Patent Publication Number: US-9853874-B2

Title: Flow-specific failure detection in SDN networks

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims the benefit and priority under U.S.C. 119(e) of U.S. Provisional Application No. 62/136,922, filed Mar. 23, 2015, entitled “INCREASING RESILIENCY IN SDN NETWORKS.” The entire contents of this provisional application are incorporated herein by reference for all purposes. 
     In addition, the present application is related to commonly-owned U.S. patent application Ser. No. 14/923,738, filed concurrently with the present application, entitled “EFFICIENT TOPOLOGY FAILURE DETECTION IN SDN NETWORKS.” The entire contents of this related application are incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     As known in the art, Software Defined Networking (SDN) is a computer networking paradigm in which the system(s) that make decisions about where traffic is sent (i.e., the control plane) are decoupled from the system(s) that forward traffic to their intended destinations (i.e., the data plane). By way of example,  FIG. 1A  depicts a simplified representation of an SDN network  100  comprising an SDN controller  102  and three network switches  104 ,  106 ,  108 . In this example, SDN controller  102  constitutes the control plane of network  100  and is responsible for, e.g.: (1) maintaining a global view of network  100 ; (2) determining (via one or more applications running on, or in communication with, controller  102 ) forwarding rules to be followed by switches  104 - 108  in order to achieve a desired network behavior; and (3) causing those rules to be programmed into the hardware forwarding tables of switches  104 - 108 . Switches  104 - 108  constitute the data plane of network  100  and are responsible for, e.g., forwarding, at line rate, network traffic in accordance with the forwarding rules determined by SDN controller  102 . 
     In current SDN networks, the detection of network faults is handled centrally by the SDN controller via Link Layer Discovery Protocol (LLDP). An example of a conventional fault detection method  150  that can be performed by SDN controller  102  of  FIG. 1A  using LLDP is depicted in  FIG. 1B . At step (1) (reference numeral  152 ), SDN controller  102  constructs and sends out an LLDP packet with a “packet_out” message to each connected switch. SDN controller  102  typically performs this step every second. 
     At step (2) (reference numeral  154 ), each switch ( 104 ,  106 ,  108 ) receives the LLDP packet sent by SDN controller  102  and forwards the packet on all of its outgoing ports (to other switches in the network). 
     Finally, at step (3) (reference numeral  156 ), each switch ( 104 ,  106 ,  108 ) receives the LLDP packets forwarded by other switches and sends those packets back to SDN controller  102 . If there are no topology failures in the network, SDN controller  102  should receive these return packets approximately every second (i.e., at the same rate that the packets were sent out at step ( 1 )). If SDN controller  102  does not receive a return packet from a particular switch within a predefined LLDP timeout period (e.g., 3 seconds), SDN controller  102  can conclude that one or more ports or links along the path from that switch have failed. 
     While the fault detection method shown in  FIG. 1B  is functional, it suffers from a number of limitations. First, since method  150  requires that SDN controller  102  send out LLDP packets on a continuous basis to switches  104 - 108  and monitor for the receipt of those packets before determining whether a fault has occurred, method  150  cannot easily scale to support a very large network or to support faster detection times. For instance, if SDN controller  102  increased the rate at which it sent out LLDP packets in order to improve detection times, SDN controller  102  would also need to be able to process the incoming return packets at that higher rate, which may not be possible. Similarly, if network  100  increased in size to encompass more switches, SDN controller  102  would need to be able to handle the greater volume of outgoing and incoming LLDP traffic caused by the additional switches. 
     Second, since SDN controller  102  acts as the point-of-detection, SDN controller  102  must communicate with the affected switch(es) upon detecting a fault into order to initiate a repair (e.g., provisioning and switch-over to a backup path). This extra communication step can slow down the overall repair process. 
     Third, method  150  of  FIG. 1B  can only be used to detect faults that affect the integrity of a network topology, such as port, link, or node failures. Method  150  cannot detect flow-specific failures that do not affect the network topology, but may nevertheless result in unexpected forwarding behavior (e.g., a mis-programmed flow or incorrect flow priorities). 
     SUMMARY 
     Techniques for performing flow-specific failure detection in SDN networks are provided. In one embodiment, a computer system (e.g., an SDN controller) can determine a flow to be monitored in a network. The computer system can then transmit first and second messages to first and second network devices in the network respectively, where the first network device is an upstream device in the flow, where the second network device is a downstream device in the flow, and where the first and second messages instruct the first and second network devices to collect local data rate information for the flow. 
