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+<span class="pre noprint docinfo top">[<a href="http://tools.ietf.org/html/">RFCs/IDs</a>] [<a href="http://tools.ietf.org/rfc/rfc3986.txt">Plain Text</a>] [From <a href="http://tools.ietf.org/html/draft-fielding-uri-rfc2396bis">draft-fielding-uri-rfc2396bis</a>] </span><br>
+<span class="pre noprint docinfo"> </span><br>
+<span class="pre noprint docinfo"> STANDARD</span><br>
+<span class="pre noprint docinfo"> </span><br>
+<pre>Network Working Group T. Berners-Lee
+Request for Comments: 3986 W3C/MIT
+STD: 66 R. Fielding
+Updates: <a href="http://tools.ietf.org/html/rfc1738">1738</a> Day Software
+Obsoletes: <a href="http://tools.ietf.org/html/rfc2732">2732</a>, <a href="http://tools.ietf.org/html/rfc2396">2396</a>, <a href="http://tools.ietf.org/html/rfc1808">1808</a> L. Masinter
+Category: Standards Track Adobe Systems
+ January 2005
+
+
+ <span class="h1"><h1>Uniform Resource Identifier (URI): Generic Syntax</h1></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.
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (2005).
+
+Abstract
+
+ A Uniform Resource Identifier (URI) is a compact sequence of
+ characters that identifies an abstract or physical resource. This
+ specification defines the generic URI syntax and a process for
+ resolving URI references that might be in relative form, along with
+ guidelines and security considerations for the use of URIs on the
+ Internet. The URI syntax defines a grammar that is a superset of all
+ valid URIs, allowing an implementation to parse the common components
+ of a URI reference without knowing the scheme-specific requirements
+ of every possible identifier. This specification does not define a
+ generative grammar for URIs; that task is performed by the individual
+ specifications of each URI scheme.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 1]</span>
+<a name="page-2" id="page-2" href="#page-2"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+Table of Contents
+
+ <a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
+ <a href="#section-1.1">1.1</a>. Overview of URIs . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
+ <a href="#section-1.1.1">1.1.1</a>. Generic Syntax . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
+ <a href="#section-1.1.2">1.1.2</a>. Examples . . . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
+ <a href="#section-1.1.3">1.1.3</a>. URI, URL, and URN . . . . . . . . . . . . . . . <a href="#page-7">7</a>
+ <a href="#section-1.2">1.2</a>. Design Considerations . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
+ <a href="#section-1.2.1">1.2.1</a>. Transcription . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
+ <a href="#section-1.2.2">1.2.2</a>. Separating Identification from Interaction . . . <a href="#page-9">9</a>
+ <a href="#section-1.2.3">1.2.3</a>. Hierarchical Identifiers . . . . . . . . . . . . <a href="#page-10">10</a>
+ <a href="#section-1.3">1.3</a>. Syntax Notation . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
+ <a href="#section-2">2</a>. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
+ <a href="#section-2.1">2.1</a>. Percent-Encoding . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
+ <a href="#section-2.2">2.2</a>. Reserved Characters . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
+ <a href="#section-2.3">2.3</a>. Unreserved Characters . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
+ <a href="#section-2.4">2.4</a>. When to Encode or Decode . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
+ <a href="#section-2.5">2.5</a>. Identifying Data . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
+ <a href="#section-3">3</a>. Syntax Components . . . . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
+ <a href="#section-3.1">3.1</a>. Scheme . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
+ <a href="#section-3.2">3.2</a>. Authority . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
+ <a href="#section-3.2.1">3.2.1</a>. User Information . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
+ <a href="#section-3.2.2">3.2.2</a>. Host . . . . . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
+ <a href="#section-3.2.3">3.2.3</a>. Port . . . . . . . . . . . . . . . . . . . . . . <a href="#page-22">22</a>
+ <a href="#section-3.3">3.3</a>. Path . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-22">22</a>
+ <a href="#section-3.4">3.4</a>. Query . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-23">23</a>
+ <a href="#section-3.5">3.5</a>. Fragment . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-24">24</a>
+ <a href="#section-4">4</a>. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-25">25</a>
+ <a href="#section-4.1">4.1</a>. URI Reference . . . . . . . . . . . . . . . . . . . . . <a href="#page-25">25</a>
+ <a href="#section-4.2">4.2</a>. Relative Reference . . . . . . . . . . . . . . . . . . . <a href="#page-26">26</a>
+ <a href="#section-4.3">4.3</a>. Absolute URI . . . . . . . . . . . . . . . . . . . . . . <a href="#page-27">27</a>
+ <a href="#section-4.4">4.4</a>. Same-Document Reference . . . . . . . . . . . . . . . . <a href="#page-27">27</a>
+ <a href="#section-4.5">4.5</a>. Suffix Reference . . . . . . . . . . . . . . . . . . . . <a href="#page-27">27</a>
+ <a href="#section-5">5</a>. Reference Resolution . . . . . . . . . . . . . . . . . . . . . <a href="#page-28">28</a>
+ <a href="#section-5.1">5.1</a>. Establishing a Base URI . . . . . . . . . . . . . . . . <a href="#page-28">28</a>
+ <a href="#section-5.1.1">5.1.1</a>. Base URI Embedded in Content . . . . . . . . . . <a href="#page-29">29</a>
+ <a href="#section-5.1.2">5.1.2</a>. Base URI from the Encapsulating Entity . . . . . <a href="#page-29">29</a>
+ <a href="#section-5.1.3">5.1.3</a>. Base URI from the Retrieval URI . . . . . . . . <a href="#page-30">30</a>
+ <a href="#section-5.1.4">5.1.4</a>. Default Base URI . . . . . . . . . . . . . . . . <a href="#page-30">30</a>
+ <a href="#section-5.2">5.2</a>. Relative Resolution . . . . . . . . . . . . . . . . . . <a href="#page-30">30</a>
+ <a href="#section-5.2.1">5.2.1</a>. Pre-parse the Base URI . . . . . . . . . . . . . <a href="#page-31">31</a>
+ <a href="#section-5.2.2">5.2.2</a>. Transform References . . . . . . . . . . . . . . <a href="#page-31">31</a>
+ <a href="#section-5.2.3">5.2.3</a>. Merge Paths . . . . . . . . . . . . . . . . . . <a href="#page-32">32</a>
+ <a href="#section-5.2.4">5.2.4</a>. Remove Dot Segments . . . . . . . . . . . . . . <a href="#page-33">33</a>
+ <a href="#section-5.3">5.3</a>. Component Recomposition . . . . . . . . . . . . . . . . <a href="#page-35">35</a>
+ <a href="#section-5.4">5.4</a>. Reference Resolution Examples . . . . . . . . . . . . . <a href="#page-35">35</a>
+ <a href="#section-5.4.1">5.4.1</a>. Normal Examples . . . . . . . . . . . . . . . . <a href="#page-36">36</a>
+ <a href="#section-5.4.2">5.4.2</a>. Abnormal Examples . . . . . . . . . . . . . . . <a href="#page-36">36</a>
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 2]</span>
+<a name="page-3" id="page-3" href="#page-3"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ <a href="#section-6">6</a>. Normalization and Comparison . . . . . . . . . . . . . . . . . <a href="#page-38">38</a>
+ <a href="#section-6.1">6.1</a>. Equivalence . . . . . . . . . . . . . . . . . . . . . . <a href="#page-38">38</a>
+ <a href="#section-6.2">6.2</a>. Comparison Ladder . . . . . . . . . . . . . . . . . . . <a href="#page-39">39</a>
+ <a href="#section-6.2.1">6.2.1</a>. Simple String Comparison . . . . . . . . . . . . <a href="#page-39">39</a>
+ <a href="#section-6.2.2">6.2.2</a>. Syntax-Based Normalization . . . . . . . . . . . <a href="#page-40">40</a>
+ <a href="#section-6.2.3">6.2.3</a>. Scheme-Based Normalization . . . . . . . . . . . <a href="#page-41">41</a>
+ <a href="#section-6.2.4">6.2.4</a>. Protocol-Based Normalization . . . . . . . . . . <a href="#page-42">42</a>
+ <a href="#section-7">7</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-43">43</a>
+ <a href="#section-7.1">7.1</a>. Reliability and Consistency . . . . . . . . . . . . . . <a href="#page-43">43</a>
+ <a href="#section-7.2">7.2</a>. Malicious Construction . . . . . . . . . . . . . . . . . <a href="#page-43">43</a>
+ <a href="#section-7.3">7.3</a>. Back-End Transcoding . . . . . . . . . . . . . . . . . . <a href="#page-44">44</a>
+ <a href="#section-7.4">7.4</a>. Rare IP Address Formats . . . . . . . . . . . . . . . . <a href="#page-45">45</a>
+ <a href="#section-7.5">7.5</a>. Sensitive Information . . . . . . . . . . . . . . . . . <a href="#page-45">45</a>
+ <a href="#section-7.6">7.6</a>. Semantic Attacks . . . . . . . . . . . . . . . . . . . . <a href="#page-45">45</a>
+ <a href="#section-8">8</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . . <a href="#page-46">46</a>
+ <a href="#section-9">9</a>. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-46">46</a>
+ <a href="#section-10">10</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-46">46</a>
+ <a href="#section-10.1">10.1</a>. Normative References . . . . . . . . . . . . . . . . . . <a href="#page-46">46</a>
+ <a href="#section-10.2">10.2</a>. Informative References . . . . . . . . . . . . . . . . . <a href="#page-47">47</a>
+ A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . <a href="#page-49">49</a>
+ B. Parsing a URI Reference with a Regular Expression . . . . . . <a href="#page-50">50</a>
+ C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . <a href="#page-51">51</a>
+ D. Changes from <a href="http://tools.ietf.org/html/rfc2396">RFC 2396</a> . . . . . . . . . . . . . . . . . . . . <a href="#page-53">53</a>
+ D.1. Additions . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-53">53</a>
+ D.2. Modifications . . . . . . . . . . . . . . . . . . . . . <a href="#page-53">53</a>
+ Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-56">56</a>
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-60">60</a>
+ Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . <a href="#page-61">61</a>
+
+
+
+
+
+
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+
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+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 3]</span>
+<a name="page-4" id="page-4" href="#page-4"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h2"><h2><a name="section-1">1</a>. Introduction</h2></span>
+
+ A Uniform Resource Identifier (URI) provides a simple and extensible
+ means for identifying a resource. This specification of URI syntax
+ and semantics is derived from concepts introduced by the World Wide
+ Web global information initiative, whose use of these identifiers
+ dates from 1990 and is described in "Universal Resource Identifiers
+ in WWW" [<a href="http://tools.ietf.org/html/rfc1630" title=""Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web"">RFC1630</a>]. The syntax is designed to meet the
+ recommendations laid out in "Functional Recommendations for Internet
+ Resource Locators" [<a href="http://tools.ietf.org/html/rfc1736" title=""Functional Recommendations for Internet Resource Locators"">RFC1736</a>] and "Functional Requirements for Uniform
+ Resource Names" [<a href="http://tools.ietf.org/html/rfc1737" title=""Functional Requirements for Uniform Resource Names"">RFC1737</a>].
+
+ This document obsoletes [<a href="http://tools.ietf.org/html/rfc2396" title=""Uniform Resource Identifiers (URI): Generic Syntax"">RFC2396</a>], which merged "Uniform Resource
+ Locators" [<a href="http://tools.ietf.org/html/rfc1738" title=""Uniform Resource Locators (URL)"">RFC1738</a>] and "Relative Uniform Resource Locators"
+ [<a href="http://tools.ietf.org/html/rfc1808" title=""Relative Uniform Resource Locators"">RFC1808</a>] in order to define a single, generic syntax for all URIs.
+ It obsoletes [<a href="http://tools.ietf.org/html/rfc2732" title=""Format for Literal IPv6 Addresses in URL&#39;s"">RFC2732</a>], which introduced syntax for an IPv6 address.
+ It excludes portions of <a href="http://tools.ietf.org/html/rfc1738">RFC 1738</a> that defined the specific syntax of
+ individual URI schemes; those portions will be updated as separate
+ documents. The process for registration of new URI schemes is
+ defined separately by [<a href="#ref-BCP35" title=""Registration Procedures for URL Scheme Names"">BCP35</a>]. Advice for designers of new URI
+ schemes can be found in [<a href="http://tools.ietf.org/html/rfc2718" title=""Guidelines for new URL Schemes"">RFC2718</a>]. All significant changes from <a href="http://tools.ietf.org/html/rfc2396">RFC</a>
+ <a href="http://tools.ietf.org/html/rfc2396">2396</a> are noted in Appendix D.
+
+ This specification uses the terms "character" and "coded character
+ set" in accordance with the definitions provided in [<a href="#ref-BCP19" title=""IANA Charset Registration Procedures"">BCP19</a>], and
+ "character encoding" in place of what [<a href="#ref-BCP19" title=""IANA Charset Registration Procedures"">BCP19</a>] refers to as a
+ "charset".
+
+<span class="h3"><h3><a name="section-1.1">1.1</a>. Overview of URIs</h3></span>
+
+ URIs are characterized as follows:
+
+ Uniform
+
+ Uniformity provides several benefits. It allows different types
+ of resource identifiers to be used in the same context, even when
+ the mechanisms used to access those resources may differ. It
+ allows uniform semantic interpretation of common syntactic
+ conventions across different types of resource identifiers. It
+ allows introduction of new types of resource identifiers without
+ interfering with the way that existing identifiers are used. It
+ allows the identifiers to be reused in many different contexts,
+ thus permitting new applications or protocols to leverage a pre-
+ existing, large, and widely used set of resource identifiers.
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 4]</span>
+<a name="page-5" id="page-5" href="#page-5"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ Resource
+
+ This specification does not limit the scope of what might be a
+ resource; rather, the term "resource" is used in a general sense
+ for whatever might be identified by a URI. Familiar examples
+ include an electronic document, an image, a source of information
+ with a consistent purpose (e.g., "today's weather report for Los
+ Angeles"), a service (e.g., an HTTP-to-SMS gateway), and a
+ collection of other resources. A resource is not necessarily
+ accessible via the Internet; e.g., human beings, corporations, and
+ bound books in a library can also be resources. Likewise,
+ abstract concepts can be resources, such as the operators and
+ operands of a mathematical equation, the types of a relationship
+ (e.g., "parent" or "employee"), or numeric values (e.g., zero,
+ one, and infinity).
+
+ Identifier
+
+ An identifier embodies the information required to distinguish
+ what is being identified from all other things within its scope of
+ identification. Our use of the terms "identify" and "identifying"
+ refer to this purpose of distinguishing one resource from all
+ other resources, regardless of how that purpose is accomplished
+ (e.g., by name, address, or context). These terms should not be
+ mistaken as an assumption that an identifier defines or embodies
+ the identity of what is referenced, though that may be the case
+ for some identifiers. Nor should it be assumed that a system
+ using URIs will access the resource identified: in many cases,
+ URIs are used to denote resources without any intention that they
+ be accessed. Likewise, the "one" resource identified might not be
+ singular in nature (e.g., a resource might be a named set or a
+ mapping that varies over time).
+
+ A URI is an identifier consisting of a sequence of characters
+ matching the syntax rule named <URI> in <a href="#section-3">Section 3</a>. It enables
+ uniform identification of resources via a separately defined
+ extensible set of naming schemes (<a href="#section-3.1">Section 3.1</a>). How that
+ identification is accomplished, assigned, or enabled is delegated to
+ each scheme specification.
+
+ This specification does not place any limits on the nature of a
+ resource, the reasons why an application might seek to refer to a
+ resource, or the kinds of systems that might use URIs for the sake of
+ identifying resources. This specification does not require that a
+ URI persists in identifying the same resource over time, though that
+ is a common goal of all URI schemes. Nevertheless, nothing in this
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 5]</span>
+<a name="page-6" id="page-6" href="#page-6"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ specification prevents an application from limiting itself to
+ particular types of resources, or to a subset of URIs that maintains
+ characteristics desired by that application.
+
+ URIs have a global scope and are interpreted consistently regardless
+ of context, though the result of that interpretation may be in
+ relation to the end-user's context. For example, "<a href="http://localhost/">http://localhost/</a>"
+ has the same interpretation for every user of that reference, even
+ though the network interface corresponding to "localhost" may be
+ different for each end-user: interpretation is independent of access.
+ However, an action made on the basis of that reference will take
+ place in relation to the end-user's context, which implies that an
+ action intended to refer to a globally unique thing must use a URI
+ that distinguishes that resource from all other things. URIs that
+ identify in relation to the end-user's local context should only be
+ used when the context itself is a defining aspect of the resource,
+ such as when an on-line help manual refers to a file on the end-
+ user's file system (e.g., "file:///etc/hosts").
+
+<span class="h4"><h4><a name="section-1.1.1">1.1.1</a>. Generic Syntax</h4></span>
+
+ Each URI begins with a scheme name, as defined in <a href="#section-3.1">Section 3.1</a>, that
+ refers to a specification for assigning identifiers within that
+ scheme. As such, the URI syntax is a federated and extensible naming
+ system wherein each scheme's specification may further restrict the
+ syntax and semantics of identifiers using that scheme.
