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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
                                                             L. Masinter
Expires: November 21, 2003                                         Adobe
                                                            May 23, 2003


           Uniform Resource Identifier (URI): Generic Syntax
                    draft-fielding-uri-rfc2396bis-02

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
   <http://www.ietf.org/ietf/1id-abstracts.txt>.

   The list of Internet-Draft Shadow Directories can be accessed at
   <http://www.ietf.org/shadow.html>.

Copyright Notice

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

Abstract

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

   This document defines a grammar that is a superset of all valid URIs,
   such that an implementation can parse the common components of a URI
   reference without knowing the scheme-specific requirements of every
   possible identifier type.  This document does not define a generative
   grammar for all URIs; that task will be 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://www.apache.org/~fielding/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  . . . . . . . . . . . . . . . . . . . . . .  9
   2.    Characters . . . . . . . . . . . . . . . . . . . . . . . . . 10
   2.1   Encoding of Characters . . . . . . . . . . . . . . . . . . . 10
   2.2   Reserved Characters  . . . . . . . . . . . . . . . . . . . . 10
   2.3   Unreserved Characters  . . . . . . . . . . . . . . . . . . . 11
   2.4   Escaped Characters . . . . . . . . . . . . . . . . . . . . . 12
   2.4.1 Escaped Encoding . . . . . . . . . . . . . . . . . . . . . . 12
   2.4.2 When to Escape and Unescape  . . . . . . . . . . . . . . . . 12
   2.5   Excluded Characters  . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   3.3   Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   3.4   Query  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   3.5   Fragment . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   4.    Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   4.1   URI Reference  . . . . . . . . . . . . . . . . . . . . . . . 22
   4.2   Relative URI . . . . . . . . . . . . . . . . . . . . . . . . 22
   4.3   Absolute URI . . . . . . . . . . . . . . . . . . . . . . . . 23
   4.4   Same-document Reference  . . . . . . . . . . . . . . . . . . 23
   4.5   Suffix Reference . . . . . . . . . . . . . . . . . . . . . . 23
   5.    Relative Resolution  . . . . . . . . . . . . . . . . . . . . 25
   5.1   Establishing a Base URI  . . . . . . . . . . . . . . . . . . 25
   5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 26
   5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 26
   5.1.3 Base URI from the Retrieval URI  . . . . . . . . . . . . . . 27
   5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 27



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   5.2   Obtaining the Referenced URI . . . . . . . . . . . . . . . . 27
   5.3   Recomposition of a Parsed URI  . . . . . . . . . . . . . . . 29
   5.4   Examples of Relative Resolution  . . . . . . . . . . . . . . 30
   5.4.1 Normal Examples  . . . . . . . . . . . . . . . . . . . . . . 30
   5.4.2 Abnormal Examples  . . . . . . . . . . . . . . . . . . . . . 31
   6.    Normalization and Comparison . . . . . . . . . . . . . . . . 33
   6.1   Equivalence  . . . . . . . . . . . . . . . . . . . . . . . . 33
   6.2   Comparison Ladder  . . . . . . . . . . . . . . . . . . . . . 33
   6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 34
   6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 35
   6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 36
   6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 36
   6.3   Canonical Form . . . . . . . . . . . . . . . . . . . . . . . 36
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 38
   7.1   Reliability and Consistency  . . . . . . . . . . . . . . . . 38
   7.2   Malicious Construction . . . . . . . . . . . . . . . . . . . 38
   7.3   Rare IP Address Formats  . . . . . . . . . . . . . . . . . . 39
   7.4   Sensitive Information  . . . . . . . . . . . . . . . . . . . 39
   7.5   Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 39
   8.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 41
         Normative References . . . . . . . . . . . . . . . . . . . . 42
         Informative References . . . . . . . . . . . . . . . . . . . 43
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 45
   A.    Collected ABNF for URI . . . . . . . . . . . . . . . . . . . 46
   B.    Parsing a URI Reference with a Regular Expression  . . . . . 47
   C.    Embedding the Base URI in HTML documents . . . . . . . . . . 48
   D.    Delimiting a URI in Context  . . . . . . . . . . . . . . . . 49
   E.    Summary of Non-editorial Changes . . . . . . . . . . . . . . 51
   E.1   Additions  . . . . . . . . . . . . . . . . . . . . . . . . . 51
   E.2   Modifications from RFC 2396  . . . . . . . . . . . . . . . . 51
         Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
         Intellectual Property and Copyright Statements . . . . . . . 57



















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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"
   [RFC1737].

   This document obsoletes [RFC2396], which merged "Uniform Resource
   Locators" [RFC1738] and "Relative Uniform Resource Locators"
   [RFC1808] in order to define a single, generic syntax for all URIs.
   It excludes those portions of RFC 1738 that defined the specific
   syntax of individual URI schemes; those portions will be updated as
   separate documents. The process for registration of new URI schemes
   is defined separately by [RFC2717].

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

1.1 Overview of URIs

   URIs are characterized as follows:

   Uniform

      Uniformity provides several benefits: it allows different types of
      resource identifiers to be used in the same context, even when the
      mechanisms used to access those resources may differ; it allows
      uniform semantic interpretation of common syntactic conventions
      across different types of resource identifiers; it allows
      introduction of new types of resource identifiers without
      interfering with the way that existing identifiers are used; 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.

