Control URL Attribute ("a=control")

The control URL attribute ("a=control") MUST be specified for each media description except if the SDP contains only a single media description. The control URL attributes SHOULD be expressed as relative URLs, using the URL specified by using the control attribute at the SDP session level as the base URL.

When converting a relative URL from the media description level to an absolute URL, the URL specified by the SDP session-level control attribute MUST be used as the base URL. If the session-level control attribute is missing, the base URL MUST be determined by following the rules, as specified in [RFC2326] section C.1.1.

The following example shows the "a=control" bandwidth modifier: