ZyXEL Communications ZyWall USG20-VPN User Manual page 365

Figure 237 VPN: Transport and Tunnel Mode Encapsulation
Tunnel Mode Packet
In tunnel mode, the USG 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 USG or remote IPSec
router, whichever is the destination.
• Inside header: The inside IP header contains the IP address of the computer behind the USG 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 USG includes
part of the original IP header when it encapsulates the packet. With ESP, however, the USG 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
that you also have the choice whether or not the USG 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 USG 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 USG 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.
PFS is ignored in initial IKEv2 authentication but is used when reauthenticating.
Additional Topics for IPSec SA
This section provides more information about IPSec SA in your USG.
Authentication and the Security Parameter Index (SPI)
For authentication, the USG and remote IPSec router use the SPI, instead of pre-shared keys, ID
type and content. The SPI is an identification number.
Note: The USG and remote IPSec router must use the same SPI.
NAT for Inbound and Outbound Traffic
The USG can translate the following types of network addresses in IPSec SA.
Chapter 21 IPSec VPN
IP Header AH/ESP
Header
USG20(W)-VPN Series User's Guide
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IP Header TCP
Header
IKE SA Proposal on page
Data
359), except