The content of this page is still work in progress
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 the goal of providing 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 run ONOS locally, write a P4 program, compile it for BMv2, build and run BMv2.
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:
JSON configuration swap
Packet-ins and packet-outs
Match-action table population (via flow rule/objective/intent service)
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.
In order to enforce a given JSON configuration on a given device, applications needs 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
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 wouldn't’ be convenient. The reason is that Instructions in ONOS are modeled after OpenFlow actions (which are protocol-dependent) and treatments (to be applied as a consequence of a match) are usually defined as a list of 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 might be progra-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 configuration swap on the device 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 default interpreter implementation.
ONOS already provides means for applications to process traffic according to match and actions on non-standard header fields...
ApplicationId myAppId = ...; DeviceId myDeviceId = ...'' Bmv2DeviceContext myContext = ...; Bmv2Configuration myConfiguration = myContext.configuration(); Ip4Prefix dstPrefix = Ip4Prefix.valueOf("184.108.40.206/24"); ExtensionSelector extSelector = Bmv2ExtensionSelector.builder() .forConfiguration(myConfiguration) .matchExact("standard_metadata", "ingress_port", 10) .matchLpm("ipv4", "dstAddr", dstPrefix.address().toOctets(), dstPrefix.prefixLength()) .build(); ExtensionTreatment extTreatment = Bmv2ExtensionTreatment.builder() .forConfiguration(myConfiguration) .setActionName("next_hop") .addParameter("nhop_id", 4) .build(); FlowRule rule = DefaultFlowRule.builder() .forDevice(myDeviceId) .fromApp(myAppId) .forTable(0) .withSelector(DefaultTrafficSelector.builder() .extension(extSelector, myDeviceId) .build()) .withTreatment(DefaultTrafficTreatment.builder() .extension(extTreatment, myDeviceId) .build()) .build();