Nat Traversal; The Ah Protocol; The Esp Protocol - D-Link DFL-1660 User Manual

9.3.5. NAT Traversal

AH uses a cryptographic hash function to produce a MAC from the data in the IP packet. This MAC
is then transmitted with the packet, allowing the remote endpoint to verify the integrity of the
original IP packet, making sure the data has not been tampered with on its way through the Internet.
Apart from the IP packet data, AH also authenticates parts of the IP header.
The AH protocol inserts an AH header after the original IP header. In tunnel mode, the AH header is
inserted after the outer header, but before the original, inner IP header.
ESP (Encapsulating Security Payload)
The ESP protocol inserts an ESP header after the original IP header, in tunnel mode, the ESP header
is inserted after the outer header, but before the original, inner IP header.
All data after the ESP header is encrypted and/or authenticated. The difference from AH is that ESP
also provides encryption of the IP packet. The authentication phase also differs in that ESP only
authenticates the data after the ESP header; thus the outer IP header is left unprotected.
The ESP protocol is used for both encryption and authentication of the IP packet. It can also be used
to do either encryption only, or authentication only.
9.3.5. NAT Traversal
Both IKE and IPsec protocols present a problem in the functioning of NAT. Both protocols were not
designed to work through NATs and because of this, a technique called "NAT traversal" has
Figure 9.1. The AH protocol
Figure 9.2. The ESP protocol
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Chapter 9. VPN