VPN tunnelling involves establishing and maintaining a logical network connection. On this connection, packets constructed in a specific VPN protocol format are encapsulated within some other base or carrier protocol, then transmitted between VPN client and server, and finally de-encapsulated on the receiving side.
For internet-based VPNs, packets in one of several VPN protocols are encapsulated within internet protocol (IP) packets. VPN protocols also support authentication and encryption to keep the tunnels secure.
- Types of VPN Tunnelling:
VPN supports two types of tunnelling – voluntary and compulsory. Both types of tunnelling are commonly used.
In voluntary tunnelling: the VPN client manages connection setup. The client first makes a connection to the carrier network provider (an ISP in the case of internet VPNs). Then, the VPN client application creates the tunnel to a VPN server over this live connection.
In compulsory tunnelling: the carrier network provider manages VPN connection setup. When the client first makes an ordinary connection to the carrier, the carrier in turn immediately brokers a VPN connection between that client and a VPN server. From the client's point of view, VPN connections are set up in just one step compared to the two-step procedure required for voluntary tunnels.
Compulsory VPN tunnelling authenticates clients and associates them with specific VPN servers using logic built into the broker device. This network device is sometimes called the VPN front end processor (FEP), network access server (NAS) or point of presence server (POS). Compulsory tunnelling hides the details of VPN server connectivity from the VPN clients and effectively transfers management control over the tunnels from clients to the ISP. In return, service providers must take on the additional burden of installing and maintaining FEP devices.
- VPN Tunnelling Protocols:
Several computer network protocols have been implemented specifically for use with VPN tunnels. The three most popular VPN tunnelling protocols listed below continue to compete with each other for acceptance in the industry. These protocols are generally incompatible with each other.
- Point-to-Point Tunnelling Protocol (PPTP):
Several corporations worked together to create the PPTP specification. People generally associate PPTP with Microsoft, because nearly all flavours of Windows include built-in client support for this protocol. The initial releases of PPTP for Windows by Microsoft contained security features that some experts claimed were too weak for serious use. Microsoft continues to improve its PPTP support, though.
- Layer Two Tunnelling Protocol (L2TP):
The original competitor to PPTP for VPN tunnelling was L2F, a protocol implemented primarily in Cisco products. In an attempt to improve on L2F, the best features of it and PPTP were combined to create a new standard called L2TP. Like PPTP, L2TP exists at the data link layer (layer two) in the OSI model – thus the origin of its name.
- Internet Protocol Security (IPsec):
IPsec is actually a collection of multiple related protocols. It can be used as a complete VPN protocol solution or simply as the encryption scheme within L2TP or PPTP. IPsec exists at the network layer (layer three) of the OSI model.