Abstract:
A communication system including: a first device that includes first and second units and a first encapsulation unit encapsulating data; a second device that includes a third unit communicating data with the second unit, a fourth unit, and a second encapsulation unit encapsulating data; a third device that includes a fifth unit communicating data with the first or third unit, a sixth unit, and a third encapsulation unit encapsulating data; and a fourth device that includes a seventh unit communicating data with the first or third unit, an eighth unit communicating data with the fifth unit, and a fourth encapsulation unit encapsulating data; wherein at least two of the first to fourth devices include control units that control the units that the at least two of the first to fourth devices have, so as not to transfer received data to other devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-010380, filed on Jan. 20, 2010, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The present invention relates to a communication system and a communication method. 
       BACKGROUND 
       [0003]    In recent years, “tunneling” has been used for communication using data encapsulated by adding a further header to data including a header. 
         [0004]    For example, by communicating encapsulated data obtained by encapsulating data of a communication protocol A with a communication protocol B over a network operated with the communication protocol B, it is possible to connect two networks operated with the communication protocol A by a closed, virtually direct communication route. 
         [0005]    A communication protocol, like the above communication protocol B, for encapsulating data is called a “tunneling protocol”. The EtherIP and GRE (Generic Routing Encapsulation) may be mentioned as examples of tunneling protocols enabling tunneling even in broadcast data transmission. 
         [0006]    On the other hand, when connecting two networks of differing subnets by tunneling, routers etc. for performing segmentation are arranged at either of the two networks. In such a case, even if tunneling broadcast data transmission, the broadcast data will end up being blocked by the routers. 
         [0007]    For this reason, the networks connected by tunneling are made networks of the same subnet so as to enable the transfer of broadcast data to the two networks connected by tunneling. 
         [0008]    As related art, see the IETF, Network Working Group, Generic Routing Encapsulation, RFC (Request for Comments)  1701  and the IETF, Network Working Group, EtherIP: Tunneling Ethernet Frames in IP Datagrams, RFC (Request for Comments) 3378. 
         [0009]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
       SUMMARY 
       [0010]    Accordingly, it is an object in one aspect of the embodiment to provide a communication system including: a first communication device that includes first and second communication units and a first encapsulation unit encapsulating or decapsulating data communicated by the first communication unit; a second communication device that includes a third communication unit communicating data with the second communication unit, a fourth communication unit, and a second encapsulation unit encapsulating or decapsulating data communicated by the third communication unit; a third communication device that includes a fifth communication unit communicating data with the first or third communication unit, a sixth communication unit, and a third encapsulation unit encapsulating or decapsulating data communicated by the fifth communication unit; and a fourth communication device that includes a seventh communication unit communicating data with the first or third communication unit, an eighth communication unit communicating data with the fifth communication unit, and a fourth encapsulation unit encapsulating or decapsulating data communicated by the seventh communication unit; wherein each of the first to fourth communication devices includes a control unit controlling the communication units and the control units of at least two of the first to fourth communication devices control the communication units so as not to transfer data received from other communication devices to these other communication devices. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    These and other objects and features will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein: 
           [0012]      FIG. 1  is a block diagram illustrating one example of the configuration of a communication system; 
           [0013]      FIG. 2  is a block diagram illustrating one example of the functions of a communication device; 
           [0014]      FIG. 3A  is a block diagram illustrating one example of a communication route; 
           [0015]      FIG. 3B  is a block diagram illustrating one example of a tunnel route established by tunneling; 
           [0016]      FIG. 4  is a network diagram illustrating one example of information which a storage unit stores; 
           [0017]      FIG. 5A  is a network diagram illustrating one example of a tunnel route; 
           [0018]      FIG. 5B  is a network diagram illustrating one example of a tunnel route; 
           [0019]      FIG. 5C  is a network diagram illustrating one example of a tunnel route; 
           [0020]      FIG. 5D  is a network diagram illustrating one example of a tunnel route; 
           [0021]      FIG. 5E  is a network diagram illustrating one example of a tunnel route; 
           [0022]      FIG. 6  is a flowchart illustrating one example of communication control performed by a communication device; 
           [0023]      FIG. 7  is a network diagram illustrating one example of a network configuration at which a backbone communication device is set at a tunnel side network; 
           [0024]      FIG. 8  is a sequence diagram illustrating an example of restoration of a communication system from a fault occurring at a tunnel side network; 
           [0025]      FIG. 9  is a sequence diagram illustrating one example of restoration of a communication system from a fault occurring in a communication device; and 
           [0026]      FIG. 10  is a network diagram illustrating one example of a tunnel route after restoration from a fault. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    As described previously, the networks connected by tunneling are made networks of the same subnet. However, depending on the configurations of the networks connected by tunneling, sometimes a loop forms inside the tunneling. For example, if connecting in a ring two tunneling devices redundantly configured at one network and two tunneling devices redundantly configured at the other network, a loop-shaped communication route will be formed. 
