IP MTU on gif and gre interfaces (with and without IPSec encryption)

[Victor Sudakov]

Patrick M. Hausen wrote:
> 
> > Am 23.03.2020 um 06:00 schrieb Victor Sudakov <vas at sibptus.ru>:
> > I've noticed that a newly created gre0 interface has the expected "mtu 1476"
> > value, but a newly created gif0 interface has "mtu 1280", why would the
> > default be so low?
> 
> gif is frequently used as the innermost encapsulation like in gif tunnel
> across host mode IPsec. Then there might be PPPoE, too. Possibly a
> VLAN tag ...

Please correct me if I'm wrong:

- ESP overhead - 40 bytes
- UDP encapsulation of ESP (udp/4500): 8 bytes
- PPPoE overhead - 8 bytes (?)
- A VLAN tag just increases the max frame size, it does not reduce the IP MTU.

So we could keep the safe default for gif(4) at 1500-40-8-8=1444 bytes.
OK, at 1400 as for if_ipsec. But not at 1280!


> So most probably the idea is to go as low as possible for arbitrary additional
> encapsulations while still satisfying the minimum requirement for IPv6.
> 
> For the others: no idea, I guess historical ;-)

Looks like it. But there are times to have a fresh look at things.

-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
2:5005/49 at fidonet http://vas.tomsk.ru/
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