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Network Working Group                                     T. Berners-Lee
Request for Comments: 3986                                       W3C/MIT
STD: 66                                                      R. Fielding
Updates: 1738                                               Day Software
Obsoletes: 2732, 2396, 1808                                  L. Masinter
Category: Standards Track                                  Adobe Systems
                                                            January 2005


           Uniform Resource Identifier (URI): Generic Syntax

Status of This Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   A Uniform Resource Identifier (URI) is a compact sequence of
   characters that identifies an abstract or physical resource.  This
   specification defines the generic URI syntax and a process for
   resolving URI references that might be in relative form, along with
   guidelines and security considerations for the use of URIs on the
   Internet.  The URI syntax defines a grammar that is a superset of all
   valid URIs, allowing an implementation to parse the common components
   of a URI reference without knowing the scheme-specific requirements
   of every possible identifier.  This specification does not define a
   generative grammar for URIs; that task is performed by the individual
   specifications of each URI scheme.















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Table 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  . . . . . . . . . . . . . . . 36



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   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 . . . . . . . . . . . . . . . . . . . . . 61























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1.  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.







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   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 this





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   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.






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1.1.2.  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:4.1.2


1.1.3.  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].









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1.2.  Design Considerations

1.2.1.  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 this



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   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 observed



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   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,



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   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, the





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   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.








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      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 / "-" / "." / "_" / "~"





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   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 the



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   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 numeric



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   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:nose







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3.1.  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.




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      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 sake



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   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.




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   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 syntax



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   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.





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3.2.3.  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>




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      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.



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      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).



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   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-ref



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   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.








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4.3.  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 the



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   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.






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   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.






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   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.











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5.2.1.  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;






















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      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,








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   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.





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   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 "/./".













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5.3.  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.







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5.4.1.  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"












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   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 compatibility










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6.  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.





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6.2.  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).




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   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.





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6.2.2.3.  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 scheme



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   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).












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7.  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.






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   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.





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7.4.  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 example



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      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.  References

10.1.  Normative References

   [ASCII]    American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.





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   [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.




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   [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>.











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Appendix A.  Collected ABNF for URI

   URI           = scheme ":" hier-part [ "?" query ] [ "#" fragment ]

   hier-part     = "//" authority path-abempty
                 / path-absolute
                 / path-rootless
                 / path-empty

   URI-reference = URI / relative-ref

   absolute-URI  = scheme ":" hier-part [ "?" query ]

   relative-ref  = relative-part [ "?" query ] [ "#" fragment ]

   relative-part = "//" authority path-abempty
                 / path-absolute
                 / path-noscheme
                 / path-empty

   scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

   authority     = [ userinfo "@" ] host [ ":" port ]
   userinfo      = *( unreserved / pct-encoded / sub-delims / ":" )
   host          = IP-literal / IPv4address / reg-name
   port          = *DIGIT

   IP-literal    = "[" ( IPv6address / IPvFuture  ) "]"

   IPvFuture     = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )

   IPv6address   =                            6( h16 ":" ) ls32
                 /                       "::" 5( h16 ":" ) ls32
                 / [               h16 ] "::" 4( h16 ":" ) ls32
                 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
                 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
                 / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
                 / [ *4( h16 ":" ) h16 ] "::"              ls32
                 / [ *5( h16 ":" ) h16 ] "::"              h16
                 / [ *6( h16 ":" ) h16 ] "::"

   h16           = 1*4HEXDIG
   ls32          = ( h16 ":" h16 ) / IPv4address
   IPv4address   = dec-octet "." dec-octet "." dec-octet "." dec-octet







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   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.



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      ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
       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:



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      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#WARNING













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Appendix D.  Changes from RFC 2396

D.1.  Additions

   An ABNF rule for URI has been introduced to correspond to one common
   usage of the term: an absolute URI with optional fragment.

   IPv6 (and later) literals have been added to the list of possible
   identifiers for the host portion of an authority component, as
   described by [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:













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   +----------------+--------------------------------------------------+
   | 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 octets



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   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:



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   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  4



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   D
      dec-octet  20
      dereference  9
      dot-segments  23

   F
      fragment  16, 24

   G
      gen-delims  13
      generic syntax  6

   H
      h16  20
      hier-part  16
      hierarchical  10
      host  18

   I
      identifier  5
      IP-literal  19
      IPv4  20
      IPv4address  19, 20
      IPv6  19
      IPv6address  19, 20
      IPvFuture  19

   L
      locator  7
      ls32  20

   M
      merge  32

   N
      name  7
      network-path  26

   P
      path  16, 22, 26
         path-abempty  22
         path-absolute  22
         path-empty  22
         path-noscheme  22
         path-rootless  22
      path-abempty  16, 22, 26
      path-absolute  16, 22, 26
      path-empty  16, 22, 26



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      path-rootless  16, 22
      pchar  23
      pct-encoded  12
      percent-encoding  12
      port  22

   Q
      query  16, 23

   R
      reg-name  21
      registered name  20
      relative  10, 28
      relative-path  26
      relative-ref  26
      remove_dot_segments  33
      representation  9
      reserved  12
      resolution  9, 28
      resource  5
      retrieval  9

   S
      same-document  27
      sameness  9
      scheme  16, 17
      segment  22, 23
         segment-nz  23
         segment-nz-nc  23
      sub-delims  13
      suffix  27

   T
      transcription  8

   U
      uniform  4
      unreserved  13
      URI grammar
         absolute-URI  27
         ALPHA  11
         authority  18
         CR  11
         dec-octet  20
         DIGIT  11
         DQUOTE  11
         fragment  24
         gen-delims  13



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         h16  20
         HEXDIG  11
         hier-part  16
         host  19
         IP-literal  19
         IPv4address  20
         IPv6address  20
         IPvFuture  19
         LF  11
         ls32  20
         OCTET  11
         path  22
         path-abempty  22
         path-absolute  22
         path-empty  22
         path-noscheme  22
         path-rootless  22
         pchar  23
         pct-encoded  12
         port  22
         query  24
         reg-name  21
         relative-ref  26
         reserved  13
         scheme  17
         segment  23
         segment-nz  23
         segment-nz-nc  23
         SP  11
         sub-delims  13
         unreserved  13
         URI  16
         URI-reference  25
         userinfo  18
      URI  16
      URI-reference  25
      URL  7
      URN  7
      userinfo  18












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Authors' Addresses

   Tim Berners-Lee
   World Wide Web Consortium
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA  02139
   USA

   Phone: +1-617-253-5702
   Fax:   +1-617-258-5999
   EMail: timbl@w3.org
   URI:   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/















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Full 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.

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   The IETF takes no position regarding the validity or scope of any
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.






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