+
+ This specification defines those elements of the URI syntax that are
+ required of all URI schemes or are common to many URI schemes. It
+ thus defines the syntax and semantics needed to implement a scheme-
+ independent parsing mechanism for URI references, by which the
+ scheme-dependent handling of a URI can be postponed until the
+ scheme-dependent semantics are needed. Likewise, protocols and data
+ formats that make use of URI references can refer to this
+ specification as a definition for the range of syntax allowed for all
+ URIs, including those schemes that have yet to be defined. This
+ decouples the evolution of identification schemes from the evolution
+ of protocols, data formats, and implementations that make use of
+ URIs.
+
+ A parser of the generic URI syntax can parse any URI reference into
+ its major components. Once the scheme is determined, further
+ scheme-specific parsing can be performed on the components. In other
+ words, the URI generic syntax is a superset of the syntax of all URI
+ schemes.
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 6]</span>
+<a name="page-7" id="page-7" href="#page-7"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h4"><h4><a name="section-1.1.2">1.1.2</a>. Examples</h4></span>
+
+ The following example URIs illustrate several URI schemes and
+ variations in their common syntax components:
+
+ <a href="ftp://ftp.is.co.za/rfc/rfc1808.txt">ftp://ftp.is.co.za/rfc/rfc1808.txt</a>
+
+ <a href="http://www.ietf.org/rfc/rfc2396.txt">http://www.ietf.org/rfc/rfc2396.txt</a>
+
+ ldap://[2001:db8::7]/c=GB?objectClass?one
+
+ mailto:John.Doe@example.com
+
+ news:comp.infosystems.www.servers.unix
+
+ tel:+1-816-555-1212
+
+ telnet://192.0.2.16:80/
+
+ urn:oasis:names:specification:docbook:dtd:xml:4.1.2
+
+
+<span class="h4"><h4><a name="section-1.1.3">1.1.3</a>. URI, URL, and URN</h4></span>
+
+ A URI can be further classified as a locator, a name, or both. The
+ term "Uniform Resource Locator" (URL) refers to the subset of URIs
+ that, in addition to identifying a resource, provide a means of
+ locating the resource by describing its primary access mechanism
+ (e.g., its network "location"). The term "Uniform Resource Name"
+ (URN) has been used historically to refer to both URIs under the
+ "urn" scheme [<a href="http://tools.ietf.org/html/rfc2141" title=""URN Syntax"">RFC2141</a>], which are required to remain globally unique
+ and persistent even when the resource ceases to exist or becomes
+ unavailable, and to any other URI with the properties of a name.
+
+ An individual scheme does not have to be classified as being just one
+ of "name" or "locator". Instances of URIs from any given scheme may
+ have the characteristics of names or locators or both, often
+ depending on the persistence and care in the assignment of
+ identifiers by the naming authority, rather than on any quality of
+ the scheme. Future specifications and related documentation should
+ use the general term "URI" rather than the more restrictive terms
+ "URL" and "URN" [<a href="http://tools.ietf.org/html/rfc3305" title=""Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations"">RFC3305</a>].
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 7]</span>
+<a name="page-8" id="page-8" href="#page-8"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h3"><h3><a name="section-1.2">1.2</a>. Design Considerations</h3></span>
+
+<span class="h4"><h4><a name="section-1.2.1">1.2.1</a>. Transcription</h4></span>
+
+ The URI syntax has been designed with global transcription as one of
+ its main considerations. A URI is a sequence of characters from a
+ very limited set: the letters of the basic Latin alphabet, digits,
+ and a few special characters. A URI may be represented in a variety
+ of ways; e.g., ink on paper, pixels on a screen, or a sequence of
+ character encoding octets. The interpretation of a URI depends only
+ on the characters used and not on how those characters are
+ represented in a network protocol.
+
+ The goal of transcription can be described by a simple scenario.
+ Imagine two colleagues, Sam and Kim, sitting in a pub at an
+ international conference and exchanging research ideas. Sam asks Kim
+ for a location to get more information, so Kim writes the URI for the
+ research site on a napkin. Upon returning home, Sam takes out the
+ napkin and types the URI into a computer, which then retrieves the
+ information to which Kim referred.
+
+ There are several design considerations revealed by the scenario:
+
+ o A URI is a sequence of characters that is not always represented
+ as a sequence of octets.
+
+ o A URI might be transcribed from a non-network source and thus
+ should consist of characters that are most likely able to be
+ entered into a computer, within the constraints imposed by
+ keyboards (and related input devices) across languages and
+ locales.
+
+ o A URI often has to be remembered by people, and it is easier for
+ people to remember a URI when it consists of meaningful or
+ familiar components.
+
+ These design considerations are not always in alignment. For
+ example, it is often the case that the most meaningful name for a URI
+ component would require characters that cannot be typed into some
+ systems. The ability to transcribe a resource identifier from one
+ medium to another has been considered more important than having a
+ URI consist of the most meaningful of components.
+
+ In local or regional contexts and with improving technology, users
+ might benefit from being able to use a wider range of characters;
+ such use is not defined by this specification. Percent-encoded
+ octets (<a href="#section-2.1">Section 2.1</a>) may be used within a URI to represent characters
+ outside the range of the US-ASCII coded character set if this
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 8]</span>
+<a name="page-9" id="page-9" href="#page-9"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ representation is allowed by the scheme or by the protocol element in
+ which the URI is referenced. Such a definition should specify the
+ character encoding used to map those characters to octets prior to
+ being percent-encoded for the URI.
+
+<span class="h4"><h4><a name="section-1.2.2">1.2.2</a>. Separating Identification from Interaction</h4></span>
+
+ A common misunderstanding of URIs is that they are only used to refer
+ to accessible resources. The URI itself only provides
+ identification; access to the resource is neither guaranteed nor
+ implied by the presence of a URI. Instead, any operation associated
+ with a URI reference is defined by the protocol element, data format
+ attribute, or natural language text in which it appears.
+
+ Given a URI, a system may attempt to perform a variety of operations
+ on the resource, as might be characterized by words such as "access",
+ "update", "replace", or "find attributes". Such operations are
+ defined by the protocols that make use of URIs, not by this
+ specification. However, we do use a few general terms for describing
+ common operations on URIs. URI "resolution" is the process of
+ determining an access mechanism and the appropriate parameters
+ necessary to dereference a URI; this resolution may require several
+ iterations. To use that access mechanism to perform an action on the
+ URI's resource is to "dereference" the URI.
+
+ When URIs are used within information retrieval systems to identify
+ sources of information, the most common form of URI dereference is
+ "retrieval": making use of a URI in order to retrieve a
+ representation of its associated resource. A "representation" is a
+ sequence of octets, along with representation metadata describing
+ those octets, that constitutes a record of the state of the resource
+ at the time when the representation is generated. Retrieval is
+ achieved by a process that might include using the URI as a cache key
+ to check for a locally cached representation, resolution of the URI
+ to determine an appropriate access mechanism (if any), and
+ dereference of the URI for the sake of applying a retrieval
+ operation. Depending on the protocols used to perform the retrieval,
+ additional information might be supplied about the resource (resource
+ metadata) and its relation to other resources.
+
+ URI references in information retrieval systems are designed to be
+ late-binding: the result of an access is generally determined when it
+ is accessed and may vary over time or due to other aspects of the
+ interaction. These references are created in order to be used in the
+ future: what is being identified is not some specific result that was
+ obtained in the past, but rather some characteristic that is expected
+ to be true for future results. In such cases, the resource referred
+ to by the URI is actually a sameness of characteristics as observed
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 9]</span>
+<a name="page-10" id="page-10" href="#page-10"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ over time, perhaps elucidated by additional comments or assertions
+ made by the resource provider.
+
+ Although many URI schemes are named after protocols, this does not
+ imply that use of these URIs will result in access to the resource
+ via the named protocol. URIs are often used simply for the sake of
+ identification. Even when a URI is used to retrieve a representation
+ of a resource, that access might be through gateways, proxies,
+ caches, and name resolution services that are independent of the
+ protocol associated with the scheme name. The resolution of some
+ URIs may require the use of more than one protocol (e.g., both DNS
+ and HTTP are typically used to access an "http" URI's origin server
+ when a representation isn't found in a local cache).
+
+<span class="h4"><h4><a name="section-1.2.3">1.2.3</a>. Hierarchical Identifiers</h4></span>
+
+ The URI syntax is organized hierarchically, with components listed in
+ order of decreasing significance from left to right. For some URI
+ schemes, the visible hierarchy is limited to the scheme itself:
+ everything after the scheme component delimiter (":") is considered
+ opaque to URI processing. Other URI schemes make the hierarchy
+ explicit and visible to generic parsing algorithms.
+
+ The generic syntax uses the slash ("/"), question mark ("?"), and
+ number sign ("#") characters to delimit components that are
+ significant to the generic parser's hierarchical interpretation of an
+ identifier. In addition to aiding the readability of such
+ identifiers through the consistent use of familiar syntax, this
+ uniform representation of hierarchy across naming schemes allows
+ scheme-independent references to be made relative to that hierarchy.
+
+ It is often the case that a group or "tree" of documents has been
+ constructed to serve a common purpose, wherein the vast majority of
+ URI references in these documents point to resources within the tree
+ rather than outside it. Similarly, documents located at a particular
+ site are much more likely to refer to other resources at that site
+ than to resources at remote sites. Relative referencing of URIs
+ allows document trees to be partially independent of their location
+ and access scheme. For instance, it is possible for a single set of
+ hypertext documents to be simultaneously accessible and traversable
+ via each of the "file", "http", and "ftp" schemes if the documents
+ refer to each other with relative references. Furthermore, such
+ document trees can be moved, as a whole, without changing any of the
+ relative references.
+
+ A relative reference (<a href="#section-4.2">Section 4.2</a>) refers to a resource by describing
+ the difference within a hierarchical name space between the reference
+ context and the target URI. The reference resolution algorithm,
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 10]</span>
+<a name="page-11" id="page-11" href="#page-11"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ presented in <a href="#section-5">Section 5</a>, defines how such a reference is transformed
+ to the target URI. As relative references can only be used within
+ the context of a hierarchical URI, designers of new URI schemes
+ should use a syntax consistent with the generic syntax's hierarchical
+ components unless there are compelling reasons to forbid relative
+ referencing within that scheme.
+
+ NOTE: Previous specifications used the terms "partial URI" and
+ "relative URI" to denote a relative reference to a URI. As some
+ readers misunderstood those terms to mean that relative URIs are a
+ subset of URIs rather than a method of referencing URIs, this
+ specification simply refers to them as relative references.
+
+ All URI references are parsed by generic syntax parsers when used.
+ However, because hierarchical processing has no effect on an absolute
+ URI used in a reference unless it contains one or more dot-segments
+ (complete path segments of "." or "..", as described in <a href="#section-3.3">Section 3.3</a>),
+ URI scheme specifications can define opaque identifiers by
+ disallowing use of slash characters, question mark characters, and
+ the URIs "scheme:." and "scheme:..".
+
+<span class="h3"><h3><a name="section-1.3">1.3</a>. Syntax Notation</h3></span>
+
+ This specification uses the Augmented Backus-Naur Form (ABNF)
+ notation of [<a href="http://tools.ietf.org/html/rfc2234" title=""Augmented BNF for Syntax Specifications: ABNF"">RFC2234</a>], including the following core ABNF syntax rules
+ defined by that specification: ALPHA (letters), CR (carriage return),
+ DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal
+ digits), LF (line feed), and SP (space). The complete URI syntax is
+ collected in Appendix A.
+
+<span class="h2"><h2><a name="section-2">2</a>. Characters</h2></span>
+
+ The URI syntax provides a method of encoding data, presumably for the
+ sake of identifying a resource, as a sequence of characters. The URI
+ characters are, in turn, frequently encoded as octets for transport
+ or presentation. This specification does not mandate any particular
+ character encoding for mapping between URI characters and the octets
+ used to store or transmit those characters. When a URI appears in a
+ protocol element, the character encoding is defined by that protocol;
+ without such a definition, a URI is assumed to be in the same
+ character encoding as the surrounding text.
+
+ The ABNF notation defines its terminal values to be non-negative
+ integers (codepoints) based on the US-ASCII coded character set
+ [<a href="#ref-ASCII" title=""Coded Character Set -- 7-bit American Standard Code for Information Interchange"">ASCII</a>]. Because a URI is a sequence of characters, we must invert
+ that relation in order to understand the URI syntax. Therefore, the
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 11]</span>
+<a name="page-12" id="page-12" href="#page-12"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ integer values used by the ABNF must be mapped back to their
+ corresponding characters via US-ASCII in order to complete the syntax
+ rules.
+
+ A URI is composed from a limited set of characters consisting of
+ digits, letters, and a few graphic symbols. A reserved subset of
+ those characters may be used to delimit syntax components within a
+ URI while the remaining characters, including both the unreserved set
+ and those reserved characters not acting as delimiters, define each
+ component's identifying data.
+
+<span class="h3"><h3><a name="section-2.1">2.1</a>. Percent-Encoding</h3></span>
+
+ A percent-encoding mechanism is used to represent a data octet in a
+ component when that octet's corresponding character is outside the
+ allowed set or is being used as a delimiter of, or within, the
+ component. A percent-encoded octet is encoded as a character
+ triplet, consisting of the percent character "%" followed by the two
+ hexadecimal digits representing that octet's numeric value. For
+ example, "%20" is the percent-encoding for the binary octet
+ "00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
+ character (SP). <a href="#section-2.4">Section 2.4</a> describes when percent-encoding and
+ decoding is applied.
+
+ pct-encoded = "%" HEXDIG HEXDIG
+
+ The uppercase hexadecimal digits 'A' through 'F' are equivalent to
+ the lowercase digits 'a' through 'f', respectively. If two URIs
+ differ only in the case of hexadecimal digits used in percent-encoded
+ octets, they are equivalent. For consistency, URI producers and
+ normalizers should use uppercase hexadecimal digits for all percent-
+ encodings.
+
+<span class="h3"><h3><a name="section-2.2">2.2</a>. Reserved Characters</h3></span>
+
+ URIs include components and subcomponents that are delimited by
+ characters in the "reserved" set. These characters are called
+ "reserved" because they may (or may not) be defined as delimiters by
+ the generic syntax, by each scheme-specific syntax, or by the
+ implementation-specific syntax of a URI's dereferencing algorithm.
+ If data for a URI component would conflict with a reserved
+ character's purpose as a delimiter, then the conflicting data must be
+ percent-encoded before the URI is formed.
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 12]</span>
+<a name="page-13" id="page-13" href="#page-13"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ reserved = gen-delims / sub-delims
+
+ gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
+
+ sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
+ / "*" / "+" / "," / ";" / "="
+
+ The purpose of reserved characters is to provide a set of delimiting
+ characters that are distinguishable from other data within a URI.
+ URIs that differ in the replacement of a reserved character with its
+ corresponding percent-encoded octet are not equivalent. Percent-
+ encoding a reserved character, or decoding a percent-encoded octet
+ that corresponds to a reserved character, will change how the URI is
+ interpreted by most applications. Thus, characters in the reserved
+ set are protected from normalization and are therefore safe to be
+ used by scheme-specific and producer-specific algorithms for
+ delimiting data subcomponents within a URI.
+
+ A subset of the reserved characters (gen-delims) is used as
+ delimiters of the generic URI components described in <a href="#section-3">Section 3</a>. A
+ component's ABNF syntax rule will not use the reserved or gen-delims
+ rule names directly; instead, each syntax rule lists the characters
+ allowed within that component (i.e., not delimiting it), and any of
+ those characters that are also in the reserved set are "reserved" for
+ use as subcomponent delimiters within the component. Only the most
+ common subcomponents are defined by this specification; other
+ subcomponents may be defined by a URI scheme's specification, or by
+ the implementation-specific syntax of a URI's dereferencing
+ algorithm, provided that such subcomponents are delimited by
+ characters in the reserved set allowed within that component.
+
+ URI producing applications should percent-encode data octets that
+ correspond to characters in the reserved set unless these characters
+ are specifically allowed by the URI scheme to represent data in that
+ component. If a reserved character is found in a URI component and
+ no delimiting role is known for that character, then it must be
+ interpreted as representing the data octet corresponding to that
+ character's encoding in US-ASCII.
+
+<span class="h3"><h3><a name="section-2.3">2.3</a>. Unreserved Characters</h3></span>
+
+ Characters that are allowed in a URI but do not have a reserved
+ purpose are called unreserved. These include uppercase and lowercase
+ letters, decimal digits, hyphen, period, underscore, and tilde.
+
+ unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 13]</span>
+<a name="page-14" id="page-14" href="#page-14"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ URIs that differ in the replacement of an unreserved character with
+ its corresponding percent-encoded US-ASCII octet are equivalent: they
+ identify the same resource. However, URI comparison implementations
+ do not always perform normalization prior to comparison (see Section
+ 6). For consistency, percent-encoded octets in the ranges of ALPHA
+ (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E),
+ underscore (%5F), or tilde (%7E) should not be created by URI
+ producers and, when found in a URI, should be decoded to their
+ corresponding unreserved characters by URI normalizers.
