3Com MSR 50 Series Configuration Manual page 365

Figure 92 CHAP Authentication
Authenticator
Operating mechanism of PPP
PPP operates in the following procedure:
1 Enter the Establish phase before setting up a PPP link
2 Perform LCP negotiation in the Establish phase, which includes the operating
mode (SP or MP), the authentication mode and the maximum transmission unit
(MTU). If the negotiation is successful, LCP will enter the Opened status, indicating
that the underlying layer link has been established.
3 If the authentication (the remote verifies the local or the local verifies the remote)
is configured, the process comes to the Authenticate phase and starts the
CHAP/PAP authentication
4 If the authentication fails, it will come to the Terminate phase to remove the link
and the LCP will go down. If the authentication succeeds, it will proceed to start
the network negotiation (NCP). In this case, the LCP state is still Opened, while the
state of IP control protocol (IPCP) is changed from Initial to Request.
5 NCP negotiation supports the negotiation of IPCP, which primarily refers to the
negotiation of the IP addresses of the two parties. NCP negotiation is conducted
for the purpose of selecting and configuring a network layer protocol. Only the
network layer protocol that has been agreed upon by the two parties in the NCP
negotiation can send packets over the PPP link.
6 The PPP link will remain for communications until an explicit LCP or NCP frame
close it or some external events take place (for example, the intervention of the
user).
Figure 93 PPP operation flow chart
UP
Dead
FAIL
DOWN
Challenge
Rsponse
Ack or Not Ack
OPENED
Establish
Terminate
Introduction to PPP and MP
Authenticatee
Authenticate
FAIL
SUCCESS/NONE
CLOSING
Network
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