4.1.3. Encoding reserved characters

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. 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

                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

                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

                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.