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Date: Tue, 30 Aug 2005 07:31:22 -0500 (CDT)
From: Mark Tinguely <tinguely@casselton.net>
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To: hwh@gddsn.org.cn, questions@freebsd.org
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Subject: Re: Question about TCP packet.
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<deleted background>
>  found that I received 2 ack packets when sent data to embedded device.
>  Does It mean one ack packet is not necessary and
>  the embedded device's tcp stack is broken?
>
>  thanks,
>  --hwh
>
>
>  16:58:08.448730 192.168.168.137.1693 > 211.96.21.220.9742: P 1:22(21) ack 1 win 65535 (DF)
>  16:58:08.450458 211.96.21.220.9742 > 192.168.168.137.1693: . ack 22 win 1439 (DF)
>  16:58:08.482349 211.96.21.220.9742 > 192.168.168.137.1693: . ack 22 win 1460 (DF)

The second ack is a window update. The recieving TCP stack is telling the
sender that the data has been consumed and more space is available for
incoming data.

If you increase your recieve buffer size on the "211.96.21.220" host,
you will not see these updates for interactive data.

--Mark Tinguely