Patent Publication Number: US-7596629-B2

Title: System and method for interconnecting heterogeneous layer 2 VPN applications

Description:
STATEMENT OF RELATED APPLICATIONS 
     The present invention claims priority from U.S. Provisional App. No. 60/428,198, filed on Nov. 21, 2002, entitled “Method to Interconnect Heterogeneous Layer 2 VPN Applications,” the contents of which are herein incorporated by reference in their entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to data networking and more particularly to virtual private networks. 
     Virtual private network technology is a popular and effective way of interconnecting geographically dispersed nodes belonging to a private network operator such as an enterprise via a service provider network that is shared with other data services such as other virtual private networks or public data services. The enterprise achieves the connectivity of a private network without having to own and operate its own network infrastructure over a wide geographic region. 
     A particular type of network is known as a layer 2 virtual private network (L2VPN). Layer 2 frames such as, e.g., Ethernet frames, PPP frames, etc., are carried across the service provider network via tunnels. The service provider network may be, e.g., a packet switched network that uses IP or MPLS or a combination of both, and the tunnels can be IP or MPLS tunnels. The layer 2 virtual private network can allow remote nodes to connect to one another as if they were connected to a shared physical medium even though their connection is in fact across the service provider network cloud. For example, a virtual private LAN may be configured across the service provider network. 
     Consider the structure of a layer 2 virtual private network. The layer 2 virtual private network interconnects remote customer networks via the service provider network. The service provider network is a packet-switched network. On the provider side of the border between the provider network and the customer network there are one or more provider edge routers. On the customer side of the border there are one or more customer edge routers. 
     An “attachment circuit” is a single 2-way physical or virtual link between a provider edge router and a customer edge router. For example, an attachment circuit may be, e.g., an RS-232 serial line, a point-to-point Ethernet connection, an ATM virtual circuit, etc. 
     A “pseudowire” is an emulated 2-way circuit across the provider network. A pseudowire may be implemented as a pair of VC LSPs (Virtual Circuit Label Switched Paths), one in each direction in an MPLS network. Multiple pseudowires may share the same tunnel LSP. 
     One application that may be provided across such a network is a simple cross-connection between attachment circuits via a pseudowire. This is sometimes referred to as a virtual private wire service (VPWS). Traffic received from an attachment circuit on a provider edge router is forward to a remote provider edge router via the pseudowire. When the remote provider edge router receives traffic from the pseudowire, it selects the correct receiving attachment circuit based on an encapsulation demultiplexing identifier (e.g., an MPLS label) assigned to traffic for that pseudowire. 
     In a variant of the just-described VPWS implementation, multiple attachment circuits are aggregated into what is referred to as a “colored pool”. For example, a colored pool might contain all of the attachment circuits between a given provider edge router and a given customer edge router. A pseudowire may connect two colored pools on remote provider edge routers by connecting two arbitrary attachment circuits belonging to the two pools. 
     Another application that may be provided is a virtual private LAN service (VPLS). To implement a VPLS, each participating provider edge router is fitted with a virtual switching instance (VSI). A VSI operates as a type of virtual LAN switch between one or more attachment circuits and one or more pseudowires. When a frame arrives at a VSI via an attachment circuit or pseudowire, a layer 2 address of the frame is used to pick an output attachment circuit or pseudowire. A virtual private LAN can be constructed as a mesh of pseudowires between VSIs on participating provider edge routers. 
     Generically, a single VSI used to implement VPLS, or a colored pool or single attachment circuit used to implement VPWS, may be referred to as a “forwarding instance” or “forwarder.” Currently, a forwarder (colored pool or single attachment circuit) associated with a VPWS can only connect to another forwarder associated with a VPWS and a forwarder (VSI) associated with a VPLS can only connect with another forwarder (VSI) associated with a VPLS. It would be desirable to provide connections between heterogeneous layer 2 VPN applications, e.g., between a VPLS forwarder and a VPWS forwarder. 
     A difficulty arises in signaling the connection of such disparate forwarders via a pseudowire. On approach to signaling interconnections between forwarders is described in Martini, et al., “Transport of Layer 2 Frames Over MPLS,” IETF Internet Draft, November 2002, the contents of which are herein incorporated by reference for all purposes in their entirety. A virtual circuit identifier is used to identify a pseudowire. When forwarders wish to connect, they must be preconfigured with the same virtual circuit identifier so that they can refer to it when signaling the connection. Auto-discovery of remote forwarders does not avoid the need for a priori knowledge of the virtual circuit identifiers. Also, every time a remote forwarder must be shifted to a different provider edge router due to system maintenance needs, both the remote forwarder and a local forwarder have to be reconfigured. 
