Network Working Group                                        R. FieldingRequest for Comments: 1808                                     UC IrvineCategory: Standards Track                                      June 1995                   Relative Uniform Resource LocatorsStatus 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 contextFielding                    Standards Track                     [Page 1]RFC 1808           Relative Uniform Resource Locators          June 1995   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:Fielding                    Standards Track                     [Page 2]RFC 1808           Relative Uniform Resource Locators          June 1995      <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.Fielding                    Standards Track                     [Page 3]RFC 1808           Relative Uniform Resource Locators          June 1995   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 = "<" | ">" | "#" | "%" | <">Fielding                    Standards Track                     [Page 4]RFC 1808           Relative Uniform Resource Locators          June 19952.3.  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>.Fielding                    Standards Track                     [Page 5]RFC 1808           Relative Uniform Resource Locators          June 1995   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