Patent Publication Number: US-11038634-B2

Title: Control for BFD return path

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 14/512,259 filed Oct. 10, 2014 (now U.S. Pat. No. 9,979,515 issued May 22, 2018), which claims the benefit of U.S. Provisional Application No. 62/019,244, filed Jun. 30, 2014, which is incorporated herein by reference. 
    
    
     FIELD 
     Embodiments of the invention relate to detection of data plane failures. Specifically, the embodiments relate to a method for detecting data plane failures when explicit paths and best route paths utilized in the forward and reverse directions between two nodes in a network are not co-routed. 
     BACKGROUND 
     Bidirectional forwarding detection (BFD) is a network protocol used to detect faults between two nodes in a network connected by a link or path across the network. The Internet Engineering Task Force (IETF) Request for Comments (RFC) 5880 sets forth that the base protocol for “Bidirectional Forwarding Detection (BFD)” may apply to any network, including to Internet Protocol (IP) networks. Predominantly, path selection between nodes in IP networks is controlled through dynamic routing protocols and selection of the best available route to a given network node. Routing protocols and rules of selecting the best route are designed to ensure that an IP packet follows the best route (e.g., the route with the lowest cost measure) from one node to another node towards a destination without causing the IP packet to loop in the network. 
     The BFD protocol uses control packets to detect failures in the network. These control packets are sent between the end nodes of a BFD session, where a failure of N consecutive control packets to arrive at the opposite end indicates a failure in the link or path, where N is defined as the Detect Multiplier in RFC 5880 and may be set to 1, but more often has a value of 3. BFD control packets, like other IP data packets, will follow the best route to the destination node and thus ensure an in-band requirement that is set forth toward Operation, Administration, and Maintenance (OAM) mechanisms. Because IP routing is usually symmetrical (i.e., the best routes between two IP end points traverse the same nodes), BFD control packets in the forward and reverse directions of a BFD session between two nodes usually traverse the same nodes. As a result, any detected unidirectional failure in the link or path can be considered as a bidirectional defect and can be acted upon accordingly. However, the best route model does not allow control of traffic distribution beyond the best route paths. 
     An alternative to the best route paradigm is a method wherein an entire path is predetermined, for example, either by a controller or at the ingress node. This can be done either by directing packets into an explicitly-routed tunnel or by explicitly specifying all intermediate nodes for each packet. In general, such paths need to be viewed as unidirectional. Because BFD is predominantly used in its asynchronous mode, the far-end peer node is likely to select the best available route when sending its BFD control packets. As a result, BFD control packets may not cross the same set of nodes in the forward and reverse directions. Thus, a unidirectional failure cannot be interpreted as an indication of a bidirectional defect, and in certain scenarios, a defect would not be detected for an extended period of time. 
     SUMMARY 
     A method is implemented by a network device to establish a bidirectional forwarding detection (BFD) session with a defined return path to enable detection of data plane failures between an active node and a passive node in a network where a forward path and a reverse path between the active node and the passive node are not co-routed. The method includes receiving a label switched path (LSP) ping including a BFD discriminator type length value (TLV) of the active node and a return path TLV describing a return path to the active node. A BFD discriminator of the active node and a BFD discriminator for the passive node are associated with the BFD session. The return path is associated with the BFD session between the active node and the passive node, and BFD control packets are sent to the active node using the return path to detect a failure on the return path. 
     A network device establishes a bidirectional forwarding detection (BFD) session with a defined return path to enable detection of data plane failures between an active node and a passive node in a network where a forward path and a reverse path between the active node and the passive node are not co-routed. The network device includes a non-transitory machine-readable storage medium configured to store a BFD module and a return path module and a network processor. The network processor is communicatively coupled to the non-transitory machine-readable storage medium. The network processor is configured to execute the BFD module and the return path module. The BFD module is configured to receive an LSP ping including a BFD discriminator TLV of the active node and a return path TLV describing a return path to the active node, and to associate a BFD discriminator of the active node and a BFD discriminator for the passive node with the BFD session. The return path module is configured to associate the return path with a BFD session between the active node and the passive node, and to send BFD control packets to the active node using the return path to detect a failure on the return path. 
     A computing device executes a plurality of virtual machines for implementing network function virtualization (NFV), wherein a virtual machine from the plurality of virtual machines is configured to execute a method to establish a bidirectional forwarding detection (BFD) session with a defined return path to enable detection of data plane failures between an active node and a passive node in a network where a forward path and a reverse path between the active node and the passive node are not co-routed. The computing device includes a non-transitory machine-readable storage medium configured to store a BFD module and a return path module. The computing device also includes a processor communicatively coupled to the non-transitory machine-readable storage medium. The processor is configured to execute the virtual machine that executes the BFD module and the return path module. The BFD module is configured to receive an LSP ping including a BFD discriminator TLV of the active node and a return path TLV describing a return path to the active node, and to associate a BFD discriminator of the active node and a BFD discriminator of the passive node with the BFD session. The return path module is configured to associate the return path with a BFD session between the active node and the passive node, and to send BFD control packets to the active node using the return path to detect a failure on the return path. 
     A control plane device is configured to implement at least one centralized control plane for a software defined network (SDN). The centralized control plane is configured to execute a method to establish a bidirectional forwarding detection (BFD) session with a defined return path to enable detection of data plane failures between an active node and a passive node in a network where a forward path and a reverse path between the active node and the passive node are not co-routed. The control plane device includes a non-transitory machine-readable storage medium configured to store a BFD module and a return path module and a processor communicatively coupled to the non-transitory machine-readable storage medium. The processor is configured to execute the BFD module and the return path module. The BFD module is configured to determine a BFD discriminator of the active node and a return path to the active node, and to associate the BFD discriminator of the active node and a BFD discriminator of the passive node with the BFD session. The return path module is configured to associate the return path with a BFD session between the active node and the passive node, and to configure the passive node to send BFD control packets to the active node using the return path to detect a failure on the return path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings: 
         FIG. 1  is a diagram of one embodiment of an example network in which bidirectional forwarding detection (BFD) is implemented. 
         FIG. 2A  is a flowchart of one embodiment of a process for establishing a BFD session by a near-end node. 
         FIG. 2B  is a flowchart of one embodiment of a process for establishing a BFD session by a far-end node. 
         FIG. 3  is a diagram of one embodiment a network device (ND) implementing an asynchronous BFD session with support for explicit return paths. 
         FIG. 4A  illustrates connectivity between network devices (NDs) within an exemplary network, as well as three exemplary implementations of the NDs, according to some embodiments of the invention. 
         FIG. 4B  illustrates an exemplary way to implement the special-purpose network device  402  according to some embodiments of the invention. 
         FIG. 4C  illustrates various exemplary ways in which virtual network elements (VNEs) may be coupled according to some embodiments of the invention. 
         FIG. 4D  illustrates a network with a single network element (NE) on each of the NDs of  FIG. 4A . 
         FIG. 4E  illustrates an example where each of the NDs implements a single NE (see  FIG. 4D ), but the centralized control plane has abstracted multiple ones of the NEs in different NDs into a single NE in one of the virtual network(s) of  FIG. 4D , according to some embodiments of the invention. 
         FIG. 4F  illustrates a case where multiple VNEs are implemented on different NDs and are coupled to each other, and where the centralized control plane has abstracted these multiple VNEs such that they appear as a single VNE within one of the virtual networks of  FIG. 4D , according to some embodiments of the invention. 
         FIG. 5  illustrates a general purpose control plane device including hardware comprising a set of one or more processor(s) (which are often Commercial off-the-shelf (COTS) processors) and network interface controller(s) (NICs; also known as network interface cards) (which include physical NIs), as well as non-transitory machine readable storage media having stored therein centralized control plane (CCP) software), according to some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description describes methods and apparatus for establishing an asynchronous BFD session with support for defining an explicit return path that can be co-routed or not co-routed with a forward path. In the following description, numerous specific details such as logic implementations, opcodes, means to specify operands, resource partitioning/sharing/duplication implementations, types and interrelationships of system components, and logic partitioning/integration choices are set forth in order to provide a more thorough understanding of the present invention. It will be appreciated, however, by one skilled in the art that the invention may be practiced without such specific details. In other instances, control structures, gate level circuits and full software instruction sequences have not been shown in detail in order not to obscure the invention. Those of ordinary skill in the art, with the included descriptions, will be able to implement appropriate functionality without undue experimentation. 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Bracketed text and blocks with dashed borders (e.g., large dashes, small dashes, dot-dash, and dots) may be used herein to illustrate optional operations that add additional features to embodiments of the invention. However, such notation should not be taken to mean that these are the only options or optional operations, and/or that blocks with solid borders are not optional in certain embodiments of the invention. 
     In the following description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. “Coupled” is used to indicate that two or more elements, which may or may not be in direct physical or electrical contact with each other, co-operate or interact with each other. “Connected” is used to indicate the establishment of communication between two or more elements that are coupled with each other. 
     The operations in the flow diagrams will be described with reference to the exemplary embodiments of the other figures. However, it should be understood that the operations of the flow diagrams can be performed by embodiments of the invention other than those discussed with reference to the other figures, and the embodiments of the invention discussed with reference to these other figures can perform operations different than those discussed with reference to the flow diagrams. 
     Overview 
     The IETF RFC 5880, the IETF RFC 5881 entitled “Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop),” and the IETF RFC 5883 entitled “Bidirectional Forwarding Detection (BFD) for Multihop Paths” established the BFD protocol for Internet Protocol (IP) networks. The IETF RFC 5884 entitled “Bidirectional Forwarding Detection (BFD) for MPLS Label Switched Paths (LSPs)” describes a procedure for using BFD in asynchronous mode over IP/Multiprotocol Label Switching (MPLS) LSPs. These standards implicitly presume that the best route (e.g., the route with the lowest cost measure) between a near-end node establishing a BFD session and a far-end peer node of the BFD session will be used, regardless of the actual route used by that far-end peer node to send its BFD control packets to the near-end node. The term near-end node indicates a node that initiates a BFD session with a target node. The term far-end node indicates the target node of a BFD session. These can be any two nodes in a network that support the BFD related standards. 
     BFD has two operational modes, (1) demand mode and (2) asynchronous mode. In demand mode, no hello packets are automatically exchanged after the session is established; rather, the two endpoints can exchange hello packets as needed. Asynchronous mode provides the scheduled or periodic exchange of hello packets or similar BFD control packets between the two end nodes. When these BFD control packets fail to arrive at the destination node then a failure is assumed to have occurred along the path. The embodiments described herein contemplate a modification to the asynchronous mode of operation. 
     As discussed above, BFD is expected to monitor bidirectional co-routed paths (i.e., the forward direction path from the near-end node to the far-end node, and the return direction path from the far-end node to the near-end node). In most IP and IP/MPLS networks, the best route between two IP nodes is likely to be co-routed in a stable network environment, and thus, that implicit BFD requirement is fulfilled. However, when the forward direction path of a BFD session is explicitly routed (e.g., not using the best route) while the return direction path of the BFD session uses the best route, several scenarios present a problem:
         a. A failure detected on the return direction path cannot be interpreted as a bidirectional failure, because the forward and return direction paths are not co-routed. If it is incorrectly interpreted as a bidirectional failure, unnecessary protection switchover of the forward direction path may occur.   b. If the return direction path is in the down state, the near-end peer node would not receive an indication of a forward direction path failure from its far-end peer node.       

       FIG. 1  illustrates nodes and links of an example network wherein the BFD-monitored paths in the forward and return directions between peer nodes A and E are not co-routed. This example network is used to show the problems in BFD monitoring where the forward direction path and the return direction path are not co-routed. In the example BFD session of  FIG. 1 , peer node A is the near-end peer node, and peer node E is the far-end peer node. The path in the forward direction between peer nodes A and E is explicitly routed as A-B-C-D-E. In contrast, the path in the return direction between peer nodes E and A is the best route E-G-F-A. Thus, the BFD-monitored paths in the forward and return directions are not co-routed at any intermediate node or link. If a failure on the path in the return direction (e.g., a failure in the link between nodes F and G) is detected by peer node A, that failure should not be interpreted as a failure on the path in the forward direction, on the explicitly-routed A-B-C-D-E path. Moreover, if peer node E detects a failure on the path in the forward direction (e.g., a failure in the link between nodes C and D on the explicitly-routed A-B-C-D-E path), then peer node A should not receive an indication of this forward direction failure from peer node E, even for example, if the return direction path is not in the down state, as the peer node E can continue to function on its return path. 
     The embodiments of the invention provide a system and method to overcome these limitations of the existing standards, such that the method to use BFD in MPLS networks can properly handle non-co-routed forward direction path and return direction path. In the embodiments, a session initiating near-end peer node uses LSP Ping with a BFD Discriminator Type-Length-Value (TLV) so that the far-end peer node can properly associate the BFD session with the sending near-end peer node. In effect, LSP Ping bootstraps a BFD session between two peer nodes, e.g., Label Switch Routers (LSRs). The embodiments further introduce a mechanism for the near-end peer node to instruct the far-end peer node to use a particular path or an explicit MPLS label stack to send the far-end peer node&#39;s BFD control packets in the return direction of the BFD session that is being bootstrapped. 
     The embodiments further overcome the limitations of the prior art such that a near-end peer node (e.g., an LSR) that is to monitor an explicitly-routed unidirectional LSP uses LSP Ping to bootstrap the BFD session by informing the far-end peer node (e.g., an LSR) of the near-end peer node&#39;s BFD Discriminator (i.e., using the BFD Discriminator TLV). The near-end peer node includes an LSP identifier (ID) or lists an explicit MPLS label stack in a BFD Return Path TLV (referred to herein simply as the return path TLV) to be used by the far-end peer node to send the far-end peer node&#39;s BFD control packets in the return direction. 
     When the far-end peer node receives the LSP Ping, the far-end peer node locates the BFD Discriminator TLV from the LSP Ping and associates the LSP with the value of the near-end peer node&#39;s BFD Discriminator. The far-end peer node then locates the BFD Return Path TLV with the LSP ID or the explicit MPLS label stack from the LSP Ping. The far-end peer node verifies availability of the specified path (i.e. the path indicated by the LSP ID or the explicit MPLS label stack) and, if the availability is verified, associates this specified path with the BFD session. The far-end peer node then begins to send its BFD control packets with the My Discriminator value set to the far-end peer node&#39;s BFD Discriminator, and the Your Discriminator value set to the value received in the BFD Discriminator TLV (i.e., the near-end peer node&#39;s BFD Discriminator) over the path specified in the BFD Return Path TLV. Alternatively, if the specified path is not available, the far-end peer node notifies the near-end peer node of the error using an LSP Ping Reply and may use the best route to send BFD control packets in the return direction. 
     In other embodiments, other mechanisms may be used to instruct the far-end peer node (e.g., an LSR) to use a particular return path to send BFD control packets in the return direction. For example, the path could be collected as the packet flies by, centralized control (e.g., via an Operations Support System (OSS) or Software-Defined networking (SDN)) could be used to retrieve information about, or an indication of, the particular path to use in the reverse direction, the particular path could be encoded in an identifier, or similar techniques can be implemented. 
     The embodiments provide significant benefits over the prior art BFD systems. For example, directing the far-end peer node of the BFD session to use a particular path for sending the far-end peer node&#39;s BFD control packets in the return direction enables those BFD control packets to traverse the same nodes and links as traversed in the forward direction, even when the BFD-monitored path in the forward direction is explicitly routed. Moreover, because the BFD session is maintained over a bidirectional co-routed path when using the process described herein, detection of a failure in one direction can be reliably interpreted and treated as an indication of a bidirectional defect in the path. 
       FIG. 2A  is a flowchart of one embodiment the process for establishing a BFD session by the near-end node. The near-end node can be any node in any type of network. A node, as referred to herein, is a computing device or networking device that can communicate over a connected network with other nodes. The BFD session can be initiated as directed by a user or based on a local policy of the implementing device. The near-end node can also be referred to herein as an ‘active’ node to distinguish it from the far-end node or ‘passive’ node. The active and passive nodes are peer nodes in the network with a set of defined paths such as LSPs between them. A ‘set,’ as used herein refers to any positive whole number of items including at least one item. In this context, the active node and passive node are assumed to have at least two paths between them to enable the possibility of a forward path and a return path between the active node and passive node that are not co-located such that a BFD session can be established with differing forward and return paths. 
     The process begins with initiating the BFD session by the active node sending an LSP ping to the passive node (Block  201 ). The LSP ping is utilized to bootstrap the BFD session, since BFD does not have defined a remote initiation process. The LSP ping is a type of message and, as used herein, refers to a modified form of the message that includes a discriminator TLV and a return path TLV. The BFD discriminator TLV includes a BFD discriminator that is a value utilized in the network to distinguish between nodes in the network that are communicating with the receiving node. The BFD discriminator is locally unique to the receiving node, but may not be globally unique for the network. 
     The return path TLV can identify a path between the active node and passive node by including an LSP identifier or an MPLS stack, which is a set of MPLS labels for traversing LSRs between the active node and passive node. In further embodiments, any system or format for identifying a path between the active node and passive node can be utilized such that it can be stored in a return path TLV. The LSP ping containing the BFD discriminator TLV and return path TLV causes the passive node to establish the BFD session. 
     The passive node establishes the BFD session using the BFD discriminator provided by the BFD discriminator TLV and the return path identified by the return path TLV to send BFD control packets to the active node along the return path, such that the active node receives the BFD control packets (Block  203 ). This assumes that no error occurred at the passive node when it processed the LSP ping. If an error did occur, then an LSP ping reply would be sent by the passive node and received by the active node (Block  203 ). To properly establish the BFD session, the BFD control packets sent by the passive node must include the BFD discriminator of the active node. The active node begins receiving the BFD control packets over the return path and can thereafter detect a distinct failure in the return path or forward path when the BFD control packets are not received at either the active node or passive node. Operation of the BFD session can continue indefinitely until a failure occurs or the BFD session is terminated by either the active node or passive node. 
       FIG. 2B  is a flowchart of one embodiment of the process for establishing the BFD session as implemented by the far-end or passive node in response to receiving an LSP ping. The passive node can be any node in the network other than the active node and is assumed to have at least two paths to the active node to enable the possibility of a forward path and a return path between the active node and passive node that are not co-located such that a BFD session can be established with differing forward and return paths. The passive node is configured to support BFD and bootstrapping of BFD sessions using LSP ping messages as set forth herein. The process of establishing the BFD session is initiated at the passive node when an LSP ping is received (Block  251 ). The LSP ping can be received over any port of the passive node and from any other node in the network. In the example, the LSP ping is sent by the active node. The LSP ping is modified to include at least a BFD discriminator TLV and a return path TLV. The BFD discriminator TLV includes a BFD discriminator, which is a value that identifies the sending node and is unique at least relative to the passive node. The return path TLV includes data describing a path between the active node and the passive node. The path can be identified using an LSP identifier, a MPLS stack (i.e., a set of labels that identify how a packet is to be forwarded across a set of nodes) or similar format or mechanism for identifying a path in the network. Using an explicitly defined return path can be utilized for any purpose, for example, to monitor additional paths, to match how other data flows are handled between the active and passive nodes, or for similar reasons. 
     After receipt of the LSP ping, the passive node can access the content of the LSP ping message to retrieve the BFD discriminator for the active node from the BFD discriminator TLV of the message (Block  253 ). The information in the LSP ping is also verified by the passive node including the Target Forward Equivalency Class (FEC) Stack TLV and the encapsulation of the LSP ping. If this information is validated and if a BFD discriminator was located in the BFD discriminator TLV, then the BFD discriminator of the active node can then be associated with the BFD session being established (Block  255 ). In addition, a BFD discriminator that is locally unique for the passive node is associated with the BFD session. BFD control packets sent to the active node will include the BFD discriminator of the active node and a BFD discriminator of the passive node. The passive node can also access and retrieve the return path TLV (Block  257 ). The description of the return path can then be analyzed and associated with the BFD session. 
     The analysis of the path described by the return path TLV can include verifying the availability of the return path (Block  259 ). The verification can use any process or mechanism to check the received return path against the network topology available to the passive node. Path availability can be determined on whether the active node is reachable according to the known topology of the passive node along the specified return path or based on the reachability of intermediate nodes such as the first hop according to the return path. If the path is not available, then the active node is notified of the error by generating and sending the error as part of an LSP ping reply or using a similar mechanism (Block  261 ). The passive node then uses the best available route to send the BFD control packets to the active node instead of the return path described by the received return path TLV (Block  267 ). The best available route can be determined by using any routing algorithm, for example, the general routing algorithm utilized by the network to determine paths using the shared network topology. 
     If the received return path is available, then the passive node utilizes the specified path for sending the BFD control packets to the active node. The return path is associated with the BFD session between the active node and the passive node (Block  263 ). The sending of the BFD control packets to the active node using the return path can then continue for any period of time or during the duration of the BFD session (Block  265 ). The exchange of the BFD control packets enables the separate detection of the failure of the forward path and the return path between the active node and the passive node. 
       FIG. 3  illustrates an example of a network device  301  to implement return path control functions for BFD. 
     A network device (ND) is an electronic device that communicatively interconnects other electronic devices on the network (e.g., other network devices, end-user devices). Some network devices are “multiple services network devices” that provide support for multiple networking functions (e.g., routing, bridging, switching, Layer 2 aggregation, session border control, Quality of Service, and/or subscriber management), and/or provide support for multiple application services (e.g., data, voice, and video). 
