PATENT ABSTRACT
Mobile terminals have the problem of large packet loss during movement because the mobile terminal has no opportunity to detect movement and generate a new care-of address until a new router advertisement is received, and also cannot register the position in the home agent. The present invention is characterized in including a means to automatically collect network information on access router devices in the vicinity of a particular access router device, and a means to send information in the router advertisement including information on at least one or more adjacent access router devices. The mobile terminal selects an access router device to become the next movement destination, and the most essential characteristic is a means to register the position information beforehand in the home agent.

PATENT DESCRIPTION
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese application JP 2004-263190 filed on Sep. 10, 2004, the content of which is hereby incorporated by reference into this application.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method for preventing packet loss that accompanies movement of the mobile terminal during mobile unit communications. The invention relates in particular to a mobile detection method for use when a mobile terminal is utilizing a Mobile IP protocol for mobile communications, and a router advertisement distribution method for access routers utilized to implement that mobile detection method.  
       BACKGROUND OF THE INVENTION  
       [0003]     In recent years, many intensive studies have been made on IP (Internet Protocol) for mobile communication networks.  
         [0004]     The IETF (Internet Engineering Task Force) is proceeding with efforts to standardize Mobile IPv6 specifications. The essential elements comprising a Mobile IPv6 network are a mobile node (hereafter expressed as MN), a home agent (hereafter expressed as HA), a correspondent node (hereafter expressed as CN), and an access router (hereafter expressed as AR).  
         [0005]     The basic operation of Mobile IPv6 is described next. An IP address (home address, hereafter expressed as HoA) that does not change even if the MN has moved, is assigned to the MN 1 . Therefore, an application that starts up on the MN can continue running without stopping even if the MN is moving. A network possessing a network prefix identical to the HoA is called a home network. When the MN moves to a network (host network) other than the home network, it acquires an IP address that conforms to the communication protocol within that host network. This IP address is called the Care of Address (hereafter expressed as CoA).  
         [0006]     The MN receives a router advertisement (RA) sent periodically from the AR located in the host network. If the network prefix detected at this time that is different from the HoA, then this signifies movement has been detected and a CoA is then generated. The MN that detected movement sends to the HA, a position registration message (Binding Update: BU) requesting the relay of a packet for an MN destination sent to the home network. The HA that received the binding update creates a binding cache to link the MN home address with the care of address (CoA). The HA afterwards sends a position registration reply message (Binding Ack: BA) to the MN, and broadcasts a packet acquisition message (Gratuitous Neighbor Advertisement: G-NA) as a proxy for receiving the packet addressed to the MN moving in the host network. The CN is the communication correspondent node (other communication party) for the MN. The CN sends a packet addressed to the HoA of the MN. The HA receives as a proxy, the HoA addressed packet for the MN. The HA searches the Binding Cache and acquires a CoA corresponding to the HoA for the MN. The HA attaches (encapsulates) an IP header for the CoA that matches the original received packet and sends that packet. The MN removes (decapsulates) the encapsulated header of the CoA. The CN can then receive the packet sent addressed to the HoA of the MN, which is the original packet.  
         [0007]     However, in the technology of the related art the MN cannot identify movement of the mobile terminal until a router advertisement is received from the AR installed within the host network at the movement destination. The packet from the CN cannot be received during this time, because a new CoA cannot be generated and position registration messages (binding updates) cannot be exchanged due to failure to identify movement of the mobile terminal. 
    [Non-patent document  1 ] D. Johnson et. al. “Mobility Support in IPv6”, IETF  2003 ,     http://www.ietf.org/internet-drafts/draft-ietf-MobileIP-IPv 6-24.txt    
 
       SUMMARY OF THE INVENTION  
       [0010]     Problems that occur during handovers in the technology of the related art are described next. As shown in  FIG. 1 , the time  10  required for completing a handover in Mobile IPv6 is broadly grouped into a CoA generation time  12  for generating a CoA after receiving an RA with the wireless link setup  11 ; and a binding update time  13  up to completion of the binding update (position registration). Shortening this CoA generation time  12  and the binding update time  13  is an effective way to make the handover more efficient.  
         [0011]     In the technology of the related art however, the access router advertisement sent by the access router in the host network only contains network information for the access router itself. The mobile terminal therefore has no opportunity to detect movement and generate a new CoA until the mobile terminal receives a new router advertisement from the movement destination. Also a binding update (position registration) of the home agent cannot be performed and shortening the handover time becomes difficult. The technology of the related art has the further problem of large packet loss accompanying movement of the mobile terminal. Resolving these problems requires that the mobile terminal be capable of acquiring network information about the next access router it will connect to after the switching from currently connected access router, and complete the binding update (position registration) before moving.  
         [0012]     The present invention is characterized in including a means to automatically collect network information such as the network prefix for neighboring (or nearby) access routers near the access router device, and a means to send information in the router advertisement message including information on at least one or more adjacent access router devices. In the most essential characteristic of the present invention, the mobile terminal selects an access router device to become the next movement destination, and registers the position information (makes a binding update) beforehand in the home agent.  
