Patent Publication Number: US-11038791-B2

Title: Techniques for exposing maximum node and/or link segment identifier depth utilizing OSPF

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of International Application No. PCT/IB2016/051891, filed Apr. 1, 2016, which claims the benefit of U.S. Provisional Application No. 62/295,453, filed Feb. 15, 2016, which are all hereby incorporated by reference. 
    
    
     FIELD 
     Embodiments of the invention relate to the field of computer networking; and more specifically, to techniques for exposing maximum node and/or link segment identifier depth utilizing Open Shortest Path First (OSPF). 
     BACKGROUND 
     Segment Routing (SR) is a recently-developed routing technique leveraging the source routing paradigm. In SR networks, a node steers a packet through an ordered list of instructions called segments. A segment can represent any instruction, topological or service-based, and can have a local semantic to an SR node or global within an SR domain. 
     SR allows traffic of a flow to be passed through any topological path and service chain while maintaining per-flow state only at the ingress node to the SR domain. SR can be directly applied to the common Multiprotocol Label Switching (MPLS) architecture without requiring any change to the forwarding planes of participating nodes. For example, a segment can be encoded as an MPLS label, and an ordered list of segments can be encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. 
     SR can also be applied to the Internet Protocol (IP) version 6 (IPv6) architecture using a new type of routing extension header. A segment can be encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing extension header. The segment to process is indicated by a pointer in the routing extension header. Upon completion of a segment, the pointer is incremented. 
     SR can provide many benefits, including enabling a scalable end-to-end policy, the fact that it was designed for IP and Software Defined Networking (SDN) (i.e., has Equal-Cost Multi-path (ECMP) awareness, and balance between distributed intelligence, centralized optimization, and application-based policy creation), has operational simplicity, and can provide better utilization of the installed infrastructure thanks to centralized optimization. 
     SUMMARY 
     A method in a device implementing a network element in a SR network includes transmitting, by the network element, a Type Length Value (TLV) element including a Maximum Segment Identifier Depth (MSD) value. The TLV element is transmitted using the OSPF protocol. The MSD value identifies a maximum number of segment identifier (SID) labels that the network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The method also includes receiving, from a controller, data for a path to be utilized by the network element for forwarding the received packets through the SR network. The data includes a set of one or more SID labels to be pushed into the received packets associated with the path. The set of SID labels includes less than or equal to the MSD value number of SID labels, and the controller and the network element do not utilize the Path Computation Element Protocol (PCEP) over a southbound interface. 
     In some embodiments, the OSPF protocol utilized comprises either OSPF version 2 (OSPFv2) or OSPF version 3 (OSPFv3). 
     In some embodiments, the TLV element is a node MSD TLV and is carried by either an OSPFv2 Router Information Opaque Link State Advertisement (LSA) or an OSPFv3 Router Information Opaque LSA. 
     In some embodiments, the TLV element is a link MSD sub-TLV and is carried by either an OSPFv2 Extended Link Opaque LSA, an OSPFv2 TE LSA, or an OSPVv3 E-Router-LSA. 
     In some embodiments, the method further includes determining, by the network element, the MSD value as a smallest one of one or more MSD values of one or more line cards of the network element. 
     According to some embodiments, a non-transitory computer-readable storage medium has instructions which, when executed by one or more processors of a device, cause the device to implement a network element in a SR network and perform operations. The operations include transmitting a TLV element including a MSD value. The TLV element is transmitted using the OSPF protocol. The MSD value identifies a maximum number of SID labels that the network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The operations also include receiving, from a controller, data for a path to be utilized by the network element for forwarding the received packets through the SR network. The data includes a set of one or more SID labels to be pushed into the received packets associated with the path. The set of SID labels includes less than or equal to the MSD value number of SID labels, and the controller and the network element do not utilize the PCEP over a southbound interface. 
     In some embodiments, a device includes one or more processors and a non-transitory computer-readable storage medium. The non-transitory computer-readable storage medium has instructions which, when executed by the one or more processors, cause the device to implement a network element in a SR network and perform operations. The operations include transmitting a TLV element including a MSD value. The TLV element is transmitted using the OSPF protocol. The MSD value identifies a maximum number of SID labels that the network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The operations also include receiving, from a controller, data for a path to be utilized by the network element for forwarding the received packets through the SR network. The data includes a set of one or more SID labels to be pushed into the received packets associated with the path. The set of SID labels includes less than or equal to the MSD value number of SID labels, and the controller and the network element do not utilize the PCEP over a southbound interface. 
     A system includes a controller and a network element. The network element, which is implemented by a device, participates in a SR network and performs operations. The operations include transmitting a TLV element including a MSD value. The TLV element is transmitted using the OSPF protocol. The MSD value identifies a maximum number of SID labels that the network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The operations also include receiving, from the controller, data for a path to be utilized by the network element for forwarding the received packets through the SR network. The data includes a set of one or more SID labels to be pushed into the received packets associated with the path. The set of SID labels includes less than or equal to the MSD value number of SID labels, and the controller and the network element do not utilize the PCEP over a southbound interface. 
     According to some embodiments, a method in a controller communicatively coupled with a plurality of network elements of a SR network includes receiving, from a first network element of the plurality of network elements, a TLV element including a MSD value. The TLV element is transmitted using the OSPF protocol. The MSD value identifies a maximum number of SID labels that the first network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The method also includes determining, based upon the MSD value, a path from the first network element to a second network element of the plurality of network elements for an identifiable set of traffic that includes a set of one or more SID labels. A number of SID labels in the set of SID labels is necessarily less than or equal to the MSD value. The method also includes sending, to the first network element, data for the path to be utilized by the network element for the forwarding of the received packets through the SR network. The data includes the set of SID labels to be pushed into received packets of the identifiable set of traffic. The controller and the first network element do not utilize the PCEP over a southbound interface. 
     In some embodiments, the OSPF protocol comprises either OSPF version 2 (OSPFv2) or OSPF version 3 (OSPFv3). 
     In some embodiments, the TLV element is a node MSD TLV and is carried by either an OSPFv2 Router Information Opaque LSA, or an OSPFv3 Router Information Opaque LSA. 
     In some embodiments, the TLV element is a link MSD sub-TLV and is carried by either an OSPFv2 Extended Link Opaque LSA, an OSPFv2 TE LSA, or an OSPVv3 E-Router-LSA. 
     According to some embodiments, a non-transitory computer-readable storage medium has instructions which, when executed by one or more processors of a device, cause the device to implement a controller to be communicatively coupled with a plurality of network elements of a SR network and perform operations. The operations include receiving, from a first network element of the plurality of network elements, a TLV element including a MSD value. The TLV element was transmitted using the OSPF protocol. The MSD value identifies a maximum number of SID labels that the first network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The operations also include determining, based upon the MSD value, a path from the first network element to a second network element of the plurality of network elements for an identifiable set of traffic that includes a set of one or more SID labels. A number of SID labels in the set of SID labels is necessarily less than or equal to the MSD value. The operations also include sending, to the first network element, data for the path to be utilized by the network element for the forwarding of the received packets through the SR network. The data includes the set of SID labels to be pushed into received packets of the identifiable set of traffic. The controller and the first network element do not utilize the PCEP over a southbound interface. 
     In some embodiments, a device includes one or more processors and a non-transitory computer readable storage medium. The non-transitory computer-readable storage medium has instructions which, when executed by the one or more processors, cause the device to implement a controller to be communicatively coupled with a plurality of network elements of a SR network and perform operations. The operations include receiving, from a first network element of the plurality of network elements, a TLV element including a MSD value. The TLV element was transmitted using the OSPF protocol. The MSD value identifies a maximum number of SID labels that the first network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The operations also include determining, based upon the MSD value, a path from the first network element to a second network element of the plurality of network elements for an identifiable set of traffic that includes a set of one or more SID labels. A number of SID labels in the set of SID labels is necessarily less than or equal to the MSD value. The operations also include sending, to the first network element, data for the path to be utilized by the network element for the forwarding of the received packets through the SR network. The data includes the set of SID labels to be pushed into received packets of the identifiable set of traffic. The controller and the first network element do not utilize the PCEP over a southbound interface. 
