More Accurate ECN Feedback in TCP

The information below is for an old version of the document
Document Type Expired Internet-Draft (tcpm WG)
Authors Bob Briscoe  , Mirja K├╝hlewind  , Richard Scheffenegger 
Last updated 2020-09-06 (latest revision 2020-03-05)
Replaces draft-kuehlewind-tcpm-accurate-ecn
Stream Internet Engineering Task Force (IETF)
Expired & archived
pdf htmlized bibtex
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Stream WG state WG Document (wg milestone: Dec 2021 - Submit specification... )
On Agenda tcpm at IETF-111
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IESG IESG state Expired
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Responsible AD Martin Duke
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft can be found at


Explicit Congestion Notification (ECN) is a mechanism where network nodes can mark IP packets instead of dropping them to indicate incipient congestion to the end-points. Receivers with an ECN- capable transport protocol feed back this information to the sender. ECN is 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 Scalable Throughput (L4S) need more accurate ECN feedback information whenever more than one marking is received in one RTT. This document specifies a scheme to provide more than one feedback signal per RTT in the TCP header. Given TCP header space is scarce, it allocates a reserved header bit, that was previously used for the ECN-Nonce which has now been declared historic. 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 a new TCP option, which is never used on the TCP SYN.


Bob Briscoe (
Mirja K├╝hlewind (
Richard Scheffenegger (

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