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RFC 9768
Title: More Accurate Explicit Congestion Notification (AccECN)
Feedback in TCP
Author: B. Briscoe,
M. Kühlewind,
R. Scheffenegger
Status: Standards Track
Stream: IETF
Date: April 2026
Mailbox: [email protected],
[email protected],
[email protected]
Pages: 58
Updates: RFC 3168
I-D Tag: draft-ietf-tcpm-accurate-ecn-34.txt
URL: https://www.rfc-editor.org/info/rfc9768
DOI: 10.17487/RFC9768
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.
This document is a product of the TCP Maintenance and Minor Extensions Working
Group of the IETF.
This is now a Proposed Standard.
STANDARDS TRACK: This document specifies an Internet Standards Track
protocol for the Internet community, and requests discussion and suggestions
for improvements. Please refer to the current edition of the Official
Internet Protocol Standards (https://www.rfc-editor.org/standards) for the
standardization state and status of this protocol. Distribution of this
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