Date: Mon, 23 Mar 2020 16:21:04 +0700 From: Victor Sudakov <vas@sibptus.ru> To: "Patrick M. Hausen" <hausen@punkt.de> Cc: freebsd-net@freebsd.org Subject: Re: IP MTU on gif and gre interfaces (with and without IPSec encryption) Message-ID: <20200323092104.GA56721@admin.sibptus.ru> In-Reply-To: <7D560C6A-EB51-414A-A3A1-18587D40C218@punkt.de> References: <20200323050012.GA50490@admin.sibptus.ru> <7D560C6A-EB51-414A-A3A1-18587D40C218@punkt.de>
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[-- Attachment #1 --] Patrick M. Hausen wrote: > > > Am 23.03.2020 um 06:00 schrieb Victor Sudakov <vas@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@fidonet http://vas.tomsk.ru/ [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- iQEcBAEBAgAGBQJeeH+AAAoJEA2k8lmbXsY0CREIAIHSjKbQhoCqY7zZvJdYsaeb MlHhPJHdy0whqreC2Xr9nNVL2aEzux08znDGSk5Wai1fKmerbWaOsZOqEF/ZJibn GmlIpF/W3FG6aAxAORiMItBm6G1QrVkMvh0J1bDTZajh5i2Ywc9nMIdfR8d5jgoL awGr7X9YwaOteUv/dIqkDdwDjMFjEr+NZWnTDBy4+6puMX6CPHLgRBFA4mTbKJMO FTXV4crW/AnwkWeR4EAN+XQt7rsjUwaPlUoNM42ZMGwgxKHoaY54GkKm+bX3T1uU VXW4bBq/s4TMPP91kVAbdymH5ki9AL8VUl1E9jgRCJsaB9l/HA8Xr+96zAP+ry0= =Vvhi -----END PGP SIGNATURE-----
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