Transport Layer Connection Services

Transport Layer Connection Services

Some services can be performed at more than one layer of the OSI model. In addition to the Data Link and Network layers, the Transport layer can take on some responsibility for connection services. The Transport layer interacts with the Network layer’s connection-oriented and connectionless services and provides some of the essential quality control features. Some of the Transport layers activities include the following:

Segment sequencing. Segment sequencing is one connection-oriented service provided by the Transport layer. When large messages are divided into segments for transport, the Transport layer must resequence the segments when they are received before reassembling the original message.
Error control. When segments are lost during transmission or when segments have duplicate segment IDs, the Transport layer must initiate error recovery. The Transport layer also detects corrupted segments by managing end-to-end error control using techniques such as checksums.
End-to-end flow control. The Transport layer uses acknowledgments to manage end-to-end flow control between two connected devices. Besides negative acknowledgments, some Transport-layer protocols can request the retransmission of the most recent segments.