Re: [tsvwg] New Version Notification for draft-herbert-udp-space-hdr-01.txt

[Joe Touch <[e-mail]>]

On 2019-07-10 15:17, Tom Herbert wrote:

> On Wed, Jul 10, 2019 at 12:24 PM Joe Touch <[e-mail]> wrote: 
> 
>> On 2019-07-10 11:36, Tom Herbert wrote:
>> 
>> ...
>> The UDP surplus header provides alignment,
>> 
>> UDP options already has NOPs for that.
> The NOPs align the options to the start of surplus. If the surplus
> space is unaligned to begin with then there's no point to using NOPs
> to align individual options.

That's already supported in the UDP options. They can start with NOPs
too. 

>> integrity check,
>> 
>> UDP options already has a OCS for that.
> 
> A one byte optional checksum is next to useless as an integrity check.
> This has already been brought up in reference to OCS.

It was changed to 2 bytes before the last IETF, between v5 and v6 (we're
on v7; the doc table does need to be updated, but the text is accurate
and no longer talks about folding it over). 

>> disambiguation for other uses of surplus space,
>> 
>> This isn't needed. It won't support existing uses (not that any are known) because they won't follow the format.  UDP options already allows for experimental uses and other standards-track extensions through codepoint assignment.
> 
> If the checksum fails then surplus space isn't processed. So,
> probability of misinterpretation of some random use of surplus space
> is < 0.0015%. 
> 
>> a means to extend the UDP header,
>> 
>> Actually, it hinders UDP options by limiting them to 1020 bytes, as with all "other uses".
> That is intentional. Allowing unlimited options is a path towards DOS attack.
> 
>> friendliness with HW and SW implentations,
>> 
>> It has no benefit vs. the existing approach in this regard.
> The checksum ensures that the surplus space sums to zero so that the
> UDP checksum computation yields the same result rather it covers the
> UDP datagram only or the UDP datagram and surplus space. If we don't
> have this property, we will lose checksum offload which is important
> for UDP performance. Along similar lines, a per packet checksum is
> much better than the fragmentation checksum since we can offload the
> former and not the latter.
> 
> Tom