RFC 4612

Real-Time Facsimile (T.38) - audio/t38 MIME Sub-type Registration

📅 published August 2006, from ietf

RFC 4612: Real-Time Facsimile (T.38) - audio/t38 MIME Sub-type Registration 🦋 (Part 1 of 2)

Okay bestie, this is the RFC that registers the audio/t38 MIME type so fax can ride inside the same RTP stream as audio. ✨

Here's the title block, summarised: it's RFC 4612, published in the Network Working Group stream in August 2006. Its authors are P. Jones from Cisco Systems, Inc. and H. Tamura from Ricoh Company, LTD. Its category is Historic. The header doesn't list anything it obsoletes or updates. 💅

Status of This Memo 📜

This memo defines a Historic Document for the Internet community. It does not specify an Internet standard of any kind, and distribution of this memo is unlimited. 🌸

Copyright Notice ©️

Abstract 💖

This document defines the MIME sub-type audio/t38. The way this MIME type gets used, which is meant for the Session Description Protocol (SDP), is specified in 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 ..................................5
6. Security Considerations .........................................6
7. Normative References ............................................6
8. Informative References ..........................................6

1. Introduction 🌈

ITU-T Recommendation T.38 [1] defines the Internet Facsimile Protocol (IFP) for carrying fax 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 gives the reasoning for using RTP as a transport for fax signaling, and it specifies the MIME type associated with that 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 🌐

Okay so, when T.38 was first approved in 1998, it allowed transport over UDP (using UDP Transport Layer (UDPTL), rather than RTP) or over TCP. As of the time of this publication, UDPTL is the predominant means for transporting T.38 data over an IP network. To support that, RFC 3362 [11] was published so devices could signal their desire to use UDPTL to transport T.38. 💅

BUT. A number of issues came up about using UDPTL for the long term. The main concern was that UDPTL doesn't give the same kind of statistics reporting as RTP Control Protocol (RTCP). There are also no procedures in place for encrypting the UDPTL stream or protecting its integrity. That could be fixed inside UDPTL, but it would take a lot of effort and would mostly duplicate the security work already done within the IETF, such as Secure RTP (SRTP) [10]. 🙈

There are clear advantages to using RTP for T.38 today. RTP lets you take advantage of the redundancy [12], header compression [13][14], and other RTP-related work within the IETF. Using RTP instead of UDPTL also gives better interoperability with a wider range of devices that know and understand RTP. That includes firewalls, Network Address Translation (NAT) devices, and gateways that bridge two IP networks, which generally support RTP before most other real-time media. 🫶

Lastly, most T.38 data today is generated by gateways that bridge two Public Switched Telephone Network (PSTN) networks. So it's really natural for the transition from audio to fax to happen within the same media stream. The T.38 data is just another way of representing information received on the PSTN circuit. If T.38 data is encapsulated in RTP, gateways can move from audio to fax and back again easily, and they can just use the payload type to show what kind of media they're currently sending. ✨

With all that in mind, the ITU-T amended T.38 [1] to allow RTP to be used to transport T.38. So a new MIME registration (audio/t38) is needed to let T.38 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 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: This is the RTP timestamp clock rate. It 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: This 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: This shows the capability to remove and insert fill bits in Phase C (refer to [6]), non-ECM data to reduce bandwidth. It's a boolean parameter, where inclusion = true and exclusion = false. This parameter is defined in ITU-T Recommendation T.38. 🌸
T38FaxTranscodingMMR: This shows the ability to convert to and from MMR from and to the line format, which increases the compression of the data and reduces the bandwidth in the packet network. It's a boolean parameter, where inclusion = true and exclusion = false. This parameter is defined in ITU-T Recommendation T.38. 💜
T38FaxTranscodingJBIG: This shows the ability to convert to and from JBIG to reduce bandwidth. It's a boolean parameter, where inclusion = true and 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. If this parameter is absent, that indicates version 0. The version is expressed as an integer value. This parameter is defined in ITU-T Recommendation T.38. 🔢
T38FaxMaxBuffer: This indicates the maximum number of octets that can be stored on the remote device before an overflow condition occurs. It's 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 removed from the buffer. The value is an integer. This parameter is defined in ITU-T Recommendation T.38. 🪣
T38FaxMaxDatagram: This is the maximum size of the payload within an RTP packet that the remote device can accept. It's 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 4612. 🔐

Interoperability considerations: ITU-T Recommendation T.38 defines the procedures, syntax, and parameters for carrying 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", 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. Jones 🧑‍💻

That's the first half, bestie. The rest of the doc picks up with the SDP mapping of these parameters, then Security Considerations, then the References. 🎀

8. Informative References 📚

okay bestie, these are the "extra reading" references, the ones the spec mentions for background but doesn't require you to implement 🤓 ✨

[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, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, 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. 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 🧑‍💻

these are the people who wrote it, so you know who to thank (or blame) 💖

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.jp

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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