MSC2444: peeking over federation via /peek

proposals/2444-peeking-over-federation-peek-api.md

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+# Proposal for implementing peeking over federation (peek API)
+
+## Problem
+
+Currently you can't peek over federation, as it was never designed or
+implemented due to time constraints when peeking was originally added to Matrix
+in 2016.
+
+As well as stopping users from previewing rooms before joining, the fact that
+servers can't participate in remote rooms without joining them first is
+inconvenient in other ways:
+
+ * You can't use rooms as generic pubsub mechanisms for synchronising data like
+   profiles, groups, reputation lists, device-lists etc if you can't peek into
+   them remotely.
+ * Matrix-speaking search engines can't work if they can't peek remote rooms.
+
+A related problem (not solved by this MSC) is that servers can't participate
+in E2E encryption when peeking into a room, given the other users in the
+room do not know to encrypt for the peeking device.
+
+Another related problem (not solved by this MSC) is that invited users can't
+reliably participate in E2E encryption before joining a room, given the invited
+server doesn't currently have a way to know about new users/devices in the room
+without peeking, and so doesn't tell them if the invited user's devices changes.
+(https://github.com/vector-im/element-web/issues/2713#issuecomment-691480736
+outlines a fix to this, not covered by this MSC).
+
+## Solution
+
+We let servers participate in peekable rooms (i.e. those with `world_readable`
+`m.room.history_visibility`) without having actually joined them.
+
+Firstly, this means that a number of federation endpoints should be updated to
+allow inspection of `world_readable` rooms. This includes:
+
+ * [`GET /_matrix/federation/v1/event_auth/{roomId}/{eventId}`](https://matrix.org/docs/spec/server_server/r0.1.4#get-matrix-federation-v1-event-auth-roomid-eventid)
+ * [`GET /_matrix/federation/v1/backfill/{roomId}`](https://matrix.org/docs/spec/server_server/r0.1.4#get-matrix-federation-v1-backfill-roomid)
+ * [`POST /_matrix/federation/v1/get_missing_events/{roomId}`](https://matrix.org/docs/spec/server_server/r0.1.4#post-matrix-federation-v1-get-missing-events-roomid)
+ * [`GET /_matrix/federation/v1/state/{roomId}`](https://matrix.org/docs/spec/server_server/r0.1.4#get-matrix-federation-v1-state-roomid)
+ * [`GET /_matrix/federation/v1/state_ids/{roomId}`](https://matrix.org/docs/spec/server_server/r0.1.4#get-matrix-federation-v1-state-ids-roomid)
+ * [`GET /_matrix/federation/v1/event/{eventId}`](https://matrix.org/docs/spec/server_server/r0.1.4#get-matrix-federation-v1-event-eventid)
+
+(Of course, these apis should only allow access to `world_readable` parts of
+the history.)
+
+Secondly, we introduce a new API allowing servers to subscribe to new events.
+
+### Initiating a peek
+
+To start peeking, firstly the peeking server must pick server(s) to peek
+via. It can do this based on the `servers` parameter of the CS API `/peek`
+command (from [MSC2753](https://github.com/matrix-org/matrix-doc/pull/2753)),
+or failing that the domain of the room alias being peeked.
+
+The peeking server then makes a `/peek` request to the target server. An
+example request and response might look like:
+
+```
+PUT /_matrix/federation/v1/peek/{roomId}/{peekId}?ver=5&ver=6 HTTP/1.1
+{}
+
+200 OK
+{
+  "latest_event_state_ids": {
+    "$fwd_extremity_1": [
+        "$state_event_3",
+        "$state_event_4"
+    ],
+    "$fwd_extremity_2": [
+        "$state_event_5",
+        "$state_event_6"
+    ]
+  },
+  "common_state_ids": [
+     "$state_event_1",
+     "$state_event_2",
+  ],
+  "events": [
+    {
+      "type": "m.room.member",
+      "room_id": "!somewhere:example.org",
+      "content": { /* ... */ }
+    }
+  ],
+  "renewal_interval": 3600000
+}
+```
+
+The request takes an empty object as a body as a placeholder for future
+extension.
+
+The peeking server selects an ID for the peeking subscription for the purposes
+of idempotency. The ID must be unique for a given `{ peeking_server, room_id,
+target_server }` tuple, and should be a string consisting of the characters
+`[0-9a-zA-Z.=_-]`. Its length must not exceed 8 characters and it should not be
+empty.
+
+The request takes `?ver=` querystring parameters with the same behaviour as
+`/make_join` to advertise the room versions the peeking server supports.
