Patent Publication Number: US-2018048531-A1

Title: Management apparatus, network management method, and storage medium storing program

Description:
TECHNICAL FIELD 
     The present invention relates to a management apparatus, a network management method, and a storage medium storing a program. 
     BACKGROUND ART 
     To know the traffic volume on each of links constituting a network, statistical information regarding each link needs to be acquired. Hence, to acquire such statistical information regarding each link, methods for acquiring the statistical information from a switch by using a protocol such as Simple Network Management Protocol (SNMP), sFlow, or NetFlow are used. 
     PTL 1 describes a technique by which notification about a traffic volume is received from a router satisfying a predetermined condition, and a traffic volume on the router other than the volume as notified is estimated. 
     NPL 1 describes a technique by which information serving as the basis for acquiring a traffic volume is limited, and a traffic volume is estimated based on the limited information. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Unexamined Patent Application Publication No. 2006-013926 
     Non-Patent Literature 
     [NPL 1] Yuichi OSHITA et al, “Estimation of Current Traffic Matrices from Long Term Traffic Variations”, IEICE TRANSACTIONS on Communications, Vol. E92-B, No. 1, pp. 171-183. 
     SUMMARY OF INVENTION 
     Technical Problem 
     Note that the disclosures of the related literatures given above are incorporated by reference herein. The following analysis is made from viewpoints of the present invention. 
     As described above, to know the traffic volume on each of links constituting a network, statistical information regarding each link needs to be acquired. To monitor traffic volumes, however, statistical information regarding links has to be collected periodically. However, as the monitored network is larger in size, the traffic volume for acquiring the statistical information is increased, resulting in a greater load imposed on the network. In addition, as the statistical information is acquired at shorter time intervals, a greater load is imposed on switches. 
     The foregoing techniques described in PTL 1 and NPL 1 may cause an estimation error when a traffic volume is estimated. Thus, the techniques described in PTL 1 and NPL 1 may fail to recognize an accurate traffic volume. 
     Hence, an object of the present invention is to provide a management apparatus, a network management method, and a storage medium storing a program that contribute to reducing the load caused for acquiring a traffic volume and to knowing an accurate traffic volume. 
     Solution to Problem 
     According to a first aspect of the present invention, a management apparatus including the functions described below is provided. The management apparatus includes a topology information acquisition unit that acquires topology information regarding a group of links, each of which has a switch at an endpoint of the link, the switch being included in a group of first switches that are included in a network. 
     The management apparatus further includes a switch selection unit that selects one or more monitoring target switches forming a set of switches, each of which includes either endpoint of any of links constituting the group of links. 
     The management apparatus further includes a statistical information acquisition unit that acquires statistical information from the monitoring target switches. 
     According to a second aspect of the present invention, a network management method including the steps described below is provided. 
     The network management method includes the step of acquiring topology information regarding a group of links, each of which has a switch at an endpoint of the link, the switch being included in a group of first switches that are included in a network. 
     The network management method further includes the step of selecting one or more monitoring target switches forming a set of switches, each of which includes either endpoint of any of links constituting the group of links. 
     The network management method further includes the step of acquiring statistical information from the monitoring target switches. 
     Note that this method is associated with a certain machine, which is a management apparatus that is connected to switches and acquires statistical information from the switches. 
     According to a third aspect of the present invention, a storage medium storing a program including the processes described below is provided. The storage medium storing a program stores a program for causing a computer controlling a management apparatus connected to a group of first switches to execute the process of acquiring topology information regarding a group of links, each of which has a switch at an endpoint of the link, the switch being included in the group of first switches that are included in a network. 
     In addition, the storage medium storing a program stores a program for causing the computer to execute the process of selecting one or more monitoring target switches forming a set of switches, each of which includes either endpoint of any of links constituting the group of links. 
     Furthermore, the storage medium storing a program stores a program for causing the computer to execute the process of acquiring statistical information from the monitoring target switches. 
