3.2.3.5.5 PING

If the packet sequence is higher than all of the activity's active calls, the server MUST reply with a NOCALL without body data. Otherwise, if the activity contains no active call for the packet sequence, discard the packet.

Otherwise, the packet matches an active call. Because client packets might be duplicated and reordered in transit, the server MAY ignore the packet using implementation-specific criteria in order to avoid redundant responses.<93> If not, the server MUST check the Call State, as specified in the sections that follow.