Abstract:
Disclosed is a communication method allowing MN (mobile nodes) to acquire a home address in a visited network. A communication system comprises a home network installed with a home agent for holding mobile terminal location information and, a visited network installed with a radio communications device for communicating with that mobile terminal, and gateway equipment in the visited network for forming an interface with the home network. The radio communication device contains an access request transfer means for forwarding access requests from a mobile terminal of the visited network to the gateway equipment, and the gateway equipment makes an acquisition request to the home agent for an identifier of that mobile terminal, and forwards that acquired identifier to that mobile terminal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a mobile communication system and relates in particular to a mobile communication system, gateway equipment, communication method and mobile terminal authentication method for mobile IP protocol.  
           [0003]    2. Description of Related Art  
           [0004]    Much intense study of IP (Internet Protocol) for mobile communication networks has been made in recent years.  
           [0005]    The IETF (Internet Engineering Task Force) is working on standardized Mobile IPv6 specifications (for example, in non-patent document 1; Mobility Support in IVv6  
           [0006]    &lt;draft-ietf-mobileip-ipv6-19.txt&gt;, Work in Progress).  
           [0007]    The elements comprising the Mobile IPv6 network structure are a Mobile Node (MN), a home agent (HA), and a correspondent node (CN).  
           [0008]    An IP address (home address serving as identification information for the mobile node) is assigned to the MN. This IP address does not change even when the MN moves. A link having the same prefix as the home address is called the home link. The HA manages MN location information (Binding Cache) located other than in the home link.  
           [0009]    When the MN moves to a link (host link) other than the home link (foreign link), the MN acquires the IP address of that link. This address is called the Care of Address (CoA). The MN receives a router advertisement sent periodically by the router in the foreign link. The MN senses movement by detecting a prefix different from the home address.  
           [0010]    When the MN detects movement, it registers the location in the HA. The MN contains a Home Agent Address Discovery function capable of dynamically searching for home agent IP addresses.  
           [0011]    The MN creates a Mobile IPv6 Home-Agents Anycast Address from the prefix of the home link. The MN sends an ICMP Home Agent Address Discovery Request to that address. That ICMP Home Agent Address Discovery Request signal is sent to the HA in each of the home links. The HA that received the ICMP Home Agent Address Discovery Request sends an ICMP Home Agent Address Discovery Reply containing HA information to the MN. The MN acquires the address of the HA by extracting HA information from the ICMP Home Agent Address Discovery Reply signal. The MN makes a binding update (performs location registration) of the HA address it acquired.  
           [0012]    When the HA receives the binding update, it retains the MN home address and the CoA binding information in the binding cache. The HA next acts as a proxy (on behalf) of the MN, sending a Gratuitous Neighbor Advertisement to capture packets addressed to the home address of the MN.  
           [0013]    The CN (correspondent node) is a peer node with which a MN is communicating.  
           [0014]    The procedure used by the CN for sending a packet addressed to the MN is described next.  
           [0015]    The CN sends a packet addressed to the MN home address. The HA that received the packet addressed to the MN home address, searches the binding cache, and acquires the CoA matching the MN home address. The HA attaches an IP header for the received packet and sends the encapsulated packet. The Destination Address of the external header is a CoA.  
           [0016]    When the CoA addressed packet is received, the MN first removes (decapsulizes) the IP header that was attached and restores the original packet.  
           [0017]    The home network might reattach a prefix. The Mobile IPv6 contains a function to notify the MN of the host network of prefix information for the home network. The HA checks the binding cache and notifies the MN in the binding update process with an MPA (Mobile Prefix Advertisement).  
           [0018]    Technology for localized management of mobile communication based on Mobile IPv6 has been proposed (for example, in non-patent document 2; Hierarchical Mobile IPv6 mobility management (HMIPv6)  
           [0019]    &lt;draft-ietf-mobileip-hmipv6-07.txt&gt;Work in Progress).  
           [0020]    The MPIPv6 contains a MAP (Mobile Anchor Point) between the HA and MN. The MAP provides that network with a local HA function. The MAP may also contain an AR (access router) under its control. The MN receives a router advertisement from the AR containing a MAP option, and acquires the IP address of the MAP. The MAP option contains the global address of the MAP, the MAP prefix, the MAP preference, and the number of hops to the MAP, etc. The MAP uses either of the following methods to notify the AR of MAP options.  
           [0021]    (1) MAP distributes a router advertisement containing MAP options to the router (AR).  
           [0022]    (2) MAP expands the router renumbering function for IPv6 and notifies the AR of the MAP options.  
           [0023]    Instead of the MAP informing the AR of the MAP options, the network manager may set the MAP option information in the AR.  
           [0024]    The information with the MAP options is stored when a MN compatible with HMIPv6 standards receives the router advertisement containing the MAP options. The MN (compatible with HMIPv6) generates a RCoA (Regional Core of Address) from the MAP prefix contained in the MAP option and the MN interface identifier. The MN also generates an LCoA (on-link CoA) utilizing prefix information contained in the router advertisement. The LCoA is equivalent to the CoA of the Mobile IPv6.  
           [0025]    The (HMIPv6 compatible) MN first of all makes a binding update (location registration) on the MAP. The MAP manages the binding information along with the RCoA and LCoA of the MN. The binding update signal that the MN sends to the MAP contains bits showing the MAP binding update (registration information). The MN next performs the binding update in the HA. The HA manages the MN home address and the RCoA of the binding information. When the MN has moved within the MAP, the MN rewrites only the MAP location information.  
           [0026]    The IPv6 address is specified in RFC2373. Three types of addresses; unicast, anycast and multicast are defined as IPv6 addresses. The unicast address contains three types (global address, site local address, and link local address) of addresses. The site local address can only be used within the site and is equivalent to an IPv4 private address (for example non-patent document 3).  
           [0027]    The IPv6 Prefix Delegation Options for DHCPv6 (DHCP-PD) are also being evaluated. Here, DHCP-PD is a function that makes use of DHCP (Dynamic Host Configuration Protocol) to assign IPv6 prefix (group) to sites from the side handling address assignment.  
           [0028]    The delegating router and requesting router are the elements that make up the DHCP-PD. The requesting router makes a request to the delegating router to assign an IPv6 prefix (group). The delegating router selects an IPv6 prefix (group) and sends that prefix to the requesting router. This DHCP-PD is used for example by ISP (Internet Service Providers) when assigning a prefix to subscribers (as for example in non-patent document 4).  
