the present invention deals with a cu separation type network system . in this case , an open flow network system as one of the cu separation type network systems will be described , using it as an example . however , actually , the present invention is not limited to the open flow network system . hereinafter , exemplary embodiments of the present invention will be described with reference to the attached drawings . as shown in fig1 , the network system according to the present invention includes a controller 10 , switches 20 ( 20 - i , i = 1 to n : n is the number of switches ), and a header translating unit 30 . when detecting the switch 20 ( 20 - i , i = 1 to n ), the controller 10 calculates a packet transfer route based on topology data showing a connection state of the switches in the network , and registers an entry ( flow entry ) on a flow table of each of the switches related to the calculated route . each of the switches 20 ( 20 - i , i = 1 to n ) transfers a received packet according to an entry registered on its own flow table . in this case , it is supposed that each of the switches 20 ( 20 - i , i = 1 to n ) is a virtual switch . the virtual switch is a switch realized by a virtual machine which operates on a physical machine . however , actually , each of the switches 20 ( 20 - i , i = 1 to n ) may be a physical switch in which software to realize a function as the switch according to the present invention is installed . also , the switch 20 ( 20 - i , i = 1 to n ) cooperates with an encapsulating module 21 ( 21 - i , i = 1 to n ). the encapsulating module 21 ( 21 - i , i = 1 to n ) carries out encapsulation of an ip packet . it should be noted that the encapsulating module 21 ( 21 - i , i = 1 to n ) may be built in the physical machine in which the switch 20 ( 20 - i , i = 1 to n ) operates and may be connected with the physical machine . for example , the encapsulating module 21 ( 21 - i , i = 1 to n ) may operate on a computer which can communicate with the physical machine . the header translating unit 30 translates the header data of a packet received from a predetermined switch 20 and transfers to another switch 20 . an example of the specific hardware to realize the network system according to the present invention will be described below . as an example of the controller 10 , computers such as a pc ( personal computer ), appliance , a thin client terminal / server , a workstation , a mainframe , and a supercomputer are assumed . also , the controller 10 may be an extension board which is installed in the computer and a virtual machine ( vm ) built on a physical machine . as an example of a physical machine on which the switch 20 ( 20 - i , i = 1 to n ) operates as a virtual switch and the header translating unit 30 , a network switch , a router , a proxy , a gateway , a firewall , a load balancer , a base station , an access point , a computer which has a plurality of communication ports and so on are exemplified . each of the controller 10 and the physical machine on which the switch 20 ( 20 - i , i = 1 to n ) operates , and the header translating unit 30 is realized by a processor which executes predetermined processing based on a program , a memory which stores the program and all kinds of data , and a communication interface to connect with network . as an example of the above - mentioned processor , a cpu ( central processing unit ), a network processor ( np ), a microprocessor , a microcontroller and a semiconductor integrated circuit ( lsi ) and so on which has a function of an exclusive use are exemplified . as an example of the above - mentioned memory , a semiconductor memory device such as ram ( random access memory ), rom ( read only memory ), eeprom ( electrically erasable and programmable read only memory ) and a flash memory , an auxiliary storage such as hdd ( hard disk drive ) and ssd ( solid state drive ), a removable disk such as dvd ( digital versatile disk ), a storage medium such as an sd memory card ( secure digital memory card ) and so on are exemplified . it should be noted that the above - mentioned processor and the above - mentioned memory may be unified . for example , in recent years , a 1 - chip microcomputer is being developed . therefore , a case that the 1 - chip microcomputer installed in the computer and so on is composed of a processor and a memory is thought of . as an example of the above - mentioned communication interface , a substrate ( motherboard , and an i / o board ) corresponding to the network communication , a semiconductor integrated circuit of a chip and so on , a network adapter such as nic ( network interface card ), a similar expansion card , a communication device such as an antenna , a communication port such as a connection mouth ( connector ) and so on are exemplified . also , an an example of the network , the internet , lan ( local area network ), wireless lan ( wireless lan ), wan ( wide area network ), backbone , catv line , fixation telephone network , mobile phone network , wimax ( ieee 802 . 