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AVT Working Group G. Herlein
Internet-Draft S. Morlat
Expires: July 2, 2005 J. Jean-Marc
R. Hardiman
P. Kerr
January 01, 2005
draft-herlein-speex-rtp-profile-03
RTP Payload Format for the Speex Codec
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she 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 July 2, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Speex is an open-source voice codec suitable for use in Voice over IP
(VoIP) type applications. This document describes the payload format
for Speex generated bit streams within an RTP packet. Also included
here are the necessary details for the use of Speex with the Session
Description Protocol (SDP) and a preliminary method of using Speex
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within H.323 applications.
Table of Contents
1. Conventions used in this document . . . . . . . . . . . . . 3
2. Overview of the Speex Codec . . . . . . . . . . . . . . . . 4
3. RTP payload format for Speex . . . . . . . . . . . . . . . . 5
4. RTP Header . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Speex payload . . . . . . . . . . . . . . . . . . . . . . . 8
6. Example Speex packet . . . . . . . . . . . . . . . . . . . . 9
7. Multiple Speex frames in a RTP packet . . . . . . . . . . . 10
8. MIME registration of Speex . . . . . . . . . . . . . . . . . 11
9. SDP usage of Speex . . . . . . . . . . . . . . . . . . . . . 12
10. ITU H.323/H.245 Use of Speex . . . . . . . . . . . . . . . . 15
11. NonStandardMessage format . . . . . . . . . . . . . . . . . 16
12. RTP Payload Types . . . . . . . . . . . . . . . . . . . . . 17
13. Security Considerations . . . . . . . . . . . . . . . . . . 18
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 19
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
15.1 Normative References . . . . . . . . . . . . . . . . . . . 20
15.2 Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . 22
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1. Conventions used in this document
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 [1].
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2. Overview of the Speex Codec
Speex is based on the CELP [10] encoding technique with support for
either narrowband (nominal 8kHz), wideband (nominal 16kHz) or
ultra-wideband (nominal 32kHz), and (non-optimal) rates up to 48 kHz
sampling also available. The main characteristics can be summarized
as follows:
o Free software/open-source
o Integration of wideband and narrowband in the same bit-stream
o Wide range of bit-rates available
o Dynamic bit-rate switching and variable bit-rate (VBR)
o Voice Activity Detection (VAD, integrated with VBR)
o Variable complexity
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3. RTP payload format for Speex
For RTP based transportation of Speex encoded audio the standard RTP
header [2] is followed by one or more payload data blocks. An
optional padding terminator may also be used.
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 Speex .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| one or more frames of Speex .... | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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4. 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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RTP header begins with an octet of fields (V, P, X, and CC) to
support specialized RTP uses (see [2] and [7] for details). For
Speex the following values are used.
Version (V): 2 bits
This field identifies the version of RTP. The version used by this
specification is two [2].
Padding (P): 1 bit
If the padding bit is set, the packet contains one or more additional
padding octets at the end which are not part of the payload. P is
set if the total packet size is less than the MTU.
Extension (X): 1 bit
If the extension, X, bit is set, the fixed header MUST be followed by
exactly one header extension, with a format defined in Section 5.3.1.
of [2].
CSRC count (CC): 4 bits
The CSRC count contains the number of CSRC identifiers.
Marker (M): 1 bit
The M bit indicates if the packet contains comfort noise. This field
is used in conjunction with the cng SDP attribute and is detailed
further in section 5 below. In normal usage this bit is set if the
packet contains comfort noise.
Payload Type (PT): 7 bits
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An RTP profile for a class of applications is expected to assign a
payload type for this format, or a dynamically allocated payload type
SHOULD be chosen which designates the payload as Speex.
Sequence number: 16 bits
The sequence number increments by one for each RTP data packet sent,
and may be used by the receiver to detect packet loss and to restore
packet sequence. This field is detailed further in [2].
