May 2018 - dfdl-wg - lists.ogf.org

Tracker/Issue 233 - minor corrections
by Mike Beckerle 15 May '18

15 May '18

The phrase "When specified in a pattern, the 32-bit codepoint immediately following the pad escape is the pad character. " The term "32-bit codepoint" should be replaced by "character". In the phrase: For example, in the pattern "* #0 o''clock"... The font for the example pattern needs to be changed so that the two apostrophe/single-quotes after the lower case 'o' can be visually distinguished from the double quotes surrounding the example. 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>

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Action 302 - backup of redmine issue trackers - 2018-05-15
by Mike Beckerle 15 May '18

15 May '18

I captured xhtml from atom feed for all open trackers which includes all the descriptions, and for each tracker with comments/updates, there is a separate xhtml page (from atom feed reader) with that content. I packaged it all in a zip file. I cannot attach the zip file of this content, as gmail doesn't allow zip file attachments. Here is a link to the file on google drive: https://drive.google.com/open?id=1tEQnr2HrC7_G6hmTPxSfGN6oCYdlBq2j I have added a calendar reminder so that every 3 months the DFDL Workgroup chairpersons will get a reminder to repeat this process. With that I believe we can close the action. 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>

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Latest OGF DFDL WG Call Minutes
by Steve Hanson 15 May '18

15 May '18

Please find minutes from the latest call at https://redmine.ogf.org/projects/dfdl-wg/news Regards Steve Hanson IBM Hybrid Integration Architect, IBM DFDL, Co-Chair, OGF DFDL Working Group smh(a)uk.ibm.com tel:+44-1962-815848 mob:+44-7717-378890

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Typographical correction % -> %% - Section 31
by Mike Beckerle 15 May '18

15 May '18

Section 31.2, 31.3, 31.4 specify dfdl:escapeEscapeCharacter="%" This needs to be "%%". Or needs to be changed to use english prose to say the dfdl:escapeEscapeCharacter is '%' (requires property to be defined as dfdl:escapeEscapeCharacter="%%" since % are used for character entities syntax in DFDL string literals). Or perhaps better would be to change the usage of % in these examples to use $ or other neutral character to avoid the confusion. 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>

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clarification needed: section 14.2.1 potentially trailing group
by Mike Beckerle 15 May '18

15 May '18

Section 14.2.1 introduces the term "potentially trailing group". It is referred to only in the next paragraph which defines "Trailing or Actually Trailing". Presumably this was done because it was intended that separator suppression would consider a potentially trailing group to have some separator suppression behavior. However, all subsequent discussion of separator suppression I find discusses only separators between elements. So we should either drop "potentially trailing group" and its definition, and modify the definition of "Trailing or Actually Trailing" to specify only elements.... or fix everyplace where things are specific to elements to refer to terms (term meaning element or model group). I suggest it is ok to eliminate "potentially trailing group". First, it simplifies things. If you want separator suppression behavior, well it applies only to potentially trailing things, and those things must be elements. Since only elements can be optional (only elements can have min/max occurs in DFDL), this means if you want separator suppression behavior, the separated things must be elements, period. Any model group, regardless of whether it has framing syntax, zero-length, whatever, would be considered required, and elements prior to it within a sequence group could not be considered potentially trailing and so separatorSuppressionPolicy would never result in suppression. I do not think we lose any expressive power in DFDL by requiring separator suppression to work only on elements. It perhaps requires you to put an element decl around something you might not want to. But there are lots of things in DFDL that require use of extra tiers of elements. Daffodil does not implement potentially trailing model groups, or if it does, it's super buggy. DFDL4S is binary only so doesn't have this stuff. What does IBM DFDL do here? Do you allow a sequence like: <sequence> optional element A optional element B <sequence> optional element C </sequence> optional element D </sequence> to have separator suppression that applies to elements A and B, or only to element D ? 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>

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Confirmed: OGF DFDL Working Group Call (15 May 15:00 GDT)
by Steve Hanson 15 May '18

15 May '18

Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

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Agenda for OGF DFDL WG Call today
by Steve Hanson 15 May '18

15 May '18

Please find agenda for call on Redmine at https://redmine.ogf.org/dmsf_files/13576?download= Regards Steve Hanson IBM Hybrid Integration Architect, IBM DFDL Co-Chair, OGF DFDL Working Group smh(a)uk.ibm.com tel:+44-1962-815848 mob:+44-7717-378890

