Abstract:
A file sharing system for managing both P2P type and C/S type shard files, wherein a management server accepts from each client a file registration request and a file access request for not only a P2P type shared file, but also a C/S type shared file, and each client executes a subsequent procedure for storing or accessing a shared file according to an acknowledgement response from the management server.

Description:
CLAIM OF PRIORITY 
   The present application claims priority from Japanese application serial no. 2004-186148, filed on Jun. 24, 2004, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates to a file sharing system and client apparatus and, more particularly, to a file sharing system and client apparatus that can take the advantages of both a Client/Server (C/S) type and a Peer to Peer (P2P) type. 
   (2) Description of the Related Art 
   As conventional file sharing systems, C/S type and P2P type file sharing systems are known. In the following description, a system file denotes a file that is managed by a file sharing system and accessible to a plurality of users and a local file denotes a file for private use that is managed by a client apparatus. 
   A conventional C/S type file sharing system is comprised of a file server and a plurality of clients connected to a communication network. In the C/S type file sharing system, when a client user wishes to make one of the user&#39;s local files open as a shared file, the client sends a file upload request to the file server. When the file upload is successful, the uploaded file is stored as a system file in the file server and the file server sends a file upload confirmation message to the requester client. When the client receives the file upload confirmation message, the shared file registration sequence is completed. 
   When a client user in the C/S type file sharing system wishes to access any open shared file, the client sends the file server a file download request designating the file identifier. In reply to the download request, the file server sends back the specified file to the requester client. The downloaded file is stored as a local file in the client apparatus. 
   On the other hand, the P2P type file sharing system is comprised of a management server and a plurality of clients connected to a communication network. In the P2P type file sharing system, when a client user wishes to register one of the user&#39;s local files with the sharing system as a shared file, the client sends a file registration request message to the management server. When the client receives a file registration acknowledgement message in which the file identifier is specified from the management server, the client stores a copy of the local file to be registered as a shared file into its storage as a system file and sends the management server a message to notify that the file saving has been completed. The management server registers the file identifier designated in the notification message into a management table as a new shared file, and returns an ACK message to the client. When the client receives the ACK message, the shared file registration sequence is completed. 
   When a client user in the P2P type file sharing system wishes to access any open shared file, the client sends the management server a file download request message designating the file identifier. Upon receiving the request message, the management server judges whether the specified file can be downloaded. If the file download is permitted, the management sever sends back the requester client a response message indicating the address of a client where the requested file is located. The requester client sends a file down load request to the client identified from the response message to receive and store the downloaded file as a local file. 
   As for the P2P type file sharing system, a distributed file management type system is proposed, e.g., in Japanese Unexamined Patent Publication No. 2004-5491. This system is arranged such that the management server manages directory information and each client can register a file with the sharing system by linking a system file stored in its storage to any folder represented by the directory information. 
   SUMMARY OF THE INVENTION 
   The C/S type file sharing system is suitable for centralized management of file data, because all shared files are stored in the file server as described above. However, a total amount of data available to be open in the file sharing system depends on the storage capacity of the file server. On the other hand, the total amount of data available in the P2P type file sharing system can increase, because the shared files are stored in distributed manner in a plurality of clients located across the system. In the P2P type file sharing system, however, each user allocates a part of the private memory resources in the client apparatus to store system files. Due to this, when memory space shortage occurs on a client, it is inevitable to discard a part of the shared files. 
   Since the P2P type and the C/S type have a merit and a demerit, as above, each user has to choose one of these types for use according to a file type and the purpose of use of file sharing. Practically, since the P2P type and the C/S type file sharing systems have been at work as independent system, it has been necessary for a user who wants to take advantage of the merits of both types to subscribe both P2P type and C/S type systems. When a user wished to change a shared file already registered with the P2P type (or C/S type) system to the file for C/S type (or P2P type), the user has to erase the P2P type (or C/S type) system file after saving the registered system file as a local file in the user&#39;s apparatus, and execute a procedure for registering that file as a new system file with the C/S type (or P2P type) system. 
   It is an object of the present invention to provide a file sharing system and client apparatus that can take the advantages of both the P2P type and C/S type systems. 
   It is another object of the present invention to provide a file sharing system and client apparatus capable of registering and accessing P2P type and C/S type files through the same user interface. 
   It is yet another object of the present invention to provide a file sharing system and client apparatus in which a registered P2P type or C/S type shared file can easily be changed to the file of the other sharing type. 
   To achieve the foregoing objects, a file sharing system of the present invention is characterized in that a management server accepts a file registration request and a file access request for not only a P2P type shared file, but also a C/S type shared file, issued from each client, and each client executes a subsequent procedure for storing or accessing a file according to an acknowledgement response from the management server. 
   More specifically, the file sharing system of the present invention comprises a file server and a management server connected to a plurality of clients via a network. The file sharing system is characterized in that the file server has means for notifying the management server of a C/S type shared file registration made by one of clients, and the management server has a management table including an entry for each shared file, the entry indicating file management information, for example, correspondence of file identifier, sharing type, and shared file location information, and means for managing a new shared file in the management table, according to a P2P type shared file registration notification from one of clients or a C/S type shared file registration notification from the file server. The management server sends the file location information retrieved from the management table back to a requester client in response to a shared file access request from the clients. 
   According to the present invention, because the same management server manages P2P type and C/S type shared file management information, it is possible to easily change a registered P2P type or C/S type shared file to the other sharing type file. One feature of the file sharing system of the present invention resides in that the management server is provided with a function of accepting a request to shift an already registered shared file or a request to change the sharing type of that file. 
   Specifically, the file sharing system of the present invention is characterized in that the management server is provided with means for updating in said management table, after acknowledging a shift request for a registered shared file received from one of clients, the sharing type corresponding to the identifier of the shared file to be shifted, upon completion of a sequence performed between the requester client and the file server to change the location of the registered shared file. 
   In an embodiment of the present invention, when a C/S type shared file stored in the file server has been downloaded to a client and changed to a P2P type shared file, the management server changes in the management table the sharing type corresponding to the identifier of the shared file from C/S type to P2P type in response to a file shift complete notification issued from the client. When a P2P type shared file stored in a client has been uploaded to the file server and changed to a C/S type shared file, the management server changes in the management table the sharing type corresponding to the identifier of the shared file from P2P type to C/S type in response to a shifted file registration complete notification issued from the file server. 
   According to the present invention, there is provided a management server that allows a plurality of clients connected to the management server via a network to make use of P2P type file sharing in which shared files are stored in any of the client apparatuses and C/S type file sharing in which shared files are stored in a file server connected to the management server via the network. 
   The management server of the present invention is provided with a management table storing an entry for each shared file, the entry indicating correspondence of at least file identifier, sharing type, and shared file location information; means for acknowledging a shared file registration request from one of clients; means for storing, in response to a shared file registration notification issued from the client or the file server in association with completion of storing the acknowledged shared file, the sharing type and file location information of the shared file into the management table; and means for notifying, in response to a shared file access request from one of clients, the requester client of the file location information retrieved from the management table. 
