Audio/Video Transport M. Romaine
Internet-Draft M. Hatanaka
Expires: December 3, 2005 J. Matsumoto
SONY
June 2005
RTP Payload Format for ATRAC Family
draft-ietf-avt-rtp-atrac-family-04
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes an RTP payload format for efficient and
flexible transporting of audio data encoded with the Adaptive
TRansform Audio Codec (ATRAC) family of codecs. Recent enhancements
to the ATRAC family of codecs support high quality audio coding with
multiple channels. The RTP payload format as presented in this
document also includes support for data fragmentation, elementary
redundancy measures, and a variation on scalable streaming.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Codec Specific Details . . . . . . . . . . . . . . . . . . . . 4
4. RTP Packetization and Transport of ATRAC-Family Streams . . . 5
4.1 ATRAC Frames . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Concatenation of Frames . . . . . . . . . . . . . . . . . 5
4.3 Frame Fragmentation . . . . . . . . . . . . . . . . . . . 5
4.4 Transmission of Redundant Frames . . . . . . . . . . . . . 5
4.5 Global Structure of Payload Format . . . . . . . . . . . . 6
4.6 Scalable Lossless Streaming (High-Speed Transfer mode) . . 6
4.6.1 Scalable Multiplexed Streaming . . . . . . . . . . . . 6
4.6.2 Scalable Multi-Session Streaming . . . . . . . . . . . 7
5. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1 Usage of RTP Header Fields . . . . . . . . . . . . . . . . 7
5.2 RTP Payload Structure . . . . . . . . . . . . . . . . . . 8
5.2.1 ATRAC Header Section . . . . . . . . . . . . . . . . . 8
5.2.2 Redundant Data Section . . . . . . . . . . . . . . . . 8
5.2.3 ATRAC Frames Section . . . . . . . . . . . . . . . . . 9
6. Packetization Examples . . . . . . . . . . . . . . . . . . . . 10
6.1 Example Multi-frame Packet . . . . . . . . . . . . . . . . 10
6.2 Example Fragmented ATRAC Frame . . . . . . . . . . . . . . 11
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 12
7.1 ATRAC3 MIME Registration . . . . . . . . . . . . . . . . . 13
7.2 ATRAC-X MIME Registraion . . . . . . . . . . . . . . . . . 14
7.3 ATRAC Advanced Lossless MIME Registration . . . . . . . . 15
7.4 Channel Mapping Configuration Table . . . . . . . . . . . 17
7.5 Mapping MIME Parameters into SDP . . . . . . . . . . . . . 18
7.5.1 For MIME subtype ATRAC3 . . . . . . . . . . . . . . . 18
7.5.2 For MIME subtype ATRAC-X . . . . . . . . . . . . . . . 19
7.5.3 For MIME subtype ATRAC Advanced Lossless . . . . . . . 19
7.6 Offer-Answer Model Considerations . . . . . . . . . . . . 20
7.6.1 For All Three MIME Subtypes . . . . . . . . . . . . . 20
7.6.2 For MIME subtype ATRAC3 . . . . . . . . . . . . . . . 20
7.6.3 For MIME subtype ATRAC-X . . . . . . . . . . . . . . . 20
7.6.4 For MIME subtype ATRAC Advanced Lossless . . . . . . . 21
7.7 Usage of declarative SDP . . . . . . . . . . . . . . . . . 21
7.8 Example SDP Session Descriptions . . . . . . . . . . . . . 22
7.9 Example Offer-Answer Exchange . . . . . . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9.1 Confidentiality . . . . . . . . . . . . . . . . . . . . . 25
9.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 25
9.3 Decoding Validation . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1 Normative References . . . . . . . . . . . . . . . . . . . 25
10.2 Informative References . . . . . . . . . . . . . . . . . . 26
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . 28
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1. Introduction
The ATRAC family of perceptual audio codecs are designed to address
numerous needs for high-quality, low bit-rate audio transfer. ATRAC
technology can be found in many consumer and professional products
and applications, including MD players, CD players, voice recorders,
and mobile phones. The need for real-time streaming of audio data
has grown, and this document details our efforts in increasing the
product and application space for the ATRAC family of codecs.
Recent advances in ATRAC technology allow for multiple channels of
audio to be encoded in customizable groupings. This should allow for
future expansions in scaled streaming. To provide the greatest
flexibility in streaming any one of the ATRAC family member codecs
however, this payload format does not distinguish between the codecs
on a packet level.
This simplified payload format contains only the basic information
needed to disassemble a packet of ATRAC audio in order to decode it.
Timestamps are in sample units, with audio data currently encoded
into frames of 512, 1024 or 2048 samples depending on the ATRAC
version. There is also basic support for fragmentation and
redundancy, as ATRAC frames MAY exceed an MTU size of 1500 octets.
