Network Working Group                                     T. Berners-Lee
Internet-Draft                                                   MIT/LCS
Updates: 1738 (if approved)                                  R. Fielding
Obsoletes: 2732, 2396, 1808 (if approved)                   Day Software
Expires: September 1, 2003                                   L. Masinter
                                                           March 3, 2003

           Uniform Resource Identifier (URI): Generic Syntax

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This Internet-Draft will expire on September 1, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   A Uniform Resource Identifier (URI) is a compact string of characters
   for identifying an abstract or physical resource.  This document
   defines the generic syntax of a URI, including both absolute and
   relative forms, and guidelines for their use.

   This document defines a grammar that is a superset of all valid URIs,
   such that an implementation can parse the common components of a URI
   reference without knowing the scheme-specific requirements of every
   possible identifier type.  This document does not define a generative

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   grammar for all URIs; that task will be performed by the individual
   specifications of each URI scheme.

Editorial Note

   Discussion of this draft and comments to the editors should be sent
   to the mailing list.  An issues list and version history
   is available at <>.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1   Overview of URIs . . . . . . . . . . . . . . . . . . . . . .  4
   1.2   URI, URL, and URN  . . . . . . . . . . . . . . . . . . . . .  5
   1.3   Example URIs . . . . . . . . . . . . . . . . . . . . . . . .  6
   1.4   Hierarchical URIs and Relative Forms . . . . . . . . . . . .  6
   1.5   URI Transcribability . . . . . . . . . . . . . . . . . . . .  7
   1.6   Syntax Notation and Common Elements  . . . . . . . . . . . .  8
   2.    URI Characters and Escape Sequences  . . . . . . . . . . . .  9
   2.1   URIs and non-ASCII characters  . . . . . . . . . . . . . . .  9
   2.2   Reserved Characters  . . . . . . . . . . . . . . . . . . . . 10
   2.3   Unreserved Characters  . . . . . . . . . . . . . . . . . . . 11
   2.4   Escape Sequences . . . . . . . . . . . . . . . . . . . . . . 11
   2.4.1 Escaped Encoding . . . . . . . . . . . . . . . . . . . . . . 11
   2.4.2 When to Escape and Unescape  . . . . . . . . . . . . . . . . 11
   2.4.3 Excluded US-ASCII Characters . . . . . . . . . . . . . . . . 12
   3.    URI Syntactic Components . . . . . . . . . . . . . . . . . . 14
   3.1   Scheme Component . . . . . . . . . . . . . . . . . . . . . . 15
   3.2   Authority Component  . . . . . . . . . . . . . . . . . . . . 15
   3.2.1 Registry-based Naming Authority  . . . . . . . . . . . . . . 16
   3.2.2 Server-based Naming Authority  . . . . . . . . . . . . . . . 16
   3.3   Path Component . . . . . . . . . . . . . . . . . . . . . . . 18
   3.4   Query Component  . . . . . . . . . . . . . . . . . . . . . . 19
   4.    URI References . . . . . . . . . . . . . . . . . . . . . . . 20
   4.1   Fragment Identifier  . . . . . . . . . . . . . . . . . . . . 20
   4.2   Same-document References . . . . . . . . . . . . . . . . . . 21
   4.3   Parsing a URI Reference  . . . . . . . . . . . . . . . . . . 21
   5.    Relative URI References  . . . . . . . . . . . . . . . . . . 22
   5.1   Establishing a Base URI  . . . . . . . . . . . . . . . . . . 23
   5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 24
   5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 24
   5.1.3 Base URI from the Retrieval URI  . . . . . . . . . . . . . . 25
   5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 25
   5.2   Resolving Relative References to Absolute Form . . . . . . . 25
   6.    URI Normalization and Comparison . . . . . . . . . . . . . . 29
   6.1   URI Equivalence  . . . . . . . . . . . . . . . . . . . . . . 29
   6.2   Comparison Ladder  . . . . . . . . . . . . . . . . . . . . . 29
   6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 30

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   6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 31
   6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 32
   6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 32
   6.3   Good Practice When Using URIs  . . . . . . . . . . . . . . . 32
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 34
   7.1   Reliability and Consistency  . . . . . . . . . . . . . . . . 34
   7.2   Malicious Construction . . . . . . . . . . . . . . . . . . . 34
   7.3   Rare IP Address Formats  . . . . . . . . . . . . . . . . . . 35
   7.4   Sensitive Information  . . . . . . . . . . . . . . . . . . . 35
   7.5   Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 36
   8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
         Normative References . . . . . . . . . . . . . . . . . . . . 38
         Non-normative References . . . . . . . . . . . . . . . . . . 39
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 40
   A.    Collected BNF for URI  . . . . . . . . . . . . . . . . . . . 42
   B.    Parsing a URI Reference with a Regular Expression  . . . . . 43
   C.    Examples of Resolving Relative URI References  . . . . . . . 44
   C.1   Normal Examples  . . . . . . . . . . . . . . . . . . . . . . 44
   C.2   Abnormal Examples  . . . . . . . . . . . . . . . . . . . . . 44
   D.    Embedding the Base URI in HTML documents . . . . . . . . . . 46
   E.    Recommendations for Delimiting URI in Context  . . . . . . . 47
   F.    Abbreviated URIs . . . . . . . . . . . . . . . . . . . . . . 49
   G.    Summary of Non-editorial Changes . . . . . . . . . . . . . . 50
   G.1   Additions  . . . . . . . . . . . . . . . . . . . . . . . . . 50
   G.2   Modifications from RFC 2396  . . . . . . . . . . . . . . . . 50
         Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
         Intellectual Property and Copyright Statements . . . . . . . 55

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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 such objects dates
   from 1990 and is described in "Universal Resource Identifiers in WWW"
   [RFC1630], and 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 excludes those 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 [RFC2717].

   All significant changes from RFC 2396 are noted in Appendix G.

1.1 Overview of URIs

   URIs are characterized by the following definitions:


      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; and,
      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.


      A resource can be anything that has identity.  Familiar examples
      include an electronic document, an image, a service (e.g.,
      "today's weather report for Los Angeles"), and a collection of
      other resources.  Not all resources are network "retrievable";
      e.g., human beings, corporations, and bound books in a library can
      also be considered resources.

      The resource is the conceptual mapping to an entity or set of

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      entities, not necessarily the entity which corresponds to that
      mapping at any particular instance in time.  Thus, a resource can
      remain constant even when its content---the entities to which it
      currently corresponds---changes over time, provided that the
      conceptual mapping is not changed in the process.


      An identifier is an object that can act as a reference to
      something that has identity.  In the case of a URI, the object is
      a sequence of characters with a restricted syntax.

   Having identified a resource, a system may perform a variety of
   operations on the resource, as might be characterized by such words
   as `access', `update', `replace', or `find attributes'.

1.2 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 the 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) refers to the subset of URIs that are required to remain
   globally unique and persistent even when the resource ceases to exist
   or becomes unavailable.

   An individual scheme does not need to be cast into one of a discrete
   set of URI types such as "URL", "URN", "URC", etc.  Any given URI
   scheme may define subspaces that have the characteristics of a name,
   a locator, 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 URI scheme.  For that reason, this specification
   deprecates use of the terms URL or URN to distinguish between
   schemes, instead using the term URI throughout.

