Abstract:
A method for packet communication including: sending a packet, by a node configured to be moved among different networks, the packet including a sender address being a private sender address; changing the sender address from the private sender address to a global address, even when the sending is performed in any of the different networks.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for packet communication where a node moves between different networks, and particularly to a method for packet communication where a node is moved to networks having a network address translation (NAT) function. 
   2. Description of the Related Art 
   In Internet Protocol (IP) communication, a wide area network is divided into a plurality of sub-networks (local area network) for management. Further, each node (node) is assigned an IP address, which includes a network portion indicating a number for identifying a sub-network and a host portion for indicating a reference number of a node in the sub-network. 
   In IP communication, routing is performed by including the IP address as a sender address and a destination address. For example, a router provided in each sub-network receives a packet existed on an IP network and checks whether a network portion of the destination address matches with a sub-network number managed by itself. If they match, the router incorporates the packet to the its managing sub-network. Each node selects and receives a packet whose destination address matches with its IP address among packets incorporated to the sub-network. It should be noted that the router might be called gateway in software terms. 
   Due to the above-described configuration of the IP network, a node to which an IP address is assigned within a particular sub-network cannot receive packets addressed to itself when the node is moved to another sub-network. It is because the router managing the sub-network to which the node is moved does not incorporate packets addressed to the node to its managing network. 
   Thus, in order to solve such a problem, a mobile IP (RFC2002, “IP Mobility Support”, RFC: Request For Comment) has been proposed. The mobile IP is proposed by Internet Engineering Task Force (IETF). It is a technology for transferring an IP packet to a node without changing the IP address assigned to the node even when the node is moved across different sub-networks. 
     FIG. 1  is a construction diagram exemplifying a network concept of the mobile IP.  FIG. 1  includes a mobile node(MN)  10 , a home network  20  which is sub-network to which the mobile node  10  originally belongs, a home agent unit (HA)  16  for managing movements of the mobile node  10 , a router  14  having the HA  16 , a foreign network  21  which is a sub-network to which the mobile node  10  moves, a foreign agent unit (FA)  17  for managing the mobile node  10  which is moved into the foreign network  21 , a router  15  having the FA  17 , a correspondent node (CN)  11  for communicating with the mobile node  10 , and an IP network  22  such as Internet connected to the home network  20  and the foreign network  21 . 
   A processing procedure of the mobile IP will be described by using as an example a case where the mobile node  10  is moved to the foreign network  21 . An IP address is preset in the mobile node  10  in hardware or software manner. The IP address of the mobile node  10  includes an identification number of the home network  20  as a network portion and includes an identification number of the mobile node  10  in the home network  20  as a host portion. 
   Now, it is assumed that a packet addressed to the mobile node  10  is sent from the correspondent node  11  onto the IP network  22 . The destination address of the packet, that is, the IP address of the mobile node  10  includes the identification number of the home network  20 . Thus, the packet is incorporated to the router  14  managing the home network  20  via a path  23 . The HA  16  having the router  14  recognizes in advance that the mobile node  10  has been moved to the foreign network  21  and transfers the packet to the router  15  managing the foreign network  21 . The FA  17  having the router  15  incorporates the transferred packet to its managing sub-network, that is, the foreign network  21 . The mobile node  10  receives the packet incorporated to the foreign network  21  and determines that the destination address matches with its address in order to receive the packet. 
   It should be noted that a tunneling technology is used for transferring packets from the HA  16  to the FA  17  via path  24 . The tunneling technology adds a header including a new destination address and a transferring address to a packet when transferring. The processing that a transferring node adds the header is called encapsulation processing while the processing that a destination node removes the header is called decapsulating processing. In this case where a packet is transferred from the HA  16  to the FA  17 , an IP address D, as a transferring address, of the router  15  having the FA  17  and IP address C, as a destination address, of the router  14  having the HA  16  are added as a header. 
   It will be described a case where the mobile node  10  exists in the foreign network  21  and sends a packet to the correspondent node  11 . The mobile node  10  creates a packet including an IP address B of the correspondent node  11  and its IP address (that is, home address A) as the destination and sender addresses, respectively. The created packet is sent to the correspondent node  11  via the path  26 , the router  15 , and the path  27 . 
     FIG. 2  is a sequence diagram for exemplifying a processing sequence of a mobile IP in a case where the mobile node  10  is moved from the home network  20  to the foreign network  21 . In  FIG. 2 ,  1000  to  1002  are processing where the mobile node  10  exists in the home network  20  while  1003  to  1015  are processing where the mobile node  10  exists in the foreign network  21 . 
   First of all, the mobile node  10  in the home network  20  receives an agent advertisement from the router  14  so that it recognizes that a sub-network where it is positioned is managed by the HA  16  of the router  14  (processes  1000  to  1002 ).  FIG. 3  is an explanatory diagram for exemplifying a format of the agent advertisement. As shown, the agent advertisement includes an IP address (Care of Address) of an agent node (that is, a node having the HA and FA) managing the sub-network. It should be noted that the agent advertisement is a packet in which a router discovery of an Internet control message protocol (ICMP) is extended and broadcasted in the sub-network. 
   Next, the mobile node  10  is moved from the home network  20  into the foreign network  21  (process  1003 ). In the foreign network  21 , the mobile node  10  receives an agent advertisement from the router  15  and recognizes that it is moved into the sub-network managed by the FA  17  of the router  15  (process  1004 ). Then, the mobile node  10  requests the FA  17  of the router  15  to register its presence (process  1005 ). More specifically, a message in a format shown in  FIG. 4  is sent as a registration request message to the router  15 . As shown in  FIG. 4 , the registration request message includes a home address of the mobile node  10 , an address of a home agent managing the mobile node  10  (IP address of the router  14 , here), and an address of a foreign agent (Care of address, IP address of the router  15 , here). 
   The FA  17  receives the registration request message shown in  FIG. 4  from the mobile node  10  and registers (stores) its content. Then, the FA  17  transfers the registration request message to the HA  16  (process  1006 ). The HA  16  receives the registration request message and registers (stores) its registration content so that packet communication can be performed in the foreign network  21  as well. Further, the HA  16  responds to the FA  17  that registration has been completed (process  1007 ). More specifically, the FA  17  sends a message in a format shown in  FIG. 5  as a response message to the router  15 . As shown in  FIG. 5 , the message includes a home address of the mobile node  10  and an IP address of the home agent (IP address of the router  14 , here) FA  17  receives the response message and sends the response message to the mobile node  10 . The mobile node  10  receives the response message so that it can recognizes that communication is possible (process  1008 ). 
   Next, a method for communication between the mobile node  10  and the correspondent node  11  will be described with reference to  FIGS. 1 and 2 . The mobile node  10  creates packet data whose destination address is an IP address B of the correspondent node  11  and whose sender address is its IP address A. Then, the mobile node  10  sends the packet into the foreign network  21  (process  1009 ). The packet is sent to the correspondent node  11  on the IP network  22  through the router  15  (process  1010 ). When the correspondent node  11  responds to the received packet, that is, when the correspondent node  11  responds to the mobile node  10 , the correspondent node  11  sends to the IP address B a packet whose destination address is the address A which is a destination address of the received packet and whose sender address is its IP address B (process  1011 ). 
   The packet sent in this manner includes an identification number of the home network  20  in the network portion of its destination address A. Thus, it is incorporated to the router  14  once. The home agent  16  of the router  14  has already realized that the mobile node  10  was moved to the foreign network  21 . The home agent  16  encapsulates the received packet and tunneling-transfers it to the router  15  (processes  1012  and  1213 ). More specifically, the home agent  16  adds to the packet a new packet whose destination address is an IP address D of the router  15  and whose sender address is an IP address C of the router  14  and sends out the packet to the IP network  22 . The router  15  receives the tunnel-transferred and encapsulated packet and decapsulates the packet (process  1014 ). It should be noted that the destination and sender addresses of the packet resulted from the decapsulation are the same as those before encapsulation, and they are the IP address A of the mobile node  10  and the IP address B of the correspondent node  11 , respectively. The decapsulated packet are sent from the router  15  into the foreign network  21  and received by the mobile node  10  (process  1015 ). In this way, the communication is possible between the mobile node  10  in the foreign network  21  and the correspondent node  11  on the IP network. 
   Recently, since IP address resources must be used efficiently, a private network has been established which uses technologies including private addresses, NAT, and IP masquerades in enterprises. 
   It should be noted that when an address used within a particular network is a global address, a private address is an address used only in a particular sub-network belonging to the global network. Further, NAT translates a destination address and a sender address of each packet from a private address to a global address, or from a global address to a private address when packet communication is performed between node using the private address and node using the global address. In NAT, the private address and the global address are mapped one-to-one. 
   The IP masquerade is resulted from the extension of the NAT. The IP masquerade is a technology for mapping a plurality of private addresses to one global address by regarding one including a port number of TCP (transmission control) and/or UDP (user datagram protocol) as an address. A network using a private address and a network using a global address are called a private network and a global network, respectively, below. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a method for communication which can be applied to a case where such a private network is included in networks where a node is moved. 
   In order to achieve the object, the present invention changes a sender address of a packet sent by a node to a same global address when the node, which can be moved among different networks and sends a packet including a private address as a sender address, sends the packet in either one of the networks. 
