Network Working Group J. Spittka
Internet-Draft
Intended status: Standards Track K. Vos
Expires: January 31, 2015 vocTone
JM. Valin
Mozilla
July 30, 2014
RTP Payload Format for Opus Speech and Audio Codec
draft-ietf-payload-rtp-opus-03
Abstract
This document defines the Real-time Transport Protocol (RTP) payload
format for packetization of Opus encoded speech and audio data
necessary to integrate the codec in the most compatible way.
Further, it describes media type registrations for the RTP payload
format.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 31, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions, Definitions and Acronyms used in this document . 3
2.1. Audio Bandwidth . . . . . . . . . . . . . . . . . . . . . 3
3. Opus Codec . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Network Bandwidth . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Recommended Bitrate . . . . . . . . . . . . . . . . . 4
3.1.2. Variable versus Constant Bitrate . . . . . . . . . . 4
3.1.3. Discontinuous Transmission (DTX) . . . . . . . . . . 4
3.2. Complexity . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Forward Error Correction (FEC) . . . . . . . . . . . . . 5
3.4. Stereo Operation . . . . . . . . . . . . . . . . . . . . 6
4. Opus RTP Payload Format . . . . . . . . . . . . . . . . . . . 6
4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . 6
4.2. Payload Structure . . . . . . . . . . . . . . . . . . . . 7
5. Congestion Control . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. Opus Media Type Registration . . . . . . . . . . . . . . 9
6.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 12
6.2.1. Offer-Answer Model Considerations for Opus . . . . . 14
6.2.2. Declarative SDP Considerations for Opus . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
9. Normative References . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
The Opus codec is a speech and audio codec developed within the IETF
Internet Wideband Audio Codec working group. The codec has a very
low algorithmic delay and it is highly scalable in terms of audio
bandwidth, bitrate, and complexity. Further, it provides different
modes to efficiently encode speech signals as well as music signals,
thus making it the codec of choice for various applications using the
Internet or similar networks.
This document defines the Real-time Transport Protocol (RTP)
[RFC3550] payload format for packetization of Opus encoded speech and
audio data necessary to integrate the Opus codec in the most
compatible way. Further, it describes media type registrations for
the RTP payload format. More information on the Opus codec can be
obtained from [RFC6716].
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2. Conventions, Definitions and Acronyms used in this document
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 [RFC2119].
CBR: Constant bitrate
CPU: Central Processing Unit
DTX: Discontinuous transmission
FEC: Forward error correction
IP: Internet Protocol
samples: Speech or audio samples (per channel)
SDP: Session Description Protocol
VBR: Variable bitrate
2.1. Audio Bandwidth
Throughout this document, we refer to the following definitions:
+--------------+----------------+-----------------+-----------------+
| Abbreviation | Name | Audio Bandwidth | Sampling Rate |
| | | (Hz) | (Hz) |
+--------------+----------------+-----------------+-----------------+
| NB | Narrowband | 0 - 4000 | 8000 |
| | | | |
| MB | Mediumband | 0 - 6000 | 12000 |
| | | | |
| WB | Wideband | 0 - 8000 | 16000 |
| | | | |
| SWB | Super-wideband | 0 - 12000 | 24000 |
| | | | |
| FB | Fullband | 0 - 20000 | 48000 |
+--------------+----------------+-----------------+-----------------+
Audio bandwidth naming
Table 1
3. Opus Codec
The Opus [RFC6716] codec encodes speech signals as well as general
audio signals. Two different modes can be chosen, a voice mode or an
audio mode, to allow the most efficient coding depending on the type
of the input signal, the sampling frequency of the input signal, and
the intended application.
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The voice mode allows efficient encoding of voice signals at lower
bit rates while the audio mode is optimized for general audio signals
at medium and higher bitrates.
The Opus speech and audio codec is highly scalable in terms of audio
bandwidth, bitrate, and complexity. Further, Opus allows
transmitting stereo signals.
