Patent Publication Number: US-10320687-B2

Title: Policy enforcement for upstream flood traffic

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/793,301 filed on Jul. 7, 2015, which claims priority to U.S. Provisional Patent Application Ser. No. 62/159,100 filed May 8, 2015, the contents of which are incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present technology relates in general to the field of computer networks, and more specifically to policy enforcement via a physical network layer or hardware fabric of a data center. 
     BACKGROUND 
     As more and more enterprises are transitioning to next-generation data centers and integrating cloud computing for their business requirements, automated and robust policy management is becoming necessary to support on-demand provisioning of computing resources and dynamic scaling of applications. Conventionally, network administrators manually configure security policies in the data center using a device-centric management model. However, such an approach is likely to result in security breaches caused by policy misconfiguration. In addition to lack of automation, a primary reason for misconfiguration is lack of awareness regarding application context. For example, organizations may have tens of thousands to millions of access control lists (ACLs) and firewall rules. These organizations often lack the operational processes to remove these policies in a timely way when applications are decommissioned and/or prefer to retain policies because they are uncertain about the potential effect of removal. 
     A conventional approach for policy management utilizes manual service chaining and a static network topology that is bound to network connections, VLAN, network interface, IP addressing, etc. This model requires policy configuration across multiple security devices (e.g., firewalls and intrusion detection and prevention systems (IDSs and IPSs)), slows application deployment, and is hard to scale because applications are frequently created, moved, and decommissioned in a next-generation data center. Another conventional approach for policy management is to implement a virtualization-centric model, but this approach fails to address applications not running as virtual machines. Further, the hypervisor-based overlay approach requires that each connection pass through multiple policy enforcement points (e.g., source virtual machine, destination virtual machine, and firewall). This routing introduces overhead and complexity for each inter-application connection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features of the disclosure can be obtained, a more particular description of the principles briefly described above will be rendered by reference to specific examples thereof which are illustrated in the appended drawings. Understanding that these drawings depict only examples of the disclosure and are not therefore to be considered to be limiting of its scope, the principles herein are described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  illustrates an example network device according to some aspects of the subject technology; 
         FIGS. 2A and 2B  illustrate example system embodiments according to some aspects of the subject technology; 
         FIG. 3  illustrates a schematic block diagram of an example architecture for a network fabric according to some aspects of the subject technology; 
         FIG. 4  illustrates an example overlay network according to some aspects of the subject technology; 
         FIG. 5  illustrates an example method for enforcing policy for upstream flood traffic via a physical network layer according to some aspects of the subject technology; 
         FIG. 6  illustrates an example of a matrix of policies among endpoint groups according to some aspects of the subject technology; and 
         FIG. 7  illustrates an example of a virtual extensible local area network (VXLAN) data packet according to some aspects of the subject technology. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     The detailed description set forth below is intended as a description of various configurations of the subject technology and is not intended to represent the only configurations in which the subject technology can be practiced. The appended drawings are incorporated herein and constitute a part of the detailed description. The detailed description includes specific details for the purpose of providing a more thorough understanding of the subject technology. However, it will be clear and apparent that the subject technology is not limited to the specific details set forth herein and may be practiced without these details. In some instances, structures and components are shown in block diagram form in order to avoid obscuring the concepts of the subject technology. 
     Overview 
     Systems and methods in accordance with various embodiments of the present disclosure may overcome one or more of the aforementioned and other deficiencies experienced in conventional approaches for policy management. In various embodiments, an application-centric, unified, and automated approach can be implemented for policy management in the data center and cloud infrastructure that is decoupled from the underlying network topology, supports application mobility, offers real-time compliance lifecycle management, and reduces the risk of security breaches. This approach is based on promise theory. Promise theory is based on declarative, scalable control of intelligent objects, in comparison to conventional imperative models, which can be thought of as heavyweight, top-down management. An imperative model is a big brain system or top-down style of management. In these systems the central manager must be aware of both the configuration commands of underlying objects and the current state of those objects. Promise theory, in contrast, relies on the underlying objects to handle configuration state changes initiated by the control system as desired state changes. The objects are in turn also responsible for passing exceptions or faults back to the control system. This lightens the burden and complexity of the control system and allows for greater scale. These systems scale further by allowing for methods of underlying objects to in turn request state changes from one another and/or lower level objects. With this approach, there is a new level of abstraction between the hardware and the software and a methodology to adapt networking across various hardware platforms, capabilities, and future evolutions. This can allow automation between networking and application teams, and substantially reduce the time of deployment for applications. 
     In some embodiments, the physical network layer or hardware fabric of a data center is built on a two-tier, spine-leaf design that uses a bipartite graph where each leaf is a switch that connects to each spine switch (though a full mesh is not required in certain embodiments), and no direct connections are allowed between leaf switches and between spine switches (sometimes referred to as a two-tier Clos network). The leaves act as the connection point for all external devices and networks, and the spines act as the high-speed forwarding engine between leaves. The fabric is managed, monitored, and administered by a controller appliance (or cluster of controllers). 
     In some embodiments, a policy management model can be built on a series of one or more tenants that allow segregation of the network infrastructure administration and data flows. Tenants can be broken down into private Layer 3 networks or “contexts,” which directly relate to a Virtual Route Forwarding (VRF) instance or separate IP space. Each tenant may have one or more private Layer 3 networks or contexts depending on their business needs. Private Layer 3 networks provide a way to further separate the organizational and forwarding requirements below a given tenant. Because contexts use separate forwarding instances, IP addressing can be duplicated in separate contexts for the purpose of multi-tenancy. 
     Below the context, the policy management model incorporates a series of objects that define the application itself. These objects are called endpoint groups (EPGs). EPGs are a collection of similar endpoints representing an application tier or set of services. EPGs are connected to each other via policies. As used herein, policies are more than just a set of ACLs and can include a collection of inbound/outbound filters, traffic quality settings, marking rules/redirection rules, and Layers 4-7 service device graphs. 
     EPGs provide a logical grouping for objects that require similar policy. For example, an EPG could be the group of components that make up an application&#39;s web tier. Endpoints themselves are defined using NIC, vNIC, IP address, or DNS name with extensibility for future methods of identifying application components. EPGs can also be used to represent other entities such as outside networks, network services, security devices, network storage, etc. They are collections of one or more endpoints providing a similar function. They are a logical grouping with varying use options depending on the application deployment model in use. 