         [0028]    In transmission of broadcasts over a communication route forming such a loop, the broadcast packets will repeatedly loop around the four tunneling devices while the broadcast is transmitted to all ports of the tunneling devices, so communication will be disabled. 
         [0029]    It is however possible to prevent broadcast packets from ending up looping inside a network. 
         [0030]    Below, an embodiment of a communication system will be explained with reference to the drawings. 
         [0031]    [Configuration of Communication System] 
         [0032]    &lt;Communication Devices&gt; 
         [0033]    The communication system  300  illustrated in  FIG. 1  has communication devices  10  to  40 . The communication devices  10  and  20  form a redundant configuration. They connect with the information processing devices  110  and  120  and form a network  100 . The communication devices  30  and  40  also form a redundant configuration. They connect with the information processing devices  210  and  220  and form a network  200 . 
         [0034]    The redundantly configured communication devices  10  to  40  form active redundancy systems. All are in the operating state. In an active redundancy system, even if one of the redundantly configured communication devices starts having trouble operating, the other communication device continues to operate, so this is superior in reliability and practicality compared with a standby redundancy system in which the redundantly provided device is in a standby state at the time of normal operation. 
         [0035]    The illustrated network  100  and network  200  are networks of the same subnet. The communication devices  10  to  40  may form the network  100  and the network  200  by different VLANs (Virtual Local Area Networks) or by the same VLAN. 
         [0036]    Note that, in the following explanation, a network in which communication devices and information processing devices are connected will be referred to as an “inside network”, while a network between communication devices will be referred to as a “tunnel side network”. Further, the direction of data transmission from a communication device to the tunnel side network will be referred to as the “tunnel side”, while the direction of data transmission from a communication device to the inside network will be referred to as the “inside”. 
         [0037]    Reference numerals  1 ,  2 ,  4 , and  5  are virtual communication routes, that is, “tunnel routes” established with other communication devices by communication devices which encapsulate data. As explained later, a tunnel route is a virtual communication route established after communication devices authenticate each other on an actual network. For this reason, in an actual network configuration, the communication devices  10  to  40  may be any of ring types, star types, or full connect types, but the communication route between communication devices formed by the tunneling becomes a ring type or full connect type network configuration. The relationship between a communication route formed by such an actual network configuration and a tunnel route established by tunneling will be explained later using  FIG. 5A  to  FIG. 5E . Data to be transmitted to another communication device is encapsulated by the communication devices by, for example, the EtherIP or GRE. The communication routes  3  and  6  are communication routes between the communication device  10  and communication device  20  and decapsulated data is transferred over the communication routes. They are “crossovers” serving as cables connecting two redundantly configured communication devices. 
         [0038]    The communication device  10  and the communication device  20  form a redundant configuration. Even if one of the communication device  10  and communication device  20  breaks down, the other communication device can connect through a tunnel route to the communication device  30  or communication device  40 . 
         [0039]    The communication device  10  connects through a tunnel route  1  to the communication device  30 , through a tunnel route  2  to the communication device  40 , and through a communication route  3  to the communication device  20 . The communication device  20  connects through a communication route  3  to the communication device  10 , through a tunnel route  4  to the communication device  30 , and through a tunnel route  5  to the communication device  40 . 
         [0040]    The communication device  30  and the communication device  40  form a redundant configuration. Even if one of the communication device  30  and communication device  40  breaks down, the other communication device can connect through a tunnel route to the communication device  10  or communication device  20 . 
         [0041]    The communication device  30  connects through the tunnel route  1  to the communication device  10 , through the tunnel route  4  to the communication device  20 , and through the communication route  6  to the communication device  40 . The communication device  40  connects through the tunnel route  2  to the communication device  10 , through the tunnel route  5  to the communication device  20 , and through the communication route  6  to the communication device  30 . 
         [0042]    &lt;Information Processing Devices&gt; 
         [0043]    The information processing devices  110  and  120  are connected in the network  100 , while the information processing devices  210  and  220  are connected in the network  200 . The information processing devices  110 ,  120 ,  210 , and  220  respectively have two network interfaces. The information processing devices  110  and  120  use either of the network interfaces to connect to the communication device  10  or communication device  20 . The information processing devices  210  and  220  use either of the network interfaces to connect to the communication device  30  or communication device  40 . The network interfaces are, for example, NICs (Network Interface Cards) and use twisted pair cables or optical fiber cables to connect to the communication devices. 
         [0044]    &lt;Example of Application of Communication System&gt; 
         [0045]    The communication system  300  can, for example, be arranged at a data center. In a data center accommodating systems of a plurality of customers, the respective customer systems are isolated from each other in the data center so that they cannot be mutually recognized. The information processing devices  110  to  220  are, for example, servers and operate as customer systems. The information processing devices  110  to  220  operating as customer systems, for example, are arranged dispersed separately in racks in the data center in accordance with the timing of installation in the data center, expansion after installation (increase in number of units), etc. The information processing devices  110  to  220  are connected to each other by the networks  100  and  200  and form a system including customer system units of single servers and networks  100  and  200 . To isolate the networks, in the data center, the physical network is logically divided by a VLAN and the divided networks are allocated to the customer systems. 
         [0046]    For example, the information processing devices  110  and  120  connected to the network  100  are arranged at the same racks, while the information processing devices  210  and  220  connected to the network  200  are arranged at the same racks. The networks  100  and  200  are, for example, networks of the same subnet. They may be differentiated from each other by the VLAN. The information processing devices for running the customer systems are not formed completely in the racks. When connecting racks with each other, the networks are connected with each other by the tunnel routes  1 ,  2 ,  4 , and  5 . The communication devices  10  to  40  are arranged at racks. Tunnels are set up between the devices. 
         [0047]    [Configuration of Communication Devices] 
         [0048]    Using  FIG. 2  to  FIG. 5 , one example of the configuration of a communication device will be explained.  FIG. 2  is a block diagram illustrating one example of the component elements of the communication device. 
         [0049]    The communication device  10  has a tunnel side communication unit  11 , crossover side communication unit  12 , inside communication unit  13 , encapsulation unit  14 , storage unit  15 , control unit  16 , and input unit  17 . The communication devices  10  to  40  may have the same component elements. While not illustrated in  FIG. 2 , the other communication devices  20  to  40  may also have corresponding component elements. For example, the communication device  20  has a tunnel side communication unit  21 , crossover side communication unit  22 , inside communication unit  23 , encapsulation unit  24 , storage unit  25 , control unit  26 , and input unit  27 . The communication device  30  has a tunnel side communication unit  31 , crossover side communication unit  32 , inside communication unit  33 , encapsulation unit  34 , storage unit  35 , control unit  36 , and input unit  37 . The communication device  40  has a tunnel side communication unit  41 , crossover side communication unit  42 , inside communication unit  43 , encapsulation unit  44 , storage unit  45 , control unit  46 , and input unit  47 . The explanation of component elements of the communication device  10  explained below may also be applied to the corresponding component elements of the other communication devices  20  to  40 . 
         [0050]    &lt;Communication Unit&gt; 
         [0051]    The tunnel side communication unit  11  is a communication unit which sends and receives encapsulated packets. It is an interface device which connects with other communication devices  30  and  40  connected to the tunnel side network through tunnel routes. The tunnel side communication unit  11  adds header information to a payload provided from the encapsulation unit  14  in accordance with header information provided by the encapsulation unit  14 . 
         [0052]    The crossover side communication unit  12  is an interface device which connects to redundantly configured communication devices. 
         [0053]    The inside communication unit  13  is a connection interface device which transmits and receives decapsulated packets with information processing devices  110  and  120  both connected in the network  100 . 
