pdfrfc.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">

<?rfc toc='yes' ?><!-- add table of contents -->
<?rfc editing='no' ?><!-- paragraph numbering -->
<?rfc symrefs='yes' ?><!-- use references like [ABC] instead of [1] -->
<?rfc sortrefs='no'?><!-- sort the references alphabetically -->
<?rfc linkmailto='no'?>
<?rfc compact='yes'?>
<?rfc comments='yes'?><!-- print crefs -->
<?rfc inline='yes'?>
<?rfc-ext parse-xml-in-artwork='yes' ?>
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<rfc ipr='trust200902'
     docName='draft-iab-rfc-use-of-pdf-02' category='info' >
  <front>
    <title abbrev='PDF for RFCs'>PDF for an RFC Series Output Document Format</title>
    <author initials='T.' surname='Hansen' fullname='Tony Hansen' role='editor'>
      <organization>AT&amp;T Laboratories</organization>
      <address>
	<postal>
	  <street>200 Laurel Ave. South</street>
	  <city>Middletown</city>
	  <region>NJ</region>
	  <code>07748</code>
	  <country>USA</country>
	</postal>
	<email>tony@att.com</email>
      </address>
    </author>
    <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
      <organization>Adobe</organization>
      <address>
	<postal>
	  <street>345 Park Ave</street>
	  <city>San Jose</city>
	  <region>CA</region>
	  <code>95110</code>
	  <country>USA</country>
	</postal>
	<email>masinter@adobe.com</email>
	<uri>http://larry.masinter.net</uri>
      </address>
    </author>
    <author initials='M.' surname='Hardy' fullname='Matthew Hardy'>
      <organization>Adobe</organization>
      <address>
	<postal>
	  <street>345 Park Ave</street>
	  <city>San Jose</city>
	  <region>CA</region>
	  <code>95110</code>
	  <country>USA</country>
	</postal>
	<email>mahardy@adobe.com</email>
      </address>
    </author>
    <date />
    <keyword>Requests for Comment</keyword>
    <abstract>
      <t>
	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.
      </t>
    </abstract>
  </front>
  <middle>
    <section title='Introduction'>
      <t> The RFC Series is evolving, as outlined in
	<xref target='RFC6949'/>. Future documents will use a canonical format, XML,
	with renderings in various formats, including PDF.
      </t>

      <t>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,
	or include text layout options, hyperlinks, embedded fonts, and
	digital signatures.
	(See <xref target='I-D.hardy-pdf-mime'/>
	for a history of PDF.)
      </t>

      <t> This document explains some of the relevant options and
	makes recommendations, both for the RFC series and Internet-Drafts.
      </t>

      <t> 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.
      </t>

      <t>
	The details described in this document are expected to change based on experience
	gained in implementing the RFC production center's toolset.
	Revised documents will be published capturing those changes as the toolset is completed.
	Other implementers must not expect those changes to remain backwards-compatible with the
	details described in this document.
      </t>

      <t>NOTE: [RFC-EDITOR: This note should be removed before publication.]
      	See <eref target="https://github.com/masinter/pdfrfc"/>
	for XML source, related files, and an issue tracker for
	this document.
      </t>
    </section>

    <section title='Choosing PDF versions and Standards'>
      <t> <xref target="PDF">PDF</xref> has gone through several revisions,
	primarily for the addition of features.  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.
      </t>

      <t>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. The PDF/E 
	standard was designed for engineering documents with dynamic 
	workflows (where a document continues to be revised after 
	publication) and allows interactive media (including
	animation and 3D).
      </t>

      <t>Two additional standards families are important to the
	RFC format, though: long-term preservation (PDF/A), and user
	accessibility (<xref target="PDFUA">PDF/UA</xref>). These then have sub-profiles (PDF/A-1,
	<xref target="PDFA2">PDF/A-2</xref>, <xref target="PDFA3">PDF/A-3</xref>), each of which have conformance levels.
	These standards are then supported
	by various software libraries and tools.
      </t>

      <t>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.
      </t>

      <t>Recommendations:
	<list>
	  <t>Use PDF 1.7; although relatively recent, it is well
	    supported by widely available viewers.
	  </t>

	  <t>For RFCs, require PDF/A-3 with conformance level "U".
	    This captures the archivability and
	    long-term stability of PDF 1.7 files, mandatory Unicode mapping,
	    and many of the requirement features.
	  </t>

