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 network congestion without dropping packets. ECN is an optional feature that is only used when both endpoints support it and are willing to use it. It is only effective when supported by the underlying network.
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 though it detected a dropped packet.
Rather than responding properly or ignoring the bits, some outdated or buggy network equipment drop packets which have ECN bits set.
| Internet protocols |
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| Application layer |
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| Transport layer |
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| Routing protocols * |
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| Internet layer |
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| Link layer |
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| * Not a layer. A routing protocol belongs either to application or network layer. |
Read more about Explicit Congestion Notification: Operation, Effects On Performance, Implementations
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