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(As-is-built 2021-0910-2806)

This page describes an enhancement to the DFDL language, the Daffodil DFDL Layering extension, enabling DFDL schemas using this extension to describe data formats in such a way as to enable algorithmic computations of CRCs, Checksums, Parity bits, etc. These are checked when parsing, to insure data validity and/or well-formedness, and are recomputed and incorporated into the data infoset when unparsing data. 

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• Layer transformations are able to read and write DFDL variables, which means they need access to the parse or unparse state of the computation.
• When parsing, the DFDL variables read or written by a layer transformation must be defined or it is a runtime an SDE. 
• When unparsing, DFDL variables read by a layer transformation must either have a value, or the read must cause causes suspension of the layer transformation if the variable is not yet defined. This layer transformation must be is resumed once the variable gets its value. 
• When unparsing, computations of dfdl:outputValueCalc, or dfdl:length or other runtime-valued expressions can reference DFDL variables that will are (to be) set by a layer transform. Such computations must will suspend if the layer transformation has not yet set the variable, and resume when the variable is set. 

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