Audio/Video Transport M. Romaine Internet-Draft M. Hatanaka Expires: January 6, 2005 J. Matsumoto SONY July 8, 2004 RTP Payload Format for ATRAC Family draft-hatanaka-avt-rtp-atrac-family-02 Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I 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 January 6, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document describes an RTP payload format for efficient and flexible transporting of audio data encoded with the Adaptive TRansform Audio Codec (ATRAC) family of codecs. Recent enhancements to the ATRAC family of codecs support high quality audio coding with multiple channels. The RTP payload format as presented in this document includes support for data fragmentation and elementary redundancy measures. Romaine, et al. Expires January 6, 2005 [Page 1] Internet-Draft RTP Payload Format for ATRAC Family July 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 ATRAC Details . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1 RTP Header . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2 Payload Header . . . . . . . . . . . . . . . . . . . . . . 5 3.3 Payload Data . . . . . . . . . . . . . . . . . . . . . . . 6 4. Frame Packetization . . . . . . . . . . . . . . . . . . . . . 8 4.1 Example Fragmented ATRAC Frame . . . . . . . . . . . . . . 8 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 10 5.1 ATRAC3 MIME Registration . . . . . . . . . . . . . . . . . 10 5.2 ATRAC-X MIME Registraion . . . . . . . . . . . . . . . . . 11 5.3 Channel Mapping Configuration Table . . . . . . . . . . . 13 5.4 Mapping MIME Parameters into SDP . . . . . . . . . . . . . 13 5.4.1 For MIME subtype ATRAC3 . . . . . . . . . . . . . . . 14 5.4.2 For MIME subtype ATRAC-X . . . . . . . . . . . . . . . 14 5.5 Offer-Answer Model Considerations . . . . . . . . . . . . 14 5.5.1 For MIME subtype ATRAC3 . . . . . . . . . . . . . . . 14 5.5.2 For MIME subtype ATRAC-X . . . . . . . . . . . . . . . 14 5.6 Example SDP Session Descriptions . . . . . . . . . . . . . 15 5.7 Example Offer-Answer Exchange . . . . . . . . . . . . . . 15 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 7.1 Confidentiality . . . . . . . . . . . . . . . . . . . . . 17 7.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 17 7.3 Decoding Validation . . . . . . . . . . . . . . . . . . . 17 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 8.1 Normative References . . . . . . . . . . . . . . . . . . . . 18 8.2 Informative References . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18 Intellectual Property and Copyright Statements . . . . . . . . 20 Romaine, et al. Expires January 6, 2005 [Page 2] Internet-Draft RTP Payload Format for ATRAC Family July 2004 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, voice recorders, mobile phones, and CD players. 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 channel-basis to be concerned with. 1.1 ATRAC 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 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. Romaine, et al. Expires January 6, 2005 [Page 3] Internet-Draft RTP Payload Format for ATRAC Family July 2004 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]. Romaine, et al. Expires January 6, 2005 [Page 4] Internet-Draft RTP Payload Format for ATRAC Family July 2004 3. Payload Format 3.1 RTP Header 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 payload type can either be dynamically allocated at the application level, or an RTP profile for a class of applications is expected to assign the payload type for this format. A dynamic allocation SHOULD designate this format as ATRAC-Family. Sequence number: 16 bits This field is as defined in RFC 3550 [1]. Timestamp: 32 bits A timestamp representing the sampling time of the first sample of the first ATRAC frame in the RTP packet. The clock frequency MUST be set to the sample rate of the encoded audio data, and is conveyed out-of-band. 3.2 Payload Header 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| Rsrvd |NFrames| FrOff | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Continuous flag (C): 1 bit Set to one if this is a continuation of a fragmented packet. Romaine, et al. Expires January 6, 2005 [Page 5] Internet-Draft RTP Payload Format for ATRAC Family July 2004 Fragment Number (FrgNo): 3 bits In the event of data fragmentation, this value is 1 for the first packet, and increases sequentially for the remaining fragmented data packets. Number of Frames (NFrames): 4 bits The number of frames in this packet. This allows for a maximum of 16 ATRAC-encoded audio encapsulations per packet, with 0 indicating one frame. Keep in mind only the first frame is allowed to be fragmented. Additionally, this MUST not be anything other than 0 for subsequent packets containing the fragmented frame. Frame Offset (FrOff): 4 bits The purpose of frame offsets is to provide a basic mechanism for the transmission of redundant data. Redundant frames are sent sequentially before any new frames in the same packet. The timestamp also reflects the playback time of the first frame in a packet, even if the first frame is a redundant frame. Frame-size lengths are determined during SDP negotiations (one of either 1024 or 2048 samples), and are fixed for a given session. A "maxRedundantFrames" parameter is also sent during SDP negotations; this allows for the necessary buffer size to be calculated in advance. As an example of using Frame Offsets, refer to Figure 1, which considers a situation when FrOff 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 MUST have been established during SDP negotiations). This field SHOULD NOT be used in packets containing fragmented data. |-Fr1-|-Fr2-|-Fr3-|-Fr4-| Nth Packet, TS=1 |-Fr3-|-Fr4-|-Fr5-|-Fr6-| N+1th Packet, TS=3 |-Fr5-|-Fr6-|-Fr7-|-Fr8-| N+1th Packet, TS=5 3.3 Payload Data ATRAC payload data consists of 2 octets which represent the byte-length of encoded audio data. After that, the actual audio data follows. 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 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Block Length | Rsrvd | ATRAC data... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Romaine, et al. Expires January 6, 2005 [Page 6] Internet-Draft RTP Payload Format for ATRAC Family July 2004 Block length: 12 bits The byte length of encoded audio data until the end of the current packet. This is so that in the case of fragmentation, if only a subsequent packet is received, decoding can still occur. 