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

[Damian Lukowski <damian@tvk.rwth-aachen.de>]

> 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?

Hi,
the backoff reversion code uses __tcp_set_rto() to recalculate icsk_rto,
which itself relies on tp->srtt and rttvar.
srtt is explicitly set to 0 in tcp_create_openreq_child(), so I didn't change it.
Initializing it with TCP_TIMEOUT_INIT should also fix that specific bug,
but I don't know if there are other impacts.

Regards
 Damian

> Thanks,
> 
> Jerry
> 
>> From: David Miller <davem@davemloft.net>
>>
>> Date: Sun, Feb 21, 2010 at 7:10 PM
>> Subject: Re: [PATCH][v4] tcp: fix ICMP-RTO war
>> To: ilpo.jarvinen@helsinki.fi
>> Cc: damian@tvk.rwth-aachen.de, netdev@vger.kernel.org
>>
>>
>> From: "Ilpo Järvinen" <ilpo.jarvinen@helsinki.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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