UPTIME (NT Resource Kit) and Firewalls

Hi:
I am currently using the UPTIME utility as a valuable tool for all servers in my network to check uptime & availability.
However, some servers have been placed behind a firewall, and now UPTIME seems to fail – unable to connect to HOST.
Does anyone know the nature of how or what protocol/port UPTIME uses to connect to server?
What strategy would have to be used in this situation? And, do you believe that this is indeed the cause? Thank you much.