12.12.
1 Electronic Mail (e-mail)
Electronic mail or e-mail was originally designed to allow a pair of individuals to
communicate by sendinga text document from one computer to another. Nowadays
e-mail can alsobe used to send other media like graphics, sound, video, etc. and can be
tsimultaneously to multiple receivers. To receive e-mail auser must have a mailbox,
astorage area, that holds incominge-mail messages until the user has time to read them.
The mailbox must be identified by aunique address. Any user can send mail across the
Interner toanother user's mailbox if they knowthe mailbox address. Only the owner can
however examine the contents of themailbox and extract messages by using a password
protection scheme. Acomputer connected to the Internet needs application software
before userscan send or receiveelectronic nail. E-mail software allows a user tocompose
andsend a message or to read messages that have arrived. A user can send a reply to any
message.
S16 Principles of Multimedia
E-mail Message Ane-mail message begins with a header that specifies the
person
sent the message, the person to whom the message was sent, the date
and timensttheo
message was sent and thetopic name (Fig 12.17),. Additionallyit may also)
labeled Cc (carbon copy) that lists mailbox addresses of additional people contain aline
who recei
a copy of the message.
From: agupta@company1.com
To: pkumar@company2. com
Date. 01 Apr 2005 11:10.34 EST
Subject: Some bad news
Prabir.
Theard that your company has decided to cut
the first one fired. personnel and that you will be
April fooll
Ajay
Figure 12.17, E-mail message
E-mail Sofware E-mail software consists of two parts: the
the user to compose, read and send the e-mail, and client program which allows
the server program running on anothr
computer which receives e-mail from the client and stores
The client uses TCP/IP to send messages across the it in the recipient's mailbox
computer that is frequently powered down or disconnected Internet. A user who has apersonal
receive e-mail while the computer is inactive. from the Internet cannot
receive e-mail directly. The user Therefore most personal computers do
to
arranges have a mailbox ona large computer witha
server program that always remains
Mail Servers and are ready to accept e-mail messages. These:are called
dedicated for this purpose only. To read e-mail, a user from2
personal computer must contact the mail server and obtain a copy of the mailbos.
E-tmail Addresses Mailbox
atext string separated into addresses used to send e-mail across the Internettcoonsist
of
two parts by the at sign (@). The the usefand
thesuffix gives the domain and prefix:specifies
resides, e.g sub-domain details of the computer on which the mailbox
Most commercially akgupta@vsnl .com
allowseach userto defineavailable
as set of
e-mail software also supports e-mail alias facility thal
abbreviations for the mailbox addresses they use firequent:
john =jksmith@computerl.SOmecompany.com
When composing an e-mailthe
software will automatically user can simply enter johnin the To feld. Thee-mal
the full mailbox consult the user's alias list and replace the abbreviaionwith
address.
Network Essentials 517
Shared Aliases Asingle individual on apersonal computer may have apersonal list of
alhases. However alarge computer that many people share need amore complex mechanis.
Here the alias list may need to be shared by multiple users.
For example, a systemn administrator of a company may decide that all hardware
problems should be reported to william on computer2. SOmewhere.cOm.
Thefollowing alias could be added to a system-wide list of mail alases:
hard trouble = william@computer2.somewhere.com
When any user of the company wants to report ahardware problem, they send e-mall
to hard trouble.The mail software first reads the user's private alias-list and then the
system-wide alias list before substituting the actual e-mail address. The system wide alias
list would be needed to be stored on a private mail server within the organization.
Multiple Recipients Athough e-mail was originally designed for two people to
communicate, most e-mail systems allow auser to send amessage to muliple recipients.
To do so the sender specifies muliple mailbox addresses on the To line of a message.
The system sends one copy of the message to each recipient (Fig. 12.18).
Mailing List One of the most useful features of e-mail arises from an alias that specifies
multiple recipients. When the mal system expands the alias, and finds multiple recipients,
it sends a copy of the message to cach.
