What is the Bonjour service _acp-sync._tcp used for? And why is it accessing my Apple AirPort Base station using a long and obscure string?

Solution 1:

Bonjour advertises the services on devices

example

HOSTNAME._smb._tcp

Means the device is sharing folders via smb over TCP (as opposed to using UDP). It is also used with things like AirPrint to share printers.

Now

_acp-sync._tcp

Its over tcp.
It would seem ACP is some sort of configuration protocol used by the monitoring and configuration utilities for the AirPort.

Some people have tried to reverse engineer

https://github.com/x56/airpyrt-tools

I think natutil will tell you that its using TCP Port 5009

Apple AirPort Admin Utility, AirPort Express Assistant, Xwis (TCP/UDP) - Source SG airport-admin Apple AirPort WAP Administration (TCP) - Source NMAP

So yes its some sort of configuration protocol