So in PCAP format we find something like this: <element name="length" type="xs:int" dfdl:outputValueCalc="{ dfdl:contentLength(../payload) + 20 }"> <element name="payloadLength" type="xs:int" dfdl:inputValueCalc="{ ../length - 20 }" /> <element name="payload" dfdl:length="{ ../payloadLength }"> <complexType>.....</complexType> </element> To make this unparse we really need the payloadLength element to carry both an IVC and an OVC. Otherwise when unparsing the dfdl:length expression on the payload element can't be meaningful. We have an erratum that states that length expressions are always re-evaluated when unparsing, so as to produce a target length that can be used to insert padding/fillByte. However, the spec currently says you cannot have both IVC and OVC on the same element. I do not recall why we have this restriction, other than trying to prohibit things we didn't think were necessary, but it seems in this situation to be needed. Comments? Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com Please note: Contributions to the DFDL Workgroup's email discussions are subject to the OGF Intellectual Property Policy <http://www.ogf.org/About/abt_policies.php>