All,
The topic of discovery came up during discussions for a GSoC 2020 project to implement a GUI client that talks to the IPP System Service. Since we didn't define anything for discovery in PWG 5100.22, I wrote up a quick proposal on the IPP Sample project wiki:
https://github.com/istopwg/ippsample/wiki/IPP-System-Service-Discovery-with-DNS-SD
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Michael Sweet