Abstract:
An object of the present invention is to solve a problem that when multicast is utilized in a network configured with virtual routers, traffic in the relay network is increased. According to the present invention, the number of multicast packets via the relay network is reduced by performing a multicast packet transfer across the virtual routers within the same router. Specifically, it is allowed to register in a multicast routing table held by each virtual router, a line interface identifier of another virtual router as outgoing line interface information, whereby multicast packet transfer from a virtual router to another virtual router is made possible. In order to specify a virtual router to share the multicast packet, each virtual router holds information to specify a virtual router having a line interface to receive the multicast traffic indicated by the multicast group address.

Description:
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese application JP 2004-192530 filed on Jun. 30, 2004, the content of which is hereby incorporated by reference into this application.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a network comprising a packet relay device with a function of virtual router, and more particularly, it relates to a system to reduce multicast traffic flowing within the network, by means of sharing the multicast traffic among multiple virtual routers in a single router.  
       BACKGROUND OF THE INVENTION  
       [0003]     With speedup in access line, a demand for voice and/or video streaming service is increasing. Currently used streaming service employs a unicast system which transmits from a voice/video server, data packets respectively dedicated to multiple subscriber terminals. Therefore, the voice/video server is forced to transmit a large number of packets, causing problems such that data transmission load is high in the voice/video server, as well as increasing a load on a relay network which relays the large number of packets.  
         [0004]     In view of the situation above, telecommunication carriers, such as CDSP (content delivery service provider), and ISP (Internet Service Provider), now consider employing multicast system for the voice/video streaming service. In the multicast system, the voice/video server transmits only one data packet to multiple subscriber terminals, and a packet relay device within a relay network copies the data packet as appropriate, thereby executing a packet transfer to the multiple subscriber terminals. Therefore, the number of packets transmitted by the voice/video server is suppressed to a small number, and the load onto the server can be reduced. Furthermore, the number of packets flowing in the relay network is also suppressed to a small number and the load onto the relay network can be reduced as well.  
         [0005]     In the meantime, the telecommunication carriers may construct a network employing a virtual router (hereinafter, referred to as “VR”), for the purpose of reducing a cost in building the network or separating traffic among subscribers. VR is a technique to configure a function of multiple virtual routers within one packet transfer device. Since such function of multiple routers can be implemented with one packet transfer device, it is possible to reduce the number of installed routers in the circumstance where multiple routers are required at the same spot.  
         [0006]     For example, as for multiple ISPs, ADSL (Asynchronous Digital Subscriber Line) line service provider or FTTH (Fiber To The Home) line service provider, for providing an access network to connect the ISPs and the subscribers, may be capable of providing a function of routers dedicated to the ISPs, respectively. Alternatively, a telecommunication carrier who offers a wide area IP network service called as IP-VAN (IP Virtual Private Network) allocates a VR to each of customers, thereby providing the VPN service without interference in traffic among customers and establishing communication within each customer only.  
         [0007]      FIG. 2  shows a configuration of a conventional packet relay device in a form of functional block of VR, with which a network adapted for multicast transfer can be constructed, as well as VR function is being installed therein. The packet relay device as shown in  FIG. 2  implements VR  61   a  and VR  61   b . Each of the VRs is provided with UPLINK information  611   a ,  611   b , multicast routing tables  612   a ,  612   b , PIM  613   a ,  613   b , IGMP PROXY  614   a ,  614   b , and the like. Reference numerals  12   a  to  12   d  denote subscriber terminals, and they are connected to the packet relay device  6  via communication lines. Subscriber terminals requesting to participate in a multicast delivery target group transmit IGMP Report messages  13   a  to  13   d  to the packet relay device  6 .  
         [0008]     The packet transfer device which has received the IGMP Report messages refers to Uplink information using as a key a multicast group address included in each of the IGMP Report messages, selects an interface connected to a line being upstream of the multicast, and transmits to the upstream router a PIM protocol message to allow the terminal to participate in the multicast group. In  FIG. 2 , VR  61   a  and VR  61   b  respectively receive IGMP Report messages  13   a ,  13   b  and  13   c ,  13   d , and transmit PIM protocol messages  13   e  and  13   f  to the upstream routers.  
