Patent Publication Number: US-7715346-B2

Title: Communication system, communication terminal, routing control method, and router

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/935,265, filed Sep. 8, 2004, and is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-316011, filed on Sep. 8, 2003. The entire contents of these applications are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to a communication system, a communication terminal, a routing control method, and a router, which control a packet transfer route. 
   2. Description of the Related Art 
   Conventionally, “IP 2 ” (IP-based IMT Network Platform) has been proposed as a routing control method which is applied in the case where a communication terminal is moved using wireless communication (refer to, for example, T. Okagawa, M. Jo, K. Nishida, and A. Miura, “IP Packet Routing Mechanism Based on Mobility Management in IP-based IMT Network Platform”, International Conference on Intelligence in the next generation Networks (ICIN), 2003, and T. Okagawa et al., “IP Packet Routing Mechanism in IP 2 ”, The Institute of Electronics, Information and Communication Engineers (IEICE) Technical Report, Mobile Multimedia Communications (MoMuc) 2002-63, November 2002). 
   In IP 2 , communication with a mobile communication terminal has been achieved by using two addresses: a unique host address unique to the communication terminal; and a routing host address, which is assigned to the communication terminal by an access router (edge router) and the like for routing between routers. 
   Specifically, as shown in  FIG. 1 , the access router includes: a routing cache table (RCT)  300  which is a correspondence table between a unique host address (IPha) and a routing host address (IPra); a routing table  400  having routing information which establishes correspondence between a unique host address and an output port of an access router; and a neighbor cache  500  holding information which establishes correspondence between a MAC address (layer 2 address) and an IP address (layer 3 address). 
   When the communication terminal starts communication through the access router, or executes handover, the access router assigns a routing host address to the communication terminal, that is, the access router establishes correspondence between the routing host address and a unique host address, and, at the same time, updates the contents of RCT  300  and the routing table  400 . 
     FIG. 2  shows message procedure for a case where handover is executed for the communication terminal under control of the access router, that is, between wireless base stations (access points, base stations, and the like) connected to the same access router. Up on receiving a router advertisement which has been transmitted from the access router through a base station of a destination for handover, the communication terminal updates the neighbor cache created in the communication terminal, and transmits an activation message, which is a message of an upper layer above the layer 3, to the access router. 
   When the access router receives the activation message, the router re-assigns a routing host address to the communication terminal, and updates the contents of the routing table  400  which establishes correspondence between a unique host address and an output port. 
   The access router transmits an activate notification message to a routing manager, which is a routing management mechanism in a network, in order to register the correspondence between a routing host address and a unique host address, and updates the contents of RCT  300  according to a routing host address update (IPra update) message transmitted from the routing manager. 
     FIG. 3  shows message procedure for a case where the access router transfers a packet addressed to the communication terminal. As shown in  FIG. 3 , when the packet addressed to the communication terminal from the network side is received, the access router searches RCT  300  to convert the routing host address included in the received packet, to a unique host address. 
   The access router then selects a route to the communication terminal, using the routing table  400  based on the converted unique host address, and transmits the packet which has the converted unique host address to the communication terminal. 
   However, in IP 2 , when handover is executed between the wireless base stations connected to the same access router for the communication terminal, re-assignment of the routing host address, updating of the routing table  400 , updating of RCT  300  in the access router, and updating of RCT in the routing manager are executed as described above, though the correspondence between a unique host address and a routing host address for the communication terminal has been registered in RCT  300  of the access router. 
   Therefore, there has been a problem in that a certain period of time is required for completion of the handover for the communication terminal even when the handover is executed between the wireless base stations connected to the same access router. 
