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It is important to understand that congestion control mechanism, implemented in current TCP, controls congestion once it happens. Particularly, first of all, TCP repeatedly increases the load it imposes on the network by following the slow-start algorithm in order to find a point, which congestion occurs. That point is detected if TCP is informed that some packet is lost in the network. The size of TCP's congestion window, when congestion is detected, is the available bandwidth of the connection in view of TCP. Then, congestion control will be activated.
As opposed to TCP's strategy, a new attractive strategy is to predict when congestion is about to happen and then reduce the sending rate before data packets start being dropped. Such a mechanism is called congestion avoidance. It is implemented in either the router or the end node based on the chosen machanism.
RED (Random Early Detection) is one of congestion avoidance mechanisms. It is placed in gateways (routers). Each gateway is programmed to monitor its own queue length. When it detects that congestion is imminent, it notifys the source, which is believed to be the cause of congestion, by dropping one of its packets. Presented in RED paper by Sally Floyd and Van Jacobson, the algorithm that each gateway employs to detect whether congestion is impending:
The simulation showing an advantage of RED gateway over DropTail gateway:
Stated in RED paper by Sally Floyd and Van Jacobson, one advantage of RED gateway over DropTail gateway or Random Drop gateway is that RED gateway do not have a bias against bursty traffic. This fact will be shown by the following simulation.
Note! According to the simulation, with RED gateway, packets chosen to be dropped is not only packets from the bursty source. However, with DropTail gateway, only packets sent from the bursty source get dropped.
Tools:
1. Javis version 0.2 (Applet version)
2. ns version 2.1b6
Topology Desctiption: There are 4 nodes named "0", "1", "2", "3" and "4". "3" acts as a gateway (router) implementing RED mechanism in it. "0", "1" and "2" act as sources sending data packets to "Receiver" via "Gateway".
Link Description: There are four duplex-links. The characterization of each link is listed:
"0" & "3": duplex-link 6Mb 1ms DropTail
"1" & "3": duplex-link 6Mb 1ms DropTail
"2" & "3": duplex-link 2Mb 6ms DropTail
"3" & "4": duplex-link 2Mb 5ms RED (DropTail)
Assumptions:
1. "2" acts as a bursty source in this simulation because the link between it and "Gateway" has the highest delay and low bandwidth.
2. The maximum window size of "0" is 8.
3. The maximum window size of "1" is 8.
4. The maximum window size of "2" is 4.
5. The maximum queue size of the link between "3" and "4" is 15