HomeDocsIETF standardsRFCRFC 5500 — 5599
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Network Working Group
Request for Comments: 5584
Category: Standards Track
M. Hatanaka
J. Matsumoto
Sony Corporation
July 2009

RTP Payload Format for

the Adaptive TRansform Acoustic Coding (ATRAC) Family

Abstract

This document describes an RTP payload format for efficient and flexible transporting of audio data encoded with the Adaptive TRansform Audio Coding (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.

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

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This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

Table of Contents

   1. Introduction ....................................................3
   2. Conventions Used in This Document ...............................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 Redundant Frames ...........................4
      4.5. Scalable Lossless Streaming (High-Speed Transfer Mode) .....5
           4.5.1. Scalable Multiplexed Streaming ......................5
           4.5.2. Scalable Multi-Session Streaming ....................5
   5. Payload Format ..................................................6
      5.1. Global Structure of Payload Format .........................6
      5.2. Usage of RTP Header Fields .................................7
      5.3. RTP Payload Structure ......................................8
           5.3.1. Usage of ATRAC Header Section .......................8
           5.3.2. Usage of ATRAC Frames Section .......................9
   6. Packetization Examples .........................................12
      6.1. Example Multi-Frame Packet ................................12
      6.2. Example Fragmented ATRAC Frame ............................13
   7. Payload Format Parameters ......................................14
      7.1. ATRAC3 Media Type Registration ............................14
      7.2. ATRAC-X Media Type Registration ...........................16
      7.3. ATRAC Advanced Lossless Media Type Registration ...........18
      7.4. Channel Mapping Configuration Table .......................20
      7.5. Mapping Media Type Parameters into SDP ....................21
           7.5.1. For Media Subtype ATRAC3 ...........................21
           7.5.2. For Media Subtype ATRAC-X ..........................21
           7.5.3. For Media Subtype ATRAC Advanced Lossless ..........22
      7.6. Offer/Answer Model Considerations .........................22
           7.6.1. For All Three Media Subtypes .......................22
           7.6.2. For Media Subtype ATRAC3 ...........................23
           7.6.3. For Media Subtype ATRAC-X ..........................23
           7.6.4. For Media Subtype ATRAC Advanced Lossless ..........23
      7.7. Usage of Declarative SDP ..................................24
      7.8. Example SDP Session Descriptions ..........................24
      7.9. Example Offer/Answer Exchange .............................26
   8. IANA Considerations ............................................28
   9. Security Considerations ........................................28
   10. Considerations on Correct Decoding ............................28
      10.1. Verification of the Packets ..............................28
      10.2. Validity Checking of the Packets .........................29
   11. References ....................................................29
      11.1. Normative References .....................................29
      11.2. Informative References ...................................30

1. Introduction

The ATRAC family of perceptual audio codecs is 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.

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. There is also basic support for fragmentation and redundancy.

2. Conventions Used in This Document

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 44.1 kHz sampling frequency, with typical bit-rates between 66 kbps and 132 kbps. The latest version allows for a maximum of 8 channels of audio, up to 96 kHz in sampling frequency, and a lossless encoding option that 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 8 kbps up to 1400 kbps in lossy encoding modes.

Depending on the version of ATRAC used, the sample-frame size is 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.

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 with which to be concerned.

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 octet-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 64 kbps 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 MUST carry either 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" at 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. For further details, see Section 5.3.2.1 and [12].

4.5. 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.

4.5.1. Scalable Multiplexed Streaming

In multiplexed streaming, the base layer and enhancement layer are coupled together in each packet, utilizing only one session as illustrated in Figure 1.

The packet MUST begin with the base layer, and the two layer types MUST interleave if both of the layers exist in a packet (only base or enhancement is included in a packet at the beginning of a streaming, or during the fragmentation). 

   +----------------+  +----------------+  +----------------+
   |Base|Enhancement|--|Base|Enhancement|--|Base|Enhancement| ...
   +----------------+  +----------------+  +----------------+
   
           N                   N+1                 N+2        : Packet

Figure 1. Multiplexed Structure

4.5.2. Scalable Multi-Session Streaming

In multi-session streaming, the base layer and enhancement layer are sent over two separate sessions, allowing clients with certain bandwidth limitations to receive just the base layer for decoding as illustrated in Figure 2.

