Network congestion avoidance is a process used in computer networks to avoid congestion. The fundamental problem is that all network resources are limited, including router processing time and link throughput. Users can easily overload certain networking resources (as in a denial of service attack), making the network unusable, unless steps are taken to prevent this. Implementations of connection-oriented protocols, such as the widely-used TCP protocol, generally watch for packet errors, losses, or delays (see Quality of Service) in order to adjust the transmit speed. There are many different network congestion avoidance processes, since there are a number of different trade-offs available. A computer network is a system for communication among two or more computers. ... In data networking and queueing theory, network congestion occurs when incremental increases in offered load lead either only to small increases in network throughput, or to an actual reduction in network throughput. ... A Linksys NAT router, popular for home and small office networks A router is a computer networking device that forwards data packets toward their destinations through a process known as routing. ... BUSINESS In the business management theory of constraints, throughput is the rate at which a system produces money, in contrast to output, which may be sold or stored in a warehouse. ... A denial-of-service attack (also, DoS attack) is an attack on a computer system or network that causes a loss of service to users, typically the loss of network connectivity and services by consuming the bandwidth of the victim network or overloading the computational resources of the victim system. ... The word protocol derives from a Greek phrase meaning first leaf, referring to the first draft of a treaty. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. ... In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service (QoS) refers to the probability of the telecommunication network meeting a given traffic contract, or in many cases is used informally to refer the probability of a packet succeeding in passing between two...

TCP/IP congestion avoidance

Problems occur when many concurrent TCP-flows are experiencing port queue buffer tail-drops. Then TCP's automatic congestion avoidance is not enough. All flows that experience port queue buffer tail-drop, will begin a TCP retrain at the same moment - this is called TCP global synchronization. In computing, a port (derived from seaport) is usually a connection through which data is sent and received. ... TCP global synchronization in data networking can happen to TCP/IP flows during periods of congestion because each sender will reduce their transmission rate at the same time when packet loss occurs. ...

RED

One solution is to use RED (Random Early Detection) on network equipments port queue buffer. On network equipment ports with more than one queue buffer, WRED (Weighted Random Early Detection) could be used if available. RED indirectly signals to sender and receiver by deleting some packets, eg. when the average queue buffer lengths are more than 50% filled and deletes exponentially more and more packets, when the average queue buffer lengths are approaching 100%. The average queue buffer lengths are computed over 1 second at a time. ssssss In computing, a port (derived from seaport) is usually a connection through which data is sent and received. ... Computer networking devices are units that mediate data in a computer network. ... In computing, a port (derived from seaport) is usually a connection through which data is sent and received. ... In mathematics, a quantity that grows exponentially is one that grows at a rate proportional to its size. ...

Flowbased-RED/WRED

Some network equipment are equipped with ports that can follow and measure each flow (flowbased-RED/WRED) and are hereby able to signal to a too big bandwidth flow according to some QoS policy. A policy could divide the bandwidth among all flows by some criteria.

IP ECN

Another approach is to use IP ECN (Explicit Congestion Notification). ECN is only used when the two hosts signal that they want to use it. With this method, an ECN bit is used to signal that there is explicit congestion. This is better in some ways than the indirect packet delete congestion notification performed by the RED/WRED algorithms, but it requires explicit support by both hosts to be effective. Some outdated or buggy network equipment drops packets with the ECN bit set, rather than ignoring the bit. The Internet Protocol (IP) is a data-oriented protocol used by source and destination hosts for communicating data across a packet-switched internetwork. ...

When router receives packet marked as ECN capable and anticipates (using RED) congestion, it will set a flag notyfing sender to decrease its window (sending rate). The intent is to avoid resending packets.

Cisco AQM: Dynamic buffer limiting (DBL)

Cisco has taken a step further in their Catalyst 4000 series with engine IV. Engine IV has the possibility to classify all connection-oriented flows in "aggressive" (bad) and "adaptive" (good). It ensures that no flows fill the port queues for a long time. DBL can utilize IP ECN instead of packet-delete-signalling. See the sections in: Active Queue Management and Enabling Dynamic Buffer Limiting. Cisco may mean: Cisco (fish) Coregonus artedi, or any of several related fish species in Coregonus (Salmonidae) Cisco Systems, a computer networking company CISCO Security Private Limited, a security company in Singapore Abbreviation of Commercial and Industrial Security Corporation, a statutory board in Singapore An abbreviation for the name of...

Good things about active queue management (RED, WRED, ECN, Cisco DBL)

"Recommendations on Queue Management and Congestion Avoidance in the Internet" (RFC 2309) states that:

• Less packets will be dropped with Active Queue Management (AQM).
• The link utilization will increase because less TCP global synchronization will occur.
• By keeping the average queue size small, queue management will reduce the delays and jitter seen by flows.
• The connection bandwidth will be more equally shared among connection oriented flows, even without flow-based RED or WRED.

FLOW is a J-Pop group. ...

See also

• Traffic Shaping
• Quality of Service
• Exponential backoff

Traffic shaping is an attempt to control computer network traffic in order to optimize or guarantee performance, low-latency, and/or bandwidth. ... In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service (QoS) refers to the probability of the telecommunication network meeting a given traffic contract, or in many cases is used informally to refer the probability of a packet succeeding in passing between two... Exponential backoff is an algorithm that uses feedback to multiplicatively decrease the rate of some process, in order to gradually find an acceptable rate. ...

External links

• Sally Floyd: RED (Random Early Detection) Queue Management
• April 1998, RFC2309: Recommendations on Queue Management and Congestion Avoidance in the Internet, Original RFC2309
• RFC2001: TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms, Original RFC2001
• TCP Congestion Control, Original RFC2581
• TCP Increasing TCP's Initial Window, Original RFC3390
• The Addition of Explicit Congestion Notification (ECN) to IP, Original RFC3168
• Linktionary term: Queuing
• Comparative study of RED, ECN and TCP Rate Control (1999)
• Pierre-Francois Quet, Sriram Chellappan, Arjan Durresi, Mukundan Sridharan, Hitay Ozbay, Raj Jain, " Guidelines for optimizing Multi-Level ECN, using fluid flow based TCP model"
• Sally Floyd, Ratul Mahajan, David Wetherall: RED-PD: RED with Preferential Dropping
• TCP Tunnels: Avoiding Congestion Collapse (2000)