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).
AbstractTo 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.
1]. Recommended behaviors for Internet Fax Gateway functions are defined in . 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 . 2) The format of image data is the data format defined by "simple mode" in . This document does not apply to the gateway functions for "real-time Internet Fax", as described and defined in . 17].
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  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  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. 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. 21] or IMAP ), the message header "Message-ID" (see ) 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.
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).
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
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.
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  and OpenPGP  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  or TLS  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.  Masinter, L., "Terminology and Goals for Internet Fax", RFC 2542, March 1999.  Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004.  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP Message Format", RFC 2440, November 1998.  Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998.
 Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998.  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001.  Piper, D., "The Internet IP Security Domain of Interpretation for ISAKMP", RFC 2407, November 1998.  Thayer, R., Doraswamy, N., and R. Glenn, "IP Security Document Roadmap", RFC 2411, November 1998.  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 3546, June 2003.  Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME Security with OpenPGP", RFC 3156, August 2001.  Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631, June 1999.  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Certificate Handling", RFC 3850, July 2004.  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 3851, July 2004.  Hoffman, P., "Enhanced Security Services for S/MIME", RFC 2634, June 1999.  Mimura, K., Yokoyama, K., Satoh, T., Kanaide, C., and C. Allocchio, "Internet Fax Gateway Requirements", RFC 4160, August 2005.  Toyoda, K., Ohno, H., Murai, J., and D. Wing, "A Simple Mode of Facsimile Using Internet Mail", RFC 3965, December 2004.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  "Procedures for real-time Group 3 facsimile communication over IP networks", ITU-T Recommendation T.38, June 1998.
 Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001.  Resnick, P., "Internet Message Format", RFC 2822, April 2001.  Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996.  Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 3501, March 2003.
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