Why is multiplexing/demultiplexing necessary in the transport layer?

• A. It allows a single application to receive data from multiple remote processes.
• B. It prevents any form of multiplexing.
• C. It allows many conversations to share a single host/port resource and delivers data to the correct process.
• D. It ensures data is always delivered in order.

Answer: (C)

Multiplexing/demultiplexing is about creating separate communication channels over the same network resources. In the transport layer, data from many applications can share the same host and port because each stream is labeled with identifiers such as port numbers (and, for TCP, the full connection state). When data arrives, the receiver’s transport layer uses those identifiers to route each piece of data to the correct socket and, therefore, to the correct application. This lets a single host handle many conversations with different remote endpoints while keeping each stream distinct and delivered to the right process.

Delivering data to the correct application heartbeat relies on the port numbers acting as logical channels. Without this demultiplexing, the system wouldn’t know which app should receive a given packet or segment, even though multiple conversations are occurring through the same host/port endpoint. That’s exactly why this mechanism is essential: it enables multiple conversations to share a single host/port resource and delivers data to the right process.

The other ideas don’t fit as the primary purpose here. Ensuring data arrives in order is a reliability feature associated with the transport protocol itself, not multiplexing/demultiplexing. And the notion of preventing multiplexing is the opposite of what the transport layer needs to do. While an app might receive data from multiple remote processes, the key concept is how the transport layer distinguishes and routes many simultaneous conversations.