Network Working Group D. Crocker

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.

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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].

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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].

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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.

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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.

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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.

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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.

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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: [email protected]

Graham Klyne
Nine by Nine
UK

Phone:
EMail: [email protected]

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Full Copyright Statement

Copyright (C) The Internet Society (2005).

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