     In some embodiments, a system includes a controller that is communicatively coupled with a plurality of network elements of a SR network. The controller can be implemented by a device and configured to perform operations. The operations include receiving, from a first network element of the plurality of network elements, a TLV element including a MSD value. The TLV element was transmitted using the OSPF protocol. The MSD value identifies a maximum number of SID labels that the first network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The operations also include determining, based upon the MSD value, a path from the first network element to a second network element of the plurality of network elements for an identifiable set of traffic that includes a set of one or more SID labels. A number of SID labels in the set of SID labels is necessarily less than or equal to the MSD value. The operations also include sending, to the first network element, data for the path to be utilized by the network element for the forwarding of the received packets through the SR network. The data includes the set of SID labels to be pushed into received packets of the identifiable set of traffic. The controller and the first network element do not utilize the PCEP over a southbound interface. 
    
    
     
       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 high-level block diagram illustrating a system of network elements communicatively coupled within a SR-enabled network configured to expose maximum node and/or link segment identifier depth according to some embodiments. 
         FIG. 2  illustrates an OSPF Node MSD TLV element, an OSPFv2 Router Information Opaque LSA carrying the OSPF Node MSD TLV, and an OSPFv3 Router Information Opaque LSA carrying the OSPF Node MSD TLV, which can be used for exposing maximum node segment identifier depth according to some embodiments. 
         FIG. 3  illustrates an OSPF Link MSD sub-TLV element, an OSPFv2 Extended Link TLV carrying the OSPF Link MSD sub-TLV, and an OSPFv2 Link TLV carrying the OSPF Link MSD sub-TLV, which can be used for exposing maximum link segment identifier depth according to some embodiments. 
         FIG. 4  illustrates an OSPFv3 Router-Link TLV carrying the OSPF Link MSD sub-TLV of  FIG. 3  that can be used for exposing maximum link segment identifier depth according to some embodiments. 
         FIG. 5  illustrates an IS-IS Node MSD TLV element and an IS-IS Router Capability TLV carrying the IS-IS Node MSD TLV that which can be used for exposing maximum node segment identifier depth according to some embodiments. 
         FIG. 6  illustrates an IS-IS Link MSD sub-TLV element and several types of IS-IS TLVs, which can be used for exposing maximum link segment identifier depth according to some embodiments. 
         FIG. 7A  illustrates a flow for exposing maximum node and/or link segment identifier depth according to some embodiments. 
         FIG. 7B  illustrates a high-level block diagram of a network element that can perform the flow of  FIG. 7A  according to some embodiments. 
         FIG. 8A  illustrates a flow for utilizing an exposed maximum node and/or link segment identifier depth according to some embodiments. 
         FIG. 8B  illustrates a high-level block diagram of a controller that can perform the flow of  FIG. 8A  according to some embodiments. 
         FIG. 9A  illustrates connectivity between network devices (NDs) within an exemplary network, as well as three exemplary implementations of the NDs according to some embodiments. 
         FIG. 9B  illustrates an exemplary way to implement a special-purpose network device according to some embodiments. 
         FIG. 9C  illustrates various exemplary ways in which virtual network elements (VNEs) may be coupled according to some embodiments. 
         FIG. 9D  illustrates a network with a single network element (NE) on each of the NDs, 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. 
         FIG. 9E  illustrates the simple case of where each of the NDs implements a single NE, but a centralized control plane has abstracted multiple of the NEs in different NDs into (to represent) a single NE in one of the virtual network(s), according to some embodiments. 
         FIG. 9F  illustrates a case where multiple VNEs are implemented on different NDs and are coupled to each other, and where a centralized control plane has abstracted these multiple VNEs such that they appear as a single VNE within one of the virtual networks, according to some embodiments. 
         FIG. 10  illustrates a general purpose control plane device with centralized control plane (CCP) software), according to some embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     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 some embodiments. 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. 
     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. 
     In SR networks, a controller may be tasked with determining one or more Segment Identifiers (SIDs)(also referred to as a “label stack”) that are to be added by a network element to network traffic to allow that traffic to transit a particular path in the SR network. For example, a network element (e.g., an edge router at the edge of the SR network portion) could receive a packet to be forwarded across the SR network, determine a configured set of SIDs to be inserted into the packet, and add those labels. These SIDs can be examined by other network elements in the SR network to quickly identify how the traffic is to be forwarded. As the packets move through the SR network, these SIDs can be stripped (or “popped”) as a particular segment (or portion of the path) has been completed. Accordingly, a controller can configured a network element that initially receives the traffic to place a particular set of SIDs in the traffic to ensure that the traffic is forwarded appropriately. 
     However, many network devices (providing network element functionality) have limitations—based upon hardware and/or software—that limit how the network devices can process traffic. For example, different network devices have different capabilities to insert different numbers of SIDs into traffic, and even different components of network devices (e.g., line cards)—even within a single network device—can have different capabilities to insert different numbers of SIDs into traffic. For example, a single network device could employ a first line card only able to inject four labels, while a second line card of the single network device could inject twelve labels, and while a third line card of the single network device could inject sixteen labels. 
     Thus, when SR tunnels are computed by a centralized controller (e.g., a Path Computation Element (PCE) or SDN controller), it is crucial that the controller knows what the “Maximum SID Depth” (or “MSD”) of the particular node is, so that it does not attempt to create a path/tunnel with a set of SID that is deeper (i.e., larger) than what the node is capable of inserting into the traffic. 
     For example, if a controller selects a path for a set of traffic that requires the insertion of six labels (SIDs), this path cannot be implemented by a network element that only has the ability to insert three labels into the traffic. In such situations, a variety of problems can result, which can cause network congestion, dropped traffic, and network failures. 
     Accordingly, efforts have been made to address this problem using extensions of the Path Computation Element Communication Protocol (PCEP) (e.g., via the southbound interface between a controller and its network elements serving as the forwarding plane, perhaps using a TLV within a PCEP OPEN message) by allowing a network element to provide a MSD value for the network element to the controller. Thus, the network element can report the maximum depth of SIDs that it can inject, and the controller can take that into account when determining paths for traffic so that it does not attempt to create a path having more SIDs than can be inserted by the network element. 
     Although this development is certainly useful, it still falls short in many environments where the PCEP is not used (or cannot be used), and thus the network elements cannot report their MSDs to a controller. For example, in many environments certain network elements (e.g., a network element at a head-end of a SR tunnel) do not support PCEP. (Additionally, any effort to use Border Gateway Protocol (BGP) for providing this information similarly is problematic as network elements in many deployments may also not support or “speak” BGP.) 
     Further, in environments where a single network device may have multiple capabilities on multiple links (e.g., different capabilities from different line cards), the aforementioned PCEP approach effectively “wastes” the ability of some components of the device. For example, to be safe it is apparent that using PCEP, the network element must report the smallest/lowest MSD capability that all of its components can support as its MSD value. Thus, this “wastes” the excess capabilities of any larger-capability components, as these components could inject larger numbers of SIDs but are unable to do so because the controller is not allowed to provide such SIDs to the network element for use. For example, for a network device with a first line card able to inject three labels and with a second line card able to inject sixteen labels, the network device would need to report back a MSD value of only three, and thus the controller can only provide three-label paths to the network device—likely severely limiting the paths that it can utilize, despite the fact that the second line card could handle larger sets of SIDs. 
     Accordingly, improved techniques that can remedy these deficiencies are desirable. 
     As described herein, in some embodiments a MSD value can be reported back to a controller using a Label Distribution Protocol (LDP) such as the OSPF protocol or the Intermediate System to Intermediate System (IS-IS) protocol, thereby eliminating the need for PCEP to be utilized. Moreover, in some embodiments a network element can optionally report (for the controller) a MSD for multiple of its links (e.g., line cards), which can allow the controller to more fully utilize the capabilities of the network by considering and/or installing longer sets of SIDs for those links that can accommodate them. 
       FIG. 1  is a high-level block diagram illustrating a system  100  of network elements  102 A- 102 F communicatively coupled within a SR enabled network  120  configured to expose maximum node and/or link segment identifier depth according to some embodiments. This system includes a controller  104  and six network elements A-F ( 102 A- 102 F) serving as the forwarding plane of the SR network  120 . In this example, we assume that no PCEP channel  116  exists between the controller  104  and at least network element ‘A’  102 A. Notably, a PCEP channel  116  may or may not exist between others of the network elements B-F ( 102 B- 102 F). 
