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Link State Routing. David Holmer [email protected] Generic Link State Routing. Each node monitors neighbors/local links and advertises them to the network Usually state of local links is sent periodically Must be re-sent because of non-reliable delivery and possible joins/merges

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generic link state routing
Generic Link State Routing
  • Each node monitors neighbors/local links and advertises them to the network
    • Usually state of local links is sent periodically
    • Must be re-sent because of non-reliable delivery and possible joins/merges
  • Each node maintains the full graph by collecting the updates from all other nodes
    • The set of all links forms the complete graph
    • Routing is performed using shortest path computations on the graph
basic hello protocol description
Basic Hello Protocol Description
  • Used for neighbor discovery
  • Basic version
    • Each node sends a hello/beacon message periodically containing its id
    • Neighbors are discovered by hearing a hello from a previously un-heard from node
    • Neighbors are maintained by continuing to hear their periodic hello messages
    • Neighbors are considered lost if a number of their hello messages are no longer heard (typically two)
basic hello protocol properties
Basic Hello Protocol Properties
  • Very simple protocol agnostic method of discovering neighbors
  • Generates constant overhead
    • Basic hello packets are very small
    • N/T packets per second (N=nodes, T=period)
    • Not scalable in very dense networks where nodes have a large number of neighbors
  • Link failure only discovered after >2T
  • May discover asymmetric links
enhanced hello protocols
Enhanced Hello Protocols
  • Hello message also carries neighbor list
  • Advantages
    • Enables symmetric link discovery
      • A symmetric link neighbor will include your id in its neighbor list
    • Enables two hop topology discovery
      • Useful for connected dominating set calculation
      • Used by OLSR for Multi-Point Relay (MPR) computation
  • Disadvantages
    • Significantly larger hello message size in dense networks
      • Differential hello protocol used in TBRPF to reduce overhead
generic link state properties
Generic Link State Properties
  • Generic Link State Protocol
    • Basic hello protocol for neighbor/link discovery
    • Normal flooding for periodic link advertisement
  • Problems
    • Inefficient under mobility
      • Every link change causes two floods (from either endpoint)
      • Movement of a single node causes multiple link changes
    • Inefficient in static case
      • All links must be periodically advertised
strategies for optimizing link state
Strategies for Optimizing Link State
  • Hello protocol enhancement (differential)
  • Flooding protocol enhancement
    • Reduces the cost of flooding by using knowledge of topology
    • Multi-Point Relay (OLSR)
    • Topology Based Reverse Path Forwarding (TBRPF)
  • Topology Filtering
    • Reduce the number of advertised links to less than the complete graph
  • Nearsighted Updates (Fisheye & HSLS)
    • Scoped flooding for advertisements
    • Update nearby nodes quickly & far nodes more slowly
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