Internet DRAFT - draft-valin-opus-extension

draft-valin-opus-extension







Internet Engineering Task Force                                JM. Valin
Internet-Draft                                             T. Terriberry
Updates: 6716 (if approved)                                       Amazon
Intended status: Standards Track                           11 April 2023
Expires: 13 October 2023


                Extension Formatting for the Opus Codec
                     draft-valin-opus-extension-01

Abstract

   This document proposes a mechanism to extend the Opus codec (RFC6716)
   in a way that maintains inter-operability, while adding optional
   functionality.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 13 October 2023.

Copyright Notice

   Copyright (c) 2023 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 (https://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
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   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Extension Format  . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  ID 0: Original Padding  . . . . . . . . . . . . . . . . .   4
     2.2.  ID 1: Separator . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  IDs 2-119: Unassigned . . . . . . . . . . . . . . . . . .   4
     2.4.  IDs 120-127: I-D Experimental . . . . . . . . . . . . . .   4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Opus Media Type Update  . . . . . . . . . . . . . . . . .   5
     3.2.  Mapping to SDP Parameters . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   This document proposes a mechanism to extend the Opus codec [RFC6716]
   in a way that maintains inter-operability, while adding optional
   functionality.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Extension Format

   The Opus padding mechanism provides a safe way to extend the Opus
   codec while preserving interoperability and without having to
   transmit any extra packets.  [RFC6716] specifies that all padding
   bytes "MUST be set to zero" by the encoder, while the decoder "MUST
   accept any value for the padding bytes".  In that way, any non-zero
   padding will indicate to an extended decoder that an extension is
   present and can be processed.  On the other hand, for any all-zero
   padding, the decoder will just discard the padding like any non-
   extended decoder.  A non-extended decoder receiving a packet with an
   extension will simply discard the extension and proceed as if none
   was present.

   An extension starts with a byte that signals a 7-bit ID, as well as a
   binary flag L for length signalling.  For extension IDs 1 through 31,
   L=0 means that no data follows the extension, whereas L=1 means that



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   exactly one byte of extension data follows.  For IDs 32 to 127, L=0
   signals that the extension data takes up the rest of the padding, and
   L=1 signals that a length indicator follows.  For ID 0, L=0 has the
   same meaning as for IDs 32 to 127, but L=1 signals a length of zero
   (no length indicator follows).  In any given packet containing
   padding, the "rest of the padding" cannot appear more than once.
   When a length indicator is signalled, the following byte contains a
   length value from 0 to 254.  If the length byte is 255, then the
   length is 255 plus the length signaled from the next byte, with 255
   case being allowed to repeat as long as the size of the padding is
   not exceeded.  Any extension signalled with a length that would cause
   the decoder to read beyond the bounds of the packet MUST be ignored
   by the decoder.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      ID     |L| Length (opt.) |    extension content...       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      |                                                               |
      :                                                               :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: Extension framing

   A decoder MUST ignore any extension it does not know, decoding the
   rest of the packet as if the extension was not present.
   Additionally, a decoder MAY ignore any other extension even if it
   technically supports it.  An encoder MUST NOT alter the way it
   encodes the non-extension part of an Opus packet in such a way as to
   noticeably reduce its quality when decoded with a non-extended
   decoder.

   Open questions:

   *  Should we allow more than one of the same extension ID for the
      same frame?

   *  Should we pre-define "unsafe" extensions that must cause the
      decoder to ignore all extensions if it doesn't understand the
      unsafe extension.









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2.1.  ID 0: Original Padding

   For compatibility reasons, an ID of 0 means that the content of the
   extension is actual padding, as originally defined in [RFC6716].  As
   in its original definition, the padding bytes MUST be set to zero by
   the encoder, while the decoder MUST ignore any non-zero padding.  In
   the case where the L flag is set, the 0x01 header byte is simply
   skipped and extension decoding continues from the next byte.  This
   can be useful as a way to insert padding one byte at a time, since
   appending zeros at the end may cause an increase in size from having
   to signal a multi-byte length indicator for the last extension.

