MIME Type Registration of RTP Payload Formats
obsoleted by
- rfc-4855 — Media Type Registration of RTP Payload Formats
- rfc-4856 — Media Type Registration of Payload Formats in the RTP Profile for Audio and Video Conferences
updated by
Extracted elements (20)
The MIME namespace, originally designed for asynchronous Internet mail (RFC 2048), was reused for RTP payload type identification because it already provided a well-known media type registry, avoiding the need for a parallel RTP-specific naming scheme.
When an RTP payload format MIME type already existed for non-RTP transport, the procedure updates the existing registration's encoding considerations rather than creating a duplicate registration, keeping the MIME namespace non-redundant.
For audio/L16, the type is compatible with the encoding used in WAV (Microsoft Windows RIFF) and Apple AIFF formats, and with the 'sox' and 'rateconv' public-domain programs.
Payload format encoding names defined in the RTP Profile are case-insensitive in both RTP and MIME contexts; similarly, parameter names are case-insensitive in MIME types and in the default mapping to SDP a=fmtp.
The audio/L16 subtype had previously been registered via RFC 2586 for non-RTP transports; RFC 3555 incorporates that prior registration and augments it with RTP-specific parameters and encoding considerations.
For audio/L16 RTP transport, the recommended sample rates are 8000, 11025, 16000, 22050, 24000, 32000, 44100, and 48000 samples/second; other values are permissible but these are RECOMMENDED.
For audio/L16, the 'emphasis' parameter MUST be omitted if no analog preemphasis was applied; the 'channel-order' parameter MUST be omitted when the AIFF-C channel order convention is in use.
In audio/RED, if the content-type string within 'pt' includes a comma, then that content-type string MUST be a quoted-string; the outer list is itself a quoted-string so inner quotes MUST use backslash quoting per RFC 2045.
The encoding considerations field of an RTP MIME type registration MUST note that the type can be transferred via RTP.
The published specification for an RTP payload format MIME registration MUST include the RTP timestamp clock rate (or multiple rates for audio encodings with multiple sampling rates).
The set of allowed fmtp parameters for a payload format is defined by its RFC and MUST NOT be extended by the MIME subtype registration without a corresponding revision of the payload format specification.
For all video payload formats registered here, the RTP timestamp clock rate is always 90000 Hz; the 'rate', 'channels', 'ptime', and 'maxptime' parameters are not applicable to video types.
For audio/MPA (MPEG audio), the RTP timestamp clock rate is always 90000 Hz regardless of the audio sample rate; the 'channels' parameter does not apply, and the selected layer, sampling rate, and mode are signaled in-band in the payload.
MIME type maps to SDP 'm=' media name; MIME subtype maps to 'a=rtpmap' encoding name; 'rate' and 'channels' go in 'a=rtpmap'; 'ptime'/'maxptime' go in 'a=ptime'/'a=maxptime'; payload-format-specific parameters go in 'a=fmtp'.
The audio/RED MIME type requires a 'pt' parameter: a comma-separated quoted-string list of RTP payload types, where dynamic payload types are mapped as 'dynamic-type=content-type'. For SDP, commas in pt are replaced with slashes in a=fmtp.
The optional parameters 'ptime' and 'maxptime' specify the recommended and maximum duration in milliseconds of media in a packet; 'channels' specifies the number of audio channels. These apply to most audio payload formats.
The 'rate' parameter specifies the RTP timestamp clock rate; it is required for payload formats that do not have a fixed clock rate. Most audio formats set rate equal to the audio sampling rate.
This document registers 13 video MIME subtypes under type 'video': BT656, CelB, JPEG, H261, H263, H263-1998, H263-2000, MPV, MP2T, MP1S, MP2P, BMPEG, nv — all with a fixed RTP timestamp clock rate of 90000 Hz.
This document registers 21 audio MIME subtypes under type 'audio': DVI4, G722, G723, G726-16, G726-24, G726-32, G726-40, G728, G729, G729D, G729E, GSM, GSM-EFR, L8, L16, LPC, MPA, PCMA, PCMU, QCELP, RED, VDVI — all defined for transfer via RTP per RFC 3551.
Several audio and video encodings are suitable for hiding data via steganography. The MIME subtype registration procedure itself imposes no security risks, but implementors should consult RFC 3550 for security considerations of RTP transport, including use of encryption for confidentiality.