         [0054]    The tunnel side communication unit  11 , crossover side communication unit  12 , and inside communication unit  13  control communication in accordance with, for example, the Ethernet® or other communication protocol and use twisted pair cables or optical fiber cables to communicate with the communication devices or information processing devices. The tunnel side communication unit  11  is identified by an IP (Internet Protocol) address of the tunnel side network, while the crossover side communication unit  12  and inside communication unit  13  are identified by IP addresses of the inside network. 
         [0055]    &lt;Encapsulation Unit&gt; 
         [0056]    The encapsulation unit  14  adds a further additional header to data including a header sent from the control unit  16 , as encapsulation processing. In other words, the encapsulation unit  14  provides data received from the information processing device  110  etc., as a payload, to the tunnel side communication unit  11  and instructs the tunnel side communication unit  11  to perform processing for adding an additional header. The additional header includes, for example, the IP address of the tunnel side communication unit  11  at the tunnel side network and tunnel identification information. Tunnel identification information is prepared corresponding to the number of tunnel routes established. For example, when the communication device  10  establishes a tunnel route between the communication device  30  and communication device  40 , there are two sets of tunnel identification information: tunnel identification information for the tunnel route with the communication device  30  and tunnel identification information for the tunnel route with the communication device  40 . 
         [0057]    The encapsulation unit  14  utilizes a connection established by the tunnel side communication unit  11  by, for example, the TCP (Transmission Control Protocol), and performs authentication with the encapsulation unit of the destination using tunnel identification information included in the header, that is, performs tunnel authentication. When the tunnel is authenticated, the tunnel is established and a tunnel route is established between the communication devices. Depending on the type of the tunneling protocol, there are also protocols with no tunnel authentication procedures. In this case, there is no need for performing an authentication procedure. 
         [0058]    The encapsulation unit  14  decapsulates data received from an outside network through the tunnel side communication unit  11  by removing communication use header information at the outside network and obtains the payload from the data received at the tunnel side communication unit  11 . 
         [0059]    Using  FIG. 3A  and  FIG. 3B , one example of a tunnel route established by the encapsulation unit  14  will be explained. Reference numeral  310  illustrated in  FIG. 3A  is a physical communication route of the tunnel side network, while  320  illustrated in  FIG. 3B  is a tunnel route formed by establishment of tunneling. 
         [0060]    The encapsulation unit  14  refers to the later explained tunnel information  910  and uses the tunnel side communication unit  11  to establish a control connection for the tunnel destination address in accordance with, for example, the TCP.  FIG. 3A  is a block diagram illustrating one example of a communication route connecting communication devices. The tunnel side communication units  11 ,  31 , and  41  of the communication devices establish control connection through the physical network  310  illustrated in  FIG. 3A . 
         [0061]    The encapsulation unit  14  further refers to the data transfer table  930  ( FIG. 4 ) and performs authentication with the encapsulation unit  34  or encapsulation unit  44  of the destination other communication device so as to establish a tunnel route. The encapsulation unit  14  provides a tunnel route for the data including the authenticated tunnel identification information. For example, by using the physical communication route illustrated in  FIG. 3A  to establish a tunnel route, as illustrated in  FIG. 3B , tunnel routes  1  and  2  are established. Thus, the communication device  10  establishes a tunnel route  1  and a tunnel route  2  respectively with the communication devices  30  and  40 . Thus, the encapsulation unit  24  establishes tunnel routes separately for the number of tunnel connections for each communication device to be connected. Note that, in the case of tunneling protocol with no authentication procedures, tunnel routes can be established and tunnel routes can be provided without authentication procedures. 
         [0062]    &lt;Storage Unit&gt; 
         [0063]    The storage unit  15  stores information illustrated in  FIG. 4  such as tunnel information  910 , filter rules  920 , a data transfer table  930 , and a program  990 . 
         [0064]    &lt;Tunnel Information&gt; 
         [0065]    The tunnel information  910  is information linking the IP addresses of the tunnel side network and the tunnel identification information. The tunnel information  910  has entries (rows) for each tunnel identification information. The entries have input values of the tunnel identification information column  911 , destination IP address column  912 , and source IP address column  913 . In the example illustrated in  FIG. 4 , the tunnel identification information column  911  has the input tunnel identification information, the destination IP address column  912  has the input IP addresses of the communication device  30  and communication device  40 , while the source IP address column  913  has the input IP addresses of the communication device  10 . For example, the communication device  10  has the tunnel routes  1 ,  2  to the communication device  30  and the communication device  40 , so there are two sets of tunnel identification information. Further, the tunnel information  910  has, as input, the destination address and source address for each tunnel identification information. 