     The use of EPGs provides several benefits. EPGs can act as a single policy enforcement point for a group of contained objects. This can simplify configuration of policies and ensure their consistency. Additional policy can be applied based on EPG rather than subnet as in conventional techniques. This means that IP addressing changes to the endpoint do not necessarily change its policy, which is common in the case of conventional networks. Alternatively, moving an endpoint to another EPG applies the new policy to the leaf switch that the endpoint is connected to and defines new behavior for that endpoint based on the new EPG. 
     An additional benefit of EPGs relates to how policy is enforced for an EPG. The physical ternary content-addressable memory (TCAM) where policy is stored for enforcement is an expensive component of switch hardware and therefore tends to lower policy scale and/or raise hardware costs. In various embodiments, policy is applied via the hardware fabric based on the EPG rather than the endpoint itself. This policy size can be expressed as n×m×f, where n is the number of sources, m is the number of destinations, and f is the number of policy filters. Using this approach, sources and destinations become one entry for a given EPG, which reduces the number of total entries required. For example, if there are 5 sources, 4 destinations, and five policy filters, the conventional approach would require 100 policy entries. Using the various embodiments disclosed herein, only 5 policy entries are required because the number of sources and destinations are reduced down to 1. 
     As discussed, various advantages are provided by designing a data center such that policy is enforced via a physical network layer or hardware fabric of the data center. Further, it can be advantageous for a data center to accommodate flood traffic (e.g., broadcast, unknown unicast, or multicast traffic). For example, flooding techniques may use network infrastructure more efficiently by requiring a source endpoint to send a packet only once, and utilizing network elements (e.g., switches, routers) for replicating the packet to multiple receivers such that the packet is sent over each link of the network only once. It may also be advantageous for a data center to support virtual or overlay networking. However, upstream (i.e., traffic from an endpoint to the fabric) flood traffic from a particular endpoint can potentially be received by other virtual switches sitting on a same overlay network. 
     The subject technology provides embodiments for systems, methods, and non-transitory computer-readable storage media for enforcing policy for upstream flood traffic via a physical network layer or hardware fabric of a data center. This can be accomplished, in an embodiment, by transmitting upstream flood traffic using a special multicast group to which only nodes of the data center fabric (e.g., physical switches, routers) are subscribed. That is, upstream traffic is assigned to the special multicast group, resulting in unintended endpoints not receiving the flood traffic. When the hardware fabric receives the flood traffic, applicable policies can be enforced to route the packets to intended endpoints. 
     DETAILED DESCRIPTION 
     A computer network is a geographically distributed collection of nodes interconnected by communication links and segments for transporting data between endpoints, such as personal computers and workstations. Many types of networks are available, with the types ranging from local area networks (LANs) and wide area networks (WANs) to overlay and software-defined networks, such as virtual extensible local area networks (VXLANs). 
     LANs typically connect nodes over dedicated private communications links located in the same general physical location, such as a building or campus. WANs, on the other hand, typically connect geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines, optical lightpaths, synchronous optical networks (SONET), or synchronous digital hierarchy (SDH) links. LANs and WANs can include layer 2 (L2) and/or layer 3 (L3) networks and devices. 
     The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes on various networks. The nodes typically communicate over the network by exchanging discrete frames or packets of data according to predefined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP). In this context, a protocol can refer to a set of rules defining how the nodes interact with each other. Computer networks may be further interconnected by an intermediate network node, such as a router, to extend the effective “size” of each network. 
     Overlay networks generally allow virtual networks to be created and layered over a physical network infrastructure. Overlay network protocols, such as Virtual Extensible LAN (VXLAN), Network Virtualization using Generic Routing Encapsulation (NVGRE), Network Virtualization Overlays (NVO3), and Stateless Transport Tunneling (STT), provide a traffic encapsulation scheme which allows network traffic to be carried across L2 and L3 networks over a logical tunnel. Such logical tunnels can be originated and terminated through virtual tunnel end points (VTEPs). 
     Overlay networks can also include virtual segments, such as VXLAN segments in a VXLAN overlay network, which can include virtual L2 and/or L3 overlay networks over which virtual machines (VMs) communicate. The virtual segments can be identified through a virtual network identifier (VNI), such as a VXLAN network identifier, which can specifically identify an associated virtual segment or domain. 
     Network virtualization allows hardware and software resources to be combined in a virtual network. For example, network virtualization can allow multiple numbers of VMs to be attached to the physical network via respective virtual LANs (VLANs). The VMs can be grouped according to their respective VLAN, and can communicate with other VMs as well as other devices on the internal or external network. 
     Network segments, such as physical or virtual segments, networks, devices, ports, physical or logical links, and/or traffic in general can be grouped into a bridge or flood domain. A bridge domain or flood domain can represent a broadcast domain, such as an L2 broadcast domain. A bridge domain or flood domain can include a single subnet, but can also include multiple subnets. Moreover, a bridge domain can be associated with a bridge domain interface on a network device, such as a switch. A bridge domain interface can be a logical interface which supports traffic between an L2 bridged network and an L3 routed network. In addition, a bridge domain interface can support internet protocol (IP) termination, VPN termination, address resolution handling, MAC addressing, etc. Both bridge domains and bridge domain interfaces can be identified by a same index or identifier. 
     Cloud computing can also be provided in one or more networks to provide computing services using shared resources. Cloud computing can generally include Internet-based computing in which computing resources are dynamically provisioned and allocated to client or user computers or other devices on-demand, from a collection of resources available via the network (e.g., “the cloud”). Cloud computing resources, for example, can include any type of resource, such as computing, storage, and network devices, virtual machines (VMs), etc. For instance, resources may include service devices (firewalls, deep packet inspectors, traffic monitors, load balancers, etc.), compute/processing devices (servers, CPUs, memory, brute force processing capability), storage devices (e.g., network attached storages, storage area network devices), etc. In addition, such resources may be used to support virtual networks, virtual machines (VM), databases, applications (“apps”), etc. 