     Note that the program can be recorded in a computer-readable storage medium. The storage medium may be a non-transient medium such as a semiconductor memory, a hard disk, a magnetic recording medium, or an optical recording medium. The present invention can also be embodied in a computer program product. 
     Advantageous Effects of Invention 
     According to the aspects of the present invention, there are provided a management apparatus, a network management method, and a storage medium storing a program that contribute to reducing the load caused for acquiring a traffic volume and to knowing an accurate traffic volume. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating an overview of one example embodiment. 
         FIG. 2  is a block diagram illustrating an example internal configuration of a management apparatus  100 . 
         FIG. 3  is a block diagram illustrating an example internal configuration of a network control unit  105 . 
         FIG. 4  is a flowchart illustrating an example of operations of the management apparatus  100  according to a first example embodiment. 
         FIG. 5  illustrates an example of connections among switches  301  to  308  and paths connecting among switches  301  to  308 . 
         FIG. 6  illustrates an example of an updated topology. 
         FIG. 7  illustrates correspondence relations between links and switches used for acquiring statistical information regarding the respective links. 
         FIG. 8  is a flowchart illustrating an example of operations of the management apparatus  100  according to a second example embodiment. 
         FIG. 9  illustrates an example of an updated topology. 
         FIG. 10  illustrates an example of an updated topology. 
         FIG. 11  illustrates an example of an updated topology. 
         FIG. 12  illustrates an example of an updated topology. 
         FIG. 13  illustrates correspondence relations between links and switches used for acquiring statistical information regarding the respective links. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     To begin with, an overview of one example embodiment is given below with reference to  FIG. 1 . Note that drawing reference symbols for the overview are added to respective elements as an example for convenience to aid in understanding, and that descriptions of the overview are not intended for any limitation. 
     As described above, there is a demand for a management apparatus that contributes to reducing the load caused for acquiring a traffic volume and to knowing an accurate traffic volume. 
     To meet this demand, a management apparatus  10  illustrated in  FIG. 1  is provided as an example. The management apparatus  10  includes a topology information acquisition unit  11 , a switch selection unit  12 , and a statistical information acquisition unit  13 . 
     The topology information acquisition unit  11  acquires topology information regarding a group of links, each of which has a switch at an endpoint of the link, the switch being included in a group of first switches that are included in a network. 
     The switch selection unit  12  selects one or more monitoring target switches forming a set of switches, each of which includes either endpoint of one of the links constituting the group of links included in the topology information. The one or more monitoring target switches as selected are switches connected to any of the links within a network composed of a predetermined group of links. 
     For example, among the links constituting the network, the switch selection unit  12  may select one or more monitoring target switches forming a set of switches, each of which includes at least one of the endpoints of a link. In this case, the selected monitoring target switches are connected to any of the links throughout the network. 
     It is preferable here that the switch selection unit  12  selects the monitoring target switches so that the total number of selected monitoring target switches is less than the total number of switches included in the group of first switches. In other words, it is preferable that the switch selection unit  12  selects, as the monitoring target switches, some of the first switches. 
     The statistical information acquisition unit  13  acquires statistical information from the monitoring target switches selected by the switch selection unit  12 . In other words, the statistical information acquisition unit  13  acquires statistical information from some of the first switches (that is, the monitoring target switches). Note that “statistical information”, as used herein, means statistical information regarding traffic moving along a link. 
     As a result, the management apparatus  10  contributes to reducing the load caused for acquiring a traffic volume. In addition, as described above, the selected monitoring target switches are connected to any of the links within a network from which statistical information is to be acquired. Therefore, concerning the links from which statistical information is to be acquired, the management apparatus  10  can easily find accurate traffic volumes from the switches connected to the links. Consequently, the management apparatus  10  contributes to reducing the load caused for acquiring a traffic volume and to knowing an accurate traffic volume. 
     First Example Embodiment 
     A first example embodiment will now be described in more detail with reference to the drawings. 