         [0013]     The mobile terminal of the present invention is capable of acquiring network information on the movement destination prior to movement. The invention therefore has the merit that packet loss after movement is prevented since the binding update (position registration) can be made beforehand. The invention has the further merit that management such as of settings that accompanies adding or removing access routers by the administrator can be simplified since the access router automatically collects information on nearby access routers.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a drawing for showing details of the time required for implementing the IETF standard MobileIPv6 handover in the technology of the related art;  
         [0015]      FIG. 2 -A is a diagram showing the network structure for the present invention (first embodiment);  
         [0016]      FIG. 2B  is a diagram showing the network structure for the present invention (second embodiment);  
         [0017]      FIG. 3 -A is a drawing showing the structure of the software for the access router device;  
         [0018]      FIG. 3 -B is a drawing showing the structure of the software for the access router device;  
         [0019]      FIG. 4 -A is a block diagram showing the structure of the hardware of the mobile terminal;  
         [0020]      FIG. 4 -B is a block diagram showing the structure of the software of the mobile terminal;  
         [0021]      FIG. 5 -A is a block diagram showing the structure of the hardware of the home agent and the correspondent terminal (other party terminal);  
         [0022]      FIG. 5 -B is a block diagram showing the structure of the software of the home agent and the correspondent terminal (other party terminal);  
         [0023]      FIG. 6  is a diagram showing the sequence in the handover method of the present invention;  
         [0024]      FIG. 7  is a diagram showing the RIPng message format;  
         [0025]      FIG. 8  is a diagram showing the message format of the IPv6 router advertisement;  
         [0026]      FIG. 9  is a diagram showing the message format requesting MobileIPv6 position registration (binding update);  
         [0027]      FIG. 10 -A is a drawing showing the structure of the router information management table;  
         [0028]      FIG. 10 -B is a drawing showing the structure of the terminal information management table;  
         [0029]      FIG. 11 -A is a drawing showing the structure of the Binding Update management table;  
         [0030]      FIG. 11 -B is a drawing showing the structure of the Binding Cache management table;  
         [0031]      FIG. 12  is a flow chart showing the process flow during sending of the RIPng message by the access router;  
         [0032]      FIG. 13  is a flow chart showing the process flow during receiving of the RIPng message by the access router;  
         [0033]      FIG. 14  is a flow chart showing the process flow during sending of the router advertisement by the access router;  
         [0034]      FIG. 15  is a flow chart showing the process flow during receiving of the router advertisement by the mobile terminal;  
         [0035]      FIG. 16  is a flow chart showing the process flow for movement detection after receiving of the router advertisement by the mobile terminal;  
         [0036]      FIG. 17  is a flow chart showing the process flow during receiving of the binding update message by the home agent and the correspondent terminal (other party terminal);  
         [0037]      FIG. 18  is a flow chart showing the process flow during relaying of the packet by the home agent;  
         [0038]      FIG. 19  is a flow chart showing the process flow during receiving of the encapsulated packet by the mobile terminal;  
         [0039]      FIG. 20  is a flow chart showing the process flow for sending the packet after route optimization by the correspondent terminal (other party terminal); and  
         [0040]      FIG. 21  is a flow chart showing the process flow for sending the packet by the mobile terminal. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     The embodiment of the present invention is described next while referring to the accompanying drawings. The present invention is represented by two embodiments. The difference in the embodiments lies in the network connection method used by the mobile terminal. These respective connection methods are shown in  FIG. 2 -A and  FIG. 2B .  
       First Embodiment  
       [0042]     Networks connecting to each device are all comprised of IP networks as shown in the network structure of the first embodiment in  FIG. 2 -A. An HA (home agent)  3  for relaying the packet addressed to MN 1 , and a CN 4  as the correspondent node (other communication party) for MN 1  are connected to the home network  5   a  of MN 1 . A packet relay network  5   b  is installed between the home network  5   a  and the wireless networks (host network  5   c ,  5   d ,  5   e ) connected to movement destination MN 1 . A router  6  is installed at the boundary of the home network  5   a  and the packet relay network  5   b . The access routers ( 2 -A,  2   b ,  2   c ) are installed at the boundary of the host networks ( 5   c ,  5   d ,  5   e ) connected to the movement destination MN 1  and the packet relay network  5   b . The MN 1  is IP-connected to the host network via the access points (hereafter called AP) connected to the AR ( 2 - a ,  2   b ,  2   c ).  
         [0043]     In  FIG. 2 -B showing the network structure of the second embodiment, the devices connected to each network are the same as in the first embodiment. In the second embodiment, the MN 1  is connected by PPP (Point-to-Point Protocol) to the AR ( 2   a ,  2   b ,  2   c ) from the host networks ( 5   f ,  5   g ,  5   h ) connected to the movement destination, and the IP connection is then made.  
         [0044]     Sections not relating to the embodiments of the present invention are described next.  
         [0000]      FIG. 3 -A is a drawing showing the structure of the hardware for the AR 2  in the present invention. The AR 2  is made up of at least six hardware blocks. Each of these hardware blocks is described next.  
         [0000]    
       
         
           
              At least two or more IP interfaces ( 206   a ,  206   b ) for sending and receiving the IP packets.  
              A switch  205  for the IP interfaces  
              A hard disk  200  for storing information such as file settings that determine the operation of devices and software of the present invention  
              A memory  203  as a region temporarily utilized when executing the software of the present invention  
              A CPU  201  for managing the control of this device  
              An expansion interface  207  utilized when connecting the peripheral equipment to this device.  
           
         
       
     
         [0051]     All the above hardware blocks are connected by the bus  204 . A GPS receiver  208  is connected to the expansion interface  207  of the present invention for the purpose of collecting the physical position information of the AR 2 . This GPS receiver may be installed internally in the AR 2 . Also the physical position information may be collected by a means other than a GPS receiver.  
         [0052]      FIG. 3 -B is a drawing showing the software structure of the AR 2  of the present invention. When an IP packet arrives at AR 2 , the packet I/O processing  219  extracts the received IP packet. The packet I/O processing  219  verifies the destination address of the IP packet and if addressed to itself (AR 2 ), sorts the packet in the packet assignment processing  216 . If not addressed to itself (the AR 2 ), the packet is relayed by the packet relay processing  217 . The packet assignment processing  216  verifies the data section of the IP packet, selects the next processing, and performs the appropriate processing. The packet relay processing  217  searches the routing table  218 , determines the next destination to relay the IP packet to, and relays the IP packet via the packet I/O processing  219 .  
