WebRTC-HTTP Egress Protocol (WHEP)

FFmpeg can send WebRTC streams via WHIP but has no way to receive them. This project fills that gap by implementing a WHEP demuxer in libavformat, built on top of the existing WHIP code. The plan is to reuse SDP, ICE, DTLS, and SRTP as much as possible and just flip the direction. The end goal is making ffplay whep://server/stream work, with a clean patch series ready for ffmpeg-devel.