   Resource

      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
      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



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

   Identifier

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

   A URI is an identifier that consists of a sequence of characters
   matching the restricted syntax defined by this specification.  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.

      ftp://ftp.is.co.za/rfc/rfc1808.txt
         -- ftp scheme for File Transfer Protocol services

      gopher://gopher.tc.umn.edu:70/11/Mailing%20Lists/
         -- gopher scheme for Gopher and Gopher+ Protocol services

      http://www.ietf.org/rfc/rfc2396.txt
         -- http scheme for Hypertext Transfer Protocol services

      mailto:John.Doe@example.com
         -- mailto scheme for electronic mail addresses

      news:comp.infosystems.www.servers.unix
         -- news scheme for USENET news groups and articles

      telnet://melvyl.ucop.edu/
         -- telnet scheme for interactive TELNET services


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 the resource, provide a means of
   locating the resource by describing its primary access mechanism
   (e.g., its network "location").  The term "Uniform Resource Name"
   (URN) refers to the subset of URIs that are required to remain
   globally unique and persistent even when the resource ceases to exist
   or becomes unavailable.

   An individual scheme does not need to be 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.  This specification deprecates use of the term "URN" for
   anything but URIs in the "urn" scheme [RFC2141].  This specification
   also deprecates the term "URL".

1.2 Design Considerations

1.2.1 Transcription

   The URI syntax has been designed with global transcription as one of



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   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
   octets in a coded character set.  The interpretation of a URI depends
   only on the characters used and not how those characters are
   represented in a network protocol.

   The goal of 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
      locales.

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

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,



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   data format attribute, or natural language text in which it appears.

   Given a URI, a system may attempt to perform a variety of operations
   on the resource, as might be characterized by such words as "denote",
   "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.  Using that access mechanism to perform some action on
   the URI's resource is termed a "dereference" of 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 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.

   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
   decreasing order 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 URI syntax reserves the slash ("/"), question-mark ("?"), and
   crosshatch ("#") 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.

   An "absolute" URI refers to a resource independent of the naming
   hierarchy in which the identifier is used.  In contrast, a "relative"
   URI refers to a resource by describing the difference within a
   hierarchical name space between the current context and an absolute
   URI of the resource.  Section 4.2 defines a scheme-independent form
   of relative URI reference that can be used in conjunction with a base
   URI of a hierarchical scheme to produce the absolute URI form of that
   reference.

1.3 Syntax Notation

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

   The following core ABNF productions are used by this specification as
   defined by Section 6.1 of [RFC2234]: ALPHA, CR, CTL, DIGIT, DQUOTE,
   HEXDIG, LF, OCTET, and SP. The complete URI syntax is collected in
   Appendix A.









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2. Characters

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

      uric        = reserved / unreserved / escaped

   Within a URI, reserved characters are used to delimit syntax
   components, unreserved characters are used to describe registered
   names, and unreserved, non-delimiting reserved, and escaped
   characters are used to represent strings of data (1*OCTET) within the
   components.

2.1 Encoding of Characters

   As described above (Section 1.3), the URI syntax is defined in terms
   of characters by reference to the US-ASCII encoding of characters to
   octets.  This specification does not mandate the use of any
   particular mapping between its character set and the octets used to
   store or transmit those characters.

   URI characters representing strings of data within a component may,
   if allowed by the component production, represent an arbitrary
   sequence of octets.  For example, portions of a given URI might
   correspond to a filename on a non-ASCII file system, a query on
   non-ASCII data, numeric coordinates on a map, etc.  Some URI schemes
   define a specific encoding of raw data to US-ASCII characters as part
   of their scheme-specific requirements. Most URI schemes represent
   data octets by the US-ASCII character corresponding to that octet,
   either directly in the form of the character's glyph or by use of an
   escape triplet (Section 2.4).

   When a URI scheme defines a component that represents textual data
   consisting of characters from the Unicode (ISO 10646) character set,
   we recommend that the data be encoded first as octets according to
   the UTF-8 [UTF-8] character encoding, and then escaping any octets
   that are not in the unreserved character set.

2.2 Reserved Characters

   URIs include components and sub-components that are delimited by
   certain special characters.  These characters are called "reserved",
   since their usage within a URI component is limited to their reserved



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   purpose within that component.  If data for a URI component would
   conflict with the reserved purpose, then the conflicting data must be
   escaped (Section 2.4) before forming the URI.

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

   Reserved characters are used as delimiters of the generic URI
   components described in Section 3, as well as within those components
   for delimiting sub-components.  A component's ABNF syntax rule will
   not use the "reserved" production directly; instead, each rule lists
   those reserved characters that are allowed within that component.
   Allowed reserved characters that are not assigned a sub-component
   delimiter role by this specification should be considered reserved
   for special use by whatever software generates the URI (i.e., they
   may be used to delimit or indicate information that is significant to
   interpretation of the identifier, but that significance is outside
   the scope of this specification).  Outside of the URI's origin, a
   reserved character cannot be escaped without fear of changing how it
   will be interpreted; likewise, an escaped octet that corresponds to a
   reserved character cannot be unescaped outside the software that is
   responsible for interpreting it during URI resolution.