Timestamp: 32 bits
A timestamp representing the sampling time of the first sample of the
first Speex packet in the RTP packet. The clock frequency MUST be
set to the sample rate of the encoded audio data. Speex uses 20 msec
frames and a variable sampling rate clock. The RTP timestamp MUST be
in units of 1/X of a second where X is the sample rate used. Speex
uses a nominal 8kHz sampling rate for narrowband use, a nominal 16kHz
sampling rate for wideband use, and a nominal 32kHz sampling rate for
ultra-wideband use.
SSRC/CSRC identifiers:
These two fields, 32 bits each with one SSRC field and a maximum of
16 CSRC fields, are as defined in [2].
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5. Speex payload
For the purposes of packetizing the bit stream in RTP, it is only
necessary to consider the sequence of bits as output by the Speex
encoder [9], and present the same sequence to the decoder. The
payload format described here maintains this sequence.
A typical Speex frame, encoded at the maximum bitrate, is approx.
110 octets and the total number of Speex frames SHOULD be kept less
than the path MTU to prevent fragmentation. Speex frames MUST NOT be
fragmented across multiple RTP packets,
An RTP packet MAY contain Speex frames of the same bit rate or of
varying bit rates, since the bit-rate for a frame is conveyed in band
with the signal.
The encoding and decoding algorithm can change the bit rate at any 20
msec frame boundary, with the bit rate change notification provided
in-band with the bit stream. Each frame contains both "mode"
(narrowband, wideband or ultra-wideband) and "sub-mode" (bit-rate)
information in the bit stream. No out-of-band notification is
required for the decoder to process changes in the bit rate sent by
the encoder.
It is RECOMMENDED that values of 8000, 16000 and 32000 be used for
normal internet telephony applications, though the sample rate is
supported at rates as low as 6000 Hz and as high as 48 kHz.
The RTP payload MUST be padded to provide an integer number of octets
as the payload length. These padding bits are LSB aligned in network
octet order and consist of a 0 followed by all ones (until the end of
the octet). This padding is only required for the last frame in the
packet, and only to ensure the packet contents ends on an octet
boundary.
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6. Example Speex packet
In the example below we have a single Speex frame with 5 bits of
padding to ensure the packet size falls on an octet boundary.
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 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
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
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..speex data.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..speex data.. |0 1 1 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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7. Multiple Speex frames in a RTP packet
Below is an example of two Speex frames contained within one RTP
packet. The Speex frame length in this example fall on an octet
boundary so there is no padding.
Speex codecs [9] are able to detect the the bitrate from the payload
and are responsible for detecting the 20 msec boundaries between each
frame.
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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..speex data.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..speex data.. | ..speex data.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..speex data.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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8. MIME registration of Speex
Full definition of the MIME [3] type for Speex will be part of the
Ogg Vorbis MIME type definition application [8].
MIME media type name: audio
MIME subtype: speex
Optional parameters:
Required parameters: to be included in the Ogg MIME specification.
Encoding considerations:
Security Considerations:
See Section 6 of RFC 3047.
Interoperability considerations: none
Published specification:
Applications which use this media type:
Additional information: none
Person & email address to contact for further information:
Greg Herlein <gherlein@herlein.com>
Jean-Marc Valin <jean-marc.valin@hermes.usherb.ca>
Intended usage: COMMON
Author/Change controller:
Author: Greg Herlein <gherlein@herlein.com>
Change controller: Greg Herlein <gherlein@herlein.com>
Change controller: IETF AVT Working Group
This transport type signifies that the content is to be interpreted
according to this document if the contents are transmitted over RTP.
Should this transport type appear over a lossless streaming protocol
such as TCP, the content encapsulation should be interpreted as an
Ogg Stream in accordance with [8], with the exception that the
content of the Ogg Stream may be assumed to be Speex audio and Speex
audio only.
