DESCRIPTION MOBILE TERMINALAND NETWORK NODE TECHNICAL FIELD [0001] The present invention relates to a mobile terminal and a network node capable of performing packet communication using a wireless communication function while moving, and more particularly to a mobile terminal and a network node having two or more radio communication interfaces. BACKGROUND ART [0002] A mobile node (MN) using mobile IP (see Non-Patent Document 1 cited below. The mobile IP may also be referred to as CMIP below.) registers a care-of address (CoA) as a destination address with a home agent (HA) managing the home address (HoA) of the mobile node, enabling a request for forwarding, to the CoA, of packets destined to the HoA. In this specification, a mobile terminal capable of performing communication using a wireless communication function while moving is called a mobile node (MN). In other words, the MN in this specification does not always need to implement mobile IP. [0003] Further, if the MN can register two or more CoAs in association with one HoA at the same time, the MN having two or more interfaces (which may be abbreviated as "IF" below) can register two or more CoAs assigned to the respective interfaces to use the CoAs, enabling switching between the CoAs depending on the state of each interface. Non-Patent Document 2 cited below describes a technique for allowing a MN to register, with a HA, two or more CoAs in association with one HoA. [0004] On the other hand, when a HA receives the registration of two or more CoAs from a MN, the HA can select a CoA based on the intention of the MN if the HA can acquire, from the MN, information (flow control information, Routing Rule) for determining which CoA should be used as the destination of packets destined to the HoA of the MN. [0005] For example, Non-Patent Document 3 cited below describes a technique for causing a MN to register flow control information with a HA as a technique for enabling the HA to select a CoA according to the flow. This flow control information includes flow information (Routing Filter) consisting of pieces of information (5-tuple) such as a destination address, a source address, a destination port number, a source port number, and a protocol number, and a flow to be identified is represented by one or more pieces of information, or a combination of all pieces of information. The flow control information also includes information in which a flow ID for identifying each piece of flow information is associated with a CoA (or a Binding Unique Identifier (BID) for identifying the CoA) used as a destination. The flow ID may be information for identifying an actual flow. If desiring to set a specific IF as a forwarding destination for a certain flow, the MN notifies the HA of flow control information including a flow ID for identifying the flow and a CoA assigned to an IF desired to be set as the forwarding destination of this flow or a BID for identifying the CoA. [0006] Further, if the flow control information is notified from the MN, the HA receiving the registration of two or more CoAs forwards packets according to the flow control information. However, if the flow control information is not notified from the MN, the HA can switch CoAs as forwarding destinations according its path selection policy. In this case, the HA can consider the connection conditions of the MN, communication path conditions and the conditions of communication with other nodes to select a CoA to which packets addressed to the MN are to be sent, so that the HA can switch over to a path more effective and suited to the conditions the HA can figure out. If there is a flow whose flow control information is not notified from the MN, the HA may request the MN to register the flow control information. Alternatively, the HA may notify the MN of flow control information generated and used by itself. Thus, this is effective because, even if there is a flow for which the MN does not recognize the necessity to register the flow control information, the HA can effectively request the MN to generate and register flow control information if the HA needs the flow control information. Further, if the MN receives a notification from the HA of a flow to use a dedicated HoA, the MN can select either a common HoA or the dedicated HoA based on information notified from the HA upon starting communication. For example, when a 3GPP network operator offers a service to users to allow the users to use two or more interfaces as forwarding destinations, it permits use of the common HoA on a specific condition (e.g., in the case of communication with a specified correspondent or a node belonging to a common operator or a specified operator). On the other hand, if not, it can perform control such as to instruct use of the dedicated HoA. [0007] Further, in Non-Patent Document 4 cited below, Proxy Mobile IP (PMIP, e.g., Proxy Mobile IPv6) is defined as a network-based mobility control protocol. PMIP is a technique for performing the mobility management of the MN on the network side to eliminate the need for the MN to perform processing for mobility control. Each network in a domain (PMIP domain) providing PMIP is configured to advertise the same home prefix to a specific MN at all times, so that any MN changing its network connection does not need to change its address, enabling the MN to act simply as an IPv6 node without the need to be aware of the presence of a HA. [0008] On the other hand, on the network side, a Mobile Access Gateway (MAG) performs mobility control as a proxy node of the MN not only to advertise the home prefix to theMN, but also to register, with a Local Mobility Anchor (LMA), itsaddress (or its prefix) as location information on the destination of the MN. The LMA manages the location information on the MN in the PMIP domain, and corresponds to a HA in mobile IP. Thus, packets destined to the home address of the MN are intercepted by the LMA, forwarded from the LMA to the MAG, and further sent from the MAG to the MN. Non-Patent Document 1: D.Johnson, C.Perkins, J.Arkko, "Mobility Support in IPv6," RFC3775, June 2004. Non-Patent Document 2: R.Wakikawa, T.Ernst, K.Nagami, "Multiple Care-of Addresses Registration," draft-ietf-monami6-multiplecoa-03.txt, July 2007. Non-Patent Document 3: H.Soliman, N.Montavont, N.Fikouras, K.Kuladinithi, "Flow Bindings in Mobile IPv6 and Nemo Basic Support," draft-soliman-monami6-flow-binding-04.txt, February 2007. Non-Patent Document 4: S.Gundavelli, K.Leung, V.Devarapalli, "Proxy Mobile IPv6," draft-sgundave-mipv6-proxymipv6-00, October 2006. [0009] However, in the case of use of the technique as shown in Non-Patent Document 3 mentioned above, in which the MN registers, with the HA, flow control information for specifying the forwarding destination of a flow, the MN has to register, with the HA, flow control information on all flows each specifying a specific IF as the forwarding destination. [0010] For example, upon receiving a flow related to communication originated from a Correspondent Node (CN), if the MN wants the flow to be forwarded to a specific IF, the MN registers, with the HA, flow control information for specifying the forwarding destination of the flow. Similarly, upon starting communication with the CN, if the MN wants a flow sent from the CN in connection with the communication to be forwarded to a specific IF, the MN also needs to register flow control information for specifying the forwarding destination of the flow. [0011] Thus, the MN has to register, with the HA, flow control information on each flow whose forwarding destination the MN wants to specify. Therefore, as the number of flows whose forwarding destinations the MN wants to specify increases, the number of pieces of flow control information registered by the MN increases. The increase in the number of flows poses a problem of dramatically increasing the load on the HA for forwarding destination control and the loadon the MN for registration of flow information. [0012] Further, since the number of registration messages sent from the MN to register the flow control information and the number of messages as responses to the registration messages increase, the network traffic increases, causing a potential problem of packet loss or delay. DISCLOSURE OF THE INVENTION [0013] In order to solve the above problems, it is an object of the present invention to reduce the processing load accompanied by registration of flow control information and reduce the traffic of signaling for the registration. [0014] In order to attain the above object, the present invention provides a mobile terminal capable of attaching to a network to perform packet communication, the mobile terminal comprising: a plurality of radio communication interfaces; address holding unit configured to hold a common address assigned to the plurality of radio communication interfaces in common and a dedicated address assigned individually to each of the plurality of radio communication interfaces; and binding registration processing unit configured to process first binding information indicative of a correspondence relationship between the mobile terminal and the common address and second binding information indicative of a correspondence relationship between each of the plurality of radio communication interfaces and the dedicated address assigned to each of the plurality of radio communication interfaces in such a manner to be registered with a network node for performing forwarding processing on a packet addressed to the mobile terminal. This configuration can reduce the processing load accompanied by registration of flow control information and reduce the traffic of signaling for the registration. [0015] In addition to the above configuration, the mobile terminal of the present invention may also comprise flow determination unit configured to determine whether a flow necessary to be sent and received by using only a specific interface among the plurality of radio communication interfaces exists, and if determining that a flow necessary to be sent and received by using only a specific interface among the plurality of radio communication interfaces exists, controlling the binding registration processing unit to send the second binding information. According to this configuration, when there is a need to send and receive a flow specifying a specific interface, a dedicated address can be set and registered. [0016] In addition to the above structure, the mobile terminal of the present invention may further comprise address generation unit configured to generate the dedicated address assigned to each of the plurality of radio communication interfaces. According to this configuration, since the mobile terminal itself generates a dedicated address, the dedicated address can be acquired quickly. [0017] In addition to the above configuration, the mobile terminal of the present invention may further comprise address assignment request unit configured to request a specific network node on the network to assign the dedicated address to each of the plurality of radio communication interfaces. According to this configuration, a dedicated address can be acquired easily from the network. [0018] Further, in addition to the above configuration, the mobile terminal of the present invention may be such that the specific network node is a DHCP server or a home agent of the mobile terminal. According to this configuration, a dedicated address can be acquired easily from the DHCP server for managing the assignment of addresses or the home agent. [0019] In addition to the above configuration, the mobile terminal of the present invention may further comprise address determination unit configured to distinguish between the dedicated address and the common address by referring to classification information, wherein the classification information is added to an address provided from the specific network node to indicate as which of the dedicated address and the common address the address is usable. This configuration enables the mobile terminal to make