ZyXEL UAG715 User Guide - Page 278
Encapsulation, IPSec SA Proposal and Perfect Forward Secrecy
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Chapter 22 IPSec VPN 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 UAG and remote IPSec router (for example, for remote management), not between computers on the local and remote networks. Note: The UAG and remote IPSec router must use the same encapsulation. These modes are illustrated below. Figure 185 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 In tunnel mode, the UAG 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 UAG or remote IPSec router, whichever is the destination. • Inside header: The inside IP header contains the IP address of the computer behind the UAG 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 UAG includes part of the original IP header when it encapsulates the packet. With ESP, however, the UAG 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 272), except that you also have the choice whether or not the UAG 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 UAG 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 UAG 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. 278 UAG715 User's Guide