A Security Association (SA, German security association ) is an agreement between two communicating entities in computer networks. It describes how the two parties use security services to communicate securely with each other.
The security association is a simplex connection. So you always transmits exactly one security service. When using multiple services and multiple security connections must be established. When the security services, it may be an authentication ( by AH) or encryption ( by ESP).
A SA is described by a Security Parameter Index (German security parameter index), the destination IP address and a security protocol identifier, the communication method to be used (AH, ESP ) refers.