3.1. Network Bandwidth
Opus supports all bitrates from 6 kb/s to 510 kb/s. The bitrate can
be changed dynamically within that range. All other parameters being
equal, higher bitrates result in higher quality.
3.1.1. Recommended Bitrate
For a frame size of 20 ms, these are the bitrate "sweet spots" for
Opus in various configurations:
o 8-12 kb/s for NB speech,
o 16-20 kb/s for WB speech,
o 28-40 kb/s for FB speech,
o 48-64 kb/s for FB mono music, and
o 64-128 kb/s for FB stereo music.
3.1.2. Variable versus Constant Bitrate
For the same average bitrate, variable bitrate (VBR) can achieve
higher quality than constant bitrate (CBR). For the majority of
voice transmission applications, VBR is the best choice. One reason
for choosing CBR is the potential information leak that _might_ occur
when encrypting the compressed stream. See [RFC6562] for guidelines
on when VBR is appropriate for encrypted audio communications. In
the case where an existing VBR stream needs to be converted to CBR
for security reasons, then the Opus padding mechanism described in
[RFC6716] is the RECOMMENDED way to achieve padding because the RTP
padding bit is unencrypted.
The bitrate can be adjusted at any point in time. To avoid
congestion, the average bitrate SHOULD NOT exceed the available
network capacity. If no target bitrate is specified, the bitrates
specified in Section 3.1.1 are RECOMMENDED.
3.1.3. Discontinuous Transmission (DTX)
The Opus codec can, as described in Section 3.1.2, be operated with a
variable bitrate. In that case, the encoder will automatically
reduce the bitrate for certain input signals, like periods of
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silence. When using continuous transmission, it will reduce the
bitrate when the characteristics of the input signal permit, but will
never interrupt the transmission to the receiver. Therefore, the
received signal will maintain the same high level of quality over the
full duration of a transmission while minimizing the average bit rate
over time.
In cases where the bitrate of Opus needs to be reduced even further
or in cases where only constant bitrate is available, the Opus
encoder can use discontinuous transmission (DTX), where parts of the
encoded signal that correspond to periods of silence in the input
speech or audio signal are not transmitted to the receiver. A
receiver can distinguish between DTX and packet loss by looking for
gaps in the sequence number, as described by Section 4.1
of [RFC3551].
On the receiving side, the non-transmitted parts will be handled by a
frame loss concealment unit in the Opus decoder which generates a
comfort noise signal to replace the non transmitted parts of the
speech or audio signal. Use of [RFC3389] Comfort Noise (CN) with
Opus is discouraged. The transmitter MUST drop whole frames only,
based on the size of the last transmitted frame, to ensure successive
RTP timestamps differ by a multiple of 120 and to allow the receiver
to use whole frames for concealment.
DTX can be used with both variable and constant bitrate. It will
have a slightly lower speech or audio quality than continuous
transmission. Therefore, using continuous transmission is
RECOMMENDED unless restraints on network capacity are severe.
3.2. Complexity
Complexity can be scaled to optimize for CPU resources in real-time,
mostly as a trade-off between audio quality and bitrate. Also,
different modes of Opus have different complexity.
3.3. Forward Error Correction (FEC)
The voice mode of Opus allows for embedding "in-band" forward error
correction (FEC) data into the Opus bit stream. This FEC scheme adds
redundant information about the previous packet (N-1) to the current
output packet N. For each frame, the encoder decides whether to use
FEC based on (1) an externally-provided estimate of the channel's
packet loss rate; (2) an externally-provided estimate of the
channel's capacity; (3) the sensitivity of the audio or speech signal
to packet loss; (4) whether the receiving decoder has indicated it
can take advantage of "in-band" FEC information. The decision to
send "in-band" FEC information is entirely controlled by the encoder
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and therefore no special precautions for the payload have to be
taken.