+
+If the request is successful, the target server retuns a 200 response with the
+following fields:
+ * `latest_event_state_ids`: a map whose keys are the IDs of the events forming
+   the target server's current forward extremities in the room. The values are
+   lists of the IDs of the events forming the room state after the event in
+   question, excluding any events in `common_state_ids`.
+
+   TBD: would the state *before* the extremity event be more useful?
+
+ * `common_state_ids`: A list of the IDs of any events which are common to the room
+   states after *all* of the forward extremities in the room.
+
+ * `events`: The bodies of any events whose IDs are:
+   * listed in the keys of `latest_event_state_ids`, or:
+   * listed in the values of `latest_event_state_ids`, or:
+   * listed in the values of `common_state_ids`, or:
+   * listed in the `auth_events` field of any of the above events, or:
+   * listed in the `auth_events` of the `auth_events`, recursively.
+
+ * `renewal_interval`: a duration in milliseconds after which the target server
+   will expire the peek. The peeking server must renew the peek before that
+   time to be sure of continuing to receive events.
+
+If the room is not peekable, the target server should return a 403 error with
+ `M_FORBIDDEN`.
+
+If the room is not known to the target server, it should return a 404 error
+with `M_NOT_FOUND`.
+
+If the peek ID is not valid, the target server responds with 400 and `M_UNRECOGNIZED`.
+
+If the room version of the room being peeked isn't supported by the peeking
+server, the target server responds with 400 and `M_INCOMPATIBLE_ROOM_VERSION`.
+
+If the target server doesn't wish to honour the peek request due to server load
+or rate-limiting, it may respond with 429 and `M_LIMIT_EXCEEDED`, including a
+`retry_after_ms` value indicating when the request could be retried.
+
+The room states returned by `/peek` should be validated just as the one
+returned by the `/send_join` API. If the peeking server finds the response
+unacceptable, it should cancel the peek with a `DELETE` request (see below).
+
+XXX: it might be better to split this into two operations: first fetch the
+state data, then begin the peek operation by sending your idea of the forward
+extremities, to bring you up to date with anything you missed. This would
+reduce the chance of having to immediately cancel a peek, and would be more
+efficient in the case of rapid `peek-unpeek-peek` switches.
+
+While a peek subscription is active, the target server must relay any events
+received in that room over the [`PUT
+/_matrix/federation/v1/send/{txnId}`](https://matrix.org/docs/spec/server_server/r0.1.4#put-matrix-federation-v1-send-txnid)
+API. If a peeking server has multiple peeks active for a given room and target
+server, the target server should still only send one copy of each event, rather
+than duplicating the event for each peek.
+
+### Renewing a peek
+
+The target server will eventually expire a peek if it is not renewed. The
+peeking server can renew a peek by calling `POST
+/_matrix/federation/v1/peek/{roomId}/{peekId}/renew`:
+
+```
+POST /_matrix/federation/v1/peek/{roomId}/{peekId}/renew HTTP/1.1
+{}
+
+200 OK
+{
+  "renewal_interval": 3600000
+}
+```
+
+The target server simply returns the new `renewal_interval`.
+
+If the peek ID is not known for the `{ peeking_server, room_id, target_server }`
+tuple, the target server returns a 404 error with `M_NOT_FOUND`.
+
+### Deleting a peek
+
+The peeking server may terminate a peek by calling `DELETE
+/_matrix/federation/v1/peek/{roomId}/{peekId}`:
+
+```
+DELETE /_matrix/federation/v1/peek/{roomId}/{peekId} HTTP/1.1
+Content-Length: 0
+
+200 OK
+{}
+```
+
+The request has no body <sup id="a1">[1](#f1)</sup>. On success, the target
+server returns a 200 with an empty json object.
+
+If the peek ID is not known for the `{ peeking_server, room_id, target_server }`
+tuple, the target server returns a 404 error with `M_NOT_FOUND`.
+
+### Expiring a peek
+
+The target server should expire any peek which is not renewed before the
+`renewal_interval` elapses.
+
+It should indicate the expiry to the peeking server via `PUT
+/_matrix/federation/v1/send/`, via a new `expired_peeks` key:
+
+```
+PUT /_matrix/federation/v1/send/S0meTransacti0nId HTTP/1.1
+
+{
+  "expired_peeks": [
+    {
+      "room_id": "{roomId}",
+      "peek_id": "{peekId}"
+    }
+  ]
+}
+```
+
+### Joining a room
+
+When the user joins the peeked room, the peeking server should just emit the
+right membership event rather than calling `/make_join` or `/send_join`, to
+avoid the unnecessary burden of a full room join, given the server is already
+participating in the room.  It should also send a `DELETE` request to cancel
+any active peeks.
