Abstract:
A control apparatus including: a network interface configured to communicate with a plurality of networks including a first network and coupling each other via a plurality of relaying apparatus, the first network including a communication apparatus that receives a packet addressed to a first address provided for a specific terminal from the first network and transmits the received packet to a second address provided for the specific terminal from a second network in the plurality of networks when the specific terminal couples to the second network, and a processor configured to: identify at least one relaying apparatus lying between the communication apparatus and the specific terminal that couples to the second network from among the plurality of relaying apparatus, and transmit at least one control packet controlling the at least one relaying apparatus so that a packet addressed to the first address is relayed to the second address.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2014-094568 filed on May 1, 2014, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The present embodiments discussed herein are related to a forwarding method of a packet in a network that includes a plurality of apparatus. 
       BACKGROUND 
       [0003]    If a mobile internet protocol (IP) technology is used, then while an IP terminal coupled to an IP network keeps a communication session of an application in a coupled state, the coupling position of the IP terminal to the IP network can be changed.  FIG. 1  is a diagrammatic view depicting an example of communication that uses a mobile IP technology. The network depicted in  FIG. 1  includes subnetworks  5   a  to  5   c . The subnetwork  5   a  is a home subnetwork of a mobile terminal  10  and includes a home agent (HA)  3 . It is assumed that the subnetworks  5   a  to  5   c  are coupled in such a manner as depicted in  FIG. 1  by relaying apparatus  2   a  to  2   d . The mobile terminal  10  has a home address (HoA) allocated thereto. The home address is an unchanged IP address allocated to the mobile terminal  10  and is an address that belongs to the home subnetwork. When the mobile terminal  10  moves to a subnetwork such as the subnetwork  5   c  other than the home subnetwork (arrow mark A), the mobile terminal  10  acquires or generates a care-of address (CoA) that belongs to the subnetwork of the destination of the movement. Thereafter, the mobile terminal  10  registers the care-of address into the home agent  3  (arrow mark B). 
         [0004]    If a packet P 1  destined for the mobile terminal  10  is transmitted from a communication apparatus  15  when the mobile terminal  10  is not positioned in the home subnetwork, then the home agent  3  receives the packet P 1  by proxy. The destination address of the packet P 1  is the home address of the mobile terminal  10 , and the transmission source address is the address (CN) of the communication apparatus  15 . It is to be noted that, in the header of packets in  FIG. 1 , the transmission source address is represented as SA (Source Address) and the destination address is represented as DA (Destination Address). 
         [0005]    The home agent  3  assigns, to the packet P 1 , a header that designates the CoA of the mobile terminal  10  as the destination address and the address (HA) of the home agent  3  as the transmission source address to generate a packet P 2 . The home agent  3  forwards the packet P 2  to the mobile terminal  10  in the subnetwork  5   c . The header used for encapsulation is hereinafter referred to as outer header, and the header in the encapsulated packet is hereinafter referred to as inner header. 
         [0006]    On the other hand, the mobile terminal  10  generates a packet P 3  as a response to the packet P 2  and transmits the packet P 3  to the home agent  3 . Also the packet P 3  is in an encapsulated state, and the destination address of the outer header is designated to the home agent  3 , and the transmission source address is designated to the CoA of the mobile terminal  10 . In the inner header of the packet P 3 , the destination address is an address of the communication apparatus  15  and the transmission source address is the HoA of the mobile terminal  10 . When the home agent  3  receives the packet P 3 , it performs decapsulation of the packet P 3  to remove the outer header from the packet P 3  and transmits a resulting packet P 4  to the communication apparatus  15 . 
         [0007]    In recent years, as a technology for performing flexible packet forwarding control, OpenFlow is occasionally used. The relaying apparatus  2   a  to  2   d  depicted in  FIG. 1  may each be an apparatus used in OpenFlow (OpenFlow Switch, OFS). In this case, an OpenFlow controller (OFC)  4  notifies the relaying apparatus  2   a  to  2   d  of a condition for the identification of a flow of a control target and a process to be performed for a packet of the flow that satisfies the condition. The relaying apparatus  2   a  to  2   d  perform processes in accordance with the notification from the OFC  4 . 
         [0008]    As a related art, a mobile router has been proposed which includes a communication attribute table indicating route selection information in an associated relationship with flow conditions and a forwarding processing unit. The mobile router determines, in response to the route selection information, an optimum route to an apparatus of the communication destination as a forwarding route for a received packet and which one of mobile tunnels set with respect to a home agent is to be used (for example, Patent Document 1). Also a home agent has been proposed which distributes, when a mobile terminal moves, an IP tunnel setting that designates transmission destination information to be used for transmission to the mobile terminal to a plurality of IP tunnel routers (for example, Patent Document 2). When each IP tunnel router receives a packet whose transmission destination is the mobile terminal, the IP tunnel router forwards the packet without by way of the home agent on the basis of the IP tunnel setting information. Also a system has been proposed wherein a home agent selects a virtual home agent that provides a service better than the service provided by the home agent to a mobile terminal and the virtual home agent is notified of the CoA of the mobile terminal (for example, Patent Document 3). 
       PRIOR ART DOCUMENT 
     Patent Document 
       [0000]    
       
         [Patent Document 1] Japanese Laid-open Patent Publication No. 2006-50035 
         [Patent Document 2] International Publication No. 03/071749 
         [Patent Document 3] Japanese National Publication of International Patent Application No. 2013-532920 
       
     
       Non-Patent Document  
       [0000]    
       
         [Non-Patent Document 1] “RFC3220 IP Mobility Support for IPv4,” [online], January 2002, IETF, [searched on Apr. 2, 2014], &lt;http://www.ietf.org/rfc/rfc3220.txt&gt; 
         [Non-Patent Document 2] “RFC3775 Mobility Support in IPv6,” [online], June 2004, IETF, [searched on Apr. 2, 2014], &lt;http://www.ietf.org/rfc/rfc3775.txt&gt; 
         [Non-Patent Document 3] “OpenFlow Switch Specification Version 1.4.0,” [online], Oct. 14, 2013, OPEN NETWORKING FOUNDATION, [searched on Apr. 2, 2014], &lt;https://www.opennetworking.org/images/stories/downloads/sdn-resources/onf-specifications/openflow/openflow-spec-v1.4.0.pdf&gt; 
       
     
       SUMMARY 
       [0015]    According to an aspect of the invention, a control apparatus includes a network interface configured to communicate with a plurality of networks including a first network, the plurality of networks coupling each other via a plurality of relaying apparatus, the first network including a communication apparatus that receives a packet addressed to a first address and transmits the received packet to a second address, the first address being provided for a specific terminal from the first network, the second address being provided for the specific terminal from a second network in the plurality of networks when the specific terminal couples to the second network, and a processor configured to: identify at least one relaying apparatus from among the plurality of relaying apparatus, the at least one relaying apparatus lying between the communication apparatus and the specific terminal that couples to the second network, and transmit at least one control packet to the at least one relaying apparatus, the at least one control packet controlling the at least one relaying apparatus so that a packet addressed to the first address is relayed to the second address. 
         [0016]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0017]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a diagrammatic view depicting an example of communication that uses a mobile IP technology; 
           [0019]      FIG. 2  is a diagrammatic view depicting an example of a communication method according to an embodiment; 
           [0020]      FIG. 3  is a diagrammatic view depicting an example of a network; 
           [0021]      FIG. 4  is a block diagram depicting an example of a configuration of a conversion apparatus; 
           [0022]      FIG. 5  is a block diagram depicting an example of a configuration of a control apparatus; 
           [0023]      FIG. 6  is a view depicting an example of a topology information table; 
           [0024]      FIG. 7  is a view depicting an example of a coupling information table; 
           [0025]      FIG. 8  is a view depicting an example of a flow table; 
           [0026]      FIG. 9  is a block diagram depicting an example of a configuration of a relaying apparatus; 
           [0027]      FIG. 10  is a view depicting an example of a flow table held by a relaying apparatus; 
           [0028]      FIG. 11  is a block diagram depicting an example of a hardware configuration; 
           [0029]      FIG. 12  is a sequence diagram depicting an example of processing performed when a mobile terminal moves; 
           [0030]      FIG. 13  is a view depicting an example of authentication information; 
           [0031]      FIG. 14  is a view depicting an example of position registration information; 
           [0032]      FIG. 15  is a view depicting an example of a change of a flow table; 
           [0033]      FIG. 16  is a view depicting an example of a relaying apparatus information table; 
           [0034]      FIG. 17  is a view depicting an example of a flow table after updated; 
           [0035]      FIG. 18  is a sequence diagram depicting an example of processing performed upon transmission and reception of a packet to and from a mobile terminal; 
           [0036]      FIG. 19  is a flow chart illustrating an example of processing of a conversion apparatus; 
           [0037]      FIG. 20  is a flow chart illustrating an example of processing of a control apparatus; 
           [0038]      FIG. 21  is a flow chart illustrating an example of processing of a relaying apparatus; 
           [0039]      FIG. 22  is a block diagram depicting an example of a configuration of a conversion apparatus according to a second embodiment; 
           [0040]      FIG. 23  is a sequence diagram depicting an example of processing performed when a flow table is updated using a transmission process of a packet destined for a mobile terminal as a trigger; 
           [0041]      FIG. 24  is a sequence diagram depicting an example of processing performed when a flow table is updated using a transmission process of a packet from a mobile terminal as a trigger; 
           [0042]      FIG. 25  is a flow chart illustrating an example of processing of a conversion apparatus; 
           [0043]      FIG. 26  is a block diagram depicting an example of a configuration of a control apparatus according to a third embodiment; 
           [0044]      FIG. 27  is a view depicting an example of a flow table; 
           [0045]      FIG. 28  is a view depicting an example of a flow table held by a relaying apparatus; 
           [0046]      FIG. 29  is a block diagram depicting an example of a configuration of a conversion apparatus according to the third embodiment; 
           [0047]      FIG. 30  is a sequence diagram depicting an example of processing performed upon change of a flow table in the third embodiment; 
           [0048]      FIG. 31  is a view depicting an example of a change of a flow table; 
           [0049]      FIG. 32  is a view depicting an example of a flow table after updated; and 
           [0050]      FIG. 33  is a flow chart illustrating an example of processing of a control apparatus. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0051]    When a mobile terminal performs communication on a subnetwork other than a home subnetwork, since a data packet to be transmitted to or received by the mobile terminal is forwarded through a home agent, the data packet has an encapsulated form. Accordingly, in comparison with an alternative case in which a packet is transmitted without being encapsulated, the data amount to be transmitted and received increases by an amount of data of a header used for packet forwarding along a path between the home subnetwork and the subnetwork of the destination of the movement. Whichever one of the technologies described as related art hereinabove is used, the problem of the transmission amount by encapsulation remains unsolved. For example, with the method of determining whether or not the forwarding route of a packet is to include the home agent in accordance with route selection information, a flow that is not set as route selection information is not forwarded without passing the home agent. 
         [0052]    The present embodiments disclosed herein are effective to forward a packet without encapsulation to a mobile terminal positioned in a subnetwork of a destination of movement of the mobile terminal. 
