Network Working Group G S. Massey
Internet-Draft APT Ltd
Intended status: Standards Track April 15, 2008
Expires: October 17, 2008
"RTP Payload Format for apt-X"
draft-gmassey-avt-rtp-aptx-01.txt
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Abstract
This document describes an RTP payload format for transporting apt-X
encoded audio. It details the RTP encapsulation mechanism for
standard, enhanced apt-X and apt-X Live data. Also included within
this memo are media type registrations, and the details necessary for
the use of apt-X, enhanced apt-X and apt-X Live with the Session
Description Protocol.
Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Encapsulation of apt-X and Enhanced apt-X Streams . . . . . . 5
3.1. RTP header usage . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. Standard apt-X . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Enhanced apt-X 16 Bit . . . . . . . . . . . . . . . . . . 7
4.3. Enhanced apt-X 24 Bit . . . . . . . . . . . . . . . . . . 8
4.4. apt-X Live . . . . . . . . . . . . . . . . . . . . . . . . 9
5. SDP Related Considerations . . . . . . . . . . . . . . . . . . 11
5.1. Mapping Media Type Parameters into SDP . . . . . . . . . . 11
5.2. SDP Example . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Usage with the SDP Offer/Answer Model . . . . . . . . . . 12
6. Usage with the SDP Offer/Answer Model . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Normative References . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Requirements notation
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 [RFC2119].
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2. Introduction
apt-X, enhanced apt-X and apt-X Live are proprietary audio
compression algorithms. No prior RFC or standard has been used to
describe how to packetise this audio data. The compression algorithm
yields a fixed bit rate scaled according to the sample frequency of
the input audio. The fixed rate is one quarter of the original bit
rate. This rule also applies to the resolution of the audio.
Example, a 1.536MB/s stereo audio stream using 16 bit PCM is reduced
to 384 kb/s. The apt-X and enhanced apt-X algorithms can carry an
ancillary data stream and synchronisation information within the
compressed audio stream without additional overhead. The ancillary
data stream can be used to transport data or timecode data for
synchronisation with video. The bandwidth of the ancillary data
channel is one quarter the sample frequency, example at Fs = 48kHz
the ancillary data stream is 12 kb/s. The similarities between the
packetisation is such that it does each variant does not warrant a
separate RFC.
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3. Encapsulation of apt-X and Enhanced apt-X Streams
apt-X Live and Enhanced apt-X is a non framed based algorithm and
produces as its output a sixteen bit word representing four sixteen
bit PCM samples. The packetisation of the data shall be on a byte by
byte basis. Packet size must be a multiple of the number of n x
64kbps channels in the resultant bitrate. For example a 384kbps
channel would result in a multiple of 6. Therefore the resultant
payload size would be N x 6 bytes. Likewise a 576kbps channel would
require a payload N * 9 bytes.
3.1. RTP header usage
apt-X Live and Enhanced apt-X require no special considerations be
placed in the RTP header. The timestamp used in the RTP header shall
comply with the same timestamp specified in RFC3550. Payload Header
(payload format dependent) Payload
V - As per RFC3550
P - As per RFC3550
X - As per RFC3550
CC - As per RFC3550
M - As per RFC3550
PT - Payload Number to be defined according to MIME allocation:
audio/aptX audio/EaptX audio/aptXLive
SN - As per RFC3550
Payload See description below
V - Version Number
P - Padding
X - Extensions
CC - Count of contributing sources
M - Marker
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PT - Payload Type
PS - Payload Structure
The payload data for apt-X Live and Enhanced apt-X shall be
delineated as Left channel data/ Right Channel data. Data is word
aligned big endian format. For multi-channel packets the data is
allocated as per the following rules.
1) N bytes (LL/RR) format per channel
2) N is defined by the number of N x 64kb/s channels present in
the overall audio bit stream.
3) Channels are paired for simplicity .i.e 12/34/56
Multiple channels are grouped together for transmission to preserve
phase between channels.
Example 6.912 mb/s 5.1 Enhanced apt-X 24bit 3 * 576/64 = N = 9
17..Channels 1/2....0 17..Channels 3/4....0 17...Channels 5/6...0
For multi-channel distribution the format becomes,
17..Channels L/R... 0 17..Channels Ls/Rs..0 17..Channels C/B... 0
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4. IANA Considerations
4.1. Standard apt-X
MIME media type name: audio
MIME subtype name: aptX
Required parameters: rate/Channels The RTP timestamp clock rate,
which is equal to the sampling rate. The typical rate is 48000,
but other rates may be specified.
Optional parameters: audio-mode
audio-mode: (stereo/mono/dualmono)
Encoding considerations: This type is only defined for transfer
via RTP [RFC3550].
Security considerations: See Section 5 of [RFC4855][RFC4856]
Interoperability considerations: none
Published specification: RFC XXXX [RFC Editor: please replace by
the RFC number of this memo, when published]
Applications which use this media type: Audio streaming.
Additional information: none
Person & email address to contact for further information:
Gregory Massey
Intended usage: COMMON
Author/Change controller: Gregory Massey
4.2. Enhanced apt-X 16 Bit
MIME media type name: audio
MIME subtype name: EaptX 16
Required parameters: rate, channels The RTP timestamp clock
rate, which is equal to the sampling rate. The typical rate is
48000, but other rates may be specified.
