Patent Publication Number: US-11044191-B2

Title: Coupling reactive routing with predictive routing in a network

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to computer networks, and, more particularly, to coupling reactive routing with predictive routing in a network. 
     BACKGROUND 
     Software-defined wide area networks (SD-WANs) represent the application of software-defined networking (SDN) principles to WAN connections, such as connections to cellular networks, the Internet, and Multiprotocol Label Switching (MPLS) networks. The power of SD-WAN is the ability to provide consistent service level agreement (SLA) for important application traffic transparently across various underlying tunnels of varying transport quality and allow for seamless tunnel selection based on tunnel performance characteristics that can match application SLAs. 
     Failure detection in a network has traditionally been reactive, meaning that the failure must first be detected before rerouting the traffic along a secondary (backup) path. In general, failure detection leverages either explicit signaling from the lower network layers or using a keep-alive mechanism that sends probes at some interval T that must be acknowledged by a receiver (e.g., a tunnel tail-end router). Typically, SD-WAN implementations leverage the keep-alive mechanisms of Bidirectional Forwarding Detection (BFD), to detect tunnel failures and to initiate rerouting the traffic onto a backup (secondary) tunnel, if such a tunnel exits. While this approach is somewhat effective at mitigating tunnel failures in an SD-WAN, reactive failure detection is also predicated on a failure first occurring. This means that traffic will be affected by the failure, until the traffic is moved to another tunnel. 
     With the recent evolution of machine learning, predictive failure detection in an SD-WAN now becomes possible through the use of machine learning techniques. Thus, a choice now exists between reactive and predictive routing approaches. However, as proposed herein, a third approach also becomes possible whereby reactive routing is coupled with predictive routing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1B  illustrate an example communication network; 
         FIG. 2  illustrates an example network device/node; 
         FIG. 3  illustrates an example architecture for predicting failures in a network; 
         FIGS. 4A-4C  illustrate examples of feedback for failure predictions; 
         FIGS. 5A-5E  illustrate examples of coupling predictive and reactive routing in a network; and 
         FIG. 6  illustrates an example simplified procedure for coupling predictive and reactive routing in a network. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     According to one or more embodiments of the disclosure, a device in a network predicts failure of a network element in the network using a machine learning-based failure prediction model. The device updates, based on the predicted failure of the network element, a topology of the network to remove the network element from the topology of the network. The device recomputes a reactive routing table of the device using the updated topology of the network. The device notifies one or more other devices of the network of the predicted failure using a reactive routing protocol message. 
     DESCRIPTION 
     A computer network is a geographically distributed collection of nodes interconnected by communication links and segments for transporting data between end nodes, such as personal computers and workstations, or other devices, such as sensors, etc. Many types of networks are available, with the types ranging from local area networks (LANs) to wide area networks (WANs). LANs typically connect the nodes over dedicated private communications links located in the same general physical location, such as a building or campus. WANs, on the other hand, typically connect geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines, optical lightpaths, synchronous optical networks (SONET), or synchronous digital hierarchy (SDH) links, or Powerline Communications (PLC) such as IEEE 61334, IEEE P1901.2, and others. The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes on various networks. The nodes typically communicate over the network by exchanging discrete frames or packets of data according to predefined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP). In this context, a protocol consists of a set of rules defining how the nodes interact with each other. Computer networks may be further interconnected by an intermediate network node, such as a router, to extend the effective “size” of each network. 
     Smart object networks, such as sensor networks, in particular, are a specific type of network having spatially distributed autonomous devices such as sensors, actuators, etc., that cooperatively monitor physical or environmental conditions at different locations, such as, e.g., energy/power consumption, resource consumption (e.g., water/gas/etc. for advanced metering infrastructure or “AMI” applications) temperature, pressure, vibration, sound, radiation, motion, pollutants, etc. Other types of smart objects include actuators, e.g., responsible for turning on/off an engine or perform any other actions. Sensor networks, a type of smart object network, are typically shared-media networks, such as wireless or PLC networks. That is, in addition to one or more sensors, each sensor device (node) in a sensor network may generally be equipped with a radio transceiver or other communication port such as PLC, a microcontroller, and an energy source, such as a battery. Often, smart object networks are considered field area networks (FANs), neighborhood area networks (NANs), personal area networks (PANs), etc. Generally, size and cost constraints on smart object nodes (e.g., sensors) result in corresponding constraints on resources such as energy, memory, computational speed and bandwidth. 
       FIG. 1A  is a schematic block diagram of an example computer network  100  illustratively comprising nodes/devices, such as a plurality of routers/devices interconnected by links or networks, as shown. For example, customer edge (CE) routers  110  may be interconnected with provider edge (PE) routers  120  (e.g., PE-1, PE-2, and PE-3) in order to communicate across a core network, such as an illustrative network backbone  130 . For example, routers  110 ,  120  may be interconnected by the public Internet, a multiprotocol label switching (MPLS) virtual private network (VPN), or the like. Data packets  140  (e.g., traffic/messages) may be exchanged among the nodes/devices of the computer network  100  over links using predefined network communication protocols such as the Transmission Control Protocol/Internet Protocol (TCP/IP), User Datagram Protocol (UDP), Asynchronous Transfer Mode (ATM) protocol, Frame Relay protocol, or any other suitable protocol. Those skilled in the art will understand that any number of nodes, devices, links, etc. may be used in the computer network, and that the view shown herein is for simplicity. 