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9. SDP usage of Speex
When conveying information by SDP [4], the encoding name MUST be set
to "speex". An example of the media representation in SDP for
offering a single channel of Speex at 8000 samples per second might
be:
m=audio 8088 RTP/AVP 97
a=rtpmap:97 speex/8000
Note that the RTP payload type code of 97 is defined in this media
definition to be 'mapped' to the speex codec at an 8kHz sampling
frequency using the 'a=rtpmap' line. Any number from 96 to 127 could
have been chosen (the allowed range for dynamic types).
The value of the sampling frequency is typically 8000 for narrow band
operation, 16000 for wide band operation, and 32000 for ultra-wide
band operation.
If for some reason the offerer has bandwidth limitations, the client
may use the "b=" header, as explained in SDP [4]. The following
example illustrates the case where the offerer cannot receive more
than 10 kbit/s.
m=audio 8088 RTP/AVP 97
b=AS:10
a=rtmap:97 speex/8000
In this case, if the remote part agrees, it should configure its
Speex encoder so that it does not use modes that produce more than 10
kbit/s. Note that the "b=" constraint also applies on all payload
types that may be proposed in the media line ("m=").
An other way to make recommendations to the remote Speex encoder is
to use its specific parameters via the a=fmtp: directive. The
following parameters are defined for use in this way:
ptime: duration of each packet in milliseconds.
sr: actual sample rate in Hz.
ebw: encoding bandwidth - either 'narrow' or 'wide' or 'ultra'
(corresponds to nominal 8000, 16000, and 32000 Hz sampling rates).
vbr: variable bit rate - either 'on' 'off' or 'vad' (defaults
to off). If on, variable bit rate is enabled. If off, disabled.
If set to 'vad' then constant bit rate is used but silence will be
encoded with special short frames to indicate a lack of voice for
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that period.
cng: comfort noise generation - either 'on' or 'off'. If off
then silence frames will be silent; if 'on' then those frames will
be filled with comfort noise.
mode: Speex encoding mode. Can be {1,2,3,4,5,6,any} defaults to
3 in narrowband, 6 in wide and ultra-wide.
penh: use of perceptual enhancement. 1 indicates to the decoder
that perceptual enhancement is recommended, 0 indicates that it is
not. Defaults to on (1).
Examples:
m=audio 8008 RTP/AVP 97
a=rtpmap:97 speex/8000
a=fmtp:97 mode=4
This examples illustrate an offerer that wishes to receive a Speex
stream at 8000Hz, but only using speex mode 3.
The offerer may suggest to the remote decoder to activate its
perceptual enhancement filter like this:
m=audio 8088 RTP/AVP 97
a=rtmap:97 speex/8000
a=fmtp:97 penh=1
Several Speex specific parameters can be given in a single a=fmtp
line provided that they are separated by a semi-colon:
a=fmtp:97 mode=any;penh=1
The offerer may indicate that it wishes to send variable bit rate
frames with comfort noise:
m=audio 8088 RTP/AVP 97
a=rtmap:97 speex/8000
a=fmtp:97 vbr=on;cng=on
The "ptime" attribute is used to denote the packetization interval
(ie, how many milliseconds of audio is encoded in a single RTP
packet). Since Speex uses 20 msec frames, ptime values of multiples
of 20 denote multiple Speex frames per packet. Values of ptime which
are not multiples of 20 MUST be ignored and clients MUST use the
default value of 20 instead.
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In the example below the ptime value is set to 40, indicating that
there are 2 frames in each packet.
m=audio 8008 RTP/AVP 97
a=rtpmap:97 speex/8000
a=ptime:40
Note that the ptime parameter applies to all payloads listed in the
media line and is not used as part of an a=fmtp directive.
Values of ptime not multiple of 20 msec are meaningless, so the
receiver of such ptime values MUST ignore them. If during the life
of an RTP session the ptime value changes, when there are multiple
Speex frames for example, the SDP value must also reflect the new
value.
Care must be taken when setting the value of ptime so that the RTP
packet size does not exceed the path MTU.
