SSTHRESH

ssthresh is a congestion control parameter used in TCP (Transmission Control Protocol) implementations. It stands for Slow Start Threshold. The ssthresh value is dynamically adjusted by TCP to manage the rate at which data is sent over a network connection.

When a TCP connection is established, the congestion window (cwnd), which limits the amount of data that can be sent without acknowledgment, starts small. Initially, TCP operates in a "slow start" phase. During slow start, the cwnd doubles with each Round Trip Time (RTT) until it reaches a certain threshold. This threshold is where ssthresh comes into play.

Once the cwnd reaches or exceeds the ssthresh, TCP transitions from the slow start phase to the "congestion avoidance" phase. In congestion avoidance, the cwnd increases more slowly, typically by one segment per RTT. This prevents overwhelming the network path.

The ssthresh value is typically set to half of the cwnd when congestion is detected. Congestion is usually inferred when a packet loss occurs, indicated by duplicate acknowledgments or a retransmission timeout. Upon detecting congestion, TCP reduces its cwnd significantly (often to 1 segment) and sets the ssthresh to half of the cwnd before the loss occurred. This mechanism helps TCP to probe for available bandwidth without causing excessive congestion. The value of ssthresh is crucial for TCP's ability to adapt to changing network conditions and maintain efficient data transfer.