On the receiving side, the decoder can take advantage of this
additional information when it loses a packet and the next packet is
available. In order to use the FEC data, the jitter buffer needs to
provide access to payloads with the FEC data. The receiver can then
configure its decoder to decode the FEC data from the packet rather
than the regular audio data. If no FEC data is available for the
current frame, the decoder will consider the frame lost and invoke
frame loss concealment.
If the FEC scheme is not implemented on the receiving side, FEC
SHOULD NOT be used, as it leads to an inefficient usage of network
resources. Decoder support for FEC SHOULD be indicated at the time a
session is set up.
3.4. Stereo Operation
Opus allows for transmission of stereo audio signals. This operation
is signaled in-band in the Opus payload and no special arrangement is
needed in the payload format. Any implementation of the Opus decoder
MUST be capable of receiving stereo signals, although it MAY decode
those signals as mono.
If a decoder can not take advantage of the benefits of a stereo
signal this SHOULD be indicated at the time a session is set up. In
that case the sending side SHOULD NOT send stereo signals as it leads
to an inefficient usage of network resources.
4. Opus RTP Payload Format
The payload format for Opus consists of the RTP header and Opus
payload data.
4.1. RTP Header Usage
The format of the RTP header is specified in [RFC3550]. The use of
the fields of the RTP header by the Opus payload format is consistent
with that specification.
The payload length of Opus is an integer number of octets and
therefore no padding is necessary. The payload MAY be padded by an
integer number of octets according to [RFC3550].
The timestamp, sequence number, and marker bit (M) of the RTP header
are used in accordance with Section 4.1 of [RFC3551].
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The RTP payload type for Opus has not been assigned statically and is
expected to be assigned dynamically.
The receiving side MUST be prepared to receive duplicate RTP packets.
The receiver MUST provide at most one of those payloads to the Opus
decoder for decoding, and MUST discard the others.
Opus supports 5 different audio bandwidths, which can be adjusted
during a call. The RTP timestamp is incremented with a 48000 Hz
clock rate for all modes of Opus and all sampling rates. The unit
for the timestamp is samples per single (mono) channel. The RTP
timestamp corresponds to the sample time of the first encoded sample
in the encoded frame. For data encoded with sampling rates other
than 48000 Hz, the sampling rate has to be adjusted to 48000 Hz using
the corresponding multiplier in Table 2.
+--------------------+------------+
| Sampling Rate (Hz) | Multiplier |
+--------------------+------------+
| 8000 | 6 |
| | |
| 12000 | 4 |
| | |
| 16000 | 3 |
| | |
| 24000 | 2 |
| | |
| 48000 | 1 |
+--------------------+------------+
Table 2: Timestamp multiplier
4.2. Payload Structure
The Opus encoder can output encoded frames representing 2.5, 5, 10,
20, 40, or 60 ms of speech or audio data. Further, an arbitrary
number of frames can be combined into a packet, up to a maximum
packet duration representing 120 ms of speech or audio data. The
grouping of one or more Opus frames into a single Opus packet is
defined in Section 3 of [RFC6716]. An RTP payload MUST contain
exactly one Opus packet as defined by that document.
Figure 1 shows the structure combined with the RTP header.
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+----------+--------------+
|RTP Header| Opus Payload |
+----------+--------------+
Figure 1: Payload Structure with RTP header
Table 3 shows supported frame sizes in milliseconds of encoded speech
or audio data for the speech and audio modes (Mode) and sampling
rates (fs) of Opus and shows how the timestamp is incremented for
packetization (ts incr). If the Opus encoder outputs multiple
encoded frames into a single packet, the timestamp increment is the
sum of the increments for the individual frames.
