1. PPTP (Point to Point Tunneling Protocol)
   • Layer: Data link layer of OSI.
   • Connection: Supports only one single point-to-point connection per session.
   • Use Case: Commonly used for dial-up networks.
   • Security: Uses PPP for authentication and tunneling but does not support EAP and sends initial packets in plaintext.
2. L2F (Layer 2 Forwarding)
   • Developer: Cisco.
   • Layer: Data link layer of OSI.
   • Security: Does not offer encryption.
   • Use Case: A mutual authentication tunneling mechanism, but was replaced by L2TP.
3. L2TP (Layer 2 Tunneling Protocol)
   • Layer: Data link layer of OSI.
   • Connection: Single point-to-point connection per session.
   • Use Case: Commonly used for dial-up networks.
   • Security: Uses IPsec for encryption and operates on port 115.
4. IPsec (Internet Protocol Security)
   • Layer: Network layer of OSI.
   • Connection: Supports multiple and simultaneous tunnels.
   • Security: Provides encryption and authentication, built into IPv6.
   • Protocols: AH (Authentication Header) and ESP (Encapsulated Security Payload).
   • Modes: Transport (encrypts data only) and Tunnel (encrypts entire IP packet).
   • Use Case: Network-to-network connections.
5. TLS (Transport Layer Security)
   • Function: Encrypts and protects transactions to prevent sniffing while data is in transit, along with VPN and IPsec.
   • Security: Most effective control against session hijacking.
   • Key Management: Uses ephemeral session keys to encrypt communications between a web server and client.
   • Standard: TLS is the most current standard, not SSL.
6. PVC (Permanent Virtual Circuit)
   • Function: A dedicated logical circuit that always exists and is waiting for the customer to send data, similar to a walkie-talkie.
7. SVC (Switched Virtual Circuit)
   • Function: Requires tuning the transmitter and receiver to a new frequency every time communication is needed, similar to shortwave or ham radio.

These VPN protocols and technologies provide various methods for securing data transmission across public and private networks, each with its own strengths and specific use cases.