Inter-Access Point Protocol, from a withdrawn IEEE trial-use standard for AP-to-AP roaming handoff.
IANA registered 3517 in 2002 for the IEEE 802.11 working group's Inter-Access Point Protocol, the mechanism by which access points on the same subnet told each other that a station had roamed and handed over its context. It was published as IEEE 802.11F, a trial-use standard, and withdrawn in 2006; vendors moved to controller-based architectures and CAPWAP instead. If you see it, you are looking at an old autonomous access point on a management VLAN, not at anything current.
Do not expose
This is wireless infrastructure signalling between APs on a trusted segment, with no authentication worth the name — it has no business crossing a routed boundary, let alone reaching the internet.
$ nmap -Pn -sU -sT -p 3517 TARGETSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.