Abstract:
A Layer 2 switch manages distribution destinations of a multicast packet and distributes a copy of the multicast packet, so that the bandwidth wasted in a network is reduced. The Layer 2 switch is disposed between a user terminal and a packet transfer apparatus, which are connected by P2P, has a function to manage connected multicast users, receives a multicast packet on behalf of the users, and distributes a copy, to reduce the bandwidth wasted between the Layer 2 switch and the packet transfer apparatus.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to Layer 2 switches, and more specifically, to a Layer 2 switch for copying a multicast packet, in multicast packet transfer used for content information distribution such as broadcasting.  
         [0003]     2. Description of the Related Art  
         [0004]     In the initial stage of the spread of the Internet among the general public, Internet access was obtained in most cases by a direct dial-up connection to an access point of an Internet service provider (ISP) and authentication. A commonly used protocol is Point-to-Point Protocol (PPP), which supports an authentication function and a compression function. PPP is a Layer-2-class protocol for making a one-to-one connection between a user terminal and an ISP access point. In this age of broadband, a connection by an access carrier network using Internet Protocol (IP) has become the mainstream of connection between a user terminal and an ISP, instead of a connection by a telephone network. Layer-1 protocols such as Ethernet (registered trademark) and Asynchronous Transfer Mode (ATM) are used in the access carrier network. For these n-to-n protocols, PPP, which was originally developed for one-to-one authentication, cannot be used directly. PPP on Ethernet (registered trademark (PPPoE) and PPP on ATM (PPPoA) have been developed for those applications.  
         [0005]     As broadband networks have advanced, multicast technologies for broadcasting and other content distribution have also been receiving attention. The conventional multicast technologies utilizing IP include an IP multicast method and a PPP multicast method. Efficient information distribution can be conducted by using an IP-multicast router or an IP-multicast access server for implementing a broadcast service on an IP network built for communication. As technologies for multicast communication for distributing data to a plurality of specific destinations, the Internet Engineering Task Force (IETF) has standardized TCP/IP details in Internet Group Membership Protocol (IGMP: RFC 1112, chapter 4, chapter 7, appendix 1 (non-patent document 1), RFC 2236, chapter 2, chapter 3, chapter 6, chapter 7 (non-patent document 2)) for IPv4, and Multicast Listener Discovery (MLD: RFC 2710, chapter 3, chapter 4, chapter 5, chapter 6 (non-patent document 3) for IPv6.  
         [0006]     IGMP and MLD are used between a user terminal and a packet transfer apparatus (such as a gateway and a router). The protocols control a group of user terminals (a multicast group) made for receiving multicast distribution, which distributes identical data to a plurality of user terminals efficiently. IGMP and MLD are used when a user terminal makes a request to join a multicast group (multicast data distribution request) or when a user terminal makes a request to leave a multicast group (request to stop multicast data distribution). When a content distribution server distributes information in packets, by a request sent from a user terminal included in a multicast group, a content distribution service is performed as follows: A low-level packet transfer apparatus (such as a gateway or a router) receives the packet, makes as many copies of the packet as the number of user terminals included in the requesting multicast group, and transfers the copies to the user terminals. Copying a multicast packet by an apparatus closest to the user terminal is ideal for minimizing a network load in multicasting. Japanese Unexamined Patent Application Publication No. 2003-69639 (patent document 1) describes an x Digital Subscriber Line (xDSL) apparatus including a cache server and a method of storing received content data in the cache server of the xDSL apparatus on behalf of the clients and distributing data copied by the xDSL apparatus to each client. This method requires a large-capacity cache that can hold a moving image and other data. Japanese Unexamined Patent Application Publication Nos. 2003-32287 (patent document 2), 2003-348148 (patent document 3), and 2003-152796 (patent document 4), and others are also known to the public.  
         [0007]     In a general access carrier network, a user terminal is generally connected to a broadband access server (BAS) for the purpose of authentication and further connected through a router of an ISP network to a content distribution server. In some other cases, a user terminal is connected to a Layer 2 Tunneling Protocol (L2TP) access concentrator (LAC) and further connected through an L2TP network server (LNS) to an ISP network. The BAS or LNS (hereafter referred to as a packet transfer apparatus) keeps track of user terminals connected to it by IGMP or MLD and performs multicast data distribution by making as many copies of each multicast packet received from the content distribution server as the number of user terminals and sending the copies at the same time.  
         [0008]     If the packet transfer apparatus used in the network has a small number of ports, the user terminals may be connected by a Layer 2 switch, and the user terminals and the packet transfer apparatus may be connected by PPPoE, VLAN, and others. IGMP or MLD is used to keep track of the user terminals in this network as well, but IGMP or MLD gives layer information of IP or a higher level. A Layer 2 switch does not support packet processing of IP or a higher level and cannot keep track of the user terminals connected to it. Accordingly, a packet transfer apparatus supporting Layer 3 or above must be used to manage the user terminals and distributes packets to the user terminals connected to it. However, since a point-to-point (P2P) connection is used between the user terminal and the packet transfer apparatus, unicast communication performed for distribution to the user terminals would use up an n-fold bandwidth. So, a sufficient bandwidth must be reserved between the Layer 2 switch and BAS, resulting in a high cost.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, it is an object of the present invention to minimize a bandwidth wasted between the Layer 2 switch and the packet transfer apparatus by managing the user terminals and implementing multicast copy distribution by means of the Layer 2 switch.  