     In this example, as illustrated by circle ‘ 1 ’, network element ‘A’  102 A determines an MSD value  112  (e.g., two), and advertises the MSD value  112  within a TLV element  110 . In some embodiments, this TLV element  110  is sent as part of an IGP advertisement, such as within an OSPF LSA or IS-IS TLV. As is known those of skill in the art, this advertisement can travel a variety of physical communication paths, as the network element ‘A’  102 A likely is not in direct communication with the controller, and instead, the advertised TLV  110  may be sent out one or more (or all) of its interfaces leading to other IGP-participating nodes, and eventually the advertised TLV  110  can be provided to the controller  104  at circle ‘ 2 ’. 
     At this point, the controller  104  can use the MSD value  112  of network element ‘A’  102 A when determining paths for traffic that is to transit the SR network  120  that is first received at network element ‘A’  102 A. Accordingly, when attempting to create a path for an identifiable group of traffic (e.g., a flow or other uniquely identifiable type of traffic, such as packets  150 A), the controller  104  can limit its consideration of candidate paths to be programmed to only those requiring a same number (or smaller number) of SIDs to be injected by network element ‘A’  102 A. Thus, in some embodiments the MSD value  112  can be treated by the controller  104  similar to a TE requirement/constraint, thus limiting the path possibilities that can ultimately be selected. 
     In this example, the controller  104  at circle ‘ 3 ’ will program network element ‘A’  102 A (e.g., send a message to network element ‘A’  102 A with data including path information such as the set of SIDs  152 ) with a path  114  having a number of SIDs that do not exceed the reported MSD value  112  of network element ‘A’  102 A—here, the valid path  122  is selected, which could utilize one or two SIDs, for example, to transit network element ‘C’  102 C and network element ‘E’  102 E. Thereafter, upon receipt of traffic (e.g., packets  150 A at circle ‘ 4 ’), the network element ‘A’  102 A can identify the set of SIDs and inject the set of SIDs into that traffic, resulting in the traffic (forwarded packets  150 B) being sent using the valid path  122  toward their destination at circle ‘ 5 ’. 
     For the sake of illustration,  FIG. 1  also shows an invalid path  124  that might have been considered by the controller  104  but for the fact that it requires a larger SID label stack than the reported MSD value  112 —for example, four labels. Notably, prior systems might have attempted to utilize this path due to a controller  104  not being aware of the capabilities of the network element ‘A’  102 A, which would have led to network problems that embodiments described herein can avoid. 
     As described earlier, the MSD value  112  can be advertised using a variety of IGP protocols, including but not limited to OSPF and IS-IS. 
     OSPF 
     For example,  FIG. 2  illustrates an OSPF Node MSD TLV element  110 A, an OSPFv2 Router Information Opaque LSA  210  carrying the OSPF Node MSD TLV  110 A, and an OSPFv3 Router Information Opaque LSA  250  carrying the OSPF Node MSD TLV  110 A, which can be used for exposing maximum node segment identifier depth according to some embodiments. 
     In some embodiments, the OSPF Node MSD TLV  110 A can be utilized to advertise a “node” MSD value  112 A that is of a node (or network element) level of granularity. In some embodiments where multiple different supported SID depths may apply to a single network element (e.g., a network device has multiple different line cards with different injectable SID depths), the MSD value can be the lowest value of the multiple SID depths supported. This OSPF Node MSD TLV  110 A includes a 2 byte type value  202 , which can be a unique value (e.g., unique within a particular set of TLVs utilized in a particular system or according to a particular standard) that identifies the OSPF Node MSD TLV  110 A as such, a 2 byte length value  204  indicating a length of the “value” part of the TLV  110 A—the Node MSD value  112 A. 
     The Node MSD value  112 A, in some embodiments, is a 2 byte value including a value representing a number in the range of zero (“0”) to 254. In some embodiments, a value of zero can represent a lack of ability of the sending network element to push a SIDs of any depth, and any other value can represent that particular number of SIDs supported by the node. Again, in some embodiments this value can represent the lowest value supported by node when multiple different link-level values of supported SID injectable-depth are possible (e.g., because of different types of line cards in a network element). Notably, in some embodiments the use of this OSPF Node MSD TLV  110 A is optional, and thus it may or may not be advertised. 
     However, in some embodiments the OSPF Node MSD TLV  110 A can be carried by an OSPFv2 Router Information Opaque LSA  210 , which is described by Internet Engineering Task Force (IETF) Request for Comments (RFC) 4970. This OSPFv2 Router Information Opaque LSA  210  can include a 2 byte link state (LS) age  212  field, a 1 byte options  214  field, 1 byte carrying a representation of ‘9’ or ‘10’ or ‘11’, 1 byte carrying a representation of ‘4’ (an Opaque type value), 3 bytes carrying a representation of ‘0’ (an Opaque ID value), 4 bytes of an advertising router  216  identifier, 4 bytes of a LS sequence number  218 , 2 bytes of a LS checksum  220 , 2 bytes of a length  222  of the size of the following TLV(s)  224 , and the TLV(s)  224 , which can carry this new Node MSD TLV  110 A. 
     Similarly, in some embodiments OSPF Node MSD TLV  110 A can be carried by an OSPFv3 Router Information Opaque LSA  250 , which is also described by IETF RFC 4970. This OSPFv3 Router Information Opaque LSA  250  includes some similar and some different fields/values as the OSPFv2 Router Information Opaque LSA  210 , and similarly carries the new Node MSD TLV  110 A in its TLV(s)  224  field. 
     As alluded to above, in some embodiments a network element can report different capabilities for SID injection sizes of the network element. Turning to  FIG. 3 , an illustration is provided of an OSPF Link MSD sub-TLV element  110 B, an OSPFv2 Extended Link TLV  308  carrying the OSPF Link MSD sub-TLV, and an OSPFv2 Link TLV  330  carrying the OSPF Link MSD sub-TLV, which can be used for exposing maximum link segment identifier depth according to some embodiments. 
     In some embodiments, a network element can advertise one or more OSPF Link MSD sub-TLVs  110 B—e.g., one for each link of the network element. The Link MSD sub-TLV  110 B can include a 2 byte type  302  field including a unique identifier that identifies the Link MSD sub-TLVs  110 B as such within a particular context, a 2 byte length value  304  indicating a length of the “value” part of the TLV—the link MSD value  112 B. In some embodiments, the link MSD value  112 B can be a 2 byte value that is a number in the range of 0-254, where 0 represents lack of ability to push SIDs of any depth, and any other value represents that number of SIDs for that particular link. 
     Accordingly, in some embodiments a network element can advertise different MSD values (i.e., link MSD values  112 B) using multiple OSPF Link MSD sub-TLVs  110 B. 
     One example OSPF message that can be used to carry one or more Link MSD sub-TLVs  110 B is the OSPFv2 Extended Link TLV  308  described by IETF RFC 7684, and which itself can be carried within an OSPFv2 Extended Link Opaque LSA. The OSPFv2 Extended Link TLV  308  can include a 2 byte type  310 , a 2 byte length  312 , a 1 byte link type  314 , 3 bytes of reserved space, a 4 byte link identifier  318 , a 4 byte link data  320  value, and zero or more sub-TLVs  324 , where the one or more Link MSD sub-TLVs  110 B can optionally be carried. 
     Similarly, in some embodiments a network element can use an OSPFv2 Link TLV  330  described by IETF RFC 3630, which itself can be carried by a TE LSA. The OSPFv2 Link TLV  330  can include a type  332 , length  334 , and a variable number of sub-TLVs  336 , including a link type sub-TLV  338 , a link identifier sub-TLV  340 , and optionally: the Link MSD sub-TLV  110 B, a local interface IP address  342 , a remote interface IP address  344 , a TE metric  346 , a maximum bandwidth  348 , a maximum reservable bandwidth  350 , an unreserved bandwidth  352 , and/or an admin group  354  value. In some embodiments, each OSPFv2 Link TLV  330  may carry only one Link MSD sub-TLV  110 B, and thus, two advertise multiple Link MSD sub-TLVs  110 B, multiple OSPFv2 Link TLVs  330  can be sent by the network element (e.g., within a TE LSA). 