   The slash ("/"), question-mark ("?"), and crosshatch ("#") characters
   are reserved in all URI for the purpose of delimiting components that
   are significant to the generic parser's hierarchical interpretation
   of an identifier.  The hierarchical prefix of a URI, wherein the
   slash ("/") character signifies a hierarchy delimiter, extends from
   the scheme (Section 3.1) through to the first question-mark ("?"),
   crosshatch ("#"), or the end of the URI string. In other words, the
   slash ("/") character is not treated as a hierarchical separator
   within the query (Section 3.4) and fragment (Section 3.5) components
   of a URI, but is still considered reserved within those components
   for purposes outside the scope of this specification.

2.3 Unreserved Characters

   Data 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, and a limited set of punctuation marks and
   symbols.

      unreserved  = ALPHA / DIGIT / mark

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

   Unreserved characters can be escaped without changing the semantics
   of a URI, but this should not be done unless the URI is being used in



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   a context that does not allow the unescaped character to appear. URI
   normalization processes may unescape sequences in the ranges of ALPHA
   (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), underscore
   (%5F), or tilde (%7E) without fear of creating a conflict, but
   unescaping the other mark characters is usually counterproductive.

2.4 Escaped Characters

   Data must be escaped if it does not have a representation using an
   unreserved character; this includes data that does not correspond to
   a printable character of the US-ASCII coded character set or
   corresponds to a US-ASCII character that delimits the component from
   others, is reserved in that component for delimiting sub-components,
   or is excluded from any use within a URI (Section 2.5).

2.4.1 Escaped Encoding

   An escaped octet is encoded as a character triplet, consisting of
   the percent character "%" followed by the two hexadecimal digits
   representing that octet's numeric value.  For example, "%20" is the
   escaped encoding for the US-ASCII space character (SP).  This is
   sometimes referred to as "percent-encoding" the octet.

      escaped     = "%" 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 escaped octets
   are equivalent.  For consistency, we recommend that uppercase digits
   be used by URI generators and normalizers.

2.4.2 When to Escape and Unescape

   Under normal circumstances, the only time that characters within a
   URI string are escaped is during the process of generating the URI
   from its component parts.  Each component may have its own set of
   characters that are reserved, so only the mechanism responsible for
   generating or interpreting that component can determine whether or
   not escaping a character will change its semantics.  The exception is
   when a URI is being used within a context where the unreserved "mark"
   characters might need to be escaped, such as when used for a
   command-line argument or within a single-quoted attribute.

   Once generated, a URI is always in an escaped form.  When a URI is
   resolved, the components significant to that scheme-specific
   resolution process (if any) must be parsed and separated before the
   escaped characters within those components can be safely unescaped.




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   In some cases, data that could be represented by an unreserved
   character may appear escaped; for example, some of the unreserved
   "mark" characters are automatically escaped by some systems.  A URI
   normalizer may unescape escaped octets that are represented by
   characters in the unreserved set.  For example, "%7E" is sometimes
   used instead of tilde ("~") in an "http" URI path and can be
   converted to "~" without changing the interpretation of the URI.

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

2.5 Excluded Characters

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

      excluded    = invisible / delims / unwise

   The control characters (CTL) in the US-ASCII coded character set are
   not used within a URI, both because they are non-printable and
   because they are likely to be misinterpreted by some control
   mechanisms. The space character (SP) is excluded because significant
   spaces may disappear and insignificant spaces may be introduced when
   a URI is transcribed, typeset, or subjected to the treatment of
   word-processing programs.  Whitespace is also used to delimit a URI
   in many contexts. Characters outside the US-ASCII set are excluded as
   well.

      invisible   = CTL / SP / %x80-FF

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

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

   Other characters are excluded because gateways and other transport
   agents are known to sometimes modify such characters.

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



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

















































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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
   fragment.

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

      hier-part   = net-path / abs-path / rel-path

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

   The scheme and path components are required, though path may be empty
   (no characters).  An ABNF-driven parser of hier-part will find that
   the three productions in the rule are ambiguous: they are
   disambiguated by the "first-match-wins" (a.k.a. "greedy") algorithm.
   In other words, if the string begins with two slash characters ("//
   "), then it is a net-path; if it begins with only one slash
   character, then it is an abs-path; otherwise, it is a rel-path.  Note
   that rel-path does not necessarily contain any slash ("/")
   characters; a non-hierarchical path will be treated as opaque data by
   a generic URI parser.

   The authority component is only present when a string matches the
   net-path production.  Since the presence of an authority component
   restricts the remaining syntax for path, we have not included a
   specific "path" rule in the syntax.  Instead, what we refer to as the
   URI path is that part of the parsed URI string matching the abs-path
   or rel-path production in the syntax above, since they are mutually
   exclusive for any given URI and can be parsed as a single component.

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



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   the sake of robustness, but should only generate 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
   [RFC2717].  The scheme registry maintains the mapping between scheme
   names and their specifications.

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 domain
   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
   crosshatch ("#") character, or by the end of the URI.

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

   The parts "<userinfo>@" and ":<port>" may be omitted.

   Some schemes do not allow the userinfo and/or port sub-components.
   When presented with a URI that violates one or more scheme-specific
   restrictions, the scheme-specific URI 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 7.5).

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 server.  The user information, if
   present, is followed by a commercial at-sign ("@") that delimits it
   from the host.

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

   Some URI schemes use the format "user:password" in the userinfo



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

3.2.2 Host

   The host sub-component of authority is identified by an IPv6 literal
   encapsulated within square brackets, an IPv4 address in
   dotted-decimal form, or a domain name.

      host        = [ IPv6reference / IPv4address / hostname ]

   If host is omitted, a default may be defined by the scheme-specific
   semantics of the URI.  For example, the "file" URI scheme defaults to
   "localhost", whereas the "http" URI scheme does not allow host to be
   omitted.

