Chapter 6: Network OS

We are now ready to move from a single switch with purely local state, to the global, network-wide view maintained by a Network Operating System. The best way to think about a NOS is that it is like any other horizontally scalable cloud application. It consists of a collection of loosely coupled subsystems—as is often associated with a micro-service architecture—including a scalable and highly available key/value store.

This chapter describes the overall structure of a NOS using ONOS as a reference implementation. The focus is on the core abstractions that have emerged from experience implementing a wide range of control applications on top of ONOS, and using ONOS to manage an equally wide range of network devices. This chapter also discusses the critically important issues of scalable performance and high availability.

6.1 Architecture

Using ONOS as our model, the architecture of a Network OS is shown in Figure 30. It consists of three main layers:

  1. A collection of Northbound Interfaces (NBI) that applications use to stay informed about the network state (e.g. traverse topology graph, intercept network packets) and also to control the network data plane (e.g. program flow objectives via FlowObjective API introduced in Chapter 3).

  2. A Distributed Core that is responsible for managing network state and notifying applications about relevant changes in that state. Internal to the core is a scalable key/value store called Atomix.

  3. A Southbound Interface (SBI) constructed from a collection of plugins including shared protocol libraries and device-specific drivers.


Figure 30. Three-layer architecture of ONOS, hosting a set of control applications.

As Figure 30 suggests, the design is highly modular, with a given deployment configured to include the subset of modules it requires. We delay a discussion about the exact form for the modularity (e.g., Karaf, Kubernetes) until the concluding section, where we take up the issue of scalability. Until then, the focus is on the functional organization of ONOS.

There are three other things to note about Figure 30 before we get into the details about each layer. The first is the breadth of the NBI. If you think of ONOS as an operating system, this makes sense: All access to the underlying hardware, whether by a control program or a human operator, is mediated by ONOS. This means the union of all northbound APIs must be sufficient to configure, operate, and control the network. For example, the NBI includes gNMI and gNOI for configuration and operations, respectively. It also means the NBI includes a Topology API that Control Applications use to learn about changes in the underlying network state (e.g., ports coming up and down), along with the FlowObjective API used to control the underlying switches.

As an aside, while we generally characterize the applications that run on top of a Network OS as implementing the network control plane, there is actually a wide assortment of apps running on ONOS, implementing everything from a GUI that can be used to monitor the state of the network, to a traditional CLI that operators can use to issue directives.

Among the applications sitting on top of ONOS is a zero-touch management plane that provisions new hardware added to the network, making sure the right software, certificates, configuration parameters, and pipeline definition are installed. This example is illustrated in Figure 31, where one takeaway is that ONOS does not have a fixed NBI: there are potentially multiple layers of applications and services running on ONOS, each providing some value on top of the applications and services below it. Declaring zero-touch provisioning to be in ONOS versus on ONOS is an arbitrary statement, which points to an important way in which ONOS is not like a conventional operating system: There is no syscall-equivalent interface demarking the boundary between a privileged kernel and multiple user domains. In other words, ONOS currently operates in a single trust domain.


Figure 31. Example of a Zero-Touch Provisioning (ZTP) application taking a “role spec” for a switch being installed as input, with ONOS provisioning the switch accordingly.

The second thing to note about Figure 30 is that ONOS maps an abstract specification of behavior the control application wants to impose on the network onto the concrete instructions that need to be communicated to each switch in the network. Applications can select from a variety of means to affect the network operation. Some applications use high-level Intents, which are network-wide, topology-independent programming constructs. Others that require finer-grained control use Flow Objectives, which are device-centric programming constructs. Flow Objectives are much like Flow Rules, except they are pipeline-independent. Applications use them to control both fixed-function and programmable pipelines. As highlighted in Figure 32, doing this job in the face of varied forwarding pipelines is a complexity ONOS is explicitly designed to address.


Figure 32. ONOS manages the mapping of an abstract specification of network-wide behavior to a collection of per-device instructions.

