Encapsulation; Ipsec Protocols - D-Link DFL-900 User Manual

Part V
Virtual Private Network
1024-bit (Group 2 – DH2) Diffie-Hellman groups are supported. Upon completion of the Diffie-Hellman exchange, the two peers
have a shared secret, but the IKE SA is not authenticated. For authentication, use pre-shared keys.
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Perfect Forward Secrecy (PFS)
Enabling PFS means that the key is transient. The key is thrown away and replaced by a brand new key using a new Diffie-Hellman
exchange for each new IPSec SA setup. With PFS enabled, if one key is compromised, previous and subsequent keys are not
compromised, because subsequent keys are not derived from previous keys. The (time-consuming) Diffie-Hellman exchange is the
trade-off for this extra security.
This may be unnecessary for data that does not require such security, so PFS is disabled (None) by default in the DFL-1500.
Disabling PFS means new authentication and encryption keys are derived from the same root secret (which may have security
implications in the long run) but allows faster SA setup (by bypassing the Diffie-Hellman key exchange).

12.2.6 Encapsulation

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Transport Mode
Transport mode is used to protect upper layer protocols and only affects the data in the IP packets. In Transport mode, the IP packets
contains the security protocol (AH or ESP) located after the original IP header and options, but before any upper layer protocols
contains 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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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 system. 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
communication have two sets of IP headers：
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Outside header： The outside IP header contains the destination IP address of the VPN gateway.
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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.

12.2.7 IPSec Protocols

The ESP and AH protocols are necessary to create a Security Association (SA), the foundation of an IPSec VPN. An SA is built
from the authentication provided by AH and ESP protocols. The primary function of key management is to establish and maintain
the SA between systems. Once the SA is established, the transport of data may commence.
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AH (Authentication Header) Protocol
AH protocol (RFC 2402) was designed for integrity, authentication, sequence integrity (replay resistance), and non-repudiation but
not for confidentiality, for which the ESP was designed.
In applications where confidentiality is not required or not sanctioned by government encryption restrictions, an AH can be
employed to ensure integrity. This type of implementation does not protect the information from dissemination but will allow for
verification of the integrity of the information and authentication of the originator.
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ESP (Encapsulating Security Payload) Protocol
The ESP protocol (RFC 2406) provides encryption as well as some of the services offered by AH. ESP authenticating properties are
limited compared to the AH due to the non-inclusion of the IP header information during the authentication process. However, ESP
is sufficient if only the upper layer protocols need to be authenticated.
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