   The production for host is ambiguous because it does not completely
   distinguish between an IPv4address and a hostname.  Again, the
   "first-match-wins" algorithm applies: If host matches the production
   for IPv4address, then it should be considered an IPv4 address literal
   and not a hostname.

   A hostname takes the form described in Section 3 of [RFC1034] and
   Section 2.1 of [RFC1123]: a sequence of domain labels separated by
   ".", each domain label starting and ending with an alphanumeric
   character and possibly also containing "-" characters.  The rightmost
   domain label of a fully qualified domain name may be followed by a
   single "." if it is necessary to distinguish between the complete
   domain name and some local domain.

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

   A host identified by an IPv4 literal address is represented in
   dotted-decimal notation (a sequence of four decimal numbers in the
   range 0 to 255, separated by "."), as described in [RFC1123] by
   reference to [RFC0952].  Note that other forms of dotted notation may
   be interpreted on some platforms, as described in Section 7.3, but
   only the dotted-decimal form of four octets is allowed by this
   grammar.

      IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet




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      dec-octet   = DIGIT                 ; 0-9
                  / %x31-39 DIGIT         ; 10-99
                  / "1" 2DIGIT            ; 100-199
                  / "2" %x30-34 DIGIT     ; 200-249
                  / "25" %x30-35          ; 250-255

   A host identified by an IPv6 literal address [RFC3513] is
   distinguished by enclosing the IPv6 literal within square-brackets
   ("[" and "]").  This is the only place where square-bracket
   characters are allowed in the URI syntax.

      IPv6reference = "[" IPv6address "]"

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

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

      h4          = 1*4HEXDIG

   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
   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 creator.

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

   If port is omitted, a default may be defined by the scheme-specific
   semantics of the URI.  Likewise, the type of network port designated
   by the port number (e.g., TCP, UDP, SCTP, etc.) is defined by the URI
   scheme. For example, the "http" URI scheme defines a default of TCP



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   port 80.

3.3 Path

   The path component contains hierarchical data that, along with data
   in the optional query (Section 3.4) component, serves to identify a
   resource within the scope of that URI's scheme and naming authority
   (if any).  There is no specific "path" syntax production in the
   generic URI syntax.  Instead, what we refer to as the URI path is
   that part of the parsed URI string matching either the abs-path or
   the rel-path production, since they are mutually exclusive for any
   given URI and can be parsed as a single component. The path is
   terminated by the first question-mark ("?") or crosshatch ("#")
   character, or by the end of the URI.

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

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

   The 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) or opaque (not containing any
   "/" delimiters).  For example, the URI <mailto:fred@example.com> has
   a path of "fred@example.com".

   Within a path segment, 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
   generator 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 meaning
   of a parameter is specific to the URI originator. Parameters are not
   significant to the parsing of relative references.

   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
   relative position within the hierarchical tree of names, with a
   similar effect to how they are used within some operating systems'
   file directory structure to indicate the current directory and parent
   directory, respectively.  Unlike a file system, however, these
   dot-segments are only interpreted within the URI path hierarchy and
   must be removed as part of the URI normalization or resolution
   process, in accordance with the process described in Section 5.2.



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3.4 Query

   The query component contains non-hierarchical data that, along with
   data in the path (Section 3.3) component, serves to identify a
   resource within the scope of that URI's scheme and naming authority
   (if any). The query component is indicated by the first question-mark
   ("?") character and terminated by a crosshatch ("#") character or by
   the end of the URI.

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

   The characters slash ("/") and question-mark ("?") are allowed to
   represent data within the query component, but such use is
   discouraged; incorrect implementations of relative URI resolution
   often fail to distinguish them from hierarchical separators, thus
   resulting in non-interoperable results while parsing relative
   references.  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 include those characters unescaped.

3.5 Fragment

   The fragment identifier component allows indirect identification of
   a secondary resource by reference to a primary resource and
   additional identifying information that is selective within that
   resource. 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 that is merely named within
   the primary resource.  A fragment identifier component is indicated
   by the presence of a crosshatch ("#") character and terminated by the
   end of the URI string.

      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.  Therefore, the format and interpretation of a
   fragment identifier component is dependent on the media type
   [RFC2046] of a potential retrieval result.  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 fragments by
   that media type.  If the primary resource is represented by multiple
   media types, as is often the case for resources whose representation
   is selected based on attributes of the retrieval request, then
   interpretation of the given fragment identifier must be consistent
   across all of those media types in order for it to be viable as an



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   identifier.

   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.  However, if
   that URI is used in a context that does call for retrieval and is not
   a same-document reference (Section 4.4), the fragment identifier is
   only valid as a reference if a retrieval action on the primary
   resource succeeds and results in a representation that defines the
   fragment.

   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
   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 such use is
   discouraged for the same reasons as described above for query.























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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 production. 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

   The ABNF rule URI-reference is used to denote the most common usage
   of a resource identifier.

      URI-reference = URI / relative-URI

   A URI-reference may be absolute or relative: if the reference
   string'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 the reference is relative or absolute, 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 hier-part syntax
   (Section 3) in order to express a reference that is relative to the
   name space of another hierarchical URI.

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

   The URI referred to by a relative URI reference is obtained by
   applying the relative resolution algorithm of Section 5.