	  <t>Use PDF/A-3 for embedding additional data (including
	    the XML source file) in RFCs and Internet-Drafts.
	  </t>

	  <t>Use PDF/UA for user accessibility.
	  </t>
	</list>
      </t>
    </section>

    <section title='Options and Requirements for PDF RFCs'
	     anchor='requirements'>
      <t>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
	whether a document has been digitally signed.)
      </t>

      <t>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
	<xref target='tooling'/>.
      </t>
      <!-- t>NOTE: each option in this section will eventually outline the nature
	of the design choice, outline the pros and cons, and make a
	recommendation.
      </t -->

      <section title='"Visible" Requirements'>
	<t> PDF supports rich visible layout of fixed-sized pages.
	</t>

	<section title='General Visible Requirements'>
	  <t> 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.
	  </t>
	</section>

	<section title='Page Size, Margins'>
	  <t>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.
	    There is some controversy, but the argument that A4 is
	    an international standard is compelling.
	    However, if the margins and header positioning are chosen
	    appropriately, the document can be printed without any
	    scaling.
	  </t>

	  <t>Recommendation:
	    The Internet-Draft and RFC processors should produce A4 size
	    by default.
	    However, the margins and header positioning need to be chosen
	    to look good on both paper sizes without scaling.
	    Following the advice found in <xref target='RFC2346'/>, this
	    means that we should use A4 portrait mode with left and right
	    margins of 20 mm, and top and bottom margins of 33 mm.
	  </t>
	</section>

	<section title='Headers and Footers'>
	  <t>
	    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.
	  </t>

	  <t>
	    Recommendation: 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 way so as to be unobtrusive.
	    The page headers and footers should be placed into the PDF
	    in a way not to interfere with screen readers. 
	  </t>
	</section>

	<section title='Paragraph Numbering'>
	  <t>One common feature of the Internet-draft 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.
	  </t>
	  <t>Recommendation: When the XML "editing=yes" option has been chosen,
	    show paragraph numbers in the right margin, typeset in a way so
	    as to be unobtrusive.
	    (The right margin instead of the left margin prevents the paragraph
	    numbers from being confused with the section numbers.)
	    If possible, the paragraph numbers should be coded in a way
	    that they do not interfere with screen readers.
	  </t>
	</section>

	<section title="Paged Content Layout">
	  <t>
	    By its nature, PDF is paginated, so pagination issues must
	    be considered.
	    This is reflected in two areas: running headers and footers, 
	    and how text is layed out on a page for optimal reading.
	  </t>
	  <t>
	    <xref target="page-layout"/> describes
	    the process of creating a paged document from running text
	    such that related material is present
	    on the same page together and artifacts of pagination
	    don't interfere with easy reading of the document.
	  </t>
	  <t>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 ensure, for example, that a text line intended
	    as a heading is "kept" with the text it is heading for.
	  </t>
	  <t>Recommendations:
	  <list style='symbols'>
	    <t>
	    Headers and footers should be printed on each page.
	    The information should include the RFC number or internet-draft name,
	    the page number, the category (informational, etc.),
	    a shortened version of the authors' names,
	    the date of the RFC or internet-draft, and
	    the short form of the document title.
	    </t>
	  <t><!-- Minimize the requirements for authors
				  to add specific markup to the XML source for assisting with
				  manually enhancing layout for Internet-Drafts. -->
	    Choose a layout engine so that manual
	    intervention is minimized, and that widow and
	    orphan processing, heading and title contiguation
	    are automatic.
	  </t>
	  </list>
	  </t>
	</section>

	<section title="Typeface Choices">
	  <t>
	    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.
	  </t>
	  <t>
	    In addition, the PDF/A standards mandate the embedding
	    of fonts. Instead of using additional software to embed the fonts, 
	    the software generating the PDF files should produce PDF/A-conforming 
	    files directly, thus ensuring that all glyphs include Unicode 
	    mappings and embedded fonts from the outset.
	  </t>

	  <t>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.
	  </t>
	  <t> The typefaces used by Internet-Drafts and by RFCs
	    need not be identical.
	  </t>
	  <t> 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.
	    The RFC Editor is defining where Unicode characters
	    may be used within
	    RFCs.<xref target='I-D.flanagan-nonascii'/>
	  </t>
	  <t>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.
	  </t>
	  <t> Recommendations:
	    <list style='symbols'>
	      <t>
		For consistent viewing, all fonts should be embedded.
		The fonts used must be available for use by the IETF community.
		Some discussion of available typefaces can be found in 
		<xref target="typefaces"/>.
	      </t>

	      <t>
		The choice of type faces with respect to serif, sans serif,
		monospace, etc., should follow the recommendations for
		HTML and CSS rendering <xref target='I-D.hildebrand-html-rfc'/>
		and  <xref target='I-D.flanagan-rfc-css'/>.
	      </t>

	      <t>
		The range of Unicode characters allowed in
		the XML source for Internet-Drafts and RFCs may
		be bounded by the availability of embeddable fonts with
		appropriate glyphs <xref target='I-D.flanagan-nonascii'/>.
	      </t>
	    </list>
	  </t>
	</section>

	<section title='Hyphenation and Line Breaks'>
	  <t>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, inserting line breaks mid-word can be harmful
	    when the "word" is actually a sequence of characters
	    representing a protocol element or protocol sequence.
	  </t>
	  <t>
	    Recommendation: avoid introducing hyphenated
	    line breaks mid-word into the visual display,
	    consistent with requirements for plain text
	    and HTML.
	  </t>
	</section>

	<section title='Hyperlinks'>
	  <t>PDF supports hyperlinks both to sections of the same
	    document and to other documents.
	  </t>
	  <t>The conversion to PDF can generate:
	    <list style='symbols'>
	      <t>hyperlinks within the document
	      </t>
	      <t>hyperlinks to other RFCs and Internet-Drafts
	      </t>
	      <t>hyperlinks to external locations
	      </t>
	      <t>hyperlinks within a table of contents
	      </t>
	      <t>
		hyperlinks within an index
	      </t>
	    </list>
	  </t>

	  <t>Recommendations:
	    <list style='symbols'>
	      <t>All hyperlinks available in the HTML rendition of the
		RFC should also be visible and active in the PDF
		produced.
		This includes both internal hyperlinks and hyperlinks to
		external resources.
	      </t>
	      <t>The table of contents, including page numbers, are useful
		when printed. These should also be hyperlinked to their
		respective sections.
	      </t>
	      <t>
	      	As specified in the section on Referencing RFCs in
	      	<xref target="RFC7322"/>, hyperlinks to RFCs from the
		references section should point to the RFC "info" page, which
		then links to the various formats available.
	      </t>
	      <t>Hyperlinks to Internet-Drafts from the
		references section should point to the datatracker entry
		page for the draft, which
		then links to the various formats available.
	      </t>
	    </list>
	  </t>
	</section>

	<section title='Similarity to Other Outputs'>
	  <t>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
	    <list style='numbers'>
	      <t>could look like the text version of the document, or
	      </t>
	      <t>could look like the text version of the document but
		with pictures rendered as pictures instead of using their
		ASCII-art equivalent, or
	      </t>
	      <t>could look like the HTML version.
	      </t>
	    </list>
	  </t>

	  <t>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.
	  </t>

	  <t>
	    Most of the choices used for the <xref target='I-D.hildebrand-html-rfc'/>
	    rendering and  <xref target='I-D.flanagan-rfc-css'/>  are thus applicable.
	    See those documents for specifics on the rendering of the specific XML elements.
	    Some notes are:
	    <list>
	      <t>
		Every place in the document that would receive an HTML ID would be given
		an identical PDF named destination.
		In addition, a named destination will be created for each page with the form
		"pg-#", as in "pg-35".
	      </t>
	      <t>
		No pilcrows are generated or made visible.
	      </t>
	      <t>
		The table of contents (generated if the XML's &lt;rfc&gt;
		element's tocInclude attribute has the value "true") 
		will have the section number linked to that section named destination, but
		will also include a page number that is linked to the page named destination.
		The section title and the page number will be separated by a
		visually-appropriate separator and the page numbers will be aligned
		with each other.
	      </t>
	      <t>
		The index (generated if the XML's &lt;rfc&gt;
		element's indexInclude attribute has the value "true")
		will have the section number linked to that section named destination, but
		will also include a page number that is linked to the page named destination.
	      </t>
	      <t>
		The running header in one line (on page 2 and all subsequent
		pages) has the RFC number on the left (RFC NNNN), the (possibly
		shortened form) title centered, and the date (Month Year) on the
		right.
		The text is rendered in a way that is visually unobtrusive.
	      </t>
	      <t>
		The running footer in one line (on all pages) has the author's
		last name on the left, category centered, and the page number on
		the right ([Page N]).
		The text is rendered in a way that is visually unobtrusive.
	      </t>
	      <t>
		We should not attempt to replicate in PDF the feature of the
		HTML format that includes a dynamic block that displays
		up-to-date information on updates, obsoletions and errata.
	      </t>
	    </list>
	  </t>
	</section>
      </section>

      <section title='"Invisible" Options and Requirements'>
	<t>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.
	</t>

	<section title='Internal Text Representation' anchor='internal'>
	  <t> The contents of a PDF file can be represented in many
	    ways.  The PDF file could be generated:
	    <list style='symbols'>

	      <t>as an image of the visual representation, such as a
		JPEG image of the word "IETF". That is, there might be no internal
		representation of letters, words or paragraphs at all.
	      </t>

	      <t>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". That is, there might be
		no internal representation of words or paragraphs at all.
	      </t>

	      <t>placing words in position on the page, such as keeping
		the word "IETF" would be kept together. That is, there might be
		no internal representation of paragraphs at all.
	      </t>

	      <t>ensuring that the running order of text in the content
		stream matches the logical reading order. That is, a sentence
		such as 'The Internet Engineering Task Force (IETF)
		supports the Internet.' would be kept together as a sentence,
		and multiple sentences within a paragraph would be kept together.
	      </t>
	    </list>
	  </t>

	  <t>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 auxiliary uses.
	  </t>

	  <t>In addition, the "role map" feature of PDF
	    (<eref target="http://help.adobe.com/en_US/acrobat/X/pro/using/WS58a04a822e3e50102bd615109794195ff-7cd8.w.html"/>)
	    would additionally allow for the mapping of the logical tags
	    found in the original XML into tags in the PDF.
	  </t>

	  <t>Recommendations:
	    <list style='symbols'>

	      <t>Text in content streams should follow the XML
		document's logical order (in the order of tags) to the
		extent possible.  This will provide optimal reuse by
		software that does not understand Tagged PDF.
		(PDF/UA requires this.)
	      </t>

	      <t>
		It might be possible to use the "role map" annotation
		to capture enough of the xml2rfc source structure,
		to the point where it is possible to reconstruct
		the XML source structure completely.
		However, there is not a compelling case to do so
		over embedding the original XML, as described in
		<xref target='embedXML' />.
	      </t>
	    </list>
	  </t>
	</section>

	<section title='Unicode Support'>
	  <t> PDF itself does not require use of Unicode.  Text is
	    represented as a sequence of glyphs which then can be
	    mapped to Unicode.
	  </t>
	  <t>Recommendations:
	    <list>
	      <t>PDF files generated must have the full text,
		as it appears in the original XML.
	      </t>
	      <t>Unicode normalization may occur.
	      </t>
	      <t>Text within SVG for SVG images should
		also have Unicode mappings.
	      </t>
	      <t>Alt-text for images should also support Unicode.
	      </t>
	    </list>
	  </t>
	</section>

	<section title='Image Processing (Artwork)'>
	  <t>
	    The XML allows both ASCII art and SVG to be used for artwork.
	  </t>
	  <t>Recommendations:
	    <list>
	      <t>
		If both ASCII art and SVG are available for a picture,
		the SVG artwork should be the preferred over the ASCII artwork.
	      </t>
	      <t>
		ASCII artwork must be rendered using a monospace font.
	      </t>
	    </list>
	  </t>
	</section>

	<section title='Text Description of Images (Alt-Text)'>
	  <t>
	    Guidelines for accessibility of PDF 
	    <eref target="http://www.w3.org/TR/WCAG20-TECHS/PDF1.html"/>
	    recommend that images, formulas, and other non-text items
	    provide textual alternatives, using the '/Alt' Tag in
	    PDF to provide human-readable text that can be vocalized
	    by text-to-speech technology.
	  </t>
	  <t>
	    Recommendation: Any alt-text for artwork and figures
	    available in the XML source should be stored using the PDF
	    /Alt property. Internet draft authors and the RFC editor
	    should ensure inclusion of alt-text for all SVG or images,
	    within the XML source.
	  </t>
	</section>

