Probing the Path
The congestion window is the sender estimate of how much data the network can absorb without dropping packets. Because that capacity is unknown and changing, the window is constantly adjusted.
Two Phases
After the rapid climb of slow start, the sender enters a gentler phase.
- Slow start grows the window quickly until a threshold called the slow start threshold.
- Congestion avoidance then grows the window by about one segment per round trip, probing slowly.
- A loss signals the path is full, so the sender cuts the window and lowers the threshold.
This pattern of slow rise and sharp drop is often drawn as a sawtooth.
Loss as a Signal
Classic algorithms treat a dropped packet as the main hint of congestion. Newer approaches such as the bottleneck bandwidth method instead estimate delay and rate directly, which can avoid filling buffers.
Key idea
The congestion window climbs fast in slow start, then probes gently in congestion avoidance, and a loss cuts it sharply, producing the sawtooth that lets a sender track changing path capacity.