1.3. Brief history of link-state routing technology

OSPF is a link state routing protocol. Such protocols are also referred to in the literature as SPF-based or distributed- database protocols. This section gives a brief description of the developments in link-state technology that have influenced the OSPF protocol.

The first link-state routing protocol was developed for use in the ARPANET packet switching network. This protocol is described in [McQuillan]. It has formed the starting point for all other link-state protocols. The homogeneous Arpanet environment, i.e., single-vendor packet switches connected by synchronous serial lines, simplified the design and implementation of the original protocol.

Modifications to this protocol were proposed in [Perlman]. These modifications dealt with increasing the fault tolerance of the routing protocol through, among other things, adding a checksum to the link state advertisements (thereby detecting database corruption). The paper also included means for reducing the routing traffic overhead in a link-state protocol. This was accomplished by introducing mechanisms which enabled the interval between link state advertisement originations to be increased by an order of magnitude.

A link-state algorithm has also been proposed for use as an ISO IS-IS routing protocol. This protocol is described in [DEC]. The protocol includes methods for data and routing traffic reduction when operating over broadcast networks. This is accomplished by election of a Designated Router for each broadcast network, which then originates a link state advertisement for the network.

The OSPF subcommittee of the IETF has extended this work in developing the OSPF protocol. The Designated Router concept has been greatly enhanced to further reduce the amount of routing traffic required. Multicast capabilities are utilized for additional routing bandwidth reduction. An area routing scheme has been developed enabling information hiding/protection/reduction. Finally, the algorithm has been modified for efficient operation in TCP/IP internets.