Patent Publication Number: US-2009238140-A1

Title: Gateway apparatus and handover method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-73227, filed on Mar. 21, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a gateway apparatus and a handover method. 
     BACKGROUND 
     In recent years, the IEEE (Institute of Electrical and Electronic Engineers) has been working ongoingly in the standardization of WiMAX (Worldwide Interoperability for Microwave Access), which is a wireless communication scheme (see, for instance, IEEE 802.16e-2005. WiMAX extended to mobile stations is referred to also as “mobile WiMAX”). 
       FIG. 10  is a diagram illustrating a network configuration example of a WiMAX-based mobile communication system  100 . The mobile communication system  100  has a home agent (HA)  120 ; access service network gateways (ASN-GWs, hereinafter “gateways”)  130 ,  140 ; base stations (BS)  150 ,  160 ; and a mobile station (MS)  170 . The home agent  120  and the two gateways  150 ,  160  are connected to one another. The MS and BS are specified in IEEE 802.16e-2005. ASN-GW is specified in “WiMAX Forum Network Architecture Stage 2/3 Release 1 Version 1.2”, 11 Jan. 2008. HA is governed by Mobile IP (hereinafter “MIP”) (for instance, as in “IP Mobility Support for IPv4” RFC3344, August 2002). 
     In the example illustrated in  FIG. 10 , the mobile station  170  is depicted moving (handover) from being covered by the base station  150  to being covered by the base station  160 . The numerals in brackets denote the route sequence of the packets being exchanged. 
     Specifically, when the mobile station  170  is under the base station  150 , packets are exchanged with the home agent  120  via the base station  150  and the source gateway  130  (route ( 1 )). The destination gateway  140  carries out exchange of handover-request messages and so forth with the source gateway  130 , and temporarily buffers the packets received from the home agent  120  via the source gateway  130  (route ( 2 )). Thereafter, when the mobile station  170  actually moves on under the coverage of the base station  160 , the destination gateway  140  sends the buffered packets and receives packets from the mobile station  170 . The mobile station  170  exchanges packets with the home agent  120  via the source gateway  130  (route ( 3 )). With an appropriate timing, the destination gateway  140  issues a formal registration to the home agent  120  (post-registration), and switches the route, whereupon the mobile station  170  exchanges packets with the home agent  120  via the destination gateway  140  (route ( 4 )). 
     Switching from route ( 3 ) to route ( 4 ) is done on condition that the switching timing is the timing estimated by the destination gateway  140  when there are no packets on route ( 3 ). For instance, should packets not be exchanged for a given time, it is estimated that there are no packets on route ( 3 ). 
     However, when it is not possible to find out the timing at which there are no packets on route ( 3 ), or when the timing estimation is erroneous, there exists the possibility of loss of uplink packets (hereinafter “UL packets”) residing in route ( 3 ) when the destination gateway  140  switches to route ( 4 ). 
     To prevent packet loss during such a handover, there is a conventional technology called simultaneous registration (Simultaneous Binding) (see, for instance, “Simultaneous Bindings for Mobile IPv6 Fast Handovers &lt;draft-elmaki-mobileip-bicasting-v6-0.6txt&gt;”, July 2005). In simultaneous registration, the home agent  120  is requested simultaneous registration of a bi-directional tunnel (a state in which both uplink and downlink transmission of UL packets and DL packets is possible) between the source and destination gateways  130 ,  140  and the home agent  120 . As a result, a reverse tunnel (state in which UL packets can be transmitted to the home agent  120 ) can be maintained between the source and destination gateways  130 ,  140  and the home agent  120 , thereby preventing UL packet loss. 
     Japanese Patent Application Laid-open No. 2006-246481 discloses another technology for preventing packet loss during handover. 
     However, the home agent  120  copies and sends downlink packets (hereinafter “DL packets”), when simultaneous registration is carried out. That is, the home agent  120  sends the DL packets via route ( 3 ) and the copied DL packets via route ( 4 ). As a result, the mobile station  170  receives overlappingly two identical DL packets via route ( 3 ) and route ( 4 ). 
