Chapter 26
SMTP
and
FTP
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Email is used for sending a single message that includes
text, voice, video, or graphics to one or more recipients.
Simple Mail Transfer Protocol (SMTP).
Sending Mail
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User creates mail that looks very similar to postal mail.
Envelope: sender and receiver address
Message:
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Header: Define the sender, receiver, subject of the message, ...
Body:contains the actual information to be read by the recipient.
Receiving mail
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Email system periodically checks the mailboxes.
If a user has mail, it informs the user with a notice
If the user is ready to read the mail, a list is displayed in which
each line contains a summary of the information about a particular
message in the mailbox. Summary contains the sender mail
address, the subject, and time the mail was sent or received.
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Figure 26.1
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Format of an email
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Figure 26.2
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Mail handling system must use an addressing system
with unique addresses.
Local part: Name of a special file, called the user
mailbox, where all the mail received for a user is
stored for retrieval by the user agent.
Domain Name:
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Email address
Organization usually selects one or more hosts to receive
and send email; they are sometimes called mail exchangers.
Domain name assigned to each mail exchanger either comes
from the DNS database or is a logical name (e.g., the name
of the organization).
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Figure 26.3
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First component of an electronic mail system is the
user agent (UA); Mail Reader.
Service provided by a user agent
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User agent is a software package (program) that composes,
reads, replies to, and forward messages.
It also handles mailboxes.
Composing messages:
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User agent
Responsible for composing the email msg to be sent out.
Some of them provide a template and some provide with builtin editor; OR else user can type in an editor and paste into the
user agent.
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Reading messages
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Replying to messages
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After the reading a message, user can use the user agent to reply
to a message.
User agent allows the user to reply to the original sender or to
reply to all recipients of the message.
Reply contains the original message (for quick reference) and new
message.
Forwarding messages: Can forward with or without extra
comments.
Handling Mailboxes:
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User agent is used to read incoming messages.
When a user invokes a user agent, it first checks the mail in the
incoming mailbox. A one-line summary [number, flag field
indicating if the mail is new OR already read but not replied to OR
read and replied to OR …, size of message, sender, subject if
subject line in msg is not empty] of each received mail is shown.
Inbox: Keeps all received emails until they are deleted by the user.
Outbox: Keeps all sent emails until the user deletes them.
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User Agent Types
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Command-Driven
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GUI-Based
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Normally accepts a one-character command from the
keyboard to perform its task.
mail, pine, elm, …
Contain GUI components that allow the user to interact with
the software by using both the keyboard and the mouse.
Eudora, Microsoft’s Outlook, Netscape,…
SMTP is a simple mail transfer protocol
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Can send messages only in 7-bit ASCII format.
Cannot be used to send binary files or video or audio.
Cannot support few languages that are not supported by ASCII
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Figure 26.4
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Multipurpose Internet Mail Extensions (MIME)
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MIME
Supplementary protocol that allows non-ASCII data to be
sent through SMTP.
Only an extension to SMTP.
MIME transforms non-ASCII data at the sender side to ASCII
data and delivers them to client SMTP to sent through the
Internet. Reverse is done at the receiver side.
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Figure 26.5
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MIME defines five headers that can be added to original SMTP
header:
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MIME header
MIME-Version
Content-Type: Type of data used in the body; has subtypes
separated with slashes.
Content-Transfer-Encoding
Content-Id: Uniquely identifies the whole message in a multiplemessage environment.
Content-Description: Whether the body is image, audio, or video.
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Content Types
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Text
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7-bit ASCII; no transformation by MIME
Multipart
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Multiple, independent parts.
Needs to define the boundary between each part, as a
parameter.
Boundary is terminated using two hyphens.
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Mixed: Present the parts in exact order as they are in message.
Parallel: Order of parts is unimportant.
Digest: Same as mixed but default type/subtype is
message/RFC822
Alternative: Same message is repeated using different formats.
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Message:
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Body is itself a whole mail message, a part of a mail
message, or a pointer to a message.
Subtypes:
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RFC822: Used if the body is encapsulating another message
(including header and the body).
Partial: Used if the original message has been fragmented into
different mail messages and this message is one of the
fragments.
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Fragments must be reassembled at the destination by MIME.
Three parameters must be added: id, number, total
Id: identifies the message and is present in all the fragments.
Number: sequence order of the fragment
Total: number of fragments that comprise the original message
External-body.
