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RTP Payload Format for a 64 kbit/s Transparent Call

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 4040.
Author Ruediger Kreuter
Last updated 2018-12-20 (Latest revision 2004-04-22)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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IESG IESG state Became RFC 4040 (Proposed Standard)
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Responsible AD Allison J. Mankin
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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). 
   Internet-Drafts are working documents of the Internet Engineering 
   Task Force (IETF), its areas, and its working groups.  Note that 
   other groups may also distribute working documents as Internet-
   Internet-Drafts are draft documents valid for a maximum of six months 
   and may be updated, replaced, or obsoleted by other documents at any 
   time.  It is inappropriate to use Internet-Drafts as reference 
   material or cite them other than as "work in progress". 
   The list of current Internet-Drafts can be accessed at 
   The list of Internet-Draft Shadow Directories can be accessed at 
   This document is a submission of the IETF AVT WG. Comments should be 
   directed to the AVT WG mailing list, 
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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Internet-Draft      64kbit/s voice band data call          April 2004 
   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. 
   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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Internet-Draft      64kbit/s voice band data call          April 2004 
   [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 
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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