         [0010]     Another object of the present invention is to implement a multicast packet distribution function including multicast packet distribution destination management by the Layer 2 switch, by taking out an IGMP packet or an MLD packet sent from the user terminal selectively from a PPP connection. A further object of the present invention is to reduce the internal load of the access carrier network and suppress the equipment cost, by making a dynamic or static multicast connection between the Layer 2 switch and the packet transfer apparatus and letting the Layer 2 switch receive a multicast packet as a representative of the plurality of multicast user terminals connected to it.  
         [0011]     A Layer 2 switch of the present invention has a snoop function to check the contents of a multicast packet and manages the user information of a multicast group. The Layer 2 switch also includes a termination means of IGMP packets or MLD packets sent from the user, so that the packet transfer apparatus will not make as many copies as the number of users of an IGMP packet or an MLD packet sent to the packet transfer apparatus, such as an IGMP join packet and an MLD join packet issued by the user to join a multicast group when multicast packet distribution is performed. A multicast connection is made, and the Layer 2 switch receives a multicast packet from the packet transfer apparatus through the connection. Multicast packet transfer is carried out through the multicast connection. According to the present invention, the Layer 2 switch including the means described above can solve the problems as described earlier. The apparatus of the present invention includes a plurality of channel interfaces, a channel interface control block, and a processor for performing packet analysis and editing, so that those means can be implemented. Memory holds a user management table for storing the information of a connected user terminal and a multicast connection management table for managing a multicast connection made to a packet transfer apparatus. An IGMP packet snoop function and an MLD packet snoop function to check a packet coming from the user, an IGMP packet termination function and an MLD packet termination function to terminate a packet coming from the user, a function to make a multicast connection to a packet transfer apparatus, a multicast packet copy function, and a header edit function are provided.  
         [0012]     The conventional Layer 2 switch cannot perform multicast packet distribution together with multicast packet distribution destination management. A Layer 2 switch of the present invention can implement multicast packet distribution involving multicast packet distribution destination management by taking out an IGMP packet or an MLD packet coming from the user selectively from a PPP connection. A dynamic or static multicast connection is made between the Layer 2 switch and a packet transfer apparatus, and the Layer 2 switch receives a multicast packet as a representative of the plurality of multicast user terminals connected to it, so that the internal load of the access carrier network can be minimized and the facility cost can be reduced.  
         [0013]     According to the solving means of the invention, there is provided a Layer 2 switch disposed between a user terminal and a packet transfer apparatus, for passing a packet, the Layer 2 switch comprising: 
        a user management table for storing an address of a multicast group which the user terminal is going to join, a channel interface number connected to the user terminal, and a MAC address of the user terminal;     a multicast connection management table for storing a channel interface number of a multicast connection to be made to the packet transfer apparatus and a MAC address of the packet transfer apparatus, corresponding to an address of a multicast group which the user terminal is going to join;     a channel interface connected to a network; and     a processor which has access to each of the tables and transfers a multicast packet between the user terminal and the packet transfer apparatus through the channel interface;     the processor comprising: 
            a means for receiving a packet of a multicast reception request message for joining a multicast group having a multicast group address, from a user terminal desiring to receive a multicast packet through the channel interface;     a means for terminating the received packet without transferring the packet to the packet transfer apparatus of high-level if it is judged from the header or the data portion that the packet from a user terminal contains a multicast reception request message;     a means for determining whether a multicast connection has already been made, in accordance with a multicast group address obtained from the received multicast reception request message, with reference to the multicast connection management table;     a means for adding a multicast group address and a channel interface number and a MAC address of a user terminal from which the multicast reception request message is sent, if the multicast reception connection has already been made;     a means for receiving a multicast packet from the packet transfer apparatus and determining whether the multicast packet is received through the multicast connection, with reference to the multicast connection management table, in accordance with a multicast group address and a transmission source MAC address included in the received packet;     a means for referencing the user management table in accordance with a group address included in the received packet, if the received packet is determined as being a multicast packet; and     a means for copying the multicast packet for each contained user terminal while obtaining the information of a channel interface to each user terminal from the user management table, and distributing the multicast packet to each user terminal, if the user management table contains the information of a user terminal joining a group having the same group address as included in the received packet.   
               
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  shows an example configuration of a network of an embodiment.  
         [0027]      FIG. 2  shows the flow of a multicast packet in the embodiment.  
         [0028]      FIG. 3  shows an internal configuration of a Layer 2 switch of the present invention.  
         [0029]      FIG. 4  shows a configuration of a user management table.  
         [0030]      FIG. 5  shows a configuration of the user management table.  
         [0031]      FIG. 6  shows a configuration of a multicast connection management table.  
         [0032]      FIG. 7  shows the configuration of a packet transferred between a user terminal and a packet transfer apparatus.  
         [0033]      FIG. 8  shows an example of a multicast packet which the Layer 2 switch receives from the packet transfer apparatus.  
         [0034]      FIG. 9  shows an example of a multicast packet copied by the Layer 2 switch and transferred to a user terminal.  
         [0035]      FIG. 10  shows an example of a multicast packet copied by the Layer 2 switch and transferred to a user terminal.  
         [0036]      FIG. 11  shows a sequence of joining a multicast group.  
