linux-kernel - Re: [PATCH] tcp: don't use timestamp from repaired skb-s to calculate RTT

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Date:	Tue, 12 Aug 2014 05:15:01 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrey Vagin <avagin@...nvz.org>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] tcp: don't use timestamp from repaired skb-s to
 calculate RTT

On Tue, 2014-08-12 at 13:45 +0400, Andrey Vagin wrote:
> We don't know right timestamp for repaired skb-s. Wrong RTT estimations
> isn't good, because some congestion modules heavily depends on it.
> 
> This patch adds the TCPCB_REPAIRED flag, which is included in
> TCPCB_RETRANS.

...

> +
> +				/* All packets are restored as if they have
> +				 * already been sent. skb_mstamp isn't set to
> +				 * avoid wrong rtt estimation.
> +				 */
> +				if (tp->repair) {
> +					TCP_SKB_CB(skb)->sacked |= TCPCB_REPAIRED;
> +					TCP_SKB_CB(skb)->when = tcp_time_stamp;
> +				}
>  			}
>  
>  			/* Try to append data to the end of skb. */


Are you sure TCP_SKB_CB(skb)->when needs to be set ?

It should not anymore.

If yes, I believe a comment would help a lot here.

Also, please include this tag to ease stable backports (3.15+) :

Fixes: 431a91242d8d ("tcp: timestamp SYN+DATA messages")
Fixes: 740b0f1841f6 ("tcp: switch rtt estimations to usec resolution")

Thanks !


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