Network Working Group                                     T. Berners-Lee
Request for Comments: 2396                                       MIT/LCS
Updates: 1808, 1738                                          R. Fielding
Category: Standards Track                                    U.C. Irvine
                                                            L. Masinter
                                                      Xerox Corporation
                                                            August 1998


          Uniform Resource Identifiers (URI): Generic Syntax

Status of this Memo

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

Copyright Notice

  Copyright (C) The Internet Society (1998).  All Rights Reserved.

IESG Note

  This paper describes a "superset" of operations that can be applied
  to URI.  It consists of both a grammar and a description of basic
  functionality for URI.  To understand what is a valid URI, both the
  grammar and the associated description have to be studied.  Some of
  the functionality described is not applicable to all URI schemes, and
  some operations are only possible when certain media types are
  retrieved using the URI, regardless of the scheme used.

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 URI, including both absolute and
  relative forms, and guidelines for their use; it revises and replaces
  the generic definitions in RFC 1738 and RFC 1808.

  This document defines a grammar that is a superset of all valid URI,
  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 URI; that task will be performed by the individual
  specifications of each URI scheme.




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1. Introduction

  Uniform Resource Identifiers (URI) provide a simple and extensible
  means for identifying a resource.  This specification of URI syntax
  and semantics is derived from concepts introduced by the World Wide
  Web global information initiative, whose use of such objects dates
  from 1990 and is described in "Universal Resource Identifiers in WWW"
  [RFC1630].  The specification of URI 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 updates and merges "Uniform Resource Locators"
  [RFC1738] and "Relative Uniform Resource Locators" [RFC1808] in order
  to define a single, generic syntax for all URI.  It excludes those
  portions of RFC 1738 that defined the specific syntax of individual
  URL schemes; those portions will be updated as separate documents, as
  will the process for registration of new URI schemes.  This document
  does not discuss the issues and recommendation for dealing with
  characters outside of the US-ASCII character set [ASCII]; those
  recommendations are discussed in a separate document.

  All significant changes from the prior RFCs are noted in Appendix G.

1.1 Overview of URI

  URI are characterized by the following definitions:

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



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

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

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

1.2. URI, URL, and URN

  A URI can be further classified as a locator, a name, or both.  The
  term "Uniform Resource Locator" (URL) refers to the subset of URI
  that identify resources via a representation of their primary access
  mechanism (e.g., their network "location"), rather than identifying
  the resource by name or by some other attribute(s) of that resource.
  The term "Uniform Resource Name" (URN) refers to the subset of URI
  that are required to remain globally unique and persistent even when
  the resource ceases to exist or becomes unavailable.

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

  Although many URL schemes are named after protocols, this does not
  imply that the only way to access the URL's resource is via the named
  protocol.  Gateways, proxies, caches, and name resolution services
  might be used to access some resources, independent of the protocol
  of their origin, and the resolution of some URL may require the use
  of more than one protocol (e.g., both DNS and HTTP are typically used
  to access an "http" URL's resource when it can't be found in a local
  cache).





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  A URN differs from a URL in that it's primary purpose is persistent
  labeling of a resource with an identifier.  That identifier is drawn
  from one of a set of defined namespaces, each of which has its own
  set name structure and assignment procedures.  The "urn" scheme has
  been reserved to establish the requirements for a standardized URN
  namespace, as defined in "URN Syntax" [RFC2141] and its related
  specifications.

  Most of the examples in this specification demonstrate URL, since
  they allow the most varied use of the syntax and often have a
  hierarchical namespace.  A parser of the URI syntax is capable of
  parsing both URL and URN references as a generic URI; once the scheme
  is determined, the scheme-specific parsing can be performed on the
  generic URI components.  In other words, the URI syntax is a superset
  of the syntax of all URI schemes.

1.3. Example URI

  The following examples illustrate URI that are in common use.

