This document specifies the addition of Explicit Congestion Notification (ECN) to TCP and IP, including the use of two bits in the IP header. ECN is designed to provide a congestion indication to transport protocols, allowing them to react more efficiently to transient congestion. The document outlines the mechanisms for ECN in IP and TCP, including the use of the Congestion Experienced (CE) codepoint in the IP header and new flags in the TCP header. It discusses the benefits of ECN, such as reducing the impact of packet losses on latency-sensitive flows, and addresses issues related to non-compliance by end nodes and routers, as well as the handling of ECN in IP tunnels and IPsec tunnels. The document also provides guidelines for differentiating between ECT(0) and ECT(1) codepoints and discusses the implications of subverting end-to-end congestion control.This document specifies the addition of Explicit Congestion Notification (ECN) to TCP and IP, including the use of two bits in the IP header. ECN is designed to provide a congestion indication to transport protocols, allowing them to react more efficiently to transient congestion. The document outlines the mechanisms for ECN in IP and TCP, including the use of the Congestion Experienced (CE) codepoint in the IP header and new flags in the TCP header. It discusses the benefits of ECN, such as reducing the impact of packet losses on latency-sensitive flows, and addresses issues related to non-compliance by end nodes and routers, as well as the handling of ECN in IP tunnels and IPsec tunnels. The document also provides guidelines for differentiating between ECT(0) and ECT(1) codepoints and discusses the implications of subverting end-to-end congestion control.