Explicit Congestion Notification

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 if it detected a dropped packet.

Rather than responding properly or ignoring the bits, some outdated or faulty network equipment has historically dropped or mangled packets that have ECN bits set. As of 2015, measurements suggested that the fraction of web servers on the public Internet for which setting ECN prevents network connections had been reduced to less than 1%.

Passive support has existed in Ubuntu Linux since 12.04 and in Windows Server since 2012. Passive support in the most popular websites has increased from 8.5% in 2012 to over 70% in May 2017. Adoption across the Internet now requires clients to actively request ECN. In June 2015, Apple announced that ECN will be enabled by default on its supported and future products, to help drive the adoption of ECN signaling industry-wide.

Note that this article describes classic ECN as originally defined; the L4S protocol, published in 2023 and described in RFC 9330, has altered the interpretation of the ECN bits and the expected treatment of traffic compared to classic ECN behavior.

Operation

ECN requires specific support at both the Internet layer and the transport layer for the following reasons:

• In TCP/IP, routers operate within the Internet layer, while the transmission rate is handled by the endpoints at the transport layer.
• Congestion may be handled only by the transmitter, but since it is known to have happened only after a packet was sent, there must be an echo of the congestion indication by the receiver to the transmitter.

Without ECN, congestion indication echo is achieved indirectly by the detection of lost packets. With ECN, congestion is indicated by setting the ECN field within an IP packet to CE (Congestion Experienced) and is echoed back by the receiver to the transmitter by setting proper bits in the header of the transport protocol. For example, when using TCP, the congestion indication is echoed back by setting the ECE bit.

Operation of ECN with IP

Note: this describes the operation of ECN prior to the definition of L4S in RFC 9330, which reserves the use of the codepoint 01 for L4S traffic.

ECN uses the two least significant (right-most) bits of the Traffic Class field in the IPv4 or IPv6 header to encode four different code points:

• 00 – Not ECN-Capable Transport, Not-ECT
• 01 – ECN Capable Transport(1), ECT(1)
• 10 – ECN Capable Transport(0), ECT(0)
• 11 – 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 supports ECN, it may change the code point to CE instead of dropping the packet. This act is referred to as marking, and its purpose is to inform the receiving endpoint of impending 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 TCP, that support congestion control and have a method for echoing the CE indication to the transmitting endpoint.

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 (tcp[13] & 0xc0 != 0).

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 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.

Operation of ECN with other transport protocols

ECN is also defined for other transport layer protocols that perform congestion control, notably DCCP and Stream Control Transmission Protocol (SCTP). The general principle is similar to TCP, although the details of the on-the-wire encoding differ.

It is possible to use ECN with protocols layered above UDP. However, UDP requires that congestion control be performed by the application, and early UDP-based protocols such as DNS did not use ECN. More recent UDP-based protocols such as QUIC are using ECN for congestion control.