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rfc:rfc7310

Internet Engineering Task Force (IETF) J. Lindsay Request for Comments: 7310 H. Foerster Category: Standards Track APT Ltd ISSN: 2070-1721 July 2014

  RTP Payload Format for Standard apt-X and Enhanced apt-X Codecs

Abstract

 This document specifies a scheme for packetizing Standard apt-X or
 Enhanced apt-X encoded audio data into Real-time Transport Protocol
 (RTP) packets.  The document describes a payload format that permits
 transmission of multiple related audio channels in a single RTP
 payload and a means of establishing Standard apt-X and Enhanced apt-X
 connections through the Session Description Protocol (SDP).

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7310.

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Lindsay & Foerster Standards Track [Page 1] RFC 7310 apt-X RTP Format July 2014

Table of Contents

 1. Introduction ....................................................2
 2. Conventions .....................................................3
 3. Standard apt-X and Enhanced apt-X Codecs ........................3
 4. Payload Format Capabilities .....................................5
    4.1. Use of Forward Error Correction (FEC) ......................5
 5. Payload Format ..................................................5
    5.1. RTP Header Usage ...........................................5
    5.2. Payload Structure ..........................................6
    5.3. Default Packetization Interval .............................7
    5.4. Implementation Considerations ..............................8
    5.5. Payload Example ............................................8
 6. Payload Format Parameters ......................................10
    6.1. Media Type Definition .....................................10
    6.2. Mapping to SDP ............................................12
         6.2.1. SDP Usage Examples .................................13
         6.2.2. Offer/Answer Considerations ........................14
 7. IANA Considerations ............................................14
 8. Security Considerations ........................................14
 9. Acknowledgements ...............................................14
 10. References ....................................................15
    10.1. Normative References .....................................15
    10.2. Informative References ...................................15

1. Introduction

 This document specifies the payload format for packetization of audio
 data encoded with the Standard apt-X or Enhanced apt-X audio coding
 algorithms into the Real-time Transport Protocol (RTP) [RFC3550].
 The document outlines some conventions, a brief description of the
 operating principles of the audio codecs, and the payload format
 capabilities.  The RTP payload format is detailed, and a relevant
 example of the format is provided.  The media type, its mappings to
 SDP [RFC4566], and its usage in the SDP offer/answer model are also
 specified.  Finally, some security considerations are outlined.
 This document registers a media type (audio/aptx) for the RTP payload
 format for the Standard apt-X and Enhanced apt-X audio codecs.

Lindsay & Foerster Standards Track [Page 2] RFC 7310 apt-X RTP Format July 2014

2. Conventions

 This document uses the normal IETF bit-order representation.  Bit
 fields in figures are read left to right and then down.  The leftmost
 bit in each field is the most significant.  The numbering starts from
 0 and ascends, where bit 0 will be the most significant.
 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 [RFC2119].

3. Standard apt-X and Enhanced apt-X Codecs

 Standard apt-X and Enhanced apt-X are proprietary audio coding
 algorithms, which can be licensed from CSR plc. and are widely
 deployed in a variety of audio processing equipment.  For commercial
 reasons, the detailed internal operations of these algorithms are not
 described in standards or reference documents.  However, the data
 interfaces to implementations of these algorithms are very simple and
 allow easy RTP packetization of data coded with the algorithms
 without detailed knowledge of the actual coded audio stream syntax.
 Both the Standard apt-X and Enhanced apt-X coding algorithms are
 based on Adaptive Differential Pulse Code Modulation principles.
 They produce a constant coded bit rate that is scaled according to
 the sample frequency of the uncoded audio.  This constant rate is 1/4
 of the bit rate of the uncoded audio, irrespective of the resolution
 (number of bits) used to represent an uncoded audio sample.  For
 example, a 1.536-Mbit/s stereo audio stream composed of two channels
 of 16-bit Pulse Code Modulated (PCM) audio that is sampled at a
 frequency of 48 kHz is encoded at 384 kbit/s.
 Standard apt-X and Enhanced apt-X do not enforce a coded frame
 structure, and the coded data forms a continuous coded sample stream
 with each coded sample capable of regenerating four PCM samples when
 decoded.  The Standard apt-X algorithm encodes four successive 16-bit
 PCM samples from each audio channel into a single 16-bit coded sample
 per audio channel.  The Enhanced apt-X algorithm encodes four
 successive 16-bit or 24-bit PCM samples from each audio channel and
 respectively produces a single 16-bit or 24-bit coded sample per
 channel.  The same RTP packetization rules apply for each of these
 algorithmic variations.
 Standard apt-X and Enhanced apt-X coded data streams can optionally
 carry synchronization information and an auxiliary data channel
 within the coded audio data without additional overhead.  These
 mechanisms can, for instance, be used when the IP system is cascaded
 with another transportation system and the decoder is acting as a

