Re: [tsvwg] RDMA Support by UDP FRAG Option

[Joseph Touch <[e-mail]>]

> On Jun 20, 2021, at 9:11 PM, Tom Herbert <[e-mail]> wrote:
> 
> 
> 
> On Sun, Jun 20, 2021, 8:58 PM Joseph Touch <[e-mail] <mailto:[e-mail]>> wrote:
> 
> 
>> On Jun 20, 2021, at 8:43 PM, Tom Herbert <[e-mail] <mailto:[e-mail]>> wrote:
>> 
>> 
>> 
>> On Sun, Jun 20, 2021, 8:04 PM Joseph Touch <[e-mail] <mailto:[e-mail]>> wrote:
>> 
>> 
>>> On Jun 20, 2021, at 6:24 PM, Tom Herbert <[e-mail] <mailto:[e-mail]>> wrote:
>> …
>> 
>> Length  is the length  in octets  of this user datagram  including  this
>> header  and the data.   (This  means  the minimum value of the length is
>> eight.)
>> 
>> Please stop repeating any claim to the contrary, except to note an error in an existing implementation (which should be corrected, not propagated).
>> 
>> I am describing how real implementation actually *works*.
> 
> I do not doubt that, but you *are* propagating an error.
> 
> That is your opinion.

It is explicit in the text cited above. 

> 
> 
>> There is no error because the correct checksum per RFC768 can be produced.
> 
> That logic is equivalent to “we do math in base 6” because you only ever tried to add 2+2=4. 
> 
> It’s both faulty as logic and incorrect.
> 
> No, it's not. The algorithms I described are provably correct. One's complement addition is commutative and associative so:
> 
> Sum (whole_packet_from_UDP_header) = Sum(UDP_packet) + Sum(Surplus_area)

Agreed, where the first one is “whole ***IP** packet from UDP header”. The UDP packet has a length and that’s not it.

> So if Sum(Surplus_area == 0)

That is not a requirement anywhere in UDP, but note that there are two zeroes in ones complement arithmetic.

> then Sum (whole_packet_from_UDP_header) = Sum(UDP_packet),

Sure. And if Sum(Surplus_area == 1, then Sum(IP payload) == Sum(UDP packet) + 1.

> and therefore rather an offload performs the computation over the just the UDP packet or includes the surplus area they get the same correct result.

Said more specifically, an offload that is implemented INCORRECTLY will still work.

Yes.

Again, we already support that when OCS is included for many versions of the draft now. We don’t support it when it isn’t, but that’s only allowed when UDP CS=0. These two are coupled for reasons other than making erroneous checksum work.

What’s your point?

Joe