     In one embodiment, the process of  FIG. 2A  and/or  FIG. 2B  is implemented by a router  301  or network device or similar computing device. The router  301  can have any structure that enables it to receive data traffic and forward it toward its destination. The router  301  can include a network processor  303  or a set of network processors that execute the functions of the router  301 . The router  301  or network element can execute BFD including the explicit return path definition functionality via a network processor  303  or other components of the router  301 . The network processor  303  can implement the BFD and return path definition functions stored as a BFD module  307  and the return path module  351 , wherein the functions include the BFD session initiation, bootstrapping, return path definition and verification described herein above. The return path module  351  may be part of the BFD module  307 , as illustrated in  FIG. 3 , or separate from the BFD module  307 . The network processor  303  can also service the routing information base  305 A and similar functions related to data traffic forwarding and network topology maintenance. 
     The BFD and return path definition functions can be implemented as modules in any combination of software, including firmware, and hardware within the router. The functions of the BFD and return path definition processes that are executed and implemented by the router  301  include those described further herein above including the bootstrapping using the LSP ping to provide return path information as part of the establishment of a BFD session between two nodes. 
     In one embodiment, the router  301  can include a set of line cards  317  that process and forward the incoming data traffic toward the respective destination nodes by identifying the destination and forwarding the data traffic to the appropriate line card  317  having an egress port that leads to or toward the destination via a next hop. These line cards  317  can also implement the forwarding information base  305 B, or a relevant subset thereof. The line cards  317  can also implement or facilitate the BFD and return path definition functions described herein above. For example, the line cards  317  can implement LSP ping and LSP ping reply functions and similar functions. The line cards  317  are in communication with one another via a switch fabric  311  and communicate with other nodes over attached networks  321  using Ethernet, fiber optic or similar communication links and media. 
     The operations of the flow diagrams have been described with reference to the exemplary embodiment of the block diagrams. However, it should be understood that the operations of the flowcharts can be performed by embodiments of the invention other than those discussed, and the embodiments discussed with reference to block diagrams can perform operations different than those discussed with reference to the flowcharts. While the flowcharts show a particular order of operations performed by certain embodiments, it should be understood that such order is exemplary (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.). 
     As described herein, operations performed by the router may refer to specific configurations of hardware such as application specific integrated circuits (ASICs) configured to perform certain operations or having a predetermined functionality, or software instructions stored in memory embodied in a non-transitory computer readable storage medium. Thus, the techniques shown in the figures can be implemented using code and data stored and executed on one or more electronic devices (e.g., an end station, a network element). Such electronic devices store and communicate (internally and/or with other electronic devices over a network) code and data using computer-readable media, such as non-transitory computer-readable storage media (e.g., magnetic disks; optical disks; random access memory; read only memory; flash memory devices; phase-change memory) and transitory computer-readable communication media (e.g., electrical, optical, acoustical or other form of propagated signals—such as carrier waves, infrared signals, digital signals). In addition, such electronic devices typically include a set of one or more processors coupled to one or more other components, such as one or more storage devices (non-transitory machine-readable storage media), user input/output devices (e.g., a keyboard, a touchscreen, and/or a display), and network connections. The coupling of the set of processors and other components is typically through one or more busses and bridges (also termed as bus controllers). Thus, the storage device of a given electronic device typically stores code and/or data for execution on the set of one or more processors of that electronic device. One or more parts of an embodiment of the invention may be implemented using different combinations of software, firmware, and/or hardware. 
     An electronic device stores and transmits (internally and/or with other electronic devices over a network) code (which is composed of software instructions and which is sometimes referred to as computer program code or a computer program) and/or data using machine-readable media (also called computer-readable media), such as machine-readable storage media (e.g., magnetic disks, optical disks, read only memory (ROM), flash memory devices, phase change memory) and machine-readable transmission media (also called a carrier) (e.g., electrical, optical, radio, acoustical or other form of propagated signals—such as carrier waves, infrared signals). Thus, an electronic device (e.g., a computer) includes hardware and software, such as a set of one or more processors coupled to one or more machine-readable storage media to store code for execution on the set of processors and/or to store data. For instance, an electronic device may include non-volatile memory containing the code since the non-volatile memory can persist code/data even when the electronic device is turned off (when power is removed), and while the electronic device is turned on that part of the code that is to be executed by the processor(s) of that electronic device is typically copied from the slower non-volatile memory into volatile memory (e.g., dynamic random access memory (DRAM), static random access memory (SRAM)) of that electronic device. Typical electronic devices also include a set or one or more physical network interface(s) to establish network connections (to transmit and/or receive code and/or data using propagating signals) with other electronic devices. One or more parts of an embodiment of the invention may be implemented using different combinations of software, firmware, and/or hardware. 
       FIG. 4  illustrates connectivity between network devices (NDs) within an exemplary network, as well as three exemplary implementations of the NDs, according to some embodiments of the invention.  FIG. 4A  shows NDs  400 A-H, and their connectivity by way of lines between A-B, B-C, C-D, D-E, E-F, F-G, and A-G, as well as between H and each of A, C, D, and G. These NDs are physical devices, and the connectivity between these NDs can be wireless or wired (often referred to as a link). An additional line extending from NDs  400 A, E, and F illustrates that these NDs act as ingress and egress points for the network (and thus, these NDs are sometimes referred to as edge NDs; while the other NDs may be called core NDs). 
     Two of the exemplary ND implementations in  FIG. 4A  are: 1) a special-purpose network device  402  that uses custom application-specific integrated-circuits (ASICs) and a proprietary operating system (OS); and 2) a general purpose network device  404  that uses common off-the-shelf (COTS) processors and a standard OS. 
     The special-purpose network device  402  includes networking hardware  410  comprising compute resource(s)  412  (which typically include a set of one or more processors), forwarding resource(s)  414  (which typically include one or more ASICs and/or network processors), and physical network interfaces (NIs)  416  (sometimes called physical ports), as well as non-transitory machine readable storage media  418  having stored therein networking software  420 . A physical NI is hardware in a ND through which a network connection (e.g., wirelessly through a wireless network interface controller (WNIC) or through plugging in a cable to a physical port connected to a network interface controller (NIC)) is made, such as those shown by the connectivity between NDs  1200 A-H. During operation, the networking software  420  may be executed by the networking hardware  410  to instantiate a set of one or more networking software instance(s)  422 . Each of the networking software instance(s)  422 , and that part of the networking hardware  410  that executes that network software instance (be it hardware dedicated to that networking software instance and/or time slices of hardware temporally shared by that networking software instance with others of the networking software instance(s)  422 ), form a separate virtual network element  430 A-R. Each of the virtual network element(s) (VNEs)  430 A-R includes a control communication and configuration module  432 A-R (sometimes referred to as a local control module or control communication module) and forwarding table(s)  434 A-R, such that a given virtual network element (e.g.,  430 A) includes the control communication and configuration module (e.g.,  432 A), a set of one or more forwarding table(s) (e.g.,  434 A), and that portion of the networking hardware  410  that executes the virtual network element (e.g.,  430 A). In some embodiments, the control communication and configuration module  432 A encompasses the BFD module  433 A that establishes BFD sessions over connected links with remote nodes in the network. A return path module  492 A can similarly manage the handling of the explicit definition and exchange of return paths where they differ from the forward paths of the BFD sessions. 
     A network interface (NI) may be physical or virtual; and in the context of IP, an interface address is an IP address assigned to a NI, be it a physical NI or virtual NI. A virtual NI may be associated with a physical NI, with another virtual interface, or stand on its own (e.g., a loopback interface, a point-to-point protocol interface). A NI (physical or virtual) may be numbered (a NI with an IP address) or unnumbered (a NI without an IP address). A loopback interface (and its loopback address) is a specific type of virtual NI (and IP address) of a NE/VNE (physical or virtual) often used for management purposes; where such an IP address is referred to as the nodal loopback address. The IP address(es) assigned to the NI(s) of a ND are referred to as IP addresses of that ND; at a more granular level, the IP address(es) assigned to NI(s) assigned to a NE/VNE implemented on a ND can be referred to as IP addresses of that NE/VNE. 