           [0029]    [Non-patent document 1] 
           [0030]    D. Johnson and others, Mobility Support in IPv6, [online], Oct. 29, 2002, Internet&lt;URL: http://www.ietf.org/internet-drafts/draft-ietf-mobileip-ipv6-19.txt&gt; 
           [0031]    [Non-patent document 2] 
           [0032]    Claude Castelluccia and others, Hierarchical Mobile IPv6 mobility management (HMIPv6), [online], October 2002, Internet&lt;URL: http://www.ietf.org/internet-drafts/draft-ietf-mobileip-hmipv6-07.txt&gt; 
           [0033]    [Non-patent document 3] 
           [0034]    R. Hinden and others, IP Version 6 Addressing Architecture, [online], July 1988, Internet&lt;URL: http://www.ietf.org/rfc/rfc2373.txt&gt; 
           [0035]    [Non-patent document 4] 
           [0036]    O. Troan and others, IPv6 Prefix Options for DHCPv6, [online], Nov. 27, 2002, Internet&lt;URL: http://www.ietf.org/internet-drafts/draft-ietf-dhc-dhcpv 6-opt-prefix-delegation-01.txt&gt; 
           [0037]    The zone A and zone B are mutually connected and when a mobile node (MN) belonging to zone A has moved to zone B, the HA holding the location information (binding cache) for mobile nodes in zone A functions as a proxy for the MN.  
           [0038]    The zone A and the zone B utilize site local addresses so a conflict might arise between the MN home address and the visited network address.  
           [0039]    The Mobile IPv6 ensures the arrival of communications at the home address by supplying a permanent IP address (home address) to the MN that does not change even if the MN moves its location. The MN user can change the network settings containing the IP address if the MN is a personal computer, etc. However, when the home address of the MN is changed (rewritten), the problem arises that the arrival of communications at the MN cannot be guaranteed. Methods allowing the MN to acquire the home address have been considered in order to resolve this problem. However, Mobile IPv6 does not contain a function to dynamically assign home addresses to the MN.  
           [0040]    The Mobile IPv6 prefix discovery function is provided to the MN registering the location information (binding update) in the HA. So prior to making the binding update (location registration), the MN cannot receive the signal (MPA) notifying it of home network prefix information.  
         SUMMARY OF THE INVENTION  
         [0041]    The present invention has the object of providing a mobile communication method capable of allowing the MN to acquire the home address in the visited network. The present invention in particular has the object of providing a mobile communications method for distributing home address prefix information to a mobile terminal X by utilizing the DHCP-PD function, when the home network of the mobile terminal X with an HA belonging to zone A has moved to zone B.  
           [0042]    The present invention has the further object of providing a mobile communication method for a mobile terminal X not performing a binding update (location registration) in the HA, to acquire home address prefix information in zone B.  
           [0043]    The present invention has the still further object of providing a mobile communication method to send functions and information required for distributing home address prefix information to a mobile terminal from a network management equipment via HA and gateway equipment of zone B.  
           [0044]    The present invention has the yet further object of providing a mobile communication method for starting authentication processing and DHCP-PD functions for the binding update signal the MN sends to the MAP, when the gateway equipment of zone B is an HMIPv6 compatible MAP. A yet further object is to provide a highly safe mobile terminal communication method by having the MAP accept only the binding update request of the MN whose results from authentication processing were approved by the MAP.  
           [0045]    The present invention is a communication system comprising a home network installed with a home agent for holding mobile terminal location information and, a visited network installed with a radio communications device for communicating with that mobile terminal, and gateway equipment in the visited network for forming an interface with the home network, wherein the radio communication device contains an access request transfer means for forwarding access requests from a mobile terminal of the visited network to the gateway equipment, and the gateway equipment makes an acquisition request to the home agent for an identifier of that mobile terminal, and forwards that acquired identifier to that mobile terminal.  
           [0046]    The present invention is in other words characterized in comprising the following means.  
           [0047]    (1) The home agent (HA) contains a means for reporting (notifying) prefix information to a gateway equipment containing a DHCP-PD requesting router function and further contains a delegating router function for the DHCPv6 Prefix Delegation Option (DHCP-PD).  
           [0048]    (2) The gateway equipment (GW) belonging to the visited network contains a DHCP-PD requesting router function and an authentication function. The gateway equipment contains a means for requesting the sending of authentication information on a packet transmission source when the gateway equipment receives packets from an un-authenticated transmission source; a means for sending the authentication request to the authentication server when an authentication request is received from the mobile terminal (MN); and a means for requesting distribution of prefix information to an HA containing a DHCP-PD function when determined from the authentication results that the mobile terminal is authentic; and a means to notify the mobile terminal with prefix information when prefix information was received from the HA.  
           [0049]    (3) The HA and gateway equipment (GW) further contain a means for receiving the DHCP-PD function described in (1) and (2) from the network management equipment.  
           [0050]    (4) The gateway equipment (MAP) may instead contain a means to request distribution of prefix information to the HA when the gateway equipment receives a binding update request from the MN if the gateway equipment contains an HMIPv6 compatible MAP function.  
           [0051]    (5) The gateway equipment (MAP) may instead contain a means to approve the binding update only for authenticated MN if the gateway equipment contains an HMIPv6 compatible MAP function.  
           [0052]    The present invention is a communication system comprising a home network installed with a home agent for holding mobile terminal location information and, a visited network installed with a radio communications device for communicating with that mobile terminal, and gateway equipment in the visited network for forming an interface with the home network, wherein the radio communication device contains an access request transfer means for forwarding access requests from a mobile terminal of the visited network to the gateway equipment, and the gateway equipment makes an acquisition request to the home agent for an identifier of that mobile terminal, and forwards that acquired identifier to that mobile terminal so that the mobile terminal can acquire the home address in networks (visited networks) other than the home network.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0053]    [0053]FIG. 1 is a structural view of the communication network of the first embodiment of the present invention;  
         [0054]    [0054]FIG. 2 is a block diagram of the home agent (HA) of the first embodiment of the present invention;  
         [0055]    [0055]FIG. 3 is a drawing showing the structure of the binding cache management table contained in the home agent (HA) of the first embodiment of the present invention;  
         [0056]    [0056]FIG. 4 is a drawing showing the structure of the prefix management table contained in the home agent (HA) of the first embodiment of the present invention;  
         [0057]    [0057]FIG. 5 is a block diagram of the gateway equipment (GW) of the first embodiment of the present invention;  
         [0058]    [0058]FIG. 6 is a drawing showing the structure of the prefix management table contained in the gateway equipment (GW) of the first embodiment of the present invention;  
         [0059]    [0059]FIG. 7 is a drawing showing the structure of the acceptable IP address list contained in the gateway equipment (GW) of the first embodiment of the present invention;  
         [0060]    [0060]FIG. 8 is a drawing showing the structure of the zone file contained in the DNS server of the first embodiment of the present invention;  
         [0061]    [0061]FIG. 9 is a drawing showing the sequence of the binding update processing of the first embodiment of the present invention;  
         [0062]    [0062]FIG. 10 is a flowchart of the authentication processing routine of the gateway equipment (GW) of the first embodiment of the present invention;  
         [0063]    [0063]FIG. 11 is a flowchart of the prefix request processing of the gateway equipment (GW) of the first embodiment of the present invention;  
         [0064]    [0064]FIG. 12 is a flowchart of the prefix delegation processing routine of the first embodiment of the present invention;  
         [0065]    [0065]FIG. 13 is a drawing showing the format of the IPv6 packet of the first embodiment of the present invention;  
         [0066]    [0066]FIG. 14 is a drawing showing the format of the DHCPv6 packet of the first embodiment of the present invention;  
         [0067]    [0067]FIG. 15 is a drawing showing the IA_PD options format of the first embodiment of the present invention;  
         [0068]    [0068]FIG. 16 is a drawing showing the IA_PD prefix options format of the first embodiment of the present invention;  
         [0069]    [0069]FIG. 17 is a drawing showing the Home Agent Address Discovery Request message format of the first embodiment;  
         [0070]    [0070]FIG. 18 is a drawing showing the Home Agent Address Discovery Reply message format of the first embodiment;  
         [0071]    [0071]FIG. 19 is a drawing showing the Binding Update message format of the first embodiment;  
         [0072]    [0072]FIG. 20 is a drawing showing the Binding Acknowledgment message format of the first embodiment;  
         [0073]    [0073]FIG. 21 is a diagram showing the structure of the communication network of the second embodiment of the present invention;  
         [0074]    [0074]FIG. 22 is a block diagram of the home agent (HA) of the second embodiment of the present invention;  
         [0075]    [0075]FIG. 23 is a block diagram of the gateway equipment (GW) of the second embodiment of the present invention;  
         [0076]    [0076]FIG. 24 is an illustrative drawing showing the structure of the communication network of the third embodiment of the present invention FIG. 25 is a block diagram of the gateway equipment (MAP) of the third embodiment of the present invention;  
         [0077]    [0077]FIG. 26 is a sequence diagram of the binding update processing routine of the third embodiment of the present invention;  
         [0078]    [0078]FIG. 27 is a sequence diagram of the binding update processing routine of the fourth embodiment of the present invention; and  
         [0079]    [0079]FIG. 28 is a flowchart of the prefix request processing for the MAP of the fourth embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0080]    The embodiment of the present invention is described next while referring to the accompanying drawings. The method for acquiring the home address when the Mobile IPv6-compatible node (MN) has moved to a network (hereafter visited network) other than the home link (hereafter home network) is described in detail as an example.  
         [0081]    [0081]FIG. 1 is a drawing showing the structure of the communication network of the first embodiment.  
         [0082]    The communication network is comprised of an MN 3  home network  6 , and IP network  7  and a visited network  5  ( 5   a ,  5   b ). In the first embodiment, the home network  6 , the IP network  7  and the visited network  5  are an IPv6 network. The MN 3  is a mobile node (MN) compatible with Mobile IPv6. The visited network  5  and the IP network  7  are connected by way of gateway equipment (or routers) functioning as connection devices. The IP network  7  and the home network  6  are connected by way of routers (or gateway equipment). The visited network  5  and the home network  6  may also be connected by way of routers or gateway equipment.  
         [0083]    The home network  6  is comprised of HA  1  and a DNS server  10 . The visited network  5  ( 5   a ,  5   b ) is comprised of a radio communications device (access point) connected to MN 3 , a router  4  ( 4   a ,  4   b ), and a gateway equipment  2  ( 2   a ,  2   b ) functioning as an interface between the visited network  5  and the IP network  7 . The IP network  7  is comprised of an authentication server  8 . The gateway equipment  2  and the authentication server  8  may be directly connected.  
         [0084]    Here, the HA  1  is a home agent (HA) compatible with Mobile IPv6. The HA 1  manages the location information other than in the home network  6 . The HA 1  contains a function to supplement the packet addressed to the MN 3  home address that was sent by the other communication terminal (CN)  9 , and to forward the packet to the MN 3  present within the visited network  5   b . The DNS server  10  for example, replies with an IP address to the domain name of NM 3  sent from the CN 9 .  
         [0085]    [0085]FIG. 2 is a block diagram showing the structure of the home agent (HA)  1  installed in the home network  6 .  
         [0086]    The HA 1  is comprised of an interface section (IF)  19  ( 19   a ,  19   b ,  19   m ,  19   n ) containing a server section  11  ( 11   a ,  11   b ), server section  12 , and line  18  ( 18   a ,  18   b ,  18   m ,  18   n ) as well as a switching section  17  ( 17   a ,  17   b ).  
         [0087]    The server section  11  is comprised of a packet send-receive processing section  13 , an encapsulation-decapsulation processing section  14 , and a Mobile IP processing section  15 .  
         [0088]    The packet send-receive processing section  13  contains a function to send or receive data packets. The encapsulation-decapsulation processing section  14  contains a function to add or delete the IP header from the data packet. The Mobile IP processing section  15  has a home agent (HA) function for Mobile IPv6 and contains a binding cache management table  330  (FIG. 3).  
         [0089]    The server section  12  contains the packet send-receive processing section  13  and DHCP-PD function section  16 . The DHCP-PD function section  16  is comprised of a delegating router function for DHCP-PD, contains a prefix management table  310  (FIG. 4) and executes the prefix delegation processing routine  80  (FIG. 12).  
         [0090]    [0090]FIG. 3 is a drawing showing the structure of the binding cache management table.  
         [0091]    The binding cache management table  330  matches at least a Care of Address (CoA)  332  acquired in the visited network by MN 3  and a lifetime  333  showing the effective period of the binding cache with the MN 3  home address  331  and stores them ( 332  and  333 ).  
         [0092]    [0092]FIG. 4 is a drawing showing the structure of the prefix management table  310 . The prefix management table  310  matches at least an IAID  312  showing the prefix (group), and a distributed prefix  313 , and a lifetime (effective period)  314  of the distributed prefix with the DHCP Client identifier  311  and stores them ( 312 ,  313 ,  314 ).  
         [0093]    [0093]FIG. 5 is a block diagram of the gateway equipment (GW)  2  installed in the visited network  5 .  
         [0094]    The gateway equipment  2  is comprised of an interface section (IF)  23  ( 23   a ,  23   b ) containing a CPU  21 , a memory  22 , and a line  24  ( 24   a ,  24   b ) mutually connected to a bus  25 .  
         [0095]    The memory  22  contains a DHCP-PD function section  26  and an authentication processing section  27 . The DHCP-PD function section  26  has a requesting router function for the DHCP-PD, contains a prefix management table  320  (FIG. 6) and executes a prefix request processing routine  60  (FIG. 11) The authentication processing section  27  contains an acceptable IP address list  340  (FIG. 7) and executes an authentication processing routine  70  (FIG. 10).  
         [0096]    [0096]FIG. 6 is a drawing showing the structure of the prefix management table  320 .  
         [0097]    The prefix management table  320  matches the distributed prefix  323 , the lifetime (effective period)  324  of the distributed prefix, and the DHCP server identifier  325 , with the IAID322 showing the IA_PD321 prefix (group) in the gateway equipment  2  and stores them ( 323 ,  324 ,  325 ).  
         [0098]    [0098]FIG. 7 is a drawing showing the table structure of the acceptable IP address list  340 .  
         [0099]    The acceptable IP address list  340  matches the authenticated IP address  341  and the IP address  342  installed in the authentication processing  27  of gateway equipment  2  and stores them ( 341  and  342 ).  
         [0100]    The structure of the DNS server is described next. The DNS server  10  is comprised of a CPU, a memory, a storage section for storing a zone file (FIG. 8) and an interface section connected to a network (home network  6 ).  
         [0101]    [0101]FIG. 8 is a drawing showing the structure of the zone file  350  contained in the storage (memory) section of the DNS server. The zone file  350  stores the FQDN (Fully Qualified Domain Name)  351 , and the IP address  352  corresponding to that domain name. If there request is made to the DNS server for an IP address via the domain name, the DNS server searches the zone file  350  and replies with an IP address matching that domain name. A node that acquired a new IP address can add the new entry by making a request to the DNS server  10 .  
         [0102]    [0102]FIG. 9 is a drawing showing the sequence from the MN 3  in the visited network  5   b  creating a home address shown in FIG. 5, to the completion of the binding update processing.  
         [0103]    The MN 3  receives (101) the router advertisement from router  4   c  belonging to the network  5   b . The MN 3  searches the M bit of the router advertisement and sets the method for acquiring the CoA (Care of Address). In other words, if the M bit of the router advertisement is set to 1, then the MN 3  acquires the CoA by using the IPv6 statefull address autoconfiguration. If the M bit of the router advertisement is not set, then the MN 3  acquires the CoA by using the IPv6 stateless address autoconfiguration ( 102 ).  
         [0104]    The MN 3  sends the CoA acquired in the visited network  5   b  to the IP packet that was set in the source IP address ( 103 ). This IP packet is conveyed via the gateway equipment (GW)  2   b . The gateway equipment  2   b  searches the acceptable address list  340  using the source IP address (CoA). If an entry for that CoA is not present in the acceptable address list  340 , then a request to send authentication information to source IP address of the packet ( 104 ).  
         [0105]    The MN 3  that received the authentication information sends an authentication request containing the requested information (ID, password, etc.) to the gateway equipment  2   b  ( 105 ). The gateway equipment  2   b  performs the authentication processing routine  70  when the authentication request is received (See FIG. 10.).  
         [0106]    The gateway equipment  2   b  then sends an authentication request to the authentication server  8  ( 106 ). The authentication server  8  authenticates the user ( 107 ). If the authentication was a success then a user home network identifier is included in that authentication reply.  
         [0107]    The gateway equipment  2  that received the authentication reply  108  executes the prefix request processing routine  60  (See FIG. 11.). The gateway equipment  2   b  creates a prefix and an IA_PD showing the correspondence, from the home network identifier. The gateway equipment  2  sends a DHCP Solicit message addressed to All_DHCP_Relay_Agents_and_Servers address to discover a DHCP server capable of distributing the prefix ( 109 ). This solicit message contains a Client Identifier Option for gateway equipment  2   b  and IA_PD options. The IA_PD options are set in that IAID.  
         [0108]    The server section  12  of the HA 1  that received the DHCP solicit message ( 109 ), starts the Prefix Delegation processing routine  80  (FIG. 12), and sends a DHCP.advertise message to the gateway equipment  2   b  ( 110 ).  
         [0109]    The gateway equipment  2   b  receives the DHCP advertise message ( 110 ). A check is then made that the IA_PD options and client identifier option of the received message contain correct values and that the server identifier option is included in the received message.  
         [0110]    The gateway equipment  2  then sends a DHCP request message containing the IA_PD options to that server section  12  in order to request distribution of an IPv6 prefix ( 111 ).  
         [0111]    The IPv6 prefix to be distributed is designated when the server section  12  of the HA 1  that received the DHCP solicit message is capable of distributing the prefix to the IAID contained in the IAID file of the IA_PD options. When that request message contains IA_PD prefix options in the IA_PD options, the server section  12  searches the IPv6 prefix field and approves the prefix that the gateway  2   b  wishes to use.  
         [0112]    The server section  12  next searches DHCP Client identifier and IAID contained in that DHCP request message and the prefix management table  310  in the IAID. The server section  12  detects the entry created in step  82 . The server section  12  then sends a DHCP reply message to the gateway equipment  2   b  ( 112 ).  
         [0113]    The gateway equipment  2  checks that the Client Identifier option, IA_PD options and Server Identifier option contain appropriate values when that DHCP reply message is received. The gateway equipment  2  then creates a new entry in the acceptable address list  340 , and sends an authentication replay containing prefix information to MN 3  ( 113 ).  
         [0114]    In step  74 , when a send authentication request to authentication server message containing an error notice from the prefix request processing routine is received, the gateway equipment  2   b  sends an authentication reply containing the error notification to MN 3  ( 77 ) and this routine then ends.  
         [0115]    When the MN 3  receives an authentication reply containing prefix information, the MN 3  creates a home address from the prefix information that was received and from the interface identifier ( 114 ).  
         [0116]    The MN 3  next sends a Home Agent Address Discovery Request for designating the IP address of the HA ( 115 ).  
         [0117]    The MN 3  sets the CoA acquired in step  102 , into the (transmit) source address  41   a  of the home agent address discovery request message S 11  (See FIG. 17.). The MN 3  sets the prefix received in step  113  as the home network prefix for the Mobile IPv6 Home-Agents Anycast Address, into the destination address  41   b  of the home agent address discovery request message S 1 .  
         [0118]    Any HA having the same prefix as that Mobile IPv6 Home-Agents Anycast Address, receives the home agent address discovery request.  
         [0119]    The server section of HA 1  here stores a prefix identical to the prefix of the Mobile IPv6 Home-Agents Anycast Address and receives the Home Agent Address Discovery Request. The server section  11   a  sends a Home Agent Address Discovery Reply to MN 3  ( 116 ).  
         [0120]    The server section of HA 1  stores the following values in the Home Agent Address Discovery Reply message (FIG. 18) to send to the MN 3 . The HA address (HA list) is stored in the Home Agent Addresses field of IPv6 ICMP411. The source address for the Home Agent Address Discovery Request is set in the destination address  41   b . The global unicast address for the server section  11   a  is set in the (transmit) source address  41   a.    
         [0121]    The MN 3  acquires the HA address from the Home Agent Address Discovery Reply ( 117 ). The MN 3  first of all, checks whether or not the source address of the Home Agent Address Discovery Reply is in the HA address (HA list). If the source address is contained in the HA list then the MN 3  performs a binding update (location registration) for addresses recorded in the HA list. If the source address is not contained in the HA list then the MN 3  performs a binding update of the source address.  
         [0122]    The HA list for this Home Agent Address Discovery Reply does not contain a source address. The MN 3  performs the binding update (location registration) in the server section  11   a  of HA 1 .  
         [0123]    The MN 3  sends the binding update message to the server section  11   a  of HA 1  ( 118 ). The MN 3  temporarily records the server section  11   a  address in the binding update list management table ( 119 ).  
         [0124]    The following values are stored as shown in FIG. 19 when MN 3  sends the binding update to the server section  11   a . The CoA of MN 3  is set in the source address  41   a  of the IPv6 header. The home address created by MN 3  in step  114  is set in the home address field of the IPv6 Destination Options Header  401 .  
         [0125]    The server section  11   a  of HA 1  receives the binding update message and extracts the home address contained in the Pv6 Destination Options Header  401 . The server section  11   a  searches the binding cache management table  330  to find if that home address entry is present. If that MN 3  entry is not found in the binding cache management table  330 , then the MN 3  entry is added to the binding cache management table  330  ( 120 ). The MN 3  sets the CoA acquired in the visited network  5   b  into the Care of Address  332  of that entry. The server section  11   a  functions as a proxy for the MN 3 .  
         [0126]    The server section  11   a  sends a binding acknowledgment to the MN 3  ( 121 ). The binding acknowledgment that the server section  11   a  sends to the MN 3  is stored as the following values as shown in FIG. 20. The (transmit) source address of the binding update received in step  118  is stored in the destination address  41   b  of the IPv6 packet header. The MN 3  home address is stored in the home address field of the IPv6 routing header  403  when storing the values other than the MN 3  home address in the destination address  41   b.    
         [0127]    When a binding acknowledgment is received showing that the binding update ended correctly, the MN 3  registers the entry temporarily stored in step  119 , into the binding update list management table ( 122 ).  
         [0128]    The MN 3  may at this point also register the identification information (for example, FQDN) and corresponding information for the home address acquired in step  114 , into the location information management device (for example, DNS server) belonging to the home network  6 , visited network  5  or the IP network  7 .  
         [0129]    The processing in the gateway equipment  2   b  from 106 through 113 in FIG. 9 is described next. FIG. 10 is a flowchart of the authentication processing routine  70 . This authentication process is implemented in the authentication processing section  27 .  
         [0130]    The gateway equipment  2   b  sends an authentication request to the authentication server  8  ( 106  in FIGS. 9 and 71). The authentication server  8  sends the authentication results to the gateway equipment  2   b  ( 108  in FIG. 9). If authentication was successful, then the home network identifier of the user is included in that authentication reply.  
         [0131]    The gateway equipment  2   b  that received the authentication reply, searches the authentication results and determines if authentication was a success or not (72). If the authentication succeeded, the prefix request processing routine  60  (in FIG. 11) starts up ( 73 ). On the other hand, if the authentication in step  72  failed, then the gateway equipment  2   b  sends an authentication reply containing an error notice to the MN 3  ( 77 ) and this routine then ends.  
         [0132]    When an authentication reply transmit request containing the prefix is received from the prefix request processing routine ( 74 ), a new entry is created in the acceptable address list  340  (FIG. 7) and the IP address (CoA of MN 3 )  341  and effective period  342  are then stored ( 75 ). The gateway equipment  2   b  sends an authentication reply containing prefix information to the MN 3  ( 113  in FIG. 9, and  76 ) and this routine then ends.  
         [0133]    However, when the gateway equipment  2   b  receives an authentication response request containing an error notice from the prefix request processing routine in step  74 , the gateway equipment  2   b  sends an authentication response ( 77 ) containing an error notification to MN 3  and this routine then ends.  
         [0134]    The server section  12  may also use the DHCP Reply message ( 112  in FIG. 9) to notify the gateway equipment  2   b  with information such as the IP address of the DNS server  10 . The gateway equipment  2   b  sends an authentication response containing information received from the server section  12  ( 76 ).  
         [0135]    [0135]FIG. 11 is a flowchart of the prefix request processing routine  60 . This routine is executed on DHCP-PD function section of the gateway equipment  2 .  
         [0136]    The gateway equipment  2   b  designates an IA_PD linked with the prefix, from the home network identifier contained in the authentication request send from the authentication server  8 . A DHCP Solicit message is sent addressed to All_DHCP_Relay_Agents_and_Servers addressed ( 109  of FIG. 9, 61). This solicit message contains a client identifier option for the gateway equipment  2  and an IA_PD options (FIG. 15). The IA_PD options ( 533  of FIG. 15) are set in that IAID.  