         [0037]      FIG. 12  shows a sequence of leaving the multicast group when the intention to leave is explicitly indicated.  
         [0038]      FIG. 13  shows a sequence of leaving a multicast group when the intention to leave is not explicitly indicated.  
         [0039]      FIG. 14  shows a determination procedure performed when the user management table is updated.  
         [0040]      FIG. 15  shows a procedure for updating the user management table.  
         [0041]      FIG. 16  shows a procedure for updating the multicast connection management table. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]     An embodiment of the present invention will be described below with reference to the drawings. The embodiment uses IPv4 IGMP. An embodiment using IPv6 MLD will not be described because the basic operation is similar to that described here. The present invention and embodiment can be applied to another multicast data distribution method or another protocol.  
         [0000]     1. Hardware Configuration  
         [0043]      FIG. 1  shows an example configuration of a network to which the present invention is applied.  
         [0044]     User terminals H 1 - i  and H 2 - i  are connected to Layer 2 switches  100  and  101  respectively and then connected through access carrier networks NW 1  and NW 2  respectively to a packet transfer apparatus  200 . The packet transfer apparatus is generally a BAS or LAC-LNS. The packet transfer apparatus is connected through a router  300  in an ISP network NW 3  to the Internet  400  and to a content distribution server S 1 . The connections between the user terminals H 1 - i  and H 2 - i  and the packet transfer apparatus  200  are made by PPPoE, PPPoA, VLAN, and others.  
         [0045]      FIG. 2  shows the flow of an IGMP packet and a multicast packet.  
         [0046]     The user terminals H 1 - 1  and H 1 - 2  have already subscribed to a content distribution agent and have a right to join a multicast group having group address “224.10.10.10”. The user terminal H 1 - 3  has not yet subscribed and does not have a right to join the multicast group having group address “224.10.10.10”. The user terminals H 1 - 1  and H 1 - 2 , having a right to join the multicast group having group address “224.10.10.10”, indicate their intention to join or leave the multicast group having group address “224.10.10.10”, by sending IGMP packets  600 - 1  and  600 - 2  respectively to the Layer 2 switch. The Layer 2 switch establishes a multicast connection  500  on the access carrier network NW 1  and sends an IGMP packet  600 - 3  to the packet transfer apparatus on behalf of the user terminals H 1 - 1  and H 1 - 2 . Their intention to join or leave the multicast group having group address “224.10.10.10” is transferred through the router  300  to the content distribution server S 1 .  
         [0047]     A multicast packet distributed from the content distribution server S 1  is transferred in the inverse route. The content distribution server S 1  sends a multicast packet  600 - 4  via the router  300  and the packet transfer apparatus  200  to the Layer 2 switch. The Layer 2 switch makes copies  600 - 5  and  600 - 6  and sends the copies to the user terminals H 1 - 1  and H 1 - 2 .  
         [0048]      FIG. 3  shows the internal configuration of the Layer 2 switch  100  of the embodiment.  
         [0049]     The Layer 2 switch includes a plurality of channel interfaces  100 - 1 - i  for input and output channels, a channel interface control block  100 - 2 , a processor  100 - 3 , a memory  100 - 4 , a control terminal interface  100 - 5 , and a transmission-and-reception buffer  100 - 7 . The channel interface control block  100 - 2  controls the channel interfaces  100 - 1 - i.  The processor  100 - 3  performs packet analysis, editing, and other processing by using the memory  100 - 4 . The control terminal interface  100 - 5  interfaces with an external control terminal  100 - 6 . The transmission-and-reception buffer  100 - 7  temporarily stores a packet sent or received through the channel interface  100 - 1 - i.  The memory  100 - 4  stores a program  100 - 4 - 3  to be executed by the processor, a user management table  100 - 4 - 1  for managing connected multicast users, and a multicast connection management table  100 - 4 - 2  for managing multicast connections made to the packet transfer apparatus  200 . The transmission-and-reception buffer  100 - 7  includes a transmission buffer  100 - 7 - 1  and a reception buffer  100 - 7 - 2 .  
         [0050]     The channel interface  100 - 1 -i restores an IP packet from a signal received from an IP network, transfers the IP packet to the channel interface control block  100 - 2 , and sends an output IP packet received from the channel interface control block to the IP network in a communication frame format matching the communication protocol of an input or output channel, such as Ethernet (registered trademark) and ATM. Each channel interface  100 - 1 - i  has a separate MAC address. In the shown embodiment, the channel interface # 1   100 - 1 - 1  has a MAC address of 00-00-87-11-11-11; the channel interface # 2   100 - 1 - 2  has a MAC address of 00-00-87-22-22-22; the channel interface # 3   100 - 1 - 3  has a MAC address of 00-00-87-33-33-33.  
         [0051]      FIG. 4  and  FIG. 5  show the user management table  100 - 4 - 1 .  