     The special-purpose network device  402  is often physically and/or logically considered to include: 1) a ND control plane  424  (sometimes referred to as a control plane) comprising the compute resource(s)  412  that execute the control communication and configuration module(s)  432 A-R; and 2) a ND forwarding plane  426  (sometimes referred to as a forwarding plane, a data plane, or a media plane) comprising the forwarding resource(s)  414  that utilize the forwarding table(s)  434 A-R and the physical NIs  416 . By way of example, where the ND is a router (or is implementing routing functionality), the ND control plane  424  (the compute resource(s)  412  executing the control communication and configuration module(s)  432 A-R) is typically responsible for participating in controlling how data (e.g., packets) is to be routed (e.g., the next hop for the data and the outgoing physical NI for that data) and storing that routing information in the forwarding table(s)  434 A-R, and the ND forwarding plane  426  is responsible for receiving that data on the physical NIs  416  and forwarding that data out the appropriate ones of the physical NIs  416  based on the forwarding table(s)  434 A-R. 
       FIG. 4B  illustrates an exemplary way to implement the special-purpose network device  402  according to some embodiments of the invention.  FIG. 4B  shows a special-purpose network device including cards  438  (typically hot pluggable). While in some embodiments the cards  438  are of two types (one or more that operate as the ND forwarding plane  426  (sometimes called line cards), and one or more that operate to implement the ND control plane  424  (sometimes called control cards)), alternative embodiments may combine functionality onto a single card and/or include additional card types (e.g., one additional type of card is called a service card, resource card, or multi-application card). A service card can provide specialized processing (e.g., Layer 4 to Layer 7 services (e.g., firewall, Internet Protocol Security (IPsec) (RFC 4301 and 4309), Secure Sockets Layer (SSL)/Transport Layer Security (TLS), Intrusion Detection System (IDS), peer-to-peer (P2P), Voice over IP (VoIP) Session Border Controller, Mobile Wireless Gateways (Gateway General Packet Radio Service (GPRS) Support Node (GGSN), Evolved Packet Core (EPC) Gateway)). By way of example, a service card may be used to terminate IPsec tunnels and execute the attendant authentication and encryption algorithms. These cards are coupled together through one or more interconnect mechanisms illustrated as backplane  436  (e.g., a first full mesh coupling the line cards and a second full mesh coupling all of the cards). 
     Returning to  FIG. 4A , the general purpose network device  404  includes hardware  440  comprising a set of one or more processor(s)  442  (which are often COTS processors) and network interface controller(s)  444  (NICs; also known as network interface cards) (which include physical NIs  446 ), as well as non-transitory machine readable storage media  448  having stored therein software  450 . During operation, the processor(s)  442  execute the software  450  to instantiate a hypervisor  454  (sometimes referred to as a virtual machine monitor (VMM)) and one or more virtual machines  462 A-R that are run by the hypervisor  454 , which are collectively referred to as software instance(s)  452 . A virtual machine is a software implementation of a physical machine that runs programs as if they were executing on a physical, non-virtualized machine; and applications generally do not know they are running on a virtual machine as opposed to running on a “bare metal” host electronic device, though some systems provide para-virtualization which allows an operating system or application to be aware of the presence of virtualization for optimization purposes. Each of the virtual machines  462 A-R, and that part of the hardware  440  that executes that virtual machine (be it hardware dedicated to that virtual machine and/or time slices of hardware temporally shared by that virtual machine with others of the virtual machine(s)  462 A-R), forms a separate virtual network element(s)  460 A-R. In some embodiments, the virtual machine module  462 A encompasses a BFD module  463 A that manages the configuration of BFD session between the network device and another network device. The return path module  491 B can similarly facilitate the establishment of BFD sessions, in particular, where it is desirable to have separate forward and return paths for the BFD sessions. 
     The virtual network element(s)  460 A-R perform similar functionality to the virtual network element(s)  430 A-R. For instance, the hypervisor  454  may present a virtual operating platform that appears like networking hardware  410  to virtual machine  462 A, and the virtual machine  462 A may be used to implement functionality similar to the control communication and configuration module(s)  432 A and forwarding table(s)  434 A (this virtualization of the hardware  440  is sometimes referred to as network function virtualization (NFV)). Thus, NFV may be used to consolidate many network equipment types onto industry standard high volume server hardware, physical switches, and physical storage, which could be located in Data centers, NDs, and customer premise equipment (CPE). However, different embodiments of the invention may implement one or more of the virtual machine(s)  462 A-R differently. For example, while embodiments of the invention are illustrated with each virtual machine  462 A-R corresponding to one VNE  460 A-R, alternative embodiments may implement this correspondence at a finer level granularity (e.g., line card virtual machines virtualize line cards, control card virtual machine virtualize control cards, etc.); it should be understood that the techniques described herein with reference to a correspondence of virtual machines to VNEs also apply to embodiments where such a finer level of granularity is used. 
     In certain embodiments, the hypervisor  454  includes a virtual switch that provides similar forwarding services as a physical Ethernet switch. Specifically, this virtual switch forwards traffic between virtual machines and the NIC(s)  444 , as well as optionally between the virtual machines  462 A-R; in addition, this virtual switch may enforce network isolation between the VNEs  460 A-R that by policy are not permitted to communicate with each other (e.g., by honoring virtual local area networks (VLANs)). 
     The third exemplary ND implementation in  FIG. 4A  is a hybrid network device  06 , which includes both custom ASICs/proprietary OS and COTS processors/standard OS in a single ND or a single card within an ND. In certain embodiments of such a hybrid network device, a platform VM (i.e., a VM that that implements the functionality of the special-purpose network device  402 ) could provide for para-virtualization to the networking hardware present in the hybrid network device  406 . 
     Regardless of the above exemplary implementations of an ND, when a single one of multiple VNEs implemented by an ND is being considered (e.g., only one of the VNEs is part of a given virtual network) or where only a single VNE is currently being implemented by an ND, the shortened term network element (NE) is sometimes used to refer to that VNE. Also in all of the above exemplary implementations, each of the VNEs (e.g., VNE(s)  430 A-R, VNEs  460 A-R, and those in the hybrid network device  406 ) receives data on the physical NIs (e.g.,  416 ,  446 ) and forwards that data out the appropriate ones of the physical NIs (e.g.,  416 ,  446 ). For example, a VNE implementing IP router functionality forwards IP packets on the basis of some of the IP header information in the IP packet; where IP header information includes source IP address, destination IP address, source port, destination port (where “source port” and “destination port” refer herein to protocol ports, as opposed to physical ports of a ND), transport protocol (e.g., user datagram protocol (UDP) (RFC 768, 2460, 2675, 4113, and 5405), Transmission Control Protocol (TCP) (RFC 793 and 1180), and differentiated services (DSCP) values (RFC 2474, 2475, 2597, 2983, 3086, 3140, 3246, 3247, 3260, 4594, 5865, 3289, 3290, and 3317). 
