What caused traffic on port 631?

After I turn on my computer, something triggers tcp packets from localhost with source port 631. I replicated the packets using the command ipptools.

Assuming I got it right, is there a way for me to find out which file/application triggers the call of this command?

The packets are triggered randomly, i.e., x minutes after I turn on my computer.

SIDENOTES: my internet adapters are disabled, and I’ve denied all incoming traffic and port 631 in my firewall. lsof -i does not list port 631.