+
+<span class="h3"><h3><a name="section-2.4">2.4</a>. When to Encode or Decode</h3></span>
+
+ Under normal circumstances, the only time when octets within a URI
+ are percent-encoded is during the process of producing the URI from
+ its component parts. This is when an implementation determines which
+ of the reserved characters are to be used as subcomponent delimiters
+ and which can be safely used as data. Once produced, a URI is always
+ in its percent-encoded form.
+
+ When a URI is dereferenced, the components and subcomponents
+ significant to the scheme-specific dereferencing process (if any)
+ must be parsed and separated before the percent-encoded octets within
+ those components can be safely decoded, as otherwise the data may be
+ mistaken for component delimiters. The only exception is for
+ percent-encoded octets corresponding to characters in the unreserved
+ set, which can be decoded at any time. For example, the octet
+ corresponding to the tilde ("~") character is often encoded as "%7E"
+ by older URI processing implementations; the "%7E" can be replaced by
+ "~" without changing its interpretation.
+
+ Because the percent ("%") character serves as the indicator for
+ percent-encoded octets, it must be percent-encoded as "%25" for that
+ octet to be used as data within a URI. Implementations must not
+ percent-encode or decode the same string more than once, as decoding
+ an already decoded string might lead to misinterpreting a percent
+ data octet as the beginning of a percent-encoding, or vice versa in
+ the case of percent-encoding an already percent-encoded string.
+
+<span class="h3"><h3><a name="section-2.5">2.5</a>. Identifying Data</h3></span>
+
+ URI characters provide identifying data for each of the URI
+ components, serving as an external interface for identification
+ between systems. Although the presence and nature of the URI
+ production interface is hidden from clients that use its URIs (and is
+ thus beyond the scope of the interoperability requirements defined by
+ this specification), it is a frequent source of confusion and errors
+ in the interpretation of URI character issues. Implementers have to
+ be aware that there are multiple character encodings involved in the
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 14]</span>
+<a name="page-15" id="page-15" href="#page-15"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ production and transmission of URIs: local name and data encoding,
+ public interface encoding, URI character encoding, data format
+ encoding, and protocol encoding.
+
+ Local names, such as file system names, are stored with a local
+ character encoding. URI producing applications (e.g., origin
+ servers) will typically use the local encoding as the basis for
+ producing meaningful names. The URI producer will transform the
+ local encoding to one that is suitable for a public interface and
+ then transform the public interface encoding into the restricted set
+ of URI characters (reserved, unreserved, and percent-encodings).
+ Those characters are, in turn, encoded as octets to be used as a
+ reference within a data format (e.g., a document charset), and such
+ data formats are often subsequently encoded for transmission over
+ Internet protocols.
+
+ For most systems, an unreserved character appearing within a URI
+ component is interpreted as representing the data octet corresponding
+ to that character's encoding in US-ASCII. Consumers of URIs assume
+ that the letter "X" corresponds to the octet "01011000", and even
+ when that assumption is incorrect, there is no harm in making it. A
+ system that internally provides identifiers in the form of a
+ different character encoding, such as EBCDIC, will generally perform
+ character translation of textual identifiers to UTF-8 [<a href="#ref-STD63" title=""UTF-8, a transformation format of ISO 10646"">STD63</a>] (or
+ some other superset of the US-ASCII character encoding) at an
+ internal interface, thereby providing more meaningful identifiers
+ than those resulting from simply percent-encoding the original
+ octets.
+
+ For example, consider an information service that provides data,
+ stored locally using an EBCDIC-based file system, to clients on the
+ Internet through an HTTP server. When an author creates a file with
+ the name "Laguna Beach" on that file system, the "http" URI
+ corresponding to that resource is expected to contain the meaningful
+ string "Laguna%20Beach". If, however, that server produces URIs by
+ using an overly simplistic raw octet mapping, then the result would
+ be a URI containing "%D3%81%87%A4%95%81@%C2%85%81%83%88". An
+ internal transcoding interface fixes this problem by transcoding the
+ local name to a superset of US-ASCII prior to producing the URI.
+ Naturally, proper interpretation of an incoming URI on such an
+ interface requires that percent-encoded octets be decoded (e.g.,
+ "%20" to SP) before the reverse transcoding is applied to obtain the
+ local name.
+
+ In some cases, the internal interface between a URI component and the
+ identifying data that it has been crafted to represent is much less
+ direct than a character encoding translation. For example, portions
+ of a URI might reflect a query on non-ASCII data, or numeric
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 15]</span>
+<a name="page-16" id="page-16" href="#page-16"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ coordinates on a map. Likewise, a URI scheme may define components
+ with additional encoding requirements that are applied prior to
+ forming the component and producing the URI.
+
+ When a new URI scheme defines a component that represents textual
+ data consisting of characters from the Universal Character Set [<a href="#ref-UCS" title=""Information Technology - Universal Multiple-Octet Coded Character Set (UCS)"">UCS</a>],
+ the data should first be encoded as octets according to the UTF-8
+ character encoding [<a href="#ref-STD63" title=""UTF-8, a transformation format of ISO 10646"">STD63</a>]; then only those octets that do not
+ correspond to characters in the unreserved set should be percent-
+ encoded. For example, the character A would be represented as "A",
+ the character LATIN CAPITAL LETTER A WITH GRAVE would be represented
+ as "%C3%80", and the character KATAKANA LETTER A would be represented
+ as "%E3%82%A2".
+
+<span class="h2"><h2><a name="section-3">3</a>. Syntax Components</h2></span>
+
+ The generic URI syntax consists of a hierarchical sequence of
+ components referred to as the scheme, authority, path, query, and
+ fragment.
+
+ URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
+
+ hier-part = "//" authority path-abempty
+ / path-absolute
+ / path-rootless
+ / path-empty
+
+ The scheme and path components are required, though the path may be
+ empty (no characters). When authority is present, the path must
+ either be empty or begin with a slash ("/") character. When
+ authority is not present, the path cannot begin with two slash
+ characters ("//"). These restrictions result in five different ABNF
+ rules for a path (<a href="#section-3.3">Section 3.3</a>), only one of which will match any
+ given URI reference.
+
+ The following are two example URIs and their component parts:
+
+ foo://example.com:8042/over/there?name=ferret#nose
+ \_/ \______________/\_________/ \_________/ \__/
+ | | | | |
+ scheme authority path query fragment
+ | _____________________|__
+ / \ / \
+ urn:example:animal:ferret:nose
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 16]</span>
+<a name="page-17" id="page-17" href="#page-17"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h3"><h3><a name="section-3.1">3.1</a>. Scheme</h3></span>
+
+ Each URI begins with a scheme name that refers to a specification for
+ assigning identifiers within that scheme. As such, the URI syntax is
+ a federated and extensible naming system wherein each scheme's
+ specification may further restrict the syntax and semantics of
+ identifiers using that scheme.
+
+ Scheme names consist of a sequence of characters beginning with a
+ letter and followed by any combination of letters, digits, plus
+ ("+"), period ("."), or hyphen ("-"). Although schemes are case-
+ insensitive, the canonical form is lowercase and documents that
+ specify schemes must do so with lowercase letters. An implementation
+ should accept uppercase letters as equivalent to lowercase in scheme
+ names (e.g., allow "HTTP" as well as "http") for the sake of
+ robustness but should only produce lowercase scheme names for
+ consistency.
+
+ scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
+
+ Individual schemes are not specified by this document. The process
+ for registration of new URI schemes is defined separately by [<a href="#ref-BCP35" title=""Registration Procedures for URL Scheme Names"">BCP35</a>].
+ The scheme registry maintains the mapping between scheme names and
+ their specifications. Advice for designers of new URI schemes can be
+ found in [<a href="http://tools.ietf.org/html/rfc2718" title=""Guidelines for new URL Schemes"">RFC2718</a>]. URI scheme specifications must define their own
+ syntax so that all strings matching their scheme-specific syntax will
+ also match the <absolute-URI> grammar, as described in <a href="#section-4.3">Section 4.3</a>.
+
+ When presented with a URI that violates one or more scheme-specific
+ restrictions, the scheme-specific resolution process should flag the
+ reference as an error rather than ignore the unused parts; doing so
+ reduces the number of equivalent URIs and helps detect abuses of the
+ generic syntax, which might indicate that the URI has been
+ constructed to mislead the user (<a href="#section-7.6">Section 7.6</a>).
+
+<span class="h3"><h3><a name="section-3.2">3.2</a>. Authority</h3></span>
+
+ Many URI schemes include a hierarchical element for a naming
+ authority so that governance of the name space defined by the
+ remainder of the URI is delegated to that authority (which may, in
+ turn, delegate it further). The generic syntax provides a common
+ means for distinguishing an authority based on a registered name or
+ server address, along with optional port and user information.
+
+ The authority component is preceded by a double slash ("//") and is
+ terminated by the next slash ("/"), question mark ("?"), or number
+ sign ("#") character, or by the end of the URI.
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 17]</span>
+<a name="page-18" id="page-18" href="#page-18"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ authority = [ userinfo "@" ] host [ ":" port ]
+
+ URI producers and normalizers should omit the ":" delimiter that
+ separates host from port if the port component is empty. Some
+ schemes do not allow the userinfo and/or port subcomponents.
+
+ If a URI contains an authority component, then the path component
+ must either be empty or begin with a slash ("/") character. Non-
+ validating parsers (those that merely separate a URI reference into
+ its major components) will often ignore the subcomponent structure of
+ authority, treating it as an opaque string from the double-slash to
+ the first terminating delimiter, until such time as the URI is
+ dereferenced.
+
+<span class="h4"><h4><a name="section-3.2.1">3.2.1</a>. User Information</h4></span>
+
+ The userinfo subcomponent may consist of a user name and, optionally,
+ scheme-specific information about how to gain authorization to access
+ the resource. The user information, if present, is followed by a
+ commercial at-sign ("@") that delimits it from the host.
+
+ userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
+
+ Use of the format "user:password" in the userinfo field is
+ deprecated. Applications should not render as clear text any data
+ after the first colon (":") character found within a userinfo
+ subcomponent unless the data after the colon is the empty string
+ (indicating no password). Applications may choose to ignore or
+ reject such data when it is received as part of a reference and
+ should reject the storage of such data in unencrypted form. The
+ passing of authentication information in clear text has proven to be
+ a security risk in almost every case where it has been used.
+
+ Applications that render a URI for the sake of user feedback, such as
+ in graphical hypertext browsing, should render userinfo in a way that
+ is distinguished from the rest of a URI, when feasible. Such
+ rendering will assist the user in cases where the userinfo has been
+ misleadingly crafted to look like a trusted domain name
+ (<a href="#section-7.6">Section 7.6</a>).
+
+<span class="h4"><h4><a name="section-3.2.2">3.2.2</a>. Host</h4></span>
+
+ The host subcomponent of authority is identified by an IP literal
+ encapsulated within square brackets, an IPv4 address in dotted-
+ decimal form, or a registered name. The host subcomponent is case-
+ insensitive. The presence of a host subcomponent within a URI does
+ not imply that the scheme requires access to the given host on the
+ Internet. In many cases, the host syntax is used only for the sake
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 18]</span>
+<a name="page-19" id="page-19" href="#page-19"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ of reusing the existing registration process created and deployed for
+ DNS, thus obtaining a globally unique name without the cost of
+ deploying another registry. However, such use comes with its own
+ costs: domain name ownership may change over time for reasons not
+ anticipated by the URI producer. In other cases, the data within the
+ host component identifies a registered name that has nothing to do
+ with an Internet host. We use the name "host" for the ABNF rule
+ because that is its most common purpose, not its only purpose.
+
+ host = IP-literal / IPv4address / reg-name
+
+ The syntax rule for host is ambiguous because it does not completely
+ distinguish between an IPv4address and a reg-name. In order to
+ disambiguate the syntax, we apply the "first-match-wins" algorithm:
+ If host matches the rule for IPv4address, then it should be
+ considered an IPv4 address literal and not a reg-name. Although host
+ is case-insensitive, producers and normalizers should use lowercase
+ for registered names and hexadecimal addresses for the sake of
+ uniformity, while only using uppercase letters for percent-encodings.
+
+ A host identified by an Internet Protocol literal address, version 6
+ [<a href="http://tools.ietf.org/html/rfc3513" title=""Internet Protocol Version 6 (IPv6) Addressing Architecture"">RFC3513</a>] or later, is distinguished by enclosing the IP literal
+ within square brackets ("[" and "]"). This is the only place where
+ square bracket characters are allowed in the URI syntax. In
+ anticipation of future, as-yet-undefined IP literal address formats,
+ an implementation may use an optional version flag to indicate such a
+ format explicitly rather than rely on heuristic determination.
+
+ IP-literal = "[" ( IPv6address / IPvFuture ) "]"
+
+ IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
+
+ The version flag does not indicate the IP version; rather, it
+ indicates future versions of the literal format. As such,
+ implementations must not provide the version flag for the existing
+ IPv4 and IPv6 literal address forms described below. If a URI
+ containing an IP-literal that starts with "v" (case-insensitive),
+ indicating that the version flag is present, is dereferenced by an
+ application that does not know the meaning of that version flag, then
+ the application should return an appropriate error for "address
+ mechanism not supported".
+
+ A host identified by an IPv6 literal address is represented inside
+ the square brackets without a preceding version flag. The ABNF
+ provided here is a translation of the text definition of an IPv6
+ literal address provided in [<a href="http://tools.ietf.org/html/rfc3513" title=""Internet Protocol Version 6 (IPv6) Addressing Architecture"">RFC3513</a>]. This syntax does not support
+ IPv6 scoped addressing zone identifiers.
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 19]</span>
+<a name="page-20" id="page-20" href="#page-20"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ A 128-bit IPv6 address is divided into eight 16-bit pieces. Each
+ piece is represented numerically in case-insensitive hexadecimal,
+ using one to four hexadecimal digits (leading zeroes are permitted).
+ The eight encoded pieces are given most-significant first, separated
+ by colon characters. Optionally, the least-significant two pieces
+ may instead be represented in IPv4 address textual format. A
+ sequence of one or more consecutive zero-valued 16-bit pieces within
+ the address may be elided, omitting all their digits and leaving
+ exactly two consecutive colons in their place to mark the elision.
+
+ IPv6address = 6( h16 ":" ) ls32
+ / "::" 5( h16 ":" ) ls32
+ / [ h16 ] "::" 4( h16 ":" ) ls32
+ / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
+ / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
+ / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
+ / [ *4( h16 ":" ) h16 ] "::" ls32
+ / [ *5( h16 ":" ) h16 ] "::" h16
+ / [ *6( h16 ":" ) h16 ] "::"
+
+ ls32 = ( h16 ":" h16 ) / IPv4address
+ ; least-significant 32 bits of address
+
+ h16 = 1*4HEXDIG
+ ; 16 bits of address represented in hexadecimal
+
+ A host identified by an IPv4 literal address is represented in
+ dotted-decimal notation (a sequence of four decimal numbers in the
+ range 0 to 255, separated by "."), as described in [<a href="http://tools.ietf.org/html/rfc1123" title=""Requirements for Internet Hosts - Application and Support"">RFC1123</a>] by
+ reference to [<a href="http://tools.ietf.org/html/rfc0952" title=""DoD Internet host table specification"">RFC0952</a>]. Note that other forms of dotted notation may
+ be interpreted on some platforms, as described in <a href="#section-7.4">Section 7.4</a>, but
+ only the dotted-decimal form of four octets is allowed by this
+ grammar.
+
+ IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
+
+ dec-octet = DIGIT ; 0-9
+ / %x31-39 DIGIT ; 10-99
+ / "1" 2DIGIT ; 100-199
+ / "2" %x30-34 DIGIT ; 200-249
+ / "25" %x30-35 ; 250-255
+
+ A host identified by a registered name is a sequence of characters
+ usually intended for lookup within a locally defined host or service
+ name registry, though the URI's scheme-specific semantics may require
+ that a specific registry (or fixed name table) be used instead. The
+ most common name registry mechanism is the Domain Name System (DNS).
+ A registered name intended for lookup in the DNS uses the syntax
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 20]</span>
+<a name="page-21" id="page-21" href="#page-21"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ defined in <a href="#section-3.5">Section 3.5</a> of [<a href="http://tools.ietf.org/html/rfc1034" title=""Domain names - concepts and facilities"">RFC1034</a>] and <a href="#section-2.1">Section 2.1</a> of [<a href="http://tools.ietf.org/html/rfc1123" title=""Requirements for Internet Hosts - Application and Support"">RFC1123</a>].
+ Such a name consists of a sequence of domain labels separated by ".",
+ each domain label starting and ending with an alphanumeric character
+ and possibly also containing "-" characters. The rightmost domain
+ label of a fully qualified domain name in DNS may be followed by a
+ single "." and should be if it is necessary to distinguish between
+ the complete domain name and some local domain.
