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TCP/IP Services and Client/Server Operation
(Page 3 of 3)
Understanding TCP/IP Client and Server Roles
The terms client and
server can be confusing in TCP/IP because they are used
in several different ways, sometimes simultaneously:
- Hardware Roles: The terms client
and server usually refer to the primary roles played by
networked hardware. A client computer is usually something
like a PC or Macintosh computer used by an individual, and primarily
initiates conversations by sending requests. A server is
usually a very high-powered machine dedicated to responding to client
requests, sitting in a computer room somewhere that nobody but its administrator
- Software Roles: As mentioned earlier,
TCP/IP uses different pieces of software for many protocols to implement
client and server roles. A Web browser is a
piece of client software, while Web server software is completely different.
Client software is usually found on client hardware and server software
on server hardware, but not always. Some devices may run
both client and server software.
- Transactional Roles: In any exchange of
information, the client is normally the device that initiates communication
or sends a query; the server responds, usually providing information.
Again, usually the client software on a client device initiates the
transaction, but this is not always the case.
So, in a typical organization there
will be many smaller individual computers designated clients,
and a few larger ones that are servers. The servers normally
run server software, and the clients run client software. But servers
can also be set up with client software, and clients with server software.
For example, suppose you are an administrator
working in the computer room on server #1 and need to transfer a file
to server #2. You fire up FTP to initiate a file-transfer session with
server #2. In this transaction, server #1 is playing the role of the
client, since it is initiating communication using an FTP client program.
Theoretically, you could even start an FTP transfer from server #1 to
a particular client, if that client had FTP server software to answer
the server's request. (This is less common, because server software
is often not installed on client machines.)
Transactional roles come into play
when communication occurs between servers in certain protocols. For
example, when two SMTP
servers communicate to exchange electronic mail, even though they are
both server programs running on server hardware, during any transaction
one device acts as the client while the other acts as the server. In
some cases, devices can even swap client and server roles in the middle
of a session!
I should conclude by making clear
that the client and server roles I have discussed above are the traditional
ones. The rise of powerful personal computers and widespread Internet
access (especially always-on broadband connectivity) has
led to a significant blurring of client and server hardware and software.
Many client machines now include server software to allow them to, for
example, respond to World Wide Web queries from other clients. There
are also many file sharing programs around that allow clients to communicate
using the peer-to-peer
structural model. However, most TCP/IP
communication is still client/server in nature, so its important
to keep these roles in mind.
Key Concept: Understanding client/server computing concepts in TCP/IP is made more complex due to the very different meanings that the terms client and server can have in various contexts. The two terms can refer to hardware rolesdesignations given to hardware devices based on whether they usually function as clients or as servers. The terms can also refer to software roles, meaning whether protocol software components function as clients or servers. Finally, they can refer to transactional roles, meaning whether a device and program functions as a client or server in any given exchange of data.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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