Editing Explicit Congestion Notification (section)

===Operation of ECN with IP===
ECN uses the two [[Least significant bit|least significant]] (right-most) bits of the [[IPv6 packet#Fixed header|Traffic Class]] field in the [[IPv4 header|IPv4]] or [[IPv6 header]] to encode four different code points:
* <code>00</code> &ndash; Not ECN-Capable Transport, Not-ECT
* <code>01</code> &ndash; ECN Capable Transport(1), ECT(1)
* <code>10</code> &ndash; ECN Capable Transport(0), ECT(0)
* <code>11</code> &ndash; Congestion Experienced, CE.

When both endpoints support ECN they mark their packets with ECT(0) or ECT(1). Routers treat the ECT(0) and ECT(1) codepoints as equivalent. If the packet traverses an [[active queue management]] (AQM) queue (e.g., a queue that uses [[random early detection]] (RED)) that is experiencing congestion and the corresponding [[router (computing)|router]] supports ECN, it may change the code point to <code>CE</code> instead of [[packet loss|dropping the packet]]. This act is referred to as "marking" and its purpose is to inform the receiving endpoint of impending [[network congestion|congestion]]. At the receiving endpoint, this congestion indication is handled by the upper layer protocol ([[transport layer]] protocol) and needs to be echoed back to the transmitting node in order to signal it to reduce its transmission rate.

Because the CE indication can only be handled effectively by an upper layer protocol that supports it, ECN is only used in conjunction with upper layer protocols, such as [[Transmission Control Protocol|TCP]], that support congestion control and have a method for echoing the CE indication to the transmitting endpoint.