WO2018150223A1

WO2018150223A1 - A method and system for identification of traffic flows causing network congestion in centralized control plane networks

Info

Publication number: WO2018150223A1
Application number: PCT/IB2017/050831
Authority: WO
Inventors: Vinayak Joshi; Faseela K; Ashvin Lakshmikantha
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2017-02-14
Filing date: 2017-02-14
Publication date: 2018-08-23

Abstract

A method and a central network controller for identification of traffic flows causing network congestion are described. In response to a detection of congestion at an egress network interface of a network element, the network element is configured to include a forwarding pipeline including a plurality of forwarding table entries. The forwarding table entries include: an egress forwarding table entry, a group table entry, and a meter table entry. In response to the configuring the network element to include the forwarding table entries, a subset of packets from a copy of the packets is received at the central network controller from the network element at a predetermined threshold rate. A packet flow causing the congestion at the egress network interface is identified from the received subset of packets.

Description

A METHOD AND SYSTEM FOR IDENTIFICATION OF TRAFFIC FLOWS CAUSING NETWORK CONGESTION IN CENTRALIZED CONTROL PLANE NETWORKS

TECHNICAL FIELD

[0001] Embodiments of the invention relate to the field of packet networking; and more specifically, to the identification of traffic flows causing network congestion in centralized control plane networks.

BACKGROUND

[0002] Software-Defined Networking (SDN) is an approach to computer networking that allows network administrators to manage network services through abstraction of lower-level functionality. This is done by decoupling the system that makes decisions about where traffic is sent (the control plane) from the underlying systems that forward traffic to the selected destination (the data plane). In such a system, a network controller, which can be deployed as a cluster of server nodes, has the role of the control plane and is coupled to one or more network elements (NEs) that have the role of the data plane. Each network element may be implemented on one or multiple network devices (NDs). A network device (ND) is an electronic device that communicatively interconnects other electronic devices on the network (e.g., other network devices, end-user devices). The control connection between the network controller and network elements is generally a TCP/UDP based communication. The network controller communicates with the network elements using an SDN protocol (e.g., OpenFlow, I2RS, etc.).

[0003] For implementing SDN, the Open Networking Foundation (ONF), an industrial consortium focusing on commercializing SDN and its underlying technologies, has defined a set of open commands, functions, and protocols. The defined protocol suites are known as the OpenFlow (OF) protocol. The network controller, acting as the control plane, may then program the data plane on the network elements by causing packet handling rules to be installed on the forwarding network elements using OF commands and messages. These packet handling rules may have criteria to match various packet types as well as actions that may be performed on those packets. The forwarding plane includes forwarding tables (e.g., flow tables, group tables) which may be distributed across multiple data-path network elements.

[0004] Network interface congestion is a common problem that occurs in network elements, when a network interface (NI) of the NE carries more data than it can handle. Several mechanisms are known for preventing and mitigating network congestion. In centralized control plane (CCP) networks (e.g., SDN networks), the network controller has a centralized view of the entire network and can accomplish a more intelligent congestion avoidance than in traditional distributed networks. In contrast to traditional distributed networks, which are able through various techniques to determine the rate of flows at a given network interface and to drop or mark packets from flows that are responsible for congestion at the network interface, CCP networks are able to identify the network elements that are the origin of the traffic causing the congestion in the CCP network. The identification of the origin network elements may enable the network controller to limit the rate of these flows at the ingress of the network prior to them reaching the network interface. This avoids having packets travelling all the way to the network element, consuming network resources, just to be dropped at that network element.

[0005] However, identification of a flow causing congestion in a network element of a CCP network, in particular SDN/OpenFlow networks, is a non-trivial problem and presents several challenges. Some of the challenges include: 1) Hidden Flow Problem identification problem; and 2) Tunneling. The forwarding pipeline of a network element maintains packet/byte counters only for entries of the flow match table. However, in general the flow match entries do not correspond to 5-tuple flows (i.e., flows as identified with their Source IP address/port number, Destination IP address/port number, and the protocol in use) creating a hidden flow

identification problem. For example, in a network element there may be a single entry in a flow match table which matches the destination media access control (MAC) address. Many 5-tuple flows (which may have entered the NE through different ingress network interfaces) may match the entry identified with the MAC address. Thus, the packet/byte counters in the flow table would be the sum of all 5-tuple flows (possibly millions) that match a flow entry in the table and the values of these counters cannot be directly used to identify culprit flows causing the congestion.

[0006] In addition, even when there is a one to one correspondence between flow entries in the forwarding pipeline the original 5-tuple packet may be encapsulated in a tunnel (e.g., GPRS (General Packet Radio Service) Tunneling Protocol (GTP)) and hence not visible to the forwarding pipeline as only the outer header (tunnel header) can be examined in the pipeline. It may be desirable to know the inner 5-tuple if congestion information has to be used for further intelligence such as providing feedback to a subscriber management system.

[0007] Several solutions exist for identifying flows that cause congestion while attempting to solve the hidden flow problem. These solutions rely on sampling packets of the flows, transmitting them to a packet analyzer (which can be part of the network controller or not) and analyzing these packets to extract information (5-tuple) identifying the flows. In traditional flow analyzers, when the NE cannot locally detect elephant flows (i.e., heavy flows which cause the network congestions), port mirroring is performed to redirect copies of packet streams to an analyzer. The analyzers then examine the received packets to identify them. These solutions provide several disadvantages. Packets which are mirrored from the egress network interfaces do not contain information on the ingress network interface. Therefore, in order to obtain the ingress information and identify the culprit flows, the mirroring is to be performed from multiple ingress NIs of the network element. Consequently, this solution is not very scalable. In addition, the analyzer and the network have to handle the packet rate of the mirrored packets. Typically, the analyzer is part of the network controller; however, receiving the mirrored packets can bring down the network controller as it is designed for handling control plane packets that are typically received at much lower rates than the rates of traffic packets (like the mirrored packets) in an NE.

[0008] Other approaches may use sampled Flow (sFlow) for sampling flows. sFlow is an industry standard for packet export at the Layer 2. It enables export of truncated packets, together with interface counters to a network controller/packet analyzer. However, sFlow does not allow for inner packet inspection in a tunneling scenario. In addition, sFlow is a standard that is distinct from OpenFlow and cannot be used with standard OpenFlow network elements and SDN controllers. It requires complex implementation on the network element.

[0009] Another approach that has been proposed is to enable OpenFlow support for packet sampling by adding new extensions to the protocol. For example, the approach described in "FleXam: flexible sampling extension for monitoring and security applications in OpenFlow" by Sajad Shirali-Shahreza and Yashar Ganjali, Proceedings of the second ACM SIGCOMM workshop on Hot topics in software defined networking (HotSDN Ί3), ACM, New York, NY, USA, 167-168., discloses that in order to invoke the sampling process on a network element for a specific flow entry, the ofp_stats_type message is extended by the new type

OFPST_SAMPLING. On reception of this message, a network element marks every matching wildcard entry with the requested sampling probability and duration. In the message transmitted to the NE, sampling period specifies the average sampling rate. However, these approaches (which require OpenFlow protocol extensions for packet sampling) are not compliant to standard OpenFlow protocol, and mandate very complex implementations on the network elements.

SUMMARY

[0010] One general aspect includes a method, in a central network controller of a centralized control plane network, of identification of traffic flows causing network congestion. The method includes: in response to a detection of congestion at an egress network interface of a network element, configuring the network element to include the following forwarding table entries: an egress forwarding table entry, where all packets forwarded towards the egress network interface are matched to the egress forwarding table entry and caused to be transmitted to a group table entry, the group table entry, where packets matching the group table entry are forwarded towards the egress network interface for output, and a copy of the packets matching the group table entry are forwarded to a meter table entry, the meter table entry indicating a predetermined threshold rate, where a first packet from the copy of the packets matching the meter table entry is forwarded to the central network controller when a rate of the copy of the packets has not reached the predetermined threshold rate, and a second packet from the copy of the packets matching the meter table entry is dropped when the rate of the copy of the packets has reached the predetermined threshold rate. The method also includes receiving, as a result of the configuring the network element to include the forwarding table entries, from the network element a subset of packets from the copy of the packets at the predetermined threshold rate. The method also includes identifying from the received subset of packets a packet flow causing congestion at the egress network interface.

[0011] One general aspect includes a central network controller, including: a non-transitory computer readable medium to store instructions; and a processor coupled with the non-transitory computer readable medium to process the stored instructions to: in response to a detection of congestion at an egress network interface of a network element, configure the network element to include the following forwarding table entries: an egress forwarding table entry, where all packets forwarded towards the egress network interface are matched to the egress forwarding table entry and caused to be transmitted to a group table entry, the group table entry, where packets matching the group table entry are forwarded towards the egress network interface for output, and a copy of the packets matching the group table entry are forwarded to a meter table entry, and the meter table entry indicating a predetermined threshold rate, where a first packet from the copy of the packets matching the meter table entry is forwarded to the central network controller when a rate of the copy of the packets has not reached the predetermined threshold rate, and a second packet from the copy of the packets matching the meter table entry is dropped when the rate of the copy of the packets has reached the predetermined threshold rate. The processor is further to receive, as a result of the configuration of the network element to include the forwarding table entries, from the network element a subset of packets from the copy of the packets at the predetermined threshold rate, and identify from the received subset of packets a packet flow causing congestion at the egress network interface.

