Congestion Avoidance
Congestion avoidance is a mechanism used to control service flows. Congestion avoidance allows the device to monitor network resource usage, such as queues and memory buffers. The system uses tail drop or WRED to discard packets when congestion occurs or worsens.
Tail drop is used to solve the congestion problem.
When the queue length reaches its maximum value, the packets that were added last (at the tail of the queue) are discarded. This packet drop policy may cause global TCP synchronization. As a result, TCP connections cannot be set up.
Below, three colors represent three TCP connections. When packets from multiple TCP connections are discarded, these TCP connections enter the congestion avoidance and slow start state. Traffic reduces, and then reaches a peak. Traffic volume varies greatly.