	<section title='Metadata Support'>
	  <t>
	    Metadata encodes information about the document authors,
	    the document series, date created, etc.  Having this
	    metadata within the PDF file allows it to be used by
	    search engines, viewers and other reuse tools.
	    PDF supports embedded metadata in a variety of
	    ways, including using XMP
	    <xref target='XMP' />, the Extensible Metadata Platform (XMP).
	    The RFC editor maintains metadata about an RFC on its
	    info page.
	  </t>
	  <t>
	    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. This information should be consistent with
	    the RFC editor info page at the time of publication.
	  </t>
	</section>

	<section title='Document Structure Support'>
	  <t>PDF supports an "outline" feature where sections of the
	    document are marked; this could be used in addition to the
	    table of contents as a navigation aid.
	  </t>
	  <t>
	    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.
	  </t>
	  <t>
	    Recommendation: 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.
	  </t>
	</section>

	<!--
	<section title='Tagged PDF'>
	  <t>
	    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.
	    (See also
	    <eref target="http://www.pdfa.org/2011/10/the-value-of-tagged-pdf/"/>.)
	  </t>
	</section>
	-->

	<section title='Embedded Files' anchor='embedXML'>
	  <t>PDF has the capability of including other files;
	    the files may be labeled both by a media type and a
	    role, the AFRelationship key <xref target='PDFA3'/>.
	    In this way, the PDF file acts also as a container.
	  </t>
	  <t>Embedded content may be compressed.
	  </t>
	  <t>Many PDF viewers support the ability to
	    view and extract embedded files, although this capability
	    is not universal.
	  </t>
	  <t>Embedding content in the PDF file allows the PDF to act as
	    a complete package, which can be transformed, archived,
	    and digitally signed. 
	    (Some sample code illustrating how items can be attached to a 
	    PDF file and subsequently extracted can be found
	    at <eref target='https://github.com/Aiybe/xmptest'/>.)
	    Useful possibilities:

	    <list>
	      <t>Embed the source XML input file itself within the PDF.
		If the source SVG and images for illustrations are also
		embedded, this would make the PDF file totally
		self-referential.
	      </t>
	      <t>Embed directly extractable components that are useful for
		independent processing, including ABNF, MIBs, source code
		for reference implementations.  This capability might be
		supported through other mechanisms from the XML source
		files, but could also be supported within the PDF.
	      </t>
	      <t>Finding, extracting and embedding other components
		may require additional markup to clearly identify them,
		and additional review to ensure the correctness of
		embedded files that are not visible.
	      </t>
	    </list>
	  </t>
	  <t>
	    Recommendations:
	    <list>
	      <t>Embed the XML source and all illustrations,
		for RFCs, as a standard
		feature for xml2rfc's PDF output.
	      </t>
	      <t>If possible, make this a standard feature for
		Internet-Drafts as well.
	      </t>
	      <t>
		Named &lt;sourcecode&gt; entries should be embedded.
	      </t>
	      <t>
		Bitmap images (SVG sources, JPEGs, PNGs, etc) should be embedded.
	      </t>
	    </list>
	  </t>
	</section>

      </section>
      <section title='Digital Signatures'>
	<t>
	  The RFC Editor and staff are at times called to provide evidence
	  that a particular RFC is the "original" and has not been modified;
	  digital signatures can provide that verification.
	  As signatures also apply to embedded content, embedding the
	  XML source will provide a way of signing the source XML
	  that was used to product the PDF file as well.
	</t>
	<t>
	  PDF has supported digital signatures since PDF 1.2, and there
	  are multiple methods and options available for signing PDF files.
	  The signing of internet-drafts and RFCs will be guided by
	  <xref target='I-D.housley-rfc-and-id-signatures'/>.
	</t>
      </section>
    </section>

  </middle>
  <back>
    <references>
      <!-- ?rfc include='reference.ISO.32000-1.2008' ? -->
      <reference anchor="PDF">
	<front>
	  <title>
	    Portable document format -- Part 1: PDF 1.7
	  </title>
	  <author>
	    <organization>ISO</organization>
	  </author>
	  <date month="" year="2008"/>
	</front>
	<seriesInfo name="ISO" value="32000-1"/>
	<annotation>Also available free from Adobe.</annotation>
      </reference>

