The VPN technology is based on the idea of tunneling. Packets constructed in a specific protocol format can be encapsulated with carrier protocol headers and transparently transmitted over tunnels on the VPN backbone network.
The tunneling technology uses one protocol to encapsulate the packets of another protocol, and the carrier protocol itself can be encapsulated or carried by other protocols. From the perspective of a user, a tunnel is a logical extension of a public switched telephone network (PSTN) or integrated services digital network (ISDN) link and functions in the same way as a physical link.
A VPN tunnel provides the following functions:
Encapsulates user data.
Establishes a link between two endpoints.
Periodically checks link connectivity.
Guarantees data transmission security.
Provides QoS guarantee.