ITU-T Model
- International
Federation for Information Processing developed a model for message
handling.
- This model was
eventually adopted and expanded by the International Telecommunication
Union –Telecommunication(ITU-T),which developed the X.400 series
recommendations, Message Handling System(MHS)
- Internet Mail is
not based on ITU-T standards
- E-mail messages
are transported by a message transfer system(MTS),which is composed of one
or more message transfer agents(MTAs)
- At the borders of
the system, User Agent(UA) acts on behalf of a user and interface to its
local message transfer agent.
- The E-mail message
being sent is called the content.
- All delivery
information associated with the message is the envelope.
- The MTS is not aware
of the structure of the content it transports.
- There are two
types of content in each e-mail message
- Control
information(often called the headers)
- Data
information(often called the body)
- The envelope is
meaningful to the message transfer agents.
- The headers are
meaningful to the user agents.
- The body is
meaningful to the users.(people or programs)
When E-mail is sent
from one user to another, the following activities occur
a) The
originating user indicates to the UA the address of the recipient
b) The
UA places the destination address and the senders address into the envelope and
then posts the message through a posting slot to a message transfer agent, which
involves a posting protocol in which the validity of those addresses and the
syntax of the e-mail message considered.
c) Upon
successful completion of the submission protocol, the MTA accepts the
responsibility to deliver the e-mail message or if delivery fails, to inform
the originating user of the failure by generating an error report.
d) After
accepting responsibility to deliver the e-mail message, an MTA must decide if
it can deliver the message directly to the recipient; if so it delivers the
e-mail message through a delivery slot to the recipients UA, using a delivery
protocol.
e) If
not, it contacts an adjacent MTA that is closer to the recipient and negotiates
transfer of the e-mail message.
f) This
process repeats until some MTA is able to deliver the e-mail message or some
MTA determines that the message is undeliverable
Given
this model for e-mail, one realizes that
a) E-mail
transfer is third –party in nature.
b) E-mail
transfer is store and forward in nature
There are three general protocols involved in the
model
a) A
messaging protocol used between two UAs
b) A
relaying protocol used between two MTAs
c) A
submission/delivery protocol used between an MTA and a UA
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