JP2011166360A - Multicast-tree calculation device, calculation method, and network system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently find a loop which is required, when constituting a redundant multicast tree. <P>SOLUTION: In a network system in which a plurality of nodes are connected through a link, such a loop as an originator node acts as both a base point, an end point is configured, and a path is configured in forward and reverse directions of the loop, which procedure is repeated to calculate two multicast trees containing all the modes. Such a method is applied to a multicast-tree calculation device. The multicast-tree calculation device includes a node grouping part which classifies, for grouping, nodes in lower stream for each next transfer destination node of the originator node, and an inter-group link extracting part which extracts a link whose nodes at both ends belong respectively to a different group as a candidate of the link used for constituting a loop, using the groups classified by the node grouping part. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a technique for calculating two redundant multicast trees.

  Multicast is a communication method in which a packet sent from a transmission source is copied in a network and distributed to a plurality of recipients, and is used to realize a broadcast type service such as video distribution on the network.

  However, in multicast, if a packet loss occurs in the network, it affects a large number of recipients, so a highly reliable technique is required.

  Therefore, a technique called reliable multicast has been developed in which a delivery confirmation function and a retransmission function are added to a normal multicast. However, the approach used to perform retransmission when packet loss occurs, which is used in reliable multicast, has the problem that viewing video stops during retransmission. Not suitable.

  As another approach to achieve high reliability, there is a method to ensure reliability by configuring two multicast trees to be redundant with each other and distributing packets redundantly on each multicast tree To do. Here, the term “redundant with each other” means that even if any node or link other than the source node is lost due to a failure, all nodes except the failed node are in one of the two multicast trees. Is connected to the transmission source node.

  FIG. 7 is a diagram illustrating an example of two multicast trees configured to be redundant with each other on a network in which nodes 11 to 20 are connected via a link.

  For example, when the node 12 fails, the node 13 cannot receive the packet via the tree 1. However, the node 13 can receive the packet via the tree 2 through which the packet flows in the order of the nodes 17, 16, 15, and 14.

  As described above, by performing video distribution using two multicast trees that are redundant to each other, even when a single node or link in the network has a single failure, it is possible to continue providing services without packet loss. .

  A method for calculating the mutually redundant multicast trees is disclosed in Non-Patent Document 1. In this method, a loop having a transmission source node as a base point and an end point is configured, and a path of tree 1 is extended along the loop, and a path of tree 2 is extended in the opposite direction.

  FIG. 8A is a diagram showing a loop configuration with a transmission source as a base point and an end point. In FIG. 8A, a loop of node 11 → node 12 → node 15 → node 16 → node 17 → node 11 is configured. Along this loop, the constructed tree is shown in FIG. 8B. In FIG. 8B, the path of the tree 1 is configured in the order of node 11 → node 12 → node 15 → node 16 → node 17. Further, in the opposite direction to this path, the path of the tree 2 is constructed in the order of node 11 → node 17 → node 16 → node 15 → node 12.

  In FIG. 8B, nodes 13, 14, 18, 19, and 20 have not yet been incorporated into any tree. At this time, the nodes (nodes 11, 12, 15, 16, and 17) that have already been incorporated in the tree at the end of FIG. 8B are regarded as virtual source nodes 21, and node 21 → node 13 → node 14 → A loop called node 21 is formed (FIG. 9A). Furthermore, the paths of trees 1 and 2 are formed along the forward and reverse directions of the loop (FIG. 9B).

  In this way, basically, two multicast trees including all nodes are formed by repeating a procedure of configuring a loop with a source node as a base point and an end point, and configuring paths in the forward and reverse directions of the loop. Can be configured.

Medard, M. and Finn, SG and Barry, RA: "Redundant trees for preplanned recovery in arbitrary vertex-redundant or edge-redundant graphs," IEEE / ACM Transactions on Networking (TON), Vol.7, No.5, pp .641-652, 1999.

  However, the method disclosed in Non-Patent Document 1 has the following problems.

  The first problem is that a specific method for how to find a loop used in the configuration of two multicast trees (redundant multicast trees) that are mutually redundant is not shown.

