linux-kernel - Re: Misalignment, MIPS, and ip

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Date:   Sun, 11 Dec 2016 10:47:43 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Willy Tarreau <w@....eu>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>, linux-mips@...ux-mips.org,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>
Subject: Re: Misalignment, MIPS, and ip_hdr(skb)->version

Willy Tarreau <w@....eu> 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@...emloft.net> 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