Patent Publication Number: US-2007121549-A1

Title: Fast handover method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of Korean Application No. 2005-113488, filed Nov. 25, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      Aspects of the present invention relate to a fast handover method and apparatus of a mobile node between sub-networks and a handover supporting method and apparatus. More particularly, aspects of the present invention relate to a fast handover method and apparatus of a mobile node between sub-networks using a predictive algorithm and a handover supporting method and apparatus.  
      2. Description of the Related Art  
       FIG. 1  is a schematic showing a mobile communication environment according to a related art mobile “Internet Protocol version 6” (IPv6) standard. Referring to  FIG. 1 , the mobile communication environment includes a mobile node (MN)  1 , access points (APs) AP 1   21  and AP 2   22 , a previous access router (PAR)  31 , a new access router (NAR)  32 , a home agent (HA)  4 , and a correspondent node (CN)  5 .  
      As shown, the AP 1   21  manages a basic service set (BSS) BSS 1  and wirelessly communicates with the MN  1  included in the BSS 1 . The PAR  31  manages a sub-network  1  and wirelessly communicates with the AP 1   21  included in the sub-network a. Accordingly, the MN  1  can access the Internet via the AP 1   21  and the PAR  31 , and communicates with the HA  4  or the CN  5  which also access the Internet. In addition, the AP 2   22  manages a BSS 2  and wirelessly communicates with the MN  1  included in the BSS 2 . The NAR  32  manages a sub-network b and communicates with the AP 2   22  included in the sub-network b. Accordingly, the MN  1  can access the Internet via the AP 2   22  and the NAR  32 , and communicates with the HA  4  or the CN  5  which also access the Internet. Accordingly, the MN  1  is able to move from the BSS 1  managed by the AP 1   21  to the BSS 2  managed by the AP 2   22  and maintain communication with HA  4  and/or CN 5  via the internet.  
      As shown, the HA  4  is a router that has the registration information of the MN  1 . HA  4  is one of the routers located in a home network of the MN  1 . Particularly, when the MN  1  is not located in a home network thereof, the HA  4  intercepts packets which are directed to the home address of the MN  1  and tunnels the packets to a care-of-address (COA) of the MN  1  used in a sub-network where the MN  1  is currently located. Accordingly, although the MN  1  moves between the sub-networks, the MN  1  can receive the packets which are directed to the home address of the MN  1  regardless of which of the sub-networks the MN 1  is located. However, in order to implement such a configuration, the MN  1  must notify the HA  4  of the current COA thereof.  
       FIG. 2  is a diagram of a handover method according to the related art mobile IPv6 standard. The handover method is disclosed in “Request For Comments” (RFCs)  3775 . Referring to  FIG. 2 , the handover method includes operations which are sequentially carried out in the mobile communication environment shown in  FIG. 1 .  
      As shown, in operation  201 , the NAR  32  multicasts a router advertisement (RA) message, and the MN  1  receives the multicasted RA message from the NAR  32 . The MN  1  recognizes that the MN  1  is moving from the sub-network a to the sub-network b because of a prefix of the sub-network b included in the RA message from the NAR  32 . The RA message from the NAR  32  may be a message which is periodically multicasted by the NAR  32  or a message in response to a router solicitation (RS) message from the MN  1 .  
      In operation  202 , the MN  1  generates a COA of the MN  1  by combining the prefix of the sub-network b included in the received RA message and an Interface ID of the MN  1 , and performs a duplicate address detection (DAD) operation for the generated COA.  
      In operation  203 , when the DAD result of operation  202  indicates that the generated COA for the MN  1  is not used by an arbitrary node included in the sub-network b, the MN  1  transmits to the HA  4  a binding update (BU) message to request binding of the generated COA with the home address of the MN  1 . The HA  4  then receives the BU message.  
      In operation  204 , the HA  4  performs a binding update (BU) for the MN  1  by binding the generated COA included in the received BU message received in operation  203  with the home address of the MN  1 .  
      In operation  205 , the HA 4  transmits a binding acknowledgement (BAck) message to acknowledge that the BU of the MN  1  was normally performed, and the MN  1  receives the transmitted BAck message.  
      In operation  206 , since the CN  5  cannot recognize that the MN  1  moves from the sub-network a to the sub-network b, the CN  5  continues to transmit the packets which are directed to the home address of the MN  1 . Subsequently, the HA  4  intercepts the packets which are directed to the home address of the MN  1  and tunnels the packets to the COA that has been bound to the home address of the MN  1  through the BU in operation  205 . Accordingly, the MN  1  receives the packets.  
      In the above, operations  201  to  206  are carried out during a wireless overlap interval, which is an interval from a time when the MN  1  is completely apart from the sub-network a to a time when the MN  1  completely enters the sub-network b. Meanwhile, in order to perform the DAD of the COA of the MN  1 , a neighbor solicitation (NS) message and a neighbor advertisement (NA) message must be transmitted and received. However, it takes up to one second to perform the transmission and reception of the NA and NS messages. As a result, handover delay occurs after the wireless overlap interval, and the communication is disconnected between the MN  1  and the sub-networks a and b.  
       FIG. 3  is a diagram of a fast handover method according to the related art mobile IPv6 standard. The fast handover method shown in  FIG. 3  is disclosed in RFC 4068 as a technique to solve the aforementioned problem. Referring to  FIG. 3 , the fast handover method according to the related art mobile IPv6 standard includes operations as follows.  
      In operation  301 , when the MN  1  detects a new sub-network b, the MN  1  transmits a router-solicitation-for-proxy-advertisement (RtSolPr) message to the PAR  31  to request information on the handover from the sub-network a to the sub-network b, and the PAR  31  receives the RtSolPr message.  
      In operation  302 , the PAR  31  transmits to the MN  1  a proxy-router-advertisement (PrRtAdv) message that includes the information on the handover from the sub-network a to the sub-network b as a response to the RtSolPr message that was received in operation  301 , and the MN  1  receives the PrRtAdv message. The information on the handover from the sub-network a to the sub-network b may include the prefix of the sub-network b and the like. Accordingly, the MN  1  can generate the COA thereof when the MN  1  is still connected to the PAR  31 .  
      In operation  303 , when the MN  1  detects the connection to the sub-network b, the MN  1  transmits a fast binding update (BU) message to the PAR  31  to request binding of the newly generated COA with the previous COA of the MN  1  with reference to the PrRtAdv message received in operation  302 , and the PAR  31  receives the fast BU message.  
      In operation  304 , the PAR  31  binds the new COA included in the fast BU message received in operation  303  with the previous COA of the MN  1 . The PAR  31  transmits a handover initiate (HI) message to the NAR  31  to enquire whether the new COA can be used, and the NAR  32  receives the Hi message. Accordingly, the NAR  32  checks whether or not the new COA included in the fast BU message can be used.  
      In operation  305 , the NAR  32  transmits to the PAR  31  a handover acknowledge (HAck) message to the PAR  31  indicating whether or not the new COA for the MN  1  can be used as a response to the Hi message transmitted in operation  304 , and the PAR  31  receives the HAck message.  
      In operation  306 , when the received HAck message of operation  305  indicates that the new COA for the MN  1  can be used, the PAR  31  transmits a fast binding acknowledge (FBAck) message as a response to the received fast BU message received of operation  303 . The FBAck message is sent to and received by the PAR  31  and the NAR  32 .  
      In operation  307 , when the MN  1  and the PAR  31  are disconnected from each other, the NAR  32  transmits packets received from the MN  1  using the BU of operation  304 .  
      In operation  308 , when the MN  1  does not receive the fast BAck message when the MN  1  and the NAR  32  are connected to each other, the MN  1  transmits to the NAR  32  a fast neighbor advertisement (FNA) message to confirm that the new COA is usable, and uses the new COA.  
      However, in the aforementioned related art fast handover method of  FIG. 3 , the RtSolPr, PrRtAdv, HI message, and HAck messages are newly-introduced messages different from the messages which are used in related art handover method of  FIG. 2 . Accordingly, a problem of increased load of the MN  1  occurs. In addition, since mobile IP stacks for the router and the MN  1  must be modified, a problem of increased cost involved in employing the fast handover method occurs.  
     SUMMARY OF THE INVENTION  
      According to aspects of the present invention, a fast handover method and apparatus of a mobile node between sub-networks uses only existing message types of the mobile IPv6 standard instead of using separately defined message types, and also includes and a fast handover supporting method and apparatus.  
      Aspects of the present invention also include one or more computer readable recording media having embodied thereon more or more computer programs for the fast handover method and the fast handover supporting method.  
      According to an aspect of the present invention, a handover method includes: identifying a sub-network which is likely to be connected to a mobile node; requesting for detecting whether or not an address of the mobile node which is to be used in the sub-network is used; and selectively requesting for performing binding update of the mobile node based on a response to the request.  
      According to another aspect of the present invention, a handover apparatus includes: a prediction unit which predicts a sub-network which is likely to be connected to a mobile node; a request unit which requests for detecting whether or not an address of the mobile node which is to be used in the sub-network is used; and a binding request unit which selectively requests for performing binding update of the mobile node based on a response to the request.  
      According to another aspect of the present invention, a computer readable recording medium having embodied thereon a computer program for a computer to perform the aforementioned handover method.  
      According to another aspect of the present invention, a handover supporting method includes: detecting whether or not an address of a mobile node which is to be used in a sub-network which is predicted to be likely to be connected to the mobile node is used according to a request of the mobile node; transmitting a response including a result of the detecting; and performing binding update of the mobile node according to a request of the mobile node which receives the response.  
      According to another aspect of the present invention, a handover supporting apparatus includes: a detection unit which detects whether or not an address of a mobile node which is to be used in a sub-network which is predicted to be likely to be connected to the mobile node is used according to a request of the mobile node; a responding unit which transmits a response including a result of the detecting of the detection unit; and a binding update unit which performs binding update of the mobile node according to a request of the mobile node which receives the response.  
