On the State of ECN and TCP Options on the Internet

Explicit Congestion Notification (ECN) is a TCP/IP extension that can avoid packet loss and thus improve network performance. Though standardized in 2001, it is barely used in today’s Internet. This study, following on previous active measurement studies over the past decade, shows marked and continued increase in the deployment of ECNcapable servers, and usability of ECN on the majority of paths to such servers. We additionally present new measurements of ECN on IPv6, passive observation of actual ECN usage from flow data, and observations on other congestion-relevant TCP options (SACK, Timestamps and Window Scaling). We further present initial work on burst loss metrics for loss-based congestion control following from our findings.