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Versions: (RFC 2396) 00 01 02 03 04 05 06 07 RFC 3986

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: August 16, 2004                                     L. Masinter
                                                       February 16, 2004

           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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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on August 16, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   A Uniform Resource Identifier (URI) is a compact string of characters
   for identifying 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, such that an implementation can 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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Editorial Note

   Discussion of this draft and comments to the editors should be sent
   to the uri@w3.org mailing list.  An issues list and version history
   is available at <http://gbiv.com/protocols/uri/rev-2002/issues.html>.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1   Overview of URIs . . . . . . . . . . . . . . . . . . . . . .  4
   1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . . . .  5
   1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   1.1.3 URI, URL, and URN  . . . . . . . . . . . . . . . . . . . . .  6
   1.2   Design Considerations  . . . . . . . . . . . . . . . . . . .  6
   1.2.1 Transcription  . . . . . . . . . . . . . . . . . . . . . . .  6
   1.2.2 Separating Identification from Interaction . . . . . . . . .  7
   1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . . . .  9
   1.3   Syntax Notation  . . . . . . . . . . . . . . . . . . . . . . 10
   2.    Characters . . . . . . . . . . . . . . . . . . . . . . . . . 11
   2.1   Percent Encoding . . . . . . . . . . . . . . . . . . . . . . 11
   2.2   Reserved Characters  . . . . . . . . . . . . . . . . . . . . 12
   2.3   Unreserved Characters  . . . . . . . . . . . . . . . . . . . 12
   2.4   When to Encode or Decode . . . . . . . . . . . . . . . . . . 13
   3.    Syntax Components  . . . . . . . . . . . . . . . . . . . . . 15
   3.1   Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   3.2   Authority  . . . . . . . . . . . . . . . . . . . . . . . . . 16
   3.2.1 User Information . . . . . . . . . . . . . . . . . . . . . . 16
   3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   3.3   Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   3.4   Query  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   3.5   Fragment . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   4.    Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   4.1   URI Reference  . . . . . . . . . . . . . . . . . . . . . . . 24
   4.2   Relative URI . . . . . . . . . . . . . . . . . . . . . . . . 24
   4.3   Absolute URI . . . . . . . . . . . . . . . . . . . . . . . . 25
   4.4   Same-document Reference  . . . . . . . . . . . . . . . . . . 25
   4.5   Suffix Reference . . . . . . . . . . . . . . . . . . . . . . 25
   5.    Reference Resolution . . . . . . . . . . . . . . . . . . . . 27
   5.1   Establishing a Base URI  . . . . . . . . . . . . . . . . . . 27
   5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 27
   5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 28
   5.1.3 Base URI from the Retrieval URI  . . . . . . . . . . . . . . 28
   5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 28

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   5.2   Relative Resolution  . . . . . . . . . . . . . . . . . . . . 28
   5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . . . . 29
   5.2.2 Transform References . . . . . . . . . . . . . . . . . . . . 29
   5.2.3 Merge Paths  . . . . . . . . . . . . . . . . . . . . . . . . 30
   5.2.4 Remove Dot Segments  . . . . . . . . . . . . . . . . . . . . 30
   5.3   Component Recomposition  . . . . . . . . . . . . . . . . . . 32
   5.4   Reference Resolution Examples  . . . . . . . . . . . . . . . 33
   5.4.1 Normal Examples  . . . . . . . . . . . . . . . . . . . . . . 33
   5.4.2 Abnormal Examples  . . . . . . . . . . . . . . . . . . . . . 33
   6.    Normalization and Comparison . . . . . . . . . . . . . . . . 35
   6.1   Equivalence  . . . . . . . . . . . . . . . . . . . . . . . . 35
   6.2   Comparison Ladder  . . . . . . . . . . . . . . . . . . . . . 36
   6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 36
   6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 37
   6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 38
   6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 39
   6.3   Canonical Form . . . . . . . . . . . . . . . . . . . . . . . 39
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 41
   7.1   Reliability and Consistency  . . . . . . . . . . . . . . . . 41
   7.2   Malicious Construction . . . . . . . . . . . . . . . . . . . 41
   7.3   Back-end Transcoding . . . . . . . . . . . . . . . . . . . . 42
   7.4   Rare IP Address Formats  . . . . . . . . . . . . . . . . . . 42
   7.5   Sensitive Information  . . . . . . . . . . . . . . . . . . . 43
   7.6   Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 43
   8.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 45
         Normative References . . . . . . . . . . . . . . . . . . . . 46
         Informative References . . . . . . . . . . . . . . . . . . . 47
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 48
   A.    Collected ABNF for URI . . . . . . . . . . . . . . . . . . . 50
   B.    Parsing a URI Reference with a Regular Expression  . . . . . 52
   C.    Delimiting a URI in Context  . . . . . . . . . . . . . . . . 53
   D.    Summary of Non-editorial Changes . . . . . . . . . . . . . . 55
   D.1   Additions  . . . . . . . . . . . . . . . . . . . . . . . . . 55
   D.2   Modifications from RFC 2396  . . . . . . . . . . . . . . . . 55
         Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
         Intellectual Property and Copyright Statements . . . . . . . 62