+
+ reg-name = *( unreserved / pct-encoded / sub-delims )
+
+ If the URI scheme defines a default for host, then that default
+ applies when the host subcomponent is undefined or when the
+ registered name is empty (zero length). For example, the "file" URI
+ scheme is defined so that no authority, an empty host, and
+ "localhost" all mean the end-user's machine, whereas the "http"
+ scheme considers a missing authority or empty host invalid.
+
+ This specification does not mandate a particular registered name
+ lookup technology and therefore does not restrict the syntax of reg-
+ name beyond what is necessary for interoperability. Instead, it
+ delegates the issue of registered name syntax conformance to the
+ operating system of each application performing URI resolution, and
+ that operating system decides what it will allow for the purpose of
+ host identification. A URI resolution implementation might use DNS,
+ host tables, yellow pages, NetInfo, WINS, or any other system for
+ lookup of registered names. However, a globally scoped naming
+ system, such as DNS fully qualified domain names, is necessary for
+ URIs intended to have global scope. URI producers should use names
+ that conform to the DNS syntax, even when use of DNS is not
+ immediately apparent, and should limit these names to no more than
+ 255 characters in length.
+
+ The reg-name syntax allows percent-encoded octets in order to
+ represent non-ASCII registered names in a uniform way that is
+ independent of the underlying name resolution technology. Non-ASCII
+ characters must first be encoded according to UTF-8 [<a href="#ref-STD63" title=""UTF-8, a transformation format of ISO 10646"">STD63</a>], and then
+ each octet of the corresponding UTF-8 sequence must be percent-
+ encoded to be represented as URI characters. URI producing
+ applications must not use percent-encoding in host unless it is used
+ to represent a UTF-8 character sequence. When a non-ASCII registered
+ name represents an internationalized domain name intended for
+ resolution via the DNS, the name must be transformed to the IDNA
+ encoding [<a href="http://tools.ietf.org/html/rfc3490" title=""Internationalizing Domain Names in Applications (IDNA)"">RFC3490</a>] prior to name lookup. URI producers should
+ provide these registered names in the IDNA encoding, rather than a
+ percent-encoding, if they wish to maximize interoperability with
+ legacy URI resolvers.
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 21]</span>
+<a name="page-22" id="page-22" href="#page-22"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h4"><h4><a name="section-3.2.3">3.2.3</a>. Port</h4></span>
+
+ The port subcomponent of authority is designated by an optional port
+ number in decimal following the host and delimited from it by a
+ single colon (":") character.
+
+ port = *DIGIT
+
+ A scheme may define a default port. For example, the "http" scheme
+ defines a default port of "80", corresponding to its reserved TCP
+ port number. The type of port designated by the port number (e.g.,
+ TCP, UDP, SCTP) is defined by the URI scheme. URI producers and
+ normalizers should omit the port component and its ":" delimiter if
+ port is empty or if its value would be the same as that of the
+ scheme's default.
+
+<span class="h3"><h3><a name="section-3.3">3.3</a>. Path</h3></span>
+
+ The path component contains data, usually organized in hierarchical
+ form, that, along with data in the non-hierarchical query component
+ (<a href="#section-3.4">Section 3.4</a>), serves to identify a resource within the scope of the
+ URI's scheme and naming authority (if any). The path is terminated
+ by the first question mark ("?") or number sign ("#") character, or
+ by the end of the URI.
+
+ If a URI contains an authority component, then the path component
+ must either be empty or begin with a slash ("/") character. If a URI
+ does not contain an authority component, then the path cannot begin
+ with two slash characters ("//"). In addition, a URI reference
+ (<a href="#section-4.1">Section 4.1</a>) may be a relative-path reference, in which case the
+ first path segment cannot contain a colon (":") character. The ABNF
+ requires five separate rules to disambiguate these cases, only one of
+ which will match the path substring within a given URI reference. We
+ use the generic term "path component" to describe the URI substring
+ matched by the parser to one of these rules.
+
+ path = path-abempty ; begins with "/" or is empty
+ / path-absolute ; begins with "/" but not "//"
+ / path-noscheme ; begins with a non-colon segment
+ / path-rootless ; begins with a segment
+ / path-empty ; zero characters
+
+ path-abempty = *( "/" segment )
+ path-absolute = "/" [ segment-nz *( "/" segment ) ]
+ path-noscheme = segment-nz-nc *( "/" segment )
+ path-rootless = segment-nz *( "/" segment )
+ path-empty = 0<pchar>
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 22]</span>
+<a name="page-23" id="page-23" href="#page-23"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ segment = *pchar
+ segment-nz = 1*pchar
+ segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
+ ; non-zero-length segment without any colon ":"
+
+ pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
+
+ A path consists of a sequence of path segments separated by a slash
+ ("/") character. A path is always defined for a URI, though the
+ defined path may be empty (zero length). Use of the slash character
+ to indicate hierarchy is only required when a URI will be used as the
+ context for relative references. For example, the URI
+ <mailto:fred@example.com> has a path of "fred@example.com", whereas
+ the URI <foo://info.example.com?fred> has an empty path.
+
+ The path segments "." and "..", also known as dot-segments, are
+ defined for relative reference within the path name hierarchy. They
+ are intended for use at the beginning of a relative-path reference
+ (<a href="#section-4.2">Section 4.2</a>) to indicate relative position within the hierarchical
+ tree of names. This is similar to their role within some operating
+ systems' file directory structures to indicate the current directory
+ and parent directory, respectively. However, unlike in a file
+ system, these dot-segments are only interpreted within the URI path
+ hierarchy and are removed as part of the resolution process (Section
+ 5.2).
+
+ Aside from dot-segments in hierarchical paths, a path segment is
+ considered opaque by the generic syntax. URI producing applications
+ often use the reserved characters allowed in a segment to delimit
+ scheme-specific or dereference-handler-specific subcomponents. For
+ example, the semicolon (";") and equals ("=") reserved characters are
+ often used to delimit parameters and parameter values applicable to
+ that segment. The comma (",") reserved character is often used for
+ similar purposes. For example, one URI producer might use a segment
+ such as "name;v=1.1" to indicate a reference to version 1.1 of
+ "name", whereas another might use a segment such as "name,1.1" to
+ indicate the same. Parameter types may be defined by scheme-specific
+ semantics, but in most cases the syntax of a parameter is specific to
+ the implementation of the URI's dereferencing algorithm.
+
+<span class="h3"><h3><a name="section-3.4">3.4</a>. Query</h3></span>
+
+ The query component contains non-hierarchical data that, along with
+ data in the path component (<a href="#section-3.3">Section 3.3</a>), serves to identify a
+ resource within the scope of the URI's scheme and naming authority
+ (if any). The query component is indicated by the first question
+ mark ("?") character and terminated by a number sign ("#") character
+ or by the end of the URI.
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 23]</span>
+<a name="page-24" id="page-24" href="#page-24"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ query = *( pchar / "/" / "?" )
+
+ The characters slash ("/") and question mark ("?") may represent data
+ within the query component. Beware that some older, erroneous
+ implementations may not handle such data correctly when it is used as
+ the base URI for relative references (<a href="#section-5.1">Section 5.1</a>), apparently
+ because they fail to distinguish query data from path data when
+ looking for hierarchical separators. However, as query components
+ are often used to carry identifying information in the form of
+ "key=value" pairs and one frequently used value is a reference to
+ another URI, it is sometimes better for usability to avoid percent-
+ encoding those characters.
+
+<span class="h3"><h3><a name="section-3.5">3.5</a>. Fragment</h3></span>
+
+ The fragment identifier component of a URI allows indirect
+ identification of a secondary resource by reference to a primary
+ resource and additional identifying information. The identified
+ secondary resource may be some portion or subset of the primary
+ resource, some view on representations of the primary resource, or
+ some other resource defined or described by those representations. A
+ fragment identifier component is indicated by the presence of a
+ number sign ("#") character and terminated by the end of the URI.
+
+ fragment = *( pchar / "/" / "?" )
+
+ The semantics of a fragment identifier are defined by the set of
+ representations that might result from a retrieval action on the
+ primary resource. The fragment's format and resolution is therefore
+ dependent on the media type [<a href="http://tools.ietf.org/html/rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>] of a potentially retrieved
+ representation, even though such a retrieval is only performed if the
+ URI is dereferenced. If no such representation exists, then the
+ semantics of the fragment are considered unknown and are effectively
+ unconstrained. Fragment identifier semantics are independent of the
+ URI scheme and thus cannot be redefined by scheme specifications.
+
+ Individual media types may define their own restrictions on or
+ structures within the fragment identifier syntax for specifying
+ different types of subsets, views, or external references that are
+ identifiable as secondary resources by that media type. If the
+ primary resource has multiple representations, as is often the case
+ for resources whose representation is selected based on attributes of
+ the retrieval request (a.k.a., content negotiation), then whatever is
+ identified by the fragment should be consistent across all of those
+ representations. Each representation should either define the
+ fragment so that it corresponds to the same secondary resource,
+ regardless of how it is represented, or should leave the fragment
+ undefined (i.e., not found).
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 24]</span>
+<a name="page-25" id="page-25" href="#page-25"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ As with any URI, use of a fragment identifier component does not
+ imply that a retrieval action will take place. A URI with a fragment
+ identifier may be used to refer to the secondary resource without any
+ implication that the primary resource is accessible or will ever be
+ accessed.
+
+ Fragment identifiers have a special role in information retrieval
+ systems as the primary form of client-side indirect referencing,
+ allowing an author to specifically identify aspects of an existing
+ resource that are only indirectly provided by the resource owner. As
+ such, the fragment identifier is not used in the scheme-specific
+ processing of a URI; instead, the fragment identifier is separated
+ from the rest of the URI prior to a dereference, and thus the
+ identifying information within the fragment itself is dereferenced
+ solely by the user agent, regardless of the URI scheme. Although
+ this separate handling is often perceived to be a loss of
+ information, particularly for accurate redirection of references as
+ resources move over time, it also serves to prevent information
+ providers from denying reference authors the right to refer to
+ information within a resource selectively. Indirect referencing also
+ provides additional flexibility and extensibility to systems that use
+ URIs, as new media types are easier to define and deploy than new
+ schemes of identification.
+
+ The characters slash ("/") and question mark ("?") are allowed to
+ represent data within the fragment identifier. Beware that some
+ older, erroneous implementations may not handle this data correctly
+ when it is used as the base URI for relative references (Section
+ 5.1).
+
+<span class="h2"><h2><a name="section-4">4</a>. Usage</h2></span>
+
+ When applications make reference to a URI, they do not always use the
+ full form of reference defined by the "URI" syntax rule. To save
+ space and take advantage of hierarchical locality, many Internet
+ protocol elements and media type formats allow an abbreviation of a
+ URI, whereas others restrict the syntax to a particular form of URI.
+ We define the most common forms of reference syntax in this
+ specification because they impact and depend upon the design of the
+ generic syntax, requiring a uniform parsing algorithm in order to be
+ interpreted consistently.
+
+<span class="h3"><h3><a name="section-4.1">4.1</a>. URI Reference</h3></span>
+
+ URI-reference is used to denote the most common usage of a resource
+ identifier.
+
+ URI-reference = URI / relative-ref
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 25]</span>
+<a name="page-26" id="page-26" href="#page-26"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ A URI-reference is either a URI or a relative reference. If the
+ URI-reference's prefix does not match the syntax of a scheme followed
+ by its colon separator, then the URI-reference is a relative
+ reference.
+
+ A URI-reference is typically parsed first into the five URI
+ components, in order to determine what components are present and
+ whether the reference is relative. Then, each component is parsed
+ for its subparts and their validation. The ABNF of URI-reference,
+ along with the "first-match-wins" disambiguation rule, is sufficient
+ to define a validating parser for the generic syntax. Readers
+ familiar with regular expressions should see Appendix B for an
+ example of a non-validating URI-reference parser that will take any
+ given string and extract the URI components.
+
+<span class="h3"><h3><a name="section-4.2">4.2</a>. Relative Reference</h3></span>
+
+ A relative reference takes advantage of the hierarchical syntax
+ (<a href="#section-1.2.3">Section 1.2.3</a>) to express a URI reference relative to the name space
+ of another hierarchical URI.
+
+ relative-ref = relative-part [ "?" query ] [ "#" fragment ]
+
+ relative-part = "//" authority path-abempty
+ / path-absolute
+ / path-noscheme
+ / path-empty
+
+ The URI referred to by a relative reference, also known as the target
+ URI, is obtained by applying the reference resolution algorithm of
+ <a href="#section-5">Section 5</a>.
+
+ A relative reference that begins with two slash characters is termed
+ a network-path reference; such references are rarely used. A
+ relative reference that begins with a single slash character is
+ termed an absolute-path reference. A relative reference that does
+ not begin with a slash character is termed a relative-path reference.
+
+ A path segment that contains a colon character (e.g., "this:that")
+ cannot be used as the first segment of a relative-path reference, as
+ it would be mistaken for a scheme name. Such a segment must be
+ preceded by a dot-segment (e.g., "./this:that") to make a relative-
+ path reference.
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 26]</span>
+<a name="page-27" id="page-27" href="#page-27"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h3"><h3><a name="section-4.3">4.3</a>. Absolute URI</h3></span>
+
+ Some protocol elements allow only the absolute form of a URI without
+ a fragment identifier. For example, defining a base URI for later
+ use by relative references calls for an absolute-URI syntax rule that
+ does not allow a fragment.
+
+ absolute-URI = scheme ":" hier-part [ "?" query ]
+
+ URI scheme specifications must define their own syntax so that all
+ strings matching their scheme-specific syntax will also match the
+ <absolute-URI> grammar. Scheme specifications will not define
+ fragment identifier syntax or usage, regardless of its applicability
+ to resources identifiable via that scheme, as fragment identification
+ is orthogonal to scheme definition. However, scheme specifications
+ are encouraged to include a wide range of examples, including
+ examples that show use of the scheme's URIs with fragment identifiers
+ when such usage is appropriate.
+
+<span class="h3"><h3><a name="section-4.4">4.4</a>. Same-Document Reference</h3></span>
+
+ When a URI reference refers to a URI that is, aside from its fragment
+ component (if any), identical to the base URI (<a href="#section-5.1">Section 5.1</a>), that
+ reference is called a "same-document" reference. The most frequent
+ examples of same-document references are relative references that are
+ empty or include only the number sign ("#") separator followed by a
+ fragment identifier.
+
+ When a same-document reference is dereferenced for a retrieval
+ action, the target of that reference is defined to be within the same
+ entity (representation, document, or message) as the reference;
+ therefore, a dereference should not result in a new retrieval action.
+
+ Normalization of the base and target URIs prior to their comparison,
+ as described in Sections 6.2.2 and 6.2.3, is allowed but rarely
+ performed in practice. Normalization may increase the set of same-
+ document references, which may be of benefit to some caching
+ applications. As such, reference authors should not assume that a
+ slightly different, though equivalent, reference URI will (or will
+ not) be interpreted as a same-document reference by any given
+ application.
+
+<span class="h3"><h3><a name="section-4.5">4.5</a>. Suffix Reference</h3></span>
+
+ The URI syntax is designed for unambiguous reference to resources and
+ extensibility via the URI scheme. However, as URI identification and
+ usage have become commonplace, traditional media (television, radio,
+ newspapers, billboards, etc.) have increasingly used a suffix of the
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 27]</span>
+<a name="page-28" id="page-28" href="#page-28"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ URI as a reference, consisting of only the authority and path
+ portions of the URI, such as
+
+ www.w3.org/Addressing/
+
+ or simply a DNS registered name on its own. Such references are
+ primarily intended for human interpretation rather than for machines,
+ with the assumption that context-based heuristics are sufficient to
+ complete the URI (e.g., most registered names beginning with "www"
+ are likely to have a URI prefix of "http://"). Although there is no
+ standard set of heuristics for disambiguating a URI suffix, many
+ client implementations allow them to be entered by the user and
+ heuristically resolved.
+
+ Although this practice of using suffix references is common, it
+ should be avoided whenever possible and should never be used in
+ situations where long-term references are expected. The heuristics
+ noted above will change over time, particularly when a new URI scheme
+ becomes popular, and are often incorrect when used out of context.
+ Furthermore, they can lead to security issues along the lines of
+ those described in [<a href="http://tools.ietf.org/html/rfc1535" title=""A Security Problem and Proposed Correction With Widely Deployed DNS Software"">RFC1535</a>].
+
+ As a URI suffix has the same syntax as a relative-path reference, a
+ suffix reference cannot be used in contexts where a relative
+ reference is expected. As a result, suffix references are limited to
+ places where there is no defined base URI, such as dialog boxes and
+ off-line advertisements.
+
+<span class="h2"><h2><a name="section-5">5</a>. Reference Resolution</h2></span>
+
+ This section defines the process of resolving a URI reference within
+ a context that allows relative references so that the result is a
+ string matching the <URI> syntax rule of <a href="#section-3">Section 3</a>.