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

  gopher://spinaltap.micro.umn.edu/00/Weather/California/Los%20Angeles
     -- gopher scheme for Gopher and Gopher+ Protocol services

  http://www.math.uio.no/faq/compression-faq/part1.html
     -- http scheme for Hypertext Transfer Protocol services

  mailto:[email protected]
     -- 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 services via the TELNET Protocol

1.4. Hierarchical URI and Relative Forms

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






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  Some URI schemes support a hierarchical naming system, where the
  hierarchy of the name is denoted by a "/" delimiter separating the
  components in the scheme. This document defines a scheme-independent
  `relative' form of URI reference that can be used in conjunction with
  a `base' URI (of a hierarchical scheme) to produce another URI. The
  syntax of hierarchical URI is described in Section 3; the relative
  URI calculation is described in Section 5.

1.5. URI Transcribability

  The URI syntax was designed with global transcribability as one of
  its main concerns. A URI is a sequence of characters from a very
  limited set, i.e. the letters of the basic Latin alphabet, digits,
  and a few special characters.  A URI may be represented in a variety
  of ways: e.g., ink on paper, pixels on a screen, or a sequence of
  octets in a coded character set.  The interpretation of a URI depends
  only on the characters used and not how those characters are
  represented in a network protocol.

  The goal of transcribability can be described by a simple scenario.
  Imagine two colleagues, Sam and Kim, sitting in a pub at an
  international conference and exchanging research ideas.  Sam asks Kim
  for a location to get more information, so Kim writes the URI for the
  research site on a napkin.  Upon returning home, Sam takes out the
  napkin and types the URI into a computer, which then retrieves the
  information to which Kim referred.

  There are several design concerns revealed by the scenario:

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

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

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

  These design concerns are not always in alignment.  For example, it
  is often the case that the most meaningful name for a URI component
  would require characters that cannot be typed into some systems.  The
  ability to transcribe the resource identifier from one medium to
  another was considered more important than having its URI consist of
  the most meaningful of components.  In local and regional contexts



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  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.

1.6. Syntax Notation and Common Elements

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

     <first>/<second>;<third>?<fourth>

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

  The second convention is a BNF-like grammar, used to define the
  formal URI syntax.  The grammar is that of [RFC822], except that "|"
  is used to designate alternatives.  Briefly, rules are separated from
  definitions by an equal "=", indentation is used to continue a rule
  definition over more than one line, literals are quoted with "",
  parentheses "(" and ")" are used to group elements, optional elements
  are enclosed in "[" and "]" brackets, and elements may be preceded
  with <n>* to designate n or more repetitions of the following
  element; n defaults to 0.

  Unlike many specifications that use a BNF-like grammar to define the
  bytes (octets) allowed by a protocol, the URI grammar is defined in
  terms of characters.  Each literal in the grammar corresponds to the
  character it represents, rather than to the octet encoding of that
  character in any particular coded character set.  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 which contains it.

  The following definitions are common to many elements:

     alpha    = lowalpha | upalpha

     lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
                "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
                "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"

     upalpha  = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
                "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
                "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"




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     digit    = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                "8" | "9"

     alphanum = alpha | digit

  The complete URI syntax is collected in Appendix A.

2. URI Characters and Escape Sequences

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

     uric          = reserved | unreserved | escaped

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

2.1 URI and non-ASCII characters

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

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

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

  A URI scheme may define a mapping from URI characters to octets;
  whether this is done depends on the scheme. Commonly, within a
  delimited component of a URI, a sequence of characters may be used to
  represent a sequence of octets. For example, the character "a"
  represents the octet 97 (decimal), while the character sequence "%",
  "0", "a" represents the octet 10 (decimal).




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

  In the simplest case, the original character sequence contains only
  characters that are defined in US-ASCII, and the two levels of
  mapping are simple and easily invertible: each 'original character'
  is represented as the octet for the US-ASCII code for it, which is,
  in turn, represented as either the US-ASCII character, or else the
  "%" escape sequence for that octet.

  For original character sequences that contain non-ASCII characters,
  however, the situation is more difficult. Internet protocols that
  transmit octet sequences intended to represent character sequences
  are expected to provide some way of identifying the charset used, if
  there might be more than one [RFC2277].  However, there is currently
  no provision within the generic URI syntax to accomplish this
  identification. An individual URI scheme may require a single
  charset, define a default charset, or provide a way to indicate the
  charset used.

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

2.2. Reserved Characters

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

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

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







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

2.3. Unreserved Characters

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

     unreserved  = alphanum | mark

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

  Unreserved characters can be escaped without changing the semantics
  of the URI, but this should not be done unless the URI is being used
  in a context that does not allow the unescaped character to appear.

