The OSI Transport layer protocol (ISO-TP) manages end-to-end control and error checking to ensure complete data transfer. It performs transport address to network address mapping, makes multiplexing and splitting of transport connections, also provide functions such as Sequencing, Flow Control and Error detection and recover.

In carrying out its duties, the Transport layer performs a range of support activities, including maintaining data integrity through flow control techniques, multiplexing the data from upper layer applications, setting up and tearing down any virtual circuits established to transport the data over the network, hiding any network-dependent information from the upper layers (which will only confuse them), breaking down Session layer (layer 5) datagrams into segments, monitoring the error-free delivery of the data to its destination, providing for general connection management and data transfer services, providing for the reliable (but, not guaranteed) delivery of data.

  • TCP (Transmission Control Protocol)are The protocol primarily concerned with the reliable delivery of packets that requires an acknowledgement of a packet's arrival at its destination.
  • UDP (User Datagram Protocol): The TCP/IP best-effort protocol that isn't concerned with the reliable delivery of packets and doesn't bother with overhead such as acknowledgments.
  • SPX (Sequence Package Exchange): The Novell protocol most akin to TCP. It guarantees data delivery. NWLink (NetWare Link): Microsoft's version of Novell's IPX/SPX.
  • ATP/NBP (AppleTalk Transaction Protocol/Name Binding Protocol): AppleTalk's data transport protocols.
  • NetBIOS/NetBEUI (Network Basic Input/Output System/NetBIOS extended User Interface): Microsoft's network protocols that work together to manage communications and provide data transport services.
You use three primary methods for error-checking. They are parity bit, check-sum, and CRC.

The three basic forms of flow control are buffering , congestion avoidance, windowing . Five transport layer protocols exist in the OSI suite, ranging from Transport Protocol Class 0 through Transport Protocol Class 4 (TP0, TP1, TP2, TP3 & TP4). 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 both connection-oriented and connectionless communications.

  • Transport Protocol Class 0 (TP0) performs segmentation (fragmentation) and reassembly functions. 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.
  • Transport Protocol Class 1 (TP1) performs segmentation (fragmentation) and reassembly, plus error recovery. TP1 sequences protocol data units (PDUs) and will retransmit PDUs or reinitiate the connection if an excessive number of PDUs are unacknowledged.
  • Transport Protocol Class 2 (TP2) performs segmentation and reassembly, as well as multiplexing and demultiplexing of data streams over a single virtual circuit.
  • Transport Protocol Class 3 (TP3) offers error recovery, segmentation and reassembly, and multiplexing and demultiplexing of data streams over a single virtual circuit. TP3 also sequences PDUs and retransmits them or reinitiates the connection if an excessive number are unacknowledged.
  • Transport Protocol Class 4 (TP4) offers error recovery, performs segmentation and reassembly, and supplies multiplexing and demultiplexing of data streams over a single virtual circuit. TP4 sequences PDUs and retransmits them or reinitiates the connection if an excessive number are unacknowledged. TP4 provides reliable transport service and functions with either connection-oriented or connectionless network service. TP4 is the most commonly used of all the OSI transport protocols, which is similar to the Transmission Control Protocol (TCP) in the TCP/IP suite. Both TP4 and TCP are built to provide a reliable connection oriented end-to-end transport service on top of an unreliable network service. The network service may loose packets, store them, deliver them in the wrong order or even duplicate packets. Both protocols have to be able to deal with the most severe problems e.g. a subnetwork stores valid packets and sends them at a later date. TP4 and TCP have a connect, transfer and a disconnect phase. The principles of doing this are also quite similar.

Comments (0)Add Comment

Write comment

security code
Write the displayed characters


busy