PPP Phase Diagram

The process of establishing, maintaining, and terminating PPP links passes through several distinct phases, during which PPP frames are exchanged between the endpoints of the link.

• Link establishment phase

• Peer authentication phase (optional)

• Network layer protocol phase

• Link termination phase

Figure A-1 shows the transitions between these phases.

Figure A-1 PPP Link Operating Phases

Link Establishment Phase

The Link Control Protocol (LCP) is used to configure the PPP link during the link establishment phase. The configuration parameters are negotiated by an exchange of LCP frames. If the negotiation converges, the link is established and either the authentication protocol or network layer protocol phase is started. If the endpoints fail to negotiate a common configuration for the link, it is closed immediately.

During the link establishment phase only LCP frames should be transmitted in the PPP frames. All other frames are discarded.

Peer Authentication Phase (optional)

Optionally, one or both of the endpoints can request peer authentication. Solstice PPP supports peer authentication based on the Password Authentication Protocol (PAP) and the Challenge-Handshake Authentication Protocol (CHAP). If authentication is successful, the link remains up and the network layer protocol phase is started. If authentication fails, the link termination phase is started to close the link.

During the peer authentication phase only LCP and authentication frames should be transmitted in the PPP frames. All other frames are discarded.

Network Layer Protocol Phase

The Network Control Protocols (NCP) are used to configure the appropriate network layer. Solstice PPP implements the Internet Protocol Control Protocol (IPCP) to configure IP over PPP.

Once the network layer is configured by an exchange of NCP frames, IP datagrams can be encapsulated for transmission across the link. LCP frames may be exchanged periodically to test and maintain the link.

During the network layer protocol phase, LCP, NPC, and IP frames should be transmitted in the PPP frames. All other frames are discarded.

Link Termination Phase

The link may be terminated at any time, at the request of either endpoint. Termination may occur due to authentication failure, carrier loss, or the expiration of the inactivity timeout. Link control protocol (LCP) frames are exchanged to terminate the link.

During the link termination phase only LCP frames should be transmitted in the PPP frames. All other frames are discarded.