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Re: [DFDL-WG] Action 287: Find a way to handle a variable path step in DFDL expression - Re: DFDL4S use of wildcard/regex in length path expression
by Steve Hanson 15 May '18

15 May '18

Hi Michele Any update for the DFDL WG on this action item ? Regards Steve Hanson IBM Hybrid Integration, Hursley, UK Architect, IBM DFDL Co-Chair, OGF DFDL Working Group smh(a)uk.ibm.com tel:+44-1962-815848 mob:+44-7717-378890 Note: I work Tuesday to Friday From: Michele Zundo <michele.zundo(a)esa.int> To: Steve Hanson <smh(a)uk.ibm.com> Cc: "dfdl-wg(a)ogf.org" <dfdl-wg(a)ogf.org>, Joaquim Oliveira <joaquim.oliveira(a)deimos.com.pt>, Maurizio De Bartolomei <Maurizio.De.Bartolomei(a)esa.int>, Mike Beckerle <mbeckerle.dfdl(a)gmail.com>, dfdl4s(a)eopp.esa.int Date: 13/11/2017 19:27 Subject: Re: [DFDL-WG] Action 287: Find a way to handle a variable path step in DFDL expression - Re: DFDL4S use of wildcard/regex in length path expression Dear Steve, You will have to bear with us. We have been very busy closing the 2017 contract as well as setting up the one for 2018 covering DFDL4S that we just released fixing quite few problems affecting our users. We plan in the next month to go through the pending new technical issues (the one you mention is one of them) to see if/how they can fit in DFDL4S evolution. Of interest might also be that we secure funding to implement a C++ version of our library (called UMBRA) to address performance and C++ language compatibility. Will be a native C++ version with associated tool to work with schemas. Activity will start Jan 2018 for 2 years. I will come back when we finalise the wildcard/regex story. Sent from my iPhone On 13 Nov 2017, at 20:00, Steve Hanson <smh(a)uk.ibm.com> wrote: Michele, Joaquim Any update on this for the DFDL WG call tomorrow (Tuesday 14th) ? Regards Steve Hanson IBM Hybrid Integration, Hursley, UK Architect, IBM DFDL Co-Chair, OGF DFDL Working Group smh(a)uk.ibm.com tel:+44-1962-815848 mob:+44-7717-378890 From: Michele Zundo <michele.zundo(a)esa.int> To: Steve Hanson <smh(a)uk.ibm.com> Cc: Mike Beckerle <mbeckerle.dfdl(a)gmail.com>, "dfdl-wg(a)ogf.org" < dfdl-wg(a)ogf.org>, Maurizio De Bartolomei <Maurizio.De.Bartolomei(a)esa.int>, Joaquim Oliveira <joaquim.oliveira(a)deimos.com.pt> Date: 04/10/2017 09:02 Subject: Re: [DFDL-WG] Action 287: Find a way to handle a variable path step in DFDL expression - Re: DFDL4S use of wildcard/regex in length path expression Dear Steve, nice to hear from you. We are in process of compiling tasks to evolve our DFDL implementation in 2018 and have been discussing a new task for this. I will check with others and let you know the status regarding these aspects. Regards PS note that the responsible on our contractor side now has now changed and is Mr. Olivera (in copy to this e-mail) On 3 Oct 2017, at 19:25 , Steve Hanson <smh(a)uk.ibm.com> wrote: Hi Michele This problem was discussed on the DFDL WG call today. We wondered whether you had made any progress with your contractor? We also discussed an alternative solution, which is to use XPath 2.0 wildcards. This works as long as the wildcard can only ever match one element at that level in the infoset. You would need to change your xsd so that the variability occurred at a specific level in the model. Your example might then look something like: "contentLength(/Packet_Data_Field/*/Packet_Secondary_Header, 'bytes')" Does that work for you? Regards Steve Hanson IBM Hybrid Integration, Hursley, UK Architect, IBM DFDL Co-Chair, OGF DFDL Working Group smh(a)uk.ibm.com tel:+44-1962-815848 mob:+44-7717-378890 From: Michele Zundo <michele.zundo(a)esa.int> To: Mike Beckerle <mbeckerle.dfdl(a)gmail.com> Cc: "Rui Mestre $DME$" <rui.mestre(a)deimos.com.pt>, " dfdl-wg(a)ogf.org" <dfdl-wg(a)ogf.org>, Maurizio De Bartolomei < Maurizio.De.Bartolomei(a)esa.int> Date: 26/01/2017 04:36 Subject: Re: [DFDL-WG] Action 287: Find a way to handle a variable path step in DFDL expression - Re: DFDL4S use of wildcard/regex in length path expression Sent by: "dfdl-wg" <dfdl-wg-bounces(a)ogf.org> Dear Mike, Thanks for the elaborate expkanation and analysis. I will pass this to our contractor and see what are the possibilities and time scales for any intervention. Note that in the past we had discussed the implementation of variabkes in out parser but put it on hold due to lack of resources. I will let you and the rest of the WG know the output of thr discussion, although might take some time. Regards Michele Zundo Sent from my iPhone On 19 Jan 2017, at 18:16, Mike Beckerle <mbeckerle.dfdl(a)gmail.com> wrote: I have analyzed this use of (.