2.4. Escape Sequences

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

2.4.1. Escaped Encoding

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

     escaped     = "%" hex hex
     hex         = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                           "a" | "b" | "c" | "d" | "e" | "f"

2.4.2. When to Escape and Unescape

  A URI is always in an "escaped" form, since escaping or unescaping a
  completed URI might change its semantics.  Normally, the only time
  escape encodings can safely be made is when the URI is being created
  from its component parts; each component may have its own set of
  characters that are reserved, so only the mechanism responsible for
  generating or interpreting that component can determine whether or



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  not escaping a character will change its semantics. Likewise, a URI
  must be separated into its components before the escaped characters
  within those components can be safely decoded.

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

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

2.4.3. Excluded US-ASCII Characters

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

  The control characters 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.

  control     = <US-ASCII coded characters 00-1F and 7F hexadecimal>

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

  space       = <US-ASCII coded character 20 hexadecimal>

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

  delims      = "<" | ">" | "#" | "%" | <">



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

  unwise      = "{" | "}" | "|" | "\" | "^" | "[" | "]" | "`"

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

3. URI Syntactic Components

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

     <scheme>:<scheme-specific-part>

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

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

     <scheme>://<authority><path>?<query>

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

     absoluteURI   = scheme ":" ( hier_part | opaque_part )

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

     hier_part     = ( net_path | abs_path ) [ "?" query ]

     net_path      = "//" authority [ abs_path ]

     abs_path      = "/"  path_segments




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

     opaque_part   = uric_no_slash *uric

     uric_no_slash = unreserved | escaped | ";" | "?" | ":" | "@" |
                     "&" | "=" | "+" | "$" | ","

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

3.1. Scheme Component

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

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

     scheme        = alpha *( alpha | digit | "+" | "-" | "." )

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

3.2. Authority Component

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

     authority     = server | reg_name

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



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  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.

3.2.1. Registry-based Naming Authority

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

     reg_name      = 1*( unreserved | escaped | "$" | "," |
                         ";" | ":" | "@" | "&" | "=" | "+" )

3.2.2. Server-based Naming Authority

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

     <userinfo>@<host>:<port>

  where <userinfo> may consist of a user name and, optionally, scheme-
  specific information about how to gain authorization to access the
  server.  The parts "<userinfo>@" and ":<port>" may be omitted.

     server        = [ [ userinfo "@" ] hostport ]

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

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

  Some URL schemes use the format "user:password" in the userinfo
  field. This practice is NOT RECOMMENDED, because the passing of
  authentication information in clear text (such as URI) has proven to
  be a security risk in almost every case where it has been used.

  The host is a domain name of a network host, or its IPv4 address as a
  set of four decimal digit groups separated by ".".  Literal IPv6
  addresses are not supported.

     hostport      = host [ ":" port ]
     host          = hostname | IPv4address
     hostname      = *( domainlabel "." ) toplabel [ "." ]
     domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
     toplabel      = alpha | alpha *( alphanum | "-" ) alphanum



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     IPv4address   = 1*digit "." 1*digit "." 1*digit "." 1*digit
     port          = *digit

  Hostnames take the form described in Section 3 of [RFC1034] and
  Section 2.1 of [RFC1123]: a sequence of domain labels separated by
  ".", each domain label starting and ending with an alphanumeric
  character and possibly also containing "-" characters.  The rightmost
  domain label of a fully qualified domain name will never start with a
  digit, thus syntactically distinguishing domain names from IPv4
  addresses, and may be followed by a single "." if it is necessary to
  distinguish between the complete domain name and any local domain.
  To actually be "Uniform" as a resource locator, a URL hostname should
  be a fully qualified domain name.  In practice, however, the host
  component may be a local domain literal.

     Note: A suitable representation for including a literal IPv6
     address as the host part of a URL is desired, but has not yet been
     determined or implemented in practice.

  The port is the network port number for the server.  Most schemes
  designate protocols that have a default port number.  Another port
  number may optionally be supplied, in decimal, separated from the
  host by a colon.  If the port is omitted, the default port number is
  assumed.

