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Explicit Congestion Notification

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  1. Explicit Congestion Notification
Explicit Congestion Notification
OSI layerNetwork layer / Transport layer
IP numberIP (IPv4, IPv6), TCP, DCCP
RFCs3168 (Core Specification)
8311 (Relaxing Restrictions)
9768 (More Accurate Feedback)
HardwareSwitches and Routers

Preview warning: Page using Template:Infobox networking protocol with unknown parameter "acronym"

Explicit Congestion Notification (ECN) is an extension to the Internet Protocol and to the Transmission Control Protocol and is defined in RFC 3168 (2001). ECN allows end-to-end notification of without dropping packets. ECN is an optional feature that may be used between two ECN-enabled endpoints when the underlying network infrastructure also supports it.

network congestion

Conventionally, TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a mark in the IP header instead of dropping a packet in order to signal impending congestion. The receiver of the packet echoes the congestion indication to the sender, which reduces its transmission rate as if it detected a dropped packet.

Rather than responding properly or ignoring the bits, some outdated or faulty network equipment has historically dropped or mangled packets that have ECN bits set.123 As of 2015, measurements suggested that the fraction of web servers on the public Internet for which setting ECN prevents network connections had been reduced to less than 1%.4

Passive support has existed in Ubuntu Linux since 12.04 and in Windows Server since 2012.5 Passive support in the most popular websites has increased from 8.5% in 2012 to over 70% in May 2017.5 Adoption across the Internet now requires clients to actively request ECN. In June 2015, Apple announced that ECN will be enabled by default on its supported and future products, to help drive the adoption of ECN signaling industry-wide.6

Note that this article describes classic ECN as originally defined; the L4S protocol, published in 2023 and described in RFC 9330, has altered the interpretation of the ECN bits and the expected treatment of traffic compared to classic ECN behavior.

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General
  • Recent changes
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This article
Reading time
9 min
Topic
Computer Science
Quality
C-class
Views
630 in 14 days
Last edited
8 Nov 2026
Print/export
  • Printable version
In other projects
  • Wikidata item
Last edited
8 Nov 2026