16a ), 3g ( 3rd generation ), dedicated line ( leased line ), irda ( infrared data association ), bluetooth ( registered trademark ), serial communication line , data bus and so on are exemplified . as an example of the encapsulating module 21 ( 21 - i , i = 1 to n ), software or virtual machine which operates on the same physical machine as the switch 20 ( 20 - i , i = 1 to n ) are assumed . however , actually , the encapsulating module 21 ( 21 - i , i = 1 to n ) may be the software or the virtual machine which operates on an external server to which each of the switches 20 ( 20 - i , i = 1 to n ) can access . also , the encapsulating module 21 ( 21 - i , i = 1 to n ) may be an exclusive use device , an expansion board or a peripheral device which is installed in the physical machine , an intermediate equipment ( middle box ) on the network in addition to the software . however , actually , the present invention is not limited to these examples . the controller 10 previously sets “ the encapsulation of a packet in a predetermined flow ” to the encapsulating module 21 - 1 of the switch 20 - 1 . specifically , the controller 10 previously sets “ duplicating the ip header of the packet in a predetermined flow , encapsulating a packet with the same header as the ip header , and transferring the encapsulated packet ” to the encapsulating module 21 - 1 of the switch 20 - 1 . in this case , an example of using the ip header is described , but the ip header is only an example of the header . actually , all the headers from layer 2 to layer 4 may be dealt with without limiting to the ip header of layer 3 . specifically , “ the ip header ” may be read as “ a part or all of data which can function as a rule of the flow entry ”. also , the controller 10 previously sets “ transmitting a pair of a header before translation and a header after the translation as header translation data to the controller and cancelling capsulation of a received packet by removing the header after the translation if there is the header after the translation , and replacing the header before the translation with the header after the translation if there is the header before the translation ” to the encapsulating module 21 - 2 of the switch 20 - 2 . it should be noted that “ removing all the headers from the reception packet once and then giving only the header after the translation ” may be set . also , “ removing the second stage header ( the header before the translation ) in the reception packet if there is it ” may be set . next , the switch 20 - 1 hands an inputted packet to the encapsulating module 21 - 1 . here , the inputted packet has the structure of “ the ip header 1 - the payload ”. the encapsulating module 21 - 1 duplicates the ip header 1 of the packet as set from the controller 10 , encapsulates the packet with the same header as the ip header 1 and transfers the packet . that is , the ip header 1 is given to the payload doubly ( two stages ) in the transferred packet . therefore , the packet transferred from the switch 20 - 1 has the structure of “ the ip header 1 - the ip header 1 - the payload ”. in this case , “ the ip header 1 ” at the head of “ the ip header 1 - the ip header 1 - the payload ” is the ip header of the encapsulated packet , and the remaining part of “ the ip header 1 - the payload ” ( original packet ) is a payload of the encapsulated packet . it should be noted that as for the encapsulation of the packet , for example , a method such as gre ( generic routing encapsulation ) shown in rfc1701 and rfc2784 may be used . next , the header translating unit 30 refers to the ip header 1 of the inputted packet , carries out the address translation processing to translate the ip header 1 into the ip header 2 . for example , when the destination ip address is a private address of “ 192 . 168 . 0 . 10 ”, the header translating unit 30 translates the ip header 1 of the inputted packet into the ip header 2 having the global address of “ 10 . 0 . 0 . 10 ” as the destination ip address . in this way , the header translating unit 30 translates the ip header 1 of the inputted packet into the ip header 2 having a different value and transfers the packet . in this case , the header translating unit 30 removes or translates a head part of the ip header 1 given doubly , and then transfers the packet after after giving the ip header 2 to the payload . therefore , the transferred packet has the structure of “ the ip header 2 - the ip header 1 - the payload ”. that is , when receiving the encapsulated packet from the switch 20 - 1 , the header translating unit 30 translates “ the ip header 1 ” of “ the ip header 1 - the ip header 1 - the payload ” at the head of of the ip header of the encapsulated packet into “ the ip header 2 ”. in this case , “ the ip header 2 ” at the head of “ the ip header 2 - the ip header 1 - the payload ” is set as the ip header of the transferred packet , and the remaining part ( original packet ) of “ the ip header 1 - the payload ” is set as the payload of the transferred packet . next , the switch 20 - 2 hands an inputted packet to the encapsulating module 21 - 2 . “ transmitting a pair of the header after the translation and the header before translation as header translation data to the controller , carrying out processing of removing the header after the translation if it is present , and replacing the header before the translation with the header after the translation if it is preset ” is set to the encapsulating module 21 - 2 from the controller 10 . or , “ giving only the header after the translation after removing all the headers from the reception packet once ” may be set . or , “ if there is a second - stage header ( the header before the translation ) in the reception packet , removing it ” may be set . in this case , the encapsulating module 21 - 2 checks whether or not the inputted packet is an encapsulated packet . if being the encapsulated packet , the encapsulating module 21 - 2 refers to the ip header 2 and the ip header 1 of “ the ip header 2 - the ip header 1 - the payload ”, and generates header translation data based on a pair ( or set ) of them and transmits it to the controller 10 . in the above example , the header translation data indicates that the source ip address of “ 192 . 