The second byte in an IPv4 header is called the Differentiated Services Field. Quoting RFC 2474: 2. Terminology Used in This Document ... Differentiated Services Field: the IPv4 header TOS octet or the IPv6 Traffic Class octet when interpreted in conformance with the definition given in this document. Also DS field. It's a historical accident that the ECN bits were shoehorned into this octet. -- Regards Brian Carpenter _______________________________________________ Ietf mailing list Ietf@xxxxxxxx https://www.ietf.org/mailman/listinfo/ietf