TP0-TP4 (transport protocols 0 to 4) are the five protocols in the Transport layer of the Open Systems Interconnection (OSI) model for telecommunication. The Transport layer manages end-to-end control and error checking to ensure complete data transfer. The protocols increase in complexity from 0-4. TP0-3 work only with connection-oriented communications, in which a session connection must be established before any data is sent; TP4 also works with connectionless communications, which don't require an established session connection before sending data.
- TP0 performs segmentation and reassembly (SAR) tasks. To respond to restrictions in a particular communications channel or to reduce latency, it may be necessary to break a packet into smaller pieces before transmitting. This process is known as "segmentation" (in TCP/IP, the same process is known as fragmentation). TP0 discerns the size of the smallest maximum protocol data unit (PDU) supported by any of the underlying networks, and segments the packets accordingly. The packet segments are reassembled at the receiver.
- TP1 performs SAR tasks, and adds error recovery. It assigns numbers to identify each PDU and resends any whose receipt is not acknowledged (NAK). If there are very many unacknowledged PDUs, TP1 can reinitiate the connection.
- TP2 adds multiplexing and demultiplexing capabilities to the SAR tasks performed by TP0 and TP1.
- TP3 combines all the features of the three lower protocols.
- TP4 is the OSI equivalent of Transmission Control Protocol (TCP). Based on TCP, TP4 adds reliable transport to the services featured by TP3. TP4 is the most commonly used of all the OSI transport protocols.