168 . 0 . 10 ” is changed to “ 10 . 0 . 0 . 10 ”. also , the encapsulating module 21 - 2 removes the ip header 2 at the head of “ the ip header 2 - the ip header 1 - the payload ” to decapsulate the inputted encapsulated packet , and replaces “ the ip header 1 ” with “ the ip header 2 ” of the remaining portion of “ the ip header 1 - the payload ” ( original packet ). in this way , the encapsulating module 21 - 2 removes the ip header 1 from the original packet and adds the ip header 2 to the payload , and then transfers the packet . therefore , the configuration of the transferred packet is of “ the ip header 2 - the payload ”. by operating as mentioned above , the controller 10 can acquire the header translation data without improving the header translating unit 30 , and trace the flow by using the data . it should be noted that the controller 10 itself may carry out the processing of the switch 20 ( 20 - i , i = 1 to n ) (“ encapsulation ” on the transmission side and “ decapsulation on the reception side ”). in this case , the controller 10 cooperates with the encapsulating module 21 ( 21 - i , i = 1 to n ). also , the controller 10 and the encapsulating module ( 21 - i , i = 1 to n ) may be an identical unit . the switch 20 ( 20 - i , i = 1 to n ) transmits the whole of inputted packet to the controller 10 and receives the processing result as a response . for example , it is supposed that the controller 10 cooperates with the encapsulating module 21 - 1 and the encapsulating module 21 - 2 in fig1 , and that they are unified . the switch 20 - 1 transmits “ the reception packet ” to the controller 10 and receives “ an encapsulated packet ” from the controller 10 as the response . also , the switch 20 - 2 transmits the “ encapsulated packet ” to the controller 10 and receives a “ packet having only the translated header ” from the controller 10 as the response . therefore , if the necessary processing can be carried out in a necessary place , the location of the encapsulating module 21 ( 21 - i , i = 1 to n ) may be on the side of the controller 10 or on the side of the switch 20 ( 20 - i , i = 1 to n ). of course , the processing may be carried out in both of the switch 20 ( 20 - i , i = 1 to n ) and the controller 10 . for example , usual packets are processed on the switch 20 ( 20 - i , i = 1 to n ), and packets of an important flow may be processed on the controller 10 . at this time , the encapsulating module 21 ( 21 - i , i = 1 to n ) may operate on a computer which can communicate with each of the controller 10 and the switches 20 ( 20 - i , i = 1 to n ). also , an example that the ip packet is translated by using nat has been described here for simplification of description . a case that a mac frame is translated by using mat ( mac address translation ) and a case that a tcp / udp packet is translated by using napt can be carried out in the same way . not only the ip header of layer 3 but also the headers from layer 2 to layer 4 can be dealt with for the translation . specifically , “ the ip header ( or header )” in the above description of the present invention is read as “ a part or whole of data for a rule of a flow entry ”. referring to a flow chart of fig2 , an operation of the encapsulating module 21 - 1 will be described . the encapsulating module 21 - 1 receives a packet . in this case , the configuration of the reception packet is of “ the ip header 1 - the payload ”. next , the encapsulating module 21 - 1 checks whether or not the reception packet is a packet of an encapsulation target . in case of being the packet of the encapsulation target , the encapsulating module 21 - 1 refers to the ip header 1 , duplicates the ip header 1 and carries out encapsulation by using the same ip header 1 . in this case , the configuration of the packet is of “ the ip header 1 - ip header 1 - the payload ”. the encapsulating module 21 - 1 transfers the packet through the switch 20 - 1 . in case of being the packet of the encapsulation target , the configuration of the packet is of “ the ip header 1 - the ip header 1 - the payload ”. in case of not being the packet of the encapsulation target , the configuration of the packet is of “ the ip header 1 - the payload ”. the header translating unit 30 translates the ip header of the transferred packet . in case of being the encapsulated packet , the header translating unit 30 translates the configuration of the packet from “ the ip header 1 - the ip header 1 - the payload ” into “ the ip header 2 - the ip header 1 - the payload ”. in case of not being the encapsulated packet , the header translating unit 30 translates the configuration of the packet from “ the ip header 1 - the payload ” into “ the ip header 2 - the payload ”. because the processing of the header translating unit 30 is not