     In Japanese Patent Application Laid-open No. 2006-246481, a PAR (Previous Access Router: equivalent to the source gateway  130  in  FIG. 10 ) sends a transmission complete notification message (flush message) to a NAR (New Access Router; the destination gateway  140  in  FIG. 10 ). This takes into account only DL packets, but not the UL packets reaching the PAR from the NAR. Therefore, there exists the possibility of loss of UL packets residing between the NAR and the PAR when routes are switched as described above. 
     Even if route ( 3 ) is switched to route ( 4 ) by post-registration, the distances in the two routes are different, and hence the mobile station  170  receives the DL packets via route ( 4 ) earlier than via route ( 3 ), which gives rise to the problem of reception of DL packets having an inverted order. 
     SUMMARY 
     In the light of the above problems, it is an object of the present invention to provide a gateway apparatus, and a handover method, in which packet loss is prevented. 
     Another object of the present invention is to provide a gateway apparatus and so forth in which packet order inversion is prevented. 
     Furthermore, another object of the present invention is to provide a gateway apparatus and so forth in which duplicated reception of packets is prevented. 
     To achieve the above-described objects, according to one embodiment of the present invention, a gateway apparatus for exchanging packet between a home agent and a mobile station, and being a destination gateway apparatus to which the mobile station is connected by handover, has a transmission unit which transmits to the home agent a simultaneous registration request requesting simultaneous registration of a bi-directional tunnel at both an old route via a source gateway apparatus to which the mobile station is connected before handover, and a new route via the destination gateway apparatus; and a packet control unit which discards one downlink packet of a duplicated downlink packet addressed to the mobile station, when the simultaneous registration is maintained. 
     Additionally, to achieve the above-described objects, according to another embodiment of the present invention, a handover method in a gateway apparatus for exchanging packets between a home agent and a mobile station, and being a destination gateway apparatus to which the mobile station is connected by handover, the method has the steps of transmitting to the home agent a simultaneous registration request requesting simultaneous registration of a bi-directional tunnel at both an old route via a source gateway apparatus to which the mobile station is connected before handover, and a new route via the destination gateway apparatus; and discarding one downlink packet of a duplicated downlink packet addressed to the mobile station, when the simultaneous registration is maintained. 
     The present invention succeeds in providing a gateway apparatus and a handover method in which packet loss is prevented. The present invention succeeds also in providing a gateway apparatus and so forth in which packet order inversion is prevented. The present invention succeeds moreover in providing a gateway apparatus and so forth in which duplicated reception of packets is prevented. 
     Additional objects and advantages of the invention (embodiments) will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a network configuration structure example of a mobile communication system; 
         FIG. 2  is a diagram illustrating a configuration example of the destination gateway; 
         FIG. 3  is a sequence diagram illustrating a hand over operation example; 
         FIG. 4  is a sequence diagram illustrating a hand over operation example; 
         FIG. 5  is a diagram illustrating a configuration example of the gateways and the home agent; 
         FIG. 6  is a sequence diagram illustrating a handover operation example; 
         FIG. 7  is a sequence diagram illustrating a handover operation example; 
         FIG. 8  is a sequence diagram illustrating a handover operation example; 
         FIG. 9  is a sequence diagram illustrating a handover operation example; and 
         FIG. 10  is a diagram illustrating a network configuration example of a conventional mobile communication system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention are explained below with reference to accompanying drawings. 
     Embodiment 1 
     Embodiment 1 will be explained first.  FIG. 1  is a diagram illustrating a network configuration structure example of a mobile communication system  10  according to Embodiment 1. The mobile communication system  10  has a home agent (HA)  20 ; two gateway apparatuses (serving ASN-GW, target ASN-GW, hereinafter “gateways”)  30 ,  40 ; two base stations (BS 1 , BS 2 )  50 ,  60 ; and a mobile station  70 . 
