Method and device for multicast communication path calculation

BACKGROUND OF THE INVENTION

1. Field of the Invention

0001The present invention relates to a technology for calculating multicast paths in a multicast communication network. More particularly, the present invention relates to a technology for calculating efficient multicast paths from a source node to each destination node for transmitting multicast traffic.

2. Description of the Related Art

0002Attention is being given to multicast communication for delivering moving images and voices to a plurality of specific users on a computer network. In the multicast communication network, a source node is connected to destination nodes via a rendezvous point node at which a path is divided into a plurality of paths to the destination nodes. At the rendezvous point node, multicasted data are copied and transferred over the plurality of paths to the destination nodes.

0003As for unicast communication in which a source node is connected to each destination node in a one-to-one relationship, the source node needs to prepare copies of information the number of which is the same as that of the destination nodes. Thus, by using multicast communication, the amount of information transmitted over the network can be decreased.

0004In multicast communication, a set of destination nodes is called a multicast group, and a set of multicast paths are established from the source node to each node of the multicast group. The multicast paths are established such that the source node is connected to all of the destination nodes that belong to the multicast group. A user who wants to obtain information that is transferred to a multicast group joins the multicast group. Thus, the multicast paths change according to the joining state of users.

0005There are a plurality of applications that utilize multicast communication, such as videoconferences, on-line games, and applications for delivering moving images such as movies and television programs. As to the videoconferences and the on-line games, destination nodes correspond to a plurality of users, in addition, each of the plurality of users may become a source node when responding to received data. As for such applications, attention is being given to a technology for equalizing times at which each user responds to data transmitted via the multicast network so that occasion of response is equalized for each user. For equalizing the response times, there is a technology in which multicast paths from the source node to each destination node are designed such that similar delay arises in each multicast path.

0006Differences among delays along the paths between the source node and each destination node are called delay variation among users. However, currently, algorithms for calculating minimum delay paths are mainly used, and there is no communication method adopting an algorithm for calculating paths in consideration of the delay variation among users. There are following conventional examples of algorithms for decreasing delay variation among users: <ul id="ul0001" list-style="none" compact="compact"><li>Document 1: <nplcit id="ncit0001" npl-type="s"><text>G,Rouskas, et.al., "Multicast Routing With End-to-End Delay and Delay Variation Constraints", IEEE Journal on Selected Areas in Communication, Vol 15, NO.3, Apr. 1997</text></nplcit>.</li><li>Document 2: <nplcit id="ncit0002" npl-type="s"><text>Pi-Rong Sheu, et.al., "A Fast and Efficient Heuristic Algorithm for the Delay and Delay Variation Bound Multicast Tree Problem", IEEEICC, 2001</text></nplcit>.</li></ul>

0007The document 1 is the first document that try to solve the above-mentioned problem. Following is the method for calculating paths according to the document 1.

0008First, each minimum delay path between a source node and each destination node is calculated, and a delay w of a path that has maximum delay among the minimum delay paths is checked. Assuming that <img file="EP1515495B1_D0001.tif" /> is delay variation acceptable by an application (this parameter means that the difference of the maximum end-to-end delay and the minimum end-to-end delay among the paths from the source node to all the destination nodes has to be kept within <img file="EP1515495B1_D0002.tif" />), paths which satisfy di ≦ w - <img file="EP1515495B1_D0003.tif" /> are adopted in which di indicates a delay between the source node and the destination node i. As for a path that is not adopted, a path that satisfies the condition is searched for by using an algorithm for searching for a k-th shortest path. Then, the searched path is adopted. Assuming that the number of the destination nodes is m and the number of nodes in the network is n, then the computation time complexity of this method is O (kmn³).

0009In the document 2, a method is proposed in which the time complexity is smaller than that of the document 1, so that calculation time for obtaining paths is shortened. The method of calculation is as follows.

0010The multicast paths include an one-to-one unicast path between the source node and a rendezvous point node (that can be also called as a central node) and one-to-many paths between the rendezvous point node and each destination node. In the method of document 2, the minimum delay between each destination node and each other node in the network is calculated first. Next, for each node in the network, associated multicast delay variation between the node and each destination node is calculated. Then, a node with the minimum multicast delay variation is selected as the rendezvous point node. Finally, each destination node is connected to this rendezvous point node through the minimum delay path, the source node is also connected to the rendezvous point node through the minimum delay path. The time complexity of this method is O(n³), and calculation faster than document 1 can be realized.

0011There are following problems in the above-mentioned conventional technologies.

0012According to the above-mentioned two calculation methods, the time complexity becomes large. Since applications that require decrease of the delay variation generally need real-time processing, it is desirable to decrease time complexity. In addition, it is requested to decrease transmission delay for the real-time applications. As for the method of the document 2 in which rendezvous point is provided, it is pointed out that realizing minimum delay path is generally difficult. Therefore, there is a problem in that the transmission delay to each destination node becomes generally large according to the method of the document 2.

0013When establishing the multicast communication network, from the viewpoint of establishing multicast paths efficiently between a source node and each destination node, a minimal tree problem is known in which total cost of the multicast paths is minimized. This problem is called a Steiner tree problem. The Steiner tree problem is known as NP problem in which it is impossible to calculate paths that form an optimal minimal tree from the source node to each destination node in a finite calculation time if the size of the network is large.

0014Although to obtain the ideal solution of the Steiner problem is NP hard, a calculation method for heuristically deriving an approximated solution is proposed as KMB communication method (refer to document 3 : <nplcit id="ncit0003" npl-type="s"><text>L.Kou, G.Markowsky, and L.Berman, "A Fast Algorithm for Steiner Tree," Acta Informatica 15, 1981,pp.141-145</text></nplcit>.).