      According to another aspect of the present invention, one or more computer readable recording media having embodied thereon a computer program for one or more computers to perform the aforementioned handover supporting method.  
      According to another aspect of the present invention, a fast handover method between one sub-network and another sub-network and involving a mobile node and a home agent includes: the mobile node detecting the another sub-network as a next visited sub-network; the mobile node generating a care-of-address for use in the another sub-network; the mobile node forwarding the care-of-address and a duplicate address detection request based on the care-of-address to the home agent; the home agent performing the duplicate address detection to determine if the care-of-address is usable in the another sub-network; and if usable, the mobile node requesting a binding update of the care-of-address to a home address of the mobile node.  
      According to another aspect of the present invention, a system of a fast handover method involving a mobile node and a home agent includes: the mobile node identifying at a sub-network which is likely to be connected to a mobile node; the mobile node requesting detection by the home agent of whether an address of the mobile node which is to be used in the sub-network is already used in the sub-network; the home agent detecting whether the address of the mobile node is already used according to the detection request of the mobile node; the home agent transmitting a response to the detection request including a result of the detection to the mobile node; the mobile node selectively requesting performance of a binding update of the address based on the response to the detection request generated by the home agent; and the home agent performing the binding update of the address of the mobile node according to the request of the mobile node which receives the response.  
      Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the invention will become apparent more readily appreciated from the following description of the aspects, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a schematic showing a mobile communication environment according to a related art mobile IPv6 standard;  
       FIG. 2  is a diagram of a handover method according to a related art mobile IPv6 standard;  
       FIG. 3  is a diagram of a fast handover method according to another related art mobile IPv6 standard;  
       FIG. 4  is a schematic showing a mobile communication environment according to an aspect of the present invention;  
       FIG. 5  is a diagram of a handover method and a handover supporting method according to an aspect of the present invention;  
       FIG. 6A  is a format of a related art BU message, and  FIG. 6B  is a format of a fast BU message according to an aspect of the present invention;  
       FIG. 7  is a format of a BAck message according to an aspect of the present invention;  
       FIG. 8  is a schematic of configurations of MN and HA according to aspects of the present invention;  
       FIG. 9  is a flowchart of a handover method according to an aspect of the present invention; and  
       FIG. 10  is a flowchart of a handover supporting method according to another aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the aspects of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The aspects are described below in order to explain the present invention by referring to the figures.  
       FIG. 4  is a schematic showing a mobile communication environment according to an aspect of the present invention. Referring to  FIG. 4 , the mobile communication environment includes a mobile node (MN)  6 , access points (APs) AP 1   71  and AP 2   72 , a previous access router (PAR)  81 , a new access router (NAR)  82 , a home agent (HA)  9 , and a correspondent node (CN)  10 .  
      As shown, the AP 1   71  manages a basic service set (BSS) BSS 1  and wirelessly communicates with the MN  6  included in the BSS 1 . The PAR  81  manages a sub-network A and wirelessly communicates with the AP 1   71  included in the sub-network A. Accordingly, the MN  6  can access the Internet via the AP 1   71  and the PAR  81 , and can communicate with the HA  9  or the CN  10  which also access the Internet. In addition, the AP 2   72  manages a basic service set (BSS) BSS 2  and wirelessly communicates with the MN  6  included in the BSS 2 . The NAR  82  manages a sub-network B and communicates with the AP 2   72  included in the sub-network B. Accordingly, the MN  6  can access the Internet via the AP 2   72  and the NAR  82 , and can communicate with the HA  9  or the CN  10  which also access the Internet. Accordingly, the MN  6  is able to move from the BSS 1  managed by the AP 1   71  to the BSS 2  managed by the AP 2   72 .  
      As shown, the HA  9  includes a router that has the registration information of the MN  6 . The HA  9  is one of the routers located in a home network of the MN  6 . Particularly, when the MN  6  is not located in a home network thereof, the HA  9  intercepts packets which are directed to the home address of the MN  6  and tunnels the packets to a care-of-address (COA) of the MN  6  used in a sub-network (visited network) where the MN  6  is currently located. Accordingly, although the MN  6  moves between the sub-networks, the MN  6  can receive the packets which are directed to the home address of the MN  6 . However, in order to implement such a configuration, the MN  6  must notify the HA  9  of the current COA thereof.  
      During a movement of the MN  6 , when the MN  6  is located in an overlap interval of the BSS 1  and the BSS 2 , (the overlap interval being an interval where wireless signals transmitted from the AP 1   71  and the AP 2   72  overlap with each other) the MN  6  can simultaneously receive information of the sub-network A provided by the AP 1   71  and the information of the sub-network B provided by the AP 2   72 . In the following aspects, the HA  9  performs the duplicate address detection (DAD) of the COA for the MN  6  during the wireless overlap interval. Such is different from a related art DAD where it is the MN that performs the DAD. In general, the HA  9  is more robust and has a much better performance than the MN  6 . Accordingly, the HA  9  can perform the DAD of the. COA for the MN  6  much faster than the MN  6 .  
