Network Working Group T. Hansen, Ed.
Internet-Draft AT&T Laboratories
Intended status: Informational L. Masinter
Expires: December 28, 2014 M. Hardy
Adobe Systems
June 26, 2014
PDF for an RFC Series Output Document Format
draft-hansen-rfc-use-of-pdf-00
Abstract
<1>
This document discusses options and requirements for the PDF
rendering of RFCs in the RFC Series, as outlined in RFC 6989. It
also discusses the use of PDF for Internet Drafts, and available or
needed software tools for producing and working with PDF.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 28, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. History and current use of PDF with RFCs and Internet Drafts 3
2.1. RFCs . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Internet Drafts . . . . . . . . . . . . . . . . . . . . . 3
3. Options and Requirements for PDF RFCs . . . . . . . . . . . . 3
3.1. "Visible" requirements . . . . . . . . . . . . . . . . . 4
3.1.1. General visible requirements . . . . . . . . . . . . 4
3.1.2. Page size, margins, headings . . . . . . . . . . . . 4
3.1.3. Similarity to other outputs . . . . . . . . . . . . . 4
3.1.4. Typeface choices . . . . . . . . . . . . . . . . . . 5
3.1.5. Hyperlinks . . . . . . . . . . . . . . . . . . . . . 5
3.2. "Invisible" options and requirements . . . . . . . . . . 5
3.2.1. Internal Text Representation . . . . . . . . . . . . 6
3.2.2. Unicode Support . . . . . . . . . . . . . . . . . . . 7
3.2.3. Metadata Support . . . . . . . . . . . . . . . . . . 7
3.2.4. Document Structure Support . . . . . . . . . . . . . 7
3.2.5. Tagged PDF . . . . . . . . . . . . . . . . . . . . . 8
3.2.6. Embedded Files . . . . . . . . . . . . . . . . . . . 8
3.2.7. Document Signatures . . . . . . . . . . . . . . . . . 8
4. Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Picking a PDF Profile . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Informative References . . . . . . . . . . . . . . . . . 9
6.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. A Synopsis of PDF Format History . . . . . . . . . . 10
A.1. PDF/A . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. PDF/UA . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.3. Additional Reading . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
<2>
The RFC Series is evolving, as outlined in [RFC6949]. Future
documents will use an archival format of XML with renderings in
various formats, including PDF.
<3>
PDF has a wide range of capabilities and alternatives; not all PDFs
are "equal". (See Appendix A for a brief history of PDF and its
options.) For example, visually similar documents could be scanned
or rasterized images, include text layout options, hyperlinks,
embedded fonts, digital signatures. This document explains the
options and also makes recommendations for choices, both for the RFC
series and also Internet Drafts.
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<4>
The PDF format and the tools to manipulate it are not as well known
as those for other formats. This document discusses some of the
processes for creating and using PDFs and both open source and
commercial products.
<5>
NOTE: see [1] for XML source, related files, and an issue tracker.
2. History and current use of PDF with RFCs and Internet Drafts
<6>
NOTE: this section is meant as an overview to give some background.
2.1. RFCs
<7>
The RFC series has for a long time accepted Postscript renderings of
RFCs, either in addition to or instead of the text renderings of
those same RFCs. These have usually been produced when there was a
complicated figure or mathematics within the document. For example,
consider the figures and mathematics found in RFCs 1119 and RFC 1142,
and compare the figures found in the text version of RFC 3550 with
those in the Postscript version. The RFC editor has provided a PDF
rendering of RFCs. Usually, this has been a print of the text file
that does not take advantage of any of the broader PDF functionality,
unless there was a Postscript version of the RFC, which would then be
used by the RFC editor to generate the PDF.
2.2. Internet Drafts
<8>
In addition to PDFs generated and published by the RFC editor, the
IETF tools community has also long supported PDF for Internet Drafts.
Most RFCs start with Internet Drafts, edited by individual authors.
The Internet drafts submission tool at https://datatracker.ietf.org/
submit/ accepts PDF and Postscript files in addition to the
(required) text submission and (currently optional) XML. If a PDF
wasn't submitted for a particular version of an Internet Draft, the
tools would generate one from the Postscript, HTML, or text.
3. Options and Requirements for PDF RFCs
<9>
This section lays out options and requirements for PDFs produced by
the RFC editor for RFCs. There are two sections: "Visible" options
are related to how the PDF appears when it is viewed with a PDF
viewer. "Internal Structure" options affect the ability to process
PDFs in other ways, but do not change the way the document looks.
