Media Type Registration of Payload Formats in the RTP Profile for Audio and Video Conferences
The standard media type names for the audio and video encodings used in RTP conferencing sessions.
Casner, S., "Media Type Registration of Payload Formats in the RTP Profile for Audio and Video Conferences", RFC 4856, DOI 10.17487/RFC4856, February 2007, <https://www.rfc-editor.org/info/rfc4856>.
@misc{rfc4856,
series = {Request for Comments},
number = 4856,
howpublished = {RFC 4856},
publisher = {RFC Editor},
doi = {10.17487/RFC4856},
url = {https://www.rfc-editor.org/info/rfc4856},
author = {S. Casner},
title = {{Media Type Registration of Payload Formats in the RTP Profile for Audio and Video Conferences}},
pagetotal = 29,
year = 2007,
month = feb,
} RFC text
Contents
Network Working Group S. Casner
Request for Comments: 4856 Packet Design
Obsoletes: 3555 March 2007
Category: Standards Track
Media Type Registration of Payload Formats in the
RTP Profile for Audio and Video Conferences
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies media type registrations for the RTP payload
formats defined in the RTP Profile for Audio and Video Conferences.
Some of these may also be used for transfer modes other than RTP.
Table of Contents
1. Introduction ....................................................2
1.1. IANA Considerations ........................................2
1.2. Terminology ................................................3
2. Registrations for "Audio/Video Profile" .........................3
2.1. Audio Type Registrations ...................................3
2.2. Video Type Registrations ..................................24
3. Changes from RFC 3555 ..........................................25
4. Security Considerations ........................................26
5. References .....................................................27
5.1. Normative References ......................................27
5.2. Informative References ....................................27
1. Introduction
This document updates the media type registrations initially
specified in RFC 3555 for the Real-time Transport Protocol (RTP)
payload formats defined in the RTP Profile for Audio and Video
Conferences, RFC 3551 [1], as subtypes under the "audio" and "video"
media types. This document does not include media type registrations
for the RTP payload formats that are referenced in RFC 3551 but
defined in other RFCs. The media type registrations for those
payload formats are intended to be updated by including them in
revisions of the individual RFCs defining the payload formats.
The media type registrations specified here conform to the updated
template format and procedures in RFC 4288 [2] and RFC 4855 [3].
This update makes no technical changes in the registrations.
Together with RFC 4855, this document obsoletes RFC 3555.
1.1. IANA Considerations
As a consequence of the generalized applicability of the media types
registry as specified in RFC 4288, some changes in nomenclature are
needed in the RTP Payload Format section of the registry. In the
registry title "RTP Payload Format MIME types" and the introductory
text, "MIME" should be changed to "media". "MIME" should be deleted
from the table headings, leaving just "media type" and "subtype".
This document updates the media type registrations listed below to
conform to the revised registration format specified in RFC 4288 and
RFC 4855, so the reference for these media types should be changed
from RFC 3555 to this document. Some media type registrations
contained in RFC 3555 are omitted from this document; the existing
registrations for those types continue to be valid until updated by
other RFCs. There are no new registrations contained here.
audio/DVI4
audio/G722
audio/G723
audio/G726-16
audio/G726-24
audio/G726-32
audio/G726-40
audio/G728
audio/G729
audio/G729D
audio/G729E
audio/GSM
audio/GSM-EFR
audio/L8
audio/L16
audio/LPC
audio/PCMA
audio/PCMU
audio/VDVI
video/nv
Media type audio/L16 was initially registered via RFC 2586 for
transports other than RTP. That registration is incorporated here
and augmented with additional information for RTP transport.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4] and
indicate requirement levels for implementations compliant with this
specification.
2. Registrations for "Audio/Video Profile"
In the following sections, the RTP payload formats defined in the RTP
Profile for Audio and Video Conferences, RFC 3551 [1], are registered
as media types.
