Re: [tsvwg] Fwd: New Version Notification for draft-touch-tsvwg-udp-options-05.txt

Joe Touch <touch@isi.edu> Tue, 28 February 2017 22:20 UTC

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From: Joe Touch <touch@isi.edu>
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Date: Tue, 28 Feb 2017 14:20:07 -0800
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Subject: Re: [tsvwg] Fwd: New Version Notification for draft-touch-tsvwg-udp-options-05.txt
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Hi, Derek,


On 2/28/2017 1:36 PM, Derek Fawcus wrote:
> On Tue, Feb 28, 2017 at 01:16:07p.m. -0800, Joe Touch wrote:
>>> Sorry,  I've lost the thread here.
>>>
>>> I was not suggesting altering how the normal UDP checksum works.
>>>
>>> I was mainly driving at how in implementing this in a host stack I'd
>>> like to verify the options.  Namely that in the absense of the Lite
>>> option,  it would allow one to use the existing checksum validation
>>> routine to run across the whole of the surplus area, and if it summed
>>> to the usual (1's complement) zero,  it is good.
>> I'm not sure I see that.
>> The existing checksum runs over the IP pseudoheader, the UDP header, and
>> the UDP body.
>>
>> OCS runs over just the option area.
>>
>> Even in the absence of LITE, the UDP checksum wouldn't cover the option
>> area.
> We're at cross purposes here...
>
> I'm not suggesting altering the UDP checksum,  I'm referring to replacing
> (discarding) the OCS option.
>
> So in section 4, figure 3 where you describe the surplus area, which is
> essentially an array of bytes used for options.  I'm proposing instead to
> give it a structure:
>
>     +------+-------+
>     |   checksum   |
>     +------+-------+
>     ... options ...
>     +------+-------+
>
> Where options is zero or more options as you describe (omitting OCS).
We discussed that before. The issue is whether OCS is optional or not.
If it isn't, yes - it can come first as you note.

> Where checksum is the usual checksum algorith over the whole of the
> surplus area.  i.e. such that when the receiver sums the whole of the
> area checksum through to the end of options, it should sum to 1s-complement
> zero.  (taking in to consideration the length of options being odd).
>
> Then for your LITE header scheme,  still require that the LITE header
> be the first option,  and rather than swap 4 bytes (the LITE header)
> as you describe,  instead swap 6 bytes (the checksum followed by the
> LITE header).  Hence why I was the referring to partial sums.

I see now. Yes, that's another possibility. It depends on what the WG
wants in terms of whether OCS is optional or not.

Joe