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166 CHAPTER 12: CONFIGURING PPP AND MP
The authenticator sends some randomly generated packets to the requester
(challenge), and at the same time it sends its configured username to the
requester.
When the requester receives the challenge, it will look for the user password
according to the authenticator's username and its own user list. If it finds the user
in the user list with the same name as the authenticator's username, the requester
builds the response with its own password, serial number of packet using MD5
algorithm, and sends the generated response and its configured username to the
authenticator (response).
After receiving the response from the requester, the authenticator does the same
encryption as the requester with the saved password, serial number of packet
using MD5 algorithm. Then it compares the encryption result with the response
from requester, and returns the response according to the comparison result
(Acknowledge or Not Acknowledge).
3 Phases of PPP negotiation:
a When the physical layer is unavailable, the link is in Dead phase. A link shall start
from the Dead phase. When the physical layer becomes available, PPP link enters
the Establish phase.
b In Establish phase, PPP link carries out LCP negotiation, including negotiation
of working mode (SP or MP), authentication mode and maximum transmission
unit etc. After the successful LCP negotiation, the status of LCP is Open,
indicating that the link has been established.
c If the authentication is not configured, it begins NCP negotiation. At this time,
the status of LCP is still Open, while the status of NCP is changed from Initial
to Request-sent.
d If the authentication is configured (the remote verifies the local or the local
verifies the remote), it enters Authenticate phase to start CHAP or PAP
authentication. If the authentication fails, it enters Terminate phase, the link is
removed and LCP turns down. After successful authentication, the NCP
negotiation begins. At this time, the status of LCP is still Open, while the status
of NCP is changed from Initial to Request-sent.
e NCP negotiation supports the negotiations of IPCP and IPXCP, of which IPCP
negotiation mainly includes the IP addresses of two partners. One or more
network layer protocols is selected and configured through NCP negotiation.
The selected network layer protocol must be configured successfully before this
network layer protocol sends packets through this link.
f PPP link will remain in communication status until a specific LCP or NCP frame
closes this link or some external events take place (for example, the
intervention of user).
Phases of PPP negotiation are shown in the following diagram.