In contrast, connectionless sessions communicate without receipt acknowledgements or sequence numbers. Connectionless protocols aren’t without merit; they just don’t have any reliability mechanisms built in, since they’re mainly built for speed. Their lower overhead means faster communication, but also means that reliability is left to a higher layer in the model. For example, if a file was being transferred with a connectionless protocol, any missing parts would need to be re-requested (if necessary) by the receiving application – the protocol would not account for these missing parts automatically. Connectionless protocols are most often used by applications with a high degree of time sensitivity. Consider a streaming audio application – it would have little to gain from acknowledgements, but benefits greatly (in terms of speed) from the lower overhead.
Examples of Transport layer protocols:
- Transmission Control Protocol (TCP) Connection-oriented TCP/IP sessions
- User Datagram Protocol (UDP) Connectionless TCP/IP sessions
- Sequenced Packet Exchange (SPX) Connection-oriented sessions in IPX/SPX protocol suite.
Tip: Remember that the protocol data unit (PDU) of the Transport layer is a segment.