     Another method of signaling interconnections between forwarders is described in Rosen, et al., “LDP-Based Signaling for L2VPNs,” IETF Internet Draft, September 2002, the contents of which are herein incorporated by reference for all purposes in their entirety. Each forwarder is assigned an “attachment identifier” that is unique on a particular provider edge router. The combination of the provider edge router&#39;s address and the attachment identifier provides a globally unique identifier for the forwarder. However, the signaling procedures do not use the globally unique form of the identifier but instead use various components of the globally unique identifier and the components that are used change depending on which application is being supported and, for VPWS, whether the connection is to be between colored pools. It is therefore difficult to interconnect forwarders corresponding to disparate layer 2 virtual private network applications because the signaling mechanism lacks a generalized application-independent method for uniquely specifying a forwarder. A further obstacle to interconnection is that different VPN applications are assigned different multiprotocol BGP subsequent address family identifiers, making auto-discovery of disparate types of forwarders impossible. 
     Systems and methods for interconnecting forwarders belonging to heterogeneous layer 2 virtual private network applications are needed. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide systems and methods for interconnecting heterogeneous layer 2 virtual private network applications. To facilitate such interconnections, a common addressing scheme for forwarders is provided. All current pseudowire signaling protocols can incorporate this addressing scheme, and therefore establish connectivity among forwarders of different applications. Auto-discovery of remote forwarders is also facilitated by use of a common address family identifier (and subsequent address family identifier) for BGP. 
     One aspect of the present invention provides a method for operating a first provider edge node in a virtual private network. The method includes: providing a source forwarder in the first provider edge node wherein a target forwarder is provided at a second provider edge node of the virtual private network, sending a request from the first provider edge node to the second provider edge node for connection of the source forwarder to the target forwarder via a pseudowire, and including in the request an identifier of the target forwarder. The identifier includes a portion belonging to an address space shared by forwarders that are connected to multiple attachment circuits to customer nodes and forwarders that are connected to a customer node via exactly one attachment circuit. The identifier is globally unique within the virtual private network. 
     Further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a network reference model for a layer 2 virtual private network. 
         FIG. 2  depicts a layer 2 virtual private network using virtual private wire service (VPWS). 
         FIG. 3  depicts a layer 2 virtual private network using virtual private LAN service (VPLS). 
         FIG. 4  depicts a layer 2 virtual private network employing both VPWS and VPLS according to one embodiment of the present invention. 
         FIG. 5  depicts structure of an identifier of a forwarder operating on a provider edge router according to one embodiment of the present invention. 
         FIG. 6  is a flowchart describing steps of establishing pseudowires between two provider edge routers according to one embodiment of the present invention. 
         FIG. 7  depicts a network device useful in implementing embodiments of the present invention. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Embodiments of the present invention find application in layer 2 virtual private networks. Layer 2 virtual private networks carry layer 2 frames within tunnels across a packet-switched network. Background information about layer 2 virtual private networking is found above and in the following documents, the contents of which are herein incorporated by reference in their entirety for all purposes: 
     Lau, et al. “Layer Two Tunneling Protocol (Version 3),” IETF Internet Draft, January 2003. (hereinafter “Lau, et al.”) 
     Townsley, “Pseudowires and L2TPv3,” IETF Internet Draft, June 2002. 
     Townsley, et al., “Layer Two Tunneling Protocol “L2TP,” IETF Request for Comments 2661, August 1999. 
     Rosen, “LDP-based signaling for L2VPNs,” IETF Internet Draft, September 2002. 
     Martini, et al. “Transport of Layer 2 Frames Over MPLS,” IETF Internet Draft, November 2002. 
     Andersson “L2VPN Framework,” IETF Internet Draft, January 2003. 
       FIG. 1  depicts a layer 2 virtual private network reference model. Two provider edge routers  102  and  104  are interconnected by packet-switched network. Provider edge router  102  is connected to a customer edge router  106  via an attachment circuit. Provider edge router  104  is connected to customer edge router  108  via another attachment circuit. A pseudowire extends between provider edge router  102  and provider edge router  104  across the packet-switched provider network. 