 Body does not contain the actual message but is only a
reference (pointer) to the original message.
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Image
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Original message is a stationary image, indicating that there is no
animation.
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Video
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Original message is a time-varying image (animation).
Motion Picture Experts Group (MPEG)
If the animated image contains sounds, it must be sent separately
using the audio content type.
Audio
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Joint Photographic Experts Group (JPEG)
Graphics Interchange Format (GIF)
Original message is a sound.
Only subtype is basic, which uses 8-KHz standard audio data.
Application
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Original message is a type of data not previously defined.
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Octet-stream is used when the data must be interpreted as a
sequence of 8-bit bytes (binary file).
PostScript is used when the data are in Adobe PostScript format.
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Table 26.1
Type
Data types and subtypes in MIME
Subtype
Description
Plain
Unformatted text; 7-bit ASCII
Mixed
Body contains ordered parts of different data types
Parallel
Same as above, but no order
Digest
Similar to mixed, but the default is message/RFC822
Alternative
Parts are different versions of the same message
RFC822
Body is an encapsulated message
Partial
Body is a fragment of a bigger message
Ext. Body
Body is a reference to another message
JPEG
Image is in JPEG
GIF
Video is in GIF format
Video
MPEG
Video is in MPEG format
Audio
Basic
Single-channel encoding of voice at 8 KHz
PostScript
Adobe PostScript
Octet-Stream
General binary data (8-bit bytes)
Text
Multipart
Message
Image
Application
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Content-Transfer-Encoding
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Method to encode the messages into 0s and 1s for transport.
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7bit:
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8bit: [NOT Recommended]
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8-bit encoding
Non-ASCII characters can be sent, and the length of the line can exceed 1000
characters.
MIME does not do any encoding here: SMTP must be able to transfer binary data.
Base64:
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8-bit encoding; can send non-ASCII characters.
Length of line should not exceed 1000 characters
MIME does not do any encoding here
SMTP must be able to transfer 8-bit non-ASCII characters.
Binary: [NOT Recommended]
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7-bit ASCII encoding
Length of line should not exceed 1000 characters
Solution for sending data made of bytes when the highest bit is not necessarily
zero.
Base64 transforms this type of data to printable characters, which can then be
sent as ASCII characters or any type of character set supported by underlying
mail transfer mechanism.
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Table 26.2 Content-transfer encoding
Category Description
Type
ASCII characters and short lines
7bit
Non-ASCII characters and short lines
8bit
Non-ASCII characters with unlimited-length lines
Binary
6-bit blocks of data are encoded into 8-bit ASCII characters
Base64
Non-ASCII characters are encoded as an equal sign followed by an ASCII
code
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Figure 26. 6
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Base64
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Table 26.3 Base64 encoding table
Value
Code
Value
Code
Value
Code
Value
Code
Value
Code
Value
Code
0
A
11
L
22
W
33
h
44
s
55
3
1
B
12
M
23
X
34
i
45
t
56
4
2
C
13
N
24
Y
35
j
46
u
57
5
3
D
14
O
25
Z
36
k
47
v
58
6
4
E
15
P
26
a
37
l
48
w
59
7
5
F
16
Q
27
b
38
m
49
x
60
8
6
G
17
R
28
c
39
n
50
y
61
9
7
H
18
S
29
d
40
o
51
z
62
+
8
I
19
T
30
e
41
p
52
0
63
/
9
J
20
U
31
f
42
q
53
1
10
K
21
V
32
g
43
r
54
2
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Figure 26.7
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Quoted-Printable
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Base64 is a redundant encoding scheme; that is, 24 bits
becomes four characters and eventually is sent as 32 bits.
If the data consist mostly of ASCII characters with a small
non-ASCII portion, we can use quoted-printable encoding.
If a character is:
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Quoted-printable
ASCII, it is sent as it is.
Non-ASCII, sent as three characters. First character is the
equals sign (=). Next two characters are the hex of the byte.
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Figure 26.8
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McGraw-Hill
MTA client and server
A mail transfer agent or MTA (also called a mail
server, or a mail exchange server is a computer
program or software agent which transfers electronic
mail messages from one computer to another. The
MTA works behind the scenes, while the user usually
interacts with another program, the mail user agent
(MUA), which contacts an MTA for actual delivery of
the mail.
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Typical MTAs:
 Sendmail
Postfix
Exim
QMail
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Figure 26.9
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SMTP uses commands and responses to transfer messages
between an MTA client and an MTA server.
