Patent Publication Number: US-2007104143-A1

Title: Communication handover method, communication message processing method, program for executing these methods by use of computer, and communication system

Description:
TECHNICAL FIELD  
      The present invention relates to a communication handover method and a communication message processing method, which concern the handover of a mobile terminal (mobile terminal) that performs radio communication, and a program for executing these methods by use of a computer, and a communication system, and more particularly, to a communication handover method and a communication message processing method, which concern the handover of a mobile node that performs radio communication using the mobile IPv6 (Mobile Internet Protocol version 6) protocol that is the next generation Internet protocol, and a program for executing these methods by use of a computer, and a communication system.  
     BACKGROUND ART  
      As a technology which provides a user who accesses a communication network like the Internet over a radio network from a mobile node while moving with seamless connection to the communication network, one which uses the mobile IPv6 as the next generation Internet protocol becomes popular. A radio communication system which uses the mobile IPv6 will be explained with reference to  FIG. 1 . The technology regarding the mobile IPv6 which will be explained below is disclosed in, for example, Non-patent Document 1 described below.  
      The radio communication system illustrated in  FIG. 1  includes an IP network (communication network)  15  like the Internet, a plurality of subnets  20  and  30  (also called sub networks) which are connected to the IP network  15 , and a mobile node (MN: Mobile Node)  10  which can be connected to any of the plurality of subnets  20  and  30 . In  FIG. 1  and two subnets  20  and  30  are illustrated as the plurality of subnets  20  and  30 .  
      The subnet  20  comprises an access router (AR: Access Router)  21  which performs routing of IP packets (packet data), and a plurality of access points (AP: Access Point)  22  and  23  which respectively constitute unique radio coverage areas (communication available areas)  24  and  25 . Each of those APs  22  and  23  is connected to the AR  21 , which is connected to the IP network  15 . In  FIG. 1 , two APs  22  and  23  are illustrated as the plurality of APs  22  and  23 . The subnet  30  is constituted in the same connection mode as that of the subnet  20  by an AR  31 , and a plurality of APs  32  and  33 .  
      The AR  21 , which is a component of the subnet  20 , and the AR  31 , which is a component of the subnet  30 , can communicate with each other over the IP network  15 , i.e., the subnet  20  and the subnet  30  are connected together over the IP network  15 .  
      Suppose that in the radio communication system illustrated in  FIG. 1 , the MN  10  has started radio communication with the AP  23  in the radio coverage area  25 . In this situation, in a case where an IPv6 address assigned to the MN  10  is not adequate for the IP address system of the subnet  20 , the MN  10  present in the radio coverage area  25  acquires an IPv6 address adequate for the subnet  20 , i.e., a care-of address (CoA: Care of Address) through the radio communication with the AP  23 .  
      Methods for the MN  10  to acquire a CoA include a method by which a DHCP server assigns it in a stateful manner by a technique, such as the DHCPv6 (Dynamic Host Configuration Protocol for IPv6), and a method of acquiring the network prefix and prefix length of the subnet  20  from the AR  21 , and combining the network prefix and the prefix length, both obtained from the AR  21 , with the link layer address of the MN  10  at the MN  10 , thus automatically generating a CoA in a stateless manner.  
      The MN  10  registers (Binding Update: BU) the acquired CoA at a router (home agent) on its home network, and a certain communication party (Correspondent Node: CN), thereby ensuring packet data transmission and reception in the subnet  20 .  
      Accordingly, packet data transmitted from a predetermined communication party to the MN  10  is transferred to the MN  10  through the AR  21  and the AP  23  based on the CoA of the MN  10 , and packet data transmitted to a desired communication party by the MN  10  is transferred to the desired communication party through the AP  23  and the AR  21 . Packet data which is addressed to the MN  10  and is transmitted to the home network is sent to the AR  21  of the subnet  20  based on the CoA of the MN  10  registered at the home agent, and is transferred to the MN  10  through the AP  23 .  
      As mentioned above, the radio communication system using the mobile IPv6 and illustrated in  FIG. 1  is structured in such a way that radio communication at the MN  10  continues by using the CoA even if the MN  10  performs handover from one subnet to another. As a technology which speeds up such a handover process, a fast handover technology disclosed in, for example, Non-patent Document 2 described below is known.  
      According to the fast handover technology, before the MN  10  performs L2 handover, the MN  10  preacquires a new (New) CoA (hereinafter called NCoA) which is used at the subnet  30 , and notifies the AR  21  of the NCoA, so that a tunnel can be created between the AR  21  and the AR  31 , and even during a period from the time when the MN  10  performs L2 handover to switch over connection from the AP  23  to AP  32 , to the time when it moves to the subnet  30 , and formally registers (BU) the NCoA preacquired, packet data transmitted to an old (Previous) CoA (hereinafter called PCoA) used in the subnet  20  is forwarded to the MN  10  through the AR  31  and the AP  32  via the tunnel, while packet data transmitted from the MN  10  arrives at the AR  21  through the AP  32  and the AR  31  via the tunnel, and is then transmitted to a communication party from the AR  21 .  
      In a communication using a network, there are services including a QoS (Quality of Service) guarantee (in the specification, such a service is called additional service), and there are various communication protocols to realize the additional services. Among those various communication protocols, a protocol for performing QoS guarantee is, for example, RSVP (Resource Reservation Protocol) (see, for example, Non-patent Document 3 to be described later). The RSVP allows a band to be reserved over a path (flow) from a transmission side communication terminal which transmits data to a reception side communication terminal which receives data, thereby ensuring smooth data transmission from the transmission side communication terminal to the reception side communication terminal.  