*) notation in the Sentinel2X-bandTMISPData.xsd file. Below is my discussion of what this is, why it is used, the alternative to it, and why it is more problematic than it seems at first, as an addition to DFDL. There are two instances of (.*) in the Sentinel schema files. What it expresses is a partial step-name wildcard. This is not a variation on the XPath * notation, as that matches any step, and can't match a partial name. In the usage in the Sentinel schema, this wild card is a regex that matches against a step name, and it matches exactly a single name. It is not used in a way where it can result in a node-set instead of a single node. The question really is why is this (.*) needed - that is what is it trying to achieve, and whether there is an acceptable alternative for what it is doing. I find that this wildcard (.*) is used to achieve a parameterization of the types TypeISPData and TypeISPData_HKTM. These types are polymorphic, in that their exact behavior depends on aspects of their surrounding context. Each of these types incorporates a length of some content that is outside of its own definition. As the types are used now, the length comes from a thing outside of them in the schema that happens to have a particular suffix on its name, which is "Packet_Secondary_Header" or just "_Secondary_Header". So these names, while outside of the type, are in some sense being hard-coded in these types. Even though they are outside of these types, you cannot change the names of these elements without breaking the ability of the type to find them via this (.*) notation and a name suffix. These types, TypeISPData and TypeISPData_HKTM are placed inside other types, and those are then placed in context with various packet-header structures. Those structures have various distinguishing prefixes of sub-elemetnts such as MSI_Packet_Secondary_Header. There are a variety of different things instead of "MSI_", but there's always something with suffix "Packet_Secondary_Header" or "Secondary_Header" in it. The use of this (.*) name wildcard seems convenient, but doesn't offer anything that isn't better captured by a true parameterization mechanism which decouples the names used outside the type from those used inside it. DFDL provides a general mechanism for this sort of parameterization using variables and dfdl:newVariableInstance. Let's look at just one of the two instances, TypeISPData. To use variables, a variable is created which represents this parameter to the TypeISPData. It is declared in the schema file where TypeISPData is defined. <dfdl:defineVariable name="Additional_Content_Length" type="xs:int"/> The expression within TypeISPData that currently contains "contentLength(/Packet_Data_Field/(.*)Packet_Secondary_Header, 'bytes')" That part of the expression instead references the parameter variable $Additional_Content_Length At the point of use, where this packet secondary header element is combined with the element that contains the TypeISPData, at that location, a dfdl:newVariableInstance is created and bound like so: ... <xs:element name="MSI_Packet_Secondary_Header" type="TypePacketData_MSI"/> <xs:sequence> <xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl"> <dfdl:newVariableInstance ref="Additional_Content_Length" value="{ dfdl:contentLength(../MSI_Packet_Secondary_Header, 'bytes') }"/> </xs:appinfo></xs:annotation> <xs:element name="MSI_User_Data_Field" type="TypeUserData"/> </xs:sequence> ... This binds the parameter to the needed length around the point of use of the type that needs it (which is down inside the TypeUserData) This seems bulky due to the XML and XSD-annotation notational overheads, but it is just parameter-binding as would occur in an ordinary programming language when passing an argument. Using variables decouples the names of the two parts of the schema entirely. For example, if MSI_Packet_Secondary_Header were to be renamed to MSI_PSH, it would change only the