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

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
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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
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   tain to the implementation or use of the technology described in this
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   The IETF invites any interested party to bring to its attention any
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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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