Which OSI layer handles end-to-end communication and reliable data transfer between applications?

• A. Application Layer (Layer 7)
• B. Presentation Layer (Layer 6)
• C. Transport Layer (Layer 4)
• D. Network Layer (Layer 3)

Answer: (C)

End-to-end communication and reliable data transfer between applications is the job of the Transport Layer. This layer sits between the applications and the network, using ports to identify the sending and receiving processes and enabling two programs on different hosts to communicate. It provides reliability through mechanisms like segmentation, sequencing, acknowledgments, and retransmission of lost data, along with flow control to prevent overwhelming the receiver. TCP, the common protocol at this layer, guarantees delivery, preserves order, and checks for errors, which is what “reliable data transfer” means in practice. The Network Layer handles routing and addressing to move packets across networks, not the guarantees about delivery. The Presentation Layer deals with data formatting, encryption, and compression, while the Application Layer is where programs access network services; neither ensures end-to-end reliability on its own.