Information Assets with Identifiers in Public Namespaces. 
      [RFC4452] (section 3) defines an "info" registry 
        of public namespaces, which is maintained by NISO and can be accessed 
        from [http://info-uri.info/].
Access to data available from mail servers
Real-Time Streaming Protocol (RTSP) over TLS
Real-Time Streaming Protocol (RTSP) over unreliable datagram transport
Extensible Messaging and Presence Protocol

[RFC7232, Section 4.2][RFC8144, Section 3.2]