Each mobile node, foreign agent, and home agent supports a mobility security association between the various Mobile IP components. The security association is indexed by the security parameter index (SPI) and IP address. In the instance of the mobile node, this address is the home address of the mobile node. Registration messages between a mobile node and the home agent are authenticated with the Mobile-home authentication extension. In addition to Mobile-home authentication, which is mandatory, you can use the optional Mobile-foreign agent and Home-foreign agent authentications.