matrix-spec/content/rooms/v3.md

title	type	weight
Room Version 3	docs	30

This room version builds on version 2 with an improved event format.

Client considerations

This room version changes the format for event IDs sent to clients. Clients should be aware that these event IDs may contain slashes and other potentially problematic characters. Clients should be treating event IDs as opaque identifiers and should not be attempting to parse them into a usable form, just like with other room versions.

Clients should expect to see event IDs changed from the format of $randomstring:example.org to something like $acR1l0raoZnm60CBwAVgqbZqoO/mYU81xysh1u7XcJk (note the lack of domain and the potentially problematic slash).

Server implementation components

{{% boxes/warning %}} The information contained in this section is strictly for server implementors. Applications which use the Client-Server API are generally unaffected by the intricacies contained here. The section above regarding client considerations is the resource that Client-Server API use cases should reference. {{% /boxes/warning %}}

Room version 3 uses the state resolution algorithm defined in room version 2, and the event format defined here.

Event IDs

{{% boxes/rationale %}} In other room versions (namely version 1 and 2) the event ID is a distinct field from the remainder of the event, which must be tracked as such. This leads to complications where servers receive multiple events with the same ID in either the same or different rooms where the server cannot easily keep track of which event it should be using. By removing the use of a dedicated event ID, servers are required to track the hashes on an event to determine its ID. {{% /boxes/rationale %}}

The event ID is the reference hash of the event encoded using Unpadded Base64, prefixed with $. A resulting event ID using this approach should look similar to $CD66HAED5npg6074c6pDtLKalHjVfYb2q4Q3LZgrW6o.

Event IDs should not be sent over federation to servers when the room uses this room version. On the receiving end of an event, the server should compute the relevant event ID for itself.

Additionally, the auth_events and prev_events have had a format change compared to other room versions to make it easier to handle. Instead of a tuple of values, they are now plain lists of events.

{{definition_ss_pdu_v3}}

Changes to APIs

Due to the event ID being removed from the event, some APIs need to change. All APIs which currently accept an event ID must do so with the new format. Servers must append the calculated event ID to all events sent to clients where an event ID would normally be expected.

Because the format of events has changed, servers must be aware of the room version where the event resides so that the server may parse and handle the event. The federation API has taken this concern into consideration by ensuring that servers are aware of (or can find) the room version during a request.

Authorization rules for events

The authorization rules for a given event have changed in this room version due to the change in event format:

• The event no longer needs to be signed by the domain of the event ID (as there is no domain in the event ID), but still needs to be signed by the sender's domain.
• In past room versions, redactions were only permitted to enter the DAG if the sender's domain matched the domain in the event ID being redacted, or the sender had appropriate permissions per the power levels. Due to servers now not being able to determine where an event came from during event authorization, redaction events are always accepted (provided the event is allowed by events and events_default in the power levels). However, servers should not apply or send redactions to clients until both the redaction event and original event have been seen, and are valid. Servers should only apply redactions to events where the sender's domains match, or the sender of the redaction has the appropriate permissions per the power levels.

The remaining rules are the same as room version 1.