3.3. Path Component

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

     path          = [ abs_path | opaque_part ]

     path_segments = segment *( "/" segment )
     segment       = *pchar *( ";" param )
     param         = *pchar

     pchar         = unreserved | escaped |
                     ":" | "@" | "&" | "=" | "+" | "$" | ","

  The path may consist of a sequence of path segments separated by a
  single slash "/" character.  Within a path segment, the characters
  "/", ";", "=", and "?" are reserved.  Each path segment may include a
  sequence of parameters, indicated by the semicolon ";" character.
  The parameters are not significant to the parsing of relative
  references.





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3.4. Query Component

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

     query         = *uric

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

4. URI References

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

     URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]

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

4.1. Fragment Identifier

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

     fragment      = *uric

  The semantics of a fragment identifier is a property of the data
  resulting from a retrieval action, regardless of the type of URI used
  in the reference.  Therefore, the format and interpretation of
  fragment identifiers is dependent on the media type [RFC2046] of the
  retrieval result.  The character restrictions described in Section 2



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  for URI also apply to the fragment in a URI-reference.  Individual
  media types may define additional restrictions or structure within
  the fragment for specifying different types of "partial views" that
  can be identified within that media type.

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

4.2. Same-document References

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

4.3. Parsing a URI Reference

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

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

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

5. Relative URI References

  It is often the case that a group or "tree" of documents has been
  constructed to serve a common purpose; the vast majority of URI in
  these documents point to resources within the tree rather than





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  outside of it.  Similarly, documents located at a particular site are
  much more likely to refer to other resources at that site than to
  resources at remote sites.

  Relative addressing of URI 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 URI.
  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 URI.

  The syntax for relative URI takes advantage of the <hier_part> syntax
  of <absoluteURI> (Section 3) in order to express a reference that is
  relative to the namespace of another hierarchical URI.

     relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]

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

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

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

     rel_path      = rel_segment [ abs_path ]

     rel_segment   = 1*( unreserved | escaped |
                         ";" | "@" | "&" | "=" | "+" | "$" | "," )

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

  Authors should be aware that a path segment which contains a colon
  character cannot be used as the first segment of a relative URI path
  (e.g., "this:that"), because it would be mistaken for a scheme name.




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  It is therefore necessary to precede such segments with other
  segments (e.g., "./this:that") in order for them to be referenced as
  a relative path.

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

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

5.1. Establishing a Base URI

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

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

     .----------------------------------------------------------.
     |  .----------------------------------------------------.  |
     |  |  .----------------------------------------------.  |  |
     |  |  |  .----------------------------------------.  |  |  |
     |  |  |  |  .----------------------------------.  |  |  |  |
     |  |  |  |  |       <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        |
     `----------------------------------------------------------'



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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) [RFC1866] is provided in Appendix D.

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

5.1.2. Base URI from the Encapsulating Entity

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

5.1.3. Base URI from the Retrieval URI

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

5.1.4. Default Base URI

  If none of the conditions described in Sections 5.1.1--5.1.3 apply,
  then the base URI is defined by the context of the application.
  Since this definition is necessarily application-dependent, failing





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

5.2. Resolving Relative References to Absolute Form

  This section describes an example algorithm for resolving URI
  references that might be relative to a given base URI.

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

  For each URI reference, the following steps are performed in order:

  1) The URI reference is parsed into the potential four components and
     fragment identifier, as described in Section 4.3.

  2) If the path component is empty and the scheme, authority, and
     query components are undefined, then it is a reference to the
     current document and we are done.  Otherwise, the reference URI's
     query and fragment components are defined as found (or not found)
     within the URI reference and not inherited from the base URI.

  3) If the scheme component is defined, indicating that the reference
     starts with a scheme name, then the reference is interpreted as an
     absolute URI and we are done.  Otherwise, the reference URI's
     scheme is inherited from the base URI's scheme component.

     Due to a loophole in prior specifications [RFC1630], some parsers
     allow the scheme name to be present in a relative URI if it is the
     same as the base URI scheme.  Unfortunately, this can conflict
     with the correct parsing of non-hierarchical URI.  For backwards
     compatibility, an implementation may work around such references
     by removing the scheme if it matches that of the base URI and the
     scheme is known to always use the <hier_part> syntax.  The parser




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     can then continue with the steps below for the remainder of the
     reference components.  Validating parsers should mark such a
     misformed relative reference as an error.