   According to the construction, a sender address of a packet received in a correspondent node does not change before and after the movement of the node. Therefore, it reduces interruptions of communication due to that the correspondent node determines a packet from a same node as a packet from a different node. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a configuration diagram for exemplifying a communication system for a mobile IP; 
       FIG. 2  is a diagram for exemplifying a communication sequence of the mobile IP; 
       FIG. 3  is a diagram for exemplifying a format for an agent advertisement message used in the mobile IP; 
       FIG. 4  is a diagram for exemplifying a format for a registration request message used in the mobile IP; 
       FIG. 5  is a diagram for exemplifying a format for a registration response message used in the mobile IP; 
       FIG. 6  is a configuration diagram for exemplifying a communication system according to a first embodiment of the present invention; 
       FIG. 7  is a diagram for exemplifying a communication sequence if registration and data transfer according to the first embodiment of the present invention; 
       FIG. 8  is a diagram for exemplifying address translation information according to the first embodiment of the present invention; 
       FIG. 9  is a diagram for exemplifying a format for a message notifying address translation information according to a first embodiment of the present invention; 
       FIG. 10  is a diagram for exemplifying a communication sequence according to a second embodiment of the present invention; 
       FIG. 11  is a diagram for exemplifying a format for a message requesting address translation information according to the second embodiment of the present invention; 
       FIG. 12  is a diagram for exemplifying a communication sequence according to a third embodiment of the present invention; 
       FIG. 13  is a diagram for exemplifying a communication sequence according to a fourth embodiment of the present invention; 
       FIG. 14  is a construction diagram for exemplifying a communication system according to a fifth embodiment of the present invention; 
       FIG. 15  is a diagram for exemplifying a communication sequence according to the fifth embodiment of the present invention; 
       FIG. 16  is a diagram for exemplifying a communication sequence according to a sixth embodiment of the present invention; 
       FIG. 17  is a diagram for exemplifying a communication sequence according to a seventh embodiment of the present invention; 
       FIG. 18  is a construction diagram for exemplifying a communication system according to a eighth embodiment of the present invention; 
       FIG. 19  is a diagram for exemplifying a communication sequence according to the eighth embodiment of the present invention; and 
       FIG. 20  is a diagram for exemplifying an address translation list according to the eighth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   When a mobile node is moved from a private network to a private network, from a private network to a global network, or from a global network to a private network, communication may be interrupted due to some setting in a correspondent node. 
   For example, in a case where a mobile node is moved from a private network to a private network, that is, where both home network and foreign network are private networks, and when the mobile node exists in the home network, address translation is performed by a NAT in the home network. When the mobile node is moved to the foreign network, the address translation is performed by another NAT in the foreign network. So, an IP address resulted from the translation by the NAT in the home network and an IP address resulted from the translation by the NAT in the foreign network are not translated into the same address. 
   Therefore, when the IP address resulted from the translation by the NAT in the private network the mobile node is moved from and the IP address resulted from the translation by the NAT in the private network the mobile node is moved to are different, the correspondent node determines those packets as packets from different mobile nodes, which causes interruption of communication. 
   Thus, according to the present invention, the communication is performed in a following manner. 
   First Embodiment 
   A method for packet communication according to the present invention will be described below with reference to figures.  FIG. 6  is an explanatory diagram for exemplifying a network construction and its packet transfer path according to a first embodiment of the present invention. In  FIG. 6 , identical reference numerals will be given to identical or corresponding parts, and their description will be omitted here. In  FIG. 6 , both home network  20  and foreign network  21  are private networks using private addresses. A router  12  has functions of HA  16  and a NAT  18  and connects an IP network  22  and a home network  20 . A router  13  has functions of the FA  17  and the NAT  19  and connects the IP network  22  and the foreign network  21 . Assigned to a home address of the mobile node  10 , the correspondent node  11 , the router  12 , and the router  13  are a private address A, a global address B, a global address C, and a global address D. While a case where the NAT  18  assigns the global address C of the router  12  itself to the mobile node  10 , another global address may be assigned to the mobile node  10 . 
   As described above, the NAT maps a particular global address to a private address of a node. Such a mapping relationship will be referred to address translation information below.  FIG. 8  is an explanatory diagram for exemplifying address translation information for the NAT  18 .  FIG. 8  includes a private address  28  which is assigned to the mobile node  10  as a home address, and a global address  29  which is assigned as a translated address of the home address. Here, a global address C is mapped to a private address A. That is, the NAT  18  translates a sender address of a packet sent from the mobile node  10  from the private address A to the global address C. Further, the NAT  18  translates a destination address of a packet sent from the correspondent node  11  from the global address C to the private address A. 
   The first embodiment is characterized in that the NAT  19  of the foreign network  21  has identical address translation information to that of the NAT  18  in the home network  20  with respect to the mobile node  10 . More specifically, the NAT  18  and the NAT  19  communicates each address translation information with each other periodically so that the identical global address are mapped for the mobile node  10 . Thus, like the NAT  18 , the NAT  19  translates a sender address of a packet sent from the mobile node  10  from the private address A to the global address c in order to send it to the correspondent node  11 . Therefore, sender addresses of the packets sent from the home network  20  and the foreign network  21 , respectively, can be matched, which can improve interruption of communication due to changes in sender addresses during communication. 
   A packet in a format shown in  FIG. 9  is used for notifying address translation information.  FIG. 9  shows message type  30 , which indicates that it is a message for notifying address translation information here. Further,  FIG. 9  shows an unused reserved area  31 , a number of address translation information to be notified, a home address  33  (for example, provide address A) of the mobile node  10 , and a global address  34  (for example, global address C) mapped to the home address of the mobile node  10 . The home address  33  and the global address  34  make a pair, and a number of this pair is used as the number of the address translation information  32 . 
     FIG. 7  is an explanatory diagram for exemplifying a sequence in a case where the mobile node  10  is moved from the home network  20  to the foreign network  21  during communication with the correspondent node  11  according to the network construction shown in  FIG. 6 .  FIG. 7  includes processes  100  to  105  in a case where the mobile node  10  exists in the home network  20 , and processes  107  to  123  in a case where the mobile node  10  exists in the foreign network  21 . In the process  100  in  FIG. 7 , the mobile node  10  in the home network  20  sends packet data to the router  12  so that it can send data to the correspondent node  11  (process  100 ). A destination address of the packet is the global address B of the correspondent node  11 , while its sender address is the private address A of the mobile node  10 . It should be noted that packets having the destination address B and the sender address A are referred to Data (B, A) in  FIG. 7 . The other packets are similarly referred to Data (destination address, sender address). 