         [0137]    The authentication processing section  27  of gateway equipment  2   b  is comprised of a user prefix management table (FIG. 4). This table may hold a user ID received in step  105  and information corresponding to the prefix information sent to MN 3  in step  113 .  
         [0138]    The gateway equipment  2   b  receives the DHCP Advertise message and checks ( 110  in FIG. 9) that appropriate values are set in the IA_PD options and Client Identifier option of the received message and contained in the Server Identifier option of the received message. The gateway equipment  2   b  next searches the prefix management table  320  (FIG. 6) for the IAID contained in the IA_PD options of the DHCP Advertise message. If there is no applicable entry, the gateway equipment  2  creates a new entry in the prefix management table  320  and stores the DHCP Advertise message of the IAID and the identifier of the DHCP server. The gateway equipment  2  also stores home network identifier acquired in step  108  in IA_PD321 ( 62 ).  
         [0139]    The gateway equipment  2  next sends a DHCP Request message containing IA_PD options to the server section  12  for requesting distribution of the IPv6 prefix ( 63 ,  111 ).  
         [0140]    If the Advertise message received in step  110  is contained in the IA_PD Prefix options (FIG. 16), then that Request message contains a prefix in IPv6 prefix field  535  of IA_PD Prefix Options that the gateway equipment  2  wishes to use.  
         [0141]    When the DHCP reply message is received, the gateway equipment  2  checks that the Client Identifier option, IA_PD options and Server Identifier option have appropriate values ( 64 ). If the values are appropriate, the gateway equipment  2   b  searches the prefix management table  320  using the IAID and DHCP Server identifier contained in the DHCP replay message. The gateway equipment  2  detects the entry made in step  62 , stores the IPv6 Prefix and lifetime of the prefix distributed to the applicable entry and rewrites the entry. After rewriting the applicable entry, the gateway equipment  2   b  requests ( 65 ) sending of an authentication reply containing IPv6 Prefix information from an authentication processing section  27  and this routine then ends.  
         [0142]    However, when a DHCP advertisement containing appropriate parameters could not be received in step  62  or a DHCP reply message containing a prefix could not be received in step  64  (including when a DHCP reply message without the applicable entry was received in the prefix management table  320 ) the gateway equipment  2   b  requests ( 66 ) an authentication reply containing an error notice from the authentication processing section  27 , and this process ends.  
         [0143]    The HA 1  processing from 110 to 112 in FIG. 9 is described next.  
         [0144]    [0144]FIG. 12 is a flowchart of the prefix delegation processing routine  80 . The DHCP-PD function section  16  of HA 1  executes this routine.  
         [0145]    The server section  12  of HA 1  that received the DHCP Solicit message, searches the IA_PD file of IA_PD options for the DHCP Solicit message. The server section  12  then decides whether or not a prefix can be distributed to the received IAID ( 81 ).  
         [0146]    If the prefix can be distributed, the server section  12  searches the DHCP client identifiers contained in the solicit message and the prefix management table  310  (FIG. 4). If there is no applicable entry, the server section  12  creates a new entry in the prefix management table  310  and stores the DHCP client identifier  311  contained in the solicit message and the IAID  312 . The server section  12  then sends the DHCP advertise message to the gateway equipment  2   b  ( 110  of FIGS. 9 and 82).  
         [0147]    The advertise message contains an identifier for the server section  12  (Server Identifier option) and identifier for gateway equipment  2   b  (Client Identifier option) and the IA_PD option received in step  109 . The server section  12  may also include IPv6 prefix information distributable to IAID, in the DHCP Advertise message. The IPv6 prefix information is set in IPv6 prefix field  535  of the IA_PD options contained in the IA_PD options  533  of the advertise message. The server section  12  designates a distributable IPv6 prefix using IAID.  
         [0148]    When the server section  12  then receives the DHCP request message ( 111  of FIG. 9, and  83 ), it searches the IAID file of IA_PD options. If the prefix can be distributed to IAID, then the server section  12  designates a distributable IPv6 prefix. When the request message contains IA_PD prefix options in the IA_PD options, the server section  12  searches the IPv6 prefix field and approves the prefix that the gateway equipment  2   b  wishes to use.  
         [0149]    The server section  12  searches the DHCP client identifiers contained in the request message and the prefix management table  310  with IAID. The server section  12  detects the entry created in step  82  and stores the prefix Lifetime and IPv6 prefix distributable to the applicable entry. The server section  12  sends the DHCP reply message to the gateway equipment  2   b  ( 112  in FIGS. 9 and 84) and this routine then ends. The reply message contains the IPv6 prefix information stored in the prefix management table  310  in the IA_PD Prefix options field of IA_PD options  533 .  
         [0150]    If the server section  12  cannot distribute the IPv6 Prefix to the IAID in step  81 , then the server section  12  sends an advertise message containing a status code option showing the prefix cannot be distributed to the gateway equipment  2   b , and this routine then ends ( 85 ).  
         [0151]    When a prefix for distribution cannot be designated in step  83  or when there is no applicable entry in the prefix management table  310  in step  84 , then the server section  12  sends a reply message containing error notice to the gateway equipment  2   b  ( 86 ) and this routine then ends).  
         [0152]    [0152]FIG. 13 is a drawing showing the format of the IPv6 packet.  
         [0153]    The source address  41   a  and the destination address  41   b  are contained in the IPv6 header  41 . Information such as IPv6 path information can be stored in the extension header  42 , and in this embodiment it can store the IPv6 destination options header  401  (FIG. 19), IPv6 mobility header  402  (FIG. 19, FIG. 20) and the IPv6 routing header  403  (FIG. 20). The DHCP message S 1  (FIG. 14) can be stored in the payload  43 .  
         [0154]    [0154]FIG. 14 is a drawing showing the format containing the DHCPv6 message S 1 .  
         [0155]    In the packet with DCHPv6 message, the DHCPv6 is an application protocol using UDP/IP in the transport layer. The DHCP message is specified by a value in the message type field  51 . The DHCP message option parameter is set in the options field  53 .  
         [0156]    [0156]FIG. 15 is a drawing showing the format of IA_PD options S 2 .  
