PPP defines an extensible LCP that allows negotiation of an authentication protocol for authenticating its peer before allowing network layer protocols to transmit over the link. RFC 1334 defines two protocols for authentication, PAP and CHAP, as shown in the figure.
PAP is a very basic two-way process. There is no encryption. The username and password are sent in plaintext. If it is accepted, the connection is allowed. CHAP is more secure than PAP. It involves a three-way exchange of a shared secret.
The authentication phase of a PPP session is optional. If used, the peer is authenticated after LCP establishes the link and chooses the authentication protocol. If it is used, authentication takes place before the network layer protocol configuration phase begins.
The authentication options require that the calling side of the link enter authentication information. This helps to ensure that the user has the permission of the network administrator to make the call. Peer routers exchange authentication messages.