0015In this method, the source node and the destination node group are extracted, and a graph formed by edges each corresponding to a shortest path between extracted nodes is constructed first. From the constructed graph, a minimal spanning tree is constructed. Then, each edge of the spanning tree is replaced by shortest path of the input graph to construct a subgraph. Then, a minimal spanning tree is found from the subgraph. Finally, multicast paths (Steiner tree) are constructed by deleting unnecessary edges from the spanning tree.

0016However, according to the multicast path calculation method of the document 3, although the transfer cost of the whole multicast paths can be minimized, the method does not consider the cost (delay) variation from the source node to each destination node. Thus, the cost variation becomes very large. Therefore, when applying this calculation method to the real-time application that does not permit large delay variation, there is a large problem in that large delay variation occurs among multicast receivers.

SUMMARY OF THE INVENTION

0017The invention provides a multicast communication path calculation method as defined in Claim 1, multicast communication path calculation apparatus as defined in Claim 4, and a corresponding computer program and a computer readable medium as defined in Claims 8 and 9.

0018The present invention can improve computation time complexity for calculating multicast paths and decrease delay variation among users. It can decrease delay variation among users while suppressing total cost of multicast paths.

0019According to the present invention, the method includes a calculation process for minimizing total tree cost, and a calculation process for shaping the tree such that delay variation from the source node to each destination node is small. Thus, multicast paths can be obtained in which the delay variation can be decreased while total cost of the multicast paths is minimized.

0020In addition, in the calculated minimal tree, the rendezvous point node for optimizing the delay variation can be dynamically set, which is different from the conventional technology.

BRIEF DESCRIPTION OF THE DRAWINGS

0021Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: <ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a figure for explaining the outine of an example;</li><li><figref idref="f0002">Fig.2</figref> is a block diagram of a multicast communication path calculation apparatus according to the first example;</li><li><figref idref="f0003">Fig.3</figref> shows a configuration example of.a path calculation module 121 of the first example;</li><li><figref idref="f0004">Fig.4</figref> shows a configuration of a multicast communication path setting apparatus according to the first example;</li><li><figref idref="f0005">Fig.5</figref> is a flowchart showing processes for calculating paths in the multicast communication path calculation apparatus according to the first example;</li><li><figref idref="f0006">Fig.6</figref> shows an example of a multicast network according to the first example;</li><li><figref idref="f0007">Fig.7</figref> shows minimum delay paths from a source node to each destination node;</li><li><figref idref="f0008">Fig.8</figref> shows paths connecting multicast communication path setting apparatus 20, nodes A, C, E, G, I and destination node 3;</li><li><figref idref="f0009">Fig.9</figref> shows a minimum delay path from the multicast communication path setting apparatus 20 to the node E and minimum delay paths from the node E to each destination node 1-5;</li><li><figref idref="f0010">Fig.10</figref> shows an outline of the procedure for multicast communication path calculation according to a first embodiment;</li><li><figref idref="f0011">Fig.11</figref> shows a configuration of a path calculation module of the multicast communication path calculation apparatus according to the embodiment;</li><li><figref idref="f0012">Fig.12</figref> is a flowchart of the multicast communication path calculation method according to the embodiment;</li><li><figref idref="f0013">Fig.13</figref> shows an example of a network graph to which the procedure of the embodiment is applied;</li><li><figref idref="f0013">Fig.14</figref> shows a network graph in which the source node is excluded;</li><li><figref idref="f0014">Fig.15</figref> shows a shortest path graph among destination nodes;</li><li><figref idref="f0014">Fig.16</figref> is a minimal spanning tree obtained from the shortest path graph among destination nodes;</li><li><figref idref="f0015">Fig.17</figref> shows an intermediate result for constructing a minimal spanning tree considering intermediate nodes;</li><li><figref idref="f0015">Fig.18</figref> shows a subgraph that forms a minimal spanning tree considering intermediate nodes;</li><li><figref idref="f0016">Fig.19</figref> shows minimal spanning tree of the subgraph;</li><li><figref idref="f0016">Fig.20</figref> is a minimal tree including destination nodes;</li><li><figref idref="f0017">Fig.21</figref> shows multicast paths from the source node to each destination node in which a rendezvous point node is provided;</li><li><figref idref="f0018">Fig.22</figref> shows a configuration example of a multicast communication path calculation system according to the embodiment;</li><li><figref idref="f0019">Fig.23</figref> shows a network for evaluating the multicast communication, path calculation system according to the embodiment;</li><li><figref idref="f0020">Fig.24</figref> shows a performance evaluation graph of multicast communication path cost;</li><li><figref idref="f0020">Fig.25</figref> shows a performance evaluation graph of transfer delay variation.</li></ul>

DETAILED DESCRIPTON OF THE PREFERRED EMBODIMENT

0022In the following, an embodiment of the present invention is described with reference to <figref idref="f0010 f0011 f0012 f0013 f0014 f0015 f0016 f0017 f0018 f0019 f0020">Figures 10-25</figref>.

0023First, an example of a multicast communication path establishment method is described with reference to figures. This is outside the scope of the invention and is provided to assist in understanding the invention.

0024<figref idref="f0001">Fig.1</figref> is a figure for explaining the outline of the example. A multicast network indicates a plurality of nodes each including a multicast transfer apparatus. A multicast communication path calculation apparatus is provided in a node, and a multicast communication path setting apparatus is provided in a node. The multicast communication path calculation apparatus may be separate from the nodes that form the multicast network. In addition, the multicast communication path calculation apparatus and the multicast communication path setting apparatus may be included in one node.