The third thing to notice about Figure 30 is that information flows both “down” and “up” through ONOS. It’s easy to focus on applications using the ONOS NBI to control the network, but it is also the case that the southbound plugins pass information about the underlying network up to the ONOS core. This includes intercepting packets, discovering devices and their ports, reporting link quality, and so on. These interactions between the ONOS core and the network devices are handled by a set of adaptors (e.g., OpenFlow, P4Runtime), which hide the details of communicating with the devices, thereby insulating the ONOS core and the applications running on top of it from the diversity of network devices. For example, ONOS is being used to control proprietary switches, bare-metal switches, optical devices, and cellular base stations.

6.2 Distributed Core

The ONOS core is comprised of a number of subsystems, each responsible for a particular aspect of network state (e.g. topology, host tracking, packet intercept, flow programming). Each subsystem maintains its own service abstraction, where its implementation is responsible for propagating the state throughout the cluster.

Many ONOS services are built using distributed tables (maps), which are in turn implemented using a distributed key/value store. The store itself will be familiar to anyone who has looked at how modern cloud services are designed—it scales across a distributed set of servers, and implements a consensus algorithm to achieve fault tolerance in the event of failures. The specific algorithm used in ONOS is Raft, which is well described in a paper by Diego Ongaro and John Ousterhout. The website also provides a helpful visualization tool.

Further Reading

D. Ongaro and J. Ousterhout. The Raft Consensus Algorithm.

ONOS uses Atomix as its store. Atomix goes beyond the core Raft algorithm to provide a rich set of programming primitives that ONOS uses to manage the distributed state and to provide easy access to that state by the control apps.

This distributed approach is a common design paradigm, which results in a system that is both scalable (runs on enough virtualized instances to handle the request workload) and highly available (runs on enough instances to continue offering service in the face of failure). What’s specific to ONOS—or any Network OS, for that matter—is the set of maps it defines: the semantics of the keys it stores and the types of the values associated with those keys. It is this data model that makes a Network OS a Network OS (and not, say, a ride-share application or a social network). This section mostly focuses on this set of data models and the corresponding services built around them, although we start with a brief introduction to the primitives that Atomix supports.

6.2.1 Atomix Primitives

The preceding discussion introduced Atomix as a key/value store, which it is, but it is also accurate to describe Atomix as a general tool for building distributed systems. It is a Java-based system that includes support for:

  • Distributed data structures, including maps, sets, trees, and counters.

  • Distributed communication, including direct messaging and publish/subscribe.

  • Distributed coordination, including locks, leader elections, and barriers.

  • Managing group membership.

For example, Atomix includes AtomicMap and DistributedMap primitives. Both extend Java’s Map utility with additional methods. In the case of AtomicMap, the primitive performs atomic updates using optimistic locks, such that all operations are guaranteed to be atomic (and each value in a map has a monotonically increasing version number). In contrast, the DistributedMap primitive supports eventual consistency rather than guaranteed consistency. Both primitives support event-based notifications of changes to the corresponding map. Clients can listen for inserted/updated/removed entries by registering event listeners on a map.

Maps are the workhorse primitive used by ONOS, as we will see in the next subsection. We conclude this section by looking at another role that Atomix plays in ONOS: coordinating all the ONOS instances.1 There are two aspects to this coordination.


For the purpose of this discussion, assume ONOS is packaged as a whole and then scaled across multiple virtualized instances. An alternative partitioning of ONOS functionality into independently scaled microservices is discussed in Section 6.5.

First, as a horizontally scalable service, the number of ONOS instances running at any given time depends on the workload and the level of replication needed to guarantee availability in the face of failures. The Atomix group membership primitive is used to determine the set of available instances, making it possible to detect new instances that have been spun up and existing instances that have failed. (Note that the set of ONOS instances is distinct from the set of Atomix instances, with both able to scale independently. This and the next paragraph are focused on the ONOS instances.)

Second, the primary job of each instance is to monitor and control a subset of the physical switches in the network. The approach ONOS takes is to elect a master instance for each switch, where only the master issues (writes) control instructions to a given switch. All the instances are able to monitor (read) switch state. The instances then use the Atomix leader-election primitive to determine the master for each switch. Should an ONOS instance fail, the same primitive is used to elect a new master for the switches. The same approach is applied when a new switch comes online.

6.2.2 Services

ONOS builds on Atomix by defining a core set of tables (maps), which are in turn packaged as a collection of services available to control applications (and other services). A table and a service are two ways of looking at the same things: one is a collection of key/value pairs, and the other is the interface through which applications and other services interact with those pairs. Figure 33 depicts the respective layers, where the middle three components—Topology, Link, and Device—are example ONOS services.


Figure 33. ONOS provides a set of services, such as the Topology, Device, and Link Services, on top of a corresponding table (Map) implemented in Atomix.

Note that the Topology Service in Figure 33 does not have an associated map but instead indirectly accesses the maps defined by the Link and Device Services. The Topology Service caches the resulting network graph in memory, which gives applications a low-latency, read-only way to access the network state. The Topology Service also computes a spanning tree of the graph to ensure that all applications see the same broadcast tree.

As a whole, ONOS defines an interconnected graph of services, with Figure 33 showing just a small subgraph. Figure 34 expands on that view to illustrate some other aspects of the ONOS core, this time simplified to show the Atomix maps as an attribute of some (but not all) of the services.


Figure 34. Dependency graph of services (some with their own key/value maps) involved in building a Path Service.

There are several things to note about this dependency graph. First, the Path Service, which applications can query to learn end-to-end paths between host pairs, depends on both the Topology Service (which tracks the network graph) and a Host Service (which tracks the hosts connected to the network). Note that arrow directionality implies dependency, but as we saw in Figure 34, information flows in both directions.

Second, the Host Service has both a north-bound and a south-bound interface. The Path Service uses its north-bound interface to read host-related information, while the Host Location Provider uses its south-bound interface to write host-related information. The Host Service itself is little more than a wrapper around the Atomix Map that stores information about hosts. We return to the Provider abstraction in Section 6.4, but, in a nutshell, they are modules that interact with the underlying network devices.

Third, the Host Location Provider snoops network traffic—for example, intercepting ARP, NDP, and DHCP packets—to learn about hosts connected to the network, which it then provides to the Host Service. The Host Location Provider, in turn, depends on a Packet Service to help it intercept those packets. The Packet Service defines a device-independent means for other ONOS services to instruct the underlying switches to capture and forward selected packets to the control plane. ONOS services can also use the Packet Service to inject packets into the data plane.

Finally, while the service graph depicted in Figure 34 is designed to discover the network topology, there are many scenarios where the topology is fixed and known a priori. This often happens when the control plane is tailored for a particular topology, as is the case for the leaf-spine topology discussed throughout this book. For such scenarios, the Topology Service accepts configuration instructions from a control application (or high-level service) sitting above it in the dependency graph.2 ONOS includes such a configuration service, called Network Config, as depicted in Figure 35. Network Config, in turn, accepts configuration directives from either a human operator or an automated orchestrator, such as the example ZTP control application from Figure 31.


The Topology Service still collects ground-truth information from the underlying network, verifies that it matches the configuration directives passed in from above, and notifies the Network Config Service when there is a discrepancy.


Figure 35. Network Config Service, supporting both provisioning applications and human operators.

The sequence of examples we just walked through (Figures 33, 34, and 35) illustrates the basics of how ONOS is built from parts. For completeness, the following gives a summary of the most commonly used ONOS services:

Host: Records end systems (machine or virtual machine) connected to the network. Populated by one or more host discovery apps, generally by intercepting ARP, NDP, or DHCP packets.

Device: Records infrastructure device-specific information (switches, ROADMs, etc.), including ports. Populated by one or more device discovery apps.

Link: Records attributes of links connecting pairs of infrastructure devices/ports. Populated by one or more link discovery apps (e.g., by emitting and intercepting LLDP packets).

Topology: Represents the network as a whole using a graph abstraction. It is built on top of the Device and Link services and provides a coherent graph comprised of infrastructure devices as vertices and infrastructure links as edges. The graph converges on the network topology using an eventual consistency approach as events about device and link inventory are received.

Mastership: Runs leadership contests (using the Atomix leader-election primitive) to elect which ONOS instance in the cluster should be the master for each infrastructure device. In cases when an ONOS instance fails (e.g., server power failure), it makes sure a new master is elected as soon as possible for all devices left without one.