     Cloud computing resources may include a “private cloud,” a “public cloud,” and/or a “hybrid cloud.” A “hybrid cloud” can be a cloud infrastructure composed of two or more clouds that inter-operate or federate through technology. In essence, a hybrid cloud is an interaction between private and public clouds where a private cloud joins a public cloud and utilizes public cloud resources in a secure and scalable manner. Cloud computing resources can also be provisioned via virtual networks in an overlay network, such as a VXLAN. 
     The disclosed technology addresses the need in the art for improving policy enforcement. Disclosed are systems and methods for implementing and enforcing network policies of upstream flood traffic via a physical network layer or hardware fabric of a data center. A brief introductory description of exemplary systems and networks, as illustrated in  FIGS. 1-4 , is disclosed herein. A detailed description of policy enforcement for upstream flood traffic via the fabric, and example variations, will then follow. These variations shall be described as the various embodiments are set forth. The disclosure now turns to  FIG. 1 . 
       FIG. 1  illustrates an exemplary network device  110  suitable for implementing the present invention. Network device  110  includes a master central processing unit (CPU)  162 , interfaces  168 , and a bus  115  (e.g., a PCI bus). When acting under the control of appropriate software or firmware, the CPU  162  is responsible for executing packet management, error detection, and/or routing functions, such policy enforcement, for example. The CPU  162  preferably accomplishes all these functions under the control of software including an operating system and any appropriate applications software. CPU  162  may include one or more processors  163  such as a processor from the Motorola family of microprocessors or the MIPS family of microprocessors. In an alternative embodiment, processor  163  is specially designed hardware for controlling the operations of router  110 . In a specific embodiment, a memory  161  (such as non-volatile RAM and/or ROM) also forms part of CPU  162 . However, there are many different ways in which memory could be coupled to the system. 
     The interfaces  168  are typically provided as interface cards (sometimes referred to as “line cards”). Generally, they control the sending and receiving of data packets over the network and sometimes support other peripherals used with the network device  110 . Among the interfaces that may be provided are Ethernet interfaces, frame relay interfaces, cable interfaces, DSL interfaces, token ring interfaces, and the like. In addition, various very high-speed interfaces may be provided such as fast token ring interfaces, wireless interfaces, Ethernet interfaces, Gigabit Ethernet interfaces, ATM interfaces, HSSI interfaces, POS interfaces, FDDI interfaces and the like. Generally, these interfaces may include ports appropriate for communication with the appropriate media. In some cases, they may also include an independent processor and, in some instances, volatile RAM. The independent processors may control such communications intensive tasks as packet switching, media control, and management. By providing separate processors for the communications intensive tasks, these interfaces allow the master microprocessor  162  to efficiently perform routing computations, network diagnostics, security functions, etc. 
     Although the system shown in  FIG. 1  is one specific network device of the present invention, it is by no means the only network device architecture on which the present invention can be implemented. For example, an architecture having a single processor that handles communications as well as routing computations, etc., is often used. Further, other types of interfaces and media could also be used with the router. 
     Regardless of the network device&#39;s configuration, it may employ one or more memories or memory modules (including memory  161 ) configured to store program instructions for the general-purpose network operations and mechanisms for roaming, route optimization and routing functions described herein. The program instructions may control the operation of an operating system and/or one or more applications, for example. The memory or memories may also be configured to store tables such as mobility binding, registration, and association tables, etc. 
       FIG. 2A  and  FIG. 2B  illustrate systems that can be used in various embodiments. The more appropriate embodiment will be apparent to those of ordinary skill in the art when practicing the present technology. Persons of ordinary skill in the art will also readily appreciate that other embodiments are possible. 
       FIG. 2A  illustrates a conventional system bus computing system architecture  200  wherein the components of the system are in electrical communication with each other using a bus  205 . Exemplary system  200  includes a processing unit (CPU or processor)  210  and a system bus  205  that couples various system components including the system memory  215 , such as read only memory (ROM)  220  and random access memory (RAM)  225 , to the processor  210 . The system  200  can include a cache of high-speed memory connected directly with, in close proximity to, or integrated as part of the processor  210 . The system  200  can copy data from the memory  215  and/or the storage device  230  to the cache  212  for quick access by the processor  210 . In this way, the cache can provide a performance boost that avoids processor  210  delays while waiting for data. These and other modules can control or be configured to control the processor  210  to perform various actions. Other system memory  215  may be available for use as well. The memory  215  can include multiple different types of memory with different performance characteristics. The processor  210  can include any general purpose processor and a hardware module or software module, such as module  1   232 , module  2   234 , and module  3   236  stored in storage device  230 , configured to control the processor  210  as well as a special-purpose processor where software instructions are incorporated into the actual processor design. The processor  210  may essentially be a completely self-contained computing system, containing multiple cores or processors, a bus, memory controller, cache, etc. A multi-core processor may be symmetric or asymmetric. 
     To enable user interaction with the computing device  200 , an input device  245  can represent any number of input mechanisms, such as a microphone for speech, a touch-sensitive screen for gesture or graphical input, keyboard, mouse, motion input, speech, etc. An output device  235  can also be one or more of a number of output mechanisms known to those of skill in the art. In some instances, multimodal systems can enable a user to provide multiple types of input to communicate with the computing device  200 . The communications interface  240  can generally govern and manage the user input and system output. There is no restriction on operating on any particular hardware arrangement and therefore the basic features here may easily be substituted for improved hardware or firmware arrangements as they are developed. 
     Storage device  230  is a non-volatile memory and can be a hard disk or other types of computer readable media which can store data that are accessible by a computer, such as magnetic cassettes, flash memory cards, solid state memory devices, digital versatile disks, cartridges, random access memories (RAMs)  225 , read only memory (ROM)  220 , and hybrids thereof. 
     The storage device  230  can include software modules  232 ,  234 ,  236  for controlling the processor  210 . Other hardware or software modules are contemplated. The storage device  230  can be connected to the system bus  205 . In one aspect, a hardware module that performs a particular function can include the software component stored in a computer-readable medium in connection with the necessary hardware components, such as the processor  210 , bus  205 , display  235 , and so forth, to carry out the function. 
