When a user dials in to a PPP-compatible server, three things happen:
The PPP Data Link Control layer is a slightly modified version of the HDLC layer. The HDLC format, extensively used by IBM and others for synchronous data transfer, was modified by adding a 16-bit protocol field that allows PPP to multiplex traffic for several Network Control Protocol layers. This encapsulation frame has a 16-bit checksum, but the size of this field can be negotiated.
LCP can negotiate with any of these authentication protocols: