TPKT is officially defined as "ISO Transport Service on top of the TCP." "TCP" and "ISO" relate to two rival suites of networking protocols. TPKT enables translation between these two groups.
The International Standards Organization, or ISO, sponsored the definition of a suite of protocols for networking: the "Open Systems Interconnection (OSI) model. A rival suite of protocols is called "TCP/IP": "Transmission Control Protocol/Internet Protocol." Both layer groups of protocols in a "stack." In TCP/IP, "TCP" is at the "Transport Layer" and the Internet Protocol (IP) is at the "Internet Layer." TCP/IP is more widely implemented than OSI. TPKT provides a method to carry OSI data over TCP/IP networks.
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TPKT is an "encapsulation" protocol. It carries the OSI packet in its own packet's data payload and then passes the resulting structure to TCP, from then on, the packet is processed as a TCP/IP packet. The OSI programs passing data to TPKT are unaware that their data will be carried over TCP/IP because TPKT emulates the OSI protocol Transport Service Access Point (TSAP).
Like most networking protocols, TPKT also works in reverse. When a TPKT packet arrives, TPKT strips off its packet structure and passes the carried packet up the protocol stack. The OSI protocols receiving the data are unaware that TCP/IP was involved in data transmission.