         [0053]     The L 2  (layer  2 ) connection processing  211  is required in the second embodiment. The L 2  connection processing  211  mainly controls the PPP termination processing and connection management. The route information processing  212  is used during replacement of route information utilized in the routing protocol typically shown for RIPng in the example in  FIG. 7 . The router advertisement processing  213  controls the distribution of the router advertisement message sent from the AR 2  as shown in the example in  FIG. 8 . The position information processing  214  controls the collection and management of the physical position information made up of the AR 2  longitude, latitude and altitude. The present invention is characterized by the route information processing  212  and router advertisement processing  213  and the position information management processing  215 . The information acquired in each of the different processing is stored in the router information processing table  214  as shown in the example in  FIG. 10 -A. The AR 2  establishes the contents of the processing for the AR 2  to perform and operation with information stored in the IP packet by searching the router information management table  215 .  
         [0054]      FIG. 4 -A is a block diagram showing the hardware structure of the MN 1  that is the mobile terminal of the present invention. Each of these hardware blocks is described next. 
        At least one or more IP interfaces  106  for sending and receiving the IP packets.     At least one or more wireless IP interfaces  107  for sending and receiving the IP packets.     A switch  205  for the IP interfaces     A hard disk  100  for storing information such as file settings that determine the operation of devices and software of the present invention     A memory  103  as a region temporarily utilized when executing the software of the present invention     A CPU  101  for managing the control of this device     An expansion interface  208  utilized when connecting the peripheral equipment to this device.          
         [0062]     All the above hardware blocks are connected by the bus  104 . A GPS receiver  109  is connected to the expansion interface  108  of the present invention for the purpose of collecting the physical position information of the MN 1 . This GPS receiver may be installed internally in the MN 1 . Also the physical position information may be collected by a means other than a GPS receiver. A structure not utilizing the hard disk  100  may be contrived by storing the information in the memory  103  instead of the hard disk  100 .  
         [0063]      FIG. 4 -B is a block diagram showing the structure of the software of the MN- 1  (mobile terminal) of the present invention. The Packet I/O processing  121  extracts IP packet received at the MN 1 . The packet I/O processing  121  verifies the destination address of the IP packet and if addressed to itself (MN 1 ), sorts the packet in the packet assignment processing  118 . If not addressed to itself (the MN 1 ), the packet is relayed by the packet relay processing  119 . The packet assignment processing  118  verifies the data section of the IP packet, selects the next processing, and performs the appropriate processing. The packet relay processing  119  searches the routing table  120 , determines the next destination to relay the IP packet, and relays the IP packet via the packet I/O processing  121 . The L 2  (layer  2 ) connection management processing  111  is required in the second embodiment for making PPP connections to the AR 2 . The position information processing  112  controls the collection and management of the physical position information made up of the MN 1  longitude, latitude and altitude. The acquired information is stored in the router information management table  116  shown in the example in  FIG. 10 -A and the terminal information management table  115  shown in the example in  FIG. 10B . The router advertisement processing  113  controls the movement detection control of the MN 1  and receiving the router advertisement message sent from AR 2  and shown in the example in  FIG. 8 . The acquired information is stored in the router information management table  116  and the terminal information management table  115 . The Mobile IP processing  114  controls the relay of the IP encapsulated packet and the position registration processing (binding update) using the Mobile IP binding update (request) message shown in the example in  FIG. 9 . The Mobile IP processing  114  also manages the information such as the CoA and HoA relating to the Mobile IP, in the binding update management table in the example shown in  FIG. 11 -A.  
         [0064]      FIG. 5 -A is a block diagram showing the hardware structure of the HA 3  and the CN 4  in the present invention.  
         [0065]     The HA 3  and the CN 4  are comprised of at least six hardware blocks. Each of these hardware blocks is described next. 
        At least two or more IP interfaces ( 306   a ,  306   b ) for sending and receiving the IP packets.     A switch  305  for the IP interfaces     A hard disk  300  for storing information such as file settings that determine the operation of devices and software of the present invention     A memory  303  as a region temporarily utilized when executing the software of the present invention     A CPU  301  for managing the control of this device     An expansion interface  308  utilized when connecting the peripheral equipment to this device.        
 
         [0072]     All the above hardware blocks are connected by the bus  304 . A GPS receiver  309  is connected to the expansion interface  208  of the present invention for the purpose of collecting the physical position information of the HA 3  and the CN 4 . This GPS receiver may be installed internally in the HA 3  and the CN 4 . The physical position information may also be collected by a means other than a GPS receiver.  
         [0073]      FIG. 5 -B is a block diagram showing the structure of the software of the HA 3  and the CN 4  of the present invention. When an IP packet arrives at the HA 3  and the CN 4 , the packet I/O processing  317  extracts the received IP packet. The packet I/O processing  317  verifies the destination address of the IP packet and if addressed to itself (HA 3  or CN 4 ), sorts the packet in the packet assignment processing  314 . If not addressed to itself (HA 3  or CN 4 ), the packet is relayed by the packet relay processing  315 . The packet assignment processing  314  verifies the data section of the IP packet, selects the next processing, and performs the appropriate processing. The packet relay processing  315  searches the routing table  316 , determines the next destination to relay the IP packet to, and relays the IP packet via the packet I/O processing  317 . The Mobile IP processing  312  controls the relay of the IP encapsulated packet and the receive processing for position registration (binding update) using the binding update (request) message shown in the example in  FIG. 9 . The Mobile IP processing  312  also manages the information such as the CoA and HoA relating to the Mobile IP, in the binding cache management table in the example shown in  FIG. 11 -B.  
         [0074]     The operation sequence for mobile communication in the present invention is described next. In a typical example,  FIG. 6  shows the operating sequence for the MN 1  to communicate with the CN 4  installed in the home network  5   a , while the MN 1  is moving from the host network  5   d  administered by AR 2  ( 2   b ), to the host network  5   e  administered by AR- 3  ( 2   c ) as shown in  FIG. 2 -A.  
         [0075]     The MN 1  and the AR ( 2   a ,  2   b ,  2   c ) in the figure include a means to hold physical position information and their own route information.  