2.1. Audio Type Registrations
For most audio payload formats, the RTP timestamp clock rate is equal
to the sampling rate. Some payload formats operate only at one fixed
sampling rate, while others are adjustable.
These audio formats also include the optional parameters "ptime" to
specify the recommended length of time in milliseconds represented by
the media in a packet, and "maxptime" to specify the maximum amount
of media that can be encapsulated in each packet, expressed as time
in milliseconds. The "ptime" and "maxptime" parameters are defined
in the Session Description Protocol (SDP), RFC 4566 [5].
2.1.1. Registration of Media Type audio/DVI4
Type name: audio
Subtype name: DVI4
Required parameters:
rate: The RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000, but other rates may
be specified.
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550 [6]). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.2. Registration of Media Type audio/G722
Type name: audio
Subtype name: G722
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.3. Registration of Media Type audio/G723
Type name: audio
Subtype name: G723
Required parameters: none
Optional parameters:
ptime, maxptime: see RFC 4566
bitrate: the data rate in kb/s used or preferred for the audio
bit stream, with permissible values 5.3 or 6.3. If
unspecified, the bitrate may change from frame to frame as
indicated inband.
annexa: indicates that Annex A, voice activity detection, is
used or preferred. Permissible values are "yes" and "no"
(without the quotes); "yes" is implied if this parameter is
omitted.
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.4. Registration of Media Type audio/G726-16
Type name: audio
Subtype name: G726-16
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.5. Registration of Media Type audio/G726-24
Type name: audio
Subtype name: G726-24
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.6. Registration of Media Type audio/G726-32
Type name: audio
Subtype name: G726-32
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.7. Registration of Media Type audio/G726-40
Type name: audio
Subtype name: G726-40
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.8. Registration of Media Type audio/G728
Type name: audio
Subtype name: G728
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.9. Registration of Media Type audio/G729
Type name: audio
Subtype name: G729
Required parameters: none
Optional parameters:
ptime, maxptime: see RFC 4566
annexb: indicates that Annex B, voice activity detection, is
used or preferred. Permissible values are "yes" and "no"
(without the quotes); "yes" is implied if this parameter is
omitted.
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.10. Registration of Media Type audio/G729D
Type name: audio
Subtype name: G729D
Required parameters: none
Optional parameters:
ptime, maxptime: see RFC 4566
annexb: indicates that Annex B, voice activity detection, is
used or preferred. Permissible values are "yes" and "no"
(without the quotes); "yes" is implied if this parameter is
omitted.
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.11. Registration of Media Type audio/G729E
Type name: audio
Subtype name: G729E
Required parameters: none
Optional parameters:
ptime, maxptime: see RFC 4566
annexb: indicates that Annex B, voice activity detection, is
used or preferred. Permissible values are "yes" and "no"
(without the quotes); "yes" is implied if this parameter is
omitted.
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.12. Registration of Media Type audio/GSM
Type name: audio
Subtype name: GSM
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.13. Registration of Media Type audio/GSM-EFR
Type name: audio
Subtype name: GSM-EFR
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.14. Registration of Media Type audio/L8
Type name: audio
Subtype name: L8
Required parameters:
rate: the RTP timestamp clock rate
Optional parameters:
channels: how many audio streams are interleaved -- defaults
to 1; stereo would be 2, etc. Interleaving takes place
between individual one-byte samples. The channel order is as
specified in RFC 3551.
ptime, maxptime: see RFC 4566
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.15. Registration of Media Type audio/L16
Media type audio/L16 was initially registered via RFC 2586 [10] for
transports other than RTP. That registration is incorporated here
and augmented with additional information for RTP transport.
Type name: audio
Subtype name: L16
Required parameters:
rate: number of samples per second -- For non-RTP transport,
the permissible values for rate are 8000, 11025, 16000, 22050,
24000, 32000, 44100, and 48000 samples per second. For RTP
transport, other values are permissible but the aforementioned
values are RECOMMENDED. For RTP, the rate parameter indicates
the RTP timestamp clock rate, which is equal to the sample
rate.
