AIMD Algorithm

AIMD (Additive Increase Multiplicative Decrease) is a fundamental congestion control algorithm used in computer networks to dynamically adjust data transmission rates. It helps prevent network congestion while maximizing throughput by proactively increasing sending rates during low congestion and aggressively reducing them when congestion is detected.

AIMD is most notably implemented in TCP (Transmission Control Protocol) congestion control, making it one of the most widely deployed network algorithms on the Internet today.

How AIMD Works

The AIMD algorithm operates on two core principles:

• Additive Increase When no congestion is detected, incrementally increase the congestion window size by a fixed amount (typically 1 MSS per RTT).

• Multiplicative Decrease When congestion is detected (packet loss), reduce the congestion window size by a multiplicative factor (typically by half).

Congestion Detection

AIMD algorithms detect congestion through various signals:

• Packet Loss Missing acknowledgments indicate network congestion and buffer overflow.

• Timeout Events Retransmission timeouts suggest severe congestion.

• Duplicate ACKs Multiple duplicate acknowledgments indicate out-of-order packet delivery.

TCP AIMD Implementation

In TCP, AIMD is implemented through the congestion window (cwnd) mechanism:

// Additive Increase (per RTT)
if (no_packet_loss) {
    cwnd = cwnd + 1 MSS
}

// Multiplicative Decrease
if (packet_loss_detected) {
    cwnd = cwnd / 2
}

The algorithm transitions through different phases:

• Slow Start Exponential increase until reaching ssthresh (slow start threshold).

• Congestion Avoidance Linear increase using AIMD principles.

• Fast Recovery Rapid recovery from isolated packet losses.

Advantages and Disadvantages

Real-World Applications

AIMD is fundamental to modern Internet protocols:

• Web Browsing HTTP connections use TCP's AIMD for reliable data transfer.

• File Transfer FTP and peer-to-peer applications rely on AIMD for efficient large file transfers.

• Email SMTP uses TCP's congestion control for message delivery.

Conclusion

AIMD algorithm provides essential congestion control through its additive increase and multiplicative decrease strategy. While conservative in nature, it ensures network stability and fair bandwidth allocation among competing flows, making it a cornerstone of Internet reliability.