A transport endpoint specifies a communication path between a transport user and a specific transport provider, which is identified by a local file descriptor (fd). When a user opens a transport endpoint, a local file descriptor fd is returned which identifies the endpoint. A transport provider is defined to be the transport protocol that provides the services of the transport layer. All requests to the transport provider must pass through a transport endpoint. The file descriptor fd is returned by the function t_open() and is used as an argument to the subsequent functions to identify the transport endpoint. A transport endpoint can support only one established transport connection at a time.
To be active, a transport endpoint must have a transport address associated with it by the t_bind() function. A transport connection is characterized by the association of two active endpoints, made by using the transport connection establishment functions t_listen(), t_accept(), t_connect(), and t_rcvconnect().