It's difficult to take advantage of ECN nowadays, too. Some ISPs seem to clobber the ECN field when packets enter their AS. This is measured and discussed here:

It's difficult to take advantage of ECN nowadays, too. Some ISPs seem to clobber the ECN field when packets enter their AS. This is measured and discussed here: http://dl.acm.org/citation.cfm?id=2068833 http://dl.acm.org/citation.cfm?id=2068833. Notably: """Via large-scale path measurements, we find the ECN feedback loop failing in the core of the network 40% of the time, typically at AS boundaries.""" So, unfortunately, we've got a ways to go on this.