<pre>Network Working Group                                           E. Wilde
Request for Comments: 5147                                   UC Berkeley
Updates: <a href="./rfc2046">2046</a>                                                  M. Duerst
Category: Standards Track                       Aoyama Gakuin University
                                                              April 2008


         <span class="h1">URI Fragment Identifiers for the text/plain Media Type</span>

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This memo defines URI fragment identifiers for text/plain MIME
   entities.  These fragment identifiers make it possible to refer to
   parts of a text/plain MIME entity, either identified by character
   position or range, or by line position or range.  Fragment
   identifiers may also contain information for integrity checks to make
   them more robust.


























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Table of Contents

   <a href="#section-1">1</a>.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-3">3</a>
     <a href="#section-1.1">1.1</a>.  What Is text/plain?  . . . . . . . . . . . . . . . . . . .  <a href="#page-3">3</a>
     <a href="#section-1.2">1.2</a>.  What Is a URI Fragment Identifier? . . . . . . . . . . . .  <a href="#page-4">4</a>
     <a href="#section-1.3">1.3</a>.  Why text/plain Fragment Identifiers? . . . . . . . . . . .  <a href="#page-4">4</a>
     <a href="#section-1.4">1.4</a>.  Incremental Deployment . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
     <a href="#section-1.5">1.5</a>.  Notation Used in This Memo . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
   <a href="#section-2">2</a>.  Fragment Identification Methods  . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
     <a href="#section-2.1">2.1</a>.  Fragment Identification Principles . . . . . . . . . . . .  <a href="#page-6">6</a>
       <a href="#section-2.1.1">2.1.1</a>.  Positions and Ranges . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
       <a href="#section-2.1.2">2.1.2</a>.  Characters and Lines . . . . . . . . . . . . . . . . .  <a href="#page-7">7</a>
     <a href="#section-2.2">2.2</a>.  Combining the Principles . . . . . . . . . . . . . . . . .  <a href="#page-7">7</a>
       <a href="#section-2.2.1">2.2.1</a>.  Character Position . . . . . . . . . . . . . . . . . .  <a href="#page-7">7</a>
       <a href="#section-2.2.2">2.2.2</a>.  Character Range  . . . . . . . . . . . . . . . . . . .  <a href="#page-8">8</a>
       <a href="#section-2.2.3">2.2.3</a>.  Line Position  . . . . . . . . . . . . . . . . . . . .  <a href="#page-8">8</a>
       <a href="#section-2.2.4">2.2.4</a>.  Line Range . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-8">8</a>
     <a href="#section-2.3">2.3</a>.  Fragment Identifier Robustness . . . . . . . . . . . . . .  <a href="#page-8">8</a>
   <a href="#section-3">3</a>.  Fragment Identification Syntax . . . . . . . . . . . . . . . .  <a href="#page-9">9</a>
     <a href="#section-3.1">3.1</a>.  Integrity Checks . . . . . . . . . . . . . . . . . . . . .  <a href="#page-9">9</a>
   <a href="#section-4">4</a>.  Fragment Identifier Processing . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
     <a href="#section-4.1">4.1</a>.  Handling of Line Endings in text/plain MIME Entities . . . <a href="#page-10">10</a>
     <a href="#section-4.2">4.2</a>.  Handling of Position Values  . . . . . . . . . . . . . . . <a href="#page-11">11</a>
     <a href="#section-4.3">4.3</a>.  Handling of Integrity Checks . . . . . . . . . . . . . . . <a href="#page-11">11</a>
     <a href="#section-4.4">4.4</a>.  Syntax Errors in Fragment Identifiers  . . . . . . . . . . <a href="#page-12">12</a>
   <a href="#section-5">5</a>.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
   <a href="#section-6">6</a>.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
   <a href="#section-7">7</a>.  Security Considerations  . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
   <a href="#section-8">8</a>.  References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
     <a href="#section-8.1">8.1</a>.  Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
     <a href="#section-8.2">8.2</a>.  Informative References . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
   <a href="#appendix-A">Appendix A</a>.  Acknowledgements  . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>



















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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This memo updates the text/plain media type defined in <a href="./rfc2046">RFC 2046</a> [<a href="#ref-3" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types&quot;">3</a>]
   by defining URI fragment identifiers for text/plain MIME entities.
   This makes it possible to refer to parts of a text/plain MIME entity.
   Such parts can be identified by either character position or range,
   or by line position or range.  Integrity checking information can be
   added to a fragment identifier to make it more robust, enabling
   applications to detect changes of the entity.

