Audio/Video Transport M. Romaine
Internet-Draft M. Hatanaka
Expires: November 17, 2005 J. Matsumoto
SONY
May 16, 2005
RTP Payload Format for ATRAC Family
draft-ietf-avt-rtp-atrac-family-03
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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, auxiliary
data, and elementary redundancy measures.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Codec Specific Details . . . . . . . . . . . . . . . . . . . . 3
4. RTP Packetization and Transport of ATRAC-Family Streams . . . 4
4.1 ATRAC Frames . . . . . . . . . . . . . . . . . . . . . . . 4
4.2 Concatenation of Frames . . . . . . . . . . . . . . . . . 4
4.3 Frame Fragmentation . . . . . . . . . . . . . . . . . . . 4
4.4 Transmission of Auxiliary Information . . . . . . . . . . 4
4.5 Transmission of Redundant Frames . . . . . . . . . . . . . 5
4.6 Global Structure of Payload Format . . . . . . . . . . . . 5
5. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1 Usage of RTP Header Fields . . . . . . . . . . . . . . . . 6
5.2 RTP Payload Structure . . . . . . . . . . . . . . . . . . 6
5.2.1 ATRAC Header Section . . . . . . . . . . . . . . . . . 6
5.2.2 Auxiliary Data Section . . . . . . . . . . . . . . . . 7
5.2.3 Redundant Data Section . . . . . . . . . . . . . . . . 8
5.2.4 ATRAC Frames Section . . . . . . . . . . . . . . . . . 8
6. Packetization Examples . . . . . . . . . . . . . . . . . . . . 9
6.1 Example Multi-frame Packet . . . . . . . . . . . . . . . . 9
6.2 Example Fragmented ATRAC Frame . . . . . . . . . . . . . . 10
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 10
7.1 ATRAC3 MIME Registration . . . . . . . . . . . . . . . . . 11
7.2 ATRAC-X MIME Registraion . . . . . . . . . . . . . . . . . 12
7.3 Channel Mapping Configuration Table . . . . . . . . . . . 14
7.4 Mapping MIME Parameters into SDP . . . . . . . . . . . . . 14
7.4.1 For MIME subtype ATRAC3 . . . . . . . . . . . . . . . 15
7.4.2 For MIME subtype ATRAC-X . . . . . . . . . . . . . . . 15
7.5 Offer-Answer Model Considerations . . . . . . . . . . . . 15
7.5.1 For Both MIME Subtypes . . . . . . . . . . . . . . . . 16
7.5.2 For MIME subtype ATRAC3 . . . . . . . . . . . . . . . 16
7.5.3 For MIME subtype ATRAC-X . . . . . . . . . . . . . . . 16
7.6 Usage of declarative SDP . . . . . . . . . . . . . . . . . 17
7.7 Example SDP Session Descriptions . . . . . . . . . . . . . 17
7.8 Example Offer-Answer Exchange . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9.1 Confidentiality . . . . . . . . . . . . . . . . . . . . . 19
9.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 19
9.3 Decoding Validation . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1 Normative References . . . . . . . . . . . . . . . . . . . 19
10.2 Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . 22
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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 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.
To anticipate the need for transportation of additional system-
related information in the future, an auxiliary field can be
configured that may carry any such data. However, this document does
not prescribe how to transcode or map such information in the
payload. Any such processing is left to the discretion of the
application.
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 up to 8 channels of audio
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at 96kHz sampling frequency. The feasible bit-rate range has also
expanded, allowing from 8kbps to 1400kbps.
Depending on the version of ATRAC used, the sample-frame size is
either 1024 or 2048. Actual bit-rates are determined by specifying a
fixed encoded frame-size. In other words, instead of requesting a
stereo 44.1kHz stream at, say, 64kbps, one would tell the encoder to
create encoded frame-sizes of 364bytes.
