3.5 Dynamic Alternate Routing

Dynamic Alternate Routing (DAR) was a project initiated by British Telecom. This is a distributed, adaptive routing scheme. Like other dynamic routing schemes, DAR is limited to maximum two-link routing and employs trunk reservation. DAR has no crankback and PCC is used.

For each destination switch j, the originating switch i maintains an alternate path k in its cache. A newly arrived call first attempts the direct link i-j. If there is no capacity available on the direct link, it tries the alternate path in its cache. If the call succeeds on this alternate path, this alternate path remains in the cache. However, if the call cannot set up a call through the alternate path due to nonavailability of capacity, the call is lost, meaning the user has to retry it.

In addition, the originating node i pick an intermediate node randomly and stick this in the cache for the next call to use. This means an alternate path remains in the cache as long as any calls using this alternate path are successfully connected; a new alternate path is picked randomly the instant the current alternate route cannot connect a call using this path.

The elegance of this routing scheme lies in its simplicity. There is another important observation about this routing scheme: it does not require any network link status to be updated for computing routes. Such approaches are sometimes referred to as learning automata. Sometimes, a routing scheme such as DAR is referred to as event driven routing.