         [0052]     The user management table  100 - 4 - 1  manages the connection information of the user terminal H 1 - i  connected to the Layer 2 switch  100 , such as the multicast group to which the user terminal belongs and the channel type. The user management table  100 - 4 - 1  includes these fields: multicast group address  100 - 4 - 1 - 1 , channel interface number  100 - 4 - 1 - 2 , channel type  100 - 4 - 1 - 3 , session ID  100 - 4 - 1 - 4 , VPI  100 - 4 - 1 - 5 , VCI  100 - 4 - 1 - 6 , tag ID  100 - 4 - 1 - 7 , and terminal MAC address  100 - 4 - 1 - 8 . The session ID field  100 - 4 - 1 - 4  is used just when the channel type is PPPoE. Otherwise, the session ID field is set to “don&#39;t care”. The VPI field  100 - 4 - 1 - 5  and the VCI field  100 - 4 - 1 - 6  are used just when the channel type is PPPoA. Otherwise, the VIP and VCI fields are set to “don&#39;t care”. The tag ID field  100 - 4 - 1 - 7  is used just when the channel type is VLAN. Otherwise, the tag ID field is set to “don&#39;t care”. If the user management table  100 - 4 - 1  contains no data, the Layer 2 switch  100  has no multicast user terminals.  
         [0053]      FIG. 4  shows that one user terminal is connected by PPPoE.  FIG. 5  shows that one more user terminal joins the multicast group.  
         [0054]      FIG. 6  shows the multicast connection management table  100 - 4 - 2 .  
         [0055]     The multicast connection management table  100 - 4 - 2  manages which multicast group data is received through which connection from the packet transfer apparatus. The multicast connection management table includes these fields: multicast group address  100 - 4 - 2 - 1 , channel interface number  100 - 4 - 2 - 2 , channel type  100 - 4 - 2 - 3 , session ID  100 - 4 - 2 - 4 , VPI  100 - 4 - 2 - 5 , VCI  100 - 4 - 2 - 6 , tag ID  100 - 4 - 2 - 7 , and packet transfer apparatus MAC address  100 - 4 - 2 - 8 . The session ID field  100 - 4 - 2 - 4  is used just when the channel type is PPPoE. Otherwise, the session ID field is set to “don&#39;t care”. The VPI field  100 - 4 - 2 - 5  and the VCI field  100 - 4 - 2 - 6  are used just when the channel type is PPPoA. Otherwise, the VPI and VCI fields are set to “don&#39;t care”. The tag ID field  100 - 4 - 2 - 7  is used just when the channel type is VLAN. Otherwise, the tag ID field is set to “don&#39;t care”. If the multicast connection management table  100 - 4 - 2  contains no data, no multicast connection is made to the packet transfer apparatus  200 .  
         [0056]      FIG. 7  shows the configuration of a packet  600 - i  transferred between the user terminal H 1 - i  and the packet transfer apparatus  200 .  
         [0057]     The packet  600 - i  transferred between the user terminal H 1 - i  and the packet transfer apparatus  200  includes a MAC DA field  700 , a MAC SA field  701 , a PPPoE header information field  702 , a PPP header information field  703 , an IP SA field  704 , an IP DA field  705 , and a data field  706 . The MAC DA field  700  contains the physical address of the transmission destination. The MAC SA field  701  contains the physical address of the transmission source. The IP SA field  704  contains the IP address of the transmission source. The IP DA field  705  contains the IP address of the transmission destination.  
         [0058]      FIG. 8  shows an example of a multicast packet  600 - 4  transferred from the packet transfer apparatus to the Layer 2 switch.  FIG. 9  shows an example of a multicast packet  600 - 6  copied by the Layer 2 switch and transferred to the terminal H 1 - 1 .  FIG. 10  shows an example of a multicast packet  600 - 5  copied by the Layer 2 switch and transferred to the terminal H 1 - 2 . In  FIGS. 8, 9 , and  10 , the IP DA field  705  contains the multicast group address “224.10.10.10”.  
         [0000]     2. Operation  
         [0059]      FIG. 11  shows a sequence since when the user terminal H 1 - 1  and the user terminal H 1 - 2  indicate their intention to join the multicast group having group address “224.10.10.10” until when the user terminals receive a multicast packet.  
         [0060]      FIG. 12  shows a sequence since when the user terminal H 1 - 1  and the user terminal H 1 - 2  indicate their intention to leave the multicast group having group address “224.10.10.10” until when the multicast packet reception stops.  
         [0061]      FIG. 13  shows the operation when the user terminal H 1 - 1  and the user terminal H 1 - 2  leave the multicast group having group address “224.10.10.10”, without indicating their intention to leave, because of a terminal failure or others. The figure shows a sequence since when the Layer 2 switch  100  recognizes that the user terminal H 1 - 1  and the user terminal H 1 - 2  have left until when the multicast packet transfer stops.  
         [0062]      FIG. 14  shows a flow chart for adding information to or deleting information from the user management table  100 - 4 - 1 .  FIG. 15  shows a detailed flow chart for adding user information to the user management table  100 - 4 - 1 .  FIG. 16  shows a detailed flow chart for adding multicast connection information to the multicast connection management table  100 - 4 - 2 .  
         [0000]     (Joining a Multicast Group)  
         [0063]     The operation of the Layer 2 switch  100  performed when the user terminal H 1 - 1  and the user terminal H 1 - 2  join the content distribution server S 1  of the multicast group having group address “224.10.10.10” will be described first with reference to  FIG. 11 . Suppose that the user terminal H 1 - 1 , the user terminal H 1 - 2 , and the packet transfer apparatus  200  are connected respectively to channel interface # 1   100 - 1 - 1 , channel interface # 2   100 - 1 - 2 , and channel interface # 3   100 - 1 - 3 , of the Layer 2 switch  100 . The user terminal H 1 - 1  wants to join the multicast group having group address “224.10.10.10” and sends an IGMP join packet  600 - 1  to indicate the intention to join (SQ 1 - 1 ). The IGMP join packet  600 - 1  reaches channel interface # 1  of the Layer 2 switch  100  and is transferred to the channel interface control block  100 - 2 , which transfers the packet to the reception buffer  100 - 7 - 2  and generates an interrupt to notify the processor  100 - 3  that the packet has been received. The processor  100 - 3  receives the interrupt, reads the packet from the reception buffer  100 - 7 - 2 , and updates the user management table accordingly (SQ 1 - 2 ).  