     The two gateways  30 ,  40  have also the function of FA (foreign agent) governed by the MIP protocol, and of AR (access router) governed by a fast MIP protocol. The mobile station  70  has also the function of MN (mobile node) governed by the MIP protocol. In the case where the network is controlled under a proxy MIP protocol, the two gateways  30 ,  40  have the function of MN governed by the Mobile IP protocol. 
     The home agent  20  is a router at the network to which the mobile station  70  is to be connected. The home agent  20  is connected to the two gateways  30 ,  40 , and also to a backbone gateway. During simultaneous registration (Simultaneous Binding), the home agent  20 , which has a copy unit  21 , transmits DL packets from a backbone gateway to the gateway  30 , and transmits DL packets copied by the copy unit  21  to the gateway  40 . 
     The two gateways  30 ,  40 , which are connected to each other, manage a plurality of respective base stations. In the present embodiment, the gateways  30 ,  40  manage base stations  50 ,  60 , respectively. 
     The two base stations  50 ,  60  are connected to the gateways  30 ,  40 , respectively, transmits DL packet outputted from the gateway  30 ,  40  to the mobile station  70 , and outputs UL packets outputted from the mobile station  70  to the gateway  30 ,  40 . 
     The mobile station  70  communicates wirelessly to the base station  50 ,  60 , receives DL packet, and transmits UL packet. 
     The example illustrated in  FIG. 1  is an example in which the mobile station  70  is handed over from the base station  50  to the base station  60 . In the figure, the dashed double-dotted line represents the route of the packets. In the example illustrated in  FIG. 1 , the operations of route ( 1 ) and route ( 2 ) illustrated in  FIG. 10  have been already performed. Hereinafter, the gateway  30  will be referred to as the source gateway (or serving gateway (Serving ASN-GW)) and the gateway  40  will be referred to as the destination gateway (or target gateway (Target ASN-GW)). 
       FIG. 2  is a diagram illustrating a configuration example of the destination gateway  40 . The source gateway  30  has the same configuration as the destination gateway  40 . The destination gateway  40  ( 30 ) has a MIP control unit  41  ( 31 ), a packet buffer control unit  42  ( 32 ), a packet buffer  43  ( 33 ) and a packet transmission complete notification unit  44  ( 34 ). 
     The MIP control unit  41  generates a new route addition request message and an old route release request message, and exchanges these messages with the home agent  20 . The MIP control unit  41  transmits the old route release request to other gateways (for instance, the source gateway  30 ) via the packet buffer control unit  42 . 
     The packet buffer control unit  42 , exchanges UL packets and DL packets with the home agent  20 , the base stations  50 ,  60  and other gateways (for instance, the source gateway), and writes and reads packets to and from the packet buffer  43 , as the case may require. In accordance with the state of the packet buffer  43 , the packet buffer control unit  42  instructs the packet transmission complete notification unit  44  to issue an old route-addressed packet transmission complete notification. The packet buffer control unit  42  receives the old route-addressed packet transmission complete notification from another gateway, and outputs the notification to the MIP control unit  41 . 
     The packet buffer  43  stores DL packets and UL packets, under the control of the packet buffer control unit  42 . 
     The packet transmission complete notification unit  44  generates the old route-addressed packet transmission complete notification, on the basis of the instruction from the packet buffer control unit  42 , and transmits the notification to another gateway (source gateway  30  or the like). 
     The handover operation of the mobile communication system  10  is explained next.  FIG. 3  is a sequence diagram illustrating an operation example of the mobile communication system  10  in Embodiment 1. The operation example is an operation example after the mobile station  70  has actually moved on under the coverage of the base station  60 . A bi-directional tunnel is maintained between the source gateway  30  and the home agent  20 . 
     Firstly, the destination gateway  40  establishes an old route (in the present example, route ( 3 )) between the destination gateway  40  and the mobile station  70  (S 11 ). 
     Next, the destination gateway  40  sends a new route addition request (Registration Request) for a new route (in the present example, route ( 4 )) to the home agent  20  (S 12 ). This new route addition request is accompanied by simultaneous registration (Simultaneous Binding) (Sbit=1). For instance, the MIP control unit  41  generates the request and sends it to the home agent  20 . 
