Patent Publication Number: US-9906438-B2

Title: Communication node, control apparatus, communication system, packet processing method, communication node controlling method and program

Description:
DESCRIPTION OF RELATED APPLICATION 
     The present application asserts priority rights based on JP Patent Application 2012-276733 filed in Japan on Dec. 19, 2012, the total contents thereof being incorporated by reference into the present application. 
     TECHNICAL FIELD 
     This invention relates to a communication node, a control apparatus, a communication system, a packet processing method, a communication node controlling method and a program. More particularly, it relates to a communication node, a control apparatus, a communication system, a packet processing method, a communication node controlling method and a program, in which a packet is processed in accordance with contents obtained on making an inquiry at the control apparatus. 
     BACKGROUND 
     A technique known as OpenFlow has been proposed in Patent Literatures 1 to 3 and in Non-Patent Literatures 1, 2. The OpenFlow comprehends communication as an end-to-end flow and manages path control, recovery from malfunctions, load balancing and optimization on the flow-by-flow basis. An OpenFlow switch, specified in Non-Patent Literature 2, includes a secure channel over which to communicate with an OpenFlow controller, and operates in accordance with a flow table an addition to or a rewriting in which is instructed from time to time from the OpenFlow controller. In the flow table, a set of match conditions (Match Fields) to be matched against a packet header, flow statistics information (Counters) and instructions that define the processing contents (Instructions) is defined from one flow to the next. See ‘5.2 Flow Table’ of Non-Patent Literature 2. 
     On receipt of a packet, the OpenFlow switch searches from the flow table an entry having the match condition conforming to the header information of the received packet. See ‘5.3 Matching’ of Non-Patent Literature 2. If, as a result of the search, the entry matched to the received packet is found, the OpenFlow switch updates the flow statistics information (Counter), at the same time as it executes processing contents stated in an instruction field of the matched entry, such as packet transmission at a specified port, flooding or dropping. If conversely no entry matched to the received packet is found, the OpenFlow switch sends an entry setting request, that is, a request for transmission of the control information to process the control information (Packet-In message), to the OpenFlow controller over the secure channel. The OpenFlow switch receives the flow entry, in which processing contents are stated, and updates the flow table. In this manner, the OpenFlow switch uses the entry, stored in the flow table, as the control information, in order to forward the packet. 
     Patent Literature 1 
     
         
         Reissued WO2010/103909 
       
    
     Patent Literature 2 
     
         
         Japanese Patent Kokai Publication No. JP2011-166384A 
       
    
     Patent Literature 3 
     
         
         Japanese Patent Kokai Publication No. JP2011-146982A 
       
    
     NON-PATENT LITERATURE 
     Non-Patent Literature 1 
     
         
         Nick McKeown and three others: “OpenFlow: Enabling Innovation in Campus Networks”, [online], [searched on November 24, Heisei24 (2012), Internet URL: http://www.openflow.org/documents/openflow-wp-latest.pdf 
       
    
     Non-Patent Literature 2 
     
         
         “OpenFlow Switch Specification”, Version 1.3.1 (Wire Protocol Ox04), [online], [searched on December 11, Heisei24 (2012), Internet URL: https://www.opennetworking.org/images/stories/downloads/specification/openflow-spec-v1.3.1.pdf 
       
