Internet Protocol Security (IPsec) is an IETF standard that provides a variety of security services at layer three. IPsec can be used to create an encrypted tunnel for message confidentiality. Alternatively - or in conjunction - IPsec can also verify message integrity.
For confidentiality purposes, IPsec supports a format called Encapsulating Security Payloads (ESP). Likewise, for integrity assurance, IPsec incorporates a format called Authentication Header (AH). These options can be used separately or together. Either option improves resistance to replay attacks.
IPsec also supports a variety of authentication and key exchange options under the Internet Security Association and Key Management Protocol (ISAKMP) framework. One such option is Internet Key Exchange (IKE).
Reference: http://www.unixwiz.net/techtips/iguide-ipsec.html