   The router  12  receives the packet from the mobile node  10 . Then, the router  12  uses the NAT  18  to translate the sender address of the packet from the private address A to the global address C of the router  12  (process  101 ). The packet whose address has been translated is sent onto the IP network  22  and received by the correspondent node  11  (process  102 ). The correspondent node  11  sends the packet to the sender address of the packet received in the above-described process  102  in order to return the packet to the mobile node  10  (process  103 ). That is, the correspondent node  11  sends onto the IP network  22  the packet whose destination address is the global address C of the router  12  and whose sender address is the global address B of the correspondent node  11 . 
   The router  12  receives the packet returned from the correspondent node  11 . Then, the router  12  uses the NAT  18  to translate the destination address of the packet from the global address C of the router  12  to the private address A of the mobile node  10  (process  104 ). The packet having translated address is sent onto the home network  20  and received by the mobile node  10  (process  105 ). This way allows data exchanges between the mobile node  10  in the home network  20  and the correspondent node  11  on the IP network. 
   Next, it will be described a case where the mobile node  10  is moved from the home network  20  to the foreign network  21  during communication as described above. In the process  106  in  FIG. 7 , the mobile node  10  is moved from the home network  20  to the foreign network  21  (process  106 ). The mobile node  10  receives an agent advertisement in the foreign network  21  so that the mobile node  10  can detect that it is moved to the foreign network  21  (process  107 ). As described with reference to  FIG. 3 , the agent advertisement includes an IP address of the FA  17  (IP address D of the router  13 , here). 
   The mobile node  10  having detected the movement requests registration to the FA  17  by using a format shown in  FIG. 4  as described above. The FA  17  receives the registration request and notices that the mobile node  10  has been moved into the foreign network  21  (process  108 ). Further, the FA  17  transfers the registration request to the HA  16  (process  109 ). The HA  16  receives the registration request from the FA  17  and notices that the mobile node  10  has been moved to the foreign network  21 . Then, the HA  16  returns a registration response in a format shown in  FIG. 5  described above to the FA  17  (process  110 ). Further, the registration response is returned from the FA  17  to the mobile node  10  so that the registration will be completed (process  111 ). 
   After the registration process as described above, the NAT  18  and the NAT  19  notify address translation information regarding the mobile node  10  to each other periodically. It will be described in detail. The NAT  18  has a timer, which times out periodically (process  112 ). Corresponding to the time-out, address translation information is notified from the NAT  18  to the NAT  19  in a format shown in  FIG. 9  described above (process  113 ). Thus, the NAT  19  notices that the home address A of a packet of the mobile node  10  is translated to the global address C in the home network  20 . Similarly, when a timer provided in the NAT  19  times out (process  114 ), address translation information is notified from the NAT  19  to the NAT  18  (process  115 ). 
   Next, a case will be described where data is sent from the mobile node  10  in the foreign network  21  to the correspondent node  11 . The mobile node  10  in the foreign network sends a packet to the router  13  in order to send data to the correspondent node  11  (process  116 ). A destination address and sender address of the to-be-sent packet are the global address B of the correspondent node  11  and the private address A of the mobile node  10 , respectively. The NAT  19  of the router  13  having received the packet from the mobile node  10  uses address translation information received in process  113  to translate a sender address of the packet from the private address A to the global address C (process  117 ). The destination address and the sender address of the packet whose address is translated in this manner are both global addresses. Therefore, the packet can be sent onto the IP network and transmitted to the correspondent node  11  (process  118 ). 
   The correspondent node  11  having received the packet send the packet whose destination address is the global address C which is the sender address of the earlier received packet in order to return the packet to the mobile node  10  (process  119 ). The router having received the packet from the correspondent node  11  translates the destination address of the packet from the global address C to the private address A in the NAT  18  (process  120 ). Further, HA  16  adds a new header to the packet so that the destination address and the sender address become the global address D of the router  13  and the global address C of the router  12 , respectively (that is, IP-encapsulated). The encapsulated packet is translated, that is, tunneled to the FA  17  of the router  13  (process  121 ). The FA  17  of the router  13  having received the tunneled packet decapsulates the packet (process  122 ) and sends it to the mobile node  10  (process  123 ). This allows communication between the mobile node  10  in the foreign network  21  and the correspondent node  11  on the IP network. 
   As described above, according to the first embodiment, even when a mobile node to which a private address is assigned is moved between different sub-networks, interruption of communication due to changes in a sender address during communication can be improved since mapping between a private address and a global address with respect to the mobile node in a foreign network is the same as mapping in a home network. 
   Second Embodiment 
   In the first embodiment, as shown in he processes  112  to  115  in  FIG. 7 , address translation information is sent by using time-out of a timer as a trigger. In a second embodiment, a registration request to be sent from be mobile node  10  to the router  13  is used as a trigger in order to send a request message for address translation information from the NAT  19  to the NAT  18 .  FIG. 10  is an explanatory diagram for exemplifying a sequence where, in a network construction shown in  FIG. 6 , a registration request from the mobile node  10  is used as a trigger so that the NAT  19  of the router  13  obtains address translation information from the NAT  18  of the router  12 .  FIG. 10  includes each of processes  200  to  207 . 
