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IETF RFC 9768
[ACTIVE]

More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP

Metadata

Publisher
IETF
Doc Type
Standard
Pages
58
Abstract
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.
Publication Date
2026-04-01
Status Note
Proposed Standard
DOI
10.17487/RFC9768
Link
https://doi.org/10.17487/RFC9768
Author(s)
Bob Briscoe
Mirja Kühlewind
Richard Scheffenegger
Keyword(s)
Congestion Control, Congestion Notification, Protocol, ECN
Latest version of IETF RFC 9768

Document Versions — IETF RFC 9768

  • ▶ IETF RFC 9768 (2026-04-01)
    THIS DOC [ACTIVE] [LATEST VERSION]

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IETF RFC 9768, More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP. Available at https://doi.org/10.17487/RFC9768
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IETF RFC 9768, More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP. Available at https://doi.org/10.17487/RFC9768

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IETF RFC 9768, More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP. Available at https://doi.org/10.17487/RFC9768
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<span class="citation"><cite>IETF RFC 9768</cite>, More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP. Available at <a href="https://doi.org/10.17487/RFC9768" target="_blank" rel="noopener">https://doi.org/10.17487/RFC9768</a></span>

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IETF RFC 9768, More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP
doi: 10.17487/RFC9768
url: https://doi.org/10.17487/RFC9768
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<li>
<cite id="bib-rfc9768">IETF RFC 9768</cite>, More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP
<span class="doi">10.17487/RFC9768</span>
</li>