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Network Working Group                                     T. Berners-Lee
INTERNET-DRAFT                                                   MIT/LCS
<draft-fielding-url-syntax-04>                               R. Fielding
Expires six months after publication date.                   U.C. Irvine
                                                             L. Masinter
                                                       Xerox Corporation
                                                          March 26, 1997

    Uniform Resource Locators (URL): Generic Syntax and Semantics

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
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   To learn the current status of any Internet-Draft, please check the
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   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net
   (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
   Coast), or ftp.isi.edu (US West Coast).

      1. We need to define a mechanism for using IPv6 addresses in the
         URL hostname which will not break existing systems too badly.
        Proposal: *hex *["." *hex] ".ipv6"
        I.e., treat the top level domain of "ipv6" as special.
      2. Examples should include one with multiple parameters and
         one with multiple queries.


   A Uniform Resource Locator (URL) is a compact string representation
   of a location for use in identifying an abstract or physical
   resource.  This document defines the general syntax and semantics
   of URLs, including both absolute and relative locators, and
   guidelines for their use. It revises and replaces the generic
   definitions in RFC 1738 and RFC 1808.

1. Introduction

   Uniform Resource Locators (URLs) provide a simple and extensible
   means for identifying a resource by its location.  This
   specification of URL 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 URLs is designed to meet the recommendations laid
   out in "Functional Recommendations for Internet Resource Locators"

   This document updates and merges "Uniform Resource Locators"
   [RFC1738] and "Relative Uniform Resource Locators" [RFC1808] in
   order to define a single, general syntax for all URLs.  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 URL schemes.

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

   URLs are characterized by the following definitions:

         Uniformity of syntax and semantics allows the mechanism for
         referencing resources to be independent of the mechanism used
         to locate those resources and the operations applied to those
         resources once they have been located.  New types of resources,
         access mechanisms, and operations can be introduced without
         changing the protocols and data formats that use URLs.

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

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

         A locator is an object that identifies a resource by its
         location.  In the case of URLs, the object is a sequence of
         characters with a restricted syntax.  An absolute locator
         identifies a location independent of any context, whereas a
         relative locator identifies a location relative to the
         context in which it is found.

   URLs are used to `locate' resources by providing an abstract
   identification of the resource location.  Having located 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'.  This specification is only
   concerned with the issue of identifying a resource by its location.

1.1. URL, URN, and URI

   URLs are a subset of Uniform Resource Identifiers (URI), which also
   includes the notion of Uniform Resource Names (URN).  A URN differs
   from a URL in that it identifies a resource in a location-independent
   fashion (see [RFC1737]).  URNs are defined by a separate set of

   Although this specification restricts its discussion to URLs, the
   syntax defined is that of URI in general.  Any requirements placed on
   the URL syntax also apply to the URI syntax.  This uniform syntax for
   all resource identifiers allows a URN to be used in any data field
   that might otherwise hold a URL.

1.2. Example URLs

   The following examples illustrate URLs which are in common use.

      -- ftp scheme for File Transfer Protocol services

      -- gopher scheme for Gopher and Gopher+ Protocol services

      -- http scheme for Hypertext Transfer Protocol services

      -- mailto scheme for electronic mail addresses

      -- news scheme for USENET news groups and articles

      -- telnet scheme for interactive services via the TELNET Protocol

   Many other URL schemes have been defined.

   The scheme defines the namespace of the URL.  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 URLs 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

1.3. URL Transcribability

   The URL syntax has been designed to promote transcribability as one
   of its main concerns. A URL is a sequence of characters from a very
   limited set, i.e. the letters of the basic Latin alphabet, digits,
   and some special characters.  A URL 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 URL
   depends only on the characters used and not how those characters
   are represented on the wire.

   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 URL
   for the research site on a napkin.  Upon returning home, Sam takes
   out the napkin and types the URL into a computer, which then
   retrieves the information to which Kim referred.

