Session Management

TCP is one of several protocols operating at the host-to-host layer of the DoD 4-layer model. I call them session management protocols.

The network-layer protocols of the Internet are fundamentally datagram-oriented and unreliable. It is the responsibility of the Transport and Session layer protocols to enhance the quality of service to that desired by a particular application. In Internet terminology, these are the protocols of the Host-to-Host Layer, but I think Session Management better describes their role. These protocols function as an intermediary between the application and network layers.

Currently, there are three major Internet session management protocols:

• UDP (User Datagram Protocol) provides almost no additional functionality over IP. It performs fast, unreliable, datagram delivery. UDP is the Internet's version of the no-look pass.

• TCP (Transmission Control Protocol) provides reliable, stream-oriented delivery for applications such as file transfers and remote logins. TCP is a sliding-window protocol that takes steps to insure reliable data transfer, resending if needed due to network overloads or malfunctions. TCP, one of the most important Internet protocols, is also one of the most complex. Although it often works well unattended, a good understanding of TCP is essential for diagnosing performance problems.

• RPC (Remote Procedure Call) is designed for programs to make subroutine calls on other systems. Essentially a request-reply protocol, RPC usually makes heavy use of UDP datagrams, adding its own facilities for insuring data transfer. RPC implementations generally do not yield TCP-quality performance, so its use is mostly limited to local area networks. Its most important application is the file sharing via the NFS protocol.