2.2.2.4 ASF Payload Extension Systems
If the RTP Payload Format for WMA and WMV [WMRTP] is used, or if the Generic RTP Payload Format defined in section 2.2.2.3 is used, then the RTP packets can include the ASF Payload Extension Systems extension defined in section 2.2.1.1. If the ASF Payload Extension Systems extension will be used, then the ASF Payload Extension Systems (see [ASF], section 7.3) MUST be listed on the a=fmtp line.
An ASF Payload Extension Systems is listed by appending the extsys parameter to the a=fmtp line. The extsys parameter is defined as follows:
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extsys = ";extsys=" ExtensionID "/" ExtensionLength [ "/" ExtensionInfo ] ExtensionLength = ( 1*3DIGIT ) / 65535 ExtensionInfo = 1 * ( 2HEXDIG )
The value of the ExtensionID parameter is a UUID, as specified in [RFC4122], section 3, identifying the ASF Extension System. See [ASF] section 10.13 for a list of ASF Extension System IDs.
The value of the ExtensionLength field is the number of bytes used by the ASF Extension System data in the RTP packet. If the data is of variable size, this field MUST be set to 65535. If the data is of fixed size, the field MUST be set to a value between 0 and 255, inclusive.
If the value of the ExtensionSystemInfoLength field in the definition ASF Payload Extension System is greater than zero, then the ExtensionInfo field MUST be present. Otherwise, the ExtensionInfo field MUST be omitted. See section 4.1 in [ASF] for details about how to locate the ExtensionSystemInfoLength field in the ASF file.
If the ExtensionInfo field is present, then it MUST be set to the value of the ExtensionSystemInfo field from the definition of the ASF Payload Extension System. Each byte in the ExtensionSystemInfo field MUST be written as two hexadecimal digits in the ExtensionInfo field. See section 4.1 in [ASF] for details about how to locate the ExtensionSystemInfo field in the ASF file.