In this case, it is REQUIRED to determine which sessions are paired together in receiver side. For paired base and enhancement layer sessions, the CNAME bindings in the RTP Control Protocol (RTCP) session MUST be applied using the same CNAME to ensure correct mapping to the RTP source.

While there may be alternative methods for synchronization of the layers, the timestamp SHOULD be used for synchronizing the base layer with its enhancement. The two sessions MUST be synchronized using the information in RTCP SR packets to align the RTP timestamps.

If the enhancement layer's session data cannot arrive until the presentation time, the decoder MUST decode the base layer session's data only, ignoring the enhancement layer's data. 

         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

Figure 2. Multi-Session Streaming

5. Payload Format

5.1. Global Structure of Payload Format

The structure of ATRAC Payload is illustrated in Figure 3. The RTP payload following the RTP header contains two octet-aligned data sections. 

            +------+--------------+-----------------------------+
            |RTP   | ATRAC Header |   ATRAC Frames Section      |
            |Header| Section      | (including redundant data)  |
            +------+--------------+-----------------------------+
            < ---------------- RTP Packet Payload ------------- >

Figure 3. Structure of RTP Payload of ATRAC Family

The first data section is the ATRAC Header, containing just one header with information for the whole packet. The second 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. When using the redundancy mechanism described in Section 5.3.2.1, the redundant frame data can be included in this section and timestamp MUST be set to the oldest redundant frame's timestamp.

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.

5.2. 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              |
   |                             .....                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4. RTP Standard Header Part

The structure of the RTP Standard Header Part is illustrated in Figure 4.

Version(V): 2 bits
Set to 2.

Padding(P): 1 bit
If the padding bit is set, the packet contains one or more additional padding octets at the end, which are not part of the payload. The last octet of the padding contains a count of how many padding octets should be ignored, including itself. Padding may be needed by some encryption algorithms with fixed block sizes or for carrying several RTP packets in a lower-layer protocol data unit (see [1]).

Extension(X): 1 bit
Defined by the RTP profile used.

CSRC count(CC): 4 bits
See RFC 3550 [1].

Marker (M): 1 bit
Set to 1 if the packet is the first packet after a silence period; otherwise, it MUST be set to 0.

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 the Session Description Protocol (SDP)).

sequence number: 16 bits
A sequential number for the RTP packet. It ranges from 0 to 65535 and repeats itself periodically.

Timestamp: 32 bits
A timestamp representing the sampling time of the first sample of the first ATRAC frame in the current RTP packet.
When using SDP, the clock rate of the RTP timestamp MUST be expressed using the "rtpmap" attribute. For ATRAC3 and ATRAC Advanced Lossless, the RTP timestamp rate MUST be 44100 Hz. For ATRAC-X, the RTP timestamp rate is 44100 Hz or 48000 Hz, and it will be selected by out-of-band signaling.

SSRC: 32 bits
See RFC 3550 [1].

CSRC list: 0 to 15 items, 32 bits each
See RFC 3550 [1].

5.3. RTP Payload Structure

5.3.1. Usage of ATRAC Header Section

The ATRAC header section has the fixed length of one byte as illustrated in Figure 5. 

                     0 1 2 3 4 5 6 7
                    +-+-+-+-+-+-+-+-+
                    |C|FrgNo|NFrames|
                    +-+-+-+-+-+-+-+-+

Figure 5. ATRAC RTP Header

Continuation Flag (C) : 1 bit
The packet that corresponds to the last part of the audio frame data in a fragmentation MUST have this bit set to 0; otherwise, it's set to 1.

Fragment Number (FrgNo): 3 bits
In the event of data fragmentation, this value is one for the first packet, and increases sequentially for the remaining fragmented data packets. This value MUST be zero for an unfragmented frame. (Note: 3 bits is sufficient to avoid Fragment Number rollover given the current maximum supported bit-rate in the ATRAC specification. If that changes, the choice of 3 bits for the Fragment Number should be revisited.)