  4) If the authority component is defined, then the reference is a
     network-path and we skip to step 7.  Otherwise, the reference
     URI's authority is inherited from the base URI's authority
     component, which will also be undefined if the URI scheme does not
     use an authority component.

  5) If the path component begins with a slash character ("/"), then
     the reference is an absolute-path and we skip to step 7.

  6) If this step is reached, then we are resolving a relative-path
     reference.  The relative path needs to be merged with the base
     URI's path.  Although there are many ways to do this, we will
     describe a simple method using a separate string buffer.

     a) 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.

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

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

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

     e) 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.

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

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



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     h) The remaining buffer string is the reference URI's new path
        component.

  7) The resulting URI components, including any inherited from the
     base URI, are recombined to give the absolute form of the URI
     reference.  Using pseudocode, this would be

        result = ""

        if scheme is defined then
            append scheme to result
            append ":" to result

        if authority is defined then
            append "//" to result
            append authority to result

        append path to result

        if query is defined then
            append "?" to result
            append query to result

        if fragment is defined then
            append "#" to result
            append fragment to result

        return result

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

  The above algorithm is intended to provide an example by which the
  output of implementations can be tested -- implementation of the
  algorithm itself is not required.  For example, some systems may find
  it more efficient to implement step 6 as a pair of 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 in step 6 above.  This may result in
     a loss of information if the query component contains the strings
     "/../" or "/./".

  Resolution examples are provided in Appendix C.



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6. URI Normalization and Equivalence

  In many cases, different URI strings may actually identify the
  identical resource. For example, the host names used in URL are
  actually case insensitive, and the URL <http://www.XEROX.com> is
  equivalent to <http://www.xerox.com>. In general, the rules for
  equivalence and definition of a normal form, if any, are scheme
  dependent. When a scheme uses elements of the common syntax, it will
  also use the common syntax equivalence rules, namely that the scheme
  and hostname are case insensitive and a URL with an explicit ":port",
  where the port is the default for the scheme, is equivalent to one
  where the port is elided.

7. Security Considerations

  A URI does not in itself pose a security threat.  Users should beware
  that there is no general guarantee that a URL, which at one time
  located a given resource, will continue to do so.  Nor is there any
  guarantee that a URL will not locate a different resource at some
  later point in time, due to the lack of any constraint on how a given
  authority apportions its namespace.  Such a guarantee can only be
  obtained from the person(s) controlling that namespace and the
  resource in question.  A specific URI scheme may include additional
  semantics, such as name persistence, if those semantics are required
  of all naming authorities for that scheme.

  It is sometimes possible to construct a URL such that an attempt to
  perform a seemingly harmless, idempotent operation, such as the
  retrieval of an entity associated with the resource, will in fact
  cause a possibly damaging remote operation to occur.  The unsafe URL
  is typically constructed by specifying a port number other than that
  reserved for the network protocol in question.  The client
  unwittingly contacts a site that is in fact running a different
  protocol.  The content of the URL contains instructions that, when
  interpreted according to this other protocol, cause an unexpected
  operation.  An example has been the use of a gopher URL to cause an
  unintended or impersonating message to be sent via a SMTP server.

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

  Care should be taken when a URL contains escaped delimiters for a
  given protocol (for example, CR and LF characters for telnet
  protocols) that these are not unescaped before transmission.  This
  might violate the protocol, but avoids the potential for such





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  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 to be performed.

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

8. Acknowledgements

  This document was derived from RFC 1738 [RFC1738] and RFC 1808
  [RFC1808]; the acknowledgements in those specifications still apply.
  In addition, contributions by Gisle Aas, Martin Beet, Martin Duerst,
  Jim Gettys, Martijn Koster, Dave Kristol, Daniel LaLiberte, Foteos
  Macrides, James Marshall, Ryan Moats, Keith Moore, and Lauren Wood
  are gratefully acknowledged.

9. 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, Editors,
            "Uniform Resource Locators (URL)", RFC 1738, December 1994.

  [RFC1866] Berners-Lee T., and D. Connolly, "HyperText Markup Language
            Specification -- 2.0", RFC 1866, November 1995.