     Next, the destination gateway  40  receives a new route addition response (Registration Reply) from the home agent  20  (S 13 ). Thereby, the bi-directional tunnel is maintained simultaneously not only between the source gateway  30  and the home agent  20 , but also between the destination gateway  40  and the home agent  20 . For instance, the MIP control unit  41  receives the response, and outputs the response to the packet buffer control unit  42 . 
     Next, the destination gateway  40  receives DL packets from the home agent  20  via the new route (route ( 4 )) (S 14 ). The source gateway  30  as well receives the DL packets from the home agent  20  (S 14 ). For instance, the packet buffer control units  32 ,  42  receive the DL packets. 
     Upon receiving the DL packets from the home agent  20 , the destination gateway  40  discards the DL packets (S 15 ), to prevent duplicated packets from being sent to the mobile station  70 . For instance, the packet buffer control unit  42  discards the DL packets received by the new route from the time at which the new route addition response (S 13 ) is inputted from the MIP control unit  41 . Since, for instance, the DL packets include route information (or information on whether or not the DL packet is copied), or since the receiving interface differs for the new route or the old route, the packet buffer control unit  42  can identify the DL packets of the new route and can easily discard the DL packets. Of course, the packet buffer control unit  42  may discard the old route DL packets instead of the new route DL packets. 
     Next, the destination gateway  40  checks completion of transmission of UL packets addressed to the old route (s 16 ). That is, the destination gateway  40  checks that there are held no UL packets to be sent (UL packets to be sent to the source gateway  30 ) on route ( 3 ). 
     For instance, the packet buffer control unit  42  accesses the packet buffer  43 , and checks that there are stored no UL packets sent by the old route, i.e. route ( 3 ). For instance, the packet buffer control unit  42  confirms UL packet transmission completion when UL packets cannot be read from a certain storage area of the packet buffer  43  where the UL packets are stored. Alternatively, the packet buffer control unit  42  stores in the packet buffer  43 , together with the UL packets, a flag indicating the UL direction, and confirms UL packet transmission completion by checking that no such flag is stored in the packet buffer  43 . 
     Upon confirming completion of transmission of UL packets addressed to the old route, the destination gateway  40  sends an old route-addressed UL packet transmission complete notification to the source gateway  30  (S 17 ). For instance, the packet buffer control unit  42  outputs a completion notification instruction to the packet transmission complete notification unit  44 , whereupon the latter sends the old route-addressed UL packet transmission complete notification. 
     Next, the source gateway  30  checks completion of transmission of UL packets addressed to the old route (S 18 ). The source gateway  30  checks that no UL packets to be sent to route ( 3 ) (UL packets to be sent to the home agent  20 ) are still held. For instance, the packet buffer control unit  32  of the source gateway  30  accesses the packet buffer  33  and checks whether there are any stored UL packets. This check follows the same process of S 16 . 
     Upon confirming transmission completion, the source gateway  30  sends an old route-addressed UL packet transmission complete notification to the destination gateway  40  (S 19 ). For instance, the packet buffer control unit  32  outputs a completion notification instruction to the packet transmission complete notification unit  34 , whereupon the latter outputs the old route-addressed UL packet transmission complete notification to the destination gateway  40 . 
     The purpose of the processing from S 16  to S 19  is to confirm that there are no UL packets residing in route ( 3 ). In the processing of S 16 , the destination gateway  40  checks that there are no UL packets to be sent to the source gateway  30 , while in the processing of S 18 , the source gateway  30  checks that there are no UL packets to be sent to the home agent  20 . As a result, there are no UL packets residing in route ( 3 ), and packet loss can be prevented in that UL packets residing in route ( 3 ) do not reach the home agent  20 , when the route is switched only to route ( 4 ). 
     Upon receiving from the source gateway  30  the old route-addressed UL packet transmission complete notification (S 19 ), the destination gateway  40  sends a release request (Registration Request (Sbit=0)) to the source gateway  30  (S 20 ) for releasing the old route (route ( 3 )). For instance, upon receiving the notification, the packet buffer control unit  42  outputs the notification to the MIP control unit  41 , whereupon the MIP control unit  41  generates the release request message, and sends the release request message to the source gateway  30  via the packet buffer control unit  42 . 