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10. ITU H.323/H.245 Use of Speex
Application is underway to make Speex a standard ITU codec. However,
until that is finalized, Speex MAY be used in H.323 [5] by using a
non-standard codec block definition in the H.245 [6] codec capability
negotiations.
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11. NonStandardMessage format
For Speex use in H.245 [6] based systems, the fields in the
NonStandardMessage should be:
t35CountryCode = Hex: B5
t35Extension = Hex: 00
manufacturerCode = Hex: 0026
[Length of the Binary Sequence (8 bit number)]
[Binary Sequence consisting of an ASCII string, no NULL
terminator]
The binary sequence is an ascii string merely for ease of use. The
string is not null terminated. The format of this string is
speex [optional variables]
The optional variables are identical to those used for the SDP a=fmtp
strings discussed in section 5 above. The string is built to be all
on one line, each key-value pair separated by a semi-colon. The
optional variables MAY be omitted, which causes the default values to
be assumed. They are:
ebw=narrow;mode=3;vbr=off;cng=off;ptime=20;sr=8000;penh=no;
The fifth octet of the block is the length of the binary sequence.
NOTE: this method can result in the advertising of a large number of
Speex 'codecs' based on the number of variables possible. For most
VoIP applications, use of the default binary sequence of 'speex' is
RECOMMENDED to be used in addition to all other options. This
maximizes the chances that two H.323 based applications that support
Speex can find a mutual codec.
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12. RTP Payload Types
Dynamic payload type codes MUST be negotiated 'out-of-band' for the
assignment of a dynamic payload type from the range of 96-127. H.323
applications MUST use the H.245 H2250LogicalChannelParameters
encoding to accomplish this.
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13. 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. 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.
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14. Acknowledgments
The authors would like to thank Equivalence Pty Ltd of Australia for
their assistance in attempting to standardize the use of Speex in
H.323 applications, and for implementing Speex in their open source
OpenH323 stack. The authors would also like to thank Brian C. Wiles
<brian@streamcomm.com> of StreamComm for his assistance in developing
the proposed standard for Speex use in H.323 applications.
The authors would also like to thank the following members of the
Speex and AVT communities for their input: Ross Finlayson, Federico
Montesino Pouzols, Henning Schulzrinne, Magnus Westerlund.
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15. References
15.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119.
[2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for real-time applications", RFC
3550.
[3] "Multipurpose Internet Mail Extensions (MIME) Part One: Format
of Internet Message Bodies", RFC 2045.
[4] Jacobson, V. and M. Handley, "SDP: Session Description
Protocol", RFC 2327.
[5] "Packet-based Multimedia Communications Systems", ITU-T
Recommendation H.323.
[6] "Control of communications between Visual Telephone Systems and
Terminal Equipment", ITU-T Recommendation H.245.
[7] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control.", RFC 3551.
[8] Walleij, L., "The application/ogg Media Type", RFC 3534.
15.2 Informative References
[9] "Speexenc/speexdec, reference command-line encoder/decoder",
Speex website http://www.speex.org/.
[10] "CELP, U.S. Federal Standard 1016.", National Technical
Information Service (NTIS) website http://www.ntis.gov/.
Authors' Addresses
Greg Herlein
2034 Filbert Street
San Francisco, California 94123
United States
EMail: gherlein@herlein.com
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Simon Morlat
35, av de Vizille App 42
Grenoble 38000
France
EMail: simon.morlat@linphone.org
Jean-Marc Valin
Department of Electrical and Computer Engineering
University of Sherbrooke
2500 blvd Universite
Sherbrooke, Quebec J1K 2R1
Canada
EMail: jean-marc.valin@hermes.usherb.ca
Roger Hardiman
49 Nettleton Road
Cheltenham, Gloucestershire GL51 6NR
England
EMail: roger@freebsd.org
Phil Kerr
England
EMail: phil@plus24.com
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Acknowledgment
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Herlein, et al. Expires July 2, 2005 [Page 22]
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