From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mout.gmx.net (mout.gmx.net [212.227.15.18]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "mout.gmx.net", Issuer "TeleSec ServerPass DE-1" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id 4C5CE21F1BC for ; Tue, 28 Apr 2015 08:35:18 -0700 (PDT) Received: from u-084-c190.eap.uni-tuebingen.de ([134.2.84.190]) by mail.gmx.com (mrgmx001) with ESMTPSA (Nemesis) id 0MbOoG-1Z3h1I2NSm-00Ip2J; Tue, 28 Apr 2015 17:35:12 +0200 Content-Type: text/plain; charset=windows-1252 Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) From: Sebastian Moeller In-Reply-To: <553F8932.6050009@darbyshire-bryant.me.uk> Date: Tue, 28 Apr 2015 17:32:32 +0200 Content-Transfer-Encoding: quoted-printable Message-Id: <0EB27232-6810-4B4F-A9BD-4C9466BF650F@gmx.de> References: <553F8932.6050009@darbyshire-bryant.me.uk> To: Kevin Darbyshire-Bryant X-Mailer: Apple Mail (2.1878.6) X-Provags-ID: V03:K0:7PP44BEyLOP4U4SmagXCHEU9RPFuIJaCR3eTluK3UB4vLPHmkHp 5A87SJr5g3nbVKOJ7R9TNuXbx9jSk/Xo6qlyP49aj+gNRBeB8bV1XQMYpl6Ol/RgLWt60V+ y+wWLTA56jjylHrGKFUjVpQ2cTvFI5mW0SjGNXh/5eoim6rtGUEL+hCJKTOtD3k2+JmlV88 Gnn2dclur3WopvZJlTRJA== X-UI-Out-Filterresults: notjunk:1; Cc: "cerowrt-devel@lists.bufferbloat.net" Subject: Re: [Cerowrt-devel] Link layer adaptation VDSL2 - Basis for 8 byte overhead? X-BeenThere: cerowrt-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: Development issues regarding the cerowrt test router project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 28 Apr 2015 15:35:51 -0000 Hi Kevin, On Apr 28, 2015, at 15:20 , Kevin Darbyshire-Bryant = wrote: > Hi, >=20 > Enquiring mind here :-) Reading = http://www.bufferbloat.net/projects/cerowrt/wiki/Everything_you_wanted_to_= know_about_Link_Layer_Adaptation and the linked to email discussion, I'm = curious as to where the 8 byte overhead recommendation for VDSL2 comes = from. Is this assumption that PPPoE is being used? =20 Not exactly, for all we know using PPPoE and a VLAN on a VDSL2 = link results in: VDSL2 header: VDSL (IEEE 802.3-2012 61.3 relevant for VDSL2): 2 Byte PPP + 6 Byte = PPPoE + 4 Byte VLAN + 1 Byte Start of Frame (S), 1 Byte End of Frame = (Ck), 2 Byte TC-CRC (PTM-FCS), =3D 16 Byte Or in other words, 8 byte either just reflect PPPoE or the real VDSL2 = headers plus a VLAN (I am still unsure what to do with the ethernet = FCS). But I think you are right that initially the 8 byte came as a = recommendation jus to handle PPPoE overhead ;) > Based on that assumption it raises further questions in my mind: >=20 > My ISP supplier (Sky in the UK) provide straight ethernet over PTM, = with DHCP to obtain a public IP address, so in theory no PPPoE overhead = unlike other ISPs offering 'fibre' (ha!) broadband in the UK. There = appears to be a tagged VLAN on the WAN port, therefore I think the = correct overhead in my case is 4 (VLAN) and for everyone else it should = be 12 (VLAN + PPPoE) See above. BUT this might or might not be relevant; my ISP = actually throttles my link to a speed below the VDSL2 link speed and = accounts for 16 bytes overhead at the BRAS level, so ymmv=85 I had a = nice way to figure out the per packet overhead on ATM links (actually = only ATM links using AAL5, but that should be all of them ;) ), but for = PTM I have no real idea... >=20 > Please correct my assumptions :-) The other thing I am uncertain of is the VLAN tag, if your = router terminates it will the kernel account for it or not? Not that I = can test this currently as my modem terminates the VLAN =93silently" Best Regards & hope that helps Sebastian >=20 > Kevin >=20 > --=20 > Thanks, >=20 > Kevin@Darbyshire-Bryant.me.uk >=20 >=20 > _______________________________________________ > Cerowrt-devel mailing list > Cerowrt-devel@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/cerowrt-devel