         [0009]     The multicast system has a function to reduce the load on the relay network, but there is a possibility the load on the relay network is increased if the multicast system is employed in the network utilizing a VR. In order to clarify a problem to be solved by the present invention, as a reference example, an example in which a network is constructed using the packet relay device as shown in  FIG. 2  and data delivery in the network is preformed through multicast system will be described with reference to  FIG. 3 . In  FIG. 3 , multicast traffic  32  comprising data packets is delivered from the multicast server  3 , via ISP  41   a , router  2   a , VR  61   a  and VR  61   b , and via SP  41   b , router  2   b , VR  61   c  and VR  61   d , respectively to the subscriber terminals  12   a  to  12   d  and  12   e  to  12   h . Here, VR  61   a  and VR  61   b  are located within the packet relay device  6   a , and VR  61   c  and VR  61   d  are located within the packet relay device  6   b.    
         [0010]     A multicast packet is copied by a router on a path from the multicast server to the subscriber terminal, and then the copy is delivered to the subscriber terminal. In  FIG. 3 , a data packet from the multicast server  3  is copied by a router (not illustrated) in the Internet  4  and delivered to the routers  2   a  and  2   b . Subsequently, the copied data packets are delivered to VR  61   a  and VR  61   c  from the router  2   a , and another copied data packets are delivered to VR  61   b  and VR  61   d  from the router  2   b.    
         [0011]     In the multicast delivery, it is desirable to carry out copying in a router located as close as possible to a subscriber, thereby reducing the number of data packets transferred within the network and also reducing the load onto the relay network. However, in the example of  FIG. 3 , the data packets belonging to the identical multicast delivery (packets having the same destination address and data) are redundantly delivered from the router  2   a  and the router  2   b , to the VR  61   a  and VR  61   b  in the packet relay device  6   a , and to the VR  61   c  and VR  61   d  in the packet relay device  6   b.    
         [0012]     The situation above occurs since the VRs with conventional function have to operate independently even if they are located within the same packet relay device, and the multicast traffic cannot be shared between the VRs. Therefore, as it is shown in  FIG. 3 , the router  2   a  and the router  2   b  have to transmit the multicast traffic to all the VRs within the packet relay devices  6   a  and  6   b , increasing the load onto the relay network.  
       SUMMARY OF THE INVENTION  
       [0013]     Considering the problem above, the object of the present invention is to provide a communications network when a multicast system is utilized in a network comprising a packet relay device mounting VR function, the communications network being capable of reducing the load onto a relay network lower than before, and a packet relay device which is capable of implementing the communications network.  
         [0014]     When a network has been configured employing a packet transfer device with the VR function, for example, in the network having the configuration as shown in  FIG. 3 , in order to suppress to the minimum the number of multicast packets via the relay network  5 , it is sufficient to transmit one packet to each of the packet relay devices  6   a  and  6   b , from either of the routers  2   a  and  2   b , the packet being copied within each of the packet relay devices  6   a  and  6   b , shared between the VRs, and to transmit those copied packets to the subscriber terminals.  
         [0015]     Therefore, in the present invention, packet transfer across the VRs within the packet relay device can be executed, thereby reducing the number of multicast packets via the relay network. Specifically, the present invention allows a multicast routing table held by each VR to register a line interface identifier of another VR as outgoing line interface information indicating a destination address of the packet. Here, the multicast routing table includes information comprising a combination of a multicast group address and multiple line interface identifiers. When the VR transfers a multicast packet, the VR refers to the multicast routing table using as a key the multicast group address held by the packet, and obtains an outgoing line interface identifier to transmit the packet.  
         [0016]     In the case of conventional VR, each VR operates independently. Therefore, it has been imperative that the outgoing interface identifier of the multicast routing table held by each VR corresponds to a line interface identifier belonging to the VR itself which holds the multicast routing table. On the other hand, the VR according to the present invention allows an interface identifier indicating a line interface of another VR to be registered as an outgoing line interface identifier. Accordingly, a multicast packet received by an arbitrary one VR within the same packet relay device can be shared among multiple VRs within the same relay device.  
         [0017]     By configuring a network by employing the packet relay device implementing the features above, traffic in multicast packet transfer is concentrated, whereby the traffic volume can be reduced than before. Internal configuration of the packet relay device as described above and details of a packet transfer method will be explained in the following preferred embodiments of the present invention.  
