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CSE-538-Fisheye

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					CSE-538 Fisheye State
      Routing
     Dr. R. B. Patel
           Link State Routing
• Similar to existing protocols of Internet routers
• Routing table update at each node
  – Periodically or on link addition/removal: flood “link
    state” – list of neighbors
  – Re-broadcasts link state information received from
    neighbors
  – Employ “sequence numbers” to distinguish new
    from stale updates
• Each node maintains a routing table with all
  the “link state” of all the nodes in the system
• Routing
  – The destination is stored in the message header
  – Each forwarding node finds the shortest path to
    the destination according to its routing table
                Link State Routing
                                                        0   {1}
 At node 5, based on the link
  state packets, topology table
  is constructed:
                                     {0,2,3}    1
        0   1   2   3   4   5
    0   1   1   0   0   0   0
    1   1   1   1   1   0   0                                         3     {1,4}
    2   0   1   1   0   1   1
    3   0   1   0   1   1   0
    4   0   0   1   1   1   1     {1,4,5}   2
    5   0   0   1   0   1   1
                                                                  4       {2,3,5}

 Dijkstra’s algorithm can then
  be used for the shortest path                         5
                                                {2,4}
 Link State Routing drawbacks
• As number of nodes grows / mobility
  increases
  – Routing tables grow linearly (assuming
    constant density)
  – Link state control traffic grow linearly
• Not scalable
  Fisheye State Routing (FSR)
• A variant of Link State routing
• Aimed at reducing control traffic (link state
  updates)
  – At the expense of routing table accuracy
• Which entries in the routing table can be
  updated less frequently?
  – The ones corresponding to a more distant
    nodes
        Fisheye State Routing
• The network is logically divided into “Fisheye”
  circles with respect to each node. The scope of
  the circle may be defined in terms of number of
  hops
• Reduction in update message achieved by using
  different exchange periods for different entries in
  the table
• Creates larger latencies for stations that are far
• Routes become progressively accurate as the
  packet gets closer to the destination
• FSR scales well to large networks, by keeping
  the Link State exchange overhead low
        Fisheye State Routing
– Maintain accurate information in immediate
  neighborhood (hop=1)
– Progressively less detail as distance increases
– Entries of nearby nodes are exchanged more
  frequently
– Frequency decreases proportionally to distance
       Routing - as packet
        gets closer to
        destination, routing
        accuracy increases
       Topology information
        is exchanged between
        neighbors via Unicast
              Fisheye State Routing
       LST       HOP
                                0
     0:{1}       1                              LST       HOP
     1:{0,2,3}   0                            0:{1}       2
     2:{5,1,4}   1                            1:{0,2,3}   1
     3:{1,4}     1          1                 2:{5,1,4}   2
     4:{5,2,3}   2                            3:{1,4}     0
     5:{2,4}     2                            4:{5,2,3}   1
                                        3     5:{2,4}     2


•Entries in black are
exchanged more                                LST       HOP
                        2
frequently
                                            0:{1}       2
•Information in {}                  4       1:{0,2,3}   2
gives nodes which                           2:{5,1,4}   1
are reachable from                          3:{1,4}     1
the node just                   5           4:{5,2,3}   0
outside the {}.                             5:{2,4}     1
                 Summary
• Major scalability benefit: control traffic
  decreases significantly
• Unsolved problems
  – Route table size still grows linearly with
    network size
  – Out of date routes to remote destinations

				
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