Download - Emails on internet
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Hello.. ladies and gentlemen!!!
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ailE-
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ailE-
Electronic mail, or e-mail has been around for
over two decades.
has its own conventions and styles
little ASCII symbols called smileys or
emoticons in e-mail
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rotating the book 90 degrees clockwise will
make them clearer
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The first e-mail systems simply consisted of
file transfer protocols
recipient's address
1. Sending a message to a group of people was
inconvenient
2. Messages had no internal structure, making
computer processing difficult.3. The originator (sender) never knew if a message
arrived or not.
Limitations of the above approach:
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4. If someone was planning to be away on
business for several weeks and wanted all
incoming e-mail to be handled by his
secretary, this was not easy to arrange.
5. poor user interface
6. no multimedia support
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Architecture and Services
overview of what e-mail systems can do and
how they are organized
consist of two subsystems:
user agents
message transfer agents
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e-mail systems support five basic functions
Composition refers to the process of creatingmessages and answers
Transfer refers to moving messages from theoriginator to the recipient
Reporting has to do with telling the originatorwhat happened to the message
Displaying incoming messages is needed sopeople can read their e-mail
Disposition is the final step and concerns whatthe recipient does with the message afterreceiving it.
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Additional services:
mailboxes
mailing list
carbon copies blind carbon copies
high-priority e-mail
secret (i.e., encrypted) e-mail
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Key ideas:
envelope contains all the information needed
for transporting the message, such as the
destination address, priority, and security level
message:
header
body
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Envelopes and messages. (a) Paper mail. (b) Email.
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The User Agent
A program (sometimes called a mail reader)that accepts a variety of commands for
composing
receiving replying to messages
manipulating mailboxes
User agent interface: fancy menu- or icon-driven
1-character commands from the keyboard
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Sending E-mail:
the destination address
a word processing program, or possibly with a
specialized text editor built into the user agent
destination address format: user@dns-address
Most e-mail systems support mailing lists:
locally remotely
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Reading E-mail
K:read or unread
A:answered(replied)
F:forwarded
Flags: Command user interface:D:deleteK:read or unread
A:answered(replied)
F:forwarded
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Message Formats
RFC 822:
basic e-mail system using ASCII encoding
designed decades ago no clear distinction between envelope fields
and header fields
user agent builds a message and passes it tothe message transfer agent
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RFC 822 header fields related to
message transport
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Some fields used in the RFC 822
message header
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MIMEThe Multipurpose Internet Mail Extensions
Earlier e-mail systems: English and ASCII using RFC 822
Problems include sending and receiving:
1. Messages in languages with accents (e.g., French
and German). 2. Messages in non-Latin alphabets (e.g., Hebrew and
Russian).
3. Messages in languages without alphabets (e.g.,
Chinese and Japanese). 4. Messages not containing text at all (e.g., audio or
images).
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RFC 822 headers added by MIME
ASCII String
message id
Type of encoding
7 types
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MIME header, continued
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Message Transfer : SMTP
transferring the message between sender and
receiver
SMTP : simple mail transfer protocol
TCP connection to port 25 of the destination
machine
No checksums are needed because TCP
provides a reliable byte stream
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Problems with SMTP:
older implementations cannot handle
messages exceeding 64 KB
different timeouts for client and server
solution extended SMTP (ESMTP)
clients wanting to use it should send an EHLO
message instead ofHELO initially
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POP3(Post Office Protocol V3):
simple download-and-delete requirements foraccess to remote mailboxes
POP3 begins when the user starts the mail
reader TCP connection to port 110
Goes through 3 states:
Authorization
Transactions
Update
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POP3
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IMAP (Internet Message Access Protocol)
all the e-mail will remain on the server
indefinitely in multiple mailboxes
reading even parts of messages for slower
modem connections
allows for creating, destroying, and
manipulating multiple mailboxes on the server
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Comparison b/w POP3 and IMAP:
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Thank
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