      <!-- rfc include='reference.ISO.16684-1.2012' XMP -->
      <reference anchor="XMP">
	<front>
	  <title>Extensible metadata platform (XMP) specification --
	    Part 1: Data model, serialization and core properties
	  </title>
	  <author><organization>ISO</organization></author>
	  <date month="" year="2012"/>
	</front>
	<seriesInfo name="ISO" value="16684-1"/>
	<annotation>Not available free, but there
	  are a number of descriptive resources, e.g.,
	  <eref target="http://en.wikipedia.org/wiki/Extensible_Metadata_Platform"/>
	</annotation>
      </reference>

      <!-- rfc include='reference.ISO.19005-2.2012' PDF/A2 -->
      <reference anchor="PDFA2">
	<front>
	  <title>Electronic document file format for long-term
	    preservation -- Part 2: Use of ISO 32000-1 (PDF/A-2).
	  </title>
	  <author><organization>ISO</organization></author>
	  <date month="" year="2011" />
	</front>
	<seriesInfo name="ISO" value="19005-2" />
      </reference>

      <!-- rfc include='reference.ISO.19005-3.2012' PDF/A3 -->
      <reference anchor="PDFA3">
	<front>
	  <title>Electronic document file format for long-term
	    preservation -- Part 3: Use of ISO 32000-1 with support for
	    embedded files (PDF/A-3)
	  </title>
	  <author><organization>ISO</organization></author>
	  <date month="" year="2012" />
	</front>
	<seriesInfo name="ISO" value="19005-3" />
      </reference>

      <!-- rfc include='reference.ISO.14289-1.2012' PDF/UA -->
      <reference anchor="PDFUA">
	<front>
	  <title>Electronic document file format enhancement for
	    accessibility -- Part 1: Use of ISO 32000-1
	    (PDF/UA-1)</title>
	  <author><organization>ISO</organization></author>
	  <date month="" year="2012" />
	</front>
	<seriesInfo name="ISO" value="19005-3" />
      </reference>
    </references>
    <references title='Informative References'>
      <!-- ?rfc include='reference.RFC.3778' ? -->
      <?rfc include='reference.RFC.2346' ?>
      <?rfc include='reference.RFC.6949' ?>
      <?rfc include='reference.RFC.7322' ?>
      <?rfc include='reference.I-D.flanagan-nonascii' ?>
      <?rfc include='reference.I-D.flanagan-rfc-css' ?>
      <?rfc include='reference.I-D.hardy-pdf-mime' ?>
      <?rfc include='reference.I-D.hildebrand-html-rfc' ?>
      <?rfc include='reference.I-D.housley-rfc-and-id-signatures' ?>
    </references>

    <section title='History and Current Use of PDF with RFCs and Internet-Drafts'>
      <t>NOTE: this section is meant as an overview
	to give some background.
      </t>
      <section title='RFCs'>
	<t>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.
	</t>
      </section>
      <section title='Internet-Drafts'>
	<t>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.
	</t>
      </section>
    </section>
    
    <section title="Paged Content Layout Quality" anchor="page-layout">
      <t>The process of creating a paged document from running text
	typically involves ensuring 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:
	<list style='hanging'>
	  <t hangText='Widow and Orphan Management:'>
	    Widows and orphans (<eref target='https://en.wikipedia.org/wiki/Widows_and_orphans'/>)
	    should be avoided automatically (unless the entire paragraph is only one line).
	    Ensure that a page break does not occur after the first
	    line of a paragraph (orphans), if necessary, using slightly longer
	    page sizes.
	    Similarly, ensure that a page break does not occur before
	    the last line of a paragraph (widows).
	  </t>
	  <t hangText='Keep Section Heading Contiguous:'>
	    Do not insert a page break immediately after a section heading.
	    If there isn't room on a page for the first (two)
	    lines of a section after the section heading,
	    insert a page break before the heading.
	  </t>
	  <t hangText='Avoid Splitting Artwork:'>
	    Figures should not be split from figure titles.
	    If possible, keep the figure on the same page
	    as the (first) mention of the figure.
	  </t>
	  <t hangText='Headers for Long Tables after Page Breaks:'>
	    Another common option in producing paginated documents
	    is to include the column headings of a table if
	    the table cannot be displayed on a single page.
	    Similarly, tables should not be split from the table titles.
	  </t>
	  <t hangText='keepWithNext and keepWithPrevious:'>
	    The XML attributes of "keepWithNext" and "keepWithPrevious"
	    should be followed whenever possible.
	  </t>
	  <t hangText='Whitespace Preservation:'>
	    The XML entities such as NBSP and NBHYPHEN should be
	    followed as directed whenever possible.
	  </t>
	</list>
      </t>
    </section>

