In traditional telephone network CAC was one of the main components to assure quality of service. In the internet world it might be best to say SAC (service admission control). Admission control is not that easy in IP world, packet network is different than circuit switching. There is a solution with RSVP to reserve bandwidth but it will not scale when you need to reserve 64k on a 40G link for some millions of calls.
One option could be to use ISIS TE extension to announce core link utilization. As soon as a core link reaches a preconfigured threshold, ISIS sends an update with this information to the edge of the network. At the edge RSVP could be used to signal this to the application or Customer edge (CPE).