netdev - Re: [TCP_CA_CWR] Causes for entering TCP_CA

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Message-ID: <46bf9ede0907141249x7823c199uf3867b4d8284eb1c@mail.gmail.com>
Date:	Tue, 14 Jul 2009 15:49:52 -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

On Tue, Jul 14, 2009 at 3:15 PM, David Miller<davem@...emloft.net> wrote:
> From: Yinglin Sun <yinglinsun@...pitt.edu>
> Date: Tue, 14 Jul 2009 15:12:16 -0400
>
>> 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.
>
> Great, that if you're reading the code you also see that there
> are many code paths that invoke tcp_enter_cwr() that can occur
> without any retransmissions. :-)
>
> One such case is when ECN congestion notification bits are
> seen in an ACK packet.
>

Hi David,

Thanks for your reminding. I have checked all paths to
tcp_enter_cwr(). Two of them are FRTO and ECN. Both frto and ECN on my
sender and receiver are NOT enabled, so these two are not causes.
Another one is tcp_transmit_skb(). It seems that tcp_transmit_skb
detects if TX Queue is full. If so, it calls tcp_enter_cwr. I double
checked these invocation paths. If  I found all paths, the cause for
me is that TX Q is full, that is, local congestion. But it is also
possible that I missed other possible causes. Do you have any idea
about other invocation to tecp_enter_cwr?

Thanks a lot.

Yinglin
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