       FIG. 2B  illustrates a computer system  250  having a chipset architecture that can be used in executing the described method and generating and displaying a graphical user interface (GUI). Computer system  250  is an example of computer hardware, software, and firmware that can be used to implement the disclosed technology. System  250  can include a processor  255 , representative of any number of physically and/or logically distinct resources capable of executing software, firmware, and hardware configured to perform identified computations. Processor  255  can communicate with a chipset  260  that can control input to and output from processor  255 . In this example, chipset  260  outputs information to output  265 , such as a display, and can read and write information to storage device  270 , which can include magnetic media, and solid state media, for example. Chipset  260  can also read data from and write data to RAM  275 . A bridge  280  for interfacing with a variety of user interface components  285  can be provided for interfacing with chipset  260 . Such user interface components  285  can include a keyboard, a microphone, touch detection and processing circuitry, a pointing device, such as a mouse, and so on. In general, inputs to system  250  can come from any of a variety of sources, machine generated and/or human generated. 
     Chipset  260  can also interface with one or more communication interfaces  290  that can have different physical interfaces. Such communication interfaces can include interfaces for wired and wireless local area networks, for broadband wireless networks, as well as personal area networks. Some applications of the methods for generating, displaying, and using the GUI disclosed herein can include receiving ordered datasets over the physical interface or be generated by the machine itself by processor  255  analyzing data stored in storage  270  or  275 . Further, the machine can receive inputs from a user via user interface components  285  and execute appropriate functions, such as browsing functions by interpreting these inputs using processor  255 . 
     It can be appreciated that exemplary systems  200  and  250  can have more than one processor  210  or be part of a group or cluster of computing devices networked together to provide greater processing capability. 
       FIG. 3  illustrates a schematic block diagram of an example architecture  300  for a network fabric  312  that can be used in various embodiments. The network fabric  312  can include spine switches  302   A ,  302   B , . . . ,  302   N  (collectively “ 302 ”) connected to leaf switches  304   A ,  304   B ,  304   C  . . .  304   N  (collectively “ 304 ”) in the network fabric  312 . 
     Spine switches  302  can be L3 switches in the fabric  312 . However, in some cases, the spine switches  302  can also, or otherwise, perform L2 functionalities. Further, the spine switches  302  can support various capabilities, such as 40 or 10 Gbps Ethernet speeds. To this end, the spine switches  302  can include one or more 40 Gigabit Ethernet ports. Each port can also be split to support other speeds. For example, a 40 Gigabit Ethernet port can be split into four 10 Gigabit Ethernet ports. 
     In some embodiments, one or more of the spine switches  302  can be configured to host a proxy function that performs a lookup of the endpoint address identifier to locator mapping in a mapping database on behalf of leaf switches  304  that do not have such mapping. The proxy function can do this by parsing through the packet to the encapsulated, tenant packet to get to the destination locator address of the tenant. The spine switches  302  can then perform a lookup of their local mapping database to determine the correct locator address of the packet and forward the packet to the locator address without changing certain fields in the header of the packet. 
     When a packet is received at a spine switch  302   i , the spine switch  302   i  can first check if the destination locator address is a proxy address. If so, the spine switch  302   i  can perform the proxy function as previously mentioned. If not, the spine switch  302   i  can look up the locator in its forwarding table and forward the packet accordingly. 
     Spine switches  302  connect to leaf switches  304  in the fabric  312 . Leaf switches  304  can include access ports (or non-fabric ports) and fabric ports. Fabric ports can provide uplinks to the spine switches  302 , while access ports can provide connectivity for devices, hosts, endpoints, VMs, or external networks to the fabric  312 . 
     Leaf switches  304  can reside at the edge of the fabric  312 , and can thus represent the physical network edge. In some cases, the leaf switches  304  can be top-of-rack (“ToR”) switches configured according to a ToR architecture. In other cases, the leaf switches  304  can be aggregation switches in any particular topology, such as end-of-row (EoR) or middle-of-row (MoR) topologies. The leaf switches  304  can also represent aggregation switches, for example. 
     The leaf switches  304  can be responsible for routing and/or bridging the tenant packets and applying network policies. In some cases, a leaf switch can perform one or more additional functions, such as implementing a mapping cache, sending packets to the proxy function when there is a miss in the cache, encapsulate packets, enforce ingress or egress policies, etc. 
     Moreover, the leaf switches  304  can contain virtual switching functionalities, such as a virtual tunnel endpoint (VTEP) function as explained below in the discussion of VTEP  408  in  FIG. 4 . To this end, leaf switches  304  can connect the fabric  312  to an overlay network, such as overlay network  400  illustrated in  FIG. 4 . 
     Network connectivity in the fabric  312  can flow through the leaf switches  304 . Here, the leaf switches  304  can provide servers, resources, endpoints, external networks, or VMs access to the fabric  312 , and can connect the leaf switches  304  to each other. In some cases, the leaf switches  304  can connect EPGs to the fabric  312  and/or any external networks. Each EPG can connect to the fabric  312  via one of the leaf switches  304 , for example. 
     Endpoints  310 A-E (collectively “ 310 ”) can connect to the fabric  312  via leaf switches  304 . For example, endpoints  310 A and  310 B can connect directly to leaf switch  304 A, which can connect endpoints  310 A and  310 B to the fabric  312  and/or any other one of the leaf switches  304 . Similarly, endpoint  310 E can connect directly to leaf switch  304 C, which can connect endpoint  310 E to the fabric  312  and/or any other of the leaf switches  304 . On the other hand, endpoints  310 C and  310 D can connect to leaf switch  304 B via L2 network  306 . Similarly, the wide area network (WAN) can connect to the leaf switches  304 C or  304 D via L3 network  308 . 
     Endpoints  310  can include any communication device, such as a computer, a server, a switch, a router, etc. In some cases, the endpoints  310  can include a server, hypervisor, or switch configured with a VTEP functionality which connects an overlay network, such as overlay network  400  below, with the fabric  312 . For example, in some cases, the endpoints  310  can represent one or more of the VTEPs  408 A-D illustrated in  FIG. 4 . Here, the VTEPs  408 A-D can connect to the fabric  312  via the leaf switches  304 . The overlay network can host physical devices, such as servers, applications, EPGs, virtual segments, virtual workloads, etc. In addition, the endpoints  310  can host virtual workload(s), clusters, and applications or services, which can connect with the fabric  312  or any other device or network, including an external network. For example, one or more endpoints  310  can host, or connect to, a cluster of load balancers or an EPG of various applications. 
