Guidelines for Optional Services for Internet Fax Gateways
RFC 4161

Network Working Group                                         K. Mimura
Request for Comments: 4161                                  K. Yokoyama
Category: Informational                                        T. Satoh
                                                            K. Watanabe
                                                             C. Kanaide
                                           TOYO Communication Equipment
                                                            August 2005

       Guidelines for Optional Services for Internet Fax Gateways

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   To allow connectivity between the general switched telephone network
   facsimile service (GSTN fax) and the e-mail-based Internet Fax
   service (i-fax), an "Internet Fax Gateway" is required.  This
   document provides guidelines for the optional functionality of
   Internet Fax Gateways.  In this context, an "offramp gateway"
   provides facsimile data transmission from i-fax to GSTN fax; vice
   versa, an "onramp gateway" provides data transmission from GSTN fax
   to i-fax.  The recommendations in this document apply to the
   integrated service including Internet Fax terminals, computers with
   i-fax software on the Internet, and GSTN fax terminals on the GSTN.

   This document supplements the recommendation for minimal features of
   an Internet Fax Gateway.  In particular, it covers techniques for
   dropping duplicated fax messages, automatic fax re-transmission,
   error, return notice, and log handling, and possible authorization
   methods by DTMF (Dual Tone Multi-Frequency) for onramp gateways.

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1.  Introduction

   An Internet Fax Gateway can be classified as either an offramp
   gateway or an onramp gateway.  This document provides guidelines for
   optional services and examples of Internet Fax Gateway operations.
   In particular, it covers techniques for dropping duplicated fax
   messages, automatic fax re-transmission, error, return notice, and
   log handling, and possible authorization methods by DTMF (Dual Tone
   Multi-Frequency) for onramp gateways.

   A more detailed definition of onramps and offramps is provided in
   [1].  Recommended behaviors for Internet Fax Gateway functions are
   defined in [15].

   This document provides recommendations only for the specific cases
   hereunder:

   1) the operational mode of the Internet Fax is "store and forward",
      as defined in Section 2.5 of [1].

   2) The format of image data is the data format defined by "simple
      mode" in [16].

   This document does not apply to the gateway functions for "real-time
   Internet Fax", as described and defined in [18].

1.1.  Key Words

   The key words "MUST", "SHOULD", "SHOULD NOT", and "MAY" in this
   document are to be interpreted as described in [17].

2.  Optional Services for an Offramp Gateway

2.1.  Drop Duplicated GSTN Fax Transmission

   Electronic mail transport agents (MTA) deliver an Internet Fax
   message into either the recipient's mailbox or an offramp gateway
   mailbox.  Hence, the message is retrieved for further action, which
   in the case of the offramp gateway, will result in its delivery to
   the GSTN fax service.

   The offramp gateway mailbox will thus receive all messages which the
   gateway will process, regardless of their final, distinct GSTN
   destinations.  As such, addresses like

      Fax=+12224567654@example.com
      Fax=+38155234578@example.com
      Fax=+3904567437777@example.com

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   will all end up in the offramp gateway mailbox corresponding to the
   "example.com" domain.

   However, the handling of e-mail messages (including those of Internet
   Faxes) that contain more than one recipient, but are directed to the
   same final MTA, can be different, depending on the MTA configuration
   or features.  A single message with multiple recipients in the SMTP
   envelope [19] is likely to be the most common case on the mail
   transport system, but it may happen that multiple copies of the same
   message are transmitted, one per recipient.  Or it may happen that
   the final MTA is set to deliver a separate copy of the message per
   recipient into the final mailbox, supposing it is delivering messages
   to real mailboxes of distinct endusers.

   Thus, it may happen that the offramp gateway receives multiple copies
   of the same Internet Fax message that is to be delivered to different
   GSTN destinations, which are listed together and repeatedly in the
   e-mail message headers [20] of the Internet Fax.  In such cases, the
   offramp gateway SHOULD implement techniques to avoid duplicate or
   multiple transmission over GSTN of the same fax message to the same
   recipient.

   Here are some possible, but non-exclusive, examples of these
   techniques.

