Description Title of Invention: COMMUNICATION SYSTEM, FORWARDING NODE, CONTROL DEVICE, COMMUNICATION CONTROL METHOD, AND PROGRAM Technical Field t000ll (CROSS-REFERENCETO RELATED APPLTCATTONS) The present invention claims priority from Japanese Patent Application 2OLL-016323 (filed on January 28,201I ) the content of which is hereby incorporated in its entirety by reference into this specification. The present invention relates to a communication system, a forwarding node, a control device, a communication contrcl method, and a program, and more particularly to a communication system having a control device for integrally controlling forwarding node(s) arranged in a networt, a forwarding node, a control device, a communication control method, and a program. Background Art t00021 In a communication system where a control device integrally controls forwarding node(s) for carrying out communication, the forwarding node(s) and the control device must be synchronized with each other. This is because, if the control device and the forwarding node(s) are not synchronized, a packet-forwarding method instruction transmitted from the control device to a forwarding node becomes inconsistent with the packet forwarding processing performed by the forwarding node, with the result that the packet forwarding not intended by the control device is performed. t00031 As a communication system where the control device integrally controls the forwarding node(s) as described above, the technology called OpenFlow is known (see Patent Literature 1 and Non Patent Literaturcs I and 2). OpenFlow identifies communications as end-to-end flows and performs path control, failure recovery, load balancing, and optimization on a per-flow basis. An OpenFlow switch, which is specified in Non Patent Literature 2, has a secure channel for communication with an OpenFlow controller that serves as a control device, and operates according to the flow table to which information is added, and whose contents are rewritten, according to an instruction from the OpenFlow controller as necessary. In the flow table, a set of the following three is defined for each flow: a matching rule (Header Fields) against which a packet header is matched, flow statistical information (Counters), and an action(s) (Actions) that defines processing contents (see FIG. 15). t00041 For example, when a packet is rcceived, the OpenFlow switch searches the flow table for an entry that has a matching rule (see Header tields in FIG. 15) that matches the wo 20r2l101689 PCT/JP201 1/005303 header information of the rcceived packet. If an entry matching the received packet is found as a result of the search, the OpenFlow switch updates the flow statistical infbrmation (Counters) and, at the same time, pertbrms the processing contents (packet transmission firom a specified port, flooding, drop, etc.), described in the Actions field of the entry, for the received packet. On the other hand, if an entry matching the received packet is not found as a result of the sealch, the OpenFlow switch forwards the received packet to the OpenFlow confioller via the secure channel, requests the OpenFlow controller to determine a packet path based on the transmission source/ destination of the received packet, receives a flow entry for the packet path, and updates the flow table. In this way, the OpenFlow switch forwards a packet using an eno!, stored in the flow table, as the processing rule. 100051 However, Patent Literature 1 and Non Patent Literatures I and2 described above do not include a practical study on how to confirm the synchronization between the OpenFlow controller and the OpenFlow switch. t00061 In addition, Patent Literature 2 discloses the technology for confirming if data held in a mobile device and data stored in the database to which the mobile device is connected are synchroniznd. According to this literature, a mobile device generates a hash for the data held in the mobile device and forwards the generated hash to the synchronization server to request it to confirm if the data is synchroniz.ed. The synchronization server generates a hash for the data for which synchronization confirmation is requested. The synchronization server compares the hash, transmitted from the mobile device, with the hash generated by ttre synchronization server. The synchronization server confirms data synchronization based on the comparison result. This literarure describes that the transmission time and the bandwidth required for synchronization confirmation may be reduced by generating a hash from the data for which synchronization confirmation is required and performing synchronization confirmation based on the hash. Citation List Patent Literature t00071 PTL 1: International Publication No. WO2008/095010 PTL2: Japanese Patent Tokuhyo Publication No. JP2009-510566,{ Non Patent Literature 100081 NPL 1: Nick McKeown and seven other authors, "OpenFlow: Enabling lnnovation in Campus Networks, [online], [Searched on December 22,2010], Intemet NPL 2: "Openflow Switch Specification" Version 1.0.0. (Wire Protocol0x0l) I S earched on Decemb er 22, 20 l0), Internet Summary of Invention Technical Problem t0009] The following analysis is given by the present invention. As described above, Patent Literature I and non-Patent Literatures I and 2 describe a technology that only provides control messages, called Stats RequesUReply, fbr retrieving the contents of flow tables, or used to transmit the contents of flow entries as a response (see "5.3.5 Read State Messages" in Non Patent Literature 2) but not a method for efficiently confirming synchronization between a forwarding node and the control device. t00101 The technology disclosed in Patent Literature 2 compares the hash data (database hashes) each of which represents respective database and, if a mismatch is detected as a result of the comparison, performs confirmation of synchronization (comparison of record hashes) per each piece of data held in a mobile device. [0011] In a communication system where a control device integrally controls forwarding nodes for can'ying out communication, synchronization confirmation, if performed by the method such as Stats Request/Reply disclosed in Non Patent Literature 2 or by the method disclosed in Patent Literature 2, will generate a problem that network traffic is generated for confrming synchronization between the control device and the forwarding node(s) and, therefore, the load of the control device or the forwarding nodes will further increase. In addition, it should be taken into consideration that an increase in the number of forwarding nodes to be controlled will result in an exponential increase in the load of the control device. t00121 In view of the foregoing, it is an object of the present invention to provide an efficient method for confirming synchronization between a forwarding node and the control device in a communication system where the control device integrally controls the forwarding nodes for communication. Solution to Problem t00l3l According to a tirst aspect of the present invention, there is provided a communication system comprising: a control device that sets a forwarding path of a packet and determines processing rules corresponding to the forwarding path; a forwalding node that includes a processing unit that processes a received packet by a processing rule transmitted from the control device, and a matching information processing unit that adds matching information. The matching information represents information on processing for the received packet, to the received packet and forwards the received packet. The communication system further comprises a confirmation unit that confirms if the forwarding node and the control device are synchronized according to whether the matching information added to the received packet matches pre-calculated wo 2012/101689 PCT/JP2011/00s303 matching information. l00l4l According to a second?spect of the present invention, there is provided a f