     The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of particular embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  depicts an example SDN network. 
         FIG. 1B  depicts an example LLDP workflow within the SDN network of  FIG. 1A . 
         FIG. 2  depicts an SDN network that supports active path tracing and flow-specific failure detection according to an embodiment. 
         FIG. 3  depicts a workflow for performing active path tracing according to an embodiment. 
         FIG. 4  depicts a flowchart that provides additional details regarding the workflow of  FIG. 3  according to an embodiment. 
         FIG. 5  depicts a workflow for performing flow-specific failure detection according to an embodiment. 
         FIG. 6  depicts a flowchart that provides additional details regarding the workflow of  FIG. 5  according to an embodiment. 
         FIG. 7  depicts a network switch according to an embodiment. 
         FIG. 8  depicts a computer system according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous examples and details are set forth in order to provide an understanding of various embodiments. It will be evident, however, to one skilled in the art that certain embodiments can be practiced without some of these details, or can be practiced with modifications or equivalents thereof. 
     1. Overview 
     Embodiments of the present disclosure provide techniques for improving the resiliency of SDN networks against various types of network faults. In one set of embodiments, these techniques include an improved fault detection method (referred to as “active path tracing”) in which an SDN controller instructs first and second switches at the endpoints of a link/path to respectively transmit, and monitor for, probe packets along the link/path. If the second switch determines that it has not received a probe packet from the first switch within a predetermined timeout period (or has not received a predetermined number of consecutive probe packets), the second switch can transmit a notification to the SDN controller indicating that the link or ports between the two switches have failed. With this approach, there is no need for the SDN controller itself to send out probe (e.g., LLDP) packets and monitor for the return of those packets in order to detect faults; instead, the controller can effectively offload these tasks to the switches in the network. As a result, the amount of control traffic exchanged between the SDN controller and the switches can be significantly reduced when compared to traditional LLDP fault detection, which in turn can allow for greater efficiency/scalability and faster detection times. 
     In another set of embodiments, the techniques described herein include a method for detecting flow-specific failures. In these embodiments, the SDN controller can instruct a first switch involved in a unidirectional flow (e.g., a downstream switch) to keep track of its local flow data rate and to communicate this flow rate information to a second switch involved in the flow (e.g., an upstream switch) via special packets. If the upstream switch determines that its local flow data rate is not consistent with the data rate information received from the downstream switch, the upstream switch can transmit a message to the SDN controller indicating that there is a flow disruption. Thus, this method can detect “soft” failures where there is no change to the network topology, but there are nevertheless flow problems due to, e.g., system issues (packet forwarding from ingress to egress port), mis-programmed flows, and the like. 
     In yet another set of embodiments, the techniques described herein can include methods for reducing the time needed to repair a detected fault (either a topology failure or a flow-specific failure). At a high level, this can involve pre-provisioning, at the SDN controller, backup paths for switches in the network and transmitting this backup path information to the switches prior to the detection of any fault. For example, this backup path information can be sent as part of the instructions transmitted by the SDN controller for initiating active path tracing or flow data rate monitoring as described above. If a particular switch determines that there is a topology or flow-specific failure, that switch can immediately failover to the backup path provided by the SDN controller (referred to as a “local repair”), without having to communicate again with the SDN controller. It should be noted that this local repair is performed on a per-switch basis; in other words, each switch (with the exception of the last hop) can have a backup path pre-provisioned by the SDN controller. Upon detecting a topology or flow-specific failure, the switch can automatically failover to the pre-provisioned backup path. 
     These and other aspects of the present disclosure are described in further detail in the sections that follow. 
     2. System Environment 
       FIG. 2  depicts architectural components of an SDN controller  200  and a network switch  202  that support the resiliency features described herein according to an embodiment. SDN controller  200  can be implemented using, e.g., a general purpose or specialized computer system. Network switch  202  can be a physical (i.e., hardware-based) or virtual (i.e., software-based) switch. 
