ARP and UDP

[Paolo Simoncelli]

Hi all,

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 ;-) ...

Ciao.
Paolo


 
 
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[Nathan Moore-5]

Hi Paolo,

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 :( ...
>

If you can send one of the network dumps to this list maybe someone
will have an idea of what is happening.  It would be best if the time stamps
were either absolute (time of day) or relative for only the packets of
interest
("seconds since previously displayed packet" in wireshark).



Nathan
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