Lindsay & Foerster Standards Track [Page 3] RFC 7310 apt-X RTP Format July 2014

 simple bridge between the two systems.  Since auxiliary data channel
 and synchronization information are carried within the coded audio
 data without additional overhead, RTP payload format rules do not
 change if they are present.  Out-of-band signaling is required,
 however, to notify the receiver end when autosync and auxiliary data
 have been embedded in the apt-X stream.
 Embedded auxiliary data is typically used to transport non-audio data
 and timecode information for synchronization with video.  The bit
 rate of the auxiliary data channel is 1/4 of the sample frequency.
 For example, with a single audio channel encoded at Fs = 48 kHz, an
 auxiliary data bit rate of 12 kbit/s can be embedded.
 apt-X further provides a means of stereo-pairing apt-X channels so
 that the embedded autosync and auxiliary data channel can be shared
 across the channel pair.  In the case of a 1.536-Mbit/s stereo audio
 stream composed of two channels of 16-bit PCM audio that is sampled
 at 48 kHz, a byte of auxiliary data would typically be fed into the
 Standard apt-X or Enhanced apt-X encoder once every 32 uncoded left
 channel samples.  By default, apt-X channel-pairing is not enabled.
 Out-of-band signaling is required to notify the receiver when the
 option is being used.
 Standard apt-X and Enhanced apt-X decoders that have not been set up
 with the correct embedded autosync, auxiliary data, and
 stereo-pairing information will play out uncoded PCM samples with a
 loss of decoding quality.  In the case of Standard apt-X, the loss of
 quality can be significant.
 Further details on the algorithm operation can be obtained from
 CSR plc.
    Corporate HQ
    Churchill House
    Cambridge Business Park
    Cowley Road
    Cambridge
    CB4 0WZ
    UK
    Tel: +44 1223 692000
    Fax: +44 1223 692001
    <http://www.csr.com>

Lindsay & Foerster Standards Track [Page 4] RFC 7310 apt-X RTP Format July 2014

4. Payload Format Capabilities

 This RTP payload format carries an integer number of Standard apt-X
 or Enhanced apt-X coded audio samples.  When multiple related audio
 channels are being conveyed within the payload, each channel
 contributes the same integer number of apt-X coded audio samples to
 the total carried by the payload.

4.1. Use of Forward Error Correction (FEC)

 Standard apt-X and Enhanced apt-X do not inherently provide any
 mechanism for adding redundancy or error-control coding into the
 coded audio stream.  Generic schemes for RTP, such as forward error
 correction as described in RFC 5109 [RFC5109] and RFC 2733 [RFC2733],
 can be used to add redundant information to Standard apt-X and
 Enhanced apt-X RTP packet streams, making them more resilient to
 packet losses at the expense of a higher bit rate.

5. Payload Format

 The Standard apt-X and Enhanced apt-X algorithms encode four
 successive PCM samples from each audio channel and produce a single
 compressed sample for each audio channel.  The encoder MUST be
 presented with an integer number S of input audio samples, where S is
 an arbitrary multiple of 4.  The encoder will produce exactly S/4
 coded audio samples.  Since each coded audio sample is either 16 or
 24 bits, the amount of coded audio data produced upon each invocation
 of the encoding process will be an integer number of bytes.  RTP
 packetization of the encoded data SHALL be on a byte-by-byte basis.