0025(1) A multicast transfer apparatus (node) in the network collects network measurement information such as delay of data transfer on each link. (2) Then, each multicast transfer apparatus sends the network measurement information to the multicast communication path calculation apparatus and the multicast communication path setting apparatus. (3) When the need for setting multicast paths for transferring multicast data arises, the multicast communication path setting apparatus and the multicast communication path calculation apparatus performs setting of the multicast paths by performing after-mentioned processes. In the first embodiment, the multicast transfer apparatus has a capability for collecting network measurement information. The multicast communication path calculation apparatus has a capability for calculating the paths. The multicast communication path setting apparatus has capability for setting multicast paths on the network.

0026When the multicast communication path setting apparatus and the multicast communication path calculation apparatus are separate apparatuses, the multicast communication path setting apparatus requests the multicast communication path calculation apparatus to calculate the multicast paths. (4) When the multicast communication path setting apparatus and the multicast communication path calculation apparatus are the same apparatus, the multicast communication path setting apparatus instructs its path calculation module to calculate the paths. (5) Then, the path calculation module of the multicast communication path setting apparatus or the multicast communication path calculation apparatus calculates the multicast paths according to the collected information. (6) The calculation result is sent to a path setting module of the multicast communication path setting apparatus. (7) Then, the multicast communication path setting apparatus that receives the calculation result sets the multicast paths.

0027In the capability for collecting the network measurement information, the network measurement information is collected by using a protocol having functions for exchanging network measurement information between adjacent nodes, such as OSPF-TE (Open Shortest Path First-Traffic Engineering) and IS-IS-TE (Intermediate system-Intermediate system-Traffic Engineering) and the like.

0028The multicast communication path calculation apparatus includes a function for receiving network measurement information from a multicast transfer apparatus, a packet transfer function for sending the calculation result, a program for realizing an algorithm used for path calculation, a recording medium for storing the network measurement information, the path calculation program and path calculation result, and a path calculation function for realizing path calculation. The multicast communication path calculation apparatus may receive the network measurement information from the multicast communication path calculation apparatus instead of collecting the network measurement information by itself.

0029The path calculation program used in this example has a function for calculating minimum delay paths from a source node to each destination node, a function for calculating delays from a candidate node to each destination node in which the candidate node exists on a path having the maximum delay among the minimum delay paths, and a function for selecting a rendezvous point node among candidate nodes.

0030According to the above-mentioned functions, time complexity for selecting a candidate node as the rendezvous point node can be decreased comparing with the method of document 2.

0031In addition, by selecting the rendezvous point node according to a selection criterion effective for decreasing delay variation, the delay variation can be decreased comparing with conventional multicast path calculation method in which minimum delay paths are selected as multicast paths.

0032In addition, since the existing capability of collecting network measurement information can be used, the multicast paths can be calculated easily. It is easy for the multicast communication path calculation apparatus to obtain the network measurement information, and there is an advantage in that it is unnecessary to develop a new protocol to collect the network measurement information.

0033In the following, the multicast communication path calculation apparatus and the multicast communication path setting apparatus are described.

0034<figref idref="f0002">Fig.2</figref> is a block diagram of the multicast communication path calculation apparatus. In <figref idref="f0002">Fig.2</figref>, the symbol 10 shows the multicast communication path calculation apparatus. The multicast communication path calculation apparatus 10 includes an information management part 11, a path calculation part 12 and a packet process part 13. The information management part 11 is for managing network measurement information on delay or cost arising in each node and each link between nodes in the network. The path calculation part 12 is for calculating multicast paths. The packet process part 13 is for processing packets to be sent or received. In addition, the packet process part 13 receives the network measurement information and a path calculation request, and sends calculation result to the multicast communication path setting apparatus.

0035The information management part 11 includes a routing protocol module 111 and a measurement information storing part 112. The routing protocol module 111 processes an information exchange protocol used in a routing protocol such as OSPF and IS-IS used for collecting information on traffic state and the like. The measurement information storing part 112 manages network measurement information, obtained by the protocol, such as topology, and delay or cost. In addition the path calculation part 12 includes a path calculation module 121 for calculating multicast paths and a calculation result storing part 122 for storing calculation results.

0036The packet process part 13 includes a packet process module 131, a packet transfer table storing part 132 and network interfaces 133. The packet process module 131 determines the type of an arriving packet, transfers the packet to another node or sends the packet to the information management part 11. The packet transfer table storing part 132 stores transfer destinations.

0037<figref idref="f0003">Fig.3</figref> shows an example of a configuration of the path calculation module 121 of this example. Functions of each part of the path calculation module 121 can be realized by hardware such as CPU and memory, and software.

0038As shown in <figref idref="f0003">Fig.3</figref>, the path calculation module 121 includes an input part 141, a minimum delay path calculation part 142, a candidate node selection part 143, a difference calculation part 144, a rendezvous point node selection part 145, and an output part 146. The input part 141 receives topology information and delay information of the network. The minimum delay path calculation part 142 calculates minimum delay paths from a source node to each destination node by using the topology information and the delay information. The candidate node selection part 143 selects nodes, as candidate nodes of the rendezvous point node, that reside on a minimum delay path among the plurality of minimum delay paths from the source node to each destination node. The difference calculation part 144 calculates, for each of the candidate nodes, minimum delay paths from the candidate node to each destination node, and calculates, for each of the candidate nodes, a difference between the maximum value and the minimum value among delays of the calculated minimum delay paths. The rendezvous point node selection part 145 selects a candidate node, as the rendezvous point node, of which the calculated difference is smallest. The output part 146 outputs, as the multicast paths, a minimum delay path from the source node to the rendezvous point node and each minimum delay path from the rendezvous point node to each destination node.