Although streaming of multi-channel audio is supported depending on
the ATRAC version used, all encoded audio for a given time period is
contained within a single frame. Therefore, there is no interleaving
nor splitting of audio data on a per-channel basis to be concerned
with.
2. Conventions
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 RFC 2119 [4].
3. Codec Specific Details
Early versions of the ATRAC codec handled only two channels of audio
at 44.1kHz sampling frequency, with typical bit-rates between 66kbps
and 132kbps. The latest version allows for a maximum 8 channels of
audio, up to 96kHz in sampling frequency, and a lossless encoding
option which can be transmitted in either a scalable (also known as
High-Speed Transfer mode) or standard (aka Standard mode) format.
The feasible bit-rate range has also expanded, allowing from a low of
8kbps up to 1400kbps in lossy encoding modes.
Depending on the version of ATRAC used, the sample-frame size is
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either 512, 1024 or 2048 samples. While the lossy and Standard mode
lossless formats are encoded as sequential single audio frames, High-
Speed Transfer mode lossless data comprises two layers -- a lossy
base layer and an enhancement layer.
4. RTP Packetization and Transport of ATRAC-Family Streams
4.1 ATRAC Frames
For transportation of compressed audio data, ATRAC uses the concept
of frames. ATRAC frames are the smallest data unit for which timing
information is attributed. Frames are octect-aligned by definition.
4.2 Concatenation of Frames
It is often possible to carry multiple frames in one RTP packet.
This can be useful in audio, where on a LAN with a 1500 byte MTU, an
average of 7 complete 64kbps ATRAC frames could be carried in a
single RTP packet, as each ATRAC frame would be approximately 200
bytes. ATRAC frames may be of fixed or variable length. To
facilitate parsing in the case of multiple frames in one RTP packet,
the size of each frame is made known to the receiver by carrying "in
band" the frame size for each contained frame in an RTP packet.
However, to simplify the implementation of RTP receivers, it is
required that when multiple frames are carried in an RTP packet, each
frame MUST be complete, i.e., the number of frames in an RTP packet
MUST be integral.
4.3 Frame Fragmentation
The ATRAC codec can handle very large frames. As most IP networks
have significantly smaller MTU sizes than the frame sizes ATRAC can
handle, this payload format allows for the fragmentation of an ATRAC
frame over multiple RTP packets. However, to simplify the
implementation of RTP receivers, an RTP packet SHALL either carry one
or more complete ATRAC frames or a single fragment of one ATRAC
frame. In other words, RTP packets MUST NOT contain fragments of
multiple ATRAC frames and MUST NOT contain a mix of complete and
fragmented frames.
4.4 Transmission of Redundant Frames
As RTP does not guarantee reliable transmission, receipt of data is
not assured. Loss of a packet can result in a "decoding gap" by the
receiver. One method to remedy this problem is to allow time-shifted
copies of ATRAC frames to be sent along with current data. For a
modest cost in latency and implementation complexity, error
resiliency to packet loss can be achieved.
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4.5 Global Structure of Payload Format
The RTP payload following the RTP header contains three octet-aligned
data sections, of which the second MAY be empty:
+------+--------------+--------------+--------------+
|RTP | ATRAC Header | Redundant | ATRAC Frames |
|Header| Section | Data Section | Section |
+------+--------------+--------------+--------------+
< ----------- RTP Packet Payload --------- >
The first data section is the ATRAC Header, containing just one
header with information for the whole packet. The second section is
for redundant ATRAC frames; this section MAY be empty. The third
section is where the encoded ATRAC frames are stored. This may
contain either a single fragment of one ATRAC frame, or one or more
complete ATRAC frames. The ATRAC Frames Section MUST NOT be empty.
To benefit from ATRAC's High-Speed Transfer mode lossless encoding
capability, the RTP payload can be split across two sessions, with
one transmitting an essential base layer and the other transmitting
enhancement data. However in either case, the above structure still
applies.
4.6 Scalable Lossless Streaming (High-Speed Transfer mode)
As ATRAC supports a variation on scalable encoding, this payload
format provides a mechanism for transmitting essential data (also
referred to as the base layer) with its enhancement data in two ways
-- multiplexed through one session or separated over two sessions.
In either method, only the base layer is essential in producing audio
data. The enhancement layer carries the remaining audio data needed
to decode lossless audio data. So in situations of limited
bandwidth, the sender may choose not to transmit enhancement data yet
still provide a client with enough data to generate lossily-encoded
audio through the base layer.
4.6.1 Scalable Multiplexed Streaming
In multiplexed streaming, the base layer and enhancement layer are
coupled together in each packet, utilizing only one session. While
the packet may begin with either layer type, the two layer types MUST
interleave.
+----------------+ +----------------+ +----------------+
|Base|Enhancement|--|Base|Enhancement|--|Base|Enhancement| ...