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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 identifiers
   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"

   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]. 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 "character
   encoding" in accordance with the definitions provided in [RFC2978].

1.1 Overview of URIs

   URIs are characterized as follows:


      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.


      Anything that can be named or described can be a resource.
      Familiar examples include an electronic document, an image, a
      service (e.g., "today's weather report for Los Angeles"), and a

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      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 or the types of a relationship (e.g.,
      "parent" or "employee").


      An identifier embodies the information required to distinguish
      what is being identified from all other things within its scope of

   A URI is an identifier that consists of a sequence of characters
   matching the syntax defined by the syntax rule named "URI" in Section
   3. A URI can be used to refer to a resource. This specification does
   not place any limits on the nature of a resource or the reasons why
   an application might wish to refer to a resource.  URIs have a global
   scope and should be interpreted consistently regardless of context,
   but that interpretation may be defined in relation to the user's
   context (e.g., "http://localhost/" refers to a resource that is
   relative to the user's network interface and yet not specific to any
   one user).

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 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.  Likewise, protocols and data
   formats that make use of URI references can refer to this
   specification as defining the range of syntax allowed for all URIs,
   including those schemes that have yet to be defined.

   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.

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1.1.2 Examples

   The following examples illustrate URIs that are in common use.






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 need 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 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].

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
   integers from 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.

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   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 to be able to be
      entered into a computer, within the constraints imposed by
      keyboards (and related input devices) across languages and

   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 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 in 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 such
   representation is defined by the scheme or by the protocol element in
   which the URI is referenced; such a definition will specify the
   character encoding scheme 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.  In fact, the URI alone only provides
   identification; access to the resource is neither guaranteed nor
   implied by the presence of a URI.  Instead, an operation (if any)
   associated with a URI reference is defined by the protocol element,
   data format attribute, or natural language text in which it appears.

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   Given a URI, a system may attempt to perform a variety of operations
   on the resource, as might be characterized by such words 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; such 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 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 that
   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 systems are designed to be
   late-binding: the result of an access is generally determined at the
   time it is accessed and may vary over time or due to other aspects of
   the interaction. When an author creates a reference to such a
   resource, they do so with the intention that the reference 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 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 such a URI 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, 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 origin server
   when a representation isn't found in a local cache).

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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 for the purpose of delimiting 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
   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 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 using relative references. Furthermore, such
   document trees can be moved, as a whole, without changing any of the
   relative references.

   A relative URI 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, presented in Section 5, defines how such a reference is
   transformed to the target URI. Since 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.

   All URIs are parsed by generic syntax parsers when used. A URI scheme
   that wishes to remain opaque to hierarchical processing must disallow
   the use of slash and question mark characters.  However, since a
   non-relative URI reference is only modified by the generic parser if
   it contains complete path segments of "." or ".." (see Section 3.3),

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   URIs may safely use "/" for other purposes if they do not allow

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),
   CTL (control characters), DIGIT (decimal digits), DQUOTE (double
   quote), HEXDIG (hexadecimal digits), LF (line feed), and SP (space).
   The complete URI syntax is collected in Appendix A.

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2. Characters

   Although ABNF notation defines its terminal values to be non-negative
   integers (codepoints) based on the US-ASCII coded character set
   [ASCII], we must invert that relation in order to understand the URI
   syntax, since URIs are defined as strings of characters independent
   of any particular encoding.  Therefore, the integer values must be
   mapped back to their corresponding characters via US-ASCII in order
   to complete the syntax rules.