         [0018]     According to the present invention, as for packet relay devices arranged dispersedly in subscriber accommodation stations or the like, it is possible to share the multicast traffic among VRs within a single packet relay device, thereby reducing the multicast traffic in a multicast packet relay network. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a schematic diagram showing an example of multicast delivery through an access network employing a router adapted for VR according to the present invention;  
         [0020]      FIG. 2  is a conceptual diagram showing an existing router adapted for VR;  
         [0021]      FIG. 3  is a schematic diagram showing an example of multicast delivery through an access network employing an existing router adapted for VR;  
         [0022]      FIG. 4  is a schematic diagram showing a hardware configuration of the router adapted for VR according to the present invention;  
         [0023]      FIG. 5  is conceptual diagram of the router adapted for VR according to the present invention;  
         [0024]      FIG. 6  shows Uplink information;  
         [0025]      FIG. 7  shows Uplink VR information;  
         [0026]      FIG. 8  shows a multicast routing table of the router adapted for VR according to the present invention;  
         [0027]      FIGS. 9A and 9B  show multicast routing tables of the existing router adapted for VR;  
         [0028]      FIG. 10  shows VR configuration information;  
         [0029]      FIG. 11  shows an operational flowchart of the router adapted for VR according to the present invention;  
         [0030]      FIG. 12  shows a conceptual diagram of multicast by the router adapted for VR according to the present invention;  
         [0031]      FIG. 13  shows an IGMP message format;  
         [0032]      FIG. 14  shows a conceptual diagram of multicast by the existing router adapted for VR;  
         [0033]      FIG. 15  shows a conceptual diagram of a router adapted for VR according to another embodiment of the present invention;  
         [0034]      FIGS. 16A and 16B  show multicast routing tables of a router adapted for VR according to another embodiment of the present invention; and  
         [0035]      FIG. 17  shows a conceptual diagram of multicast by a router adapted for VR according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0036]     Hereinafter, with reference to the accompanying drawings, preferred embodiments of the present invention will be explained.  FIG. 1  is a configuration diagram of Internet access network (hereinafter, referred to as “access network”) which has been constructed by applying packet relay devices  1   a ,  1   b  according to the present embodiment, to routers for accommodating subscriber circuits. The packet relay devices  1   a ,  1   b  are arranged dispersedly in a subscriber accommodation station on the subscriber side in the access network, and the packet relay devices respectively accommodates subscriber terminals  12   a  to  12   d , and  12   e  to  12   h . In  FIG. 1 , the packet relay device  1   a  includes therein VR  11   a  and VR  11   b , and the packet relay device  1   b  includes therein VR  11   c  and VR  11   d . The VR  11   a  and VR  11   c  are allocated to ISP  41   a , and the VR  11   b  and VR  11   d  are allocated to ISP  41   b.    
         [0037]     With this configuration, it is not necessary for the subscriber accommodation station to install routers with respect to each ISP, whereby the number of installed routers are reduced. The ISP  41   a  is connected to VR  11   a  in the packet relay device  1   a  and VR  11   c  in the packet relay device  1   b , via the router  2   a  and the relay network  5 . The ISP  41   b  is connected to VR  11   b  in the packet relay device  1   a  and VR  11   d  in the packet relay device  1   b , via the router  2   b  and the relay network  5 . The VR  11   a  accommodates the subscriber terminals  12   a  and  12   b , the VR  11   b  accommodates the subscriber terminals  12   c  and  12   d . Furthermore, VR  11   c  accommodates the subscriber terminals  12   e  and  12   f , the VR  11   d  accommodates the subscriber terminals  12   g  and  12   h . The relay network  5  may be a wide area network, for example, constructed by ATM (Asynchronous Transfer Mode) network and the like.  
         [0038]      FIG. 1  shows an example that multicast server  3  within the Internet  4  performs multicast data delivery to the subscriber terminals  12   a  to  12   h . Multicast traffic  31  from the multicast server  3  goes through the ISP  41   a , router  2   a , relay network  5 , and then it is transmitted to the VR  11   a  in the packet relay device  1   a , and to the VR  11   c  in the packet relay device  1   b . Subsequently, the multicast traffic  31  is copied between the VR  11   a  and VR  11   b  within the packet relay device  1   a , and also it is copied between the VR  11   c  and VR  11   d  within the packet relay device  1   b . Then, the copied multicast traffic is delivered from VR  11   a , VR  11   b , VR  11   c , and VR  11   d  to the subscribers  12   a  to  12   h.    
