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Network Working Group                                        R. FieldingRequest for Comments: 1808                                     UC IrvineCategory: Standards Track                                      June 1995Relative 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 toRFC 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   inSection 2.2):      scheme ":"   ::= scheme name, as perSection 2.1 of RFC 1738 [2].      "//" net_loc ::= network location and login information, as perSection 3.1 of RFC 1738 [2].      "/" path     ::= URL path, as perSection 3.1 of RFC 1738 [2].      ";" params   ::= object parameters (e.g., ";type=a" as inSection 3.2.2 of RFC 1738 [2]).      "?" query    ::= query information, as perSection 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 ofRFC 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 inRFC 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 Sections2.1 and2.2.  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 inRFC 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 perSection 4 of RFC 1738 [2].2.4.  Parsing a URL   An accepted method for parsing URLs is useful to clarify the   generic-RL syntax ofSection 2.2 and to describe the algorithm for   resolving relative URLs presented inSection 4.  This section   describes the parsing rules for breaking down a URL (relative or   absolute) into the component parts described inSection 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   listed in the order in which they would be applied by the parser.2.4.1.  Parsing the Fragment Identifier   If the parse string contains a crosshatch "#" character, then the   substring after the first (left-most) crosshatch "#" and up to the   end of the parse string is the <fragment> identifier.  If the   crosshatch is the last character, or no crosshatch is present, then   the fragment identifier is empty.  The matched substring, including   the crosshatch character, is removed from the parse string before   continuing.   Note that the fragment identifier is not considered part of the URL.   However, since it is often attached to the URL, parsers must be able   to recognize and set aside fragment identifiers as part of the   process.2.4.2.  Parsing the Scheme   If the parse string contains a colon ":" after the first character   and before any characters not allowed as part of a scheme name (i.e.,   any not an alphanumeric, plus "+", period ".", or hyphen "-"), the   <scheme> of the URL is the substring of characters up to but not   including the first colon.  These characters and the colon are then   removed from the parse string before continuing.2.4.3.  Parsing the Network Location/Login   If the parse string begins with a double-slash "//", then the   substring of characters after the double-slash and up to, but not   including, the next slash "/" character is the network location/login   (<net_loc>) of the URL.  If no trailing slash "/" is present, the   entire remaining parse string is assigned to <net_loc>.  The double-   slash and <net_loc> are removed from the parse string beforeFielding                    Standards Track                     [Page 6]

RFC 1808           Relative Uniform Resource Locators          June 1995   continuing.2.4.4.  Parsing the Query Information   If the parse string contains a question mark "?" character, then the   substring after the first (left-most) question mark "?" and up to the   end of the parse string is the <query> information.  If the question   mark is the last character, or no question mark is present, then the   query information is empty.  The matched substring, including the   question mark character, is removed from the parse string before   continuing.2.4.5.  Parsing the Parameters   If the parse string contains a semicolon ";" character, then the   substring after the first (left-most) semicolon ";" and up to the end   of the parse string is the parameters (<params>).  If the semicolon   is the last character, or no semicolon is present, then <params> is   empty.  The matched substring, including the semicolon character, is   removed from the parse string before continuing.2.4.6.  Parsing the Path   After the above steps, all that is left of the parse string is the   URL <path> and the slash "/" that may precede it.  Even though the   initial slash is not part of the URL path, the parser must remember   whether or not it was present so that later processes can   differentiate between relative and absolute paths.  Often this is   done by simply storing the preceding slash along with the path.3.  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 embedded   relative URLs; 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.Fielding                    Standards Track                     [Page 7]

RFC 1808           Relative Uniform Resource Locators          June 1995   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:      .----------------------------------------------------------.      |  .----------------------------------------------------.  |      |  |  .----------------------------------------------.  |  |      |  |  |  .----------------------------------------.  |  |  |      |  |  |  |   (3.1) Base URL embedded in the       |  |  |  |      |  |  |  |         document's content             |  |  |  |      |  |  |  `----------------------------------------'  |  |  |      |  |  |   (3.2) Base URL of the encapsulating entity |  |  |      |  |  |         (message, document, or none).        |  |  |      |  |  `----------------------------------------------'  |  |      |  |   (3.3) URL used to retrieve the entity            |  |      |  `----------------------------------------------------'  |      |   (3.4) Base URL = "" (undefined)                        |      `----------------------------------------------------------'3.1.  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) [3] is provided in an Appendix (Section 10).   Messages 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 inRFC 822 [5], we recommend   that the format of this header be:      base-header  = "Base" ":" "<URL:" absoluteURL ">"   where "Base" is case-insensitive and any whitespace (including that   used for line folding) inside the angle brackets is ignored.  For   example, the header fieldFielding                    Standards Track                     [Page 8]

