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From what I studied, congestion avoidance phase sets CWND = CWND + MSS * (MSS/CWND) every time a new acknowledgment is received. This is assuming we don't encounter duplicate ACKS or timeouts. But what happens if there are delayed acknowledgements ?

Here's what I think from research on delayed acks (no idea if this is correct):

Basically Delayed ACK is the destination retaining the ACK segment for a period of time expecting one of two things. Either there will be more ACKS will be required to be sent before the timer is up because of new packets recieved by the receiver. OR the receiver will need to send some data back to the sender in which case it can piggy back the message on that packet. How does this affect the congestion avoidance phase ? This would be bad for congestion avoidance phase of TCP which depends on new Acks to increase CWND. This would cause delays in CWND window size change thus causing delay in the sending of packets. This means by the time that TCP could be sending packets to the receiver, it is actually not because acknowledgments are being delayed.

Eugène Adell
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Daniel Kobe
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2 Answers2

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This affects the congestion avoidance phase the same way it affects the other phase (SS) : it will slow down the traffic. However, keep in mind there are two different network uses, the interactive one (such as telnet), and the bulk one. Delayed Acks are likely to be used with interactive protocols sending very small amounts of data, but this can bring new problems if Nagle's algorithm is used by the other side. When unsure, just disable delayed Acks.

Eugène Adell
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That is a really good question. Since they keep the bottleneck buffer full, delayed Ack is not a big problem for traditional congestion control algorithms such as Reno and CUBIC. For TCP Vegas which tries to keep the queue small, it is still not a problem because if it faces delayed Ack, it will reduce cwnd very slowly (one unit every RTT). Therefore a Vegas connection will not suffer from under-utilization unless delayed ack lasts for a very long time.

Mahdi
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