firewall authentication problem


I have two group of users connected to internet through a firewall for
traffic control. Both should be authenticated by firewall through an
AAA server before connecting to internet and there are different user
policies for different users in firewall.

1) LAN users.
2) Dial-up users which are connected to a router, and are
authenticated by an AAA server (for logging in)
The problem is that I don't want a second authentication. I want the
users logged in, can be automatically authenticated by the firewall.
Firewall should somehow know their username to apply the policies.
There should be a mapping from IP to username in AAA server. I think
it should be a function of the router to send the IP to AAA server
after successful authentication. AAA server can be a RADIUS or LDAP

Is this possible? If yes, please tell me how.