Network Working Group                                S. Hardcastle-Kille
Request for Comments: 1328                     University College London
                                                                May 1992


                     X.400 1988 to 1984 downgrading

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This document considers issues of downgrading from X.400(1988) to
   X.400(1984) [MHS88a, MHS84].  Annexe B of X.419 specifies some
   downgrading rules [MHS88b], but these are not sufficient for
   provision of service in an environment containing both 1984 and 1988
   components.  This document defines a number of extensions to this
   annexe.

   This specification is not tutorial.  COSINE Study 8.2 by J.A.I.
   Craigie gives a useful overview [Cra88].

1.  The need to Downgrade

   It is expected that X.400(1988) systems will be extensively deployed,
   whilst there is still substantial use of X.400(1984).  If 1988
   features are to be used, it it important for there to be a clear
   approach to downgrading.  This document specifies an approach to
   downgrading for the Internet and COSINE communities.  As 1988 is a
   strict superset of 1984, the mapping is a one-way problem.

2.  Avoiding Downgrading

   Perhaps the most important consideration is to configure systems so
   as to minimise the need for downgrading.  Use of 1984 systems to
   interconnect 1988 systems should be strenuously avoided.

   In practice, many of the downgrading issues will be avoided.  When a
   1988 originator sends to a 1984 recipient, 1988 specific features
   will not be used as they will not work!  For distribution lists with
   1984 and 1988 recipients, messages will tend to be "lowest common
   denominator".




Hardcastle-Kille                                                [Page 1]

RFC 1328             X.400 1988 to 1984 downgrading             May 1992


3.  Addressing

   In general there is a problem with O/R addresses which use 88
   specific features.  The X.419 downgrade approach will mean that
   addresses using these features cannot be specified from 84 systems.
   Worse, a message originating from such an address cannot be
   transferred into X.400(1984).  This is unacceptable.  Two approaches
   are defined.  The first is a general purpose mechanism, which can be
   implemented by the gateway only.  The second is a special purpose
   mechanism to optimise for a form of X.400(88) address which is
   expected to be used frequently (Common Name).  The second approach
   requires cooperation from all X.400(88) UAs and MTAs which are
   involved in these interactions.

3.1  General Approach

   The first approach is to use a DDA "X400-88".  The DDA value is an
   std-or encoding of the address as defined in RFC 1327 [Kil92].  This
   will allow source routing through an appropriate gateway.  This
   solution is general, and does not require co-operation.  For example:

88:
     PD-ADDRESS=Empire State Building;  PRMD=XX; ADMD=ZZ; C=US;

84:
     O=MHS-Relay; PRMD=UK.AC; C=GB;
     DD.X400-88=/PD-ADDRESS=Empire State Building/PRMD=XX/ADMD=ZZ/C=US/;

   The std-or syntax can use IA5 characters not in the printable string
   set (typically to handle teletext versions).  To enable this to be
   handled, the std-or encoded in encapsulated into printable string
   using the mappings of Section 3.4 of RFC 1327.  Where the generated
   address is longer than 128 characters, up to three overflow domain
   defined attributes are used:  X400-C1; X400-C2; X400-C3.

3.2  Common Name

   Where a common name attribute is used, this is downgraded to the
   Domain Defined Attribute "Common".  For example:

   88:
       CN=Postmaster; O=A; ADMD=B; C=GB;

   84:
       DD.Common=Postmaster; O=A; ADMD=B; C=GB;

   The downgrade will always happen correctly.  However, it will not
   always be possible for the gateway to do the reverse mapping.



Hardcastle-Kille                                                [Page 2]

RFC 1328             X.400 1988 to 1984 downgrading             May 1992


   Therefore, this approach requires that all 1988 MTAs and UAs which
   wish to interact with 1984 systems through gateways following this
   specification will need to understand the equivalence of these two
   forms of address.

4.  MTS

   Annexe B of X.419 is sufficient, apart from the addressing.

   The discard of envelope fields is unfortunate.  However, the
   criticality mechanism ensures that no information the originator
   specifies to be critical is discarded.  There is no sensible
   alternative.  If mapping to a system which support the MOTIS-86 trace
   extensions, it is recommended that the internal trace of X.400(88) is
   mapped on to this, noting the slight differences in syntax.

