On Thu, 18 Feb 2010, Andreas Petlund wrote:

quoted text
> This patch enables fast retransmissions after one dupACK for
> TCP if the stream is identified as thin. This will reduce
> latencies for thin streams that are not able to trigger fast
> retransmissions due to high packet interarrival time. This
> mechanism is only active if enabled by iocontrol or syscontrol
> and the stream is identified as thin.
> 
> 
> Signed-off-by: Andreas Petlund <apetlund@simula.no>
> ---
>  Documentation/networking/ip-sysctl.txt |   12 ++++++++++++
>  include/linux/tcp.h                    |    4 +++-
>  include/net/tcp.h                      |    1 +
>  net/ipv4/sysctl_net_ipv4.c             |    7 +++++++
>  net/ipv4/tcp.c                         |    7 +++++++
>  net/ipv4/tcp_input.c                   |   12 ++++++++++++
>  6 files changed, 42 insertions(+), 1 deletions(-)
> 
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index f147310..2571a62 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -499,6 +499,18 @@ tcp_thin_linear_timeouts - BOOLEAN
>  	Documentation/networking/tcp-thin.txt
>  	Default: 0
>  
> +tcp_thin_dupack - BOOLEAN
> +	Enable dynamic triggering of retransmissions after one dupACK
> +	for thin streams. If set, a check is performed upon reception
> +	of a dupACK to determine if the stream is thin (less than 4
> +	packets in flight). As long as the stream is found to be thin,
> +	data is retransmitted on the first received dupACK. This
> +	improves retransmission latency for non-aggressive thin
> +	streams, often found to be time-dependent.
> +	For more information on thin streams, see
> +	Documentation/networking/tcp-thin.txt
> +	Default: 0
> +
>  UDP variables:
>  
>  udp_mem - vector of 3 INTEGERs: min, pressure, max
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 3ba8b07..a778ee0 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -104,6 +104,7 @@ enum {
>  #define TCP_MD5SIG		14	/* TCP MD5 Signature (RFC2385) */
>  #define TCP_COOKIE_TRANSACTIONS	15	/* TCP Cookie Transactions */
>  #define TCP_THIN_LINEAR_TIMEOUTS 16      /* Use linear timeouts for thin streams*/
> +#define TCP_THIN_DUPACK         17      /* Fast retrans. after 1 dupack */
>  
>  /* for TCP_INFO socket option */
>  #define TCPI_OPT_TIMESTAMPS	1
> @@ -343,7 +344,8 @@ struct tcp_sock {
>  	u8	frto_counter;	/* Number of new acks after RTO */
>  	u8	nonagle     : 4,/* Disable Nagle algorithm?             */
>  		thin_lto    : 1,/* Use linear timeouts for thin streams */
> -		unused      : 3;
> +		thin_dupack : 1,/* Fast retransmit on first dupack      */
> +		unused      : 2;
>  
>  /* RTT measurement */
>  	u32	srtt;		/* smoothed round trip time << 3	*/
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 6278fc7..56f0aec 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -245,6 +245,7 @@ extern int sysctl_tcp_slow_start_after_idle;
>  extern int sysctl_tcp_max_ssthresh;
>  extern int sysctl_tcp_cookie_size;
>  extern int sysctl_tcp_thin_linear_timeouts;
> +extern int sysctl_tcp_thin_dupack;
>  
>  extern atomic_t tcp_memory_allocated;
>  extern struct percpu_counter tcp_sockets_allocated;
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index e6a2460..c1bc074 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -582,6 +582,13 @@ static struct ctl_table ipv4_table[] = {
>  		.mode           = 0644,
>  		.proc_handler   = proc_dointvec
>  	},
> +        {
> +		.procname       = "tcp_thin_dupack",
> +		.data           = &sysctl_tcp_thin_dupack,
> +		.maxlen         = sizeof(int),
> +		.mode           = 0644,
> +		.proc_handler   = proc_dointvec
> +	},
>  	{
>  		.procname	= "udp_mem",
>  		.data		= &sysctl_udp_mem,
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 21bae9a..5901010 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2236,6 +2236,13 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
>  			tp->thin_lto = val;
>  		break;
>  
> +	case TCP_THIN_DUPACK:
> +		if (val < 0 || val > 1)
> +			err = -EINVAL;
> +		else
> +			tp->thin_dupack = val;
> +		break;
> +
>  	case TCP_CORK:
>  		/* When set indicates to always queue non-full frames.
>  		 * Later the user clears this option and we transmit
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 3fddc69..8d950b9 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -89,6 +89,8 @@ int sysctl_tcp_frto __read_mostly = 2;
>  int sysctl_tcp_frto_response __read_mostly;
>  int sysctl_tcp_nometrics_save __read_mostly;
>  
> +int sysctl_tcp_thin_dupack __read_mostly;
> +
>  int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
>  int sysctl_tcp_abc __read_mostly;
>  
> @@ -2447,6 +2449,16 @@ static int tcp_time_to_recover(struct sock *sk)
>  		return 1;
>  	}
>  
> +	/* If a thin stream is detected, retransmit after first
> +	 * received dupack. Employ only if SACK is supported in order
> +	 * to avoid possible corner-case series of spurious retransmissions
> +	 * Use only if there are no unsent data.
> +	 */
> +	if ((tp->thin_dupack || sysctl_tcp_thin_dupack) &&
> +	    tcp_stream_is_thin(tp) && tcp_dupack_heuristics(tp) > 1 &&
> +	    tcp_is_sack(tp) && sk->sk_send_head == NULL)

Use tcp_send_head(sk) instead.

quoted text
> +		return 1;
> +
>  	return 0;
>  }

Other than that,

Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>

-- 
 i.