Occasionally packets lose their sequencing across a complicated internetwork. Say, for example that a PNS sends packets 0 to 5 to a PAC. Because of rerouting in the internetwork, packet 4 arrives at the PAC before packet 3. The PAC acknowledges packet 4, and may assume packet 3 is lost. This acknowledgment grants window credit beyond packet 4.
When the PAC does receive packet 3, it MUST not attempt to transmit it to the corresponding PPP client. To do so could cause problems, as proper PPP protocol operation is premised upon receiving packets in sequence. PPP does properly deal with the loss of packets, but not with reordering so out of sequence packets between the PNS and PAC MUST be silently discarded, or they may be reordered by the receiver. When packet 5 comes in, it is acknowledged by the PAC since it has a higher sequence number than 4, which was the last highest packet acknowledged by the PAC. Packets with duplicate sequence numbers should never occur since the PAC and PNS never retransmit GRE packets. A robust implementation will silently discard duplicate GRE packets, should it receive any.