netdev - Re: [PATCH][TCP]: simplify tcp_mark_lost

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Message-ID: <Pine.LNX.4.64.0901071301460.27307@wrl-59.cs.helsinki.fi>
Date:	Wed, 7 Jan 2009 13:03:55 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Arnd Hannemann <hannemann@...s.rwth-aachen.de>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH][TCP]: simplify tcp_mark_lost_retrans()

On Wed, 7 Jan 2009, Ilpo Järvinen wrote:

> On Wed, 7 Jan 2009, Arnd Hannemann wrote:
> 
> > I noticed
> 
> Good that somebody else is looking TCP code besides me... :-)
> 
> > that in tcp_mark_lost_retrans the for-loop is only entered
> > if tcp_is_fack(tp) evaluates to true:
> > 
> >           if (!tcp_is_fack(tp) || !tp->retrans_out ||
> >               !after(received_upto, tp->lost_retrans_low) ||
> >               icsk->icsk_ca_state != TCP_CA_Recovery)
> >                   return;
> > 
> > Therefore the following check in the for-loop seems to be redundant,
> > because it always evaluates to true:
> > 
> >                      (tcp_is_fack(tp) ||
> >                       !before(received_upto,
> >                               ack_seq + tp->reordering * tp->mss_cache))
> > 
> > Did I miss something?
> 
> It was just a left over from the RFC3517 SACK addition which added that 
> !tcp_is_fack(tp) there above. ...It would have been nice to have similar 
> lost rexmit feature without FACK as well but calculating that wasn't 
> trivial (or I didn't find that too trivial) and could end up being 
> extremely expensive in case of large holes. (So I also left it there as 
> sort of reminder).
> 
> On the second thought, it would be possible to count skbs we pass while 
> walking from the beginning and use that a remaining_sacked counter 
> to get rid of all heurestics too and base the counting only on sacked 
> stuff which aligns with the spirit of rfc3517 much better than 
> sacked+holes used by fack.

Nah, tried to do that that wasn't working nicely either... Since there is 
a need to know how many sack blocks reside between ack_seq and 
received_upto, not the number of sack blocks between skb and 
received_upto...

-- 
 i.