ZyXEL 202H 91-003-194001B User Manual

P-202H Plus v2 User’s Guide

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Chapter 10 Introduction to IPSec

Figure 48   Transport and Tunnel Mode IPSec Encapsulation

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.

Tunnel mode encapsulates the entire IP packet to transmit it securely. A Tunnel mode is 
required for gateway services to provide access to internal systems. Tunnel mode is 
fundamentally an IP tunnel with authentication and encryption. This is the most common 
mode of operation. Tunnel mode is required for gateway to gateway and host to gateway 
communications. Tunnel mode communications have two sets of IP headers:

• Outside header: The outside IP header contains the destination IP address of the VPN 

gateway.

• Inside header: The inside IP header contains the destination IP address of the final 

system behind the VPN gateway. The security protocol appears after the outer IP header 
and before the inside IP header. 

Read this section if you are running IPSec on a host computer behind the ZyXEL Device.