     Next, upon receiving the release request, the source gateway  30  sends the received release request to the home agent  20  (S 21 ). For instance, upon receiving the release request, the packet buffer control unit  32  outputs the release request to the MIP control unit  31 , and the MIP control unit  31  sends the release request to the home agent  20 . 
     Thus, the destination gateway  40  sends the old route release request to the home agent  20  not directly, but via (diversion) the source gateway  30  (S 20  to S 21 ). Since route ( 4 ) has a shorter distance than route ( 3 ), in the case where the destination gateway  40  sends the release request directly to the home agent  20  there is a possibility that UL packets residing in route ( 3 ) reach the home agent  20  after the release request has reached the home agent  20 . The purpose of the diversion is to avoid such an occurrence. Loss of UL packets residing in the old route (route ( 3 )) is thus reliably prevented by diverting the release request so as to track (without overtaking) the UL final packet that is transmitted via the old route. 
     Next, the destination gateway  40  receives an old route release response (Registration Reply) from the home agent  20  (S 22 ). As a result, only the new route (route ( 4 )) of the two bi-directional tunnels maintained by simultaneous registration is maintained now. For instance, the MIP control unit  41  receives the old route release response and outputs it to the packet buffer control unit  42 . 
     Next, the destination gateway  40  stops discarding the DL packets of the new route (route ( 4 )) being discarded theretofore, and buffers the DL packets in the packet buffer  43  (S 23 , S 24 ). Discarding of DL packets addressed to the new route is now discontinued because in the processing of S 22  the old route is released and there is only maintained the bi-directional tunnel of the new route, so that the problem of duplicated transmission of DL packet does not arise. For instance, the packet buffer control unit  42  stores the DL packets received from the home agent  20  in the packet buffer  43 . 
     Next, the source gateway  30  receives from the home agent  20  an old route-addressed DL packet transmission complete notification (Revocation Request/Response) (S 25 ). For instance, when the home agent  20  checks that no DL packets to be sent are held in the old route (route ( 3 )), the home agent  20  sends the notification. The MIP control unit  31  of the source gateway  30  receives the notification and sends it to the packet buffer control unit  32 . 
     Next, the source gateway  30  checks that no old route DL packets are held (S 26 ). For instance, the packet buffer control unit  32  accesses the packet buffer  33  and checks that there are stored no DL packets to be sent via the old route. 
     Next, the source gateway  30  sends the old route-addressed DL packet transmission complete notification to the destination gateway  40  (S 27 ). For instance, when the packet buffer control unit  32  confirms that no old route DL packets are stored in the packet buffer  33 , the packet buffer control unit  32  sends the old route-addressed DL packet transmission complete notification to the destination gateway  40  via the packet transmission complete notification unit  34 . 
     Next, the destination gateway  40  sends the DL packet stored in the packet buffer  43  to the base station  60  (S 28 ). If the destination gateway  40  sends to the base station  60  the DL packets transmitted by the new route (route ( 4 )) after old route release (after S 23 ), there exists the possibility that the DL packets residing in the old route (route ( 3 )) reach the destination gateway  40  later than the DL packets transmitted via the new route (route ( 4 )). That is, there exists the possibility of sending inverted-order DL packets. Therefore, the destination gateway  40  stores in the packet buffer  43  the DL packets transmitted via the new route (route ( 4 )) until receiving all the DL packets residing in route ( 3 ) (S 24 ). After confirming that there are no DL packets residing in the old route (route ( 3 )) (S 27 ), the destination gateway  40  sends the DL packets transmitted via the new route (route ( 4 )). As a result, the destination gateway  40  sends DL packets without order inversion to the mobile station  70 , so that the mobile station  70  can receive DL packets without order inversion. 
     This concludes the above series of processes. From then on the mobile station  70  exchanges packets with the destination base station  60 . 