   Each URI scheme (Section 3.1) defines the namespace of the URI, and
   thus may further restrict the syntax and semantics of identifiers
   using that scheme.  This specification defines those elements of the
   URI syntax that are either required of all URI schemes or are common
   to many URI schemes.  It thus defines the syntax and semantics that
   are needed to implement a scheme-independent parsing mechanism for
   URI references, such that the scheme-dependent handling of a URI can
   be postponed until the scheme-dependent semantics are needed.

   Although many URI schemes are named after protocols, this does not
   imply that use of such a URI will result in access to the resource
   via the named protocol.  URIs are often used in contexts that are

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   purely for identification, just like any other identifier.  Even when
   a URI is used to obtain a representation of a resource, that access
   might be through gateways, proxies, caches, and name resolution
   services that are independent of the protocol of the resource origin,
   and 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 resource when it can't be found in a local cache).

   A parser of the generic URI syntax is capable of parsing 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.

1.3 Example URIs

   The following examples illustrate URIs that are in common use.
         -- ftp scheme for File Transfer Protocol services

         -- gopher scheme for Gopher and Gopher+ Protocol services
         -- http scheme for Hypertext Transfer Protocol services
         -- mailto scheme for electronic mail addresses

         -- news scheme for USENET news groups and articles

         -- telnet scheme for interactive TELNET services

1.4 Hierarchical URIs and Relative Forms

   An absolute identifier refers to a resource independent of the
   context in which the identifier is used.  In contrast, a relative
   identifier refers to a resource by describing the difference within a
   hierarchical namespace between the current context and an absolute
   identifier of the resource.

   Some URI schemes support a hierarchical naming system, where the
   hierarchy of the name is denoted by a "/" delimiter separating the
   components in the scheme. This document defines a scheme-independent

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   `relative' form of URI reference that can be used in conjunction with
   a `base' URI of a hierarchical scheme to produce the `absolute' URI
   form of the reference. The syntax of a hierarchical URI is described
   in Section 3; the relative URI calculation is described in Section 5.

1.5 URI Transcribability

   The URI syntax was designed with global transcribability as one of
   its main concerns. A URI is a sequence of characters from a very
   limited set, i.e. 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
   octets in a coded character set.  The interpretation of a URI depends
   only on the characters used and not how those characters are
   represented in a network protocol.

   The goal of transcribability 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 concerns revealed by the scenario:

   o  A URI is a sequence of characters, which is not always represented
      as a sequence of octets.

   o  A URI may be transcribed from a non-network source, and thus
      should consist of characters that are most likely to be able to be
      typed into a computer, within the constraints imposed by keyboards
      (and related input devices) across languages and locales.

   o  A URI often needs to be remembered by people, and it is easier for
      people to remember a URI when it consists of meaningful

   These design concerns 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 the resource identifier from one medium to
   another was considered more important than having its URI consist of
   the most meaningful of components.  In local and regional contexts
   and with improving technology, users might benefit from being able to
   use a wider range of characters; such use is not defined in this

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1.6 Syntax Notation and Common Elements

   This document uses two conventions to describe and define the syntax
   for URI.  The first, called the layout form, is a general description
   of the order of components and component separators, as in


   The component names are enclosed in angle-brackets and any characters
   outside angle-brackets are literal separators.  Whitespace should be
   ignored.  These descriptions are used informally and do not define
   the syntax requirements.

   The second convention is a formal grammar defined using the Augmented
   Backus-Naur Form (ABNF) notation of [RFC2234]. Although the ABNF
   defines syntax in terms of the ASCII character encoding [ASCII], the
   URI syntax should be interpreted in terms of the character that the
   ASCII-encoded octet represents, rather than the octet encoding
   itself.  How a URI is represented in terms of bits and bytes on the
   wire is dependent upon the character encoding of the protocol used to
   transport it, or the charset of the document that contains it.

   The complete URI syntax is collected in Appendix A.

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2. URI Characters and Escape Sequences

   A URI consists of a restricted set of characters, primarily chosen
   to aid transcribability and usability both in computer systems and in
   non-computer communications. Characters used conventionally as
   delimiters around a URI are excluded.  The restricted set of
   characters consists of digits, letters, and a few graphic symbols
   chosen from those common to most of the character encodings and input
   facilities available to Internet users.

      uric          = reserved / unreserved / escaped

   Within a URI, characters are either used as delimiters or to
   represent strings of data (octets) within the delimited portions.
   Octets are either represented directly by a character (using the
   US-ASCII character for that octet [ASCII]) or by an escape encoding.
   This representation is elaborated below.

2.1 URIs and non-ASCII characters

   The relationship between URIs and characters has been a source of
   confusion for characters that are not part of US-ASCII. To describe
   the relationship, it is useful to distinguish between a "character"
   (as a distinguishable semantic entity) and an "octet" (an 8-bit
   byte). There are two mappings, one from URI characters to octets, and
   a second from octets to original characters:

   URI character sequence->octet sequence->original character sequence

   A URI is represented as a sequence of characters, not as a sequence
   of octets. That is because a URI might be "transported" by means that
   are not through a computer network, e.g., printed on paper, read over
   the radio, etc.

   Within a delimited component of a URI, a sequence of characters is
   used to represent a sequence of octets. For example, the character
   "a" represents the octet 97 (decimal), while the character sequence
   "%", "0", "a" represents the octet 10 (decimal).

   There is a second translation for some resources: the sequence of
   octets defined by a component of the URI is subsequently used to
   represent a sequence of characters. A 'charset' defines this mapping.
   There are many charsets in use in Internet protocols. For example,
   UTF-8 [UTF-8] defines a mapping from sequences of octets to sequences
   of characters in the repertoire of ISO 10646.

   In the simplest case, the original character sequence contains only
   characters that are defined in US-ASCII, and the two levels of

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   mapping are simple and easily invertible: each 'original character'
   is represented as the octet for the US-ASCII code for it, which is,
   in turn, represented as either the US-ASCII character, or else the
   "%" escape sequence for that octet.

   For original character sequences that contain non-ASCII characters,
   however, the situation is more difficult. Internet protocols that
   transmit octet sequences intended to represent character sequences
   are expected to provide some way of identifying the charset used, if
   there might be more than one [RFC2277].  However, there is currently
   no provision within the generic URI syntax to accomplish this
   identification. An individual URI scheme may require a single
   charset, define a default charset, or provide a way to indicate the
   charset used.  For example, a new scheme "foo" might be defined such
   that any escaped octet is keyed to the UTF-8 encoding in order to
   determine the corresponding Unicode character.

   It is expected that a systematic treatment of character encoding
   within URIs will be developed as a future modification of this

2.2 Reserved Characters

   Many URI include components consisting of or delimited by, certain
   special characters.  These characters are called "reserved", since
   their usage within the URI component is limited to their reserved
   purpose.  If the data for a URI component would conflict with the
   reserved purpose, then the conflicting data must be escaped before
   forming the URI.

      reserved    = "[" / "]" / ";" / "/" / "?" /
                    ":" / "@" / "&" / "=" / "+" / "$" / ","

   The "reserved" syntax class above refers to those characters that are
   allowed within a URI, but which may not be allowed within a
   particular component of the generic URI syntax; they are used as
   delimiters of the components described in Section 3.

   Characters in the "reserved" set are not reserved in all contexts.
   The set of characters actually reserved within any given URI
   component is defined by that component. In general, a character is
   reserved if the semantics of the URI changes if the character is
   replaced with its escaped US-ASCII encoding.

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2.3 Unreserved Characters

   Data characters that are allowed in a URI but do not have a reserved
   purpose are called unreserved.  These include upper and lower case
   letters, decimal digits, and a limited set of punctuation marks and

      unreserved  = ALPHA / DIGIT / mark

      mark        = "-" / "_" / "." / "!" / "~" / "*" / "'" / "(" / ")"

   Unreserved characters can be escaped without changing the semantics
   of the URI, but this should not be done unless the URI is being used
   in a context that does not allow the unescaped character to appear.
   URI normalization processes may unescape sequences in the ranges of
   ALPHA (%41-%5A and %61-%7A), DIGIT (%30-%39), underscore (%5F), or
   tilde (%7E) without fear of creating a conflict, but unescaping the
   other mark characters is usually counterproductive.

2.4 Escape Sequences

   Data must be escaped if it does not have a representation using an
   unreserved character; this includes data that does not correspond to
   a printable character of the US-ASCII coded character set, or that
   corresponds to any US-ASCII character that is disallowed, as
   explained below.

2.4.1 Escaped Encoding

   An escaped octet is encoded as a character triplet, consisting of
   the percent character "%" followed by the two hexadecimal digits
   representing the octet code in . For example, "%20" is the escaped
   encoding for the US-ASCII space character.

      escaped     = "%" HEXDIG HEXDIG

2.4.2 When to Escape and Unescape

   A URI is always in an "escaped" form, since escaping or unescaping a
   completed URI might change its semantics.  Normally, the only time
   escape encodings can safely be made is when the URI is being created
   from its component parts; each component may have its own set of
   characters that are reserved, so only the mechanism responsible for
   generating or interpreting that component can determine whether or
   not escaping a character will change its semantics. Likewise, a URI
   must be separated into its components before the escaped characters
   within those components can be safely decoded.

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   In some cases, data that could be represented by an unreserved
   character may appear escaped; for example, some of the unreserved
   "mark" characters are automatically escaped by some systems.  If the
   given URI scheme defines a canonicalization algorithm, then
   unreserved characters may be unescaped according to that algorithm.
   For example, "%7e" is sometimes used instead of "~" in an http URI
   path, but the two are equivalent for an http URI.

   Because the percent "%" character always has the reserved purpose of
   being the escape indicator, it must be escaped as "%25" in order to
   be used as data within a URI.  Implementers should be careful not to
   escape or unescape the same string more than once, since unescaping
   an already unescaped string might lead to misinterpreting a percent
   data character as another escaped character, or vice versa in the
   case of escaping an already escaped string.

2.4.3 Excluded US-ASCII Characters

   Although they are disallowed within the URI syntax, we include here a
   description of those US-ASCII characters that have been excluded and
   the reasons for their exclusion.

   The control characters (CTL) in the US-ASCII coded character set are
   not used within a URI, both because they are non-printable and
   because they are likely to be misinterpreted by some control

   The space character (SP) is excluded because significant spaces may
   disappear and insignificant spaces may be introduced when a URI is
   transcribed or typeset or subjected to the treatment of
   word-processing programs.  Whitespace is also used to delimit a URI
   in many contexts.

   The angle-bracket "<" and ">" and double-quote (") characters are
   excluded because they are often used as the delimiters around a URI
   in text documents and protocol fields.  The character "#" is excluded
   because it is used to delimit a URI from a fragment identifier in a
   URI reference (Section 4). The percent character "%" is excluded
   because it is used for the encoding of escaped characters.

      delims      = "<" / ">" / "#" / "%" / DQUOTE

   Other characters are excluded because gateways and other transport
   agents are known to sometimes modify such characters, or they are
   used as delimiters.

      unwise      = "{" / "}" / "|" / "\" / "^" / "`"

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   Data corresponding to excluded characters must be escaped in order to
   be properly represented within a URI.

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3. URI Syntactic Components

   The URI syntax is dependent upon the scheme.  In general, absolute
   URIs are written as follows:


   An absolute URI contains the name of the scheme being used (<scheme>)
   followed by a colon (":") and then a string (the
   <scheme-specific-part>) whose interpretation depends on the scheme.

   The URI syntax does not require that the scheme-specific-part have
   any general structure or set of semantics which is common among all
   URIs.  However, a subset of URI do share a common syntax for
   representing hierarchical relationships within the namespace.  This
   "generic URI" syntax consists of a sequence of four main components:


   each of which, except <scheme>, may be absent from a particular URI.
   For example, some URI schemes do not allow an <authority> component,
   and others do not use a <query> component.

      absolute-URI  = scheme ":" ( hier-part / opaque-part )

   URIs that are hierarchical in nature use the slash "/" character for
   separating hierarchical components.  For some file systems, a "/"
   character (used to denote the hierarchical structure of a URI) is the
   delimiter used to construct a file name hierarchy, and thus the URI
   path will look similar to a file pathname.  This does NOT imply that
   the resource is a file or that the URI maps to an actual filesystem

      hier-part     = [ net-path / abs-path ] [ "?" query ]

      net-path      = "//" authority [ abs-path ]

      abs-path      = "/"  path-segments

   URIs that do not make use of the slash "/" character for separating
   hierarchical components are considered opaque by the generic URI

      opaque-part   = uric-no-slash *uric

      uric-no-slash = unreserved / escaped / "[" / "]" / ";" / "?" /
                      ":" / "@" / "&" / "=" / "+" / "$" / ","

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   We use the term <path> to refer to both the <abs-path> and
   <opaque-part> constructs, since they are mutually exclusive for any
   given URI and can be parsed as a single component.

3.1 Scheme Component

   Just as there are many different methods of access to resources,
   there are a variety of schemes for identifying such resources.  The
   URI syntax consists of a sequence of components separated by reserved
   characters, with the first component defining the semantics for the
   remainder of the URI string.

   Scheme names consist of a sequence of characters beginning with a
   lower case letter and followed by any combination of lower case
   letters, digits, plus ("+"), period ("."), or hyphen ("-").  For
   resiliency, programs interpreting a URI should treat upper case
   letters as equivalent to lower case in scheme names (e.g., allow
   "HTTP" as well as "http").

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

   Relative URI references are distinguished from absolute URI in that
   they do not begin with a scheme name.  Instead, the scheme is
   inherited from the base URI, as described in Section 5.2.

3.2 Authority Component

   Many URI schemes include a top hierarchical element for a naming
   authority, such that the namespace defined by the remainder of the
   URI is governed by that authority.  This authority component is
   typically defined by an Internet-based server or a scheme-specific
   registry of naming authorities.

      authority     = server / reg-name

   The authority component is preceded by a double slash "//" and is
   terminated by the next slash "/", question-mark "?", or by the end of
   the URI.  Within the authority component, the characters ";", ":",
   "@", "?", "/", "[", and "]" are reserved.

   An authority component is not required for a URI scheme to make use
   of relative references.  A base URI without an authority component
   implies that any relative reference will also be without an authority

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3.2.1 Registry-based Naming Authority

   The structure of a registry-based naming authority is specific to
   the URI scheme, but constrained to the allowed characters for an
   authority component.

      reg-name      = 1*( unreserved / escaped / ";" /
                          ":" / "@" / "&" / "=" / "+" / "$" / "," )

3.2.2 Server-based Naming Authority

   URI schemes that involve the direct use of an IP-based protocol to a
   specified server on the Internet use a common syntax for the server
   component of the URI's scheme-specific data:


   where <userinfo> may consist of a user name and, optionally,
   scheme-specific information about how to gain authorization to access
   the server.  The parts "<userinfo>@" and ":<port>" may be omitted. If
   <host> is omitted, the default host is defined by the scheme-specific
   semantics of the URI (e.g., the "file" URI scheme defaults to
   "localhost", whereas the "http" URI scheme does not allow host to be

      server        = [ [ userinfo "@" ] hostport ]

   The user information, if present, is followed by a commercial
   at-sign "@".

      userinfo      = *( unreserved / escaped / ";" /
                         ":" / "&" / "=" / "+" / "$" / "," )

   Some URI schemes use the format "user:password" in the userinfo
   field. This practice is NOT RECOMMENDED, because the passing of
   authentication information in clear text has proven to be a security
   risk in almost every case where it has been used. Note also that
   userinfo which is crafted to look like a trusted domain name might be
   used to mislead users, as described in Section 7.5.

   The server is identified by a network host --- as described by an
   IPv6 literal encapsulated within square brackets, an IPv4 address in
   dotted-decimal form, or a domain name --- and an optional port
   number. The server's port, if any is required by the URI scheme, can
   be specified by a port number in decimal following the host and
   delimited from it by a colon (":") character.  If no explicit port
   number is given, the default port number, as defined by the URI

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   scheme, is assumed.  The type of network port identified by the URI
   (e.g., TCP, UDP, SCTP, etc.) is defined by the scheme-specific
   semantics of the URI scheme.

      hostport      = host [ ":" port ]
      host          = IPv6reference / IPv4address / hostname
      port          = *DIGIT

   A hostname takes the form described in Section 3 of [RFC1034] and
   Section 2.1 of [RFC1123]: 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 will never start with a
   digit, thus syntactically distinguishing domain names from IPv4
   addresses, and may be followed by a single "." if it is necessary to
   distinguish between the complete domain name and any local domain.

      hostname      = domainlabel qualified
      qualified     = *( "." domainlabel ) [ "." toplabel "." ]
      domainlabel   = alphanum [ 0*61( alphanum | "-" ) alphanum ]
      toplabel      = alpha    [ 0*61( alphanum | "-" ) alphanum ]
      alphanum      = ALPHA / DIGIT

   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 7.3, but
   only the dotted-decimal form of four octets is allowed by this

      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

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   A host identified by an IPv6 literal address [RFC2373] is
   distinguished by enclosing the IPv6 literal within square-brakets
   ("[" and "]").  This is the only place where square-bracket
   characters are allowed in the hierarchical URI syntax.

      IPv6reference = "[" IPv6address "]"

      IPv6address   = (                          6( h4 ":" ) ls32 )
                    / (                     "::" 5( h4 ":" ) ls32 )
                    / ( [              h4 ] "::" 4( h4 ":" ) ls32 )
                    / ( [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32 )
                    / ( [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32 )
                    / ( [ *3( h4 ":" ) h4 ] "::"    h4 ":"   ls32 )
                    / ( [ *4( h4 ":" ) h4 ] "::"             ls32 )
                    / ( [ *5( h4 ":" ) h4 ] "::"             h4   )
                    / ( [ *6( h4 ":" ) h4 ] "::"                  )

      ls32          = ( h4 ":" h4 ) / IPv4address
                    ; least-significant 32 bits of address

      h4            = 1*4HEXDIG

3.3 Path Component

   The path component contains data, specific to the authority (or the
   scheme if there is no authority component), identifying the resource
   within the scope of that scheme and authority.

      path          = [ abs-path / opaque-part ]

      path-segments = segment *( "/" segment )
      segment       = *pchar

      pchar         = unreserved / escaped / ";" /
                      ":" / "@" / "&" / "=" / "+" / "$" / ","

   The path may consist of a sequence of path segments separated by a
   single slash "/" character.  Within a path segment, the characters "/
   ", ";", "=", and "?" are reserved.  The semicolon (";") and equals
   ("=") characters have the reserved purpose of delimiting parameters
   and parameter values within a path segment.  However, parameters are
   not significant to the parsing of relative references.

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3.4 Query Component

   The query component is a string of information to be interpreted by
   the resource.

      query         = *( pchar / "/" / "?" )

   Within a query component, the characters ";", "/", "?", ":", "@",
   "&", "=", "+", ",", and "$" are reserved.

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4. URI References

   The term "URI-reference" is used here to denote the common usage of
   a resource identifier.  A URI reference may be absolute or relative,
   and may have additional information attached in the form of a
   fragment identifier.  However, "the URI" that results from such a
   reference includes only the absolute URI after the fragment
   identifier (if any) is removed and after any relative URI is resolved
   to its absolute form.  Although it is possible to limit the
   discussion of URI syntax and semantics to that of the absolute
   result, most usage of URI is within general URI references, and it is
   impossible to obtain the URI from such a reference without also
   parsing the fragment and resolving the relative form.

      URI-reference = [ absolute-URI / relative-URI ] [ "#" fragment ]

   Many protocol elements allow only the absolute form of a URI with an
   optional fragment identifier.

      absolute-URI-reference = absolute-URI [ "#" fragment ]

   The syntax for a relative URI is a shortened form of that for an
   absolute URI, where some prefix of the URI is missing and certain
   path components ("." and "..") have a special meaning when, and only
   when, interpreting a relative path.  The relative URI syntax is
   defined in Section 5.

4.1 Fragment Identifier

   When a URI reference is used to perform a retrieval action on the
   identified resource, the optional fragment identifier, separated from
   the URI by a crosshatch ("#") character, consists of additional
   reference information to be interpreted by the user agent after the
   retrieval action has been successfully completed.  As such, it is not
   part of a URI, but is often used in conjunction with a URI.

      fragment      = *( pchar / "/" / "?" )

   The semantics of a fragment identifier is a property of the data
   resulting from a retrieval action, regardless of the type of URI used
   in the reference.  Therefore, the format and interpretation of
   fragment identifiers is dependent on the media type [RFC2046] of the
   retrieval result.  The character restrictions described in Section 2
   for a URI also apply to the fragment in a URI-reference.  Individual
   media types may define additional restrictions or structure within
   the fragment for specifying different types of "partial views" that
   can be identified within that media type.

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   A fragment identifier is only meaningful when a URI reference is
   intended for retrieval and the result of that retrieval is a document
   for which the identified fragment is consistently defined.

