The OSI Reference Model
17
Figure 1.5 depicts a typical reliable session taking place between sending
and receiving systems. By looking at it, you can see that both hosts' applica-
tion programs begin by notifying their individual operating systems that a
connection is about to be initiated. The two operating systems communicate
by sending messages over the network confirming that the transfer is approved
and that both sides are ready for it to take place. Once all this required
synchronization takes place, a connection is fully established and the data
transfer begins.
F I G U R E 1 . 5
Establishing a connection-oriented session
While the information is being transferred between hosts, the two
machines periodically check in with each other, communicating through
their protocol software to ensure that all is going well and that the data is
being received properly.
Let me sum up the steps in the connection-oriented session--the three-
way handshake--pictured in Figure 1.5:
The first "connection agreement" segment is a request for
synchronization.
The second and third segments acknowledge the request and establish
connection parameters--the rules--between hosts.
Synchronize
Negotiate connection
Synchronize
Acknowledge
Connection established
Data transfer
(Send segments)
Sender
Receiver
Copyright ©2002 SYBEX, Inc., Alameda, CA
www.sybex.com