Network Working Group Internet Draft P. Jones Cisco Systems, Inc. Expires: December 2004 H. Tamura Ricoh Company, Ltd. June 2004 Real-Time Facsimile (T.38) - audio/t38 MIME Sub-type Registration Status of this Memo By submitting this Internet-Draft, we certify that any applicable patent or other IPR claims of which we am aware have been disclosed and any of which we become aware will be disclosed, in accordance with RFC 3668 (BCP 79). By submitting this Internet-Draft, we 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 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.] Abstract This document defines the MIME sub-type audio/t38. The 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 Jones Expires - December 2004 [Page 1] MIME Sub-Type audio/t38 March 2004 3. Mechanisms for Transporting T.38 over an IP Network............2 4. IANA Considerations............................................3 5. SDP Mapping of MIME Parameters.................................5 6. Security Considerations........................................5 7. Normative References...........................................6 8. Informative References.........................................6 9. Author's Address...............................................7 10. Full Copyright Statement......................................7 1. Introduction ITU-T Recommendation T.38 [1] defines the Internet Facsimile Protocol (IFP) for carriage of facsimile data 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 [11] 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 and protecting the integrity of 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 completed within the IETF, e.g. Secure RTP (SRTP) [10]. There are clear advantages in using RTP for T.38 today. For example, using RTP allows one to take advantage of the redundancy [12], header compression [13][14], 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 Jones Expires - December 2004 [Page 2] MIME Sub-Type audio/t38 March 2004 understand RTP. This includes applications such as firewalls, NAT devices, and gateways that bridge two IP networks, which are generally support RTP before most other real-time media. 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 SHOULD be 8000Hz. The clock frequency MAY be set to any value, but SHOULD be set to the same value as for any audio packets in the same RTP stream in order to avoid RTP timestamp rate switching. T38FaxRateManagement: Indicates the fax rate management model as defined in T.38. Values may be "localTCF" or "transferredTCF". This parameter is defined in ITU-T Recommendation T.38. Optional parameters: T38FaxFillBitRemoval: Indicates the capability to remove and insert fill bits in Phase C (refer to [6]), non-ECM data to reduce bandwidth. This is a boolean parameter (inclusion = true, exclusion = false). This parameter is defined in ITU-T Recommendation T.38. T38FaxTranscodingMMR: Indicates the ability to convert to/from MMR from/to the line format for increasing the compression of the data Jones Expires - December 2004 [Page 3] MIME Sub-Type audio/t38 March 2004 and reducing the bandwidth in the packet network. This is a boolean parameter (inclusion = true, exclusion = false). This parameter is defined in ITU-T Recommendation T.38. T38FaxTranscodingJBIG: Indicates the ability to convert to/from JBIG to reduce bandwidth. This is a boolean parameter (inclusion = true, exclusion = false). This parameter is defined in ITU-T Recommendation T.38. T38FaxVersion: This is the version number of ITU-T Rec. T.38. New versions shall be compatible with previous versions. Absence of this parameter indicates version 0. The version is expressed as an integer value. This parameter is defined in ITU-T Recommendation T.38. T38FaxMaxBuffer: Indicates the maximum number of octets that can be stored on the remote device before an overflow condition occurs. It is the responsibility of the transmitting application to limit the transfer rate to prevent an overflow. The negotiated data rate should be used to determine the rate at which data is being removed from the buffer. Value is an integer. This parameter is defined in ITU-T Recommendation T.38. T38FaxMaxDatagram: The maximum size of the payload within an RTP packet that can be accepted by the remote device. This is an integer value. This parameter is defined in ITU-T Recommendation T.38. 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 6 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 [8], SIP [9], and H.248 [7] systems. Published specification: ITU-T Recommendation T.38, "Procedures for real-time Group 3 facsimile communication over IP networks", (2002) with Amendment 2, January 2004. Jones Expires - December 2004 [Page 4] MIME Sub-Type audio/t38 March 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 described in section 4 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. The MIME type defines several required and optional parameters to qualify the operation of T.38; these are to be used as defined in RFC 3555 [5], Section 2. The parameters are provided as a semi-colon separated list of "parameter" or "parameter=value" pairs using the "a=fmtp" parameter defined in SDP [2]; the "parameter" form is used for boolean values, where presence equals "true" and absence "false". Consider the following example, which describes a media stream that allows the transport of G.711 audio and T.38 fax information: m=audio 6800 RTP/AVP 0 98 a=rtpmap:98 t38/8000 a=fmtp:98 T38FaxVersion=2;T38FaxRateManagement=transferredTCF 6. Security Considerations Jones Expires - December 2004 [Page 5] MIME Sub-Type audio/t38 March 2004 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 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 specification and any applicable RTP profile (for example, [10]) 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", 2002 with Amendment 2, 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] Casner, S., Hoschka, P., "MIME Type Registration of RTP Payload Formats", RFC 3555, July 2003. [6] ITU-T Recommendation T.30, "Procedures for document facsimile transmission in the general switched telephone network", July 2003. 8. Informative References [7] ITU-T Recommendation H.248, "Gateway Control Protocol", May 2002. [8] ITU-T Recommendation H.323, "Packet-based multimedia communications systems", May 2003. [9] 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. [10] Baugher, et al., "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. Jones Expires - December 2004 [Page 6] MIME Sub-Type audio/t38 March 2004 [11] Parsons, G., "Real-time Facsimile (T.38) - image/t38 MIME Sub- type Registration", RFC 3362, August 2002. [12] Perkins, C., et al., "RTP Payload for Redundant Audio Data", RFC 2198, September 1997. [13] Casner, S., Jacobson, V., "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC 2508, February 1999. [14] Koren, T., et al, "Enhanced Compressed RTP (CRTP) for Links with High Delay, Packet Loss and Reordering", RFC 3545, July 2003. 9. Author's Address Paul E. Jones Cisco Systems, Inc. 7025 Kit Creek Rd. Research Triangle Park, NC 27709, USA Phone: +1 919 392 6948 Email: paulej@packetizer.com Hiroshi Tamura Ricoh Company, Ltd. 1-3-6 Nakamagome, Ohta-ku, Tokyo 143-8555 Japan Phone: +81-3-3777-8124 Fax: +81-3-5742-8859 Email: tamura@toda.ricoh.co.jp 10. Full Copyright Statement Copyright (C) The Internet Society (2004). 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. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 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 at http://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. Jones Expires - December 2004 [Page 8]