netdev - Re: [TCP_CA_CWR] Causes for entering TCP_CA

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Message-ID: <46bf9ede0907141212x7a7a84f8l1882ffefbaf21e14@mail.gmail.com>
Date:	Tue, 14 Jul 2009 15:12:16 -0400
From:	Yinglin Sun <yinglinsun@...pitt.edu>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [TCP_CA_CWR] Causes for entering TCP_CA_CWR state with 0 
	retransmissions

Hi David,

I use the total number of retransmissions reported by tcp_info. The
field name is tcpi_total_retrans. From the kernel source code, I found
that this number is for both fast retransmit and timeout
retransmissions. On the other hand, if fast retransmission happened,
ca_state should be TCP_CA_Recovery or TCP_CA_Disorder, but it's
TCP_CA_CWR.

Thanks.

Yinglin

On Tue, Jul 14, 2009 at 2:42 PM, David Miller<davem@...emloft.net> wrote:
> From: Yinglin Sun <yinglin.s@...il.com>
> Date: Tue, 14 Jul 2009 14:38:07 -0400
>
>> I have a question about tcp_ca_state "TCP_CA_CWR".  Under what
>> circumstances does it enter this state?
>> I'm doing some experiments and have trace from tcp_info. From tcp_info
>> trace, I found that many places show ca_state is 2 (TCP_CA_CWR). Under
>> this state, the congestion window size decreases but the total number
>> of retransmissions is still 0.
>
> What kind of "retransmissions"?  I bet the counter you are looking
> at is counting timeout based retransmissions, which means it isn't
> including the most predominant type of retransmissions, which are
> fast retransmissions which are triggered in response to ACKs.
>
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