     In some implementations, a router or a set of routers may be connected to a private network (e.g., dedicated leased lines, an optical network, etc.) or a virtual private network (VPN), such as an MPLS VPN thanks to a carrier network, via one or more links exhibiting very different network and service level agreement characteristics. For the sake of illustration, a given customer site may fall under any of the following categories: 
     1.) Site Type A: a site connected to the network (e.g., via a private or VPN link) using a single CE router and a single link, with potentially a backup link (e.g., a 3G/4G/5G/LTE backup connection). For example, a particular CE router  110  shown in network  100  may support a given customer site, potentially also with a backup link, such as a wireless connection. 
     2.) Site Type B: a site connected to the network by the CE router via two primary links (e.g., from different Service Providers), with potentially a backup link (e.g., a 3G/4G/5G/LTE connection). A site of type B may itself be of different types: 
     2a.) Site Type B1: a site connected to the network using two MPLS VPN links (e.g., from different Service Providers), with potentially a backup link (e.g., a 3G/4G/5G/LTE connection). 
     2b.) Site Type B2: a site connected to the network using one MPLS VPN link and one link connected to the public Internet, with potentially a backup link (e.g., a 3G/4G/5G/LTE connection). For example, a particular customer site may be connected to network  100  via PE-3 and via a separate Internet connection, potentially also with a wireless backup link. 
     2c.) Site Type B3: a site connected to the network using two links connected to the public Internet, with potentially a backup link (e.g., a 3G/4G/5G/LTE connection). 
     Notably, MPLS VPN links are usually tied to a committed service level agreement, whereas Internet links may either have no service level agreement at all or a loose service level agreement (e.g., a “Gold Package” Internet service connection that guarantees a certain level of performance to a customer site). 
     3.) Site Type C: a site of type B (e.g., types B1, B2 or B3) but with more than one CE router (e.g., a first CE router connected to one link while a second CE router is connected to the other link), and potentially a backup link (e.g., a wireless 3G/4G/5G/LTE backup link). For example, a particular customer site may include a first CE router  110  connected to PE-2 and a second CE router  110  connected to PE-3. 
       FIG. 1B  illustrates an example of network  100  in greater detail, according to various embodiments. As shown, network backbone  130  may provide connectivity between devices located in different geographical areas and/or different types of local networks. For example, network  100  may comprise local/branch networks  160 ,  162  that include devices/nodes  10 - 16  and devices/nodes  18 - 20 , respectively, as well as a data center/cloud environment  150  that includes servers  152 - 154 . Notably, local networks  160 - 162  and data center/cloud environment  150  may be located in different geographic locations. 
     Servers  152 - 154  may include, in various embodiments, a network management server (NMS), a dynamic host configuration protocol (DHCP) server, a constrained application protocol (CoAP) server, an outage management system (OMS), an application policy infrastructure controller (APIC), an application server, etc. As would be appreciated, network  100  may include any number of local networks, data centers, cloud environments, devices/nodes, servers, etc. 
     In some embodiments, the techniques herein may be applied to other network topologies and configurations. For example, the techniques herein may be applied to peering points with high-speed links, data centers, etc. 
     According to various embodiments, a software-defined WAN (SD-WAN) may be used in network  100  to connect local network  160 , local network  162 , and data center/cloud  150 . In general, an SD-WAN uses a software defined networking (SDN)-based approach to instantiate tunnels on top of the physical network and control routing decisions, accordingly. For example, as noted above, one tunnel may connect router CE-2 at the edge of local network  160  to router CE-1 at the edge of data center/cloud  150  over an MPLS or Internet-based service provider network in backbone  130 . Similarly, a second tunnel may also connect these routers over a 4G/5G/LTE cellular service provider network. SD-WAN techniques allow the WAN functions to be virtualized, essentially forming a virtual connection between local network  160  and data center/cloud  150  on top of the various underlying connections. Another feature of SD-WAN is centralized management by a supervisory service that can monitor and adjust the various connections, as needed. 
       FIG. 2  is a schematic block diagram of an example node/device  200  that may be used with one or more embodiments described herein, e.g., as any of the computing devices shown in  FIGS. 1A-1B , particularly the PE routers  120 , CE routers  110 , nodes/device  10 - 20 , servers  152 - 154  (e.g., a network controller/supervisory service located in a data center, etc.), any other computing device that supports the operations of network  100  (e.g., switches, etc.), or any of the other devices referenced below. The device  200  may also be any other suitable type of device depending upon the type of network architecture in place, such as IoT nodes, etc. Device  200  comprises one or more network interfaces  210 , one or more processors  220 , and a memory  240  interconnected by a system bus  250 , and is powered by a power supply  260 . 
     The network interfaces  210  include the mechanical, electrical, and signaling circuitry for communicating data over physical links coupled to the network  100 . The network interfaces may be configured to transmit and/or receive data using a variety of different communication protocols. Notably, a physical network interface  210  may also be used to implement one or more virtual network interfaces, such as for virtual private network (VPN) access, known to those skilled in the art. 
