Network Working Group M. Zanaty
Internet-Draft S. Nandakumar
Intended status: Informational Cisco
Expires: 11 January 2024 P. Thatcher
Microsoft
10 July 2023
Low Overhead Media Container
draft-mzanaty-moq-loc-01
Abstract
This specification describes a media container format for encoded and
encrypted audio and video media data to be used primarily for
interactive Media over QUIC Transport (MOQT) [MoQTransport], with the
goal of it being a low-overhead format. It also defines the MOQ
Catalog format for publishers to announce their tracks and for
subscribers to consume them.
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 11 January 2024.
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Table of Contents
1. Introduction
1.1. Requirements Notation and Conventions
2. Payload Format
2.1. MOQ Object Mapping
2.2. LOC Header Metadata
2.2.1. Common Header Data
2.2.2. Video Header Data
2.2.3. Audio Header Data
2.2.4. Header Data Registration
3. Catalog
3.1. Catalog Fields
3.1.1. Base Fields
3.1.2. Extension Fields
3.1.3. Track Quality Profile
3.1.4. Track Relations
3.2. Catalog Retrieval
3.3. Catalog Examples
3.3.1. Lip Sync Audio/Video Tracks with single quality
3.3.2. Simulcast video tracks - 3 qualities
4. Payload Encryption
5. Container Serialization
6. Security Considerations
7. IANA Considerations
8. Normative References
Appendix A. Acknowledgements
Authors' Addresses
1. Introduction
This specification describes a low-overhead media container format
for encoded and encrypted audio and video media data, as well as a
MOQ Catalog format to describe such tracks.
"Low-overhead" refers to minimal extra encapsulation as well as
minimal application overhead when interfacing with WebCodecs
[WebCodecs].
The container format description is specified for all audio and video
codecs defined in the WebCodecs Codec Registry
[WEBCODECS-CODEC-REGISTRY]. The audio and video payload bitstream is
identical to the "internal data" inside an EncodedAudioChunk and
EncodedVideoChunk, respectively, specified in the registry.
In addition to the media payloads, critical metadata is also
specified for audio and video payloads. (Note: Align with MOQT
terminology of either "metadata" or "header".)
A primary motivation is to align with media formats used in WebCodecs
to minimize extra encapsulation and application overhead when
interfacing with WebCodecs. Other container formats like CMAF or RTP
would require more extensive application overhead in format
conversions, as well as larger encapsultion overhead which may burden
some use cases like low bitrate audio scenarios.
This specification can also be used by applications outside the
context of WebCodecs or a web browser. While the media payloads are
defined by referring to the "internal data" of an EncodedAudioChunk
or EncodedVideoChunk in the WebCodecs Codec Registry, this "internal
data" is the elementary bitstream format of codecs without any
encapsulation. Referring to the WebCodecs Codec Registry avoids
duplicating it in an identical IANA registry.
* Section 2 defines the core media payload formats.
* Section 2.2 defines the metadata associated with audio and video
payloads.
* Section 3 describes the MoQ Catalog format including examples.
1.1. Requirements Notation and Conventions
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
[RFC2119].
2. Payload Format
The WebCodecs Codec Registry defines the contents of an
EncodedAudioChunk and EncodedVideoChunk for the audio and video codec
formats in the registry. The "internal data" in these chunks is used
directly in this specification as the "LOC Payload" bitstream. This
"internal data" is the elementary bitstream format of each codec
without any encapsulation.
For video formats with multiple bitstream formats in the WebCodecs
Registry, such as H.264/AVC or H.265/HEVC, the LOC Payload uses the
"canonical" format ("avcc" or "hevc", not "annexB") with the
following additions: * Parameter sets are sent in the bitstream
before key frames. * 4 byte lengths are sent before each NAL Unit. *
No start codes or emulation prevention are used in the bitstream. *
No additional codec configuration information ("extradata") is
needed.
