Abstract:
A mechanism that provides congruent forwarding paths for unicast and multicast data traffic over a service provider core network includes issuing, by a receiver edge node, a request to join a multicast tree structure. A unicast path from the receiver edge node to a source node of the provider network is then established using a special message that contains an identifier. The identifier allows the unicast path through the core network to be aligned with the multicast tree structure. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/704,817 filed Aug. 1, 2005, entitled “Multicast Mechanism For VPLS”. The present application is also related to co-pending application entitled, “Optimal Bridging Over MPLS/IP Through Alignment of Multicast and Unicast Paths” filed concurrently herewith, which application is assigned to the assignee of the present application. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to digital computer network technology; more particularly, to methods and apparatus for providing Local Area Network (LAN) emulation services over Internet protocol (IP) networks. 
     BACKGROUND OF THE INVENTION 
     A LAN is a high-speed network (typically 10 to 1000 Mbps) that supports many computers connected over a limited distance (e.g., under a few hundred meters). Typically, a LAN spans a single building. U.S. Pat. No. 6,757,288 provides a general description of a LAN segment. A Virtual Local Area Network (VLAN) is mechanism by which a group of devices on one or more LANs are configured using management software so that they can communicate as if they were attached to the same LAN, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible. 
     Virtual Private Network (VPN) services provide secure network connections between different locations. A company, for example, can use a VPN to provide secure connections between geographically dispersed sites that need to access the corporate network. There are three types of VPN that are classified by the network layer used to establish the connection between the customer and provider network: Layer 1, VPNs, which are simple point-to-point connections using Layer 1 circuits such as SONET; Layer 2 VPNs (L2VPNs), where the provider delivers Layer 2 circuits to the customer (one for each site) and provides switching of the customer data; and Layer 3 VPNs (L3VPNs), where the provider edge (PE) device participates in the customer&#39;s routing by managing the VPN-specific routing tables, as well as distributing routes to remote sites. In a Layer 3 IP VPN, customer sites are connected via IP routers, e.g., provider edge (PE) and intermediate provider (P) nodes, that can communicate privately over a shared backbone as if they are using their own private network. Multi-protocol label switching (MPLS) Border Gateway Protocol (BGP) networks are one type of L3VPN solution. An example of an IP-based Virtual Private Network is disclosed in U.S. Pat. No. 6,693,878. U.S. Pat. No. 6,665,273 describes a MPLS system within a network device for traffic engineering. 
     Virtual Private LAN Service (VPLS) has recently emerged as a L2VPN to meet the need to connect geographically dispersed locations with a protocol-transparent, any-to-any, full-mesh service. VPLS is an architecture that delivers Layer 2 service that in all respects emulates an Ethernet LAN across a wide area network (WAN) and inherits the scaling characteristics of a LAN. All customer sites in a VPLS appear to be on the same LAN, regardless of their locations. In other words, with VPLS, customers can communicate as if they were connected via a private Ethernet LAN segment. The basic idea behind VPLS is to set up a full-mesh of label switched paths (LSPs) between each PE router so that Media Access Control (MAC) frames received on the customer side can be switched based on their MAC addresses and then encapsulated into MPLS/IP packets on the P node side and sent across the VPLS domain over the full mesh. Conceptually, VPLS can therefore be thought of as an emulated Ethernet LAN segment connected by a set of virtual bridges or virtual Ethernet switches. 
     In multicast data transmission, data packets originating from a source node are delivered to a group of receiver nodes through a tree structure. (In contrast, unicast communications take place between a single sender and a single receiver.) Various mechanisms, such as the Protocol Independent Multicast (PIM) protocol, have been developed for establishing multicast distribution trees and routing packets across service provider (SP) networks. One commonly used approach uses a dynamic routing algorithm to build the multicast tree by allowing group member receiver nodes to join one-by-one. When a new receiver node attempts to join, it sends a Join request message along a computed path to join the group. The routing algorithm/protocol then connects the new receiver to the exiting tree (rooted at the source) without affecting the other tree member nodes. 
     By way of further background, U.S. Pat. No. 6,078,590 teaches a method of routing multicast packets in a network. Content-based filtering of multicast information is disclosed in U.S. Pat. No. 6,055,364. 