         [0039]     Here are two multicast traffic flows which pass through the relay network  5 , i.e., a traffic flow from the router  2   a  to the VR  11   a  in the packet relay device  1   a , and a traffic flow from the router  2   a  to the VR  11   c  in the packet relay device  1   b . In general, when there is m number of packet relay devices, the number of multicast traffic flows passing through the relay network  5  is m. It is to be noted that multiple number of VRs for receiving the multicast packet may exist among the VRs mounted on the packet relay device  1   a  or  1   b  as shown in  FIG. 1 . Furthermore, VR prepared for receiving is not fixed constantly, and it may be changed according to a type of the multicast packet. In  FIG. 7 , for example, the VR for receiving the multicast addressed to MC address  1  is defined as VR 1 . When there is a request from a subscriber to participate in the multicast addressed to MC Address  1 , Uplink VR information is referred to, so as to inform an upstream router of the request. Consequently, the VR 1  becomes the VR for receiving the multicast. As to the Uplink VR information, there will be a detailed explanation in the following, with reference to  FIG. 7 .  
         [0040]     In order to see an effect brought about by reducing the traffic in the network as shown in  FIG. 1 , traffic volume in the network as shown in  FIG. 3  will be explained as a comparative example.  FIG. 3  is a configuration diagram showing an access network constructed by applying to a subscriber accommodation station, conventional packet relay devices  6   a ,  6   b  which do not have VR function as provided by the present embodiment. The network configuration in  FIG. 3  is the same as that of  FIG. 1 , except the packet relay devices  6   a  and  6   b . Similar to the case of  FIG. 1 , there is shown an example to perform multicast data delivery from the multicast server  3  within the Internet  4  to the subscriber terminals  12   a  to  12   h . Here, the multicast traffic  32  from the multicast server  3  branches out within the Internet  4 , and each reaches ISP  41   a  and ISP  41   b.    
         [0041]     From the ISP  41   a , the multicast traffic  3 . 2  goes through the router  2   a  and the relay network  5 , and then it is transmitted to the VR  61   a  in the packet relay device  6   a  and to the VR  61   c  in the packet relay device  6   b . From the ISP  41   b , the multicast traffic is transmitted to the VR  61   b  in the packet relay device  6   a , and the VR  61   d  in the packet relay device  6   b . Then, the multicast traffic is delivered to the subscriber terminals  12   a  and  12   b  from the VR  61   a  in the packet relay device  6   a , to the subscriber terminals  12   c  and  12   d  from the VR  61   b , to the subscriber terminals  12   e  and  12   f  from the VR  61   c  in the router  6   b , and to the subscriber terminals  12   g  and  12   h  from the VR  61   d.    
         [0042]     Here, there are four multicast traffic flows which pass through the relay network  5 , i.e., a traffic flow from the router  2   a  to the VR  61   a , a traffic flow from the router  2   a  to the VR  61   c , a traffic flow from the router  2   b  to the VR  61   b , and a traffic flow from the router  2   b  to the VR  61   d . In general, if there is m number of packet relay devices and n number of ISPs, the number of multicast traffic flows passing through the relay network  5  is m×n.  
         [0043]     As described above, it is found that by constructing a network by employing the packet relay devices  1   a ,  1   b  having the multicast function according to the present embodiment, the number of traffic flows through the relay network  5  can be reduced to m from m×n.  
         [0044]     Next, a configuration of the packet relay device according to the present embodiment will be explained.  FIG. 4  shows a schematic diagram of a hardware configuration of the packet relay device  1   a  or  1   b  according to the present embodiment. The packet relay device according to the present embodiment comprises a control function unit  81 , a memory unit  82 , and a packet transfer function unit  83 , and these units are connected via a control bus. The memory unit  82  holds the aforementioned Uplink information  111 , the multicast routing table  112 , and Uplink VR information  115 . Furthermore, the memory unit  82  holds VR configuration information  821  indicating VR configuration. When the packet transfer function unit  83  receives a multicast data packet via the line interface units  84   a  to  84   e , the packet transfer function unit  83  refers to the multicast routing table  112 , and transfers the data packet to the line interface indicated by the outgoing IF identifier in the table. At this stage, if there are multiple line interfaces indicated by the outgoing IF identifier, the packet transfer function unit  83  copies the data packet and thus copied packets are transmitted from those line interfaces respectively.  
