tinc - Jan 2013 - Ethernet frame header size of TAP device and apropriate MTU calculation

Hello, all! I have many questions about tap device architecture.

What is a right way to calc mtu on TAP device to avoid fragmentation on 
real eth device?

I suppose TAP MTU = 1500-8(UDP)-20(IP)-18(Ethernet) = 1454.
So I'd set 1454 for tap device: "ip link set mtu 1454 dev eth0"

I'm not shure about what is the exact size of ethernet frame header, 
which tap device use in switch mode?
Is there FCS field? - Sure it should be.
But is there InterFrame Space and Preamble in the "switch
environment"?

And how this correlate with PMTU and PMTUDiscovery options?
Should I really manually change MTU for tap device?

I very appreciate your help.
Thanks!

My understanding is that PMTUDiscovery determines what mtu to use for
tunneled frames in switch mode so that they will fit without fragmentation.


On Sat, Jan 19, 2013 at 5:30 AM, SVM <svm7 at mail15.com> wrote:
> Hello, all! I have many questions about tap device architecture.
>
> What is a right way to calc mtu on TAP device to avoid fragmentation on
> real eth device?
>
> I suppose TAP MTU = 1500-8(UDP)-20(IP)-18(**Ethernet) = 1454.
> So I'd set 1454 for tap device: "ip link set mtu 1454 dev
eth0"
>
> I'm not shure about what is the exact size of ethernet frame header,
which
> tap device use in switch mode?
> Is there FCS field? - Sure it should be.
> But is there InterFrame Space and Preamble in the "switch
environment"?
>
> And how this correlate with PMTU and PMTUDiscovery options?
> Should I really manually change MTU for tap device?
>
> I very appreciate your help.
> Thanks!
>
> ______________________________**_________________
> tinc mailing list
> tinc at tinc-vpn.org
>
http://www.tinc-vpn.org/cgi-**bin/mailman/listinfo/tinc<http://www.tinc-vpn.org/cgi-bin/mailman/listinfo/tinc>
>
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