Willy Tarreau [off-list ref] writes:

Hi Jason,

On Thu, Dec 08, 2016 at 11:20:04PM +0100, Jason A. Donenfeld wrote:

quoted

Hi David,

On Thu, Dec 8, 2016 at 1:37 AM, David Miller [off-list ref] wrote:

quoted

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