MSC4163: Make ACLs apply to EDUs (#4163)

proposals/4163-make-acls-apply-to-edus.md

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+# MSC4163: Make ACLs apply to EDUs
+
+[Access Control Lists](https://spec.matrix.org/v1.11/client-server-api/#server-access-control-lists-acls-for-rooms)
+(also known as ACLs) are used to prevent other servers from participating in a room at a federation level,
+covering many federation API endpoints, including
+[`/send`](https://spec.matrix.org/v1.11/server-server-api/#put_matrixfederationv1sendtxnid). However, while ACLs
+are applied on a per-PDU basis on this endpoint, they are not applied to EDUs at all. Considering that some EDUs
+are specific to certain rooms (e.g. read receipts & typing indicators), it makes sense to apply ACLs to them as well.
+
+
+## Proposal
+
+All EDUs which are local to a specific room MUST have ACLs applied to them. This means that for the EDUs currently
+in the spec, ACLs would only apply to receipts and typing notifications. Examples of how ACLs should be enforced 
+at the point of receiving a transaction for those two types of EDUs are as follows:
+  - For
+[typing notifications (`m.typing`)](https://spec.matrix.org/v1.11/server-server-api/#typing-notifications),
+the `room_id` field inside `content` should be checked, with the typing notification ignored if the `origin`
+of the request is a server which is forbidden by the room's ACL. Ignoring the typing notification means that the EDU
+MUST be dropped upon receipt.
+  - For [read receipts (`m.receipt`)](https://spec.matrix.org/v1.11/server-server-api/#receipts), all receipts
+inside a `room_id` inside `content` should be ignored if the `origin` of the request is forbidden by the
+room's ACL.
+
+## Potential issues
+
+None considered.
+
+## Alternatives
+
+Leave things as-is, which wouldn't be that big of a deal when you consider that this would only apply
+to typing notifications and read receipts currently, which don't allow for very significant disruption inside
+a room. However, as ACLs are meant to prevent certain servers from participating in a room at all, it makes
+sense to apply ACLs to EDUs which are local to certain rooms, as they are a form of participation.
+
+## Security considerations
+
+None considered.
+
+## Unstable prefix
+
+None required, as no new fields or endpoints are added.
+
+## Dependencies
+
+None.