Encapsulation - ZyXEL Communications P-660HN-51 - V1.10 Manual

Chapter 13 IPSec

13.4.2 Encapsulation

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 82 Transport and Tunnel Mode IPSec Encapsulation
Transport Mode
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
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.
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P-660HN-51 User's Guide