RE: [ebxml-msg] Compression issues

["Pim van der Eijk" <pvde@sonnenglanz.net>]

 

There are actually two things:

(1)  Expressing whether or not the message is compressed.  This looks like a Boolean type to me.  The name of the property "Compressed" seems to refer to this. 

(2)  Expressing which compression algorithm is used.  This is more an enumeration type.  In AS4 there is only one choice, GZIP,  so expressing the selected algorithm in the message is actually not needed.

(3)  Expressing the compressed data format as a MIME type.

 

In section 4.2.4 of ebMS 3.0 Part 2, we define a message compression feature. (1) is encoded by the presence of a "CompressionAlgorithm" element and (2) by a particular value.  It uses values from the IANA content-coding token list, as done in section 3.5 of HTTP (IETF RFC 2616).  Some IANA content-coding values are relevant for compression:  gzip, compress, deflate, and identity.

 

http://docs.oasis-open.org/ebxml-msg/ebms/v3.0/part2/201004/cs01/ebms-v3.0-part2-cs01.html#__RefHeading__29502104

 

So here are the options:

 

A.  The original proposal was to stick to the property "Compressed" but add a value "true" for (1) and not include (2) in the message, but assume AS4 processors are hard-wired to use GZIP.  Absence of the property,  or presence with a value "false" means no compression.

 

<eb:Property name="Compressed">true</eb:Property>

 

B.  Rename the property to something like "CompressionAlgorithm" or "ContentCoding" and use the content-coding token values.  Absence of the property means no compression or equivalently "identity" content-coding.

 

<eb:Property name="CompressionAlgorithm">gzip</eb:Property>

 

The IANA registry of content-coding values is available at:

http://www.iana.org/assignments/http-parameters/http-parameters.xml

 

This works for AS4 as gzip is a value on this list.

 

C.  The third option is what Dale proposes.  A variant is that the property might perhaps be renamed to something like:

 

<eb:Property name="CompressedMediaType">application/gzip</eb:Property>

 

The advantage of using MIME types is that some newer compression methods have a MIME type but not (yet) a content encoding, and that there is the convention of adding new (unregistered) types using a "x-" prefix that does not exist for content coding. An example is LZMA used by 7-Zip, which has the MIME type "application/x-7z-compressed".   But this is not an issue with AS4 as it is limited to gzip.

 

Pim, 

 

---

From: ebxml-msg@lists.oasis-open.org [mailto:ebxml-msg@lists.oasis-open.org] On Behalf Of Moberg Dale
Sent: 03 November 2011 22:10
To: ebxml-msg@lists.oasis-open.org
Subject: [ebxml-msg] Compression issues

Here are some observations about AS4 compression section, and the issues faced in fixing the specification to align with the ebMS3 schema.

 

tns:non-empty-string is the base type extended by the complexType Property.

 

     <xsd:complexType name="Property">

           <xsd:simpleContent>

                <xsd:extension base="tns:non-empty-string">

                     <xsd:attribute name="name" type="tns:non-empty-string" use="required"/>

                </xsd:extension>

           </xsd:simpleContent>

     </xsd:complexType>

 

 

So, a Property element of the Property type included in a PartProperties sequence of a PartInfo cannot be empty; it needs a text node with at least one character in it.

 

A good choice for a value when the Property@name is “Compressed” is the content-type of the attachment, which MUST be “application/gzip” according to the profile.

 

Already the Property@MimeType value is to be the mime type of the original that has been compressed (application/edi-consent e.g.) , and it is already recommended

that this mime type information of the original be included.

 

The other proposal on the table is to use a Boolean value, as mentioned in describing PMode.PayloadServiceCompression…

But PMode values are really for a MSH’s local operation. Information conveyed in the SOAP message header does not necessarily have to repeat PMode information or values.

 

When a sender includes the PartInfo, I guess the compression Property  metadata’s  value can be useful to the receiver. (Though in my opinion, it should be in an attachment  MIME content- header in the first place.)

 

On the other hand, the PMode information about PayloadServiceCompression tells the sender to compress a part by using the processing of an application/gzip. It is not used to describe a compressed data type like a jpeg or mpeg, and having a PMode PayloadServiceCompression false does not imply that the content has not been compressed in some way as part of its application layer processing definition. At least that is my understanding of what should be understood. Possibly we should clarify the PMode values so that we say:

 

True:  the sending MSH is to compress at least one attachment payload over this MEP segment.

False: no attachments are to be compressed by the sending MSH using application/gzip processing.

 

The defaulting of the PMode PayloadServiceCompression to”false” carries no implication about what an omitted Property@name=Compression has as its value.  

 

If the PMode of PayloadServiceCompression is “false,” and the MSH adds an ebMS3 Property element to a PartInfo saying it has compressed an attachment, the MSH is clearly broken.

 

So the example in section 3.1 needs to change. My proposal  is that the line:

 

<eb:Property name=”Compressed”>application/gzip</eb:Property>

 

replace the current:

 

<eb:Property name=”Compressed”/>

 

I would also urge consideration of rewording the explanatory text around PMode so that no one interprets it to mean that having some payload whose data has been compressed at the application layer requires a PMode value of “true”.  That is simply not what was intended.  We should not confuse an imperative with a declarative semantic!

“True” in the  PMode value here must mean “Make it so!”