+
+<span class="h3"><h3><a name="section-5.1">5.1</a>. Establishing a Base URI</h3></span>
+
+ The term "relative" implies that a "base URI" exists against which
+ the relative reference is applied. Aside from fragment-only
+ references (<a href="#section-4.4">Section 4.4</a>), relative references are only usable when a
+ base URI is known. A base URI must be established by the parser
+ prior to parsing URI references that might be relative. A base URI
+ must conform to the <absolute-URI> syntax rule (<a href="#section-4.3">Section 4.3</a>). If the
+ base URI is obtained from a URI reference, then that reference must
+ be converted to absolute form and stripped of any fragment component
+ prior to its use as a base URI.
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 28]</span>
+<a name="page-29" id="page-29" href="#page-29"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ The base URI of a reference can be established in one of four ways,
+ discussed below in order of precedence. The order of precedence can
+ be thought of in terms of layers, where the innermost defined base
+ URI has the highest precedence. This can be visualized graphically
+ as follows:
+
+ .----------------------------------------------------------.
+ | .----------------------------------------------------. |
+ | | .----------------------------------------------. | |
+ | | | .----------------------------------------. | | |
+ | | | | .----------------------------------. | | | |
+ | | | | | <relative-reference> | | | | |
+ | | | | `----------------------------------' | | | |
+ | | | | (5.1.1) Base URI embedded in content | | | |
+ | | | `----------------------------------------' | | |
+ | | | (5.1.2) Base URI of the encapsulating entity | | |
+ | | | (message, representation, or none) | | |
+ | | `----------------------------------------------' | |
+ | | (5.1.3) URI used to retrieve the entity | |
+ | `----------------------------------------------------' |
+ | (5.1.4) Default Base URI (application-dependent) |
+ `----------------------------------------------------------'
+
+<span class="h4"><h4><a name="section-5.1.1">5.1.1</a>. Base URI Embedded in Content</h4></span>
+
+ Within certain media types, a base URI for relative references can be
+ embedded within the content itself so that it can be readily obtained
+ by a parser. This can be useful for descriptive documents, such as
+ tables of contents, which may be transmitted to others through
+ protocols other than their usual retrieval context (e.g., email or
+ USENET news).
+
+ It is beyond the scope of this specification to specify how, for each
+ media type, a base URI can be embedded. The appropriate syntax, when
+ available, is described by the data format specification associated
+ with each media type.
+
+<span class="h4"><h4><a name="section-5.1.2">5.1.2</a>. Base URI from the Encapsulating Entity</h4></span>
+
+ If no base URI is embedded, the base URI is defined by the
+ representation's retrieval context. For a document that is enclosed
+ within another entity, such as a message or archive, the retrieval
+ context is that entity. Thus, the default base URI of a
+ representation is the base URI of the entity in which the
+ representation is encapsulated.
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 29]</span>
+<a name="page-30" id="page-30" href="#page-30"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ A mechanism for embedding a base URI within MIME container types
+ (e.g., the message and multipart types) is defined by MHTML
+ [<a href="http://tools.ietf.org/html/rfc2557" title=""MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)"">RFC2557</a>]. Protocols that do not use the MIME message header syntax,
+ but that do allow some form of tagged metadata to be included within
+ messages, may define their own syntax for defining a base URI as part
+ of a message.
+
+<span class="h4"><h4><a name="section-5.1.3">5.1.3</a>. Base URI from the Retrieval URI</h4></span>
+
+ If no base URI is embedded and the representation is not encapsulated
+ within some other entity, then, if a URI was used to retrieve the
+ representation, that URI shall be considered the base URI. Note that
+ if the retrieval was the result of a redirected request, the last URI
+ used (i.e., the URI that resulted in the actual retrieval of the
+ representation) is the base URI.
+
+<span class="h4"><h4><a name="section-5.1.4">5.1.4</a>. Default Base URI</h4></span>
+
+ If none of the conditions described above apply, then the base URI is
+ defined by the context of the application. As this definition is
+ necessarily application-dependent, failing to define a base URI by
+ using one of the other methods may result in the same content being
+ interpreted differently by different types of applications.
+
+ A sender of a representation containing relative references is
+ responsible for ensuring that a base URI for those references can be
+ established. Aside from fragment-only references, relative
+ references can only be used reliably in situations where the base URI
+ is well defined.
+
+<span class="h3"><h3><a name="section-5.2">5.2</a>. Relative Resolution</h3></span>
+
+ This section describes an algorithm for converting a URI reference
+ that might be relative to a given base URI into the parsed components
+ of the reference's target. The components can then be recomposed, as
+ described in <a href="#section-5.3">Section 5.3</a>, to form the target URI. This algorithm
+ provides definitive results that can be used to test the output of
+ other implementations. Applications may implement relative reference
+ resolution by using some other algorithm, provided that the results
+ match what would be given by this one.
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 30]</span>
+<a name="page-31" id="page-31" href="#page-31"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h4"><h4><a name="section-5.2.1">5.2.1</a>. Pre-parse the Base URI</h4></span>
+
+ The base URI (Base) is established according to the procedure of
+ <a href="#section-5.1">Section 5.1</a> and parsed into the five main components described in
+ <a href="#section-3">Section 3</a>. Note that only the scheme component is required to be
+ present in a base URI; the other components may be empty or
+ undefined. A component is undefined if its associated delimiter does
+ not appear in the URI reference; the path component is never
+ undefined, though it may be empty.
+
+ Normalization of the base URI, as described in Sections 6.2.2 and
+ 6.2.3, is optional. A URI reference must be transformed to its
+ target URI before it can be normalized.
+
+<span class="h4"><h4><a name="section-5.2.2">5.2.2</a>. Transform References</h4></span>
+
+ For each URI reference (R), the following pseudocode describes an
+ algorithm for transforming R into its target URI (T):
+
+ -- The URI reference is parsed into the five URI components
+ --
+ (R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
+
+ -- A non-strict parser may ignore a scheme in the reference
+ -- if it is identical to the base URI's scheme.
+ --
+ if ((not strict) and (R.scheme == Base.scheme)) then
+ undefine(R.scheme);
+ endif;
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 31]</span>
+<a name="page-32" id="page-32" href="#page-32"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ if defined(R.scheme) then
+ T.scheme = R.scheme;
+ T.authority = R.authority;
+ T.path = remove_dot_segments(R.path);
+ T.query = R.query;
+ else
+ if defined(R.authority) then
+ T.authority = R.authority;
+ T.path = remove_dot_segments(R.path);
+ T.query = R.query;
+ else
+ if (R.path == "") then
+ T.path = Base.path;
+ if defined(R.query) then
+ T.query = R.query;
+ else
+ T.query = Base.query;
+ endif;
+ else
+ if (R.path starts-with "/") then
+ T.path = remove_dot_segments(R.path);
+ else
+ T.path = merge(Base.path, R.path);
+ T.path = remove_dot_segments(T.path);
+ endif;
+ T.query = R.query;
+ endif;
+ T.authority = Base.authority;
+ endif;
+ T.scheme = Base.scheme;
+ endif;
+
+ T.fragment = R.fragment;
+
+<span class="h4"><h4><a name="section-5.2.3">5.2.3</a>. Merge Paths</h4></span>
+
+ The pseudocode above refers to a "merge" routine for merging a
+ relative-path reference with the path of the base URI. This is
+ accomplished as follows:
+
+ o If the base URI has a defined authority component and an empty
+ path, then return a string consisting of "/" concatenated with the
+ reference's path; otherwise,
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 32]</span>
+<a name="page-33" id="page-33" href="#page-33"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ o return a string consisting of the reference's path component
+ appended to all but the last segment of the base URI's path (i.e.,
+ excluding any characters after the right-most "/" in the base URI
+ path, or excluding the entire base URI path if it does not contain
+ any "/" characters).
+
+<span class="h4"><h4><a name="section-5.2.4">5.2.4</a>. Remove Dot Segments</h4></span>
+
+ The pseudocode also refers to a "remove_dot_segments" routine for
+ interpreting and removing the special "." and ".." complete path
+ segments from a referenced path. This is done after the path is
+ extracted from a reference, whether or not the path was relative, in
+ order to remove any invalid or extraneous dot-segments prior to
+ forming the target URI. Although there are many ways to accomplish
+ this removal process, we describe a simple method using two string
+ buffers.
+
+ 1. The input buffer is initialized with the now-appended path
+ components and the output buffer is initialized to the empty
+ string.
+
+ 2. While the input buffer is not empty, loop as follows:
+
+ A. If the input buffer begins with a prefix of "../" or "./",
+ then remove that prefix from the input buffer; otherwise,
+
+ B. if the input buffer begins with a prefix of "/./" or "/.",
+ where "." is a complete path segment, then replace that
+ prefix with "/" in the input buffer; otherwise,
+
+ C. if the input buffer begins with a prefix of "/../" or "/..",
+ where ".." is a complete path segment, then replace that
+ prefix with "/" in the input buffer and remove the last
+ segment and its preceding "/" (if any) from the output
+ buffer; otherwise,
+
+ D. if the input buffer consists only of "." or "..", then remove
+ that from the input buffer; otherwise,
+
+ E. move the first path segment in the input buffer to the end of
+ the output buffer, including the initial "/" character (if
+ any) and any subsequent characters up to, but not including,
+ the next "/" character or the end of the input buffer.
+
+ 3. Finally, the output buffer is returned as the result of
+ remove_dot_segments.
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 33]</span>
+<a name="page-34" id="page-34" href="#page-34"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ Note that dot-segments are intended for use in URI references to
+ express an identifier relative to the hierarchy of names in the base
+ URI. The remove_dot_segments algorithm respects that hierarchy by
+ removing extra dot-segments rather than treat them as an error or
+ leaving them to be misinterpreted by dereference implementations.
+
+ The following illustrates how the above steps are applied for two
+ examples of merged paths, showing the state of the two buffers after
+ each step.
+
+ STEP OUTPUT BUFFER INPUT BUFFER
+
+ 1 : /a/b/c/./../../g
+ 2E: /a /b/c/./../../g
+ 2E: /a/b /c/./../../g
+ 2E: /a/b/c /./../../g
+ 2B: /a/b/c /../../g
+ 2C: /a/b /../g
+ 2C: /a /g
+ 2E: /a/g
+
+ STEP OUTPUT BUFFER INPUT BUFFER
+
+ <a href="#section-1">1</a> : mid/content=5/../6
+ 2E: mid /content=5/../6
+ 2E: mid/content=5 /../6
+ 2C: mid /6
+ 2E: mid/6
+
+ Some applications may find it more efficient to implement the
+ remove_dot_segments algorithm by using two segment stacks rather than
+ strings.
+
+ Note: Beware that some older, erroneous implementations will fail
+ to separate a reference's query component from its path component
+ prior to merging the base and reference paths, resulting in an
+ interoperability failure if the query component contains the
+ strings "/../" or "/./".
+
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 34]</span>
+<a name="page-35" id="page-35" href="#page-35"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h3"><h3><a name="section-5.3">5.3</a>. Component Recomposition</h3></span>
+
+ Parsed URI components can be recomposed to obtain the corresponding
+ URI reference string. Using pseudocode, this would be:
+
+ result = ""
+
+ if defined(scheme) then
+ append scheme to result;
+ append ":" to result;
+ endif;
+
+ if defined(authority) then
+ append "//" to result;
+ append authority to result;
+ endif;
+
+ append path to result;
+
+ if defined(query) then
+ append "?" to result;
+ append query to result;
+ endif;
+
+ if defined(fragment) then
+ append "#" to result;
+ append fragment to result;
+ endif;
+
+ return result;
+
+ Note that we are careful to preserve the distinction between a
+ component that is undefined, meaning that its separator was not
+ present in the reference, and a component that is empty, meaning that
+ the separator was present and was immediately followed by the next
+ component separator or the end of the reference.
+
+<span class="h3"><h3><a name="section-5.4">5.4</a>. Reference Resolution Examples</h3></span>
+
+ Within a representation with a well defined base URI of
+
+ http://a/b/c/d;p?q
+
+ a relative reference is transformed to its target URI as follows.
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 35]</span>
+<a name="page-36" id="page-36" href="#page-36"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h4"><h4><a name="section-5.4.1">5.4.1</a>. Normal Examples</h4></span>
+
+ "g:h" = "g:h"
+ "g" = "<a href="http://a/b/c/g">http://a/b/c/g</a>"
+ "./g" = "<a href="http://a/b/c/g">http://a/b/c/g</a>"
+ "g/" = "<a href="http://a/b/c/g/">http://a/b/c/g/</a>"
+ "/g" = "<a href="http://a/g">http://a/g</a>"
+ "//g" = "http://g"
+ "?y" = "http://a/b/c/d;p?y"
+ "g?y" = "<a href="http://a/b/c/g?y">http://a/b/c/g?y</a>"
+ "#s" = "http://a/b/c/d;p?q#s"
+ "g#s" = "<a href="http://a/b/c/g#s">http://a/b/c/g#s</a>"
+ "g?y#s" = "<a href="http://a/b/c/g?y#s">http://a/b/c/g?y#s</a>"
+ ";x" = "http://a/b/c/;x"
+ "g;x" = "http://a/b/c/g;x"
+ "g;x?y#s" = "http://a/b/c/g;x?y#s"
+ "" = "http://a/b/c/d;p?q"
+ "." = "<a href="http://a/b/c/">http://a/b/c/</a>"
+ "./" = "<a href="http://a/b/c/">http://a/b/c/</a>"
+ ".." = "<a href="http://a/b/">http://a/b/</a>"
+ "../" = "<a href="http://a/b/">http://a/b/</a>"
+ "../g" = "<a href="http://a/b/g">http://a/b/g</a>"
+ "../.." = "<a href="http://a/">http://a/</a>"
+ "../../" = "<a href="http://a/">http://a/</a>"
+ "../../g" = "<a href="http://a/g">http://a/g</a>"
+
+<span class="h4"><h4><a name="section-5.4.2">5.4.2</a>. Abnormal Examples</h4></span>
+
+ Although the following abnormal examples are unlikely to occur in
+ normal practice, all URI parsers should be capable of resolving them
+ consistently. Each example uses the same base as that above.
+
+ Parsers must be careful in handling cases where there are more ".."
+ segments in a relative-path reference than there are hierarchical
+ levels in the base URI's path. Note that the ".." syntax cannot be
+ used to change the authority component of a URI.
+
+ "../../../g" = "<a href="http://a/g">http://a/g</a>"
+ "../../../../g" = "<a href="http://a/g">http://a/g</a>"
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 36]</span>
+<a name="page-37" id="page-37" href="#page-37"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ Similarly, parsers must remove the dot-segments "." and ".." when
+ they are complete components of a path, but not when they are only
+ part of a segment.
+
+ "/./g" = "<a href="http://a/g">http://a/g</a>"
+ "/../g" = "<a href="http://a/g">http://a/g</a>"
+ "g." = "<a href="http://a/b/c/g">http://a/b/c/g</a>."
+ ".g" = "<a href="http://a/b/c/.g">http://a/b/c/.g</a>"
+ "g.." = "<a href="http://a/b/c/g">http://a/b/c/g</a>.."
+ "..g" = "<a href="http://a/b/c/..g">http://a/b/c/..g</a>"
+
+ Less likely are cases where the relative reference uses unnecessary
+ or nonsensical forms of the "." and ".." complete path segments.
+
+ "./../g" = "<a href="http://a/b/g">http://a/b/g</a>"
+ "./g/." = "<a href="http://a/b/c/g/">http://a/b/c/g/</a>"
+ "g/./h" = "<a href="http://a/b/c/g/h">http://a/b/c/g/h</a>"
+ "g/../h" = "<a href="http://a/b/c/h">http://a/b/c/h</a>"
+ "g;x=1/./y" = "http://a/b/c/g;x=1/y"
+ "g;x=1/../y" = "<a href="http://a/b/c/y">http://a/b/c/y</a>"
+
+ Some applications fail to separate the reference's query and/or
+ fragment components from the path component before merging it with
+ the base path and removing dot-segments. This error is rarely
+ noticed, as typical usage of a fragment never includes the hierarchy
+ ("/") character and the query component is not normally used within
+ relative references.
+
+ "g?y/./x" = "<a href="http://a/b/c/g?y/./x">http://a/b/c/g?y/./x</a>"
+ "g?y/../x" = "<a href="http://a/b/c/g?y/../x">http://a/b/c/g?y/../x</a>"
+ "g#s/./x" = "<a href="http://a/b/c/g#s/./x">http://a/b/c/g#s/./x</a>"
+ "g#s/../x" = "<a href="http://a/b/c/g#s/../x">http://a/b/c/g#s/../x</a>"
+
+ Some parsers allow the scheme name to be present in a relative
+ reference if it is the same as the base URI scheme. This is
+ considered to be a loophole in prior specifications of partial URI
+ [<a href="http://tools.ietf.org/html/rfc1630" title=""Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web"">RFC1630</a>]. Its use should be avoided but is allowed for backward
+ compatibility.
