netdev - Re: [PATCH] make _minimum_ TCP retransmission timeout configurable

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Date:	Wed, 29 Aug 2007 15:09:58 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Ian McDonald <ian.mcdonald@...di.co.nz>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] make _minimum_ TCP retransmission timeout configurable

Ian McDonald wrote:
> Hmmm... RFC2988 says:
>    (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.
> 
> I went and had a look and this RFC has not been obsoleted. RFC3390
> also backs this assertion up.
> 
> So I'm suspecting that the default should be changed to 1000 to match
> the RFC which would solve this issue. I note that the RFC is a SHOULD
> rather than a MUST. I had a quick look around and not sure why Linux
> overrides the RFC on this one.

If nothing else, 200 ms is a "principle of least surprise" thing since 
that is the current value (in MS) for TCP_RTO_MIN.

rick jones
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