      The MN  10  which performs handover between the subnets  20  and  30  is demanded to continuously get additional services, gotten before the handover, including the QoS guarantee after the handover, but the above-described RSVP cannot meet the above-described demand in terms of the following points, and cannot cope with the movement of the MN  10 .  FIG. 6  is an exemplary diagram for explaining that the RSVP of the conventional technology cannot cope with the movement of an MN.  
      According to the RSVP, a QoS path is set in an end-to-end path from a communication destination terminal (CN: Correspondent Node)  60  of the MN  10 , and data transfer is carried out by a plurality of relay nodes  61  which connect the end-to-end path. Therefore, the MN  10  should perform handover between the subnets  20  and  30 , and when the CoA of the MN  10  is changed, it is necessary to perform a process relating to address change in addition to changing the flow in the QoS path. However, the RSVP cannot cope with such a change, resulting in failure of the QoS guarantee (first problem: difficulty in changing a QoS path). Further, in a case where a new QoS path is set, when there is an overlapping portion of the QoS path before and after handover, there may occur double reservation in the overlapping portion (second problem; double reservation).  
      To overcome the problems, there are discussions on standardization of a new protocol called NSIS (Next Step in Signaling) in the IETF (Internet Engineering Task Force) are currently underway (see the following Non-Patent Document 4). The NSIS is expected to be effective particularly in various additional services including QoS guarantee, and there are documents describing the conditions for realizing QoS guarantee and mobility support in NSIS and how to realize them (e.g., see the following Non-Patent Documents 5 to 7). While NSIS coverages multifarious functions in an ordinary static network as well as under a mobile environment, attention is paid to the function of establishing a mobility-supported additional service, one of the NSIS functions, and implementation of NSIS achieves establishment of mobility-supported additional service. 
      Non-Patent Document 1: D. Johnson, C. Perkins and J. Arkko, “Mobility Support in IPv6”, draft-ietf-mobileip-ipv6-24, June 2003     Non-Patent Document 2: Rajeev Koodli “Fast Handovers for Mobile IPv6”, draft-ietf-mobileip-fast-mipv6-08, October 2003     Non-Patent Document 3: R. Braden, L. Zhang, S. Berson, S. Herzog and S. Jamin, “Resource ReSerVation Protocol-Version 1 Functional Specification”, RFC 2205, September 1997.     Non-Patent Document 4: NSIS WG (http://www.ietf.org/html.charters/nsis-charter.html)     Non-Patent Document 5: H. Chaskar, Ed, “Requirements of a Quality of Service (QoS) Solution for Mobile IP”, RFC3583. September 2003     Non-Patent Document 6: Sven Van den Bosch, Georgios Karagiannis and Andrew McDonald “NSLP for Quality-of-Service signalling”, draft-ietf-nsis-qos-nslp-01.txt, October 2003     Non-Patent Document 7: X. Fu, H. Schulzrinne, H. Tschofenig, “Mobility issues in Next Step signaling”, draft-fu-nsis-mobility-01.txt, October 2003    

     DISCLOSURE OF THE INVENTION  
      Problems to be Solved by the Invention  
      For example, let us consider that the MN  10  which is subject to QoS guarantee in the subnet  20  connected before handover performs handover to the subnet  30 , and keeps getting QoS guarantee gotten before handover in the subnet  30  to connect with after handover.  
      In this case, the time from the point when the MN  10  executes handoff from the subnet  20  connected before handover to the point when the MN 10  gets QoS guarantee in the subnet  30  to connect with after handover becomes the time where the MN  10  cannot get QoS guarantee, so that the MN  10  either gets no QoS guarantee at all or the default QoS transfer process is executed.  
      As mentioned above, therefore, the MN  10  after handover should be quickly provided with an additional service, but the current discussions on NSIS in the IETF have not given any specific proposal on the timing for initiation of an additional service after handover (e.g., timing to reconstruct a QoS path). While Non-patent document 5 describes that the number of packets to be subject to the default QoS transfer at the time of handover should be minimized, it discloses no specific solving means at all.  
      There may be a situation where an AR which has a mobility-supported additional service realizing function, such as NSIS, implemented, and an AR which does not both exist in a network. In such a network, a consideration should be given to prevention of, as much as possible, a wasteful increase in communication traffic, e.g., transmission of a message understandable only by an AR which has a mobility-supported additional service realizing function like NSIS implemented to an AR which has no mobility-supported additional service realizing function implemented.  
      In view of the above-described problems, it is an object of the present invention to provide a communication handover method and a communication message processing method, which can enable a mobile node that performs handover to promptly and continuously get an additional service gotten before handover even after the handover, and a program for executing these methods by use of a computer, and a communication system.  
