Mobile IPv4 White Paper

Location Updates - Registering the COA
When a Mobile Node attaches to a new foreign network, it sends
a Registration Request to its Home Agent to register its Care-of
Address. If the Mobile Node is using a Foreign Agent Care-of
Address, the Registration Request is sent via the Foreign Agent.
The Registration Request includes an extension with a
cryptographic authentication value (HMAC-MD5 or keyed
MD5). The Mobile Node calculates the authentication value
based on fields in the Registration Request and a static key
(shared secret key) that is specified on both the Home Agent and
the Mobile Node.
The Home Agent authenticates the Mobile Node’s Registration
Request by calculating its own authentication value and
comparing it with the authentication value from the Mobile
Node. After the Home Agent authenticates the Registration
Request, it sends a Registration Reply message to the Mobile
Node. The Registration Reply also includes a lifetime for the
registration.
Deregistering the Care-of Address
A Mobile Node, upon returning to its home network or upon
session termination, sends the Home Agent a Mobile IP
Registration Request message for deregistration. The Home
Agent removes its mobility binding for the Mobile Node.
Deregistration with the Foreign Agent occurs automatically
when the registration lifetime expires.
5 HP-UX Mobile IPv4 White Paper