JP2007096737A - Communication controller and its control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a communication controller capable of correctly recognizing a faulty packet and increasing error resistance, and to provide its control method. <P>SOLUTION: A USB controller 48 stores a packet size which is received in a size register 84, counts re-transmission to a re-transmission counter 86, inputs the packet size received by a size inspecting part 104 and the counts, compares the predetermined packet size with the received packet size, adopts at least one of a comparison result and the counts as a size condition, inspects the received packet with respect to the size condition, determines whether no response is made to a handshake to a transmission side in response to an inspection result, performs control in response to the determination so as to make no response to the handshake, and then, allows the transmission side to adequately perform re-transmission relative to the erroneous recognition of the packet size. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a communication control device and a control method therefor. The communication control apparatus according to the present invention particularly relates to a technique for improving resistance to errors that occur in USB (Universal Serial Bus) communication. The communication control method of the present invention relates to a procedure for accurately determining packet misrecognition caused by, for example, a short packet of USB communication and controlling packet communication in accordance with this determination.

In USB (Universal Serial Bus) communication, various conditions such as clock quality, signal quality due to the influence of signal waveform, jitter, skew, etc., and noise countermeasures are defined to ensure communication quality. Also, USB communication employs a mechanism for detecting errors by providing redundant fields such as CRC (Cyclic Redundancy Check) in each field of a packet even for communication errors. By using handshaking properly, USB communication not only allows flow control, but also provides a mechanism for retransmitting detected errors.
JP 2001-148719 A Japanese Patent Laid-Open No. 2001-211210

  As described above, the USB communication is provided with various regulations relating to communication quality, packet error detection, and retransmission. However, the packet length is recognized as the end of the packet by detecting EOP (End Of Packet). Therefore, the packet does not have a field indicating the packet length. For this reason, if the EOP is detected incorrectly, the packet length cannot be known even for a normal packet. Furthermore, the packet length in USB communication is variable.

  Here, when packet corruption occurs and thus EOP is misrecognized, the packet is treated as a short packet. In this case, the packet corruption depends on detection based on the error detection function of the field by the redundant part. For example, if a packet is treated as a short packet, there is no restriction on the data sequence in the data field, so the CRC value by chance in the CRC check by the part that was successfully transferred and the part that was treated as a CRC field. May match. When this occurs, the corrupted packet is recognized as a normal packet as a result.

  By the way, at the full speed of USB communication specified in version l.1 of the USB communication standard and inherited in version 2.0 of the USB communication standard, EOP is difficult to be reproduced elsewhere as a bus condition representing the type of signal. It was defined as the SE0 (Send End 0) state of bit time. Since the normal signal is a differential signal, the phase is reversed between the two data lines, but in SEO, the two data lines are both low level signals. That is, it will continue for 2 bit times. As a result, the possibility of hindering the detection of such a damaged packet cannot be completely eliminated, but the possibility has been kept low.

  On the other hand, EOP is a high-speed communication added in version 2.0 of the USB communication standard, and the definition differs from full-speed communication. In high-speed communication, EOP was defined as 8-bit time continuation in J state or K state, which is the same differential signal as normal data bits. The J and K states are the same states as normal data bits, but in USB communication, if the same state continues for 7 or more bits due to the mechanism of bit stuff, there is a rule that the J and K states toggle, and the 8-bit time continues The J and K states do not occur as data sequences, and EOP can be identified.

  Here, when an error with respect to this bit stuff, that is, a bit stuff error occurs, it is detected as an error in full-speed communication, and the packet of the detected error is discarded. However, it is specified that in high-speed communication, the bus condition immediately becomes the same as that of EOP, and it is treated as EOP even in version 2.0 of the USB communication standard. As a result, a bit stuff error cannot be detected. Therefore, EOP was not misrecognized unless an error of 2 bits occurred in full-speed communication. However, in high-speed communication, EOP was misrecognized even with an error of 1 bit depending on the data arrangement. . As a result, error tolerance for EOP detection is lower in high-speed communication than in full-speed communication. As a result, there is an increased risk of handling a short packet that is damaged due to erroneous recognition of EOP as a normal packet.