point of use where the variable binding occurs, and would not affect the type definition. This also facilitates testing the types in isolation. It eliiminates the need to have the complete data structure surrounding them. Use of (*.) name matching might seem convenient, but really it depends on an unstated invariant about the way names are chosen in the Sentinel schema. If the names weren't constructed so uniformly, this notational trick would be unable to make the necessary distinctions, and you'd have to fall back on using variables. So, the above explains an alternative, already in DFDL, that can achieve the parameterization of types that is needed. There is an additional issue that makes this (.*) notation problematic. The issue is thes semantics relative to QNames and ordinary XML namespace management for name conflict control, The Sentinel schema is all a "no namespace" schema. In that world a path step is just an identifier with no structure. However, more generally in DFDL, a path step might be a QName, and those have a namespace prefix part, and a name part. Name collisions and conflicts can be managed by use of different target namespaces and namespace prefixes. Using (.*) prefix regex-matching against QNames is problematic, as it subverts the ability to use namespace prefixes to eliminate name conflicts. E.g., does (.*)_Secondary_Header match "foo:My_Secondary_Header" given that it is in the foo namespace? Matching against things without taking namespace prefixes into consideration breaks XSD's ability to use prefixes to manage name collsions. So the answer of does "(.*)_Secondary_Header" match "foo:My_Secondary_Header" would have to be that it does not match, and that if one wanted to match that one would have to write "bar:(.*)_Secondary_Header" where bar prefix is bound to the same namespace as foo in the schema file where My_Secondary_Header is defined. This mixture of some plain-textual regex matching, and some name-prefix-qualified namespace-sensitive matching is not very attractive. There is certainly nothing like this in XPath. Last issue is implementation complexity: So, if one wants to do without the (.*) notation, in terms of making DFDL4S more complex to implement, you would need to add variables. Fortunately variables are really one of the simpler things in DFDL to implement at least for parsing, and is very simple if the dfdl:setVariable annotation is not implemented since it is not needed in this case. Each variable has a stack. the dfdl:newVariableInstance pushes a new entry onto the stack, and in this case, populates it immediately with a value. Reference to the variable from an expression always takes the value of the top-of-stack location. Exiting the scope of the annotation where the dfdl:newVariableInstance appeared pops the stack. In summary, given (a) the availability of a robust parameterization mechanism using parameter variables and newVariableInstance to bind them (b) the ease of implementing variables, and (c) the complexity of working out interactions with XML/XSD namespaces/prefixes and name management,..... given those reasons, I would be disinclined to advocate something like this (.*) notation to DFDL unless it was first added to XPath, where all necessary details were worked out for us. 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 On Tue, Sep 13, 2016 at 10:18 AM, Michele Zundo <michele.zundo(a)esa.int> wrote: Dear Steve, sorry for the delay due to Summer break and other projects. Here is the .zip. Please note that we realised that one of our previous replies date 25 July 2016 at 17:45:05 GMT+2 was a misunderstanding from our part and not applicable. Regarding your questions please find answers interleaved below Michele This message and any attachments are intended for the use of the addressee or addressees only. The unauthorised disclosure, use, dissemination or copying (either in whole or in part) of its content is not permitted. If you received this message in error, please notify the sender and delete it from your system. Emails can be altered and their integrity cannot be guaranteed by the sender. Please consider the environment before printing this email. Regards From: Steve Hanson [smh(a)uk.ibm.com] Sent: Tuesday, July 26, 2016 3:20 AM To: Michele Zundo Cc: Mike Beckerle; rui.mestre(a)deimos.com.pt Subject: Re: Fwd: OGF DFDL WG Call Minutes 2016-07-05 ….snip dfdl:length="{/Packet_Primary_Header/Packet_Data_Length + 1 - contentLength( /Packet_Data_Field/(.