       FIG. 5  is a diagram of a handover method and a handover supporting method according to an aspect of the present invention. Referring to  FIG. 5 , the handover method and the handover supporting method according to an aspect of the present invention includes operations which are sequentially carried out in the mobile communication environment shown in  FIG. 4 .  
      As shown, in operation  501 , the MN  6  predicts (senses, detects, determines, and/or identifies) the sub-network B, which is a sub-network that is likely to be connected to the MN  6 . Operation  501  is performed at a point of time  401  that is within the wireless overlap interval shown in  FIG. 4 . The time  401  is when the MN  6  is closer to the BSS 2  than to the BSS 1 . That is, time  401  is when the MN  6  draws closer to the sub-network B and away from the sub-network A.  
      In operation  502 , the MN  6  makes or forwards a request for the DAD of the COA for the MN  6 . The HA  9  then receives the request. Namely, in operation  502 , the MN  6  transmits a message requesting detection or determination of whether or not the COA for the MN  6  to be used in the sub-network B is already used by an arbitrary node (a node) included in the sub-network B. The HA  9  then receives the request message. The request message may be in the fast BU message format shown in  FIG. 6B .  
       FIG. 6A  is a view showing a format of a related art BU message, and  FIG. 6B  is a format of a fast BU message according to an aspect of the present invention. Referring to  FIG. 6A , the related art BU message format includes an acknowledge field A  601 , a home registration field H  602 , a link-local address compatibility field L  603 , a key management mobility capability field K  604 , a reserved field  605 , a sequence number field  606 , a lifetime field  607 , and a mobility option field  608 .  
      Recorded in the field A  601  is a value or an indicator requesting to receive the BU message and to transmit the BAck message. Recorded in the field H  602  is a value or an indicator requesting a node that receives the BU message to act as a HA. Recorded in the field L  603  is a value or an indicator that refers to the home address of the MN  6  that includes an interface ID of a link-local address of the MN. Recorded in the field K  604  is a value or an indication that refers to whether or not a manual “Internet Protocol SECurity” (IPSEC) is set.  
      The reserved field  605  is not used but kept in reserve. Recorded in the sequence number field  606  is a value or an indication that refers to a sequence number of the BU message. The sequence number field  606  is used to match the BU message with the BAck message that is a response to the BU message. Recorded in the lifetime field  607  is a value or an indication that refers to a lifetime of the binding. When the value of the lifetime field  607  is 0, for example, a binding cache entry of the MN is eliminated. Recorded in the mobility option field  608  is a value or an indication that refers to mobility options such as a binding authorization data option, a nonce indices option, and/or an alternative COA option.  
      Referring to  FIG. 6B , the fast BU message according to an aspect of the present invention includes an acknowledge field A  611 , a home registration field H  612 , a link-local address compatibility field L  613 , a key management mobility capability field K  614 , a fast field F  615 , a reserved field  616 , a sequence number field  617 , a lifetime field  618 , and a mobility option field  619 .  
      As show in  FIG. 6B , the fast BU message includes the field F  615 . In another aspect, the field F  615  may be added to the reserved field  605  of the related art BU message of  FIG. 6A . Recorded in the F field  615  is a value or an indicator that refers to a request to detect whether or not the COA for the MN  6  is used by an arbitrary node (e.g., any of the nodes) included in the sub-network B. In particular, if the COA for the MN  6  is used by an arbitrary node, then in the mobility option field  619 , the COA for the MN  6  is recorded by using an alternative COA for the MN  6 .  
      In operation  503 , the HA  9  performs the DAD of the COA for the MN  6  in response to the request received in operation  502 . Namely, in operation  503 , in response to the request received in operation  502 , the HA  9  detects whether or not the COA for the MN  6  to be used in the sub-network B is already used by an arbitrary node.  
      In operation  504 , the HA  9  transmits a response message that includes the DAD determination result of operation  503 , and the MN  6  receives the response message. Namely, in operation  504 , the HA  9  transmits the response message that includes the detected result of whether or not the COA included in the fast BU message is already used, and the MN  6  receives the response message. The response message may be a BAck message shown in  FIG. 7 , which is the same format as a related art BAck message format. Since the response message is a BAck message for the fast BU message, the response message is referred as a fast BAck message.  
       FIG. 7  is a format of the related art BAck message according to an aspect of the present invention. Referring to  FIG. 7 , the BAck message includes a status field  701 , a key management mobility capability field K  702 , a reserved field  703 , a sequence number field  704 , a lifetime field  705 , and a mobility option field  706 .  
      Recorded in the status field  701  is a value or an indication that refers to the result of the BU of the MN  6 . For example, when the value recorded in the status field  701  is 0, the value refers to the success of the BU of the MN  6 . When the value recorded in the status field  701  is 136, for example, the value refers to the failure of the DAD of the COA for the MN  6  (i.e., the COA is already used). Particularly, recorded in the status field  701  of the fast BAck message according to an aspect is a value that refers to the result of the DAD request in operation. Namely, when the value recorded in the status field  701  is 0, for example, the value refers to the fact that the COA for the MN  6  is detected as not being used by an arbitrary node. When the value recorded in the status field  701  is 136, for example, the value refers to the fact that the COA for the MN  6  is detected as being used by an arbitrary node. In various aspects, values other than 0 or 136 may be used for the various indication of the status field  701 .  