Number of Frames (NFrames): 4 bits
The number of audio frames in this packet are field value + 1. This allows for a maximum of 16 ATRAC-encoded audio frames per packet, with 0 indicating one audio frame. Each audio frame MUST be complete in the packet if fragmentation is not applied. In the case of fragmentation, the data for only one audio frame is allowed to be fragmented, and this value MUST be 0.

5.3.2. Usage of ATRAC Frames Section

The ATRAC Frames Section contains an integer number of complete ATRAC frames or a single fragment of one ATRAC frame, as illustrated in Figure 6. Each ATRAC frame is preceded 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 in order of their frame number. 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           |...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6. ATRAC Frame Section Format

Layer Type Flag (E): 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.

ATRAC frame: The encoded ATRAC audio data.

5.3.2.1. Support of Redundancy

This payload format provides a rudimentary scheme to compensate for occasional packet loss. As every packet's timestamp corresponds to the first audio frame regardless of whether or not it is redundant, 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 warns it to expand its buffer size if necessary. As an example of using the Redundant Data, refer to Figures 7 and 8.

In this example, the server has determined that for the next few 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 (of which the client may choose to dispose). 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. 

   [Sender]
   
   |-Fr0-|-Fr1-|-Fr2-|                         Packet: N,   TS=0
         |-Fr1-|-Fr2-|-Fr3-|                   Packet: N+1, TS=1024
               |-Fr2-|-Fr3-|-Fr4-|             Packet: N+2, TS=2048
                     |-Fr3-|-Fr4-|-Fr5-|       Packet: N+3, TS=3072
                           |-Fr4-|-Fr5-|-Fr6-| Packet: N+4, TS=4096
   
   -----------> Packet "N+2" and "N+3" not arrived  ------------->
   
   [Receiver]
   
   |-Fr0-|-Fr1-|-Fr2-|                         Packet: N,   TS=0
         |-Fr1-|-Fr2-|-Fr3-|                   Packet: N+1, TS=1024
                           |-Fr4-|-Fr5-|-Fr6-| Packet: N+4, TS=4096

The receiver can decode from FR4 to Fr6 by using Packet "N+4" data even if the packet loss of "N+2" and "N+3" has occurred.

Figure 7. Redundant Example 

    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 (= start sample time of Fr1)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            synchronization source (SSRC) identifier           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           contributing source (CSRC) identifiers              |
   |                             .....                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|  0  |   3   |0|         Block Length        |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         (redundant)  ATRAC frame (Fr1) data  ...              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|       Block Length          |(redundant) ATRAC frame (Fr2)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    (cont.)  |0|   Block Length          |  ATRAC frame (Fr3)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       (cont.)                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 8. Packet Structure Example with Redundant Data

(Case of Packet "N+1")

5.3.2.2. Frame Fragmentation

Each RTP packet MUST 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 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 MUST be fragmented.

The start of a fragmented frame gets placed in its own RTP packet with its Continuation 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 Continuation bit (C) consistently set to one. As subsequent packets do not contain any new frames, the Number of Frames field MUST be ignored. The last packet of fragmented data MUST have the Continuation bit (C) set to zero.

Packets containing related fragmented frames MUST have identical timestamps. Thus, while the Continuous bit and Fragment Number fields indicate fragmentation and a means to reorder the packets, the timestamp can be used to determine which packets go together.

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 as illustrated in Figure 9. 

    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          | enhancement frame 3 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | (cont.) |0|       Block Length          |  base layer frame 4 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | (cont.) |1|       Block Length          | enhancement frame 4 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9. Example Multi-Frame Packet

6.2. Example Fragmented ATRAC Frame

The encoded audio data frame is split over three RTP packets as illustrated in Figure 10. The following points are highlighted in the example below:

  • transition from one to zero of the Continuation bit (C)
  • 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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           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                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 10. Example Fragmented ATRAC Frame

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, ATRAC-X, and ATRAC Advanced Lossless are defined here as part of the media subtype registration process. A mapping of these parameters into the Session Description Protocol (SDP) (RFC 4566) [2] is also provided for applications that utilize SDP. These registrations use the template defined in RFC 4288 [5] and follow RFC 4855 [6].

