The Window

Threshold Changes the Rule

The threshold marks where slow start gives way to add-one growth.

highlighted = computed this step

The threshold is a rule switch

The threshold is pinned at 8. Below it, the row doubles. At it or above it, the row stops doubling.

ssthresh=8\text{ssthresh}=8
Threshold changes the ruleToy congestion window rows recomputed from exact integer rules.Threshold changes the rulecwnd in MSSlossthreshold12345678910RTTRTTcwndssthreshphaseeventnext118slow startdouble2228slow startdouble4348slow startdouble8488avoidanceadd one9598avoidanceadd one106108losshalve5755avoidanceadd one6865avoidanceadd one7975avoidanceadd one81085avoidanceadd one9

Cwnd reaches the threshold

RTT 4 starts with cwnd 8, exactly the threshold. The next step is no longer a double.

8=88=8
Threshold changes the ruleToy congestion window rows recomputed from exact integer rules.Threshold changes the rulecwnd in MSSlossthreshold12345678910RTTRTTcwndssthreshphaseeventnext118slow startdouble2228slow startdouble4348slow startdouble8488avoidanceadd one9598avoidanceadd one106108losshalve5755avoidanceadd one6865avoidanceadd one7975avoidanceadd one81085avoidanceadd one9

The next value is one larger

At the threshold, congestion avoidance takes over: 8 becomes 9, not 16.

8+1=98+1=9
Threshold changes the ruleToy congestion window rows recomputed from exact integer rules.Threshold changes the rulecwnd in MSSlossthreshold12345678910RTTRTTcwndssthreshphaseeventnext118slow startdouble2228slow startdouble4348slow startdouble8488avoidanceadd one9598avoidanceadd one106108losshalve5755avoidanceadd one6865avoidanceadd one7975avoidanceadd one81085avoidanceadd one9

Summary

The threshold keeps slow start from doubling forever. In this pinned sequence, cwnd reaches 8 and then switches to add-one growth.

switch at 8\text{switch at }8
Threshold changes the ruleToy congestion window rows recomputed from exact integer rules.Threshold changes the rulecwnd in MSSlossthreshold12345678910RTTRTTcwndssthreshphaseeventnext118slow startdouble2228slow startdouble4348slow startdouble8488avoidanceadd one9598avoidanceadd one106108losshalve5755avoidanceadd one6865avoidanceadd one7975avoidanceadd one81085avoidanceadd one9