Within IPsec, the SPI (Security Parameter Index) is a critical component that uniquely identifies a Security Association (SA) for the IPsec session. The SPI is used in the IPsec headers to help the receiving party determine which SA has been agreed upon for processing the incoming packets. This identification is crucial for the proper operation and management of security policies applied to the encrypted data flows.
Reference:
RFC 4301, "Security Architecture for the Internet Protocol," which discusses the structure and use of the SPI in IPsec communications.