         [0066]    The control unit  16  takes tunnel information  910  received through the input unit or outside or an inside network and stores it in the storage unit  15  and provides it to the encapsulation unit  14 . The encapsulation unit  14  uses the tunnel information  910  to perform tunnel authentication with the communication device establishing the tunnel route, by using the tunnel identification information corresponding to the IP address of the communication device. 
         [0067]    &lt;Filter Rules&gt; 
         [0068]    The filter rules  920  are information specifying the input/output units which stop data transfer. The control unit  16  follows the filter rules  920  to control communication at the tunnel side communication unit  11  and crossover side communication unit  12 . The filter rules  920  have entries (rows) for each set of input/output for which data transfer is controlled. Each entry has input values of the input/output columns  921  and  922 . As illustrated in  FIG. 4 , the input/output columns  921  and  922  have tunnel identification information of the tunnel side communication unit  11 , that is, “TUN 11 ” or “TUN 12 ”, and the name of the crossover side communication unit  12 , that is, “eth 0 ”, input to them. 
         [0069]    &lt;Data Transfer Table&gt; 
         [0070]    The data transfer table  930  includes destination addresses of data received from the information processing device and information linking them with the communication unit. The control unit  16  refers to the data transfer table  930  and performs processing for transferring the received data to the corresponding communication unit. The data transfer table  930  has entries for each MAC (Media Access Control) address. The entries have the input values of the destination MAC address column  931  and destination name column  932 . As illustrated in  FIG. 4 , the destination MAC address column  931  has the destination MAC addresses input to it, while the destination name column  932  has the tunnel identification information of the communication unit or encapsulation unit, that is, “TUN 11 ” or “TUN 12 ”, and the crossover side communication unit, that is, “eth 0 ”, input to it. 
         [0071]    &lt;Control Unit&gt; 
         [0072]    The control unit  16  controls the tunnel side communication unit  11  and the crossover side communication unit  12  in accordance with the data transfer table  930  and filter rules  920  in the storage unit  15 . For example, the control unit  16  performs communication control processing to control the tunnel side communication unit  11  and crossover side communication unit  12  so as not to transfer data received from another communication device to another communication device. Note, the communication control processing performed by the control unit  16  and the control of other devices can be realized by running the program  990 .  FIG. 5A  illustrates one example of a loop  1100  formed among the communication device  10  to communication device  40 . To avoid the formation of such a loop  1100 , at least two of the communication devices  10  to  40  control the transmission of received data in accordance with filter rules  920  stored in the storage unit. 
         [0073]      FIG. 5B  to  FIG. 5E  is a network diagram illustrating one example of a tunnel route. For example, the filter rule indicated by  920 - 10  prohibits data communication, through the communication device  10 , between the communication device  20  and communication device  30 , data communication between the communication device  20  and communication device  40 , and data communication between the communication device  30  and communication device  40 . The filter rule indicated by  920 - 20  prohibits data communication, through the communication device  20 , between the communication device  10  and communication device  40 , data communication between the communication device  10  and communication device  30 , and data communication between the communication device  30  and communication device  40 . Further, the filter rule indicated by  920 - 30  prohibits data communication, through the communication device  30 , between the communication device  10  and communication device  40 , data communication between the communication device  10  and communication device  20 , and data communication between the communication device  20  and communication device  40 . Furthermore, the filter rule indicated by  920 - 40  prohibits data communication, through the communication device  40 , between the communication device  10  and communication device  20 , data communication between the communication device  10  and communication device  30 , and data communication between the communication device  20  and communication device  30 . 
         [0074]    As illustrated by the data communication  54 , the tunnel routes  1 ,  2 ,  4 , and  5  are established so as to enable a communication of encapsulated data between the communication device  10  or communication device  20  and the communication device  30  or communication device  40 . 
         [0075]      FIG. 5C  illustrates the case where the filter rule  920 - 10  and filter rule  920 - 20  are applied to the communication device  10  and communication device  20 .  FIG. 5D  illustrates the case where the filter rule  920 - 10  and filter rule  920 - 30  are applied to the communication device  10  and communication device  30 .  FIG. 5E  illustrates the case where the filter rule  920 - 10  and filter rule  920 - 40  are applied to the communication device  10  and communication device  40 . 