Optional parameters:
channels: how many audio streams are interleaved -- defaults
to 1; stereo would be 2, etc. Interleaving takes place
between individual two-byte samples. The channel order is as
specified in RFC 3551 unless a channel-order parameter is also
present.
emphasis: analog preemphasis applied to the signal before
quantization. The only emphasis value defined here is
emphasis=50-15 to indicate the 50/15 microsecond preemphasis
used with Compact Discs. This parameter MUST be omitted if no
analog preemphasis was applied. Note that this is a stream
property parameter, not a receiver configuration parameter.
Thus, if parameters are negotiated, it may not be possible for
the sender to comply with a receiver request for a particular
setting.
channel-order: specifies the sample interleaving order for
multiple-channel audio streams (see RFC 3190 [7], Section 7).
Permissible values are DV.LRLsRs, DV.LRCS, DV.LRCWo,
DV.LRLsRsC, DV.LRLsRsCS, DV.LmixRmixTWoQ1Q2,
DV.LRCWoLsRsLmixRmix, DV.LRCWoLs1Rs1Ls2Rs2, DV.LRCWoLsRsLcRc.
For interoperation with DV video systems, only a subset of
these channel combinations is specified for use with 20-bit
linear encoding in the DV video specification [9]; those are
DV.LRLsRs, DV.LRCS, DV.LmixRmixTWoQ1Q2. This parameter MUST
be omitted when the AIFF-C channel order convention (see RFC
3551) is in use.
For RTP, ptime: RECOMMENDED duration of each packet in
milliseconds.
For RTP, maxptime: maximum duration of each packet in
milliseconds.
Encoding considerations:
Audio data is binary data, and must be encoded for non-binary
transport; the Base64 encoding is suitable for Email. Note
that audio data does not compress easily using lossless
compression.
Security considerations:
Audio/L16 data is believed to offer no security risks. This
media type does not carry active content. The encoding is not
compressed. See Section 4 of RFC 4856.
Interoperability considerations:
This type is compatible with the encoding used in the WAV
(Microsoft Windows RIFF) and Apple AIFF union types, and with
the public domain "sox" and "rateconv" programs.
Published specification:
RFC 2586 for non-RTP transports, RFC 3551 for RTP
Applications that use this media type:
The public domain "sox" and "rateconv" programs accept this
type.
Additional information:
Magic number(s): none
File extension(s): WAV L16
Macintosh file type code: AIFF
Person to contact for further information:
James Salsman <jps-L16@bovik.org>
Intended usage:
Common
It is expected that many audio and speech applications will
use this type. Already the most popular platforms provide
this type with the rate=11025 parameter, referred to as "radio
quality speech".
Restrictions on usage:
In addition to file-based transfer methods, this type is also
defined for transfer via RTP (RFC 3550).
Author:
James Salsman for non-RTP transports.
Stephen Casner for RTP transport.
Change controller:
James Salsman for non-RTP transports.
For RTP transport, IETF Audio/Video Transport working group
delegated from the IESG.
2.1.16. Registration of Media Type audio/LPC
Type name: audio
Subtype name: LPC
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.17. Registration of Media Type audio/PCMA
Type name: audio
Subtype name: PCMA
Required parameters:
rate: The RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000, but other rates may
be specified.
Optional parameters:
channels: how many audio streams are interleaved -- defaults
to 1; stereo would be 2, etc. Interleaving takes place
between individual one-byte samples. The channel order is as
specified in RFC 3551.
ptime, maxptime: see RFC 4566
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.18. Registration of Media Type audio/PCMU
Type name: audio
Subtype name: PCMU
Required parameters:
rate: The RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000, but other rates may
be specified.
