How does window scaling extend the TCP receive window?

• A. By increasing the maximum segment size
• B. By negotiating a shift factor in the window scale option
• C. By using larger acknowledgement numbers
• D. By enabling SACK

Answer: (B)

Window scaling enlarges the receive window by negotiating a shift factor in the Window Scale option during the TCP handshake. The 16-bit receive window field can only express up to 65,535 bytes, so to handle large bandwidth–delay paths TCP uses a scale factor S (0–14). If both sides agree to a scale, the actual receive window becomes the 16-bit value multiplied by 2^S, effectively left-shifting the window. This lets you advertise a much larger window without changing the 16-bit field. The shift factor is established during connection setup, and both ends must support this option. Increasing the maximum segment size changes the amount of data per segment, not the window size; larger acknowledgement numbers are just values, and SACK is about reporting missing data, not widening the window.