3Com 3C13636 Configuration Manual page 294

3Com Router 3000 Ethernet Family
Configuration Guide
After receiving the Response, the authenticator looks up its local user database
for a match according to the username of the authenticatee in the Response.
When a match is found, it encrypts the original randomly generated packet with
the authenticatee password and the MD5 algorithm, compares the encryption
result with the received ciphertext, and returns an Acknowledge or Not
Acknowledge packet depending on the comparison result.
Initiate an Initiate an
authentication authentication
request request
Encrypt the packet Encrypt the packet
w ith the passw ord w ith the passw ord
of Router 2 of Router 2
Figure 1-1 CHAP Authentication
II. Operating mechanism of PPP
Following is how PPP operates:
1) Before setting up a PPP link, enter the Establish phase.
2) Carry out LCP negotiation in the Establish phase, which includes the negotiation
in operating mode (SP or MP), authentication mode and maximum receive unit
(MRU). If the negotiation is successful, LCP will enter the Opened status,
indicating the setup of the bottom layer link.
3) If the authentication (the remote verifies the local or the local verifies the remote)
is configured, it enters the Authenticate phase and starts the CHAP/PAP
authentication
4) If the authentication fails, it will enter 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).
Authenticator Authenticator
Router1, pass1 Router1, pass1
3Com Corporation
Chapter 1 PPP and MP Configuration
Authenticatee Authenticatee
Router2, pass2 Router2, pass2
1-2
Look for the matched passw ord Look for the matched passw ord
and encrypt the packet and encrypt the packet