         [0053]      FIG. 2  is a diagrammatic view depicting an example of a communication method according to an embodiment. Also  FIG. 2  depicts an example wherein a network includes subnetworks  5   a  to  5   c  and the subnetwork  5   a  is a home network of a mobile terminal  10 . Furthermore, also in the example of  FIG. 2 , it is assumed that the mobile terminal  10  moves to the subnetwork  5   c . Also it is assumed that a communication apparatus  15  that communicates with the mobile terminal  10  is positioned in the subnetwork  5   b . Further, it is assumed that a control apparatus  50  uses a technology such as OpenFlow to control the behavior of relaying apparatus  80  ( 80   a  to  80   d ) in the network. 
         [0054]    A conversion apparatus  20  (may referred to as a communication apparatus) is provided in the subnetwork  5   a . It is assumed that the conversion apparatus  20  operates as a home agent of the mobile terminal  10 . When the mobile terminal  10  moves to the subnetwork  5   c , the mobile terminal  10  issues a request for position registration to the conversion apparatus  20 . After the position registration of the mobile terminal  10  is completed, the conversion apparatus  20  notifies the control apparatus  50  of a care-of address (CoA) and a home address (HoA) allocated to the mobile terminal  10  (step S 1 ). 
         [0055]    It is assumed that the control apparatus  50  stores in advance therein information representative of a topology in the network and coupling destinations of ports of each relaying apparatus  80 . Further, the control apparatus  50  stores therein also information of a flow table used by each relaying apparatus  80 . In the following description, it is assumed that the flow table is information which associates conditions for determination of an output port for a packet of a forwarding target and output ports with each other. 
         [0056]    If the control apparatus  50  is notified of the care-of address of the mobile terminal  10 , then the control apparatus  50  uses the care-of address to specify the subnetwork in which the mobile terminal  10  is positioned. Then, the control apparatus  50  selects those relaying apparatus  80  which are included in a route from the conversion apparatus  20  to the mobile terminal  10 . In the example of  FIG. 2 , the control apparatus  50  determines that the relaying apparatus  80   a ,  80   c , and  80   d  are included in the route from the conversion apparatus  20  to the mobile terminal  10 . 
         [0057]    The control apparatus  50  requests the selected relaying apparatus  80  to set an output port for a packet destined for the home address of the mobile terminal  10  to a port that is used when a packet is forwarded to an apparatus in the subnetwork  5   c  (steps S 2  to S 4 ). For example, it is assumed that the relaying apparatus  80   a  outputs a packet destined for an apparatus in the subnetwork  5   c  to the port  3  thereof. In this case, the control apparatus  50  requests the relaying apparatus  80   a  to write it into the flow table of the relaying apparatus  80   a  that a packet destined for the home address of the mobile terminal  10  is to be outputted from the port  3  of the relaying apparatus  80   a . A similar process is performed also for the relaying apparatus  80   c  and  80   d.    
         [0058]    After change of the flow tables in the relaying apparatus  80   a ,  80   c , and  80   d  comes to an end, it is assumed that a packet P 5  destined for the mobile terminal  10  is transmitted from the communication apparatus  15 . The destination of the packet P 5  is the home address of the mobile terminal  10 . The relaying apparatus  80   b  forwards the packet P 5  to the relaying apparatus  80   c  in accordance with the flow table thereof (step S 5 ). 
         [0059]    The relaying apparatus  80   c  receives the packet P 5  and searches the flow table to specify an output port for the packet P 5 . In step S 3 , it is recorded in the flow table that a packet destined for the home address of the mobile terminal  10  is to be outputted from a port same as the port from which a packet destined for an apparatus in the subnetwork  5   c  is to be outputted. Therefore, the relaying apparatus  80   c  forwards the packet P 5  to the relaying apparatus  80   d  (step S 6 ). 
         [0060]    Upon reception of the packet P 5 , the relaying apparatus  80   d  operates similarly to the relaying apparatus  80   c , and therefore, the relaying apparatus  80   d  forwards the packet P 5  to the subnetwork  5   c  (step S 7 ). Therefore, the mobile terminal  10  can receive the packet P 5 . 
         [0061]    In this manner, with the method according to the present embodiment, after movement of the mobile terminal  10 , the control apparatus  50  acquires the care-of address and the home address of the mobile terminal  10  in an associated relationship with each other from the conversion apparatus  20  and changes the setting of the relaying apparatus  80  in the route from the conversion apparatus  20  to the mobile terminal  10 . Therefore, even if a packet transmitted from the communication apparatus  15  to the mobile terminal  10  is not in an encapsulated state, the packet is transmitted to the mobile terminal  10 . 
         [0062]    &lt;Example of the Configuration of the Network and the Apparatus&gt; 
         [0063]      FIG. 3  is a diagrammatic view depicting an example of a network. The following description is given taking communication in the network depicted in  FIG. 3  as an example. The network depicted in  FIG. 3  includes subnetworks  5   a  to  5   c , and communication between the subnetworks is relayed by relaying apparatus  80   a  to  80   d . It is assumed that the subnetworks have subnetwork addresses allocated thereto such that 10.0.10.0/24 is allocated to the subnetwork  5   a ; 10.0.20.0/24 is allocated to the subnetwork  5   b ; and 10.0.30.0/24 is allocated to the subnetwork  5   c . Further, it is assumed that the address allocated to the conversion apparatus  20  is 10.0.10.1 and the home address (HoA) of the mobile terminal  10  is 10.0.10.10. Meanwhile, since the mobile terminal  10  moves to the subnetwork  5   c , it is assumed that the care-of address (CoA) allocated to the mobile terminal  10  is 10.0.30.10. 
         [0064]    The control apparatus  50  can change the substance of a relaying process to be performed by the relaying apparatus  80   a  to  80   d  by transmitting a control packet to each of the relaying apparatus  80   a  to  80   d . Further, the control apparatus  50  transmits and receives a control packet to and from the conversion apparatus  20 . In the example of  FIG. 3 , a line used for transmission and reception of a control packet is indicated by a broken line, and a line used for transmission and reception of a data packet is indicated by a solid line. A port number of each relaying apparatus  80  is indicated by a square including a numeral. For example, the relaying apparatus  80   a  is coupled to the subnetwork  5   a  through the port  2  thereof and coupled to the relaying apparatus  80   c  through the port  1  thereof. The relaying apparatus  80   b  is coupled to the subnetwork  5   b  through the port  1  thereof and coupled to the relaying apparatus  80   c  through the port  2  thereof. Further, the relaying apparatus  80   d  is coupled to the subnetwork  5   c  through the port  2  thereof and coupled to the relaying apparatus  80   c  through the port  1  thereof. The port  1  of the relaying apparatus  80   c  is coupled to the relaying apparatus  80   b ; the port  2  is coupled to the relaying apparatus  80   a ; and the port  3  is coupled to the relaying apparatus  80   d.    
         [0065]    It is to be noted that  FIG. 3  depicts an example of the network, and the number and the coupling scheme of the subnetworks  5 , the relaying apparatus  80  and so forth included in the network can be changed arbitrarily in response to the implementation of the network. 
         [0066]      FIG. 4  is a block diagram depicting an example of a configuration of a conversion apparatus. The conversion apparatus depicted in  FIG. 4  may be the conversion apparatus  20  depicted in  FIG. 2 . The conversion apparatus  20  includes a transmission and reception unit  21 , a packet identification unit  24 , a storage unit  30 , a packet processing unit  35 , and a message processing unit  40 . The transmission and reception unit  21  includes a transmission unit  22  and a reception unit  23 . The packet processing unit  35  includes a forwarding processing unit  36 , an encapsulation unit  37  and a decapsulation unit  38 . The message processing unit  40  includes a detection unit  41  and a setting requesting unit  42 . The storage unit  30  has authentication information  31  and position registration information  32  stored therein. The authentication information  31  is used for authentication of a legitimate user of the mobile terminal  10 . The position registration information  32  associates the home address and the care-of address of the mobile terminal  10  positioned in a subnetwork other than the home subnetwork with each other. 
         [0067]    The transmission unit  22  transmits a packet to the control apparatus  50  or a different apparatus. The reception unit  23  receives a packet from the control apparatus  50  or a different apparatus and outputs the received packet to the packet identification unit  24 . The packet identification unit  24  outputs a data packet to the forwarding processing unit  36  and outputs a position registration request message transmitted thereto from the mobile terminal  10  to the detection unit  41 . The detection unit  41  updates the position registration information  32  on the basis of the position registration request message and notifies the setting requesting unit  42  that the mobile terminal  10  has moved. The setting requesting unit  42  generates a flow table setting request on the basis of the notification from the detection unit  41  and transmits the flow table setting request to the control apparatus  50  through the transmission unit  22 . 
         [0068]    The forwarding processing unit  36  outputs a data packet destined for the mobile terminal  10  to the encapsulation unit  37  and outputs a data packet transmitted thereto from the mobile terminal  10  to the decapsulation unit  38 . The encapsulation unit  37  encapsulates a packet inputted thereto. At this time, the encapsulation unit  37  sets an address associated with the home address of the mobile terminal  10  in the position registration information  32  to the destination of the outer header and sets the address of the conversion apparatus  20  to the transmission source of the outer header. The decapsulation unit  38  decapsulates a packet inputted thereto. The decapsulation unit  38  outputs, where the mobile terminal  10  of the transmission source of the data packet is set to the position registration information  32 , the packet after decapsulation to the transmission unit  22 . 
         [0069]      FIG. 5  is a block diagram depicting an example of a configuration of a control apparatus. The control apparatus depicted in  FIG. 5  may be the control apparatus  50  depicted in  FIG. 2 . The control apparatus  50  includes a transmission and reception unit  51 , an identification unit  54 , a control message processing unit  55 , a storage unit  60 , and a setting request processing unit  70 . The transmission and reception unit  51  includes a transmission unit  52  and a reception unit  53 . The reception unit  53  outputs a received packet to the identification unit  54 . The identification unit  54  outputs, when the received packet is a flow table setting request transmitted from the conversion apparatus  20 , the flow table setting request to an extraction unit  71  of the setting request processing unit  70 . On the other hand, if the transmission source of the received packet is a relaying apparatus  80 , the identification unit  54  determines that a control message is received and outputs the inputted packet to the control message processing unit  55 . The control message processing unit  55  processes the control message received from the relaying apparatus  80  and suitably performs a process for controlling operation of the relaying apparatus  80  of the transmission source of the control message such as notification of an output port for a packet held by the relaying apparatus  80 . 
         [0070]    The setting request processing unit  70  includes the extraction unit  71 , a selection unit  72 , and a generation unit  73 . The extraction unit  71  extracts the home address and the care-of address of the mobile terminal  10  from the flow table setting request. The selection unit  72  uses the extracted care-of address and a topology information table  61  ( FIG. 6 ) and a coupling information table  62  ( FIG. 7 ) of the storage unit  60  to specify the relaying apparatus  80  included in the route from the home subnetwork to the subnetwork of the destination of movement in which the mobile terminal  10  is positioned from among the relaying apparatus  80  included in the network. In other words, the selection unit  72  selects those relaying apparatus  80  in the route from the home subnetwork to the subnetwork of the destination of movement in which the mobile terminal  10  is positioned as relaying apparatus  80  to each of which a request for rewriting of the flow table is to be issued. The generation unit  73  changes a flow table  63  in the storage unit  60  in response to the movement of the mobile terminal  10  and generates a control packet for notifying the selected relaying apparatus  80  of the information in the flow table  63  after the change. The control packet is transmitted to the relaying apparatus  80  through the transmission unit  52 . It is to be noted that the control apparatus  50  presumably holds, for each of the relaying apparatus  80  of the control target, information of an address to be used for transmission of a control packet and so forth as a relaying apparatus information table  64  in the storage unit  60 . The storage unit  60  has stored therein the topology information table  61 , the coupling information table  62  and the flow table  63  in addition to the relaying apparatus information table  64 . 