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

  [RFC822]  Crocker, D., "Standard for the Format of ARPA Internet Text
            Messages", STD 11, RFC 822, August 1982.

  [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.




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

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

  [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.

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

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

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































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10. Authors' Addresses

  Tim Berners-Lee
  World Wide Web Consortium
  MIT Laboratory for Computer Science, NE43-356
  545 Technology Square
  Cambridge, MA 02139

  Fax: +1(617)258-8682
  EMail: [email protected]


  Roy T. Fielding
  Department of Information and Computer Science
  University of California, Irvine
  Irvine, CA  92697-3425

  Fax: +1(949)824-1715
  EMail: [email protected]


  Larry Masinter
  Xerox PARC
  3333 Coyote Hill Road
  Palo Alto, CA 94034

  Fax: +1(415)812-4333
  EMail: [email protected]























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A. Collected BNF for URI

     URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]
     absoluteURI   = scheme ":" ( hier_part | opaque_part )
     relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]

     hier_part     = ( net_path | abs_path ) [ "?" query ]
     opaque_part   = uric_no_slash *uric

     uric_no_slash = unreserved | escaped | ";" | "?" | ":" | "@" |
                     "&" | "=" | "+" | "$" | ","

     net_path      = "//" authority [ abs_path ]
     abs_path      = "/"  path_segments
     rel_path      = rel_segment [ abs_path ]

     rel_segment   = 1*( unreserved | escaped |
                         ";" | "@" | "&" | "=" | "+" | "$" | "," )

     scheme        = alpha *( alpha | digit | "+" | "-" | "." )

     authority     = server | reg_name

     reg_name      = 1*( unreserved | escaped | "$" | "," |
                         ";" | ":" | "@" | "&" | "=" | "+" )

     server        = [ [ userinfo "@" ] hostport ]
     userinfo      = *( unreserved | escaped |
                        ";" | ":" | "&" | "=" | "+" | "$" | "," )

     hostport      = host [ ":" port ]
     host          = hostname | IPv4address
     hostname      = *( domainlabel "." ) toplabel [ "." ]
     domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
     toplabel      = alpha | alpha *( alphanum | "-" ) alphanum
     IPv4address   = 1*digit "." 1*digit "." 1*digit "." 1*digit
     port          = *digit

     path          = [ abs_path | opaque_part ]
     path_segments = segment *( "/" segment )
     segment       = *pchar *( ";" param )
     param         = *pchar
     pchar         = unreserved | escaped |
                     ":" | "@" | "&" | "=" | "+" | "$" | ","

     query         = *uric

     fragment      = *uric



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     uric          = reserved | unreserved | escaped
     reserved      = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
                     "$" | ","
     unreserved    = alphanum | mark
     mark          = "-" | "_" | "." | "!" | "~" | "*" | "'" |
                     "(" | ")"

     escaped       = "%" hex hex
     hex           = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                             "a" | "b" | "c" | "d" | "e" | "f"

     alphanum      = alpha | digit
     alpha         = lowalpha | upalpha

     lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
                "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
                "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
     upalpha  = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
                "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
                "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
     digit    = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                "8" | "9"





























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B. Parsing a URI Reference with a Regular Expression

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

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

     ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
      12            3  4          5       6  7        8 9

  The numbers in the second line above are only to assist readability;
  they indicate the reference points for each subexpression (i.e., each
  paired parenthesis).  We refer to the value matched for subexpression
  <n> as $<n>.  For example, matching the above expression to

     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 in step 7 of Section 5.2.





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C. Examples of Resolving Relative URI References

  Within an object with a well-defined base URI of

     http://a/b/c/d;p?q

  the relative URI would be resolved as follows:

C.1.  Normal Examples

     g:h           =  g:h
     g             =  http://a/b/c/g
     ./g           =  http://a/b/c/g
     g/            =  http://a/b/c/g/
     /g            =  http://a/g
     //g           =  http://g
     ?y            =  http://a/b/c/?y
     g?y           =  http://a/b/c/g?y
     #s            =  (current document)#s
     g#s           =  http://a/b/c/g#s
     g?y#s         =  http://a/b/c/g?y#s
     ;x            =  http://a/b/c/;x
     g;x           =  http://a/b/c/g;x
     g;x?y#s       =  http://a/b/c/g;x?y#s
     .             =  http://a/b/c/
     ./            =  http://a/b/c/
     ..            =  http://a/b/
     ../           =  http://a/b/
     ../g          =  http://a/b/g
     ../..         =  http://a/
     ../../        =  http://a/
     ../../g       =  http://a/g

C.2.  Abnormal Examples

  Although the following abnormal examples are unlikely to occur in
  normal practice, all URI parsers should be capable of resolving them
  consistently.  Each example uses the same base as above.