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 octet sizes. 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 Auxiliary Information
This RTP payload format supports the definition of a specific field
to transport auxiliary data. Similar to each ATRAC frame, the
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auxiliary data field is preceded by a field that specifies the length
of the auxiliary data. This allows receivers to skip the data
without parsing it. The coding of the auxiliary data is not defined
in this document. Rather, the format, meaning, and signaling of
auxiliary data is expected to be specified in one or more future
RFCs. Auxiliary data MUST NOT be transmitted until its format,
meaning, and signaling have been specified and its use has been
signaled. Receivers that have knowledge of the auxiliary data MAY
decode the auxiliary data, but receivers without knowledge of such
data MUST skip the auxiliary data field.
4.5 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.
4.6 Global Structure of Payload Format
The RTP payload following the RTP header contains four octet-aligned
data sections, of which the second and third MAY be empty:
+------+--------------+--------------+--------------+--------------+
|RTP | ATRAC Header | Auxiliary | Redundant | ATRAC Frames |
|Header| Section | Data 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 auxiliary data; this section MAY be configured empty. The third
section is for redundant ATRAC frames; this section MAY also be
empty. The fourth 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.
5. Payload Format
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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
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-bounds.
5.2 RTP Payload Structure
5.2.1 ATRAC Header Section
The ATRAC family payload header is a scant two octets. This should
make processing very simple.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|FrgNo|A|Rsrvd|NFrames| FrOff |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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In the event of data fragmentation, this value is 1 for the first
packet, and increases sequentially for the remaining fragmented data
packets.
Auxiliary Data Flag (A): 1 bit
Indicates the existence of auxiliary data in this packet.
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.
Frame Offset (FrOff): 4 bits
As mentioned earlier, a simple mechanism for redundancy is supported
in this payload format. Frame offsets allow utilization of this
feature. Redundant frames are sent sequentially before any new
frames in the same packet. The timestamp SHOULD reflect the playback
time of the first frame in a packet, even if the first frame is a
redundant frame. In addition, a "maxRedundantFrames" parameter may
be sent "out-of-band" (i.e. SDP) to allow for a buffer size to be
calculated in advance.
5.2.2 Auxiliary Data Section
When the Auxiliary Data Flag (A) is set in the ATRAC Header Section,
it indicates the existence of an Auxiliary Data Section. The
Auxiliary Data Section consists of an auxiliary-bit-length field of
16 bits followed by the actual auxiliary data (auxiliary-data-field).
Receivers MAY parse the auxiliary-data-field; however, to facilitate
skipping of the auxiliary-data-field by receivers, the auxiliary-bit-
length field indicates the length in bits of the auxiliary-data-
field. This means that if the concatenation of the auxiliary-bit-
length and the auxiliary-data-field consumes a non-integer number of
bytes, zero-padding MUST be inserted immediately after the auxiliary
data to achieve octet-alignment. With 16 bits, the auxiliary-data-
field can hold up to 8KB of data.
+------------------------+--------------------------+-----------+
| auxiliary-bit-length | auxiliary-data-field ... | padding |
+------------------------+--------------------------+-----------+
auxiliary-bit-length: 16 bits
specifies length in bits of the auxiliary-data-field which follows
immediately.
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auxiliary-data-field:
contains data of a format not defined by this specification.
5.2.3 Redundant Data Section
As specified earlier, indicating a Frame Offset allows for
transmission of redundant ATRAC frames. As an example of its usage,
refer to Figure 1, which considers a situation when FrOff in the
ATRAC Header is 2. If a packet has 4 frames of audio, with each
frame representing 1024 samples of audio, then we can calculate that
playback begins with 2 frames (2048 samples) of redundant data, and
can allocate buffer space as necessary. (The only other necessary
variable is sampling frequency, which should have been established
during out-of-band negotiations). This field SHOULD NOT be used in
packets containing fragmented data.
|-Fr1-|-Fr2-|-Fr3-|-Fr4-| Packet N, TS=1
|-Fr3-|-Fr4-|-Fr5-|-Fr6-| Packet N+1, TS=3
|-Fr5-|-Fr6-|-Fr7-|-Fr8-| Packet N+2, TS=5
5.2.4 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 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 Block Length and
ATRAC frame pairs. This section is never 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Length | ATRAC frame... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Block length: 16 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. 16 bits allows for a maximum
block length of 65535 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.
