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- Drafts. 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 http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html 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. 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..................................................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 Kreuter Expires - October 2004 [Page 1] 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 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 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. Kreuter Expires - October 2004 [Page 2] Internet-Draft 64kbit/s voice band data call April 2004 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 otherwise. 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. Kreuter Expires - October 2004 [Page 3] Internet-Draft 64kbit/s voice band data call April 2004 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 [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. Kreuter Expires - October 2004 [Page 4] 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 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 "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. Kreuter Expires - October 2004 [Page 5]