Re: [tcpm] I-D Action:draft-ietf-tcpm-tcp-auth-opt-01.txt

["Adam Langley" <[e-mail]>]

On Wed, Jul 16, 2008 at 3:38 PM, Joe Touch <[e-mail]> wrote:
> Both of these seem strange. They end up making the end of the option align,
> but disturb the front of the option. Why don't they pad at the end?

So here's my hypothesis (and it's only a guess really):

It's good to write in 32-bit words for speed reasons. Also, some
architectures make writing non-aligned 32-bit words hard. Now,
consider the SACK and timestamp options, you want to write a 32-bit
header followed by a payload (2 32-bit words for timestamp and 2*n
words for SACK). So you want the payload to be 32-bit aligned for both
writing and reading. So you end up with:
  uint32_t *ptr;

  *ptr++ = htonl((NOP << 24) | (NOP << 16) | (TCP_OPT_TS << 8) | length)
  *ptr++ = htonl(timestamp_1)
  *ptr++ = htonl(timestamp_2)

and, from the timestamp and SACK options all the others keep the same
style of padding with NOPs before the option.


AGL

-- 
Adam Langley [e-mail] http://www.imperialviolet.org
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