| Network Working Group | T. Hansen, Ed. |
| Internet-Draft | AT&T Laboratories |
| Intended status: Informational | L. Masinter |
| Expires: July 27, 2015 | M. Hardy |
| Adobe | |
| January 23, 2015 |
PDF for an RFC Series Output Document Format
draft-hansen-rfc-use-of-pdf-04
This document discusses options and requirements for the PDF rendering of RFCs in the RFC Series, as outlined in RFC 6949. It also discusses the use of PDF for Internet Drafts, and available or needed software tools for producing and working with PDF.
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This Internet-Draft will expire on July 27, 2015.
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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.
Because PDF has a wide range of capabilities and alternatives; not all PDFs are "equal". For example, visually similar documents could consist of scanned or rasterized images, include text layout options, hyperlinks, embedded fonts, and digital signatures. (See Appendix B for a brief history of PDF.)
This document explains some of the relevant options and makes recommendations, both for the RFC series and Internet Drafts.
The PDF format and the tools to manipulate it are not as well known as those for the other RFC formats, at least in the IETF community. This document discusses some of the processes for creating and using PDFs using both open source and commercial products.
NOTE: see https://github.com/masinter/pdfrfc for XML source, related files, and an issue tracker for this document.
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 control the way the document appears. (Of course, a viewer UI might display processing capabilities, such as showing if a document has been digitally signed.)
In many cases, the choice of PDF requirements is heavily influenced by the capabilities of available tools to create PDFs. Most of the discussion of tooling is to be found in Appendix C.
NOTE: each option in this section should outline the nature of the design choice, outline the pros and cons, and make a recommendation.
PDF supports rich visible layout of fixed-sized pages.
For a consistent 'look' of RFC and good style, the PDFs produced by the RFC editor should have a clear, consistent, identifiable and easy-to-read style. They should print well on the widest range of printers, and look good on displays of varying resolution.
PDF files are laid out for a particular size of page and margins. There are two paper sizes in common use: "US Letter" (8.5 x 11 inches, 216x279 mm, in popular use in North America) and "A4" (210x297 mm, 8.27x11.7 inches, standard for the rest of the world). Usually PDF printing software is used in a "shrink to fit" mode where the printing is adjusted to fit the paper in the printer.
Recommendation: There is some controversy, but the argument for choosing A4 over US Letter is appealing. Whatever page size is chosen, the margins and header positioning will need to be chosen to look good on both paper sizes using common printing methods. In addition, for both Internet Drafts and RFCs, margins should be the smallest consistent with the above requirement.
Page headers and footers are part of the page layout. There are a variety of options. Note that page headers and footers in PDF can be typeset in a way that the entire (longer) title might fit.
Page headers and footers should contain similar information as the headings in the current text versions of documents, including page numbers, title, author, working group. However, the page headers and footers should be typeset in a lighter color, smaller typeface, so as to be inobtrusive. The page headers and footers should be placed into the PDF in a way not to interfere with screen readers.
One common feature of the RFC output formats are optional visible paragraph numbers, to aid in discussions. In the PDF and thus printed rendition, it is possible to make paragraph numbers unobtrusive, and even to impinge on the margins.
Recommendation: When the XML "editing=yes" option has been chosen, show paragraph numbeers in the right margin (not left), using a small font and light color. If possible, code them in a way as to not interfere with screen readers.
The process of creating a paged document from running text typically involves insuring that related material is present on the same page together, and that artifacts of pagination don't interfere with easy reading of the document. Typical high-quality layout processors do several things:
Layout engines differ in the quality of the algorithms used to automate these processes. In some cases, the automated processes require some manual assistance to insure, for example, that a text line intended as a heading is 'kept' with the text it is heading for.
Recommendation: Minimize requirements for authors to add specific markup to the XML source for assisting with with manually enhancing layout for Internet Drafts. Choose a layout engine for the RFC Editor so that manual intervention is minimized, and that widow and orphan processing, heading and title contiguation are automatic.
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
Recommendation: the PDF rendition should look like the HTML rendition, at least in spirit. Some differences from the HTML rendition would include different typeface and size (chosen for printing), page numbers in the table of contents and index, and the use of page headers and footers.
Most of the choices used for the HTML rendering are thus applicable:
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 fall back to something implementation dependent and usually undesirable.
Recommendation: for consistent viewing, all fonts should be embedded.
In addition, if the HTML version of the document is being visually mimicked, the font(s) chosen should have both variable width and constant width components, as well as bold and italic representations.
The typefaces used by Internet Drafts and by RFCs need not be identical.
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.
Typefaces are typically licensed and, in many cases, there is a fee for use by PDF creation tools; however, not for display or print of the embedded fonts.