   A relative reference that begins with two slash characters is termed
   a network-path reference; such references are rarely used. A relative
   reference that begins with a single slash character is termed an
   absolute-path reference.  A relative reference that does not begin



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   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 might 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 the base URI for later
   use by relative references calls for an absolute-URI production that
   does not allow a fragment.

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


4.4 Same-document Reference

   When a URI reference occurring within a document or message refers to
   a URI that is, aside from its fragment component (if any), identical
   to the base URI (Section 5), 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 crosshatch ("#") 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 that current document or message; the dereference should not
   result in a new retrieval.

4.5 Suffix Reference

   The URI syntax is designed for unambiguous reference to resources and
   extensibility via the URI scheme.  However, as URI identification and
   usage have become commonplace, traditional media (television, radio,
   newspapers, billboards, etc.) have increasingly used a suffix of the
   URI as a reference, consisting of only the authority and path
   portions of the URI, such as

      www.w3.org/Addressing/

   or simply the DNS hostname on its own.  Such references are primarily
   intended for human interpretation rather than machine, with the
   assumption that context-based heuristics are sufficient to complete
   the URI (e.g., most hostnames beginning with "www" are likely to have



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   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. It should be noted that such heuristics may
   change over time, particularly when new URI schemes are introduced.

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








































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5. Relative Resolution

   It is often the case that a group or "tree" of documents has been
   constructed to serve a common purpose; the vast majority of URIs in
   these documents point to resources within the tree rather than
   outside of it.  Similarly, documents located at a particular site are
   much more likely to refer to other resources at that site than to
   resources at remote sites.

   Relative 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 URIs.
   Furthermore, such document trees can be moved, as a whole, without
   changing any of the relative references.  Experience within the WWW
   has demonstrated that the ability to perform relative referencing is
   necessary for the long-term usability of embedded URIs.

5.1 Establishing a Base URI

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

   A document that contains relative references must have a base URI
   that contains a hierarchical path component.  In other words, a
   relative-URI cannot be used within a document that has an unsuitable
   base URI. Some URI schemes do not allow a hierarchical path component
   and are thus restricted to full URI references.

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

   The base URI of a document can be established in one of four ways,
   listed below in order of precedence.  The order of precedence can be
   thought of in terms of layers, where the innermost defined base URI
   has the highest precedence.  This can be visualized graphically as:

      .----------------------------------------------------------.
      |  .----------------------------------------------------.  |
      |  |  .----------------------------------------------.  |  |
      |  |  |  .----------------------------------------.  |  |  |



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


5.1.1 Base URI within Document Content

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

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

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

5.1.2 Base URI from the Encapsulating Entity

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





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5.1.3 Base URI from the Retrieval URI

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

5.1.4 Default Base URI

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

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

5.2 Obtaining the Referenced URI

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

   The base URI (Base) is established according to the rules 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 the
   base URI; the other components may be empty or undefined.  A
   component is undefined if its preceding separator does not appear in
   the URI reference; the path component is never undefined, though it
   may be empty.

   For each URI reference (R), the following pseudocode describes an
   algorithm for transforming R into its target URI (T):

      (R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
         -- The URI reference is parsed into the five URI components

      if ((not validating) and (R.scheme == Base.scheme)) then
         -- A non-validating parser may ignore a scheme in the



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         -- reference if it is identical to the base URI's scheme.
         undefine(R.scheme);
      endif;

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

      T.fragment = R.fragment;

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

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




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

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

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

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

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

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

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

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

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


5.3 Recomposition of a Parsed URI

   Parsed URI components can be recombined to obtain the referenced URI.
   Using pseudocode, this would be:

      result = ""

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



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

      append T.path to result;

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

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

      return result;

   Note that we are careful to preserve the distinction between a
   component that is undefined, meaning that its separator was not
   present in the reference, and a component that is empty, meaning that
   the separator was present and was immediately followed by the next
   component separator or the end of the reference.

5.4 Examples of Relative Resolution

   Within an object with a well-defined base URI of

      http://a/b/c/d;p?q

   a relative URI reference would be resolved 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"



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      "g;x?y#s"       =  "http://a/b/c/g;x?y#s"
      "."             =  "http://a/b/c/"
      "./"            =  "http://a/b/c/"
      ".."            =  "http://a/b/"
      "../"           =  "http://a/b/"
      "../g"          =  "http://a/b/g"
      "../.."         =  "http://a/"
      "../../"        =  "http://a/"
      "../../g"       =  "http://a/g"


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.

   An empty reference refers to the current base URI.

      ""              =  "http://a/b/c/d;p?q"

   Parsers must be careful in handling the case where there are more
   relative path ".." segments than there are hierarchical levels in the
   base URI's path.  Note that the ".." syntax cannot be used to change
   the authority component of a URI.

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

   Similarly, parsers should 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 uses unnecessary or
   nonsensical forms of the "." and ".." complete path segments.

      "./../g"        =  "http://a/b/g"
      "./g/."         =  "http://a/b/c/g/"
      "g/./h"         =  "http://a/b/c/g/h"
      "g/../h"        =  "http://a/b/c/h"
      "g;x=1/./y"     =  "http://a/b/c/g;x=1/y"



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      "g;x=1/../y"    =  "http://a/b/c/y"

   Some applications fail to separate the reference's query and/or
   fragment components from a relative path before merging it with the
   base path.  This error is rarely noticed, since typical usage of a
   fragment never includes the hierarchy ("/") character, and the query
   component is not normally used within relative references.

