URI, URL, and URN   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 [RFC2141], 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" [RFC3305].

最终的 URI Standard (RFC3986) 在 1.1.3 小节“URI, URL, and URN”中澄清了这一区别：

URI 可以进一步分为定位器、名称，或者二者兼具。术语“Uniform Resource Locator” (URL) 涉及的是 URI 的子集，除识别资源外，它还通过描述其最初访问机制（比如它的网络“位置”）来提供定位资源的方法。 术语“Uniform Resource Name” (URN) 在历史上曾用于引用“urn”方案 [RFC2141] 下的 URI，这个 URI 需要是全球惟一的，并且在资源不存在或不再可用时依然保持不变，对于其他任何拥有名称的一些属性的 URI，都需要使用这样的 URI。

对于单独的方案，没有必要将其分为仅仅是一个 “名称”或者是一个“定位器”。 来自任意特定方案的 URI 实例可能有名称或定位器的特征，或两者兼而有之， 这通常取决于标识符分配中的持久性和命名机构对其关注程度， 而不取决于其他方案的质量。未来的规范和相关的文档应当使用通用术语“URI”，而不是使用有更多限制的条目“URL”和“URN” [RFC3305]。

原文：www.ibm.com/developerworks/cn/xml/x-urlni.html

URI 标准

RFC3986，即“Uniform Resource Identifier (URI)：Generic Syntax”，是一个 Internet Standard。 Request for Comments (RFC) 系列是著名的档案式文档系列，该系列构成了 Internet Engineering Task Force (IETF) 标准过程的主干。 在数以千计的 RFC 中，只有很少的部分，比如 TCP (RFC793) 以及 Internet Mail 格式 (RFC821) 和协议 (RFC822)， 提高了整个 Internet Standard 的发展水平。 RFC3986 在 2005 年 1 月也提高了这个水平。

按照 URI 标准，上面的第一个例子 —— http://www.cisco.com/en/US/partners/index.html —— 实际上是一个 URI，并且它由以下几个组成部分：

• 方案名 (http)
• 域名 (www.cisco.com)
• 路径 (/en/US/partners/index.html)

IETF 达成共识，共同管理该方案。Official IANA Registry of URI Schemes（请参阅 参考资料）中包括一些大家所熟悉的方案，如 http、 https 和 mailto，还有其他许多您可能熟悉或不熟悉的方案。

URI 路径像一个典型的文件路径名。URI 按照 UNIX® 的惯例采用了正下划线 (a/b/c)，因为在 20 世纪 80 年代后期设计 URI 的时候, 在 Internet 上， UNIX 文化比 PC 文化更流行。正是那个时候，出现了几个用于访问远程文件的流行表示法。其中一个是 Ange-ftp， 它是用来编辑远程文件的 emacs 的一个扩展。它用路径名将主机名和用户名结合起来，以获取像/jbrown@freddie.ucla.edu:~mblack/这样的结果。为了跨机器进行命名，为 Web 开发的 URI 语法（按照非标准的 Apollo Domain UNIX）使用了双下划线符号，但是它也引入了方案语法，这样，来自许多不同协议的命名约定得到了统一。其中的一些例子有：

• mailto:mbox@domain
• ftp://host/file
• http://domain/path

这里介绍的第二个例子是 www.yahoo.com/sports，它不是一个真正的 URI。 它是对 http://www.yahoo.com/sports 的一种方便的简写，是一种受流行的 Web 浏览器用户界面 (UI) 支持的格式。然而，不要再犯在 XSLT 中遗漏方案这样的错误，如下所示：

<xsl:include href="http://www.mamicode.com/exslt.org/math/min/math.min.template.xsl" />

因为它将不会按照您期望的那样工作，除非您真的 想 在 XSLT 样式表之后引用 exslt.org 目录中的一个文件。XSLT 的 href 属性采用了一个 URI 引用，它可能是绝对引用，也可能是相对引用。以一个方案和一个冒号开始的 URI 引用是绝对引用；否则，该引用就是相对引用。相对的 URI 引用更像一个文件路径。例如，../noarch/config.xsd 也是一个相对的 URI 引用。

国际化的资源标志符

HTML 中的 href 属性采用了 URI 引用，这样讲有些过于简单。URI 和 URI 引用都是从有限的 ASCII 字符集合中得出的，并且 HTML 比它们更加国际化。事实上，对遵循 RFC3986 的注释的请求是符合 RFC3987 标准，即 Internationalized Resource Identifiers (IRI) 标准（请参阅 参考资料）。 此规范在 IETF 标准化过程中没有它的前辈走的远，但是技术本身已是相当成熟，并被广泛部署。除了能够使用所有 Unicode 字符，而不是仅仅能够使用 ASCII 字符之外，IRI 和 URI 是完全一样的。像 URI一样，每个 IRI 都有一个相应的编码，以防需要在只接受 URI 的协议（比如 HTTP）中使用 IRI。

用 xml:base 重写基本 URI

通常， URI 引用与在哪种文档中发现它有关。如果使用基本 URI http://exslt.org/math/min/math.min.template.xsl 查看一个文档，并看到了一个 URI 引用 ../../random/random.xml，那么引用将扩展为 http://exslt.org/random/random.xml。在 HTML 中，您可以把一个 base 元素放在文档顶端来重写基本 URI。XML Base 规范（请参阅 参考资料）在 XML 中也提供了同样的功能。

