JP3791304B2 - Gateway device and multicast communication system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gateway device and a multicast communication system that use a communication path capable of large-scale multicast of tens of thousands represented by satellite communication.
[0002]
[Prior art]
When communicating on an internetwork system where multiple different networks are connected, a protocol for data communication between different networks and a data communication between application programs located at the upper level It is widely practiced to design communication protocols in layers.
[0003]
Currently, routing protocols for exchanging packets between different networks that are widely used are designed on the assumption that data can be sent in both directions on the same route. For this reason, applications designed on the assumption that bidirectional communication can be performed using a communication path such as satellite data communication that can send data only one way from the base station to the receiving station cannot be operated. .
[0004]
One possible solution is to improve the routing protocol at the network layer so that it can handle one-way routes. In that case, the number of devices that need to be improved will be tens of thousands or more. Therefore, it is not realistic.
[0005]
Therefore, as disclosed in Japanese Patent Application Laid-Open No. 11-313109, a transmission gateway and a reception gateway are installed at both ends of a one-way communication path, packet tunneling and dynamic network address replacement at the network layer between both gateways. Thus, a communication system using an asymmetric path that is transparent to the application layer has been devised.
[0006]
With the above-described conventional technology, an Internet communication system using a satellite data communication path that is a one-way path can be constructed, and a user can access the Internet at high speed using the satellite data communication path.
[0007]
A packet used in Internet communication is an IP packet. As a communication method using an IP packet, there is a multicast communication method in which one-to-many communication is performed in addition to a unicast communication method in which one-to-one communication is performed.
[0008]
In general, on an Internet system, a router that relays packets determines a route in consideration of various conditions such as a line speed and a transmission delay time. When relaying multicast packets, the routing information is exchanged using the multicast routing protocol to determine the route. In the case of multicast, the destination address of the IP packet is used as the multicast group number, unlike unicast. The device cannot be specified from the end point address. Therefore, in addition to a protocol for controlling routing between routers, an IGMP protocol for controlling which multicast group is transmitted on the same segment is used.
[0009]
[Problems to be solved by the invention]
The satellite data communication path is a communication path that is very suitable for use in a multicast communication system because data transmitted from a satellite transmission station can be received simultaneously by a large number of satellite reception stations. However, the conventional multicast routing protocol is limited to about several thousand entries in the routing table, and the amount of data for exchanging routing information increases in proportion to the number of routers. It is not designed in consideration of multicast, and there is a problem that it is difficult to make use of the characteristics of the satellite data communication path.
[0010]
In addition, the unicast route and the multicast route are independent, and when a client PC receives a multicast packet transmitted by a server on the Internet, whether the multicast packet reaches the RGW satellite line side or the Internet side. It depends on the situation in the Internet and cannot be specified. The most reliable method for receiving the multicast packet on the satellite line side is to set the RGW not to perform multicast routing on the Internet side interface. In this case, since the multicast route to the RGW is one via the satellite line, this route is always used.
[0011]
However, in this case, when the satellite line is temporarily unavailable due to bad weather or the like, the multicast route is lost, so there is a problem that unicast communication is possible but multicast communication is not possible. is there.
[0012]
An object of the present invention is to enable a large-scale multicast routing by using a satellite line which can perform a large-scale multicast, but partially disables communication due to factors such as weather, and uses an IGMP proxy in the satellite line part. Another object of the present invention is to provide a system and a gateway device that automatically perform multicast delivery via the Internet when a satellite line is disconnected.
[0013]
[Means for Solving the Problems]
In order to solve the above problems, a multicast communication system that performs asymmetric path control has been devised.
[0014]
In the IGMP protocol for controlling multicast transmission within the same segment, the data amount of control packets necessary for control increases with a multiplier smaller than the first power of the number of machines in the segment. Since the satellite line itself looks like a large single segment, IGMP is used for multicast control within the satellite line. At this time, although the RGW originally behaves as a router, it can be controlled by IGMP alone by acting as a host instead.
[0015]
A multicast routing program is provided in the RGW (receiving gateway) to route multicast packets between different interfaces. Although there are many types of multicast routing protocols, the present invention can be applied to any routing algorithm, and therefore description of the multicast routing protocol itself is omitted.
[0016]
An IGMP proxy is provided on the RGW. IGMP proxy is
(1) Exchange multicast group distribution requests with other machines on the satellite interface using the IGMP protocol.
