netdev - Re: Antw: Re: Q: need effective backlog for listen()

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Message-Id: <54880312020000A100018453@gwsmtp1.uni-regensburg.de>
Date:	Wed, 10 Dec 2014 08:23:46 +0100
From:	"Ulrich Windl" <Ulrich.Windl@...uni-regensburg.de>
To:	<phil@...i.org>
Cc:	<netdev@...r.kernel.org>
Subject: Re: Antw: Re: Q: need effective backlog for listen()

>>> Philippe Troin <phil@...i.org> schrieb am 09.12.2014 um 16:07 in Nachricht
<1418137657.384.17.camel@...bium.home.fifi.org>:
> On Tue, 2014-12-09 at 08:01 +0100, Ulrich Windl wrote:
>> >>> Philippe Troin <phil@...i.org> schrieb am 08.12.2014 um 17:35 in Nachricht
>> <1418056540.384.5.camel@...bium.home.fifi.org>:
>>
>> > The argument to listen() specifies how many connections the system is
>> > allow to keep waiting to be accept()ed.
>> > As soon as you accept() the connection, the count is decremented.
>> > So that won't help for your use case.
>> > 
>> >> However none of the above see ms true. Even if my server delays
>> >> accept()ing new connections, no client ever sees a "connection
>> >> refused" or "connection timed out". Is there any chance to signal the
>> >> client that no more connections are accepted at the moment?
>> > 
>> > Close the listening socket.  No new connections will be accepted.
>> > When you reopen the socket for accepting new connections, you may have
>> > to use SO_REUSEADDR before bind()ing to the port.
>> 
>> This is what I had done, but those connections who are waiting to be
>> accepted: If I close the listening socket, will the clients see a
>> connection abort, or will they see a connection refused?
>> Connection aborts could confuse clients.
> 
> The clients that completed the 3-way handshake and are waiting for their
> (server-side) socket to be accept()ed will see connection reset I
> believe.
> 
> You may be able to work something out by having no listen backlog
> (listen(0)) and closing and reopening the socket when needed.

I think I'd feel happier if listen() would implement the old semantics by default, and a new ioctl() (or such) would change to the "Linux accept all connections (3-way handshake) mode" on demand... Wishful thinking...

Ulrich


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