   Further, since the routing information establishing correspondence between a unique host address and an output port is registered in the routing table  400 , and the amount of information in the routing table  400  is increased according to an increase in the number of communication terminals communicating through the access router, there has been a problem in that the time required to search for routing information is increased, and the high-speed transfer of packets becomes difficult in IP 2 . Further, since the access router is required to refer to RCT  300 , the routing table  400 , and the neighbor cache  500  when the packets are transferred, there has also been a problem in that the high-speed transfer of packets becomes difficult. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention has been made in consideration of the above circumstances and its objective is to provide a communication system, a communication terminal, a routing control method and a router, which reduces the time required for handover executed between wireless base stations connected to the same access router, and the high-speed transfer of packets can be realized when a route for packet transfer is controlled using a unique host address and a routing host address. 
   In order to solve the above-described problems, the present invention has the following features. A first aspect of the present invention is a communication system comprising a communication terminal and a router, wherein the communication terminal comprises a judgment unit configured to judge whether handover is to be executed or not between wireless base stations connected to the router based on a router advertisement transmitted from the router, and a link moving notification transmitter configured to transmit a link moving notification to the router, which indicates that handover is executed between the wireless base stations connected to the router, based on a judgment result by the judgment unit, and the router comprises a routing information updating unit configured to establish correspondence between a unique host address which is unique to the communication terminal and a routing host address used in the router, and to update a routing information which establishes correspondence between the unique host address and an output port for output of a packet addressed to the communication terminal, and wherein, the routing information updating unit only updates the routing information when the link moving notification is received. 
   According to the above-described aspect, re-assignment of a routing host address and updating of a routing cache table (RCT) can be avoided, and a reduction in the time required for completion of handover can be realized, because only routing information is updated when the router receives the link moving notification indicating that the handover is executed between the wireless base stations connected with the same access router. 
   Further, the wasting of network resources is avoided according to this aspect, because neither the access router nor a routing manager transmits and receives a message to update RCT. 
   A second aspect of the present invention is a communication terminal comprising a judgment unit configured to judge whether handover is to be executed or not between wireless base stations connected to a router based on a router advertisement transmitted from the router and a link moving notification transmitter configured to transmit a link moving notification to the router, which indicates that handover is executed between the wireless base stations connected to the router, based on a judgment result by the judgment unit. 
   A third aspect of the present invention is a routing control method which is used in a communication system comprising a communication terminal and a router, having the steps of judging to judge whether handover is to be executed or not between wireless base stations connected to the router based on a router advertisement transmitted from the router, transmitting a link moving notification to the router, which indicates that handover is executed between the wireless base stations connected to the router, based on a judgment result in the judging step, corresponding a unique host address which is unique to the communication terminal and a routing host address used in the router, updating a routing information which establishes correspondence between the unique host address and an output port for output of a packet addressed to the communication terminal, and wherein, in the updating step, only the routing information is updated when the link moving notification is received. 
   A fourth aspect of the present invention is a router comprising a routing table storage unit configured to store a routing table which establishes correspondence between a routing host address used between routers and an output port for output of a packet addressed to a communication terminal, an address processor which establishes correspondence between the routing host address and a unique host address which is unique to the communication terminal, based on the routing table, a route selector which selects the output port, based on the routing host address included in the packet and the routing table, and an address converter which converts the routing host address included in the packet into the unique host address, based on the result after establishing correspondence between the routing address and the unique host address by the address processor. 
   According to the aspect, the contents of a routing table are not updated even if the number of communication terminals which communicate through the router is increased. Therefore, an increase in the amount of information in the routing table can be avoided, and a reduction in the time required to search for routing information can be realized, because a routing table including a routing information which establishes correspondence not between the unique host address and output port but between the routing host addresses and the output port is used. 
   In other words, if the routing information establishing correspondence between the routing host address assigned to the communication terminal and the output port for a packet is expressed in an integrated manner in the routing table, for example, the routing information is expressed per network address in the routing table, the amount of the routing information in the downward direction, i.e., the routing information addressed to the communication terminal can be reduced. Thereby, a reduction in the time required for searching for the routing information, and the high-speed transfer of packets can be realized. 
