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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 Architectures and Implementation Methods 1 2 3 4 IPSec Security Associations and the Security Association Database (SAD); Security Policies and the Security Policy Database (SPD); Selectors; the Security Parameter Index (SPI)

Tunnel Mode

In this mode, IPSec is used to protect a complete encapsulated IP datagram after the IP header has already been applied to it. The IPSec headers appear in front of the original IP header, and then a new IP header is added in front of the IPSec header. That is to say, the entire original IP datagram is secured and then encapsulated within another IP datagram. This is shown in Figure 120.

Figure 120: IPSec Tunnel Mode Operation

IPSec tunnel mode is so named because it represents an encapsulation of a complete IP datagram, forming a virtual tunnel between IPSec-capable devices. The IP datagram is passed to IPSec, where a new IP header is created with the AH and/or ESP IPSec headers.added. Contrast to Figure 119.

IPSec Architectures and Implementation Methods 1 2 3 4 IPSec Security Associations and the Security Association Database (SAD); Security Policies and the Security Policy Database (SPD); Selectors; the Security Parameter Index (SPI)

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