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tcp: fix loopback slowness issue / set tcb->mss for incoming connections (thanks David du Colombier)

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David du Colombier wrote:
> The slowness issue only appears on the loopback, because
> it provides a 16384 MTU.
>
> There is an old bug in the Plan 9 TCP stack, were the TCP
> MSS doesn't take account the MTU for incoming connections.
>
> I originally fixed this issue in January 2015 for the Plan 9
> port on Google Compute Engine. On GCE, there is an unusual
> 1460 MTU.
>
> The Plan 9 TCP stack defines a default 1460 MSS corresponding
> to a 1500 MTU. Then, the MSS is fixed according to the MTU
> for outgoing connections, but not incoming connections.
>
> On GCE, this issue leads to IP fragmentation, but GCE didn't
> handle IP fragmentation properly, so the connections
> were dropped.
>
> On the loopback medium, I suppose this is the opposite issue.
> Since the TCP stack didn't fix the MSS in the incoming
> connection, the programs sent multiple small 1500 bytes
> IP packets instead of large 16384 IP packets, but I don't
> know why it leads to such a slowdown.
This commit is contained in:
cinap_lenrek 2015-05-14 21:09:12 +02:00
parent e611879eab
commit 21f97338f8
1 changed files with 5 additions and 3 deletions
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8
sys/src/9/ip/tcp.c
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@ -1770,11 +1770,13 @@ tcpincoming(Conv *s, Tcp *segp, uchar *src, uchar *dst, uchar version)
tcb->flgcnt = 0;
tcb->flags |= SYNACK;
/* set desired mss and scale */
tcb->mss = tcpmtu(s->p, s->laddr, s->ipversion, &tcb->scale);
/* our sending max segment size cannot be bigger than what he asked for */
if(lp->mss != 0 && lp->mss < tcb->mss) {
if(lp->mss != 0 && lp->mss < tcb->mss)
tcb->mss = lp->mss;
tpriv->stats[Mss] = tcb->mss;
}
tpriv->stats[Mss] = tcb->mss;
/* window scaling */
tcpsetscale(new, tcb, lp->rcvscale, lp->sndscale);