         [0071]      FIG. 6  is a view depicting an example of a topology information table. The topology information table depicted in  FIG. 6  may be the topology information table  61  depicted in  FIG. 5 . The topology information table  61  has recorded therein, for each of identifiers allocated to the individual relaying apparatus  80  (relaying apparatus IDs), information that specifies serial numbers of ports used by the relaying apparatus  80  of the coupling source. In the following description, it is assumed that the relaying apparatus  80   a  has a relaying apparatus ID=1; the relaying apparatus  80   b  has a relaying apparatus ID=2; the relaying apparatus  80   c  has a relaying apparatus ID=3; and the relaying apparatus  80   d  has a relaying apparatus ID=4. In the topology information table  61  depicted in  FIG. 6 , combinations of a coupling source and a coupling destination of a link among the relaying apparatus  80  and a port number used in the relaying apparatus  80  of the coupling source are recorded where the network is formed in such a manner as depicted in  FIG. 3 . For example, the first entry of the topology information table  61  indicates the coupling source relaying apparatus ID=1 and the coupling destination relaying apparatus ID=3, and therefore indicates that the relaying apparatus  80   a  is the coupling source and the relaying apparatus  80   c  is the coupling destination. Further, since the first entry indicates that the coupling source port is 1, it is indicated that the port  1  of the relaying apparatus  80   a  is coupled to the relaying apparatus  80   c . Also from the other entries, a coupling source and a coupling destination of a link can be specified similarly. 
         [0072]      FIG. 7  is a view depicting an example of a coupling information table. The coupling information table depicted in  FIG. 7  may be the coupling information table  62  depicted in  FIG. 5 . The coupling information table  62  of  FIG. 7  associates an ID of a relaying apparatus  80  coupled to a subnetwork  5  (coupled relaying apparatus ID) and a port of the relaying apparatus  80  with a subnetwork address of the subnetwork  5 . For example, the subnetwork address of the subnetwork  5   a  is 10.0.10.0/24, and the port  2  of the relaying apparatus  80   a  is coupled to the subnetwork  5   a . Therefore, the coupling information relating to the subnetwork  5   a  is such as indicated by the first entry in  FIG. 7 . Similarly, the subnetwork address of the subnetwork  5   b  is 10.0.20.0/24, and the port  1  of the relaying apparatus  80   b  is coupled to the subnetwork  5   b . Therefore, the coupling information relating to the subnetwork  5   b  is such as indicated by the second entry in  FIG. 7 . Furthermore, the subnetwork address of the subnetwork  5   c  is 10.0.30.0/24 and the port  2  of the relaying apparatus  80   d  is coupled to the subnetwork  5   c . Therefore, the coupling information relating to the subnetwork  5   c  is such as indicated by the third entry in  FIG. 7 . 
         [0073]      FIG. 8  is a view depicting an example of a flow table. The flow table depicted in  FIG. 8  may be the flow table  63  depicted in  FIG. 5 . The flow table  63  associates information included in the flow table used in a forwarding process by each relaying apparatus  80  with the relaying apparatus ID allocated to the relaying apparatus  80  that retains the information. It is to be noted that, in the example of  FIG. 8 , conditions used for communication with the control apparatus  50  by each relaying apparatus  80  are omitted. A process set in a column of process in an entry of the greatest bit number that indicates coincidence between a destination address of a packet and a match condition is performed for a packet of a process target. For example, in the flow table  63  of  FIG. 8 , it is used as a match condition that the destination IP address of a packet of a forwarding target coincides with the subnetwork address of one of the subnetworks  5 . Further, the process designated in each entry is to output a packet of a process target to a port directed toward the subnetwork  5  set in the match condition. Therefore, for example, from the relaying apparatus  80   a  (relaying apparatus ID=1), a packet destined for 10.0.10.0/24 is outputted to the port  2 ; a packet destined for 10.0.20.0/24 is outputted to the port  1 ; and a packet destined for 10.0.30.0/24 is outputted to the port  1 . It is to be noted that  FIG. 8  depicts an example of the flow table  63  generated where the network is formed in such a manner as depicted in  FIG. 3  and besides the mobile terminal  10  is positioned in the home network. 
         [0074]      FIG. 9  is a block diagram depicting an example of a configuration of a relaying apparatus. The relaying apparatus depicted in  FIG. 9  may be the relaying apparatus  80  described with reference to  FIG. 2 . The relaying apparatus  80  includes a transmission and reception unit  81 , a sorting unit  82 , a control message processing unit  83 , a storage unit  85 , and a data packet processing unit  90 . The storage unit  85  stores control apparatus information  86  and a flow table  87  therein. The data packet processing unit  90  includes a determination unit  91 , a packet processing unit  93 , and a decapsulation unit  94 . 
         [0075]    The transmission and reception unit  81  transmits and receives a packet to and from a different apparatus. The sorting unit  82  outputs, when the packet received through the transmission and reception unit  81  is a data packet or a position registration request message, the received packet to the determination unit  91 . On the other hand, if a control packet transmitted from the control apparatus  50  is inputted, then the sorting unit  82  outputs the inputted packet to the control message processing unit  83 . The control message processing unit  83  performs a changing process of the flow table  87 , a forwarding process for the packet and so forth in accordance with the control packet received from the control apparatus  50 . 
         [0076]    The determination unit  91  searches the flow table on the basis of header information of a packet to search for a match condition. The determination unit  91  outputs a packet, for which decapsulation is requested, to the decapsulation unit  94  together with information indicative of the substance of a process recorded in the entry that matches with the packet. However, with regard to a packet for which decapsulation is not requested, the determination unit  91  outputs information obtained from the flow table  87  to the packet processing unit  93  together with the packet of the process target. 
         [0077]    The packet processing unit  93  performs, for a packet inputted thereto, a process designated by the information received from the determination unit  91 . The decapsulation unit  94  performs decapsulation for the inputted packet and further performs a process designated by the information received from the determination unit  91 . 
         [0078]      FIG. 10  is a view depicting an example of a flow table held by a relaying apparatus. The relaying apparatus described with reference to  FIG. 10  may be the relaying apparatus  80  described with reference to  FIG. 2 , and the flow table depicted in  FIG. 10  may be the flow table  87  depicted in  FIG. 9 . It is to be noted that, in the following description, in order to facilitate distinction of a table or the like included in an individual relaying apparatus  80 , an alphabet allocated to a relaying apparatus  80  that is operating is added to a reference character. For example, a flow table  87   a  represents the flow table  87  included in the relaying apparatus  80   a . Further, when the substance of the table varies as time passes, a numeral is sometimes added next to the underscore to indicate at which point of time the table belongs. Here, the flow table  87  held by each relaying apparatus  80  is coincident with information associated with the relaying apparatus ID of the relaying apparatus  80  in the flow table  63  depicted in  FIG. 8 . In particular, in a flow table  87   a _ 1 , the information of the relaying apparatus ID=1 (relaying apparatus  80   a ) in the flow table  63  ( FIG. 8 ) is recorded. 
         [0079]    Similarly, a flow table  87   b  in  FIG. 10  is held by the relaying apparatus  80   b  and includes information of the relaying apparatus ID=2 (relaying apparatus  80   b ) in the flow table  63 . A flow table  87   c _ 1  is held by the relaying apparatus  80   c  and includes information of the relaying apparatus ID=3 (relaying apparatus  80   c ) in the flow table  63 . Further, a flow table  87   d _ 1  is held by the relaying apparatus  80   d  and includes information of the relaying apparatus ID=4 (relaying apparatus  80   d ) in the flow table  63 . 
         [0080]    The flow table  87  has processes recorded therein in an associated relationship with match conditions similarly to the flow table  63  held by the control apparatus  50 . The determination unit  91  uses an entry, in which the match condition coincides with the match condition of the destination address of the packet and which has the greatest bit number, for determination of a process to be performed for the packet. 
         [0081]      FIG. 11  is a block diagram depicting an example of a hardware configuration. All of the conversion apparatus  20 , the control apparatus  50  and the relaying apparatus  80  can be configured to have the hardware configuration depicted in  FIG. 11  and include a processor  101 , a memory  102 , a bus  105  and a network interface  109 . Any of the conversion apparatus  20 , the control apparatus  50  and the relaying apparatus  80  may further include one or more of an inputting apparatus  103 , an outputting apparatus  104 , a storage apparatus  106  and a portable storage device driving apparatus  107 . The processor  101  is an arbitrary processing circuit including a central processing unit (CPU) and can execute a program stored in the memory  102  or the storage apparatus  106 . The bus  105  couples the processor  101 , the memory  102  and the network interface  109  to each other such that data can be communication between the processor  101 , the memory  102  and the network interface  109 . 
         [0082]    The inputting apparatus  103  is an arbitrary apparatus used to input information such as a keyboard, and the outputting apparatus  104  is an arbitrary apparatus used to output data such as a display unit. The portable storage device driving apparatus  107  can output data in the memory  102  or the storage apparatus  106  to a portable storage device  108  and read out a program, data and so forth from the portable storage device  108 . Here, the portable storage device  108  may be an arbitrary recording medium that can be carried and includes a compact disc recordable (CD-R) or a digital versatile disc recordable (DVD-R). 
         [0083]    In the conversion apparatus  20 , the processor  101  implements the packet identification unit  24 , the packet processing unit  35  and the message processing unit  40 . The memory  102  operates as the storage unit  30 . The network interface  109  operates as the transmission and reception unit  21  and includes also ports. 
         [0084]    In the control apparatus  50 , the processor  101  implements the identification unit  54 , the control message processing unit  55  and the setting request processing unit  70 . The memory  102  operates as the storage unit  60 . The network interface  109  operates as the transmission and reception unit  51 . 
         [0085]    In the relaying apparatus  80 , the processor  101  implements the sorting unit  82 , the control message processing unit  83  and the data packet processing unit  90 . The memory  102  operates as the storage unit  85 . The network interface  109  operates as the transmission and reception unit  81 . 
       First Embodiment 
       [0086]      FIG. 12  is a sequence diagram depicting an example of processing performed when a mobile terminal moves. The mobile terminal described with reference to in  FIG. 12  may be the mobile terminal  10  depicted in  FIG. 1 . At steps S 11  to S 19  in  FIG. 12 , a process relating to position registration of the mobile terminal  10  is performed, and at steps beginning with step S 20 , a changing process of the flow table  87  in the relaying apparatus  80  for transmitting a packet to the mobile terminal  10  after the movement is performed. In the following, an example of processing performed when the mobile terminal  10  moves from the subnetwork  5   a  to the subnetwork  5   c  under a situation that the control apparatus  50  retains the information illustrated in  FIGS. 6 to 8 . It is to be noted that  FIG. 12  illustrates an example of the processing, and the processes at steps S 20  to S 25  may be performed in parallel to the processes at steps S 16  to S 19 . 
