HTTP 410 Gone Status

proposals/0000-http-gone-status.md

@@ -0,0 +1,59 @@
+# MSC0000: Handling 410 Gone Status in Matrix Server Discovery
+
+*This proposal aims to improve the efficiency of Matrix server discovery by introducing a mechanism
+for homeservers to handle the `410 Gone` HTTP status code. This will help reduce unnecessary
+requests to servers that are intentionally no longer available, optimising network resources and
+improving overall performance.*
+
+## Proposal
+
+Currently, when a Matrix homeserver attempts to discover another server, as a first step it checks
+that server's `/.well-known/matrix/server` endpoint for possible delegation. Any status other than
+`200 OK` is currently considered an error and should result in continuing the discovery process,
+checking for SRV records and/or trying the default port.
+
+This proposal suggests that homeservers should interpret a `410 Gone` status code as an indication
+that the server is permanently offline and should suspend further discovery attempts for a period
+of time specified by the homeserver's implementation.
+
+This will avoid unnecessary DNS lookups and requests to servers that no longer wish to federate.
+
+### Server-Server API Changes
+
+When a homeserver receives a `410 Gone` response from the `/.well-known/matrix/server` endpoint,
+it should consider the server offline and halt discovery.
+
+The period until discovery is re-attempted can be determined by the homeserver's implementation,
+such as using exponential backoff, waiting a period of days, or until the next restart.
+
+### Implementation Details
+
+- Homeservers should log the `410 Gone` response and the suspension period for transparency and
+  debugging purposes.
+- The suspension period should be configurable to allow administrators to fine-tune the behaviour
+  based on their specific needs and network conditions.
+- Homeservers should resume discovery attempts after the suspension period has elapsed, or when new
+  PDUs or EDUs are received from the server as this should signal that the server is online.
+
+## Potential Issues
+
+If the suspension period is too long, it may delay the detection of a server that has come back
+online. Conversely, too short and it may lead to unnecessary requests.
+
+## Alternatives
+
+- **Permanent Suspension**: Instead of a configurable suspension period, homeservers could
+  permanently stop discovery attempts for servers that return a `410 Gone` status. However,
+  this approach may lead to issues if the server comes back online.
+- **No Change**: Homeservers could continue to handle the `410 Gone` status as they currently do,
+  without any special treatment. This would not address the inefficiencies (and unwanted requests)
+  caused by repeated discovery attempts.
+
+## Unstable Prefix
+
+No unstable prefix has been proposed as the `410 Gone` status currently causes servers to continue
+discovery as normal.
+
+## Dependencies
+
+This MSC does not have any dependencies on other MSCs.