         [0045]      FIG. 5  shows a functional block diagram of the multicast packet relay device according to the present embodiment. In addition,  FIG. 5  shows a procedure for a subscriber terminal to participate in a target of multicast delivery in the packet relay device  1  according to the present embodiment. The packet relay device according to the present embodiment includes VRs  11   a  and  11   b . The VR  11  comprises Uplink information  111   a , multicast routing tables  112   a , PIM-SM function  113   a , IGMP Proxy function  114   a , and Uplink VR information  115   a . The VR  11   b  comprises Uplink information  111   b , multicast routing tables  112   b , PIM-SM function  113   b , IGMP Proxy function  114   b , and Uplink VR information  115   b . As the outgoing interface identifier defined in each of the multicast routing tables  112   a ,  112   b , it is possible to register not only a line interface identifier belonging to the VR holding the multicast routing table, but also a line interface identifier of arbitrary VR.  
         [0046]     The packet relay devices  1   a ,  1   b  according to the present embodiment hold Uplink VR information to indicate a VR which shares a multicast packet. The Uplink VR information is information comprising a combination of multicast group address and VR identifier. Here, the VR identifier is information which specifies a VR having a line interface for receiving a multicast packet holding the multicast group address. In other words, it is information which indicates a VR having a line interface serving as upstream of the multicast. In the packet relay device according to the present embodiment, the Uplink VR information is appropriately set in each VR, thereby allowing the multicast packet to be shared among the VRs.  
         [0047]     Transferring multicast packets is carried out on the basis of the multicast routing table. A VR which received a multicast delivery request transmitted by a subscriber terminal notifies an upstream router of the request, and simultaneously records in the multicast routing table a line interface identifier to connect the multicast group address with the subscriber terminal, whereby the multicast routing table is created. The multicast group address is included in the multicast delivery request.  
         [0048]     In addition, when the VR implemented by the packet relay devices  1   a ,  1   b  according to the present embodiment receives a multicast delivery request, the VR refers to the Uplink VR information using as a key the multicast group address included in the request, and passes to a VR indicated by thus obtained VR identifier, the multicast delivery request and a line interface identifier which received the request. The VR which has been passed the multicast delivery request and the line interface identifier, processes the multicast delivery request, and registers in the multicast routing table the multicast group address and the line interface identifier thus passed. Then, the VR performs multicast protocol processing such as IGMP (Internet Group Management Protocol) Proxy function, or PIM (Protocol Independent Multicast), and newly creates a multicast delivery request, followed by transmitting the request from the line interface being an upstream of the multicast.  
         [0049]      FIG. 6  shows Uplink information  111   a ,  111   b  respectively held by the VRs  11   a  and  11   b  in the packet relay device  1  according to the present embodiment. The Uplink information is information indicating a line as a multicast upstream, and comprises a combination of multicast group address  6111  and Uplink interface identifier  6112 .  
         [0050]      FIG. 7  shows an example of Uplink VR information  115   a ,  115   b  respectively held by the VRs  11   a ,  11   b  in the packet relay device  1  according to the present embodiment. The Uplink VR information comprises a combination of multicast group address  1111  and VR number  1112 . Upon receipt of an IGMP Report message from a subscriber terminal, each VR refers to the Uplink VR information using as a key the multicast group address set in the Group Address field of the IGMP Report message, identifies a VR (upstream VR) for receiving the multicast traffic indicated by the multicast group address, and passes to thus identified VR, the IGMP group message and an identifier of the line interface which received the message.  
         [0051]     Subsequently, in the VR which received the IGMP Group message and the line interface identifier, PIM protocol processing or IGMP Proxy protocol processing is performed.  FIG. 5  shows that the VR  11   b  which received the IGMP Report messages  13   c ,  13   d  refer to the Uplink VR information  114   b , selects the VR  11   a  as an upstream VR, and a PIM protocol message or IGMP Report message  13   e  is transmitted.  
         [0052]     The packet relay device according to the present embodiment performs a multicast control processing by use of the Uplink VR information as described above, whereby a multicast routing table across the VRs is created.  FIG. 8  shows an example of the multicast routing table  112   a  of the VR  11   a . In  FIG. 8 , the multicast group address MC Address  1  is associated with the transmission interfaces, line interface identifiers  1 ,  2 , belonging to the VR  11   a , and the line interface identifiers  3 ,  4 , belonging to the VR  11   b . The VR  11   a  transfers a multicast packet according to this routing table.  
