The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  IP Security (IPSec) Protocols

IPSec Overview, History and Standards 1 2 3 IPSec Architectures and Implementation Methods

I have a confession to make: I considered not writing about IPSec in this Guide. When you find yourself writing a tome as large as this one, you lose stamina sometimes and there's this urge to avoid writing about confusing subjects. J IPSec isn't the only difficult topic in this Guide but it is definitely a subject that baffles many because it's hard to get your hands around. Most discussions of it jump straight to describing the mechanisms and protocols without providing a general description of what it does and how the pieces fit together. Well, I recognized that IPSec is important and I don't shy away from a challenge. Thus, here's my attempt to provide a framework for understanding IPSec's various bits and pieces.

So, what exactly does IPSec do and how does it do it? In general terms, it provides security services at the IP layer for other TCP/IP protocols and applications to use. What this means is that IPSec provides the tools that devices on a TCP/IP network need in order to communicate securely. When two devices (either end user hosts or intermediate devices such as routers or firewalls) want to engage in secure communications, they set up a secure path between themselves that may traverse across many insecure intermediate systems. To accomplish this, they must perform (at least) the following tasks:

1. They must agree on a set of security protocols to use, so that each one sends data in a format the other can understand.
   
   
2. They must decide on a specific encryption algorithm to use in encoding data.
   
   
3. They must exchange keys that are used to “unlock” data that has been cryptographically encoded.
   
   
4. Once this background work is completed, each device must use the protocols, methods and keys previously agreed upon to encode data and send it across the network.

IPSec Overview, History and Standards 1 2 3 IPSec Architectures and Implementation Methods