From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?utf-8?Q?Dan_L=C3=BCdtke?= Subject: Re: Misalignment, MIPS, and ip_hdr(skb)->version Date: Sat, 10 Dec 2016 23:18:14 +0100 Message-ID: References: <095cac5b-b757-6f4a-e699-8eedf9ed7221@stressinduktion.org> <87vauvhwdu.fsf@alice.fifthhorseman.net> Mime-Version: 1.0 (Mac OS X Mail 10.1 \(3251\)) Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-mips@linux-mips.org, Netdev , LKML , Hannes Frederic Sowa , WireGuard mailing list To: Daniel Kahn Gillmor Return-path: In-Reply-To: <87vauvhwdu.fsf@alice.fifthhorseman.net> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: wireguard-bounces@lists.zx2c4.com Sender: "WireGuard" List-Id: netdev.vger.kernel.org > On 8 Dec 2016, at 05:34, Daniel Kahn Gillmor wrote: > > On Wed 2016-12-07 19:30:34 -0500, Hannes Frederic Sowa wrote: >> Your custom protocol should be designed in a way you get an aligned ip >> header. Most protocols of the IETF follow this mantra and it is always >> possible to e.g. pad options so you end up on aligned boundaries for the >> next header. > > fwiw, i'm not convinced that "most protocols of the IETF follow this > mantra". we've had multiple discussions in different protocol groups > about shaving or bloating by a few bytes here or there in different > protocols, and i don't think anyone has brought up memory alignment as > an argument in any of the discussions i've followed. > If the trade-off is between 1 padding byte and 2 byte alignment versus 3 padding bytes and 4 byte alignment I would definitely opt for 3 padding bytes. I know how that waste feels like to a protocol designer, but I think it is worth it. Maybe the padding/reserved will be useful some day for an additional feature. I remember alignment being discussed and taken very seriously in 6man a couple of times. Often, though, protocol designers did align without much discussion. Implementing unaligned protocols is a pain I've experienced first hand.