4.2 Same-document References

   A URI reference that does not contain a URI is a reference to the
   current document.  In other words, an empty URI reference within a
   document is interpreted as a reference to the start of that document,
   and a reference containing only a fragment identifier is a reference
   to the identified fragment of that document.  Traversal of such a
   reference should not result in an additional retrieval action.
   However, if the URI reference occurs in a context that is always
   intended to result in a new request, as in the case of HTML's FORM
   element [HTML], then an empty URI reference represents the base URI
   of the current document and should be replaced by that URI when
   transformed into a request.

4.3 Parsing a URI Reference

   A URI reference is typically parsed according to the four main
   components and fragment identifier in order to determine what
   components are present and whether the reference is relative or
   absolute.  The individual components are then parsed for their
   subparts and, if not opaque, to verify their validity.

   Although the BNF defines what is allowed in each component, it is
   ambiguous in terms of differentiating between an authority component
   and a path component that begins with two slash characters.  The
   greedy algorithm is used for disambiguation: the left-most matching
   rule soaks up as much of the URI reference string as it is capable of
   matching.  In other words, the authority component wins.

   Readers familiar with regular expressions should see Appendix B for a
   concrete parsing example and test oracle.

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5. Relative URI References

   It is often the case that a group or "tree" of documents has been
   constructed to serve a common purpose; the vast majority of URIs in
   these documents point to resources within the tree rather than
   outside of 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 addressing 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 using relative URIs.
   Furthermore, such document trees can be moved, as a whole, without
   changing any of the relative references.  Experience within the WWW
   has demonstrated that the ability to perform relative referencing is
   necessary for the long-term usability of embedded URIs.

   The relative URI syntax takes advantage of the <hier-part> syntax of
   <absolute-URI> (Section 3) in order to express a reference that is
   relative to the namespace of another hierarchical URI.

      relative-URI  = [ net-path / abs-path / rel-path ] [ "?" query ]

   A relative reference beginning with two slash characters is termed a
   network-path reference, as defined by <net-path> in Section 3.  Such
   references are rarely used.

   A relative reference beginning with a single slash character is
   termed an absolute-path reference, as defined by <abs-path> in
   Section 3.

   A relative reference that does not begin with a scheme name or a
   slash character is termed a relative-path reference.

      rel-path      = rel-segment [ abs-path ]

      rel-segment   = 1*( unreserved / escaped / ";" /
                          "@" / "&" / "=" / "+" / "$" / "," )

   Within a relative-path reference, the complete path segments "." and
   ".." have special meanings: "the current hierarchy level" and "the
   level above this hierarchy level", respectively.  Although this is
   very similar to their use within Unix-based filesystems to indicate
   directory levels, these path components are only considered special
   when resolving a relative-path reference to its absolute form
   (Section 5.2).

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   Authors should be aware that a path segment which contains a colon
   character cannot be used as the first segment of a relative URI path
   (e.g., "this:that"), because it would be mistaken for a scheme name.
   It is therefore necessary to precede such segments with other
   segments (e.g., "./this:that") in order for them to be referenced as
   a relative path.

   It is not necessary for all URI within a given scheme to be
   restricted to the <hier-part> syntax, since the hierarchical
   properties of that syntax are only necessary when a relative URI is
   used within a particular document.  Documents can only make use of a
   relative URI when their base URI fits within the <hier-part> syntax.
   It is assumed that any document which contains a relative reference
   will also have a base URI that obeys the syntax.  In other words, a
   relative URI cannot be used within a document that has an unsuitable
   base URI.

   Some URI schemes do not allow a hierarchical syntax matching the
   <hier-part> syntax, and thus cannot use relative references.

5.1 Establishing a Base URI

   The term "relative URI" implies that there exists some absolute "base
   URI" against which the relative reference is applied.  Indeed, the
   base URI is necessary to define the semantics of any relative URI
   reference; without it, a relative reference is meaningless.  In order
   for relative URI to be usable within a document, the base URI of that
   document must be known to the parser.

   The base URI of a document can be established in one of four ways,
   listed 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:

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      |  .----------------------------------------------------.  |
      |  |  .----------------------------------------------.  |  |
      |  |  |  .----------------------------------------.  |  |  |
      |  |  |  |  .----------------------------------.  |  |  |  |
      |  |  |  |  |       <relative-reference>       |  |  |  |  |
      |  |  |  |  `----------------------------------'  |  |  |  |
      |  |  |  | (5.1.1) Base URI embedded in the       |  |  |  |
      |  |  |  |         document's content             |  |  |  |
      |  |  |  `----------------------------------------'  |  |  |
      |  |  | (5.1.2) Base URI of the encapsulating entity |  |  |
      |  |  |         (message, document, or none).        |  |  |
      |  |  `----------------------------------------------'  |  |
      |  | (5.1.3) URI used to retrieve the entity            |  |
      |  `----------------------------------------------------'  |
      | (5.1.4) Default Base URI is application-dependent        |

5.1.1 Base URI within Document Content

   Within certain document media types, the base URI of the document can
   be embedded within the content itself such that it can be readily
   obtained by a parser.  This can be useful for descriptive documents,
   such as tables of content, which may be transmitted to others through
   protocols other than their usual retrieval context (e.g., E-Mail or
   USENET news).

   It is beyond the scope of this document to specify how, for each
   media type, the base URI can be embedded.  It is assumed that user
   agents manipulating such media types will be able to obtain the
   appropriate syntax from that media type's specification.  An example
   of how the base URI can be embedded in the Hypertext Markup Language
   (HTML) [HTML] is provided in Appendix D.

   A mechanism for embedding the base URI within MIME container types
   (e.g., the message and multipart types) is defined by MHTML
   [RFC2110].  Protocols that do not use the MIME message header syntax,
   but which do allow some form of tagged metainformation to be included
   within messages, may define their own syntax for defining the base
   URI as part of a message.

5.1.2 Base URI from the Encapsulating Entity

   If no base URI is embedded, the base URI of a document is defined by
   the document's retrieval context.  For a document that is enclosed
   within another entity (such as a message or another document), the
   retrieval context is that entity; thus, the default base URI of the

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   document is the base URI of the entity in which the document is

5.1.3 Base URI from the Retrieval URI

   If no base URI is embedded and the document is not encapsulated
   within some other entity (e.g., the top level of a composite entity),
   then, if a URI was used to retrieve the base document, 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., that which
   resulted in the actual retrieval of the document) is the base URI.

5.1.4 Default Base URI

   If none of the conditions described in Sections 5.1.1--5.1.3 apply,
   then the base URI is defined by the context of the application. Since
   this definition is necessarily application-dependent, failing to
   define the base URI using one of the other methods may result in the
   same content being interpreted differently by different types of

   It is the responsibility of the distributor(s) of a document
   containing a relative URI to ensure that the base URI for that
   document can be established.  It must be emphasized that a relative
   URI cannot be used reliably in situations where the document's base
   URI is not well-defined.

5.2 Resolving Relative References to Absolute Form

   This section describes an example algorithm for resolving URI
   references that might be relative to a given base URI.  The algorithm
   is intended to provide a definitive result that can be used to test
   the output of other implementations.  Implementation of the algorithm
   itself is not required, but the result given by an implementation
   must match the result that would be given by this algorithm.

