Smtp POP3 IMAP protocol notes

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)

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

Figure 1 First scenario

When the sender and the receiver of an email are on the same system, we

need only two user agents.

Note:Note:

Figure 2 Second scenario

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:

Figure 20.3 Third scenario

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:

Figure 4 Fourth scenario

Figure 20.5 Push vs. pull

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:

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.

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

Figure 6 User agent

Some examples of command-driven user agents are mail, pine, and elm

Note:Note:

Some examples of GUI-based user agents are Eudora, Outlook, and

Netscape.

Note:Note:

Figure 7 Format of an email

Figure 8 Email address

Figure 29 MIME

Table 1 Table 1 Data types and subtypes in MIMEData types and subtypes in MIME

Table 1 Table 1 Data types and subtypes in MIME (Continued)Data types and subtypes in MIME (Continued)

Table 2 Table 2 Content-transfer-encodingContent-transfer-encoding

Figure 11 Base64

Table 3 Table 3 Base64 encoding tableBase64 encoding table

Figure 12 Quoted-printable

3 MESSAGE TRANSFER AGENT: SMTP

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).

Commands and Responses Commands and Responses Mail Transfer Phases Mail Transfer Phases

Figure 13 SMTP range

Figure 14 Commands and responses

Figure 15 Command format

Table 4 Table 4 CommandsCommands

Table 5 Table 5 ResponsesResponses

Table 20.5 Table 20.5 Responses (Continued)Responses (Continued)

Figure 20.16 Connection establishment

Figure 17 Message transfer

Figure 18 Connection termination

4 MESSAGE ACCESS AGENT: POP AND IMAP

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.

POP3 POP3 IMAP4 IMAP4

All Messages

IMAP:Dr.Amer

Friends

….headers

Whole message

POP vs. IMAP

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

email on their system Minimizes server resources

Figure 19 POP3 and IMAP4

Figure 20 POP3

Smtp POP3 IMAP protocol notes

tcp ip protocol suite

fourth scenario

emails users

section include

topics discussed

mail server

protocol

mail

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