Abstract:
This wireless device ( 1 ) is provided with a wireless base station function ( 4 ) and a wireless terminal function ( 5 ), and is provided with: an address information acquisition means that transmits an address payout request to another wireless base station ( 3 ) by means of the wireless terminal function ( 5 ), and acquires and stores address information allocated to the device in question in response; and an address information payout means that pays out address information prioritizing that which is other than the address information stored at the address information acquisition means to a wireless terminal ( 2 ) by means of the wireless base station function ( 4 ). As a result, it is possible to provide: a wireless device that can suppress redundant allocation of address information to wireless terminals; an address determination method, a communication system, and a wireless terminal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a radio device, an address determination method, a communication system and a radio terminal, and particularly to a control technique to determine an address for the radio terminal. 
       BACKGROUND ART 
       [0002]    WLAN (Wi-Fi) has been universally put into practical use as one of wireless communication technologies, specifications of which have been developed by IEEE and Wi-Fi Alliance. WLAN-based devices include a software AP (Access Point) on which both a radio base station function and a radio terminal function are mounted (e.g., see PTLs 1 and 2). Currently, specifications of the software AP technique have been studied under the name specified as “Wi-Fi P2P” or “Wi-Fi Direct”. Both a Wi-Fi terminal function and an AP function can be implemented in equipment (Wi-Fi P2P Device) which mounts thereon a chip compatible with the Wi-Fi P2P standard. 
         [0003]    The AP in the Wi-Fi technology accepts and processes an AP search, a connection (association) request and a disconnection request, which originate from a Wi-Fi terminal. If the AP includes a DHCP server function, the AP further processes an IP offer request. The association intends Association procedures defined by the specifications. The disconnection intends that a set of procedures concerning the association and the offer of an IP address be taken back to square one. Generally, in the offer of an IP address handled by the Wi-Fi technology, there is offered a local address which is valid only within a local network. 
         [0004]    The terminal function of the Wi-Fi P2P Device is referred to as “Wi-Fi P2P Client”, and the base station function is referred to as “Wi-Fi P2P Group Owner”. 
         [0005]    DHCP (Dynamic Host Configuration Protocol) is the most common technique for offering an IP address to a terminal. While the DHCP is a client-server type address offer technique and requires a DHCP server function, a lot of broadband routers include the AP and the DHCP server. In the Wi-Fi P2P standard, it has been defined that a server function for the IP offer such as the DHCP is mounted on the Wi-Fi P2P Group Owner. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1: Japanese Unexamined Patent Application Publication No. 2004-304720 
         PTL 2: Japanese Unexamined Patent Application Publication No. 2010-268145 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    Conventionally, the following problems arise in the wireless communication environment where a plurality of local address systems are mixed. One of the problems is that if a radio base station relays packets between two IP networks, IP addresses collide and thus it is not possible to normally perform routing. Another one of the problems is that upon resolving the collision of addresses by use of NAT or NAPT, it is not possible to rewrite information in a payload, or to normally conduct communication if there is a multi-stage address conversion configuration. Yet another one of the problems is that if a radio terminal simultaneously associates with two radio base stations, two network addresses collide and thus it is not possible to normally transmit packets. 
         [0009]    An exemplary object of the present invention is to provide a radio device, an address determination method, a communication system and a radio terminal, which can suppress duplicate allocation of address information to the radio terminal. 
       Solution to Problem 
       [0010]    A radio device according to the present invention includes a radio base station function and a radio terminal function. This radio device includes: address information acquisition means for transmitting, by the radio terminal function, an address offer request to a different radio base station, and for acquiring and storing address information allocated to the radio device in response to the request; and address information offer means for preferentially offering to a radio terminal, by the radio base station function, address information other than the address information stored in the address information acquisition means. 
         [0011]    Further, an address determination method according to the present invention includes the steps of: transmitting an address offer request to a different radio base station; receiving allocated address information from the different radio base station; and preferentially offering, upon offering an address to a radio terminal, address information other than the received address information to the radio terminal. 
         [0012]    Further, a communication system according to the present invention includes a radio device that includes a radio base station function and a radio terminal function, and a radio terminal that can communicate with the radio device. The radio terminal includes means for outputting a change request for address information upon performing multi access to a radio base station other than the radio device. The radio device includes: address information acquisition means for transmitting an address offer request to a different radio base station, the address offer request being triggered when a change request for address information is received from a radio terminal to which the radio base station function is connected, and for acquiring and storing address information allocated to the radio device in response to the address offer request; and address information offer means for preferentially offering to a radio terminal, by the radio base station function, address information other than the address information stored in the address information acquisition means. 