The data format and parameters are specified for real-time transport in RTP.

7.1. ATRAC3 Media Type Registration

The media subtype for the Adaptive TRansform Codec version 3 (ATRAC3) uses the template defined in RFC 4855 [6].

Note, any unknown parameter MUST be ignored by the receiver. 

   Type name:  audio
   
   Subtype name:  ATRAC3

Required parameters: 

   rate:  Represents the sampling frequency in Hz of the original audio
   data.  Permissible value is 44100 only.
   
   baseLayer:  Indicates the encoded bit-rate in kbps for the audio data
   to be streamed.  Permissible values are 66, 105, and 132.

Optional parameters: 

   ptime:  See RFC 4566 [2].
   
   maxptime:  See RFC 4566 [2].
   The frame length of ATRAC3 is 1024/44100 = 23.22...(ms), and
   fractional value may not be applicable for the SDP definition.

So the value of the parameter MUST be a multiple of 24 (ms) considering safe transmission.

If this parameter is not present, the sender MAY encapsulate a maximum of 6 encoded frames into one RTP packet, in streaming of ATRAC3. 

   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 document.  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.
   
   Encoding considerations:  This media type is framed and contains
   binary data.
   
   Security considerations:  This media type does not carry active
   content.  See Section 9 of this document.
   
   Interoperability considerations:  none
   
   Published specification:  ATRAC3 Standard Specification [9]

Applications that use this media type:
Audio and video streaming and conferencing tools. 

   Additional information:  none
   Magic number(s):  none
   File extension(s):  'at3', 'aa3', and 'omg'
   Macintosh file type code(s):  none

Person and email address to contact for further information: Mitsuyuki Hatanaka
Jun Matsumoto
actech@jp.sony.com 

   Intended usage:  COMMON
   
   Restrictions on usage:  This media type depends on RTP framing, and
   hence is only defined for transfer via RTP.
   
   Author:
   Mitsuyuki Hatanaka
   Jun Matsumoto
   actech@jp.sony.com
   
   Change controller:  IETF AVT WG delegated from the IESG

7.2. ATRAC-X Media Type Registration

The media subtype for the Adaptive TRansform Codec version X (ATRAC-X) uses the template defined in RFC 4855 [6].

Note, any unknown parameter MUST be ignored by the receiver. 

   Type name:  audio
   
   Subtype name:  ATRAC-X

Required parameters: 

   rate:  Represents the sampling frequency in Hz of the original
   audio data.  Permissible values are 44100 and 48000.
   
   baseLayer:  Indicates the encoded bit-rate in kbps for the audio data
   to be streamed.  Permissible values are 32, 48, 64, 96, 128, 160,
   192, 256, 320, and 352.
   
   channelID:  Indicates the number of channels and channel layout
   according to the table1 in Section 7.4.  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: 

   ptime:  See RFC 4566 [2].
   maxptime:  See RFC 4566 [2].
   The frame length of ATRAC-X is 2048/44100 = 46.44...(ms) or
   2048/48000 = 42.67...(ms), but fractional value may not be applicable
   for the SDP definition.  So the value of the parameter MUST be a
   multiple of 47 (ms) or 43 (ms) considering safe transmission.

If this parameter is not present, the sender MAY encapsulate a maximum of 16 encoded frames into one RTP packet, in streaming of ATRAC-X. 

   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 document.  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.
   
   Encoding considerations:  This media type is framed and contains
   binary data.
   
   Security considerations:  This media type does not carry active
   content.  See Section 9 of this document.
   
   Interoperability considerations:  none
   
   Published specification:  ATRAC-X Standard Specification [10]

Applications that use this media type:
Audio and video streaming and conferencing tools. 