0039The multicast communication path calculation apparatus 10 may be configured as a general computer, provided separately from the multicast network, including a CPU, memory, hard disc and the like. In this case, information necessary for path calculation is provided from the outside and the path calculation is performed by using a program for multicast communication path calculation.

0040<figref idref="f0004">Fig.4</figref> shows a configuration of the multicast communication path setting apparatus. In this figure, the symbol 20 shows the multicast communication path setting apparatus. The multicast communication path setting apparatus 20 includes an information management part 21, a measurement part 22, a path setting protocol process part 23 and a packet process part 24. The information management part 21 manages information of delay or cost that arises in nodes and links in the network. The measurement part 22 measures delay or cost arises in itself. The path setting protocol process part 23 performs path setting when new data flow arises, for example. The packet process part 24 processes arriving packets.

0041Basic structure of the information management part 21 is the same as that of the information management part 11 in the multicast communication path calculation apparatus 10, and includes a routing protocol module 211 and a measurement information storing part 212. The measurement part 22 includes a measurement module for measuring status of the network interfaces 243 and information such as delay of each node on the network. The packet process part 24 includes a packet process module 241, a packet transfer table storing part 242 for storing transfer destination of packets, and network interfaces 243. Th packet process module 241 determines types of arriving packets, transfers the packets and determines whether establishing a new path. In addition, the multicast communication path setting apparatus 20 includes a path calculation part 25. The path calculation part 25 includes a calculation process module 251 for calculating multicast paths, and a calculation result storing part 252 for storing the calculation result. In the case where the multicast communication path setting apparatus 20 performs calculation of multicast paths, the path calculation part 25 performs processes the same as those of the multicast communication path calculation apparatus 10. When the multicast communication path setting apparatus 20 does not perform calculation of multicast paths, the multicast communication path setting apparatus 20 may not include the path calculation part 25.

0042The path setting protocol process part 23 receives path setting request from the packet process part 24, and sends the request to the multicast communication path calculation apparatus 10. In addition, the path setting protocol process part 23 has a function for setting multicast paths in the network according to the calculation result received from the multicast communication path calculation apparatus 10.

0043When the multicast communication path calculation apparatus 10 and the multicast communication path setting apparatus 20 are included in the same node, the node includes each process part of the multicast communication path calculation apparatus 10 and the multicast communication path setting apparatus 20.

0044Next, operations of the multicast communication path calculation apparatus 10, the multicast communication path setting apparatus 20 and the multicast transfer apparatus are described.

0045The nodes in the network exchange, with adjacent nodes, network measurement information including topology and delay of the network. Each node stores the network measurement information obtained by the exchange operation.

0046The network measurement information includes not only information measured by its own node but also includes information measured by other nodes. According to the exchange operation, each node can obtain network measurement information of whole nodes of the network.

0047A node having the capability of the multicast communication path setting apparatus 20 sends a path calculation request to a node having the capability of the multicast communication path calculation apparatus 10. The node having the capability of the multicast communication path calculation apparatus 10 calculates the multicast paths by using the network measurement information managed by the information management part 11 and information of destination nodes sent from the node that sends the path calculation request.

0048<figref idref="f0005">Fig.5</figref> is a flowchart showing processes for calculating multicast paths in the multicast communication path calculation apparatus 10.

0049First, the multicast communication path calculation apparatus 10 receives the path calculation request from the multicast communication path setting apparatus 20. At this time, the multicast communication path calculation apparatus 10 also receives information of the source node and the destination nodes from the multicast communication path setting apparatus 20. Then, the path calculation part 20 in the multicast communication path calculation apparatus 10 reads, from the measurement information storing part 112 in the information management part 11, the network measurement information indicating topology and traffic state of the network in step S1. Then, the path calculation module 121 calculates minimum delay paths between the source node to each destination node by using the network measurement information in step S2. More particularly, the path calculation module 121 calculates each minimum delay path between the source node that send the path calculation request and each destination node. For calculating the minimum delay path, Dijkstra's algorithm is used, for example.

0050Next, in step S3, the path calculation module of the multicast communication path calculation apparatus 10 selects a minimum delay path that has the maximum delay among the minimum delay paths calculated in step S2. Then, the path calculation module 121 calculates, for each node (referred to as candidate node) on the selected minimum delay path, data transfer delays from the node to each destination node in step S4. Then, the path calculation module 121 calculates, for each candidate node, a difference between the maximum delay and the minimum delay among delays calculated in step S4, and selects a candidate node by which smallest difference is calculated as the rendezvous point node in step S5. More particularly, in step S5, the difference δ between the maximum delay and the minimum delay for transferring data from a candidate node to each destination node is calculated for each candidate node, and the rendezvous point node is determined as a candidate node that has the minimum difference δ. Then, the path calculation module 121 returns the calculation result to the node that sent the path calculation request via the packet process part 13 in step S6, in which the calculation result indicates a path from the source node to the rendezvous point node and paths from the rendezvous point node to each destination node.

0051In this example, for collecting the network measurement information such as delay by the multicast transfer apparatus, a communication protocol OSPF-TE is used. In the OSPF-TE, traffic information such as delay is included in topology information exchange information of OSPF that is a unicast routing protocol.

