Simple Mode Fax: A Simple Mode of Facsimile Using Internet Mail

Network Working Group                                         K. Toyoda
Request for Comments: 2305                                      H. Ohno
Category: Standards Track                                      J. Murai
                                                           WIDE Project
                                                                D. Wing
                                                                  Cisco
                                                             March 1998



             A Simple Mode of Facsimile Using Internet Mail


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 (1998).  All Rights Reserved.

SUMMARY

   This specification provides for "simple mode" carriage of facsimile
   data over the Internet.  Extensions to this document will follow.
   The current specification employs standard protocols and file formats
   such as TCP/IP, Internet mail protocols [1, 2, 3], MIME [4, 16, 17],
   and TIFF for Facsimile [5,6,19].  It can send images not only to
   other Internet-aware facsimile devices but also to Internet-native
   systems, such as PCs with common email readers which can handle MIME
   mail and TIFF for Facsimile data.  The specification facilitates
   communication among existing facsimile devices, Internet mail agents,
   and the gateways which connect them.

   The key words "MUST", "SHOULD", "SHOULD NOT", and "MAY" in this
   document are to be interpreted as described in [7].

1  SCOPE

   This specification defines a message-based facsimile communication
   over the Internet.  It describes a minimum set of capabilities,
   taking into account those of typical facsimile devices and PCs that
   can generate facsimile data.

   A G3Fax device has substantial restrictions due to specifications in
   the standards, such as for timers. This specification defines a
   profile for Internet mail, rather than creating a distinct "facsimile
   over the Internet" service.  The semantics resulting from the profile
   are designed to be compatible with facsimile operation over the
   general switched telephone network, so that gateways between
   facsimile and Internet mail can operate with very high fidelity.

   The reason for developing this capability as an email profile is to
   permit interworking amongst facsimile and email users.  For example
   it is intended that existing email users be able to send normal
   messages to lists of users, including facsimile-based recipients, and
   that other email recipients shall be able to reply to the original
   and continue to include facsimile recipients.  Similarly it is
   intended that existing email software work without modification and
   not be required to process new, or different data structures, beyond
   what is normal for Internet mail users.  Existing email service
   standards are used, rather than replicating mechanisms which are more
   tailored to existing facsimile standards, to ensure this
   compatibility with existing email service.

1.1 Services

   A facsimile-capable device that uses T.4 [8] and the general switched
   telephone network (GSTN) is called a "G3Fax device" in this
   specification.  An "IFax device" is an Internet- accessible device
   capable of sending, receiving or forwarding Internet faxes.  A
   message can be sent to an IFax device using  an Internet mail
   address. A message can be sent to a G3Fax device  using an Internet
   mail address; the message MAY be forwarded via an IFax offramp
   gateway.

1.2 Cases

   This specification provides for communication between each of the
   following combinations:

   Internet mail             =>  Network printer
   Internet mail             =>  Offramp gateway (forward to
                                 G3Fax)
   Network scanner           =>  Network printer
   Network scanner           =>  Offramp gateway (forward to
                                 G3Fax)
   Network scanner           =>  Internet mail

2  COMMUNICATION PROTOCOLS

   The set of conventions necessary to achieve facsimile- compatible
   service covers basic data transport, document data formats, message
   (document) addressing, delivery confirmation, and message security.
   In this section, the first 4 are covered.  The remainder are covered
   in following sections, along with additional details for addressing
   and formats.

2.1 Transport

   This section describes mechanisms involved in the transport between
   IFAX devices.

2.1.1     Relay

   Data transfer MAY be achieved using standard Internet mail transfer
   mechanisms[1, 3].  The format of addresses MUST conform to the RFC
   821 <addr-spec> and RFC 822 <mailbox> Internet mail standards [1, 2,
   3].

2.1.2     Gateway

   A gateway translates between dissimilar environments.  For IFax, a
   gateway connects between Internet mail and the T.4/GSTN facsimile.
   Gateways can service multiple T.4/GSTN facsimile users or can service
   only one.  In the former case, they serve as a classic "mail transfer
   agent" (MTA) and in the latter as a classic "mail user agent" (UA).

   An onramp is a gateway which connects from T.4/GSTN facsimile to
   Internet mail.  An offramp is a gateway which connects from Internet
   mail to T.4/GSTN facsimile. Behavior of onramps is out of scope for
   this specification.

   This specification describes the Internet mail service portion of
   offramp addressing, confirmation and failure notification.  Details
   are provided in later sections.

2.1.3     Mailbox protocols

   An offramp gateway that operate as an MTA serving multiple users
   SHOULD use SMTP; a gateway that operates as a UA serving a single
   mail recipient MAY use a mailbox access protocol such as POP or IMAP
   [9, 10].