2.1.1.  SMTP Envelope Addresses Check

   Using the SMTP [19] envelope destination address given in the "RCPT
   TO" field is usually the best technique to ensure that a received
   message is delivered to that address, and to avoid duplicate
   deliveries.

   If the offramp gateway has the "RCPT TO" information still available
   during processing, then it MUST use it to determine the recipients
   over GSTN fax service.

2.1.2 Message-ID Check

   If the SMTP "RCPT TO" information is not available (for example, in
   the case where the offramp gateway retrieves messages from its
   mailbox using either POP [21] or IMAP [22]), the message header
   "Message-ID" (see [20]) MAY be used to check if a message has already
   been processed, and hence avoid retransmission to all its GSTN
   recipients handled by the offramp gateway.

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2.2.  Error Handling

2.2.1.  Recoverable Errors

   Recoverable errors that happen during GSTN transmission are those
   where there is good chance that the error may not occur at the next
   attempt.  This category includes "busy signal", "no line/carrier
   signal", etc.

   For all these errors, the offramp gateway SHOULD re-queue the message
   and perform a retransmission attempt later on, as specified in
   Section 2.3.

2.2.2.  Non-Recoverable Errors

   If the error that occurs during GSTN transmission is likely non-
   recoverable, the offramp gateway SHOULD NOT attempt retransmission,
   and an error Message Delivery Notification (MDN) with appropriate
   error codes MUST be generated for the Internet Fax message sender.
   Examples of non-recoverable errors include paper-related errors (such
   as a jam, an empty tray, etc.) at a remote device, no response from a
   remote destination, voice response errors, data modem response
   errors, and stop event errors.

2.3.  Automatic Re-Transmission Handling

   An offramp gateway SHOULD implement a function that automatically
   tries to send facsimile data again if recoverable delivery failure
   occurs.  If this function is implemented, then:

   - the retry times and retry interval MAY be specified as options by
     the administrator of the offramp gateway;

   - any error return notice SHOULD be sent only when the maximum number
     of retries has been completed without success;

   - if transmission is suspended due to an error, then the subsequent
     transmission attempt SHOULD avoid retransmitting the pages already
     delivered successfully, if any.

2.4.  Multiple Return Notice Handling

   An offramp gateway can receive an Internet Fax for delivery to
   multiple GSTN recipients.  If errors occur, which require the
   Internet Fax sender to be informed about them, or if the Internet Fax
   sender requested delivery notifications, then the offramp gateway has
   various ways to handle these multiple return notices:

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   1) An offramp gateway sends a return notice as soon as an error or a
      successful delivery occurs, per single GSTN recipient.

   2) An offramp gateway gathers all information about the message, but
      sends a return notice only after all or a number of GSTN
      recipients have been handled (successfully or not).

   If Case 2 is implemented, then the offramp gateway MAY also choose to
   send separate success and failure notices, or to limit the number of
   GSTN recipients handled per single return note (for example, no more
   than 10 recipients per return note).

2.5.  Handling Transmission Errors for a Return Notice

   When an offramp gateway fails in the transmission of a return notice,
   the Internet Fax Gateway SHOULD process the notice in either of the
   following ways:

   1) The return notices SHOULD be re-queued, and delivery retried
      later.  The number of retry attempts and the time interval between
      them MAY be a feature configured by the offramp gateway
      administrator.  This is the preferred method to implement;
      however, if all the retransmission attempts fail, processing
      SHOULD continue as in Case 2.

   2) If the gateway does not have enough capabilities to handle notice
      re-queuing, but has a log information preservation function, the
      error information SHOULD be recorded to a log, and processing
      SHOULD end.  At this time, the administrator of the gateway system
      SHOULD be notified of these errors using a specific method (for
      example, by an e-mail message).

   3) If the gateway does not even have a log information preservation
      function, the administrator SHOULD be notified about the failure
      (for example, via an e-mail message), and processing SHOULD end.

2.6.  Offramp Gateway Log

   An offramp gateway SHOULD have a function that keeps information
   listed as a log, either specific to the fax gateway or in a log file
   that exists locally on the gateway or remotely.  If the fax gateway
   or the remote system are equipped with recording media, the log
   information SHOULD be saved as a log file.  As a last resort, if no
   recording media are available, the log MAY be printed.

