Transport Mode; Tunnel Mode; Ike Phases - ZyXEL Communications SBG5500-A User Manual

Figure 140 Transport 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 ESP, 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 mode 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:
• Outside header: The outside IP header contains the destination IP address of the VPN gateway.
• Inside 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.

10.9.3 IKE Phases

There are two phases to every IKE (Internet Key Exchange) negotiation – phase 1 (Authentication) and
phase 2 (Key Exchange). A phase 1 exchange establishes an IKE SA and the second one uses that SA to
negotiate SAs for IPsec.
Chapter 10 VPN
SBG5500 Series User's Guide
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