     Additionally,  FIG. 4  illustrates an OSPFv3 Router-Link TLV  400  carrying the OSPF Link MSD sub-TLV of  FIG. 3  that can be used for exposing maximum link segment identifier depth according to some embodiments. The OSPFv3 Router-Link TLV  400  can include a 2 byte router-link type  210  (e.g., with a value of ‘1’), a 2 byte length  212 , a 1 byte type  214 , a 1 byte representation of zero, a 2 byte metric  402 , a 4 byte interface identifier  404 , a 4 byte neighbor interface identifier  406 , a 4 byte neighbor router identifier  408 , and a variable amount of sub-TLVs  418 , which can carry a Link MSD sub-TLV  110 B. 
     IS-IS 
     As described earlier, various IGP protocols can be utilized for exposing maximum node and/or link segment identifier depth of network elements to a controller without the use of PCEP. For example, some embodiments utilize IS-IS.  FIG. 5  illustrates an IS-IS Node MSD sub-TLV element  110 C and an IS-IS Router Capability TLV  506  carrying the IS-IS Node MSD TLV that which can be used for exposing maximum node segment identifier depth according to some embodiments. 
     In some embodiments, the IS-IS Node MSD sub-TLV element  110 C includes a 1 byte type  502  carrying a value that uniquely identifies the IS-IS Node MSD sub-TLV element  110 C as being a IS-IS Node MSD sub-TLV element  110 C within a particular context (e.g., is unique across all types of sub-TLVs in a particular usage scenario). The IS-IS Node MSD sub-TLV element  110 C can also include a 1 byte length  504  value indicating a length of the “value” portion of the sub-TLV that carries the Node MSD value  112 A, which is 2 bytes in some embodiments. 
     Similar to the Node MSD value  112 A presented with regard to  FIG. 2 , this Node MSD value  112 A can also be a 2 byte value representing a number in the range of zero (“0”) to 254. In some embodiments, a value of zero can represent a lack of ability of the sending network element to push/inject a SIDs of any depth, and any other value can represent that particular number of SIDs that can be pushed/injected by the node. Again, in some embodiments this value can represent the lowest value supported by node when multiple different link-level values of supported SID injectable-depth are possible (e.g., because of different types of line cards in a network element). 
     In systems utilizing IS-IS, the IS-IS Node MSD sub-TLV element  110 C can be advertised using, for example, an IS-IS Router Capability TLV  506  as defined by IETF RFC 4971. This IS-IS Router Capability TLV  506 , in some embodiments, includes a 1 byte type  508 , a 1 byte length  510  value, and a variable length “value”  512  portion. In some embodiments, the variable length value  512  portion can include a 4 byte router identifier  514 , 1 byte of flags  515 , and a variable amount of sub-TLVs  324 , which can (optionally) carry the IS-IS Node MSD sub-TLV element  110 C. 
     Like in OSPF, link-level MSD values can similarly be advertised in systems utilizing IS-IS.  FIG. 6  illustrates an IS-IS Link MSD sub-TLV element  110 D and several types of IS-IS TLVs ( 606 / 630 ) that can be used for exposing maximum link segment identifier depth according to some embodiments. 
     In some embodiments, a new IS-IS Link MSD sub-TLV  110 D can be used to carry per-link MSD values. For example, the IS-IS Link MSD sub-TLV  110 D can include 1 byte of a type  602  that uniquely identifies the type of this sub-TLV, a 1 byte length  604  value of the “value” portion of the sub-TLV, which is the 2 byte link MSD value  112 B. 
     The IS-IS Link MSD sub-TLV  110 D can be carried by a variety of different IS-IS TLVs, depending upon context of use. For example, one or more IS-IS Link MSD sub-TLVs  110 D can be carried by an IS-IS “Type 22” Extended IS Reachability TLV  606 A, as described by IETF RFC 5305, or by an IS-IS “Type 23” IS Neighbor Attribute TLV  606 B, as described by IETF RFC 5311. 
     These TLVs  606 A- 606 B can include a 1 byte type  608  (of ‘22’ or ‘23’), a 1 byte length  610  of the “value” of the TLV: namely, the 7 byte system ID and pseudonode number  612 , a 3 byte default metric  614 , a 1 byte sub-TLV length  616 , and a variable number of sub-TLVs, which can include the one or more IS-IS Link MSD sub-TLVs  110 D. 
     Similarly, the IS-IS Link MSD sub-TLV  110 D can be carried by a “Type 222” IS-IS Multi-Topology (MT) IS TLV  630 A, as described by IETF RFC 5120, or in a “Type 223” IS-IS MT IS Neighbor Attribute TLV  630 B, as described by IETF RFC 5311. These TLVs  630 A- 630 B can include a 1 byte type  632  (of ‘222’ or ‘223’), a 1 byte length  634 , 4 reserved bits, 12 bits of a MT identifier  636 , and the TLV  606  (e.g., the IS-IS Extended IS Reachability TLV  606 A illustrated above). 
     In some embodiments, only one IS-IS Link MSD sub-TLV  110 D exists for a particular link. Although in some embodiments only one IS-IS Link MSD sub-TLV  110 D is carried by a particular TLV (e.g., a Type 22/23/222/223 TLV as described above), in some embodiments multiple IS-IS Link MSD sub-TLVs  110 D—i.e., for multiple links—can be packed into just one of these TLVs (e.g., Type 22/23/222/223). 
     Flows and System Components 
       FIG. 7A  illustrates a flow  700  for exposing maximum node and/or link segment identifier depth according to some embodiments. The operations in this flow diagram and the flow diagram of  FIG. 8A  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 other than those discussed with reference to the other figures, and the embodiments discussed with reference to these other figures can perform operations different than those discussed with reference to the flow diagrams. In some embodiments, though, flow  700  can be performed by a network element of  FIG. 1 , such as network element ‘A’  102 A, which can act as an “edge” router at the edge of a SR enabled network  120 /act as a “head” end of a SR tunnel/path. 
     At block  705 , the flow  700  includes transmitting a TLV element including a MSD value. This transmission can be performed by a network element in a SR network, and can be sent out one or more (or all) of its links leading to other network elements in the SR network. The MSD value identifies a maximum number of SID labels that the network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. In some embodiments, the TLV element is a sub-TLV, and in some embodiments the TLV element is a “regular” TLV element. 
     In some embodiments, the TLV element is part of an OSPF advertisement (e.g., an LSA), and in some embodiments the TLV element is part of an IS-IS advertisement (e.g., a TLV). The TLV element may directly or indirectly (e.g., via other network elements) ultimately be provided to a controller. 
     In various embodiments, the TLV element could be carried by an OSPFv2 Router Information Opaque LSA  210 , an OSPFv3 Router Information Opaque LSA  250 , an OSPFv2 Extended Link TLV  308 , an OSPFv2 Link TLV  330 , an OSPFv3 Router-Link TLV  400 , an IS-IS Router Capability TLV  506 , an IS-IS “Type 22” Extended IS Reachability TLV  606 A, an IS-IS “Type 23” IS Neighbor Attribute TLV  606 B, a “Type 222” IS-IS Multi-Topology (MT) IS TLV  630 A, and/or a “Type 223” IS-IS MT IS Neighbor Attribute TLV  630 B. 
     At block  710 , the flow  700  also includes receiving, from the controller, data for a path to be utilized for forwarding the received packets through the SR network. The data includes a set of one or more SID labels to be pushed into any received packets associated with the path. Notably, the set of SID labels includes less than or equal to the MSD value number of SID labels initially transmitted at block  705 . 
     Optionally, at blocks  715 - 720 - 725 , the flow  700  can include receiving one or more packets (e.g., that can be associated with the path described with regard to block  710 ), pushing the set of one or more labels (e.g., the set of SIDs) into the received packets to yield modified packets, and transmitting the modified packets according to the path. 
       FIG. 7B  illustrates a high-level block diagram  750  of a network element  102 A that can perform the flow  700  of  FIG. 7A  according to some embodiments. 
     It is not strictly necessary that each module depicted herein be implemented as physically separate units. Some or all modules may be combined in a physical unit. Also, the modules need not be implemented strictly in hardware. It is envisioned that the units may be implemented through a combination of hardware and software. For example, the electronic device  752  may include one or more central processing units executing program instructions stored in a non-transitory storage medium or in firmware to perform the functions of the modules. 
     The electronic device  752  can implement a network element  102 A and include a transmission module  754  and receiving module  756 . 