     Although the fabric  312  is illustrated and described herein as an example leaf-spine architecture, one of ordinary skill in the art will readily recognize that the subject technology can be implemented based on any network fabric, including any data center or cloud network fabric. Indeed, other architectures, designs, infrastructures, and variations are contemplated herein. 
       FIG. 4  illustrates an exemplary overlay network  400 . Overlay network  400  uses an overlay protocol, such as VXLAN, NVGRE, VO3, or STT, to encapsulate traffic in L2 and/or L3 packets which can cross overlay L3 boundaries in the network. As illustrated in  FIG. 4 , overlay network  400  can include hosts  406 A-D interconnected via network  402 . 
     Network  402  can include a packet network, such as an IP network, for example. Moreover, network  402  can connect the overlay network  400  with the fabric  312  in  FIG. 3 . For example, VTEPs  408 A-D can connect with the leaf switches  304  in the fabric  312  via network  402 . 
     Hosts  406 A-D include virtual tunnel end points (VTEP)  408 A-D, which can be virtual nodes or switches configured to encapsulate and de-encapsulate data traffic according to a specific overlay protocol of the network  400 , for the various virtual network identifiers (VNIDs)  410 A-I. Moreover, hosts  406 A-D can include servers containing a VTEP functionality, hypervisors, and physical switches, such as L3 switches, configured with a VTEP functionality. For example, hosts  406 A and  406 B can be physical switches configured to run VTEPs  408 A-B. Here, hosts  406 A and  406 B can be connected to servers  404 A-D, which, in some cases, can include virtual workloads through VMs loaded on the servers, for example. 
     In some embodiments, network  400  can be a VXLAN network, and VTEPs  408 A-D can be VXLAN tunnel end points (VTEP). However, as one of ordinary skill in the art will readily recognize, network  400  can represent any type of overlay or software-defined network, such as NVGRE, STT, or even overlay technologies yet to be invented. 
     The VNIDs can represent the segregated virtual networks in overlay network  400 . Each of the overlay tunnels (VTEPs  408 A-D) can include one or more VNIDs. For example, VTEP  408 A can include VNIDs 1 and 2, VTEP  408 B can include VNIDs 1 and 2, VTEP  408 C can include VNIDs 1 and 2, and VTEP  408 D can include VNIDs 1-3. As one of ordinary skill in the art will readily recognize, any particular VTEP can, in other embodiments, have numerous VNIDs, including more than the 3 VNIDs illustrated in  FIG. 4 . 
     The traffic in overlay network  400  can be segregated logically according to specific VNIDs. This way, traffic intended for VNID 1 can be accessed by devices residing in VNID 1, while other devices residing in other VNIDs (e.g., VNID2 and VNID3) can be prevented from accessing such traffic. In other words, devices or endpoints connected to specific VNIDs can communicate with other devices or endpoints connected to the same specific VNIDs, while traffic from separate VNIDs can be isolated to prevent devices or endpoints in other specific VNIDs from accessing traffic in different VNIDs. 
     Servers  404 A-D and VMs  404 E-I can connect to their respective VNID or virtual segment, and communicate with other servers or VMs residing in the same VNID or virtual segment. For example, server  404 A can communicate with server  404 C and VMs  404 E and  404 G because they all reside in the same VNID, i.e., VNID 1. Similarly, server  404 B can communicate with VMs  404 F and  404 H because they all reside in VNID 2. VMs  404 E-I can host virtual workloads, which can include application workloads, resources, and services, for example. However, in some cases, servers  404 A-D can similarly host virtual workloads through VMs hosted on the servers  404 A-D. Moreover, each of the servers  404 A-D and VMs  404 E-I can represent a single server or VM, but can also represent multiple servers or VMs, such as a cluster of servers or VMs. 
     VTEPs  408 A-D can encapsulate packets directed at the various VNIDs 1-3 in the overlay network  400  according to the specific overlay protocol implemented, such as VXLAN, so traffic can be properly transmitted to the correct VNID and recipient(s). Moreover, when a switch, router, or other network device receives a packet to be transmitted to a recipient in the overlay network  400 , it can analyze a routing table, such as a lookup table, to determine where such packet needs to be transmitted so the traffic reaches the appropriate recipient. For example, if VTEP  408 A receives a unicast packet from endpoint  404 B that is intended for endpoint  404 H, VTEP  408 A can analyze a routing table that maps the intended endpoint, endpoint  404 H, to a specific switch that is configured to handle communications intended for endpoint  404 H. VTEP  408 A might not initially know, when it receives the packet from endpoint  404 B, that such packet should be transmitted to VTEP  408 D in order to reach endpoint  404 H. Accordingly, by analyzing the routing table, VTEP  408 A can lookup endpoint  404 H, which is the intended recipient, and determine that the packet should be transmitted to VTEP  408 D, as specified in the routing table based on endpoint-to-switch mappings or bindings, so the packet can be transmitted to, and received by, endpoint  404 H as expected. 
     However, in some instances, VTEP  408 A may analyze the routing table and fail to find any bindings or mappings associated with the intended recipient, e.g., endpoint  404 H. Here, the routing table may not yet have learned routing information regarding endpoint  404 H. In this situation, the packet is treated as an unknown unicast packet or a flood packet. In some conventional networks, flood traffic (e.g., broadcast, unknown unicast, or multicast traffic) transmitted over an overlay network is handled via IP multicasting. In IP multicasting, a unique IP address range is assigned as multicast group IP addresses. This range is a Class D address range from 224.0.0.0 to 239.255.255.255. Each address in this range represents a multicast group, although some addresses are reserved. A device in the network can join a multicast group using Internet Group Management Protocol (IGMP). After IGMP subscribe requests, when an IP packet with a destination IP address of a multicast group is transmitted, the packet gets forwarded to every device that has subscribed to the multicast group. The network devices (e.g., Layer 2 switches and Layer 3 routers) run multicast protocols to provide optimal delivery of packets to the intended endpoints. 