    
     SUMMARY 
     The following analysis is given by the present invention. In case there is no entry in a flow table of an OpenFlow switch matching to the header information of a received packet, the OpenFlow switch requests a control apparatus, termed an OpenFlow controller, to set a flow entry in the switch, that is, to send the control information (Packet-In message) thereto. This is shown in the Patent Literatures 1 through to 3 and in Non-Patent Literatures 1, 2. In case it is instructed, in an entry found out as a result of flow table search, that a request is to be made to the OpenFlow controller to set a flow entry, the OpenFlow switch likewise requests the OpenFlow controller to set the flow entry. 
     Thus, if larger numbers of packets, consistent with the condition of making flow entry setting requests as described above, are received, the OpenFlow switch makes a flow entry setting request to the OpenFlow controller each time such packet is received. As a result, such a problem may arise in which the above described situation pressures the band of a secure channel provided between the OpenFlow switch and the OpenFlow controller. In addition, if the OpenFlow controller takes charge of larger numbers of the OpenFlow switches, another problem may also arise in which the load on the OpenFlow controller increases to deteriorate e.g., the response performance. By the way, it is stated in ‘6. OpenFlow Channel’ of Non-Patent Literature 2 that the secure channel uses TLS over TCP (Transport Layer Security over Transmission Control Protocol). 
     It is an object of the present invention to provide a communication node, a control apparatus, a communication system, a packet processing method, a communication node controlling method and a program which will contribute to solution of a variety of problems derived from sending a control information entry setting request triggered by reception of a preset packet according to the technique termed the OpenFlow. 
     In a first aspect, there is provided a communication node comprising an entry memory capable of retaining a control information entry stipulating a processing to be applied to a packet received, a packet processor referencing the entry memory to process the packet received, and a request suppressing unit. Triggered by reception of a packet, the request suppressing unit requests a preset control apparatus to set the control information entry and subsequently suppresses a control information entry setting request(s) for a preset time for such packet(s) having a characteristic in common with the packet for which the control information entry setting request was made to the preset control apparatus. 
     In a second aspect, there is provided a control apparatus that controls a communication node including an entry memory capable of retaining a control information entry stipulating a processing to be applied to a packet received and a packet processor referencing the entry memory to process the packet received. The control apparatus includes a request suppressing unit that suppresses a control information entry setting request(s) from a controller of the communication node for a preset time as concerns a packet(s) having a characteristic in common with the packet for which the control information entry setting request was made from the communication node. 
     In a third aspect, there is provided a communication system comprising a control apparatus that causes a control information entry to be set in a pertinent communication node in response to a control information entry setting request from a communication node, and the communication node(s). The communication node includes an entry memory capable of retaining the control information entry that stipulates a processing to be applied to a packet received, a packet processor that references the entry memory to process the packet received, and a request suppressing unit. Triggered by reception of a packet, the request suppressing unit requests a preset control apparatus to set the control information entry and subsequently suppresses a control information entry setting request(s) for a preset time for such packet(s) having a characteristic in common with the packet for which the control information entry setting request was made to the preset control apparatus. 
     In a fourth aspect, there is provided a method for processing a packet comprising requesting, in response to reception of a first packet, for which a control information entry setting request is to be made to a preset control apparatus, the preset control apparatus to set a control information entry, and dropping a second packet received within a preset time as from reception of the first packet in case the second packet has a characteristic in common with the first packet. The present method is bound up with a specified machine which is a communication node that references the control information entry to process the packet. 
     In a fifth aspect, there is provided a method for controlling a communication node(s) comprising: a control apparatus configured to control the communication node(s) setting a control information entry in a pertinent communication node in response to a control information entry setting request from the communication node; and the control apparatus controlling the communication node so as to suppress the control information entry setting request(s) for a preset time as concerns a packet(s) having a characteristic in common with the packet for which the control information entry setting request was made. The present method is bound up with a particular machine which is a control apparatus that controls the communication node. 
     In a sixth aspect, there is provided a program for implementing the functions of the above mentioned communication node or control apparatus. It is noted that the program may be recorded on a computer-readable non-transient recording medium. That is, the present invention may be implemented as a computer program product. 
     The present invention may contribute to solution of problems derived from transmission of a control information entry setting request triggered by reception of a preset packet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of an exemplary embodiment. 
         FIG. 2  is a table for illustrating the operation of the exemplary embodiment. 
         FIG. 3  is a block diagram showing a configuration of an exemplary embodiment 1. 
         FIG. 4  is a table showing an example flow entry retained by a flow table of a communication node according to the exemplary embodiment 1. 
         FIG. 5  is a flowchart showing the operation of a communication node of the exemplary embodiment 1. 
         FIG. 6  is a table showing an example flow entry added to a flow table by a controller of the communication node of the exemplary embodiment 1. 
         FIG. 7  is a sequence diagram showing a global operation of the exemplary embodiment 1. 
         FIG. 8  is a block diagram showing a configuration of an exemplary embodiment 2. 
         FIG. 9  is a table showing an example configuration of a suppressive cache of a communication node of the exemplary embodiment 2. 
         FIG. 10  is a block diagram showing a configuration of an exemplary embodiment 3. 
         FIG. 11  is a block diagram showing a configuration of an exemplary embodiment 4. 
         FIG. 12  is a block diagram showing a modification of the exemplary embodiment 4. 
     