         [0157]    The option code field  531  shows the type of DHCP message option parameter. The IAID is set in the IAID field  532 . The IA_PD Prefix options are included in the IA_PD options  533 .  
         [0158]    [0158]FIG. 16 is a drawing showing format of the IA_PD prefix options S 3 .  
         [0159]    The IPv6 prefix field  536  contains prefix information capable of being distributed to the IAID.  
         [0160]    [0160]FIG. 17 is a drawing showing the format of the Home Agent Address Discovery Request message S 11 .  
         [0161]    The Home Agent Address Discovery Request message S 11  is stored in the payload  43  of the IPv6 packet.  
         [0162]    [0162]FIG. 18 is a drawing showing the format of the Home Agent Address Discovery Reply message S 12 . The Home Agent Address Discovery Reply message S 12  is stored in the payload  43  of the IPv6 packet.  
         [0163]    [0163]FIG. 19 is a drawing showing the format of the Binding Update message S 13 . The IPv6 Destination Options Header  401  and IPv6 Mobility Header  402  are stored in the extension header  42  of the IPv6 packet.  
         [0164]    [0164]FIG. 20 is a drawing showing the format of the Binding Acknowledgment message S 14 . The IPv6 Routing Header  403  and the IPv6 Mobility Header  402  are stored in the IPv6 packet extension header  42 .  
         [0165]    In the first embodiment of the present invention as described above, the HA 1  and the gateway equipment contain a DHCP-PD function. The gateway device  2  starts this DHCP-PD function when the authentication request is received. The HA 1  in other words, contains a means for distributing a prefix to the gateway equipment  2  belonging to visited network  5 . Since this gateway equipment  2  belonging to visited network  5  contains a means to notify MN 3  of the prefix, the gateway equipment  2  can therefore notify MN 3  of the prefix information distributed from HA 1  so that MN 3  outside the home network can acquire the home address.  
         [0166]    Since the MN 3  contains a means to register identification information (for example, FQDN) and (mapping) information linked to the home address acquired in the visited network  5 , in the DNS server, a call destination service can also be provided in the visited network  5  for MN 3  that acquired a home address.  
         [0167]    The second embodiment of the present invention is described next. In addition to the first embodiment, the second embodiment is characterized in that the HA 1  and gateway device  2  receive programs from a network management equipment  31 .  
         [0168]    [0168]FIG. 21 is a diagram showing the structure of the communication network of the second embodiment.  
         [0169]    The second embodiment has a network management equipment  31  in addition to the structure (FIG. 1) of the previously described first embodiment. This network management equipment  31  is connected to the HA 1  and the gateway equipment  2 . The network management equipment  31  contains a means to send programs to the HA 1  and the gateway equipment  2 . The structure is the same as previously described for the first embodiment (FIG. 1) so identical reference numerals are assigned and a detailed description of those sections is omitted.  
         [0170]    [0170]FIG. 22 is a block diagram of the structure of the home agent (HA)  1  of the second embodiment.  
         [0171]    In addition to the structure (FIG. 2) of the first embodiment, the HA 1  contains a server section  32  having a management function section  33 . In the present embodiment, the program and data used for the authentication operation are received from the network management equipment  31 . Sections having the same structure as the first embodiment (FIG. 2) are assigned the same reference numerals and their detailed description is omitted here.  
         [0172]    [0172]FIG. 23 is a block diagram of the gateway equipment (GW)  2  of the second embodiment of the present invention.  
         [0173]    In addition to the structure (FIG. 5) of the first embodiment, the gateway device  2  contains a management function section  28  in a memory  22 .  
         [0174]    The HA 1  and the gateway equipment  2  receive from the network management equipment  31 , the respective programs and information needed for the prefix distribution shown in the first embodiment. The HA 1  installs the received programs and information into the DHCP-PD function of the server section  12 , and runs the applicable program with the DHCP-PD function section  16 . The gateway device  2  installs the received programs and information into the DHCP-PD function  26  of the memory  22  and the authentication processing section  27 , and runs the applicable program with the DHCP-PD function section  26  and the authentication processing section  27 . Sections identical in structure to the first embodiment (FIG. 5) are assigned the same reference numerals and their detailed description is omitted here.  
         [0175]    The second embodiment of the present invention as described above is comprised of a network management equipment  31  to provide the HA 1  and the gateway equipment  2  with programs and data needed for notifying the MN 3  of the prefix. Providing the HA 1  and gateway equipment  2  with a function to receive the applicable programs and data from the network management equipment  31  allows functions to be automatically installed and the authentication operation executed without having to install these functions beforehand as was described for the first embodiment, and the functions of the mobile communication network (of the second embodiment) can easily be improved.  
         [0176]    The third embodiment of the present invention is described next. The third embodiment is characterized in that the gateway equipment  2  contains a MAP function for HMIPv6.  
         [0177]    [0177]FIG. 24 is a diagram showing the structure of the communication network of the third embodiment.  
         [0178]    The third embodiment differs from the above described structure (FIG. 1) of the first embodiment in that MN 3  is a HMIPv6 mobile terminal and that the gateway equipment (MAP)  2  comprises a MAP function for HMIPv6. Sections identical in structure to the first embodiment (FIG. 1) are assigned the same reference numerals and their detailed description is omitted here.  
         [0179]    [0179]FIG. 25 is a block diagram showing the gateway equipment (MAP)  2  of the third embodiment.  
         [0180]    In addition to the structure (FIG. 5) of the first embodiment, the gateway device  2  contains an HMIPv6 processing section  29  in the memory  22 . This HMIPv6 processing section  29  provides a MAP function and contains a binding cache management table for holding information linked to the RCoA and LCoA. Sections identical in structure to the first embodiment (FIG. 5) are assigned the same reference numerals and their detailed description is omitted here.  
         [0181]    [0181]FIG. 26 is a sequence diagram of the processing routine from the MN 3  in the visited network  5   b  shown in FIG. 24 creating a home address, to the completion of the binding update (location registration) in the MAP 2   b.    
         [0182]    The MN 3  receives a router advertisement containing a MAP option from the router (AR: Access Router)  4   c  belonging to the network  5   b  ( 151 ). When the router advertisement containing the MAP option is received, the MN 3  specifies the gateway equipment (MAP)  2   b  containing the HMIPv6 processing section  29 , from the information contained in the MAP option. The MN 3  next creates the RCoA and LCoA by using the information contained in the router advertisement MAP option ( 152 ).  
