Checked Existing

• I have checked the repository for duplicate issues.

What happened?

When playing Splatfests - currently through third-party orgs and manually replacing the BOSS files, but this was observed during the official Pretendo fest as well - you can lobby up with your team in groups of 4, but when the time comes to join up with an opposing team, 118-0551 (a PIA error) is received.

Screenshot of the state of the game just before the error - a full 4 person team searching for an opposing team to join:

What did you expect to happen?

Another team joins and the game starts normally.

Steps to reproduce?

1. Cause a Splatfest (e.g. adding Splatfestival files, other fests from Gamebanana)
2. Lobby up
3. You can't join with an opposing team

Other relevant information. (OPTIONAL)

Fest matchmaking works by having each team matchmake into a lobby of 4 (friendly people on your team) and then merging with another gathering of 4 to start the match. I am unclear on how that occurs, but it seems the bug is in this step.

My pet theory is that the NEX SwitchGathering notification is used to inform the host of one gathering to disband and move everyone into another, making one 8-person gathering.

It's also possible that the PIA Session protocol is involved in migrating the users around.

It might also be that the NEX server is supposed to do NAT probes for the participants who are moving to the final gathering; or create a new, third gathering that both teams will migrate into.

Needs research, packet dumps of the error occurring, and analysis of the protocols used. Server logs from a failed Fest attempt would also be helpful. Will accept PRs once the nature of the issue is understood.

Originally reported by @ yellowpurple on Discord, thanks!