Internet DRAFT - draft-ietf-mlcodec-opus-extension
draft-ietf-mlcodec-opus-extension
mlcodec T. Terriberry
Internet-Draft Xiph.Org
Updates: 6716 (if approved) JM. Valin
Intended status: Standards Track Amazon
Expires: 25 April 2024 23 October 2023
Extension Formatting for the Opus Codec
draft-ietf-mlcodec-opus-extension-01
Abstract
This document updates RFC6716 to extend the Opus codec (RFC6716) in a
way that maintains interoperability, 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.
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This Internet-Draft will expire on 25 April 2024.
Copyright Notice
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document authors. All rights reserved.
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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 120-126: I-D Experimental . . . . . . . . . . . . . . 4
2.5. ID 127: Extended Extensions . . . . . . . . . . . . . . . 5
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
3.1. Opus Media Type Update . . . . . . . . . . . . . . . . . 6
3.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
This document updates RFC6716 to extend the Opus codec (RFC6716) in a
way that maintains interoperability, 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.
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An extension starts with a byte that signals a 7-bit ID, as well as a
binary flag L for length signaling. For extension IDs 1 through 31,
L=0 means that no data follows the extension, whereas L=1 means that
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 signaled, 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 signaled 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.
A given extension ID may appear multiple times and extension ID
ordering is significant (see, e.g., Section 2.2). A particular
extension ID definition MAY place further restrictions on count and
ordering of these extensions (see Section 3).
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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 Opus 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 Opus
frame in the packet. An extension with ID=1 acts as a separator
between extensions from different Opus frames. When parsing
extensions sequentially, any time a separator with L=0 is
encountered, 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 does not 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 120-126: I-D Experimental
We reserve these 7 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
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experiments.
2.5. ID 127: Extended Extensions
The last ID is reserved for future extensions to the extension
mechanism. As with all other IDs in the range 32...127, the meaning
of the L flag is pre-defined to ensure decoders can skip extended
extensions they do not know about. The contents of the payload for
this extension will be defined by a future specification.
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] |
+-----------+------------------+------------------------------------+
| 120-126 | Experimental | Defined in Section 2.4, |
| | Internet-Draft | following the "Experimental |
| | implementations | Use" policy [RFC8126] |
+-----------+------------------+------------------------------------+
| 127 | Reserved for | Reserved in Section 2.5 |
| | future | |
| | extensions | |
+-----------+------------------+------------------------------------+
Table 1
For forward compatibility, any extension MUST use the definition of
the L flag dictated by its ID value (see Section 2). Extension
definitions MUST specify whether or not it is permitted for the
extension to appear multiple times for a given Opus frame within the
packet.
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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 ID, a hyphen, and the parameter name.
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.
Extension IDs 0 and 1 MUST be supported by any receiver that
recognizes Opus extensions, and do not need to be included in the
extensions or sprop-extensions lists.
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]. As per [RFC5576] Section 6.3, media-level format
parameters MUST be explicitly specified and MUST NOT be carried over
blindly from another offer or answer. 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
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[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>.
[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,
<https://www.rfc-editor.org/info/rfc5576>.
Authors' Addresses
Timothy B. Terriberry
Xiph.Org
United States of America
Email: tterribe@xiph.org
Jean-Marc Valin
Amazon
Canada
Email: jmvalin@amazon.com
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