When a TCP sender reduces cwnd due to CE marks, it is possible to end
up with very small cwnd (<2 mss). When the next packet is sent, with
the CWR flag, the receiver will often wait for another packet before
sending an ACK after the delack timer expires.
This can effectively drive up the high-percentile latency on request-
response type interactions.
The above was found specifically for flows using dctcp, most likely as
the cwnd in dctcp environments is more likely to collapse to very small
values – but this patch is generic and also addresses rfc3168 ECN