   This section gives an introduction to the general concepts of text/
   plain MIME entities and URI fragment identifiers, and it discusses
   the need for fragment identifiers for text/plain and deployment
   issues.  <a href="#section-2">Section 2</a> discusses the principles and methods on which this
   memo is based.  <a href="#section-3">Section 3</a> defines the syntax, and <a href="#section-4">Section 4</a> discusses
   processing of text/plain fragment identifiers.  <a href="#section-5">Section 5</a> shows some
   examples.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  What Is text/plain?</span>

   Internet Media Types (often referred to as "MIME types"), as defined
   in <a href="./rfc2045">RFC 2045</a> [<a href="#ref-2" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies&quot;">2</a>] and <a href="./rfc2046">RFC 2046</a> [<a href="#ref-3" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types&quot;">3</a>], are used to identify different
   types and sub-types of media.  <a href="./rfc2046">RFC 2046</a> [<a href="#ref-3" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types&quot;">3</a>] and <a href="./rfc3676">RFC 3676</a> [<a href="#ref-6" title="&quot;The Text/Plain Format and DelSp Parameters&quot;">6</a>] specify
   the text/plain media type, which is used for simple, unformatted
   text.  Quoting from <a href="./rfc2046">RFC 2046</a> [<a href="#ref-3" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types&quot;">3</a>]: "Plain text does not provide for or
   allow formatting commands, font attribute specifications, processing
   instructions, interpretation directives, or content markup.  Plain
   text is seen simply as a linear sequence of characters, possibly
   interrupted by line breaks or page breaks".

   The text/plain media type does not restrict the character encoding;
   any character encoding may be used.  In the absence of an explicit
   character encoding declaration, US-ASCII [<a href="#ref-13" title="&quot;Coded Character Set - 7-Bit American National Standard Code for Information Interchange&quot;">13</a>] is assumed as the
   default character encoding.  This variability of the character
   encoding makes it impossible to count characters in a text/plain MIME
   entity without taking the character encoding into account, because
   there are many character encodings using more than one octet per
   character.

   The biggest advantage of text/plain MIME entities is their ease of
   use and their portability among different platforms.  As long as they
   use popular character encodings (such as US-ASCII or UTF-8 [<a href="#ref-12" title="&quot;UTF-8, a transformation format of ISO 10646&quot;">12</a>]),
   they can be displayed and processed on virtually every computer
   system.  The only remaining interoperability issue is the
   representation of line endings, which is discussed in <a href="#section-4.1">Section 4.1</a>.






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<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  What Is a URI Fragment Identifier?</span>

   URIs are the identification mechanism for resources on the Web.  The
   URI syntax specified in <a href="./rfc3986">RFC 3986</a> [<a href="#ref-7" title="&quot;Uniform Resource Identifier (URI): Generic Syntax&quot;">7</a>] optionally includes a so-called
   "fragment identifier", separated by a number sign ('#').  The
   fragment identifier consists of additional reference information to
   be interpreted by the user agent after the retrieval action has been
   successfully completed.  The semantics of a fragment identifier are a
   property of the data resulting from a retrieval action, regardless of
   the type of URI used in the reference.  Therefore, the format and
   interpretation of fragment identifiers is dependent on the media type
   of the retrieval result.

   The most popular fragment identifier is defined for text/html
   (defined in <a href="./rfc2854">RFC 2854</a> [<a href="#ref-10" title="&quot;The 'text/html' Media Type&quot;">10</a>]) and makes it possible to refer to a
   specific element (identified by the value of a 'name' or 'id'
   attribute) of an HTML document.  This makes it possible to reference
   a specific part of a Web page, rather than a Web page as a whole.