         [0064]     The user management table is updated (SQ 1 - 2 ) as shown in the flow chart of  FIG. 14 . The processor  100 - 3  checks whether the packet read from the reception buffer  100 - 7 - 2  is a PPP packet ( 5001 ). If not, normal packet transfer is performed. If yes, the processor  100 - 3  further checks whether the packet is an IGMP packet ( 5002 ). If not, normal packet transfer is performed. If yes, the processor  100 - 3  further checks whether the packet is an IGMP join packet or an IGMP leave packet ( 5003 ). If the packet is an IGMP join packet, the user information is added to the user management table  100 - 4 - 1  ( 5004 ).  
         [0065]     The user information is added ( 5004 ) as shown in the flow chart of  FIG. 15 . The processor  100 - 3  checks the channel type from the received packet ( 5004 - 1 ), takes necessary information successively from the contents of the packet, and stores the information in the user management table  100 - 4 - 1 . If the channel type is PPPoE, the processor  100 - 3  takes the group address, the channel interface number, the channel type, the session ID, and the terminal MAC address from the packet and stores them in the group address field  100 - 4 - 1 - 1 , the channel interface number field  100 - 4 - 1 - 2 , the channel type field  100 - 4 - 1 - 3 , the session ID field  100 - 4 - 1 - 4 , and the terminal MAC address field  100 - 4 - 1 - 8  of the user management table  100 - 4 - 1  ( 5004 - 2 ). This state is shown in  FIG. 4 . The group address field  100 - 4 - 1 - 1  indicates that the user terminal H 1 - 1  joins the group having group address “224.10.10.10”. The channel interface number field  100 - 4 - 1 - 2  indicates that the user terminal H 1 - 1  is connected to channel interface # 1   100 - 1 - 1  of the Layer 2 switch  100 . The table also shows that the channel type is PPPoE, the session ID is  10 , and the MAC address of the user terminal H 1 - 1  is 00-00-87-00-00-01.  
         [0066]     Back to  FIG. 11 , after the user table is updated (SQ 1 - 2 ), the processor  100 - 3  searches through the multicast connection management table  100 - 4 - 2 , using group address “224.10.10.10” of the join request as a search key, and checks whether a multicast connection has already been established to the group address (SQ 1 - 2 - 1 ). The processor  100 - 3  can check whether the multicast connection has been established by searching for an entry corresponding to the group address through the multicast connection management table  100 - 4 - 2 , for instance. If the corresponding multicast connection has not yet been established, the Layer 2 switch  100  establishes a multicast connection  500  to the packet transfer apparatus  200  (SQ 1 - 3 ). If the corresponding multicast connection has already been established, no new connection is established, that is, the processing of multicast connection establishment (SQ 1 - 3 ) through a PIM join process (SQ 1 - 7 ) in  FIG. 11  will be skipped. When the multicast connection is established, the Layer 2 switch  100  updates the multicast management table (SQ 1 - 4 ) by adding the information of the established connection to the multicast connection management table  100 - 4 - 2 .  
         [0067]     The multicast management table is updated as shown in the flow chart of  FIG. 16 . If the channel type of the established connection is PPPoE, the group address, the channel interface number, the channel type, the session ID, and the MAC address of the packet transfer apparatus are put in the group address field  100 - 4 - 2 - 1 , the channel interface number field  100 - 4 - 2 - 2 , the channel type field  100 - 4 - 2 - 3 , the session ID field  100 - 4 - 2 - 4 , and the packet transfer apparatus MAC address field ( 100 - 4 - 2 - 8 ) of the multicast connection management table  100 - 4 - 2 . This state is shown in  FIG. 6 .  
         [0068]     Back to  FIG. 11 , the Layer 2 switch  100  sends an IGMP join packet to the packet transfer apparatus  200  on behalf of the user terminal H 1 - 1  (SQ 1 - 6 ). The packet transfer apparatus  200  sends a PIM join packet through the router  300  to the content distribution server S 1  (SQ 1 - 7 ). Then, the content distribution server S 1  starts distributing a multicast packet through the router  300  to the packet transfer apparatus  200 . The packet transfer apparatus  200  transfers a multicast packet  600 - 4  through a multicast connection  500  to the Layer 2 switch  100  (SQ 1 - 8 ). The multicast packet reaches channel interface # 3   100 - 1 - 3  of the Layer 2 switch  100  and is transferred to the channel interface control block  100 - 2 . The channel interface control block  100 - 2  stores the received multicast packet in the reception buffer  100 - 7 - 2  and notifies the processor  100 - 3  that the packet has been received. The processor  100 - 3  searches through the multicast connection management table  100 - 4 - 2  by using as keys the group address, the session ID, and the transmission source MAC address stored respectively in the IP DA field  705 , and the PPPoE field  702 , and the MAC SA field ( 701 ) of the received packet. If a connection of exact matching is found, the user management table  100 - 4 - 1  is further searched through, by using the group address as a search key. The user management table  100 - 4 - 1  contains the data of a single user terminal in this state, as shown in  FIG. 4 . If the group address of the received multicast packet is “224.10.10.10”, the received multicast packet is transferred to a user terminal having terminal MAC address “00-00-87-00-00-01”, that is, the terminal H 1 - 1 . In the meantime, the processor  100 - 3  edits the header information of the received multicast packet.  