+----------------+ +----------------+ +----------------+
N N+1 N+2 : Packet
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4.6.2 Scalable Multi-Session Streaming
In multi-session streaming, the base layer and enhancement layer are
sent over two seperate sessions, allowing clients with certain
bandwidth limitations to receive just the base layer for decoding.
While there may be alternative methods for synchronization of the
layers, it is RECOMMENDED that the timestamp be used for
synchronizing the base layer with its enhancement. Applications can
determine which sessions are paired together through use of the
Session Description Protocol (SDP) (RFC 2327) [2]. Further details
are discussed in the section titled "Usage of declarative SDP".
Session 1:
+------+ +------+ +------+ +------+
| Base |--| Base |--| Base |--| Base | ...
+------+ +------+ +------+ +------+
N N+1 N+2 N+3 : Packet
Session 2:
+-------------+ +-------------+ +-------------+
| Enhancement |--| Enhancement |--| Enhancement | ...
+-------------+ +-------------+ +-------------+
N N+1 N+2 : Packet
5. Payload Format
5.1 Usage of RTP Header Fields
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| contributing source (CSRC) identifiers |
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker (M): 1 bit
Set to zero as silence suppression is currently not used.
Payload Type (PT): 7 bits
The assignment of an RTP payload type for this packet format is
outside the scope of this document; it is specified by the RTP
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profile under which this payload format is used, or signaled
dynamically out-of-band (e.g., using SDP).
Timestamp: 32 bits
A timestamp representing the sampling time of the first sample of the
first ATRAC frame in the RTP packet. When using SDP, the clock rate
of the RTP timestamp MUST be expressed using the "rtpmap" attribute.
For ATRAC3 the RTP timestamp rate MUST be 44100Hz. For ATRAC-X the
RTP timestamp rate is defined out-of-band.
5.2 RTP Payload Structure
5.2.1 ATRAC Header Section
The ATRAC family payload header is one byte.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|C|FrgNo|NFrames|
+-+-+-+-+-+-+-+-+
Continuous flag (C): 1 bit
Set to 1 if this is part of a fragmented packet. The last packet in
a series would have this bit set to 0.
Fragment Number (FrgNo): 3 bits
In the event of data fragmentation, this value is 1 for the first
packet, and increases sequentially for the remaining fragmented data
packets.
Number of Frames (NFrames): 4 bits
The number of frames in this packet. This allows for a maximum of 16
ATRAC-encoded audio frames per packet, with 0 indicating one frame.
Each frame must be complete. Only the first frame is allowed to be
fragmented, in which case this MUST NOT be anything other than 0 for
subsequent packets containing the fragmented frame.
5.2.2 Redundant Data Section
The Redundant Data Section provides a rudimentary mechanism to
compensate for occasional packet loss. As every packet's timestamp
corresponds to the first audio frame regardless of whether it is
redundant or not, and because we know how many frames of audio each
packet encapsulates, if two successive packets are successfully
transmitted, we can calculate the number of redundant frames being
sent. The result gives the client a sense of how the server is
responding to RTCP reports and to expand its buffer size if
necessary.
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As an example of using the Redundant Data Section, refer to Figure 1.
In this example, the server has determined that for the next few
number of packets, it should send the last two frames from the
previous packet due to recent RTCP reports. Thus, between packets N
and N+1, there is a redundancy of two frames (which the client may
choose to dispose of). The benefit arises when packets N+2 and N+3
do not arrive at all, after which eventually packet N+4 arrives with
successive necessary audio frame data. This field SHOULD NOT be used
in packets containing fragmented data.
|-Fr1-|-Fr2-|-Fr3-| Packet N, TS=1
|-Fr2-|-Fr3-|-Fr4-| Packet N+1, TS=2
|-Fr5-|-Fr6-|-Fr7-| Packet N+4, TS=5
5.2.3 ATRAC Frames Section
The ATRAC Frames Section contains an integer number of complete ATRAC
frames or a single fragment of one ATRAC frame. Each ATRAC frame is
preceeded by a one-bit flag indicating the layer type and a Block
Length field indicating the size in bytes of the ATRAC frame. If
more than one ATRAC frame is present, then the frames are
concatenated into a contiguous string of bit-flag, Block Length, and
ATRAC frame. This section MUST NOT be empty.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Block Length | ATRAC frame |...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Layer Type Flag: 1 bit
Set to 1 if the corresponding ATRAC frame is from an enhancement
layer. 0 indicates a base layer encoded frame.
Block length: 15 bits
The byte length of encoded audio data for the following frame. This
is so that in the case of fragmentation, if only a subsequent packet
is received, decoding can still occur. 15 bits allows for a maximum
block length of 32,767 bytes. If there are multiple frames in a
packet, a block-length field exists before each frame data.
ATRAC frame: The encoded ATRAC audio data.
