Minimal FAX address format in Internet Mail
A standard format for putting fax numbers in email addresses.
Allocchio, C., "Minimal FAX address format in Internet Mail", RFC 2304, DOI 10.17487/RFC2304, March 1998, <https://www.rfc-editor.org/info/rfc2304>.
@misc{rfc2304,
series = {Request for Comments},
number = 2304,
howpublished = {RFC 2304},
publisher = {RFC Editor},
doi = {10.17487/RFC2304},
url = {https://www.rfc-editor.org/info/rfc2304},
author = {C. Allocchio},
title = {{Minimal FAX address format in Internet Mail}},
pagetotal = 8,
year = 1998,
month = mar,
} RFC text
Contents
Network Working Group C. Allocchio
Request for Comments: 2304 GARR-Italy
Category: Standards Track March 1998
Minimal FAX address format in 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.
IESG NOTE
This memo describes a simple method of encoding PSTN addresses of
facsimile devices in the local-part of Internet email addresses.
As with all Internet mail addresses, the left-hand-side (local- part)
of an address generated according to this specification, is not to be
interpreted except by the MTA that is named on the right-hand-side
(domain).
1. Introduction
Since the very first e-mail to fax gateway objects appeared, a number
of different methods to specify a fax address as an e-mail address
have been used by implementors. Two major objectives for this were
- enable an e-mail user to send faxes from his/her e-mail
interface;
- enable some kind of "fax over e-mail service" transport, to
reduce the costs of fax transmissions, and use the existing
e-mail infrastructure.
This memo describes the MINIMAL addressing method and standard
extensions to encode FAX addresses in e-mail addresses, as required
in reference [13]. The opposite problem, i.e. to allow a traditional
numeric-only fax device user to access the e-mail transport service,
is not discussed here.
All implementations supporting this FAX over e-mail address format
MUST support as a minimum the specification described in this
document. The generic complex case of converting the whole PSTN
addressing in e-mail is out of scope in this minimal specification:
there is some work in progress in the field, where also a number of
standard optional extensions are being defined.
In this document the formal definitions are described using ABNF
syntax, as defined into [7]. We will also use some of the "CORE
DEFINITIONS" defined in "APPENDIX A - CORE" of that document. The
exact meaning of the capitalised words
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", "OPTIONAL"
is defined in reference [6].
2. Minimal Fax address
The "service-selector" defined in section 2 of reference [13] for the
fax service is:
service-selector = "FAX"
The minimal addressing for the fax service also requires support for
a "qualif-type1" element (see section 2 of reference [13]). This
element is an OPTIONAL element of the fax address, but its support,
when present, is REQUIRED:
qualif-type1 = "/" t33-sep "=" sub-addr
where
t33-sep = "T33S"
sub-addr = 1*( DIGIT )
Thus, the minimal specification of a fax in e-mail address is:
fax-address = fax-mbox [ "/T33S=" sub-addr ]
fax-mbox = "FAX=" global-phone
Note:
See section 4.1 in case multiple sub-addr per fax-mbox need to be
specified.
The Minimal supported syntax for global-phone (as described in
section reference [13]) is:
global-phone = "+" 1*( DIGIT , written-sep )
written-sep = ( "-" / "." )
The use of other dialling schemas for PSTN numbers (like private
numbering plans or local dialling conventions) is also allowed.
However, this does not preclude nor remove the minimal compulsory
requirement to support the "global-phone" syntax as defined above.
Any non "global-phone" dialling schema MUST NOT use the leading "+"
between the "=" sign and the dialling string. The "+" sign is
strictly reserved for the standard "global-phone" syntax.
Note:
The specification of these different dialling schemas is out of
scope for this minimal specification.
User specification of PSTN e-mail addresses will be facilitated if
they can insert these separators between dial elements like digits
etc. For this reason we allow them in the syntax the written-sep
element.
Implementors' note:
Use of the written-sep elements is allowed, but not recommended.