   In process  200  in  FIG. 10 , the mobile node  10  in moved from the home network  20  to the foreign network  21  (process  200 ). The mobile node  10  receives an agent advertisement so that it detects that it has been moved to the foreign network  21  (process  201 ). The mobile node  10  having detected the movement uses a format shown in  FIG. 4  in order to request a registration to the FA  17  (process  202 ). The receipt of the translation request is used as a trigger, and the NAT  19  of the router  13  requests address translation information regarding the mobile node  10  to the NAT  18  of the router  12  (process  203 ).  FIG. 11  is an explanatory diagram for exemplifying a format of a message sent to the NAT  18  as the address translation information request.  40  indicates a message type, which includes a code indicating that it is a message for requesting address translation information here.  41  indicates that a reserved area is unused, and  42  indicates a number of address translation information to be requested.  43  indicates a home address of the mobile node  10 . 
   The NAT  18  having received the address translation information request in the process  203  returns address translation information regarding the mobile node  10  to the NAT  19  (process  204 ). That is, a global address mapped to the home address of the mobile node  10  (a private address here) is sent to the NAT  19  by using a format shown in  FIG. 9 . Next, a registration is requested from the FA  17  to the HA  16  (process  205 ) and a response indicating that a registration has been done, that is a registration response is returned from the HA  16  to the FA  17  (process  206 ). Further, the registration response is returned from the FA  17  to the mobile node  10  (process  207 ). 
   In the manner as described above, the mobile node  10  is registered to the HA  16  and the FA  17 , and address translation information of the mobile node  10  is set in the NAT  19 . Thus, a sender address of a packet sent from the mobile node  10  is translated to a global address, which is identical to that of the NAT  18  of the router  12 , in the NAT  19  of the router  13 . 
   Third Embodiment 
   In the second embodiment, a registration request sent from the mobile node  10  to the router  13  is used as a trigger in order to request address translation information. In a third embodiment, a registration response sent from the NAT  18  to the NAT  19  is used as a trigger in order to request address translation information from the NAT  19  to the NAT  18 .  FIG. 12  is an explanatory diagram for exemplifying a sequence where, in a network construction shown in  FIG. 6 , a registration response from the HA  16  is used as a trigger so that the NAT  19  of the router  13  obtains address translation information from the NAT  18  of the router  12 .  FIG. 12  includes each of processes  300  to  307 . In process  300  in  FIG. 12 , the mobile node  10  is moved from the home network  20  to the foreign network  21  (process  300 ). The mobile node  10  receives an agent advertisement so that it detects that it has been moved to the foreign network  21  (process  301 ). The mobile node  10  having detected the movement uses a format shown in  FIG. 4  in order to request a registration to the FA  17  (process  302 ). The FA  17  having received the registration request requests a translation further to the HA  16  after the registration(process  303 ). The HA  16  having received the registration request from the FA  17  returns a response indicating that it has registered, that is, a registration response to the FA  17  after the registration (process  304 ). 
   When the router  13  receives the registration response in the process  304  from the HA  16 , it requests address translation information to the NAT  18  of the router  12  by using the receipt of the registration response as a trigger (process  305 ). More specifically, a message requesting address translation information regarding the mobile node  10  is sent by using a format shown in  FIG. 11 . The NAT  18  having received the request message returns the address translation information by using a format shown in  FIG. 9  to the NAT  19  (process  306 ). The NAT  19  of the router  13  receives the address translation information and then returns a translation response to the mobile node  10  (process  307 ). 
   As described above, address translation information is requested after an HA (home agent) returns a translation response, which eliminates a need for address translation information requests when a registration is failed. In this case, an unnecessary message does not have to be sent to a network. 
   Fourth Embodiment 
   In a fourth embodiment, the address translation information request shown in  FIG. 11  is added into a message of the registration request and the registration response in the processes  109  and  110  shown in  FIG. 7 .  FIG. 13  is an explanatory diagram for exemplifying a sequence where, in a network construction shown in  FIG. 6 , an address translation information request is added into a translation request message and address translation information is added into a translation response message so that the NAT  10  of the router  13  obtains address translation information from the NAT  18  of the router  12 . 
   In a process  400  in  FIG. 13 , the mobile node  10  is moved from the home network  20  to the foreign network  21  (process  400 ). The mobile node  10  receives an agent advertisement so that it detects that it has moved to the foreign network  21  (process  401 ). The mobile node  10  having detected the movement requests a registration to the FA  17  by using the format shown in  FIG. 4  (process  402 ). The FA  17  having received the translation request requests an extended registration to the HA  16  after translation. More specifically, the FA  17  sends an extended registration request message, which is made by adding a number of address translation information requests  42  and a home address  43  shown in  FIG. 11  to the format shown in  FIG. 4  (process  403 ). 
