Re: [lwip-devel] [mqtt] Disconnection caused by a keep-alive timeout

[Benjamin Kalytta]

I don't know if my problem is related since I relied on Netconn API.

In that case, my embedded device was running a HTTP web server. Clients 
connected through Wi-FI to my device. AJAX requests were heavily used i.e. many 
connections opened and closed very often. As soon as the client  (running on 
Microsoft Windows) loses the Wi-FI connection Windows automatically shuts down 
all active TCP/IP connections since this network interface wasn't available any 
more (This should  not be conform to the TCP/IP specification!). However, the 
embedded device did not notice that the connections on the opposite side were 
already closed, so no pool memory were ever freed. When the Wi-FI connection 
reestablished, and the client tries to reconnect, no more memory was available 
to handle that client requests.

My solution was to enable TCP/IP keep-alive per connection and setting a 
reasonable short timeout. Following configuration has to be set for that to 
work reliably:

#define LWIP_NETCONN_FULLDUPLEX                 1
#define LWIP_NETCONN_SEM_PER_THREAD             0