+
+ "http:g" = "http:g" ; for strict parsers
+ / "<a href="http://a/b/c/g">http://a/b/c/g</a>" ; for backward compatibility
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 37]</span>
+<a name="page-38" id="page-38" href="#page-38"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h2"><h2><a name="section-6">6</a>. Normalization and Comparison</h2></span>
+
+ One of the most common operations on URIs is simple comparison:
+ determining whether two URIs are equivalent without using the URIs to
+ access their respective resource(s). A comparison is performed every
+ time a response cache is accessed, a browser checks its history to
+ color a link, or an XML parser processes tags within a namespace.
+ Extensive normalization prior to comparison of URIs is often used by
+ spiders and indexing engines to prune a search space or to reduce
+ duplication of request actions and response storage.
+
+ URI comparison is performed for some particular purpose. Protocols
+ or implementations that compare URIs for different purposes will
+ often be subject to differing design trade-offs in regards to how
+ much effort should be spent in reducing aliased identifiers. This
+ section describes various methods that may be used to compare URIs,
+ the trade-offs between them, and the types of applications that might
+ use them.
+
+<span class="h3"><h3><a name="section-6.1">6.1</a>. Equivalence</h3></span>
+
+ Because URIs exist to identify resources, presumably they should be
+ considered equivalent when they identify the same resource. However,
+ this definition of equivalence is not of much practical use, as there
+ is no way for an implementation to compare two resources unless it
+ has full knowledge or control of them. For this reason,
+ determination of equivalence or difference of URIs is based on string
+ comparison, perhaps augmented by reference to additional rules
+ provided by URI scheme definitions. We use the terms "different" and
+ "equivalent" to describe the possible outcomes of such comparisons,
+ but there are many application-dependent versions of equivalence.
+
+ Even though it is possible to determine that two URIs are equivalent,
+ URI comparison is not sufficient to determine whether two URIs
+ identify different resources. For example, an owner of two different
+ domain names could decide to serve the same resource from both,
+ resulting in two different URIs. Therefore, comparison methods are
+ designed to minimize false negatives while strictly avoiding false
+ positives.
+
+ In testing for equivalence, applications should not directly compare
+ relative references; the references should be converted to their
+ respective target URIs before comparison. When URIs are compared to
+ select (or avoid) a network action, such as retrieval of a
+ representation, fragment components (if any) should be excluded from
+ the comparison.
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 38]</span>
+<a name="page-39" id="page-39" href="#page-39"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h3"><h3><a name="section-6.2">6.2</a>. Comparison Ladder</h3></span>
+
+ A variety of methods are used in practice to test URI equivalence.
+ These methods fall into a range, distinguished by the amount of
+ processing required and the degree to which the probability of false
+ negatives is reduced. As noted above, false negatives cannot be
+ eliminated. In practice, their probability can be reduced, but this
+ reduction requires more processing and is not cost-effective for all
+ applications.
+
+ If this range of comparison practices is considered as a ladder, the
+ following discussion will climb the ladder, starting with practices
+ that are cheap but have a relatively higher chance of producing false
+ negatives, and proceeding to those that have higher computational
+ cost and lower risk of false negatives.
+
+<span class="h4"><h4><a name="section-6.2.1">6.2.1</a>. Simple String Comparison</h4></span>
+
+ If two URIs, when considered as character strings, are identical,
+ then it is safe to conclude that they are equivalent. This type of
+ equivalence test has very low computational cost and is in wide use
+ in a variety of applications, particularly in the domain of parsing.
+
+ Testing strings for equivalence requires some basic precautions.
+ This procedure is often referred to as "bit-for-bit" or
+ "byte-for-byte" comparison, which is potentially misleading. Testing
+ strings for equality is normally based on pair comparison of the
+ characters that make up the strings, starting from the first and
+ proceeding until both strings are exhausted and all characters are
+ found to be equal, until a pair of characters compares unequal, or
+ until one of the strings is exhausted before the other.
+
+ This character comparison requires that each pair of characters be
+ put in comparable form. For example, should one URI be stored in a
+ byte array in EBCDIC encoding and the second in a Java String object
+ (UTF-16), bit-for-bit comparisons applied naively will produce
+ errors. It is better to speak of equality on a character-for-
+ character basis rather than on a byte-for-byte or bit-for-bit basis.
+ In practical terms, character-by-character comparisons should be done
+ codepoint-by-codepoint after conversion to a common character
+ encoding.
+
+ False negatives are caused by the production and use of URI aliases.
+ Unnecessary aliases can be reduced, regardless of the comparison
+ method, by consistently providing URI references in an already-
+ normalized form (i.e., a form identical to what would be produced
+ after normalization is applied, as described below).
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 39]</span>
+<a name="page-40" id="page-40" href="#page-40"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ Protocols and data formats often limit some URI comparisons to simple
+ string comparison, based on the theory that people and
+ implementations will, in their own best interest, be consistent in
+ providing URI references, or at least consistent enough to negate any
+ efficiency that might be obtained from further normalization.
+
+<span class="h4"><h4><a name="section-6.2.2">6.2.2</a>. Syntax-Based Normalization</h4></span>
+
+ Implementations may use logic based on the definitions provided by
+ this specification to reduce the probability of false negatives.
+ This processing is moderately higher in cost than character-for-
+ character string comparison. For example, an application using this
+ approach could reasonably consider the following two URIs equivalent:
+
+ example://a/b/c/%7Bfoo%7D
+ eXAMPLE://a/./b/../b/%63/%7bfoo%7d
+
+ Web user agents, such as browsers, typically apply this type of URI
+ normalization when determining whether a cached response is
+ available. Syntax-based normalization includes such techniques as
+ case normalization, percent-encoding normalization, and removal of
+ dot-segments.
+
+<span class="h5"><h5><a name="section-6.2.2.1">6.2.2.1</a>. Case Normalization</h5></span>
+
+ For all URIs, the hexadecimal digits within a percent-encoding
+ triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
+ should be normalized to use uppercase letters for the digits A-F.
+
+ When a URI uses components of the generic syntax, the component
+ syntax equivalence rules always apply; namely, that the scheme and
+ host are case-insensitive and therefore should be normalized to
+ lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
+ equivalent to <http://www.example.com/>. The other generic syntax
+ components are assumed to be case-sensitive unless specifically
+ defined otherwise by the scheme (see <a href="#section-6.2.3">Section 6.2.3</a>).
+
+<span class="h5"><h5><a name="section-6.2.2.2">6.2.2.2</a>. Percent-Encoding Normalization</h5></span>
+
+ The percent-encoding mechanism (<a href="#section-2.1">Section 2.1</a>) is a frequent source of
+ variance among otherwise identical URIs. In addition to the case
+ normalization issue noted above, some URI producers percent-encode
+ octets that do not require percent-encoding, resulting in URIs that
+ are equivalent to their non-encoded counterparts. These URIs should
+ be normalized by decoding any percent-encoded octet that corresponds
+ to an unreserved character, as described in <a href="#section-2.3">Section 2.3</a>.
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 40]</span>
+<a name="page-41" id="page-41" href="#page-41"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h5"><h5><a name="section-6.2.2.3">6.2.2.3</a>. Path Segment Normalization</h5></span>
+
+ The complete path segments "." and ".." are intended only for use
+ within relative references (<a href="#section-4.1">Section 4.1</a>) and are removed as part of
+ the reference resolution process (<a href="#section-5.2">Section 5.2</a>). However, some
+ deployed implementations incorrectly assume that reference resolution
+ is not necessary when the reference is already a URI and thus fail to
+ remove dot-segments when they occur in non-relative paths. URI
+ normalizers should remove dot-segments by applying the
+ remove_dot_segments algorithm to the path, as described in
+ <a href="#section-5.2.4">Section 5.2.4</a>.
+
+<span class="h4"><h4><a name="section-6.2.3">6.2.3</a>. Scheme-Based Normalization</h4></span>
+
+ The syntax and semantics of URIs vary from scheme to scheme, as
+ described by the defining specification for each scheme.
+ Implementations may use scheme-specific rules, at further processing
+ cost, to reduce the probability of false negatives. For example,
+ because the "http" scheme makes use of an authority component, has a
+ default port of "80", and defines an empty path to be equivalent to
+ "/", the following four URIs are equivalent:
+
+ http://example.com
+ http://example.com/
+ <a href="http://example.com/">http://example.com:/</a>
+ <a href="http://example.com/">http://example.com:80/</a>
+
+ In general, a URI that uses the generic syntax for authority with an
+ empty path should be normalized to a path of "/". Likewise, an
+ explicit ":port", for which the port is empty or the default for the
+ scheme, is equivalent to one where the port and its ":" delimiter are
+ elided and thus should be removed by scheme-based normalization. For
+ example, the second URI above is the normal form for the "http"
+ scheme.
+
+ Another case where normalization varies by scheme is in the handling
+ of an empty authority component or empty host subcomponent. For many
+ scheme specifications, an empty authority or host is considered an
+ error; for others, it is considered equivalent to "localhost" or the
+ end-user's host. When a scheme defines a default for authority and a
+ URI reference to that default is desired, the reference should be
+ normalized to an empty authority for the sake of uniformity, brevity,
+ and internationalization. If, however, either the userinfo or port
+ subcomponents are non-empty, then the host should be given explicitly
+ even if it matches the default.
+
+ Normalization should not remove delimiters when their associated
+ component is empty unless licensed to do so by the scheme
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 41]</span>
+<a name="page-42" id="page-42" href="#page-42"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ specification. For example, the URI "http://example.com/?" cannot be
+ assumed to be equivalent to any of the examples above. Likewise, the
+ presence or absence of delimiters within a userinfo subcomponent is
+ usually significant to its interpretation. The fragment component is
+ not subject to any scheme-based normalization; thus, two URIs that
+ differ only by the suffix "#" are considered different regardless of
+ the scheme.
+
+ Some schemes define additional subcomponents that consist of case-
+ insensitive data, giving an implicit license to normalizers to
+ convert this data to a common case (e.g., all lowercase). For
+ example, URI schemes that define a subcomponent of path to contain an
+ Internet hostname, such as the "mailto" URI scheme, cause that
+ subcomponent to be case-insensitive and thus subject to case
+ normalization (e.g., "mailto:Joe@Example.COM" is equivalent to
+ "mailto:Joe@example.com", even though the generic syntax considers
+ the path component to be case-sensitive).
+
+ Other scheme-specific normalizations are possible.
+
+<span class="h4"><h4><a name="section-6.2.4">6.2.4</a>. Protocol-Based Normalization</h4></span>
+
+ Substantial effort to reduce the incidence of false negatives is
+ often cost-effective for web spiders. Therefore, they implement even
+ more aggressive techniques in URI comparison. For example, if they
+ observe that a URI such as
+
+ http://example.com/data
+
+ redirects to a URI differing only in the trailing slash
+
+ http://example.com/data/
+
+ they will likely regard the two as equivalent in the future. This
+ kind of technique is only appropriate when equivalence is clearly
+ indicated by both the result of accessing the resources and the
+ common conventions of their scheme's dereference algorithm (in this
+ case, use of redirection by HTTP origin servers to avoid problems
+ with relative references).
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 42]</span>
+<a name="page-43" id="page-43" href="#page-43"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h2"><h2><a name="section-7">7</a>. Security Considerations</h2></span>
+
+ A URI does not in itself pose a security threat. However, as URIs
+ are often used to provide a compact set of instructions for access to
+ network resources, care must be taken to properly interpret the data
+ within a URI, to prevent that data from causing unintended access,
+ and to avoid including data that should not be revealed in plain
+ text.
+
+<span class="h3"><h3><a name="section-7.1">7.1</a>. Reliability and Consistency</h3></span>
+
+ There is no guarantee that once a URI has been used to retrieve
+ information, the same information will be retrievable by that URI in
+ the future. Nor is there any guarantee that the information
+ retrievable via that URI in the future will be observably similar to
+ that retrieved in the past. The URI syntax does not constrain how a
+ given scheme or authority apportions its namespace or maintains it
+ over time. Such guarantees can only be obtained from the person(s)
+ controlling that namespace and the resource in question. A specific
+ URI scheme may define additional semantics, such as name persistence,
+ if those semantics are required of all naming authorities for that
+ scheme.
+
+<span class="h3"><h3><a name="section-7.2">7.2</a>. Malicious Construction</h3></span>
+
+ It is sometimes possible to construct a URI so that an attempt to
+ perform a seemingly harmless, idempotent operation, such as the
+ retrieval of a representation, will in fact cause a possibly damaging
+ remote operation. The unsafe URI is typically constructed by
+ specifying a port number other than that reserved for the network
+ protocol in question. The client unwittingly contacts a site running
+ a different protocol service, and data within the URI contains
+ instructions that, when interpreted according to this other protocol,
+ cause an unexpected operation. A frequent example of such abuse has
+ been the use of a protocol-based scheme with a port component of
+ "25", thereby fooling user agent software into sending an unintended
+ or impersonating message via an SMTP server.
+
+ Applications should prevent dereference of a URI that specifies a TCP
+ port number within the "well-known port" range (0 - 1023) unless the
+ protocol being used to dereference that URI is compatible with the
+ protocol expected on that well-known port. Although IANA maintains a
+ registry of well-known ports, applications should make such
+ restrictions user-configurable to avoid preventing the deployment of
+ new services.
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 43]</span>
+<a name="page-44" id="page-44" href="#page-44"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ When a URI contains percent-encoded octets that match the delimiters
+ for a given resolution or dereference protocol (for example, CR and
+ LF characters for the TELNET protocol), these percent-encodings must
+ not be decoded before transmission across that protocol. Transfer of
+ the percent-encoding, which might violate the protocol, is less
+ harmful than allowing decoded octets to be interpreted as additional
+ operations or parameters, perhaps triggering an unexpected and
+ possibly harmful remote operation.
+
+<span class="h3"><h3><a name="section-7.3">7.3</a>. Back-End Transcoding</h3></span>
+
+ When a URI is dereferenced, the data within it is often parsed by
+ both the user agent and one or more servers. In HTTP, for example, a
+ typical user agent will parse a URI into its five major components,
+ access the authority's server, and send it the data within the
+ authority, path, and query components. A typical server will take
+ that information, parse the path into segments and the query into
+ key/value pairs, and then invoke implementation-specific handlers to
+ respond to the request. As a result, a common security concern for
+ server implementations that handle a URI, either as a whole or split
+ into separate components, is proper interpretation of the octet data
+ represented by the characters and percent-encodings within that URI.
+
+ Percent-encoded octets must be decoded at some point during the
+ dereference process. Applications must split the URI into its
+ components and subcomponents prior to decoding the octets, as
+ otherwise the decoded octets might be mistaken for delimiters.
+ Security checks of the data within a URI should be applied after
+ decoding the octets. Note, however, that the "%00" percent-encoding
+ (NUL) may require special handling and should be rejected if the
+ application is not expecting to receive raw data within a component.
+
+ Special care should be taken when the URI path interpretation process
+ involves the use of a back-end file system or related system
+ functions. File systems typically assign an operational meaning to
+ special characters, such as the "/", "\", ":", "[", and "]"
+ characters, and to special device names like ".", "..", "...", "aux",
+ "lpt", etc. In some cases, merely testing for the existence of such
+ a name will cause the operating system to pause or invoke unrelated
+ system calls, leading to significant security concerns regarding
+ denial of service and unintended data transfer. It would be
+ impossible for this specification to list all such significant
+ characters and device names. Implementers should research the
+ reserved names and characters for the types of storage device that
+ may be attached to their applications and restrict the use of data
+ obtained from URI components accordingly.
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 44]</span>
+<a name="page-45" id="page-45" href="#page-45"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+<span class="h3"><h3><a name="section-7.4">7.4</a>. Rare IP Address Formats</h3></span>
+
+ Although the URI syntax for IPv4address only allows the common
+ dotted-decimal form of IPv4 address literal, many implementations
+ that process URIs make use of platform-dependent system routines,
+ such as gethostbyname() and inet_aton(), to translate the string
+ literal to an actual IP address. Unfortunately, such system routines
+ often allow and process a much larger set of formats than those
+ described in <a href="#section-3.2.2">Section 3.2.2</a>.
+
+ For example, many implementations allow dotted forms of three
+ numbers, wherein the last part is interpreted as a 16-bit quantity
+ and placed in the right-most two bytes of the network address (e.g.,
+ a Class B network). Likewise, a dotted form of two numbers means
+ that the last part is interpreted as a 24-bit quantity and placed in
+ the right-most three bytes of the network address (Class A), and a
+ single number (without dots) is interpreted as a 32-bit quantity and
+ stored directly in the network address. Adding further to the
+ confusion, some implementations allow each dotted part to be
+ interpreted as decimal, octal, or hexadecimal, as specified in the C
+ language (i.e., a leading 0x or 0X implies hexadecimal; a leading 0
+ implies octal; otherwise, the number is interpreted as decimal).