Optional parameters:
channels: how many audio streams are interleaved -- defaults
to 1; stereo would be 2, etc. Interleaving takes place
between individual one-byte samples. The channel order is as
specified in RFC 3551.
ptime, maxptime: see RFC 4566
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.1.19. Registration of Media Type audio/VDVI
Type name: audio
Subtype name: VDVI
Required parameters: none
Optional parameters: ptime, maxptime (see RFC 4566)
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
2.2. Video Type Registrations
For most video payload formats, including the one registered here,
the RTP timestamp clock rate is always 90000 Hz, so the "rate"
parameter is not applicable. Likewise, the "channel" parameter is
not used with video, while "ptime" and "maxptime" could be but
typically are not.
2.2.1. Registration of Media Type video/nv
Type name: video
Subtype name: nv
Required parameters: none
Optional parameters: none
Encoding considerations:
This media type is framed binary data (see Section 4.8 in RFC
4288).
Security considerations:
This media type does not carry active content. It does
transfer compressed data. See Section 4 of RFC 4856.
Interoperability considerations: none
Published specification: RFC 3551
Applications that use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Stephen Casner <casner@acm.org>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP (RFC 3550). Transfer within
other framing protocols is not defined at this time.
Author:
Stephen Casner
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
3. Changes from RFC 3555
RFC 3555 is obsoleted by the combination of RFC 4855 [3] and this
document. RFC 4855 retains the specification of procedures and
requirements from RFC 3555, while the media type registrations from
RFC 3555 were extracted into this document. The media type
registrations for the RTP payload formats that are referenced in RFC
3551 [1], but defined in other RFCs, have been elided from this
document because those registrations are intended to be updated by
including them in revisions of the individual RFCs defining the
payload formats.
The media type registrations in this document have been updated to
conform to the revised media type registration procedures in RFC 4288
[2] and RFC 4855. Whereas RFC 3555 required the encoding
considerations to specify transfer via RTP, that is now specified
under restrictions on usage. The encoding considerations now warn
that these types are framed binary data. The change controller is
also now identified according to current conventions. The optional
parameter "channels" was clarified for audio subtypes L8, PCMA, and
PCMU. Finally, reference [9], which was missing from RFC 3555, has
been corrected.
4. Security Considerations
This memo specifies media type registrations for the transfer of
several compressed audio and video data encodings via RTP, so
implementations using these media types are subject to the security
considerations discussed in the RTP specification [8].
None of these media types carry "active content" that could impose
malicious side-effects upon the receiver. The content consists
solely of compressed audio or video data to be decoded and presented
as sound or images. However, several audio and video encodings are
perfect for hiding data using steganography.
A potential denial-of-service threat exists for data encodings using
compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the stream, which are complex to decode and cause the receiver
to be overloaded. However, none of the encodings registered here has
an expansion factor greater than about 20, and all are considered
relatively simple by modern standards (some are implemented on
handheld devices and most were implemented on general-purpose
computers ten years ago).
As with any IP-based protocol, in some circumstances a receiver may
be overloaded simply by the receipt of too many packets, either
desired or undesired. Network-layer authentication MAY be used to
discard packets from undesired sources, but the processing cost of
the authentication itself may be too high.
RTP may be sent via IP multicast, which provides no direct means for
a sender to know all the receivers of the data sent and therefore no
measure of privacy. Rightly or not, users may be more sensitive to
privacy concerns with audio and video communication than they have
been with more traditional forms of network communication.
Therefore, the use of security mechanisms with RTP to provide
confidentiality and integrity of the data is important. Because the
data compression used with these media types is applied end-to-end,
encryption may be performed after compression so there is no conflict
between the two operations.
5. References
5.1. Normative References
[1] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 3551, July 2003.
[2] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[3] Casner, S., "Media Type Registration of RTP Payload Types", RFC
4855, January 2007.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[6] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.
[7] Kobayashi, K., Ogawa, A., Casner, S. and C. Bormann, "RTP
Payload Format for 12-bit DAT Audio and 20- and 24-bit Linear
Sampled Audio", RFC 3190, January 2002.
[8] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.