+---------+-----------------+-----+-----+-----+-----+------+------+
| Mode | fs | 2.5 | 5 | 10 | 20 | 40 | 60 |
+---------+-----------------+-----+-----+-----+-----+------+------+
| ts incr | all | 120 | 240 | 480 | 960 | 1920 | 2880 |
| | | | | | | | |
| voice | NB/MB/WB/SWB/FB | | | x | x | x | x |
| | | | | | | | |
| audio | NB/WB/SWB/FB | x | x | x | x | | |
+---------+-----------------+-----+-----+-----+-----+------+------+
Table 3: Supported Opus frame sizes and timestamp increments
5. Congestion Control
The target bitrate of Opus can be adjusted at any point in time, thus
allowing efficient congestion control. Furthermore, the amount of
encoded speech or audio data encoded in a single packet can be used
for congestion control, since the transmission rate is inversely
proportional to the packet duration. A lower packet transmission
rate reduces the amount of header overhead, but at the same time
increases latency and loss sensitivity, so it ought to be used with
care.
It is RECOMMENDED that senders of Opus encoded data apply congestion
control.
6. IANA Considerations
One media subtype (audio/opus) has been defined and registered as
described in the following section.
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6.1. Opus Media Type Registration
Media type registration is done according to [RFC4288] and [RFC4855].
Type name: audio
Subtype name: opus
Required parameters:
rate: the RTP timestamp is incremented with a 48000 Hz clock rate
for all modes of Opus and all sampling rates. For data encoded
with sampling rates other than 48000 Hz, the sampling rate has to
be adjusted to 48000 Hz using the corresponding multiplier in
Table 2.
Optional parameters:
maxplaybackrate: a hint about the maximum output sampling rate that
the receiver is capable of rendering in Hz. The decoder MUST be
capable of decoding any audio bandwidth but due to hardware
limitations only signals up to the specified sampling rate can be
played back. Sending signals with higher audio bandwidth results
in higher than necessary network usage and encoding complexity, so
an encoder SHOULD NOT encode frequencies above the audio bandwidth
specified by maxplaybackrate. This parameter can take any value
between 8000 and 48000, although commonly the value will match one
of the Opus bandwidths (Table 1). By default, the receiver is
assumed to have no limitations, i.e. 48000.
sprop-maxcapturerate: a hint about the maximum input sampling rate
that the sender is likely to produce. This is not a guarantee
that the sender will never send any higher bandwidth (e.g. it
could send a pre-recorded prompt that uses a higher bandwidth),
but it indicates to the receiver that frequencies above this
maximum can safely be discarded. This parameter is useful to
avoid wasting receiver resources by operating the audio processing
pipeline (e.g. echo cancellation) at a higher rate than necessary.
This parameter can take any value between 8000 and 48000, although
commonly the value will match one of the Opus bandwidths
(Table 1). By default, the sender is assumed to have no
limitations, i.e. 48000.
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maxptime: the maximum duration of media represented by a packet
(according to Section 6 of [RFC4566]) that a decoder wants to
receive, in milliseconds rounded up to the next full integer
value. Possible values are 3, 5, 10, 20, 40, 60, or an arbitrary
multiple of an Opus frame size rounded up to the next full integer
value, up to a maximum value of 120, as defined in Section 4. If
no value is specified, the default is 120. This value is a
recommendation by the decoding side to ensure the best performance
for the decoder. The decoder MUST be capable of accepting any
allowed packet sizes to ensure maximum compatibility.
ptime: the preferred duration of media represented by a packet
(according to Section 6 of [RFC4566]) that a decoder wants to
receive, in milliseconds rounded up to the next full integer
value. Possible values are 3, 5, 10, 20, 40, 60, or an arbitrary
multiple of an Opus frame size rounded up to the next full integer
value, up to a maximum value of 120, as defined in Section 4. If
no value is specified, the default is 20. If ptime is greater
than maxptime, ptime MUST be ignored. This parameter MAY be
changed during a session. This value is a recommendation by the
decoding side to ensure the best performance for the decoder. The
decoder MUST be capable of accepting any allowed packet sizes to
ensure maximum compatibility.
