Chapter 22 IPSec VPN
SBG3500-N Series User’s Guide
288
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) 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.
Key Management
Key management allows you to determine whether to use IKE (ISAKMP) or manual key
configuration in order to set up a VPN.
22.7.2 Encapsulation
The two modes of operation for IPSec VPNs are Transport mode and Tunnel mode. At the time of
writing, the SBG3500-N Series supports Tunnel mode only.
Figure 149 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.
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