       FIG. 2  is a block diagram illustrating an example internal configuration of a management apparatus  100  according to the present example embodiment. The management apparatus  100  is configured to include a communication unit  101 , a path information management unit  102 , a topology information management unit  103 , a statistical information management unit  104 , and a network control unit  105 . 
     The communication unit  101  controls communications between individual network nodes and another system. The path information management unit  102  manages path information regarding paths on a network. The topology information management unit  103  manages topology information regarding a network configuration. The statistical information management unit  104  holds statistical information regarding traffic. The network control unit  105  controls the network managed by the management apparatus  100 . 
     The following describes the network control unit  105  in detail. 
       FIG. 3  is a block diagram illustrating an example internal configuration of the network control unit  105 . The network control unit  105  is configured to include a topology information acquisition unit  1051 , a path information acquisition unit  1052 , a topology information updating unit  1053 , a switch selection unit  1054 , a table generation unit  1055 , and a statistical information acquisition unit  1056 . 
     The topology information acquisition unit  1051  identifies topology information regarding the network configuration. Specifically, the topology information acquisition unit  1051  acquires topology information regarding a group of links, each of which has a switch at an endpoint of the link, the switch being included in a group of first switches that are included in the network. 
     Then, the topology information acquisition unit  1051  identifies one or more switches included in the network (the foregoing group of first switches) and one or more links (the foregoing group of links), each of which has any of the switches at an endpoint of the link. 
     The path information acquisition unit  1052  acquires path information. Specifically, the path information acquisition unit  1052  acquires path information from the path information management unit  102 . 
     Based on the path information and the topology information, the topology information updating unit  1053  selects one or more second switches satisfying a predetermined condition from the group of first switches, and generates a group of links, each of which has any of the selected second switches at an endpoint of the link. Then, based on the generated group of links, the topology information updating unit  1053  updates the topology information. 
     The topology information updating unit  1053  performs the update of the topology information so that the number of switches included in the updated topology information (that is, the number of second switches) is less than the number of switches included in the topology information existing prior to the update (that is, the number of switches included in a group of first switches). In other words, the topology information updating unit  1053  updates the topology information so that the number of links included in the updated topology information is less than the number of links included in the topology information existing prior to the update. 
     In short, the topology information updating unit  1053  updates the topology information so that the topology information becomes degenerate; provided, however, the topology information updating unit  1053  updates the topology information so that the network bandwidth is kept unchanged before and after the update of the topology information. 
     The following describes the aforementioned second switches in detail. 
     The topology information updating unit  1053  selects any switch that satisfies at least one of the following conditions 1 to 3, as a second switch: 
     (Condition 1) 
     A switch that serves as a contact point between the network configured to include a group of first switches and another network external to the network, and is included in the path information. 
     (Condition 2) 
     A switch that has links with another switch in the group of first switches, the number of links being greater than a predetermined number (three, for example), and is included in the path information. 
     (Condition 3) 
     A switch at which a path is generated or terminated in the group of first switches according to the path information. 
     The switch selection unit  1054  selects one or more monitoring target switches forming a set of switches, each of which includes either endpoint of one of the links constituting the group of links included in the topology information. The term “monitoring target switches”, as used herein, means the switches from which the statistical information acquisition unit  1056  collects statistical information. And, a set of monitoring target switches is equivalent to a set of vertices in vertex cover in a graph composed of a group of links. 
     In addition, when the topology information updating unit  1053  updates the topology information, the switch selection unit  1054  selects one or more monitoring target switches based on the group of links included in the updated topology information. That is, when the topology information is updated so that the network becomes degenerate, the switch selection unit  1054  selects, based on the degenerate topology information, switches from which statistical information is to be collected. 
     The table generation unit  1055  generates a table in which a link having a switch included in a group of first switches at an endpoint of the link is associated with a monitoring target switch from which statistical information regarding the link is to be collected. 
