Examples of Routing Algorithms

Developed for ARPANET.

A distributed adaptive routing algorithm using estimated delays as cost criteria.
Each node holds an estimate of the delay, and the next destination, for each possible target.
Periodically, each node gets the estimated delay vectors of its neighbors, and its own delay information.
Each node modifies its estimates and destinations.
Shortcomings
- Queue length depends not just on speed of delivery between nodes, but also on other factors like processing time within the nodes
- The algorithm responds slowly to congestion within the network

Measures delay time between adjacent nodes, by looking at the time it takes to positive acknowledgments to come back.
Uses flooding to inform other nodes when significant changes in delay occur
Delay time is a good criteria for low traffic loads, but not for high traffic loads where the algorithm can easily cause oscillations.

Observation: oscillations are implied by the conflicting objectives of providing the best routes for all destinations
Solution: provide good path to the average route instead of providing the best path to all routes.
Algorithm to calculate link cost:
1. Measure average delay t on link
2. Get service time
  s = (average packet size) / (data rate on link) = (600 bits) / (data rate on link)
3. Use queuing theory to calculate the utilization
  = (2s - 2t)/(s - 2t)
4. Let the cost be the average of the old and new utilization:
  U_new = (U_old + )/2
The averaging is to reduce the effect of oscillation.