# blax-webrtc-sfu — the Rust data plane

The sidecar binary behind `Blax\WebRtc\Media\RustMediaEngine`: **backend-hosted
rooms**. PHP/Laravel owns the control plane (rooms, auth, signaling
orchestration, billing hooks); this process owns everything per-packet.

- **ICE** (Interactive Connectivity Establishment — finds a working network
  path to each browser), **DTLS** (the UDP TLS handshake that negotiates keys)
  and **SRTP** (the encrypted media packets) are terminated here via
  [str0m](https://github.com/algesten/str0m), a sans-IO Rust WebRTC stack.
- **SFU forwarding** (Selective Forwarding Unit): each peer's Opus audio is
  received once and forwarded to the other peers of its room **without
  decoding** — one loop thread, one shared UDP socket, `rtc.accepts()`
  demultiplexing.
- **Recording**: per-peer Ogg/Opus written straight from the depayloaded
  stream — playable by ffmpeg/browsers, no transcoding, no client cooperation.
- **Negotiation**: the initial offer/answer arrives via the control socket
  (from PHP signaling). Renegotiation as peers join rides each browser's
  WebRTC **data channel** directly — PHP never relays it (see str0m's `chat`
  example, which this follows).

## Control protocol (PHP ⇄ sidecar)

JSON-lines over a Unix socket. Requests carry an `id` echoed by the reply;
notices are pushed without one.

```
→ {"id":1,"cmd":"add_peer","room":"lobby","peer":"alice","offer":"v=0…","record":"/rec/alice.ogg"}
← {"id":1,"ok":true,"answer":"v=0…"}
→ {"id":2,"cmd":"mute_peer","room":"lobby","peer":"alice","muted":true}
→ {"id":3,"cmd":"record_start","room":"lobby","peer":"alice","path":"/rec/a.ogg"}   (record_stop too)
→ {"id":4,"cmd":"remove_peer","room":"lobby","peer":"alice"}
→ {"id":5,"cmd":"stats"}      ← rooms → peers → {connected, muted, seconds}
→ {"id":6,"cmd":"ping"} / {"id":7,"cmd":"shutdown"}
← {"event":"peer_connected","room":"lobby","peer":"alice"}
← {"event":"peer_left","room":"lobby","peer":"alice","seconds":42.5}
```

## Build & test

```bash
cargo build && cargo test          # 7 tests incl. an end-to-end control-socket run
./build-release.sh                 # static musl release + .sha256 into dist/
```

On NixOS: `nix-shell -p gcc pkg-config openssl perl gnumake musl --run ./build-release.sh`.

## Releasing

`BinaryManager::VERSION` (PHP) pins the release tag it downloads from. To ship:
build `dist/` artifacts (linux-x86_64 at minimum), attach both files to the
Forgejo release with that tag, and bump `VERSION` when the protocol changes.
PHP resolves binaries in this order: `webrtc.sfu.binary` override → a local
`target/{release,debug}` build (dev) → the downloaded install.

## CLI

```
blax-webrtc-sfu --socket /run/sfu.sock --udp-ip 0.0.0.0 --udp-port 41000 --public-ip 203.0.113.9
```

`--udp-port 0` (default) picks an ephemeral port — fine on one host, pin it in
production and publish it (UDP) on the container. `--public-ip` is what
browsers are told to reach in the ICE host candidate; it defaults to the
default-route interface address.