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Chapter 20 IPSec VPN
ZyWALL 110/310/1100 Series User’s Guide
311
Note: The ZyWALL and remote IPSec router must use the same active protocol.
Usually, you should select ESP. AH does not support encryption, and ESP is more suitable with NAT.
Encapsulation
There are two ways to encapsulate packets. Usually, you should use tunnel mode because it is more
secure. Transport mode is only used when the IPSec SA is used for communication between the
ZyWALL and remote IPSec router (for example, for remote management), not between computers
on the local and remote networks.
Note: The ZyWALL and remote IPSec router must use the same encapsulation.
These modes are illustrated below.
In tunnel mode, the ZyWALL uses the active protocol to encapsulate the entire IP packet. As a
result, there are two IP headers:
• Outside header: The outside IP header contains the IP address of the ZyWALL or remote IPSec
router, whichever is the destination.
• Inside header: The inside IP header contains the IP address of the computer behind the ZyWALL
or remote IPSec router. The header for the active protocol (AH or ESP) appears between the IP
headers.
In transport mode, the encapsulation depends on the active protocol. With AH, the ZyWALL includes
part of the original IP header when it encapsulates the packet. With ESP, however, the ZyWALL
does not include the IP header when it encapsulates the packet, so it is not possible to verify the
integrity of the source IP address.
IPSec SA Proposal and Perfect Forward Secrecy
An IPSec SA proposal is similar to an IKE SA proposal (see IKE SA Proposal on page 306 ), except
that you also have the choice whether or not the ZyWALL and remote IPSec router perform a new
DH key exchange every time an IPSec SA is established. This is called Perfect Forward Secrecy
(PFS).
If you enable PFS, the ZyWALL and remote IPSec router perform a DH key exchange every time an
IPSec SA is established, changing the root key from which encryption keys are generated. As a
result, if one encryption key is compromised, other encryption keys remain secure.
Figure 191
VPN: Transport and Tunnel Mode Encapsulation
Original Packet IP Header TCP
Header Data
Transport Mode Packet IP Header AH/ESP
Header TCP
Header Data
Tunnel Mode Packet IP Header AH/ESP
Header IP Header TCP
Header Data