minptime: the minimum duration of media represented by a packet
(according to Section 6 of [RFC4566]) that SHOULD be encapsulated
in a received packet, in milliseconds rounded up to the next full
integer value. Possible values are 3, 5, 10, 20, 40, and 60 or an
arbitrary multiple of Opus frame sizes rounded up to the next full
integer value up to a maximum value of 120 as defined in
Section 4. If no value is specified, the default is 3. This
value is a recommendation by the decoding side to ensure the best
performance for the decoder. The decoder MUST be capable to
accept any allowed packet sizes to ensure maximum compatibility.
maxaveragebitrate: specifies the maximum average receive bitrate of
a session in bits per second (b/s). The actual value of the
bitrate can vary, as it is dependent on the characteristics of the
media in a packet. Note that the maximum average bitrate MAY be
modified dynamically during a session. Any positive integer is
allowed, but values outside the range 6000 to 510000 SHOULD be
ignored. If no value is specified, the maximum value specified in
Section 3.1.1 for the corresponding mode of Opus and corresponding
maxplaybackrate is the default.
stereo: specifies whether the decoder prefers receiving stereo or
mono signals. Possible values are 1 and 0 where 1 specifies that
stereo signals are preferred, and 0 specifies that only mono
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signals are preferred. Independent of the stereo parameter every
receiver MUST be able to receive and decode stereo signals but
sending stereo signals to a receiver that signaled a preference
for mono signals may result in higher than necessary network
utilization and encoding complexity. If no value is specified,
the default is 0 (mono).
sprop-stereo: specifies whether the sender is likely to produce
stereo audio. Possible values are 1 and 0, where 1 specifies that
stereo signals are likely to be sent, and 0 specifies that the
sender will likely only send mono. This is not a guarantee that
the sender will never send stereo audio (e.g. it could send a pre-
recorded prompt that uses stereo), but it indicates to the
receiver that the received signal can be safely downmixed to mono.
This parameter is useful to avoid wasting receiver resources by
operating the audio processing pipeline (e.g. echo cancellation)
in stereo when not necessary. If no value is specified, the
default is 0 (mono).
cbr: specifies if the decoder prefers the use of a constant bitrate
versus variable bitrate. Possible values are 1 and 0, where 1
specifies constant bitrate and 0 specifies variable bitrate. If
no value is specified, the default is 0 (vbr). When cbr is 1, the
maximum average bitrate can still change, e.g. to adapt to
changing network conditions.
useinbandfec: specifies that the decoder has the capability to take
advantage of the Opus in-band FEC. Possible values are 1 and 0.
Providing 0 when FEC cannot be used on the receiving side is
RECOMMENDED. If no value is specified, useinbandfec is assumed to
be 0. This parameter is only a preference and the receiver MUST
be able to process packets that include FEC information, even if
it means the FEC part is discarded.
usedtx: specifies if the decoder prefers the use of DTX. Possible
values are 1 and 0. If no value is specified, the default is 0.
Encoding considerations:
The Opus media type is framed and consists of binary data
according to Section 4.8 in [RFC4288].
Security considerations:
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See Section 7 of this document.
Interoperability considerations: none
Published specification: none
Applications that use this media type:
Any application that requires the transport of speech or audio
data can use this media type. Some examples are, but not limited
to, audio and video conferencing, Voice over IP, media streaming.
Person & email address to contact for further information:
SILK Support silksupport@skype.net
Jean-Marc Valin jmvalin@jmvalin.ca
Intended usage: COMMON
Restrictions on usage:
For transfer over RTP, the RTP payload format (Section 4 of this
document) SHALL be used.
Author:
Julian Spittka jspittka@gmail.com
Koen Vos koenvos74@gmail.com
Jean-Marc Valin jmvalin@jmvalin.ca
Change controller: TBD
6.2. Mapping to SDP Parameters
The information described in the media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[RFC4566], which is commonly used to describe RTP sessions. When SDP
is used to specify sessions employing the Opus codec, the mapping is
as follows:
o The media type ("audio") goes in SDP "m=" as the media name.