   This specification does not mandate the use of any particular
   character encoding scheme 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; absent such a definition, a URI is assumed to use the same
   character encoding as the surrounding text.

   A URI is composed from a limited set of characters consisting of
   digits, letters, and a few graphic symbols. A reserved (Section 2.2)
   subset of those characters may be used to delimit syntax components
   within a URI, while the remaining characters, including both the
   unreserved (Section 2.3) set and those reserved characters not acting
   as delimiters, define each component's 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).

      pct-encoded = "%" HEXDIG HEXDIG

   The uppercase hexadecimal digits 'A' through 'F' are equivalent to
   the lowercase digits 'a' through 'f', respectively.  Two URIs that
   differ only in the case of hexadecimal digits used in percent-encoded
   octets are equivalent.  For consistency, URI producers and
   normalizers should use uppercase hexadecimal digits for all

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2.2 Reserved Characters

   URIs include components and sub-components 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 forming the URI.

      reserved    = gen-delims / sub-delims

      gen-delims  = ":" / "/" / "?" / "#" / "[" / "]" / "@"

      sub-delims  = "!" / "$" / "&" / "'" / "(" / ")"
                  / "*" / "+" / "," / ";" / "="

   A subset of the reserved characters (gen-delims) are 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 those reserved
   characters that are allowed within that component (i.e., not
   delimiting it).  The allowed reserved characters, including those in
   the sub-delims set and any of the gen-delims that are not a delimiter
   of that component, are reserved for use as sub-component delimiters
   within the component.  Only the most common sub-components are
   defined by this specification; other sub-components may be defined by
   a URI scheme's specification, or by the implementation-specific
   syntax of a URI's dereferencing algorithm, provided that such
   sub-components are delimited by characters in that component's
   reserved set.  If no such delimiting role has been assigned, then a
   reserved character appearing in a component represents the data octet
   corresponding to its encoding in US-ASCII.

   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.

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 octet are equivalent: they identify
   the same resource.  However, percent-encoded unreserved characters
   may change the result of some URI comparisons (Section 6),
   potentially leading to incorrect or inefficient behavior. 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 character by URI normalizers.

2.4 When to Encode or Decode

   Under normal circumstances, the only time that octets within a URI
   are percent-encoded is during the process of producing the URI from
   its component parts.  It is during that process that an
   implementation determines which of the reserved characters are to be
   used as sub-component 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 sub-components
   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, since 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 software; 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" in order
   for that octet to be used as data within a URI.  Implementations must
   not percent-encode or decode the same string more than once, since
   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

   URI characters serve as an external interface for identification
   between systems.  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 [RFC3629] (or some other superset of the US-ASCII character
   encoding) at an internal interface, since that results in more
   meaningful identifiers than simply percent-encoding the original
   octets. When interpreting an incoming URI on such an interface,

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   percent-encoded octets must be decoded before the reverse transcoding
   can be applied.

   In some cases, the interface between a URI component and the
   identifying data 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, numeric coordinates on a
   map, etc.  Likewise, a URI scheme may define components with
   additional encoding requirements, such as base64, that are applied
   prior to forming the component and producing the URI.

   When a URI scheme defines a component that represents textual data
   consisting of characters from the Unicode (ISO/IEC 10646-1) character
   set, the data should be encoded first as octets according to the
   UTF-8 character encoding [RFC3629], and 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".

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3. Syntax Components

   The generic URI syntax consists of a hierarchical sequence of
   components referred to as the scheme, authority, path, query, and

      URI = scheme ":" ["//" authority] path ["?" query] ["#" fragment]

   The scheme and path components are required, though path may be empty
   (no characters).  An ABNF-driven parser will find that the border
   between authority and path is ambiguous; they are disambiguated by
   the "first-match-wins" (a.k.a. "greedy") algorithm.  In other words,
   if authority is present then the first segment of the path must be

   The following are two example URIs and their component parts:

         \_/   \______________/\_________/ \_________/ \__/
          |           |            |            |        |
       scheme     authority       path        query   fragment
          |   _____________________|__
         / \ /                        \

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 scheme is
   case-insensitive, the canonical form is lowercase and documents that
   specify schemes must do so using 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

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   [RFC2717].  The scheme registry maintains the mapping between scheme
   names and their specifications. Advice for designers of new URI
   schemes can be found in [RFC2718].

