1. A computer implemented method in a foreign agent comprising: in response to a first mobile network registration request from a mobile node, accessing a remote authentication facility to retrieve a security association for the mobile node for authenticating and providing a first network connectivity to the mobile node, wherein the security association is associated with a lifespan that specifies the time period in which the security association will be valid; inserting the security association in a local security association database in the foreign agent to create a security association entry that is active, wherein the security association entry includes the lifespan; transmitting the first mobile network registration request to a home agent; responsive to receiving from the home agent a first authentication reply that indicates that the mobile node is authenticated, providing the mobile node with the first network connectivity by binding a first Care-of-Address for the mobile node to a home address associated with the mobile node; terminating the first network connectivity including removing the binding of the first Care-of-Address and the home address; responsive to the step of terminating the first network connectivity, modifying the security association entry to indicate that the security association is idle; receiving a second mobile network registration request from the mobile node after the first network connectivity has been terminated; responsive to determining that the lifespan associated with the security association is valid, using the security association entry in the local security association database that corresponds to the mobile node to provide authentication of the mobile node without having to access the remote authentication facility; transmitting the second mobile network registration request to the home agent; and responsive to receiving from the home agent a second authentication reply that indicates that the mobile node is authenticated, providing the mobile node with a second network connectivity by binding a second Care-of-Address for the mobile node to the home address associated with mobile node, and modifying the security association entry to indicate that the security association is active.