Abstract:
A method for maintaining data transmission from a corresponding node when a mobile node moves from a first access router to a second access router in a network system, the method comprising: transmitting using the mobile node a home address of the mobile node and a one care-of address (CoA) assigned from the first access router to the corresponding node and a home agent, when the mobile node is located in the first access router; and transmitting using the mobile node the home address, the one CoA, and another CoA assigned from the second access router to the first access router so that data from the corresponding node is received via the first access router and the second access router, when the mobile node moves from the first access router to the second access router and the second access router is located within a predetermined distance from the first access router.

Description:
PRIORITY 
   This application claims priority under 35 U.S.C. § 119 to an application entitled “System and Method for Supporting Mobility of Mobile Node Using Regional Anchor Point in Future Internet” filed in the Korean Intellectual Property Office on Aug. 6, 2002 and assigned Serial No. 2002-46293, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a system and method for managing mobility based on MIPv6 (Mobile Internet Protocol Version 6), and in particular, to a system and method for supporting regional mobility using access routers each having an anchor point function. 
   2. Description of the Related Art 
   Internet users desire to use a high-quality Internet service anytime and anyplace, and with the performance improvement of mobile terminals such as portable computers and PDAs (Personal Digital Assistants) and the development of wireless communication technology, the number of users has remarkably increased. 
   An IP (Internet Protocol) address in an Internet addressing system is comprised of a network identifier field and a host identifier field. The network identifier field is a part for identifying a network, while the host identifier field is a part for identifying a host within the network. If a mobile terminal moves to another network, the network identifier is changed and accordingly, an IP address of the mobile terminal is also changed. In an IP layer, since packets are routed according to a network identifier of a destination address, the mobile terminal cannot receive packets when it moves to another network. If a mobile terminal desires to continue communication even in another network, the mobile terminal should change its IP address so that it has a network identifier of the network each time it moves to another network. In this case, upper layer connection such as TCP (Transmission Control Protocol) connection is not guaranteed. Therefore, a protocol called “Mobile IP” capable of guaranteeing mobility is used to enable communication while maintaining an existing address intact. 
   If the number of wireless Internet users is increased, as is the current tendency, the increasing IP address demands cannot be satisfied with the existing IPv4 (Internet Protocol version 4) address system. Therefore, active searches have been carried out on a method for supporting mobility using a MIPv6 protocol that has recently attracted public attention as a future Internet protocol. 
   A fundamental operation of the MIPv6 will be described. If a mobile node (MN) moves from a home network to an external network, the mobile node acquires a care-of address (CoA) from an agent of a subnet where it is currently located. Also, when a mobile node moves from an external one subnet to a new subnet, the mobile node acquires a new CoA from the new subnet. The mobile node binds the CoA with a home address and registers the binding result in corresponding nodes (CNs) with which a home agent of the home network and the mobile node itself communicate. Thereafter, the corresponding nodes set a destination of a packet which is to be transmitted to the mobile node at the CoA, and transmit the packet to the mobile node. The home agent of the home network intercepts the packet being transmitted to the mobile node using the original home address as a destination address, and tunnels the intercepted packet to the mobile node. 
   If the mobile node is geographically or topologically remote from the home agent or the corresponding nodes, a time required for binding update is increased. During the binding update time, packets to be transmitted to the mobile node may be lost in an access router. A concept called “localized mobility management (LMM)” has been introduced as a scheme for solving this problem. LMM refers to a method in which even though a mobile node moves to a new subnet, a packet can be routed to the mobile node without affecting the binding registered in a home agent or the corresponding nodes. In this method, a mobile node can move to a new subnet while maintaining an IP address of the mobile node, as seen by the home agent of the mobile node and by the corresponding nodes. 
   Hierachical MIPv6 (HMIPv6) has been proposed as a conventional technique that satisfies the LMM condition. In HMIPv6, a new node called a “mobile anchor point (MAP)” is defined. The MAP is a router located in a domain visited by a mobile node, and can also be located in any layer among routers in a hierachical structure. 
