TCP Congestion Handling and Congestion Avoidance Algorithms
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TCP Congestion Handling Mechanisms
We must then decide what to do with this information. The main TCP standard, RFC 793, includes very little information about TCP congestion handling issues. That is because early versions of TCP based solely on this standard didn't include congestion handling measuresit was problems with these early implementations that lead to the discovery that congestion was an important issue. The measures used in modern devices were developed over the years, and eventually documented in RFC 2001, TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms.
Here is a simplified description of each of the four techniques that comprise the name of this standard.
In the original implementation of TCP, as soon as a connection was established between two devices, they could each go hog wild, sending segments as fast as they liked as long as there was room in the other device's receive window. In a busy internetwork the sudden appearance of a large amount of new traffic could exacerbate any existing congestion.
To alleviate this, modern TCP devices are restrained in the rate at which they initially send segments. Each sender is at first restricted to sending only an amount of data equal to one full-sized segmentthat is, equal to the MSS value for the connection. Each time an acknowledgment is received, the amount of data the device can send is increased by the size of another full-sized segment. Thus, the device starts slow in terms of how much data it can send, with the amount it sends increasing until either the full window size is reached or congestion is detected on the link. In the latter case, the congestion avoidance feature, described below, is used.
When potential congestion is detected on a TCP link, a device responds by throttling back the rate at which it sends segments. A special algorithm is used that allows the device to drop the rate at which segments are sent quickly when congestion occurs. The device then uses the Slow Start algorithm just above to gradually increase the transmission rate back up again to try to maximize throughput without congestion occurring again.
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