On-vehicle repeater, flow control method, and computer program

Abstract

An on-vehicle repeater according to one embodiment of the present disclosure comprises: a first port that is a physical port for a first frame described below; a second port that is a physical port for a second frame described below; and a relay processing unit that performs relay processing for determining an output port for a received frame and protocol conversion for converting the format of the received frame into a transmission-side format, wherein if the transmission buffer amount of the first port is equal to or greater than a threshold value, the relay processing unit transmits, from the second port, a second frame in which an identifier indicating the transmission priority is set to a predetermined value.　First frame: a communication frame that conforms to a first communication protocol and includes an identifier indicating the transmission priority. Second frame: a communication frame that conforms to a second communication protocol having a higher maximum transmission speed than that of the first communication protocol and includes an identifier indicating the transmission priority.

Description

In-vehicle relay device, flow control method, and computer program

[0001] The present disclosure relates to an in-vehicle relay device, a flow control method, and a computer program. This application claims priority based on Japanese Application No. 2024-221617 filed on December 18, 2024, and incorporates all the descriptions described in the above Japanese application.

[0002] Patent Document 1 describes an in-vehicle communication system that enables the mixed accommodation of an ECU (Electric Control Unit) compliant with CAN (Control Area Network: registered trademark) and an ECU compliant with Ethernet (registered trademark) by adopting an in-vehicle relay device that involves protocol conversion between CAN and Ethernet (registered trademark). Patent Document 2 describes a flow control device that has a buffer for each priority level, sets a threshold value for each buffer, and sends stop and restart instructions for transmission.

[0003] Japanese Unexamined Patent Application Publication No. 2021-138263 Japanese Unexamined Patent Application Publication No. 2006-20027

[0004] The device according to one aspect of the present disclosure includes a first port that is a physical port of the following first frame, a second port that is a physical port of the following second frame, a relay process for determining an output port of a received frame, and a relay processing unit that performs protocol conversion for converting the format of the received frame into the format of the transmission side. The relay processing unit transmits the second frame in which an identifier indicating the transmission priority order is set to a predetermined value from the second port when the transmission buffer amount of the first port is equal to or greater than a threshold value.

[0005] First frame: A communication frame compliant with a first communication protocol and including an identifier indicating the transmission priority order Second frame: A communication frame compliant with a second communication protocol whose maximum transmission speed is higher than that of the first communication protocol and including an identifier indicating the transmission priority order

[0006] This disclosure can be implemented not only as a system and apparatus having the characteristic configuration described above, but also as a program for causing a computer to execute such characteristic configuration. Furthermore, this disclosure can be implemented as a semiconductor integrated circuit that implements part or all of the system and apparatus.

[0007] Figure 1 is a network configuration diagram showing an example of an in-vehicle communication system configuration. Figure 2 is a block diagram showing an example of the internal configuration of a CGW. Figure 3 is an explanatory diagram showing an example of a CGW problem and its solution. Figure 4 is a flowchart showing an example of flow control. Figure 5 is a diagram showing an example of a correspondence table.

[0008] <Problems this disclosure aims to solve> Patent Document 1 does not consider buffer leakage in the low-speed transmit buffer, which is a concern when the flow from the high-speed Ethernet to the low-speed CAN increases. As a measure to prevent the above buffer leakage, it is conceivable to apply the flow control device of Patent Document 2 to an in-vehicle communication system. However, such an in-vehicle communication system would require modifying the software of numerous ECUs to enable the determination of stop and restart instructions according to priority, which would result in high costs.

[0009] In view of the aforementioned conventional problems, this disclosure aims to provide an in-vehicle relay device, etc., that can easily suppress buffer leakage in the low-speed transmission buffer.

[0010] <Effects of this disclosure> According to this disclosure, an in-vehicle relay device, etc., can be obtained that can easily suppress buffer leakage in the low-speed transmission buffer.

[0011] <Outline of Embodiments of the Disclosure> The embodiments of the disclosure are outlined below.

[0012] (1) An in-vehicle relay device according to one aspect of this embodiment includes a first port which is the physical port of the first frame described below, a second port which is the physical port of the second frame described below, a relay processing unit which performs relay processing to determine the output port of the received frame and protocol conversion to convert the format of the received frame to the format of the transmitting side, wherein the relay processing unit transmits the second frame from the second port when the amount of the transmit buffer of the first port is equal to or greater than a threshold, with an identifier representing the transmission priority set to a predetermined value.

[0013] First frame: A communication frame conforming to the first communication protocol, containing an identifier representing the transmission priority. Second frame: A communication frame conforming to the second communication protocol, which has a higher maximum transmission speed than the first communication protocol, containing an identifier representing the transmission priority.

[0014] According to the in-vehicle relay device of this embodiment, when the amount of the transmit buffer of the first port is greater than or equal to a threshold, the relay processing unit transmits a second frame with an identifier set to a predetermined value from the second port. As a result, the in-vehicle communication device (e.g., ECU) connected to the second port will not transmit a second frame with a lower priority than the predetermined value. Therefore, pressure transmission to the in-vehicle communication device connected to the second port becomes possible without modifying the software of the said in-vehicle communication device. Thus, buffer leakage in the transmit buffer of the first port, which is the low-speed side, can be easily suppressed.

[0015] (2) In the in-vehicle relay device described in (1) above, the first port is a plurality of ports, and the relay processing unit may transmit the second frame from the second port if the transmit buffer amount of any of the plurality of first ports is equal to or greater than a threshold. In this way, buffer leakage can be easily suppressed for the transmit buffers of all first ports.

[0016] (3) In the in-vehicle relay device described in (1) or (2) above, the second port may be a plurality of ports, and the relay processing unit may determine the value of the identifier to be applied to the second frame according to the amount of the receive buffer of the plurality of second ports. In this way, the degree of transmission suppression to the in-vehicle communication device (e.g., ECU) connected to the second port can be flexibly adjusted compared to the case where the value of the identifier is a fixed value.

[0017] (4) In the in-vehicle relay device described in (3) above, the relay processing unit may determine the value of the identifier such that the transmission priority increases for the second port which has a larger receive buffer amount. In this way, the degree of transmission suppression for in-vehicle communication devices (e.g., ECUs) connected to the second port which are estimated to have a high transmission frequency can be increased in stages.

[0018] (5) In the in-vehicle relay device described in (4) above, the relay processing unit may increase the transmission frequency of the second frame for the second port which has a larger receive buffer size. In this way, the degree of transmission suppression to the in-vehicle communication device (e.g., ECU) connected to the second port can be increased compared to the case where the transmission frequency of the second frame is kept constant.

[0019] (6) In the in-vehicle relay devices described in (1) to (5) above, the payload of the second frame may contain an identifier different from the identifier and the actual data portion. In this way, the second frame for transmission suppression can also be used for data transmission from the in-vehicle relay device to an in-vehicle communication device (e.g., ECU) connected to the second port.

[0020] (7) In the in-vehicle relay devices described in (1) to (6) above, the first communication protocol may be CAN, and the second communication protocol may be CAN-FD (CAN with flexible data rate). In this case, pressure transmission to the in-vehicle communication device (e.g., ECU) can be performed without modifying the software of the in-vehicle communication device (e.g., ECU) that complies with CAN-FD. Therefore, buffer leakage in the transmission buffer of the CAN port, which is the low-speed side, can be easily suppressed.

[0021] (8) In the in-vehicle relay device described in (7) above, the identifier may be a CAN ID. This is because in CAN and CAN-FD, a CAN ID is used as an identifier to indicate the transmission priority.

[0022] (9) A method according to one aspect of this embodiment is a flow control method executed by the in-vehicle relay device described in (1) to (8) above. Therefore, the flow control method of this embodiment has the same effects as the in-vehicle relay device described in (1) to (8) above.

[0023] (10) A computer program according to one aspect of this embodiment is a computer program for causing a computer to function as an in-vehicle relay device as described in (1) to (8) above. Therefore, the computer program of this embodiment has the same effects as the in-vehicle relay devices described in (1) to (8) above.

[0024] <Details of Embodiments of the Invention> Hereinafter, details of embodiments of the invention will be described with reference to the drawings. At least some of the embodiments described below may be combined in any way.

[0025] [Example of In-Vehicle Communication System Configuration] Figure 1 is a network configuration diagram showing an example of the configuration of an in-vehicle communication system 100. As shown in Figure 1, the in-vehicle communication system 100 according to this embodiment is an in-vehicle LAN (Local Area Network) constructed inside the vehicle 1. The in-vehicle communication system 100 includes a CGW (Central Gateway) 10, a TCU (Telematics Control Unit) 20, and a plurality of ECUs 30, 40 as communication nodes that constitute the network.

[0026] The TCU 20 is a wireless communication unit that performs data communication with external servers operated by the vehicle manufacturer. The external server communicates with the TCU 20 to share information with the vehicle 1 in near real-time. The ECUs 30 and 40 are electronic control units for vehicles that control various in-vehicle equipment such as sensors or actuators within the vehicle 1. The ECUs 30 and 40 are communication nodes that constitute the in-vehicle communication system 100, and from a communication standpoint, they are a type of "in-vehicle communication device".

[0027] Focusing on the controlled objects, the types of ECUs 30 and 40 include engine control ECUs, transmission control ECUs, power steering control ECUs, air conditioning control ECUs, and AV (Audio / Visual) system control ECUs. ECUs 30 and 40 take measurement information from sensors connected to them (such as speed sensors, acceleration sensors, temperature sensors, and pressure sensors) into the system and control various actuators connected to them (such as electric motors) based on the measurement information.

[0028] Focusing on the communication protocol, the in-vehicle communication system 100 is a network in which ECUs 30 that perform communication compliant with the "first communication protocol" and ECUs 40 that perform communication compliant with the "second communication protocol" coexist. In this embodiment, both the first communication protocol and the second communication protocol are assumed to meet the following conditions.

[0029] Condition 1: A protocol is adopted that uses arbitration to determine transmission priority based on the number of identifiers in the communication frame. Condition 2: The maximum transmission speed of the second communication protocol is greater than the maximum transmission speed of the first communication protocol.

[0030] In this embodiment, as an example, the first communication protocol is CAN and the second communication protocol is CAN-FD. In CAN and CAN-FD, collision resolution is performed in the CAMA / CR (Carrier Sense Multiple Access with Collision Resolution) of each communication node, prioritizing the transmission of data with smaller CANIDs. Therefore, CAN and CAN-FD satisfy condition 1.

[0031] The maximum transmission speed of CAN-FD is 8 Mbps in standard mode, while the maximum transmission speed of CAN is typically 1 Mbps. Therefore, both CAN and CAN-FD satisfy condition 2. However, CAN and CAN-FD are just examples, and any protocol other than CAN may be used for the first communication protocol and a protocol other than CAN-FD for the second communication protocol, as long as conditions 1 and 2 are met.

[0032] ECU 30 is an ECU that performs communication compliant with CAN (First Communication Protocol). Hereinafter, a communication frame compliant with CAN will be referred to as a "CAN frame" or "First frame," and an ECU that performs communication compliant with CAN will be referred to as "C-ECU" or "First ECU." ECU 40 is an ECU that performs communication compliant with CAN-FD (Second Communication Protocol). Hereinafter, a communication frame compliant with CAN-FD will be referred to as a "CAN-FD frame" or "Second frame," and an ECU that performs communication compliant with CAN-FD will be referred to as "F-ECU" or "Second ECU."

[0033] C-ECU 30 is connected to CGW 10 by CAN bus 50 (hereinafter referred to as "first bus 50"). F-ECU 40 is connected to CGW 10 by CAN bus 60 (hereinafter referred to as "second bus 60"). The first bus 50 and the second bus 60 are twisted pair cables including high and low transmission lines. Multiple first ECUs 30 can be connected in a line configuration to one first bus 50. Multiple second ECUs 40 can be connected in a line configuration to one second bus 60.

[0034] The CGW10 has the following multiple types of communication ports: First port CPi: A physical port for CAN frames (first frames). "i" is a port number from 1 to m. Second port FPj: A physical port for CAN-FD frames (second frames). "j" is a port number from 1 to n. Third port TP: A physical port for communicating with external communication devices of vehicle 1.

[0035] Therefore, the first bus 50 is connected to the first port CPi. The second bus 60 is connected to the second port FPj. In addition, a communication cable (for example, a LAN cable) used for communication with the TCU 20 is connected to the third port TP.

[0036] The CGW10 has routing functions to distribute the destination of communication frames received from the ECUs 30 and 40, protocol conversion functions to convert the format of communication protocols, and security functions to prevent unauthorized access from the outside. Therefore, the CGW10 is a type of "in-vehicle relay device" that performs routing of communication frames based on CANID and protocol conversion of communication frames transmitted and received between the C-ECU 30 and the F-ECU 40.

[0037] [Example of CGW Configuration] Figure 2 is a block diagram showing an example of the internal configuration of CGW 10. As shown in Figure 2, CGW 10 includes a relay processing unit 11, a first transceiver 12C, and a second transceiver 12F as electronic components housed in the enclosure. CGW 10 also has physical ports provided on the wall of the enclosure: a first port CPi (i = 1, 2...m), a second port FPj (j = 1, 2...n), and a third port TP.

[0038] The relay processing unit 11 is a communication module that performs Layer 2 signal processing on CAN frames and CAN-FD frames, and includes a controller 13, a memory 14, and a plurality of buffers 15. The plurality of buffers 15 include a transmit buffer CTBi and a receive buffer CRBi that correspond one-to-one with the first port CPi, and a transmit buffer FTBj and a receive buffer FRBj that correspond one-to-one with the second port FPi.

[0039] The controller 13 controls the retrieval of data from the transmit buffer CTBi, receive buffer CRBi, transmit buffer FTBj, and receive buffer FRBj.

[0040] The first transceiver 12C is a transceiver that performs Layer 1 signal processing related to CAN frames. The first transceiver 12C encodes the CAN frame input from the transmit buffer CTBi to generate a differential signal and outputs the differential signal to the first port CPi. The first transceiver 12C reconstructs the CAN frame from the differential signal input from the first port CPi and outputs the CAN frame to the receive buffer CRBi.

[0041] The second transceiver 12F is a transceiver that performs Layer 1 signal processing related to CAN-FD frames. The second transceiver 12F encodes the CAN-FD frame input from the transmit buffer FTBj to generate a differential signal and outputs the differential signal to the second port FPj. The second transceiver 12F reconstructs the CAN-FD frame from the differential signal input from the second port FPj and outputs the CAN-FD frame to the receive buffer FRBj.

[0042] The controller 13 may consist of, for example, one or more CPUs (Central Processing Units). However, the controller 13 may also include an FPGA (Field Programmable Gate Array), or it may consist of an FPGA without implementing a CPU. The controller 13 reads a computer program 16 from the memory 14 and executes predetermined communication processing according to the read program 16.

[0043] The memory 14 may be composed of a non-volatile recording medium such as an EEPROM (Electrically Erasable Programmable ROM). In addition to the computer program 16 for communication processing, the memory 14 stores a relay table 17 used for routing and a correspondence table 18 used for flow control. A specific example of the correspondence table 18 (see Figure 5) will be described later.

[0044] The computer program 16 stored in the memory 14 includes a program that causes the controller 13 to execute the following processes: 1) Relay process; 2) Protocol conversion; 3) Arbitration; 4) Flow control

[0045] The "relay process" is a process of determining the output port of the received frame based on the output destination for each CAN ID defined by the relay table 17. The "protocol conversion" is a process of converting the format of the received frame into the format of the transmission destination when the communication protocols at the reception source and the transmission destination are different. Therefore, the protocol conversion includes a process of converting a CAN frame into a CAN-FD frame and a process of converting a CAN-FD frame into a CAN frame.

[0046] "Arbitration" is a process of determining the transmission priority based on the identifier (CAN ID) of the communication frame. In the arbitration of CAN and CAN-FD, it is determined that the smaller the value of the CAN ID, the higher the transmission priority. "Flow control" is control for suppressing buffer overflow of the communication frame. In the present embodiment, the transmission buffer CTBi corresponding to the first port CPi on the low-speed side is the suppression target. The details of the flow control (see FIG. 4) will be described later.

[0047] 〔Problems of CGW and Solutions Thereof〕FIG. 3 is an explanatory diagram showing an example of the problems of the CGW 10 and solutions therefor. As shown in FIG. 3, here, it is assumed that the CGW 10 relays a communication frame from the second bus 60 of CAN-FD on the high-speed side to the second bus 50 of CAN on the low-speed side.

[0048] In this case, for example, when burst transmission occurs on the second bus 60, the capacity of the transmission buffer CTBi of the first port CPi connected to the first bus 50 may be exceeded, and the CAN frame (first frame) may be lost. Therefore, in the CGW 10 that accommodates a mixture of CAN and CAN-FD, it is necessary to perform flow control to suppress the transmission of CAN-FD frames (second frames) to the high-speed side F-ECU (second ECU) 40.

[0049] Therefore, in this embodiment, when the amount of data accumulated in the transmit buffer CTBi exceeds a predetermined threshold Th, the CGW 10 performs a "pressure transmission" from the second port FPj connected to the second bus 60. Specifically, the CGW 10 continuously transmits the "suppression frame SF" defined below from the second port FPj. Suppression frame SF: A CAN-FD frame (second frame) in which a predetermined value (for example, "0x300" in this case) is written in CANID.

[0050] Upon receiving the suppression frame SF described above, the F-ECU (second ECU) 40 will, through communication arbitration in accordance with CAN-FD, refrain from transmitting CAN-FD frames (second frames) with a lower priority and a CANID value greater than 0x300. Therefore, by continuously transmitting the suppression frame SF to the high-speed side, buffer leakage in the transmission buffer CTBi of the CAN frame (first frame), which is on the low-speed side, can be suppressed.

[0051] In this way, by continuously transmitting suppression frames SF and applying transmission suppression to the high-speed F-ECU (second ECU) 40, it is possible to prevent the buffer size of the low-speed transmission buffer CTBi from becoming excessive. In order to transmit the suppression frames SF as quickly as possible, it is preferable for the controller 13 of the CGW 10 to output the suppression frames SF to the second transceiver 12F without putting them into the transmission buffer FTBj.

[0052] Incidentally, in Ethernet® backpressure using "PAUSE frames," the communication node that receives the frame temporarily suspends the transmission process. In contrast, in the pressure transmission of this embodiment, the transmission of CAN-FD frames (second frames) with a higher priority than the CANID value written in the suppression frame SF is permitted. Furthermore, since the arbitration function of CAN-FD is utilized, there is the advantage that flow control can be achieved without modifying the software of the F-ECU (second ECU) 40.

[0053] As a suppression frame SF, for example, a meaningless frame with padding data stored in the payload can be used. However, as shown in Figure 3, a CAN-FD frame may be used in which a CANID different from the CANID in the header section (=0x300) and the actual data section are stored in the payload. If a suppression frame SF of this data format is used, the suppression frame SF can also be used for data transmission from CGW 10 to F-ECU (second ECU) 40.

[0054] [Flow Control Details] Figure 4 is a flowchart showing an example of the flow control performed by the controller 13 of the CGW 10. The controller 13 of the CGW 10 performs the processing shown in the flowchart in Figure 4 at predetermined control cycles C (for example, 10 seconds).

[0055] As shown in Figure 4, the controller 13 first obtains the buffer amount BA of all low-speed transmit buffers CTBi that are in operation (step ST11). Next, the controller 13 determines whether or not there are any transmit buffers CTBi whose obtained buffer amount BA is equal to or greater than a predetermined threshold Th (for example, 90%) (step ST12).

[0056] If the result of the determination in step ST12 is positive, the controller 13 determines the value of the CANID to be written in the suppression frame SF (hereinafter referred to as the "ID value") based on the buffer amount of the high-speed receiving buffer FRBi and the correspondence table 18 (step ST13). Next, the controller 13 starts pressure transmission using the suppression frame SF (step ST14) and terminates the process. As a result, the suppression frame SF with the above ID value written in it is continuously transmitted from the second port FPj.

[0057] If the result of step ST12 is negative, the controller 13 determines whether or not pressure transmission is currently being performed (step ST15). If the result of step ST15 is positive, the controller 13 stops the ongoing pressure transmission (step ST16) and terminates the process. As a result, the pressure transmission that was being performed in the previous control cycle C is stopped in the current control cycle C.

[0058] If the result of step ST15 is negative, the controller 13 skips step ST16 and terminates the process.

[0059] [Method for determining ID values ​​using a correspondence table] Figure 5 shows an example of a correspondence table 18. As shown in Figure 5, either the first table 18A or the second table 18B can be used as the correspondence table 18 used to determine the ID value of CANID.

[0060] Table 18A is a table-formatted data set containing a column for the "Buffer Amount BA Range" and a column for the "CANID" in the receive buffer FRBj. Table 18A defines the ID value of CANID for each range of buffer amount BA as follows: 80% ≤ BA < 90% → ID value = 0x200 90% ≤ BA < 95% → ID value = 0x100 95% ≤ BA → ID value = 0x50

[0061] If the first table 18A is adopted, the controller 13 will select a smaller ID value (a higher priority ID value) the larger the buffer amount BA of the high-speed receive buffer FRBj. Consequently, the degree of transmission suppression for the F-ECU (second ECU) 40 will increase. This is because the larger the buffer amount BA of the receive buffer FRBj, the higher the estimated transmission frequency of CAN-FD frames (second frames), and therefore the degree of transmission suppression should be increased.

[0062] Table 18B, in contrast to Table 18A, is a table-formatted data set that also includes a "Bus Load L" column. In Table 18B, the ID value of CANID and the bus load L value (hereinafter referred to as "L value") are defined as follows for each range of buffer amount BA: 80% ≤ BA < 90% → ID value = 0x200, L value = 60% 90% ≤ BA < 95% → ID value = 0x100, L value = 70% 95% ≤ BA → ID value = 0x50, L value = 100%

[0063] When the second table 18B is adopted, the controller 13 selects a smaller ID value (higher priority ID value) as the buffer amount BA of the high-speed receive buffer FRBj increases, and increases the transmission frequency of the suppression frame SF to achieve the defined L value. Therefore, the degree of transmission suppression to the F-ECU (second ECU) 40 can be increased compared to when the transmission frequency of the suppression frame SF is kept constant.

[0064] The transmission frequency F of suppression frames SF is calculated by the following equation (1): F = (L × B) / D ……(1) where L: bus load (% / 100) D: data length per frame (bits) F: frame transmission frequency (frames / second) B: bus bandwidth (bits / second)

[0065] [Other Variations] The embodiments disclosed herein are illustrative and not restrictive in all respects. The scope of the present invention is not limited to the embodiments described above, and includes all modifications within the scope of equivalence to the configurations described in the claims. In the embodiments described above, in addition to CAN and CAN-FD, FlexRay (Flexible Time-Triggered Protocol for Automotive Applications) may be employed.

[0066] FlexRay satisfies condition 1 because it employs a protocol that determines transmission priority according to the number of identifiers. Furthermore, FlexRay is a high-speed communication protocol suitable for real-time requests and has a higher maximum transmission speed than CAN-FD. Therefore, in the above embodiment, the first communication protocol may be CAN-FD and the second communication protocol may be FlexRay. Alternatively, the first communication frame may be CAN and the second communication protocol may be FlexRay.

[0067] 1 Vehicle 10 CGW (On-board relay device) 11 Relay processing unit 12C First transceiver 12F Second transceiver 13 Controller 14 Memory 15 Buffer 16 Computer program 17 Relay table 18 Correspondence table 18A First table 18B Second table 30 C-ECU (First ECU) 40 F-ECU (Second ECU) 50 CAN bus (First bus) 60 CAN bus (Second bus) 100 On-board communication system CPi First port (physical port for CAN frames) FPj Second port (physical port for CAN-FD frames) TP Third port (physical port for external communication) CTBi Transmit buffer for first port CRBi Receive buffer for first port FTBj Transmit buffer for second port FRBj Receive buffer for second port

Claims

1. An in-vehicle relay device comprising: a first port which is the physical port for the first frame described below; a second port which is the physical port for the second frame described below; and a relay processing unit which performs relay processing to determine the output port of a received frame and protocol conversion to convert the format of the received frame to the format of the transmitting side, wherein the relay processing unit transmits the second frame from the second port when the amount of the transmit buffer of the first port is greater than or equal to a threshold, the identifier representing the transmission priority is set to a predetermined value. First frame: A communication frame that conforms to a first communication protocol and includes an identifier representing the transmission priority. Second frame: A communication frame that conforms to a second communication protocol whose maximum transmission speed is greater than that of the first communication protocol and includes an identifier representing the transmission priority.

2. The in-vehicle relay device according to claim 1, wherein the first port is a plurality of ports, and the relay processing unit transmits the second frame from the second port when the transmit buffer amount of any of the plurality of first ports is equal to or greater than a threshold.

3. The in-vehicle relay device according to claim 1 or 2, wherein the second port is a plurality of ports, and the relay processing unit determines the value of the identifier to be applied to the second frame according to the amount of the receive buffer of the plurality of second ports.

4. The in-vehicle relay device according to claim 3, wherein the relay processing unit determines the value of the identifier such that the transmission priority increases for the second port which has a larger receive buffer amount.

5. The in-vehicle relay device according to claim 4, wherein the relay processing unit increases the transmission frequency of the second frame for the second port which has a larger receive buffer size.

6. The in-vehicle relay device according to any one of claims 1 to 5, wherein the payload of the second frame stores an identifier different from the identifier and an actual data section.

7. The in-vehicle relay device according to any one of claims 1 to 6, wherein the first communication protocol is CAN and the second communication protocol is CAN-FD.

8. The in-vehicle relay device according to claim 7, wherein the identifier is a CANID.

9. A flow control method to be executed in an in-vehicle relay device comprising: a first port which is the physical port of the first frame described below; a second port which is the physical port of the second frame described below; and a relay processing unit which performs relay processing to determine the output port of a received frame and protocol conversion to convert the format of the received frame to the format of the transmitting side, the flow control method comprising: a step of acquiring the amount of the transmit buffer of the first port; and a step of transmitting the second frame from the second port, if the acquired amount of the transmit buffer is greater than or equal to a threshold, setting an identifier representing the transmission priority to a predetermined value. First frame: A communication frame that conforms to a first communication protocol and includes an identifier representing the transmission priority. Second frame: A communication frame that conforms to a second communication protocol whose maximum transmission speed is greater than that of the first communication protocol and includes an identifier representing the transmission priority.

10. A computer program for causing a computer to function as an in-vehicle relay device comprising: a first port which is the physical port for the first frame described below; a second port which is the physical port for the second frame described below; and a relay processing unit which performs relay processing to determine the output port of a received frame and protocol conversion to convert the format of the received frame to the format of the transmitting side, the computer program comprising: a step of acquiring the amount of the transmit buffer of the first port; and a step of transmitting the second frame from the second port, with an identifier representing the transmission priority set to a predetermined value, if the acquired amount of the transmit buffer is equal to or greater than a threshold. First frame: A communication frame conforming to the first communication protocol, containing an identifier representing the transmission priority. Second frame: A communication frame conforming to the second communication protocol, which has a higher maximum transmission speed than the first communication protocol, containing an identifier representing the transmission priority.

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