From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from emea01-am1-obe.outbound.protection.outlook.com (mail-am1on0074.outbound.protection.outlook.com [157.56.112.74]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client CN "mail.protection.outlook.com", Issuer "MSIT Machine Auth CA 2" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id 84EFB21F18E for ; Tue, 28 Apr 2015 09:08:07 -0700 (PDT) Received: from HE1PR07MB0937.eurprd07.prod.outlook.com (25.162.27.143) by HE1PR07MB1097.eurprd07.prod.outlook.com (25.163.177.147) with Microsoft SMTP Server (TLS) id 15.1.148.16; Tue, 28 Apr 2015 16:08:01 +0000 Authentication-Results: lists.bufferbloat.net; dkim=none (message not signed) header.d=none; Received: from [IPv6:2001:470:183f:da2b::717c:e11c] (2001:470:183f:da2b::717c:e11c) by HE1PR07MB0937.eurprd07.prod.outlook.com (25.162.27.143) with Microsoft SMTP Server (TLS) id 15.1.148.16; Tue, 28 Apr 2015 16:07:58 +0000 Message-ID: <553FB04F.6020209@darbyshire-bryant.me.uk> Date: Tue, 28 Apr 2015 17:07:43 +0100 From: Kevin Darbyshire-Bryant User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Sebastian Moeller References: <553F8932.6050009@darbyshire-bryant.me.uk> <0EB27232-6810-4B4F-A9BD-4C9466BF650F@gmx.de> In-Reply-To: <0EB27232-6810-4B4F-A9BD-4C9466BF650F@gmx.de> Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary="------------ms010205020506090405080303" X-Originating-IP: [2001:470:183f:da2b::717c:e11c] X-ClientProxiedBy: CO2PR06CA044.namprd06.prod.outlook.com (10.141.242.44) To HE1PR07MB0937.eurprd07.prod.outlook.com (25.162.27.143) X-Microsoft-Antispam: UriScan:; BCL:0; PCL:0; RULEID:; SRVR:HE1PR07MB0937; UriScan:; BCL:0; PCL:0; RULEID:; SRVR:HE1PR07MB1097; X-Forefront-Antispam-Report: BMV:1; SFV:NSPM; SFS:(10009020)(6009001)(479174004)(24454002)(51704005)(1720100001)(122386002)(65806001)(77156002)(87266999)(74482002)(64126003)(65956001)(50986999)(65816999)(54356999)(15975445007)(568964001)(512944002)(62966003)(76176999)(80316001)(46102003)(40100003)(42186005)(36756003)(84326002)(19580395003)(2950100001)(33656002)(4001350100001)(87976001)(110136001)(86362001)(92566002)(19580405001)(83506001)(5890100001)(7059030)(3826002); DIR:OUT; SFP:1101; SCL:1; SRVR:HE1PR07MB0937; H:[IPv6:2001:470:183f:da2b::717c:e11c]; FPR:; SPF:None; MLV:sfv; LANG:en; X-Microsoft-Antispam-PRVS: X-Exchange-Antispam-Report-Test: UriScan:; X-Exchange-Antispam-Report-CFA-Test: BCL:0; PCL:0; RULEID:(601004)(5002010)(5005006)(3002001); SRVR:HE1PR07MB0937; BCL:0; PCL:0; RULEID:; SRVR:HE1PR07MB0937; X-Forefront-PRVS: 0560A2214D X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 Apr 2015 16:07:58.9831 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-Transport-CrossTenantHeadersStamped: HE1PR07MB0937 X-OriginatorOrg: darbyshire-bryant.me.uk 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 16:08:38 -0000 --------------ms010205020506090405080303 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable On 28/04/2015 16:32, Sebastian Moeller wrote: > Hi Kevin, > > On Apr 28, 2015, at 15:20 , Kevin Darbyshire-Bryant wrote: > >> Hi, >> >> Enquiring mind here :-) Reading http://www.bufferbloat.net/projects/c= erowrt/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 overh= ead recommendation for VDSL2 comes from. Is this assumption that PPPoE i= s being used? =20 > Not exactly, for all we know using PPPoE and a VLAN on a VDSL2 link re= sults in: > VDSL2 header: > VDSL (IEEE 802.3-2012 61.3 relevant for VDSL2): 2 Byte PPP + 6 Byte PPP= oE + 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 h= eaders 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 recommend= ation jus to handle PPPoE overhead ;) Ha! Think to myself "See Kevin, *this* is what happens when you start di= gging and asking questions!" :-) Hmm, I've a feeling that for most in the UK the value should be 16 to cop= e with PPP, PPPoE, VLAN, S, Ck, PTM-FCS. Skyt VDSL2 customers can use 8.= Sky don't use the PPP/PPPoE (& CHAP for login) - rather it is DHCP opti= on 60 used as the customer ID for login purposes. > >> Based on that assumption it raises further questions in my mind: >> >> My ISP supplier (Sky in the UK) provide straight ethernet over PTM, wi= th DHCP to obtain a public IP address, so in theory no PPPoE overhead unl= ike 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 overh= ead 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 th= rottles 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 A= AL5, but that should be all of them ;) ), but for PTM I have no real idea= =2E.. > >> Please correct my assumptions :-) > The other thing I am uncertain of is the VLAN tag, if your router term= inates it will the kernel account for it or not? Not that I can test this= currently as my modem terminates the VLAN =93silently" I was looking at the modem (HG612) which appears to bridge an ethernet po= rt to a PTM port and is set to tag the WAN bound packets with a VLAN tag.= Based on what I can see, the ethernet port is untagged so can only assu= me the vlan tag goes out on WAN side interface. Therefore the kernel in = OpenWrt doesn't know the tag is there or not so cannot automatically acco= unt for it. The VLAN id is curious too. It's VLAN 101 which is same as = the ATM VPI/VCI (0.101) that Sky UK used for a while with their ADSL 'MER= ' implementation. They used VC-Mux but did NOT want ethernet FCS include= d, if you did include the ethernet FCS the other end just went 'huh?' Ag= ain I've a feeling but no real proof that passing on FCS is unpopular. > > Best Regards & hope that helps > Sebastian It helps prove what a complicated beast this is :-) and yes it did help = (I'm setting the overhead to 8, closing my eyes & shutting my ears!) 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