MTUs, fragment, mssfix

[giraffedata]

I'm trying to make sense of the whole packet size issue, and finding a lot of incomplete information. Can someone who understands enlighten me?

First, I read that sometimes connections just won't transmit large packets because of packet size issues, and I did see that once on my Openvpn link (the problem came, and then it went without me doing anything!). But why? IP fragmentation is supposed to transparently take care of too-big packets. I know there's the Don't Fragment IP flag, but does something in a typical Openvpn link use that? I would expect no, since it breaks things so badly.

But I do understand that packet size can affect speed, so all these controls have to exist nonetheless.

My second question is what is the difference between --link-mtu and --fragment?

[lightman]

i also wish for an good explenation, going crazy testing the multiple settings...