netdev - Re: [PATCH 2.6.22-rc5] TCP: Make TCP_RTO

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Date:	Tue, 26 Jun 2007 10:18:46 +1200
From:	"Ian McDonald" <ian.mcdonald@...di.co.nz>
To:	"OBATA Noboru" <noboru.obata.ar@...achi.com>
Cc:	"David Miller" <davem@...emloft.net>,
	"Stephen Hemminger" <shemminger@...ux-foundation.org>,
	netdev@...r.kernel.org
Subject: Re: [PATCH 2.6.22-rc5] TCP: Make TCP_RTO_MAX a variable

On 6/26/07, OBATA Noboru <noboru.obata.ar@...achi.com> wrote:
> From: OBATA Noboru <noboru.obata.ar@...achi.com>
>
> Make TCP_RTO_MAX a variable, and allow a user to change it via a
> new sysctl entry /proc/sys/net/ipv4/tcp_rto_max.  A user can
> then guarantee TCP retransmission to be more controllable, say,
> at least once per 10 seconds, by setting it to 10.  This is
> quite helpful on failover-capable network devices, such as an
> active-backup bonding device.  On such devices, it is desirable
> that TCP retransmits a packet shortly after the failover, which
> is what I would like to do with this patch.  Please see
> Background and Problem below for rationale in detail.
>
RFC2988 says this:
   (2.4) Whenever RTO is computed, if it is less than 1 second then the
         RTO SHOULD be rounded up to 1 second.

         Traditionally, TCP implementations use coarse grain clocks to
         measure the RTT and trigger the RTO, which imposes a large
         minimum value on the RTO.  Research suggests that a large
         minimum RTO is needed to keep TCP conservative and avoid
         spurious retransmissions [AP99].  Therefore, this
         specification requires a large minimum RTO as a conservative
         approach, while at the same time acknowledging that at some
         future point, research may show that a smaller minimum RTO is
         acceptable or superior.

   (2.5) A maximum value MAY be placed on RTO provided it is at least 60
         seconds.

Your code doesn't seem to meet requirements of section 2.5 as your
minimum is 1 second.

I think if you're trying to solve the bonding issue then you should
solve that issue, not hack the TCP implementation as that opens it up
to abuse in other ways.

Ian
-- 
Web: http://wand.net.nz/~iam4/
Blog: http://iansblog.jandi.co.nz
WAND Network Research Group
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