rfc3191.txt

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Network Working Group                                       C. Allocchio
Request for Comments: 3191                                    GARR-Italy
Obsoletes: 2303                                             October 2001
Updates: 2846
Category: Standards Track


              Minimal GSTN 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 (2001).  All Rights Reserved.

Abstract

   This memo describes a simple method of encoding Global Switched
   Telephone Network (GSTN) addresses (commonly called "telephone
   numbers") 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 GSTN-based services.

1. Introduction

   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 an MTA that handles messages for the domain
   given in the right-hand-side.

   Since the very first e-mail to GSTN services gateway appeared, a
   number of different methods to specify a GSTN address as an e-mail
   address have been used by implementors.  Several objectives for this
   methods have been identified, like to enable an e-mail user to access
   GSTN services from his/her e-mail interface, to allow some kind of
   "GSTN over e-mail service" transport (possibly reducing the costs of
   GSTN long distance transmissions) while using the existing e-mail
   infrastructure.







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RFC 3191      Minimal GSTN address format in Internet Mail  October 2001


   This memo describes the MINIMAL addressing method to encode GSTN
   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 GSTN device user
   to access the e-mail transport service, is not discussed here.

   The IANA registration templates which MUST be used to register any
   standard element defined according to this specification are given in
   the "IANA Considerations" chapter (section 7 of this document).

   All implementations supporting this GSTN over e-mail service MUST
   support as a minimum the specification described in this document.
   The generic complex case of converting the entirety of GTSN
   addressing into e-mail is out of scope in this minimal specification.

1.1 Terminology and Syntax conventions

   In this document the formal definitions are described using ABNF
   syntax, as defined into [7].  This memo also uses some of the "CORE
   DEFINITIONS" defined in "APPENDIX A - CORE" of that document.  The
   exact meaning of the capitalized words

      "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
      "SHOULD NOT", "RECOMMENDED", "MAY", "OPTIONAL"

   is defined in reference [6].

   In this document the following new terms are also defined:

      I-pstn device:
         a device which has an Internet domain name and it is able to
         communicate either directly or indirectly with the GSTN
         network;

      mta-I-pstn:
         the Internet domain name which identifies uniquely an I-pstn
         device over the Internet;

      pstn-email:
         the complete Internet e-mail address structure which is used to
         transport a GSTN address over the Internet e-mail service.

2. Minimal GSTN address

   The minimal specification of a GSTN address within an e-mail address
   is as follows:





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      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.

   Other specifications adopting the "pstn-address" definition MUST
   define and register with IANA a unique case insensitive
   "service-selector" element to identify the specific messaging service
   involved.

   These specifications and registrations MUST also define which minimal
   "qualif-type1" extensions, if any, MUST be supported for the
   specified messaging service.

   Implementations confirming to this minimal requirements specification
   are allowed to ignore any other non-minimal extensions address
   element which is 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" extension elements MUST be defined in the
   "qualif-type1" form at the time of registration with IANA.

2.1 Minimal "global-phone" definition

   The purpose of global-phone element is to represent standard E.164
   numeric addresses [10] within a syntax for electronic mail addressing
   that is compliant with standard e-mail specifications given in [1]
   and [2].







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   The minimal supported syntax for global-phone element is as follows:

      global-phone = "+" 1*( DIGIT / written-sep )

      written-sep = ( "-" / "." )

   The use of other dialing schemes for GSTN numbers (like private
   numbering plans or local dialing conventions) is also allowed.
   However, this does not preclude nor remove the mandatory requirement
   for support to the "global-phone" syntax within the minimal GSTN
   address format.

   Any other dialing schemes MUST NOT use the leading "+" defined here
   between the "=" sign and the dialing string.  The "+" sign is
   strictly reserved for the standard "global-phone" syntax.

   Note:

      The specification of alternate dialing schemas is out of scope for
      this minimal specification.

   This document also permits the use of written-sep elements in order
   to improve human readability of GSTN e-mail addresses.  The
   written-sep are elements which can be placed between dial elements
   such as digits etc.