     Ideally, a network would be configured such that there is a one-to-one mapping of a logical network (e.g., VXLAN segment) to an IP multicast group address. It may be necessary, however, for some networks to have multiple virtual networks share a single multicast group address. For example, with a one-to-one mapping between VXLAN segments and IP multicast groups, an increase in the number of VXLAN segments concomitantly causes an increase in the required multicast address space and the amount of forwarding states on the physical network devices. At some point, multicast scalability in the transport network can become untenable. Therefore, mapping multiple VXLAN segments to a single multicast group can help conserve multicast control plane resources on the network devices and achieve the desired VXLAN scalability. This mapping, however, creates inefficiencies in multicast forwarding. For instance, packets forwarded to the multicast group for one tenant are now sent to the VTEPs of other tenants that are sharing the same multicast group address. This causes sub-optimal utilization of multicast data plane resources. Therefore, this conventional approach is a trade-off between control plane scalability and data plane efficiency. 
     Systems and approaches in accordance with various embodiments of the present disclosure overcome these limitations. As discussed, in various embodiments, a network can be configured to operate with a special multicast group to which only the physical network or hardware fabric is subscribed. In an embodiment, at least one IP address in the multicast address space can be reserved for the special multicast group. The network can be further configured such that no endpoints can join or subscribe to the special multicast group. When a flood packet is received to a virtual switch from a virtual endpoint, the switch will encapsulate the destination address of the packet with the IP address for the special multicast group, and forward the packet until it is ultimately received by the network fabric, e.g., a spine switch. At that point, the fabric may enforce applicable policies and can update the packet with the multicast group address designated to the virtual network of the endpoint from which the packet originated. The packet is then forwarded “downstream” (i.e., from the fabric to the designated multicast group) where it is eventually received only by intended endpoints. 
     As discussed, when multiple logical networks are mapped to a single multicast group address in the conventional manner, flood traffic for a particular logical network will be sent to that logical network as well as other logical network(s) sharing the same multicast group address. This can result in the flood traffic being received by unintended endpoints. Various embodiments avoid such deficiencies of the conventional approach by causing “upstream” traffic (i.e., traffic flowing from an endpoint to the fabric) to be routed to the fabric before being routed to a designated multicast group address. This ensures that policy is enforced by the fabric, and that only the intended endpoints receive the traffic. 
     As one of ordinary skill in the art will readily recognize, the examples and technologies provided above are simply for clarity and explanation purposes, and can include many additional concepts and variations. 
     Having disclosed some basic system components and concepts, the disclosure now turns to an example process that can be used in an embodiment as shown in  FIG. 5 . For the sake of clarity, the method is described in terms of a network fabric  312 , as shown in  FIG. 3 , configured to practice the method. The steps outlined herein are exemplary and can be implemented in any combination thereof, including combinations that exclude, add, or modify certain steps. 
     Method  500  begins at step  502  in which a special multicast group is configured such that only elements of a physical network of a data center (e.g., physical switches, routers) are joined or subscribed. Alternatively or additionally, the network is configured such that endpoints in the network are prohibited from joining the special multicast group. The method can continue at step  504  by receiving network traffic from a source virtual endpoint to a virtual switch. The virtual endpoint and virtual switch are “downstream” from the network fabric, and network traffic originating from the virtual endpoint is propagated “upstream” towards the virtual switch and the network fabric. A determination is made  506  whether the network traffic corresponds to flood traffic, such as one or more broadcast, unknown unicast, or multicast frames. If the network traffic comprises flood traffic, the flood packets are encapsulated with destination information corresponding to the special multicast group  508 , such as an IP address in the multicast group address space that is reserved for the special multicast group. As discussed, this ensures that the flood traffic will be routed to the fabric before being routed to a designated multicast group address (and unintended endpoints). Further, this enables the fabric to enforce policy, and limits transmission of the flood traffic to intended endpoints. Subsequently, the flood packets are forwarded to a next hop and ultimately received by the network fabric  510 . In some embodiments, the virtual switch may be a virtual tunnel endpoint (VTEP) that performs the encapsulation and forwarding. In some embodiments, when the encapsulated flood traffic is received by the fabric at an ingress leaf node, the encapsulated flood traffic may be de-encapsulated and re-encapsulated using a specific network virtualization encapsulation format, such as VXLAN. In certain embodiments, if the encapsulated flood traffic is already in VXLAN format, there is no de-encapsulation and re-encapsulation. 
     In one example, the flood traffic can be received at a leaf switch, such as leaf switch  304   N  of  FIG. 3 , connected to a physical network layer, such as network fabric  312 . The leaf switch  304   N  can be responsible for performing routing and policy enforcement functions. The leaf switch  304   N  can include a memory device where it stores a policy table. In one example, the flood traffic can be received at a leaf switch  304   N  that serves as the ingress switch for the flood traffic to network fabric  312 . 
     Upon the flood traffic being received by the leaf switch, the method continues to step  512  where the network fabric can determine one or more destination endpoint groups (dEPGs) by applying one or more applicable policies to the flood traffic. As discussed, an endpoint group (EPG) is a collection of similar endpoints that require similar policy. A policy defines the allowed communication between endpoints according to the EPGs to which they belong. A collection of endpoints can be associated with an endpoint group based on a number of different characteristics. For example, an endpoint group can be based on a subnet, an internet protocol (IP) address, a virtual local area network (VLAN), a virtual extensible local area network (VXLAN), a media access control (MAC) address, a domain name server (DNS) name or range, network services, security services, network storage, etc. or any combination thereof. Those that are skilled in the art will recognize that endpoint groups are very flexible and can be defined based on any number of different factors. Endpoint groups can be used to efficiently define policy within a network. By defining policies according to EPGs rather than individual endpoints, the scalability of the policy table can be greatly increased. 
     In one example, a leaf switch can determine a source EPG (sEPG) and one or more dEPGs by analyzing the flood traffic to identify the originating endpoint and the destination endpoint(s) and their respective EPGs. As will be appreciated by one of ordinary skill in the art, a destination EPG can also include a multicast group corresponding to the virtual network of the sEPG. A network device serving as the ingress leaf switch can identify the sEPG for all endpoints that are attached to it. Furthermore, in some instances the ingress leaf switch may also be able to identify the dEPG(s) based on the characteristics of previous network traffic saved or cached at the particular ingress switch. Alternatively, if the ingress switch is not able to identify the dEPG(s), it can tag the flood traffic data with the appropriate sEPG and forward it to a spine switch  302   N  for further routing. The flood traffic will then be routed through the fabric and arrive at one or more appropriate egress leaf switches that are associated with the destination endpoint(s). The egress leaf switch can extract the sEPG information from the data packet because it was previously added by the ingress leaf switch. The egress leaf switches can also identify the dEPGs because the destination endpoints are attached to the egress leaf switches. 
