Virtual Private Network

networking, security (VPN) The use of encryption in the

lower protocol layers to provide a secure connection through

an otherwise insecure network, typically the Internet. VPNs

are generally cheaper than real private networks using private

lines but rely on having the same encryption system at both

ends. The encryption may be performed by firewall software

or possibly by routers.

Link-level (layer 2 and 3) encryption provides extra

protection by encrypting all of each datagram except the

link-level information. This prevents a listener from

obtaining information about network structure. While

link-level encryption prevents traffic analysis (a form of

attack), it must encrypt/decrypt on every hop and every

path.

Protocol-level encryption (layer 3 and 4) encryption encrypts

protocol data but leaves protocol and link headers clear.

While protocol-level encryption requires you to

encrypt/decrypt data only once, and it encrypts/decrypts only

those sessions that need it, headers are sent as clear text,

allowing traffic analysis.

Application (layer 5 up) encryption is based on a particular

application and requires that the application be modified to

incorporate encryption.

Cisco.

(1999-11-15)