<10>
In many cases, the choice of PDF requirements is heavily influenced
by the utility of available tools to create PDFs. Most of the
discussion of tooling is to be found in Section 4.
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<11>
NOTE: each option in this section should outline the nature of the
design choice, outline the pros and cons, and make a recommendation.
3.1. "Visible" requirements
<12>
PDF supports rich visible layout of fixed-sized pages.
3.1.1. General visible requirements
<13>
For a consistent 'look' of RFC and good style, the PDFs produced by
the RFC editor should have a clear, easy-to-read style. They should
print well on the widest range of printers, and look good on displays
of varying resolution.
3.1.2. Page size, margins, headings
<14>
PDF files are laid out for a particular size of page, margins, and
any headers and footers part of the layout. Recommendations or ideas
for further study:
<15>
Page size US Letter page size, but margins chosen so it will print
and look good on A4 paper.
<16>
Margins The smallest margin consistent with above requirements.
<17>
Headings The same information from the text version of the document,
but set in a smaller font in a lighter color.
3.1.3. Similarity to other outputs
<18>
There is some advantage to having the PDF files look like the text or
HTML renderings of the same document. There are several options even
so. The PDF
<19>
1. could look like the text version of the document, or
<20>
2. could look like the text version of the document but with
pictures rendered as pictures instead of using their ASCII-art
equivalent, or
<21>
3. could look like the HTML version.
<22>
(Note that numbers 1 and 2 are what are currently produced by the RFC
Editor on their web site.)
<23>
Recommendation: the PDF rendition should look like the HTML
rendition, at least in spirit -- for example, visually searching or
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scanning should be facilitated. The typeface and size for printing
should be chosen.
3.1.4. Typeface choices
<24>
A PDF may refer to a font by name, or it may use an embedded font.
When a font is not embedded, a PDF viewer will attempt to locate a
locally installed font of the same name. If it can not find an exact
match, it will find a "close match". If a close match is not
available, it will fallback to something. This is highly
implementation dependent.
<25>
Recommendation: for consistent viewing, all fonts should be embedded.
<26>
In addition, since the HTML version of the document is being visually
replicated, the font(s) chosen should have both variable width and
constant width components, as well as bold and italic
representations.
<27>
Few fonts have glyphs for the entire repertoire of Unicode
characters; for this purpose, the PDF generation tool may need a set
of fonts and a way of choosing them.
<28>
Recommendation: ... TBD ...
<29>
For readability, the main body text should be in a serif font and the
headings in a sans-serif font.
<30>
Code, BNF, and other text could use a fixed-width font to aid in
insuring alignment, e.g., in BNF.
3.1.5. Hyperlinks
<31>
PDF supports hyperlinks both to sections of the same document and to
other documents.
<32>
Recommendation: All hyperlinks available in the HTML rendition of the
RFC should also be visible and active in the PDF produced.
3.2. "Invisible" options and requirements
<33>
There are many things going on under the cover:
<34>
o What is usable in an RFC rendered as PDF?
<35>
o Where can we improve on the past?
<36>
o Where must we improve?
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<37>
o What must be supported in PDF?
<38>
These all turn into requirements for the conversion tools that are
used to generate the PDF rendering of the visible representation.
<39>
The areas we'll discuss are oriented around:
<40>
1. Text Structure
<41>
2. Unicode support
<42>
3. Metadata
<43>
4. Document Structure
<44>
5. Tagged PDF
<45>
6. Extractable Sections
<46>
7. Document Signatures
3.2.1. Internal Text Representation
<47>
The contents of a PDF file can be represented in many ways. The PDF
file could be generated:
<48>
o as an image of the visual representation, such as a picture (e.g.,
a GIF) of the word 'IETF'
<49>
o placing individual characters in position on the page, such as
saying "put an 'F' here", then "put an 'T' before it", then "put
an 'E' before that", then "put an 'I' before that" to render the
word 'IETF'
<50>
o placing words in position on the page, such as keeping the word
'IETF' would be kept together, and
<51>
o using higher-level constructs for sets of words, such as keeping
the sentence 'The Internet Engineering Task Force (IETF) supports
the Internet.' together as a sentence.
<52>
All of these end up with the same visual representation of the
output. However, each level has trade offs for auxiliary usage of
the format. For example, the higher-level construct would allow you
to search for the word "IETF" or phrases including the word "IETF"",
whereas using word placement would only allow you to search for
"IETF", and the other representations would not easily support search
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at all. As another example, when using an annotation tool to review
a PDF file, it is harder to place a comment on a location within an
image compared to attaching a comment to a given word. Attaching a
comment to a set of words (such as a bracketed set of words) is
easier when using higher-level constructs. Another example where
higher-level constructs are needed are for accessibility purposes:
text to speech needs the sentences to be presented as a whole and in
the proper order.
