17. Conformance - Interaction with SHOULD and MAY following implementations

17.3 Interaction With Other Implementations In order to be conformant, an implementation MUST be able to interoperate with any implementation that satisfies all MUST and REQUIRED level requirements. Where the implementation has implemented optional behaviors, the implementation MUST be able to fall back to mandated behaviors if the implementation it is interacting with has not implemented those same behaviors. Where the other implementation has implemented optional behaviors not implemented by this implementation, the conformant implementation MUST be able to provide the mandated level behaviors that allow the other implementation to fall back to using only mandated behaviors.