      Means for Solving the Problems  
      To achieve the object, a communication handover method for use in a mobile node in a communication system in which a plurality of access routers each constituting a subnet are connected together over a communication network and at least one or more of access points forming a unique communication available area are connected to each of the plurality of access routers, the mobile node being so structured as to communicate with the access router connected with the access points, through radio communication with the access points within the communication available area, the communication handover method comprising:  
      a storing step of storing correspondence information describing a correspondence relationship between information on the access points and information on the access router connected to the access points into a predetermined information storage means of the mobile node;  
      a reception step of receiving information on another access point from the another access point when communication is switched over from an access point currently in communication to the another access point;  
      an acquisition step of acquiring information on that access router to which the another access point is connected from the correspondence information based on the information on the another access point received at the reception step;  
      a determination step of determining from the information on the access router acquired at the acquisition step whether or not changing address information currently assigned in connection of the subnet is necessary when communication is switched from the access point currently in communication to the another access point;  
      an address hold control step of performing such control as to continuously use the currently assigned address information upon determination that it is not necessary to change the address information at the determination step;  
      an address generation step of generating address information in the subnet constituted by the access router from the information on the access router acquired at the acquisition step upon determination that it is necessary to change the address information at the determination step; and  
      an address information transmission step of acquiring address information on the access router from the correspondence information, creating a message including the address information generated at the address generation step, and transmitting the message to the access router through the access point currently in communication.  
      This structure permits a mobile node that performs handover to promptly and continuously get an additional service gotten before handover even after the handover.  
      Further, the communication handover method of the invention comprises a process switching step of performing a process based on conventional handover when the information on the access router to which the another access point is connected cannot be acquired from the correspondence information at the acquisition step.  
      The structure can ensure switching to a process by the conventional fast handover to surely perform a process associated with a handover when a mobile node cannot generate address information in a subnet from correspondence information or cannot acquire address information of an access router for transmitting a message to the access router.  
      Further, the communication handover method of the invention comprises:  
      a correspondence information reception step of receiving information relating to a change in the correspondence information from a predetermined communication apparatus which manages the correspondence information or the access router; and  
      a correspondence information update step of updating the correspondence information stored in the predetermined information storage means with the information relating to the change in the correspondence information.  
      With the structure, when correspondence information is updated, a mobile node can receive the updated contents of the correspondence information, and can always hold latest correspondence information.  
      Further, the communication handover method of the invention comprises an information check step of periodically checking the predetermined communication apparatus or the access router to see whether or not there is information relating to a new change of the correspondence information.  
      The structure can allow a mobile node to dynamically check in a given cycle whether or not correspondence information has been updated.  
      Further, the communication handover method of the invention uses a link layer address of the access point as the information on the access point, and uses a link layer address of the access router, a prefix length of the subnet constituted by the access router, and an IP address of the access router as the information on the access router.  
      The structure can allow a mobile node to surely execute an efficient handover process, and improves the compatibility with a communication system using the fast handover technology of the mobile IPv6.  
      Further, in the communication handover method of the invention, the correspondence information describes a correspondence relationship between the information on the access point in the subnet to which the mobile node is currently connected, and the information on the access router, and a correspondence relationship between the information on the access point in the subnet present in a neighborhood of the subnet to which the mobile node is currently connected and the information on the access router.  
      With the structure, a mobile node can store only least necessary correspondence information, thus reducing the data capacity for correspondence information, and can relieve loads on a process of reading correspondence information, a search process for desired information, etc.  
      Further, the communication handover method of the invention is structured in such a way that the correspondence information describes whether or not an additional service early establishment function of realizing early establishment of a mobility supported additional service is implemented in the access router, and  
      in such a way as to determine whether or not, at the address information transmission step, the access router has the additional service early establishment function implemented therein, and transmit the message only to the access router having the additional service early establishment function implemented therein.  
      The structure can allow a mobile node to switch connection to a subnet capable of statelessly generating address information, and transmit a message only when an access router constituting the subnet has an additional service early establishment function, thereby reducing wasteful communication traffic.  
      Further, the communication handover method of the invention is structured in such a manner as to determine whether or not NSIS which enables early establishment of the mobility supported additional service is implemented in the access router.  
      The structure can allow a mobile node to determine whether an access router has NSIS implemented, and transmit a message to an access router which has NSIS implemented in addition to an additional service early establishment function, thereby reducing wasteful communication traffic.  
      Further, the communication handover method of the invention is structured in such a way that the additional service is a QoS guarantee.  
      The structure can allow a mobile node to determine whether an access router has is implemented with a function of starting a process of establishing a QoS path relating to QoS guarantee, and transmit a message only to an access router which is implemented with a function relating to QoS guarantee, thereby reducing wasteful communication traffic and promptly and continuously establishing QoS guarantee after handover in the mobile node.  
      The invention provides a communication handover program for allowing a computer to execute the above-described communication handover method.  
      To achieve the object, a communication message processing method according to the invention is for use in at least one of a plurality of access routers in a communication system in which the plurality of access routers each constituting a subnet are connected together over a communication network, at least one or more of access points forming a unique communication available area are connected to each of the plurality of access routers, and a mobile node present in the communication available area is structured in such a manner as to communicate with the access router connected with the access points, through radio communication with the access points, and comprises:  
      a validity checking step of, when a message including address information in a subnet generated by the mobile node is received from the mobile node not in present in the subnet constituted by the access router, checking a validity of the address information included in the message; and  
      an additional service starting step of starting an establishment process of an additional service to the mobile node when it is checked that the address information is valid at the validity checking step.  
      This structure can ensure reception of a message from a mobile node and confirmation of address information, as well as initiation of an additional service establishment function relating to the mobile node, and permits a mobile node that performs handover to promptly and continuously get an additional service gotten before handover even after the handover.  
      Further, in the communication message processing method of the invention, the access router has NSIS implemented which enables establishment of a mobility supported additional service.  
      This structure can ensure reception of a message from a mobile node and confirmation of address information, and allows an access router to start an additional service establishment function relating to the mobile node, so that the access router itself can perform an additional service establishment function relating to the mobile node by using NSIS.  