     Thus, the destination gateway  40  discards new route DL packets (S 15  to S 24 ) while bi-directional tunnels are maintained by simultaneous registration between the gateways  30 ,  40  and the home agent  20 . As a result, the mobile station  70  does not receive duplicated DL packets. 
     Also, both gateways  30 ,  40  check completion of transmission of UL packets addressed to the old route, and send the diverted release request to the home agent  20  (S 16  to S 22 ). Packet loss of UL packets residing in the old route (route ( 3 )) can be prevented thereby. 
     Moreover, the destination gateway  40  buffers the new route DL packets (S 24  to S 28 ), and hence the mobile station  70  does not receive order-inverted DL packets. 
     The above-described processes of Embodiment 1 are carried out in the source and destination gateways  30 ,  40 , and hence the embodiment can be realized without adding new functions to the home agent  20 . 
     Embodiment 2 
     Embodiment 2 is explained next. The configurations of the network of the mobile communication system  10  and of the gateways  30 ,  40  are identical to those of Embodiment 1 ( FIGS. 1 and 2 ). The present Embodiment 2 is an example in which a message based on Embodiment 1 is mapped to a message used by so-called WiMAX (inter-gateway tunnel-release message). As a result, the mobile communication system  10  can perform handover using a WiMAX-compliant communication scheme. 
       FIG. 4  is a sequence diagram illustrating an operation example of the mobile communication system  10  in Embodiment 2. In the present Embodiment 2, the old route-addressed UL packet transmission complete notification (S 17 ,S 19 ) and the old route-addressed DL packet transmission complete notification (S 27 ) described in Embodiment 1 are performed using tunnel-release messages (R4-DP-Release-Req, R4-DP-Release-Rsp) defined in WiMAX. Herein, “R4” is the denomination of an interface between gateways. 
     Specifically, the destination gateway  40  sends an uplink-direction tunnel-release request message (R4-DP-Release-Req (Direction=UL)) (S 17 ′). This tunnel-release request message has superimposed thereon the meaning of “old route-addressed UL packet transmission complete notification” (S 17  of Embodiment 1), and is thus used as that notification in Embodiment 2. 
     The source gateway  30  sends, to the destination gateway  40 , a response message (R4-DP-Release-Rsp (Direction=UL)) responding to the notification (S 19 ′). The tunnel response message is used also as the “old route-addressed UL packet transmission complete notification” (S 19  in Embodiment 1). 
     The destination gateway  40  sends a DL tunnel release request message (R4-DP-Release-Req (Direction=DL)) requesting that the source gateway  30  sends the old route-addressed DL packet transmission complete notification as a DL tunnel release request response message (S 25   a ). The source gateway  30  sends to the destination gateway  40  the DL tunnel release response message (R4-DP-Release-Rsp (Direction=DL)) as the response message (old route-addressed DL packet transmission complete notification) (S 27 ′). 
     The packet buffer control units  42 ,  32  generate these tunnel release request messages and send them to the gateways  30 ,  40 , via the packet transmission complete notification units  44 ,  34 . 
     Upon receiving the new route addition response from the home agent  20  (S 13 ), the destination gateway  40  stores the UL packets addressed to the new route (route ( 4 )) in the packet buffer  43  (S 13   a ), while upon receiving the old route release response from the home agent  20  (S 22 ), the destination gateway  40  sends the UL packets to the home agent  20  via the new route (S 22   a ), so that, in the process from S 13   a  to S 22   a,  bi-directional tunnels are maintained for both the new route and the old route, and UL packets are sent to the home agent  20  without packet order inversion. For instance, the packet buffer control unit  42  performs buffer control of the UL packets. 
     Embodiment 3 
     Embodiment 3 is explained next. Embodiment 3 is an example in which the home agent  20  buffers UL packets routed via the new route. 
       FIG. 5  is a diagram illustrating a configuration example of the gateways  30 ,  40  and the home agent  20  of Embodiment  3 . The home agent  20  has a buffer  22  and a buffer control unit  23 . 
