Encryption can happen at different layers of an operating system and network stack. Where does PPTP encryption take place?

A.
Data link layer

B.
Within applications

C.
Transport layer

D.
Data link and physical layers

Explanation:
A: The Point-to-Point Tunneling Protocol (PPTP) is a method for implementing virtual private networks (VPNs). It is a Microsoft-proprietary VPN protocol that works at the data link
layer of the OSI model. PPTP can only provide a single connection and can only work over PPP connections.
B is incorrect because end-to-end encryption takes place within the applications. End-to-end encryption means that only the data payload is encrypted. If encryption works at any
layer of the OSI model, then headers and trailers can also be encrypted. Since PPTP works at the data link layer, headers and trailers from the upper layers can be encrypted and
protected along with the data payload.
C is incorrect because SSL is an example of an encryption technology that works at the transport layer, not PPTP. SSL uses public key encryption and provides data encryption,
server authentication, message integrity, and optional client authentication to display secured portions of a Web site to a user. When HTTP runs over SSL, you have HTTP Secure
(HTTPS). HTTP works at the application layer, but SSL still works at the transport layer.
D is incorrect because PPTP works at the data link layer, but not the physical layer. The physical layer technologies convert the bits from the data link layer into some type of
transmission format. If the data transmission is taking place over a UTP connection, then the data is converted into electronic voltage at the physical layer. If data transmission is taking
place over fiber lines, then the data is converted into photons. Specifications for the physical layer include the timing of voltage changes, voltage levels, and the physical connectors for
electrical, optical, and mechanical transmission.