Gary C. Kessler's "An Overview of Cryptography: IP Security (IPSec) Protocol"
Read section 5.6 about Internet Protocol for securing communications. After reading this section, describe the two modes for IPSec: AH and ESP. Also try to explain how to use AH and ESP to build VPN (tunnel mode and transport mode).
Click http://web.archive.org/web/20160702191723/http://www.garykessler.net/library/crypto.html link to open resource.