A number of different protocols exist that each satisfies the key characteristics of a VPN. The most commonly recognized protocols are:

SOCKS

SSL

· SSL (Secure Sockets Layer), working only with TCP/IP protocols is the primary protocol for secure connections to web servers, requiring a valid site certificate issued from an authorized certificate authority.

· SSL (Secure Sockets Layer) is effectively a VPN protocol at the application layer that allows web browsers to securely connect to web servers, usually for secure credit card connections.

PPTP

· PPTP (Point to Point Tunneling Protocol) is compatible with most network protocols and is characterized by being generally easy to set up. Clients for PPTP tend to be in many operating systems with an abundance of 3rd-party clients available.

· PPTP uses GRE, generic routing encapsulation. PPTP wraps IP packets in GRE packets before sending them down the tunnel.

IPSEC (often written as IPSec)

· IPSec (short for IP Security) is the deFacto standard for Virtual Private Networks and is a TCP/IP-based protocol.

· IPSec is way to authenticate and optionally encrypt IP packets. IPSec transport mode allows for authenticated and encrypted sessions between two nodes and can carry any kind of IP traffic (except multicast).

· Most vendors have gone along with the inevitable and have moved towards emphasizing IPSec. Both Microsoft and Cisco have announced current and future products supporting IPSec.

L2TP

· Originally Microsoft developed PPTP (along with a few other vendors) while Cisco pushed a protocol called L2F which was primarily implemented in Cisco products. A combination of the best features of PPTP and L2F was formed to create L2TP.

· L2TP (Layer Two Tunneling Protocol) supports non-TCP/IP clients and protocols (such as Frame Relay, ATM and SONET) but fails to define any encryption standard.

Although L2TP (Layer 2 Tunneling Protocol) is compatible with most network protocols it is not widely deployed but is common in certain Telco and ISP networks.

For general VPN use the most important are IPSec, PPTP, and L2TP. More information on each of these protocols can be found below.