Minimal PSTN address format in Internet Mail
How to write a phone number as an email address for gateways
Allocchio, C., "Minimal PSTN address format in Internet Mail", RFC 2303, DOI 10.17487/RFC2303, March 1998, <https://www.rfc-editor.org/info/rfc2303>.
@misc{rfc2303,
series = {Request for Comments},
number = 2303,
howpublished = {RFC 2303},
publisher = {RFC Editor},
doi = {10.17487/RFC2303},
url = {https://www.rfc-editor.org/info/rfc2303},
author = {C. Allocchio},
title = {{Minimal PSTN address format in Internet Mail}},
pagetotal = 8,
year = 1998,
month = mar,
} RFC text
Contents
Network Working Group C. Allocchio
Request for Comments: 2303 GARR-Italy
Category: Standards Track March 1998
Minimal PSTN 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 in the
local-part of Internet email addresses, along with an extension
mechanism to allow encoding of additional standard attributes needed
for email gateways to PSTN-based services.
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 PSTN services gateway appeared, a
number of different methods to specify a PSTN address as an e-mail
address have been used by implementors. Two major objectives for this
were
- enable an e-mail user to access these services from his/her
e-mail interface;
- enable some kind of "PSTN over e-mail service" transport, to
reduce the costs of PSTN long distance transmissions, and use the
existing e-mail infrastructure.
This memo describes the MINIMAL addressing method to encode PSTN
addresses into e-mail addresses and the standard extension mechanism
to allow definition of further standard elements. The opposite
problem, i.e. to allow a traditional numeric-only PSTN device user to
access the e-mail transport service, is not discussed here.
All implementations supporting this PSTN over e-mail service MUST
support as a minimum the specification described in this document.
The generic complex case of converting the whole PSTN addressing into
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 PSTN address
The minimal specification of a PSTN address in e-mail address is as
follows:
pstn-address = pstn-mbox [ qualif-type1 ]
pstn-mbox = service-selector "=" global-phone
service-selector = 1*( DIGIT / ALPHA / "-" )
; note that SP (space) is not allowed in
; service-selector.
; service-selector MUST be handled as a case
; INSENSITIVE string by implementations.
Specifications adopting the "pstn-address" definition MUST define a
unique case insensitive "service-selector" element to identify the
specific messaging service involved.
These specifications MUST also define which minimal "qualif-type1"
extensions, if any, MUST be supported for the specified service.
Implementations confirming to these minimal requirements
specification are allowed to ingnore any other non-minimal extensions
address element which can be present in the "pstn-address". However,
conforming implementations MUST preserve all "qualif-type1" address
elements they receive.
The generic "qualif-type1" element is defined as:
qualif-type1 = "/" keyword "=" string
keyword = 1*( DIGIT / ALPHA / "-" )
; note that SP (space) is not allowed in keyword
string = PCHAR
; note that printable characters are %x20-7E
As such, all "pstn-address" extensions elements MUST be defined in
the "qualif-type1" form.
2.1 Minimal "global-phone" definition
We now define the minimal supported syntax for global-phone:
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 "pstn-address"
VOICE=+3940226338
FAX=+12027653000/T33S=6377
SMS=+33-1-88335215
3. The e-mail address of the I-pstn device: mta-I-pstn
An "I-pstn 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-pstn"
mta-I-pstn = 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 pstn-email
The complete structure used to transfer a minimal PSTN address over
the Internet e-mail transport system is called "pstn-email". This
object is a an e-mail address which conforms to RFC822 [2] and
RFC1123 [3] "addr-spec" syntax, with some extra structure which
allows the PSTN number to be identified.
pstn-email = ["/"] pstn-address ["/"] "@" mta-I-pstn
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 "pstn-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 subaddresses (in any
form defined by the specific standard specification for that
service), and these subaddresses need to be given on the same "pstn-
mbox", multiple "pstn-email" elements will be used.
Implementors' note:
The UA could accept multiple subaddress elements for the same
global-phone, but it must generate multiple "pstn-mbox" elements
when passing the message to the MTA.
4.2 Some examples of "pstn-email"
VOICE=+3940226338@worldvoice.com
FAX=+1.202.7653000/T33S=6377@faxserv.org
/SMS=+33-1-88335215/@telecom.com
5. Conclusions
This proposal creates a minimal standard encoding for PSTN addresses
within the global e-mail transport system and defines the standard
extension mechanism to be used to introduce specific new elements.
The proposal requires no changes to existing e-mail software. Each
specific PSTN service using this proposal MUST define its own
"service-selector" specification and MUST define the eventual other
"qualif-type1" elements to be supported for its minimal addressing
specification. An example is in reference [13].
6. Security Considerations
This document specifies a means by which PSTN 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 2304, 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.
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AI ELI5
On this page
§1Introduction
This RFC describes a minimal standard format for encoding PSTN (Public Switched Telephone Network) addresses into email addresses. The goal is to let email users send messages to phones through email gateways, and to enable email networks to be used as a transport for messages destined for phone numbers, reducing the cost of long-distance PSTN calls.
Unlike full PSTN-to-email conversion, which is more complex, this specification provides only the minimum required addressing method. It includes an extension mechanism so service-specific specifications can add additional fields as needed.
The standard is a simple recipe for putting a phone number in an email address so gateways can send the email to the right phone service and phone number.
§2PSTN Address Format
The minimal PSTN address has two parts: a service selector and a global phone number, separated by an equals sign. The service selector is a name like VOICE, FAX, or SMS (case-insensitive) that tells the gateway which service to use. The global phone number starts with a + and contains digits, with optional separators (hyphens or dots) for readability.
The format looks like VOICE=+1-202-765-3000. Each service type must have its own specification defining what service selector it uses and what optional fields it may include. The separators in the phone number are for user convenience; implementations must remove them before processing.
The address has a service name and a phone number starting with a plus sign. It is like writing a label for the type of phone service and then the number to reach.
§3Extensions and Options
Each PSTN address can include optional extensions in the form /keyword=value. These extensions let service-specific standards add extra information. For example, RFC 2303 mentions a FAX extension T33S=6377 that adds additional routing information for fax services, following ITU T.33 recommendations.
When implementations receive extensions they do not recognize, they must preserve them. Implementations that only support the minimal specification can ignore non-minimal extensions. This design lets the standard evolve without breaking existing systems.
You can add extra instructions by using a slash and a keyword, like /T33S=6377 for fax machines that need a subaddress to route the call to the right place.
§4Complete Email Address
The complete email address, called pstn-email, combines the PSTN address with an email domain. The format is VOICE=+1-202-765-3000@example.com. The part before the @ sign (the local part) contains the encoded phone number and follows the quoting rules of RFC 822 and RFC 1123. The part after the @ (the domain) must be a valid email domain that identifies the PSTN gateway service.
Optional slashes can appear at the start or end of the PSTN address for compatibility with other gateway standards, but implementations should not generate them. When multiple subaddresses are needed for one service, multiple email addresses must be used.
The complete address looks like a normal email address, but instead of a person's name you put the phone number with a service type before the @ sign.
§5Security
The main security concern is that email routing is determined by DNS lookups. An attacker who compromises a DNS server, forges a DNS response, or returns incorrect DNS information could force email messages destined for a phone gateway through an untrusted intermediary, where mail security could be compromised.
To mitigate this risk, clients should ensure mail routing is based only on authoritative DNS answers. As DNS Security mechanisms become more widely deployed, clients should use them to verify the authenticity and integrity of mail routing records.
If someone tricks the email system about which server is the phone gateway, they could intercept the message. Use trusted DNS to find the real gateway.