相关原理：
2. The recovery measures described in this annex are intended to minimize the
impacts due to runaway calls (ongoing calls for which the PPC SLP is not
executing) or runaway charging (calls which have ended but for which the PPC
SLP has not received disconnect information).
7. This annex proposes the use of a sanity check procedure and the use of the
CallControlDirective operation by the PPC SLP as a heartbeat check of call
status at reasonable time intervals. This mechanism can be used to address
runaway calls, runaway charging, and to provide a limited retry mechanism for
sequencing notifications when the threshold has been reached during the
temporary failure. If used, the sanity check time interval should be configurable and chosen to provide a suitable time interval for status checking and to avoid overloading the network with heartbeat messages.
相关规范：
3GPP2 N.S0018： TIA/EIA-41-D Pre-Paid Charging