PPP can terminate the link at any time. This might happen because of the loss of carrier, authentication failure, link quality failure, the expiration of an idle-period timer, or the administrative closing of the link.
LCP is used to close the link through an exchange of Terminate packets. When the link is closing, PPP informs the network-layer protocols so that they may take appropriate action.
After the exchange of Terminate packets, the implementation SHOULD signal the physical-layer to disconnect in order to enforce the termination of the link, particularly in the case of an authentication failure. The sender of the Terminate-Request SHOULD disconnect after receiving a Terminate-Ack, or after the Restart counter expires. The receiver of a Terminate-Request SHOULD wait for the peer to disconnect, and MUST NOT disconnect until at least one Restart time has passed after sending a Terminate-Ack. PPP SHOULD proceed to the Link Dead phase.
Any non-LCP packets received during this phase MUST be silently discarded.