processing peculiar to the present invention but general header translation processing , the description is omitted . next , referring to a flow chart of fig3 , the processing of the encapsulating module 21 - 2 will be described . next , the encapsulating module 21 - 2 checks whether or not the reception packet is an encapsulated packet . in case of being the encapsulated packet , the configuration of the packet is of “ the ip header 2 - the ip header 1 - the payload ”. in case of not being the encapsulated packet , the configuration of the packet is of “ the ip header 2 - the payload ”. in case of being the encapsulated packet , the encapsulating module 21 - 2 refers to the ip headers 2 and the ip headers 1 and generates header translation data . the header translation data is a pair ( set ) of “ the ip header 1 ” before the translation and “ the ip header 2 ” after the translation . next , the encapsulating module 21 - 2 transmits the generated header translation data to the controller 10 . next , the encapsulating module 21 - 2 removes “ the ip header 2 ” from the encapsulated packet of “ the ip header 2 - the ip header 1 - the payload ” and replaces “ the ip header 1 ” of “ the ip header 1 - the payload ” with “ the ip header 2 ”. at this time , the encapsulating module 21 - 2 retains “ the ip header 2 ”, which has been first removed , as the header after the translation , removes “ the ip header 1 ” as the header before the translation from “ the ip header 1 - the payload ”, and adds “ the ip header 2 ” as the header after the translation to “ the payload ”. that is , “ the ip header 1 ” as the header before the translation is substituted by “ the ip header 2 ” as the header after the translation . or , after all the headers of “ the ip header 2 - the ip header 1 ” are removed from the encapsulated packet of “ the ip header 2 - the ip header 1 - the payload ” once , only the header of “ the ip header 2 ” is added to the payload after the translation . or , processing is carried out to remove the second header (“ the ip header 1 ” as the header before the translation ) from the reception packet . thus , the configuration of the packet is of “ the ip header 2 - the payload ”. the encapsulating module 21 - 2 transfers the packet through the switch 20 - 2 . the configuration of the packet is of “ the ip header 2 - the payload ”. it should be noted that it is enough that the controller 10 can acquire the header translation data once . the encapsulation of the packet may be stopped after the acquisition . as mentioned above , by the encapsulating module 21 - 1 and the encapsulating module 21 - 2 carrying out encapsulation of the packet and acquisition of the header translation data , the controller grasps a corresponding relation of the flows before the translation and after the translation , and the trace of the flow can be carried out even when passing through the header translating unit . in the present invention , a technique is disclosed which identifies flows in the front and back of the header translating unit by using the encapsulating function of the packet which is installed in “ openv switch ” as one of the switches in the open flow . in the present invention , the switch in the front of the header translating unit duplicates the same header as the header of a packet and encapsulates the packet , as shown in fig4 . here , it is supposed that the status that the packet pkt 0 has the header h 0 and the payload p 0 is represented as being “ the packet pkt 0 =( h 0 , p 0 )”. when receiving the packet pkt 0 , the switch s 0 refers to the header h 0 to duplicate the same header h 0 , and encapsulates the packet pkt 0 to generate the packet pkt 1 . that is , because the packet pkt 1 has the header h 0 and the payload ( h 0 , p 0 ), the packet pkt 1 is represented as “ the packet pkt 1 =( h 0 , ( h 0 , p 0 ))”. supposing that the packet after this packet pkt 1 passes through the header translating unit to is pkt 2 , the packet pkt 2 is represented as “ the packet pkt 2 =( h 1 , ( h 0 , p 0 ))” because the header of the packet pkt 2 is changed from the header h 0 into header h 1 . next , when receiving the packet pkt 2 , the switch s 1 compares ( compares and refers ) the header h 1 and the header h 0 in the payload . thus , the switch s 1 recognizes that the original header h 0 is changed into the header h 1 by the header translating unit to . the switch s 1 decapsulates the packet pkt 2 , removes the header h 1 from the packet pkt 2 , replaces the header h 0 in the payload with the header h 1 , and forms the packet pkt 3 . that is , the switch s 1 replaces “ the packet pkt 2 =( h 1 , ( h 0 , p 0 ))” with “ the packet pkt 3 =( h 1 , p 0 )”. after that , the switch s 1 transmits the packet pkt 3 . in this way , the present invention can acquire the corresponding relation of the flows in the front and back of the header translating unit without remodeling the header translating unit to . therefore , the trace of the flow becomes possible . at mentioned above , in the present invention , a packet is encapsulated in the switch ( in the controller which receives a request from the switch ). in such a case , by encapsulating by using the same header as the current header , two kinds of headers before the translation and after the translation are attached to the packet after the packet has passed through the network