<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  Why text/plain Fragment Identifiers?</span>

   Referring to specific parts of a resource can be very useful because
   it enables users and applications to create more specific references.
   Users can create references to the part they really are interested in
   or want to talk about, rather than always pointing to a complete
   resource.  Even though it is suggested that fragment identification
   methods are specified in a media type's MIME registration (see [<a href="#ref-15" title="&quot;Media Type Specifications and Registration Procedures&quot;">15</a>]),
   many media types do not have fragment identification methods
   associated with them.

   Fragment identifiers are only useful if supported by the client,
   because they are only interpreted by the client.  Therefore, a new
   fragment identification method will require some time to be adopted
   by clients, and older clients will not support it.  However, because
   the URI still works even if the fragment identifier is not supported
   (the resource is retrieved, but the fragment identifier is not
   interpreted), rapid adoption is not highly critical to ensure the
   success of a new fragment identification method.

   Fragment identifiers for text/plain, as defined in this memo, make it
   possible to refer to specific parts of a text/plain MIME entity,
   using concepts of positions and ranges, which may be applied to
   characters and lines.  Thus, text/plain fragment identifiers enable
   users to exchange information more specifically, thereby reducing the
   time and effort that is necessary to manually search for the relevant
   part of a text/plain MIME entity.





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   The text/plain format does not support the embedding of links, so in
   most environments, text/plain resources can only serve as targets for
   links, and not as sources.  However, when combining the text/plain
   fragment identifiers specified in this memo with out-of-line linking
   mechanisms such as XLink [<a href="#ref-14" title="&quot;XML Linking Language (XLink) Version 1.0&quot;">14</a>], it becomes possible to "bind" link
   resources to text/plain resources and thereby "embed" links into
   text/plain resources.  Thus, the text/plain fragment identifiers
   specified in this memo open a path for text/plain files to become
   bidirectionally navigable resources in hypermedia systems such as the
   Web.

<span class="h3"><a class="selflink" id="section-1.4" href="#section-1.4">1.4</a>.  Incremental Deployment</span>

   As long as text/plain fragment identifiers are not supported
   universally, it is important to consider the implications of
   incremental deployment.  Clients (for example, Web browsers) not
   supporting the text/plain fragment identifier described in this memo
   will work with URI references to text/plain MIME entities, but they
   will fail to locate the sub-resource identified by the fragment
   identifier.  This is a reasonable fallback behavior, and in general,
   users should take into account the possibility that a program
   interpreting a given URI will fail to interpret the fragment
   identifier part.  Since fragment identifier evaluation is local to
   the client (and happens after retrieving the MIME entity), there is
   no reliable way for a server to determine whether a requesting client
   is using a URI containing a fragment identifier.

<span class="h3"><a class="selflink" id="section-1.5" href="#section-1.5">1.5</a>.  Notation Used in This Memo</span>

   The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in <a href="./rfc2119">RFC</a>
   <a href="./rfc2119">2119</a> [<a href="#ref-4" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">4</a>].

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Fragment Identification Methods</span>

   The identification of fragments of text/plain MIME entities can be
   based on different foundations.  Since it is not possible to insert
   explicit, invisible identifiers into a text/plain MIME entity (for
   example, as used in HTML documents, implemented through dedicated
   attributes), fragment identification has to rely on certain inherent
   properties of the MIME entity.  This memo specifies fragment
   identification using four different methods, which are character
   positions and ranges, and line positions and ranges, augmented by an
   integrity check mechanism for improving the robustness of fragment
   identifiers.





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   When interpreting character or line numbers, implementations MUST
   take the character encoding of the MIME entity into account, because
   character count and octet count may differ for the character encoding
   being used.  For example, a MIME entity using the UTF-16 encoding (as
   specified in <a href="./rfc2781">RFC 2781</a> [<a href="#ref-11" title="&quot;UTF-16, an encoding of ISO 10646&quot;">11</a>]) uses two octets per character in most
   cases, and sometimes four octets per character.  It can also have a
   leading BOM (Byte-Order Mark), which does not count as a character
   and thus also affects the mapping from a simple octet count to a
   character count.

