There is one in place from 1999. Mueller was INVITED by Putin to come over to Russia and be part of the interview of the 12 Russian hackers. When questioned about it, Mueller and team said “No Comment”. They got called out.
It’s actually required by a treaty which got the Consent of the Senate:
https://www.state.gov/documents/organization/123676.pdf
Russia (and the US) can ask for this process at any time, it’s not something that Trump can refuse.
If you read it, you can see that both the US and Russia can request questioning of suspects in criminal cases.
Of course Mueller won’t, because he doesn’t have any actual evidence and would look foolish when he asks one of the indicted Russians a question and they have an iron-tight alibi (i.e. I wasn’t working at GRU in this time period).
Mueller’s people get sloppy when they don’t think there will be any court cases where they have to present evidence.