| Mobile IP is a set of protocols defined in RFC 2002 by IETF Mobile IP Group, which mainly aims at mobile node seamless roaming in Internet. The most important feature of Mobile IP is that it can make mobile terminal roam in Internet with the same and not changed IP address. At the same time, communication cannot be interrupted all the time. The specific solution includes two parts: one is the solution of mobile node, and the other is the solution of the home agent.In this thesis, we carry out a scheme for agent searching process, registration & registration reply process, and routing of the data packets towards the double protocol stack IPv4/IPv6.Agent Discovery Agent discovery helps mobile node to confirm its position, and the mobile node must know which link it is, the home link or the foreign link. Through this process, the mobile node can make sure whether it has moved after the last registration. Agent searching is the most important precondition that cannot be absent to offer the roaming communication, for only by knowing it's position, can mobile node registers its care-of address obtained on foreign link on home agent properly. Then home agent can route the packets properly to mobile node according to the new registration.Registration After agent searching process, there comes the registration, which keep the continuous contact between the mobile node and the home agent. Commonly, the home agent is a router in the home link of the mobile node, the mainly effect of which is transmitting the packets from other nodes to the roaming mobile node. When the mobile node is away from home link, it registers its care-of address to its home agent. This is done through exchange of a Registration Request and Registration Reply messages (which are sent with UDP using well known port number 434). Depending on the method of attachment, the mobile node will register either directly with its home agent, or through a foreign agent, which forwards the registration to the home agent. The care-of address can either be determined from a foreign agent's advertisements (a foreign agent care-of address), or by some external assignment mechanism such as DHCP (a co-located care-of address). When the home agent receives the register message, it will maintain a register message table in the server, where the original registered care of address will be replaced by the new registered care-of address. Then a reply message will be sent to the mobile node also by well-known port 434. OK, all the register and reply process finish.Packet routingAfter the register, the packet can be routed in IP layer. Among all the routing process, our thesis emphasizes on the mobile node. The work of the mobile node mainly involves two parts, the one is that mobile node receives packets from other net nodes, and the other is the mobile node actively communicates with other nodes. If the mobile node receives packets from other nodes, there comes two condition: one way is that its position is on the home route, then it will discover that the destination address of the packets is the home address, so it receives packets. The other way is that the mobile node is on the roaming foreign link, at this time, the packet's destination address it receives will be the care-of address transmitted by the home agent. In order to communicate with other nodes only with its home address, the mobile node will dispose the packets like this: First, it will shuck off the IP head, so comes out the home address which be added by home agent. Second, the mobile node can use home address to send packets to the communication opposite. Generally, the home agent substitutes the mobile node to receive packets in home link, then send all the packets through the channel to the care-of address, so the roaming mobile node receives the packets on the foreign link.On the other hand, the mobile node maybe actively communicates with other nodes. Here also two conditions: the one is the mobile node discovers that it is on the home link through agent searching, then no...
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