Cluster: Manages ONOS cluster configuration. It provides information about the Atomix cluster nodes as well as about all peer ONOS nodes. Atomix nodes form the actual cluster that is the basis for consensus, while the ONOS nodes are effectively mere clients used to scale control logic and I/O to network devices. Entries are set by ONOS using the Atomix membership primitive.

Network Config: Prescribes meta-information about the network, such as devices and their ports, hosts, links, etc. Provides outside information about the network and how the network should be treated by ONOS core and applications. Set by orchestrator apps, the ZTP control application, or manually by an operator.

Component Config: Manages configuration parameters for various software components in the ONOS core and applications. Such parameters (i.e., how to treat foreign flow rules, address or DHCP server, polling frequency, and so on) allow for tailoring the behavior of the software. Set by the operator according to the needs of the deployment.

Packet: Allows the core services and applications to intercept packets (packet in) and to emit packets back into the network. This is the basis for most of the host and link discovery methods (e.g., ARP, DHCP, LLDP).

The above services are used by nearly every application because they offer information about the network devices and their topology. There are, however, many more services, including ones that allow applications to program the behavior of the network using different constructs and different levels of abstraction. We discuss some of these in more depth in the next section, but for now, we note that they include:

Route: Defines a prefix to next-hop mapping. Set either by a control app or manually configured by an operator.

Mcast: Defines group IP, source and sink locations. Set by a control app or manually configured by an operator.

Group: Aggregates ports or actions in a device. Flow entries can point to a defined group to allow sophisticated means of forwarding, such as load-balancing between ports in a group, failover among ports in a group, or multicast to all ports specified in a group. A group can also be used for aggregating common actions of different flows, so that in some scenarios only one group entry is required to be modified for all the referencing flow entries instead of having to modify all of them.

Meter: Expresses a rate-limit to enforce a quality of service for selected network traffic handled by a device.

Flow Rule: Provides a device-centric, match/action pair for programming the data-plane forwarding behavior of a device. It requires that flow rule entries be composed in accordance with the device’s table pipeline structure and capabilities.

Flow Objective: Provides a device-centric abstraction for programming the forwarding behavior of a device in a pipeline-agnostic manner. It relies on the Pipeliner subsystem (see next section) to implement the mapping between table-agnostic flow objectives and table-specific flow rules or groups.

Intent: Provides a topology-agnostic way to establish flows across the network. High-level specifications, call intents, indicate various hints and constraints for the end-to-end path, including the type of traffic and the source and destination hosts, or ingress and egress ports to request connectivity. The service provisions this connectivity over the appropriate paths, and then continuously monitors the network, changing the paths over time to continue meeting the objectives prescribed by the intent in the face of varying network conditions.

Each of the above services comprises its own distributed store and notification capabilities. Individual applications are free to extend this set with their own services and to back their implementations with their own distributed stores. This is why ONOS provides applications with direct access to Atomix primitives, such as AtomicMaps and DistributedMaps. We will see examples of such extensions in the next Chapter when we take a closer look at SD-Fabric.

6.3 Northbound Interface

The ONOS NBI has multiple parts. First, there is a corresponding API for every service included in a given configuration of ONOS. For example, the “Topology” interface shown in Figure 30 is exactly the API offered by the Topology Service shown in Figure 33. Second, because ONOS permits applications to define and use their own Atomix tables, it is fair to consider the Atomix programmatic interface as another part of the ONOS NBI. Third, the ONOS NBI includes gNMI and gNOI. These are standardized interfaces, defined independent of ONOS but supported as part of the ONOS NBI. Note that the implementation sitting behind gNMI and gNOI are also ONOS services wrapped around Atomix maps. Finally, and most interestingly, ONOS offers a set of interfaces for controlling the underlying switches. Figure 30 depicts two: Flow Rules and Flow Objectives. The first is borrowed from OpenFlow and hence, is pipeline-aware. The second is pipeline-agnostic and the focus of the rest of this section.

