MSC3885: Sliding Sync Extension: To-Device messages

proposals/3885-sliding-sync-to-device.md

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+# MSC3885: Sliding Sync Extension: To-Device
+
+This MSC is an extension to [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575)
+which includes support for to-device messages in the `/sync` response.
+
+## Proposal
+
+MSC3575 does not include support for to-device events. This extension will add support for
+to-device events. There is a **critical** problem in the current to-device implementation, which is
+that events are implicitly acknowledged when the user advances the `/sync` token. This causes problems
+when clients need to have 2 or more sync streams open at a time, e.g a push notification process _and_
+a main process. This can cause the two processes to race to fetch the to-device events, resulting in
+the need for complex synchronisation rules to ensure the token is correctly and atomically exchanged
+between processes. Sliding Sync's implementation of to-device events removes this requirement by
+associating an _explicit_ token **just for to-device events**. This token uses the same terminology
+of the current `/sync` implementation: `since` and `next_batch`.
+
+The proposal is to introduce a new extension called `to_device` with the following request parameters:
+```js
+{
+    "enabled": true, // sticky
+    "limit": 100, // sticky, max number of events to return, server can override
+    "since": "some_token" // optional, can be omitted on initial sync / when extension is enabled
+}
+```
+If `enabled` is `true`, then the sliding sync response includes the following response fields in
+the `to_device` extension response:
+```js
+{
+    "next_batch": "some_token", // REQUIRED: even if no changes
+    "events": [ // optional, only if there are events since the last `since` value.
+        { ... to device event ... }
+    ]
+}
+```
+
+The semantics of the `events` field is exactly the same as the current `/sync` implementation, as implemented
+in v1.3 of the Client-Server Specification. The server MUST NOT send more than `limit` events: it may send
+less than `limit` events if the value provided was too large.
+
+The `events` are treated as "acknowledged" when the server receives a new request with the `since`
+value set to the previous response's `next_batch` value. When this occurs, acknowledged events are
+permanently deleted from the server, and MUST NOT be returned to the client should another request
+with an older `since` value be sent.
+
+If a response is lost, the client will retry the request with the same `since` value. When this happens,
+the server MAY bundle together more events if they arrived in the interim. 
+
+## Potential issues
+
+This proposal introduces a new sync token type for clients to manage. It is done for good reasons but
+it does slightly increase the complexity of designing a correct client implementation. 
+
+## Alternatives
+
+The alternative is to include to-device events like normal events in a different section, without
+making use of dedicated `since` and `next_batch` tokens, instead relying on the `pos` value. This
+would revert to-device events to be implicitly acknowledged, which has caused numerous [issues](https://github.com/vector-im/element-ios/issues/3817) in
+the past.
+
+## Security considerations
+
+No additional security considerations beyond what the current `/sync` implementation provides.
+
+## Unstable prefix
+
+No unstable prefix as Sliding Sync is still in review. To enable this extension, just add this to
+your request JSON:
+```js
+{
+    "extensions": {
+        "to_device": {
+            "enabled": true
+        }
+    }
+}
+```
+
+## Dependencies
+
+This MSC builds on MSC3575, which at the time of writing has not yet been accepted into the spec.