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Chapter 20 IPSec VPN
ZyWALL USG 300 User’s Guide
293
Usually, you should select ESP. AH does not support encryption, and ESP is more suitable
with NAT.
20.1.1.3 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.
" 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.
20.1.1.4 IPSec SA Proposal and Perfect Forward Secrecy
An IPSec SA proposal is similar to an IKE SA proposal (see Section 20.4.1.2 on page 307),
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).
Figure 196 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