MODEL FOR MESSAGE HANDLING

 

ITU-T Model

  1. International Federation for Information Processing developed a model for message handling.
  2. 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)
  3. Internet Mail is not  based on ITU-T standards
  4. E-mail messages are transported by a message transfer system(MTS),which is composed of one or more message transfer agents(MTAs)
  5. At the borders of the system, User Agent(UA) acts on behalf of a user and interface to its local message transfer agent.
  6. The E-mail message being sent is called the content.
  7. All delivery information associated with the message is the envelope.
  8. The MTS is not aware of the structure of the content it transports.
  9. There are two types of content in each e-mail message
    1. Control information(often called the headers)
    2. Data information(often called the body)
  10. The envelope is meaningful to the message transfer agents.
  11. The headers are meaningful to the user agents.
  12. 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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