   There are several design concerns revealed by the scenario:

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

      o  A URL may be transcribed from a non-network source, and thus
         should consist of characters which 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

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

   These design concerns are not always in alignment.  For example, it
   is often the case that the most meaningful name for a URL component
   would require characters which cannot be typed on most keyboards.
   The ability to transcribe the resource location from one medium to
   another was considered more important than having its URL consist
   of the most meaningful of components.  In local and regional
   contexts and with improving technology, users might benefit from
   being able to use a wider range of characters.  However, such use
   is not guaranteed to work, and should therefore be avoided.

   In a few cases, exceptions were made for characters already in
   widespread use within URLs: the "~", "$" and "#" characters might
   have otherwise been excluded from URLs.

1.4. Syntax Notation and Common Elements

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


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

   The second convention is a BNF-like grammar, used to define the
   formal URL 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 which use a BNF-like grammar to define the
   bytes (octets) allowed by a protocol, the URL 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 URL 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"

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

      alphanum = alpha | digit

   The complete URL syntax is collected in Appendix A.

2. URL Characters and Character Escaping

   All URLs consist of a restricted set of characters, primarily chosen
   to aid transcribability and usability both in computer
   systems and in non-computer communications. In addition, characters
   used conventionally as delimiters around URLs were excluded.  The
   restricted set of characters consists of digits, letters, and a few
   graphic symbols corresponding to a subset of the graphic printable
   characters of the US-ASCII coded character set [ASCII]; they are
   common to most of the character encodings and input facilities
   available to Internet users.

   Within a URL, characters are either used as delimiters, or to
   represent strings of data (octets) within delimited portions.  When
   used to represent data directly, the character denotes the octet
   corresponding to the US-ASCII code for that character.  In
   addition, an octet may be represented by an escaped encoding.

   Thus, the set of "characters" allowed within URLs can be described in
   three categories: reserved, unreserved, and escaped.

      urlc        = reserved | unreserved | escaped

2.1. Characters, octets, and encodings

   URLs are sequences of characters. Parts of those sequences of
   characters are then used to represent sequences of octets. In turn,
   sequences of octets are (frequently) used (with a character
   encoding scheme) to represent characters. This means that when
   dealing with URLs it's necessary to work at three levels:

    represented characters <-> octets <-> URL characters

   where one mapping (a character encoding) is used to convert a
   sequence of characters to a sequence of octets, and another mapping
   (using ASCII or the escape encoding) is used to convert between a
   sequence of octets and a sequence of characters.  This looks more
   complicated than necessary if all one is dealing with is file names
   in ASCII, but it is necessary when dealing with the wide variety of
   systems in use.

   In current practice, many different character encoding schemes are
   used in the first mapping (between sequences of represented
   characters and sequences of octets) and there is generally no
   representation in the URL itself of which mapping was used.  For
   this reason, a client without knowledge of the origination
   mechanism cannot reliably unescape characters for display.

2.2. Reserved Characters

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

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

   The "reserved" syntax class above refers to those
   characters which are allowed within a URL, but which may not be
   allowed within a particular component of the generic URL syntax; they
   are used as delimiters of the components described in Section 4.3.

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

2.3. Unreserved Characters

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

      unreserved  = alphanum | mark

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

   Unreserved characters can be escaped without changing the semantics
   of the URL, but this should not be done unless the URL is being used
   in a context which 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, and
   also data that corresponds to characters used to delimit a URL from
   its context.

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 URL is always in an escaped form, since escaping or unescaping a
   completed URL might change its semantics.  Normally, the only time
   escape encodings can safely be made is when the URL is
   being created from its component parts.  Each component may have its
   own set of characters which are reserved, so only the mechanism
   responsible for generating or interpreting that component can
   determine whether or not escaping a character will change its
   semantics. Likewise, a URL 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. It
   is safe to unescape these within the body of a URL.
   For example, "%7e" is sometimes used instead of "~" in http URL
   path, but the two can be used interchangably.