     The transmission module  754  can be adapted to transmit a TLV element including a MSD value, where the MSD value identifies a maximum number of SID labels that the network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. The TLV element can be transmitted utilizing an OSPF protocol. In some embodiments, the transmission module  754  can perform block  705  of flow  700  of  FIG. 7A . 
     The receiving module  756  can be adapted to receive, from a controller, data for a path to be utilized for forwarding the received packets through the SR network, where the data includes a set of one or more SID labels to be pushed into the received packets associated with the path, and where the set of SID labels has fewer than or equal to the MSD value number of SID labels. In some embodiments, the receiving module  756  can perform block  710  of flow  700  of  FIG. 7A . 
     In some embodiments, the electronic device  752  can include other non-illustrated modules, such as one or modules to perform blocks  715 ,  720 , and/or  725  of flow  700  of  FIG. 7A , and possibly can include other non-illustrated modules to perform other operations disclosed herein. Thus, the depicted embodiment is illustrative and not limiting. 
       FIG. 8A  illustrates a flow  800  for utilizing an exposed maximum node and/or link segment identifier depth according to some embodiments. In some embodiments, flow  800  can be performed by a controller such as the controller  104  of  FIG. 1 . 
     At block  805 , the flow  800  includes receiving, from a first network element of a plurality of network elements of a SR network, a TLV element including a MSD value. The MSD value identifies a maximum number of SID labels that the first network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network. 
     In some embodiments, the TLV element is part of an OSPF advertisement (e.g., an LSA), and in some embodiments the TLV element is part of an IS-IS advertisement (e.g., a TLV). The TLV element may directly or indirectly (e.g., via other network elements) be provided to the controller from the originating network element. 
     In some embodiments, the TLV element is a sub-TLV, and in some embodiments the TLV element is a “regular” TLV element. 
     In various embodiments, the TLV element could be carried by an OSPFv2 Router Information Opaque LSA  210 , an OSPFv3 Router Information Opaque LSA  250 , an OSPFv2 Extended Link TLV  308 , an OSPFv2 Link TLV  330 , an OSPFv3 Router-Link TLV  400 , an IS-IS Router Capability TLV  506 , an IS-IS “Type 22” Extended IS Reachability TLV  606 A, an IS-IS “Type 23” IS Neighbor Attribute TLV  606 B, a “Type 222” IS-IS Multi-Topology (MT) IS TLV  630 A, and/or a “Type 223” IS-IS MT IS Neighbor Attribute TLV  630 B. 
     At block  810 , the flow  800  includes determining, based upon the MSD value, a path from the first network element to a second network element of the plurality of network elements for an identifiable set of traffic. The path can be represented by a set of one or more SID labels. The number of SID labels in the set of segment identifier labels is necessarily less than or equal to the MSD value received in block  805 . 
     In some embodiments, block  810  includes performing a path-finding process using the received MSD value as a constraint such that any selected path will not have a corresponding number of SIDs that exceeds the MSD value. For example, in some embodiments, block  810  includes identifying a candidate set of paths, and eliminating from consideration one or more of the candidate paths based upon determining that the path would require a number of SIDs to be pushed into traffic (e.g., by the first network element) that exceeds the MSD value reported by the first network element. 
     In embodiments where the MSD value is a “link” MSD value for a first link of the first network element, block  805  may also include receiving one or more additional MSD values for one or more additional links of the first network element. In this case, the block  810  determination can be based upon one or more of these MSD values, such that the MSD capabilities of the various links can be considered when attempting to identify paths using those various links. 
     At block  815 , the flow  800  includes sending, to the first network element, data for the path to be utilized by the network element for the forwarding of the received packets through the SR network. The data includes the set of SID labels to be pushed into received packets of the identifiable set of traffic. 
       FIG. 8B  illustrates a high-level block diagram  850  of a controller  104 , which can perform the flow  800  of  FIG. 8A  according to some embodiments. 
     It is not strictly necessary that each module depicted herein be implemented as physically separate units. Some or all modules may be combined in a physical unit. Also, the modules need not be implemented strictly in hardware. It is envisioned that the units may be implemented through a combination of hardware and software. For example, the electronic device  852  may include one or more central processing units executing program instructions stored in a non-transitory storage medium or in firmware to perform the functions of the modules. 
     The electronic device  852  can implement the controller  104 , which can include a receiving module  854 , determination module  856 , and/or sending module  858 . 
     The receiving module  854  can be adapted to receive, from a first network element of a plurality of network elements of a segment routing network, a TLV element including a MSD value that identifies a maximum number of SID labels that the first network element is able to push into packet headers of received packets to enable forwarding of the received packets through a SR network. The TLV element can be sent utilizing an OSPF protocol. In some embodiments, the receiving module  854  can perform block  805  of flow  800  of  FIG. 8A . 
     The determination module  856  can be adapted to determine, based upon the MSD value, a path from the first network element to a second network element of the plurality of network elements for an identifiable set of traffic that includes a set of one or more SID labels, wherein a number of SID labels in the set of SID labels is less than or equal to the MSD value. In some embodiments, the determination module  856  can perform block  810  of flow  800  of  FIG. 8A . 
     The sending module  858  can be adapted to send, to the first network element, data for the path to be utilized by the network element for the forwarding of the received packets through the SR network, where the data includes the set of SID labels to be pushed into received packets of the identifiable set of traffic. In some embodiments, the sending module  858  can perform block  815  of flow  800  of  FIG. 8A . 
     In some embodiments, the controller  104  can include other non-illustrated modules to perform other operations as disclosed herein. Thus, the depicted embodiment is illustrative and not limiting. 
     As described herein, some embodiments utilize electronic devices. An electronic device can store and/or transmit (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, many electronic devices (e.g., a computer or “computing device”) can include 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 example, an electronic device can include non-volatile memory containing code, as non-volatile memory can persist code/data even when the electronic device is turned off (when power is removed). 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. Many 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) or otherwise communicate with other electronic devices. One or more parts of various embodiments described herein can be implemented using different combinations of software, firmware, and/or hardware. 
     A network device (ND) is an electronic device that communicatively interconnects other electronic devices (e.g., other network devices, end-user devices) in one or more networks. 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). 
       FIG. 9A  illustrates connectivity between network devices (NDs) within an exemplary network, as well as three exemplary implementations of the NDs, according to some embodiments.  FIG. 9A  shows NDs  900 A- 900 H, and their connectivity by way of lines between  900 A- 900 B,  900 B- 900 C,  900 C- 900 D,  900 D- 900 E,  900 E- 900 F,  900 F- 900 G, and  900 A- 900 G, as well as between  900 H and each of  900 A,  900 C,  900 D, and  900 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  900 A,  900 E, and  900 F (and not illustrating as ending at a terminus) 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. 9A  are: 1) a special-purpose network device  902  that can include custom application—specific integrated—circuits (ASICs) and a special-purpose operating system (OS); and 2) a general-purpose network device  904  that can use common off-the-shelf (COTS) components (e.g., processors) and a potentially a “standard” OS. 
     The special-purpose network device  902  includes networking hardware  910  comprising compute resource(s)  912  (typically including a set of one or more processors), forwarding resource(s)  914  (which typically include one or more ASICs and/or network processors), and physical network interfaces (NIs)  916  (sometimes called physical ports), as well as non-transitory machine readable storage media  918  having stored therein networking software  920 . 
     The networking software  920  can, in some embodiments, include MSD advertisement module code  990 A useful for implementing an MSD advertisement module  933 A that can perform operations disclosed herein, including the operations of flow  700  of  FIG. 7A  and discussed throughout this description. For example, MSD advertisement module  933 A can comprise an instance of an OSPF and/or IS-IS routing engine configured to send MSD values as described herein. These MSD values can be determined based upon, for example, the capabilities of the physical network interfaces  916 . 
     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  900 A- 900 H. 
     During operation, the networking software  920  may be executed by the networking hardware  910  to instantiate a set of one or more networking software instance(s)  922 . Each of the networking software instance(s)  922 , and that part of the networking hardware  910  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)  922 ), can form a (separate) virtual network element  930 A- 930 R. Each of the virtual network element(s) (VNEs)  930 A- 930 R includes a control communication and configuration module  932 A- 932 R (sometimes referred to as a local control module or control communication module) and forwarding table(s)  934 A- 934 R, such that a given virtual network element (e.g.,  930 A) includes the control communication and configuration module (e.g.,  932 A), a set of one or more forwarding table(s) (e.g.,  934 A), and that portion of the networking hardware  910  that executes the virtual network element (e.g.,  930 A). 