   NOTE: An offramp gateway that relays mail based on addressing
   information needs to ensure that it uses addresses supplied in the
   MTA envelope, rather than from elsewhere, such as addresses listed in
   the message content headers.

2.2 Formats

2.2.1     Headers

   IFax devices MUST be compliant with RFC 822 and RFC1123, which define
   the format of mail headers.  The header of an IFax message SHOULD
   include Message-ID and MUST include all fields required by [2, 3],
   such as DATE and FROM.

2.2.2     MIME

   IFax devices MUST be compliant with MIME [4], except as noted in
   Appendix A.

2.2.3     Content

   The data format of the facsimile image is based on the minimum set of
   TIFF for Facsimile[6], also known as the S profile.   Such facsimile
   data are included in a MIME object by use of the image/TIFF sub-type
   [19].  Additional rules for the use of TIFF for Facsimile, for the
   message-based Internet facsimile application, are defined later.

2.2.4     Multipart

   A single multi-page document SHOULD be sent as a single multi- page
   TIFF file, even though recipients MUST process multipart/mixed
   containing multiple TIFF files. If multipart content is present and
   processing of any part fails, then processing for the entire message
   is treated as failing, per [Processing failure] below.

2.3 Error Handling

2.3.1     Delivery failure

   This section describes existing requirements for Internet mail,
   rather than indicating special requirements for IFax devices.

   In the event of relay failure, the sending relay MUST generate a
   failure message, which SHOULD be in the format of a DSN. [14,15]

        NOTE:  Internet mail transported via SMTP MUST contain a MAIL
        FROM address appropriate for delivery of return notices [Also
        see section 5.2.6]

2.3.2     Processing failure

   IFax devices with limited capabilities might be unable to process the
   content of a message.  If this occurs it is important to ensure that
   the message is not lost without any notice. Notice MAY be provided in
   any appropriate fashion, and the exact handling is a local matter.
   (Also see Appendix A, second bullet.)

3  ADDRESSING

3.1 Classic Email Destinations

   Messages being sent to normal Internet mail recipients will use
   standard Internet mail addresses, without additional constraints.

3.2 G3Fax Devices

   G3Fax devices are accessed via an IFAX offramp gateway, which
   performs any authorized telephone dial-up.

3.3 Address Formats Used by Offramps

   When a G3Fax device is identified by a telephone number, the entire
   address used for the G3fax device, including the number and offramp
   host reference MUST be contained within standard Internet mail
   transport fields, such as RCPT TO and MAIL FROM [1, 3].  The address
   MAY be contained within message content fields, such as <authentic>
   and <destination> [2, 3], as appropriate.

   As for all Internet mail addresses, the left-hand-side (local- part)
   of an address is not to be interpreted except by the MTA that is
   named on the right-hand-side (domain).

   The telephone number format SHOULD conform to [11, 12].  Other
   formats MUST be syntactically distinct from [11, 12].

4  IMAGE FILE FORMAT

   Sending IFax devices MUST be able to write minimum set TIFF files,
   per the rules for creating minimum set TIFF files defined in TIFF for
   Facsimile (the S profile) [6], which is also compatible with the
   specification for the minimum subset of TIFF-F in [5].  Receiving
   IFax devices MUST be able to read minimum set TIFF files.

   A sender SHOULD NOT use TIFF fields and values beyond the minimum
   subset of TIFF for Facsimile unless the sender has prior knowledge of
   other TIFF fields or values supported by the recipient.  The
   mechanism for determining capabilities of recipients is beyond the
   scope of this document.

5  SECURITY CONSIDERATIONS

5.1 General Directive

   This specification is based on use of existing Internet mail.  To
   maintain interoperability with Internet mail, any security to be
   provided should be part of the of the Internet security
   infrastructure, rather than a new mechanism or some other mechanism
   outside of the Internet infrastructure.

5.2 Threats and Problems

   Both Internet mail and G3Fax standards and operational services have
   their own set of threats and countermeasures.  This section attends
   only to the set of additional threats which ensue from integrating
   the two services. This section reviews relevant concerns about
   Internet mail for IFax environments, as well as considering the
   potential problems which can result of integrating the existing G3Fax
   service with Internet mail.

5.2.1     Spoofed sender

   The actual sender of the message might not be the same as that
   specified in the Sender or From fields of the message content headers
   or the MAIL FROM address from the SMTP envelope.

   In a tightly constrained environment, sufficient physical and
   software controls may be able to ensure prevention of this problem.
   The usual solution is through encryption-based authentication, either
   for the channel or associated with the object, as discussed below.