         [0087]    (1) Process for Position Registration 
         [0088]    At step S 11 , the mobile terminal  10  detects that it has moved to the subnetwork  5   c  different from the home subnetwork thereof. For example, the mobile terminal  10  can detect the movement using a dynamic host configuration protocol (DHCP) message broadcasted by a DHCP server, a router advertisement message broadcasted by the relaying apparatus  80   d  or the like. Further, the mobile terminal  10  can detect that the mobile terminal  10  has moved also from the fact that the network address in an IP package transmitted to or received from an apparatus in the subnetwork  5   c  is different from the network address (10.0.10.0/24) in the home address. After detecting that the mobile terminal  10  has moved from the subnetwork  5   a , the mobile terminal  10  acquires an IP address to be used as a care-of address. In the following example, as depicted in  FIG. 3 , it is assumed that the care-of address acquired by the mobile terminal  10  is 10.0.30.10. After acquiring the care-of address, the mobile terminal  10  transmits a position registration request message destined for the conversion apparatus  20 . The position registration request message includes the home address (10.0.10.10) and the care-of address of the mobile terminal  10 , authentication information and so forth. It is to be noted that, in the IP header of the position registration request message, the transmission source address is the care-of address of the mobile terminal  10  and the destination address is the address (10.0.10.1) of the conversion apparatus  20 . 
         [0089]    At step S 12 , a transmission and reception unit  81   d  of the relaying apparatus  80   d  receives and outputs the position registration request message to a sorting unit  82   d . The sorting unit  82   d  outputs the position registration request message to a determination unit  91   d . Since the destination address of the position registration request message is 10.0.10.1, the determination unit  91   d  selects the first entry in the flow table  87   d _ 1  ( FIG. 10 ). Since the process included in the first entry is to output from the port  1 , the determination unit  91   d  outputs the substance of the process and the position registration request message to a packet processing unit  93   d . The packet processing unit  93   d  designates the output port to the port  1  and outputs the packet inputted thereto from the determination unit  91   d  to the transmission and reception unit  81   d . Then, the transmission and reception unit  81   d  outputs the position registration request message from the port  1  whereby the position registration request message is forwarded to the relaying apparatus  80   c.    
         [0090]    At step S 13 , the relaying apparatus  80   c  receives the position registration request message and performs a process similar to that performed by the relaying apparatus  80   d . Therefore, the position registration request message is outputted from the port  2  of the relaying apparatus  80   c  in accordance with the first entry in the flow table  87   c _ 1  ( FIG. 10 ). Since the port  2  of the relaying apparatus  80   c  is coupled to the relaying apparatus  80   a , the position registration request message is forwarded to the relaying apparatus  80   a.    
         [0091]    At step S 14 , the relaying apparatus  80   a  receives the position registration request message and performs a process similar to that performed by the relaying apparatus  80   d . The position registration request message is outputted from the port  2  of the relaying apparatus  80   a  in accordance with the first entry in the flow table  87   a _ 1  and forwarded to the conversion apparatus  20  in the subnetwork  5   a.    
         [0092]    At step S 15 , the reception unit  23  of the conversion apparatus  20  outputs the position registration request message to the packet identification unit  24 . The packet identification unit  24  outputs the position registration request message to the detection unit  41 . The detection unit  41  performs an authentication process and position registration using the information in the position registration request message. 
         [0093]      FIG. 13  is a view depicting an example of authentication information. The authentication information depicted in  FIG. 13  may be the authentication information  31  depicted in  FIG. 4 . In the example of  FIG. 13 , the authentication information  31  includes a security parameter index (SPI), the home address of the mobile terminal  10 , an authentication algorithm, a bit length of an encryption key, the encryption key and so forth. The SPI is an identifier regarding an authentication parameter set. The authentication algorithm is a kind of algorithm used for calculation of authentication information, and the encryption key is key data to be used to calculate authentication information using the authentication algorithm. The detection unit  41  performs an authentication process using the authentication information and the home address included in the position registration request message. Here, it is assumed that the detection unit  41  succeeds in the authentication process of the mobile terminal  10 . 
         [0094]      FIG. 14  is a view depicting an example of position registration information. The position registration information depicted in  FIG. 14  may be the position registration information  32  depicted in  FIG. 4 . The position registration information  32  includes a care-of address (CoA) of the mobile terminal  10  and Lifetime in association with the home address (HoA) of the mobile terminal  10 . If the detection unit  41  succeeds in the authentication of the mobile terminal  10 , then the detection unit  41  records the home address included in the position registration request message and the care-of address in an associated relationship with each other in the position registration information  32  as depicted in  FIG. 14 . The Lifetime is a valid time period (seconds) of the information included in the entry. If a desired value of the Lifetime is included in the position registration request message, then the detection unit  41  sets the value of the Lifetime to a value lower than the desired value received from the mobile terminal  10 . In the example of  FIG. 14 , the Lifetime is set to 300 seconds. 
         [0095]    Step S 16  of  FIG. 12  indicates transmission of a position registration response message. After the position registration is completed, the detection unit  41  generates a position registration response message and transmits the position registration response message toward the mobile terminal  10  through the transmission unit  22 . At this time, the destination of the position registration response message is set to the care-of address (10.0.30.10) of the mobile terminal  10 . The position registration response message is forwarded from the conversion apparatus  20  to the relaying apparatus  80   a.    
         [0096]    At step S 17 , the process of the relaying apparatus  80   a  that receives the position registration response message is similar to that at step S 14 . Therefore, the position registration response message is outputted toward the relaying apparatus  80   c  from the port  1  of the relaying apparatus  80   a  in accordance with the flow table  87   a _ 1 . At step S 18 , also the relaying apparatus  80   c  performs a process similar to that at step S 13 . Therefore, the position registration response message is outputted from the port  3  of the relaying apparatus  80   c  toward the relaying apparatus  80   d . At step S 19 , the relaying apparatus  80   d  performs a process similar to that at step S 12  to forward the position registration response message to the mobile terminal  10  in the subnetwork  5   c.    
         [0097]    (2) Process for Update of the Flow Table 
         [0098]    In the following, a selecting method of a relaying apparatus  80  that becomes a target for update of the flow table  87 , an updating method of the flow table  87  and so forth are described with reference to the steps beginning with step S 20  of  FIG. 12 . 
         [0099]    At step S 20 , the detection unit  41  of the conversion apparatus  20  performs position registration regarding the mobile terminal  10  and thereby determines that a movement of the mobile terminal  10  is detected. The detection unit  41  notifies the setting requesting unit  42  that a movement of the mobile terminal  10  is detected. The setting requesting unit  42  generates a message (flow table setting request) for requesting the control apparatus  50  to perform rewriting of the flow table  87  held by one or more relaying apparatus  80  in the network in response to the movement of the mobile terminal  10 . The flow table setting request includes the home subnetwork of the mobile terminal  10 , and information usable to specify a subnetwork in which the mobile terminal  10  is positioned. In the following description, the subnetwork in which the mobile terminal  10  is positioned is sometimes referred to as “movement destination subnetwork.” The information to be used for the specification of the home subnetwork of the mobile terminal  10  may be the home address of the mobile terminal  10  or may be the subnetwork address of the subnetwork  5   a  that is the home network of the mobile terminal  10 . Similarly, also the information to be used for the specification of the movement destination subnetwork may be the care-of address of the mobile terminal  10  or the subnetwork address of the subnetwork  5   c  that is the movement destination subnetwork. Note that it is assumed that, even if the flow table setting request includes the subnetwork address of the home subnetwork, the flow table setting request includes the home address of the mobile terminal  10 . 
         [0100]    Here, a case in which the home address is used as the information to be used for the specification of the home subnetwork and the care-of address is used as the information to be used for the specification of the movement destination subnetwork is taken as an example. Accordingly, the flow table setting request generated by the setting requesting unit  42  includes 
         [0101]    the HoA of the mobile terminal  10 : 10.0.10.10, and 
         [0102]    the CoA of the mobile terminal  10 : 10.0.30.10. 
         [0103]    The setting requesting unit  42  transmits the generated flow table setting request to the control apparatus  50  through the transmission unit  22 . 
         [0104]    At step S 21 , the reception unit  53  of the control apparatus  50  outputs the flow table setting request to the identification unit  54 . If the identification unit  54  determines that the packet inputted thereto is a flow table setting request, then the identification unit  54  outputs the flow table setting request to the extraction unit  71 . The extraction unit  71  extracts, from the flow table setting request, information for specifying the home subnetwork and the movement destination subnetwork of the mobile terminal  10 . Here, the extraction unit  71  extracts the home address and the care-of address of the mobile terminal  10  from the flow table setting request. The extraction unit  71  specifies from the extracted address that the home subnetwork address of the mobile terminal  10  is 10.0.10.0/24 and the address of the movement destination subnetwork is 10.0.30.0/24. In other words, the extraction unit  71  specifies that the home subnetwork of the mobile terminal  10  is the subnetwork  5   a  and that the movement destination subnetwork is the subnetwork  5   c . It is to be noted that, when the extraction unit  71  specifies the home subnetwork and the movement destination subnetwork, the extraction unit  71  may suitably use the coupling information table  62  ( FIG. 7 ) to specify the address of the subnetwork to which the extracted address belongs. The extraction unit  71  outputs the obtained subnetwork address to the selection unit  72 . 
         [0105]    The selection unit  72  uses the subnetwork address inputted thereto from the extraction unit  71  to search for the relaying apparatus  80  included in the route from the home subnetwork to the movement destination subnetwork of the mobile terminal  10 . At this time, the selection unit  72  uses the topology information table  61  and the coupling information table  62  to specify the position of each relaying apparatus  80  in the network and a coupling relationship between the relaying apparatus  80 . Here, the selection unit  72  uses the topology information table  61  ( FIG. 6 ) and the coupling information table  62  ( FIG. 7 ) to recognize that the network is such as depicted in  FIG. 3 . Accordingly, the selection unit  72  selects the relaying apparatus  80   a ,  80   c , and  80   d  as a target to which a change of the flow table  87  is requested. 
         [0106]    The generation unit  73  changes the flow table  63  such that, in each of the relaying apparatus  80  selected by the selection unit  72 , the forwarding destination of a packet destined for the home address of the mobile terminal  10  becomes same as the output port for a packet destined for the movement destination subnetwork of the mobile terminal  10 . Further, the generation unit  73  sets such that decapsulation of the packet received from the mobile terminal  10  is to be performed by the relaying apparatus  80  coupled to the movement destination subnetwork. 
         [0107]      FIG. 15  is a view depicting an example of a change of a flow table. The flow table described with reference to  FIG. 15  may be the flow table  63  depicted in  FIG. 5 . As depicted in  FIG. 15 , the selection unit  72  adds, to the match condition for the relaying apparatus  80   a  (relaying apparatus ID=1), a setting that the output port for a packet destined for the home address (10.0.10.10) of the mobile terminal  10  is the port  1 . It is to be noted that, in the relaying apparatus  80   a , also the output port for a packet destined for the subnetwork  5   c  is the port  1 . Similarly, the generation unit  73  adds, to the match condition for the relaying apparatus  80   c  (relaying apparatus ID=3), that the output port for a packet destined for 10.0.10.10 that is the HoA of the mobile terminal  10  is the port  3 . Further, the generation unit  73  adds, to the match condition for the relaying apparatus  80   d  (relaying apparatus ID=4), that the output port for a packet destined for 10.0.10.10 that is the HoA of the mobile terminal  10  is the port  1 . 