     The statistical information acquisition unit  1056  acquires statistical information from the monitoring target switches. Specifically, when a link about which statistical information is to be collected is specified, the statistical information acquisition unit  1056  refers to the table generated by the table generation unit  1055  to identify the monitoring target switch corresponding to the link. Then, the statistical information acquisition unit  1056  acquires the outgoing traffic volume and the incoming traffic volume at a port on the identified monitoring target switch. Then, based on the acquired outgoing and incoming traffic volumes, the statistical information acquisition unit  1056  calculates statistical information, which is a statistical value representing the traffic volume on a switch. 
     Operations of the management apparatus  100  according to the present example embodiment will now be described. 
       FIG. 4  is a flowchart illustrating an example of operations of the management apparatus  100 . 
     In step S 1 , the topology information updating unit  1053  selects one or more second switches satisfying a predetermined condition from a group of switches (a group of first switches) included in a network. 
     Specifically, the topology information acquisition unit  1051  acquires path information regarding the network from the path information management unit  102 . In addition, the topology information acquisition unit  1051  acquires topology information regarding the network from the topology information management unit  103 . Then, based on the topology information, the topology information acquisition unit  1051  identifies the group of first switches. In addition, the topology information acquisition unit  1051  identifies one or more links, each of which has a switch at an endpoint of the link, the switch being included in the identified group of first switches. Then, the topology information updating unit  1053  selects any switch that satisfies at least one of the above-described conditions 1 to 3, as a second switch. 
     In step S 2 , the topology information updating unit  1053  generates a group of links, each of which has any of the selected second switches at an endpoint of the link. 
     In step S 3 , the switch selection unit  1054  selects one or more monitoring target switches from the generated group of links. Next, the table generation unit  1055  generates a table in which a link having a switch included in the group of first switches at an endpoint of the link is associated with a monitoring target switch from which statistical information regarding the link is to be collected. 
     In step S 4 , the statistical information acquisition unit  1056  acquires statistical information from the monitoring target switches. 
     In step S 5 , the statistical information acquisition unit  1056  calculates traffic volumes on the network, based on the acquired statistical information. Then, the statistical information acquisition unit  1056  stores the acquired statistical information into the statistical information management unit  104 . The management apparatus  100  repeats the processes in steps S 4  and S 5  to accumulate statistical information. 
     The following describes in detail an example of the process of selecting monitoring target switches with reference to the drawings. 
       FIG. 5  illustrates an example of connections among switches  301  to  308  and paths connecting the switches  301  to  308 . In  FIG. 5 , each circle represents a communication device (switch). In addition, in  FIG. 5 , the number in each circle represents a reference symbol identifying a switch for illustrative purposes. Furthermore, switches  301  to  308  in  FIG. 5  correspond to the above-mentioned first switches. The same applies to  FIG. 6  and  FIGS. 9 to 12 . 
       FIG. 5  shows path L 1  (solid line in  FIG. 5 ), path L 2  (dotted line in  FIG. 5 ), path L 3  (dashed line in  FIG. 5 ), path L 4  (dot-dash line in  FIG. 5 ), and path L 5  (dot-dot-dash line in  FIG. 5 ). More specifically, paths L 1  to L 5  in  FIG. 5  are configured to include links having the following switches at the respective endpoints of the links: 
     Path L 1 : switch  301 -switch  302 , switch  302 -switch  303 , switch  303 -switch  305 , switch  305 -switch  306 , switch  306 -switch  308 , switch  308 -switch  304 , switch  304 -switch  301 , switch  301 -switch  307 , and switch  307 -switch  308  Path L 2 : switch  302 -switch  303 , switch  303 -switch  305 , and switch  305 -switch  306   
     Path L 3 : switch  301 -switch  304 , switch  304 -switch  308 , and switch  308 -switch  306   
     Path L 4 : switch  301 -switch  307 , switch  307 -switch  308 , and switch  308 -switch  306   
     Path L 5 : switch  301 -switch  307   
     For the network illustrated in  FIG. 5 , in this example, the topology information management unit  103  stores relations of connection among switches  301  to  308 . In addition, the path information management unit  102  stores information regarding paths L 1  to L 5 . 