         [0053]     For comparison purposes,  FIGS. 9A and 9B  show an example of multicast routing tables which are used in a conventional packet relay device  6  as shown in  FIG. 2 . In this multicast routing table, the multicast group address  6122  is associated with multiple outgoing interface identifiers  6123  serving as transmission interfaces of the multicast packet.  
         [0054]     In the conventional multicast routing table, only the line interface belonging to the VR which holds the multicast routing table can be specified as the transmission interface. On the other hand, the multicast routing table according to the present embodiment can specify a line interface belonging to a VR other than the VR which holds the multicast routing table. Accordingly, multicast transfer across VRs is made possible.  
         [0055]      FIG. 10  shows VR configuration information  821 . The VR configuration information  821  comprises a combination of interface identifier  8211  and VR number  8212 , and indicates a correspondence between the line interface and VR. For example,  FIG. 10  shows that the interfaces  1  and  2  belong to the VR having the VR number  1 , and the interfaces  3  and  4  belong to the VR having the VR number  2 . As shown in  FIG. 4 , the control function unit  81  is connected to the management terminal  9 . An administrator of the packet relay device uses the management terminal  9 , and sets VR configuration information  821 , Uplink information  111 , and Uplink VR information  115 . The packet transfer function  83  has line interface units  84   a  to  84   e , copies multicast packets received from those line interfaces as appropriate, with reference to the multicast routing table  112 , and then, transfers those copied packets to another line interfaces.  
         [0056]      FIG. 11  shows a flow diagram to provide a summary of processing for creating the multicast routing table in the packet relay device  1  according to the present embodiment. By use of the flow diagram, operations of the VR in the packet relay device  1  according to the present embodiment will be explained in the following.  
         [0057]     The VR that has received an IGMP Report message ( 70 ) refers to the Uplink VR information ( 71 ). If the Uplink VR corresponds to own VR, it performs PIM protocol processing or an IGMP Proxy processing ( 72 ,  73 ). At this stage, the VR refers to the Uplink information ( 74 ), and transmits a PIM protocol packet or an IGMP Report message to an upstream line ( 75 ). In addition, the VR registers in the multicast routing table, an identifier of the line interface which received the IGMP Report message, as an outgoing interface identifier associated with the multicast group address included in the IGMP Report message ( 76 ).  
         [0058]     On the other hand, if the Uplink VR is another VR, the IGMP Report message thus received is passed to an upstream VR together with the line interface identifier which received the IGMP Report message ( 72 ,  78 ). In the upstream VR, a processing for receiving the IGMP Report message is newly performed, by use of the IGMP Report message which has been passed ( 79 ).  
         [0059]     According to the processing as described above, the multicast traffic is shared among VRs within the same packet relay device, and the multicast delivery as shown in  FIG. 1  is carried out. In  FIG. 1 , at first, the router  2   a  transmits multicast packets to the VR  11   a  in the router  1   a  and to the VR  11   c  in the router  1   b . Subsequently, multicast delivery is performed from the VR  11   a  to the subscriber terminals  12   a ,  12   b  accommodated in the VR  11   a , and to the subscriber terminals  12   c ,  12   d  accommodated in the VR  11   b . Furthermore, multicast delivery is performed from the VR  11   c  to the subscriber terminals  12   e ,  12   f  accommodated in the VR  11   c , and to the subscriber terminals  12   g ,  12   h  accommodated in the VR  11   d .  
         [0060]      FIG. 12  shows a situation where multicast packets are transmitted to the subscriber terminals  12   c ,  12   d  accommodated in the VR  11   b , in addition to the subscriber terminals  12   a ,  12   b  accommodated in the VR  11   a . Next, a method for a subscriber terminal to participate in a multicast group in the multicast packet transfer will be explained.  
         [0061]      FIG. 13  shows a format of IGMP Ver.  2  message. The IGMP Ver.  2  is a multicast control protocol between a subscriber terminal and a packet relay device, currently used most frequently. The IGMP Ver.  2  message includes Type field, Maximum Response Time field, Checksum field, and Group Address field. The IGMP Report message includes a value of 0×16 in the Type field, and multicast group address indicating a participating subscriber terminal group is set in the Group Address field. It is to be noted that Checksum is information used for data error detection. It is also to be noted that the Maximum Response Time field is not used for the IGMP Report message.  
