Tunneling protocol used between MS and PDSN:
- 'notunnel' indicates no tunnel encapsulation
- 'l2tp' indicates tunnel encapsulation using l2tp protocol
- 'l2f' indicates tunnel encapsulation using l2f protocol
- 'gre' indicates tunnel encapsulation using
GRE protocol for mobileip
- 'ip-in-ip' indicates tunnel encapsulation using
IP-in-IP protocol for mobile
- 'other' in other cases