         [0069]     The multicast packet  600 - 4  received from the packet transfer apparatus  200  is configured as shown in  FIG. 8 . The MAC DA field  700  holds MAC address “00-00-87-33-33-33” assigned to channel interface # 3   100 - 1 - 3  of the Layer 2 switch to which the multicast connection is made; the MAC SA field  701  holds MAC address “00-00-87-00-00-10” assigned to the packet transfer apparatus  200  from which the multicast connection is made; the PPPoE header field  701  holds session ID “30” of the multicast connection  500 ; and the IP DA field  705  holds group IP address “224.10.10.10” of the transmission destination.  
         [0070]     To transfer the packet to the user terminal H 1 - 1 , the processor  100 - 3  obtains the transmission destination MAC address and the ID of the session between the user terminal H 1 - 1  and the Layer 2 switch  100 , from the terminal MAC address field  100 - 4 - 1 - 8  and the session ID field  100 - 4 - 1 - 4  of the user management table  100 - 4 - 1 , and replaces the corresponding items of the multicast packet received from the packet transfer apparatus with these items. MAC address “00-00-87-11-11-11” assigned to the channel interface  100 - 1 - 1  connected to the user terminal H 1 - 1  is held in the MAC SA field  701 . This state is shown in  FIG. 9 .  
         [0071]     The processor  100 - 3  stores the created multicast packet  600 - 6  in the transmission buffer  100 - 7 - 1  and activates the channel interface control block  100 - 2 . The channel interface control block  100 - 2  takes out the packet from the transmission buffer  100 - 7 - 1  and sends the multicast packet  600 - 6  through channel interface # 1   100 - 1 - 1  to the user terminal H 1 - 1  (SQ 1 - 10 ). Now, the user terminal H 1 - 1  receives a multicast packet from the content distribution server S 1 .  
         [0072]     Suppose that the user terminal H 1 - 2  indicates its intention to join the group having group address “224.10.10.10” by sending an IGMP join packet  600 - 2  to the Layer 2 switch  100  (SQ 1 - 11 ). The IGMP join packet  600 - 2  reaches channel interface # 2  of the Layer 2 switch  100  and is transferred to the channel interface control block  100 - 2 , which transfers the received packet to the reception buffer  100 - 7 - 2  and generates an interrupt to notify the processor  100 - 3  that the packet has been received. The processor  100 - 3  receives the interrupt, reads the packet from the reception buffer  100 - 7 - 2 , and updates the user management table (SQ 10 - 12 ).  
         [0073]     The user management table is updated (SQ 10 - 12 ) in the same way as described above, as shown in the flow chart of  FIG. 14 . The processor  100 - 3  checks whether the packet read from the reception buffer  100 - 7 - 2  is a PPP packet ( 5001 ). If not, normal packet transfer is performed. If yes, the processor  100 - 3  further checks whether the packet is an IGMP packet ( 5002 ). If not, normal packet transfer is performed. If yes, the processor  100 - 3  further checks whether the packet is an IGMP join packet or an IGMP leave packet ( 5003 ). If an IGMP join packet is received, user information is added to the user management table  100 - 4 - 1  ( 5004 ).  
         [0074]     The user information is added ( 5004 ) in the same way as described above, as shown in the flow chart of  FIG. 15 . The channel type is judged from the received packet ( 5004 - 1 ), and necessary information is successively taken from the contents of the packet and stored in the user management table  100 - 4 - 1 . If the channel type is PPPoE, the processor  100 - 3  takes the group address, the channel interface number, the channel type, the session ID, and the terminal MAC address from the packet, and puts the items in the group address field  100 - 4 - 1 - 1 , the channel interface number field  100 - 4 - 1 - 2 , the channel type field  100 - 4 - 1 - 3 , the tag ID field  100 - 4 - 1 - 7 , and the terminal MAC address field  100 - 4 - 1 - 8  of the user management table  100 - 4 - 1  ( 5004 - 2 ). This state is shown in  FIG. 5 .  
         [0075]     In the shown example, the user terminal H 1 - 2  has MAC address “00-00-87-00-00-02”; the channel type is PPPoE; the session ID is  20 ; a connection is made to channel interface # 2  of the Layer 2 switch  100 ; and a request to join the multicast group having group address “224.10.10.10” is made. After the user management table is updated (SQ 1 - 12 ), the multicast connection management table  100 - 4 - 2  is searched through by using the group address as a search key, and whether a multicast connection has already been established is checked (SQ 1 - 12 - 1 ).  
         [0076]     The user terminal H 1 - 1  has already joined the multicast group having group address “224.10.10.10”, and the Layer 2 switch  100  has already received a multicast packet of the multicast group from the content distribution server S 1 , as shown in  FIG. 5 . Accordingly, the Layer 2 switch  100  does not make a new multicast connection  500  and does not send a new IGMP join packet to the packet transfer apparatus  200 .  
