AMINUL ISLAM SHAMIM

IPSec is a framework for authentication and encryption of the network layer, it is often used for VPNs (Virtual Private Network). IPSec can be used in transport and tunnel mode. With transport mode, the payload of the IP packet is encrypted but the header remains in clear text. Tunnel mode encapsulates the original IP packet, encrypting the payload and IP header. In these lessons, IPSec is used in one form or another.

https://networklessons.com/tag/ipsec

https://raymii.org/s/tutorials/IPSEC_vpn_with_Ubuntu_16.04.html