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Link State Routing

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

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  1. Link State Routing David Holmer dholmer@jhu.edu

  2. 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

  3. 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)

  4. 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

  5. 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

  6. 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

  7. 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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