sure to distinguish between an address usable as a dedicated address and an address usable as a common address. [0020] Further, in addition to the above configuration, the mobile terminal of the present invention may be such that the common address and the dedicated address are home addresses, and the binding registration processing unit is configured to register the first binding information and the second binding information with a home agent of the mobile terminal. This configuration enables a mobile terminal running mobile IP to reduce the processing load accompanied by registration of flow control information and reduce the traffic of signaling for the registration. [0021] Further, in addition to the above configuration, the mobile terminal of the present invention may be such that the second binding information includes information indicative of association between the dedicated address and a care-of address assigned to a radio communication interface corresponding to the dedicated address. According to this configuration, information indicative of association between a home address as a dedicated address and a care-of address assigned to a radio communication interface corresponding to the dedicated address can be registered. [0022] In addition to the above configuration, the mobile terminal of the present invention may further comprise external network determination unit configured to determine whether at least two radio communication interfaces among the plurality of radio communication interfaces are connected to different external networks, respectively, and if determining that at least two radio communication interfaces among the plurality of radio communication interfaces are connected to different external networks, respectively, controlling the binding registration processing unit to send the second binding information. According to this configuration, when there are two or more packet transmission paths from a home agent to a mobile terminal via external networks, packet transmission can be achieved in such a manner that a dedicated address is set and registered to specify any one of packet transmission paths. [0023] Further, in addition to the above configuration, the mobile terminal of the present invention may be such that PMIP is executed on the network, and the binding registration processing unit is configured to request a proxy node, to which a radio communication interface corresponding to the dedicated address is connected, to notify a location information management device in the network of identification information of the proxy node and the second binding information including information indicative of association with the dedicated address. According to this configuration, flow control can be achieved in PMIP without registering flow control information with a location information management device (LMA), enabling reduction in processing load accompanied by registration of flow control information and reduction in traffic of signaling for the registration. [0024] In addition to the above configuration, the mobile terminal of the present invention may further comprise proxy node determination unit configured to determine whether at least two radio communication interfaces among the plurality of radio communication interfaces are connected to different proxy nodes, respectively, and if determining that at least two radio communication interfaces among the plurality of radio communication interfaces are connected to different proxy nodes, respectively, controlling the binding registration processing unit to send the second binding information. According to this configuration, when there are two or more packet transmission paths to a mobile terminal via different MAGs, packet transmission can be achieved in such a manner that a dedicated address is set and registered to specify any one of packet transmission paths. [0025] In addition to the above configuration, the mobile terminal of the present invention may further comprise address setting unit configured to determine whether, upon transmission of the packet, the packet is a packet belonging to a flow necessary to be sent and received by using only a specific interface among the plurality of radio communication interfaces, and if determining that the packet is a packet belonging to a flow necessary to be sent and received by using only the specific interface, using the dedicated address corresponding to the specific interface as the source address of the packet According to this configuration, not only a packet sent from the mobile terminal but also a packet returned as its response can be sent and received via a radio communication interface corresponding to the dedicated address. [0026] In addition to the above configuration, the mobile terminal of the present invention may further comprise address setting unit configured to determine whether, upon transmission of the packet, the packet is a packet belonging to a flow necessary to be sent and received by using only a specific interface among the plurality of radio communication interfaces, and if determining that the packet is not a packet belonging to a flow necessary to be sent and received by using only the specific interface, using the common address as the source address of the packet. According to this configuration, packets on which flow control does not need to be performed can be sent and received by using a common address. [0027] Further, in addition to the above configuration, the mobile terminal of the present invention may be such that when switching is so performed that a flow using the common address as the source address of the packet will be sent and received by using only the specific interface, the binding registration processing unit is configured to perform processing for registering flow control information, explicitly indicative of a relationship between the flow and the specific interface, with a network node for performing forwarding processing on a packet addressed to the mobile terminal. According