Recently, with the improvements of the Ethernet (registered trade mark) and IP technologies, use of IP in the network has been more and more popular. This trend has been popular among network providers, so that the replacement of an SDH (Synchronous Digital Hierarchy) transmission method by a packet transmission method has been started in order to improve the transmission efficiency to respond to an increasing demand of IP traffic of a carrier network and reduce a cost. A difference between the SDH transmission method and the packet transmission method is described below.
The SDH transmission method is a TDM (Time Division Multiplexing) based technology, which means time slots may be occupied (used) when there are no data to be transmitted. On the other hand, in the packet transmission method, when there are no data to be transmitted, another service may use the time slots; therefore the line (use) efficiency may be improved.
In a carrier network, from a viewpoint of operational management, even when a packet transmission method is used, it is required to conduct path management (i.e., static path setting). To fulfill this requirement, a method called an MPLS-TP (Multi Protocol Label Switching-Transport Profile) packet-based transport method has been developed.
FIG. 1 illustrates an example of a ring network employing the MPLS-TP scheme for conducting multicast communications. As illustrated in FIG. 1, nodes N1, N2, N3, N4, N5, N6, N7, and N8 are connected in a ring shape to form a ring network. In the ring network, an LSP#WA is set which is a working LSP (Label Switch Path) set along an “A” direction (clockwise direction) illustrated as a solid line between the node N1 and the node N5.
Also, an LSP#WB is set which is a working LSP (Label Switch Path) set along a “B” direction (counterclockwise direction) illustrated as a solid line between the node N1 and the node N7.
In this case, a signal added (inserted) at the node N1 is duplicated and branched into two. As a result, one of the branched signals is distributed (transmitted) from the node N1 to the nodes N2, N3, N4, and N5 in this order and dropped (extracted) at the nodes N2, N4, and N5. Similarly, the other of the branched signals is distributed from the node N1 to the nodes N8 and N7 in this order and dropped (extracted) at the nodes N8 and N7.
When a failure (typically, a loss of continuity) occurs in the ring network of FIG. 1, it is necessary to respond to the failure to make it possible to use the ring network (hereinafter may be referred to as “relieve” or “backup”).
For example, to “relieve” the LSP#WA in the A direction (clockwise direction), an LSP#PB illustrated as the dotted line along the B direction (counterclockwise direction) is set. Also, to relieve the LSP#WB in the B direction (counterclockwise direction), an LSP#PA illustrated as the dotted line along the A direction (clockwise direction) is set.
Generally, by monitoring the reception of a CCM (Continuity Check Message) packet of an OAM (Operation Administration and Maintenance) packet which is a monitor/control packet of the working and backup (preliminary) LSP at nodes N5 and N7 at terminals points, a failure is relieved (“backuped”) separately for each of the LSPs.
There has been known a technique in which in a ring-type network, in multicast communications, the entry information indicating the nodes relevant to the multicast communications is shared among the node transmitting data and the nodes receiving the data. Then, the transmitting node determines the transmission direction along which the multicast data are to be transmitted by referring the entry information and topology information, and, on the other hand, the receiving nodes discard (destroy) the multicast data when determining that there is no farther receiving node to received the multicast data beyond in the transmission direction (see, for example, International Publication Pamphlet No. WO2004/064335).
Further, there has been known a technique in which communications signals are transmitted from an ingress node to plural egress nodes along two or more paths on a network; a primary path through which a multicast communication signal is transmitted in advance along a communication link before a failure occurs in the network and a backup path through which the multicast communication signal is transmitted along a communication link are set; when a failure occurs a multicast communication signal is transmitted on the primary path and a copy of the multicast signal is transmitted on the backup path (see, for example, Japanese National Publication of International Patent Application No. 2010-515314).
Further, there has been known a technique in which a multi-point logical path is set in two systems (i.e., a working system and a backup system). The multi-point logical path is the path in which a frame transmitted from a transmission terminal node is not only transmitted to multicast frame receiving terminal nodes but also transmitted back to the transmission terminal node after transmitting along the ring so that the frame is terminated by the transmission terminal node as well. Then, a node detecting a failure transmits a forward failure report frame to the multicast (multi-point) logical path where the failure occurs, and the transmission terminal node receiving the forward failure report frame stops using the multicast (multi-point) logical path and transmits the frame by using a path that no forward failure report frame is received from yet (see, for example, Japanese Laid-open Patent Publication No. 2007-282153).
Further, there has been known a technique in which the transmission terminal node hierarchically divides the transmission frame into two, appends a header including plural addresses and the terminal No. of the terminal which becomes the end terminal, and separately transmits in both directions of the ring transmission path. By doing this, a multicast communication having higher confidentiality and band use efficiency in the ring transmission path is achieved (see, for example, Japanese Laid-open Patent Publication No. 2007-53484).