The Transport Layer

The transport layer (layer 4) is the lowest layer of the model that provides applications and higher layers with end-to-end service. This layer hides the topology and characteristics of the underlying network from users. The transport layer also defines a set of services common to many contemporary protocol suites including the ISO protocols, Transmission Control Protocol and TCP/IP Internet Protocol Suite, Xerox Network Systems (XNS), and Systems Network Architecture (SNA).

In RPC programming, the term "network" is frequently used as a synonym for transport or transport type.