    <section title='Tooling' anchor='tooling'>

      <t>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.
      </t>

      <section title='PDF Viewers'>
	<t>As with most file formats, PDF files are experienced through
	a reader or viewer of PDF files. For most of the common
	platforms in use (iOS, OS X, Windows, Android, ChromeOS,
	Kindle) and for most browsers (Edge, Safari, Chrome,
	Firefox), PDF viewing is built in. In addition there are
	many PDF viewers available for download and install.
	</t>

	<t>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
	  <xref target='requirements'/>).
	</t>

      </section>
      <section title='Printers'>
	<t>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.
	</t>
      </section>

      <section title='PDF Generation Libraries'>
	<t>Because the xml2rfc format is a unique format,
	  software for converting XML source documents to
	  the various formats will be needed, including
	  PDF generation.
	</t>
	<t>
	  One promising
	  direction is suggested in
	  <eref target="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.
	</t>
	<t>
	  Several libraries are also available for generating PDF signatures.
	  The choice of library to use for xml2pdf will depend on many factors:
	  programming language, quality of implementation, quality of
	  PDF generated, support, cost, availability, 
	  and so forth. 

	</t>
      </section>

      <section title='Typefaces' anchor='typefaces'>
	<t>This section is intended to discuss available typefaces
	  that might satisfy requirements. Some openly available
	  fixed-width typefaces (without extensive Unicode support,
	  however) include:
	  <list style='symbols'>
	    <t><eref target="https://sourceforge.net/projects/sourcesans.adobe/?source=directory">Source Sans</eref></t>
	    <t><eref target="https://sourceforge.net/projects/sourceserifpro.adobe/?source=directory">Source Serif Pro</eref></t>
	    <t><eref target="https://sourceforge.net/projects/sourcecodepro.adobe/?source=drectory">Source Code Pro</eref></t>
	  </list>
	  A font that looks promising for its broad Unicode support is
	  <eref target="https://www.rosettatype.com/Skolar">Skolar</eref>,
	  but it requires licensing.
	  Another potentially useful set of typefaces is the <eref target="https://www.google.com/get/noto/">Noto</eref> family from Google.
	</t>
      </section>

      <section title='Other Tools'>
	<t>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.
	</t>
	<t>Validation of an arbitrary author-generated PDF file
	  would be quite difficult; there are few PDF validation
	  tools. However, if RFCs and Internet-Drafts are generated by
	  conversion from XML via xml2rfc, then explicit validation of PDF
	  and adherence to expected profiles would mainly be useful to ensure
	  that xml2rfc has functioned properly.
	</t>
	<t>Recommendations:
	  <list style='symbols'>
	    <t>Discourage (but allow) submission of a PDF representation
	      for Internet-Drafts. In most cases, the PDF for an Internet-Draft
	      should be produced automatically when XML is submitted,
	      with an opportunity to verify the conversion.
	    </t>
	  </list>
	</t>
      </section>

    </section>
    <section title='Acknowledgements'>
      <t>
	The input of the following people is gratefully acknowledged:
   	Nevil Brownlee (ISE), 
	Brian Carpenter,
	Chris Dearlove,
	Martin Duerst,
	Heather Flanagan (RSE),
	Joe Hildebrand,
	Paul Hoffman,
	Duff Johnson,
   	Ted Lemon, 
	Henrik Levkowetz,
   	Julian Reschke, 
   	Adam Roach,
	Leonard Rosenthol,
   	Alice Russo,
   	Robert Sparks,
	Andrew Sullivan,
	and
   	Dave Thaler.
	<!--
	  Sean Leonard &lt;dev+ietf@seantek.com&gt;

	  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]

	-->
      </t>
    </section>
  </back>
</rfc>