Tcp Ip Protocol Questions Long
Congestion control is an essential aspect of the TCP/IP protocol suite, which aims to prevent network congestion and ensure efficient data transmission. TCP's congestion avoidance algorithms play a crucial role in achieving this objective. The process of congestion control using TCP's congestion avoidance algorithms can be described as follows:
1. Slow Start: When a TCP connection is established, the sender starts in the slow start phase. In this phase, the sender initially sends a small number of packets, known as the congestion window (cwnd), to the receiver. The cwnd size is typically set to a conservative value, such as one or two segments.
2. Congestion Window Growth: As the sender receives acknowledgments for the transmitted packets, it increases the cwnd size exponentially. This exponential growth allows TCP to probe the network capacity quickly. The cwnd size is doubled after each round-trip time (RTT), resulting in a rapid increase in the number of packets sent.
3. Congestion Detection: During the transmission, TCP monitors the network for signs of congestion. It does so by observing the occurrence of packet loss or receiving duplicate acknowledgments. Packet loss is considered an indication of network congestion, as it suggests that the network is unable to handle the current traffic load.
4. Congestion Response: Upon detecting congestion, TCP enters the congestion avoidance phase. In this phase, the cwnd growth rate is reduced to prevent further congestion. TCP achieves this by increasing the cwnd size linearly rather than exponentially. The cwnd is incremented by a smaller value for each RTT, ensuring a more conservative approach to network capacity probing.
5. Fast Retransmit and Recovery: In case of packet loss, TCP employs a fast retransmit mechanism to recover from the loss quickly. When TCP receives three duplicate acknowledgments for the same packet, it assumes that the packet has been lost and retransmits it immediately, without waiting for the retransmission timer to expire. This helps in reducing the delay caused by waiting for the timer to expire.
6. Timeout and Recovery: If a packet loss is not detected through duplicate acknowledgments, TCP relies on a retransmission timer. If the timer expires before an acknowledgment is received, TCP assumes that the packet has been lost and retransmits it. Additionally, TCP reduces the cwnd size to a conservative value and restarts the slow start phase to avoid overwhelming the network.
7. Additive Increase, Multiplicative Decrease (AIMD): TCP's congestion avoidance algorithms follow the principle of additive increase and multiplicative decrease. During congestion avoidance, TCP increases the cwnd size linearly by a small value for each RTT. However, upon detecting congestion, TCP reduces the cwnd size multiplicatively by halving it. This approach ensures a conservative response to congestion, gradually increasing the sending rate while quickly reducing it upon congestion detection.
By employing these congestion avoidance algorithms, TCP effectively manages network congestion, ensuring reliable and efficient data transmission. The algorithms allow TCP to adapt to varying network conditions, dynamically adjusting the sending rate to prevent congestion and maintain network stability.