netdev - Re: [PATCH 1/1] net: tcp: RTO restart

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Date:	Wed, 19 Feb 2014 18:17:12 +0100
From:	Andreas Petlund <apetlund@...ula.no>
To:	Per Hurtig <per.hurtig@....se>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	Anna Brunström <anna.brunstrom@....se>,
	Michael Welzl <michawe@....uio.no>, ilpo.jarvinen@...sinki.fi
Subject: Re: [PATCH 1/1] net: tcp: RTO restart

I’ve applied and tested the patch on VMs with both 32 bit and 64 bit kernel.

Acked-by: Andreas Petlund <apetlund@...ula.no>
Tested-by: Andreas Petlund <apetlund@...ula.no>

Best regards,
Andreas Petlund

On 18 Feb 2014, at 19:12, Per Hurtig <per.hurtig@....se> wrote:

> This patch implements the RTO restart modification described in
> http://tools.ietf.org/html/draft-ietf-tcpm-rtorestart-02
> 
> RTO Restart's goal is to provide quicker loss recovery for segments lost in the
> end of a burst/connection. To accomplish this the algorithm adjusts the RTO
> value on each rearm of the retransmission timer to be exactly RTO ms after the
> earliest outstanding segment was sent. The offsetting against the earliest
> outstanding segment is not done by the regular rearm algorithm, which causes
> RTOs to occur, on average, after RTO+RTT ms.
> 
> As a faster timeout is only beneficial in scenarios where fast retransmit/early
> retransmit cannot be triggered the algorithm will only kick in when there is a
> small amount of outstanding segments.
> 
> The RTO Restart proposal is accepted as a working group item in the IETF TCP
> Maintenance and Minor Extensions (tcpm) TCP wg and is intended for experimental
> RFC status.
> 
> Signed-off-by: Per Hurtig <per.hurtig@....se>
> ---
> include/net/tcp.h          |    1 +
> net/ipv4/sysctl_net_ipv4.c |    7 +++++++
> net/ipv4/tcp_input.c       |   11 +++++++++++
> 3 files changed, 19 insertions(+)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 56fc366..575e82a 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -278,6 +278,7 @@ extern int sysctl_tcp_slow_start_after_idle;
> extern int sysctl_tcp_thin_linear_timeouts;
> extern int sysctl_tcp_thin_dupack;
> extern int sysctl_tcp_early_retrans;
> +extern int sysctl_tcp_rto_restart;
> extern int sysctl_tcp_limit_output_bytes;
> extern int sysctl_tcp_challenge_ack_limit;
> extern unsigned int sysctl_tcp_notsent_lowat;
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index 44eba05..c605f4f 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -717,6 +717,13 @@ static struct ctl_table ipv4_table[] = {
> 		.extra2		= &four,
> 	},
> 	{
> +		.procname	= "tcp_rto_restart",
> +		.data		= &sysctl_tcp_rto_restart,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec,
> +	},
> +	{
> 		.procname	= "tcp_min_tso_segs",
> 		.data		= &sysctl_tcp_min_tso_segs,
> 		.maxlen		= sizeof(int),
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 227cba7..450ee30 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -98,6 +98,7 @@ int sysctl_tcp_thin_dupack __read_mostly;
> 
> int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
> int sysctl_tcp_early_retrans __read_mostly = 3;
> +int sysctl_tcp_rto_restart __read_mostly = 1;
> 
> #define FLAG_DATA		0x01 /* Incoming frame contained data.		*/
> #define FLAG_WIN_UPDATE		0x02 /* Incoming ACK was a window update.	*/
> @@ -2972,6 +2973,16 @@ void tcp_rearm_rto(struct sock *sk)
> 			 */
> 			if (delta > 0)
> 				rto = delta;
> +		} else if (icsk->icsk_pending == ICSK_TIME_RETRANS &&
> +			   sysctl_tcp_rto_restart &&
> +			   sk->sk_send_head == NULL &&
> +			   tp->packets_out < 4) {
> +			struct sk_buff *skb = tcp_write_queue_head(sk);
> +			const u32 rto_time_stamp = TCP_SKB_CB(skb)->when;
> +			s32 delta = (s32)(tcp_time_stamp - rto_time_stamp);
> +
> +			if (delta > 0)
> +				rto -= delta;
> 		}
> 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
> 					  TCP_RTO_MAX);
> -- 
> 1.7.9.5
> 

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