5.2.3.1 Frame Fragmentation
Each RTP packet SHALL contain either an integer number of ATRAC
encoded audio frames (with a maximum of 16), or one ATRAC frame
fragment. In the former case, as many complete ATRAC frames as can
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fit in a single path-MTU SHOULD be placed in an RTP packet. However,
if even a single ATRAC frame will not fit into a complete RTP packet,
the ATRAC frame SHOULD be fragmented.
The start of a fragmented frame gets placed in its own RTP packet,
its Continuous bit (C) set to one, and its Fragment Number (FragNo)
set to one. As the frame must be the only one in the packet, the
Number of Frames field is zero. Subsequent packets are to contain
the remaining fragmented frame data, with the Fragment Number
increasing sequentially and the Continuous bit (C) consistently set
to one. As subsequent packets do not contain any new frames, the
Number of Frames field SHOULD be ignored. The last packet of
fragmented data MUST have the Continuous bit (C) set to zero.
In addition to the Continuous bit and Fragment Number fields
indicating fragmentation and a means to reorder the packets, the
timestamp can be used to determine which packets go together. Thus,
packets containing related fragmented frames MUST have identical
timestamps.
In the event of fragmentation, the basic redundancy measures MUST NOT
be used. This means the Frame Offset field MUST be ignored.
6. Packetization Examples
6.1 Example Multi-frame Packet
Multiple encoded audio frames are combined into one packet. Note how
for this example, only base layer frames are sent redundantly, but
are followed by interleaved base layer and enhancement layer frames.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| contributing source (CSRC) identifiers |
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| 0 | 5 |0| Block Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (redundant) base layer frame 1 data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Block Length |(redundant) base layer frame 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (cont.) |0| Block Length | base layer frame 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (cont.) |1| Block Length | enhancment frame 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Block Length | base layer frame 4... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.2 Example Fragmented ATRAC Frame
The encoded audio data frame is split over three RTP packets. The
following points are highlighted in the example below:
o transition from one to zero of the Continuous bit (C)
o sequential increase in the Fragment Number
Packet 1:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| contributing source (CSRC) identifiers |
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| 1 | 0 |1| Block Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| enhancement data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Packet 2:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| contributing source (CSRC) identifiers |
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| 2 | 0 |1| Block Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...more enhancement data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Packet 3:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| contributing source (CSRC) identifiers |
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| 3 | 0 |1| Block Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...the last of the enhancement data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7. Payload Format Parameters
Certain parameters will need to be defined before ATRAC family
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encoded content can be streamed. Other optional parameters may also
be defined to take advantage of specific features relevant to certain
ATRAC versions. Parameters for ATRAC3, ATRAC-X, and ATRAC Advanced
Lossless are defined here as part of the MIME subtype registration
process. A mapping of these parameters into the Session Description
Protocol (SDP) (RFC 2327) [2] is also provided for applications that
utilize SDP.
The data format and parameters are specified for real-time transport
in RTP.
7.1 ATRAC3 MIME Registration
The MIME subtype for the Adaptive TRansform Codec version 3 (ATRAC3)
is allocated from the Vendor tree since this codec is intended to be
used with commercial products, and use of any ATRAC family codec
requires a license from Sony Corporation, the vendor.
Note, any unspecified parameter MUST be ignored by the receiver.
Media Type name: audio
Media subtype name: vnd.sony.atrac3
Required parameters:
baseLayer: Indicates the encoded bit-rate for the audio data to be
streamed. Permissible values are 66, 105, and 132.
Optional parameters:
maxRedundantFrames: The maximum number of redundant frames that may
be sent during a session in any given packet under the redundant
framing mechanism detailed in the draft. Allowed values are integers
in the range of 0 to 15, inclusive. If this parameter is not used, a
default of 15 MUST be assumed.
maxptime: The maximum amount of media which can be encapsulated in
each packet, expressed as time in milliseconds. The time SHALL be
calculated as the sum of the time the media present in the packet
represents. For frame based codecs, the time MUST be an integer
multiple of the frame size. If this parameter is not present, the
sender MAY encapsulate a maximum of 16 encoded frames into one RTP
packet.
ptime: see RFC 2327 [2]
Encoding considerations: This type is defined for transfer via RTP
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RFC 3550 [1].
Security considerations:
Please refer to section 7 of this draft.
Public specifications:
Please refer to section 7 of this draft.
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Mitsuyuki Hatanaka
hatanaka@av.crl.sony.co.jp
Intended usage: COMMON
Author/Change controller:
hatanaka@av.crl.sony.co.jp
7.2 ATRAC-X MIME Registraion
The MIME subtype for the Adaptive TRansform Codec version X (ATRAC-X)
is allocated from the Vendor tree since this codec is intended to be
used with commercial products, and use of any ATRAC family codec
requires a license from Sony Corporation, the vendor.
Note, any unspecified parameter MUST be ignored by the receiver.
