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+# MSC 2002 - Rooms V4
+
+This MSC proposes creating a new room version named v4 to allow servers to switch
+event ID grammar to that proposed in MSC1884.
+
+## Proposal
+
+The new room version is called "4". The only difference between v4 and v3 is
+that v4 rooms use the grammar for defining event IDs as proposed in MSC1884 -
+expressing event IDs as url-safe base64 rather than plain base64 for the
+convenience of client implementors.
+
+It is not proposed that servers change the default room version used when
+creating new rooms, and it is not proposed that servers recommend upgrading
+existing rooms to v4.
+
+## Rationale and Context
+
+We would like to default to creating rooms with a reasonably secure room
+algorithm in upcoming Matrix 1.0.  We do not want to default to creating v3
+rooms due to the inconvenience the v3 event ID grammar might cause, so instead
+we are proposing favouring v4.
+
+Ideally we would also include other room algorithm changes in v4 (e.g.
+honouring server signing key validity periods, as per
+https://github.com/matrix-org/synapse/issues/4364), but as spec &
+implementation work is still ongoing there, we are proposing using v4 as a
+room version which can be supported in the wild before Matrix 1.0 and then
+used as the initial default for creating rooms.  The expectation is for the
+versions of the spec which coincide with 1.0 to also support v5 rooms, but in
+practice v5 will not be marked as default until it has sufficient adoption on
+the public network.
+
+The expectation is never to recommend upgrading existing
+rooms to v4, but instead v5 (once ready), to avoid overburdening room admins
+with upgrade notifications.
+
+To conclude, the proposed plan is:
+ 1. Define room v4 as MSC1884
+ 2. Introduce servers with v4 support into the public network as rapidly as possible
+ 3. Wait for enough servers to speak v4.  Meanwhile:
+    1. Define an MSC for honouring server signing key validity periods
+    2. Implement this MSC
+    3. Define room v5 as this MSC
+ 4. Release Matrix 1.0, defining room v4 as the new default for creating rooms,
+    but also shipping support for room v5 for the first time.
+ 5. Wait for enough servers to speak v5 rooms.
+ 6. Define room v5 as the new default for creating rooms.
+ 7. Define room versions prior to v5 as unsafe, thus prompting users to upgrade their
+    rooms to v5.
+
+The reason we don't wait for v5 to be widespread before releasing 1.0 is to avoid
+delaying the 1.0 yet further.  It is good enough for 1.0 to support v5 without it
+also being the default for creating rooms.