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

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Date:	Thu, 12 Jul 2007 21:54:36 +1200
From:	"Ian McDonald" <ian.mcdonald@...di.co.nz>
To:	"OBATA Noboru" <noboru.obata.ar@...achi.com>
Cc:	rick.jones2@...com, davem@...emloft.net,
	shemminger@...ux-foundation.org, yoshfuji@...ux-ipv6.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 2.6.22-rc5] TCP: Make TCP_RTO_MAX a variable

On 7/12/07, OBATA Noboru <noboru.obata.ar@...achi.com> wrote:
> > Ian McDonald wrote:
> > > 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.
> >
> > (At the risk of having another Emily Litella moment entering a
> > discussion late...)
> >
> > I thought that those sorts of things were generally referring to the
> > _default_ setting?
>
> I believe so.  And the requirement of section 2.5 is rather weak
> (it says "MAY").
>
It is weak in saying you don't have to have a maximum, but if you do
have one IT IS AT LEAST 60 seconds (emphasis mine). So the time period
is a strong requirement if you decide to implement - which is a weak
requirement.

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