3.2.2.5.11 QUACK
The client SHOULD check the following conditions before taking prescribed actions:
If the F_CANCELED flag is false, the packet MUST be discarded. No further processing is necessary.
If the packet has body data of length 0, this indicates that the server has orphaned the Current Call. F_CANCELED flag MUST be set to false and the call MUST be transitioned to STATE_FAULT.
The following conditions indicate protocol errors; the packet MUST be discarded with no additional processing:
If the packet has body data, and the length of the body data is less than 9 bytes.
The body version is not zero.
The packet's event ID does not match the call's CANCEL_EVENT_ID.
Having received a valid QUACK where the packet's event ID matches the call's CANCEL_EVENT_ID, F_CANCELED flag MUST be set to false and the call MUST be transitioned to STATE_FAULT.