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Link State Routing. David Holmer dholmer@jhu.edu. 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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Link state routing l.jpg

Link State Routing

David Holmer

dholmer@jhu.edu


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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


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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)


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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


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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


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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


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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