The functions of the Transport Layer Interface are adapted to the transport layer according to the International Standards Organization( ISO). It was first introduced with UnixSystem V Release 3.0 (SVID) and its specifications are defined by SVID. It is a transport mechanism for network interfaces with a preference for sockets.
The Transport Layer Interface (TLI) and the X/Open Transport Layer Interface(XTI) were used extensively until Unix 98, which preferred the programming interface over POSIX sockets. They are still supported by Unix System Release 4.0 (SVR4).