netdev - Re: sendfile()? Re: SO_LINGER dead: I get an immediate RST on 2.6.24?

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Message-Id: <20090113003108.72860b5c.billfink@mindspring.com>
Date:	Tue, 13 Jan 2009 00:31:08 -0500
From:	Bill Fink <billfink@...dspring.com>
To:	=?ISO-8859-1?Q? "Ilpo_J=E4rvinen" ?= <ilpo.jarvinen@...sinki.fi>
Cc:	Evgeniy Polyakov <zbr@...emap.net>,
	bert hubert <bert.hubert@...herlabs.nl>,
	"H. Willstrand" <h.willstrand@...il.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: sendfile()? Re: SO_LINGER dead: I get an immediate RST on
 2.6.24?

On Mon, 12 Jan 2009, Ilpo Järvinen wrote:

> On Sun, 11 Jan 2009, Bill Fink wrote:
> 
> > On Mon, 12 Jan 2009, Evgeniy Polyakov wrote:
> > 
> > > On Mon, Jan 12, 2009 at 12:08:24AM +0100, bert hubert (bert.hubert@...herlabs.nl) wrote:
> > > > I fully understand. Sometimes I have to talk to stupid devices though. What
> > > > I do find is the TCP_INFO ioctl, which offers this field in struct tcp_info:
> > > > 
> > > >         __u32   tcpi_unacked;
> > > > 
> > > > Which comes from:
> > > > 
> > > > struct tcp_sock {
> > > > ...
> > > >         u32     packets_out;    /* Packets which are "in flight"        */
> > > > ...
> > > > }
> > > > 
> > > > If this becomes 0, perhaps this might tell me everything I sent was acked?
> > > 
> > > 0 means that there are noin-flight packets, which is effectively number
> > > of unacked packets. So if your application waits for this field to
> > > become zero, it will wait for all sent packets to be acked.
> > 
> > I use this type of strategy in nuttcp, and it seems to work fine.
> > I have a loop with a small delay and a check of tcpi_unacked, and
> > break out of the loop if tcpi_unacked becomes 0 or a defined timeout
> > period has passed.
> 
> Checking tcpi_unacked alone won't be reliable. The peer might be slow 
> enough to advertize zero window for a short period of time and during 
> that period you would have packets_out zero...

I'll keep this in mind for the future, although it doesn't seem to
be a significant issue in practice.  I use this scheme to try and
account for the tcpi_total_retrans for the data stream, so if this
corner case was hit, it would mean an under reporting of the total
TCP retransmissions for the nuttcp test.

If I understand you correctly, to hit this corner case, just after
the final TCP write, there would have to be no packets in flight
together with a zero TCP window.  To make it more bullet-proof, I
guess after seeing a zero tcpi_unacked, an additional small delay
should be performed, and then rechecking for a zero tcpi_unacked.
I don't see anything else obvious (to me anyway) in the tcp_info
that would be particularly helpful in handling this.

						-Thanks

						-Bill
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