rfc1808.txt

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Network Working Group                                        R. Fielding
Request for Comments: 1808                                     UC Irvine
Category: Standards Track                                      June 1995


                   Relative Uniform Resource Locators

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.

Abstract

   A Uniform Resource Locator (URL) is a compact representation of the
   location and access method for a resource available via the Internet.
   When embedded within a base document, a URL in its absolute form may
   contain a great deal of information which is already known from the
   context of that base document's retrieval, including the scheme,
   network location, and parts of the url-path.  In situations where the
   base URL is well-defined and known to the parser (human or machine),
   it is useful to be able to embed URL references which inherit that
   context rather than re-specifying it in every instance.  This
   document defines the syntax and semantics for such Relative Uniform
   Resource Locators.

1.  Introduction

   This document describes the syntax and semantics for "relative"
   Uniform Resource Locators (relative URLs): a compact representation
   of the location of a resource relative to an absolute base URL.  It
   is a companion to RFC 1738, "Uniform Resource Locators (URL)" [2],
   which specifies the syntax and semantics of absolute URLs.

   A common use for Uniform Resource Locators is to embed them within a
   document (referred to as the "base" document) for the purpose of
   identifying other Internet-accessible resources.  For example, in
   hypertext documents, URLs can be used as the identifiers for
   hypertext link destinations.

   Absolute URLs contain a great deal of information which may already
   be known from the context of the base document's retrieval, including
   the scheme, network location, and parts of the URL path.  In
   situations where the base URL is well-defined and known, it is useful
   to be able to embed a URL reference which inherits that context



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   rather than re-specifying it within each instance.  Relative URLs can
   also be used within data-entry dialogs to decrease the number of
   characters necessary to describe a location.

   In addition, 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
   Internet 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, document trees can be moved, as a whole, without
   changing any of the embedded URLs.  Experience within the World-Wide
   Web has demonstrated that the ability to perform relative referencing
   is necessary for the long-term usability of embedded URLs.

2.  Relative URL Syntax

   The syntax for relative URLs is a shortened form of that for absolute
   URLs [2], where some prefix of the URL is missing and certain path
   components ("." and "..") have a special meaning when interpreting a
   relative path.  Because a relative URL may appear in any context that
   could hold an absolute URL, systems that support relative URLs must
   be able to recognize them as part of the URL parsing process.

   Although this document does not seek to define the overall URL
   syntax, some discussion of it is necessary in order to describe the
   parsing of relative URLs.  In particular, base documents can only
   make use of relative URLs when their base URL fits within the
   generic-RL syntax described below.  Although some URL schemes do not
   require this generic-RL syntax, it is assumed that any document which
   contains a relative reference does have a base URL that obeys the
   syntax.  In other words, relative URLs cannot be used within
   documents that have unsuitable base URLs.

2.1.  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,
   there is enough uniformity in the use of URLs to allow a parser to
   resolve relative URLs based upon a single, generic-RL syntax.  This
   generic-RL syntax consists of six components:



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      <scheme>://<net_loc>/<path>;<params>?<query>#<fragment>

   each of which, except <scheme>, may be absent from a particular URL.
   These components are defined as follows (a complete BNF is provided
   in Section 2.2):

      scheme ":"   ::= scheme name, as per Section 2.1 of RFC 1738 [2].

      "//" net_loc ::= network location and login information, as per
                       Section 3.1 of RFC 1738 [2].

      "/" path     ::= URL path, as per Section 3.1 of RFC 1738 [2].

      ";" params   ::= object parameters (e.g., ";type=a" as in
                       Section 3.2.2 of RFC 1738 [2]).

      "?" query    ::= query information, as per Section 3.3 of
                       RFC 1738 [2].

      "#" fragment ::= fragment identifier.

   Note that the fragment identifier (and the "#" that precedes it) is
   not considered part of the URL.  However, since it is commonly used
   within the same string context as a URL, a parser must be able to
   recognize the fragment when it is present and set it aside as part of
   the parsing process.

   The order of the components is important.  If both <params> and
   <query> are present, the <query> information must occur after the
   <params>.

2.2.  BNF for Relative URLs

   This is a BNF-like description of the Relative Uniform Resource
   Locator syntax, using the conventions of RFC 822 [5], except that "|"
   is used to designate alternatives.  Briefly, literals are quoted with
   "", parentheses "(" and ")" are used to group elements, optional
   elements are enclosed in [brackets], and elements may be preceded
   with <n>* to designate n or more repetitions of the following
   element; n defaults to 0.