考虑一个既可以用 file:/my/doc 访问也可以用 http://my.domain/doc 访问的文档。通常，当通过文件系统访问文档时，您可能希望这些引用像 #part2 那样扩展为 file:/my/doc#part2；而通过 HTTP 访问文档时，您可能希望 #part2 扩展为 http://my.domain/doc#part2。但是在 Resource Description Framework (RDF) 模式中，为了使一些组件正常工作，展开的形式必须保持不变。 XML Base 使这种扩展变得容易（参见清单 1）。

清单 1. RDF 中的展开形式

<rdf:RDF  xmlns="&owl;"  xmlns:owl="&owl;"  xml:base="http://www.w3.org/2002/07/owl"  xmlns:rdf="&rdf;"  xmlns:rdfs="&rdfs;">...    <Class rdf:about="#Nothing"/>

在这个例子中，无论您是在哪里找到的那个文件，#Nothing 引用均被扩展为 http://www.w3.org/2002/07/owl#Nothing。

好了，关于 URI、IRI 和 URI 引用的介绍就到此结束了。下面将介绍 URL 和 URN。

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URL 和 URN

设计 URI 的目的是让它起到名称和定位器的作用。当 IETF 用它们实现标准化的时候，它们就成了通常所说的 Uniform Resource Locators，并且另一项关于 Uniform Resource Names 的独立的工作也已经开始了。

对于 Internet 主机，名称和位置都有单独的标准。主机名和域名有相同的语法（例如，zork1.example.edu）。这些主机名通过 Domain Name System (DNS) 协议和类似 192.168.300.21 的地址相连。当主机改变了在网络中的位置或重新编号之后，这种间接的做法允许主机保留其名称。

Web 中偶尔中断的链接使 Web 地址从外观上看更像是一个位置，而不是一个名称，并且在 IEIF 社区中也出现了不同的观点：

• URI:RFC1630， 发布于 1994 年 6 月，被称为“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”（请参阅 参考资料）。它是一个Informational RFC —— 也就是说，它没有获得社区的任何认可。
• URL:RFC1738，发布于 1994 年 12 月, 被称为“Uniform Resource Locators”（请参阅 参考资料）。它是一个 Proposed Standard —— 也就是说，它是一个共识过程的结果，虽然它还没有经过测试，并成熟到足以成为一个完整的 Internet Standard。
• URN:RFC1737，发布于 1994 年 12 月，被称为“Functional Requirements for Uniform Resource Names”（请参阅 参考资料）。

1997 年，紧随 Proposed Standard RFC2141（即 URN Syntax）之后发布了 RFC1737，它指定了另一个方案 —— urn: —— 来加入 http:、ftp:和其他协议中。

最终的 URI Standard (RFC3986) 在 1.1.3 小节“URI, URL, and URN”中澄清了这一区别：

URI 可以进一步分为定位器、名称，或者二者兼具。术语“Uniform Resource Locator” (URL) 涉及的是 URI 的子集，除识别资源外，它还通过描述其最初访问机制（比如它的网络“位置”）来提供定位资源的方法。 术语“Uniform Resource Name” (URN) 在历史上曾用于引用“urn”方案 [RFC2141] 下的 URI，这个 URI 需要是全球惟一的，并且在资源不存在或不再可用时依然保持不变，对于其他任何拥有名称的一些属性的 URI，都需要使用这样的 URI。

对于单独的方案，没有必要将其分为仅仅是一个 “名称”或者是一个“定位器”。 来自任意特定方案的 URI 实例可能有名称或定位器的特征，或两者兼而有之， 这通常取决于标识符分配中的持久性和命名机构对其关注程度， 而不取决于其他方案的质量。未来的规范和相关的文档应当使用通用术语“URI”，而不是使用有更多限制的条目“URL”和“URN” [RFC3305]。

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实际的持久性

持久性和可用性之间存在着一种天生的紧张关系。如果我在一台连接到 Internet 的主机上有一个文件，使其可用的最简单的方法是：在主机上运行一个 Web 服务器，并交给您一个由主机碰巧得到的名称组成的 URI，以及文件名（例如， http://dhcp324.coolISP.net/drafts/freeLunch.wsdl）。在 Dynamic Host Configuration Protocol (DHCP) 租约到期之前，它一直工作得很好，接着，我改变了 ISP，或者说我将文件从 /drafts/ 移动到了 /keepers/ 中。如果服务逐渐流行，那么我决定购买它的时候将会发生什么呢？名称中的信息越是无关紧要，它能够坚持不改变的可能性就越小。

但是一个良好的持久的名称（如 http://xyzpdq.org/2005/ls434）是不易管理的。必需要注册一个域，维护从域名到主机地址的映射，还要记住，ls434 是保存我的午餐服务描述的文件，或者是在 Web 服务器上建立一个文件映射表的地方。

PURL 项目和 Digital Object Identifier (DOI) 系统（请参阅 参考资料）代表了解决持久性问题的不同方法。Persistent URL (PURL) 在域中是一个普通的 HTTP URI，它受到强大的持久性策略的支持。例如，purl.org 由 Online Computer Library Center (OCLC)运行，OCLC 是一个全球范围的库协作组织。任何人都可以申请一个帐户并管理他或她的 PURL 设置。可以将内容发布在普通的 Web 服务器上，然后用 HTTP 重定向连接到 PURL。从 PURL 到持久性较底的 HTTP URI，这种的间接性和 DNS 提供的间接性非常相似，只要重定向的来源和目的不在同一类别中即可。在安装好一个 PURL（如 http://purl.org/net/dajobe/ 之后，就可以像使用其他任何 HTTP URI 一样使用它。更重要的是，您想要进行通信的人可以像使用其他任何 HTTP URI 一样使用它；不需要任何插件或增件。

DOI 系统使用它自身的方案 —— 比如 doi:10.123/456。Web 浏览器可以适应的支持这个带有插件的方案。DOI 基金会像 PURL 提供者（如 OCLC）一样提供策略、注册服务和 HTTP 重定向服务。当 DOI 基金会支持格式 http://dx.doi.org/10.123/456 的每个 DOI 的别名时，DOI Handbook（请参阅 参考资料）声称此系统“与分解器插件比较时有明显的缺点。” 为一个对象管理两个不同的名称似乎是我的一个明显不足之处。

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信息管理中的创造性压力

尽管持久性和可用性之间存在压力，但好的 URI 可以同时具备这两种特性；好的 URI 既是一个持久性名称，又是一个可用的位置。所以，URL 实际上是一个带有实际有用工具的 URI。

urn: 方案的支持者争辩说，他们认为这种压力在 HTTP 和 DNS 的范围内是矛盾的。我承认确实涉及到了某些领域，但每个 Web 管理者都面对着相同的问题，而且社区正在学习一些信息管理原理，以便对它们进行定位。基本的问题是：世界在不断变化，要保持事物同步就需要付出努力。

大 多数时候，使用 DNS 命名的分层结构特性是为了提供便利，但它在一个位置集中了大量的力量，并产生了复杂的管理方式问题。点对点设计，比如分布式散列表，可能用 DNS 消除一些集中问题，但谁知道使用它们将带来什么样的管理问题呢？许多不同的前沿开发展示了如何将新协议服务于现有的 http://...名称，增加现有超媒体网络的价值。对任何与 HTTP 的 GET/PUT/POST/DELETE 操作相似的远程操作而言，这种方法看起来比新方案的部署更有可能成功。 我期望目前信息管理中的最佳实践和未来协议增强使得构建在 HTTP 和 DNS 上的精选的 URI 能够持续很长一段时间。

参考资料

• 您可以参阅本文在 developerWorks 全球站点上的 英文原文。
• 请参阅“Knowledge-Domain Interoperability and an Open Hyperdocument System”，这是 Douglas Engelbart 于 1990 年在计算机支持的协作工作 (CSCW) 领域中的前沿研究成果的总结。
• 研究 Tim Berners-Lee 撰写的 esign Issues 的“Document Naming”部分，这个系列的文章始于 1991 年，并一直持续至今。
• 找到更多关于 Internet Engineering Task Force (IETF) 的信息，该机构开发了 Internet Protocol、DNS、Internet Mail和其他许多 Internet 技术。 RFC 系列是 IETF 标准过程的主干。接下来的 RFC 系列将在下列文章中讨论：
  • RFC1630 —— "Universal Resource Identifiers in WWW"
  • RFC1737 -- "Functional Requirements for Uniform Resource Names"
  • RFC1738 -- "Uniform Resource Locators" (W3C 保持了 hypertext version)
  • RFC2141 -- "URN Syntax"
  • RFC3305 -- "Report from the Joint W3C/IETF URI Planning Interest Group: URIs, URLs, and URNs"
  • RFC3986 -- "Uniform Resource Identifier (URI): Generic Syntax" (Roy Fielding 的 hypertext version 也很方便)
  • RFC3987 -- "Internationalized Resource Identifiers (IRIs)"
• 请参阅 Internet Assigned Numbers Authority (IANA)，它在其他内容中保留了 official list of URI schemes。
• 想了解更多关于 PURL 的信息以及它们的使用方法吗？请参阅 PURL project site，同时请务必阅读PURL FAQ.
• 查找关于 The DOI System 的信息以及它的持久性方法。DOI Handbook 有它自己的 chapter on resolution。
• 访问 W3C，查找更多关于 XML Schema, XML Namespaces 和 XML Base的信息。
• 在 Uche Ogbuji 撰写的 developerWorks 文章“XML 设计原则：小心使用 XML 名称空间”（2004 年 4 月）和他的 XML 标准概览: 第 1 部分（2004 年 1 月）中进一步学习 URL、URN 和 URI。
• 在 developerWorks XML 专区 上有更多 XML 方面的参考资料。
• 可以在 developerWorks Developer Bookstore 上浏览大量与 XML 相关的主题。
• 学习如何才能成为一名 IBM 认证的 XML 和相关技术的开发人员。