      Recorded in the field K  702  is a value or an indicator that refers to whether or not a manual IPSEC is set. The reserved field  703  is not used but is reserved. Recorded in the sequence number field  704  is a value or an indication that refers to a sequence number of the BU message. The sequence number field  704  is used to match the BU message with the BAck message that is a response to the BU message. Recorded in the lifetime field  705  is a value that refers to a lifetime of the binding. When a value of the lifetime field  705  is 0, for example, a binding cache entry of the MN is eliminated. Recorded in the mobility option field  706  is a value or an indication that refers to mobility options such as a binding authorization data option and a binding refresh advice option. In various aspects, values other than 0 or 136 may be used for the various indication of the lifetime field  705 .  
      Referring back to  FIG. 5 , operations  502  to  504  are performed at times  402  to  404  shown in  FIG. 4 . These times are after the time the MN  6  is completely apart from the BSS 1  and before the time when the MN  6  completely enters the BSS 2 . Namely, operations  502  to  504  are performed during an interval after the MN  6  is completely apart from the BSS 1  and before the MN  6  completely enters the BSS 2 . This is because it takes a relatively long time for the MN  6  to completely enter the new sub-network B (of the BSS 2 ) after the wireless overlap interval and because up to one second is generally spent on the DAD.  
      In operation  505 , the MN  6  is disconnected from the AP 1   71  (to which the MN  6  was previously connected to in the sub-network A), and the MN  6  is connected to the AP 2   72  (to which the MN  6  is predicted to be likely to be connected to in the sub-network B). Operation  505  is performed at the time  404  shown in  FIG. 4 . The time  404  is after the time when the MN  6  is completely apart from the BSS 1 , namely, the sub-network A.  
      In operation  506 , when the DAD result in the fast BAck message received in operation  505  refers to the fact that the COA for the MN  6  is detected to be not used, the MN  6  requests for binding of the COA with the home address of the MN  6 . The HA  6  then receives the request. Namely, in operation  506 , the MN  6  transmits the BU message that includes the COA which is detected to be not used by an arbitrary node included in the sub-network B, and the HA  9  receives the BU message. The format of the BU message is that shown in  FIG. 7 , though not required. Accordingly, the BU message has the same function as the related art BU message shown in  FIG. 6A .  
      In operation  507 , the HA  9  binds the COA of the BU message received in the operation  506  with the home address of the MN  6  and performs the BU of the MN  6 . In operation  508 , the HA  9  transmits the BAck message to acknowledge that the BU of the MN  6  has been normally (successfully) performed. The MN  6  then receives the BAck message. In operation  509 , the HA  9  intercepts the packets which are directed to the home address of the MN  6  and tunnels the packets to the COA that has been bound with the home address of the MN  6  during the BU of operation  507 . The MN  6  then receives the tunneled packets.  
      Operations  506  to  509  are carried out in an interval after the time  405  shown in  FIG. 4 . The interval shown is one where the MN  6  is connected to the AP 2   72  that is managing the BSS 2  (namely, the sub-network B). In addition, operations following operation  509  are carried out according to the mobile IPv6 standard.  
      As discussed above, the MN  6  predicts (senses, detects, determines, and/or identifies) the sub-network which is likely to be connected to the MN  6  in the interval before the time  405 , and the DAD of the COA which is to be used in the sub-network is performed. Accordingly, it is possible to remove or reduce a handover delay caused by the DAD operation being performed by an MN. In addition, the aforementioned operations are performed by using only the BU and BAck messages according to the mobile IPv6 standard without using a separately defined message. Accordingly, it is possible to reduce the load on the MN and the cost involved in a fast handover method as compared to that of the related art fast handover method.  
       FIG. 8  is a schematic of configurations of the MN and the HA according to an aspect of the present invention. Referring to  FIG. 8 , a handover apparatus in the form of the MN  6  according to an aspect of the present invention includes a mobility prediction unit  61 , a COA generation unit  62 , a DAD request unit  63 , a DAD response reception unit  64 , a mobility detection unit  65 , a binding request unit  66 , a binding response reception unit  67 , and a packet transmission/reception unit  68 . These components are loaded on the IP layer of the MN  6  shown in  FIG. 5 .  
      The mobility prediction unit  61  predicts (senses, detects, determines, and/or identifies) the sub-network B (namely, a sub-network which is likely to be connected to the MN  6 ). The mobility prediction unit  61  may use various methods to predict the sub-network as follows. Namely, the mobility prediction unit  61  may use a global positioning system (GPS) to predict the sub-network which is likely to be connected to the MN  6 . For example, the mobility prediction unit  61  may receive position information of the MN  6  from a GPS satellite and measures distances between the MN  6  and various access points (APs) based on the position information. As a result of the measurement, when the distance between the MN  6  and an AP 2   72  among the various APs is less than a threshold value, the sub-network (such as sub-network B) in which the AP 2   72  is included is determined as a sub-network which is likely to be connected to the MN  6 .  
