Section Three - Configurations
11. Overview
This section specifies how one can configure the MHS to satisfy any of a variety
of functional, physical, and organizational requirements.
This section covers the following topics:
a) Functional configurations
b) Physical configurations
c) Organizational configurations
d) The Global MHS
12. Functional Configurations
This clause specifies the possible functional configurations of the MHS. The
variety of such configurations results from the presence or absence of the Directory,
and from whether a direct user employs an MS.
12.1 Regarding the Directory
With respect to the Directory, the MHS can be configured for a particular user, or
a collection of users (e.g., see clause 14.1), in either of two ways: with or
without the Directory. A user without access to the Directory may lack the capabilities
described in section five.
Note A partially, rather than fully interconnected Directory may exist for an
interim period during which the (global) Directory made possible by Recommendations
for Directories is under construction.
12.2 Regarding the Message Store
With respect to the MS, the MHS can be configured for a particular direct user in
either of two ways: with or without an MS. A user without access to an MS lacks the
capabilities of Message Storage. A user in such circumstances depends upon his UA
for the storage of information objects, a capability that is a local matter.
The two functional configurations identified above are depicted in Figure 7/X.402
which also illustrates one possible configuration of the MTS, and its linkage to
another communication system via an AU. In the figure, user 2 is equipped with an MS
while user 1 is not.
+----+ | 01 | | 02 | | 03 | | 04 | | 05 | | 06 | | 07 | | 08 | | 09 | | 10 | | 11
| | 12 | | 13 | | 14 | | 15 | | 16 | | 17 | | 18 | | 19 | | 20 | | 21 | | 22 | | 23
| | 24 | | 25 | | 26 | | 27 | | 28 | | 29 | +----+
Figure .F.:7/X.402 Functional Configurations Regarding the MS
Note While the users depicted in the figure are people, the figure applies with
equal force and validity to users of other kinds.
13. Physical Configurations
This clause specifies the possible physical configurations of the MHS, i.e., how
the MHS can be realized as a set of interconnected computer systems. Because the
number of configurations is unbounded, the clause describes the kinds of messaging
systems from which the MHS is assembled, and identifies a few important representative
configurations.
13.1 Messaging Systems
The building blocks used in the physical construction of the MHS are called
messaging systems. A .I.gl:messaging system; is a computer system (possibly but not
necessarily an open system) that contains, or realizes, one of more functional objects.
Messaging systems are of the types depicted in Figure 8/X.402.
+----+ | 01 | | 02 | | 03 | | 04 | | 05 | | 06 | | 07 | | 08 | | 09 | | 10 | | 11
| | 12 | | 13 | | 14 | | 15 | | 16 | | 17 | | 18 | | 19 | | 20 | | 21 | | 22 | | 23
| | 24 | | 25 | | 26 | | 27 | | 28 | | 29 | +----+
Figure .F.:8/X.402 Messaging System Types
The types of messaging system, depicted in the figure, are listed in the first
column of Table 8/X.402. For each type listed, the second column indicates the kinds
of functional object--UAs, MSs, MTAs, and AUs--that may be present in such a
messaging system, whether their presence is mandatory or optional, and whether just one or
possibly several of them may be present in the messaging system.
The table is divided into two sections. Messaging systems of the types in the
first section are dedicated to single users, those of the types in the second can (but
need not) serve multiple users.
Table .T.:8/X.402 Messaging Systems
+-----------+--------------------+ | | Functional Objects | | Messaging
+--------------------+ | System | UA MS MTA AU | +-----------+-------------
--------+ | A/SYS | 1 - - - | | S/SYS | - 1 - - | |
AS/SYS | 1 1 - - | +-----------+--------------------+ | T/SYS | -
- 1 [M] | | AT/SYS | M - 1 [M] | | ST/SYS | - M 1
[M] | | AST/SYS | M M 1 [M] | +-----------+--------------------+ +-
Legend -------------------+ | M multip e [...] optional | +----------------------------
-+
The messaging system types, summarized in the table, are individually defined and
described in the clauses below.