         [0076]    As illustrated in  FIG. 5C  to  FIG. 5E , if any two of the filter rules  920 - 10 ,  920 - 20 ,  920 - 30 , and  920 - 40  are applied to the communication devices  10  to  40 , the loop  1100  illustrated in  FIG. 5A  does not occur. For this reason, without applying the filter rules to the communication devices  10  to  40 , it is also possible to apply two filter rules to any two communication devices of the communication devices  10  to  40 . 
         [0077]    The input unit  17  is an input unit which can set the tunnel information  910  and filter rules  920 , for example, a keyboard. Further, the input unit  17  may also function as a drive unit which reads out a program  990  from a storage medium  980  ( FIG. 2 ) storing the program  990 . Note, the tunnel information  910  and the filter rules  920  may also be received from an outside information processing device. 
         [0078]      FIG. 6  is a flowchart illustrating an example of the communication control performed by a communication device. Either of the inside communication unit  13 , crossover side communication unit  12 , or tunnel side communication unit  11  receives data (S 601 ). When data received by the tunnel side communication unit  11  (S 602 , Y), the tunnel side communication unit  11  transfers the data to the encapsulation unit  14  of the destination address of the received data (S 603 ). The tunnel side communication unit  11  decapsulates the packets to take out the packets at the payload part (S 604 ), then the routine proceeds to step S 605 . 
         [0079]    When no data is received at the tunnel side communication unit  11  (S 602 , N), the routine proceeds to step S 605 . The control unit  16  refers to the data transfer table  930  and determines the destination of the received data (S 605 ). The control unit  16  refers to the filter rules  920  and judges if the destination of the received data is a destination to which transmission is prohibited by the filter rules (S 606 ). When the destination of the received data is prohibited by the filter rules (S 606  Y), the control unit  16  discards the received data (S 607 ) and ends the processing. When the destination of the received data is not prohibited by the filter rules (S 606  N), the control unit  16  proceeds to step S 608 . 
         [0080]    The control unit  16  judges if the destination determined at step S 605  is a communication unit or encapsulation unit (S 608 ). When the determined destination is an encapsulation unit (S 608 , Y), the control unit  16  transfers the received data to the encapsulation unit. The encapsulation unit  14  encapsulates the received data (S 610 ), then the routine proceeds to step S 611 . When the determined destination is not the encapsulation unit (S 608 , N), the routine proceeds to step S 611 . At step S 611 , the tunnel side communication unit  11  or crossover side communication unit  12  transmits the data and ends the processing. 
         [0081]    [Processing of Abnormality of Communication System] 
         [0082]    The operation for monitoring the communication system  300  and the operation for dealing with trouble when it occurs will be explained for each abnormality envisioned. Note, the communication devices  10  to  40  performs the monitoring operation and troubleshooting operation in parallel with the above data communication processing. 
         [0083]      FIG. 7  is a network diagram illustrating one example of the network configuration in which backbone communication devices are set at the tunnel side network. As illustrated in  FIG. 7 , the communication devices  10  to  40  are connected to the backbone communication devices R 1  and R 2  through a network  310   a . Reference numeral  310   a  corresponds to the network  310  illustrated in  FIG. 3A  and is not a tunnel route. As illustrated in  FIG. 7 , the tunnel side network and the inside network differ in subnets. The tunnel side communication unit has, at the tunnel side, an IP address of the address system of the tunnel side network. Further, the inside communication unit has an IP address of the address system of the inside network at the inside. The backbone communication devices R 1  and R 2  are communication devices used for segmentation of the tunnel side network and, for example, consist of a router or L3 (Layer 3) switch. 
         [0084]    &lt;Monitoring of Communication Device&gt; 
         [0085]    After the establishment of a tunnel, a communication device transmits a heart beat packet to the other communication device to thereby monitor if the other communication device is operating normally or if the backbone communication device is operating normally. To separate faults of the backbone communication device and other communication devices, for example, it may designate their respective IP addresses and judge if there is a PING response. When not receiving a confirmation packet from the destination communication device, it may stop the processing for continuing to send packets to a communication device which has gone down due to a fault. Furthermore, it generates a transmission table so as to send data to the redundantly configured communication device side. Further, the communication device monitors the statuses of the communication units and uses the states to monitor if the backbone communication device is operating normally. 
