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
