[En-Nut-Discussion] ARP and UDP
simonp at email.it
Fri Nov 6 13:27:51 CET 2009
Following my previous post about this question, after a lot of
packet sniffing with ettercap, wireshark and tcpdump.
i can definitively say that if you throw out an UDP packet
very early (seems about before 500 ms) after opening the socket
it gets lost into the TCP guts of NutOs :( ...
In his reply Ole pointed out that if was possible that
ethernut fired the packet too early for the UDP server
to complete his ARP table, but it seems to me that the
problem is on the ethernut side, :( things go like this:
ARP request starts from ethernut
the server replies with its IP
then two cases:
if you send an UDP packet within 500 ms, just
nothing comes out from the etherenet
if you send an UDP packet after 500 ms all is fine
probably ethernut has not built its ARP table
and just silently discards the packet, second one
goes like a breeze !
NutSleep(500); before sending out the first packet
resolves the issue
maybe TCP seems not affected by this behaviour due to
protocol retries that mask the problem
Sorry to appear pedantic about this question,
but i'm developing an application that sends out
just one packet ;-) ...
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