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

     <>            =  (current document)

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




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

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

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

     /./g          =  http://a/./g
     /../g         =  http://a/../g
     g.            =  http://a/b/c/g.
     .g            =  http://a/b/c/.g
     g..           =  http://a/b/c/g..
     ..g           =  http://a/b/c/..g

  Less likely are cases where the relative URI uses unnecessary or
  nonsensical forms of the "." and ".." complete path segments.

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

  All client applications remove the query component from the base URI
  before resolving relative URI.  However, 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










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  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.

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












































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D. Embedding the Base URI in HTML documents

  It is useful to consider an example of how the base URI of a document
  can be embedded within the document's content.  In this appendix, we
  describe how documents written in the Hypertext Markup Language
  (HTML) [RFC1866] 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 "-//IETF//DTD HTML//EN">
     <HTML><HEAD>
     <TITLE>An example HTML document</TITLE>
     <BASE href="http://www.ics.uci.edu/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.ics.uci.edu/Test/a/x>

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





















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E. Recommendations for Delimiting URI in Context

  URI are often transmitted through formats that do not provide a clear
  context for their interpretation.  For example, there are many
  occasions when URI are 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://test.com/", angle brackets
  <http://test.com/>, or just using whitespace

                            http://test.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, linebreaks, tabs, etc.) may
  need to be added to break 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 URI that contain whitespace.

  The prefix "URL:" (with or without a trailing space) was recommended
  as a way to used to help distinguish a URL from other bracketed
  designators, although this is not common in practice.

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

  For example, the text:







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

  The URL syntax was designed for unambiguous reference to network
  resources and extensibility via the URL scheme.  However, as URL
  identification and usage have become commonplace, traditional media
  (television, radio, newspapers, billboards, etc.) have increasingly
  used abbreviated URL references.  That is, a reference consisting of
  only the authority and path portions of the identified resource, 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 URL (e.g., most hostnames beginning with "www" are likely to have
  a URL prefix of "http://").  Although there is no standard set of
  heuristics for disambiguating abbreviated URL references, many client
  implementations allow them to be entered by the user and
  heuristically resolved.  It should be noted that such heuristics may
  change over time, particularly when new URL schemes are introduced.

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
























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G. Summary of Non-editorial Changes

G.1. Additions

  Section 4 (URI References) was added to stem the confusion regarding
  "what is a URI" and how to describe fragment identifiers given that
  they are not part of the URI, but are part of the URI syntax and
  parsing concerns.  In addition, it provides a reference definition
  for use by other IETF specifications (HTML, HTTP, etc.) that have
  previously attempted to redefine the URI syntax in order to account
  for the presence of fragment identifiers in URI references.

  Section 2.4 was rewritten to clarify a number of misinterpretations
  and to leave room for fully internationalized URI.

  Appendix F on abbreviated URLs was added to describe the shortened
  references often seen on television and magazine advertisements and
  explain why they are not used in other contexts.

G.2. Modifications from both RFC 1738 and RFC 1808

  Changed to URI syntax instead of just URL.

  Confusion regarding the terms "character encoding", the URI
  "character set", and the escaping of characters with %<hex><hex>
  equivalents has (hopefully) been reduced.  Many of the BNF rule names
  regarding the character sets have been changed to more accurately
  describe their purpose and to encompass all "characters" rather than
  just US-ASCII octets.  Unless otherwise noted here, these
  modifications do not affect the URI syntax.

