Windowing and Acknowledgments ACK Services

Windowing and acknowledgment services are used to indicate that packets have been received (ACK) and how many packets are expected before any acknowledgment is required (Windowing). The window size (amount of data that can be sent without an acknowledgment) is negotiated at connection time by connection-oriented protocols, such as TCP.

There must be mechanisms to tell any end device how many packets you can receive without overflowing your buffer; otherwise, packets will be lost, and sessions will time out. The window size can be adjusted during a connection if both end systems have more buffer memory available or if memory is decreasing. To illustrate, Figure 2.5 shows a simple windowing flow.

Let's say that you have two end systems that have negotiated that only one packet will be sent before any acknowledgment (ACK) is required. This session would be inefficient, because acknowledgment packets would traverse the link unnecessarily. This form of acknowledgment is basically a form of flow control so that end systems do not become overwhelmed with data.

Now, look at Figure 2.6. Figure 2.6 shows the same flow as discussed earlier except that the window size is set to three packets.

In Figure 2.6, the session will perform better than the earlier one-packet ACK scenario, because only one acknowledgment is required for every three packets

Table 2.2 Characteristics of connection-oriented and conectionless services.





Path setup, path connection,


information transfer,

teardown connection


Data packaged and sent

IP, Ethernet, Token Ring,

frame relay

Device A

Device B



Send Packet 1

Receive ACK 1

Receive Packet 1

Send Packet 2

Send ACK 1

Receive ACK 2

Receive Packet 2

Send Packet 3

Send ACK 2

Receive ACK 3

Receive Packet 3

Send ACK 3

Figure 2.5 Simple windowing.

Device A

Device B



Send Packets

1, 2, 3


~ :)

Receive Packets

Receive ACK 4

1, 2, 3

Send ACK 4

Send Packets

4, 5, 6


Receive Packets

Receive ACK 6

4, 5, 6

Send ACK 6

Figure 2.6 Advanced windowing. In TCP this is called a sliding window.

sent. Notice that Device B sends an acknowledgment for the next expected packet. TCP uses this same model for Telnet sessions, for example. This form of acknowledgment is known as a sliding window or advanced windowing.

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