   The base URI is established according to the rules of Section 5.1 and
   parsed into the four main components as described in Section 3.  Note
   that only the scheme component is required to be present in the base
   URI; the other components may be empty or undefined.  A component is
   undefined if its preceding separator does not appear in the URI
   reference; the path component is never undefined, though it may be
   empty.  The base URI's query component is not used by the resolution
   algorithm and may be discarded.

   For each URI reference (R), the following pseudocode describes an
   algorithm for transforming R into its target (T), which is either an
   absolute URI or the current document, and R's optional fragment:

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      (R.scheme, R.authority, R.path, R.query, fragment) = parse(R);
         -- The URI reference is parsed into the four components and
         -- fragment identifier, as described in Section 4.3.

      if ((not validating) and (R.scheme == Base.scheme)) then
         -- A non-validating parser may ignore a scheme in the
         -- reference if it is identical to the base URI's scheme.

      if defined(R.scheme) then
         T.scheme    = R.scheme;
         T.authority = R.authority;
         T.path      = R.path;
         T.query     = R.query;
         if defined(R.authority) then
            T.authority = R.authority;
            T.path      = R.path;
            T.query     = R.query;
            if (R.path == "") then
               if defined(R.query) then
                  T.path  = Base.path;
                  T.query = R.query;
                  -- An empty reference refers to the current document
                  return (current-document, fragment);
               if (R.path starts-with "/") then
                  T.path = R.path;
                  T.path = merge(Base.path, R.path);
               T.query = R.query;
            T.authority = Base.authority;
         T.scheme = Base.scheme;

      return (T, fragment);

   The pseudocode above refers to a merge routine for merging a
   relative-path reference with the path of the base URI to obtain the
   target path.  Although there are many ways to do this, we will
   describe a simple method using a separate string buffer:

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   1.  All but the last segment of the base URI's path component is
       copied to the buffer.  In other words, any characters after the
       last (right-most) slash character, if any, are excluded. If the
       base URI's path component is the empty string, then a single
       slash character ("/") is copied to the buffer.

   2.  The reference's path component is appended to the buffer string.

   3.  All occurrences of "./", where "." is a complete path segment,
       are removed from the buffer string.

   4.  If the buffer string ends with "." as a complete path segment,
       that "." is removed.

   5.  All occurrences of "<segment>/../", where <segment> is a complete
       path segment not equal to "..", are removed from the buffer
       string.  Removal of these path segments is performed iteratively,
       removing the leftmost matching pattern on each iteration, until
       no matching pattern remains.

   6.  If the buffer string ends with "<segment>/..", where <segment> is
       a complete path segment not equal to "..", that "<segment>/.." is

   7.  If the resulting buffer string still begins with one or more
       complete path segments of "..", then the reference is considered
       to be in error.  Implementations may handle this error by
       retaining these components in the resolved path (i.e., treating
       them as part of the final URI), by removing them from the
       resolved path (i.e., discarding relative levels above the root),
       or by avoiding traversal of the reference.

   8.  The remaining buffer string is the target URI's path component.

   Some systems may find it more efficient to implement the merge
   algorithm as a pair of path segment stacks being merged, rather than
   as a series of string pattern replacements.

      Note: Some WWW client applications will fail to separate the
      reference's query component from its path component before merging
      the base and reference paths.  This may result in a loss of
      information if the query component contains the strings "/../" or

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   The resulting target URI components and fragment can be recombined to
   provide the absolute form of the URI reference. Using pseudocode,
   this would be:

      result = ""

      if defined(T.scheme) then
         append T.scheme to result;
         append ":" to result;

      if defined(T.authority) then
         append "//" to result;
         append T.authority to result;

      append T.path to result;

      if defined(T.query) then
         append "?" to result;
         append T.query to result;

      if defined(fragment) then
         append "#" to result;
         append fragment to result;

      return result;

   Note that we must be 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.

   Resolution examples are provided in Appendix C.

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6. URI Normalization and Comparison

   One of the most common operations on URIs is simple comparison:
   determining if 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 reduce
   duplication of request actions and response storage.

   URI comparison is performed in respect to some particular purpose,
   and software with differing purposes will often be subject to
   differing design trade-offs in regards to how much effort should be
   spent in reducing duplicate identifiers.  This section describes a
   variety of methods that may be used to compare URIs, the trade-offs
   between them, and the types of applications that might use them.

6.1 URI Equivalence

   Since URIs exist to identify resources, presumably they should be
   considered equivalent when they identify the same resource.  However,
   such a definition of equivalence is not of much practical use, since
   there is no way for software to compare two resources without
   knowledge of their origin.  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,
   it is never possible to be sure that two URIs identify different
   resources. Therefore, comparison methods are designed to minimize
   false negatives while strictly avoiding false positives.

   In testing for equivalence, it is generally unwise to directly
   compare relative URI references; they should be converted to their
   absolute forms before comparison.  Furthermore, when URI references
   are being compared for the purpose of selecting (or avoiding) a
   network action, such as retrieval of a representation, it is often
   necessary to separate fragment identifiers from the URIs prior to

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

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   processing required and the degree to which the probability of false
   negatives is reduced.  As noted above, false negatives cannot in
   principle 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 those 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, 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 of strings for
   equality is normally based on pairwise comparison of the characters
   that make up the strings, starting from the first and proceeding
   until both strings are exhausted and all characters found to be
   equal, or a pair of characters compares unequal or one of the strings
   is exhausted before the other.

   Such character comparisons require 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 be in a Java String
   object, bit-for-bit comparisons applied naively will produce both
   false-positive and false-negative errors.  Thus, in principle, it is
   better to speak of equality on a character-for-character rather than
   byte-for-byte or bit-for-bit basis.

   Unicode defines a character as being identified by number
   ("codepoint") with an associated bundle of visual and other
   semantics. At the software level, it is not practical to compare
   semantic bundles, so in practical terms, character-by-character
   comparisons are done codepoint-by-codepoint.

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6.2.2 Syntax-based Normalization

   Software may use logic based on the definitions provided by this
   specification to reduce the probability of false negatives.  Such
   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:


   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, escape normalization, and removal of leftover
   relative path segments. Case Normalization

   When a URI scheme uses elements of the common syntax, it will also
   use the common syntax equivalence rules, namely that the scheme and
   hostname are case insensitive and therefore can be normailized to
   lowercase.  For example, the URI <HTTP://> is
   equivalent to <>. Escape Normalization

   The %-escape mechanism described in Section 2.4 is a frequent source
   of variance among otherwise identical URIs.  One cause is the choice
   of upper-case or lower-case letters for the hexadecimal digits within
   the escape sequence (e.g., "%3a" versus "%3A").  Such sequences are
   always equivalent; for the sake of uniformity, URI generators and
   normalizers are strongly encouraged to use upper-case letters for the
   hex digits A-F.

   Only characters that are excluded from or reserved within the URI
   syntax must be escaped when used as data.  However, some URI
   generators go beyond that and escape characters that do not require
   escaping, resulting in URIs that are equivalent to their unescaped
   counterparts. Such URIs can be normalized by unescaping sequences
   that represent the unreserved characters, as described in Section
   2.3. Path Segment Normalization

   The complete path segments "." and ".." have a special meaning within
   hierarchical URI schemes.  As such, they should not appear in

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   absolute URI paths; if they are found, they can be removed by
   splitting the URI just after the "/" that starts the path, using the
   left half as the base URI and the right as a relative reference, and
   normalizing the URI by merging the two in in accordance with the
   relative URI processing algorithm (Section 5).

