Add MSC2540 for stricter validation of event JSON.

proposals/2540-stricter-event-validation.md

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+# MSC2540: Stricter event validation: JSON compliance
+
+## Background
+
+There has been [prior discussions](https://github.com/matrix-org/matrix-doc/issues/1646)
+about validating events more strictly. This MSC proposes fixing a small piece of
+this: JSON compliance.
+
+The [Canonical JSON](https://matrix.org/docs/spec/appendices#canonical-json)
+specification requires that numbers that are serialized in JSON are integers in
+the range of [-2 ^ 53 + 1, 2 ^ 53 - 1], which matches the requirements of
+[section 6 of RFC 7159](https://tools.ietf.org/html/rfc7159). Note that it is
+not explicit, but all floats are invalid.
+
+It is worth mentioning that there are common extensions to JSON which produce 
+invalid JSON according to the Matrix specification, some programming langauges
+even support these by default. One common additional feature is handling
+"special" float values: `Infinity`, `-Infinity`, and `NaN`.
+
+
+## Proposal
+
+In a future room version, Matrix server implementations should strictly enforce
+the Canonical JSON specification for events.
+
+The rationale for doing this in a future room version is to avoid a split brain
+room -- where some federated servers believe an event is valid and others reject
+it as invalid. Rooms will be able to opt into this behavior as part of a room
+version upgrade.
+
+
+## Potential issues
+
+N/A
+
+
+## Alternatives
+
+It could be argued that this MSC is unnecessary since it does not add any new
+requirements for handling of JSON data. Unfortunately starting to enforce these 
+requirements in current rooms could cause federation to break as homeservers
+will disagree on whether events are valid.
+
+
+## Security considerations
+
+N/A
+
+
+## Unstable prefix
+
+A room versions of `org.matrix.strict_canonicaljson` until a future room version
+is available. This room version will use
+[room version 5](https://matrix.org/docs/spec/rooms/v5) as base and include the
+above modifications.