Published on: 6 June, 2023

Briefly describe the congestion control, slow start and fast retransmit mechanism.


Congestion occurs when the load network i.e, the number of packets sent to the network is greater than the capacity of the network. Congestion control refers to the mechanisms and techniques to control the congestion and control the load below the capacity. The congestion control mechanism are divided into two categories: 1) open loop congestion control (prevention) 2) closed loop congestion control(removal). the congestion control in TCP consists of three phases: 1) slow start phase(exponential incease) 2) congestion avoidance(additive increase) 3)congestion detection(multiplicative decrease). In the slow star phase the sender starts with a very slow rate of transmission, but increases the rate rapidly to reach a threshold. When the threshold is reached, the data rate is reduced to avoid congestion. Finally if congestion is detected, the sender goes back to the slow start phase or the congestion avoidance phase based on how the congestion is detected.

The slow start phase starts with a size of the congestion window equal to one maximum segment size(MSS). The MSS is determined during connection establishment. The size of the window increases one MSS each time an acknowledgement is received. The window size starts slowly but grows exponentially until it reaches a threshold called slow start threshold(ssthresh). When the threshold of slow start phase is reached, the congestion avoidance or the additive increase phase starts.

The main method to control errors is retransmission of segments. The segment is retransmitted in case of two situations: 1) when a retransmission timer expires 2) when the sender receives two duplicate ACKs. the first method is suitable when the size of the RTO is not very large. but there are situations when the size of the RTO is very large, in that case fast retransmission mechanisms are used. In this mechanism, one segment is lost and the receiver receives so many out of order segments that they cannot be saved. To remove this situation, after receiving three duplicate ACKSs, the missing segment is retransmitted immediately.

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