     The memory  240  comprises a plurality of storage locations that are addressable by the processor(s)  220  and the network interfaces  210  for storing software programs and data structures associated with the embodiments described herein. The processor  220  may comprise necessary elements or logic adapted to execute the software programs and manipulate the data structures  245 . An operating system  242  (e.g., the Internetworking Operating System, or IOS®, of Cisco Systems, Inc., another operating system, etc.), portions of which are typically resident in memory  240  and executed by the processor(s), functionally organizes the node by, inter alia, invoking network operations in support of software processors and/or services executing on the device. These software processors and/or services may comprise a routing process  248 , as described herein, any of which may alternatively be located within individual network interfaces. 
     It will be apparent to those skilled in the art that other processor and memory types, including various computer-readable media, may be used to store and execute program instructions pertaining to the techniques described herein. Also, while the description illustrates various processes, it is expressly contemplated that various processes may be embodied as modules configured to operate in accordance with the techniques herein (e.g., according to the functionality of a similar process). Further, while processes may be shown and/or described separately, those skilled in the art will appreciate that processes may be routines or modules within other processes. 
     In general, routing process (services)  248  contains computer executable instructions executed by the processor  220  to perform functions provided by one or more routing protocols. These functions may, on capable devices, be configured to manage a routing/forwarding table (a data structure  245 ) containing, e.g., data used to make routing/forwarding decisions. In various cases, connectivity may be discovered and known, prior to computing routes to any destination in the network, e.g., link state routing such as Open Shortest Path First (OSPF), or Intermediate-System-to-Intermediate-System (ISIS), or Optimized Link State Routing (OLSR). Conversely, neighbors may first be discovered (i.e., a priori knowledge of network topology is not known) and, in response to a needed route to a destination, send a route request into the network to determine which neighboring node may be used to reach the desired destination. Example protocols that take this approach include Ad-hoc On-demand Distance Vector (AODV), Dynamic Source Routing (DSR), DYnamic MANET On-demand Routing (DYMO), etc. Notably, on devices not capable or configured to store routing entries, routing process  248  may consist solely of providing mechanisms necessary for source routing techniques. That is, for source routing, other devices in the network can tell the less capable devices exactly where to send the packets, and the less capable devices simply forward the packets as directed. 
     In various embodiments, as detailed further below, routing process  248  may also include computer executable instructions that, when executed by processor(s)  220 , cause device  200  to predict failures of network elements in the network (e.g., a link or node/device), thereby allowing device  200  to proactively reroute traffic to avoid the failed element. To do so, in some embodiments, routing process  248  may utilize machine learning. In general, machine learning is concerned with the design and the development of techniques that take as input empirical data (such as network statistics and performance indicators), and recognize complex patterns in these data. One very common pattern among machine learning techniques is the use of an underlying model M, whose parameters are optimized for minimizing the cost function associated to M, given the input data. For instance, in the context of classification, the model M may be a straight line that separates the data into two classes (e.g., labels) such that M=a*x+b*y+c and the cost function would be the number of misclassified points. The learning process then operates by adjusting the parameters a,b,c such that the number of misclassified points is minimal. After this optimization phase (or learning phase), the model M can be used very easily to classify new data points. Often, M is a statistical model, and the cost function is inversely proportional to the likelihood of M, given the input data. 
     In various embodiments, routing process  248  may employ one or more supervised, unsupervised, or semi-supervised machine learning models. Generally, supervised learning entails the use of a training set of data, as noted above, that is used to train the model to apply labels to the input data. For example, the training data may include sample network telemetry that has been labeled as indicative of a network element failure, such as failure of a link or node/device, or indicative of normal operation. On the other end of the spectrum are unsupervised techniques that do not require a training set of labels. Notably, while a supervised learning model may look for previously seen patterns that have been labeled as such, an unsupervised model may instead look to whether there are sudden changes or patterns in the behavior. Semi-supervised learning models take a middle ground approach that uses a greatly reduced set of labeled training data. 
     Example machine learning techniques that routing process  248  can employ may include, but are not limited to, nearest neighbor (NN) techniques (e.g., k-NN models, replicator NN models, etc.), statistical techniques (e.g., Bayesian networks, etc.), clustering techniques (e.g., k-means, mean-shift, etc.), neural networks (e.g., reservoir networks, artificial neural networks, etc.), support vector machines (SVMs), logistic or other regression, Markov models or chains, principal component analysis (PCA) (e.g., for linear models), singular value decomposition (SVD), multi-layer perceptron (MLP) artificial neural networks (ANNs) (e.g., for non-linear models), replicating reservoir networks (e.g., for non-linear models, typically for time series), random forest classification, or the like. 
     The performance of a machine learning model can be evaluated in a number of ways based on the number of true positives, false positives, true negatives, and/or false negatives of the model. For example, the false positives of the model may refer to the number of times the model incorrectly predicted that a network element will fail. Conversely, the false negatives of the model may refer to the number of times the model predicted normal network element operations, when the network element actually fails. True negatives and positives may refer to the number of times the model correctly predicted whether a network element will perform normally or will fail, respectively. Related to these measurements are the concepts of recall and precision. Generally, recall refers to the ratio of true positives to the sum of true positives and false negatives, which quantifies the sensitivity of the model. Similarly, precision refers to the ratio of true positives the sum of true and false positives. 
