Routing Metrics Neil Tang 11/03/2009

1. Routing MetricsNeil Tang11/03/2009 CS440 Computer Networks

2. Outline • Weaknesses of Hop-count Based Metric • Delay Based Metric • Utilization Based Metric CS440 Computer Networks

3. Weaknesses of Hop-Count Metric • It does not distinguish between links on a delay basis. • It does not distinguish between links on a capacity basis. • It does not distinguish between links on a load basis. CS440 Computer Networks

4. Delay Based Metric Delay = queuing delay + transmission delay + prop delay • queuing delay = depart time – arrival time • transmission delay = message size/bandwidth • propagation delay = distance/speed of light CS440 Computer Networks

5. Delay Based Metric Problems • Instability: traffic switch back and forth from certain links frequently. • A large range of link cost values: e.g., a heavily loaded 9.6kpbs link may be 127 more costly than a lightly loaded 56kbps link. CS440 Computer Networks

6. Utilization Based Metric • Link utilization is measured based on the average of the previous reported utilization. • Link cost is given be a function which takes the measured link utilization, link type (e.g., wireless link, wired link) and link capacity as parameters. • A hard limit is usually set for the range of link values. CS440 Computer Networks

7. Utilization Based Metric CS440 Computer Networks

8. Routing Metrics • None of metrics are instantaneous. Link cost is usually given based on an average value over a period of time. • In order to reduce the overhead, link cost update messages are normally sent out only when the change exceeds some threshold. CS440 Computer Networks