   Implementors' note:

      Use of the written-sep elements is allowed, but not recommended
      for transmission.  Any occurrences of written-sep elements in a
      pstn-mbox MUST be ignored by all conformant implementations.

2.2 The minimal "pstn-address" examples

   Some examples of minimal pstn-address are:

      VOICE=[phone]

      FAX=[phone]/T33S=6377

      SMS=[phone]

   Note:

      these examples are given as illustrations only; they do not
      necessarily represent valid pstn-addresses.





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3. The e-mail address of the I-pstn device: mta-I-pstn

   An "I-pstn device" has, among its characteristics, a unique Internet
   domain name which identifies it on the Internet.  Within Internet
   mail, this is the Right Hand Side (RHS) part of the address, i.e.,
   the part on the right of the "@" sign.  For purposes of this document
   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 standards <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:

      the use of "domain names" or "domain literals" is permitted in
      addresses in both the SMTP envelope and message header fields.

4. The pstn-email

   The complete structure used to transfer a minimal GSTN address over
   the Internet e-mail transport system is called "pstn-email".  This
   object is a an e-mail address which conforms to [2] and [3]
   "addr-spec" syntax, with structure refinements which allows the GSTN
   number to be identified.

      pstn-email =  ["""] ["/"] pstn-address ["/"] ["""] "@" mta-I-pstn

   Implementors' note:

      The optional "/" characters can result from translations from
      other transport gateways (such as some X.400 gateways) which have
      included the "/" as 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.  The relevant standard [2], [3] define exactly
      when the optional "quotes" characters surrounding the entire local
      part (i.e., the part on the left of the "@" character into the
      pstn-email) MUST be added.

4.1 Multiple subaddresses

   There are some instances in GSTN applications where multiple
   subaddresses are used.  On the other hand in e-mail practice a
   separate and unique e-mail address is always used for each recipient.



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   In the event a particular GSTN service requires multiple subaddresses
   (in any form defined by the standard specification for that GSTN
   service) that are associated with the same "pstn-mbox", then the use
   of multiple "pstn-email" elements is REQUIRED.

   Implementors' note:

      The UA may accept multiple subaddress elements for the same
      global-phone, but it MUST generate multiple "pstn-mbox" elements
      when submitting the message to the MTA.

4.2 Some examples of minimal "pstn-email" addresses

   Some examples of minimal pstn-email addresses follows:

         VOICE=[e-mail]

         FAX=+1.202.7653000/T33S=[e-mail]

         /SMS=[phone]/@telecom.com

   Note:

      these examples are given as illustrations only; they do not
      necessarily represent valid pstn-addresses.

5. Conclusions

   This proposal creates a minimal standard encoding for GSTN addresses
   within the global e-mail transport system.  It also defines the
   standard extension mechanism to be used to introduce new elements for
   GSTN addresses.

   The proposal is consistent with existing e-mail standards.  Each
   specific GSTN service using this proposal MUST define and register
   with IANA its own "service-selector" specification and MUST define
   and register the eventual other "qualif-type1" elements needed for
   its specific application.  An example of such an application is
   contained in reference [13].

6. Security Considerations

   This document specifies a means by which GSTN addresses can be
   encoded into e-mail addresses.  Since e-mail routing is determined by
   Domain Name System (DNS) data, a successful attack to DNS could
   disseminate tampered information, which causes e-mail messages to be
   diverted via some MTA or Gateway where the security of the software
   has been compromised.



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   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. IANA Considerations

   As the service-selector and qualif-type1 elements values are
   extensible, they MUST be registered with IANA.

   To register a service-selector or a qualif-type1 element, the
   registration form templates given in 7.1 and 7.2 MUST be used. Any
   new registration MUST fulfill the "Specification Required" criteria,
   as defined in RFC 2434, section 2 [16]:

     "Specification Required - Values and their meaning MUST be
      documented in an RFC or other permanent and readily available
      reference, in sufficient detail so that interoperability between
      independent implementations is possible."