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When will a host ACK?

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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gheistCommented:
No.
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dissolvedAuthor Commented:
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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gheistCommented:
No
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dissolvedAuthor Commented:
Does anyone know?  
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gheistCommented:
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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dissolvedAuthor Commented:
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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gheistCommented:
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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