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Network Working Group                                           P. JonesRequest for Comments: 4612                           Cisco Systems, Inc.Category: Historic                                             H. Tamura                                                     Ricoh Company, LTD.                                                             August 2006Real-Time Facsimile (T.38) - audio/t38MIME Sub-type RegistrationStatus of This Memo   This memo defines a Historic Document for the Internet community.  It   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   This document defines the MIME sub-type audio/t38.  The usage of this   MIME type, which is intended for use within Session Description   Protocol (SDP), is specified within ITU-T Recommendation T.38.Table of Contents1. Introduction ....................................................22. Conventions Used in This Document ...............................23. Mechanisms for Transporting T.38 over an IP Network .............24. IANA Considerations .............................................35. SDP Mapping of MIME Parameters ..................................56. Security Considerations .........................................67. Normative References ............................................68. Informative References ..........................................6Jones & Tamura                  Historic                        [Page 1]

RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 20061.  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 inRFC 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 UDP Transport Layer (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 RTP Control Protocol (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   understand RTP.  This includes applications such as firewalls,   Network Address Translation (NAT) devices, and gateways that bridge   two IP networks, which generally support RTP before most other real-   time media.Jones & Tamura                  Historic                        [Page 2]

RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006   Lastly, since today most T.38 data is generated by gateways that   bridge two Public Switched Telephone Network (PSTN) networks, it is   quite natural to expect that 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 has been   registered, by the IANA 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   Required parameters:      rate:  The RTP timestamp clock rate, which SHOULD be 8000Hz.  The      clock frequency MAY be set to any value, but it SHOULD be set to      the same value as that 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.Jones & Tamura                  Historic                        [Page 3]

RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006      T38FaxTranscodingMMR: Indicates the ability to convert to/from MMR      from/to the line format for increasing the compression of the data      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:      SeeSection 6 of RFC 4612.   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.Jones & Tamura                  Historic                        [Page 4]

RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006   Published specification:      ITU-T Recommendation T.38, "Procedures for real-time Group 3      facsimile communication over IP networks", September 2005   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. Jones5.  SDP Mapping of MIME Parameters   The MIME information described inSection 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 inRFC3555 [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=transferredTCFJones & Tamura                  Historic                        [Page 5]

RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 20066.  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   manipulated in ways that are more obtrusive than audio.  For example,   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", September 2005.   [2]  Handley, M. and V. Jacobson, "SDP: Session Description        Protocol",RFC 2327, April 1998.   [3]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,        "RTP: A Transport Protocol for Real-Time Applications", STD 64,RFC 3550, July 2003.   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [5]  Casner, S. and P. Hoschka, "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.Jones & Tamura                  Historic                        [Page 6]

RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006   [10] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.        Norrman, "The Secure Real-time Transport Protocol (SRTP)",RFC3711, 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",RFC2198, September 1997.   [13] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for        Low-Speed Serial Links",RFC 2508, February 1999.   [14] Koren, T., Casner, S., Geevarghese, J., Thompson, B., and P.        Ruddy, "Enhanced Compressed RTP (CRTP) for Links with High        Delay, Packet Loss and Reordering",RFC 3545, July 2003.Authors' Addresses   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@cs.ricoh.co.jpJones & Tamura                  Historic                        [Page 7]

RFC 4612        Real-time Facsimile (T.38) - audio /t38      August 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   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 provided by the IETF   Administrative Support Activity (IASA).Jones & Tamura                  Historic                        [Page 8]