Engress path / congestion detection and avoidance

Hi All,
i have few doubts related to congestion and engress path.

1) What happens when congestion hits and how it affects engress path?.
2) How the engress traffic slow down when the congestion hits?
3) what are the variables updated when the congestion hits?
4) is it good to invoke netif_stop_queue when congestion hits?

thanks
Jeba