User's Manual

Chapter 16 IPSec VPN 122
16.3.2 Encapsulation
The two modes of operation for IPSec VPNs are Transport mode and Tunnel mode. At the time of
writing, the Router supports Tunnel mode only.
Figure 79 T
ransport and Tunnel Mode IPSec Encapsulation
Transport Mode
Transport
mode is used to protect upper layer protocols and only affects the data in the IP packet.
In Transport mode, the IP packet contains the security protocol (AH or ESP) located after the
original IP header and options, but before any upper layer protocols contained in the packet (such
as TCP and UDP).
With ES
P, protection is applied only to the upper layer protocols contained in the packet. The IP
header information and options are not used in the authentication process. Therefore, the
originating IP address cannot be verified for integrity against the data.
With the use of AH as
the security protocol, protection is extended forward into the IP header to
verify the integrity of the entire packet by use of portions of the original IP header in the hashing
process.
Tunnel Mode
Tunnel mo
de encapsulates the entire IP packet to transmit it securely. A Tunnel mode is required
for gateway services to provide access to internal systems. Tunnel mode is fundamentally an IP
tunnel with authentication and encryption. This is the most common mode of operation. Tunnel
mode is required for gateway to gateway and host to gateway communications. Tunnel mode
communications have two sets of IP headers:
• Ou
tside header: The outside IP header contains the destination IP address of the VPN gateway.
• In
side header: The inside IP header contains the destination IP address of the final system
behind the VPN gateway. The security protocol appears after the outer IP header and before the
inside IP header.