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Network Layer – Routing 1

This article provides an introduction to the network layer in computer networks, specifically focusing on routing. It explains the importance of routing in delivering packets from source to destination and discusses the two types of network layer services: connectionless and connection-oriented. The article also covers the elements of routing techniques and various routing algorithms, including Dijkstra's algorithm.

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Network Layer – Routing 1

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  1. Network Layer – Routing 1 Dr. Sanjay P. Ahuja, Ph.D. Fidelity National Financial Distinguished Professor of CIS School of Computing, UNF

  2. Introduction • Network Layer is responsible for delivering packets from source to destination over multiple links (routing). • Network layer provides a service to the Transport Layer. • The Transport Layer is shielded from number, type, and topology of the subnets present. • Two types of network layer services considered: connectionless (using datagrams) and connection-oriented (using Virtual Circuits). Application Transport Network Link Physical

  3. Routing • Routing is one of the most complex and crucial aspects of a packet switched network. • Desirable attributes of routing include: • Correctness • Simplicity • Robustness • Stability

  4. Elements of Routing Techniques • Performance Criteria: • Least number of hops • Least cost based on: • Delay • Throughput • Decision Time • Datagram • Virtual Circuit • Decision Place • Each node (distributed) • Central node (centralized) • Originating node

  5. Least-Cost Algorithms • Virtually all packet switched networks base their routing decision on some form of least-cost routing. • E.g. least cost α current load on link (delay) • E.g. least cost 1/α link capacity (or bandwidth) • The least-cost routing algorithm states that: “Given a network of nodes connected by bidirectional links, where each link has a cost associated with it in each direction, define the cost of a path between nodes as the sum of the costs of the links traversed. For each pair of nodes find the path with the least cost.”

  6. Dijkstra’s Algorithm • This finds the shortest paths from a given source node to all other nodes by developing the paths in order of increasing path length. • Definitions: • N = set of nodes in the network • s = source node • M = set of nodes so far incorporated by the algorithm • dij = link cost from node i to node j; dii = 0; dij = ∞ if the two nodes are not directly connected; dij >= 0 if the two nodes are directly connected. • Dn = cost of least-cost path from node s to node n that is currently known to the algorithm.

  7. Dijkstra’s Algorithm • The algorithm has 3 steps; step 2 and 3 are repeated until M = N. 1. Initialize: M = {s} i.e. set of nodes incorporated is only the source node Dn= dsnfor n ≠ s i.e. initial path costs to neighboring nodes are simply the link costs 2. Find the neighboring node not in M that has the least-cost path from node s and incorporate that node in M: Find w ε M such that Dw = min Dj j ε M Add w to M. 3. Update least-cost paths: Dn= min [Dn,Dw + dwn] for all n ε M. If the latter term is the minimum, the path from s to n is now the path from s to w concatenated with the link from w to n.

  8. Dijkstra’s Algorithm - Example

  9. Dijkstra’s Algorithm - Example • Source node = 1

  10. Routing Strategies – Static or Fixed Routing • Static or Fixed Routing Route selected for each s-d pair using Dijkstra’s algorithm. Routes are fixed and can only change if there is change in topology. Costs based on link capacity and not traffic conditions. Advantage: Simplicity; works well in a network with steady load. Disadvantage: Does not react to congestion or failures.

  11. Routing Strategies – Flooding • Requires no network information at all. • A packet is sent by source node to all its neighbors. At each node, incoming packet is retransmitted on all outgoing links except for the link that it arrived from. • To prevent incessant retransmissions, a hop count field is included with each packet. The count is sent to some maximum value. • Advantage: All possible routes between s and d tried. So a packet always gets through as long as at least one path exists between s and d. • Disadvantage: Traffic load generated increases with network connectivity; could cause traffic delays.

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