   The MAP has a function of intercepting all packets to be delivered to a mobile node registered therein and directly tunneling the intercepted packets to a current CoA, or on-line CoA (LCoA), of the mobile node. The mobile node, when it moves to a new MAP domain, binding-registers a region or a regional CoA (RCoA), acquired from the new MAP, and its home address in the corresponding nodes or in the home agent. However, when the mobile node moves within the MAP domain, the mobile node binding-updates the RCoA and the LCoA only in the MAP without binding-updating them in the corresponding nodes or in the home agent. 
   A boundary of the MAP domain is defined by access routers that advertise MAP information to connected mobile nodes.  FIG. 1  illustrates an example of a conventional network topology with one MAP domain. Referring to  FIG. 1 , MAP  101  is connected to a plurality of access routers (ARs)  103 , and each access router  103  is connected again to one or multiple access routers (ARs)  105 . The access routers  105  advertise information on the MAP  101  to connected mobile nodes through a MAP optima message. With introduction of the MAP concept, a waiting time caused by handoff between two access routers is minimized. In addition, the MAP reduces signals that must be transmitted and received to/from the exterior of a regional domain in MIPv6, and smoothly performs handoff of a mobile node. 
   As described above, the MAP can be located in any layer among routers in a hierachical structure or access routers. However, once a position of the MAP is determined, only the access routers located in a lower layer of the MAP can use the MAP as anchor point. That is, the HMIPv6 can be undesirably realized only in a fixed hierachical network topology. Accordingly, there is a demand for a method capable of satisfying the LMM condition without restriction of the network topology like in the conventional HMIPv6. 
   In addition, in the hierachical network topology, when a mobile node is connected to an access router of a lower layer in a MAP domain, a length of a tunnel through which a packet transmitted to the access node is tunneled can become excessively long. An increase in tunnel length means that encapsulation and decapsulation of a packet transmitted to the mobile node must be repeated several times, causing an increase in a transmission time of a packet and a load of the routers that perform tunneling. Thus, there is a demand for an LMM realization method capable of restricting an excessive increase in tunnel length. 
   The HMIPv6 is divided into a basic mode and an extended mode according to a method in which a mobile node acquires RCoA. In the basic mode, RCoA is formed from (1) a subnet prefix of MAP broadcasted in a MAP option and (2) an interface identifier of a mobile node. In the extended mode, a mobile node receives RCoA assigned to any one of the interfaces of a MAP through a MAP option and uses the received RCoA intact. In either mode, the mobile node must acquire both LCoA and RCoA. Particularly, in order for the mobile node to acquire RCoA, a MAP option must be advertised from each router. Therefore, an RCoA acquisition procedure becomes a primary factor of increasing overhead in network operation. Thus, it is necessary to reduce the overhead caused by the RCoA acquisition procedure. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide an apparatus and method for realizing MIPv6 that satisfies an LMM condition without restriction of network topology. 
   It is another object of the present invention to provide an apparatus and method that uses an anchor point for preventing an excessive increase in a length of a tunnel through which a packet is tunneled in MIPv6. 
   It is further another object of the present invention to provide an apparatus and method that uses an anchor point for reducing overhead caused by an RCoA acquisition procedure in MIPv6. 
   To achieve the above and other objects, the invention provides a method for supporting mobility of a mobile node in the future Internet using the mobile node, which is a host for supporting a mobile service; a home agent for managing an address of the mobile node; and corresponding nodes in communication with the mobile node. The method comprises the steps of acquiring a care-of address (CoA) from a current access router where it is currently located; determining whether there is an access router capable of functioning as an anchor point among access routers where the mobile node was previously located; designating the access router capable of functioning as the anchor point as a regional anchor point (RAP); binding the CoA acquired in the RAP and the CoA from the current access router as a RAP address and a final CoA, respectively, and registering them in the RAP; and intercepting, using the RAP, packets being transmitted with the mobile node designated as a destination, and tunneling the packets to the final CoA of the mobile node. 