Note The following major principles governed the admission of messaging system
types:
a) An AU and the MTA with which it interacts are typically co-located because no
protocol to govern their interaction is standardized.
b) An MTA is typically co-located with multiple UAs or MSs because, of the
standardized protocols, only that for transfer simultaneously conveys a message to
multiple recipients. The serial delivery of a message to multiple recipients served by a
messaging system, which the delivery protocol would require, would be inefficient.
c) No purpose is served by co-locating several MTAs in a messaging system because
a single MTA serves multiple users, and the purpose of an MTA is to convey objects
between, not within such systems. (This is not intended to exclude the possibility
of several MTA-related processes co-existing within a single computer system.)
d) The co-location of an AU with an MTA does not affect that system's behavior
with respect to the rest of the MHS. A single messaging system type, therefore,
encompasses the AU's presence and absence.
13.1.1 Access Systems
An .I.gl:access system; (.I.ab:A/SYS;) contains one UA and neither an MS, an MTA,
nor an AU.
An A/SYS is dedicated to a single user.
13.1.2 Storage Systems
A .I.gl:storage system; (.I.ab:S/SYS;) contains one MS and neither a UA, an MTA,
nor an AU.
An S/SYS is dedicated to a single user.
13.1.3 Access and Storage Systems
An .I.gl:access and storage system; (.I.ab:AS/SYS;) contains one UA, one MS, and
neither an MTA nor an AU.
An AS/SYS is dedicated to a single user.
13.1.4 Transfer Systems
A .I.gl:transfer system; (.I.ab:T/SYS;) contains one MTA; optionally, one or more
AUs; and neither a UA nor an MS.
A T/SYS can serve multiple users.
13.1.5 Access and Transfer Systems
An .I.gl:access and transfer system; (.I.ab:AT/SYS;) contains one or more UAs; one
MTA; optionally, one or more AUs; and no MS.
An AT/SYS can serve multiple users.
13.1.6 Storage and Transfer Systems
A .I.gl:storage and transfer system; (.I.ab:ST/SYS;) contains one or more MSs; one
MTA; optionally, one or more AUs; and no UA.
An ST/SYS can serve multiple users.
13.1.7 Access, Storage, and Transfer Systems
An .I.gl:access, storage, and transfer system; (.I.ab:AST/SYS;) contains one or
more UAs; one or more MSs; one MTA; and optionally, one or more AUs.
An AST/SYS can serve multiple users.
13.2 Representative Configurations
Messaging systems can be combined in various ways to form the MHS. The possible
physical configurations are unbounded in number and thus cannot be enumerated.
Several important representative configurations, however, are described below and in
Figure 9/X.402.
+----+ | 01 | | 02 | | 03 | | 04 | | 05 | | 06 | | 07 | | 08 | | 09 | | 10 | | 11
| | 12 | | 13 | | 14 | | 15 | | 16 | | 17 | | 18 | | 19 | | 20 | | 21 | | 22 | | 23
| | 24 | | 25 | | 26 | | 27 | | 28 | | 29 | +----+
Figure .F.:9/X.402 Representative Physical Configurations
Notes
1. While the users depicted in the figure are people, the figure applies with
equal force and validity to users of other kinds.
2. Besides the physical configurations that result from the "pure" approaches
below, many "hybrid" configurations can be constructed.
13.2.1 Fully Centralized
The MHS may be fully centralized (panel a of the figure). This design is realized
by a single AST/SYS which contains functional objects of all kinds and which can
serve multiple users.
13.2.2 Centralized Message Transfer and Storage
The MHS may provide both Message Transfer and Message Storage centrally but user
access distributedly (panel b of the figure). This design is realized by a single
ST/SYS and, for each user, an A/SYS.
13.2.3 Centralized Message Transfer
The MHS may provide Message Transfer centrally but Message Storage and user access
distributedly (panel c of the figure). This design is realized by a single T/SYS
and, for each user, either an AS/SYS alone or an S/SYS and an associated A/SYS.
13.2.4 Fully Distributed
The MHS may provide even Message Transfer distributedly (panel d of the figure).
This design involves multiple ST-SYNs or T-SYNs.
14. Organizational Configurations
This clause specifies the possible organizational configurations of the MHS, i.e.,
how the MHS can be realized as interconnected but independently managed sets of
messaging systems (which are themselves interconnected). Because the number of
configurations is unbounded, the clause describes the kinds of management domains from
which the MHS is assembled, and identifies a few important representative
configurations.
