RFC 4160

Internet Fax Gateway Requirements 📠✨

okay bestie, this is RFC 4160 and it's giving old-school fax-meets-email energy 💌 It's written by K. Mimura, K. Yokoyama, T. Satoh, and C. Kanaide from TOYO Communication Equipment, plus C. Allocchio from Consortium GARR. It's dated August 2005 and it's Informational, so it's a "here's some info" doc, not an official Internet standard 🫶 It doesn't list anything it obsoletes or updates.

---

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 🌸

okay so the whole point is connecting the old-school phone fax service (GSTN fax) with e-mail-based Internet Fax (i-fax) 📠💌 that needs an "Internet Fax Gateway" in the middle. This doc gives recommendations for how those gateways should work 🎀

Quick terms: an "offramp gateway" takes faxes from i-fax and sends them out to GSTN fax, and an "onramp gateway" goes the other way, taking GSTN fax and sending it into i-fax 🔄 The recommendations apply to the whole combined service, which includes Internet Fax terminals, computers running i-fax software on the Internet, and GSTN Fax terminals on the GSTN 💖

1. Introduction 🧚

An Internet Fax Gateway handles the connection and translation between GSTN fax and i-fax 💅 This doc spells out how one should behave 👑 A gateway is "onramp" when a fax goes from GSTN fax to Internet Fax, and "offramp" when it goes from Internet Fax to GSTN fax 🔥 For a deeper dive on what onramp and offramp mean inside i-fax, check [1] 🫶

This doc only gives recommendations for these specific cases:

1. the Internet Fax operational mode 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 [4].

This doc does not cover the gateway functions for "real-time Internet Fax", as described and defined in [3]. Extra recommendations for optional functionality are in [24] ✨

1.1. Key Words 🔑

The key words "MUST", "SHOULD", "SHOULD NOT", and "MAY" in this document are to be interpreted as described in [5]. 💜

2. Internet Fax Gateway Operations 📬

An onramp gateway gets a fax from a GSTN fax device (which might even be an offramp gateway itself), and makes an Internet Fax over the Internet that gets sent to any Internet Fax device 🌈

An offramp gateway gets an Internet Fax over the Internet from any Internet Fax-capable device (which might be an onramp gateway or a PC), and makes a GSTN fax that gets sent to any GSTN fax device 💖

In both cases, the Internet side of the gateway acts like an Internet Fax device, as described in [4], while the GSTN side acts like a GSTN fax device, as described in [6] 🎀

In this document we will only thus recommend the actions that occur while:

1. the onramp gateway converts a fax received from GSTN and forwards it to the Internet Fax service;
2. the offramp gateway converts a fax received from the Internet and forwards it to the GSTN fax service.

3. The Offramp Gateway Operations 📤

An offramp gateway MUST, as a minimal requirement, perform the following functions:

• address translation/mapping,
• image format conversion, and
• error/return notification handling

and MAY also perform

• user authorization.

3.1. User Authorization 🔐

An offramp gateway MAY have a user authorization function to confirm that a user is allowed to send their Internet Fax to the GSTN fax service 🫶

Since an Internet Fax is sent as a MIME e-mail message to the offramp gateway, digital signatures can be used to authenticate and authorize the user 💌 S/MIME is one example of a protocol with digital signature services, and it's described in [9][10][11][12][13] 🦋 Other ways of adding a digital signature to a mail message (like OpenPGP [17] [25]) MAY also be used to authenticate and authorize the user ✨

The agent sending the Internet Fax (which might be an onramp gateway) sends the digitally-signed S/MIME or OpenPGP Fax message to the offramp gateway. Then the offramp gateway compares the credentials of the user to figure out if he/she is authorized to send faxes to the GSTN fax service 👀 If the authorization fails, the offramp gateway MUST generate an error delivery notification for the sender of the Internet Fax 🙅‍♀️

3.2. Addressing 📍

An Internet Fax can have multiple e-mail addresses, both as originators and as recipients 💌 For its forwarding function to GSTN fax service, an offramp gateway MUST only consider those addresses which are explicitly itself, i.e., those where the right-hand side of the e-mail address corresponds to the offramp gateway 🎯

Since addresses on the Internet Fax service are e-mail addresses, the offramp gateway MUST convert e-mail addresses into GSTN addresses in order to reach a destination in the GSTN fax service 🔄

The GSTN destination address SHOULD normally be encoded inside the left-hand side of the e-mail address, according to [7]. However, an offramp gateway MAY use locally implemented translation rules to map left-hand side strings into GSTN addresses 🗺️

In any case, the offramp gateway MUST process the resultant GSTN address and convert it to a "local-phone", in accordance with local dialing rules 📞

"Global-phone" is defined in Section 2 of [7]. "Local-phone" is defined in Section 2 of [8]. "Exit-code" is defined in Section 2.1 of [8] 💜