         [0108]    Here, when communicating with the communication apparatus  15 , the mobile terminal  10  transmits an encapsulated packet because the mobile terminal  10  tries to transmit a packet through the conversion apparatus  20 . Therefore, with regard to the relaying apparatus  80   d , since the relaying apparatus  80   d  is coupled to the movement destination subnetwork, the generation unit  73  sets such that, after a packet received from the mobile terminal  10  is decapsulated, an output destination is searched out again from within the flow table  63 . Accordingly, in the relaying apparatus ID=4 in  FIG. 15 , decapsulation and re-search of an output destination are recorded as processes for IPinIP encapsulated packet whose inner header designates that the transmission source is 10.0.10.10. 
         [0109]    After the update of the flow table  63  is completed, the generation unit  73  generates a control packet for requesting an update of the flow table  87  to the relaying apparatus  80  associated with the relaying apparatus IDs including the updated entries. At this time, the generation unit  73  suitably uses the relaying apparatus information table  64 . 
         [0110]      FIG. 16  is a view depicting an example of a relaying apparatus information table. The relaying apparatus information table depicted in  FIG. 16  may be the relaying apparatus information table  64  depicted in  FIG. 5 . The relaying apparatus information table  64  includes a datapath ID, an address and a transmission control protocol (TCP) port number. The datapath ID is an identifier provided for each relaying apparatus  80  of a communication destination for identifying a controlling path. In the following description, it is assumed that the communication path between the control apparatus  50  and the relaying apparatus  80   a  is identified with the datapath ID=1. Similarly, it is assumed that, to the control apparatus  50 , the datapath ID of the communication path from the relaying apparatus  80   b  is 2; the datapath ID of the communication path from the relaying apparatus  80   c  is 3; and the datapath ID of the communication path from the relaying apparatus  80   d  is 4. The address is an IP address allocated to the relaying apparatus  80  that uses a path specified by a datapath ID. Further, the TCP port number is a serial number of a TCP port used for communication with the relaying apparatus  80  specified with the datapath ID. 
         [0111]    At step S 22  of  FIG. 12 , the generation unit  73  transmits a control packet generated using the flow table  63  and the relaying apparatus information table  64  after updated to each of the relaying apparatus  80   a ,  80   c , and  80   d . The control packet transmitted to the relaying apparatus  80   a  includes information for requesting to add the substance of the entry added with regard to the relaying apparatus ID=1 to the flow table  87   a . Similarly, the control packet destined for the relaying apparatus  80   c  includes information for requesting to add the substance of the entry added with regard to the relaying apparatus ID=3, and the control packet destined for the relaying apparatus  80   d  includes information for requesting to add the substance of the entry added with regard to the relaying apparatus ID=4. Where an OpenFlow is used between the control apparatus  50  and the relaying apparatus  80 , a control packet can be transmitted as a Flow-Mod message to the relaying apparatus  80  of the notification destination. 
         [0112]      FIG. 17  is a view depicting an example of a flow table after updated. The flow table described with reference to  FIG. 17  may be the flow table  87  depicted in  FIG. 9 . At step S 23 , the control message processing unit  83   c  of the relaying apparatus  80   c  acquires a control packet through the transmission and reception unit  81   c  and the sorting unit  82   c . The control message processing unit  83   c  updates the flow table  87   c _ 1  ( FIG. 10 ) into a flow table  87   c _ 2  depicted in  FIG. 17  in accordance with the control packet. 
         [0113]    Similarly, at step S 24 , the control message processing unit  83   a  of the relaying apparatus  80   a  updates the flow table  87   a _ 1  ( FIG. 10 ) into a flow table  87   a _ 2  depicted in  FIG. 17  using the control packet. Further, the control message processing unit  83   d  of the relaying apparatus  80   d  updates the flow table  87   d _ 1  ( FIG. 10 ) into a flow table  87   d _ 2  depicted in  FIG. 17  using the control packet (step S 25 ). 
         [0114]      FIG. 18  is a sequence diagram depicting an example of processing performed upon transmission and reception of a packet to and from a mobile terminal. Steps S 31  to S 35  indicate an example of a forwarding process performed when a packet is transmitted from the communication apparatus  15  to the mobile terminal  10  after the flow table  87  is updated in a relaying apparatus  80 . Meanwhile, steps S 36  to S 40  indicate an example of a forwarding process performed when a packet is transmitted from the mobile terminal  10  to the communication apparatus  15  after the flow table  87  in the relaying apparatus  80  is updated. It is to be noted that, while  FIG. 18  depicts transmission of a packet from the mobile terminal  10  to the communication apparatus  15  at steps from step S 36  onward, the order of the processes at steps S 31  to S 35  and the processes at steps S 36  to S 40  can be changed arbitrarily and they can be processed in parallel. 
         [0115]    (3) Forwarding Process of a Packet from the Communication Apparatus  15  to the Mobile Terminal  10   
         [0116]    Since the communication apparatus  15  generates a data packet destined for the mobile terminal  10  and transmits the generated data packet toward the mobile terminal  10 , the relaying apparatus  80   b  receives the data packet (step S 31 ). It is to be noted that the destination address of the data packet is the home address of the mobile terminal  10  (10.0.10.10). 
         [0117]    Since the destination of the data packet is 10.0.10.10, the relaying apparatus  80   b  outputs the data packet from the port  2  thereof in accordance with the first entry of the flow table  87   b  ( FIG. 10 ) (step S 32 ). Accordingly, the data packet is forwarded to the relaying apparatus  80   c.    
         [0118]    The determination unit  91   c  of the relaying apparatus  80   c  acquires the data packet through the transmission and reception unit  81   c  and the sorting unit  82   c . The determination unit  91   c  refers to the flow table  87   c _ 2  ( FIG. 17 ) in order to determine a process that is performed for the data packet. The determination unit  91   c  compares the destination address of the data packet and the match condition with each other and decides that the information in the first entry in the flow table  87   c _ 2  can be used for a process of the data packet. The determination unit  91   c  outputs the data packet to the packet processing unit  93   c  together with the information in the first entry of the flow table  87   c _ 2 . In the first entry of the flow table  87   c _ 2 , it is set that a packet destined for the home address of the mobile terminal  10  is to be outputted from the port  3  (step S 33 ). Therefore, the packet processing unit  93   c  outputs the data packet from the port  3  thereof. Therefore, the data packet is forwarded from the relaying apparatus  80   c  to the relaying apparatus  80   d  (step S 34 ). 
         [0119]    The relaying apparatus  80   d  performs a forwarding process using the flow table  87   d _ 2  after updated ( FIG. 17 ) similarly to the relaying apparatus  80   c . Therefore, the determination unit  91   d  compares the destination address of the data packet and the match condition with each other and decides that the information in the first entry of the flow table  87   d _ 2  can be used for a process of the data packet. In the first entry of the flow table  87   d _ 2 , it is set that a packet destined for the home address of the mobile terminal  10  is to be outputted from the port  2 . Therefore, the packet processing unit  93   d  outputs the data packet from the port  2  thereof (step S 35 ). Since the port  2  of the relaying apparatus  80   d  is coupled to the subnetwork  5   c , the data packet is forwarded from the relaying apparatus  80   d  to the mobile terminal  10 . 
         [0120]    In this manner, since the flow table  87  of each relaying apparatus  80  in the route from the home subnetwork to the movement destination subnetwork of the mobile terminal  10  is updated, a data packet transmitted from the communication apparatus  15  and destined for the mobile terminal  10  is forwarded without being encapsulated. Therefore, in comparison with an alternative case in which a data packet is forwarded after encapsulated, the total amount of information to be communicated in the network can be reduced without changing the data amount to be communicated. Further, when a data packet is forwarded from the communication apparatus  15  to the mobile terminal  10 , the data packet does not pass through the conversion apparatus  20 . Therefore, the communication method according to the first embodiment is effective also in reduction of the length of the route and reduction of the delay time. 
         [0121]    (4) Forwarding Process of a Packet from the Mobile Terminal  10  to the Communication Apparatus  15   
         [0122]    Now, transmission of a packet from the mobile terminal  10  is described. Since the mobile terminal  10  is positioned in the movement destination subnetwork, at an outer side of a packet including data destined for the communication apparatus  15 , an outer header for forwarding the packet to the conversion apparatus  20  is added. Accordingly, the following addresses are included in a packet transmitted from the mobile terminal  10 : 
         [0123]    Destination address of the outer header: 10.0.10.1 (conversion apparatus  20 ) 
         [0124]    Transmission source address of the outer header: 10.0.30.10 (CoA) 
         [0125]    Destination address of the inner header: 10.0.20.20 (communication apparatus  15 ) 
         [0126]    Transmission source address of the inner header: 10.0.10.10 (HoA) 
         [0127]    Since the mobile terminal  10  transmits the generated packet toward the conversion apparatus  20 , the packet that is in an encapsulated form is received by the relaying apparatus  80   d  (step S 36 ). 
         [0128]    The determination unit  91   d  of the relaying apparatus  80   d  acquires the packet through the transmission and reception unit  81   d  and the sorting unit  82   d . Since the inputted packet is in an encapsulated form and the transmission source address of the inner header is the home address of the mobile terminal  10 , the determination unit  91   d  determines that the second entry of the flow table  87   d _ 2  ( FIG. 17 ) can be applied. The determination unit  91   d  outputs the encapsulated packet and the information of the second entry of the flow table  87   d _ 2  to the decapsulation unit  94   d . The decapsulation unit  94   d  determines that it is requested to perform decapsulation of the packet and re-search the forwarding destination of the packet after the decapsulation from the flow table  87   d _ 2 . Therefore, the decapsulation unit  94   d  decapsulates the packet on the basis of the inputted information. Further, the decapsulation unit  94   d  uses the destination address of the data packet after the decapsulation to search the flow table  87   d _ 2 . Since the destination address of the packet after the decapsulation is 10.0.20.20 (communication apparatus  15 ), the decapsulation unit  94   d  determines to output the data packet after the decapsulation from the port  1  thereof on the basis of the fourth entry of the flow table  87   d _ 2  (step S 37 ). Since the data packet after the decapsulation is outputted from the port  1 , the data packet is forwarded from the relaying apparatus  80   d  to the relaying apparatus  80   c  (step S 38 ). 
         [0129]    The relaying apparatus  80   c  performs a forwarding process using the flow table  87   c _ 2  ( FIG. 17 ) after updated similarly to the relaying apparatus  80   d . Therefore, a process for the data packet is performed using the information in the third entry of the flow table  87   c _ 2 . In the third entry of the flow table  87   d _ 2 , it is set that a packet destined for an apparatus in the subnetwork  5   b  is to be outputted from the port  1 . Therefore, the packet processing unit  93   d  outputs the data packet from the port  1  thereof (step S 39 ). Since the port  1  of the relaying apparatus  80   c  is coupled to the relaying apparatus  80   b , the data packet is forwarded from the relaying apparatus  80   c  to the relaying apparatus  80   b.    
         [0130]    Since the destination of the data packet is 10.0.20.20, the relaying apparatus  80   b  outputs the data packet from the port  1  thereof in accordance with the second entry of the flow table  87   b  ( FIG. 10 ) (step S 40 ). Accordingly, the data packet is forwarded to the communication apparatus  15 . 