   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 URL.  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 Characters

   Although they are not used within the URL syntax, we include here a
   description of those US-ASCII characters which have been excluded
   and the reasons for their exclusion.

      excluded    = control | space | delims | unwise | others

   The control characters in the US-ASCII coded character set are
   unsafe to use within a URL, both because they are non-printable and
   because they are likely to be misinterpreted by some control

      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 URLs are
   transcribed or typeset or subjected to the treatment of
   word-processing programs.  Whitespace is also used to delimit URLs 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 URLs in
   text documents and protocol fields.  The character "#" is excluded
   because it is used to delimit a URL from a fragment identifier in URL
   references (Section 3). The percent character "%" is excluded because
   it is used for the encoding of escaped characters.

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

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

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

   Finally, all other characters besides those mentioned in the above
   sections are excluded because they are often difficult or impossible
   to transcribe using traditional computer keyboards and software.

      others      = <Any character not in the reserved, unreserved,
                     control, space, delims, or unwise sets>

   Data corresponding to excluded characters must be escaped in order
   to be properly represented within a URL.  However, there do exist
   some systems that allow characters from the "unwise" and "others"
   sets to be used in URL references (section 3); a robust
   implementation should be prepared to handle those characters when
   it is possible to do so.

3. URL References

   A common source of confusion in the use and interpretation of Uniform
   Resource Locators is the distinction between a reference to a URL and
   the URL itself.  A URL reference may be absolute or relative, and may
   have additional information attached in the form of a fragment
   identifier.  However, "the URL" which results from such a reference
   includes only the absolute URL after the fragment identifier (if any)
   is removed and after any relative URL is resolved to its absolute
   form.  Although it is possible to limit the discussion of URL syntax
   and semantics to that of the absolute result, most usage of URLs
   is within general URL references, and it is impossible to obtain the
   URL from such a reference without also parsing the fragment and
   resolving the relative form.

      URL-reference = [ absoluteURL | relativeURL ] [ "#" fragment ]

   The syntax for relative URLs is a shortened form of that for absolute
   URLs, where some prefix of the URL is missing and certain path
   components ("." and "..") have a special meaning when interpreting a
   relative path.

   When a URL reference is used to perform a retrieval action on the
   identified resource, the optional fragment identifier, separated from
   the URL 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 URL, but is often used in conjunction with a URL.  The
   format and interpretation of fragment identifiers is dependent on the
   media type of the resource referenced by the URL.

      fragment      = *urlc

   A URL reference which does not contain a URL is a reference to the
   current document.  In other words, an empty URL 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 URL reference occurs in a context that is always
   intended to result in a new request, as in the cases of HTML's
   FORM "action" attribute and IMG "src" attribute [RFC1866], then
   an empty URL reference represents the URL of the current document
   and should be replaced by that URL when transformed into a request.

4. Generic URL Syntax

4.1. Scheme

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

   In general, absolute URLs are written as follows:


   An absolute URL 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.

   Scheme names consist of a sequence of characters.  The lower case
   letters "a"--"z", digits, and the characters plus ("+"), period
   ("."), and hyphen ("-") are allowed.  For resiliency, programs
   interpreting URLs should treat upper case letters as equivalent to
   lower case in scheme names (e.g., allow "HTTP" as well as "http").

      scheme        = 1*( alpha | digit | "+" | "-" | "." )

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

4.2. Opaque and Hierarchical URLs

   The URL syntax does not require that the scheme-specific-part have
   any general structure or set of semantics which is common among all
   URLs.  However, a subset of URLs do share a common syntax for
   representing hierarchical relationships within the locator namespace.
   This generic-URL syntax is used in interpreting relative URLs.

      absoluteURL   = generic-URL | opaque-URL

      opaque-URL    = scheme ":" *urlc

      generic-URL   = scheme ":" relativeURL

   URLs which 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 URL) is the
   delimiter used to construct a file name hierarchy, and thus the URL
   path will look similar to a file pathname.  This does NOT imply that
   the URL is a Unix pathname.