rfc 3986标准

原文  http://www.ietf.org/rfc/rfc3986

翻译  http://wiki.jabbercn.org/index.php?title=RFC3986&diff=prev&oldid=3474

Network Working Group                                     T. Berners-LeeRequest for Comments: 3986                                       W3C/MITSTD: 66                                                      R. FieldingUpdates: 1738                                               Day SoftwareObsoletes: 2732, 2396, 1808                                  L. MasinterCategory: Standards Track                                  Adobe Systems                                                            January 2005           Uniform Resource Identifier (URI): Generic SyntaxStatus 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.Berners-Lee, et al.         Standards Track                     [Page 1]RFC 3986                   URI Generic Syntax               January 2005Table of Contents   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4       1.1.  Overview of URIs . . . . . . . . . . . . . . . . . . . .  4             1.1.1.  Generic Syntax . . . . . . . . . . . . . . . . .  6             1.1.2.  Examples . . . . . . . . . . . . . . . . . . . .  7             1.1.3.  URI, URL, and URN  . . . . . . . . . . . . . . .  7       1.2.  Design Considerations  . . . . . . . . . . . . . . . . .  8             1.2.1.  Transcription  . . . . . . . . . . . . . . . . .  8             1.2.2.  Separating Identification from Interaction . . .  9             1.2.3.  Hierarchical Identifiers . . . . . . . . . . . . 10       1.3.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . 11   2.  Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 11       2.1.  Percent-Encoding . . . . . . . . . . . . . . . . . . . . 12       2.2.  Reserved Characters  . . . . . . . . . . . . . . . . . . 12       2.3.  Unreserved Characters  . . . . . . . . . . . . . . . . . 13       2.4.  When to Encode or Decode . . . . . . . . . . . . . . . . 14       2.5.  Identifying Data . . . . . . . . . . . . . . . . . . . . 14   3.  Syntax Components  . . . . . . . . . . . . . . . . . . . . . . 16       3.1.  Scheme . . . . . . . . . . . . . . . . . . . . . . . . . 17       3.2.  Authority  . . . . . . . . . . . . . . . . . . . . . . . 17             3.2.1.  User Information . . . . . . . . . . . . . . . . 18             3.2.2.  Host . . . . . . . . . . . . . . . . . . . . . . 18             3.2.3.  Port . . . . . . . . . . . . . . . . . . . . . . 22       3.3.  Path . . . . . . . . . . . . . . . . . . . . . . . . . . 22       3.4.  Query  . . . . . . . . . . . . . . . . . . . . . . . . . 23       3.5.  Fragment . . . . . . . . . . . . . . . . . . . . . . . . 24   4.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25       4.1.  URI Reference  . . . . . . . . . . . . . . . . . . . . . 25       4.2.  Relative Reference . . . . . . . . . . . . . . . . . . . 26       4.3.  Absolute URI . . . . . . . . . . . . . . . . . . . . . . 27       4.4.  Same-Document Reference  . . . . . . . . . . . . . . . . 27       4.5.  Suffix Reference . . . . . . . . . . . . . . . . . . . . 27   5.  Reference Resolution . . . . . . . . . . . . . . . . . . . . . 28       5.1.  Establishing a Base URI  . . . . . . . . . . . . . . . . 28             5.1.1.  Base URI Embedded in Content . . . . . . . . . . 29             5.1.2.  Base URI from the Encapsulating Entity . . . . . 29             5.1.3.  Base URI from the Retrieval URI  . . . . . . . . 30             5.1.4.  Default Base URI . . . . . . . . . . . . . . . . 30       5.2.  Relative Resolution  . . . . . . . . . . . . . . . . . . 30             5.2.1.  Pre-parse the Base URI . . . . . . . . . . . . . 31             5.2.2.  Transform References . . . . . . . . . . . . . . 31             5.2.3.  Merge Paths  . . . . . . . . . . . . . . . . . . 32             5.2.4.  Remove Dot Segments  . . . . . . . . . . . . . . 33       5.3.  Component Recomposition  . . . . . . . . . . . . . . . . 35       5.4.  Reference Resolution Examples  . . . . . . . . . . . . . 35             5.4.1.  Normal Examples  . . . . . . . . . . . . . . . . 36             5.4.2.  Abnormal Examples  . . . . . . . . . . . . . . . 36Berners-Lee, et al.         Standards Track                     [Page 2]RFC 3986                   URI Generic Syntax               January 2005   6.  Normalization and Comparison . . . . . . . . . . . . . . . . . 38       6.1.  Equivalence  . . . . . . . . . . . . . . . . . . . . . . 38       6.2.  Comparison Ladder  . . . . . . . . . . . . . . . . . . . 39             6.2.1.  Simple String Comparison . . . . . . . . . . . . 39             6.2.2.  Syntax-Based Normalization . . . . . . . . . . . 40             6.2.3.  Scheme-Based Normalization . . . . . . . . . . . 41             6.2.4.  Protocol-Based Normalization . . . . . . . . . . 42   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 43       7.1.  Reliability and Consistency  . . . . . . . . . . . . . . 43       7.2.  Malicious Construction . . . . . . . . . . . . . . . . . 43       7.3.  Back-End Transcoding . . . . . . . . . . . . . . . . . . 44       7.4.  Rare IP Address Formats  . . . . . . . . . . . . . . . . 45       7.5.  Sensitive Information  . . . . . . . . . . . . . . . . . 45       7.6.  Semantic Attacks . . . . . . . . . . . . . . . . . . . . 45   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 46   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46       10.1. Normative References . . . . . . . . . . . . . . . . . . 46       10.2. Informative References . . . . . . . . . . . . . . . . . 47   A.  Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 49   B.  Parsing a URI Reference with a Regular Expression  . . . . . . 50   C.  Delimiting a URI in Context  . . . . . . . . . . . . . . . . . 51   D.  Changes from RFC 2396  . . . . . . . . . . . . . . . . . . . . 53       D.1.  Additions  . . . . . . . . . . . . . . . . . . . . . . . 53       D.2.  Modifications  . . . . . . . . . . . . . . . . . . . . . 53   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56   Authors‘ Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 61Berners-Lee, et al.         Standards Track                     [Page 3]RFC 3986                   URI Generic Syntax               January 20051.  Introduction   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" [RFC1630].  The syntax is designed to meet the   recommendations laid out in "Functional Recommendations for Internet   Resource Locators" [RFC1736] and "Functional Requirements for Uniform   Resource Names" [RFC1737].   This document obsoletes [RFC2396], which merged "Uniform Resource   Locators" [RFC1738] and "Relative Uniform Resource Locators"   [RFC1808] in order to define a single, generic syntax for all URIs.   It obsoletes [RFC2732], which introduced syntax for an IPv6 address.   It excludes portions of RFC 1738 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 [BCP35].  Advice for designers of new URI   schemes can be found in [RFC2718].  All significant changes from RFC   2396 are noted in Appendix D.   