Audio/Video Transport M. Romaine Internet-Draft M. Hatanaka Expires: August 18, 2005 J. Matsumoto SONY February 14, 2005 RTP Payload Format for ATRAC Family draft-ietf-avt-rtp-atrac-family-02 Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 18, 2005. 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 Romaine, et al. Expires August 18, 2005 [Page 1] Internet-Draft RTP Payload Format for ATRAC Family February 2005 document also includes support for data fragmentation, auxiliary data, and elementary redundancy measures. 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 MIME subtype ATRAC3 . . . . . . . . . . . . . . . 16 7.5.2 For MIME subtype ATRAC-X . . . . . . . . . . . . . . . 16 7.6 Example SDP Session Descriptions . . . . . . . . . . . . . 16 7.7 Example Offer-Answer Exchange . . . . . . . . . . . . . . 17 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 9. Security Considerations . . . . . . . . . . . . . . . . . . . 18 9.1 Confidentiality . . . . . . . . . . . . . . . . . . . . . 18 9.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 18 9.3 Decoding Validation . . . . . . . . . . . . . . . . . . . 19 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 10.1 Normative References . . . . . . . . . . . . . . . . . . . 19 10.2 Informative References . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 20 Intellectual Property and Copyright Statements . . . . . . . . 21 Romaine, et al. Expires August 18, 2005 [Page 2] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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 Romaine, et al. Expires August 18, 2005 [Page 3] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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 Romaine, et al. Expires August 18, 2005 [Page 4] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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 Romaine, et al. Expires August 18, 2005 [Page 5] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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 Romaine, et al. Expires August 18, 2005 [Page 6] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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. Romaine, et al. Expires August 18, 2005 [Page 7] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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 Romaine, et al. Expires August 18, 2005 [Page 8] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Romaine, et al. Expires August 18, 2005 [Page 9] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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 Romaine, et al. Expires August 18, 2005 [Page 10] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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: Romaine, et al. Expires August 18, 2005 [Page 11] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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 Romaine, et al. Expires August 18, 2005 [Page 12] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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. encryptionMode: Indicates whether the audio frames have been encrypted using OpenMG ("OpenMG") or a third party method ("Other"). If "Other", the specific mode MUST be determined at the application level. Permissible values are "OpenMG" and "Other". 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 Romaine, et al. Expires August 18, 2005 [Page 13] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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: Romaine, et al. Expires August 18, 2005 [Page 14] Internet-Draft RTP Payload Format for ATRAC Family February 2005 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: Romaine, et al. Expires August 18, 2005 [Page 15] Internet-Draft RTP Payload Format for ATRAC Family February 2005 7.5.1 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. 7.5.2 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 parameters "delayMode" and "encryptionMode" are non-negotiable. If the Answerer cannot comply with the offered value, the session must be deemed inoperable. o The parameters "maxptime" and "ptime" should not, in most cases, affect interoperability. However, the parameter settings can affect application performance. 7.6 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 Romaine, et al. Expires August 18, 2005 [Page 16] Internet-Draft RTP Payload Format for ATRAC Family February 2005 a=fmtp:99 frameLength=1156; channelID=5 a=maxptime:30 7.7 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 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 Romaine, et al. Expires August 18, 2005 [Page 17] Internet-Draft RTP Payload Format for ATRAC Family February 2005 SDP a=fmtp attribute. 8. IANA Considerations Two new MIME subtypes, for ATRAC3 and ATRAC-X, are requested to be registered (see Section 5). 9. Security Considerations Certain security precautions may be desired to protect copyrighted material. The payload format as described in this document is subject to the security considerations defined in RFC3550 [1] and any applicable profile, for example RFC 3551 [3]. This payload format however does not implement any security mechanisms of its own. External means, such as SRTP [5], MAY be used since the audio compression scheme follows an end-to-end model. Since the data transported is audio that is already encoded, the main security issues are confidentiality, integrity, and authentication of 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. Romaine, et al. Expires August 18, 2005 [Page 18] Internet-Draft RTP Payload Format for ATRAC Family February 2005 9.3 Decoding Validation Verification of the received encoded audio packets should be performed so as to ensure a minimal level of audio quality. As a most primitive implementation, if the receiver calculates a packet size differing from the payload length based on data in the payload header fields, the receiver SHOULD discard the packet. 10. References 10.1 Normative References [1] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobsen, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, STD 64, July 2003. [2] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences 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. Romaine, et al. Expires August 18, 2005 [Page 19] Internet-Draft RTP Payload Format for ATRAC Family February 2005 Authors' Addresses Matthew Romaine Sony Corporation, Japan 6-7-35 Kitashinagawa Shinagawa-ku Tokyo 141-0001 Japan Email: Matthew.Romaine@jp.sony.com 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 Romaine, et al. Expires August 18, 2005 [Page 20] Internet-Draft RTP Payload Format for ATRAC Family February 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Romaine, et al. Expires August 18, 2005 [Page 21]