   To achieve the above and other objects, the invention provides a network system supporting mobility of a mobile node in a future Internet. The network system comprises a mobile node supporting a mobile service; a first access router for assigning a first CoA to the mobile node and transmitting a packet with the first CoA designated as a destination to the mobile node; a second access router with a first binding memory, for assigning a second CoA to the mobile node, registering the second CoA and the first CoA in the first binding memory as a regional CoA and a final CoA, respectively, intercepting a packet being transmitted to the mobile node with the regional CoA designated as a destination, and tunneling the packet data to the mobile node, when the mobile node is located in the first access router; and a corresponding node with a second binding memory, for registering a home address of the mobile node and the regional CoA in the second binding memory, designating the regional CoA as a destination of a packet, and transmitting the packet to the mobile node, when the mobile node is located in the first access router. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  illustrates an example of a conventional network topology with one MAP domain; 
       FIG. 2  illustrates a network topology according to an embodiment of the present invention; 
       FIG. 3  illustrates a method for using MIPv6 in the network topology of  FIG. 2  according to an embodiment of the present invention; 
       FIG. 4  is a flowchart illustrating a procedure performed in a mobile node when the mobile node first moves from a home network to an external network according to an embodiment of the present invention; 
       FIG. 5  is a flowchart illustrating a procedure in which a mobile node performing a second handoff first sets up a RAP; 
       FIG. 6  is a flowchart illustrating a procedure performed in a mobile node when the mobile node performs third or later handoff according to an embodiment of the present invention; 
       FIG. 7  illustrates packet transmission flow before route optimization is performed according to an embodiment of the present invention; and 
       FIG. 8  illustrates packet transmission flow after route optimization is performed according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Several preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for conciseness. 
   The invention proposes a concept of regional MIPv6 (RMIPv6) that uses a regional anchor point (RAP), as MIPv6 that satisfies an LMM condition. The regional anchor point is an access router serving as an anchor point for a mobile node among access routers, and is distinguished from MAP used in HMIPv6. 
   If a mobile node moves to another access router, either a current access router or a previous access router is designated as a RAP. The RAP, including a binding cache, binds LCoA with RCoA of a mobile node and registers the binding result in the binding cache. Like in the HMIPv6, LCoA is a care-of address (CoA) indicating a current position of a mobile node, while RCoA is a CoA indicating an anchor point. The RAP intercepts a packet targeting the mobile node and tunnels the intercepted packet to the LCoA until a binding entry in the binding cache is cleared. 
   A MIPv6 realization method will now be described with reference to the accompanying drawings by applying the above concept. 
     FIG. 2  illustrates a network topology according to an embodiment of the present invention. In the network topology, an AR 1  (Access Network  1 )  10  and an AR 4   40  can serve as an anchor point, i.e., RAP.  FIG. 3  illustrates a method for releasing MIPv6 on the assumption that in a network having the same topology as that of  FIG. 2 , a mobile node sequentially moves from an AR 1   10  to an AR 5   50 . 
   In step  1010 , a mobile node (MN) moves from a home agent (HA)  70  to the AR 1   10  of an external network. The mobile node acquires CoA 1  to be used in the AR 1   10  upon receiving an agent advertisement from the AR 1   10 . Meanwhile, the mobile node determines whether the AR 1   10  can operate as a RAP. 
   In step  1012 , the mobile node sends a binding update (BU) to the home agent  70  with the CoA 1 . In step  1013 , the mobile node sends the binding update to a corresponding node (CN)  60 , with which the mobile node itself is communicating, with the CoA 1 . That is, the home agent  70  and the corresponding node  60  bind the CoA 1  with a home address of the mobile node and register the binding result in their internal binding cache. The binding-updated corresponding node  60  directly transmits a packet targeting the mobile node to the CoA 1 . In addition, the home agent  70  intercepts a packet targeting a home address of the mobile node as a destination and directly tunnels the intercepted target to the CoA 1  when the corresponding node  60  sends a packet to the mobile node. 
   In step  1020 , the mobile node moves again to an AR 2   20 . In step  1021 , the mobile node acquires CoA 2  to be used in the AR 2   20  upon receiving an agent advertisement from the AR 2   20 . In addition, the mobile node determines whether the AR 1   10 , a previous AR, provides a RAP function. At this point, the mobile node determines not only whether the AR 1   10  has a RAP function but also whether the AR 1   10  and the AR 2   20  are located within a predetermined distance range. Herein, the distance range between the two ARs is limited, to prevent a tunneling length from becoming excessively long by using a farther AR as a RAP. Herein, a distance between two ARs is determined on the basis of a hop count, and a limitation of the distance range to an AR that can be used as a RAP is, for example, 3 hops. However, it should be noted that an actual distance between two routers and other methods can also be used. In the network topology of  FIG. 2 , since the AR 1   10  has a RAP function and a distance between the AR 1   10  and the AR 2   20  is 1 hop, the mobile node at the AR 2   20  can use the AR 1   10  as a RAP. Here, the AR 1   10  is represented by RAP 1  in order to distinguish it from other ARs having a RAP function. 
