outside of it.  Similarly, documents located at a particular site are
   much more likely to refer to other resources at that site than to
   resources at remote sites.

   Relative addressing of URI 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 URI.
   Furthermore, such document trees can be moved, as a whole, without
   changing any of the relative references.  Experience within the WWW
   has demonstrated that the ability to perform relative referencing is
   necessary for the long-term usability of embedded URI.

   The syntax for relative URI takes advantage of the  syntax
   of <absoluteURI> (Section 3) in order to express a reference that is
   relative to the namespace of another hierarchical URI.

      relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]

   A relative reference beginning with two slash characters is termed a
   network-path reference, as defined by  in Section 3.  Such
   references are rarely used.

   A relative reference beginning with a single slash character is
   termed an absolute-path reference, as defined by  in
   Section 3.

   A relative reference that does not begin with a scheme name or a
   slash character is termed a relative-path reference.

      rel_path      = rel_segment [ abs_path ]

      rel_segment   = 1*( unreserved | escaped |
                          ";" | "@" | "&" | "=" | "+" | "$" | "," )

   Within a relative-path reference, the complete path segments "." and
   ".." have special meanings: "the current hierarchy level" and "the
   level above this hierarchy level", respectively.  Although this is
   very similar to their use within Unix-based filesystems to indicate
   directory levels, these path components are only considered special
   when resolving a relative-path reference to its absolute form
   (Section 5.2).

   Authors should be aware that a path segment which contains a colon
   character cannot be used as the first segment of a relative URI path
   (e.g., "this:that"), because it would be mistaken for a scheme name.





 
RFC 2396                   URI Generic Syntax                August 1998


   It is therefore necessary to precede such segments with other
   segments (e.g., "./this:that") in order for them to be referenced as
   a relative path.

   It is not necessary for all URI within a given scheme to be
   restricted to the  syntax, since the hierarchical
   properties of that syntax are only necessary when relative URI are
   used within a particular document.  Documents can only make use of
   relative URI when their base URI fits within the  syntax.
   It is assumed that any document which contains a relative reference
   will also have a base URI that obeys the syntax.  In other words,
   relative URI cannot be used within a document that has an unsuitable
   base URI.

   Some URI schemes do not allow a hierarchical syntax matching the
    syntax, and thus cannot use relative references.

5.1. Establishing a Base URI

   The term "relative URI" implies that there exists some absolute "base
   URI" against which the relative reference is applied.  Indeed, the
   base URI is necessary to define the semantics of any relative URI
   reference; without it, a relative reference is meaningless.  In order
   for relative URI to be usable within a document, the base URI of that
   document must be known to the parser.

   The base URI 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 URI
   has the highest precedence.  This can be visualized graphically as:

      .----------------------------------------------------------.
      |  .----------------------------------------------------.  |
      |  |  .----------------------------------------------.  |  |
      |  |  |  .----------------------------------------.  |  |  |
      |  |  |  |  .----------------------------------.  |  |  |  |
      |  |  |  |  |       <relative_reference>       |  |  |  |  |
      |  |  |  |  `----------------------------------'  |  |  |  |
      |  |  |  | (5.1.1) Base URI embedded in the       |  |  |  |
      |  |  |  |         document's content             |  |  |  |
      |  |  |  `----------------------------------------'  |  |  |
      |  |  | (5.1.2) Base URI of the encapsulating entity |  |  |