         [0076]     The router information management table ( 116 ,  215 ) for storing route information as shown in the example in  FIG. 10 -A is described next. The router information management table ( 116 ,  215 ) stores the network prefix  80 , the prefix length  802 , and the gateway address  803 . When sending or relaying IP packets, this information is utilized to determine to what gateway to send the IP packet containing the destination address. The proximity flag  804  is a flag for deciding whether the network prefix  801  is for a network the AR 2  or the MN 1  themselves belong to. A proximity flag showing “0” indicates network information the AR 2  or the MN 1  themselves belong to. A proximity flag showing “1” indicates network information obtained by manual settings or a routine protocol. The AR flag  805  is a flag for deciding whether the network for network prefix  801  can be reported as an AR to the MN 1 .  
         [0077]     An AR flag showing “0” indicates that the network prefix  801  is network information belonging to the relay network  5   b  or the home network  5   a . An AR flag showing “1” indicates network information belonging to an MN 1  host network ( 5   c ,  5   d ,  5   e ). The longitude  806 , latitude  807 , and altitude  808  indicate physical position information on the AR 2  associated with the network prefix  801 . The hop count  809  is a numerical value showing via how many routers the network prefix  801  can arrive. The reference flag  810  is used to prevent redundant searches when determining the control operation while referring to the router information management table ( 116 ,  215 ). The information from  801  through  810  stored on a single record  800  and multiple records are stored on the router information management table.  
         [0078]     The terminal information management table  115  for storing physical position information is described next utilizing the example in  FIG. 10 -B. The terminal information management table  115  stores: the home address  821  of MN 1 , the list of currently used CoA list  822 , a network prefix  823  for the AR 2  currently connected with the host network, and the longitude  824 , latitude  825  and altitude  826  as current physical position information for the MN 1 , all in a single record  820 . The CoA list  822  is made up of a CoA priority sequence in a list structure for use when sending the IP packet.  
         [0079]     The description of the operating sequence for mobile communication now continues while referring back to  FIG. 6 . The MN 1  in the figure, first of all connects to the AR- 1  ( 2   a ). When connecting in the second embodiment, the MN 1  makes the PPP session setup (S 20 ) after the wireless link setup. In the case of the first embodiment, the MN 1  makes a direct IP connection to the AR- 1  ( 2   a ).  
         [0080]     The AR ( 2   a ,  2   b ,  2   c ) exchanges route (or path) information held respectively in the AR 2  using a routing protocol represented here by RIPng (S 30 ). In the present invention, route information is sent (F 4 ) in the RIPngmessage format shown in the example in  FIG. 7 . In the RIPng message format of  FIG. 7 , the AR location information  516  (physical position information area) for the AR 2  is added within the route table entry  515  in the format specified for “RIPng for IPv6” in IETF RFC2080. This AR location information  516  contains a longitude  506 , latitude  507 , altitude  508 , and an AR flag  510 , and a hop count  511  for the AR 2  associated with the IPv6 prefix  502 .  
         [0081]     The RIPng message format transmit processing (F 4 ) executed by the AR 2  ( 2   b ,  2   c ) is described next utilizing the examples in  FIG. 7  and  FIG. 12 . The AR 2  first of all generates ( 600 ) an RIPng message format as shown in  FIG. 7 . The AR 2  next searches the router information management table  215  and extracts one record at a time ( 601 ). After extracting the record ( 602 ), and adding it to the route table entry  515  ( 603 ), the AR 2  extracts information such as the network prefix  801 , the longitude  806 , latitude  807 , and the AR flag  805  from within the record ( 604 ). The AR 2  next writes ( 605 ) on the applicable area within the route table entry  515 . One record at a time is then repeatedly extracted from the router information management table  215  again so that the route information held in the AR 2  ( 2   b ,  2   c ) is utilized in the RIPng message format. The RIPng message is completed at the point in time that none of the records extracted from the router information management table  215  still remain. The AR 2  ( 2   b ,  2   c ) then sends ( 606 ) the generated RIPng message to the nearby AR 2  ( 2   a ).  
         [0082]     Returning now to  FIG. 6 , the process for receiving the RIPng message (S 30 ) that the AR 2  ( 2   b ,  2   c ) sent in the AR 2  ( 2   a ) is described while referring to the examples in  FIG. 7  and  FIG. 13 . First of all, in  FIG. 7  after receiving the RIPng message ( 610 ), the AR 2  confirms that there is an applicable route table entry  515  ( 611 ). When there is a route table entry  515 , the AR 2  extracts each items such as the IPv6 prefix  502  for the applicable entry ( 613 ). The AR 2  next searches the router information management table  215  ( 614 ) and confirms whether there is a record matching the extracted items ( 615 ). If there is a matching record, then a check is made for the next route table entry  515 , after rewriting the router information management table  215 . If there is no matching record, new information is written onto the router information management table  215  ( 617 ), and information is simultaneously written ( 618 ) onto the routing table  218  in the packet relay processing  217  shown in  FIG. 3 -B as route information required during relay of the packet. The route information held by the nearby AR 2  ( 2   b ,  2   c ) can from hereon be utilized in the AR 2  ( 2   a ) router information management table  215 , by extracting all of the route table entries  515  contained in the RIPng message.  
         [0083]     The AR 2  ( 2   a ,  2   b ,  2   c ) automatically collects physical position information and route information held in the respective routers using the routing protocol as described above.  
         [0084]     The description of the operating sequence for mobile communication now continues while referring back to  FIG. 6 . Usually, the AR- 1  ( 2   a ) sends a router advertisement (S 31 ) from the network interface connected to the host networks ( 5   c ,  5   d ,  5   e ) to which the AR 2  itself is connected, to the host networks ( 5   c ,  5   d ,  5   e ) to which the MN 1  is connected. The MN 1  that moved to the host networks ( 5   c ,  5   d ,  5   e ) can in this way be detected and a CoA valid in the host networks ( 5   c ,  5   d ,  5   e ) can be generated. The present invention is characterized in that the router AR 2  ( 2   a ) sends a router advertisement as shown in  FIG. 8 . The router advertisement message shown in  FIG. 8  is added to the AR physical position information  540  of AR 2  in the prefix information option  530  area of the format specified in IETF “Mobility Support in IPv6”.  
