12-NL-Routing - 1 Routing& I P Routing Protocols 2...

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Unformatted text preview: 1 Routing & I P Routing Protocols 2 Routing: Problem Definition • You are a router in a packet switched network and you receive a packet destined to some remote node – E.g., router A below receives a packet destined to node F • Question: How does A know where to send this packet? – Does A send it to B? C? or D? • Answer: Recall from our earlier discussion in I P that router A consults a forwarding table to make this decision • Routing Problem: How does router A build this forwarding table? – Built by a routing algorithm (protocol): The job of the routing algorithm is to determine the next hop router for ALL destinations in the network A E D C B F 2 2 1 3 1 1 2 5 3 5 Dest. Next Hop Cost B B 2 D D 1 C D 3 E D 2 F D 4 Forwarding Table in A 3 End-to-End Path Determination: Routing principles Graph abstraction for routing algorithms: • graph nodes are routers • graph edges are physical links – link cost: delay, $ cost, or congestion level (amount of traffic carried on the link) Goal: determine “good” path (sequence of routers) thru network from source to dest. Routing protocol A E D C B F 2 2 1 3 1 1 2 5 3 5 • “good” path: – typically means minimum cost path 4 Forwarding vs Routing • Distinction between “Forwarding” and “Routing” – Forwarding consists of taking a packet, looking at its destination address, consulting the forwarding table , and sending the packet in a direction determined by the table • Very easy once the forwarding table has been built – Routing is the process by which the forwarding table is built • Need a routing protocol to dynamically build and maintain the table A E D C B F 2 2 1 3 1 1 2 5 3 5 Dest. Next Hop Cost B B 2 D D 1 C D 3 E D 2 F D 4 Forwarding table in A 5 Routing Algorithm classification Global or decentralized information? Global: • all routers have complete topology, link cost info • “link state” algorithms Decentralized: • router knows physically-connected neighbors, link costs to neighbors • iterative process of computation, exchange of info with neighbors • “distance vector” algorithms 6 Link-State Algorithms: General I dea • Have each router build the complete topology of the network • Once the complete topology is built, have each router run an algorithm to compute the shortest path from itself to all other routers (nodes) in the network A E D C B F 2 2 1 3 1 1 2 5 3 5 2 I ssues • How does a router build the complete topology of the network? • How does a router compute the shortest path to all other nodes in the network using this topology information? – Dijkstra’s Shortest Path Algorithm 7 Building the Network Topology • At the heart of a link state algorithm is the discovery of a node’s links’ states – Each node is assumed to capable of finding out the state of the link to its neighbors (up or down) and the cost of each link – Each node creates an update packet, also called a link-state...
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12-NL-Routing - 1 Routing& I P Routing Protocols 2...

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