RFC 4142

Full-mode Fax Profile for Internet Mail (FFPIM)

📅 published November 2005, from ietf

RFC 4142: Full-mode Fax Profile for Internet Mail (FFPIM) 📠✨

Okay bestie, RFC 4142 is the spec that lets full-mode fax travel over Internet email, and it's giving real main character energy 💅 Here's the title block so you know who's behind it 🫶

Authors: D. Crocker, Brandenburg, and G. Klyne
Working group: Network Working Group
RFC number: 4142
Category: Standards Track 👑
Date: November 2005 🗓️
Affiliation: Nine by Nine is the affiliation listed in the header 🌸
Obsoletes or updates: The header doesn't list any RFC this one obsoletes or updates 🦋

Status of This Memo

Okay so this one is an Internet standards track protocol for the Internet community 💜 It's asking for discussion and suggestions for improvements, so bring your ideas! For the current standardization state and status of this protocol, check the "Internet Official Protocol Standards" (STD 1) 📚 And yes, distribution of this memo is unlimited 🌈

Copyright Notice

Abstract

Classic facsimile is two things at once: a set of technical specs AND a whole class of service 📠 Earlier work already copied part of that service into Internet mail as a profile. This spec defines "full mode" fax carried over the Internet, builds on that earlier work, and adds the stuff still missing for reliability and capability negotiation on Internet mail, so it lines up with classic T.30 fax 💅 The goal is the highest level of interoperability with standards-compliant email infrastructure and mail user agents, plus a level of service that's pretty close to what fax users already get ✨

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2. Content Negotiation  . . . . . . . . . . . . . . . . . . . . . . 3
      2.1. UA-based Content Negotiation. . . . . . . . . . . . . . . . 3
      2.2. ESMTP-based Content Negotiation . . . . . . . . . . . . . . 3
      2.3. Interactions between UA and ESMTP Negotiation Mechanisms. . 4
   3. Content Format . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4. Security Considerations  . . . . . . . . . . . . . . . . . . . . 4
   5. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
      5.1. Normative References. . . . . . . . . . . . . . . . . . . . 5
      5.2. Informative References. . . . . . . . . . . . . . . . . . . 6
   A. Direct Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8

1. Introduction 🌸

This spec defines "full mode" fax over the Internet, building on two earlier works: A Simple Mode of Facsimile Using Internet Mail [RFC3965] and Extended Facsimile Using Internet Mail [RFC2532] 💖 It also adds the remaining stuff needed for reliable and capable negotiation on Internet mail, on par with classic [T30] fax. These extras are designed for the highest level of interoperability with standards-compliant email infrastructure and mail user agents, while still giving a service level that closely matches what fax users already enjoy 🫶

Basic terminology is discussed in [RFC2542] 📖

Implementations that conform to this specification MUST also conform to [RFC3965] and [RFC2532]. No exceptions, bestie 🙅‍♀️

The new features are built to play nicely with the existing base of mail transfer agents (MTAs) and mail user agents (MUAs) 🦋 They also use existing standards for optional stuff, like positive delivery confirmation and disposition notification. Where possible, they reuse the existing Internet email messaging infrastructure instead of inventing fax-only features that non-fax messaging software would probably never implement 🧚

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 [RFC2119] 💜

2. Content Negotiation 💅

Okay, so classic fax is interactive. A sending station can check what the receiving station can handle before it sends a document, so the sender can pick the best quality both sides support 📠 Internet mail is store-and-forward with potentially long latency, which means before-the-fact negotiation is kind of a mess 🙈

Using a negotiation mechanism lets senders transfer a richer document form than the safer-but-universal default form. Without it, the sender can't be sure the receiving station will actually support the form. Iykyk 😭

The capabilities an FFPIM participant can negotiate are specified in [RFC2534] and [RFC2879]. Implementations that are conformant to FFPIM MUST support content negotiation as described there 🔥

2.1. UA-based Content Negotiation 🎀

One method for swapping capabilities info uses a post-hoc technique. The originator can send the best version it knows the recipient supports, and then send a better-suited version if the recipient asks for it 💖 This mechanism is specified in [RFC3297]. FFPIM implementations MUST support this mechanism ✨

2.2. ESMTP-based Content Negotiation 🧚

Another method uses an ESMTP option specified in [RFC4141]. It needs content negotiation support along the entire path the email travels 🛤️ With this mechanism, receiving ESMTP servers can report the capabilities of the addresses (mailboxes) they support, and sending email clients can signal both permission and constraints on conversions 💜

FFPIM participants MAY support this mechanism 🌈

NOTE: This specification provides for content conversion by unspecified intermediaries. Use of this mechanism carries significant risk. Although intermediaries always have the ability to perform damaging transformations, use of this specification could result in more exploitation of that potential and, therefore, more misbehavior. Use of intermediaries is discussed in [RFC3238]. 🚨

2.3. Interactions between UA and ESMTP Negotiation Mechanisms 🦋

FFPIM participants must ensure that their use of the UA and ESMTP methods for content negotiation is compatible. For example, two mechanisms might consult two different repositories of capabilities information, and those repositories might contain different information. Presumably, that means at least one of those repositories is inaccurate 😬 So the bigger problem is correctness, not synchronization.

This specification does not require a particular method of using the mechanisms together 🫶

3. Content Format 📠

FFPIM allows the transfer of enhanced TIFF data relative to [RFC3965] and [RFC2532]. The details for these enhancements are in [RFC3949]. Implementations that are conformant to FFPIM SHOULD support TIFF enhancements ✨

It should also be noted that the content negotiation mechanism lets a sender know the full range of content types the recipient supports. Therefore, requirements for support of TIFF represent a functional minimum for FFPIM 👑

Appendix A. Direct Mode 🧚

Okay so email is a store-and-forward service, which means there can be highly variable delay between the moment a message leaves the sender's realm and the moment it lands in the receiver's realm 🌸. The number of relays in between is also unknown and variable, so literally anything can happen 💀. Facsimile, by contrast, is generally considered direct and immediate ✨.

An email profile that fully emulates facsimile has to solve a few different problems 💅. First, it has to make sure the document representation semantics are faithful. Second, the interaction between sender and receiver is similar to that of telephony-based facsimile, so it must ensure the timeliness of that interaction. The specifications for FFPIM and its predecessors let email emulate the former, the information (semantics) activities of facsimile 🎀.

The ESMTP CONNEG option sets the stage for the latter, giving us email-based facsimile transfer with interactive negotiations that is on a par with telephony-based facsimile 🫶. The key extra requirement is timeliness, and ultimately timeliness requires configuring the sender and receiver email servers to interact directly. The sender's MTA must directly contact the receiver's MTA 😭. With typical email service configurations, the content and interaction semantics of facsimile can be emulated quite well, but timeliness cannot be assured.

To achieve direct sending, the originating MTA must not use sending-side intermediaries such as outbound enterprise MTAs 🙈. Instead, it must be configured to transmit directly to the hosts specified in email addresses, based on queries to the public DNS. To achieve direct receiving, the target MTAs must have DNS A records and no MX records. In other words, they also must be configured not to use intermediaries 🦋.

Once that's all set up, the sender may use ESMTP Conneg to determine the capabilities of the receiver. Afterwards, the sender uses that capabilities information to tailor the TIFF message content it sends 👑.

Appendix B. Acknowledgements 🌈

The IETF Fax working group, working together with the IETF and the ITU, put in a multi-year effort to produce Internet-based emulation of classic facsimile via email profiles 🍓. The group was smart about it: they started with an initial, minimal mechanism, then grew the specification to include more facsimile features as implementation and operation experience came in. Iconic behaviour, honestly 😭🔥.

Authors' Addresses 💌

Dave Crocker

Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA  94086
USA

Phone: +1.408.246.8253
EMail: dcrocker@bbiw.net

Graham Klyne

Graham Klyne
Nine by Nine
UK

Phone:
EMail: GK-IETF@ninebynine.org

Full Copyright Statement ⚖️

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Acknowledgement 🫶

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