Instant Messaging Meets Video Conferencing: Studying the Performance of IM Video Calls

Video conferencing applications typically use UDP and often implement their own congestion control. Research studying these custom algorithms generally finds that they do react to congestion and can hold their own against competing TCP flows. However, these works focus on applications specializing in video conferencing, neglecting those that only offer video conferencing as one of many features, such as many instant messengers. While these instant messaging-based video call applications (IMVCAs) may opt to use standardized frameworks, such as WebRTC, their actual implementations and behaviors are wildly unknown. In this paper, we thus set out to study the behavior of three popular IMVCAs, analyzing their interplay with TCP and the impact on QoE. We find that the surveyed IMVCAs (Signal, Telegram, WhatsApp) are TCP-friendly, i.e., they do not choke TCP. However, their per-app behavior differs significantly and no app equals the other: Signal and Telegram, e.g., take TCP-friendliness too far, yielding up to 90 % of their bandwidth. This results in severe QoE detriments in the form of drastically reduced sending rates and visual quality. As Signal is known to use WebRTC, this finding suggests that the current variant might be too conservative for coexisting with TCP. In contrast, WhatsApp counters congestion by filling queues to avoid losing bandwidth. Overall, IMVCAs do not keep up with the performance of specialized applications.