The OSI Reference Model
19
F I G U R E 1 . 6
Transmitting segments with flow control
Windowing
Ideally, data throughput happens quickly and efficiently. And as you can
imagine, it would be slow if the transmitting machine had to wait for an
acknowledgment after sending each segment. But because there's time avail-
able after the sender transmits the data segment and before it finishes pro-
cessing acknowledgments from the receiving machine, the sender uses the
break as an opportunity to transmit more data. The quantity of data seg-
ments (measured in bytes) the transmitting machine is allowed to send
without receiving an acknowledgment for them is called a window.
Windows are used to control the amount in outstanding, unacknowledged
data segments.
So the size of the window controls how much information is transferred
from one end to the other. While some protocols quantify information by
observing the number of packets, TCP/IP measures it by counting the num-
ber of bytes.
As you can see in Figure 1.7, there are two window sizes--one set to 1,
and one set to 3. When you've configured a window size of 1, the sending
Transmit
Transmit
Not ready--
STOP!
GO!
Segments
processed
Buffer full
Sender
Receiver
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