There are three types of flow objectives: Filtering, Forwarding, and Next. Filtering objectives determine whether or not traffic should be permitted to enter the pipeline, based on a traffic Selector. Forwarding objectives determine what traffic is to be allowed to egress the pipeline, generally by matching select fields in the packet with a forwarding table. Next objectives indicate what kind of Treatment the traffic should receive, such as how the header is to be rewritten. If this sounds like an abstract three-stage pipeline:

Filtering → Forwarding → Next

then you understand the idea behind Flow Objectives. For example, the Filter objective (stage) might specify that packets matching a particular MAC address, VLAN tag, and IP address be allowed to enter the pipeline; the corresponding Forwarding objective (stage) then looks up the IP address in a routing table; and finally the Next objective (stage) rewrites the headers as necessary and assigns the packet to an output port. All three stages, of course, are agnostic as to exactly what combination of tables in the underlying switch is used to implement the corresponding sequence of match/action pairs.

The challenge is to map these pipeline-agnostic objectives onto the corresponding pipeline-dependent rules. In ONOS, this mapping is managed by the Flow Objective Service, as depicted in Figure 36. For simplicity, the example focuses on the selector (match) specified by a Filtering objective, where the key is to express the fact that you want to select a particular input port, MAC address, VLAN tag, and IP address combination, without regard for the exact sequence of pipeline tables that implement that combination.


Figure 36. Flow Objective Service manages the mapping of pipeline-agnostic objectives onto pipeline-specific rules.

Internally, the Flow Objective Service is organized as a collection of device-specific handlers, each of which is implemented using the ONOS device driver mechanism. The device driver behavior that abstracts the implementation of how flow objective directives should map to flow rule operations is called a Pipeliner. Figure 36 shows Pipeliners for two example switch pipelines.

Pipeliners are able to map flow objectives onto both flow rules (in the case of fixed-function pipelines) and P4-programmed pipelines. The example given in Figure 36 shows the former case, which includes a mapping to OpenFlow 1.3. In the latter case, Pipeliner leverages Pipeconf, a structure that maintains associations among the following elements:

  1. A model of the pipeline for each target switch.

  2. A target-specific driver needed to deploy flow instructions to the switch.

  3. A pipeline-specific translator to map flow objectives into the target pipeline.

Pipeconf maintains these bindings using information extracted from the .p4info file output by the P4 compiler, as described in Section 5.2.

Today, the “model” identified in (1) is ONOS-defined, meaning the end-to-end workflow for a developer involves being aware of both a P4 architecture model (e.g., v1model.p4) when programming the data plane and this ONOS model when programming the control plane using flow objectives. Eventually, these various layers of pipeline models will be unified, and in all likelihood, specified in P4.

Programmatically, flow objectives are a data structure packaged with associated constructor routines. The control application builds a list of objectives and passes them to ONOS to be executed. The following code example shows flow objectives being constructed to specify an end-to-end flow through the network. The process of applying them to the underlying devices is done elsewhere and not included in the example.

public void createFlow(
      TrafficSelector originalSelector,
      TrafficTreatment originalTreatment,
      ConnectPoint ingress, ConnectPoint egress,
      int priority, boolean applyTreatment,
      List<Objective> objectives,
      List<DeviceId> devices) {
   TrafficSelector selector = DefaultTrafficSelector.builder(originalSelector)

   // Optionally apply the specified treatment
   TrafficTreatment.Builder treatmentBuilder;
   if (applyTreatment) {
      treatmentBuilder = DefaultTrafficTreatment.builder(originalTreatment);
   } else {
      treatmentBuilder =



The above example creates a Next objective and a Forwarding objective, with the Next objective applying a Treatment to the flow. Minimally, that Treatment sets the output port, but optionally, it also applies the originalTreatment passed in as an argument to createFlow.

6.4 Southbound Interface

A critical part of ONOS’s flexibility is its ability to accommodate different control protocols. While the nature of control interactions and associated abstractions was certainly inspired by the OpenFlow protocol, ONOS is designed to ensure that the core (and the applications written on top of the core) are insulated from the specifics of the control protocol.

This section takes a closer look at how the ONOS accommodates multiple protocols and heterogeneous network devices. The basic approach is based on a plugin architecture, with two types of plugins: Protocol Providers and Device Drivers. The following subsections describe each, in turn.

