After all, mobility and wireless often are found together for obvious reasons. But wireless communication is really about getting data from A to B without a wire, while mobility is about dealing with what happens when a node moves around as it communicates.
Routing to Mobile Hosts (Mobile IP)
Certainly many nodes that use wireless communication channels are not mobile, and sometimes mobile nodes will use wired communication although this is less common. Finally, in this chapter we are mostly interested in what we might call network-layer mobility. That is, we are interested in how to deal with nodes that move from one network to another. Moving from one access point to another in the same Mobile IP is the primary mechanism in today's Internet architecture to tackle the problem of routing packets to mobile hosts.
It introduces a few new capabilities but does not require any change from non-mobile hosts or most routers—thus making it incrementally deployable. The mobile host is assumed to have a permanent IP address, called its home address , which has a network prefix equal to that of its home network. This is the address that will be used by other hosts when they initially send packets to the mobile host; because it does not change, it can be used by long-lived applications as the host roams. We can think of this as the long-lived identifier of the host. When the host moves to a new foreign network away from its home network, it typically acquires a new address on that network using some means such as DHCP.
This address is going to change every time the host roams to a new network, so we can think of this as being more like the locator for the host, but it is important to note that the host does not lose its permanent home address when it acquires a new address on the foreign network. This home address is critical to its ability to sustain communications as it moves, as we'll see below. While the majority of routers remain unchanged, mobility support does require some new functionality in at least one router, known as the home agent of the mobile node.
This router is located on the home network of the mobile host. In some cases, a second router with enhanced functionality, the foreign agent, is also required. This router is located on a network to which the mobile node attaches itself when it is away from its home network. We will consider first the operation of Mobile IP when a foreign agent is used. An example network with both home and foreign agents is shown in Figure 2. Both home and foreign agents periodically announce their presence on the networks to which they are attached using agent advertisement messages.
A mobile host may also solicit an advertisement when it attaches to a new network.
Mobile IP - Wikipedia
The advertisement by the home agent enables a mobile host to learn the address of its home agent before it leaves its home network. When the mobile host attaches to a foreign network, it hears an advertisement from a foreign agent and registers with the agent, providing the address of its home agent. The foreign agent then contacts the home agent, providing a care-of address. This is usually the IP address of the foreign agent.
At this point, we can see that any host that tries to send a packet to the mobile host will send it with a destination address equal to the home address of that node. Normal IP forwarding will cause that packet to arrive on the home network of the mobile node on which the home agent is sitting.
4.4 Routing Among Mobile Devices
Thus, we can divide the problem of delivering the packet to the mobile node into three parts:. The first problem might look easy if you just look at Figure 2 , in which the home agent is clearly the only path between the sending host and the home network and thus must receive packets that are destined to the mobile node. But what if the sending correspondent node were on network 18, or what if there were another router connected to network 18 that tried to deliver the packet without its passing through the home agent?
To address this problem, the home agent actually impersonates the mobile node, using a technique called proxy ARP. It uses its own hardware address, so that all the nodes on the same network learn to associate the hardware address of the home agent with the IP address of the mobile node. One subtle aspect of this process is the fact that ARP information may be cached in other nodes on the network.
To make sure that these caches are invalidated in a timely way, the home agent issues an ARP message as soon as the mobile node registers with a foreign agent. The second problem is the delivery of the intercepted packet to the foreign agent. Here we use the tunneling technique described elsewhere. The home agent simply wraps the packet inside an IP header that is destined for the foreign agent and transmits it into the internetwork. All the intervening routers just see an IP packet destined for the IP address of the foreign agent.
Another way of looking at this is that an IP tunnel is established between the home agent and the foreign agent, and the home agent just drops packets destined for the mobile node into that tunnel. When a packet finally arrives at the foreign agent, it strips the extra IP header and finds inside an IP packet destined for the home address of the mobile node. Clearly the foreign agent cannot treat this like any old IP packet because this would cause it to send it back to the home network. Instead, it has to recognize the address as that of a registered mobile node.
It then delivers the packet to the hardware address of the mobile node e. One observation that can be made about these procedures is that it is possible for the foreign agent and the mobile node to be in the same box; that is, a mobile node can perform the foreign agent function itself.
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To make this work, however, the mobile node must be able to dynamically acquire an IP address that is located in the address space of the foreign network e. This address will then be used as the care-of address.
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In our example, this address would have a network number of This approach has the desirable feature of allowing mobile nodes to attach to networks that don't have foreign agents; thus, mobility can be achieved with only the addition of a home agent and some new software on the mobile node assuming DHCP is used on the foreign network.
What about traffic in the other direction i. This turns out to be much easier. The mobile node just puts the IP address of the fixed node in the destination field of its IP packets while putting its permanent address in the source field, and the packets are forwarded to the fixed node using normal means. Of course, if both nodes in a conversation are mobile, then the procedures described above are used in each direction. There is one significant drawback to the above approach: The route from the correspondent node to the mobile node can be significantly suboptimal.
One of the most extreme examples is when a mobile node and the correspondent node are on the same network, but the home network for the mobile node is on the far side of the Internet. The sending correspondent node addresses all packets to the home network; they traverse the Internet to reach the home agent, which then tunnels them back across the Internet to reach the foreign agent. Clearly, it would be nice if the correspondent node could find out that the mobile node is actually on the same network and deliver the packet directly.
In the more general case, the goal is to deliver packets as directly as possible from correspondent node to mobile node without passing through a home agent. This is sometimes referred to as the triangle routing problem since the path from correspondent to mobile node via home agent takes two sides of a triangle, rather than the third side that is the direct path.
The basic idea behind the solution to triangle routing is to let the correspondent node know the care-of address of the mobile node. The correspondent node can then create its own tunnel to the foreign agent. This is treated as an optimization of the process just described. If the sender has been equipped with the necessary software to learn the care-of address and create its own tunnel, then the route can be optimized; if not, packets just follow the suboptimal route. When a home agent sees a packet destined for one of the mobile nodes that it supports, it can deduce that the sender is not using the optimal route.
Therefore, it sends a "binding update" message back to the source, in addition to forwarding the data packet to the foreign agent. The source, if capable, uses this binding update to create an entry in a binding cache, which consists of a list of mappings from mobile node addresses to care-of addresses. He has more than nine years of experience with IP mobility deployments and is an expert on real-life applications of the technology.
Chandra, Ph. She earned her Ph.
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