Universidad Carlos III de Madrid Independent This document specifies an experimental modification to ECN when used with TCP. It allows the use of ECN in the IP header of the following TCP packets: SYNs, SYN/ACKs, pure ACKs, Window probes, FINs, RSTs and retransmissions. This specification obsoletes RFC5562, which described a different way to use ECN on SYN/ACKs alone.