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.
Network protocols which use aggressive retries to compensate for packet loss tend to keep systems in a state of network congestion even after the initial load has been reduced to a level which would not normally have induced network congestion. Thus, networks using these protocols can exhibit two stable states under the same level of load. The stable state with low throughput is known as congestion collapse, a term which was first defined by John Nagle in RFC 896, written in 1984, when he identified problems with early implementations of the TCP network protocol over parts of the very early Internet. Congestion collapse is a condition where a system such as a data network has settled under load into a state where traffic demand is high but little useful throughput is available, with high levels of packet loss, delay, and delay variation.
Experience of widespread congestion collapse in the Internet in 1987 led to further improvements of the TCP congestion control mechanisms.
As discussed in Section 3, congestioncollapse was first reported in the mid 1980s [RFC896], and was largely due to TCP connections unnecessarily retransmitting packets that were either in transit or had already been received at the receiver.
Congestioncollapse from undelivered packets arises when bandwidth is wasted by delivering packets through the network that are dropped before reaching their ultimate destination.
Because congestioncollapse from undelivered packets consists of packets that waste valuable bandwidth only to be dropped downstream, this form of congestioncollapse is not possible in an environment where each flow traverses only one congested link, or where only a small number of packets are dropped at links downstream of the first congested link.
Congestion is increasingly being recognized as a problem of national importance that adversely affects both the economy and quality of life.
In 2003, congestion cost an estimated $63 billion in wasted fuel and time alone.
The final rule on Work Zone Safety and Mobility was published in the Federal Register (69 FR 54562) on September 9, 2004 with an effective date of October 12, 2007.
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