netdev - Re: [RFC] tcp: allow timestamps even if SYN packet has tsval=0

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Message-Id: <20090313.142507.47563051.davem@davemloft.net>
Date:	Fri, 13 Mar 2009 14:25:07 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	dada1@...mosbay.com, netdev@...r.kernel.org
Subject: Re: [RFC] tcp: allow timestamps even if SYN packet has tsval=0

From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Thu, 12 Mar 2009 09:26:20 +0200 (EET)

> On Wed, 11 Mar 2009, David Miller wrote:
> 
> > From: Eric Dumazet <dada1@...mosbay.com>
> > Date: Wed, 11 Mar 2009 13:17:54 +0100
> > 
> > > So apparently WindowsXP sends a NULL tsval in SYN packet, then
> > > subsequent packets get a real value (60498) in this case.
> > > 
> > > This seems to work on other OS as well, so is the following patch
> > > considered evil ?  Do we have security concerns or only risking
> > > windows client to have slightly wrong rtt estimation at the begining
> > > of the tcp session ?
> > 
> > I think we'll have to accept this.
> > 
> > I don't see other systems blocking initial ts_ecn values of
> > zero like we do.
> 
> What about the fact that PAWS could bite us leaving us a hung connection 
> if timestamp changes too much when we get the first ACK? Though I doubt 
> you can get windows to run long enough for this to become a problem if 
> they always start from zero... ;-)

I really don't think it's a real issue, and Windows XP should be happy
we're willing to try timestamps at all with it :-)
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