During code inspection and testing of DCTCP in mixed
loss and ECN marking scenarios, it was found that the alpha
value of DCTCP is updated twice during windows expiriencing loss.

While RFC8257 is silent on the interaction between loss
recovery and updates of alpha, updating it one additional time
with an effective ECN marking rate of 0 reduces the effect of
ECN marks due to the lower alpha value. This in turn may result
in an overall higher loss rate than necessary.

Reported-by: Cheng Cui

A packetdrill script with the expected evolution
of cwnd is provided.