netdev - Re: [PATCH][v4] tcp: fix ICMP-RTO war

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Message-ID: <n2gd1c2719f1005071625nd3df3d72t9655a1f64f2fd825@mail.gmail.com>
Date:	Fri, 7 May 2010 16:25:46 -0700
From:	Jerry Chu <hkchu@...gle.com>
To:	damian@....rwth-aachen.de
Cc:	ilpo.jarvinen@...sinki.fi, netdev@...r.kernel.org
Subject: Re: [PATCH][v4] tcp: fix ICMP-RTO war

Hi,

I'm working on a patch that tries to measure and use the RTT for the passive
open side when the TS option is NOT enabled. My code conflicts with your
recently added "tcp_ack_update_rtt(sk, 0, 0);" Could you tell me why do you
force this call for the no-TS case when obviously "0" is not a measured RTT?
If you try to force icsk_rto to be initialized correctly, it is
already initialized to
TCP_TIMEOUT_INIT by tcp_create_openreq_child(). What am I missing?

Thanks,

Jerry

>From: David Miller <davem@...emloft.net>
>
> Date: Sun, Feb 21, 2010 at 7:10 PM
> Subject: Re: [PATCH][v4] tcp: fix ICMP-RTO war
> To: ilpo.jarvinen@...sinki.fi
> Cc: damian@....rwth-aachen.de, netdev@...r.kernel.org
>
>
> From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
> Date: Tue, 16 Feb 2010 14:45:25 +0200 (EET)
>
> > On Wed, 10 Feb 2010, Damian Lukowski wrote:
> >
> >> @@ -5783,12 +5783,10 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
> >>
> >>                              /* tcp_ack considers this ACK as duplicate
> >>                               * and does not calculate rtt.
> >> -                             * Fix it at least with timestamps.
> >> +                             * Force it here.
> >>                               */
> >> -                            if (tp->rx_opt.saw_tstamp &&
> >> -                                tp->rx_opt.rcv_tsecr && !tp->srtt)
> >> -                                    tcp_ack_saw_tstamp(sk, 0);
> >> -
> >> +                            tcp_ack_update_rtt(sk, 0, 0);
> >> +
> >
> > ...Here a zero seq_rtt is given to RTT estimator (it will be effective
> > only in the case w/o timestamps, TS case recalculates it from the stored
> > timestamps). Maybe we could use some field (timestamp related one comes to
> > my mind) in request sock to get a real RTT estimate for non-timestamp case
> > too. ...It seems possible to me, though tricky because the request_sock is
> > no longer that easily available here so some parameter passing would be
> > needed.
>
> Agreed.
>
> But even more simply I think we should make even the current
> tcp_ack_update_rtt() call here conditional on at least
> tp->srtt being zero.
>
> Damian do you at least agree with that?
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