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Lecture 17

Lecture 17. Routing versus Forwarding. Routing Table:. Forwarding Table:. Why routing protocols?. Link failures New nodes Congestion Two approaches: Distance Vector-based on local information Link State-based on global information. Distance-vector routing. Routing Loops. Example 1

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Lecture 17

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  1. Lecture 17

  2. Routing versus Forwarding Routing Table: Forwarding Table:

  3. Why routing protocols? • Link failures • New nodes • Congestion • Two approaches: • Distance Vector-based on local information • Link State-based on global information

  4. Distance-vector routing

  5. Routing Loops • Example 1 • F detects that link to G has failed • F sets distance to G to infinity and sends update t o A • A sets distance to G to infinity since it uses F to reach G • A receives periodic update from C with 2-hop path to G • A sets distance to G to 3 and sends update to F • F decides it can reach G in 4 hops via A • Example 2 • link from A to E fails • A advertises distance of infinity to E • B and C advertise a distance of 2 to E • B decides it can reach E in 3 hops; advertises this to A • A decides it can read E in 4 hops; advertises this to C • C decides that it can reach E in 5 hops…

  6. Loop-Breaking Heuristics • Set infinity to 16 • Split horizon • Split horizon with poison reverse

  7. Router Information Protocol (RIP)

  8. RIP packet

  9. Link State Routing • Each node establishes a list of directly connected neighbors and cost of each link • Floods that information in a LSP to all neighbors • Retransmits LSPs from other nodes- but does not echo to sender

  10. Propagation of LSPs

  11. LSP Information • ID of sending node • Link-state of sending node • Sequence number • Time to live

  12. Route Calculation • Each node has enough information to map the network • Dijkstra’s shorted path algorithm used to compute the routes

  13. Example: Link State Routing

  14. Routing Table Calculation

  15. Routing Table Calculation

  16. Routing Table Calculation

  17. OSPF • Authentication • Hierarchy-Domains and Areas • Load Balancing

  18. OSPF Header Format

  19. OSPF link-state advertisement

  20. Metrics • Issues • Number of Hops • Latency • Bandwidth or Capacity • Congestion • Difficult to assign a scalar cost to such a complex and changing problem

  21. ARPANET 1 • Lowest Cost=Shortest Queue

  22. ARPANET 2 • Delay=(Depart time-Arrival Time)+Transmission Time+Latency • Reliability incorporated through the Depart Time parameter • Wide spread of weights- • Oscillations

  23. ARPANET 3 • Reduce dynamic range of metric • Averaging • Hard limit on changes in metric-like the stock market

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