TP0-TP4

• 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.

Read more about TP0-TP4: - Pacific Internet's Network Operations Center offers a tutorial about "OSI Protocols."

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