ZyXEL Communications VSG1432-B101 Series Manual De Usuario

Chapter 21 IPSec

VSG1432-B101 Series User’s Guide

The two modes of operation for IPSec VPNs are Transport mode and Tunnel 
mode. At the time of writing, the ZyXEL Device supports Tunnel mode only.

Figure 121   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.