     In some embodiments, one or more of the control communication and configuration modules (e.g.,  932 A) of the virtual network element(s) (VNEs)  930 A- 930 R can include an MSD advertisement module  933 A instantiated or implemented by the execution of the MSD advertisement module code  990 A. 
     The special-purpose network device  902  is often physically and/or logically considered to include: 1) a ND control plane  924  (sometimes referred to as a control plane) comprising the compute resource(s)  912  that execute the control communication and configuration module(s)  932 A- 932 R; and 2) a ND forwarding plane  926  (sometimes referred to as a forwarding plane, a data plane, or a media plane) comprising the forwarding resource(s)  914  that utilize the forwarding table(s)  934 A- 934 R and the physical NIs  916 . By way of example, where the ND is a router (or is implementing routing functionality), the ND control plane  924  (the compute resource(s)  912  executing the control communication and configuration module(s)  932 A- 932 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)  934 A- 934 R, and the ND forwarding plane  926  is responsible for receiving that data on the physical NIs  916  and forwarding that data out the appropriate ones of the physical NIs  916  based on the forwarding table(s)  934 A- 934 R. 
       FIG. 9B  illustrates an exemplary way  937  to implement the special-purpose network device  902  according to some embodiments.  FIG. 9B  illustrates a special-purpose network device including cards  938  (typically hot pluggable). While in some embodiments the cards  938  are of two types (one or more that operate as the ND forwarding plane  926  (sometimes called line cards), and one or more that operate to implement the ND control plane  924  (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), 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  936  (e.g., a first full mesh coupling the line cards and a second full mesh coupling all of the cards). 
     Within the context of embodiments disclosed herein, the cards  938  may potentially have different depths of SIDs that they can inject into traffic, and thus may have different MSD values. For example, in some common deployment scenarios, the cards  938  can include cards manufactured by different vendors, or include cards that are different models from a same manufacturer, and these cards  938  may have different SID injection capabilities. 
     Returning to  FIG. 9A , the general purpose network device  904  includes hardware  940  comprising a set of one or more processor(s)  942  (which are often COTS processors) and network interface controller(s)  944  (NICs; also known as network interface cards) (which include physical NIs  946 ), as well as non-transitory machine readable storage media  948  having stored therein software  950 . 
     The software  950  can include MSD advertisement module code  990 B that can be executed by the processor(s)  942  to implement a MSD advertisement module (not illustrated), which may be part of application(s)  964 A- 964 R, in order to perform the operations described herein (e.g., flow  700  of  FIG. 7 ). Accordingly, this MSD advertisement module can advertise the MSD value of the “node” and/or “links,” which can be determined based upon the capabilities of the NIC(s)  944 , and can be advertised using an IGP such as OSPF or IS-IS. 
     During operation, the processor(s)  942  execute the software  950  to instantiate one or more sets of one or more applications  964 A- 964 R. While one embodiment does not implement virtualization, alternative embodiments may use different forms of virtualization. For example, in one such alternative embodiment the virtualization layer  954  represents the kernel of an operating system (or a shim executing on a base operating system) that allows for the creation of multiple instances  962 A- 962 R called software containers that may each be used to execute one (or more) of the sets of applications  964 A- 964 R; where the multiple software containers (also called virtualization engines, virtual private servers, or jails) are user spaces (typically a virtual memory space) that are separate from each other and separate from the kernel space in which the operating system is run; and where the set of applications running in a given user space, unless explicitly allowed, cannot access the memory of the other processes. In another such alternative embodiment the virtualization layer  954  represents a hypervisor (sometimes referred to as a virtual machine monitor (VMM)) or a hypervisor executing on top of a host operating system, and each of the sets of applications  964 A- 964 R is run on top of a guest operating system within an instance  962 A- 962 R called a virtual machine (which may in some cases be considered a tightly isolated form of software container) that is run on top of the hypervisor—the guest operating system and application may not know they are running on a virtual machine as opposed to running on a “bare metal” host electronic device, or through para-virtualization the operating system and/or application may be aware of the presence of virtualization for optimization purposes. In yet other alternative embodiments, one, some or all of the applications are implemented as unikernel(s), which can be generated by compiling directly with an application only a limited set of libraries (e.g., from a library operating system (LibOS) including drivers/libraries of OS services) that provide the particular OS services needed by the application. As a unikernel can be implemented to run directly on hardware  940 , directly on a hypervisor (in which case the unikernel is sometimes described as running within a LibOS virtual machine), or in a software container, embodiments can be implemented fully with unikernels running directly on a hypervisor represented by virtualization layer  954 , unikernels running within software containers represented by instances  962 A- 962 R, or as a combination of unikernels and the above-described techniques (e.g., unikernels and virtual machines both run directly on a hypervisor, unikernels and sets of applications that are run in different software containers). 
     The instantiation of the one or more sets of one or more applications  964 A- 964 R, as well as virtualization if implemented, are collectively referred to as software instance(s)  952 . Each set of applications  964 A- 964 R, corresponding virtualization construct (e.g., instance  962 A- 962 R) if implemented, and that part of the hardware  940  that executes them (be it hardware dedicated to that execution and/or time slices of hardware temporally shared), forms a separate virtual network element(s)  960 A- 960 R. 
     The virtual network element(s)  960 A- 960 R perform similar functionality to the virtual network element(s)  930 A- 930 R—e.g., similar to the control communication and configuration module(s)  932 A and forwarding table(s)  934 A (this virtualization of the hardware  940  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). While embodiments of the invention are illustrated with each instance  962 A- 962 R corresponding to one VNE  960 A- 960 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 instances  962 A- 962 R to VNEs also apply to embodiments where such a finer level of granularity and/or unikernels are used. 
     In certain embodiments, the virtualization layer  954  includes a virtual switch that provides similar forwarding services as a physical Ethernet switch. Specifically, this virtual switch forwards traffic between instances  962 A- 962 R and the NIC(s)  944 , as well as optionally between the instances  962 A- 962 R; in addition, this virtual switch may enforce network isolation between the VNEs  960 A- 960 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. 9A  is a hybrid network device  906 , which includes both custom ASICs/special-purpose 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  902 ) could provide for para-virtualization to the networking hardware present in the hybrid network device  906 . 
     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)  930 A- 930 R, VNEs  960 A- 960 R, and those in the hybrid network device  906 ) receives data on the physical NIs (e.g.,  916 ,  946 ) and forwards that data out the appropriate ones of the physical NIs (e.g.,  916 ,  946 ). 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), Transmission Control Protocol (TCP), and differentiated services (DSCP) values. 
       FIG. 9C  illustrates various exemplary ways in which VNEs may be coupled according to some embodiments of the invention.  FIG. 9C  shows VNEs  970 A. 1 - 970 A.P (and optionally VNEs  970 A.Q- 970 A.R) implemented in ND  900 A and VNE  970 H. 1  in ND  900 H. In  FIG. 9C , VNEs  970 A. 1 -P are separate from each other in the sense that they can receive packets from outside ND  900 A and forward packets outside of ND  900 A; VNE  970 A. 1  is coupled with VNE  970 H. 1 , and thus they communicate packets between their respective NDs; VNE  970 A. 2 - 970 A. 3  may optionally forward packets between themselves without forwarding them outside of the ND  900 A; and VNE  970 A.P may optionally be the first in a chain of VNEs that includes VNE  970 A.Q followed by VNE  970 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. 9C  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. 9A , 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, phablets, multimedia phones, Voice over Internet Protocol (VoIP) phones, terminals, portable media players, Global Positioning System (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. 9A  may also host one or more such servers (e.g., in the case of the general purpose network device  904 , one or more of the software instances  962 A- 962 R may operate as servers; the same would be true for the hybrid network device  906 ; in the case of the special-purpose network device  902 , one or more such servers could also be run on a virtualization layer executed by the compute resource(s)  912 ); 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. 9A ) 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 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 IETF 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., BGP/MPLS IPVPN) 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. 9D  illustrates a network with a single network element on each of the NDs of  FIG. 9A , 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. 9D  illustrates network elements (NEs)  970 A- 970 H with the same connectivity as the NDs  900 A-H of  FIG. 9A . 
