Re: fix TCP roundtrip time update code

Martin Josefsson
2003-06-03T17:27:46Z

(trimmed CC line and added netdev)

On Tue, 2003-06-03 at 17:52, David Mosberger wrote:
> One of those very-hard-to-track-down, trivial-to-fix kind of problems:
> without this patch, TCP roundtrip time measurements will corrupt the
> routing cache's RTT estimates under heavy network load (the bug causes
> RTAX_RTT to go negative, but since its type is u32, you end up with a
> huge positive value...). From there on, later TCP connections quickly
> will go south.
>
> The typo was introduced 8 months ago in v1.29 of the file by the patch
> entitled "Cleanup DST metrics and abstrct MSS/PMTU further".

I tested this patch and it looks like it has cured my mysterious TCP
stalls.

without patch:

cache mtu 1500 rtt 479411ms rttvar 953813ms cwnd 46 advmss 1460

I see that before and during the stall if not using this patch.
(rtt is never above 20ms accoring to ping)

With the patch I see normal rtt and rttvar times.
Havn't seen a stall yet (~30 kernelcompiles with distcc over a sometimes
congested link), will continue testing.

> ===== net/ipv4/tcp_input.c 1.36 vs edited =====
> --- 1.36/net/ipv4/tcp_input.c Mon Apr 28 09:27:57 2003
> +++ edited/net/ipv4/tcp_input.c Tue Jun 3 08:19:36 2003
> @@ -556,8 +556,8 @@
> if (m >= dst_metric(dst, RTAX_RTTVAR))
> dst->metrics[RTAX_RTTVAR-1] = m;
> else
> - dst->metrics[RTAX_RTT-1] -=
> - (dst->metrics[RTAX_RTT-1] - m)>>2;
> + dst->metrics[RTAX_RTTVAR-1] -=
> + (dst->metrics[RTAX_RTTVAR-1] - m)>>2;
> }
>
> if (tp->snd_ssthresh >= 0xFFFF) {

--
/Martin