The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Multicasting technology is a bandwidth-conserving network technology that reduces network traffic by simultaneously delivering a single stream of information to thousands of recipients. Applications that can use multicast include videoconferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news. Internet Protocol (IP) Multicast delivers source traffic to multiple receivers without adding any additional burden on the source or the receivers, and while using the least network bandwidth of any competing technology.
Multicasting is based on the concept of a group. An arbitrary group of receivers expresses an interest in receiving a particular data stream. Receivers may include network infrastructure elements, such as routers and switches, or end station devices, such as workstations or personal computers. A multicast group does not have any physical or geographical boundaries; receivers (also termed “hosts”) can be located anywhere in the Internet. Hosts that are interested in receiving data flowing to a particular group must join the group. In a typical approach, Internet Group Management Protocol (IGMP) is used to dynamically register individual hosts in a multicast group on a particular LAN. Hosts identify group memberships by sending IGMP messages to a local multicast-enabled router. Under IGMP, routers listen to IGMP messages and periodically send out queries to discover which groups are active or inactive on a particular subnet.
To receive a data stream associated with a group, a receiver must be a member of the group. Multicast traffic flows from the source to the multicast group using a distribution tree that connects all of the sources to all of the receivers in the group. Distribution trees define the path that multicast traffic takes through the network to arrive at group members. Distribution trees comprise either source trees or shared trees. A distribution tree may be shared by all sources (a shared tree), or a separate distribution tree can be built for each source (a source tree). The shared tree may be one-way or bi-directional.
For a source to build a distribution tree, sources such as routers implement Protocol Independent Multicast (PIM) in software. PIM software forwards IP Multicast traffic using the standard Unicast routing table that is maintained in the router. PIM uses the Unicast routing table to decide if the source of an IP multicast packet has arrived on an optimal path from the source. Typically, the router sends PIM messages upstream to the source to refresh the forwarding state once every sixty (60) seconds. Thus, receivers notify a source that they want to receive the data stream by sending the PIM messages to the source. Using PIM in this manner results in the most efficient delivery of data to multiple receivers possible.
Because of increasing usage of IP Multicast in the networks of business enterprises, network service providers who offer Layer 3 Virtual Private Network (VPN) services have moved to offer IP Multicast-enabled VPNs. A VPN provides secure, private network connectivity across a non-secure or shared infrastructure, such as the internetworks owned and operated by Internet Service Providers (ISPs). VPN multicasting offers the same multicasting technology as conventional IP multicast, but traverses shared infrastructure elements between the source and the receivers. Multicast Virtual Private Network service aims to provide the same policies and performance as a private network, without requiring enterprises to purchase dedicated lines or service.
In Multicast Virtual Private Network (MVPN) service, a service provider establishes a secure logical connection or “tunnel” between provider edge (PE) routers in the ISP network for communication of multicast information. The PE routers transmit IP multicast traffic for default Multicast Distribution Trees (MDTs) and Data MDTs across the MDT Tunnel. The MDT Tunnel provides protection and integrity for the private data involved in the multicast VPN, since that data travels over public connections of the ISP. Information identifying the PE routers involved in a VPN, and the receivers that are authorized to receive VPN traffic, are stored in a VPN routing/forwarding (“VRF”) table. The number of multicast sources in a default MDT group is equal to the number of PE routers connected to the VRF table using the default MDT group.
The approach described above for VPN multicasting was designed for applications on an Intranet, in which all sources and receivers are in the same domain and have the same administrative scope. However, there is a present need to expand VPN multicasting for use in an Extranet, in which sources and receivers are not in the same domain or same administrative scope. To extend VPN multicasting to an Extranet, a VRF can be established on an Extranet source, so that the Extranet source can communicate with each VPN associated with a PE router that wishes to receive data from the Extranet source. However, this approach to VPN Multicasting on an Extranet causes complications because of the VPN boundary line and the different administrative scopes.
In particular, in the current approach PIM messaging applied to VPN Multicasting on an Extranet causes certain participating network elements to send far too many messages. For example, by conforming to conventional PIM protocol operations, the Multicast VPN Extranet routers send PIM messages for each VRF in a PE router to a Reverse Path Forwarding (RPF) neighbor. In the case of multicast VPN, the RPF interface is the MDT Tunnel. Therefore, for every VRF receiver, a PIM message is sent through the MDT Tunnel, even though sending only one PIM message is needed.
The resulting number of PIM messages causes an unnecessary amount of network traffic. For example, if a multicast group has 100 VPNs established in a PE router, then 100 PIM messages are sent through the MDT Tunnel. Processing the PIM messages consumes resources on each router. Additionally, the MDT tunnel becomes flooded with unnecessary PIM messages, thereby consuming unnecessary bandwidth.
Based on the foregoing, there is a clear need for an approach to reduce the number of PIM messages that are sent from an Extranet receiver in a VPN multicasting network.