Section 6.5. Mobile IP

6.5. Mobile IP

The mobile IP scheme is a protocol responsible for handling the mobility of users attached to the Internet. Mobile computing allows computing devices, such as computers, to move while functioning routinely. In a mobile IP network, it is essential for both mobile and wired users to interoperate seamlessly. In most mobile IP cases, TCP cannot be used, as the congestion-control scheme would greatly reduce the throughput and the inherent delays and error bursts may result in a large number of retransmissions. Some changes have to be made to TCP to use it for internetworking wired and wireless networks. The major challenges with mobile IP are

• Mobility. A quality connection is desired for a user while it is mobile with different speeds.

• Registration. A mobile user's address must be identified and registered in different areas.

• Interoperability. A mobile user must interact with other stationary and mobile users.

• Connection reliability. TCP connections must survive in mobility cases.

• Security. A connection must be secured, especially since a wireless connection is less immune to intrusions.

A mobile user requires the same level of reliability for a TCP connection as he/she receives in a wired connection. Note that, typical Internet congestion-control schemes cannot be used in wireless networks, because the packet drop is caused mainly by poor link quality and channel distortions rather than by congestion. The channel imperfections make it difficult to implement a quality-of-service model other than the best-effort model. The varying data rates and delays make it challenging to implement high-speed and real-time applications, such as voice and video, over wireless networks.

6.5.1. Addresses and Agents

In a mobile IP network, a mobile host (mobile user) is allowed to hold two addresses simultaneously . One of the two addresses is permanent and the other is temporary. The permanent address of a host is the conventional IP address. Note that similar to regular networks, there are still MAC (at the link-layer ) addresses in wireless networks that identify physical endpoints of links. A mobile host must have a permanent IP address in its home network . This address is called the home address . A home address is an IP address and is assigned to a mobile host for an extended period of time. The home address remains unchanged even if the host moves out of its home area. In such cases, a host needs to be registered by the home mobile switching center (MSC). This MSC is a router and it iss called the home agent .

When a mobile host leaves its home network and enters a foreign network , the host must also be registered by the new network and obtain a temporary address. Changing the network typically happens when a mobile host roams in a certain city or changes the city. Once a mobile host leaves its home network for a foreign network, it is assigned a foreign address reflecting the mobile host's current point of attachment when away from its home network. In such a case, its messages from the Internet corresponding servers are still sent to the mobile's home address. Similarly, a foreign agent is a router in the mobile host's foreign network that informs a host's home agent of its current foreign address. The home agent always forwards messages to the mobile host's current location. Figure 6.16 shows two wireless networks attached to the Internet in which mobile host B has moved from its home network to a foreign network.

Figure 6.16. A mobile host moves from its home network to a foreign network

Generally, MSC routers (acting as agents) in a network are connected through high-speed links to all access points (base stations ) in a network. An MSC router maintains two databases: a home-location database and a foreign-location database. When a mobile host moves to its home network, a signal is sent to the local base station which forwards this signal to its MSC. The MSC router in turn authenticates the user and registers the user in its home-location database.

6.5.2. Agent Discovery Phase

A home agent maintains a database containing the mobile host's home address. When a mobile host moves to a foreign network, its home and foreign agents establish an association for updating registration with its home agent through the foreign agent. This association is made possible by sending agent advertisement messages. Upon receiving an agent advertisement, the mobile host can learn if it is located in its home network or in a foreign network depending on the type of message. A mobile host can detect if it is connected to its home link or foreign link. Once the host moves to a new network, it can determine whether it has changed its point of attachment to obtain a foreign address.

Advertisement messages are propagated periodically in a broadcast manner by all agents. It is always possible that a mobile host does not receive the advertisement due to restricted timing. In such a case, the mobile host needs to send a request message to the agent which it is attached to. If the agent to which the host is attached is a home agent, the registration process is the same as traditional host in a fixed place. But, if the agent is a foreign one, the agent replies with a message containing a foreign address for the agent.

6.5.3. Registration

Mobile IP acts as an interface between the mobile's home network and the foreign network where the mobile currently resides. Mobile IP keeps track of the mobile's locations, and maps a home address into a current foreign address. The mobile IP interface delivers messages from the mobile's home network to the mobile host in its current foreign location in a seamless fashion after a registration process with the foreign agent is completed. The procedure of registration with a new network is summarized as follows :

Mobile IP Registration Steps

1.	Use UDP (a transport layer protocol to be discussed in Chapter 8) and register with an agent on the new network
2.	On the home network, register with an agent to request call forwarding
3.	If any registration is about to expire, renew it
4.	When returning to the home network, cancel the registration with the new network.

A registration phase involves an exchange of two messages between the mobile host and its home agent: registration request and registration response . Once a mobile host enters a foreign network, it listens for agent advertisements and then obtains a foreign address from the foreign network it has moved to. The host's home-network agent then adds the foreign network address agent to its home-location database. This is done after the agent authenticates the host through the host's home-network agent. The host's home-network agent now forwards all calls to the host in the foreign network. On the Internet, the location management and routing are done through mobile IP.

A mobile host can also register using a collocated foreign address . A collocated foreign address is a local IP address temporarily assigned to a mobile host without using a foreign agent. In a collocated foreign addressing, a mobile host receives an assigned temporary foreign address through its own home network. In the meanwhile, as soon as the mobile host leaves the foreign network, it also requires to deregister .

Example.

Consider Figure 6.16 showing two wireless networks connected to the Internet. Network 1 is assigned a CIDR IP address (see Chapter 2) 205.101.8.0/20 and it has three active mobile hosts A, B, and C. Suppose that this network is the home network for hosts A and B and but not for host C as it appears from the home IP addresses of the agent routing entry. Consider a situation in a different time in which host A stays in this network (thus there is no foreign address for it), and host B moves out of this network (thus it obtains a foreign address). Particularly, host B has moved to network 2. Network 2 is also assigned a CIDR IP address 183.133.0.1/21 and it has three active mobile hosts D, E, and B. Network 2 is now considered a foreign network for B and is therefore assigned a foreign address of 183.133.0.1. This address appears in its both associated home and foreign agents as seen in the figure.

6.5.4. Mobile IP Routing

In mobile IP systems, datagrams are encapsuled by a mobile IP header. Figure 6.17 shows the header format of mobile IP registration. The type field determines whether the registration is a request or a reply. The flags/code field is used in the reply-message to specify forwarding details. The lifetime field gives the permitted time (in seconds) a registration is valid. The home address and temporary address fields are the two addresses explained. The home agent field specifies the home-agent address of the host. The identification field helps a mobile host prevent repeated messages.

Figure 6.17. Header format of mobile IP registration

Each datagram forwarded to the mobile host's home address is received by its home agent, and then it is forwarded to the mobile host's foreign address. In this case, mobile host's foreign agent receives the datagram and forwards it to the mobile host. If a mobile host residing in a foreign network wants to send a message to host outside of its new network, the message is not required to be passed through its home agent. In such a case, the message is handled by the foreign agent.

Mobile IP has two routing schemes: Delta routing and direct routing . In the Delta routing, a triangular path is established among the host's home agent, host's foreign agent, and a corresponding machine. Suppose that the mobile host belonging to wireless network 1 moves to foreign network 2. While in the foreign network, the mobile host is contacted for communication by a server (as a corresponding machine) fixed in a residential area network. In this case, a datagram (IP packet) from the server is first sent to the mobile's home network using standard IP routing. The host's home agent detects the message, finds the host's foreign address, and forwards the message to the host's foreign agent. The foreign agent delivers the message to the mobile host. In response, the mobile host can send its reply directly to the server through the foreign agent. This routing process forms a triangular -shape path routing and that is why it is called Delta routing.

Now, consider a case that a mobile host is the one who wants to initiate the transmission of a message with a server in the same scenario as explained above. In the first step, the mobile host informs the server of its foreign address. Then, the mobile host can send the message directly to the server through its foreign agent bypassing its home agent. This way a chunk of signaling due to routing to home agent is eliminated. We remember that the corresponding server should initiate a communication with a mobile host always starting to contact the mobile host's home agent since the server does not have a real-time knowledge of the mobile host's whereabout.