         [0062]     As another type of IGMP message, there are IGMP Leave Group message and IGMP Query message. When a subscriber terminal leaves from a multicast group, the address of the multicast group is set in the Group Address field in the IGMP Leave group message and it is transmitted to the packet relay device. The IGMP Leave Group message has a value of 0×17 in the Type field, and the Max Response Time field is not used.  
         [0063]     The packet relay device accommodating a subscriber terminal transmits an IGMP Query message to the subscriber terminal on regular basis. The subscriber terminal receives the IGMP Query message, and if the multicast group address in the Group Address field of the IGMP Query message indicates the multicast subscriber terminal group to which the subscriber terminal itself belongs, the subscriber terminal transmits an IGMP Report message. As thus described, the packet relay device accommodating the subscriber terminal prompts the subscriber terminal to transmit an IGMP Report message, thereby checking a participation status of the subscriber terminal in the multicast group. The IGMP Query message has a value of 0×11 in Type field, and in the Group Address field, there is set a multicast group address as to which it is inquired whether or not the terminal is participating. Furthermore, in the Max Response Time field, there is a setting of time period permitted until the IGMP Report message is returned from the subscriber terminal.  
         [0064]     In order to implement the multicast function, the VR  11   a  and VR  11   b  within the packet relay device  1  includes respectively, PIM functions  113   a ,  113   b  or IGMP Proxy functions  114   a ,  114   b , each being a multicast routing protocol to create a multicast routing table. The VR  11   a  and VR  11   b  further holds respectively, Uplink information  111   a ,  111   b  required for protocol operations, and the multicast routing tables  112   a ,  112   b  which are created with those protocols.  
         [0065]     A procedure for creating the multicast routing table, according to PIM function or IGMP Proxy function will be explained in the following. The PIM functions  113   a ,  113   b  and the IGMP Proxy functions  114   a ,  114   b  are protocols to create the multicast routing table. Upon receipt of an IGMP Report message from the subscriber terminal, the PIM function refers to Uplink information using as a key the multicast group address included in the IGMP Report message, selects an interface which is connected to a line being upstream of the multicast, and transmits a PIM protocol message to the upstream router for allowing the terminal to participate in the multicast group. In  FIG. 5 , the VR  11   a  receives the IGMP Report messages  13   a  and  13   b , and VR  11   b  receives the IGMP Report messages  13   c  and  13   d , and PIM protocol message  13   e  is transmitted to the upstream router. Here, as for the VR  11   a , the Uplink VR described in the Uplink VR information indicates its own VR. Therefore, the VR  11   a  refers to the Uplink information and transmits the PIM protocol message directly from its own VR to the upstream router. However, as for the VR  11   b , since the Uplink VR described in the Uplink VR information indicates the VR  11   a , the PIM protocol message is not transmitted to the upstream router directly from the VR  11   b  itself.  
         [0066]     Similarly, when the IGMP Proxy function receives an IGMP Report message from the subscriber terminal, it refers to the Uplink VR information and Uplink information using as a key the multicast group address included in the IGMP Report message, selects an interface connected to the upstream line of the multicast, and transfers the IGMP Report message from the subscriber terminal on the current line to the upstream router.  
         [0067]     There may be considered following modes for setting the Uplink information, i.e., manually setting by an administrator, and automatically setting of a line being the shortest route to the multicast server on the basis of unicast routing information. Since each VR operates as an independent router, Uplink interface identifier which is set in the Uplink interface information  6112  has to be an identifier indicating a line interface belonging to that VR. In addition, the Uplink VR information is information set by the administrator.  
         [0068]     In the conventional VR, the multicast routing table is managed independently with respect to each VR. In  FIG. 2 , the VR  61   a  has the multicast routing table  612   a , and the VR  61   b  has the multicast routing table  612   b . Here, since each VR operates independently, the outgoing interface identifier indicates any of the line interfaces belonging to the VR holding the multicast routing table. As shown in  FIG. 14 , the VR  61   a  and VR  61   b  within the router  6  transfer the multicast packets according to the respective multicast routing tables.  