   The HA  16  having received the extended registration request in the process  403  returns an extended registration response to the HA  17 . More specifically, the HA  16  returns an extended registration response message, which is made by adding a number of address translation information  32 , a home address  33 , and a translation address  34  shown in  FIG. 9  to the format shown in  FIG. 5  (process  404 ). The NAT  19  receives the extended registration response so that it can obtain address translation information regarding the mobile node  10 . Then, a registration response is returned from the FA  17  to the mobile node  10  so that the registration is completed (process  405 ). 
   As described above, an address translation information request message is added to a registration request message for transmission. Thus, it can reduces a number of message sent and time required from the registration request to a completion of obtaining address translation information. 
   Fifth Embodiment 
   In the first embodiment, both home network  20  and foreign network  21  have NATs. In a fifth embodiment, the foreign network  21  does not have a NAT. It will be described a case where the FA  17  performs address translation with respect the mobile node  10 .  FIG. 14  is an explanatory diagram for exemplifying a network construction and its packet transfer path according to the fifth embodiment. In  FIG. 14 , identical reference numerals will be given to identical or corresponding parts in  FIG. 6 , and their description will be omitted here. 
   In  FIG. 14 , the foreign network  21  is a global network using a global address. The router  15  connects an IP network  22  and a foreign network  21 . Further, the home network  20  is a private network using a private address. The router  12  connects the IP network  22  and the home network  20 . The router  15  has functions of the FA  17  but not NAT functions. The router  12  has functions of the HA  16  and the NAT  18 . A private address A, a global address B, a global address C, and a global address D are assigned to a home address of the mobile node  10 , the correspondent node  11 , the router  12 , and the router  15 , respectively. 
     FIG. 15  is an explanatory diagram for exemplifying a sequence where, in a network construction shown in  FIG. 14 , the mobile node  10  is moved from the home network  20  to the foreign network  21  during communication with the correspondent node  11 . In  FIG. 15 ,  500  to  522  indicate processes, respectively. In the process  500  in  FIG. 15 , the mobile node  10  in the home network  20  sends a packet to the router  12  in order to send data to the correspondent node  11  (process  500 ). A destination address of the packet is a global address B while its sender address is a private address A. 
   The router  12  having received the packet from the mobile node  10  uses the NAT  18  to translate the sender address of the received packet from the private address to the global address C of the router  13  (process  501 ). Then, the packet whose address is translated is sent to the correspondent node  11  (process  502 ). The correspondent node  11  receives the packet and returns the packet whose destination address is the address C of the router  12  and whose sender address is the address B of the correspondent node  11  (process  503 ). The router  12  having received the returned packet from the correspondent node  11  uses the NAT  18  to translate the destination address of the returned packet from the address C to the address A of the mobile node  10  (process  504 ). The packet whose address has been translated is sent to the mobile node  10  (process  505 ). In this way, communication is performed between the mobile node  10  in the home network  20  and the correspondent node  11  on the IP network. 
   Further, during communication as above, the mobile node  10  is moved from the home network  20  to the foreign network  21  (process  506 ). The mobile node  10  receives an agent advertisement in the foreign network  21  so that it detects that it has moved to the foreign network  21  (process  507 ). The mobile node  10  having detected the movement request a registration to the FA  17  by using the format shown in  FIG. 4  (process  508 ). Further, a registration request is performed from the FA  17  to the HA  16  (process  509 ), and a registration response is returned from the HA  16  to the FA  17  (process  510 ). As shown in  FIG. 5 , The registration response includes a home address of the mobile node  10 . 
   Here, the home address of the mobile node  10  is a private address. Thus, The FA  17  having received the registration response in the process  510  determines that address translation information regarding the mobile node  10  is required (process  511 ). Then, the FA  17  sends a message requesting for address translation information regarding the mobile node  10  to the NAT  18  by using the format shown in  FIG. 11  (process  512 ). The NAT  18  returns address translation information of the mobile node  10  in the format shown in  FIG. 9  to the FA  17  as a response to the request message in the process  512  (process  513 ). The FA  17  receives the response and then sends a registration response to the mobile node  10  (process  514 ). Thus, a registration is completed, and address translation information of the mobile node  10  is set in the FA  17 . 
   Next, a case will be described where data is sent from the mobile node  10  in the foreign network  21  to the correspondent node  11 . The mobile node  11  in the foreign network  21  sends a packet to the router  15  in order to send data to the correspondent node  11  (process  515 ). A destination address of the packet is a global address B while its sender address is a private address A. The router  15  having received the packet from the mobile node  10  translates through the FA  17  the sender address of the received packet from the private address A to the global address C based on address translation information obtained in the above-described process  513  (process  516 ). The packet whose address has been translated is sent to the correspondent node  11  (process  517 ). 
   The correspondent node  11  receives the packet and then sends as a returned packet to the mobile node  10  the packet whose destination address is an address C which is a sender address of the received packet (process  518 ). The NAT  18  of the router  12  translate the destination address of the returned packet from the correspondent node  11  from the address C of the router  12  to the address A of the mobile node  10 . Further, through the HA  16 , the NAT  18  sets the destination address and the sender address to the address D of the router  15  and the address C of the router  12  for IP-encapsulation (process  519 ). The IP-encapsulated packet is tunneled to the FA  17  of the router  15  (process  520 ). The FA  17  of the router  15  decapsulates the tunneled packet (process  521 ). The packet resulted from the decapsulation is sent to the mobile node  10  (process  522 ). In this way, communication becomes possible between the mobile node  10  in the foreign network  21  and the correspondent node  11  on the IP network. 
