More Accurate ECN Feedback in TCP

Document Type Expired Internet-Draft (tcpm WG)
Last updated 2020-01-09 (latest revision 2019-07-08)
Replaces draft-kuehlewind-tcpm-accurate-ecn
Stream IETF
Intended RFC status Experimental
Expired & archived
pdf htmlized bibtex
Stream WG state WG Document
Document shepherd No shepherd assigned
IESG IESG state Expired
Consensus Boilerplate Unknown
Telechat date
Responsible AD Magnus Westerlund
Send notices to (None)

This Internet-Draft is no longer active.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 an experimental 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. 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.)