Each command or reply is terminated by a two-character
(carriage return and line feed) end-of-line token.
Commands
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Commands are sent from the client to the server.
Consists of a keyword followed by zero and more arguments.
SMTP defines 14 commands.
Responses
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Commands and responses
Responses are sent from server to client.
Response is a three-digit code that may be followed by additional
textual information.
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Mail Transfer Phases
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Process of transferring a mail message occurs in
three phases
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Connection establishment
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Message transfer
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After connection establishment, a single message between a
sender and one or more recipients can be exchanged.
Connection termination
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After a client has made a TCP connection to the well-known
port 25, the SMTP server starts the connection phase.
After the message is transferred successfully, the client
terminates the connection.
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Mail Delivery
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Delivery of email from sender to receiver consists of
three stages
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1st stage
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2nd stage
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Email is relayed by local server, which now acts as the SMTP
client, to the remote server, which is the SMTP server.
Email is delivered to remote server and not to the remote user
agent; Email is stored in mailbox of the user for later retrieval..
Email server runs the SMTP server program.
3rd stage
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Email goes from user agent to local server
Mail does not go directly to remote server because the remote
server may not be available at all times.
Mail is stored in the local server until it can be sent.
Remote user agent uses a mail access protocol such as POP3 or
IMAP4 to access the mailbox and obtain the mail.
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Figure 26.10
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Email delivery
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Mail access protocols
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SMTP is a push protocol; it pushes the message from
the sender to the receiver even if the receiver does
not want it.
3rd stage needs a pull protocol; the operation must
start with the recipient.
Mail must stay in the mail server mailbox until the
recipient retrieves it.
POP3 [Post Office Protocol]
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Client POP3 software is installed on the recipient computer.
Server POP3 software is installed on the mail server.
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Client (user agent) opens a connection with the server on TCP
port 110.
It then sends its user name and password to access the
mailbox.
User can then list and receive mail messages, one by one.
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POP3 modes
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Delete mode: Mail is deleted from the mailbox after each retrieval.
Keep mode: Mail remains in the mailbox after retrieval.
IMAP4 (Internet Mail Access Protocol)
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User can check the email header prior to downloading
User can search the contents of email for a specific string of
characters prior to downloading
User can partially download email.
User can create, delete, or rename mailboxes on mail server
User can create a hierarchy of mailboxes in a folder for email
storage.
Web-Based Email: Hotmail or Yahoo
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POP3 assumes that each time a client accesses the server, the whole
mailbox will be cleared out (transferred).
POP3 does not allow user to organize mail on the server; user cannot
have different folders on the server; POP3 does not allow the user to
partially check the contents of the mail before downloading.
Mail transfer from client browser to mail server is via HTTP
SMTP is used for mail transfer from sending mail server to receiving
mail server.
Message from receiving mail server to browser is via HTTP
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Figure 26.11 POP3
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File Transfer Protocol
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Copying a file from one host to another
Problems in FTP
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FTP establishes two connections between client and
server.
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Two systems may use different file name conventions
Two systems may have different ways to represent text and
data.
Two systems may have different directory structures.
One connection is used for data transfer [Port 20]
Other connection for control information (commands and
responses) [Port 21]
Separation of commands and data transfer makes FTP more
efficient. Commands are very simple.
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Figure 26.12
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Client has three components
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User interface, Client control process, client data transfer process.
Server has two components
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FTP
Server control process, server data transfer process
Control connection is made between control processes; Control
connection is maintained during entire interactive FTP session.
Data connection is made between data transfer processes; Data
connection is opened & then closed for each file transferred.
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Figure 26.13
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Control Connection
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Created in same way as other application programs.
Connection remains open during the entire process.
Service type used by the IP is minimize delay, because this is
an interactive connection between a user (human) and a
server.
User types commands and expects to receive responses
without significant delay.
Data connection
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Using the control connection
Opened when data are ready to transfer. Closed when it is
not needed.
Service type used by IP is maximize throughput.
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Figure 26.14
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Communication over control connection
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Uses ASCII character set.
Each command or response is only one short line, so we
need not worry about file format or file structure.
Each line is terminated with a two-character (carriage return
and line feed) end-of-line token.
Communication over data connection
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Using the data connection
Client must define the type to be transferred, the structure
of the data, and the transmission mode.
