Editing Explicit Congestion Notification (section)

====Linux====
Since version 2.4.20 of the [[Linux kernel]], released in November 2002,<ref>{{cite web
 |url         = https://datatag.web.cern.ch/datatag/papers/tr-datatag-2004-1.pdf
 |title       = A Map of the Networking Code in Linux Kernel 2.4.20, Technical Report DataTAG-2004-1, FP5/IST DataTAG Project
 |date        = March 2004
 |access-date  = 1 September 2015
 |website     = datatag.web.cern.ch
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20151027124019/http://datatag.web.cern.ch/datatag/papers/tr-datatag-2004-1.pdf
 |archive-date = 27 October 2015
}}</ref> Linux supports three working modes of the ECN for TCP, as configured through the [[sysctl]] interface by setting parameter {{Mono|/proc/sys/net/ipv4/tcp_ecn}} to one of the following values:<ref name="linux">{{cite web
 |url         = https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
 |title       = Documentation/networking/ip-sysctl.txt: /proc/sys/net/ipv4/* Variables
 |access-date  = 2016-02-15
 |publisher   = [[kernel.org]]
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20160305080444/https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
 |archive-date = 2016-03-05
}}</ref>
* {{Mono|0}}{{snd}} disable ECN and neither initiate nor accept it
* {{Mono|1}}{{snd}} enable ECN when requested by incoming connections, and also request ECN on outgoing connection attempts
* {{Mono|2}}{{snd}} (default) enable ECN when requested by incoming connections, but do not request ECN on outgoing connections

Beginning with version 4.1 of the Linux kernel, released in June 2015, the {{Mono|tcp_ecn_fallback}} mechanism{{Ref RFC|3168|rsection=6.1.1.1}} is enabled by default<ref>{{cite web
 |url         = http://man7.org/linux/man-pages/man7/tcp.7.html
 |title       = Linux man pages
 |date        = 2015-12-05
 |access-date  = 2016-02-15
 |website     = man7.org
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20160216231741/http://man7.org/linux/man-pages/man7/tcp.7.html
 |archive-date = 2016-02-16
}}</ref> when ECN is enabled (the value of 1).  The fallback mechanism attempts ECN connectivity in the initial setup of outgoing connections, with a graceful fallback for transmissions without ECN capability, mitigating issues with ECN-intolerant hosts or firewalls.