     Once the sEPG and dEPG(s) are identified, the network device can perform a policy lookup. As discussed, policies define the nature of communications between EPGs. An example of a policy may be to allow traffic between two endpoint groups. Another example of a policy may be to make a copy of data packets between two endpoint groups. As one that is skilled in the art will recognize, a policy is a flexible tool that can be used to define numerous functions within a network. Other examples of policies include redirect, deny, change quality of service (QoS), encrypt, or drop actions. 
     A network device can perform a policy lookup by accessing the policy table stored on its local memory device. In one embodiment, a policy table can be stored using a ternary content-addressable memory (TCAM). By using a TCAM, the network device can determine if a policy is present in a single lookup operation. Alternatively, the network device may use other forms of memory such as RAM, Flash, EEPROM, etc. to store the policy table. 
     In the unicast case, the policy lookup is based on the sEPG and dEPG.  FIG. 6  illustrates an example of a matrix  600  of policies among endpoint groups. The matrix form in  FIG. 6  is used for ease of readability and to facilitate the understanding of how a policy table works. However, a policy table need not be stored in a matrix format or any particular type of data structure. In matrix  600 , each box lists the applicable policy or policies between a particular source endpoint group and a destination endpoint group. In one example, the network device can perform a lookup for sEPG=EPG 1 and destination EPG=EPG 1 to determine the appropriate policy for a packet that is traveling from an endpoint that is part of EPG 1 to an endpoint that is also part of EPG 1. Accordingly, box  600   11  dictates that “Policy A” should be applied to traffic that travels from an endpoint that is part of EPG 1 to and an endpoint that is also part of EPG 1. Policy A may correspond to a policy that allows traffic to travel between the endpoints. 
     The nature of multicast (and broadcast and unknown unicast) makes policy enforcement slightly different for such cases. Although the sEPG can easily be determined at ingress because it is never a multicast address, the destination is an abstract entity—the multicast group may comprise endpoints from multiple EPGs. In these situations, a multicast EPG can be used for policy enforcement. These groups are defined by specifying a multicast address range or ranges. Policy is then configured between the sEPG and the multicast group. 
     The multicast group (EPG group corresponding to the multicast stream) is always the destination and never used as the sEPG. Traffic sent to a multicast group is either from the multicast source or a receiver joining the stream through an Internet Group Management Protocol (IGMP) join. Because multicast streams are non-hierarchical and the stream itself is already in the forwarding table (using IGMP join), multicast policy is always enforced at ingress. This prevents the need for multicast policy to be written to egress leaves. 
     In some embodiments, the same policies are applied in a bidirectional fashion. For example, box  600   12  provides for “Policy B” to be applied to traffic from EPG 1 to EPG 2, and box  600   21  provides for “Policy B” to also be applied to traffic from EPG 2 to EPG 1. Alternatively, policies can be applied differently for data that is going in one direction versus another. For example, box  600   13  provides for both “Policy C” and “Policy D” to be applied to traffic from EPG 1 to EPG 3 while box  600   31  provides only for “Policy C” to be applied to the data that travels in the opposite direction, from EPG 3 to EPG 1. Here, “Policy C” may be used to allow traffic to flow in both directions. However, “Policy D” may be used to change the quality of service (QoS) of the traffic in only one of the directions. 
     In some embodiments, a network can control data traffic by using a whitelist model wherein a policy must be present to allow communication. For example, box  600   32  defines the policies that govern traffic from EPG 3 to EPG 2. However, because this box does not contain any policies, traffic would not be allowed to flow from EPG 3 to EPG 2 under a whitelist model. Conversely, box  600   23  includes “Policy E” that governs traffic from EPG 2 to EPG 3. Hence, under a whitelist model, this example would allow unidirectional traffic from EPG 2 to EPG3. Alternatively, a network can employ a blacklist model in which all traffic is permitted unless a particular policy exists to prevent it. 
     In addition to the sEPG and dEPG(s), a policy can be defined according to other characteristics. In some embodiments, a policy can be further defined in accordance with the protocol that is associated with the data packet. For example, a policy can dictate that hypertext transfer protocol (HTTP) traffic between EPG 1 and EPG 2 is allowed and that file transfer protocol (FTP) traffic should be redirected. Accordingly, the network device can analyze the data packet to determine that the transport layer (Layer 4) port number is 80, which is associated with HTTP, and permit the traffic. Similarly, the network device can redirect a data packet having a transport layer port number of 20 or 21, which is associated with FTP. Those that are skilled in the art will recognize that the applicable standards define many port numbers and the associated services. Thus, the network device can perform a policy lookup using criteria that includes the sEPG, the dEPG, and the port numbers associated with a transport layer service. 
     Turning back to  FIG. 5 , the network device performs the policy lookup to determine if a policy is available. As discussed, this determination can be made in a single lookup operation with the use of a TCAM. In some instances, a policy may not be available because the policy table on the memory of the network device is full and does not have room to store additional policies. Alternatively, a policy may not be available because the network device does not have the resources or capability to enforce the policy. For example, a network operator may wish to implement a new policy that requires all traffic between two EPGs to be encrypted. However, the network device may not have the hardware or software resources to perform the encryption, thus making it impractical to store the encryption policy on the network device&#39;s policy table. 
     If an appropriate policy is identified, the policy is enforced. Enforcement of the policy may involve a number of actions such as allowing the traffic to continue, redirecting the traffic, changing the quality of service, or copying the data packet. In addition, the network device may also apply a tag to the data packet or set one or more bits in the data packet to mark the enforcement of the policy. In the case where the policy is enforced at an ingress switch, the bits or tag can be read by the egress switch and allow the packet to be forwarded without duplicating the policy enforcement process. Once the policy is applied, the flood traffic can be forwarded to its appropriate endpoint(s) at  514 . 