   In an embodiment of the present invention, the management table comprises a folder management table including an entry for each folder to which at least one shared file is linked, each entry indicating correspondence of at least folder identifier, sharing type, and folder location information; and a file management table including an entry for each shared file, each entry indicating correspondence of file identifier and its registration folder identifier. 
   Another feature of the present invention resides in that a client apparatus for communicating with a management server in a file sharing system via a network and providing file data stored in the client apparatus to another client connected to the network as a P2P type shared file, comprises first means for transmitting to the management server a request message to register file data as a C/S type shared file with a file server connected to the network, and second means for uploading file data to the file server in response to a file registration acknowledgment message received from the management server. 
   In an embodiment of the present invention, the client apparatus is characterized in that the first means displays on a display screen a directory tree structure comprising a group of folder icons denoting registration folders for shared files and a group of file icons denoting files stored in each folder, and transmits to the management server a shared file registration request message when a file icon denoting a shared file to be newly registered has been linked to a particular folder icon within the directory tree structure. The shared file registration request message specifies a folder identifier pre-assigned to the particular folder icon as the registration folder of the shared file. The second means uploads file data to the filer server, using the shared file identifier specified in the file registration acknowledgment message received from the management server. 
   In an embodiment of the present invention, the file registration acknowledgment message received from the management server specifies the sharing type of the registration folder to which the shared file is registered. The second means selectively carries out either of P2P type shared file registration for storing file data in the client apparatus and C/S type shared file registration for storing file data in the file server, according to the sharing type specified in the file registration acknowledgment message received from the management server. 
   According to the present invention, C/S type file sharing and P2P type file sharing can be realized in the same file sharing system. Thus, a registered P2P type or C/S type shared file can easily be changed to the file of the other sharing type and flexible operation of the system can be performed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an entire configuration of a file sharing system according to the present invention; 
       FIG. 2  shows an example of a graphical user interface to be displayed on a display screen of a client; 
       FIG. 3  is a block diagram showing an example of a client apparatus configuration; 
       FIG. 4  shows an example of a folder management table equipped in the client apparatus; 
       FIG. 5  shows an example of a file management table equipped in the client apparatus; 
       FIG. 6  is a block diagram showing an example of a file server configuration; 
       FIG. 7  is a block diagram showing an example of a management server configuration; 
       FIG. 8  shows an example of a folder management table equipped in the management server; 
       FIG. 9  shows an example of a file management table equipped in the management server; 
       FIG. 10  shows an example of a directory management table equipped in the management server; 
       FIG. 11  shows C/S type shared file registration sequence and download sequence in the file sharing system of the present invention; 
       FIG. 12  shows a format example of a packet to be communicated over the network for the file sharing system of the present invention; 
       FIG. 13A  shows a format example of a file registration request message; 
       FIG. 13B  shows a format example of a file registration acknowledgement message; 
       FIG. 13C  shows a format example of a file upload request message; 
       FIG. 13D  shows a format example of a file registration notification message; 
       FIG. 14A  shows a format example of a file down load message; 
       FIG. 14B  shows a format example of a file download acknowledgement message; 
       FIG. 14C  shows a format example of a file download response message; 
       FIG. 15  shows P2P type shared file registration sequence and download sequence in the file sharing system of the present invention; 
       FIG. 16  shows a C/S to P2P type file shift sequence in the file sharing system of the present invention; 
       FIG. 17A  shows a format example of a file shift request message; 
       FIG. 17B  shows a format example of a file shift acknowledgement message; 
       FIG. 17C  shows a format example of a download request message for file shift; and 
       FIG. 18  shows a P2P to C/S type file shift sequence in the file sharing system of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of a file sharing system according to the present invention will be described hereinafter with reference to the drawings. 
     FIG. 1  shows an example of an entire configuration of a file sharing system according to the present invention. 
   The files sharing system according to the present invention is comprised of a plurality of clients  100  ( 100 A,  100 B, . . .  100 N), a management server  200 , and a file server  300 , interconnected by a network. Each client  100  has storage  150  for storing local files and system files to be used as P2P type shared files, for example, as shown for a client  100 A. In the storage  150 , a folder management table and a file management table necessary for accessing a shared file are prepared. Details of these tables will be described later. 
     FIG. 2  shows an example of a graphical user interface (GUI) that is output on a display screen of each client apparatus  100 . 
   A window  141  for accessing the file sharing system is comprised of a directory display area  141 A where a directory  142  of shared folders is shown and a detail display area  141 B where the contents of a folder selected in the directory display area are shown. Also, there is shown an icon  145  for a local file that the user is going to register with the system as a shared file. 
   The directory  142  of shared folders includes a plurality of folders arranged in a hierarchical structure. In the directory, for example, C/S type shared folders denoted by, e.g., icons F 42 , F 43  and P2P shared folders denoted by icons F 41  and F 433  are mixed. Folders shown in the hierarchical structure within the display area  141 A will change according to icons selected by the user. 
   In the file sharing system of the present invention, the user of each client can register a local file belonging to the user with the file sharing system by selecting the icon  145  of any local file and dragging and dropping the selected local file icon onto any folder icon in the shared folder directory  142 , as denoted by an arrow D&amp;D 1 . The local file is registered as a C/S type shared file or a P2P type shared file depending on the sharing type of the folder onto which the local file was dragged and dropped. Details of this operation will be described below. 
   The user can shift a registered system file to change the sharing type by dragging and dropping, e.g., a registered file icon Dxx under a P2P type folder F 433  onto a C/S type folder F 432 , for example, as denoted by an arrow D&amp;D 2 . By clicking a file icon selected by the cursor, the user can download the shared file corresponding to that file icon from another client or the file server to the user&#39;s apparatus. 
     FIG. 3  is a bock diagram showing the configuration of the client apparatus  100 . 
   The client apparatus  100  is comprised of the following physical components: a communication interface  110  for connection to the network; a control processor  120 ; a data input/output controller  130 ; a user interface  140 ; data storage  150  connected to the data input/output controller  130 . The user interface  140  comprises input/output devices such as a display, keyboard, and mouse. 
   The control processor  120  includes, as logical components realized by software stored in the memory, a P2P communication protocol processing unit  101 , C/S communication protocol processing unit  102 , and client control logic unit  103 . The client control logic unit  103  comprises a basic control unit  111 , client communication unit  113 , management server communication unit  114 , and file server communication unit  115 . The data storage  150  includes a system file area  151 , local file area  152 , and other areas where a folder management table  160  and a file management table  170  are stored. 
   The folder management table  160  is comprised of, for example, as shown in  FIG. 4 , a plurality of table entries each having a folder ID  161 . Each of the entries indicates, with respect to a folder identified by the folder ID  161 , folder location information  162 , date of creation of the folder  163 , and folder creator name  164 . 
   The file management table  170  is comprised of, for example, as shown in  FIG. 5 , a plurality of table entries each having a file ID  171 . Each of the entries indicates, with respect to a file identified by the file ID  171 , file location information  172 , date of creation of the file  173 , and file creator name  174 . 
   Here, the folder location information  162  and file location information  172  logically indicate the position of a folder or file in the directory structure illustrated in FIG.  2 . For instance, when the user selects a particular folder icon in the window  141  with the cursor, the basic control unit  111  of the client obtains folder location information corresponding to the icon selected by the user from a section of the hierarchical structure being displayed on the display screen and the position of the selected icon. By referring to the folder management table  160 , the basic control unit gets the folder ID  161  corresponding to the folder location information. When the user selects a file icon in the window  141 , the basic control unit obtains file location information corresponding to the selected icon in the same way. In the case where the selected icon is a file icon, the basic control unit refers to the file management table  170  and gets the file ID  171  corresponding to the file location information  172 . 
     FIG. 6  is a block diagram showing a configuration of the file server  300 . 
   The file server  300  is comprised of the following physical components: a communication interface  310  for connection to the network; a control processor  320 ; a data input/output controller  330 ; a user interface  340 ; and data storage  350  connected to the data input/output controller  330 . The user interface  340  comprises input/output devices such as a display, keyboard, and mouse for operation by an administrator of the file sharing system. 
   The control processor  320  includes, as logical components realized by software stored in the memory, an S/S communication protocol processing unit  301  for executing inter-server communication protocol processing, a C/S communication protocol processing unit  302 , and a server control logic unit  303 . The server control logic unit  303  comprises a basic control unit  311 , client communication unit  313 , and management server communication unit  314 . In the data storage  350 , a system file area  351  and an area for storing a file management table  370  are prepared. 
     FIG. 7  is a block diagram showing a configuration of the management server  200 . 
   The management server  200  is comprised of the following physical components: a communication interface  210  for connection to the network; a control processor  220 ; a data input/output controller  230 ; a user interface  240 ; and data storage  250  connected to the data input/output controller  230 . The user interface  240  comprises input/output devices such as a display, keyboard, and mouse for operation by an administrator of the file sharing system. 
   The control processor  220  includes, as logical components realized by software stored in the memory, an S/S communication protocol processing unit  201 , a C/S communication protocol processing unit  202 , and a server control logic unit  203 . The server control logic unit  203  comprises a basic control unit  211 , sharing type management unit  212 , a client communication unit  213 , and a file server communication unit  215 . In the data storage  250 , a folder management table  260 , a file management table  270 , and a directory management table  280  are stored. 
   The folder management table  260  is referred to by the basic control unit  211  of the management server  211 , and comprised of, for example, as shown in  FIG. 8 , a plurality of table entries each having a folder ID  261 . Each of the entries indicates, with respect to a folder identified by a folder ID  261 , sharing type  262 , date of creation of the folder  263 , folder creator name  264  and folder address  265 . 
   In the present embodiment, a value “01” of the sharing type  262  denotes the C/S type and a value “10” denotes the P2P type. The folder address  265  denotes address information (the IP address) of a server or a client that holds the folder. 
   The file management table  270  is comprised of, for example, as shown in  FIG. 9 , a plurality of table entries each having a file ID  271 . Each of the entries indicates, with respect to a file identified by the file ID  271 , sharing type  272 , date of registration as a shared file  273 , registrar name  274 , and folder ID  275  in which the file was registered. 
   The directory management table  280  is comprised of, for example, as shown in  FIG. 10 , a plurality of table entries each having a file ID  281 . Each of the entries indicates, with respect to a file identified by the file ID  281 , folder location information  282  for a folder in which the file was registered, folder ID  283 , and file address (folder address)  384 . 
   Next, by referring to  FIG. 11 , C/S type shared file registration sequence and download sequence in the file sharing system of the present invention will be described. Here, a communication sequence will be described in the case where a client  100 A user registers a local file belonging to the user with the file server  300  as a C/S type shared file and a client  100 B user downloads the shared file registered with the file server  300  to the client terminal. 
   (1) Registering a C/S Type Shared File 
   When the client  100 A user has started a program for the file sharing system and logged into the system, a connection between the client  100 A and the management server  200  is set up and latest folder directory information is downloaded from the management server  200  to the client  100 A. The basic control unit  111  of the client  100 A creates the folder management table  160  and the file management table  170  according to the downloaded folder directory information, and displays the directory  142  of shared folders in the directory display area  141 A in the window  141  for accessing the sharing system, described in conjunction with  FIG. 2 . 
   The client  101 A user selects a local file (file icon  145  in  FIG. 2 ) that the user wishes to register as a shared file out of local files stored in the data storage  150 . For example, the user drags and drops with a mouse the file icon  145  onto a registration folder, e.g., a C/S type folder icon F 42 , to which a file corresponding to the file icon  145  should be registered, within the folder directory  142 , (step  930  in  FIG. 11 ). 
   By this drag and drop operation, the identifiers of the file icon  145  and the registration folder icon F 42  are transferred to the basic control unit  111 . The basis control unit  111  figures out the location of the registration folder, assigns a file ID to the local file denoted by the file icon  145 , and gets the folder ID corresponding to the folder location information of the registration folder by referring to the folder management table  160  in the data storage  150 . 
   The basic control unit  111  creates a file registration request message M 1  in which the local file ID, folder ID, and other information for the file are described and passes the message M 1  to the management server communication unit  114 . The management server communication unit  114  outputs the file registration request message M 1  to the C/S communication protocol processing unit  102 . The C/S communication protocol processing unit  102  appends a packet header including the IP address of the management server  200  as the destination address to the file registration request message M 1  and transmits the message in IP packet form from the communication interface  110  to the network (step  931  in  FIG. 11 ). 
   A packet that is transferred over the network is comprised of payload part  70  including the message body and a header part  71  as shown in  FIG. 12 . The header part  71  shown here is comprised of an Ethernet header H 1 , IP header H 2 , TCP header H 3 , and HTTP header H 4 . 
   The file registration request message M 1  includes, for example, as shown in  FIG. 13A , a message type indicator  700  indicating registration request, the file ID  701  of the local file to be registered as a shared file, the folder ID  702  of the registration folder, a user name  703 , and other information  704 . The other information  704  includes, for example, a file name, file size obtainable from the OS, and file type (application program). 
   The IP packet including the file registration request message M 1  transmitted to the network is received by the communication interface  210  of the management server  200  and passed to the C/S communication protocol processing unit  202 . The C/S communication protocol processing unit  202  extracts the file registration request message M 1  from the payload of the received IP packet and passes the message M 1  to the client communication unit  213 . The client communication unit  213  transfers the file registration request message M 1  to the basic control unit  211 . 
   Having received the file registration request message M 1 , the basic control unit  211  refers to other file information  704  in the received message and determines whether the local file requested to register is acceptable as a shared file. If registering the file as a shared file is permitted, the basic control unit  211  instructs the sharing type management unit  212 , designating the folder ID  702  specified in the received message, to search for the sharing type and address of the folder. In response to the instruction, the sharing type management unit  212  retrieves the sharing type and folder address corresponding to the folder ID  702  from the folder management table  260  in the data storage  250  and informs the basic control unit  211  of the retrieval results (step  932  in  FIG. 11 ). 
   Having been informed of the sharing type and folder address, the basic control unit  211  assigns a new file ID that becomes valid in the file sharing system (hereinafter, referred to as a shared file ID) to the local file and creates a file registration acknowledgement message M 2 . The file registration acknowledgement message M 2  includes, for example, as shown in  FIG. 13B , following the message type indicator  700  indicating registration ACK, local file ID  711 , shared file ID  712 , registration folder ID  713 , sharing type  714  of the registration folder, and folder address  715 . In this instance, a code designating the C/S sharing type is set as the sharing type  714  and the IP address of the file server  300  is set as the folder address  715 . 
   The file registration acknowledgement message M 2  is output via the client communication unit  213  to the C/S communication protocol processing unit  202 . The C/S communication protocol processing unit  202  appends the headers H 1  to H 4  including the IP address of the client  100 A as the destination address to the message M 2  and transmits the message in IP packet form from the communication interface  210  to the network (step  933  in  FIG. 11 ). 
   The IP packet including the file registration acknowledgement message M 2  is received by the communication interface  110  of the client  10 A, and the file registration acknowledgement message M 2  is extracted from the payload by the C/S communication protocol processing unit  102 . The message M 2  is transferred via the management server communication unit  114  to the basic control unit  111 . 
   Having received the file registration acknowledgement message M 2 , the basic control unit  111  of the client  100 A checks the sharing type  714  of the folder. If the sharing type  714  of the folder indicates “C/S type,” the basic control unit  111  reads out file data corresponding to the local file ID  711  from the local file area  152  of the data storage  150  and creates an upload message M 3  shown in  FIG. 13C . Thereafter, the basic control unit  111  manages that file data using the shared file ID. 
   The upload message M 3  includes, as shown in  FIG. 13C , following the message type indicator  700  indicating file upload, file classification  721 , shared file ID  722 , registration folder ID  723 , user name  724 , and file data  725 . For the file to be registered with the file server  300  as a C/S type shared file, the file classification  721  indicates whether the shared file is one that is newly registered with the file sharing system or a shared file that has already been registered as a P2P type and is requested to shift its location from a client to the fileserver. The latter file will be referred to a “shift-file” hereinafter. In this instance being discussed, a code denoting a new file is set as the file classification  721  of the upload message M 3 . 
   The upload message M 3  is fed via the client communication unit  113  to the C/S communication protocol processing unit  102 . The C/S communication protocol processing unit  102  appends the headers H 1  to H 4  including the address of the file server  300  as the destination IP address to the message and transmits the message in IP packet form from the communication interface  110  to the network (step  934  in  FIG. 11 ). 
   The IP packet including the upload message M 3  is received by the communication interface  310  of the file server  300 . The C/S communication protocol processing unit  302  extracts the upload message M 3  from the payload and transfers the message via the client communication unit  313  to the basic control unit  311 . 
   The basic control unit  311  of the file server  300  checks the file classification  721  in the received message M 3 . If the file classification  721  indicates a new file, the basic control unit  311  extracts the file data  725  from the received message M 3 , stores the file as a new system file into the system file area  351  of the data storage  350 , and registers the system file location in association with the file ID obtained from the received message M 3  into the file management table  370  in the data storage  350  (step  935  in  FIG. 11 ). 
   Then, the basic control unit  311  creates a file registration notification message M 4  which is shown in  FIG. 13D  and outputs the message M 4  via the management server communication unit  314  to the S/S communication protocol processing unit  301 . The S/S communication protocol processing unit  301  appends the headers H 1  to H 4  including the IP address of the management server  200  as the destination IP address to the message and transmits the message in IP packet form from the communication interface  310  to the network (step  936  in  FIG. 11 ). The file registration notification message M 4  includes, as shown in  FIG. 13D , following the message type indicator  700  indicating notification of file registration, file classification  731 , shared file ID  732 , registration folder ID  733 , and user name  734 . 
   The IP packet including the file registration notification message M 4  is received by the communication interface  210  of the management server  200 . The S/S communication protocol processing unit  201  extracts the file registration notification message M 4  from the payload and transfers the message via the file server communication unit  215  to the basic control unit  211 . Having received the file registration notification message M 4 , the basic control unit  211  checks the file classification  731 . If the file classification  731  indicates a new file, the basic control unit  211  instructs the sharing type management unit  212 , designating the shared file ID  732 , registration folder ID  733 , and user name  734  obtained from the received message, to register a new entry into the file management table  270 , and waits for a registration complete notification from the sharing type management unit  212 . 
   The sharing type management unit  212  retrieves the sharing type (C/S type in this instance)  262  corresponding to the folder ID from the folder management table  260  in the data storage  250  via the data input/output controller  230 , and registers a new entry indicating correspondence of shared file ID, registration folder ID, user name, that are received from the basic control unit  211 , the sharing type, and date of registration into the file management table  270  (step  937  in  FIG. 11 ). The current date is automatically set as the data of registration. Then, the sharing type management unit  212  sends the registration complete notification including the sharing type (C/S type) back to the basic control unit  211 . 
   Upon receiving the registration complete notification from the sharing type management unit  212 , the basic control unit  211  checks the sharing type in the notification. If the sharing type is C/S type, the basic control unit  211  creates a registration acknowledgement message M 5  and outputs the message via the file server communication unit  215  to the S/S communication protocol processing unit  201 . The S/S communication protocol processing unit  201  appends the headers H 1  to H 4  including the IP address of the file server  300  as the destination IP address to the registration acknowledgement message M 5  and transmits the message in IP packet form from the communication interface  210  to the network (step  938  in  FIG. 11 ). The registration acknowledgement message M 5  includes, following the message type indicator  700  indicating registration acknowledgement, file classification, shared file ID, and registration folder ID, which are the same as those in the file registration notification message M 4 . 
   The IP packet including the registration acknowledgement message M 5  is received by the communication interface  310  of the file server  300 . The C/S communication protocol processing unit  302  extracts the registration acknowledgement message M 5  from the payload and transfers the message M 5  via the client communication unit  313  to the basic control unit  311 . 
   Having received the registration acknowledgement message M 5 , the basic control unit  311  of the file server  300  creates an upload acknowledgement message M 6  to be a reply to the upload request message M 3  and outputs the message M 6  via the client communication unit  313  to the C/S communication protocol processing unit  302 . The upload acknowledgement message M 6  includes the same items of information as those in the registration acknowledgement message M 5 . The C/S communication protocol processing unit  302  appends the headers H 1  to H 4  including the IP address of the client  100 A as the destination IP address to the upload acknowledgement message M 6  received and transmits the message in IP packet form from the communication interface  310  to the network (step  939  in  FIG. 11 ). When the client  100 A receives the upload acknowledgement message M 6 , the file registration sequence terminates. 
   (2) Downloading a C/S Type Shared File 
   Next, a download sequence in which the client  100 B user downloads a C/S type shared file is described with reference to  FIG. 11 . 
   The client  100 B user starts the program for the file sharing system in the same way as the client  100 A user does to display the window  141  for accessing the sharing system. The user selects any of C/S type shared folders, e.g., F 42 , F 43 , F 431  and F 432  in the directory display area  141 A illustrated in  FIG. 2  and clicks one of the file icons displayed in the detail display area  141 B. 
   In response to the file icon click, the identifier of the file icon selected by the user is transferred via the user interface  140  to the basic control unit  111 . The basic control unit  111  detects the occurrence of a download request, figures out the file location of the selected file icon, and gets the file ID corresponding to the file location information by referring to the file management table  170  in the data storage  150 . Then, the basic control unit  111  creates a file download request message M 7  including the file ID and outputs the message via the management server communication unit  114  to the C/S communication protocol processing unit  102 . 
   The file download request message M 7  includes, as shown in  FIG. 14A , following the message type indicator  700  indicating download request, the file ID  741  of the shared file to be downloaded. The C/S communication protocol processing unit  102  appends the headers H 1  to H 4  including the IP address of the management server  200  as the destination IP address to the file download request message M 7  and transmits the message in IP packet form from the communication interface  110  to the network (step  941  in  FIG. 11 ). 
   The IP packet including the file download request message M 7  is received by the communication interface  210  of the management server  200 . The C/S communication protocol processing unit  202  extracts the file download request message M 7  from the payload and transfers the message M 7  via the client communication unit  213  to the basic control unit  211 . Having received the file download request message M 7 , the basic control unit  211  instructs the sharing type management unit  212 , designating the file ID  741  specified in the received message, to search for the appropriate sharing type and folder address. 
   In response to the instruction from the basic control unit  211 , the sharing type management unit  212  retrieves the sharing type  272  and registration folder ID  275  corresponding to the file ID from the file management table  270  in the data storage  250 , retrieves the folder address  265  corresponding to the registration folder ID  275  from the folder management table  260 , and sends back the retrieval results to the basic control unit  211  (step  942  in  FIG. 11 ). 
   Based on the retrieval results, the basic control unit  211  creates a download acknowledgement message M 8  and outputs the message via the client communication unit  213  to the C/S communication protocol processing unit  202 . The download acknowledgement message M 8  includes, as shown in  FIG. 14B , following the message type indicator  700  indicating download ACK, the descriptions of shared file ID  751 , sharing type  752  of the registration folder, and folder address  753 . In this context, the folder address  753  designates the IP address of the file server  300 . 
   The C/S communication protocol processing unit  202  appends the headers H 11  to H 4  including the IP address of the requester client  100 B as the destination address to the download acknowledgement message M 8  and transmits the message in IP packet form from the communication interface  210  to the network (step  943  in  FIG. 11 ). 
   The IP packet including the download acknowledgement message M 8  is received by the communication interface  110  of the client  100 B. The C/S communication protocol processing unit  102  extracts the download acknowledgement message M 8  from the payload and transfers the message via the management server communication unit  114  to the basic control unit  111 . 
   Having received the download acknowledgement message M 8 , the basic control unit  111  creates a file download request message M 7  again and outputs the message via the file server communication unit  115  to the C/S communication protocol processing unit  102 . The C/S communication protocol processing unit  102  appends the headers H 1  to H 4  to the file download request message M 7  and transmits the message in IP packet form from the communication interface  110  to the network (step  944  in  FIG. 11 ). At this time, the IP address of the file server  300  designated with the folder address  753  of the download acknowledgement message MB is applied as the destination IP address. 
   The IP packet including the file download request message M 7  is received by the communication interface  310  of the file server  300 . The C/S communication protocol processing unit  302  extracts the file download request message M 7  from the payload and transfers the message via the client communication unit  313  to the basic control unit  311 . Having received the file download request message M 7 , the basic control unit  311  reads out, from the file management table  370  in the data storage  350 , the address of a storage location of a system file corresponding to the shared file ID  741  specified in the received message. Based on that address, the basic control unit  311  reads out file data having the shared file ID  741  from the system file area  351  (step  945  in  FIG. 11 ). 
   Then, the basic control unit  311  creates a file download response message M 9  including the file data and outputs the message via the client communication unit  313  to the C/S communication protocol processing unit  302 . The file download response message M 9  includes, as shown in  FIG. 14C , following the message type indicator  700  indicating download response, the shared file ID  762  and file data  762 . 
   The C/S communication protocol processing unit  302  appends the headers H 1  to H 4  including the IP address of the requester client  100 B as the destination IP address to the file download response message M 9  and transmits the message in IP packet form from the communication interface  310  to the network (step  946  in  FIG. 11 ). 
   The IP packet including the file download response message M 9  is received by the communication interface  110  of the client  100 B. The C/S communication protocol processing unit  102  extracts the file download response message M 9  from the payload and transfers the message via the file server communication unit  115  to the basic control unit  111 . The basic control unit  111  extracts the file data  762  from the file download response message M 9  and stores the file data into the local file area  152  as new local file data (step  947  in  FIG. 11 ). 
   Then, the basic control unit  111  creates a download complete notification message M 10  and outputs the message via the management server communication unit  114  to the C/S communication protocol processing unit  102 . The C/S communication protocol processing unit  102  appends the headers H 1  to H 4  including the IP address of the management server  200  as the destination IP address to the download complete notification message M 1  and transmits the message in IP packet form from the communication interface  110  to the network (step  948  in  FIG. 11 ). Having received the download complete notification message M 10 , the management server  200  performs internal processing in association with the use of the shared file, if necessary, and returns an ACK message (step  949  in  FIG. 11 ). When the client  100 B receives the ACK message, the file download sequence is completed. 
   Next, by referring to  FIG. 15 , a registration sequence in which the client  100 A user registers a P2P type shared file and a download sequence in which the client  100 B user downloads a P2P type shared file will be described. 
   (3) Registering a P2P Type Shared File 
   The P2P shared file registration sequence starts when the client  100 A user drags and drops a file icon  145  onto a P2P type folder icon, e.g., an icon F 433  in the above-described window  141  for accessing the file sharing system (step  950  in  FIG. 15 ). Here, the P 2 P folder corresponding to the icon F 433  in which the selected shared file should be stored is the one that the client  100 A user has registered beforehand with the management server  200  in the folder registration procedure. In the folder management table  260 , an entry including the IP address of the client  100 A as the folder address  265  corresponding to the folder ID exists. 
   When the file icon  145  is dragged and dropped onto the P2P type folder icon, the basic control unit  111  of the client  100 A assigns a file ID to the local file denoted by the file icon  145  and creates a file registration request message M 1 , as same in the case performed when registering a C/S type shared file. The file registration request message M 1  created at this time has the same format as shown in  FIG. 13A . However, only the value of the registration folder ID  702  differs from that in the registration request message for a C/S type shared file. 
   The file registration request message M 1  in IP packet form including the IP address of the management server  200  as the destination IP address is transmitted to the network (step  951  in  FIG. 15 ). The message is received by the communication interface  210  of the management server  200  and transferred to the basic control unit  211 . 
   In the same way as described for step  932  in  FIG. 11 , the basic control unit  211  assigns a shared file ID to the local file to be registered, retrieves the sharing type of the registration folder and folder address (step  952  in  FIG. 15 ), and creates a file registration acknowledgement message M 2  shown in  FIG. 13B . In this instance, a code designating P2P type is set as the sharing type  714  of the registration folder. After the C/S communication protocol processing unit  202  appends the headers including the IP address of the client  101 A as the destination IP address to the message M 2 , the message is transmitted in IP packet form to the network (step  953  in  FIG. 15 ). 
   The IP packet including the registration acknowledgement message M 2  is received by the communication interface  110  of the client  10 A. The C/S communication protocol processing unit  102  extracts the registration acknowledgement message M 2  from the payload and transfers the message via the management server communication unit  114  to the basic control unit  111 . Having received the registration acknowledgement message M 2 , the basic control unit  111  checks the sharing type  714  of the registration folder. At this time, since the sharing type is P2P type, the basic control unit  111  reads out the local file having the local file ID  711 , which should be registered, from the local file area  152  in the data storage  150 , stores the local file as a new system file with the shared file ID  721  into the system file area  151 , and adds a new entry with the shared file ID to the file management table  170  (step  954  in  FIG. 15 ). 
   Then, the basic control unit  111  creates a file registration notification message M 10  including the shared file ID and outputs the message via the management server communication unit  114  to the C/S communication protocol processing unit  102 . The C/S communication protocol processing unit  102  appends the headers H 1  to H 4  including the IP address of the management server  200  as the destination IP address to the file registration notification message M 10  and transmits the message in IP packet from the communication interface  110  to the network (step  955  in  FIG. 15 ). The format of the file registration notification message M 10  is the same as the file registration notification message M 4  shown in  FIG. 13D . 
   The IP packet including the file registration notification message M 10  is received by the communication interface  210  of the management server  200 . The C/S communication protocol processing unit  202  extracts the file registration notification message M 10  from the payload and transfers the message via the client communication unit  213  to the basic control unit  211 . Having received the file registration notification message M 10 , the basic control unit  211  checks the file classification  731 . If the file classification is a new file, the basic control unit instructs the sharing type management unit  212  to register a new entry into the file management table  270  in the same way as described for step  937  in  FIG. 11  and waits for a registration complete notification from the sharing type management unit  212 . In this instance, the sharing type management unit  212  sends the registration complete notification of the sharing type (P2P type) back to the basic control unit  211  after registering a new entry record into the file management table  270  (step  956  in  FIG. 15 ). 
   In the case where the sharing type is P2P, the basic control unit  211  creates an ACK message and outputs the message via the client communication unit  213  to the C/S communication protocol processing unit  202 . The C/S communication protocol processing unit  202  appends the headers H 1  to H 4  including the IP address of the client  100 A as the destination IP address to the ACK message and transmits the message in IP packet form from the communication interface  210  to the network (step  957  in  FIG. 15 ). When the client  100 A receives the ACK message, the P2P type shared file registration sequence terminates. 
   (4) Downloading a P2P Type Shared File 
   Next, a download sequence in which the client  100 B user downloads a P2P type shared file is described with reference to  FIG. 15 . 
   The client  100 B user selects any P2P type shared folder, e.g., F 41  or F 433  in the directory display area  141 A shown in  FIG. 2 , selects one of the file icons displayed in the detail display area, and clicks a download request button. Here, it is assumed that the user has selected a file icon denoting a shared file stored as a system file in the client apparatus  100 A. 
   In response to the click operation of the download request button, the identifier of the file icon selected by the user is transferred via the user interface  140  to the basic control unit  111 . Upon detecting the occurrence of a download request, the basic control unit  111  retrieves the file ID from the file management table  170  based on the file location information of the selected file icon, as described by referring to  FIG. 11 . The basic control unit  111  creates a file download request message M 7  including the file ID shown in  FIG. 14A , and outputs the message via the management server communication unit  114  to the C/S communication protocol processing unit  102 . The C/S communication protocol processing unit  102  converts the file download request message M 7  into an IP packet with the destination IP address of the management server and transmits the IP packet from the communication interface  110  to the network (step  961  in  FIG. 15 ). 
   The IP packet is received by the management server  200 . Upon receiving the file download request message M 7 , the basic control unit  211  instructs the sharing type management unit  212  to search for the appropriate sharing type and folder address in the same way as described for step  942  in  FIG. 11 , and receives the retrieval results from the sharing type management unit  212  (step  962  in  FIG. 15 ). The basic control unit  211  creates a download acknowledgement message M 8  shown in  FIG. 14B  based on the retrieval results. In the download acknowledgement message M 8  created at this time, the sharing type  752  of the registration folder indicates P2P type and the folder address  753  specifies the IP address of the client  10 A. The C/S communication protocol processing unit  202  converts the download acknowledgement message M 8  into an IP packet having the IP address of the requester client  100 B as the destination and transmits the IP packet from the communication interface  210  to the network (step  963  in  FIG. 15 ). 
   The IP packet is received by the client  100 B and the download acknowledgement message M 8  is transferred to the basic control unit  111 . Then, the basic control unit  111  creates a file download request message M 7  again and outputs the message via the file server communication unit  115  to the C/S communication protocol processing unit  102 . The C/S communication protocol processing unit  102  converts the file download request message M 7  into an IP packet and transmits the IP packet from the communication interface  110  to the network (step  964  in  FIG. 15 ). At this time, the IP address of the client  100 A designated with the folder address  753  of the download acknowledgement message M 8  is applied to the destination IP address of the IP packet. 
   Accordingly, the IP packet including the file download request message M 7  is received by the communication interface  110  of the client  100 A and transferred to the P2P communication protocol processing unit  101 . The P2P communication protocol processing unit  101  extracts the file download request message M 7  from the payload of the received packet and transfers the message via the client communication unit  113  to the basic control unit  111 . Having received the file download request message M 7 , the basic control unit  111  reads out file data corresponding to the shared file ID  741  specified in the file download request message M 7  from the system file area  151  of the data storage  150  and creates a file download response message M 12  including the file data. 
   The file download response message M 12  includes, following the message type indicator  700  indicating download response, the shared file ID and file data, like the file download response message M 9  that the file server  300  creates. The file download response message M 12  is output via the client communication unit  113  to the P2P communication protocol processing unit  101 , converted into IP packet form having the IP address of the client  100 B as the destination IP address, and transmitted from the communication interface  110  to the network (step  965  in  FIG. 15 ). 
   Upon receiving the file download response message M 12 , the basic control unit  111  of the client  100 B stores the file data  762  extracted from the received message into the data storage as local file data (step  966  in  FIG. 15 ) and creates a file download complete notification message M 13 . The C/S communication protocol processing unit  102  converts the file download complete notification message M 13  into an IP packet having the IP address of the management server  200  as the destination IP address and transmits the IP packet to the network (step  967  in  FIG. 15 ). 
   Upon receiving the down load complete notification message M 13 , the basic control unit  211  of the management server  200  performs internal processing in association with the use of the shared file, if necessary, and returns an ACK message to the client  100 B (step  968  in  FIG. 15 ). When the client  100 B receives the ACK message, the file download sequence is completed. 
   Next, changing the sharing type of a registered shared file in the file sharing system of the present invention will be described. First, a sequence for changing a registered C/S type shared file to a P2P type one will be described with reference to  FIG. 16 . 
   (5) Changing from C/S Type to P2P Type 
   In the state that the window  141  for accessing the file sharing system is displayed on the display screen, if the client  100 A user selects a file icon Fx of a registered C/S type shared file and drags and drops the selected file icon onto a folder icon Fy of a P2P folder created by the user, the identifier of the file icon Fx and the identifier of the folder icon Fy are transferred via the user interface  140  to the basic control unit  111  (step  970  in  FIG. 16 ). 
   Upon detecting that the file icon Fx has been dragged  110  and dropped onto the folder icon Fy, the basic control unit  111  figures out the locations of these icons Fx and Fy, based on the configuration of the folder directory being displayed, and retrieves the file ID corresponding to the file icon Fx from the file management table  170  in the data storage  150 . The basic control unit  111  further retrieves from the folder management table  160 , the ID of the “shift to” folder, which will also be referred to as the destination folder hereinafter, corresponding to the folder icon Fy, and the ID of the current registration folder (“shift from” folder, which will be also referred to as the original folder hereinafter) of the shared file denoted by the file icon Fx. 
   The basic control unit  111  creates a file shift request message M 20  based on these IDs, and outputs the message via the management server communication unit  114  to the C/S communication protocol processing unit  102 . The file shift request message M 20  includes, as shown in  FIG. 17A , following the message type indicator  700  indicating file shift request, ID of file to be shifted  771  which will be referred to as “shift-file ID” hereinafter, original folder ID  772 , and destination folder ID. 
   The C/S communication protocol processing unit  102  appends the headers H 1  to H 4  including the IP address of the management server  200  as the destination address to the file shift request message M 20  and transmits the message in IP packet from the communication interface  110  to the network (step  971  in  FIG. 16 ). 
   The IP packet including the file shift request message M 20  is received by the communication interface  210  of the management server  200 . The C/S communication protocol processing unit  202  extracts the file shift request message M 20  from the payload and transfers the message via the client communication unit  213  to the basic control unit  211 . Having received the file shift request message M 20 , the basic control unit  211  instructs the sharing type management unit  212 , designating the original folder ID and destination folder ID obtained from the received message, to search for the sharing type of each folder and the destination folder address. 
   In response to the instruction, the sharing type management unit  212  retrieves the sharing type corresponding to the original folder ID and the sharing type and folder address corresponding to the destination folder ID from the folder management table  260  in the data storage  250  (step  972  in  FIG. 16 ) and sends the retrieval results back to the basic control unit  211 . 
   Having received the retrieval results from the sharing type management unit  212 , the basic control unit  211  creates a file shift acknowledgement message M 21  and outputs the message via the client communication unit  213  to the C/S communication protocol processing unit  202 . The file shift acknowledgement message M 21  includes, as shown in  FIG. 17B , following the message type indicator  700  indicating file shift acknowledgement, shift-file ID  781 , sharing type of original folder  782 , sharing type of destination folder  783 , and destination folder address  784 . 
   The C/S communication protocol processing unit  202  appends the headers H 1  to H 4  including the IP address of the requester client  100 A as the destination address to the file shift acknowledgement message M 21  and transmits the message in IP packet form from the communication interface  210  onto the network (step  973  in  FIG. 16 ). 
   The IP packet including the file shift acknowledgement message M 21  is received by the communication interface  110  of the client  10 A. The C/S communication protocol processing unit  102  extracts the file shift acknowledgement message M 21  from the payload and transfers the message via the management server communication unit  114  to the basic control unit  111 . Having received the file shift acknowledgement message M 21 , the basic control unit  111  recognizes, from the relation between the sharing type of original folder  782  and sharing type of destination folder  783 , that the request to change the system file having the shared file ID  781  from C/S type to P2P type was accepted. 
   In this context, the basic control unit  111  creates a download request message for file shift M 22  from C/S type to P2P type, which is shown in  FIG. 17C , and outputs the message via the file server communication unit  115  to the C/S communication protocol processing unit  102 . The download request message for file shift M 22  includes, following the message type indicator  700  indicating download for file shift, file classification (shift-file in this instance)  791  and the values extracted from the shift acknowledgement message M 21 ; namely, the shared file ID to be shifted  792 , sharing type of original folder (C/S type in this instance)  793 , and sharing type of destination folder (P2P type in this instance)  794 . 
   The C/S communication protocol processing unit  102  appends the headers H 1  to H 4  including the IP address of the file server  300  as the destination IP address to the download request message M 22  and transmits the message in IP packet form from the communication interface  110  to the network (step  974  in  FIG. 16 ). 
   The IP packet including the download request message for file shift M 22  is received by the communication interface  310  of the file server  300 . The C/S communication protocol processing unit  302  extracts the download request message for file shift M 22  from the payload and transfers the message via the client communication unit  313  to the basic control unit  311 . 
   Having received the download request message for file shift M 22 , the basic control unit  311  retrieves the location of a system file corresponding to the shift-file ID specified in the message M 22  from the file management table  370  in the data storage  350  and reads out file data of the system file having the shift-file ID from the system file area  351 . At this time, since the file classification  791  in the download request message M 22  indicates a file to be shifted, it is unnecessary for the file server  30  to keep the file data of the requested system file in the data storage  350 , unlike an ordinary download. Thus, the basic control unit  311  deletes the system file from the storage  351  and deletes the entry having the shift-file ID from the file management table  370  (step  975  in  FIG. 16 ). 
   After that, the basic control unit  311  creates a download response message M 23  for file shift including the file data of the requested system file and outputs the message via the client communication unit  313  to the C/S communication protocol processing unit  302 . The download response message M 23  has the same format as the download response message M 9  shown in  FIG. 14C . 
   The C/S communication protocol processing unit  302  appends the headers H 1  to H 4  including the IP address of the requester client  100 A as the destination IP address to the download response message M 23  received and transmits the message in IP packet form from the communication interface  310  to the network (step  976  in  FIG. 16 ). 
   The IP packet including the download response message M 23  is received by the communication interface  110  of the client  10 A. The C/S communication protocol processing unit  102  extracts the download response message M 23  from the payload and transfers the message via the management server communication unit  114  to the basic control unit  111 . Having received the download response message M 23 , the basic control unit  111  stores the filed at a extracted from the received message into the system file area  151  of the data storage  150  and manages the file data with the shared file ID  751  obtained from the received message (step  977  in  FIG. 16 ). 
   Then, the basic control unit  111  creates a file shift complete notification message M 24  and outputs the message via the management server communication unit  114  to the C/S communication protocol processing unit  102 . The file shift complete notification message M 24  includes, following the message type indicator  700  indicating notification of file shift completion, the same items  731  to  734  as that in the file registration notification message M 4  shown in  FIG. 13D . In this context, a code designating a shift-file is set as the file classification  731 . 
   The C/S communication protocol processing unit  102  appends the headers H 1  to H 4  including the IP address of the management server  200  as the destination address to the file shift complete notification message M 24  and transmits the message in IP packet form from the communication interface  110  onto the network (step  978  in  FIG. 16 ). 
   The IP packet including the file shift complete notification message M 24  is received by the communication interface  210  of the management server  200 . The C/S communication protocol processing unit  202  extracts the file shift complete notification message M 24  from the payload and transfers the message via the client communication unit to the basic control unit  211 . Having received the file shift complete notification message M 24 , the basic control unit  211  checks the file classification  731 . 
   In this instance, since the file classification  731  indicates a shift-file, the basic control unit  211  instructs the sharing type management unit  212 , designating the shared file ID, parent (destination) folder ID, and sharing type, to modify the entry of the shared file in the file management table  270 . For the entry having the specified shared file ID in the file management table  270 , the sharing type management unit  212  replaces the values of the sharing type  272  and registration folder ID  275  with the sharing type and folder ID designated by the basic control unit  211  (step  979  in  FIG. 16 ), and sends a change complete notification to the basic control unit  211 . 
   Having received the change complete notification from the sharing type management unit  212 , the basic control unit  211  creates an ACK message and outputs the message via the client communication unit  213  to the C/S communication protocol processing unit  202 . The C/S communication protocol processing unit  202  appends the headers H 1  to H 4  including the IP address of the client  100 A as the destination IP address to the ACK message and transmits the message in IP packet form from the communication interface  210  to the network (step  980  in  FIG. 16 ). When the client  100 A receives the ACK message, the file shift sequence terminates. 
   (6) Changing from P2P type to C/S Type 
   Next, a sequence for changing a registered P2P type shared file to a C/S type one will be described with reference to  FIG. 18 . 
   In the state that the window  141  for accessing the file sharing system is displayed on the display screen, if the client  100 A user selects a file icon Fx of a registered P2P type shared file and drags and drops the selected file icon onto a folder icon Fz of a C/S type folder, the identifier of the file icon Fx and the identifier of the folder icon Fz are transferred via the user interface  140  to the basic control unit  111  (step  981  in  FIG. 18 ). 
   Upon detecting that the file icon Fx has been dragged and dropped onto the folder icon Fz, the basic control unit  111  figures out the locations of these icons Fx and Fz, based on the configuration of the folder directory being displayed, and retrieves the file ID corresponding to the file icon Fx from the file management table  170  in the data storage  150 . The basic control unit  111  further retrieves, from the folder management table  160 , the ID of the destination folder corresponding to the folder icon Fz and the ID of the current registration folder (original folder) of the shared file denoted by the file icon Fx. 
   Based on these IDs, the basic control unit  111  creates a file shift request message M 20  shown in  FIG. 17A . In the same procedure as described for step  971  in  FIG. 16 , the file shift request message M 20  is transmitted in IP packet form to the management server  200  (step  982  in  FIG. 16 ) and, then, transferred to the basic control unit  211  of the management server  200 . 
   Having received the file shift request message M 20 , in the same way as described for step  972  in  FIG. 16 , the basic control unit  211  instructs the sharing type management unit  212  to search for the sharing types of the original and destination folders and the destination folder address (step  983  in  FIG. 18 ). Having received the retrieval results from the sharing type management unit  212 , the basic control unit  211  creates a file shift acknowledgement message M 21  shown in  FIG. 17B  and transmits the message in IP packet form to the client  100 A (step  984  in  FIG. 18 ). 
   Having received the file shift acknowledgement message M 21 , the basic control unit  111  of the client  100 A recognizes, from the relation between the sharing type of original folder  782  and sharing type of destination folder  783 , that the request to change the system file having the shared file ID  781  from P2P type to C/S type was accepted. Then, the basic control unit  111  reads out file data having the shared file ID  781  from the system file area  151  in the data storage  150  and creates an upload request message M 3  for file shift in the message format shown in  FIG. 13C . 
   As the file classification  721  of the upload request message M 3  created at this time, a classification code indicating a shift-file is set. For P2P to C/S type system file change like this instance, it is unnecessary for the client  100 A to save the system file having the shared file ID  781  in the data storage  150 , because the storage location of the file is changed to the file server. Thus, the basic control unit  11  deletes that system file data from the system file  151  (step  985  in  FIG. 18 ). 
   The upload request message M 3  for file shift is transmitted to the network in IP packet form addressed to the file server  300  (step  986  in  FIG. 18 ). The message is received by the communication interface  310  of the file server  300  and transferred to the basic control unit  311 . 
   Having received the upload request message M 3 , the basic control unit  311  stores the file data extracted from the received message M 3  into the system file area  351  in the data storage  350  and adds a new entry to the file management table  370  in the same way as is the case when having received the upload request message M 3  described for  FIG. 11  (step  987  in  FIG. 18 ). After that, the basic control unit  311  creates a file registration message M 4  and transmits the message in IP packet form to the management server  200  (step  988  in  FIG. 18 ). In the file registration message M 4  created at this time, the file classification  731  has the same classification code designating a shift-file as in the received upload request message M 3 . 
   Having received the file registration message M 4 , the basic control unit  211  of the management server  200  judges from the file classification  731  that the message is the notification of file registration issued in association with the file shift and instructs the sharing type management unit  212  to change, in the file management table  270 , the sharing type  272  from P2P type to C/S type in a table entry corresponding to the shared file ID specified in the received message M 4  (step  989  in  FIG. 18 ). In the same way as described for step  938  in  FIG. 11 , the basic control unit  211  of the management server  200  creates a registration acknowledgement message M 5  and transmits the message to the file server  300  (step  990  in  FIG. 18 ). In the same way as described for step  939  in  FIG. 11 , in response to receiving the registration acknowledgement message M 5 , the file server  300  creates an upload acknowledgement message M 6  and transmits the message to the client  100 A (step  991  in  FIG. 18 ). 
   While the above embodiment illustrates the file sharing system comprising the management server  200  and the file server  300 , in which the file server  300  cooperates with the management server  200  to store C/S type shared files and provide a service of downloading file data to a client, the present invention can also be applied to a system configuration in which the management server  200  cooperates with a plurality of file servers. 
   According to the above embodiment, when a client issues, designating its registration folder ID, an access request (registration request or download request) for a shared file, the management server  200  can notify the client of the device address where the registration folder exists by means of a response message (registration acknowledgment M 2  or download acknowledgement M 8 ) because the management server  200  manages address information for all folders (or files). Therefore, when the client requests to access a C/S type registration folder, even if a plurality of file servers exist, the management server  200  can notify the client of the appropriate file server to which the client should get access. Consequently, according to the present invention, by distributing C/S type shared files across a plurality of file servers, a file sharing system with expanded storage capacity and improved speed of server response can be provided. 
   While the above embodiment discusses the case where a client user registers a P2P type shared file in a folder having the address of the user&#39;s client apparatus as the folder address, a P2P type shared file may be stored in a folder existing on any other client by using a distributed file management type directory presented, e.g., in patent document 1. 
   In this case, a plurality of files stored in different client apparatuses will be linked to a folder. For this reason, the management server  200  having receives a file download request for a P2P type shared file is considered impossible to identify the address of a destination client terminal to which the second file download request should be transmitted, even if the management server  200  identifies the registration folder ID of the requested file from the file management table  270  shown in  FIG. 9  and searches the folder management table  260  in  FIG. 8  according to the folder ID. Therefore, in the case where P2P type shared files are managed using the distributed file management type directory, it is advisable to add a field to the registration request message M 1  and specify the client terminal address to which the file download request should be transmitted in the field. In this case, the client address may be registered into the file management table  270  when a shared file is registered.