secsoa

We are sending large UDP packets over low-bandwidth network (9.6Kbit/s)
between two Windows XP PCs, and UDP gets fragmented into several IP
fragments.

Problem: If it takes more than 5 seconds between the fist and last IP
fragment are received by Windows, they seem to be discarded. It seems
like a reassembly timeout in Windows. However, netdiag.exe from MS
Support tools shows that "IP Reasembly Timeout" is 60 seconds on our
system. No ICMP error packets are generated by Windows.

IP packet counter in "netstat" increments for each fragment, but the
UDP counter does not. Wireshark packet sniffer show all fragments
correctly.

Any suggestions are appreciated; thank you.

brian cryer

I'm no UDP expert, and I know this isn't the answer you want: but why not
use TCP (or RUDP) rather than UDP? My understanding of UDP is that neither
the arrival nor sequencing of data packets is guaranteed. Presumably your
packets could be lost at either the sender or receiver end. I'm guessing
that if one of your IP packets gets lost then your entire UDP packet will
get discarded. Have you checked your number of IP packets sent against the
number of IP packets received?

Also, I believe that the sending queue for UDP packets is of finite size,
non-blocking and if the queue gets overloaded then packets will be lost
before they are sent. Don't know the details, but this information may be of
help.

If you require guaranteed delivery then don't use UDP. Sorry.
--
Brian Cryer
http://www.cryer.co.uk/brian