Which of the following is considered the most common VPN security protocol?

The most common VPN security protocol among the listed options is often recognized as IPSec. IPSec is widely used for securing internet protocol communications by authenticating and encrypting each IP packet in a communication session. It operates at the network layer and is robust in protecting data integrity, confidentiality, and authenticity. This normative approach has solidified IPSec as a foundational protocol for securing virtual private networks, making it the standard choice in many enterprise environments.

While SSL is a significant protocol used for encrypting data, particularly in web communications, it is generally not classified as a dedicated VPN protocol on its own and is primarily associated with securing HTTP traffic, leading to confusion regarding its rank compared to IPSec. SSTP is a specialized protocol that leverages SSL for VPN connections and is used in specific situations, while SSH is primarily focused on secure remote access to systems, not directly as a VPN protocol. Thus, the distinction clearly positions IPSec as the preeminent choice for VPN security, given its extensive application and foundational role in VPN architecture.