  The Explicit Congestion Notification (ECN) field has taken on new importance
due to Low Latency, Low Loss, and Scalable throughput (L4S) technology designed
for extremely latency-sensitive applications (such as cloud games and
cloud-rendered VR/AR). ECN and L4S need to be supported by the client and
server but also all devices in the network path. We have identified that "ECN
bleaching", where an intermediate network device clears or "bleaches" the ECN
flags, occurs and quantified how often that happens, why it happens and
identified where in the network it happens. In this research, we conduct a
comprehensive measurement study on end-to-end traversal of the ECN field using
probes deployed on the Internet across different varied clients and servers.
Using these probes, we identify and locate instances of ECN bleaching on
various network paths on the Internet. In our six months of measurements,
conducted in late 2021 and early 2022, we found the prevalence varied
considerably from network to network. One cloud provider and two cellular
providers bleach the ECN field as a matter of policy. Of the rest, we found
1,112 out of 129,252 routers, 4.17% of paths we measured showed ECN bleaching.