   Additional information:  none
   
   Magic number(s):  none
   File extension(s):  'atx', 'aa3', and 'omg'
   Macintosh file type code(s):  none

Person and email address to contact for further information: Mitsuyuki Hatanaka
Jun Matsumoto
actech@jp.sony.com 

   Intended usage:  COMMON
   
   Restrictions on usage:  This media type depends on RTP framing, and
   hence is only defined for transfer via RTP.
   Author:
   Mitsuyuki Hatanaka
   Jun Matsumoto
   actech@jp.sony.com
   
   Change controller:  IETF AVT WG delegated from the IESG

7.3. ATRAC Advanced Lossless Media Type Registration

The media subtype for the Adaptive TRansform Codec Lossless version (ATRAC Advanced Lossless) uses the template defined in RFC 4855 [6].

Note, any unknown parameter MUST be ignored by the receiver. 

   Type name:  audio
   
   Subtype name:  ATRAC-ADVANCED-LOSSLESS

Required parameters: 

   rate:  Represents the sampling frequency in Hz of the original
   audio data.  Permissible value is 44100 only for High-Speed Transfer
   mode.  Any value of 24000, 32000, 44100, 48000, 64000, 88200, 96000,
   176400, and 192000 can be used for Standard mode.
   
   baseLayer:  Indicates the encoded bit-rate in kbps for the base layer
   in High-Speed Transfer mode lossless encodings.

For Standard lossless mode, this value MUST be 0. 

   The Permissible values for ATRAC3 baselayer are 66, 105, and 132.
   For ATRAC-X baselayer, they are 32, 48, 64, 96, 128, 160, 192, 256,
   320, and 352.
   
   blockLength:  Indicates the block length.  In High-Speed Transfer
   mode, the value of 1024 and 2048 is used for ATRAC3 based and ATRAC-X
   based ATRAC Advanced Lossless streaming, respectively.

Any value of 512, 1024, and 2048 can be used for Standard mode. 

   channelID:  Indicates the number of channels and channel layout
   according to the table1 in Section 7.4.  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 in this case.  Permissible values
   are 0, 1, 2, 3, 4, 5, 6, 7.
   
   ptime:  See RFC 4566 [2].
   maxptime:  See RFC 4566 [2].
   In streaming of ATRAC Advanced Lossless, multiple frames cannot be
   transmitted in a single RTP packet, as the frame size is large.  So
   it SHOULD be regarded as the time of one encoded frame in both of the
   sender and the receiver side.  The frame length of ATRAC Advanced
   Lossless is 512/44100 = 11.6...(ms), 1024/44100 = 23.22...(ms), or
   2048/44100 = 46.44...(ms), but fractional value may not be applicable
   for the SDP definition.  So the value of the parameter MUST be
   12(ms), 24(ms), or 47(ms) considering safe transmission.
   
   Encoding considerations:  This media type is framed and contains
   binary data.
   
   Security considerations:  This media type does not carry active
   content.  See Section 9 of this document.
   
   Interoperability considerations:  none

Published specification:
ATRAC Advanced Lossless Standard Specification [11]

Applications that use this media type:
Audio and video streaming and conferencing tools. 

   Additional information:  none
   
   Magic number(s):  none
   File extension(s):  'aal', 'aa3', and 'omg'
   Macintosh file type code(s):  none

Person and email address to contact for further information: 

   Mitsuyuki Hatanaka
   Jun Matsumoto
   actech@jp.sony.com
   
   Intended usage:  COMMON
   
   Restrictions on usage:  This media type depends on RTP framing, and
   hence is only defined for transfer via RTP.
   
   Author:
   Mitsuyuki Hatanaka
   Jun Matsumoto
   actech@jp.sony.com
   
   Change controller:  IETF AVT WG delegated from the IESG

7.4. Channel Mapping Configuration Table

Table 1 explains the mapping between the channelID as passed during SDP negotiations, and the speaker mapping the value represents. 

            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            | 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                 |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Table 1. Channel Configuration

7.5. Mapping Media Type Parameters into SDP

The information carried in the 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 Media Subtype ATRAC3

  • The Media type ("audio") goes in SDP "m=" as the media name.
  • The Media 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 (0 or 1) MUST also be specified in this attribute.
  • 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".
  • The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and "a=maxptime" attributes, respectively.