         [0013]    Furthermore, a radio terminal according to the present invention outputs, to a first radio base station, a change request for address information offered by the first radio base station, when the radio terminal performs multi access to a second radio base station that differs from the first radio base station, while communicating with the first radio base station. 
       Advantageous Effects of Invention 
       [0014]    According to the present invention, it is possible to provide a radio device, an address determination method, a communication system and a radio terminal, which can suppress duplicate allocation of address information to the radio terminal. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a block diagram showing a configuration of a communication system according to a first exemplary embodiment; 
           [0016]      FIG. 2A  is a sequence diagram showing a flow of processes in the communication system according to the first exemplary embodiment; 
           [0017]      FIG. 2B  is a sequence diagram showing a flow of processes in the communication system according to the first exemplary embodiment; 
           [0018]      FIG. 3  is a block diagram showing a configuration of a communication system according to a second exemplary embodiment; 
           [0019]      FIG. 4A  is a sequence diagram showing a flow of processes in the communication system according to the second exemplary embodiment; 
           [0020]      FIG. 4B  is a sequence diagram showing a flow of processes in the communication system according to the second exemplary embodiment; 
           [0021]      FIG. 5  is a block diagram showing a configuration of a communication system according to a third exemplary embodiment; 
           [0022]      FIG. 6A  is a sequence diagram showing a flow of processes in the communication system according to the third exemplary embodiment; 
           [0023]      FIG. 6B  is a sequence diagram showing a flow of processes in the communication system according to the third exemplary embodiment; 
           [0024]      FIG. 7  is a block diagram showing a configuration of a communication system according to a fourth exemplary embodiment; 
           [0025]      FIG. 8A  is a sequence diagram showing a flow of processes in the communication system according to the fourth exemplary embodiment; and 
           [0026]      FIG. 8B  is a sequence diagram showing a flow of processes in the communication system according to the fourth exemplary embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
       [0027]    Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompany drawings. In the following drawings, primary elements in the exemplary embodiments of the present invention are illustrated by solid lines. 
         [0028]      FIG. 1  shows a configuration of a communication system according to a first exemplary embodiment. A radio device  1  includes a radio base station function  4 , a radio terminal function  5 , an unused address manager  9 , and a terminal-base station function activating manager  13 . 
         [0029]    The radio base station function  4  is processing means which serves as a radio base station over the communication system. Accordingly, the radio device  1  can conduct wireless communication with one or more radio terminals  2  by using this radio base station function  4 . The radio base station function  4  includes a radio terminal association manager  6 , a communication buffer manager  7 , and a DHCP server  8 . 
         [0030]    The radio terminal function  5  is processing means which serves as a radio terminal over the communication system. Accordingly, the radio device  1  can communicates with one or more radio base stations  3  by using this radio terminal function  5 . The radio terminal function  5  includes a radio base station connector  11  and an association canceller  12 . 
         [0031]    The radio terminal association manager  6  manages association states of radio terminals  2  under control thereof. 
         [0032]    The communication buffer manager  7  manages empty states of communication buffers for communication with radio terminals  2  under control thereof. 
         [0033]    The DHCP server  8  offers IP addresses to radio terminals  2  under control thereof. 
         [0034]    The unused address manager  9  includes an IP address pool  10 . The unused address manager  9  executes a process to manage IP addresses determined to be unused in the communication system. The IP address pool  10  stores information on IP addresses which are used or likely to be used. The unused address manager  9  holds a database which indicates the captured state of the recognized IP address pool  10 . Note that in the following description, the IP address pool  10  will be sometimes denoted as “data base  10 ” or “captured database  10 ”. 
         [0035]    The radio base station connector  11  performs/makes a base station search, an association request and an IP offer request for/to a different radio base station  3 . 
         [0036]    The association canceller  12  cancels connection to a different radio base station  3  which is established for recognizing the IP address pool  10 , every time the connection is established. 
         [0037]    The terminal-base station function activating manager  13  activates and deactivates the radio base station function and the radio terminal function so that they separately operate. 
         [0038]    Next, a flow of processes in the communication system according to this exemplary embodiment will be described with reference to  FIGS. 1 ,  2 A and  2 B.  FIGS. 2A and 2B  are sequence diagrams showing the flow of processes in the communication system. 