     In a simple cross-connect application, an attachment circuit is directly bound to the pseudowire. This is a one-to-one mapping. Layer 2 connectivity is provided between customer edge routers  106  and  108 . 
       FIG. 2  depicts a VPWS scenario where interconnectivity among customer edge routers  202  through a packet-switched network is provided by provider edge routers  204 . Each customer edge router  202  is connected to its local provider edge router  204  via an attachment circuit. Each attachment circuit is bound to a particular pseudowire that itself connects to a remote provider edge router. When traffic is received at a provider edge router via an attachment circuit, it is automatically forwarded to the corresponding pseduowire. Similarly, when traffic is received via a pseudowire, it is automatically transferred to the bound attachment circuit. 
     One can also aggregate sets of attachment circuit into colored pools. For example, a single colored pool may include all the attachment circuits between a given customer edge router and a given provider edge router. A cross-connect pseudowire that connects to a colored pool will actually connect to an arbitrary one of the attachment circuits belonging to the colored pool. 
       FIG. 3  depicts a simple VPLS scenario. A virtual private LAN has been established to interconnect customer edge routers  302 . Each of provider edge routers  304  is fitted with a single VSI. Of course in a real application, provider edge routers may be fitted with multiple VSIs. Multiple attachment circuits and pseudowires are connected to each VSI. The VSI acts as a virtual LAN switch. The VSI forwards frames based on their layer 2 identifiers. 
     In both the VPLS and VPWS scenarios, it is convenient to refer to an element to which the pseduowire connects as a “forwarder.” In the VPLS scenario, the forwarder is a VSI. In the VPWS, the forwarder is an attachment circuit, except that it may be a colored pool to which the attachment circuit belongs. 
       FIG. 4  depicts a heterogeneous layer 2 virtual private networking scenario among customer edge routers  408  according to one embodiment of the present invention. In  FIG. 4 , provider edge router  402  has a VSI and thus implements a VPLS application for its customer edge routers. By contrast, provider edge routers  404  and  406  provide one-to-one couplings between pseudowires and attachment circuits and thus provide a VPWS for their customer edge routers. 
     Embodiments of the present invention allow one to establish a virtual private network as in  FIG. 4  where pseudowires interconnect disparate types of forwarder. It will of course be appreciated that the networking scenario of  FIG. 4  is greatly simplified compared to many networks that may be achieved. For example, there may be numerous VSIs and VPWS attachment circuits at a single provider edge router and disparate applications may both be present at the same provider edge router. The overall effect is to provide a heterogeneous set of layer 2 virtual private network layer applications among a set of customer edge routers  408 . 
     To support the signaling of interconnections, each forwarder is assigned a Forwarder Identifier that is unique at a given provider edge router.  FIG. 5  depicts the structure of this Forwarder Identifier according to one embodiment of the present invention. The Forwarder Identifier is a variable length value. 
     Furthermore, each provider edge router has an associated Router ID which is a 32-bit value that is preferably globally unique within a layer 2 virtual private network. If the 32-bit Router ID value is not globally unique, it may be combined with a 64-bit Router Distinguisher as described in Rosen, et al., “BGP/MPLS VPNs,” IETF Request for Comments 2547, March 1999, the contents of which are herein incorporated by reference for all purposes in their entirety. A common layer 2 virtual private network address is defined as a concatenation of the Router ID (or globally unique extension thereof) and Forwarder Identifier. This is a globally unique address that identifies the forwarder. 
     Before discussing the signaling of pseudowires between disparate forwarders, we will introduce auto-discovery of remote forwarders and establishment of control connections between provider edge routers. The common layer 2 virtual private network address facilitates auto-discovery. For example, BGP may be used for auto-discovery of remote forwarders. For example, the techniques used in Bates, et al., “Multiprotocol Extensions for BGP-4,” IETF Request for Comments (RFC) 2858, June 2002, may be used. The contents of RFC 2858 are herein incorporated by reference in their entirety for all purposes. The Network Layer Reachability Information (NLRI) for RFC 2858 is encoded as one or more tuples of the form [Length, Prefix]: 
     Length: one octet that indicates the length in bits of the common layer 2 virtual private network address. 
     Prefix: a variable length layer 2 virtual private network address. 
     A common Address Family Identifier (AFI) and a common Subsequent Address Family Identifier (SAFI) are defined for all forwarder types used in layer 2 virtual private networking. In this way, Multiprotocol BGP advertisements can be used to learn about forwarders of all types on remote provider edge nodes and their properties such as, e.g., next-hop address, forwarder type, etc. 
