From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f43.google.com (mail-ee0-f43.google.com [74.125.83.43]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id 82ACD2022DF for ; Thu, 21 Jun 2012 10:03:44 -0700 (PDT) Received: by eeke50 with SMTP id e50so579988eek.16 for ; Thu, 21 Jun 2012 10:03:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=message-id:date:from:user-agent:mime-version:to:subject:references :in-reply-to:content-type:content-transfer-encoding; bh=qkU0nDDtpPCwJwfNVOtkFdHGyYQG4aq7W6mWxLD8DGM=; b=R+FLguD9/M30ufqepyb4SmoNBqpmTMQq5EPBXL4aLXrNPBG4oS6C/g+50naZPWh+UJ 3wVKJEZXLInH3MWdSNfhBoSPGI33F8wgkvaSoBCN7n3tMkrZKpoo5fgGmYFgVQloNJnf CyRHofjy+ZeUJvYmGjqwRo0iIJ+4R/E1yNDSkacXNF6MS+WwwCPwNSPvmd408SVfyoy7 eRtXP4gSLJ62XNLlIp7qjWCCUQ4vLOp6RwtgoJ+B6p1qfxO5W4XqOYNdgd653GUCwfzm Hl2v1UnGRnSmQfpMESWgWLy3zIlNiP+9cKmaurZ3mOKfOr+1HCqCFg/xswnLJ/jG/QxK 3yVw== Received: by 10.14.100.68 with SMTP id y44mr2990817eef.69.1340298221886; Thu, 21 Jun 2012 10:03:41 -0700 (PDT) Received: from ?IPv6:2001:5c0:1400:a::ed? ([2001:5c0:1400:a::ed]) by mx.google.com with ESMTPS id y12sm104698309eem.7.2012.06.21.10.03.39 (version=TLSv1/SSLv3 cipher=OTHER); Thu, 21 Jun 2012 10:03:40 -0700 (PDT) Message-ID: <4FE353E9.1020306@gmail.com> Date: Thu, 21 Jun 2012 18:03:37 +0100 From: Robert Bradley User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1 MIME-Version: 1.0 To: cerowrt-devel@lists.bufferbloat.net References: <1340288726.671712236@apps.rackspace.com> In-Reply-To: <1340288726.671712236@apps.rackspace.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Cerowrt-devel] Baby jumbo frames support? 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: Thu, 21 Jun 2012 17:03:45 -0000 On 21/06/12 15:25, dpreed@reed.com wrote: > I understand Dave Taht's long lecture - actually understood it years ago. But frame aggregation is not the same thing as jumbo frames in a multi-technology Ethernet LAN. Jumbo frames provide a way to exploit *end-to-end* frame sizes greater than 1500 bytes. That means the source and destination TCPs get frames that are "whole" (and not random subassemblies of frames that may arrive close together in time). > Your post makes more sense as a reply to Dave's post rather than mine, I think... Just in case replying to mine was deliberate, though: Going back to Alexander's original post, "baby" jumbo packets are quite simply a hack to get around the issues of PPPoE and reduced MTU. For UK ADSL customers without loop-unbundling, most of the infrastructure between customer and ISP is owned/managed by one provider (BT Wholesale), and either PPPoA or PPPoE may be used on this link (http://blog.farnz.org.uk/2010/02/on-pppoa-pppoe-atm-and-adsl.html). On the newer network, PPPoE is apparently(?) preferred. (I am sure that there are others reading this that know far more about BT's ADSL setup than I do; if any of this is wrong, feel free to correct me here!) Now, one of the issues of tunnelling is that you have overheads; in this case, the PPPoE header is 8 octets, with 1492 octets remaining for the tunnelled packet. In theory, you could simply set the tunnel MTU to 1492 bytes and have done with it - after all, you can always use path MTU discovery! The problem is that PMTU black holes can and do exist, so that is not a viable option. Ideally, you want your link to be 1500-byte-safe instead. One way to achieve this is to increase the frame size by just enough to contain a full 1500-octet packet within a PPPoE frame (8 octets larger in this case). Your standard 1500-byte packet now becomes a 1508-byte PPPoE packet (8 bytes PPPoE header, and 1500 bytes of content), which is sent from your router over the BT Wholesale network to your ISP's PPPoE server. The server then extracts the embedded Ethernet frame and sticks it on to the ISP's network, with no reassembly required. -- Robert Bradley