When a system uses a local addressing scheme, it is useful to provide
   a mapping from local addresses into URIs so that references to
   objects within the addressing scheme may be referred to globally, and
   possibly accessed through gateway servers.

   For a new naming scheme, any mapping scheme may be defined provided
   it is unambiguous, reversible, and provides valid URIs.  It is
   recommended that where hierarchical aspects to the local naming
   scheme exist, they be mapped onto the hierarchical URL path syntax in
   order to allow the partial form to be used.

   It is also recommended that the conventional scheme below be used in
   all cases except for any scheme which encodes binary data as opposed
   to text, in which case a more compact encoding such as pure
   hexadecimal or base 64 might be more appropriate.  For example, the
   conventional URI encoding method is used for mapping WAIS, FTP,
   Prospero and Gopher addresses in the URI specification.

   CONVENTIONAL URI ENCODING SCHEME

      Where the local naming scheme uses ASCII characters which are not
      allowed in the URI, these may be represented in the URL by a
      percent sign "%" immediately followed by two hexadecimal digits
      (0-9, A-F) giving the ISO Latin 1 code for that character.
      Character codes other than those allowed by the syntax shall not
      be used unencoded in a URI.

   REDUCED OR INCREASED SAFE CHARACTER SETS

      The same encoding method may be used for encoding characters whose
      use, although technically allowed in a URI, would be unwise due to
      problems of corruption by imperfect gateways or misrepresentation
      due to the use of variant character sets, or which would simply be
      awkward in a given environment.  Because a % sign always indicates
      an encoded character, a URI may be made "safer" simply by encoding
      any characters considered unsafe, while leaving already encoded
      characters still encoded.  Similarly, in cases where a larger set
      of characters is acceptable, % signs can be selectively and
      reversibly expanded.

      Before two URIs can be compared, it is therefore necessary to
      bring them to the same encoding level.

      However, the reserved characters mentioned above have a quite
      different significance when encoded, and so may NEVER be encoded
      and unencoded in this way.




 
RFC 1630                      URIs in WWW                      June 1994


      The percent sign intended as such must always be encoded, as its
      presence otherwise always indicates an encoding.  Sequences which
      start with a percent sign but are not followed by two hexadecimal
      characters are reserved for future extension.  (See Example 3.)

   Example 1

   The URIs

                http://info.cern.ch/albert/bertram/marie-claude

   and

                http://info.cern.ch/albert/bertram/marie%2Dclaude

   are identical, as the %2D encodes a hyphen character.

   Example 2

   The URIs

                http://info.cern.ch/albert/bertram/marie-claude

   and

                http://info.cern.ch/albert/bertram%2Fmarie-claude

   are NOT identical, as in the second case the encoded slash does not
   have hierarchical significance.

   Example 3

   The URIs

                fxqn:/us/va/reston/cnri/ietf/24/asdf%*.fred

   and

                news:12345667123%asdghfh@info.cern.ch

   are illegal, as all % characters imply encodings, and there is no
   decoding defined for "%*"  or "%as" in this recommendation.

Partial (relative) form

   Within a object whose URI is well defined, the URI of another object