Any occurences of written-sep elements in a pstn-mbox MUST be
ignored by all conformant implementations. User Agents SHOULD
remove written-sep elements before submitting messages to the
Message Transport System.
2.2 Some examples of a minimal "fax-address"
FAX=+3940226338
FAX=+12027653000/T33S=1387
FAX=+33-1-88335215
3. The e-mail address of the I-fax device: mta-I-fax
An "I-fax device" has an e-mail address, or to be more exact, a name
which enables a mail system to identify it on the e-mail global
system.
In Internet mail, this is the Right Hand Side (RHS) part of the
address, i.e. the part on the right of the "@" sign. We will call
this mta-I-fax
mta-I-fax = domain
For "domain" strings used in SMTP transmissions, the string MUST
conform to the requirements of that standard's <domain>
specifications [1], [3]. For "domain" strings used in message
content headers, the string MUST conform to the requirements of the
relevant standards [2], [3].
Note: in both cases, the standards permit use of "domain names" or
"domain literals" in addresses.
4. The fax-email
The complete structure used to transfer a minimal FAX address over
the Internet e-mail transport system is called "fax-email". This
object is an e-mail address which conforms to RFC822 [2] and RFC1123
[3] "addr-spec" syntax, with some extra structure which allows the
FAX number to be identified.
fax-email = ["/"] fax-address ["/"] "@" mta-I-fax
Implementors' note:
The optional "/" characters can result from other mail transport
services gateways, where it is also an optional element.
Implementations MUST accept the optional slashes but SHOULD NOT
generate them. Gateways are allowed to strip them off when
converting to Internet mail addressing.
It is essential to remind that "fax-address" element MUST strictly
follow the "quoting rules" spcified in the relevant standards [2],
[3]
4.1 Multiple subaddresses
In case a particular service requires multiple T.33 subaddresses, and
these subaddresses need to be given on the same "fax-mbox", multiple
"fax-email" elements will be used.
Implementors' note:
The UA could accept multiple subaddress elements for the same
global-phone, but it must generate multiple "fax-mbox" elements
when passing the message to the MTA.
4.2 Some examples of minimal "fax-email"
FAX=+3940226338@faxworld.org
FAX=+12027653000/T33S=1387@faxworld.org
/FAX=+33-1-88335215/@faxworld.org
5. Conclusion
This proposal creates a minimal standard encoding for FAX addresses
within the global e-mail transport system. The proposal requires no
changes to existing e-mail software.
6. Security Considerations
This document specifies a means by which FAX addresses can be encoded
into e-mail addresses. As routing of e-mail messages is determined by
Domain Name System (DNS) information, a successful attack on this
service could force the mail path via some particular gateway or
message transfer agent where mail security can be affected by
compromised software.
There are several means by which an attacker might be able to deliver
incorrect mail routing information to a client. These include: (a)
compromise of a DNS server, (b) generating a counterfeit response to
a client's DNS query, (c) returning incorrect "additional
information" in response to an unrelated query. Clients SHOULD ensure
that mail routing is based only on authoritative answers. Once DNS
Security mechanisms [5] become more widely deployed, clients SHOULD
employ those mechanisms to verify the authenticity and integrity of
mail routing records.
7. Author's Address
Claudio Allocchio
Sincrotrone Trieste
SS 14 Km 163.5 Basovizza
I 34012 Trieste
Italy
RFC822: Claudio.Allocchio@elettra.trieste.it
X.400: C=it;A=garr;P=Trieste;O=Elettra;
S=Allocchio;G=Claudio;
Phone: +39 40 3758523
Fax: +39 40 3758565
8. 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 1982.
[3] Braden, R., "Requirements for Internet hosts - application and
support", RFC 1123, October 1989.
[4] Malamud, C. and M. Rose, "Principles of Operation for the
TPC.INT Subdomain: Remote Printing -- Technical Procedures", RFC
1528, October 1993.