      Further, the communication message processing method is constructed in such a way that the additional service is a QoS guarantee.  
      This structure can ensure reception of a message from a mobile node and confirmation of address information, and allows an access router to start a process of establishing QoS guarantee relating to the mobile node by using the additional service early establishment function.  
      The invention provides a program for processing a communication message that allows a computer to execute the above-described communication message processing method.  
      To achieve the object, a communication system of the invention is structured in such a way that a plurality of access routers each constituting a subnet are connected together over a communication network and at least one or more of access points forming a unique communication available area are connected to each of the plurality of access routers, and a mobile node present in the communication available area communicates with the access router connected with the access points, through radio communication with the access points,  
      the mobile node has correspondence information storage means for storing correspondence information describing a correspondence relationship between information on the access points and information on the access router connected to the access points, and  
      when communication is switched over from an access point currently in communication to another access point, information on that access router to which the another access point is connected is acquired based on the information on the another access point received from the another access point by referring to the correspondence information, address information in the subnet constituted by the access router is generated from the acquired information on the access router and the address information in the subnet is transmitted to the access router through an access point currently in communication.  
      This structure permits a mobile node that performs handover to promptly and continuously get an additional service gotten before handover even after the handover.  
      Further, the communication system of the invention is structured in such a way as to execute a process by conventional handover when the mobile node cannot acquire the information on the access router to which the another access point is connected, from the correspondence information.  
      The structure can ensure switching to a process by the conventional fast handover to surely perform a process associated with a handover when a mobile node cannot generate address information in a subnet from correspondence information or cannot acquire address information of an access router for transmitting a message to the access router.  
      Further, the communication system of the invention is structured in such a way that a predetermined communication apparatus which manages the correspondence information is connected to the communication network, and is so structured as to transmit the correspondence information to the mobile node.  
      With this structure, a predetermined communication apparatus which is connected to the communication network and can grasp the network configuration manages correspondence information, and a mobile node has only to receive and store correspondence information transmitted from the predetermined communication apparatus.  
      Further, the communication system of the invention is structured in such a way that when a change in the information on the access point or the information on the access router occurs, the predetermined communication apparatus receives the information on the access point or the information on the access router after generation of the change, from the access router, updates the correspondence information managed by the predetermined communication apparatus, and informs the mobile node that the correspondence information has been changed.  
      With the structure, when correspondence information is updated, a mobile node can receive the updated contents of the correspondence information, and can always hold latest correspondence information.  
      Advantageous Effect of the Invention  
      The invention provides a communication handover method and a communication message processing method, which have the above-described structures, and a program for executing these methods by use of a computer, and a communication system, and has an advantage such that a mobile node which performs handover can promptly and continuously get an additional service gotten before handover even after the handover.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  An exemplary diagram showing the structure of a radio communication system common to the present invention and the prior art.  
       FIG. 2 A  block diagram showing the structure of an MN according to an embodiment of the invention.  
       FIG. 3  An exemplary diagram showing one example of AP-AR correspondence information which is stored in an MN according to an embodiment of the invention.  
       FIG. 4 A  block diagram showing the structure of an AR which constitutes a subnet at the destination of handover by an MN according to the embodiment of the invention.  
       FIG. 5 A  sequence chart illustrating an operational example when an MN performs handover between subnets in a radio communication system according to the embodiment of the invention.  
       FIG. 6  An exemplary diagram for explaining that an RSVP according to the prior art cannot cope with movement of an MN. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      An embodiment of the invention will now be described with reference to FIGS.  1  to  5 .  FIG. 1  is an exemplary diagram showing the structure of a radio communication system common to the present invention and the prior art, and the structure of the radio communication system shown in  FIG. 1  has been explained in the description of the prior art. The radio communication system shown in  FIG. 1  is referred to in the description of the embodiment of the invention.  
      The function of the MN  10  will be explained next.  FIG. 2  is a block diagram showing the structure of an MN according to the embodiment of the invention. While the individual functions of the MN  10  are illustrated in blocks in  FIG. 2 , the functions can be achieved by hardware and/or software. In particular, the main processes of the invention (processes in the individual steps illustrated in  FIG. 5  to be discussed later) can be achieved by a computer program.  
      The MN  10  shown in  FIG. 2  has handover decision means  101 , radio reception means  102 , radio transmission means  103 , subnet discrimination means  104 , NCoA generation means  105 , message generation means  106 , and AP-AR correspondence information storage means  107 . The handover decision means  101  is means that determines initiation of an L2 handover based on an arbitrary condition, such as execution of an L2 handover to an AP having the highest radio signal strength (switching of connection of an AP of a correspondent node) by comparing, for example, the strengths of radio signals from different APs. The radio reception means  102  and the radio transmission means  103  are means for respectively performing data reception and data transmission by radio communication, and include various functions needed for radio communication.  
      When the handover decision means  101  is means for deciding to perform an L2 handover, the subnet discrimination means  104  discriminates whether the L2 handover is a handover to a different subnet, based on information described in AP-AR correspondence information  40  in the AP-AR correspondence information storage means  107  (e.g., information on the network prefix of a subnet), and the link layer address of the AP at the L2 handover destination.  
