netdev - Re: [PATCH] tcp: Implement a two-level initial RTO as per draft RFC 2988bis-02.

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Message-Id: <20110519.001426.2119532755281545481.davem@davemloft.net>
Date:	Thu, 19 May 2011 00:14:26 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	tsunanet@...il.com
Cc:	kuznet@....inr.ac.ru, pekkas@...core.fi, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, kaber@...sh.net, hagen@...u.net,
	eric.dumazet@...il.com, alexander.zimmermann@...sys.rwth-aachen.de,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tcp: Implement a two-level initial RTO as per draft
 RFC 2988bis-02.

From: tsuna <tsunanet@...il.com>
Date: Wed, 18 May 2011 20:56:33 -0700

> On Wed, May 18, 2011 at 7:36 PM, David Miller <davem@...emloft.net> wrote:
>> From: Benoit Sigoure <tsunanet@...il.com>
>> Date: Wed, 18 May 2011 19:22:24 -0700
>>
>>> Prior to this patch, Linux would always use 3 seconds (compile-time
>>> constant) as the initial RTO.  Draft RFC 2988bis-02 proposes to tune
>>> this down to 1 second and, in case of a timeout during the TCP 3WHS,
>>> revert the RTO back up to 3 seconds when data transmission begins.
>>
>> We just had a discussion where it was determined that changes to
>> these settings are "network specific" and therefore that if it
>> is appropriate at all (I'm still not convinced) it is only suitable
>> as a routing metric.
> 
> Fair enough.  I'll take another stab at it and see if I can change
> this to be on a per network basis.  Do I need any patch that's not yet
> in Linus' tree?  I'm referring to this:

Keep in mind another thing I do not like about this knob.

The IETF draft has a requirement that we fallback to 3 seconds if the
initial RTO is 1 second.

Nothing in your facilities ensure this, or provide a way for the
kernel to make sure this is the case.

And for other values of initial RTO, what fallback is appropriate?

As a result of all of this, I do not really think this is something
the user should control at all.

I really would rather see the initial RTO be static and be set to 1
with fallback RTO of 3.
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