Encapsulation; Figure 12-3 Transport And Tunnel Mode Ipsec Encapsulation - ZyXEL Communications ZyWALL 5 User Manual

ZyWALL 5 Internet Security Appliance
12.2.1 IPSec Algorithms
The ESP (Encapsulating Security Payload) Protocol (RFC 2406) and AH (Authentication Header)
protocol (RFC 2402) describe the packet formats and the default standards for packet structure
(including implementation algorithms).
The Encryption Algorithm describes the use of encryption techniques such as DES (Data Encryption
Standard), AES (Advanced Encryption Standard) and Triple DES algorithms.
The Authentication Algorithms, HMAC-MD5 (RFC 2403) and HMAC-SHA-1 (RFC 2404, provide an
authentication mechanism for the AH and ESP protocols. Please see section 13.2 for more
information.
12.2.2 Key Management
Key management allows you to determine whether to use IKE (ISAKMP) or manual key configuration
in order to set up a VPN.

12.3 Encapsulation

The two modes of operation for IPSec VPNs are Transport mode and Tunnel mode.

Figure 12-3 Transport and Tunnel Mode IPSec Encapsulation

12.3.1 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.
12.3.2 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:
Introduction to IPSec 12-3