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rfc:rfc1808

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

Fielding 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 1995

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>.

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
 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 before

Fielding 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 in RFC 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 field

Fielding 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 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.

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 Sections 3.1 -- 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 of
         Section 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 in Section 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 in RFC 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" in RFC 1630 [1].  That description was
 expanded for inclusion as an appendix for an early draft of RFC 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.edu

10. 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]

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