Transport Layer Interface

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In computer networking, the Transport Layer Interface (TLI) was the networking API provided by AT&T UNIX System V Release 3 (SVR3) in 1987[1] and continued into Release 4 (SVR4).[2] TLI was the System V counterpart to the BSD sockets programming interface, which was also provided in UNIX System V Release 4 (SVR4).[2] TLI was later standardized as XTI, the X/Open Transport Interface.

TLI and Sockets[edit]

It was originally expected that the OSI protocols would supersede TCP/IP, thus TLI is designed from an OSI model-oriented viewpoint, corresponding to the OSI transport layer. Otherwise, TLI looks similar, API-wise, to sockets.

TLI and XTI were widely used (?) and, up to UNIX 98, may have been preferred over the POSIX Sockets API[3] with respect to existing standards. However, it was clear at least since the early 1990s that the Berkeley Socket interface would ultimately prevail.[4] TLI and XTI are still supported in SVR4-derived operating systems and operating systems conforming to branded UNIX (UNIX 95, UNIX 98 and UNIX 03 Single UNIX Specifications) such as Solaris and AIX (as well as the classic Mac OS, in the form of Open Transport). Under UNIX 95 (XPG4) and UNIX 98 (XPG5.2), XTI was the preferred and recommended supported API for new transport protocols.[3][5] As a result of deliberations by the Austin Group with the goal of bringing flavors of UNIX that do not provide STREAMS, such as BSD and Linux, under the Single UNIX Specification, the UNIX 03 Single UNIX Specification both declares STREAMS as optional, and declares POSIX Sockets as the preferred API for new transport protocols.

See also[edit]



  • Goodheart, Berny; James Cox (1994). The magic garden explained: the internals of UNIX System V Release 4, an open-systems design. Australia: Prentice Hall. ISBN 0-13-098138-9. 
  • Open Group (February 1997). "Networking Services (XNS) Issue 5". CAE Specification. XNS (5th ed.). Berkshire, U.K.: The Open Group. ISBN 1-85912-165-9. 
  • Open Group (January 2000). "Networking Services (XNS) Issue 5.2". CAE Specification. XNS (5.2 ed.). Berkshire, U.K.: The Open Group. 
  • Nuss, Branko (January 2000). "RTFM!!! Issue 5.2". CAE Specification. 1 (1.0 ed.). Akron, OH: Addison-Wesley. 
  • Pajari, George (1992) [1991]. Writing UNIX Device Drivers (2nd Printing ed.). Reading, MA: Addison-Wesley. ISBN 0-201-52374-4. 
  • Santa Cruz Operation (1997) [1995]. "System V Application Binary Interface — Intel386 Architecture Processor Supplement" (4th ed.). Santa Cruz, CA: The Santa Cruz Operation, Inc. 
  • Santa Cruz Operation (1997) [1995]. "System V Application Binary Interface" (4.1th ed.). Santa Cruz, CA: The Santa Cruz Operation, Inc. 
  • Stevens, W. Richard (1990). UNIX Network Programming. Englewood Cliffs, N.J.: Prentice-Hall. ISBN 0-13-949876-1. 
  • Stevens, W. Richard (1993). Advanced Programming in the UNIX Environment (1st Edition, 15th Printing ed.). Reading, MA: Addison-Wesley. ISBN 0-201-56317-7. 
  • Thomas, Rebecca; Lawrence R. Rogers; Jean L. Yates (1986). Advanced Programmers Guide to UNIX System V. Berkeley, CA: Osborne McGraw-Hill. ISBN 0-07-881211-9. 
  • UNIX System Laboratories (1995). "System V Application Binary Interface" (4th ed.). Englewood Cliffs, N.J.: UNIX System Laboratories. 
  • Waite Group (1987). Mitchel Waite, ed. UNIX Papers (2nd Printing ed.). Indianapolis, IN: Howard W. Sams & Company. ISBN 0-672-22578-6. 
  • Zucker, Steve; Kari Karhi (1995) [1993]. "System V Application Binary Interface—PowerPc Processor Supplement". Mountain View, CA: SunSoft. 

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