Competitive Mode Enhancement for Delay-Based Congestion Control Algorithms

Introduction Congestion control algorithms can be categorized into loss-based and delay-based algorithms based on their congestion detection mechanisms. Loss-based congestion control algorithms typically fill the link buffer until packet loss occurs, then reduce the sending rate. In contrast, delay-based congestion control algorithms proactively reduce the sending rate when queuing delay increases. Representative delay-based congestion control algorithms (e.g., Vegas, FAST, Copa) measure RTT or queuing delay, calculate the expected throughput based on delay variations, determine congestion, and adjust the congestion window size. While delay-based algorithms generally exhibit lower packet loss rates and smaller queuing delays than loss-based algorithms, they fail to fairly share link bandwidth with loss-based algorithms (e.g., Cubic) when both types of traffic coexist.

Introducing Competitive Mode into Delay-Based Congestion Control Algorithms

Method for Determining Competitive Mode Determine whether the current flow is in a coexistence/competition phase with Cubic traffic based on the magnitude of queuing delay variation. The COPA provides a method to determine whether it is in competitive mode. Specifically, upon receiving an ACK, the algorithm calculates the RTT for each flow and maintains a historical minimum RTT value(min_rtt). Based on RTT samples, it records the maximum(max_delay) and minimum(min_delay) RTT values over a 4-RTT window. Competitive Mode is triggered based on the following inequality. min_delay < min_rtt + 0.1(max_delay - min_rtt) Formula 1 Here, max_delay and min_delay represent the maximum and minimum RTT values within the last 4 RTT intervals, and min_rtt is the historical minimum RTT. The difference between min_delay and min_rtt represents the minimum queuing delay at the bottleneck link during this period, while the difference between max_delay and min_rtt represents the maximum queuing delay. If the inequality is not satisfied, it indicates that the bottleneck link's queue has not emptied during this period, suggesting likely competition from Cubic-like flows. Consequently, the algorithm enters Competitive Mode. If the inequality holds, the algorithm operates in Default Mode.

Congestion Window Compensation in Competition Mode When the algorithm determines it is in Competition Mode, it introduces an additional congestion window gain factor to moderately increase the congestion window size. Cubic persistently fills the switch buffer, and a larger window size leads to more severe delay inflation. Accordingly, the congestion window can be adjusted based on delay variations to match Cubic's window size. Since the delay gradient effectively characterizes the rise and fall of delay, the congestion control algorithm computes this gradient every round trip time(RTT) and adjusts the window size accordingly. TBD.

Examples

Copa with Competitive Mode TBD.

Vegas with Competitive Mode TBD.

IANA Considerations TBD.

Security Considerations TBD.

Contributors The following people have substantially contributed to this document:

Acknowledgements TBD.