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>.

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].