     Recent VPLS working group drafts (draft-ieff-l2vpn-vpls-ldp-07.txt and draft-ieff-l2vpn-vpls-bgp-05) have no special handling specified for multicast data within a VPLS instance. That is, multicast data within a VPLS instance is treated the same as broadcast data and it is replicated over all the pseudo-wires (PWs) belonging to that VPLS instance at the ingress provider edge (PE) device. This ingress replication is very inefficient in terms of ingress PE and MPLS/IP core network resources. Furthermore, it is not viable for high bandwidth applications where replicating the multicast data N times may exceed the throughput of the ingress PE trunk. Therefore, SPs are interested in deploying multicast mechanisms in their VPLS-enabled networks that can reduce or eliminate ingress replication, e.g., either replicating the data over the PWs to the PE devices that are member of the multicast group(s) or only sending one copy of the data over each physical link among PE and P nodes destined to the PE devices that are member of the multicast group(s). 
     Two submissions in the Internet Engineering Task Force (IETF) L2VPN Working Group attempt to solve this problem. The first one (specified in draft-serbest-l2vpn-vpls-mcast-03.txt) uses Internet Group Management Protocol (IGMP)/PIM snooping to restrain multicast traffic over a full mesh of PWs belonging to a given VPLS. IGMP is a standard for IP multicasting in the Internet, and is defined in Request For Comments 1112 (RFC1112) for IGMP version 1 (IGMPv1), in RFC2236 for IGMPv2, and in RFC3376 for IGMPv3. (IGMPv3 includes a feature called Source Specific Multicast (SSM) that adds support for source filtering.) By snooping IGMP/PIM messages, the PE (i.e., switch or router) node can populate the Layer 2 (L2) forwarding table based on the content of the intercepted packets. Thus, a PE device can determine which PWs should be included in a multicast group for a given VPLS instance and only replicate the multicast data stream over that subset of PWs. 
     Although IGMP snooping helps to alleviate replication overhead, it does not completely eliminate the replication problem at the ingress PE. Therefore, this mechanism may not be viable for multicast applications with high bandwidth requirements because the aggregate data throughput after replication may exceed the bandwidth of the physical trunk at the ingress PE. 
     The second IETF proposal (described in draft-raggarwa-l2vpn-vpls-mcast-01.txt) tries to address the shortcomings of the previous draft by using the multicast tree to transport customer multicast data of a given VPLS service instance. However, because the unicast and multicast paths for a given VPLS instance are different, this approach can result in numerous problems. The first problem involves packet re-ordering, wherein two consecutive frames are sent on two different paths, e.g., a first frame is sent on a multicast path because of unknown destination unicast MAC address, with a second frame being sent on a unicast path after the path to the destination has been learned. If the unicast path is shorter than multicast path, the second packet can arrive ahead of the first one. 
     Another problem with the second. IETF proposal is that bridged control packets typically need to take the same path as unicast and multicast data, which means the unicast and multicast path need to be aligned or congruent. If control packets are sent on unicast paths, any failure in the multicast path can go undetected. This situation is illustrated in  FIG. 1 , which shows a SP network  10  with a multicast tree  18  having a path through P nodes  14 ,  15 , and  17  that connects PE nodes  11 - 13 . A unicast path  19  is shown passing through P node  16 . In this example, failure of P node  15  may go undetected if control packets are sent via unicast path  19 . Furthermore, since unicast and multicast paths are usually different in the network core, Ethernet operations, administration, and management (OAM) connectivity check messages often cannot detect a path/node failure. Even if the failure is detected through some other means, notification of the failure to the originator of the Ethernet OAM becomes problematic. 
     What is needed therefore is a method and apparatus for eliminating ingress replication of multicast data within a VPLS instance that overcomes the aforementioned problems of the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description that follows and from the accompanying drawings, which, however, should not be taken to limit the invention to the specific embodiments shown, but are for explanation and understanding only. 
         FIG. 1  is a simplified diagram of a provider network showing one problem inherent in the prior art. 
         FIG. 2  illustrates an exemplary service provider network with an equal cost multiple path (ECMP) configuration in accordance with one embodiment of the present invention. 
         FIG. 3  shows the network diagram of  FIG. 2  after formation of a multicast tree in accordance with one embodiment of the present invention. 
         FIG. 4  shows the network-diagram of  FIG. 3  after multiple unicast paths have been established in accordance with one embodiment of the present invention. 
         FIG. 5  is a flow chart diagram of network operations in accordance with one embodiment of the present invention. 
         FIG. 6  is a generalized circuit schematic block diagram of a network node. 
     
    
    
     DETAILED DESCRIPTION 
     A mechanism for aligning unicast and multicast paths in a service provider network, and which thereby achieves shortest path (i.e., optimal) bridging, is described. In the following description specific details are set forth, such as device types, protocols, network configurations, etc., in order to provide a thorough understanding of the present invention. However, persons having ordinary skill in the networking arts will appreciate that these specific details may not be needed to practice the present invention. 