<53>
Requirement: ... TBD ...
3.2.2. Unicode Support
<54>
Unicode is being fully supported, so the RFC PDF format must
similarly have full support for Unicode. While Unicode is not
required by PDF, certain PDF profiles require its support.
<55>
Requirement: PDF files generated must have the full text, exactly as
it appears in the original XML for text, or within SVG for images.
(NOTE: What about text in image illustrations?)
3.2.3. Metadata Support
<56>
Metadata encodes information about the document authors, the document
series, date created, etc. using the RDF Dublin core (and other
elements). Having this metadata within the PDF file allows it to be
extracted by the rendering tools. It can also include additional
information such as pointers to where the document can be found on
the RFC Editor web site.
<57>
PDF supports embedded metadata using XMP. NOTE: Need a reference and
explanation.
<58>
Recommendation: The PDFs generated should have all of the metadata
from the XML version embedded directly as XMP metadata, including the
author and date information, set the document series, and a URL for
where the document can be retrieved.
3.2.4. Document Structure Support
<59>
The section structure of an RFC can be mapped into the PDF elements
for the document structure. This will allow the bookmark feature of
PDF readers to be used to quickly access sections of the document.
<60>
Requirement: The section structure of an RFC must be mapped into the
PDF elements for the document structure. This would include section
headings for the boilerplate sections such as the Abstract, Status of
the Document, Table of Contents, and Author Addresses.
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3.2.5. Tagged PDF
<61>
... say more about the use of alternative texts for images, tagging
text spans and giving them an ID, and providing replacement texts for
symbols and images
...
hyperlinks within the document, hyperlinks to external locations,
...
Where should hyperlinks to RFCs point? to the info page for the RFC?
to the PDF version of the RFC? (NOTE: the RFC Series Editor has
stated a preference for them to point to the info page for the RFC.)
...
Similar questions need to be answered on references to internet
drafts: Where should hyperlinks to internet drafts point? To the
datatracker entry? To the tools entry? To a PDF version of the
internet draft?
...
a role-map should be provided here to map the logical tags found in
the RFC XML to the standard tagset for PDF. This would be included
in the generated PDF.
3.2.6. Embedded Files
3.2.6.1. Extractable Code Segments and Artwork
<62>
It has been suggested that the source input for code segments (e.g.,
ABNF, C code, MIBs) be extractable from the PDF. This capability
might be supported through other mechanisms from the XML source
files, but could also be supported within the PDF. PDF/A-2 (based on
PDF release 1.7) allows for the embedding of some file formats, while
PDF/A-3 adds support for arbitrary files to be embedded.
3.2.6.2. Extractable XML source
<63>
Another suggestion that has been made is that the XML input file
itself could be embedded within the PDF. This would make the PDF
file totally self-referential.
3.2.7. Document Signatures
<64>
PDF has supported file signatures since PDF 1.2. It has been
suggested that the PDF files be signed by the RFC Editor on creation.
This would allow the signatures to be authenticated.
<65>
Recommendation: The RFC PDF documents created by the RFC Editor
should be digitally signed.
<66>
Recommendation: Internet drafts do not need to be digitally signed.
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4. Tooling
<67>
NOTE: This section will talk about tools for creating, manipulating,
transforming PDF files, including those currently in use by the RFC
editor and Internet drafts, as well as outlining available PDF tools
for various processes.
<68>
during either phase: generation.
<69>
during I-D phase: xml2rfc, of course for authors. Copy from PDF
files? Review and comment. Digital Signature tools. Comparing two
PDF files (versions).
<70>
During RFC publishing, xml2rdf. Editing of PDF to correct layout
errors. Nits checking, checking conformance with PDF/a and PDF/ua.
5. Picking a PDF Profile
<71>
There are profiles of PDF for specific purposes: PDF/UA and PDFA3
etc.
<72>
NOTE: add reasoning here about the recommendations.
<73>
Recommendation: use PDF/UA and also PDF/A3.
6. References
6.1. Informative References
[RFC3778] Taft, E., Pravetz, J., Zilles, S., and L. Masinter, "The
application/pdf Media Type", RFC 3778, May 2004.
[RFC6949] Flanagan, H. and N. Brownlee, "RFC Series Format
Requirements and Future Development", RFC 6949, May 2013.
