Explicit Congestion Notification

Explicit Congestion Notification (ECN) is an extension to the Internet Protocol and to the Transmission Control Protocol. ECN allows end-to-end notification of network congestion without dropping packets. ECN is an optional feature that is only used when both endpoints support it and are willing to use it. It is only effective when supported by the underlying network.

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 though it detected a dropped packet.

Operation

ECN requires specific support at the Internet and Transport layers for the following reasons:
In TCP/IP routers operate mostly on the Internet layer while 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, the congestion is indicated by setting the CE bits at an IP packet and is echoed back by the receiver to the transmitter by setting proper bits in the Transport Layer's protocol header. For example, when using TCP, the congestion indication is echoed back by setting the ECE bit.

Operation of ECN with IP

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

• 00: Non ECN-Capable Transport — Non-ECT
• 10: ECN Capable Transport — ECT(0)
• 01: ECN Capable Transport — ECT(1)
• 11: Congestion Encountered — CE

When both endpoints support ECN they mark their packets with ECT(0) or ECT(1). 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 codepoint 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 (e.g. 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 three flags in the TCP header. The first one, the Nonce Sum (NS), is used to protect against accidental or malicious concealment of marked packets from the TCP sender. The other two bits are used to echo back the congestion indication (i.e. signal the sender to reduce the amount of information it sends) and to acknowledge that the congestion-indication echoing was received. These are the ECN-Echo (ECE) and Congestion Window Reduced (CWR) bits.

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 codepoint. This allows intermediate routers that support ECN to mark those IP packets with the CE codepoint instead of dropping them in order to signal impending congestion.

Upon receiving an IP packet with the Congestion Experienced codepoint, 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.

To see affected packets with Tcpdump, use the filter predicate (tcp[13] & 0xc0 != 0).

ECN and TCP control packets

Since TCP does not perform congestion control on control packets (pure ACKs, SYN, FIN segments), control packets are usually not marked as ECN-capable.

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

Operation of ECN with other transport protocols

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

It should in principle be possible to use ECN with protocols layered above UDP. However, UDP requires that congestion control be performed by the application, and current networking APIs do not give access to the ECN bits.

Effects on performance

Since ECN is only effective in combination with an Active Queue Management (AQM) policy, the benefits of ECN depend on the precise AQM being used. A few observations, however, appear to hold across different AQMs.

As expected, ECN reduces the number of packets dropped by a TCP connection, which, by avoiding a retransmission, reduces latency and especially jitter. This effect is most drastic when the TCP connection has a single outstanding segment, when it is able to avoid an RTO timeout; this is often the case for interactive connections (such as remote logins) and transactional protocols (such as HTTP requests, the conversational phase of SMTP, or SQL requests).

Effects of ECN on bulk throughput are less clear because modern TCP implementations are fairly good at resending dropped segments in a timely manner when the sender's window is large.

Use of ECN has been found to be detrimental to performance on highly congested networks when using AQM algorithms that never drop packets. Modern AQM implementations avoid this pitfall by dropping rather than marking packets at very high load.

Implementations

Many modern implementations of the TCP/IP protocol suite have some support for ECN; however, they usually ship with ECN disabled.

ECN support in TCP by hosts

• Microsoft Windows - Windows versions since Windows Server 2008 and Windows Vista support ECN for TCP, but it is disabled by default.[9] ECN support can be enabled with the following shell command:

                       netsh interface tcp set global ecncapability=enabled

• Apple Macintosh - Mac OS X 10.5 and 10.6 implements ECN support for TCP. It is controlled using the following boolean sysctl variables:

            net.inet.tcp.ecn_negotiate_in
            net.inet.tcp.ecn_initiate_out

The first enables ECN on incoming connections that already have ECN flags set; the second tries to initiate outgoing connections with ECN enabled. Both variables default to 0, but can be set to 1 to enable the respective behavior:

            sysctl -w net.inet.tcp.ecn_negotiate_in=1
            sysctl -w net.inet.tcp.ecn_initiate_out=1

To make the settings persistent, put following lines in /etc/sysctl.conf:

            net.inet.tcp.ecn_initiate_out=1
            net.inet.tcp.ecn_negotiate_in=1

Unix-like

• Linux - The Linux kernel supports three states of ECN for TCP:

0 = No ECN
1 = Use ECN
2 = "Support ECN"/"Server Mode", i.e. only advertise ECN support when asked for

Its default behaviour is to support ECN if the other side supports it (server-mode). In most kernel versions, full ECN usage (1) can be activated through the sysctl interface: sysctl net.ipv4.tcp_ecn=1

BSD - FreeBSD 8.0 implements ECN support for TCP. It can be activated through the sysctl interface:
                 sysctl net.inet.tcp.ecn.enable=1

ECN support in IP by routers

Since ECN marking in routers is dependent on some form of active queue management, routers must be configured with a suitable queue discipline in order to perform ECN marking.

Cisco IOS routers perform ECN marking if configured with the WRED queuing discipline since version 12.2(8)T.

Linux routers perform ECN marking if configured with one of the RED or GRED queue disciplines with an explicit ecn parameter, by using the sfb discipline, or by using the CoDel Fair Queueing (fq_codel) discipline.

Modern BSD implementations, such as FreeBSD, NetBSD and OpenBSD, have support for ECN marking in the ALTQ queueing implementation for a number of queuing disciplines, notably RED and Blue.