net.inet.tcp.slowstart_flightsize in 8-STABLE

[Charles Logan]

Sorry but this is not a bug. You set bad sysctl flags.

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

Regards,
Freebsd Team

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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>>
>
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