TTL Scope is constrained by the ``Time To Live" (TTL) field in the IP packets a multicast application sends. An application sends its data with a fixed TTL. Each multicast router that the data traverses decreases the TTL on the packets by one. Each multicast router also has a threshold associated with each multicast capable interface or multicast tunnel. If the TTL on the data packets arriving at a router is lower that the threshold of the interface or tunnel, then the packet is not forwarded out of that interface.
The multicast tunnels comprising the Mbone are configured so that it is possible to constrain a conference to a particular region of the Mbone by careful setting of the TTL. Thus the multicast tunnels in Europe are generally configured with the following thresholds: