Abstract:
An IP multicast communication system includes a layer-2 switch for accommodating a plurality of recipients dynamically joining or not joining a multicast group, a layer-3 switch adapted to a subnet for receiving IP multicast data sent from a sender via an IP network and distributing the received IP multicast data to authorized recipients joining the multicast group via the layer-2 switch under control, and a controller for collectively managing recipient management information for authentication of the recipients obtained according to an Internet Group Management Protocol IGMP. The layer-3 switch authenticates the recipients according to the recipient management information adapted to its subnetwork among the recipient management information collectively managed by the controller. The layer-2 switch stops transmission of the IP multicast data or thins the IP multicast data sent to recipients that are determined to have made unauthorized accesses by the layer-3 switch.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an IP (Internet Protocol) multicast communication system, and particularly to an IP multicast communication system that is capable of preventing or disturbing reception of multicast data through unauthorized access, by utilizing information based on the IGMP (Internet Group Management Protocol).  
         [0002]     In a conventional IP multicast communication system, as shown in  FIG. 1 , a multicast router R-RT on the receiving side receives IP multicast data from a sender (strictly, including a sending terminal such as a host/server computer and its operator) through a multicast router T-RT on the sending side and an IP network NW.  
         [0003]     A switching hub R-SW-HUB for the receiving-side subnetwork (subnet) receives the IP multicast data from the receiving-side multicast router R-RT and distributes the IP multicast data to a plurality of recipients A, B, and C that gained membership of the multicast group in advance (strictly, including user terminals and the users). When the system includes a single sender, the sending-side switching hub T-SW-HUB can be omitted.  
         [0004]     In this IP multicast communication system, the IP multicast data (which is referred to also as multicast data or simply as data unless particular limitation is required) is sent to the recipients when the recipients make data reception requests or when the sender makes a data transmission request.  
         [0005]     That is to say, when the sender sends out multicast data onto the IP network, and a recipient specifies an IP multicast address and the receiving-side multicast router defines a multicast routing protocol (a routing protocol such as the PIM-SM (Protocol Independent Multicast-Sparse Mode) or the PIM-DM (Protocol Independent Multicast-Dense Mode)), then the recipient can obtain the multicast data.  
         [0006]     The multicast address is a class-D IP address and includes a multicast group ID. The multicast group ID is in a certain range of address values (e.g., 224. 0. 0.0-239. 255. 255. 255) and so it is easier to know the multicast address than to know a unicast address. It is therefore difficult to control access to multicast data from recipients and hence to prevent acquisition of multicast data by recipients making unauthorized access.  
         [0007]     Also, in video distribution, which distributes data compressed by, e.g. MPEG2 (Moving Picture Experts Group-2), encrypting video multicast data (including moving picture data and audio data) for high speed and wide-band transmission (e.g. 6 Mbps) causes delay in data encryption and decryption. Accordingly, it is difficult to use encryption techniques in streaming.  
         [0008]     On the other hand, in a conventional method in which a recipient obtains video by entering a password informed from the video sender, the communication between the video sender and the recipient is one-to-one communication and therefore traffic increases in proportion to the number of recipients, where delay in distribution of passwords may hinder provision of video. Also, this scheme requires management of recipients and passwords for each distributed program, which complicates processing on the management side.  
         [0009]     In a method according to the Simple Multicast Receiver Access Control (All Provisions of Section 10 of RFC 2026), a recipient and a proximate multicast router use a public key and a secret key so that the multicast router can check the recipient for authentication according to the Internet Group Management Protocol IGMP to decide whether to accept or reject the recipient.  
         [0010]     However, this method is very fragile when an authorized recipient is included in the same subnet; i.e. this method tends to suffer from masquerading as authorized recipients. When the Simple Multicast Receiver Access Control scheme is combined with an existing “peeping” technique called IGMP Snooping, the IGMP snooping in a switching hub causes propagation delay since a MAC (Media Access Control) address is read directly from the header of data flowing through ports and data is exchanged between ports connected with the sender and the destination.  
       SUMMARY OF THE INVENTION  
       [0011]     An object of the present invention is to provide a technique capable of preventing or disturbing reception of multicast data by unauthorized access, by utilizing information according to the Internet Group Management Protocol IGMP.  
         [0012]     In order to achieve the above object, the present invention provides an IP multicast communication system, including: 
        a layer-2 switch that accommodates a plurality of recipients capable of dynamically joining or not joining a multicast group;     a layer-3 switch, for a subnetwork, that receives IP multicast data sent from a sender through an IP network and distributes, through the layer-2 switch subordinate to the layer-3 switch, the received IP multicast data to a plurality of authorized recipients joining the multicast group; and     a controller that collectively manages recipient management information for authentication of the recipients obtained according to an Internet Group Management Protocol IGMP;     wherein the layer-3 switch checking the recipients for authentication on the basis of recipient management information for the own subnetwork that is contained in the recipient management information collectively managed by the controller, and     the layer-2 switch ceasing transfer of the IP multicast data to a recipient that is judged by the layer-3 switch as having made unauthorized access, thinning out the IP multicast data, and sending the thinned-out data.        
 
         [0018]     In the IP multicast communication system, the layer-2 switch may be a switching hub and the layer-3 switch may be a multicast router.  
         [0019]     The controller as an authentication server has a table storing the recipient management information. The recipient management information collectively managed by the controller includes, for each the recipient, a multicast group address, an IP address, a MAC address, a multicast group membership level, a subnetwork address, and a flag for specifying a recipient making unauthorized access.  
         [0020]     Further, when the layer-3 switch receives, through the layer-2 switch, a join message for joining the IP multicast group which is sent from the recipient according to the IGMP, and a subnetwork address of the recipient is absent in its own the recipient management information, then the layer-3 switch changes the direction and distributes a reporting message according to the IGMP to the layer-2 switch to cause the layer-2 switch to set a flag for specifying a recipient making unauthorized access.  
         [0021]     The Internet Group Management Protocol IGMP is a protocol for distributing IP multicast data to a particular group identified with an IP multicast group address (a single IP destination address).  
         [0022]     The present invention makes it possible to prevent or disturb reception of multicast data by recipients making unauthorized access and provides an IP multicast communication system with great security. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  shows a configuration example of a conventional IP multicast communication system.  
         [0024]      FIG. 2  shows a first configuration example of an IP multicast communication system according to the present invention.  
         [0025]      FIG. 3  shows a second configuration example of the IP multicast communication system according to the present invention.  
         [0026]      FIG. 4  is a flowchart of a process performed by an authentication server.  
         [0027]      FIG. 5  is a flowchart of a process performed by the authentication server.  
         [0028]      FIG. 6  is a flowchart of a process performed by a multicast router.  
         [0029]      FIG. 7  is a flowchart of a process performed by the multicast router.  
         [0030]      FIG. 8  is a flowchart of a process performed by the multicast router.  
         [0031]      FIG. 9  is a flowchart of a process performed by a switching hub.  
         [0032]      FIG. 10  is a flowchart of a process performed by the switching hub.  
         [0033]      FIG. 11  is a flowchart of a process performed by the switching hub.  
         [0034]      FIG. 12  is a flowchart of a process performed by recipients (recipients that desire to receive multicast data).  
         [0035]      FIG. 13  is a flowchart of a process performed by recipients (recipients that desire to receive multicast data). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     Next, an embodiment of the present invention is described referring to the drawings.  
         [0037]     [Configurations of IP Multicast Communication System] 
         [0038]     Referring to  FIGS. 2 and 3  showing system configurations according to an embodiment of the present invention, an IP multicast communication system  1  includes multicast routers  3  ( 31 ,  32 , and  33 ) connected to an IP network  2 , e.g. the Internet.  
         [0039]     These multicast routers  3  are provided for respective subnetworks (subnets) and connected to respective subordinate switching hubs (SW-HUBs)  4  ( 41 ,  42 , and  43 ). The multicast routers  3  can be replaced by other layer-3 (L3) switches that support IP multicasting.  
         [0040]     The switching hub  41  accommodates a sender  5  that sends IP multicast data (strictly, including a sending terminal such as a host/server computer and its operator). The switching hub  42  accommodates an authentication server  6 . The switching hubs  41  and  42  may be omitted. Also, the switching hubs  41  and  42  may be replaced by other layer-2 (L2) switches.  
         [0041]     The switching hub  43  accommodates a plurality of recipients  7  ( 71 ,  72 , and  73 : strictly, user terminals such as personal computers and the users) that are capable of dynamically joining or leaving (not joining) the multicast group. The switching hub  43  can be replaced by other L2 switch.  
         [0042]     In the IP multicast communication system  1 , the authentication server  6  manages authorized recipients  7  by utilizing information based on the Internet Group Management Protocol IGMP. For this purpose, the authentication server  6  has a user management information table  61  storing user management information that is authentication information about the multicast data recipients  7 .  
         [0043]     All multicast routers  31 ,  32 , and  33  in the IP network  2 , or strictly all multicast routers related to the edge of the IP network  2 , and the receiving-side switching hub  43  accommodating the recipients  7  have their respective user management information tables  34  and  44  for storing user management information.  
         [0044]     The receiving-side multicast router  33  for the recipients  7  checks for unauthorized access users (recipients) on the basis of the user management information in the user management information table  34 .  
         [0045]     In the IP multicast communication system  1  which adopts the first configuration shown in  FIG. 2 , the receiving-side switching hub  43  refers to the user management information table  44  and ceases distribution of multicast data (including moving picture data and audio data) to recipients  7  that desire data reception but are not registered to join the multicast group. This prevents unauthorized recipients  7  from receiving multicast data.  
         [0046]     In an IP multicast communication system  1  which adopts the second configuration shown in  FIG. 3 , the receiving-side switching hub  43  refers to the user management information table  44  and thins out multicast data, e.g. moving picture data, and sends the thinned out data to recipients  7  that desire data reception but are not registered to join the multicast group. The thinning out of data can disturb the reception of multicast data by unauthorized recipients  7 .  
         [0047]     The layer configurations of the multicast routers  31 ,  32 , and  33 , the switching hub  43 , and the authentication server  6  will be described in detail later.  
         [0048]     [Functions of Authentication Server] 
         [0049]      FIGS. 4 and 5  are flowcharts of processes performed by the authentication server  6  shown in  FIGS. 2 and 3 . Referring to FIGS.  2  to  5  together, the authentication server  6 , managing the recipients  7  that desire to receive multicast data, has the following functions:  
         [0050]     (1) The user registration management unit  62  in the authentication server  6  checks, on the basis of an IGMP message, to see whether data is for user registration, and performs the following process steps when the data is for user registration (S 401  in  FIG. 4 ).  
         [0051]     (2) The user registration management unit  62  refers to the user management information table  61  to check attributes of the recipient  7  that desires to receive multicast data (the attributes include an IP multicast group address, IP address, MAC address, membership level, illegality flag, and so forth), and when the user registration management unit  62  permits reception of multicast data, it registers the recipient in the user management information table  61  and updates the user management information table  61  (S 402  and S 403 ).  
         [0052]     (3) After updating the user management information table  61 , the user registration management unit  62  activates a user management information distributing process (S 404 ).  
         [0053]     (4) Activated by the user registration management unit  62 , a user management information distribution processing unit  63  cooperates with the user registration management unit  62  to distribute user management information corresponding to the contents of the user management information table  61 , to all multicast routers  33  in the receiving-side subnet, through the switching hub  42  (S 501  in  FIG. 5 ). Just a single multicast router  33  is shown herein.  
         [0054]     Also, in cooperation with the user registration management unit  62 , the user management information distribution processing unit  63  distributes, through the switching hub  42 , user management information which is part of the contents of the user management information table  61  (information required for routing) to the multicast routers  31  and  32  related to the edge of the IP network  2 .  
         [0055]     (5) When the user registration management unit  62  judges, in step S 401 , that the data is not for user registration, it then updates the user management information table  61  on the basis of a multicast group join message (IGMP Join message) or leave message (IGMP Leave message) (S 405 ).  
         [0056]     (6) When the user registration management unit  62  does not permit multicast data reception in step S 402 , it reports “not permitted” to the recipient  7  desiring reception of multicast data (S 406 ).  
         [0057]     [Functions of Multicast Router] 
         [0058]      FIGS. 6, 7 , and  8  are flowcharts of processes performed by the multicast router  33  of  FIGS. 2 and 3 . Referring to  FIGS. 2, 3 , and  6  to  8  together, the functions of the multicast router  33  are described.  
         [0059]     (1) When the multicast router  33  receives the entire user management information corresponding to the contents of the user management information table  61  that is distributed from the authentication server  6 , the user management unit  35  of the multicast router  33  extracts (specifies) only the management information about the users belonging to its subnet and updates the user management information table  34  on the basis of the specified user management information (S 601  and S 602  in  FIG. 6 ).  
         [0060]     In extracting the user management information about its own subnet, the user management unit  35  utilizes information such as the IP multicast group address (multicast address), the IP addresses of the recipients  71 ,  72 , and  73 , or the source (recipient) subnet address.  
         [0061]     On the other hand, the user management units  35  of the multicast routers  31  and  32  related to the edge of the IP network  2  receive, from the authentication server  6 , the user management information (information required for routing) that corresponds to part of the contents of the user management information table  61  and update their respective user management information tables  34  on the basis of the user management information.  
         [0062]     (2) The user management unit  35  of the multicast router  33  sends to the subordinate switching hub  43  user management information that the switching hub  43  should store (hold) in its user management information table  44  (S 603 ).  
         [0063]     The user management information corresponding to the contents of the user management information table  61  of the authentication server  6  is distributed only at the time of initial introduction, and the user management information is updated thereafter utilizing IGMP Join S messages and IGMP Leave S messages and the switching hub  43  does not search the layer-3 (network layer) information at the port level, which avoids loads on the IP network  2 .  
         [0064]     As for the IGMP Join S message, when the multicast router  33  receives an IGMP Join (Group) message sent from a recipient  7  joining the multicast group, the multicast router  33  uses the IGMP Join S message to report to the switching hub  43  that an IGMP Join message was sent.  
         [0065]     As for the IGMP Leave S message, when the multicast router  33  receives an IGMP Leave (Group) message sent from a recipient  7  leaving the multicast group, the multicast router  33  uses the IGMP Leave S message to report to the switching hub  43  that an IGMP Leave message was sent.  
         [0066]     (3) When the data receiving unit  36  of the multicast router  33  receives an IGMP Join message from the subordinate switching hub  43 , the user management unit  35  checks the subnet IP address of the message source (recipient) (which may be referred to simply as a source address) with the contents of the user management information table  34  to check the recipient  7  for authentication. Then, when the IP address is present in the user management information table  34 , the user management unit  35  directly ends the process, and when the IP address is absent, the user management unit  35  changes the direction and sends an IGMP Join S message to the switching hub  43  (S 604 , S 605 , and S 606 ).  
         [0067]     (4) When the multicast router  33  receives multicast data and at least one recipient  7  in the subnet is a member of the multicast group, then the user management unit  35  sends the data to the switching hub  43  to relay the multicast data, destined to that group, into the entire area of the subnet (S 701  and S 702  in  FIG. 7 ).  
         [0068]     (5) The user management unit  35  issues IGMP HMQ (IGMP Host Membership Query) messages to regularly inquire of the recipients  7  whether they continue membership in the multicast group (S 801  in  FIG. 8 ).  
         [0069]     (6) When the multicast router  33  receives an IGMP HMR (IGMP Host Membership Report) message within a predetermined time period, then the user management unit  35  checks the source address of the message with the contents of the user management information table  34 . When the source address is present in the user management information table  34 , the user management unit  35  directly goes to the next step, and when the source address is absent, the user management unit  35  changes the direction and sends an IGMP Join S message to the subordinate switching hub  43  (S 802 , S 803 , and S 804 ).  
         [0070]     The IGMP HMR message is a message that a recipient  7  sends to the multicast router  33  in response to the IGMP HMQ message to report the multicast address at which the recipient  7  desires to receive data.  
         [0071]     (7) When the data receiving unit  36  of the multicast router  33  receives an IGMP Leave message from the subordinate switching hub  43 , the user management unit  35  checks the source address of the message with the contents of the user management information table  34 . When the user management information table  34  defines the membership in the multicast group, the user management unit  35  deletes the membership in the multicast group and updates the user management information table  34  (S 607 , S 608 , and S 609 ).  
         [0072]     (8) When the user management information table  34  does not define the membership in the multicast group, the user management unit  35  changes the direction and sends an IGMP Leave S message to the switching hub  43  after updating the user management information table  34  (S 610 ).  
         [0073]     (9) When a plurality of multicast routers  33  are present in the receiving-side subnet, the multicast routers  33  make a selection among themselves so that the router having the largest IP address functions as a designated router. The designated router issues IGMP HMQ messages and sends to the authentication server  6  multicast group join message or leave message from the recipients  7  (S 805  and S 806 ).  
         [0074]     [Functions of Switching Hub] 
         [0075]      FIGS. 9, 10 , and  11  are flowcharts of processes performed by the switching hub  43  shown in  FIGS. 2 and 3 . Referring to  FIGS. 2, 3 , and  9  to  11  together, the functions of the switching hub  43  are described.  
         [0076]     (1) When the data receiving unit  46  of the switching hub  43  receives user management information distributed from the multicast router  33 , the user management unit  45  registers the user management information in the user management information table  44 .  
         [0077]     (2) With an IGMP Join S message received from the multicast router  33 , the user management unit  45  checks the source address with the user management information in the user management information table  44 . When the source address is absent in the user management information table  44 , the user management unit  45  regards the recipient  7  as being unauthorized and sets (to  1 ) an unauthorized recipient identify flag (an illegality flag or an unauthorized recipient flag) and updates the user management information table  44  (S 901 , S 902 , and S 903  in  FIG. 9 ).  
         [0078]     (3) The user management unit  45 , referring to the user management information table  44 , distributes intact multicast data to recipients  7  with the illegality flags being off and ceases distribution of multicast data (MPEG data) to recipients  7  with the illegality flags being on (S 1001 , S 1002 , and S 1003  in  FIG. 10 , and refer to the configuration of  FIG. 2 ). The user management unit  45  does not distribute data to recipients  7  that did not submit a multicast data reception request, i.e. to recipients  7  that did not join the multicast group in advance.  
         [0079]     (4) When it is permissible to allow recipients  7  with illegality flags being on to know the outlines of data, the user management unit  45  may delete data portions of frames, i.e. thin out moving picture data, for example, and send the thinned out data. Unauthorized recipients  7  then receive data destructed by the data thinning-out process, i.e. data deteriorated in quality (S 1101  to S 1104  in  FIG. 11 , also see the configuration of  FIG. 3 ).  
         [0080]     (5) When the data receiving unit  46  receives an IGMP HMQ message from the multicast router  33 , the user management unit  45  relays the message to all ports, i.e. to all recipients  7  ( 71 ,  72 , and  73 ) (S 904  and S 905 ).  
         [0081]     (6) When the data receiving unit  46  of the switching hub  43  receives an IGMP HMR message sent from a recipient  7 , the user management unit  45  relays the IGMP HMR message to the multicast router  33  (S 906  and S 907 ).  
         [0082]     (7) When the user management unit  45  refers to an IGMP Leave S message and judges that the source address corresponding to the recipient  7  is defined in the user management information table  44  as a member of the multicast group, the user management unit  45  deletes the membership in the group (S 908 , S 909 , and S 910 ).  
         [0083]     (8) When the user management unit  45  judges it is not defined, the user management unit  45  refers to the port information in the user management information table  44  to see whether the illegality flag is on or off. When the flag is on, the user management unit  45  unsets the flag (sets the flag too) and updates the user management information table  44  (S 908 , S 909 , S 911 , and S 912 ).  
         [0084]     (9) When the user management unit  45  judges that, in step S 908 , the message is not an IGMP Leave S message and that the source address corresponds to the authentication server  6 , then the user management unit  45  extracts the user management information corresponding to its subnet and updates the user management information table  44  (S 913  and S 914 ).  
         [0085]     [Functions of Recipients (Who Desire to Receive Multicast Data)] 
         [0086]      FIGS. 12 and 13  are flowcharts of processes performed by recipients  7  (that desire to receive multicast data) shown in  FIGS. 2 and 3 . Referring to  FIGS. 2, 3 ,  12 , and  13  together, the functions of the recipients  7 , as applicants for reception of multicast data, are described.  
         [0087]     (1) A recipient  7  that desires to receive multicast data (video including moving picture data and audio data) reports, by unicast, data (video) the recipient  7  desires to receive, the multicast membership attribute (membership level) of the recipient  7 , etc., so as to register itself in the authentication server  6  (S 1201  in  FIG. 12 ).  
         [0088]     (2) The recipient  7  issues an IGMP Join message to join the multicast group. The issued IGMP Join message is sent through the switching hub  43  to all multicast routers  33  in the receiving-side subnet (S 1301  and S 1302  in  FIG. 13 ).  
         [0089]     (3) When an applicant  7  for reception that desires to continue the membership in the multicast group receives an IGMP HMQ message, the applicant  7  issues an IGMP HMR message. The issued IGMP HMR message is sent to all multicast routers  33  through the switching hub  43  (S 1301  and S 1303 ).  
         [0090]     (4) An unauthorized recipient  7  cannot normally receive data unless it issues an IGMP Leave message. That is to say, an unauthorized recipient  7  can leave the multicast group by issuing an IGMP Leave message to all multicast routers  33 . After leaving the group, the unauthorized recipient  7  does not receive quality-deteriorated data.  
         [0091]     [First Operation Example of IP Multicast Communication System] 
         [0092]     Next, referring to  FIG. 2  and relevant flowcharts, a first example of operation of the IP multicast communication system is described.  
         [0093]     In the IP network system  1 , the authentication server  6  manages (registers, deletes, and updates) the sender  5  that sends multicast data and the recipients  7  that are authorized to receive the data. The authentication server  6  utilizes the user management information table  61  in managing the recipients  7  authorized to receive multicast data.  
         [0094]     A recipient  7 , as an applicant for reception of multicast data, applies to the authentication server  6  by unicasting information indicating data it desires to receive, multicast group membership level, etc. The multicast group membership levels include: Level 0—no sending and no receiving; Level 1—sending but no receiving; and Level 2—sending and receiving.  
         [0095]     The user registration management unit  62  of the authentication server  6  examines the application from the recipient  7  referring to the user management information previously registered in the user management information table  61 . After the examination, when permitting reception, the user registration management unit  62  registers the user management information in the user management information table  61  and updates the user management information table  61 .  
         [0096]     As shown in  FIG. 2 , the user management information table  61  stores user management information for each recipient  7 , including user ID, IP multicast group address (multicast address) IP address, MAC address, multicast group membership level, source (recipient) subnet address, TTL (Time to Live: a time after which the entry can be deleted from the table), Out router (the preceding hop router) address, In port, Out ports, state of availability of ports of the switching hub  43 , illegality flag, and so on.  
         [0097]     The user management information distribution processing unit  63  of the authentication server  6  distributes user management information contained in the user management information table  61  to the multicast routers  31 ,  32 , and  33 .  
         [0098]     The user management units  35  of all multicast routers  33  in the receiving-side subnet (a single multicast router  33  is shown herein) extract only the information about their own subnet on the basis of particular information contained in the user management information distributed from the authentication server  6  (e.g. multicast address), register the information in the corresponding user management information tables  34 , and send user management information to the subordinate switching hubs  43 .  
         [0099]     The user management unit  45  of the switching hub  43  extracts user management information about users belonging to its own subnet on the basis of MAC address contained in the user management information received from the multicast router  33 , and registers the information in the user management information table  44  in the switching hub  43 .  
         [0100]     An authorized recipient  7  declares, in order to receive multicast data, to all multicast routers  33  present in the receiving-side subnet, that the recipient  7  desires multicast group data. For this purpose, the authorized recipient  7  sends an IGMP HMR message for requesting multicast group membership.  
         [0101]     The multicast router  33  in the receiving-side subnet receives the IGMP HMR message and then the user management unit  35  checks the source address of the message with the contents of the user management information table  34 . When the source address is present in the user management information table  34 , the user management unit  35  directly goes to the next step, and when the source address is absent, it changes the direction and sends an IGMP Join S message to the switching hub  43 .  
         [0102]     The switching hub  43  receives the IGMP Join S message and the user management unit  45  checks the source address with the contents of the user management information table  44 . When the source address is absent in the user management information table  44 , the user management unit  45  regards the recipient  7  as being unauthorized, sets the illegality flag on, and updates the user management information table  44 .  
         [0103]     When the data receiving unit  36  of the multicast router  33  receives multicast data and the subnet includes at least one recipient  7  joining the multicast group, then the user management unit  35  sends the data to the switching hub  43  to relay the multicast data destined to that group into the entire area of the subnet.  
         [0104]     The user management unit  45  of the switching hub  43  refers to the user management information table  44 , and distributes the data to recipients  7  with the illegality flag being off and ceases data transfer to recipients  7  with illegality flag being on.  
         [0105]     [Second Operation Example of IP Multicast Communication System] 
         [0106]     Next, referring to  FIG. 3  and relevant flowcharts, a second example of operation of the IP multicast communication system is described.  
         [0107]     In the IP network system  1 , the authentication server  6  manages (registers, deletes, and updates) the sender  5  that sends multicast data and the recipients  7  that are authorized to receive the data. The authentication server  6  utilizes the user management information table  61  in managing the recipients  7  authorized to receive multicast data.  
         [0108]     A recipient  7 , as an applicant for reception of multicast data, applies to the authentication server  6  by unicasting information indicating data it desires to receive, multicast group membership level, etc.  
         [0109]     The user registration management unit  62  of the authentication server  6  examines the application from the recipient  7  referring to the user management information previously registered in the user management information table  61 . After the examination, when permitting reception, the user registration management unit  62  registers the user management information in the user management information table  61  and updates the user management information table  61 .  
         [0110]     The user management information distribution processing unit  63  of the authentication server  6  distributes user management information contained in the user management information table  61  to the multicast routers  31 ,  32 , and  33 .  
         [0111]     The user management units  35  of all multicast routers  33  in the receiving-side subnet (a single multicast router  33  is shown herein) extract only the information about their own subnet on the basis of particular information contained in the user management information distributed from the authentication server  6  (e.g. multicast address), register the information in the corresponding user management information tables  34 , and send user management information to the subordinate switching hubs  43 .  
         [0112]     The user management unit  45  of the switching hub  43  extracts user management information about users belonging to its own subnet on the basis of MAC address contained in the user management information received from the multicast router  33 , and registers the information in the user management information table  44  in the switching hub  43 .  
         [0113]     An authorized recipient  7  declares, in order to receive multicast data, to all multicast routers  33  present in the receiving-side subnet, that the recipient  7  desires multicast group data. For this purpose, the authorized recipient  7  sends an IGMP HMR message for requesting multicast group membership.  
         [0114]     The multicast router  33  in the receiving-side subnet receives the IGMP HMR message and then the user management unit  35  checks the source address of the message with the contents of the user management information table  34 . When the source address is present in the user management information table  34 , the user management unit  35  directly goes to the next step, and when the source address is absent, it sends an IGMP Join S message to the switching hub  43 .  
         [0115]     The switching hub  43  receives the IGMP Join S message and the user management unit  45  checks the source address with the contents of the user management information table  44 . When the source address is absent in the user management information table  44 , the user management unit  45  regards the recipient  7  as being unauthorized, sets the illegality flag on, and updates the user management information table  44 .  
         [0116]     When the data receiving unit  36  of the multicast router  34  receives multicast data and the subnet includes at least one recipient  7  joining the multicast group, then the user management unit  35  sends the data to the switching hub  43  to relay the multicast data destined to that group into the entire area of the subnet.  
         [0117]     The user management unit  45  of the switching hub  43  refers to the user management information table  44 , and distributes the data to recipients  7  with the illegality flag being off. With recipients  7  with the illegality flag being on, the user management unit  45  refers, through the data receiving unit  46 , to data thinning-out information  47  that defines, e.g. sending only two data frames out of every four frames, and sends the thinned out data.  
         [0118]     Destructing about 5% of entire multicast data deteriorates quality. An unauthorized recipient  7  thus receives quality-deteriorated data destructed by the thinning-out. The unauthorized recipient  7  continuously receives destructed data until it issues an IGMP Leave message. The unauthorized recipient  7  can reject the reception of quality-deteriorated data by issuing an IGMP Leave message to all multicast routers  33  to leave the multicast group.  
         [0119]     The multicast router  33  receives the IGMP Leave message and then checks the source address of the message with the contents of the user management information table  34 . When the user management information table  34  defines the membership in the multicast group, the multicast router  33  deletes the membership and updates the user management information.  
         [0120]     After updating the user management information in the user management information table  34 , the multicast router  33  changes the direction and sends an IGMP Leave S message to the switching hub  43 .  
         [0121]     The switching hub  43  refers to the IGMP Leave S message, and when the user management information table  44  defines the membership in the multicast group, the switching hub  43  deletes the membership, and then refers to the port information of the switching hub  43  registered in the user management information table  44 , and when the illegality flag is on, the switching hub  43  unsets the flag and updates the user management information.  
         [0122]     Among multicast routers  33 , the router having the largest IP address sends to the authentication server  6  multicast group join messages and leave messages from recipients  7 . The authentication server  6  updates the user management information on the basis of the messages. An unauthorized recipient  7  does not receive quality-deteriorated data after leaving the group.