     In this case, the topology information acquisition unit  1051  refers to the topology information management unit  103  to acquire the relations of connection among switches  301  to  308 . In addition, the path information acquisition unit  1052  refers to the path information management unit  102  to acquire information regarding paths L 1  to L 5 . Then, the topology information updating unit  1053  selects second switches satisfying the above-described conditions 1 to 3 from switches  301  to  308  illustrated in  FIG. 5 . 
     It is assumed here that, as the second condition, the topology information updating unit  1053  selects as the second switch having links to another switch in the group of first switches, the number of links being greater than three. In this case, the topology information updating unit  1053  selects as the second switches satisfying at least one of the foregoing conditions 1 to 3, namely, switch  301 , switch  302 , switch  303 , switch  306 , switch  307 , and switch  308 . Then, the topology information updating unit  1053  generates a group of links, each of which has a second switch at an endpoint of the link, and updates the topology information based on the generated group of links. Specifically, the topology information updating unit  1053  generates a group of links that include the switches illustrated in  FIG. 6 . 
       FIG. 6  illustrates an example of the updated group of links generated by the topology information updating unit  1053 . In the group of links illustrated in  FIG. 6 , switch  304  and switch  305  in  FIG. 5  are included in switch  301  and switch  303 , respectively, which are now called switch  301 ′ and switch  303 ′, respectively. As a result, the topology information updating unit  1053  generates the following group of links as illustrated in  FIG. 6 . 
     Generated group of links: switch  301 ′-switch  302 , switch  302 -switch  303 ′, switch  303 ′-switch  306 , switch  306 -switch  308 , switch  308 -switch  301 ′, switch  308 -switch  307 , and switch  307 -switch  301 ′ 
     It is assumed here that the switch selection unit  1054  selects, from the updated group of links generated by the topology information updating unit  1053 , one or more monitoring target switches that each include at least one endpoint of any of the links. By way of example, the following description assumes that the switch selection unit  1054  selects switch  301 ′, switch  303 ′, and switch  308  as the monitoring target switches. 
     In this case, the statistical information acquisition unit  1056  acquires statistical information regarding the following links that correspond to the monitoring target switches (switch  301 ′, switch  303 ′, and switch  308 ):
         Switch  301 ′: switch  301 ′-switch  302 , switch  301 ′-switch  307 , and switch  301 ′-switch  308     Switch  303 ′: switch  303 ′-switch  302  and switch  303 ′-switch  306     Switch  308 : switch  308 -switch  301 ′, switch  308 -switch  306 , and switch  308 -switch  307         

     Note that, when statistical information can be acquired from either of two switches, as seen in the link between switches  301 ′ and  308 , the statistical information acquisition unit  1056  need only acquire statistical information from either one of the switches (either switch  301 ′ or switch  308 , for example). Consequently, the statistical information acquisition unit  1056  acquires statistical information regarding all the links included in the degenerate topology information. 
     It is further assumed here that the statistical information acquisition unit  1056  is going to acquire statistical information regarding the traffic along links that are not included in the second switches. That is, with reference to  FIG. 5  illustrating switches  301  to  308 , it is assumed here that the statistical information acquisition unit  1056  is going to acquire statistical information regarding the traffic along the following links:
         Switch  301 -switch  304     Switch  304 -switch  308     Switch  303 -switch  305     Switch  305 -switch  306         

     In this case, the statistical information acquisition unit  1056  may use the statistical information regarding the link between switch  301 ′ and switch  308  as the statistical information regarding the links between switch  301  and switch  304  and between switch  304  and switch  308 . Likewise, the statistical information acquisition unit  1056  may use the statistical information regarding the link between switch  303 ′ and switch  306  as the statistical information regarding the links between switch  303  and switch  305  and between switch  305  and switch  306 . 
