UDP and IP fragmentation

Philip Lavine source_route at yahoo.com
Tue Sep 22 16:59:52 UTC 2009

To all,

I am running a Windows based high performance computing application that uses "reliable" multicast (29West) on a gigabit LAN. All systems are logically on the same VLAN and even on the same physical switch The application is set to use an 8k buffer and therefore results in IP fragmentation when datagrams are transmitted. The application is sensitive to any latency or data loss (of course) and uses a proprietary mechanism to create TCP-like retransmissions in case there is any actual data loss. Unfortunately, becasue of the fragmentation during the retransmission window all ip fragments must be resent even though only one may have been lost.

If the buffer size is tweeked to the ~1460 this may fix the fragmentation but will the side effects be less throughput and possibly more latency. Is there a sweet spot for UDP on an ethernet segment? 



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