I've finally found the answer.
Powering a computer down causes its MAC<->port entry to expire on my switch.
SYN packets sent by my device to that computer start to be visible by my
router. It responds with ICMP packets (type 5 code 1) which causes
allocations in NETBUF pool by lwip. Simple pinging my device resulted in
NETBUF saturation as well. Why? Because I had RAW ICMP socket open which I
have completely forgotten about. It was used for pinging. Data from that
socket were read only when pinging other hosts was needed, which was very
rarely. This way NETBUF pool, used also for UDP, was not freed up and
blocked UDP processing.
Thank you all who answered in this thread.
Best regards,
Grzegorz Niemirowski
http://www.grzegorz.net/
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