The offramp gateway SHOULD also have a function to apply translation to originator addresses and other addresses referred to into the Internet Fax. That way there's a possible return path from GSTN fax service to Internet Fax destinations, including other offramp gateways 🔁 These functions MUST be compliant with the address handling of onramp gateways described in Section 4.2 of this document 🫶

3.2.1. Examples of Local Dialing Rules Applied to GSTN Destination Addresses 🇬🇧

okay the first example is giving "global-phone to local-phone" 📞 The offramp gateway removes the "+" and "44" (since it recognizes the international country code is local), and then it can dial directly without an exit-code 💅

global-phone:  +441164960348

resulting in:

local-phone:   1164960348

The next example is also a "global-phone to local-phone" conversion. The offramp gateway removes the "+" and "44" (recognizing the international country code is local), and then adds the exit-code "0" in front of the string 🌸

global-phone:   +441164960348

resulting in:

local-phone:   01164960348

Another one! The offramp gateway removes the "+" and "44" (recognizing the international country code is local), and then adds the long distance "0" in front of the string 📏

global-phone:   +441164960348

resulting in:

local-phone:    01164960348

The last example is the plot twist 🙈 The offramp gateway removes the "+", recognizes the international country code is non-local, and adds the local international dialing prefix "00" in front of the string 🌍

global-phone:   +441164960348

resulting in:

local-phone:   00441164960348

3.2.2. Support for Subaddress 📮

An offramp gateway SHOULD support the subaddress. If a subaddress is encoded into the left-hand side of the e-mail address [7], then it MUST be used by the offramp gateway, as specified in T.33 [15], to reach the final GSTN fax recipient 🎯

3.3. Image Format Conversion 🖼️

An offramp gateway MUST convert the file format from TIFF Profile-S for Internet Fax (defined in [16]) into the GSTN fax image format. Other Internet Fax file formats are not considered in this document 🙅‍♀️

3.4. Error/Return Notification Handling 🚨

An offramp gateway SHOULD have a function that lets it send a return notice to the originator Internet Fax device (defined in [4]) when a transmission error happens over the GSTN fax service and the facsimile is not delivered to the destination 💌 The return notice MUST be in Message Delivery Notification (MDN) format and delivered by the offramp gateway over the Internet e-mail transport service used by Internet Fax. The MDN disposition-type MUST be set as "processed", and the disposition-modifier MUST be set as an "error" ✨

If the offramp gateway fails to transmit the MDN, the error information MAY be recorded to a log, and processing MAY end, or the administrator of the gateway system MAY be notified of these errors through a specific method (for example, by an e-mail message). 🫣

The more complex case of Delivery Status Notification (DSN) requests handling is not considered in this document 💀

4. The Onramp Gateway Operations 📥

An onramp gateway MUST, as minimal requirement, perform the following functions:

• address translation/mapping,
• image format conversion, and
• error/return notification handling,

and MAY also perform

• user authorization.

4.1. User Authorization 🔑

An onramp gateway MAY have a user authorization function to confirm that the user is authorized to transmit a facsimile to the Internet fax service. For example, user authorization may be accomplished by getting a user-ID and password received by Dual Tone Multi-Frequency (DTMF), or via a local authorization table based on the GSTN caller-ID 📞

If the authorization process fails, the onramp gateway MUST generate an error message/code for the sender of the GSTN Fax 🙅‍♀️

4.2. Address Translation/Mapping 🗺️

Addresses on Internet Fax service are e-mail addresses, so a recipient of an Internet Fax might be an e-mail user, an Internet Fax device with its own recipients/users, or an offramp gateway 💌 The onramp gateway SHOULD have a functionality to receive destination addresses from GSTN (via DTMF) 📲 However, there are two categories of destination addresses:

• e-mail users and Internet Fax recipient/users
• real GSTN addresses reached via an offramp gateway

We define "indirect address mapping" as the functionality for the first category, and "direct address mapping" as the functionality for the second category 🦋

4.2.1. Indirect Address Mapping

Okay so the onramp gateway MAY implement local address mapping mechanisms (via a table, directory lookup, or something similar) that permit translation from addresses (called "indirect address numbers") received from the GSTN fax sending device into e-mail addresses. A single e-mail address or a list of e-mail addresses MAY correspond to a single indirect address number. 🦋

Here's one mapping example, bestie 🎀:

1. An onramp gateway receives the indirect address number "1234" from the source GSTN facsimile by DTMF.

1234

2. The destination address gets looked up in the address mapping table.

address mapping table
1234 : ifax@example.com

3. An Internet Fax is sent to the address ("addr-spec").

ifax@example.com

"Addr-spec" is defined in Section 3.4.1 of [14].

