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The Application LayerMail

李嘉銘分散系統實驗室成功大學電機系

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The Application Layer

The layers below the application layer are there to provide reliable transport, but they do not do real work for users.

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Electronic Mail

Architecture and ServicesThe User AgentMessage FormatsMessage TransferFinal Delivery

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E-Mail

E-mail, like most other forms of communication, has its own conventions and styles. Jargon

BTW (By The Way), ROTFL (Rolling On The Floor Laughing)…

Smiles

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Smileys/Emoticons

Some smileys. They will not be on the final exam :-).

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The limitations of sending email using FTP (1)

Sending a message to a group of people was inconvenient.Messages had no internal structure, making computer processing difficult.The originator (sender) never knew if a message arrived or not.

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The limitations of sending email using FTP (2)

This was not easy to forwarding all incoming e-mail to be handled by his secretary.The user interface was poorly integrated.It was not possible to create and send messages containing a mixture of text, drawings, facsimile, and voice.

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E-mail Standards

In 1982, the ARPANET e-mail proposals were published as RFC 821 (transmission protocol) RFC 822 (message format) Minor revisions, RFC 2821 and RFC

2822

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ArchitectureHow e-mail systems are organized the user agents (UA), which allow

people to read and send e-mail the message transfer agents

(TA), which move the messages from the source to the destination

the message deliver agents (DA), which deliver (or forward) the mail to user’s mail box

10Mail System Architecture (1)

11Mail System Architecture (2)

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Services

E-mail systems support five basic functions Composition Transfer Reporting Displaying Disposition

13Envelopes and messages (1)

The envelope contains all the information needed for transporting the message.The header contains control information for the user agents.The body is entirely for the human recipient.

14Envelopes and messages (2)

(a) Paper mail.

(b) Electronic mail.

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Sending E-mail

To send an e-mail message, a user must provide the message, the destination address, and possibly some other parameters. user@dns-address

Most e-mail systems support mailing lists (send the same message to a list of people).

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Reading E-mail

An example display of the contents of a mailbox.

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Message Formats – RFC 822 (1)

RFC 822 header fields related to message transport.

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Message Formats – RFC 822 (2)

Some fields used in the RFC 822 message header.

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MIME – Multipurpose Internet Mail Extensions

The problems include sending and receiving Message: Languages

with accents (French, German). in non-Latin alphabets (Hebrew, Russian). without alphabets (Chinese, Japanese).

Not containing text at all (audio or images).

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RFC 822 headers added by MIME

base64 encoding Groups of 24 bits are broken up into four 6-bit units. The coding is ''A'' for 0, ''B'' for 1, and so on, followed by

the 26 lower-case letters, the ten digits, and finally + and / for 62 and 63, respectively.

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The MIME types and subtypes defined in RFC 2045

22A multipart message containing enriched and audio alternatives

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Message Transfer

• SMTP (Simple Mail Transfer Protocol).• Transferring a mes

sage from elinore@abc.com to carolyn@xyz.com.

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Mail aliases

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Mail forward

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MX in DNS ServerhostC IN MX 50 hostB.ee.ncku.edu.tw hostC IN MX 100 hostD.ee.ncku.edu.tw

RelayReceiver is not on this site

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Final Delivery (1)

Internet connection is not always on. To have a message transfer agent on

an ISP machine accept e-mail for its customers.

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Final Delivery (2)

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POP3 (Post Office Protocol Version 3)

How does the user get the e-mail from the ISP's message transfer agent? To create a protocol that allows user

transfer agents (on client PCs) to contact the message transfer agent (on the ISP's machine) and allow e-mail to be copied from the ISP to the user.

RFC 1939

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POP3 protocol states

Authorization.Transactions.Update.

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IMAP (Internet Message Access Protocol)

A protocol for people to operate their mail box and directory Many people have a single e-mail account

at work or school and want to access it from multiple computer.

IMAP assumes that all the e-mail will remain on the server indefinitely in multiple mailboxes.RFC 2060

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A comparison of POP3 and IMAP

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WebMailWebMail 就是使用者透過 browser 連到 web server, 而由 web server 上的程式負責作收信 / 送信的動作 .使用者直接透過 browser 作讀信 / 寫信等操作 , 信件其實並不存在使用者的電腦上 . 因此無論使用者用的是哪一部電腦 , 只要可以連上網路 , 都可以使用讀到之前與新收到的信件 .另外一個好處是 WebMail 不容易因為讀取含有病毒的檔案而導致中毒 , 更不會發生個人郵件系統中毒後 , 寄送大量病毒信件給其他親朋好友的這種慘劇 .

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WebMail -- Through Imap/POP3

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WebMail -- Direct Access

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OpenWebMail