Routing Metrics Neil Tang 11/03/2009 CS440 Computer Networks

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

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

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

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

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

Utilization Based Metric CS440 Computer Networks

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