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.
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This Internet-Draft will expire on 13 October 2023.
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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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