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   The information listed in the log MAY be the following:

   - Date and time when the Internet Fax is received
   - Sender address
   - Recipient address(es)
   - Start date and time of transmission over GSTN
   - End date and time of transmission over GSTN
   - Number of actually transmitted pages
   - Number of actually transmitted bytes
   - Fax resolution used
   - Error codes/text that occurred during transmission
   - Number of transmission attempts (retries)
   - Date and time of transmission of the (eventual) delivery notice

3.  Optional Services for an Onramp Gateway

3.1.  Examples of User Authorization

   An onramp gateway MAY have a user authorization function to confirm
   that the user is authorized to transmit a facsimile into the Internet
   fax service.  For example, user authorization may be accomplished by
   getting a user ID and password received by DTMF, or via a local
   authorization table based on the GSTN caller-ID.  The following
   subsections give some possible examples, but other methods are also
   possible.

3.1.1.  Authorization via GSTN Caller-ID

   The most simple method to authenticate and authorize a GSTN fax
   service user is to use the GSTN caller-ID.  If available, in fact,
   the caller-ID is generated by the GSTN network service itself, and it
   is quite difficult to produce fake caller-IDs.  In other words, the
   security related to this authentication method relies on the
   confidence that the GSTN caller-ID service is secure by itself.

   The GSTN sender MAY be authorized via a lookup into a table managed
   by the onramp gateway administrator, via complete or partial
   (wildcard) matches.

3.1.2.  Authorization via GSTN Fax "Station ID"

   During the initial GSTN fax service negotiation, the sender fax can
   send various information to the onramp gateway, including the
   "station ID" alphanumeric string.  This string MAY be used to
   transmit authentication and authorization information for subsequent
   lookup by the onramp gateway.  Thus, user ID and an eventual password
   MAY be sent inside this string.

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   However, if used as the only authentication, this method is much less
   secure than the caller-ID one because the user of the calling GSTN
   station can decide which string to send, and the string travels in
   clear form over the GSTN.  Given this security warning, this method
   allows more flexibility to the GSTN user: in fact, it is not tied to
   a single GSTN fax terminal, and authorization can be obtained from
   anywhere, provided the sender has the possibility to configure the
   "station ID" on the device being used.

   A combination of caller-ID and station ID checks MAY, on the other
   hand, result in a greatly improved level of security.

3.1.3.  Authorization via DTMF

   An onramp gateway MAY implement the Authorization function by
   requesting that a user ID and password information are sent over GSTN
   via DTMF.  For example, this function MAY be accomplished by
   requesting that the DTMF information is sent immediately after the
   connection over GSTN is established, before starting the GSTN fax
   negotiation; but other methods are also possible.

3.2.  Onramp Gateway Log

   An onramp gateway SHOULD have a function that keeps information
   listed as a log, either specific to the fax gateway or in a log file
   that exists locally on the gateway or remotely.  If the fax gateway
   or the remote system are equipped with recording media, the log
   information SHOULD be saved as a log file.  As a last resort, if no
   recording media are available, the log MAY be printed.

   The information listed in the log MAY be the following:

   - Start date and time of transmission from GSTN
   - End date and time of transmission from GSTN
   - Number of actually received pages
   - Number of actually received bytes
   - Fax resolution used
   - Sender address (if available)
   - Recipient address(es)
   - Date and time when the Internet Fax is sent
   - Error codes/text that occurred during Internet Fax transmission
   - Number of transmission attempts (retries)
   - Date and time of transmission of the (eventual) delivery notice

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4.  Security Considerations

   Refer to Section 3.1 ("User Authorization") for authentication for an
   onramp gateway.  In particular, sending user IDs and passwords in
   clear, as described in Section 3.1.2, can pose high security risks,
   and thus is NOT RECOMMENDED.

