Definition
A VPN protocol is the technical specification that defines how a VPN client and server establish a tunnel, authenticate, exchange keys, encrypt traffic, and handle reconnection. The protocol determines security properties, performance characteristics, network compatibility, and platform support. Different protocols suit different use cases, and most commercial VPN clients let users pick among several.
Major Protocols Today
WireGuard is the modern choice — fast, simple, audited, with strong cryptography baked in and minimal configuration surface. OpenVPN is the long-standing open-source standard, more flexible and slower than WireGuard but extremely well-supported. IKEv2/IPSec is fast and stable on mobile, with native OS support, but uses fixed ports easily blocked. SSL VPNs (using TLS) traverse restrictive firewalls well by running on TCP 443.
Deprecated Protocols
PPTP is broken and should never be used. L2TP/IPSec works but is largely supplanted by IKEv2. SSTP is a Microsoft proprietary protocol with limited platform support. Older configurations of OpenVPN with weak ciphers should be updated. The picture has changed substantially over the past decade: WireGuard barely existed in 2017 and is now the default for most performance-conscious deployments; PPTP was still common in 2015 and is now essentially extinct in serious use.