    
    
     PREFERRED MODES 
     A summary of a preferred mode of the present disclosure will now be described with reference to the drawings. It is noted that symbols are entered in the summary merely as examples to assist in understanding and are not intended to limit the present disclosure to the mode illustrated. 
     In a mode, the present disclosure may be implemented by a control apparatus  10  and a communication node(s)  20 A, as shown in  FIG. 1 . The control apparatus  10  sets, in a pertinent communication node, a flow entry (a control information entry) stipulating the processing to be applied to a received packet, in response to a request from the communication node  20 A for setting the flow entry therein. The communication node references the flow entry, as set by the control apparatus  10 , so as to process the packet. 
     More specifically, the communication node  20 A includes an entry memory  21 A, capable of holding the flow entry or entries, a packet processor  22 A that references the entry memory  21 A to process a packet received, and a request suppressing unit  23 A. The request suppressing unit  23 A operates, after requesting the control apparatus to set a flow entry, with reception of a packet as a clue, so as to suppress a flow entry setting request(s) for a preset time as concerns a packet(s) having a characteristic(s) in common with the packet for which the flow entry setting request was made to the preset control apparatus. 
     To suppress the request suppressing unit  23 A from making the flow entry setting request(s), such a method may be used in which a flow entry (second flow entry) is set in the entry memory  21 A, as shown for example in  FIG. 2 . In this second flow entry, a match condition to capture a packet having a characteristic(s) identical with that of or in common with the packet, for which the control apparatus  10  was requested to set the flow entry, is correlated with an instruction to drop the packet conforming to the match condition. The match condition may, for example, be a destination IP address=an IP address of a server A if a request was already made to set a flow entry as concerns a packet destined to be sent to the server A. To terminate suppression of flow entry setting requests after lapse of a certain preset time, such a scheme may be used in which the time duration elapsed as from the time of setting the second control information entry in the flow entries shown in  FIG. 2  is measured and the second control information entry is deleted after lapse of the time duration measured. 
     Also, in place of setting the second control information entry, such an implementation in which the request suppressing unit  23 A filters the flow entry setting requests transmitted from the packet processor  22 A may be used. Or, such an implementation may be used in which a cache stated as below retaining a characteristic(s) of a packet for which a flow entry setting request was already made is provided so as to suppress the flow entry setting requests. 
     Exemplary Embodiment 1 
     An exemplary embodiment 1 will now be described with reference to the drawings.  FIG. 3  shows a configuration of the exemplary embodiment 1. There are shown in  FIG. 3  a communication node  20 , connected to a communication node B and a communication node C, and a control apparatus  10  that controls the communication nodes  20  over a secure channel indicated by broken lines in  FIG. 3 . 
     The control apparatus  10  is such apparatus that controls the communication node  20  by setting a flow entry in a flow table of the communication node. As such control apparatus, an OpenFlow controller, disclosed in the Non-Patent Literatures 1, 2, can be used as an example. 
     The communication node  20  includes a flow table  21 , a packet processor  22 , a controller  23  and a secure channel connection unit  25 . Such communication node  20  may, besides the OpenFlow switch of the Non-Patent Literatures 1, 2, be a personal computer or a smartphone provided with a function of processing a packet entered to or output from an upper order application layer as an example. 
     The flow table  21  is such a table holding a flow entry or entries in which the match condition for identifying a packet being processed is correlated with the packet conforming to the match condition. 
       FIG. 4  shows example flow entries to be retained by the flow table  21 . In the example flow entries, shown in  FIG. 4 , priority levels, match conditions and instructions, as processing contents, are correlated with one another. The ‘priority level’ is referred to for selecting a flow entry in case the match condition of each of a plurality of flow entries conforms to the packet received. For example if, in the case of  FIG. 4 , the communication node  20  has received a packet a transmission source IP address of which is an IP address of a server B and which is destined to be sent to a server B, search is made, as a result of which a ‘high priority’ flow entry at the upper most field and the flow entry at the lower most field with the ‘match condition=All Any’ are selected as candidates. In this case, the priority levels of the two are compared to each other and the upper most flow entry of ‘High Priority Level’ is selected. If, on the other hand, the communication node  20  has received a packet destined to be sent to a server A, the lower most field flow entry in the flow table  21  of  FIG. 4  is selected, and a request is made for setting the flow entry to the control apparatus  10  (Output OFC (=OpenFlow Controller)). 
