Date: Mon, 23 Mar 2020 16:26:47 +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: <20200323092647.GA57111@admin.sibptus.ru> In-Reply-To: <20200323092104.GA56721@admin.sibptus.ru> References: <20200323050012.GA50490@admin.sibptus.ru> <7D560C6A-EB51-414A-A3A1-18587D40C218@punkt.de> <20200323092104.GA56721@admin.sibptus.ru>
next in thread | previous in thread | raw e-mail | index | archive | help
--6TrnltStXW4iwmi0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Victor Sudakov wrote: > Patrick M. Hausen wrote: > >=20 > > > 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 "mt= u 1476" > > > value, but a newly created gif0 interface has "mtu 1280", why would t= he > > > default be so low? > >=20 > > 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 ... >=20 > Please correct me if I'm wrong: >=20 > - 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. >=20 > So we could keep the safe default for gif(4) at 1500-40-8-8=3D1444 bytes. > OK, at 1400 as for if_ipsec. But not at 1280! I should probably have counted the 20 bytes of the additional IP header which results in 1500-40-8-8-20=3D1424. So 1400 is really safe. --=20 Victor Sudakov, VAS4-RIPE, VAS47-RIPN 2:5005/49@fidonet http://vas.tomsk.ru/ --6TrnltStXW4iwmi0 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEcBAEBAgAGBQJeeIDXAAoJEA2k8lmbXsY087YH+wVNzMozSWG8rUHqoqJLVAHw aHNKQh+mawIyVO2In+54ed3tQP06UxPYWcbtaH+zoBggdidQBw5MQKkynntSstKT 4LaYZoQO2dP6bO85l44zuR2iT69E1OXauW8jIx6B+avVB/4niedvEZfyCloHcaFg Mvvw5Gxss3OhMGX5/xggs/XPO0LWke5k8txVTrLywJ0ESv1K44TMTleSv2sXDp1X Jmwnu6/0xQVRtbSUeSPIFH3dCfHMeev+ft97iOmh4kamrCAmxa3VvavNhCmNc9FT 5N0QfdZNdKa6jS9pJyuu/rHkSBc30b/sPlZnrOsm1wytlcVQdvXggE1K/dgglKI= =Wpa6 -----END PGP SIGNATURE----- --6TrnltStXW4iwmi0--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20200323092647.GA57111>