5.2.4.1 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
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fragment. In the former case, as many complete ATRAC frames as can
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 SHOULD
NOT be used. This means the Frame Offset field SHOULD be ignored.
6. Packetization Examples
6.1 Example Multi-frame Packet
Multiple encoded audio frames are combined into one packet. For
brevity, the RTP packet header details have been omitted.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| 0 | Rsrvd | 5 | 2 | Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (Redundant) Frame 1 data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Length | (Redundant) Frame 2 data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (cont.) | Block Length | Frame 3 data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (cont.) | Block Length | Frame 4 data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Length | Frame 5 data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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6.2 Example Fragmented ATRAC Frame
The encoded audio data frame is split over three RTP packets. For
brevity, the RTP packet header details have been omitted. 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| 1 | Rsrvd | 0 | 0 | Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATRAC data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Packet 2:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| 2 | Rsrvd | 0 | 0 | Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...more ATRAC data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| 3 | Rsrvd | 0 | 0 | Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...the last of the ATRAC data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7. Payload Format Parameters
Certain parameters will need to be defined before ATRAC family
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 and ATRAC-X 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
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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:
frameLength: Indicates the size in bytes of an encoded audio frame.
In essence, this value determines the bit-rate of the encoded audio.
Permissible values are 192 (66kbps), 304 (105kbps), and 384
(132kbps).
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 SHOULD 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 SHOULD be an integer
multiple of the frame size. This attribute is probably only
meaningful for audio data, but may be used with other media types if
it makes sense. Note that this attribute was introduced after RFC
2327, and non updated implementations will ignore this attribute.
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:
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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: LIMITED USE
Only licensees of ATRAC technology may use this type.
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.
frameLength: Indicates the size in bytes of an encoded audio frame.
In essence, this value determines the bitrate of the encoded audio.
Permissible values lie within 8 ~ 8192.
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
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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 SHOULD 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 SHOULD 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: LIMITED USE
Only licensees of ATRAC technology may use this type.
Author/Change controller:
hatanaka@av.crl.sony.co.jp
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7.3 Channel Mapping Configuration Table
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.4 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:
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7.4.1 For MIME subtype ATRAC3
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. ATRAC3 supports only mono or stereo signals,
so a corresponding number of channels SHALL also be included in
this attribute.
o The "frameLength" 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.
7.4.2 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
"frameLength" 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 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:
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7.5.1 For Both MIME Subtypes
o Each combination of the RTP payload transport format configuration
parameters (frameLength, 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.
7.5.2 For MIME subtype ATRAC3
o In response to an offer, downgraded subsets of "frameLength" are
possible. However for best performance, we suggest the answer
contain the highest possible values offered.
7.5.3 For MIME subtype ATRAC-X
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",
"frameLength", 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
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deemed inoperable.
7.6 Usage of declarative SDP
In declarative usage, like SDP in RTSP [9] or SAP [10], the
parameters SHALL be interpreted as follows:
o The payload format configuration parameters (frameLength,
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.
7.7 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 frameLength=312; 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 frameLength=1156; channelID=5
a=maxptime:30
7.8 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 frameLength=1156; channelID=5
a=rtpmap:99 ATRAC-X/44100/6
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a=fmtp:99 frameLength=386; channelID=5
Answer:
m=audio 49170 RTP/AVP 99
a=rtpmap:99 ATRAC-X/44100/2
a=fmtp:99 frameLength=386; channelID=2
The following Offer/Answer example simply 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 frameLength=386; channelID=2
a=rtpmap:98 ATRAC-X/44100/6
a=fmtp:98 frameLength=386; channelID=5
a=rtpmap:99 ATRAC-X/48000/6
a=fmtp:99 frameLength=1156; channelID=5
Answer:
m=audio 49170 RTP/AVP 97 98
a=rtpmap:97 ATRAC-X/44100/2
a=fmtp:97 frameLength=386; channelID=2
a=rtpmap:98 ATRAC-X/44100/6
a=fmtp:98 frameLength=386; channelID=5
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
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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
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.
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[2] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
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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