     The packet processor  22  processes a packet received by referencing the flow table  21  in which there are stored the above mentioned flow entries. 
     It is now assumed that a number of flow entry setting requests to the control apparatus  10  were made by the packet processor  22  over the secure channel connection unit  25 . In this case, the controller  23  sets in the flow table  21  a flow entry that causes dropping of such a packet that has a priority level higher than at least the flow entry for which a flow entry setting request is to be made to the control apparatus  10  (the flow entry of ‘low priority level’ at the lower most field in  FIG. 6 ) and that has the same header information as that of the packet for which the flow entry setting request was made to the control apparatus  10 . A timeout value is set in this flow entry, such that, after lapse of a preset time, the flow entry is automatically deleted. 
     The secure channel connection unit  25  performs the processing of transmitting a flow entry setting request to the control apparatus  10  or the processing of receiving a flow entry setting message from the control apparatus  10  to deliver the message received to the controller  23 . It is noted that an OpenFlow protocol disclosed in Non-Patent Literature 2 may be used in exchanging the control message or the request between the communication node  20  and the control apparatus  10 . 
     Although the specified components of the communication nodes B, C are not shown in detail in  FIG. 3 , the components of these communication nodes B, C may be the same as those of the communication node  20 . 
     The operation of the present exemplary embodiment will now be described in detail with reference to the drawings.  FIG. 5  depicts a flowchart showing the operation at the time of reception of a packet at the communication node  20  in the exemplary embodiment 1. Referring to  FIG. 5 , the communication node  20  references the flow table  21  to search for an upper order (higher priority) flow entry having a match condition(s) conforming to the received packet (step S 001 ). If the upper order (higher priority) flow entry has been found, the communication node executes the processing contents stated in an instruction field of the flow entry, such as packet forwarding at a specified port or header rewriting (step S 002 ). 
     If, as a result of search at the flow table  21 , no upper order (higher priority) flow entry has been found, the communication node  20  increments a counter provided in for example the controller  23  (step S 003 ). 
     The communication node  20  then decides whether or not the value of the counter has exceeded a preset value (step S 004 ). If the counter value is not more than the preset value (No of the step S 004 ), the communication node  20  sends the received packet to the control apparatus  10  to request the control apparatus to set a flow entry (step S 005 ). 
     If the counter value exceeds the preset value (Yes of the step S 004 ), the communication node  20  registers a flow entry setting request suppressing flow entry (a flow entry suppressing a flow entry setting request) in the flow table  21  (step S 006 ). It is noted that the counter value is cleared when a preset time has elapsed or when the state where a flow entry setting request is not made has continued for a preset time 
       FIG. 6  depicts a flow table in which there is registered, by the controller of the communication node according to the exemplary embodiment 1, an example setting request suppressing flow entry, indicated by a broken line. If, for example, no upper order flow entry having the match condition conforming to the packet received from the server A has been set in the flow table  21 , the communication node  20  increments the counter and requests the control apparatus  10  to set a flow entry. Thereafter, if, before the flow entry to process the packet received from the server A is sent from the control apparatus  10 , a preset number of packets has been received from the server A, the communication node  20  sets a flow entry shown at a second row from above of the flow table of  FIG. 6 . As shown therein, this flow entry is such a one that instructs dropping the packet in which the transmission source IP address is the server A (setting request suppressing flow entry). In this manner, the processing of dropping the packet(s), whose transmission source IP address is the server A&#39;s IP address, is carried out for a certain time. Of course, if the packet received is a packet belonging to a different flow, for example, a packet whose transmission source IP address is a server B&#39;s IP address, the packet is forwarded in accordance with the upper most row flow entry of  FIG. 6 . If a packet received is an unknown one whose transmission source IP address is a server C, as an example, it is a target packet of the setting request suppressing flow entry and remains so until such time the counter is cleared. Hence, the packet is dropped. 