      The NCoA generation means  105  is means for statelessly configurating an NCoA which can match with a subnet constituted by an AR upper-level the AP at the L2 handover destination, based on information described in the AP-AR correspondence information  40  in the AP-AR correspondence information storage means  107  (e.g., information on the network prefix of a subnet), when the subnet discrimination means  104  discriminates that it is a handover to a different subnet.  
      The message generation means  106  is means for acquiring the IP address of an AR constituting a subnet at the handover destination referring to the information described in the AP-AR correspondence information  40  in the AP-AR correspondence information storage means  107 , and generating a message having this IP address as the transmission destination and including at least the NCoA generated by the NCoA generation means  105 .  
      The AP-AR correspondence information storage means  107  is means for storing the AP-AR correspondence information  40 . The AP-AR correspondence information  40  can be referred to by the subnet discrimination means  104 , the NCoA generation means  105 , and the message generation means  106 , as mentioned above, and includes the following information.  
      Referring to  FIG. 3 , information included in the AP-AR correspondence information  40  will be explained below.  FIG. 3  is an exemplary diagram showing one example of AP-AR correspondence information which is stored in an MN according to an embodiment of the invention. As shown in  FIG. 3 , the AP-AR correspondence information  40  to be stored in the AP-AR correspondence information storage means  107  has information indicating the connection relationship between an AP and an AR (information indicating which AP is connected to which AR, i.e., information indicating which AP is under the control of each AR), the link layer address of each AP, the IPv6 address of each AR, a network prefix and a prefix length of a subnet to which each AR belongs, and information on the functions supported by each AR (e.g., information indicating whether or not NSIS is implemented). The network prefix of a subnet to which an AR belongs is information which can easily be acquired by combining the IPv6 address of the AR and the prefix length of the subnet. Although the network prefix of a subnet to which an AR belongs should not necessarily described in the AP-AR correspondence information  40 , therefore, a description will also be given of a case where the network prefix of a subnet to which an AR belongs is described in the AP-AR correspondence information  40 .  
      Correspondence information is set in the AP-AR correspondence information  40  shown in  FIG. 3  for each connection of an AP and an AR, and in association with the link layer address of an AP, a set of the link layer address of an AR which has control over the AP, the IPv6 address of the AR which has control over the AP, the network prefix of a subnet of the AR having control over the AP, the prefix length of the subnet of the AR having control over the AP, and information on the functions supported by the AR is described in each correspondence information. With the structure of the AP-AR correspondence information  40 , when the link layer address of an AP is known, for example, it is possible to refer to the link layer address of an AR which is at an upper level of the AP, and the network prefix and prefix length of a subnet to which the upper-level AR belongs by referring to individual cells laid out horizontally with a cell where the link layer address of the AP is described as a starting point. Accordingly, the NCoA generation means  105  can configure a CoA which can match with the subnet constituting an AR upper-level the AP based on local information present in the MN  10  without acquiring information from other nodes.  
      The MN  10  can grasp the IPv6 address of an AR upper-level the AP from the link layer address of the AP in a similar manner. Further, the MN  10  can refer to the functions supported by the AR. That is, the MN  10  can know the IPv6 address for transmission of packet data to the AR, what function the AR has (e.g., whether or not it has an additional service early establishment function (to be discussed later) according to the invention), etc., based on local information present in the MN  10  without acquiring information from other nodes.  
      The structure of the AP-AR correspondence information  40  shown in  FIG. 3  is one example, and the AP-AR correspondence information  40  is not limited to this structure. Other information than the link layer address of an AP, the IPv6 address of an AR, the link layer address of the AR, the network prefix of a subnet, the prefix length of a subnet, information on other than the functions supported by the AR may be described in the AP-AR correspondence information  40 . As information on the functions supported by the AR, information on various functions, such as whether or not a CoA statelessly configured by the MN  10  can be registered, whether or not to compatible with fast handover, and whether or not to compatible with context transfer which can ensure sharing of situations among nodes, can be described in addition to information on whether or not a mobility-supported additional service realizing function, such as NSIS, is implemented.  
      Although the AP-AR correspondence information  40  shown in  FIG. 3  describes information relating to the connection relationship between an AR  21  and an AP  22  (AP 22 -AR 21  correspondence information) in relation to the arrangement in  FIG. 1 , information relating to the connection relationship between an AR  21  and an AP  23  (AP 23 -AR 21  correspondence information), information relating to the connection relationship between an AR  31  and an AP  32  (AP 32 -AR 31  correspondence information), and information relating to the connection relationship between an AR  31  and an AP  33  (AP 33 -AR 31  correspondence information), information relating to the connection relationship between an arbitrary AP and AR can be set for those information relating to the connection relationships. A method of holding the AP-AR correspondence information  40  in the MN  10  is also arbitrary. Under the local environment of the MN  10 , for example, the AP-AR correspondence information  40  stored in a portable memory medium may be coped or moved into the MN  10 , information relating to the connection relationship between the AR  21  and the AP  23  may be input directly by using operation means (a keyboard or a mouse or the like) of the MN  10 , and saved as the AP-AR correspondence information  40 . For example, the MN  10  can acquire the AP-AR correspondence information  40  over a communication network.  
      Particularly, it is expected that a mobile IPv6 network is used when services are carried out in a limited area by a company LAN (Local Area Network), a local autonomous community, a network provider or the like. In such a network system, the number of APs and ARs which are provided by each company, provider or the like is limited, it is possible to describe information relating to all the AP-AR connection relationships in the AP-AR correspondence information  40 , and store the AP-AR correspondence information  40  in the AP-AR correspondence information storage means  107  beforehand.  
