Editing Explicit Congestion Notification (section)

===Operation of ECN with TCP===
TCP supports ECN using two flags in the TCP header.  The first, ''ECN-Echo'' (ECE) is used to echo back the congestion indication (i.e., signal the sender to reduce the transmission rate).  The second, ''Congestion Window Reduced'' (CWR), to acknowledge that the congestion-indication echoing was received.  Use of ECN on a TCP connection is optional; for ECN to be used, it must be negotiated at connection establishment by including suitable options in the SYN and SYN-ACK segments.

When ECN has been negotiated on a TCP connection, the sender indicates that IP packets that carry TCP segments of that connection are carrying traffic from an ECN Capable Transport by marking them with an ECT code point. This allows intermediate routers that support ECN to mark those IP packets with the CE code point instead of dropping them in order to signal impending congestion.

Upon receiving an IP packet with the ''Congestion Experienced'' code point, the TCP receiver echoes back this congestion indication using the ECE flag in the TCP header.  When an endpoint receives a TCP segment with the ECE bit it reduces its congestion window as for a packet drop.  It then acknowledges the congestion indication by sending a segment with the CWR bit set.

A node keeps transmitting TCP segments with the ECE bit set until it receives a segment with the CWR bit set.

To see affected packets with [[tcpdump]], use the filter predicate <code>(tcp[13] & 0xc0 != 0)</code>.

====ECN and TCP control packets====
Since the [[Transmission Control Protocol]] (TCP) does not perform congestion control on control packets (pure ACKs, SYN, FIN segments), control packets are usually not marked as ECN-capable.

A 2009 proposal{{Ref RFC|5562}} suggests marking SYN-ACK packets as ECN-capable.  This improvement, known as ECN+, has been shown to provide dramatic improvements to performance of short-lived TCP connections.<ref name="kuzmanovic">Aleksandar Kuzmanovic.  The power of explicit congestion notification. In ''Proceedings of the 2005 conference on Applications, technologies, architectures, and protocols for computer communications''. 2005.</ref>