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

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Date:	Sun, 2 Mar 2014 16:16:47 -0800
From:	Nandita Dukkipati <nanditad@...gle.com>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp: fix bogus RTT on special retransmission

On Fri, Feb 28, 2014 at 4:42 PM, Yuchung Cheng <ycheng@...gle.com> 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: Nandita Dukkipati <nanditad@...gle.com>
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