to this configuration, when flow control of packets, on which flow control does not need to be performed, is started, flow control can be achieved by registering flow control information with a network node for performing packet forwarding processing. [0028] In order to attain the above object, the present invention also provides a network node for performing forwarding processing on a packet addressed to a mobile terminal, the network node comprising: binding holding unit configured to hold first binding information indicative of a correspondence relationship between a common address, assigned in common to a plurality of radio communication interfaces contained in the mobile terminal, and the mobile terminal, and second binding information indicative of a correspondence relationship between a dedicated address, assigned individually to each of the plurality of radio communication interfaces, and each of the plurality of radio communication interfaces; and packet to dedicated address forwarding unit configured to, when the dedicated address is set as the destination address of a packet addressed to the mobile terminal upon performing forwarding processing on the packet, forward the packet to a radio communication interface corresponding to the dedicated address. This configuration can reduce the processing load accompanied by registration of flow control information and reduce the traffic of signaling for the registration. [0029] In addition to the above configuration, the network node of the present invention may also comprise packet to common address forwarding unit configured to, when the common address is set as the destination address of a packet addressed to the mobile terminal upon performing forwarding processing on the packet, forward the packet to any radio communication interface among the plurality of radio communication interfaces of the mobile terminal. This configuration enables the network node to select, at its discretion, a forwarding destination address of a packet addressed to the mobile terminal and for which a common address is set as the destination address. [0030] In addition to the above configuration, the network node of the present invention may further comprise address assignment unit configured to, when the mobile terminal requests the network node to assign the dedicated address to each of the plurality of radio communication interfaces, assign the dedicated address to each of the plurality of radio communication interfaces of the mobile terminal. This configuration enables the network node to provide an address usable as a dedicated address in response to an address assignment request from the mobile terminal. [0031] The present invention has such a configuration as mentioned above to eliminate the need for a mobile terminal to register flow control information with an HA, enabling reduction in processing load accompanied by registration of flow control information and reduction in traffic of signaling for the registration. BRIEF DESCRIPTION OF THE DRAWINGS [0032] [FIG. 1] It is a diagram showing the structure of a network according to a first embodiment of the present invention. [FIG. 2] It is a diagram showing the configuration of a MN according to the first embodiment of the present invention. [FIG. 3] It is a diagram showing the configuration of a HA according to the first embodiment of the present invention. [FIG. 4] It is a flowchart showing an example of the operation of the MN in the first embodiment of the present invention. [FIG. 5] It is a flowchart showing another example of the operation of the MN in the first embodiment of the present invention. [FIG. 6] It is a flowchart showing an example of data transmission processing at the MN in the first embodiment of the present invention. [FIG. 7] It is a flowchart showing an example of packet forwarding processing at the HA in the first embodiment of the present invention. [FIG. 8] It is a sequence chart showing an example in which the MN selects a dedicated HoA as a source address in the first embodiment of the present invention. [FIG. 9] It is a sequence chart showing an example in which the MN selects a common HoA as the source address in the first embodiment of the present invention. [FIG. 10] It is a diagram showing an example of a HoA assignment request message sent from the MN to the HA in the first embodiment of the present invention. [FIG. 11] It is a diagram showing an example of a HoA assignment notification message sent from the MN to the HA in the first embodiment of the present invention. [FIG. 12] It is a diagram showing another example of the HoA assignment request message in the first embodiment of the present invention. [FIG. 13] It is a diagram showing another example of the HoA assignment notification message in the first embodiment of the present invention. [FIG. 14] It is a diagram showing an example of address binding information on the MN in the first embodiment of the present invention. [FIG. 15] It is a diagram showing an example of the configuration of a MN according to a second embodiment of the present invention. [FIG. 16] It is a diagram showing an example of the configuration of a HA according to the second embodiment of the present invention. [FIG. 17] It is a flowchart showing an example of the operation of the MN in the second embodiment of the present invention. [FIG. 18] It is a flowchart showing another example of the operation of the MN in the second embodiment of the present invention. [FIG. 19] It is a diagram showing an example of a BU message sent from the MN to the HA in the second embodiment of the present invention. [FIG. 20] It is a diagram showing an example of a BA message sent from the HA to the MN in the second embodiment of the present invention. [FIG. 21] It is a diagram showing the structure of a network according to a third embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION [0033] First to third embodiments of the present invention will now be described with reference to the accompanying drawings. [0034]