A: The DPD is to test the keepalive status of the IKE SA of the IPSec peer, synchronously updating the IKE SA at the local end. The PDP consists of two parameters, namely, idle timer and timeout timer. The idle timer determines whether to send the DPD packet. If the idle timer does not receive encapsulated packets during the timing process, the DPD verification is performed; otherwise, the idle timer is reset. The timeout timer determines whether to perform the DPD verification again. If no DPD reply is received after three consecutive requests (request > timeout > request > timeout > request > timeout), the SA is deleted. To continue sending encrypted data packets, the SA is re-negotiated.