TCP/IP Tutorial and Technical Overview - IBM Redbooks

AH format
The AH format is described in RFC 2402. Figure 22-24 shows the position of the
Authentication Header fields in the IP packet.
Figure 22-24 AH format
The fields are as follows:
Next header The next header t is an 8-bit field that identifies the type of
what follows. The value of this field is chosen from the set
of IP protocol numbers defined in the most recent
Assigned Numbers RFC from the Internet Assigned
Numbers Authority (IANA). In other words, the IP header
protocol field is set to 51, and the value that would have
gone in the protocol field goes in the AH next header field.
Payload length This field is 8 bits long and contains the length of the AH
header expressed in 32-bit words, minus 2. It does not
relate to the actual payload length of the IP packet as a
whole. If default options are used, the value is 4 (three
32-bit fixed words plus three 32-bit words of
authentication data minus two).
Reserved This field is reserved for future use. Its length is 16 bits
and it is set to zero.
Security parameter index (SPI)
This field is 32 bits in length. See “Security Associations”
on page 810 for a definition.
814 TCP/IP Tutorial and Technical Overview
IP Hdr AH Payload
Next header Payld length Reserved
Security parameter index (SPI)
Sequence number
Authentication data (variable size)
(Integrity check value)
32 bits