       FIG. 7  depicts a sequence diagram showing the global operation of the exemplary embodiment 1. For example, if a new packet devoid of a corresponding flow entry in the flow table  21  is received from an apparatus of the transmission source, such as another communication node, terminal or server, the communication node  20  requests the control apparatus  10  to set a flow entry (Packet-In of  FIG. 7 ). 
     Thereafter, if a count value of the counter has exceeded a preset value, the communication node  20  registers, in the flow table  21 , a flow entry that causes dropping of those packets for each of which flow entry setting was already requested to the control apparatus  10  (setting request suppressing flow entry). Thereafter, even if the communication node  20  should receive a packet(s), for which the flow entry setting request was already made, the communication node causes such packet(s) to be dropped based on the setting request suppressing flow entry. This suppresses flow entry setting requests for a certain time interval. If thereafter a flow entry setting message is sent from the control apparatus  10 , the communication node  20  sets the flow entry in the flow table  21 . Thereafter, the packet(s) is processed in accordance with the flow entry sent from the control apparatus  10  so as to be forwarded to a destination apparatus. 
     In the subject exemplary embodiment, described above, the frequency with which the flow entry setting requests are issued to the control apparatus  10  may be diminished to avoid pressuring the secure channel. Moreover, the load on the control apparatus  10  per communication node may be suppressed, so that it becomes possible to increase the number of communication nodes that may be handled by the control apparatus  10 . 
     In addition, in the above described exemplary embodiment 1, a preset number of times of flow entry setting requests is allowed through use of the counter, so that, even if no flow entry setting request has reached the control apparatus  10  by some or other reason, the request may get to the control apparatus  10  by a second request, a third request and so on. By doing as described above, a flow entry for a set of a plurality of packets can also be set in the control apparatus  10 . 
     Of course, if the above mentioned precautions are unnecessary, the value for comparison to the count value of the counter may be set to 1 for a case where the initial value of the counter is 0. In this case, the packet(s) for which the flow entry setting request has once been made is dropped, beginning from the next time packet, thus appreciably decreasing the load otherwise imposed on the control apparatus  10 . 
     Exemplary Embodiment 2 
     An exemplary embodiment 2, which has modified the above described exemplary embodiment 1, will now be described with reference to the drawings. In the subject exemplary embodiment 2, a dedicated cache is provided in place of registering a setting request suppressing flow entry in the flow table  21 . The dedicated cache is used for deciding whether or not a packet is a target for a flow entry setting request. Since the subject exemplary embodiment is otherwise similar in configuration to the exemplary embodiment 1, the following description is centered about this point of difference from the exemplary embodiment 1. 
       FIG. 8  depicts a configuration of the exemplary embodiment 2. The subject exemplary embodiment 2 differs from the exemplary embodiment 1, shown in  FIG. 3 , in that a suppressing cache  232  connecting to the controller  23 B is provided, and in that, as a result of providing the suppressing cache, the operation of the controller  23 B differs from that of the exemplary embodiment 1. 
     Referring to  FIG. 9 , the suppressing cache  232  is made up of a memory etc. capable of holding, for a preset time, an entry showing the correlation between a match condition and a timer field. The entry can be maintained for a preset time. The match condition uses e.g. the header information of a packet for which a flow entry setting request was made in order to discriminate the packet for which the flow entry setting request was made. The timer field indicates the time elapsed as from start of initiating the suppressive processing subject to the match condition. 
     In place of setting the setting request suppressing flow entry in the flow table  21  as in the exemplary embodiment 1, the controller  23 B of the subject exemplary embodiment executes flow entry setting request suppressive processing by referencing the suppressing cache  232 . Specifically, before making a request to the control apparatus  10  for setting a flow entry for the received packet, the controller  23 B checks to see if the received packet conforms to the match condition registered in the suppressing cache  232 . If the received packet conforms to the match condition registered in the suppressing cache  232 , the received packet is dropped. 
     The controller  23 B of the subject exemplary embodiment also performs the processing of deleting the entry which is retained by the suppressing cache  232  and the timer field value of which has exceeded a preset time. This allows controlling the time interval of suppression of the flow entry setting request from one flow to another. 
     In the subject exemplary embodiment, as in the exemplary embodiment 1, it is possible to remove problems such as pressuring of the secure channel or the necessity of restricting the number of communication nodes that can be handled by the control apparatus  10 . Moreover, in the subject exemplary embodiment, in which it is unnecessary to register the setting request suppressing flow entry in the flow table  21 , the flow table retained by the communication node  20  may effectively be used for its intrinsic purpose, that is, for controlling the forwarding of the packets. 
     It is noted that, although the configuration of providing a timer field in each entry is shown in the example of  FIG. 9 , such a method may also be used in which the suppressing cache is cleared in its entirety after lapse of a preset time. 
     Exemplary Embodiment 3 
     An exemplary embodiment 3, which is a combination of the above described exemplary embodiments 1 and 2, will now be described with reference to the drawings. In the subject exemplary embodiment 3, the registration of the setting request suppressing flow entry in the flow table  21  of the exemplary embodiment 1 and the suppression of the flow entry setting requests by the suppressing cache  232  of the exemplary embodiment 2 are combined together in a more desirable manner. The configuration as well as the operation of the subject exemplary embodiment is otherwise the same as that of the exemplary embodiments 1 and 2, and hence the following description is centered about the point of difference of the subject exemplary embodiment from the exemplary embodiments 1, 2. 
       FIG. 10  shows a configuration of the exemplary embodiment 3. The point of difference from the exemplary embodiment 2 shown in  FIG. 8  is that an entry setting unit  231  is provided within a controller  23 C, such that, when the suppressing cache  232  is full, registration of a setting request suppressing flow entry in the flow table  21  is commenced. That is, the communication node  20 C of the subject exemplary embodiment executes flow entry setting request suppression by the suppressing cache  232  in preference to setting request suppressing entry registration. 
     In the subject exemplary embodiment, should there be a smaller number of entries that may be retained in the suppressing cache  232  due to hardware restraints etc., such deficiency may be coped with by setting the setting request suppressing flow entry. In addition, since the suppressing cache  232  is provided in the subject exemplary embodiment, the time-out value of deletion of the setting request suppressing flow entry may be shorter than in the exemplary embodiment 1. It is thus possible to reduce the time during which the setting request suppressing flow entry remains registered in the flow table  21  even though the condition for flow request suppression has become valid. 
     Exemplary Embodiment 4 
     An exemplary embodiment 4 in which the flow entry setting request suppressing function is provided on the control apparatus side will now be described with reference to the drawings. 
       FIG. 11  shows the configuration of an exemplary embodiment 4. The subject exemplary embodiment differs from the exemplary embodiment 1 shown in  FIG. 3  in including, on the control apparatus  10 B side, a request suppressing unit  131  which is configured for setting a setting request suppressing flow entry in the flow table  21  of the communication node  20  based on a packet for which a flow entry setting request was made from the communication node  20 . 
     It is an advantage of the subject exemplary embodiment that flow entry setting requests can be suppressed without the necessity of providing the function of registering the setting request suppressing flow entries or providing the suppressing cache  232  on the communication node  20  side. It is also an advantage of the subject exemplary embodiment that setting of the setting request suppressing flow entries may be made on the control apparatus  10 B side from the perspective of controlling the entire system. For example, such setting request suppressing flow entry can be set that will be able to suppress flow entry setting requests not only for a packet for which a flow entry setting request was made from the communication node  20  but also for a packet anticipated to be received by the communication node  20  in time to come. In such case, it is possible for the control apparatus  10 B to aggregate setting request suppressing flow entries using a wildcard as a matching condition for the setting request suppressing flow entries. 
