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

Joe Touch <touch@ISI.EDU> Wed, 16 July 2008 21:13 UTC

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Date: Wed, 16 Jul 2008 14:13:25 -0700
From: Joe Touch <touch@ISI.EDU>
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Subject: Re: [tcpm] I-D Action:draft-ietf-tcpm-tcp-auth-opt-01.txt
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Adam Langley wrote:
> On Tue, Jul 15, 2008 at 5:36 PM, Adam Langley <agl@imperialviolet.org> wrote:
>> 2008/7/14  <Internet-Drafts@ietf.org>:
>>>        Title           : The TCP Authentication Option
>>>        Author(s)       : J. Touch, et al.
>>>        Filename        : draft-ietf-tcpm-tcp-auth-opt-01.txt
> 
> Another thought: the keyid should be at the beginning of the MAC. We
> want the options to be 4-byte aligned if at all possible and we want
> the MAC to also be 4 byte aligned because CPUs have an easier time the
> aligned data.

We can place the keyID wherever it's most useful; note that putting the 
KeyID at the front will probably make the MAC non-wordaligned.

> Currently (with the key ID at the end) the option, on the wire, looks like:
>   NOP OPT 15 <12 bytes of MAC> KeyID   (16 bytes, 96-bit MAC)
> or
>   NOP OPT 19 <16 bytes of MAC> KeyID   (20 bytes, 128-bit MAC)
> 
> Both MACs are unaligned. Moving the KeyID to the beginning means that
> the MACs would be 4-byte aligned:
>   NOP OPT 15 KeyID <12 bytes of MAC> etc

This presumes that we misalign the option to start, using the NOP. I 
would prefer to assume that the option starts word-aligned. I don't see 
a way to do that, though, without wasting another byte in the option. 
The goal is currently to assume a half-word aligned start, and let the 
next option decide whether to pad or not. At that point, it's better to 
leave the MAC half-word aligned too.

I.e., there are various concerns here, and a solution isn't very clear. 
Thoughts from anyone else on this?

Joe

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