7.5.2. For Media Subtype ATRAC-X

  • The Media type ("audio") goes in SDP "m=" as the media name.
  • The Media 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.
  • The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and "a=maxptime" attributes, respectively.
  • Any remaining parameters go in the SDP "a=fmtp" attribute by copying them directly from the Media type string as a semicolon- separated list of parameter=value pairs. The "baseLayer" parameter MUST be the first entry on this line. The "channelID" parameter MUST be the next entry. The receiver MUST assume a default value of "15" for "maxRedundantFrames".

7.5.3. For Media Subtype ATRAC Advanced Lossless

  • The Media type ("audio") goes in SDP "m=" as the media name.
  • The Media subtype (payload format name) goes in SDP "a=rtpmap" as the encoding name. This MUST be followed by the "sampleRate" (as the RTP clock rate), and then the actual number of channels regardless of the channelID parameter.
  • The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and "a=maxptime" attributes, respectively.
  • Any remaining parameters go in the SDP "a=fmtp" attribute by copying them directly from the Media type string as a semicolon- separated list of parameter=value pairs.

On this line, the parameters "baseLayer" and "blockLength" MUST be present in this order.

The value of "blockLength" MUST be one of 1024 and 2048, for using ATRAC3 and ATRAC-X as baselayer, respectively. If "baseLayer=0" (means standard mode), "blockLength" MUST be one of either 512, 1024, or 2048. The "channelID" parameter MUST 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 of the sender and receiver complying with certain specifications. In order to establish an interoperable transmission framework, an Offer/Answer negotiation in SDP MUST observe the following considerations. (See [14].)

7.6.1. For All Three Media Subtypes

  • 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 44100 kHz, more than two channels), the offerer SHOULD 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.
  • 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 RFC 3264. The "maxptime" parameter MUST be handled in the same way.

7.6.2. For Media Subtype ATRAC3

  • 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 Media Subtype ATRAC-X

  • In response to an offer, downgraded subsets of "sampleRate", "baseLayer", and "channelID" are possible. For best performance, an answer MUST NOT contain any values requiring further capabilities than the offer contains, but it SHOULD provide values as close as possible to those in the offer.
  • The "maxRedundantFrames" is a suggested minimum. This value MAY be increased in an answer (with a maximum of 15), but MUST NOT be reduced.
  • 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 Media Subtype ATRAC Advanced Lossless

  • In response to an offer, downgraded subsets of "sampleRate", "baseLayer", and "channelID" are possible. For best performance, an answer MUST NOT contain any values requiring further capabilities than the offer contains, but it SHOULD provide values as close as possible to those in the offer.
  • There are no requirements when negotiating "blockLength", other than that both parties must be in agreement.
  • The "maxRedundantFrames" is a suggested minimum. This value MAY be increased in an answer (with a maximum of 15), but MUST NOT be reduced.
  • For transmission of scalable multi-session streaming of ATRAC Advanced Lossless content, the attributes of media stream identification, group information, and decoding dependency between base layer stream and enhancement layer stream MUST be signaled in SDP by the Offer/Answer model. In this case, the attribute of

"group", "mid", and "depend" followed by the appropriate parameter MUST be used in SDP [7] [8] in order to indicate layered coding dependency. The attribute of "group" followed by "DDP" parameter is used for indicating the relationship between the base and the enhancement layer stream with decoding dependency. Each stream is identified by "mid" attribute, and the dependency of enhancement layer stream is defined by the "depend" attribute, as the enhancement layer is only useful when the base layer is available. Examples for signaling ATRAC Advanced Lossless decoding dependency are described in Sections 7.8 and 7.9.

7.7. Usage of Declarative SDP

In declarative usage, like SDP in Real-Time Streaming Protocol (RTSP) [15] or Session Announcement Protocol (SAP) [16], the parameters MUST be interpreted as follows:

  • The payload format configuration parameters (baseLayer, sampleRate, channelID) are all declarative and a participant MUST use the configuration(s) 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.
  • Any "maxptime" and "ptime" values SHOULD be selected with care to ensure that the session's participants can achieve reasonable performance.
  • The attribute of "mid", "group", and "depend" MUST be used for indicating the relationship and dependency of the base layer and the enhancement layer in scalable multi-session streaming of ATRAC ADVANCED LOSSLESS content, as described in Sections 7.6, 7.8, and 7.9.

