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P4 is a domain-specific language designed to allow programming of protocol-independent packet processors. Behavioral Model v2 (BMv2) is the reference P4 software switch. Initial support for this type of device has been included into ONOS 1.6 (Goldeneye) with 2 goals: i) provide the research community with a platform to experiment with P4-based applications and ii) define a common groundwork to support programmable data planes in the next versions of ONOS.
This document will guide you through the necessary steps to program a network of BMv2 (virtual) devices using ONOS. This document assumes you are already familiar with ONOS, P4 and BMv2. In other words, we assume you already know how to build and run ONOS, write a P4 program, build and run BMv2.
Features at a glance
By using ONOS 1.6, you'll be able to program a network of BMv2 devices with all the benefits of a logically centralized SDN platform. The following features are currently supported:
- Device discovery: connection / disconnection events
JSON configuration swap
Packet-ins and packet-outs
Match-action table population (via flow rules, flow objectives, or intents)
Port statistics collections
Flow statistics collection
The figure below sketches the high-level architecture of the BMv2 integration in ONOS (click to zoom).
On the northbound, ONOS provides a new Java API called "BMv2 Device Context Service" that can be used by applications to specify at runtime the JSON configuration of a given BMv2 device. Match-action tables can be populated using existing northbound APIs such as flow rule, flow objective or intents, with native support for non-standard P4 match and actions (via extension selectors and treatments).
On the southbound, ONOS speaks with BMv2 using Thrift. This project has been based on a customized version of the BMv2 “simple_switch” target that, differently from the original one, supports primitives to send packet-in events to the controller and to receive and transmit packet-outs. The source code of onos-bmv2 is available here.
BMv2 device context
In order to enforce a given JSON configuration on a device, applications need to provide a “BMv2 Device Context”. Device contexts are used to bind together in a Java class a BMv2 JSON configuration and an “Interpreter” implementation. Interpreters are used by ONOS to “understand” a given P4 program. They provide a mapping between ONOS objects and program-specific P4 objects (e.g. headers, actions, table names, etc.), allowing existing services and apps (e.g. host tracking, LLDP discovery, ARP proxy, reactive forwarding, etc.) to work with virtually any P4 program.
The Interpreter interface defines 3 types of mapping:
ONOS table ID ↔ P4 table name
Criterion's type ↔ P4 header instance’s field name
- Flow rule's treatment instance → BMv2 action instance
While for criteria and tables it is possible to specify a 1-to-1 relationship through a map, for Instructions the same is not possible or at least it's not convenient. The reason is that Instructions in ONOS are modeled after OpenFlow actions (which are protocol-dependent), while flow rule treatments (to be applied as a consequence of a match) are usually defined as a list of multiple instructions. In P4 instead, actions are defined as a compound of low-level protocol-independent primitives (not expressible using ONOS Instructions), and, most important, P4 allows to specify only one action per table entry. Extracting the "meaning" of a given treatment instance and mapping it to a P4 action is not straightforward and it's usually program-specific. That’s why we expect a P4 programmer using ONOS to write its own interpretation logic (i.e. Java code) that can map a given treatment instance to a BMv2 action instance.
When devices connect for the first time to ONOS a “default” context is automatically applied, triggering a device configuration swap and exposing to the system a default interpreter. Such a context is used to provide a minimum set of data plane capabilities for basic ONOS services and apps to work. The default context is based on a default.json BMv2 configuration, (compiled from default.p4) and a default interpreter implementation.
FAQ regarding interpreters
- Do I necessarily need to write an interpreter for my P4 program?
No, interpreters are optional, meaning that a context can be created with an “empty” interpreter. In this case, you can’t expect other ONOS services to work with that given context. When not using an Interpreter or when creating flow rules based on non-standard match or actions, developers can use BMv2 extension treatment and selectors.
- Do I need to provide a mapping for all the headers and actions defined in my P4 program?
No, you can provide a mapping for only some of them. The general advice is to provide a mapping for those criteria and treatments used by the other ONOS services and applications you need in order to run your applications. Most of the times, you can re-use the default interpreter (provided that you write your own P4 program as an extension to default.p4)
Non-standard match and actions
BMv2 extension selectors and treatments can be used to express flow rules with match fields or actions for which either a mapping is not provided by the interpreter, or for which such a mapping is not possible using standard ONOS types. The following code example shows how to create a flow rule that uses these extensions. Header fields and actions can be referenced using the same name used in the P4 program, while the BMv2 configuration is needed by the extension builder to properly format the values according to the specific P4 program (e.g. generate the appropriate byte representation of a given match field).
ONOS+P4 development environment
To make it easy for you to get started, we prepared a repository with all you need to build and run a Mininet network of BMv2 devices that connect to ONOS. Follow the instructions at this link to get started:
Exporting environment variables in the shell profile
To get the most from the tools and instructions discussed in the following sections, it is highly recommended that you add this line to your shell configuration profile (.bash_aliases, .profile, etc.):
bmv2.py Mininet script
- Sample commands to run a network of bmv2 devices connected to ONOS
- Usage with onos.py
BMv2 Device Context Service API
- Prototype API for device configuration management, to be promoted to core APIs in the future
- How to set a context for a given device?
- Configuration swap enforcement
- Why do I need to register the interpreter's class loader?
Known issues (WIP)