VPN is a network technology that is used to create secure network connection over a public network Corporations like government organizations , educational institutions use VPN technology so as to help remote users to securely communicate with a private network.A VPN can connect multiple sites over a large distance.

Types of VPN:

1- Secure VPN
2- Trusted VPN
3- Hybrid VPN

Requirements of Secure VPN:

1-Secure VPN requires encrypted and authenticated traffic.

2-All parties in the VPN must agree to the security properties.

3-No outsider (i.e. other than VPN) can affect the security properties of the VPN

Requirements of Trusted VPN:

1-Only trusted VPN provider can affect the creation/modification on a path in the VPN.

2-Only trusted VPN provider can delete data, change data, inject data on a path in the VPN.

3-We establish the routing and addressing before the VPN is created

4-The address boundaries of the secure VPN within the trusted VPN must be extremely clear.

Hybrid VPN requirements

The secure VPN may be subset of trusted VPN in a hybrid VPN.

NOTE: A requirement that is common to all the VPNs is that the VPN administrator must have the extent knowledge of the VPN. 

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VPN Protocols :

Below is the list of VPN protocols that make data transport secure over a public network infrastructure. Each protocol has a varied feature in respect of keeping data secure.

1-IP security (IPSec):

1-It is used for secure communications over the Internet. It is also used as a security layer for other
protocols.

2-IPSec traffic can use either transport mode or tunneling to encrypt data traffic in a VPN.

2.1-Transport mode: this mode encrypts message within the payload (i.e. data packet)

2.2-Tunneling mode: this mode encrypts the entire payload. 

2 Secure Sockets Layer (SSL) and Transport Layer Security (TLS):

1-These protocols uses cryptography for secure communications over the Internet.

2-Handshake authentication method is used that is helpful in negotiating network parameters between the
server and client machines.

3-For initating a connection we require an authentication process and certificates.

3-Point-To-Point Tunneling Protocol (PPTP):

1-This protocol is used to connect a remote client to a private server over the Internet.

2-It is a tunneling protocol (i.e. it encrypts the whole data packet while communication).

3-It is most widely used VPN protocols. We can find it easily with the Windows Operating System.

4-Layer 2 Tunneling Protocol (L2TP):

1-This protocol is used for encrypting data communications traffic between two sites.

2-L2TP is often used with IPSec to secure the transfer of L2TP payload.

3-A VPN implementation that uses L2TP/IPSec needs certificates or shared key.