0052In addition, Multicast MPLS (Multi Protocol Label Switching) protocol is used as a protocol for setting multicast paths. The Multicast MPLS protocol is an extension of RSVP-TE (Resource Reservation Protocol-Traffic Engineering) for realizing explicit path designation. The Multicast MPLS is a technology in which information element is added, to RSVP-TE used in normal MPLS, in a message for generating LSP (Label Switched Path) for storing tree topology, so that Point-to-Multipoint LSP can be established according to the topology information.

0053In the following, a process example for calculating multicast paths according to this example is described.

0054<figref idref="f0006">Fig.6</figref> shows a multicast network. In this figure, symbols 1-5 show destination nodes. A-I indicate nodes existing between the source node and the destination nodes, and each node includes the capability of the multicast transfer apparatus. A multicast network is formed by the multicast communication path setting apparatus 20 (source node), the nodes A-I, and destination nodes 1-5 that are connected by communication lines. Each number shown on each link indicates a delay (cost) of the link.

0055The multicast communication path setting apparatus 20, that is a source node, transfers data to the destination nodes 1-5 according to the result calculated by the multicast communication path calculation apparatus 10. The network measurement information such as delay that arises in each link is collected by each node by using the beforementioned OSPF-TE. The network measurement information is sent to the multicast communication path calculation apparatus beforehand.

0056<figref idref="f0007">Fig.7</figref> shows minimum delay paths from the source node to each destination node.

0057When the multicast communication path calculation apparatus 10 receives a path calculation request from the multicast communication path setting apparatus 20, the multicast communication path calculation apparatus 10 calculates the minimum delay paths from the source node (multicast communication path setting apparatus 20) to each destination node first. For calculating the minimum delay paths, the multicast communication path calculation apparatus 10 uses Dijkstra's algorithm. Dijkstra's algorithm is generally used for calculating a minimum delay path. The minimum delay paths calculated by the multicast communication path calculation apparatus 10 are, the multicast communication path setting apparatus 20 -> node A -> node C -> destination node 1, the multicast communication path setting apparatus 20 -> node A -> node C -> node E -> node G -> destination node 2, the multicast communication path setting apparatus 20 -> node A -> node C -> node E -> node G -> node I -> destination node 3, the multicast communication path setting apparatus 20 -> node A -> node C -> node D -> node F -> destination node 4, and the multicast communication path setting apparatus 20 -> node A -> node C -> node D -> destination node 5.

0058The delay for the source node -> the destination node 1 is 4, the delay for the source node -> the destination node 2 is 7, the delay for the source node -> the destination node 3 is 9, the delay for the source node -> the destination node 4 is 6, and the delay for the source node -> the destination node 5 is 5. Therefore, the minimum delay path having the maximum delay is the minimum delay path from the source node to the destination node 3.

0059Next, the multicast communication path calculation apparatus 10 selects a rendezvous point node among nodes A, C, E, G and I on the minimum delay path from the source node to the destination node 3 having maximum delay among the minimum delay paths.

0060<figref idref="f0008">Fig.8</figref> shows the minimum delay path connecting the source node, nodes A, C, E, G, I and destination node 3. In the following, the minimum delay path is called as a rendezvous point candidate path. In addition, the nodes A, C, E, G and I are called candidate nodes.

0061Next, the multicast communication path calculation apparatus 10 calculates, for each candidate node, minimum delay paths from the candidate node to each of destination nodes 1-5. For each candidate node n (n: n indicates a number of a candidate node), assuming that the maximum delay among delays of the calculated minimum delay paths is represented by Dmax(n) and the minimum delay is represented by Dmin(n), the multicast communication path calculation apparatus 10 calculates Dmax (A) and Dmin (A), Dmax (C) and Dmin (C), Dmax (E) and Dmin (E), Dmax (G) and Dmin (G), and Dmax (I) and Dmin (I).

0062For decreasing delay variation among users, it is necessary that a difference between Dmax and Dmin is small. Therefore, the multicast communication path calculation apparatus 10 selects a candidate node, as the rendezvous point node, by which difference between Dmax (n) and Dmin (n) is smallest. More particularly, the multicast communication path calculation apparatus 10 calculates each difference between Dmax (A) and Dmin (A), between Dmax (C) and Dmin (C), between Dmax (E) and Dmin (E), between Dmax (G) and Dmin (G), and between Dmax (I) and Dmin (I). Then, the multicast communication path calculation apparatus 10 selects a candidate node by which the difference is smallest as the rendezvous point node. As a result, since the candidate node by which the difference between Dmax (n) and Dmin (n) is minimum is the node E, the multicast communication path calculation apparatus 10 selects the node E as the rendezvous point node. Delay from the node E to the destination node 1 is 3, delay from the node E to the destination node 2 is 2, delay from the node E to the destination node 3 is 4, delay from the node E to the destination node 4 is 3, and delay from the node E to the destination node 5 is 4. Therefore, variation of delays from the rendezvous point node to each destination node is within a range 2-4.

0063The multicast communication path calculation apparatus 10 prepares, as the multicast communication paths, a minimum delay path from the multicast communication path setting apparatus 20 to the node E and minimum delay paths from the node E to each destination node 1-5. Then, the multicast communication path calculation apparatus 10 sends the calculation result to the multicast communication path setting apparatus 20. <figref idref="f0009">Fig.9</figref> shows the minimum delay path from the multicast communication path setting apparatus 20 to the node E and minimum delay paths from the node E to each destination node 1-5. Data are transferred from the multicast communication path setting apparatus 20 to each destination node via the node E that is the rendezvous point node. Next, the multicast communication path setting apparatus 20 that receives information of the multicast communication paths includes the information of the multicast communication paths into a transfer path setting control message, and establishes the multicast paths by using the multicast MPLS that is a protocol for setting paths and is an extended protocol of RSVP-TE. Then, the multicast communication path setting apparatus 20 transfers data via the multicast paths after establishing the multicast paths.

