seg6: add support for optional attributes in SRv6 behaviors
Before this patch, each SRv6 behavior specifies a set of required attributes that must be provided by the userspace application when such behavior is going to be instantiated. If at least one of the required attributes is not provided, the creation of the behavior fails. The SRv6 behavior framework lacks a way to manage optional attributes. By definition, an optional attribute for a SRv6 behavior consists of an attribute which may or may not be provided by the userspace. Therefore, if an optional attribute is missing (and thus not supplied by the user) the creation of the behavior goes ahead without any issue. This patch explicitly differentiates the required attributes from the optional attributes. In particular, each behavior can declare a set of required attributes and a set of optional ones. The semantic of the required attributes remains *totally* unaffected by this patch. The introduction of the optional attributes does NOT impact on the backward compatibility of the existing SRv6 behaviors. It is essential to note that if an (optional or required) attribute is supplied to a SRv6 behavior which does not expect it, the behavior simply discards such attribute without generating any error or warning. This operating mode remained unchanged both before and after the introduction of the optional attributes extension. The optional attributes are one of the key components used to implement the SRv6 End.DT6 behavior based on the Virtual Routing and Forwarding (VRF) framework. The optional attributes make possible the coexistence of the already existing SRv6 End.DT6 implementation with the new SRv6 End.DT6 VRF-based implementation without breaking any backward compatibility. Further details on the SRv6 End.DT6 behavior (VRF mode) are reported in subsequent patches. From the userspace point of view, the support for optional attributes DO NOT require any changes to the userspace applications, i.e: iproute2 unless new attributes (required or optional) are needed. Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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