Early on while we were playing this save, we were having extreme latency issues, despite having more than enough bandwidth, and ICMP ping latency being consistently low. In response, I set up QoS and traffic shaping on my router, which worked amazingly well for Factorio; he went from having 20+ tick ...

I got Wireshark out and confirmed that the problem was packet loss. By comparing a successful and unsuccessful (from my friend) connection (from me) on both ends, we determined it was a router configuration issue that was causing it to drop the middle fragments of one of the handshake messages ...

I've been playing modded Factorio multiplayer with a friend for several weeks. We started on 2.0.69, and are now on 2.0.72.

We had some trouble initially, which lead us to switch from IPv4 to IPv6, because he couldn't connect to my server over IPv4 due to it not recognizing that mods were synced ...