[0017]
(2) Based on the above information, the multicast routing information and the multicast packet are passed to the multicast routing program.
[0018]
It has a function. Details of the IGMP proxy are described in "http://www.ietf.org/internet-drafts/draft-fenner-igmp-proxy-01.txt".
[0019]
RGW has a satellite interface monitoring program. The satellite interface monitoring program checks the reception status of the satellite interface, and if it is received normally, disables the multicast routing function of the Internet side interface, and if the satellite interface side does not receive normally, the multicast routing function of the Internet side interface Is valid.
[0020]
As described above, by providing a multicast router, IGMP proxy, and satellite interface monitoring program on the RGW, the IGMP proxy is used when the satellite line is normal, and normal multicast routing processing is performed when the satellite line is abnormal As a result, a multicast communication system that performs asymmetric and dynamic routing was realized.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0022]
FIG. 1 shows the configuration of an embodiment of the present invention. The system of the present invention has a transmission gateway (SGW) 20 and an uplink station 21 on the satellite transmission station side, a reception gateway (RGW) 10, a satellite receiver (IRD) 11, and a local network on the satellite reception station side. And one or more personal computers (PC) 60 (a, b,...) Connected to the RGW via. The satellite transmitting station and the satellite receiving station can communicate from the transmitting station side to the receiving station side via the communication satellite 30 and can communicate bidirectionally via the Internet network 40. One or more servers 50 (a, b,...) Are also connected on the Internet network 40. These devices constituting the system may be operated by the same operating entity or may be a collection of devices operated by different operating entities.
[0023]
Asymmetric routing is realized by the functions of SGW and RGW. The RGW also has an asymmetric multicast routing control function.
[0024]
Here, the asymmetric routing process which is the premise of the present invention will be briefly described.
[0025]
An Internet system is a collection of interconnected routers. Routers exchange route information with each other, dynamically select a route for sending a packet, and send the packet between routers. In the currently used route information exchange protocol, the route itself is not directional, and if a route is available, it is assumed that the route can send packets in both directions. In order to use a one-way route in such an Internet system, an asymmetric route-using Internet system has been devised that can be realized only by a specific router having information about a one-way route.
[0026]
Assume that the PCa in the local network communicates with the server a on the Internet network. PCa sends an IP packet whose start address is PCa and end point address is server a, and server a sends an IP packet whose start point address is server a and end point address is PCa.
[0027]
The route information on the Internet network in the system is sent from the PCa to the server a when the communication between the PCa and the server a is not via the SGW but directly via the RGW from the Internet network. Packets are sent as PCa → RGW → Internet network → server a, and packets sent from server a to PCa are sent as server a → Internet network → RGW → PCa. This state is a normal routing path, and satellite Data communication paths are not used.
[0028]
If the path information on the Internet network in the system is that the communication between the PCa and the server a is via the SGW, the packet sent from the PCa to the server a is PCa → RGW → Internet network → SGW A packet sent from the Internet network to the server a and sent from the server a to the PCa is sent from the server a to the Internet network → SGW → uplink station → satellite → IRD → RGW → PCa, and an asymmetric path is used.
[0029]
With respect to this non-target path, a case where a packet is sent from the PCa to the server a and a case where a packet is sent from the server a to the PCa will be specifically described. First, when sending a packet from the PCa to the server a, first, the PCa creates an IP packet whose start address is PCa and whose end address is the server a, and sends it to the RGW via the local network. In the RGW, when it is desired to send a packet to the SGW, the IP packet to be sent is encapsulated in the IP packet and sent to the SGW via the Internet network (tunneling). The IP packet outside the capsule has a start address RGW and an end address SGW, and is sent according to a normal routing path. When the SGW receives the encapsulated IP packet, the SGW releases the capsule and sends it out on the Internet network. The packet sent here has an end address of server a, and arrives at server a according to a normal routing path.
[0030]
Next, the flow of packets sent from the server a to the PCa is shown. The server a returns a response (something) to the received IP packet, and the IP packet generated at this time is a packet whose start address is server a and whose end address is PCa. However, since the packet sent from the server a to the PCa is set on the Internet via the SGW, it always reaches the SGW via the Internet network. The SGW sends this packet to the uplink station, which stores the IP packet as the payload of the satellite data packet and sends it to the satellite. Data received from the satellite is received and sent to the RGW, and the RGW sends the packet onto the local network.
