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What are issues in congestion control and how is it handled in TCP/IP?
The Congestion at the network layer came across two issues which are throughput and delay.
Based on Delay
When compared to capacity of the network, if load is less, the delay is minimum.
Here the minimum delay is composed of propagation delay and processing delay and both are negligible.
Therefore, when load reaches the network capacity, the delay increases because we must add the queuing delay to the total delay.
When the load is greater than the capacity the delay becomes infinite.
Based on Throughout
When load is below the capacity of the network, the throughput increases proportional to the load.
After the load reaches the capacity, we expect the throughput to remain constant, but instead the throughput declines sharply.
The reason behind this is discarding of packets by the routers.
When sender and receiver are communicating with each other in a network with the help of packets, if sender overflows the network with a greater number of packets, that situation is called congestion.
At the time of congestion, the traffic was not able to handle properly. It leads to reduced quality of service. The symptoms of congestion are excessive packet delay, packet loss and re transmission.
TCP handling Congestion
Transmission Control Protocol (TCP) congestion control techniques prevent congestion. TCP uses the congestion window maintained at the sender side. For each TCP session, a separate congestion window is maintained. This congestion window represents the maximum amount of data that can be sent into the network without being acknowledged.
Initially, the congestion window is set to one maximum segment size. After each ACK packet is received, the sender increases congestion window size by one MSS (Maximum Segment Size).
The formula used here is,
Congestion window size = congestion window size + MSS.
This phase is known as the slow start mechanism, when the slow start threshold is reached, TCP switches from slow start phase to congestion avoidance phase.
Now the formula changes to the following −
Congestion window size=congestion window size + MSS/congestion window size.
When TCP detects any packet loss, then,
Congestion window size=congestion window size -1
In this way, congestion is handled in TCP.
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