Explicit Congestion Notification
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ECN is an extension to IP which allows advance notification of congestion.
ECN is only used when the two hosts signal that they want to use it. With this method, an ECN bit is placed in the IP header to explicitly notify the transmitter that the network link is congested. ECN is an alternative to the indirect packet delete congestion notification performed by the Random early detection (RED) and weighted random early detection (WRED) algorithms, but it requires explicit support by both hosts to be effective. Some outdated or buggy network equipment drops packets with the ECN bit set, rather than ignoring the bit.
When a router receives a packet marked as ECN capable and anticipates (using RED) congestion, it will set a flag notifying the sender to decrease its window size (sending rate). The intent is to avoid resending packets.
ECN uses two bits in the Differentiated Services field in the IP header, in the IPv4 TOS Byte or the IPv6 Traffic Class Octet. TCP adds two flags in its header to indicate, the sender to reduce the amount of information it sends.
[edit] See also
[edit] External links
- RFC 3168 – The Addition of Explicit Congestion Notification (ECN) to IP
- RFC 4774 – Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field
- http://www.icir.org/floyd/ecn.html – Sally Floyd's ECN page
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