The slow start phase is a crucial part of the TCP congestion control algorithm, which is used to prevent network congestion. During the slow start phase, the sender gradually increases the amount of data it sends to the receiver, starting from a small initial window size and doubling the window size for each round trip time (RTT) until the slow start threshold (ssthres) is reached.
Initially, the ssthres is set as high as the advertised window size of the receiver, which is the maximum amount of data that the receiver is willing to accept at any given time. This allows the sender to slowly ramp up its transmission rate and avoid overwhelming the network with too much data at once.
If congestion occurs during the slow start phase, the ssthres is updated as (window size during congestion)/2. This reduces the window size and allows the sender to slow down its transmission rate, which helps to alleviate the congestion. The slow start phase continues until the updated ssthres is reached, at which point the TCP congestion control algorithm enters the additive increase phase.
During the additive increase phase, the sender increases the window size by a small fixed amount for each RTT, up to the congestion window size. This allows the sender to gradually increase its transmission rate again, but at a slower and more controlled rate than during the slow start phase. The congestion window size is the maximum amount of data that can be sent during a given RTT without causing further congestion.
Overall, the slow start phase of the TCP congestion control algorithm is an important mechanism for preventing network congestion and ensuring that data is transmitted smoothly and efficiently across the network.