*)Packet_Secondary_Header, 'bytes') - 2}" Firstly, contentLength is a DFDL function so it needs to be in the DFDL namespace, eg, dfdl:contentLength(). Yes agree with you. We will add the dfdl: it in future releases and modify the applications accordingly. Secondly, the first argument to dfdl:contentLength() is a path, so you are effectively still using regular expressions in path steps. Yes. For now we are using it and expect this to become part of the standard. Regards Steve Hanson IBM Integration Bus, Hursley, UK Architect, IBM DFDL Co-Chair, OGF DFDL Working Group smh(a)uk.ibm.com tel:+44-1962-815848 mob:+44-7717-378890 From: Michele Zundo <michele.zundo(a)esa.int> To: Steve Hanson/UK/IBM@IBMGB Cc: Mike Beckerle <mbeckerle(a)tresys.com> Date: 25/07/2016 17:10 Subject: Fwd: OGF DFDL WG Call Minutes 2016-07-05 Dear Steve, Please find below the answer from our developers and example. Note that we have updated our implementation of DFDL to be as compliant as we can at this point in time with the exception noted below. Michele Begin forwarded message: From: "Rui Mestre (DME)" <rui.mestre(a)deimos.com.pt> Subject: Re: Fwd: OGF DFDL WG Call Minutes 2016-07-05 Date: 25 July 2016 at 17:45:05 GMT+2 Dear Michele, I believe that after our DFDL compliance effort the mentioned "use of a regex in the path step of a DFDL expression" is no longer in place. Currently the only extension implemented in DFDL4S regarding the use of regular expressions is that implementation of dfdl:contentLength is extended to support also regular expressions when specifying the node. Please find attached a schema file example containing such extension in the use of dfdl:contentLength. Best regards, Rui Begin forwarded message: From: Steve Hanson <smh(a)uk.ibm.com> Subject: OGF DFDL WG Call Minutes 2016-07-05 Date: 5 July 2016 at 17:49:13 GMT+2 To: dfdl-wg(a)ogf.org Cc: "Mike Beckerle" <mbeckerle(a)tresys.com>, "Michele Zundo" < michele.zundo(a)esa.int> Please find minutes from the above call at https://redmine.ogf.org/dmsf_files/13537?download= @Michele - please can you send to the WG a schema that shows your use of a regex in the path step of a DFDL expression ? Next call Aug 2nd Regards Steve Hanson Architect, IBM DFDL, Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh(a)uk.ibm.com tel:+44-1962-815848 ----------------------------------------- Michele Zundo Head of Ground System Definition and Verification Office EOP-PEP European Space Agency, ESTEC e-mail: michele.zundo(a)esa.int This message and any attachments are intended for the use of the addressee or addressees only. The unauthorised disclosure, use, dissemination or copying (either in whole or in part) of its content is not permitted. If you received this message in error, please notify the sender and delete it from your system. Emails can be altered and their integrity cannot be guaranteed by the sender. Please consider the environment before printing this email. ----------------------------------------- Michele Zundo Head of Ground System Definition and Verification Office EOP-PEP European Space Agency, ESTEC e-mail: michele.zundo(a)esa.int Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU #### Sentinel2X-bandTMISPData.xsd moved to MyAttachments Repository V3.8 ( Link) on 23 August 2016 by Steve Hanson. Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU ----------------------------------------- Michele Zundo Head of Ground System Definition and Verification Office EOP-PEP European Space Agency, ESTEC e-mail: michele.zundo(a)esa.int This message and any attachments are intended for the use of the addressee or addressees only. The unauthorised disclosure, use, dissemination or copying (either in whole or in part) of its content is not permitted. If you received this message in error, please notify the sender and delete it from your system. Emails can be altered and their integrity cannot be guaranteed by the sender. Please consider the environment before printing this email. -- dfdl-wg mailing list dfdl-wg(a)ogf.org https://www.ogf.org/mailman/listinfo/dfdl-wg Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