14.1 Management Domains
The primary building blocks used in the organizational construction of the MHS are
called management domains. A .I.gl:management domain; (.I.ab:MD;) (or
I.gl:domain;) is a set of messaging systems--at least one of which contains, or realizes, an
MTA--that is managed by a single organization.
The above does not preclude an organization from managing a set of messaging
systems (e.g., a single A/SYS) that does not qualify as an MD for lack of an MTA. Such a
collection of messaging systems, a secondary building block used in the MHS'
construction, "attaches" to an MD.
MDs are of several types which are individually defined and described in the
clauses below.
14.1.1 Administration Management Domains
An .I.gl:administration management domain; (.I.ab:ADMD;) comprises messaging
systems managed by an Administration. The major technical distinction between an ADMD
and a PRMD is that the former is positioned above the latter in the MHS' hierarchical
addressing (see clause 18) and routing (see clause 19) regimes.
Note An ADMD provides Message Handling to the public.
14.1.2 Private Management Domains
A .I.gl:private management domain; (.I.ab:PRMD;) comprises messaging systems
managed by an organization other than an Administration. The major technical distinction
between a PRMD and an ADMD is that the former is positioned below the latter in the
MHS' hierarchical addressing (see clause 18) and routing (see clause 19) regimes.
Note A PRMD provides Message Handling, e.g., to the employees of a company, or
to those employees at a particular company site.
14.2 Representative Configurations
MDs can be combined in various ways to form the MHS. The possible organizational
configurations are unbounded in number and thus cannot be enumerated. Several
important representative configurations, however, are described below and in Figure
10/X.402.
+----+ | 01 | | 02 | | 03 | | 04 | | 05 | | 06 | | 07 | | 08 | | 09 | | 10 | | 11
| | 12 | | 13 | | 14 | | 15 | | 16 | | 17 | | 18 | | 19 | | 20 | | 21 | | 22 | | 23
| | 24 | | 25 | | 26 | | 27 | | 28 | | 29 | +----+
Figure .F.:10/X.402 Representative Organizational Configurations
Note Besides the organizational configurations that result from the "pure"
approaches below, many "hybrid" configurations can be constructed.
14.2.1 Fully Centralized
The entire MHS may be managed by one organization (panel a of the figure). This
design is realized by a single MD.
14.2.2 Directly Connected
The MHS may be managed by several organizations, the messaging systems of each
connected to the messaging systems of all of the others (panel b of the figure). This
design is realized by multiple MDs interconnected pair-wise.
14.2.3 Indirectly Connected
The MHS may be managed by several organizations, the messaging systems of one
serving as intermediary between the messaging systems of the others (panel c of the
figure). This design is realized by multiple MDs one of which is interconnected to all
of the others.
15. The Global MHS
A major purpose of this Recommendation and others in the set is to enable the
construction of the .I.gl:Global MHS;, an MHS providing both intra- and inter-organizational, and both intra- and international Message Handling world-wide.
The Global MHS almost certainly encompasses the full variety of functional
configurations specified in clause 12.
The physical configuration of the Global MHS is a hybrid of the pure
configurations specified in clause 13, extremely complex and highly distributed physically.
The organizational configuration of the Global MHS is a hybrid of the pure
configurations specified in clause 14, extremely complex and highly distributed
organizationally.
Figure 11/X.402 gives an example of possible interconnections. It does not
attempt to identify all possible configurations. As depicted, ADMDs play a central
role in the Global MHS. By interconnecting to one another internationally, they
provide an international Message Transfer backbone. Depending upon national
regulations, by interconnecting to one another domestically, they may also provide domestic
backbones joined to the international backbone. ADMDs also serve as primary naming
authorities in the assignment of O/R addresses to users and DLs.
PRMDs play a peripheral role in the Global MHS, being connected to the ADMD
backbone which serves as an intermediary between them.
+----+ | 01 | | 02 | | 03 | | 04 | | 05 | | 06 | | 07 | | 08 | | 09 | | 10 | | 11
| | 12 | | 13 | | 14 | | 15 | | 16 | | 17 | | 18 | | 19 | | 20 | | 21 | | 22 | | 23
| | 24 | | 25 | | 26 | | 27 | | 28 | | 29 | +----+
Figure .F.:11/X.402 The Global MHS