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.  Software
   may use scheme-specific rules, at further processing cost, to reduce
   the probability of false negatives.  For example, Web spiders that
   populate most large search engines would consider the following two
   URIs to be equivalent:

   This behavior is based on the rules provided by the syntax and
   semantics of the "http" URI scheme, which defines an empty port
   component as being equivalent to the default TCP port for HTTP (port
   80).  In general, a URI scheme that uses the generic syntax of
   hostport is defined such that a URI with an explicit ":port", where
   the port is the default for the scheme, is equivalent to one where
   the port is elided.

6.2.4 Protocol-based Normalization

   Web spiders, for which substantial effort to reduce the incidence of
   false negatives is often cost-effective, are observed to implement
   even more aggressive techniques in URI comparison.  For example, if
   they observe that a URI such as

   redirects to

   they will likely regard the two as equivalent in the future.
   Obviously, this kind of technique is only appropriate in special

6.3 Good Practice When Using URIs

   It is in the best interests of everyone to avoid false-negatives in
   comparing URIs, and to only require the minimum amount of software
   processing for such comparisons.  Those who generate and make

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   reference to URIs can reduce the cost of processing and the risk of
   false negatives by consistently providing them in a form that is
   reasonably canonical with respect to their scheme.  Specifically:

      Always provide the URI scheme in lower-case characters.

      Always provide the hostname, if any, in lower-case characters.

      Only perform %-escaping where it is essential.

      Always use upper-case A-through-F characters when %-escaping.

      Use the UTF-8 character-to-octet mapping, whenever possible.

      Prevent /./ and /../ from appearing in absolute URI paths.

   The choices listed above are motivated by observations that a high
   proportion of deployed software already use these techniques in
   practice for the purposes of normalization.

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7. Security Considerations

   A URI does not in itself pose a security threat.  However, since 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

7.1 Reliability and Consistency

   There is no guarantee that, having once used a given URI to retrieve
   some information, that the same information will be retievable 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 a guarantee 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 such that an attempt to
   perform a seemingly harmless, idempotent operation, such as the
   retrieval of a representation associated with a resource, will in
   fact cause a possibly damaging remote operation to occur.  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 that is in fact running a different
   protocol.  The content of the URI contains instructions that, when
   interpreted according to this other protocol, cause an unexpected
   operation.  An example has been the use of a gopher URI to cause an
   unintended or impersonating message to be sent via a SMTP server.

   Caution should be used when using any URI that specifies a TCP port
   number other than the default for the protocol, especially when it is
   a number within the reserved space.

   Care should be taken when a URI contains escaped delimiters for a
   given protocol (for example, CR and LF characters for telnet
   protocols) that these are not unescaped before transmission.  This
   might violate the protocol, but avoids the potential for such
   characters to be used to simulate an extra operation or parameter in
   that protocol, which might lead to an unexpected and possibly harmful
   remote operation being performed.

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7.3 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 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; otherwise, 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 such filtering is performed, it is recommended that literals be
   converted to numeric form and filtered based on the numeric value,
   rather than a prefix or suffix of the string form.

7.4 Sensitive Information

   It is clearly unwise to use a URI that contains a password which is
   intended to be secret. In particular, the use of a password within
   the userinfo component of a URI is strongly discouraged except in
   those rare cases where the 'password' parameter is intended to be

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7.5 Semantic Attacks

   Because the userinfo component is rarely used and appears before the
   hostname in the authority component, it can be used to construct a
   URI that is 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

   might lead a human user to assume that the authority is
   '', whereas it is actually ''.  Note that the
   misleading userinfo could be much longer than the example above.

   A misleading URI, such as the one 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 visually distinguishing the various
   components of the URI when rendered, such as by using a different
   color or tone to render userinfo if any is present, though there is
   no general panacea. More information on URI-based semantic attacks
   can be found in [Siedzik].

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8. Acknowledgements

   This document is derived from RFC 2396 [RFC2396], RFC 1808 [RFC1808],
   and RFC 1738 [RFC1738]; the acknowledgements in those specifications
   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 Reese Anschultz, Tim Bray, Dan Connolly, Adam M.
   Costello, Jason Diamond, Martin Duerst, Henry Holtzman, Graham Klyne,
   Dan Kohn, Bruce Lilly, Michael Mealling, Julian Reschke, Tomas
   Rokicki, Miles Sabin, Ronald Tschalaer, Marc Warne, Henry Zongaro,
   and Zefram are gratefully acknowledged.

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Normative References

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

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

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Non-normative References

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [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.

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

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

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

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

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

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

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

   [RFC2373]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

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

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

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

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

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   [RFC2110]  Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
              Aggregate Documents, such as HTML (MHTML)", RFC 2110,
              March 1997.

   [RFC2717]  Petke, R. and I. King, "Registration Procedures for URL
              Scheme Names", BCP 35, RFC 2717, November 1999.

   [HTML]     Raggett, D., Le Hors, A. and I. Jacobs, "Hypertext Markup
              Language (HTML 4.01) Specification", December 1999.

   [Siedzik]  Siedzik, R., "Semantic Attacks: What's in a URL?", April

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 2279, January 1998.

Authors' Addresses

   Tim Berners-Lee
   World Wide Web Consortium
   MIT/LCS, Room NE43-356
   200 Technology Square
   Cambridge, MA  02139

   Phone: +1-617-253-5702
   Fax:   +1-617-258-5999

   Roy T. Fielding
   Day Software
   2 Corporate Plaza, Suite 150
   Newport Beach, CA  92660

   Phone: +1-949-999-2523
   Fax:   +1-949-644-5064

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   Larry Masinter
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110

   Phone: +1-408-536-3024

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Appendix A. Collected BNF for URI

   To be filled-in later.

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Appendix B. Parsing a URI Reference with a Regular Expression

   As described in Section 4.3, the generic URI syntax is not sufficient
   to disambiguate the components of some forms of URI.  Since the
   "greedy algorithm" described in that section is identical to the
   disambiguation method used by POSIX regular expressions, it is
   natural and commonplace to use a regular expression for parsing the
   potential four components and fragment identifier of a URI reference.

   The following line is the regular expression for breaking-down a URI
   reference into its components.

       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

   results in the following subexpression matches:

      $1 = http:
      $2 = http
      $3 = //
      $4 =
      $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 four components and fragment as

      scheme    = $2
      authority = $4
      path      = $5
      query     = $7
      fragment  = $9

   and, going in the opposite direction, we can recreate a URI reference
   from its components using the algorithm of Section 5.2.

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Appendix C. Examples of Resolving Relative URI References

   Within an object with a well-defined base URI of


   the relative URI would be resolved as follows:

C.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            =  (current document)#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/
      ./            =  http://a/b/c/
      ..            =  http://a/b/
      ../           =  http://a/b/
      ../g          =  http://a/b/g
      ../..         =  http://a/
      ../../        =  http://a/
      ../../g       =  http://a/g

C.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 above.

