cwnd

Cwnd, short for congestion window, is a TCP sender state variable that limits the amount of data that may be in flight. It defines the maximum number of bytes the sender may transmit before receiving acknowledgments, and it is constrained by the receiver’s advertised window (rwnd). Cwnd is typically expressed in bytes or in MSS units (the maximum segment size negotiated for the connection).

The cwnd value changes as the connection progresses in response to network conditions. At connection start, cwnd is small. In slow start, cwnd grows roughly exponentially, often doubling each round-trip time until it reaches a threshold called ssthresh. Beyond ssthresh, cwnd grows more slowly, in a linear fashion during congestion avoidance. The effective window for sending is the minimum of cwnd and rwnd.

Congestion events drive adjustments to cwnd. When packet loss is detected via triple duplicate acknowledgments, many implementations perform fast retransmit and fast recovery, decreasing cwnd and updating ssthresh to about half of the prior cwnd, with cwnd set to ssthresh and growth resuming. If a timeout occurs, cwnd is typically reset to 1 MSS and ssthresh is set to half of the previous cwnd. These mechanisms implement additive increase/multiplicative decrease (AIMD) to probe for available capacity while preventing congestion collapse.

Cwnd interacts with other TCP elements and varies across implementations. While the cwnd concept is shared, variants such as Reno, New Reno, and Cubic adjust cwnd differently during growth and in response to losses. The sender’s effective window is the smaller of cwnd and rwnd, and network path characteristics influence the achievable throughput.