     Moreover, in the subject exemplary embodiment, the control apparatus  10 B is able to discriminate the communication necessary for the control apparatus itself as well as for global system control. Hence, the packets, the flow entry setting requests for which were received from the communication node  20 , do not have to be unanimously the targets of flow entry setting request suppression. That is, the communication that is necessary may be exempted from becoming the target of flow entry setting request suppression. 
     In the subject exemplary embodiment, the request suppressing unit  131  may be provided with a counter, as in the exemplary embodiment 1, so that, after the number of packets, for which the flow entry setting requests were received from the communication node  20 , has reached a preset value, setting of the setting request suppressing flow entry will be initiated. 
       FIG. 12  shows a modification of the exemplary embodiment 4. The configuration of the subject exemplary embodiment differs from that of  FIG. 11  in including the suppressing cache  232  in the communication node  20 B. In the configuration of  FIG. 12 , even in case the number of the entries that may be retained in the suppressing cache  232  is small due to hardware constraints, such deficiency may be coped with by setting the setting request suppressing flow entry, as stated in connection with the exemplary embodiment 3. It is thus possible to obtain beneficent effects similar to those of the above described other exemplary embodiments. 
     Moreover, in the configuration of  FIG. 12 , the setting request suppressing flow entries, necessary from the perspective of controlling the entire system, may be prepared and set on the control apparatus  10 B side and, as long as the individual events of communication, which may vary diversely in shorter time periods, are concerned, flow entry setting requests may be suppressed by the suppressing cache  232  provided on the communication node  20 B side, by way of effecting more flexible management operations. 
     It is noted that, although certain preferred exemplary embodiments of the present invention are set out above, the present invention is not limited to the exemplary embodiments illustrated, such that further changes, substitutions or adjustments may be made within the range not departing from the basic technical concept of the invention. For example, the configuration of the network or components shown in the drawings is merely illustrative to aid in the understanding of the present invention and is not meant to restrict the present invention to the configurations illustrated. 
     Also, in the explanation of the above described exemplary embodiments, it is presupposed that the match condition for the setting request suppressing flow entry is prepared based on the packet for which the communication node  20 ,  20 A,  20 B or  20 C made the flow entry setting request to the control apparatus  10 . It is noted that the communication nodes  20 ,  20 A,  20 B,  20 C or the control apparatus  10 B may include the function of aggregating the setting request suppressing flow entries with the use of a wildcard or a mask for the match conditions. For example, the setting request suppressing flow entry may be set so that packets with specified IP addresses or apparently illicit packets are collected together and exempted in their entirety from becoming the subject of flow entry setting request. 
     It should be noted that various parts (processing means) of the control apparatus and communication nodes, shown in  FIG. 3 ,  FIG. 8 ,  FIG. 10 ,  FIG. 11  and  FIG. 12 , may be implemented by a computer program which causes computers making up the control apparatus or the communication nodes to execute the above mentioned processing operations with the aid of the hardware of the respective computers. 
     Finally, certain preferred modes of the present invention will be shown by way of summary. 
     [Mode 1] 
     (Reference is made to the communication node according to the above mentioned first aspect). 
     [Mode 2] 
     The communication node according to mode 1, wherein, the request suppressing unit sets in the entry memory a second control information entry that causes dropping of a packet having a characteristic in common with the packet for which the control information entry setting request was made to the preset control apparatus, so as to suppress the control information entry setting request(s) for a preset time. 
     [Mode 3] 
     The communication node according to mode 1 or 2, further comprising: 
     a suppressing cache that retains information regarding the packet for which the control information entry setting request was made to the preset control apparatus; 
     the request suppressing unit referencing the suppressing cache to suppress the control information entry setting request(s) to the preset control apparatus. 
     [Mode 4] 
     The communication node according to mode 3, wherein, 
     suppression of the control information entry setting request(s) by the suppressing cache is carried out in preference to suppression of the control information entry setting request(s) by setting in the entry memory the second control information entry that causes dropping of the packet(s) having a characteristic in common with the packet for which the control information entry setting request was made to the preset control apparatus.
 
