While debugging a network issue on my laptop with wireshark, I noticed massive local traffic to ports 9229, sending TCP SYN and getting RST back. After stopping the local Music Player Daemon (MPD) instance, that massive traffic also went to port 6600:
Also, a notification popped up telling me that the connection to MPD was lost.
I did not have any application running that controls MPD, so this was strange. Maybe an extension shipped with the MATE Desktop Environment that I use?
I know that MPRIS is used on Linux to remote control media players, so I searched for "mate mpris" and found the commit that introduced that feature.
The commit also showed that there is a setting to disable it at org.mate.SettingsDaemon.plugins.mpris.active.
So after running dconf-editor, disabling the plugin and logging off and on again, the superfluous local network traffic was gone.