+
+ These additional IP address formats are not allowed in the URI syntax
+ due to differences between platform implementations. However, they
+ can become a security concern if an application attempts to filter
+ access to resources based on the IP address in string literal format.
+ If this filtering is performed, literals should be converted to
+ numeric form and filtered based on the numeric value, and not on a
+ prefix or suffix of the string form.
+
+<span class="h3"><h3><a name="section-7.5">7.5</a>. Sensitive Information</h3></span>
+
+ URI producers should not provide a URI that contains a username or
+ password that is intended to be secret. URIs are frequently
+ displayed by browsers, stored in clear text bookmarks, and logged by
+ user agent history and intermediary applications (proxies). A
+ password appearing within the userinfo component is deprecated and
+ should be considered an error (or simply ignored) except in those
+ rare cases where the 'password' parameter is intended to be public.
+
+<span class="h3"><h3><a name="section-7.6">7.6</a>. Semantic Attacks</h3></span>
+
+ Because the userinfo subcomponent is rarely used and appears before
+ the host in the authority component, it can be used to construct a
+ URI intended to mislead a human user by appearing to identify one
+ (trusted) naming authority while actually identifying a different
+ authority hidden behind the noise. For example
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 45]</span>
+<a name="page-46" id="page-46" href="#page-46"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ ftp://cnn.example.com&story=breaking_news@10.0.0.1/top_story.htm
+
+ might lead a human user to assume that the host is 'cnn.example.com',
+ whereas it is actually '10.0.0.1'. Note that a misleading userinfo
+ subcomponent could be much longer than the example above.
+
+ A misleading URI, such as that above, is an attack on the user's
+ preconceived notions about the meaning of a URI rather than an attack
+ on the software itself. User agents may be able to reduce the impact
+ of such attacks by distinguishing the various components of the URI
+ when they are rendered, such as by using a different color or tone to
+ render userinfo if any is present, though there is no panacea. More
+ information on URI-based semantic attacks can be found in [<a href="#ref-Siedzik" title=""Semantic Attacks: What&#39;s in a URL?"">Siedzik</a>].
+
+<span class="h2"><h2><a name="section-8">8</a>. IANA Considerations</h2></span>
+
+ URI scheme names, as defined by <scheme> in <a href="#section-3.1">Section 3.1</a>, form a
+ registered namespace that is managed by IANA according to the
+ procedures defined in [<a href="#ref-BCP35" title=""Registration Procedures for URL Scheme Names"">BCP35</a>]. No IANA actions are required by this
+ document.
+
+<span class="h2"><h2><a name="section-9">9</a>. Acknowledgements</h2></span>
+
+ This specification is derived from <a href="http://tools.ietf.org/html/rfc2396">RFC 2396</a> [<a href="http://tools.ietf.org/html/rfc2396" title=""Uniform Resource Identifiers (URI): Generic Syntax"">RFC2396</a>], <a href="http://tools.ietf.org/html/rfc1808">RFC 1808</a>
+ [<a href="http://tools.ietf.org/html/rfc1808" title=""Relative Uniform Resource Locators"">RFC1808</a>], and <a href="http://tools.ietf.org/html/rfc1738">RFC 1738</a> [<a href="http://tools.ietf.org/html/rfc1738" title=""Uniform Resource Locators (URL)"">RFC1738</a>]; the acknowledgements in those
+ documents still apply. It also incorporates the update (with
+ corrections) for IPv6 literals in the host syntax, as defined by
+ Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in
+ [<a href="http://tools.ietf.org/html/rfc2732" title=""Format for Literal IPv6 Addresses in URL&#39;s"">RFC2732</a>]. In addition, contributions by Gisle Aas, Reese Anschultz,
+ Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,
+ Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin
+ Duerst, Stefan Eissing, Clive D.W. Feather, Al Gilman, Tony Hammond,
+ Elliotte Harold, Pat Hayes, Henry Holtzman, Ian B. Jacobs, Michael
+ Kay, John C. Klensin, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew
+ Main, Dave McAlpin, Ira McDonald, Michael Mealling, Ray Merkert,
+ Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin, Kai
+ Schaetzl, Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne,
+ Stuart Williams, and Henry Zongaro are gratefully acknowledged.
+
+<span class="h2"><h2><a name="section-10">10</a>. References</h2></span>
+
+<span class="h3"><h3><a name="section-10.1">10.1</a>. Normative References</h3></span>
+
+ [<a name="ref-ASCII" id="ref-ASCII">ASCII</a>] American National Standards Institute, "Coded Character
+ Set -- 7-bit American Standard Code for Information
+ Interchange", ANSI X3.4, 1986.
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 46]</span>
+<a name="page-47" id="page-47" href="#page-47"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ [<a name="ref-RFC2234" id="ref-RFC2234">RFC2234</a>] Crocker, D. and P. Overell, "Augmented BNF for Syntax
+ Specifications: ABNF", <a href="http://tools.ietf.org/html/rfc2234">RFC 2234</a>, November 1997.
+
+ [<a name="ref-STD63" id="ref-STD63">STD63</a>] Yergeau, F., "UTF-8, a transformation format of
+ ISO 10646", STD 63, <a href="http://tools.ietf.org/html/rfc3629">RFC 3629</a>, November 2003.
+
+ [<a name="ref-UCS" id="ref-UCS">UCS</a>] International Organization for Standardization,
+ "Information Technology - Universal Multiple-Octet Coded
+ Character Set (UCS)", ISO/IEC 10646:2003, December 2003.
+
+<span class="h3"><h3><a name="section-10.2">10.2</a>. Informative References</h3></span>
+
+ [<a name="ref-BCP19" id="ref-BCP19">BCP19</a>] Freed, N. and J. Postel, "IANA Charset Registration
+ Procedures", <a href="http://tools.ietf.org/html/bcp19">BCP 19</a>, <a href="http://tools.ietf.org/html/rfc2978">RFC 2978</a>, October 2000.
+
+ [<a name="ref-BCP35" id="ref-BCP35">BCP35</a>] Petke, R. and I. King, "Registration Procedures for URL
+ Scheme Names", <a href="http://tools.ietf.org/html/bcp35">BCP 35</a>, <a href="http://tools.ietf.org/html/rfc2717">RFC 2717</a>, November 1999.
+
+ [<a name="ref-RFC0952" id="ref-RFC0952">RFC0952</a>] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
+ host table specification", <a href="http://tools.ietf.org/html/rfc952">RFC 952</a>, October 1985.
+
+ [<a name="ref-RFC1034" id="ref-RFC1034">RFC1034</a>] Mockapetris, P., "Domain names - concepts and facilities",
+ STD 13, <a href="http://tools.ietf.org/html/rfc1034">RFC 1034</a>, November 1987.
+
+ [<a name="ref-RFC1123" id="ref-RFC1123">RFC1123</a>] Braden, R., "Requirements for Internet Hosts - Application
+ and Support", STD 3, <a href="http://tools.ietf.org/html/rfc1123">RFC 1123</a>, October 1989.
+
+ [<a name="ref-RFC1535" id="ref-RFC1535">RFC1535</a>] Gavron, E., "A Security Problem and Proposed Correction
+ With Widely Deployed DNS Software", <a href="http://tools.ietf.org/html/rfc1535">RFC 1535</a>,
+ October 1993.
+
+ [<a name="ref-RFC1630" id="ref-RFC1630">RFC1630</a>] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
+ Unifying Syntax for the Expression of Names and Addresses
+ of Objects on the Network as used in the World-Wide Web",
+ <a href="http://tools.ietf.org/html/rfc1630">RFC 1630</a>, June 1994.
+
+ [<a name="ref-RFC1736" id="ref-RFC1736">RFC1736</a>] Kunze, J., "Functional Recommendations for Internet
+ Resource Locators", <a href="http://tools.ietf.org/html/rfc1736">RFC 1736</a>, February 1995.
+
+ [<a name="ref-RFC1737" id="ref-RFC1737">RFC1737</a>] Sollins, K. and L. Masinter, "Functional Requirements for
+ Uniform Resource Names", <a href="http://tools.ietf.org/html/rfc1737">RFC 1737</a>, December 1994.
+
+ [<a name="ref-RFC1738" id="ref-RFC1738">RFC1738</a>] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
+ Resource Locators (URL)", <a href="http://tools.ietf.org/html/rfc1738">RFC 1738</a>, December 1994.
+
+ [<a name="ref-RFC1808" id="ref-RFC1808">RFC1808</a>] Fielding, R., "Relative Uniform Resource Locators",
+ <a href="http://tools.ietf.org/html/rfc1808">RFC 1808</a>, June 1995.
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 47]</span>
+<a name="page-48" id="page-48" href="#page-48"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ [<a name="ref-RFC2046" id="ref-RFC2046">RFC2046</a>] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
+ Extensions (MIME) Part Two: Media Types", <a href="http://tools.ietf.org/html/rfc2046">RFC 2046</a>,
+ November 1996.
+
+ [<a name="ref-RFC2141" id="ref-RFC2141">RFC2141</a>] Moats, R., "URN Syntax", <a href="http://tools.ietf.org/html/rfc2141">RFC 2141</a>, May 1997.
+
+ [<a name="ref-RFC2396" id="ref-RFC2396">RFC2396</a>] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
+ Resource Identifiers (URI): Generic Syntax", <a href="http://tools.ietf.org/html/rfc2396">RFC 2396</a>,
+ August 1998.
+
+ [<a name="ref-RFC2518" id="ref-RFC2518">RFC2518</a>] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
+ Jensen, "HTTP Extensions for Distributed Authoring --
+ WEBDAV", <a href="http://tools.ietf.org/html/rfc2518">RFC 2518</a>, February 1999.
+
+ [<a name="ref-RFC2557" id="ref-RFC2557">RFC2557</a>] Palme, J., Hopmann, A., and N. Shelness, "MIME
+ Encapsulation of Aggregate Documents, such as HTML
+ (MHTML)", <a href="http://tools.ietf.org/html/rfc2557">RFC 2557</a>, March 1999.
+
+ [<a name="ref-RFC2718" id="ref-RFC2718">RFC2718</a>] Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,
+ "Guidelines for new URL Schemes", <a href="http://tools.ietf.org/html/rfc2718">RFC 2718</a>, November 1999.
+
+ [<a name="ref-RFC2732" id="ref-RFC2732">RFC2732</a>] Hinden, R., Carpenter, B., and L. Masinter, "Format for
+ Literal IPv6 Addresses in URL's", <a href="http://tools.ietf.org/html/rfc2732">RFC 2732</a>, December 1999.
+
+ [<a name="ref-RFC3305" id="ref-RFC3305">RFC3305</a>] Mealling, M. and R. Denenberg, "Report from the Joint
+ W3C/IETF URI Planning Interest Group: Uniform Resource
+ Identifiers (URIs), URLs, and Uniform Resource Names
+ (URNs): Clarifications and Recommendations", <a href="http://tools.ietf.org/html/rfc3305">RFC 3305</a>,
+ August 2002.
+
+ [<a name="ref-RFC3490" id="ref-RFC3490">RFC3490</a>] Faltstrom, P., Hoffman, P., and A. Costello,
+ "Internationalizing Domain Names in Applications (IDNA)",
+ <a href="http://tools.ietf.org/html/rfc3490">RFC 3490</a>, March 2003.
+
+ [<a name="ref-RFC3513" id="ref-RFC3513">RFC3513</a>] Hinden, R. and S. Deering, "Internet Protocol Version 6
+ (IPv6) Addressing Architecture", <a href="http://tools.ietf.org/html/rfc3513">RFC 3513</a>, April 2003.
+
+ [<a name="ref-Siedzik" id="ref-Siedzik">Siedzik</a>] Siedzik, R., "Semantic Attacks: What's in a URL?",
+ April 2001, <<a href="http://www.giac.org/practical/gsec/Richard_Siedzik_GSEC.pdf">http://www.giac.org/practical/gsec/</a>
+ <a href="http://www.giac.org/practical/gsec/Richard_Siedzik_GSEC.pdf">Richard_Siedzik_GSEC.pdf</a>>.
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 48]</span>
+<a name="page-49" id="page-49" href="#page-49"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+Appendix A. Collected ABNF for URI
+
+ URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
+
+ hier-part = "//" authority path-abempty
+ / path-absolute
+ / path-rootless
+ / path-empty
+
+ URI-reference = URI / relative-ref
+
+ absolute-URI = scheme ":" hier-part [ "?" query ]
+
+ relative-ref = relative-part [ "?" query ] [ "#" fragment ]
+
+ relative-part = "//" authority path-abempty
+ / path-absolute
+ / path-noscheme
+ / path-empty
+
+ scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
+
+ authority = [ userinfo "@" ] host [ ":" port ]
+ userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
+ host = IP-literal / IPv4address / reg-name
+ port = *DIGIT
+
+ IP-literal = "[" ( IPv6address / IPvFuture ) "]"
+
+ IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
+
+ IPv6address = 6( h16 ":" ) ls32
+ / "::" 5( h16 ":" ) ls32
+ / [ h16 ] "::" 4( h16 ":" ) ls32
+ / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
+ / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
+ / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
+ / [ *4( h16 ":" ) h16 ] "::" ls32
+ / [ *5( h16 ":" ) h16 ] "::" h16
+ / [ *6( h16 ":" ) h16 ] "::"
+
+ h16 = 1*4HEXDIG
+ ls32 = ( h16 ":" h16 ) / IPv4address
+ IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 49]</span>
+<a name="page-50" id="page-50" href="#page-50"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ dec-octet = DIGIT ; 0-9
+ / %x31-39 DIGIT ; 10-99
+ / "1" 2DIGIT ; 100-199
+ / "2" %x30-34 DIGIT ; 200-249
+ / "25" %x30-35 ; 250-255
+
+ reg-name = *( unreserved / pct-encoded / sub-delims )
+
+ path = path-abempty ; begins with "/" or is empty
+ / path-absolute ; begins with "/" but not "//"
+ / path-noscheme ; begins with a non-colon segment
+ / path-rootless ; begins with a segment
+ / path-empty ; zero characters
+
+ path-abempty = *( "/" segment )
+ path-absolute = "/" [ segment-nz *( "/" segment ) ]
+ path-noscheme = segment-nz-nc *( "/" segment )
+ path-rootless = segment-nz *( "/" segment )
+ path-empty = 0<pchar>
+
+ segment = *pchar
+ segment-nz = 1*pchar
+ segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
+ ; non-zero-length segment without any colon ":"
+
+ pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
+
+ query = *( pchar / "/" / "?" )
+
+ fragment = *( pchar / "/" / "?" )
+
+ pct-encoded = "%" HEXDIG HEXDIG
+
+ unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
+ reserved = gen-delims / sub-delims
+ gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
+ sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
+ / "*" / "+" / "," / ";" / "="
+
+Appendix B. Parsing a URI Reference with a Regular Expression
+
+ As the "first-match-wins" algorithm is identical to the "greedy"
+ disambiguation method used by POSIX regular expressions, it is
+ natural and commonplace to use a regular expression for parsing the
+ potential five components of a URI reference.
+
+ The following line is the regular expression for breaking-down a
+ well-formed URI reference into its components.
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 50]</span>
+<a name="page-51" id="page-51" href="#page-51"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
+ 12 3 4 5 6 7 8 9
+
+ The numbers in the second line above are only to assist readability;
+ they indicate the reference points for each subexpression (i.e., each
+ paired parenthesis). We refer to the value matched for subexpression
+ <n> as $<n>. For example, matching the above expression to
+
+ <a href="http://www.ics.uci.edu/pub/ietf/uri/#Related">http://www.ics.uci.edu/pub/ietf/uri/#Related</a>
+
+ results in the following subexpression matches:
+
+ $1 = http:
+ $2 = http
+ $3 = //www.ics.uci.edu
+ $4 = www.ics.uci.edu
+ $5 = /pub/ietf/uri/
+ $6 = <undefined>
+ $7 = <undefined>
+ $8 = #Related
+ $9 = Related
+
+ where <undefined> indicates that the component is not present, as is
+ the case for the query component in the above example. Therefore, we
+ can determine the value of the five components as
+
+ scheme = $2
+ authority = $4
+ path = $5
+ query = $7
+ fragment = $9
+
+ Going in the opposite direction, we can recreate a URI reference from
+ its components by using the algorithm of <a href="#section-5.3">Section 5.3</a>.
+
+Appendix C. Delimiting a URI in Context
+
+ URIs are often transmitted through formats that do not provide a
+ clear context for their interpretation. For example, there are many
+ occasions when a URI is included in plain text; examples include text
+ sent in email, USENET news, and on printed paper. In such cases, it
+ is important to be able to delimit the URI from the rest of the text,
+ and in particular from punctuation marks that might be mistaken for
+ part of the URI.
+
+ In practice, URIs are delimited in a variety of ways, but usually
+ within double-quotes "http://example.com/", angle brackets
+ <http://example.com/>, or just by using whitespace:
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 51]</span>
+<a name="page-52" id="page-52" href="#page-52"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ http://example.com/
+
+ These wrappers do not form part of the URI.
