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).

Abstract

   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

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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [8].

3.  64 kbit/s Data Stream Handling and RTP Header Parameters

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

   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
   document.

   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.

   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
             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

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
       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.

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.

   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.

   [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

   EMail: ruediger.kreuter@siemens.com

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