   A fifth aspect of the present invention is a router according to the fourth aspect, wherein the route selector selects the output port, using an integrated table which establishes correspondence among the routing host address, the unique host address and the output port, and notifies the unique host address of the communication terminal to the address converter, and the address converter converts the routing host address into the unique host address, based on the notified unique host address. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a view showing an example of the contents of a routing cache table, a routing table, and a neighbor cache, which are used in a conventional router; 
       FIG. 2  is a view showing message procedure for a case where the communication terminal executes handover between wireless base stations connected to the same router; 
       FIG. 3  is a view showing message procedure for a case where a conventional router transfers a packet to a communication terminal; 
       FIG. 4  is a view showing a network configuration including a communication terminal and a router according to an embodiment of the present invention; 
       FIG. 5A  and  FIG. 5B  are a view showing configurations of logical blocks for the communication terminal, and the router according to the embodiment of the present invention; 
       FIG. 6  is a view showing message procedure for a case where the communication terminal executes handover between wireless base stations connected to the same router according to a first embodiment of the present invention; 
       FIG. 7  is a view showing message procedure for the communication terminal, and the router according to a second embodiment of the present invention; 
       FIG. 8  is a view showing an example of the contents of a routing table, a routing cache table (RCT), and a neighbor cache according to the second embodiment of the present invention; and 
       FIG. 9  is a view showing an example of a table into which the routing cache table and the routing table are integrated according to variant of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
   A first embodiment of the present invention will now be explained. In the first embodiment, an embodiment in which, when a communication terminal executes handover between wireless base stations connected to the same router, the communication terminal transmits a link moving notification that the communication terminal executes handover between the wireless base stations connected to the router, and the router updates only routing information, based on the received link moving notification. 
   (Network Configuration) 
     FIG. 4  shows a network configuration including a communication terminal and a router according to the first embodiment. As shown in  FIG. 4 , access routers AR 1  and AR 2 , which are the router of the embodiment are deployed in a network  1  which is the network where packets are forwarded. A routing manager RM that is a route management mechanism in the network  1  is connected to the network  1 . 
   The access router AR 1  is connected to access points AP 1  and AP 2  through a layer- 2  switch  10  for switching in the layer  2 , using a connection IF 1 . The access router AR 1  is also connected to an access point AP 3  using a connection IF 2 . The access router AR 2  is connected to an access point AP 4 . 
   The access points AP 1  through AP 4  are configured to carry out wireless communication with mobile terminals MN 1  and MN 2  which are the communication terminals of the embodiment. For example, a wireless base station in conformity with a wireless LAN system set forth in the Institute of Electrical and Electronic Engineers (IEEE) can be used as the access points AP 1  through AP 4 . Further, the access points AP 1  through AP 4  may be a wireless base station, which carries out wireless communication with cellular telephone terminals. 
   The mobile terminals MN 1  and MN 2  are communication terminals having a wireless communication function to establish wireless communication with access points AP 1  through AP 4  and, for example, a personal computer, a personal digital assistant (PDA), a cellular telephone terminal, or the like can be used as the mobile terminals. 
   (Configurations of Logical Blocks in Communication Terminal and Router) 
   The configurations of logical blocks for the communication terminal and the router of the embodiment will be explained with reference to  FIGS. 5A and 5B . 
   As shown in  FIGS. 5A and 5B , the mobile terminal MN 1  which is the communication terminal of the embodiment includes an RA receiver  201 , a storage unit  203 , a judgment processor  205  and a notification unit  207 . Here, the mobile terminal MN 2  has the same configuration as that of the mobile terminal MN 1 . 
   As shown in  FIG. 5B , the access router AR 1  which is the router of the embodiment comprises, a connection IF 1 , a connection IF 2 , a storage unit  105 , a routing controller  107 , an address converter  109 , a network connection IF 111  and a notification receiver  113 . Here, the access router AR 2  has the same configuration as that of the access router AR 1 . 