appliance . the trace of the flow becomes possible using this . in the present invention , in the flow - based network like the open flow network system , by encapsulating a packet by using the same header as the current packet header , even when passed through a network appliance such as nat and napt for changing the packet header , the corresponding relation of two flows before and after the passage can be grasped and the trace of the flows in end - to - end becomes possible . also , in the present invention , because only the packet header given by the encapsulation is translated and the translation data can be acquired by comparing with the packet header before the translation , the acquisition of the translation data of the packet header and the trace of the flows become possible without remodeling the network appliance by which a packet header is translated . a part or all of the above - mentioned exemplary embodiments can be mentioned like the following supplemental notes . however , actually , the present invention is not limited to the following specified examples . a switch having a function of ) searching a flow table which stores an entry defining a rule and an operation to uniformly control packets of a flow and carrying out the operation defined in the entry , to the reception packet matching the rule defined in the entry in case of the packet reception ; a controller having a function to receive an inquiry of the packet from the switch , and to set the entry to the flow table of the switch ; a header translating unit having a function to translate header data at a head of the reception packet and to transfer the packet , in case of the packet reception ; and an encapsulating module having a function to check whether or not the packet received by the switch is an encapsulated packet ; to duplicate header data of the packet in case of not being the encapsulated packet , to give the duplicated header data to the reception packet to encapsulate and to hand to the switch ; to remove header data at a head of the encapsulated packet and to replace header data in a remaining part with the removed header data to hand to the switch in case of being the encapsulated packet ; and to notify a set of the removed header data and the header data in the remaining part to the controller as header translation data . the network system according to supplemental note 1 , wherein the encapsulating module has a function to duplicate a first header which is the header data of the reception packet of a first switch when the switch is the first switch in a front of the header translating unit , and to give the first header to the reception packet to transfer a packet , which has a configuration called “ the first header - the first header - the payload ” in which the first header is given doubly to the payload , through the first switch as the encapsulated packet , wherein the header translating unit has a function to translate the first header at a head of the encapsulated packet into a second header when receiving the packet from the first switch , and to generate and transfer a packet which has a configuration called “ the second header - the first header - the payload ”, and wherein the encapsulating module has a function to refer to header data of a reception packet of a second switch to notify a set of the first header and the second header to the controller as the header translation data , when the switch is the second switch in a back of the header translating unit , to remove the second header at a head of the reception packet , to replace the first header in a remaining part with the second header , to generate a packet with a configuration called “ the second header - the payload ” and to transfer through the second switch . the network system according to supplemental note 1 , wherein the encapsulating module is provided on a side of at least one of the switch and the controller , and has a function to receive the packet directly from the switch when provided on the side of the switch , and to receive the packet from the switch through the controller when provided on the side of the controller . a computer having a function as an encapsulating module , and includes : a unit for checking whether or not the reception packet is an encapsulated packet ; a unit for duplicating header data of the packet when the reception packet is not the encapsulated packet ; a unit for giving the duplicated header data to the reception packet for encapsulation ; a unit for removing header data at a head of the encapsulated packet when the reception packet is the encapsulated packet ; a unit for replacing the header data in the remaining part of the encapsulated packet with the removed header data ; and a unit for generating header translation data based on a set of the header data in the remaining part and the removed header data . as such , the exemplary embodiments of the present invention have been described in detail . however , actually , the present invention is not limited to the above - mentioned exemplary embodiments and a modification which does not deviate from the features of the present invention is contained in the present invention . it should be noted that the present application claims a priority based on japan patent application no . jp 2011 - 031752 . the disclosure thereof is incorporated herein by reference .