  This danger will be briefly explained. If the CRC coincides and the ACK (ACKnowledge) response is returned before the data transmission for the rest of the error packets in which EOP is misrecognized, the transmitting side cannot receive the ACK response. For this reason, the transmission side recognizes that the packet transmission has failed, that is, determines that the abnormal transaction has been completed. However, if the ACK response is returned after the data for the remainder of the error packet is transmitted, the transmitting side receives the ACK response, so that it is erroneously recognized as normal reception. As a result, transmission data is not retransmitted.

  It is an object of the present invention to provide a communication control device and a control method therefor that can eliminate such drawbacks of the prior art, correct recognition of damaged packets, and further improve error resistance.

  In order to solve the above-mentioned problem, the present invention receives a packet supplied from the transmission side, analyzes the received packet, and based on the information obtained by the analysis, the transmission side according to the reception state of the received packet In the communication control device for controlling the communication of the received packet, the device includes an information storage means for storing the first packet size obtained by analyzing the received packet, and a packet supplied from the transmission side. Counting means for counting the number of times of retransmission, a first packet size and the number of times are input, a second packet size set in advance for the received packet is compared with the first packet size, and at least , One of the comparison result and the number of times is set as a size condition, and the received packet for this size condition is inspected, and Flip handshaking to the sender determines whether the no-response, characterized in that it comprises a size checking means for controlling.

  The communication control apparatus of the present invention stores the first packet size in the information storage means, counts the number of retransmissions in the counting means, receives the first packet size and the number of times in the size inspection means, The second packet size set in advance and the first packet size are compared, and at least one of the comparison result and the number of times is set as the size condition, and the received packet for the size condition is inspected. If the packet payload is within the specified size, the ACK is normally determined regardless of the size. (ACKnowledge) The packet size by returning no response when the handshake is not responded depending on the size condition match or the number of times. Is the correct retransmitted against erroneous recognition to the sender.

  In order to solve the above-described problem, the present invention receives a packet supplied from the transmission side, analyzes the received packet, and based on information obtained by the analysis, according to the reception state of the received packet. In the communication control method for controlling the communication of the received packet with respect to the transmission side, this method is supplied from the transmission side and a first step for obtaining the size of the packet received by analysis as the first packet size. A second step of counting the number of times a packet is retransmitted, a second packet size is set in advance, and the comparison result and the number of times between the first packet size and the second packet size are used as size conditions. The third step of inspecting the received packet for the size condition, and the handshake to the transmission side according to the inspection result of the received packet, no response And the fourth step of controlling whether or not the handshake to the transmitting side is made to be unresponsive when the number of times is smaller than the limit for allowing retransmission of the packet. If the number of times matches the limit, the transmission side is controlled as normal, and the received packet error is asserted by an interrupt to the upper layer component specified by this communication control method. It is characterized by holding a packet.

  The communication control method according to the present invention obtains the size of a packet received by analysis as the first packet size, counts the number of times the packet supplied from the transmission side is retransmitted, and sets a second packet size in advance. Then, the comparison result between the first packet size and the second packet size and the number of times counted are used as the size condition, the received packet is inspected for the size condition, and the transmission side is inspected according to the inspection result of the received packet. Determines whether or not to make the handshake unresponsive, and controls this. If the number of times is less than the limit that allows packet retransmission, the handshake to the sending side is made unresponsive and the number matches the limit. In this case, the transmission side is controlled as normal, and the received packet is received for the upper layer component specified by this communication control method. Bets errors asserted by interrupt, corrupted packets by holding this received packet, in particular, properly awareness of short packets, it improves the more error tolerant.

  According to the communication control device and the communication control method of the present invention, an error that cannot be detected by the USB protocol due to the size condition determined by the upper layer protocol can be detected and the error detection rate can be increased. In addition, the resend unit is reduced and the response speed of hardware is the same as that of normal error packets, so that the impact on endpoint transfer performance and unnecessary packet transfer can be eliminated, and the bandwidth of the entire USB bus Can also be used effectively.

  Next, an embodiment of a communication control apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

  In this embodiment, the communication control device of the present invention is applied to the USB communication device 10. The illustration and description of parts not directly related to the present invention are omitted. In the following description, the signal is indicated by the reference number of the connecting line in which it appears.

  As shown in FIG. 2, the USB communication device 10 has a hierarchical structure in which hardware 12 (HW) and software 14 (SW) for controlling a protocol are stacked. Further, from the viewpoint of the layer, it can be roughly divided into the USB bus 1 / F layer 16, the USB logical device layer 18, and the function layer 20.

  The USB bus 1 / F layer 16 includes a USB physical layer (PHYsical HW) 22 and a USB controller (link layer HW) 24. The physical layer HW (22) has functions such as clock synchronization, bus state detection, EOP detection, NRZI (Non-Return to Zero Invert) codec, and bit stuff / unstuff. NRZI is a method of inverting the polarity every time data “1” appears in a modulation code bit.

  The USB link layer HW (24) has functions such as packet error detection, transaction management, address, and endpoint management. It has an interface for accessing each endpoint as a communication path.

  The USB logical device layer 18 functions as a USB device to control PnP (Plug and Play), power management, status management, and the like using standard commands, and manage communication in units of transfer in which a plurality of transactions are collected. The USB logical device layer 18 is implemented with a protocol for realizing this function. This is the USB peripheral driver 26, for example.

  The function layer 20 has a function of controlling functions as a peripheral device. In the function layer 20, a protocol for each function in which several transfers are combined is implemented. In the present embodiment, the functions are not limited. For example, the USB class driver 28 and the application 30 are included as functions of a large-capacity storage class.

  An example of the USB communication device 10 including the above-described components is a USB HDD (Hard Disk Drive). A USB HDD 32 is shown in FIG. As shown in FIG. 3, the USB HDD 32 is arranged between the USB host 34 and the device 36, and is responsible for communication between both devices. In this case, the USB HDD 32 is mounted with a mass storage class 38 defined using the USB device class as a class driver. In addition, as an application, a command analysis AP (APplication) 40 is analyzed according to software, and a USB to IDE (Integrated Drive Electronics) bridge function is implemented and realized by processing data.

  Here, IDE is one of hard disk interfaces for compatible computers of personal computers.

  The USB event control software 42 is a specific example of the USB logical device layer 18 in the USB HDD 32. The device driver 44 is a driver on the device 36 side.

  The control unit 46 includes a USB device controller 48 and a device controller 50. The USB device controller 48 has a function of controlling transmission / reception of USB packets and preventing a reduction in error resistance caused by recognizing a short packet as a normal packet by erroneous recognition by applying the present invention. A configuration example for realizing this function will be described below. The device controller 50 has a function of controlling communication with the device side.

  The device 36 includes ATA (Advanced Technology Attachment) / SCSI (Small Computer System Interface) standard HDD / CD-ROM (Compact Disk-Read Only Memory) / FDD (Floppy Disk Driver) (registered trademark).

  Next, FIG. 1 shows a schematic configuration example of the USB device controller 48 in the present embodiment. The USB controller 48 includes a plurality of endpoint processing units, for example, as endpoints A (52), B (54),..., A transaction control unit 56, MUX (MUltipleXer) 58 and 60, and data packet transmission Processing unit 62, handshake transmission processing unit 64, token transmission processing unit 66, PID (Packet Identifier) generation unit 68, data packet transmission processing unit 62, handshake transmission processing unit 64, token transmission processing unit 66, PID (Packet Identifier) generation Unit 68, data packet reception processing unit 70, handshake reception processing unit 72, token reception processing unit 74, and PID inspection unit 76.

  The endpoint A (52) is shown on behalf of the endpoint processing unit. The endpoint A (52) includes a first-in first-out (FIFO) memory 78, a transmission / reception counter 80, a status register 82, and a size register 84 and a retransmission counter 86 as features. The size register 84 is used for both transmission and reception for each endpoint, and has the function of storing the transmission size specified in the known and predictable size at the time of transmission, and the reception size and size check circuit valid / invalid at the time of reception It also has a function to specify. When invalid is designated by the latter designation function, the USB controller 48 can be operated in the same manner as a normal USB controller. The USB controller 48 can dynamically set the size register 84 from a register interface created on an MCU (Micro-Controller Unit) interface. The size register 84 operates to implement each designated function.

  The retransmission counter 86 has a function of measuring the number of times a packet has been retransmitted by using it exclusively for reception. Endpoints A, B,... Are connected to MUX 58. In particular, the size register 84 and the retransmission counter 86 supply the packet size and the number of retransmissions to the data packet reception processing unit 70, respectively.

  The transaction control unit 56 has a function of controlling an operation by supplying an enable signal 86 to the MUXs 58 and 60 and controlling each state in transmission and reception. In order to realize this function, the transaction control unit 56 includes a data packet transmission processing unit 62 as a transmission processing block, a handshake transmission processing unit 64, a token transmission processing unit 66, a PID generation unit 68, and a data packet as a reception processing block. Control signals 88 to 102 are supplied to the reception processing unit 70, the handshake reception processing unit 72, the token reception processing unit 74, and the PID checking unit 76.

  Although not shown, data packet transmission processing unit 62 includes a CRC generation unit and a toggle value generation unit. The data packet transmission processing unit 62 is connected to the MUX 60. The handshake transmission processing unit 64 is connected to the PID generation unit 68, and the token transmission processing unit 66 is also connected to the PID generation unit 68. Although not shown, the token transmission processing unit 66 includes an endpoint number generation unit and an address generation unit. The PID generation unit 68 is connected to the MUX 60. In the transmission processing block, each field value of the packet is generated. Each processing block of the data packet is connected to each register of the endpoint by a data path.

  The data packet reception processing unit 70 includes a size inspection unit 104, a CRC inspection unit 106, and a toggle value inspection unit 106. The size inspection unit 104 has a function of detecting a packet size error. The size checking unit 104 includes not only the transaction control unit 56 but also a data path to the size register 84 for each end point, the reception counter for retransmission and the retransmission counter 86. As a result, the size inspection unit 104 acquires values of the size register 84, the reception counter, and the retransmission counter 86. The size checking unit 104 can set a value for the retransmission counter 86 based on the acquired value, and checks the size based on the acquired value. Even in the existing USB controller, as the size check, only the maximum packet size of the corresponding endpoint is checked, but in this embodiment, in addition to this, the packet size specified by the size register 84 and the actual packet are also checked. Compare with size and inspect. If the comparison inspection does not match, the transaction control unit 56 is notified as an invalid packet. As a result, the handshake can be made unresponsive. As a reception processing block, the data packet reception processing unit 70, the handshake reception processing unit 72, the token reception processing unit 74, and the PID inspection unit 76 have a function of inspecting each field.

  The above-described retransmission counter 86 has a function of storing the number of consecutive retransmissions that are incremented when an error determination is made by the size inspection unit 104 and cleared when a normal determination is made. The stored number of retransmissions is also used when selecting a handshake to be performed after determination of size inspection.

  The operation of the data packet reception processing unit 70 will be briefly described. The data packet reception processing unit 70 performs toggle value inspection and CRC inspection, and when the received packet is determined to be a normal packet after completion of these inspections, the size inspection unit 104 executes size inspection. In the size inspection, the designation of validity / invalidity of the size inspection is acquired from the size register 84 of the endpoint corresponding to the received packet. If the acquired size inspection result is valid, the size inspection is executed, and if it is invalid, the size inspection is not executed.

  When performing the size inspection, the size inspection unit 104 acquires the reception size from the size register 84 and the reception counter of the transmission / reception counter 80, respectively. The size inspection unit 104 compares the acquired reception sizes. If the values of the reception sizes match, the size inspection unit 104 determines that it is normal. Based on the determination result, the data reception packet processing unit 70 asserts reception of a normal packet to the transaction control unit 56. Along with this, the retransmission counter 86 of the corresponding endpoint is returned to zero, and the process ends.

  If the size inspection unit 104 does not match the size values, the size inspection is determined to be abnormal. Then, the retransmission counter 86 is further confirmed. If the count value of the retransmission counter 86 is 2 or less, the data reception packet processing unit 70 asserts reception of an abnormal packet to the transaction control 56. On the other hand, if the count value of the retransmission counter 86 is 3, the data reception packet processor 70 asserts reception of a normal packet. Along with this assertion, the data reception packet processing unit 70 asserts a short packet reception interrupt as an external interrupt request to the MCU. Then, the retransmission counter 86 is returned to zero and the processing is terminated.

  A communication process including a software operation in the USB controller 48 of the present embodiment, particularly a packet reception procedure will be described with reference to FIGS. 4 and 5. FIG. FIG. 4 shows the procedure of the USB control software of the receiving endpoint. First, initialization or the like is performed so that the USB controller 48 can operate (step S10). Thereafter, the setting is made for the endpoint assigned to data reception (step S12). This setting is performed by software for the size register 84 of the end point. The size register 84 sets the packet size expected for the supplied packet and whether or not reception of a short packet is permitted.

  Next, it is determined whether or not there is a transfer request from the application (step S14). If there is a transfer request (YES), the process proceeds to transfer acceptance / analysis processing (to step S16). If there is no transfer request (NO), this procedure is terminated.

  The transfer acceptance / analysis process (step S16) accepts a transferred packet and analyzes the packet. In this analysis, the USB controller 48 obtains the size of the received packet. Thereafter, the process proceeds to transfer reception processing (subroutine SUB1). In the transfer reception process, the size of the received packet is evaluated and received according to normal evaluation.

  Next, it is determined whether or not there is a transfer request (step S18). If there is no transfer request (YES), it is determined that there is no packet to be received, and data reception is terminated. If there is a transfer request (NO), the process returns to the transfer acceptance / analysis process (to step S16).

  Next, the transfer reception process (subroutine SUB1) will be briefly described. It is determined whether the size of the received packet is known or predictable (substep SS10). If the size of the received packet is either known or predictable (YES), the process proceeds to the setting of the packet size (go to substep SS12). If the packet size is either indefinite or variable length (NO), the process proceeds to packet reception (go to sub-step SS14).

  Next, the size of the received packet is set (substep SS12). Next, a size error of the received packet is detected (substep SS16). When the size check unit 104 receives a size error, the USB controller 48 compares the value of the setting register, that is, the size register 84, with the received packet size obtained from the result of packet analysis when data is received. If the compared values are different, it is determined as an invalid packet. At this time, the USB controller 48 discards the packet without changing the data toggle, and the handshake does not respond.

  However, the size inspection unit 104 receives the number of retransmissions from the retransmission counter 86 and controls communication according to the number of retransmissions. For example, when the receiving side makes no response for three consecutive times, the size checking unit 104 determines that the transmitting side cannot communicate, and therefore controls no response up to two times. When the packet size is different three times consecutively, the size checking unit 104 returns an ACK handshake and receives it as a valid packet (substep SS14). In this case, an interrupt indicating reception of a size error packet is generated.

  If the size comparison results are the same value, the size checking unit 104 returns a handshake according to the situation and generates an interrupt indicating packet reception.

  Next, it is determined whether or not there is an interrupt (substep SS18). If there is no interrupt, return to substep SS18 and wait. If there is an interrupt (YES), it is determined whether or not the reception is normal (substep SS20). In the case of normal reception (YES), the process proceeds to transfer completion determination (go to sub-step SS22). In the case of abnormal reception (NO), it is determined as a size error and the process proceeds to return. Then, as software control when an error packet reception interrupt occurs at this time, a transfer error is notified to the upper layer.

  Next, it is determined whether or not the transfer is completed (substep SS22). If the transfer is not completed (NO), it is determined that there are still packets to be received, and the process returns to the determination process of whether the packet size is known and predictable (go to sub-step SS10). When the transfer is completed (YES), it is determined that the transfer reception is completed, and the process proceeds to return.

  If the USB controller 48 can know the packet size to be transferred depending on the host application and class driver in this way, it can realize retransmission of only error packets by adding simple hardware and controlling the software. it can. In this case, in particular, since retransmission is performed in units of packets, which is the minimum retransmission unit in USB, by software, it is preferable to perform error determination and response selection by hardware as a retransmission method in units of packets.

  On the other hand, a method of executing retransmission at an early stage by changing a response by software is also conceivable, but in this case, a retransmission unit becomes large. This is because, in general, in a hierarchical protocol, the higher the hierarchy, the larger the data unit handled.

  To summarize the configuration and operation specifically described above, the USB controller 48 has been improved so that it can set the size condition for each endpoint, determine the error according to the size, and select the handshake of the transaction according to this, and respond to short packets Has a function that can be set before receiving a packet. The USB controller 48 sets validity / invalidity of handshake selection as an optional function via the register interface, together with determination of the size condition and the size error, by software. In addition, the USB controller 48 allows this setting to be changed dynamically and allows this function to be controlled dynamically.

  In this software, it is preferable to perform control so that the size error retransmission function is enabled / disabled and the size condition is reset according to the transfer size condition determined by the upper layer protocol. The size condition should be set for each packet.

  In the case of bulk transfer, all but the last packet in one transfer is transferred with the payload size, and only the last packet is the number of remaining bytes in the transfer. Therefore, control is performed so that the size condition is set for the last packet of the transfer and others. It is desirable.

  Then, regarding the third retransmission, which is the maximum number of continuous retransmissions, the USB controller 48 determines that there is no error in the lower protocol, the hardware responds with an ACK, receives normally, and interrupts the short packet error to the MCU. Is generated. As a result, it is possible to cope with a short packet cancellation in which the transfer size is smaller than the expected size.

  With the above configuration, it is possible to increase the error detection rate by detecting an error that cannot be detected by the USB protocol based on the size condition determined by the upper layer protocol. Naturally, if the retransmission unit is large, the retransmission time will be longer and not only the transfer performance of the endpoint to be transferred will be degraded, but also the USB bandwidth will be consumed due to the transfer of useless packets, and the entire USB bus will be consumed. Reduces bandwidth. According to the present embodiment to which the present invention is applied, the resend unit can be reduced, and the response speed can be reduced by hardware in the same manner as a normal error packet, thereby eliminating the influence on the performance.

It is a block diagram which shows schematic structure of the USB controller in the USB communication apparatus to which the communication control apparatus which concerns on this invention is applied. It is a block diagram which shows the schematic hierarchical structure in the USB communication apparatus of FIG. FIG. 2 is a block diagram illustrating a schematic configuration of a USB HDD as the USB communication device in FIG. 1. 2 is a flowchart for explaining a data reception procedure in the USB controller of FIG. 1. 5 is a flowchart for explaining a procedure of a transfer reception process in FIG. 4.

Explanation of symbols

10 USB communication device
48 USB controller
52, 54 Endpoint A, B
84 Size register
86 Retransmission counter
104 Size inspection section

Claims (10)

Receives a packet supplied from the transmission side, analyzes the received packet, and controls the communication of the received packet to the transmission side according to the reception state of the received packet based on information obtained by the analysis In the communication control device, the device
Information storage means for storing a first packet size obtained by analyzing the received packet;
Counting means for counting the number of times the packet supplied from the transmission side is retransmitted;
The first packet size and the number of times are input, the second packet size set in advance for the received packet is compared with the first packet size, and at least one of the comparison result and the number of times Size check means for inspecting the received packet for the size condition, determining whether or not to make no response to the handshake to the transmitting side according to the inspection result, A communication control device.   The communication control apparatus according to claim 1, wherein the second packet size is a known size and a predictable size.   3. The apparatus according to claim 1, wherein the size inspection unit returns a response to the packet supplied at a limit that allows the retransmission of the packet, and sets the reception of the packet as normal to the transmission side. A communication control device that controls, asserts an error of the received packet to an upper layer component of the device by an interrupt, and holds the received packet.   4. The communication control device according to claim 3, wherein the information storage unit and the counting unit are arranged in a circuit managed for each end point.   5. The communication control apparatus according to claim 3, wherein the apparatus controls communication in accordance with a USB (Universal Serial Bus) standard. Receives a packet supplied from the transmission side, analyzes the received packet, and controls the communication of the received packet to the transmission side according to the reception state of the received packet based on information obtained by the analysis In the communication control method, the method includes:
A first step of determining the size of the received packet as a first packet size by the analysis;
A second step of counting the number of times the packet supplied from the transmission side is retransmitted;
A third step in which a second packet size is set in advance, the comparison result between the first packet size and the second packet size and the number of times are set as size conditions, and the received packet is inspected with respect to the size conditions When,
Determining whether or not to make no response to the handshake to the transmission side according to the inspection result of the received packet, and a fourth step of controlling,
In the fourth step, when the number of times is smaller than the limit for allowing retransmission of the packet, the handshake to the transmission side is controlled to be no response, and when the number of times matches the limit, the transmission side A communication control method, comprising: controlling as normal, asserting an error of the received packet with an interrupt to an upper layer component defined by the communication control method, and holding the received packet .   7. The communication control method according to claim 6, wherein the third step operates according to whether the size inspection preset as the size condition is valid or invalid.   The method according to claim 6 or 7, wherein, in the case of bulk transfer, the method transfers a packet other than the last packet in one transfer with a payload size, and sets the remaining packet number of the transfer only for the last packet. And a communication control method characterized by setting and controlling size conditions for other packets.   The communication control method according to claim 8, wherein the method sets and controls the switching of the size condition and retransmission valid / invalid for each packet.   10. The communication control method according to claim 8, wherein the method controls communication according to a USB (Universal Serial Bus) standard.

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