   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 that might indicate 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, such 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.

      authority   = [ userinfo "@" ] host [ ":" port ]

   URI producers and normalizers should omit the "@" delimiter that
   separates userinfo from host if the userinfo component is empty (zero
   length) and 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 sub-components.

3.2.1 User Information

   The userinfo sub-component 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
   sub-component unless such data is the empty string (indicating no
   password) or "anonymous". Applications may choose to ignore or reject
   such data when received as part of a reference, and should reject the

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   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

3.2.2 Host

   The host sub-component of authority is identified by an IP literal
   encapsulated within square brackets, an IPv4 address in
   dotted-decimal form, or a host name.

      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.  Again, the
   "first-match-wins" algorithm applies: 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 host 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 optional version flag may be used to indicate such a format
   explicitly rather than relying on heuristic determination.

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

      IPvFuture  = "v" 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 existing IPv4
   and IPv6 literal addresses. 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".

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   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]. 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

      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 a registered name is a string of characters that
   is intended for lookup within a locally-defined host or service name
   registry.  The most common of such registry mechanisms is the Domain
   Name System (DNS), as defined by Section 3 of [RFC1034] and Section
   2.1 of [RFC1123].  A DNS 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 followed by one if it
   is necessary to distinguish between the complete domain name and some
   local domain.

      reg-name    = 0*255( unreserved / pct-encoded / sub-delims )

   If the host component is defined and the registered name is empty
   (zero length), then the name defaults to "localhost" (Section 6.2.3
   discusses how this should be normalized). If "localhost" is not
   determined by a host name lookup, then it should be interpreted to
   mean the machine on which the URI is being resolved.

   This specification does not mandate a particular registered name
   lookup technology and therefore does not restrict the syntax of
   reg-name beyond that necessary for interoperability.  Instead, it
   delegates the issue of host 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 host and service names. However, a globally-scoped naming system,
   such as DNS fully-qualified domain names, is necessary for URIs that
   are intended to have global scope. URI producers should use host
   names that conform to the DNS syntax, even when use of DNS is not
   immediately apparent.

   The reg-name syntax allows percent-encoded octets in order to
   represent non-ASCII host or service names in a uniform way that is
   independent of the underlying name resolution technology; such octets
   must represent characters encoded in the UTF-8 character encoding
   [RFC3629] prior to being percent-encoded. When a non-ASCII host name
   represents an internationalized domain name intended for resolution
   via DNS, the name must be transformed to the IDNA encoding [RFC3490]
   prior to name lookup. URI producers should provide such host names in
   the IDNA encoding, rather than a percent-encoding, if they wish to
   maximize interoperability with legacy URI resolvers.

   The presence of host 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 of reusing the existing

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   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.

3.2.3 Port

   The port sub-component 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, etc.) is defined by the URI scheme.  URI producers
   and normalizers should omit the port component and its ":" delimiter
   if port is empty or its value would be the same as the scheme's

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).  If a URI contains an
   authority component, then the initial path segment must be empty
   (i.e., the path must begin with a slash ("/") character or be
   entirely empty).  The path is terminated by the first question mark
   ("?") or number sign ("#") character, or by the end of the URI.

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

      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 ".." 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) for indicating

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   relative position within the hierarchical tree of names.  This is
   similar to their role within some operating systems' file directory
   structure to indicate the current directory and parent directory,
   respectively. However, unlike 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 for the
   purpose of delimiting scheme-specific or dereference-handler-specific
   sub-components. For example, the semicolon (";") and equals ("=")
   reserved characters are often used for delimiting 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 like "name;v=1.1" to indicate a
   reference to version 1.1 of "name", whereas another might use a
   segment like "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.

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

   The characters slash ("/") and question mark ("?") may represent data
   within the query component, but should not be used as such within a
   URI that is expected to be the base for relative references (Section
   5.1).  Incorrect implementations of reference resolution often fail
   to distinguish query data from path data when looking for
   hierarchical separators, thus resulting in non-interoperable results.
   However, since 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.

