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Layer 4: Transport

Two trains passing each other over many tracks.  Source: Microsoft.

The Transport layer is primarily responsible for ensuring that data is transferred from one point to another reliably and without errors. For example, the Transport layer makes sure data are sent and received in the correct sequence.

The Transport layer provides flow control and error handling, and participates in solving problems concerned with the transmission and reception of packets. Common examples of Transport layer protocols are Transmission Control Protocol (TCP), User Datagram Protocol (UDP) and Sequenced Packet Exchange (SPX).

Layer 4 PDU: Segment

Next: Layer 3: Network