      When there are large numbers of APs and ARs, AP-AR correspondence information  40  describing only information relating to the AP-AR connection relationship in a subnet to which the MN  10  is currently connected, or information relating to the AP-AR connection relationship in a subnet located in the neighborhood (a subnet which is a possible target to which the subnet the MN  10  is currently connected to is to be changed) can be saved in the MN  10 . In this case, as one example, the MN  10  may download only the necessary AP-AR correspondence information  40  from a predetermined communication apparatus (AP-AR correspondence information managing apparatus) provided by the provider of the connection service to the mobile IPv6 network, or may receive broadcast information including the necessary AP-AR correspondence information  40  from the AP-AR correspondence information managing apparatus.  
      An AR may be installed with the function of the AP-AR correspondence information managing apparatus. In this case, as the AR has executed a process of acquiring information relating to the AP-AR connection relationship in a neighboring subnet beforehand to generate AP-AR correspondence information  40 , the MN  10  can acquire the AP-AR correspondence information  40  from the AR currently in connection.  
      When the AP-AR correspondence information managing apparatus manages AP-AR correspondence information, it is possible to cope with a dynamic change in network system. That is, even in a case where the contents of the AP-AR correspondence information  40  are changed, such as a case where information on the IPv6 address of an AR is changed, a case where a function supported by the AR is updated, or a case where an AP or AR has failed, or is newly added to the network, the MN  10  can flexibly cope with a dynamic change in network as the MN  10  periodically checks the AP-AR correspondence information  40  in the AP-AR correspondence information managing apparatus, or the AP-AR correspondence information managing apparatus notifies the MN  10  of update of the correspondence information, so that the MN  10  can always hold latest AP-AR correspondence information  40 .  
      The functions of an AR (AR  31 ) to which the MN  10  is to be connected after handover will be described next.  FIG. 4  is a block diagram showing the structure of an AR according to the embodiment of the invention. The individual functions of the AR  31  shown in  FIG. 4 , like those of the MN  10  shown in  FIG. 2 , can be achieved by hardware and/or software. In particular, the main processes of the invention (processes in the individual steps illustrated in  FIG. 5  to be discussed later) can be achieved by a computer program.  
      The AR  31  shown in  FIG. 4  has reception means  311 , transmission means  312 , message processing means  313 , and QoS path establishing means  314 . The reception means  311  and the transmission means  312  are connected to APs  32 ,  33  which are under the control of the AR  31 , and an IP network  15  which is an external network, and respectively perform data reception and data transmission.  
      The message processing means  313  is means for processing a message when the reception means  311  receives the message generated by the message generation means  106  of the MN  10 . Specific processes to be executed by the message processing means  313  include, for example, checking of the validity of an NCoA included in the message (checking if it is usable by the subnet  30  constituted by the AR  31 ). When the validity of the NCoA is acknowledged, the message processing means  313  requests the QoS path establishing means  314  to establish a QoS path relating the MN  10  which is expected to move to the subnet  30 .  
      The QoS path establishing means  314  is means capable of starting a process of changing the QoS path of the MN  10  by some method (e.g., a method which is expected by be defined by NSIS) upon reception of a request to establish the QoS path relating to the MN  10  from the message processing means  313 . Although the AR  31  has the QoS path establishing means  314  capable of performing QoS guarantee, one of additional services, in an example given here, the QoS path establishing means  314  can be expanded to means that can realize an arbitrary additional service to be supported by, for example, the NSIS. The QoS path establishing means  314  is means capable of starting a process of changing the QoS path of the MN  10 , and should not itself necessarily have a function of changing the QoS path of the MN  10 . That is, upon reception of a request to establish a QoS path relating to the MN  10  from the message processing means  313 , the QoS path establishing means  314  may request other nodes having a function of changing the QoS path of the MN  10  to start the process of changing the QoS path of the MN  10 .  
      As described above, the AR  31  shown in  FIG. 4  is configured in such a way that upon reception of a message generated by the message generation means  106  of the MN  10  (a message including at least an NCoA configured by the MN  10 ), the AR  31  checks the NCoA, and triggered by the reception of the message, the QoS path establishing means  314  starts establishment of the QoS path of the MN  10 . The function of starting a process of establishing an additional service relating to the MN  10  with reception of a predetermined message from the MN  10  as a trigger is called an additional service early establishment function.  
      Referring to a sequence chart in  FIG. 5 , a description will be given of the operation in a case where the MN  10  shown in  FIG. 2  stores the AP-AR correspondence information  40  shown in  FIG. 3  in the AP-AR correspondence information storage means  107 , and performs handover to the subnet  30  from the subnet  20 . While the AR  31  needs to have the additional service early establishment function as mentioned above, it is premised hereinafter that the AR  31  has an additional service realizing function capable of establishing an additional service relating to the MN  10  itself in addition to the additional service early establishment function. Hereinafter, QoS guarantee will be explained as one example of additional services, and it is premised that the AR  31  is implemented with a mobility-supported QoS-path establishment function, such as NSIS (hereinafter called mobility QoS function).  
       FIG. 5  is a sequence chart illustrating an operational example when an MN performs handover between subnets in the radio communication system according to the embodiment of the invention. The sequence chart shown in  FIG. 5  shows, along the time axis, individual processes of the MN  10 , the AR  21 , and the AR  31 , which perform handover from the subnet  20  to the subnet  30  when the MN  10  moves into a radio coverage area  34  formed by the AP  32  from within a radio coverage area  25  formed by the AP  23 , passing through an overlap area  26 , in the radio communication system shown in  FIG. 1 .  
