IPSec Architectures and Implementation Methods
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The main reason that IPSec is so powerful is that it provides security to IP, the basis for all other TCP/IP protocols. In protecting IP, we are protecting pretty much everything else in TCP/IP as well. An important issue, then, is how exactly we get IPSec into IP? There are several implementation methods for deploying IPSec, which represent different ways that IPSec may modify the overall layer architecture of TCP/IP.
Three different implementation architectures are defined for IPSec in RFC 2401. Which one we use depends on various factors including the version of IP used (v4 versus v6), the requirements of the application and other factors. These in turn rest on a primary implementation decision: whether IPSec should be programmed into all hosts on a network, or just into certain routers or other intermediate devices.
This implementation decision is one that must be based on the requirements of the network. There are two options: to implement IPSec in end hosts, or in routers.
Putting IPSec into all host devices provides the most flexibility and security. It enables end-to-end security between any two devices on the network. However, there are many hosts on a typical network, so this means far more work than just implementing IPSec in routers.
This option is much less work because it means we only make changes to a few routers instead of hundreds or thousands of clients. It only provides protection between pairs of routers that implement IPSec, but this may be sufficient for certain applications such as virtual private networks (VPNs). The routers can be used to provide protection only for the portion of the route that datagrams take outside the organization, leaving connections between routers and local hosts unsecured (or possibly, secured by other means).
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