rfc1808.txt

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   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



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   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.













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   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




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      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.



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   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.









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   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.