Re: First UDP message to a specific IP gets lost

Correct.
And I can also see the outgoing "first" messages in wireshark on sender side but not incoming on receiver side.


"Dave Lowther" <davel@xxxxxxxxx> wrote in message news:OubeBzG$IHA.3756@xxxxxxxxxxxxxxxxxxxxxxx
"Ole Dittmann" <ole@xxxxxxxxxxxxxx> wrote in message news:eVDfIsG$IHA.5336@xxxxxxxxxxxxxxxxxxxxxxx
I made a test program that sends just one datagram and then waits for keyboard input. Also that single message never arrives if it is the first one after some time.
And I also used wireshark but it tells me only what I can see on my own: First messages are send but they never arrive.

It's not clear, but I think you mean that you call send() for the "First messages" and send returns indicating success, but you do not see the "First messages" in wireshark ?

"Dave Lowther" <davel@xxxxxxxxx> wrote in message news:eYYIv4D$IHA.2056@xxxxxxxxxxxxxxxxxxxxxxx
"Ole Dittmann" <ole@xxxxxxxxxxxxxx> wrote in message news:uBcOZs8%23IHA.984@xxxxxxxxxxxxxxxxxxxxxxx
Hi,
I am working on a show controlling system based on UDP Communication.

Problem is:
When I send a message specifically to an IP adress I did not communicate
with for some time then it gets lost.
The next messages after a short time will work. But every time I wait again
for some minutes the first message after waiting gets lost again.

Sounds like it might be caused by ARP but I have no direct experience of this situation on Windows.

Perhaps something like this is happening, you could check with WireShark.

a) First UDP send stores the send data in the socket.
b) Then windows has to ARP to get destination MAC
c) If you try to send too much more here before ARP reply some data may be lost.
d) Windows gets ARP reply now knows destination MAC and sends stored data.
e) Time passes and ARP cache entry is deleted
f) Go to a)





.

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