MSC4373: Server opt-out of specific EDU types

proposals/4373-edu-types.md

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+# MSC4373: Server opt-out of specific EDU types
+
+Some servers may wish to not receive specific types of EDUs, such as presence, to
+cut down on the amount of bandwidth used (as an example).
+
+## Proposal
+
+### `GET /_matrix/federation/v1/edutypes`
+
+This endpoint dictates what types of EDUs the server wishes to receive.
+
+The response for this endpoint is shaped like the following:
+
+```json
+{
+    "m.presence": false,
+    "m.receipt": true,
+    "m.typing": true
+}
+```
+
+The allowed types are:
+* `m.presence`
+* `m.receipt`
+* `m.typing`
+
+Other types of EDUs (signing key updates, device lists, to-device messaging, etc)
+are likely unsafe to opt-out of.
+
+This endpoint SHOULD NOT require authentication, but if provided, follow the normal
+verification process.
+
+If the EDU type is listed, and is set to `false`, any EDUs of that type
+SHOULD NOT be sent to the target homeserver. The target server MUST ignore
+EDUs of these types.
+
+If the EDU type is set to `true` or absent, the EDU MAY be sent to the target 
+homeserver, unless other factors disallow it (such as room ACLs, where that is
+relevant).
+
+The endpoint response SHOULD be cached. Servers SHOULD read the `Cache-Control`
+HTTP header, and max it out at a week, using a week if the header is absent
+or malformed.
+
+## Potential issues
+
+Older homeservers (or simply non-compliant ones) will still send unwanted EDUs,
+although these can just be ignored.
+
+## Alternatives
+
+An explicit allowlist is recommended over allowing everything with a list of keys
+that SHOULD NOT be put in the list, to ensure important information in new EDU types
+is not ignored or not sent to homeservers.
+
+While the alternative design is more flexible, it introduces unnecessary risk by
+defaulting to allowing filtering of all EDU types, which is bad for the reasons
+described above.
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+`/_matrix/federation/unstable/io.fsky.vel/edutypes`
+
+## Dependencies
+
+None.