VPN / RAS problem Server 2003

I have 2 Mac users that access a source code server on the network. They access the network through Windows 2003 Server VPN/RAS. They can connect and find the servers on the network, but they have to be constantly pinging the server to keep the connection open. Otherwise it shuts down. My question is, is this a settings problem on the server? Or is this a problem with Mac clients? Has anyone else heard of such a problem?