       FIG. 4C  illustrates various exemplary ways in which VNEs may be coupled according to some embodiments of the invention.  FIG. 4C  shows VNEs  470 A. 1 - 470 A.P (and optionally VNEs  470 A.Q- 470 A.R) implemented in ND  400 A and VNE  470 H. 1  in ND  400 H. In  FIG. 4C , VNEs  470 A. 1 -P are separate from each other in the sense that they can receive packets from outside ND  400 A and forward packets outside of ND  400 A; VNE  470 A. 1  is coupled with VNE  470 H. 1 , and thus they communicate packets between their respective NDs; VNE  470 A. 2 - 470 A. 3  may optionally forward packets between themselves without forwarding them outside of the ND  400 A; and VNE  470 A.P may optionally be the first in a chain of VNEs that includes VNE  470 A.Q followed by VNE  470 A.R (this is sometimes referred to as dynamic service chaining, where each of the VNEs in the series of VNEs provides a different service—e.g., one or more layer 4-7 network services). While  FIG. 4C  illustrates various exemplary relationships between the VNEs, alternative embodiments may support other relationships (e.g., more/fewer VNEs, more/fewer dynamic service chains, multiple different dynamic service chains with some common VNEs and some different VNEs). 
     The NDs of  FIG. 4A , for example, may form part of the Internet or a private network; and other electronic devices (not shown; such as end user devices including workstations, laptops, netbooks, tablets, palm tops, mobile phones, smartphones, multimedia phones, Voice Over Internet Protocol (VOIP) phones, terminals, portable media players, GPS units, wearable devices, gaming systems, set-top boxes, Internet enabled household appliances) may be coupled to the network (directly or through other networks such as access networks) to communicate over the network (e.g., the Internet or virtual private networks (VPNs) overlaid on (e.g., tunneled through) the Internet) with each other (directly or through servers) and/or access content and/or services. Such content and/or services are typically provided by one or more servers (not shown) belonging to a service/content provider or one or more end user devices (not shown) participating in a peer-to-peer (P2P) service, and may include, for example, public webpages (e.g., free content, store fronts, search services), private webpages (e.g., username/password accessed webpages providing email services), and/or corporate networks over VPNs. For instance, end user devices may be coupled (e.g., through customer premise equipment coupled to an access network (wired or wirelessly)) to edge NDs, which are coupled (e.g., through one or more core NDs) to other edge NDs, which are coupled to electronic devices acting as servers. However, through compute and storage virtualization, one or more of the electronic devices operating as the NDs in  FIG. 4A  may also host one or more such servers (e.g., in the case of the general purpose network device  404 , one or more of the virtual machines  462 A-R may operate as servers; the same would be true for the hybrid network device  406 ; in the case of the special-purpose network device  402 , one or more such servers could also be run on a hypervisor executed by the compute resource(s)  412 ); in which case the servers are said to be co-located with the VNEs of that ND. 
     A virtual network is a logical abstraction of a physical network (such as that in  FIG. 4A ) that provides network services (e.g., L2 and/or L3 services). A virtual network can be implemented as an overlay network (sometimes referred to as a network virtualization overlay) that provides network services (e.g., layer 2 (L2, data link layer) and/or layer 3 (L3, network layer) services) over an underlay network (e.g., an L3 network, such as an Internet Protocol (IP) network that uses tunnels (e.g., generic routing encapsulation (GRE), layer 2 tunneling protocol (L2TP), IPSec) to create the overlay network). 
     A network virtualization edge (NVE) sits at the edge of the underlay network and participates in implementing the network virtualization; the network-facing side of the NVE uses the underlay network to tunnel frames to and from other NVEs; the outward-facing side of the NVE sends and receives data to and from systems outside the network. A virtual network instance (VNI) is a specific instance of a virtual network on a NVE (e.g., a NE/VNE on an ND, a part of a NE/VNE on a ND where that NE/VNE is divided into multiple VNEs through emulation); one or more VNIs can be instantiated on an NVE (e.g., as different VNEs on an ND). A virtual access point (VAP) is a logical connection point on the NVE for connecting external systems to a virtual network; a VAP can be physical or virtual ports identified through logical interface identifiers (e.g., a VLAN ID). 
     Examples of network services include: 1) an Ethernet LAN emulation service (an Ethernet-based multipoint service similar to an Internet Engineering Task Force (IETF) Multiprotocol Label Switching (MPLS) or Ethernet VPN (EVPN) service) in which external systems are interconnected across the network by a LAN environment over the underlay network (e.g., an NVE provides separate L2 VNIs (virtual switching instances) for different such virtual networks, and L3 (e.g., IP/MPLS) tunneling encapsulation across the underlay network); and 2) a virtualized IP forwarding service (similar to IETF IP VPN (e.g., Border Gateway Protocol (BGP)/MPLS IPVPN RFC 4364) from a service definition perspective) in which external systems are interconnected across the network by an L3 environment over the underlay network (e.g., an NVE provides separate L3 VNIs (forwarding and routing instances) for different such virtual networks, and L3 (e.g., IP/MPLS) tunneling encapsulation across the underlay network)). Network services may also include quality of service capabilities (e.g., traffic classification marking, traffic conditioning and scheduling), security capabilities (e.g., filters to protect customer premises from network—originated attacks, to avoid malformed route announcements), and management capabilities (e.g., full detection and processing). 
       FIG. 4D  illustrates a network with a single network element on each of the NDs of  FIG. 4A , and within this straight forward approach contrasts a traditional distributed approach (commonly used by traditional routers) with a centralized approach for maintaining reachability and forwarding information (also called network control), according to some embodiments of the invention. Specifically,  FIG. 4D  illustrates network elements (NEs)  470 A-H with the same connectivity as the NDs  400 A-H of  FIG. 4A . 
       FIG. 4D  illustrates that the distributed approach  472  distributes responsibility for generating the reachability and forwarding information across the NEs  470 A-H; in other words, the process of neighbor discovery and topology discovery is distributed. 
     For example, where the special-purpose network device  402  is used, the control communication and configuration module(s)  432 A-R of the ND control plane  424  typically include a reachability and forwarding information module to implement one or more routing protocols (e.g., an exterior gateway protocol such as Border Gateway Protocol (BGP) (RFC  4271 ), Interior Gateway Protocol(s) (IGP) (e.g., Open Shortest Path First (OSPF) (RFC 2328 and 5340), Intermediate System to Intermediate System (IS-IS) (RFC 1142), Routing Information Protocol (RIP) (version 1 RFC 1058, version 2 RFC 2453, and next generation RFC 2080)), Label Distribution Protocol (LDP) (RFC 5036), Resource Reservation Protocol (RSVP) (RFC 2205, 2210, 2211, 2212, as well as RSVP-Traffic Engineering (TE): Extensions to RSVP for LSP Tunnels RFC 3209, Generalized Multi-Protocol Label Switching (GMPLS) Signaling RSVP-TE RFC 3473, RFC 3936, 4495, and 4558)) that communicate with other NEs to exchange routes, and then selects those routes based on one or more routing metrics. Thus, the NEs  470 A-H (e.g., the compute resource(s)  412  executing the control communication and configuration module(s)  432 A-R) perform their responsibility for participating in controlling how data (e.g., packets) is to be routed (e.g., the next hop for the data and the outgoing physical NI for that data) by distributively determining the reachability within the network and calculating their respective forwarding information. Routes and adjacencies are stored in one or more routing structures (e.g., Routing Information Base (RIB), Label Information Base (LIB), one or more adjacency structures) on the ND control plane  424 . The ND control plane  424  programs the ND forwarding plane  426  with information (e.g., adjacency and route information) based on the routing structure(s). For example, the ND control plane  424  programs the adjacency and route information into one or more forwarding table(s)  434 A-R (e.g., Forwarding Information Base (FIB), Label Forwarding Information Base (LFIB), and one or more adjacency structures) on the ND forwarding plane  426 . For layer 2 forwarding, the ND can store one or more bridging tables that are used to forward data based on the layer 2 information in that data. While the above example uses the special-purpose network device  402 , the same distributed approach  472  can be implemented on the general purpose network device  404  and the hybrid network device  406 . 
