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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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• IPv4 packets - See the daffodil-test src/test/resources/org/apache/daffodil/layers/IPv4.tdml test, related IPv4.dfdl.xsd schema. This is fake, but very much like IPv4. 
  
  • There is also a PR to update the ethernetIP github DFDL schema project to compute checksums.  https://github.com/DFDLSchemas/ethernetIP/pull/1. However, there are checksums in the IPv4Header, UDP, TCP, ICMP, etc. So the work involved here is extensive.
  • This work likely should be done only after

    Jira
    server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key DAFFODIL-1927

    is complete so that the pluggable layers that compute these checksums can be created as a plugin. 
• GPS data with parity - See the GPS-SPS DFDL schema on the DFDL schemas github site.
  • TBD: currently a pull request: https://github.com/DFDLSchemas/gps-sps/pull/1
  • This work likely should be done only after

    Jira
    server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key DAFFODIL-1927

    is complete so that the pluggable layers for this parity can be done as a plugin. 
• Check-digit computation - See the daffodil-test src/test/resources/org/apache/daffodil/layers/TestCheckDigit.tdml test file and related checkDigit.dfdl.xsd schema. 

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