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Default for channels is 2 (Stereo)
Optional parameters: audio-mode
audio-mode: (stereo/mono/dualmono)
Encoding considerations: This type is only defined for transfer
via RTP [RFC3550].
Security considerations: See Section 5 of [RFC4855][RFC4856]
Interoperability considerations: none
Published specification: RFC XXXX [RFC Editor: please replace by
the RFC number of this memo, when published]
Applications which use this media type: Audio streaming.
Additional information: none
Person & email address to contact for further information:
Gregory Massey
Intended usage: COMMON
Author/Change controller: Gregory Massey
4.3. Enhanced apt-X 24 Bit
MIME media type name: audio
MIME subtype name: EaptX 24
Required parameters: rate/Channels The RTP timestamp clock rate,
which is equal to the sampling rate. The typical rate is 48000,
but other rates may be specified.
Optional parameters: audio-mode
audio-mode: stereo/mono/dualmono
Encoding considerations: This type is only defined for transfer
via RTP [RFC3550].
Security considerations: See Section 5 of [RFC4855][RFC4856]
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Interoperability considerations: none
Published specification: RFC XXXX [RFC Editor: please replace by
the RFC number of this memo, when published]
Applications which use this media type: Audio streaming.
Additional information: none
Person & email address to contact for further information:
Gregory Massey
Intended usage: COMMON
Author/Change controller: Gregory Massey
4.4. apt-X Live
MIME media type name: audio
MIME subtype name: aptXLive
Required parameters: rate/Channels The RTP timestamp clock rate,
which is equal to the sampling rate. The typical rate is 48000,
but other rates may be specified.
Optional parameters: audio-mode/scale
audio-mode: stereo/mono
scale: Scale factors for aptX-Live algorithm (S20/S25/S30)
Encoding considerations: This type is only defined for transfer
via RTP [RFC3550].
Security considerations: See Section 5 of [RFC4855][RFC4856]
Interoperability considerations: none
Published specification: RFC XXXX [RFC Editor: please replace by
the RFC number of this memo, when published]
Applications which use this media type: Audio streaming.
Additional information: none
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Person & email address to contact for further information:
Gregory Massey
Intended usage: COMMON
Author/Change controller: Gregory Massey
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5. SDP Related Considerations
The following paragraphs defines the mapping of the parameters
described in the IANA considerations section and their usage in the
Offer/Answer Model [RFC3264].
5.1. Mapping Media Type Parameters into SDP
The information carried in the Media Type 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 specify sessions the mapping 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 clock rate.
o The parameter "channels" also goes in "a=rtpmap" as channel
count.
o The optional parameter "audio-mode", "aux", "scale", when
present, MUST be included in the SDP "a=fmtp" attribute and MUST
follow the delivery-method that applies.
5.2. SDP Example
The following example shows a basic SDP single stream. The first
configuration packet is inlined in the sdp, other configurations
could be fetched at any time from the first provided uri using or all
the known configuration could be downloaded using the second uri.
c=IN IP4 192.0.2.x
m=audio RTP/AVP 98
a=rtpmap:98 aptX/44100/2
a=fmtp:98 audio-mode=stereo; aux=auxoff;
Note that the payload format (encoding) names are commonly shown in
upper case. Media subtypes are commonly shown in lower case. These
names are case-insensitive in both places. Similarly, parameter
names are case-insensitive both in media types and in the default
mapping to the SDP a=fmtp attribute.
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5.3. Usage with the SDP Offer/Answer Model
The only parameter negotiable is the delivery method. All the others
are declarative: the offer, as described in An Offer/Answer Model
Session Description Protocol offer answer model [RFC3264], may
contain a large number of delivery methods per single fmtp attribute,
the answerer MUST remove every delivery-method and configuration-uri
not supported. All the parameters MUST not be altered on answer
otherwise.
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6. Usage with the SDP Offer/Answer Model
The only parameter negotiable is the delivery method. All the others
are declarative: the offer, as described in An Offer/Answer Model
Session Description Protocol [RFC3264], may contain a large number of
delivery methods per single fmtp attribute, the answerer MUST remove
every delivery-method and configuration-uri not supported. All the
parameters MUST not be altered on answer otherwise.
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7. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [2], and any appropriate RTP profile (for example
[RFC3550]). This implies that confidentiality of the media streams
is achieved by encryption. Because the data compression used with
this payload format is applied end-to-end, encryption may be
performed after compression so there is no conflict between the two
operations. A potential denial-of-service threat exists for data
encodings using compression techniques that have non-uniform
receiver-end computational load. The attacker can inject
pathological datagrams into the stream which are complex to decode
and cause the receiver to be overloaded. However, this encoding does
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.
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8. 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.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[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.
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Author's Address
Gregory S. Massey
APT Ltd
Edgewater Road
Belfast, CA BT3 9JQ
UK
Phone: +44 2890 371110
Email: gmassey@aptx.com
URI: http://www.aptx.com/
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