         [0039]    Assume that a user powers up the radio device  1  in order to use it as a radio base station. When the radio device  1  is booted, the terminal-base station function activating manager  13  firstly activates the radio terminal function  5  (step S 101 ), thereby performing/making a base station search, an association request and an IP offer request for/to a different radio base station  3  which is provided in the vicinity of the radio device  1 . The offer of IP address is performed for recognizing an IP address pool in a neighboring network, and therefore the offer is immediately disconnected. This process is also similarly repeated with respect to other radio base stations (step S 102 ). As a result, the unused address manager  9  creates and holds the database  10  indicating the captured state of the IP address pool (step S 103 ). 
         [0040]    At this time, the terminal-base station function activating manager  13  deactivates the radio terminal function  5 , and activates the radio base station function  4  (step S 104 ). The radio base station function  4  accepts an association request originating from a different radio terminal  2 , and offers an unused IP address to the radio terminal  2  (step S 105 ). 
         [0041]    Assume that a policy is hastily changed such that the radio device  1  is simultaneously connected to a different network, wirelessly (step S 106 ). The terminal-base station function activating manager  13  activates the radio terminal function  5  separate from the active radio base station function  4  (step S 107 ), thereby performing a search for a neighboring radio base station  3 , associating with the neighboring radio base station  3 , receiving an offered IP address, and then canceling the association. This process is similarly repeated with respect to other radio base stations (step S 108 ). According to this process, the radio device  1  recognizes the IP address offered by a different radio base station  3 . The state of this IP address once becomes an unused state, since the association is canceled after the offer thereof. Meanwhile, the state of this IP address is determined to be likely to become a used state, since the radio base station  3  may offer this IP address again at a later time. Accordingly, in this exemplary embodiment, the IP address recognized in such a manner is recognized as address information which is likely to be used, and stored in the captured database  10  for the IP address pool to update the database  10 . 
         [0042]    The unused address manager  9  acquires the address information which is likely to be used from the updated captured database  10 , and checks whether or not the IP address pool which the radio base station function  4  has already offered to radio terminals  2  under control thereof is stored in the captured database  10  as IP addresses which are likely to be used (step S 109 ). 
         [0043]    There is no problem if the IP addresses which the radio base station function  4  has offered to radio terminals  2  under control thereof do not coincide with the IP addresses stored in the captured database  10  as the ones which are likely to be used. Therefore, the operation procedure is terminated at this timing. 
         [0044]    Meanwhile, if the IP addresses which the radio base station function  4  has offered to radio terminals  2  under control thereof coincide with the IP addresses stored in the captured database  10  as the ones which are likely to be used, the unused address manager  9  notifies the radio base station function  4  about new unused IP addresses (step S 110 ), and the radio base station function  4  checks communication buffers for communication with all associated radio terminals under control thereof and waits for the communication buffers to become empty (step S 111 ). 
         [0045]    After the communication buffers become empty, the radio base station function  4  cancels the association with all radio terminals and the offer of IP addresses (step S 112 ), and changes its configuration so as to subsequently perform DHCP offer using the unused IP addresses received at step S 110 . After that, all radio terminals, whose association has been canceled, are re-connected to the radio base station function  4  (step S 113 ). 
         [0046]    As described above, the radio device according to the first exemplary embodiment activates the radio terminal function  5  to receive the offer of IP addresses from other radio base stations  3 , thereby acquiring the IP address information which is likely to be used, and preferentially offering, by the radio base station function  4 , the IP address other than the acquired IP address to the radio terminal  2 . Therefore, it is possible to suppress that the IP address offered by the radio device is a duplication of the IP address offered by the different radio base station  3 . Particularly in the case where the radio base station relays packets between two IP networks, it is possible to solve the problem that IP addresses collide and thus routing cannot be normally performed. At this time, there are not caused secondary problems that information in a payload cannot be rewritten upon resolving the collision of addresses by use of NAT or NAPT, and that communication cannot be normally conducted if there is multi-stage address conversion configuration. 
       Second Exemplary Embodiment 
       [0047]      FIG. 3  shows a configuration of a communication system according to a second exemplary embodiment of the present invention. A radio device  21  includes a radio base station function  24 , a radio terminal function  25 , an unused address manager  29 , and a terminal-base station function activating manager  33 . Moreover, the radio base station function  24  includes a radio terminal association manager  26 , a communication buffer manager  27 , and a DHCP server  28 . The radio terminal function  25  includes a radio base station connector  31  and an association canceller  32 . The unused address manager  29  includes an IP address pool  30 . The radio device  21  is connected to one or more radio terminals  22  and one or more radio base stations  23  by using a specific radio technology, and the radio terminal  22  includes therein an IP address pool change requester  34 . 
         [0048]    The radio terminal association manager  26  manages association states of radio terminals  22  under control thereof. 