         [0131]    In this manner, in the first embodiment, a relaying apparatus  80  coupled to the movement destination subnetwork of the mobile terminal  10  decapsulates a packet transmitted thereto from the mobile terminal  10 . In other words, in a route beginning with the relaying apparatus  80  that performs decapsulation, a data packet destined for the communication apparatus  15  is forwarded without being encapsulated. Therefore, in comparison with an alternative case in which a data packet is forwarded in an encapsulated form, the total amount of information to be communicated in the network can be reduced without changing the data amount to be communicated. Further, when a data packet is forwarded from the mobile terminal  10  to the communication apparatus  15 , the data packet does not pass the conversion apparatus  20 . Therefore, the communication method according to the first embodiment is effective also in reduction of the length of the route and reduction of the delay time. 
         [0132]      FIG. 19  is a flow chart illustrating an example of processing of a conversion apparatus. The conversion apparatus described with reference to  FIG. 19  may be the conversion apparatus  20  depicted in  FIG. 2 . It is to be noted that the order of steps S 57  to S 59  can be changed arbitrarily. When the reception unit  23  receives a packet forwarded from the relaying apparatus  80 , the packet identification unit  24  decides whether the received packet is a data packet (steps S 51  and S 52 ). If the received packet is a data packet, then the forwarding processing unit  36  determines one of the encapsulation unit  37  and the decapsulation unit  38  as an output destination of the packet in response to the destination of the packet (Yes at step S 52 ). The encapsulation unit  37  encapsulates and forwards the packet destined for the mobile terminal  10 . On the other hand, the decapsulation unit  38  decapsulates and forwards the packet destined for the communication apparatus  15  transmitted thereto from the mobile terminal  10  in the movement destination subnetwork (step S 53 ). 
         [0133]    If the received packet is not a data packet, then the detection unit  41  decides that a position registration request message is received and extracts such parameters as the HoA, CoA, Lifetime and authentication information from within the position registration request message (No at step S 52 , and steps S 54  and S 55 ). The detection unit  41  performs an authentication process and then, if failing in the authentication, ends the processing (No at step S 56 ). On the other hand, if the authentication of the mobile terminal  10  is performed successfully, then the detection unit  41  performs position registration of the mobile terminal  10  and transmits a position registration response message to the mobile terminal  10  (Yes at step S 56 , and steps S 57  and S 58 ). Further, the setting requesting unit  42  transmits a flow table setting request to the control apparatus  50  (step S 59 ). 
         [0134]      FIG. 20  is a flow chart illustrating an example of processing of a control apparatus. The control apparatus described with reference to  FIG. 20  may be the control apparatus  50  depicted in  FIG. 2 . If the control apparatus  50  receives a packet, then the identification unit  54  determines whether a flow table setting request is received (steps S 71  and S 72 ). The identification unit  54  outputs a packet other than the flow table setting request to the control message processing unit  55 , and the control message processing unit  55  processes the packet inputted thereto (No at step S 72 , and step S 73 ). 
         [0135]    On the other hand, if the packet received by the control apparatus  50  is a flow table setting request, then the extraction unit  71  extracts the home address and the care-of address of the mobile terminal  10  from within the flow table setting request (Yes at step S 72 , and step S 74 ). The extraction unit  71  specifies the home subnetwork and the movement destination subnetwork of the mobile terminal  10  from the extracted address (step S 75 ). The selection unit  72  specifies the relaying apparatus on the route from the home subnetwork to the movement destination subnetwork of the mobile terminal  10  (step S 76 ). The generation unit  73  transmits a control packet for requesting setting update of the flow table  87  to the specified relaying apparatus  80  in accordance with the update substance of the flow table  63  (step S 77 ). 
         [0136]      FIG. 21  is a flow chart illustrating an example of processing of a relaying apparatus. The relaying apparatus described with reference to  FIG. 21  may be the relaying apparatus  80  described with reference to  FIG. 2 . If the relaying apparatus  80  receives a packet, then the sorting unit  82  decides whether the received packet includes information for control by the relaying apparatus  80  (steps S 81  and S 82 ). If the received packet does not include control information, then since the received packet is one of a data packet and a position registration request message, a forwarding process of the received packet by the data packet processing unit  90  is performed (No at step S 82 , and step S 83 ). If the received packet is a control packet, then the control message processing unit  83  decides whether the received packet is a control packet that requests update of the flow table  87  (Yes at step S 82 , and step S 84 ). If the received packet is a control packet that requests update of the flow table  87 , the control message processing unit  83  adds the substance conveyed thereto by the control packet to the flow table  87  (Yes at step S 84 , and step S 85 ). On the other hand, if the received packet is not a control packet that requests update of the flow table  87 , then the control message processing unit  83  performs a forwarding process and so forth of the packet on the basis of the substance conveyed thereto by the received packet (step S 86 ). 
         [0137]    In this manner, with the method according to the first embodiment, after the mobile terminal  10  moves, the control apparatus  50  changes a setting of the relaying apparatus  80  in the route from the conversion apparatus  20  to the mobile terminal  10 . Therefore, even if a packet transmitted from the communication apparatus  15  toward the mobile terminal  10  is not in an encapsulated form, the packet is transmitted to the mobile terminal  10 . Also a packet transmitted from the mobile terminal  10  is decapsulated by the relaying apparatus  80  that is coupled to the movement destination subnetwork and then forwarded to the communication apparatus  15 . Therefore, the number of links through which an encapsulated packet is communicated is minimized. Accordingly, the amount of information of a header to be used for forwarding from within information communicated through the network can be reduced, and the communication can be streamlined. 
       Second Embodiment 
       [0138]    A second embodiment to be described below is directed to an example wherein, after detecting that communication with a mobile terminal is started, a conversion apparatus changes a flow table of a relaying apparatus on a route from the conversion apparatus to a movement destination subnetwork. It is to be noted that, also in the second embodiment, a position registration process involved in a movement of the mobile terminal is performed in a similar manner as in the processes at steps S 11  to S 19  of  FIG. 12 . 
         [0139]      FIG. 22  is a block diagram depicting an example of a configuration of a conversion apparatus according to a second embodiment. A conversion apparatus  45  includes a transmission and reception unit  21 , a packet identification unit  24 , a storage unit  30 , a message processing unit  43  and a packet processing unit  44 . The message processing unit  43  includes a setting requesting unit  42 . The packet processing unit  44  includes a forwarding processing unit  36 , an encapsulation unit  37 , a decapsulation unit  38  and a detection unit  39 . The processes performed by the transmission and reception unit  21 , the packet identification unit  24 , the storage unit  30 , the forwarding processing unit  36 , the encapsulation unit  37  and the decapsulation unit  38  are similar to those in the conversion apparatus  20  of the first embodiment. Note that it is assumed that, in the second embodiment, position registration and authentication processes are performed by the message processing unit  43 . 
         [0140]    The detection unit  39  detects starting of communication of the mobile terminal  10  if one of the destination of a packet outputted from the forwarding processing unit  36  to the encapsulation unit  37  or the transmission source of a packet outputted from the forwarding processing unit  36  to the decapsulation unit  38  is the mobile terminal  10  after the position registration. After starting of communication is detected, the detection unit  39  notifies the setting requesting unit  42  of the starting of communication. The setting requesting unit  42  generates a flow table setting request on the basis of the notification from the detection unit  39  and transmits the flow table setting request to the control apparatus  50  through a transmission unit  22 . 
         [0141]      FIG. 23  is a sequence diagram depicting an example of processing performed when a flow table is updated using a transmission process of a packet destined for a mobile terminal as a trigger. It is assumed that, in the description given with reference to  FIG. 23 , the flow table  87  of each relaying apparatus  80  in the network is such as depicted in  FIG. 10  and the flow table  63  of the control apparatus  50  is such as depicted in  FIG. 8  when the data packet destined for the mobile terminal  10  is transmitted. 
         [0142]    If the communication apparatus  15  transmits a data packet destined for the mobile terminal  10  after the position registration process of the mobile terminal  10 , then the relaying apparatus  80   b  receives the data packet (step S 91 ). Here, since the destination of the data packet is set to the home address of the mobile terminal  10 , the relaying apparatus  80   b  outputs the data packet from the port  2  thereof using the flow table  87   b  ( FIG. 10 ). Therefore, the data packet is forwarded from the relaying apparatus  80   b  to the relaying apparatus  80   c  (step S 92 ). When receiving the data packet destined for the mobile terminal  10 , the relaying apparatus  80   c  outputs the data packet from the port  2  thereof in accordance with the flow table  87   c _ 1  ( FIG. 10 ). Therefore, the data packet is forwarded from the relaying apparatus  80   c  to the relaying apparatus  80   a  (step S 93 ). The relaying apparatus  80   a  receives the data packet destined for the mobile terminal  10  and outputs the data packet from the port  2  thereof in accordance with the flow table  87   a _ 1  ( FIG. 10 ). Therefore, the data packet is forwarded to the conversion apparatus  45  (step S 94 ). 
         [0143]    The forwarding processing unit  36  in the conversion apparatus  45  acquires the data packet destined for the mobile terminal  10  through a reception unit  23  and the packet identification unit  24 . The forwarding processing unit  36  outputs the data packet to the encapsulation unit  37  because the destination of the data packet is the mobile terminal  10 . The encapsulation unit  37  acquires the care-of address (CoA) allocated to the mobile terminal  10 , which is the destination of the data packet, from the position registration information  32  ( FIG. 14 ). The encapsulation unit  37  encapsulates the data packet using the acquired care-of address. Therefore, the destination address in the outer header of the data packet after the encapsulation is the care-of address (10.0.30.10) of the mobile terminal  10 . On the other hand, since the encapsulation of the data packet is performed, the detection unit  39  detects starting of communication with the mobile terminal  10  that resides in a different subnetwork (step S 95 ). The encapsulation unit  37  transmits the data packet after the encapsulation through the transmission unit  22 . By this process, the data packet after the encapsulation is forwarded to the relaying apparatus  80   a  (step S 96 ). In  FIG. 23 , the forwarding process of the encapsulated packet is indicated by a double line arrow mark. 
         [0144]    Since the destination of the outer header of the data packet after the encapsulation is the care-of address of the mobile terminal  10 , the relaying apparatus  80   a  applies the process for a packet destined for an apparatus in the subnetwork  5   c  using the flow table  87   a _ 1 . Since the relaying apparatus  80   a  outputs the data packet after the encapsulation from the port  1  thereof, the encapsulated data packet is forwarded to the relaying apparatus  80   c  (step S 97 ). Also, since the relaying apparatus  80   c  processes the data packet after the encapsulation similarly to the relaying apparatus  80   a  using the flow table  87   c _ 1 , the encapsulated data packet is forwarded to the relaying apparatus  80   d  (step S 98 ). The relaying apparatus  80   d  forwards the data packet after the encapsulation to the mobile terminal  10  in accordance with the flow table  87   d _ 1  (step S 99 ). Therefore, the mobile terminal  10  can acquire the data destined for the mobile terminal  10  from the payload of the packet obtained by decapsulation of the received packet. 
         [0145]    At step S 100 , the detection unit  39  in the conversion apparatus  45  notifies the setting requesting unit  42  of starting of communication with the mobile terminal  10 . The setting requesting unit  42  generates a flow table setting request in order to request the control apparatus  50  to perform rewriting of the flow table  87  held by one or more ones of the relaying apparatus  80  in the network in accordance with the request of the detection unit  39 . The information factors included in the flow table setting request are similar to those in the first embodiment. The setting requesting unit  42  transmits the flow table setting request to the control apparatus  50  through the transmission unit  22 . 
