FAST TCP

FAST TCP (Fast Active queue size management Scaling Transmission Control Protocol) is a sender-side-only congestion control algorithm designed for high-speed networks with large bandwidth-delay products. Developed at Caltech, FAST TCP aims to achieve high utilization and fairness in networks where traditional TCP variants, such as TCP Reno, often struggle to perform optimally.

The core idea behind FAST TCP is to adjust the congestion window size based on a combination of round-trip time (RTT) measurements and a target queue delay. Unlike loss-based congestion control algorithms that react to packet loss events, FAST TCP proactively adjusts its sending rate based on network congestion signals inferred from RTT variations.

The congestion window update rule in FAST TCP typically involves two components: an additive increase term and a multiplicative decrease term. The additive increase component encourages the sender to increase its sending rate until the RTT reaches a target value. The multiplicative decrease component reacts to increased RTTs, suggesting congestion, by reducing the sending rate.

The algorithm utilizes two parameters: α and γ. α represents the gain for the additive increase term, and it is typically set to a small value. γ is used in the multiplicative decrease term and contributes to controlling the aggressiveness of the rate reduction when congestion is detected. The precise values of these parameters are crucial for achieving optimal performance and fairness.

A key advantage of FAST TCP is its ability to quickly adapt to changing network conditions and recover from congestion. It also exhibits better fairness compared to some other TCP variants, especially in heterogeneous network environments.

However, FAST TCP also has some challenges. Tuning the parameters α and γ appropriately can be complex and require careful consideration of the specific network environment. Furthermore, its reliance on RTT measurements makes it susceptible to inaccuracies caused by phenomena such as route changes or variations in background traffic. The effectiveness of FAST TCP depends on accurate RTT estimation and the responsiveness of the network infrastructure.