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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

IPSec Authentication Header (AH) 1 2 3 4 IPSec Key Exchange (IKE)

Encapsulating Security Payload Operations and Field Use

Let's try to explain this procedurally, by considering three basic steps performed by ESP.

The first thing to consider is how the ESP header is placed; this is similar to how AH works:

• IPv6: The ESP Header is inserted into the IP datagram as an extension header, following the normal IPv6 rules for extension header linking. In transport mode, it appears before a Destination Options header containing options intended for the final destination, but after any other extension headers, if present. In tunnel mode, it appears as an extension header of the new IP datagram that encapsulates the original one being tunneled. This can be seen in Figure 124.
  

  Figure 124: IPv6 Datagram Format With IPSec Encapsulating Security Payload (ESP)

  At top is the same example IPv6 datagram with two extension headers shown in Figure 121. When ESP is applied in transport mode, the ESP Header is added to the existing datagram as in AH, and the ESP Trailer and ESP Authentication Data are placed at the end. In tunnel mode, the ESP Header and Trailer bracket the entire encapsulated IPv6 datagram. Note the encryption and authentication coverage in each case, and also how the Next Header field “points back” into the datagram since it appears in the ESP Trailer.

  
  
  
  
• IPv4: As with the AH, the ESP Header is placed after the normal IPv4 header. In transport mode, it appears after the original IP header; in tunnel mode, after the IP header of the new datagram encapsulating the original. This is shown in Figure 125.
  

  Figure 125: IPv4 Datagram Format With IPSec Encapsulating Security Payload (ESP)

  At top is the same sample IPv4 datagram shown in Figure 122. When this datagram is processed by ESP in transport mode, the ESP Header is placed between the IPv4 header and data, with the ESP Trailer and ESP Authentication Data following. In tunnel mode, the entire original IPv4 datagram is surrounded by these ESP components, rather than just the IPv4 data. Again, as in Figure 124, note the encryption and authentication coverage, and how the Next Header field “points back” to specify the identity of the encrypted data/datagram.

  
  

IPSec Authentication Header (AH) 1 2 3 4 IPSec Key Exchange (IKE)