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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. Individual media types may define their own
   restrictions on, or structure 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 such that it corresponds to the same
   secondary resource, regardless of how it is represented, or the
   fragment should be left undefined by the representation (i.e., not

   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

   Fragment identifiers have a special role in information systems as
   the primary form of client-side indirect referencing, allowing an
   author to specifically identify those aspects of an existing resource
   that are only indirectly provided by the resource owner. As such,
   interpretation of the fragment identifier during a retrieval action
   is performed solely by the user agent; the fragment identifier is not
   passed to other systems during the process of retrieval. Although
   this is often perceived to be a loss of information, particularly in
   regards to accurate redirection of references as content moves over
   time, it also serves to prevent information providers from denying

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   reference authors the right to selectively refer to information
   within a resource.

   The characters slash ("/") and question mark ("?") are allowed to
   represent data within the fragment identifier, but should not be used
   as such within a URI that is expected to be the base for relative
   references (Section 5.1) for the same reasons as described above for

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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. In order to
   save space and take advantage of hierarchical locality, many Internet
   protocol elements and media type formats allow an abbreviation of a
   URI, while 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

      URI-reference = URI / relative-URI

   A URI-reference may be relative: if the reference's prefix matches
   the syntax of a scheme followed by its colon separator, then the
   reference is a URI rather than a relative-URI.

   A URI-reference is typically parsed first into the five URI
   components, in order to determine what components are present and
   whether or not the reference is relative, and 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 URI

   A relative URI reference takes advantage of the hierarchical syntax
   (Section 1.2.3) in order to express a reference that is relative to
   the name space of another hierarchical URI.

      relative-URI  = ["//" authority] path ["?" query] ["#" fragment]

   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

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   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
   because 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.

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 ":" ["//" authority] path ["?" query]

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 the purpose of 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 Section 6.2.2 and Section 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

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,

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   newspapers, billboards, etc.) have increasingly used a suffix of the
   URI as a reference, consisting of only the authority and path
   portions of the URI, such as


   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 host 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.

   While this practice of using suffix references is common, it should
   be avoided whenever possible and never 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

   Since 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
   those places where there is no defined base URI, such as dialog boxes
   and off-line advertisements.

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5. Reference Resolution

   This section defines the process of resolving a URI reference within
   a context that allows relative references, such 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 there exists a "base URI" 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.

   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

      |  .----------------------------------------------------.  |
      |  |  .----------------------------------------------.  |  |
      |  |  |  .----------------------------------------.  |  |  |
      |  |  |  |  .----------------------------------.  |  |  |  |
      |  |  |  |  |       <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 within Document Content

   Within certain media types, a base URI for relative references 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 specification to specify how, for each

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   media type, a base URI can be embedded.  The appropriate syntax, when
   available, is described by each media type's specification.

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.

   A mechanism for embedding a 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 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. Since this definition is
   necessarily application-dependent, failing to define a base URI using
   one of the other methods may result in the same content being
   interpreted differently by different types of application.

   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

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 componets
   of the reference's target.  The components can then be recomposed, as
   described in Section 5.3, to form the target URI. This algorithm

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   provides definitive results that can be used to test the output of
   other implementations.  Applications may implement relative reference
   resolution using some other algorithm, provided that the results
   match what would be given by this algorithm.

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 Section 6.2.2 and
   Section 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

      if defined(R.scheme) then
         T.scheme    = R.scheme;
         T.authority = R.authority;
         T.path      = remove_dot_segments(R.path);
         T.query     = R.query;
         if defined(R.authority) then
            T.authority = R.authority;
            T.path      = remove_dot_segments(R.path);
            T.query     = R.query;
            if (R.path == "") then
               T.path = Base.path;

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               if defined(R.query) then
                  T.query = R.query;
                  T.query = Base.query;
               if (R.path starts-with "/") then
                  T.path = remove_dot_segments(R.path);
                  T.path = merge(Base.path, R.path);
                  T.path = remove_dot_segments(T.path);
               T.query = R.query;
            T.authority = Base.authority;
         T.scheme = Base.scheme;

      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,

   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 a two string

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   1.  The input buffer is initialized with the now-appended path
       components and the output buffer is initialized to the empty

   2.  Replace any prefix of "./" or "../" at the beginning of the input
       buffer with "/".

   3.  While the input buffer is not empty, loop:

       1.  If the input buffer begins with a prefix of "/./" or "/.",
           where "." is a complete path segment, then replace that
           prefix with "/"; otherwise

       2.  If the input buffer begins with a prefix of "/../" or "/..",
           where ".." is a complete path segment, then replace that
           prefix with "/" and remove the last segment and its preceding
           "/" (if any) from the output buffer; otherwise

       3.  Remove the first segment and its preceding "/" (if any) from
           the input buffer and append them to the output buffer.