This specification uses the terms "character" and "coded character   set" in accordance with the definitions provided in [BCP19], and   "character encoding" in place of what [BCP19] refers to as a   "charset".1.1.  Overview of URIs   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.Berners-Lee, et al.         Standards Track                     [Page 4]RFC 3986                   URI Generic Syntax               January 2005   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 Section 3.  It enables   uniform identification of resources via a separately defined   extensible set of naming schemes (Section 3.1).  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 thisBerners-Lee, et al.         Standards Track                     [Page 5]RFC 3986                   URI Generic Syntax               January 2005   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, "http://localhost/"   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").1.1.1.  Generic Syntax   Each URI begins with a scheme name, as defined in Section 3.1, 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.Berners-Lee, et al.         Standards Track                     [Page 6]RFC 3986                   URI Generic Syntax               January 20051.1.2.  Examples   The following example URIs illustrate several URI schemes and   variations in their common syntax components:      ftp://ftp.is.co.za/rfc/rfc1808.txt      http://www.ietf.org/rfc/rfc2396.txt      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:4.1.21.1.3.  URI, URL, and URN   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 [RFC2141], 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" [RFC3305].Berners-Lee, et al.         Standards Track                     [Page 7]RFC 3986                   URI Generic Syntax               January 20051.2.  Design Considerations1.2.1.  Transcription   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 (Section 2.1) may be used within a URI to represent characters   outside the range of the US-ASCII coded character set if thisBerners-Lee, et al.         Standards Track                     [Page 8]RFC 3986                   URI Generic Syntax               January 2005   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.1.2.2.  Separating Identification from Interaction   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 observedBerners-Lee, et al.         Standards Track                     [Page 9]RFC 3986                   URI Generic Syntax               January 2005   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).1.2.3.  Hierarchical Identifiers   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 (Section 4.2) 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,Berners-Lee, et al.         Standards Track                    [Page 10]RFC 3986                   URI Generic Syntax               January 2005   presented in Section 5, 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 Section 3.3),   URI scheme specifications can define opaque identifiers by   disallowing use of slash characters, question mark characters, and   the URIs "scheme:." and "scheme:..".1.3.  Syntax Notation   This specification uses the Augmented Backus-Naur Form (ABNF)   notation of [RFC2234], 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.2.  Characters   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   [ASCII].  Because a URI is a sequence of characters, we must invert   that relation in order to understand the URI syntax.  Therefore, theBerners-Lee, et al.         Standards Track                    [Page 11]RFC 3986                   URI Generic Syntax               January 2005   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.2.1.  Percent-Encoding   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).  Section 2.4 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.2.2.  Reserved Characters   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.Berners-Lee, et al.         Standards Track                    [Page 12]RFC 3986                   URI Generic Syntax               January 2005      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 Section 3.  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.2.3.  Unreserved Characters   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 / "-" / "." / "_" / "~"Berners-Lee, et al.         Standards Track                    [Page 13]RFC 3986                   URI Generic Syntax               January 2005   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.2.4.  When to Encode or Decode   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.2.5.  Identifying Data   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 theBerners-Lee, et al.         Standards Track                    [Page 14]RFC 3986                   URI Generic Syntax               January 2005   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 [STD63] (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 numericBerners-Lee, et al.         Standards Track                    [Page 15]RFC 3986                   URI Generic Syntax               January 2005   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 [UCS],   the data should first be encoded as octets according to the UTF-8   character encoding [STD63]; 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".3.  Syntax Components   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 (Section 3.3), 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:noseBerners-Lee, et al.         Standards Track                    [Page 16]RFC 3986                   URI Generic Syntax               January 20053.1.  Scheme   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 [BCP35].   The scheme registry maintains the mapping between scheme names and   their specifications.  Advice for designers of new URI schemes can be   found in [RFC2718].  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 Section 4.3.   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 (Section 7.6).3.2.  Authority   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.Berners-Lee, et al.         Standards Track                    [Page 17]RFC 3986                   URI Generic Syntax               January 2005      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.3.2.1.  User Information   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   (Section 7.6).3.2.2.  Host   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 sakeBerners-Lee, et al.         Standards Track                    [Page 18]RFC 3986                   URI Generic Syntax               January 2005   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   [RFC3513] 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 [RFC3513].  This syntax does not support   IPv6 scoped addressing zone identifiers.Berners-Lee, et al.         Standards Track                    [Page 19]RFC 3986                   URI Generic Syntax               January 2005   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 [RFC1123] by   reference to [RFC0952].  Note that other forms of dotted notation may   be interpreted on some platforms, as described in Section 7.4, 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 syntaxBerners-Lee, et al.         