Which TCP header fields are critical for reliability and flow control, and how?

• A. Source Port and Destination Port define reliability.
• B. Sequence Number and Acknowledgment Number enable reliable, ordered delivery; the Window field provides flow control by advertising receiver capacity.
• C. Data Offset alone determines reliability.
• D. Urgent Pointer alone provides flow control.

Answer: (B)

TCP ensures reliable, in-order delivery and proper pacing through how it tracks data and signals capacity. The Sequence Number marks the position of each byte of data, letting the receiver reorder segments and detect any missing pieces. The Acknowledgment Number communicates the next byte the receiver expects, enabling the sender to know what has been received and to retransmit any gaps. The Window field, or receive window, tells the sender how much data the receiver can buffer, which controls how fast the sender can send data and prevents overwhelming the receiver—this is flow control.

Other header fields don’t drive reliability or flow control: the Source Port and Destination Port identify which endpoints are communicating; the Data Offset is just the header length; the Urgent Pointer indicates urgent data but doesn’t establish reliability or flow control.