     A computer network is a geographically distributed collection of interconnected subnetworks for transporting data between nodes, such as intermediate nodes and end nodes. A local area network (LAN) is an example of such a subnetwork; a plurality of LANs may be further interconnected by an intermediate network node, such as a router, bridge, or switch, to extend the effective “size” of the computer network and increase the number of communicating nodes. Examples of the end nodes may include servers and personal computers. The nodes typically communicate by exchanging discrete frames or packets of data according to predefined protocols. In this context, a protocol consists of a set of rules defining how the nodes interact with each other. 
     As shown in  FIG. 6 , each node  50  typically comprises a number of basic subsystems including a processor subsystem  51 , a main memory  52  and an input/output (I/O) subsystem  55 . Data is transferred between main memory (“system memory”)  52  and processor subsystem  51  over a memory bus  53 , and between the processor and I/O subsystems over a system bus  56 . Examples of the system bus may include the conventional lightning data transport (or hyper transport) bus and the conventional peripheral component [computer] interconnect (PCI) bus. Node  50  may also comprise other hardware units/modules  54  coupled to system bus  56  for performing additional functions. Processor subsystem  51  may comprise one or more processors and a controller device that incorporates a set of functions including a system memory controller, support for one or more system buses and direct memory access (DMA) engines. In general, the single-chip device is designed for general-purpose use and is not heavily optimized for networking applications. 
     In a typical networking application, packets are received from a framer, such as an Ethernet media access control (MAC) controller, of the I/O subsystem attached to the system bus. A DMA engine in the MAC controller is provided a list of addresses (e.g., in the form of a descriptor ring in a system memory) for buffers it may access in the system memory. As each packet is received at the MAC controller, the DMA engine obtains ownership of (“masters”) the system bus to access a next descriptor ring to obtain a next buffer address in the system memory at which it may, e.g., store (“write”) data contained in the packet. The DMA engine may need to issue many write operations over the system bus to transfer all of the packet data. 
     According to one embodiment of the present invention, congruent (i.e., aligned) unicast and multicast paths through a MPLS/IP network are achieved in the presence of either an asymmetrical path cost or an Equal Cost Multiple Paths (ECMP) through a protocol that allows the multicast tree identifier to be used during establishment of the unicast path. In a particular embodiment, a multicast tree structure is first build using a standard algorithm or protocol. After the multicast tree has been built, a modification to the PIM protocol (for IP) allows unicast paths to be established that align with or follow the multicast tree paths. The modification involves providing the multicast tree identifier information inside the PIM message used during unicast path construction. 
     In a specific implementation the PIM protocol (with the modification described above) is utilized to set up both the unicast and multicast trees across the P-domain of the SP network. Since in the PIM protocol a join request is initiated from the receiver PE node to the source address of the PE node for the setup of both unicast and multicast paths, the unicast and multicast paths are ensured to be congruent, eliminating the problem of different paths even in the presence of ECMP. 
     It is appreciated that other protocols, including similar modifications to existing routing protocols such as the Label Distribution Protocol (LDP), for Point-to-Multipoint LSPs as outlined in IETF draft-minei-mpls-ldp-p2mp-00.txt or draft-wijnands-mpls-ldp-mcast-ext-00.txt, can also be used to achieve congruent multicast and unicast paths over the SP network. LDP is a known protocol that uses Transmission Control Protocol (TCP) to provide reliable connections between Label Switching Routers (LSRs) to exchange protocol messages to distribute labels and to set up Label Switched Paths (LSPs). LDP is specified in RFC3479. 
     The multicast tree identifier utilized when establishing the unicast path may include the multicast group destination address (GDA), which is an IP address from 224.0.0.0 to 239.255.255.255. The tree identifier may also comprise the MAC address associated with each GDA. This MAC address is formed by 01-00-5e, followed by the last 23 bits of the GDA translated into hex (e.g., 230.20.20.20 corresponds to MAC 01-00-5e-14-14-14; and 224.10.10.10 corresponds to MAC 01-00-5e-0a-0a-0a). In other embodiments, the multicast tree identifier may include other information of the multicast tree structure that enables the unicast path to be built congruent with the multicast path. 
     In one embodiment, the multicast tree is built in a direction opposite to that of data forwarding; that is, if data packets are forwarded in a direction across the SP network from west to east (or left to right in  FIGS. 2-5 ), then the multicast and unicast paths are built in a direction from east to west (right to left). 