         [0049]    The communication buffer manager  27  manages empty states of communication buffers for communication with radio terminals  22  under control thereof. 
         [0050]    The DHCP server  28  offers IP addresses to radio terminals  22  under control thereof. 
         [0051]    The unused address manager  29  holds a database which indicates the captured state of the recognized IP address pool  30 . Note that in the following description, the IP address pool  30  will be sometimes denoted as “data base  30 ” or “captured database  30 ”. 
         [0052]    The radio base station connector  31  performs/makes a base station search, an association request and an IP offer request for/to a different radio base station  23 . 
         [0053]    The association canceller  32  cancels connection to a different radio base station  23  which is established for recognizing the IP address pool  30 , every time the connection is established. 
         [0054]    The terminal-base station function activating manager  33  activates and deactivates the radio base station function and the radio terminal function so that they separately operate. 
         [0055]    The IP address pool change requester  34  requests to change the IP address pool. 
         [0056]    Next, a flow of processes in the communication system according to the second exemplary embodiment will be described with reference to  FIGS. 3 ,  4 A and  4 B.  FIGS. 4A and 4B  are sequence diagrams showing the flow of processes in the communication system according to the second exemplary embodiment. 
         [0057]    Assume that a user powers up the radio device  21  in order to use it as a radio base station. When the radio device  21  is booted, the terminal-base station function activating manager  23  firstly activates the radio terminal function  25  (step S 201 ), thereby performing/making a base station search, an association request and an IP offer request for/to a different radio base station  23  which is provided in the vicinity of the radio device  21 . The offer of IP address is performed for recognizing an IP address pool in a neighboring network, and therefore the offer is immediately disconnected. This process is also similarly repeated with respect to other radio base stations (step S 202 ). As a result, the unused address manager  29  creates and holds the database  30  indicating the captured state of the IP address pool (step S 203 ). 
         [0058]    At this time, the terminal-base station function activating manager  23  deactivates the radio terminal function  25 , and activates the radio base station function  24  (step S 204 ). The radio base station function  24  accepts an association request originating from a different radio terminal  22 , and offers an unused IP address to the radio terminal  22  (step S 205 ). 
         [0059]    Assume that there is caused, in the above different associated radio terminal  22 , a demand for starting multi access for which a plurality of IP addresses are simultaneously used, or for changing the used IP address. In the case of the multi access, since the used IP address may also be a duplication of an IP address offered by a different AP to which the radio terminal  22  is connected, it is necessary to perform procedures for address change. Therefore, the IP address pool change requester  34  in the different radio terminal  22  issues a change request to the radio base station function  24  in the radio device  21  (step S 206 ). The terminal-base station function activating manager  33  activates the radio terminal function  25  separate from the active radio base station function  24  (step S 207 ), thereby performing a search for a neighboring radio base station  23 , associating with the neighboring radio base station  23 , receiving an offered IP address, and then canceling the association. This process is similarly repeated with respect to other radio base stations (step S 208 ). 
         [0060]    As a result, the unused address manager  29  updates the captured database  30  for the IP address pool, and checks whether or not the already offered IP address pool is used by other IP address pools (step S 209 ). There is no problem if the IP addresses pool is not used. Therefore, the operation procedure is terminated at this timing. Meanwhile, if the IP address pool has already been used, the unused address manager  29  notifies the radio base station function  24  about new unused IP addresses (step S 210 ), and the radio base station function  24  checks communication buffers for communication with all associated radio terminals under control thereof and waits for the communication buffers to become empty (step S 211 ). 
         [0061]    After the communication buffers become empty, the radio base station function  24  cancels the association with all radio terminals and the offer of IP addresses (step S 212 ), and changes its configuration so as to subsequently perform DHCP offer using the unused IP addresses received at step S 210 . Moreover, the terminal-base station function activating manager  33  deactivates the radio terminal function  25  (step S 213 ). After that, all radio terminals, whose association has been canceled, are re-connected to the radio base station function  24  (step S 214 ). 
         [0062]    Thus, in the case where the radio terminal simultaneously associates with two radio base stations, it is possible to solve the problem that two IP addresses collide and thus packets cannot be normally transmitted. 
