Early Packet Loss Feedback for WebRTC-based Mobile Video Telephony over Wi-Fi

We consider improving the performance of WebRTC-based mobile video telephony over a Wi-Fi link, which is susceptible to channel-caused or congestion-caused packet losses. Existing reactive packet loss mitigation mechanisms rely on end-to-end feedback, which takes about one round-trip time (RTT), causing excessive artifacts or video freeze in the received video in typical network scenarios. To address this problem, we propose early packet loss feedback (EPLF), where the MAC layer of the local Wi-Fi link sends a spoofed NACK to the Realtime Transport Protocol (RTP) layer if and only if the transmission failure is caused by a channel error, and the NACK immediately triggers an RTP layer retransmission. This significantly reduces the loss-feedback delay while improving the effectiveness of the RTP-layer congestion control. Analytic modeling and experimental results show that EPLF almost completely eliminates channel-caused video freezes.