The Transport LayerInterface or Transport Level Interface (TLI) is a protocol-independent programming interface (API) via which the application programs can access network services
. The functions of the Transport Layer Interface are adapted to the transport layer according to
. 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).