       Third Exemplary Embodiment 
       [0063]      FIG. 5  shows a configuration of a communication system according to a third exemplary embodiment of the present invention. This configuration is based on the configuration of the communication system according to the first exemplary embodiment. A Wi-Fi P2P Device  41  includes a Wi-Fi P2P Group Owner  44 , a Wi-Fi P2P Client  45 , an unused address manager  49 , and a terminal-base station function activating manager  53 . The Wi-Fi P2P Group Owner  44  includes a radio terminal association manager  46 , a communication buffer manager  47 , and a DHCP server  48 . The Wi-Fi P2P Client  45  includes a radio base station connector  51  and an association canceller  52 . The unused address manager  49  includes an IP address pool  50 . The Wi-Fi P2P Device  41  is connected to one or more Wi-Fi terminals  42  and one or more access points (hereinafter, APs)  43 , by using a specific radio technology. The Wi-Fi P2P Device  41  is a radio device which is compatible with the Wi-Fi P2P. The Wi-Fi terminal  42  and the AP  43  may or may not be compatible with the Wi-Fi P2P. 
         [0064]    The radio terminal association manager  46  manages association states of Wi-Fi terminals  42  under control thereof. 
         [0065]    The communication buffer manager  47  manages empty states of communication buffers for communication with Wi-Fi terminals  42  under control thereof. 
         [0066]    The DHCP server  48  offers IP addresses to Wi-Fi terminals  42  under control thereof. 
         [0067]    The unused address manager  49  holds a database which indicates the captured state of the recognized IP address pool  50 . Note that in the following description, the IP address pool  50  will be sometimes denoted as “data base  50 ” or “captured database  50 ”. 
         [0068]    The radio base station connector  51  performs/makes a base station search, an association request and an IP offer request for/to a different AP  43 . 
         [0069]    The association canceller  52  cancels connection to a different AP  43  which is established for recognizing the IP address pool  50 , every time the connection is established. 
         [0070]    The terminal-base station function activating manager  53  activates and deactivates the Wi-Fi P2P Group Owner  64  and the WiFi P2P Client  65  so that they separately operate. 
         [0071]    Next, a flow of processes in the communication system according to the third exemplary embodiment will be described with reference to  FIGS. 5 ,  6 A and  6 B.  FIGS. 6A and 6B  are sequence diagrams showing the flow of processes in the communication system according to the third exemplary embodiment. 
         [0072]    Assume that a user powers up the Wi-Fi P2P Device  41  in order to use it as a radio base station. When the Wi-Fi P2P Device  41  is booted, the terminal-base station function activating manager  53  firstly activates the Wi-Fi P2P Client  45  (step S 301 ), thereby performing/making a base station search, an association request and an IP offer request for/to a different AP  43  which is provided in the vicinity of the Wi-Fi P2P Device  41 . The offer of IP address is performed for recognizing an IP address pool in a neighboring network, and therefore the offer is immediately disconnected. This process is also similarly repeated with respect to other APs (step S 302 ). As a result, the unused address manager  49  creates and holds the database  50  indicating the captured state of the IP address pool (step S 303 ). 
         [0073]    At this time, the terminal-base station function activating manager  53  deactivates the Wi-Fi P2P Client  45 , and activates the Wi-Fi P2P Group Owner  44  (step S 304 ). The Wi-Fi P2P Group Owner  44  accepts an association request originating from a different Wi-Fi terminal  42 , and offers an unused IP address to the Wi-Fi terminal  42  (step S 305 ). 
         [0074]    Assume that a policy is hastily changed such that the Wi-Fi P2P Device  41  is simultaneously connected to a different network, wirelessly (step S 306 ). The terminal-base station function activating manager  53  activates the Wi-Fi P2P Client  45  separate from the active Wi-Fi P2P Group Owner  44  (step S 307 ), thereby performing a search for a neighboring AP  43 , associating with the neighboring AP  43 , receiving an offered IP address, and then canceling the association. This process is similarly repeated with respect to other APs (step S 308 ). 
         [0075]    As a result, the unused address manager  49  updates the captured database  50  for the IP address pool, and checks whether or not the already offered IP address pool is used by other IP address pools (step S 309 ). There is no problem if the IP addresses pool is not used. Therefore, the operation procedure is terminated at this timing. Meanwhile, if the IP address pool has already been used, the unused address manager  59  notifies the Wi-Fi P2P Group Owner  44  about new unused IP addresses (step S 310 ), and the Wi-Fi P2P Group Owner  44  checks communication buffers for communication with all Wi-Fi terminals under control thereof and waits for the communication buffers to become empty (step S 311 ). 
         [0076]    After the communication buffers become empty, the Wi-Fi P2P Group Owner  44  cancels the association with all Wi-Fi terminals and the offer of IP addresses (step S 312 ), and changes its configuration so as to subsequently perform DHCP offer using the unused IP addresses received at step S 310 . After that, all Wi-Fi terminals, whose association has been canceled, are re-connected to the Wi-Fi P2P Group Owner  44  (step S 313 ). 