7.8. Example SDP Session Descriptions

Example usage of ATRAC-X with stereo at 44100 Hz: 

   v=0
   o=atrac 2465317890 2465317890 IN IP4 service.example.com
   s=ATRAC-X Streaming
   c=IN IP4 192.0.2.1/127
   t=3409539540 3409543140
   m=audio 49120 RTP/AVP 99
   a=rtpmap:99 ATRAC-X/44100/2
   a=fmtp:99 baseLayer=128; channelID=2; delayMode=2
   a=maxptime:47

Example usage of ATRAC-X with 5.1 setup at 48000 Hz: 

   v=0
   o=atrac 2465317890 2465317890 IN IP4 service.example.com
   s=ATRAC-X 5.1ch Streaming
   c=IN IP4 192.0.2.1/127
   t=3409539540 3409543140
   m=audio 49120 RTP/AVP 99
   a=rtpmap:99 ATRAC-X/48000/6
   a=fmtp:99 baseLayer=320; channelID=5
   a=maxptime:43

Example usage of ATRAC-Advanced-Lossless in multiplexed High-Speed Transfer mode: 

   v=0
   o=atrac 2465317890 2465317890 IN IP4 service.example.com
   s=AAL Multiplexed Streaming
   c=IN IP4 192.0.2.1/127
   t=3409539540 3409543140
   m=audio 49200 RTP/AVP 96
   a=rtpmap:96 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:96 baseLayer=128; blockLength=2048; channelID=2
   a=maxptime:47

Example usage of ATRAC-Advanced-Lossless in multi-session High-Speed Transfer mode. In this case, the base layer and the enhancement layer stream are identified by L1 and L2, respectively, and L2 depends on L1 in decoding. 

   v=0
   o=atrac 2465317890 2465317890 IN IP4 service.example.com
   s=AAL Multi Session Streaming
   c=IN IP4 192.0.2.1/127
   t=3409539540 3409543140
   a=group:DDP L1 L2
   m=audio 49200 RTP/AVP 96
   a=rtpmap:96 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:96 baseLayer=128; blockLength=2048; channelID=2
   a=maxptime:47
   a=mid:L1
   m=audio 49202 RTP/AVP 97
   a=rtpmap:97 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:97 baseLayer=0; blockLength=2048; channelID=2
   a=maxptime:47
   a=mid:L2
   a=depend:97 lay L1:96

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:24

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/44100/2
   a=fmtp:99 baseLayer=160; channelID=2

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 options: multi-session High-Speed Transfer mode, multiplexed High- Speed Transfer mode, and Standard mode.

Offer: 

// Multi-session High-Speed Transfer mode, L1 and L2 correspond
   to the base layer and the enhancement layer, respectively, and L2
   depends on L1 in decoding.

   a=group:DDP L1 L2
   m=audio 49200 RTP/AVP 96
   a=rtpmap:96 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:96 baseLayer=132; blockLength=1024; channelID=2
   a=maxptime:24
   a=mid:L1

   m=audio 49202 RTP/AVP 97
   a=rtpmap:97 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:97 baseLayer=0; blockLength=2048; channelID=2
   a=maxptime:24
   a=mid:L2
   a=depend:97 lay L1:96

// Multiplexed High-Speed Transfer mode
   m=audio 49200 RTP/AVP 98
   a=rtpmap:98 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:98 baseLayer=256; blockLength=2048; channelID=2
   a=maxptime:47

// Standard mode 

   m=audio 49200 RTP/AVP 99
   a=rtpmap:99 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:99 baseLayer=0; blockLength=2048; channelID=2
   a=maxptime:47

Answer: 

   a=group:DDP L1 L2
   m=audio 49200 RTP/AVP 94
   a=rtpmap:94 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:94 baseLayer=132; blockLength=1024; channelID=2
   a=maxptime:24
   a=mid:L1
   m=audio 49202 RTP/AVP 95
   a=rtpmap:95 ATRAC-ADVANCED-LOSSLESS/44100/2
   a=fmtp:95 baseLayer=0; blockLength=2048; channelID=2
   a=maxptime:24
   a=mid:L2
   a=depend:95 lay L1:94

Note that the names of payload format (encoding) and Media subtypes are case-insensitive in both places. Similarly, parameter names are case-insensitive both in Media types and in the default mapping to the SDP a=fmtp attribute.