2.1. MOQ Object Mapping
An application object when transported as a [MoQTransport] object is
composed of a MOQ Object Header and its Payload. Media objects
encoded using the container format defined in this specification
populate the MOQ Object Payload with a LOC Header and LOC Payload as
shown below.
The LOC Payload is the "internal data" of an EncodedAudioChunk or
EncodedVideoChunk.
+--------------+----------+-----------+
| MOQ Object | LOC | LOC |
| Header | Header | Payload |
+--------------+----------------------+
<---------------------->
MOQ Object Payload
MOQ Object with LOC Container
2.2. LOC Header Metadata
The LOC Header carries metadata for the corresponding LOC Payload.
This metadata provides necessary information for intermediaries such
as media switches to perform their media switching decisions when the
payload is inaccessible due to encryption.
Section Section 2.2.4 provides a framework for registering new LOC
Header fields that aren't defined by this specification.
2.2.1. Common Header Data
The following metadata MUST be captured for each media frame.
Sequence Number: Identifies a sequentially increasing variable length
integer that is incremented per encoded media frame. This may be
replaced with the Object Sequence from the MOQ Object Header in cases
where a MOQ Object is exactly one frame.
Capture Timestamp in Microseconds: Captures the wall-clock time of
the encoded media frame in a 64-bit unsigned integer.
2.2.2. Video Header Data
Flags for frames which are independent, discardable, or base layer
sync points, as well as temporal and spatial layer identification.
[Framemarking] .
2.2.3. Audio Header Data
Audio Level: Captures the magnitude of the audio level of the
corresponding audio frame encoded in 7 bits as defined in section 3
of [RFC6464].
2.2.4. Header Data Registration
This section details the procedures to register header data fields
that might be useful for a particular class of media applications.
Registering a given metadata field requires the following attributes
to be specified.
Shortname: Short name for the metadata. (Not sent on the wire.)
Description: Detailed description for the metadata. (Not sent on the
wire.)
ID: Identifier assigned by the registry. (varint)
Length: Length of metadata value in bytes. (varint)
Value: Value of metadata. (length bytes)
Registration of type "Specification Required" is followed for
registering new metadata in the LOC Header.
3. Catalog
A Catalog is a MOQT Object that provides information about tracks
from a given publisher. Catalog is used by subscribers for consuming
tracks and for publishers to advertise the tracks. The content of
"Catalog" is opaque to the Relays and may be end to end encrypted.
Catalog provides the details of tracks such as Track IDs and
corresponding media configuration details (audio/video codec detail,
gamestate encoding details, for example)
3.1. Catalog Fields
At the minumum catalog MUST provide enough information about MOQ
Tracks, such as its full name, information about media for the track
and mode of usage of the underlying QUIC transport. Following
subsections identify the mandatory Section 3.1.1 fields and optional
Section 3.1.2 fields that describe a given publisher's track in the
catalog. The applications is free to add further fields to the
catalog that is deemed necessary than the ones defined in this
specification and they don't need to be standardized.
TODO: Describe mechanics for preventing field name conflicts for
future extensions and for application specific extensions.
3.1.1. Base Fields
This section identifies the mandatory fields needs to be defined per
track listed in the catalog.
* Track Namespace: See section 2.3 of [MoQTransport]
* Track Name: See section 2.3 of [MoQTransport]
* Track Quality Profile: See Section 3.1.3
* Relation: See Section 3.1.4
Table 1 provides an overview of all base fields defined by this
document.
+=================+=======+===================+===========+
| Name | Label | Media Type | JSON Type |
+=================+=======+===================+===========+
| Track Namespace | ns | AV | String |
+-----------------+-------+-------------------+-----------+
| Track Name | tn | AV | String |
+-----------------+-------+-------------------+-----------+
| QualityProfile | qp | See Section 3.1.3 | |
+-----------------+-------+-------------------+-----------+
Table 1
3.1.2. Extension Fields
Following optional extension fields may be supported by the
application.
* Temporal ID: Identifies the temporal layer/sub-layer encoded,
starting with 0 for the base layer, and increasing with higher
temporal fidelity.