   The RA receiver  201  is configured to receive a router advertisement (RA) transmitted from the access router AR 1 . The RA receiving unit  201  forwards the received router advertisement to the judgment processor  205 . 
   The storage unit  203  is configured to store the contents of a neighbor cache that includes information establishing correspondence between a MAC address and an IP address. 
   The judgment processor  205  is configured to generate an activation message to be transmitted to the access router AR 1  (access router AR 2 ) to start communication, or to execute handover, up on receiving the router advertisement forwarded by the RA receiver  201 . 
   In the present embodiment, the judgment processor  205  also forms a judgment unit which judges, based on the router advertisement transmitted from the access router AR 1  (access router AR 2 ), whether handover is to be executed or not between access points (wireless base stations) connected to the same access router. 
   Specifically, the judgment processor  205  judges whether the router advertisement received through an access point under current wireless communication, and the router advertisement received through an access point of a destination for handover have been transmitted from the same access router. 
   When the router advertisement received through an access point under current wireless communication, and the router advertisement received through an access point of a destination for handover have been transmitted from the same access router, the judgment processor  205  generates a link moving notification (LMN) which indicates that a link for communication is moved under control of the same access router. 
   Further, the judgment processor  205  updates the contents of the neighbor cache stored in the storage unit  203 , based on the received router advertisement. 
   The notification unit  207  is configured to transmit the activation message generated by the judgment processor  205  to the access router AR 1  (access router AR 2 ). 
   In the present embodiment, the notification unit  207  is also configured to transmit the LMN, which indicates that handover is executed between the access points (wireless base stations) connected to the same access router, to the access router AR 1  (access router AR 2 ) based on the judgment result by the judgment processor  205 , and forms a link moving notification transmitter. 
   Other than the above-described blocks, the mobile terminal MN 1  (mobile terminal MN 2 ) has a block, by which data packets are transmitted and received. 
   Functions of each block of the access router AR 1  will now be explained. The connection units IF 1  and IF 2  are an interface which connects the access points and the layer-2 switch  10 , and transmits and receives packets between the units and mobile terminal MN 1  (mobile terminal MN 2 ). For example, a LAN interface such as a 100 BASE-TX or the like can be used as the units. Here, the connection units IF 1  and IF 2  form an output port which outputs packets addressed to the mobile terminal MN 1  (mobile terminal MN 2 ). 
   The storage unit  105  stores RCT  300 , the routing table  400  and the neighbor cache  500  as shown in  FIG. 1 . 
   The routing controller  107  is configured to select any one of the connection IF 1 , the connection IF 2 , or the network connection IF 111  to which packets received through the connecting units IF 1 , IF 2 , or the network communication IF 111  are output, based on the routing table  400  stored in the storage unit  105 . 
   The routing controller  107  also updates the contents of the neighbor cache  500  based on the received activation message, when it is notified from the notification receiver  113  that the activation message has been received. 
   The routing controller  107  assigns a routing host address to the mobile terminal MN 1  (mobile terminal MN 2 ), that is, establishes correspondence between the unique host address and the routing host address of the mobile terminal MN 1  (mobile terminal MN 2 ), and transmits an activate notification message to a routing manager RM to update the contents of RCT  300  based on an IPra Update message transmitted from the routing manager RM. 
   The routing controller  107  then updates the contents of the routing table  400  based on the unique host address assigned to the mobile terminal MN 1  (mobile terminal MN 2 ). 
   In the present embodiment, the routing controller  107  updates only the routing information on the routing table  400  when the access router AR 1  receives the link moving notification (LMN) from the mobile terminal MN 1  (mobile terminal MN 2 ). Here, in the present embodiment, the routing controller  107  forms a routing information updating unit. 
