netdev - Re: Established sockets remain open after iface down or address lost

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Message-ID: <524AEDD1.9010709@hp.com>
Date:	Tue, 01 Oct 2013 08:44:17 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Chris Verges <cverges@...tient-energy.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>, davem@...emloft.net,
	kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
	kaber@...sh.net, netdev@...r.kernel.org
Subject: Re: Established sockets remain open after iface down or address lost

On 10/01/2013 06:27 AM, Chris Verges wrote:
> The client establishes a connection to the server.  It requests some
> data and gets a response.  The socket remains open.  The server then
> decides, through some asynchronous process, that the radio needs to be
> duty cycled.  So the radio is turned off.
>
> The client attempts to make another request to the device, but
> determines that the connection is dead through the normal retry
> mechanisms.  It's write() operation returns something like EPIPE.  So on
> the client's side, the connection is dead.
>
> But on the server side, the socket is still open and waiting for some
> more data.  The interface and IP address and even the remote client are
> long gone, but the socket still persists and uses system resources.

The protocol between client and server needs to have an 
application-layer "keepalive" mechanism added, and then the server will 
be able to detect a dangling connection without need of any further 
kernel modifications.

If that is not possible, the server can/should set SO_KEEPALIVE and 
perhaps tweak the TCP keepalive settings.  Not as good (IMO) as an 
application-layer keepalive because it only shows that the connection is 
good as far as TCP, but I suppose it could do in a pinch.

rick jones

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