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More Accurate Explicit Congestion Notification (ECN) Feedback in TCP

Document Type Expired Internet-Draft (tcpm WG)
Expired & archived
Authors Bob Briscoe , Mirja Kühlewind , Richard Scheffenegger
Last updated 2024-05-20 (Latest revision 2023-11-17)
Replaces draft-kuehlewind-tcpm-accurate-ecn
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources FreeBSD implementation source with AccECN TCP options
FreeBSD implementation source without AccECN TCP options
Linux implementation source
XML source of draft
Mailing list discussion
Stream WG state WG Consensus: Waiting for Write-Up
Associated WG milestone
Dec 2022
Submit specification of more accurate ECN feedback in TCP to the IESG for publication as a Proposed Standard RFC
Document shepherd Michael Tüxen
IESG IESG state Expired
Consensus boilerplate Yes
Telechat date (None)
Responsible AD (None)
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft is available in these formats:


Explicit Congestion Notification (ECN) is a mechanism where 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). Recent new TCP 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 in RFC 3168 to specify a scheme that provides more than one feedback signal per RTT in the TCP header. 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 exploited to feed back the IP-ECN field received during the 3-way handshake as well. 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.


Bob Briscoe
Mirja Kühlewind
Richard Scheffenegger

(Note: The e-mail addresses provided for the authors of this Internet-Draft may no longer be valid.)