OSPF Extension for Manets Using Reliable Flooding

1
OSPF Extension for Manets Using Reliable Flooding
IETF Meeting - MANET WG

• Richard Ogier
• SRI International
• November 13, 2003

2
Objectives and Issues

• OSPF generates too much control traffic in
  manets.
• Link State ACKs
• Database Exchange with each new neighbor
• Unreliable, periodic flooding is wasteful of
  bandwidth.
• To reduce control traffic, we need an efficient
  mechanism for reliable flooding of LSA in manets.
• Minimize changes to OSPF make only necessary
  changes (otherwise we are really desiging a new
  protocol).
• Some questions
• How to select relay nodes used for flooding LSAs?
• How to flood LSAs reliably?

3
Choice for Relay Nodes

• Connected Dominating Set (CDS)
• Independent of originator
• Called overlapping relays in Fred Bakers
  presentation.
• Multipoint Relays (MPRs)
• Dependent on originator
• Useful if a min-hop path must exist that uses
  only MPRs as intermediate nodes. (Not necesary
  for OSPF extension.)
• Can be neighbor-selected (as in OLSR)
• Can be self-selected (avoids having to include
  MPRs in Hellos)
• Non-leaf nodes of min-hop tree rooted at the
  originator.
• Feasible, but requires more computation for
  router to determine if it is a leaf for each
  originator.

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Advantages of Using a CDS vs. MPRs

• OSPF Hello packets can be used without
  modification (the DR field would indicate a CDS
  node, which can be the sending node itself).
• Each node decides whether it is a CDS node based
  on Hellos and LSAs originated by neighbors.
• Using a CDS results in fewer transmissions (or
  approximately the same, depending on the
  scenario) as MPRs.
• Using a CDS is simpler since it is
  originator-independent. Each CDS node is
  responsible for relaying all LSAs.
• A CDS selects itself, similar to a DR in OSPF.
  This can result in faster recovery from link
  failures. (MPRs are selected by neighbors.)

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Reliable Flooding Using Periodic Database
Description Packets

• Each router originates an LSA when it changes.
• To flood the LSA, each CDS node transmits each
  LSA once.
• Duplicate detection is done as in OSPF - based on
  the LSA sequence number in the database.
• No ACKs, no NACKs.
• No new message type is required. OSPF Database
  Description (DD) and LS Request packets are used,
  but in a more efficient manner.
• Each CDS node periodically (e.g., every 5 or 10
  sec) broadcasts a complete set of DD packets on
  each manet interface. Similar to the Periodic
  Complete Sequence Numbers packets of IS-IS.
• Since a DD packet contains only LSA headers, this
  generates much less overhead than periodic
  flooding of LSAs.
• Continued...

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Reliable Flooding Using Periodic Database
Description Packets (cont.)

• Periodic DD packets accomplish two purposes
• The reliable delivery of LSAs to neighbors
  (without ACKs).
• The exchange of LSAs between each CDS node and
  its neighbors (without requiring a separate
  Database Exchange with each neighbor)
• If a node receives a DD packet that indicates a
  CDS neighbor has a more recent instance of an
  LSA, the node sends a LS Request for that LSA to
  the CDS neighbor.
• LSAs and DD packets are always broadcast (to all
  neighbors) on manet interfaces. LS Requests are
  unicast to a single CDS neighbor.
• The number of LS Requests can be reduced by
  having each CDS node transmit each LSA multiple
  times. (The number of times can depend on recent
  history of LS Requests.)
• The number of LS Requests can also be reduced by
  using a redundant CDS, so that each node is a
  neighbor of at least two CDS nodes.

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Differential LSAs

• A differential LSA reports only differences (new
  links lost links, and metric changes) between the
  current instance and a previous instance of an
  LSA, instead of reporting the entire LSA.
• Example If a node has 100 neighbors and loses
  one of them, it would be wasteful to generate a
  full LSA with 99 neighbor IDs, when the loss
  could be reported in a differential LSA
  containing a single neighbor ID.
• A differential LSA includes two LS sequence
  numbers sn1 the LSA includes the changes from instance sn1 to
  instance sn2.
• The checksum in the differential LSA is the
  checksum for the entire LSA with sequence number
  sn2.
• LS Request would also include a sequence number
  to indicate the current instance.

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Conclusions

• One approach was presented for extending OSPF to
  support manet interfaces.
• Many questions need to be answered before
  deciding on a final design, including
• Whether to use a CDS, and if so, how to select
  CDS nodes.
• Whether to use MPRs, and if so, how to select
  them.
• Whether to use reliable flooding, and if so, how
  to achieve it.
• Additional discussions, analysis, and simulations
  are needed to answer these and other questions.