* Spatial ID: Identifies the spatial and quality layer encoded,
starting with 0 for the base layer, and increasing with higher
fidelity.
* Depend: Identifies track dependencies for a given track.
* Relation: See Section 3.1.4.
Table 2 provides label and type identification for the extension
fields
+=============+===================+============+===========+
| Name | Label | Media Type | JSON Type |
+=============+===================+============+===========+
| Temporal ID | tid | V | String |
+-------------+-------------------+------------+-----------+
| Spatial ID | lid | V | String |
+-------------+-------------------+------------+-----------+
| Depend | dep | V | Array |
+-------------+-------------------+------------+-----------+
| Relation | See Section 3.1.4 | | |
+-------------+-------------------+------------+-----------+
Table 2
3.1.3. Track Quality Profile
Each track has an associated quality profile that describes the media
objects for that track. Following properties identify a track's
quality profile.
* Codec: Codec information as defined by the codec registrations
listed in [WEBCODECS-CODEC-REGISTRY].
* Framerate: As defined in section 7.8 of
[WEBCODECS-CODEC-REGISTRY].
* Bitrate: As defined in section 7.7 and 7.8 of
[WEBCODECS-CODEC-REGISTRY].
* SampleRate: As defined in section 7.7 of
[WEBCODECS-CODEC-REGISTRY].
* Width,Height: As defined in section 7.8 of
[WEBCODECS-CODEC-REGISTRY].
* ChanelCount: As defined in section 7.7 of
[WEBCODECS-CODEC-REGISTRY].
* DisplayWidth, DisplayHeight: As defined in section 7.7 of
[WEBCODECS-CODEC-REGISTRY].
Table 3 provides an overview of all QualityProfile fields defined by
this document with their respective labels, applicable media types
and data types.
+===============+=======+============+===========+
| Name | Label | Media Type | JSON Type |
+===============+=======+============+===========+
| Codec | cs | AV | String |
+---------------+-------+------------+-----------+
| Framerate | fr | V | Number |
+---------------+-------+------------+-----------+
| Bitrate | br | AV | Number |
+---------------+-------+------------+-----------+
| Width | wd | V | Number |
+---------------+-------+------------+-----------+
| Height | ht | V | Number |
+---------------+-------+------------+-----------+
| SampleRate | sr | A | Number |
+---------------+-------+------------+-----------+
| ChanelCount | cc | A | Number |
+---------------+-------+------------+-----------+
| DisplayWidth | dw | V | Number |
+---------------+-------+------------+-----------+
| DisplayHeight | dh | V | Number |
+---------------+-------+------------+-----------+
Table 3
3.1.4. Track Relations
Tracks can express dependency on other tracks via relations property.
Following relation types are defined in this document.
* time-aligned: Indicates set of tracks that share the same time
offset when producing the media as well as considered as having
same time offset when consuming the media. Typical example would
be simulcasting a camera capture across multiple encoding
qualities.
* lip-sync: Indicates a synchronized playout of the media from the
tracks identified. Example audio and video media synced for
playout in a conference setting.
* layered: Indicates tracks are dependent via layered encoding and
applies to video tracks. Each track that is part of the layered
relation set MUST include depend field listing the dependencies.
Table 4 lists relation fields defined by this document with their
respective labels, applicable media types and data types.
+==============+=======+============+===========+
| Name | Label | Media Type | JSON Type |
+==============+=======+============+===========+
| lip-sync | ls | AV | Array |
+--------------+-------+------------+-----------+
| layered | ly | V | Number |
+--------------+-------+------------+-----------+
| time-aligned | ta | AV | Number |
+--------------+-------+------------+-----------+
Table 4
3.2. Catalog Retrieval
On a successful connection setup, subscribers proceed by retrieving
the catalog (if not already retrieved), subscribing to the tracks of
their interest and consuming the data published as detailed below.