      In other aspects, the mobility prediction unit  61  may use intensities of wireless LAN signals radiated from the various APs to predict the sub-network which is likely to be connected to the MN  6 . For example, the mobility prediction unit  61  may measure the intensities of the wireless LAN signals radiated from the various APs. As a result of the measurement, when the intensity of the wireless LAN signal radiated from one of the APs among the various APs (such as the AP 2   72 ) is more than a threshold value, the sub-network in which the AP 2   72  is included is determined as a sub-network which is likely to be connected to the MN  6 .  
      The COA generation unit  62  receives a router advertisement (RA) message from the NAR  82  that manages the sub-network B which is predicted to be likely to be connected to the MN  6 . The RA message includes a prefix of the sub-network B. The COA generation unit  62  generates the COA for the MN  6  by combining the prefix of the sub-network included in the RA message with the Interface ID of the MN  6 . In addition, the COA generation unit  62  generates another COA that is different from the COA generated previously when the status field  701  of the fast BAck message received by the DAD response reception unit  64  indicates that the COA generated by the COA generation unit  62  is detected to be in use. In various aspects, though not required, the value that indicates use may be 136, for example.  
      The DAD request unit  63  forwards or makes a request to the HA  9  for the DAD of the COA generated by the COA generation unit  62 . Namely, the DAD request unit  63  request the HA  9  to detect whether or not the COA for the MN  6  to be used in the sub-network (which is predicted to be likely to be connected to the MN  6 ) is used by an arbitrary node included in the sub-network B. That is, the DAD request unit  63  transmits the fast BU message including the COA for the MN  6  to the HA  9  to request the DAD.  
      The DAD response reception unit  64  receives a response to the request for DAD that was forwarded by the DAD request unit  63 . That is, the DAD response reception unit  64  receives a fast BAck message that includes a detected result of whether or not the COA generated by the COA generation unit  62  is already used or is usable.  
      Thereafter, the mobility detection unit  65  is notified that the MN  6  is disconnected from the AP 1   71  that is included in the sub-network A, and which was previously connected to the MN  6  in the link layer. The link layer is the lowest layer of the IP layers and is connected to the AP 2   72  included in the sub-network B so that the connection of the MN  6  to the sub-network B is detected.  
      When the mobility detection unit  65  detects or determines that the MN  6  is likely to be connected to the sub-network B, the binding request unit  66  selectively requests the BU of the MN  6  based on the response to the request forwarded by the DAD request unit  63  upon the MN  6  becoming connected to the sub-network B. More specifically, the binding request unit  66  forwards or makes a request to the HA  6  to bind the home address of the MN  6  with the COA generated by the COA generation unit  62  when the status field  701  of the fast BAck message received by the DAD response reception unit  64  indicates that the COA generated by the COA generation unit  62  is not in use. In various aspects, though not required, the value that indicates non use is 0, for example. Particularly, the binding request unit  66  transmits the BU message to the HA  9  to request for the BU of the MN  6 . The BU message includes the COA which is detected to be not used by an arbitrary node included in the sub-network B.  
      The binding response reception unit  67  receives a response to the BU request that was forwarded by the binding request unit  66 . Particularly, the binding response reception unit  67  receives the BAck message to acknowledge that the BU of the MN  6  requested by the binding request unit  66  is normally (or successfully) performed. The BU message has the same function as the related art BU message shown in  FIG. 6A . The format of the BU message is that shown in  FIG. 6B .  
      Based on the BAck message received from the binding response reception unit  67 , the packet transmission/reception unit  68  receives packets which are tunneled to the COA of the MN  6  by the HA  9 . The packets are ones which are directed to the home address of the MN  6  by the CN  10 . In various aspects, the DAD request unit  63 , the DAD response reception unit  64 , the binding request unit  66 , the binding response reception unit  67 , and packet transmission/reception unit  68  may be a single input/output unit. Also, in other aspects, the mobility prediction unit  61 , the COA generation unit  62 , and the mobility detection unit  65  may be performed by a single processing unit. In various aspects of the present invention, the units  61 - 68  may be integrated into a single unit.  
      Referring again to  FIG. 8 , a handover supporting apparatus in the form of the HA  9  includes a DAD request reception unit  91 , a DAD execution unit  92 , a DAD responding unit  93 , a binding request reception unit  94 , a BU unit  95 , a binding responding unit  96 , and a packet transmission/reception unit  97 . These components are loaded on the IP layer of the HA  9  shown in  FIG. 5 .  
      The DAD request reception unit  91  receives from the MN  6  a DAD request for the COA for the MN  6  used in the sub-network B. Namely, the DAD request reception unit  91  receives from the MN  6  a fast BU message (a DAD request message) that includes the COA for the MN  6  which is used in the sub-network B.  
      The DAD execution unit  92  detects whether or not the COA for the MN  6  is already used or is usable according to the request received from the DAD request reception unit  91 . Namely, when a value or an indication that refers to the request to detect whether or not the COA for the MN  6  is used is recorded in the field F  615  of the BU message received from the DAD request reception unit  91 , the DAD execution unit  92  determines that the BU message received by the DAD request reception unit  91  is the fast BU message. Next, the DAD execution unit  92  detects whether or not the COA recorded in the mobility option field  619  of the fast BU message is used or usable.  