4.3. URL Syntactic Components

   The URL syntax is dependent upon the scheme.  Some schemes use
   reserved characters like "?" and ";" to indicate special components,
   while others just consider them to be part of the path.  However,
   most URL schemes use a common sequence of four main components to
   define the location of a resource


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

4.3.1. Site Component

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


   Some or all of the parts "<user>:<password>@", ":<password>", and
   ":<port>" may be excluded.  The <site> component is preceded by a
   double slash "//" and is terminated by the next slash "/" or by the
   end of the URL.  Within the <site> component, the characters ":",
   "@", "?", and "/" are reserved.

      <site>        = [ [ user [ ":" password ] "@" ] hostport ]

   The user name and password, if present, are followed by a commercial
   at-sign "@".

      user          = *( unreserved | escaped | ";" | "&" | "=" | "+" )

      password      = *( unreserved | escaped | ";" | "&" | "=" | "+" )

   Note that an empty user name or password is different than no user
   name or password; there is no way to specify a password without
   specifying a user name.  E.g., <ftp://@host.com/> has an empty
   user name and no password, <ftp://host.com/> has no user name,
   while <ftp://foo:@host.com/> has a user name of "foo" and an
   empty password.

   The host is a domain name of a network host, or its IPv4 address as a
   set of four decimal digit groups separated by ".".  A suitable
   representation for IPv6 addresses has not yet been determined.

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

   Hostnames take the form as described in Section 3.5 of [RFC1034]
   and Section 2.1 of [RFC1123]: a sequence of domain labels separated
   by ".", each domain label starting and ending with an
   alphanumerical character and possibly also containing "-"
   characters.  The rightmost domain label will never start with a
   digit, though, which syntactically distinguishes all domain names
   from hostnumbers. To actually be "Uniform" as a resource locator,
   a URL hostname should be a fully qualified domain names. In practice,
   however, the host component may be a local domain literal.

   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

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

4.3.2. Path Component

   The path component contains data, specific to the scheme or site,
   regarding the details of how the resource can be accessed.

      path          = [ "/" ] path_segments

      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

4.3.3. Query Component

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

      query         = *urlc

   Within a query component, the characters "/", "&", "=", and "+" are

4.4. Parsing a URL Reference

   A URL reference is typically parsed according to the four main
   components in order to determine what components are present and
   whether or not the reference is relative or absolute.  The individual
   components are then parsed for their subparts and to verify their
   validity.  A reference is parsed as if it is a generic-URL, even
   though it might be considered opaque by later processes.

   Although the BNF defines what is allowed in each component, it is
   ambiguous in terms of differentiating between a site 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 URL reference string as it is capable of
   matching.  In other words, the site component wins.

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

5. Relative URL 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 URLs in
   these documents point to locations within the tree rather than
   outside of it.  Similarly, documents located at a particular site
   are much more likely to refer to other resources at that site than
   to resources at remote sites.

   Relative addressing of URLs 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 URLs.
   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 URLs.

      relativeURL   = net_path | abs_path | rel_path

   A relative reference beginning with two slash characters is termed a
   network-path reference.  Such references are rarely used.

      net_path      = "//" site [ abs_path ]

   A relative reference beginning with a single slash character is
   termed an absolute-path reference.

      abs_path      = "/"  rel_path

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

      rel_path      = [ path_segments ] [ "?" query ]

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

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

5.1. Establishing a Base URL

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

   The base URL 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 URL
   has the highest precedence.  This can be visualized graphically as:

      |  .----------------------------------------------------.  |
      |  |  .----------------------------------------------.  |  |
      |  |  |  .----------------------------------------.  |  |  |
      |  |  |  |  .----------------------------------.  |  |  |  |
      |  |  |  |  |       <relative_reference>       |  |  |  |  |
      |  |  |  |  `----------------------------------'  |  |  |  |
      |  |  |  | (5.1.1) Base URL embedded in the       |  |  |  |
      |  |  |  |         document's content             |  |  |  |
      |  |  |  `----------------------------------------'  |  |  |
      |  |  | (5.1.2) Base URL of the encapsulating entity |  |  |
      |  |  |         (message, document, or none).        |  |  |
      |  |  `----------------------------------------------'  |  |
      |  | (5.1.3) URL used to retrieve the entity            |  |
      |  `----------------------------------------------------'  |
      | (5.1.4) Base URL = "" (undefined)                        |

5.1.1. Base URL within Document Content

   Within certain document media types, the base URL 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 URL 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 URL can be embedded in the Hypertext Markup Language
   (HTML) [RFC1866] is provided in Appendix D.

