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Network Working Group                                         R. KreuterRequest for Comments: 4040                                    Siemens AGCategory: Standards Track                                     April 2005RTP Payload Format for a 64 kbit/s Transparent CallStatus 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 Contents1.  Introduction..................................................22.  Conventions Used in This Document.............................23.  64 kbit/s Data Stream Handling and RTP Header Parameters......34.  IANA Considerations...........................................35.  Mapping to Session Description Protocol (SDP) Parameters......56.  Security Considerations.......................................57.  References....................................................67.1. Normative References.....................................67.2. Informative References...................................68.  Acknowledgements..............................................7Kreuter                     Standards Track                     [Page 1]

RFC 4040             64 kbit/s Voice Band Data Call           April 20051.  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 toRFC 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 withRFC 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 toRFC 2045 [2] andRFC 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 inRFC 2119 [8].Kreuter                     Standards Track                     [Page 2]

RFC 4040             64 kbit/s Voice Band Data Call           April 20053.  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 inRFC3551 [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 inRFC 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 inRFC 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: audioKreuter                     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 inRFC2327 [6].             "maxptime" represents the maximum amount of media, which             can be encapsulated in each packet, expressed as time in             milliseconds, as described inRFC 3267 [9].   Encoding considerations:             This type is only defined for transfer via RTP [4].   Security considerations:             SeeSection 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 IESGKreuter                     Standards Track                     [Page 4]

RFC 4040             64 kbit/s Voice Band Data Call           April 20055.  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   theRFC 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 toRFC 3551 [5], section 2, for a   discussion of this issue.7.  References7.1.  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",BCP13,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)",RFC3711, 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.comKreuter                     Standards Track                     [Page 7]

RFC 4040             64 kbit/s Voice Band Data Call           April 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Kreuter                     Standards Track                     [Page 8]