MSC4143: MatrixRTC – Real-time communication over Matrix

proposals/4143-matrix-rtc.md

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+# MSC4143: MatrixRTC
+
+This MSC defines the modules with which the MatrixRTC (Matrix Real Time Communication) signalling system is built.
+
+The MatrixRTC specification is separated into different modules.
+
+- The MatrixRTC room state that defines the state of the real time application.\
+  It is the source of truth for:
+  - Who is part of a session
+  - Who is connected via what technology/backend
+  - Metadata per device used by other participants to decide whether the streams
+    from this source are of interest / need to be subscribed.
+- The RTC backend.
+  - It defines how to connect the participating peers.
+  - Livekit is the standard for this as of writing.
+  - Defines how to connect to a server/other peers, how to update the connection,
+    how to subscribe to different streams...
+  - Another planned backend is a full mesh implementation based on MSC3401.
+- The RTCSession types (application) have their own per application spec.
+  - Calls can be done with an application of type `m.call` see (TODO: link call msc)
+  - The application defines all the details of the RTC experience:
+    - How to interpret the metadata of the member events.
+    - What streams to connect to.
+    - What data in which format to sent over the RTC channels.
+
+This MSC will focus on the Matrix room state, which can be seen as the most high
+level signalling of a call:
+
+## Proposal
+
+Each RTC session is made out of a collection of `m.rtc.member` state events.
+Each `m.rtc.member` event defines the application type: `application`
+and a `call_id`.
+The first element of the state key is the `userId` and the second the `deviceId`.
+(see [this proposal for state keys](https://github.com/matrix-org/matrix-spec-proposals/pull/3757#issuecomment-2099010555)
+for context about second/first state key.)
+
+### The MatrixRTC room state
+
+Everything required for working MatrixRTC
+(current session, sessions history, join/leave events, ...) only
+require one event type.
+
+A complete `m.rtc.member` state event looks like this:
+
+```json5
+// event type: "m.rtc.member"
+// event key: "@user:matrix.domain_DEVICEID"
+{
+  "application": "m.my_session_type",
+  "call_id": "",
+  "device_id": "DEVICEID",
+  "created_ts": Time | undefined,
+  "expires_after": Duration,
+  "focus_active": {...FOCUS_A},
+  "foci_preferred": [
+    {...FOCUS_1},
+    {...FOCUS_2}
+  ]
+}
+```
+
+> [!NOTE]  
+> This relies on [MSC3757](https://github.com/matrix-org/matrix-spec-proposals/pull/3757).
+> We need to have one state event per device, hence multiple "non-overwritable" state
+> events per user.
+
+This gives us the information, that user: `@user:matrix.domain` with device `DEVICEID`
+is part of an RTCSession of type `m.call` in the scope/sub-session `""` (empty
+string as call id) connected over `FOCUS_A`. This is all information that is needed
+for another room member to detect the running session and join it.
+
+We include the device_id in the member content to not rely on the exact format of the state key.
+In case [MSC3757](https://github.com/matrix-org/matrix-spec-proposals/pull/3757) is used it would not
+be the second element of the state key array.
+
+`created_ts` is an optional property that caches the time of creation. It is not required
+for an event that, has not yet been updated, there the `origin_server_ts` is used.
+
+> [!NOTE]
+> We introduce `created_ts()` as the notation for `created_ts ?? origin_server_ts`
+
+Once the event gets updated, the origin_server_ts needs to be copied into the `created_ts` field.
+An existing `created_ts` field implies that this is a state event updating the current session
+and a missing `created_ts` field implies that it is a join state event.
+All membership events that belong to one member session can be grouped with the index
+`created_ts()`+`device_id`. This is why the `m.rtc.member` events deliberately do NOT include a `membership_id`.
+
+Other then the membership sessions, there is **no event** to represent a rtc session (containing all members).
+Such an event would include shared information, and deciding who has authority over that is not trivial.
+Instead the session is a computed value based on `m.rtc.member` events.
+The list of events with the same `application` and `m.call_id` represent one session.
+This array allows to compute fields such as participant count, start time, etc.
+
+Sending an empty `m.rtc.member` event represents a leave action.
+Sending a well formatted `m.rtc.member` represents a join action.
+
+Based on the value of `application`, the event might include additional parameters
+required to provide additional session parameters.