     BGP is not the only method for discovering information about remote forwarders. Directory-based discovery mechanisms, such as RADIUS, may also be used. It is also possible to manually configure information about remote forwarders. 
     Pseudowires between provider edge routers are established within tunnels. An individual tunnel between two provider edge routers can carry multiple pseudowires. Also, between two provider edge routers there will be a control channel. The control channel is used for signaling of pseudowires. Embodiments of the present invention operate in the context of pseudowire signaling protocols such as e.g., Layer 2 Tunneling Protocol or Label Distribution Protocol. Pseudowire signaling and operation of the control channel in the context of the Layer 2 Tunneling Protocol is described in detail in Lau, et al. Pseudowire signaling and operation of the control channel in an LDP context is described in Andersson, et al., “LDP Specificiation,” IETF Request for Comments 3036, January 2001, the contents of which are herein incorporated by reference for all purposes. 
     An overview of layer 2 virtual private network signaling procedures will now be presented. The procedures apply generally to situations where forwarders of the same type are being connected and to situations where forwarders of different types are being connected. The procedures are generic to the various pseudowire signaling schemes provided by, e.g., Layer 2 Tunneling Protocol, LDP, etc. 
     Assume a provider edge router knows that it wants to set up a pseudowire between a local forwarder having an assigned Forwarder Identifier and a remote forwarder on a remote provider edge router that has a certain Forwarder Identifier. Before establishing the intended pseudowire, the pair of provider edge routers exchanges control connection messages to establish a control connection. Each may advertise in these control connection messages its supported pseudowire types (e.g., Ethernet, ATM, Frame Relay, etc.) in a pseudowire capabilities list. Each may examine whether the remote provider edge router supports the pseudowire type it intends to set up. 
     After the control connection is established, either of the two provider edge router may send a control message to one another to initiate a pseudowire. The control message includes a local and remote forwarder identifier specifying the two forwarders which intend to be interconnected. 
     When the local provider edge router receives this control message, it determines whether it has a local forwarder assigned with an ID value specified by this remote forwarder identifier. The local provider edge router also examines whether the remote forwarder is allowed to connect with this specified local forwarder. If both conditions are met, the local provider edge router accepts the connection request. If either of the two conditions fails, it rejects the connection request. 
     Now a procedure will be defined for establishing a layer 2 virtual private network among forwarders on multiple provider edge routers. Again, the forwarders may either be of the same type or of different types. 
     Each provider edge router forms a list of source forwarders consisting of all local forwarders to be used by the virtual private network that is being established. The provider edge router then establishes a list of target forwarders which includes all of the local forwarders for the given virtual private network and all of the remote forwarders on other provider edge routers of the same virtual private network. Formation of the network topology depends on the contents of these lists of source forwarders and target forwarders. The two lists can be constructed by manual configuration and/or by auto-discovery. 
       FIGS. 6A-6B  are flow charts describing steps to be performed at a single provider edge router in establishing a layer 2 virtual private network according to one embodiment of the present invention. This procedure of  FIGS. 6A-6B  is repeated on every provider edge router involved in the layer 2 virtual private network. The procedure of  FIGS. 6A-6B  begin with the lists of source forwarders and target forwarders having already been defined. Each forwarder is specified as a combination of Router ID and Forwarder Identifier. 
     At step  602 , the provider edge router picks the next forwarder from the list of source forwarders. Step  604  tests whether there are in fact any source forwarders to pick from. If no forwarder is available, processing is complete. Step  606  picks the next target forwarder from the list of target forwarders. Step  608  tests whether there are any target forwarders to pick from. If no target forwarders remain, then processing proceeds back to step  602  to pick the next source forwarder. 
     Step  610  compares the Router IDs of the selected source forwarder and target forwarder. If the Router IDs are different, then processing proceeds to step  612  since a pseudowire should be established between the two forwarders. Step  612  tests whether a pseudowire has already been established between the source forwarder and target forwarder. If a pseudowire has already been established, then processing proceeds back to step  606  to pick the next target forwarder. 
     If a pseudowire has not already been established between the source and target processing proceeds to step  614 . At step  614 , the local provider edge router obtains the address of the remote provider edge router, e.g., from previous manual configuration, BGP, directory look-up, etc., and establishes a control connection to the remote provider edge router if one does not already exist. Step  614  then sends a connection request to the remote provider edge router hosting the target forwarder. The Forwarder Identifiers of the source and target forwarders are encoded as the Local Forwarder Identifier and Remote Forwarder Identifier respectively in the connection request message. 
