Which OSI model layer is primarily responsible for end-to-end communication and reliability between hosts?

• A. Session
• B. Network
• C. Data Link
• D. Transport

Answer: (D)

The Transport layer handles end-to-end communication between processes on different hosts and provides reliability for that transfer. It distinguishes the communicating applications using ports and ensures data arrives intact and in order by using sequencing, acknowledgments, retransmission of lost data, and flow control (as seen with TCP). This end-to-end responsibility is what sets it apart from other layers: the Network layer is concerned with routing and delivering packets across multiple networks, not guaranteeing delivery; the Data Link layer handles frames on a single physical link with its own error checking; and the Session layer manages dialog and synchronization between applications. So, for reliable, end-to-end host-to-host communication, the Transport layer is the best fit.