     As noted above, failure detection in a network has traditionally been reactive, meaning that the failure must first be detected before rerouting the traffic along a secondary (backup) path. This is true, not only for IP and MPLS networks, but also for optical networks (with protection and restoration) such as SONET and SDH networks. Typically, failure detection leverages either explicit signaling from the lower network layers (e.g., optical failures signaled to the upper layers) or using a keep-alive mechanism that sends probes at some interval T that must be acknowledged by a receiver (e.g., a tunnel tail-end router). For example, routing protocols such as Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (ISIS) use keep-alive signals over routing adjacencies or MPLS traffic engineering (TE) tunnels. Protocols such as Bidirectional Forwarding Detection (BFD) also make use of keep-alive mechanisms. 
     Traditionally, failure detection in an SD-WAN has relied on the keep-alive mechanisms of BFD over tunnels, such as IPSec tunnels. When the BFD signaling times out, the tunnel is flagged as failed and traffic is rerouted onto another tunnel. While this approach does help to mitigate the effects of the failure, the reactive nature of this approach also means that at least some of the traffic will be lost. 
       FIG. 3  illustrates an example architecture  300  for predicting failures in network, according to various embodiments. In general, architecture  300  may include a supervisory service  310  located in the cloud or implemented locally in the network under scrutiny, such as a supervisory service for an SD-WAN. As shown, supervisory service  310  may include the following components: a telemetry collection module  302 , a machine learning failure forecasting (MLFF) module  304 , and/or a model retraining module  306 . These components  302 - 306  may be implemented in a distributed manner or implemented as their own stand-alone services, either as part of the network under observation or as a remote service. In addition, the functionalities of the components of architecture  300  may be combined, omitted, or implemented as part of other processes, as desired. 
     Supervisory service  310  may be in communication with any number of devices  308  (e.g., a first through n th  device), which may be CE routers  110  and/or PE routers  120 , described previously, or other forms of networking devices configured to convey traffic through the network. In various embodiments, devices  308  may be part of the same SD-WAN or, in cases in which supervisory service  310  is implemented as a cloud-based service, part of any number of different SD-WANs or other networks. 
     In general, there are many circumstances in a network that can lead to failures in various areas of the network between a head-end and tail-end router (e.g., between routers  110 , etc.). An objective of MLFF  304 , as detailed below, is to learn early signs (networking behavioral) that have some predictive power, allowing the model to predict/forecast a tunnel failure or a failure of a network element supporting the tunnel. It is expected that some failures are predictable (i.e., there exist early signs of an upcoming failure) while others will not be non predictable (e.g., fiber cut, router crash, etc.). More specifically, almost all failures exhibit early signs, but those signs may appear only a few milliseconds (or even nanoseconds), prior to the failure (e.g. fiber cut), thereby making forecasting an almost impossible task. Some non-predictable failures may be due to the absence of signaling back to the device  308  involved and may be localized to the core of the service provider network (e.g., the underlying IP, 4G, 5G, etc. network), in which case the failure is non-predicable from the perspective of the device  308 . 
     A first aspect of architecture  300  relates to telemetry collection module  302  obtaining the telemetry data required for model training by MLFF module  304 . As used herein, the term ‘relevant telemetry’ refers to a telemetry measurement variable with predictive power to predict failures, which can be determined dynamically by MLFF module  304 . Indeed, failures may be predictable, yet not successfully predicted, due to a lack of relevant telemetry, the inability of the model to predict the failure, or the telemetry is sampled at too coarse of a time granularity. In some embodiments, to obtain relevant telemetry from devices  308 , supervisory service  310  may send a custom request to one or more of devices  308  with the objective of obtaining the list of events of interest along with the set of candidate telemetry variables with potential predictive power to predict failures. In further embodiments, devices  308  may instead provide the telemetry data to supervisory service  310  on a push basis (e.g., without service  310  first requesting the telemetry data). 
     In various embodiments, telemetry collection module  302  may adjust the set of telemetry variables/parameters obtained from the device(s)  308  and/or their sampling frequency. If, for example, MLFF module  304  determines that a particular telemetry variable has a strong predictive power (according to the feature importance, Shapley values, etc.), the frequency at which such a variable may be gathered may be higher compared to a variable with lower predictive power. MLFF module  304  may also determine the predictive power of a particular telemetry variable by assessing the conditional probabilities involved, in further embodiments. 
     MLFF module  304  may also select the set of most relevant telemetry variables. In turn, telemetry collection module  302  may request that devices  308  measure and send these variables to supervisory service  310  periodically, since real-time variations of such telemetry is needed for forecasting failure events. For example, based on the above conclusion, MLFF module  304  may determine that the CPU and memory utilizations of one or more networking devices that support a given tunnel should be sent periodically (e.g., every 1 second) by devices  308 . 
     Other telemetry variables, such as during a rekey failure when a device  308  is not able to successfully exchange the security keys with the controller, may also be requested to be sent to supervisory service  310 , on occurrence of the event. Since such events are rare and the states of the variables remain the same for longer periods of time, telemetry collection module  302  may request an event-based push request, rather than periodic messages. In other words, telemetry collection module  302  may instruct one or more of devices  308  to report certain telemetry variables only after occurrence of certain events. For example, Table 1 below shows some example telemetry variables and when a device 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Relevant Telemetry 
                 Request Type 
               