+
+### Encrypted rooms
+
+It is considered a feature that you cannot peek into encrypted rooms, given
+the act of peeking would leak the identity of the peeker to the joined users
+in the room (as they'd need to encrypt for the peeker).  This also feels
+acceptable given there is little point in encrypting something intended to be
+world-readable.
+
+## Alternatives
+
+ * simply use `room_id` for idempotency rather than requiring a separate
+   `peek_id`. One reason not to do this is to allow a future extension where
+   there are multiple subscriptions active, each filtering out different event
+   types. Another reason that `peek_id` is useful is to improve idempotency
+   with rapid peek/unpeek/peek cycles, to ensure we aren't `DELETE`ing the
+   wrong peek.
+
+ * An earlier rejected solution is
+   [MSC1777](https://github.com/matrix-org/matrix-doc/pull/1777), which
+   proposed joining a pseudouser (`@:server`) to a room in order to peek into
+   it. However:
+
+   * being forced to write to a room DAG (by joining such a user) in order to
+     perform a read-only operation (peeking) is somewhat inefficient.
+
+   * it also constitutes a privacy violation - tantamount to a tracking
+     pixel. If I'm on a smallish server and I happen to briefly peek into
+     `#nsfw:matrix.org`, I don't want every random server in that room to know
+     that I did so. It's even worse than sending read-receipts.
+
+   * In the interests of empowering the user, it's actually quite nice to
+     (theoretically at least) have some control over which servers you trust to
+     peek via.
+
+   * That said, a P2P world is going to need a totally different approach,
+     which might be back towards MSC1777 but using
+     [MSC1228](https://github.com/matrix-org/matrix-doc/pull/1228)-style IDs to
+     protect privacy. We need to solve scalability of "which nodes are
+     participating in the room" irrespectively of whether those nodes are
+     active or passive. So it's worth noting this solution (MSC2444) is very
+     much for today's federated architecture.
+
+   * MSC1777 offers a solution for EDU transmission which this MSC does not,
+     given we don't currently have any data flows for mirroring other servers'
+     EDUs.
+
+ * Rather than attempting to maintain a local replica of peeked traffic, have
+   the peeking server proxy any peek requests from the client-server API onto
+   the target server, somehow.  This couples room availability to the peeked
+   server and means we don't have a local replica we can index or serve
+   independently etc - and also means you could have problems deduplicating
+   peeks between local clients.
+
+## Future extensions
+
+These features are explicitly descoped for the purposes of this MSC.
+
+ * It may be useful to allow peeking servers to "filter" the events to be
+   returned - for example, if you only care about particular events, or
+   particular servers - e.g. if load-balancing peeking via multiple servers.
+
+   It's worth noting that this would make it very hard for peeking servers to
+   reliably track state changes and detect missing events.
+
+## Security considerations
+
+ * A malicious server could set up multiple peeks to multiple target servers by
+   way of attempting a denial-of-service attack. Server implementations should
+   rate-limit requests to establish peeks, as well as limiting the absolute
+   number of active peeks each server may have, to mitigate this.
+
+ * The peeked server becomes a centralisation point which could conspire
+   against the peeking server to withhold events.  This is not that dissimilar
+   to trying to join a room via a malicious server, however, and can be
+   mitigated somewhat if the peeking server tries to query missing events from
+   other servers.  The peeking server could also peek to multiple servers for
+   resilience against this sort of attack.
+
+ * The peeked server will be able to track the metadata surrounding which
+   servers are peeking into which of its rooms, and when.  This could be
+   particularly sensitive for single-person servers peeking at profile rooms.
+
+## Design considerations
+
+This doesn't solve the problem that rooms wink out of existence when all
+participants leave (https://github.com/matrix-org/matrix-doc/issues/534),
+unlike other approaches to peeking (e.g. MSC1777).
+
+How do we handle backpressure or rate limiting on the event stream (if at
+all?)
+
+## Dependencies
+
+This unblocks [MSC1769](https://github.com/matrix-org/matrix-doc/pull/1769)
+(profiles as rooms) and
+[MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772) (Matrix Spaces),
+and is required for
+[MSC2753](https://github.com/matrix-org/matrix-doc/pull/2753) (peeking via
+/sync) to be of any use.
+
+This would close https://github.com/matrix-org/matrix-doc/issues/913.
+
+## Footnotes
+
+<a id="f1"/>[1]: per
+https://www.ietf.org/archive/id/draft-ietf-httpbis-semantics-12.html#name-delete:
+"A client SHOULD NOT generate a body in a DELETE request."  [↩](#a1)