     As shown in  FIG. 2 , SDN controller  200  executes one or more SDN applications  204  that are configured to determine forwarding rules to be followed by network switch  202  according to the applications&#39; respective requirements. SDN controller  200  can transmit the application-determined forwarding rules, via an appropriate southbound SDN protocol, to a SDN protocol agent  206  running on network switch  202 . SDN protocol agent  206  can then program the forwarding rules into one or more of the switch&#39;s flow tables (e.g., tables  208 ). 
     In one embodiment, the southbound SDN protocol used for communication between SDN controller  200  and agent SDN protocol agent  206  can be the OpenFlow protocol. In other embodiments, the southbound SDN protocol can be any other standard or proprietary protocol known in the art. 
     As noted in the Background section, one deficiency with existing SDN network implementations is that they perform network fault detection using an LLDP flooding/timeout mechanism that requires the SDN controller to send out, and monitor for, LLDP packets—in other words, the SDN controller is the point-of-detection. This means that the processing capabilities of the SDN controller act as a limit on the scalability of the solution. Further, LLDP-based fault detection can only detect topology failures, and cannot detect flow-specific failures. 
     To address these and other similar issues, SDN controller  200  and network switch  202  of  FIG. 2  implement a novel resiliency application  210  (as part of SDN applications  204 ) and a novel SDN protocol helper component  212  respectively. As described in further detail below, resiliency application  210  and SDN protocol helper  212  can interoperate in a manner that: (1) enables SDN controller  200  to offload the transmission and monitoring of fault detection probe packets to network switches like switch  202  (referred to herein as “active path tracing”); and (2) enables switch  202  and other similar switches to locally detect flow-specific failures via a flow statistics monitoring mechanism. Features (1) and (2) can be enabled without implementing any additional protocols beyond the SDN communication protocol already in use between SDN controller  200  and network switch  202 . These features do not entirely replace LLDP (which may still be used for topology building at the SDN controller), but instead offload the fault detection function of LLDP to the switches, thereby providing faster detection, scalability, and ability to detect other kinds of failures that cannot be detected with LLDP (e.g., flow-specific failures). 
     Further, since features (1) and (2) above effectively make network switch  202  (rather than SDN controller  200 ) the point-of-detection for faults, in certain embodiments components  210  and  212  can work in concert to achieve local repair at switch  202  (i.e., failover of data traffic to a backup path in the case of a fault). This can significantly improve failover times, because there is no need for a roundtrip communication between network switch  202  and SDN controller  200  before initiating the repair process. 
     It should be appreciated that  FIG. 2  is illustrative and not intended to limit embodiments of the present invention. For example, the various components/modules shown in  FIG. 2  may have sub-components or functions that are not specifically described. One of ordinary skill in the art will recognize other modifications, variations, and alternatives. 
     3. Active Path Tracing 
       FIG. 3  depicts a high-level workflow  300  that can be performed by an SDN controller  302  and two network switches  304  and  306  for implementing active path tracing according to an embodiment. SDN controller  302  can be implemented using SDN controller  200  of  FIG. 2 , and each network switch  304 / 306  can be implemented using network switch  202  of  FIG. 2 . Active path tracing enables SDN controller  302  to advantageously offload the transmission and monitoring of probe packets for network fault detection to switches  304  and  306 , thereby allowing for greater scalability and potentially faster detection times. 
     Starting with step (1) of workflow  300  (reference numeral  308 ), the resiliency application running on SDN controller  302  can transmit a special packet/message to switch  304  instructing the switch to begin sending probe packets to switch  306  for the purpose of monitoring the health of the link between the two switches. In embodiments where SDN controller  302  uses OpenFlow to communicate with switch  304 , the special packet/message can be sent in the form of an OpenFlow “Experimenter” (in OpenFlow v. 1.3) or “Vendor” (in OpenFlow v. 1.0) message. In these embodiments, the Experimenter or Vendor message can include a payload that identifies the purpose of the packet/message (i.e., initiate active path tracing), as well as supporting parameters such as path details, probe packet transmission rate, etc. Alternatively, the special packet/message can be sent in the form of an OpenFlow message that has been created and standardized for this specific purpose. In yet other embodiments, SDN controller  302  can use any other southbound protocol to communicate the special packet/message. Note that SDN controller  302  only needs to send this special packet/message once to switch  304  in order to initiate active path tracing. 