  The second problem is that depending on the loop configuration method, the difference in path length between the tree 1 and the tree 2 from the transmission source node to a certain node may become large. In this case, when a packet transmitted from the transmission source node reaches the node via the path on each tree, the difference in reception time becomes large. If it is considered to complement a packet lost on one path with a packet received on the other path, it is desirable that the difference in reception time is small in video distribution.

  Therefore, the present invention is to provide a multicast tree calculation device and calculation method, and a network system that can solve any of the above-described problems.

The multicast tree calculation apparatus of the present invention
In a network system in which a plurality of nodes are connected to each other via a link, a loop having a source node as a base point and an end point is configured, and a path is configured in each of forward and reverse directions of the loop, thereby repeating the plurality of the plurality of nodes. A multicast tree computing device for computing two multicast trees including all nodes,
For each next transfer destination node of the transmission source node, a node grouping unit that classifies and classifies each node downstream thereof, and
An inter-group link extraction unit that uses each group classified by the node grouping unit, and extracts a link in which each node at both ends of the link belongs to a different group as a link candidate used when configuring the loop; It is characterized by having.

The multicast tree calculation method of the present invention is:
In a network system in which a plurality of nodes are connected to each other via a link, a loop having a source node as a base point and an end point is configured, and a path is configured in each of forward and reverse directions of the loop, thereby repeating the plurality of the plurality of nodes. A multicast tree calculation method by a multicast tree calculation device for calculating two multicast trees including all nodes,
A grouping step for classifying and grouping each downstream node for each next transfer destination node of the transmission source node;
A link extracting step of using each of the classified groups and extracting a link in which each node at both ends of the link belongs to a different group as a link candidate to be used when configuring the loop, To do.

The network system of the present invention
A plurality of nodes connected to each other via a link;
Multicast tree calculation apparatus for calculating two multicast trees including all of the plurality of nodes by repeating a procedure of forming a loop having a transmission source node as a base point and an end point and forming paths in forward and reverse directions of the loop. And having
The multicast tree calculation device includes:
For each next transfer destination node of the transmission source node, a node grouping unit that classifies and classifies each node downstream thereof, and
An inter-group link extraction unit that uses each group classified by the node grouping unit, and extracts a link in which each node at both ends of the link belongs to a different group as a link candidate used when configuring the loop; It is characterized by having.

  According to the present invention, each node is grouped for each next transfer destination node of the transmission source node, and the link to which the nodes at both ends of the link belong to different groups is extracted as a link candidate to be used when forming a loop. .

  Therefore, an effect is obtained that it is possible to efficiently find a loop necessary when configuring a redundant multicast tree.

It is a block diagram which shows the structure of the multicast tree calculation apparatus of the 1st and 2nd embodiment of this invention. It is a flowchart which shows the process which calculates the redundant multicast tree in the multicast tree calculation apparatus of the 1st Embodiment of this invention. It is a figure which shows one state of the process which comprises the redundant multicast tree in the multicast tree calculation apparatus of the 1st Embodiment of this invention. It is a figure which shows the example by which the shortest path tree is comprised on the network where the cost value was provided to each link. It is a figure which shows one state of the process which comprises the redundant multicast tree in the multicast tree calculation apparatus of the 2nd Embodiment of this invention. It is a flowchart which shows the process which calculates | requires the path length evaluation value in the multicast tree calculation apparatus of the 2nd Embodiment of this invention. It is a figure which shows the example by which the redundant multicast tree is comprised on the network. It is a figure which shows one state of the process in which the redundant multicast tree in the method of a nonpatent literature 1 is comprised. It is a figure which shows one state of the process in which the redundant multicast tree in the method of a nonpatent literature 1 is comprised. It is a figure which shows one state of the process in which the redundant multicast tree in the method of a nonpatent literature 1 is comprised. It is a figure which shows one state of the process in which the redundant multicast tree in the method of a nonpatent literature 1 is comprised.