   4.  Finally, the output buffer is returned as the result of

   The following illustrates how the above steps are applied for two
   example merged paths, showing the state of the two buffers after each


       1 :                         /a/b/c/./../../g
       3c:   /a                    /b/c/./../../g
       3c:   /a/b                  /c/./../../g
       3c:   /a/b/c                /./../../g
       3a:   /a/b/c                /../../g
       3b:   /a/b                  /../g
       3b:   /a                    /g
       3c:   /a/g


       1 :                         mid/content=5/../6
       3c:   mid                   /content=5/../6
       3c:   mid/content=5         /../6
       3b:   mid                   /6
       3c:   mid/6

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   Some applications may find it more efficient to implement the
   remove_dot_segments algorithm using two segment stacks rather than

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

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;

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

      append path to result;

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

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

      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.

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5.4 Reference Resolution Examples

   Within a representation with a well-defined base URI of


   a relative URI reference is transformed to its target URI as follows.

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 above.

   Parsers must be careful in handling cases 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.

      "../../../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 URI reference uses
   unnecessary or nonsensical forms of the "." and ".." complete path

      "./../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 and removing dot-segments.  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
   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

      "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 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 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 the implementation-specific syntax of each URI's
   dereferencing algorithm. 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. 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 URI references; the references should be converted to their
   target URI forms before comparison.  When URIs are being compared for
   the purpose of selecting (or avoiding) a network action, such as
   retrieval of a representation, the 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 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
   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, 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, 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 (UTF-16), bit-for-bit comparisons applied naively will produce
   both false-positive and false-negative errors.  It is better to speak
   of equality on a character-for-character rather than 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.

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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, encoding normalization, empty-component
   normalization, and removal of dot-segments. Case Normalization

   When a URI scheme uses components of the generic syntax, it will also
   use the common syntax equivalence rules, 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/>.  Applications should not
   assume anything about the case sensitivity of other URI components,
   since that is dependent on the implementation used to handle a

   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. Encoding Normalization

   The percent-encoding mechanism (Section 2.1) is a frequent source of
   variance among otherwise identical URIs. In addition to the
   case-insensitivity 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. Such
   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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   Components of the generic URI syntax are delimited from other
   components by optional separators.  For example, a query component is
   separated from the path by a question mark ("?") and a port
   sub-component is separated from host by a colon (":").  A URI in
   which a delimiter is present and the (sub-)component it delimits is
   empty is equivalent to the same URI without that delimiter.  For
   example, the following are all equivalent:


   URI producers and normalizers should omit a delimiter if the
   component it delimits is empty, as exemplified by the first URI
   above, with one exception: a double-slash delimiter indicating an
   authority component should not be removed, even when the authority is
   empty, since doing so can lead to misinterpreting the path. Path Segment Normalization

   The complete path segments "." and ".." have a special meaning within
   hierarchical URI schemes.  As such, they should not appear in
   absolute paths; if they are found, they can be removed by applying
   the remove_dot_segments algorithm to the path, as described in
   Section 5.2.

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, since 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:


   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", where the port is empty or the default for the

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   scheme, is equivalent to one where the port and its ":" delimiter are
   elided. In other words, the second of the above URI examples is the
   normal form for the "http" scheme.

   Another case where normalization varies by scheme is in the handling
   of an empty authority component.  For many scheme specifications, an
   empty authority is considered an error; for others, it is considered
   equivalent to "localhost".  For the sake of uniformity, future scheme
   specifications should define an empty authority as being equivalent
   to "localhost", and URI producers and normalizers should use
   "localhost" instead of an empty authority.