       FIG. 4D  illustrates that a centralized approach  474  (also known as software defined networking (SDN)) that decouples the system that makes decisions about where traffic is sent from the underlying systems that forwards traffic to the selected destination. The illustrated centralized approach  474  has the responsibility for the generation of reachability and forwarding information in a centralized control plane  476  (sometimes referred to as a SDN control module, controller, network controller, OpenFlow controller, SDN controller, control plane node, network virtualization authority, or management control entity), and thus the process of neighbor discovery and topology discovery is centralized. The centralized control plane  476  has a south bound interface  482  with a data plane  480  (sometime referred to the infrastructure layer, network forwarding plane, or forwarding plane (which should not be confused with a ND forwarding plane)) that includes the NEs  470 A-H (sometimes referred to as switches, forwarding elements, data plane elements, or nodes). The centralized control plane  476  includes a network controller  478 , which includes a centralized reachability and forwarding information module  479  that determines the reachability within the network and distributes the forwarding information to the NEs  470 A-H of the data plane  480  over the south bound interface  482  (which may use the OpenFlow protocol). Thus, the network intelligence is centralized in the centralized control plane  476  executing on electronic devices that are typically separate from the NDs. 
     For example, where the special-purpose network device  402  is used in the data plane  480 , each of the control communication and configuration module(s)  432 A-R of the ND control plane  424  typically include a control agent that provides the VNE side of the south bound interface  482 . In this case, the ND control plane  424  (the compute resource(s)  412  executing the control communication and configuration module(s)  432 A-R) performs its responsibility for participating in controlling how data (e.g., packets) is to be routed (e.g., the next hop for the data and the outgoing physical NI for that data) through the control agent communicating with the centralized control plane  476  to receive the forwarding information (and in some cases, the reachability information) from the centralized reachability and forwarding information module  479  (it should be understood that in some embodiments of the invention, the control communication and configuration module(s)  432 A-R, in addition to communicating with the centralized control plane  476 , may also play some role in determining reachability and/or calculating forwarding information—albeit less so than in the case of a distributed approach; such embodiments are generally considered to fall under the centralized approach  474 , but may also be considered a hybrid approach). In some embodiments, the centralized reachability and forwarding information module  479  encompasses BFD functions in a BFD module  481  that manages the establishment of BFD sessions between the network devices. A return path module  491  can similarly manage the establishment of explicit return paths between these network devices. In this embodiment, the BFD session does not need to be bootstrapped using the LSP ping, thus, the BFD module  481  and return path module  491  have visibility to identify the forward path and return path and to configure each of the end nodes to send the BFD control packets along the appropriate path (i.e., the forward path or return path). Similarly, the return path module  491  can verify the availability of these return paths. 
     While the above example uses the special-purpose network device  402 , the same centralized approach  474  can be implemented with the general purpose network device  404  (e.g., each of the VNE  460 A-R performs its responsibility for controlling how data (e.g., packets) is to be routed (e.g., the next hop for the data and the outgoing physical NI for that data) by communicating with the centralized control plane  476  to receive the forwarding information (and in some cases, the reachability information) from the centralized reachability and forwarding information module  479 ; it should be understood that in some embodiments of the invention, the VNEs  460 A-R, in addition to communicating with the centralized control plane  476 , may also play some role in determining reachability and/or calculating forwarding information—albeit less so than in the case of a distributed approach) and the hybrid network device  406 . In fact, the use of SDN techniques can enhance the NFV techniques typically used in the general purpose network device  404  or hybrid network device  406  implementations as NFV is able to support SDN by providing an infrastructure upon which the SDN software can be run, and NFV and SDN both aim to make use of commodity server hardware and physical switches. 
       FIG. 4D  also shows that the centralized control plane  476  has a north bound interface  484  to an application layer  486 , in which resides application(s)  488 . The centralized control plane  476  has the ability to form virtual networks  492  (sometimes referred to as a logical forwarding plane, network services, or overlay networks (with the NEs  470 A-H of the data plane  480  being the underlay network)) for the application(s)  488 . Thus, the centralized control plane  476  maintains a global view of all NDs and configured NEs/VNEs, and it maps the virtual networks to the underlying NDs efficiently (including maintaining these mappings as the physical network changes either through hardware (ND, link, or ND component) failure, addition, or removal). In some embodiments, the PCE  499  as described herein above and/or an associated PCA (not shown) can be implemented at the application layer  486 . 
     While  FIG. 4D  shows the distributed approach  472  separate from the centralized approach  474 , the effort of network control may be distributed differently or the two combined in certain embodiments of the invention. For example: 1) embodiments may generally use the centralized approach (SDN)  474 , but have certain functions delegated to the NEs (e.g., the distributed approach may be used to implement one or more of fault monitoring, performance monitoring, protection switching, and primitives for neighbor and/or topology discovery); or 2) embodiments of the invention may perform neighbor discovery and topology discovery via both the centralized control plane and the distributed protocols, and the results compared to raise exceptions where they do not agree. Such embodiments are generally considered to fall under the centralized approach  474 , but may also be considered a hybrid approach. 
     While  FIG. 4D  illustrates the simple case where each of the NDs  400 A-H implements a single NE  470 A-H, it should be understood that the network control approaches described with reference to  FIG. 4D  also work for networks where one or more of the NDs  400 A-H implement multiple VNEs (e.g., VNEs  430 A-R, VNEs  460 A-R, those in the hybrid network device  406 ). Alternatively or in addition, the network controller  478  may also emulate the implementation of multiple VNEs in a single ND. Specifically, instead of (or in addition to) implementing multiple VNEs in a single ND, the network controller  478  may present the implementation of a VNE/NE in a single ND as multiple VNEs in the virtual networks  492  (all in the same one of the virtual network(s)  492 , each in different ones of the virtual network(s)  492 , or some combination). For example, the network controller  478  may cause an ND to implement a single VNE (a NE) in the underlay network, and then logically divide up the resources of that NE within the centralized control plane  476  to present different VNEs in the virtual network(s)  492  (where these different VNEs in the overlay networks are sharing the resources of the single VNE/NE implementation on the ND in the underlay network). 
     On the other hand,  FIGS. 4E and 4F  respectively illustrate exemplary abstractions of NEs and VNEs that the network controller  478  may present as part of different ones of the virtual networks  492 .  FIG. 4E  illustrates the simple case of where each of the NDs  400 A-H implements a single NE  470 A-H (see  FIG. 4D ), but the centralized control plane  476  has abstracted multiple of the NEs in different NDs (the NEs  470 A-C and G-H) into (to represent) a single NE  470 I in one of the virtual network(s)  492  of  FIG. 4D , according to some embodiments of the invention.  FIG. 4E  shows that in this virtual network, the NE  470 I is coupled to NE  470 D and  470 F, which are both still coupled to NE  470 E. 