   MIME messages [RFC2045] are considered to be composite documents.
   The base URL of a message can be specified within the message
   headers (or equivalent tagged metainformation) of the message.  For
   protocols that make use of message headers like those described in
   MIME [RFC2045], the base URL can be specified by the Content-Base
   or Content-Location[RFC2068] header fields.

      Content-Base      = "Content-Base" ":" absoluteURL

      Content-Location  = "Content-Location" ":"
                          ( absoluteURL | relativeURL )

   The field names are case-insensitive and any whitespace inside
   the field value (including that used for line folding) is ignored.
   Content-Base takes precedence over any Content-Location.  If the
   latter is relative, it must be resolved to its absolute form (like
   any relative URL) before it can be used as the base URL for other

   For example, the header field

      Content-Base: http://www.ics.uci.edu/Test/a/b/c

   would indicate that the base URL for that message is the string
   "http://www.ics.uci.edu/Test/a/b/c".  The base URL for a message
   serves as both the base for any relative URLs within the message
   headers and the default base URL for documents enclosed within the
   message, as described in the next section.

   Protocols which do not use the RFC 822 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
   URL as part of a message.

5.1.2. Base URL from the Encapsulating Entity

   If no base URL is embedded, the base URL 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 URL of the
   document is the base URL of the entity in which the document is

   Composite media types, such as the "multipart/*" and "message/*"
   media types defined by MIME[RFC2046], define a hierarchy of
   retrieval context for their enclosed documents.  In other words, the
   retrieval context of a component part is the base URL of the
   composite entity of which it is a part.  Thus, a composite entity can
   redefine the retrieval context of its component parts via the
   inclusion of a Content-Base or Content-Location header, and this
   redefinition applies recursively for a hierarchy of composite parts.
   Note that this might not change the base URL of the components, since
   each component may include an embedded base URL or base-header that
   takes precedence over the retrieval context.

5.1.3. Base URL from the Retrieval URL

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

5.1.4. Default Base URL

   If none of the conditions described in Sections 5.1.1--5.1.3 apply,
   then the base URL is considered to be the empty string and all
   URL references within that document are assumed to be absolute URLs.

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

5.2. Resolving Relative References to Absolute Form

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

   The base URL is established according to the rules of Section 5.1 and
   parsed into the four main components as described in Section 4.4.
   Note that only the scheme component is required to be present in the
   base URL; the other components may be empty or undefined.  A
   component is undefined if its preceding separator does not appear in
   the URL reference; the path component is never undefined, though it
   may be empty.  The base URL's query component is not used by the
   resolution algorithm and may be discarded.

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

   1) The URL reference is parsed into the potential four components and
      fragment identifier, as described in Section 4.4.

   2) If the path component is empty and the scheme, site, and query
      components are undefined, then it is a reference to the current
      document and we are done.

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

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

      c) If the reference's query component is defined, then a "?"
         character is appended to the buffer string, followed by the
         query component.

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

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

      f) 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.

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

      h) If the buffer string contains a question-mark "?" character,
         then the reference URL's query component is the substring after
         the first (left-most) question-mark.  Otherwise, the reference
         URL's query component is set undefined.

      i) The reference URL's new path component is the buffer string up
         to, but not including, the first question-mark character or the
         end of the buffer string.

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

         result = ""

         if scheme is defined then

             append scheme to result
             append ":" to result

         if site is defined then
             append "//" to result
             append site 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.

   Resolution examples are provided in Appendix C.

6. URL Normalization and Equivalence

   In many cases, different URL strings may actually identify the
   identical resource. For example, the host names used in URLs 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 host
   name is case independent, 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 URL 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
   site apportions its namespace.  Such a guarantee can only be
   obtained from the person(s) controlling that namespace and the
   resource in question.

