Re: very old rtt calculation bug?

[yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)]

hi,

> Thor Lancelot Simon <tls%coyotepoint.com@localhost> writes:
> 
>> A coworker is looking at some ad-hoc stack tuning for heavy load which
>> has parameters that depend on rtt.  We are seeing the smoothed rtt vary
>> for TCP connections on loopback in ways we don't expect.
>>
>> We have begun to suspect that the changes in tcp_input.c 1.16 and 1.27
>> are wrong.  In particular, as my coworker points out:
>>
>> | I am pretty sure the NetBSD algo is wrong:
>> | 
>> |         if (tp->t_srtt != 0) {
>> |                 /*
>> |                  * srtt is stored as fixed point with 3 bits after the
>> |                  * binary point (i.e., scaled by 8).  The following magic
>> |                  * is equivalent to the smoothing algorithm in rfc793 with
>> |                  * an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
>> |                  * point).  Adjust rtt to origin 0.
>> |                  */
>> |                 delta = (rtt << 2) - (tp->t_srtt >> TCP_RTT_SHIFT);
>> |                 if ((tp->t_srtt += delta) <= 0)
>> |                         tp->t_srtt = 1 << 2;
>> | 
>> | The (rtt << 2) means that we are adding 1/2 the RTT to srtt, rather than 
>> 1/8.
>> | Given that srtt is << 3, if we do not shift rtt by anything, adding it to
>> | srtt adds rtt/8 to it.
>>
>> FreeBSD shifts rtt by two here, but they keep 5 bits of precision vs our 3,
>> so there it's correct.
>>
>> Are we missing something?
> 
> Bev Schwartz, Laura Ma, and I have been looking at this.  This is from
> memory -- defuzzed version and patch to follow probably next week.
> 
> There are several problems in the RTT code.
> 
>   value is stored with a +1 to let 0 be special, but it's not subtracted
>   before use
> 
>   rounding errors when dealing with very low RTTs
> 
>   "very low" is not that low because this is based on a long timer tick
> 
> We have a fix for the first two problems; with the fix TCP performs
> better under loss due to faster RTO processing.

what's the status of this?

YAMAMOTO Takashi