The following article is a brief introduction to IPSec VPN that is utilized to provide a logical connection between 2 sites (Site to Site) or a client and a site (Client to Site). The article is written to provide the key terms behind IPSec VPN implementation in a Cisco ASA Firewall or any other similar device.
IPSec Virtual Private Network (VPN) provides the following services:
ISAKMP: Internet Security Association and Key Management Protocol
IKE: Internet Key Exchange
IPSec: Internet Protocol Security
ISAKMP is the framework for secure communication channel establishment. IKE is the actual implementation of ISAKMP framework. There are 2 main versions of IKE – IKEv1 and IKEv2. In this article, we will stick with IKEv1.
IKE runs over UDP 500 and consists of 2 Phases.IKE Phase I can utilize either one of 2 modes – Main or Aggressive mode in order to establish a ISAKMP SA. IKE Phase II utilizes Quick Mode in order to establish IPSec SA. ISAKMP SA established in IKE Phase I is bidirectional. Two unidirectional IPSec SAs are established in IKE Phase II.