netdev - Re: [PATCH] tcp: fixing TLP's FIN recovery

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Date:	Sat, 07 Jun 2014 19:58:25 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Per Hurtig <per.hurtig@....se>
Cc:	netdev@...r.kernel.org, anna.brunstrom@....se,
	mohammad.rajiullah@....se, ncardwell@...gle.com,
	nanditad@...gle.com, sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH] tcp: fixing TLP's FIN recovery

On Sat, 2014-06-07 at 16:34 +0200, Per Hurtig wrote:
> Fix to a problem observed when losing a FIN segment that does not
> contain data.  In such situations, TLP is unable to recover from
> *any* tail loss and instead adds at least PTO ms to the
> retransmission process, i.e., RTO = RTO + PTO.
> 
> Signed-off-by: Per Hurtig <per.hurtig@....se>
> ---
>  net/ipv4/tcp_output.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index d463c35..6573765 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2130,8 +2130,10 @@ void tcp_send_loss_probe(struct sock *sk)
>  	if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
>  		goto rearm_timer;
>  
> -	/* Probe with zero data doesn't trigger fast recovery. */
> -	if (skb->len > 0)
> +	/* Probe with zero data doesn't trigger fast recovery, if FIN
> +	 * flag is not set.
> +	 */
> +	if ((skb->len > 0) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
>  		err = __tcp_retransmit_skb(sk, skb);
>  
>  	/* Record snd_nxt for loss detection. */


You know, I believe the test was exactly to avoid sending data less FIN
packets.

If you write :

    if (A  || !A)

Better remove the condition, completely ;)


Nandita, why FIN packet wont trigger fast retransnmits ?

It sounds like if the timer is the issue you want to fix, you might
simply rearm a timer with RTO-PTO instead of RTO ?



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