Recommendations:
Typically, when doing page layout of running text, especially with narrow page width and long words, layout processors of English text often have the option of hyphenating words, or using existing hyphens as a place to introduce word breaks. However, line breaks mid-word where the 'word' is actually technically a sequence of characters representing a protocol element or protocol sequence is actively harmful.
Recommendation: do not introduce any hyphens into the visual display or any line breaks mid-word even for hyphens in the text.
The PDF/A standards mandate the embedding of fonts. Preferably, the software generating the files would produce PDF/A-conforming files directly, thus ensuring that all glyphs include Unicode mappings and embedded fonts from the outset.
PDF supports hyperlinks both to sections of the same document and to other documents.
The conversion to PDF can generate:
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?
Recommendations:
PDF offers a number of features which improve the utility of PDF files in a variety of workflows, at the cost of extra effort in the xml2rfc conversion process; the tradeoffs may be different for the RFC editor production of RFCs and for Internet Drafts.
The contents of a PDF file can be represented in many ways. The PDF file could be generated:
All of these end up with essentially the same visual representation of the output. However, each level has tradeoffs for auxiliary uses, such as searching or indexing, commenting and annotation, and accessibility (text-to-speech). Keeping the running order of text in the content stream in the proper order supports all of these auxiliar uses.
In addition, the "role map" feature of PDF would additionally allow for the mapping of the logical tags found in the original XML into tags in the PDF.
Recommendations:
PDF itself does not require use of Unicode. Text is represented as a sequence of glyphs which then can be mapped to Unicode.
Recommendations:
The XML allows both ASCII art and SVG to be used for artwork.
Recommendations:
NOTE: This section should describe how alt-text for images is presented in PDF....TBD
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 used by search engines, viewers and other reuse tools.
PDF supports embedded metadata using XMP [XMP], the Extensible Metadata Platform (XMP).
Recommendation: The PDFs generated should have all of the metadata from the XML version embedded directly as XMP metadata, including the author, date, the document series, and a URL for where the document can be retrieved.
PDF supports an 'outline' feature where sections of the document are marked; this oould be used in addition to the table of contents as a navigation aid.
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.
Requirement: The section structure of an RFC should 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, plus the obvious section headings that are normally included in the Table of Contents. If possible, this should be done in a way that the same fragment identifiers for the HTML version of the RFC will work for the PDF version.
NOTE: say more about the use of alternative texts for images, tagging text spans, and providing replacement texts for symbols and images. A role-map could be provided here to map the logical tags found in the RFC XML to the standard tagset for PDF. This could be included in the generated PDF.
PDF has the capability of including other files; the files may be labeled both by a media type and a role, the AFRelationship key [PDFA3]. In this way, the PDF file acts also as a container.
Embedded content may be compressed.
Many PDF viewers support the ability to view and extract embedded files, although this capability is not universal.
Embedding content in the PDF file allows the PDF to act as a complete package, which can be transformed, archived, and digitally signed. Useful possibilities:
Recommendations:
PDF has supported digital signatures since PDF 1.2. There are multiple methods for signing PDF files. The signature is intended to apply not only to the bits in the file (that they haven't been modified) but also to lock down the visual presentation as well.
Normally, the authenticity of RFC files is not an issue, since the RFC editor maintains a repository of all RFCs which is widely replicated. However, the RFC Editor and staff are at times called to provide evidence that a particular RFC is the 'original' and has not been visually modified, and there may be other use cases. As signatures also apply to embedded content, embedding the XML source will provide a way of signing the source XML as well.
Recommendation: PDFs produced by the RFC editor SHOULD be signed. The management of certificates for the RFC editor function needs further review.
There is no need for digital signatures on Internet Drafts.
PDF has gone through several revisions, primarily for the addition of features, as noted in in Appendix B. PDF features have generally been added in a way that older viewers 'fail gracefully', but even so, the older the PDF version produced, the more legacy viewers will support that version, but the fewer features will be enabled.
As PDF has evolved a broad set of capabilities, additional standards for PDF files are applicable. These standards establish ground rules that are important for specific applications. For example PDF/X was specifically designed for Prepress digital data exchange, with careful attention to color management and printing instructions, while the PDF/E standard was designed for engineering documents.
Two additional standards families are important to the RFC format, though: long-term preservation (PDF/A), and user acessibility (PDF/UA). These then have sub-profiles (PDF/A-1, PDF/A-2, PDF/A-3), each of which have conformance levels. These standards are then supported by various software libraries and tools.
It is effective and useful to use these standards to capture PDF for RFC requirements, and they will make the PDF files useful in workflows that expect them.