     At step (2) (reference numeral  310 ), the resiliency application of SDN controller  302  can also transmit a special packet/message to switch  306  instructing the switch to begin listening for the probe packets from switch  304 , and to alert controller  302  in case such packets are not received from switch  304  within a predefined timeout period (and/or for a certain number of times) Like the special packet/message sent at step ( 1 ), this packet/message can take the form of an OpenFlow Experimenter/Vendor message or a new, standardized OpenFlow message (not yet defined), and only needs to be transmitted to switch  306  once. 
     At step (3) (reference numeral  312 ), the SDN protocol helper running on switch  304  can interpret the special packet/message received from SDN controller  302  and can cause switch  304  to begin sending probe packets to switch  306 . Generally speaking, the frequency at which the probe packets are sent will determine how quickly faults can be detected, and this frequency can be configured by the resiliency application of SDN controller  302  (via the “probe packet transmission rate” parameter mentioned above). In one embodiment, switch  304  can be configured to send out the probe packets at a rate faster than one per second (which is the typical rate for LLDP fault detection). Since the probe packets are transmitted by switch  304  instead of SDN controller  302 , controller  302  does not incur any additional stress or computational load by increasing this frequency value. 
     Concurrently with step (3), at step (4) (reference numeral  314 ), the SDN protocol helper running on switch  306  can interpret the special packet/message received from SDN controller  302  and can begin listening for the probe packets sent by switch  304 . 
     Finally, at step (5) (reference numeral  316 ), if the SDN protocol helper on switch  306  determines that probe packets have not been received from switch  304  within a preconfigured interval (or for a certain number of times), the SDN protocol helper can cause switch  306  to send a single notification message to SDN controller  306  indicating that the path between the two switches has experienced a failure. 
     With workflow  300  of  FIG. 3 , a number of advantages can be realized over conventional LLDP fault detection. First, SDN controller  302  only sends a single instruction packet/message to switch  304  and  306  respectively in order to initiate the detection process; switches  304  and  306  then autonomously handle the tasks of sending, and monitoring for, probe packets over the path between the switches. Further, SDN controller  302  only receives a single notification (from switch  306 ) when a fault has been detected. This is contrast to LLDP fault detection, which requires the controller itself to transmit and monitor for LLDP packets on a continuous basis. Thus, workflow  300  is far more scalable, and can be configured to reduce detection latency (by increasing the probe packet transmission rate at switch  304 ) without impacting SDN controller  302 . 
     Further, since switch  306  becomes the point-of-detection in workflow  300 , this opens up the possibility of performing local repair directly at switch  306 , without having to contact SDN controller  302  (described in Section 5 below). 
     Although not shown in  FIG. 3 , in certain embodiments, rather than sending special packets/messages to both switches  304  and  306  at steps (1) and (2) of workflow  300  respectively, SDN controller  302  may send such a packet/message to switch  304  only. This packet/message may include instructions to initiate active path tracing on the path to switch  306 , as well as a timeout and/or miss count parameter. Switch  304  may then begin sending probe packets to switch  306  over the path and may monitor for reply packets from switch  306  in response to the probe packets. If the SDN protocol helper on switch  304  determines that a reply packet is not received from switch  306  within a time period m (where m corresponds to the timeout parameter) and/or there are n consecutive misses of the reply packet from switch  306  (where n corresponds to the miss count parameter), switch  304  can conclude that the path to switch  306  has been disrupted or has gone down and can send an appropriate notification to SDN controller  302 . Thus, in these embodiments, switch  304  can act as both the sender of probe packets along the monitored path and the point-of-detection of network faults for that path. 
       FIG. 4  depicts a flowchart  400  that provides additional details regarding the active path tracing workflow of  FIG. 3  according to an embodiment. Starting with block  402 , the resiliency application running on an SDN controller (e.g., controller  302  of  FIG. 3 ) can identify a path in a network that should be monitored via active path tracing and can determine one or more parameters for configuring the tracing process. These parameters can include, e.g., the details of the path, a probe packet transmission rate parameter indicating how often probe packets should be sent out along the path, a timeout parameter indicating a time-based threshold for concluding that the path has become nonoperational, a miss count value indicating a packet-based threshold for concluding that the path has become nonoperational, and/or others. In a particular embodiment, the resiliency application may receive desired values for one or more of these parameters from an administrator or user. 
