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Lecture-12 - Lecture#12 Today’s Lecture Objectives •...

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Unformatted text preview: Lecture #12 Today’s Lecture Objectives • Network Layer – Routing • Small note going before the lecture – Lots more slides today, there mostly to help you review Routing vs. Forwarding • Forwarding – Decided once a packet arrives – Given a packet’s destination, which outgoing interface gets it closer to its destination – Uses the routing table • Routing – The process of discovering neighbors and gathering information about in which direction destinations lie • THE Question – On what outgoing interface should a packet be sent to get it closer to its destination – Not an end-to-end decision: the tool is always the routing table routing algorithm local forwarding table header value output link 0100 0101 0111 001 3 2 2 Routing vs. Forwarding 1 2 3 0111 value in arriving packet’s header 1001 1 • Forwarding plane • Routing speed (CPU) • Buffering (Memory) • Interfaces Routing Algorithm Classification Centralized, Global or Decentralized? Centralized: • One central node gathers information, makes decisions, and informs routers lobal: Static or Dynamic? Static: • Routes are set and not changed • Works well when routes change slowly over time ynamic: Global: • All routers have complete topology, link cost info Decentralized: • Router knows physically- connected neighbors, link costs to neighbors • Iterative process of computation, exchange of info with neighbors Dynamic: • Routes are set and then periodically updated • Works well when routes change more quickly – periodic update – in response to link cost changes Routing Basics • THE Question – On what outgoing interface should a packet be sent to get it closer to its destination – Not an end-to-end decision: the tool is always the routing table asic Process • Basic Process – Router interface is configured for a range of addresses – Routers discover neighbors – Routers start advertising “local prefixes” to the world Distance Vector Link State 1. Discover neighbors 2. Build routing table 3. Advertise to neighbors 4. Receive 1. Discover neighbors 2. Build routing table 3. Flood through network 4. Receive advertisements 5. Run Bellman-Ford algorithm—update table advertisements 5. Run Dijkstra’s algorithm—update table u y x w v z 2 2 1 3 1 1 2 5 3 5 Graph Abstraction seful for understanding Bellman ord and Dijkstra • Useful for understanding Bellman-Ford and Dijkstra • Graph: G = (N,E) • N = set of routers = { u, v, w, x, y, z } – “routers” translate into real-world address ranges • E = set of links ={ (u,v),(u,x),(v,x),(v,w),(x,w),(x,y),(w,y),(w,z),(y,z) } Examples • Bellman-Ford ijkstra • Dijkstra Distance Vector Algorithm: Bellman-Ford Iterative, asynchronous: • Each local iteration caused by: – local link cost change – DV update message from neighbor istributed: wait for (change in local link cost of msg from neighbor) Each node: Distributed: • Each node notifies neighbors only when its DV changes – neighbors then notify their neighbors if necessary recompute estimates if DV to any dest has changed, notify neighbors x...
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Lecture-12 - Lecture#12 Today’s Lecture Objectives •...

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