What is the effect of the TCP window scale option and how do you verify its use?

• A. Window scaling expands the receiver window beyond 65k; verify by inspecting the TCP options for Window Scale value in the handshake.
• B. It improves DNS performance.
• C. It reduces MSS.
• D. It limits the number of concurrent connections.

Answer: (A)

The window scale option lets TCP advertise a larger receive window by negotiating a scale factor during the initial three-way handshake. This expands the amount of data that can be in flight beyond the 65,535-byte limit, which is especially helpful on networks with high latency relative to bandwidth.

You verify its use by inspecting the initial handshake packets for the Window Scale option. The value you see is the exponent (the scale factor) that will be applied to the 16-bit window size. If both sides include this option, scaling is in effect, and the actual usable window is the 16-bit window size multiplied by 2^scale. In a packet analyzer, you’ll commonly see the Window Scale value in the TCP options of the SYN and SYN-ACK, and the subsequent window size will reflect the scaled value.