchange Summary List Optimization draft-ogier-ospf-dbex-opt--00.txt

1. IETF Meeting - OSPF WG OSPF Database ExchangeSummary List Optimizationdraft-ogier-ospf-dbex-opt--00.txt Richard Ogier Presented by Acee Lindem July 12, 2006

2. 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.

3. 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).

4. Example • Assumptions of example: • RT1 and RT2 already have identical databases when they start database exchange. • The list of LSA headers for the database fits into two (but not one) DD packets. • RT1 is the first to change the neighbor state to ExStart. • The standard database exchange for this example is shown on the next slide. Note that each router must send two full DD packets.

5. Standard Exchange for Example RT2 RT1 ExStart Empty DD (Seq=x, I, M, Master) --------------------------------------------> Empty DD (Seq=y, I, M, Master) ExStart <------------------------------------------- Exchange Full DD (Seq=y, M, Slave) --------------------------------------------> Full DD (Seq=y+1, M, Master) Exchange <------------------------------------------- Full DD (Seq=y+1, Slave) --------------------------------------------> Full DD (Seq=y+2, Master) <------------------------------------------- Full Empty DD (Seq=y+2, Slave) --------------------------------------------> Full

6. Applying Optimization to Example • When RT2 receives the first full DD packet from RT1, it removes from its summary list all LSAs that are listed in the DD packet, and sends a DD packet that lists the remaining LSAs (since all LSA headers fit into two DD packets). • When RT1 receives this DD packet, it removes these remaining LSAs from its summary list (causing it to be empty) and sends an empty DD packet to RT2. • With the optimization, each router sends only one full DD packet instead of two, as shown on the next slide.

7. Optimized Exchange for Example RT2 RT1 ExStart Empty DD (Seq=x, I, M, Master) --------------------------------------------> Empty DD (Seq=y, I, M, Master) ExStart <------------------------------------------- Exchange Full DD (Seq=y, M, Slave) --------------------------------------------> Full DD (Seq=y+1, Master) Exchange <------------------------------------------- Full Empty DD (Seq=y+1, Slave) --------------------------------------------> Full