Which TCP option provides RTT measurements and protection against reordering?

• A. MSS
• B. Window Scale
• C. Timestamps
• D. SACK

Answer: (C)

RTT measurements and protection against reordering come from the TCP timestamps option. Each segment is stamped with the sender’s TSval, and the receiver echoes this value back in the ACK as TSecr. When the sender receives that ACK, it can compute a precise RTT by comparing the current time to the original TSval, updating its RTT estimate over time. For protection against reordering, the PAWS mechanism uses these timestamps to detect and discard old or duplicate segments that arrive out of order, preventing them from being mistaken for new data. The other options serve different purposes: MSS negotiates the maximum segment size, Window Scale extends the usable window size, and SACK allows reporting of noncontiguous received blocks, but they do not provide RTT timing or PAWS protection.