Subject: Re: RE: A Linux TCP SACK Question

From: Wenji Wu <[email-masked]>
Date: Tue, 08 Apr 2008 09:30:14 -0500

> > Yes, the adaptive tp->reordering will play a role here. 
> 
> ...What is not clear to me why NewReno does not go to recovery at 
> least 
> once near the beginning, or at least it won't result in a retransmission.

The problem cause me two weeks' time to debug!

With 3 DupACKs, tcp_ack() calls tcp_fastretrans_alert(), and which in turn calls tcp_xmit_retransmit_queue().

Within tcp_xmit_retransmit_queue(), there is a line of code that would cause the problem above:

......................................................................................................
 /* Forward retransmissions are possible only during Recovery. */
1999        if (icsk->icsk_ca_state != TCP_CA_Recovery)
2000                return;

2001
2002        /* No forward retransmissions in Reno are possible. */
2003        if (tcp_is_reno(tp))
2004                return;

.....................................................................................................

if you look at "tcp_is_reno", you would see that with SACK off, Reno does not do retransmit, it will return!!!

Really do not understand why these two lines of code exist there!!!

Also, this code still in 2.6.25.

> In which kernel version this dump comes from? 2.6.24 newreno is 
> crippled 
> with TSO as was recently discovered, ie., it won't mark lost super 
> skbs 
> at head and thus won't retransmit them. Also 2.6.25-rcs are still 
> broken 
> (though they'll transmit too much, I'll not go detail in here), DaveM 
> now 
> has the fix for 2.6.25-rcs in net-2.6.

The dumped file is from 2.6.24. 2.6.25's is similiar.

> > You can reverse the order of the tests, with SACK option on/off. The 
> 
> > results are still the same.
> 
> Ok. I just wanted to make sure so that we don't end up trace some test 
> 
> setup issue :-).
> 
> > Also, according to the source code, tp->reordering will be 
> initialized 
> > to "/proc/sys/net/ipv4/tcp_reordering" (default 3), when the new 
> > connection is established.
> 
> In addition, in tcp_init_metrics():
> 
> 	if (dst_metric(dst, RTAX_REORDERING) &&
>             tp->reordering != dst_metric(dst, RTAX_REORDERING)) {
>                 tcp_disable_fack(tp);
>                 tp->reordering = dst_metric(dst, RTAX_REORDERING);
>         }

Good to know this, thanks

wenji