Which of the following functionalities must be implemented by a transport…

Correct answer: End to end connectivity

Explanation: The primary role of the transport layer is to provide logical end-to-end communication between applications running on different hosts. This includes services such as segmentation and reassembly of application data, and multiplexing/demultiplexing using port numbers.

• End-to-end connectivity and process-to-process communication (using port numbers) are fundamental transport-layer responsibilities that go beyond the network layer's host-to-host routing.

• Recovery from packet losses, detection of duplicate packets, and in-order delivery are reliability features that some transport protocols (for example TCP) implement, but they are not mandatory for every transport protocol. Examples: TCP provides these features; UDP provides end-to-end datagram delivery without reliability or ordering.

Conclusion: End-to-end connectivity is the core functionality that a transport protocol must provide above the network protocol; the other listed features are optional services that specific transport protocols may choose to implement.