3.3. Where IPsec Can Be Implemented

There are many ways in which IPsec may be implemented in a host, or in conjunction with a router or firewall to create a security gateway, or as an independent security device.

Implementation Approaches

a. Native IP Stack Integration

IPsec may be integrated into the native IP stack. This requires access to the IP source code and is applicable to both hosts and security gateways, although native host implementations benefit the most from this strategy, as explained later (Section 4.4.1, paragraph 6; Section 4.4.1.1, last paragraph).

b. Bump-in-the-Stack (BITS)

In a "bump-in-the-stack" (BITS) implementation, IPsec is implemented "underneath" an existing implementation of an IP protocol stack, between the native IP and the local network drivers. Source code access for the IP stack is not required in this context, making this implementation approach appropriate for use with legacy systems. This approach, when it is adopted, is usually employed in hosts.

c. Bump-in-the-Wire (BITW)

The use of a dedicated, inline security protocol processor is a common design feature of systems used by the military, and of some commercial systems as well. It is sometimes referred to as a "bump-in-the-wire" (BITW) implementation. Such implementations may be designed to serve either a host or a gateway. Usually, the BITW device is itself IP addressable. When supporting a single host, it may be quite analogous to a BITS implementation, but in supporting a router or firewall, it must operate like a security gateway.

Implementation Flexibility

This document often talks in terms of use of IPsec by a host or a security gateway, without regard to whether the implementation is native, BITS, or BITW. When the distinctions among these implementation options are significant, the document makes reference to specific implementation approaches.

A host implementation of IPsec may appear in devices that might not be viewed as "hosts". For example, a router might employ IPsec to protect routing protocols (e.g., BGP) and management functions (e.g., Telnet), without affecting subscriber traffic traversing the router. A security gateway might employ separate IPsec implementations to protect its management traffic and subscriber traffic. The architecture described in this document is very flexible. For example, a computer with a full-featured, compliant, native OS IPsec implementation should be capable of being configured to protect resident (host) applications and to provide security gateway protection for traffic traversing the computer. Such configuration would make use of the forwarding tables and the SPD selection function described in Sections 5.1 and 5.2.