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

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Date:	Mon, 12 Jan 2009 00:08:24 +0100
From:	bert hubert <bert.hubert@...herlabs.nl>
To:	Evgeniy Polyakov <zbr@...emap.net>
Cc:	"H. Willstrand" <h.willstrand@...il.com>, netdev@...r.kernel.org
Subject: Re: sendfile()? Re: SO_LINGER dead: I get an immediate RST on 2.6.24?

On Mon, Jan 12, 2009 at 01:54:27AM +0300, Evgeniy Polyakov wrote:
> You can try to work with tcp cork options, uncorking the socket means
> that stack has sent data to the hardware, there are no other guarantees.

Ah, smart.

> > It appears that if one asks sendfile() to transmit a million bytes, it will
> > only return when the ACK for the millionth byte is in.
> 
> No it is not, it returns when it believes it has sent all the requested
> data, but in practice it can be even not sent but waiting in some
> hardware queue.

Ah ok.

> > I know that TCP will never be fully fully reliable, but I would love to have
> > a way to know that the millionth byte was ACKed, or alternatively, that an
> > error prevented that.
> 
> There is no way to get a notification when data is acked by the remote
> side. Generally you should invent some kind of own explicit acks.

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?

	Bert

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