2.2.  ID 1: Separator

   In the case where multiple frames are packed inside the same packet,
   there may be a need to specify which extension(s) apply to which
   frame.  By default, all extensions apply to the first frame in the
   packet.  Any time a separator with L=0 is encountered when parsing
   extensions sequentially, the associated frame is increased by one.
   If L=1 is used, the following data byte indicates the increment
   applied for the new associated frame.  The associated frame value
   MUST NOT exceed the bound equal to the number of frames in the
   packet, minus one (indexing starts at zero).  Similarly, L=0
   separators MUST NOT cause the associated frame to exceed the above
   bound.  The decoder MUST ignore all extensions associated with an
   out-of-bound frame index.

2.3.  IDs 2-119: Unassigned

   These extensions are to be define in their own respective documents
   and the IDs are to be assigned by IANA.  Note that the definition of
   the L flag is already defined for all these unassigned IDs because a
   decoder must know how to skip extensions it doesn't know about.  Due
   to potential for interaction between extensions, new extensions are
   to be assigned with the "Standards Action" policy defined by
   [RFC8126].

2.4.  IDs 120-127: I-D Experimental

   We reserve these 8 IDs for experimental extensions, such that
   extensions defined in Internet-Drafts can be tested before they
   become RFC without causing possible interoperability issues should
   their bitstream definitions change.  When using an experimental ID,
   it is RECOMMENDED to use a two-byte prefix that attempts to encode an
   experiment number (first byte) and a version number (second byte).
   Experimental extension documents SHOULD attempt to choose an
   experiment number that does not collide with other ongoing
   experiments.



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3.  IANA Considerations

   This document defines a new registry "Opus Extension IDs" in a new
   "Opus" group, that allocates individual IDs to individual extensions
   to be defined in the future.  The existing "Opus Channel Mapping
   Families" registry will also be moved to the newly created "Opus"
   group.  Moreover, this document already defines the following IDs:

   +===========+==================+====================================+
   | Extension | Description      | Reference                          |
   | ID        |                  |                                    |
   +===========+==================+====================================+
   | 0         | Original padding | Defined in Section 2.1             |
   |           | definition       |                                    |
   +-----------+------------------+------------------------------------+
   | 1         | Frame separator  | Defined in Section 2.2.            |
   +-----------+------------------+------------------------------------+
   | 2-119     | Unassigned       | To be assigned with the            |
   |           |                  | "Standards Action" policy          |
   |           |                  | [RFC8126]                          |
   +-----------+------------------+------------------------------------+
   | 120-127   | Experimental     | Defined in Section 2.4,            |
   |           | Internet-Draft   | following the "Experimental        |
   |           | implementations  | Use" policy [RFC8126]              |
   +-----------+------------------+------------------------------------+

                                  Table 1

   Note that for forward compatibility, any extension defined in the
   future MUST use the definition of the L flag that is dictated
   (Section 2) by its ID value.

3.1.  Opus Media Type Update

   This document updates the audio/opus media type registration
   [RFC7587] to add the following two optional parameters:

   extensions: specifies a comma-separated list of supported extension
   IDs on the receiver side.

   sprop-extensions: specifies a comma-separated list of supported
   extension IDs on the sender side.

   extN-*: To facilitate parameter forwarding, extension document that
   require receiver extension parameters SHOULD name them "ext",
   followed by the extension number, a hyphen, and the paramter name.





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   sprop-extN-*: Extension-specific sender-side parameters defined
   similarly as above.

   All names starting with "ext" and "sprop-ext" are reserved for use by
   Opus extensions.

3.2.  Mapping to SDP Parameters

   The media type parameters described above map to declarative SDP and
   SDP offer-answer in the same way as other optional parameters in
   [RFC7587].  Regardless of any a=fmtp SDP attribute specified, the
   receiver MUST be capable of receiving any signal.

4.  Security Considerations

   This document does not add security considerations beyond those
   already documented in [RFC6716].  Future Opus extensions may have
   their own security implications.

5.  References

5.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC7587]  Spittka, J., Vos, K., and JM. Valin, "RTP Payload Format
              for the Opus Speech and Audio Codec", RFC 7587,
              DOI 10.17487/RFC7587, June 2015,
              <https://www.rfc-editor.org/info/rfc7587>.

Authors' Addresses




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   Jean-Marc Valin
   Amazon
   Canada
   Email: jmvalin@amazon.com


   Timothy B. Terriberry
   Amazon
   United States of America
   Email: territim@amazon.com









































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