     At block  404 , the resiliency application can generate and send a first special packet/message to a first switch along the path (e.g., switch  304  of  FIG. 3 ) that includes some (or all) of the parameters determined at block  402  and that instructs the first switch to begin sending out probe packets to a second switch along the path (e.g., switch  306  of  FIG. 3 ). For example, in one embodiment, this first special packet/message can include the timer parameter described above so that the first switch knows the frequency at which it should send out the probe packets. 
     At approximately the same time as block  404 , SDN controller  302  can also generate and send a second special packet/message to the second switch along the path that includes some (or all) of the parameters determined at block  402  and that instructs the second switch to begin monitoring for probe packets from the first switch (block  406 ). In one embodiment, this second special packet/message can include the timeout and/or miss count parameters described above so that the second switch knows how to determine when the path between the first and second switches has gone down. 
     Then, at blocks  408  and  410 , the first switch can send out the probe packets to the second switch at the specified transmission rate, and the second switch can monitor for and receive the probe packets. If, at block  412 , the second network switch detects a fault by, e.g., determining that it has not received n consecutive probe packets from the first switch (where n is the miss count parameter described above) or has not received a probe packet for m seconds (where m is the timeout parameter described above), the second switch can send an error notification to the SDN controller (block  414 ). 
     Finally, at block  416 , the resiliency application on the SDN controller can receive the error notification from the second switch and take one or more steps to address the fault (e.g., reroute or trap the flows along the path). 
     4. Flow-Specific Fault Detection 
     In addition to enabling faster fault detection, certain embodiments can also enable the detection of flow-specific failures.  FIG. 5  depicts high-level workflow  500  that can be performed by an SDN controller  502  and two network switches  504  and  506  for implementing such a process according to an embodiment. In various embodiments, SDN controller  502  and network switches  504 / 506  can be implemented using SDN controller  200  and network switch  202  of  FIG. 2  respectively. 
     Starting with step (1) (reference numeral  508 ), the resiliency application running on SDN controller  502  can determine that a unidirectional flow between switches  504  and  506  should be monitored, and can send out special packets/messages to switches  504  and  506  instructing them to begin flow rate monitoring. In this example, switch  504  is upstream of switch  506  with respect to the flow, and thus switch  504  is considered an upstream device and switch  506  is considered a downstream device. Like the special packets/messages described with respect to workflow  300  of  FIG. 3 , the packets/messages sent at step (1) of workflow  500  can be OpenFlow Experimenter/Vendor messages or a new, standardized OpenFlow message (not yet defined). 
     In response to these packets/messages, the SDN protocol helper running on downstream switch  506  can begin sending flow rate information for the flow to upstream switch  504  via special packets (step (2), reference numeral  510 ). In various embodiments, this flow rate information can reflect the local data rate for the flow as measured at downstream switch  506 . Switch  506  can send this flow rate information at a regular interval (e.g., once a second) that may be defined in the special packets/message received from SDN controller  502 . 
     At step (3) (reference numeral  512 ), the SDN protocol helper running on upstream switch  504  can receive the flow rate information sent by downstream switch  506  and can compare that rate (i.e., the downstream rate) to the local rate determined at upstream switch  504 . In this particular example, the downstream flow has been disrupted, and thus the downstream rate is 0 kbps (while the upstream rate is 100 kbps). Upon detecting this discrepancy in rates, the SDN protocol helper can conclude that there has been a flow disruption. 
     Finally, at step (4) (reference numeral  514 ), upstream switch  504  can transmit a message to SDN controller  502  identifying the flow failure. 
     With workflow  500  of  FIG. 5 , switches  504  and  506  can advantageously detect “soft” failures that affect a network flow, such as a flow congestion at a particular switch, flow mis-programming, or the like. Switch  504  can then communicate this information SDN controller  502  so that controller  502  can take steps to address the problem. This type of soft failure detection is not possible with conventional LLDP, which is only designed to detect failures that affect the physical network topology. 
     Further, since switch  504  handle the flow failure detection locally, there is no need for external monitors and/or SDN controller  502  to check for traffic loss, thereby significantly reducing the amount of northbound traffic that is needed between switches  504 / 506  and such monitors and/or controller  502 . 
       FIG. 6  depicts a flowchart  600  that provides additional details regarding the flow-specific fault detection workflow of  FIG. 5  according to an embodiment. Starting with block  602 , the resiliency application running on an SDN controller (e.g., controller  502  of  FIG. 5 ) can identify a flow in a network that should be monitored for faults and can determine one or more parameters for configuring the detection process. These parameters can include, e.g., flow details and a threshold parameter indicating a degree of difference in flow rates that would signal a flow disruption. In a particular embodiment, the resiliency application may receive desired values for one or more of these parameters from an administrator or user. 
     At block  604 , the resiliency application can generate and send a special packet/message to each of two switches along the path of the flow (e.g., upstream switch  504  and downstream switch  506  of  FIG. 5 ) that includes some (or all) of the parameters determined at block  602  and that instructs the downstream switch to collect local flow rate information indicating the incoming and/or outgoing data rate(s) for the flow and send out this local flow rate to the upstream switch in the form of special flow rate packets, and instructs the upstream switch to monitor for the special packets from the downstream switch. The SDN protocol helper of each switch can then process these special packets/messages and being local flow rate monitoring as instructed (block  606 ). 
     At block  608 , upon receiving a special packet from the downstream switch with flow rate information, the upstream switch can compare the received flow rate information with the switch&#39;s local flow rate information. For example, the upstream switch can compare the outgoing flow data rate with the incoming flow data rate specified in the packet. Based on this comparison, the upstream switch can check whether the difference in flow data rates exceeds a threshold (as specified by the threshold parameter discussed at block  602 ) (block  610 ). If not, the switch can determine that there is no flow disruption and flowchart  600  can cycle back to block  608 . 
     However, if the different in flow data rates does exceed the threshold, the upstream switch can determine that a flow disruption has occurred and can send an error notification to the SDN controller (block  612 ). The resiliency app of the SDN controller can then take appropriate steps to address the disruption, such as by redirecting the flow (block  614 ). 
     It should be appreciated that the workflows and flowcharts of  FIGS. 3-6  are illustrative and not intended to limit embodiments of the present disclosure. For example, although only two switches are shown in  FIGS. 3 and 5 , SDN controller  302 / 502  can potentially interact with many switches simultaneously using the general concepts outlined in the workflows. Further, in some embodiments, the special/probe packets exchanged between switch  304 / 504  and switch  306 / 506  can be tunneled to make their format opaque to any intermediate switches that may not implement the SDN protocol helper described herein. One of ordinary skill in the art will recognize many variations, modifications, and alternatives. 
     5. Local Repair 
     As mentioned previously, in certain embodiments the switches shown in  FIGS. 3 and 5  can perform a “local repair” upon detecting a topology or flow-specific failure. This is in contrast to conventional repair methods, which require the central SDN controller to be notified of (or detect) the failure, and then provision and push backup path information to the switches to implement the repair. There are existing techniques can perform local repair at an in-band switch, but those existing techniques generally can only react to port down failures (not other topology or flow-specific failures). 
     To enable local repair in response to a topology or flow-specific failures, the resiliency application running on the SDN controller can pre-provision backup paths and transmit this information to connected switches as part of the special packet/messages described with respect to workflows  300  and  500 . In a particular embodiment, this can be facilitated by using the “fast-failover group” functionality available in OpenFlow 1.3. Then, when a given switch detects a topology failure (in the case of workflow  300 ) or a flow-specific failure (in the case of workflow  500 ), the switch can automatically failover traffic to the pre-provisional backup path(s) without contacting the SDN controller again. 
     6. Network Switch 
       FIG. 7  is a simplified block diagram of an example network switch  700  according to an embodiment. Network switch  700  can be used to implement, e.g., switches  202 ,  304 ,  306 ,  504 , and  506  of  FIGS. 2, 3, and 5  respectively. 
     As shown, network switch  700  includes a management module  702 , a switch fabric module  704 , and a number of I/O modules  706 ( 1 )- 706 (N). Management module  702  includes one or more management CPUs  708  for managing/controlling the operation of the device. Each management CPU  708  can be a general purpose processor, such as a PowerPC, Intel, AMD, or ARM-based processor, that operates under the control of software stored in an associated memory (not shown). In one embodiment, management CPU  708  can carry out the operations attributed to SDN protocol helper  212  and SDN protocol agent  206  in the foregoing disclosure. 
