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Network Working Group R. Kreuter Request for Comments: 4040 Siemens AG Category: Standards Track April 2005

        RTP Payload Format for a 64 kbit/s Transparent Call

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).


 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..................................................  2
 2.  Conventions Used in This Document.............................  2
 3.  64 kbit/s Data Stream Handling and RTP Header Parameters......  3
 4.  IANA Considerations...........................................  3
 5.  Mapping to Session Description Protocol (SDP) Parameters......  5
 6.  Security Considerations.......................................  5
 7.  References....................................................  6
     7.1. Normative References.....................................  6
     7.2. Informative References...................................  6
 8.  Acknowledgements..............................................  7

Kreuter Standards Track [Page 1] RFC 4040 64 kbit/s Voice Band Data Call April 2005

1. Introduction

 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
 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 [10] 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; i.e. 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 on or allow any data processing within the VoIP
 Media Gateway except for packetization or depacketization.  ISDN data
 terminals i.e. will produce data streams that are not compatible with
 a non-linear encoding as used for voice.
 For such applications, there is a necessity for a transparent relay
 of 64 kbit/s data streams in real-time transport protocol (RTP) [4]
 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 [1], 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 2045 [2] and RFC 2048 [3], 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 [8].

Kreuter Standards Track [Page 2] RFC 4040 64 kbit/s Voice Band Data Call April 2005

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.
 The payload packetization/depacketization for Clearmode is similar to
 the Pulse Code Modulation (PCMU or PCMA) handling described in RFC
 3551 [5].  Each Clearmode octet SHALL be octet-aligned in an 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 [4].
 The marker bit is always zero.  Silence suppression is not applicable
 for Clearmode data streams.
 The payload type is dynamically assigned and is not presented in this
 RTP header fields not mentioned here SHALL be used as specified in
 RFC 3550 [4] 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.
 Subject: Registration of MIME media type audio/clearmode
 MIME media type name: audio

Kreuter Standards Track [Page 3] RFC 4040 64 kbit/s Voice Band Data Call April 2005

 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
           2327 [6].
           "maxptime" represents the maximum amount of media, which
           can be encapsulated in each packet, expressed as time in
           milliseconds, as described in RFC 3267 [9].
 Encoding considerations:
           This type is only defined for transfer via RTP [4].
 Security considerations:
           See Section 6 of RFC 4040
 Interoperability considerations: none
 Published specification: RFC 4040
 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 [6], [7], for
           one example.  This sharing is an important application and
           would not be possible otherwise.
 Additional information: none
 Intended usage: COMMON
 Author/Change controller:
           IETF Audio/Video transport working group
           delegated from the IESG

Kreuter Standards Track [Page 4] RFC 4040 64 kbit/s Voice Band Data Call April 2005

5. Mapping to Session Description Protocol (SDP) Parameters

 Parameters are mapped to SDP [6] 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
 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 [4].  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 [11].
 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.

Kreuter Standards Track [Page 5] RFC 4040 64 kbit/s Voice Band Data Call April 2005

 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 [5], section 2, for a
 discussion of this issue.

7. References

7.1. Normative References

 [1]  Handley, M. and C. Perkins, "Guidelines for Writers of RTP
      Payload Format Specifications", BCP 36, RFC 2736, December 1999.
 [2]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part One: Format of Internet Message Bodies",
      RFC 2045, November 1996.
 [3]  Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
      Mail Extensions (MIME) Part Four: Registration Procedures", BCP
      13, RFC 2048, November 1996.
 [4]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
      "RTP: A Transport Protocol for Real-Time Applications", STD 64,
      RFC 3550, July 2003.
 [5]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
      Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
 [6]  Handley, M. and V. Jacobson, "SDP: Session Description
      Protocol", RFC 2327, April 1998.
 [7]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
      Session Description Protocol (SDP)", RFC 3264, June 2002.
 [8]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [9]  Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "Real-
      Time Transport Protocol (RTP) Payload Format and File Storage
      Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate
      Wideband (AMR-WB) Audio Codecs", RFC 3267, June 2002.

7.2. Informative References

 [10] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L.,
      Lin, H., Juhasz, I., Holdrege, M., and C. Sharp, "Framework
      Architecture for Signaling Transport", RFC 2719, October 1999.

Kreuter Standards Track [Page 6] RFC 4040 64 kbit/s Voice Band Data Call April 2005

 [11] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
      Norrman, "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.

Author's Address

 Ruediger Kreuter
 Siemens AG
 81730 Munich, Germany

Kreuter Standards Track [Page 7] RFC 4040 64 kbit/s Voice Band Data Call April 2005

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 Copyright (C) The Internet Society (2005).
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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Kreuter Standards Track [Page 8]

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