     Referring again to step  610 , if the Router IDs of the source forwarder and target forwarder are the same, processing proceeds to a step  611 . Step  611  tests whether the Forwarder Identifiers are also the same. If the Forwarder Identifiers are the same, the source forwarder and target forwarder are the same forwarder and processing returns to step  606  to pick the next target forwarder. If the Forwarder Identifiers are not the same, the source forwarder and target forwarder are not the same forwarder but are on the same provider edge router. Instead of a pseudowire, a local cross-connect is established between the source forwarder and target forwarder at step  613 . 
     Following step  614 , at step  616  the local provider edge router awaits a response from the remote provider edge router. The local provider edge router could receive either a response to its connection request or a separate connection request from the remote provider edge router. Step  618  represents receipt of a response from the remote provider edge router. Step  620  tests whether the request has been accepted or rejected. If the connection has been rejected, processing proceeds back to step  606  to pick the next target forwarder. If the connection is accepted, then at step  622  the local provider edge router binds the source forwarder to the pseudowire. Step  622  also completes the remaining signaling, if any, to the remote provider edge router and returns processing to step  606  to select the next target forwarder. 
     Step  624  corresponds to the case where the local provider edge router receives another pseudowire connection request from the remote provider edge router while awaiting a response to its own request. In this case, a step  626  performs pseudowire tie detection. Pseudowire tie detection is performed to avoid setting up duplicated pseudowires between two forwarders. If the received connection request and the transmitted connection request both specify the same pair of forwarders then a tie is detected. Step  628  tests whether or not a tie has been detected in step  626 . 
     If a tie has been detected, then tie breaking is performed at step  630  to determine which of the requested connections should proceed. The tie-breaking procedure is specified by the operative pseudowire signaling protocol and does not form a part of the present invention. A step  632  tests whether the local provider edge router or remote provider edge router won the tie breaking procedure. If the local provider edge router won the tie then at step  634  it ignores the remote connection request and proceeds back to step  616 . If the remote provider edge router won the tie, then processing proceeds to step  636  where the local provider edge router sends a disconnect message to the remote provider edge router to withdraw its previous connection request. Processing then proceeds to step  638  to test whether the received remote connection request should be accepted. 
     The remote connection request should be accepted only if there is a local forwarder having the Forwarder Identifier value specified in the remote request and the remote forwarder is allowed to connect with a specified local forwarder. If either of these conditions are not met, then at step  640 , the remote request is rejected by sending a disconnect message to the provider edge router. If both conditions are met, the local provider edge router binds the source forwarder to a pseudowire to the target forwarder at step  642 . After either step  640  or step  642 , processing proceeds back to step  606 . 
     Network Device Details 
       FIG. 7  depicts a network device  700  that may be used to implement, e.g., the provider edge routers of  FIG. 4  and/or perform any of the steps of  FIGS. 6A-6B . In one embodiment, network device  700  is a programmable machine that may be implemented in hardware, software or any combination thereof. A processor  702  executes code stored in a program memory  704 . Processor  702  may perform the encapsulation, de-encapsulation, and flow control operations referred to above. Program memory  704  is one example of a computer-readable storage medium. Program memory  704  can be a volatile memory. Another form of computer-readable storage medium storing the same codes would be some type of non-volatile storage such as floppy disks, CD-ROMs, DVD-ROMs, hard disks, flash memory, etc. A carrier wave that carries the code across a network is an example of a transmission medium. 
     Network device  700  interfaces with physical media via a plurality of linecards  706 . Some of linecards  706  may interface to customer edge routers while others may act as interfaces into the packet-switched network. It will be appreciated that linecards  706  may also implement virtual interfaces. The attachment circuits and pseudowires themselves may be hosted on the linecards  706 . As packets are received, processed, and forwarded by network device  700 , they may be stored in a packet memory  708 . 
     Network device  700  may implement all of the network protocols and extensions thereof described above as well as the data networking features provided by the present invention. Much of the functionality may be left to the linecards  706 . It will be understood the linecards  706  may themselves contain processing resources, software, and other resources as referred to in reference to the description of network device  700  as a whole. 
     It is understood that the examples and embodiments that are described herein are for illustrative purposes only and that various modifications and changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims and their full scope of equivalents.