Relay device, time synchronization method, and program

Abstract

To facilitate time synchronization among multiple communication instruments while maintaining efficient communication.SOLUTION: A relay device 102 relays communication between a communication terminal 101 and a time server 105. The relay device 102 comprises a first communication unit 201, a second communication unit 202, a determination unit 203, an extraction unit 204 and a time counting unit 205. The determination unit 203 determines whether or not a time synchronization response is included in an IP communication frame transmitted from the time server 105, and transfers the IP communication frame to the first communication unit 201 and the extraction unit 204 when the time synchronization response is included in the IP communication frame. The extraction unit 204 extracts time data from the time synchronization response included in the IP communication frame and then notifies the time counting unit 205 of the extracted time data. The time counting unit 205 synchronizes the time counted by the time counting unit 205 based on the extracted time data with the time of the time data counted by the time server 105.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a relay device, a time synchronization method, and a program therefor. [Background technology]

[0002] Conventionally, a communication system has been known in which a communication device (communication terminal) acquires time data from a time server via a relay device (for example, a bridge device) to perform time synchronization. In one example of such a communication system, when a communication terminal performs time synchronization, the relay device transmits an acquisition request for the time data transmitted from the communication terminal to the time server. Upon receiving the acquisition request, the time server transmits the time data clocked by the device itself to a wireless network by including it in a reply frame, and this reply frame is received by the communication terminal via the wireless network and the relay device. Thereafter, the communication terminal sets the time in the device itself based on the time data included in the received reply frame, and synchronizes the time with the time server. Patent Document 1 describes an example of a network system that performs such synchronization processing. In such a communication system, when the relay device has a clock unit that clocks the time in the device itself, it is required that the relay device synchronizes the time of the device itself with the time clocked by the time server so that the relay device also clocks the correct time. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2013-168811 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in a conventional network system such as that described in Patent Document 1, when a time server that transmits a reply frame only to a communication terminal with a combination of a set of IP address and MAC address, or a base station device that allows communication only to a communication terminal with a combination of a set of IP address and MAC address, is included, only the communication terminal that transmitted the acquisition request can receive the reply frame, and a relay device that shares the IP address and MAC address with the communication terminal and relays the communication cannot perform time synchronization. In such a communication system, when both the communication terminal and the relay device want to synchronize the time, each communication device that desires to synchronize the time must have an individual IP address and transmit an acquisition request, which causes a problem that communication cannot be performed efficiently.

[0005] The present invention has been made in consideration of the above circumstances, and its main objective is to provide a relay device or the like that can reliably perform time synchronization processing while maintaining efficient communications, even in a communication system that has a time server that transmits reply frames only to communication terminals with a combination of a set of IP address and MAC address, or in a communication system that has a base station device that allows communication only with communication devices with a combination of a set of IP address and MAC address. [Means for solving the problem]

[0006] A relay device according to one embodiment of the present invention comprises a communication terminal, a relay device, and a time server, the communication terminal and the time server being connected via the relay device, and the communication terminal is a relay device in a communication system that performs time synchronization based on time data clocked by the time server and included in a communication frame transmitted by the time server, and comprises a first communication unit that communicates with the communication terminal, a second communication unit that receives the communication frame from the time server, an extraction unit that extracts time data from the communication frame, a determination unit that determines whether the communication frame received by the second communication unit contains time data and, if the time data is included, transfers the communication frame to the first communication unit and the extraction unit, and a timing unit that performs time synchronization based on the time data.

[0007] According to this, the relay device receives a communication frame sent from a time server to a communication terminal, extracts the time data stored in the communication frame, and transmits the communication frame to the communication terminal. This allows the relay device and the communication terminal to perform time synchronization using one communication frame, and the relay device, which wishes to perform time synchronization while improving communication efficiency, can easily perform time synchronization.

