First, the network must be able to detect link failures. Link failure detection can be performed by dedicated hardware or software by the end nodes and of the failed link. Second, nodes that detect the link failure must notify certain nodes in the network of the failure. Which nodes are actually notified of the failure depends on the rerouting technique. Third, a backup path must be computed. In pre-planned rerouting schemes however, this step is performed before link failure detection. Fourth, instead of sending traffic on the primary, failed path, a node called Path Switching Node must send traffic on the backup path. This step in the rerouting process is called switchover. Switchover completes the repairing of the network after a link failure.
When the failed link is physically repaired, traffic can be rerouted to the primary path, or keep being sent on the backup path. In the latter case, no further mechanism is necessary to reroute traffic to the primary path while three additional steps are needed to complete rerouting in the former case. First, a mechanism must detect the link repair. Second, nodes of the network must be notified of the recovery, and third the Path Switching Node must send traffic back on the primary path in the so-called switchback step.
Consider a unicast communication. When a link of the path between the sender and the receiver fails, users experience service interruption until the path is repaired. The length of the interruption is the time between the instant the last bit that went through the failed link before the failure is received, and the instant when the first bit of the data that uses the backup path after the failure arrives at the receiver. Let denote the time to detect the failure, the notification time,
the switchover time, and the sum of the queuing, transmission and propagation delay needed to send a bit of data between two nodes and . Then, for the example given in Figure 2.1, the total service interruption time for the communication is given by: