RFC 9768: More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP
In plain English — editorial summary, not part of the RFC
Explicit Congestion Notification (ECN) is a mechanism by which network nodes can mark IP packets instead of dropping them to indicate incipient congestion to the endpoints. Receivers with an ECN-capable transport protocol feed back this information to the sender. ECN was originally specified for TCP in such a way that only one feedback signal can be transmitted per Round-Trip Time (RTT). More recently defined mechanisms like Congestion Exposure (ConEx), Data Center TCP (DCTCP), or Low Latency, Low Loss, and Scalable Throughput (L4S) need more accurate ECN feedback information whenever more than one marking is received in one RTT. This document updates the original ECN specification defined in RFC 3168 by specifying a scheme that provides more than one feedback signal per RTT in the TCP header, called More Accurate ECN (AccECN). Given TCP header space is scarce, it allocates a reserved header bit previously assigned to the ECN-nonce. It also overloads the two existing ECN flags in the TCP header. The resulting extra space is additionally exploited to feed back the IP-ECN field received during the TCP connection establishment. Supplementary feedback information can optionally be provided in two new TCP Option alternatives, which are never used on the TCP SYN. The document also specifies the treatment of this updated TCP wire protocol by middleboxes.
Document record
- Document ID
- RFC9768
- Published
- April 2026
- Authors
- B. Briscoe; M. Kühlewind; R. Scheffenegger
- Status
- PROPOSED STANDARD
- Stream
- IETF
- Area
- wit
- Pages
- 58
- Also known as
- —
- Updates:
- RFC 3168
Topics
Related documents
Ranked automatically by shared keywords, IETF area and stream — not by editorial selection.
- RFC 9601Propagating Explicit Congestion Notification across IP Tunnel Headers Separated by a ShimCurrent
August 2024
- RFC 9599Guidelines for Adding Congestion Notification to Protocols that Encapsulate IPCurrent
August 2024
- RFC 6040Tunnelling of Explicit Congestion NotificationUpdated
November 2010
- RFC 6660Encoding Three Pre-Congestion Notification (PCN) States in the IP Header Using a Single Diffserv Codepoint (DSCP)Current
July 2012
- RFC 9332Dual-Queue Coupled Active Queue Management (AQM) for Low Latency, Low Loss, and Scalable Throughput (L4S)Current
January 2023
- RFC 9331The Explicit Congestion Notification (ECN) Protocol for Low Latency, Low Loss, and Scalable Throughput (L4S)Current
January 2023
- RFC 9330Low Latency, Low Loss, and Scalable Throughput (L4S) Internet Service: ArchitectureCurrent
January 2023
- RFC 9134RTP Payload Format for ISO/IEC 21122 (JPEG XS)Current
October 2021
Also filed under
About this page
The document record above — title, authors, date, status, stream, area, relationships, DOI and errata — is imported verbatim from the public RFC Editor index. The “in plain English” section is editorial: written by The metasystema editorial team, not part of the RFC. Where the two differ, the RFC text governs.
Last checked against the RFC Editor index on . RFCs are never revised after publication; changes are issued as new documents.
Data sources · Editorial policy · Report a correction · What is an RFC?