DATA TRANSFER DEVICE TECHNICAL FIELD [0001] The present invention relates to a data transfer device that transfers data received from one external device to another external device. BACKGROUND ART [0002] A data transfer device that transfers data (e.g., a frame) received from one external device to another external device is known. A data transfer device described in Patent Document 1 as one of data transfer devices of this type is equipped with a communication establishment processor. [0003] The communication establishment processor transfers and receives communication establishing information for establishing communication with an external device connected via a communication port, to and from the external device. Moreover the communication establishment processor transmits communication maintaining information for maintaining the establishment of communication with an external device to the external device every time a predetermined transmission period elapses. [0004] Thus, communication between the data transfer device and the external device is established, and the establishment of the communication can be maintained. [0005] [Patent Document 1] Japanese Unexamined Patent Application Publication No. JP-A 2008-160227 [0006] There is a case that, in the data transfer device described above, an operation of the communication establishment processor must be stopped (e.g., when a firmware program is updated in a case that the communication establishment processor is configured by a CPU that executes the firmware program, or when failure occurs in the communication establishment processor). [0007] In this case, the data transfer device cannot transmit communication maintaining information. Therefore, in such a case, there is a problem that it is impossible to maintain the establishment of communication with an external device. SUMMARY [0008] Accordingly, an object of the present invention is to provide a data transfer device capable of solving the aforementioned problem, "in a case that an operation of the communication establishment processor has stopped, it is impossible to maintain the establishment of communication with an external device." [0009] In order to achieve the object, a data transfer device of an embodiment of the present invention is equipped with: a plurality of communication ports; a communication establishment processor configured to transmit and receive communication establishing information for establishing communication with an external device connected via the communication port to and from the external device; and a communication maintenance processor configured to operate independently of the communication establishment processor, and transmit communication maintaining information for maintaining establishment of the communication with the external device, to the external device, every time a predetermined transmission period elapses The data transfer device is configured to, in a state that communication with each of a plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices. [0010] Further, a communication maintenance processing circuit of another embodiment of the present invention is configured to, in a case that the communication maintenance processing circuit is applied to a data transfer device, which is equipped with a plurality of communication ports and a communication establishment processor configured to transmit and receive communication establishing information for establishing communication with an external device connected via the communication port to and from the external device, and which is configured to, in a state that communication with each of a plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices: operate independently of the communication establishment processor; and transmit communication maintaining information for maintaining establishment of communication with the external device to the external device ever y time a predetermined transmission period elapses. [0011] Further, a data transfer system of another embodiment of the present invention is a system includes a data transfer device and a plurality of external devices. [0012] Furthermore, the data transfer device is equipped with: a plurality of communication ports; a communication establishment processor configured to transmit and receive communication establishing information for establishing communication with the external device connected via the communication port to and from the external device; and a communication maintenance processor configured to operate independently of the communication establishment processor, and transmit communication maintaining information for maintaining establishment of communication with the external device to the external device every time a predetermined transmission period elapses. The data transfer device is configured to, in a state that communication with each of the plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices. [0013] Furthermore, the external device is configured to transmit and receive the communication establishing information to and from the data transfer device. The external device is configured to maintain establishment of communication with the data transfer device in a case that a predetermined standby time has not elapsed since a latest reception time point that is a latest time point of reception of the communication maintaining information from the data transfer device, whereas cancel the establishment of the communication with the data transfer device in a case that the standby time has elapsed since the latest reception time point. [0014] Further, a data transfer method of another embodiment of the present invention is a method including: transmitting and receiving communication establishing information for establishing communication with an external device connected via a communication port to and from the external device, by a communication establishment processor; transmitting communication maintaining information for maintaining establishment of communication with the external device to the external device every time a predetermined transmission period elapses, by a communication maintenance processor configured to operate independently of the communication establishment processor; and, in a state that communication with each of a plurality of external devices via a plurality of communication ports is established, transmitting data received from one of the plurality of external devices to the other one of the plurality of external devices. [0015] Further, a communication maintenance processing program of another embodiment of the present invention is a computer program for causing a processing device to realize a communication maintenance processing means. The processing device is configured to operate independently of a communication establishment processor in a case that the communication maintenance processing program is applied to a data transfer device. The data transfer device is equipped with a plurality of communication ports and the communication establishment processor that is configured to transmit and receive communication establishing information for establishing communication with an external device connected via the communication port to and from the external device, and is configured to, in a state that communication with each of a plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices. The communication maintenance processing means is configured to transmit communication maintaining information for maintaining establishment of communication with the external device to the external device every time a predetermined transmission period elapses. [0016] With the configurations as described above, the present invention can maintain the establishment of communication with an external device even if an operation of the communication establishment processor has stopped. BRIEF DESCRIPTION OF DRAWINGS [0017] Fig. 1 is a diagram showing a schematic configuration of a data transfer system according to an exemplary embodiment of the present invention; Fig. 2 is an explanation view conceptually showing an LACP frame transmitted as communication maintaining information; Fig. 3 is a sequence diagram showing an operation of the data transfer system when communication between two L2SWs is established; Fig. 4 is a sequence diagram showing an operation of the data transfer system when a CPU stops an operation; Fig. 5 is a sequence diagram showing an operation of the data transfer system when transmission of communication maintaining information from an L2SW with which communication has been established is stopped; and Fig. 6 is a diagram showing a schematic configuration of a data transfer system according to a second exemplary embodiment of the present invention. EXEMPLARY EMBODIMENT [0018] Below, the respective exemplary embodiments of a data transfer device, a communication maintenance processing circuit, a data transfer system, a data transfer method and a communication maintenance processing program according to the present invention will be described with reference to Figs. 1 to 6. [0019] Next, a data transfer system according to a second exemplary embodiment of the present invention will be described with reference to Fig. 6. A data transfer system 200 according to the second exemplary embodiment includes a data transfer device 210, and a plurality of external devices 220 and 23 0. [0060] The data transfer device 210 is equipped with a plurality of communication ports 211a and 211b, a communication establishment processor 212, and a communication maintenance processor 213. [0061] The data transfer device 210 and the external device 220 are connected via the communication port 211a. Moreover, the data transfer device 210 and the external device 230 are connected via the communication port 211b. [0062] The communication establishment processor 212 transmits and receives communication establishing information for establishing communication with the external device 220 to and from the external device 220. In a like manner, the communication establishment processor 212 transmits and receives communication establishing information for establishing communication with the external device 230 to and from the external device 230. [0063] The communication maintenance processor 213 is configured to operate independently of the communication establishment processor 212. Every time a predetermined transmission period elapses, the communication maintenance processor 213 transmits communication maintaining information for maintaining the establishment of communication with the external device 220, to the external device 220. In a like manner, every time a predetermined transmission period elapses, the communication maintenance processor 213 transmits communication maintaining information for maintaining the establishment of communication with the external device 230, to the external device 230. [0064] In a state that communication with each of the plurality of external devices 220 and 230 via the plurality of communication ports 211a and 211b is established, the data transfer device 210 transmits data received from one (e.g., the external device 220) of the plurality of external devices to the other (e.g., the external device 230) of the plurality of external devices. [0065] According to the second exemplary embodiment, the communication maintenance processor 213 also can transmit communication maintaining information in a case that an operation of the communication establishment processor 212 stops. As a result, it is possible to maintain the establishment of communication with the external device 220,230. [0066] In this case, it is preferred that: the communication maintenance processor includes a transmission instruction information storing part configured to store transmission instruction information, which includes the communication maintaining information and communication port specification information for specifying the communication port used for transmitting the communication maintaining information; and the communication maintenance processor is configured to transmit the communication maintaining information included in the stored transmission instruction information to the external device via the communication port specified by the communication port specification information included in the transmission instruction information. [0067] In this case, it is preferred that the transmission instruction information storing part is configured to store the transmission instruction information for each of the plurality of communication ports. According to this, it is possible to transmit different communication maintaining information for each communication port. [0068] In this case, it is preferred that: the transmission instruction information includes transmission period information representing the transmission period; and the communication maintenance processor is configured to transmit the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information elapses. [0069] According to this, in a case that a standby time during which the establishment of communication can be maintained even if an external device does not receive communication maintaining information is set to any time (e.g., one second, thirty seconds, etc.,), it is also possible to securely maintain the establishment of the communication with the external device. [0070] In this case, it is preferred that the communication establishment processor is configured to generate the transmission instruction information based on the communication establishing information and cause the transmission instruction information storing part to store the generated transmission instruction information. [0071] In this case, it is preferred that the communication establishment processor is configured to, every time a predetermined transmission instruction update period elapses, generate the transmission instruction information and cause the transmission instruction information storing part to store the generated transmission instruction information. [0072] According to this, it is possible to securely update transmission instruction information every time a transmission instruction update period elapses. As a result, it is possible to securely maintain the establishment of communication with an external device. [0073] In this case, it is preferred that the communication establishment processor is configured to, in a case that communication maintaining information to be transmitted for maintaining the establishment of the communication has changed, generate the transmission instruction information and cause the transmission instruction information storing part to store the generated transmission instruction information. [0074] According to this, it is possible to securely update transmission instruction information in a case that communication maintaining information to be transmitted for maintaining the establishment of communication has changed. As a result, it is possible to securely maintain the establishment of communication with an external device. [0075] In this case, it is preferred that the communication establishment processor is configured to acquire the transmission instruction information stored in the transmission instruction information storing part when the communication establishment processor starts an operation. [0076] According to this, in a case that the communication establishment processor restarts an operation after stopping, the communication establishment processor can execute a process for changing a state of communication with an external device or a process for maintaining the establishment of communication with an external device, based on the latest transmission instruction information. [0077] In this case, it is preferred that the data transfer device is configured to perform link aggregation, which is transmission and reception of data to and from one external device via the plurality of communication ports. [0078] In this case, it is preferred that the data transfer device is configured to transmit and receive the data in accordance with a protocol of a data link layer in OSI (Open Systems Interconnection) reference model. [0079] Further, a communication maintenance processing circuit of another exemplary embodiment of the present invention is configured to, in a case that the communication maintenance processing circuit is applied to a data transfer device, which is equipped with a plurality of communication ports and a communication establishment processor configured to transmit and receive communication establishing information for establishing communication with an external device connected via the communication port to and from the external device, and which is configured to, in a state that communication with each of a plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices: operate independently of the communication establishment processor; and transmit communication maintaining information for maintaining establishment of communication with the external device to the external device ever y time a predetermined transmission period elapses. [0080] In this case, it is preferred that: the communication maintenance processing circuit includes a transmission instruction information storing part configured to store transmission instruction information, which includes the communication maintaining information and communication port specification information for specifying the communication port used for transmitting the communication maintaining information; and the communication maintenance processing circuit is configured to transmit the communication maintaining information included in the stored transmission instruction information to the external device via the communication port specified by the communication port specification information included in the transmission instruction information. [0081] In this case, it is preferred that the transmission instruction information storing part is configured to store the transmission instruction information for each of the plurality of communication ports. [0082] In this case, it is preferred that: the transmission instruction information includes transmission period information representing the transmission period; and the communication maintenance processing circuit is configured to transmit the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information elapses. [0083] Further, a data transfer system of another exemplary embodiment of the present invention is a system including a data transfer device and a plurality of external devices. [0084] Furthermore, the data transfer device is equipped with: a plurality of communication ports; a communication establishment processor configured to transmit and receive communication establishing information for establishing communication with the external device connected via the communication port to and from the external device; and a communication maintenance processor configured to operate independently of the communication establishment processor, and transmit communication maintaining information for maintaining establishment of communication with the external device to the external device every time a predetermined transmission period elapses. The data transfer device is configured to, in a state that communication with each of the plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices. [0085] Furthermore, the external device is configured to transmit and receive the communication establishing information to and from the data transfer device. The external device is configured to maintain establishment of communication with the data transfer device in a case that a predetermined standby time has not elapsed since a latest reception time point that is a latest time point of reception of the communication maintaining information from the data transfer device, whereas cancel the establishment of the communication with the data transfer device in a case that the standby time has elapsed since the latest reception time point. [0086] In this case, it is preferred that: the communication maintenance processor includes a transmission instruction information storing part configured to store transmission instruction information, which includes the communication maintaining information and communication port specification information for specifying the communication port used for transmitting the communication maintaining information; and the communication maintenance processor is configured to transmit the communication maintaining information included in the stored transmission instruction information to the external device via the communication port specified by the communication port specification information included in the transmission instruction information. [0087] In this case, it is preferred that the transmission instruction information storing part is a configured to store the transmission instruction information for each of the plurality of communication ports. [0088] In this case, it is preferred that: the transmission instruction information includes transmission period information representing the transmission period; and the communication maintenance processor is configured to transmit the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information elapses. [0089] Further, a data transfer method of another exemplary embodiment of the present invention is a method including: transmitting and receiving communication establishing information for establishing communication with an external device connected via a communication port to and from the external device, by a communication establishment processor; transmitting communication maintaining information for maintaining establishment of communication with the external device to the external device every time a predetermined transmission period elapses, by a communication maintenance processor configured to operate independently of the communication establishment processor; and, in a state that communication with each of a plurality of external devices via a plurality of communication ports is established, transmitting data received from one of the plurality of external devices to the other one of the plurality of external devices. [0090] In this case, it is preferred that the data transfer method includes: causing a transmission instruction information storing part included in the communication maintenance processor to store transmission instruction information, which includes the communication maintaining information and communication port specification information for specifying the communication port used for transmitting the communication maintaining information; and transmitting the communication maintaining information included in the stored transmission instruction information to the external device via the communication port specified by the communication port specification information included in the transmission instruction information. [0091] In this case, it is preferred that the data transfer method includes causing the transmission instruction information storing part to store the transmission instruction information for each of the plurality of communication ports. [0092] In this case, it is preferred that: the transmission instruction information includes transmission period information representing the transmission period; and the data transfer method includes transmitting the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information elapses, by the communication maintenance processor. [0093] Further, a communication maintenance processing program of another exemplary embodiment of the present invention is a computer program for causing a processing device to realize a communication maintenance processing means. The processing device is configured to operate independently of a communication establishment processor in a case that the communication maintenance processing program is applied to a data transfer device. The data transfer device is equipped with a plurality of communication ports and the communication establishment processor that is configured to transmit and receive communication establishing information for establishing communication with an external device connected via the communication port to and from the external device, and is configured to, in a state that communication with each of a plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices. The communication maintenance processing means is configured to transmit communication maintaining information for maintaining establishment of communication with the external device to the external device every time a predetermined transmission period elapses. [0094] In this case, it is preferred that the communication maintenance processing means is configured to transmit the communication maintaining information included in transmission instruction information stored in a transmission instruction information storing part included in the data transfer device, to the external device via a communication port specified by communication port specification information included in the transmission instruction information. [0095] In this case, it is preferred that the transmission instruction information storing part is configured to store the transmission instruction information for each of the plurality of communication ports. [0096] In this case, it is preferred that: the transmission instruction information includes transmission period information representing the transmission period; and the communication maintenance processing means is configured to transmit the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information. [0097] Inventions of a communication maintenance processing circuit, a data transfer system, a data transfer method or a communication maintenance processing program having the abovementioned configurations have like actions as the abovementioned data transfer device, and therefore, can achieve the object of the present invention mentioned above. [0098] Although the present invention has been described with reference to the respective exemplary embodiments described above, the present invention is not limited to the abovementioned exemplary embodiments. The configuration and details of the present invention can be altered within the scope of the present invention in various manners that can be understood by those skilled in the art. [0099] For example, the communication maintenance processor is configured by the communication maintenance processing circuit 10 in the abovementioned exemplary embodiments, but may be configured by a processing device and a program executed by the processing device. Moreover, in this case, the program may be stored in a storing device, or may be stored in a recording medium that can be read by a computer. For example, the recording medium is a portable recording medium such as a flexible disk, an optical disk, a magneto-optical disk, and a semiconductor memory. [0100] Further, the external device is an L2SW in the abovementioned exemplary embodiment, but may be another device (e.g., a server device, etc.). [0101] Further, as a modified example of the abovementioned exemplary embodiments, any combination of the abovementioned exemplary embodiments and modified examples may be employed. [0102] The present invention is based upon and claims the benefit of priority from Japanese patent application No. 2008-318149, filed on December 15, 2008, the disclosure of which is incorporated herein in its entirety by reference. [0103] The present invention can be applied to a data transfer system connected with a plurality of layer 2 switches executing link aggregation, and so on. CLAIMS 1. A data transfer device comprising: a plurality of communication ports; a communication establishment processor configured to transmit and receive communication establishing information for establishing communication with an external device connected via the communication port to and from the external device; and a communication maintenance processor configured to operate independently of the communication establishment processor, and transmit communication maintaining information for maintaining establishment of the communication with the external device, to the external device, every time a predetermined transmission period elapses, the data transfer device being configured to, in a state that communication with each of a plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices. 2. The data transfer device according to Claim 1, wherein: the communication maintenance processor includes a transmission instruction information storing part configured to store transmission instruction information, which includes the communication maintaining information and communication port specification information for specifying the communication port used for transmitting the communication maintaining information; and the communication maintenance processor is configured to transmit the communication maintaining information included in the stored transmission instruction information to the external device via the communication port specified by the communication port specification information included in the transmission instruction information. 3. The data transfer device according to Claim 2, wherein the transmission instruction information storing part is configured to store the transmission instruction information for each of the plurality of communication ports. 4. The data transfer device according to Claim 2 or 3, wherein: the transmission instruction information includes transmission period information representing the transmission period; and the communication maintenance processor is configured to transmit the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information elapses. 5. The data transfer device according to any of Claims 2 to 4, wherein the communication establishment processor is configured to generate the transmission instruction information based on the communication establishing information and cause the transmission instruction information storing part to store the generated transmission instruction information. 6. The data transfer device according to Claim 5, wherein the communication establishment processor is configured to, every time a predetermined transmission instruction update period elapses, generate the transmission instruction information and cause the transmission instruction information storing part to store the generated transmission instruction information. 7. The data transfer device according to Claim 5 or 6, wherein the communication establishment processor is configured to, in a case that communication maintaining information to be transmitted for maintaining the establishment of the communication has changed, generate the transmission instruction information and cause the transmission instruction information storing part to store the generated transmission instruction information. 8. The data transfer device according to any of Claims 2 to 7, wherein the communication establishment processor is configured to acquire the transmission instruction information stored in the transmission instruction information storing part when the communication establishment processor starts an operation. 9. The data transfer device according to any of Claims 1 to 8 being configured to perform link aggregation, which is transmission and reception of data to and from one external device via the plurality of communication ports. 10. The data transfer device according to any of Claims 1 to 9 being configured to transmit and receive the data in accordance with a protocol of a data link layer in OSI (Open Systems Interconnection) reference model. 11. A communication maintenance processing circuit configured to, in a case that the communication maintenance processing circuit is applied to a data transfer device, which is equipped with a plurality of communication ports and a communication establishment processor configured to transmit and receive communication establishing information for establishing communication with an external device connected via the communication port to and from the external device, and which is configured to, in a state that communication with each of a plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices: operate independently of the communication establishment processor; and transmit communication maintaining information for maintaining establishment of communication with the external device to the external device ever y time a predetermined transmission period elapses. 12. The communication maintenance processing circuit according to Claim 11 including a transmission instruction information storing part configured to store transmission instruction information, which includes the communication maintaining information and communication port specification information for specifying the communication port used for transmitting the communication maintaining information, the communication maintenance processing circuit being configured to transmit the communication maintaining information included in the stored transmission instruction information to the external device via the communication port specified by the communication port specification information included in the transmission instruction information. 13. The communication maintenance processing circuit according to Claim 12, wherein the transmission instruction information storing part is configured to store the transmission instruction information for each of the plurality of communication ports. 14. The communication maintenance processing circuit according to Claim 12 or 13, wherein: the transmission instruction information includes transmission period information representing the transmission period; and the communication maintenance processing circuit is configured to transmit the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information elapses. 15. A data transfer system comprising a data transfer device and a plurality of external devices, wherein: the data transfer device is equipped with; a plurality of communication ports; a communication establishment processor configured to transmit and receive communication establishing information for establishing communication with the external device connected via the communication port to and from the external device; and a communication maintenance processor configured to operate independently of the communication establishment processor, and transmit communication maintaining information for maintaining establishment of communication with the external device to the external device every time a predetermined transmission period elapses; the data transfer device is configured to, in a state that communication with each of the plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices; the external device is configured to transmit and receive the communication establishing information to and from the data transfer device; and the external device is configured to maintain establishment of communication with the data transfer device in a case that a predetermined standby time has not elapsed since a latest reception time point that is a latest time point of reception of the communication maintaining information from the data transfer device, whereas cancel the establishment of the communication with the data transfer device in a case that the standby time has elapsed since the latest reception time point. 16. The data transfer system according to Claim 15, wherein: the communication maintenance processor includes a transmission instruction information storing part configured to store transmission instruction information, which includes the communication maintaining information and communication port specification information for specifying the communication port used for transmitting the communication maintaining information; and the communication maintenance processor is configured to transmit the communication maintaining information included in the stored transmission instruction information to the external device via the communication port specified by the communication port specification information included in the transmission instruction information. 17. The data transfer system according to Claim 16, wherein the transmission instruction information storing part is configured to store the transmission instruction information for each of the plurality of communication ports. 18. The data transfer system according to Claim 16 or 17, wherein: the transmission instruction information includes transmission period information representing the transmission period; and the communication maintenance processor is configured to transmit the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information elapses. 19. A data transfer method comprising: transmitting and receiving communication establishing information for establishing communication with an external device connected via a communication port to and from the external device, by a communication establishment processor; transmitting communication maintaining information for maintaining establishment of communication with the external device to the external device ever y time a predetermined transmission period elapses, by a communication maintenance processor configured to operate independently of the communication establishment processor; and in a state that communication with each of a plurality of external devices via a plurality of communication ports is established, transmitting data received from one of the plurality of external devices to the other one of the plurality of external devices. 20. The data transfer method according to Claim 19 comprising: causing a transmission instruction information storing part included in the communication maintenance processor to store transmission instruction information, which includes the communication maintaining information and communication port specification information for specifying the communication port used for transmitting the communication maintaining information; and transmitting the communication maintaining information included in the stored transmission instruction information to the external device via the communication port specified by the communication port specification information included in the transmission instruction information. 21. The data transfer method according to Claim 20 comprising: causing the transmission instruction information storing part to store the transmission instruction information for each of the plurality of communication ports. 22. The data transfer method according to Claim 20 or 21, wherein the transmission instruction information includes transmission period information representing the transmission period, the data transfer method comprising: transmitting the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information elapses, by the communication maintenance processor. 23. A communication maintenance processing program for causing a processing device to realize a communication maintenance processing means, the processing device being configured to operate independently of a communication establishment processor in a case that the communication maintenance processing program is applied to a data transfer device, the data transfer device being equipped with a plurality of communication ports and the communication establishment processor that is configured to transmit and receive communication establishing information for establishing communication with an external device connected via the communication port to and from the external device, and being configured to, in a state that communication with each of a plurality of external devices via the plurality of communication ports is established, transmit data received from one of the plurality of external devices to the other one of the plurality of external devices, and the communication maintenance processing means being configured to transmit communication maintaining information for maintaining establishment of communication with the external device to the external device every time a predetermined transmission period elapses. 24. The communication maintenance processing program according to Claim 23, wherein the communication maintenance processing means is configured to transmit the communication maintaining information included in transmission instruction information stored in a transmission instruction information storing part included in the data transfer device, to the external device via a communication port specified by communication port specification information included in the transmission instruction information. 25. The communication maintenance processing program according to Claim 24, wherein the transmission instruction information storing part is configured to store the transmission instruction information for each of the plurality of communication ports. 26. The communication maintenance processing program according to Claim 24 or 25, wherein: the transmission instruction information includes transmission period information representing the transmission period; and the communication maintenance processing means is configured to transmit the communication maintaining information every time the transmission period represented by the transmission period information included in the stored transmission instruction information.