netdev - Re: [PATCH v2 1/1] TCP: increase default initial receive window.

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Message-ID: <AANLkTinobEo0yu-a_tfk1qS4cfg2ByhnGqsKjkV0HGYc@mail.gmail.com>
Date:	Mon, 20 Dec 2010 16:23:24 -0800
From:	Nandita Dukkipati <nanditad@...gle.com>
To:	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <shemminger@...tta.com>,
	Rick Jones <rick.jones2@...com>
Cc:	netdev@...r.kernel.org, Tom Herbert <therbert@...gle.com>,
	Laurent Chavey <chavey@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>
Subject: Re: [PATCH v2 1/1] TCP: increase default initial receive window.

As per the comments, I have resubmitted the patch with a longer
explanation. Please let me know if the preference is to have a sysctl
for the initial default receive window, as I am ok either way.

Thanks,
-Nandita

On Mon, Dec 20, 2010 at 4:15 PM, Nandita Dukkipati <nanditad@...gle.com> wrote:
> This patch changes the default initial receive window to 10 mss
> (defined constant). The default window is limited to the maximum
> of 10*1460 and 2*mss (when mss > 1460).
>
> draft-ietf-tcpm-initcwnd-00 is a proposal to the IETF that recommends
> increasing TCP's initial congestion window to 10 mss or about 15KB.
> Leading up to this proposal were several large-scale live Internet
> experiments with an initial congestion window of 10 mss (IW10), where
> we showed that the average latency of HTTP responses improved by
> approximately 10%. This was accompanied by a slight increase in
> retransmission rate (0.5%), most of which is coming from applications
> opening multiple simultaneous connections. To understand the extreme
> worst case scenarios, and fairness issues (IW10 versus IW3), we further
> conducted controlled testbed experiments. We came away finding minimal
> negative impact even under low link bandwidths (dial-ups) and small
> buffers.  These results are extremely encouraging to adopting IW10.
>
> However, an initial congestion window of 10 mss is useless unless a TCP
> receiver advertises an initial receive window of at least 10 mss.
> Fortunately, in the large-scale Internet experiments we found that most
> widely used operating systems advertised large initial receive windows
> of 64KB, allowing us to experiment with a wide range of initial
> congestion windows. Linux systems were among the few exceptions that
> advertised a small receive window of 6KB. The purpose of this patch is
> to fix this shortcoming.
>
> References:
> 1. A comprehensive list of all IW10 references to date.
> http://code.google.com/speed/protocols/tcpm-IW10.html
>
> 2. Paper describing results from large-scale Internet experiments with IW10.
> http://ccr.sigcomm.org/drupal/?q=node/621
>
> 3. Controlled testbed experiments under worst case scenarios and a
> fairness study.
> http://www.ietf.org/proceedings/79/slides/tcpm-0.pdf
>
> 4. Raw test data from testbed experiments (Linux senders/receivers)
> with initial congestion and receive windows of both 10 mss.
> http://research.csc.ncsu.edu/netsrv/?q=content/iw10
>
> 5. Internet-Draft. Increasing TCP's Initial Window.
> https://datatracker.ietf.org/doc/draft-ietf-tcpm-initcwnd/
>
> Signed-off-by: Nandita Dukkipati <nanditad@...gle.com>
> ---
> Changelog since v1:
> - A longer and better commit message.
>
>  include/net/tcp.h     |    3 +++
>  net/ipv4/tcp_output.c |   11 ++++++++---
>  2 files changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index b448030..38509f0 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -60,6 +60,9 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
>  */
>  #define MAX_TCP_WINDOW         32767U
>
> +/* Offer an initial receive window of 10 mss. */
> +#define TCP_DEFAULT_INIT_RCVWND        10
> +
>  /* Minimal accepted MSS. It is (60+60+8) - (20+20). */
>  #define TCP_MIN_MSS            88U
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 2d39066..dc7c096 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -228,10 +228,15 @@ void tcp_select_initial_window(int __space, __u32 mss,
>                }
>        }
>
> -       /* Set initial window to value enough for senders, following RFC5681. */
> +       /* Set initial window to a value enough for senders starting with
> +        * initial congestion window of TCP_DEFAULT_INIT_RCVWND. Place
> +        * a limit on the initial window when mss is larger than 1460.
> +        */
>        if (mss > (1 << *rcv_wscale)) {
> -               int init_cwnd = rfc3390_bytes_to_packets(mss);
> -
> +               int init_cwnd = TCP_DEFAULT_INIT_RCVWND;
> +               if (mss > 1460)
> +                       init_cwnd =
> +                       max_t(u32, (1460 * TCP_DEFAULT_INIT_RCVWND) / mss, 2);
>                /* when initializing use the value from init_rcv_wnd
>                 * rather than the default from above
>                 */
> --
> 1.7.3.1
>
>
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