[0012] One general aspect includes a non-transitory computer readable storage medium that provide instructions, which when executed by a processor of a central network controller, cause said processor to perform operations including: in response to a detection of congestion at an egress network interface of a network element, configuring the network element to include the following forwarding table entries: an egress forwarding table entry, where all packets forwarded towards the egress network interface are matched to the egress forwarding table entry and caused to be transmitted to a group table entry, the group table entry, where packets matching the group table entry are forwarded towards the egress network interface for output, and a copy of the packets matching the group table entry are forwarded to a meter table entry, the meter table entry indicating a predetermined threshold rate, where a first packet from the copy of the packets matching the meter table entry is forwarded to the central network controller when a rate of the copy of the packets has not reached the predetermined threshold rate, and a second packet from the copy of the packets matching the meter table entry is dropped when the rate of the copy of the packets has reached the predetermined threshold rate. The operations also include receiving, as a result of the configuring the network element to include the forwarding table entries, from the network element a subset of packets from the copy of the packets at the predetermined threshold rate; and identifying from the received subset of packets a packet flow causing congestion at the egress network interface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:

[0014] Figure 1 illustrates a block diagram of an exemplary network performing operations for identification of traffic flows causing network congestion in centralized control plane networks, in accordance with some embodiments of the invention.

[0015] Figure 2A illustrates a block diagram of an exemplary forwarding pipeline within a network element for enabling identification of traffic flows causing network congestion, in accordance with some embodiments of the invention.

[0016] Figure 2B illustrates a block diagram of an exemplary meter table in a forwarding pipeline within a network element for enabling identification of traffic flows causing network congestion in accordance with some embodiments of the invention.

[0017] Figure 3 illustrates a flow of exemplary operations for enabling identification of traffic flows causing network congestion in centralized control plane networks, in accordance with some embodiments of the invention.

[0018] Figure 4A illustrates connectivity between network devices (NDs) within an exemplary network, as well as three exemplary implementations of the NDs, according to some embodiments of the invention. [0019] Figure 4B illustrates an exemplary way to implement a special-purpose network device according to some embodiments of the invention.

[0020] Figure 4C illustrates various exemplary ways in which virtual network elements (VNEs) may be coupled according to some embodiments of the invention.

[0021] Figure 4D illustrates a network with a single network element (NE) on each of the NDs, and within this straight forward approach contrasts a traditional distributed approach (commonly used by traditional routers) with a centralized approach for maintaining reachability and forwarding information (also called network control), according to some embodiments of the invention.

[0022] Figure 4E illustrates the simple case of where each of the NDs implements a single NE, but a centralized control plane has abstracted multiple of the NEs in different NDs into (to represent) a single NE in one of the virtual network(s), according to some embodiments of the invention.

[0023] Figure 4F illustrates a case where multiple VNEs are implemented on different NDs and are coupled to each other, and where a centralized control plane has abstracted these multiple VNEs such that they appear as a single VNE within one of the virtual networks, according to some embodiments of the invention.

[0024] Figure 5 illustrates a general purpose control plane device with centralized control plane (CCP) software 550), according to some embodiments of the invention.

DETAILED DESCRIPTION

[0025] The following description describes methods and apparatuses for enabling

identification of traffic flows causing congestions in centralized control plane networks. In the following description, numerous specific details such as logic implementations, opcodes, means to specify operands, resource partitioning/sharing/duplication implementations, types and interrelationships of system components, and logic partitioning/integration choices are set forth in order to provide a more thorough understanding of the present invention. It will be appreciated, however, by one skilled in the art that the invention may be practiced without such specific details. In other instances, control structures, gate level circuits and full software instruction sequences have not been shown in detail in order not to obscure the invention. Those of ordinary skill in the art, with the included descriptions, will be able to implement appropriate functionality without undue experimentation.

[0026] References in the specification to "one embodiment," "an embodiment," "an example embodiment," etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0027] Bracketed text and blocks with dashed borders (e.g., large dashes, small dashes, dot- dash, and dots) may be used herein to illustrate optional operations that add additional features to embodiments of the invention. However, such notation should not be taken to mean that these are the only options or optional operations, and/or that blocks with solid borders are not optional in certain embodiments of the invention.

[0028] In the following description and claims, the terms "coupled" and "connected," along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. "Coupled" is used to indicate that two or more elements, which may or may not be in direct physical or electrical contact with each other, co-operate or interact with each other. "Connected" is used to indicate the establishment of communication between two or more elements that are coupled with each other.

[0029] An electronic device stores and transmits (internally and/or with other electronic devices over a network) code (which is composed of software instructions and which is sometimes referred to as computer program code or a computer program) and/or data using machine -readable media (also called computer-readable media), such as machine -readable storage media (e.g., magnetic disks, optical disks, solid state drives, read only memory (ROM), flash memory devices, phase change memory) and machine-readable transmission media (also called a carrier) (e.g., electrical, optical, radio, acoustical or other form of propagated signals - such as carrier waves, infrared signals). Thus, an electronic device (e.g., a computer) includes hardware and software, such as a set of one or more processors (e.g., wherein a processor is a microprocessor, controller, microcontroller, central processing unit, digital signal processor, application specific integrated circuit, field programmable gate array, other electronic circuitry, a combination of one or more of the preceding) coupled to one or more machine-readable storage media to store code for execution on the set of processors and/or to store data. For instance, an electronic device may include non-volatile memory containing the code since the non-volatile memory can persist code/data even when the electronic device is turned off (when power is removed), and while the electronic device is turned on that part of the code that is to be executed by the processor(s) of that electronic device is typically copied from the slower nonvolatile memory into volatile memory (e.g., dynamic random access memory (DRAM), static random access memory (SRAM)) of that electronic device. Typical electronic devices also include a set or one or more physical network interface(s) (NI(s)) to establish network connections (to transmit and/or receive code and/or data using propagating signals) with other electronic devices. For example, the set of physical NIs (or the set of physical NI(s) in combination with the set of processors executing code) may perform any formatting, coding, or translating to allow the electronic device to send and receive data whether over a wired and/or a wireless connection. In some embodiments, a physical NI may comprise radio circuitry capable of receiving data from other electronic devices over a wireless connection and/or sending data out to other devices via a wireless connection. This radio circuitry may include transmitter(s), receiver(s), and/or transceiver(s) suitable for radiofrequency communication. The radio circuitry may convert digital data into a radio signal having the appropriate parameters (e.g., frequency, timing, channel, bandwidth, etc.). The radio signal may then be transmitted via antennas to the appropriate recipient(s). In some embodiments, the set of physical NI(s) may comprise network interface controller(s) (NICs), also known as a network interface card, network adapter, or local area network (LAN) adapter. The NIC(s) may facilitate in connecting the electronic device to other electronic devices allowing them to communicate via wire through plugging in a cable to a physical port connected to a NIC. One or more parts of an embodiment of the invention may be implemented using different combinations of software, firmware, and/or hardware.

[0030] A network device (ND) is an electronic device that communicatively interconnects other electronic devices on the network (e.g., other network devices, end-user devices). Some network devices are "multiple services network devices" that provide support for multiple networking functions (e.g., routing, bridging, switching, Layer 2 aggregation, session border control, Quality of Service, and/or subscriber management), and/or provide support for multiple application services (e.g., data, voice, and video).

[0031] Methods and apparatuses for identification of traffic flows causing congestions in centralized control planes networks are described. The operations described herein are performed in a network including multiple network elements and a centralized control plane (which is referred to herein as a central network controller). In response to the detection of congestion at an egress network interface of a network element, the central network controller configures the network element to include the following forwarding table entries: an egress forwarding table entry, where all packets forwarded towards the egress network interface are matched to the egress forwarding table entry and caused to be transmitted to a group table entry; the group table entry, where packets matching the group table entry are forwarded towards the egress network interface for output, and a copy of the packets matching the group table entry are forwarded to a meter table entry; and the meter table entry indicating a predetermined threshold rate, where a first packet from the copy of the packets matching the meter table entry is forwarded to the central network controller when a rate of the copy of the packets has not reached the predetermined threshold rate, and a second packet from the copy of the packets matching the meter table entry is dropped when the rate of the copy of the packets has reached the predetermined threshold rate. As a result of the configuration, the central network controller receives, from the network element a subset of packets from the copy of the packets at the predetermined threshold rate. The central network controller identifies from the received subset of packets a packet flow causing congestion at the egress network interface.

[0032] The embodiments of the present invention enable an efficient sampling mechanism for receiving at the central network controller a subset of packets causing the congestion at the network interface enabling the central network controller to identify the culprit packets. The embodiments present clear advantages with respect to the prior sampling approaches. This will enable the central network controller to identify the source of the packet flows and to act on the source network elements in order to reduce congestion and/or inform a higher layer application. The methods described are an efficient mechanism for sending sampled traffic to the central network controller without the need for OpenFlow extensions, and enable back tracking sources that generate heavy traffic in cloud deployments. The solution presented herein does not involve intensive OpenFlow operations such as querying the statistics of one or more flow tables. The solution is independent of the number of flows in the Open Flow table.

[0033] Figure 1 illustrates a block diagram of an exemplary network performing operations for identification of traffic flows causing network congestion in centralized control plane networks, in accordance with some embodiments of the invention. Figure 1 illustrates a network 103 including a first network device ND 111 A, a second network device ND 11 IB and a third network device ND 111C. Each of the NDs 111 A-C includes a single network element NEs 101 A-C. Each of the NDs 111 A-l 11C, and the respective network elements NEs 101 A-C may be implemented as described in further details with reference to Figure 4A-F. While the example of Figure 1 illustrates a single NE being part of an ND, in other embodiments, each ND may include more than one NE. Alternatively, in other embodiments, a single NE may be distributed over two or more network devices as described with reference to Figure 4F. The network 103 further includes a central network controller 121 operative to maintain reachability and forwarding information according to a centralized approach (also called network control), according to some embodiments of the invention.

