Security parameters index (SPI): This is a 32-bit pseudo-random value identifying the security association for this datagram. The SPI value 0 is reserved to indicate that "no security association exists."

The set of SPI values in the range 1 through 255 are reserved to the Internet Assigned Numbers Authority (IANA) for future use. A reserved SPI value will not normally be assigned by IANA unless the use of that particular assigned SPI value is openly specified in an RFC.