   Specifically, the routing control unit  107  neither assigns a new routing host address to the mobile terminal MN 1  (mobile terminal MN 2 ), nor updates the contents of RCT  300  when it is notified from the notification receiver  113  that the link moving notification (LMN) has been received. 
   When it is notified from the notification receiver  113  that the link moving notification (LMN) has been received, the routing controller  107  updates only the routing information on the routing table  400  because the output port of the packet addressed to the mobile terminal MN 1  is changed due to handover executed for the mobile terminal MN 1  (mobile terminal MN 2 ) between the access points connected to the access router AR 1 . 
   The address converter  109  is configured to execute conversion between the routing host address included in the packet received through the network connection IF 111  and the unique host address. The address converter  109  also executes conversion between the unique host address included in the packet received through the connection units IF 1 /IF 2  and the routing host address. 
   The network connection IF 111  is an interface, by which packets are transmitted and received between the network connection IF 111  and the network  1 . For example, a LAN interface such as a 100 BASE-TX or the like can be used as the network connection IF 111 . 
   The notification receiver  113  is configured to receive the activation message from the mobile terminal MN 1  (mobile terminal MN 2 ), and to notify the routing controller  107  that the activation message has been received. The notification receiver  113  is also configured to receive the activate notification message from the routing manager RM, and to notify the routing controller  107  that the activate notification message has been received. 
   Further, in the embodiment, the notification receiver  113  is configured to receive the link moving notification (LMN) from the mobile terminal MN 1  (mobile terminal MN 2 ), and to notify the routing controller  107  that the LMN has been received. 
   (Routing Control Method Using Communication Terminal and Router) 
   A routing control method using the communication terminal and the router according to the first embodiment will now be explained. 
     FIG. 6  shows message procedure for a case in which the mobile terminal MN 1  executes handover from the access point AP 2  connected to the connection IF 1  of the access router AR 1  to the access point AP 3  connected to the connection IF 2  of the access router AR 1 . 
   At first, in STEP S 10 , the mobile terminal MN 1  receives a router advertisement from the access router AR 1  through the access point AP 3  of a destination for handover. 
   In STEP S 20 , the mobile terminal MN 1  judges, based on the router advertisement received in STEP S 10  through the access point AP 3  and the router advertisement received through the access point AP 2  under current wireless communication, that handover is executed between the access points connected to the access router AR 1 . 
   In STEP S 30 , the mobile terminal MN 1  updates the contents of the neighbor cache, based on the advertisement information received in STEP S 10 . 
   In STEP S 40 , the mobile terminal MN 1  transmits the link moving notification (LMN), which indicates that handover is executed from the access point AP 2  to the access point AP 3 , both of which are connected to the access router AR 1 , to the access router AR 1 . 
   In STEP S 50 , the access router AR 1  updates only the correspondence between the unique host address of the mobile terminal MN 1  and the output port based on the received link moving notification (LMN) in the routing table  400 . Accordingly, the access router AR 1  neither assigns a new routing host address to the mobile terminal MN 1 , nor updates the contents of RCT  300  when the link moving notification (LMN) is received. 
   Here, the mobile terminal MN 1  may transmit the link moving notification (LMN) either by multicast or by unicast. Further, the timing at which the mobile terminal MN 1  transmits the link moving notification (LMN) to the access router AR 1  may not be the same timing at which the router advertisement is received from the access router AR 1  through the access point AP 3 . The timing at which the mobile terminal MN 1  transmits the LMN to the access router AR 1  may be the timing at which the mobile terminal MN 1  switches a radio link to the access router AR 1 , or to an access point to be connected to the access router AR 1 . 
   (Effects) 
   According to the above-described first embodiment of the present invention, re-assignment of a routing host address and updating of a routing cache table (RCT) can be avoided, and a reduction in the time required for completion of handover can be realized, because only the routing information is updated when the access router AR 1  (access router AR 2 ) receives the link moving notification indicating that the handover is executed between the access points connected with the same access router. 