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o The media subtype ("opus") goes in SDP "a=rtpmap" as the encoding
name. The RTP clock rate in "a=rtpmap" MUST be 48000 and the
number of channels MUST be 2.
o The OPTIONAL media type parameters "ptime" and "maxptime" are
mapped to "a=ptime" and "a=maxptime" attributes, respectively, in
the SDP.
o The OPTIONAL media type parameters "maxaveragebitrate",
"maxplaybackrate", "minptime", "stereo", "cbr", "useinbandfec",
and "usedtx", when present, MUST be included in the "a=fmtp"
attribute in the SDP, expressed as a media type string in the form
of a semicolon-separated list of parameter=value pairs (e.g.,
maxaveragebitrate=20000). They MUST NOT be specified in an SSRC-
specific "fmtp" source-level attribute (as defined in Section 6.3
of [RFC5576]).
o The OPTIONAL media type parameters "sprop-maxcapturerate", and
"sprop-stereo" MAY be mapped to the "a=fmtp" SDP attribute by
copying them directly from the media type parameter string as part
of the semicolon-separated list of parameter=value pairs (e.g.,
sprop-stereo=1). These same OPTIONAL media type parameters MAY
also be specified using an SSRC-specific "fmtp" source-level
attribute as described in Section 6.3 of [RFC5576]. They MAY be
specified in both places, in which case the parameter in the
source-level attribute overrides the one found on the "a=fmtp"
line. The value of any parameter which is not specified in a
source-level source attribute MUST be taken from the "a=fmtp"
line, if it is present there.
Below are some examples of SDP session descriptions for Opus:
Example 1: Standard mono session with 48000 Hz clock rate
m=audio 54312 RTP/AVP 101
a=rtpmap:101 opus/48000/2
Example 2: 16000 Hz clock rate, maximum packet size of 40 ms,
recommended packet size of 40 ms, maximum average bitrate of 20000
bps, prefers to receive stereo but only plans to send mono, FEC is
desired, DTX is not desired
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m=audio 54312 RTP/AVP 101
a=rtpmap:101 opus/48000/2
a=fmtp:101 maxplaybackrate=16000; sprop-maxcapturerate=16000;
maxaveragebitrate=20000; stereo=1; useinbandfec=1; usedtx=0
a=ptime:40
a=maxptime:40
Example 3: Two-way full-band stereo preferred
m=audio 54312 RTP/AVP 101
a=rtpmap:101 opus/48000/2
a=fmtp:101 stereo=1; sprop-stereo=1
6.2.1. Offer-Answer Model Considerations for Opus
When using the offer-answer procedure described in [RFC3264] to
negotiate the use of Opus, the following considerations apply:
o Opus supports several clock rates. For signaling purposes only
the highest, i.e. 48000, is used. The actual clock rate of the
corresponding media is signaled inside the payload and is not
restricted by this payload format description. The decoder MUST
be capable of decoding every received clock rate. An example is
shown below:
m=audio 54312 RTP/AVP 100
a=rtpmap:100 opus/48000/2
o The "ptime" and "maxptime" parameters are unidirectional receive-
only parameters and typically will not compromise
interoperability; however, some values might cause application
performance to suffer. [RFC3264] defines the SDP offer-answer
handling of the "ptime" parameter. The "maxptime" parameter MUST
be handled in the same way.
o The "minptime" parameter is a unidirectional receive-only
parameters and typically will not compromise interoperability;
however, some values might cause application performance to suffer
and ought to be used with care.
o The "maxplaybackrate" parameter is a unidirectional receive-only
parameter that reflects limitations of the local receiver. When
sending to a single destination, a sender MUST NOT use an audio
bandwidth higher than necessary to make full use of audio sampled
at a sampling rate of "maxplaybackrate". Gateways or senders that
are sending the same encoded audio to multiple destinations SHOULD
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NOT use an audio bandwidth higher than necessary to represent
audio sampled at "maxplaybackrate", as this would lead to
inefficient use of network resources. The "maxplaybackrate"
parameter does not affect interoperability. Also, this parameter
SHOULD NOT be used to adjust the audio bandwidth as a function of
the bitrate, as this is the responsibility of the Opus encoder
implementation.
