MSC4192: Comparison of proposals for ignoring invites

proposals/xxxx-ignoring-invites.md

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+# Comparison of proposals for ignoring invites
+
+Matrix supports ignoring specific users via the eponymous [module] and the
+`m.ignored_user_list` account data event.
+
+```json5
+{
+  "type": "m.ignored_user_list"
+  "content": {
+    "ignored_users": {
+      "@someone:example.org": {}
+    }
+  },
+}
+```
+
+Servers MUST NOT forward invites from ignored users to clients. For the
+purposes of ignoring invites, this feature is, however, quite limiting
+because:
+
+* Ignoring users also hides their events in existing rooms which might not
+  be desirable
+* There's no support for globs or wildcards
+* The module only supports block-list but no allow-list semantics
+* Users cannot review or unignore specific ignored invites
+* There's no built-in concept of sharing ignore lists between users
+
+Several attempts at designing a scheme for ignoring invites specifically
+have been made so far. This documents provides a brief comparison of the
+different proposals and serves as an entry point for further research.
+
+
+## [MSC2270] – Proposal for ignoring invites
+
+A new account data event `m.ignored_room_list` is introduced. The event
+has the exact same semantics as `m.ignored_user_list` but tracks room IDs
+rather than user IDs.
+
+```json5
+{
+  "type": "m.ignored_room_list"
+  "content": {
+    "ignored_rooms": {
+      "!something:example.org": {}
+    }
+  },
+}
+```
+
+A key drawback of this scheme is that it's not possible to blanketly ignore
+invites from specific users or servers. Spammers could, therefore, send
+invites repeatedly.
+
+Additionally, the ignore list is subject to the 64 KiB event size limit.
+
+
+## [MSC3840] – Ignore invites
+
+A new account data event `m.ignored_invites` is introduced. The event has
+separate fields for ignoring invites:
+
+- From specific users
+- From specific servers
+- Into specific rooms
+- Into specific rooms by specific users
+
+```json5
+{
+  "type": "m.ignored_invites"
+  "content": {
+    "ignored_user_ids": {
+      "@someone:example.org": {
+        "ts": 1726138169160,
+        "reason": "..."
+      },
+    },
+    "ignored_servers": {
+      "example.org": {
+        "ts": 1726138169160,
+        "reason": "..."
+      },
+    },
+    "ignored_room_ids": {
+      "!something:example.org": {
+        "ts": 1726138169160,
+        "reason": "..."
+      },
+    },
+    "ignored_invites": {
+      "!something:example.org": {
+        "@someone:example.org": {
+          "ts": 1726138169160,
+          "reason": "..."
+        },
+      },
+    },
+  },
+}
+```
+
+A key improvement over [MSC2270] is the ability to ignore entire users
+and servers.
+
+Similar to [MSC2270], however, the ignore configuration is still subject
+to the 64 KiB event size limit.
+
+Contrary to `m.ignored_user_list` and [MSC2270], `m.ignored_invites` is
+handled exclusively on the client side. This allows users to review and
+unignore ignored invites but also means that ignored invites will be
+repeated in `/sync` which can cause performance issues over time.
+
+
+## [MSC4155]: Invite filtering
+
+A new account data event `m.invite_permission_config` is introduced to
+maintain a configuration for invite filtering with:
+
+- A global default to allow / block all invites
+- Per-user & per-server exceptions that override the default
+
+```json5
+{
+  "type": "m.invite_permission_config",
+  "content": {
+    "default": "allow | block",
+    "user_exceptions": {
+      "@someone:example.org": {},
+      ...
+    },
+    "server_exceptions": {
+      "example.org": {},
+      ...
+    }
+  }
+}
+```
+
+Similar to [MSC3840], this scheme allows to ignore entire users and servers.
+A key advantage though is the ability to create both block-list and allow-list
+configurations.
+
+Unlike [MSC3840], processing the ignore configuration can happen on either the
+client or the server.
+
+Similar to [MSC2270] and [MSC3840], the ignore configuration is subject to
+the 64 KiB event size limit.
+
+
+## [MSC3847] – Ignoring invites with policy rooms
+
+The invite filtering configuration is tracked in [moderation policy lists].
+These are already part of the spec and allow formulating glob rules against
+users, rooms and servers via state events in a policy room.
+
+```json5
+{
+  "type": "m.policy.rule.user | m.policy.rule.room | m.policy.rule.server",
+  "state_key": "...arbitrary...",
+  "content": {
+    "entity": "@someone:example.org | !abcdefg:example.org | example.org | *example.org",
+    "reason": "...",
+    "recommendation": "m.ban"
+  }
+}
+```
+
+Users can hook into policy rooms for invite filtering via a new account data
+event `m.policies` which defines:
+
+- A room into which the user will write their own custom policy rules (`target`)
+- A list of policy rooms to be executed against invites (`sources`). This
+  should generally include `target` but may also list other rooms such as
+  policy rooms recommended by the home server.
