PPP link establishment process
Figure 2 shows the PPP link establishment process.
Figure 2: PPP link establishment process
Initially, PPP is in Link Dead phase. After the physical layer goes up, PPP enters the Link Establishment phase (Establish).
In the Link Establishment phase, the LCP negotiation is performed. The LCP configuration options include Authentication-Protocol, Async-Control-Character-Map (ACCM), Protocol-Field-Compression (PFC), Address-and-Control-Field-Compression (ACFC), and MP. If the negotiation fails, LCP reports a Fail event, and PPP returns to the Dead phase. If the negotiation succeeds, LCP enters the Opened state and reports an Up event, indicating that the underlying layer link has been established. (At this time, the PPP link is not established for the network layer, and network layer packets cannot be transmitted over the link.)
If authentication is configured, the PPP link enters the Authentication phase, where PAP, CHAP, MS-CHAP, or MS-CHAP-V2 authentication is performed. If the supplicant fails to pass the authentication, the link reports a Fail event and enters the Link Termination phase, where the link is torn down and LCP goes down. If the supplicant passes the authentication, a Success event is reported.
If a network layer protocol is configured, the PPP link enters the Network-Layer Protocol phase for NCP negotiation, such as IPCP negotiation or IPv6CP negotiation. If the NCP negotiation succeeds, the link goes up and becomes ready to carry negotiated network-layer protocol packets. If the NCP negotiation fails, NCP reports a down event and enters the Link Termination phase.
If the interface is configured with an IP address, the IPCP negotiation is performed. IPCP configuration options include IP addresses of the two ends, IP compression protocol, and DNS server IP address. After the IPCP negotiation succeeds, the link can carry IP packets.
After the NCP negotiation is performed, the PPP link remains active until explicit LCP or NCP frames close the link, or until some external events take place (for example, the intervention of a user).
For more information about PPP, see RFC 1661.