netdev - Re: [PATCH net] tcp: fix bogus RTT on special retransmission

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Message-ID: <1393636422.26794.91.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 28 Feb 2014 17:13:42 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	davem@...emloft.net, ncardwell@...gle.com, edumazet@...gle.com,
	nanditad@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] tcp: fix bogus RTT on special retransmission

On Fri, 2014-02-28 at 16:42 -0800, Yuchung Cheng wrote:
> RTT may be bogus with tall loss probe (TLP) when a packet
> is retransmitted and latter (s)acked without TCPCB_SACKED_RETRANS flag.
> 
> For example, TLP calls __tcp_retransmit_skb() instead of
> tcp_retransmit_skb(). The skb timestamps are updated but the sacked
> flag is not marked with TCPCB_SACKED_RETRANS. As a result we'll
> get bogus RTT in tcp_clean_rtx_queue() or in tcp_sacktag_one() on
> spurious retransmission.
> 
> The fix is to apply the sticky flag TCP_EVER_RETRANS to enforce Karn's
> check on RTT sampling. However this will disable F-RTO if timeout occurs
> after TLP, by resetting undo_marker in tcp_enter_loss(). We relax this
> check to only if any pending retransmists are still in-flight.
> 
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> ---

Acked-by: Eric Dumazet <edumazet@...gle.com>


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