      "g?y/./x"       =  "http://a/b/c/g?y/./x"
      "g?y/../x"      =  "http://a/b/c/g?y/../x"
      "g#s/./x"       =  "http://a/b/c/g#s/./x"
      "g#s/../x"      =  "http://a/b/c/g#s/../x"

   Some parsers allow the scheme name to be present in a relative URI if
   it is the same as the base URI scheme.  This is considered to be a
   loophole in prior specifications of partial URI [RFC1630]. Its use
   should be avoided, but is allowed for backward compatibility.

      "http:g"        =  "http:g"         ; for validating 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 their origin.  For this reason, determination of
   equivalence or difference of URIs is based on string comparison,
   perhaps augmented by reference to additional rules provided by URI
   scheme definitions. We use the terms "different" and "equivalent" to
   describe the possible outcomes of such comparisons, but there are
   many application-dependent versions of equivalence.

   Even though it is possible to determine that two URIs are equivalent,
   it is never possible to be sure that two URIs identify different
   resources. Therefore, comparison methods are designed to minimize
   false negatives while strictly avoiding false positives.

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

6.2 Comparison Ladder

   A variety of methods are used in practice to test URI equivalence.
   These methods fall into a range, distinguished by the amount of



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   processing required and the degree to which the probability of false
   negatives is reduced.  As noted above, false negatives cannot in
   principle be eliminated.  In practice, their probability can be
   reduced, but this reduction requires more processing and is not
   cost-effective for all applications.

   If this range of comparison practices is considered as a ladder, the
   following discussion will climb the ladder, starting with those that
   are cheap but have a relatively higher chance of producing false
   negatives, and proceeding to those that have higher computational
   cost and lower risk of false negatives.

6.2.1 Simple String Comparison

   If two URIs, considered as character strings, are identical, then it
   is safe to conclude that they are equivalent.  This type of
   equivalence test has very low computational cost and is in wide use
   in a variety of applications, particularly in the domain of parsing.

   Testing strings for equivalence requires some basic precautions. This
   procedure is often referred to as "bit-for-bit" or "byte-for-byte"
   comparison, which is potentially misleading.  Testing of strings for
   equality is normally based on pairwise comparison of the characters
   that make up the strings, starting from the first and proceeding
   until both strings are exhausted and all characters found to be
   equal, a pair of characters compares unequal, or one of the strings
   is exhausted before the other.

   Such character comparisons require that each pair of characters be
   put in comparable form.  For example, should one URI be stored in a
   byte array in EBCDIC encoding, and the second be in a Java String
   object, bit-for-bit comparisons applied naively will produce both
   false-positive and false-negative errors.  Thus, in principle, it is
   better to speak of equality on a character-for-character rather than
   byte-for-byte or bit-for-bit basis.

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










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6.2.2 Syntax-based Normalization

   Software may use logic based on the definitions provided by this
   specification to reduce the probability of false negatives.  Such
   processing is moderately higher in cost than character-for-character
   string comparison.  For example, an application using this approach
   could reasonably consider the following two URIs equivalent:

      example://a/b/c/%7A
      eXAMPLE://a/./b/../b/c/%7a

   Web user agents, such as browsers, typically apply this type of URI
   normalization when determining whether a cached response is
   available. Syntax-based normalization includes such techniques as
   case normalization, escape normalization, and removal of leftover
   relative path segments.

6.2.2.1 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
   hostname are case insensitive and therefore can be normalized to
   lowercase.  For example, the URI <HTTP://www.EXAMPLE.com/> is
   equivalent to <http://www.example.com/>.

6.2.2.2 Escape Normalization

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

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

6.2.2.3 Path Segment Normalization

   The complete path segments "." and ".." have a special meaning within
   hierarchical URI schemes.  As such, they should not appear in
   absolute URI paths; if they are found, they can be removed by



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   splitting the URI just after the "/" that starts the path, using the
   left half as the base URI and the right as a relative reference, and
   normalizing the URI by merging the two in in accordance with the
   relative URI processing algorithm (Section 5).

6.2.3 Scheme-based Normalization

   The syntax and semantics of URIs vary from scheme to scheme, as
   described by the defining specification for each scheme.  Software
   may use scheme-specific rules, at further processing cost, to reduce
   the probability of false negatives.  For example, Web spiders that
   populate most large search engines would consider the following two
   URIs to be equivalent:

      http://example.com/
      http://example.com:80/

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

6.2.4 Protocol-based Normalization

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

      http://example.com/data

   redirects to

      http://example.com/data/

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

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 generate and make reference to URIs can
   reduce the cost of processing and the risk of false negatives by



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   consistently providing them in a form that is reasonably canonical
   with respect to their scheme.  Specifically:

      Always provide the URI scheme in lowercase characters.

      Always provide the hostname, if any, in lowercase characters.

      Only perform percent-escaping where it is essential.

      Always use uppercase A-through-F characters when percent-escaping.

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

   The good practices listed above are motivated by observations that a
   high proportion of deployed software use these techniques for the
   purposes of normalization.



































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7. Security Considerations

   A URI does not in itself pose a security threat.  However, since URIs
   are often used to provide a compact set of instructions for access to
   network resources, care must be taken to properly interpret the data
   within a URI, to prevent that data from causing unintended access,
   and to avoid including data that should not be revealed in plain
   text.

