Internet-Security.com TCP port 1990 details

TCP port 1990 is used by Cisco Serial Tunnel (STUN), an IOS feature for tunneling IBM SNA/SDLC/QLLC traffic over IP networks.
IANA registers 1990/tcp as stun-p1; Cisco STUN commonly uses 1990–1993 for priority queues, with 1990 as priority 1.
Real-world use: enterprises historically used Cisco 2500/2600/7200 routers running STUN to link IBM 3174/3745 front-end controllers and remote 3270 terminal networks across IP WANs.
You’ll still see tcp/1990 in legacy environments where STUN peers form long-lived TCP sessions between branch and data-center routers.

TCP 1990