wdiff rfc7310.original rfc7310.txt

Internet-Draft
Internet Engineering Task Force (IETF) J. Lindsay
A/V Transport Payloads Working Group H.Foerster
Intended status:
Request for Comments: 7310 H. Foerster
Category: Standards Track APT Ltd
Expires: Aug 05, 2014 February 05,
ISSN: 2070-1721 July 2014

RTP Payload Format for Standard apt-X and Enhanced apt-X Codecs
draft-ietf-payload-rtp-aptx-05

Abstract

This document specifies a scheme for packetizing Standard apt-X, apt-X or
Enhanced apt-X, 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,
payload and a means of establishing Standard apt-X and Enhanced apt-X
connections through the Session Description Protocol (SDP).

Status of this This Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 ....................................................2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5 .....................................................3
3. Standard apt-X and Enhanced apt-X Codecs . . . . . . . . . . . 6 ........................3
4. Payload Format Capabilities . . . . . . . . . . . . . . . . . 8 .....................................5
4.1. Use of Forward Error Correction (FEC) . . . . . . . . . . 8 ......................5
5. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 9 ..................................................5
5.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . . 9 ...........................................5
5.2. Payload Structure . . . . . . . . . . . . . . . . . . . . 10 ..........................................6
5.3. Default Packetization Interval . . . . . . . . . . . . . . 11 .............................7
5.4. Implementation Considerations . . . . . . . . . . . . . . 11 ..............................8
5.5. Payload Example . . . . . . . . . . . . . . . . . . . . . 11 ............................................8
6. Payload Format Parameters . . . . . . . . . . . . . . . . . . 14 ......................................10
6.1. Media Type Definition . . . . . . . . . . . . . . . . . . 14 .....................................10
6.2. Mapping to SDP . . . . . . . . . . . . . . . . . . . . . . 16 ............................................12
6.2.1. SDP Usage Example . . . . . . . . . . . . . . . . . . 16 Examples .................................13
6.2.2. Offer/Answer Considerations . . . . . . . . . . . . . 17 ........................13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 ............................................14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 ........................................14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20 ...............................................14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 ....................................................15
10.1. Normative References . . . . . . . . . . . . . . . . . . . 21 .....................................15
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22 ...................................15

1. Introduction

This document specifies the payload format for packetization of audio
data,
data encoded with the Standard apt-X or Enhanced apt-X audio coding
algorithms,
algorithms into the Real-time Transport Protocol (RTP). (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 detailed, and a relevant
example of the format is provided. The media type, its mappings to
SDP [RFC4566] [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.

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, simple and
allow easy RTP packetization of data coded with the algorithms, algorithms
without a 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 1.536-Mbit/s stereo audio stream, stream composed of 2 two channels
of 16-bit Pulse Code Modulated (PCM) audio that is sampled at a
frequency of 48 kHz, 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 4 four PCM samples when
decoded. The Standard apt-X algorithm encodes 4 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 4 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 packetisation packetization rules apply for each of these
algorithmic variations.

Standard apt-X and Enhanced apt-X coded data streams can optionally
carry synchronisation 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
simple bridge between the two systems. Since auxiliary data channel
and synchronisation 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 signalling signaling is required
however 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, data
and timecode information for synchronisation synchronization with video. The bit
rate of the auxiliary data channel is 1/4 of the sample frequency.
For example example, with a single audio channel encoded at Fs =
48kHz, 48 kHz, an
auxiliary data bit rate of 12 kbit/s can be embedded.

apt-X further provides a means of stereo pairing 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 1.536-Mbit/s stereo audio
stream,
stream composed of 2 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 default, apt-X channels pairing channel-pairing is not enabled.
Out-of-band signalling signaling is required to notify the receiver when the
option is being used.

Standard apt-X and Enhanced apt-X decoders that have not be been set up
with the correct embedded autosync, auxiliary data data, and stereo pairing
stereo-pairing information will playout play out uncoded PCM samples with a
loss of decoding quality. In the case of standard apt-X 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
<http://www.csr.com>

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 forward error correction schemes for RTP RTP, such as RFC 2198 [RFC2198] and 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 4 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
o PT - A dynamic payload type, type; MUST be used. 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 order, and starting with the most significant bit.
Coded samples are packed into the packet in time sequence 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 E-APTX 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, ordering [RFC3551]. Coded sample
blocks are then packed into the packet in time sequence beginning
with the oldest coded sample block.

l left
r right
c center
S surround
F front
R rear

channels, description,

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 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 milliseconds 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
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 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
apt-X applications based applications. on
apt-X. All implementations MUST support this default packetization
interval.

