CSCI 465 D ata Communications and Networks Lecture 17

1. CSCI 465Data Communications and NetworksLecture 17 Martin van Bommel CSCI 465Data Communications & Networks

2. ARPANET Routing Strategies1st Generation Distance Vector Routing – 1969 • version of Bellman-Ford algorithm • distributed adaptive algorithm using estimated delay • queue length used as estimate of delay • node exchanges delay vector with neighbors (128ms) • update routing table based on incoming information • doesn't consider line speed, just queue length • higher capacity links no favored • responds slowly to congestion • packet seeks low congestion, no focus on destination CSCI 465Data Communications & Networks

3. ARPANET Routing Strategies2nd Generation Link State Routing – 1979 • distributed adaptive algorithm using delay criterion • using timestamps of arrival, departure & ACK times • node re-computes average delays every 10 seconds • any changes are flooded to all other nodes • re-computes routing using Dijkstra’s algorithm • good under light and medium loads • under heavy loads, little correlation between delays before and after routing table updates CSCI 465Data Communications & Networks

4. Link State Routing Oscillation • Bi-partite network – must pass through A or B • Routes all use A – high delay – flood changes • All will switch to B – new high delay CSCI 465Data Communications & Networks

5. ARPANET Routing Strategies3rd Generation Average Distance Vector Routing – 1987 • link cost calculation function changed • damp routing oscillations • reduce routing overhead • measure average delay over last 10 seconds and transform into link utilization estimate • normalize this based on current value and previous results – increases period of oscillations • set link cost as function of average utilization • transformed value of delay CSCI 465Data Communications & Networks

6. ARPANET Delay Metrics • Delay normalized to idle line • Queue delay rises exponentially • Cost estimate kept at normalized minimum value until threshold • Reduces routing overhead • Cost level capped at 3x minimum • Can avoid high delay line by no more than two additional hops • Minimum higher for satellite • Higher propagation delay CSCI 465Data Communications & Networks