What is a Transport Control Block (TCB) and what data does it store?

• A. A per-connection data structure storing sequence numbers, window sizes, timers, and state
• B. A buffer for application data
• C. A security token
• D. An IP header field

Answer: (A)

A Transport Control Block is the per-connection data structure that TCP uses to track the state and control information for a single connection. It holds the sequence numbers that show what data has been sent and what has been acknowledged (for example, what has been sent but not yet acknowledged and what the next byte to send is), the window sizes that govern how much data can be in flight, and timers that handle retransmission, timeouts, and other timing-related actions. It also stores the current state of the connection (the TCP state machine, such as established, listen, closing, etc.) so the protocol knows what steps to take next. In addition, the TCB contains information used for receiving data and controlling congestion, like receive next and receive window, as well as data needed for RTT estimation and retransmission decisions. This is distinct from a simple data buffer for application data, a security token, or a field in the IP header, none of which are managed by the per-connection control block. So the option describing a per-connection structure with sequence numbers, window sizes, timers, and state best captures what a TCB stores.