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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  IP Security (IPSec) Protocols

Previous Topic/Section
IPSec Security Associations and the Security Association Database (SAD); Security Policies and the Security Policy Database (SPD); Selectors; the Security Parameter Index (SPI)
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
IPSec Encapsulating Security Payload (ESP)
Next Topic/Section

IPSec Authentication Header (AH)
(Page 3 of 4)

IPv4 Authentication Header Placement and Linking

A method that is similar to the IPv6 header linking technique is employed. In an IPv4 datagram, the Protocol field indicates the identity of the higher layer protocol (typically TCP or UDP) carried in the datagram. As such, this field “points” to the next header, which is at the front of the IP payload. AH takes this value and puts it into its Next Header field, and then places the protocol value for AH itself (51 decimal) into the IP Protocol field. This makes the IP header “point” to the AH, which then “points” to whatever the IP datagram pointed to before.

Again, in transport mode, the authentication header is added after the main IP header of the original datagram; in tunnel mode it is added after the new IP header that encapsulates the original datagram being tunneled. This is shown in Figure 122.


Figure 122: IPv4 Datagram Format With IPSec Authentication Header (AH)

At top is an example IPv4 datagram; it may or may not contain IPv4 options (which are not distinct entities as they are in IPv6). In transport mode, the authentication header is added between the IP header and the IP data; the Protocol field of the IP header points to it, while its Next Header field contains the IP header’s prior protocol value (in this case 6, for TCP.) In tunnel mode the IPv4 datagram is encapsulated into a new IPv4 datagram that includes the AH header. Note in tunnel mode, the AH header’s use of the value 4 (which means IPv4) in its Next Header.

 


Key Concept: The IPSec Authentication Header (AH) protocol allows the recipient of a datagram to verify its authenticity. It is implemented as a header added to an IP datagram that contains an integrity check value computed based on the values of the fields in the datagram. This value can be used by the recipient to ensure that the data has not been changed in transit. The Authentication Header does not encrypt data and thus does not ensure the privacy of transmissions.



Previous Topic/Section
IPSec Security Associations and the Security Association Database (SAD); Security Policies and the Security Policy Database (SPD); Selectors; the Security Parameter Index (SPI)
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
IPSec Encapsulating Security Payload (ESP)
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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