[WIP] MSC3898: Native Matrix VoIP signalling for cascaded foci (SFUs, MCUs...)

proposals/3898-sfu.md

@@ -164,7 +164,7 @@ between itself and the SFU to use later for rapid signalling.
 
 The client uses the established data channel connection to the SFU to perform
 low-latency signalling to rapidly (un)subscribe/(un)publish streams, send
-keep-alive messages, metadata, cascade and perform re-negotiation.
+ping messages, metadata, cascade and perform re-negotiation.
 
 See the section about the [rationale](#the-use-of-the-data-channels-for-signaling)
 behind the use of the data channels for signaling.
@@ -294,17 +294,29 @@ This event works very similarly to the 1:1 call `m.call.sdp_stream_metadata`.
 }
 ```
 
-##### `m.call.keep_alive`
-
-Clients should send an `m.call.keep_alive` event to foci every so often. If
-the client does not send an `m.call.keep_alive` event for 30 seconds, the
-focus should hang up.
-
-- **TODO: should this be configurable somehow?**
+##### `m.call.ping`, `m.call.pong`
+A ping message must be sent by the focus to the client at an interval
+no greater than 30 seconds. On receiving a ping message, a client must respond
+immediately with a pong message. A client may therefore detect that the
+connection has failed after an amount of time of its choosing (greater than
+30 seconds) has elapsed since it last saw a ping message. A server may deem a
+client unresponsive after not receiving a pong some amount of time after it
+has sent a ping, again the amount of time the server waits is up to the
+implementation. Either send should hang up once deeming the other side
+unresponsive.
+
+focus -> client:
+```json
+{
+    "type": "m.call.ping",
+    "content": {}
+}
+```
 
+client -> focus:
 ```json
 {
-    "type": "m.call.keep_alive",
+    "type": "m.call.pong",
     "content": {}
 }
 ```