nothing much to investigate. if default packet size is 1500, this is what the ipsec endpoint uses, inserts it's headers and rest is for data. I think for ipsec the data was 1460 or 1463 or something like that.

now, fragmented packages are be re-combined at the other end. in some special cases though, the delays caused by this or weird software having no-fragment bit on, this will cause connection issues.

supposedly even RDP does this:
http://www.thincomputing.net/2011/06/28/mtu-size-mismatch-a-major-cause-of-disconnections/
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