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• 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 criterions 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 upon default.p4) 

Anchor
extensions extensions

Non-standard match and actions (WIP)

ONOS already provides means for applications to process traffic according to match and actions on non-standard header fields..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).

ApplicationId myAppId = ...;
DeviceId myDeviceId = ...''
Bmv2DeviceContext myContext = ...;

Bmv2Configuration myConfiguration = myContext.configuration();

Ip4Prefix dstPrefix = Ip4Prefix.valueOf("192.16.184.0/24");

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();

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