       FIG. 9D  illustrates that the distributed approach  972  distributes responsibility for generating the reachability and forwarding information across the NEs  970 A- 970 H; in other words, the process of neighbor discovery and topology discovery is distributed. 
     For example, where the special-purpose network device  902  is used, the control communication and configuration module(s)  932 A- 932 R of the ND control plane  924  typically include a reachability and forwarding information module to implement one or more routing protocols (e.g., an exterior gateway protocol such as BGP, Interior Gateway Protocol(s) (IGP) (e.g., OSPF, IS-IS, Routing Information Protocol (RIP), LDP, Resource Reservation Protocol (RSVP) (including RSVP-TE: Extensions to RSVP for LSP Tunnels and Generalized Multi-Protocol Label Switching (GMPLS) Signaling RSVP-TE)) that communicate with other NEs to exchange routes, and then selects those routes based on one or more routing metrics. Thus, the NEs  970 A- 970 H (e.g., the compute resource(s)  912  executing the control communication and configuration module(s)  932 A- 932 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  924 . The ND control plane  924  programs the ND forwarding plane  926  with information (e.g., adjacency and route information) based on the routing structure(s). For example, the ND control plane  924  programs the adjacency and route information into one or more forwarding table(s)  934 A- 934 R (e.g., Forwarding Information Base (FIB), Label Forwarding Information Base (LFIB), and one or more adjacency structures) on the ND forwarding plane  926 . 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  902 , the same distributed approach  972  can be implemented on the general purpose network device  904  and the hybrid network device  906 . 
       FIG. 9D  illustrates that a centralized approach  974  (also known as 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  974  has the responsibility for the generation of reachability and forwarding information in a centralized control plane  976  (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  976  has a south bound interface  982  with a data plane  980  (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  970 A- 970 H (sometimes referred to as switches, forwarding elements, data plane elements, or nodes). The centralized control plane  976  includes a network controller  978 , which includes a centralized reachability and forwarding information module  979  that determines the reachability within the network and distributes the forwarding information to the NEs  970 A- 970 H of the data plane  980  over the south bound interface  982  (which may use the OpenFlow protocol). Thus, the network intelligence is centralized in the centralized control plane  976  executing on electronic devices that are typically separate from the NDs. 
     In some embodiments, the centralized reachability and forwarding information module  979  includes an MSD-aware path selection module  981  that is operable to perform operations disclosed herein, such as the operations of flow  800  of  FIG. 8A . 
     For example, where the special-purpose network device  902  is used in the data plane  980 , each of the control communication and configuration module(s)  932 A- 932 R of the ND control plane  924  typically include a control agent that provides the VNE side of the south bound interface  982 . In this case, the ND control plane  924  (the compute resource(s)  912  executing the control communication and configuration module(s)  932 A- 932 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  976  to receive the forwarding information (and in some cases, the reachability information) from the centralized reachability and forwarding information module  979  (it should be understood that in some embodiments of the invention, the control communication and configuration module(s)  932 A- 932 R, in addition to communicating with the centralized control plane  976 , 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  974 , but may also be considered a hybrid approach). 
     While the above example uses the special-purpose network device  902 , the same centralized approach  974  can be implemented with the general purpose network device  904  (e.g., each of the VNE  960 A- 960 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  976  to receive the forwarding information (and in some cases, the reachability information) from the centralized reachability and forwarding information module  979 ; it should be understood that in some embodiments of the invention, the VNEs  960 A- 960 R, in addition to communicating with the centralized control plane  976 , 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  906 . In fact, the use of SDN techniques can enhance the NFV techniques typically used in the general purpose network device  904  or hybrid network device  906  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. 9D  also shows that the centralized control plane  976  has a north bound interface  984  to an application layer  986 , in which resides application(s)  988 . The centralized control plane  976  has the ability to form virtual networks  992  (sometimes referred to as a logical forwarding plane, network services, or overlay networks (with the NEs  970 A- 970 H of the data plane  980  being the underlay network)) for the application(s)  988 . Thus, the centralized control plane  976  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). 
     While  FIG. 9D  shows the distributed approach  972  separate from the centralized approach  974 , the effort of network control may be distributed differently or the two combined in certain embodiments. For example: 1) embodiments may generally use the centralized approach (SDN)  974 , 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  974 , but may also be considered a hybrid approach. 
     While  FIG. 9D  illustrates the simple case where each of the NDs  900 A-H implements a single NE  970 A- 970 H, it should be understood that the network control approaches described with reference to  FIG. 9D  also work for networks where one or more of the NDs  900 A- 900 H implement multiple VNEs (e.g., VNEs  930 A- 930 R, VNEs  960 A- 960 R, those in the hybrid network device  906 ). Alternatively or in addition, the network controller  978  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  978  may present the implementation of a VNE/NE in a single ND as multiple VNEs in the virtual networks  992  (all in the same one of the virtual network(s)  992 , each in different ones of the virtual network(s)  992 , or some combination). For example, the network controller  978  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  976  to present different VNEs in the virtual network(s)  992  (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. 9E and 9F  respectively illustrate exemplary abstractions of NEs and VNEs that the network controller  978  may present as part of different ones of the virtual networks  992 .  FIG. 9E  illustrates the simple case of where each of the NDs  900 A-H implements a single NE  970 A- 970 H (see  FIG. 9D ), but the centralized control plane  976  has abstracted multiple of the NEs in different NDs (the NEs  970 A- 970 C and  970 G- 970 H) into (to represent) a single NE  9701  in one of the virtual network(s)  992  of  FIG. 9D , according to some embodiments.  FIG. 9E  shows that in this virtual network, the NE  9701  is coupled to NE  970 D and  970 F, which are both still coupled to NE  970 E. 
       FIG. 9F  illustrates a case where multiple VNEs (VNE  970 A. 1  and VNE  970 H. 1 ) are implemented on different NDs (ND  900 A and ND  900 H) and are coupled to each other, and where the centralized control plane  976  has abstracted these multiple VNEs such that they appear as a single VNE  970 T within one of the virtual networks  992  of  FIG. 9D , according to some embodiments of the invention. Thus, the abstraction of a NE or VNE can span multiple NDs. 
     While some embodiments implement the centralized control plane  976  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  976 , and thus the network controller  978  including the centralized reachability and forwarding information module  979 , 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) 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. 10  illustrates a general purpose control plane device  1004  including hardware  1040  comprising a set of one or more processor(s)  1042  (which are often COTS processors) and network interface controller(s)  1044  (NICs; also known as network interface cards) (which include physical NIs  1046 ), as well as non-transitory machine readable storage media  1048  having stored therein centralized control plane (CCP) software  1050 . 
     In some embodiments, the CCP software  1050  includes MSD-aware path selection module code  1081  which, when executed by the processor(s)  1042 , can provide a MSD-aware path selection module  981  component within the centralized reachability and forwarding information module instance  1079  and/or CCP application layer  1080 , depending upon the particular implementation. 
     In embodiments that use compute virtualization, the processor(s)  1042  typically execute software to instantiate a virtualization layer  1054  (e.g., in one embodiment the virtualization layer  1054  represents the kernel of an operating system (or a shim executing on a base operating system) that allows for the creation of multiple instances  1062 A-R called software containers (representing separate user spaces and also called virtualization engines, virtual private servers, or jails) that may each be used to execute a set of one or more applications; in another embodiment the virtualization layer  1054  represents a hypervisor (sometimes referred to as a virtual machine monitor (VMM)) or a hypervisor executing on top of a host operating system, and an application is run on top of a guest operating system within an instance  1062 A-R called a virtual machine (which in some cases may be considered a tightly isolated form of software container) that is run by the hypervisor; in another embodiment, an application is implemented as a unikernel, which can be generated by compiling directly with an application only a limited set of libraries (e.g., from a library operating system (LibOS) including drivers/libraries of OS services) that provide the particular OS services needed by the application, and the unikernel can run directly on hardware  1040 , directly on a hypervisor represented by virtualization layer  1054  (in which case the unikernel is sometimes described as running within a LibOS virtual machine), or in a software container represented by one of instances  1062 A-R). Again, in embodiments where compute virtualization is used, during operation an instance of the CCP software  1050  (illustrated as CCP instance  1076 A) is executed (e.g., within the instance  1062 A) on the virtualization layer  1054 . In embodiments where compute virtualization is not used, the CCP instance  1076 A is executed, as a unikernel or on top of a host operating system, on the “bare metal” general purpose control plane device  1004 . The instantiation of the CCP instance  1076 A, as well as the virtualization layer  1054  and instances  1062 A-R if implemented, are collectively referred to as software instance(s)  1052 . 
