Editing Mobile IP (section)

== Operational principles ==
The goal of IP Mobility is to maintain the TCP connection between a mobile host and a static host while reducing the effects of location changes while the mobile host is moving around, without having to change the underlying TCP/IP.<ref>Internet Protocol</ref> To solve the problem, the RFC allows for a kind of proxy agent to act as a ''middle-man'' between a mobile host and a correspondent host.

A mobile node has two addresses – a permanent home address and a [[care-of address]] (CoA), which is associated with the network the mobile node is visiting. Two kinds of entities comprise a Mobile IP implementation:
* A ''home agent'' (HA) stores information about mobile nodes whose permanent home address is in the home agent's network.  The HA acts as a router on a mobile host's (MH) home network which tunnels datagrams for delivery to the MH when it is away from home, maintains a location directory (LD) for the MH.
* A ''foreign agent'' (FA) stores information about mobile nodes visiting its network. Foreign agents also advertise care-of addresses, which are used by Mobile IP. If there is no foreign agent in the host network, the mobile device has to take care of getting an address and advertising that address by its own means.  The FA acts as a router on a MH’s visited network which provides routing services to the MH while registered. FA detunnels and delivers datagrams to the MH that were tunneled by the MH’s HA

The so-called ''Care of Address'' is a termination point of a tunnel toward a MH, for datagrams forwarded to the MH while it is away from home.
* Foreign agent care-of address: the address of a foreign agent that MH registers with
* co-located care-of address: an externally obtained local address that a MH gets.

A Mobile Node (MN) is responsible for discovering whether it is connected to its home network or has moved to a foreign network. HA’s and FA’s broadcast their presence on each network to which they are attached. They are not solely ''responsible'' for discovery, they only play a part. {{IETF RFC|2002}} specified that MN use agent discovery to locate these entities.  When connected to a foreign network,  a MN has to determine the foreign agent care-of-address being offered by each foreign agent on the network.

A node wanting to communicate with the mobile node uses the permanent home address of the mobile node as the destination address to send packets to.  Because the home address logically belongs to the network associated with the home agent, normal IP routing mechanisms forward these packets to the home agent. Instead of forwarding these packets to a destination that is physically in the same network as the home agent, the home agent redirects these packets towards the remote address through an [[IP tunnel]] by encapsulating the datagram with a new IP header using the care of address of the mobile node.

When acting as transmitter, a mobile node sends packets directly to the other communicating node, without sending the packets through the home agent, using its permanent home address as the source address for the IP packets. This is known as [[triangular routing]] or "route optimization" (RO) mode. If needed, the foreign agent could employ ''reverse tunneling'' by tunneling the mobile node's packets to the home agent, which in turn forwards them to the communicating node. This is needed in networks whose gateway routers check that the source IP address of the mobile host belongs to their subnet or discard the packet otherwise. In Mobile IPv6 (MIPv6), "reverse tunneling" is the default behaviour, with RO being an optional behaviour.