         [0085]     An N bit  531  is added to the prefix  532  contained in the prefix information option  530  area to indicate network information associated with the AR 2  itself, or nearby network information. The AR position information  540  contains information on longitude  541 , latitude  542 , and altitude  543  as physical position information on the AR 2  connected to the prefix  532 . Multiple prefix information option  530  areas can be stored within the router advertisement message. However, in the present invention there is a limit on the number of prefix information option  530  areas due to the need to store router advertisement messages within the maximum length range of the IP packet. Methods to set this limit include a method to keep adding areas as long as the maximum length of the IP packet is not reached, and a method to set limits on static settings made by the service personnel. In view of the need for efficiency during the MN 1  handover, the AR 2  ( 2   a ) preferably sends at least three items of network information on the nearest AR 2  ( 2   b ,  2   c ) and the network to which the AR 2  ( 2   a ) itself belongs.  
         [0086]     The process (F 6 ) for the AR- 1  ( 2   a ) to send the router advertisement message is described next while referring to  FIG. 14 . The AR- 1  first of all generates the router advertisement message format shown in  FIG. 8  ( 620 ). Next, The AR- 1  searches the router information management table  215  ( 621 ) shown in  FIG. 10 -A, and after extracting a record with a proximity flag  804  of “0”, utilizes each of the prefix information option  530  areas ( 622 ). The AR 2  sets the N bit  531  to “0” to clearly show network information associated with the AR 2  ( 2   a ) itself. Next, the AR- 2  stores network information for the nearby AR 2  ( 2   b ,  2   c ) in the router information management table  215 , into the prefix information option  530  areas. This operation extracts one record at a time from the router information management table  215  up to the most recent record ( 623 ). When there is a (matching) record, the AR- 2  checks that the proximity flag  804  is a “1” and also that the AR flag  805  is a “1” ( 626 ).  
         [0087]     When there is no (matching) record matching this condition, then a decision is made that there is no valid network information on the MN 1  and the process shifts to extracting the next record. If there is a record matching the condition, then the longitude  806 , latitude  807 , altitude  808 , are extracted from the record in the router information management table  215  and are compared with the longitude  541 , latitude  542 , altitude  543  already stored in the prefix information option  530  ( 627 ). If farther away than any of the prefix information option  530  information, then the prefix information option  530  storage limit is checked ( 629 ) and if over the storage limit then the process shifts to extracting the next record. If the information can be stored then a new prefix information option  530  is written. However if determined to be nearer than the already stored position information ( 628 ), then the prefix information option  530  storage limit is checked ( 631 ). If the information can be stored then a new prefix information option  530  is written ( 633 ). If at the limit, then a substitution is made with the farthest record and it is written ( 632 ). The AR 2  ( 2   a ) can therefore send an optimal router advertisement message by repeating the above process. After checking all the router information management table  215  records, the AR then sends the router advertisement message that was created ( 624 )  
         [0088]     As a result of the above process, the AR 2  can select from within its own router advertisement, network information on the host network where the nearest AR is connected, and send it to the MN 1 .  
         [0089]     Returning now to  FIG. 6 , the process for receiving (F 7 ) the router advertisement message (S 31 ) that the AR 2  ( 2   a ) sent to the MN 1  is described while referring to the examples in  FIG. 15  and  FIG. 16 . The MN 1  first of all receives ( 640 ) the router advertisement shown in  FIG. 8 . The MN 1  next checks whether there is a prefix information option  530  within the received router advertisement message ( 641 ). If present, then the information contained within the prefix information option  530  is extracted ( 642 ). A search is next made of the router information management table  116  and a comparison made with the extracted information ( 643 ). A decision is made whether the N bit  531  within the prefix information option  530  is a “0” ( 644 ). If the N bit  531  is a “1” at this time then the network information stored in that prefix information option  530  can be identified as information on the host network  5   d  to where the MN 1  has currently moved. If the N bit  531  is a “0” then a search is made for a record where the proximity flag  804  “0” within the router information management table  116 . Next a decision is made whether the prefix length  802  and the network prefix  801  of the applicable record, match the prefix length and network prefix  532  extracted from within the prefix information option  530  ( 645 ).  
         [0090]     If these (record info and option  530  info) are a match, then there is no change from the router advertisement that was received the previous time and so the AR determines that the MN 1  has not moved. Then a “1” is stored in the reference flag  810  for a record in the router information management table  116  ( 651 ) to record that the table was searched, and the prefix option  530  then extracted. If the network information does not match, then the MN 1  is identified as having moved to a host network different from the previous time. Therefore, after storing the detected movement in the flag ( 646 ), the “0” recorded in the proximity flag  804  is changed to a “1” ( 647 ).  
         [0091]     Next, a decision is made whether the prefix length in records where the proximity flag  804  router information management table  116  is a “1” is a match with (the prefix length of) the network prefix  532  extracted from within the prefix information option  530  ( 649 ). If there is a matching record then the proximity flag  804  for that applicable record is changed to a “0” ( 648 ). When there is no matching record, a new entry is made to the router information management table  116  ( 650 ). A “1” is then recorded in the reference flag  810  ( 651 ) that records an addition or changing of the proximity flag  804  to “0”, to show that it was referred to. The prefix information option  530  is then extracted.  
         [0092]     However, when decided whether or not the N bit  531  in the prefix information option  530  is a “0” ( 644 ), when the N bit  531  is “1”, the network information stored within the applicable prefix information option  530  area allows specifying network information for the host network  5   c  or the host network  5   e  which are the next movement destination candidates. In this case, a decision is made whether or not the prefix length in records where the proximity flag  804  within the router information management table  116  is “1”, matches the prefix length of the network prefix  532  extracted from within the prefix information option  530  ( 649 ). When there is a matching record, the applicable record is updated to the most recent information ( 648 ). When there is no matching record, a new addition is made to the router information management table  116  ( 650 ). A “1” is later stored in the reference flag  810  (for additions or updated records) to record the fact that a search was made. The prefix information option  530  is then extracted.  