0064The time complexity O of Dijkstra's algorithm is generally n² if the number of nodes is n. Since the multicast paths are calculated by applying Dijkstra's algorithm to each of n nodes according to the conventional technology, the time complexity is O=n³ according to the conventional technology. However, according to the present invention, since Dijkstra's algorithm is applied to p nodes (p&lt;n) on a minimum delay path between the source node and an destination node, the time complexity is O=pn². Therefore, compared with the conventional technology, the time complexity can be decreased.

0065Each of the multicast communication path calculation apparatus and the multicast communication path setting apparatus includes a computer system in the inside. The above-mentioned processes of this example can be realized by reading a program from a computer readable medium and executing the program. The computer readable medium may be a magnetic disk, an optical magnetic disk, a CD-ROM, a DVD-ROM, a semiconductor memory and the like. The computer program can be delivered to a computer via a communication line, and the computer can function as the multicast communication path calculation apparatus or the multicast communication path setting apparatus.

0066As mentioned above, by adopting a system including a node for calculating paths that can executes the path calculation algorithm in consideration of the delay variation among users, delays can be equalized among users. Accordingly, a service can be provided for satisfying fairness of among users on delay, which was difficult according to the conventional technology. In addition, faster calculation time can be achieved comparing with a conventional apparatus that provide a service for satisfying fairness among users on delay. Accordingly, path setting time can be decreased.

(First embodiment)

0067In the following, an embodiment of the present invention is described with reference to figures.

0068In the first embodiment, the calculation algorithm for calculating the multicast paths is different from that of the first example. The other configurations can be the same as those of the first example.

0069<figref idref="f0010">Fig.10</figref> shows an outline of the procedure of the multicast communication path calculation method according to the embodiment.

0070First, a distance graph is input into the multicast communication path calculation apparatus in step S11, in which the distance graph represents multicast network topology and network transfer cost by using link information connecting between nodes and transfer cost information necessary for transferring data on the link. Instead of inputting the distance graph from the outside, the distance graph can be collected in the apparatus beforehand and can be read from a recording medium in the apparatus.

0071Next, source node information and destination node group information are input in step S12. From the input information, a first distance subgraph is established in which the source node is deleted in step S13.

0072After that, the destination node group is selected from the first distance subgraph, and a second distance subgraph is constructed that is formed by edges corresponding to shortest paths among the destination nodes in step S14. Then, a minimal spanning tree is constructed from the second distance subgraph in step S15. At this time, if a plurality of minimal spanning trees exist, pick an arbitrary one.

0073Then, each edge that corresponds to a shortest path in the minimal spanning tree is reconstructed into a path formed by nodes of the input distance graph, so that a subgraph is constructed in step S16. Then, a minimal spanning tree is constructed again from the reconstructed subgraph in step S17. If there are a plurality of spanning trees, pick an arbitrary one. From the constructed spanning tree, unnecessary edges are deleted such that all destination nodes becomes a part of the spanning tree, so that multicast paths including all destination nodes is established in step S18.

0074Nodes in the constructed multicast paths are candidate nodes of a rendezvous point node. Then, for each candidate node, distances from a candidate node to each destination node are calculated, and a difference between the maximum distance and the minimum distance in the distances is calculated. Then, a candidate node is selected as the rendezvous point node in which the difference is minimum among the candidate nodes in step S19. Then, the multicast paths formed by the destination nodes is connected to the source node at the rendezvous point node, and multicast paths including the source node and all destination nodes are constructed in step S20.

0075<figref idref="f0011">Fig.11</figref> shows a configuration of a path calculation module of the multicast communication path calculation apparatus according to the embodiment of the present invention.

0076The path calculation module shown in <figref idref="f0011">Fig.11</figref> includes an information input part 300, a first distance subgraph constructing part 311, a second distance subgraph constructing part 312, a first minimal spanning tree constructing part 313, a path reconstruction part 314, a second minimal spanning tree constructing part 315, a first multicast path constructing part 316, a rendezvous point selection part 317, and a second multicast communication path constructing part 318.

0077The information input part 300 receives the distance graph, in which the distance graph represents multicast network topology and network transfer cost by using link information connecting between nodes and transfer cost information necessary for transferring data on the link. In addition, the information input part 300 receives source node information and destination node group information. The network transfer cost in the second embodiment corresponds to "delay" in the first example.

0078The first distance subgraph constructing part 311 constructs a first distance subgraph in which the source node is deleted from the input distance graph.

0079The second distance subgraph constructing part 312 selects the destination node group from the first distance subgraph, and constructs a second distance subgraph that is formed by shortest paths each between two nodes in the destination node group.

0080The first minimal spanning tree constructing part 313 constructs a minimal spanning tree from the second distance subgraph, at this time, if a plurality of minimal spanning trees exist, pick an arbitrary one.

0081The transfer path reconstruction part 314 reconstructs each edge that corresponds to a shortest path in the minimal spanning tree into a path formed by nodes of the input distance graph, so that a subgraph is established.

0082The second minimal spanning tree constructing part 315 constructs a minimal spanning tree again from the subgraph reconstructed by the transfer path reconstruction part 314. If there are a plurality of spanning trees, pick an arbitrary one.