   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 which is in fact running a different
   protocol.  The content of the URL contains instructions which, when
   interpreted according to this other protocol, cause an unexpected
   operation.  An example has been the use of gopher URLs to cause an
   unintended or impersonating message to be sent via a SMTP server.

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

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

   It is clearly unwise to use a URL that contains a password which is
   intended to be secret.

7. Acknowledgements

   This document was derived from RFC 1738 [RFC1738] and RFC 1808
   [RFC1808]; the acknowledgements in those specifications still
   apply.  In addition, contributions by Lauren Wood, Martin Duerst,
   Gisle Aas, Martijn Koster, Ryan Moats and Foteos Macrides are
   gratefully acknowledged.

8. References

[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,
   CERN, June 1994.

[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, Editors,
   "Uniform Resource Locators (URL)", RFC 1738, CERN, Xerox
   Corporation, University of Minnesota, December 1994.

[RFC1866] Berners-Lee T., and D. Connolly, "HyperText Markup Language
   Specification -- 2.0", RFC 1866, MIT/W3C, November 1995.

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

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

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

[RFC2045] N. Freed & N. Borenstein, "Multipurpose Internet Mail
   Extensions (MIME) Part One: Format of Internet Message Bodies," RFC
   2045, November 1996.

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

[RFC1736] Kunze, J., "Functional Recommendations for Internet Resource
   Locators", RFC 1736, IS&T, UC Berkeley, February 1995.

[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
   STD 13, RFC 1034, USC/Information Sciences Institute, November

[RFC2110] Palme, J., Hopmann, A. "MIME E-mail Encapsulation of
   Agregate Documents, such as HTML (MHTML)", RFC 2110, Stockholm
   University/KTH, Microsoft Corporation, March 1997.

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

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

9. 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: timbl@w3.org

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

   Fax: +1(714)824-4056
   EMail: fielding@ics.uci.edu

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

   Fax: +1(415)812-4333
   EMail: masinter@parc.xerox.com


A. Collected BNF for URLs

      URL-reference = [ absoluteURL | relativeURL ] [ "#" fragment ]
      absoluteURL   = generic-URL | opaque-URL
      opaque-URL    = scheme ":" *urlc
      generic-URL   = scheme ":" relativeURL

      relativeURL   = net_path | abs_path | rel_path
      net_path      = "//" site [ abs_path ]
      abs_path      = "/"  rel_path
      rel_path      = [ path_segments ] [ "?" query ]

      scheme        = 1*( alpha | digit | "+" | "-" | "." )

      site          = [ [ user [ ":" password ] "@" ] hostport ]
      user          = *( unreserved | escaped | ";" | "&" | "=" | "+" )
      password      = *( unreserved | escaped | ";" | "&" | "=" | "+" )
      hostport      = host [ ":" port ]
      host          = hostname | hostnumber
      hostname      = *( domainlabel "." ) toplabel
      domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
      toplabel      = alpha | alpha *( alphanum | "-" ) alphanum
      hostnumber    = 1*digit "." 1*digit "." 1*digit "." 1*digit
      port          = *digit

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

      query         = *urlc

      fragment      = *urlc

      urlc          = reserved | unreserved | escaped
      reserved      = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
      unreserved    = alpha | digit | 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"

B. Parsing a URL Reference with a Regular Expression

   As described in Section 4.4, the generic-URL syntax is not sufficient
   to disambiguate the components of some forms of URL.  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 URL reference.

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

       12            3  4          5       6  7        8 9

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


   results in the following subexpression matches:

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

   where <undefined> indicates that the component is not present, as is
   the case for the query component in the above example.  Therefore, we
   can determine the value of the four components and fragment as

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

   and, going in the opposite direction, we can recreate a URL reference
   from its components using the algorithm in step 7 of Section 5.2.

