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rfc:rfc4161

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.

Mimura, et al. Informational [Page 1] RFC 4161 Optional Services for Internet Fax Gateways August 2005

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

Mimura, et al. Informational [Page 2] RFC 4161 Optional Services for Internet Fax Gateways August 2005

 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.

Mimura, et al. Informational [Page 3] RFC 4161 Optional Services for Internet Fax Gateways August 2005

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:
  1. the retry times and retry interval MAY be specified as options by

the administrator of the offramp gateway;

  1. any error return notice SHOULD be sent only when the maximum number

of retries has been completed without success;

  1. 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:

Mimura, et al. Informational [Page 4] RFC 4161 Optional Services for Internet Fax Gateways August 2005

 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.

Mimura, et al. Informational [Page 5] RFC 4161 Optional Services for Internet Fax Gateways August 2005

 The information listed in the log MAY be the following:
  1. Date and time when the Internet Fax is received
  2. Sender address
  3. Recipient address(es)
  4. Start date and time of transmission over GSTN
  5. End date and time of transmission over GSTN
  6. Number of actually transmitted pages
  7. Number of actually transmitted bytes
  8. Fax resolution used
  9. Error codes/text that occurred during transmission
  10. Number of transmission attempts (retries)
  11. 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.

Mimura, et al. Informational [Page 6] RFC 4161 Optional Services for Internet Fax Gateways August 2005

 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:
  1. Start date and time of transmission from GSTN
  2. End date and time of transmission from GSTN
  3. Number of actually received pages
  4. Number of actually received bytes
  5. Fax resolution used
  6. Sender address (if available)
  7. Recipient address(es)
  8. Date and time when the Internet Fax is sent
  9. Error codes/text that occurred during Internet Fax transmission
  10. Number of transmission attempts (retries)
  11. Date and time of transmission of the (eventual) delivery notice

Mimura, et al. Informational [Page 7] RFC 4161 Optional Services for Internet Fax Gateways August 2005

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.

Mimura, et al. Informational [Page 8] RFC 4161 Optional Services for Internet Fax Gateways August 2005

 [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.

Mimura, et al. Informational [Page 9] RFC 4161 Optional Services for Internet Fax Gateways August 2005

 [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.

Mimura, et al. Informational [Page 10] RFC 4161 Optional Services for Internet Fax Gateways August 2005

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

Mimura, et al. Informational [Page 11] RFC 4161 Optional Services for Internet Fax Gateways August 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.
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 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Acknowledgement

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

Mimura, et al. Informational [Page 12]

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