      First, the initial state is set wherein the MN  10  shown in  FIG. 2  is present in the radio coverage area  25 , and is connected to the AR  21  via the AP  23 . When the radio signal from the AP  23  currently in communication becomes weaker with the movement in the radio coverage area  25 , the MN  10  starts searching for another communicatable AP. When it enters the overlap area  26  where the radio coverage area  25  overlaps the radio coverage area  34  (the hatched area in  FIG. 1 ), it becomes possible to listen to the radio wave (radio signal) from the AP  32  (step S 501 : receive a radio signal), i.e., it finds the AP  32 . It is to be noted that in the overlap area  26  the can listen to both the radio signal from the AP  23  and the radio signal from the AP  32 .  
      Then, when the handover decision means  101  of the MN  10  compares the strength of the radio signal from the AP  23  with the strength of the radio signal from the AP  32 , and finds out that the radio signal from the AP  32  is stronger, for example, it decides to switch connection of the AP at the transmission destination (L2 handover) (step S 503 : decide to do L2 handover to AP  32 ). In the embodiment, the condition for deciding execution of an L2 handover has been explained as the use of the result of comparison of radio signal strengths by the handover decision means  101 , the condition is not limited, and execution of an L2 handover may be determined based on another condition.  
      When the handover decision means  101  decides to perform an L2 handover, the handover decision means  101  supplies the subnet discrimination means  104  with a request to refer to the AP-AR correspondence information  40 . Meanwhile, the subnet discrimination means  104  receives the link layer address of the AP  32  acquired from a beacon or the like received from the AP  32 , searches the AP-AR correspondence information  40  for the link layer address of the AP  32  referring to the AP-AR correspondence information storage means  107  (step S 505 : refer to AP-AR correspondence information  40 ), and acquires information on the AR  31  associated with the link layer address of the AP  32 .  
      Then, the subnet discrimination means  104  discriminates whether or not the L2 handover from the AP  23  to the AP  32  will cause handover to a different subnet by referring to, for example, the network prefix of the subnet  30  associated with the link layer address of the AP  32  at the L2 handover destination. When it is determined that the L2 handover from the AP  23  to the AP  32  is to be carried out between different subnets, the MN  10  performs processes at and following step S 507 . When it is determined that the L2 handover from the AP  23  to the AP  32  is to be carried out within the same subnet, the MN  10  does not perform the processes at and following step S 507 , but performs only an L2 handover and keeps using the CoA currently in use.  
      The subnet discrimination means  104 , which grasps that the L2 handover from the AP  23  to the AP  32  is a handover between different subnets (handover from the subnet  20  to the subnet  30 ), requests the NCoA generation means  105  to configure an NCoA. The NCoA generation means  105  combines the network prefix and prefix length of the subnet  30  associated with the link layer address of the AP  32  in the AP-AR correspondence information  40  and the link layer address of the MN  10  to configure an NCoA which matches with the subnet constituted by the AR  31  (step S 507 : autoconfigure NCoA from AP-AR correspondence information  40 ). It is to be noted that, as mentioned above, it is possible to acquire the network prefix of the subnet  30  constituted by the AR  31  by combining the IPv6 address of the AR  31  and the prefix length of the subnet  30 , and configure an NCoA using the network prefix of the subnet  30 . The NCoA generation means  105  supplies the message generation means  106  with the NCoA configured at step S 507  together with a request to generate a predetermined message (hereinafter called message A).  
      The message generation means  106  having received the request to generate the message A from the NCoA generation means  105  first discriminates whether the AR  31  upper-level the AP  32  at the handover destination has a mobility QoS function, such as NSIS (step S 509 : check function of AR  31 ), and proceeds to step S 511  when the AR  31  has the mobility QoS function. Although it is premised that as mentioned above, the AR  31  upper-level the AP  32  at the handover destination of the MN  10  has a mobility QoS function, it is desirable that generation of the message A at step S 511  to be discussed later should not be performed when the AR  31  does not have a mobility QoS function. In this case, the MN  10  can switch the process to a process defined by the conventional fast handover technique, or inform the AR  21  of the NCoA configured at step S 507 . Although it is checked if the AR  31  has the mobility QoS function after configuration of an NCoA in  FIG. 5 , it is also possible to make the checking before configuration of an NCoA (e.g., the subnet discrimination means  104  checks it).  
      After checking that the AR  31  has the mobility QoS function, the message generation means  106  acquires the IP address of the AR  31  held in association with the link layer address of the AP  32  in the AP-AR correspondence information  40 , and generates the message A addressed to the IP address of the AR  31  including at least the NCoA configured at step S 507  (step S 511 : generate message A). Then, as the MN  10  sends the message A to the AP  23  by radio communication, the message is transferred from the MN  10  to the AR  31  via the AP  23 , the AR  21  and the IP network  15  (step S 513 : send message A).  
      The AR  31  having received the message A from the MN  10  checks if the NCoA included in the message A is valid (step S 515 : check if NCoA is valid) in the message processing means  313 . When it is determined that the NCoA is valid at this time, the AR  31  performs a registration process of assigning the NCoA to the MN  10 , and requests the QoS path establishing means  314  to establish the QoS path of the MN  10 . When it is determined that the NCoA is not valid, it is desirable to send a message notifying that the NCoA is not valid to the MN  10 .  