         [0183]    The MN 3  sends the binding update (location registration) signal showing the location registration to the MAP in the MAP  2   b  ( 153 ). The following information is stored in the binding update (location registration) signal. The LCoA created in step  152  by the MN 3  is set in the source address  141   a . The MAP 2   b  address is set in the destination address  41   b . The RCoA created by the MN 3  in step  152  is set in the home address option of the Destination Options Header  401 .  
         [0184]    The MAP 2   b  that received the binding update (location registration) signal then starts the authentication processing. This MAP 2   b  that received the binding update (location registration) signal, searches the acceptable IP address list  340  in the source IP address (LCoA). If there is no LCoA entry for the MN 3  in the acceptable IP address list  340 , then a request to send authorization information is sent ( 104 ) to the packet source (NN 3 ). The processing from here onwards from step  104  to step  113  is the same as in the first embodiment (FIG. 9).  
         [0185]    In the third embodiment, the MAP  2   b  adds an LCoA and RCoA for the MN 3  to the acceptable IP address list  340  (FIG. 7).  
         [0186]    If the authentication processing ends normally, then the binding information for the RCoA and LCoA of the MN 3  is stored in the binding cache management table of HMIPv6 processing section  29  ( 154 ). The MAP 2   b  then sends the binding acknowledgment to the MN 3  ( 155 ). A MAP  2   b  address is next set in the source address  41   a  of this binding acknowledgment. An LCoA for MN 3  is set in the destination address  41   b , and an RCoA for MN 3  is set in the routing header  403 .  
         [0187]    However, if the authentication processing does not end normally, then the MAP 2   b  sends a binding acknowledgment containing an error notification to the MN 3  ( 155 ).  
         [0188]    If a binding acknowledgment ( 155 ) was received showing that the binding update (location registration) ended normally, then a home address is created from the prefix information received in step  113  and the MN 3  interface identifier ( 114 ). The processing from here onwards from step  114  to step  122  is the same as in the first embodiment (FIG. 9).  
         [0189]    The MN 3  registers the home address created in step  114  and the RCoA binding information created in step  152 , into the HA 1 . The source address  41   a  of the binding update  118  is set based on the MAP option contained in the router advertisement received by the MN 3  in step  151 . If the binding update  118  source address  41   a  is not the RCoA, then the RCoA is set in the Alternate-CoA option of binding update  118 .  
         [0190]    In the third embodiment of the present invention as described above, the HA 1  and the gateway equipment contain a DHCP-PD function. The gateway equipment  2  contains an HMIPv6 MAP function. The gateway equipment  2  starts the MN authentication processing when an HMIPv6 binding update (location registration) signal is received, sends a prefix distribution request to the HA, and requests the sending of authentication information. The gateway equipment  2  can therefore start up the DHCP-PD function when the authentication request signal is received, and can notify the MN 3  of the prefix information distributed from HA 1 . Those MN 3  located outside of the home network can therefore acquire the home address.  
         [0191]    The gateway equipment  2  comprising the HMIPv6 function only permits binding update (location registration) of MN 3  whose authentication processing ended normally so a highly safe communication service can be provided.  
         [0192]    The fourth embodiment of the present invention is described next. The fourth embodiment is characterized in that the MN 3  acquires home network prefix information in the binding update (location registration) response message from the gateway equipment  2 . In other words, the DHCP-PD function section  26  of the gateway equipment  2  executes the prefix request processing routine  90  (FIG. 28) instead of the prefix request processing routine  60  (FIG. 11).  
         [0193]    [0193]FIG. 27 is a sequence diagram of the processing routine from the MN 3  in the visited network  5   b  shown in FIG. 24 creating a home address, to the completion of the binding update (location registration) in the MAP 2   b.    
         [0194]    The processing from step  151  to step  152  are identical to the same steps in the previously described third embodiment (FIG. 26).  
         [0195]    When the RCoA and LCoA are created, the MN 3  sends a binding update (location registration) signal to the gateway equipment (MAP)  2   b  to show location registration in the MAP  2   b  ( 171 ). The binding update ( 171 ) is implemented so that the home network identifier of MN 3  is contained in the Mobility Options of the Mobility Header  402 .  
         [0196]    The MAP 2  that received the binding update, starts the prefix request processing routine  90  (FIG. 28).  
         [0197]    [0197]FIG. 28 is a flowchart of the prefix request processing routine  90  of the fourth embodiment.  
         [0198]    The MAP  2   b  specifies an IA_PD showing a link to the prefix, from the home network identifier contained in the binding update  171 , and creates the IAID. The MAP  2   b  sends a DHCP solicit message addressed to All_DHCP_Relay_Agents_and _Servers in order to discover a DHCP server capable of prefix distribution ( 91 ,  109 ). Hereafter, step  92  through step  94  (from step  110  to step  112  in FIG. 27) are performed by the same processing as in prefix request processing routine  60  of the first embodiment (from step  62  to step  64 ).  
         [0199]    When a DHCP reply message ( 112 ) containing the prefix is received, and the MAP 2   b  rewrites the applicable entry in the prefix management table  320 , it requests a binding acknowledgment containing IPv6 prefix information from the HMIPv6 processing section  29  ( 95 ), and this routine then ends ( 95 ). After the MAP 2   b  stores the MN 3  location information in the binding cache management table ( 154 ), it sends a binding acknowledgment ( 172 ) to the MN 3 . The binding acknowledgment ( 172 ) is implemented so that the IPv6 prefix information is contained in Mobility Options of the Mobility Header  402 .  
         [0200]    On the other hand, when the processing in step  92  or step  94  did not end correctly, or in other words, when a DHCP advertise message containing the correct parameters could not be received in step  92 , or when a DHCP replay message containing a prefix could not be received in step  94  (including the case where a DHCP reply message was received not having an applicable entry in the prefix management table  320 ); then the MAP 2   b  sends a request ( 96 ) for sending of a binding acknowledgment containing an error notice to the HMIPv6 processing section  29  ( 96 ) and this routine then ends.  
         [0201]    The processing from step  114  to step  122  are identical to the same steps in the previously described third embodiment.  
         [0202]    In the fourth embodiment of the present invention as described above, the MAP 2  starts the DHCP-PD function when the binding update (location registration) signal is received, and the home network prefix of the MN 3  can be acquired from the HA 1 .