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rfc:rfc4142

Network Working Group D. Crocker Request for Comments: 4142 Brandenburg Category: Standards Track G. Klyne

                                                          Nine by Nine
                                                         November 2005
          Full-mode Fax Profile for Internet Mail (FFPIM)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 Classic facsimile document exchange represents both a set of
 technical specifications and a class of service.  Previous work has
 replicated some of that service class as a profile within Internet
 mail.  The current specification defines "full mode" carriage of
 facsimile data over the Internet, building upon that previous work
 and adding the remaining functionality necessary for achieving
 reliability and capability negotiation for Internet mail, on a par
 with classic T.30 facsimile.  These additional features are designed
 to provide the highest level of interoperability with the
 standards-compliant email infrastructure and mail user agents, while
 providing a level of service that approximates what is currently
 enjoyed by fax users.

Crocker & Klyne Standards Track [Page 1] RFC 4142 FFPIM November 2005

Table of Contents

 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 specification defines "full mode" carriage of facsimile data
 over the Internet, building upon previous work in A Simple Mode of
 Facsimile Using Internet Mail [RFC3965] and Extended Facsimile Using
 Internet Mail [RFC2532].  This specification also adds the remaining
 functionality necessary to achieve reliable and capable negotiation
 for Internet mail, on par with classic [T30] facsimile.  These
 additional features are designed to provide the highest level of
 interoperability with the standards-compliant email infrastructure
 and mail user agents, while providing a level of service that closely
 approximates the level of service currently enjoyed by fax users.
 Basic terminology is discussed in [RFC2542].  Implementations that
 conform to this specification MUST also conform to [RFC3965] and
 [RFC2532].
 The new features are designed to be interoperable with the existing
 base of mail transfer agents (MTAs) and mail user agents (MUAs), and
 to take advantage of existing standards for optional functionality
 (e.g., positive delivery confirmation and disposition notification).
 Enhancements described in this document utilize the existing Internet
 email messaging infrastructure, where possible, instead of creating
 fax-specific features that are unlikely to be implemented in non-fax
 messaging software.
 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].

Crocker & Klyne Standards Track [Page 2] RFC 4142 FFPIM November 2005

2. Content Negotiation

 Classic facsimile service is interactive, such that a sending station
 can discover the capabilities of the receiving station, prior to
 sending a facsimile of a document.  This permits the sender to
 transmit the best quality of facsimile supported by both the sending
 station and the receiving station.  Internet mail is
 store-and-forward, with potentially long latency, such that
 before-the-fact negotiation is problematic.
 Use of a negotiation mechanism permits senders to transfer a richer
 document form than is permitted when using the safer-but-universal
 default form.  Without this mechanism, the sender of a document
 cannot be certain that the receiving station will be able to support
 the form.
 The capabilities that can be negotiated by an FFPIM participant 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 exchanging the capabilities information uses a
 post-hoc technique, which permits an originator to send the best
 version known to be supported by the recipient, and to also send a
 better suited version if the recipient requests 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
 requires support for content negotiation along the entire path the
 email travels.  Using this mechanism, receiving ESMTP servers are
 able to report capabilities of the addresses (mailboxes) they
 support, and sending email clients are able to 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].

Crocker & Klyne Standards Track [Page 3] RFC 4142 FFPIM November 2005

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, this means that at least one of these
 repositories is inaccurate.  Therefore, the larger problem is one of
 correctness, rather than 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 contained in
 [RFC3949].  Implementations that are conformant to FFPIM SHOULD
 support TIFF enhancements.
 It should also be noted that the content negotiation mechanism
 permits a sender to know the full range of content types that are
 supported by the recipient.  Therefore, requirements for support of
 TIFF represent a functional minimum for FFPIM.

4. Security Considerations

 As this document is an extension of [RFC3965] and [RFC2532], the
 Security Considerations sections of [RFC3965] and [RFC2532] apply to
 this document, including discussion of PGP and S/MIME use for
 authentication and privacy.
 It appears that the mechanisms added by this specification do not
 introduce new security considerations.  However, the concerns raised
 in [RFC2532] are particularly salient for these new mechanisms.
 Use of this specification should occur with particular attention to
 the following security concerns:
  • Negotiation can be used as a denial of service attack.
  • Negotiation may lead to the use of an unsafe data format.
  • Negotiation discloses information and therefore raises privacy

concerns.

