Cisco Certified Network Professional 2026 – 400 Free Practice Questions to Pass the Exam

During the slow start phase, TCP (Transmission Control Protocol) employs a mechanism where the congestion window size is initially set to one maximum segment size (MSS). This means that the first segment transmitted will be of size one MSS. As acknowledgments are received for this initial segment, the congestion window increases exponentially, allowing TCP to increase the amount of data sent into the network more quickly, as it discovers the available bandwidth.

This mechanism is crucial for avoiding network congestion, especially after a connection is established or after a timeout. By starting with a small window size, TCP can safely probe the network for its capacity and adjust accordingly. As more acknowledgments are received, the congestion window grows, allowing more data to flow through until a threshold condition is met, leading TCP to switch to the congestion avoidance phase where it alters the growth rate of the congestion window.

The initial segmentation size reset to one maximizes the chances for all packets to be received successfully before significantly increasing the load on the network, which helps to prevent congestion during the start of a session.