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Figure 8.83 CCA and its objects.
Control Objects
CCA
Display Object
Lower ISO Layers
Lower ISO Layers
VT Association Terminal
Application Mapping
Device Objects
Control Objects
CCA
Display Object
Mapping Device
Objects
8.3.2. Message Handling Systems
Electronic mail e-mail represents one of the most successful classes of network applications currently enjoyed by many users. Early e-mail systems were network
dependent, and their use was limited to the private networks of individual organizations. The CCITT X.400 and the ISO 10021 series of standards for
Message Handling Systems MHS have paved the way for standardized and widely-available e-mail services.
Figure 8.85 illustrates the X.400 view of MHS architecture. At the center of
MHS is a Message Transfer System MTS which handles the delivery of messages. The system consists of the following components:
•
A Message Transfer Agent MTA is responsible for the routing of complete e-mail messages called envelopes through the MTS. MTAs handle envelopes
in a store-and-forward fashion.
•
A User Agent UA manages a user’s mailbox. It enables the user to create, submit, and receive messages. The UA may serve an application or provide a
user interface for direct interaction. UAs typically run on multi-user systems e.g., mainframes.
•
A Message Store MS acts on behalf of a UA running on a system which may not be available on a continuous basis e.g., personal computers. MSs are
typically used within a LAN environment, serving a collection of personal computers.
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101
Figure 8.84 MHS architecture.
MTA
Message Transfer System MTS
UA
UA UA
MTA
MTA MTA
MS UA
UAs and MSs make it possible for users to receive messages when they are not personally present, and while even their terminal or personal computer is not
switched on. They simply store the messages and notify the user at the earliest opportunity.
Each user has its own UA. Furthermore, each user is identified by a unique address which is structured in a hierarchical fashion similar to a postal address. The
address structure reflects the division of MTAs into domains. Domains exist at various level of abstraction: country, organization, unit, etc.
Figure 8.85 illustrates the general structure of an envelope. It consists of contents
and addressing information. The contents consists of two parts: heading and body. A heading is comprised of fields such as recipients’ addresses, addresses to which
the message should be copied, originator’s address, subject, etc. Some of the heading information is provided by the user e.g., recipients’ addresses and subject,
others are automatically inserted by the UA e.g., date and originator’s address. The body
contains the message information itself. It may consist of more than one part, each of which may be of a different type e.g., text, digitized voice, digitized image.
An envelope is constructed by a UA by deriving envelope addressing
information from the heading and adding it to the contents. MTAs only deal with envelopes and their addressing. They have no interest in the contents of an envelope.
Each receiving MTA looks at the addressing of an envelope, records on it the last leg of the route so far, time stamps it, and hands it over to the next MTA. The envelope
therefore bears a complete trace of its route through the network of MTAs.
102 Communication Networks
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Figure 8.85 Envelope structure.
To: Cc:
From: Subject:
...
Heading
Body Contents
Envelope
Envelope addressing info...
As would be expected, the MHS service is defined by a set of service primitives. Figure 8.35 summarizes the primitives and their purpose.
Figure 8.86 MHS service primitives. Primitive
Issued By Types Purpose
LOGON UA
request response
To initiate a session. If successful, the UA is informed of any messages waiting.
MTA indicate
confirm LOGOFF
UA request
To terminate a session. MTA
confirm CHANGE
UA request
To change the UAs logon password. PASSWORD
MTA indicate
confirm REGISTER
UA request
To change the UAs profile maintained by the MTA
confirm MTA.
SUBMIT UA
request To submit a message an envelope to the MTA.
MTA confirm
DELIVER MTA
indicate To deliver a submitted message to the UA.
NOTIFY MTA
indicate To notify the UA of message delivery.
CANCEL UA
request To cancel a message delivery to a UA.
MTA confirm
PROBE UA
request To check if a SUBMIT would be successful.
MTA confirm
CONTROL UA
request response
To alter the ways in which message storage and delivery is controlled by the MTA.
MTA indicate
confirm