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 signalling signaling protocol MUST support all
parameters. Implementation 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 6 six channels of 24-bit PCM
data, sampled at a rate of 48 kHz and packetized on a an RTP packet
interval of 4 milliseconds. The total bit rate before audio coding
is 6 * 24 * 48000 = 6.912 Mbits/s. Mbit/s. Applying Enhanced apt-X coding, coding
with a coded sample size of 24 bits, bits results in a transmitted coded
bit rate of 1/4 of the uncoded bit rate, i.e. 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 centre center (lc) = 2, centre
center (c) = 3, right (r) = 4, right centre center (rc) = 5,
and surround (S) = 6.

T:
Sample Block time index: The first sample block is 1, 1; the final
sample is 48.

S(C)(T):
The Tth sample from channel C C.

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
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,
audio/aptx, which is registered in accordance with RFC 4855 [RFC4855]
and using the template of RFC 6838 [RFC6838] [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 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. (i.e., Standard or Enhanced) that is being
used. The following variants can be signalled: signaled:

variant=standard
variant=enhanced

bitresolution:
The number of bits used by the algorithm to encode 4 four PCM
samples. This value MAY only be set to 16 for Standard apt-X
and 16 or 24 for Enhanced apt-X.

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 length amount of time (in milliseconds) media that can be encapsulated in a packet. 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. Absence 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 are
Embedded-autosync-channels is defined as a list of
comma-separated values that correspond to channel numbers in
the range 1..
channels. 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 are
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-autosync-channels
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
Published specification: RFC XXXX 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 email:lindsay@worldcastsystems.com <Lindsay@worldcastsystems.com>

Intended usage: Common 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 IETF Payload Working Group delegated
from the IESG" 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 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" "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", "embedded-aux-channels"
"embedded-autosync-channels", and "embedded-aux-channels", when
present, MUST be included in the SDP "a=fmtp" attribute.

o The parameter "ptime", when present, goes in a separate SDP
attribute field and is signalled 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 "a=fmtp" lines are folded to meet
the column width constraints of this document.

Example 1: A standard Standard apt-X stream that encodes two independent
44.1kHz
44.1-kHz 16-bit PCM channels into a 4 milliseconds 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 Enhanced apt-X stream that encodes two 48kHz 48-kHz 24-bit
stereo channels into a 4 milliseconds 4-millisecond RTP packet and that 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 Enhanced apt-X stream that encodes six 44.1kHz 44.1-kHz 24-bit
channels into a 6 milliseconds 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

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
attribute. The answerer MUST remove every delivery method and
configuration uri 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 defined and needs registration registered in the media types "Media Types"
registry. See Section 6.1 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], [RFC3550] and any appropriate RTP profile (for example 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, 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
which
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.
[draft-ietf-avtcore-srtp-vbr-audio] [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 Hunter, and Derrick Rea Rea, along with
practical tests carried out by Paul McCambridge of APT Ltd.

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.

[RFC3551] H. Schulzrinne, "RTP profile for audio and video
conferences with minimal control", RFC 3551, July 2003.

10.2. Informative References

[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A.,

[RFC2733] Rosenberg, J. and S. Fosse-
Parisis, "RTP H. Schulzrinne, "An RTP Payload Format
for Redundant Audio Data", Generic Forward Error Correction", RFC 2198,
September 1997. 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.

[RFC6838] Freed, N.,J. Klensin, and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, January 2013.

[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.

[draft-ietf-avtcore-srtp-vbr-audio]

[RFC6562] Perkins, C., Valins, JM., C. and JM. Valin, "Guidelines for the Use of
Variable Bit Rate Audio with Secure RTP", draft-ietf-avtcore-srtp-vbr-audio, RFC 6562,
March 2012.

11.

[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:
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
EMail: Foerster@worldcastsystems.com