Hello again: Stabilization of the Hello protocol in OSPF

The Hello protocol in OSPF allows each router in a network to check whether it can exchange messages with neighboring routers in its network. This check is carried out by making each router periodically send hello messages to every neighboring router in the network. Associated with the Hello protocol are two time periods: the hello period and the dead period. The hello period is the time period between sending two successive hello messages to the same neighbor. The dead period is the time period after which a router can declare a neighbor dead if during this period the router does not receive any hello messages from that neighbor. The original Hello protocol restricts the hello and dead periods to be fixed over time and to be identical in all routers. Simulation studies have shown that these restrictions contribute to network instabilities and even to network collapse. To improve network stability, we present a flexible Hello protocol where the hello and dead periods change over time and become consistent (rather than identical) in all routers. To ensure the fault-tolerance of our Hello protocol, the protocol is designed to be stabilizing. That is, when started from an arbitrary initial state, the protocol converges to a legitimate state, and remains in legitimate states throughout the remainder of its execution.