12 bits allows for a maximum block length of 4096 bytes. In the event a data block is larger than 4096 bytes but would still fit within MTU limits, fragmentation MUST occur. Romaine, et al. Expires January 6, 2005 [Page 7] Internet-Draft RTP Payload Format for ATRAC Family July 2004 4. Frame Packetization Each RTP packet contains either an integer number of ATRAC encoded audio frames, with a maximum of 16, or one ATRAC frame fragment. As many complete ATRAC frames as can fit in a single path-MTU SHOULD be placed in an RTP packet, with the aforementioned maximum of 16. However, if an ATRAC frame will not fit into an RTP packet, it MUST 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 the event of fragmentation, the basic redundancy measures should NOT be used. 4.1 Example Fragmented ATRAC Frame An example of a fragmented ATRAC frame is presented below. The encoded audio data frame is split over three RTP packets. For brevity, the RTP packet header details have been excluded. Romaine, et al. Expires January 6, 2005 [Page 8] Internet-Draft RTP Payload Format for ATRAC Family July 2004 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 | Rsrvd | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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 | Rsrvd | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ...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 | Rsrvd | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ...the last of the ATRAC data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The following points highlight important characteristics of the example above: o the transition from one to zero of the Continuous bit (C) o a sequential increase in the Fragment Number Romaine, et al. Expires January 6, 2005 [Page 9] Internet-Draft RTP Payload Format for ATRAC Family July 2004 5. 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 in RTP. 5.1 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. 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] Romaine, et al. Expires January 6, 2005 [Page 10] Internet-Draft RTP Payload Format for ATRAC Family July 2004 Encoding considerations: This type is defined for transfer via RTP RFC 3550 [1]. Security considerations: Audio data is believed to offer no security risks. 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 5.2 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 Romaine, et al. Expires January 6, 2005 [Page 11] Internet-Draft RTP Payload Format for ATRAC Family July 2004 in Section 4.1 of [3] could be used. Permissible values are 0, 1, 2, 3, 4, 5, 6, 7. Optional parameters: maxRedundantFrames: The maximum number of redundant frames that may be sent during a session in any given packet under the redundant framing mechanism detailed in the draft. If this parameter is not used, a default of "16" 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: Audio data is believed to offer no security risks. 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: Romaine, et al. Expires January 6, 2005 [Page 12] Internet-Draft RTP Payload Format for ATRAC Family July 2004 hatanaka@av.crl.sony.co.jp 5.3 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 5.4 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 January 6, 2005 [Page 13] Internet-Draft RTP Payload Format for ATRAC Family July 2004 5.4.1 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. 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 "16". o The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and "a=maxptime" attributes, respectively. 5.4.2 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 total number of channels. 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 "16" for "maxRedundantFrames". o The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and "a=maxptime" attributes, respectively. 5.5 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: 5.5.1 For MIME subtype ATRAC3 o Downgraded subsets of "frameLength" are possible. However for best performance, we suggest the Answerer respond with the highest possible values offered. 5.5.2 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 offerer also propose a configuration with lower requirements, such as a stereo only option. Although multiple alternative configurations may be offered, care should be taken to not offer too many payload types. o Downgraded subsets of "sampleRate", "frameLength", and "channelID" are possible. However for best performance, we suggest the Answerer respond with the highest possible values offered. Romaine, et al. Expires January 6, 2005 [Page 14] Internet-Draft RTP Payload Format for ATRAC Family July 2004 o The "maxRedundantFrames" is a suggested minimum. The Answerer MAY use a higher value, but MUST NOT use a lower value. o The optional parameters "delayMode" and "encryptionMode" are non-negotiable. Thus, 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 the interoperability. However, the parameter settings can affect application performance. 5.6 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 a=fmtp:99 frameLength=1156; channelID=5 a=maxptime:30 5.7 Example Offer-Answer Exchange An example Offer/Answer model (assuming ATRAC Family's PT is 99). Alice's Offer: m=audio 49170 RTP/AVP 99 a=rtpmap:98 ATRAC-X/44100/6 a=fmtp:99 frameLength=1156; channelID=5 a=rtpmap:99 ATRAC-X/44100/6 a=fmtp:99 frameLength=386; channelID=5 Bob's Answer: m=audio 49170 RTP/AVP 99 a=rtpmap:99 ATRAC-X/44100/2 a=fmtp:99 frameLength=386; channelID=2 Romaine, et al. Expires January 6, 2005 [Page 15] Internet-Draft RTP Payload Format for ATRAC Family July 2004 6. IANA Considerations New MIME subtypes for ATRAC3 and ATRAC-X are currently being registered (see Section 5). Romaine, et al. Expires January 6, 2005 [Page 16] Internet-Draft RTP Payload Format for ATRAC Family July 2004 7. 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]. 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. 7.1 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. 7.2 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. 7.3 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. Romaine, et al. Expires January 6, 2005 [Page 17] Internet-Draft RTP Payload Format for ATRAC Family July 2004 8. References 8.1 Normative References [1] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobsen, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, 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, July 2003. [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels, BCP 14", RFC 2119, March 1997. 8.2 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. Authors' Addresses Matthew Romaine Sony Corporation, Japan 6-7-35 Kitashinagawa Shinagawa-ku Tokyo 141-0001 Japan EMail: Matthew.Romaine@jp.sony.com Romaine, et al. Expires January 6, 2005 [Page 18] Internet-Draft RTP Payload Format for ATRAC Family July 2004 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 January 6, 2005 [Page 19] Internet-Draft RTP Payload Format for ATRAC Family July 2004 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. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Romaine, et al. Expires January 6, 2005 [Page 20]