I am learning IPSec. My class notes define a security association as a one way relationship between sender and receiver that (1) affords security for traffic flow and (2) is identified by the destination address in the IP header, the SPI and the enclosed extension header (AH or ESP). Wikipedia describes SA as a "logical group of security parameters that enable the sharing of information to another entity." After reading the wikipedia article, I imagine SAs as a set of security parameters.
What is a security association within the context of IPSEC?