      More specifically, the DAD execution unit  92  multicasts a neighbor solicitation (NS) message whose target address is the received COA recorded in the mobility option field  619  of the fast BU message. Subsequently, the DAD execution unit  92  receives a neighbor advertisement (NA) message, which is a response message for the NS message, and detects from the NA message whether or not the received COA recorded in the mobility option field  619  of the fast BU message is used by an arbitrary node (other or any of the nodes) included in the sub-network B.  
      The DAD responding unit  93  transmits a response that includes a result of the above DAD from the DAD execution unit  92  to the MN  6 . Namely, when the DAD execution unit  92  detects that the COA included in the fast BU message is not used by an arbitrary node, the DAD responding unit  93  transmits a fast BAck message including the detection result that refers to the COA as not being used. In this case, the value  0  is usable, for example, as a value that refers to the COA included in the fast BU message as usable. The value is recorded in the status field  701  of the fast BAck message.  
      On the other hand, when the DAD execution unit  92  detects that the COA included in the fast BU message is used by an arbitrary node, the DAD responding unit  93  transmits a fast BAck message that includes the detection result that refers to the COA as being used. In this case, the value  136  is usable, for example, as a value that refers to that the COA included in the fast BU message is detected as not usable. The value is recorded in the status field  701  of the fast BAck message.  
      The binding request reception unit  94  receives of the MN  6  from the MN  6 . Namely, the binding request reception unit  94  receives the BU request from the MN  6  which had received the fast BAck message transmitted from the DAD responding unit  93 . The BU message includes the COA which is detected as not used by an arbitrary node included in the sub-network B.  
      The BU unit  95  performs the BU in response to the request from the MN  6 . Namely, the BU unit  95  performs the BU of the MN  6  by binding the COA included in the BU message received by the binding request reception unit  94  with the home address of the MN  6 .  
      In response to the BU request received by the binding request reception unit  94 , the binding responding unit  96  transmits a response to the BU request of the MN  6  . Namely, the binding responding unit  96  transmits to the MN  6  the BAck message to acknowledge that the BU of the MN  6  has been normally (successfully) performed by the BU unit  95 .  
      When packets are received by the HA, the packet transmission/reception unit  97  tunnels the packets to the COA which is bound with the home address of the MN  6  due to the BU operation by the BU unit  95 .  
       FIG. 9  is a flowchart of a handover method according to another aspect of the present invention. Referring to  FIG. 9 , the handover method according to an aspect of the present invention includes operations which are sequentially performed by the handover apparatus in the form of the MN  6  shown in  FIG. 8 . Accordingly, although omitted in the following description, the above-described components in the handover apparatus of the MN  6  shown in  FIG. 8  will be applied to the handover method according to the aspect.  
      In operation  901 , the MN  6  predicts (senses, detects, determines, and/or identifies) the sub-network B, namely, a sub-network which is likely to be connected to the MN  6 . In operation  902 , the MN  6  receives a router advertisement (RA) message from the NR  82  managing the sub-network B that includes the prefix of the sub-network B . The MN  6  also generates the COA for the MN  6  by combining the prefix of the sub-network B included in the RA message with the interface ID of the MN  6 .  
      In operation  903 , the MN  6  forwards or makes a request of the HA  9  for the DAD the COA generated in operation  902 . Namely, in operation  903 , the MN  6  transmits to the HA  9  the fast BU message that includes the COA generated in operation  902  to request the HA  9  to detect whether or not the COA for the MN  6  is already used by an arbitrary node included in the sub-network B.  
      In operation  904 , the MN  6  receives a response to the DAD request of operation  903 . Namely, in operation  904 , the MN  6  receives the fast BAck message that includes a detected result of whether or not the COA generated in operation  902  is already used.  
      In operation  905 , the MN  6  determines where or not the COA generated in operation  902  is already used by checking the value recorded in the status field  701  of the fast BAck message received in operation  904 . According to a result of the determination, the operation proceeds to operation  906  (no in operation  905 ), or the operation of the method returns to operation  902  (yes in operation  905 ). Namely, in operation  905 , when the value recorded in the status field  701  of the fast BAck message received in operation  904  refers to the COA as detected as not being used, the operation  906  is performed. In this case, the value is 0, for example. On the other hand, in operation  905 , when the value recorded in the status field  701  of the fast BAck message received in operation  904  refers to the COA as detected as being used, , the procedure of the MN  6  returns to operation  902 . In this case, when the value is  136 , for example.  
      In operation  906 , the MN  6  is notified that the MN  6  is disconnected from the AP 1   71  included in the sub-network A which was previously connected to the MN  6  in the link layer. The link layer is the lowest layer of the IP layers. Also in operation  906 , the MN  6  is connected to the AP 2   72  included in the sub-network B. Accordingly, the connection of the MN  6  to the sub-network B is detected.  
