Group call protocol
Three topologies for multiple participants: mesh (peer-to-peer between all, max 3-4), MCU (server transcodes and mixes, high CPU), SFU (server forwards without transcoding — today's standard).
WebRTC-based calls: all major platforms (Meet, Zoom, Teams, Discord) use SFU. Simulcast: each participant sends 2-3 quality versions; the SFU picks the right one per recipient. Cascaded SFU for global scaling (server network across continents). E2EE in group calls is hard — Signal solves it via a Group MLS-like key tree. Zoom's early E2EE claim was false (the server had the keys) — fixed in 2021. Jitsi uses insertable streams for end-to-end E2EE while the SFU just routes packets.