         [0077]     Back to  FIG. 11 , suppose that the Layer 2 switch  100  now receives a multicast packet of group address “224.10.10.10” from the content distribution server S 1  (SQ 1 - 13 ). As described above, the multicast packet reaches channel interface # 3   100 - 1 - 3  of the Layer 2 switch  100  and is transferred to the channel interface control block  100 - 2 , which stores the multicast packet in the reception buffer  100 - 7 - 2  and generates an interrupt to notify the processor  100 - 3  that a multicast packet has been received. The processor  100 - 3  searches through the multicast connection management table  100 - 4 - 2  by using as keys the group address, the session ID, and the transmission source MAC address stored respectively in the IP DA field  705 , the PPPoE field  702 , and the MAC SA field  701  of the received packet. If a connection of exact matching is found, the user management table  100 - 4 - 1  is searched through, by using the group address as a key. Because two user terminals join the group having group address “224.10.10.10”, as shown in  FIG. 5 , the Layer 2 switch  100  copies the multicast packet (SQ 1 - 14 ) and transfers the copied multicast packet to the user terminal H 1 - 1  and the user terminal H 1 - 2  (SQ 1 - 15  and SQ 1 - 16 ). The packet is edited as when the packet is transferred to the user terminal H 1 - 1 , which is described earlier. An example of a packet transferred to the user terminal H 1 - 2  is shown in  FIG. 10 . When the edited packets are transferred to the user terminal H 1 - 1  and the user terminal H 1 - 2 , the user terminals can receive a multicast packet.  
         [0000]     (Case 1 of Leaving a Multicast Group)  
         [0078]     The operation of the Layer 2 switch performed when a user terminal leaves a multicast group will next be described. The sequence of leaving a multicast group depends on whether the user terminal makes a request of leaving explicitly or not. The user terminal is assumed not to make a request of leaving explicitly when the user terminal becomes out of order, for instance.  
         [0079]      FIG. 12  shows a sequence in which the user terminal H 1 - 1  and the user terminal H 1 - 2  make a request of leaving explicitly.  
         [0080]     The user terminal H 1 - 2  wants to leave the multicast group with group address “224.10.10.10” and indicates the intention to leave by sending an IGMP leave packet (SQ 2 - 1 ). The IGMP leave packet reaches channel interface # 2  of the Layer 2 switch  100  and is transferred to the channel interface control block  100 - 2 , which transfers the packet to the reception buffer  100 - 7 - 2  and generates an interrupt to notify the processor  100 - 3  that the packet has been received. The processor  100 - 3  receives the interrupt, reads the packet from the reception buffer  100 - 7 - 2 , and updates the user management table (SQ 2 - 2 ).  
         [0081]     The user management table is updated (SQ 2 - 2 ) as shown in the flowchart of  FIG. 14 . The processor  100 - 3  checks whether the packet read from the reception buffer  100 - 7 - 2  is a PPP packet ( 5001 ). If not, normal packet transfer is performed. If yes, the processor  100 - 3  further checks whether the packet is an IGMP packet ( 5002 ). If not, normal packet transfer is performed. If yes, the processor  100 - 3  further checks whether the packet is an IGMP join packet or an IGMP leave packet ( 5003 ). If the packet is an IGMP leave packet, the user information is deleted from the user management table  100 - 4 - 1  ( 5005 ). To delete the user information ( 5005 ), the processor  100 - 3  takes the channel type from the contents of the received IGMP leave packet. If the channel type is PPPoE, the session ID, the terminal MAC address, and the group address are also taken. The processor  100 - 3  searches through the user management table  100 - 4 - 1  by using the taken information as a key, and deletes the corresponding user terminal information. After the user terminal information is deleted from the user management table  100 - 4 - 1 , the user management table  100 - 4 - 1  is searched through by using the group address as a key, and it is checked whether any user still belongs to the corresponding group (SQ 2 - 2 - 1 ). If any user is found, the multicast connection corresponding to the group address is kept. The state after the user information of the user terminal H 1 - 2  is deleted is shown in  FIG. 4 . The figure shows that a user terminal still belongs to the group with group address “224.10.10.10”, so that the multicast connection for the group address is held as shown in  FIG. 6 .  
         [0082]     If the Layer 2 switch  100  now receives a multicast packet of group address “224.10.10.10” from the content distribution server S 1 , the Layer 2 switch  100  does not transfer the multicast packet to the user terminal H 1 - 2  and transfers the packet just to the user terminal H 1 - 1  contained in the user management table  100 - 4 - 1 .  
         [0083]     Back to  FIG. 12 , the user terminal H 1 - 1  then sends an IGMP leave packet to indicate the intention to leave (SQ 2 - 3 ). The IGMP leave packet reaches channel interface # 1  of the Layer 2 switch  100  and is transferred to the channel interface control block  100 - 2 , which transfers the packet to the reception buffer  100 - 7 - 2  and generates an interrupt to notify the processor  100 - 3  that the packet has been received. The processor  100 - 3  receives the interrupt, reads the packet from the reception buffer  100 - 7 - 2 , and updates the user management table (SQ 2 - 4 ).  