[Mode 5]
 
     The communication node according to any one of modes 1 to 4, wherein, the request suppressing unit commences an operation of control information entry setting suppression after a number of the packets for which the control information entry setting requests were made to the preset control apparatus has reached a preset value. 
     [Mode 6] 
     The communication node according to mode any one of modes 1 to 5, wherein, 
     a wildcard is used in a match condition for the second control information entry that causes dropping of a packet(s) having a characteristic in common with the packet for which the control information entry setting request was made to the preset control apparatus, whereby a packet(s) other than the packet that triggered the control information entry setting request is also dropped for a preset time.
 
[Mode 7]
 
     (Reference is made to the control apparatus according to the second aspect). 
     [Mode 8] 
     The control apparatus according to mode 7, wherein, 
     the request suppressing unit sets, in the entry memory of the communication node, a second control information entry that causes dropping of the packet(s) having a characteristic in common with the packet for which the control information entry setting request was made to the preset control apparatus, so as to suppress the control information entry setting request(s) for a preset time.
 
[Mode 9]
 
     The control apparatus according to mode 7 or 8, wherein, 
     the request suppressing unit commences the control information entry setting suppressing operation after a number of the packets for which the control information entry setting requests were made from the communication node has reached a preset value. 
     [Mode 10] 
     The control apparatus according to any one of modes 7 to 9, wherein, 
     a wildcard is used in a match condition in the second control information entry that causes dropping of the packet(s) having a characteristic in common with the packet for which the control information entry setting request was made, whereby the packet(s) other than the packet for which the control information entry setting request was made is also dropped for a preset time.
 
[Mode 11]
 
     (Reference is made to the above mentioned communication system according to the third aspect). 
     [Mode 12] 
     (Reference is made to the above mentioned packet processing method according to the fourth aspect). 
     [Mode 13] 
     (Reference is made to the above mentioned communication node controlling method according to the fifth aspect). 
     [Mode 14] 
     (Reference is made to the above mentioned program according to the sixth aspect). 
     It is noted that, like the mode 1, the modes 11 to 14 may be extended to the modes 2 to 6. 
     The disclosures of the above mentioned Patent Documents as well as non-Patent Documents are to be incorporated herein by reference. The exemplary embodiments or Examples may be modified or adjusted within the concept of the total disclosures of the present invention, inclusive of claims, based on the fundamental technical concept of the invention. A wide variety of combinations or selections of elements herein disclosed (elements of claims, Examples and drawings) may be made within the context of the claims of the present invention. That is, the present invention may include a wide variety of changes or corrections that may occur to those skilled in the art in accordance with the total disclosures inclusive of the claims and the drawings as well as the technical concept of the invention. In particular, it should be understood that any optional numerical figures or sub-ranges contained in the ranges of numerical values set out herein ought to be construed to be specifically stated even in the absence of explicit statements.