Audio/Video Transport
Internet Draft R. Kreuter
Document: draft-ietf-avt-rtp-clearmode-00.txt Siemens AG
Expires: October 2003 April 2003
RTP payload format for a 64 kbit/s transparent call
Status of this Memo
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Abstract
This document describes how to carry 64 kbit/s data streams trans-
parently in RTP packets, using a pseudo-codec called "Clearmode".
It also serves as registration for a related MIME type called
"audio/clearmode".
"Clearmode" is a basic feature of VoIP media gateways.
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 RFC2119 [2].
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Table of Contents
1. Introduction..................................................2
2. 64 kbit/s data stream handling and RTP header parameters......2
3. Registration of audio/clearmode...............................3
4. Mapping to SDP parameters.....................................4
5. Security Considerations.......................................5
6. References....................................................5
7. Author's Address..............................................6
8. IPR Notice....................................................6
9. Full Copyright Statement......................................6
1. Introduction
Voice over IP media gateways need to carry all data streams generated
by analog or ISDN terminals via an IP network.
ISDN wideband speech terminals do not rely on a voice data processing
(e.g. echo cancellation or DTMF detection) within a Voice over IP
media gateway. And ISDN data terminals e.g. will produce data streams
that are not compatible with a non-linear encoding as is used for
voice.
For such applications, there exists a necessity for a transparent
relay of 64 kbit/s data streams in RTP packets. This mode is often
referred to as "clear-channel data" or "64 kbit/s unrestricted". No
encoder/decoder is needed in that case, but a unique RTP [6] payload
type is necessary and a related MIME type is to be registered for
signaling purposes.
This payload format document describes a pseudo-codec called
"Clearmode", for sample-oriented 64 kbit/s data streams with 8 bits
per sample. It is in accordance with RFC 2736 [3], which provides a
guideline for the specification of new RTP payload formats.
Examples for the use of Clearmode in current VoIP media gateways are
the transfer of "ISDN 7 kHz voice" and "ISDN data".
This document also serves as the MIME type registration according RFC
2048 [5], which defines procedures for registration of new MIME types
within the IETF tree.
2. 64 kbit/s data stream handling and RTP header parameters
Clearmode does not use any encoding or decoding. It just provides
packetization.
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Clearmode assumes that the data to be handled is sample oriented with
one octet (8bits) per sample. There is no restriction on the number
of samples per packet.
The payload handling for Clearmode is similar to the PCMU or PCMA
handling described in [7]. Each Clearmode octet SHALL be octet-
aligned in a RTP packet.
A sample rate of 8000 Hz is used.
This calculates to a 64 kbit/s transmission rate per channel.
The Timestamp SHALL be set according to [6].
The marker bit is always zero.
Silence suppression is not applicable for Clearmode data streams.
The payload type is dynamically assigned by means outside the scope
of this document.
This document specifies the use of RTP over unicast and multicast UDP
as well as TCP.
(This does not preclude the use of this definition when RTP is
carried by other lower-layer protocols.)
3. Registration of audio/clearmode
To: ietf-types@iana.org
Subject: Registration of MIME media type audio/clearmode
MIME media type name: audio
MIME subtype name: clearmode
Required parameters: none
Optional parameters: ptime, maxptime
"ptime" gives the length of time in milliseconds represented
by the media in a packet [9].
"maxptime" represents the maximum amount of media which can be
encapsulated in each packet, expressed as time in milliseconds
[9].
Encoding considerations:
This type is defined for transfer via RTP [6].
Security considerations:
Implementations using the profile defined in this
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specification are subject to the security considerations
discussed in the RTP specification [6].
Interoperability considerations: none
Published specification: This document
Applications which use this media type:
Voice over IP Media Gateways, transferring "ISDN 64 kb/s data"
or "ISDN 7 kHz voice" or other VoIP-related 64 kbit/s data
streams via a RTP connection.
Additional information: none
Intended usage: COMMON
Author/Change controller:
This registration is part of the IETF registration tree.
4. Mapping to SDP parameters
Parameters are mapped to SDP [9] in a standard way.
o The MIME type (audio) goes in SDP "m=" as the media name.
o The MIME subtype (clearmode) goes in SDP "a=rtpmap" as the
encoding name.
o The optional parameters "ptime" and "maxptime" go in the SDP
"a=ptime" and "a=maxptime" attributes, respectively.
An example mapping is as follows:
audio/clearmode; ptime=10
m=audio 12345 RTP/AVP 97
a=rtpmap:97 CLEARMODE/8000
a=ptime:10
Note that the payload format (encoding) names defined in the RTP
Profile are commonly shown in upper case. MIME subtypes are commonly
shown in lower case. These names are case-insensitive in both
places.
Similarly, parameter names are case-insensitive both in MIME types
and in the default mapping to the SDP a=fmtp attribute.
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5. Security Considerations
Implementations using the profile defined in this specification are
subject to the security considerations discussed in the RTP
specification [6].
This profile does not specify any different security services. The
primary function of this profile is to add a transparent transport
for a 64 kbit/s data stream.
Confidentiality of the media streams is achieved by encryption. Since
there is no processing of the data stream other than packetization
and depacketization, there is no interference to an end-to-end
encryption mechanism.
6. References
[1] S. Bradner, "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[3] M. Handley and C. Perkins, "Guidelines for Writers of RTP
Payload Format Specifications", RFC 2736, December 1999
[4] N. Freed, N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies ", RFC 2045,
November 1996.
[5] N. Freed, J. Klensin and J. Postel, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures", RFC
2048, November 1996.
[6] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson, "RTP:
transport protocol for real-time applications", revision of RFC
1889 work in Progress.
[7] H. Schulzrinne, S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", revision of RFC 1890 work in
Progress
[8] S. Casner and P. Hoschka, "MIME type registration of RTP payload
formats", Work in Progress.
[9] M. Handley, V. Jacobson and C. Perkins, "SDP: Session Description
Protocol", revision of 2327, work in progress.
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[10] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson, "RTP:
a transport protocol for real-time applications", RFC 1889,
January 1996.
[11] H. Schulzrinne "RTP Profile for Audio and Video Conferences
with Minimal Control", RFC 1890, January 1996.
[12] M. Handley and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
7. Author's Address
Ruediger Kreuter
Siemens AG
81359 Munich, Germany
Phone: +49 89 722 62553
Email: ruediger.kreuter@siemens.com
8. IPR Notice
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to per-
tain to the implementation or use of the technology described in this
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this standard. Please address the information to the IETF Executive
Director.
9. Full Copyright Statement
"Copyright (C) The Internet Society (date). All Rights Reserved.
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