What is the function of the transport layer, and which protocols operate there?

• A. The transport layer provides end-to-end data transport, flow control, and error handling; TCP and UDP operate there.
• B. The transport layer handles routing between networks; ICMP operates there.
• C. The transport layer defines how data is formatted for display; HTML protocols operate there.
• D. The transport layer manages physical addressing and framing; ARP operates there.

Answer: (A)

The transport layer is responsible for end-to-end delivery of data between processes on different hosts, providing mechanisms for reliable transfer, error handling, and flow control. It also multiplexes multiple applications using port numbers so several processes can share the same network connection.

Protocols that operate there include TCP, which is connection-oriented and guarantees reliable, ordered delivery with flow and congestion control, and UDP, which is connectionless and offers a lightweight, best-effort delivery without the overhead of reliability mechanisms.

Other items mentioned belong to different layers: routing occurs at the network layer; ICMP is a network-layer control protocol; HTML is used by application-layer protocols; ARP operates at the link layer to map IP addresses to MAC addresses.