Network Layer – Routing 1

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.