RFC 3986 Uniform Resource Identifier (URI): Generic Syntax

[RFCs/IDs] [Plain Text] [From draft-fielding-uri-rfc2396bis]
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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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    RFC 3986                   URI Generic Syntax               January 2005

       [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", RFC 2234, November 1997.

       [STD63]    Yergeau, F., "UTF-8, a transformation format of
                  ISO 10646", STD 63, RFC 3629, November 2003.

       [UCS]      International Organization for Standardization,
                  "Information Technology - Universal Multiple-Octet Coded
                  Character Set (UCS)", ISO/IEC 10646:2003, December 2003.

    10.2.  Informative References

       [BCP19]    Freed, N. and J. Postel, "IANA Charset Registration
                  Procedures", BCP 19, RFC 2978, October 2000.

       [BCP35]    Petke, R. and I. King, "Registration Procedures for URL
                  Scheme Names", BCP 35, RFC 2717, November 1999.

       [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
                  host table specification", RFC 952, October 1985.

       [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
                  STD 13, RFC 1034, November 1987.

       [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
                  and Support", STD 3, RFC 1123, October 1989.

       [RFC1535]  Gavron, E., "A Security Problem and Proposed Correction
                  With Widely Deployed DNS Software", RFC 1535,
                  October 1993.

       [RFC1630]  Berners-Lee, T., "Universal Resource Identifiers in WWW: A
                  Unifying Syntax for the Expression of Names and Addresses
                  of Objects on the Network as used in the World-Wide Web",
                  RFC 1630, June 1994.

       [RFC1736]  Kunze, J., "Functional Recommendations for Internet
                  Resource Locators", RFC 1736, February 1995.

       [RFC1737]  Sollins, K. and L. Masinter, "Functional Requirements for
                  Uniform Resource Names", RFC 1737, December 1994.

       [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
                  Resource Locators (URL)", RFC 1738, December 1994.

       [RFC1808]  Fielding, R., "Relative Uniform Resource Locators",
                  RFC 1808, June 1995.

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    RFC 3986                   URI Generic Syntax               January 2005

       [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part Two: Media Types", RFC 2046,
                  November 1996.

       [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

       [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                  Resource Identifiers (URI): Generic Syntax", RFC 2396,
                  August 1998.

       [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
                  Jensen, "HTTP Extensions for Distributed Authoring --
                  WEBDAV", RFC 2518, February 1999.

       [RFC2557]  Palme, J., Hopmann, A., and N. Shelness, "MIME
                  Encapsulation of Aggregate Documents, such as HTML
                  (MHTML)", RFC 2557, March 1999.

       [RFC2718]  Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,
                  "Guidelines for new URL Schemes", RFC 2718, November 1999.

       [RFC2732]  Hinden, R., Carpenter, B., and L. Masinter, "Format for
                  Literal IPv6 Addresses in URL's", RFC 2732, December 1999.

       [RFC3305]  Mealling, M. and R. Denenberg, "Report from the Joint
                  W3C/IETF URI Planning Interest Group: Uniform Resource
                  Identifiers (URIs), URLs, and Uniform Resource Names
                  (URNs): Clarifications and Recommendations", RFC 3305,
                  August 2002.

       [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
                  "Internationalizing Domain Names in Applications (IDNA)",
                  RFC 3490, March 2003.

       [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
                  (IPv6) Addressing Architecture", RFC 3513, April 2003.

       [Siedzik]  Siedzik, R., "Semantic Attacks: What's in a URL?",
                  April 2001, <http://www.giac.org/practical/gsec/
                  Richard_Siedzik_GSEC.pdf>.

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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.
          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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    RFC 3986                   URI Generic Syntax               January 2005

       corresponding to unreserved characters.  In general, the terms
       "escaped" and "unescaped" have been replaced with "percent-encoded"
       and "decoded", respectively, to reduce confusion with other forms of
       escape mechanisms.

       The ABNF for URI and URI-reference has been redesigned to make them
       more friendly to LALR parsers and to reduce complexity.  As a result,
       the layout form of syntax description has been removed, along with
       the uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path,
       path_segments, rel_segment, and mark rules.  All references to
       "opaque" URIs have been replaced with a better description of how the
       path component may be opaque to hierarchy.  The relativeURI rule has
       been replaced with relative-ref to avoid unnecessary confusion over
       whether they are a subset of URI.  The ambiguity regarding the
       parsing of URI-reference as a URI or a relative-ref with a colon in
       the first segment has been eliminated through the use of five
       separate path matching rules.

       The fragment identifier has been moved back into the section on
       generic syntax components and within the URI and relative-ref rules,
       though it remains excluded from absolute-URI.  The number sign ("#")
       character has been moved back to the reserved set as a result of
       reintegrating the fragment syntax.

       The ABNF has been corrected to allow the path component to be empty.
       This also allows an absolute-URI to consist of nothing after the
       "scheme:", as is present in practice with the "dav:" namespace
       [RFC2518] and with the "about:" scheme used internally by many WWW
       browser implementations.  The ambiguity regarding the boundary
       between authority and path has been eliminated through the use of
       five separate path matching rules.

       Registry-based naming authorities that use the generic syntax are now
       defined within the host rule.  This change allows current
       implementations, where whatever name provided is simply fed to the
       local name resolution mechanism, to be consistent with the
       specification.  It also removes the need to re-specify DNS name
       formats here.  Furthermore, it allows the host component to contain
       percent-encoded octets, which is necessary to enable
       internationalized domain names to be provided in URIs, processed in
       their native character encodings at the application layers above URI
       processing, and passed to an IDNA library as a registered name in the
       UTF-8 character encoding.  The server, hostport, hostname,
       domainlabel, toplabel, and alphanum rules have been removed.

       The resolving relative references algorithm of [RFC2396] has been
       rewritten with pseudocode for this revision to improve clarity and
       fix the following issues:

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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.

    Intellectual Property

       The IETF takes no position regarding the validity or scope of any
       Intellectual Property Rights or other rights that might be claimed to
       pertain to the implementation or use of the technology described in
       this document or the extent to which any license under such rights
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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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       this standard.  Please address the information to the IETF at ietf-
       ipr@ietf.org.

    Acknowledgement

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

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