       FIG. 7  shows correspondence relations between the links in  FIG. 5  and the switches used for acquiring statistical information regarding the respective links. Concerning the network illustrated in  FIG. 5 , the switches used for acquiring statistical information regarding the links are switch  301 , switch  303 , and switch  308 , as seen in  FIG. 7 . In short, the statistical information acquisition unit  1056  collects statistical information from the selected monitoring target switches, and consequently the management apparatus  100  can recognize statistical information regarding traffic along every link in the network. 
     As described above, the management apparatus  100  according to the present example embodiment selects (narrows down to) switches from which statistical information is to be acquired, from the switches included in a network. Furthermore, the management apparatus  100  according to the present example embodiment selects a switch connected to any link included in the network, as the switch from which statistical information is to be acquired. Consequently, the management apparatus  100  according to the present example embodiment contributes to reducing the load caused for acquiring a traffic volume and to knowing an accurate traffic volume. 
     Second Example Embodiment 
     A second example embodiment will now be described in more detail with reference to the drawings. 
     In the present example embodiment, two or more sets of monitoring target switches are created. Note that descriptions of the present example embodiment in common with those of the above-described example embodiment are omitted. In addition, in the description of the present example embodiment, reference symbols identical to those of the foregoing example embodiment are given to components identical to those of the foregoing example embodiment, and descriptions of these components are omitted. Furthermore, operational effects identical to those provided by the foregoing example embodiment are also omitted in the descriptions of the present example embodiment. 
     The internal configuration of the management apparatus  100  is the same as that illustrated in  FIG. 2 . The internal configuration of the network control unit  105  is also the same as that illustrated in  FIG. 3 . The following describes differences from the first example embodiment in detail. 
     The switch selection unit  1054  according to the present example embodiment creates two or more sets of monitoring target switches. Then, every time statistical information is acquired, the statistical information acquisition unit  1056  according to the present example embodiment selects a set of monitoring target switches, and selects a switch included in the selected set of monitoring target switches as the monitoring target switch. Then, the statistical information acquisition unit  1056  acquires statistical information from the selected monitoring target switch. 
     Additionally, the statistical information acquisition unit  1056  according to the present example embodiment may calculate a frequency of acquiring statistical information for every switch at an endpoint of a link included in the updated group of links generated by the topology information updating unit  1053 . In this case, the statistical information acquisition unit  1056  selects a set of monitoring target switches in accordance with the frequency of acquisition calculated for every switch. The statistical information acquisition unit  1056  then selects a switch included in the selected set of monitoring target switches, as the monitoring target switch. Then, the statistical information acquisition unit  1056  acquires statistical information from the selected monitoring target switch. 
     Operations of the management apparatus  100  according to the present example embodiment will now be described. 
       FIG. 8  is a flowchart illustrating an example of operations of the management apparatus  100  according to the present example embodiment. Note that steps S 101  and S 102  in  FIG. 8  are the same as steps S 1  and S 2  in  FIG. 4 , thus their detailed descriptions are omitted. 
     In step S 103 , the switch selection unit  1054  creates two or more sets of monitoring target switches. 
     In step S 104 , the switch selection unit  1054  determines, based on the loads on switches, a specific set of monitoring target switches from which statistical information is to be acquired. 
     In step S 105 , the statistical information acquisition unit  1056  acquires statistical information from a switch included in the determined set of monitoring target switches. 
     In step S 106 , information about traffic on the network is created based on the acquired statistical information. 
     The following describes in detail an example embodiment of the process of selecting monitoring target switches with reference to the drawings. 
     To begin with, suppose that the topology information updating unit  1053  has selected second switches as described above, namely switches  301 ,  302 ,  303 ,  306 ,  307 , and  308  from switches  301  to  308  on paths L 1  to L 5  illustrated in  FIG. 5 . 
