net.inet.tcp.slowstart_flightsize in 8-STABLE

[Andre Oppermann]

On 06.08.2010 11:00, Charles Logan wrote:
> Sorry but this is not a bug. You set bad sysctl flags.

Care to explain in more detail?  For example which sysctl flag was set wrong?

> We won't add it to our database and this is the final decison.

Which database?

> Regards,
> Freebsd Team

Which team?  What is your @FreeBSD.ORG address?

-- 
Andre

> On Fri, Aug 6, 2010 at 10:56 AM, Andre Oppermann<andre at freebsd.org>  wrote:
>> On 13.07.2010 16:01, Maxim Dounin wrote:
>>>
>>> Hello!
>>>
>>> On Wed, May 12, 2010 at 04:47:02PM +0400, Igor Sysoev wrote:
>>>
>>>> It seems that net.inet.tcp.slowstart_flightsize does not work in
>>>> 8-STABLE.
>>>> For a long time I used slowstart_flightsize=2 on FreeBSD 4, 6, and 7
>>>> hosts.
>>>> However, FreeBSD-8 always starts with the single packet.
>>>> I saw this on different versions of 8-STABLE since 8 Oct 2009 till
>>>> 04 Apr 2010.
>>>
>>> Finally I had some time to look into it (sorry for long delay).
>>>
>>> 1. Slow start isn't used on recent FreeBSD versions for initial snd_cwnd
>>> calculations as long as you have rfc3390 support switched on (default
>>> since
>>> Jan 06 23:29:46 2004, at least in 7.*).  It effectively sets initial
>>> snd_cwnd to 3*MSS on common networks and shouldn't cause any problems.
>>> Slowstart_flightsize only affects connection restarts.
>>>
>>> 2. Due to bug in syncache code (patch below) all accepted connections has
>>> their snd_cwnd reset to 1*MSS (since r171639, 7.0+ AFAIR).
>>>
>>> 3. Support for rfc3465 introduced in r187289 uncovered (2) as
>>> ACK to SYN/ACK no longer causes snd_cwnd increase by MSS (actually, this
>>> increase shouldn't happen as it's explicitly forbidden by rfc 3390, but
>>> it's another issue).  Snd_cwnd remains really small (1*MSS + 1) and this
>>> causes really bad interaction with delayed acks on other side.
>>>
>>> As a workaround to delayed acks interaction problems you may disable
>>> rfc3465 by setting net.inet.tcp.rfc3465 to 0.  Correct fix would be to
>>> apply
>>> the patch below.
>>>
>>> To Andre Oppermann: could you please take a look at the patch and
>>> commit it if found appropriate?
>>
>> I've committed your fix with svn r210666. In a few days I will MFC it back
>> to the stable branches.  Thanks for reporting the bug and a patch for it.
>>
>> --
>> Andre
>>
>>> # HG changeset patch
>>> # User Maxim Dounin<mdounin at mdounin.ru>
>>> # Date 1279028684 -14400
>>> # Node ID 93699203f408fa8e22c971769bde9c26bd14d410
>>> # Parent  e2cf8c51a6294a0d09d5628d96c6ab3ab626957e
>>> Fix cwnd resetting problem introduced in r171639.
>>>
>>> Sc_rxmits counts timer engagements, not actual retransmits, and value 1
>>> corresponds to no retransmits.  Revert check to correct one as present
>>> before r171639.
>>>
>>> diff --git a/netinet/tcp_syncache.c b/netinet/tcp_syncache.c
>>> --- a/netinet/tcp_syncache.c
>>> +++ b/netinet/tcp_syncache.c
>>> @@ -804,8 +804,10 @@ syncache_socket(struct syncache *sc, str
>>>
>>>         /*
>>>          * If the SYN,ACK was retransmitted, reset cwnd to 1 segment.
>>> +        * Note that sc_rxmits counts timer engagements, not actual
>>> +        * retransmits.
>>>          */
>>> -       if (sc->sc_rxmits)
>>> +       if (sc->sc_rxmits>    1)
>>>                 tp->snd_cwnd = tp->t_maxseg;
>>>         tcp_timer_activate(tp, TT_KEEP, tcp_keepinit);
>>>
>>>
>>> Maxim Dounin
>>> _______________________________________________
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>>> http://lists.freebsd.org/mailman/listinfo/freebsd-net
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>>>
>>>
>>
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