   S/MIME [2][11][12][13][14] and OpenPGP [3][10] can also be used to
   encrypt an Internet Fax message.  A signed or encrypted message is
   protected while transported along the network; however, when a
   message reaches an Internet Fax Gateway, either onramp or offramp,
   this kind of protection cannot be applied anymore.  In this
   situation, security must rely on trusted operations of the gateway
   itself.  A gateway might have its own certificate/key to improve
   security operations when sending Internet Faxes, but, as with any
   gateway, it breaks the end-to-end security pattern of both S/MIME and
   OpenPGP.

   Other security mechanisms, like IPsec [4][5][6][7][8] or TLS [9] also
   do not ensure a secure gateway operation.

   Denial-of-service attacks are beyond the scope of this document.
   Host compromise caused by flaws in the implementation is beyond the
   scope of this document.

5.  Acknowledgments

   Thanks to Claudio Allocchio (Consortium GARR, Italy) for its final
   review of this document, and for contributing the authorization and
   security sections of this document.

6.  References

6.1.  Informative References

   [1]  Masinter, L., "Terminology and Goals for Internet Fax", RFC
        2542, March 1999.

   [2]  Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
        July 2004.

   [3]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP
        Message Format", RFC 2440, November 1998.

   [4]  Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

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   [5]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
        November 1998.

   [6]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
        Explicit Congestion Notification (ECN) to IP", RFC 3168,
        September 2001.

   [7]  Piper, D., "The Internet IP Security Domain of Interpretation
        for ISAKMP", RFC 2407, November 1998.

   [8]  Thayer, R., Doraswamy, N., and R. Glenn, "IP Security Document
        Roadmap", RFC 2411, November 1998.

   [9]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
        T. Wright, "Transport Layer Security (TLS) Extensions", RFC
        3546, June 2003.

   [10] Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME
        Security with OpenPGP", RFC 3156, August 2001.

   [11] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631,
        June 1999.

   [12] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
        (S/MIME) Version 3.1 Certificate Handling", RFC 3850, July 2004.

   [13] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
        (S/MIME) Version 3.1 Message Specification", RFC 3851, July
        2004.

   [14] Hoffman, P., "Enhanced Security Services for S/MIME", RFC 2634,
        June 1999.

6.2.  Normative References

   [15] Mimura, K., Yokoyama, K., Satoh, T., Kanaide, C., and C.
        Allocchio, "Internet Fax Gateway Requirements", RFC 4160, August
        2005.

   [16] Toyoda, K., Ohno, H., Murai, J., and D. Wing, "A Simple Mode of
        Facsimile Using Internet Mail", RFC 3965, December 2004.

   [17] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [18] "Procedures for real-time Group 3 facsimile communication over
        IP networks", ITU-T Recommendation T.38, June 1998.

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   [19] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
        2001.

   [20] Resnick, P., "Internet Message Format", RFC 2822, April 2001.

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

   [22] Crispin, M., "Internet Message Access Protocol - Version 4rev1",
        RFC 3501, March 2003.

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Authors' Addresses

   Katsuhiko Mimura
   TOYO Communication Equipment CO., LTD.
   2-1-1 Koyato, Samukawa-machi, Koza-gun
   Kanagawa-pref., Japan

   Fax: +81 467 74 5743
   EMail: mimu@miyabi-labo.net

   Keiichi Yokoyama
   TOYO Communication Equipment CO., LTD.
   2-1-1 Koyato, Samukawa-machi, Koza-gun
   Kanagawa-pref., Japan

   Fax: +81 467 74 5743
   EMail: keiyoko@msn.com

   Takahisa Satoh
   TOYO Communication Equipment CO., LTD.
   2-1-1 Koyato, Samukawa-machi, Koza-gun
   Kanagawa-pref., Japan

   Fax: +81 467 74 5743
   EMail: zsatou@t-ns.co.jp

   Ken Watanabe
   TOYO Communication Equipment CO., LTD.
   2-1-1 Koyato, Samukawa-machi, Koza-gun
   Kanagawa-pref., Japan

   Fax: +81 467 74 5743
   EMail: knabe@ad.cyberhome.ne.jp

   Chie Kanaide
   TOYO Communication Equipment CO., LTD.
   2-1-1 Koyato, Samukawa-machi, Koza-gun
   Kanagawa-pref., Japan

   Fax: +81 467 74 5743
   EMail: icemilk77@yahoo.co.jp

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