lect-09 - Internetworking: Routing ECE/CS 438 Fall 2011...

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CS/ECE 438 1 Internetworking: Routing ECE/CS 438 Fall 2011 Slides from Peterson & Davies
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Routing Forwarding versus Routing Forwarding : to select an output port based on destination address and routing table Routing : process by which routing table is built
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Routing Forwarding table VS Routing table Forwarding table Used when a packet is being forwarded and so must contain enough information to accomplish the forwarding function A row in the forwarding table contains the mapping from a network number to an outgoing interface and some MAC information, such as Ethernet Address of the next hop Routing table Built by the routing algorithm as a precursor to build the forwarding table Generally contains mapping from network numbers to next hops
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CS/ECE 438 Kravets and Hou 4 Routing n Factors ¡ Network topology can change ¡ Traffic conditions can change n Design elements ¡ Performance criteria ¡ Decision time and place ¡ Information source n Goals ¡ Correctness ¡ Simplicity ¡ Robustness ¡ Fairness ¡ High throughput ¡ Low end-to-end latency
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CS/ECE 438 Kravets and Hou 5 Features of Good Solutions n Solution ¡ Dynamic n Periodically recalculate routes ¡ Distributed n No single point of failure n Reduced computation per node ¡ Abstract Metric n Distance may combine many factors n Use heuristics
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Routing Example rows from (a) routing and (b) forwarding tables
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Routing Network as a Graph The basic problem of routing is to find the lowest-cost path between any two nodes Where the cost of a path equals the sum of the costs of all the edges that make up the path
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CS/ECE 438 Kravets and Hou 8 Routing Overview n Static shortest path algorithms ¡ Bellman-Ford n Based on local iteration ¡ Dijkstra s algorithm n Build tree from source n Distributed, dynamic routing protocols ¡ Distance vector routing n Implement distributed version of Bellman-Ford algorithm ¡ Link state routing n Implement Dijkstra s algorithm at each node
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Routing For a simple network, we can calculate all shortest paths and load them into some nonvolatile storage on each node. Such a static approach has several shortcomings It does not deal with node or link failures It does not consider the addition of new nodes or links It implies that edge costs cannot change What is the solution? Need a distributed and dynamic protocol Two main classes of protocols Distance Vector Link State
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Distance Vector n Each node constructs a one dimensional array (a vector) containing the distances (costs) to all other nodes and distributes that vector to its immediate neighbors n Starting assumption is that each node knows the cost of the link to each of its directly connected neighbors
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Distance Vector Initial distances stored at each node (global view)
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Initial routing table at node A Distance Vector
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Distance Vector Final routing table at node A
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Distance Vector Final distances stored at each node (global view)
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lect-09 - Internetworking: Routing ECE/CS 438 Fall 2011...

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