Media Type name: audio
Media subtype name: vnd.sony.atrac-x
Required parameters:
sampleRate: Represents the sampling frequency in Hz of the original
audio data. Permissible values are 32000, 44100, 48000, 88200,
96000.
baseLayer: Indicates the encoded bit-rate for the audio data to be
streamed. Permissible values are 48, 64, 96, 128, 160, 192, 256, and
320.
channelID: Indicates the number of channels and channel layout
according to the table in Section 5.3. Note that this layout is
different from that proposed in RFC 3551 [3]. However, as channelID
= 0 defines an ambiguous channel layout, the channel mapping defined
in Section 4.1 of [3] could be used. Permissible values are 0, 1, 2,
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3, 4, 5, 6, 7.
Optional parameters:
maxRedundantFrames: The maximum number of redundant frames that may
be sent during a session in any given packet under the redundant
framing mechanism detailed in the draft. Allowed values are integers
in the range 0 to 15, inclusive. If this parameter is not used, a
default of 15 MUST be assumed.
delayMode: Indicates a desire to use low-delay features, in which
case the decoder will process received data accordingly based on this
value. Permissible values are 2 and 4.
maxptime: The maximum amount of media which can be encapsulated in a
payload packet, expressed as time in milliseconds. The time is
calculated as the sum of the time the media present in the packet
represents. The time MUST be a multiple of the frame size. If this
parameter is not present, the sender MAY encapsulate a maximum of 16
encoded frames into one RTP packet.
ptime: see RFC 2327 [2]
Encoding considerations: This type is defined for transfer via RTP
(RFC 3550) [1].
Security considerations:
Please refer to section 7 of this draft.
Public specifications:
Please refer to section 7 of this draft.
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Mitsuyuki Hatanaka
hatanaka@av.crl.sony.co.jp
Intended usage: COMMON
Author/Change controller:
hatanaka@av.crl.sony.co.jp
7.3 ATRAC Advanced Lossless MIME Registration
The MIME subtype for the Adaptive TRansform Codec Lossless version
(ATRAC Advanced Lossless) is allocated from the Vendor tree since
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this codec is intended to be used with commercial products, and use
of any ATRAC family codec requires a license from Sony Corporation,
the vendor.
Note, any unspecified parameter MUST be ignored by the receiver.
Media Type name: audio
Media subtype name: vnd.sony.atrac-advanced-lossless
Required parameters:
sampleRate: Represents the sampling frequency in Hz of the original
audio data. Permissible values are 32000, 44100, 48000, 88200,
96000.
baseLayer: Indicates the encoded bit-rate for the base layer in
High-Speed Transfer mode lossless encodings. For Standard lossless
mode this value MUST be 0. Permissible values are 0, 48, 64, 66, 96,
105, 128, 132, 160, 192, 256, and 320.
blockLength: Indicates the block length being used for Standard
lossless mode. When "baseLayer=0", this value MUST be one of either
512, 1024, or 2048. If the "baseLayer" parameter is not "0", this
parameter MUST be ignored.
channelID: Indicates the number of channels and channel layout
according to the table in Section 5.3. Note that this layout is
different from that proposed in RFC 3551 [3]. However, as channelID
= 0 defines an ambiguous channel layout, the channel mapping defined
in Section 4.1 of [3] could be used. Permissible values are 0, 1, 2,
3, 4, 5, 6, 7.
Optional parameters:
maxRedundantFrames: The maximum number of redundant frames that may
be sent during a session in any given packet under the redundant
framing mechanism detailed in the draft. Allowed values are integers
in the range 0 to 15, inclusive. If this parameter is not used, a
default of 15 MUST be assumed.
maxptime: The maximum amount of media which can be encapsulated in a
payload packet, expressed as time in milliseconds. The time is
calculated as the sum of the time the media present in the packet
represents. The time MUST be a multiple of the frame size. If this
parameter is not present, the sender MAY encapsulate a maximum of 16
encoded frames into one RTP packet.
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ptime: see RFC 2327 [2]
Encoding considerations: This type is defined for transfer via RTP
(RFC 3550) [1].
Security considerations:
Please refer to section 7 of this draft.
Public specifications:
Please refer to section 7 of this draft.