   Meanwhile, it should be noted that in a preferred embodiment of the present invention, the mobile node located in the AR 2  can be realized so that it operates using a method defined in the conventional MIPv6 without using the MIPv6 proposed in the invention even though the AR 1  provides a RAP function. 
   In step  1022 , the mobile node designates CoA 1  as RCoA and CoA 2  as LCoA, and registers them in the RAP 1 . That is, CoA 1  acquired by the mobile node from the RAP 1  is used as RCoA for the mobile node within the RAP 1  domain. Meanwhile, the mobile node determines whether the AR 2   20  can operate as a RAP. In step  1023 - 10 , the AR 1   10  operating as the RAP 1  performs a binding update on CoA 1  and CoA 2  as RCoA and LCoA, respectively, for a mobile node, and transmits a binding acknowledgement (BA) to the mobile node. As a result, the AR 1   10  operates as a RAP for a mobile node. 
   However, upon failure to receive a binding acknowledgement BA from the AR 1   10 , the mobile node operates in a manner defined in MIPv6. In steps  1023 - 20  and  1023 - 21 , the mobile node sends a binding update BU to the home agent  70  ( 1023 - 20 ) and the corresponding node  60  ( 1023 - 21 ) by using CoA 2  as CoA. If the mobile node receives a binding acknowledgement BA including a deny code for denying the binding update BU from the AR 1   10 , or fails to succeed in the other binding update BU, the mobile node performs the steps  1023 - 20  and  1023 - 21 . The deny code follows the MIPv6 specification. 
   In step  2010 , the mobile node moves again to an AR 3   30 . In step  2011 , the mobile node acquires CoA 3  to be used in the AR 3   30  upon receiving an agent advertisement from the AR 3   30 , and then determines whether it can use the AR 1   10  as a RAP. In the network topology of  FIG. 2 , the AR 1   10  is at a 4-hop distance from the AR 3   30 , exceeding the distance range, so that the mobile node located in the AR 3   30  cannot use the AR 1   10  as a RAP. Since the mobile node fails to be supported with the RAP function, it operates in a manner defined in the conventional MIPv6. Therefore, in steps  2012  and  2013 , the mobile node binding-registers CoA 3  and its own home address in the home agent  70  ( 2012 ) and the corresponding node  60  ( 2013 ) by using CoA 3  as CoA. Meanwhile, the mobile node determines whether a RAP function exists in the AR 3   30 . 
   In step  3010 , the mobile node moves again from the AR 3   30  to the AR 4   40 . The mobile node acquires CoA 4  to be used in the AR 4  upon receiving an agent advertisement from the AR 4   40 , and then determines whether it can use the AR 3   30  as a RAP. Here, since no RAP function exists in the AR 3   30 , the mobile node cannot use the AR 3   30  as a RAP. Therefore, in steps  3012  and  3013 , the mobile node designates CoA 4  as CoA, and transmits a binding update BU to the home agent  70  ( 3012 ) and the corresponding node  60  ( 3013 ). Meanwhile, the mobile node determines whether a RAP function exists in the AR 4   40 . 
   In step  3020 , the mobile node moves to the AR 5   50 . In step  3021 , the mobile node acquires CoA 5  to be used in the AR 5   50 , and determines whether it can use the AR 4   40 , a previous AR, as a RAP. In  FIG. 2 , since a RAP function exists in the AR 4   40  and the AR 4   40  is at a 2-hop distance from the AR 5   50 , the mobile node located in the AR 5   50  can use the AR 4   40  as a RAP. Herein, the AR 4   40  is represented by RAP 2  in order to distinguish it from the RAP 1 . 
   In step  3022 , the mobile node designates CoA 4  as RCoA and CoA 5  as LCoA, and registers them in the RAP 2 . Meanwhile, the mobile node determines whether the AR 5   50  can operate as a RAP. In step  3023 , the AR 4   40  operating as a RAP performs binding update on CoA 4  and CoA 5  with the RCoA and the LCoA for a mobile node, and thereafter, transmits a binding acknowledgement BA to the mobile node. Thus, the AR 4   40  operates as a RAP for a mobile node. 
   However, when the mobile node fails to receive the binding acknowledgement BA from the AR 4   40 , the mobile node operates in a manner defined in MIPv6. That is, in steps  3023 - 20  and  3023 - 21 , the mobile node sends a binding update BU to the home agent  70  ( 3023 - 20 ) and the corresponding node  60  ( 3023 - 21 ) by using CoA 5  as CoA. 
   A concept of the MIPv6 using a RAP has been described so far with reference to  FIGS. 2 and 3 . Compared with the conventional HMIPv6, the MIPv6 proposed by the present invention has the following advantages. First, the MIPv6 realizes a RAP irrespective of a network topology layer, thereby satisfying an LMM condition without restriction of a network topology, unlike the conventional HMIPv6. Second, the MIPv6 uses only an AR located within a predetermined distance range, thereby preventing an excessive increase in tunnel length. Third, the MIPv6 uses CoA acquired in a previous AR as RcoA, thereby removing overhead caused by acquisition of RCoA. 
   Shown in Table 1 below are binding entries registered in binding caches of the access routers, the home agent (HA)  70  and the corresponding node (CN)  60  according to a position of a mobile node when the mobile node is sequentially handed off between the access routers in the order shown in  FIG. 3 . 
   
