kaustubh, read this link to help you understand the overall standards of VPN, the answer is there - IPsec.
http://www.cisco.com/en/US/tech/tk583/tk372/technologies_tech_note09186a0080094203.shtml
Ipsec VPN involves two phases - phase-1 and phase-2
IPSec Phase 1 Internet Key Exchange Security Association policy to be used to negotiate the tunnel, which consists of the following:
-Encryption algorithm for IPSec VPN tunnel, which must be the same for both devices-DES, 3DES, AES-128, AES-192, or AES-256. The default is 3DES.
-Authentication algorithm for the IPSec VPN tunnel, which must be the same for both devices-MD5 or SHA. The default is SHA.
Diffie Hellman Group, which must be the same for both devices-group 1, group 2, group 5, or group 7. The default is group 2.
4. IPSec Phase 2 Encryption and Authentication policy to be applied to the VPN tunnel. The parameters and options consist of the following:
-Encryption algorithm for IPSec VPN tunnel, which must be the same for both devices-DES, 3DES, AES-128, AES-192, or AES-256. The default is 3DES.
-Authentication algorithm for the IPSec VPN tunnel, which must be the same for both devices-MD5 or SHA. The default is SHA
Regards
Jorge Rodriguez
