(Transport Level Interface) A common interface for transport services (layer 4 of the OSI model). It provides a common language to a transport protocol and allows client/server applications to be used in different networking environments. Instead of directly calling NetWare's SPX, for example, the application calls the TLI library. Thus, any transport protocol that is TLI compliant (SPX, TCP, etc.) can provide transport services to that application. TLI is part of Unix System V. It is also supported by NetWare 3.x. See STREAMS.