Care of address
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Used in Internet routing, a care-of-address (usually referred to as CoA) is a temporary IP address for a mobile device. This allows a Home Agent to forward messages to the mobile device.
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[edit] Acquiring
The care-of address can be acquired by the Mobile Node (usually referred to as MN) in two different ways:
- FACoA (Foreign Agent Coa): The MN receives the same CoA as the Foreign Agent, implementing a kind of NAT network. All the Mobile Nodes in the Foreign Network are given the same CoA.
- Collocated CoA: Each Mobile Node in the Foreign Network is assigned his own CoA. This is usually done by means of a DHCP server managed by the Foreign Agent.
[edit] IPv4 Restriction
Given the lack of IP addresses with the current IPv4, the first solution is more frequently chosen, because it does not waste a public IP address for every Mobile Node which changes its network location, as the collocated CoA does.
[edit] Foreign Networks
The care-of address has to be a valid IP address within the Foreign Network, so that it allows the Mobile Node to receive and make connections with any host in the outside. To send outgoing packets, the Mobile Node may as well use its Home Address but, since it is not a valid IP address in the actual network, some routers in the way might prevent the packets from reaching the destination.
[edit] IPv4
That is why, in IPv4, the outgoing information is transported to the Home Agent (which is in the Home Network) by means of an IP tunnel. From the Home Network, the packets of the Mobile Node can be sent using its original Home Address, without any routing problem. The Correspondent Node will send its information again to the Home Network. Thus, it has to be sent on through the tunnel to the Foreign Agent and then to the Mobile Node.
[edit] Home Agent
In mobile IP a home agent is a router on a mobile node’s home network which tunnels datagrams for delivery to the mobile node when it is away from home, and maintains current location information for the mobile node. It is used with one or more foreign agents.
[edit] Foreign Agent
A foreign agent in Mobile IP is a router which stores information about mobile nodes visiting its network. Foreign agents also advertise care-of addresses which are used by Mobile IP.
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