One feature of the PPTP sliding window protocol is that it allows the acknowledgment of multiple packets with a single acknowledgment. All outstanding packets with a sequence number lower or equal to the acknowledgment number are considered acknowledged. Time-out calculations are performed using the time the packet corresponding to the highest sequence number being acknowledged was transmitted.
Adaptive time-out calculations are only performed when an Acknowledgment is received. When multi-packet acknowledgments are used, the overhead of the adaptive time-out algorithm is reduced. The PAC is not required to transmit multi-packet acknowledgments; it can instead acknowledge each packet individually as it is delivered to the PPP client.