linux-kernel - Re: [PATCH 1/1] tcp: Wrong timeout for SYN segments

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Message-ID: <CAFbMe2NVbjGyEF9VRj9p_ejx3mGKTPT2urnguLtfRZEMShsO4g@mail.gmail.com>
Date:	Wed, 22 Aug 2012 09:44:10 -0700
From:	"H.K. Jerry Chu" <hkjerry.chu@...il.com>
To:	Alex Bergmann <alex@...lab.net>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] tcp: Wrong timeout for SYN segments

On Tue, Aug 21, 2012 at 4:29 PM, Alex Bergmann <alex@...lab.net> wrote:
> Hi David,
>
> I'm not 100% sure, but it looks like I found an RFC mismatch with the
> current default values of the TCP implementation.
>
> Alex
>
> From 8b854a525eb45f64ad29dfab16f9d9f681e84495 Mon Sep 17 00:00:00 2001
> From: Alexander Bergmann <alex@...lab.net>
> Date: Wed, 22 Aug 2012 00:29:08 +0200
> Subject: [PATCH 1/1] tcp: Wrong timeout for SYN segments
>
> Commit 9ad7c049 changed the initRTO from 3secs to 1sec in accordance to
> RFC6298 (former RFC2988bis). This introduced a gap with RFC1122 that
> defines a minimum retransmission window for SYN segments of at least
> 180secs.
>
> Prior to 9ad7c049 the timeout was defined with 189secs. Now we have only
> a timeout of 63secs.
>
>         ((2 << 5) - 1) * 3 secs = 189 secs
>         ((2 << 5) - 1) * 1 secs = 63 secs
>
> To fulfill the MUST constraint in RFC1122 section 4.2.3.5 about R2 for
> SYN segments, the values of TCP_SYN_RETRIES and TCP_SYNACK_RETRIES must
> be changed to 7 reties.
>
>         ((2 << 7) - 1) * 1 secs = 255 secs
>
> This would result in an ETIMEDOUT of 4 minutes 15 seconds.

This issue occurred to me right after I submitted the patch for RFC6298.
I did not commit any more change because RFC compliance aside, 180secs
just seem like eternity in the Internet age.

(See my past post on this at
http://marc.info/?l=linux-netdev&m=130759078118866&w=2)

Jerry

>
> Signed-off-by: Alexander Bergmann <alex@...lab.net>
> ---
>  include/net/tcp.h |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 1f000ff..7eaae19 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -98,10 +98,10 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
>                                  * 15 is ~13-30min depending on RTO.
>                                  */
>
> -#define TCP_SYN_RETRIES         5      /* number of times to retry active opening a
> +#define TCP_SYN_RETRIES         7      /* number of times to retry active opening a
>                                  * connection: ~180sec is RFC minimum   */
>
> -#define TCP_SYNACK_RETRIES 5   /* number of times to retry passive opening a
> +#define TCP_SYNACK_RETRIES 7   /* number of times to retry passive opening a
>                                  * connection: ~180sec is RFC minimum   */
>
>  #define TCP_TIMEWAIT_LEN (60*HZ) /* how long to wait to destroy TIME-WAIT
> --
> 1.7.8.6
>
> --
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