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Action 289 - example of backward reference to OVC element (from PCAP)
by Mike Beckerle 14 May '18

14 May '18

This is taken from the PCAP schema on DFDLSchemas. However, the element OrigLen trivially refers to the prior element InclLen, using dfdl:outputValueCalc. These two fields hold the same value. (There must be a case where they don't, but we're not yet modeling that case in this schema.) <xs:element name="Packet" maxOccurs="unbounded"> <xs:complexType> <xs:sequence> <xs:element name="PacketHeader"> <xs:complexType> <xs:sequence> <xs:element name="Seconds" type="pcap:uint32"/> <xs:element name="USeconds" type="pcap:uint32"/> <xs:element name="InclLen" type="pcap:uint32" dfdl:outputValueCalc="{ if (dfdl:valueLength(../../pcap:LinkLayer/pcap:Ethernet, 'bytes') le 60) then 60 else dfdl:valueLength(../../pcap:LinkLayer/pcap:Ethernet, 'bytes') }"/> </xs:sequence> <xs:element name="OrigLen" type="pcap:uint32" dfdl:outputValueCalc="{ ../InclLen }"/> </xs:complexType> </xs:element> <xs:element ref="pcap:LinkLayer" dfdl:lengthUnits="bytes" dfdl:lengthKind="explicit" dfdl:length="{ ../PacketHeader/InclLen }"/> </xs:sequence> </xs:complexType> </xs:element> It's not a very interesting case, but it is one where an element refers backward to a prior element from an OVC, where that element is also not computed because it is an OVC that is referencing forward. A related issue that may require a separate action item is this: Here's the same DFDL schema fragment, enhanced to avoid the redundant subexpression in the InclLen outputValueCalc: <xs:element name="Packet" maxOccurs="unbounded"> <xs:complexType> <xs:sequence> <xs:element name="PacketHeader"> <xs:complexType> <xs:sequence> <xs:element name="Seconds" type="pcap:uint32"/> <xs:element name="USeconds" type="pcap:uint32"/> <xs:sequence> <xs:annotation><xs:appinfo source=" http://www.ogf.org/dfdl/"> <dfdl:newVariableInstance ref="tns:ethernetValueLength" value="{ dfdl:valueLength(../../pcap:LinkLayer/pcap:Ethernet, 'bytes') } " </xs:appinfo></xs:annotation> <xs:element name="InclLen" type="pcap:uint32" dfdl:outputValueCalc="{ if ($tns:ethernetValueLength le 60) then 60 else $tns:ethernetValueLength }"/> </xs:sequence> <xs:element name="OrigLen" type="pcap:uint32" dfdl:outputValueCalc="{ ../InclLen }"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element ref="pcap:LinkLayer" dfdl:lengthUnits="bytes" dfdl:lengthKind="explicit" dfdl:length="{ ../PacketHeader/InclLen }"/> </xs:sequence> </xs:complexType> </xs:element> Now consider when parsing, this dfdl:newVariableInstance statement will be encountered, and there is no information to tell the parser that this variable is not needed at parse time. So it will attempt to evaluate the expression, and that expression will not be able to be evaluated, because it is forward referencing. Without dfdl:newVariableInstance, one can accomplish the same thing by using a hidden group with a zero-length element having dfdl:outputValueCalc. However, note in that case we know, by the dfdl:outputValueCalc, that the computation is ONLY needed when unparsing. With dfdl:newVariableInstance, there is no such knowledge. I think a parse/unparse flag is needed for dfdl:newVariableInstance to prevent the parser from encountering an error. It is quite possible that at parse time there will be analogous newVariableInstance expressions only needed at parse time. However, as there cannot be forward references at parse time, those expressions have to work. So the flag is *in principle* only needed to tell the parser to ignore the expression because it is only used for unparsing. Here's another example of the same phenominon. This group is hidden and appears before the ../IPSrc it refers to. In the Infoset, ../IPSrc will contain a typical IPAddress string e.g., 10.20.30.40. This group picks that string apart at the "." dots into 4 binary bytes which are the representation. <xs:group name="IPSrcGrp"> <xs:sequence> <xs:annotation><xs:appinfo="http://www.ogf.org/dfdl/"> <dfdl:newVariableInstance ref="tns:afterIPSRCByte1" value="{ fn:substring-after(../IPSrc, '.') "/> <dfdl:newVariableInstance ref="tns:afterIPSRCByte2" value="{ fn:substring-after($tns:afterIPSRCByte1, '.')}" /> <dfdl:newVariableInstance ref="tns:afterIPSRCByte3" value="{ fn:substring-after($tns:afterIPSRCByte2, '.')}" /> </xs:appinfo></xs:annotation> <xs:element name="IPSrcByte1" type="xs:unsignedByte" dfdl:outputValueCalc="{ xs:unsignedByte(fn:substring-before(../IPSrc, '.')) }"/> <xs:element name="IPSrcByte2" type="xs:unsignedByte" dfdl:outputValueCalc="{ xs:unsignedByte(fn:substring-before($tns:afterIPSRCByte1, '.')) }"/> <xs:element name="IPSrcByte3" type="xs:unsignedByte" dfdl:outputValueCalc="{ xs:unsignedByte(fn:substring-before($tns:afterIPSRCByte2, '.')) }"/> <xs:element name="IPSrcByte4" type="xs:unsignedByte" dfdl:outputValueCalc="{ xs:unsignedByte($tns:tns:afterIPSRCByte3) }"/> </xs:sequence> </xs:group> 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>

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