Crocker & Klyne Standards Track [Page 4] RFC 4142 FFPIM November 2005

 Use of the ESMTP CONNEG option permits content transformation by an
 intermediary, along the mail transfer path.  When the contents are
 encrypted, the intermediary cannot perform the conversion, because it
 is not expected to have access to the relevant secret keying
 material.  When the contents are signed, but not encrypted,
 conversion will invalidate the signature.  Therefore, permission to
 convert SHOULD NOT normally be used with signed or sealed messages.

5. References

5.1. Normative References

 [RFC4141] Toyoda, K. and D. Crocker, "SMTP and MIME Extensions for
           Content Conversion", RFC 4141, November 2005.
 [RFC3949] Buckley, R., Venable, D., McIntyre, L., Parsons, G., and J.
           Rafferty, "File Format for Internet Fax", RFC 3949,
           February 2005.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2532] Masinter, L. and D. Wing, " Extended Facsimile Using
           Internet Mail", RFC 2532, March 1999.
 [RFC2534] Masinter, L., Wing, D., Mutz, A., and K. Holtman, "Media
           Features for Display, Print, and Fax", RFC 2534, March
           1999.
 [RFC2542] Masinter, L., "Terminology and Goals for Internet Fax", RFC
           2542, March 1999.
 [RFC2879] Klyne, G. and L. McIntyre, "Content Feature Schema for
           Internet Fax (V2)", RFC 2879, August 2000.
 [RFC3297] Klyne, G., Iwazaki, R., and D. Crocker, "Content
           Negotiation for Messaging Services based on Email", RFC
           3297, July 2002.
 [RFC3965] Toyoda, K., Ohno, H., Murai, J., and D. Wing, "A Simple
           Mode of Facsimile Using Internet Mail", RFC 3965, December
           2004.

Crocker & Klyne Standards Track [Page 5] RFC 4142 FFPIM November 2005

5.2. Informative References

 [RFC3238] Floyd, S. and L. Daigle, "IAB Architectural and Policy
           Considerations for Open Pluggable Edge Services", RFC 3238,
           January 2002.
 [T30]     ITU-T (CCITT), "Procedures for Document Facsimile
           Transmission in the General Switched Telephone Network",
           Recommendation T.30, July 1996.

Crocker & Klyne Standards Track [Page 6] RFC 4142 FFPIM November 2005

Appendix A. Direct Mode

 Email is a store-and-forward service, typically with highly variable
 delay between the time a message leaves the sender's realm and the
 time it arrives in the receiver's realm.  The number of relays
 between sender and receiver is also unknown and variable.  By
 contrast, facsimile is generally considered to be direct and
 immediate.
 An email profile that fully emulates facsimile must solve several
 different problems.  One is to ensure that the document
 representation semantics are faithful.  Another is that the
 interaction between sender and receiver is similar to that of
 telephony-based facsimile.  In particular, it must ensure the
 timeliness of the interaction.  The specifications for FFPIM and its
 predecessors enable email to emulate the former, the information
 (semantics) activities of facsimile.
 The ESMTP CONNEG option sets the stage for achieving the latter, with
 email-based facsimile transfer that has interactive negotiations, on
 a par with telephony-based facsimile.  The key, additional
 requirement is to achieve timeliness.  Ultimately, timeliness
 requires configuring 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 do transmissions directly to hosts
 specified in email addresses, based on queries to the public DNS.  To
 achieve direct receiving, the target MTAs must have DNS A records,
 without MX records.  That is, they also must be configured not to use
 intermediaries.
 The sender may then use ESMTP Conneg to determine the capabilities of
 the receiver.  Afterwards the sender will use the capabilities
 information to tailor the TIFF message content it sends.

Crocker & Klyne Standards Track [Page 7] RFC 4142 FFPIM November 2005

Appendix B. Acknowledgements

 The IETF Fax working group, in collaboration with the IETF and the
 ITU, has diligently participated in a multi-year effort to produce
 Internet-based emulation of classic facsimile via email profiles.
 The effort benefited from the group's willingness to provide an
 initial, minimal mechanism, and then develop the specification to
 include more facsimile features as implementation and operation
 experience was gained.

Authors' Addresses

 Dave Crocker
 Brandenburg InternetWorking
 675 Spruce Drive
 Sunnyvale, CA  94086
 USA
 Phone: +1.408.246.8253
 EMail: dcrocker@bbiw.net
 Graham Klyne
 Nine by Nine
 UK
 Phone:
 EMail: GK-IETF@ninebynine.org

Crocker & Klyne Standards Track [Page 8] RFC 4142 FFPIM November 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 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.
 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.

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Crocker & Klyne Standards Track [Page 9]

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