RFC 1808           Relative Uniform Resource Locators          June 1995      Base: <URL: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 theRFC 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.3.2.  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   encapsulated.   Composite media types, such as the "multipart/*" and "message/*"   media types defined by MIME (RFC 1521, [4]), 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 base-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.3.3.  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.3.4.  Default Base URL   If none of the conditions described in Sections3.1 -- 3.3 apply,   then the base URL is considered to be the empty string and all   embedded URLs within that document are assumed to be absolute URLs.Fielding                    Standards Track                     [Page 9]

RFC 1808           Relative Uniform Resource Locators          June 1995   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.4.  Resolving Relative URLs   This section describes an example algorithm for resolving URLs within   a context in which the URLs may be relative, such that the result is   always a URL in absolute form.  Although this algorithm cannot   guarantee that the resulting URL will equal that intended by the   original author, it does guarantee that any valid URL (relative or   absolute) can be consistently transformed to an absolute form given a   valid base URL.   The following steps are performed in order:   Step 1: The base URL is established according to the rules ofSection 3.  If the base URL is the empty string (unknown),           the embedded URL is interpreted as an absolute URL and           we are done.   Step 2: Both the base and embedded URLs are parsed into their           component parts as described inSection 2.4.           a) If the embedded URL is entirely empty, it inherits the              entire base URL (i.e., is set equal to the base URL)              and we are done.           b) If the embedded URL starts with a scheme name, it is              interpreted as an absolute URL and we are done.           c) Otherwise, the embedded URL inherits the scheme of              the base URL.   Step 3: If the embedded URL's <net_loc> is non-empty, we skip to           Step 7.  Otherwise, the embedded URL inherits the <net_loc>           (if any) of the base URL.   Step 4: If the embedded URL path is preceded by a slash "/", the           path is not relative and we skip to Step 7.Fielding                    Standards Track                    [Page 10]

RFC 1808           Relative Uniform Resource Locators          June 1995   Step 5: If the embedded URL path is empty (and not preceded by a           slash), then the embedded URL inherits the base URL path,           and           a) if the embedded URL's <params> is non-empty, we skip to              step 7; otherwise, it inherits the <params> of the base              URL (if any) and           b) if the embedded URL's <query> is non-empty, we skip to              step 7; otherwise, it inherits the <query> of the base              URL (if any) and we skip to step 7.   Step 6: The last segment of the base URL's path (anything           following the rightmost slash "/", or the entire path if no           slash is present) is removed and the embedded URL's path is           appended in its place.  The following operations are           then applied, in order, to the new path:           a) All occurrences of "./", where "." is a complete path              segment, are removed.           b) If the path ends with "." as a complete path segment,              that "." is removed.           c) All occurrences of "<segment>/../", where <segment> is a              complete path segment not equal to "..", are removed.              Removal of these path segments is performed iteratively,              removing the leftmost matching pattern on each iteration,              until no matching pattern remains.           d) If the path ends with "<segment>/..", where <segment> is a              complete path segment not equal to "..", that              "<segment>/.." is removed.   Step 7: The resulting URL components, including any inherited from           the base URL, are recombined to give the absolute form of           the embedded URL.   Parameters, regardless of their purpose, do not form a part of the   URL path and thus do not affect the resolving of relative paths.  In   particular, the presence or absence of the ";type=d" parameter on an   ftp URL does not affect the interpretation of paths relative to that   URL.  Fragment identifiers are only inherited from the base URL when   the entire embedded URL is empty.Fielding                    Standards Track                    [Page 11]

RFC 1808           Relative Uniform Resource Locators          June 1995   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 matches.5.  Examples and Recommended Practice   Within an object with a well-defined base URL of      Base: <URL:http://a/b/c/d;p?q#f>   the relative URLs would be resolved as follows:5.1.  Normal Examples      g:h        = <URL:g:h>      g          = <URL:http://a/b/c/g>      ./g        = <URL:http://a/b/c/g>      g/         = <URL:http://a/b/c/g/>      /g         = <URL:http://a/g>      //g        = <URL:http://g>      ?y         = <URL:http://a/b/c/d;p?y>      g?y        = <URL:http://a/b/c/g?y>      g?y/./x    = <URL:http://a/b/c/g?y/./x>      #s         = <URL:http://a/b/c/d;p?q#s>      g#s        = <URL:http://a/b/c/g#s>      g#s/./x    = <URL:http://a/b/c/g#s/./x>      g?y#s      = <URL:http://a/b/c/g?y#s>      ;x         = <URL:http://a/b/c/d;x>      g;x        = <URL:http://a/b/c/g;x>      g;x?y#s    = <URL:http://a/b/c/g;x?y#s>      .          = <URL:http://a/b/c/>      ./         = <URL:http://a/b/c/>      ..         = <URL:http://a/b/>      ../        = <URL:http://a/b/>      ../g       = <URL:http://a/b/g>      ../..      = <URL:http://a/>      ../../     = <URL:http://a/>      ../../g    = <URL:http://a/g>5.2.  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.Fielding                    Standards Track                    [Page 12]