         [0086]    Further, the communication device transmits a heart beat packet through the crossover side communication unit to the redundantly configured other communication device to monitor if the redundantly configured other communication device is normally operating. Further, the communication device may monitor the state of the communication unit and, from that state, monitor if the redundantly configured other communication device is normally operating. 
         [0087]    In this way, the communication devices  10  to  40  can constantly monitor if the other communication devices are normally operating. 
         [0088]    Note, the communication devices  10  to  40  detect the MAC addresses of the sources and destinations to generate communication tables linking the sources and destinations of MAC addresses. For this reason, when the other communication devices stop operating, the communication devices  10  to  40  discard the communication tables and perform processing for preparing the communication tables again. 
         [0089]    &lt;Occurrence of Fault at Tunnel Side Network&gt; 
         [0090]    As faults at the tunnel side network, an open circuit at the tunnel side network, a fault at the backbone communication devices R 1 , R 2 , etc. may be mentioned. 
         [0091]      FIG. 8  is a sequence diagram illustrating an example of restoration of a communication system from a fault occurring at the tunnel side network. 
         [0092]    At step S 701 , if a fault occurs at the backbone communication device R 1  or a cable connecting to the backbone communication device R 1  is disconnected, the communication device  10  detects a fault at the backbone communication device R 1  by its monitoring and stops the operation (S 702 ). The other communication device and information processing device  110  detect the stopping of the communication device  10  by monitoring (S 703 ). When the communication devices  20 ,  30 , and  40  detect the stopping of the communication device  10 , they discard their communication tables (S 704 ). When the information processing device  110  detects the stopping of the communication device  10 , it switches from the NIC connected to the communication device  10  to the other NIC (S 705 ) and sends the information processing device  210  an ARP (Address Resolution Protocol) request (S 706 ). An “ARP request” is a request for transmission of the MAC address from the destination. When the communication device  20  receives an ARP request, it generates a communication table (S 707 ) and transfer the ARP request to other communication devices (S 708 ). 
         [0093]    When the information processing device  210  receives an ARP request from the information processing device  110 , it generates a table (S 709 ) and sends an ARP response to the information processing device  110  (S 710 ). The communication devices  30  and  20  to which the ARP response is transferred update the tables (S 711 ). 
         [0094]    In this way, if open circuit in the network or fault in a backbone communication device etc. occurs, the communication device  10  stops the operation. Due to this, the information processing device  110  which had transferred data through the communication device  10  switches to the communication device  20  for transfer of data. 
         [0095]    &lt;Occurrence of Fault at Communication Device&gt; 
         [0096]    As a fault at the inside network, an open circuit at the inside network, a down state of a information processing device, etc. may be mentioned. The control unit monitors the state of the inside communication units in preparation for the occurrence of such faults. However, even when such faults occur and the inside communication unit goes down, the device subsequently continues operation without doing anything. As a result, the affected information processing device switches to the redundant system. 
         [0097]      FIG. 9  is a sequence diagram illustrating one example of restoration of a communication system from a fault occurring at the communication device. At step S 801  indicated in  FIG. 9 , a fault occurs at the communication device  10 , so another communication device detects the occurrence of the fault at the communication device  10  by monitoring. The other steps S 703  to S 711  illustrated in  FIG. 9  are the same as the steps explained in  FIG. 8 , so explanations will be omitted. 
         [0098]    As illustrated in  FIG. 8  and  FIG. 9 , the information processing device  110  can switch the transfer of data to the other communication device forming the redundant configuration so as to continue operation. 
         [0099]      FIG. 10  is a network diagram illustrating one example of a tunnel route after restoration from a fault. Even when the filter rule  920 - 10  has been set for an ended communication device  10 , if a filter rule is set for the other communication device, that is, the communication device  20 , it will be understood that no loop is formed. In this way, even when a filter rule is set for a communication device which stops at the time of a fault, the communication system  300  enables data communication without formation of a loop after the communication device stops. Note, when the filter rule  920 - 10  has not been set for a communication device to be stopped, filter rules are set for at least two other communication devices, so the communication system  300  does not form a loop. 
         [0100]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding 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 illustrating 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.