[0034] As will be described in further details below with reference to Figure 4D, and Figure 5, the centralized approach (also known as software defined networking (SDN)) decouples the system that makes decisions about where traffic is sent from the underlying systems that forwards traffic to the selected destination. The illustrated central network controller 121 has the responsibility for the generation of reachability and forwarding information in a centralized control plane 176 (sometimes referred to as a SDN control module, controller, network controller, OpenFlow controller, SDN controller, control plane node, network virtualization authority, or management control entity), and thus the process of neighbor discovery and topology discovery is centralized. The centralized control plane 176 has a south bound interface with a data plane 180 (sometime referred to the infrastructure layer, network forwarding plane, or forwarding plane (which should not be confused with a ND forwarding plane)) that includes the NEs 101A-C (sometimes referred to as switches, forwarding elements, data plane elements, or nodes). The centralized control plane 176 includes the central network controller 121, which includes a centralized reachability and forwarding information module (not illustrated) that determines the reachability within the network and distributes the forwarding information to the NEs 101 A-C of the data plane 180 over the south bound interface (which may use the OpenFlow protocol). Thus, the network intelligence is centralized in the centralized control plane 176 executing on electronic devices that are typically separate from the NDs.

[0035] The system of Figure 1 is operative to identify flows of traffic that cause congestion at a network interface (e.g., NI IOC of NE 101C) of one of the network elements. In the illustrated example, a set of packet flows 131 is forwarded from NI 10A of NE 101 A towards the NE 101C and output at the NI IOC. A set of packet flows 132 is forwarded from NI 10B of NE 101B towards the NE 101C and output at the NI IOC. The flows 131 and 132 may traverse multiple network devices/elements within the network 103 prior to reaching NE 101C. To simplify the description only the ingress NEs (that represent edge network devices of the network 103) from which the flows originate are illustrated. When they reach NE 101C, the flows are matched in a forwarding table towards the NI IOC and cause congestion at the NI IOC.

[0036] The embodiments of the present invention enable identification of the flows that cause congestion at the NI IOC with the use of conventional OpenFlow and without overwhelming the central network controller 121 with excessive packet rates received from the congested NI. The solution is triggered with the detection of heavy traffic on an egress network interface (or as it may be referred to as an egress port). Several mechanisms can be used to detect the congestion at the NI IOC without departing from the scope of the present invention. In one embodiment, the detection of the congestion can be performed through the normal OpenFlow statistics query on flow table entries redirecting to the NI IOC. In one example, the central network controller 121 may observe and keep track of sustained drop counters on the NI IOC (i.e., monitor counters associated with the NI IOC that record a number of dropped packets at the NI). This is not an expensive operation as the counters are monitored for each port. In other embodiments, the detection of congestion can be performed by a northbound application which monitors the status of the NE 101C and its respective output ports/network interfaces. This external source can detect congestion (e.g. observing application counters) and may request the central network controller 121 to identify the culprit 5-tuple flows that cause the congestion. Several mechanisms can be used to detect the congestion at a given network interface. Upon determining that a congestion is occurring at a given NI (by either explicitly identifying the NI or by receiving the NI's identification from a north bound application), the central network controller 121 is operative to identify the flows that cause this congestion.

[0037] In order to identify the culprit flows causing the congestion, the packet flows output at the NI IOC are sampled and the samples are transmitted to the central network controller 121 to be analyzed.

[0038] At operation 1), in response to a detection of the congestion at the NI IOC, the central network controller 121 configures NE 101C to output packets forwarded towards the NI IOC and to transmit a copy of a subset of these packets to the central network controller 121 according to a threshold rate.

[0039] At operation 2), as a result of the configuration, the NE 101C performs the following operations: a) it outputs packets (the flows 133) forwarded towards the NI IOC (i.e., the original packets egress the NE 101C), and b) transmits a copy of a subset of these packets to the central network controller 121 according to the threshold rate (i.e., the copied packets are mirrored to the central network controller). The copied packets are metered to the central network controller 121 at a constant rate to enable the central network controller 121 to handle and analyze the flow of packets received at that rate. The duration and metering rate can be configured based on the central network controller's resource availability and may be dynamically updated according to the availability of the resources. In some embodiments, the mirrored packets reach the controller as standard OpenFlow PACKET_IN messages.

[0040] The central network controller receives, at operation 3), from NE 101C the subset of packets at the predetermined threshold rate, and identifies, at operation 4), one or more packet flows causing congestion at the egress network interface. For example, the central network controller 121 identifies the flows provided from NI 10A and NI 10B. In some embodiments, when the central network controller 121 receives the packets in the standard Open Flow PACKET_IN message, the information about the ingress network interface from which the packets originate is available in the in_port field of the packet.

[0041] Once the central network controller 121 receives the sampled packets, it can parse the packets to identify the 5-tuple identifying a flow of packets (i.e., the Source IP address/port number, Destination IP address/port number, and the protocol in use). In some embodiments, the central network controller 121 maintains a packet counter against the parsed 5-tuple, and uses the values of the counters to determine and identify the packet flows that cause the congestion. Generally, congestion is caused by a few elephant flows. The sampled packets received at the central network controller would generally belong to such elephant flows. Once the packet flows are identified, the ingress NI information can be used to determine the upstream NE from which the packet flows originate in the network 103. In some embodiments, the upstream NEs are connected over point to point overlay tunnels (e.g., VxLAN), and therefore, an ingress port number of the identified 5-tuple directly points to the upstream NE from which the flow originates. If the upstream NE is connected over broadcast media, the central network controller 121 has the information about the NE connectivity and can derive the potential subset of upstream NEs for the 5-tuple flows.

[0042] Figure 2A illustrates a block diagram of an exemplary forwarding pipeline within a network element for enabling identification of traffic flows causing network congestion, in accordance with some embodiments of the invention. In some embodiments, the central network controller 121 configures at operation 1 of Figure 1, the NE 101C to include a succession of forwarding table entries (e.g., OpenFlow forwarding table entries) that enable the NE 101C to output the original packets to the NI IOC and to transmit a subset of a copy of these packets to the central network controller 121 (in other words to send sampled packets towards the CNC 121). When a packet is received at the NE 101C, packet classification is performed in order to determine a forwarding pipeline to be used for forwarding the packet within the NE.

[0043] Packet classification involves executing a lookup in memory to classify the packet by determining which entry (also referred to as a forwarding table entry or flow entry) in the forwarding tables best matches the packet based upon the match structure, or key, of the forwarding table entries. It is possible that many flows represented in the forwarding table entries can correspond/match to a packet; in this case the system is typically configured to determine one forwarding table entry from the many according to a defined scheme (e.g., selecting a first forwarding table entry that is matched). Forwarding table entries include both a specific set of match criteria (a set of values or wildcards, or an indication of what portions of a packet should be compared to a particular value/values/wildcards, as defined by the matching capabilities - for specific fields in the packet header, or for some other packet content), and a set of one or more actions for the data plane to take on receiving a matching packet. For example, an action may be to push a header onto the packet, for the packet using a particular port, flood the packet, or simply drop the packet. Thus, a forwarding table entry for IPv4/IPv6 packets with a particular transmission control protocol (TCP) destination port could contain an action specifying that these packets should be dropped. [0044] Making forwarding decisions and performing actions occur, based upon the forwarding table entry identified during packet classification, by executing the set of actions identified in the matched forwarding table entry on the packet.

[0045] In particular, the central network controller 121 configures NE 101C (upon determination that a congestion occurs at the NI IOC) to include an egress forwarding table entry 204A in an egress forwarding table 204, such that all packets forwarded towards the egress network interface (e.g., packets 212A-C) are matched to the egress forwarding table entry 204A and caused to be transmitted to a group table entry (Group 1) of a group table 206.

[0046] While some embodiments herein will be described with reference to OpenFlow, the embodiments of the present invention are not so limiting; any future version/modification to OpenFlow, or any similar south bound communication interface and protocol between the central network controller and the NEs, are within the scope of the present invention. In OpenFlow, an egress forwarding table enables processing of packet flows to be done in the context of the output port. When a packet is output to a network interface/port, it will start processing at a first egress forwarding table where flow entries can define processing and redirect the packet to other egress forwarding tables. A new OXM field,

OXM_OF_ACTSET_OUTPUT, enables egress flow entry to match the outgoing interface/port. In some embodiments, a packet may be forwarded through an ingress pipeline (e.g., ingress forwarding table 202) prior to being matched to an entry of the egress forwarding table 204.

[0047] The central network controller 121 further configures NE 101C to include the group table entry (Group 1). All packets matching the group table entry are caused to perform two actions: 1) to be forwarded (through bucket 206B) towards the egress network interface (210) for output, and 2) to be copied and forwarded to a meter table entry 208A in the meter table 208 (through bucket 206 A).

[0048] In OpenFlow, a single group bucket (e.g., bucket 206A) can contain an ordered list of actions. A packet would be subjected to all actions in the bucket in the order specified. In Figure 2A, there are two actions in bucket 206A: a meter action and a punt to central network controller action. The meter action and the punt to central network controller action, cause all packets that cross the rates indicated in the meter entry associated with the meter action to be dropped and the remaining packets are caused to be subjected to the next action (i.e., punt to the central network controller). Thus only packets surviving the metering would be punted to the central network controller.

