Mobile IPv4 White Paper

Agent Advertisements
Home Agents and Foreign Agents, at regular intervals (every
few seconds), broadcast on their subnet, messages known as
Agent Advertisements. The Agent Advertisement is designed as
an extension of the already existing ICMP router advertisement
[RFC 1256] message. The agent advertisement conveys the
following information:
o Whether the agent is a Home Agent or a Foreign Agent.
o A list of available Care-of Addresses (in case of the
Foreign Agents).
Agent Solicitations
The Mobile Node may also broadcast or multicast an Agent
Solicitation message. Any Home Agent or Foreign Agent that
receives the agent solicitation message responds with an Agent
Advertisement.
How a Mobile Node gets the Care-of Address
A Mobile Node, when attaching to a foreign network, must
acquire a Care-of Address on that network. There are two ways
of achieving this:
1. Foreign Agent Care-of Address: A Foreign Agent Care-
of Address is a Care-of Address acquired by the Mobile
Node from a Foreign Agent’s advertisement broadcast.
The foreign Care-of Address is registered with the Home
Agent and the Foreign Agent serves as the endpoint in
the tunnel for encapsulated packets sent from the Home
Agent to the Mobile Node.
2. Co-located Care-of Address: A co-located Care-of
Address is a Care-of Address acquired by the Mobile
Node as a local IP address through some external means,
such as DHCP [RFC2131], that the Mobile Node then
associates with one of its own network interfaces. When
using a co-located Care-of Address, the Mobile Node
serves as the endpoint of the tunnel and performs
decapsulation of datagrams tunneled to it.
HP-UX Mobile IPv4 White Paper 4