TT#89352 document Janus replacement mode in README

Change-Id: I95832d1916643b0ec7602b34775785397789441e

README.md

@@ -67,6 +67,9 @@ the following additional features are available:
 * Media forking
 * Publish/subscribe mechanism for N-to-N media forwarding
 
+There is also limited support for *rtpengine* to be used as a drop-in
+replacement for *Janus* using the native Janus control protocol (see below).
+
 *Rtpengine* does not (yet) support:
 
 * ZRTP, although ZRTP passes through *rtpengine* just fine
@@ -2327,3 +2330,40 @@ Prometheus Stats Exporter
 -------------------------
 
 The Prometheus metrics can be found under the URI `/metrics`.
+
+*Janus* Interface and Replacement Functionality
+===============================================
+
+*Rtpengine* supports a limited and narrow subset of the features provided by
+[Janus](https://janus.conf.meetecho.com/), specifically the basic business
+logic behind the *videoroom* plugin. This makes it possible to use *rtpengine*
+as a drop-in replacement for *Janus* for this one specific use case, and has
+the benefit of being able to use all the extra features that *rtpengine*
+provides, such as transcoding, in-kernel packet forwarding for improved
+performance, etc.
+
+The required subset of the *Janus* API is exposed via *rtpengine*'s HTTP/WS
+interface. The HTTP admin API is connected to the `/admin` URI path using a
+JSON payload (same as *Janus* does), while the module communication happens on
+the WS protocol `janus-protocol`, also with JSON payloads (same as *Janus*
+does). Unlike *Janus*, both HTTP and WS endpoints are running on the same port.
+In fact, there is no real distinction between both interfaces, therefore both
+admin and non-admin messages can be sent via either interface. HTTPS and WSS
+are also supported.
+
+Token-based plugin authentication works similar to how it works in *Janus*
+except that only the single *videoroom* plugin is supported. The configuration
+setting `janus-secret` must be set to enable clients to connect to this
+simulated *Janus* interface and make use of its features.
+
+Under the hood the functionality of the *videoroom* plugin is facilitated using
+*rtpengine*'s `publish` and `subscribe` methods, which are mapped directly to
+the respective *Janus* methods. One *Janus* video room becomes one *rtpengine*
+call, with a distinctive and unique call ID based on the video room ID.
+
+There's currently no support for customising the SDP features and options used
+within the *Janus* drop-in mode, and, as *Janus* is WebRTC-specific, all SDPs
+produced from this mode can be used directly by WebRTC clients. Non-WebRTC
+clients can participate in the same video room as *Janus* clients if the
+respective mapped `publish` and `subscribe` methods are used, and with the call
+ID mapped to the video room ID.