  Both RFC 1738 and RFC 1808 refer to the "reserved" set of characters
  as if URI-interpreting software were limited to a single set of
  characters with a reserved purpose (i.e., as meaning something other
  than the data to which the characters correspond), and that this set
  was fixed by the URI scheme.  However, this has not been true in
  practice; any character that is interpreted differently when it is
  escaped is, in effect, reserved.  Furthermore, the interpreting
  engine on a HTTP server is often dependent on the resource, not just
  the URI scheme.  The description of reserved characters has been
  changed accordingly.

  The plus "+", dollar "$", and comma "," characters have been added to
  those in the "reserved" set, since they are treated as reserved
  within the query component.






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  The tilde "~" character was added to those in the "unreserved" set,
  since it is extensively used on the Internet in spite of the
  difficulty to transcribe it with some keyboards.

  The syntax for URI scheme has been changed to require that all
  schemes begin with an alpha character.

  The "user:password" form in the previous BNF was changed to a
  "userinfo" token, and the possibility that it might be
  "user:password" made scheme specific. In particular, the use of
  passwords in the clear is not even suggested by the syntax.

  The question-mark "?" character was removed from the set of allowed
  characters for the userinfo in the authority component, since testing
  showed that many applications treat it as reserved for separating the
  query component from the rest of the URI.

  The semicolon ";" character was added to those stated as being
  reserved within the authority component, since several new schemes
  are using it as a separator within userinfo to indicate the type of
  user authentication.

  RFC 1738 specified that the path was separated from the authority
  portion of a URI by a slash.  RFC 1808 followed suit, but with a
  fudge of carrying around the separator as a "prefix" in order to
  describe the parsing algorithm.  RFC 1630 never had this problem,
  since it considered the slash to be part of the path.  In writing
  this specification, it was found to be impossible to accurately
  describe and retain the difference between the two URI
     <foo:/bar>   and   <foo:bar>
  without either considering the slash to be part of the path (as
  corresponds to actual practice) or creating a separate component just
  to hold that slash.  We chose the former.

G.3. Modifications from RFC 1738

  The definition of specific URL schemes and their scheme-specific
  syntax and semantics has been moved to separate documents.

  The URL host was defined as a fully-qualified domain name.  However,
  many URLs are used without fully-qualified domain names (in contexts
  for which the full qualification is not necessary), without any host
  (as in some file URLs), or with a host of "localhost".

  The URL port is now *digit instead of 1*digit, since systems are
  expected to handle the case where the ":" separator between host and
  port is supplied without a port.




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  The recommendations for delimiting URI in context (Appendix E) have
  been adjusted to reflect current practice.

G.4. Modifications from RFC 1808

  RFC 1808 (Section 4) defined an empty URL reference (a reference
  containing nothing aside from the fragment identifier) as being a
  reference to the base URL.  Unfortunately, that definition could be
  interpreted, upon selection of such a reference, as a new retrieval
  action on that resource.  Since the normal intent of such references
  is for the user agent to change its view of the current document to
  the beginning of the specified fragment within that document, not to
  make an additional request of the resource, a description of how to
  correctly interpret an empty reference has been added in Section 4.

  The description of the mythical Base header field has been replaced
  with a reference to the Content-Location header field defined by
  MHTML [RFC2110].

  RFC 1808 described various schemes as either having or not having the
  properties of the generic URI syntax.  However, the only requirement
  is that the particular document containing the relative references
  have a base URI that abides by the generic URI syntax, regardless of
  the URI scheme, so the associated description has been updated to
  reflect that.

  The BNF term <net_loc> has been replaced with <authority>, since the
  latter more accurately describes its use and purpose.  Likewise, the
  authority is no longer restricted to the IP server syntax.

  Extensive testing of current client applications demonstrated that
  the majority of deployed systems do not use the ";" character to
  indicate trailing parameter information, and that the presence of a
  semicolon in a path segment does not affect the relative parsing of
  that segment.  Therefore, parameters have been removed as a separate
  component and may now appear in any path segment.  Their influence
  has been removed from the algorithm for resolving a relative URI
  reference.  The resolution examples in Appendix C have been modified
  to reflect this change.

  Implementations are now allowed to work around misformed relative
  references that are prefixed by the same scheme as the base URI, but
  only for schemes known to use the <hier_part> syntax.








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RFC 2396                   URI Generic Syntax                August 1998


H.  Full Copyright Statement

  Copyright (C) The Internet Society (1998).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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