5.1. RTP Header Usage

 Utilization of the Standard apt-X and Enhanced apt-X coding
 algorithms does not create any special requirements with respect to
 the contents of the RTP packet header.  Other RTP packet header
 fields are defined as follows.
 o  V - As per [RFC3550]
 o  P - As per [RFC3550]
 o  X - As per [RFC3550]
 o  CC - As per [RFC3550]
 o  M - As per [RFC3550] and [RFC3551] Section 4.1

Lindsay & Foerster Standards Track [Page 5] RFC 7310 apt-X RTP Format July 2014

 o  PT - A dynamic payload type; MUST be used [RFC3551]
 o  SN (sequence number) - As per [RFC3550]
 o  Timestamp - As per [RFC3550].  The RTP timestamp reflects the
    instant at which the first audio sample in the packet was sampled,
    that is, the oldest information in the packet.
 Header field abbreviations are defined as follows.
 o  V - Version Number
 o  P - Padding
 o  X - Extensions
 o  CC - Count of contributing sources
 o  M - Marker
 o  PT - Payload Type
 o  PS - Payload Structure

5.2. Payload Structure

 The RTP payload data for Standard apt-X and Enhanced apt-X MUST be
 structured as follows.
 Standard apt-X and Enhanced apt-X coded samples are packed
 contiguously into payload octets in "network byte order", also known
 as big-endian order, and starting with the most significant bit.
 Coded samples are packed into the packet in time sequence, beginning
 with the oldest coded sample.  An integer number of coded samples
 MUST be within the same packet.
 When multiple channels of Standard apt-X and Enhanced apt-X coded
 audio, such as in a stereo program, are multiplexed into a single RTP
 stream, the coded samples from each channel, at a single sampling
 instant, are interleaved into a coded sample block according to the
 following standard audio channel ordering [RFC3551].  Coded sample
 blocks are then packed into the packet in time sequence beginning
 with the oldest coded sample block.

Lindsay & Foerster Standards Track [Page 6] RFC 7310 apt-X RTP Format July 2014

    l left
    r right
    c center
    S surround
    F front
    R rear
    channels   description     channel
                               1   2   3   4   5   6
    ___________________________________________________
    2          stereo          l   r
    3                          l   r   c
    4                          l   c   r   S
    5                          Fl  Fr  Fc  Sl  Sr
    6                          l   lc  c   r   rc  S
 For the two-channel encoding example, the sample sequence is (left
 channel, first sample), (right channel, first sample), (left channel,
 second sample), (right channel, second sample).  Coded samples for
 all channels, belonging to a single coded sampling instant, MUST be
 contained in the same packet.  All channels in the same RTP stream
 MUST be sampled at the same frequency.

5.3. Default Packetization Interval

 The default packetization interval MUST have a duration of
 4 milliseconds.  When an integer number of coded samples per channel
 cannot be contained within this 4-millisecond interval, the default
 packet interval MUST be rounded down to the nearest packet interval
 that can contain a complete integer set of coded samples.  For
 example, when encoding audio with either Standard apt-X or Enhanced
 apt-X, sampled at 11025 Hz, 22050 Hz, or 44100 Hz, the packetization
 interval MUST be rounded down to 3.99 milliseconds.
 The packetization interval sets limits on the end-to-end delay;
 shorter packets minimize the audio delay through a system at the
 expense of increased bandwidth, while longer packets introduce less
 header overhead but increase delay and make packet loss more
 noticeable.  A default packet interval of 4 milliseconds maintains an
 acceptable ratio of payload to header bytes and minimizes the
 end-to-end delay to allow viable interactive applications based on
 apt-X.  All implementations MUST support this default packetization
 interval.

Lindsay & Foerster Standards Track [Page 7] RFC 7310 apt-X RTP Format July 2014

5.4. Implementation Considerations

 An application implementing this payload format MUST understand all
 the payload parameters that are defined in this specification.  Any
 mapping of these parameters to a signaling protocol MUST support all
 parameters.  Implementations can always decide whether they are
 capable of communicating based on the entities defined in this
 specification.

