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.


   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

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

     - 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.

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RFC 2303             Minimal PSTN in Internet Mail            March 1998

   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


   is defined in reference [6].

2. Minimal PSTN address

   The minimal specification of a PSTN address in e-mail address is as

      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,

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   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.

     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

   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.

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2.2 Some examples of a minimal "pstn-address"




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

   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.

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   It is essential to remind that "pstn-address" element MUST strictly
   follow the "quoting rules" spcified in the relevant standards [2],

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"



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

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RFC 2303             Minimal PSTN in Internet Mail            March 1998

   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

   X.400:  C=it;A=garr;P=Trieste;O=Elettra;
   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

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RFC 2303             Minimal PSTN in Internet Mail            March 1998

   [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.

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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

   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

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