AVT R. Even
Internet-Draft Polycom
Intended status: Standards Track P. Luthi
Expires: November 23, 2008 Tandberg
May 22, 2008
RTP Payload Format for ITU-T Recommendation G.722.1
draft-ietf-avt-rfc3047-bis-06.txt
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Abstract
International Telecommunication Union (ITU-T) Recommendation G.722.1
is a wide-band audio codec. This document describes the payload
format for including G.722.1 generated bit streams within an RTP
packet. The document also describes the syntax and semantics of the
SDP parameters needed to support G.722.1 audio codec.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. RTP usage for G.722.1 . . . . . . . . . . . . . . . . . . . . 5
3.1. RTP G.722.1 Header Fields . . . . . . . . . . . . . . . . 5
3.2. RTP payload format for G.722.1 . . . . . . . . . . . . . . 5
3.3. Multiple G.722.1 frames in a RTP packet . . . . . . . . . 7
3.4. Computing the number of G.722.1 frames . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
4.1. Media Type Registration . . . . . . . . . . . . . . . . . 8
4.1.1. Registration of media type audio/G7221 . . . . . . . . 8
5. SDP Parameters . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Usage with the SDP Offer Answer Model . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Changes from RFC 3047 . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. RFC editor note . . . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
ITU-T G.722.1 [ITU.G7221] is a low complexity coder, it compresses 50
Hz - 14 kHz audio signals into one of the following bit rates, 24
kbit/s, 32 kbit/s or 48 kbit/s.
The coder may be used for speech, music and other types of audio.
Some of the applications for which this coder is suitable are:
o Real-time communications such as videoconferencing and telephony.
o Streaming audio
o Archival and messaging
ITU-T G.722.1 [ITU.G7221] uses 20 ms frames and a sampling rate clock
of 16 kHz or 32kHz. The encoding and decoding algorithm can change
the bit rate at any 20ms frame boundary, but no bit rate change
notification is provided in-band with the bit stream.
For any given bit rate the number of bits in a frame is a constant.
Within this fixed frame, G.722.1 uses variable length coding (e.g.
Huffman coding) to represent most of the encoded parameters. All
variable length codes are transmitted in order from the left most
(most significant - MSB) bit to the right most (least significant -
LSB) bit, see [ITU.G7221] for more details.
The ITU-T standardized bit rates for G.722.1 are 24 kbit/s or
32kbit/s at 16 Khz sample rate, and 24 kbit/s, 32 kbit/s or 48 kbit/s
at 32 Khz sample rate. However, the coding algorithm itself has the
capability to run at any user specified bit rate (not just 24, 32 and
48 kbit/s) while maintaining an audio bandwidth of 50 Hz to 14 kHz.
This rate change is accomplished by a linear scaling of the codec
operation, resulting in frames with size in bits equal to 1/50 of the
corresponding bit rate.
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2. Terminology
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 [RFC2119] and
indicate requirement levels for compliant RTP implementations.
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3. RTP usage for G.722.1
3.1. RTP G.722.1 Header Fields
The RTP header is defined in the RTP specification [RFC3550]. This
section defines how fields in the RTP header are used.
Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is
specified by the RTP profile under which this payload format is
used, or signaled dynamically out-of-band (e.g., using SDP).
Marker (M) bit: The M bit is set to zero.
Extension (X) bit: Defined by the RTP profile used.
Timestamp: A 32-bit word that corresponds to the sampling instant
for the first frame in the RTP packet.
3.2. RTP payload format for G.722.1
The RTP payload for G.722.1 has the format shown in Figure 1. No
additional header fields specific to this payload format are
required.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
+ one or more frames of G.722.1 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP payload for G.722.1
Because bit rate is not signaled in-band, a separate out-of-band
method is REQUIRED to indicate the bit rate (see section 5 for an
example of signaling bit rate information using SDP). For the
payload format specified here, the bit rate MUST remain constant for
a particular payload type value. An application MAY switch bit rates
from packet to packet by defining two payload type values and
switching between them.
The use of Huffman coding means that it is not possible to identify
the various encoded parameters/fields contained within the bit stream
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without first completely decoding the entire frame. For the purposes
of packetizing the bit stream in RTP, it is only necessary to
consider the sequence of bits as output by the G.722.1 encoder, and
present the same sequence to the decoder. The payload format
described here maintains this sequence.
When operating at 24 kbit/s, 480 bits (60 octets) are produced per
frame. When operating at 32 kbit/s, 640 bits (80 octets) are
produced per frame. When operating at 48 kbit/s, 960 bits (120
octets) are produced per frame. Thus, all three bit rates allow for
octet alignment without the need for padding bits.
Figure 2 illustrates how the G.722.1 bit stream MUST be mapped into
an octet aligned RTP payload.
first bit last bit
transmitted transmitted
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ sequence of bits (480, 640 or 960) generated by the |
| G.722.1 encoder for transmission |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | |
| | | ... | |
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... +-+-+-+-+-+-+-+-+-+-+-+-+
|MSB... LSB|MSB... LSB| |MSB... LSB|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... +-+-+-+-+-+-+-+-+-+-+-+-+
RTP RTP RTP
octet 1 octet 2 octet
60, 80, 120
Figure 2: The G.722.1 encoder bit stream is split into
a sequence of octets (60, 80 or 120 depending on
the bit rate), and each octet is in turn
mapped into an RTP octet.
When operating at non-standard rates the payload format MUST follow
the guidelines illustrated in Figure 2. It is RECOMMENDED that
values in the range 16000 to 48000 be used. Non standard rates MUST
have a value that is a multiple of 400 (this maintains octet
alignment and does not then require (undefined) padding bits for each
frame if not octet aligned). For example, a bit rate of 16.4 kbit/s
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will result in a frame of size 328 bits or 41 octets which are mapped
into RTP per Figure 2.
