Registered as PARK AGENT, but when it turns up it is usually a consumer router's own service.
IANA has 5431 registered as park-agent on TCP and UDP, an old entry with no published protocol description. The registered service is effectively extinct. What people do report finding on 5431 is a listener on consumer broadband routers — Broadcom-based Belkin, Linksys, and D-Link gateways — that disappears when UPnP is switched off, which points at the gateway's UPnP IGD control service; no vendor documentation confirms the port, so treat that as a field observation.
Do not expose
If this is your router answering, it is a device control surface — it must not be reachable from the WAN side.
$ curl -sS -o /dev/null -w '%{http_code}\n' http://TARGET:5431/Seen 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.