Editing Explicit Congestion Notification (section)

===ECN support in TCP by hosts===

====Microsoft Windows====
Windows versions since Windows Server 2008 and Windows Vista support ECN for TCP.<ref name=win>{{cite web | url = https://technet.microsoft.com/en-us/library/bb726965.aspx | title = New Networking Features in Windows Server 2008 and Windows Vista | url-status = live | archive-url = https://web.archive.org/web/20100115195304/http://technet.microsoft.com/en-us/library/bb726965.aspx | archive-date = 2010-01-15 }}</ref> Since Windows Server 2012, it is enabled by default in Windows Server versions, because [[#DCTCP|Data Center Transmission Control Protocol]] (DCTCP) is used.<ref>{{cite web | url = https://technet.microsoft.com/en-us/library/hh997028.aspx | title = Data Center Transmission Control Protocol (DCTCP) (Windows Server 2012) | url-status = live | archive-url = https://web.archive.org/web/20170826132305/https://technet.microsoft.com/en-us/library/hh997028.aspx | archive-date = 2017-08-26 }}</ref> In previous Windows versions and non-server versions it is disabled by default.

ECN support can be enabled using a shell command such as {{code|netsh interface tcp set global ecncapability{{=}}enabled}}.

====BSD====
On [[FreeBSD]], ECN for TCP can be configured using the {{Mono|net.inet.tcp.ecn.enable}} [[sysctl]]. By default, it is enabled only for incoming connections that request it. It can also be enabled for all connections or disabled entirely.<ref>{{cite web |title=tcp(4) - Internet Transmission Control Protocol |url=https://www.freebsd.org/cgi/man.cgi?query=tcp |website=FreeBSD Kernel Interfaces Manual |access-date=3 April 2020}}</ref>

[[NetBSD]]&nbsp;4.0 implements ECN support for TCP; it can be activated through the [[sysctl]] interface by setting 1 as value for the {{code|sysctl net.inet.tcp.ecn.enable}} parameter.<ref>{{cite web|url=http://netbsd.org/releases/formal-4/NetBSD-4.0.html|title=Announcing NetBSD 4.0|date=2007-12-19|access-date=2014-10-13|url-status=live|archive-url=https://web.archive.org/web/20141031073922/http://netbsd.org/releases/formal-4/NetBSD-4.0.html|archive-date=2014-10-31}}</ref>

Likewise, the sysctl {{Mono|net.inet.tcp.ecn}} can be used in [[OpenBSD]].<ref>{{cite book|url=https://books.google.com/books?id=PN6Xy9zWAbsC&pg=PA347 |title=Absolute OpenBSD: UNIX for the Practical Paranoid |author=Michael Lucas |year=2013 |publisher=No Starch Press |isbn=9781593274764 |access-date=2014-03-22}}</ref>

====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.

====Mac OS X====
[[Mac OS X]] 10.5 and 10.6 implement ECN support for TCP. It is controlled using the Boolean [[sysctl]] variables {{Mono|net.inet.tcp.ecn_negotiate_in}} and {{Mono|net.inet.tcp.ecn_initiate_out}}.<ref name="mac">{{cite web | url = http://www.icir.org/floyd/ecn.html#implementations | title = ECN (Explicit Congestion Notification) in TCP/IP | url-status = live | archive-url = https://web.archive.org/web/20120619200913/http://www.icir.org/floyd/ecn.html#implementations | archive-date = 2012-06-19 }}</ref>  The first variable enables ECN on incoming connections that already have ECN flags set; the second one tries to initiate outgoing connections with ECN enabled. Both variables default to {{Mono|0}},{{Failed verification|date=February 2025}} but can be set to {{Mono|1}} to enable the respective behavior.

In June 2015, [[Apple Inc.]] announced that [[OS X 10.11]] would have ECN turned on by default,<ref name="apple-wwdc-719" /> but the OS shipped without that default behavior. In macOS Sierra, ECN is enabled for half of TCP sessions.<ref>{{cite web|url=https://arstechnica.com/gadgets/2016/09/macos-10-12-sierra-the-ars-technica-review/6/|title=macOS 10.12 Sierra: The Ars Technica review|website=Ars Technica|date=20 September 2016|access-date=25 April 2018|url-status=live|archive-url=https://web.archive.org/web/20180426012040/https://arstechnica.com/gadgets/2016/09/macos-10-12-sierra-the-ars-technica-review/6/|archive-date=26 April 2018}}</ref>

====iOS====
In June 2015, [[Apple Inc.]] announced that [[iOS 9]], its next version of iOS, would support ECN and have it turned on by default.<ref name="apple-wwdc-719" /> TCP ECN negotiation is enabled on 5% of randomly selected connections over Wi-Fi / Ethernet in iOS 9 and 50% of randomly selected connections over Wi-Fi / Ethernet and a few cellular carriers in [[iOS 10]]<ref>{{cite web|url=https://developer.apple.com/videos/play/wwdc2016/714/|title=Networking for the Modern Internet - WWDC 2016 - Videos - Apple Developer|first=Apple|last=Inc.|website=Apple Developer|access-date=18 April 2018|url-status=live|archive-url=https://web.archive.org/web/20180418161153/https://developer.apple.com/videos/play/wwdc2016/714/|archive-date=18 April 2018}}</ref><ref>{{cite web |url=https://www.ietf.org/proceedings/98/slides/slides-98-maprg-tcp-ecn-experience-with-enabling-ecn-on-the-internet-padma-bhooma-00.pdf |title=TCP ECN — Experience with enabling ECN on the Internet |first=Padma|last=Bhooma |date=March 2017 |access-date=2017-05-03 |url-status=live |archive-url=https://web.archive.org/web/20180509153808/https://www.ietf.org/proceedings/98/slides/slides-98-maprg-tcp-ecn-experience-with-enabling-ecn-on-the-internet-padma-bhooma-00.pdf |archive-date=2018-05-09 }}</ref> and 100% for [[iOS 11]]<ref>{{cite web|url=https://developer.apple.com/videos/play/wwdc2017/707/|title=Advances in Networking, Part 1 - WWDC 2017 - Videos - Apple Developer|first=Apple|last=Inc.|website=Apple Developer|access-date=18 April 2018|url-status=live|archive-url=https://web.archive.org/web/20180131043815/https://developer.apple.com/videos/play/wwdc2017/707/|archive-date=31 January 2018}}</ref>

====Solaris====
The [[Solaris (operating system)|Solaris]] kernel supports three states of ECN for TCP:<ref>{{cite web |title=ipadm(8) |url=https://docs.oracle.com/cd/E88353_01/html/E72487/ipadm-8.html |website=Oracle Solaris 11.4 Information Library |publisher=Oracle |access-date=6 May 2021}}</ref>
* {{Mono|never}}{{snd}} no ECN
* {{Mono|active}}{{snd}} use ECN
* {{Mono|passive}}{{snd}} only advertise ECN support when asked for.

As of Solaris 11.4, the default behavior is {{Mono|active}}. ECN usage can be modified via {{Mono|ipadm set-prop -p ecn{{=}}active tcp}}.<ref>{{cite web |title=Administering TCP/IP Networks, IPMP, and IP Tunnels in Oracle® Solaris 11.4, Using the TCP ECN Feature |url=https://docs.oracle.com/cd/E37838_01/html/E60991/gqyxf.html |website=Oracle Solaris 11.4 Information Library |publisher=Oracle |access-date=6 May 2021}}</ref>