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.
Challenges for Mobile Networking
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. 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.
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Normal IP forwarding will cause that packet to arrive on the home network of the mobile node on which the home agent is sitting. 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.
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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. 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.
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This address will then be used as the care-of address. In our example, this address would have a network number of Data Communications and Networking. This is a good textbook for introductory courses to networking. The chapters include detailed yet easy to grasp explanations of the topics covered, often with figures and demonstrations Language is easy to understand.. Networking All-in-One For Dummies, 6ed. Book is very good and covers almost all the aspects of networking.
This book contains basic as well as advanced level topics too. Computer Networks, 5e 5th Edition. No complaints about the content of the book: it's an excellent, enjoyable and thoroughly readable. My only complaint about this Indian adaptation is that a table of contents is missing and sections in chapters are named without the chapter numbers chapters are not numbered at all!
Nice book i like this book its very helping for networker. Cryptography and Network Security - Principles and Practice. Preferred text book for Cryptography. Language is easy to understand. Contents are organized well and provides Complete and detailed understanding. Hi Martin and Goran.
I do appreciate you responses and suggestions. I will definitely check on them. I think it's a very good book for starters. However, I noticed that it was published in What do you think of that book? I went straight to CCNA , with 0 "null" knowledge about networking. And its not that difficult, just need to read a lot, make notes and practice. So in my opinion I think that Cisco books are the best for beginners. It's an academic textbook, so it's fairly rigorous, but its intended audience is beginners. He also covers network security.
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What I most like about Tanenbaum is his writing style. He's informed, accurate and engaging.
The book isn't cheap, but neither are Cisco Press books. Computer Networks 5th Edition : Andrew S. Tanenbaum, David J. Wetherall: Amazon. Thank you for that. Indeed, it isn't cheap.