+
+> A [thirdroom](https://thirdroom.io)-like experience could include the information of an approximate position
+> on the map, so that clients can omit connecting to participants that are not in their
+> area of interest.
+
+#### Reliability requirements for the room state
+
+Room state is a very well suited place to store the data for a MatrixRTC session, as
+it allows:
+
+- The client to determine current ongoing sessions without loading history for every room,
+  or doing additional work other than the sync loop that needs to run anyway.
+- The client can compute/access data of past sessions without any additional redundant data.
+- Sessions (start/end/participant count) are federated and there is not redundant data storage that
+  could result in conflicts, or can get out of sync. The room state events are part of the dag and this
+  is solved like any other PDU in matrix.
+
+A challenge with using the room state to represent a session is disconnection behaviour.
+If the client disconnects from a call because of a network issue,
+an application crash, or a user forcefully quitting the client - then the room state cannot be updated any more.
+The client is required to leave by sending a new empty state which cannot happen once connection is lost.
+
+If the state is not updated correctly we end up with incorrect session end timestamps, and a room state that is not
+correctly representing the current RTC session state. Historic and current MatrixRTC session data would be broken.
+
+For an acceptable solution, the following requirements need to be taken into consideration:
+
+- Room state is set to empty if the client loses connection. (A heardbeat like system is desired)
+- The best source of truth for a call participation is a working connection to the SFU.
+  It is desired that the disconnect of the SFU is connected to the room state.
+- It should be possible to update the room state without the client being online.
+- All of this should still work when Matrix uses cryptographic identities (e.g.
+  [MSC4080](https://github.com/matrix-org/matrix-spec-proposals/pull/4080)).
+
+[MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140) proposes a concept to
+delay the leave events until one of the leave conditions (heartbeat or SFU disconnect) occur
+and fulfil all of the these requirements.
+
+A MatrixRTC client has to first send/schedule the following delayed leave event:
+
+```json5
+// event type: "m.rtc.member"
+// event key: "@user:matrix.domain_DEVICEID"
+{
+  "leave_reason": "CONNECTION_LOST"
+}
+```
+
+Subsequently, the actual state event can be sent, so that we guarantee that the state will be empty eventually.
+The `leave_reason` is added so clients can be more verbal about why a user disconnected from a call.
+
+Receiving clients will be able to detect if the delayed event request was recognised by the presence of the `has_delayed_overwrite: true`
+unsigned property. If the property is missing the event is invalid.
+
+This also ensures that delayed leave events that are incorrectly sent with a non-empty membership content are invalidated,
+as they will not contain the `has_delayed_overwrite: true` unsigned property.
+
+#### Historic sessions
+
+Since there is no single entry for a historic session (because of the ownership ambiguity),
+historic sessions need to be computed on the client.
+
+Each state event can either mark a join or leave:
+
+- join: `prev_state.application != current_state.application` &&
+  `prev_state.m.call_id != current_state.m.call_id` &&
+  `current_state.application != undefined`
+  (where an empty `m.rtc.member` event would imply `state.application == undefined`)
+- leave: `prev_state.application != current_state.application` &&
+  `prev_state.m.call_id != current_state.m.call_id` &&
+  `current_state.application == undefined`
+
+Based on this one can find user sessions. The range between a join and a leave
+event gives the specific times and duration of the session.
+The collection of all overlapping user sessions with the same `call_id` and
+`application` define one MatrixRTC history event.
+
+### The RTC backend
+
+`foci_active` and `foci_preferred` are used to communicate:
+
+- how a user is connected to the session (`foci_active`)
+- what connection method this user knows about would like to connect with.
+
+The only enforced parameter of a `foci_preferred` or `foci_active` is `type`.
+Based on the focus type a different amount of parameters might be needed to,
+communicate how to connect to other users.
+`foci_preferred` and `foci_active` can have different parameters so that it is,
+possible to use a combination of the two to figure our that everyone is connected
+with each other.
+
+Only users with the same type can connect in one session. If a frontend does
+not support the used type they cannot connect.
+
+Each focus type will get its own MSC, describing how to get from the foci
+information to establishing WebRTC connections for all participants.
+
+- [`livekit`](www.example.com) TODO: create `livekit` focus MSC and add link here.
