Historical Perspective

When ISO came to define the services to be offered by their layers, they were heavily influenced by the existing 'X.25' service developed by the PTTs and consequently defined Virtual Circuit type services. (Unfortunately ISO treat 'Connection Oriented' as synonymous with 'Virtual Circuit Oriented' ignoring the additional reliability and flow control features which the latter implies). Thus the original OSI model was CO from top to bottom. The first addendum to the OSI reference model introduced CL working. Gradually CL layer standards have been developed and these now exist for the Datalink and Network layers. CL Transport layer standards are under development. These developments are important for Open Distributed Systems since a large part of the non-OSI work which has been carried out on distributed systems has assumed an underlying CL service. The Internet is the largest network in the world, and is CL, and was built in some senses independently of OSI.