   This BNF also describes the generic-RL syntax for valid base URLs.
   Note that this differs from the URL syntax defined in RFC 1738 [2] in
   that all schemes are required to use a single set of reserved
   characters and use them consistently within the major URL components.






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   URL         = ( absoluteURL | relativeURL ) [ "#" fragment ]

   absoluteURL = generic-RL | ( scheme ":" *( uchar | reserved ) )

   generic-RL  = scheme ":" relativeURL

   relativeURL = net_path | abs_path | rel_path

   net_path    = "//" net_loc [ abs_path ]
   abs_path    = "/"  rel_path
   rel_path    = [ path ] [ ";" params ] [ "?" query ]

   path        = fsegment *( "/" segment )
   fsegment    = 1*pchar
   segment     =  *pchar

   params      = param *( ";" param )
   param       = *( pchar | "/" )

   scheme      = 1*( alpha | digit | "+" | "-" | "." )
   net_loc     =  *( pchar | ";" | "?" )
   query       =  *( uchar | reserved )
   fragment    =  *( uchar | reserved )

   pchar       = uchar | ":" | "@" | "&" | "="
   uchar       = unreserved | escape
   unreserved  = alpha | digit | safe | extra

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

   alpha       = lowalpha | hialpha
   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"
   hialpha     = "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"

   safe        = "$" | "-" | "_" | "." | "+"
   extra       = "!" | "*" | "'" | "(" | ")" | ","
   national    = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`"
   reserved    = ";" | "/" | "?" | ":" | "@" | "&" | "="
   punctuation = "<" | ">" | "#" | "%" | <">



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2.3.  Specific Schemes and their Syntactic Categories

   Each URL scheme has its own rules regarding the presence or absence
   of the syntactic components described in Sections 2.1 and 2.2.  In
   addition, some schemes are never appropriate for use with relative
   URLs.  However, since relative URLs will only be used within contexts
   in which they are useful, these scheme-specific differences can be
   ignored by the resolution process.

   Within this section, we include as examples only those schemes that
   have a defined URL syntax in RFC 1738 [2].  The following schemes are
   never used with relative URLs:

      mailto     Electronic Mail
      news       USENET news
      telnet     TELNET Protocol for Interactive Sessions

   Some URL schemes allow the use of reserved characters for purposes
   outside the generic-RL syntax given above.  However, such use is
   rare.  Relative URLs can be used with these schemes whenever the
   applicable base URL follows the generic-RL syntax.

      gopher     Gopher and Gopher+ Protocols
      prospero   Prospero Directory Service
      wais       Wide Area Information Servers Protocol

   Users of gopher URLs should note that gopher-type information is
   almost always included at the beginning of what would be the
   generic-RL path.  If present, this type information prevents
   relative-path references to documents with differing gopher-types.

   Finally, the following schemes can always be parsed using the
   generic-RL syntax.  This does not necessarily imply that relative
   URLs will be useful with these schemes -- that decision is left to
   the system implementation and the author of the base document.

      file       Host-specific Files
      ftp        File Transfer Protocol
      http       Hypertext Transfer Protocol
      nntp       USENET news using NNTP access

   NOTE: Section 5 of RFC 1738 specifies that the question-mark
         character ("?") is allowed in an ftp or file path segment.
         However, this is not true in practice and is believed to be an
         error in the RFC.  Similarly, RFC 1738 allows the reserved
         character semicolon (";") within an http path segment, but does
         not define its semantics; the correct semantics are as defined
         by this document for <params>.



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   We recommend that new schemes be designed to be parsable via the
   generic-RL syntax if they are intended to be used with relative URLs.
   A description of the allowed relative forms should be included when a
   new scheme is registered, as per Section 4 of RFC 1738 [2].

2.4.  Parsing a URL

   An accepted method for parsing URLs is useful to clarify the
   generic-RL syntax of Section 2.2 and to describe the algorithm for
   resolving relative URLs presented in Section 4.  This section
   describes the parsing rules for breaking down a URL (relative or
   absolute) into the component parts described in Section 2.1.  The
   rules assume that the URL has already been separated from any
   surrounding text and copied to a "parse string".  The rules are