What does the Window field advertise in TCP headers?

• A. The receiver's available buffer space.
• B. The sender's maximum segment size.
• C. The round-trip time estimate.
• D. The IP address.

Answer: (A)

TCP uses flow control to prevent overwhelming the receiver, and that relies on the window mechanism. The Window field in the TCP header tells the sender how much data the receiver currently has buffer space for—the receiver’s available capacity. This value, the receive window, limits how much data the sender may have in transit before waiting for more acknowledgments. As the receiver processes data and frees space, it advertises a larger window in subsequent ACKs; if it can’t accept any more data yet, it may advertise a window of zero and the sender should pause until the window opens again. The other items aren’t conveyed by this field: the maximum segment size is negotiated with a TCP option during setup, RTT estimates come from ACK timing, and IP addresses live in the IP header, not the TCP header.