Encapsulation; Transport Mode; Figure 65 Transport And Tunnel Mode Ipsec Encapsulation - Nortel BSR252 Configuration - Basics

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206 Chapter 13 VPN

Encapsulation

The two modes of operation for IPSec VPNs are Transport mode and Tunnel

Transport mode is used to protect upper layer protocols and only affects the data

in the IP packet. In Transport mode, the IP packet contains the security protocol

(AH or ESP) located after the original IP header and options, but before any

upper layer protocols contained in the packet (such as TCP and UDP).

With ESP, protection is applied only to the upper layer protocols contained in the

packet. The IP header information and options are not used in the authentication

process. Therefore, the originating IP address cannot be verified for integrity

against the data.

With the use of AH as the security protocol, protection is extended forward into

the IP header to verify the integrity of the entire packet by use of portions of the

original IP header in the hashing process.