beautypg.com

How ppp works – H3C Technologies H3C WA2600 Series WLAN Access Points User Manual

Page 13

background image

4-3

5) When the authenticatee receives the authentication request, it encrypts the packet using the MD5

algorithm, with the packet ID and the default CHAP password as the parameters, and then sends

the encrypted packet and its own username to the authenticator (Response).

6) The authenticator encrypts the original randomly-generated packet using the MD5 algorithm, with

the password of the authenticatee it maintains as the parameter, compares the encrypted packet

with the one received from the authenticatee, and returns an Acknowledge or Not Acknowledge

packet depending on the comparison result.

Figure 4-2 CHAP Authentication

Authenticator

Authenticatee

Challenge

Rsponse

Ack or Not Ack

How PPP Works

Figure 4-3

illustrates the PPP operating mechanism.

1) A PPP link is in the Establish phase when it is about to be established. In this phase, LCP

negotiation is performed, where LCP-related settings are determined, including operating mode

(SP or MP), the authentication mode, and the Maximum Transmission Unit (MTU). If the

negotiation is successful, the link enters the Opened state, indicating that the underlying layer link

has been established.

2) If the authentication (the remote authenticates the local or the local authenticates the remote) is

configured, the PPP link goes to the Authenticate phase, where CHAP or PAP authentication is

performed.

3) If the authenticate fails to pass the authentication, the link goes to the Terminate phase, where the

link is torn down and LCP goes down. If the authenticatee passes the authentication, the link goes

to the Network phase. In this phase, NCP negotiation is performed, the LCP state remains Opened,

and the state of IP Control Protocol (IPCP) transitions from Initial to Request.

4) NCP negotiation supports the negotiation of IPCP, through which the IP addresses of both sides

can be determined. NCP negotiation also determines and configures the network layer protocol to

be used. Note that a PPP link can carry a network layer protocol only after the NCP negotiation is

successful.

5) After the NCP negotiation is performed, the PPP link remains active until an LCP or NCP frame

closes it explicitly or some external events take place (for example, the intervention of a user).

This manual is related to the following products: