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

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OSPF Database ExchangeSummary List
Optimizationdraft-ogier-ospf-dbex-opt--00.txt
IETF Meeting - OSPF WG

• Richard Ogier
• Presented by Acee Lindem
• July 12, 2006

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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).

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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.

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Standard Exchange for Example
RT2
RT1
Empty DD (Seqx, I, M, Master)
ExStart
--------------------------------------------gt
Empty DD (Seqy, I, M, Master)
ExStart
lt-------------------------------------------
Full DD (Seqy, M, Slave)
Exchange
--------------------------------------------gt
Full DD (Seqy1, M, Master)
Exchange
lt-------------------------------------------
Full DD (Seqy1, Slave)
--------------------------------------------gt
Full DD (Seqy2, Master)
lt-------------------------------------------
Empty DD (Seqy2, Slave)
Full
--------------------------------------------gt
Full
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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.

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Optimized Exchange for Example
RT2
RT1
Empty DD (Seqx, I, M, Master)
ExStart
--------------------------------------------gt
Empty DD (Seqy, I, M, Master)
ExStart
lt-------------------------------------------
Full DD (Seqy, M, Slave)
Exchange
--------------------------------------------gt
Full DD (Seqy1, Master)
Exchange
lt-------------------------------------------
Full
Empty DD (Seqy1, Slave)
--------------------------------------------gt
Full