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[binaryXML-30] Binary XML problem statement.

From: Chris Lilley <chris@w3.org>
Date: Mon, 17 Feb 2003 15:33:29 +0100
Message-ID: <54664828.20030217153329@w3.org>
To: www-tag@w3.org

Hello folks,

Action CL 2002/12/02: Write up problem statement about binary XML;
send to www-tag.

My thanks to those who reviewed earlier drafts of this email and sent
me comments; errors are however my own.

---------------------------------------------------------------------

Some people would say that binary XML or perhaps more accurately
'binary infosets' is a problem all in itself.

However, people still seem to want them, or indeed to be using them,
or indeed making standards out of them. So it would be useful to
consider why them might do this, what requirements they see for this,
rather than merely dismissing it or hoping it goes away.

The primary reason that people give for using Binary XML (binary
representations of an XML Infoset) is size efficiency - both in
network transmission and in storage on the receiving device. It is
generally asserted, or assumed, that a binary form is more compact
than a textual form (which it may well be, until its extension
mechanism if any has been used a few times....)

Network Efficiency
------------------

In terms of network efficiency, compression is often used as a work
around. Put another way, the binary form is presented as a MIME
content-encoding rather than as a new format. For example, gzip
compression[1] is used in HTTP 1.1 [2]; SVG 1.0 required SVG
implementations that use HTTP 1.1 to support gzipped SVG [3] and
implementations routinely use this; the resulting gzipped files are
considerably smaller than the raw XML versions, yet the XML can be
readily obtained (provided one has space for the requisite 32k buffer
for decoding, sufficient CPU power to decode, and space to store the
result).

XMill [4] is another alternative compression method, which separates
the structure and the content and uses different compression methods
for these two different partitions.

There was a good paper at XML 2002 USA [5] regarding compression for
XML messages (in a military, highly bandwidth constrained
environment). However, the sender and receiver were desktop-class
machines, not mobile class machines. They were able to allocate
significant processing on a per-instance basis to ensure small message
size.

Robin Berjon suggested [6] that other, more XML Specific binary forms
might also be registered as content encodings.

The argument that the Binary XML proponents make, however, is that
bandwidth is not the primary problem. Sure, they want efficient
transmission (unless they are selling bandwidth by the packet to end
users) but the network performance problem they face is latency over
satellite based networks, not bandwidth as such; and the place where
they really want efficiency is the memory footprint needed to display
the document on the device.

Storage Efficiency
------------------

In terms of storage, I have often heard a desire to avoid having both
the full-strings version (for example, to answer DOM queries about
attribute values in their full, leading and trailing space and
cr-including glory) and a 'working set'.

However, this argument is sometimes overstated. As an example,
consider this valid document

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
<!ELEMENT foo EMPTY>
<!ATTLIST foo toto (yes | no)  #REQUIRED>
]> 
<foo toto="yes"/>

and this invalid one

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
<!ELEMENT foo EMPTY>
<!ATTLIST foo toto (yes | no)  #REQUIRED>
]> 
<foo toto="   yes      "/> 

Certainly a binary XML representation could encode the value of the
toto attribute in one bit, but equally the maximum of three characters
(three bytes in UTF-8) for the attribute value is not excessive.

Typing and PSVIs
----------------

There is also sometimes a desire to have a concrete representation of
a PSVI; this seems to be a factor in the MPEG-7 BiM[7] and Expway
Bin-XML[8] forms. BiM appears to require a W3C XML Schema, which may
make it less than universally suitable; Bin-XML, like XMill, can make
use of extra information from a schema but does not require it. This
issue of generality also affects WAP Binary XML [9], a fixed
encoding scheme for a particular set of elements and attributes.

Streaming and Random Access
---------------------------

Lastly, the need for streaming and random access is sometimes cited as
a reason for preferring a binary representation (particularly for BiM
- see [7.1]). This has a relationship with work on XML fragments and on
packaging, both of which W3C has sort of been interested in.....

A key paper [7.2] about BiM has a good description of why it exists, in
the abstract:
 
  "In the course of the work on the MPEG-7 standard a binary format
  with special features for the encoding of XML data was required.
  These required key features are a high data compression ratio,
  provision for streaming, dynamic update of the document structure
  and fast random access of data entities in the compressed stream. To
  support these features we propose a novel, schema-aware approach
  which exploits the knowledge of the standardized MPEG-7 syntax
  definition of the encoded XML document on the encoder and decoder
  side. The technique is part of the MPEG-7 standard. This paper gives
  an overview of the coding algorithm, including a comparison to
  standard (XML-) compression tools."

An overview paper [7.1] describes the relationship between the XML and
BiM encodings:

  "MPEG-7 description can be represented either in textual format
  (XML), in binary format (BiM), or a mixture of the two formats,
  depending on application usage. MPEG-7 defines a unique mapping
  between the binary format and the textual format. A bi-directional
  loss-less mapping between the textual representation and the
  binary representation is possible. Still, it shall not always
  be used: some applications may not want to transmit all the
  information contained in the textual representation and may
  prefer to use a lossy transmission that is more efficient in
  terms of bandwidth."