5.2. Informative References
[9] IEC 61834, Helical-scan digital video cassette recording system
using 6,35 mm magnetic tape for consumer use (525-60, 625-50,
1125-60, and 1250-50 systems), August 1998.
[10] Salsman, J. and H. Alvestrand, "The Audio/L16 MIME content
type", RFC 2586, May 1999.
Author's Address
Stephen L. Casner
Packet Design
3400 Hillview Avenue, Building 3
Palo Alto, CA 94304
United States
Phone: +1 650 739-1843
EMail: casner@acm.org
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contained in BCP 78, and except as set forth therein, the authors
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ISO Plain Language
On this page
§1Introduction
RFC 4856 updates the media type registrations initially specified in RFC 3555 for the RTP payload formats defined in the RTP Profile for Audio and Video Conferences, RFC 3551. They are registered as subtypes under the audio and video media types.
The document deliberately leaves out the payload formats that RFC 3551 references but that other RFCs define. Those registrations are intended to be updated by including them in revisions of the individual RFCs that define the payload formats.
The registrations follow the updated template format and procedures in RFC 4288 and RFC 4855. The update makes no technical changes in the registrations, and together with RFC 4855 this document obsoletes RFC 3555. No new registrations are contained here; the existing registrations for the omitted types remain valid until updated elsewhere.
This RFC lists the names for the encodings that the RTP conferencing profile uses. The names and the parameters attached to them are the same as before; only the format of the registration and the place it lives have changed.
§1.1Registry Nomenclature
Because the media types registry is now generalized, as specified in RFC 4288, some names in the registry have to change. In the registry title "RTP Payload Format MIME types" and in the introductory text, "MIME" should be changed to "media". "MIME" should be deleted from the table headings, leaving just "media type" and "subtype".
The registrations listed in this document are updated to conform to the revised registration format, so the reference for these media types changes from RFC 3555 to this document.
The registry used to be called a MIME registry. Since the registry now covers media types in general, the label changes to "media type" and the word MIME is dropped from the table headings.
§2The Registered Media Types
- Media typeNames the encoding and its parameters, for example audio/PCMA with rate=8000
- RTPCarries the framed binary data of the named encoding
- IPDelivers the packets, unicast or multicast
Section 2 registers the RTP payload formats of RFC 3551 as media types. The audio subtypes are DVI4, G722, G723, G726-16, G726-24, G726-32, G726-40, G728, G729, G729D, G729E, GSM, GSM-EFR, L8, L16, LPC, PCMA, PCMU and VDVI. The video subtype is nv, which has no required or optional parameters.
Each registration states the encoding considerations, the security considerations, the interoperability considerations, the published specification, the applications that use the media type, the intended usage and the restrictions on usage. For the audio types the encoding considerations say the media type is framed binary data, as described in Section 4.8 of RFC 4288. Their restrictions on usage say that the media type depends on RTP framing and is therefore only defined for transfer via RTP, as given in RFC 3550, and that transfer within other framing protocols is not defined at this time. The intended usage is COMMON, and the change controller is the IETF Audio/Video Transport working group delegated from the IESG.
Media type audio/L16 was initially registered by RFC 2586 for transports other than RTP. That registration is incorporated here and augmented with additional information for RTP transport.
Each name gets the same set of notes: how the data is encoded, what it is used for, who looks after it, and where it may be carried. Nearly all of them are only defined for RTP, because the format depends on RTP framing.