Heterogeneity is solved by: file type, data structure,
transmission mode.
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File type
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ASCII file:
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EBCDIC file
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If one or both ends of connection use EBCDIC encoding (used
in IBM computers) the file can be transferred using EBCDIC
encoding.
Image file
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By default for transferring text files.
Sender transforms the file from its own representation to ASCII
characters
Receiver transforms the ASCII characters to its own
representation.
Default format for transferring binary files
File is sent as continuous streams of bits without any
interpretation or encoding
Used for compiled programs or images.
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File is encoded in ASCII or EBCDIC, another attribute
must be added to define the printability of the file.
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Nonprint
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TELNET
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Default format for transferring a text file.
File contains no vertical specifications for printing.
File cannot be printed without further processing because there
are no characters to be interpreted for vertical movement of
the print head.
This format is used for files that will be stored and processed
later.
File contains ASCII vertical characters such as CR (carriage
return), LF (line feed), NL (new line) and VT (vertical tab).
File is printable after transfer.
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Data structure
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FTP can transfer a file across the data connection using one
of the following interpretations about structure of data:
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File structure (default)
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Record structure
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File is divided into records (or structs in C).
Used only with text files.
Page structure
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File has no structure.
Continuous stream of bytes
File is divided into pages, with each page having a
page number and a page header.
Pages can be stored or accessed randomly or
sequentially.
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Transmission mode: FTP can transfer a file across the data connection
by using one of following three transmission modes:
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Stream mode
 Default mode
 Data is delivered from FTP to TCP as a continuous stream of bytes;
TCP is responsible for chopping data into segments of appropriate size.
 End-of-file is closing of data connection by sender.
 If data is divided into records, each record will have 1-byte end-ofrecord (EOR) character, and the end of the file will have a 1-byte endof-file (EOF) character.
Block mode
 Data can be delivered from FTP to TCP in blocks.
 Block is preceded by 3-byte header.
st byte is called the block descriptor; next 2 bytes defines the size of
 1
block in bytes.
Compressed mode
 If file is big, data can be compressed.
 Commonly used compression method is run-length encoding.
 Consecutive appearance of a data unit are replaced by one occurrence
and number of repetitions. Usually, Blanks in text file and null
characters in binary file are compressed.
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Figure 26.15
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Means of File Transfer
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File transfer
Retrieving a file: Copy file from server to client.
Storing a file: copy file from client to server.
List of directory or file names is to be sent from the server
to the client. Note that FTP treats a list of directory or file
names as a file. It is sent over the data connection.
©The McGraw-Hill Companies, Inc., 2004
Example 1
Figure 26.16 (next slide) shows an example of how a file is stored.
1. The control connection is created, and several control
commands and responses are exchanged.
2. Data are transferred record by record.
3. A few commands and responses are exchanged to close the
connection.
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Figure 26.16
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Example 1
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Table 26.4 List of FTP commands in UNIX
Commands
!, $, account, append, ascii, bell, binary, bye, case, cd, cdup,
close, cr, delete, debug, dir, discount, form, get, glob, hash, help,
lcd, ls, macdef, mdelete, mdir, mget, mkdir, mls, mode, mput,
nmap, ntrans, open, prompt, proxy, sendport, put, pwd, quit,
quote, recv, remotehelp, rename, reset, rmdir, runique, send,
status, struct, sunique, tenex, trace, type, user, verbose,?
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Example 2
We show some of the user interface commands that accomplish the same task as in
Example 1. The user input is shown in boldface. As shown below, some of the commands
are provided automatically by the interface. The user receives a prompt and provides only
the arguments.
$ ftp challenger.atc.fhda.edu
Connected to challenger.atc.fhda.edu
220 Server ready
Name: forouzan
Password: xxxxxxx
ftp > ls /usr/user/report
200 OK
150 Opening ASCII mode
...........
...........
226 transfer complete
ftp > close
221 Goodbye
ftp > quit
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Example 3
We show an example of using anonymous [public access OR without an account] FTP.
We connect to internic.net, where we assume there are some public data available.
Sometimes, anonymous as user name and guest as password
$ ftp internic.net
Connected to internic.net
220 Server ready
Name: anonymous
331 Guest login OK, send "guest" as password
Password: guest
ftp > pwd
257 '/' is current directory
ftp > ls
200 OK
150 Opening ASCII mode
bin
...
ftp > close
221 Goodbye
ftp > quit
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