   As described above, when an NAT does not exist in a foreign network, an FA (foreign agent) performs address translation for a packet received/sent by a mobile node. Thus, the mobile node can continue communication even in the foreign network. That is, even when a home network is a private network and a foreign network to which it is moved is a global network, communication can be continued. 
   Sixth Embodiment 
   In the fifth embodiment, the FA  17  performs address translation with respect to the mobile node  10 . In a sixth embodiment, the mobile node  10  itself performs address translation. Further, the mobile node  10  uses a registration response sent from the FA  17  as a trigger to obtain address translation information.  FIG. 16  shows in the network construction shown in  FIG. 14  a sequence where the mobile node  10  itself uses a registration response as a trigger to obtain address translation information from the NAT  18  of the router  12 . 
   In a process  600  in  FIG. 16 , the mobile node  10  is moved from the home network  20  to the foreign network  21  (process  600 ). The mobile node  10  receives an agent advertisement so that it detects that it has moved to the foreign network  21  (process  601 ). The mobile node  10  having detected its movement uses the format shown in  FIG. 3  to request a registration to FA  17  (process  602 ). After the registration, the FA  17  further requests a registration to the HA  16  (process  603 ). After the registration, the HA  16  returns a registration response to the FA  17  (process  604 ). The FA  17  receives the registration response and sends the registration response further to the mobile node  10  (process  605 ). In this way, the registration is completed. 
   The mobile node  10  having received the registration response from the FA  17  uses it as a trigger in order to send a message requesting its address translation information to the NAT  18  by using the format shown in  FIG. 11  (process  606 ). The NAT  18  sends address translation information of the mobile node  10  to the mobile node  10  by using the format shown in  FIG. 9  as a response to the request message (process  607 ). The mobile node  10  receives the response and notices its address translation information. 
   Next, it will be described a case where data is sent from the mobile node  10  in the foreign network  21  to the correspondent node  11 . In process  608 , the mobile node  10  creates packet data to be sent to the correspondent node  11 . A sender address of the created packet is not the private address A of the mobile node  10  but is set to the global address C which is mapped to the private address A. The created packet is sent, and then it is received by the correspondent node  11  (process  609 ). The correspondent node  11  sends a packet by using the address C which is a sender address of the received packet for its destination address as a response to the received packet (process  610 ). The packet whose destination address is the address C is received by the router  12 . The router  12  uses the NAT  18  in order to translate the destination address of the received packet from the global address C to the private address A (process  611 ). Further, the HA  16  of the router  12  recognizes by a registration request in the process  603  that the mobile node  10  is under a management network of the FA  17 . The HA  16  IP-encapsulates the address translated packet (process  611 ) and tunneling-transfers it to the FA  17  of the router  15  (process  612 ). The FA  17  of the router  15  decapsulates the tunneling-transferred packet in order to obtain the original packet whose destination address is the private address A (process  613 ). The original packet obtained by the decapsulation is sent to the mobile node  10  (process  614 ). 
   In this way, communication becomes possible between the mobile node  10  in the foreign network  21  and the correspondent node  11  on the IP network. Particularly, when the mobile node itself to which a private address is assigned obtains address translation information, communication is possible in a foreign network which is a global network without addition of functions to an FA (foreign network) as much as possible thereby. 
   Seventh Embodiment 
   In the sixth embodiment, the mobile node  10  uses a registration response as a trigger in order to obtain address translation information. In a seventh embodiment, a registration request sent from the mobile node  10  to the FA  17 , a registration request sent from the FA  17  to the HA  16 , a registration response sent from the HA  16  to the FA  17 , and a registration response sent from the FA  17  to the mobile node  10  are extended respectively so that the mobile node  10  can obtain address translation information. 
     FIG. 17  shows a sequence where, in the network construction shown in  FIG. 14 , an address translation information request is added to a registration request message, and the address translation information is added to a registration response message, so that the mobile node  10  obtains address translation information from the NAT  18  of the router  12 . In a process  700  in  FIG. 17 , the mobile node  10  is moved from the home network  20  to the foreign network  21  (process  700 ). The mobile node  10  receives an agent advertisement so that it can detect that it has moved to the foreign network  21  (process  701 ). 
   The mobile node  10  having detected the movement in the process  701  performs an extended registration request in order to request its address translation information to the FA  17  (process  702 ). More particularly, an extended registration request message which is made by adding the number of address translation information requests  42  and the home address  43  shown in  FIG. 11  to the format shown in  FIG. 4  is sent to the FA  17 . The FA  17  receives the extended registration request message in order to perform a predetermined registration. The, the message is sent to the router  12  (process  703 ). The router  12  receives the message and performs a predetermined registration in the HA  16 . Also, the extended registration response is sent to the FA  17  by including address translation information regarding the mobile node  10  in the message (process  704 ). More specifically, an extended registration response message by adding the address translation information  32 , the home address  33 , and the translation address  34  shown in  FIG. 9  to the format shown in  FIG. 5  is sent to the FA  17 . the extended registration response message is further sent from the FA  17  to the mobile node  10  (process  705 ). Thus, the registration is completed, and address translation information regarding the mobile node  10  is set in the mobile node  10 . 