     Switch fabric module  704  and I/O modules  706 ( 1 )- 706 (N) collectively represent the data, or forwarding, plane of network switch  700 . Switch fabric module  704  is configured to interconnect the various other modules of network switch  700 . Each I/O module  706 ( 1 )- 706 (N) can include one or more input/output ports  710 ( 1 )- 710 (N) that are used by network switch  700  to send and receive data packets. Each I/O module  706 ( 1 )- 706 (N) can also include a packet processor  712 ( 1 )- 712 (N). Packet processor  712 ( 1 )- 712 (N) is a hardware processing component (e.g., an FPGA or ASIC) that can make wire speed decisions on how to handle incoming or outgoing data packets. In a particular embodiment, each packet processor can incorporate the flow tables  208  described with respect to  FIG. 2 . 
     It should be appreciated that network switch  700  is illustrative and not intended to limit embodiments of the present invention. Many other configurations having more or fewer components than switch  700  are possible. 
     7. Computer System 
       FIG. 8  is a simplified block diagram of an example computer system  800  according to an embodiment. Computer system  800  can be used to implement SDN controllers  200 ,  302 , and  502  of  FIGS. 2, 3, and 5  respectively. As shown in  FIG. 8 , computer system  800  can include one or more processors  802  that communicate with a number of peripheral devices via a bus subsystem  804 . These peripheral devices can include a storage subsystem  806  (comprising a memory subsystem  808  and a file storage subsystem  810 ), user interface input devices  812 , user interface output devices  814 , and a network interface subsystem  816 . 
     Bus subsystem  804  can provide a mechanism for letting the various components and subsystems of computer system  800  communicate with each other as intended. Although bus subsystem  804  is shown schematically as a single bus, alternative embodiments of the bus subsystem can utilize multiple busses. 
     Network interface subsystem  816  can serve as an interface for communicating data between computer system  800  and other computing devices or networks. Embodiments of network interface subsystem  816  can include wired (e.g., coaxial, twisted pair, or fiber optic Ethernet) and/or wireless (e.g., Wi-Fi, cellular, Bluetooth, etc.) interfaces. 
     User interface input devices  812  can include a keyboard, pointing devices (e.g., mouse, trackball, touchpad, etc.), a scanner, a barcode scanner, a touch-screen incorporated into a display, audio input devices (e.g., voice recognition systems, microphones, etc.), and other types of input devices. In general, use of the term “input device” is intended to include all possible types of devices and mechanisms for inputting information into computer system  800 . 
     User interface output devices  814  can include a display subsystem, a printer, a fax machine, or non-visual displays such as audio output devices, etc. The display subsystem can be a cathode ray tube (CRT), a flat-panel device such as a liquid crystal display (LCD), or a projection device. In general, use of the term “output device” is intended to include all possible types of devices and mechanisms for outputting information from computer system  800 . 
     Storage subsystem  806  can include a memory subsystem  808  and a file/disk storage subsystem  810 . Subsystems  808  and  810  represent non-transitory computer-readable storage media that can store program code and/or data that provide the functionality of various embodiments described herein. 
     Memory subsystem  808  can include a number of memories including a main random access memory (RAM)  818  for storage of instructions and data during program execution and a read-only memory (ROM)  820  in which fixed instructions are stored. File storage subsystem  810  can provide persistent (i.e., non-volatile) storage for program and data files and can include a magnetic or solid-state hard disk drive, an optical drive along with associated removable media (e.g., CD-ROM, DVD, Blu-Ray, etc.), a removable flash memory-based drive or card, and/or other types of storage media known in the art. 
     It should be appreciated that computer system  800  is illustrative and not intended to limit embodiments of the present invention. Many other configurations having more or fewer components than computer system  800  are possible. 
     The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. For example, although certain embodiments have been described in the context of SDN networks, the techniques described herein may also be used to increase resiliency and improve fault detection in other types of networks that may include a controller-like device and data forwarding devices (e.g., Ethernet or SAN fabrics, etc.). Further, although certain embodiments have been described with respect to particular process flows and steps, it should be apparent to those skilled in the art that the scope of the present invention is not strictly limited to the described flows and steps. Steps described as sequential may be executed in parallel, order of steps may be varied, and steps may be modified, combined, added, or omitted. As another example, although certain embodiments have been described using a particular combination of hardware and software, it should be recognized that other combinations of hardware and software are possible, and that specific operations described as being implemented in software can also be implemented in hardware and vice versa. 
     The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. Other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the invention as set forth in the following claims.