TFRC

TFRC, or TCP-Friendly Rate Control, is a congestion control mechanism designed for streaming multimedia and other UDP-based traffic that seeks to share network bottlenecks fairly with TCP flows. Unlike TCP’s window-based control, TFRC models the sending rate as a function of network loss and delay, providing a steady, fluid rate that responds to congestion signals rather than reacting to each acknowledgement.

The sender maintains estimates of the loss-event rate p, the round-trip time RTT, and the maximum segment size MSS. The sending rate R is computed from a TCP throughput model, typically expressed as R = MSS / ( RTT * sqrt(2p/3) + RTO * min(1, 3 sqrt(3p/8) p) ). When a loss event is detected, the rate is reduced according to the model and then gradually increased according to the TCP-friendly rule until the next loss event. RTT is smoothed to reflect current latency; p is estimated from observed loss events over time.

TFRC is designed to be fair with TCP flows sharing the same bottleneck link and similar RTT; since it uses the TCP throughput model, it tends to yield throughput similar to a TCP flow under equivalent conditions, while allowing smoother sending rates that are better suited for streaming. It is commonly used to regulate UDP-based data transfers that require predictable bandwidth.

Limitations include sensitivity to accurate estimation of p and RTT, potential underutilization in highly variable networks, and the assumption of relatively steady-state congestion. TFRC continues to influence congestion control discussions for real-time and streaming applications.