Enabling Internet-Wide Deployment of Explicit Congestion Notification

\ac{ECN} is an TCP/IP extension to signal network congestion without packet loss, which has barely seen deployment though it was standardized and implemented more than a decade ago. On-going activities in research and standardization aim to make the usage of \ac{ECN} more beneficial. This measurement study provides an update on deployment status and newly assesses the marginal risk of enabling \ac{ECN} negotiation by default on client end-systems. Additionally, we dig deeper into causes of connectivity and negotiation issues linked to \ac{ECN}. We find that about five websites per thousand suffer additional connection setup latency when fallback per RFC 3168 is correctly implemented; we provide a patch for Linux to properly perform this fallback. Moreover, we detect and explore a number of cases in which \ac{ECN} brokenness is clearly path-dependent, i.e. on middleboxes beyond the access or content provider network. Further analysis of these cases can guide their elimination, further reducing the risk of enabling \ac{ECN} by default.