               
                   
                   
               
             
            
               
                   
                 Memory_utilization 
                 Requested from head and 
               
               
                   
                 CPU Utilization 
                 tail edge routers. 
               
               
                   
                 BFD Probe Latency, 
                 Periodically once every 1 second. 
               
               
                   
                 Loss and Jitter 
                   
               
               
                   
                 Queue statistics (%-age 
                   
               
               
                   
                 drops for different queues) 
                   
               
               
                   
                 Interface down event 
                 Requested from both head and tail 
               
               
                   
                 Rekey exchange failure 
                 edge routers 
               
               
                   
                 Router crash logs 
                 Upon event occurrence. 
               
               
                   
                   
               
            
           
         
       
     
     In a further embodiment, MLFF module  304  may also attempt to optimize the load imposed on the device(s)  308  reporting the telemetry variables to supervisory service  310 . For example, MLFF module  304  may determine that the CPU and memory usages should be measured and reported every minute to supervisory service  310 . 
     A key functionality of MLFF module  304  is to train any number of machine learning-based models to predict network element failures. Preferably, the models are time-series models trained centrally (e.g., in the cloud) using the telemetry collected by telemetry collection module  302 . In one instantiation of MLFF module  304 , the models may be trained on a per network instance or per-SD-WAN basis. Testing has shown that model performance may be influenced by parameters specific to a given network instantiation, thus promoting an implementation whereby MLFF module  304  trains a model for a specific network deployment. In further embodiments, MLFF module  304  may even train certain models on a per-tunnel basis. Although such an approach may be of limited scalability, it may be highly valuable for tunnels carrying a very large amount of potentially very sensitive traffic (e.g., inter-cloud/data center traffic). 
     As pointed out earlier, with current reactive routing approaches, recall (i.e., the proportion of failures being successfully predicted) is simply equal to 0, since rerouting is always reactive. In other words, the system reacts a posteriori. As a result, any recall &gt;0 is a significant gain. One performance metric that MLFF module  304  may consider is the maximum recall (Max_Recall) achieved by the model given a precision &gt;P_Min. For example, MLFF module  304  may evaluate the variability of Max_Recall across datasets, should a single model be trained across all datasets, to determine whether an SD-WAN specific or even a tunnel specific model should be trained. 
     In various embodiments, MLFF module  304  may dynamically switch between per-tunnel, per-network/SD-WAN, and global (multiple SD-WAN) approaches to model training. For example, MLFF module  304  may start with the least granular approach (e.g., a global model across all networks/SD-WANs) and then evaluate the performance of the global model versus that of per-customer/SD-WAN models. Such model performance comparison could be easily evaluated by comparing their related precision-recall curves (PRCs)/area under the curve (AUCs), or the relative Max_Recall, given that Precision&gt;P_min. 
     In some cases, MLFF module  304  may employ a policy to trigger per-customer/SD-WAN specific model training, if the Max_Recall value improvement is greater than a given threshold. In another embodiment, a similar policy approach may be used to specifically require a dedicated model for a given tunnel according to its characteristic (between router A and router B), the type of traffic being carried out (e.g., sensitive traffic of type T, etc.), or the performance of the global or SD-WAN specific model for that tunnel. In such a case, the devices  308  may be in charge of observing the routed traffic and, on detecting a traffic type matching the policy, request specific model training by MLFF module  304 , to start per-tunnel model training for that tunnel. 
     Prototyping of the techniques herein using simple models and input features based on coarse telemetry, such as 1-minute averages of loss, latency, jitter, traffic, as well as CPU/memory of CE routers, lead to recalls in the range of a few percent with a precision of 80% or more. More advanced time-series models, such as long short-term memories (LSTMs), especially with attention mechanisms, will achieve even better performance. More importantly, using richer and more fine-grained telemetry is an important driver of the forecasting performance. 
     Once MLFF module  304  has trained a prediction model, different options exist for its inference location (e.g., where the model is executed to predict failures). In a first embodiment, model inference is performed centrally (in the cloud), thus co-located with the model training. In such a case, once MLFF module  304  identifies the set of telemetry variables with predictive power (used for prediction), telemetry collection module  302  may send a custom message to the corresponding device(s)  308  listing the set of variables along with their sampling/reporting frequencies. Note that sampling is a dynamic parameter used by MLFF module  304  computed so as to optimize the PRC of the model against the additional overhead of the device  308  pushing additional data to the cloud (and also generating additional logging of data on the router). 
     In another embodiment, MLFF module  304  may push the inference task, and the corresponding prediction model, to a specific device  308 , so that the prediction is preformed on-premise. Such an approach may be triggered by the frequency of sampling required to achieve the required model performance. For example, some failure types are known to provide signal a few seconds, or even milliseconds, before the failure. In such cases, performing the inference in the cloud is not a viable option, making on-premise execution of the model the better approach. Inference/model execution is usually not an expensive task on premise, especially when compared to model training. That being said, it may require fast processing on local event with an impact on the local CPU. In yet another embodiment, some models may be executed on premise, if the local resources on the router/device  308  are sufficient to feed the local model. 
     Thus, in some cases, the techniques herein support centralized model training (e.g., in the cloud), combined with the ability to perform local (on-premise) inference based on the required sampling frequency, local resources available on the device  308 , as well as the bandwidth required to send the telemetry for input to a model in the cloud. For example, one failure prediction model may require a slow sampling rate but a large amount of data, due to a high number of input features with predictive power. Thus, reporting these telemetry variables to the cloud for prediction may consume too much WAN bandwidth on the network. In such a case, MLFF module  304  may take this constraint into account by evaluating the volume of required telemetry, according to the sampling frequency, and the WAN bandwidth allocated on the network for the telemetry traffic. To that end, MLFF module  304  may analyze the topology of the network and the available bandwidth for telemetry reporting (e.g., according to the QoS policy). If the bandwidth available for the telemetry used for the inference of the model exceeds the capacity, MLFF module  304  may decide to activate a local inference by pushing a prediction model to one or more of devices  308 . 
     In yet another embodiment, MLFF module  304  may take a mixed approach whereby some of devices  308  perform the inferences locally, while others rely on supervisory service  310  to perform the predictions. 
     A further embodiment of the techniques herein introduces a feedback mechanism whereby feedback regarding the predictions by a trained model is provided to model retraining module  306 . In cases in which the model is executed on device  308 , the device  308  may report the rate of false positives and/or false negatives to model retraining module  308 . Optionally, the reporting can also include additional context information about each false positive and/or false negative, such as the values of the telemetry variables that led to the incorrect prediction. If the performance of the model is below a designated threshold, model retraining module  306  may trigger MLFF module  304  to retrain the model, potentially increasing the granularity of the model, as well (e.g., by training a tunnel-specific model, etc.). In cases in which MLFF module  304  trains multiple prediction models, model retraining module  306  may evaluate the performance of each model and, based on their performances, decide that a particular one of the models should be used. Such an approach allows MLFF module  304  to dynamically switch between models, based on the data pattern currently being observed. 
     When failures are predicted in the cloud by supervisory service  310 , model retraining module  306  may similarly receive feedback from devices  308  regarding the predictions. For example, once a model M predicts the failure of a tunnel at a given time, MLFF module  304  may send a notification to the affected device  308  indicating the (list of) tunnel(s) for which a failure is predicted, along with the predicted time for the failure, and other parameters such as the failure probability P f  (which can be a simple flag, a categorical variable (low, medium, high) or a real number). The device  308  may use P f  to determine the appropriate action, such as pro-actively rerouting the traffic that would be affected by the failure onto a backup tunnel. In one embodiment, the predicted failure may be signaled to the device  308  using a unicast message for one or more tunnels, or a multicast messages signaling a list of predicted failure to a set of devices  308 . 
     Regardless of how model retraining module  306  receives its feedback, either from the device  308  executing the prediction model or from MLFF module  304  executing the model, model retraining module  306  may dynamically trigger MLFF module  304  to retrain a given model. In one embodiment, the model re-training may be systematic. In another embodiment, upon reaching a plateau in terms of improvement for Max_Recall or Max_Precision, model retraining module  306  may reduce the frequency of the model training. 
       FIGS. 4A-4C  illustrate examples of feedback for failure predictions, in various embodiments. As shown in example implementation  400  in  FIGS. 4A-4B , assume that the trained model is executed in the cloud by supervisory service. In such a case, supervisory service  310  may send a sampling request  402  to a device  308  that indicates the telemetry variables to sample and report, as well as the determined sampling/reporting period(s) for those variables. In turn, device  308  may report the requested telemetry  404  to supervisory service  310  for analysis. For example, supervisory service  310  may request that device  308  report is CPU load every minute to supervisory service  310 , to predict whether the tunnel associated with device  308  is predicted to fail. More specifically, supervisory service  310  may use telemetry  404  as input to its trained prediction model, to determine whether telemetry  404  is indicative of a failure that will occur in the future. 
     When supervisory service  310  determines that a failure of a network element is predicted, it may send a predicted failure notification  406  to device  308  that identifies the element predicted to fail, the time at which the failure is expected to occur, and potentially the probability of failure, as well. Depending on the timing and probability of failure, device  308  may opt to reroute the affected traffic, or a portion thereof. In turn, device  308  may monitor the element predicted to fail and provide feedback  408  to supervisory service  310  indicating whether the element actually failed and, if so, when. Supervisory service  310  can then use feedback  408  to determine whether model retraining should be initiated, such as by training a more granular model for the SD-WAN instance or the specific tunnel or path under scrutiny. 
       FIG. 4C  illustrates an alternate implementation  410  in which supervisory service  310  pushes the failure prediction model to device  308  for local/on-premise inference. For example, supervisory service  310  may opt for device  308  to perform the local inferences, such as when model  412  requires too much bandwidth to send the needed telemetry to supervisory service  310  for cloud-based prediction. In turn, device  308  may use the corresponding telemetry measurements as input to trained model  412  and, if a failure is predicted, perform a corrective measure such as proactively rerouting the traffic to one or more other tunnels or other paths. In addition, device  308  may provide feedback  414  to supervisory service  310  that indicates false positives and/or false negatives by the model. For example, if device  308  reroutes traffic away from a element predicted by model  412  to fail, and the tunnel does not actually fail, device  308  may inform supervisory service  310 . Supervisory service  310  may use feedback  414  to determine whether model  412  requires retraining, such as by adjusting which telemetry variables are used as input to the model, adjusting the granularity of the training (e.g., by using only training telemetry data from the tunnel, etc.), or the like. 
     As noted above, predictive routing can be achieved in an SD-WAN through the use of machine learning, to predict failures. However, these predictions are contingent on capturing the necessary telemetry data from the networking devices (e.g., routers) and in advance of the failure actually occurring. 
     Coupling Reactive Routing with Predictive Routing in a Network 
