Hello Baljit,
you can find here all the details about IPCP
http://www.faqs.org/rfcs/rfc1332.html
IPCP can assign an ip address taken from a pool like DHCP.
DHCP can assign always the same ip address to a specific MAC address (reservation).
IPCP can receive the ip address conditioned on the authentication phase that identify the user.
IPCP scalability is achieved by using external AAA servers (typically RADIUS) instead of storing username /pwd pairs on the local device.
the mechanism is quite simple with the two parts that exchange IPCP frames in order to reach an agreement on what ip addresses to use on the PPP link.
The client side accepts the server proposal and can install a default route.
Hope to help
Giuseppe
