Security Association (SA)This is an instance of security policy and keying material applied to a data flow. Both IKE and IPsec use SAs, although SAs are independent of one another. IPsec SAs are unidirectional and they are unique in each security protocol. A set of SAs are needed for a protected data pipe, one per direction per protocol. For example, if you have a pipe that supports ESP between peers, one ESP SA is required for each direction. SAs are uniquely identified by destination (IPsec endpoint) address, security protocol (AH or ESP), and security parameter index (SPI). IKE negotiates and establishes SAs on behalf of IPsec. A user can also establish IPsec SAs manually. An IKE SA is used by IKE only. Unlike the IPsec SA, it is bi-directional.
Authentication Header (AH)This is a security protocol that provides authentication and optional replay-detection services. AH is embedded in the data to be protected, for example, a full IP datagram. AH can be used either by itself or with Encryption Service Payload (ESP). Refer to the RFC 2402.
Encapsulating Security Payload (ESP)A security protocol that provides data confidentiality and protection with optional authentication and replay-detection services. ESP completely encapsulates user data. ESP can be used either by itself or in conjunction with AH. Refer to RFC 2406: IP Encapsulating Security Payload (ESP).