E-mail
E-mail
or Electronic mail is a computer based method of sending
messages from one computer user to another. These messages
usually consist of individual pieces of text which you
can send to another computer user even if the other
user is not logged in (i.e. using the computer) at the
time you send your message. The message can then be
read at a later time. This procedure is analogous to
sending and receiving a letter.
Originally,
email messages were restricted to simple text, but now
many systems can handle more complicated formats, such
as graphics and word processed documents.
When
mail is received on a computer system, it is usually
stored in an electronic mailbox for the recipient to
read later. Electronic mailboxes are usually special
files on a computer which can be accessed using various
commands. Each user normally has their individual mailbox.
It
is straightforward to send electronic mail between users
of different computer systems which are connected to
major networks. Most major academic and research institutions
and companies throughout the world can now be reached
by electronic mail. In addition, a growing number of
individuals can be contacted in this way. This is effectively
part of the Internet, so email can be exchanged with
most national and international networks.
2 MAIN PROTOCOLS
There
are 2 main protocols or items used here for either the
retrieval or delivery of e-mail.
The
first one is called Simple Mail Transfer Protocol (or
SMTP for short), which is the Internet's standard host-to-host
mail transport protocol. Simply put, SMTP is what allow
for the sending of your email, either when using your
ISP or using the servers here (mail.example.com).
The
second is called Version 3 of the Post Office Protocol
(or POP-3 for short), which is designed for user-to-mailbox
access. Simply put, POP-3 is what allow for the receiving
of your email, either when using your ISP or using the
servers here (mail.example.com).
These
are the main protocols used today in most Computer-based
email programs such as Outlook or Eudora and many others.
The
tutorials is this section will show you how to setup
the various email clients to utilize the email accounts
that you have setup through your hosting account to
send email through the server and also to "grab"
your email for both online and offline viewing.
FTP
FTP
is an abbreviation for File Transfer Protocol. It is
a standard for moving files from one computer to another
over a network. Files are moved in either ASCII (text)
or BINARY form.
Files such as Simple text, HTML, Postscript files should
be moved in ASCII (text) form. Word processor, spreadsheet,
images, executables, and other files should be moved
in BINARY form.
SIMPLE FTP SESSION
The client program connects to an FTP sever on the
internet. Once connected, the FTP server sends a welcome
message to the client over the open socket (network)
connection.
Server: 220 Sample FTP server ready. Please give user-name
Client: USER anonymous
Server: 331 User name OK. Please give your email address
as password
Client: PASS joe@nowhere.comm
Server: 230 User logged in
As you can see, the client and server are communicating
in plain text. The digits in the server replies are
'reply-codes' defined by the FTP protocol. The uppercase
words in the beginning of the client commands, are command
verbs that also are defined by the RFC. The protocol
is designed in a way that makes it easy for machines
and humans to understand the dialog. In most cases,
the client program doesn't have to interoperate the
text after the reply code.
Now, the user wants to see the available files and
directories, and issues a DIR command in the client
program.
Client: TYPE A
Server: 200 Type set to A
Client: PASV
Server: 227 Entering passive mode (193,91,161,12,28,46)
Client: LIST
Server: 150 Opening ASCII mode data connection for /bin/ls
Server: 226 Transfer complete
The TYPE command tells the server to send the directory
/ file listing as plain ASCII, where each line is separated
by a CRLF sequence. The PASV command tells the server
to prepare for a new socket connection by creating a
new socket and then listening for a connection from
the client. Now, as you see, things are getting a little
more complicated. The server reply includes a IP address
and a port number, encoded as 6 different number values,
separated by commas. The client must find and understand
this address in order to receive the listing.
The LIST command tells the server to give a directory
/ file listing. Now the server replies with two reply
lines... What's happening is that the first line tells
the client that the listing is ready and that the client
can go on and make a new connection to the server. The
client connects to the IP address given by the PASV
reply and receives data until there is no more data
to get. Then it closes the temporary data connection
and switches back to the control connection to get the
second reply line, which tells if the server transferred
the whole listing.
In order to receive a directory listing, the client
and server use two socket connections, one for the control
flow (server sends commands, the server replies in plain
text) and one for the data connection (which is continuous
and goes in one direction only). Every time a directory-listing
is sent, the server and client will use another new
(temporary) socket connection for the transfer.
The user finds an interesting file, and gives the FTP
client the command to get it.
Client: TYPE I
Server: 200 Type set to I
Client: PASV
Server: 227 Entering passive mode (193,91,161,12,28,46)
Client: RETR test.zip
Server: 150 Opening BINARY mode data connection for
test.zip
Server: 226 Transfer complete
As
you see, the server and client use the exact same method
to get a file, as they do to get a directory listing.
The only change is that the RETR command is used in
stead of the LIST command. In this case, the file was
a .zip archive. Since such files can't be translated
to text-files (at least not in a safe manner), the FTP
client switched to binary mode (TYPE I). File and directory
listings can be transferred in both binary and text-mode.
Since different operating systems use different means
to separate lines in text-files, it is generally a good
idea to let the FTP server and FTP client do the conversion.
Both know the rules for a ASCII (text) transfer and
the client always knows the rules for the local storage
of text-files.
