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rfc:rfc2303

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
  1. enable an e-mail user to access these services from his/her

e-mail interface;

  1. 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.

Allocchio Standards Track [Page 1] 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
    "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,

Allocchio Standards Track [Page 2] RFC 2303 Minimal PSTN in Internet Mail March 1998

 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.

Allocchio Standards Track [Page 3] RFC 2303 Minimal PSTN in Internet Mail March 1998

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.

Allocchio Standards Track [Page 4] RFC 2303 Minimal PSTN in Internet Mail March 1998

 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

Allocchio Standards Track [Page 5] 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
 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)

Allocchio Standards Track [Page 6] 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.

Allocchio Standards Track [Page 7] RFC 2303 Minimal PSTN in Internet Mail March 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
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Allocchio Standards Track [Page 8]

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