Room IDs in Matrix are generally not routable on their own. In the room ID grammar !opaque_id:domain,
the domain is the server that created the room. There is, however, no guarantee that this server is
still joined to the room at a later time. Therefore, room IDs don't provide a reliable resident server
to send requests to. Critically, the domain is not to be used as a routing server. It is purely a namespace.
The spec partially addresses this issue by defining a via query parameter on room URIs that can be
used to list servers that have a high probability of being in the room in the distant future. Additionally,
some APIs such as /_matrix/client/v3/join/{roomIdOrAlias} can take a server_name query parameter
that has the same effect as via.
The terminology difference between server_name and via can be slightly confusing which is why this
proposal attempts to standardize on via.
The server_name query parameter on /_matrix/client/v3/join/{roomIdOrAlias} and
/_matrix/client/v3/knock/{roomIdOrAlias} is deprecated and a new parameter via: [string] is
introduced.
Clients SHOULD use via when the homeserver they're talking to supports it. To do this, they MAY either
detect server support through the supported spec versions in /_matrix/client/versions or always include
both parameters (with identical values).
Homeservers MUST ignore all server_name parameters if any via parameters are supplied.
As with any migration, some effort will be required to update client and server implementations. Additionally, while the transitions isn't completed, the concurrent existence of both query parameters might lead to further confusion.
None other than accepting status quo.
A client that supplies different via and server_name parameters could be served a different room depending
on which set of parameters the server uses to resolve the room ID. Tricking a client into doing this seems very
difficult though because Matrix URIs, for instance, only have a single documented via parameter.
Until this proposal is accepted into the spec, implementations SHOULD refer to via as org.matrix.msc4156.via.
None.