funknetz/docs/adr/0005-protocol-3-with-explicit-2-fallback.md

Announce protocol 3.0, fall back only on an explicit 2.x maximum

Every TCP connection announces protocol 3.0. The station opens one 2.0 session only when the reflector sends a downgrade, or an unsupported-version error, whose maximum is at least 2.0 and below 3.0. The connection after that 2.0 session ends announces 3.0 again. A maximum whose major is below 2 stops the station with the reflector’s message. A reflector MsgError stops the station until the operator quits. A bad CA signature, a TLS or hostname failure, a heartbeat timeout, and a dropped connection retry on 3.0, and the window keeps the reason between attempts. The Protocol line shows 2.0 or 3.0 while the session is up, so a plaintext session is visible. Staying on 3.0 after every failure was rejected because a reflector that still expects 2.0 would never accept the station. Falling back on any handshake failure was rejected because a TLS or CA problem would then send plaintext audio.