matrix_client_information spec

Author: Kerry Archibald

spec/matrix_client_information.md

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+# io.element.matrix_client_information.<device_id>: Storing additional client information per device
+
+## Problem
+
+Currently, sessions are only easily recognisable by their `display_name`. Depending on client implementation, this may
+include some stringified information about the session. (For example, Element Web uses `'%(appName)s (%(browserName)s,
+%(osName)s)'`). This information can become stale, and if edited by the user any device detail is lost.
+
+By saving structured and up to date session information, users will be able to more easily recognise their sessions.
+This gives users  more confidence in removing stale or suspicious sessions.
+
+
+## Proposal
+
+We introduce the account_data event `io.element.matrix_client_information.<device_id>`.
+
+An example event for a device with `device_id` of `abc123`:
+
+```json5
+{
+  "type": "io.element.matrix_client_information.abc123",
+  "content": {
+    "name": "Element Web",
+    "version": "1.2.3",
+    "url": "app.element.io"
+  }
+}
+```
+
+All properties are strings. `url` property is optional.
+
+## Client advisory
+
+When starting the client after login or update:
+1. Upsert an account data event with the type `io.element.matrix_client_information.<device_id>` for the current device
+
+When rendering device application information:
+
+1. Read account data event of type `io.element.matrix_client_information.<device_id>` for each given device.
+
+These events should be pruned periodically.
+
+## MSC
+Pruning of events depends on implementation of [MSC3391: Removing account
+data](https://github.com/matrix-org/matrix-spec-proposals/pull/3391)
+
+## Security considerations
+N/A
+
+## Implementations
+
+* The `matrix-react-sdk` which powers Element Web and Desktop implemented this in September 2022.
+  * https://github.com/matrix-org/matrix-react-sdk/pull/9314