+- [`full_mesh`](https://github.com/matrix-org/matrix-spec-proposals/pull/3401)
+  TODO: create `full-mesh` focus MSC based on[MSC3401](https://github.com/matrix-org/matrix-spec-proposals/pull/3401)
+  and add link here.
+
+#### Sourcing `foci_preferred`
+
+At some point participants have to decide/propose which focus they use.
+Based on the focus type and use case choosing a `foci_preferred` can be different.
+If possible these guidelines should be obeyed:
+
+- If there is a relation between the `focus_active` and a preferred focus (`type: livekit` is an example for this)
+  it is recommended to copy _the preferred focus that relates to the current `focus_active`_ of other participants to
+  the start of the `foci_preferred` array of the member event.
+  (The exact definition of: _the preferred focus that relates to the current `focus_active`_ is part of the 
+  specification for each focus type. For `full_mesh` for example there is no such thing as: _the preferred focus that 
+  relates to the current `focus_active`_ )
+- Homeservers can proposes `preferred_foci` via the well known. An array of preferred foci is provided behind the well 
+  known key `m.rtc_foci`. This is defined in [MSC4158](https://github.com/matrix-org/matrix-spec-proposals/pull/4158). 
+  They are related and it is recommended to also read
+  [MSC4158](https://github.com/matrix-org/matrix-spec-proposals/pull/4158) with this MSC.
+  Those proposals from **your own** homeserver should come next in the `foci_preferred` list of the member event.
+- Clients also have the option to configure a preferred foci even though this is not recommended (see below).
+  Those come last in the list.
+
+The rationale for these guidelines are:
+
+- It is always desired to have as few focus switches as possible.
+  That is why the highest priority is to prefer the focus that is already in use.
+- MatrixRTC is designed around the same architecture as the rest of Matrix, with
+  conversations being powered by many homeservers from across the network.
+  MatrixRTC has the same goal. To achieve a stable and healthy ecosystem
+  RTC infrastructure should be thought of as a part of a homeserver. It is very similar
+  to a turn server: mostly traffic and little cpu load.
+  To not end up in a world where each user is only using one central SFU but where the traffic
+  is split over multiple SFU's it is important that we leverage the SFU distribution similarly to the
+  distribution of homeservers.
+  For this reason the second guideline is to lookup the preferred foci from the homeserver's well_known.
+- Looking up the preferred foci from a client is toxic to a federated system. If the majority of users
+  decide to use the same client all of the users will use one focus. This destroys the passive security mechanism that 
+  each instance is not an interesting attack vector since it is only a fraction of the network.
+  Additionally it will result in poor performance if every user on Matrix would use the same focus.
+  There are cases where this is acceptable:
+  - Transitioning to MatrixRTC. Here it might be beneficial to have a client that has a fallback focus
+    so calls also work with homeservers not supporting it.
+  - For testing purposes where a different focus should be tested but one does not want to touch the .well_known
+  - For custom deployments that benefit from having the Focus configuration on a per client basis instead of per homeserver.
+
+### The RTC Session types (application)
+
+Each session type can have its own specification in how the different streams
+are interpreted and even what focus type to use. This makes this proposal extremely
+flexible. For instance, a Jitsi conference could be added by introducing a new `application`
+and a new focus type and would be MatrixRTC compatible. It would not be compatible
+with applications that do not use the Jitsi focus but clients would know that there
+is an ongoing session of unknown type and unknown focus and could display/represent
+this in the user interface.
+
+To make it easy for clients to support different RTC session types, the recommended
+approach is to provide a Matrix widget for each session type, so that client developers
+can use the widget as the first implementation if they want to support this RTC
+session type.
+
+Each application should get its own MSC in which the all the additional
+fields are explained and how the communication with the possible foci is
+defined:
+
+- [`m.call`](www.example.com) TODO: create `m.call` MSC and add link here.
+
+## Potential issues
+
+## Alternatives
+
+## Security considerations
+
+## Unstable prefix
+
+The state events and the well_known key introduced in this MSC use the unstable prefix
+`org.matrix.msc4143.` instead of `m.` as used in the text.
+
+Possible values inside the `m.rtc.member` event (like `m.call`) will use a prefix defined in the
+related PR (TODO create and link `m.call` application type PR)