   An empty reference refers to the start of the current document.

      <>            =  (current document)

   Parsers must be careful in handling the case where there are more
   relative path ".." segments 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.

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      ../../../g    =  http://a/../g
      ../../../../g =  http://a/../../g

   In practice, some implementations strip leading relative symbolic
   elements (".", "..") after applying a relative URI calculation, based
   on the theory that compensating for obvious author errors is better
   than allowing the request to fail.  Thus, the above two references
   will be interpreted as "http://a/g" by some implementations.

   Similarly, parsers must avoid treating "." and ".." as special when
   they are not complete components of a relative path.

      /./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 URI 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 a relative path before merging it with the
   base path.  This error is rarely noticed, since 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 URI 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 backwards compatibility.

      http:g        =  http:g           ; for validating parsers
                    /  http://a/b/c/g   ; for backwards compatibility

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Appendix D. Embedding the Base URI in HTML documents

   It is useful to consider an example of how the base URI of a document
   can be embedded within the document's content.  In this appendix, we
   describe how documents written in the Hypertext Markup Language
   (HTML) [HTML] can include an embedded base URI.  This appendix does
   not form a part of the URI specification and should not be considered
   as anything more than a descriptive example.

   HTML defines a special element "BASE" which, when present in the
   "HEAD" portion of a document, signals that the parser should use the
   BASE element's "HREF" attribute as the base URI for resolving any
   relative URI.  The "HREF" attribute must be an absolute URI.  Note
   that, in HTML, element and attribute names are case-insensitive.  For

      <!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN">
      <TITLE>An example HTML document</TITLE>
      <BASE href="">
      ... <A href="../x">a hypertext anchor</A> ...

   A parser reading the example document should interpret the given
   relative URI "../x" as representing the absolute URI


   regardless of the context in which the example document was obtained.

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Appendix E. Recommendations for Delimiting 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 electronic mail, USENET news messages, and, most importantly,
   printed on 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, URI are delimited in a variety of ways, but usually
   within double-quotes "", angle brackets <http://>, or just using whitespace

   These wrappers do not form part of the URI.

   In the case where a fragment identifier is associated with a URI
   reference, the fragment would be placed within the brackets as well
   (separated from the URI with a "#" character).

   In some cases, extra whitespace (spaces, linebreaks, tabs, etc.) may
   need to be added to break a long URI across lines. The whitespace
   should be ignored when extracting the URI.

   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 a line, the interpreter of a
   URI containing a line break immediately after a hyphen should ignore
   all unescaped 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 URI that contains 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.

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   For example, the text:

      Yes, Jim, I found it under "",
      but you can probably pick it up from <ftp://ds.internic.
      net/rfc/>.  Note the warning in <

   contains the URI references

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Appendix F. Abbreviated URIs

   The URI syntax was designed for unambiguous reference to network
   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 abbreviated URI references.  That is, a reference consisting of
   only the authority and path portions of the identified resource, such

   or simply the DNS hostname on its own.  Such references are primarily
   intended for human interpretation rather than machine, with the
   assumption that context-based heuristics are sufficient to complete
   the URI (e.g., most hostnames beginning with "www" are likely to have
   a URI prefix of "http://").  Although there is no standard set of
   heuristics for disambiguating abbreviated URI references, many client
   implementations allow them to be entered by the user and
   heuristically resolved.  It should be noted that such heuristics may
   change over time, particularly when new URI schemes are introduced.

   Since an abbreviated URI has the same syntax as a relative URI path,
   abbreviated URI references cannot be used in contexts where relative
   URIs are expected.  This limits the use of abbreviated URIs to places
   where there is no defined base URI, such as dialog boxes and off-line

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Appendix G. Summary of Non-editorial Changes

G.1 Additions

   IPv6 literals have been added to the list of possible identifiers for
   the host portion of a server component, as described by [RFC2732],
   with the addition of "[" and "]" to the reserved, uric, and
   uric-no-slash sets.  Square brackets are now specified as reserved
   for the authority component, allowed within the opaque part of an
   opaque URI, and not allowed in the hierarchical syntax except for
   their use as delimiters for an IPv6reference 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 rather than continuing to be
   defined in terms of uric.

   Since [RFC2732] defers to [RFC2373] for definition of an IPv6 literal
   address, which unfortunately has an incorrect ABNF description of
   IPv6address, we created a new ABNF rule for IPv6address that matches
   the text representations defined by Section 2.2 of [RFC2373].
   Likewise, the definition of IPv4address has been improved in order to
   limit each decimal octet to the range 0-255, and the definition of
   hostname has been improved to better specify length limitations and
   partially-qualified domain names.

   Section 6 on URI normalization and comparison has been completely
   rewritten and extended using input from Tim Bray and discussion
   within the W3C Technical Architecture Group.

G.2 Modifications from RFC 2396

   The ad-hoc BNF syntax 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. Likewise, absoluteURI
   and relativeURI have been changed to absolute-URI and relative-URI,
   respectively, for consistency.

   The ABNF of hier-part and relative-URI (Section 3) has been corrected
   to allow a relative URI path 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 the "about:" URI
   used by many browser implementations.

   The ABNF of qualified has been simplified to remove a parsing
   ambiguity without changing the allowed syntax.

   The resolving relative references algorithm of [RFC2396] has been
   rewritten using pseudocode for this revision to improve clarity and

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   fix the following issues:

   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 the reference
      contains an empty path and a defined query component, then the
      target URI inherits the base URI's path component.

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   abs-path  14
   absolute-URI  14
   absolute-URI-reference  20
   alphanum  17
   authority  15

   dec-octet  17
   delims  12
   domainlabel  17

   escaped  11

   fragment  20

   h4  18
   hier-part  14
   host  16
   hostname  17
   hostport  16

   IPv4  17
   IPv4address  17
   IPv6  18
   IPv6address  18
   IPv6reference  18

   ls32  18

   mark  11

   net-path  14

   opaque-part  14

   path  18

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   path-segments  18
   pchar  18
   port  16

   qualified  17
   query  19

   reg-name  16
   rel-path  22
   rel-segment  22
   relative-URI  22
   reserved  10

   scheme  15
   segment  18
   server  16

   toplabel  17

   unreserved  11
   unwise  12
   URI grammar
      abs-path  14
      absolute-URI  14
      absolute-URI-reference  20
      alphanum  17
      authority  15
      dec-octet  17
      delims  12
      domainlabel  17
      escaped  11
      fragment  20
      h4  18
      hier-part  14
      host  17
      hostname  17
      hostport  17
      IPv4address  17
      IPv6address  18
      IPv6reference  18
      ls32  18
      mark  11
      net-path  14

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      opaque-part  14
      path  18
      path-segments  18
      pchar  18
      port  17
      qualified  17
      query  19
      reg-name  16
      rel-path  22
      rel-segment  22
      relative-URI  22
      reserved  10
      scheme  15
      segment  18
      server  16
      toplabel  17
      unreserved  11
      unwise  12
      URI-reference  20
      uric  9
      uric-no-slash  14
      userinfo  16
   URI-reference  20
   uric  9
   uric-no-slash  14
   userinfo  16

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Intellectual Property Statement

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

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