On Sat, Dec 10, 2016 at 11:18:14PM +0100, Dan Lüdtke wrote: > > > On 8 Dec 2016, at 05:34, Daniel Kahn Gillmor <d...@fifthhorseman.net> 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.
Note, if you do do this (hint, I think it is a good idea), require that these reserved/pad fields always set to 0 for now, so that no one puts garbage in them and then if you later want to use them, it will be a mess. thanks, greg k-h