| Media type | Required parameters | Optional parameters |
|---|---|---|
| audio/DVI4 | rate | ptime, maxptime |
| audio/G722 | none | ptime, maxptime |
| audio/G723 | none | ptime, maxptime, bitrate, annexa |
| audio/G726-16 | none | ptime, maxptime |
| audio/G726-24 | none | ptime, maxptime |
| audio/G726-32 | none | ptime, maxptime |
| audio/G726-40 | none | ptime, maxptime |
| audio/G728 | none | ptime, maxptime |
| audio/G729 | none | ptime, maxptime |
| audio/G729D | none | ptime, maxptime |
| audio/G729E | none | ptime, maxptime |
| audio/GSM | none | ptime, maxptime |
| audio/GSM-EFR | none | ptime, maxptime |
| audio/L8 | rate | channels, ptime, maxptime |
| audio/L16 | rate | channels, emphasis, channel-order, ptime, maxptime |
| audio/LPC | none | ptime, maxptime |
| audio/PCMA | rate | channels, ptime, maxptime |
| audio/PCMU | rate | channels, ptime, maxptime |
| audio/VDVI | none | ptime, maxptime |
| video/nv THIS RFC | none | none |
§2.1Audio Parameters
For most audio payload formats the RTP timestamp clock rate is equal to the sampling rate. Some payload formats operate only at one fixed sampling rate, while others are adjustable.
Five audio types require a rate parameter: audio/DVI4, audio/L8, audio/L16, audio/PCMA and audio/PCMU. For DVI4, PCMA and PCMU the rate is the RTP timestamp clock rate, which equals the sampling rate, and the typical rate is 8000, though other rates may be specified. For L8 the rate is the RTP timestamp clock rate, and for L16 it is the number of samples per second, which for RTP indicates the RTP timestamp clock rate and equals the sample rate.
The audio types also include the optional parameters ptime, which specifies the recommended length of time in milliseconds represented by the media in a packet, and maxptime, which specifies the maximum amount of media that can be encapsulated in each packet, also expressed in milliseconds. Both are defined in the Session Description Protocol, RFC 4566.
Some types add parameters of their own. audio/G723 has bitrate, the data rate in kb/s used or preferred for the audio bit stream, with permissible values 5.3 or 6.3, and if it is unspecified the bitrate may change from frame to frame as indicated inband. It also has annexa, which indicates that Annex A voice activity detection is used or preferred, with permissible values "yes" and "no" and "yes" implied if the parameter is omitted. The channels parameter, which says how many audio streams are interleaved and defaults to 1, with stereo being 2, appears on L8, L16, PCMA and PCMU, and interleaving takes place between individual one-byte samples for L8, PCMA and PCMU and between individual two-byte samples for L16.
The rate parameter says how many samples per second the audio has, and it is also the clock the receiver uses for the packet timestamps. Two more optional parameters, ptime and maxptime, say how much audio should be in one packet and how much is the most that may be put in one. Some codecs add a preferred bit rate or a switch for voice activity detection.
- rateThe sampling rate, which for most audio types is also the RTP timestamp clock rate.
- channelsHow many audio streams are interleaved. Defaults to 1; stereo would be 2.
- ptimeThe recommended length of media in a packet, in milliseconds.
- maxptimeThe most media that may be put in one packet, in milliseconds.
- bitrateThe data rate in kb/s used or preferred for the bit stream. G723 allows 5.3 or 6.3.
- annexaWhether Annex A voice activity detection is used or preferred. Values are yes and no.
- emphasisThe analog preemphasis applied before quantization. Only 50-15 is defined.
- channel-orderThe sample interleaving order for multiple-channel audio, such as DV.LRLsRs.
§2.1.15The audio/L16 Registration
The registration of audio/L16 shows what a full entry looks like, because it is the one type that also carries parameters for transport other than RTP.
The required parameter is rate, the number of samples per second. For non-RTP transport the permissible values are 8000, 11025, 16000, 22050, 24000, 32000, 44100 and 48000 samples per second. For RTP transport other values are permissible, but those values are RECOMMENDED, and the rate parameter indicates the RTP timestamp clock rate, which equals the sample rate.