   It should be recognized that SMTP implementations do not provide
   inherent authentication of the senders of messages, nor are sites
   under obligation to provide such authentication. End-to-end
   approaches such as S/MIME and PGP/MIME are currently being developed
   within the IETF. These technologies can provide such authentication.

5.2.2     Resources consumed by dialout

   In addition to the resources normally consumed for email (CPU cycles
   and disk), offramp facsimile causes an outdial which often imposes
   significant resource consumption, such as financial cost. Techniques
   for establishing authorization of the sender are essential to those
   offramp facsimile services that need to manage such consumption.

   Due to the consumption of these resources by dialout, unsolicited
   bulk email which causes an outdial is undesirable.

   Offramp gateways SHOULD provide the ability to authorize senders in
   some manner to prevent unauthorized use of the offramp. There are no
   standard techniques for authorization using Internet protocols.

   Typical solutions use simple authentication of the originator to
   establish and verify their identity and then check the identity
   against a private authorization table.

   Originator authentication entails the use of weak or strong
   mechanisms, such as cleartext keywords or encryption-based data-
   signing, respectively, to determine and validate the identify of the
   sender and assess permissions accordingly.

   Other control mechanisms which are common include source filtering
   and originator authentication.  Source filtering entails offramp
   gateway verification of the host or network originating the message
   and permitting or prohibiting relaying accordingly.

5.2.3     GSTN authorization information

   Confidential information about the sender necessary to dial a G3Fax
   recipient, such as sender's calling card authorization number, might
   be disclosed to the G3Fax recipient (on the cover page), such as
   through parameters encoded in the G3Fax recipients address in the To:
   or CC: fields.

   Senders SHOULD be provided with a method of preventing such
   disclosure.  As with mechanisms for handling unsolicited faxes, there
   are not yet standard mechanisms for protecting such information.
   Out-of-band communication of authorization information or use of
   encrypted data in special fields are the available non-standard
   techniques.

   Typically authorization needs to be associated to specific senders
   and specific messages, in order to prevent a "replay" attack which
   causes and earlier authorization to enable a later dial-out by a
   different (and unauthorized) sender.  A non-malicious example of such
   a replay would be to have an email recipient reply to all original
   recipients -- including an offramp IFax recipient -- and have the
   original sender's authorization cause the reply to be sent.

5.2.4     Sender accountability

   In many countries, there is a legal requirement that the "sender" be
   disclosed on a facsimile message.  Email From addresses are trivial
   to fake, so that using only the MAIL FROM [1, 3]  or From [2, 3]
   header is not sufficient.

   Offramps SHOULD ensure that the recipient is provided contact
   information about the offramp, in the event of problems.

   The G3Fax recipient SHOULD be provided with sufficient information
   which permits tracing the originator of the IFax message.  Such
   information might include the contents of the MAIL FROM, From, Sender
   and Reply-To headers, as well as Message-Id and Received headers.

5.2.5     Message disclosure

   Users of G3Fax devices have an expectation of a level of message
   privacy which is higher than the level provided by Internet mail
   without security enhancements.

   This expectation of privacy by G3Fax users SHOULD be preserved as
   much as possible.

   Sufficient physical and software control may be acceptable in
   constrained environments.  The usual mechanism for ensuring data
   confidentially entail encryption, as discussed below.

5.2.6     Non private mailboxes

   With email, bounces (delivery failures) are typically returned to the
   sender and not to a publicly-accessible email account or printer.
   With facsimile, bounces do not typically occur.  However, with IFax,
   a bounce could be sent elsewhere (see section [Delivery Failure]),
   such as a local system administrator's account, publicly-accessible
   account, or an IFax printer (see also [Traffic Analysis]).

5.2.7     Traffic analysis

   Eavesdropping of senders and recipients is easier on the Internet
   than GSTN.  Note that message object encryption does not prevent
   traffic analysis, but channel security can help to frustrate attempts
   at traffic analysis.

5.3 Security Techniques

   There are two, basic approaches to encryption-based security which
   support authentication and privacy:

5.3.1     Channel security

   As with all email, an IFax message can be viewed as it traverses
   internal networks or the Internet itself.

   Virtual Private Networks (VPN) which make use of encrypted tunnels,
   such as via IPSec technology [18] or transport layer security, can be
   used to prevent eavesdropping of a message as it traverses such
   networks.   It also provides some protection against traffic
   analysis, as described above.

5.3.2     Object security

   As with all email, an IFax message can be viewed while it resides on,
   or while it is relayed through, an intermediate Mail Transfer Agent.

   Message encryption, such as PGP-MIME [13] and S/MIME, can be used to
   provide end-to-end encryption.

6  REFERENCES

   [1]  Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
        821, August 1982.

   [2]  Crocker, D., "Standard for the Format of ARPA Internet
        Text Messages", STD 11, RFC 822, August l982.

   [3]  Braden, R., 1123 "Requirements for Internet hosts -
        application and support", RFC 1123, October 1989.

   [4]  Borenstein, N., and N. Freed, " Multipurpose Internet
        Mail Extensions (MIME) Part Five:  Conformance Criteria and
        Examples ", RFC 2049, November 1996.

   [5]  Parsons, G., and J. Rafferty, "Tag Image File Format
        (TIFF) -- F Profile for Facsimile", RFC 2306, March 1998.

   [6]  McIntyre, L., Zilles, S., Buckley, R., Venable, D.,
        Parsons, G., and J. Rafferty, "File Format for Internet Fax",
        RFC 2301, March 1998.

   [7]  Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", RFC 2119, March 1997.

   [8]  ITU-T (CCITT), "Standardization of Group 3 facsimile
        apparatus for document transmission", ITU-T (CCITT),
        Recommendation T.4.

   [9]  Myers, J., and M. Rose, "Post Office Protocol - Version
        3", STD 53, RFC 1939, May 1996.

   [10] Crispin, M., "Internet Message Access Protocol - Version
        4Rev1", RFC 2060, December 1996.

   [11] Allocchio, C., "Minimal PSTN address format for Internet
        mail", RFC 2303, March 1998.

   [12] Allocchio, C., "Minimal fax address format for Internet
        mail", RFC 2304, March 1998.

   [13] Elkins, M., "MIME Security with Pretty Good Privacy
        (PGP)", RFC 2015, October 1996.

   [14] Moore, K., and G. Vaudreuil, "An Extensible Message
        Format for Delivery Status Notifications", RFC 1894, January
        1996.

   [15] Moore, K., "SMTP Service Extension for Delivery Status
        Notifications", RFC 1891, January 1996.

   [16] Freed, N., and N. Borenstein, "Multipurpose Internet
        Mail Extensions (MIME) Part Two: Media Types", RFC 2046,
        November 1996.

   [17] Moore, K., "Multipurpose Internet Mail Extensions (MIME)
        Three: Representation of Non-ASCII Text in Internet ge Headers",
        RFC 2047, November 1996.

   [18] Atkinson, R., "Security Architecture for the Internet
        Protocol", RFC 1825, Naval Research Laboratory, August 1995.

   [19] Parsons, G. and Rafferty, J. "Tag Image File Format
        (TIFF) -- image/TIFF: MIME Sub-type Registration", RFC 2302,
        March 1998.

7  ACKNOWLEDGEMENTS

   This specification was produced by the Internet Engineering Task
   Force Fax Working Group, over the course of more than one year's
   online and face-to-face discussions.  As with all IETF efforts, many
   people contributed to the final product.

   Active for this document were: Steve Huston, Jeffrey Perry, Greg
   Vaudreuil, Richard Shockey, Charles Wu, Graham Klyne, Robert A.
   Rosenberg, Larry Masinter, Dave Crocker, Herman Silbiger, James
   Rafferty.

8  AUTHORS' ADDRESSES

   Kiyoshi Toyoda
   Matsushita Graphic Communication Systems, Inc.
   2-3-8 Shimomeguro, Meguro-ku
   Tokyo 153 Japan
   Fax: +81 3 5434 7166
   Email: ktoyoda@rdmg.mgcs.mei.co.jp

   Hiroyuki Ohno
   Tokyo Institute of Technology
   2-12-1 O-okayama, Meguro-ku
   Tokyo 152 Japan
   FAX: +81 3 5734 2754
   Email: hohno@is.titech.ac.jp

   Jun Murai
   Keio University
   5322 Endo, Fujisawa
   Kanagawa 252 Japan
   Fax: +81 466 49 1101
   Email: jun@wide.ad.jp

   Dan Wing
   Cisco Systems, Inc.
   101 Cooper Street
   Santa Cruz, CA 95060 USA
   Phone: +1 408 457 5200
   Fax: +1 408 457 5208
   Email: dwing@cisco.com

9 APPENDIX A:  Exceptions to MIME

   *    IFax senders are NOT REQUIRED to be able to send
        text/plain messages (RFC 2049 requirement 4), although IFax
        recipients are required to accept such messages, and to process
        them.

   *    IFax recipients are NOT REQUIRED to offer to put results
        in  a file. (Also see 2.3.2.)

   *    IFax recipients MAY directly print/fax  the received
        message rather  than "display" it, as indicated in RFC 2049.

10  Full Copyright Statement

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