In practice a KasmVNC browser desktop; on paper, Novell's Jetstream messaging protocol.
IANA registered 6901/tcp as "jetstream" (Novell Jetstream messaging protocol) in 2010, but that is not what you find on a modern scan. KasmVNC and the Kasm Workspaces container images publish their web desktop on 6901 — the documented run line is docker run -p 6901:6901 kasmweb/firefox and the desktop is then reachable at https://host:6901 with a container user such as kasm_user. Kasm's own fixed-infrastructure docs use the same port when registering a standalone KasmVNC server. Seeing it open means someone is running a browser-accessible remote desktop.
Do not expose
It is a full remote desktop, and the stock container images ship with well-known default credentials; publish it through the Kasm gateway or a reverse proxy with real authentication, not directly.
$ curl -kI https://TARGET:6901/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.