Create webrtc.md

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+# WebRTC
+This is a new alternative video transfer mode available for Raspberry Pi 4 users with an HDMI-CSI bridge.
+It uses H.264 encoding instead of MJPEG and provides significantly less traffic consumption.
+
+# How to use
+If you use an OS image built after 2021.06.10, this mode will be available to you by default.
+If you are upgrading to an older version of the OS, you will need to manually enable the WebRTC gateway:
+```
+# systemctl enable --now kvmd-janus
+```
+Then restart the Web UI and then in the System menu you will see the video mode switch.
+
+# Subtleties
+The MJPEG video stream uses the same HTTP connection that you use to download the web interface.
+This means that for remote access, you just need to forward ports 80 and 443 on your router.
+
+In contrast, WebRTC is a completely different way of transmitting video.
+It uses a P2P connection and UDP. This reduces network load, but makes it difficult to configure
+- the server needs to know your network configuration in order to use it correctly.
+
+To implement this, the Pi-KVM checks which of the network interfaces is used for the default gateway,
+and also tries to find out your external address using the Google [STUN](https://en.wikipedia.org/wiki/STUN) server.
+This is necessary when using [Tailscale](tailscale.md) or so that you can connect to your Pi-KVM from the external Internet,
+since simply forwarding ports 80 and 443 for WebRTC is not enough - it requires a direct connection.
+
+If you don't like using Google (it was chosen as the default for reliability reasons) for this purpose,
+you can choose [any other STUN server](https://www.voip-info.org/stun/) at your discretion, or start your own.
+Use `/etc/kvmd/override.yaml` to this:
+```yaml
+janus:
+    stun:
+        host: stun.stunprotocol.org
+        port: 3478
+```
+And restart `systemctl restart kvmd-janus`.
+
+# KVMD-Janus vs. KVMD-Janus-Static
+[Janus](https://janus.conf.meetecho.com) is a WebRTC gateway that is used to transmit the [uStreamer](https://github.com/pikvm/ustreamer) video.
+KVMD-Janus is a wrapper around it, configuring and monitoring changes in the network configuration. This is suitable for most users.
+However, if your Pi-KVM is not connected to the Internet and/or you want to use a custom Janus configuration,
+then you should use the `kvmd-janus-static` service instead. You should first change the Janus configuration yourself.
+You can find it in `/etc/kvmd/janus/janus.jcfg`.
+```
+# systemctl disable --now kvmd-janus
+...
+# systemctl enable --now kvmd-janus-static
+```