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Re: [lwip-users] Handling sudden disappearance of network i/f

From: Kieran Mansley
Subject: Re: [lwip-users] Handling sudden disappearance of network i/f
Date: Mon, 25 Sep 2006 13:42:33 +0100

On Mon, 2006-09-25 at 14:29 +0200, Janusz U. wrote:
> isn't the TCP keep alive responsible for that? I have similar problem with 
> PPP. Link failed in middle of physical connection and lwIP does not detect 
> the situation. All other systems have timeouts!

TCP keep-alives are a "MAY" rather than "MUST" or even "SHOULD" part of
the TCP specification:


but lwIP does include support for them, both as a sender and receiver of
such probes.  

However, to be useful, the probes must be enabled on that connection - I
think the default is "off", but haven't checked, and the connection must
be idle for a certain time before the probe is sent, and this is a long
time by default - something like an hour or two.  The times can be
configured with socket options if you're using the sockets API.

They're not designed for detecting and responding rapidly to a sudden
physical layer death, more for cleaning up connections that have been
idle and no response received to the probes for a long time. 


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