Re: [tsvwg] RDMA Support by UDP FRAG Option

["C. M. Heard" <[e-mail]>]

On Sat, Jun 19, 2021 at 6:01 PM Tom Herbert wrote:

> On Sat, Jun 19, 2021 at 4:39 PM C. M. Heard wrote:
>
>> 2.) Assuming that the trailer checksum (OCS) is present, it is not clear
>> how offload for the inner packet checksum could be expected to work
>> properly without change. Remember, OCS does not cause the data in the
>> trailer to sum to zero. It causes it to sum to the ones-complement of the
>> trailer length.
>>
>> The modifications needed to handle point #2 are small -- specifically,
>> adding the trailer length to whatever would normally be preloaded into the
>> inner packet. The FRAG proposal
>> in draft-ietf-tsvwg-udp-options-13#section-5.5, which proposes to sandwich
>> the payload in between option headers, would make it a whole lot harder.
>>
>
> Protocol trailers are a fundamental problem. At this point it's not
> clear to me that there is a way to make them work correctly in all use
> cases with deployed HW.  Protocol headers, like those used per FRAG,
> shouldn't be a problem,


I believe that fear is unjustified. In every case analyzed in detail so far
-- including this one -- trailers involve only minor rework at most.  I
believe that I proved my case conclusively for the usual case (UDP used as
a straight transport, not as an encapsulation) in
https://mailarchive.ietf.org/arch/msg/tsvwg/RZULHOKRgrSYIvsI-5Hg7sNtSS8/.
Additionally, while an encapsulator that uses UDP options may need to be
modified in order to use hardware checksum offload, I think it's clear that
the modifications needed are bounded and small. Actually for the case of a
TCP inner packet the pseudo-header will be OK to use with a trailer that
has OCS included if instead of the actual RCP length the pseudo-header
includes TCP + trailer length. The driver still has to remember to copy
only UDP length - 8 bytes. Arguably, if it were written correctly in the
first place, it would be ready now.

The proposal in the -13 draft does have problems in this regard. I'll reply
to that thread in the next day or so with an explanation.

Thanks,

Mike Heard