Which transport-layer protocols provide end-to-end communication and port multiplexing?

• A. TCP
• B. Both TCP and UDP
• C. UDP
• D. None

Answer: (B)

At the transport layer, end-to-end communication means delivering data directly between the processes running on two hosts, using a socket identified by a port number on each end. Port multiplexing is what lets a single host run many applications at once by giving each conversation its own port number, so data can be correctly routed to the right process on both ends.

Both TCP and UDP operate this way. They establish communications between processes across hosts and use source and destination ports to multiplex multiple conversations on the same machine and demultiplex them at the other end. TCP adds reliability and an established connection, while UDP is connectionless and best-effort, but both still rely on port numbers to identify the talking processes. That’s why both protocols provide end-to-end communication and port multiplexing.