(TLI, or "Transport Level Interface") A protocol-independent interface for accessing network facilities, modelled after the ISO transport layer (level 4), that first appeared in Unix SVR3.

TLI is defined by SVID as transport mechanism for networking interfaces, in preference to sockets, which are biased towards IP and friends. A disavantage is that a process cannot use read/write directly, but has to use backends using stdin and stdout to communicate with the network connection. TLI is implemented in SVR4 using the STREAMS interface. It adds no new system calls, just a library, libnsl_s.a. The major functions are t_open, t_bind, t_connect, t_listen, t_accept, t_snd, t_rcv, read, write.

According to the Solaris t_open man page, XTI (X/OPEN Transport Interface) evolved from TLI, and supports the TLI API for compatibility, with some variations on semantics.