Chapter 3 - Routing Protocols

Chapter 3 - Routing Protocols - Routing Protocols Chapter 3...

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Lawrence Wong 1 Routing Protocols Chapter 3
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Routing in Circuit Switched Network Many connections will need paths through more than one switch Need to find a route Efficiency Resilience Public telephone switches are a tree structure Static routing uses the same approach all the time Dynamic routing allows for changes in routing depending on traffic Uses a peer structure for nodes Lawrence Wong 2
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Routing in Packet Switched Network Complexity is a crucial aspect of packet switched networks Characteristics required Correctness Simplicity Robustness Stability Fairness Optimality Efficiency Lawrence Wong 3
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Routing Performance Performance criteria used for selection of route: Minimum hop Least cost To select a path involves 2 issues: The path selection algorithm itself The cost of a path is a function of: hop count and available bandwidth . Each interface has associated a metric which indicates the amount of remaining available bandwidth. This metric is combined with the hop count to provide a cost value, whose goal is to pick the path with the minimum number of hops supporting the requested bandwidth . When several paths are available, the path whose availability is maximal is selected. This way the balance load is maximized. Observe that this algorithm has double objective optimization. When the algorithm is actually invoked. 2 options: On-demand, i.e. computation is triggered for each new request. Could be computationally expensive Using some form of pre-computation. This option amortizes computational cost over multiple requests, but each computation instance is usually more expensive because paths to all destination should be recomputed. Also the final accuracy of the selected path may be lower. Another important issue is when pre-computation should take place. There are two options: Periodic re-computation. Pre-computation after 'N' number of updates have been received Lawrence Wong 4
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Routing Strategies Fixed Alternate Flooding Random Adaptive Lawrence Wong 5
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Fixed Routing Single permanent route for each source to destination pair Determine routes using a least cost algorithm Route fixed, at least until a change in network topology Simple but inflexible 6 Lawrence Wong
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Alternate Routing Possible routes between end offices predefined Originating switch selects appropriate route Routes listed in preference order Different sets of routes may be used at different times 7 Lawrence Wong
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Flooding Requires no information about the network A packet that is being sent from A to B is Sent to all the nearest neighbors of A Each neighbor receives the packet, then transmits the packet to all it’s own nearest neighbors, except the one it received the packet from The packet takes all possible paths through network to B Multiple copies of the packet will arrive at B, the first copy of the packet will arrive along the minimum cost path through the network.
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