         [0077]    As described above, the radio device according to the third exemplary embodiment activates the Wi-Fi P2P Client  45  to receive the offer of IP addresses from other APs  43 , thereby acquiring the IP address information which is likely to be used, and preferentially offering, by the Wi-Fi P2P Group Owner  44 , the IP address other than the acquired IP address to the Wi-Fi terminal  42 . Therefore, it is possible to suppress that the IP address offered by the radio device is a duplication of the IP address offered by the different AP  43 . Particularly in the case where the radio base station relays packets between two IP networks, it is possible to solve the problem that IP addresses collide and thus routing cannot be normally performed. At this time, there are not caused secondary problems that information in a payload cannot be rewritten upon resolving the collision of addresses by use of NAT or NAPT, and that communication cannot be normally conducted if there is multi-stage address conversion configuration. 
       Fourth Exemplary Embodiment 
       [0078]      FIG. 7  shows a configuration of a communication system according to a fourth exemplary embodiment of the present invention. This configuration is based on the configuration of the communication system according to the second exemplary embodiment. A Wi-Fi P2P Device  61  includes a Wi-Fi P2P Group Owner  64 , a Wi-Fi P2P Client  65 , an unused address manager  69 , and a terminal-base station function activating manager  73 . Moreover, the Wi-Fi P2P Group Owner  64  includes a radio terminal association manager  66 , a communication buffer manager  67 , and a DHCP server  68 . The Wi-Fi P2P Client  65  includes a radio base station connector  71  and an association canceller  72 . The unused address manager  69  includes an IP address pool  70 . The Wi-Fi P2P Device  61  is connected to one or more Wi-Fi P2P Clients  62  and one or more APs  63  by using a specific radio technology, and the Wi-Fi P2P Client  62  includes therein an IP address pool change requester  74 . The Wi-Fi P2P Device  61  and the Wi-Fi P2P Client  62  are radio devices which are compatible with the Wi-Fi P2P. The AP  63  may or may not be compatible with the Wi-Fi P2P. 
         [0079]    The radio terminal association manager  66  manages association states of Wi-Fi terminals  62  under control thereof. 
         [0080]    The communication buffer manager  67  manages empty states of communication buffers for communication with Wi-Fi terminals  62  under control thereof. 
         [0081]    The DHCP server  68  offers IP addresses to Wi-Fi terminals  62  under control thereof. 
         [0082]    The unused address manager  69  holds a database which indicates the captured state of the recognized IP address pool  70 . Note that in the following description, the IP address pool  70  will be sometimes denoted as “data base  70 ” or “captured database  70 ”. 
         [0083]    The radio base station connector  71  performs/makes a base station search, an association request and an IP offer request for/to a different AP  63 . 
         [0084]    The association canceller  72  cancels connection to a different AP  63  which is established for recognizing the IP address pool  70 , every time the connection is established. 
         [0085]    The terminal-base station function activating manager  73  activates and deactivates the Wi-Fi P2P Group Owner  64  and the WiFi P2P Client  65  so that they separately operate. 
         [0086]    The IP address pool change requester  74  requests to change the IP address pool. 
         [0087]    Next, a flow of processes in the communication system according to the fourth exemplary embodiment will be described with reference to  FIGS. 7 ,  8 A and  8 B.  FIGS. 8A and 8B  are sequence diagrams showing the flow of processes in the communication system according to the fourth exemplary embodiment. 
         [0088]    Assume that a user powers up the Wi-Fi P2P Device  61  in order to use it as a radio base station. When the Wi-Fi P2P Device  61  is booted, the terminal-base station function activating manager  73  firstly activates the Wi-Fi P2P Client  65  (step S 401 ), thereby performing/making a base station search, an association request and an IP offer request for/to a different AP  63  which is provided in the vicinity of the Wi-Fi P2P Device  61 . The offer of IP address is performed for recognizing an IP address pool in a neighboring network, and therefore the offer is immediately disconnected. This process is also similarly repeated with respect to other APs (step S 402 ). As a result, the unused address manager  69  creates and holds the database  70  indicating the captured state of the IP address pool (step S 403 ). 
         [0089]    At this time, the terminal-base station function activating manager  73  deactivates the Wi-Fi P2P Client  65 , and activates the Wi-Fi P2P Group Owner  64  (step S 404 ). The Wi-Fi P2P Group Owner  64  accepts an association request originating from a different Wi-Fi P2P Client  62 , and offers an unused IP address to the Wi-Fi P2P Client  62  (step S 405 ). 