[0049] The central network controller further configures the NE IOC to include the meter table entry 208A. The meter action applied on the replicated packets to be sent to the central network controller enables sampling of the packets at a desired frequency. The meter table entry 208A indicates a predetermined threshold rate (desired frequency), such that a packet matching the meter table entry 208A is forwarded to the central network controller 121 when a rate of the replicated packets has not reached the predetermined threshold rate, and a packet matching the meter table entry 208A is dropped (e.g., not forwarded to the central network controller 121, removed from storage) when the rate of the replicated packets has reached the predetermined threshold rate. In other words, all packets that cross the predetermined/desired rate would be dropped and the remaining would be subjected to the next action (i.e., to be transmitted to the central network controller 121).

[0050] In some embodiments, the meter is used to obtain uniform byte sampling of the packets. Figure 2B illustrates a block diagram of an exemplary meter table in a forwarding pipeline within a network element for enabling identification of traffic flows causing network congestion in accordance with some embodiments of the invention. A Meter band in Open Flow is defined based on the following parameters: Band type, which indicates how to process the packets that are metered; rate, which defines the rate that the band applies; burst, which indicates the granularity of the meter band; counters, which are updated when packets are processed by the Meter Band; and type Specific Arguments, which are optional arguments that may be defined for a band. The meter band may make use of a token bucket with a burst size and a rate. The rate determines how often the tokens are refreshed. The burst size indicates the number of tokens that are put into the bucket (and the bucket's maximum size). The arriving traffic (i.e., a packet matched to the meter) will decrement the tokens in the token bucket. So long as tokens exist in the bucket, traffic is allowed to go through. These packets are called unmetered traffic. When the bucket becomes empty the packets are metered. Metered packets in Open Flow can be either marked or dropped.

[0051] In some embodiments, to sample packets through the meter table entry 208A, the meter rate 214 is set to a small value (e.g., 1% of the congested port's output capacity). The burst size is configured to hold 1 packet of a flow. In some embodiments, in order to identify elephant flows in a network, the burst size can be configured to be the maximum transmission unit (MTU) in the network (since elephant flows almost always transfer data in chunks of MTU) or a small multiple of the MTU. The packets that cause the rate to exceed the rate specified in a band can either be dropped or be remarked with different DSCP value. In the example of Figure 2B, replicated packets 212A'-C are received at the meter table entry 208A. The meter action is configured to be a DROP action, and the meter band 216 can be chosen by the central network controller 121 based on the required frequency of sampling. The central network controller 121 further determines the period of time over which sampling of the packets is performed in order to obtain information regarding the packet flows causing the congestion. The next action in the meter table entry 208A is "punt" to central network controller 121, such that only packets that pass the metering action are transmitted/punted to the central network controller 121.

[0052] Referring back to the example, where the meter rate is 1%, the packets that get through unmetered are sent to the central network controller; while the metered packets are dropped. Since the rate will be configured to be around 1% of the congested port's traffic, 99% of the incoming traffic will be dropped due to metering. Only 1% of the bytes will be sampled and set towards the central network controller 121.

[0053] When congestion occurs at the NI, given that the majority of the bytes of the traffic is expected to belong to elephant flows, the sampling mechanism makes it more likely to sample elephant flows (over mice flows). For example, if 90% of the traffic belongs to elephant flows, the sampled packets will belong to elephant flows with 90% probability, and the sampled packets will belongs to one or more mice flows with probability 10%. The sampling of the packets through the meter action avoids having to congest the central network controller 121 with information about congested flows as only a subset of the replicated packets is transmitted to the central network controller.

[0054] As a result of the configuration of the forwarding pipeline of NE 101C, the central network controller 121 receives from the network element (e.g., NE 101) a subset of packets from the replicated packets at the predetermined threshold rate, and identifies from the received subset of packets a packet flow causing congestion at the egress network interface. Thus, the replication group (group 1) in the forwarding pipeline sends a metered copy of the packet (forwarded towards the egress NI IOC) to the central network controller 121.

[0055] In some embodiments, relying solely on the packets that are received at the central network controller 121 as a result of the sampling mechanism, to identify the packets causing the congestion, could lead to false positives. For example, as discussed above, even when 90% of the traffic were to belong to elephant flows, the sampling mechanism also would result in a false positive rate of 10%, which is very high.

[0056] Thus, the central network controller may maintain counters per flows that are used to prune all the mice flows. Typically, mice flows are a collection of tens/thousands of flows with small amount of data to transfer. When 100 packets are sampled from mice flows, it is statistically very likely that all the 100 packets will be from 100 different flows. Thus, the central network controller would include 100 entries in a monitoring table where each has a packet count of 1.

[0057] On the other hand, elephant flows are very small in number and transmit a large number of bytes. For example, when 100 packets are sampled from elephant flows, and there are 10 elephant flows, on average, the central network controller would include 10 entries with a sampled packet count of 10 each.

[0058] There is a significant disparity between the packet counts of mice flows and elephant flows. Thus, in some embodiments, instead of marking all the sampled flows (i.e., all flows identified from the packets received at the central network controller as a result of the sampling mechanism) as elephant flows, the central network controller may declare that flows with a certain minimum packet count threshold are classified as elephant and that these flows are the cause of the congestion at the NE 101C. The rest of the flow samples are ignored and are not subjected to any action. In some embodiments, the central network controller can maintain a hash table of interesting five tuples (i.e. ones with more number of packets sampled) with the counters against them. If the number of such 5-tuples goes beyond the scale that can be managed by the controller it can even store the hash table in a secondary storage.

[0059] The operations in the flow diagrams will be described with reference to the exemplary embodiments of the other figures. However, it should be understood that the operations of the flow diagrams can be performed by embodiments of the invention other than those discussed with reference to the other figures, and the embodiments of the invention discussed with reference to these other figures can perform operations different than those discussed with reference to the flow diagrams.

[0060] Figure 3 illustrates a flow of exemplary operations for enabling identification of traffic flows causing network congestion in centralized control plane networks, in accordance with some embodiments of the invention. At operation 310, the central network controller 121 configures NE 101C (upon determination that a congestion occurs at the NI IOC) to include an egress forwarding table entry 204A, such that all packets forwarded towards the egress network interface (e.g., packets 212A-C) are matched to the egress forwarding table entry 204A and caused to be transmitted to a group table entry (Group 1). The central network controller 121 further configures NE 101C to include the group table entry (Group 1). All packets matching the group table entry are caused to perform two actions: 1) to be forwarded (through bucket 206B) towards the egress network interface (210) for output, and 2) to be copied and forwarded to a meter table entry 208 A in the meter table 208. The meter table entry 208 A indicates a predetermined threshold rate, such that a first packet matching the meter table entry 208A is forwarded to the central network controller 121 when a rate of the duplicate packets has not reached the predetermined threshold rate, and a second packet matching the meter table entry 208A is dropped when the rate of the duplicate packets has reached the predetermined threshold rate. [0061] As a result of this configuration, the central network controller 121 receives, at operation 315, from the network element (e.g., NE 101) a subset of packets from the copy of the packets at the predetermined threshold rate, and identifies, at operation 320, from the received subset of packets a packet flow causing congestion at the egress network interface.

[0062] Identifying the packet flow causing congestion at the egress network interface includes determining a source Internet Protocol (IP) address, a source media access control (MAC) address, a destination IP address, a destination MAC address and a forwarding protocol of the packet flow. In some embodiments, the central network controller may increment a counter associated with the packet flow each time a packet is received from the network element for the packet flow, where the packet flow is identified by a corresponding 5-tuple including a source IP address, a source MAC address, a destination IP address, a destination MAC address, and a forwarding protocol.

[0063] Figure 4A illustrates connectivity between network devices (NDs) within an exemplary network, as well as three exemplary implementations of the NDs, according to some embodiments of the invention. Figure 4A shows NDs 400A-H, and their connectivity by way of lines between 400A-400B, 400B-400C, 400C-400D, 400D-400E, 400E-400F, 400F-400G, and 400A-400G, as well as between 400H and each of 400A, 400C, 400D, and 400G. These NDs are physical devices, and the connectivity between these NDs can be wireless or wired (often referred to as a link). An additional line extending from NDs 400A, 400E, and 400F illustrates that these NDs act as ingress and egress points for the network (and thus, these NDs are sometimes referred to as edge NDs; while the other NDs may be called core NDs).

[0064] Two of the exemplary ND implementations in Figure 4 A are: 1) a special-purpose network device 402 that uses custom application-specific integrated-circuits (ASICs) and a special-purpose operating system (OS); and 2) a general purpose network device 404 that uses common off-the-shelf (COTS) processors and a standard OS.

[0065] The special-purpose network device 402 includes networking hardware 410 comprising a set of one or more processor(s) 412, forwarding resource(s) 414 (which typically include one or more ASICs and/or network processors), and physical network interfaces (NIs) 416 (through which network connections are made, such as those shown by the connectivity between NDs 400 A-H), as well as non-transitory machine readable storage media 418 having stored therein networking software 420. During operation, the networking software 420 may be executed by the networking hardware 410 to instantiate a set of one or more networking software instance(s) 422. Each of the networking software instance(s) 422, and that part of the networking hardware 410 that executes that network software instance (be it hardware dedicated to that networking software instance and/or time slices of hardware temporally shared by that networking software instance with others of the networking software instance(s) 422), form a separate virtual network element 430A-R. Each of the virtual network element(s) (VNEs) 430A-R includes a control communication and configuration module 432A-R (sometimes referred to as a local control module or control communication module) and forwarding table(s) 434A-R, such that a given virtual network element (e.g., 430A) includes the control communication and configuration module (e.g., 432A), a set of one or more forwarding table(s) (e.g., 434A), and that portion of the networking hardware 410 that executes the virtual network element (e.g., 430A).

