Which of the following is NOT a commonly used VPN protocol?

A. IPsec
B. L2TP
C. SMTP
D. OpenVPN

Answer: C

Explanation:

A. IPsec (Internet Protocol Security)

IPsec is a suite of protocols used to secure IP communications by authenticating and encrypting each IP packet in a communication session.  

• How it works: It operates at the Network Layer (Layer 3). It includes sub-protocols like AH (Authentication Header) for integrity and ESP (Encapsulating Security Payload) for confidentiality.  

• Usage: It is the primary choice for "Site-to-Site" VPNs (connecting two offices).

B. L2TP (Layer 2 Tunneling Protocol)

L2TP is a tunneling protocol that does not provide any encryption or confidentiality by itself.  

•  The Catch: Because L2TP lacks encryption, it is almost always paired with IPsec (referred to as L2TP/IPsec). L2TP creates the "tunnel," and IPsec provides the "security."  

• OSI Layer: As the name suggests, it operates at the Data Link Layer (Layer 2).

C. SMTP (Simple Mail Transfer Protocol)

SMTP is an application-layer protocol used specifically for the transmission of email.  

• Why it's the Answer: SMTP has nothing to do with creating a secure network tunnel. It is used by mail servers and clients to send messages to one another.  

• Security Nuance: While you can secure SMTP using STARTTLS or S/MIME, it remains a messaging protocol, not a Virtual Private Network protocol.

D. OpenVPN

OpenVPN is a popular open-source VPN protocol that uses the OpenSSL library to provide encryption.  

•  How it works: It can operate over either TCP or UDP and uses SSL/TLS for its key exchange.  

• The Benefit: Because it can run on port 443 (the same port as standard web traffic), it is very effective at bypassing restrictive firewalls that might block traditional IPsec traffic.