         [0069]     On the other hand, in the VR according to the present embodiment, the multicast routing table is managed across the VRs.  FIG. 5  and  FIG. 12  show that the VR  11   a  holds the multicast routing table  112   a , and the VR  11   b  holds the multicast routing table  112   b . As shown in  FIG. 12 , it is possible for any of the multicast routing tables to have an identifier indicating a line interface belonging to a VR which is different from the VR having that multicast routing table. Accordingly, as shown in  FIG. 5  and  FIG. 12 , the VR  11   a  and VR  11   b  within the router  1  are allowed to transfer the multicast packets across the VRs.  
         [0070]     In  FIG. 6 , the multicast routing table using as a key the multicast group address is illustrated as an example. However, when a protocol which can specify a multicast server is used, such as IGMP v 3 , PIM-SSM (Source Specific Multicast), a combination of a sender address (address of the multicast server) and the multicast group address is used as a key.  
         [0071]     With the packet relay device which performs multicast packet transfer across the VRs, it is possible to carry out a multicast transfer service without increasing the load of the network data packet transfer, even in the network configuration employing VRs.  
       Second Embodiment  
       [0072]     In the present embodiment, a VR implementation method, in a type of internal link, will be explained.  FIG. 15  shows a functional block diagram of the packet relay device according to the present embodiment. In addition, the hardware configuration of the packet relay device according to the present embodiment can be implemented in the same configuration as shown in  FIG. 4 . The packet transfer device  9  used in the present embodiment has the same configuration as that of the conventional packet transfer device  6 , except that the packet transfer device  9  has an internal line  92 . In other words, the Uplink information  911   a ,  911   b , multicast routing table  912   a ,  912   b , PIM-SM functions  913   a ,  913   b , IGMP Proxy functions  914   a ,  914   b  respectively correspond to Uplink information  611   a ,  611   b , multicast routing tables  612   a ,  612   b , PIM-SM functions  613   a ,  613   b , and IGMP Proxy functions  614   a ,  614   b  of the conventional packet transfer device  6 . The internal line  92  is a logical line to establish connection between the VRs.  FIG. 15  shows that the internal line  92  connects the VR  91   a  and the VR  91   b  within the router  9 .  
         [0073]     With reference to  FIGS. 15, 16 , and  17 , an operation of the router adapted for VR according to the present embodiment will be explained.  FIG. 15  shows that VR  91   b  holds Uplink information  911   b  which uses the internal line  92  as a line connecting to the upstream router, and uses the VR  91   a  as an upstream VR. In  FIG. 15 , the VR  91   b  receives IGMP Report messages  13   c  and  13   d  from the subscriber terminals  12   c ,  12   d . The VR  91   b  which received the IGMP Report messages  13   c  and  13   d  performs PIM or IGMP Proxy protocol processing, and creates a multicast routing table. When the PIM function is used, the VR  91   b  transmits a PIM protocol message  13   d  to the VR  91   a  via the internal line  92 . When the IGMP Proxy function is used, the VR  91   b  transmits an IGMP Report message to the VR  91   a  via the internal line  92 . This processing is the same as the multicast protocol processing performed by a usual router.  
         [0074]     Next, the VR  91   a , which has received the PIM protocol message or the IGMP Report message from the VR  91   b , performs a protocol processing according to the PIM function or IGMP Proxy function, as in the case of the usual router, and transmits the PIM protocol message or the IGMP Report message  13   e  to the upstream router.  
         [0075]      FIG. 16A  and  FIG. 16B  show the multicast routing tables  912   a  and  912   b  in the present embodiment, which are created respectively by the VR  91   a  and the VR  91   b  according to the procedure as described above. In the multicast routing table  912   a , the internal line  92  is set in the interface information associated with the multicast group address MC Address  1 , and this is a point different from the multicast routing table held by the existing router  6  adapted for VR. In the present embodiment, as shown in  FIG. 17 , the multicast routing tables  912   a  and  912   b  use the line interfaces 1, 2, 3, and 4, so as to transmit the multicast traffic flows to the subscriber terminals  12   a ,  12   b ,  12   c , and  12   d , respectively.  
         [0076]     In the present embodiment, since an upstream VR is specified by use of the Uplink information, it is not necessary to specify the Uplink VR information, which is required in the first embodiment. Therefore, it is possible for a VR administrator to obtain an effect of the present invention, even if its operation is closer to the operation of a conventional router.