   Further, the wasting of network resources is avoided according to the first embodiment, since neither the access router nor a routing manager transmits and receives a message to update RCT. 
   Though IP 2  has been explained as an example in the above-described first embodiment, the present invention can also be applied to other communication methods such as mobile IPv6 (MIPv6). In MIPv6, DAD (Duplicated Address Detection) process is performed when a communication terminal executes handover. In the DAD process, “care of address (CoA)” is automatically generated based on a router advertisement received from an access router and no duplication of the CoA on a network is confirmed using multicast. 
   Further, in MIPv6, control messages (Neighbor Solicitation (NS), and Neighbor Advertisement (NA)) using the multicast are exchanged between the communication terminal and the access router in order to update the neighbor caches in the communication terminal and the access router. 
   That is, there is a problem being that the network resources are wasted in MIPv6, since the messages using the multicast are exchanged between the communication terminal and the access router. Especially, a measure against impersonations of NS and NA is required, because wireless communication is used between the communication terminal and the access router. 
   Accordingly, the above-described problem can be solved by transmitting the link moving notification (LMN) using unicast, which indicates that the communication terminal executes handover between the access points connected to the same access router, only to the access router concerned, as described above. 
   Second Embodiment 
   A second embodiment of the present invention will now be explained. In the second embodiment, an embodiment in which, a router selects an output port for a packet addressed to a communication terminal based on the routing host address included in the packet using a routing table, which establishes correspondence between a routing host address and an output port, and the router converts the routing host address into a unique host address after selecting the output port to transmit the packet. 
   Further, in the second embodiment, the router pools a plurality of routing host addresses and uses a routing table establishing correspondence between pooled routing host addresses and output ports in order to avoid updating of the contents of the routing table even when the communication terminal executes handover. 
   (Network Configuration and Block Configuration of Router) 
   In the second embodiment, the same network configuration as that of the above described first embodiment is used. Further, the logical block configuration of the router of the embodiment is the same as that of the access router AR 1  explained in the first embodiment. 
   Here, a block which has different functions from the block of the access router AR 1  in the first embodiment will be mainly explained with reference to  FIG. 5B . 
   A storage unit  105  of the present embodiment is configured to store a routing table establishing correspondence between a routing host address used for the access routers AR 1  or AR 2  which forms a network  1 , and the connection IF 1  (or, connection IF 2 ) which outputs packets addressed to the mobile terminal. The storage unit  105  of the present embodiment forms a routing table storage unit. 
   Specifically, the routing table, in which the contents are different from the conventional ones shown in  FIG. 1 , and a routing cache table are stored in the storage unit  105 . Here,  FIG. 8  shows an example of the contents of a routing cache table (RCT)  310 , a routing table  410 , and a neighbor cache  500 , which are stored in the storage unit  105  of the access router AR 1  according to the present embodiment. As shown in  FIG. 8 , the routing table  410  establishes correspondence between the routing host addresses and the output ports, not between the unique host addresses and the output port. 
   In the present embodiment, the routing controller  107  forms an address processor which establishes correspondence between the routing host addresses and the unique host address of the mobile terminal MN 1  (mobile terminal MN 2 ) based on the routing table  410 . 
   Specifically, the routing table  410  includes a routing information establishing correspondence between a plurality of routing host addresses (P 1 _ra through P 4 _ra) pooled in the access router AR 1  and the output ports (connection IF 1  or connection IF 2 ), as shown in  FIG. 8 . 
   Here, the pooled host addresses for routing may be expressed, for example, per network addresses in the routing table  410 . The access router AR 1  may decide the routing host address assigned to the mobile terminal MN 1  (mobile terminal MN 2 ) based on the routing table  410  expressed by the network address. 
   For example, when the routing host address is assigned to the mobile terminal MN 1 , the routing controller  107  assigns the pooled routing host address to the mobile terminal MN 1 , based on the output port (connection IF 1  or IF 2 ), which is connected with an access point where the mobile terminal MN 1  establishes wireless communication, and the routing table  410 . 
