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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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22.4.1 Concepts<br />

<strong>IP</strong>Sec adds integrity checking, authentication, encryption, <strong>and</strong> replay protection<br />

to <strong>IP</strong> packets. It is used for end-to-end security <strong>and</strong> also for creating secure<br />

tunnels between gateways.<br />

<strong>IP</strong>Sec was designed for interoperability. When correctly implemented, it does not<br />

affect networks <strong>and</strong> hosts that do not support it. <strong>IP</strong>Sec is independent of the<br />

current cryptographic algorithms; it can accommodate new ones as they become<br />

available. It works both with <strong>IP</strong>v4 <strong>and</strong> <strong>IP</strong>v6. In fact, <strong>IP</strong>Sec is a m<strong>and</strong>atory<br />

component of <strong>IP</strong>v6.<br />

<strong>IP</strong>Sec uses state-of-the-art cryptographic algorithms. The specific<br />

implementation of an algorithm for use by an <strong>IP</strong>Sec protocol is often called a<br />

transform. For example, the DES algorithm used by ESP is called the ESP<br />

DES-CBC transform. The transforms, like the protocols, are published in the<br />

RFCs.<br />

Two major <strong>IP</strong>Sec concepts need to be clarified: Security Associations <strong>and</strong><br />

tunneling. We describe these concepts in the following sections.<br />

Security Associations<br />

The concept of a Security Association (SA) is fundamental to <strong>IP</strong>Sec. An SA is a<br />

unidirectional (simplex) logical connection between two <strong>IP</strong>Sec systems, uniquely<br />

identified by the following triple:<br />

<br />

The definition of the members is as follows:<br />

► Security parameter index (SPI)<br />

This is a 32-bit value used to identify different SAs with the same destination<br />

address <strong>and</strong> security protocol. The SPI is carried in the header of the security<br />

protocol (AH or ESP). The SPI has only local significance, as defined by the<br />

creator of the SA. SPI values in the range 1 to 255 are reserved by the<br />

Internet Assigned Numbers Authority (IANA). The SPI value of 0 must be<br />

used for local implementation-specific purposes only. RFC 2406 states that a<br />

value of 0 must not be transmitted. Generally, the SPI is selected by the<br />

destination system during SA establishment.<br />

► <strong>IP</strong> destination address<br />

This address can be a unicast, broadcast, or multicast <strong>IP</strong> address. However,<br />

currently SA management mechanisms are defined only for unicast<br />

addresses.<br />

► Security protocol<br />

This can be either AH or ESP.<br />

810 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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