5.5. Payload Example

 As an example payload format, consider the transmission of an
 arbitrary 5.1 audio signal consisting of six channels of 24-bit PCM
 data, sampled at a rate of 48 kHz and packetized on an RTP packet
 interval of 4 milliseconds.  The total bit rate before audio coding
 is 6 * 24 * 48000 = 6.912 Mbit/s.  Applying Enhanced apt-X coding
 with a coded sample size of 24 bits results in a transmitted coded
 bit rate of 1/4 of the uncoded bit rate, i.e., 1.728 Mbit/s.  On
 packet intervals of 4 milliseconds, packets contain 864 bytes of
 encoded data that contain 48 Enhanced apt-X coded samples per
 channel.
 For the example format, the diagram below shows how coded samples
 from each channel are packed into a sample block and how sample
 blocks 1, 2, and 48 are subsequently packed into the RTP packet.
    C:
    Channel index: Left (l) = 1, left center (lc) = 2,
    center (c) = 3, right (r) = 4, right center (rc) = 5,
    and surround (S) = 6.
    T:
    Sample Block time index: The first sample block is 1; the final
    sample is 48.
    S(C)(T):
    The Tth sample from channel C.

Lindsay & Foerster Standards Track [Page 8] RFC 7310 apt-X RTP Format July 2014

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    S(1)(1)                    |    S(2)(1)    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    S(2)(1)    |            S(3)(1)            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    S(3)(1)    |                   S(4)(1)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    S(5)(1)                    |    S(6)(1)    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    S(6)(1)    |            S(1)(2)            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    S(2)(2)    |                   S(3)(2)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    S(4)(2)                    |    S(5)(2)    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    S(5)(2)    |            S(6)(2)            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    S(6)(2)    |                   S(1)(3)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            S(6)(47)           |            S(1)(48)           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    S(1)(48)   |                   S(2)(48)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    S(3)(48)                   |    S(4)(48)   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   S(4)(48)    |           S(5)(48)            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    S(5)(48)   |                   S(6)(48)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 For the example format, the diagram below indicates the order that
 coded bytes are packed into the packet payload in terms of sample
 byte significance.  The following abbreviations are used.
    MSB:
    Most Significant Byte of a 24-bit coded sample
    MB:
    Middle Byte of a 24-bit coded sample
    LSB:
    Least Significant Byte of a 24-bit coded sample

Lindsay & Foerster Standards Track [Page 9] RFC 7310 apt-X RTP Format July 2014

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      MSB      |       MB      |      LSB      |               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6. Payload Format Parameters

 This RTP payload format is identified using the media type
 audio/aptx, which is registered in accordance with RFC 4855 [RFC4855]
 and using the template of RFC 6838 [RFC6838].

6.1. Media Type Definition

 Type name: audio
 Subtype name: aptx
 Required parameters:
    rate:
    RTP timestamp clock rate, which is equal to the sampling rate
    in Hz.  RECOMMENDED values for rate are 8000, 11025, 16000,
    22050, 24000, 32000, 44100, and 48000 samples per second.  Other
    values are permissible.
    channels:
    The number of logical audio channels that are present in the
    audio stream.
    variant:
    The variant of apt-X (i.e., Standard or Enhanced) that is being
    used.  The following variants can be signaled:
       variant=standard
       variant=enhanced
    bitresolution:
    The number of bits used by the algorithm to encode four PCM
    samples.  This value MAY only be set to 16 for Standard apt-X
    and 16 or 24 for Enhanced apt-X.

Lindsay & Foerster Standards Track [Page 10] RFC 7310 apt-X RTP Format July 2014

 Optional parameters:
    ptime:
    The recommended length of time (in milliseconds) represented by
    the media in a packet.  Defaults to 4 milliseconds.
    See Section 6 of [RFC4566].
    maxptime:
    The maximum amount of media that can be encapsulated in each
    packet, expressed as time in milliseconds.  See Section 6 of
    [RFC4566].
    stereo-channel-pairs:
    Defines audio channels that are stereo paired in the stream.
    See Section 3.  Each pair of audio channels is defined as two
    comma-separated values that correspond to channel numbers in
    the range 1..channels.  Each stereo channel pair is preceded
    by a '{' and followed by a '}'.  Pairs of audio channels are
    separated by a comma.  A channel MUST NOT be paired with more
    than one other channel.  The absence of this parameter signals
    that each channel has been independently encoded.
    embedded-autosync-channels:
    Defines channels that carry embedded autosync.
    Embedded-autosync-channels is defined as a list of
    comma-separated values that correspond to channel numbers in
    the range 1..channels.  When a channel is stereo paired, embedded
    autosync is shared across channels in the pair.  The first channel
    as defined in stereo-channel-pairs MUST be specified in the
    embedded-autosync-channels list.
    embedded-aux-channels:
    Defines channels that carry embedded auxiliary data.
    Embedded-aux-channels is defined as a list of comma-separated
    values that correspond to channel numbers in the range
    1..channels.  When a channel is stereo paired, embedded auxiliary
    data is shared across channels in the pair.  The second channel
    as defined in stereo-channel-pairs MUST be specified in the
    embedded-aux-channels list.
 Encoding considerations: This media type is framed in RTP and
    contains binary data; see Section 4.8 of [RFC6838].
 Security considerations: See Section 5 of [RFC4855] and Section 4
    of [RFC4856].
 Interoperability considerations: none

