System Administration Guide: IP Services


Current versions of the Internet Protocol (IP) assume that the point at which a computer attaches to the Internet or a network is fixed. IP also assumes that the IP address of the computer identifies the network to which the computer is attached. Datagrams that are sent to a computer are based on the location information that is contained in the IP address. Many Internet Protocols that are in use require that a node's IP address remain unchanged. If any of these applications are active on a Mobile IP computing device, the applications fail. Even HTTP would fail if not for the short-lived nature of its TCP connections. Updating an IP address and refreshing the Web page is not a burden.

If a mobile computer, or mobile node, moves to a new network while its IP address is unchanged, the mobile node address does not reflect the new point of attachment. Consequently, routing protocols that exist cannot route datagrams to the mobile node correctly. You must reconfigure the mobile node with a different IP address that represents the new location. Assigning a different IP address is cumbersome. Thus, under the current Internet Protocol, if the mobile node moves without changing its address, it loses routing. If the mobile node does change its address, it loses connections.

Mobile IP solves this problem by allowing the mobile node to use two IP addresses. The first address is a fixed home address. The second address is a care-of address that changes at each new point of attachment. Mobile IP enables a computer to roam freely on the Internet. Mobile IP also enables a computer to roam freely on an organization's network while still maintaining the same home address. Consequently, communication activities are not disrupted when the user changes the computer's point of attachment. Instead, the network is updated with the new location of the mobile node. See the Glossary for definitions of terms that are associated with Mobile IP.

The following figure illustrates the general Mobile IP topology.

Figure 23–1 Mobile IP Topology

Shows a mobile node's relationship between it's home agent's home network and a foreign agent's foreign network.

By using the previous illustration's Mobile IP topology, the following scenario shows how a datagram moves from one point to another within the Mobile IP framework.

  1. The Internet host sends a datagram to the mobile node by using the mobile node's home address (normal IP routing process).

  2. If the mobile node is on its home network, the datagram is delivered through the normal IP process to the mobile node. Otherwise, the home agent receives the datagram.

  3. If the mobile node is on a foreign network, the home agent forwards the datagram to the foreign agent. The home agent must encapsulate the datagram in an IP-in-IP manner so that the foreign agent's IP address appears in the outer IP header.

  4. The foreign agent delivers the datagram to the mobile node.

  5. Datagrams from the mobile node to the Internet host are sent by using normal IP routing procedures. If the mobile node is on a foreign network, the packets are delivered to the foreign agent. The foreign agent forwards the datagram to the Internet host.

  6. In situations with ingress filtering present, the source address must be topologically correct for the subnet that the datagram is coming from, or a router cannot forward the datagram. If this scenario is the situation on links between the mobile node and the correspondent node, the foreign agent needs to provide reverse tunneling support. Then the foreign agent can deliver every datagram that the mobile node sends to its home agent. The home agent then forwards the datagram through the path that the datagram would have taken had the mobile node resided on the home network. This process guarantees that the source address is correct for all links that the datagram must traverse.

In the instance of wireless communications, the illustrations depict the use of wireless transceivers to transmit the datagrams to the mobile node. Also, all datagrams between the Internet host and the mobile node use the home address of the mobile node. The home address is used even when the mobile node is located on the foreign network. The care-of address is used only for communication with mobility agents. The care-of address is invisible to the Internet host.