         [0090]    Assume that there is caused, in the above different associated Wi-Fi P2P Client  62 , a demand for starting multi access for which a plurality of IP addresses are simultaneously used, or for changing the used IP address. In the case of the multi access, since the used IP address may also be a duplication of an IP address offered by a different AP to which the Wi-Fi P2P Client  62  is connected, it is necessary to perform procedures for address change. Therefore, the IP address pool change requester  74  in the different Wi-Fi P2P Client  62  issues a change request to the Wi-Fi P2P Group Owner  64  (step S 406 ). The terminal-base station function activating manager  73  activates the Wi-Fi P2P Client  65  separate from the active Wi-Fi P2P Group Owner  64  (step S 407 ), thereby performing a search for a neighboring AP  63 , associating with the neighboring AP  63 , receiving an offered IP address, and then canceling the association. This process is similarly repeated with respect to other APs (step S 408 ). 
         [0091]    As a result, the unused address manager  69  updates the captured database  70  for the IP address pool, and checks whether or not the already offered IP address pool is used by other IP address pools (step S 409 ). There is no problem if the IP addresses pool is not used. Therefore, the operation procedure is terminated at this timing. Meanwhile, if the IP address pool has already been used, the unused address manager  69  notifies the Wi-Fi P2P Group Owner  64  about new unused IP addresses (step S 410 ), and the Wi-Fi P2P Group Owner  64  checks communication buffers for communication with all associated Wi-Fi P2P Clients under control thereof and waits for the communication buffers to become empty (step S 411 ). 
         [0092]    After the communication buffers become empty, the Wi-Fi P2P Group Owner  64  cancels the association with all Wi-Fi P2P Clients and the offer of IP addresses (step S 412 ), and changes its configuration so as to subsequently perform DHCP offer using the unused IP addresses received at step S 410 . Moreover, the terminal-base station function activating manager  73  deactivates the Wi-Fi P2P Client  65  (step S 413 ). After that, all Wi-Fi P2P Clients, whose association has been canceled, are re-connected to the Wi-Fi P2P Group Owner  64  (step S 414 ). 
         [0093]    Thus, in the case where the radio terminal simultaneously associates with two radio base stations, it is possible to solve the problem that two IP addresses collide and thus packets cannot be normally transmitted. 
         [0094]    Note that the present invention is not limited to the above-mentioned exemplary embodiments, and can be modified as appropriate in the range not departing from the gist thereof. 
         [0095]    The whole or part of the exemplary embodiments disclosed above can be described as, but not limited to, the following supplementary notes. 
         [0096]    (Supplementary Note 1) 
         [0097]    A radio device that includes a radio base station function and a radio terminal function, the radio device comprising: 
         [0098]    address information acquisition means for transmitting, by the radio terminal function, an address offer request to a different radio base station, and for acquiring and storing address information allocated to the radio device in response to the request; and 
         [0099]    address information offer means for preferentially offering to a radio terminal, by the radio base station function, address information other than the address information stored in the address information acquisition means. 
         [0100]    (Supplementary Note 2) 
         [0101]    The radio device according to Supplementary note 1, wherein the address information acquisition means is configured to: 
         [0102]    start a process for acquiring the address information, the process being triggered when the radio terminal function is wirelessly connected to a different radio base station. 
         [0103]    (Supplementary Note 3) 
         [0104]    The radio device according to Supplementary note 1, wherein the address information acquisition means is configured to: 
         [0105]    start a process for acquiring the address information, the process being triggered when a change request for the address information is received from a radio terminal to which the radio base station function is connected. 
         [0106]    (Supplementary Note 4) 
         [0107]    The radio device according to any one of Supplementary notes 1 to 3, wherein the address information acquisition means is configured to: 
         [0108]    start a process for acquiring the address information, the process being triggered when the radio device is booted. 
         [0109]    (Supplementary Note 5) 
         [0110]    The radio device according to any one of Supplementary notes 1 to 4, wherein the address information acquisition means is configured to: 
         [0111]    transmit an address offer request to a different radio base station; 
         [0112]    acquire address information allocated to the radio device in response to the request to associate with the radio base station; and 
         [0113]    then cancel the association. 
         [0114]    (Supplementary Note 6) 
         [0115]    The radio device according to any one of Supplementary notes 1 to 5, wherein the address information offer means is configured to: 
         [0116]    disconnect, upon offering address information to a radio terminal with which the radio base station function communicates, communication with the radio terminal after a communication buffer for the communication with the radio terminal becomes empty; and 
         [0117]    perform the offer of the address information. 
