path: root/doc/rfc/rfc7587.txt

Internet Engineering Task Force (IETF)                        J. Spittka
Request for Comments: 7587
Category: Standards Track                                         K. Vos
ISSN: 2070-1721                                                  vocTone
                                                               JM. Valin
                                                                 Mozilla
                                                               June 2015


         RTP Payload Format for the Opus Speech and Audio Codec

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.  It also
   provides an applicability statement for the use of Opus over RTP.
   Further, it describes media type registrations for the RTP payload
   format.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7587.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions, Definitions, and Acronyms Used in This Document    3
   3.  Opus Codec  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     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)  . . . . . . . . . .   5
     3.2.  Complexity  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Forward Error Correction (FEC)  . . . . . . . . . . . . .   6
     3.4.  Stereo Operation  . . . . . . . . . . . . . . . . . . . .   6
   4.  Opus RTP Payload Format . . . . . . . . . . . . . . . . . . .   7
     4.1.  RTP Header Usage  . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Payload Structure . . . . . . . . . . . . . . . . . . . .   7
   5.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Opus Media Type Registration  . . . . . . . . . . . . . .   9
   7.  SDP Considerations  . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  SDP Offer/Answer Considerations . . . . . . . . . . . . .  13
     7.2.  Declarative SDP Considerations for Opus . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Opus [RFC6716] 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 Opus in the most compatible way.
   It also provides an applicability statement for the use of Opus over
   RTP.  Further, it describes media type registrations for the RTP
   payload format.







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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].

   audio bandwidth:  The range of audio frequencies being coded

   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

   SSRC:  Synchronization source

   VBR:  Variable bitrate

   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      |
   +--------------+----------------+-----------------+-----------------+

                      Table 1: Audio Bandwidth Naming







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3.  Opus Codec

   Opus 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.

   The voice mode allows efficient encoding of voice signals at lower
   bitrates while the audio mode is optimized for general audio signals
   at medium and higher bitrates.

   Opus is highly scalable in terms of audio bandwidth, bitrate, and
   complexity.  Further, Opus allows transmitting stereo signals with
   in-band signaling in the bitstream.

3.1.  Network Bandwidth

   Opus supports bitrates from 6 kbit/s to 510 kbit/s.  The bitrate can
   be changed dynamically within that range.  All other parameters being
   equal, higher bitrates result in higher audio 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 kbit/s for NB speech,

   o  16-20 kbit/s for WB speech,

   o  28-40 kbit/s for FB speech,

   o  48-64 kbit/s for FB mono music, and

   o  64-128 kbit/s for FB stereo music.

3.1.2.  Variable versus Constant Bitrate

   For the same average bitrate, variable bitrate (VBR) can achieve
   higher audio 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, the Opus padding mechanism




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   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 bandwidth.  If no target bitrate is specified, the bitrates
   specified in Section 3.1.1 are RECOMMENDED.

3.1.3.  Discontinuous Transmission (DTX)

   Opus 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 silence.  When
   using continuous transmission, it will reduce the bitrate when the
   characteristics of the input signal permit, but it will never
   interrupt the transmission to the receiver.  Therefore, the received
   signal will maintain the same high level of audio quality over the
   full duration of a transmission while minimizing the average bitrate
   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.  Using Comfort Noise as defined in [RFC3389]
   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 constraints on available network bandwidth are
   severe.








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3.2.  Complexity

   Complexity of the encoder 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 bitstream.  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; and (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;
   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.  Instead of performing
   loss concealment for a missing packet, the receiver can then
   configure its decoder to decode the FEC data from the next packet.

   Any compliant Opus decoder is capable of ignoring FEC information
   when it is not needed, so encoding with FEC cannot cause
   interoperability problems.  However, if FEC cannot be used on the
   receiving side, then 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 bitstream and no special arrangement
   is needed in the payload format.  An Opus decoder is capable of
   handling a stereo encoding, but an application might only be capable
   of consuming a single audio channel.

   If a decoder cannot 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.





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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; therefore,
   no padding is necessary.  The payload MAY be padded by an integer
   number of octets according to [RFC3550], although the Opus internal
   padding is preferred.

   The timestamp, sequence number, and marker bit (M) of the RTP header
   are used in accordance with Section 4.1 of [RFC3551].

   The RTP payload type for Opus is 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 it MUST discard the others.

   Opus supports 5 different audio bandwidths, which can be adjusted
   during a stream.  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.

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: Packet Structure with RTP Header

   Table 2 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 it 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  |  o  |  o  |  o   |  o   |
    |         |                 |     |     |     |     |      |      |
    |  audio  |   NB/WB/SWB/FB  |  o  |  o  |  o  |  o  |  x   |  x   |
    +---------+-----------------+-----+-----+-----+-----+------+------+

     Table 2: Supported Opus frame sizes and timestamp increments are
         marked with an o.  Unsupported ones are marked with an x.

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.

   Since UDP does not provide congestion control, applications that use
   RTP over UDP SHOULD implement their own congestion control above the
   UDP layer [RFC5405].  Work in the RMCAT working group [rmcat]
   describes the interactions and conceptual interfaces necessary
   between the application components that relate to congestion control,
   including the RTP layer, the higher-level media codec control layer,
   and the lower-level transport interface, as well as components
   dedicated to congestion control functions.






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6.  IANA Considerations

   One media subtype (audio/opus) has been defined and registered as
   described in the following section.

