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doc/faq: tcpCleanup #4314

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Feb 7, 2018
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doc/faq: tcpCleanup #4314

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28 changes: 28 additions & 0 deletions doc/faq/readme.rst
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Expand Up @@ -88,3 +88,31 @@ This error may pop up after switching between
`stable <https://github.com/esp8266/Arduino#stable-version->`__ esp8266
/ Arduino package installations, or after upgrading the package version
:doc:`Read more <a04-board-generic-is-unknown>`.


How to clear TCP PCBs in time-wait state ?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is needed with lwIP-v1.4, less needed with lwIP-v2 but timeout is still
too high.

Time-wait PCB state helps TCP not confusing two consecutive connections with the
same (s-ip,s-port,d-ip,d-port) when the first is already closed but still
having duplicate packets lost in internet arriving later during the second.
Artificially clearing them is a workaround to help saving precious heap.

The following lines are compatible with both lwIP versions:

.. code:: cpp

// no need for #include
struct tcp_pcb;
extern struct tcp_pcb* tcp_tw_pcbs;
extern "C" void tcp_abort (struct tcp_pcb* pcb);

void tcpCleanup (void) {
while (tcp_tw_pcbs)
tcp_abort(tcp_tw_pcbs);
}

Ref. `#1923 <https://github.com/esp8266/Arduino/issues/1923>`__