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

[Giuseppe Modugno]

Il 03/12/2020 10:54, Benjamin Kalytta ha scritto:
> 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.

Yes, I think it's related. TCP/IP stack tries to close all connections 
in the best way it can, even if this means keeping data in memory for a 
very long time. During this time your system has a lower quantity of 
free memory to use for new connections. Because in embedded systems 
often we don't have big memories, this could be a critical issue.

> 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

I didn't know the keep-alive feature of TCP stack. In my case I already 
have an application protocol (MQTT) that can be configured to use 
keep-alives (at application layer). If the application detects a faulty 
connection with the peer (no reply to the keep-alive request), it closes 
the TCP socket. In this situation, I think it is safe (maybe better) to 
use tcp_abort(), that free everything related to the connection, instead 
of tcp_close(), that will try to send unacked data.