     Also suppose that the switch selection unit  1054  according to the present example embodiment creates the sets of switches 1 to 4 listed below, as sets of monitoring target switches (each set of monitoring target switches is hereinafter called a “switches set”).  FIGS. 9 to 12  identify monitoring target switches belonging to switches sets 1 to 4, respectively. In  FIGS. 9 to 12 , the switches in thick lines represent monitoring target switches constituting a switches set. 
     Switches set 1 (see  FIG. 9 ): switch  301 ′, switch  303 ′, and switch  308  Switches set 2 (see  FIG. 10 ): switch  302 , switch  306 , switch  307 , and switch  308   
     Switches set 3 (see  FIG. 11 ): switch  301 ′, switch  303 ′, switch  306 , and switch  307   
     Switches set 4 (see  FIG. 12 ): switch  302 , switch  303 ′, switch  307 , and switch  308   
     Further,  FIG. 13  shows, for every switches set, correspondence relations between the links in  FIG. 5  and the switches used for acquiring statistical information regarding the respective links. When the switch selection unit  1054  creates a switches set, the table generation unit  1055  may create a table as illustrated in  FIG. 13 . That is, the table generation unit  1055  may generate, for every switches set, a table in which a link having a switch included in the set at an endpoint of the link is associated with a monitoring target switch from which statistical information regarding the link is to be acquired. The management apparatus  100  can efficiently determine a switch from which statistical information is to be acquired, by generating and storing a table as illustrated in  FIG. 13 . 
     Then, the switch selection unit  1054  selects a switches set every time statistical information is acquired. Then, the switch selection unit  1054  selects a monitoring target switch from the selected switches set. Next, the statistical information acquisition unit  1056  refers to the table generated by the table generation unit  1055 , and, according to the link for which statistical information is to be acquired, determines the switch from which statistical information is to be acquired. 
     For example, when acquiring statistical information for the first time, the statistical information acquisition unit  1056  acquires statistical information using the aforementioned switches set 1. Subsequently, when acquiring statistical information next time, the statistical information acquisition unit  1056  may acquire statistical information using the aforementioned switches set 1. 
     Referring now to the table in  FIG. 13 , concerning the link between switches  301  and  307 , for example, the switch from which statistical information is acquired depends on the switches set. Specifically, concerning switches sets 1 and 3, the switch which corresponds to the link between switches  301  and  307  and from which statistical information is to be acquired is switch  301 . Concerning switches sets 2 and 4, the switch which corresponds to the link between switches  301  and  307  and from which statistical information is to be acquired is switch  307 . 
     Hence, the switch selection unit  1054  may count the number of acquisitions of statistical information with respect to each switch. Then, the statistical information acquisition unit  1056  may adjust statistical values representing traffic volumes in accordance with the number of acquisitions of statistical information with respect to each switch. 
     Alternatively, the switch selection unit  1054  may calculate the frequency of acquisition of statistical information with respect to each switch, and select a switches set in accordance with the frequency of acquisition with respect to each switch. Then, the switch selection unit  1054  may select a monitoring target switch from the selected switches set. 
     Alternatively, the management apparatus  100  may acquire loads on individual switches. Then, the switch selection unit  1054  may select a switches set in accordance with the loads on switches. By way of example, suppose the case where the load on switch  303  has increased. In this case, the switch selection unit  1054  may select switches set 2, which does not include switch  303 . As a result, the management apparatus  100  can reduce the load on switch  303 . 
     As described above, the management apparatus  100  according to the present example embodiment creates two or more sets of monitoring target switches. Thus, the management apparatus  100  according to the present example embodiment can prevent an increase in access to a particular switch (that is, the process of acquiring statistical information) by changing to another switch from which statistical information is to be acquired, according to the situation. As a result, the management apparatus  100  according to the present example embodiment further contributes to reducing the load caused for acquiring a traffic volume and to knowing an accurate traffic volume. 
     The whole or part of the example embodiments disclosed above can be described as, but not limited to, the following supplementary notes. 
     (Supplementary note 1) The management apparatus according to the first aspect described above. 
