Chapter 3 Authentication for FortiOS 5.0 : Introduction to authentication : Types of authentication : VPN authentication
  
VPN authentication
Authentication involves authenticating the user. In IPsec VPNs authenticating the user is optional, but authentication of the peer device is required.
This section includes:
Authenticating IPsec VPN peers (devices)
Authenticating IPsec VPN users
Authenticating SSL VPN users
Authenticating PPTP and L2TP VPN users