Link maintenance frames are transmitted periodically to test and maintain the link. The link termination frames are:
Code-reject
Code 0x07. Rejects an LCP frame that has an invalid code field.
Protocol-reject
Code 0x08. Rejects a PPP frame that has an invalid protocol id.
Echo-request
Code 0x09. Requests a response, in the form of an Echo-reply frame, from the remote end-point. Used to test that the link is still up.
Echo-reply
Code 0x10. Responds to a valid Echo-request frame. Used to test that the link is still up.
Discard-request
Code 0x11. Sends a frame which is silently discarded at the remote endpoint. Used as a debugging mechanism.