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+# MSC3672 - Sharing ephemeral streams of location data
+
+## Problem
+
+[MSC3489](https://github.com/matrix-org/matrix-doc/blob/matthew/location-streaming/proposals/3489-location-streaming.md) 
+focuses on streaming persistent location data for applications that require
+historical knowledge. 
+
+We also need the ability to share this data in a non-persistent way for cases in 
+which privacy is a concern, like user locations.
+
+## Proposal
+
+This MSC adds the ability to publish short-lived location beacons through the 
+the use of custom Ephemeral Data Units (EDUs) by building on top of [MSC2476](https://github.com/ananace/matrix-doc/blob/user-defined-edus/proposals/2477-user-defined-ephemeral-events.md).
+
+In order to do so we will start by introducing a new boolean property on 
+`m.beacon_info` called `live` which will mark the start of an user's 
+intent to share ephemeral location information.
+
+```json5
+{
+    "type": "m.beacon_info.@stefan:matrix.org",
+    "state_key": "@stefan:matrix.org",
+    "content": {
+        "m.beacon_info": {
+            "description": "Stefan's live location",
+            "timeout": 600000, // how long from the last event until we consider the beacon inactive in milliseconds
+            "live": true // this is a live location beacon
+        },
+        "m.ts": 1436829458432, // creation timestamp of the beacon on the client
+        "m.asset": {
+            "type": "m.self.live" // live user location tracking
+        }
+    }
+}
+```
+
+We define this new property in order to allow clients to distinguish between 
+share types without potentially overwriting static ones.
+
+Multiple live beacons on the same timeline have the option of either aggregating
+all available location EDUs or just the ones referencing a particular 
+`beacon_info`.
+
+Subsequently clients will start sending beacon data EDUs to the new 
+`rooms/{roomId}/ephemeral/{eventType}/{txnId}` endpoint where `eventType` equals 
+`m.beacon` with the same location payload as defined in [MSC3489](https://github.com/matrix-org/matrix-doc/blob/matthew/location-streaming/proposals/3489-location-streaming.md).
+
+
+```json5
+{
+    "m.relates_to": { // from MSC2674: https://github.com/matrix-org/matrix-doc/pull/2674
+        "rel_type": "m.reference", // from MSC3267: https://github.com/matrix-org/matrix-doc/pull/3267
+        "event_id": "$beacon_info"
+    },
+    "m.location": {
+        "uri": "geo:51.5008,0.1247;u=35",
+        "description": "Arbitrary beacon information"
+    },
+    "m.ts": 1636829458432,
+}
+```
+
+These will reach clients through `/sync`s `ephemeral` dictionary with the same 
+payload but with the addition of a `sender` which the clients can aggregate user 
+locations on.
+
+When the user decides they would like to stop sharing their live location the 
+original `m.beacon_info`'s `live` property should be set to `false`.
+
+## Encryption
+
+E2E encryption for EDUs isn't currently defined but as we're dealing with 
+privacy sensitive data we propose to wrap them inside the standard encryption
+envelope:
+
+```json5
+{
+    "algorithm": "m.megolm.v1.aes-sha2",
+    "sender_key": "<sender_curve25519_key>",
+    "device_id": "<sender_device_id>",
+    "session_id": "<outbound_group_session_id>",
+    "ciphertext": "<encrypted_payload_base_64>"
+}
+```
+
+in which the `ciphertext` will contain the `m.beacon` payload defined above and
+which will be sent to `rooms/{roomId}/ephemeral/m.room.encrypted/{txnId}`
+
+The Megolm keys required to decrypt this EDU should be shared over Olm just like
+regular encrypted timeline events.
+
+## Alternatives
+
+Alternatively, we could negotiate a WebRTC data channel using [MSC3401](https://github.com/matrix-org/matrix-doc/blob/matthew/group-voip/proposals/3401-group-voip.md) 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).
+
+## Unstable prefix
+
+ * `m.beacon_info.*` should be referred to as `org.matrix.msc3489.beacon_info.*` until this MSC lands.
+ * `m.beacon` should be referred to as `org.matrix.msc3489.beacon` until this MSC lands.
+ * `m.location` should be referred to as `org.matrix.msc3488.location.*` until MSC3488 lands.
+ * `m.ts` should be referred to as `org.matrix.msc3488.ts.*` until MSC3488 lands.
+ * `m.asset` should be referred to as `org.matrix.msc3488.asset.*` until MSC3488 lands.
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