An alias specifying multiple recipients is usually callecd amailing list.
friends = bob@company1.com.jim@company2. com, tom@company3.com
Any message sent to friends willbe delivered to all the recipients.
System administrators can cstablish public mailing lists, e.g a computer named
compl.somewhere.com may offer a public mailing ist named sales to denote the sales
department. To send mail to everyone in the sales department an individual mails to the
address: sales@compl.somewhere.com
From: agupta@company1.com
To: pkumar@company2.com, tksen@company3.com, bpal@cormpany4.com
Date: 01 Apr 2005 11:10:34 EST
Subject: Some bad news
Folks,
Iheard that your company has decided to cut personnel and that you will be
the first one fired.
April fool!
Ajay
igure 12.18, E-mail with multiple recipients
S18
Principles ofMultimedia
Bencfits ofE-mail The impact ofe-mail has been dramatic. In many cases, C-mailL
replaced postal mail as the primary communication mechanism. Some benefits are.
1. Provides high speed transfer of mails over entire globe. is notified.
2 High reliability. E-mail is seldom lost. Even then the sender
3. Convenient mechanism of sending same mailto multiple recipients.
4. User is not interrupted when mail arrives but reads at own convenience.
5. Can include other media clements apart from text as attachments.
6. Acomputer can be programmed to respond to and automatically reply to mail
Post Offce Protocol version 3(POP3) An application layer protocol used to retrieve
cmail from aremote mail server to alocalclient over a TCP/IP connection. POP3 has
been designed to allow endusers with intermittent connections ike dial-up connections
to retrieve mail. E-mail clients using POP3 generally connect, retrieve all messages, store
them on the user's PC as new mcssages, delete them from the server anddisconnect.
Internet Message Acess Protocol (IMAP) An application layer protocol used to retrieve
e-mail from aremote mail server to alocal client over a TCP/IP connection, designed as
an improvement over POP3. In contrast to POP3, IMAP allow simultaneous access by
multiple clients tothe same mailbox and provides mechanisms to detect changes made
by cach other. Through the use of flags IMAP clients can keep track of message states,
i.c. which messages have been read and replied to. Flags are stored on the server. IMAP
clients can create, rename and deletemuliple mailboxes on the server and move
between mailboxes. IMAP provides a mechanism for a client to ask the server tomessages
search
for messages meeting avariety of criteria without
downloading the messages.
Simple Mail Transfer Protocol (SMTP) Arelatively simple
totransmit e-mailmessages between mail servers oOver text-based protocol used
TCP/IP
applicationlayer. SMTP supports only 7-bit ASCII chaacters whichconnection. Resides in
limitse-mail messages
to text only, MIME (Multipurpose Internet Mail
dealing with non-textual media.SMTP, first usedExtensions) defines mechanisms tor
in the early 1980s, was designed to
transfer nmail between servers which remain
is a 'push'protocol which does not connected to the network allthe time. SMI
demand. SMTP delivery are triggered allow clients to 'pull' messages from servers
by servers themselves.
Multipurpose Internet Mail Extensions (MIME) An standard for the format
of e mail
Imessages, Defines mechanisms for
Internet:
with e-mail transmission of non-textual content along
"The generalmessages.
syntax is
MIME defines acollection of headers
specifying message? attributes.
content-type/sub-type,
MIME is extensible with cg, applicatiorn/msword or image/jpeg
provisions for registering new content types and additional
attributes. Although originally
supported by1TTP ormat fordefined for e-nails, MIME headers are also used and
content type header include 'type' and 'subtype'fieelds.
Some registered types and
subtypes are given below:
Network Essentials 519
Ipe
application
Subtyjpes
msword,pdf, rtf, zip
audio mpeg, 32kadpcm
image git, jpeg, png, tiff
text html,plain, xml, sgml
video h261, h263, mpeg, quicktim
