Willy Tarreau <w@xxxxxx> writes: > Hi Jason, > > On Thu, Dec 08, 2016 at 11:20:04PM +0100, Jason A. Donenfeld wrote: >> Hi David, >> >> On Thu, Dec 8, 2016 at 1:37 AM, David Miller <davem@xxxxxxxxxxxxx> wrote: >> > You really have to land the IP header on a proper 4 byte boundary. >> > >> > I would suggest pushing 3 dummy garbage bytes of padding at the front >> > or the end of your header. >> >> Are you sure 3 bytes to get 4 byte alignment is really the best? > > It's always the best. However there's another option which should be > considered : maybe it's difficult but not impossible to move some bits > from the current protocol to remove one byte. That's not always easy, > and sometimes you cannot do it just for one bit. However after you run > through this exercise, if you notice there's really no way to shave > this extra byte, you'll realize there's no room left for future > extensions and you'll more easily accept to add 3 empty bytes for > this, typically protocol version, tags, qos or flagss that you'll be > happy to rely on for future versions of your protocol. Always include some way of extending the protocol in the future. A single bit is often enough. Require a value of zero initially, then if you ever want to change anything, setting it to one can indicate whatever you want, including a complete redesign of the header. Alternatively, a one-bit field can indicate the presence of an extended header yet to be defined. Then old software can still make sense of the basic header. -- Måns Rullgård