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 a URI differing only in the trailing slash


   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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6.3 Canonical Form

   It is in the best interests of everyone to avoid false-negatives in
   comparing URIs and to minimize the amount of software processing for
   such comparisons.  Those who produce and make 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:

   o  Always provide the URI scheme in lowercase characters.

   o  Always provide the host, if any, in lowercase characters.

   o  Only perform percent-encoding where it is essential.

   o  Always use uppercase A-through-F characters when percent-encoding.

   o  Prevent /./ and /../ from appearing in non-relative URI paths.

   o  Omit delimiters when their associated (sub-)component is empty.

   o  For schemes that define an empty authority to be equivalent to
      "localhost", use "localhost".

   o  For schemes that define an empty path to be equivalent to a path
      of "/", use "/".

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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, 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 name space or maintains it
   over time.  Such a guarantee can only be obtained from the person(s)
   controlling that name space 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

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, 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
   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.

   When a URI contains percent-encoded octets that match the delimiters
   for a given resolution or dereference protocol (for example, CR and

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   LF characters for the TELNET protocol), such percent-encoded octets
   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 sub-components prior to decoding the octets, since
   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 filesystem or related system
   functions. Filesystems typically assign an operational meaning to
   special characters, such as the "/", "\", ":", "[", and "]"
   characters, and 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 application 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 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, literals should be converted to
   numeric form and filtered based on the numeric value, rather than 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 which 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.

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7.6 Semantic Attacks

   Because the userinfo sub-component is rarely used and appears before
   the host 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 host is
   'trusted.example.com', whereas it is actually ''.  Note that
   a misleading userinfo sub-component 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 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. Acknowledgments

   This specification is derived from RFC 2396 [RFC2396], RFC 1808
   [RFC1808], and RFC 1738 [RFC1738]; the acknowledgments 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, Tony Hammond, Pat Hayes,
   Henry Holtzman, Ian B. Jacobs, Michael Kay, John C. Klensin, Graham
   Klyne, Dan Kohn, Bruce Lilly, Andrew Main, Ira McDonald, Michael
   Mealling, Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin,
   Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne, Stuart
   Williams, and Henry Zongaro 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.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

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Informative References

   [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

   [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]  Masinter, L. and K. Sollins, "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.

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

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

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

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

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

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

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

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, October 2000.

   [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/

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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
   EMail: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/

   Roy T. Fielding
   Day Software
   5251 California Ave., Suite 110
   Irvine, CA  92612-3074

   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

   Phone: +1-408-536-3024
   EMail: LMM@acm.org
   URI:   http://larry.masinter.net/

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Appendix A. Collected ABNF for URI

    URI = scheme ":" ["//" authority] path ["?" query] ["#" fragment]

    URI-reference = URI / relative-URI

    relative-URI  = ["//" authority] path ["?" query] ["#" fragment]

    absolute-URI  = scheme ":" ["//" authority] path ["?" query]

    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" 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

    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      = 0*255( unreserved / pct-encoded / sub-delims )

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

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

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

    pct-encoded   = "%" HEXDIG HEXDIG

    pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"

    unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"
    reserved      = gen-delims / sub-delims
    gen-delims    = ":" / "/" / "?" / "#" / "[" / "]" / "@"
    sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"
                  / "*" / "+" / "," / ";" / "="

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Appendix B. Parsing a URI Reference with a Regular Expression

   Since 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.

       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 = //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 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 5.3.

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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 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, URIs are delimited in a variety of ways, but usually
   within double-quotes "http://example.com/", angle brackets <http://
   example.com/>, or just using whitespace


   These wrappers do not form part of the URI.

   In some cases, extra whitespace (spaces, line-breaks, 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 whitespace around the line break, and should be aware that the
   hyphen may or may not actually be part of the URI.

   Using <> angle brackets around each URI is especially recommended as
   a delimiting style for a reference that contains embedded whitespace.

   The prefix "URL:" (with or without a trailing space) was formerly
   recommended as a way to help distinguish a URI from other bracketed
   designators, though it is not commonly used in practice and is no
   longer recommended.

   For robustness, software that accepts user-typed URI should attempt
   to recognize and strip both delimiters and embedded whitespace.

   For example, the text:

      Yes, Jim, I found it under "http://www.w3.org/Addressing/",
      but you can probably pick it up from <ftp://foo.example.
      com/rfc/>.  Note the warning in <http://www.ics.uci.edu/pub/

   contains the URI references

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Appendix D. Summary of Non-editorial Changes

D.1 Additions

   IPv6 (and later) literals have been added to the list of possible
   identifiers for the host portion of a 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 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 rather than be defined in terms of uric.