[0031]
The above is the operation when performing asymmetric routing.
[0032]
Here, the IP address system will be briefly described. The IP address is a fixed-length numerical value for uniquely identifying a device that communicates using the Internet protocol. The IP protocol version 4 has a binary number of 32 bits and the IP protocol version 6 has a binary number of 128 bits. It is. The IP address is divided into a network part and a host part, and an IP address whose host part value is all 0 is determined to represent the network itself. Which part of the fixed-length IP address is used as the network part is variable, and a value indicating the delimiter is called a subnet mask value. In addition, it is determined that an IP address having a specific value in the network section indicates a multicast address. In this case, the IP address does not indicate a specific device but indicates a multicast group. Refer to RFC for details of the IP address system.
[0033]
Now, when realizing such asymmetric routing, the RGW and SGW need to operate differently from the normal Internet system, so the details of the operation of these devices will be described below.
[0034]
First, the SGW 20 will be described.
[0035]
FIG. 2 shows a hardware configuration diagram of the SGW 20.
[0036]
The SGW 20 is a computer system that operates as a router, and includes a CPU 2001, a memory 2002, a disk controller 2003, a hard disk 2100, a console controller 2004, a console 2200, a network controller 2005a, and a network controller 2005b.
[0037]
The CPU 2001 controls the overall operation of the SGW 20. The memory 2002 stores programs and data. A disk controller 2003 controls the hard disk 2100. The hard disk 2100 stores programs and data such as the route information table 2150 shown in FIG. 4 and the multicast transfer information table 2250 shown in FIG. A console controller 2004 controls the console 2200. The console 2200 performs input / output with the user. The network controller 2005a communicates with the Internet network. The network controller 2005b performs communication with the uplink station 21.
[0038]
The uplink station 21 converts the IP packet into a data format used by the communication satellite 30 and transmits it to the communication satellite 30.
[0039]
Next, the operation of the SGW 20 will be described.
[0040]
The operation of the SGW 20 is realized by the CPU 2001 executing an SGW processing program 2500 described in detail later.
[0041]
FIG. 4 is a data configuration diagram of the route information table list 2100. The route information table list 2100 is a list in which zero or more route information tables 2150 are collected. If there is no route information table 2150 in the list, it indicates that there is no route information.
[0042]
The route information table 2150 includes a network address field 2151, a netmask field 2152, a destination address field 2153, and a destination controller name 2154.
[0043]
A network address field 2151 and a net mask field 2152 collectively indicate a network to be routed, a destination address field 2153 indicates an address of a router next to a route when a packet is sent to the network, and a destination controller name 2154 indicates Indicates the name of the controller that should send the packet when sending the packet to the network. In this embodiment, the value of the destination controller name field 2154 is either the network controller a or the network controller b. If the destination network can send a packet directly without going through a router, the value of the destination address field 2153 is zero.
[0044]
FIG. 5 is a data configuration diagram of the multicast forwarding information table list 2200. The multicast transfer information table list 2200 is a list in which zero or more multicast transfer information tables 2250 are collected. If there is no multicast transfer information table 2250 in the list, it indicates that there is no multicast transfer information.
[0045]
The multicast transfer information table 2250 includes a group address field 2251, a source address field 2252, a receiving controller name field 2253, and a copy destination controller name list 2254.
[0046]
A group address field 2251 indicates a multicast group address for packet transfer. The source address field 2252 indicates the IP address of a device that is sending a multicast packet to the group address. When this value is 0, it indicates an arbitrary source address. The source controller name field 2253 indicates the name of the controller that received the multicast packet, and the copy destination controller name list 2254 indicates zero or more lists of controller names that should be copied and transferred.
[0047]
FIG. 6 is a flowchart of the SGW processing program 2500.
[0048]
The SGW processing program 2500 first receives an IP packet from the network controller 2005a in step 2501, checks whether the IP packet received in step 2502 is a unicast packet, and proceeds to step 2510 if it is not unicast (multicast). Perform the process.