   More specifically, for example, in the case where wireless communication is established by the mobile terminal MN 1  through the access point AP 2 , the access router AR 1  refers to the routing table  410 , and the routing host address (P 1 _ra) in correspondence with the output port (connection IF 1 ), to which the access point AP 2  is connected, is assigned to the mobile terminal MN 1  by the access router AR 1 . 
   The routing controller  107  then establishes correspondence between the unique host address (MN 1 _ha) of the mobile terminal MN 1  and the routing host address (P 1 _ra) as shown in RCT  310 . The routing controller  107  then transmits an activate notification message to a routing manager RM and updates the contents of RCT  310 , based on a IPra Update message transmitted from the routing manager RM. 
   In the present embodiment, the routing controller  107  forms a route selector in which the output port (connection IF 1  or connection IF 2 ) is selected, based on the routing host address included in the packet addressed to the mobile terminal MN 1  (mobile terminal MN 2 ) and the routing table  410 . 
   For example, the routing controller  107  selects the output port (connection IF 1 ), based on the routing host address (P 1 _ra) included in the packet addressed to the mobile terminal MN 1  and the routing table  410  as shown in  FIG. 8 . Then, based on RCT  310 , the routing controller  107  acquires the unique host address (MN 1 _ha) of the mobile terminal MN 1  in correspondence with the routing host address (P 1 _ra) included in the packet addressed to the mobile terminal MN 1 . 
   Further, the routing controller  107  transmits the packet addressed to the mobile terminal MN 1  which has been converted into the unique host address (MN 1 _ha) by the address converter  109  to the mobile terminal MN 1 , referring to the contents of the neighbor cache  500 . 
   In the present embodiment, the address converter  109  converts the routing host address into the unique host address as a result of correspondence between the routing host address and the unique host address by the routing controller  107 , that is, based on the contents of RCT  310 . 
   Here, in the above-described second embodiment, the transmitted packet addressed to the network  1  from the mobile terminal MN 1  (mobile terminal MN 2 ), is converted from the unique host address into the routing host address in the address converter  109 , based on RCT  310 . The routing controller  107  then transmits the packet to the network  1  through a network connection IF 111 , based on the converted routing host address. 
   (Routing Control Method Using Router) 
   A routing control method using a router according to the second embodiment will now be explained with reference to  FIG. 7 . Here, the mobile terminal MN 1  starts communication through the access point AP 2  as shown in  FIG. 4 . 
   At first, in STEP S 110  the mobile terminal MN 1  receives a router advertisement transmitted from the access router AR 1  through the access point AP 2  as shown in  FIG. 7 . 
   In STEP S 120 , the mobile terminal MN 1  updates the contents of the neighbor cache, based on the router advertisement received in STEP S 110 . 
   In STEP S 130 , the mobile terminal MN 1  transmits an activation message to the access router AR 1  for communication through the access router AR 1 . 
   In STEP S 140 , the access router AR 1  assigns the routing host address (for example, P 1 _ra) to the mobile terminal MN 1 , and establishes correspondence between the routing host address (P 1 _ra) and the unique host address (MN 1 _ha) of the mobile terminal MN 1 , based on the routing table  410 , and that the mobile terminal MN 1  establishes communication through the access point AP 2 , that is, communication is established by the mobile terminal MN 1  through the connection IF 1 . 
   In STEP S 150 , the access router AR 1  transmits the activate notification message to the routing manager RM. 
   In STEP S 160 , the routing manager RM establishes correspondence between the unique host address (MN 1 _ha) to the mobile terminal MN 1  and the routing host address (P 1 _ra), based on the received activate notification message, and updates the contents of the routing cache table (RCT). 
   In STEP S 170 , the routing manager RM transmits the IPra update message to the access router AR 1 . 