Lindsay & Foerster Standards Track [Page 11] RFC 7310 apt-X RTP Format July 2014

 Published specification: RFC 7310
 Applications which use this media type: Audio streaming
 Fragment identifier considerations: None
 Additional information: none
 Person & email address to contact for further information:
    John Lindsay <Lindsay@worldcastsystems.com>
 Intended usage: COMMON
 Restrictions on usage: This media type depends on RTP framing,
    and hence is only defined for transfer via RTP [RFC3550].
 Author/Change controller: IETF Payload Working Group delegated
    from the IESG.

6.2. Mapping to SDP

 The information carried in the media type specification has a
 specific mapping to fields in the Session Description Protocol (SDP)
 [RFC4566] that is commonly used to describe RTP sessions.  When SDP
 is used to describe sessions, the media type mappings are as follows.
 o  The type name ("audio") goes in SDP "m=" as the media name.
 o  The subtype name ("aptx") goes in SDP "a=rtpmap" as the encoding
    name.
 o  The parameter "rate" also goes in "a=rtpmap" as the clock rate.
 o  The parameter "channels" also goes in "a=rtpmap" as the channel
    count.
 o  The parameter "maxptime", when present, MUST be included in the
    SDP "a=maxptime" attribute.
 o  The required parameters "variant" and "bitresolution" MUST be
    included in the SDP "a=fmtp" attribute.
 o  The optional parameters "stereo-channel-pairs",
    "embedded-autosync-channels", and "embedded-aux-channels", when
    present, MUST be included in the SDP "a=fmtp" attribute.

Lindsay & Foerster Standards Track [Page 12] RFC 7310 apt-X RTP Format July 2014

 o  The parameter "ptime", when present, goes in a separate SDP
    attribute field and is signaled as "a=ptime:<value>", where
    <value> is the number of milliseconds of audio represented by
    one RTP packet.  See Section 6 of [RFC4566].

6.2.1. SDP Usage Examples

 Some example SDP session descriptions utilizing apt-X encodings
 follow.  In these examples, long "a=fmtp" lines are folded to meet
 the column width constraints of this document.
 Example 1: A Standard apt-X stream that encodes two independent
 44.1-kHz 16-bit PCM channels into a 4-millisecond RTP packet.
    m=audio 5004 RTP/AVP 98
    a=rtpmap:98 aptx/44100/2
    a=fmtp:98 variant=standard; bitresolution=16;
    a=ptime:4
 Example 2: An Enhanced apt-X stream that encodes two 48-kHz 24-bit
 stereo channels into a 4-millisecond RTP packet and carries both an
 embedded autosync and auxiliary data channel.
    m=audio 5004 RTP/AVP 98
    a=rtpmap:98 aptx/48000/2
    a=fmtp:98 variant=enhanced; bitresolution=24;
    stereo-channel-pairs={1,2}; embedded-autosync-channels=1;
    embedded-aux-channels=2
    a=ptime:4
 Example 3: An Enhanced apt-X stream that encodes six 44.1-kHz 24-bit
 channels into a 6-millisecond RTP packet.  Channels 1,2 and 3,4 are
 stereo pairs.  Both stereo pairs carry both an embedded autosync and
 auxiliary data channel.
    m=audio 5004 RTP/AVP 98
    a=rtpmap:98 aptx/44100/6
    a=fmtp:98 variant=enhanced; bitresolution=24;
    stereo-channel-pairs={1,2},{3,4}; embedded-autosync-channels=1,3;
    embedded-aux-channels=2,4
    a=ptime:6

Lindsay & Foerster Standards Track [Page 13] RFC 7310 apt-X RTP Format July 2014

6.2.2. Offer/Answer Considerations

 The only negotiable parameter is the delivery method.  All other
 parameters are declarative.  The offer, as described in [RFC3264],
 may contain a large number of delivery methods per single fmtp
 attribute.  The answerer MUST remove every delivery method and
 configuration URI that is not supported.  Apart from this exceptional
 case, all parameters MUST NOT be altered on answer.