[0049]
In the case of unicast, it is checked in step 2503 whether the end address is itself. If it is itself, the process proceeds to step 2504 to check whether the protocol type is an encapsulated IP packet. In this case, the encapsulated IP packet is extracted (step 2505), and the process returns to step 2502 to process the extracted IP packet. If the packet is not an encapsulated IP packet, the process proceeds to step 2506 to process the received packet. In step 2506, various processes are performed according to the Internet protocol. Since this part of the process is not different from packet processing in a normal router, detailed description thereof is omitted. After step 2506, the process returns to step 2501 to process the next packet.
[0050]
If the end point address is other than itself, the route information table list 2100 is searched in step 2507 to search for the packet destination. Here, the routing information table 2150 including the end point address of the packet to be sent by the network indicated by the network address field 2151 and the net mask field 2152 is searched, and the value of the destination controller name field 2154 of the table becomes the destination. If the destination is found, the process proceeds to step 2509, where the packet is transmitted to the corresponding network controller. If not found, the packet is discarded and the process proceeds to the next packet processing (step 2514).
[0051]
If the packet is a multicast packet, the multicast transfer information table list 2520 is searched in step 2510 to search for a copy destination of the packet. Here, the copy destination controller name of the multicast forwarding information table 2250 in which the value in the group address field 2251 matches the end point address of the packet to be copied and the value in the source address field 2252 matches the start point address of the packet to be copied or 0. The value in the list field 2254 is the copy destination. If a copy destination is found, the packet is copied and transmitted to one or more network controllers corresponding to the copy destination controller name list, and the process proceeds to step 2513. If the copy destination is not found, the packet is not transmitted and the process proceeds to step 2513 as it is.
[0052]
In step 2513, the received multicast packet is processed according to the Internet protocol. Since this part of processing is not different from packet processing in a normal router, detailed description is omitted.
[0053]
The contents of the route information table list 2100 and the multicast forwarding information table list 2200 are updated with the packet processing contents performed in steps 2506 and 2513. The above is the processing content of the SGW processing program 2500.
[0054]
Next, the RGW 10 will be described.
[0055]
FIG. 3 shows a hardware configuration diagram of the RGW 10.
[0056]
The RGW 10 is a computer system that operates as a router, and includes a CPU 1001, a memory 1002, a disk controller 1003, a hard disk 1100, a console controller 1004, a console 1200, a network controller 1005a, a network controller 1005b, and an IRD controller 1007.
[0057]
The CPU 1001 controls the overall operation of the RGW 10. The memory 1002 stores programs and data. The disk controller 1003 controls the hard disk 1100. The hard disk 1100 stores programs and data such as the route information table 1150 shown in FIG. 7, the multicast transfer information table 1250 shown in FIG. 8, the multicast operation state management table shown in FIG. A console controller 1004 controls the console 1200. The console 1200 performs input / output with a user. The network controller 1005a communicates with the Internet network. The network controller 1005b communicates with the local network. The IRD controller 1007 controls the IRD 11.
[0058]
The IRD 11 receives data sent from the communication satellite 30, extracts an IP packet from the data, and sends it to the RGW 10.
[0059]
Next, the operation of the RGW 10 will be shown. The operation of the RGW 10 is realized by the CPU 1001 executing an RGW processing program (described later).
[0060]
FIG. 7 is a data configuration diagram of the route information table list 1100. Since the route information table list 1100 has the same configuration as the route information table list 2100 possessed by the SGW, description thereof is omitted.
[0061]
FIG. 8 is a data configuration diagram of the multicast forwarding information table list 1200. Since the multicast forwarding information table list 1200 also has the same configuration as the multicast forwarding information table list 2200 possessed by the SGW, description thereof is omitted.
[0062]
FIG. 9 is a data configuration diagram of the multicast operation state management table 1300. The multicast operation state management table 1300 holds an operation state indicating whether the network controller 1005a transmits / receives multicast packets.
[0063]
FIG. 10 is a flowchart of the RGW processing program 1500.
[0064]
In step 1501, the RGW processing program 1500 first receives IP packets from the network controller 1005a, the network controller 1005b, and the IRD controller 1007. If the packet received in step 1502 is a multicast packet, the process proceeds to step 1520. If it is a unicast packet, the process advances to step 1503 to determine whether the end point address is itself. In the case of itself, the process proceeds to step 1508, and the received packet is processed. Since the contents of step 1508 are not different from those of a normal router, the details are omitted. When the process is completed, the process returns to step 1501 to process the next packet.