     Alternatively, if the network device determines that a policy is not available, the flood traffic data is forwarded to a policy enforcement proxy. The policy enforcement proxy can provide a failsafe mechanism for data packets that would otherwise be dropped in a network employing a whitelist model. The policy enforcement proxy can store network policies that could not be stored on the network device&#39;s local memory either because the memory was full or because the network device does not have the capability to enforce the policy. In some embodiments, the policy enforcement proxy can be a server that has significantly more processing and memory resources than a leaf switch. Alternatively, the policy enforcement proxy can be a separate switch that has excess computing/memory resources that can be allocated for performing the policy enforcement. Although the policy enforcement proxy can be part of the data center, it need not be physically collocated with the network, so long as it is communicatively coupled to the network device. 
     In some embodiments, the network device can modify the data packet prior to forwarding it to the policy enforcement proxy. As discussed above, the applicable policy is dictated by the sEPG and dEPG(s), as well as the transport protocol. When the policy lookup is performed at an egress switch, the egress switch determines the sEPG from the tag that was applied by the ingress switch and it determines the dEPG(s) according to the locally stored information that relates to all of its attached endpoints. If the policy is not available at the egress switch, it can modify the data packet to also include a tag that identifies the dEPG(s) so that the policy enforcement proxy can discern all necessary information directly from the contents of the packet. 
     In some embodiments, the data packets are encapsulated using a virtual extensible local area network (VXLAN) packet format.  FIG. 7  illustrates an example of a VXLAN data packet  700 . VXLAN defines a MAC-in-UDP (media access control in user datagram protocol) encapsulation scheme where the original Layer 2 frame  710  has a VXLAN header  708  added and is then placed in a UDP-IP packet. The VXLAN header  708  is an 8 byte field which has 1 byte allocated for VXLAN flags, 3 bytes allocated to the VNID field, and 4 bytes allocated to two separate reserved fields (3 bytes and 1 byte, respectively). The VXLAN header  708  together with the original Ethernet (L2) frame is stored in the UDP payload. To avoid growing the size of the data packet, the network device can utilize the reserved fields in the VXLAN header  708  to include the tags for the sEPG, dEPG. 
     For clarity of explanation, in some instances the present technology may be presented as including individual functional blocks including functional blocks comprising devices, device components, steps or routines in a method embodied in software, or combinations of hardware and software. 
     In some embodiments the computer-readable storage devices, mediums, and memories can include a cable or wireless signal containing a bit stream and the like. However, when mentioned, non-transitory computer-readable storage media expressly exclude media such as energy, carrier signals, electromagnetic waves, and signals per se. 
     Methods according to the above-described examples can be implemented using computer-executable instructions that are stored or otherwise available from computer readable media. Such instructions can comprise, for example, instructions and data which cause or otherwise configure a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. Portions of computer resources used can be accessible over a network. The computer executable instructions may be, for example, binaries, intermediate format instructions such as assembly language, firmware, or source code. Examples of computer-readable media that may be used to store instructions, information used, and/or information created during methods according to described examples include magnetic or optical disks, flash memory, USB devices provided with non-volatile memory, networked storage devices, and so on. 
     Devices implementing methods according to these disclosures can comprise hardware, firmware and/or software, and can take any of a variety of form factors. Typical examples of such form factors include laptops, smart phones, small form factor personal computers, personal digital assistants, rackmount devices, standalone devices, and so on. Functionality described herein also can be embodied in peripherals or add-in cards. Such functionality can also be implemented on a circuit board among different chips or different processes executing in a single device, by way of further example. 
     The instructions, media for conveying such instructions, computing resources for executing them, and other structures for supporting such computing resources are means for providing the functions described in these disclosures. 
     Although a variety of examples and other information was used to explain aspects within the scope of the appended claims, no limitation of the claims should be implied based on particular features or arrangements in such examples, as one of ordinary skill would be able to use these examples to derive a wide variety of implementations. Further and although some subject matter may have been described in language specific to examples of structural features and/or method steps, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to these described features or acts. For example, such functionality can be distributed differently or performed in components other than those identified herein. Rather, the described features and steps are disclosed as examples of components of systems and methods within the scope of the appended claims. Moreover, claim language reciting “at least one of” a set indicates that one member of the set or multiple members of the set satisfy the claim. 
     For clarity of explanation, in some instances the present technology may be presented as including individual functional blocks including functional blocks comprising devices, device components, steps or routines in a method embodied in software, or combinations of hardware and software. 
     Note that in certain example implementations, the optimization and/or placement functions outlined herein may be implemented by logic encoded in one or more tangible, non-transitory media (e.g., embedded logic provided in an application specific integrated circuit (ASIC), digital signal processor (DSP) instructions, software (potentially inclusive of object code and source code) to be executed by a processor, or other similar machine, etc.). The computer-readable storage devices, mediums, and memories can include a cable or wireless signal containing a bit stream and the like. However, when mentioned, non-transitory computer-readable storage media expressly exclude media such as energy, carrier signals, electromagnetic waves, and signals per se. 
     Methods according to the above-described examples can be implemented using computer-executable instructions that are stored or otherwise available from computer readable media. Such instructions can comprise, for example, instructions and data which cause or otherwise configure a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. Portions of computer resources used can be accessible over a network. The computer executable instructions may be, for example, binaries, intermediate format instructions such as assembly language, firmware, or source code. Examples of computer-readable media that may be used to store instructions, information used, and/or information created during methods according to described examples include magnetic or optical disks, flash memory, USB devices provided with non-volatile memory, networked storage devices, and so on. 
     Devices implementing methods according to these disclosures can comprise hardware, firmware and/or software, and can take any of a variety of form factors. Typical examples of such form factors include laptops, smart phones, small form factor personal computers, personal digital assistants, and so on. Functionality described herein also can be embodied in peripherals or add-in cards. Such functionality can also be implemented on a circuit board among different chips or different processes executing in a single device, by way of further example. 
     The instructions, media for conveying such instructions, computing resources for executing them, and other structures for supporting such computing resources are means for providing the functions described in these disclosures. 
     Although a variety of examples and other information was used to explain aspects within the scope of the appended claims, no limitation of the claims should be implied based on particular features or arrangements in such examples, as one of ordinary skill would be able to use these examples to derive a wide variety of implementations. Further and although some subject matter may have been described in language specific to examples of structural features and/or method steps, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to these described features or acts. For example, such functionality can be distributed differently or performed in components other than those identified herein. Rather, the described features and steps are disclosed as examples of components of systems and methods within the scope of the appended claims.