         [0093]     When decided that there is no prefix information option  530  area that can be extracted ( 641 ), a check is made of whether the movement detection flag is standing ( 652 ). When no movement was detected, a search is made of records for reference flag  810  that is “0” within the router information management table  116  ( 653 ). The applicable record that was not information within the router advertisement and so is deleted from the router information management table  116  ( 654 ). Also there is no longer any need to use a CoA matching the network prefix  801  whose record was deleted, so the CoA is deleted from the network prefixes ( 106 ,  107 ). The CoA is at the same time also deleted from the CoA list used within the terminal information management table  115  shown in  FIG. 10B  ( 656 ).  
         [0094]     The movement detection processing for MN 1  shown in  FIG. 16  is implemented ( 657 ) when decided that movement was detected while checking if the movement detection flag is standing ( 652 ). This process for detecting MN 1  movement is processing for generating a CoA and making a binding update (position registration) onto the HA 3  installed in the host network  5 , and onto the CN 4  during route optimizing. The processing is next described in more specific detail. A search is first of all made once again of the record within the router information management table  116  ( 660 ). One record at a time is extracted ( 662 ), and a CoA is generated ( 663 ) based on the prefix length  802  and the network prefix of the applicable record. A check is next made ( 664 ) on whether the generated CoA is already set within the MN 1  network interface ( 106 ,  107 ). If not yet set, then the generated CoA is set into the network interfaces ( 106 ,  107 ) ( 665 ).  
         [0095]     A check is next made within the extracted record for a proximity flag  804  of “1”. When the proximity flag  804  is “0”, then the applicable record can be identified as information on the host network  5   d  where the current MN 1  has moved. The CoA that was generated from the applicable record information is used at highest priority. Therefore, the information generated or extracted from the applicable record, is utilized in the network prefix  823  of the connected AR and the priority  1  ( 827   a ) of the CoA list  822  used in the terminal information management table  115  shown in  FIG. 10 -B ( 667 ,  668 ). On the other hand, when the proximity flag  804  in the extracted record is a “1”, the CoA generated from the applicable record is a valid IP address in the host network  5   c  or the host network  5   e  that are the next movement destination candidates, and its level of priority is determined and stored in the CoA list  822  used in the terminal information management table  115 . In this processing, the MN 1  physical position information is extracted from the longitude  824 , latitude  825 , and altitude  826  of the terminal information management table  115  ( 669 ).  
         [0096]     The physical position information of an AR 2  capable of using an already registered CoA is next compared with the extracted physical position information ( 670 ). The information is sorted in order of proximity to the MN 1  physical position information and stored in the generated CoA ( 671 ).  
         [0097]     After all records are extracted from within the router information management table  116 , the CoA generated and the information stored, the MN 1  sends a binding update message (position registration request message) to the HA 3  and the CN 4  within the optimized route ( 673 ). This processing is performed by sending a packet attached with a Mobile IPv6 Binding Update message shown in the example in  FIG. 9  immediately after the IPv6 packet format of the IPv6 header. This patent is characterized in that an area is newly added area for storing information relating to the next period CoA, in the mobility options  560 , in the Mobile IPv6 binding message update format as specified in IETF “Mobility Support in IPv6”. During generation of the binding message, the CoA ( 827   a ) with the highest priority level is stored as the currently used CoA in the transmit source address of the IPv6 header. The IP address of the HA 3  or the CN 4  is the destination address.  
         [0098]     The IPv6 destination header  550  of  FIG. 9  is stored in the expansion header area of the IPv6 packet format. The home address  821  of the MN 1  shown in  FIG. 10B  is stored in the home address option area within this header. An MH type  556  showing the binding update message request and a Lifetime  557  as the effective period for the reported CoA are stored in the IPv6 Mobility header area  555 . The CoA ( 827   b ) at the second priority level from the CoA list in the terminal information management table  115  is stored as the next period CoA in the Mobility Options  560  area. The S bit  562  within the Mobility Options  560  area is used for requesting relay of the IP capsule for both the current CoA and the next period CoA, when the HA 3  or the CN 4  are sending an IP packet addressed to the MN 1 . The MN 1  decides if itself is moving or not ( 672 ), when decided to be moving, the MN 1  sets the S bit  562  to “1”. When communicating while stopped in a host network the MN 1  sets the S bit  562  to “0” to prevent IP packet copies from being sent from the HA 3  or the CN 4 .  
         [0099]     The MN 1  as described above, searches the binding update table  117  shown in the example in  FIG. 11 -A ( 673 ), and after generating the binding update request message and sending it to the HA 3  of binding update destination which is the HA 3  or the CN 4  during optimizing of the route ( 674 ), store the state of the binding update (position registration) in the binding update table  117  ( 675 ). The IP address  831  as the transmit destination for the binding update and the home address  832  of the MN 1  stored in the binding update (request) message, the current CoA 833 , the next period CoA 835  and effective period  835  of the CoA and the control flag  836  containing the S-bit are stored in the binding update table  117 .  
         [0100]     After rewriting the binding update table  117 , the MN 1  searches for a record with a reference flag  810  of “0” within the router information management table  116  ( 676 ). That applicable record is deleted from the router information management table  116  ( 677 ) because the applicable record is information that is not present within router advertisement message. There is also no need to use the CoA matching the network prefix  801  of the deleted record so it is deleted from the network interface ( 106 ,  107 ), and is also deleted at the same time from the CoA (CoA in use) list within the terminal information management table  115  shown in  FIG. 10 -B ( 678 ).  
