TCP/IP Protocol Suite 1 Upon completion you will be able to: Electronic Mail: Electronic Mail: SMTP, POP, and IMAP SMTP, POP, and IMAP • Understand four configurations of email architecture • Understand the functions and formats of a user agent • Understand MIME and its capabilities and data types • Understand the functions and commands of an MTA • Understand the function of POP3 and IMAP4 Objectives
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TCP/IP Protocol Suite 1
Upon completion you will be able to:
Electronic Mail:Electronic Mail:SMTP, POP, and IMAPSMTP, POP, and IMAP
• Understand four configurations of email architecture• Understand the functions and formats of a user agent• Understand MIME and its capabilities and data types• Understand the functions and commands of an MTA• Understand the function of POP3 and IMAP4
Objectives
SMTP Protocol originated in 1982 (RFC821, Jon Postel) Standard message format (RFC822,2822, D.
Crocker) Goal: To transfer mail reliably and efficiently
SMTP SMTP clients and
servers have two main components
User Agents – Prepares the message, encloses it in an envelope. (ex. Thunderbird, Eudora)
Mail Transfer Agent – Transfers the mail across the internet (ex. Sendmail, Exim)
Analogous to the postal system in many ways
SMTP SMTP also allows
the use of Relays allowing other MTAs to relay the mail
Mail Gateways are used to relay mail prepared by a protocol other than SMTP and convert it to SMTP
Limitations in SMTP
Only uses NVT 7 bit ASCII format How to represent other data types?
No authentication mechanisms Messages are sent un-encrypted Susceptible to misuse (Spamming,
faking sender address)
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20.1 ARCHITECTURE
To explain the architecture of email, we give four scenarios. We begin To explain the architecture of email, we give four scenarios. We begin with the simplest situation and add complexity as we proceed. The fourth with the simplest situation and add complexity as we proceed. The fourth scenario is the most common in the exchange of email.scenario is the most common in the exchange of email.
The topics discussed in this section include:The topics discussed in this section include:
First Scenario First Scenario Second Scenario Second Scenario Third Scenario Third Scenario Fourth Scenario Fourth Scenario
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Figure 1 First scenario
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When the sender and the receiver of an email are on the same system, we
need only two user agents.
Note:Note:
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Figure 2 Second scenario
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When the sender and the receiver of an email are on different systems, we
need two UAs and a pair of MTAs (client and server).
Note:Note:
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Figure 20.3 Third scenario
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When the sender is connected to the mail server via a LAN or a WAN, we need two UAs and two pairs of MTAs
(client and server).
Note:Note:
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Figure 4 Fourth scenario
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Figure 20.5 Push vs. pull
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When both sender and receiver are connected to the mail server via a LAN or a WAN, we need two UAs, two pairs
of MTAs (client and server), and a pair of MAAs (client and server). This
is the most common situation today.
Note:Note:
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2 USER AGENT
The user agent (UA) provides service to the user to make the process of The user agent (UA) provides service to the user to make the process of sending and receiving a message easier.sending and receiving a message easier.
The topics discussed in this section include:The topics discussed in this section include:
Services Provided by a User Agent Services Provided by a User Agent User Agent Types User Agent Types Sending Mail Sending Mail Receiving Mail Receiving Mail Addresses Addresses Mailing List Mailing List MIME MIME
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Figure 6 User agent
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Some examples of command-driven user agents are mail, pine, and elm
Note:Note:
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Some examples of GUI-based user agents are Eudora, Outlook, and
Netscape.
Note:Note:
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Figure 7 Format of an email
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Figure 8 Email address
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Figure 29 MIME
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Table 1 Table 1 Data types and subtypes in MIMEData types and subtypes in MIME
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Table 1 Table 1 Data types and subtypes in MIME (Continued)Data types and subtypes in MIME (Continued)
The actual mail transfer requires message transfer agents (MTAs). The The actual mail transfer requires message transfer agents (MTAs). The protocol that defines the MTA client and server in the Internet is called protocol that defines the MTA client and server in the Internet is called Simple Mail Transfer Protocol (SMTP).Simple Mail Transfer Protocol (SMTP).
The topics discussed in this section include:The topics discussed in this section include:
Commands and Responses Commands and Responses Mail Transfer Phases Mail Transfer Phases
The third stage of mail delivery uses a message access agent; the client The third stage of mail delivery uses a message access agent; the client must pull messages from the server. Currently two message access must pull messages from the server. Currently two message access protocols are available: Post Office Protocol, version 3 (POP3) and protocols are available: Post Office Protocol, version 3 (POP3) and Internet Mail Access Protocol, version 4.Internet Mail Access Protocol, version 4.
The topics discussed in this section include:The topics discussed in this section include:
POP3 POP3 IMAP4 IMAP4
All Messages
POP3:
IMAP:Dr.Amer
Friends
….headers
Whole message
POP vs. IMAP
Internet
Internet
Post Office Protocol v3
Simple Allows the user to obtain a list of
their Emails Users can retrieve their emails Users can either delete or keep the