JP2008160377A - Band securing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a band securing device which secures a band to avoid congestion at the time of data sending and is capable of achieving high-speed communication without cooperative operation with a communication path control device when a communication device using this invention performs data transmission through a network. <P>SOLUTION: When an iSCSI server 10 receives a read request for data equal to or larger than a prescribed size from a client, consequently, when one embodiment 20 of this invention receives the read request, one embodiment 20 of this invention performs dummy communication in a band equal to or approximately equal to a band of actual data communication of the iSCSI server 10 with respect to a sending source IP address of the read request till the iSCSI server 10 sends data in response to the read request. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a bandwidth securing device used by a communication device that performs data communication via a network such as the Internet to secure a bandwidth (communication speed).

  In the Internet where a plurality of users' communication shares one line, there is generally no guarantee that the band necessary for communication can be used. However, operation of various servers such as a storage server and an iSCSI (Internet small computer system interface) server may be substantially difficult with a communication performance of a certain band or less depending on the contents of the provided service. Therefore, some band adjustment is required.

  TCP (transmission control protocol), which is a standard communication method on the Internet, is a method of appropriately adjusting the band used for each communication according to the current network conditions. That is, in the TCP communication, when congestion occurs in the communication path, each communication takes an operation of once narrowing the band rapidly and then gradually increasing the band to search for an available band.

  In addition, when packets occur in the transmission data due to congestion, the lost packets must be retransmitted, so the effective bandwidth becomes smaller than the bandwidth that can be used originally and communication performance is degraded. It will end up.

  As described above, in the TCP communication, when congestion occurs, adjustment for narrowing the band and retransmission of the lost packet are performed, which cause high-speed communication. This is more conspicuous as the delay of the communication path is larger, and the influence is great when a large band is used in a long-distance network.

  As a technique for securing a bandwidth in the Internet, a technology (QoS [quality of service]) that reserves a bandwidth for a specific communication and guarantees a certain bandwidth has been proposed. However, in order to realize this, all the routers in the route to be used have to be set correctly according to the method, and it is not widely used at present.

  In addition, the Internet was limited to low functions such as route control where the network itself had no state, and high-dimensional functions that required session management could be expanded and expanded by guaranteeing the communication end nodes. . From this point of view, it can be expected that an existing method that requires negotiation of a bandwidth guarantee session with all routers in the route before starting communication is difficult to spread.

  For example, in the case of a communication method such as ATM (asynchronous transfer mode) in which a network band is allocated to a user in advance, stable communication can be performed without affecting each other between a plurality of communications. However, in this case, all routers need a function of controlling the bandwidth and must be managed in a unified manner. Therefore, it is difficult to construct and operate a large-scale network.

  In addition, even in an IP (internet protocol) network, when a function such as RSVP (resource reservation protocol) that can reserve a bandwidth up to a transmission destination is used, it is possible to secure a bandwidth and perform stable communication. In this case, the RSVP function is necessary for the router on the way, and it is necessary to be managed in a unified manner. This is difficult when the entire route cannot be managed by itself, as in the Internet.

Here, there is a technique described in Patent Document 1 as a technique for improving the performance of a communication technique in which a bandwidth is allocated in advance. This is because the delay required for changing the bandwidth allocation can be concealed by using a freely available bandwidth called a pool, but the basic property that network bandwidth management / allocation is necessary is RSVP. And no different.
JP-A-5-14410

  In view of such points, the present invention enables a communication apparatus using the present invention to transmit data via a network, and in certain cases, to secure a bandwidth and avoid congestion at the time of data transmission, It is an object of the present invention to provide a bandwidth securing device that can achieve high-speed communication without a cooperative operation with a communication path control device.

  The present invention is a bandwidth securing device connected between one communication device and a network, and causes a dummy communication to be transmitted from the other communication device connected to the network to the one communication device. A first means for detecting a data request; a second means for detecting data transmission from the one communication apparatus to the other communication apparatus in response to the data request; and the first means After the request is detected, a third means for transmitting dummy data to the other communication device is provided until the second means detects the data transmission.

  In the present invention, after the first means detects a data request that causes a dummy communication to be transmitted from the other communication device to the one communication device, the second means detects the one communication device. The third means transmits dummy data to the other communication device until the transmission of data to the other communication device is detected.

  In this case, when congestion occurs in the communication path in the network, there is a possibility that another congestion control communication using the communication path in which the congestion has occurred performs congestion control by reducing the bandwidth. In this case, the one communication device can secure the bandwidth of the communication path, avoid congestion at the time of data transmission, and achieve high-speed communication.

  In this way, the present invention enables dummy communication to other communication devices before the communication device using the present invention transmits data in response to a data request that causes dummy communication from the other communication devices. If the communication path is congested due to this, the congestion control of other congestion control communication using the congested communication path is expected, so that the communication apparatus using the present invention The bandwidth is guaranteed to avoid congestion at the time of sending this data, and high-speed communication is achieved without cooperative operation with the communication path control device.

  FIG. 1 is a diagram showing a usage mode of an embodiment of the present invention. In FIG. 1, 10 is an iSCSI server that employs a TCP communication system that is a congestion control communication system, 20 is an embodiment of the present invention, and 30 is the Internet.

  An embodiment 20 of the present invention is used by the iSCSI server 10 to secure a bandwidth on the Internet 30. In this example, the embodiment 20 is installed on a communication path between the iSCSI server 10 and the Internet 30. However, it may be incorporated as part of the iSCSI server 10.

  FIG. 2 is a block circuit diagram showing a main part of one embodiment 20 of the present invention. An embodiment 20 of the present invention includes an iSCSI command observation device 21, an iSCSI data observation device 22, a dummy communication generation device 23, a status table 24, a status table management device 25, and a timer 26.

  The iSCSI command observation device 21 observes and interprets the iSCSI command sent from the client to the iSCSI server 10 via the Internet 30, and a read request (sector read command) from the same client requests data having a predetermined size or more. If it is determined that this is a data request, this is detected as a data request that causes a dummy communication, and this detection result is transmitted to the dummy communication generator 23 together with the read request source IP address. .

  Here, the data of a predetermined size or more is data of a size for which the iSCSI server 10 desires high-speed communication while avoiding congestion at the time of data transmission to the Internet 30. The predetermined size is arbitrarily determined in advance. For example, if the requested sectors are consecutive, if the requested sector number described in the instruction is greater than or equal to the predetermined number, there is a request to read data larger than the predetermined size. It is possible to adopt a method for judging, and when the requested sectors are not continuous, the data of a predetermined size or more is obtained when the total number of consecutively requested sectors is a predetermined number. It is possible to adopt a method for determining that there has been a read request.

  When the iSCSI data observation device 22 observes data transmission from the iSCSI server 10 and detects data transmission from the iSCSI server 10, the iSCSI data observation device 22 transmits the detection result to the dummy communication generation device 23 together with the data transmission destination IP address. When the end of data transmission from the iSCSI server 10 is detected, the detection result is transmitted to the dummy communication generator 23 together with the data transmission destination IP address.

  The dummy communication generator 23 is configured to detect data transmission from the iSCSI server 10 in response to the read request after the iSCSI command observation device 21 detects a read request for data of a predetermined size or more. During this period, dummy data is sent to the client that has made the read request, and dummy communication is generated.

  That is, when the iSCSI server 10 receives a read request from a client, it reads data from its own disk in response to this and sends it to the client. Therefore, when the iSCSI server 10 transmits data of a predetermined size or more using this time, the embodiment 20 of the present invention transmits dummy data before that. It is to do.

  The dummy communication by the dummy communication generator 23 is a protocol that does not perform congestion control, such as UDP (user datagram protocol) or ICMP (internet control message protocol), and is the same or approximately the same as the data communication of the iSCSI server 10. It is assumed that it is performed in the bandwidth. The dummy data uses a client discard port as a destination port. In this way, no special device is required for the client.

  The status table 24 shows a source IP address of a read request for data of a predetermined size or more and a communication status on the iSCSI server 10 side with respect to the source IP address (dummy communication status by the dummy communication generator 23 / main data by the iSCSI server 10). Communication state) and the update time of the item are registered in association with each other.

  When the state table management device 25 receives an interrupt of a certain period from the timer 26 and there is an update time that is not updated for a certain period as a management operation of the state table 24, the update time, the corresponding IP address, and communication The state is deleted.

  FIG. 3 is a diagram showing the configuration of the state table 24. The status table 24 includes an IP address field, a communication status field, and an update time field. In the IP address field, the IP address of the transmission source of a read request for data of a predetermined size or larger is registered, and in the communication status field. The communication state on the iSCSI server 10 side with respect to the IP address registered in the IP address column is registered, and the current time when the IP address registration or communication state registration / update is performed is registered in the update time column.

  As a communication state, when dummy communication by the dummy communication generator 23 is started, “Dummy” is described in the communication state column, and when this data communication by the iSCSI server 10 is started, a communication state column. Is described as “Data”.

  Therefore, in the example of FIG. 3, in the state table 24, dummy communication is started by the dummy communication generator 23 for the IP address Addr1 at time T1, and main data communication by the iSCSI server 10 for the IP address Addr2 is started at time T2. This indicates that dummy communication by the dummy communication generator 23 for the IP address Addr3 is started at time T3.

  FIG. 4 is a flowchart showing a first operation of the dummy communication generator 23. In the twentieth embodiment of the present invention, the dummy communication generator 23 repeats the operations from the first operation repetition start to the first operation repetition end shown in FIG. 4 as the first operation. It is determined whether or not the command observation device 21 has detected a read request for data of a predetermined size or larger (step S101).

  Here, when the iSCSI command observation device 21 does not detect a read request for data of a predetermined size or larger (in the case of NO in step S101), that is, the dummy communication generator 23 receives data of a predetermined size or larger from the iSCSI command monitoring device 21. If the detection result indicating that the read request has been detected is not received, the dummy communication generator 23 returns to step S101 and determines whether or not the iSCSI instruction observation device 21 has detected a read request for data of a predetermined size or larger. Continue.

  On the other hand, when the iSCSI command observation device 21 detects a read request for data of a predetermined size or larger (in the case of YES in step S101), that is, the dummy communication generator 23 receives a data of a predetermined size or larger from the iSCSI command monitoring device 21. When receiving the detection result indicating that the data read request has been detected, the dummy communication generator 23 registers the IP address of the source of the read request given from the iSCSI command observation device 21 in the status table 24. It is determined whether or not (step S102).

  Here, if the IP address given from the iSCSI command observation device 21 is already registered in the state table 24 (YES in step S102), the dummy communication generating device 23 returns to the operation in step S101.

  On the other hand, when the IP address given from the iSCSI command observation device 21 is not registered in the state table 24 (NO in step S102), the dummy communication generator 23 displays the IP address field in the state table 24. The IP address given from the iSCSI command observation device 21 is registered in the communication status column, “Dummy” indicating the start of dummy communication is registered in the communication status column, and the current time is registered in the update time column (step S103).

  Then, the dummy communication generator 23 starts transmission of dummy data toward the IP address given from the iSCSI command observation device 21, generates a dummy communication state (step S104), and returns to the operation of step S101.

  FIG. 5 is a flowchart showing the second operation of the dummy communication generator 23. In the twentieth embodiment of the present invention, the dummy communication generator 23 repeats the operations from the second operation repetition start to the second operation repetition end shown in FIG. 5 as the second operation. It is determined whether or not the data observation device 22 has detected data transmission from the iSCSI server 10 (step S201).

  Here, if the iSCSI data observation device 22 does not detect data transmission from the iSCSI server 10 (NO in step S201), that is, the dummy communication generator 23 sends data from the iSCSI data observation device 22 to the iSCSI server 10. If the detection result is not received, the dummy communication generator 23 returns to the operation of step S201, and determines whether the iSCSI data observation device 22 has detected the data transmission from the iSCSI server 10 or not. to continue.

  On the other hand, when the iSCSI data observation device 22 detects data transmission from the iSCSI server 10 (YES in step S201), that is, the dummy communication generation device 23 is sent from the iSCSI data observation device 22 to the iSCSI server 10. When receiving the detection result indicating that the data transmission has been detected, the dummy communication generating device 23 determines whether the IP address of the data transmission destination given from the iSCSI data observation device 22 is already registered in the status table 24. Is determined (step S202).

  Here, when the IP address of the data transmission destination given from the iSCSI data observation device 22 is not registered in the status table 24 (NO in step S202), the dummy communication generator 23 causes the iSCSI data observation device 22 to It is determined that the data transmission from the iSCSI server 10 detected by is not a data transmission related to a read request for data of a predetermined size or more, and the process returns to the operation of step S201.

  On the other hand, when the IP address of the data transmission destination given from the iSCSI data observation device 22 is already registered in the status table 24 (YES in step S202), the dummy communication generator 23 sets the dummy communication status. (Step S203).

  Here, in the dummy communication state (in the case of YES in step S203), the dummy communication generator 23 ends the dummy communication for the IP address given from the iSCSI data observation device 22 (step S204), and the state The corresponding part of Table 24 is updated (step S205). In this case, “Dummy” in the communication status column is updated to “Data”, and the update time is updated to the current time.

  On the other hand, if the dummy communication generating device 23 determines that the dummy communication state has ended in step S203 (NO in step S203), that is, if it determines that the data communication state is present, the state table 24 corresponding parts are updated (step S205). In this case, the update time corresponding to the IP address given from the iSCSI data observation device 22 is updated to the current time.

  FIG. 6 is a flowchart showing a third operation of the dummy communication generator 23. In the twentieth embodiment of the present invention, the dummy communication generator 23 repeats the operation from the third operation repetition start to the third operation repetition end shown in FIG. 6 as the third operation. It is determined whether or not the data observation device 22 detects the end of data transmission from the iSCSI server 10 (step S301).

  Here, if the iSCSI data observation device 22 does not detect the end of data transmission from the iSCSI server 10 (NO in step S301), the dummy communication generator 23 returns to step S301.

  On the other hand, when the iSCSI data observation device 22 detects the end of data transmission from the iSCSI server 10 (in the case of YES in step S301), the dummy communication generation device 23 reads the corresponding part from the state table 24. Is deleted (step S302). That is, in this case, the IP address of the data transmission destination given from the iSCSI data observation device 22, the “Data” in the communication status column corresponding to this IP address, and the update time in the update time column are deleted.

  FIG. 7 is a flowchart showing the operation of the state table management device 25. The state table management device 25 receives an interrupt from the timer 26 at a constant period (for example, 0.01 second period), and determines whether or not there is an update time in the state table 24 that has not been updated for a certain period. (Step S401).

  If there is no update time that has not been updated for a certain period in the state table 24 (NO in step S401), the state table management device 25 waits for the next interruption of the timer 26. On the other hand, when there is an update time that has not been updated for a certain period in the status table 24, the status table management device 25 deletes this update time, the corresponding IP address and communication status registration ( Step S402).

  FIG. 8 is a time chart showing the operation of one embodiment 20 of the present invention. When the iSCSI server 10 receives a read request for data of a predetermined size or more from the client, and when the embodiment 20 of the present invention receives the read request, the embodiment 20 of the present invention responds to the read request. Until the data is transmitted from the iSCSI server 10 in response, dummy communication in the same or the same band as the data communication by the iSCSI server 10 is performed for the source IP address of the read request.

  In this case, when congestion occurs in the communication path in the Internet 30, it is possible to perform congestion control by reducing the bandwidth of congestion control communication such as other TCP communication using the communication path in which the congestion has occurred. In this case, the iSCSI server 10 can carry out the main data communication by taking over the band secured by the dummy communication by the dummy communication generating device 23, so that the bandwidth of the path necessary for the main data communication is secured. Thus, congestion at the time of data communication can be avoided.

  FIG. 9 is a diagram for explaining the effect of the embodiment 20 of the present invention. In FIG. 9, 31 to 36 are routers in the Internet 30, 40 is a client using the Internet 30, 37 is a communication path from the iSCSI server 10 side to the client 40, and 38 is a communication path for other congestion control communication.

  Here, FIG. 9 shows that there is a request for reading data of a predetermined size or more from the client 40 to the iSCSI server 10, and the dummy communication by the dummy communication generator 23 and the data communication by the iSCSI server 10 to the client 40 in response to this request. Is shown using the communication path 37. FIG. 10 shows an example of the state of the bandwidth of the communication path 39 between the routers 33 and 34 in the example of FIG.

  Here, if a read request for data of a predetermined size or larger is made from the client 40 to the iSCSI server 10, the dummy communication generating device 23 will wait until the data is sent from the iSCSI server 10 in response to this request. On the other hand, dummy communication is performed in the same or the same band as the data communication by the iSCSI server 10.

  In this case, if the dummy communication by the dummy communication generator 23 is performed on the communication path 37, the dummy communication is performed using a protocol that does not perform congestion control, such as UDP or ICMP, so the communication path 38 is used. Other congestion control communications such as TCP communications reduce the bandwidth. As a result, the iSCSI server 10 secures the bandwidth of the communication path 37 necessary for this data communication, avoids congestion at the time of sending this data, and achieves high-speed communication without cooperative operation with the router that is the communication path control device. can do.

  FIG. 11 is a time chart for explaining another effect of the embodiment 20 of the present invention. FIG. 11A shows a case where the iSCSI server 10 does not use the embodiment 20 of the present invention, and FIG. 11B shows a case where the iSCSI server 10 uses the embodiment 20 of the present invention.

  Here, the solid line 50 shown in FIG. 11A is a bandwidth of the data communication when no packet loss due to congestion occurs at the start of the data communication, and the solid line 51 indicates a packet loss due to congestion at the start of the data communication. In this case, the bandwidth of the present data communication and the solid line 52 indicate the bandwidth due to retransmission of the lost packet.

  Also, the solid line 53 shown in FIG. 11B is the bandwidth of dummy communication when no packet loss due to congestion occurs at the start of dummy communication, and the solid line 54 is dummy communication when packet loss due to congestion occurs at the start of dummy communication. Indicates the bandwidth.

  That is, when the iSCSI server 10 does not use the embodiment 20 of the present invention, when the iSCSI server 20 receives a read request from the client, the data communication is started from the beginning. Since other congestion control communications do not perform congestion control, there is a high possibility that packet loss will occur. When packet loss occurs, the iSCSI server 10 retransmits the packet in order to guarantee the arrival of data. As a result, a bandwidth for the retransmitted packet is required in addition to the original communication on the communication path.

  On the other hand, when the iSCSI server 10 uses the embodiment 20 of the present invention, when the iSCSI server 10 receives a read request of a predetermined size or more from the client, first, the dummy according to the embodiment 20 of the present invention is used. Although communication is performed, dummy communication packets are sent to the client's discard port, so even if they are lost, they do not need to be retransmitted, and communication can be performed without placing an extra bandwidth on the communication path. It can be carried out.

  As described above, according to the twentieth embodiment of the present invention, the iSCSI server 10 using the twentieth embodiment of the present invention causes a data request (a read request for data of a predetermined size or more) that causes a dummy communication from the client. ), Perform dummy communication with the client before data transmission, and if this causes congestion in the communication path, other TCP communication using the communication path in which this congestion has occurred By expecting the congestion control of the congestion control communication such as the above, it is possible to guarantee the bandwidth for the iSCSI server 10 using the embodiment 20 of the present invention to avoid the congestion at the time of sending this data, and to control the communication path. High-speed communication can be achieved without cooperative operation with a router as a device.

  In the example shown in FIG. 1, one embodiment 20 of the present invention is arranged for one iSCSI server 10, but one embodiment of the present invention is provided for a plurality of iSCSI servers. It can also be set as the structure which arrange | positions the form 20. FIG.

  Further, the embodiment 20 of the present invention can be used not only for communication between the iSCSI server and the client but also for communication between the iSCSI servers. In this case, it is used in the iSCSI servers at both ends of the communication. be able to.

  FIG. 12 is a block circuit diagram showing the main part of another embodiment of the present invention. In FIG. 12, 60 is another embodiment of the present invention, and the other embodiment 60 of the present invention is used by the streaming server 70 to secure a bandwidth on the Internet 30, and in this example Although installed in the path between the streaming server 70 and the Internet 30, it may be incorporated in the streaming server 70 as a part thereof.

  Another embodiment 60 of the present invention is an HTTP (hypertext transfer protocol) instruction observing device 61 and an HTTP data observing device 62 instead of the iSCSI instruction observing device 21 and the iSCSI data observing device 22 provided in the twentieth embodiment of the present invention. The others are configured in the same manner as in the embodiment 20 of the present invention.

  The HTTP command observation device 61 registers a specific URL (uniform resource locator) that causes dummy communication generation by the dummy communication generation device 23, and is sent from the client to the streaming server 70 via the Internet 30. Is observed and interpreted, a data read request from a specific URL registered in advance is detected, and this detection result is transmitted to the dummy communication generator 23 together with the data request source IP address. Note that all URLs in the streaming server 70 may be targets for generating dummy communication.

  When the HTTP data observation device 62 observes data transmission from the streaming server 70 and detects data transmission from the streaming server 70, the HTTP data observation device 62 transmits this detection result to the dummy communication generator 23 together with the data transmission destination IP address. When the end of data transmission from the streaming server 70 is detected, the detection result is transmitted to the dummy communication generator 23 together with the data transmission destination IP address.

  Accordingly, in another embodiment 60 of the present invention, the dummy communication generating device 23 is configured such that the HTTP data observation device 62 detects that the HTTP command observation device 61 detects a read request for a specific URL registered in advance from a client. Until the transmission of the data read from the URL is detected, the dummy data is transmitted toward the transmission source IP address of the read request to generate dummy communication.

  According to another embodiment 60 of the present invention, the streaming server 70 using the other embodiment 60 of the present invention causes a data request that causes a dummy communication from the client (a request to read data of a URL registered in advance). Before performing data transmission according to the above, perform dummy communication to the client, and if this causes congestion in the communication path, other TCP communication using the communication path in which this congestion has occurred By expecting the congestion control communication of the present embodiment, the bandwidth is ensured for the streaming server 70 using the other embodiment 60 of the present invention to avoid the congestion at the time of sending this data, and the communication path control High-speed communication can be achieved without cooperative operation with a router as a device.

  In the example shown in FIG. 12, one other embodiment 60 of the present invention is arranged for one streaming server 70, but one other embodiment of the present invention is provided for a plurality of streaming servers. Embodiment 60 may be arranged.

  In addition, the other embodiment 60 of the present invention can be used not only for communication between the streaming server and the client but also for communication between the streaming servers. In this case, it is used in the streaming servers at both ends of the communication. can do.

  In the embodiment 20 of the present invention, the iSCSI server is used by the present invention. In the other embodiment 60 of the present invention, the streaming server is used by the present invention. And can be applied to various communication devices that perform congestion control.

It is a figure which shows the usage condition of one Embodiment of this invention. It is a block circuit diagram which shows the principal part of one Embodiment of this invention. It is a figure which shows the structure of the state table with which one Embodiment of this invention is provided. It is a flowchart which shows 1st operation | movement of the dummy communication generator with which one Embodiment of this invention is provided. It is a flowchart which shows 2nd operation | movement of the dummy communication generator with which one Embodiment of this invention is provided. It is a flowchart which shows 3rd operation | movement of the dummy communication generator with which one Embodiment of this invention is provided. It is a flowchart which shows operation | movement of the state table management apparatus with which one Embodiment of this invention is provided. It is a time chart which shows operation | movement of one Embodiment of this invention. It is a figure for demonstrating the effect by one Embodiment of this invention. It is a time chart for demonstrating the effect by one Embodiment of this invention. It is a time chart for demonstrating the other effect by one Embodiment of this invention. It is a block circuit diagram which shows the principal part of other embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... iSCSI server 20 ... One Embodiment of this invention 21 ... iSCSI command observation apparatus 22 ... iSCSI data observation apparatus 23 ... Dummy communication generator 24 ... State table 25 ... State table management apparatus 26 ... Timer 30 ... Internet 31-36 ... Router 37, 38, 39 ... Communication path 40 ... Client 60 ... Other embodiment of the present invention 61 ... HTTP command observation device 62 ... HTTP data observation device

Claims (3)

A bandwidth securing device connected between a communication device and a network,
A first means for detecting a data request that causes a dummy communication to be transmitted from another communication device connected to the network to the one communication device;
Second means for detecting data transmission from the one communication device to the other communication device in response to the data request;
A third means for transmitting dummy data to the other communication device until the second means detects the data transmission after the first means detects the data request; Bandwidth securing device characterized by this.   The band securing apparatus according to claim 1, wherein the third means performs transmission of the dummy data in the same band as data transmission by the one communication apparatus.   The bandwidth securing device according to claim 1 or 2, wherein the third means transmits the dummy data using a protocol that does not perform congestion control.

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