[0065]
If the end address is not itself, the destination of the packet is searched using the route information table 1150 shown in FIG. Since the destination search process is the same as that in the SGW, details are omitted. If found, the process proceeds to step 1506 to check whether the destination is an IRD controller. In the case of the IRD controller, the packet is encapsulated for SGW (step 1509), and the process returns to step 1502 to process the encapsulated packet.
[0066]
If the destination is other than the IRD controller, the process proceeds to step 1507, the packet is transmitted to the corresponding network controller, and the process returns to step 1501. In step 1505, if the destination is not found, the packet is discarded.
[0067]
In the case of a multicast packet, the process proceeds to step 1520, and it is checked whether the source of the packet is the network controller 1005a. If so, the value of the multicast operation status management table 1300 is checked (step 1521). If the IRD controller 1007 is not operating normally, that is, if the IRD controller 1007 has not received an IP packet, the process proceeds to step 1522, and is stopped. If the IRD controller 1007 normally receives an IP packet, the packet is discarded in step 1531 and the process returns to step 1501.
[0068]
In step 1522, the packet copy destination is searched using the multicast forwarding information table 1250 shown in FIG. Since the copy destination search process is the same as that of the SGW, the details are omitted. If the copy destination is not found, the process proceeds to step 1530 immediately.
[0069]
When the copy destination is found, it is first checked whether or not the IRD controller is included in the copy destination (step 1524). If it is included, the packet is encapsulated to the SGW, and the encapsulated packet is put in the reception queue. So that it can be taken out in step 1501 (step 1525). If the copy destination is not found, the packet is discarded.
[0070]
Next, it is checked whether or not the network controller 1005a is included in the copy destination (step 1526). If it is included, the value of the multicast operation state management table 1300 is checked (step 1527). To the network controller 1005a, and proceeds to step 1529. If it is stopped, the process proceeds to step 1529 as it is.
[0071]
In step 1529, the packet is copied and transmitted to another corresponding network controller (in this case, the network controller 1005b), the packet received in step 1530 is processed, and the process returns to step 1501.
[0072]
Since the received packet processing content in step 1530 is the same as that of a normal router having an IGMP proxy function, the details are omitted.
[0073]
The above is the processing content of the RGW processing program 1500.
[0074]
FIG. 11 is a flowchart of the RGW monitoring program 1600.
[0075]
The RGW monitoring program 1600 always monitors whether or not the packet reception from the IRD controller is normally performed (step 1601). If the packet is normal, the contents of the multicast operation state management table 1300 are set to “stopped” (step 1603). If not normal, the contents of the multicast operation state management table 1300 are set to “in operation”.
[0076]
There are several methods for determining whether or not the packet reception from the IRD controller is normal. If the received signal level is below a certain threshold value for a certain period of time, it is regarded as abnormal. There is a method that considers an abnormality when the received data is not received for a certain period, but the present invention can be realized by using any method.
[0077]
Regarding the threshold value of the received signal level, it is appropriate to set the threshold value to 40 db, which is a signal level at which video can be normally received by CS satellite broadcasting.
[0078]
In the case of detection because periodic data cannot be received, the InternetDraft is a draft-ietf-udrr-ltunnel-02. It is desirable to adopt the rule prescribed in txt, “if pilot data sent periodically from the uplink station three times or more is not received, it is regarded as a line abnormality”. In this case, the memory 1002 of the RGW 10 stores a period in which pilot data is transmitted in advance, and the CPU determines that the IRD controller 1007 does not receive pilot data for a period corresponding to three periods.
[0079]
The above is the processing content of the RGW 10.
[0080]
Because of the functions of RGW and SGW as described above, multicast packets do not flow between the RGW and the Internet network while the RGW is normally receiving packets from the satellite, so the multicast route addressed to the RGW is only via the SGW. Thus, large-scale multicast processing can be performed using an IGMP proxy via the SGW. Further, while a specific RGW cannot normally receive a packet from a satellite, an asymmetric multicast routing is realized in which a multicast packet flows between the RGW and the Internet network and multicast routing via the Internet is performed.
[0081]
According to the present embodiment, a large-scale multicast called a satellite line can be performed, but a line that is partially incapable of communication due to factors such as weather is used, and a large-scale multicast routing is performed by using an IGMP proxy in the satellite line part. In addition, it is possible to realize a system that automatically performs multicast delivery via the Internet when the satellite line is disconnected.