3.3. Multiple G.722.1 frames in a RTP packet
A sender may include more than one consecutive G.722.1 frame in a
single RTP packet.
Senders have the following additional restrictions:
o Sender SHOULD NOT include more G.722.1 frames in a single RTP
packet than will fit in the MTU of the RTP transport protocol.
o All frames contained in a single RTP packet MUST be of the same
length and sampled at the same clock rate. They MUST have the
same bit rate (octets per frame).
o Frames MUST NOT be split between RTP packets.
It is RECOMMENDED that the number of frames contained within an RTP
packet be consistent with the application. For example, in a
telephony application where delay is important, then the fewer frames
per packet the lower the delay, whereas for a delay insensitive
streaming or messaging application, many frames per packet would be
acceptable.
3.4. Computing the number of G.722.1 frames
Information describing the number of frames contained in an RTP
packet is not transmitted as part of the RTP payload. The only way
to determine the number of G.722.1 frames is to count the total
number of octets within the RTP packet, and divide the octet count by
the number of expected octets per frame. This expected octet per
frame count is derived from the bit rate, and is therefore a function
of the payload type.
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4. IANA Considerations
This document updates the G7221 media type described in RFC3047.
4.1. Media Type Registration
This section describes the media types and names associated with this
payload format. The section registers the media types, as per
RFC4288 [RFC4288]
4.1.1. Registration of media type audio/G7221
Media type name: audio
Media subtype name: G7221
Required parameters:
bitrate: the data rate for the audio bit stream. This parameter
is mandatory because the bit rate is not signaled within the
G.722.1 bit stream. At the standard G.722.1 bit rates, the value
MUST be either 24000 or 32000 at 16 Khz sample rate, and 24000,
32000 or 48000 at 32 Khz sample rate. If using the non-standard
bit rates, then it is RECOMMENDED that values in the range 16000
to 48000 be used. Non standard rates MUST have a value that is a
multiple of 400 (this maintains octet alignment and does not then
require (undefined) padding bits for each frame if not octet
aligned).
Optional parameters:
ptime: see RFC 4566. SHOULD be a multiple of 20 msec.
maxptime: see RFC 4566. SHOULD be a multiple of 20 msec.
Encoding considerations:
This media type is framed and binary, see section 4.8 in
[RFC4288].
Security considerations: See Section 6
Interoperability considerations:
Terminals SHOULD offer a media type at 16 Khz sample rate in order
to interoperate with terminals that do not support the new 32 Khz
sample rate.
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Published specification: RFC yyy [see RFCeditor notes].
Applications which use this media type:
Audio and Video streaming and conferencing applications.
Additional information: none
Person and email address to contact for further information :
Roni Even: roni.even@polycom.co.il
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only defined
for transfer via RTP [RFC3550]. Transport within other framing
protocols is not defined at this time.
Author: Roni Even
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
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5. SDP Parameters
The media types audio/G7221 are mapped to fields in the Session
Description Protocol (SDP) [RFC4566] as follows:
o The media name in the "m=" line of SDP MUST be audio.
o The encoding name in the "a=rtpmap" line of SDP MUST be G7221 (the
media subtype).
o The clock rate in the "a=rtpmap" line MUST be 16000 or 32000.
o Only one bitrate SHALL be defined (using the "bitrate=" parameter
in the a=fmtp line) for each payload type.
5.1. Usage with the SDP Offer Answer Model
When offering G.722.1 over RTP using SDP in an Offer/Answer model
[RFC3264] the following considerations are necessary.
The combination of the clock rate in the rtpmap and the bitrate
parameter defines the configuration of each payload type. Each
configuration intended to be used, MUST be declared.
There are two sampling clock rates defined for G.722.1 in this
document. RFC3047 [RFC3047] supported only the 16 Khz clock rate.
Therefore a system that wants to use G.722.1 SHOULD offer a payload
type with clock rate of 16000 for backward interoperability.
An example of an offer that includes a 16000 and 32000 clock rate is:
m=audio 49000 RTP/AVP 121 122
a=rtpmap:121 G7221/16000
a=fmtp:121 bitrate=24000
a=rtpmap:122 G7221/32000
a=fmtp:122 bitrate=48000
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6. 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. This
implies that confidentiality of the media streams is achieved by
encryption.
A potential denial-of-service threat exists for data encoding 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.
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7. Changes from RFC 3047
The new draft updates RFC3047 adding the support for the Wideband
audio support defined in the new revision of ITU-T G.722.1.
Other changes
Update the text to be in line with the current rules for RFC and with
media type registration conforming to RFC 4288.
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8. Acknowledgements
The authors would like to thank Tom Taylor for his contribution to
this work.
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9. RFC editor note
Please replace RFC yyy giving the number assigned to this RFC.
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10. References
10.1. Normative References
[ITU.G7221]
International Telecommunications Union, "Low-complexity
coding at 24 and 32 kbit/s for hands-free operation in
systems with low frame loss", ITU-T Recommendation
G.722.1, 2005.
[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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
10.2. Informative References
[RFC3047] Luthi, P., "RTP Payload Format for ITU-T Recommendation
G.722.1", RFC 3047, January 2001.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
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Authors' Addresses
Roni Even
Polycom
94 Derech Em Hamoshavot
Petach Tikva 49130
Israel
Email: roni.even@polycom.co.il
Patrick Luthi
Tandberg
Philip Pedersens vei 22
Lysaker 1366
Norway
Email: patrick.luthi@tandberg.no
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