{"draft":"draft-ietf-tcpm-accurate-ecn-34","doc_id":"RFC9768","title":"More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP","authors":["B. Briscoe","M. K\u00fchlewind","R. Scheffenegger"],"format":["HTML","TEXT","PDF","XML"],"page_count":"58","pub_status":"PROPOSED STANDARD","status":"PROPOSED STANDARD","source":"TCP Maintenance and Minor Extensions","abstract":"Explicit Congestion Notification (ECN) is a mechanism by which\r\nnetwork nodes can mark IP packets instead of dropping them to\r\nindicate incipient congestion to the endpoints. Receivers with an\r\nECN-capable transport protocol feed back this information to the\r\nsender. ECN was originally specified for TCP in such a way that only\r\none feedback signal can be transmitted per Round-Trip Time (RTT).\r\nMore recently defined mechanisms like Congestion Exposure (ConEx),\r\nData Center TCP (DCTCP), or Low Latency, Low Loss, and Scalable\r\nThroughput (L4S) need more accurate ECN feedback information whenever\r\nmore than one marking is received in one RTT. This document updates\r\nthe original ECN specification defined in RFC 3168 by specifying a\r\nscheme that provides more than one feedback signal per RTT in the TCP\r\nheader, called More Accurate ECN (AccECN). Given TCP header space is\r\nscarce, it allocates a reserved header bit previously assigned to the\r\nECN-nonce. It also overloads the two existing ECN flags in the TCP\r\nheader. The resulting extra space is additionally exploited to feed\r\nback the IP-ECN field received during the TCP connection\r\nestablishment. Supplementary feedback information can optionally be\r\nprovided in two new TCP Option alternatives, which are never used on\r\nthe TCP SYN. The document also specifies the treatment of this\r\nupdated TCP wire protocol by middleboxes.","pub_date":"April 2026","keywords":["Congestion Control and Management","Congestion Notification","Feedback","Reliable","Ordered","Protocol","ECN"],"obsoletes":[],"obsoleted_by":[],"updates":["RFC3168"],"updated_by":[],"see_also":[],"doi":"10.17487\/RFC9768","errata_url":null}