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  Fragment Identification Principles</span>

   Fragment identification can be done by combining two orthogonal
   principles, which are positions and ranges, and characters and lines.
   This section describes the principles themselves, while <a href="#section-2.2">Section 2.2</a>
   describes the combination of the principles.

<span class="h4"><a class="selflink" id="section-2.1.1" href="#section-2.1.1">2.1.1</a>.  Positions and Ranges</span>

   A position does not identify an actual fragment of the MIME entity,
   but a position inside the MIME entity, which can be regarded as a
   fragment of length zero.  The use case for positions is to provide
   pointers for applications that may use them to implement
   functionalities such as "insert some text here", which needs a
   position rather than a fragment.  Positions are counted from zero;
   position zero being before the first character or line of a text/
   plain MIME entity.  Thus, a text/plain MIME entity having one
   character has two positions, one before the first character (position
   zero), and one after the first character (position 1).

   Since positions are fragments of length zero, applications SHOULD use
   other methods than highlighting to indicate positions, the most
   obvious way being the positioning of a cursor (if the application
   supports the concept of a cursor).

   Ranges, on the other hand, identify fragments of a MIME entity that
   have a length that may be greater than zero.  As a general principle
   for ranges, they specify both a lower and an upper bound.  The start
   or the end of a range specification may be omitted, defaulting to the
   first or last position of the MIME entity, respectively.  The end of
   a range must have a value greater than or equal to the start.  A
   range with identical start and end is legal and identifies a range of
   length zero, which is equivalent to a position.

   Applications that support a concept such as highlighting SHOULD use
   such a concept to indicate fragments of lengths greater than zero to
   the user.




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   For positions and ranges, it is implicitly assumed that if a number
   is greater than the actual number of elements in the MIME entity,
   then it is referring to the last element of the MIME entity (see
   <a href="#section-4">Section 4</a> for details).

<span class="h4"><a class="selflink" id="section-2.1.2" href="#section-2.1.2">2.1.2</a>.  Characters and Lines</span>

   The concept of positions and ranges can be applied to characters or
   lines.  In both cases, positions indicate points between these
   entities, while ranges identify zero or more of these entities by
   indicating positions.

   Character positions are numbered starting with zero (ignoring initial
   BOM marks or similar concepts that are not part of the actual textual
   content of a text/plain MIME entity), and counting each character
   separately, with the exception of line endings, which are always
   counted as one character (see <a href="#section-4.1">Section 4.1</a> for details).

   Line positions are numbered starting with zero (with line position
   zero always being identical with character position zero);
   <a href="#section-4.1">Section 4.1</a> describes how line endings are identified.  Fragments
   identified by lines include the line endings, so applications
   identifying line-based fragments MUST include the line endings in the
   fragment identification they are using (e.g., the highlighted
   selection).  If a MIME entity does not contain any line endings, then
   it consists of a single (the first) line.

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  Combining the Principles</span>

   In the following sections, the principles described in the preceding
   section (positions/ranges and characters/lines) are combined,
   resulting in four use cases.  The schemes mentioned below refer to
   the fragment identifier syntax, described in detail in <a href="#section-3">Section 3</a>.

<span class="h4"><a class="selflink" id="section-2.2.1" href="#section-2.2.1">2.2.1</a>.  Character Position</span>

   To identify a character position (i.e., a fragment of length zero
   between two characters), the 'char' scheme followed by a single
   number is used.  This method identifies a position between two
   characters (or before the first or after the last character), rather
   than identifying a fragment consisting of a number of characters.
   Character position counting starts with zero, so the character
   position before the first character of a text/plain MIME entity has
   the character position zero, and a MIME entity containing n distinct
   characters has n+1 distinct character positions, the last one having
   the character position n.





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<span class="h4"><a class="selflink" id="section-2.2.2" href="#section-2.2.2">2.2.2</a>.  Character Range</span>

   To identify a fragment of one or more characters (a character range),
   the 'char' scheme followed by a range specification is used.  A
   character range is a consecutive region of the MIME entity that
   extends from the starting character position of the range to the
   ending character position of the range.

