Re: How can i detect if a received UDP got fragmented from userland?

On Tue, Mar 21, 2006 at 04:15:44AM +0100, Sten Daniel Sørsdal wrote:
Our first assumption was that adding DF to UDP would solve it, and it
does in our small tests, but it has a noticable negative effect on the
network.

Sounds like you need to implement Path MTU Discovery in userland for your
application. In FreeBSD, PMTU-D only happens in the kernel for TCP traffic.
So you would need to implement it yourself to avoid fragmentation along
the path.

Are there any way i can read whether a message's packet was fragmented
into smaller pieces and preferably how large the largest fragment was?

I think the MSG_TRUNC flag can tell you about this (data discarded before
delivery). You'd have to experiment to see if UDP will set this flag, I'm
not sure if it will.

Are there any feasible way to do this?

I don't think it would be feasible to report this per datagram. Take
a look at Ping Pan's paper on PF_IPOPTION if you wish to explore further.

Regards,
BMS
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