IPsec uses two main protocols to secure network communications: Authentication Header (AH) and Encapsulating Security Payload (ESP).
Both AH and ESP use a Security Parameters Index (SPI), which is a critical component of their headers. The SPI is a unique identifier that enables the receiver to select the correct security association for processing incoming packets.
AH provides authentication and integrity, while ESP provides confidentiality, in addition to authentication and integrity. Both protocols use the SPI to manage these functions securely.
Reference
"IPsec Security Architecture," RFC 4302 (AH) and RFC 4303 (ESP).
"IPsec Explained," by Juniper Networks.