The ACK train length is measured by calculating the sum of inter-arrival times of all the closely spaced ACKs within an RTT round. The train length is strongly affected by the bottleneck bandwidth, routing delays and buffer sizes along the path, and is easily stretched out by congestion caused by cross traffic in the path, so by estimating the train length we can reliably find a safe exit point of Slow Start.
Increase in packet delays
Increase in packet delays during Slow Start may indicate the possibility of the bottleneck router being congested.
但是Increase in packet delays的测量会受到bursty transmission的影响，所以只测一个RTT中刚开始的几个数据包的往返时间来避免bursty transission的影响，在后面给出的code中会看到