2.2.2.2.3 Marker Attribute (a=marker)
The marker attribute is specified for use at the SDP session level. If the content is stored in an ASF file, the ASF file can contain a Marker Object (defined in [ASF], section 3.7). The purpose of the marker attribute is to convey the information in the ASF Marker Object.
The syntax of the attribute is defined as follows:
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marker = "a=marker:-" 1*( SP marker-entry ) marker-entry = presentation-time ";" marker-desc presentation-time = 1*10DIGIT marker-desc = *ALPHA ; see below for restrictions on this field
Each marker-entry syntax element corresponds to one Marker entry in the ASF Marker Object.
The presentation-time syntax element is equal to the value of the Presentation Time field of the ASF Marker entry, subtracted by the value of the Preroll field in the ASF File Properties Object (see [ASF], section 3.2). The presentation-time syntax element uses millisecond time units. The valid numerical range of the presentation-time syntax element is 0 to 4294967265, inclusive.
The marker-desc syntax element is equal to the value of the Marker Description field of the ASF Marker entry.
Because the Marker Description field in the ASF Marker entry is an array of Unicode characters, the mapping of this field to the marker-desc syntax element MUST follow these rules:
The Unicode characters MUST be converted to UTF-8 characters, as specified in [RFC3629].
Any UTF-8 characters that cannot be used in a Uniform Resource Identifier (URI) MUST be encoded by using percent-encoding, as specified in [RFC3986] section 2.1. The resulting characters are used as the value of the marker-desc syntax element.