<span class="h4"><a class="selflink" id="section-2.2.3" href="#section-2.2.3">2.2.3</a>.  Line Position</span>

   To identify a line position (i.e., a fragment of length zero between
   two lines), the 'line' scheme followed by a single number is used.
   This method identifies a position between two lines (or before the
   first or after the last line), rather than identifying a fragment
   consisting of a number of lines.  Line position counting starts with
   zero, so the line position before the first line of a text/plain MIME
   entity has the line position zero, and a MIME entity containing n
   distinct lines has n+1 distinct line positions, the last one having
   the line position n.

<span class="h4"><a class="selflink" id="section-2.2.4" href="#section-2.2.4">2.2.4</a>.  Line Range</span>

   To identify a fragment of one or more lines (a line range), the
   'line' scheme followed by a range specification is used.  A line
   range is a consecutive region of the MIME entity that extends from
   the starting line position of the range to the ending line position
   of the range.

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  Fragment Identifier Robustness</span>

   It is easily possible that a modification of the referenced resource
   will break a fragment identifier.  If applications want to create
   more robust fragment identifiers, they may do so by adding integrity-
   check information to fragment identifiers.  Such information is used
   to detect changes in the resource.  Applications can then warn users
   about the possibility that a fragment identifier might have been
   broken by a modification of the resource.

   Fragment identifiers are interpreted by clients, and therefore
   integrity-check information is defined on MIME entities rather than
   on the resource itself.  This means that the integrity-check
   information is specific to a certain entity.  Specifically, content
   encodings and/or content transfer encodings must be removed before
   using integrity-check information.

   Integrity-check information may specify the character encoding that
   has been used when creating the information, and if such a
   specification is present, clients MUST check whether the character



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   encoding specified and the character encoding of the retrieved MIME
   entity are equal, and clients MUST NOT use the integrity check
   information if these values differ.  However, clients MAY choose to
   transcode the retrieved MIME entity in the case of differing
   character encodings, and after doing so, apply integrity checks.
   Please note that this method is inherently unreliable because certain
   characters or character sequences may have been lost or normalized
   due to restrictions in one of the character encodings used.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Fragment Identification Syntax</span>

   The syntax for the text/plain fragment identifiers is
   straightforward.  The syntax defines four schemes, 'char', 'line',
   and integrity check (which can either be 'length' or 'md5').  The
   'char' and 'line' schemes can be used in two different variants,
   either the position variant (with a single number), or the range
   variant (with two comma-separated numbers).  An integrity check can
   either use the 'length' or the 'md5' scheme to specify a value.
   'length' in this case serves as a very weak but easy to calculate
   integrity check.

   The following syntax definition uses ABNF as defined in <a href="./rfc5234">RFC 5234</a> [<a href="#ref-9" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">9</a>],
   including the rules DIGIT and HEXDIG.  The mime-charset rule is
   defined in <a href="./rfc2978">RFC 2978</a> [<a href="#ref-5" title="&quot;IANA Charset Registration Procedures&quot;">5</a>].

   NOTE:  In the descriptions that follow, specified text values MUST be
      used exactly as given, using exactly the indicated lower-case
      letters.  In this respect, the ABNF usage differs from [<a href="#ref-9" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">9</a>].


   text-fragment   =  text-scheme 0*( ";" integrity-check )
   text-scheme     =  ( char-scheme / line-scheme )
   char-scheme     =  "char=" ( position / range )
   line-scheme     =  "line=" ( position / range )
   integrity-check =  ( length-scheme / md5-scheme )
                        [ "," mime-charset ]
   position        =  number
   range           =  ( position "," [ position ] ) / ( "," position )
   number          =  1*( DIGIT )
   length-scheme   =  "length=" number
   md5-scheme      =  "md5=" md5-value
   md5-value       =  32HEXDIG

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Integrity Checks</span>

   An integrity check can either specify a MIME entity's length, or its
   MD5 fingerprint.  In both cases, it can optionally specify the
   character encoding that has been used when calculating the integrity



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   check, so that clients interpreting the fragment identifier may check
   whether they are using the same character encoding for their
   calculations.  For lengths, the character encoding can be necessary
   because it can influence the character count.  As an example, Unicode
   includes precomposed characters for writing Vietnamese, but in the
   windows-1258 encoding, also used for writing Vietnamese, some
   characters have to be encoded with separate diacritics, which means
   that two characters will be counted.  Applying Unicode terminology,
   this means that the length of a text/plain MIME entity is computed
   based on its "code points".  For MD5 fingerprints, the character
   encoding is necessary because the MD5 algorithm works on the binary
   representation of the text/plain resource.

   To allow future changes to this specification to address developments
   in cryptography, implementations MUST ignore new types of integrity
   checks, with names other than 'length' and 'md5'.  If several
   integrity checks are present, an application can use whatever
   integrity checks it understands, and among these, those integrity
   checks that provide an appropriate trade-off between performance and
   the need for integrity checking.  Please see <a href="#section-4.3">Section 4.3</a> for further
   details.

   The length of a text/plain MIME entity is calculated by using the
   principles defined in <a href="#section-2.1.2">Section 2.1.2</a>.  The MD5 fingerprint of a text/
   plain MIME entity is calculated by using the algorithm presented in
   [<a href="#ref-1" title="&quot;The MD5 Message-Digest Algorithm&quot;">1</a>], encoding the result in 32 hexadecimal digits (using uppercase or
   lowercase letters) as a representation of the 128 bits that are the
   result of the MD5 algorithm.  Calculation of integrity checks is done
   after stripping any potential content-encodings or content-transfer-
   encodings of the transport mechanism.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Fragment Identifier Processing</span>

   Applications implementing support for the mechanism described in this
   memo MUST behave as described in the following sections.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Handling of Line Endings in text/plain MIME Entities</span>

   In Internet messages, line endings in text/plain MIME entities are
   represented by CR+LF character sequences (see <a href="./rfc2046">RFC 2046</a> [<a href="#ref-3" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types&quot;">3</a>] and <a href="./rfc3676">RFC</a>
   <a href="./rfc3676">3676</a> [<a href="#ref-6" title="&quot;The Text/Plain Format and DelSp Parameters&quot;">6</a>]).  However, some protocols (such as HTTP) additionally allow
   other conventions for line endings.  Also, some operating systems
   store text/plain entities locally with different line endings (in
   most cases, Unix uses LF, MacOS traditionally uses CR, and Windows
   uses CR+LF).

   Independent of the number of bytes or characters used to represent a
   line ending, each line ending MUST be counted as one single



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   character.  Implementations interpreting text/plain fragment
   identifiers MUST take into account the line ending conventions of the
   protocols and other contexts that they work in.

   As an example, an implementation working in the context of a Web
   browser supporting http: URIs has to support the various line ending
   conventions permitted by HTTP.  As another example, an implementation
   used on local files (e.g., with the file: URI scheme) has to support
   the conventions used for local storage.  All implementations SHOULD
   support the Internet-wide CR+LF line ending convention, and MAY
   support additional conventions not related to the protocols or
   systems they work with.

   Implementers should be aware of the fact that line endings in plain
   text entities can be represented by other characters or character
   sequences than CR+LF.  Besides the abovementioned CR and LF, there
   are also NEL and CR+NEL.  In general, the encoding of line endings
   can also depend on the character encoding of the MIME entity, and
   implementations have to take this into account where necessary.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Handling of Position Values</span>

   If any position value (as a position or as part of a range) is
   greater than the length of the actual MIME entity, then it identifies
   the last character position or line position of the MIME entity.  If
   the first position value in a range is not present, then the range
   extends from the start of the MIME entity.  If the second position
   value in a range is not present, then the range extends to the end of
   the MIME entity.  If a range scheme's positions are not properly
   ordered (i.e., the first number is less than the second), then the
   fragment identifier MUST be ignored.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Handling of Integrity Checks</span>

   Clients are not required to implement the handling of integrity
   checks, so they MAY choose to ignore integrity check information
   altogether.  However, if they do implement integrity checking, the
   following applies:

   If a fragment identifier contains one or more integrity checks, and a
   client retrieves a MIME entity and, using some integrity check(s),
   detects that the entity has changed (observing the character encoding
   specification as described in <a href="#section-3.1">Section 3.1</a>, if present), then the
   client SHOULD NOT interpret the text/plain fragment identifier.  A
   client MAY signal this situation to the user.