   As described above, an address translation information request message is added to a registration request message for transmission, which can reduced a number and time for sending a message required from the registration request to obtaining the address translation information. 
   Eighth Embodiment 
   In the first embodiment, both home network  20  and foreign network  21  have NATs. In a eighth embodiment, it will be described a case where the home network  20  does not have a NAT while the foreign network  21  has the NAT. A global address is assigned to the mobile node  10  here.  FIG. 18  is an explanatory diagram for exemplifying a network construction and its packet transfer path according to the eighth embodiment. In  FIG. 18 , identical reference numerals will be given to identical or corresponding parts in  FIG. 6 , and their description will be omitted here. 
   In  FIG. 18 , the home network  20  is a global network using a global address. The router  14  does not have a NAT function. More specifically, the router  14  has functions of the HA  16  and connects the IP network  22  and the home network  20 . The foreign network  21  is a private network using a private address. The router  15  has functions of the FA  17  and NAT  19  and connects the IP network  22  and the foreign network  21 . A global address A, a global address B, a global address C, and a global address D are assigned to a home address of the mobile node  10 , the correspondent node  11 , the router  14 , and the router  13 , respectively. 
     FIG. 19  is an explanatory diagram for exemplifying a sequence where, in a network construction shown in  FIG. 18 , the mobile node  10  is moved from the home network  20  to the foreign network  21  during communication with the correspondent node  11 . In FIG  19 ,  800  to  818  indicate processes, respectively. In the process  800  in  FIG. 18 , the mobile node  10  in the home network  20  sends a packet to the router  14  in order to send data to the correspondent node  11 . A destination address of the packet is a global address B while its sender address is a private address A. The router  14  receives the packet and send it to the correspondent node  11  (process  801 ). The correspondent node  11  sends the packet whose destination address is a global address A which is a sender address of the received packet as a response to the packet from the router  14  (process  802 ). The packet is received by the router  14  (process  802 ) and then is sent from the router  14  to the correspondent node  11  (process  803 ). In this way, communication is performed between the mobile node  10  in the home network  20  and the correspondent node  11  on the IP network. 
   During communication as above, the mobile node  10  is moved from the home network  20  to the foreign network  21  (process  804 ). The mobile node  10  receives an agent advertisement so that it detects that it has moved to the foreign network  21  (process  805 ). The mobile node  10  having detected the movement requests a registration to the FA  17  of the router  13  by using the format shown in  FIG. 4  (process  806 ). FA  17  receives the registration request and further performs a similar registration request to the HA  16  (process  807 ). HA  16  returns a registration response to the registration request by using the format shown in  FIG. 5  (process  808 ). According to the home address of the mobile node  10  included in the response, the FA  17  can recognize that a global address is assigned to the mobile node  10 . That is, it recognizes that address translation is not performed by a NAT in the home network  20  and address translation is not necessary for the packet from the mobile node  10 . 
   The FA  17  of the router  13  which has recognized that address translation is not necessary for the home address A of the mobile node  10  registers it in an address non-translated list (process  809 ). The address non-translated list is a list for registering IP addresses on which the NAT  19  of the router  13  perform address translation.  FIG. 20  is an explanatory diagram for exemplifying the address non-translated list. Here, an address A is registered, and the NAT  19  does not perform NAT translation on the address A registered in the list. After that, a registration response is returned from the FA  17  to the mobile node  10  in order to complete the registration (process  810 ). 
   Next, it will be described a case where data is sent from the mobile node  10  in the foreign network  21  to the correspondent node  11 . In a process  811 , the mobile node  10  in the foreign network  21  sends a packet to the router  13  in order to send data to the correspondent node  11  (process  811 ). A destination address of the packet is a global address B while its sender address is a global address A. The NAT  18  of the router  13  having received the packet from the mobile node  10  does not perform address translation on the global address A based on the address non-translated list shown in  FIG. 20  (process  812 ). The non-translated packet is sent onto the IP network  22  and received by the correspondent node  11  (process  813 ). 
   The correspondent node  11  sets the address A which is a sender address of the earlier received packet as its destination address and transmits the packet in order to transmit the packet to the mobile node  10  ( 814 ). The HA  16  of the router  14  having received the packet from the correspondent node  11  sets address D of the router  13  and address C of the router  14  in the destination address and the sender address, respectively, and performs IP-encapsulation thereon (process  815 ). The IP encapsulated packet is tunneled to the FA  17  of the router  13  (process  816 ). The router  13  of the FA  17  decapsulates the tunneled packet while does not perform address translation through the NAT  19  based on the address non-translated list (process  817 ). Then, the decapsulated packet is sent to the mobile node  10  (process  818 ). In this way, communication becomes possible between the mobile node  10  in the foreign network  21  and the correspondent node  11  on the IP network. 
   The global addresses which are assigned to the router  12 , HA  16 , and NAT  18  in  FIG. 6  has been described above as a single address C. However, different global addresses may be assigned, respectively. Similarly, different global addresses may be assigned to the router  13 , the FA  17 , and the NAT  19 , respectively. 
   A method such as for example the above is stored on computer readable medium such as HARD DISK, FLOPPY DISK, CDROM, DVDROM, MEMORY.