Currently Matrix uses matrix.to URIs to reference rooms and other entities in a permanent manner. With just a room ID, users can't get into rooms if their server is not already aware of the room. This makes permalinks to rooms or events difficult as the user won't actually be able to join. A matrix.to link generated using a room's alias is not a permanent link due to aliases being transferable.
In lieu of an improved way to reference entities permanently in Matrix, a new parameter is to be added to matrix.to URIs to assist clients and servers receiving permanent links in joining the room.
For reference, existing permalinks look like this:
https://matrix.to/#/!somewhere:example.org
https://matrix.to/#/!somewhere:example.org/$something:example.org
By adding a new parameter to the end, receivers can more easily join the room:
https://matrix.to/#/!somewhere:example.org?via=example-1.org&via=example-2.org
https://matrix.to/#/!somewhere:example.org/$something:example.org?via=example-1.org&via=example-2.org
Clients can pass the servers directly to /join in the form of server_name
parameters.
When generating the permalinks, clients should pick servers that have a reasonably high chance of being in the room in the distant future. The current recommendation is to pick up to 3 unique servers where the first one is that of the user with the highest power level in the room, provided that power level is 50 or higher. The other 2 servers should be the most popular servers in the room based on the number of joined users. This same heuristic should apply to the first server if no user meets the power level requirements. Servers blocked by server ACLs should not be picked because they are unlikely to continue being residents of the room. Similarly, IP addresses should not be picked because they cannot be redirected to another location like domain names can, making them a higher risk option.