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<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Syntax Errors in Fragment Identifiers</span>

   If a fragment identifier contains a syntax error (i.e., does not
   conform to the syntax specified in <a href="#section-3">Section 3</a>), then it MUST be
   ignored by clients.  Clients MUST NOT make any attempt to correct or
   guess fragment identifiers.  Syntax errors MAY be reported by
   clients.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Examples</span>

   The following examples show some usages for the fragment identifiers
   defined in this memo.

   http://example.com/text.txt#char=100

   This URI identifies the position after the 100th character of the
   text.txt MIME entity.  It should be noted that it is not clear which
   octet(s) of the MIME entity this will be without retrieving the MIME
   entity and thus knowing which character encoding it is using (in case
   of HTTP, this information will be given in the Content-Type header of
   the response).  If the MIME entity has fewer than 100 characters, the
   URI identifies the position after the MIME entity's last character.

   http://example.com/text.txt#line=10,20

   This URI identifies lines 11 to 20 of the text.txt MIME entity.  If
   the MIME entity has fewer than 11 lines, it identifies the position
   after the last line.  If the MIME entity has less than 20 but at
   least 11 lines, it identifies the range from line 11 to the last line
   of the MIME entity.

   https://example.com/text.txt#line=,1

   This URI identifies the first line.  Please note that the URI scheme
   has been changed to https.

   ftp://example.com/text.txt#line=10,20;length=9876,UTF-8

   As in the second example, this URI identifies lines 11 to 20 of the
   text.txt MIME entity.  The additional length integrity check
   specifies that the MIME entity has a length of 9876 characters when
   encoded in UTF-8.  If the client supports the length scheme, it may
   test the retrieved MIME entity for its length, but only if the
   retrieved MIME entity uses the UTF-8 encoding or has been locally
   transcoded into this encoding.






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   Please note that the FTP protocol, as well as some other protocols
   underlying some other URI schemes, do not provide explicit
   information about the media type of the resource being retrieved.
   Using fragment identifiers with such URI schemes is therefore
   inherently unreliable.  Current user agents use various heuristics to
   infer some media type for further processing.  Processing of the
   fragment identifier according to this memo is only appropriate if the
   inferred media type is text/plain.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  IANA Considerations</span>

   IANA has added a reference to this specification in the text/plain
   Media Type registration.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Security Considerations</span>

   The fact that software implementing fragment identifiers for plain
   text and software not implementing them differs in behavior, and the
   fact that different software may show documents or fragments to users
   in different ways, can lead to misunderstandings on the part of
   users.  Such misunderstandings might be exploited in a way similar to
   spoofing or phishing.

   In particular, care has to be taken if fragment identifiers are used
   together with a mechanism that allows showing only the part of a
   document identified by a fragment.  One scenario may be the use of a
   fragment identifier to hide small-print legal text.  Another scenario
   may be the inclusion of site-key-like material, which may give the
   user the impression of using the real site rather than a fake site;
   other scenarios may also be possible.  Possible countermeasures may
   include but are not limited to displaying the included content within
   clearly visible boundaries and limiting inclusion to material from
   the same security realm or from realms that give explicit permission
   to be included in another realm.

   Please note that the above issues all apply to the client side;
   fragment identifiers are not used when resolving a URI to retrieve
   the representation of a resource, but are only applied on the client
   side.

   Implementers and users of fragment identifiers for plain text should
   also be aware of the security considerations in <a href="./rfc3986">RFC 3986</a> [<a href="#ref-7" title="&quot;Uniform Resource Identifier (URI): Generic Syntax&quot;">7</a>] and <a href="./rfc3987">RFC</a>
   <a href="./rfc3987">3987</a> [<a href="#ref-8" title="&quot;Internationalized Resource Identifiers (IRI)&quot;">8</a>].








