Which TCP header field is used to manage end-to-end flow control during transmission?

• A. Sequence Number
• B. Acknowledgment Number
• C. Window Size
• D. Source Port

Answer: (C)

Flow control in TCP is handled through the sliding window mechanism. The Window Size field in each segment tells the sender how much data the receiver can currently accept, so the sender paces its transmission accordingly. If the window is zero, the sender must stop until the receiver advertises more space, and when space is available again, transmission resumes with the new window size. This end-to-end control keeps the receiver’s buffer from overflowing and coordinates data flow between sender and receiver. The Acknowledgment Number mainly confirms which bytes have been received and supports reliability, while the Sequence Number tracks the order of bytes. The Source Port is just for identifying the sending application. Therefore, the field used to manage end-to-end flow control is the Window Size.