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Mitsuyuki Hatanaka
hatanaka@av.crl.sony.co.jp
Intended usage: COMMON
Author/Change controller:
hatanaka@av.crl.sony.co.jp
7.4 Channel Mapping Configuration Table
The following is a table explaining the mapping between the channelID
as passed during SDP negotiations, and the speaker mapping the value
represents.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| channelID | Number of | Default Speaker |
| | Channels | Mapping |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | max 64 | undefined |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 1 | front: center |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 2 | front: left, right |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 3 | front: left, right |
| | | front: center |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | 4 | front: left, right |
| | | front: center |
| | | rear: surround |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 5+1 | front: left, right |
| | | front: center |
| | | rear: left, right |
| | | LFE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 | 6+1 | front: left, right |
| | | front: center |
| | | rear: left, right |
| | | rear: center |
| | | LFE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 7 | 7+1 | front: left, right |
| | | front: center |
| | | rear: left, right |
| | | side: left, right |
| | | LFE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.5 Mapping MIME Parameters into SDP
The information carried in the MIME media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[2], which is commonly used to describe RTP sessions. When SDP is
used to specify sessions employing the ATRAC family of codecs, the
following mapping rules according to the ATRAC codec apply:
7.5.1 For MIME subtype ATRAC3
o The MIME type ("audio") goes in SDP "m=" as the media name
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o The MIME subtype (payload format name) goes in SDP "a=rtpmap" as
the encoding name. ATRAC3 supports only mono or stereo signals,
so a corresponding number of channels SHALL also be included in
this attribute.
o The "baseLayer" parameter goes in SDP "a=fmtp". This parameter
MUST be present. "maxRedundantFrames" may follow, but if no value
is transmitted, the receiver SHOULD assume a default value of
"15".
o The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and
"a=maxptime" attributes, respectively.
7.5.2 For MIME subtype ATRAC-X
o The MIME type ("audio") goes in SDP "m=" as the media name
o The MIME subtype (payload format name) goes in SDP "a=rtpmap" as
the encoding name. This should be followed by the "sampleRate"
(as the RTP clock rate), and then the actual number of channels
regardless of the channelID parameter.
o The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and
"a=maxptime" attributes, respectively.
o Any remaining parameters go in the SDP "a=fmtp" attribute by
copying them directly from the MIME media type string as a
semicolon separated list of parameter=value pairs. The
"baseLayer" parameter must be the first entry on this line. It is
recommened that the "channelID" parameter be the next entry. The
receiver MUST assume a default value of "15" for
"maxRedundantFrames".
7.5.3 For MIME subtype ATRAC Advanced Lossless
o The MIME type ("audio") goes in SDP "m=" as the media name
o The MIME subtype (payload format name) goes in SDP "a=rtpmap" as
the encoding name. This should be followed by the "sampleRate"
(as the RTP clock rate), and then the actual number of channels
regardless of the channelID parameter.
o The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and
"a=maxptime" attributes, respectively.
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o Any remaining parameters go in the SDP "a=fmtp" attribute by
copying them directly from the MIME media type string as a
semicolon separated list of parameter=value pairs. The
"baseLayer" parameter must be the first entry on this line. If
"baseLayer=0", the "blockLength" parameter MUST follow and be one
of either 512, 1024, or 2048. It is recommended that the
"channelID" parameter be the next entry. The receiver MUST assume
a default value of "15" for "maxRedundantFrames".
7.6 Offer-Answer Model Considerations
Some options for encoding and decoding ATRAC audio data will require
either or both the sender and receiver to comply with certain
specifications. In order to establish an interoperable transmission
framework, an Offer-Answer negotiation in SDP should observe the
following considerations:
7.6.1 For All Three MIME Subtypes
o Each combination of the RTP payload transport format configuration
parameters (baseLayer and blockLength, sampleRate, channelID) is
unique in its bit-pattern and not compatible with any other
combination. When creating an offer in an application desiring to
use the more advanced features (sample rates above 44100kHz, more
than two channels), the offerer is RECOMMENDED to also offer a
payload type containing only the lowest set of necessary
requirements. If multiple configurations are of interest to the
application they may all be offered, however care should be taken
not to offer too many payload types.
o The parameters "maxptime" and "ptime" will in most cases not
affect interoperability, however the setting of the parameters can
affect the performance of the application. The SDP offer-answer
handling of the "ptime" parameter is described in RFC3264. The
"maxptime" parameter MUST be handled in the same way.
7.6.2 For MIME subtype ATRAC3
o In response to an offer, downgraded subsets of "baseLayer" are
possible. However for best performance, we suggest the answer
contain the highest possible values offered.
7.6.3 For MIME subtype ATRAC-X
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o When creating an offer with considerably high requirements (such
as 8 channels at 96kHz), it is RECOMMENDED that the offer also
contain a configuration with lower requirements (such as a stereo
only option). Although multiple alternative configurations may be
offered, care should be taken not to offer too many payload types.
o In response to an offer, downgraded subsets of "sampleRate",
"baseLayer", and "channelID" are possible. For best performance,
we suggest an answer SHALL NOT contain any values requiring
further capabilities than the offer contains, but is RECOMMENDED
to provide values as close as possible to those in the offer.
o The "maxRedundantFrames" is a suggested minimum. This value MAY
be increased in an answer (with a maximum of 15), but SHALL NOT be
reduced.
o The optional parameter "delayMode" is non-negotiable. If the
Answerer cannot comply with the offered value, the session must be
deemed inoperable.