Standards Track                    [Page 20]RFC 3986                   URI Generic Syntax               January 2005   defined in Section 3.5 of [RFC1034] and Section 2.1 of [RFC1123].   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 [STD63], 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 [RFC3490] 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.Berners-Lee, et al.         Standards Track                    [Page 21]RFC 3986                   URI Generic Syntax               January 20053.2.3.  Port   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.3.3.  Path   The path component contains data, usually organized in hierarchical   form, that, along with data in the non-hierarchical query component   (Section 3.4), 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   (Section 4.1) 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>Berners-Lee, et al.         Standards Track                    [Page 22]RFC 3986                   URI Generic Syntax               January 2005      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   (Section 4.2) 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.3.4.  Query   The query component contains non-hierarchical data that, along with   data in the path component (Section 3.3), 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.Berners-Lee, et al.         Standards Track                    [Page 23]RFC 3986                   URI Generic Syntax               January 2005      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 (Section 5.1), 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.3.5.  Fragment   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 [RFC2046] 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).Berners-Lee, et al.         Standards Track                    [Page 24]RFC 3986                   URI Generic Syntax               January 2005   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).4.  Usage   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.4.1.  URI Reference   URI-reference is used to denote the most common usage of a resource   identifier.      URI-reference = URI / relative-refBerners-Lee, et al.         Standards Track                    [Page 25]RFC 3986                   URI Generic Syntax               January 2005   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.4.2.  Relative Reference   A relative reference takes advantage of the hierarchical syntax   (Section 1.2.3) 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   Section 5.   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.Berners-Lee, et al.         Standards Track                    [Page 26]RFC 3986                   URI Generic Syntax               January 20054.3.  Absolute URI   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.4.4.  Same-Document Reference   When a URI reference refers to a URI that is, aside from its fragment   component (if any), identical to the base URI (Section 5.1), 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.4.5.  Suffix Reference   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 theBerners-Lee, et al.         Standards Track                    [Page 27]RFC 3986                   URI Generic Syntax               January 2005   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 [RFC1535].   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.5.  Reference Resolution   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 Section 3.5.1.  Establishing a Base URI   The term "relative" implies that a "base URI" exists against which   the relative reference is applied.  Aside from fragment-only   references (Section 4.4), 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 (Section 4.3).  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.Berners-Lee, et al.         Standards Track                    [Page 28]RFC 3986                   URI Generic Syntax               January 2005   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)         |         `----------------------------------------------------------‘5.1.1.  Base URI Embedded in Content   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.5.1.2.  Base URI from the Encapsulating Entity   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.Berners-Lee, et al.         Standards Track                    [Page 29]RFC 3986                   URI Generic Syntax               January 2005   A mechanism for embedding a base URI within MIME container types   (e.g., the message and multipart types) is defined by MHTML   [RFC2557].  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.5.1.3.  Base URI from the Retrieval URI   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.5.1.4.  Default Base URI   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.5.2.  Relative Resolution   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 Section 5.3, 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.Berners-Lee, et al.         Standards Track                    [Page 30]RFC 3986                   URI Generic Syntax               January 20055.2.1.  Pre-parse the Base URI   The base URI (Base) is established according to the procedure of   Section 5.1 and parsed into the five main components described in   Section 3.  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.5.2.2.  Transform References   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;Berners-Lee, et al.         Standards Track                    [Page 31]RFC 3986                   URI Generic Syntax               January 2005      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;5.2.3.  Merge Paths   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,Berners-Lee, et al.         Standards Track                    [Page 32]RFC 3986                   URI Generic Syntax               January 2005   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).5.2.4.  Remove Dot Segments   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.Berners-Lee, et al.         Standards Track                    [Page 33]RFC 3986                   URI Generic Syntax               January 2005   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       1 :                         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 "/./".Berners-Lee, et al.         Standards Track                    [Page 34]RFC 3986                   URI Generic Syntax               January 20055.3.  Component Recomposition   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.5.4.  Reference Resolution Examples   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.Berners-Lee, et al.         Standards Track                    [Page 35]RFC 3986                   URI Generic Syntax               January 20055.4.1.  Normal Examples      "g:h"           =  "g:h"      "g"             =  "http://a/b/c/g"      "./g"           =  "http://a/b/c/g"      "g/"            =  "http://a/b/c/g/"      "/g"            =  "http://a/g"      "//g"           =  "http://g"      "?y"            =  "http://a/b/c/d;p?y"      "g?y"           =  "http://a/b/c/g?y"      "#s"            =  "http://a/b/c/d;p?q#s"      "g#s"           =  "http://a/b/c/g#s"      "g?y#s"         =  "http://a/b/c/g?y#s"      ";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"      "."             =  "http://a/b/c/"      "./"            =  "http://a/b/c/"      ".."            =  "http://a/b/"      "../"           =  "http://a/b/"      "../g"          =  "http://a/b/g"      "../.."         =  "http://a/"      "../../"        =  "http://a/"      "../../g"       =  "http://a/g"5.4.2.  Abnormal Examples   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"    =  "http://a/g"      "../../../../g" =  "http://a/g"Berners-Lee, et al.         Standards Track                    [Page 36]RFC 3986                   URI Generic Syntax               January 2005   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"          =  "http://a/g"      "/../g"         =  "http://a/g"      "g."            =  "http://a/b/c/g."      ".g"            =  "http://a/b/c/.g"      "g.."           =  "http://a/b/c/g.."      "..g"           =  "http://a/b/c/..g"   Less likely are cases where the relative reference uses unnecessary   or nonsensical forms of the "." and ".." complete path segments.      "./../g"        =  "http://a/b/g"      "./g/."         =  "http://a/b/c/g/"      "g/./h"         =  "http://a/b/c/g/h"      "g/../h"        =  "http://a/b/c/h"      "g;x=1/./y"     =  "http://a/b/c/g;x=1/y"      "g;x=1/../y"    =  "http://a/b/c/y"   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"       =  "http://a/b/c/g?y/./x"      "g?y/../x"      =  "http://a/b/c/g?y/../x"      "g#s/./x"       =  "http://a/b/c/g#s/./x"      "g#s/../x"      =  "http://a/b/c/g#s/../x"   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   [RFC1630].  Its use should be avoided but is allowed for backward   compatibility.      "http:g"        =  "http:g"         ; for strict parsers                      /  "http://a/b/c/g" ; for backward compatibilityBerners-Lee, et al.         Standards Track                    [Page 37]RFC 3986                   URI Generic Syntax               January 20056.  Normalization and Comparison   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.6.1.  Equivalence   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.Berners-Lee, et al.         Standards Track                    [Page 38]RFC 3986                   URI Generic Syntax               January 20056.2.  Comparison Ladder   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.6.2.1.  Simple String Comparison   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).Berners-Lee, et al.         Standards Track                    [Page 39]RFC 3986                   URI Generic Syntax               January 2005   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.6.2.2.  Syntax-Based Normalization   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.6.2.2.1.  Case Normalization   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 Section 6.2.3).6.2.2.2.  Percent-Encoding Normalization   The percent-encoding mechanism (Section 2.1) 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 Section 2.3.Berners-Lee, et al.         Standards Track                    [Page 40]RFC 3986                   URI Generic Syntax               January 20056.2.2.3.  Path Segment Normalization   The complete path segments "." and ".." are intended only for use   within relative references (Section 4.1) and are removed as part of   the reference resolution process (Section 5.2).  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   Section 5.2.4.6.2.3.  Scheme-Based Normalization   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/      http://example.com:/      http://example.com:80/   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 schemeBerners-Lee, et al.         Standards Track                    [Page 41]RFC 3986                   URI Generic Syntax               January 2005   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.6.2.4.  Protocol-Based Normalization   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).Berners-Lee, et al.         Standards Track                    [Page 42]RFC 3986                   URI Generic Syntax               January 20057.  Security Considerations   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.7.1.  Reliability and Consistency   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.7.2.  Malicious Construction   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.Berners-Lee, et al.         Standards Track                    [Page 43]RFC 3986                   URI Generic Syntax               January 2005   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.7.3.  Back-End Transcoding   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.Berners-Lee, et al.         Standards Track                    [Page 44]RFC 3986                   URI Generic Syntax               January 20057.4.  Rare IP Address Formats   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 Section 3.2.2.   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.7.5.  Sensitive Information   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.7.6.  Semantic Attacks   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 exampleBerners-Lee, et al.         Standards Track                    [Page 45]RFC 3986                   URI Generic Syntax               January 2005      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 [Siedzik].8.  IANA Considerations   URI scheme names, as defined by <scheme> in Section 3.1, form a   registered namespace that is managed by IANA according to the   procedures defined in [BCP35].  No IANA actions are required by this   document.9.  Acknowledgements   This specification is derived from RFC 2396 [RFC2396], RFC 1808   [RFC1808], and RFC 1738 [RFC1738]; 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   [RFC2732].  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.10.  References10.1.  Normative References   [ASCII]    American National Standards Institute, "Coded Character              Set -- 7-bit American Standard Code for Information              Interchange", ANSI X3.4, 1986.Berners-Lee, et al.         Standards Track                    [Page 46]RFC 3986                   URI Generic Syntax               January 2005   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF", RFC 2234, November 1997.   [STD63]    Yergeau, F., "UTF-8, a transformation format of              ISO 10646", STD 63, RFC 3629, November 2003.   [UCS]      International Organization for Standardization,              "Information Technology - Universal Multiple-Octet Coded              Character Set (UCS)", ISO/IEC 10646:2003, December 2003.10.2.  Informative References   [BCP19]    Freed, N. and J. Postel, "IANA Charset Registration              Procedures", BCP 19, RFC 2978, October 2000.   [BCP35]    Petke, R. and I. King, "Registration Procedures for URL              Scheme Names", BCP 35, RFC 2717, November 1999.   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet              host table specification", RFC 952, October 1985.   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",              STD 13, RFC 1034, November 1987.   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application              and Support", STD 3, RFC 1123, October 1989.   [RFC1535]  Gavron, E., "A Security Problem and Proposed Correction              With Widely Deployed DNS Software", RFC 1535,              October 1993.   [RFC1630]  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",              RFC 1630, June 1994.   [RFC1736]  Kunze, J., "Functional Recommendations for Internet              Resource Locators", RFC 1736, February 1995.   [RFC1737]  Sollins, K. and L. Masinter, "Functional Requirements for              Uniform Resource Names", RFC 1737, December 1994.   [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform              Resource Locators (URL)", RFC 1738, December 1994.   [RFC1808]  Fielding, R., "Relative Uniform Resource Locators",              RFC 1808, June 1995.Berners-Lee, et al.         Standards Track                    [Page 47]RFC 3986                   URI Generic Syntax               January 2005   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail              Extensions (MIME) Part Two: Media Types", RFC 2046,              November 1996.   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform              Resource Identifiers (URI): Generic Syntax", RFC 2396,              August 1998.   [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.              Jensen, "HTTP Extensions for Distributed Authoring --              WEBDAV", RFC 2518, February 1999.   [RFC2557]  Palme, J., Hopmann, A., and N. Shelness, "MIME              Encapsulation of Aggregate Documents, such as HTML              (MHTML)", RFC 2557, March 1999.   [RFC2718]  Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,              "Guidelines for new URL Schemes", RFC 2718, November 1999.   [RFC2732]  Hinden, R., Carpenter, B., and L. Masinter, "Format for              Literal IPv6 Addresses in URL‘s", RFC 2732, December 1999.   [RFC3305]  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", RFC 3305,              August 2002.   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,              "Internationalizing Domain Names in Applications (IDNA)",              RFC 3490, March 2003.   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6              (IPv6) Addressing Architecture", RFC 3513, April 2003.   [Siedzik]  Siedzik, R., "Semantic Attacks: What‘s in a URL?",              April 2001, <http://www.giac.org/practical/gsec/              Richard_Siedzik_GSEC.pdf>.Berners-Lee, et al.         Standards Track                    [Page 48]RFC 3986                   URI Generic Syntax               January 2005Appendix 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-octetBerners-Lee, et al.         Standards Track                    [Page 49]RFC 3986                   URI Generic Syntax               January 2005   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.Berners-Lee, et al.         Standards Track                    [Page 50]RFC 3986                   URI Generic Syntax               January 2005      ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?       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      http://www.ics.uci.edu/pub/ietf/uri/#Related   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 Section 5.3.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:Berners-Lee, et al.         Standards Track                    [Page 51]RFC 3986                   URI Generic Syntax               January 2005      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 "http://www.w3.org/Addressing/",      but you can probably pick it up from <ftp://foo.example.      com/rfc/>.  Note the warning in <http://www.ics.uci.edu/pub/      ietf/uri/historical.html#WARNING>.   contains the URI references      http://www.w3.org/Addressing/      ftp://foo.example.com/rfc/      http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNINGBerners-Lee, et al.         Standards Track                    [Page 52]RFC 3986                   URI Generic Syntax               January 2005Appendix D.  Changes from RFC 2396D.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 [RFC2732], 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 [RFC2732] defers to [RFC3513] 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 Section 2.2 of [RFC3513].   Likewise, the definition of IPv4address has been improved in order to   limit each decimal octet to the range 0-255.   Section 6, 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 RFC 2396 has been replaced with the ABNF of   [RFC2234].  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:Berners-Lee, et al.         Standards Track                    [Page 53]RFC 3986                   URI Generic Syntax               January 2005   +----------------+--------------------------------------------------+   | 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.   Section 2, 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 octetsBerners-Lee, et al.         Standards Track                    [Page 54]RFC 3986                   URI Generic Syntax               January 2005   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   [RFC2518] 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 [RFC2396] has been   rewritten with pseudocode for this revision to improve clarity and   fix the following issues:Berners-Lee, et al.         Standards Track                    [Page 55]RFC 3986                   URI Generic Syntax               January 2005   o  [RFC2396] section 5.2, step 6a, failed to account for a base URI      with no path.   o  Restored the behavior of [RFC1808] 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 RFC 2396, 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  4Berners-Lee, et al.         Standards Track                    [Page 56]RFC 3986                   URI Generic Syntax               January 2005   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, 26Berners-Lee, et al.         Standards Track                    [Page 57]RFC 3986                   URI Generic Syntax               January 2005      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  13Berners-Lee, et al.         Standards Track                    [Page 58]RFC 3986                   URI Generic Syntax               January 2005         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  18Berners-Lee, et al.         Standards Track                    [Page 59]RFC 3986                   URI Generic Syntax               January 2005Authors‘ 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:   http://www.w3.org/People/Berners-Lee/   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:   http://roy.gbiv.com/   Larry Masinter   Adobe Systems Incorporated   345 Park Ave   San Jose, CA  95110   USA   Phone: +1-408-536-3024   EMail: LMM@acm.org   URI:   http://larry.masinter.net/Berners-Lee, et al.         Standards Track                    [Page 60]RFC 3986                   URI Generic Syntax               January 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained in BCP 78, 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 BCP 78 and BCP 79.   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   http://www.ietf.org/ipr.   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]

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