     
       
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               MN 
                 
                 
                 
                 
                 
                 
             
             
               Po- 
                 
               AR1(RAP1) 
                 
                 
               AR4(RAP2) 
             
             
               sition 
               HA/CN 
               (LCoA:RCoA) 
               AR2 
               AR3 
               (LCoA:RCoA) 
               AR5 
             
             
                 
             
           
           
             
               AR1 
               MN:CoA1 
               — 
               — 
               — 
               — 
               — 
             
             
               AR2 
               MN:CoA1 
               CoA2:CoA1: 
               — 
               — 
               — 
               — 
             
             
                 
                 
               MN 
             
             
               AR3 
               MN:CoA3 
               — 
               — 
               — 
               — 
               — 
             
             
               AR4 
               MN:CoA4 
               — 
               — 
               — 
               — 
               — 
             
             
               AR5 
               MN:CoA4 
               — 
               — 
               — 
               CoA5:CoA4: 
               — 
             
             
                 
                 
                 
                 
                 
               MN 
             
             
                 
             
           
        
       
     
   
     FIGS. 4 to 6  illustrate detailed procedures performed in a mobile node to realize RMIPv6 according to a preferred embodiment of the present invention. Specifically,  FIG. 4  is a flowchart illustrating a procedure performed in a mobile node when the mobile node first moves from a home network to an access router on an external network ARi, and  FIG. 5  is a flowchart illustrating a procedure in which a mobile node performing a second handoff first sets up a RAP. Further,  FIG. 6  is a flowchart illustrating a procedure performed in a mobile node when the mobile node performs a third or later handoff. The drawings are separated according to moving order of the mobile node for the convenience of explanation. 
   With reference to  FIG. 4 , a description will now be made of a procedure for first finding a RAP using a mobile node when the mobile node first moves from a home network to an access router on an external network ARi. 
   In step  110 , the mobile node determines whether it has moved from a home network to an access router on an external network ARi. That is, the mobile node determines whether it has moved from the home agent to an external access router ARi. If the mobile node has moved to the external access router ARi, the mobile node acquires CoAi from a current access router ARi in step  120 . Here, CoAi can be acquired in a common stateful method or stateless method. 
   In step  130 , the mobile node determines whether a RAP function exists in the access router ARi. If the ARi has a RAP function, the mobile node designates the ARi as fRAP in step  140 . However, if no RAP function exists in the ARi, the mobile node designates fRAP as null in step  141 . Here, the fRAP is a parameter for indicating whether an immediately previous access router ARi can be used as a RAP when the mobile node moves to another access router, and the mobile node manages the fRAP by allocating a specific area of a memory. 
   In step  150 , the mobile node requests the home agent and the corresponding node to binding-update a current CoAi and its own home address, and then ends the RAP setting. 
   With reference to  FIG. 5 , a description will now be made of a procedure performed in a mobile node when the mobile node performs a second handoff, i.e., moves to an ARj. 
   In step  200 , the mobile node is handed off to an access router on an external network ARj and acquires CoAj from the current access router on the external network ARj, or the current external access router. In step  210 , the mobile node determines whether fRAP, i.e. a previous access router providing a RAP function, exists. If fRAP exists, the mobile node determines in step  220  whether a distance between the previous access router fRAP and the current access router ARj falls within a predetermined range. This is to prevent an excessive increase in tunnel length because of the excessively long distance between the RAP and the current mobile node. 
