Billion Electric Company BiGuard 2 - Manual (Page 142)

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A typical AH packet looks like this:
E.2.1.2 Encapsulating Security Payload (ESP)
Encapsulating Security Payload (ESP) provides privacy for data through encryption.
An encryption algorithm combines the data with a key to encrypt it. It then
repackages the data using a special format, and transmits it to the destination. The
receiver then decrypts the data using the same algorithm. ESP is usually used with
AH to provide added data security.
ESP divides its fields into three components…
ESP Header: Placed before encrypted data, the ESP Header contains the SPI and
Sequence Number. Its placement depends on whether ESP is used in transport
mode or tunnel mode.
ESP Trailer: Placed after the encrypted data, the ESP Trailer contains padding that is
used to align the encrypted data.
ESP Authentication Data: This contains an Integrity Check Value (ICV) for when
ESP's optional authentication feature is used.
ESP provides authentication, integrity, and confidentiality, which provides data
content protection, and protects against data tampering. A typical ESP packet looks
Sequence Number
Authentication Data
SPI
Next
Header
Payload
Length
Reserved