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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  References</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Normative References</span>

   [<a id="ref-1">1</a>]   Rivest, R., "The MD5 Message-Digest Algorithm", <a href="./rfc1321">RFC 1321</a>,
         April 1992.

   [<a id="ref-2">2</a>]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies",
         <a href="./rfc2045">RFC 2045</a>, November 1996.

   [<a id="ref-3">3</a>]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part Two: Media Types", <a href="./rfc2046">RFC 2046</a>,
         November 1996.

   [<a id="ref-4">4</a>]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-5">5</a>]   Freed, N. and J. Postel, "IANA Charset Registration
         Procedures", <a href="https://www.rfc-editor.org/bcp/bcp19">BCP 19</a>, <a href="./rfc2978">RFC 2978</a>, October 2000.

   [<a id="ref-6">6</a>]   Gellens, R., "The Text/Plain Format and DelSp Parameters",
         <a href="./rfc3676">RFC 3676</a>, February 2004.

   [<a id="ref-7">7</a>]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, <a href="./rfc3986">RFC 3986</a>,
         January 2005.

   [<a id="ref-8">8</a>]   Duerst, M. and M. Suignard, "Internationalized Resource
         Identifiers (IRI)", <a href="./rfc3987">RFC 3987</a>, January 2005.

   [<a id="ref-9">9</a>]   Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", STD 68, <a href="./rfc5234">RFC 5234</a>, January 2008.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Informative References</span>

   [<a id="ref-10">10</a>]  Connolly, D. and L. Masinter, "The 'text/html' Media Type",
         <a href="./rfc2854">RFC 2854</a>, June 2000.

   [<a id="ref-11">11</a>]  Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646",
         <a href="./rfc2781">RFC 2781</a>, February 2000.

   [<a id="ref-12">12</a>]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         STD 63, <a href="./rfc3629">RFC 3629</a>, November 2003.

   [<a id="ref-13">13</a>]  ANSI X3.4-1986, "Coded Character Set - 7-Bit American National
         Standard Code for Information Interchange", 1986.




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   [<a id="ref-14">14</a>]  DeRose, S., Maler, E., and D. Orchard, "XML Linking Language
         (XLink) Version 1.0", World Wide Web Consortium Recommendation,
         June 2001, &lt;<a href="http://www.w3.org/TR/xlink/">http://www.w3.org/TR/xlink/</a>&gt;.

   [<a id="ref-15">15</a>]  Freed, N. and J. Klensin, "Media Type Specifications and
         Registration Procedures", <a href="https://www.rfc-editor.org/bcp/bcp13">BCP 13</a>, <a href="./rfc4288">RFC 4288</a>, December 2005.













































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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Acknowledgements</span>

   Thanks for comments and suggestions provided by Marcel Baschnagel,
   Stephane Bortzmeyer, Tim Bray, Iain Calder, John Cowan, Spencer
   Dawkins, Lisa Dusseault, Benja Fallenstein, Ted Hardie, Sam Hartman,
   Sandro Hawke, Jeffrey Hutzelman, Cullen Jennings, Graham Klyne, Dan
   Kohn, Henrik Levkowetz, Chris Newman, Mark Nottingham, Conrad Parker,
   and Tim Polk.

Authors' Addresses

   Erik Wilde
   UC Berkeley
   School of Information, 311 South Hall
   Berkeley, CA 94720-4600
   U.S.A.

   Phone: +1-510-6432253
   EMail: dret@berkeley.edu
   URI:   <a href="http://dret.net/netdret/">http://dret.net/netdret/</a>


   Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
                 possible, for example as "D&amp;#252;rst" in XML and HTML.)
   Aoyama Gakuin University
   5-10-1 Fuchinobe
   Sagamihara, Kanagawa  229-8558
   Japan

   Phone: +81 42 759 6329
   Fax:   +81 42 759 6495
   EMail: duerst@it.aoyama.ac.jp
   URI:   <a href="http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/">http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/</a>


















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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
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   This document and the information contained herein are provided on an
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   The IETF invites any interested party to bring to its attention any
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