If the address mapping lookup fails, an error MUST be reported to the originating GSTN fax device. 🚨 That one is non-negotiable, periodt 💅

4.2.2. Direct Address Mapping

If the indirect address mapping specified in 4.2.1 is not implemented, then only "direct address mapping" can be used. The GSTN sending device SHOULD send the full numeric destination address to the onramp gateway via DTMF. Direct address mapping can also be used if indirect address mapping is implemented. ✨

Example time 🌸:

1. An onramp gateway receives the destination telephone number "441164960348" from the source facsimile by DTMF.

441164960348

2. The destination number gets encoded as a "global-phone", so "+" is added to the head of the string.

+441164960348

3. "FAX=" is added in order to build the "fax-mbox" address item.

FAX=+441164960348

4. The destination address is completed by adding the specification of the appropriate offramp gateway, which is supposed to handle the delivery of the fax message to a global-phone address.

FAX=+441164960348@example.com

The procedure for choosing the domain name of an offramp gateway is defined in Section 4.3 ("Relay Function").

"Global-phone", "fax-mbox", and "fax-address" are defined in Section 2 of [7]. "Mta-I-fax" is defined in Section 3 of [7]. "Fax-email" is defined in Section 4 of [7]. 📮

4.2.3. Sender Address Handling

The onramp gateway SHOULD gather information about the GSTN fax sender address (for example, via Caller-ID, if available) and encode it as the sender of the Internet Fax, using the direct address mapping (see Section 4.2.2 of this document). The sender address SHOULD be completed using the onramp gateway address, unless the onramp gateway has additional information with which to specify a different return path. 🕵️‍♀️

If the onramp gateway does not have any sender address information, the Internet Fax sender address SHOULD be set to either a "no-reply" address or an appropriate default mailbox. 💌

4.2.4. Support for Subaddress

An onramp gateway SHOULD support the subaddress. In the case of direct address mapping, the subaddress is specified using the T.33 [15] specification, and encoded as given in [7]. In the case of indirect address mapping, the subaddress MAY be contained inside the address mapping table. 🧚

4.3. Relay Function

The onramp gateway SHOULD provide functionality for choosing the destination offramp gateway by analyzing a destination fax number. A possible method to expand or acquire information from the onramp gateway about offramp gateways MAY include keeping cached information about sender addresses that was sent by other onramp gateways. 🦋

4.4. File Format Conversion

An onramp gateway MUST convert the file format from a facsimile over the GSTN to the file format TIFF Profile-S for Internet Fax, as defined in [16]. The format glow-up is mandatory, no cap 📠✨

4.6. Return Notice Handling

When an onramp gateway receives and analyzes a return notice from the Internet Fax destination, it MAY have the functionality to send the delivery status to a suitable facsimile device on the GSTN through an appropriate offramp gateway. The generated notice sent via GSTN fax SHOULD contain both the human-readable notice information, and the original delivery codes. 📨

If the onramp gateway fails in the transmission of the return notice back to GSTN fax service, the information MAY be recorded into a log, and processing MAY end. As an alternate, the administrator of the gateway system MAY be notified of this notice with a specific method (for example, by sending an e-mail message to a mailbox). 🙈

5. Security Considerations 🔐

Okay, this section is serious, so buckle up 😭

Refer to Section 3.1 ("User Authorization") for authentication for an offramp gateway. OpenPGP [17] [25] can be used to provide authorization services instead of S/MIME. Refer to Section 4.1 ("User Authorization") for authentication for an onramp gateway.

S/MIME and OpenPGP can also be used to encrypt a 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. Here, 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 PGP. 💔

Other security mechanisms, like IPsec [18][19][20][21][2] or TLS [23] 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.

⚠️ Translation: the gateway itself is the trust point, and a fancy protocol on the wire doesn't change that. Plan accordingly, bestie.

6. References 📚

6.1. Informative References

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

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

6.2. Normative References

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

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

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

[6] "Procedures for document facsimile transmission in the general switched telephone network", ITU-T Recommendation T.30, April 1999.

[7] Allocchio, C., "Minimal FAX address format in Internet Mail", RFC 3192, October 2001.

[8] Allocchio, C., "GSTN Address Element Extensions in E-mail Services", RFC 2846, June 2000.

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

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

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

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

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

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

[15] "Facsimile routing utilizing the subaddress", ITU recommendation T.33, July 1996.

[16] Buckley, R., Venable, D., McIntyre, L., Parsons, G., and J. Rafferty, "File Format for Internet Fax", RFC 3949, February 2005.

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

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

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

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

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

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

[24] Mimura, K., Yokoyama, K., Satoh, T., Watanabe, K., and C. Kanaide, "Guidelines for Optional Services for Internet Fax Gateways", RFC 4161, August 2005.

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

Authors' Addresses 💜

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

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


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

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


Claudio Allocchio
Consortium GARR
Viale Palmiro Togliatti 1625
00155 Roma, Italy

Fax: +39 040 3758565
EMail: Claudio.Allocchio@garr.it

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.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property 🪪

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.

Acknowledgement 🫶

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