T.120 conferencing — in practice Microsoft NetMeeting's application sharing, whiteboard, chat, and file transfer.
ITU-T T.123 specifies TCP 1503 as the transport port for the T.120 multipoint data conferencing stack, and IANA registers it as "imtc-mcs" to Databeam, one of T.120's authors. The only implementation most people ever met is Microsoft NetMeeting, which used 1503 for program sharing, whiteboard, chat, and file transfer while H.323 audio/video signalling ran on 1720. Microsoft carried the same stack into Windows Live Messenger's sharing features, and both products are long dead. A listener on 1503 today means legacy conferencing software on an old Windows box.
Do not expose
T.120 program sharing hands a remote party control of the desktop, with authentication that is per-conference at best — this belongs on no internet-facing host, and the software behind it has been unpatched for over a decade.
$ nmap -sV -p 1503 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.