Distinguish rwnd and cwnd.

• A. rwnd is the sender's window; cwnd is the receiver's.
• B. rwnd is the receiver's advertised capacity for incoming data; cwnd is the sender's limit based on network congestion; actual window is the minimum of the two.
• C. rwnd and cwnd are the same.
• D. rwnd is TTL; cwnd is hop count.

Answer: (B)

In TCP, data flow is governed by two windows: the receiver window (rwnd) and the congestion window (cwnd). rwnd is the receiver’s advertised capacity for incoming data; the receiver includes this value in ACKs to tell the sender how much more data it can store without overflowing its buffers. cwnd is the sender’s limit based on network congestion and path conditions; it is adjusted by the sender through congestion-control algorithms and may grow as the path is deemed healthy or shrink after packet loss. The actual amount of data the sender may have in flight at any moment is the minimum of rwnd and cwnd, ensuring you don’t overwhelm the receiver or the network. The other options mix up who controls which window or bring in unrelated terms, so they don’t fit.