         [0118]    (Supplementary Note 7) 
         [0119]    An address determination method for a radio terminal, comprising the steps of: 
         [0120]    transmitting an address offer request to a different radio base station; 
         [0121]    receiving allocated address information from the different radio base station; and 
         [0122]    preferentially offering, upon offering an address to a radio terminal, address information other than the received address information to the radio terminal. 
         [0123]    (Supplementary Note 8) 
         [0124]    The address determination method according to Supplementary note 7, wherein the address offer request is transmitted by using, as a trigger, wireless connection to a different radio base station. 
         [0125]    (Supplementary Note 9) 
         [0126]    The address determination method according to Supplementary note 7, wherein the address offer request is transmitted by using, as a trigger, reception of a change request for the address information from a connected radio terminal. 
         [0127]    (Supplementary Note 10) 
         [0128]    The address determination method according to any one of Supplementary notes 7 to 9, wherein the address offer request is transmitted by using a boot as a trigger. 
         [0129]    (Supplementary Note 11) 
         [0130]    The address determination method according to any one of Supplementary notes 7 to 10, including the step of cancelling association with the radio base station after the step of receiving the allocated address information. 
         [0131]    (Supplementary Note 12) 
         [0132]    The address determination method according to any one of Supplementary notes 7 to 11, wherein upon offering address information to a radio terminal in communication, communication with the radio terminal is disconnected after a communication buffer for the communication with the radio terminal becomes empty, and then the offer of the address information is performed. 
         [0133]    (Supplementary Note 13) 
         [0134]    A communication system comprising: 
         [0135]    a radio device that includes a radio base station function and a radio terminal function; and 
         [0136]    a radio terminal that can communicate with the radio device, 
         [0137]    wherein the radio terminal includes means for outputting a change request for address information upon performing multi access to a radio base station other than the radio device, 
         [0138]    wherein the radio device comprises: 
         [0139]    address information acquisition means for transmitting an address offer request to a different radio base station, the address offer request being triggered when a change request for address information is received from a radio terminal to which the radio base station function is connected, and for acquiring and storing address information allocated to the radio device in response to the address offer request; and 
         [0140]    address information offer means for preferentially offering to a radio terminal, by the radio base station function, address information other than the address information stored in the address information acquisition means. 
         [0141]    (Supplementary Note 14) 
         [0142]    A radio terminal that outputs, to a first radio base station, a change request for address information offered by the first radio base station, when the radio terminal performs multi access to a second radio base station that differs from the first radio base station, while communicating with the first radio base station. 
         [0143]    This application is based upon and claims the benefit of priority from Japanese patent application No. 2012-082671, filed on Mar. 30, 2012, the disclosure of which is incorporated herein in its entirety by reference. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1 ,  21  RADIO DEVICE 
           2 ,  22  RADIO TERMINAL 
           3 ,  23  RADIO BASE STATION 
           4 ,  24  RADIO BASE STATION FUNCTION 
           5 ,  25  RADIO TERMINAL FUNCTION 
           6 ,  26  RADIO TERMINAL ASSOCIATION MANAGER 
           7 ,  27  COMMUNICATION BUFFER MANAGER 
           8 ,  28  DHCP SERVER 
           9 ,  29  UNUSED ADDRESS MANAGER 
           10 ,  30  IP ADDRESS POOL 
           11 ,  31  RADIO BASE STATION CONNECTOR 
           12 ,  32  ASSOCIATION CANCELLER 
           13 ,  33  TERMINAL-BASE STATION FUNCTION ACTIVATING MANAGER 
           34  IP ADDRESS POLL CHANGE REQUESTER 
           41 ,  61  Wi-Fi P2P Device 
           42 ,  62  Wi-Fi TERMINAL 
           43 ,  63  ACCESS POINT 
           44 ,  64  Group Owner 
           45 ,  65  Wi-Fi P2P Client 
           46 ,  66  RADIO TERMINAL ASSOCIATION MANAGER 
           47 ,  67  COMMUNICATION BUFFER MANAGER 
           48 ,  68  DHCP SERVER 
           49 ,  69  UNUSED ADDRESS MANAGER 
           50 ,  70  IP ADDRESS POOL 
           51 ,  71  RADIO BASE STATION CONNECTOR 
           52 ,  72  ASSOCIATION CANCELLER 
           53 ,  73  TERMINAL-BASE STATION FUNCTION ACTIVATING MANAGER 
           74  IP ADDRESS POLL CHANGE REQUESTER