EMBODIMENT OF THE INVENTION Below, the form for implementing this invention is demonstrated with reference to drawings.
(1) First Embodiment FIG. 1 is a block diagram showing a functional configuration of a multicast tree calculation apparatus according to an embodiment of the present invention. The multicast tree calculation apparatus of the present invention is realized by a server or the like inside a node constituting the network system or outside the node.

  As shown in FIG. 1, the multicast tree calculation apparatus 100 of this embodiment includes a shortest path tree calculation unit 101, a node grouping unit 102, an inter-group link extraction unit 103, a link evaluation value calculation unit 104, a loop configuration unit 105, and a tree path configuration. Unit 106, topology information database 111, and redundant multicast tree storage unit 112.

  The shortest path tree calculation unit 101 refers to the topology information database 111 and calculates the shortest path tree from the transmission source node to each node.

  The node grouping unit 102 classifies each downstream node of each transmission source next transfer destination node, and performs grouping.

  The inter-group link extraction unit 103 refers to the topology information database 111 based on the result of grouping by the node grouping unit 102, and uses a link in which nodes at both ends of the link belong to different groups to form a loop. To extract as a link candidate.

  The link evaluation value calculation unit 104 evaluates each link extracted by the inter-group link extraction unit 103 according to a standard. Here, the evaluation is based on the path length from the transmission source node. That is, the link evaluation value calculation unit 104 calculates a path length evaluation value for each link, and selects a link having the lowest path length evaluation value.

  The loop forming unit 105 uses the link selected by the link evaluation value calculating unit 104 to form a necessary loop in the redundant multicast tree construction stage.

  The tree path configuration unit 106 configures the paths of the trees 1 and 2 along the forward and reverse directions of the loop configured by the loop configuration unit 105.

  The topology information database 111 is a database in which topology information of a network to be calculated is stored. The storage of topology information in the topology information database 111 may be manually input in advance, or information collected and managed in another system by network management software or the like may be input.

  When all the nodes in the network are included in the trees 1 and 2 configured by the tree path configuration unit 106, the redundant multicast tree storage unit 112 stores the trees 1 and 2 as the final output result of the redundant multicast tree.

  Hereinafter, the process of calculating a redundant multicast tree in the multicast tree calculation apparatus 100 of the present embodiment will be described with reference to the flowchart of FIG.

  As shown in FIG. 2, first, the shortest path tree calculation unit 101 refers to the topology information database 111 and calculates the shortest path tree from the transmission source node to each node using the Dijkstra method (step A1). Here, another shortest path tree calculation algorithm may be used instead of the Dijkstra method.

  The subsequent processing is performed based on the shortest path tree calculated in step A1, but it is not always necessary to use the shortest path tree. That is, as long as all nodes are included and the transmission source node is a base point, the subsequent processing may be performed using another tree.

  Next, the node grouping unit 102 groups each node except the transmission source node for each next transfer destination of the transmission source node (step A2). The arrows in FIG. 3 represent the shortest path tree with the transmission source node 11 as a base point. Here, the two nodes of the node 12 and the node 17 are the next transfer destinations of the transmission source node 11, and each node is classified into either a group 31 including the node 12 or a group 32 including the node 17. Whether each node other than the nodes 12 and 17 is classified into any group is determined as follows. The nodes on the downstream side of the node 12 on the shortest path tree are classified into the same group 31 as the node 12. That is, the nodes 13, 14, 15, and 20 are classified into the group 31. Similarly, the nodes 16, 18, and 19 on the downstream side of the node 17 are classified into the group 32.

  Next, the inter-group link extraction unit 103 extracts links in which nodes at both ends of the link belong to different groups (step A3). In the example of FIG. 3, the link between the nodes 15 and 16 and the link between the nodes 19 and 20 correspond to this condition.