0083The first multicast communication path constructing part 316 deletes unnecessary edges from the constructed spanning tree such that all destination nodes becomes a part of the spanning tree so that multicast paths including all destination nodes is established.

0084Nodes in the multicast paths are candidate nodes of a rendezvous point node. The rendezvous point selection part 317 calculates, for each candidate node, transfer distances from a candidate node to each destination node, and calculates a difference between the maximum distance and the minimum distance in the transfer distances. Then, the rendezvous point selection part 317 selects a candidate node as the rendezvous point node in which the difference is minimum among the candidate nodes.

0085The second multicast communication path constructing part 318 connects the constructed multicast paths formed by the destination nodes to the source node at the rendezvous point node so as to establish multicast paths including the source node and all destination nodes.

0086In the following, the procedure for calculating the multicast paths according to this embodiment is described in detail. In this embodiment, the following information is input from the measurement result storing part, for example. The following information can be collected as the network measurement information by using the existing protocol described in the first example. <ul id="ul0003" list-style="none" compact="compact"><li>(1) One directed distance graph that describes topology of the whole network: G=(V,E,d), wherein V : nodes, E: links (edges) between nodes, d: costs of links (corresponding to delay of the first embodiment)</li><li>(2) Multicast traffic source node: s (⊆ v)</li><li>(3) Multicast traffic destination nodes (destination node group) : S (⊆V)</li></ul>

0087By inputting the above information, multicast communication paths : T is output by performing following calculation.

0088<figref idref="f0012">Fig.12</figref> is a flowchart of the multicast communication path calculation method according to the embodiment.

0089Step 101) A subgraph G'=(V-s,E-Es,d-ds) is constructed by deleting the source node from the input graph G=(V,E,d).

0090Step 102) The destination node group S is selected from the subgraph G, and a distance subgraph G1=(V1 ,E1 d1) is constructed, wherein the distance subgraph G1 =(V1 ,E1 ,d1) is formed by the destination node group S and shortest paths each between two nodes in the destination node group S.

0091Step 103) A minimal spanning tree T1 is constructed from the distance subgraph G1. When there exist a plurality of spanning trees, arbitrary spanning tree is selected.

0092Step 104) By replacing each edge of the spanning tree T1 with corresponding shortest path in the graph G, a subgraph Ga is constructed. When there exist a plurality of shortest paths, arbitrary one is selected.

0093Step 105) A minimal spanning tree Ts is constructed from the subgraph Ga. When there exist a plurality of minimal spanning trees, arbitrary one is selected.

0094Step 106) Multicast paths that include the destination node group S are constructed by deleting unnecessary edges from the minimal spanning tree Ts such that all destination nodes are included.

0095Step 107) Nodes that forms the multicast paths constructed in step 106 are selected as candidates of a rendezvous point (RP) node. Then, distances d(RPx,NODEy) between RPx and each destination node are calculated, and a difference between dmax and dmin is calculated in which dmin is a shortest distance from RPx to the destination node, and dmin is a longest distance from RPx to the destination node. The distance d(RPx,NODEy) can be shortest distance between RPx and NODEy. Then, a candidate by which the difference is the smallest is selected as the rendezvous point (RP) node. That is, a candidate that satisfies [min(dmax (RPx,NODEy)-dmin (RPx.NODEz))] is selected as the rendezvous point node. .

0096Step 108) The multicast communication paths constructed in step 106 is connected to the source node s via the rendezvous point node, so that multicast paths including the source node s and the destination node group S are constructed.

0097In the following, an example for calculating the multicast paths by using the above-mentioned procedure is described. <figref idref="f0013">Fig.13</figref> shows a network graph of a multicast network to which the procedure is applied in this example.

0098<figref idref="f0013">Fig.13</figref> shows a network formed by 10 nodes from V0 to V9. Each node is connected by a link, and the number on each link shows a transfer cost. For example, since 1 is assigned to the link between the node V0 and the node V1, the transfer cost for transferring data from the node V0 to the node V1 is 1.

0099For constructing the multicast communication paths from the source node V0 to the destination node group V1, V2m V3 and V4, following calculation procedure is performed according to the embodiment.

0100When the path calculation module receives network graph information of <figref idref="f0013">Fig.13</figref> and source node information V0 and destination node group information V1, V2, V3 and V4, the node V0 and links V0V1, V0V9 and V0V4 that are connected to V0 are deleted from the input graph. <figref idref="f0013">Fig.14</figref> shows the network graph in which the source node and the links are deleted. That is, <figref idref="f0013">Fig.14</figref> shows a network graph after performing the process of step 101.

0101Then, a shortest path graph among destination nodes is constructed by the step 102. <figref idref="f0014">Fig.15</figref> shows the result of step 102.

0102After that, by the step 103, the minimal spanning tree formed by edges that are shortest paths among destination nodes is constructed. <figref idref="f0014">Fig.16</figref> shows the result. After that, each edge forming the spanning tree is replaced by original path of the input graph. <figref idref="f0015">Fig.17</figref> shows an intermediate result of the process. <figref idref="f0015">Fig.18</figref> shows the result. As shown in the figure, since the shortest path between the node V1 and the node V4 is formed by nodes V1V9V5V4, the nodes V9 and V5 are inserted between V1 and V4. In the same way, nodes that forms the shortest paths are inserted between nodes V1 and V2 and between nodes V2 and V3. In this example, since the nodes V9, V6 and V5 are overlapping for a plurality of paths, the subgraph shown in <figref idref="f0015">Fig.18</figref> is obtained by shaping the graph in consideration of actual physical connections.