RFC 1808           Relative Uniform Resource Locators          June 1995   An empty reference resolves to the complete base URL:      <>            = <URL:http://a/b/c/d;p?q#f>   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 <net_loc> of a URL.      ../../../g    = <URL:http://a/../g>      ../../../../g = <URL:http://a/../../g>   Similarly, parsers must avoid treating "." and ".." as special when   they are not complete components of a relative path.      /./g          = <URL:http://a/./g>      /../g         = <URL:http://a/../g>      g.            = <URL:http://a/b/c/g.>      .g            = <URL:http://a/b/c/.g>      g..           = <URL:http://a/b/c/g..>      ..g           = <URL: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        = <URL:http://a/b/g>      ./g/.         = <URL:http://a/b/c/g/>      g/./h         = <URL:http://a/b/c/g/h>      g/../h        = <URL:http://a/b/c/h>   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   [1] and should be avoided by future parsers.      http:g        = <URL:http:g>      http:         = <URL:http:>5.3.  Recommended Practice   Authors should be aware that path names which contain a colon ":"   character cannot be used as the first component of a relative URL   path (e.g., "this:that") because they will likely be mistaken for a   scheme name.  It is therefore necessary to precede such cases with   other components (e.g., "./this:that"), or to escape the colon   character (e.g., "this%3Athat"), in order for them to be correctly   parsed.  The former solution is preferred because it does not affect   the absolute form of the URL.Fielding                    Standards Track                    [Page 13]

RFC 1808           Relative Uniform Resource Locators          June 1995   There is an ambiguity in the semantics for the ftp URL scheme   regarding the use of a trailing slash ("/") character and/or a   parameter ";type=d" to indicate a resource that is an ftp directory.   If the result of retrieving that directory includes embedded relative   URLs, it is necessary that the base URL path for that result include   a trailing slash.  For this reason, we recommend that the ";type=d"   parameter value not be used within contexts that allow relative URLs.6.  Security Considerations   There are no security considerations in the use or parsing of   relative URLs.  However, once a relative URL has been resolved to its   absolute form, the same security considerations apply as those   described inRFC 1738 [2].7.  Acknowledgements   This work is derived from concepts introduced by Tim Berners-Lee and   the World-Wide Web global information initiative.  Relative URLs are   described as "Partial URLs" inRFC 1630 [1].  That description was   expanded for inclusion as an appendix for an early draft ofRFC 1738,   "Uniform Resource Locators (URL)" [2].  However, after further   discussion, the URI-WG decided to specify Relative URLs separately   from the primary URL draft.   This document is intended to fulfill the recommendations for Internet   Resource Locators as stated in [6].  It has benefited greatly from   the comments of all those participating in the URI-WG.  Particular   thanks go to Larry Masinter, Michael A. Dolan, Guido van Rossum, Dave   Kristol, David Robinson, and Brad Barber for identifying   problems/deficiencies in earlier drafts.8.  References   [1] 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.   [2] Berners-Lee, T., Masinter, L., and M. McCahill, Editors, "Uniform       Resource Locators (URL)",RFC 1738, CERN, Xerox Corporation,       University of Minnesota, December 1994.   [3] Berners-Lee T., and D. Connolly, "HyperText Markup Language       Specification -- 2.0", Work in Progress, MIT, HaL Computer       Systems, February 1995.       <URL:http://www.ics.uci.edu/pub/ietf/html/>Fielding                    Standards Track                    [Page 14]

RFC 1808           Relative Uniform Resource Locators          June 1995   [4] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail       Extensions): Mechanisms for Specifying and Describing the Format       of Internet Message Bodies",RFC 1521, Bellcore, Innosoft,       September 1993.   [5] Crocker, D., "Standard for the Format of ARPA Internet Text       Messages", STD 11,RFC 822, UDEL, August 1982.   [6] Kunze, J., "Functional Recommendations for Internet Resource       Locators",RFC 1736, IS&T, UC Berkeley, February 1995.9.  Author's Address   Roy T. Fielding   Department of Information and Computer Science   University of California   Irvine, CA  92717-3425   U.S.A.   Tel: +1 (714) 824-4049   Fax: +1 (714) 824-4056   EMail: fielding@ics.uci.edu10.  Appendix - 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) [3] 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   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>Fielding                    Standards Track                    [Page 15]

RFC 1808           Relative Uniform Resource Locators          June 1995   A parser reading the example document should interpret the given   relative URL "../x" as representing the absolute URL      <URL:http://www.ics.uci.edu/Test/a/x>   regardless of the context in which the example document was obtained.Fielding                    Standards Track                    [Page 16]