[0008] In the above communication system, the communication frame transmitted by the time server is a response to a time synchronization request, which is an inquiry sent from a communication terminal to the time server to synchronize the time of the communication terminal.

[0009] According to this, the relay device relays a time synchronization request transmitted from a communication terminal to a time server, receives a communication frame in which the time server responds as a response to the time synchronization request, and not only transmits the communication frame to the communication terminal, but also extracts the time data contained in the time synchronization response stored in the communication frame. This allows the relay device and the communication terminal to perform time synchronization based on one time synchronization request transmitted by the communication terminal, and allows the relay device, which wishes to perform time synchronization while improving communication efficiency, to easily perform time synchronization.

[0010] In addition, the above communication system may further include a base station device between the relay device and the time server, and the communication terminal and the relay device may share an IP address, and the base station device may be configured not to allow sharing of IP addresses among multiple communication devices.

[0011] According to this, a base station device that does not allow sharing of IP addresses communicates with each of a plurality of communication devices only by associating one IP address with one MAC address that each communication device has, whereas the relay device of the present invention extracts time data from a communication frame directed only to a terminal device. As a result, the relay device and the communication terminal can perform time synchronization based on one time synchronization request sent by the communication terminal while maintaining security in the communication system, and the relay device can further improve the efficiency of communication related to time synchronization.

[0012] Furthermore, the determination unit included in the relay device may be configured to transfer the communication frame only to the first communication unit when the communication frame does not include time data.

[0013] With this, the relay device can transmit a communication frame to the communication terminal even if the communication frame does not include time data, and normal communication is not affected.

[0014] Furthermore, the second communication unit provided in the relay device may be configured not to transmit a receipt confirmation response for the communication frame, even in the case of communication that requires a receipt confirmation response, which is a response to confirm whether the communication frame has arrived at the receiving side.

[0015] With this, the relay device can synchronize the time of its own device with the time server using the time data contained in the time synchronization response originally sent by the time server to the communication terminal, without changing the communication between the time server and the communication terminal.

[0016] Furthermore, a time synchronization method according to one aspect of the present invention is a time synchronization method for a relay device in a communication system comprising a communication terminal, a relay device, and a time server, wherein the communication terminal and the time server are connected via the relay device, and the communication terminal performs time synchronization based on time data clocked by the time server and contained in a communication frame transmitted by the time server, the time synchronization method including a receiving step of receiving the communication frame, a determination step of determining whether the communication frame contains time data, an extraction step of extracting the time data from the communication frame if the communication frame contains time data, and a time synchronization step of performing time synchronization based on the time data.

[0017] This provides the same effects as the relay device described above.

[0018] Furthermore, a program according to an aspect of the present invention may be configured to cause a computer to execute the above-described time synchronization method.

[0019] This provides the same effects as the relay device described above. Effect of the Invention

[0020] According to the present invention, it is possible to easily perform time synchronization among a plurality of communication devices while maintaining efficient communication. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram showing an overview of a communication system according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a block diagram showing a functional configuration of a relay device according to an embodiment of the present invention. [Diagram 3] FIG. 3 illustrates an example of an IP communication frame including a time synchronization response according to an embodiment of the present invention. [Figure 4] FIG. 4 is a flowchart showing an example of a series of processes performed by the relay device according to the embodiment of the present invention when receiving a time synchronization request from a communication terminal. [Diagram 5]FIG. 5 is a sequence diagram showing an example of a process until the communication terminal and the relay device complete time synchronization in the communication system according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, the embodiment will be specifically described with reference to the drawings. The embodiments described below each show a preferred specific example of the present invention. The numerical values, shapes, components, the arrangement and connection of the components, steps, and the order of steps shown in the following embodiments are merely examples and are not intended to limit the present invention. Furthermore, among the components in the following embodiments, those components that are not described in the independent claims showing the highest concept of the present invention are described as any components that constitute a more preferred embodiment. Note that the same reference numerals are used for the same components, and descriptions thereof may be omitted. [Embodiment Mode]

[0023] FIG. 1 is a schematic diagram showing an overview of a communication system according to an embodiment of the present invention. 1, the communication system 100 includes a communication terminal 101, a relay device 102, a base station device 104, and a time server 105. The communication system 100 is, for example, a network system in which the communication terminal 101 used by a user accesses the time server 105 connected to the base station device 104 via the relay device 102 and a wireless link 103. The relay device 102 and the base station device 104 are connected to each other via a wireless link 103 so as to be able to communicate wirelessly. The communication terminal 101 and the relay device 102 are connected to each other via a communication link 106 so as to be able to communicate wired or wirelessly. The base station device 104 and the time server 105 are connected to each other via a communication link 107 so as to be able to communicate wired.

[0024] The communication terminal 101 is realized by, for example, a PC (Personal Computer) or a tablet computer, and is used when a user accesses the communication system 100. The communication terminal 101 includes a clock unit (not shown) and clocks the time for use within the device itself. The communication terminal 101 also communicates with a time server 105 to synchronize the time of the device itself. The communication terminal 101 may include a wired communication interface that complies with a communication standard such as IEEE (Institute of Electrical and Electronics Engineers) 802.3, or may include a wireless interface that performs wireless communication that complies with a communication standard such as IEEE 802.11ah.

[0025] The relay device 102 is a communication device that relays communication between different communication links. The relay device 102 is connected to a communication link 106 that complies with a communication standard such as IEEE802.3 or IEEE802.11ah, and is also connected to a wireless link 103 that complies with a communication standard such as IEEE802.11ac. In order to appropriately relay communication between each interface, the relay device 102 transmits and receives communication data while appropriately converting the communication data into a protocol that each interface can communicate with, and relays communication between the different communication interfaces. Note that the relay device 102 and the communication terminal 101 are configured to share an IP address and a MAC address, and appear as a single communication device to a base station device 104 connected to the wireless link 103 side, a time server 105, and the like.

[0026] The wireless link 103 is a communication path between the relay device 102 and the base station device 104. The wireless link 103 is realized by wireless communication conforming to a communication standard such as IEEE802.11ac.

[0027] The base station device 104 relays communication between the relay device 102 and the time server 105. The base station device 104 has a wireless interface and a wired interface, and transmits and receives communication data to and from the relay device 102 via the wireless link 103, and transmits and receives data to and from the time server 105 via the communication link 107. In the present embodiment, one relay device 102 is connected to the base station device 104, but the present invention is not limited to this, and a plurality of communication devices such as relay devices 102 may be connected to the base station device 104. In addition, the base station device 104 may operate with a setting that allows only communication in which only one IP address is associated with one MAC address that each communication device has (i.e., does not allow sharing of an IP address between a plurality of communication devices) for a plurality of communication devices that may be connected to the base station device itself via the wireless link 103. This allows the base station device 104 to exclude communication from unknown communication devices, and maintains the security of communication in the communication system 100.

[0028] The time server 105 is a server that distributes time information (time data) including standard time, such as an NTP (Network Time Protocol) server. The time server 105 includes a clock unit (not shown) and clocks the time to be distributed to other devices. In response to an acquisition request (time synchronization request) that is an inquiry for a device (e.g., the communication terminal 101) connected to the network to set the time of its own device, the time server 105 provides the time data in a reply frame (time synchronization response) that is a response to the acquisition request. The time data may be provided in accordance with NTP or the like, or may be provided by a protocol or the like that is uniquely determined in advance between the communication terminal 101 and the time server 105. The time server 105 in this embodiment is connected to the base station device 104 by wired communication using the communication link 107, but is not limited to this, and may be connected to the base station device 104 by using the wireless link 103 by providing a wireless interface.

[0029] The communication link 106 is a communication path between the communication terminal 101 and the relay device 102. The communication link 106 may be realized by wired communication conforming to a communication standard such as IEEE802.3, or wireless communication conforming to a communication standard such as IEEE802.11ah and using a frequency band different from that of the wireless link 103 (e.g., 920 MHz band, etc.).

[0030] The communication link 107 is a communication path between the base station device 104 and the time server 105. The communication link 107 is realized by wired communication conforming to a communication standard such as IEEE802.3.

[0031] FIG. 2 is a block diagram showing a functional configuration of relay device 102 according to the embodiment of the present invention. As shown in FIG. 2, the relay device 102 includes a first communication unit 201, a second communication unit 202, a determination unit 203, an extraction unit 204, and a timekeeping unit 205. The above-mentioned functions of the relay device 102 are realized by using hardware such as a central processing unit (CPU), a random access memory (RAM), a storage device (e.g., a non-volatile storage device such as a read only memory (ROM), an embedded multimedia card (eMMC), a hard disk drive (HDD), or a solid state drive (SSD)), and a communication module, which are included in the relay device 102. Specifically, these functions are realized by the CPU reading out a program stored in the storage device or the like and executing it in cooperation with the above-mentioned hardware. In addition, transmission and reception of an IP communication frame 300 (described later) between the first communication unit 201 and the second communication unit 202 is performed by appropriately converting the type of communication packet or the like according to a communication protocol that each communication unit can process and the structure of the communication frame.

[0032] The first communication unit 201 is realized by hardware such as a communication module having a wired interface or a wireless interface and a communication program. The first communication unit 201 transmits and receives data such as IP communication frames to and from the communication terminal 101 through communication conforming to a wired communication standard such as IEEE802.3 or a wireless communication standard such as IEEE802.11ah. When the first communication unit 201 receives a time synchronization request from the communication terminal 101, the first communication unit 201 transfers the time synchronization request to the second communication unit 202.

[0033] The second communication unit 202 is realized by a communication module or the like having a wireless interface and a communication program. The second communication unit 202 transmits and receives data such as IP communication frames between the relay device 102 and the base station device 104 via the wireless link 103 by communication conforming to a wireless communication standard such as IEEE802.11ac, and can receive an IP communication frame 300 from the time server 105. When the second communication unit 202 receives a time synchronization request from the first communication unit 201, the second communication unit 202 transmits the time synchronization request to the time server 105 via the wireless link 103. The second communication unit 202 determines whether or not it has received the IP communication frame 300 transmitted from the time server 105 via the wireless link 103.

[0034] The determination unit 203 determines whether or not a time synchronization response is included in the IP communication frame 300 received by the second communication unit 202. Specifically, the determination unit 203 reads an identifier (e.g., a communication port number included in an IP header or protocol version information included in a data field) indicating that the data stored in the data field of the IP communication frame 300 is a time synchronization response, and determines whether or not the data is a time synchronization response. When the determination unit 203 determines that the IP communication frame 300 includes a time synchronization response, the determination unit 203 transfers the IP communication frame 300 to the extraction unit 204 and the first communication unit 201. On the other hand, when the determination unit 203 determines that the IP communication frame 300 does not include a time synchronization response, the determination unit 203 transfers the IP communication frame 300 only to the first communication unit 201.

[0035] When the extraction unit 204 receives the IP communication frame 300 transferred from the determination unit 203, it extracts time data from the time synchronization response included in the IP communication frame 300. Specifically, when the extraction unit 204 refers to the time data in the time synchronization response, it extracts the time data to a temporary storage area provided on a RAM or the like (for example, a buffer assigned to a communication interface, or a specific storage area assigned by a program such as an OS (Operating System)). The extraction unit 204 transfers the extracted time data to the clock unit 205.

[0036] The clock unit 205 keeps time. Furthermore, when the clock unit 205 receives time data from the extraction unit 204, the clock unit 205 corrects or sets the time currently being kept by the clock unit 205 based on the received time data. The clock unit 205 is realized by, for example, an RTC (Real Time Clock) or the like, and corrects or sets the time information via a time setting program or the like.

[0037] FIG. 3 shows, as an example, an example in which a time synchronization response is included in an IP communication frame 300 transmitted from the time server 105 according to the embodiment of the present invention to the communication terminal 101. As shown in FIG. 3, an IP communication frame 300 includes an IP header 301 , a data field 302 , and a time synchronization response 303 .

[0038] The IP header 301 stores information related to the communication of the IP communication frame 300. The information stored in the IP header 301 is, for example, the IP address of the time server 105 that is the source of the IP communication frame 300, the IP address of the communication terminal 101 that is the destination of the IP communication frame 300, and the size of the entire IP communication frame 300.

[0039] The data field 302 stores data used by a higher processing layer such as an application. For example, character string data conforming to HTTP (Hyper Text Transport Protocol) and describing a command for displaying a web page on a web browser, character string data conforming to SMTP (Simple Mail Transfer Protocol) used for sending and receiving e-mail, binary data, etc. are stored. The data field 302 may also include a time synchronization response 303.

[0040] The time synchronization response 303 is a response to the time synchronization request transmitted by the communication terminal 101. The time synchronization response 303 includes time data clocked by the time server 105. The relay device 102 and the communication terminal 101 correct or set the time clocked by the clock unit 205 of the relay device 102 and the clock unit (not shown) of the communication terminal 101 based on the time data clocked by the time server 105 included in the time synchronization response 303 using a method described in this specification. The time synchronization response 303 may conform to a standard protocol such as NTP, or may have a response format uniquely defined by an application or the like. Furthermore, it does not need to be included in a frame conforming to IP communication, as long as it is included in a frame of an appropriate format.

[0041] [Explanation of flow according to embodiment] 4 is a flowchart showing an example of a series of processes performed by the relay device 102 according to the embodiment of the present invention when receiving a time synchronization request from the communication terminal 101. The processes related to time synchronization of the relay device 102 are executed when the relay device 102 receives the time synchronization request from the communication terminal 101.

[0042] In step S401, the first communication unit 201 receives a time synchronization request from the communication terminal 101. Upon receiving the time synchronization request, the first communication unit 201 transfers the time synchronization request to the second communication unit 202.

[0043] In step S402, the second communication unit 202 transmits the time synchronization request received in step S401 to the time server 105 via the wireless link 103.

[0044] Step S403 is a step for determining whether or not the second communication unit 202 has received the IP communication frame 300 transmitted from the time server 105 via the wireless link 103. When the second communication unit 202 has received the IP communication frame 300, the second communication unit 202 transfers the received IP communication frame 300 to the determination unit 203 (Yes in step S403). On the other hand, when the second communication unit 202 has not received the IP communication frame 300, the second communication unit 202 repeatedly executes step S403 (No in step S403).

[0045] Step S404 is a step in which the determination unit 203 determines whether or not the time synchronization response 303 is included in the IP communication frame 300. If the time synchronization response 303 is included in the data field 302 of the IP communication frame 300 transferred from the second communication unit 202, the determination unit 203 transfers the IP communication frame 300 to the extraction unit 204 and step S405 is executed (Yes in step S404). On the other hand, if the time synchronization response 303 is not included in the data field 302, step S406 is executed (No in step S404).

[0046] In step S405, the extraction unit 204 extracts time data from the time synchronization response 303 included in the IP communication frame 300 transferred from the determination unit 203 in step S404, notifies the timekeeping unit 205, and the timekeeping unit 205 performs time synchronization based on the notified time data. Specifically, for example, the timekeeping unit 205 synchronizes with the time server 105 by correcting the currently measured time based on the notified time data.

[0047] Step S406 is a step of transferring the IP communication frame 300 to the first communication unit 201. Specifically, the determination unit 203 transfers the IP communication frame 300 to the first communication unit 201 regardless of whether the data field 302 of the IP communication frame 300 includes the time synchronization response 303 or not.

[0048] In step S407, the first communication unit 201 transmits to the communication terminal 101 the IP communication frame 300 transferred in step S406.

[0049] Thereafter, when the communication terminal 101 receives the transmitted IP communication frame 300 in accordance with the communication protocol, the communication terminal 101 may transmit a reception acknowledgment response (for example, an Ack (Acknowledgement) frame in TCP communication) which is a notification that reception is completed, to the relay device 102. In this case, the relay device 102 may transmit the reception acknowledgment response received from the communication terminal 101 to the time server 105 which is the source of the IP communication frame 300 via the wireless link 103.

[0050] Thereafter, the communication terminal 101 synchronizes its own time with the time of the time server 105 based on the received IP communication frame 300 .

[0051] [Sequence Description] 5 is a sequence diagram showing an example of a process until the communication terminal 101 and the relay device 102 complete time synchronization in the communication system 100 according to the embodiment of the present invention. Note that the sequence diagram of FIG. 5 will be described on the premise that the IP communication frame 300 received by the relay device 102 includes a time synchronization response 303.

[0052] First, a user uses a communication terminal 101 to transmit a time synchronization request to the time server 105 via the relay device 102 (step S501). The time synchronization request transmitted by the communication terminal 101 is received by the relay device 102 and transmitted to the time server 105 via the wireless link 103 and the base station device 104.

[0053] Next, the time server 105 receives the time synchronization request transmitted in step S501 (step S502).

[0054] Next, in response to the time synchronization request received in step S502, the time server 105 transmits a time synchronization response 303 including time data clocked by a clock unit (not shown) of the time server 105 to the communication terminal 101 (step S503). The time synchronization response 303 is stored in an IP communication frame 300 and transmitted to the communication terminal 101.

[0055] Next, the relay device 102 receives the IP communication frame 300 including the time synchronization response 303 via the base station device 104 (step S504). When the determination unit 203 included in the relay device 102 confirms that the received IP communication frame 300 includes the time synchronization response 303, the relay device 102 transfers the IP communication frame 300 to the extraction unit 204.

[0056] Next, the extraction unit 204 provided in the relay device 102 extracts the time data clocked by the time server 105 and contained in the time synchronization response 303 from the IP communication frame 300 transferred in step S504, and transfers the time data to the clock unit 205 (step S505).

[0057] Next, the timer unit 205 included in the relay device 102 performs a time synchronization process based on the time data transferred from the extraction unit 204 (step S506). In step S506, even in the case of communication requiring a response (receipt acknowledgement) to confirm whether the received IP communication frame 300 has arrived at the receiving side (for example, communication using the TCP protocol), the relay device 102 does not transmit a receipt acknowledgement for the IP communication frame 300 received by the relay device 102 from the second communication unit 202. This allows the relay device 102 to synchronize its own time with the time server 105 using the time data included in the time synchronization response 303 originally transmitted by the time server 105 to the communication terminal 101, without changing the communication between the time server 105 and the communication terminal 101.

[0058] Next, the determination unit 203 included in the relay device 102 transfers the IP communication frame 300 including the time synchronization response 303 received in step S504 to the first communication unit 201. Then, the first communication unit 201 transmits it to the communication terminal 101 via the communication link 106 (step S507). Note that step S507 may be performed immediately after step S504.

[0059] Next, the communication terminal 101 receives the IP communication frame 300 including the time synchronization response 303 from the relay device 102. At this time, if a time synchronization-related reception confirmation response is required from the time server 105 (for example, TCP protocol, etc.), the communication terminal 101 transmits a reception confirmation notification (reception confirmation response) indicating that reception has been completed to the time server 105 (step S508). The reception confirmation response is transmitted from the communication terminal 101 to the time server 105 via the relay device 102 and the base station device 104.

[0060] Next, the time server 105 detects that the communication terminal 101 has received the time synchronization response 303 (that the transmission has been completed) by receiving the reception confirmation response (step S509).

[0061] Finally, the communication terminal 101 extracts the time synchronization response 303 from the IP communication frame 300, and performs time synchronization processing of its own device (communication terminal 101) based on the time data contained in the time synchronization response 303, completing a series of time synchronization processing (step S510).

[0062] In this way, the relay device 102 relays the time synchronization request transmitted by the communication terminal 101 to the time server 105, receives the IP communication frame 300 in response from the time server 105 as a response to the time synchronization request, extracts the time data contained in the time synchronization response 303 stored in the IP communication frame 300, and transmits the IP communication frame 300 to the communication terminal 101. This allows the relay device 102 and the communication terminal 101 to each perform time synchronization based on one time synchronization request transmitted by the communication terminal 101, and allows the relay device 102, which wishes to perform time synchronization while improving communication efficiency, to easily perform time synchronization.

[0063] The present invention can be realized not only as an apparatus, but also as a method in which the processing means constituting the apparatus are steps, as a program for causing a computer to execute those steps, as a computer-readable storage medium such as a CD-ROM on which the program is recorded, or as information, data or signals indicating the program.These programs, information, data and signals may be distributed via a communication network such as the Internet. [Industrial Applicability]

[0064] The present invention can be applied to a relay device or the like that relays communication between a wired network or a wireless network and a wireless network that uses a frequency band different from that of the aforementioned wireless network. [Explanation of symbols]

[0065] 100 Communication Systems 101 Communication terminal 102 Relay device 103 Wireless Link 104 Base station equipment 105 Time Server 106, 107 Communication links 201 1st Communications Department 202 2nd Communications Department 203 Judgment section 204 Extraction part 205 Timing section 300 IP communication frames 301 IP Header 302 Data Field 303 Time Synchronization Response

Claims

1. A relay device in a communication system including a communication terminal, a relay device, and a time server, the communication terminal and the time server being connected via the relay device, the communication terminal performing time synchronization based on time data measured by the time server and included in a communication frame transmitted by the time server, A first communication unit that communicates with the communication terminal; a second communication unit that receives the communication frame from the time server; an extraction unit that extracts the time data from the communication frame; a determination unit that determines whether or not time data is included in the communication frame received by the second communication unit, and transfers the communication frame to the first communication unit and the extraction unit when the time data is included; a timekeeping unit that performs time synchronization based on the time data; Equipped with Relay device.

2. The communication frame transmitted by the time server is a response to a time synchronization request, which is an inquiry for synchronizing the time of the communication terminal, transmitted from the communication terminal to the time server. The relay device according to claim 1 .

3. The communication system further includes a base station device between the relay device and the time server, The communication terminal and the relay device share an IP address, The base station device does not permit sharing of IP addresses among multiple communication devices; The relay device according to claim 1 or 2.

4. Furthermore, the determination unit transfers the communication frame only to the first communication unit when the communication frame does not include time data. The relay device according to claim 1 or 2.

5. Furthermore, the second communication unit does not transmit a reception confirmation response of the communication frame even in the case of a communication that requires a reception confirmation response, which is a response for confirming whether or not the communication frame has arrived at the receiving side. The relay device according to claim 1 or 2.

6. A time synchronization method for a relay device in a communication system including a communication terminal, a relay device, and a time server, the communication terminal and the time server being connected via the relay device, the communication terminal performing time synchronization based on time data clocked by the time server and included in a communication frame transmitted by the time server, comprising: a receiving step of receiving the communication frame; a determination step of determining whether or not time data is included in the communication frame; an extraction step of extracting the time data from the communication frame when the time data is included in the communication frame; a time synchronization step of performing time synchronization based on the time data; Including, Time synchronization method.

7. A program for causing a computer to execute the time synchronization method according to claim 6.

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