   In STEP S 180 , the access router AR 1  updates the contents of RCT  310 , based on the received IPra update message in order to reflect the correspondence between the routing host address (P 1 _ra) transmitted in STEP S 150  and the unique host address (MN 1 _ha) of the mobile terminal MN 1 . 
   Next, message procedure in which the access router AR 1  receives the packet addressed to the mobile terminal MN 1 , and transmits the packet to the mobile terminal MN 1  will be explained. 
   In STEP S 210 , the access router AR 1  receives the packet addressed to the mobile terminal MN 1  through the network  1 . 
   Based on the routing host address (P 1 _ra) included in the received packet addressed to the mobile terminal MN 1 , in STEP S 220 , the access router AR 1  searches for the routing table  410 , and selects the output port (connection IF 1 ) through which the packet is output. 
   In STEP S 230 , the access router AR 1  searches RCT  310 , based on the routing host address (P 1 _ra) included in the received packet addressed to the mobile terminal MN 1 , to acquire the unique host address (MN 1 _ha) of the mobile terminal MN 1 . 
   In STEP S 240 , the access router AR 1  converts the routing host address (P 1 _ra) included in the packet addressed to the mobile terminal MN 1  into the unique host address (MN 1 _ha) of the mobile terminal MN 1 . 
   In STEP S 250 , the access router AR 1  transmits the packet that was addressed to the mobile terminal MN 1 , and has been converted into the unique host address (MN 1 _ha) of the mobile terminal MN 1 , referring to the neighbor cache  500 , to the mobile terminal MN 1 . 
   (Effects) 
   According to the above-described second embodiment of the present invention, the contents of the routing table  410  are not updated even if the number of mobile terminals which communicate through the access router AR 1  (access router AR 2 ) is increased, an increase in the amount of information in the routing table  410  can be avoided, and a reduction in the time required for searching for the routing information can be realized, because correspondence between the routing host address and the unique host address is established, using the routing table  410  including the routing information which establishes correspondence not between the unique host address and the output port, but between the routing host address and the output port. 
   That is, if the routing information establishing correspondence between the routing host address assigned to the mobile terminal MN 1 , MN 2  and the output port (connection IF 1  or IF 2 ) of the packet is expressed in an integrated manner in the routing table  410 , for example, if the routing information is expressed per network address, the amount of the routing information in the downward direction, that is, the amount of routing information addressed to the mobile terminal MN 1  and the mobile terminal MN 2  can be reduced. Thereby, a reduction in the time required for searching for routing information, and the high-speed transfer of packets can be realized. 
   [Variant] 
   Though the routing cache table (RCT) and the routing table have been separately provided in the above-described first and second embodiments, a table in which the routing cache table (RCT) and the routing table are integrated together can be used. 
   For example, if there is correspondence established among a unique host address (IPha), a routing host address (IPra) and an output port (connection IF 1  or connection IF 2 ) as shown in  FIG. 9 , the above-described first and second embodiments can be realized by using an access router having an integrated table  600 . 
   For example, when the access router AR 1  uses the integrated table  600  in the above-described second embodiment, the routing controller  107  selects an output port by using the integrated table  600 , and at the same time, notifies a unique host address (MN 1 _ha) of the mobile terminal MN 1  to the address converter  109 . Further, the address converter  109  converts a routing host address included in a packet addressed to the mobile terminal MN 1  into the unique host address (MN 1 _ha), based on the notified unique host address (MN 1 _ha). 
   That is, further high-speed transfer of packets can be achieved, since the access router AR 1  can search the correspondence among routing information, host addresses for routing, and unique host addresses one at a time, using the integrated table  600 . 
   According to the present invention, there can be provided communication system, a communication terminal, a routing control method and a router, which reduces the time required for handover executed between wireless base stations connected to the same access router, and the high-speed transfer of packets can be realized when a route for packet transfer is controlled using a unique host address and a routing host address.