   Since [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 (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.

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

D.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.

   Section 2 on characters has been rewritten to explain what characters
   are reserved, when they are reserved, and why they are reserved even
   when 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 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 corresponding to unreserved characters.
   In general, the terms "escaped" and "unescaped" have been replaced
   with "percent-encoded" and "decoded", respectively, to reduce

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   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 significantly reduce complexity. As
   a result, the layout form of syntax description has been removed,
   along with the uric, uric_no_slash, hier_part, 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
   ambiguity regarding the parsing of URI-reference as a URI or a
   relative-URI with a colon in the first segment is now explained and
   disambiguated in the section defining relative-URI.

   The fragment identifier has been moved back into the section on
   generic syntax components and within the URI and relative-URI 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 a relative 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:" scheme used internally by many WWW browser
   implementations. The ambiguity regarding the boundary between
   authority and path is now explained and disambiguated in the same

   Registry-based naming authorities that use the generic syntax are now
   defined within the host rule and limited to 255 path characters. This
   change allows current implementations, where whatever name provided
   is simply fed to the local name resolution mechanism, to be
   consistent with the specification and removes the need to re-specify
   DNS name formats here.  It also 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 using pseudocode for this revision to improve clarity and
   fix the following issues:

   o  [RFC2396] section 5.2, step 6a, failed to account for a base URI
      with no path.

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   o  Restored the behavior of [RFC1808] where, if the reference
      contains an empty path and a defined query component, then the
      target URI inherits the base URI's path component.

   o  Removed the special-case treatment of same-document references
      within the URI parser in favor of a section that explains when a
      reference should be interpreted by a dereferencing engine as a
      same-document reference: when the target URI and base URI,
      excluding fragments, match.  This change does not modify the
      behavior of existing same-document references as defined by RFC
      2396 (fragment-only references); it merely adds the same-document
      distinction to other references that refer to the base URI and
      simplifies the interface between applications and their URI
      parsers, as is consistent with the internal architecture of
      deployed URI processing implementations.

   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 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.

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   ABNF  10
   absolute  25
   absolute-path  24
   absolute-URI  25
   access  7
   authority  15, 16

   base URI  27

   characters  11

   dec-octet  18
   dereference  7
   dot-segments  20

   fragment  22

   gen-delims  12
   generic syntax  5

   h16  17
   hierarchical  9
   host  17

   identifier  5
   IP-literal  17
   IPv4  18
   IPv4address  18
   IPv6  17
   IPv6address  17
   IPvFuture  17

   locator  6
   ls32  17

   merge  30

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   name  6
   network-path  24

   path  15, 20
   pchar  20
   pct-encoded  11
   percent-encoding  11
   port  20

   query  21

   reg-name  19
   registered name  19
   relative  9, 27
   relative-path  24
   relative-URI  24
   remove_dot_segments  30
   representation  8
   reserved  12
   resolution  7, 27
   resource  4
   retrieval  8

   same-document  25
   sameness  8
   scheme  15
   segment  20
   sub-delims  12
   suffix  25

   transcription  6

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   uniform  4
   unreserved  12
   URI grammar
      absolute-URI  25
      ALPHA  10
      authority  15, 16
      CR  10
      CTL  10
      dec-octet  18
      DIGIT  10
      DQUOTE  10
      fragment  15, 22, 24
      gen-delims  12
      h16  18
      HEXDIG  10
      host  16, 17
      IP-literal  17
      IPv4address  18
      IPv6address  17, 18
      IPvFuture  17
      LF  10
      ls32  18
      mark  12
      OCTET  10
      path  15
      path-segments  20
      pchar  20, 21, 22
      pct-encoded  11
      port  16, 20
      query  15, 21, 24, 25
      reg-name  19
      relative-URI  24, 24
      reserved  12
      scheme  15, 15, 25
      segment  20
      SP  10
      sub-delims  12
      unreserved  12
      URI  15, 24
      URI-reference  24
      userinfo  16, 16
   URI  15
   URI-reference  24
   URL  6
   URN  6
   userinfo  16

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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights. Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11. Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
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Internet-Draft             URI Generic Syntax              February 2004



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