C. Examples of Resolving Relative URL References

   Within an object with a well-defined base URL of

      Content-Base: http://a/b/c/d;p?q

   the relative URLs 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 URL 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 URL's path.  Note that the ".." syntax cannot be used to change
   the site component of a URL.

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

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

   Finally, some older parsers allow the scheme name to be present in a
   relative URL if it is the same as the base URL scheme.  This is
   considered to be a loophole in prior specifications of partial URLs
   [RFC1630] and should be avoided by future parsers.

      http:g        =  http:g
      http:         =  http:

D. Embedding the Base URL in HTML documents

   It is useful to consider an example of how the base URL 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 URL.  This appendix does not
   form a part of the relative URL 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 URL for resolving any
   relative URLs.  The "HREF" attribute must be an absolute URL.  Note
   that, in HTML, element and attribute names are case-insensitive.  For

      <!doctype html public "-//IETF//DTD HTML//EN">
      <TITLE>An example HTML document</TITLE>
      <BASE href="http://www.ics.uci.edu/Test/a/b/c">
      ... <A href="../x">a hypertext anchor</A> ...

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


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

E. Recommendations for Delimiting URLs in Context

   URLs are often transmitted through formats which do not provide a
   clear context for their interpretation.  For example, there are many
   occasions when URLs 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 URL from the rest of the text, and in particular from
   punctuation marks that might be mistaken for part of the URL.

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


   These wrappers do not form part of the URL.

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

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

   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
   URL 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 URL.

   Using <> angle brackets around each URL is especially recommended
   as a delimiting style for URLs 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 pratice.

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


      Yes, Jim, I found it under "http://www.w3.org/pub/WWW/",
      but you can probably pick it up from <ftp://ds.internic.
      net/rfc/>.  Note the warning in <http://ds.internic.net/

F. Summary of Non-editorial Changes

F.1. Additions

   Section 1 (Introduction) is entirely new.  Design rationale for the
   scope of URLs and the chosen URL character set has been added in
   order to address common misconceptions about what would and would not
   be appropriate for additional URL schemes, and why the allowed
   character set is limited to US-ASCII characters.  A definition of URI
   is also given, and how the URI syntax equates to the URL syntax, so
   that other IETF specifications (e.g., HTTP, HTML, etc.) can refer to
   a single definition of URI.

   Section 3 (URL References) was added to stem the confusion regarding
   "what is a URL" and how to describe fragment identifiers given that
   they are not part of the URL, but are part of the URL syntax and
   parsing concerns.  In addition, it provides a reference definition
   for use by other IETF specifications (HTML, HTTP, etc.) which have
   previously attempted to redefine the URL syntax in order to account
   for the presence of fragment identifiers in URL references.

   Section 2.3.2 (When to Escape and Unescape) was added in response to
   many (mis)implementation questions on the subject.

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

   Confusion regarding the terms "character encoding", the URL
   "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 URL syntax.

   Both RFC 1738 and RFC 1808 refer to the "reserved" set of
   characters as if URL-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 URL scheme.  However, this has
   not been true in practice; any character which 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 URL scheme.  The
   description of reserved characters has been changed accordingly.

   The plus "+" character was added to those in the "reserved" set,
   since it is treated as reserved within some URL components.

   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 question-mark "?" character was removed from the set of allowed
   characters for the user and password in the site component, since
   testing showed that many applications treat it as reserved for
   separating the query component from the rest of the URL.

   RFC 1738 specified that the path was separated from the site
   portion of a URL 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 URLs
      <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.

F.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.

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

F.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 3.

   The description of the mythical Base header field has been replaced
   with the Content-Base and Content-Location header fields defined by
   HTTP/1.1 and MHTML.[palme]

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

   The BNF term <net_loc> has been replaced with <site>, since the
   latter more accurately describes its use and purpose.

   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 URL
   reference.  The resolution examples in Appendix C have been modified
   to reflect this change.

   Testing has also revealed that most client applications remove the
   query component from the base URL before resolving relative URLs, and
   append the reference's query component to a relative path before
   merging it with the base path.  The resolution algorithm has been
   changed accordingly.

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