Source : Free On-Line Dictionary of Computing

Transport Layer Interface
     
         (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
        toward {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.
     
        (1999-06-10)