      The QoS path establishing means  314 , which has received the request to establish the QoS path of the MN  10  from the message processing means  313 , starts alteration of the QoS path of the MN  10  (step S 517 : start QoS-path establishing process). As the method relating to alteration of the QoS path, an arbitrary method can be used; for example, NSIS or methods defined by other protocols are available.  
      For example, the following method is possible as one example of the QoS-path establishing process. First, the AR  31  utilizes NSIS to change a QoS path between the AR  21  and a CN  60  in a QoS path before handover set in the MN  10  (e.g., the QoS path before handover between the MN  10  and the CN  60  shown in  FIG. 6 ), and establish the QoS path between the AR  31  and the CN  60 . Meanwhile, the MN  10  performs an L2 handover from the AP  23  to the AP  32  at an arbitrary timing after transmission of the message A (step S 519 : L2 handover), and further through a predetermined process, the handover is completed and connection between the MN  10  and the AR  31  is established (step S 521 : establish connection between MN  10  and AR  31 ), after which the QoS path between the MN  10  and the AR  31  is established, and is combined with the previously-established QoS path between the AR  21  and the CN  60  to establish the QoS path between the MN  10  and the CN  60  (step S 523 : establish QoS path after establishment of QoS path between MN  10  and AR  31 ). Accordingly, QoS guarantee set in the MN  10  before handover can be given promptly after handover, making it possible to minimize the number of packets to be subject to the default QoS transfer process or make it zero.  
      Another possible example of the QoS-path establishing process is a method of, first, changing a QoS path between an arbitrary node (node having a mobility QoS function) present on a QoS path between the MN  10  and the CN  60  which has been established before handover and the AR  21 , to a QoS path between the arbitrary node and the AR  31 , then establishing a QoS path between the MN  10  and the AR  31 . In this case, as the MN  10  describes the IPv6 address of the arbitrary node (particularly, the node having mobility QoS function on the QoS path and in the vicinity of the MN  10 ) or the like in the message A, for example, the AR  31  having received the message A can grasp the IPv6 address of the arbitrary node and can request the arbitrary node to establish the QoS path.  
      The timing at which the AR  31  starts the process of establishing the QoS path at step S 517  depends on the timing at which the MN  10  sends the message A at step S 513 , not on the timing at which the MN  10  performs an L2 handover at step S 519 . In the method mentioned as one example of the QoS-path establishing process, before establishment of a QoS path between the AR  21  and the CN  60 , the MN  10  may perform an L2 handover to establish connection between the MN  10  and the AR  31 . In this case, after the QoS path between the MN  10  and the AR  21  is established first, the QoS path between the AR  21  and the CN  60  is established, and the QoS path between the MN  10  and the CN  60  is established. However, the message A to be sent to the AR  31  from the MN  10  serves as a message to request the AR  31  to start the QoS-path establishing process, and nonetheless the AR  31  can start the QoS-path establishing process at an early stage of the handover operation of the MN  10 , i.e., the number of packets to be subject to the default QoS transfer process can be minimized.  
      According to the embodiment of the invention, as described above, as the MN  10  autoconfigure the NCoA of the subnet  30  constituted by the AR  31  which has the AP  32  under control by referring to the AP-AR correspondence information  40  stored in the AP-AR correspondence information storage means  107  immediately after determining the AP  32  to which an L2 handover is to be performed, and sends the NCoA to the AR  31  directly, the AR  31  can check the NCoA configured in a stateless manner by the MN  10  at an early stage of the handover operation of the MN  10 , and can start a process of establishing various additional services including QoS guarantee. As a result, even after handover, the MN  10  can promptly and continuously get the additional service that has been gotten before handover. The MN  10  can precheck the function of the AR  31  to which the message A is to be sent, and determine whether or not the message A is to be sent to the AR  31 , thereby making it possible to prevent a wasteful increase in communication traffic and a wasteful increase in processing load, such as sending the message A to a message A-unaware AR.  
      In a case where the MN  10  is connected to an AR which uses only the stateful CoA allocation system, an NCoA generated by the MN  10  does not match a subnet to which that AR belongs. With regard to, particularly, information on the AR which uses only the stateful CoA allocation system and on an AP which is under the control of the AR, therefore, it is desirable that they should not be described in the AP-AR correspondence information  40 , or should clarify that only the stateful CoA allocation system is adopted as the function of the AR. Accordingly, with regard to the AR which uses only the stateful CoA allocation system, the MN  10  can acquire a CoA allocated in a stateful manner by performing, for example, the processes by the conventional fast handover.  
      In the sequence chart shown in  FIG. 5 , the AR  31  has an additional service realizing function such as NSIS in addition to the additional service early establishment function, and the AR  31  itself establishes a new QoS path relating to the MN  10 . However, the AR  31  may request other nodes having an additional service realizing function to start a process of changing the QoS path of the MN  10 . In this case, likewise, a process of establishing various additional services relating to the MN  10  is started at an early stage of the handover operation of the MN  10 .  
     INDUSTRIAL APPLICABILITY  
      The communication handover method, the communication message processing method, and the program for executing these methods by use of a computer, and a communication system, according to the invention, can allow a mobile node which performs handover to promptly and continuously get an additional service gotten before handover even after the handover, are applied to the technical field relating to the handover of a mobile node which performs radio communication, and more particularly, are applicable to a technical field relating to the handover of a mobile node which performs radio communication using the mobile IPv6 protocol that is the next generation Internet protocol.