AVT Working Group Internet Draft P. Jones Cisco Systems, Inc. Expires: February 2004 November 2003 Real-Time Facsimile (T.38) - audio/t38 MIME Sub-type Registration Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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 to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. [Note to RFC Editor: All references to RFC XXXX are to be replaced by references to the RFC number of this memo, when published. Also, the reference to ITU-T Recommendation T.38 Amendment YYYY must be replaced with the appropriate amendment number.] Abstract This document defines the MIME sub-type audio/t38. The packetization and usage of this MIME type, which is intended for use within SDP, is specified within ITU-T Recommendation T.38. Table of Contents 1. Introduction...................................................2 2. Conventions used in this document..............................2 3. Mechanisms for Transporting T.38 over an IP Network............2 4. IANA Considerations............................................3 5. SDP Mapping of MIME Parameters.................................4 6. Security Considerations........................................4 Jones Expires - February 2004 [Page 1] MIME Sub-Type audio/t38 November 2003 7. Normative References...........................................5 8. Informative References.........................................5 9. Author's Address...............................................6 10. Full Copyright Statement......................................6 1. Introduction ITU-T Recommendation T.38 [1] defines the Internet Facsimile Protocol (IFP) and packetization of that protocol for carriage over IP networks. As one option, IFP packets may be carried within an RTP [3] stream, either as the only content within the media stream or switched with other audio payload types. This memo provides rationale for using RTP as a transport for fax signaling and specifies the MIME type associated with said signaling. 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 [4]. 3. Mechanisms for Transporting T.38 over an IP Network When T.38 was first approved in 1998, it allowed for the transport of T.38 via UDP (using UDPTL, rather than RTP) or TCP. As of the time of this publication, UDPTL is the predominant means for transporting T.38 data over an IP network. In support of that, RFC 3362 was published in order to allow devices to signal their desire to use UDPTL to transport T.38. A number of issues were raised with respect to the usage of UDPTL for the long-term, though. Specifically, there were concerns over the fact that UDPTL does not provide the same kind of statistics reporting as RTCP. Further, there are no procedures in place for encrypting the UDPTL stream. While the latter could be addressed in UDPTL, doing so would require a lot of effort and would largely be a duplication of the security work already on-going within the IETF. There are clear advantages in using RTP for T.38 today. For example, using RTP allows one to take advantage of the redundancy [11], header compression [11][12], and other RTP-related work within the IETF. Using RTP, as opposed to UDPTL, for transport provides better interoperability with a wider range of devices that know and understand RTP. This includes applications such as firewall and NAT devices, multipoint control units (MCUs), and media gateways that bridge two IP networks. Jones Expires - February 2004 [Page 2] MIME Sub-Type audio/t38 November 2003 Lastly, since most T.38 data today is generated by gateways that bridge two PSTN networks, it is quite natural to expect the transition from audio to fax should happen within the same media stream. The reason is that the T.38 data is simply an alternative representation of information received on the PSTN circuit. If the T.38 data is encapsulated in RTP, the gateways can easily transition from audio to fax and back again and can simply use the payload type to indicate the type of media that it is currently transmitting. With these considerations in mind, the ITU-T amended T.38 [1] to allow RTP to be used to transport T.38. With that, a new MIME registration (audio/t38) is needed to allow for T.38 to be switched along with audio within the same RTP session. 4. IANA Considerations One new MIME type and associated RTP payload format is to be registered, as described below. To: ietf-types@iana.org Subject: Registration of Standard MIME media type audio/t38 MIME media type name: audio MIME subtype name: t38 Require parameters: rate: The RTP timestamp clock rate, which is equal to the sampling rate. If the T.38 stream is associated with other audio streams in an RTP session, the chosen rate SHOULD match that of the other streams. Otherwise, a rate of 8000 Hz SHOULD be used. Optional parameters: none Encoding considerations: The encoding of the IFP RTP packets is defined in ITU-T Recommendation T.38. This sub-type is not intended for use with e-mail. Security considerations: See Section 5 of RFC XXXX. Interoperability considerations: ITU-T Recommendation T.38 defines the procedures, syntax, and parameters for the carriage of T.38 over RTP within the context of H.323 [7], SIP [8], and H.248 [6] systems. Jones Expires - February 2004 [Page 3] MIME Sub-Type audio/t38 November 2003 Published specification: ITU-T Recommendation T.38, "Procedures for real-time Group 3 facsimile communication over IP networks", with Amendment YYYY, January 2004. Applications which use this media type: Real-time facsimile (fax) Additional information: Magic number(s): File extension(s): Macintosh File Type Code(s): Person & email address to contact for further information: Paul E. Jones paulej@packetizer.com Intended usage: COMMON Author/Change controller: Paul E. Jones 5. SDP Mapping of MIME Parameters The MIME information is described in section 5 is utilized in SDP in order to establish T.38 media streams. Specifically: o The MIME type ("audio") goes in SDP "m=" as the media name. o The MIME subtype ("t38") goes in SDP "a=rtpmap" as the encoding name. o The parameter "rate" also goes in "a=rtpmap" as clock rate. Consider the following example, which describes a media stream that allows the transport of G.711 audio and T.38 fax information: m=audio 6808 RTP/AVP 0 98 a=rtpmap:98 t38/8000 6. Security Considerations T.38 is vulnerable to attacks that are common to other types of RTP and SRTP payloads. However, unlike audio, T.38 data may be Jones Expires - February 2004 [Page 4] MIME Sub-Type audio/t38 November 2003 manipulated in ways that are more obtrusive than audio. As examples, rogue packets may cause transmission failure and manipulated packets may alter terminal identity. The security considerations discussed in the RTP [3] and SRTP [5] specification and any applicable RTP profile (for example, [9]) are applicable to T.38. Regarding SRTP configuration, fax payloads SHOULD NOT use an HMAC-SHA1 authentication tag that is shorter than 80 bits. 7. Normative References [1] ITU-T Recommendation T.38, "Procedures for real-time Group 3 facsimile communication over IP networks",Amendment YYYY, January 2004. [2] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [3] Schulzrinne, et al., "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003. [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [5] Baugher, et al., "The Secure Real-time Transport Protocol", draft-ietf-avt-srtp-09.txt, July 2003. 8. Informative References [6] ITU-T Recommendation H.248, "Gateway Control Protocol", May 2002. [7] ITU-T Recommendation H.323, "Packet-based multimedia communications systems", May 2003. [8] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [9] Schulzrinne, H. and Casner, S. "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3551, July 2003. [10] Parsons, G., " Real-time Facsimile (T.38) - image/t38 MIME Sub- type Registration", August 2002. [11] Perkins, C., et al., "RTP Payload for Redundant Audio Data", September 1997. Jones Expires - February 2004 [Page 5] MIME Sub-Type audio/t38 November 2003 [11] Casner, S., Jacobson, V., "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", February 1999. [12] Koren, T., et al, "Enhanced Compressed RTP (CRTP) for Links with High Delay, Packet Loss and Reordering", July 2003. 9. Author's Address Paul E. Jones Cisco Systems, Inc. 7025 Kit Creek Rd. Research Triangle Park, NC 27709 Phone: +1 919 392 6948 Email: paulej@packetizer.com 10. Intellectual Property Right Considerations The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication 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 implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. 11. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Jones Expires - February 2004 [Page 6] MIME Sub-Type audio/t38 November 2003 Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. 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