0103After that, by the step 105, a minimal spanning tree (Ts) is formed from the subgraph shown in <figref idref="f0015">Fig.18</figref>. <figref idref="f0016">Fig.19</figref> shows the result.

0104Then, by the step 106, multicast paths (a minimal tree) that include the destination nodes are established by deleting unnecessary edges such that the destination nodes are included in the multicast paths. The result is shown in <figref idref="f0016">Fig.20</figref>. As shown in <figref idref="f0016">Fig.20</figref>, a minimal tree V1V9V5V6V2V3V4 including the destination nodes V1, V2, V3 and V4 is formed.

0105After that, by the step 107, a rendezvous point node is selected among nodes in the minimal tree such that variation of transfer distances to each node becomes minimum. For selecting the rendezvous point node, every node that forms the minimal tree is a candidate of the rendezvous point node. For each candidate, transfer distances from the candidate to each destination node are calculated, and a difference of the maximum distance and the minimum distance in the transfer distances are obtained. In the example of <figref idref="f0016">Fig.20</figref>, when the node V9 is a candidate of the rendezvous point node, since distances are as follows : <maths id="math0001"><math display="block"><mi>RP</mi><mo mathvariant="normal">⋅</mo><mi mathvariant="normal">V</mi><mo>⁢</mo><mn mathvariant="normal">1</mn><mo mathvariant="normal">=</mo><mn mathvariant="normal">1</mn><mo mathvariant="normal">,</mo></math><img file="EP1515495B1_D0004.tif" /></maths><maths id="math0002"><math display="block"><mi>RP</mi><mo mathvariant="normal">⋅</mo><mi mathvariant="normal">V</mi><mo>⁢</mo><mn mathvariant="normal">2</mn><mo mathvariant="normal">=</mo><mn mathvariant="normal">3</mn><mo mathvariant="normal">,</mo></math><img file="EP1515495B1_D0005.tif" /></maths><maths id="math0003"><math display="block"><mi>RP</mi><mo mathvariant="normal">⋅</mo><mi mathvariant="normal">V</mi><mo>⁢</mo><mn mathvariant="normal">3</mn><mo mathvariant="normal">=</mo><mn mathvariant="normal">3</mn><mo mathvariant="normal">,</mo></math><img file="EP1515495B1_D0006.tif" /></maths><maths id="math0004"><math display="block"><mi>RP</mi><mo mathvariant="normal">⋅</mo><mi mathvariant="normal">V</mi><mo>⁢</mo><mn mathvariant="normal">4</mn><mo mathvariant="normal">=</mo><mn mathvariant="normal">2</mn><mo mathvariant="normal">,</mo></math><img file="EP1515495B1_D0007.tif" /></maths> the difference between the maximum distance (dmax) and the minimum distance (dmin) is 2. In the example of <figref idref="f0017">Fig.21</figref>, determining that the node V5 is RP, every transfer distance from RP to the destination nodes V1, V2, V3 and V4 is 2. This means that the node V5 is the optimal rendezvous point in which the difference is 0.

0106In addition, by step 108, the source node V0 is connected to the rendezvous point node so that optimal multicast paths from the source node to the destination nodes are constructed.

0107<figref idref="f0018">Fig.22</figref> shows a configuration example of a multicast communication path calculation system according to the embodiment. That is, the system can be configured not only as shown in <figref idref="f0001">Fig.1</figref> but also as shown in <figref idref="f0018">Fig.22</figref>. The system shown in <figref idref="f0018">Fig.22</figref> performs the above-mentioned multicast calculation procedure.

0108The network traffic information database 120 collects network traffic information by using a routing protocol. The multicast calculation engine 310 receives node information, destination node group information, requirements and constraints. Then, the multicast calculation engine 310 calculates and outputs the optimal multicast paths by using the network traffic information database 120 according to the calculation method shown in <figref idref="f0011">Fig.11</figref>.

0109In the following, an example showing performance of the calculation method of this embodiment is described.

0110<figref idref="f0019">Fig.23</figref> shows a network for evaluating the multicast communication path calculation system. As shown in the figure, by setting a random graph including 400 nodes and bandwidth for each link, a multicast tree including 40 destination nodes are constructed. <figref idref="f0020">Fig.24</figref> shows a performance evaluation graph of multicast paths, and <figref idref="f0020">Fig.25</figref> shows a performance evaluation graph of delay variation. As shown in the evaluation result, although transfer cost increases slightly according to the method of the present invention compared with the KMB communication method, delay variation of the whole tree can be suppressed according to the method of the present invention.

0111As mentioned above, by using the calculation method of the present invention, an optimal multicast communication paths can be established according to QoS requirement of each multicast traffic, and bandwidth can be used efficiently in the whole network, so that a high performance multicast communication network can be established.

0112In the same way as the first example, the operation shown in <figref idref="f0011">Fig.11</figref> can be realized by installing a program of the present invention in a computer that functions as the multicast communication path calculation apparatus. In addition, the program can be sent and received via a network.

0113The program can be stored in a hard disk of a computer, or in a movable recording medium such as a flexible disk and CD-ROM and the like. The stored program can be installed to the computer.

0114In addition, the multicast paths can be established on the network in the same way as the first example.

0115As mentioned above, according to the calculation method of this embodiment, multicast paths can be established in which delay variation can be suppressed while suppressing cost of the whole network. Thus, an efficient and high-performance multicast communication network can be constructed.

0116The present invention is not limited to the specifically disclosed embodiment, and variations and modifications may be made without departing from the scope of the present invention as claimed.