6.2. URIs
[1] https://github.com/masinter/pdfrfc
[2] http://www.adobe.com/devnet/pdf/pdf_reference_archive.html
[3] http://en.wikipedia.org/wiki/PDF
[4] http://www.pdflib.com/fileadmin/pdflib/pdf/whitepaper/Whitepaper-
Technical-Introduction-to-PDFA.pdf
[5] http://www.pdfa.org/wp-content/uploads/2011/08/
tn0003_metadata_in_pdfa-1_2008-03-128.pdf
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[6] http://www.pdfa.org/wp-content/uploads/2011/08/PDFA-in-
a-Nutshell_1b.pdf
[7] http://www.pdfa.org/2011/08/pdfa-%E2%80%93-a-look-at-the-
technical-side/
[8] http://pdf.editme.com/pdfa
Appendix A. A Synopsis of PDF Format History
<74>
[RFC3778] contains some history of PDF. This is a capsule view, plus
additional information on events that have occurred since the
publication of [RFC3778]. NOTE: currently doesn't talk about the
handoff of change control to ISO and the evolution as an ISO standard
32000. Plans are to update the application/pdf MIME registration to
include this information, and then point to that.
<75>
The Portable Document Format (PDF) family of document formats was
invented by Adobe Systems in the early 1990s. At the time, it was a
proprietary format that underwent a variety of revisions that matched
the release of different versions of the Adobe Acrobat products. For
example, Acrobat 1 supported PDF version 1.0, Acrobat 2 supported PDF
version 1.1, Acrobat 5 supported PDF version 1.4, etc. [2]
<76>
Each release (and extension level) introduced new features. For
example, (1.0) character, word and image rendering, externally-
referenced or embedded fonts, (1.1) passwords, encryption, device-
independent color, (1.2) interactive forms, unicode, signatures,
compression, (1.3) web semantic capture, embedded files, Adobe
javascript, (1.4) metadata streams, tagged PDF, (1.5) controllable
hiding of sections, slideshows, (1.6) 3D artwork, OpenType font
embedding, linking into embedded files, and (1.7) video and audio
support. After release 1.7, additional Extension Levels have been
introduced. Each release also provided enhancements to the previous
support. For example, encryption was introduced in 1.1, but AES
encryption wasn't supported until 1.7 extension level 3. A PDF
reader for PDF 1.1 is not able to read and display a PDF 1.7 file,
but a PDF reader for PDF 1.7 can also handle all previous versions of
PDF. The wikipedia page at [3] has a nice summary table going into
further details.
<77>
Certain profiles or subsets of PDF have been standardized. PDF/X (X
for Exchange), PDF/A (A for Archive), PDF/E (E for Engineering), PDF/
VT (VT for Variables and Transactions), and PDF/UA (UA for Universal
Access) all have ISO standards associated with them. Of particular
potential interest to the RFC community are PDF/A and PDF/UA.
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A.1. PDF/A
<78>
PDF/A in turn has nuances, as there have been a couple updates to it
and conformance levels within each version. PDF/A-1 was based on PDF
release 1.4. PDF/A-2 was based on PDF release 1.7, and PDF/A-3 adds
embedded arbitrary files. PDF/A is considered a profile because it
mandates that certain optional features be used. At a high level,
the conformance levels are B (basic), U (mandatory unicode mapping
[not in PDF/A-1]) and A (accessible). The requirements for
conformance level A are that: the document structure must be
represented within the PDF (e.g., section headings, table cells,
paragraph divisions), tagged PDF is used (e.g., element anchors) and
that language tags be used where appropriate. When referring to PDF/
A, you would refer to the version and conformance level. So PDF/A-1A
would be the profile for the Accessible conformance level of version
1 of PDF/A, which was based on PDF 1.4.
A.2. PDF/UA
<79>
The PDF/UA (Universal Access) profile is orthogonal to the other
profiles, specifying user accessibility requirements. It places some
restrictions on the other profiles, such as requiring the use of
higher-level constructs for the textual representation and adds
additional requirements for programatic access (think automatic
readers for the blind).
A.3. Additional Reading
<80>
[4] [5] [6] [7] [8]
Authors' Addresses
Tony Hansen (editor)
AT&T Laboratories
200 Laurel Ave. South
Middletown, NJ 07748
USA
Email: tony+rfc2pdf@maillennium.att.com
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Larry Masinter
Adobe Systems
345 Park Ave
San Jose, CA 95110
USA
Email: masinter@adobe.com
URI: http://larry.masinter.net
Matthew Hardy
Adobe Systems
345 Park Ave
San Jose, CA 95110
USA
Email: mahardy@adobe.com
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