(registered by RFC 3016; updated by RFC 6416, published October 2011) Type name: audio Subtype name: MP4A-LATM Required parameters: "rate": the "rate" parameter indicates the RTP timestamp "clock rate". The default value is 90000. Other rates MAY be indicated only if they are set to the same value as the audio sampling rate (number of samples per second). In the presence of SBR, the sampling rates for the core encoder/ decoder and the SBR tool are different in most cases. Therefore, this parameter SHALL NOT be considered as the definitive sampling rate. If this parameter is used, the server must follow the rules below: * When the presence of SBR is not explicitly signaled by the optional SDP parameters such as "object", "profile-level-id", or "config", this parameter SHALL be set to the core codec sampling rate. * When the presence of SBR is explicitly signaled by the optional SDP parameters such as "object", "profile-level-id", or "config", this parameter SHALL be set to the SBR sampling rate. NOTE: The optional parameter "SBR-enabled" in SDP "a=fmtp" is useful for implicit HE AAC / HE AAC v2 signaling. But the "SBR-enabled" parameter can also be used in the case of explicit HE AAC / HE AAC v2 signaling. Therefore, its existence (in itself) is not the criteria to determine whether or HE AAC / HE AAC v2 signaling is explicit. Optional parameters: "profile-level-id": a decimal representation of MPEG-4 Audio Profile Level indication value defined in [14496-3]. This parameter indicates which MPEG-4 Audio tool subsets the decoder is capable of using. If this parameter is not specified in the capability exchange or session setup procedure, its default value of 30 (Natural Audio Profile/Level 1) is used. "MPS-profile-level-id": a decimal representation of the MPEG Surround Profile Level indication as defined in [14496-3]. This parameter indicates the support of the MPEG Surround profile and level by the decoder to be capable to decode the stream. "object": a decimal representation of the MPEG-4 Audio Object Type value defined in [14496-3]. This parameter specifies the tool to be used by the decoder. It CAN be used to limit the capability within the specified "profile-level-id". "bitrate": the data rate for the audio bitstream. "cpresent": a boolean parameter that indicates whether audio payload configuration data has been multiplexed into an RTP payload (see Section 6.1). A 0 indicates the configuration data has not been multiplexed into an RTP payload, and in that case, the "config" parameter MUST be present; a 1 indicates that it has been multiplexed. The default if the parameter is omitted is 1. If this parameter is set to 1 and the "config" parameter is present, the multiplexed configuration data and the value of the "config" parameter SHALL be consistent. "config": a hexadecimal representation of an octet string that expresses the audio payload configuration data "StreamMuxConfig", as defined in [14496-3]. Configuration data is mapped onto the octet string in an MSB-first basis. The first bit of the configuration data SHALL be located at the MSB of the first octet. In the last octet, zero-padding bits, if necessary, SHALL follow the configuration data. Senders MUST set the StreamMuxConfig elements taraBufferFullness and latmBufferFullness to their largest respective value, indicating that buffer fullness measures are not used in SDP. Receivers MUST ignore the value of these two elements contained in the "config" parameter. "MPS-asc": a hexadecimal representation of an octet string that expresses audio payload configuration data "AudioSpecificConfig", as defined in [14496-3]. If this parameter is not present, the relevant signaling is performed by other means (e.g., in-band or contained in the "config" string). The same mapping rules as for the "config" parameter apply. "ptime": duration of each packet in milliseconds. "SBR-enabled": a boolean parameter that indicates whether SBR-data can be expected in the RTP-payload of a stream. This parameter is relevant for an SBR-capable decoder if the presence of SBR cannot be detected from an out-of-band decoder configuration (e.g., contained in the "config" string). If this parameter is set to 0, a decoder MAY expect that SBR is not used. If this parameter is set to 1, a decoder CAN up-sample the audio data with the SBR tool, regardless of whether or not SBR data is present in the stream. If the presence of SBR cannot be detected from out-of-band configuration and the "SBR-enabled" parameter is not present, the parameter defaults to 1 for an SBR-capable decoder. If the resulting output sampling rate or the computational complexity is not supported, the SBR tool can be disabled or run in down-sampled mode. The timestamp resolution at the RTP layer is determined by the "rate" parameter. Published specification: Encoding specifications are provided in [14496-3]. The RTP payload format specification is described in [RFC6416]. Encoding considerations: This type is only defined for transfer via RTP. Security considerations: See Section 10 of [RFC6416]. Interoperability considerations: MPEG-4 Audio provides a large and rich set of tools for the coding of audio objects. For effective implementation of the standard, subsets of the MPEG-4 Audio tool sets similar to those used in MPEG-4 Visual have been provided (see Section 7.1). The audio stream SHALL be compliant with the MPEG-4 Audio Profile@ Level specified by the parameters "profile-level-id" and "MPS-profile-level-id". Interoperability between a sender and a receiver may be achieved by specifying the parameters "profile-level-id" and "MPS-profile-level-id" or by arranging in the capability exchange procedure to set this parameter mutually to the same value. Furthermore, the "object" parameter can be used to limit the capability within the specified Profile@Level in the capability exchange. Applications that use this media type: Audio and video streaming and conferencing tools. Additional information: none Personal and email address to contact for further information: See Authors' Addresses section at the end of [RFC6416]. Intended usage: COMMON Author: See Authors' Addresses section at the end of [RFC6416]. Change controller: IETF Audio/Video Transport Payloads working group delegated from the IESG.