  Next, the link evaluation value calculation unit 104 calculates a path length evaluation value for each link extracted in step A3, and selects a link having the lowest path length evaluation value (step A4). Here, the path length evaluation value is a value representing the path length of a loop configured using this link, and the lower this value, the smaller the difference in path length from the transmission source in the two configured trees. . This path length evaluation value is calculated as follows. First, for each node at both ends of the link, the path length from the transmission source node is obtained on the shortest path tree. The sum of these values is the path length evaluation value for that link. In the example of FIG. 3, regarding the link between the nodes 15 and 16, the path length from the transmission source node 11 to the node 15 is 2, and the path length to the node 16 is 2, so the path length evaluation value of this link is 4. On the other hand, for the link between the nodes 19 and 20, the path length from the transmission source node 11 to the node 19 is 3, and the path length to the node 20 is 4, so the path length evaluation value of this link is 7. In this case, the link between the nodes 15 and 16 having a lower path length evaluation value is selected.

  Next, the loop configuration unit 105 configures a loop using the link selected in step A4 (step A5). In the example of FIG. 3, since the link between the nodes 15 and 16 is selected, a loop of node 11 → node 12 → node 15 → node 16 → node 17 → node 11 is formed.

  Next, the tree path configuration unit 106 configures the trees 1 and 2 along the loop configured in step A5 (step A6). In the example of FIG. 3, the tree 1 is configured as node 11 → node 12 → node 15 → node 16 → node 17, and the tree 2 is configured as node 11 → node 17 → node 16 → node 15 → node 12.

  Here, when all the nodes in the network are incorporated in the tree (Yes in step A7), the redundant multicast tree storage unit 112 converts the trees 1 and 2 configured in the tree path configuration unit 106 into the redundant multicast tree. Is stored as the final output result, and the process ends.

  On the other hand, when all the nodes in the network are not incorporated in the tree (No in step A7), the process proceeds to step A8.

  In step A8, the node grouping unit 102 regards the node group incorporated in the trees 1 and 2 in the previous steps as one virtual source node, and groups each node for each next transfer destination of the source node. After that, the process returns to step A3. This step A8 is the same operation as step A2 except that a virtual source node is used instead of the actual source node.

  In this embodiment, the link evaluation value calculation unit 104 uses the path length evaluation value that is the length of the path from the transmission source node as a reference for evaluating the link. You may use the cost evaluation value which is the total value of the cost value of a link. For example, in a network in which a cost value is assigned to each link as shown in FIG. 4, the cost evaluation value of the link between the node 15 and the node 16 is obtained as follows. The total cost of the links on the path from the node 11 to the node 15 is 7 with 2 + 5. Further, the total cost of the links on the path from the node 11 to the node 16 is 6 with 2 + 4. That is, the cost evaluation value of the link between the node 15 and the node 16 is 13.

  Further, a link delay value may be used instead of the link cost value. In this case, the delay of each link is measured in another system, the value is substituted for the cost value, and an evaluation value obtained by performing the same calculation as in the case of the cost value is used.

  As described above, in this embodiment, based on a certain tree, each node is grouped for each next transfer destination node of the transmission source node, and a link is configured such that nodes at both ends of the link belong to different groups. This is extracted as a link candidate to be used.

  Therefore, an effect is obtained that it is possible to efficiently find a loop necessary when configuring a redundant multicast tree.

  Further, in the present embodiment, for each link extracted above, evaluation is performed using the path length from the transmission source node of the nodes at both ends of the link, the cost value and the delay of each link included in the path, and Based on the evaluation result, the link to be used when configuring the loop is selected.

As a result, it is possible to find a loop in which the difference between the two path lengths from the source node to each node is as small as possible.
(2) Second Embodiment The configuration itself of the multicast tree calculation apparatus 100 of this embodiment is the same as that of the first embodiment.

  In the first embodiment, the path length from the transmission source is used as the evaluation value. However, when a virtual transmission source node is considered in step A8 in FIG. 2, a packet transmitted from this virtual transmission source node may be transmitted at a different timing depending on the path.

  In the example of FIGS. 9A and 9B, a virtual source node 21 → node 13 → node 14 is configured as a loop, and the trees 1 and 2 are extended along this loop. At this time, the packet actually reaches the node 13 via the node 11 and the node 12 via the tree 1, and the packet reaches the node 14 via the nodes 17, 16, and 15 via the tree 2. Reach. However, the path length included in the virtual transmission source node 21 is not considered in the path length evaluation value used in the first embodiment.