+
+ In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may
+ have to be added to break a long URI across lines. The whitespace
+ should be ignored when the URI is extracted.
+
+ No whitespace should be introduced after a hyphen ("-") character.
+ Because some typesetters and printers may (erroneously) introduce a
+ hyphen at the end of line when breaking it, the interpreter of a URI
+ containing a line break immediately after a hyphen should ignore all
+ whitespace around the line break and should be aware that the hyphen
+ may or may not actually be part of the URI.
+
+ Using <> angle brackets around each URI is especially recommended as
+ a delimiting style for a reference that contains embedded whitespace.
+
+ The prefix "URL:" (with or without a trailing space) was formerly
+ recommended as a way to help distinguish a URI from other bracketed
+ designators, though it is not commonly used in practice and is no
+ longer recommended.
+
+ For robustness, software that accepts user-typed URI should attempt
+ to recognize and strip both delimiters and embedded whitespace.
+
+ For example, the text
+
+ Yes, Jim, I found it under "<a href="http://www.w3.org/Addressing/">http://www.w3.org/Addressing/</a>",
+ but you can probably pick it up from <ftp://foo.example.
+ http://www.ics.uci.edu/pub/
+ <a href="http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING">ietf/uri/historical.html#WARNING</a>>.
+
+ contains the URI references
+
+ <a href="http://www.w3.org/Addressing/">http://www.w3.org/Addressing/</a>
+ ftp://foo.example.com/rfc/
+ <a href="http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING">http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING</a>
+
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 52]</span>
+<a name="page-53" id="page-53" href="#page-53"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+Appendix D. Changes from <a href="http://tools.ietf.org/html/rfc2396">RFC 2396</a>
+
+D.1. Additions
+
+ An ABNF rule for URI has been introduced to correspond to one common
+ usage of the term: an absolute URI with optional fragment.
+
+ IPv6 (and later) literals have been added to the list of possible
+ identifiers for the host portion of an authority component, as
+ described by [<a href="http://tools.ietf.org/html/rfc2732" title=""Format for Literal IPv6 Addresses in URL&#39;s"">RFC2732</a>], with the addition of "[" and "]" to the
+ reserved set and a version flag to anticipate future versions of IP
+ literals. Square brackets are now specified as reserved within the
+ authority component and are not allowed outside their use as
+ delimiters for an IP literal within host. In order to make this
+ change without changing the technical definition of the path, query,
+ and fragment components, those rules were redefined to directly
+ specify the characters allowed.
+
+ As [<a href="http://tools.ietf.org/html/rfc2732" title=""Format for Literal IPv6 Addresses in URL&#39;s"">RFC2732</a>] defers to [<a href="http://tools.ietf.org/html/rfc3513" title=""Internet Protocol Version 6 (IPv6) Addressing Architecture"">RFC3513</a>] for definition of an IPv6 literal
+ address, which, unfortunately, lacks an ABNF description of
+ IPv6address, we created a new ABNF rule for IPv6address that matches
+ the text representations defined by <a href="#section-2.2">Section 2.2</a> of [<a href="http://tools.ietf.org/html/rfc3513" title=""Internet Protocol Version 6 (IPv6) Addressing Architecture"">RFC3513</a>].
+ Likewise, the definition of IPv4address has been improved in order to
+ limit each decimal octet to the range 0-255.
+
+ <a href="#section-6">Section 6</a>, on URI normalization and comparison, has been completely
+ rewritten and extended by using input from Tim Bray and discussion
+ within the W3C Technical Architecture Group.
+
+D.2. Modifications
+
+ The ad-hoc BNF syntax of <a href="http://tools.ietf.org/html/rfc2396">RFC 2396</a> has been replaced with the ABNF of
+ [<a href="http://tools.ietf.org/html/rfc2234" title=""Augmented BNF for Syntax Specifications: ABNF"">RFC2234</a>]. This change required all rule names that formerly
+ included underscore characters to be renamed with a dash instead. In
+ addition, a number of syntax rules have been eliminated or simplified
+ to make the overall grammar more comprehensible. Specifications that
+ refer to the obsolete grammar rules may be understood by replacing
+ those rules according to the following table:
+
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 53]</span>
+<a name="page-54" id="page-54" href="#page-54"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ +----------------+--------------------------------------------------+
+ | obsolete rule | translation |
+ +----------------+--------------------------------------------------+
+ | absoluteURI | absolute-URI |
+ | relativeURI | relative-part [ "?" query ] |
+ | hier_part | ( "//" authority path-abempty / |
+ | | path-absolute ) [ "?" query ] |
+ | | |
+ | opaque_part | path-rootless [ "?" query ] |
+ | net_path | "//" authority path-abempty |
+ | abs_path | path-absolute |
+ | rel_path | path-rootless |
+ | rel_segment | segment-nz-nc |
+ | reg_name | reg-name |
+ | server | authority |
+ | hostport | host [ ":" port ] |
+ | hostname | reg-name |
+ | path_segments | path-abempty |
+ | param | *<pchar excluding ";"> |
+ | | |
+ | uric | unreserved / pct-encoded / ";" / "?" / ":" |
+ | | / "@" / "&" / "=" / "+" / "$" / "," / "/" |
+ | | |
+ | uric_no_slash | unreserved / pct-encoded / ";" / "?" / ":" |
+ | | / "@" / "&" / "=" / "+" / "$" / "," |
+ | | |
+ | mark | "-" / "_" / "." / "!" / "~" / "*" / "'" |
+ | | / "(" / ")" |
+ | | |
+ | escaped | pct-encoded |
+ | hex | HEXDIG |
+ | alphanum | ALPHA / DIGIT |
+ +----------------+--------------------------------------------------+
+
+ Use of the above obsolete rules for the definition of scheme-specific
+ syntax is deprecated.
+
+ <a href="#section-2">Section 2</a>, on characters, has been rewritten to explain what
+ characters are reserved, when they are reserved, and why they are
+ reserved, even when they are not used as delimiters by the generic
+ syntax. The mark characters that are typically unsafe to decode,
+ including the exclamation mark ("!"), asterisk ("*"), single-quote
+ ("'"), and open and close parentheses ("(" and ")"), have been moved
+ to the reserved set in order to clarify the distinction between
+ reserved and unreserved and, hopefully, to answer the most common
+ question of scheme designers. Likewise, the section on
+ percent-encoded characters has been rewritten, and URI normalizers
+ are now given license to decode any percent-encoded octets
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 54]</span>
+<a name="page-55" id="page-55" href="#page-55"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ corresponding to unreserved characters. In general, the terms
+ "escaped" and "unescaped" have been replaced with "percent-encoded"
+ and "decoded", respectively, to reduce confusion with other forms of
+ escape mechanisms.
+
+ The ABNF for URI and URI-reference has been redesigned to make them
+ more friendly to LALR parsers and to reduce complexity. As a result,
+ the layout form of syntax description has been removed, along with
+ the uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path,
+ path_segments, rel_segment, and mark rules. All references to
+ "opaque" URIs have been replaced with a better description of how the
+ path component may be opaque to hierarchy. The relativeURI rule has
+ been replaced with relative-ref to avoid unnecessary confusion over
+ whether they are a subset of URI. The ambiguity regarding the
+ parsing of URI-reference as a URI or a relative-ref with a colon in
+ the first segment has been eliminated through the use of five
+ separate path matching rules.
+
+ The fragment identifier has been moved back into the section on
+ generic syntax components and within the URI and relative-ref rules,
+ though it remains excluded from absolute-URI. The number sign ("#")
+ character has been moved back to the reserved set as a result of
+ reintegrating the fragment syntax.
+
+ The ABNF has been corrected to allow the path component to be empty.
+ This also allows an absolute-URI to consist of nothing after the
+ "scheme:", as is present in practice with the "dav:" namespace
+ [<a href="http://tools.ietf.org/html/rfc2518" title=""HTTP Extensions for Distributed Authoring -- WEBDAV"">RFC2518</a>] and with the "about:" scheme used internally by many WWW
+ browser implementations. The ambiguity regarding the boundary
+ between authority and path has been eliminated through the use of
+ five separate path matching rules.
+
+ Registry-based naming authorities that use the generic syntax are now
+ defined within the host rule. This change allows current
+ implementations, where whatever name provided is simply fed to the
+ local name resolution mechanism, to be consistent with the
+ specification. It also removes the need to re-specify DNS name
+ formats here. Furthermore, it allows the host component to contain
+ percent-encoded octets, which is necessary to enable
+ internationalized domain names to be provided in URIs, processed in
+ their native character encodings at the application layers above URI
+ processing, and passed to an IDNA library as a registered name in the
+ UTF-8 character encoding. The server, hostport, hostname,
+ domainlabel, toplabel, and alphanum rules have been removed.
+
+ The resolving relative references algorithm of [<a href="http://tools.ietf.org/html/rfc2396" title=""Uniform Resource Identifiers (URI): Generic Syntax"">RFC2396</a>] has been
+ rewritten with pseudocode for this revision to improve clarity and
+ fix the following issues:
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 55]</span>
+<a name="page-56" id="page-56" href="#page-56"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ o [<a href="http://tools.ietf.org/html/rfc2396" title=""Uniform Resource Identifiers (URI): Generic Syntax"">RFC2396</a>] <a href="#section-5.2">section 5.2</a>, step 6a, failed to account for a base URI
+ with no path.
+
+ o Restored the behavior of [<a href="http://tools.ietf.org/html/rfc1808" title=""Relative Uniform Resource Locators"">RFC1808</a>] where, if the reference
+ contains an empty path and a defined query component, the target
+ URI inherits the base URI's path component.
+
+ o The determination of whether a URI reference is a same-document
+ reference has been decoupled from the URI parser, simplifying the
+ URI processing interface within applications in a way consistent
+ with the internal architecture of deployed URI processing
+ implementations. The determination is now based on comparison to
+ the base URI after transforming a reference to absolute form,
+ rather than on the format of the reference itself. This change
+ may result in more references being considered "same-document"
+ under this specification than there would be under the rules given
+ in <a href="http://tools.ietf.org/html/rfc2396">RFC 2396</a>, especially when normalization is used to reduce
+ aliases. However, it does not change the status of existing
+ same-document references.
+
+ o Separated the path merge routine into two routines: merge, for
+ describing combination of the base URI path with a relative-path
+ reference, and remove_dot_segments, for describing how to remove
+ the special "." and ".." segments from a composed path. The
+ remove_dot_segments algorithm is now applied to all URI reference
+ paths in order to match common implementations and to improve the
+ normalization of URIs in practice. This change only impacts the
+ parsing of abnormal references and same-scheme references wherein
+ the base URI has a non-hierarchical path.
+
+Index
+
+ A
+ ABNF 11
+ absolute 27
+ absolute-path 26
+ absolute-URI 27
+ access 9
+ authority 17, 18
+
+ B
+ base URI 28
+
+ C
+ character encoding 4
+ character 4
+ characters 8, 11
+ coded character set 4
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 56]</span>
+<a name="page-57" id="page-57" href="#page-57"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ D
+ dec-octet 20
+ dereference 9
+ dot-segments 23
+
+ F
+ fragment 16, 24
+
+ G
+ gen-delims 13
+ generic syntax 6
+
+ H
+ h16 20
+ hier-part 16
+ hierarchical 10
+ host 18
+
+ I
+ identifier 5
+ IP-literal 19
+ IPv4 20
+ IPv4address 19, 20
+ IPv6 19
+ IPv6address 19, 20
+ IPvFuture 19
+
+ L
+ locator 7
+ ls32 20
+
+ M
+ merge 32
+
+ N
+ name 7
+ network-path 26
+
+ P
+ path 16, 22, 26
+ path-abempty 22
+ path-absolute 22
+ path-empty 22
+ path-noscheme 22
+ path-rootless 22
+ path-abempty 16, 22, 26
+ path-absolute 16, 22, 26
+ path-empty 16, 22, 26
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 57]</span>
+<a name="page-58" id="page-58" href="#page-58"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ path-rootless 16, 22
+ pchar 23
+ pct-encoded 12
+ percent-encoding 12
+ port 22
+
+ Q
+ query 16, 23
+
+ R
+ reg-name 21
+ registered name 20
+ relative 10, 28
+ relative-path 26
+ relative-ref 26
+ remove_dot_segments 33
+ representation 9
+ reserved 12
+ resolution 9, 28
+ resource 5
+ retrieval 9
+
+ S
+ same-document 27
+ sameness 9
+ scheme 16, 17
+ segment 22, 23
+ segment-nz 23
+ segment-nz-nc 23
+ sub-delims 13
+ suffix 27
+
+ T
+ transcription 8
+
+ U
+ uniform 4
+ unreserved 13
+ URI grammar
+ absolute-URI 27
+ ALPHA 11
+ authority 18
+ CR 11
+ dec-octet 20
+ DIGIT 11
+ DQUOTE 11
+ fragment 24
+ gen-delims 13
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 58]</span>
+<a name="page-59" id="page-59" href="#page-59"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+ h16 20
+ HEXDIG 11
+ hier-part 16
+ host 19
+ IP-literal 19
+ IPv4address 20
+ IPv6address 20
+ IPvFuture 19
+ LF 11
+ ls32 20
+ OCTET 11
+ path 22
+ path-abempty 22
+ path-absolute 22
+ path-empty 22
+ path-noscheme 22
+ path-rootless 22
+ pchar 23
+ pct-encoded 12
+ port 22
+ query 24
+ reg-name 21
+ relative-ref 26
+ reserved 13
+ scheme 17
+ segment 23
+ segment-nz 23
+ segment-nz-nc 23
+ SP 11
+ sub-delims 13
+ unreserved 13
+ URI 16
+ URI-reference 25
+ userinfo 18
+ URI 16
+ URI-reference 25
+ URL 7
+ URN 7
+ userinfo 18
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 59]</span>
+<a name="page-60" id="page-60" href="#page-60"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+Authors' Addresses
+
+ Tim Berners-Lee
+ World Wide Web Consortium
+ Massachusetts Institute of Technology
+ 77 Massachusetts Avenue
+ Cambridge, MA 02139
+ USA
+
+ Phone: +1-617-253-5702
+ Fax: +1-617-258-5999
+ EMail: timbl@w3.org
+ URI: <a href="http://www.w3.org/People/Berners-Lee/">http://www.w3.org/People/Berners-Lee/</a>
+
+
+ Roy T. Fielding
+ Day Software
+ 5251 California Ave., Suite 110
+ Irvine, CA 92617
+ USA
+
+ Phone: +1-949-679-2960
+ Fax: +1-949-679-2972
+ EMail: fielding@gbiv.com
+ URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a>
+
+
+ Larry Masinter
+ Adobe Systems Incorporated
+ 345 Park Ave
+ San Jose, CA 95110
+ USA
+
+ Phone: +1-408-536-3024
+ EMail: LMM@acm.org
+ URI: <a href="http://larry.masinter.net/">http://larry.masinter.net/</a>
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+<span class="grey">Berners-Lee, et al. Standards Track [Page 60]</span>
+<a name="page-61" id="page-61" href="#page-61"><span class="break"> </span></a>
+<span class="grey"><a href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> URI Generic Syntax January 2005</span>
+
+
+Full Copyright Statement
+
+ Copyright (C) The Internet Society (2005).
+
+ This document is subject to the rights, licenses and restrictions
+ contained in <a href="http://tools.ietf.org/html/bcp78">BCP 78</a>, and except as set forth therein, the authors
+ retain all their rights.
+
+ This document and the information contained herein are provided on an
+ "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+ OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
+ ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
+ INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
+ INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+ WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Intellectual Property
+
+ The IETF takes no position regarding the validity or scope of any
+ Intellectual Property Rights or other rights that might be claimed to
+ pertain to the implementation or use of the technology described in
+ this document or the extent to which any license under such rights
+ might or might not be available; nor does it represent that it has
+ made any independent effort to identify any such rights. Information
+ on the IETF's procedures with respect to rights in IETF Documents can
+ be found in <a href="http://tools.ietf.org/html/bcp78">BCP 78</a> and <a href="http://tools.ietf.org/html/bcp79">BCP 79</a>.
+
+ Copies of IPR disclosures made to the IETF Secretariat and any
+ assurances of licenses to be made available, or the result of an
+ attempt made to obtain a general license or permission for the use of
+ such proprietary rights by implementers or users of this
+ specification can be obtained from the IETF on-line IPR repository at
+ <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.
+
+ The IETF invites any interested party to bring to its attention any
+ copyrights, patents or patent applications, or other proprietary
+ rights that may cover technology that may be required to implement
+ this standard. Please address the information to the IETF at ietf-
+ ipr@ietf.org.
+
+
+Acknowledgement
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
+
+
+
+
+
+Berners-Lee, et al. Standards Track [Page 61]
+<span class="break"> </span>
+
+</pre><br>
+<span class="noprint"><small><small>Html markup produced by rfcmarkup 1.46, available from
+<a href="http://tools.ietf.org/tools/rfcmarkup/">http://tools.ietf.org/tools/rfcmarkup/</a>
+</small></small></span>
+
+</body></html>
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