From: Rick Jones <redacted>
Subject: Re: [PATCH 2.6.22-rc5] TCP: Make TCP_RTO_MAX a variable
Date: Mon, 25 Jun 2007 15:29:26 -0700

Ian McDonald wrote:

quoted

On 6/26/07, OBATA Noboru [off-list ref] wrote:

quoted

From: OBATA Noboru <redacted>

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").

Any comments from others?

-- 
OBATA Noboru (noboru.obata.ar@hitachi.com)