  Therefore, in this embodiment, a new path length evaluation value that takes into account the path length of the portion included in the virtual transmission source node is introduced.

  Hereinafter, the process for obtaining the path length evaluation value of each link in the multicast tree calculation apparatus 100 of the present embodiment will be described with reference to the flowchart of FIG. The process in FIG. 6 is a process performed by the link evaluation value calculation unit 104 in step A4 in FIG.

  As shown in FIG. 6, first, assuming that the nodes at both ends of the link l are n1 and n2, respectively, the path lengths p (n1) and p (n2) from the virtual source node to n1 and n2 are respectively calculated. Obtain (step B1). In the example of FIG. 5, for example, when the link between the nodes 13 and 14 is 1, the node 13 is n1, and the node 14 is n2. At this time, p (n1) and p (n2) are each 1.

  Next, nodes that reach the upstream side from n1 and n2 on the shortest path tree and find nodes already included in the redundant multicast tree are set as n'1 and n'2 (step B2). In the example of FIG. 5, when the node 13 which is n1 is traced along the shortest path tree, the node 12 in the virtual transmission source node 21 is reached. Therefore, in this case, the node 12 is set to n′1. Similarly, the node 15 becomes n′2.

  Next, the path lengths from the actual transmission source node to the node n′1 on the redundancy trees 1 and 2 are set to p1 (n′1) and p2 (n′1) (step B3). In the example of FIG. 5, the path length from the actual transmission source node 11 to the node 12 that is n′1 is 1 on the tree 1 and 4 on the tree 2, so these values are p1 (n′1 ), P2 (n′1).

  Next, the same processing as in step B3 is performed for n′2, and the path lengths from the actual transmission source node to the node n′2 on the redundancy trees 1 and 2 are represented by p1 (n′2), p2 ( n′2) (step B4). In the example of FIG. 5, the path length from the real transmission source node 11 to the node 15 that is n′2 is 2 on the tree 1 and 3 on the tree 2, so these values are p1 (n′2 ), P2 (n′2).

  Next, the value of p1 (n′1) + p2 (n′2) is compared with the value of p1 (n′2) + p2 (n′1) (step B5). In the example of FIG. 5, if the link between the links 13 and 14 is selected later to form a loop, the path from the node 12 to the node 13 needs to extend the tree 1 instead of the tree 2. On the other hand, the path from the node 15 to the node 14 needs to extend the tree 2 instead of the tree 1. If this is reversed, the link between the nodes 12 and 15 becomes a single point of failure, and if this fails, the nodes 13 and 14 cannot receive packets from either of the trees 1 and 2. Such a state depends on which node in step B1 is n1. In order to prevent such a situation, in the situation where a single failure point occurs, in order to use the fact that the sum of the path lengths becomes larger than the situation where no failure occurs, in this step B5, the path from the real node is used. Comparing sums of lengths.

  In step B5, if the former value is smaller, the difference between the path lengths of p1 (n'1) and p2 (n'2) is taken, and the value is set to d (step B6). In the example of FIG. 5, since the path lengths are 1 and 4, the value of d is 3.

  In step B5, if the latter value is larger or the same value, the difference between the path lengths of p1 (n′2) and p2 (n′1) is taken, and the value is set as d (step B7). ).

  Finally, the path length evaluation value E (l) of the link l is obtained as E (l) ← p (n1) + p (n2) + d (step B8). In the example of FIG. 5, since the values of p (n1), p (n2), and d are 1, 1, and 4, respectively, E (l) is 6.

  As described above, in the first embodiment, only the values of p (n1) and p (n2) are used. However, the present embodiment is different in that the value of d is also used.

As described above, in the present embodiment, each extracted link is evaluated in consideration of the path length included in the virtual source node, so two paths from the source node to each node are performed. The effect is obtained that a loop with a small difference in length can be found with higher accuracy.
(Appendix 1)
In a network system in which a plurality of nodes are connected to each other via a link, a loop having a source node as a base point and an end point is configured, and a path is configured in each of forward and reverse directions of the loop, thereby repeating the plurality of the plurality of nodes. A multicast tree computing device for computing two multicast trees including all nodes,
For each next transfer destination node of the transmission source node, a node grouping unit that classifies and classifies each node downstream thereof, and
An inter-group link extraction unit that uses each group classified by the node grouping unit, and extracts a link in which each node at both ends of the link belongs to a different group as a link candidate used when configuring the loop; A multicast tree calculation device characterized by comprising:
(Appendix 2)
It further comprises a link evaluation value calculation unit that evaluates each link extracted by the inter-group link extraction unit according to a criterion and selects a link to be used when configuring the loop based on the evaluation result. The multicast tree calculation apparatus according to Supplementary Note 1.
(Appendix 3)
The multicast tree calculation apparatus according to appendix 2, wherein the link evaluation value calculation unit uses a path length of each path from the transmission source node to each node at both ends of the link as a reference for evaluating the link.
(Appendix 4)
The link evaluation value calculation unit also uses a path length difference of each path from the transmission source node to each node upstream of each node at both ends of the link as a reference for evaluating the link. Multicast tree calculator.
(Appendix 5)
The link evaluation value calculation unit uses a cost value of each link included in each path from the transmission source node to each node at both ends of the link as a reference for evaluating the link. Multicast tree calculator.
(Appendix 6)
The link evaluation value calculation unit uses a link delay of each link included in each path from the transmission source node to each node at both ends of the link as a reference for evaluating the link. Multicast tree calculator.
(Appendix 7)
A shortest path tree calculation unit for calculating a shortest path tree for calculating a shortest path tree from the source node to each node;
7. The multicast tree calculation apparatus according to any one of appendices 1 to 6, wherein the node grouping unit performs the grouping by using the shortest path tree calculated by the shortest path tree calculation unit.
(Appendix 8)
The multicast tree calculation apparatus according to appendix 7, wherein the shortest path tree calculation unit calculates a shortest path tree using a Dijkstra method.
(Appendix 9)
In a network system in which a plurality of nodes are connected to each other via a link, a loop having a source node as a base point and an end point is configured, and a path is configured in each of forward and reverse directions of the loop, thereby repeating the plurality of the plurality of nodes. A multicast tree calculation method by a multicast tree calculation device for calculating two multicast trees including all nodes,
A grouping step for classifying and grouping each downstream node for each next transfer destination node of the transmission source node;
A link extracting step of using each of the classified groups and extracting a link in which each node at both ends of the link belongs to a different group as a link candidate to be used when configuring the loop, Multicast tree calculation method to be performed.
(Appendix 10)
The multicast tree according to appendix 9, further comprising a link selection step of evaluating each of the extracted links according to a criterion and selecting a link to be used when configuring the loop based on the evaluation result. Method of calculation.
(Appendix 11)
11. The multicast tree calculation method according to appendix 10, wherein in the link selection step, a path length of each path from the transmission source node to each node at both ends of the link is used as a reference for evaluating the link.
(Appendix 12)
The multicast according to appendix 11, wherein the link selection step also uses a path length difference of each path from the transmission source node to each node upstream of each node at both ends of the link as a reference for evaluating the link. Tree calculation method.
(Appendix 13)
The multicast according to appendix 10, wherein the link selection step uses a cost value of each link included in each path from the transmission source node to each node at both ends of the link as a reference for evaluating the link. Tree calculation method.
(Appendix 14)
The multicast according to appendix 10, wherein the link selection step uses a link delay of each link included in each path from the transmission source node to each node at both ends of the link as a reference for evaluating the link. Tree calculation method.
(Appendix 15)
A shortest path tree calculation step of calculating a shortest path tree for calculating a shortest path tree from the source node to each node;
15. The multicast tree calculation method according to any one of appendices 9 to 14, wherein, in the grouping step, the grouping is performed using the calculated shortest path tree.
(Appendix 16)
16. The multicast tree calculation method according to appendix 15, wherein the shortest path tree calculation unit calculates a shortest path tree using a Dijkstra method.
(Appendix 17)
A plurality of nodes connected to each other via a link;
Multicast tree calculation apparatus for calculating two multicast trees including all of the plurality of nodes by repeating a procedure of forming a loop having a transmission source node as a base point and an end point and forming paths in forward and reverse directions of the loop. And having
The multicast tree calculation device includes:
For each next transfer destination node of the transmission source node, a node grouping unit that classifies and classifies each node downstream thereof, and
An inter-group link extraction unit that uses each group classified by the node grouping unit, and extracts a link in which each node at both ends of the link belongs to a different group as a link candidate used when configuring the loop; And a network system.