[0066] The special-purpose network device 402 is often physically and/or logically considered to include: 1) a ND control plane 424 (sometimes referred to as a control plane) comprising the processor(s) 412 that execute the control communication and configuration module(s) 432A-R; and 2) a ND forwarding plane 426 (sometimes referred to as a forwarding plane, a data plane, or a media plane) comprising the forwarding resource(s) 414 that utilize the forwarding table(s) 434A-R and the physical NIs 416. By way of example, where the ND is a router (or is implementing routing functionality), the ND control plane 424 (the processor(s) 412 executing the control communication and configuration module(s) 432A-R) is typically responsible for participating in controlling how data (e.g., packets) is to be routed (e.g., the next hop for the data and the outgoing physical NI for that data) and storing that routing information in the forwarding table(s) 434A-R, and the ND forwarding plane 426 is responsible for receiving that data on the physical NIs 416 and forwarding that data out the appropriate ones of the physical NIs 416 based on the forwarding table(s) 434A-R.

[0067] Figure 4B illustrates an exemplary way to implement the special-purpose network device 402 according to some embodiments of the invention. Figure 4B shows a special-purpose network device including cards 438 (typically hot pluggable). While in some embodiments the cards 438 are of two types (one or more that operate as the ND forwarding plane 426

(sometimes called line cards), and one or more that operate to implement the ND control plane 424 (sometimes called control cards)), alternative embodiments may combine functionality onto a single card and/or include additional card types (e.g., one additional type of card is called a service card, resource card, or multi-application card). A service card can provide specialized processing (e.g., Layer 4 to Layer 7 services (e.g., firewall, Internet Protocol Security (IPsec), Secure Sockets Layer (SSL) / Transport Layer Security (TLS), Intrusion Detection System (IDS), peer-to-peer (P2P), Voice over IP (VoIP) Session Border Controller, Mobile Wireless Gateways (Gateway General Packet Radio Service (GPRS) Support Node (GGSN), Evolved Packet Core (EPC) Gateway)). By way of example, a service card may be used to terminate IPsec tunnels and execute the attendant authentication and encryption algorithms. These cards are coupled together through one or more interconnect mechanisms illustrated as backplane 436 (e.g., a first full mesh coupling the line cards and a second full mesh coupling all of the cards).

[0068] Returning to Figure 4A, the general purpose network device 404 includes hardware 440 comprising a set of one or more processor(s) 442 (which are often COTS processors) and physical NIs 446, as well as non-transitory machine readable storage media 448 having stored therein software 450. During operation, the processor(s) 442 execute the software 450 to instantiate one or more sets of one or more applications 464A-R. While one embodiment does not implement virtualization, alternative embodiments may use different forms of virtualization. For example, in one such alternative embodiment the virtualization layer 454 represents the kernel of an operating system (or a shim executing on a base operating system) that allows for the creation of multiple instances 462A-R called software containers that may each be used to execute one (or more) of the sets of applications 464A-R; where the multiple software containers (also called virtualization engines, virtual private servers, or jails) are user spaces (typically a virtual memory space ) that are separate from each other and separate from the kernel space in which the operating system is r n; and where the set of applications running in a given user space, unless explicitly allowed, cannot access the memory of the other processes. In another such alternative embodiment the virtualization layer 454 represents a hypervisor (sometimes referred to as a virtual machine monitor (VMM)) or a hypervisor executing on top of a host operating system, and each of the sets of applications 464A-R is run on top of a guest operating system within an instance 462A-R called a virtual machine (which may in some cases be considered a tightly isolated form of software container) that is run on top of the hypervisor - the guest operating system and application may not know they are running on a virtual machine as opposed to running on a "bare metal" host electronic device, or through para-virtualization the operating system and/or application may be aware of the presence of virtualization for optimization purposes. In yet other alternative embodiments, one, some or all of the applications are implemented as unikernel(s), which can be generated by compiling directly with an application only a limited set of libraries (e.g., from a library operating system (LibOS) including drivers/libraries of OS services) that provide the particular OS services needed by the application. As a unikernel can be implemented to run directly on hardware 440, directly on a hypervisor (in which case the unikernel is sometimes described as running within a LibOS virtual machine), or in a software container, embodiments can be implemented fully with unikernels running directly on a hypervisor represented by virtualization layer 454, unikernels running within software containers represented by instances 462A-R, or as a combination of unikernels and the above-described techniques (e.g. , unikernels and virtual machines both run directly on a hypervisor, unikernels and sets of applications that are run in different software containers).

[0069] The instantiation of the one or more sets of one or more applications 464A-R, as well as virtualization if implemented, are collectively referred to as software instance(s) 452. Each set of applications 464A-R, corresponding virtualization construct (e.g., instance 462A-R) if implemented, and that part of the hardware 440 that executes them (be it hardware dedicated to that execution and/or time slices of hardware temporally shared), forms a separate virtual network element(s) 460A-R.

[0070] The virtual network element(s) 460A-R perform similar functionality to the virtual network element(s) 430A-R - e.g., similar to the control communication and configuration module(s) 432A and forwarding table(s) 434A (this virtualization of the hardware 440 is sometimes referred to as network function virtualization (NFV)). Thus, NFV may be used to consolidate many network equipment types onto industry standard high volume server hardware, physical switches, and physical storage, which could be located in Data centers, NDs, and customer premise equipment (CPE). While embodiments of the invention are illustrated with each instance 462A-R corresponding to one VNE 460A-R, alternative embodiments may implement this correspondence at a finer level granularity (e.g., line card virtual machines virtualize line cards, control card virtual machine virtualize control cards, etc.); it should be understood that the techniques described herein with reference to a correspondence of instances 462A-R to VNEs also apply to embodiments where such a finer level of granularity and/or unikernels are used.

[0071] In certain embodiments, the virtualization layer 454 includes a virtual switch that provides similar forwarding services as a physical Ethernet switch. Specifically, this virtual switch forwards traffic between instances 462A-R and the physical NI(s) 446, as well as optionally between the instances 462A-R; in addition, this virtual switch may enforce network isolation between the VNEs 460A-R that by policy are not permitted to communicate with each other (e.g., by honoring virtual local area networks (VLANs)).

[0072] The third exemplary ND implementation in Figure 4A is a hybrid network device 406, which includes both custom ASICs/special-purpose OS and COTS processors/standard OS in a single ND or a single card within an ND. In certain embodiments of such a hybrid network device, a platform VM (i.e., a VM that that implements the functionality of the special-purpose network device 402) could provide for para-virtualization to the networking hardware present in the hybrid network device 406.

[0073] Regardless of the above exemplary implementations of an ND, when a single one of multiple VNEs implemented by an ND is being considered (e.g., only one of the VNEs is part of a given virtual network) or where only a single VNE is currently being implemented by an ND, the shortened term network element (NE) is sometimes used to refer to that VNE. Also in all of the above exemplary implementations, each of the VNEs (e.g., VNE(s) 430A-R, VNEs 460A-R, and those in the hybrid network device 406) receives data on the physical NIs (e.g., 416, 446) and forwards that data out the appropriate ones of the physical NIs (e.g., 416, 446). For example, a VNE implementing IP router functionality forwards IP packets on the basis of some of the IP header information in the IP packet; where IP header information includes source IP address, destination IP address, source port, destination port (where "source port" and "destination port" refer herein to protocol ports, as opposed to physical ports of a ND), transport protocol (e.g., user datagram protocol (UDP), Transmission Control Protocol (TCP), and differentiated services code point (DSCP) values.

[0074] Figure 4C illustrates various exemplary ways in which VNEs may be coupled according to some embodiments of the invention. Figure 4C shows VNEs 470A.1-470A.P (and optionally VNEs 470A.Q-470A.R) implemented in ND 400A and VNE 470H.1 in ND 400H. In Figure 4C, VNEs 470A.1-P are separate from each other in the sense that they can receive packets from outside ND 400 A and forward packets outside of ND 400 A; VNE 470 A.1 is coupled with VNE 470H.1, and thus they communicate packets between their respective NDs; VNE 470A.2-470A.3 may optionally forward packets between themselves without forwarding them outside of the ND 400 A; and VNE 470 A. P may optionally be the first in a chain of VNEs that includes VNE 470A.Q followed by VNE 470A.R (this is sometimes referred to as dynamic service chaining, where each of the VNEs in the series of VNEs provides a different service - e.g., one or more layer 4-7 network services). While Figure 4C illustrates various exemplary relationships between the VNEs, alternative embodiments may support other relationships (e.g., more/fewer VNEs, more/fewer dynamic service chains, multiple different dynamic service chains with some common VNEs and some different VNEs).