+
+```json5
+{
+  "type": "m.policies",
+  "content": {
+    "m.ignore.invites": {
+      "target": "!my-policies:example.org",
+      "sources": [
+        "!my-policies:example.org",
+        "!bobs-policies:example.org"
+      ]
+    }
+  }
+}
+```
+
+Unlike the previous proposals, the ignore configuration is not subject to
+the 64 KiB event size limit because each policy rule is stored in a separate
+event.
+
+Additionally, ignore configurations can easily be shared and reused across
+different users.
+
+Another key advantage is the ability to match users, rooms and servers against
+glob expressions. This could probably be added into the other proposals as well,
+however.
+
+Similar to [MSC3840], the ignore configuration is processed exclusively on the
+client which comes with the same advantages and disadvantages.
+
+Finally, when combined with [MSC4150], this scheme also allows for allow-list
+configurations.
+
+
+## [MSC3659] – Invite Rules
+
+A system akin to push rules is defined. Rules are stored as a flat list
+in a new account data event `m.invite_rules`.
+
+Each rule has a type that is accompanied with additional data to determines
+how the rule matches against invites. Rule types include:
+
+- `m.user`, accompanied with a field `user_id` that matches against a user
+  ID glob expression
+- `m.target_room_id`, accompanied with a field `room_id` that matches against
+  a room ID glob expression
+- `m.shared_room`, accompanied with a field `room_id` that matches if the
+  inviter and invitee already share a specific room
+
+Each rule is accompanied with two actions. One is to be executed if the rule
+matches, the other if the rule fails. Actions must have one of the values
+`allow`, `deny` and `continue`. `allow` and `deny` permit and reject the
+invite, respectively, and terminate the rulelist evaluation. `continue` skips
+ahead to the next rule in the list.
+
+```json5
+{
+  "type": "m.invite_rules",
+  "content": {
+    "rules": [{
+      "type": "m.user",
+      "user_id": "*:badguys.com",
+      "pass": "deny",
+      "fail": "continue"
+    }, {
+      "type": "m.target_room_id",
+      "room_id": "*:badguys.com",
+      "pass": "deny",
+      "fail": "continue"
+    }, {
+      "type": "m.user",
+      "user_id": "@someone:example.org",
+      "pass": "allow",
+      "fail": "continue"
+    }]
+  }
+}
+```
+
+Unlike [MSC3847], the ignore configuration is subject to the 64 KiB event size
+limit because all rules are stored in a single event.
+
+Similar to [MSC3847], however, glob expressions as well as block-list and
+allow-list configurations are supported.
+
+
+## Summary
+
+The table below compares the proposals in terms of different features. It's
+worth noting that some features such as glob matching could easily be extended
+to all proposals.
+
+|                      | [MSC2270] | [MSC3840] | [MSC4155]     | [MSC3847]           | [MSC3659] |
+|----------------------|-----------|-----------|---------------|---------------------|-----------|
+| Ignore room          | ✅        | ✅        | ❌            | ✅                  | ✅        |
+| Ignore user          | ❌        | ✅        | ✅            | ✅                  | ✅        |
+| Ignore server        | ❌        | ✅        | ✅            | ✅                  | ✅        |
+| Glob matching        | ❌        | ❌        | ❌            | ✅                  | ✅        |
+| Can exceed 64 KiB    | ❌        | ❌        | ❌            | ✅                  | ❌        |
+| Client-side          | ❌        | ✅        | ✅ (optional) | ✅                  | ❌        |
+| Block-list semantics | ✅        | ✅        | ✅            | ✅                  | ✅        |
+| Allow-list semantics | ❌        | ❌        | ✅            | ✅ (with [MSC4150]) | ✅        |
+
+
+[moderation policy lists]: https://spec.matrix.org/v1.11/client-server-api/#moderation-policy-lists
+[module]: https://spec.matrix.org/v1.10/client-server-api/#ignoring-users
+[MSC2270]: https://github.com/matrix-org/matrix-spec-proposals/pull/2270
+[MSC3659]: https://github.com/matrix-org/matrix-spec-proposals/pull/3659
+[MSC3840]: https://github.com/matrix-org/matrix-spec-proposals/pull/3840
+[MSC3847]: https://github.com/matrix-org/matrix-spec-proposals/pull/3847
+[MSC4150]: https://github.com/matrix-org/matrix-spec-proposals/pull/4150
+[MSC4155]: https://github.com/matrix-org/matrix-spec-proposals/pull/4155