Patrick M. Hausen wrote: > > > Am 23.03.2020 um 06:00 schrieb Victor Sudakov <[email protected]>: > > 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@fidonet http://vas.tomsk.ru/
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