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The Transport Level Interface (TLI) is a UNIX programming interface that lets applications request transport-layer networking services through a transport provider. It is an API, not a network protocol: it aims to shield programs from some provider-specific details, but portability depends on the provider and platform conforming to TLI.
What is the Transport Level Interface?
TLI defines an interface between application programs and protocols at the transport layer—layer four—of the OSI Reference Model. UNIX System V documentation describes its purpose as letting programs use transport services without needing to know special characteristics of the underlying network protocols. The provider supplies the actual transport service; TLI defines how a program accesses it. (UNIX System V Release 4 Migration Guide)
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TLI is therefore best understood as a programming model for accessing network transport, not as a protocol that carries data itself. Its portability goal applies when a transport provider conforms to the interface. It does not mean that every provider supports identical options, behavior, or guarantees.
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What services does TLI provide?
The System V guide describes two service styles. Which one is available, and the details of its behavior, depend on the transport provider.
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Connection-oriented service
In connection-oriented service, endpoints establish a connection before exchanging data. The cited System V guide characterizes this mode as reliable and sequenced: data is delivered in order under the service described there.
Connectionless service
In connectionless service, an application sends independent data units, commonly described as datagrams. Each unit does not depend on a logical relationship with the others. This is a different service model, not an established connection with the same guarantees.
How TLI relates to OSI, STREAMS, and transport providers
TLI corresponds to the OSI transport layer, also called layer four. Oracle’s Solaris glossary describes it as a standard UNIX user-level interface to OSI level-four data communications features and says it is modeled on ISO 8072. SCO/Xinuos documentation likewise connects the interface to ISO 8072, the ISO Transport Service Definition. (Oracle Sun Global Glossary; SCO/Xinuos: Transport interface (XTI and TLI))
On STREAMS-based UNIX systems, TLI is the user-level interface through which an application accesses a transport provider. It should not be confused with the provider itself or with the lower-level Transport Provider Interface (TPI) mechanisms used within the system. (SCO/Xinuos: Transport interface (XTI and TLI); Oracle Sun Global Glossary)
Novell documentation gives examples of protocol suites supported through TLI in its product context, including ISO protocols, TCP/IP, XNS, and SNA. These are historical, vendor-specific examples—not a current or universal compatibility list. (Novell: TLI Messages)
TLI vs. XTI: what is the difference?
XTI is the X/Open-sponsored successor to TLI. In the cited UnixWare documentation, their syntax and semantics are described as nearly identical; TLI routines remain there for backward compatibility, while XTI is preferred for new applications in that environment. Those statements describe the documented systems, not a universal rule for every UNIX-like operating system. (SCO/Xinuos: Transport interface (XTI and TLI))
| Aspect | TLI | XTI |
|---|---|---|
| Lineage | Earlier UNIX transport interface | X/Open-sponsored successor to TLI, as described by SCO/Xinuos |
| API behavior | Nearly identical syntax and semantics in the cited UnixWare documentation | |
| Compatibility in cited environment | Routines retained for backward compatibility | Preferred for new applications in that documentation |
| Headers and libraries | Implementation-specific; cited UnixWare documentation records tiuser.h and libnsl |
Implementation-specific; consult the target system’s documentation |
Do not assume those header or library names are portable build instructions. The correct API, headers, and linking requirements depend on the operating system and its version. A separate, current platform-specific comparison would be needed to make a general recommendation between TLI/XTI and other networking APIs.
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When was TLI introduced?
SCO/Xinuos documentation dates TLI’s introduction to AT&T UNIX System V Release 3.0 in 1986. That places it in the history of UNIX System V networking and STREAMS; it does not establish how widely TLI is used or which present-day systems still support it. (SCO/Xinuos: Transport interface (XTI and TLI))
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Further reading for legacy System V networking
Stephen A. Rago’s UNIX System V Network Programming includes a dedicated Chapter 4, “The Transport Layer Interface,” on endpoint management and connection-oriented and connectionless services. Addison-Wesley Professional’s catalog lists the book’s publication date as March 1993. (Addison-Wesley Professional / O’Reilly catalog)
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