     The techniques herein allow for the coupling of reactive routing (e.g., IGP, optical networks, MPLS Traffic engineering, etc.) with predictive routing where failure of network elements, such as link and node failures, are predicted at time T in the future with a probability P. In some aspects, both reactive and predictive routing co-exist on a given network device. In further aspects, upon predicting failures, the reactive routing mechanism of the device triggers the recomputation of backup paths using the updated (predicted) topology, optionally signal a backup path (e.g., Optical and MPLS restoration). In further aspects, the updated topology can be advertised to one or more other devices in the network. Such decision may be governed by the likelihood that the predicted failure takes place (e.g., based on the precision of the failure prediction model), potentially coupled with policy-driven decisions. 
     Specifically, according to one or more embodiments herein, a device in a network predicts failure of a network element in the network using a machine learning-based failure prediction model. The device updates, based on the predicted failure of the network element, a topology of the network to remove the network element from the topology of the network. The device recomputes a reactive routing table of the device using the updated topology of the network. The device notifies one or more other devices of the network of the predicted failure using a reactive routing protocol message. 
     Illustratively, the techniques described herein may be performed by hardware, software, and/or firmware, such as in accordance with the routing process  248 , which may include computer executable instructions executed by the processor  220  (or independent processor of interfaces  210 ) to perform functions relating to the techniques described herein. 
     Operationally,  FIGS. 5A-5E  illustrate examples of coupling predictive and reactive routing in a network, according to various embodiments. To better illustrate the techniques herein, assume that a network  500  comprises a set of devices  308  (e.g., routers), such as devices  308   a - 308   f  that are interconnected by links  504 , as shown in  FIG. 5A . 
     As stated above, predictive routing will not be able to predict all failure types simply because some of these failures may not be predictable (e.g., fiber cut in a service provider network), signals may not be seen by the device in charge of performing the model inference/prediction (e.g., due to a lack of telemetry data, or simply due to poor performance of the failure prediction model. 
     According to various embodiments, a first aspect of the techniques herein consists in co-hosting both a predictive and a reactive routing mechanism on a network device. For the sake of illustration, consider a core router running both an interior gateway protocol (IGP) like OSPF (Reactive Routing) along with a predictive routing component (e.g., trained model  412  shown in  FIG. 4C ) in charge of predicting failures, whereas OSPF makes use of lower signal indication. Such an indication could, for example, come from the lower link layer such as wireless, optical link (e.g., GMPLS), or fast keepalive signaling (e.g. BFD, Fast OSPF Hello), for detecting a loss of routing adjacency (e.g., due to a link failure. In such a deployment scenario, the reactive and predictive routing mechanisms of the device may be asynchronous. 
     For example, assume that device  308   a  uses a path P 1  to send traffic to device  308   d  and that device  308   a  executes a prediction model trained to predict failures along a path. Based on telemetry data collected from path P 1 , device  308   a  may predict a failure of an element along path P 1 , such as a particular device  308  or link  504 . For example, device  308   a  may predict that its link  504  with device  308   b  will fail. 
     In some embodiments, when a prediction mechanism of a network device predicts a failure of a network entity, it may trigger the computation of one or more alternate backup paths, using the reactive mechanism of the device. For example, as shown in  FIG. 5B , prediction of a failure along path P 1  by device  308   a  may trigger the reactive routing mechanism of device  308   a  to compute an alternate backup path for P 1 , such as path P 2 . In one embodiment, device  308   a  may do so by first updating its computed topology of network  500  by removing the link  504  with device  308   b  from the topology. In doing so, path P 1  can no longer be formed between device  308   a  and device  308   d.    
     If the reactive routing approach of the device relies on restoration, as opposed to protection, the reception of such a failure signal may trigger the reactive mechanism of the device to recompute the routing table(s) of the device using the locally updated topology. In other words, the reactive routing mechanism of the device (e.g. OSPF) is triggered based on the predicted failure that was predicted by the predictive routing mechanism of the device. In one embodiment, if the reactive routing is IGP-based, this may entail computing a new shortest path first (SPF) computation. In another embodiment, in the case of MPLS Traffic Engineering (MPLS-TE), all tunnels going through the predicted failed network element may trigger the computation of a new constrained SPF using the updated traffic engineering database (TED). 
     In a further embodiment, the device computes one or more secondary/backup paths that requires signaling, such as in the case of MPLS-TE, then the device may signal the secondary TE label switched path (LSP), using a make-before-break approach to avoid double bandwidth accounting. 
     In yet another embodiment, the device may proactively reroute the traffic that would be affected by the predicted failure onto the alternate path(s), without waiting for the confirmation of the predicted failure. For example, as shown in  FIG. 5C , after computing path P 2  to device  308   d , device  308   a  may proactively reroute traffic from path P 1  to path P 2 , to avoid the predicted link failure. Such a decision may be governed according to various criteria. For example, device  308   a  may use the probability (precision) of the failure prediction model as a decision factor. In such a case, if the model predicted a failure with a precision or probability below a defined threshold, device  308   a  may simply compute the alternate path P 2  without rerouting traffic from P 1  onto P 2 . Conversely, if the precision or probability exceeds the thresholds, device  308   a  may proactively reroute the traffic from P 1  onto P 2 , in advance of the predicted failure actually occurring. 
     The device predicting a failure can also proactively advertise the prediction to one or more other devices in the network, potentially with its associated probability, as well. For example, as shown in  FIG. 5D , device  308   a  may send a notification  502 , to device  308   e  indicative of the predicted link failure of the link  504  between device  308   a  and device  308   b . In some embodiments, the device that predicted the failure may do so by generating a new link state advertisement (LSA) Type 1, or a new LSP (or new LSP for ISIS) reflecting the new topology, along with a custom field indicative of the prediction. For example, notification  502  may take the form of a Type 1 LSA that includes the probability for the newly advertised LSA to be “correct” in a near future. Optionally, notification  502  may also include the predicted time of failure, if the predictive model of device  308   a  is also configured to make such a prediction. As would be appreciated, this allows the other device(s)  308  in network  500  to potentially trigger computation of alternate paths, similarly to device  308   a . For example, as shown in  FIG. 5E , notification  502  may trigger device  308   e  to also update its routing table(s), to avoid the network element predicted to fail. 
     Note that in the case device  308   a  being an edge router connected to an SD-WAN using IPSec tunnels, the edge router may be using an IGP for the LAN. In this case, device  308   a  may issue an LSA Type 5 signal, so as to reflect the potential (future) loss of connectivity via itself, based on the forecasted link failure. 
     In another embodiment, device(s) receiving notifications of predictive routing topologies, such as via a new LSA or LSP with a “predictive flag” set decide to proactively update their routing table after considering the probability of forecasted failure, as well as the potential cost increase for all destinations in the SPT computed on the new topology. For example, in response to device  308   e  receiving notification  502  from device  308   a  regarding the predicted failure of the link  504  between device  308   a  and device  308   b , device  308   e  may also compute the potential cost increase associated with rerouting its traffic to avoid the failure. Indeed, after pruning the link state database to remove the link, this may result in higher cost paths to reach all leaves. In such cases, the device may consider the average, maximum, percentile, etc., path cost increase to all leaves, in addition to the failure prediction probability, to decide whether to proactively update its routing tables. In yet another embodiment, device  308   e  may inspect the amount of traffic and the criticality of such traffic to indeed proactively reroute its traffic. For example, a core router notified of a predicted failure with probability P that would trigger a large path cost increase for a high volume of critical traffic may be decide not to perform any form of proactive rerouting. 
     The predictive routing techniques herein may be used to predict a wide range of failures such as link failures, as above, next hop device failures, or even device failures, in various implementations. When a predictive model is used to forecast local device failures, the device may report a new LSA Type 1 (or ISIS LSP) reporting its probability to fail. In the case of ISIS, the device may also set the overload bit to 1, thus triggering a recomputation of the routing tables on all other nodes in the network. Such a decision may be governed by the precision of the failure prediction model, in some cases, which can be optionally advertised to the other device(s) by the device performing the prediction. In yet another embodiment, the forecasted failure may be propagated to next-hop routers, only. For example, if device  308   a  predicts that it is likely to fail at time TI, it may notify one or more of its neighbors using a protocol extension, such as the BFD protocol. 
       FIG. 6  illustrates an example simplified procedure for coupling predictive and reactive routing in a network, in accordance with one or more embodiments described herein. For example, a non-generic, specifically configured device (e.g., device  200 ) may perform procedure  600  by executing stored instructions (e.g., process  248 ). The procedure  600  my start at step  605  and continue on to step  610  where, as described in greater detail above, the device may predict failure of a network element in the network using a machine learning-based failure prediction model. In various embodiments, the network element predicted to fail may comprise a link between nodes/devices, a particular device in the network, or even the device performing procedure  600 , itself. 
     At step  615 , as detailed above, the device may update, based on the predicted failure of the network element, a topology of the network to remove the network element from the topology of the network. For example, the prediction of the failure may trigger a reactive routing mechanism of the device to update its network topology to remove the network element predicted to fail. 
     At step  620 , the device may recompute one or more routing tables of the device using its updated network topology, as described in greater detail above. For example, if device A predicts a failure along a path to device B, the device may update a reactive routing table of the device using the updated topology of the network. If the device uses MPLS-TE, it may also signal a TE-LSP in the network that bypasses the network element predicted to fail using a make-before-break approach, to avoid double bandwidth accounting. In further embodiments, the device may trigger a new SPF computation, if the device is IGP-based. 
     At step  625 , as detailed above, the device may notify one or more other devices of the network of the predicted failure using a reactive routing protocol message. In some embodiments, the device may do so by sending a new LSA Type 1 message, or new LSP for ISIS, to the one or more other devices indicative of the predicted failure. The LSA message may further include a probability of the predicted failure of the network element computed by the machine learning-based failure prediction model. In further cases, such as when the device itself is the network element predicted to fail, the device may also advertise a time associated with the predicted failure, such as via a BFD message. Procedure  600  then ends at step  630 . 
     It should be noted that while certain steps within procedure  600  may be optional as described above, the steps shown in  FIG. 6  are merely examples for illustration, and certain other steps may be included or excluded as desired. Further, while a particular order of the steps is shown, this ordering is merely illustrative, and any suitable arrangement of the steps may be utilized without departing from the scope of the embodiments herein. 
     The techniques described herein, therefore, allow for the use of both predictive and reactive routing mechanisms on a device. In doing so, performance of the network is enhanced, as the device can proactively reroute traffic around a network element predicted to fail. Further, by still supporting reactive routing, the device can also react to failures that cannot be predicted by its predictive mechanism, such as fiber cuts and the like. 
     While there have been shown and described illustrative embodiments that provide for coupling reactive and predictive routing, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the embodiments herein. For example, while certain embodiments are described herein with respect to using certain models for purposes of predicting failures, the models are not limited as such and may be used for other types of predictions, in other embodiments. In addition, while certain protocols are shown, other suitable protocols may be used, accordingly. 
     The foregoing description has been directed to specific embodiments. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. For instance, it is expressly contemplated that the components and/or elements described herein can be implemented as software being stored on a tangible (non-transitory) computer-readable medium (e.g., disks/CDs/RAM/EEPROM/etc.) having program instructions executing on a computer, hardware, firmware, or a combination thereof. Accordingly, this description is to be taken only by way of example and not to otherwise limit the scope of the embodiments herein. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the embodiments herein.