transport layer

In computing and telecommunications, the transport layer is the second highest layer in the four and five layer TCP/IP reference models, where it responds to service requests from the application layer and issues service requests to the Internet layer. It is also the name of layer four of the seven layer OSI model, where it responds to service requests from the session layer and issues service requests to the network layer. The definitions of the transport layer are slightly different in these two models. This article primarily refers to the TCP/IP model. See also the OSI model definition of the transport layer.
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See Open Systems Interconnection--Reference Model.

<networking> (Or "host-host layer") The middle layer in the OSI seven layer model. The transport layer determines how to use the network layer to provide a virtual error-free, point to point connection so that host A can send messages to host B and they will arrive un-corrupted and in the correct order. It establishes and dissolves connections between hosts. It is used by the session layer.
An example transport layer protocol is Transmission Control Protocol (TCP).
OSI documents: ITU Rec. X.214 (ISO 8072), ITU Rec. X.224 (ISO 8073).
(1997-12-07)

<networking, programming> (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)