   If a distance condition of the fRAP is satisfied, the mobile node designates the CoA and the CoAj acquired in the fRAP as RCoA and LCoA, respectively, in step  230 . The mobile node binding-updates the designated RCoA and LCoA in the fRAP in step  240 , and then determines in step  250  whether a binding acknowledgement BA is received from the fRAP. If the BA is received, the fRAP starts providing a RAP function for the mobile node. Therefore, in step  260 , the mobile node designates the fRAP as cRAP in order to designate the fRAP as a current anchor point, and then proceeds to step  400 . The cRAP, like the fRAP, is a parameter managed by the mobile node by allocating a specific area of a memory. 
   In step  400 , the mobile node determines whether the current access router ARj can operate as the next access router RAP. This is to determine whether the current access router ARj can be used as an anchor point when the next access router cannot use a current anchor point cRAP, i.e. a current access router providing a RAP function. As a result of the determination in step  400 , if a RAP function exists in the ARj, the mobile node designates the ARj as fRAP in step  410 , and ends the RAP setting procedure in step  420 . However, if no RAP function exists in the ARj, the mobile node sets the fRAP to null in step  411 . 
   Meanwhile, if the fRAP is null in step  210 , i.e., if no RAP function exists in a previous access router, or if it is determined in step  220  that even though a RAP function exists in the previous access router a distance between the previous access router having the RAP function and the current access router ARj is too far, then the mobile node cannot be provided with a RAP function from the previous access router. In addition, even when no BA is received from the fRAP in step  250 , the mobile node cannot be provided with a RAP function from the previous access router. In this case, the mobile node proceeds to step  310  where it transmits a binding update BU to the home agent and the corresponding node with the CoAj and operates in a method defined in the conventional MIPv6. 
   Next, with reference to  FIG. 6 , a description will be made of an operation performed when a mobile node performs a third handoff, i.e., moves to ARk. 
   In step  500 , the mobile node moves to another external network, and acquires CoAk from an access router ARk of the external network. In step  510 , the mobile node determines whether cRAP, i.e., a current anchor point, exists. If the cRAP exists, the mobile node determines in step  520  whether a distance between the cRAP and the current access router ARk falls within a predetermined range. This is to prevent an excessive increase in a tunnel length because of the excessively long distance between the cRAP and the current mobile node. 
   As a result of the determination in step  520 , if a distance condition of the cRAP is satisfied, the mobile node designates the CoA and the CoAk acquired in the cRAP as RCoA and LCoA, respectively, in step  610 . In step  620 , the mobile node binding-updates the designated RCoA and LCoA in the cRAP, and then proceeds to step  900 . 
   However, as a result of the determination in step  520 , if the cRAP does not exist or the distance condition is not satisfied, the mobile node determines in step  710  whether a previous access router can be used as an anchor point. That is, it is determined whether fRAP exists. If fRAP exists as illustrated in  FIG. 5 , the mobile node determines in step  720  whether a distance between the fRAP and the current access router ARk falls within a predetermined range. As a result of the determination, if the fRAP satisfies a distance condition, the mobile node designates the CoA and the CoAk acquired in the fRAP as RCoA and LCoA, respectively, in step  730 . The mobile node binding-updates the designated RCoA and LCoA in the fRAP in step  740 , and determines in step  750  whether a response BA for the binding update is received from the fRAP. If the BA is received, the fRAP starts providing a RAP function for the mobile node. Therefore, in step  760 , the mobile node designates the fRAP as a new cRAP, and then proceeds to step  900 . At this moment, since binding registration of a previous cRAP must be released, the mobile node sets a lifetime field of a binding update message to ‘0’, and then transmits the binding update message to the previous cRAP. 
   In step  900 , the mobile node determines whether the current access router ARk can operate as a RAP in the next access router. As a result of the determination, if a RAP function exists in the ARk, the mobile node designates fRAP as ARk in step  910 , and then ends the RAP setting procedure in step  920 . However, if no RAP function exists in the ARk, the mobile node designates the fRAP as null in step  911 . 
