What is Windowing?

Windowing 

 Ideally, data throughput happens quickly and efficiently. And as you can imagine, it would 

be painfully slow if the transmitting machine had to actually wait for an acknowledgment 

after sending each and every segment! The quantity of data segments, measured in bytes, 

that the transmitting machine is allowed to send without receiving an acknowledgment is 

called a window . 

The size of the window controls how much information is transferred from one end to 

the other before an acknowledgment is required. While some protocols quantify informa-

tion depending on the number of packets, TCP/IP measures it by counting the number of 

bytes.

If you’ve confi gured a window size of 1, the sending machine will wait for an acknowl-

edgment for each data segment it transmits before transmitting another one. A window set 

to 3 allows three to be transmitted before receiving an acknowledgment. In this simplifi ed 

example, both the sending and receiving machines are workstations. Remember that in 

reality, the transmission isn’t based on simple numbers but on the amount of bytes that can 

be sent. 

Acknowledgments 

 Reliable data delivery ensures the integrity of a stream of data sent from one machine to 

the other through a fully functional data link. It guarantees that the data won’t be dupli-

cated or lost. This is achieved through something called positive acknowledgment with 

retransmission —a technique that requires a receiving machine to communicate with the 

transmitting source by sending an acknowledgment message back to the sender when it 

receives data. The sender documents each segment measured in bytes, then sends and waits 

for this acknowledgment before sending the next segment. Also important is that when it 

sends a segment, the transmitting machine starts a timer and will retransmit if the timer 

expires before it gets an acknowledgment back from the receiving end. Figure  pictures 

the process I just described.

In the figure, the sending machine transmits segments 1, 2, and 3. The receiving node 

acknowledges that it has received them by requesting segment 4 (what it is expecting next). 

When it receives the acknowledgment, the sender then transmits segments 4, 5, and 6. If 

segment 5 doesn’t make it to the destination, the receiving node acknowledges the event 

with a request for the segment to be re-sent. The sending machine will then resend the lost 

segment and wait for an acknowledgment, which it must receive in order to move on to the 

transmission of segment 7.

The Transport layer, working in tandem with the Session layer, also separates the data 

from different applications, an activity known as session multiplexing, and it happens 

when a client connects to a server with multiple browser sessions open. This is exactly 

what’s taking place when you go someplace online like Amazon and click multiple links, 

opening them simultaneously to get information when comparison shopping. The client 

data from each browser session must be separate when the server application receives it, 

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