         [0084]     The user management table is updated (SQ 2 - 4 ) as shown in the flow chart of  FIG. 14 . The processor  100 - 3  checks whether the packet received from the reception buffer  100 - 7 - 2  is a PPP packet ( 5001 ). If not, normal packet transfer is performed. If yes, the processor  100 - 3  further checks whether the received packet is an IGMP packet ( 5002 ). If not, normal packet transfer is performed. If yes, the processor  100 - 3  further checks whether the packet is an IGMP join packet or an IGMP leave packet ( 5003 ). If the packet is an IGMP leave packet, the user information is deleted from the user management table  100 - 4 - 1  ( 5005 ). To delete the user information ( 5005 ), the processor  100 - 3  first takes the channel type from the contents of the IGMP leave packet. If the channel type is PPPoE, the session ID, the terminal MAC address, and the group address are also taken. The processor  100 - 3  searches through the user management table  100 - 4 - 1  by using the taken information as a key, and deletes the corresponding user terminal information. After the user terminal information is deleted from the user management table  100 - 4 - 1 , the user management table  100 - 4 - 1  is searched through by using the group address as a key, and it is checked whether any user still belongs to the group (SQ 2 - 4 - 1 ). Because both the user terminal H 1 - 1  and the user terminal H 1 - 2  have left the multicast group, the user management table  100 - 4 - 1  indicates that no user belongs to the group with group address “224.10.10.10”.  
         [0085]     Back to  FIG. 12 , the Layer 2 switch  100  sends an IGMP leave packet to the packet transfer apparatus  200  in order to stop multicast packet reception from the content distribution server S 1  (SQ 2 - 6 ). The packet transfer apparatus  200  receives the IGMP leave packet and sends a PIM leave packet through the router  300  to the content distribution server (SQ 2 - 7 ). The content distribution server S 1  receives the PIM leave packet and stops multicast packet distribution to the Layer 2 switch  100 .  
         [0086]     Now, the multicast packet distribution to the user terminal H 1 - 1  and the user terminal H 1 - 2  stops. Because the multicast connection to the group with group address “224.10.10.10” becomes unnecessary, the connection is closed (SQ 2 - 8 ). The corresponding connection information is deleted from the multicast connection management table  100 - 4 - 2 .  
         [0000]     (Case 2 of Leaving a Multicast Group)  
         [0087]      FIG. 13  shows a sequence of stopping distribution from the content distribution server S 1  when the user terminal H 1 - 1  and the user terminal H 1 - 2  leave the multicast group with group address “224.10.10.10”, without indicating their intention to leave explicitly, such as when a user terminal failure occurs.  
         [0088]     The Layer 2 switch  100  sends an IGMP query packet to the user terminals included in the user management table  100 - 4 - 1  successively in order to check whether they are connected (SQ 3 - 1 ). If the user terminal H 1 - 2  operates normally, the user terminal H 1 - 2  returns an IGMP response packet (SQ 3 - 2 ). If no response is returned, the Layer 2 switch  100  considers that the user terminal H 1 - 2  is not connected and deletes the user information of the user terminal H 1 - 2  from the user management table  100 - 4 - 1  (SQ 3 - 3 ). After the user terminal information is deleted from the user management table  100 - 4 - 1 , the user management table  100 - 4 - 1  is searched through by using the group address as a key and it is checked whether any user still belongs to the group (SQ 3 - 3 - 1 ). If any user still belongs to the group, the multicast connection corresponding to the group address is kept. The Layer 2 switch  100  sends an IGMP query packet to the user terminal H 1 - 1  as well (SQ 3 - 4 ). If an IGMP response packet is not returned, the Layer 2 switch  100  considers that the user terminal H 1 - 1  is not connected and deletes the user information of the user terminal H 1 - 1  from the user management table  100 - 4 - 1  (SQ 3 - 6 ). After the user terminal information is deleted from the user management table  100 - 4 - 1 , the user management table  100 - 4 - 1  is searched through by using the group address as a key, and it is checked whether any user still belongs to the group (SQ 3 - 6 - 1 ). Because both the user terminal H 1 - 1  and the user terminal H 1 - 2  have already left the multicast group, the user management table  100 - 4 - 1  contains no user information, in this example.  
         [0089]     Accordingly, the Layer 2 switch  100  edits the header of an IGMP leave packet including the group address “224.10.10.10” (SQ 3 - 7 ), so that multicast packet distribution from the content distribution server S 1  will be stopped. The Layer 2 switch  100  sends the IGMP leave packet to the packet transfer apparatus  200  (SQ 3 - 8 ). The packet transfer apparatus  200  receives the IGMP leave packet and sends a PIM leave packet through the router  300  to the content distribution server (SQ 3 - 9 ). The content distribution server S 1  receives the PIM leave packet and stops multicast packet distribution to the Layer 2 switch  100 .  
         [0090]     Now, the multicast packet distribution to the user terminal H 1 - 1  and the user terminal H 1 - 2  stops. Because a multicast connection to the group with group address “224.10.10.10” becomes unnecessary, the connection is closed (SQ 3 - 10 ). The corresponding connection information is deleted from the multicast connection management table  100 - 4 - 2 .  
         [0091]     The embodiment controlled by software has been described. The processing of the present invention may also be implemented by hardware. PPPoE has been mainly described above, but the embodiment can also be applied to a point-to-point connection by VLAN, PPPoA, PPPoEoA, and others.