      When the MN  6  is detected to be connected to the sub-network B in operation  906 , the MN  6  selectively requests for the BU of the MN  6  based on the response to the request in operation  903  so that the MN  6  is connected to the sub-network B. More specifically, based on the determination of the MN  6  in operation  905  that the COA generated in operation  902  is not used, in operation  906 , the MN  6  transmits the BU message including the COA to the HA  9  to request the HA  9  to bind the home address of the MN  6  with the COA that was generated in operation  902 .  
      In operation  908 , the MN  6  receives a response to the BU request made in operation  907 . Namely, in operation  908 , the MN  6  receives the BAck message to acknowledge that the BU of the MN  6  requested in operation  907  has been normally (successfully) performed.  
      In operation  909 , based on the BAck message received in operation  908 , the MN  6  receives the packets that are tunneled to the COA for the MN  6  by the HA  9 .  
       FIG. 10  is a flowchart of a handover supporting method according to another aspect of the present invention.  
      Referring to  FIG. 10 , the handover supporting method according to the aspect of the present invention includes operations which are sequentially carried out in the handover supporting apparatus in the form of the HA  9  shown in  FIG. 8 . Accordingly, although omitted in the following description, the above-described components in the handover supporting apparatus in the form of the MN  6  shown in  FIG. 8  are applicable to the handover method according to the aspect.  
      In operation  1001 , the HA  9  receives from the MN  6  the DAD request for the COA of the MN  6  to be used in the sub-network predicted likely to be connected to the MN  6 . Namely, in operation  1001 , the HA  9  receives from the MN  6  the fast BU message, which is the DAD request message that includes the COA for the MN  6 .  
      In operation  1002 , according to the request received in operation  1001 , the HA  9  detects whether or not the COA for the MN  6  is used. Namely, when a value or an indication that refers to a request to detect whether or not the COA for the MN  6  is used as included in operation  1002 , the HA  9  determines that the BU message received by the DAD request reception unit  91  is the fast BU message and detects whether or not the COA recorded in the mobility option field  619  of the fast BU message (of  FIG. 6B ) is used by an arbitrary node of the sub-network B.  
      In operation  1003 , the HA  9  transmits a response that includes the DAD result of operation  1002  to the MN  6 . Namely, when the COA included in the fast BU message is detected not to be used in operation  1002 , in operation  1003 , the HA 9  transmits the fast BAck message that includes the detection result that indicates that the COA is detected as not used. In addition, when the COA included in the fast BU message is detected to be used in operation  1002 , in operation  1003 , the HA 9  transmits the fast BAck message that includes a detection result that indicates that the COA is detected as used.  
      In operation  1004 , the HA  9  receives the BU request from the MN  6  which had received the response transmitted in operation  1003 . Namely, in operation  1004 , based on the fast BAck message transmitted in operation  1003 , the HA  9  receives the BU message from the MN  6  that includes the COA which is detected as not used by an arbitrary node included in the sub-network B.  
      In operation  1005 , the HA  9  performs the BU of the MN  6  according to the request of the MN  6  that had received the response transmitted in operation  1003 . Namely, in operation  1005 , the HA  9  performs the BU of the MN  6  by binding the COA included in the BU message received in operation  1004  with the home address of the MN  6 .  
      In response to the BU request received in operation  1004 , in operation  1006 , the HA  9  transmits the response to the BU request of the MN  6  to the MN  6 . Namely, in operation  1006 , the HA  9  transmits to the MN  6  the BAck message to acknowledge that the BU of the MN  6  was normally (or successfully) performed in operation  1005 .  
      In operation  1007 , when the packets, which are directed to the home address of the MN  6 , are received from the CN  10 , the HA  9  tunnels the packets to the COA that is bound with the home address of the MN  6  that occurred during the BU in operation  1005 .  
      The aspects of the present invention can be written as computer programs and can be implemented in general-use digital computers that execute the programs using a computer readable recording medium to perform the method. In addition, a data structure used in the aspects of the present invention may be recorded in a computer readable recording medium though various means to perform the method.  
      Examples of the computer readable recording medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical recording media (e.g., CD-ROMs, or DVDs), and storage media such as carrier waves (e.g., transmission through the Internet).  
      While not required, it is understood that the mobile node can be a computer, a phone, a PDA, and/or a mobile media player, and that aspects can be used in non-mobile devices accessing a network connection.  
      According to aspects of the present invention, a sub-network which is likely to be connected to a mobile node is predicted, and duplicate address detection for a care-of-address of the mobile node which is to be used in the sub-network is performed by a home agent instead of the mobile node. Accordingly, it is possible to prevent or reduce a handover delay caused by the duplicate address detection being performed by the mobile node.  
      In addition, according to the present invention, a mobile node makes or forwards a request to a home agent to perform a duplicate address detection by using only BU and BAck messages according to the mobile IPv6 standard without using a separately defined message, and receives a response to the request. Accordingly, it is possible to reduce load in the mobile node and cost involved in fast handover in comparison to a related art fast handover method using the separately defined message.  
      Although a few aspects of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in the aspects without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.