[0075] The NDs of Figure 4A, for example, may form part of the Internet or a private network; and other electronic devices (not shown; such as end user devices including workstations, laptops, netbooks, tablets, palm tops, mobile phones, smartphones, phablets, multimedia phones, Voice Over Internet Protocol (VOIP) phones, terminals, portable media players, GPS units, wearable devices, gaming systems, set-top boxes, Internet enabled household appliances) may be coupled to the network (directly or through other networks such as access networks) to communicate over the network (e.g., the Internet or virtual private networks (VPNs) overlaid on (e.g., tunneled through) the Internet) with each other (directly or through servers) and/or access content and/or services. Such content and/or services are typically provided by one or more servers (not shown) belonging to a service/content provider or one or more end user devices (not shown) participating in a peer-to-peer (P2P) service, and may include, for example, public webpages (e.g., free content, store fronts, search services), private webpages (e.g.,

username/password accessed webpages providing email services), and/or corporate networks over VPNs. For instance, end user devices may be coupled (e.g., through customer premise equipment coupled to an access network (wired or wirelessly)) to edge NDs, which are coupled (e.g., through one or more core NDs) to other edge NDs, which are coupled to electronic devices acting as servers. However, through compute and storage virtualization, one or more of the electronic devices operating as the NDs in Figure 4A may also host one or more such servers (e.g., in the case of the general purpose network device 404, one or more of the software instances 462A-R may operate as servers; the same would be true for the hybrid network device 406; in the case of the special-purpose network device 402, one or more such servers could also be run on a virtualization layer executed by the processor(s) 412); in which case the servers are said to be co-located with the VNEs of that ND.

[0076] A virtual network is a logical abstraction of a physical network (such as that in Figure 4A) that provides network services (e.g., L2 and/or L3 services). A virtual network can be implemented as an overlay network (sometimes referred to as a network virtualization overlay) that provides network services (e.g., layer 2 (L2, data link layer) and/or layer 3 (L3, network layer) services) over an underlay network (e.g., an L3 network, such as an Internet Protocol (IP) network that uses tunnels (e.g., generic routing encapsulation (GRE), layer 2 tunneling protocol (L2TP), IPSec) to create the overlay network).

[0077] A network virtualization edge (NVE) sits at the edge of the underlay network and participates in implementing the network virtualization; the network-facing side of the NVE uses the underlay network to tunnel frames to and from other NVEs; the outward-facing side of the NVE sends and receives data to and from systems outside the network. A virtual network instance (VNI) is a specific instance of a virtual network on a NVE (e.g., a NE/VNE on an ND, a part of a NE/VNE on a ND where that NE/VNE is divided into multiple VNEs through emulation); one or more VNIs can be instantiated on an NVE (e.g., as different VNEs on an ND). A virtual access point (VAP) is a logical connection point on the NVE for connecting external systems to a virtual network; a VAP can be physical or virtual ports identified through logical interface identifiers (e.g., a VLAN ID).

[0078] Examples of network services include: 1) an Ethernet LAN emulation service (an Ethernet-based multipoint service similar to an Internet Engineering Task Force (IETF) Multiprotocol Label Switching (MPLS) or Ethernet VPN (EVPN) service) in which external systems are interconnected across the network by a LAN environment over the underlay network (e.g., an NVE provides separate L2 VNIs (virtual switching instances) for different such virtual networks, and L3 (e.g., IP/MPLS) tunneling encapsulation across the underlay network); and 2) a virtualized IP forwarding service (similar to IETF IP VPN (e.g., Border Gateway Protocol (BGP)/MPLS IP VPN) from a service definition perspective) in which external systems are interconnected across the network by an L3 environment over the underlay network (e.g., an NVE provides separate L3 VNIs (forwarding and routing instances) for different such virtual networks, and L3 (e.g., IP/MPLS) tunneling encapsulation across the underlay network)). Network services may also include quality of service capabilities (e.g., traffic classification marking, traffic conditioning and scheduling), security capabilities (e.g., filters to protect customer premises from network - originated attacks, to avoid malformed route announcements), and management capabilities (e.g., full detection and processing).

[0079] Fig. 4D illustrates a network with a single network element on each of the NDs of Figure 4A, and within this straight forward approach contrasts a traditional distributed approach (commonly used by traditional routers) with a centralized approach for maintaining reachability and forwarding information (also called network control), according to some embodiments of the invention. Specifically, Figure 4D illustrates network elements (NEs) 470A-H with the same connectivity as the NDs 400A-H of Figure 4A.

[0080] Figure 4D illustrates that the distributed approach 472 distributes responsibility for generating the reachability and forwarding information across the NEs 470A-H; in other words, the process of neighbor discovery and topology discovery is distributed.

[0081] For example, where the special-purpose network device 402 is used, the control communication and configuration module(s) 432A-R of the ND control plane 424 typically include a reachability and forwarding information module to implement one or more routing protocols (e.g., an exterior gateway protocol such as Border Gateway Protocol (BGP), Interior Gateway Protocol(s) (IGP) (e.g., Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), Routing Information Protocol (RIP), Label Distribution Protocol (LDP), Resource Reservation Protocol (RSVP) (including RSVP-Traffic Engineering (TE): Extensions to RSVP for LSP Tunnels and Generalized Multi-Protocol Label Switching

(GMPLS) Signaling RSVP-TE)) that communicate with other NEs to exchange routes, and then selects those routes based on one or more routing metrics. Thus, the NEs 470A-H (e.g., the processor(s) 412 executing the control communication and configuration module(s) 432A-R) perform their responsibility for participating in controlling how data (e.g., packets) is to be routed (e.g., the next hop for the data and the outgoing physical NI for that data) by

distributively determining the reachability within the network and calculating their respective forwarding information. Routes and adjacencies are stored in one or more routing structures (e.g., Routing Information Base (RIB), Label Information Base (LIB), one or more adjacency structures) on the ND control plane 424. The ND control plane 424 programs the ND forwarding plane 426 with information (e.g., adjacency and route information) based on the routing structure(s). For example, the ND control plane 424 programs the adjacency and route information into one or more forwarding table(s) 434A-R (e.g., Forwarding Information Base (FIB), Label Forwarding Information Base (LFIB), and one or more adjacency structures) on the ND forwarding plane 426. For layer 2 forwarding, the ND can store one or more bridging tables that are used to forward data based on the layer 2 information in that data. While the above example uses the special-purpose network device 402, the same distributed approach 472 can be implemented on the general purpose network device 404 and the hybrid network device 406.

[0082] Figure 4D illustrates that a centralized approach 474 (also known as software defined networking (SDN)) that decouples the system that makes decisions about where traffic is sent from the underlying systems that forwards traffic to the selected destination. The illustrated centralized approach 474 has the responsibility for the generation of reachability and forwarding information in a centralized control plane 476 (sometimes referred to as a SDN control module, controller, network controller, OpenFlow controller, SDN controller, control plane node, network virtualization authority, or management control entity), and thus the process of neighbor discovery and topology discovery is centralized. The centralized control plane 476 has a south bound interface 482 with a data plane 480 (sometime referred to the infrastructure layer, network forwarding plane, or forwarding plane (which should not be confused with a ND forwarding plane)) that includes the NEs 470A-H (sometimes referred to as switches, forwarding elements, data plane elements, or nodes). The centralized control plane 476 includes a network controller 478, which includes a centralized reachability and forwarding information module 479 that determines the reachability within the network and distributes the forwarding information to the NEs 470A-H of the data plane 480 over the south bound interface 482 (which may use the OpenFlow protocol). Thus, the network intelligence is centralized in the centralized control plane 476 executing on electronic devices that are typically separate from the NDs.

[0083] For example, where the special-purpose network device 402 is used in the data plane 480, each of the control communication and configuration module(s) 432A-R of the ND control plane 424 typically include a control agent that provides the VNE side of the south bound interface 482. In this case, the ND control plane 424 (the processor(s) 412 executing the control communication and configuration module(s) 432A-R) performs its responsibility for participating in controlling how data (e.g., packets) is to be routed (e.g., the next hop for the data and the outgoing physical NI for that data) through the control agent communicating with the centralized control plane 476 to receive the forwarding information (and in some cases, the reachability information) from the centralized reachability and forwarding information module 479 (it should be understood that in some embodiments of the invention, the control communication and configuration module(s) 432A-R, in addition to communicating with the centralized control plane 476, may also play some role in determining reachability and/or calculating forwarding information - albeit less so than in the case of a distributed approach; such embodiments are generally considered to fall under the centralized approach 474, but may also be considered a hybrid approach).

[0084] While the above example uses the special-purpose network device 402, the same centralized approach 474 can be implemented with the general purpose network device 404 (e.g., each of the VNE 460A-R performs its responsibility for controlling how data (e.g., packets) is to be routed (e.g., the next hop for the data and the outgoing physical NI for that data) by communicating with the centralized control plane 476 to receive the forwarding information (and in some cases, the reachability information) from the centralized reachability and forwarding information module 479; it should be understood that in some embodiments of the invention, the VNEs 460A-R, in addition to communicating with the centralized control plane 476, may also play some role in determining reachability and/or calculating forwarding information - albeit less so than in the case of a distributed approach) and the hybrid network device 406. In fact, the use of SDN techniques can enhance the NFV techniques typically used in the general purpose network device 404 or hybrid network device 406 implementations as NFV is able to support SDN by providing an infrastructure upon which the SDN software can be run, and NFV and SDN both aim to make use of commodity server hardware and physical switches.

[0085] Figure 4D also shows that the centralized control plane 476 has a north bound interface 484 to an application layer 486, in which resides application(s) 488. The centralized control plane 476 has the ability to form virtual networks 492 (sometimes referred to as a logical forwarding plane, network services, or overlay networks (with the NEs 470A-H of the data plane 480 being the underlay network)) for the application(s) 488. Thus, the centralized control plane 476 maintains a global view of all NDs and configured NEs/VNEs, and it maps the virtual networks to the underlying NDs efficiently (including maintaining these mappings as the physical network changes either through hardware (ND, link, or ND component) failure, addition, or removal).

[0086] While Figure 4D shows the distributed approach 472 separate from the centralized approach 474, the effort of network control may be distributed differently or the two combined in certain embodiments of the invention. For example: 1) embodiments may generally use the centralized approach (SDN) 474, but have certain functions delegated to the NEs (e.g., the distributed approach may be used to implement one or more of fault monitoring, performance monitoring, protection switching, and primitives for neighbor and/or topology discovery); or 2) embodiments of the invention may perform neighbor discovery and topology discovery via both the centralized control plane and the distributed protocols, and the results compared to raise exceptions where they do not agree. Such embodiments are generally considered to fall under the centralized approach 474, but may also be considered a hybrid approach.

[0087] While Figure 4D illustrates the simple case where each of the NDs 400A-H implements a single NE 470A-H, it should be understood that the network control approaches described with reference to Figure 4D also work for networks where one or more of the NDs 400 A-H implement multiple VNEs (e.g., VNEs 430A-R, VNEs 460A-R, those in the hybrid network device 406). Alternatively or in addition, the network controller 478 may also emulate the implementation of multiple VNEs in a single ND. Specifically, instead of (or in addition to) implementing multiple VNEs in a single ND, the network controller 478 may present the implementation of a VNE/NE in a single ND as multiple VNEs in the virtual networks 492 (all in the same one of the virtual network(s) 492, each in different ones of the virtual network(s) 492, or some combination). For example, the network controller 478 may cause an ND to implement a single VNE (a NE) in the underlay network, and then logically divide up the resources of that NE within the centralized control plane 476 to present different VNEs in the virtual network(s) 492 (where these different VNEs in the overlay networks are sharing the resources of the single VNE/NE implementation on the ND in the underlay network).

[0088] On the other hand, Figures 4E and 4F respectively illustrate exemplary abstractions of NEs and VNEs that the network controller 478 may present as part of different ones of the virtual networks 492. Figure 4E illustrates the simple case of where each of the NDs 400A-H implements a single NE 470A-H (see Figure 4D), but the centralized control plane 476 has abstracted multiple of the NEs in different NDs (the NEs 470A-C and G-H) into (to represent) a single NE 4701 in one of the virtual network(s) 492 of Figure 4D, according to some embodiments of the invention. Figure 4E shows that in this virtual network, the NE 4701 is coupled to NE 470D and 470F, which are both still coupled to NE 470E.

[0089] Figure 4F illustrates a case where multiple VNEs (VNE 470A.1 and VNE 470H.1) are implemented on different NDs (ND 400A and ND 400H) and are coupled to each other, and where the centralized control plane 476 has abstracted these multiple VNEs such that they appear as a single VNE 470T within one of the virtual networks 492 of Figure 4D, according to some embodiments of the invention. Thus, the abstraction of a NE or VNE can span multiple NDs.

[0090] While some embodiments of the invention implement the centralized control plane 476 as a single entity (e.g., a single instance of software running on a single electronic device), alternative embodiments may spread the functionality across multiple entities for redundancy and/or scalability purposes (e.g., multiple instances of software running on different electronic devices).

[0091] Similar to the network device implementations, the electronic device(s) running the centralized control plane 476, and thus the network controller 478 including the centralized reachability and forwarding information module 479, may be implemented a variety of ways (e.g., a special purpose device, a general-purpose (e.g., COTS) device, or hybrid device). These electronic device(s) would similarly include processor(s), a set or one or more physical NIs, and a non-transitory machine-readable storage medium having stored thereon the centralized control plane software. For instance, Figure 5 illustrates, a general purpose control plane device 504 including hardware 540 comprising a set of one or more processor(s) 542 (which are often COTS processors) and physical NIs 546, as well as non-transitory machine readable storage media 548 having stored therein centralized control plane (CCP) software 550.

[0092] In embodiments that use compute virtualization, the processor(s) 542 typically execute software to instantiate a virtualization layer 554 (e.g., in one embodiment the virtualization layer 554 represents the kernel of an operating system (or a shim executing on a base operating system) that allows for the creation of multiple instances 562A-R called software containers (representing separate user spaces and also called virtualization engines, virtual private servers, or jails) that may each be used to execute a set of one or more applications; in another embodiment the virtualization layer 554 represents a hypervisor (sometimes referred to as a virtual machine monitor (VMM)) or a hypervisor executing on top of a host operating system, and an application is run on top of a guest operating system within an instance 562A-R called a virtual machine (which in some cases may be considered a tightly isolated form of software container) that is run by the hypervisor ; in another embodiment, an application is implemented as a unikernel, which can be generated by compiling directly with an application only a limited set of libraries (e.g., from a library operating system (LibOS) including dri vers/libraries of OS services) that provide the particular OS services needed by the application, and the unikernel can run directly on hardware 540, directly on a hypervisor represented by virtualization layer 554 (in which case the unikernel is sometimes described as running within a LibOS virtual machine), or in a software container represented by one of instances 562A-R). Again, in embodiments where compute virtualization is used, during operation an instance of the CCP software 550 (illustrated as CCP instance 576A) is executed (e.g., within the instance 562A) on the virtualization layer 554. In embodiments where compute virtualization is not used, the CCP instance 576A is executed, as a unikernel or on top of a host operating system, on the "bare metal" general purpose control plane device 504. The instantiation of the CCP instance 576A, as well as the virtualization layer 554 and instances 562A-R if implemented, are collectively referred to as software instance(s) 552.

[0093] In some embodiments, the CCP instance 576A includes a network controller instance 578. The network controller instance 578 includes a centralized reachability and forwarding information module instance 579 (which is a middleware layer providing the context of the network controller 478 to the operating system and communicating with the various NEs), and an CCP application layer 580 (sometimes referred to as an application layer) over the middleware layer (providing the intelligence required for various network operations such as protocols, network situational awareness, and user - interfaces). At a more abstract level, this CCP application layer 580 within the centralized control plane 476 works with virtual network view(s) (logical view(s) of the network) and the middleware layer provides the conversion from the virtual networks to the physical view.

[0094] The centralized control plane 476 transmits relevant messages to the data plane 480 based on CCP application layer 580 calculations and middleware layer mapping for each flow. A flow may be defined as a set of packets whose headers match a given pattern of bits; in this sense, traditional IP forwarding is also flow-based forwarding where the flows are defined by the destination IP address for example; however, in other implementations, the given pattern of bits used for a flow definition may include more fields (e.g., 10 or more) in the packet headers. Different NDs/NEs/VNEs of the data plane 480 may receive different messages, and thus different forwarding information. The data plane 480 processes these messages and programs the appropriate flow information and corresponding actions in the forwarding tables (sometime referred to as flow tables) of the appropriate NE/VNEs, and then the NEs/VNEs map incoming packets to flows represented in the forwarding tables and forward packets based on the matches in the forwarding tables.

[0095] Standards such as OpenFlow define the protocols used for the messages, as well as a model for processing the packets. The model for processing packets includes header parsing, packet classification, and making forwarding decisions. Header parsing describes how to interpret a packet based upon a well-known set of protocols. Some protocol fields are used to build a match structure (or key) that will be used in packet classification (e.g., a first key field could be a source media access control (MAC) address, and a second key field could be a destination MAC address).

[0096] Packet classification involves executing a lookup in memory to classify the packet by determining which entry (also referred to as a forwarding table entry or flow entry) in the forwarding tables best matches the packet based upon the match structure, or key, of the forwarding table entries. It is possible that many flows represented in the forwarding table entries can correspond/match to a packet; in this case the system is typically configured to determine one forwarding table entry from the many according to a defined scheme (e.g., selecting a first forwarding table entry that is matched). Forwarding table entries include both a specific set of match criteria (a set of values or wildcards, or an indication of what portions of a packet should be compared to a particular value/values/wildcards, as defined by the matching capabilities - for specific fields in the packet header, or for some other packet content), and a set of one or more actions for the data plane to take on receiving a matching packet. For example, an action may be to push a header onto the packet, for the packet using a particular port, flood the packet, or simply drop the packet. Thus, a forwarding table entry for IPv4/IPv6 packets with a particular transmission control protocol (TCP) destination port could contain an action specifying that these packets should be dropped.

[0097] Making forwarding decisions and performing actions occurs, based upon the forwarding table entry identified during packet classification, by executing the set of actions identified in the matched forwarding table entry on the packet.

[0098] However, when an unknown packet (for example, a "missed packet" or a "match-miss" as used in OpenFlow parlance) arrives at the data plane 480, the packet (or a subset of the packet header and content) is typically forwarded to the centralized control plane 476. The centralized control plane 476 will then program forwarding table entries into the data plane 480 to accommodate packets belonging to the flow of the unknown packet. Once a specific forwarding table entry has been programmed into the data plane 480 by the centralized control plane 476, the next packet with matching credentials will match that forwarding table entry and take the set of actions associated with that matched entry.

[0099] A network interface (NI) may be physical or virtual; and in the context of IP, an interface address is an IP address assigned to a NI, be it a physical NI or virtual NI. A virtual NI may be associated with a physical NI, with another virtual interface, or stand on its own (e.g., a loopback interface, a point-to-point protocol interface). A NI (physical or virtual) may be numbered (a NI with an IP address) or unnumbered (a NI without an IP address). A loopback interface (and its loopback address) is a specific type of virtual NI (and IP address) of a NE/VNE (physical or virtual) often used for management purposes; where such an IP address is referred to as the nodal loopback address. The IP address(es) assigned to the NI(s) of a ND are referred to as IP addresses of that ND; at a more granular level, the IP address(es) assigned to NI(s) assigned to a NE/VNE implemented on a ND can be referred to as IP addresses of that NE/VNE.

[00100] Each VNE (e.g., a virtual router, a virtual bridge (which may act as a virtual switch instance in a Virtual Private LAN Service (VPLS) is typically independently administrable. For example, in the case of multiple virtual routers, each of the virtual routers may share system resources but is separate from the other virtual routers regarding its management domain, AAA (authentication, authorization, and accounting) name space, IP address, and routing database(s). Multiple VNEs may be employed in an edge ND to provide direct network access and/or different classes of services for subscribers of service and/or content providers.

[00101] Within certain NDs, "interfaces" that are independent of physical NIs may be configured as part of the VNEs to provide higher-layer protocol and service information (e.g., Layer 3 addressing). The subscriber records in the AAA server identify, in addition to the other subscriber configuration requirements, to which context (e.g., which of the VNEs/NEs) the corresponding subscribers should be bound within the ND. As used herein, a binding forms an association between a physical entity (e.g., physical NI, channel) or a logical entity (e.g., circuit such as a subscriber circuit or logical circuit (a set of one or more subscriber circuits)) and a context's interface over which network protocols (e.g., routing protocols, bridging protocols) are configured for that context. Subscriber data flows on the physical entity when some higher-layer protocol interface is configured and associated with that physical entity.

[00102] Some NDs provide support for implementing VPNs (Virtual Private Networks) (e.g., Layer 2 VPNs and/or Layer 3 VPNs). For example, the ND where a provider's network and a customer's network are coupled are respectively referred to as PEs (Provider Edge) and CEs (Customer Edge). In a Layer 2 VPN, forwarding typically is performed on the CE(s) on either end of the VPN and traffic is sent across the network (e.g., through one or more PEs coupled by other NDs). Layer 2 circuits are configured between the CEs and PEs (e.g., an Ethernet port, an ATM permanent virtual circuit (PVC), a Frame Relay PVC). In a Layer 3 VPN, routing typically is performed by the PEs. By way of example, an edge ND that supports multiple VNEs may be deployed as a PE; and a VNE may be configured with a VPN protocol, and thus that VNE is referred as a VPN VNE.

[00103] Some NDs provide support for VPLS (Virtual Private LAN Service). For example, in a VPLS network, end user devices access content/services provided through the VPLS network by coupling to CEs, which are coupled through PEs coupled by other NDs. VPLS networks can be used for implementing triple play network applications (e.g., data applications (e.g., highspeed Internet access), video applications (e.g., television service such as IPTV (Internet Protocol Television), VoD (Video-on-Demand) service), and voice applications (e.g., VoIP (Voice over Internet Protocol) service)), VPN services, etc. VPLS is a type of layer 2 VPN that can be used for multi-point connectivity. VPLS networks also allow end use devices that are coupled with CEs at separate geographical locations to communicate with each other across a Wide Area Network (WAN) as if they were directly attached to each other in a Local Area Network (LAN) (referred to as an emulated LAN).

[00104] In VPLS networks, each CE typically attaches, possibly through an access network (wired and/or wireless), to a bridge module of a PE via an attachment circuit (e.g., a virtual link or connection between the CE and the PE). The bridge module of the PE attaches to an emulated LAN through an emulated LAN interface. Each bridge module acts as a "Virtual Switch Instance" (VSI) by maintaining a forwarding table that maps MAC addresses to pseudowires and attachment circuits. PEs forward frames (received from CEs) to destinations (e.g., other CEs, other PEs) based on the MAC destination address field included in those frames.

[00105] For example, while the flow diagrams in the figures show a particular order of operations performed by certain embodiments of the invention, it should be understood that such order is exemplary (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.).

[00106] While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described, can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.

Claims

CLAIMS What is claimed is:

1. A method, in a central network controller of a centralized control plane network, of identification of traffic flows causing network congestion, the method comprising:

in response to a detection of congestion at an egress network interface of a network element, configuring (310) the network element to include the following forwarding table entries:

an egress forwarding table entry, wherein all packets forwarded towards the egress network interface are matched to the egress forwarding table entry and caused to be transmitted to a group table entry, the group table entry, wherein packets matching the group table entry are

forwarded towards the egress network interface for output, and a copy of the packets matching the group table entry are forwarded to a meter table entry, and

the meter table entry indicating a predetermined threshold rate, wherein a first packet from the copy of the packets matching the meter table entry is forwarded to the central network controller when a rate of the copy of the packets has not reached the predetermined threshold rate, and a second packet from the copy of the packets matching the meter table entry is dropped when the rate of the copy of the packets has reached the predetermined threshold rate;

receiving (315), as a result of the configuring the network element to include the

forwarding table entries, from the network element a subset of packets from the copy of the packets at the predetermined threshold rate; and

identifying (320) from the received subset of packets a packet flow causing congestion at the egress network interface.

2. The method of claim 1, wherein identifying the packet flow causing congestion at the egress network interface includes determining a source Internet Protocol (IP) address, a source media access control (MAC) address, a destination IP address, a destination MAC address and a forwarding protocol of the packet flow.

3. The method of claim 1 further comprising incrementing a counter associated with the packet flow each time a packet is received from the network element for the packet flow, wherein the packet flow is identified by a corresponding 5-tuple including a source Internet Protocol (IP) address, a source media access control (MAC) address, a destination IP address, a destination MAC address, and a forwarding protocol.

4. The method of claim 3, wherein identifying the packet flow causing congestion at the egress network interface further includes determining whether the counter associated with the packet flow has reached a congestion threshold.

5. The method of claim 1, wherein the predetermined threshold rate is determined based on a capacity of the central network controller to receive and process packets.

6. The method of claim 1, wherein the predetermined threshold rate indicates a predetermined percentage of number of bytes output at the egress network interface over a period of time.

7. The method of claim 1, wherein the central network controller is a Software Defined Networking (SDN) network controller.

8. The method of claim 1, wherein each one of the subset of packets is received from the network element in an OpenFlow packet-in.

9. A central network controller, comprising:

a non-transitory computer readable medium to store instructions; and

a processor coupled with the non-transitory computer readable medium to process the stored instructions to:

in response to a detection of congestion at an egress network interface of a network element, configure (310) the network element to include the following forwarding table entries:

an egress forwarding table entry, wherein all packets forwarded towards the egress network interface are matched to the egress forwarding table entry and caused to be transmitted to a group table entry, the group table entry, wherein packets matching the group table entry are forwarded towards the egress network interface for output, and a copy of the packets matching the group table entry are forwarded to a meter table entry, and

the meter table entry indicating a predetermined threshold rate, wherein a first packet from the copy of the packets matching the meter table entry is forwarded to the central network controller when a rate of the copy of the packets has not reached the predetermined

threshold rate, and a second packet from the copy of the packets matching the meter table entry is dropped when the rate of the copy of the packets has reached the predetermined threshold rate, receive (315), as a result of the configuration of the network element to include the forwarding table entries, from the network element a subset of packets from the copy of the packets at the predetermined threshold rate, and identify (320) from the received subset of packets a packet flow causing

congestion at the egress network interface.

10. The central network controller of claim 9, wherein to identify the packet flow causing congestion at the egress network interface includes to determine a source Internet Protocol (IP) address, a source media access control (MAC) address, a destination IP address, a destination MAC address and a forwarding protocol of the packet flow.

11. The central network controller of claim 9, wherein the processor is further to increment a counter associated with the packet flow each time a packet is received from the network element for the packet flow, wherein the packet flow is identified by a corresponding 5-tuple including a source Internet Protocol (IP) address, a source media access control (MAC) address, a destination IP address, a destination MAC address, and a forwarding protocol.

12. The central network controller of claim 11, wherein to identify the packet flow causing congestion at the egress network interface further includes to determine whether the counter associated with the packet flow has reached a congestion threshold.

13. The central network controller of claim 9, wherein the predetermined threshold rate is determined based on a capacity of the central network controller to receive and process packets.

14. The central network controller of claim 9, wherein the predetermined threshold rate indicates a predetermined percentage of number of bytes output at the egress network interface over a period of time.

15. The central network controller of claim 9, wherein the central network controller is a Software Defined Networking (SDN) network controller.

16. The central network controller of claim 9, wherein each one of the subset of packets is received from the network element in an OpenFlow packet-in.

17. A non-transitory computer readable storage medium that provide instructions, which when executed by a processor of a central network controller, cause said processor to perform operations comprising:

18. The non-transitory computer readable storage medium of claim 17, wherein identifying the packet flow causing congestion at the egress network interface includes determining a source Internet Protocol (IP) address, a source media access control (MAC) address, a destination IP address, a destination MAC address and a forwarding protocol of the packet flow.

19. The non-transitory computer readable storage medium of claim 17, wherein the operations further comprise incrementing a counter associated with the packet flow each time a packet is received from the network element for the packet flow, wherein the packet flow is identified by a corresponding 5-tuple including a source Internet Protocol (IP) address, a source media access control (MAC) address, a destination IP address, a destination MAC address, and a forwarding protocol.

20. The non-transitory computer readable storage medium of claim 19, wherein identifying the packet flow causing congestion at the egress network interface further includes determining whether the counter associated with the packet flow has reached a congestion threshold.

21. The non-transitory computer readable storage medium of claim 17, wherein the predetermined threshold rate is determined based on a capacity of the central network controller to receive and process packets.

22. The non-transitory computer readable storage medium of claim 17, wherein the predetermined threshold rate indicates a predetermined percentage of number of bytes output at the egress network interface over a period of time.

23. The non-transitory computer readable storage medium of claim 17, wherein the central network controller is a Software Defined Networking (SDN) network controller.

24. The non-transitory computer readable storage medium of claim 17, wherein each one of the subset of packets is received from the network element in an OpenFlow packet-in.