     (Supplementary note 2) The management apparatus according to supplementary note 1, further comprising: 
     a path information acquisition unit that acquires path information; and 
     topology information updating unit that selects, based on the path information and the topology information, one or more second switches satisfying a predetermined condition from the group of first switches, generates a group of links, each of which has any of the selected second switches at an endpoint of the link, and updates the topology information based on the generated group of links, 
     wherein the switch selection unit selects the one or more monitoring target switches based on the group of links generated by the topology information updating unit. 
     (Supplementary note 3) The management apparatus according to supplementary note 2, 
     wherein the topology information updating unit selects, as any of the second switches, a switch that serves as a contact point between a network configured to include the group of first switches and another network external to the network, and is included in the path information. 
     (Supplementary note 4) The management apparatus according to supplementary note 2 or 3, 
     wherein the topology information updating unit selects, as any of the second switches, a switch that has a number of links with another switch in the group of first switches, and is included in the path information, the number being greater than a predetermined number. 
     (Supplementary note 5) The management apparatus according to any one of supplementary notes 2 to 4,
         wherein the topology information updating unit selects, as any of the second switches, and based on the path information, a switch at which a path is generated or terminated in the group of first switches.       

     (Supplementary note 6) The management apparatus according to any one of supplementary notes 1 to 5, further comprising: 
     a table generation unit that generates a table in which a link having a switch included in the group of first switches at an endpoint of the link is associated with any of the monitoring target switches from which statistical information regarding the link is collected. 
     (Supplementary note 7) The management apparatus according to any one of supplementary notes 1 to 6, 
     wherein the switch selection unit creates two or more sets of the monitoring target switches, and, every time the statistical information is acquired, selects one of the sets and selects the monitoring target switches from the selected set. 
     (Supplementary note 8) The management apparatus according to supplementary note 7, 
     wherein the switch selection unit creates two or more sets of the monitoring target switches, calculates a frequency of acquisition of statistical information with respect to each switch at an endpoint of a link included in the group of links, selects one of the sets based on the frequency of acquisition with respect to each switch, and selects the monitoring target switches from the selected set. 
     (Supplementary note 9) The management apparatus according to supplementary note 7 or 8, wherein the table generation unit generates, for each of the sets, a table in which a link having a switch included in the set at an endpoint of the link is associated with the monitoring target switch from which statistical information regarding the link is acquired. 
     (Supplementary note 10) The network management method according to the second aspect described above. 
     (Supplementary note 11) The program according to the third aspect described above. 
     Note that the embodiments described in supplementary notes 10 and 11 can be turned into the embodiments described in supplementary notes 2 to 9 as with the embodiment described in supplementary note 1. 
     The disclosures of the patent and non-patent literatures described above are incorporated herein by reference. Modifications and adjustments of example embodiments may be made within the bounds of the entire disclosure (including the scope of the claims) of the present invention, and also based on fundamental technological concepts thereof. 
     Furthermore, various combinations and selections of various disclosed elements (including respective elements of the respective claims, respective elements of the respective example embodiments, respective elements of the respective drawings, and the like) are possible within the bounds of the entire disclosure of the present invention. That is, the present invention clearly includes every type of transformation and modification that those skilled in the art can achieve according to the entire disclosure of the present invention including the claims and to technological concepts thereof. In particular, it should be understood that a range of values appearing herein includes any value and sub-range falling in the range as if they are specifically set forth herein unless otherwise stated. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2015-051223, filed on Mar. 13, 2015, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS List 
     
         
           10 ,  100  Management apparatus 
           11 ,  1051  Topology information acquisition unit 
           12 ,  1054  Switch selection unit 
           13 ,  1056  Statistical information acquisition unit 
           101  Communication unit 
           102  Path information management unit 
           103  Topology information management unit 
           104  Statistical information management unit 
           105  Network control unit 
           301  to  308  Switch 
           1052  Path information acquisition unit 
           1053  Topology information updating unit 
           1055  Table generation unit 
         L 1  to L 5  Path