3.1.5.9 Acknowledgment Time-Outs
The PPTP profile does not implement acknowledgment-related time-outs as specified in [RFC2637] section 4.4. The PAC/PNS SHOULD start the Packet Acknowledgment Timer (section 3.1.2) when it receives a GRE data packet and there are no outstanding GRE packets to be sent. When the timer expires and there are no outstanding GRE data packets to send, the PAC/PNS SHOULD send the acknowledgment GRE packet to the peer; otherwise, the sent GRE data packets will also carry the acknowledgment and the Packet Acknowledgment Timer SHOULD be stopped.