As we see, the routing of mobile users may involve many different challenges. For example, in the previous scenario, if the mobile host moves to yet a new foreign network, say network 3. In this case, the mobile host can inform its previous foreign agent about its new foreign address, so that datagrams (IP packets) routed to the old location can now be routed to the new foreign location.

Virtual Registration and Routing

In order to reduce the cost and the amount of registration with the home agent, the mobile Internet protocol also offers facility called virtual registration . In each region, virtual agents instead of just a home agent can be defined. Virtual regions are then defined based on statistics and the density of traffic. Each virtual agent covers services over a local virtual region. When a mobile host enters the virtual region, it registers with the virtual agent. Thus, in a scenario of routing messages between a mobile host and a corresponding server described in the previous section, datagrams from the corresponding server are sent to the mobile's home address, and then routed to the mobile's foreign address. Datagrams are then sent from the home agent to the virtual agent first and, from there, to the foreign agent. In such cases, the mobile host has typically no knowledge of the network for routing decision making.

Tree-Based Routing

The amount of registration between a home network and a foreign network can also be reduced by a carefully designed hierarchy of foreign agents. In a hierarchical structure, multiple foreign agents are advertised in the agent advertisement. With this scheme, a mobile host has to configure to what upper level at the tree its new registration has to go. The mobile host should then transmit the registration to each level of the hierarchy between itself and the closest common parent between its new and previous foreign addresses. If a mobile host currently using the services of one foreign agent moves to a different foreign agent, it may not involve a direct registration with its home agent.

Figure 6.18 shows a tree-based hierarchy of foreign agents. Suppose that a mobile host is currently using the service of foreign agent A16 while at location L1. The mobile host receives agent advertisements from foreign agents A1, A2, A4, A7, A11, and A16. Registration messages are sent to each of these foreign agents and its home agent. However, the home agent of the mobile host can only identify foreign agents in its outside world as far as to foreign agent A1. This means that, the topology of the hierarchy beyond A1 may stay unknown for the home agent even though it receives messages from other agents. The same thing is true for agent A1 which can see only up to its nearest neighbors A2 and A3, and so on for others. In fact, no agent knows exactly where the mobile host is located except for foreign agent A16.

Figure 6.18. Routing in a tree-based structure of foreign agents

When the mobile host moves to the vicinity of foreign agent A17 at location L2, the host needs a new registration valid to travel upto the vicinity of A11. If the mobile moves to the vicinity of foreign agent A19 at location L3, the situation is different as A17 and A19 are linked directly to a common node as was the case for A16 and A17. In this case, the mobile host receives advertisements specifying the hierarchy of A4, A8, A13, and A19. The mobile host then compares the previous hierarchy and this new one and determines that it has caused the registration to move to as high as level A4 in the tree-based scheme. The same procedure occurs when the mobile host decides to move to location L4.

Mobile Routing with IPv6

Mobile IPv6 offers a simpler mobile routing scheme. With IPv6, no foreign agent is required. A mobile host should use the address autoconfiguration procedure embedded in IPv6 to obtain a foreign address on a foreign network. The procedure to route with mobile IPv6 is summarized as follows:

Mobile IPv6 Routing Steps

1.	A host informs its home agent and also corresponding machines about its foreign address.
2.	If a corresponding machine knows the mobile's current foreign address, it can send packets directly to the mobile host by using the IPv6 routing header; Otherwise the corresponding machine sends packets without the IPv6 routing header.
3.	Packets are routed to the mobile host's home agent
4.	Packets are forwarded to the mobile host's foreign address
5.	If the mobile host moves back to its home network, the host notifies its home agent.

It is clear that the routing steps are similar to IPv4 ones except for the elimination of foreign agent in IPv6. Overall, routing with IPv6 is simpler and the option of source routing is also available.

6.5.5. Security

Wireless links are susceptible to eavesdropping or passive traffic monitoring. The inherent broadcast paradigm in wireless networks make them more susceptible to various attacks. The security for these networks involve:

• Network security

• Radio link security

• Hardware security

Radio link security involves preventing the interception of radio signals, defense against jamming attacks, and encrypting traffic to ensure privacy of user location. The security portion of wireless system must prevent the misuse of mobile units by making them tamper resistant. The hardware component of wireless security is especially complex. The details of security in wireless networks are discussed in Chapter 10.