7.6.4 For MIME subtype ATRAC Advanced Lossless
o When creating an offer with considerably high requirements (such
as 8 channels at 96kHz), it is RECOMMENDED that the offer also
contain a configuration with lower requirements (such as a stereo
only option). Although multiple alternative configurations may be
offered, care should be taken not to offer too many payload types.
o In response to an offer, downgraded subsets of "sampleRate",
"baseLayer", and "channelID" are possible. For best performance,
we suggest an answer SHALL NOT contain any values requiring
further capabilities than the offer contains, but is RECOMMENDED
to provide values as close as possible to those in the offer.
o There are no requirements when negotiating "blockLength", other
than that both parties must be in agreement.
o The "maxRedundantFrames" is a suggested minimum. This value MAY
be increased in an answer (with a maximum of 15), but SHALL NOT be
reduced.
7.7 Usage of declarative SDP
In declarative usage, like SDP in RTSP [9] or SAP [10], the
parameters SHALL be interpreted as follows:
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o The payload format configuration parameters (baseLayer,
sampleRate, channelID) are all declarative and a participant MUST
use the configuration(s) that is provided for the session. More
than one configuration may be provided if necessary by declaring
multiple RTP payload types, however the number of types should be
kept small.
o Any "maxptime" and "ptime" values should be selected with care to
ensure that the session's participants can achieve reasonable
performance.
For transmission of scalable multi-session streaming of ATRAC
Advanced Lossless content, section 6 of the Session Description
Protocol (RFC 2327) [2] defines attributes for notifying applications
of hierarchically encoded streams. For multicast sessions, the base
layer and enhancement layer are transmitted over seperate multicast
groups, thus requiring multiple multicast addresses. For this
scenario, SDP slash notation as defined in RFC 2327 [2] for the "c="
field should be followed. For IP unicast addresses, it will be
necessary to specify multiple transport ports. This is done with
slash notation in the "m=" field similarly defined in RFC 2327 [2].
7.8 Example SDP Session Descriptions
Example usage of ATRAC-X with stereo at 44100Hz:
m=audio 49120 RTP/AVP 99
a=rtpmap:99 ATRAC-X/44100/2
a=fmtp:99 baseLayer=128; channelID=2; delayMode=2
a=maxptime:20
Example usage of ATRAC-X with 5.1 setup at 48000Hz:
m=audio 49120 RTP/AVP 99
a=rtpmap:99 ATRAC-X/48000/6
a=fmtp:99 baseLayer=320; channelID=5
a=maxptime:30
Example usage of ATRAC-Advanced-Lossless in Standard mode:
m=audio 49200 RTP/AVP 99
a=rtpmap:99 ATRAC-ADVANCED-LOSSLESS/44100/2
a=fmtp:99 baseLayer=0; blockLength=1024; channelID=2
a=maxptime:30
Example usage of ATRAC-Advanced-Lossless in High-Speed mode (note
slash notation for multiple port-pairings):
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m=audio 49200/2 RTP/AVP 99
a=rtpmap:99 ATRAC-ADVANCED-LOSSLESS/48000/2
a=fmtp:99 baseLayer=128; blockLength=0; channelID=2
a=maxptime:30
7.9 Example Offer-Answer Exchange
The following Offer/Answer example shows how a desire to stream
multi-channel content is turned down by the receiver, who answers
with only the ability to receive stereo content:
Offer:
m=audio 49170 RTP/AVP 98 99
a=rtpmap:98 ATRAC-X/44100/6
a=fmtp:98 baseLayer=320; channelID=5
a=rtpmap:99 ATRAC-X/44100/6
a=fmtp:99 baseLayer=160; channelID=5
Answer:
m=audio 49170 RTP/AVP 99
a=rtpmap:99 ATRAC-X/44100/2
a=fmtp:99 baseLayer=160; channelID=2
The following Offer/Answer example shows the receiver answering with
a selection of supported parameters:
Offer:
m=audio 49170 RTP/AVP 97 98 99
a=rtpmap:97 ATRAC-X/44100/2
a=fmtp:97 baseLayer=128; channelID=2
a=rtpmap:98 ATRAC-X/44100/6
a=fmtp:98 baseLayer=128; channelID=5
a=rtpmap:99 ATRAC-X/48000/6
a=fmtp:99 baseLayer=320; channelID=5
Answer:
m=audio 49170 RTP/AVP 97 98
a=rtpmap:97 ATRAC-X/44100/2
a=fmtp:97 baseLayer=128; channelID=2
a=rtpmap:98 ATRAC-X/44100/6
a=fmtp:98 baseLayer=128; channelID=5
The following Offer/Answer example shows an exchange in trying to
resolve using ATRAC-Advanced-Lossless. The offer contains three
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options: multi-session High-Speed Transfer mode, multiplexed High-
Speed Transfer mode, and Standard mode.
