Explicit Congestion Notification (ECN) is an extension to the Internet Protocol The Internet Protocol is a protocol used for communicating data across a packet-switched internetwork using the Internet Protocol Suite, also referred to as TCP/IP and to the Transmission Control Protocol The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite (the other being Internet Protocol, or IP), so the entire suite is commonly referred to as TCP/IP. Whereas IP handles lower-level transmissions from computer to computer as a message makes its way and is defined in RFC 3168 (2001). ECN allows end-to-end notification of network congestion In data networking and queueing theory, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections. A consequence of these latter two is that incremental increases in offered load lead either only to small without dropping packets which results in better network efficiency and fewer packet retransmissions. ECN is an optional feature that is only used when both endpoints support it and are willing to use it. It also requires support from the underlying network.
Traditionally, 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 impeding congestion. The receiver of the packet echoes the congestion indication to the sender, which must react as though a packet was dropped.
ECN uses two bits next to[1] the Differentiated Services field in the IP header, in the IPv4 TOS octet or the IPv6 Traffic Class octet. These two bits initially encoded one of the values ECN-Capable Transport (ECT), Not-ECN-Capable Transport (Not-ECT) or Congestion Experienced (CE) [2] but were latter changed to encode four different codepoints (Non-ECT, ECT, ECT(1), CE) [3].
Some outdated or buggy network equipment drops packets with ECN bits set, rather than ignoring the bits.[4]
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Operation
ECN requires specific support at the internet and transport layers.
Operation of ECN with IP
ECN uses the two less significant (right-most) bits of the DiffServ field in the IP header to encode four different codepoints:
- 00: Non ECN-Capable Transport - Non-ECT
- 10: ECN Capable Transport - ECT
- 01: ECN Capable Transport - ECT(1)
- 11: Congestion Encountered - CE
When both endpoints support ECN they mark their packets with ECT 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 impeding 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 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 (a) support congestion control and (b) have a method for echoing the CE indication to the transmitting endpoint. This echoing is described as ECN support of the upper layer protocol.
Operation of ECN with TCP
TCP supports ECN using two flags in the TCP header. Those 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.
A node keeps transmitting TCP segments with the ECE bit set until it receives a segment with the CWR bit set.
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[5] 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[6].
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[7], 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[8] 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[9]. 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 hosts
Windows Server 2008 and Windows Vista support ECN but it is disabled by default.[10] ECN support can be enabled with the following shell command: netsh interface tcp set global ecncapability=enabled
Mac OS X 10.5 implements ECN. It is controlled using the following sysctl interface:[11] net.inet.tcp.ecn_negotiate_in, net.inet.tcp.ecn_initiate_out.
The Linux kernel has supported ECN. Its default behaviour is to support ECN if the other side supports it (server-mode). Most Linux distributions disable ECN. In most kernel versions, it can be activated through the sysctl interface: sysctl net.ipv4.tcp_ecn=1
FreeBSD 8.0 implements ECN. It can be activated through the sysctl interface: sysctl net.inet.tcp.ecn.enable=1
ECN support in 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, or by using the sfb 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.
See also
References
- ^ Grossman, D. (April 2002) New Terminology and Clarifications for Diffserv, IETF. RFC 3260.
- ^ Ramakrishnan, K (January 1999) A Proposal to add Explicit Congestion Notification (ECN) to IP], IETF. RFC 2481
- ^ Ramakrishnan, K, Floyd, S, Black, D, (September 2001) The Addition of Explicit Congestion Notification (ECN) to IP], IETF. RFC 3168
- ^ Measuring Interactions Between Transport Protocols and Middleboxes. Alberto Medina, Mark Allman, and Sally Floyd. Internet Measurement Conference 2004, August 2004.
- ^ Adding Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK Packets. Sally Floyd. Work in Progress. 2008
- ^ Aleksandar Kuzmanovic. The power of explicit congestion notification. In Proceedings of the 2005 conference on Applications, technologies, architectures, and protocols for computer communications. 2005.
- ^ Jamal Hadi Salim and Uvaiz Ahmed. Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks. RFC 2884. July 2000
- ^ Marek Malowidzki, Simulation-based Study of ECN Performance in RED Networks, In Proc. SPECTS'03. 2003.
- ^ Aleksandar Kuzmanovic. The power of explicit congestion notification. In Proceedings of the 2005 conference on Applications, technologies, architectures, and protocols for computer communications. 2005
- ^ ""New Networking Features in Windows Server 2008 and Windows Vista". http://technet.microsoft.com/en-us/library/bb726965.aspx.
- ^ ""ECN (Explicit Congestion Notification) in TCP/IP"". http://www.icir.org/floyd/ecn.html#implementations.
External links
Categories: Network performance | Transport layer protocols