7.1 Reliability and Consistency

   There is no guarantee that, having once used a given URI to retrieve
   some information, that the same information will be 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 scheme.

7.2 Malicious Construction

   It is sometimes possible to construct a URI such that an attempt to
   perform a seemingly harmless, idempotent operation, such as the
   retrieval of a representation, 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.  The content of the URI
   contains instructions that, when interpreted according to this other
   protocol, cause an unexpected operation.  An example has been the use
   of a gopher URI to cause an unintended or impersonating message to be
   sent via a SMTP server.

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

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



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7.3 Rare IP Address Formats

   Although the URI syntax for IPv4address only allows the common,
   dotted-decimal form of IPv4 address literal, many implementations
   that process URIs make use of platform-dependent system routines,
   such as gethostbyname() and inet_aton(), to translate the string
   literal to an actual IP address.  Unfortunately, such system routines
   often allow and process a much larger set of formats than those
   described in Section 3.2.2.

   For example, many implementations allow dotted forms of three
   numbers, wherein the last part is interpreted as a 16-bit quantity
   and placed in the right-most two bytes of the network address (e.g.,
   a Class B network). Likewise, a dotted form of two numbers means the
   last part is interpreted as a 24-bit quantity and placed in the right
   most three bytes of the network address (Class A), and a single
   number (without dots) is interpreted as a 32-bit quantity and stored
   directly in the network address.  Adding further to the confusion,
   some implementations allow each dotted part to be interpreted as
   decimal, octal, or hexadecimal, as specified in the C language (i.e.,
   a leading 0x or 0X implies hexadecimal; otherwise, a leading 0
   implies octal; otherwise, the number is interpreted as decimal).

   These additional IP address formats are not allowed in the URI syntax
   due to differences between platform implementations.  However, they
   can become a security concern if an application attempts to filter
   access to resources based on the IP address in string literal format.
   If such filtering is performed, it is recommended that literals be
   converted to numeric form and filtered based on the numeric value,
   rather than a prefix or suffix of the string form.

7.4 Sensitive Information

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

7.5 Semantic Attacks

   Because the userinfo component is rarely used and appears before the
   hostname in the authority component, it can be used to construct a
   URI that is intended to mislead a human user by appearing to identify
   one (trusted) naming authority while actually identifying a different
   authority hidden behind the noise.  For example

      http://www.example.com&story=breaking_news@10.0.0.1/top_story.htm



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   might lead a human user to assume that the host is 'www.example.com',
   whereas it is actually '10.0.0.1'.  Note that the misleading userinfo
   could be much longer than the example above.

   A misleading URI, such as the one above, is an attack on the user's
   preconceived notions about the meaning of a URI, rather than an
   attack on the software itself.  User agents may be able to reduce the
   impact of such attacks by visually distinguishing the various
   components of the URI when rendered, such as by using a different
   color or tone to render userinfo if any is present, though there is
   no general panacea. More information on URI-based semantic attacks
   can be found in [Siedzik].







































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8. Acknowledgments

   This document is derived from RFC 2396 [RFC2396], RFC 1808 [RFC1808],
   and RFC 1738 [RFC1738]; the acknowledgments in those specifications
   still apply. It also incorporates the update (with corrections) for
   IPv6 literals in the host syntax, as defined by Robert M. Hinden,
   Brian E. Carpenter, and Larry Masinter in [RFC2732]. In addition,
   contributions by Reese Anschultz, Tim Bray, Rob Cameron, Dan
   Connolly, Adam M. Costello, Jason Diamond, Martin Duerst, Stefan
   Eissing, Clive D.W. Feather, Pat Hayes, Henry Holtzman, Graham Klyne,
   Dan Kohn, Bruce Lilly, Andrew Main, Michael Mealling, Julian Reschke,
   Tomas Rokicki, Miles Sabin, Ronald Tschalaer, 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.











































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Informative References

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

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

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

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

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

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

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

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

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

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

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

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

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

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




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

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

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

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

   [Siedzik]  Siedzik, R., "Semantic Attacks: What's in a URL?", April
              2001.

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

































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Authors' Addresses

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

   Phone: +1-617-253-5702
   Fax:   +1-617-258-5999
   EMail: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/


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

   Phone: +1-949-999-2523
   Fax:   +1-949-644-5064
   EMail: roy.fielding@day.com
   URI:   http://www.apache.org/~fielding/


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

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















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Appendix A. Collected ABNF for URI

   To be filled-in later.
















































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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

      http://www.ics.uci.edu/pub/ietf/uri/#Related

   results in the following subexpression matches:

      $1 = http:
      $2 = http
      $3 = //www.ics.uci.edu
      $4 = www.ics.uci.edu
      $5 = /pub/ietf/uri/
      $6 = <undefined>
      $7 = <undefined>
      $8 = #Related
      $9 = Related

   where <undefined> indicates that the component is not present, as is
   the case for the query component in the above example.  Therefore, we
   can determine the value of the 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. Embedding the Base URI in HTML documents

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

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

      <!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN">
      <HTML><HEAD>
      <TITLE>An example HTML document</TITLE>
      <BASE href="http://www.example.com/Test/a/b/c">
      </HEAD><BODY>
      ... <A href="../x">a hypertext anchor</A> ...
      </BODY></HTML>

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

      <http://www.example.com/Test/a/x>

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





















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

      http://example.com/

   These wrappers do not form part of the URI.