[5] Eastlake, D. and C. Kaufman, "Domain Name System Security
Extensions", RFC 2065, January 1997.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[7] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications", RFC 2234, November 1997.
[8] ITU F.401 - Message Handling Services: Naming and Addressing for
Public Message Handling Service; recommendation F.401 (August
1992)
[9] ITU F.423 - Message Handling Services: Intercommunication
Between the Interpersonal Messaging Service and the Telefax
Service; recommendation F.423 (August 1992)
[10] ITU E.164 - Numbering plan for the ISDN era; recommendation
E.164/I.331 (August 1991)
[11] ITU T.33 - Facsimile routing utilizing the subaddress;
recommendation T.33 (July, 1996)
[12] ETSI I-ETS 300,380 - Universal Personal Telecommunication
(UPT): Access Devices Dual Tone Multi Frequency (DTMF) sender
for acoustical coupling to the microphone of a handset telephone
(March 1995)
[13] Allocchio, C., " Minimal FAX address format in Internet Mail",
RFC 2303, March 1998.
[14] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
between X.400 and RFC 822/MIME", RFC 2156, January 1998.
9. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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AI ELI5
On this page
§1Why Fax over Email
Email users wanted to send faxes from their email interface, and organizations wanted to save money by routing fax traffic through cheaper email networks instead of the traditional phone system. To make this work, someone had to invent a standard way to write a fax number that looks like an email address, so routers and gateways would recognize it and deliver it to the right fax machine.
RFC 2304 describes the minimal set of rules needed for this to work. It does not try to solve the entire problem of PSTN (traditional telephone) addressing, just the core case that email systems could handle reliably.
This RFC answers: how do I write a fax number so an email program can read it and send it to a fax machine?
§2The Format of a Fax Email Address
A fax email address has three parts. The local part (left of the @) contains the fax information: it starts with the word FAX, then an equals sign, then a phone number in the format + followed by digits and optional separators like hyphens or dots. Optionally it can include /T33S= followed by a subaddress, which is a number that identifies a department or device on the same fax line.
The domain part (right of the @) is a normal Internet domain name that identifies the fax gateway service. Examples are FAX=+3940226338@faxworld.org or FAX=+12027653000/T33S=1387@faxworld.org.
The RFC allows optional slashes before and after the fax information (like /FAX=+33-1-88335215/@faxworld.org) to work with other gateway systems, but senders should omit them if possible.
Write it like: FAX=+phonenumber@domain.com. The plus sign means international format. You can add /T33S=number at the end to reach a specific device on the same fax line, like an extension.
§3Phone Number Rules
The RFC defines two ways to write phone numbers in a fax address. The standard way is global format: start with a plus sign, then the country code and the number, with optional hyphens or dots for readability (like +33-1-88335215). The plus sign tells systems this is a standard international number.
Alternative dialling schemas (like private company extensions or local numbering) are allowed, but they MUST NOT use a plus sign, and other systems can ignore them. The RFC does not define these alternatives; it only reserves the plus sign for the standard global format.
Implementers should accept hyphens and dots but remove them before processing, because they are only for humans to read.
Phone numbers in fax email look like international phone numbers: +1-202-765-3000. The plus sign means it is a real country code. Hyphens and dots are OK for readability but the system ignores them.
§4How Gateways Deliver Fax Messages
A fax email gateway is an Internet domain that accepts fax email addresses and converts them to real fax transmissions. It sits between the email system and the fax network. When an email arrives addressed to FAX=+3940226338@faxworld.org, the gateway extracts the phone number, verifies the format, and sends the message content as a fax to that number.
Multiple subaddresses for the same fax number are handled by sending separate messages to the MTA (mail transfer agent) for each subaddress. The user agent (email client) may accept multiple T33S values from the user, but must generate separate fax-email elements when passing the message to the mail system.
The gateway service sits at the @domain.com part. It reads the fax number from your email and converts it to a real fax transmission on the phone network.