User Level Authentication on VPN's

I'm doing some auditing on a SonicWall IPsec VPN.

I'm seeing that, in addition to the IKE Preshared Key, it wants a
username/password.  I'm really confused: Doesn't IPsec have no concept
of username/password?  Is this some type of SonicWall extension?  Or
another protocol?

Are there any open source vpn clients that can connect to these type
of VPN's (assuming you have a username/password, of course)?  Any
tools to audit them?

Also, I noticed that when you configure a VPN Client in the standard
Windows XP, it also asks for a username/password.  Is this for the VPN
itself, or so that Windows can do a domain login?  Is this the same
thing that's going on with the SonicWall?

Any help, comments, or links appreciated.

PS Please CC me on all responses, if you don't mind.  Thanks.