It seems like this aspect might impact the Timed Text activity, and
also SVG 1.2 if it tries to get better streaming behavior for large
files.

Trust boundaries
----------------

Dan Connolly notes [10] that where the two parties communicating
information are in a highly trusted relationship that does not involve
interoperability with multiple parties (an example of such a
relationship being the communication between a mobile phone and
whatever proxy the phone vendor uses to give the appearance of
wireless browsing of Web pages) then the precise mechanism used is
outside the realm of standardization. However, when interoperability
is needed between multiple parties that do not necessarily trust one
another (the usual case) then Binary XML of whatever form raises the
same security issues that binary RPC or other such mechanisms  are
continually facing. Dan asserts that the overhead of doing this
security checking is greater than that of doing the XML parsing. That
may be true; or it may not. Once some XML is parsed into a DOM, is
that a security issue? If that binary data structure is sent to
someone else, is that a security issue/ When XML is seen as a
transitory serialization medium used to move an infoset/a DOM/whatever
between two computers, it is less clear that the form of that
serialization affects security in any major way; its more what access
is given to the host machine by any programatic elements such as
script that matters.

Closing remarks
---------------

There has been previous. discussion on this subject on www-tag in
regard to a casual mention of binary infosets [11]

Additional examples of such binary infosets are Sharp NVA [12]; the
CVG proposal made to the 3GPP for transmission of SVG Tiny files [13],
the Millau encoding format presented at the WWW9 conference [14] and
IBM Xtalk [15].

The Cover Pages have an overview of this area [16].

It is entirely possible that I have omitted some major benefit that
people see in a binary infoset representation, in which case I am sure
they will be quick to tell me.

Please discuss, in a focused manner, and with a view to what the
wording of a TAG finding should be.


[1] Gzip compression
http://www.gzip.org/zlib/zlib.html
http://www.gzip.org/zlib/zlib_docs.html

[2] Hypertext Transfer Protocol -- HTTP/1.1
ftp://ftp.isi.edu/in-notes/rfc2616.txt

[3] G.7 Conforming SVG Viewers
http://www.w3.org/TR/SVG/conform.html#ConformingSVGViewers

[4] XMill: An Efficient Compressor for XML
http://www.research.att.com/sw/tools/xmill/
Paper on this by Hartmut Liefke, Dan Suciu
http://citeseer.nj.nec.com/261815.html

[5] XML Sizing and Compression Study for Military Wireless Data
http://www.xmlconference.org/xmlusa/2002/friday.asp#17

[6] XML-specific content encodings
http://lists.w3.org/Archives/Public/www-tag/2002Oct/0189.html

[7] MPEG-7 and MPEG-4 BiM
[7.1] Overview of MPEG-7 systems (including BiM)
http://gps-tsc.upc.es/imatge/pub/ps/IEEE_CSVT2001_Avaro_Salembier.pdf
[7.2] AN MPEG-7 TOOL FOR COMPRESSION AND STREAMING OF XML DATA
http://www.lnt.de/~kaup/paper/icme-2002.pdf

[8] Expway Bin-XML
Bin-XML for encoding XML documents
http://expway.tv/graph/Bin-XMLTechnical%20White%20Paper.pdf

[9] WAP Binary XML Content Format
http://www.w3.org/TR/wbxml/
Staff comment
http://www.w3.org/Submission/1999/07/Comment

[10] binaryXML, marshalling, and and trust boundaries
http://lists.w3.org/Archives/Public/www-tag/2002Dec/0022.html

[11] Discussion on www-tag regarding mention of Binary Infoset
in the arch document
http://lists.w3.org/Archives/Public/www-tag/2002Oct/0193.html 

[12] Sharp NVA
Overview of NVA  in Sharp Motion Art/e-animator
http://www.galamo.com/sharpmotionart/business/technology.pdf
[12.1] Sharp Motion Art site
http://galamo.com/sharpmotionart/
[12.2] Example of use
http://anime.galamo.com/eanime/JSP/index.jsp (in Japanese)
[12.3] JPhone support for NVA
http://www.ktlink.jp/hlp/SupportModelJPhone.html (in Japanese)

[13] CVG
Documents from 3GPP EMS meeting, Paris (zipped)
Comparisons of Bijitec, iSketch and CVG file sizes for SVG Tiny content
http://www.3gpp.org/ftp/tsg_t/WG2_Capability/SWG3/SWG3_EMS_03_Paris/Docs/

[14] Millau: an encoding format for efficient representation
and exchange of XML over the Web
http://www9.org/w9cdrom/154/154.html

[15] Discussion of IBM XTalk
http://www.oreillynet.com/lpt/wlg/1858
[15.1] Vinci: A Service-Oriented Architecture for
Rapid Development of Web Applications
http://www10.org/cdrom/papers/506/


[16] XML and Compression
http://xml.coverpages.org/xmlAndCompression.html






-- 
 Chris                          mailto:chris@w3.org
Received on Monday, 17 February 2003 09:33:34 GMT

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