Recommendations:
| [PDF] | ISO, "Portable document format -- Part 1: PDF 1.7", ISO 32000-1, 2008. Also available free from Adobe. |
| [XMP] | ISO, "Extensible metadata platform (XMP) specification -- Part 1: Data model, serialization and core properties", ISO 16684-1, 2012. Not available free, but there are a number of descriptive resources, e.g., |
| [PDFA2] | ISO, "Electronic document file format for long-term preservation -- Part 2: Use of ISO 32000-1 (PDF/A-2).", ISO 19005-2, 2011. |
| [PDFA3] | ISO, "Electronic document file format for long-term preservation -- Part 3: Use of ISO 32000-1 with support for embedded files (PDF/A-3)", ISO 19005-3, 2012. |
| [PDFUA] | ISO, "Electronic document file format enhancement for accessibility -- Part 1: Use of ISO 32000-1 (PDF/UA-1)", ISO 19005-3, 2012. |
| [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. |
NOTE: this section is meant as an overview to give some background.
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 RFC 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.
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.
[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.
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. http://www.adobe.com/devnet/pdf/pdf_reference_archive.html
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 http://en.wikipedia.org/wiki/PDF has a nice summary table going into further details.
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.
PDF/A in turn has nuances, as there have been a few 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.
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).
This section discusses tools for viewing, comparing, 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.
As with most file formats, PDF files are experienced through a reader or viewer of PDF files, and there are numerous viewers. One partial list of PDF viewers can be found at http://en.wikipedia.org/wiki/List_of_PDF_software#Viewers.
PDF viewers vary in capabilities, and it is important to note which PDF viewers support the features utilized in PDF RFCs and Internet drafts (features such as links, digital signatures, Tagged PDF and others mentioned in Section 2).
A survey of the IETF community might broaden the list of viewers in common use, but an initial list to consider include some that are currently maintained and supported viewers and legacy systems. Maintained viewers include:
Several 'legacy' viewers to consider include: Ghostview, Xpdf.
While almost all viewers also support printing of PDF files, printing is one of the most important use cases for PDFs. Some printers have direct PDF support.
Because the xml2rfc format is a unique format, software for converting XML source documents to the various formats will be needed, including PDF generation.
One promising direction is suggested in http://greenbytes.de/tech/webdav/rfc2629xslt/rfc2629xslt.html#output.pdf.fop: using XSLT to generate XSL-FO which is then processed by a formatting object processor such as Apache FOP.
Several libraries are also available for generating PDF signatures.
This section is intended to discuss available typefaces that might satisfy requirements. Some openly available fixed-width typefaces (without extensive Unicode support, however) include: Skolar, but it requires licensing. Another potentially useful set of typefaces is the Noto family from Google.
A font that looks promising for its broad Unicode support is
In addition to generating and viewing PDF, other categories of PDF tools are available and may be useful both during specification development and for published RFCs. These include tools for comparing two PDFs, checkers that could be used to validate the results of conversion, reviewing and commentary tools that attach annotations to PDF files, and digital signature creation and validation.
Validation of an arbitrary author-generated PDF file would be quite difficult; there are few PDF validation tools. However, if internet drafts and RFCs are generated by conversion from XML via xml2rfc, then explicit validation of PDF and adherance to expected profiles would mainly be useful to insure that xml2rfc has functioned properly.
Recommendations:
http://www.pdflib.com/fileadmin/pdflib/pdf/whitepaper/Whitepaper-Technical-Introduction-to-PDFA.pdf http://www.pdfa.org/wp-content/uploads/2011/08/tn0003_metadata_in_pdfa-1_2008-03-128.pdf http://www.pdfa.org/wp-content/uploads/2011/08/PDFA-in-a-Nutshell_1b.pdf http://www.pdfa.org/2011/08/pdfa-%E2%80%93-a-look-at-the-technical-side/ http://pdf.editme.com/pdfa
The input of the following people is gratefully acknowledged: Brian Carpenter, Chris Dearlove, Martin Duerst, Heather Flanagan, Joe Hildebrand, Duff Johnson, Leonard Rosenthal, .... [CREF1]Sean Leonard <dev+ietf@seantek.com> To make IETF docs look good, it would be nice to have a quality tool that captures all of the nuances of the vocabulary in PDF format. Desired features include: bookmarks for sections [x] observing pagination controls (see Improvement #2) [x] observing standardized headers and footers (compare with Tool Request #B, forthcoming) [x] preserving intra-document and extra-document hyperlinks [x] formatting choices that allow documents to be printed on Letter or A4 page sizes at 100% resolution [????] including comments and other annotations in the native PDF format [cref] observing whitespace and line break preservation as directed by the input (e.g., NBSP, NBHYPHEN, don't break this range of text, don't collapse multiple spaces) [x] vector artwork [SVG] preserving text flow for accessibility purposes [x] font embedding [x] preserving "files" and other incorporated blobs as document-level or page-level "File Attachments" [TBD] metadata preservation [x]