6.4.1 Provider Plugins

ONOS defines a Southbound Interface (SBI) plugin framework, where each plugin defines some southbound (network-facing) API. Each plugin, called a Protocol Provider, serves as a proxy between the SBI and the underlying network, where there is no limitation on what control protocol each can use to communicate with the network. Providers register themselves with the SBI plugin framework, and can start acting as a conduit for passing information and control directives between ONOS applications and core services (above) and the network environment (below), as illustrated in Figure 37.


Figure 37. ONOS Southbound Interface (SBI) is extended by Provider Plugins.

Figure 37 includes two general kinds of Provider plugins. The first type is protocol-specific, with OpenFlow and gNMI being typical examples. Each of these Providers effectively bundles the API with the code that implements the corresponding protocol. The second type—of which DeviceProvider, HostProvider, and LinkProvider are the examples shown in the figure—interact indirectly with the environment using some other ONOS service. We saw an example of this in Section 6.2.2, where Host Location Provider (an ONOS service) sits behind HostProvider (an SBI plugin); the latter defines the API for host discovery, and the former defines one specific approach to discovering hosts (e.g., using Packet Service to intercept ARP, NDP and DHCP packets). Similarly, the LLDP Link Provider Service (corresponding to the LinkProvider SBI plugin) uses Packet Service to intercept LLDP and BDDP packets to surmise links between infrastructure devices.

6.4.2 Device Drivers

In addition to insulating the core from protocol specifics, the SBI framework also supports Device Drivers plugins as a mechanism to insulate code (including Providers) from device-specific variations. A Device Driver is a collection of modules, each of which implements a very narrow facet of control or configuration capabilities. As with the Protocol Providers, no limitations are placed on how the device driver chooses to implement those capabilities. Device drivers are also deployed as ONOS applications, which allows them to be installed and uninstalled dynamically, allowing operators to introduce new device types and models on the fly.

6.5 Scalable Performance

ONOS is a logically centralized SDN controller and must ensure that it is able to respond to a scalable number of control events in a timely way. It must also remain available in the face of failures. This section describes how ONOS scales to meet these performance and availability requirements. We start with some scale and performance numbers to provide a sense of the state-of-the-art in centralized network control (at the time of writing):

  • Scale: ONOS supports up to 50 network devices; 5000 network ports; 50k subscribers, 1M routes; and 5M flow rules/groups/meters.

  • Performance: ONOS supports up to 10k configuration ops/day; 500k flow ops/sec (sustained); 1k topology events/sec (peak); 50ms to detect port/switch up events; 5ms to detect port/switch down events; 3ms for flow ops; and 6ms for hand-over events (RAN).

Production deployments run at least three instances of ONOS, but this is more for availability than performance. Each instance runs on a 32-Core/128GB-RAM server and is deployed as a Docker container using Kubernetes. Each instance bundles an identical (but configurable) collection of core services, control applications, and protocol providers, and ONOS uses Karaf as its internal modularity framework. The bundle also includes Atomix, although ONOS supports an optional configuration that scales the key/value store independently from the rest of ONOS.


Figure 38. Multiple ONOS instances, all sharing network state via Atomix, provide scalable performance and high availability.

Figure 38 illustrates ONOS scaling across multiple instances, where the set of instances shares network state via Atomix Maps. The figure also shows that each instance is responsible for a subset of the underlying hardware switches. Should a given instance fail, the remaining instances use the Atomix leader-election primitive to select a new instance to take its place, thereby ensuring high availability.

A refactoring of ONOS to more closely adhere to a microservice architecture is also underway. The new version, called µONOS, leverages ONOS’s existing modularity but packages and scales different subsystems independently. Although, in principle, each of the core services introduced in this chapter could be packaged as an independent microservice, doing so is much too fine-grained to be practical. Instead, µONOS adopts the following approach. First, it encapsulates Atomix in its own microservice. Second, it runs each control application and southbound adaptor as a separate microservice. Third, it partitions the core into four distinct microservices: (1) a Topology Management microservice that exports a Network Graph API; (2) a Control Management microservice that exports a P4Runtime API; (3) a Configuration Management microservice that exports a gNMI API; and (4) an Operations Management microservice that exports a gNOI API.