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Sending and receiving X.400 messages is implemented as a two-stage process. One stage is responsible for converting between RFC 822 and MIME, the addressing format and message representation used by PMDF, and the P1/P2/P22 representation used by X.400. This stage is realized by the X400_TO_MIME and MIME_TO_X400 channels. The second stage is responsible for the actual transfer of messages to and from the remote X.400 MTA. The second stage is realized by the X400_LOCAL channel.
The PMDF CONFIGURE X400 (OpenVMS) or pmdf configure x400
utility will create the file x400.chans in the PMDF table
directory1 containing the definitions of the above-mentioned
channels.
41.1.1 X400_TO_MIME Channel
Messages are submitted to the X400_TO_MIME channel from the X400_LOCAL
channel and converted from P1/P2/P22 into RFC 822 and MIME and then
resubmitted to the appropriate PMDF channel for delivery. Note that
this could be the local channel, if the message is to be delivered to a
local mail user, or it could be a channel to some other host or gateway.
The X400_TO_MIME channel interprets the FROM_X400 rules database during address conversion; this database, stored in the PMDF table directory, is pointed to by the PMDF_FROM_X400_DATABASE logical (OpenVMS) or tailor file option (UNIX). See Section 41.4.6 for more details on this database. These rules govern the form of RFC 822 address produced by a given X.400 address and thereby determine the further disposition of the message for recipients for which the PMDF/PMDF-X400 MTA is responsible.
41.1.2 MIME_TO_X400 Channel
Messages submitted to the MIME_TO_X400 channel from any other PMDF
channel are converted into P1/P2/P22 and then resubmitted to the
X400_LOCAL channel for transfer to the remote X.400 MTA.
The address conversion process performed by the MIME_TO_X400 channel is governed by the rules contained in the TO_X400 database; this database, stored in the PMDF table directory, is pointed to by the PMDF_TO_X400_DATABASE logical (OpenVMS) or tailor file option (UNIX). See Section 41.4.6 for more details on this database. These rules determine the form of X.400 address produced by a given RFC 822 address. If multiple remote X.400 MTAs are accessible to PMDF-X400, the MIME_TO_X400 channel also makes the determination, by applying the X400-MTAID mapping to the X.400 address, of which X.400 MTA needs to be contacted for message transfer.
41.1.3 X400_LOCAL Channel
Messages submitted to the X400_LOCAL channel have already been encoded
into the X.400 P1/P2/P22 representation and need only be
bulk-transferred to the target MTA. In principle, any sort of reliable
data transfer mechanism could be used at this stage. The X.400
recommendations, however, specify that the Reliable Transfer Service
Element (RTSE) of the OSI network protocol suite be used for the
transfer of P1/P2/P22 message data between X.400 MTAs. The X400_LOCAL
channel implements RTSE and is responsible for transferring message
data between the local PMDF-X400 and a remote RTSE service provider.
The X400_LOCAL channel implements RTSE as specified in the 1992, 1988, and 1984 X.400 recommendations. RTSE parameters and modes currently supported by PMDF-X400 are described in Table 41-1.
| Transfer mode: | 1988-Normal, 1988-X410, or 1984 |
| Dialog mode: | MONOLOGUE |
| Transport class: | 0, 2, or 4 |
| MTA password form: | IA5 or octet-stream |
There are a number of X.400 RTSE parameters normally negotiated between the two communicating X.400 MTAs. These parameters are not user settable in PMDF-X400; PMDF-X400 negotiates them with the other side. A partial listing is shown in Table 41-2.
| Maximum TPDU size: | PMDF-X400 accepts any size |
| Default TPDU size: | PMDF-X400 proposes 2048 |
| Checkpoint size: | PMDF-X400 defaults to 63 |
| Window size: | PMDF-X400 defaults to 3 |
The X400_LOCAL channel is also responsible for accepting incoming network connections and receiving P1/P2/P22 data over the network connection from the remote X.400 MTA. Received P1/P2/P22 data is placed into a file without interpretation and submitted to the X400_TO_MIME channel for conversion and delivery.
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