matrix-spec-proposals/proposals/3672-ephemeral-location-streaming.md

MSC3672 - Sharing ephemeral streams of location data

Problem

MSC3489 focuses on streaming persistent location data for applications that require historical knowledge.

While that's perfect for situations in which long term storage of the data is a non-issue and implied (e.g. flight paths, strava style exercises, fleet tracking), there are also cases in doing so is undesirable for either privacy or performance reasons.

Sharing user live location updates is one of the cases in which privacy is paramount and where we need the ability to share data in a safe and non-persistent fashion.

Proposal

This MSC adds the ability to publish short-lived location beacons through the use of Ephemeral Data Units (EDUs). It is almost identical to MSC3489, apart from this change.

Ephemeral data units (EDUs) are Matrix's default mechanism for broadcasting short-lived data to a group of users. They are intended to be non-persistent, not take part in a room's history and are currently used for typing notifications, event receipts, and presence updates.

To allow the use of EDUs for live location sharing, this MSC depends on MSC2477 for user-defined EDUs, and MSC3673 for encrypted EDUs.

Clients announce a share, and stop a share exactly as in MSC3489, by emitting an m.beacon_info state event.

Clients share locations by emitting an m.beacon event which is identical to those described in MSC3489, except that it is an EDU (whereas in MSC3489 the events are PDUs).

All other behaviour is the same as in MSC3489. The only difference is that m.beacon events are EDUs.

Delivery guarantees

MSC2477 specifically avoids defining delivery guarantees for user-defined EDUs, so we avoid making hard recommendations here. We expect that future MSCs will clarify the available options. When that happens, clients should choose the available option that suits their use case, which we expect will be broadly in line with the following paragraph.

Servers should attempt to deliver m.beacon EDUs to all clients, and delete them as soon as possible. Servers should implement a timeout mechanism to ensure they are always deleted, even if undelivered. For the expected use cases, we suggest 30 minutes would be a suitable timeout.

Security considerations

The security considerations are identical to MSC3489, except that EDUs provide a better privacy situation because they are not persisted long-term.

Alternatives

Alternatively, we could negotiate a WebRTC data channel using MSC3401 and stream low-latency geospatial data over the participating devices in the room. However it would be useful to support plain HTTP like the rest of Matrix and requiring a WebRTC stack is prohibitively complicated for simple clients (e.g. basic IOT devices reporting spatial telemetry).

Another alternative is to use to-device events but that comes with disadvantages of its own as they're 1:1, single message per transaction and not intended for conversational data.

Dependencies

This proposal relies on the following MSCs:

• MSC2477: User-defined ephemeral events in rooms
• MSC3673: Encrypted ephemeral data units
• MSC3489: Sharing streams of location data with history

For live location shares to work in appservices, these MSCs will also be required:

• MSC2409: Proposal to send EDUs to appservices
• MSC3202: Encrypted appservices

Unstable prefix

This MSC uses the same unstable prefixes as MSC3489.

Note that MSC3489 actually uses prefixes that match this MSC's numbering. This is an historical artifact of the various revision histories of these proposals.