         [0101]     As a result of the above processing, the MN 1  detects movement by receiving the router advertisement (S 31 ) sent from the AR 2 , generates a CoA for use in the host network the MN 1  moves to in the next period, and can send a binding update (position registration request) message containing CoA for both the HA 3  and the CN 4  (S 32 ). The MN 1  can in this way continue to communicate without packet loss even during repeated movement.  
         [0102]     Returning now to  FIG. 6 , the process for receiving (F 8 ) the binding update message (S 31 ) that the MN 1  sent to the HA 3  and the CN 4  carrying out route optimization is described while referring to the example in  FIG. 17 . The HA 3  or the CN 4  receive the binding update message shown in  FIG. 9  ( 680 ). After receiving the message, the HA 3  or the CN 4  extract the CoA currently used by the MN 1 , from the transmit source address of the IPv6 header area ( 681 ). The HoA of the MN 1  is next extracted from the IPv6 destination header  550  area shown in  FIG. 9  ( 682 ). After extracting other information such as the lifetime  557  ( 683 ), a check is made on whether there is a next period CoA option in the Mobility Option  560  ( 684 ). If there is a next period CoA option, then the next period CoA  563  and the S bit  562  comprising the control flag are extracted ( 685 ). The next extracted information is stored in the binding cache table  313  shown in  FIG. 11 -B ( 686 ). The present invention is characterized in that the next period CoA of the MN 1  can be registered (binding entry) in the binding cache table  313 . During storage, a check is first of all made of the binding cache table  313  for a record matching the extracted HoA ( 687 ). If there is a matching record, then the current CoA 842  and next period CoA 843  and lifetime  844  of the applicable record are rewritten ( 688 ). If there is no matching record, then a new entry is made in the binding cache management table  313  ( 689 ) and a binding update (ACK) reply message is sent to the MN 1  (S 33 ).  
         [0103]     The HA 3  that accepted the binding update (request) receives as a proxy, the packet addressed to the MN 1 . In order to relay this packet to the MN 1 , the HA 3  sends a false NA message to the home network  5   a  to change the HoA address for the MAC address of the MN 1 , to the MAC address of the HA 3  (S 34 ). The HA 3  can from hereon receive as a proxy, IP packets addressed to the MN 1  (S 35 ). The process for the HA 3  to relay the IP packet to the MN 1  is described next while referring to the example in  FIG. 18  (F 9 ) The HA 3  receives as a proxy, those IP packets sent from the CN 4  and addressed to the HoA of the MN 1  ( 700 ). The HoA which is the transmit destination address of the received IP packet is extracted ( 701 ), the binding cache management table  313  shown in  FIG. 11 -B is searched ( 702 ) and a check is made for HoA matching that HoA (for MN 1 ) ( 703 ). If there is no matching HoA, then the received IP packet is discarded and the process terminated ( 704 ). If there is a matching HoA, then the HA 3  relays an encapsulated IP packet to the MN 1 .  
         [0104]     In this processing, the current CoA  842  of the MN 1  is extracted from the binding cache management table  313  ( 705 ). To encapsulate and relay the IP packet, the HA 3  newly stores its own IP address in the transmit source address of the IP header area, stores the extracted current CoA of the MN 1  in the destination address  402 , and generate an IP header. Attaching the received IP packet to the IP header that was generated completes the encapsulated packet ( 706 ). The generated encapsulated packet is relayed to the MN 1  ( 707 ). Next, the HA 3  checks the binding cache table  313  to find whether or not a next period CoA 843  for MN 1  has been entered (binding entry) ( 708 ). If not registered, then the process for relaying the packet to the MN 1  terminates. When a next period CoA 843  has been registered, then the HA 3  checks whether the S bit within the control flag  845  is set to “1” ( 709 ). An S bit that is set to “1” specifies that the MN 1  is also requesting encapsulating and relay of the next period CoA 843 , so the next period CoA required for encapsulating and relay is extracted ( 710 ). An IP header is newly generated the same as in the previously described encapsulating process, and an encapsulated packet attached with the IP packet is generated ( 711 ). The next period CoA 843  that was extracted is stored in the destination address  402  area within the newly generated IP header. Finally, then encapsulated packet that was generated is relayed to the MN 1  ( 712 ) and the process terminates.  
         [0105]     Therefore, when the HA 3  receives a packet addressed to the MN 1 , the HA 3  encapsulates and sends the packet to the current CoA and next period CoA registered (binding entry) by the MN 1  (S 36 , S 37 ). The MN 1  can in this way receive an IP packet immediately after moving to the host network Se which is its movement destination.  
         [0106]     Next, when the MN 1  has received the encapsulated packet (S 36 , S 37 ) from the HA 3 , the MN 1  usually notifies the CN 4  of the CoA currently in use and from hereon communications between the MN 1  and the CN 4  are performed by route optimization that allows direct communication rather than communicating via the HA 3 . This patent is characterized in that the MN 1  includes the next period CoA in the binding update (position registration) message sent to the CN 4 , the same as with the binding update message sent to the HA 3 . This optimization route processing is described while referring to  FIG. 19 . The MN 1  decapsulates the encapsulated packet that was received ( 720 ).  
         [0107]     Next, a search is made of the binding update table  117  ( 721 ) to find if there is a record matching the transmit source address of the original packet ( 722 ). If there is a matching record, then route optimization processing for the CN 4  is determined to have already been performed and this processing terminates. When there is no matching record, the MN 1  implements route optimizing to notify the CN 4  of the CoA currently in use. First of all, Return Routability communication as specified in IETF “Mobility Support in IPv6” is implemented before sending the BU (binding update) message ( 723 , S 43 ). After completion of Return Routability, the MN 1  searches the terminal information management table  115  ( 724 ), extracts the next period CoA and the current CoA ( 725 ), decides movement was performed ( 726 ), and extracts the information required to generate a BU (binding update) message, the same as performed when sending a BU message to the HA 3 . After generating the BU (binding update) message, the MN sends it to the CN 4  ( 727 , S 44 ), and after rewriting the binding update table  117  ( 728 ), terminates the route optimizing.  