Three optional parameters are defined. channels says how many audio streams are interleaved, defaults to 1, and for L16 the interleaving takes place between individual two-byte samples. emphasis says what analog preemphasis was applied to the signal before quantization; the only value defined here is emphasis=50-15, indicating the 50/15 microsecond preemphasis used with Compact Discs, and the parameter MUST be omitted if no analog preemphasis was applied. It is a stream property parameter and not a receiver configuration parameter, so if parameters are negotiated it may not be possible for the sender to comply with a receiver request for a particular setting. channel-order specifies the sample interleaving order for multiple-channel audio streams, with values such as DV.LRLsRs, DV.LRCS and DV.LmixRmixTWoQ1Q2; only a subset of these is specified for use with 20-bit linear encoding in the DV video specification, and the parameter MUST be omitted when the AIFF-C channel order convention of RFC 3551 is in use.
The encoding considerations note that audio data is binary data and must be encoded for non-binary transport, for which Base64 is suitable for email, and that audio data does not compress easily using lossless compression. The type is compatible with the encoding used in the WAV and Apple AIFF union types, and with the public domain "sox" and "rateconv" programs. The published specification is RFC 2586 for non-RTP transports and RFC 3551 for RTP, and the file extensions are WAV and L16 with the Macintosh file type code AIFF.
L16 is uncompressed linear audio, so it needs to say how fast the samples come. For email and other file transfers only a fixed list of speeds is allowed, while over RTP any speed is allowed but the usual ones are recommended. It can also say how many channels are interleaved, what preemphasis was applied, and in what order the channels are laid out.
§3Changes from RFC 3555
RFC 3555 is obsoleted by the combination of RFC 4855 and this document. RFC 4855 retains the specification of procedures and requirements from RFC 3555, while the media type registrations from RFC 3555 were extracted into this document. The registrations for RTP payload formats that are referenced in RFC 3551 but defined in other RFCs have been elided, because those registrations are intended to be updated by including them in revisions of the individual RFCs defining the payload formats.
The registrations in this document have been updated to conform to the revised media type registration procedures in RFC 4288 and RFC 4855. Whereas RFC 3555 required the encoding considerations to specify transfer via RTP, that is now specified under restrictions on usage. The encoding considerations now warn that these types are framed binary data. The change controller is also now identified according to current conventions. The optional parameter "channels" was clarified for the audio subtypes L8, PCMA and PCMU. Finally, reference [9], which was missing from RFC 3555, has been corrected.
RFC 3555 did two jobs: it described how the media types should be registered, and it held the registrations themselves. The two jobs are now split between RFC 4855 and this RFC. The registrations themselves did not change in substance, only in how they are written down.
§4Security Considerations
The document specifies media type registrations for the transfer of several compressed audio and video data encodings via RTP, so implementations using these media types are subject to the security considerations discussed in the RTP specification.
None of these media types carries active content that could impose malicious side effects on the receiver. The content consists solely of compressed audio or video data to be decoded and presented as sound or images. However, several audio and video encodings are well suited to hiding data using steganography.
A potential denial-of-service threat exists for encodings that use compression techniques with a non-uniform receiver-end computational load, because an attacker can inject pathological datagrams that are complex to decode and overload the receiver. None of the encodings registered here has an expansion factor greater than about 20, and all are considered relatively simple by modern standards; some are implemented on handheld devices and most were implemented on general-purpose computers ten years ago. As with any IP-based protocol, a receiver may also be overloaded simply by the receipt of too many packets, wanted or unwanted, and network-layer authentication MAY be used to discard packets from undesired sources, although the processing cost of the authentication itself may be too high.
RTP may be sent by IP multicast, which gives a sender no direct way to know who receives the data and therefore offers no measure of privacy. The RFC notes that users may be more sensitive to privacy concerns with audio and video communication than with more traditional forms of network communication, and that using the security mechanisms of RTP to provide confidentiality and integrity of the data is therefore important. Because the data compression used with these media types is applied end to end, encryption may be performed after compression, so there is no conflict between the two operations.
None of these encodings can run code on the receiver, but a compressed picture or sound can carry a hidden message. The other risk is that a carefully crafted packet is expensive to decode and slows the receiver down. Because the data is compressed end to end, encryption can be applied after compression, so the two do not get in each other's way.