     In some embodiments, the CCP instance  1076 A includes a network controller instance  1078 . The network controller instance  1078  includes a centralized reachability and forwarding information module instance  1079  (which is a middleware layer providing the context of the network controller  978  to the operating system and communicating with the various NEs), and an CCP application layer  1080  (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  1080  within the centralized control plane  976  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 centralized control plane  976  transmits relevant messages to the data plane  980  based on CCP application layer  1080  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  980  may receive different messages, and thus different forwarding information. The data plane  980  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  980 , the packet (or a subset of the packet header and content) is typically forwarded to the centralized control plane  976 . The centralized control plane  976  will then program forwarding table entries into the data plane  980  to accommodate packets belonging to the flow of the unknown packet. Once a specific forwarding table entry has been programmed into the data plane  980  by the centralized control plane  976 , the next packet with matching credentials will match that forwarding table entry and take the set of actions associated with that matched entry. 
     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. 
     Next hop selection by the routing system for a given destination may resolve to one path (that is, a routing protocol may generate one next hop on a shortest path); but if the routing system determines there are multiple viable next hops (that is, the routing protocol generated forwarding solution offers more than one next hop on a shortest path—multiple equal cost next hops), some additional criteria is used—for instance, in a connectionless network, ECMP (also known as Equal Cost Multi Pathing, multipath forwarding and IP multipath) may be used (e.g., typical implementations use as the criteria particular header fields to ensure that the packets of a particular packet flow are always forwarded on the same next hop to preserve packet flow ordering). For purposes of multipath forwarding, a packet flow is defined as a set of packets that share an ordering constraint. As an example, the set of packets in a particular TCP transfer sequence need to arrive in order, else the TCP logic will interpret the out of order delivery as congestion and slow the TCP transfer rate down. 
     A virtual circuit (VC), synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication. Virtual circuit communication resembles circuit switching, since both are connection oriented, meaning that in both cases data is delivered in correct order, and signaling overhead is required during a connection establishment phase. Virtual circuits may exist at different layers. For example, at layer 4, a connection oriented transport layer datalink protocol such as Transmission Control Protocol (TCP) may rely on a connectionless packet switching network layer protocol such as IP, where different packets may be routed over different paths, and thus be delivered out of order. Where a reliable virtual circuit is established with TCP on top of the underlying unreliable and connectionless IP protocol, the virtual circuit is identified by the source and destination network socket address pair, i.e. the sender and receiver IP address and port number. However, a virtual circuit is possible since TCP includes segment numbering and reordering on the receiver side to prevent out-of-order delivery. Virtual circuits are also possible at Layer 3 (network layer) and Layer 2 (datalink layer); such virtual circuit protocols are based on connection oriented packet switching, meaning that data is always delivered along the same network path, i.e. through the same NEs/VNEs. In such protocols, the packets are not routed individually and complete addressing information is not provided in the header of each data packet; only a small virtual channel identifier (VCI) is required in each packet; and routing information is transferred to the NEs/VNEs during the connection establishment phase; switching only involves looking up the virtual channel identifier in a table rather than analyzing a complete address. Examples of network layer and datalink layer virtual circuit protocols, where data always is delivered over the same path: X.25, where the VC is identified by a virtual channel identifier (VCI); Frame relay, where the VC is identified by a VCI; Asynchronous Transfer Mode (ATM), where the circuit is identified by a virtual path identifier (VPI) and virtual channel identifier (VCI) pair; General Packet Radio Service (GPRS); and Multi-protocol label switching (MPLS), which can be used for IP over virtual circuits (Each circuit is identified by a label). 
     Certain NDs (e.g., certain edge NDs) use a hierarchy of circuits. The leaf nodes of the hierarchy of circuits are subscriber circuits. The subscriber circuits have parent circuits in the hierarchy that typically represent aggregations of multiple subscriber circuits, and thus the network segments and elements used to provide access network connectivity of those end user devices to the ND. These parent circuits may represent physical or logical aggregations of subscriber circuits (e.g., a virtual local area network (VLAN), a permanent virtual circuit (PVC) (e.g., for Asynchronous Transfer Mode (ATM)), a circuit-group, a channel, a pseudo-wire, a physical NI of the ND, and a link aggregation group). A circuit-group is a virtual construct that allows various sets of circuits to be grouped together for configuration purposes, for example aggregate rate control. A pseudo-wire is an emulation of a layer 2 point-to-point connection-oriented service. A link aggregation group is a virtual construct that merges multiple physical NIs for purposes of bandwidth aggregation and redundancy. Thus, the parent circuits physically or logically encapsulate the subscriber circuits. 
     Each VNE (e.g., a virtual router, a virtual bridge (which may act as a virtual switch instance in a Virtual Private LAN Service (VPLS) is typically independently administrable. For example, in the case of multiple virtual routers, each of the virtual routers may share system resources but is separate from the other virtual routers regarding its management domain, AAA (authentication, authorization, and accounting) name space, IP address, and routing database(s). Multiple VNEs may be employed in an edge ND to provide direct network access and/or different classes of services for subscribers of service and/or content providers. 
     Within certain NDs, “interfaces” that are independent of physical NIs may be configured as part of the VNEs to provide higher-layer protocol and service information (e.g., Layer 3 addressing). The subscriber records in the AAA server identify, in addition to the other subscriber configuration requirements, to which context (e.g., which of the VNEs/NEs) the corresponding subscribers should be bound within the ND. As used herein, a binding forms an association between a physical entity (e.g., physical NI, channel) or a logical entity (e.g., circuit such as a subscriber circuit or logical circuit (a set of one or more subscriber circuits)) and a context&#39;s interface over which network protocols (e.g., routing protocols, bridging protocols) are configured for that context. Subscriber data flows on the physical entity when some higher-layer protocol interface is configured and associated with that physical entity. 
     Some NDs provide support for implementing VPNs (Virtual Private Networks) (e.g., Layer 2 VPNs and/or Layer 3 VPNs). For example, the ND where a provider&#39;s network and a customer&#39;s network are coupled are respectively referred to as PEs (Provider Edge) and CEs (Customer Edge). In a Layer 2 VPN, forwarding typically is performed on the CE(s) on either end of the VPN and traffic is sent across the network (e.g., through one or more PEs coupled by other NDs). Layer 2 circuits are configured between the CEs and PEs (e.g., an Ethernet port, an ATM permanent virtual circuit (PVC), a Frame Relay PVC). In a Layer 3 VPN, routing typically is performed by the PEs. By way of example, an edge ND that supports multiple VNEs may be deployed as a PE; and a VNE may be configured with a VPN protocol, and thus that VNE is referred as a VPN VNE. 
     Some NDs provide support for VPLS (Virtual Private LAN Service). For example, in a VPLS network, end user devices access content/services provided through the VPLS network by coupling to CEs, which are coupled through PEs coupled by other NDs. VPLS networks can be used for implementing triple play network applications (e.g., data applications (e.g., high-speed Internet access), video applications (e.g., television service such as IPTV (Internet Protocol Television), VoD (Video-on-Demand) service), and voice applications (e.g., VoIP service)), VPN services, etc. VPLS is a type of layer 2 VPN that can be used for multi-point connectivity. VPLS networks also allow end use devices that are coupled with CEs at separate geographical locations to communicate with each other across a Wide Area Network (WAN) as if they were directly attached to each other in a Local Area Network (LAN) (referred to as an emulated LAN). 
     In VPLS networks, each CE typically attaches, possibly through an access network (wired and/or wireless), to a bridge module of a PE via an attachment circuit (e.g., a virtual link or connection between the CE and the PE). The bridge module of the PE attaches to an emulated LAN through an emulated LAN interface. Each bridge module acts as a “Virtual Switch Instance” (VSI) by maintaining a forwarding table that maps MAC addresses to pseudowires and attachment circuits. PEs forward frames (received from CEs) to destinations (e.g., other CEs, other PEs) based on the MAC destination address field included in those frames. 
     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.). 
     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.