
Chapter 6
Meters, queues and IPQoS
E-NIT-CTC-20041213-0013v0.5 77
Ackfiltering
Example The figure below illustrates how ack filtering is done.
An upload data stream is exists (data packets). Meanwhile a download TCP 
connection is generated as well. TCP-based downloads can only continue if the 
remote site receives ACK packets for each data packet it sends. As we can see in 
the figure above there are two ACK packets in the queue. To avoid delay we will only 
send the second ACK packet and drop the first ACK packet. If the remote site 
receives ACK2 it will know that everything that was sent before was OK. If ACK 
filtering is turned off both the ACK will be send, causing delay.
ackfiltering value Description
enabled If the ackfiltering option is enabled duplicate ACK 
packets in a queue will only be sent once. Meaning 
that the last duplicate ACK packet will be sent and the 
other ACK packets will be dropped
disabled If the ackfiltering option is disabled all ACK packets 
will be sent in their original sequence.
ACK filtering happens on a per TCP-connection base.
Data ACK1Data DataACK2