Transport Layer of the OSI Model

Connectionless Sessions

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.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.