ZyXEL Communications FMG3025-D10A ユーザーズマニュアル

 Chapter 17 VPN

FMG3024-D10A / FMG3025-D10A Series User’s Guide

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. 

There are two phases to every IKE (Internet Key Exchange) negotiation – phase 1 (Authentication) 
and phase 2 (Key Exchange). A phase 1 exchange establishes an IKE SA and the second one uses 
that SA to negotiate SAs for IPSec.