Chapter 14 IPSec VPN
ZyWALL 2 Plus User’s Guide
254
" 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.
14.6.5 IPSec SA Proposal and Perfect Forward Secrecy
An IPSec SA proposal is similar to an IKE SA proposal (see Section 14.3.1 on page 239),
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.
If you do not enable PFS, the ZyWALL and remote IPSec router use the same root key that
was generated when the IKE SA was established to generate encryption keys.
The DH key exchange is time-consuming and may be unnecessary for data that does not
require such security.
Figure 167 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
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