8. IANA Considerations

   Three new Media subtypes, audio/ATRAC3, audio/ATRAC-X, and
   audio/ATRAC-ADVANCED-LOSSLESS, have been registered (see Section 7).

9. Security Considerations

The payload format as described in this document is subject to the security considerations defined in RFC 3550 [1] and any applicable profile, for example, RFC 3551 [3]. Also, the security of Media type registration MUST be taken into account as described in Section 5 of RFC 4855 [6].

The payload for ATRAC family consists solely of compressed audio data to be decoded and presented as sound, and the standard specifications of ATRAC3, ATRAC-X, and ATRAC Advanced Lossless [9] [10] [11] strictly define the bit stream syntax and the buffer model in decoder side for each codec. So they can not carry "active content" that could impose malicious side effects upon the receiver, and they do not cause any problem of illegal resource consumption in receiver side, as far as the bit streams are conforming to their standard specifications.

This payload format does not implement any security mechanisms of its own. Confidentiality, integrity protection, and authentication have to be provided by a mechanism external to this payload format, e.g., SRTP RFC 3711 [13].

10. Considerations on Correct Decoding

10.1. Verification of the Packets

Verification of the received encoded audio packets MUST be performed so as to ensure correct decoding of the packets. As a most primitive implementation, the comparison of the packet size and payload length can be taken into account. If the UDP packet length is longer than the RTP packet length, the packet can be accepted, but the extra bytes MUST be ignored. In case of receiving a shorter UDP packet or improperly encoded packets, the packets MUST be discarded.

10.2. Validity Checking of the Packets

Also, validity checking of the received audio packets MUST be performed. It can be carried out by the decoding process, as the ATRAC format is designed so that the validity of data frames can be determined by decoding the algorithm. The required decoder response to a malformed frame is to discard the malformed data and conceal the errors in the audio output until a valid frame is detected and decoded. This is expected to prevent crashes and other abnormal decoder behavior in response to errors or attacks.

11. References

11.1. Normative References

   [1]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.
   
   [2]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.
   
   [3]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
         Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
   
   [4]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.
   
   [5]   Freed, N. and J. Klensin, "Media Type Specifications and
         Registration Procedures", BCP 13, RFC 4288, December 2005.
   
   [6]   Casner, S., "Media Type Registration of RTP Payload Formats",
         RFC 4855, February 2007.
   
   [7]   Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
         "Grouping of Media Lines in the Session Description Protocol
         (SDP)", RFC 3388, December 2002.
   
   [8]   Schierl, T., and S. Wenger, "Signaling Media Decoding
         Dependency in the Session Description Protocol (SDP)", RFC
         5583, July 2009.
   
   [9]   ATRAC3 Standard Specification ver.1.1, Sony Corporation, 2003.
   [10]  ATRAC-X Standard Specification ver.1.2, Sony Corporation, 2004.
   
   [11]  ATRAC Advanced Lossless Standard Specification ver.1.1, Sony
         Corporation, 2007.

11.2. Informative References

   [12]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley,
         M., Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP
         Payload for Redundant Audio Data", RFC 2198, September 1997.
   
   [13]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
         3711, March 2004.
   
   [14]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.
   
   [15]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", RFC 2326, April 1998.
   
   [16]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
         Protocol", RFC 2974, October 2000.

Authors' Addresses

   Mitsuyuki Hatanaka
   Sony Corporation, Japan
   1-7-1 Konan
   Minato-ku
   Tokyo  108-0075
   Japan
   
   EMail: actech@jp.sony.com
   
   Jun Matsumoto
   Sony Corporation, Japan
   1-7-1 Konan
   Minato-ku
   Tokyo  108-0075
   Japan
   
   EMail: actech@jp.sony.com