         [0146]    The processes at steps S 101  to S 105  of  FIG. 23  are similar to those at steps S 21  to S 25  described hereinabove with reference to  FIG. 12 , respectively. Therefore, using it as a trigger that a packet is transmitted from the communication apparatus  15  to the mobile terminal  10  whose position is registered already, the flow tables  87  of the relaying apparatus  80  included in the route from the conversion apparatus  45  to the movement destination subnetwork are updated. Further, transmission and reception of a packet after the flow tables  87  in the relaying apparatus  80  are updated are such as described hereinabove with reference to  FIG. 18 . 
         [0147]      FIG. 24  is a sequence diagram depicting an example of processing performed when a flow table is updated using a transmission process of a packet from a mobile terminal as a trigger. Also in  FIG. 24 , the forwarding process of an encapsulated packet is indicated by a double line arrow mark. 
         [0148]    It is assumed that, at step S 110 , after the position registration process of the mobile terminal  10 , the mobile terminal  10  generates a packet by adding an outer header destined for the conversion apparatus  45  to the data packet destined for the communication apparatus  15  and transmits the generated packet. It is to be noted that the information of the addresses in the packet generated by the mobile terminal  10  is such as described just below: 
         [0149]    Destination address of the outer header: 10.0.10.1 (conversion apparatus  45 ) 
         [0150]    Transmission source address of the outer header: 10.0.30.10 (CoA) 
         [0151]    Destination address of the inner header: 10.0.20.20 (communication apparatus  15 ) 
         [0152]    Transmission source address of the inner header: 10.0.10.10 (HoA) 
         [0153]    Since the mobile terminal  10  transmits the generated packet toward the conversion apparatus  45 , the packet that is in an encapsulated form is received by the relaying apparatus  80   d.    
         [0154]    Since the destination of the outer header of the received packet is the address of the conversion apparatus  45 , the relaying apparatus  80   d  applies a process for a packet destined for an apparatus in the subnetwork  5   a  using the flow table  87   d _ 1 . The relaying apparatus  80   d  outputs the received packet from the port  1  thereof to forward the received packet to the relaying apparatus  80   c  (step S 111 ). Also the relaying apparatus  80   c  processes the received packet similarly to the relaying apparatus  80   d  using the flow table  87   c _ 1 , and therefore, the packet in an encapsulated form is forwarded to the relaying apparatus  80   a  (step S 112 ). The relaying apparatus  80   a  forwards the received packet to the conversion apparatus  45  in accordance with the flow table  87   a _ 1  (step S 113 ). 
         [0155]    The forwarding processing unit  36  in the conversion apparatus  45  acquires the data packet through the reception unit  23  and the packet identification unit  24 . The forwarding processing unit  36  outputs the acquired packet to the decapsulation unit  38  in order to remove the outer header destined for the conversion apparatus  45  from the packet. The decapsulation unit  38  decapsulates the packet inputted thereto. Further, the decapsulation unit  38  refers to the position registration information  32  using the transmission source address of the inner IP header as a key to confirm that position registration of the mobile terminal  10  is performed. If the position registration is performed, then it is decided that the detection unit  39  detects starting of communication with the mobile terminal  10  (step S 114 ). The decapsulation unit  38  transmits the data packet after the decapsulation through the transmission unit  22 . By this process, the data packet after the decapsulation is forwarded to the relaying apparatus  80   a  (step S 115 ). It is to be noted that, in the data packet after the decapsulation, the destination address is the IP address of the communication apparatus  15 . 
         [0156]    The relaying apparatus  80   a  outputs the data packet from the port  1  thereof using the flow table  87   a _ 1  to forward the data packet to the relaying apparatus  80   c  (step S 116 ). The relaying apparatus  80   c  receives the data packet destined for the communication apparatus  15  and outputs the data packet from the port  1  thereof in accordance with the flow table  87   c _ 1  to forward the data packet to the relaying apparatus  80   b  (step S 117 ). The relaying apparatus  80   b  receives the data packet and outputs the data packet from the port  1  thereof in accordance with the flow table  87   b  to forward the data packet to the communication apparatus  15  (step S 118 ). 
         [0157]    At step S 119 , the detection unit  39  notifies the setting requesting unit  42  of the starting of communication with the mobile terminal  10 . The setting requesting unit  42  generates a flow table setting request in response to the request of the detection unit  39  and transmits the flow table setting request to the control apparatus  50  through the transmission unit  22 . 
         [0158]    The processes at steps S 120  to S 124  in  FIG. 24  are similar to those at steps S 21  to S 25  described hereinabove with reference to  FIG. 12 , respectively. Therefore, using it as a trigger that the mobile terminal  10  whose position registration is completed transmits a packet to a different apparatus, the flow tables  87  of the relaying apparatus  80  included in the route from the conversion apparatus  45  to the movement destination subnetwork are updated. Further, transmission and reception of a packet after the flow tables  87  in the relaying apparatus  80  are updated are such as described hereinabove with reference to  FIG. 18 . 
         [0159]      FIG. 25  is a flow chart illustrating an example of processing of a conversion apparatus. The conversion apparatus described with reference to  FIG. 25  may be the conversion apparatus  45  depicted in  FIG. 22 . When the conversion apparatus  45  receives a data packet, the forwarding processing unit  36  acquires the data packet through the reception unit  23  and the packet identification unit  24  (step S 131 ). The forwarding processing unit  36  decides whether the destination IP address of the data packet is the home address of the mobile terminal  10  (step S 132 ). When the destination IP address of the data packet is the home address of the mobile terminal  10 , the encapsulation unit  37  encapsulates and then forwards the data packet (Yes at step S 132 , and steps S 133  and  134 ). On the other hand, when the destination address of the data packet is not the HoA of the mobile terminal  10 , the forwarding processing unit  36  decides whether the data packet is an encapsulated packet transmitted from the mobile terminal  10  (No at step S 132 , and step S 135 ). It is to be noted that, in an encapsulated packet transmitted from the mobile terminal  10 , the transmission source IP address in the inner header is set to the home address of the mobile terminal  10 . If the data packet is an encapsulated packet transmitted from the mobile terminal  10 , then the decapsulation unit  38  decapsulates the data packet and then forwards the decapsulated data packet (Yes at step S 135 , and steps S 136  and S 137 ). After the process at step S 134  or step S 137  is performed, the setting requesting unit  42  transmits a flow table setting request toward the control apparatus  50  (step S 138 ). On the other hand, if it is decided at step S 135  that the data packet is not an encapsulated packet transmitted from the mobile terminal  10 , then the forwarding processing unit  36  performs a forwarding process in accordance with the destination of the data packet (step S 139 ). 
         [0160]    If the second embodiment is applied, then even if the mobile terminal  10  moves from the home subnetwork, the changing process of the flow table  87  is not performed until communication is started. Therefore, where the mobile terminal  10  frequently moves between subnetworks and besides the possibility that the mobile terminal  10  may not perform communication in the subnetwork of the destination of the movement is high, a useless setting process can be omitted. 
       Third Embodiment 
       [0161]    Now, a case is described in which it is detected by a control apparatus ( FIG. 26 ) that a mobile terminal transmits a position registration request to a conversion apparatus ( FIG. 29 ) and update of a flow table held by a relaying apparatus is performed using transmission of the position registration request as a trigger. 
         [0162]      FIG. 26  is a block diagram depicting an example of a configuration of a control apparatus according to a third embodiment. A control apparatus  75  includes a transmission and reception unit  51 , an identification unit  54 , a control message processing unit  55 , a storage unit  60 , and a setting request processing unit  76 . The transmission and reception unit  51 , the identification unit  54 , the control message processing unit  55  and the storage unit  60  operate similarly to those of the control apparatus  50  used in the first and second embodiments. The setting request processing unit  76  includes an extraction unit  71 , a selection unit  72 , a generation unit  73  and a position registration forwarding unit  74 . 
         [0163]    In the third embodiment, it is assumed that a relaying apparatus  80  coupled to a subnetwork can specify a position registration request message. The relaying apparatus  80  transmits a position registration request message to the control apparatus  75  using a controlling line. The control apparatus  75  transmits the position registration request message to a conversion apparatus  25  using the controlling line. The conversion apparatus  25  transmits a position registration response message to the control apparatus  75  using the controlling line. 
         [0164]    The identification unit  54  outputs, when a packet inputted thereto is a position registration request message, the position registration request message to the position registration forwarding unit  74 . The position registration forwarding unit  74  forwards the position registration request message to the conversion apparatus  25  through a transmission unit  52 . Therefore, the position registration request message is transmitted from the control apparatus  75  to the conversion apparatus  25  through the controlling line. 
         [0165]    Meanwhile, the extraction unit  71  in the control apparatus  75  can specify the home subnetwork and the movement destination subnetwork of the mobile terminal  10  using the position registration response message. It is to be noted that the position registration response message includes the home address and the care-of address of the mobile terminal  10  and is transmitted as a registration notification for notifying the mobile terminal  10  that the care-of address of the mobile terminal  10  is registered from the conversion apparatus  25  to the mobile terminal  10 . The extraction unit  71  notifies the selection unit  72  of the home subnetwork and the movement destination subnetwork of the mobile terminal  10 . The processes by the selection unit  72  and the generation unit  73  are similar to those in the first and second embodiments. 
         [0166]      FIG. 27  is a view depicting an example of a flow table.  FIG. 27  depicts an example of a flow table  63  held by the control apparatus  75  when the mobile terminal  10  is positioned in the subnetwork  5   a  that is the home network of the mobile terminal  10 . Since the network also in the third embodiment is such as depicted in  FIG. 3 , in the relaying apparatus  80   a ,  80   b , and  80   d  each coupled to one of the subnetworks  5   a  to  5   c , a condition to be used for forwarding of a position registration request message is set. It is assumed that, in the position registration request message, the user datagram protocol (UDP) is used and the UDP destination port number is fixed to  434 . Further, in a packet other than the position registration request message, the UDP destination port number is set to any other value than  434 . Accordingly, if the UDP destination port number is  434 , then any relaying apparatus  80  coupled to the subnetwork  5  can decide that a position registration request message is received. Further, the process for a packet whose UDP destination port number is  434  is to output to the controlling line coupled to the control apparatus  75 . Therefore, when a position registration request message is received, the relaying apparatus  80  can forward the position registration request message to the control apparatus  75 . The information other than the information of the first entry in each of the relaying apparatus  80   a ,  80   b , and  80   d  is similar to the information of the flow table  63  depicted in  FIG. 8 . 
         [0167]      FIG. 28  is a view depicting an example of a flow table held by a relaying apparatus. The relaying apparatus described with reference to  FIG. 28  may be the relaying apparatus  80  described with reference to  FIG. 2 , and the flow table depicted in  FIG. 28  may be the flow table  87  depicted in  FIG. 9 . It is assumed that, at a stage prior to movement of the mobile terminal  10 , the relaying apparatus  80   a  holds the flow table  87   a _ 3 ; the relaying apparatus  80   b  holds the flow table  87   b _ 2 ; the relaying apparatus  80   c  holds the flow table  87   c _ 3 ; and the relaying apparatus  80   d  holds the flow table  87   d _ 3 . It is to be noted that the flow table  87   a _ 3  is information of the relaying apparatus ID=1 in the flow table  63  depicted in  FIG. 27 . Similarly, the flow table  87   b _ 2  is information of the relaying apparatus ID=2; the flow table  87   c _ 3  is information of the relaying apparatus ID=3; and the flow table  87   d _ 3  is information of the relaying apparatus ID=4, extracted from the flow table  63  depicted in  FIG. 27 . 
         [0168]      FIG. 29  is a block diagram depicting an example of a configuration of a conversion apparatus according to the third embodiment. A conversion apparatus  25  includes a transmission and reception unit  21 , a packet identification unit  24 , a storage unit  30 , a packet processing unit  35  and a message processing unit  46 . The processes by the transmission and reception unit  21 , the packet identification unit  24 , the storage unit  30  and packet processing unit  35  are such as described hereinabove with reference to  FIG. 4  and so forth. 
         [0169]    The message processing unit  46  performs, when a position registration request message is acquired by communication through the controlling line, an authentication process and a position registration. At this time, the message processing unit  46  suitably uses the authentication information  31  to update the position registration information  32 . When completing the position registration, the message processing unit  46  designates a forwarding destination of the position registration response message to the control apparatus  75  and then outputs the position registration response message to a transmission unit  22 . Therefore, the message processing unit  46  can forward the position registration response message destined for the mobile terminal  10  to the control apparatus  75  using the controlling line through the transmission unit  22 . 
         [0170]      FIG. 30  is a sequence diagram depicting an example of processing performed upon change of a flow table in the third embodiment. In the following, an example wherein the mobile terminal  10  moves from the subnetwork  5   a  that is the home subnetwork of the mobile terminal  10  to the subnetwork  5   c  is described. It is to be noted that  FIG. 30  illustrates an example of the processing, and, for example, the processes at steps S 158  to S 162  may be performed after the processes at step S 163  and S 164 , or the processes at steps S 158  to S 162  may be performed in parallel to the processes at steps S 163  and S 164 . 
         [0171]    At step S 151 , if the mobile terminal  10  detects that the mobile terminal  10  moves from the home subnetwork to the different subnetwork  5   c , then the mobile terminal  10  transmits a position registration request message toward the conversion apparatus  25  as a destination. Since the relaying apparatus  80   d  is positioned on the route from the subnetwork  5   c  to the conversion apparatus  25  and is coupled to the subnetwork  5   c , the relaying apparatus  80   d  receives the position registration request message. 
         [0172]    At step S 152 , the determination unit  91   d  of the relaying apparatus  80   d  acquires the position registration request message through the transmission and reception unit  81   d  and the sorting unit  82   d . Since the serial number of the UDP destination port in the position registration request message is  434 , the determination unit  91   d  selects the first entry in the flow table  87   d _ 3  ( FIG. 28 ) as a condition to be used for forwarding of the packet. The determination unit  91   d  outputs the information indicative of the substance of the process recorded in the selected entry and the position registration request message to the packet processing unit  93   d.    
         [0173]    At step S 153 , the packet processing unit  93   d  forwards the position registration request message inputted thereto from the determination unit  91   d  to the control apparatus  75  through the transmission and reception unit  81   d . Note that it is assumed that the position registration request message is forwarded from the relaying apparatus  80   d  to the control apparatus  75  through a path for controlling. The position registration request message is forwarded, for example, using Packet-in of OpenFlow. 
         [0174]    At step S 154 , the identification unit  54  in the control apparatus  75  acquires the position registration request message through the reception unit  53 . The identification unit  54  outputs the position registration request message to the position registration forwarding unit  74 . The position registration forwarding unit  74  forwards the position registration request message to the conversion apparatus  25  through the transmission unit  52  (step S 155 ). Note that it is assumed that the position registration request message is forwarded from the control apparatus  75  to the conversion apparatus  25  through a path for controlling. 
         [0175]    At step S 156 , the packet identification unit  24  of the conversion apparatus  25  acquires the position registration request message through a reception unit  23 . The packet identification unit  24  outputs the position registration request message to the message processing unit  46 . The message processing unit  46  performs an authentication process using the parameters included in the position registration request message and performs position registration if the authentication results in success. When the position registration comes to an end, the message processing unit  46  generates a position registration response message destined for the mobile terminal  10 . Here, as the header of the position registration response message, the care-of address of the mobile terminal  10  is designated. Further, the home address of the mobile terminal  10  is recorded in the payload of the position registration response message. Accordingly, the position registration response message includes the care-of address and the home address of the mobile terminal  10 . The message processing unit  46  designates the forwarding destination for the position registration response message to the control apparatus  75  and then outputs the position registration response message to the transmission unit  22 . The transmission unit  22  forwards the position registration response message to the control apparatus  75  in accordance with the designation from the message processing unit  46  (step S 157 ). Note that it is assumed that the position registration response message is forwarded from the conversion apparatus  25  to the control apparatus  75  through a path for controlling. 
         [0176]    At step S 158 , the identification unit  54  in the control apparatus  75  acquires the position registration response message through the reception unit  53 . The identification unit  54  outputs the position registration response message to the extraction unit  71 . The extraction unit  71  extracts the home address and the care-of address of the mobile terminal  10  from the position registration response message and specifies the home subnetwork and the movement destination subnet of the mobile terminal  10 . The selection unit  72  searches for the relaying apparatus  80  included in the route from the home subnetwork to the movement destination subnetwork of the mobile terminal  10  using the topology information table  61  and the coupling information table  62 . Also in the present example, it is assumed that the relaying apparatus  80   a ,  80   c , and  80   d  are selected as a target for requesting a change of the flow table  87  by the selection unit  72 . Thereafter, the flow table  63  is changed by the generation unit  73 . 
         [0177]      FIG. 31  is a view depicting an example of a change of a flow table. The flow table depicted in  FIG. 31  may be the flow table  63  depicted in  FIG. 26 . The process for change of the flow table  63  is similar to the process for change of the flow table  63  in the first embodiment. The generation unit  73  changes the flow table  63  such that, in each relaying apparatus  80  selected by the selection unit  72 , the forwarding destination of a packet destined for the home address of the mobile terminal  10  becomes same as the output port for a packet destined for the movement destination subnet of the mobile terminal  10 . By this process, the first entry is added with regard to each of the relaying apparatus ID=1 (relaying apparatus  80   a ), the relaying apparatus ID=3 (relaying apparatus  80   c ) and the relaying apparatus ID=4 (relaying apparatus  80   d ). Further, the generation unit  73  adds an entry such that the relaying apparatus  80  coupled to the movement destination subnetwork performs a forwarding process of an encapsulated packet transmitted from the mobile terminal  10  after the relaying apparatus  80  decapsulates the encapsulated packet. By this process, the second entry of the relaying apparatus ID=4 (relaying apparatus  80   d ) is added. 
         [0178]    At step S 159  of  FIG. 30 , update of the flow table  87  in the relaying apparatus  80  is requested in response to the update of the flow table  63 . The process at step S 159  is similar to the process at the step S 22  ( FIG. 12 ) described in connection with the first embodiment. Also the processes at steps S 160  to S 162  of  FIG. 30  are similar to the processes at steps S 23  to S 25  described hereinabove with reference to  FIG. 12 , respectively. 
         [0179]      FIG. 32  is a view depicting an example of a flow table after updated. The flow table described with reference to  FIG. 32  may be the flow table  87  depicted in  FIG. 28 . At step S 160 , the flow table  87   c _ 3  ( FIG. 28 ) is changed into a flow table  87   c _ 4 . Similarly, at step S 161 , the flow table  87   a _ 3  ( FIG. 28 ) is changed into a flow table  87   a _ 4 . Further, at step S 162 , the flow table  87   d _ 3  ( FIG. 28 ) is changed into a flow table  87   d _ 4 . 
         [0180]    At step S 163  of  FIG. 30 , the position registration forwarding unit  74  of the control apparatus  75  outputs the position registration response message to the relaying apparatus  80   d  coupled to the movement destination subnetwork. At this time, the position registration forwarding unit  74  notifies the relaying apparatus  80   d  also that the position registration response message is to be outputted from the port  2  of the relaying apparatus  80   d . For example, the position registration forwarding unit  74  can forward the position registration response message to the relaying apparatus  80   d  using the Packet-Out instruction of OpenFlow. Since the relaying apparatus  80   d  outputs the position registration response message from the port  2  thereof in accordance with the instruction from the control apparatus  75 , the position registration response message is forwarded to the mobile terminal  10  (step S 164 ). 
         [0181]      FIG. 33  is a flow chart illustrating an example of processing of a control apparatus. The control apparatus depicted in  FIG. 33  may be the control apparatus  75  depicted in  FIG. 26 . If the control apparatus  75  receives a packet, then the identification unit  54  determines whether a position registration response message is received (steps S 171  and S 172 ). The identification unit  54  outputs a packet other than the position registration response message to the control message processing unit  55 , and the control message processing unit  55  processes the packet inputted thereto (No at step S 172 , and step S 173 ). 
         [0182]    On the other hand, if the received packet is a position registration response message, then the extraction unit  71  extracts the home address and the care-of address of the mobile terminal  10  from the position registration response message (Yes at step S 172 , and step S 174 ). The processes at steps S 175  to S 177  are similar to the processes at steps S 75  to S 77  described hereinabove with reference to  FIG. 20 , respectively. Thereafter, the control apparatus  75  forwards the position registration response message to the relaying apparatus  80  coupled to the movement destination subnetwork (step S 178 ). 
         [0183]    In the third embodiment, since the setting of the flow table  87  is changed using it as a trigger that position registration is performed, the flow table  87  can be changed prior to starting of communication with the mobile terminal  10 . Therefore, packets to be used for communication with the mobile terminal  10  can be transmitted and received in a non-encapsulated state beginning with the first packet, and increase of the communication amount by the outer header can be prevented. 
         [0184]    Further, since the conversion apparatus  25  receives a position registration request message from the control apparatus  75 , the conversion apparatus  25  may be installed also in any other subnetwork than the home subnetwork of the mobile terminal  10 . Therefore, the degree of freedom in arrangement of the apparatus when the network is to be formed is enhanced. 
         [0185]    Furthermore, if the relaying apparatus  80  detects a position registration request message, then the relaying apparatus  80  transmits the position registration request message to the control apparatus  75  using a line for transmission and reception of a control message. Then, the position registration request message from the control apparatus  75  is forwarded to the conversion apparatus  25 . Therefore, even in a situation in which the transmission delay is great because the amount of packets transmitted and received along a route used for transmission and reception of a data packet is large, the delay amount that is generated in a forwarding process of a position registration request message can be reduced. Also the changing process of the flow table  87  is performed rapidly after the position registration process. 
       OTHERS 
       [0186]    It is to be noted that the embodiments are not limited to the embodiments described above and can be modified in various manners. In the following, several modifications are described. 
         [0187]    The information factors in the tables and packets described above are exemplary and can be changed in accordance with installation. 
         [0188]    For example, the second embodiment may be modified such that, after the conversion apparatus  45  detects starting of communication with the mobile terminal  10 , rewriting of the flow table  87  by the relaying apparatus  80  is performed before a forwarding process of the received packet is performed. In this case, in the example of  FIG. 23 , the processes at steps S 100  to S 105  are performed before the processes at steps S 96  to S 99 . On the other hand, in the example of  FIG. 24 , the processes at steps S 119  to S 124  are performed before the processes at steps S 115  to S 118 . 
         [0189]    It is to be noted that one or more apparatus in the network may be implemented by a single computer. For example, one or more of the control apparatus  50  and the relaying apparatus  80  may be implemented by a single computer. 
         [0190]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.