[0082]
【The invention's effect】
As described above, according to the present invention, it is possible to realize a multicast communication system capable of switching a route on a large scale and dynamically without changing the routing method in the existing Internet network.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an embodiment of the present invention.
FIG. 2 is a hardware configuration diagram of the SGW.
FIG. 3 is a hardware configuration diagram of an RGW.
FIG. 4 is a configuration diagram of a route information table list (SGW).
FIG. 5 is a configuration diagram of a multicast transfer information table list (SGW).
FIG. 6 is a flowchart of an SGW processing program.
FIG. 7 is a configuration diagram of a route information table list (RGW).
FIG. 8 is a configuration diagram of a multicast transfer information table list (RGW).
FIG. 9 is a configuration diagram of a multicast operation state management table.
FIG. 10 is a flowchart of an RGW processing program.
FIG. 11 is a flowchart of an RGW processing program.
FIG. 12 is a flowchart of an RGW monitoring program.
[Explanation of symbols]
10 ... RGW,
11 ... IRD
20 ... SGW,
21 ... Uplink station,
30 ... Communication satellite,
40 ... Internet network,
50 ... Server
60 ... PC.

Claims (6)

Control to monitor the first controller to send and receive IP packets via the satellite receiver, and a second controller for sending and receiving IP packets over the Internet, whether the first controller is operating normally Having means,
When the control unit receives an IP packet whose type is multicast in a period in which the first controller does not normally receive an IP packet, a plurality of destinations to which the received IP packet is to be transferred to the communication device, to generate a copy of the IP packet thus received, the second through the controller sends the copy to the plurality of communication devices, and the first controller successfully receives IP packets A gateway device that discards the received IP packet when an IP packet of type multicast is received from the second controller in a certain period.   The gateway device according to claim 1, wherein the first controller is an IGMP proxy. A first controller that transmits and receives IP packets via a satellite receiver; a second controller that transmits and receives IP packets via the Internet ; a third controller that transmits and receives IP packets via a local network; Control means for monitoring whether or not one controller is operating normally,
The control means, the first, the type of IP packets received via the second or third controller is multicast, and the IP packet IP the received packet was received via the second controller in some cases, when it said first controller is determined to be operating normally discards the received packet, when the abnormality in the first controller determines that has occurred, the received The controller list corresponding to the addresses of a plurality of communication devices to be transferred is searched from the end point address and the start point address included in the received IP packet, and the received IP packet is copied and transmitted to the plurality of searched controllers. to, the gateway device. The gateway device according to claim 3 , wherein the control unit determines that the monitoring of whether or not the first controller is operating normally is abnormal when the reception level of the first controller is 40 db or less. The control means monitors whether or not the first controller is operating normally when a pilot signal periodically received by the first controller has not been received for a period corresponding to three cycles. The gateway device according to claim 3, wherein the gateway device is determined to be abnormal. In a multicast communication system comprising the Internet , a transmission-side gateway connected to the Internet and a communication satellite, a plurality of servers connected to the Internet , and a satellite receiver station connected to the communication satellite and the Internet ,
The satellite receiving station has a satellite receiver that receives a signal from the communication satellite, a receiving gateway device, and a local network,
The receiving-side gateway device includes a first controller that transmits and receives IP packets via the satellite receiver, a second controller that transmits and receives IP packets via the Internet , and IP packets via the local network. A third controller for transmitting and receiving, and a control means for monitoring whether or not the first controller is operating normally ,
The satellite transmission station is
Upon receiving the IP packets transmitted to the plurality of servers from said local network, if the Internet is transmitted to the plurality of servers via receives an IP packet transmitted to the local network from the server Set non-target path information received via the satellite transmitter station, the communication satellite and the satellite receiver;
The control means, the first, the type of IP packets received via the second or third controller is multicast, and the IP packet IP the received packet was received via the second controller in some cases, when if it is determined that the first controller is operating normally discards the IP packet thus received, abnormality in the first controller determines that has occurred, the Find the controller list corresponding to the address of the plurality of communication devices to be transferred from the end point address and source address included in the received IP packet, copies and transmits IP packet thus received against retrieved plurality of controllers A multicast communication system.

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