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

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

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

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

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

   For example, the text:

      Yes, Jim, I found it under "http://www.w3.org/Addressing/",
      but you can probably pick it up from <ftp://ds.internic.



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      net/rfc/>.  Note the warning in <http://www.ics.uci.edu/pub/
      ietf/uri/historical.html#WARNING>.

   contains the URI references

      http://www.w3.org/Addressing/
      ftp://ds.internic.net/rfc/
      http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING











































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Appendix E. Summary of Non-editorial Changes

E.1 Additions

   IPv6 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 and uric sets.
   Square brackets are now specified as reserved within the authority
   component and not allowed outside their use as delimiters for an
   IPv6reference within host.  In order to make this change without
   changing the technical definition of the path, query, and fragment
   components, those rules were redefined to directly specify the
   characters allowed rather than 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, and the definition of
   hostname has been improved to better specify length limitations and
   partially-qualified domain names.

   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.  Likewise,
   Section 2.1 on the encoding of characters has been replaced.

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

E.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 2.2 on reserved characters has been rewritten to clearly
   explain what characters are reserved, when they are reserved, and why
   they are reserved even when not used as delimiters by the generic
   syntax. Likewise, the section on escaped characters has been
   rewritten, and URI normalizers are now given license to unescape any
   octets corresponding to unreserved characters.  The crosshatch ("#")
   character has been moved back from the excluded delims to the
   reserved set.

   The ABNF for URI and URI-reference has been redesigned to make them
   more friendly to LALR parsers and significantly reduce complexity. As



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   a result, the layout form of syntax description has been removed,
   along with the uric-no-slash, opaque-part, and rel-segment
   productions. All references to "opaque" URIs have been replaced with
   a better description of how the path component may be opaque to
   hierarchy. The fragment identifier has been moved back into the
   section on generic syntax components and within the URI and
   relative-URI productions, though it remains excluded from
   absolute-URI. 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 ABNF of hier-part and relative-URI has been corrected to allow a
   relative URI path to be empty.  This also allows an absolute-URI to
   consist of nothing after the "scheme:", as is present in practice
   with the "DAV:" namespace [RFC2518] and the "about:" URI used by many
   browser implementations. The ambiguity regarding the parsing of
   net-path, abs-path, and rel-path is now explained and disambiguated
   in the same section.

   Registry-based naming authorities that use the hierarchical authority
   syntax component are now limited to DNS hostnames, since those have
   been the only such URIs in deployment.  This change was necessary to
   enable internationalized domain names to be processed in their native
   character encodings at the application layers above URI processing.
   The reg_name, server, and hostport productions have been removed to
   simplify parsing of the URI syntax.

   The ABNF of qualified has been simplified to remove a parsing
   ambiguity without changing the allowed syntax.  The toplabel
   production has been removed because it served no useful purpose. The
   ambiguity regarding the parsing of host as IPv4address or hostname is
   now explained and disambiguated in the same section.

   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.

   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 in
      favor of a section that explains that a new retrieval action
      should not be made if the target URI and base URI, excluding
      fragments, match.



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Index

A
   ABNF  9
   abs-path  15
   absolute  9
   absolute-path  22
   absolute-URI  23
   access  7
   alphanum  17
   authority  15, 16

D
   dec-octet  17
   delims  13
   dereference  8
   domainlabel  17
   dot-segments  19

E
   escaped  12
   excluded  13

F
   fragment  20

G
   generic syntax  5

H
   h4  18
   hier-part  15
   hierarchical  9
   host  17
   hostname  17

I
   identifier  5
   invisible  13
   IPv4  17
   IPv4address  17
   IPv6  18
   IPv6address  18
   IPv6reference  18

L
   locator  6
   ls32  18



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M
   mark  11

N
   name  6
   net-path  15
   network-path  22

P
   path  15, 19
   path-segments  19
   pchar  19
   port  18

Q
   qualified  17
   query  20

R
   rel-path  15
   relative  9
   relative-path  22
   relative-URI  22
   representation  8
   reserved  10
   resolution  8
   resource  4
   retrieval  8

S
   same-document  23
   sameness  8
   scheme  15
   segment  19
   suffix  23

T
   transcription  6

U
   uniform  4
   unreserved  11
   unwise  13
   URI grammar
      abs-path  15
      absolute-URI  23
      ALPHA  9
      alphanum  17



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      authority  15, 16
      CR  9
      CTL  9
      dec-octet  17
      DIGIT  9
      domainlabel  17
      DQUOTE  9
      escaped  12
      fragment  15, 20, 22
      h4  18
      HEXDIG  9
      hier-part  15, 22, 23
      host  16, 17
      hostname  17
      IPv4address  17
      IPv6address  18
      IPv6reference  18
      LF  9
      ls32  18
      mark  11
      net-path  15
      OCTET  9
      path-segments  15, 19
      pchar  19, 20, 20
      port  16, 18
      qualified  17
      query  15, 20, 22, 23
      rel-path  15
      relative-URI  22, 22
      reserved  11
      scheme  15, 16, 23
      segment  19
      SP  9
      unreserved  11
      URI  15, 22
      URI-reference  22
      uric  10
      userinfo  16, 16
   URI  15
   URI-reference  22
   uric  10
   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
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   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard. Please address the information to the IETF Executive
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Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION



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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgement

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











































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