o The "maxaveragebitrate" parameter is a unidirectional receive-only
parameter that reflects limitations of the local receiver. The
sender of the other side MUST NOT send with an average bitrate
higher than "maxaveragebitrate" as it might overload the network
and/or receiver. The "maxaveragebitrate" parameter typically will
not compromise interoperability; however, some values might cause
application performance to suffer, and ought to be set with care.
o The "sprop-maxcapturerate" and "sprop-stereo" parameters are
unidirectional sender-only parameters that reflect limitations of
the sender side. They allow the receiver to set up a reduced-
complexity audio processing pipeline if the sender is not planning
to use the full range of Opus's capabilities. Neither "sprop-
maxcapturerate" nor "sprop-stereo" affect interoperability and the
receiver MUST be capable of receiving any signal.
o The "stereo" parameter is a unidirectional receive-only parameter.
When sending to a single destination, a sender MUST NOT use stereo
when "stereo" is 0. Gateways or senders that are sending the same
encoded audio to multiple destinations SHOULD NOT use stereo when
"stereo" is 0, as this would lead to inefficient use of network
resources. The "stereo" parameter does not affect
interoperability.
o The "cbr" parameter is a unidirectional receive-only parameter.
o The "useinbandfec" parameter is a unidirectional receive-only
parameter.
o The "usedtx" parameter is a unidirectional receive-only parameter.
o Any unknown parameter in an offer MUST be ignored by the receiver
and MUST be removed from the answer.
6.2.2. Declarative SDP Considerations for Opus
For declarative use of SDP such as in Session Announcement Protocol
(SAP), [RFC2974], and RTSP, [RFC2326], for Opus, the following needs
to be considered:
o The values for "maxptime", "ptime", "minptime", "maxplaybackrate",
and "maxaveragebitrate" ought to be selected carefully to ensure
that a reasonable performance can be achieved for the participants
of a session.
o The values for "maxptime", "ptime", and "minptime" of the payload
format configuration are recommendations by the decoding side to
ensure the best performance for the decoder. The decoder MUST be
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capable of accepting any allowed packet sizes to ensure maximum
compatibility.
o All other parameters of the payload format configuration are
declarative and a participant MUST use the configurations that are
provided for the session. More than one configuration can be
provided if necessary by declaring multiple RTP payload types;
however, the number of types ought to be kept small.
7. Security Considerations
All RTP packets using the payload format defined in this
specification are subject to the general security considerations
discussed in the RTP specification [RFC3550] and any profile from,
e.g., [RFC3711] or [RFC3551].
This payload format transports Opus encoded speech or audio data.
Hence, security issues include confidentiality, integrity protection,
and authentication of the speech or audio itself. The Opus payload
format does not have any built-in security mechanisms. Any suitable
external mechanisms, such as SRTP [RFC3711], MAY be used.
This payload format and the Opus encoding do not exhibit any
significant non-uniformity in the receiver-end computational load and
thus are unlikely to pose a denial-of-service threat due to the
receipt of pathological datagrams.
8. Acknowledgements
TBD
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", RFC 4288, December 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, February 2007.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC6562] Perkins, C. and JM. Valin, "Guidelines for the Use of
Variable Bit Rate Audio with Secure RTP", RFC 6562, March
2012.
[RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the
Opus Audio Codec", RFC 6716, September 2012.
Authors' Addresses
Julian Spittka
Email: jspittka@gmail.com
Koen Vos
vocTone
Email: koenvos74@gmail.com
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Jean-Marc Valin
Mozilla
331 E. Evelyn Avenue
Mountain View, CA 94041
USA
Email: jmvalin@jmvalin.ca
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