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Versions: 00 01 02 03 04 05 rfc4040                      Standards Track
   Audio/Video Transport
   Internet Draft                                            R. Kreuter
   Document: draft-ietf-avt-rtp-clearmode-05.txt             Siemens AG
   Expires: October 2004                                     April 2004

            RTP payload format for a 64 kbit/s transparent call

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware has been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668 (BCP 79).

   By submitting this Internet-Draft, I accept the provisions of Section
   3 of RFC 3667 (BCP 78).

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   This document is a submission of the IETF AVT WG. Comments should be
   directed to the AVT WG mailing list, avt@ietf.org.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document describes how to carry 64 kbit/s channel data
   transparently 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.

Table of Contents

   1. Introduction..................................................1
   2. Conventions used in this document.............................2
   3. 64 kbit/s data stream handling and RTP header parameters......2
   4. IANA Considerations...........................................3
   5. Mapping to Session Description Protocol (SDP) parameters......3
   6. Security Considerations.......................................4
   7. References....................................................4
   8. Acknowledgements..............................................5
   9. Author's Address..............................................5
   10. Full Copyright Statement.....................................5
   11. Disclaimer...................................................5

1. Introduction

   [Note to the RFC Editor: This paragraph is to be deleted when this
   draft is published as an RFC.  All references to RFC yyyy in section
   4 should be replaced by the RFC number of this draft, when published.
   All references to RFC XXXX in sections 4 and 5 should be replaced by
   the RFC number of the revision of RFC 2327, when published.]

   Voice over IP (VoIP) media gateways need to carry all possible data
   streams generated by analog terminals or integrated services digital
   network (ISDN) terminals via an IP network. Within this document a

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   VoIP media gateway is a device that converts a (digital or analog)
   linear data stream to a digital packetized data stream or vice versa.
   Refer to RFC 2719 [12] for an introduction into the basic
   architecture of a media gateway based network.

   Usually a VoIP media gateway does some processing on the data it
   converts besides packetization or depacketization; e.g. echo
   cancellation or dual tone multifrequency (DTMF) detection, and
   especially a coding/decoding. But there is a class of data streams
   that does not rely or even does not allow any data processing within
   the VoIP media gateway except for packetization or depacketization.
   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 real-time transport protocol (RTP)
   [6] 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 payload type is necessary and a related MIME
   type is to be registered for signaling purposes.

   Clearmode is not restricted to the examples described above.  It can
   be used by any application, that does not need a special encoding /
   decoding for transfer via a RTP connection.

   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 current use of Clearmode are the transfer of "ISDN 7
   kHz voice" and "ISDN data" in VoIP media gateways.

   This document also serves as the MIME type registration according to
   RFC 2048 [5], which defines procedures for registration of new MIME
   types within the IETF tree.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [11].

3. 64 kbit/s data stream handling and RTP header parameters

   Clearmode does not use any encoding or decoding. It just provides

   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 other than the 64 kbyte limit imposed by the IP
   protocol.  The number of samples SHOULD be less than the path maximum
   transmission unit (MTU) minus combined packet header length. If the
   environment is expected to have tunnels or security encapsulation as
   part of operation, the number of samples SHOULD be reduced to allow
   for the extra header space used for those.

   The payload packetization/depacketization for Clearmode is similar to
   the Pulse Code Modulation (PCMU or PCMA) handling described in RFC
   3551 [7].  Each Clearmode octet SHALL be octet-aligned in a RTP
   packet.  The sign bit of each octet SHALL correspond to the most
   significant bit of the octet in the RTP packet.

   A sample rate of 8000 Hz MUST be used.
   This calculates to a 64 kbit/s transmission rate per channel.

   The Timestamp SHALL be set as described in RFC 3550 [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.

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   RTP header fields not mentioned here SHALL be used as specified in
   RFC 3550 [6] and any applicable profile.

   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.)

4. IANA Considerations

   This document registers the following MIME subtype: 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, as described in RFC xxxx [9].

          "maxptime" represents the maximum amount of media, which can
          be encapsulated in each packet, expressed as time in
          milliseconds, as described in RFC xxxx [9].

   Encoding considerations:

          This type is only defined for transfer via RTP [6].

   Security considerations:

          See Section 6 of RFC yyyy

   Interoperability considerations: none

   Published specification: RFC yyyy

   Applications, which use this media type:

          Voice over IP Media Gateways, transferring "ISDN 64 kb/s
          data", "ISDN 7 kHz voice", or other 64 kbit/s data streams via
          an RTP connection

          Note: the choice of the "audio" top-level MIME type was made
          because the dominant uses of this pseudo-codec are expected to
          telephony and voice-gateway-related.  The "audio" type allows
          the use of sharing of the port in the SDP "m=" line with
          codecs such as audio/g711 [9], [10], for one example.  This
          sharing is an important application and would not be possible

   Additional information: none

   Intended usage: COMMON

   Author/Change controller:

          IETF Audio/Video transport working group

5. Mapping to Session Description Protocol (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.

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   An example mapping is as follows:

           audio/clearmode; ptime=10

           m=audio 12345 RTP/AVP 97
           a=rtpmap:97 CLEARMODE/8000

   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

6. Security Considerations

   Implementations using the payload format defined in this
   specification are subject to the security considerations discussed in
   the RFC 3550 [6].  The payload format described in this document does
   not specify any different security services.  The primary function of
   this payload format is to add a transparent transport for a 64 kbit/s
   data stream.

   Confidentiality of the media streams is achieved by encryption, for
   example by application of the Secure RTP profile [13].

   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.  Overload can also occur,
   if the sender chooses to use a smaller packetization period, than the
   receiver can process.  The ptime parameter can be used to negotiate
   an appropriate packetization during session setup.

   In general RTP is not an appropriate transfer protocol for reliable
   octet streams.  TCP is better in those cases.  Besides that, packet
   loss due to congestion is as much an issue for clearmode, as for
   other payload formats.  Refer to RFC 3551 [7], section 2, for a
   discussion of this issue.

7. References

   Normative References

   [1]  Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3667,
        February 2004.

   [2]  Bradner, S., Ed., "Intellectual Property Rights in IETF
        Technology", BCP 79, RFC 3668, February 2004.

   [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", BCP 13,
        RFC 2048, November 1996.

   [6]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", RFC
        3550, July 2003.

   [7]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control", RFC 3551, July 2003.

   [8]  Casner, S. and P. Hoschka, "MIME Type Registration of RTP
        Payload Types", RFC 3555, July 2003.

   [9] M. Handley, V. Jacobson and C. Perkins, draft-ietf-mmusic-sdp-
        new-xx.txt "SDP: Session Description Protocol", revision of
        2327, work in progress.

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   [10] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        SDP", RFC 3264, June 2002

   [11] S. Bradner, "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997

   Informational References

   [12] L. Ong, et. al., "Framework Architecture for Signaling
        Transport", RFC 2719, October 1999.

   [13] Baugher, et al., "The Secure Real-time Transport Protocol
        (SRTP)", RFC 3711, March 2004

8. Acknowledgements

   The editor would like to acknowledge the help of the IETF AVT Working
   Group and, in particular the help of Colin Perkins and Magnus
   Westerlund for their intensive reviews and comments.

9. Author's Address

   Ruediger Kreuter
   Siemens AG
   81730 Munich, Germany
   Email: ruediger.kreuter@siemens.com

10. Full Copyright Statement

   Copyright (C) The Internet Society (year).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

11. Disclaimer

   This document and the information contained herein are provided on an

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