7. IANA Considerations

 One media type (audio/aptx) has been registered in the "Media Types"
 registry.  See Section 6.1.

8. Security Considerations

 RTP packets using the payload format defined in this specification
 are subject to the security considerations discussed in the RTP
 specification [RFC3550] and any appropriate RTP profile (for example,
 [RFC3551]).  This implies that confidentiality of the media streams
 is achieved by encryption.  Because the audio coding used with this
 payload format is applied end to end, encryption may be performed
 after audio coding so there is no conflict between the two
 operations.  A potential denial-of-service threat exists for audio
 coding techniques that have non-uniform receiver-end computational
 load.  The attacker can inject pathological datagrams into the stream
 that are complex to decode and cause the receiver to be overloaded.
 However, the Standard apt-X and Enhanced apt-X audio coding
 algorithms do not exhibit any significant non-uniformity.  As with
 any IP-based protocol, in some circumstances a receiver may be
 overloaded simply by the receipt of too many packets, either desired
 or undesired.  Network-layer authentication may be used to discard
 packets from undesired sources, but the processing cost of the
 authentication itself may be too high.  In a multicast environment,
 pruning of specific sources may be implemented in future versions of
 IGMP [RFC3376] and in multicast routing protocols to allow a receiver
 to select which sources are allowed to reach it.  [RFC6562] has
 highlighted potential security vulnerabilities of Variable Bit Rate
 (VBR) codecs using Secure RTP transmission methods.  As the Standard
 apt-X and Enhanced apt-X codecs are Constant Bit Rate (CBR) codecs,
 this security vulnerability is therefore not applicable.

9. Acknowledgements

 This specification was facilitated by earlier documents produced by
 Greg Massey, David Trainer, James Hunter, and Derrick Rea, along with
 practical tests carried out by Paul McCambridge of APT Ltd.

Lindsay & Foerster Standards Track [Page 14] RFC 7310 apt-X RTP Format July 2014

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264,
            June 2002.
 [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
            Jacobson, "RTP: A Transport Protocol for Real-Time
            Applications", STD 64, RFC 3550, July 2003.
 [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
            Video Conferences with Minimal Control", STD 65, RFC 3551,
            July 2003.
 [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
            Description Protocol", RFC 4566, July 2006.

10.2. Informative References

 [RFC2733]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format
            for Generic Forward Error Correction", RFC 2733,
            December 1999.
 [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
            Thyagarajan, "Internet Group Management Protocol,
            Version 3", RFC 3376, October 2002.
 [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
            Formats", RFC 4855, February 2007.
 [RFC4856]  Casner, S., "Media Type Registration of Payload Formats in
            the RTP Profile for Audio and Video Conferences",
            RFC 4856, February 2007.
 [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
            Correction", RFC 5109, December 2007.

Lindsay & Foerster Standards Track [Page 15] RFC 7310 apt-X RTP Format July 2014

 [RFC6562]  Perkins, C. and JM. Valin, "Guidelines for the Use of
            Variable Bit Rate Audio with Secure RTP", RFC 6562,
            March 2012.
 [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
            Specifications and Registration Procedures", BCP 13,
            RFC 6838, January 2013.

Authors' Addresses

 John Lindsay
 APT Ltd
 729 Springfield Road
 Belfast
 Northern Ireland
 BT12 7FP
 UK
 Phone: +44 2890 677200
 EMail: Lindsay@worldcastsystems.com
 Hartmut Foerster
 APT Ltd
 729 Springfield Road
 Belfast
 Northern Ireland
 BT12 7FP
 UK
 Phone: +44 2890 677200
 EMail: Foerster@worldcastsystems.com

Lindsay & Foerster Standards Track [Page 16]

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