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When will a host ACK?

Posted on 2004-09-26
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Last Modified: 2013-12-23
I'm trying to find the rules out when a host will ACK.
Tell me if this is correct.

A host will ACK when:

-it has received 2 full sized packets and none have been acknowledged

-the delayed timer has gone off

-it has data ready in its send buffer

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Question by:dissolved
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by:gheist
ID: 12156022
No.
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by:dissolved
ID: 12156085
What is incorrect?  I know that most implementations will fire an ACK regardless after 200ms. My question is, does the receiver always go into delayed ack mode upon receiving a segment.
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by:gheist
ID: 12157484
No
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by:dissolved
ID: 12159161
Does anyone know?  
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by:gheist
ID: 12159764
I know - ack/retry algorithm is much more complex than you think.

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ip.htm#xtocid16

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Author Comment

by:dissolved
ID: 12160827
That's not what I'm talking about.  My question is in regards to the delayed acknowledgement algorithm, not retransmission or sliding window. I know what those are.

I was trying to get confirmation on the delayed ack algorithm.

A host is supposed to ACK if:

-it has received 2 full size MSS packets that have not yet been acknowledged.

-If 200ms has passed since receiving a packet

-If data is immediately ready for transmission to sender, upon receiving a segment (such as in interactive sessions ie: telnet)
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Accepted Solution

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gheist earned 500 total points
ID: 12162353
1) this is selective acknowledgement, there can be one or even more packets (window size / mss)
2) timer is 100ms, same as in Nagle's algorythm
3) this is true, basic working of delayed ack.
Basically this means that it increases efficiency of interactive sessions sending huge bulks of data in interactive manner like database connections, no big change with telnet.
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