Internet-Security.com UDP port 1993 details

UDP port 1993 is registered by IANA as stun-p4 for Cisco’s Serial Tunnel (STUN) protocol, used to carry legacy IBM SNA/SDLC traffic over IP between routers.
In real deployments, Cisco IOS routers (e.g., 2500/7200/ISR series) configured for STUN use UDP 1990–1993 (stun-p1 through stun-p4) between defined peers to tunnel traffic from devices like IBM 3270 terminals or 3745 communications controllers across IP networks.
This is largely legacy today but still encountered in some mainframe and industrial environments.

UDP 1993