RFC 1328 (rfc1328) - Page 2 of 5

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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
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