[DFDL-WG] newVariableInstance direction property discussion - PCAP example

[Steve Hanson]

Hi Marcos

Your email is in our DFDL WG call calendar invite, which also has all the 
links; are you not receiving them? Next call is Thurs 15/4 @ 4pm UK.

OGF DFDL WG call dial-in details. https://ibm.webex.com/meet/smh

GitHub DFDL: https://github.com/OpenGridForum/DFDL

Regards
 
Steve Hanson
IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890
Note: I work Tuesday to Friday 



From:   "Marcos Bento (external)" <Marcos.Bento at esa.int>
To:     DFDL-WG <dfdl-wg at ogf.org>
Date:   13/04/2021 08:54
Subject:        [EXTERNAL] Re: [DFDL-WG] newVariableInstance direction 
property discussion - PCAP example
Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>



Hi,

We are currently working on supporting newVariableInstance in DFDL4S,
so I'd like attend the next conference call and tune in on this 
discussion.

Can anyone help me, and pointing me to the next call(s) 
schedule/coordinates?
I've noticed that there was a move to Github, and I couldn't locate
this information.

--Marcos

On Mon, Apr 12, 2021 at 7:26 PM Mike Beckerle <mbeckerle.dfdl at gmail.com> 
wrote:
>
> For our next conference call, I said I would walk through the Apache 
Daffodil newVariableInstance feature, and how for unparsing we had to add 
a direction property to variables so they can be evaluated with forward 
referencing for unparsing.
>
> The code I'd like to discuss is here:
>
> 
https://github.com/DFDLSchemas/PCAP/blob/cb9ee66aa565d982f01130918bcd785def51b58d/src/main/resources/com/tresys/pcap/xsd/pcap.dfdl.xsd 

>
> The defineVariable that is the best motivating example is defined at 
line 104, and is used at line 213. This is a simple common-subexpression 
elimination, but it is used to avoid redundant calls to dfdl:valueLength 
in the dfdl:outputValueCalc of the stored length field named InclLen.
>
> There is a second example of using newVariableInstance at unparse time 
in this schema also, with the variable definitions starting at line 115, 
and usage in the group defined starting at line 320. This usage is far 
more complex, and in hindsight I think a DFDL parse of PCAP data should 
not be creating IP addresses in the Infoset that look like "1.2.3.4", 
because it requires all this complexity to unparse from that back to the 
binary representation. However, it is a good exercise in how the 
newVariableInstance feature works.
>
> -mikeb
>
> Mike Beckerle | OGF DFDL Workgroup Co-Chair | Owl Cyber Defense | 
www.owlcyberdefense.com
> Please note: Contributions to the DFDL Workgroup's email discussions are 
subject to the OGF Intellectual Property Policy
>
> --
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Marcos Bento
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System Support Division
Earth Observation Projects Department
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