BASIC TCP/IP SERVICES
INTRODUCTION:
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Application Layer protocols are invariably associated with some kind of service e.g file
transfer, accessing email, accessing other computers etc. These services are an essential
part of modern day communication
The TCP/IP services are based on Client/ Server Model.
The Client Requests a service and the Server provides this service.
A Client is usually a PC and the Server is a powerful computer.
There are also situations where Server to server communication takes place for the
purpose of load sharing and sharing multiple copies of vital data
The Client /Server Model works by using a REQUEST/RESPONSE procedure
The Client Request s the service using a REQUEST MESSAGE and the Server provides
the service using a REPLY MESSAGE e.g HTTP protocol (to be studied later) that
underlies the World Wide Web, uses a Web browser as a Client and the specified Web
Server Software acting a Server.
Other Protocols such as FTP, Telnet, SMTP, SNMP, TFTP etc use REQUEST / REPLY
structure
FTP (File Transfer Protocol): ( Chapter 20 Text):
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One of the most common activity that takes place over the Internet is the transfer of a file
from a remote location
FTP provides a powerful mechanism to transfer files from a remote location.
FTP establishes two TCP connection between the hosts- one for Data transfer on port 20
of the Server and the other for control operation on port 21
Fig below shows the basic elements of an FTP session. ( Fig 20.1 P556 Text). Draw the
picture on the board.
The Control Connection is made between the Control Processes and remains active
during the entire interactive FTP Session (on port 21)
The Data transfer Connection is made between the data transfer Processes and is opened
and then closed for each file transfer.
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The Server uses well known Port 21 for Control and port 20 for Data .
The Client uses dynamic ports.
UI (User Interface):
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Offers the Visual front to the User e.g WINDOWS FTP.EXE files open an MS-DOS
based UI with only with only a prompt to guide you through the FTP process. Third party
protocols such as Ipswitch’s WS.FTP Pro product, offers an FTP Client application with
a graphical intuitive interface. For more information visit http://www.ipswitch.com
Client Control process Protocol Interface:
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Interprets the user’s commands
Initiates the Control Connection from the user’s dynamic port to the Server’s well known
Port.
Initiates the FTP Commands ( See Table 6-2 P257 E.T)
FTP Commands:
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Uses a series of Commands (Table 6-2 P257 E.T) that follows directly after the TCP
header.
These Commands are received and processed by the Server Process Interface (PI).
The File System:
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The file system at either end of the communication may consists of any format such as
ASCII or EBCDIC
Data Transfer Process:
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The DTP runs over a TCP connection made between the FTP Client and FTP Server after
the FTP connection is made.
The Server creates data transfer Connection. If the user issues the PASV command, the
client can also create a data transfer connection.
FTP relies on TCP to establish the underlying connection, track the order of packets,
define and adjust the data transfer Window according to rules of congestion control and
recover from data loss.
FTP Communications:
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Fig below shows the encapsulation FTP application data. Draw the picture in the space
below.
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Fig 6-2 P259 E.T shows a decode of the FTP session
Fig below (Table 6-3 p260-261) shows a packet by packet sequence that occurs in an
FTP file transfer. Study the FTP screen shown in Table 6-3 P261-262 E.T
FTP help shows a list of Commands
FTP help (command ) explains a specific command
Go through examples 1,2,3 P566,567,568 Text.
Go through example 3 P569 Text in the class.
To use FTP, a user needs an account (User name ) and a Password on the remote Server
Some sites have asset of files available for public access.
To access these files, a user does not need to have an account or a password
The user can log in as an anonymous user by using anonymous as the user id (or your
email address) and Guest as a Password
Go through Example 4 P570 Text in the class