       FIG. 4F  illustrates a case where multiple VNEs (VNE  470 A. 1  and VNE  470 H. 1 ) are implemented on different NDs (ND  400 A and ND  400 H) and are coupled to each other, and where the centralized control plane  476  has abstracted these multiple VNEs such that they appear as a single VNE  470 T within one of the virtual networks  492  of  FIG. 4D , according to some embodiments of the invention. Thus, the abstraction of a NE or VNE can span multiple NDs. 
     While some embodiments of the invention implement the centralized control plane  476  as a single entity (e.g., a single instance of software running on a single electronic device), alternative embodiments may spread the functionality across multiple entities for redundancy and/or scalability purposes (e.g., multiple instances of software running on different electronic devices). 
     Similar to the network device implementations, the electronic device(s) running the centralized control plane  476 , and thus the network controller  478  including the centralized reachability and forwarding information module  479 , may be implemented a variety of ways (e.g., a special purpose device, a general-purpose (e.g., COTS) device, or hybrid device). These electronic device(s) would similarly include compute resource(s), a set or one or more physical NICs, and a non-transitory machine-readable storage medium having stored thereon the centralized control plane software. For instance,  FIG. 5  illustrates, a general purpose control plane device  504  including hardware  540  comprising a set of one or more processor(s)  542  (which are often COTS processors) and network interface controller(s)  544  (NICs; also known as network interface cards) (which include physical NIs  546 ), as well as non-transitory machine readable storage media  548  having stored therein centralized control plane (CCP) software  550 . 
     In embodiments that use compute virtualization, the processor(s)  542  typically execute software to instantiate a hypervisor  554  (sometimes referred to as a virtual machine monitor (VMM)) and one or more virtual machines  562 A-R that are run by the hypervisor  554 ; which are collectively referred to as software instance(s)  552 . A virtual machine is a software implementation of a physical machine that runs programs as if they were executing on a physical, non-virtualized machine; and applications generally are not aware they are running on a virtual machine as opposed to running on a “bare metal” host electronic device, though some systems provide para-virtualization which allows an operating system or application to be aware of the presence of virtualization for optimization purposes. Again, in embodiments where compute virtualization is used, during operation an instance of the CCP software  550  (illustrated as CCP instance  576 A) on top of an operating system  564 A are typically executed within the virtual machine  562 A. In embodiments where compute virtualization is not used, the CCP instance  1576 A on top of operating system  564 A is executed on the “bare metal” general purpose control plane device  504 . 
     The operating system  564 A provides basic processing, input/output (I/O), and networking capabilities. In some embodiments, the CCP instance  576 A includes a network controller instance  578 . The network controller instance  578  includes a centralized reachability and forwarding information module instance  579  (which is a middleware layer providing the context of the network controller  578  to the operating system  564 A and communicating with the various NEs), and an CCP application layer  580  (sometimes referred to as an application layer) over the middleware layer (providing the intelligence required for various network operations such as protocols, network situational awareness, and user—interfaces). At a more abstract level, this CCP application layer  580  within the centralized control plane  476  works with virtual network view(s) (logical view(s) of the network), and the middleware layer provides the conversion from the virtual networks to the physical view. The CCP application layer  580  can encompass the functionality of the BFD module  581  and the return path module  591  as described herein above. 
     The centralized control plane  476  transmits relevant messages to the data plane  480  based on CCP application layer  580  calculations and middleware layer mapping for each flow. A flow may be defined as a set of packets whose headers match a given pattern of bits; in this sense, traditional IP forwarding is also flow-based forwarding where the flows are defined by the destination IP address for example; however, in other implementations, the given pattern of bits used for a flow definition may include more fields (e.g.,  10  or more) in the packet headers. Different NDs/NEs/VNEs of the data plane  480  may receive different messages, and thus different forwarding information. The data plane  480  processes these messages and programs the appropriate flow information and corresponding actions in the forwarding tables (sometime referred to as flow tables) of the appropriate NE/VNEs, and then the NEs/VNEs map incoming packets to flows represented in the forwarding tables and forward packets based on the matches in the forwarding tables. 
     Standards such as OpenFlow define the protocols used for the messages, as well as a model for processing the packets. The model for processing packets includes header parsing, packet classification, and making forwarding decisions. Header parsing describes how to interpret a packet based upon a well-known set of protocols. Some protocol fields are used to build a match structure (or key) that will be used in packet classification (e.g., a first key field could be a source media access control (MAC) address, and a second key field could be a destination MAC address). 
     Packet classification involves executing a lookup in memory to classify the packet by determining which entry (also referred to as a forwarding table entry or flow entry) in the forwarding tables best matches the packet based upon the match structure, or key, of the forwarding table entries. It is possible that many flows represented in the forwarding table entries can correspond/match to a packet; in this case the system is typically configured to determine one forwarding table entry from the many according to a defined scheme (e.g., selecting a first forwarding table entry that is matched). Forwarding table entries include both a specific set of match criteria (a set of values or wildcards, or an indication of what portions of a packet should be compared to a particular value/values/wildcards, as defined by the matching capabilities—for specific fields in the packet header, or for some other packet content), and a set of one or more actions for the data plane to take on receiving a matching packet. For example, an action may be to push a header onto the packet, for the packet using a particular port, flood the packet, or simply drop the packet. Thus, a forwarding table entry for IPv4/IPv6 packets with a particular transmission control protocol (TCP) destination port could contain an action specifying that these packets should be dropped. 
     Making forwarding decisions and performing actions occurs, based upon the forwarding table entry identified during packet classification, by executing the set of actions identified in the matched forwarding table entry on the packet. 
     However, when an unknown packet (for example, a “missed packet” or a “match-miss” as used in OpenFlow parlance) arrives at the data plane  480 , the packet (or a subset of the packet header and content) is typically forwarded to the centralized control plane  476 . The centralized control plane  476  will then program forwarding table entries into the data plane  480  to accommodate packets belonging to the flow of the unknown packet. Once a specific forwarding table entry has been programmed into the data plane  480  by the centralized control plane  476 , the next packet with matching credentials will match that forwarding table entry and take the set of actions associated with that matched entry. 
     For example, while the flow diagrams in the figures show a particular order of operations performed by certain embodiments of the invention, it should be understood that such order is exemplary (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.). 
     Those skilled in the art will appreciate that the use of the term “exemplary” is used herein to mean “illustrative,” or “serving as an example,” and is not intended to imply that a particular embodiment is preferred over another or that a particular feature is essential. Likewise, the terms “first” and “second,” and similar terms, are used simply to distinguish one particular instance of an item or feature from another, and do not indicate a particular order or arrangement, unless the context clearly indicates otherwise. Further, the term “step,” as used herein, is meant to be synonymous with “operation” or “action.” Any description herein of a sequence of steps does not imply that these operations must be carried out in a particular order, or even that these operations are carried out in any order at all, unless the context or the details of the described operation clearly indicates otherwise. 
     Of course, the present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. One or more of the specific processes discussed above may be carried out using one or more appropriately configured processing circuits. In some embodiments, these processing circuits may comprise one or more microprocessors, microcontrollers, and/or digital signal processors programmed with appropriate software and/or firmware to carry out one or more of the operations described above, or variants thereof. In some embodiments, these processing circuits may comprise customized hardware to carry out one or more of the functions described above. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 
     While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described, can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.