Explicit Congestion Notification (ECN) was first added to the Internet
Protocol in 2001, it was hit by a succession of deployment problems.
Studies in recent years have concluded that path traversal of ECN has
become close to universal.
In this article, we test whether the performance enhancement called
ECN++ would face a similar deployment struggle to base ECN.
For this, we assess the feasibility of ECN++ deployment over mobile as
well as fixed networks.
In the process, we discovered bad news for the base ECN protocol:
contrary to accepted beliefs, more than half the mobile carriers we
tested wipe the ECN field at the first upstream hop.
All packets still get through and congestion control still functions,
just without the benefits of ECN.
This throws into question whether previous studies used representative
This article also reports the good news that, wherever ECN gets through,
we found no deployment problems for the '++' enhancement to ECN.
The article includes the results of other in-depth tests that check
whether servers that claim to support ECN actually respond correctly to
explicit congestion feedback.