   Meanwhile, if the fRAP is null in step  710 , i.e., if no RAP function exists in a previous access router, or if it is determined in step  720  that even though a RAP function exists in the previous access router, a distance between the previous access router having the RAP function and the current access router ARk is too far, then the mobile node cannot be provided with a RAP function from the previous access router. In addition, even when no BA is received from the fRAP in step  750 , the mobile node cannot be provided with a RAP function from the previous access router. In this case, the mobile node proceeds to step  810  where it transmits a binding update BU to the home agent and the corresponding node with the CoAk. At this moment, the mobile node sets a lifetime field of a binding update message to ‘0’, and then transmits the binding update message to the cRAP thereby to release the binding registered in the cRAP. However, it is determined in step  510  that cRAP does not exist, it is not necessary to cancel registration in the cRAP. 
     FIG. 7  illustrates packet transmission flow to a mobile node  51  connected to an AR 5   50  in a MIPv6-based network built as described in conjunction with  FIG. 3  according to an embodiment of the present invention. Particularly,  FIG. 7  illustrates packet flow from a corresponding node  60  to the mobile node  51  before route optimization is performed. That is, in the packet flow of  FIG. 7 , binding for the mobile node  51  is not set up yet in the corresponding node  60  and a packet for the mobile node  51  is transmitted via a home agent  70 . 
   In step  701 , the corresponding node  60  desiring to transmit a packet to the mobile node  51  designates a source and a destination of a packet header as the corresponding node  60  and the mobile node  51 , respectively, and then transmits the packet to the mobile node  51 . In step  703 , the home agent  70  intercepts the transmitted packet having the mobile node  51  managed by the home agent itself as its destination. The home agent  70  encapsulates an external header of the packet with the home agent  70  designated as a source and RCoA designated as a destination, and transmits the packet to a RAP 2   40 . The RCoA is CoA 4  which is CoA of the RAP 2   40 , previously binding-registered in the home agent  70  together with a home address of the mobile node  51  at the request of the mobile node  51 , when the mobile node  51  moves to the AR 4  which is the RAP 2   40 . 
   In step  705 , the RAP 2   40  receives the packet. The RAP 2   40  removes an external header of the received packet, and encapsulates again a header with LCoA designated as a destination and RCoA designated as a source. Thereafter, in step  707 , the packet is tunneled to the mobile node  51  via the AR 5   50  connected thereto. Here, the LCoA and RCoA are binding-updated in the RAP 2   40  when the mobile node  51  moves to the AR 5   50 . 
     FIG. 8  illustrates packet flow transmitted from a corresponding node  60  to a mobile node  51  after route optimization for the corresponding node  60  is completed. 
   In step  801 , the corresponding node  60  designates the corresponding node  60  as a source of a packet header and CoA of the mobile node  51  as a destination of the packet header, attaches a home address of the mobile node  51  to a routing header, and transmits the packet to the mobile node  51 . Here, CoA of the mobile node  51  designated as a destination is current RCoA of the mobile node  51 , i.e., CoA 4  binding-updated in the corresponding node  60  together with a home address of the mobile node  51  when the mobile node  51  moves to the RAP 2   40 . In step  803 , the RAP 2   40  intercepts the packet, and designates LCoA of the mobile node  51  as a destination of the packet header and an address of RAP as a source of the packet header. Here, the LCoA is CoA 5  acquired from the AR 5   50  where the mobile node  51  is currently located, while the RCoA is CoA 4  acquired from the RAP 2   40 . Unlike in  FIG. 7 , after route optimization, a packet from the corresponding node  60  does not pass through the home agent  70 . Finally, in step  805 , the packet is tunneled again to the mobile node  51  via the AR 5   50 . 
   As described above, the invention can overcome fixation of a network topology, which is a shortcoming of HMIPv6, by realizing MIPv6 using an anchor point. In addition, the invention can prevent an excessive increase in tunnel length by using only an access router located within a predetermined distance range as an anchor point. Moreover, the invention uses CoA acquired from a previous access router as RCoA thereby removing overhead caused by acquisition of RCoA. 
   While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.