Congestion window. TCP state variable.
This variable limits the amount of data a TCP can send. At any given time, a TCP MUST NOT send data with a sequence number higher than the sum of the highest acknowledged sequence number and the minimum of cwnd and rwnd.
TCP uses two algorithms for increasing the congestion window. During steady-state, TCP uses the Congestion Avoidance algorithm to linearly increase the value of cwnd. At the beginning of a transfer, after a retransmission timeout or after a long idle period (in some implementations), TCP uses the Slow Start algorithm to increase cwnd exponentially. Slow Start bases the cwnd increase on the number of incoming acknowledgments. During congestion avoidance RFC 2581 allows more latitude in increasing cwnd, but traditionally implementations have based the increase on the number of arriving ACKs.
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