  The present invention can be applied to an apparatus for calculating a path configured on a carrier network or the like.

11 to 20 nodes 100 multicast tree calculation device 101 shortest path tree calculation unit 102 node grouping unit 103 intergroup link extraction unit 104 link evaluation value calculation unit 105 loop configuration unit 106 tree path configuration unit 111 topology information database 112 redundant multicast tree storage unit

Claims (10)

In a network system in which a plurality of nodes are connected to each other via a link, a loop having a source node as a base point and an end point is configured, and a path is configured in each of forward and reverse directions of the loop, thereby repeating the plurality of the plurality of nodes. A multicast tree computing device for computing two multicast trees including all nodes,
For each next transfer destination node of the transmission source node, a node grouping unit that classifies and classifies each node downstream thereof, and
An inter-group link extraction unit that uses each group classified by the node grouping unit, and extracts a link in which each node at both ends of the link belongs to a different group as a link candidate used when configuring the loop; A multicast tree calculation device characterized by comprising:   It further comprises a link evaluation value calculation unit that evaluates each link extracted by the inter-group link extraction unit according to a criterion and selects a link to be used when configuring the loop based on the evaluation result. The multicast tree calculation device according to claim 1.   The multicast tree calculation apparatus according to claim 2, wherein the link evaluation value calculation unit uses a path length of each path from the transmission source node to each node at both ends of the link as a reference for evaluating the link. .   The link evaluation value calculation unit also uses a path length difference of each path from a transmission source node to each node upstream of each node at both ends of the link as a reference for evaluating a link. 3. Multicast tree calculation device.   The link evaluation value calculation unit uses a cost value of each link included in each path from the transmission source node to each node at both ends of the link as a reference for evaluating the link. The multicast tree calculation device described.   The link evaluation value calculation unit uses a link delay of each link included in each path from the transmission source node to each node at both ends of the link as a reference for evaluating the link. The multicast tree calculation device described. A shortest path tree calculation unit for calculating a shortest path tree for calculating a shortest path tree from the source node to each node;
7. The multicast tree calculation apparatus according to claim 1, wherein the node grouping unit performs the grouping using the shortest path tree calculated by the shortest path tree calculation unit.   The multicast tree calculation apparatus according to claim 7, wherein the shortest path tree calculation unit calculates a shortest path tree using a Dijkstra method. In a network system in which a plurality of nodes are connected to each other via a link, a loop having a source node as a base point and an end point is configured, and a path is configured in each of forward and reverse directions of the loop, thereby repeating the plurality of the plurality of nodes. A multicast tree calculation method by a multicast tree calculation device for calculating two multicast trees including all nodes,
A grouping step for classifying and grouping each downstream node for each next transfer destination node of the transmission source node;
A link extracting step of using each of the classified groups and extracting a link in which each node at both ends of the link belongs to a different group as a link candidate to be used when configuring the loop, Multicast tree calculation method to be performed. A plurality of nodes connected to each other via a link;
Multicast tree calculation apparatus for calculating two multicast trees including all of the plurality of nodes by repeating a procedure of forming a loop having a transmission source node as a base point and an end point and forming paths in forward and reverse directions of the loop. And having
The multicast tree calculation device includes:
For each next transfer destination node of the transmission source node, a node grouping unit that classifies and classifies each node downstream thereof, and
An inter-group link extraction unit that uses each group classified by the node grouping unit, and extracts a link in which each node at both ends of the link belongs to a different group as a link candidate used when configuring the loop; And a network system.

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