5.  IPM Downgrading

   The IPM service in X.400(1984) is usually provided by content type 2.
   In many cases, it will be useful for a gateway to downgrade P2 from
   content type 22 to 2.  This will clearly need to be made dependent on
   the destination, as it is quite possible to carry content type 22
   over P1(1984).  The decision to make this downgrade will be on the
   basis of gateway configuration.

   When a gateway downgrades from 22 to 2, the following should be done:

   1.  Strip any 1988 specific headings (language indication, and
       partial message indication).

   2.  Downgrade all O/R addresses, as described in Section 3.

   3.  If a directory name is present, there is no method to preserve
       the semantics within a 1984 O/R Address.  However, it is
       possible to pass the information across, so that the information
       in the Distinguished Name can be informally displayed to the
       end user.  This is done by appendend a text representation of
       the Distinguished Name to the Free Form Name enclosed in round
       brackets.  It is recommended that the "User Friendly Name"
       syntax is used to represent the Distinguished Name [Kil90].  For
       example:

       (Steve Hardcastle-Kille, Computer Science,
        University College London, GB)

   4.  The issue of body part downgrade is discussed in Section 6.





Hardcastle-Kille                                                [Page 3]

RFC 1328             X.400 1988 to 1984 downgrading             May 1992


5.1  RFC 822 Considerations

   A message represented as content type 22 may have originated from RFC
   822 [Cro82].  The downgrade for this type of message can be improved.
   This is discussed in RFC 1327 [Kil92].

6.  Body Part downgrading

   The issue of body part downgrade is very much linked up with the
   whole issue of body part format conversion.  If no explicit
   conversion is requested, conversion depends on the MTA knowing the
   remote UA's capabilities.  The following options are available for
   body part conversion in all cases, including this one.  It is assumed
   that body part conversion is avoided where possible.

   1.  Downgrade to a standard 1984 body part, without loss of
       information

   2.  Downgrade to a standard 1984 body part, with loss of information

   3.  Discard the body part, and replace with a (typically IA5 text)
       message.  For example:

       **********************************************
       *
       *  There was a hologram here which could
       *  not be converted
       *
       **********************************************

   4.  Bounce the message

   If conversion is prohibited, 4) must be done.  If conversion-with-
   loss is prohibited, 1) should be done if possible, otherwise 4).  In
   other cases 2) should be done if possible.  If it is not possible,
   the choice between 3) and 4) should be a configuration choice.  X.419
   only recognises 4).  3) Seems to be a useful choice in practice,
   particularly where the message contains other body parts.  Another
   option is available when downgrading:

      1.  Encapsulate the body part as a Nationally Defined 1984
          body part (body part 7).

   This should be used when configured for the recipient UA.







Hardcastle-Kille                                                [Page 4]

RFC 1328             X.400 1988 to 1984 downgrading             May 1992


References

   [Cra88]  Craigie, J., "Migration strategy for x.400(84) to
            x.400(88)/MOTIS", COSINE Specification Phase 8.2, RARE, 1988.

   [Cro82]  Crocker, D., "Standard of the Format of ARPA Internet Text
            Messages", RFC 822, UDEL, August 1982.

   [Kil90]  Kille, S., "Using the OSI directory to achieve user friendly
            naming", Research Note RN/90/29, Department of Computer
            Science, University College London, February 1990.

   [Kil92]  Kille, S., "Mapping between X.400(1988) / ISO 10021 and RFC
            822", RFC 1327, University College London, May 1992.

   [MHS84]  Recommendations X.400, October 1984. CCITT SG 5/VII, Message
            Handling Systems:  System Model - Service Elements.

   [MHS88a] CCITT recommendations X.400 / ISO 10021, April 1988. CCITT
            SG 5/VII / ISO/IEC JTC1, Message Handling:  System and
            Service Overview.

   [MHS88b] CCITT recommendations X.419/ ISO 10021, April 1988.
            CCITT SG 5/VII / ISO/IEC JTC1, Message Handling:  Protocol
            Specifications.

7.  Security Considerations

   Security issues are not discussed in this memo.

8.  Author's Address

   Steve Hardcastle-Kille
   Department of Computer Science
   University College London
   Gower Street
   WC1E 6BT
   England

   Phone:  +44-71-380-7294
   EMail:  S.Kille@CS.UCL.AC.UK










Hardcastle-Kille                                                [Page 5]