Offer:
m=audio 49170/2 RTP/AVP 97
a=rtpmap:97 ATRAC-ADVANCED-LOSSLESS/44100/2
a=fmtp:97 baseLayer=64; blockLength=0; channelID=2
m=audio 49170 RTP/AVP 98
a=rtpmap:98 ATRAC-ADVANCED-LOSSLESS/44100/2
a=fmtp:98 baseLayer=256; blockLength=0; channelID=2
m=audio 49170 RTP/AVP 99
a=rtpmap:99 ATRAC-ADVANCED-LOSSLESS/48000/2
a=fmtp:99 baseLayer=0; blockLength=1024; channelID=2
Answer:
m=audio 49170/2 RTP/AVP 97
a=rtpmap:97 ATRAC-ADVANCED-LOSSLESS/44100/2
a=fmtp:97 baseLayer=64; blockLength=0; channelID=2
m=audio 49170 RTP/AVP 98
a=rtpmap:98 ATRAC-ADVANCED-LOSSLESS/44100/2
a=fmtp:98 baseLayer=256; blockLength=0; channelID=2
m=audio 0 RTP/AVP 99
Note that payload format (encoding) names are commonly shown in upper
case. MIME subtypes are commonly shown in lower case. These names
are case-insensitive in both places. Similarly, parameter names are
case-insensitive both in MIME types and in the default mapping to the
SDP a=fmtp attribute.
8. IANA Considerations
Two new MIME subtypes, for ATRAC3 and ATRAC-X, are requested to be
registered (see Section 5).
9. Security Considerations
Certain security precautions may be desired to protect copyrighted
material. The payload format as described in this document is
subject to the security considerations defined in RFC3550 [1] and any
applicable profile, for example RFC 3551 [3]. This payload format
however does not implement any security mechanisms of its own.
External means, such as SRTP [5], MAY be used since the audio
compression scheme follows an end-to-end model.
Since the data transported is audio that is already encoded, the main
security issues are confidentiality, integrity, and authentication of
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the actual audio.
9.1 Confidentiality
To ensure confidentiality of ATRAC encoded audio, the audio frames
will have to be encrypted. Encryption of the payload header,
however, is not as neccessary, and in fact may not be preferrable if
the information could be useful to some third party application.
Because the audio compression scheme follows an end-to-end model,
encryption may be performed after packet encapsulation. As multi-
channel transmissions are contained in single encoded audio frames,
there is no concern for encryption affecting interleaving data.
9.2 Authentication
Transmitted data may be tampered or altered due malicious attempts,
such as man-in-the-middle attacks. Such attacks may result in
depacketization and/or decoding errors that could decimate audio
quality.
As this payload format does not include its own means for sender
authentication and integrity protection, an external mechanism must
be used. It is RECOMMENDED, however, that the chosen mechanism
protect more than just the audio data bits. For example, to protect
against a man-in-the-middle attack, the payload header and RTP header
SHOULD be protected.
9.3 Decoding Validation
Verification of the received encoded audio packets should be
performed so as to ensure a minimal level of audio quality. As a
most primitive implementation, if the receiver calculates a packet
size differing from the payload length based on data in the payload
header fields, the receiver SHOULD discard the packet.
10. References
10.1 Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobsen,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, STD 64, July 2003.
[2] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
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with Minimal Control", RFC 3551, STD 65, July 2003.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels, BCP 14", RFC 2119, March 1997.
10.2 Informative References
[5] Kerr, P., "RTP Payload Format for Vorbis Encoded Audio",
October 2003.
[6] Sjoberg, J., "Real-Time Transport Protocol (RTP) Payload Format
and File Storage Format for the Adaptive Multi-Rate (AMR) and
Adpative Multi-Rate Wideband (AMR-WB) Audio Codecs", RFC 3267,
June 2002.
[7] Baugher, M., Carrara, E., McGrew, D., Naslund, M., and Norrman,
"The Secure Real Time Transport Protocol", July 2003.
[8] Rosenberg, J. and Schulzrinne, "An Offer/Answer Model with the
Session Description Protocl (SDP)", RFC 3264, June 2002.
[9] Schulzrinne, H., Rao, and Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[10] Handley, M., Perkins, and Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
Authors' Addresses
Matthew Romaine
Sony Corporation, Japan
6-7-35 Kitashinagawa
Shinagawa-ku
Tokyo 141-0001
Japan
Email: Matthew.Romaine@jp.sony.com
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Mitsuyuki Hatanaka
Sony Corporation, Japan
6-7-35 Kitashinagawa
Shinagawa-ku
Tokyo 141-0001
Japan
Email: hatanaka@av.crl.sony.co.jp
Jun Matsumoto
Sony Corporation, Japan
6-7-35 Kitashinagawa
Shinagawa-ku
Tokyo 141-0001
Japan
Email: jun@av.crl.sony.co.jp
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