       FIG. 6  is a sequence diagram illustrating a handover operation example according to Embodiment 3. Triggered by the new route addition (S 12 , S 13 ), the home agent  20  initiates buffering of UL packet routed via the new route (S 13   a ′). For instance, the buffer control unit  23  stores in the buffer  22  the UL packets from the time at which the buffer control unit  23  transmits the new route addition response. 
     Triggered by the old route release (S 22 ) indicating that there are no residing UL packets in the old route, the buffer control unit  23  in the home agent  20  initiates transmission of the buffered UL packets (S 22   a ′). 
     In the present Embodiment 3, the home agent  20  buffers the UL packets, and the destination gateway  40  buffers the DL packets (S 24 , S 28 ), sharing thus packet storage. This allows reducing as a result buffer size in the destination gateway  40 . 
     Embodiment 4 
     Embodiment 4 is explained next. Embodiment 4 is an example in which UL packets addressed to the new route (route ( 4 )) and DL packets routed via the new route are not buffered but sent as soon as the packets become transmittable. 
       FIG. 7  is a sequence diagram illustrating a handover operation example according to Embodiment 4. Processes identical to those of Embodiment 2 ( FIG. 4 ) and so forth are denoted with identical reference numerals. Upon receiving the new route addition response (S 13 ), the destination gateway  40  sends to the home agent  20  the UL packet addressed to the new route, without buffering the UL packet (S 13   a ″). 
     Upon receiving the old route release response (S 22 ), the destination gateway  40  stops discarding duplicated DL packets and sends to the base station  60  the DL packets routed via the new route (route ( 4 )) (S 24 ″). This is done for instance in the packet buffer control unit  42 . 
     In Embodiment 2, the destination gateway  40  buffers the UL packets and the DL packets (S 13   a,  S 24  in  FIG. 4 ) to prevent packet order inversion. However, jitter characteristics degrade on account of packet buffering. Jitter characteristics are an indicator of inter-packet delay variance. Depending on the used application (for instance, real-time video and audio), it may be preferable to improve jitter rather than packet order inversion. In Embodiment 4, the destination gateway  40  transmits the packets immediately, without packet buffering. This allows improving jitter characteristics. 
     Embodiment 5 
     Embodiment 5 is explained next. Embodiment 5 is an example in which the destination gateway  40  and the source gateway  30  share a security context that is required for message authentication. 
     An Authentication Extension field, for checking counterfeiting of a transmission-source IP address, is added to the MIP protocol message. This field cannot be generated in the absence of an encryption key. Message authentication is carried out in the destination gateway  40  and the home agent  20 , which beforehand share an encryption key. Therefore, the destination gateway  40  can send, for instance, the new route addition request (S 12 ) directly to the home agent  20 . However, the source gateway  30 , which is a transmission source different from the transmission source of the new route addition request, does not share with the home agent  20  security information such as the encryption key that is shared by the destination gateway  40  and the home agent  20 . Accordingly, the source gateway  30  cannot send, directly to the home agent  20 , the release request message corresponding to the new route addition request sent by the destination gateway  40 . In Embodiment 5, therefore, the source gateway  30  and the destination gateway  40  share a security context. 
       FIG. 8  is a sequence diagram illustrating an example of handover according to Embodiment 5. Processes identical to those of Embodiment 1 and so forth are denoted with identical reference numerals. In Embodiment 1 and so forth, the old route release request (Registration Request (Sbit=0)) is diverted by the destination gateway  40  (S 20 , S 21  in  FIG. 3  or the like). In Embodiment 5, the source gateway  30  sends the old route release request directly to the home agent  20  (S 21 α). The source gateway  30  sends this request with its source address set to the destination gateway  40 . In response to this request, the home agent  20  sends the old route release response (Registration Reply) to the destination gateway  40  (S 22   a ). The destination gateway  40  forwards the response to the source gateway  30 . 
     Next, the source gateway  30  requests tunnel release between the source gateway  30  and the destination gateway  40  (S 19 ′). 
     The security context, for instance, includes security information (such as an encryption key) for authentication, and is stored in a memory of the MIP control units  41 ,  31  ( FIG. 2 ). The processes of S 21 α, S 22 α are carried out, for instance, in the MIP control units  41 ,  31 . 
     As an alternative way, the destination gateway  40  need not forward the old route release response (S 22 α) to the source gateway  30 . That is because the destination gateway  40  may send the UL packets addressed to the new route with the timing with which the destination gateway  40  receives the response from the home agent  20 , and thus the source gateway  30  does not imperatively require information of the response. 
     The destination gateway  40  may initiate transmission of UL packets addressed to the new route when triggered by either S 22 α or S 19 ′. That is because the destination gateway  40  can detect completion of transmission of UL packets addressed to the old route on the basis of either S 22 α or S 19 ′. 
     In Embodiment 5, the destination gateway  40  can send the old route release request without diverting the latter. Also, both the source and destination gateways  30 ,  40  complete transmission of UL packets addressed to the old route (S 16 , S 18 ), whereafter the source gateway  30  requests old route release to the home agent  20  (S 21 α). In Embodiment 5, therefore, both packet order inversion and UL packet loss can be prevented, as is the case in Embodiment 1 and so forth, without the old route release request overtaking the final UL packet routed through the old route. 
     Embodiment 6 
     Embodiment 6 is explained next. Embodiments 1 to 5 have been explained as examples of the Proxy MIP protocol. The Proxy MIP protocol is a protocol according to which the gateways  30 ,  40  assume as proxies the function of the MN of the mobile station  70  (function of carrying out exchange of messages on the basis of the MIP protocol). Embodiment 6 is an example of a client MIP in which the mobile station  70  has the MN function. 
     During handover, the mobile station  70  is aware of packet exchange via the home agent  20  and the destination gateway  40  alone, without awareness of inter-gateway tunnels between the source gateway  30  and the destination gateway  40 . It is thus not realistic for the mobile station  70  to be involved in release of tunnels between the gateways, or in simultaneous registration from the gateways  30 ,  40 , which are the endpoints of the inter-gateway tunnels. Message exchange for simultaneous registration, therefore, is assumed by the destination gateway  40 , which rewrites route switching requests from the mobile station  70 . The mobile station  70  cannot see simultaneous registration being carried out between the gateways  30 ,  40  and the home agent  20 . 
       FIG. 9  is a sequence diagram illustrating a handover operation example according to Embodiment 6. Processes identical to those of Embodiment 1 and so forth are denoted with identical reference numerals. The mobile station  70 , the destination gateway  40  and the home agent  20  share a security context  80  (Embodiment 5). As in Embodiment 5, the MIP control unit  41 , the mobile station  70  and the home agent  20  hold the security context  80  beforehand. 
     The destination gateway  40  sends an Agent Advertisement to the mobile station  70  (S 11 β). For instance, the MIP control unit  41  generates the message and sends it to the mobile station  70  via the packet buffer control unit  42 . 
     The mobile station  70  receives the message and detects thereby a new FA (=destination gateway  40 ). The mobile station  70  sends a route switching request (Registration Request (without Sbit field)) requesting switching to a route via the new FA (S 12 β). 
     Next, the destination gateway  40  rewrites the received route switching request to the new route addition request (Registration Request (Sbit=1)) and sends the latter to the home agent  20  (S 12 ). The MIP control unit  41  carries out this processing. 
     Thereafter, upon receiving from the home agent  20  the old route release response (Registration Reply) (S 22 β), the destination gateway  40  rewrites the old route release response to a route switching response (Registration Reply), and sends the latter to the mobile station  70  (S 22 β). Rewriting is performed by the MIP control unit  41 , and the response is sent to the mobile station  70  via the packet buffer control unit  42 . 
     The mobile station  70  simply sends the route switching request (S 12 β) and receives the route switching response (S 22 β). Therefore, the mobile station  70  simply confirms that the old route has been switched to the new route. 
     The mobile station  70  can thus be realized in the same way as in Embodiment 1 and so forth, also in the case of being a client MIP provided with a MIP. 
     The above features can be summarized in the claims below. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.