6.1.  Opus Media Type Registration

   Media type registration is done according to [RFC6838] 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.

   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 prerecorded 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.

   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.

   maxaveragebitrate:  specifies the maximum average receive bitrate of
      a session in bits per second (bit/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
      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
      prerecorded 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).




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   cbr:  specifies if the decoder prefers the use of a constant bitrate
      versus a 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 of [RFC6838].

   Security considerations:

      See Section 8 of this document.

   Interoperability considerations: none

   Published specification: RFC 7587

   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, and media
      streaming.

   Fragment identifier considerations: N/A

   Person & email address to contact for further information:

      SILK Support, silksupport@skype.net

      Jean-Marc Valin, jmvalin@jmvalin.ca

   Intended usage: COMMON




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   Restrictions on usage:

      For transfer over RTP, the RTP payload format (Section 4 of this
      document) SHALL be used.

   Authors:

      Julian Spittka, jspittka@gmail.com

      Koen Vos, koenvos74@gmail.com

      Jean-Marc Valin, jmvalin@jmvalin.ca

   Change controller: IETF Payload working group delegated from the IESG

7.  SDP Considerations

   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 Opus, the mapping is as
   follows:

   o  The media type ("audio") goes in SDP "m=" as the media name.

   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", "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.,
      maxplaybackrate=48000).  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



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      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 that 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
   bit/s, prefers to receive stereo but only plans to send mono, FEC is
   desired, DTX is not desired

       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

7.1.  SDP Offer/Answer Considerations

   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





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   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 "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
      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.





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   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.

   The Opus parameters in an SDP offer/answer exchange are completely
   orthogonal, and there is no relationship between the SDP offer and
   the answer.

7.2.  Declarative SDP Considerations for Opus

   For declarative use of SDP such as in the Session Announcement
   Protocol (SAP) [RFC2974] and the Real Time Streaming Protocol (RTSP)
   [RFC2326] for Opus, the following needs to be considered:

   o  The values for "maxptime", "ptime", "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 of the payload format
      configuration are recommendations by the decoding side to ensure
      the best performance for the decoder.

   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.

8.  Security Considerations

   Use of VBR is subject to the security considerations in [RFC6562].

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550] and in any applicable RTP profile such as
   RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/
   SAVPF [RFC5124].  However, as "Securing the RTP Framework: Why RTP
   Does Not Mandate a Single Media Security Solution" [RFC7202]
   discusses, it is not an RTP payload format's responsibility to
   discuss or mandate what solutions are used to meet the basic security
   goals like confidentiality, integrity, and source authenticity for
   RTP in general.  This responsibility lies on anyone using RTP in an
   application.  They can find guidance on available security mechanisms



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   and important considerations in "Options for Securing RTP Sessions"
   [RFC7201].  Applications SHOULD use one or more appropriate strong
   security mechanisms.

   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.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2326]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
              Streaming Protocol (RTSP)", RFC 2326,
              DOI 10.17487/RFC2326, April 1998,
              <http://www.rfc-editor.org/info/rfc2326>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <http://www.rfc-editor.org/info/rfc3264>.

   [RFC3389]  Zopf, R., "Real-time Transport Protocol (RTP) Payload for
              Comfort Noise (CN)", RFC 3389, DOI 10.17487/RFC3389,
              September 2002, <http://www.rfc-editor.org/info/rfc3389>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <http://www.rfc-editor.org/info/rfc3551>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <http://www.rfc-editor.org/info/rfc3711>.





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   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <http://www.rfc-editor.org/info/rfc4566>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <http://www.rfc-editor.org/info/rfc4855>.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
              <http://www.rfc-editor.org/info/rfc5576>.

   [RFC6562]  Perkins, C. and JM. Valin, "Guidelines for the Use of
              Variable Bit Rate Audio with Secure RTP", RFC 6562,
              DOI 10.17487/RFC6562, March 2012,
              <http://www.rfc-editor.org/info/rfc6562>.

   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the
              Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
              September 2012, <http://www.rfc-editor.org/info/rfc6716>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

9.2.  Informative References

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
              October 2000, <http://www.rfc-editor.org/info/rfc2974>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <http://www.rfc-editor.org/info/rfc4585>.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <http://www.rfc-editor.org/info/rfc5124>.








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   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405,
              DOI 10.17487/RFC5405, November 2008,
              <http://www.rfc-editor.org/info/rfc5405>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <http://www.rfc-editor.org/info/rfc7201>.

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
              2014, <http://www.rfc-editor.org/info/rfc7202>.

   [rmcat]    "RTP Media Congestion Avoidance Techniques (rmcat)
              Documents", <https://datatracker.ietf.org/wg/rmcat/
              documents/>.

Acknowledgements

   Many people have made useful comments and suggestions contributing to
   this document.  In particular, we would like to thank Tina le Grand,
   Cullen Jennings, Jonathan Lennox, Gregory Maxwell, Colin Perkins, Jan
   Skoglund, Timothy B. Terriberry, Martin Thompson, Justin Uberti,
   Magnus Westerlund, and Mo Zanaty.

Authors' Addresses

   Julian Spittka

   Email: jspittka@gmail.com


   Koen Vos
   vocTone

   Email: koenvos74@gmail.com


   Jean-Marc Valin
   Mozilla
   331 E. Evelyn Avenue
   Mountain View, CA  94041
   United States

   Email: jmvalin@jmvalin.ca





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