       FIG. 2  illustrates an exemplary SP network  20  arranged with an equal cost multiple path (ECMP) between PE nodes  31 - 34  across the P-domain comprising P nodes  21 - 29 . In this example, PE node  31  is shown as the source (S) and PE nodes  32 - 34  each comprise receiver (R) nodes. (In the context of the present application, a receiver or destination node refers to a router, switch, or other node device that has a multicast group member in its subnet irrespective of how the receiver joins or leaves the group. Similarly, a source node refers to a router, switch, or other node device that has a host in its subnet that is a multicast traffic source.) 
       FIG. 3  shows SP network  20  after a multicast tree  36  having three branches has been established from each of receiver PE nodes  32 ,  33 , and  34  back to source PE node  31 . For example, multicast tree  36  includes a branch path from PE node  33  to PE node  31  that passes through P nodes  26 ,  23  and  21 . Similarly, the branch path from receiver PE node  34  to source PE node  31  passes through P nodes  29 ,  27 ,  25 ,  23  and  21 . Finally, the branch of multicast tree  36  between PE node  32  and PE node  31  passes through P nodes  24 ,  22 , and  21 . In one possible implementation, multicast tree  36  is built by allowing group members to join one-by-one. For instance, the routing algorithm/protocol in use may operate with a receiver join mechanism in which a receiver sends a Join request or message along a computed path to join the multicast tree routed at the source. In this example, each of the branches of tree  36  is established in a direction from right to left in the diagram, i.e., from each receiver PE node to the source root PE node, utilizing receiver-initiated join messages, e.g., PIM-SSM. 
       FIG. 4  shows the network diagram of  FIG. 3  after three unicast paths  37 - 39  (shown by dashed lines as single branch trees from receiver to source) have been established in the same direction that multicast tree  36  was built. That is, in accordance with one embodiment of the present invention, for each unicast path between the receiver node and the source node a separate multicast tree having a single branch is built. (Practitioners will understand that a single-branch tree is the same as a P2P tunnel in MPLS/IP.) Since both unicast and multicast trees are built the same way and in the same direction, both take the same path through the core network of P nodes. As discussed above, this is achieved through a modification to the PIM protocol wherein the multicast tree identifier is provided in the PIM message when establishing each of the unicast paths. 
     For example, when establishing unicast path  37  from receiver PE node  34  the multicast tree identifier provided in the PIM message provides information on the multicast path such that unicast path  37  is built as a single branch tree from receiver node  34  to source PE node  31  that passes through P nodes  29 ,  27 ,  25 ,  23 , and  21  congruent with the corresponding branch of multicast tree  36 . Likewise, unicast path  38  is built as a single branch tree from receiver node  33  to source PE node  31  through P nodes  26 ,  23 , and  21 ; and unicast path  39  is built as a single branch tree from receiver node  32  to source PE node  31  through P nodes  24 ,  22 , and  21 . 
       FIG. 5  is a flow chart diagram of network operations in accordance with the above-described embodiment of the present invention. The process begins at block  41  with the set up of a multicast distribution tree structure in a direction from each receiver node to the source root node using a known protocol/algorithm, such as PIM (for IP) or LDP (for MPLS). After the multicast tree has been built, a single branch unicast “tree” is built from each receiver to the source node using the same protocol, with the multicast tree identifier information being provided to each intermediate P node in order to establish the unicast tunnels along the identical path taken by the multicast tree (block  42 ). As previously discussed, the multicast tree identifier information used to establish the unicast tunnels may be included in a PIM message (for IP) or LDP signaling (for MPLS). 
     It is appreciated that the above-described tree identifier used to associate the unicast path with a given multicast path may comprise any general identifier used for such association. For example, the identifier can simply identify one of the equal cost paths in an ECMP network. Such an identifier may be used in both the unicast and multicast path setup such that both the unicast and multicast paths will be the same in the presence of ECMP toward the source PE node. 
     It should be understood that elements of the present invention may also be provided as a computer program product which may include a “machine-readable medium” having stored thereon instructions which may be used to program a computer (e.g., a processor or other electronic device) to perform a sequence of operations. A machine-readable medium” may include any computer program product, apparatus and/or device (e.g., magnetic discs, optical disks, memory, Programmable Logic Devices (PLDs) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions as a machine-readable signal. Alternatively, the operations may be performed by a combination of hardware and software. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or optical cards, propagation media or other type of media/machine-readable medium suitable for storing electronic instructions. For example, elements of the present invention may be downloaded as a computer program product, wherein the program may be transferred from a remote computer or telephonic device to a requesting process by way of data signals embodied in a carrier wave or other propagation medium via a communication link (e.g., a modem or network connection). 
     Although the present invention has been described with reference to specific exemplary embodiments, it should be understood that numerous changes in the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit and scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.