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Setting Up a VPN Tunnel Between Gateways

An SA, frequently called a tunnel, is the set of information that allows two entities (networks, PCs, routers, firewalls, gateways) to "trust each other" and communicate securely as they pass information over the Internet.

Figure 2-5

The SA contains all the information necessary for Gateway A to negotiate a secure and encrypted communication stream with Gateway B. This communication is often referred to as a "tunnel." The gateways contain this information so that it does not have to be loaded onto every computer connected to the gateways.

Each gateway must negotiate its Security Association with another gateway using the parameters and processes established by IPSec. As illustrated below, the most common method of accomplishing this process is via the Internet Key Exchange (IKE) protocol which automates some of the negotiation procedures. Alternatively, you can configure your gateways using manual key exchange, which involves manually configuring each paramter on both gateways.

Figure 2-6

The IPSec software on Host A initiates the IPSec process in an attempt to communicate with Host B. The two computers then begin the Internet Key Exchange (IKE) process.

IKE Phase I.

The two parties negotiate the encryption and authentication algorithms to use in the IKE SAs.
The two parties authenticate each other using a predetermined mechanism, such as preshared keys or digital certificates.
A shared master key is generated by the Diffie-Hellman Public key algorithm within the IKE framework for the two parties. The master key is also used in the second phase to derive IPSec keys for the SAs.

IKE Phase II.

The two parties negotiate the encryption and authentication algorithms to use in the IPSec SAs.
The master key is used to derive the IPSec keys for the SAs. Once the SA keys are created and exchanged, the IPSec SAs are ready to protect user data between the two VPN gateways.

Data transfer. Data is transferred between IPSec peers based on the IPSec parameters and keys stored in the SA database.

IPSec tunnel termination. IPSec SAs terminate through deletion or by timing out.