http://www.rfc-editor.org/rfc/rfc3168.txt
RFC 3168: The Addition of Explicit Congestion Notification (ECN) to IP, K. Ramakrishnan, S. Floyd, D. Black2001-09-01T23:00:00-00:00This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header. [STANDARDS-TRACK]