         [0108]     The CN 4  receives the binding update message from the MN 1  the same the HA 3  (F 8 ). After the route optimizing, the CN 4  sends packets directly to the current CoA and next period CoA of the MN 1  when sending packets to the MN 1  (S 46 ). The processing (F 11 ) by the CN 4  for sending IP packets addressed to the MN 1  is described next using the example in  FIG. 20 . The IP packet sent by the CN 4  is first of all generated in the application  311  shown in  FIG. 5B . The HoA of the MN 1  is stored in the IP header transmit destination source address of the original packet generated in the application  111 , and the IP address of CN 4  is stored in the transmit source address. This packet is sent to the Mobile IP processing  312  and IP packet conversion is performed. The Mobile IP processing  312  extracts the destination address of the IP packet that was sent ( 730 ).  
         [0109]     Next, the CN 4  searches the binding cache management table  313  shown in  FIG. 11 -B ( 731 ) and checks whether or not there is a matching HoA for the MN 1  in the destination address ( 732 ). If there is no matching HoA, then the transmit IP packet is sent unchanged. This packet is then relayed to the MN 1  via the HA ( 733 ). If there is a matching HoA, then route optimizing is determined to have been implemented between the MN 1  and CN 4 , and the current CoA 842  of the MN 1  is extracted from the binding cache management table  313  ( 734 ). The current CoA that was extracted is rewritten into the transmit destination address (=HoA) stored in the IP header. The HoA which is the source for the transmit destination address is stored in the route control expansion header ( 735 ). The CN 4  in this way, converts its own IP packet for transmit, generates a new transmit IP packet, and transmits it to the MN 1  ( 736 ).  
         [0110]     Next, the CN 4  checks whether or not the next period CoA 843  of MN 1  is registered (entered) in the binding cache management table  313  ( 737 ). If not registered, then the process for relaying the packet terminates. If the next period CoA 843  has been registered, then a check is further made of whether the S bit within the control flag  845  is set to a “1” ( 738 ). If the S bit is set to a “1”, then this specifies that the MN 1  is requesting the sending of an IP packet to the next period CoA, and the next period CoA 843  required for the IP packet is then extracted ( 739 ). The IP packet conversion is performed the same as previously described ( 740 ). The CN 4  relays the packet to the MN 1  ( 741 ) and the processing terminates.  
         [0111]     As a result of the above processing, the packet loss that accompanies MN 1  movement can be reduced even communication where the route optimizing was performed between the MN 1  and the CN 4 .  
         [0112]     Next, returning to  FIG. 6 , when the MN 1  has moved from AR- 1  ( 2   a ) to the area administered by AR- 2  ( 2   b ), the AR- 2  ( 2   b ) receives the router advertisement that was sent (S 38 ). The MN 1  performs movement detection and makes a binding update (position registration) request (S 39 , F 7 ). After also having performed the binding update (F 8 ) processing, the HA 3  prepares to relay the packet to MN 1  (S 40 ).  
         [0113]     The processing (F 12 ) when the MN 1  sends an IP packet to the CN 4  from the host network is described next while referring to the example in  FIG. 21 . The IP packet sent by the MN 1  is first of all generated in the application  110  shown in  FIG. 4 -B. This packet is relayed to the Mobile IP processing  114  for IP encapsulation. The Mobile IP processing  114  inputs the original packet generated in the application  110  ( 750 ). The HoA of the MN 1  is stored in the transmit source address of the IP header of the original packet, and the IP address of the CN 4  is stored in the transmit destination address. The transmit destination address of the original packet is extracted next ( 751 ). A search is made of the binding update management table  117  ( 752 ) shown in  FIG. 11 -A to confirm whether or not there is a BU (binding update) transmit destination address  831  matching the extracted transmit destination address ( 753 ). If there is no matching transmit destination address, then a check is made whether or not there is a matching HoA 832  in the transmit source address ( 754 ), if there is no matching HoA 832 , then the original packet is discarded ( 755 ) and the packet transmit process terminates. If there is a HoA 832  matching the transmit source address, then transmission is determined to be via the HA 4 . When there is a BU transmit destination address  831  matching the transmit destination address, then route optimization with the CN 4  is determined to have been implemented and the transmit flag to the CN 4  is raised ( 756 ).  
         [0114]     The MN 1  extracts current CoA and the next period CoA 834  ( 757 ). The MN 1  then searches the list of CoA in use in the terminal information management table  124  shown in  FIG. 10 -B ( 758 ), and selects a high priority CoA ( 759 ). The MN 1  next checks whether or not the transmit flag to the CN 4  is raised (present) in order to perform conversion to the original packet ( 760 ). If the flag is not raised at this point then the transmit process is via the HA 3  and therefore a new IP header is generated the same as described for the encapsulating process ( 761 ) and attached to the original IP packet to generate the encapsulated packet. The selected CoA is stored in the transmit source address of the newly generated IP header, and the IP address of the HA 3  is stored in the destination address, and finally the generated encapsulated packet is sent ( 763 ). On the other hand, when the transmit flag to the CN 4  is raised, then IP header conversion is performed and a destination address expansion header is attached for optimized route transmission to the CN 4  ( 762 ). This processing rewrites the selected CoA into the transmit source address (=HoA) stored in the IP header, and stores the HoA which is the original transmit source address, into the destination address expansion header. The converted IP packet is sent directly to the CN 4  ( 763 ) and the processing terminates.  
         [0115]     As clearly shown in the above embodiments, because packets can be received immediately after the mobile terminal has moved, the present invention allows smooth real-time communication that is otherwise normally susceptible to effects of packet transfer delays and packet loss. Further, since the presence of access routers in proximity to the network connected to the mobile terminal can be confirmed the present invention can also be used in application where the mobile terminal user constantly selects movement destinations where communication is possible and schedules mobile terminal movement. The present invention can also be used in applications for evaluating the structure of the mobile communications network since the provider of the mobile communication network can easily grasp the installation status of the access router.