Catalogs are identified as a special track, with its Track Name as
"catalog". Catalog objects are retrieved by subscribing to its Full
Track Name over its own MoQ control channel (Bidirectional QUIC
Stream). I
A successful subscription will lead to one or more catalog objects
being published and implies authorization for subscribing to the
tracks in the catalog.
Unsuccessful subscriptions MUST result in closure of the MOQT
session, followed by reporting the error obtained to the application.
Catalog Objects obtained MUST parse successfully, otherwise MUST be
treated as error, thus resulting the closure of the WebTransport
session.
3.3. Catalog Examples
The following section provides JSON examples of the catalog.
3.3.1. Lip Sync Audio/Video Tracks with single quality
This example shows catalog for the media sender, Alice, capable of
sending audio and video tracks and share lip-sync relation.
{
"ls": ["audio", "video"],
[
{
"ns": "conference.example.com/conference123/alice",
"n": "video",
"qp": "cs=av01.0.08M.10.0.110.09,wd=1920,ht=1080,fr=30"
},
{
"ns": "conference.example.com/conference123/alice",
"n": "audio",
"qp": "cs=opus,sr=48000,cc=2"
}
],
}
3.3.2. Simulcast video tracks - 3 qualities
This example shows catalog for the media sender, Alice, capable of
sending 3 video tracks for high definition, low definition and medium
definition qualities in time-aligned relation.
{
"ta": ["hd", "sd", "md"],
[
{
"ns": "conference.example.com/conference123/alice",
"n": "hd",
"qp": "cs=av01,wd=1920,ht=1080,fr=30"
},
{
"ns": "conference.example.com/conference123/alice",
"n": "md",
"qp": "cs=av01,wd=720,ht=640,fr=30"
},
{
"ns": "conference.example.com/conference123/alice",
"n": "sd",
"qp": "cs=av01,wd=192,ht=144,fr=30"
}
],
}
4. Payload Encryption
When end to end encryption is supported, the encoded payload is
encrypted with keys from symmetric keying mechanisms, such a MLS, and
the payload itself is protected using SFrame or other schemes similar
schemes.
TODO: Future revisions shall expand on the details on setting up such
a scheme.
5. Container Serialization
The wire encoding of the payload conforming to this specification is
a set of length delimited values as shown below.
The Bytes is obtained as output of AEAD operation for encrypting the
Payload with the header data as additional data input.
+--------+------------+-------+------------+
| Payload | Bytes | Payload | Bytes |
| Len | (0) | Len (1) | (1) | ...
+--------+------------+-------+------------+
6. Security Considerations
TODO
7. IANA Considerations
A new IANA registry for LOC Header Metadata is defined and populated
with the information in section Section 2.2.4. Specification
required for new metadata registration.
8. Normative References
[MoQTransport]
Curley, L., Pugin, K., Nandakumar, S., and V. Vasiliev,
"Media over QUIC Transport", Work in Progress, Internet-
Draft, draft-ietf-moq-transport-00, 5 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-moq-
transport-00>.
[Framemarking]
Zanaty, M., Berger, E., and S. Nandakumar, "Video Frame
Marking RTP Header Extension", Work in Progress, Internet-
Draft, draft-ietf-avtext-framemarking-14, 27 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-avtext-
framemarking-14>.
[WebCodecs]
"WebCodecs", July 2023,
<https://www.w3.org/TR/webcodecs/>.
[WEBCODECS-CODEC-REGISTRY]
"WebCodecs Codec Registry", July 2023,
<https://www.w3.org/TR/webcodecs-codec-registry/>.
[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/rfc/rfc2119>.
[RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464,
DOI 10.17487/RFC6464, December 2011,
<https://www.rfc-editor.org/rfc/rfc6464>.
Appendix A. Acknowledgements
Thanks to Cullen Jennings for suggestions and review.
Authors' Addresses
Mo Zanaty
Cisco
Email: mzanaty@cisco.com
Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
Peter Thatcher
Microsoft
Email: pthatcher@microsoft.com